# drafts

name: draft-khatri-sipcore-call-transfer-fail-response
rev: 05
title: A SIP Response Code (497) for Call Transfer Failure
time: 2024-01-25T08:14:01Z
group: none
group_uri: /api/v1/group/group/1027/
expires: 2024-01-25T17:05:27Z
states: ["/api/v1/doc/state/2/", "/api/v1/doc/state/150/"]
categories: []
tags: []
fetched_at: 2026-03-04T00:00:06.348342+00:00
source: ietf
abstract:
  This document defines the 497 (Call Transfer Failure) SIP
     response code, allowing Call Pull and Call Push parties to
     indicate that the operation was rejected. Optional warning codes
     are defined to carry granular information. SIP entities may use
     this information to adjust how subsequent calls can be handled
     intelligently.

full_text:
  Network Working Group                           Shrawan Kumar Khatri
  Internet-Draft                                          Vikram Singh                                                       
                                                      Cherng-Shung Hsu 
                                                          
  Intended status: Standard Track                Qualcomm Incorporated
  Expires: January 2024                                  July 22, 2023


            A SIP Response Code (497) for Call Transfer Failure
          draft-khatri-sipcore-call-transfer-fail-response-05.txt


  Status of this Memo

     This Internet-Draft is submitted in full conformance with the
     provisions of BCP 78 and BCP 79.

     Internet-Drafts are working documents of the Internet Engineering
     Task Force (IETF), its areas, and its working groups.  Note that
     other groups may also distribute working documents as Internet-
     Drafts.

     Internet-Drafts are draft documents valid for a maximum of six
     months and may be updated, replaced, or obsoleted by other
     documents at any time.  It is inappropriate to use Internet-
     Drafts as reference material or to cite them other than as "work
     in progress."

     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.txt

     The list of Internet-Draft Shadow Directories can be accessed at
     http://www.ietf.org/shadow.html

     This Internet-Draft will expire on January 22, 2024.

  Copyright Notice

     Copyright (c) 2023 IETF Trust and the persons identified as the
     document authors. All rights reserved.

     This document is subject to BCP 78 and the IETF Trust's Legal
     Provisions Relating to IETF Documents
     (http://trustee.ietf.org/license-info) in effect on the date of
     publication of this document. Please review these documents
     carefully, as they describe your rights and restrictions with
     respect to this document.

  Abstract



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     This document defines the 497 (Call Transfer Failure) SIP
     response code, allowing Call Pull and Call Push parties to
     indicate that the operation was rejected. Optional warning codes
     are defined to carry granular information. SIP entities may use
     this information to adjust how subsequent calls can be handled
     intelligently.

  Table of Contents


     1. Introduction................................................. 2
     2. Normative Language........................................... 4
     3. Motivation................................................... 4
     4. Theory of Operation.......................................... 5
     5. IANA Considerations.......................................... 8
        5.1. SIP Response Code....................................... 8
        5.2. Warning codes........................................... 8
     6. Security Considerations...................................... 9
     7. References................................................... 9
        7.1. Normative References.................................... 9
     8. Acknowledgments............................................. 10



  1. Introduction

    Packet switch calls for voice, video and text applications using
    IP Multimedia Subsystems (IMS) are anchored in the IMS core
    network. The signaling plane and media plane of IMS calls
    established on one device can be pushed ("Call Push") to another
    device. Similarly, IMS calls established on one device can be
    pulled ("Call Pull") by another device using SIP signaling.

    The call status during the SIP transaction can be conveyed through
    SIP response codes. RFC 3261 has defined many SIP response codes.
    The IMS core network MAY define a policy to allow or reject the
    Call Pull or Call Push operation. There are numerous reasons why
    the Call Pull or Call Push operation can fail. In case of call
    transfer failure due to policy defined by the IMS core network,
    the IMS core network MAY want to convey the failure to the user
    agent (UA) through a response code. Based on the response code,
    the UA MAY determine whether and how to establish a subsequent
    call.



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    The existing response codes in RFC 3261 are not sufficient to
    convey the information about the call transfer failure to the UA.
    Overloading an existing response code could lead to unnecessary
    subsequent signaling which could burden the IMS core network. To
    avoid possible signaling overload in the IMS core network and to
    accurately convey the call transfer failure to the UA, a new
    response code along with associated optional warning codes to be
    included in a Warning header field are proposed in this RFC.

      The following call flows illustrate the successful Call Pull.

     UA                                                  Core Network
      |                                                         |
      |                                                         |
      |     1. INVITE:                                          |
      |        Contact-Header: +g.3gpp.iut-focus                |
      |-------------------------------------------------------->|
      |                                                         |
      |     2. 200 OK                                           |
      |<--------------------------------------------------------|
      |                                                         |
            Figure 1: Call Pull Success

      1. The UA sends an INVITE to Pull the call from another device.
         Feature Tag: +g.3gpp.iut-focus [RFC6809] is added in the
         Contact Header field.
      2. The call transfer request satisfied. The Core Network sends
         200 OK.

      The following call flows illustrate the successful Call Push.

      UA                                                 Core Network
      |                                                         |
      |     1. REFER:                                           |
      |        Contact-Header: +g.3gpp.iut-focus                |
      |-------------------------------------------------------->|
      |                                                         |
      |     2. 202 Accepted                                     |
      |<--------------------------------------------------------|


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      |                                                         |
            Figure 2: Call Push Success

      1. The UA sends a REFER to Push the call to another device.
         Feature Tag: +g.3gpp.iut-focus is added in the Contact Header
         field.
      2. The call transfer request satisfied. The Core Network sends
         202 Accepted.


  2. Normative Language

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
    NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
    "MAY", and "OPTIONAL" in this document are to be interpreted as
    described in BCP 14 [RFC2119] [RFC8174] when, and only when, they
    appear in all capitals, as shown here.

  3. Motivation

    Seamless transfer of the media plane of an on-going voice call
    between devices is a legacy behavior. The signaling plane in the
    legacy behavior resides on the originating device.

    If the two devices can run IMS over SIP signaling, the signaling
    plane and media plane can be transferred between these devices
    with minimal media flow interruption. The control plane and media
    plane transfer procedures are beyond the scope of this RFC.

    There are various reasons why an on-going call cannot be
    transferred between the devices. Some of these reasons are policy
    driven, for instance: the call to be transferred is in the circuit
    switched (CS) domain and the operator's policy does not allow
    transfer of a CS call, the call is an emergency call and the
    operator's policy does not allow transfer of an emergency call,
    the call is a mobile-terminated call and the operator's policy
    does not allow transfer of a mobile-terminated call, or the call
    is a video call and the operator's policy does not allow transfer
    of a video call.

    The UA initiating the call transfer procedure will be notified of
    any failure through a SIP response code. However the existing SIP
    response codes are not suitable to adequately convey the
    information regarding why the call transfer request is not
    accepted by the network: handling of these existing response codes


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    has already been implemented by various devices, with an
    associated device behavior defined for a specific purpose not
    related to call transfer. For instance, upon receiving some of
    these response code, such as 403 (Forbidden), the device MAY
    initiate IMS re-registration procedure, which is not needed in
    case of Call Pull/Call Push failure and will result in unnecessary
    SIP signaling.

    Consequently, to accurately convey the information about the call
    transfer failure to the UA, a new response code is required along
    with an optional warning code in a Warning header field to convey
    the exact reason why the call could not be transferred, so that
    the UA can determine the subsequent steps (e.g. call termination)
    and optionally provide an indication of the reason for the failure
    to the user.

    Backward compatibility is maintained in both the UE and Network
    side. The Network component that has not implemented this RFC
    shall use the existing response code. The UE that has not
    implemented this RFC shall handle this new response code as an
    unknown response code.

  4. Theory of Operation

    Response code:

    A new SIP response code 497 is defined.

    Description: Call Transfer Failure

    The server understood the call transfer request but is refusing
    the service. The SIP response with SIP response code 497 MAY
    include a Warning header field [RFC3261] with a warning code set
    to one of the values listed below and the associated warning text
    conveying granular information about the reason for the call
    transfer failure, so as to enable the UA to develop extra logic
    for subsequent call transfer procedure.

    Warning header:

    An optional Warning header will carry more granular failure
    information as follows:


    380. Inactive state
    381. Local Receive-only state


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    382. Local Transmit-only state
    383. Remote Receive-only state
    384. Remote Transmit-only state
    385. Hold state
    386. Mortal state
    387. Conference call
    388. Emergency call
    389. Video call
    390. Real Time Text call
    391. Circuit Switch call
    392. Non existing call
    393. Single Radio Voice Call Continuity in progress

    Feature-tag:

       media feature-tag:+g.3gpp.iut-focus [3GPP TS 24.337]

       In Call Pull, INVITE method is used and media feature tag
       "+g.3gpp.iut-focus" is included in the Contact Header field.

       In Call Push, REFER method is used and media feature tag
       "+g.3gpp.iut-focus" is included in the Contact Header field.

    Call Transfer failure

       If call transfer using INVITE or REFER method fails and
       response code 497 is supported by the network, the SIP response
       SHALL include SIP response code 497.

       If a call transfer fails and response code 497 is not supported
       by the network, the SIP response code should be chosen from
       existing
       SIP response codes defined in RFC 3261

       Example:

       Warning: 388 proxy.example.com "Call is an emergency call"





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       The following call flow illustrates the usage of SIP response
       code 497 and the associated warning codes:

     UA                                                  Core Network
      |                                                         |
      |                                                         |
      |     1. INVITE:                                          |
      |        Contact-Header: +g.3gpp.iut-focus                |
      |-------------------------------------------------------->|
      |                                                         |
      |     2. 497 Call Transfer Failure                        |
      |        Warning: 388 "Call is an emergency call"         |
      |<--------------------------------------------------------|
      |                                                         |
            Figure 3: Usage of SIP response code 497 Call Pull

      1. The UA sends an INVITE to Pull the call from another device.
         Feature Tag: +g.3gpp.iut-focus is added in the Contact Header
         field.
      2. The call transfer request cannot be satisfied due to the call
         requested to be transferred being an emergency call. Since
         the core network supports SIP response code 497, the core
         network sends a 497 Call Transfer Failure with a Warning
         header field set to: 388 "Call is an emergency call"



      UA                                              Core Network
      |                                                         |
      |     1. REFER:                                           |
      |        Contact-Header: +g.3gpp.iut-focus                |
      |-------------------------------------------------------->|
      |                                                         |
      |     2. 497 Call Transfer Failure                        |
      |        Warning: 388 "Call is an emergency call"         |
      |<--------------------------------------------------------|
      |                                                         |
            Figure 4: Usage of SIP response code 497 Call Push




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  Internet-Draft          Call Transfer Failure          July 2023

      1. The UA sends a REFER to Push the call to another device.
         Feature Tag: +g.3gpp.iut-focus is added in the Contact Header
         field.
      2. The call transfer request cannot be satisfied due to the call
         requested to be transferred being an emergency call. Since
         the core network supports SIP response code 497, the core
         network sends a 497 Call Transfer Failure with a Warning
         header field set to: 388 "Call is an emergency call"


  5. IANA Considerations

  5.1. SIP Response Code

    This document registers a new SIP response code. This response
    code is defined by the following information, which has been added
    to the "Response Codes" sub-registry under the "Session Initiation
    Protocol (SIP) Parameters" registry
    <http://www.iana.org/assignments/sip-parameters>.



    Response Code: 497

    Description: CALL TRANSFER FAILURE

    The server understood the request to transfer the call but is
    refusing the service. This response MAY include a Warning header
    field [RFC3261] with a warning code set to one of the values
    listed in section 5.2 and the associated warning text conveying
    granular information about the reason for the call transfer
    failure.

    Reference: [RFCxxxx]



  5.2. Warning codes

    This document registers new warning codes. These warning codes are
    defined by the following information, which has been added to the
    "Warning Codes (warn-codes)" sub-registry under the "Session
    Initiation Protocol (SIP) Parameters" registry
    <http://www.iana.org/assignments/sip-parameters>.

    380. Inactive state


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  Internet-Draft          Call Transfer Failure         July 2023

    381. Local Receive-only state

    382. Local Transmit-only state

    383. Remote Receive-only state

    384. Remote Transmit-only state

    385. Hold state

    386. Mortal state

    387. Conference call

    388. Emergency call

    389. Video call

    390. Real Time Text call

    391. Circuit Switch call

    392. Non existing call

    393. Single Radio Voice Call Continuity in progress



  6. Security Considerations

     The general discussion in [RFC3261] applies.



  7. References

  7.1. Normative References

     [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

     [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G.,
               Johnston, A., Peterson, J., Sparks, R., Handley, M.,
               and E. Schooler, "SIP: Session Initiation Protocol",
               RFC 3261, DOI 10.17487/RFC3261, June 2002,
               <http://www.rfc-editor.org/info/rfc3261>.




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     [RFC5407] Hasebe, M., Koshiko, J., Suzuki, Y., Yoshikawa, T.,
               Kyzivat, P., "Example Call Flows of Race Conditions in
               the Session Initiation Protocol (SIP)", BCP 147, RFC
               5407, DOI 10.17487/RFC5407, December 2008,
               <http://www.rfc-editor.org/info/rfc5407>.

     [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
               2119 Key Words", BCP 14, FC 8174, DOI 10.17487/RFC8174,
               May 2017, <http://www.rfc-editor.org/info/rfc8174>.

     [RFC6809] Holmberg, C., Sedlacek, I., and H. Kaplan, "Mechanism
               to Indicate Support of Features and Capabilities in the
               Session Initiation Protocol (SIP)", RFC 6809, DOI
               10.17487/RFC6809, November 2012, <https://www.rfc-
               editor.org/info/rfc6809>.

     [3GPP TS 24.337] "Universal Mobile Telecommunications System
               (UMTS); LTE; IP Multimedia (IM) Core Network
               (CN)subsystem IP Multimedia Subsystem (IMS) inter-UE
               transfer

  8. Acknowledgments

    Lena Chaponniere, Qualcomm Inc. and Waqar Zia, Qualcomm Inc. for
    the technical guidance.

























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  Authors' Addresses

     Shrawan Khatri
     5775 Morehouse Drive
     San Diego, CA 92121
     United States of America
     Email: skhatri@qualcomm.com

     Vikram Singh
     5775 Morehouse Drive
     San Diego, CA 92121
     United States of America
     viksingh@qualcomm.com

     Cherng-Shung Hsu
     5775 Morehouse Drive
     San Diego, CA 92121
     United States of America
     simonh@qualcomm.com




















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name: draft-templin-intarea-aero
rev: 67
title: Automatic Extended Route Optimization (AERO)
time: 2024-02-16T18:40:04Z
group: none
group_uri: /api/v1/group/group/1027/
expires: 2024-08-19T18:40:04Z
states: ["/api/v1/doc/state/4/", "/api/v1/doc/state/150/"]
categories: []
tags: []
fetched_at: 2026-03-04T00:00:06.352370+00:00
source: ietf
abstract:
  This document specifies an Automatic Extended Route Optimization
     (AERO) service for IP internetworking over Overlay Multilink Network
     (OMNI) interfaces.  AERO/OMNI use an IPv6 unique-local address format
     for IPv6 Neighbor Discovery (IPv6 ND) messaging over the OMNI virtual
     link.  Router discovery and neighbor coordination are employed for
     network admission and to manage the OMNI link forwarding and routing
     systems.  Secure multilink path selection, multinet traversal,
     mobility management, multicast forwarding, multihop operation and
     route optimization are naturally supported through dynamic neighbor
     cache updates.  AERO is a widely-applicable mobile internetworking
     service especially well-suited for air/land/sea/space mobility
     applications including aviation, intelligent transportation systems,
     mobile end user devices, space exploration and many others.

full_text:
  Network Working Group                                 F. L. Templin, Ed.
  Internet-Draft                              Boeing Research & Technology
  Intended status: Standards Track                        16 February 2024
  Expires: 19 August 2024


                Automatic Extended Route Optimization (AERO)
                       draft-templin-intarea-aero-67

  Abstract

     This document specifies an Automatic Extended Route Optimization
     (AERO) service for IP internetworking over Overlay Multilink Network
     (OMNI) interfaces.  AERO/OMNI use an IPv6 unique-local address format
     for IPv6 Neighbor Discovery (IPv6 ND) messaging over the OMNI virtual
     link.  Router discovery and neighbor coordination are employed for
     network admission and to manage the OMNI link forwarding and routing
     systems.  Secure multilink path selection, multinet traversal,
     mobility management, multicast forwarding, multihop operation and
     route optimization are naturally supported through dynamic neighbor
     cache updates.  AERO is a widely-applicable mobile internetworking
     service especially well-suited for air/land/sea/space mobility
     applications including aviation, intelligent transportation systems,
     mobile end user devices, space exploration and many others.

  Status of This Memo

     This Internet-Draft is submitted in full conformance with the
     provisions of BCP 78 and BCP 79.

     Internet-Drafts are working documents of the Internet Engineering
     Task Force (IETF).  Note that other groups may also distribute
     working documents as Internet-Drafts.  The list of current Internet-
     Drafts is at https://datatracker.ietf.org/drafts/current/.

     Internet-Drafts are draft documents valid for a maximum of six months
     and may be updated, replaced, or obsoleted by other documents at any
     time.  It is inappropriate to use Internet-Drafts as reference
     material or to cite them other than as "work in progress."

     This Internet-Draft will expire on 19 August 2024.

  Copyright Notice

     Copyright (c) 2024 IETF Trust and the persons identified as the
     document authors.  All rights reserved.





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  Internet-Draft                    AERO                     February 2024


     This document is subject to BCP 78 and the IETF Trust's Legal
     Provisions Relating to IETF Documents (https://trustee.ietf.org/
     license-info) in effect on the date of publication of this document.
     Please review these documents carefully, as they describe your rights
     and restrictions with respect to this document.  Code Components
     extracted from this document must include Revised BSD License text as
     described in Section 4.e of the Trust Legal Provisions and are
     provided without warranty as described in the Revised BSD License.

  Table of Contents

     1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  17
     4.  Automatic Extended Route Optimization (AERO)  . . . . . . . .  17
       4.1.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .  17
       4.2.  The AERO Service over OMNI Links  . . . . . . . . . . . .  19
         4.2.1.  AERO/OMNI Reference Model . . . . . . . . . . . . . .  19
         4.2.2.  Addressing and Node Identification  . . . . . . . . .  23
         4.2.3.  AERO Routing System . . . . . . . . . . . . . . . . .  24
         4.2.4.  Segment Routing Topologies (SRTs) . . . . . . . . . .  26
         4.2.5.  Segment Routing For OMNI Link Selection . . . . . . .  27
       4.3.  OMNI Interface Characteristics  . . . . . . . . . . . . .  27
       4.4.  OMNI Interface Initialization . . . . . . . . . . . . . .  30
         4.4.1.  AERO Proxy/Server and Relay Behavior  . . . . . . . .  30
         4.4.2.  AERO Client Behavior  . . . . . . . . . . . . . . . .  31
         4.4.3.  AERO Host Behavior  . . . . . . . . . . . . . . . . .  31
         4.4.4.  AERO Gateway Behavior . . . . . . . . . . . . . . . .  32
       4.5.  OMNI Interface Neighbor Cache Maintenance . . . . . . . .  32
         4.5.1.  OMNI ND Messages  . . . . . . . . . . . . . . . . . .  35
         4.5.2.  OMNI Neighbor Advertisement Message Flags . . . . . .  37
         4.5.3.  OMNI Neighbor Window Synchronization  . . . . . . . .  38
       4.6.  OMNI Interface Encapsulation and Fragmentation  . . . . .  38
       4.7.  OMNI Interface Decapsulation  . . . . . . . . . . . . . .  41
       4.8.  OMNI Interface Data Origin Authentication . . . . . . . .  42
       4.9.  OMNI Interface MTU  . . . . . . . . . . . . . . . . . . .  42
       4.10. OMNI Interface Forwarding Algorithm . . . . . . . . . . .  43
         4.10.1.  Host Forwarding Algorithm  . . . . . . . . . . . . .  45
         4.10.2.  Client Forwarding Algorithm  . . . . . . . . . . . .  45
         4.10.3.  Proxy/Server and Relay Forwarding Algorithm  . . . .  47
         4.10.4.  Gateway Forwarding Algorithm . . . . . . . . . . . .  49
       4.11. OMNI Interface Error Handling . . . . . . . . . . . . . .  51
       4.12. AERO Mobility Service Coordination  . . . . . . . . . . .  54
         4.12.1.  AERO Service Model . . . . . . . . . . . . . . . . .  54
         4.12.2.  AERO Host and Client Behavior  . . . . . . . . . . .  55
         4.12.3.  AERO Proxy/Server Behavior . . . . . . . . . . . . .  56
       4.13. AERO Address Resolution, Multilink Forwarding and Route
              Optimization . . . . . . . . . . . . . . . . . . . . . .  63



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         4.13.1.  Multilink Address Resolution . . . . . . . . . . . .  65
         4.13.2.  Multilink Forwarding . . . . . . . . . . . . . . . .  70
         4.13.3.  Mobile Ad-hoc Network (MANET) Forwarding . . . . . .  84
         4.13.4.  Client/Gateway Route Optimization  . . . . . . . . .  87
         4.13.5.  Client/Client Route Optimization . . . . . . . . . .  89
         4.13.6.  Intra-ANET/ENET Route Optimization for AERO Peers  .  91
       4.14. Neighbor Unreachability Detection (NUD) . . . . . . . . .  91
       4.15. Mobility Management and Quality of Service (QoS)  . . . .  93
         4.15.1.  Mobility Update Messaging  . . . . . . . . . . . . .  93
         4.15.2.  Announcing Link-Layer Information Changes  . . . . .  94
         4.15.3.  Bringing New Links Into Service  . . . . . . . . . .  95
         4.15.4.  Deactivating Existing Links  . . . . . . . . . . . .  95
         4.15.5.  Moving Between Proxy/Servers . . . . . . . . . . . .  95
       4.16. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  97
         4.16.1.  Source-Specific Multicast (SSM)  . . . . . . . . . .  97
         4.16.2.  Any-Source Multicast (ASM) . . . . . . . . . . . . .  98
         4.16.3.  Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . .  99
       4.17. Operation over Multiple OMNI Links  . . . . . . . . . . .  99
       4.18. DNS Considerations  . . . . . . . . . . . . . . . . . . . 100
       4.19. Transition/Coexistence Considerations . . . . . . . . . . 100
       4.20. Proxy/Server-Gateway Bidirectional Forwarding
              Detection  . . . . . . . . . . . . . . . . . . . . . . . 101
       4.21. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . . 101
     5.  Implementation Status . . . . . . . . . . . . . . . . . . . . 102
     6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 102
     7.  Security Considerations . . . . . . . . . . . . . . . . . . . 102
     8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 105
     9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 107
       9.1.  Normative References  . . . . . . . . . . . . . . . . . . 107
       9.2.  Informative References  . . . . . . . . . . . . . . . . . 108
     Appendix A.  Non-Normative Considerations . . . . . . . . . . . . 115
       A.1.  Implementation Strategies for Route Optimization  . . . . 115
       A.2.  Implicit Mobility Management  . . . . . . . . . . . . . . 115
       A.3.  Direct Underlying Interfaces  . . . . . . . . . . . . . . 116
       A.4.  AERO Critical Infrastructure Considerations . . . . . . . 116
       A.5.  AERO Server Failure Implications  . . . . . . . . . . . . 117
       A.6.  AERO Client / Server Architecture . . . . . . . . . . . . 118
     Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . . 120
     Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 120

  1.  Introduction

     Automatic Extended Route Optimization (AERO) fulfills the
     requirements of Distributed Mobility Management (DMM) [RFC7333] and
     route optimization [RFC5522] for air/land/sea/space mobility
     applications including aeronautical networking intelligent
     transportation systems, enterprise mobile device users space
     exploration and many others.  AERO is a secure internetworking and



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     mobility management service that employs the Overlay Multilink
     Network Interface (OMNI) [I-D.templin-intarea-omni] Non-Broadcast,
     Multiple Access (NBMA) virtual link model.  The OMNI link is an
     adaptation layer virtual overlay manifested by IPv6 encapsulation
     over a network-of-networks concatenation of underlay Internetworks.
     Nodes on the link can exchange original IP packets or parcels
     [I-D.templin-6man-parcels2][I-D.templin-intarea-parcels2] as single-
     hop neighbors - both IP protocol versions (IPv4 and IPv6) are
     supported.  The OMNI Adaptation Layer (OAL) supports multilink
     operation for increased reliability and path optimization while
     providing fragmentation and reassembly services to support improved
     performance and Maximum Transmission Unit (MTU) diversity.  This
     specification provides a mobility service architecture companion to
     the OMNI specification.

     The AERO service connects Hosts and Clients as OMNI link end systems
     via Proxy/Servers and Relays as intermediate systems as necessary;
     AERO further employs Gateways that interconnect diverse Internetworks
     as OMNI link segments through OAL forwarding at a layer below IP.
     Each node's OMNI interface uses an IPv6 unique-local address format
     that supports operation of the IPv6 Neighbor Discovery (IPv6 ND)
     protocol [RFC4861].  A Client's OMNI interface can be configured over
     multiple underlay interfaces, and therefore appears as a single
     interface with multiple link layer addresses.  Each link layer
     address is subject to change due to mobility and/or multilink
     fluctuations, and link layer address changes are signaled by ND
     messaging the same as for any IPv6 link.

     AERO provides a secure cloud-based service where mobile node Clients
     use Proxy/Servers acting as proxys and/or designated routers while
     fixed nodes may use any Relay on the link for efficient
     communications.  Fixed nodes forward original IP packets/parcels
     destined to other AERO nodes via the nearest Relay, which forwards
     them through the cloud.  Mobile node Clients discover shortest paths
     to OMNI link neighbors through AERO route optimization.  Both unicast
     and multicast communications are supported, and Clients may
     efficiently move between locations while maintaining continuous
     communications with correspondents using stable IP Addresses not
     subject to dynamic fluctuations.

     AERO Gateways peer with Proxy/Servers in a secured private BGP
     overlay routing instance to establish a Segment Routing Topology
     (SRT) virtual spanning tree over the underlay Internetworks of one or
     more disjoint administrative domains concatenated as a single unified
     OMNI link.  Each OMNI link instance is characterized by a set of
     Mobility Service Prefixes (MSPs) common to all mobile nodes.  Relays
     provide an optimal route from (fixed) correspondent nodes on underlay
     Internetworks to (mobile or fixed) nodes on the OMNI link.  From the



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     perspective of underlay Internetworks, each Relay appears as the
     source of a route to the MSP; hence uplink traffic to mobile nodes is
     naturally routed to the nearest Relay.

     AERO can be used with OMNI links that span private-use Internetworks
     and/or public Internetworks such as the global IPv4 and IPv6
     Internets.  In both cases, Clients may be located behind Network
     Address Translators (NATs) on the path to their associated Proxy/
     Servers and/or peers.  A means for robust traversal of NATs while
     avoiding "triangle routing" and critical infrastructure traffic
     concentration through a service known as route optimization is
     therefore provided.

     AERO assumes the use of PIM Sparse Mode in support of multicast
     communication.  In support of Source Specific Multicast (SSM) when a
     Mobile Node is the source, AERO route optimization ensures that a
     shortest-path multicast tree is established with provisions for
     mobility and multilink operation.  In all other multicast scenarios
     there are no AERO dependencies.

     AERO provides a secure aeronautical internetworking service for both
     manned and unmanned aircraft, where the aircraft is treated as a
     mobile node (MN) that can connect airborne Internet of Things (IoT)
     sub-networks.  AERO is also applicable to a wide variety of other use
     cases.  For example, it can be used to coordinate the links of mobile
     nodes (e.g., cellphones, tablets, laptop computers, etc.) that
     connect into a home enterprise network via public access networks
     with Virtual Private Network (VPN) or open Internetwork services
     enabled according to the appropriate security model.  AERO also
     supports terrestrial vehicular, urban air mobility and mobile
     pedestrian communication services for intelligent transportation
     systems [I-D.ietf-ipwave-vehicular-networking].  Other applicable use
     cases are also in scope.

     Along with OMNI, AERO provides secured optimal routing support for
     the "6 M's of Modern Internetworking", including:

     1.  Multilink - a mobile node's ability to coordinate multiple
         diverse underlay data links as a single logical unit (i.e., the
         OMNI interface) to achieve the required communications
         performance and reliability objectives.

     2.  Multinet - the ability to span the OMNI link over a segment
         routing topology with multiple diverse administrative domain
         network segments while maintaining seamless end-to-end
         communications between mobile Clients and correspondents such as
         air traffic controllers, fleet administrators, other mobile
         Clients, etc.



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     3.  Mobility - a mobile node's ability to change network points of
         attachment (e.g., moving between wireless base stations) which
         may result in an underlay interface address change, but without
         disruptions to ongoing communication sessions with peers over the
         OMNI link.

     4.  Multicast - the ability to send a single network transmission
         that reaches multiple nodes belonging to the same interest group,
         but without disturbing other nodes not subscribed to the interest
         group.

     5.  Multihop - a mobile node vehicle-to-vehicle relaying capability
         useful when multiple forwarding hops between vehicles may be
         necessary to "reach back" to an infrastructure access point
         connection to the OMNI link.

     6.  (Performance) Maximization - the ability to exchange large
         packets/parcels between peers without loss due to a link size
         restriction, and to adaptively adjust packet/parcel sizes to
         maintain the best performance profile for each independent
         traffic flow.

     The following numbered sections present the AERO specification.  The
     appendices at the end of the document are non-normative.

  2.  Terminology

     The terminology in the normative references applies; especially, the
     OMNI specification terminology [I-D.templin-intarea-omni] and the
     IPv6 Neighbor Discovery (IPv6 ND) [RFC4861] node variables, protocol
     constants and message types (including Router Solicitation (RS),
     Router Advertisement (RS), Neighbor Solicitation (NS), Neighbor
     Advertisement (NA), unsolicited NA (uNA) and Redirect) are cited
     extensively throughout.

     OMNI interfaces normally limit the size of their IPv6 ND control
     plane messages to the minimum IPv6 link MTU, but some messages may
     exceed this size if there are sufficient OMNI parameters and/or IP
     packet/parcel attachments.  These larger messages can still travel
     over secured underlying network control plane paths that include
     IPsec tunnels [RFC4301] and/or secured direct point-to-point links
     without loss due to a size restriction by engaging OMNI IPv6
     encapsulation/fragmentation as necessary up to a maximum size of
     65535 octets.

     Throughout the document, the simple terms "Host", "Client", "Proxy/
     Server", "Gateway" and "Relay" refer to "AERO/OMNI Host", "AERO/OMNI
     Client", "AERO/OMNI Proxy/Server", "AERO/OMNI Gateway" and "AERO/OMNI



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     Relay", respectively.  Capitalization is used to distinguish these
     terms from other common Internetworking uses in which they appear
     without capitalization, and implies that the node in question both
     configures an OMNI interface and engages the OMNI Adaptation Layer.

     The terms "All-Routers multicast", "All-Nodes multicast", "Solicited-
     Node multicast" and "Subnet-Router anycast" are defined in [RFC4291].

     The term "IP" refers generically to either Internet Protocol version
     (IPv4 [RFC0791] or IPv6 [RFC8200]) for specification elements that
     apply equally to both.

     The terms "application layer (L5 and higher)", "transport layer
     (L4)", "network layer (L3)", "(data) link layer (L2)" and "physical
     layer (L1)" are used consistently with common Internetworking
     terminology, with the understanding that reliable delivery protocol
     users of UDP are considered as transport layer elements.  The OMNI
     specification further defines an "adaptation layer" positioned below
     the network layer but above the link layer, which may include
     physical links and Internet- or higher-layer tunnels.  A (network)
     interface is a node's attachment to a link (via L2), and an OMNI
     interface is therefore a node's attachment to an OMNI link (via the
     adaptation layer).

     The terms "IP jumbogram", "advanced jumbo (AJ)" and "IP parcel" refer
     to special packet formats that enable a new link model for the
     Internet as discussed in [I-D.templin-6man-parcels2]
     [I-D.templin-intarea-parcels2].

     The following terms are defined within the scope of this document:

     IPv6 Neighbor Discovery (IPv6 ND)
        a control message service for coordinating neighbor relationships
        between nodes connected to a common link.  AERO uses the IPv6 ND
        messaging service specified in [RFC4861] in conjunction with the
        OMNI extensions specified in [I-D.templin-intarea-omni].

     IPv6 Prefix Delegation
        a networking service for delegating IPv6 prefixes to nodes on the
        link.  The nominal service is DHCPv6 [RFC8415], however alternate
        services (e.g., based on IPv6 ND messaging) are also in scope.  A
        minimal form of prefix delegation known as "prefix registration"
        can be used if the Client knows its prefix in advance and can
        represent it in the source address of an IPv6 ND message.

     L3
        The Network layer in the OSI network model.  Also known as "layer
        3", "IP layer", etc.



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     L2
        The Data Link layer in the OSI network model.  Also known as
        "layer 2", "link layer", "sub-IP layer", etc.

     Adaptation layer
        An encapsulation mid-layer that adapts L3 to a diverse collection
        of L2 underlay interfaces and their encapsulations.  (No layer
        number is assigned, since numbering was an artifact of the legacy
        reference model that need not carry forward in the modern
        architecture.)  The adaptation layer sees the network layer as
        "L3" and sees all link layer encapsulations as "L2
        encapsulations", which may include UDP, IP and true link layer
        (e.g., Ethernet, etc.) headers.

     Access Network (ANET)
        a connected network region (e.g., an aviation radio access
        network, satellite service provider network, cellular operator
        network, WiFi network, etc.) that joins Clients to the Mobility
        Service.  Physical and/or data link level security is assumed, and
        sometimes referred to as "protected spectrum".  Private enterprise
        networks and ground domain aviation service networks may provide
        multiple secured IP hops between the Client's point of connection
        and the nearest Proxy/Server.

     Internetwork (INET)
        a connected network region with a coherent IP addressing plan that
        provides transit forwarding services between ANETs and AERO/OMNI
        nodes that coordinate with the Mobility Service over unprotected
        media.  No physical and/or data link level security is assumed,
        therefore security must be applied by the network and/or higher
        layers.  The global public Internet itself is an example.

     End-user Network (ENET)
        a simple or complex "downstream" network tethered to a Client as a
        single logical unit that travels together.  The ENET could be as
        simple as a single link connecting a single Host, or as complex as
        a large network with many links, routers, bridges and end user
        devices.  The ENET provides an "upstream" link for arbitrarily
        many low-, medium- or high-end devices dependent on the Client for
        their upstream connectivity, i.e., as Internet of Things (IoT)
        entities.  The ENET can also support a recursively-descending
        chain of additional Clients such that the ENET of an upstream
        Client is seen as the ANET of a downstream Client.

     ANET/INET/ENET interface
        a node's attachment to a link in an ANET/INET/ENET.





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     underlay network/interface
        an ANET/INET/ENET network/interface over which an OMNI interface
        is configured.  The OMNI interface is seen as a network layer (L3)
        interface by the IP layer, and the OMNI adaptation layer sees the
        underlay interface as a data link layer (L2) interface.  The
        underlay interface either connects directly to the physical or
        virtual communications media or coordinates with another node that
        hosts the media.

     Mobile Ad-hoc NETwork (MANET)
        a connected network region that shares the same properties as an
        ANET except that links often have undetermined connectivity
        properties, physical and/or data link layer security cannot always
        be assumed and multihop forwarding between Clients acting as MANET
        routers may be necessary.  Proxy/Servers that connect the MANET to
        outside networks act as Clients on their MANET interfaces and act
        as ordinary Proxy/Servers on their ANET/INET interfaces, while
        Clients configure MANET interfaces and provide multihop forwarding
        services for other Clients as necessary.

     MANET Interface
        a node's underlay interface connection to a local network with
        indeterminant neighborhood properties over which multihop relaying
        may be necessary.  All MANET interfaces used by AERO/OMNI are IPv6
        interfaces and therefore must configure a Maximum Transmission
        Unit (MTU) at least as large as the IPv6 minimum MTU (1280 octets)
        even if lower-layer fragmentation is needed.

     OMNI link
        the same as defined in [I-D.templin-intarea-omni].  The OMNI link
        employs IPv6 encapsulation to traverse intermediate systems in a
        spanning tree over underlay network segments the same as a bridged
        campus LAN.  AERO nodes on the OMNI link appear as single-hop
        neighbors at the network layer even though they may be separated
        by many underlay network hops; AERO nodes can employ Segment
        Routing [RFC8402] to navigate between different OMNI links, and/or
        to cause packets/parcels to visit selected waypoints within the
        same OMNI link.

     OMNI Adaptation Layer (OAL)
        an OMNI interface sublayer service that encapsulates original IP
        packets/parcels admitted into the interface in an IPv6 header and/
        or subjects them to fragmentation and reassembly.  The OAL is also
        responsible for generating MTU-related control messages as
        necessary, and for providing addressing context for spanning
        multiple segments of an extended OMNI link.





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     OMNI Interface
        a node's attachment to an OMNI link.  Since OMNI interface
        addresses are managed for uniqueness, OMNI interfaces do not
        require Duplicate Address Detection (DAD) and therefore set the
        administrative variable 'DupAddrDetectTransmits' to zero
        [RFC4862].

     (network) partition
        frequently, underlay networks such as large corporate enterprise
        networks are sub-divided internally into separate isolated
        partitions (a technique also known as "network segmentation").
        Each partition is fully connected internally but disconnected from
        other partitions, and there is no requirement that separate
        partitions maintain consistent Internet Protocol and/or addressing
        plans.  (Each partition is seen as a separate OMNI link segment as
        discussed throughout this document.)

     (OMNI) L2 encapsulation
        the OMNI protocol encapsulation of OAL packets/fragments in an
        outer header or headers to form carrier packets that can be routed
        within the scope of the local ANET/INET/ENET underlay network
        partition.  Common L2 encapsulation combinations include UDP/IP/
        Ethernet, etc. using a port/protocol/type number for OMNI.

     L2 address (L2ADDR)
        an address that appears in the L2 encapsulation for an underlay
        interface and also in IPv6 ND message OMNI options.  L2ADDR can be
        either an IP address for IP encapsulations or an IEEE EUI address
        [EUI] for direct data link encapsulation.  (When UDP/IP
        encapsulation is used, the UDP port number is considered an
        ancillary extension of the IP L2ADDR.)

     original IP packet/parcel
        a whole IP packet/parcel or fragment admitted into the OMNI
        interface by the network layer prior to OAL encapsulation and
        fragmentation, or an IP packet delivered to the network layer by
        the OMNI interface following OAL reassembly/decapsulation.

     OAL packet
        an original IP packet/parcel encapsulated in an OAL IPv6 header
        with an IPv6 Extended Fragment Header extension that includes an
        8-octet (64-bit) OAL Identification value.  Each OAL packet is
        then subject to OAL fragmentation and reassembly.

     OAL fragment
        a portion of an OAL packet following fragmentation but prior to L2
        encapsulation/fragmentation, or following L2 reassembly/
        decapsulation but prior to OAL reassembly.



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     (OAL) atomic fragment
        an OAL packet that can be forwarded without fragmentation, but
        still includes an IPv6 Extended Fragment Header with an 8-octet
        (64-bit) OAL Identification value and with Fragment Offset and
        More Fragments both set to 0.

     (L2) carrier packet
        an encapsulated OAL packet/fragment following L2 encapsulation or
        prior to L2 decapsulation.  OAL sources and destinations exchange
        carrier packets over underlay interfaces, and may be separated by
        one or more OAL intermediate systems.  OAL intermediate systems
        re-encapsulate OAL packets/fragments during forwarding by removing
        the L2 headers of carrier packets from a previous hop underlay
        network and replacing them with new L2 headers for the next hop
        underlay network.  Carrier packets may themselves be subject to
        fragmentation and reassembly in L2 underlay networks at a layer
        below the OAL.  Carrier packets sent over unsecured paths use OMNI
        protocol L2 encapsulations, while those sent over the secured
        paths use L2 security encapsulations such as IPsec [RFC4301], etc.

     OAL source
        an OMNI interface acts as an OAL source when it encapsulates
        original IP packets/parcels to form OAL packets, then performs OAL
        fragmentation and L2 encapsulation to create carrier packets.
        Every OAL source is also an OAL end system.

     OAL destination
        an OMNI interface acts as an OAL destination when it decapsulates
        carrier packets, then performs OAL reassembly/decapsulation to
        restore the original IP packet/parcel.  Every OAL destination is
        also an OAL end system.

     OAL intermediate system
        an OMNI interface acts as an OAL intermediate system when it
        performs L2 reassembly/decapsulation for carrier packets received
        from a previous hop, then performs L2 encapsulation/fragmentation
        on the enclosed OAL packets/fragments and forwards these new
        carrier packets to the next hop.  OAL intermediate systems
        decrement the OAL Hop Limit during forwarding, and discard the OAL
        packet/fragment if the Hop Limit reaches 0.  OAL intermediate
        systems do not decrement the TTL/Hop Limit of the original IP
        packet/parcel.

     Mobility Service Prefix (MSP)
        an aggregated IP Global Unicast Address (GUA) prefix (e.g.,
        2001:db8::/32, 192.0.2.0/24, etc.) assigned to the OMNI link and
        from which more-specific Mobile Network Prefixes (MNPs) are
        delegated.  OMNI link administrators typically obtain MSPs from an



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        Internet address registry, however private-use prefixes can
        alternatively be used subject to certain limitations (see:
        [I-D.templin-intarea-omni]).  OMNI links that connect to the
        global Internet advertise their MSPs to interdomain routing peers.

     Mobile Network Prefix (MNP)
        a longer IP prefix derived from an MSP (e.g.,
        2001:db8:1000:2000::/56, 192.0.2.8/30, etc.) and delegated to an
        AERO Client or Relay.

     Interface Identifier (IID)
        the least significant 64 bits of an IPv6 address, as specified in
        the IPv6 addressing architecture [RFC4291].

     Link Local Address (LLA)
        an IPv6 address beginning with fe80::/64 per the IPv6 addressing
        architecture [RFC4291] and with either a 64-bit MNP (LLA-MNP) or a
        56-bit random value (LLA-RND) encoded in the IID as specified in
        [I-D.templin-intarea-omni].

     Unique Local Address (ULA)
        an IPv6 address beginning with fd00::/8 followed by a 40-bit
        Global ID followed by a 16-bit Subnet ID per [RFC4193] and with
        either a 64-bit MNP (ULA-MNP) or a 56-bit random value (ULA-RND)
        encoded in the IID as specified in [I-D.templin-intarea-omni].
        (Note that [RFC4193] specifies a second form of ULAs based on the
        prefix fc00::/8, which are referred to as "ULA-C" throughout this
        document to distinguish them from the ULAs defined here.)

     Temporary Local Address (TLA)
        a ULA beginning with fd00::/16 followed by a 48-bit randomly-
        initialized value followed by an MNP-based (TLA-MNP) or random
        (TLA-RND) IID as specified in [I-D.templin-intarea-omni].  Clients
        use TLAs to bootstrap autoconfiguration in the presence of OMNI
        link infrastructure or for sustained communications in the absence
        of infrastructure.  (Note that in some environments Clients can
        instead use a (Hierarchical) Host Identity Tag ((H)HIT) instead of
        a TLA - see: [I-D.templin-intarea-omni].)

     eXtended Local Address (XLA)
        a ULA beginning with fd00::/64 followed by an MNP-based (XLA-MNP)
        or random (XLA-RND) IID as specified in
        [I-D.templin-intarea-omni].  An XLA can be used to supply a stable
        address for IPv6 ND messaging, a routing table entry for the OMNI
        link routing system, etc.  (Note that XLAs can also be statelessly
        formed from LLAs (and vice-versa) simply by inverting prefix bits
        7 and 8.)




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     AERO node
        a node that is connected to an OMNI link and participates in the
        AERO internetworking and mobility service.

     (AERO) Host
        an AERO node that configures an OMNI interface over an ENET
        underlay interface serviced by an upstream Client.  The Host does
        not assign an LLA or ULA to the OMNI interface, but instead
        assigns the address taken from the ENET underlay interface.  When
        an AERO host forwards an original IP packet/parcel to another AERO
        node on the same ENET, it uses simple IP-in-L2 OMNI encapsulation
        without including an OAL encapsulation header.  The Host is
        therefore an OMNI link termination endpoint.  (Note: as an
        implementation matter, the Host may instead configure the "OMNI
        interface" as a virtual sublayer of the underlay interface
        itself.)

     (AERO) Client
        an AERO node that configures an OMNI interface over one or more
        underlay interfaces and requests MNP delegation/registration
        service from AERO Proxy/Servers.  The Client assigns an XLA-MNP
        (as well as Proxy/Server-specific ULA-MNPs) to the OMNI interface
        for use in IPv6 ND exchanges with other AERO nodes and forwards
        original IP packets/parcels to correspondents according to OMNI
        interface neighbor cache state.  The Client coordinates with
        Proxy/Servers and/or other Clients over upstream ANET/INET
        interfaces and may also provide Proxy/Server services for Hosts
        and/or other Clients over downstream ENET interfaces.

     (AERO) Proxy/Server
        an AERO node that provides a proxying service between AERO Clients
        and external peers on its Client-facing ANET interfaces (i.e., in
        the same fashion as for an enterprise network proxy) as well as
        designated router services for coordination with correspondents on
        its INET-facing interfaces.  (Proxy/Servers in the open INET
        instead configure only a single INET interface and no ANET
        interfaces.)  The Proxy/Server configures an OMNI interface and
        assigns a ULA-RND to support the operation of IPv6 ND services,
        while advertising any associated MNPs for which it is acting as a
        hub via BGP peerings with AERO Gateways.

     (AERO) Relay
        an AERO Proxy/Server that provides forwarding services between
        nodes reached via the OMNI link and correspondents on other links/
        networks.  AERO Relays assign a ULA-RND to an OMNI interface and
        maintain BGP peerings with Gateways the same as Proxy/Servers.
        Relays also run a dynamic routing protocol to discover any non-MNP
        IP GUA routes in service on other links/networks, advertise OMNI



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        link MSP(s) to other links/networks, and redistribute routes
        discovered on other links/networks into the OMNI link BGP routing
        system.  (Relays that connect to major Internetworks such as the
        global IPv6 or IPv4 Internets can also be configured to advertise
        "default" routes into the OMNI link BGP routing system.)

     (AERO) Gateway
        a BGP hub autonomous system node that also provides OAL forwarding
        services for nodes on an OMNI link.  Gateways forward OAL packets/
        fragments between OMNI link segments as OAL intermediate systems
        while decrementing the OAL IPv6 header Hop Limit but without
        decrementing the network layer IP TTL/Hop Limit.  Gateways peer
        with Proxy/Servers and other Gateways to form an IPv6-based OAL
        spanning tree over all OMNI link segments and to discover the set
        of all MNP and non-MNP prefixes in service.  Gateways process OAL
        packets/fragments received over the secured spanning tree that are
        addressed to themselves, while forwarding all other OAL packets/
        fragments to the next hop also via the secured spanning tree.
        Gateways forward OAL packets/fragments received over the unsecured
        spanning tree to the next hop either via the unsecured spanning
        tree or via direct encapsulation if the next hop is on the same
        OMNI link segment.

     First-Hop Segment (FHS) Client
        a Client that initiates communications with a target peer by
        sending an NS message to establish reverse-path multilink
        forwarding state in OMNI link intermediate systems on the path to
        the target.  Note that in some arrangements the Client's (FHS)
        Proxy/Server (and not the Client itself) initiates the NS.

     Last-Hop Segment (LHS) Client
        a Client that responds to a communications request from a source
        peer's NS by returning an NA response to establish forward-path
        multilink forwarding state in OMNI link intermediate systems on
        the path to the source.  Note that in some arrangements the
        Client's (LHS) Proxy/Server (and not the Client itself) returns
        the NA.

     First-Hop Segment (FHS) Proxy/Server
        a Proxy/Server for an FHS Client's underlay interface that
        forwards the Client's OAL packets into the segment routing
        topology.  FHS Proxy/Servers also act as intermediate forwarding
        systems to facilitate RS/RA exchanges between a Client and its Hub
        Proxy/Server.







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     Last-Hop Segment (LHS) Proxy/Server
        a Proxy/Server for an underlay interface of an LHS Client that
        forwards OAL packets received from the segment routing topology to
        the Client over that interface.

     Hub Proxy/Server
        a single Proxy/Server selected by a Client that injects the
        Client's XLA-MNP into the BGP routing system and provides a
        designated router service for all of the Client's underlay
        interfaces.  Clients often select the first FHS Proxy/Server they
        coordinate with to serve in the Hub role (as all FHS Proxy/Servers
        are equally capable candidates to serve as a Hub), however the
        Client can also select any available Proxy/Server for the OMNI
        link (as there is no requirement that the Hub must also be one of
        the Client's FHS Proxy/Servers).

     Segment Routing Topology (SRT)
        a Multinet OMNI link forwarding region between FHS and LHS Proxy/
        Servers.  FHS/LHS Proxy/Servers and SRT Gateways span the OMNI
        link on behalf of FHS/LHS Client pairs.  The SRT maintains a
        spanning tree established through BGP peerings between Gateways
        and Proxy/Servers.  Each SRT segment includes Gateways in a "hub"
        and Proxy/Servers in "spokes", while adjacent segments are
        interconnected by Gateway-Gateway peerings.  The BGP peerings are
        configured over both secured and unsecured underlay network paths
        such that a secured spanning tree is available for critical
        control messages while other messages can use the unsecured
        spanning tree.

     Mobile Node (MN)
        an AERO Client and all of its downstream-attached networks that
        move together as a single unit, i.e., an end system and its
        connected IoT sub-networks.

     Mobile Router (MR)
        a MN's on-board router that forwards original IP packets/parcels
        between any downstream-attached networks and the OMNI link.  The
        MR is the MN entity that hosts the AERO Client.

     Address Resolution Source (ARS)
        the node nearest the original source that initiates OMNI link
        address resolution.  The ARS may be a Proxy/Server or Relay for
        the source, or may be the source Client itself.  The ARS is often
        (but not always) also the same node that becomes the FHS source
        during route optimization.






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     Address Resolution Target (ART)
        the node toward which address resolution is directed.  The ART may
        be a Relay or the target Client itself.  The ART is often (but not
        always) also the same node that becomes the LHS target during
        route optimization.

     Address Resolution Responder (ARR)
        the node that responds to address resolution requests on behalf of
        the ART.  The ARR may be a Relay, the ART itself, or the ART's
        current Hub Proxy/Server.  Note that a Hub Proxy/Server can assume
        the ARR role even if it is located on a different SRT segment than
        the ART.  The Hub Proxy/Server assumes the ARR role only when it
        receives an RS message from the ART with the 'ARR' flag set (see:
        [I-D.templin-intarea-omni]).

     Potential Router List (PRL)
        a geographically and/or topologically referenced list of addresses
        of all Proxy/Servers within the same OMNI link.  Each OMNI link
        has its own PRL.

     Distributed Mobility Management (DMM)
        a BGP-based overlay routing service coordinated by Proxy/Servers
        and Gateways that tracks all Proxy/Server-to-Client associations.

     Mobility Service (MS)
        the collective set of all Proxy/Servers, Gateways and Relays that
        provide the AERO Service to Clients.

     AERO Forwarding Information Base (AFIB)
        A forwarding table on each OAL source, destination and
        intermediate system that includes AERO Forwarding Vectors (AFV)
        with both multilink forwarding instructions and context for
        reconstructing compressed headers for specific communicating peer
        underlay interface pairs.  The AFIB also supports route
        optimization where one or more OAL intermediate systems in the
        path can be "skipped" to reduce path stretch and decrease load on
        critical infrastructure elements.

     AERO Forwarding Vector (AFV)
        An AFIB entry that includes soft state (including addressing and
        Identification information) for each underlay interface pairwise
        communication session between peer OAL nodes.  AFVs are identified
        by both a forward and reverse path AFV Index (AFVI).  OAL nodes
        establish reverse path AFVIs when they forward an IPv6 ND unicast
        NS message and establish forward path AFVIs when they forward the
        solicited IPv6 ND unicast NA response.





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     AERO Forwarding Vector Index (AFVI)
        A locally-unique 2-octet or 4-octet value automatically generated
        by an OAL node when it creates an AFV.  OAL intermediate systems
        assign two distinct 4-octet AFVIs (called "A" and "B") to each
        AFV, with "A" representing the forward path and "B" representing
        the reverse path.  Meanwhile, the OAL source assigns a single "B"
        AFVI, and the OAL destination assigns a single "A" AFVI.  Each OAL
        node advertises its "A" AFVI to previous hop nodes on the reverse
        path toward the source and advertises its "B" AFVI to next hop
        nodes on the forward path toward the destination.  Clients in
        MANETs also assign distinct 2-octet AFVIs (called "C" and "D") to
        support local multihop forwarding.  The same as for the A/B AFVIs,
        the "C" AFVI represents the forward path and the "D" AFVI
        represents the reverse path.  For unidirectional MANET paths, only
        the forward path ("C") AFVI is used.

     AERO Forwarding Parameters (AFP)
        An OMNI option sub-option that appears in IPv6 ND NS/NA messages
        and includes all parameters necessary for establishing AFV state
        in OAL nodes in the path (see: [I-D.templin-intarea-omni]).

  3.  Requirements

     The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
     "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
     "OPTIONAL" in this document are to be interpreted as described in BCP
     14 [RFC2119][RFC8174] when, and only when, they appear in all
     capitals, as shown here.

  4.  Automatic Extended Route Optimization (AERO)

     The following sections specify the operation of IP over OMNI links
     using the AERO service:

  4.1.  AERO Node Types

     AERO Hosts configure an OMNI interface over an underlay interface
     connected to a Client's ENET and coordinate with both other AERO
     Hosts and Clients over the ENET.  As an implementation matter, the
     Host either assigns the same (MNP-based) IP address from the underlay
     interface to the OMNI interface, or configures the "OMNI interface"
     as a virtual sublayer of the underlay interface itself.  AERO Hosts
     treat the ENET as an ANET, and treat the upstream Client for the ENET
     as a Proxy/Server.  AERO Hosts are seen as OMNI link termination
     endpoints.






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     AERO Clients can be deployed as fixed infrastructure nodes close to
     end systems, or as Mobile Nodes (MNs) that can change their network
     attachment points dynamically.  AERO Clients configure OMNI
     interfaces over underlay interfaces with addresses that may change
     due to mobility.  AERO Clients register their Mobile Network Prefixes
     (MNPs) with the AERO service, and distribute the MNPs to ENETs (which
     may connect AERO Hosts and other Clients).  AERO Clients provide
     Proxy/Server-like services for Hosts and other Clients on downstream-
     attached ENETs.

     AERO Gateways, Proxy/Servers and Relays are critical infrastructure
     elements in fixed (i.e., non-mobile) ANET/INET boundary (or
     standalone INET) deployments and hence have permanent and unchanging
     INET addresses.  Together, they provide access to the AERO service
     OMNI link virtual overlay for connecting AERO Clients and Hosts.
     AERO Gateways (together with Proxy/Servers and Relays) provide the
     secured backbone supporting infrastructure for a Segment Routing
     Topology (SRT) spanning tree for the OMNI link.

     AERO Gateways are OMNI link intermediate systems that forward packets
     both within the same SRT segment and between disjoint SRT segments
     based on an IPv6 encapsulation mid-layer known as the OMNI Adaptation
     Layer (OAL).  The OMNI interface and OAL provide an adaptation layer
     forwarding service that the network layer perceives as L2 bridging,
     since the inner IP TTL/Hop Limit is not decremented.  Each Gateway
     peers with Proxy/Servers, Relays and other Gateways in a dynamic
     routing protocol instance to provide a Distributed Mobility
     Management (DMM) service for the list of active MNPs (see
     Section 4.2.3).  Gateways assign one or more Mobility Service
     Prefixes (MSPs) to the OMNI link and configure secured tunnels with
     Proxy/Servers, Relays and other Gateways; they further maintain
     forwarding table entries for each MNP or non-MNP prefix in service on
     the OMNI link.

     AERO Proxy/Servers distributed across one or more SRT segments
     provide default forwarding and mobility/multilink services for AERO
     Client mobile nodes.  The Proxy/Server acts as either OMNI link
     intermediate systems or end systems according to the service model
     selected by each Client.  Proxy/Server also peer with Gateways in an
     adaptation layer dynamic routing protocol instance to advertise its
     list of associated MNPs (see Section 4.2.3).  Hub Proxy/Servers
     provide prefix delegation/registration services and track the
     mobility/multilink profiles of each of their associated Clients,
     where each delegated prefix becomes an MNP taken from an MSP.  Proxy/
     Servers at ANET/INET boundaries provide a primary forwarding service
     for ANET Client/Host communications with peers in external INETs,
     while Proxy/Servers in open INETs provide an authentication service
     for IPv6 ND messages but should be used only a last resort data plane



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     forwarding service when a Client cannot forward directly to an INET
     peer or Gateway.  Source Clients securely coordinate with target
     Clients by sending control messages via a First-Hop Segment (FHS)
     Proxy/Server which forwards them over the SRT spanning tree to a
     Last-Hop Segment (LHS) Proxy/Server which finally forwards them to
     the target.

     AERO Relays are Proxy/Servers that provide forwarding services to
     exchange original IP packets/parcels between the OMNI link and fixed
     or mobile nodes on other links/networks.  Relays run a dynamic
     routing protocol to discover any non-MNP prefixes in service on other
     links/networks, and Relays that connect to larger Internetworks (such
     as the Internet) may originate default routes.  The Relay
     redistributes OMNI link MSP(s) into other links/networks, and
     redistributes non-MNP prefixes via OMNI link Gateway BGP peerings.

  4.2.  The AERO Service over OMNI Links

  4.2.1.  AERO/OMNI Reference Model

     Figure 1 presents the basic OMNI link reference model:






























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                           +-----------------+
                           | AERO Gateway G1 |
                           | Nbr: S1, S2, P1 |
                           |(X1->S1; X2->S2) |
                           |      MSP M1     |
                           +--------+--------+
         +--------------+           |            +--------------+
         |  AERO P/S S1 |           |            |  AERO P/S S2 |
         |  Nbr: C1, G1 |           |            |  Nbr: C2, G1 |
         |  default->G1 |           |            |  default->G1 |
         |    X1->C1    |           |            |    X2->C2    |
         +-------+------+           |            +------+-------+
                 |       OMNI link  |                   |
         X===+===+==================+===================+===+===X
             |                                              |
       +-----+--------+                            +--------+-----+
       |AERO Client C1|                            |AERO Client C2|
       |    Nbr: S1   |                            |   Nbr: S2    |
       | default->S1  |                            | default->S2  |
       |    MNP X1    |                            |    MNP X2    |
       +------+-------+                            +-----+--------+
              |                                          |
             .-.                                        .-.
          ,-(  _)-.     +-------+     +-------+      ,-(  _)-.
       .-(_  IP   )-.   |  AERO |     |  AERO |    .-(_  IP   )-.
     (__    ENET     )--|Host H1|     |Host H2|--(__    ENET     )
        `-(______)-'    +-------+     +-------+     `-(______)-'

                      Figure 1: AERO/OMNI Reference Model

     In this model:

     *  the OMNI link is an overlay network service configured over one or
        more underlay SRT segments which may be managed by diverse
        administrative domains using incompatible protocols and/or
        addressing plans.

     *  AERO Gateway G1 aggregates Mobility Service Prefix (MSP) M1,
        discovers Mobile Network Prefixes (MNPs) X* and advertises the MSP
        via BGP peerings over secured tunnels to Proxy/Servers (S1, S2).
        Gateways provide the backbone for an SRT spanning tree for the
        OMNI link.









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     *  AERO Proxy/Servers S1 and S2 configure secured tunnels with
        Gateway G1 and also provide mobility, multilink, multicast and
        default router services for the MNPs of their associated Clients
        C1 and C2.  (Proxy/Servers that act as Relays can also advertise
        non-MNP routes for non-mobile correspondent nodes the same as for
        MNP Clients.)

     *  AERO Clients C1 and C2 associate with Proxy/Servers S1 and S2,
        respectively.  They receive MNP delegations X1 and X2, and also
        act as default routers for their associated physical or internal
        virtual ENETs.

     *  AERO Hosts H1 and H2 attach to the ENETs served by Clients C1 and
        C2, respectively.

     An OMNI link configured over a single underlay network appears as a
     single unified link with a consistent addressing plan; all nodes on
     the link can exchange carrier packets via simple L2 encapsulation
     (i.e., following any necessary NAT traversal) since the underlay is
     connected.  In common practice, however, OMNI links are often
     configured over an SRT spanning tree that bridges multiple distinct
     underlay network segments managed under different administrative
     authorities (e.g., as for worldwide aviation service providers such
     as ARINC, SITA, Inmarsat, etc.).  Individual underlay networks may
     also be partitioned internally, in which case each internal partition
     appears as a separate segment.

     The addressing plan of each SRT segment is consistent internally but
     will often bear no relation to the addressing plans of other
     segments.  Each segment is also likely to be separated from others by
     network security devices (e.g., firewalls, proxys, packet filtering
     gateways, etc.), and disjoint segments often have no common physical
     link connections.  Therefore, nodes can only be assured of exchanging
     carrier packets directly with correspondents in the same segment, and
     not with those in other segments.  The only means for joining the
     segments therefore is through inter-domain peerings between AERO
     Gateways.

     The OMNI link spans multiple SRT segments using the OMNI Adaptation
     Layer (OAL) to provide the network layer with a virtual abstraction
     similar to a bridged campus LAN.  The OAL is an OMNI interface
     sublayer that inserts a mid-layer IPv6 encapsulation header for
     inter-segment forwarding (i.e., bridging) without decrementing the
     network layer TTL/Hop Limit of the original IP packet/parcel.  An
     example OMNI link SRT is shown in Figure 2:






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      . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
    .                                                                           .
   .                                                                             .
   .     .-(::::::::)                .-(::::::::)               .-(::::::::)     .
   .  .-(::::::::::::)-.   +-+    .-(::::::::::::)-.   +-+   .-(::::::::::::)-.  .
   . (::::    FHS    :::)--|G|--(::: Intermediate ::)--|G|--(::::    LHS    :::) .
   .  `-(::::::::::::)-'   +-+    `-(::Segments::)-'   +-+   `-(::::::::::::)-'  .
   .     `-(::::::)-'                `-(::::::)-'               `-(::::::)-'     .
   .           |                                                      |          .
   .         +---+                                                  +---+        .
   .         |P/S|                                                  |P/S|        .
   .         +---+                                                  +---+        .
   .           |                                                      |          .
   .     .-(::::::::)                                          .-(::::::::)      .
   .  .-(: First Hop :)-.  +-------+             +-------+   .-(: Last Hop :)-.  .
   . (::::  Access  ::::)--| Source|             | Target|--(::::  Access  ::::) .
   .  `-(:: Network ::)-'  | Client|             | Client|     (:: Network ::)-' .
   .     `-(::::::)-'      +-------+             +-------+      `-(::::::)-'     .
   .                                                                             .
   .                                                                             .
   .         <--  Segment Routing Topology (SRT) Spanned by OMNI Link -->        .
     .                                                                          .
       . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

            Figure 2: OMNI Link Segment Routing Topology (SRT)

     In the Segment Routing Topology, a source Client connects via a first
     hop access network served by a First Hop Segment (FHS) Proxy/Server.
     The FHS Proxy/Server then forwards to an FHS Gateway which connects
     to an arbitrarily complex set of Intermediate Segments.  Adjacent
     intermediate Segments are joined by intermediate Gateways (not shown)
     that serve as adaptation layer IPv6 routers, with the final segment
     connected by a Last Hop Segment (LHS) Gateway.  The LHS Gateway then
     forwards to an LHS Proxy/Server which in turn connects to the last
     hop access network where the target Client resides.
















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     Gateway, Proxy/Server and Relay OMNI interfaces are configured over
     both secured tunnels and open INET underlay interfaces within their
     respective SRT segments.  Within each segment, Gateways configure
     "hub-and-spokes" BGP peerings with Proxy/Servers and Relays as
     "spokes".  Adjacent SRT segments are joined by Gateway-to-Gateway
     peerings to collectively form a spanning tree over the entire SRT.
     The "secured" spanning tree supports authentication and integrity for
     critical control plane messages (and any trailing data plane message
     extensions).  The "unsecured" spanning tree conveys ordinary carrier
     packets without security codes and that must be treated by
     destinations according to data origin authentication procedures.
     AERO nodes can employ route optimization to cause carrier packets to
     take more direct paths between OMNI link neighbors without having to
     follow strict spanning tree paths.

     The AERO Multinet service concatenates SRT segments to form a larger
     network through Gateway-to-Gateway peerings as originally suggested
     in the "Catenet Model for Internetworking" [IEN48]; especially
     Figure 2 follows directly from the illustrations in [IEN48-2].  The
     Catenet concept suggested a "network-of-networks" concatenation of
     independent and diverse Internetwork "segments" to form a much larger
     network supporting end-to-end services.

     The Catenet concept first articulated in the 1970's was distorted
     through the evolution of the Internet in the decades that followed,
     since the adaptation layer was a critical element missing from the
     architecture.  As a result, while the Internet has been successful
     beyond measure it has evolved as a monolithic public routing and
     addressing service interconnecting private domains instead of a true
     network-of-networks which has impeded flexibility and inhibited end-
     to-end services.  The adaptation layer manifested by AERO and OMNI
     now provides the means to address these limitations as well as the
     other "6 Ms of Modern Internetworking" according to the original
     Catenet network-of-networks vision.

  4.2.2.  Addressing and Node Identification

     AERO nodes on OMNI links use the Link-Local Address (LLA) prefix
     fe80::/64 [RFC4291] to assign LLAs to the OMNI interface to satisfy
     the requirements of [RFC4861].  AERO Clients configure LLAs
     constructed from MNPs (i.e., "LLA-MNPs") while AERO infrastructure
     nodes construct LLAs based on 56-bit random values ("LLA-RNDs") per
     [I-D.templin-intarea-omni].  Non-MNP routes are also represented the
     same as for MNPs, but may include a prefix that is not properly
     covered by an MSP.






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     AERO nodes also use the Unique Local Address (ULA) prefix fd00::/8
     followed by a pseudo-random 40-bit Global ID to form the prefix
     {ULA}::/48, then include a 16-bit Subnet ID '*' to form the prefix
     {ULA*}::/64 [RFC4291].  The AERO node then uses the prefix
     {ULA*}::/64 to form "ULA-MNPs" or "ULA-RNDs" as specified in
     [I-D.templin-intarea-omni] to support OAL addressing.  (The prefix
     {ULA*}::/64 appearing alone and with no suffix represents "default"
     for that prefix.)

     AERO Clients also use Temporary Local Addresses (TLAs) and eXtended
     Local Addresses (XLAs) constructed per [I-D.templin-intarea-omni],
     where TLAs are distinguished from ordinary ULAs based on the prefix
     fd00::/16 and XLAs are distinguished from ULAs/TLAs based on the
     prefix fd00::/64.  Clients use TLA-RNDs only in initial control
     message exchanges until a stable MNP is assigned, but may sometimes
     also use them for sustained communications within a local routing
     region.  AERO nodes use XLA-MNPs to provide forwarding information
     for the global routing table as well as IPv6 ND message addressing
     information.

     AERO MSPs, MNPs and non-MNP routes are typically based on Global
     Unicast Addresses (GUAs), but in some cases may be based on IPv4
     private addresses [RFC1918] or IPv6 ULA-C's [RFC4193].  A GUA block
     is also reserved for OMNI link anycast purposes.  See
     [I-D.templin-intarea-omni] for a full specification of LLAs, ULAs,
     TLAs, XLAs, GUAs and anycast addresses used by AERO nodes on OMNI
     links.

     Finally, AERO Clients and Proxy/Servers configure node Identification
     values as specified in [I-D.templin-intarea-omni].

  4.2.3.  AERO Routing System

     The AERO routing system comprises a private Border Gateway Protocol
     (BGP) [RFC4271] service coordinated between Gateways and Proxy/
     Servers (Relays also engage in the routing system as simplified
     Proxy/Servers).  The service supports OAL packet/fragment forwarding
     at a layer below IP and does not interact with the public Internet
     BGP routing system, but supports redistribution of information for
     other links and networks connected by Relays.

     In a reference deployment, each Proxy/Server is configured as an
     Autonomous System Border Router (ASBR) for a stub Autonomous System
     (AS) using a 32-bit AS Number (ASN) [RFC4271] that is unique within
     the BGP instance, and each Proxy/Server further uses eBGP to peer
     with one or more Gateways but does not peer with other Proxy/Servers.
     Each SRT segment in the OMNI link must include one or more Gateways
     in a "hub" AS, which peer with the Proxy/Servers within that segment



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     as "spoke" ASes.  All Gateways within the same segment are members of
     the same hub AS, and use iBGP to maintain a consistent view of all
     active routes currently in service.  The Gateways of different
     segments peer with one another using eBGP.

     Gateways maintain forwarding table entries only for ULA prefixes for
     infrastructure elements and XLA-MNPs corresponding to MNP and non-MNP
     routes that are currently active; Gateways also maintain black-hole
     routes for the OMNI link MSPs so that OAL packets/fragments destined
     to non-existent more-specific routes are flushed from the routing
     system.  In this way, Proxy/Servers and Relays have only partial
     topology knowledge (i.e., they only maintain routing information for
     their directly associated Clients and non-AERO links) and they
     forward all other OAL packets/fragments to Gateways which have full
     topology knowledge.

     Each OMNI link segment assigns a unique sub-prefix of {ULA}::/48
     known as the "SRT prefix".  For example, a first segment could assign
     {ULA}:1000::/56, a second could assign {ULA}:2000::/56, a third could
     assign {ULA}:3000::/56, etc.  Within each segment, each Proxy/Server
     configures a ULA-RND within the segment's SRT prefix with a 56-bit
     random value in the interface identifier as specified in
     [I-D.templin-intarea-omni].

     The administrative authorities for each segment must therefore
     coordinate to assure mutually-exclusive ULA prefix assignments, but
     internal provisioning of ULAs is an independent local consideration
     for each administrative authority.  For each ULA prefix, the
     Gateway(s) that connect that segment assign the all-zero's address of
     the prefix as a Subnet Router Anycast address.  For example, the
     Subnet Router Anycast address for {ULA}:1023::/64 is simply
     {ULA}:1023::/64.

     ULA prefixes are statically represented in Gateway forwarding tables.
     Gateways join multiple SRT segments into a unified OMNI link over
     multiple diverse network administrative domains.  They support a
     virtual bridging service by first establishing forwarding table
     entries for their ULA prefixes either via standard BGP routing or
     static routes.  For example, if three Gateways ('A', 'B' and 'C')
     from different segments serviced {ULA}:1000::/56, {ULA}:2000::/56 and
     {ULA}:3000::/56 respectively, then the forwarding tables in each
     Gateway appear as follows:

     A:  {ULA}:1000::/56->local, {ULA}:2000::/56->B, {ULA}:3000::/56->C

     B:  {ULA}:1000::/56->A, {ULA}:2000::/56->local, {ULA}:3000::/56->C

     C:  {ULA}:1000::/56->A, {ULA}:2000::/56->B, {ULA}:3000::/56->local



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     These forwarding table entries rarely change, since they correspond
     to fixed infrastructure elements in their respective segments.

     MNP (and non-MNP) routes are instead dynamically advertised in the
     AERO routing system by Proxy/Servers and Relays that provide service
     for their corresponding MNPs.  The routes are advertised as XLA-MNP
     prefixes, i.e., as fd00::{MNP} (see: [I-D.templin-intarea-omni]).
     For example, if three Proxy/Servers ('D', 'E' and 'F') service the
     MNPs 2001:db8:1000:2000::/56, 2001:db8:3000:4000::/56 and
     2001:db8:5000:6000::/56 then the routing system would include:

     D:  fd00::2001:db8:1000:2000/120

     E:  fd00::2001:db8:3000:4000/120

     F:  fd00::2001:db8:5000:6000/120

     Note: the MNP length found in an OMNI Neighbor Control sub-option
     encodes a Preflen between 1 and 64, but the corresponding XLA-MNP is
     entered into the routing system with length (64 + MNP length).  A
     full discussion of the BGP-based routing system used by AERO is found
     in [I-D.ietf-rtgwg-atn-bgp].

  4.2.4.  Segment Routing Topologies (SRTs)

     The distinct {ULA}::/48 prefixes in an OMNI link domain identify
     distinct Segment Routing Topologies (SRTs).  Each SRT is a mutually-
     exclusive OMNI link overlay instance using a distinct set of ULAs,
     and emulates a bridged campus LAN service for the OMNI link.  In some
     cases (e.g., when redundant topologies are needed for fault tolerance
     and reliability) it may be beneficial to deploy multiple SRTs that
     act as independent overlay instances.  A communication failure in one
     instance therefore will not affect communications in other instances.

     Each SRT is identified by a distinct value in the 40-bit ULA Global
     ID field and assigns an OMNI IPv6 anycast address used for OMNI
     interface determination in Safety-Based Multilink (SBM) as discussed
     in [I-D.templin-intarea-omni].  Each OMNI interface further applies
     Performance-Based Multilink (PBM) internally.

     The Gateways and Proxy/Servers of each independent SRT engage in BGP
     peerings to form a spanning tree with the Gateways in non-leaf nodes
     and the Proxy/Servers in leaf nodes.  The spanning tree is configured
     over both secured and unsecured underlay network paths.  The secured
     spanning tree is used to convey secured control messages (and
     sometimes data message extensions) between Proxy/Servers and
     Gateways, while the unsecured spanning tree forwards bulk data
     messages and/or unsecured control messages.



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     Each SRT segment is identified by a unique ULA prefix used by all
     Proxy/Servers and Gateways in the segment.  Each AERO node must
     therefore discover an SRT prefix that correspondents can use to
     determine the correct segment, and must publish the SRT prefix in
     IPv6 ND messages.

     Note: The distinct ULA prefixes in an OMNI link domain can be carried
     either in a common BGP routing protocol instance for all OMNI links
     or in distinct BGP routing protocol instances for different OMNI
     links.  In some SBM environments, such separation may be necessary to
     ensure that distinct OMNI links do not include any common
     infrastructure elements as single points of failure.  In other
     environments, carrying the ULAs of multiple OMNI links within a
     common routing system may be acceptable.

  4.2.5.  Segment Routing For OMNI Link Selection

     Original IPv6 sources can direct IPv6 packets/parcels to an AERO node
     by including a standard IPv6 Segment Routing Header (SRH) [RFC8754]
     with the OMNI IPv6 anycast address for the selected OMNI link as
     either the IPv6 destination or as an intermediate hop within the SRH.
     This allows the original source to determine the specific OMNI link
     SRT an original IPv6 packet/parcel will traverse when there may be
     multiple alternatives.

     When an AERO node processes the SRH and forwards the original IPv6
     packet/parcel to the correct OMNI interface, the OMNI interface
     writes the next IPv6 address from the SRH into the IPv6 destination
     address and decrements Segments Left.  If decrementing would cause
     Segments Left to become 0, the OMNI interface deletes the SRH before
     forwarding.  This form of Segment Routing supports Safety-Based
     Multilink (SBM).

  4.3.  OMNI Interface Characteristics

     OMNI interfaces are virtual interfaces configured over one or more
     underlay interfaces classified as follows:

     *  ANET interfaces connect to a protected and secured ANET that is
        separated from open INETs by Proxy/Servers.  The ANET interface
        may be either on the same L2 link segment as a Proxy/Server, or
        separated from a Proxy/Server by multiple IP hops.  (Note that
        NATs may appear internally within an ANET and may require NAT
        traversal on the path to the Proxy/Server the same as for the INET
        case.)






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     *  INET interfaces connect to an INET either natively or through one
        or several IPv4 Network Address Translators (NATs).  Native INET
        interfaces have global IP addresses that are reachable from
        correspondent on the same INET.  NATed INET interfaces typically
        have private IP addresses and connect to a private network behind
        one or more NATs with the outermost NAT providing INET access.

     *  ENET interfaces connect a Client's downstream-attached networks,
        where the Client provides forwarding services for ENET Host and
        Client communications to remote peers.  An ENET can be as simple
        as a small IoT sub-network that travels with a mobile Client to as
        complex as a large private enterprise network that the Client
        connects to a larger ANET or INET.

     *  VPN interfaces use security encapsulations (e.g.  IPsec tunnels)
        over underlay networks to connect Clients, Proxy/Servers and/or
        Gateways.  VPN interfaces provide security services at lower
        layers of the architecture (L2/L1) the same as for Direct point-
        to-point interfaces.

     *  Direct point-to-point interfaces securely connect Clients, Proxy/
        Servers and/or Gateways over physical or virtual media that does
        not transit any open Internetwork paths.  Examples include a line-
        of-sight link between a remote pilot and an unmanned aircraft, a
        fiberoptic link between Gateways, etc.

     OMNI interfaces use OAL encapsulation and fragmentation as discussed
     in Section 4.6.  OMNI interfaces use L2 encapsulation (see:
     Section 4.6) to exchange carrier packets with OMNI link neighbors
     over INET interfaces and IPsec tunnels as well as over ANET
     interfaces for which the Client and FHS Proxy/Server may be multiple
     IP hops away.  OMNI interfaces use link layer encapsulation only
     (i.e., and no other L2 encapsulations) over Direct underlay
     interfaces or ANET interfaces when the Client and FHS Proxy/Server
     are known to be on the same underlay link.

     OMNI interfaces maintain an adaptation layer neighbor cache for
     tracking per-neighbor state.  OMNI interfaces use IPv6 ND messages
     including Router Solicitation (RS), Router Advertisement (RA),
     Neighbor Solicitation (NS), Neighbor Advertisement (NA), unsolicited
     Neighbor Advertisement (uNA) and Redirect to manage the neighbor
     cache.  In environments where spoofing may be a threat, OMNI
     neighbors should invoke OAL Identification window synchronization in
     their IPv6 ND message exchanges.

     OMNI interfaces send IPv6 ND messages with an OMNI option formatted
     as specified in [I-D.templin-intarea-omni].  The OMNI option includes
     prefix registration information, Interface Attributes and/or AERO



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     Forwarding Parameters (AFPs) containing link information parameters
     for the OMNI interface's underlay interfaces (as well as any other
     per-neighbor information).  The presence of the OMNI option
     identifies each IPv6 ND message as an adaptation layer (i.e., and not
     a network layer) control message.

     A Host's OMNI interface is configured over an underlay interface
     connected to an ENET provided by an upstream Client.  From the Host's
     perspective, the ENET appears as an ANET and the upstream Client
     appears as a Proxy/Server.  The Host does not provide OMNI
     intermediate system services and is therefore a logical termination
     point for the OMNI link.

     A Client's OMNI interface may be configured over multiple ANET/INET
     underlay interfaces.  For example, common mobile handheld devices
     have both wireless local area network ("WLAN") and cellular wireless
     links.  These links are often used "one at a time" with low-cost WLAN
     preferred and highly-available cellular wireless as a standby, but a
     simultaneous-use capability could provide benefits.  In a more
     complex example, aircraft frequently have many wireless data link
     types (e.g.  satellite-based, cellular, terrestrial, air-to-air
     directional, etc.) with diverse performance and cost properties.

     If a Client's multiple ANET/INET underlay interfaces are used "one at
     a time" (i.e., all other interfaces are in standby mode while one
     interface is active), then successive IPv6 ND messages all include
     OMNI option Interface Attributes, Traffic Selector and/or AFP sub-
     options with the same underlay interface ifIndex.  In that case, the
     Client would appear to have a single underlay interface but with a
     dynamically changing link layer address.

     If the Client has multiple active ANET/INET underlay interfaces, then
     from the perspective of IPv6 ND it would appear to have multiple link
     layer addresses.  In that case, IPv6 ND message OMNI options MAY
     include sub-options with different underlay interface ifIndexes.

     Proxy/Servers on the open Internet include only a single INET
     underlay interface.  INET Clients therefore discover only the L2ADDR
     information for the Proxy/Server's INET interface.  Proxy/Servers on
     an ANET/INET boundary include both an ANET and INET underlay
     interface.  ANET Clients therefore must discover both the ANET and
     INET L2ADDR information for their Proxy/Servers.

     Gateway and Proxy/Server OMNI interfaces are configured over underlay
     interfaces that provide both secured tunnels for carrying IPv6 ND and
     BGP protocol control plane messages and open INET access for carrying
     unsecured data plane messages.  The OMNI interface configures a ULA-
     RND and acts as an OAL source to encapsulate original IP packets/



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     parcels, then fragments the resulting OAL packets, performs L2
     encapsulation/fragmentation and sends the resulting carrier packets
     over the secured or unsecured underlay paths.  Note that Gateway and
     Proxy/Server end-to-end transport protocol sessions used by the BGP
     run directly over the OMNI interface and use ULA-RND source and
     destination addresses.  The ULA-RND addresses that appear in the
     original IP packets/parcels of a BGP protocol session may therefore
     be the same as those that appear in the OAL IPv6 encapsulation
     header.

  4.4.  OMNI Interface Initialization

     AERO Proxy/Servers, Clients and Hosts configure OMNI interfaces as
     their point of attachment to the OMNI link.  AERO nodes assign the
     MSPs for the link to their OMNI interfaces (i.e., as a "route-to-
     interface") to ensure that original IP packets/parcels with
     destination addresses covered by an MNP not explicitly associated
     with another interface are directed to an OMNI interface.

     OMNI interface initialization procedures for Proxy/Servers, Clients
     Hosts and Gateways are discussed in the following sections.

  4.4.1.  AERO Proxy/Server and Relay Behavior

     When a Proxy/Server enables an OMNI interface, it assigns a ULA-RND
     appropriate for the given OMNI link SRT segment.  The Proxy/Server
     also configures secured underlay interface tunnels and engages in BGP
     routing protocol sessions over the OMNI interface with one or more
     neighboring Gateways.

     The OMNI interface provides a single interface abstraction to the
     network layer, but internally serves as an NBMA nexus for sending
     carrier packets to OMNI interface neighbors over underlay interfaces
     and/or secured tunnels.  The Proxy/Server further configures a
     service to facilitate IPv6 ND exchanges with AERO Clients and manages
     per-Client neighbor cache entries and IP forwarding table entries
     based on control message exchanges.

     Relays are simply Proxy/Servers that run a dynamic routing protocol
     to redistribute routes between the OMNI interface and INET/ENET
     interfaces (see: Section 4.2.3).  The Relay provisions MNPs to
     networks on the INET/ENET interfaces (i.e., the same as a Client
     would do) and advertises the MSP(s) for the OMNI link over the INET/
     ENET interfaces.  The Relay further provides an OMNI link attachment
     point for non-MNP-based topologies.






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  4.4.2.  AERO Client Behavior

     When a Client enables an OMNI interface, it assigns either an XLA-MNP
     or a TLA and sends OMNI-encapsulated RS messages over its ANET/INET
     underlay interfaces to an FHS Proxy/Server, which coordinates with a
     Hub Proxy/Server that returns an RA message with corresponding
     parameters.  The RS/RA messages may pass through one or more NATs in
     the path between the Client and FHS Proxy/Server.  (Note: if the
     Client used a TLA in its initial RS messages, it may discover ULA-
     MNPs in the corresponding RAs that it receives from FHS Proxy/Servers
     and begin using these new addresses.  If the Client is operating
     outside the context of AERO infrastructure such as in a Mobile Ad-hoc
     Network (MANET), however, it may continue using TLAs for Client-to-
     Client communications either indefinitely or at least until it
     encounters an infrastructure element that can delegate MNPs.)

     A Client can further extend the OMNI link over its (downstream) ENET
     interfaces where it provides a first-hop router for Hosts and other
     AERO Clients connected to the ENET.  A downstream Client that
     connects via the ENET serviced by an upstream Client can in turn
     service further downstream ENETs that connect other Hosts and
     Clients.  This OMNI link extension can be applied recursively over a
     "chain" of ENET Clients.

  4.4.3.  AERO Host Behavior

     When a Host enables an OMNI interface, it assigns an address taken
     from the ENET underlay interface which may itself be a GUA delegated
     by the upstream Client.  The Host does not assign a link-local
     address to the OMNI interface, since no autoconfiguration is
     necessary on that interface.  (As an implementation matter, the Host
     could instead configure the "OMNI interface" as a virtual sublayer of
     the ENET underlay interface itself.)

     The Host sends OMNI-encapsulated RS messages over its ENET underlay
     interface to the upstream Client, which returns encapsulated RAs and
     provides routing services in the same fashion that Proxy/Servers
     provides services for Clients.  Hosts represent the leaf end systems
     in recursively-nested chain of concatenated ENETs, i.e., they
     represent terminating endpoints for the OMNI link.











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  4.4.4.  AERO Gateway Behavior

     AERO Gateways configure an OMNI interface and assign a ULA-RND and
     corresponding Subnet Router Anycast address for each of their OMNI
     link SRT segments.  Gateways configure underlay interface secured
     tunnels with Proxy/Servers in the same SRT segment and other Gateways
     in the same (or an adjacent) SRT segment.  Gateways then engage in a
     BGP routing protocol session with neighbors over the secured spanning
     tree (see: Section 4.2.3).

  4.5.  OMNI Interface Neighbor Cache Maintenance

     Each Client, Proxy/Server and Gateway OMNI interface maintains a
     network layer conceptual neighbor cache per [RFC1256] or [RFC4861]
     the same as for any IP interface.  The OMNI interface network layer
     neighbor cache is maintained through static and/or dynamic neighbor
     cache entry configurations.

     Each OMNI interface also maintains a separate internal adaptation
     layer conceptual neighbor cache that includes a Neighbor Cache Entry
     (NCE) for each of its active OAL neighbors per [RFC4861].  IPv6 ND
     messages that update the adaptation layer neighbor cache include ULA
     addresses as well as one or more OMNI options.  Throughout this
     document, the terms "neighbor cache" and "NCE" refer to this
     adaptation layer neighbor cache unless otherwise specified.

     Each OMNI interface NCE is indexed by the ULA of a neighbor found in
     the ND message IPv6 header and determines the context for
     Identification verification.  Clients and Proxy/Servers maintain NCEs
     through dynamic RS/RA message exchanges, and also maintain NCEs for
     any active correspondent peers through dynamic NS/NA message
     exchanges.

     Hosts maintain NCEs for Clients and other Hosts through the exchange
     of RS/RA, NS/NA or Redirect messages.  Each NCE is indexed by the IP
     address assigned to the Host ENET interface, which is the same
     address used for L2 encapsulation (i.e., without the insertion of an
     OAL header).  This encapsulation format identifies the NCE as a Host-
     based entry where the Host is a leaf end system in the recursively
     extended OMNI link.

     Gateways maintain NCEs for Clients within their local segments based
     on NS/NA route optimization messaging (see: Section 4.13.4).  When a
     Gateway creates/updates a NCE for a local segment Client based on NS/
     NA route optimization, it also maintains AFIB state for messages
     destined to this local segment Client.





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     Clients establish NCEs for their associated FHS and Hub Proxy/Servers
     through the exchange of RS/RA messages.  When a Client and Proxy/
     Server establish NCEs, they set a ReachableTime timer to
     REACHABLE_TIME seconds.  Clients determine the service profiles for
     their FHS and Hub Proxy/Servers by setting the NUD/ARR/RPT flags in
     RS messages and also by setting/clearing the FMT-Forward and FMT-Mode
     flags in the Interface Attributes sub-option.  When the NUD/ARR/RPT
     flags are clear, Proxy/Servers forward all NS/NA messages to the
     Client, while the Client performs mobility update signaling through
     the transmission of uNA messages to all active neighbors following a
     mobility event.  However, in some environments this may result in
     excessive NS/NA control message overhead especially for Clients
     connected to low-end data links.

     Clients can therefore set the NUD/ARR/RPT flags in RS messages they
     send to select their Proxy/Server service profiles.  If the NUD flag
     is set, the FHS Proxy/Server that forwards the RS message assumes the
     role of responding to NS messages and maintains peer NCEs associated
     with the NCE for this Client.  If the ARR flag is set, the Hub Proxy/
     Server that processes the RS message assumes the role of responding
     to NS(AR) messages on behalf of this Client NCE.  If the RPT flag is
     set, the Hub Proxy/Server that processes the RS message becomes
     responsible for maintaining a "Report List" for each Client NCE for
     the source addresses of NS(AR) messages it forwards on behalf of this
     Client.

     When a Client sets the RPT flag, the Hub Proxy/Server maintains
     Report List entries based on a ReportTime timer initialized to
     REACHABLE_TIME seconds upon receipt of an NS(AR) and decremented once
     per second while no additional NS(AR)s arrive.  The Hub Proxy/Server
     then sends uNA messages to each Report List entry when it receives a
     Client mobility update indication (e.g., through receipt of an RS
     with updated Interface Attributes and/or Traffic Selectors).  When a
     Report List entry ReportTime timer expires, the Hub Proxy/Server
     deletes the entry.  When a Client NCE timer expires, the Hub Proxy/
     Server deletes the NCE along with its associated Report List.

     Clients can also set/clear the FMT-Forward and FMT-Mode flags in the
     Interface Attributes sub-option of each RS message to express their
     desired service profile from each FHS Proxy/Server for a specific
     underlay interface.  The FHS Proxy/Server will consider the Client's
     preferences and either accept or override by setting/clearing the
     flags in the corresponding RA message reply.  Implications for these
     bit settings are discussed in [I-D.templin-intarea-omni].

     Both the Client and its Hub Proxy/Server have full knowledge of the
     Client's current underlay Interface Attributes and Traffic Selectors,
     while FHS Proxy/Servers acting in "proxy" mode have knowledge of only



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     the individual Client underlay interfaces they service.  Clients
     determine their FHS and Hub Proxy/Server service models by setting
     the NUD/ARR/RPT flags in the RS messages they send as discussed
     above.

     When an Address Resolution Source (ARS) sends an NS(AR) message
     toward an Address Resolution Target (ART) Client/Relay, the OMNI link
     routing system directs the NS(AR) to a Hub Proxy/Server for the ART.
     The Hub then either acts as an Address Resolution Responder (ARR) on
     behalf of the ART or forwards the NS(AR) to the ART which acts as an
     ARR on its own behalf.  The ARR returns an NA(AR) response to the
     ARS, which creates or updates a NCE for the ART while caching L3 and
     L2 addressing information.  The ARS then (re)sets ReachableTime for
     the NCE to REACHABLE_TIME seconds and performs unicast NS/NA
     exchanges over specific underlay interface pairs to determine paths
     for sending carrier packets directly to the ART.  The ARS otherwise
     decrements ReachableTime while no further solicited NA messages
     arrive.

     Proxy/Servers add an additional state DEPARTED to the list of NCE
     states found in Section 7.3.2 of [RFC4861].  When a Client terminates
     its association, the Proxy/Server OMNI interface sets a DepartTime
     variable for the NCE to DEPART_TIME seconds.  DepartTime is
     decremented unless a new IPv6 ND message causes the state to return
     to REACHABLE.  While a NCE is in the DEPARTED state, the Proxy/Server
     forwards OAL packets/fragments destined to the target Client to the
     Client's new FHS/Hub Proxy/Server instead.

     It is RECOMMENDED that REACHABLE_TIME be set to the default constant
     value 30 seconds as specified in [RFC4861].  It is RECOMMENDED that
     DEPART_TIME be set to the default constant value 10 seconds to accept
     any carrier packets that may be in flight.  When ReachableTime or
     DepartTime decrement to 0, the NCE is deleted.

     AERO nodes also use the value MAX_UNICAST_SOLICIT to limit the number
     of NS messages sent when a correspondent may have gone unreachable,
     the value MAX_RTR_SOLICITATIONS to limit the number of RS messages
     sent without receiving an RA and the value MAX_NEIGHBOR_ADVERTISEMENT
     to limit the number of uNAs that can be sent based on a single event.
     It is RECOMMENDED that MAX_UNICAST_SOLICIT, MAX_RTR_SOLICITATIONS and
     MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the same as specified in
     [RFC4861].

     Different values for the above constants MAY be administratively set;
     however, if different values are chosen, all nodes on the link MUST
     consistently configure the same values.





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  4.5.1.  OMNI ND Messages

     OMNI interfaces use IPv6 ND messages as the secured control plane
     messaging service for all adaptation layer neighbor coordination
     exchanges.  OMNI interfaces prepare IPv6 ND messages the same as for
     standard IPv6 ND, but also include a new option type termed the OMNI
     option [I-D.templin-intarea-omni].  OMNI interfaces use ULAs instead
     of LLAs as adaptation layer IPv6 ND message source and destination
     addresses.  This allows multiple different OMNI links to be joined
     into a single link at some future time without requiring a global
     renumbering event.

     OMNI interfaces normally limit the size of the IPv6 ND messages they
     send to the IPv6 minimum link MTU, but messages that include a
     substantial amount of OMNI parameters and/or IP packet/parcel
     attachments may occasionally exceed that size.  The OMNI interface
     engages IPv6 encapsulation followed by fragmentation to break IPv6 ND
     messages as large as 65535 octets into fragments no larger than 1280
     octets.  Whenever possible, OMNI interfaces should send multiple
     smaller IPv6 ND messages instead of singleton larger messages to
     minimize fragmentation.

     For each IPv6 ND message, the OMNI interface includes one or more
     OMNI options (and any other ND message options) then completely
     populates all option information.  If the OMNI interface includes an
     authentication option, it first writes the value 0 into the
     authentication signature field then calculates the signature
     beginning with the first IPv6 ND message octet following the header
     Checksum field and continuing over the entire length of the message.
     The OMNI interface next writes the authentication signature value
     into the appropriate OMNI authentication option field, then
     calculates the IPv6 ND message checksum per [RFC4443] beginning with
     a pseudo-header of the IPv6 header and writes the value into the
     Checksum field.  The IPv6 ND message checksum therefore provides
     integrity assurance for the message, while the authentication
     signature covers the entire packet or super-packet.  OMNI interfaces
     verify integrity and authentication of each message received, and
     process the message further only following successful verification.

     OMNI options include per-neighbor information that provides multilink
     forwarding, link layer address and traffic selector information for
     the neighbor's underlay interfaces.  This information is stored in
     both the neighbor cache and AERO Forwarding Information Base (AFIB)
     as basis for the forwarding algorithm specified in Section 4.10.  The
     information is cumulative and reflects the union of the OMNI
     information from the most recent IPv6 ND messages received from the
     neighbor.




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     The OMNI option is distinct from any Source/Target Link-Layer Address
     Options (S/TLLAOs) that may appear in an IPv6 ND message according to
     the appropriate IPv6 over specific link layer specification (e.g.,
     [RFC2464]).  If both OMNI options and S/TLLAOs appear, the former
     pertains to the adaptation layer to underlay interface address
     mappings while the latter pertains to the native L2 address format of
     the underlay media.

     OMNI interface IPv6 ND messages may also include other IPv6 ND
     options.  In particular, solicitation messages may include a Nonce
     option if required for verification of advertisement replies.  If an
     OMNI IPv6 ND solicitation message includes a Nonce option, the
     advertisement reply must echo the same Nonce.  If an OMNI IPv6 ND
     solicitation message includes a Timestamp option, the recipient must
     also include a Timestamp option in its advertisement reply.  All
     unsolicited advertisement and redirect messages should include a
     Timestamp option.

     AERO Clients send RS messages to the link-scoped All-Routers
     multicast address or a ULA-RND while using unicast or anycast OAL/L2
     addresses.  AERO Proxy/Servers respond by returning unicast RA
     messages.  During the RS/RA exchange, AERO Clients and Proxy/Servers
     include state synchronization parameters to establish Identification
     windows and other state.

     AERO Hosts and Clients on ENET underlay networks send RS messages to
     the link-scoped All-Routers multicast address, a ULA-RND of a remote
     Hub Proxy/Server or the ULA-MNP of an upstream Client while using
     unicast or anycast OAL/L2 addresses.  The upstream AERO Client
     responds by returning a unicast RA message.

     AERO nodes use NS/NA messages for the following purposes:

     *  NS/NA(AR) messages are used for address resolution and optionally
        to establish sequence number windows.  The ARS sends an NS(AR) to
        the solicited-node multicast address of the ART, and an ARR with
        addressing information for the ART returns a unicast NA(AR) that
        contains current, consistent and authentic target address
        resolution information.  NS(AR) messages include a solicited-node
        multicast destination address to distinguish them from ordinary NS
        messages.  NS/NA(AR) messages must be secured.

     *  Ordinary NS/NA messages are used determine target reachability,
        establish and maintain NAT state, and/or establish multilink
        forwarding (i.e., AFIB) state.  The source sends an NS to the
        unicast address of the target while optionally including an OMNI
        AERO Forwarding Parameters (AFP) sub-option naming a specific
        underlay interface pair, and the target returns a unicast NA that



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        includes a responsive AFP if necessary.  NS/NA messages that use
        an in-window sequence number and do not update any other state
        need not include an authentication signature but must include an
        IPv6 ND message checksum.  NS/NA messages used to establish window
        synchronization and/or AFIB state must be secured.

     *  Unsolicited NA (uNA) messages are used to signal addressing and/or
        other neighbor state changes (e.g., address changes due to
        mobility, signal degradation, traffic selector updates, etc.). uNA
        messages can also be also used to acknowledge receipt of non-
        solicitation IPv6 ND messages (see below).  uNA messages that
        update state information must be secured.

     *  NS/NA(DAD) messages are not used in AERO, since Duplicate Address
        Detection is not required.

     AERO and OMNI together support an added reliability feature not
     available in ordinary IPv6 ND messaging.  In particular, nodes can
     set the OMNI Neighbor Coordination SNR flag or Window Synchronization
     SYN flag in unicast non-solicitation IPv6 ND messages (including RA,
     NA and Redirect) to request a synchronous (but "unsolicited") uNA
     response (see: [I-D.templin-intarea-omni]).

     The node that processes an SNR/SYN message prepares the response the
     same as for an ordinary uNA as specified in [RFC4861], including the
     setting of the R/S/O flags as discussed below.  The node sets the uNA
     Target Address to the unicast destination and uNA destination address
     to the unicast source of the original message.

     The node then sets the uNA source address to its own address and
     includes any necessary OMNI sub-options but MUST NOT itself set the
     SNR/SYN flags.  If the SNR/SYN message included a Nonce and/or
     Timestamp option, the node includes matching Nonce/Timestamp options
     in the uNA response.  The node finally returns the uNA message to the
     source of the SNR/SYN message.

  4.5.2.  OMNI Neighbor Advertisement Message Flags

     As discussed in Section 4.4 of [RFC4861] NA messages include three
     flag bits R, S and O.  OMNI interface NA messages treat the flags as
     follows:

     *  R: The R ("Router") flag is set to 1 in the NA messages sent by
        all AERO forwarding nodes on the OMNI link.  (AERO Hosts are by
        definition the only non-forwarding nodes on the OMNI link and
        therefore set the R flag to 0.)





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     *  S: The S ("Solicited") flag is set exactly as specified in
        Section 4.4. of [RFC4861], i.e., it is set to 1 for Solicited NAs
        and set to 0 for uNAs (both unicast and multicast).

     *  O: The O ("Override") flag is set to 0 for solicited NAs returned
        by a Proxy/Server ARR and set to 1 for all other solicited and
        unsolicited NAs.  For further study is whether solicited NAs for
        anycast targets apply for OMNI links.  Since XLA-MNPs must be
        uniquely assigned to Clients to support correct IPv6 ND protocol
        operation, however, no role is currently seen for assigning the
        same XLA-MNP to multiple Clients.

  4.5.3.  OMNI Neighbor Window Synchronization

     In secured environments (e.g., between secured spanning tree
     neighbors, between neighbors on the same secured ANET, etc.), OMNI
     interface neighbors can exchange OAL packets that include randomly-
     initialized and monotonically-increasing (extended) Identification
     values (modulo 2**64) without window synchronization.  In
     environments where spoofing is considered a threat, OMNI interface
     neighbors instead invoke window synchronization by including OMNI
     Window Synchronization sub-options in RS/RA or NS/NA message
     exchanges to maintain send/receive window state in their respective
     neighbor cache and AFIB entries as specified in
     [I-D.templin-intarea-omni].

     In common arrangements, OAL Identification window synchronization is
     necessary between Clients and Proxy/Servers or other Clients since
     their message exchanges are typically conducted over unsecured
     Internetworks.  Conversely, Proxy/Server to Proxy/Server, Proxy/
     Server to Gateway and Gateway to Gateway message exchanges carried
     over the secured spanning tree do not require window synchronization.

     All OAL nodes must verify Identification values of OAL packets
     addressed to themselves when window synchronization is required; OAL
     Intermediate systems forward OAL packets/fragments not addressed to
     themselves without examining their Identification values.

  4.6.  OMNI Interface Encapsulation and Fragmentation

     When the network layer forwards an original IP packet/parcel into an
     OMNI interface, the interface locates or creates a Neighbor Cache
     Entry (NCE) that matches the destination.  The OMNI interface then
     invokes the OMNI Adaptation Layer (OAL) as discussed in
     [I-D.templin-intarea-omni] which encapsulates the packet/parcel in an
     IPv6 header to produce an OAL packet.  For example, an original IP
     packet/parcel with source address 2001:db8:1:2::1 and destination
     address 2001:db8:1234:5678::1 might cause the OAL encapsulation



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     header to include source address {XLA*}::2001:db8:1:2 (i.e., an XLA-
     MNP) and destination address {ULA*}::0012:3456:789a:bcde (i.e., a
     ULA-RND).

     Following encapsulation, the OAL source then fragments the OAL packet
     while including an identical Identification value for each fragment
     that must be within the window for the neighbor.  The OAL source
     includes any necessary OAL IPv6 extension headers including an
     identical Compressed Routing Header with 32-bit ID fields (CRH-32)
     [I-D.ietf-6man-comp-rtg-hdr] with each fragment containing AERO
     Forwarding Vector Indexes (AFVIs) as discussed in Section 4.13.  The
     OAL source can instead invoke OAL header compression by replacing the
     OAL IPv6 header, CRH-32 and Extended Fragment Header with an OAL
     Compressed Header (OCH).

     For messages that will traverse unsecured paths, the OAL source
     finally performs L2 encapsulation/fragmentation on each resulting OAL
     fragment to form a carrier packet, with source address set to its own
     L2 address (e.g., 192.0.2.100) and destination set to the L2 address
     of the next hop OAL intermediate system or destination (e.g.,
     192.0.2.1).  The carrier packet encapsulation format in the above
     example is shown in Figure 3:

          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |           L2 Headers          |
          ~       src = 192.0.2.100       ~
          |        dst = 192.0.2.1        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ~   L2 IPv6 Extension Headers   ~
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |        OAL IPv6 Header        |
          ~  src = {XLA*}::2001:db8:1:2   ~
          |dst={ULA*}::0012:3456:789a:bcde|
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ~   OAL IPv6 Extension Headers  ~
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |       Original IP Header      |
          ~     (first-fragment only)     ~
          ~    src = 2001:db8:1:2::1      ~
          |  dst = 2001:db8:1234:5678::1  |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               |
          ~                               ~
          ~ Original Packet Body/Fragment ~
          ~                               ~
          |                               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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                        Figure 3: Carrier Packet Format

     (Note that carrier packets exchanged by Hosts on ENETs do not include
     the OAL IPv6 or CRH-32 headers, i.e., the OAL encapsulation is NULL
     and only the L2 encapsulations including any L2 IPv6 extension
     headers are included.)

     In this format, the OAL source encapsulates the original IP header
     and packet/parcel body/fragment in an OAL IPv6 header, the CRH-32 is
     a Routing Header extension of the OAL header, the Extended Fragment
     Header identifies each fragment, and the L2 headers are prepared as
     discussed in [I-D.templin-intarea-omni].  The OAL source sends each
     such carrier packet into the SRT unsecured spanning tree, where they
     may be forwarded over multiple OAL intermediate systems until they
     arrive at the OAL destination.  These carrier packets may themselves
     be subject to L2 fragmentation and reassembly along the path.

     The OMNI link control plane service distributes Client XLA-MNP prefix
     information that may change occasionally due to regional node
     mobility, as well as XLA-MNP prefix information for Relay non-MNPs
     and per-segment ULA prefix information that rarely changes.  OMNI
     link Gateways and Proxy/Servers use the information to establish and
     maintain a forwarding plane spanning tree that connects all nodes on
     the link.  The spanning tree supports a virtual bridging service
     according to link layer (instead of network layer) information, but
     may often include longer paths than necessary.

     Each OMNI interface therefore also includes an AERO Forwarding
     Information Base (AFIB) that caches AERO Forwarding Vectors (AFVs)
     which can provide both carrier packet Identification context and more
     direct forwarding "shortcuts" that avoid strict spanning tree paths.
     As a result, the spanning tree is always available but OMNI
     interfaces can often use the AFIB entries established through route
     optimization to greatly improve performance and reduce load on
     critical infrastructure elements.

     For OAL packets/fragments undergoing L2 re-encapsulation at an OAL
     intermediate system, the OMNI interface performs L2 reassembly/
     decapsulation followed by Identification verification and OAL
     reassembly only if the OAL packet/fragment is addressed to itself.
     The OMNI interface then decrements the OAL IPv6 header Hop Limit and
     discards the packet/fragment if the Hop Limit reaches 0.  Otherwise,
     the OMNI interface updates the OAL addresses if necessary, includes
     an appropriate Identification, performs OAL fragmentation then for
     each OAL fragment performs L2 encapsulation/fragmentation to produce
     carrier packets appropriate for next segment forwarding.





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     When an FHS Gateway forwards an OAL packet/fragment to an LHS Gateway
     over the unsecured spanning tree, it reconstructs the OAL header
     based on AFV state, inserts a CRH-32 immediately following the OAL
     header and adjusts the OAL payload length and destination address
     field.  The FHS Gateway includes a single AFVI in the CRH-32 that the
     LHS Gateway can use to search its AFIB, then forwards the OAL packet/
     fragment over the unsecured spanning tree.  When the LHS Gateway
     receives the OAL packet/fragment, it locates the AFV for the next hop
     based on the CRH-32 AFVI then re-applies header compression
     (resulting in the removal of the CRH-32) and forwards the OAL packet/
     fragment to the next hop.

     OAL packets/fragments that travel over secured spanning tree hops do
     not include OMNI L2 encapsulations.  They are instead admitted into
     secured links such as IPsec tunnels or direct links where they may be
     subject to L2 security encapsulations as secured carrier packets.
     (Note that OMNI protocol L2 encapsulations could be used above the L2
     security services, but this could result in excessive encapsulation
     in some instances.)

  4.7.  OMNI Interface Decapsulation

     When an OAL node receives OAL packets/fragments addressed to another
     node, it discards the L2 headers and includes new L2 headers
     appropriate for the next hop in the forwarding path to the OAL
     destination (after first performing any necessary L2 fragmentation or
     reassembly).  The node then sends these new carrier packets into the
     next hop underlay interface.

     When an OAL node receives OAL packets/fragments addressed to itself,
     it performs L2 reassembly/decapsulation, verifies the Identification,
     then performs OAL reassembly/decapsulation to obtain the original OAL
     packet or super-packet (see: [I-D.templin-intarea-omni]).  Next, if
     the enclosed original IP packet(s)/parcel(s) are destined either to
     itself or to a destination reached via an interface other than the
     OMNI interface, the OAL node discards the OAL encapsulation and
     forwards the original IP packet(s)/parcel(s) to the network layer.

     If the original IP packet(s)/parcel(s) are destined to another node
     reached by the OMNI interface, the OAL node instead changes the OAL
     source to its own address, changes the OAL destination to the ULA of
     the next-hop node over the OMNI interface, decrements the Hop Limit,
     then performs L2 encapsulation/fragmentation and forwards these new
     carrier packets into the next hop underlay interface.

     Further OMNI link decapsulation details are specified in
     [I-D.templin-intarea-omni].  Further OMNI link forwarding procedures
     are specified in Section 4.10.



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  4.8.  OMNI Interface Data Origin Authentication

     AERO nodes employ simple data origin authentication procedures.  In
     particular:

     *  AERO Gateways and Proxy/Servers accept carrier packets received
        from the secured spanning tree.

     *  AERO Proxy/Servers and Clients accept carrier packets and original
        IP packets/parcels that originate from within the same secured
        ANET.

     *  AERO Clients and Relays accept original IP packets/parcels from
        downstream network correspondents based on ingress filtering.

     *  AERO Hosts, Clients, Relays, Proxy/Servers and Gateways verify
        carrier packet L2 encapsulation addresses according to
        [I-D.templin-intarea-omni].

     *  AERO nodes that invoke window synchronization accept OAL packets/
        fragments with Identification values within the current window for
        the OAL source neighbor for a specific underlay interface pair and
        drop any packets with out-of-window Identification values.

     AERO nodes silently drop any packets/parcels that do not satisfy the
     above data origin authentication procedures.  Further security
     considerations are discussed in Section 7.

  4.9.  OMNI Interface MTU

     The OMNI interface observes the link nature of tunnels, including the
     Maximum Transmission Unit (MTU), Effective MTU to Receive (EMTU_R)
     and the role of fragmentation and reassembly
     [I-D.ietf-intarea-tunnels].  The OMNI interface employs an OMNI
     Adaptation Layer (OAL) that accommodates multiple underlay links with
     diverse MTUs.  OMNI interface packet sizing considerations are
     specified in [I-D.templin-intarea-omni], where the OMNI interface MTU
     can essentially be considered "unlimited".













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     When the network layer presents an original IP packet/parcel to the
     OMNI interface, the OAL source encapsulates and fragments the packet/
     parcel if necessary.  When the network layer presents the OMNI
     interface with multiple original IP packets/parcels bound to the same
     OAL destination, the OAL source can concatenate them as a single OAL
     super-packet as discussed in [I-D.templin-intarea-omni] before
     applying fragmentation.  The OAL source then submits each OAL
     fragment for L2 encapsulation/fragmentation for transmission as
     carrier packets over an underlay interface connected to either a
     physical link (e.g., Ethernet, WiFi, Cellular, etc.) or a virtual
     link such as an Internet or higher-layer tunnel.

     Note: Although a CRH-32 may be inserted or removed by a Gateway in
     the path (see: Section 4.10.4), this does not interfere with the
     destination's ability to reassemble since the CRH-32 is not included
     in the fragmentable part and its removal/transformation does not
     invalidate fragment header information.

  4.10.  OMNI Interface Forwarding Algorithm

     Original IP packets/parcels enter a node's OMNI interface either from
     the network layer (i.e., from a local application or the IP
     forwarding system) while carrier packets enter from the link layer
     (i.e., from an OMNI interface neighbor).  All original IP packets/
     parcels and carrier packets entering a node's OMNI interface first
     undergo data origin authentication as discussed in Section 4.8.
     Those that satisfy data origin authentication are processed further,
     while all others are dropped silently.

     Original IP packets/parcels that enter the OMNI interface from the
     network layer are forwarded to an OMNI interface neighbor using OAL
     encapsulation and fragmentation to produce carrier packets for
     transmission over underlay interfaces.  (If forwarding state
     indicates that the original IP packet/parcel should instead be
     forwarded back to the network layer, the packet/parcel is dropped to
     avoid looping).  Carrier packets that enter the OMNI interface from
     the link layer are either re-encapsulated and re-admitted into the
     link layer, or reassembled and forwarded to the network layer where
     they are subject to either local delivery or IP forwarding.

     When the network layer forwards an original IP packet/parcel into the
     OMNI interface, it decrements the TTL/Hop Limit following standard IP
     router conventions.  Once inside the OMNI interface, however, the OAL
     does not further decrement the original IP packet/parcel TTL/Hop
     Limit since its adaptation layer forwarding actions occur below the
     network layer.  The original IP packet/parcel's TTL/Hop Limit will
     therefore be the same when it exits the destination OMNI interface as
     when it first entered the source OMNI interface.



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     When an OAL intermediate system receives a carrier packet, it
     performs L2 reassembly/decapsulation to obtain the enclosed OAL
     packet/fragment.  When the intermediate system forwards an OAL
     packet/fragment not addressed to itself, it decrements the OAL Hop
     Limit without decrementing the network layer IP TTL/Hop Limit.  If
     decrementing would cause the OAL Hop Limit to become 0, the OAL
     intermediate system drops the OAL packet/fragment.  This ensures that
     original IP packet(s)/parcel(s) cannot enter an endless loop.

     OMNI interfaces may have multiple underlay interfaces and/or neighbor
     cache entries for neighbors with multiple underlay interfaces (see
     Section 4.3).  The OAL uses Interface Attributes and/or Traffic
     Selectors to select an outbound underlay interface for each OAL
     packet and also to select segment routing and/or link layer
     destination addresses based on the neighbor's target underlay
     interfaces.  AERO implementations SHOULD permit network management to
     dynamically adjust Traffic Selector values at runtime.

     If an OAL packet/fragment matches the Interface Attributes and/or
     Traffic Selectors of multiple outgoing interfaces and/or neighbor
     interfaces, the OMNI interface replicates the packet and sends a
     separate copy via each of the (outgoing / neighbor) interface pairs;
     otherwise, it sends a single copy via an interface with the best
     matching attributes/selectors.  (While not strictly required, the
     likelihood of successful reassembly may improve when the OMNI
     interface sends all fragments of the same fragmented OAL packet/
     fragment consecutively over the same underlay interface pair to avoid
     complicating factors such as delay variance and reordering.)  AERO
     nodes keep track of which underlay interfaces are currently
     "reachable" or "unreachable", and only use "reachable" interfaces for
     forwarding purposes.

     In addition to standard forwarding based on Interface Attributes and/
     or Traffic Selectors, nodes may employ a policy engine that would
     provide further guidance to the forwarding algorithm.  For example
     the policy engine may suggest a load balancing profile over multiple
     underlay interface pairs, with portions of a traffic flow spread
     between multiple paths according to Equal Cost MultiPath or Link
     Aggregation Groups (LAGs) [RFC6438] (note that Interface Attributes
     include an underlay interface group identifier).  Other policies may
     suggest the use of paths with the least cost, best performance, etc.
     This document therefore specifies mechanisms without mandating any
     particular policies.

     The ULA Subnet ID value is used only for subnet coordination within a
     local OMNI link segment.  When a node forwards an OAL packet/fragment
     addressed to a ULA with a foreign Global and/or Subnet ID value, it
     forwards the OAL packet/fragment based solely on the OMNI link



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     routing information.  For this reason, OMNI link routing and
     forwarding table entries always include both ULA-RNDs with their
     associated prefix lengths and XLA-MNPs which encode an MNP while
     leaving the Global and Subnet ID values set to 0.

     The following sections discuss the OMNI interface-specific forwarding
     algorithms for Hosts, Clients, Proxy/Servers and Gateways.  In the
     following discussion, an original IP packet/parcel's destination
     address is said to "match" if it is the same as a cached address, or
     if it is covered by a cached prefix (which may be encoded in an
     {ULA,XLA}-MNP).

  4.10.1.  Host Forwarding Algorithm

     When an original IP packet/parcel enters a Host's OMNI interface from
     the network layer the Host searches for a NCE that matches the
     destination.  If there is a matching NCE, the Host performs OMNI L2
     encapsulation/fragmentation as discussed in Section 6.13 of
     [I-D.templin-intarea-omni] then forwards the resulting carrier
     packets into the ENET addressed to the L2 address of the neighbor.
     If there is no match, the host instead sends the carrier packets to
     its upstream Client.

     After sending carrier packets, the Host may receive an OAL Redirect
     message from its upstream Client to inform it of another AERO node on
     the same ENET that would provide a better first hop.  The Host
     authenticates the Redirect message, then updates its neighbor cache
     accordingly.

  4.10.2.  Client Forwarding Algorithm

     When an original IP packet/parcel enters a Client's OMNI interface
     from the network layer the Client searches for a NCE that matches the
     destination.  If there is a matching NCE for a neighbor reached via
     an ANET/INET interface (i.e., an upstream interface), the Client
     selects one or more "reachable" neighbor interfaces in the entry for
     forwarding purposes.  Otherwise, the Client performs OAL
     encapsulation and fragmentation, forwards the resulting OAL packet/
     fragment to an FHS Proxy/Server, then either invokes address
     resolution and multilink forwarding procedures per Section 4.13 or
     allows the FHS Proxy/Server to invoke these procedures on its behalf.
     If there is a matching NCE for a neighbor reached via an ENET
     interface (i.e., a downstream interface), the Client instead forwards
     the original IP packet/parcel to the downstream Host or Client using
     encapsulation and fragmentation if necessary.






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     When a carrier packet enters a Client's OMNI interface from the link
     layer, the Client performs L2 reassembly/decapsulation to obtain the
     OAL packet/fragment then examines the OAL destination.  If the OAL
     destination matches one of the Client's ULAs the Client (acting as an
     OAL destination) verifies that the Identification is in-window for
     the matching AFV, then reassembles/decapsulates as necessary and
     delivers the original IP packet/parcel to the network layer.  If the
     OAL destination matches a NCE for a peer Client on an ENET interface,
     the Client instead forwards the OAL packet/fragment to the peer while
     decrementing the OAL Hop Limit.  If the OAL destination matches a NCE
     for a Host on an ENET interface, the Client instead reassembles then
     forwards the original IP packet/parcel to the Host while using L2
     encapsulation/fragmentation (i.e., without invoking the OAL) if
     necessary.  If the OAL destination does not match, the Client drops
     the original IP packet/parcel and MAY return a network layer ICMP
     Destination Unreachable message subject to rate limiting (see:
     Section 4.11).

     When a Client forwards an OAL packet/fragment from an ENET Host to a
     neighbor connected to the same ENET, it also returns a Redirect
     message to inform the Host that it can reach the neighbor directly as
     an ENET peer.

     Note: Clients and their FHS Proxy/Server (and other Client) peers can
     exchange original IP packets/parcels over ANET underlay interfaces
     using OMNI L2 encapsulation/fragmentation without invoking the OAL,
     since the ANET is secured at the link and physical layers.  By
     forwarding original IP packets/parcels without invoking the OAL, the
     ANET peers use the same L2 encapsulation/fragmentation procedures as
     specified for Hosts above.

     Note: The forwarding table entries established in peer Clients of a
     multihop forwarding region are based on ULA-MNPs and/or TLAs used to
     seed the multihop routing protocols.  When ULA-MNPs are used, the ULA
     /64 prefix provides topological relevance for the multihop forwarding
     region, while the 64-bit Interface Identifier encodes the Client MNP.
     Therefore, Clients can forward atomic fragments with compressed OAL
     headers that do not include ULA or AFVI information by examining the
     MNP-based addresses in the original IP packet/parcel header.  In
     other words, each forwarding table entry contains two pieces of
     forwarding information - the ULA information in the prefix and the
     MNP information in the interface identifier.









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  4.10.3.  Proxy/Server and Relay Forwarding Algorithm

     When the network layer admits an original IP packet/parcel into a
     Proxy/Server's OMNI interface, the OAL drops the packet/parcel to
     avoid looping if forwarding state indicates that it should be
     forwarded back to the network layer.  Otherwise, the OAL examines the
     IP destination address to determine if it matches the ULA of a
     neighboring Gateway found in the OMNI interface's network layer
     neighbor cache.  If so, the Proxy/Server performs OAL encapsulation
     and fragmentation then performs L2 encapsulation/fragmentation and
     forwards the resulting carrier packets to the Gateway over a secured
     link (e.g., an IPsec tunnel, Direct link, etc.) to support control
     plane functions such as the operation of the BGP routing protocol.
     If the destination is a non-ULA, the Proxy/Server instead assumes the
     Relay role and forwards the original IP packet/parcel in a similar
     manner as for Clients.  Specifically, if there is a matching NCE the
     Proxy/Server selects one or more "reachable" neighbor interfaces in
     the entry for forwarding purposes; otherwise, the Proxy/Server
     performs OAL encapsulation/fragmentation followed by L2
     encapsulation/fragmentation and forwards the resulting carrier
     packets while invoking address resolution and multilink forwarding
     procedures per Section 4.13.

     When the Proxy/Server receives/reassembles carrier packets on
     underlay interfaces that contain OAL packets/fragments with both a
     source and destination OAL address that correspond to the same
     Client's delegated MNP, the Proxy/Server drops the carrier packets
     regardless of their OMNI link point of origin.  The Proxy/Server also
     drops original IP packets/parcels received on underlay interfaces
     either directly from an ANET Client or following reassembly of
     carrier packets received from an ANET/INET Client if the original IP
     destination corresponds to the same Client's delegated MNP.  Proxy/
     Servers also drop carrier packets that contain OAL packets/fragments
     with foreign OAL destinations that do not match their own ULA, the
     ULA of one of their Clients or a ULA corresponding to one of their
     GUA routes.  These checks are essential to prevent forwarding
     inconsistencies from accidentally or intentionally establishing
     endless loops that could congest nodes and/or ANET/INET links.

     Proxy/Servers process carrier packets that contain OAL packets/
     fragments with OCH headers or with destinations that match their ULA
     and also include a CRH-32 header that encodes AFVI information.  The
     Proxy/Server examines the AFVI to locate the corresponding AFV entry
     in the AFIB.  If the carrier packets were not received from the
     secured spanning tree, the Proxy/Server must then verify that the L2
     addresses are "trusted" according to the AFV.  If the carrier packets
     were trusted, the Proxy/Server then forwards them according to the
     AFV state while decrementing the OAL packet/fragment Hop Limit.



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     For OAL packets/fragments with destinations that match their ULA but
     do not include a CRH-32/OCH, the Proxy/Server instead performs L2
     reassembly/decapsulation, verifies the Identification and performs
     OAL reassembly to obtain the original IP packet/parcel.  For data
     packets/parcels addressed to its own ULA that arrived via the secured
     spanning tree, the Proxy/Server delivers the original IP packet/
     parcel to the network layer to support secured BGP routing protocol
     control messaging.  For data packets/parcels originating from one of
     its dependent Clients, the Proxy/Server instead performs OAL
     encapsulation/fragmentation followed by L2 encapsulation/
     fragmentation and sends the resulting carrier packets while invoking
     address resolution and multilink forwarding procedures per
     Section 4.13.  For IPv6 ND control messages, the Proxy/Server instead
     authenticates the message and processes it as specified in later
     sections of this document while updating neighbor cache and/or AFIB
     state accordingly.

     When the Proxy/Server receives a carrier packet that contains an OAL
     packet/fragment with OAL destination set to a {ULA,XLA}-MNP of one of
     its Client neighbors established through RS/RA exchanges, it accepts
     the carrier packet only if data origin authentication succeeds.  If
     the NCE state is DEPARTED, the Proxy/Server changes the OAL
     destination address to the ULA of the new Proxy/Server, decrements
     the OAL Hop Limit, then performs L2 encapsulation/fragmentation and
     forwards the resulting carrier packets into the spanning tree which
     will eventually deliver them to the new Proxy/Server.  If the
     neighbor cache state for the Client is REACHABLE and the Proxy/Server
     is a Hub responsible for serving as the Client's address resolution
     responder and/or default router, it verifies the Identification then
     submits the OAL packet/fragment for reassembly then decapsulates and
     processes the resulting IPv6 ND message or original IP packet/parcel
     accordingly.  Otherwise, the Proxy/Server decrements the OAL Hop
     Limit, performs L2 encapsulation/fragmentation and sends the carrier
     packets to the Client which must then perform data origin
     verification and reassembly.  (In the latter case, the Client may
     receive fragments of the same original IP packet/parcel from
     different Proxy/Servers but this will not interfere with reassembly.)

     When the Proxy/Server receives a carrier packet that contains an OAL
     packet/fragment with OAL destination set to a {ULA,XLA}-MNP that does
     not match the MSP, it accepts the carrier packet only if data origin
     authentication succeeds and if there is a network layer forwarding
     table entry for a GUA route that matches the MNP.  The Proxy/Server
     then performs L2 reassembly/decapsulation, verifies the
     Identification, performs OAL reassembly/decapsulation to obtain the
     original IP packet/parcel, then presents it to the network layer (as
     a Relay) where it will be delivered according to standard IP
     forwarding.



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     Clients and their FHS Proxy/Server peers can exchange original IP
     packets/parcels over ANET underlay interfaces using L2 encapsulation
     IPv6 Extended Fragment Headers only and no OAL addressing
     information, since the ANET is secured at the link and physical
     layers.  (For packets that do not require fragmentation, the peers
     can even omit the Extended Fragment Header.)  FHS Proxy/Servers will
     then supply an OAL Full (OFH) or Compressed (OCH) header when they
     forward ANET Client original IP packets/parcels toward final
     destinations located in other networks.

     Proxy/Servers forward OAL packets/fragments received in secure
     control plane carrier packets via the SRT secured spanning tree and
     forward other OAL packets/fragments via the unsecured spanning tree.
     When a Proxy/Server receives a carrier packet from the secured
     spanning tree, it considers the message as authentic without having
     to verify network or higher layer authentication signatures.  When a
     Proxy/Server receives a carrier packet from the unsecured spanning
     tree, it applies data origin authentication itself and/or forwards
     the enclosed unsecured OAL contents toward the destination which must
     apply data origin authentication on its own behalf.

     If the Proxy/Server has multiple original IP packets/parcels to send
     to the same neighbor, it can concatenate them as a single OAL super-
     packet [I-D.templin-intarea-omni].

  4.10.4.  Gateway Forwarding Algorithm

     When the network layer admits an original IP packet/parcel into the
     Gateway's OMNI interface, the OAL drops the packet if routing
     indicates that it should be forwarded back to the network layer to
     avoid looping.  Otherwise, the Gateway examines the IP destination
     address to determine if it matches the ULA of a neighboring Gateway
     or Proxy/Server by examining the OMNI interface's network layer
     neighbor cache.  If so, the Gateway performs OAL encapsulation and
     fragmentation followed by L2 encapsulation/fragmentation and forwards
     the resulting carrier packets to the neighboring Gateway or Proxy/
     Server over a secured link (e.g., an IPsec tunnel, etc.) to support
     the operation of control plane functions (including the BGP routing
     protocol) between OAL neighbors.

     Gateways forward OAL packets/fragments reassembled from spanning tree
     carrier packets while decrementing the OAL Hop Limit but not the
     original IP header TTL/Hop Limit.  Gateways send carrier packets that
     contain OAL packets/fragments with critical IPv6 ND control messages
     or BGP routing protocol control messages via the SRT secured spanning
     tree, and may send other carrier packets via the secured/unsecured
     spanning tree or via more direct paths according to AFIB information.
     When the Gateway receives a carrier packet, it reassembles/



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     decapsulates to obtain the OAL packet/fragment then searches for an
     AFIB entry that matches the OAL header AFVI or an IP forwarding table
     entry that matches the OAL destination address.

     Gateways process carrier packets that contain OAL packets/fragments
     with OAL destinations that do not match their ULA or the SRT Subnet
     Router Anycast address in the same manner as for traditional IP
     forwarding within the OAL, i.e., they forward packets not explicitly
     addressed to themselves.  Gateways locally process OAL packets/
     fragments with OCH headers or full OAL headers with their ULA or the
     SRT Subnet Router Anycast address as the OAL destination.  If the OAL
     packet/fragment contains an OCH or a full OAL header with a CRH-32
     extension, the Gateway examines the AFVI to locate the AFV entry in
     the AFIB for next hop forwarding.  If an AFV is found, the Gateway
     uses the next hop AFVI to forward the OAL packet/fragment to the next
     hop while decrementing the OAL Hop Limit but without reassembling.
     If the Gateway has a NCE for the target Client with an entry for the
     target underlay interface and current L2 addresses, the Gateway
     instead forwards the OAL packet/fragment directly to the target
     Client while using the final hop AFVI instead of the next hop (see:
     Section 4.13.4).

     If the OAL packet/fragment includes a full OAL header addressed to
     itself but does not include an AFVI, the Gateway instead reassembles
     if necessary and processes the OAL packet further.  The Gateway first
     determines whether the OAL packet includes an NS/NA message then
     processes the message according to the multilink forwarding
     procedures discussed in Section 4.13.  If the carrier packets arrived
     over the secured spanning tree and the enclosed OAL packets/fragments
     are addressed to its ULA, the Gateway instead reassembles then
     discards the OAL header and forwards the original IP packet/parcel to
     the network layer to support secured BGP routing protocol control
     messaging.  The Gateway instead drops all other OAL packets.

     Gateways forward OAL packets/fragments received in carrier packets
     that arrived from a first segment via the secured spanning tree to
     the next segment also via the secured spanning tree.  Gateways
     forward OAL packets/fragments received in carrier packets that
     arrived from a first segment via the unsecured spanning tree to the
     next segment also via the unsecured spanning tree.  Gateways
     configure a single IPv6 routing table that determines the next hop
     for a given OAL destination, where the secured/unsecured spanning
     tree is determined through the selection of the underlay interface to
     be used for transmission (e.g., an IPsec tunnel or an open INET
     interface).






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     As for Proxy/Servers, Gateways must verify that the L2 addresses of
     carrier packets not received from the secured spanning tree are
     "trusted" before forwarding according to an AFV (otherwise, the
     carrier packet must be dropped).

  4.11.  OMNI Interface Error Handling

     When an AERO node admits an original IP packet/parcel into the OMNI
     interface, it may receive link and/or network layer error
     indications.  The AERO node may also receive OMNI link error
     indications in OAL-encapsulated uNA messages that include
     authentication signatures.

     A link layer error indication is an ICMP error message generated by a
     router in an underlay network on the path to the neighbor or by the
     neighbor itself.  The message includes an IP header with the address
     of the node that generated the error as the source address and with
     the link layer address of the AERO node as the destination address.

     The IP header is followed by an ICMP header that includes an error
     Type, Code and Checksum.  Valid type values include "Destination
     Unreachable", "Time Exceeded", "Parameter Problem" etc.
     [RFC0792][RFC4443].

     The ICMP header is followed by the leading portion of the carrier
     packet that generated the error, also known as the "packet-in-error".
     For ICMPv6, [RFC4443] specifies that the packet-in-error includes:
     "As much of invoking packet as possible without the ICMPv6 packet
     exceeding the minimum IPv6 MTU" (i.e., no more than 1280 bytes).  For
     ICMPv4, [RFC0792] specifies that the packet-in-error includes:
     "Internet Header + 64 bits of Original Data Datagram", however
     [RFC1812] Section 4.3.2.3 updates this specification by stating: "the
     ICMP datagram SHOULD contain as much of the original datagram as
     possible without the length of the ICMP datagram exceeding 576
     bytes".

     The link layer error message format is shown in Figure 4:














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          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               |
          ~    IP Header of link layer    ~
          ~         error message         ~
          |                               |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ~          ICMP Header          ~
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
          |                               |   P
          ~   carrier packet L2 and OAL   ~   a
          ~     encapsulation headers     ~   c
          |                               |   k
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   e
          |                               |   t
          ~original IP packet/parcel hdrs ~
          ~    (first-fragment only)      ~   i
          |                               |   n
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                               |   e
          ~    Portion of the body of     ~   r
          ~ the original IP packet/parcel ~   r
          ~       (all fragments)         ~   o
          |                               |   r
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

            Figure 4: OMNI Interface Link-Layer Error Message Format

     The AERO node rules for processing these link layer error messages
     are as follows:

     *  When an AERO node receives a link layer Parameter Problem message,
        it processes the message the same as described as for ordinary
        ICMP errors in the normative references [RFC0792][RFC4443].

     *  When an AERO node receives persistent link layer Time Exceeded
        messages, the IP ID field may be wrapping before earlier fragments
        awaiting reassembly have been processed.  In that case, the node
        should begin including integrity checks and/or institute rate
        limits for subsequent carrier packets.












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     *  When an AERO node receives persistent link layer Destination
        Unreachable messages in response to carrier packets that it sends
        to one of its neighbor correspondents, the node should process the
        message as an indication that a path may be failing, and
        optionally initiate NUD over that path.  If it receives
        Destination Unreachable messages over multiple paths, the node
        should allow future carrier packets destined to the correspondent
        to flow through a default route and re-initiate route
        optimization.

     *  When an AERO Client receives persistent link layer Destination
        Unreachable messages in response to carrier packets that it sends
        to one of its neighbor Proxy/Servers, the Client should mark the
        path as unusable and use another path.  If it receives Destination
        Unreachable messages on many or all paths, the Client should
        associate with a new Proxy/Server and release its association with
        the old Proxy/Server as specified in Section 4.15.5.

     *  When an AERO Proxy/Server receives persistent link layer
        Destination Unreachable messages in response to carrier packets
        that it sends to one of its neighbor Clients, the Proxy/Server
        should mark the underlay path as unusable and use another underlay
        path.

     *  When an AERO Proxy/Server receives link layer Destination
        Unreachable messages in response to a carrier packet that it sends
        to one of its permanent neighbors, it treats the messages as an
        indication that the path to the neighbor may be failing.  However,
        the dynamic routing protocol should soon re-converge and correct
        the temporary outage.

     When an AERO Gateway receives a carrier packet for which the network
     layer destination address is covered by an MSP assigned to a black-
     hole route, the Gateway drops the carrier packet if there is no more-
     specific routing information for the destination and returns an OMNI
     interface Destination Unreachable message subject to rate limiting.

     When an AERO node receives a carrier packet for which OAL reassembly
     is currently congested, it returns an OMNI interface Packet Too Big
     (PTB) message as discussed in [I-D.templin-intarea-omni] (note that
     the PTB messages could indicate either "hard" or "soft" errors).

     AERO nodes include ICMPv6 error messages intended for an OAL source
     as sub-options in the OMNI option of secured uNA messages.  When the
     OAL source receives the uNA message, it can extract the ICMPv6 error
     message enclosed in the OMNI option and either process it locally or
     translate it into a network layer error to return to the original
     source.



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  4.12.  AERO Mobility Service Coordination

     AERO nodes observes the Router Discovery and Prefix Registration
     specifications found in [I-D.templin-intarea-omni].  AERO nodes
     further coordinate their autoconfiguration actions with the mobility
     service as discussed in the following sections.

  4.12.1.  AERO Service Model

     Each AERO Proxy/Server on the OMNI link is configured to respond to
     Client prefix delegation/registration requests.  Each Proxy/Server is
     provisioned with a database of MNP-to-Client ID mappings for all
     Clients enrolled in the AERO service, as well as any information
     necessary to authenticate each Client.  The Client database is
     maintained by a central administrative authority for the OMNI link
     and securely distributed to all Proxy/Servers, e.g., via the
     Lightweight Directory Access Protocol (LDAP) [RFC4511], via static
     configuration, etc.  Clients receive the same service regardless of
     the Proxy/Servers they select.

     Clients associate each of their ANET/INET underlay interfaces with
     FHS Proxy/Servers.  Each FHS Proxy/Server locally services one or
     more of the Client's underlay interfaces, and the Client typically
     selects one among them to serve as the Hub Proxy/Server (the Client
     may instead select a "third-party" Hub Proxy/Server that does not
     directly service any of its underlay interfaces).  All of the
     Client's other FHS Proxy/Servers forward proxyed copies of RS/RA
     messages between the Hub Proxy/Server and Client without assuming the
     Hub role functions themselves.

     Each Client associates with a single Hub Proxy/Server at a time,
     while all other Proxy/Servers are candidates for providing the Hub
     role for other Clients.  An FHS Proxy/Server assumes the Hub role
     when it receives an RS message with its own ULA or link-scoped All-
     Routers multicast as the destination.  An FHS Proxy/Server assumes
     the proxy role when it receives an RS message with the ULA of another
     Proxy/Server as the destination.  (An FHS Proxy/Server can also
     assume the proxy role when it receives an RS message addressed to
     link-scoped All-Routers multicast if it can determine the ULA of a
     better candidate Proxy/Server to serve as a Hub.)

     Hosts and Clients on ENET interfaces associate with an upstream
     Client on the ENET the same as a Client would associate with an ANET
     Proxy/Server.  Specifically, the Host/Client sends an RS message via
     the ENET which directs the message to the upstream Client.  The
     upstream Client then responds to the RS message by returning an RA.
     In this way, the downstream nodes see the ENET as an ANET and see the
     upstream Client as a Proxy/Server for that ANET.



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     AERO Hosts, Clients and Proxy/Servers use IPv6 ND messages to
     maintain adaptation layer NCEs.  AERO Proxy/Servers configure their
     OMNI interfaces as advertising NBMA interfaces, and therefore send
     unicast RA messages with a short Router Lifetime value (e.g.,
     ReachableTime seconds) in response to a Client's RS message.
     Thereafter, Clients send additional RS messages to keep Proxy/Server
     state alive.

     AERO Clients and Hub Proxy/Servers include prefix delegation and/or
     registration parameters in RS/RA messages.  The IPv6 ND messages are
     exchanged between the Client and Hub Proxy/Server (via any FHS Proxy/
     Servers acting as proxys) according to the prefix management schedule
     required by the service.  If the Client knows its MNP in advance, it
     can employ prefix registration by including its XLA-MNP as the source
     address of an RS message and with an OMNI option with valid prefix
     registration information for the MNP.  If the Hub Proxy/Server
     accepts the Client's MNP assertion, it injects the MNP into the
     routing system and establishes the necessary neighbor cache state.
     If the Client does not have a pre-assigned MNP, it can instead employ
     prefix delegation by including a TLA as the source address of an RS
     message and with an OMNI option with prefix delegation parameters to
     request an MNP.

     The following sections outline Host, Client and Proxy/Server
     behaviors based on the Router Discovery and Prefix Registration
     specifications found in Section 15 of [I-D.templin-intarea-omni].
     These sections observe all of the OMNI specifications, and include
     additional specifications of the interactions of Client-Proxy/Server
     RS/RA exchanges with the AERO mobility service.

  4.12.2.  AERO Host and Client Behavior

     AERO Hosts and Clients discover the addresses of candidate Proxy/
     Servers by resolving the Potential Router List (PRL) in a similar
     manner as described in [RFC5214].  Discovery methods include static
     configuration (e.g., a flat-file map of Proxy/Server addresses and
     locations), or through an automated means such as Domain Name System
     (DNS) name resolution [RFC1035].  Alternatively, the Host/Client can
     discover Proxy/Server addresses through a data link layer login
     exchange, or through an RA response to a multicast/anycast RS as
     described below.  In the absence of other information, the Host/
     Client can resolve the DNS Fully-Qualified Domain Name (FQDN)
     "linkupnetworks.[domainname]" where "linkupnetworks" is a constant
     text string and "[domainname]" is a DNS suffix for the OMNI link
     (e.g., "example.com").  The name resolution returns a set of resource
     records with Proxy/Server address information.





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     The Host/Client then performs RS/RA exchanges over each of its
     underlay interfaces to associate with (possibly multiple) FHS Proxy/
     Serves and a single Hub Proxy/Server as specified in Section 15 of
     [I-D.templin-intarea-omni].  The Host/Client sends each RS (either
     directly via Direct interfaces, via an IPsec tunnel for VPN
     interfaces, via an access router for ANET interfaces or via INET
     encapsulation for INET interfaces) and waits up to RetransTimer
     milliseconds for an RA message reply (see Section 4.12.3) while
     retrying up to MAX_RTR_SOLICITATIONS if necessary.  If the Host/
     Client receives no RAs, or if it receives an RA with Router Lifetime
     set to 0, the Client SHOULD abandon attempts through the first
     candidate Proxy/Server and try another Proxy/Server.

     After the Host/Client registers its underlay interfaces, it may wish
     to change one or more registrations, e.g., if an interface changes
     address or becomes unavailable, if traffic selectors change, etc.  To
     do so, the Host/Client prepares an RS message to send over any
     available underlay interface as above.  The RS includes an OMNI
     option with prefix registration/delegation information and with an
     Interface Attributes sub-option specific to the selected underlay
     interface.  When the Host/Client receives the Hub Proxy/Server's RA
     response, it has assurance that both the Hub and FHS Proxy/Servers
     have been updated with the new information.

     If the Host/Client wishes to discontinue use of a Hub Proxy/Server it
     issues an RS message over any underlay interface with an OMNI Proxy/
     Server Departure sub-option that encodes the (old) Hub Proxy/Server's
     ULA.  When the Hub Proxy/Server processes the message, it releases
     the MNP, sets the NCE state for the Host/Client to DEPARTED and
     returns an RA reply with Router Lifetime set to 0.  After a short
     delay (e.g., 2 seconds), the Hub Proxy/Server withdraws the MNP from
     the routing system.  (Alternatively, when the Host/Client associates
     with a new FHS/Hub Proxy/Server it can include an OMNI "Proxy/Server
     Departure" sub-option in RS messages with the ULAs of the Old FHS/Hub
     Proxy/Servers.)

  4.12.3.  AERO Proxy/Server Behavior

     AERO Proxy/Servers act as both IP routers and IPv6 ND proxys, and
     support a prefix delegation/registration service for Clients.  Proxy/
     Servers arrange to add their ULAs to the PRL maintained in a static
     map of Proxy/Server addresses for the link, the DNS resource records
     for the FQDN "linkupnetworks.[domainname]", etc. before entering
     service.  The PRL should be arranged such that Clients can discover
     the addresses of Proxy/Servers that are geographically and/or
     topologically "close" to their underlay network connections.





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     When a FHS/Hub Proxy/Server receives a prospective Client's secured
     RS message, it SHOULD return an immediate RA reply with Router
     Lifetime set to 0 if it is currently too busy or otherwise unable to
     service the Client; otherwise, it processes the RS as specified in
     Section 15 of [I-D.templin-intarea-omni].  When the Hub Proxy/Server
     receives the RS, it determines the correct MNPs for the Client by
     processing the XLA-MNP prefix parameters and/or the DHCPv6 OMNI sub-
     option.  When the Hub Proxy/Server returns the MNPs, it also creates
     an XLA-MNP forwarding table entry for the MNP resulting in a BGP
     update (see: Section 4.2.3).  The Hub Proxy/Server then returns an RA
     to the Client with destination set to the source of the RS (if an FHS
     Proxy/Server on the return path proxys the RA, it changes the
     destination to the Client's ULA-MNP).

     After the initial RS/RA exchange, the Hub Proxy/Server maintains a
     ReachableTime timer for each of the Client's underlay interfaces
     individually (and for the Client's NCE collectively) set to expire
     after ReachableTime seconds.  If the Client (or an FHS Proxy/Server)
     issues additional RS messages, the Hub Proxy/Server sends an RA
     response and resets ReachableTime.  If the Hub Proxy/Server receives
     an IPv6 ND message with a prefix release indication it sets the
     Client's NCE to the DEPARTED state and withdraws the XLA-MNP route
     from the routing system after a short delay (e.g., 2 seconds).  If
     ReachableTime expires before a new RS is received on an individual
     underlay interface, the Hub Proxy/Server marks the interface as DOWN.
     If ReachableTime expires before any new RS is received on any
     individual underlay interface, the Hub Proxy/Server sets the NCE
     state to STALE and sets a 10 second timer.  If the Hub Proxy/Server
     has not received a new RS or uNA message with a prefix release
     indication before the 10 second timer expires, it deletes the NCE and
     withdraws the XLA-MNP from the routing system.

     The Hub Proxy/Server processes any IPv6 ND messages pertaining to the
     Client while forwarding to the Client or responding on the Client's
     behalf as necessary.  The Hub Proxy/Server may also issue unsolicited
     RA messages, e.g., with reconfigure parameters to cause the Client to
     renegotiate its prefix delegation/registrations, with Router Lifetime
     set to 0 if it can no longer service this Client, etc.  The Hub
     Proxy/Server may also receive carrier packets via the secured
     spanning tree that contain initial data sent while route optimization
     is in progress.  The Hub Proxy/Server reassembles the enclosed OAL
     packets/fragments, then re-encapsulates/re-fragments and sends the
     carrier packets to the target Client via an FHS Proxy/Server if
     necessary.  Finally, If the NCE is in the DEPARTED state, the old Hub
     Proxy/Server forwards any OAL packets/fragments it receives from the
     secured spanning tree and destined to the Client to the new Hub
     Proxy/Server, then deletes the entry after DepartTime expires.




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     Note: Clients SHOULD arrange to notify former Hub Proxy/Servers of
     their departures, but Hub Proxy/Servers are responsible for expiring
     neighbor cache entries and withdrawing XLA-MNP routes even if no
     departure notification is received (e.g., if the Client leaves the
     network unexpectedly).  Hub Proxy/Servers SHOULD therefore set Router
     Lifetime to ReachableTime seconds in solicited RA messages to
     minimize persistent stale cache information in the absence of Client
     departure notifications.  A short Router Lifetime also ensures that
     proactive RS/RA messaging between Clients and FHS Proxy/Servers will
     keep any NAT state alive (see above).

     Note: All Proxy/Servers on an OMNI link MUST advertise consistent
     values in the RA Cur Hop Limit, M and O flags, Reachable Time and
     Retrans Timer fields the same as for any link, since unpredictable
     behavior could result if different Proxy/Servers on the same link
     advertised different values.

  4.12.3.1.  Additional Proxy/Server Considerations

     AERO Clients register with FHS Proxy/Servers for each underlay
     interface.  Each of the Client's FHS Proxy/Servers must inform a
     single Hub Proxy/Server of the Client's underlay interface(s) that it
     services.  For Clients on Direct and VPN/IPsec underlay interfaces,
     the FHS Proxy/Server for each interface is directly connected, for
     Clients on ANET underlay interfaces the FHS Proxy/Server is located
     on the ANET/INET boundary, and for Clients on INET underlay
     interfaces the FHS Proxy/Server is located somewhere in the connected
     Internetwork.  When FHS Proxy/Server "B" processes a Client
     registration, it must either assume the Hub role or forward a proxyed
     registration to another Proxy/Server "A" acting as the Hub. Proxy/
     Servers satisfy these requirements as follows:




















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     *  when FHS Proxy/Server "B" receives a Client RS message, it first
        verifies that the OAL Identification is within the window for the
        NCE that matches the {ULA,XLA}-MNP in the RS source address for
        this Client neighbor and authenticates the message.  If no NCE was
        found, Proxy/Server "B" instead creates one in the STALE state and
        caches the Client-supplied Interface Attributes, Origin Indication
        and OMNI Window Synchronization sub-option parameters as well as
        the Client's observed L2 addresses (noting that they may differ
        from the Origin addresses if there were NATs on the path).  Proxy/
        Server "B" then examines the RS destination address.  If the
        destination address is the ULA of a different Proxy/Server "A",
        Proxy/Server "B" prepares a separate proxyed version of the RS
        message with an OAL header with source set to its own ULA and
        destination set to Proxy/Server A's ULA.  Proxy/Server "B" also
        writes its own information over the Interface Attributes sub-
        option supplied by the Client, omits or zeros the Origin
        Indication sub-option then forwards the message into the OMNI link
        secured spanning tree.

     *  when Hub Proxy/Server "A" receives the RS, it assumes the Hub
        role, delegates an MNP for the Client if necessary according to
        the Preflen in a Neighbor Control sub-option included by the
        Client, and creates/updates a NCE indexed by the Client's XLA-MNP
        with FHS Proxy/Server "B"'s Interface Attributes as the link layer
        address information for this FHS ifIndex.  Hub Proxy/Server "A"
        then prepares an RA message with source set to its own ULA,
        destination set to the source of the RS message, and with a
        Neighbor Control sub-option with Preflen set to the actual MNP
        length it will delegate to the Client.  Hub Proxy/Server "A" then
        encapsulates the RA in an OAL header with source set to its own
        ULA and destination set to the ULA of FHS Proxy/Server "B", then
        finally performs fragmentation if necessary and sends the
        resulting carrier packets into the secured spanning tree.

     *  when FHS Proxy/Server "B" reassembles the RA, it locates the
        Client NCE based on the RA destination.  If the RA message
        includes an OMNI "Proxy/Server Departure" sub-option with non zero
        old FHS/Hub Proxy/Server ULAs that do not match its own ULA, FHS
        Proxy/Server "B" first sends a uNA to the old FHS/Hub Proxy/
        Servers named in the sub-option.  If the RA message delegates a
        new XLA-MNP, Proxy/Server "B" then resets the RA destination to
        the corresponding ULA-MNP for this interface.  Proxy/Server "B"
        then re-encapsulates the message with OAL source set to its own
        ULA and OAL destination set to ULA that appeared in the Client's
        RS message OAL source, with an appropriate Identification value,
        with an authentication signature if necessary, with the Client's
        Interface Attributes sub-option echoed and with the cached
        observed L2 addresses written into an Origin Indication sub-



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        option.  Proxy/Server "B" sets the P flag in the RA flags field to
        indicate that the message has passed through a proxy [RFC4389],
        includes responsive window synchronization parameters, then
        fragments the RA if necessary and returns the fragments to the
        Client.

     *  The Client repeats this process over each of its additional
        underlay interfaces while treating each additional FHS Proxy/
        Server "C", "D", "E", etc. as a proxy to facilitate RS/RA
        exchanges between Hub "A" and the Client.  The Client creates/
        updates NCEs for each such FHS Proxy/Server as well as the Hub
        Proxy/Server in the process.

     After the initial RS/RA exchanges each FHS Proxy/Server forwards any
     of the Client's carrier packets that contain OAL packets/fragments
     with destinations for which there is no matching NCE to a Gateway
     using OAL encapsulation with its own ULA as the source and with
     destination determined by the Client.  The Proxy/Server instead
     forwards any OAL packets/fragments destined to a neighbor cache
     target directly to the target according to the OAL or link layer
     information - the process of establishing neighbor cache entries is
     specified in Section 4.13.

     While the Client is still associated with FHS Proxy/Servers "B", "C",
     "D", "E", etc., each FHS Proxy/Server can send NS, RS and/or uNA
     messages to update the neighbor cache entries of other AERO nodes on
     behalf of the Client based on changes in Interface Attributes,
     Traffic Selectors, etc.  This allows for higher-frequency Proxy-
     initiated RS/RA messaging over well-connected INET infrastructure
     supplemented by lower-frequency Client-initiated RS/RA messaging over
     constrained ANET data links.

     If the Hub Proxy/Server "A" ceases to send solicited RAs, FHS Proxy/
     Servers "B", "C", "D", "E", etc. can send unsolicited RAs over the
     Client's underlay interface with destination set to (link-local) All-
     Nodes multicast and with Router Lifetime set to zero to inform
     Clients that the Hub Proxy/Server has failed.  Although Proxy/Servers
     "B", "C", "D", "E", etc. can engage in IPv6 ND exchanges on behalf of
     the Client, the Client can also send IPv6 ND messages on its own
     behalf, e.g., if it is in a better position to convey state changes.
     The IPv6 ND messages sent by the Client include the Client's XLA-MNP
     as the source in order to differentiate them from the IPv6 ND
     messages sent by a FHS Proxy/Server.

     If the Client becomes unreachable over all underlay interfaces it
     serves, the Hub Proxy/Server sets the NCE state to DEPARTED and
     retains the entry for DepartTime seconds.  While the state is
     DEPARTED, the Hub Proxy/Server forwards any OAL packets/fragments



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     destined to the Client to a Gateway via OAL encapsulation.  When
     DepartTime expires, the Hub Proxy/Server deletes the NCE, withdraws
     the XLA-MNP route and discards any further carrier packets that
     contain OAL packets/fragments destined to the former Client.

     In some ANETs that employ a Proxy/Server, the Client's MNP can be
     injected into the ANET routing system.  In that case, the Client can
     send original IP packets/parcels without invoking the OAL so that the
     ANET routing system transports the original IP packets/parcels to the
     Proxy/Server.  This can be beneficial, e.g., if the Client connects
     to the ANET via low-end data links such as some aviation wireless
     links.

     If the ANET first-hop access router is on the same underlay link as
     the Client and recognizes the AERO/OMNI protocol, the Client can
     avoid OAL encapsulation for both its control and data messages.  When
     the Client connects to the link, it can send an unencapsulated RS
     message with source address set to its own XLA-MNP (or to a TLA), and
     with destination address set to the ULA of the Client's selected
     Proxy/Server or to link-scoped All-Routers multicast.  The Client
     includes an OMNI option formatted as specified in
     [I-D.templin-intarea-omni].  The Client then sends the unencapsulated
     RS message, which will be intercepted by the AERO-aware ANET access
     router.

     The ANET access router then performs OAL encapsulation on the RS
     message and forwards it to a Proxy/Server at the ANET/INET boundary.
     When the access router and Proxy/Server are one and the same node,
     the Proxy/Server would share an underlay link with the Client but its
     message exchanges with outside correspondents would need to pass
     through a security gateway at the ANET/INET border.  The method for
     deploying access routers and Proxys (i.e. as a single node or
     multiple nodes) is an ANET-local administrative consideration.

     Note: When a Proxy/Server alters the IPv6 ND message contents before
     forwarding (e.g., such as altering the OMNI option contents), the
     original IPv6 ND message checksum and authentication signature values
     are invalidated and must be re-calculated.

     Note: When a Proxy/Server receives a secured Client NS message, it
     performs the same proxying procedures as for described for RS
     messages above.  The proxying procedures for NS/NA message exchanges
     is specified in Section 4.13.








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  4.12.3.2.  Detecting and Responding to Proxy/Server Failures

     In environments where fast recovery from Proxy/Server failure is
     required, FHS Proxy/Servers SHOULD use proactive Neighbor
     Unreachability Detection (NUD) to track Hub Proxy/Server reachability
     in a fashion that parallels Bidirectional Forwarding Detection (BFD)
     [RFC5880].  Each FHS Proxy/Server can then quickly detect and react
     to failures so that cached information is re-established through
     alternate paths.  The NS/NA control messaging is carried only over
     well-connected ground domain networks (i.e., and not low-end
     aeronautical radio links) and can therefore be tuned for rapid
     response.

     FHS Proxy/Servers can perform continuous NS/NA exchanges with the Hub
     Proxy/Server, e.g., one exchange per N seconds.  The FHS Proxy/Server
     sends the NS message via the spanning tree with its own ULA as the
     source and the ULA of the Hub Proxy/Server as the destination, and
     the Hub Proxy/Server responds with an NA.  When the FHS Proxy/Server
     also sends RS messages to a Hub Proxy/Server on behalf of Clients,
     the resulting RA responses can be considered as equivalent hints of
     forward progress.  This means that the FHS Proxy/Server need not also
     send a periodic NS if it has already sent an RS within the same
     period.  If the Hub Proxy/Server fails (i.e., if the FHS Proxy/Server
     ceases to receive advertisements), the FHS Proxy/Server can quickly
     inform Clients by sending unsolicited RA messages

     The FHS Proxy/Server sends unsolicited RA messages with source
     address set to the Hub Proxy/Server's address, destination address
     set to (link-local) All-Nodes multicast, and Router Lifetime set to
     0.  The FHS Proxy/Server SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA
     messages separated by small delays [RFC4861].  Any Clients that had
     been using the failed Hub Proxy/Server will receive the RA messages
     and select a different Proxy/Server to assume the Hub role (i.e., by
     sending an RS with destination set to the ULA of the new Hub).

  4.12.3.3.  DHCPv6-Based Prefix Registration

     When a Client is not pre-provisioned with an MNP, it will need for
     the Hub Proxy/Server to select one or more MNPs on its behalf and set
     up the correct state in the AERO routing service.  (A Client with a
     pre-provisioned MNP may also request the Hub Proxy/Server to select
     additional MNPs.)  Clients can use the DHCPv6 service [RFC8415] to
     satisfy this requirement.








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     When a Client needs to have the Hub Proxy/Server select MNPs, it
     sends an RS message with source address set to a TLA and with an OMNI
     option that includes a DHCPv6 message sub-option with DHCPv6 Prefix
     Delegation (DHCPv6-PD) parameters.  When the Hub Proxy/Server
     receives the RS message, it extracts the DHCPv6-PD message from the
     OMNI option.

     The Hub Proxy/Server then acts as a "Proxy DHCPv6 Client" in a
     message exchange with the locally-resident DHCPv6 server, which
     delegates MNPs and returns a DHCPv6-PD Reply message.  (If the Hub
     Proxy/Server wishes to defer creation of MN state until the DHCPv6-PD
     Reply is received, it can instead act as a Lightweight DHCPv6 Relay
     Agent per [RFC6221] by encapsulating the DHCPv6-PD message in a
     Relay-forward/reply exchange with Relay Message and Interface ID
     options.)

     When the Hub Proxy/Server receives the DHCPv6-PD Reply, it creates an
     XLA based on the delegated MNP adds an XLA-MNP route to the routing
     system.  The Hub Proxy/Server then sends an RA back to the Client
     either directly or via an FHS Proxy/Server acting as a proxy.  The
     Proxy/Server that returns the RA directly to the Client sets the
     (newly-created) ULA-MNP as the destination address and with a
     DHCPv6-PD Reply message sub-option coded in the OMNI option.  When
     the Client receives the RA, it creates a default route, assigns the
     Subnet Router Anycast address and sets its {ULA,XLA}-MNP based on the
     delegated MNP.

     Note: Further details of the DHCPv6-PD based MNP registration (as
     well as a minimal MNP delegation alternative that avoids including a
     DHCPv6 message sub-option in the RS) are found in
     [I-D.templin-intarea-omni].

     Note: when the Hub Proxy/Server forwards an RA to the Client via a
     different node acting as a FHS Proxy/Server, the Hub sets the RA
     destination to the same address that appeared in the RS source.  The
     FHS Proxy/Server then subsequently sets the RA destination to the
     ULA-MNP when it forwards the Proxyed version of the RA to the Client
     - see [I-D.templin-intarea-omni] for further details.

  4.13.  AERO Address Resolution, Multilink Forwarding and Route
         Optimization

     AERO nodes invoke address resolution, multilink forwarding and route
     optimization when they need to forward initial original IP packets/
     parcels to new neighbors over ANET/INET interfaces and for ongoing
     multilink forwarding coordination with existing neighbors.  Address
     resolution is based on an IPv6 ND NS/NA(AR) messaging exchange
     between an Address Resolution Source (ARS) and the target neighbor as



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     the Address Resolution Target (ART).  Either the ART itself or the
     ART's current Hub Proxy/Server serves as the Address Resolution
     Responder (ARR).

     Address resolution is initiated by the first eligible ARS closest to
     the original source as follows:

     *  For Clients on VPN/IPsec and Direct interfaces, the Client's FHS
        Proxy/Server is the ARS.

     *  For Clients on ANET interfaces, either the FHS Proxy/Server or the
        Client itself may be the ARS.

     *  For Clients on INET interfaces, the Client itself is the ARS.

     *  For correspondent nodes on INET/ENET interfaces serviced by a
        Relay, the Relay is the ARS.

     *  For Clients that engage the Hub Proxy/Server in "mobility anchor"
        mode, the Hub Proxy/Server is the ARS.

     *  For peers within the same ANET/ENET, address resolution and route
        optimization is through receipt of Redirect messages.

     The AERO routing system directs an address resolution request sent by
     the ARS to the ARR.  The ARR then returns an address resolution reply
     which must include information that is complete, current, consistent
     and authentic.  Both the ARS and ARR are then jointly responsible for
     periodically refreshing the address resolution, and for quickly
     informing each other of any changes.  Following address resolution,
     the ARS and ART perform continuous multilink forwarding and route
     optimization exchanges to maintain optimal forwarding profiles.

     During address resolution, multilink forwarding and/or route
     optimization an NS/NA message source may attach a small number of
     original IP packets/parcels associated with the message exchange as
     super-packet extensions per [I-D.templin-intarea-omni].  The
     authentication signatures and/or lower-layer security features
     employed at the OAL source and each OAL intermediate system will
     provide authorization and integrity services for both the NS/NA
     messages and their IP packet/parcel attachments.  When an OAL source
     or intermediate system forwards a secured NS/NA super-packet, it
     should perform OAL encapsulation followed by fragmentation using a
     fragment size no larger than 1280 octets to ensure that the fragments
     will traverse any possible secured spanning tree paths.  The final
     OAL intermediate system in the path will then securely forward the
     NS/NA message IP packet/parcel attachments to the OAL target.




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     The address resolution, multilink forwarding and route optimization
     procedures are specified in the following sections.

  4.13.1.  Multilink Address Resolution

     When one or more original IP packets/parcels from a source node
     destined to a target node arrive, the ARS checks for a NCE with an
     XLA-MNP that matches the target destination.  If there is a NCE in
     the REACHABLE state, the ARS invokes the OAL and sends the resulting
     carrier packets according to the cached state then returns from
     processing.

     Otherwise, if there is no NCE the ARS creates one in the INCOMPLETE
     state.  The ARS then prepares an NS message for Address Resolution
     (NS(AR)) to send toward an ART while attaching the original IP
     packet(s)/parcel(s) to the end of the NS(AR) as an OAL super-packet
     (see above).  The resulting NS(AR) message must be sent securely, and
     includes:

     *  the ULA of the ARS as the source address.

     *  the XLA corresponding to the original IP packet/parcel's
        destination as the Target Address, e.g., for 2001:db8:1:2::10:2000
        the Target Address is fd00::2001:db8:1:2.

     *  the Solicited-Node multicast address [RFC4291] formed from the
        lower 24 bits of the original IP packet/parcel's destination as
        the destination address, e.g., for 2001:db8:1:2::10:2000 the
        NS(AR) destination address is ff02:0:0:0:0:1:ff10:2000.

     The NS(AR) message also includes an OMNI option with an
     authentication sub-option (if necessary), with Interface Attributes
     and/or Traffic Selectors for all of the source Client's underlay
     interfaces and with a Neighbor Control sub-option with a valid
     Preflen for its claimed MNP.  The ARS then calculates and includes
     the authentication signature (if necessary) followed by the checksum,
     then submits the NS(AR) message for OAL encapsulation.  The ARS sets
     the OAL source to its own ULA and sets the OAL destination according
     to the Client's RS message "RPT" flag (see:
     [I-D.templin-intarea-omni]).  If the "RPT" flag was set, the ARS sets
     the OAL destination to the ULA of its Hub Proxy/Server which
     maintains a Report List; otherwise, the ARS sets the destination to
     the XLA-MNP corresponding to the ART.  The ARS then includes an
     appropriate Identification value, performs OAL fragmentation and L2
     encapsulation/fragmentation, then sends the resulting carrier packets
     into the SRT secured spanning tree without decrementing the network
     layer TTL/Hop Limit field.




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     When the ARS is a Client, it must instead use the ULA of one of its
     FHS Proxy/Servers as the OAL destination.  The ARS Client then
     performs OAL fragmentation followed by L2 encapsulation/fragmentation
     then forwards the carrier packets to the FHS Proxy/Server.  The FHS
     Proxy/Server then performs L2 reassembly/decapsulation, verifies the
     Identification, performs OAL reassembly, verifies the NS(AR)
     checksum/authentication signature and confirms that the Client's
     claimed Neighbor Control Preflen is valid for its ULA-MNP source
     address.  The FHS Proxy/Server then changes the OAL source to its own
     ULA and changes the OAL destination to the ULA of the Hub Proxy/
     Server or XLA-MNP corresponding to the ART as specified above.  The
     FHS Proxy/Server next includes an appropriate Identification,
     performs OAL fragmentation, performs L2 encapsulation/fragmentation
     and sends the resulting carrier packets into the secured spanning
     tree on behalf of the Client.

     Note: both the source and target Client/Relay and their Hub Proxy/
     Servers include current and accurate information for their multilink
     Interface Attributes profile.  The Hub Proxy/Servers can be trusted
     to provide an authoritative ARR response and/or mobility update
     message on behalf of the source/target should the need arise.  While
     the source or target itself has no such trust basis, any attempt to
     mount an attack by providing false Interface Attributes information
     would only result in black-holing of return traffic, i.e., the
     "attack" could only result in denial of service to the source/target
     itself.  The source/target's asserted Interface Attributes therefore
     do not need to be validated by the Hub Proxy/Server.

  4.13.1.1.  ARS Hub Proxy/Server NS(AR) Processing

     If the ARS Client's Hub Proxy/Server maintains a Report List, the
     carrier packets containing the NS(AR) will first arrive at the Hub
     due to the OAL destination address supplied by the ARS (see above).
     This source Hub then performs L2 reassembly/decapsulation, verifies
     the Identification, performs OAL reassembly and records the NS Target
     Address in the Report List for this source Client.  The Hub then
     leaves the OAL source address unchanged, but changes the OAL
     destination address to the XLA corresponding to the NS Target
     Address.  The Hub then decrements the OAL header Hop Limit, includes
     an appropriate Identification, performs OAL fragmentation followed by
     L2 encapsulation/fragmentation and sends the resulting carrier
     packets into the secured spanning tree.









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  4.13.1.2.  Relaying the NS(AR)

     When a Gateway receives carrier packets containing the NS(AR), it
     performs L2 reassembly/decapsulation and determines the next hop by
     consulting its standard IPv6 forwarding table for the OAL header XLA
     destination address.  The Gateway next decrements the OAL header Hop
     Limit, performs L2 encapsulation/fragmentation and sends the carrier
     packet(s) via the secured spanning tree the same as for any IPv6
     router where they may traverse multiple OMNI link segments.  The
     final-hop Gateway will deliver the carrier packets via the secured
     spanning tree to the Hub Proxy/Server (or Relay) that services the
     ART.

  4.13.1.3.  NS(AR) Processing at the ARR/ART

     When the Hub Proxy/Server of the ART receives the NS(AR) secured
     carrier packets with the XLA-MNP of the ART as the OAL destination,
     it performs L2 reassembly/decapsulation followed by OAL reassembly
     then either forwards the NS(AR) to the ART or processes it locally if
     it is acting as a Relay or as the ART's designated ARR.  The Hub
     Proxy/Server processes the message as follows:

     *  if the NS(AR) target matches a Client NCE in the DEPARTED state,
        the (old) Hub Proxy/Server resets the OAL destination address to
        the ULA of the Client's new Hub Proxy/Server.  The old Hub Proxy/
        Server then decrements the OAL header Hop Limit, performs OAL
        fragmentation followed by L2 encapsulation/fragmentation and
        forwards the resulting carrier packets over the secured spanning
        tree.

     *  If the NS(AR) target matches a Client NCE in the REACHABLE state,
        the Hub Proxy/Server notes whether the NS(AR) arrived from the
        secured spanning tree.  If the message arrived via the secured
        spanning tree the Hub Proxy/Server verifies the NS checksum only;
        otherwise, it must also verify the message authentication
        signature.  If the Hub Proxy/Server maintains a Report List for
        the ART, it next records the NS source address in the Report List
        for this ART.  If the Hub Proxy/Server is the ART's designated
        ARR, it forwards any original IP packet(s)/parcel(s) attached to
        the NS(AR) super-packet to the ART and prepares to return an
        NA(AR) as discussed below; otherwise, the Hub Proxy/Server
        determines the underlay interface for the ART and proceeds as
        follows:

        -  If the Hub Proxy/Server is also the FHS Proxy/Server on the
           underlay interface used to convey the NS(AR) to the ART, it
           includes an authentication signature if necessary then
           recalculates the NS(AR) checksum.  The Hub then changes the OAL



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           source to its own ULA and OAL destination to the ULA-MNP of the
           ART, decrements the OAL Hop Limit, includes an appropriate
           Identification value, performs OAL fragmentation followed by L2
           encapsulation/fragmentation and forwards the resulting carrier
           packets over the underlay interface to the ART.

        -  If the Hub Proxy/Server is not the FHS Proxy/Server on the
           underlay interface used to convey the NS(AR) to the ART, it
           instead recalculates the NS(AR) checksum, changes the OAL
           source to its own ULA and changes the OAL destination to the
           ULA of the FHS Proxy/Server for this ART interface.  The Hub
           Proxy/Server next decrements the OAL Hop Limit, includes an
           appropriate Identification value, performs OAL fragmentation
           followed by L2 encapsulation/fragmentation and forwards the
           resulting carrier packets over the secured spanning tree.

        -  When the FHS Proxy/Server receives the carrier packets, it
           performs L2 reassembly/decapsulation, reassembles the NS(AR)
           and verifies the checksum, then forwards to the ART the same as
           described above.

     *  If the NS(AR) target matches one of its non-MNP routes, the Hub
        Proxy/Server serves as both a Relay and an ARR, since the Relay
        forwards original IP packets/parcels toward the (fixed network)
        target at the network layer.

     If the ARR is a Relay or the ART itself, it first creates or updates
     a NCE for the NS(AR) source address while caching all Interface
     Attributes and Traffic Selector information.  Next, the ARR prepares
     a solicited NA(AR) message to return to the ARS with the source
     address set to the ART's XLA, the destination address set to the
     NS(AR) ULA source address and the Target Address set to the same
     value that appeared in the NS(AR) Target Address.

     The ARR then includes Interface Attributes and Traffic Selector sub-
     options for all of the ART's underlay interfaces with current
     information for each interface and includes a Neighbor Control sub-
     option with the Preflen to apply to the ART's MNP.  The ARR next sets
     the NA(AR) message R flag to 1 (as a router) and S flag to 1 (as a
     response to a solicitation) and sets the O flag to 1 (as an
     authoritative responder).  The ARR finally includes an authentication
     signature if necessary, calculates the NA message checksum, then
     submits the NA(AR) for OAL encapsulation with source set to its own
     ULA and destination set to the ULA that appeared in the NS(AR) OAL
     source while including an appropriate Identification.  The ARR then
     performs OAL fragmentation followed by L2 encapsulation/
     fragmentation, and forwards the resulting carrier packets.




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     When the ART Proxy/Server receives carrier packets sent by an ART
     acting as an ARR on its own behalf, it performs L2 reassembly and
     decapsulation, verifies the Identification, performs OAL reassembly,
     then verifies the checksum/authentication signature.  The Proxy/
     Server then verifies that the Neighbor Control Preflen is acceptable,
     changes the OAL source address to its own ULA and changes the OAL
     destination to the ULA corresponding to the NA(AR) destination.  The
     Proxy/Server next decrements the OAL Hop Limit, includes an
     appropriate Identification then recalculates the NA checksum.  The
     Proxy/Server finally performs OAL fragmentation followed by L2
     encapsulation/fragmentation and forwards the resulting carrier
     packets into the secured spanning tree.

  4.13.1.4.  Relaying the NA(AR)

     When a Gateway receives NA(AR) carrier packets, it performs L2
     reassembly/decapsulation and determines the next hop by consulting
     its standard IPv6 forwarding table for the OAL header destination
     address.  The Gateway then decrements the OAL header Hop Limit,
     performs L2 encapsulation/fragmentation and forwards the resulting
     carrier packets via the SRT secured spanning tree where they may
     traverse multiple OMNI link segments.  The final-hop Gateway will
     deliver the carrier packets via the secured spanning tree to a Proxy/
     Server for the ARS.

  4.13.1.5.  Processing the NA(AR) at the ARS

     When the ARS receives NA(AR) carrier packets, it performs L2
     reassembly/decapsulation, verifies the Identification, performs OAL
     reassembly, then searches for a NCE that matches the NA(AR) target
     address.  The ARS then processes the message the same as for standard
     IPv6 Address Resolution [RFC4861].  In the process, it caches all
     OMNI option information in the NCE for the ART (including Interface
     Attributes, Traffic Selectors, etc.), and caches the NA(AR) XLA
     source address as the address of the ART.
















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     When the ARS is a Client, the SRT secured spanning tree will first
     deliver the solicited NA(AR) message to the FHS Proxy/Server, which
     re-adjusts the OAL header and forwards the message to the Client.  If
     the Client is on a well-managed ANET, physical security and protected
     spectrum ensures security for the NA(AR) without needing an
     additional authentication signature; if the Client is on the open
     INET the Proxy/Server must instead include an authentication
     signature (while adjusting the OMNI option size, if necessary).  The
     Proxy/Server uses its own ULA as the OAL source and the ULA-MNP of
     the Client as the OAL destination when it forwards the NA(AR).  The
     Proxy/Server then decrements the OAL Hop Limit, includes an
     appropriate Identification, performs OAL fragmentation followed by L2
     encapsulation/fragmentation and forwards the resulting carrier
     packets over the underlay interface to the Client.

  4.13.1.6.  Reliability

     After the ARS transmits the first NS(AR), it should wait up to
     RETRANS_TIMER seconds to receive a responsive NA(AR).  The ARS can
     then retransmit the NS(AR) up to MAX_UNICAST_SOLICIT times before
     giving up.

  4.13.2.  Multilink Forwarding

     Following address resolution, the ARS and ART (or their respective
     FHS Proxy/Servers) can assert multilink forwarding paths through
     underlay interface pairs serviced by the same source/destination ULAs
     by sending unicast NS/NA messages with OMNI AERO Forwarding Parameter
     (AFP) sub-options.  The unicast NS/NA messages establish multilink
     forwarding state in OAL intermediate systems in the path between the
     ARS and ART.  Note that either the ARS or ART can independently
     initiate multilink forwarding by sending unicast NS messages on
     behalf of specific underlay interface pairs.  (Underlay interface
     directionality (i.e., in/out) must also be factored into the paths
     established for multilink forwarding.)

     When an OAL source asserts a multilink forwarding path through the
     transmission of a unicast NS message, it includes an IPv6 Minimum
     Path MTU Hop-by-Hop option for the (adaptation layer) IPv6 header per
     [RFC9268].  Each OAL intermediate node and OAL IPv6 router along the
     path then updates the minimum MTU per the specification.  When the
     OAL destination responds with a unicast NA message, it returns an
     IPv6 Minimum Path MTU Hop-by-Hop option based on the one it received
     in the NS message per [RFC9268].  This allows the OAL source to
     discover any OAL Fragment Size (OFS) limitations for this OAL
     destination (see: [I-D.templin-intarea-omni]).  For this reason, IPv6
     routers that connect SRT segments MUST implement [RFC9268].




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     The multilink forwarding profile provides support for redundant paths
     that each OAL node can harness to its best advantage.  For example,
     OAL nodes can use traffic selectors to guide the dispersal of
     different traffic types over available multilink paths, while other
     factors such as metrics, cost, provider, etc. can also provide useful
     decision points.  OAL nodes can also employ multilink forwarding for
     fault tolerance by sending redundant data over multiple paths
     simultaneously, or for load balancing where the individual packets of
     a single traffic flow are spread across multiple independent paths.
     OAL nodes that engage in multilink forwarding therefore must
     incorporate a policy engine that selects both inbound and outbound
     multilink paths for a given traffic profile at a given point in time.
     This specification therefore provides multilink forwarding mechanisms
     without mandating any specific multilink policy.

     Nodes that configure OMNI interfaces and engage in multilink
     coordination include an additional forwarding table termed the AERO
     Forwarding Information Base (AFIB) that supports OAL packet/fragment
     forwarding based on OMNI neighbor underlay interface pairs.  The AFIB
     contains per-interface-pair AERO Forwarding Vectors (AFVs) identified
     by locally-unique values known as AFV Indexes (AFVIs).  The AFVs
     cache uncompressed OAL header information as well as the previous/
     next-hop addressing and AFVI information.  The AFVs also cache window
     synchronization state for the specific underlay interface pair.
     Using the window synchronization state, simple Identification-based
     data origin authentication is enabled at each OAL source,
     intermediate system and target node.

     OMNI interfaces manage the AFIB in conjunction with their internal
     Neighbor Cache.  OMNI interface NCEs link to (possibly) multiple
     AFVs, with one AVF per underlay interface pair (according to
     directionality).  When OMNI interface peers need to coordinate, they
     locate a NCE for the peer then use the NCE as a nexus that aggregates
     potentially many AVFs.  In particular, the NCE caches the AFVI to be
     used to index the local AFV at the head end of the path.

     OAL source, intermediate system and target nodes create AFVs/AFVIs
     when they process an NS/NA message with an AFP sub-option with Job
     code '00' (Initialize; Build B) or a responsive NA message with Job
     code '01' (Follow B; Build A) (see: [I-D.templin-intarea-omni]).  The
     OAL source of the NS/NA (which is also the OAL destination of the
     responsive NA) is considered to reside in the "First Hop Segment
     (FHS)", while the OAL destination of the NS/NA (which is also the OAL
     source of the responsive NA) is considered to reside in the "Last Hop
     Segment (LHS)".






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     The FHS and LHS roles are determined on a per-interface-pair basis.
     After address resolution, either peer is equally capable of
     initiating multilink forwarding on behalf of a specific FHS/LHS
     underlay interface pair.  The peer that sends the initiating NS/NA
     with Job code '00' message for a specific pair becomes the FHS peer
     while the one that returns the responsive NA becomes the LHS peer for
     that pair only.  It is therefore commonplace that peers may assume
     the FHS role for some pairs while assuming the LHS role for other
     pairs, i.e., even though each peer maintains only a single NCE.

     When an OAL node initiates or forwards an NS/NA with Job code '00',
     it creates an AFV, records the NS/NA source and destination ULAs then
     generates and assigns a locally-unique "B" AFVI (while also caching
     the "B" values for all previous OAL hops on the path from the FHS OAL
     source).  When the OAL node receives future OAL packets/fragments
     that include "B", it can unambiguously locate the correct AFV and
     determine directionality without examining addresses.  When the AFV
     is indexed by its "B" AFVI, it returns the ULAs in (dst,src) order
     the opposite of how they appeared in the OAL header of the original
     NS to support full header reconstruction for reverse-path forwarding.
     (If the NS message included a nested OAL encapsulation, the ULAs of
     both OAL headers are returned.)

     When an OAL node initiates or forwards a responsive NA with Job code
     '01', it uses the "B" AFVI to locate the AFV created by the NS then
     generates and assigns a locally-unique "A" AFVI (while also caching
     the "A" values for all previous OAL hops on the path from the LHS OAL
     source).  When the OAL node receives future carrier packets that
     include "A", it can unambiguously locate the correct AFV and
     determine directionality without examining addresses.  When the AFV
     is indexed by its "A" AFVI, it returns the ULAs in (src,dst) order
     the same as they appeared in the OAL header of the original NS to
     support full header reconstruction for forward-path forwarding.  (If
     the NS message included a nested OAL encapsulation, the ULAs of both
     OAL headers are returned.)

     OAL nodes generate random non-zero 32-bit values as candidate AFVIs
     which must first be tested for local uniqueness.  If a candidate AFVI
     is already in use, the OAL node repeats the random generation process
     until it obtains a unique non-zero value.  Since the number of AFVs
     in service at each OAL node is likely to be much smaller than 2**32,
     the process will generate a unique value after a small number of
     tries.  Since the uniqueness property is node-local only, an AFVI
     locally generated by a first OAL node must not be tested for
     uniqueness by other OAL nodes.






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     OAL nodes cache AFVs for up to ReachableTime seconds following their
     initial creation.  If the node processes another NS or NA message
     specific to an AFV, it resets ReachableTime to REACHABLE_TIME
     seconds, i.e., the same as for NCEs.  If ReachableTime expires, the
     node deletes the AFV and frees its associated AFVIs so they can be
     reused for future AFVs.

     The following sections provide the detailed specifications of these
     NS/NA exchanges for all nodes along the forward and reverse paths.

  4.13.2.1.  FHS Client-Proxy/Server NS Forwarding

     When an FHS OAL source has an original IP packet/parcel to send
     toward an LHS OAL target, it first performs multilink address
     resolution resulting in the creation of a NCE for the XLA of the
     target then selects a source and target underlay interface pair.  The
     FHS source then uses its cached information for the target interface
     as LHS information then prepares an NS message with an AFP sub-option
     with Job code '00', includes window synchronization information, then
     sets the NS source to the XLA of the FHS Client and the NS target to
     the XLA of the LHS Client.  The FHS source next creates an AFV then
     generates and assigns a locally-unique "B" AFVI to the AFV while also
     including it as the first "B" entry in the AFP AFVI List.  The FHS
     source then includes any FHS/LHS addressing information it knows
     locally in the AFP sub-option, i.e., based on information discovered
     through address resolution.

     If the FHS source is the FHS Proxy/Server, it then examines the LHS
     FMT-Forward code.  If FMT-Forward is clear the FHS Proxy/Server sets
     the NS destination to the ULA of the LHS Proxy/Server; otherwise, it
     sets the NS destination to the same address as the target.  The FHS
     Proxy/Server then performs OAL encapsulation while setting the OAL
     source to its own ULA and setting the OAL destination to the FHS
     Subnet Router Anycast ULA determined by applying the FHS SRT prefix
     length to its ULA.  The FHS Proxy/Server then includes an appropriate
     Identification value, performs OAL fragmentation followed by L2
     encapsulation/fragmentation then forwards the resulting carrier
     packets into the secured spanning tree which will deliver them to a
     Gateway interface that assigns the FHS Subnet Router Anycast ULA.

     If the FHS source is the FHS Client, it instead includes an
     authentication signature if necessary.  If LHS FMT-Forward is clear,
     the FHS Client sets the NS destination to the ULA of the LHS Proxy/
     Server; otherwise, it sets the NS destination to the same address as
     the target.  The FHS Client then calculates the NS message checksum,
     performs OAL encapsulation, sets the OAL source to its own ULA-MNP
     and sets the OAL destination to the ULA of the FHS Proxy/Server.  The
     FHS Client finally includes an appropriate Identification value for



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     the FHS Proxy/Server, performs OAL fragmentation followed by L2
     encapsulation/fragmentation and forwards the resulting carrier
     packets to the FHS Proxy/Server.

     When the FHS Proxy/Server receives the carrier packets, it performs
     L2 reassembly/decapsulation, verifies the Identification, performs
     OAL reassembly if necessary and verifies the NS checksum or
     authentication signature.  The FHS Proxy/Server then creates an AFV
     (i.e., the same as the FHS Client had done) while caching the AFP "B"
     entry along with the FHS Client addressing information as previous
     hop information for this AFV.  The FHS Proxy/Server next generates a
     new locally-unique "B" AFVI, then assigns it as the AFV index and
     writes it as the next "B" entry in the AFP AFVI List (while also
     writing any FHS Client and Proxy/Server addressing information).  The
     FHS Proxy/Server then calculates the NS checksum and sets the OAL
     source address to its own ULA and destination address to the FHS
     Subnet Router Anycast ULA.  The FHS Proxy/Server finally decrements
     the OAL Hop Limit and includes an Identification appropriate for the
     secured spanning tree.  The FHS Proxy/Server finally performs OAL
     fragmentation followed by L2 encapsulation/fragmentation and forwards
     the resulting carrier packets into the secured spanning tree.

  4.13.2.2.  FHS/intermediate/LHS Gateway NS Forwarding

     Gateways in the spanning tree forward OAL packets/fragments not
     explicitly addressed to themselves, while forwarding those that
     arrived via the secured spanning tree to the next hop also via the
     secured spanning tree and forwarding all others via the unsecured
     spanning tree.  When an FHS Gateway receives an OAL packet/fragment
     over the secured spanning tree addressed to its ULA or the FHS Subnet
     Router Anycast ULA, it instead performs L2 reassembly/decapsulation,
     verifies the Identification, then finally performs OAL reassembly to
     obtain the NS then verifies the NS checksum.  The FHS Gateway next
     creates an AFV (i.e., the same as the FHS Proxy/Server had done)
     while caching the AFP FHS Client and Proxy/Server addressing
     information, window synchronization information and corresponding
     AFVI List "B" values in the AFV to enable future reverse path
     forwarding to this FHS Client.  The FHS Gateway then generates a
     locally-unique "B" AFVI for the AFV and writes it as the next "B"
     entry in the NS AFP AFVI List.

     The FHS Gateway then examines the SRT prefixes corresponding to both
     FHS and LHS.  If the FHS Gateway has a local interface connection to
     both the FHS and LHS (whether they are the same or different
     segments), the FHS/LHS Gateway caches the NS AFP LHS information in
     the AFV, writes its LHS ULA and L2ADDR into the NS AFP LHS fields,
     then sets its LHS ULA as the OAL source and the ULA of the LHS Proxy/
     Server as the OAL destination.  If the FHS and LHS prefixes are



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     different, the FHS Gateway instead sets its FHS ULA as the OAL source
     and the LHS Subnet Router Anycast ULA as the OAL destination.  The
     FHS Gateway then decrements the OAL Hop Limit, includes an
     appropriate Identification, recalculates the NS checksum, performs
     OAL fragmentation followed by L2 encapsulation/fragmentation and
     forwards the resulting carrier packets into the secured spanning
     tree.

     When the FHS and LHS Gateways are different, the LHS Gateway will
     receive carrier packets over the secured spanning tree from the FHS
     Gateway, noting there may be many intermediate Gateways in the path
     between FHS and LHS which will simply forward the enclosed IPv6 OAL
     packets/fragments without further processing.  The LHS Gateway then
     performs L2 reassembly/decapsulation, verifies the Identification,
     performs OAL reassembly to obtain the NS, verifies the NS checksum
     then creates an AFV (i.e., the same as the FHS Gateway had done)
     while caching the AFP "B" AFVIs and addressing information of
     previous OAL forwarding hops along with window synchronization
     information.  In particular, the LHS Gateway caches the ULA of the
     FHS Gateway as the spanning tree address for the previous-hop, caches
     the LHS information then generates a locally-unique "B" AFVI for the
     AFV.  The LHS Gateway then writes its own LHS ULA and L2ADDR into the
     AFP sub-option while also writing "B" as the next entry in the AFP
     AFVI List.  The LHS Gateway then sets its own ULA as the OAL source
     and the ULA of the LHS Proxy/Server as the OAL destination,
     decrements the OAL Hop Limit, includes an appropriate Identification,
     recalculates the NS checksum, performs OAL fragmentation followed by
     L2 encapsulation/fragmentation and forwards the resulting carrier
     packets into the secured spanning tree.

  4.13.2.3.  LHS Proxy/Server-Client NS Receipt and NA Forwarding

     When the LHS Proxy/Server receives the carrier packets from the
     secured spanning tree, it performs L2 reassembly/decapsulation,
     verifies the Identification, performs OAL reassembly, verifies the NS
     checksum then verifies that the LHS information supplied by the FHS
     source is consistent with its own cached information.  If the
     information is consistent, the LHS Proxy/Server then creates an AFV
     and caches the AFP "B" AFVIs and addressing information of previous
     OAL forwarding hops the same as for the prior hop.  The LHS Proxy/
     Server next caches the NS window synchronization parameters in the
     AFV.  If the NS destination is the XLA of the LHS Client, the LHS
     Proxy/Server also generates a locally-unique "B" AFVI and assigns it
     both to the AFV and as the next "B" entry in the NS AFVI List.

     If the NS destination matches its own ULA, the LHS Proxy/Server next
     prepares to return a responsive NA with Job code '01'.  The LHS
     Proxy/Server next creates or updates an NCE for the NS source address



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     (if necessary) with state set to STALE and with an AFVI pointer to
     the new AFV state.  When the LHS Proxy/Server forwards future carrier
     packets based on the cached information, it can populate forwarding
     information in a CRH-32 routing header to enable forwarding based on
     the cached AFVI List "B" entries.

     The LHS Proxy/Server then creates an NA with Job code '01' while
     copying the NS AFP sub-option into the NA and including responsive
     window synchronization information.  The LHS Proxy/Server then
     generates a locally-unique "A" AFVI and both assigns it to the AFV
     and includes it as the first "A" entry in the AFP sub-option AFVI
     List (see: [I-D.templin-intarea-omni] for details on AFVI List A/B
     processing).  The LHS Proxy/Server then encapsulates the NA with OAL
     source set to its own ULA and OAL destination set to the ULA of the
     LHS Gateway.  The LHS Proxy/Server then includes an appropriate
     Identification value, calculates the NA checksum, performs OAL
     fragmentation followed by L2 encapsulation/fragmentation and forwards
     the resulting carrier packets into the secured spanning tree.

     If the NS destination was the XLA of the LHS Client, the LHS Proxy/
     Server includes an authentication signature in the NS if necessary,
     then recalculates the NS checksum, changes the OAL source to its own
     ULA and changes the OAL destination to the ULA-MNP of the LHS Client.
     The LHS Proxy/Server then decrements the OAL Hop Limit, includes an
     appropriate Identification value, performs OAL fragmentation followed
     by L2 encapsulation/fragmentation and forwards the resulting carrier
     packets to the LHS Client.  When the LHS Client receives the carrier
     packets, it performs L2 reassembly/decapsulation, verifies the
     Identification, performs OAL reassembly then verifies the NS
     checksum/authentication signature.  The LHS Client then creates a NCE
     for the NS ULA source address (if necessary) in the STALE state and
     examines the AFP sub-option.  The Client then caches the NS OMNI AFP
     sub-options in the NCE corresponding to the NS ULA source, then
     creates an AFV, caches the addressing information and "B" entries of
     the previous OAL hops then finally generates and assigns a locally-
     unique "A" AFVI the same as for previous hops.  The Client finally
     caches the new AFVI in the NCE so that future communications can
     locate the correct AFV.

     The LHS Client then prepares an NA using exactly the same procedures
     as for the LHS Proxy/Server above (while including responsive window
     synchronization information), except that it uses its XLA as the NA
     source and the NS source as the NA destination.  The LHS Client also
     includes an authentication signature if necessary, calculates the NA
     message checksum, then encapsulates the NA with OAL source set to its
     own ULA-MNP and OAL destination set to the ULA of the LHS Proxy/
     Server.  The LHS Client finally includes an appropriate
     Identification, performs OAL fragmentation followed by L2



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     encapsulation/fragmentation and forwards the resulting carrier
     packets to the LHS Proxy/Server.  When the LHS Proxy/Server receives
     the carrier packets, it performs L2 reassembly/decapsulation,
     verifies the Identification, performs OAL reassembly, verifies the NA
     checksum/authentication signature, then uses the current AFP AFVI
     List "B" entry to locate the AFV.  The LHS Proxy/Server then caches
     the addressing and "A" information for the LHS Client in the AFV,
     then generates a locally-unique "A" AFVI and both assigns it to the
     AFV and writes it as the next AFP AFVI List "A" entry.  The LHS
     Proxy/Server then calculates the NA checksum, sets the OAL source to
     its own ULA and destination to the ULA of the LHS Gateway, decrements
     the OAL Hop Limit, includes an appropriate Identification, performs
     OAL fragmentation followed by L2 encapsulation/fragmentation and
     forwards the resulting carrier packets into the secured spanning
     tree.

  4.13.2.4.  LHS/intermediate/FHS Gateway NA Forwarding

     When the LHS Gateway receives the carrier packets containing the NA
     message, it performs L2 reassembly/decapsulation, verifies the
     Identification, performs OAL reassembly, verifies the NA checksum
     then uses the current NA AFP AFVI List "B" entry to locate the AFV.
     The LHS Gateway then caches the AFP addressing and AFVI List "A"
     information for the previous hops in the AFV, then generates a
     locally-unique "A" AFVI and both assigns it to the AFV and writes it
     as the next AFP AFVI List "A" entry.  The LHS Gateway then
     recalculates the NA checksum.  If the LHS Gateway is connected
     directly to both the FHS and LHS segments (whether the segments are
     the same or different), the LHS Gateway will have already cached the
     FHS/LHS information based on the original NS; the LHS Gateway then
     sets the OAL source to its FHS ULA and OAL destination to the ULA of
     the FHS Proxy/Server.  Otherwise, the LHS Gateway sets the OAL source
     to its LHS ULA and OAL destination to the ULA of the FHS Gateway.
     The LHS Gateway then decrements the OAL Hop Limit, includes an
     appropriate Identification, performs OAL fragmentation followed by L2
     encapsulation/fragmentation and forwards the resulting carrier
     packets into the secured spanning tree.

     When the FHS and LHS Gateways are different, the FHS Gateway will
     receive carrier packets containing the NA message from the LHS
     Gateway over the secured spanning tree, where there may have been
     many intermediate Gateway forwarding hops.  The FHS Gateway performs
     L2 reassembly/decapsulation, verifies the Identification, performs
     OAL reassembly, verifies the NA checksum and locates the AFV based on
     the current AFP AFVI List "B" entry.  The FHS Gateway then caches the
     addressing and "A" information for the previous hops in the AFV and
     generates a locally-unique "A" AFVI.  The FHS Gateway then assigns
     the new "A" value to the AFV, records "A" in the AFP AFVI List then



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     writes its FHS ULA and L2ADDR into the AFP FHS Gateway fields.  The
     FHS Gateway then recalculates the NA checksum, sets its FHS ULA as
     the OAL source and sets the ULA of the FHS Proxy/Server as the OAL
     destination.  The FHS Gateway then decrements the OAL Hop Limit,
     includes an appropriate Identification value, performs OAL
     fragmentation followed by L2 encapsulation/fragmentation and forwards
     the resulting carrier packets into the secured spanning tree.

  4.13.2.5.  FHS Proxy/Server-Client NA Receipt

     When the FHS Proxy/Server receives the carrier packets from the
     secured spanning tree, it performs L2 reassembly/decapsulation,
     verifies the Identification, performs OAL reassembly, verifies the NA
     checksum then locates the AFV based on the current AFP AFVI List "B"
     entry.  The FHS Proxy/Server then caches the AFP addressing and "A"
     information for the previous hops.  If the NA destination matches its
     own ULA, the FHS Proxy/Server locates the NCE for the ULA of the LHS
     Proxy/Server or XLA of the LHS Client and sets the state to
     REACHABLE.  The FHS Proxy/Server then caches the window
     synchronization parameters and prepares to return an acknowledgement,
     if necessary.

     If the NA destination is the XLA of the FHS Client, the FHS Proxy/
     Server instead generates a locally-unique "A" AFVI and assigns it
     both to the AFV and as the next AFP AFVI List "A" entry, then
     includes an authentication signature/checksum in the NA message.  The
     FHS Proxy/Server then sets the OAL source to its own ULA and sets the
     OAL destination to the ULA-MNP of the FHS Client.  The FHS Proxy/
     Server then decrements the OAL Hop Limit, includes an appropriate
     Identification value, performs OAL fragmentation followed by L2
     encapsulation/fragmentation and finally forwards the resulting
     carrier packets to the FHS Client.

     When the FHS Client receives the carrier packets, it performs L2
     reassembly/decapsulation, verifies the Identification, performs OAL
     reassembly, verifies the NA checksum/authentication signature, then
     locates the AFV based on the current AFP AFVI List "B" entry.  The
     FHS Client then caches the previous hop addressing and "A"
     information the same as for prior hops.  The FHS Client then locates
     the NCE for the NS source address and sets the state to REACHABLE,
     then caches the window synchronization parameters and prepares to
     return a uNA acknowledgement, if necessary.









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  4.13.2.6.  Returning Window Acknowledgements

     If either the FHS Client or FHS Proxy/Server needs to return an
     acknowledgement to complete window synchronization, it prepares a uNA
     message with an AFP sub-option with Job code set to '10' (Follow A;
     Record B).  The FHS node sets the uNA source to its own ULA or XLA,
     then sets the uNA destination to the ULA or XLA of the LHS node.  The
     FHS node next sets the AFP AFVI List to the cached list of "A"
     entries received in the Job code '01' NA, but need not set any other
     FHS/LHS information.  The FHS node then encapsulates the uNA message
     in an OAL header with its own ULA as the OAL source.  If the FHS node
     is the Client, it next sets the ULA of the FHS Proxy/Server as the
     OAL destination, includes an authentication signature/checksum,
     includes an appropriate Identification value, performs OAL
     fragmentation followed by L2 encapsulation/fragmentation and forwards
     the resulting carrier packets to the FHS Proxy/Server.  The FHS
     Proxy/Server then verifies the Identification, performs OAL
     reassembly, verifies the uNA checksum/authentication signature, then
     uses the current AFVI List "A" entry to locate the AFV.

     The FHS Proxy/Server then writes its "B" AFVI as the next AFP AFVI
     List "B" entry, recalculates the uNA checksum then sets its own ULA
     as the OAL source and the ULA of the FHS Gateway as the OAL
     destination, The FHS Proxy/Server finally decrements the OAL Hop
     Limit, includes an appropriate Identification then finally performs
     OAL fragmentation followed by L2 encapsulation/fragmentation and
     forwards the resulting carrier packets into the secured spanning
     tree.  When the FHS Gateway receives the carrier packets, it performs
     L2 reassembly/decapsulation, verifies the Identification, performs
     OAL reassembly, verifies the uNA checksum then uses the current AFVI
     List "A" entry to locate the AFV.  The FHS Gateway then writes its
     "B" AFVI as the next AFP AFVI List "B" entry, then sets the OAL
     source to its own ULA.  If the FHS Gateway is also the LHS Gateway,
     it sets the OAL destination to the ULA of the LHS Proxy/Server;
     otherwise it sets the OAL destination to the ULA of the LHS Gateway.
     The FHS Gateway recalculates the uNA checksum then decrements the OAL
     Hop Limit, includes an appropriate Identification, performs OAL
     fragmentation followed by L2 encapsulation/fragmentation and forwards
     the resulting carrier packets into the secured spanning tree.  If an
     LHS Gateway receives the carrier packets, it processes them exactly
     the same as the FHS Gateway had done while re-setting the OAL
     destination to the ULA of the LHS Proxy/Server.

     When the LHS Proxy/Server receives the carrier packets, it performs
     L2 reassembly/decapsulation, verifies the Identification, performs
     OAL reassembly, then verifies the uNA checksum.  The LHS Proxy/Server
     then locates the AFV based on the current AFP AFVI List "A" entry.
     If the uNA destination matches its own ULA, the LHS Proxy/Server next



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     updates the NCE/AFV for the source ULA based on the uNA window
     synchronization parameters and MAY compare the AFVI List to the
     version it had cached in the AFV based on the original NS.

     If the uNA destination is the XLA of the LHS Client, the LHS Proxy/
     Server instead writes its "B" AFVI as the next AFP AFVI List "B"
     entry and includes an authentication signature/checksum.  The LHS
     Proxy/Server then writes its own ULA as the OAL source and the ULA-
     MNP of the Client as the OAL destination, then decrements the OAL Hop
     Limit and includes an appropriate Identification.  The LHS Proxy/
     Server finally performs OAL fragmentation followed by L2
     encapsulation/fragmentation and forwards the resulting carrier
     packets to the LHS Client.  When the LHS Client receives the carrier
     packets, it performs L2 reassembly/decapsulation, verifies the
     Identification, performs OAL reassembly, verifies the uNA checksum/
     authentication signature then processes the message exactly the same
     as for the LHS Proxy/Server case above.

     Note: If either the LHS Client or LHS Proxy/Server needs to return an
     acknowledgement to complete window synchronization, it prepares a uNA
     message with an AFP sub-option with Job code set to '11' (Follow B;
     Record A).  All other procedures are exactly the opposite as per the
     FHS case specified above.

  4.13.2.7.  OAL End System Exchanges Following Synchronization

     Following the initial NS/NA exchange with AFP sub-options, OAL end
     systems can begin exchanging ordinary carrier packets that include
     "A/B" AFVIs and with Identification values within their respective
     send/receive windows without requiring security signatures and/or
     secured spanning tree traversal.  OAL end and intermediate systems
     can also consult their AFIBs when they receive carrier packets that
     contain OAL packets/fragments with "A/B" AFVIs to unambiguously
     locate the correct AFV and can use any discovered "A/B" values of
     other OAL nodes to forward OAL packets/fragments to nodes that
     configure the corresponding AFVIs.  OAL end systems must then perform
     continuous NS/NA exchanges to update window state, register new
     interface pairs for optimized multilink forwarding, confirm
     reachability and/or refresh AFIB cache state in the path before
     ReachableTime expires.

     While the OAL end systems continue to actively exchange OAL packets,
     they are jointly responsible for updating cache state and per-
     interface reachability before expiration.  Window synchronization
     state is performed on a per-interface-pair basis and tracked in the
     AFVs which are also linked to the appropriate NCE.  However, the
     window synchronization exchange only confirms target Client
     reachability over the specific underlay interface pair.  Reachability



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     for other underlay interfaces that share the same window
     synchronization state must be determined individually using
     additional NS/NA messages.

     To update AFIB state in the path, the FHS node that sent the original
     NS message with AFP Job code '00' can send additional NS messages
     with AFP sub-options with Job code '10' (Follow "A"; Record "B") and
     with window synchronization parameters.  The message will be
     processed by all intermediate systems which will refresh AFV timers,
     cache window synchronization parameters and forward the NS onward
     toward the LHS node that returned the original NA message.  When the
     LHS node receives the NS, it returns an NA message with AFP Job code
     '11' (Follow "B"; Record "A").

     At the same time, the LHS node that received the original NS message
     with Job code '00' can send additional NS messages with Job code '11'
     in order to cause the FHS node to return an NA message with AFP Job
     code '10'.  The process can therefore be coordinated asynchronously
     with the FHS/LHS nodes initiating an NS/NA exchange independently of
     one another.  The exchanges will succeed as long as the AFIB state in
     the path remains active.  Note that all intermediate system
     processing of Job code '10' and '11' NS/NA messages is conducted the
     same as for the initial NS/NA exchange according to the detailed
     specifications above.

     OAL sources can also begin including CRH-32s in OAL packets/fragments
     with AFVI information that OAL intermediate systems can use for
     shortest-path forwarding based on AFVIs instead of spanning tree
     addresses.  OAL sources and intermediate systems can instead forward
     OAL packets/fragments with OAL compressed headers termed "OCH" (see:
     [I-D.templin-intarea-omni]) that include only a single "A/B" AFVI
     meaningful to the next hop, since all OAL nodes in the path up to
     (and sometimes including) the OAL destination have already
     established AFVs.  Note that when an FHS OAL source receives a
     responsive NA with Job code '01', the AFP sub-option will contain an
     AFVI List with "A" entries populated in the reverse order needed for
     populating a CRH-32 routing header.  The FHS OAL source must
     therefore write the AFP AFVI List "A" entries last-to-first when it
     populates a CRH-32, or must select the correct "A" entry to include
     in an OCH header based on the intended OAL intermediate system or
     destination.










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     When a Gateway receives unsecured carrier packets that contain OAL
     packets/fragments destined to a local SRT segment Client that has
     asserted direct reachability, the Gateway performs direct forwarding
     while bypassing the local Proxy/Server based on the Client's
     advertised AFVIs and discovered NATed L2ADDR information (see:
     Section 4.13.4).  If the Client cannot be reached directly (or if NAT
     traversal has not yet converged), the Gateway instead forwards OAL
     packets/fragments directly to the local segment Proxy/Server.

     When a Proxy/Server receives OAL packets/fragments destined to a
     local SRT segment Client or forwards OAL packets/fragments received
     from a local segment Client, it first locates the correct AFV.  If
     the OAL packet/fragment includes a secured IPv6 ND message, the
     Proxy/Server uses the Client's NCE established through RS/RA
     exchanges to re-encapsulate/re-fragment while sending outbound
     secured carrier packets via the secured spanning tree and sending
     inbound secured carrier packets while including an authentication
     signature/checksum.  For ordinary OAL packets/fragments, the Proxy/
     Server uses the same AFV if directed by AFVI and/or OAL addressing.
     Otherwise it locates an AFV established through an NS/NA exchange
     between the Client and the remote SRT segment peer, and forwards the
     OAL packet/fragments without first reassembling/decapsulating.

     When a source Client forwards OAL packets/fragments it can employ
     header compression according to the AFVIs established through an NS/
     NA exchange with a remote or local peer.  When the source Client
     forwards to a remote peer, it can forward OAL packets/fragments to a
     local SRT Gateway (following the establishment of L2ADDR information)
     while bypassing the Proxy/Server following route optimization (see:
     Section 4.13.4).  When a target Client receives carrier packets that
     contain OAL packets/fragments that match a local AFV, the Client
     first verifies the Identification then decompresses the headers if
     necessary, reassembles to obtain the OAL packet then decapsulates and
     delivers the original IP packet/parcel to the network layer.

     When synchronized peer Clients in the same SRT segment with FMT-
     Forward and FMT-Mode set discover each other's NATed L2ADDR
     addresses, they can exchange carrier packets that contain OAL
     packets/fragments directly with header compression using AFVIs
     discovered as above (see: Section 4.13.5).  The FHS Client will have
     cached the "A" AFVI for the LHS Client, which will have cached the
     "B" AFVI for the FHS Client.









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     When the FHS Client or FHS Proxy/Server sends an NS for the purpose
     of establishing multilink forwarding state, it should wait up to
     RETRANS_TIMER seconds to receive a responsive NA.  The FHS node can
     then retransmit the NS up to MAX_UNICAST_SOLICIT times before giving
     up.  Note that each successive attempt establishes new AFV state in
     the OAL intermediate systems, but that any abandoned stale AFV state
     will be quickly reclaimed.

  4.13.2.8.  Rapid Commit Multilink Forwarding

     Multilink forwarding can often be invoked in conjunction with Address
     Resolution in order to reduce control message overhead and round-trip
     delays.  When an ART acting as an ARR receives an NS(AR) with a set
     of Interface Attributes for the ARS source Client, it can perform
     "rapid commit" by immediately invoking multilink forwarding as above
     at the same time as returning the NA(AR).

     In order to perform rapid commit, the ARR includes an AFP sub-option
     with Job code '00' and a Window Synchronization sub-option as though
     it were initiating a multilink coordination NS/NA exchange as
     specified above.  The ARR then includes any Interface Attributes and/
     or Traffic Selector sub-options as necessary to satisfy the address
     resolution request.  The ARR then returns the NA(AR) to the ARS using
     the same hop-by-hop OAL addressing disciplines as specified above for
     an ordinary multilink NS/NA exchange.  This will cause the NA(AR) to
     visit all FHS/LHS OAL intermediate systems on the path towards the
     ARS.

     When the NA(AR) traverses the return path to the ARS, OAL
     intermediate systems in the path process the NS AFP information
     exactly the same as for an ordinary multilink forwarding exchange as
     specified above, i.e., without examining the remaining NA(AR) message
     contents.  This results in the ARR node now assuming the FHS role and
     the ARS assuming the LHS role from the perspective of multilink
     forwarding coordination.  When the NA(AR) arrives, the ARS processes
     the AFP and window synchronization parameters while also processing
     all other NA(AR) OMNI option information, thereby eliminating an
     extraneous message transmission and associated delay.  The ARS (now
     acting as an LHS peer) then completes the exchange by returning a
     responsive NA with an AFP sub-option with Job code '01'; if no NA
     response is received within RETRANS_TIMER seconds, the ARR can
     retransmit the NA(AR) up to MAX_NEIGHBOR_ADVERTISEMENT times before
     giving up.








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  4.13.3.  Mobile Ad-hoc Network (MANET) Forwarding

     Clients with OMNI interfaces configured over underlay interfaces with
     indeterminant neighborhood properties may be connected to ANETs
     coordinated as Mobile Ad-hoc NETworks (MANETs).  Each MANET may be
     either completely outside of the range of any OMNI link Proxy/Servers
     or may require multihop traversal between Clients acting as MANET
     routers to reach Proxy/Servers that connect to the rest of the OMNI
     link.  The former class of MANETs must operate in isolation solely
     based on the unique IPv6 addresses they configure locally, including
     TLAs and HHITs.  The latter class allows MANET routers to extend
     infrastructure-based addressing information including MNPs over
     multiple OMNI link hops as discussed in the OMNI specification.

     MANET Clients configure their OMNI interfaces over one or more MANET
     interfaces where multihop forwarding may be necessary.  Routing
     protocols suitable for use over MANET interfaces include OSPFv3
     [RFC5340] with MANET Designated Router (OSPF-MDR) extensions
     [RFC5614], OLSR [RFC7181], AODV [I-D.perkins-manet-aodvv2] and
     others.  Other services specific to MANET link-local and/or site-
     local operations (including SMF [RFC6621], DLEP [RFC8175] and others)
     are also considered in-scope.  These services strive for optimal use
     of available radio bandwidth and power consumption in their control
     message transmissions, but efficient data plane operation is also
     essential.

     Clients must therefore reduce overhead through minimal encapsulation
     and effective header compression whenever possible.  For this reason,
     when the MANET routing protocol discovers a new route the Client
     configures a lesser-preferred forwarding table entry over the
     corresponding MANET interface and a more-preferred forwarding table
     entry over the OMNI interface.  This will cause the network layer to
     direct outbound packets to the OMNI interface, which can apply header
     compression and underlay MANET interface selection.

     When two Clients within the same MANET communicate using IP addresses
     that are advertised in the MANET routing protocol, their OMNI
     interfaces can avoid OAL encapsulation and treat the IP header
     supplied by the network layer as if it were an OAL encapsulation
     header.  This includes the application of OAL fragmentation and
     header compression as discussed in the OMNI specification.










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     Proxy/Servers that connect a MANET to the rest of the OMNI link act
     as regular Proxy/Servers for exchanges with external INETs, but act
     as Clients over their MANET interfaces.  Each such Proxy/Server
     therefore has at least two underlay interfaces, including an INET
     interface and a MANET interface.  The Proxy/Server therefore services
     the MANET as if it were an ordinary Client but presents itself as a
     Proxy/Server to external facing INETs.

     The process for a multihop Client to establish header compression
     state in the MANET is conducted as a MANET-local aspect of the NS/NA
     multilink forwarding message exchange discussed in Section 4.13.2.
     The process can be used to establish either asymmetric or symmetric
     path header compression state.  In the asymmetric case, the forward
     path from the source Client to the destination Client or a MANET
     border Proxy/Server may be different than the reverse path.  In the
     symmetric case, both the forward and reverse paths traverse the same
     set of MANET routers.

     When the OMNI interface of a MANET source Client sends an NS to
     establish asymmetric path header compression state, it also includes
     a CRH-16 extension header and Window Synchronization parameters.  The
     source Client selects a non-zero 16-bit "C" AFVI that is unique for
     the L2 address of the next MANET forwarding hop for the NS message
     and writes that value into the first SID field of the CRH-16 while
     writing the value 0 into the second SID field.  The source Client
     then caches the full OAL header in an AFV for the destination and
     sends the NS to the next hop.

     When the next MANET forwarding hop's OMNI interface receives the NS,
     it creates an AFV and caches the full OAL header as well as the
     previous hop's "C" AFVI, L2 address and Window Synchronization
     parameters for the forward path.  The OMNI interface then selects its
     own non-zero/unique "C" AFVI and over-writes that value into the
     first SID field of the CRH-16.  Consecutive MANET forwarding hops
     then repetitively forward the NS to their respective next hops, which
     perform the same procedures as above.  The process continues until
     the NS reaches either a final destination within the same MANET or a
     MANET border Proxy/Server that can forward to destinations in other
     networks.

     When the final destination is within the same MANET, the destination
     OMNI interface returns an NA with a CRH-16 and uses the same non-
     zero/unique "C" AVFI discipline described above in the reverse path
     which may travel over a completely different set of MANET routers
     than those in the forward path.  Otherwise, the Proxy/Server that
     receives the NS forwards it to other networks according to the same
     multilink forwarding procedures discussed in Section 4.13.2.  When
     the Proxy/Server eventually receives an NA to return to the original



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     source, the Proxy/Server inserts a CRH-16 (while removing the CRH-32
     if present) and performs the same reverse path forwarding that an
     ordinary MANET destination would perform as described above.  When
     the original source receives the NA, header compression state will
     have been completely populated in both the forward and reverse paths
     and the source and destination nodes can begin sending ordinary
     packets with OCH headers instead of full OAL headers.

     The same procedures that appear above also apply when an NS
     originating from a remote network arrives at a MANET border Proxy/
     Server for a MANET that contains the final destination.  The Proxy/
     Server assumes the source role, inserts a CRH-16 with a non-zero/
     unique "C" AFVI and forwards it to the next MANET forwarding hop
     toward the final destination.  The forwarding process continues
     between successive MANET routers until the final destination receives
     the NS.  The final destination then prepares a responsive NA again
     while inserting a CRH-16 with a non-zero/unique "C" AFVI and returns
     the NA through the MANET toward the same Proxy/Server that forwarded
     the NS.  Note that it is important that the NA message contains the
     OAL address of the same Proxy/Server, since that is the only location
     where state resides to enable the return of the NA message to the
     original source.

     In order to establish symmetric MANET paths, the initiating Client
     can instead send an NS that includes a CRH-16 with a non-zero/unique
     2-octet "D" AFVI written into the second SID field and 0 written into
     the first SID field.  The Client then forwards the NS message to the
     next MANET forwarding hop toward the destination.  When the next
     MANET forwarding hop receives the NS, it creates an AFV and caches
     the (previous hop) "D" AFVI, then overwrites the second CRH-16 SID
     field with a newly-generated (next hop) non-zero/unique "D" AFVI
     value.  Consecutive MANET forwarding hops then repetitively forward
     the NS and create new AFVs in the same fashion until the NS reaches
     either a final destination within the same MANET or a MANET border
     Proxy/Server.

     The destination or Proxy/Server then returns an NA along the reverse
     path with the (previous hop) "D" AFVI in the second CRH-16 SID field,
     and with a newly-generated (next hop) non-zero/unique "C" AFVI in the
     first CRH-16 SID field.  When the previous MANET hop processes the
     NA, it locates the AFV based on the "D" AFVI, caches the "C" AFVI and
     generates a new non-zero/unique "C" AFVI.  The MANET node then
     overwrites the second CRH-16 SID with its cached previous hop "D"
     value and overwrites the first CRH-16 SID with the new "C" AFVI value
     and returns the NA to the previous hop.  The process continues until
     the NA message reaches the original multihop Client that transmitted
     the NS, at which point header compression state is established in
     both the forward and reverse directions of the MANET symmetric path.



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     Following the NS/NA exchanges in both the asymmetric and symmetric
     cases discussed above, each MANET router in the path in both the FHS
     and LHS MANETs will have established AFVs containing header
     compression state.  The AFVs determine AFVI-based forwarding based on
     the OCH header contents, and each MANET router only forwards packet
     with in-window Identification values.  MANET routers maintain AFVs
     for up to ReachableTime seconds unless they are refreshed by either a
     new NS/NA exchange or the transmission of any data packet with a full
     OAL header with an in-window Identification value and a CRH-16
     extension.  New window synchronization exchanges must also be
     performed periodically to avoid window exhaustion and/or spoofing
     based on predictable Identifications.

     Note: while the MANET routing protocol runs directly over the node's
     MANET interfaces to discover routing information, the node configures
     lesser-preferred forwarding table entries over the MANET interface
     and corresponding more-preferred forwarding table entries over the
     OMNI interface.  This causes the network layer to forward outbound
     packets via the OMNI interface which applies encapsulation,
     fragmentation and/or header compression as necessary before
     forwarding over the underlying MANET interface.  The OMNI protocol
     designator in the UDP port, IP protocol or Ethernet EtherType field
     will then cause the packets to visit the OMNI interface of each
     successive next-hop MANET node.

  4.13.4.  Client/Gateway Route Optimization

     Following multilink route optimization for specific underlay
     interface pairs, FHS/LHS Clients located on open INETs can invoke
     Client/Gateway route optimization to improve performance and reduce
     load and congestion on their respective Proxy/Servers.  To initiate
     Client/Gateway route optimization, the Client prepares an NS message
     with its own XLA address as the source and the ULA of its Gateway as
     the destination while creating a NCE for the Gateway if necessary.
     The NS message must be encapsulated as an atomic fragment and not
     subject to OAL fragmentation.

     The Client then includes an Interface Attributes sub-option for its
     underlay interface as well as an authentication signature but does
     not include window synchronization parameters.  The Client then
     performs OAL encapsulation with its own ULA-MNP as the source and the
     ULA of the Gateway as the destination while including a randomly-
     chosen Identification value, then performs L2 encapsulation/
     fragmentation on the OAL atomic fragment and forwards the resulting
     carrier packets directly to the Gateway.






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     When the Gateway receives the carrier packets, it performs L2
     decapsulation/reassembly to recover the OAL atomic fragment then
     verifies the NS checksum/authentication signature and creates a NCE
     for the Client.  The Gateway then caches the L2 encapsulation
     addresses (which may have been altered by one or more NATs on the
     path) as well as the Interface Attributes for this Client ifIndex,
     and marks this Client underlay interface as "trusted".  The Gateway
     then prepares an NA reply with its own ULA as the source and the XLA
     of the Client as the destination where the NA again must be an atomic
     fragment.

     The Gateway then echoes the Client's Interface Attributes, includes
     an Origin Indication with the Client's observed L2 addresses and
     includes an authentication signature.  The Gateway then performs OAL
     encapsulation with its own ULA as the source and the ULA-MNP of the
     Client as the destination while using the same Identification value
     that appeared in the NS, then performs L2 encapsulation/fragmentation
     on the OAL atomic fragment and forwards the resulting carrier packets
     directly to the Client.

     When the Client receives the NA reply, it caches the carrier packet
     L2 source address information as the Gateway target address via this
     underlay interface while marking the interface as "trusted".  The
     Client also caches the Origin Indication L2 address information as
     its own (external) source address for this underlay interface.

     After the Client and Gateway have established NCEs as well as
     "trusted" status for a particular underlay interface pair, each node
     can begin sending ordinary carrier packets intended for this
     multilink route optimization directly to one another while omitting
     the Proxy/Server from the forwarding path while the status is
     "trusted".  The NS/NA messaging will have established the correct
     state in any NATs in the path so that NAT traversal is naturally
     supported.  The Client and Gateway must maintain timers that watch
     for activity on the path; if no carrier packets and/or NS/NA messages
     are sent or received over the path before NAT state is likely to have
     expired, the underlay interface pair status becomes "untrusted".

     Thereafter, when the Client sends a carrier packet that contains an
     OAL packet/fragment toward the Gateway as the next hop, the Client
     includes the AFVI for the Gateway (discovered during multilink route
     optimization) instead of the AFVI for its Proxy/Server; the Gateway
     will accept the OAL packet/fragment from the Client if and only if
     the AFVI matches the correct AFV and the underlay interface status is
     trusted.  (The same is true in the reverse direction when the Gateway
     sends carrier packets directly to the Client.)





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     Note: the Client and Gateway each maintain a single NCE, but the NCE
     may aggregate multiple underlay interface pairs.  Each underlay
     interface pair may use differing source and target L2 addresses
     according to NAT mappings, and the "trusted/untrusted" status of each
     pair must be tested independently.  When no "trusted" pairs remain,
     the NCE is deleted.

     Note: the above method requires Gateways to participate in NS/NA
     message authentication signature application and verification.  In an
     alternate approach, the Client could instead exchange NS/NA messages
     with authentication signatures via its Proxy/Server but addressed to
     the ULA of the Gateway, and the Proxy/Server and Gateway could relay
     the messages over the secured spanning tree.  However, this would
     still require the Client to send additional messages toward the L2
     address of the Gateway to populate NAT state; hence the savings in
     complexity for Gateways would result in increased message overhead
     for Clients.

  4.13.5.  Client/Client Route Optimization

     When the FHS/LHS Clients are both located on the same SRT segment,
     Client-to-Client route optimization is possible following the
     establishment of any necessary state in NATs in the path.  Both
     Clients will have already established state via their respective
     shared segment Proxy/Servers (and possibly also the shared segment
     Gateway) and can begin sending carrier packets directly via NAT
     traversal while avoiding any Proxy/Server and/or Gateway hops.

     When the FHS/LHS Clients on the same SRT segment perform the initial
     NS/NA exchange to establish AFIB state, they first examine the FMT-
     Forward and FMT-Mode settings to determine whether direct-path
     forwarding is even possible for one or both Clients (direct-path
     forwarding is only possible for one or both when FMT-Forward and FMT-
     Mode are both 1).  The NS/NA messages then include an Origin
     Indication (i.e., in addition to an AFP sub-option) with the mapped
     addresses discovered during the RS/RA exchanges with their respective
     Proxy/Servers.  After the AFV paths have been established, both
     Clients can begin sending carrier packets via strict AFV paths while
     establishing a direct path for Client-to-Client route optimization.

     To establish the direct path, either Client (acting as the source)
     transmits a bubble to the mapped L2 address for the target Client
     which primes its local chain of NATs for reception of future carrier
     packets from that L2 address (see: [RFC4380] and
     [I-D.templin-intarea-omni]).  The source Client then prepares an NS
     message with its own XLA as the source, with the XLA of the target as
     the destination and with an OMNI option with an Interface Attributes
     sub-option.  The source Client then encapsulates the NS in an OAL



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     header with its own ULA-MNP as the source, with the ULA-MNP of the
     target Client as the destination and with an in-window Identification
     for the target.  The source Client then performs OAL fragmentation
     followed by L2 encapsulation/fragmentation with L2 headers addressed
     to its Proxy/Server then sends the resulting carrier packets to the
     Proxy/Server.

     When the Proxy/Server receives the carrier packets, it re-
     encapsulates and sends them as unsecured carrier packets according to
     AFIB state where they will eventually arrive at the target Client
     which can perform L2 reassembly/decapsulation, verify the
     Identification and perform OAL reassembly.  Following reassembly, the
     target Client prepares an NA message with its own XLA as the source,
     with the XLA of the source Client as the destination and with an OMNI
     option with an Interface Attributes sub-option.  The target Client
     then encapsulates the NA in an OAL header with its own ULA-MNP as the
     source, with the ULA-MNP of the source Client as the destination and
     with an in-window Identification for the source Client.  The target
     Client then performs OAL fragmentation followed by L2 encapsulation/
     fragmentation then forwards the resulting carrier packets directly to
     the source Client.

     Following the initial NS/NA exchange, both Clients mark their
     respective (source, target) underlay interface pairs as "trusted" for
     no more than ReachableTime seconds.  The Clients can then begin
     exchanging ordinary data packets as OCH encapsulated carrier packets.
     While the Clients continue to exchange packets via the direct path
     avoiding all Proxy/Servers and Gateways, they should perform
     additional NS/NA exchanges via their local Proxy/Servers to refresh
     NCE state as well as send additional bubbles to the peer's Origin
     address information if necessary to refresh NAT state.

     Note: these procedures are suitable for a widely-deployed but basic
     class of NATs.  Procedures for advanced NAT classes are outlined in
     [RFC6081], which provides mechanisms that can be employed equally for
     AERO using the corresponding sub-options specified by OMNI.

     Note: each communicating pair of Clients may need to maintain NAT
     state for peer to peer communications via multiple underlay interface
     pairs.  It is therefore important that Origin Indications are
     maintained with the correct peer interface and that the NCE may cache
     information for multiple peer interfaces.









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     Note: the source and target Client exchange Origin information during
     the secured NS/NA multilink route optimization exchange.  This allows
     for subsequent NS/NA exchanges to proceed using only the
     Identification value as a data origin confirmation.  However, Client-
     to-Client peerings that require stronger security may also include
     authentication signatures for mutual authentication.

  4.13.6.  Intra-ANET/ENET Route Optimization for AERO Peers

     When a Client forwards an OAL packet (or an original IP packet/
     parcel) from a Host or another Client connected to one of its
     downstream ENETs to a peer within the same downstream ENET, the
     Client returns an IPv6 ND Redirect message to inform the source that
     that target can be reached directly.  The contents of the Redirect
     message are the same as specified in [RFC4861], and should also
     include a Neighbor Control sub-option with the Preflen of the MNP
     found in the Target Address field.

     In the same fashion, when a Proxy/Server forwards an OAL packet (or
     original IP packet/parcel) from a Host or Client connected to one of
     its downstream ANETs to a peer within the same downstream ANET, the
     Proxy/Server returns an IPv6 ND Redirect message.

     All other route optimization functions are conducted per the NS/NA
     messaging discussed in the previous sections.

  4.14.  Neighbor Unreachability Detection (NUD)

     AERO nodes perform Neighbor Unreachability Detection (NUD) per
     [RFC4861] either reactively in response to persistent link layer
     errors (see Section 4.11) or proactively to confirm reachability.
     The NUD algorithm is based on periodic control message exchanges and
     may further be seeded by IPv6 ND hints of forward progress, but care
     must be taken to avoid inferring reachability based on spoofed
     information.  For example, IPv6 ND message exchanges that include
     authentication codes and/or in-window Identifications may be
     considered as acceptable hints of forward progress, while spurious
     random carrier packets should be ignored.

     AERO nodes can perform NS/NA exchanges over the OMNI link secured
     spanning tree (i.e. the same as described above) to test reachability
     without risk of DoS attacks from nodes pretending to be a neighbor.
     These NS/NA messages use the unicast XLAs/ULAs of the parties
     involved in the NUD test.  When only reachability information is
     required without updating any other NCE state, AERO nodes can instead
     perform NS/NA exchanges directly between neighbors without employing
     the secured spanning tree as long as they include in-window
     Identifications and an authentication signature/checksum.



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     After route optimization directs a source FHS peer to a target LHS
     peer with one or more link layer addresses, either node may invoke
     multilink forwarding state initialization to establish authentic
     intermediate system state between specific underlay interface pairs
     which also tests their reachability.  Thereafter, either node acting
     as the source may perform additional reachability probing through NS
     messages over the SRT secured or unsecured spanning tree, or through
     NS messages sent directly to an underlay interface of the target
     itself.  While testing a target underlay interface, the source can
     optionally continue to forward OAL packets/fragments via alternate
     interfaces or maintain a small queue of carrier packets until target
     reachability is confirmed.

     NS messages are encapsulated, fragmented and transmitted as carrier
     packets the same as for ordinary original IP data packets/parcels,
     however the encapsulated destinations are either the ULA or XLA of
     the source and either the ULA of the LHS Proxy/Server or the XLA of
     the target itself.  The source encapsulates the NS message the same
     as described in Section 4.13.2 and includes an Interface Attributes
     sub-option with ifIndex set to identify its underlay interface used
     for forwarding.  The source then includes an in-window
     Identification, performs OAL fragmentation followed by L2
     encapsulation/fragmentation then forwards the resulting carrier
     packets into the unsecured spanning tree, either directly to the
     target if it is in the local segment or directly to a Gateway in the
     local segment.

     When the target receives the NS carrier packets, it performs L2
     reassembly/decapsulation, verifies that it has a NCE for this source
     and that the Identification is in-window then performs OAL
     reassembly.  The target next verifies the NS checksum/authentication
     signature, then searches for Interface Attributes in its NCE for the
     source that match the NS for the NA reply.  The target then prepares
     the NA with the source and destination addresses reversed,
     encapsulates and sets the OAL source and destination, includes an
     Interface Attributes sub-option in the NA to identify the ifIndex of
     the underlay interface the NS arrived on and sets the Target Address
     to the same value included in the NS.  The target next sets the R
     flag to 1, the S flag to 1 and the O flag to 1, then includes an in-
     window Identification for the source.  The node then performs OAL
     fragmentation followed by L2 encapsulation/fragmentation and forwards
     the resulting carrier packets into the unsecured spanning tree either
     directly to the source if it is in the local segment or directly to a
     Gateway in the local segment.

     When the source receives the NA, it marks the target underlay
     interface tested as "trusted".  Note that underlay interface states
     are maintained independently of the overall NCE REACHABLE state, and



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     that a single NCE may have multiple target underlay interfaces in
     various "trusted/untrusted" states while the NCE state as a whole
     remains REACHABLE.

  4.15.  Mobility Management and Quality of Service (QoS)

     AERO is a fully Distributed Mobility Management (DMM) service in
     which each Proxy/Server is responsible for only a small subset of the
     Clients on the OMNI link.  This is in contrast to a Centralized
     Mobility Management (CMM) service where there are only one or a few
     network mobility collective entities for large Client populations.
     Clients coordinate with their associated FHS and Hub Proxy/Servers
     via RS/RA exchanges to maintain the DMM profile, and the AERO routing
     system tracks all current Client/Proxy/Server peering relationships.

     Hub Proxy/Servers provide a designated router service for their
     dependent Clients, while FHS Proxy/Servers provide a proxy conduit
     between the Client and both the Hub and OMNI link in general.
     Clients are responsible for maintaining neighbor relationships with
     their Proxy/Servers through periodic RS/RA exchanges, which also
     serves to confirm neighbor reachability.  When a Client's underlay
     interface attributes change, the Client is responsible for updating
     the Hub Proxy/Server through new RS/RA exchanges using the FHS Proxy/
     Server as a first-hop conduit.  The FHS Proxy/Server can also act as
     a proxy to perform some IPv6 ND exchanges on the Client's behalf
     without consuming bandwidth on the Client underlay interface.

     Note: when a Client's underlay interface address changes, the Client
     and/or its (former) FHS Proxy/Server for this interface must
     invalidate any AFVs based on the (changed) interface.  Future data
     packet forwarding will then trigger a new multilink forwarding NS/NA
     exchange to re-seed new AFVs in the path.

     Mobility management considerations are specified in the following
     sections.

  4.15.1.  Mobility Update Messaging

     Mobile Clients (and/or their Hub Proxy/Servers) accommodate mobility
     and/or multilink change events by sending secured uNA messages to
     each active neighbor.  When a node sends a uNA message to each
     specific neighbor on behalf of a mobile Client, it sets the IPv6
     source address to its own ULA or XLA, sets the destination address to
     the neighbor's ULA or XLA and sets the Target Address to the mobile
     Client's XLA.  The uNA also includes an OMNI option with OMNI
     Interface Attributes and Traffic Selector sub-options for the mobile
     Client's underlay interfaces and includes an authentication signature
     if necessary.  The node then sets the uNA R flag to 1, S flag to 0



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     and O flag to 1, then encapsulates the message in an OAL header with
     source set to its own ULA and destination set to either the specific
     neighbor's ULA or the FHS Proxy/Server's ULA.  Following OAL
     fragmentation and L2 encapsulation/fragmentation, the carrier packets
     containing the uNA message will then follow the secured spanning tree
     and arrive at the specific neighbor.

     As discussed in Section 7.2.6 of [RFC4861], the transmission and
     reception of uNA messages is unreliable but provides a useful
     optimization.  In well-connected Internetworks with robust data links
     uNA messages will be delivered with high probability, but in any case
     the node can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT uNAs to
     each neighbor to increase the likelihood that at least one will be
     received.  Alternatively, the node can set the SNR flag in the uNA
     OMNI option header to request a uNA response (see: Section 4.5.1).

     When the FHS/LHS Proxy/Server receives a secured uNA message prepared
     as above, if the uNA destination was its own ULA the Proxy/Server
     uses the included OMNI option information to update its NCE for the
     target but does not reset ReachableTime since the receipt of a uNA
     message does not provide confirmation that any forward paths to the
     target Client are working.  If the destination was the XLA of the
     FHS/LHS Client, the Proxy/Server instead changes the OAL source to
     its own ULA, includes an authentication signature if necessary, and
     includes an in-window Identification for this Client.  Finally, if
     the uNA message SNR flag was set, the node that processes the uNA
     also returns a uNA response (see: Section 4.5.1).

  4.15.2.  Announcing Link-Layer Information Changes

     When a Client needs to change its underlay Interface Attributes and/
     or Traffic Selectors for one or more underlay interfaces (e.g., due
     to a mobility event), the Client sends RS messages to its Hub Proxy/
     Server (via first-hop FHS Proxy/Servers if necessary).  Each RS
     includes an OMNI option with Interface Attributes and/or Traffic
     Selector sub-options for the ifIndex in question.

     Note that the first FHS Proxy/Server may change due to the underlay
     interface change.  If the Client RS includes an OMNI Proxy/Server
     Departure sub-option for the former FHS Proxy/Server, the new FHS
     Proxy/Server can send a departure indication (see Section 4.15.5);
     otherwise, any stale state in the former FHS Proxy/Server will simply
     expire after ReachableTime expires with no effect on the Hub Proxy/
     Server.







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     Up to MAX_RTR_SOLICITATIONS RS messages MAY be sent in parallel with
     sending carrier packets containing user data in case one or more RAs
     are lost.  If all RAs are lost, the Client SHOULD re-associate with a
     new Proxy/Server.

     After performing the RS/RA exchange, the Client sends uNA messages to
     all neighbors the same as described in the previous section.

  4.15.3.  Bringing New Links Into Service

     When a Client needs to bring new underlay interfaces into service
     (e.g., when it activates a new data link), it sends an RS message to
     the Hub Proxy/Server via a FHS Proxy/Server for the underlay
     interface (if necessary) with an OMNI option that includes an
     Interface Attributes sub-option with interface parameters and with
     link layer address information for the new link.  The Client then
     again sends uNA messages to all neighbors the same as described
     above.

  4.15.4.  Deactivating Existing Links

     When a Client needs to deactivate an existing underlay interface, it
     sends a uNA message toward the Hub Proxy/Server via an FHS Proxy/
     Server with an OMNI option with appropriate Interface Attributes
     values for the deactivated link.

     If the Client needs to send uNA messages over an underlay interface
     other than the one being deactivated, it MUST include Interface
     Attributes for any underlay interfaces being deactivated.  The Client
     then again sends uNA messages to all neighbors the same as described
     above.

     Note that when a Client deactivates an underlay interface, neighbors
     that receive the ensuing uNA messages need not purge all references
     for the underlay interface from their neighbor cache entries.  The
     Client may reactivate or reuse the underlay interface and/or its
     ifIndex at a later point in time, when it will send new RS messages
     to an FHS Proxy/Server with fresh interface parameters to update any
     neighbors.

  4.15.5.  Moving Between Proxy/Servers

     The Client performs the procedures specified in Section 4.12.2 when
     it first associates with a new Hub Proxy/Server or renews its
     association with an existing Hub Proxy/Server.






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     When a Client associates with a new Hub Proxy/Server, it sends RS
     messages to register its underlay interfaces with the new Hub while
     including the old Hub's ULA in the "Old Hub Proxy/Server ULA" field
     of a Proxy/Server Departure OMNI sub-option.  When the new Hub Proxy/
     Server returns the RA message via the FHS Proxy/Server (acting as a
     proxy), the FHS Proxy/Server sends a uNA to the old Hub Proxy/Server
     (i.e., if the ULA is non-zero and different from its own).  The uNA
     has the XLA of the Client as the source and the ULA of the old hub as
     the destination and with an OMNI Proxy/Server Departure sub-option as
     above.  The FHS Proxy/Server encapsulates the uNA in an OAL header
     with the ULA of the new Hub as the source and the ULA of the old Hub
     as the destination, then performs OAL fragmentation followed by L2
     encapsulation/fragmentation and forwards the resulting carrier
     packets via the secured spanning tree.

     When the old Hub Proxy/Server receives the carrier packets, it
     decapsulates and reassembles if necessary to obtain the uNA then
     changes the Client's NCE state to DEPARTED, resets DepartTime and
     caches the new Hub Proxy/Server ULA.  After a short delay (e.g., 2
     seconds) the old Hub Proxy/Server withdraws the Client's MNP from the
     routing system.  While in the DEPARTED state, the old Hub Proxy/
     Server forwards any carrier packets received via the secured spanning
     tree destined to the Client's ULA-MNP to the new Hub Proxy/Server's
     ULA.  When DepartTime expires, the old Hub Proxy/Server deletes the
     Client's NCE.

     Mobility events may also cause a Client to change to a new FHS Proxy/
     Server over a specific underlay interface at any time such that a
     Client RS/RA exchange over the underlay interface will engage the new
     FHS Proxy/Server instead of the old.  The Client can arrange to
     inform the old FHS Proxy/Server of the departure by including a
     Proxy/Server Departure sub-option with a ULA for the "Old FHS Proxy/
     Server ULA", and the new FHS Proxy/Server will issue a uNA using the
     same procedures as outlined for the Hub above while using its own ULA
     as the source address.  This can often result in successful delivery
     of carrier packets that would otherwise be lost due to the mobility
     event.

     Clients SHOULD NOT move rapidly between Hub Proxy/Servers in order to
     avoid causing excessive oscillations in the AERO routing system.
     Examples of when a Client might wish to change to a different Hub
     Proxy/Server include a Hub Proxy/Server that has become unresponsive,
     topological movements of significant distance, movement to a new
     geographic region, movement to a new OMNI link segment, etc.







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  4.16.  Multicast

     Each Client provides an IGMP (IPv4) [RFC2236] or MLD (IPv6) [RFC3810]
     proxy service for its ENETs and/or hosted applications [RFC4605] and
     acts as a Protocol Independent Multicast - Sparse-Mode (PIM-SM, or
     simply "PIM") Designated Router (DR) [RFC7761] on the OMNI link.
     Proxy/Servers act as OMNI link PIM routers for Clients on ANET, VPN/
     IPsec or Direct interfaces, and Relays also act as OMNI link PIM
     routers on behalf of nodes on other links/networks.

     Clients on VPN/IPsec, Direct or ANET underlay interfaces for which
     the ANET has deployed native multicast services forward IGMP/MLD
     messages into the ANET.  The IGMP/MLD messages may be further
     forwarded by a first-hop ANET access router acting as an IGMP/MLD-
     snooping switch [RFC4541], then ultimately delivered to an ANET (FHS)
     Proxy/Server.  The FHS Proxy/Server then acts as an ARS to send
     NS(AR) messages to an ARR for the multicast source.  Clients on ANET/
     INET underlay interfaces without native multicast services instead
     send NS(AR) messages as an ARS to cause their FHS Proxy/Server to
     forward the message to an ARR.  When the ARR prepares an NA(AR)
     response, it initiates PIM protocol messaging according to the
     Source-Specific Multicast (SSM) and Any-Source Multicast (ASM)
     operational modes as discussed in the following sections.

  4.16.1.  Source-Specific Multicast (SSM)

     When an ARS "X" (i.e., either a Client or Proxy/Server) acting as PIM
     router receives a Join/Prune message from a node on its downstream
     interfaces containing one or more ((S)ource, (G)roup) pairs, it
     updates its Multicast Routing Information Base (MRIB) accordingly.
     For each S belonging to a prefix reachable via X's non-OMNI
     interfaces, X then forwards the (S, G) Join/Prune to any PIM routers
     on those interfaces per [RFC7761].

     For each S belonging to a prefix reachable via X's OMNI interface, X
     sends an NS(AR) message (see: Section 4.13) using its own ULA or XLA
     as the source address, the solicited node multicast address
     corresponding to S as the destination and the XLA of S as the target
     address.  X then encapsulates the NS(AR) in an OAL header with source
     address set to its own ULA and destination address set to the ULA for
     S, then forwards the message into the secured spanning tree which
     delivers it to ARR "Y" that services S.  Y will then return an NA(AR)
     that includes an OMNI option with Interface Attributes for any
     underlay interfaces that are currently servicing S.

     When X processes the NA(AR) it selects one or more underlay
     interfaces for S and performs an NS/NA multilink forwarding exchange
     over the secured spanning tree while including a PIM Join/Prune



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     message for each multicast group of interest in the OMNI option.  If
     S is located behind any Proxys "Z"*, each Z* then updates its MRIB
     accordingly and maintains the XLA of X as the next hop in the reverse
     path.  Since Gateways forward messages not addressed to themselves
     without examining them, this means that the (reverse) multicast tree
     path is simply from each Z* (and/or S) to X with no other multicast-
     aware routers in the path.

     Following the initial combined Join/Prune and NS/NA messaging, X
     maintains a NCE for each S the same as if X was sending unicast data
     traffic to S.  In particular, X performs additional NS/NA exchanges
     to keep the NCE alive for up to t_periodic seconds [RFC7761].  If no
     new Joins are received within t_periodic seconds, X allows the NCE to
     expire.  Finally, if X receives any additional Join/Prune messages
     for (S,G) it forwards the messages over the secured spanning tree.

     Client C that holds an MNP for source S may later depart from a first
     Proxy/Server Z1 and/or connect via a new Proxy/Server Z2.  In that
     case, Y sends a uNA message to X the same as specified for unicast
     mobility in Section 4.15.  When X receives the uNA message, it
     updates its NCE for the XLA for source S and sends new Join messages
     in NS/NA exchanges addressed to the new target Client underlay
     interface connection for S.  There is no requirement to send any
     Prune messages to old Proxy/Server Z1 since source S will no longer
     source any multicast data traffic via Z1.  Instead, the multicast
     state for (S,G) in Proxy/Server Z1 will soon expire since no new
     Joins will arrive.

  4.16.2.  Any-Source Multicast (ASM)

     When an ARS "X" acting as a PIM router receives Join/Prune messages
     from a node on its downstream interfaces containing one or more (*,G)
     pairs, it updates its Multicast Routing Information Base (MRIB)
     accordingly.  X first performs an NS/NA(AR) exchange to receive
     address resolution information for Rendezvous Point (RP) "R" for each
     G.  X then includes a copy of each Join/Prune message in the OMNI
     option of an NS message with its own ULA or XLA as the source address
     and the ULA or XLA for R as the destination address, then
     encapsulates the NS message in an OAL header with its own ULA as the
     source and the ULA of R's Proxy/Server as the destination then sends
     the message into the secured spanning tree.

     For each source "S" that sends multicast traffic to group G via R,
     Client S* that aggregates S (or its Proxy/Server) encapsulates the
     original IP packets/parcels in PIM Register messages, includes the
     PIM Register messages in the OMNI options of uNA messages, performs
     OAL encapsulation and fragmentation with Identification values within
     the receive window for Client R* that aggregates R, then performs L2



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     encapsulation/fragmentation and forwards the resulting carrier
     packets.  Client R* may then elect to send a PIM Join to S* in the
     OMNI option of a uNA over the secured spanning tree.  This will
     result in an (S,G) tree rooted at S* with R as the next hop so that R
     will begin to receive two copies of the original IP packet/parcel;
     one native copy from the (S, G) tree and a second copy from the pre-
     existing (*, G) tree that still uses uNA PIM Register encapsulation.
     R can then issue a uNA PIM Register-stop message over the secured
     spanning tree to suppress the Register-encapsulated stream.  At some
     later time, if Client S* moves to a new Proxy/Server, it resumes
     sending original IP packets/parcels via uNA PIM Register
     encapsulation via the new Proxy/Server.

     At the same time, as multicast listeners discover individual S's for
     a given G, they can initiate an (S,G) Join for each S under the same
     procedures discussed in Section 4.16.1.  Once the (S,G) tree is
     established, the listeners can send (S, G) Prune messages to R so
     that multicast original IP packets/parcels for group G sourced by S
     will only be delivered via the (S, G) tree and not from the (*, G)
     tree rooted at R.  All mobility considerations discussed for SSM
     apply.

  4.16.3.  Bi-Directional PIM (BIDIR-PIM)

     Bi-Directional PIM (BIDIR-PIM) [RFC5015] provides an alternate
     approach to ASM that treats the Rendezvous Point (RP) as a Designated
     Forwarder (DF).  Further considerations for BIDIR-PIM are out of
     scope.

  4.17.  Operation over Multiple OMNI Links

     An AERO Client can connect to multiple OMNI links the same as for any
     data link service.  In that case, the Client maintains a distinct
     OMNI interface for each link, e.g., 'omni0' for the first link,
     'omni1' for the second, 'omni2' for the third, etc.  Each OMNI link
     would include its own distinct set of Gateways and Proxy/Servers,
     thereby providing redundancy in case of failures.

     Each OMNI link could utilize the same or different ANET/INET link
     layer connections.  The links can be distinguished at the link layer
     via the SRT prefix in a similar fashion as for Virtual Local Area
     Network (VLAN) tagging (e.g., IEEE 802.1Q) and/or through assignment
     of distinct sets of MSPs on each link.  This gives rise to the
     opportunity for supporting multiple redundant networked paths (see:
     Section 4.2.4).






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     The Client's network layer can select the outbound OMNI interface
     appropriate for a given traffic profile while (in the reverse
     direction) correspondent nodes must have some way of steering their
     original IP packets/parcels destined to a target via the correct OMNI
     link.

     In a first alternative, if each OMNI link services different MSPs the
     Client can receive a distinct MNP from each of the links.  IP routing
     will therefore assure that the correct OMNI link is used for both
     outbound and inbound traffic.  This can be accomplished using
     existing technologies and approaches, and without requiring any
     special supporting code in correspondent nodes or Gateways.

     In a second alternative, if each OMNI link services the same MSP(s)
     then each link could assign a distinct "OMNI link Anycast" address
     that is configured by all Gateways on the link.  Correspondent nodes
     can then perform Segment Routing to select the correct SRT, which
     will then direct the original IP packet/parcel over multiple hops to
     the target.

  4.18.  DNS Considerations

     AERO Client MNs and INET correspondent nodes consult the Domain Name
     System (DNS) the same as for any Internetworking node.  When
     correspondent nodes and Client MNs use different IP protocol versions
     (e.g., IPv4 correspondents and IPv6 MNs), the INET DNS must maintain
     A records for IPv4 address mappings to MNs which must then be
     populated in Relay NAT64 mapping caches.  In that way, an IPv4
     correspondent node can send original IPv4 packets/parcels to the IPv4
     address mapping of the target MN, and the Relay will translate the
     IPv4 header and destination address into an IPv6 header and IPv6
     destination address of the MN.

     When an AERO Client registers with an AERO Proxy/Server, the Proxy/
     Server can return the address(es) of DNS servers in RDNSS options
     [RFC6106].  The DNS server provides the IP addresses of other MNs and
     correspondent nodes in AAAA records for IPv6 or A records for IPv4.

  4.19.  Transition/Coexistence Considerations

     OAL encapsulation ensures that dissimilar INET partitions can be
     joined into a single unified OMNI link, even though the partitions
     themselves may have differing protocol versions and/or incompatible
     addressing plans.  However, a commonality can be achieved by
     incrementally distributing globally routable (i.e., native) IP
     prefixes to eventually reach all nodes (both mobile and fixed) in all
     OMNI link segments.  This can be accomplished by incrementally
     deploying AERO Gateways on each INET partition, with each Gateway



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     distributing its MNPs and/or discovering non-MNP IP GUA prefixes on
     its INET links.

     This gives rise to the opportunity to eventually distribute native IP
     addresses to all nodes, and to present a unified OMNI link view even
     if the INET partitions remain in their current protocol and
     addressing plans.  In that way, the OMNI link can serve the dual
     purpose of providing a mobility/multilink service and a transition/
     coexistence service.  Alternatively, if an INET partition is
     transitioned to a native IP protocol version and addressing scheme
     compatible with the OMNI link MNP-based addressing scheme, the
     partition and OMNI link can be joined by Gateways.

     Relays that connect INETs/ENETs with dissimilar IP protocol versions
     may need to employ a network address and protocol translation
     function such as NAT64 [RFC6146].

  4.20.  Proxy/Server-Gateway Bidirectional Forwarding Detection

     In environments where rapid failure recovery is essential, Proxy/
     Servers and Gateways SHOULD use Bidirectional Forwarding Detection
     (BFD) [RFC5880].  Nodes that use BFD can quickly detect and react to
     failures so that cached information is re-established through
     alternate nodes.  BFD control messaging is carried only over well-
     connected ground domain networks (i.e., and not low-end radio links)
     and can therefore be tuned for rapid response.

     Proxy/Servers and Gateways can maintain BFD sessions in parallel with
     their BGP peerings.  If a Proxy/Server or Gateway fails, BGP peers
     will quickly re-establish routes through alternate paths the same as
     for common BGP operational practice.

  4.21.  Time-Varying MNPs

     In some use cases, it is desirable, beneficial and efficient for the
     Client to receive a constant MNP that travels with the Client
     wherever it moves.  For example, this would allow air traffic
     controllers to easily track aircraft, etc.  In other cases, however
     (e.g., intelligent transportation systems), the MN may be willing to
     sacrifice a modicum of efficiency in order to have time-varying MNPs
     that can be changed every so often to defeat adversarial tracking.

     The DHCPv6 service offers a way for Clients that desire time-varying
     MNPs to obtain short-lived prefixes (e.g., on the order of a small
     number of minutes).  In that case, the identity of the Client would
     not be bound to the MNP but rather to a Node Identification value
     (see: [I-D.templin-intarea-omni]) that can serve as a Client ID seed
     for MNP prefix delegation.  The Client would then be obligated to



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     renumber its internal networks whenever its MNP (and therefore also
     its XLA) changes.  This should not present problems for Clients with
     automated network renumbering services, however it can limit the
     durations of ongoing sessions that would prefer to use a constant
     address.

  5.  Implementation Status

     An early AERO implementation based on OpenVPN (https://openvpn.net/)
     was announced on the v6ops mailing list on January 10, 2018 and an
     initial public release of the AERO proof-of-concept source code was
     announced on the intarea mailing list on August 21, 2015.

     Many AERO/OMNI functions are implemented and undergoing final
     integration.  OAL fragmentation/reassembly buffer management code has
     been cleared for public release.

     Implementation of AERO/OMNI functions specified in more recent
     document versions is considered work in progress.

  6.  IANA Considerations

     The IANA has assigned the UDP port number "8060" for an experimental
     first edition of AERO [RFC6706].  This document together with OMNI
     [I-D.templin-intarea-omni] reclaims UDP port number "8060" as the
     service port for AERO/OMNI UDP/IP encapsulation.  This document makes
     no IANA request, since the OMNI specification already provides IANA
     guidance.  (Note: although [RFC6706] was not widely implemented or
     deployed, it need not be obsoleted since its messages use the invalid
     ICMPv6 message type number '0' which implementations of this
     specification can easily distinguish and ignore.)

     No further IANA actions are required.

  7.  Security Considerations

     AERO Gateways establish security associations with AERO Proxy/Servers
     and Relays within their local OMNI link segments using secured
     tunnels over underlay interfaces.  The AERO Gateways of all OMNI link
     segments in turn configure secured tunnels with neighboring AERO
     Gateways for other OMNI link segments in a secured spanning tree
     topology.  Applicable security services include IPsec [RFC4301] with
     IKEv2 [RFC7296], etc.  (Note that secured direct point-to-point links
     can also be used instead of or in addition to network layer
     security.)  Together, these services are responsible for assuring
     connectionless integrity and data origin authentication with optional
     protection against replays for control messages that traverse the
     secured spanning tree.



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     To prevent unauthorized local applications from congesting the
     secured spanning tree, Proxy/Servers and Gateways configure local
     access controls to permit only the BGP protocol service daemon to
     source routing protocol control messages with the ULA assigned to the
     OMNI interface as the source over the secured spanning tree.  This
     could be implemented as a port/address filtering configuration that
     permits only TCP port 179 (as defined in the IANA "Service Names and
     Port Numbers" registry) when using the ULA assigned to the OMNI
     interface.  To prevent malicious Clients from congesting the secured
     spanning tree, Proxy/Servers should also rate-limit the secured IPv6
     ND NS/NA messages they process for the same (source, target) pair,
     e.g., by applying IPv6 ND MAX_UNICAST_SOLICIT;
     MAX_NEIGHBOR_ADVERTISEMENT limits.

     To prevent spoofing, Proxy/Servers MUST silently discard without
     responding to any unsecured IPv6 ND messages with OMNI sub-options
     that would otherwise affect state.  Also, Proxy/Servers MUST silently
     discard without forwarding any original IP packets/parcels received
     from one of their own Clients (whether directly or following OAL
     reassembly) with a source address that does not match the Client's
     MNP and/or a destination address that does match the Client's MNP.
     Finally, Proxy/Servers MUST silently discard without forwarding any
     carrier packets that include an OAL packet/fragment with source and
     destination that both match the same MNP.

     AERO Clients that connect to secured ANETs need not apply additional
     security to their IPv6 ND messages, since the messages will be
     accepted and forwarded by a perimeter Proxy/Server that applies
     security over its INET-facing interface to the secured spanning tree
     (see above).  AERO Clients that connect to the open INET can use
     network and/or transport layer security services such as VPNs (e.g.,
     IPsec tunnels) or can by some other means establish a secured direct
     link to a Proxy/Server.  When a VPN or direct link may be
     impractical, however, INET Clients and Proxy/Servers SHOULD include
     and verify authentication signatures for IPv6 ND messages as
     specified in [I-D.templin-intarea-omni].

     End systems SHOULD apply transport or higher layer security services
     such as QUIC-TLS [RFC9000], TLS/SSL [RFC8446], DTLS [RFC6347], etc.
     to provide a level of protection comparable to critical secured
     Internet services.  End systems that require host-based VPN services
     SHOULD use network and/or transport layer security services such as
     IPsec, TLS/SSL, DTLS, etc.  AERO Proxy/Servers and Clients can also
     provide a network-based VPN service on behalf of end systems, e.g.,
     if the end system is located within a secured enclave and cannot
     establish a VPN on its own behalf.





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     For INET partitions that require strong network layer security in the
     data plane, two options for securing communications include 1)
     disable route optimization and direct all traffic over the secured
     spanning tree, or 2) enable on-demand secure tunnel establishment
     between Client neighbors.  Option 1) would result in longer routes
     than necessary and impose traffic concentration on critical
     infrastructure elements.  Option 2) could be coordinated between
     Clients using NS/NA messages with OMNI Host Identity Protocol (HIP)
     "Initiator/Responder" message sub-options [RFC7401]
     [I-D.templin-intarea-omni] or QUIC-TLS protocol message sub-options
     [RFC9000][RFC9001] [RFC9002] to establish secured sessions.

     AERO Proxy/Servers and Gateways present targets for traffic
     amplification Denial of Service (DoS) attacks.  This concern is no
     different than for widely-deployed VPN security gateways in the
     Internet, where attackers could send spoofed packets to the gateways
     at high data rates.  This can be mitigated through the AERO/OMNI data
     origin authentication procedures, as well as connecting Proxy/Servers
     and Gateways over dedicated links with no connections to the Internet
     and/or when connections to the Internet are only permitted through
     well-managed firewalls.  Traffic amplification DoS attacks can also
     target an AERO Client's low data rate links.  This is a concern not
     only for Clients located on the open Internet but also for Clients in
     secured enclaves.  AERO Proxy/Servers and Proxys can institute rate
     limits that protect Clients from receiving carrier packet floods that
     could DoS low data rate links.

     AERO Relays must implement ingress filtering to avoid a spoofing
     attack in which spurious messages with ULA addresses are injected
     into an OMNI link from an outside attacker.  AERO Clients MUST ensure
     that their connectivity is not used by unauthorized nodes on their
     ENETs to gain access to a protected network, i.e., AERO Clients that
     act as routers MUST NOT provide routing services for unauthorized
     nodes.  (This concern is no different than for ordinary hosts that
     receive an IP address delegation but then "share" the address with
     other nodes via some form of Internet connection sharing such as
     tethering.)

     The AERO service for open INET Clients depends on a public key
     distribution service in which Client public keys and identities are
     maintained in a shared database accessible to all open INET Proxy/
     Servers.  Similarly, each Client must be able to determine the public
     key of each Proxy/Server, e.g. by consulting an online database.








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     The PRL contains only public information, but MUST be well-managed
     and secured from unauthorized tampering.  The PRL can be conveyed to
     the Client in a similar fashion as in [RFC5214] (e.g., through data
     link layer login messaging, secure upload of a static file, DNS
     lookups, etc.).

     Security considerations for IPv6 fragmentation and reassembly are
     discussed in [I-D.templin-intarea-omni].  In environments where
     spoofing is considered a threat, all OAL nodes SHOULD employ
     Identification window synchronization and OAL end systems SHOULD
     configure an (end-system-based) firewall.

     Security considerations for accepting link layer ICMP messages and
     reflected carrier packets are discussed throughout the document.

  8.  Acknowledgements

     Discussions in the IETF, aviation standards communities and private
     exchanges helped shape some of the concepts in this work.
     Individuals who contributed insights include Mikael Abrahamsson, Mark
     Andrews, Fred Baker, Bob Braden, Stewart Bryant, Scott Burleigh,
     Brian Carpenter, Wojciech Dec, Pavel Drasil, Ralph Droms, Adrian
     Farrel, Nick Green, Sri Gundavelli, Brian Haberman, Bernhard Haindl,
     Joel Halpern, Tom Herbert, Bob Hinden, Sascha Hlusiak, Lee Howard,
     Christian Huitema, Zdenek Jaron, Andre Kostur, Hubert Kuenig, Eliot
     Lear, Ted Lemon, Andy Malis, Satoru Matsushima, Tomek Mrugalski,
     Thomas Narten, Madhu Niraula, Alexandru Petrescu, Behcet Saikaya,
     Michal Skorepa, Dave Thaler, Joe Touch, Bernie Volz, Ryuji Wakikawa,
     Tony Whyman, Lloyd Wood and James Woodyatt.  Members of the IESG also
     provided valuable input during their review process that greatly
     improved the document.  Special thanks go to Stewart Bryant, Joel
     Halpern and Brian Haberman for their shepherding guidance during the
     publication of the AERO first edition.

     This work has further been encouraged and supported by Boeing
     colleagues including Akash Agarwal, Kyle Bae, M.  Wayne Benson, Dave
     Bernhardt, Cam Brodie, John Bush, Balaguruna Chidambaram, Irene Chin,
     Bruce Cornish, Claudiu Danilov, Sean Dickson, Don Dillenburg, Joe
     Dudkowski, Wen Fang, Samad Farooqui, Anthony Gregory, Jeff Holland,
     Seth Jahne, Brian Jaury, Greg Kimberly, Ed King, Madhuri Madhava
     Badgandi, Laurel Matthew, Gene MacLean III, Kyle Mikos, Rob
     Muszkiewicz, Sean O'Sullivan, Satish Raghavendran, Vijay Rajagopalan,
     Kristina Ross, Greg Saccone, Ron Sackman, Bhargava Raman Sai Prakash,
     Rod Santiago, Madhanmohan Savadamuthu, Kent Shuey, Brian Skeen, Mike
     Slane, Carrie Spiker, Katie Tran, Brendan Williams, Amelia Wilson,
     Julie Wulff, Yueli Yang, Eric Yeh and other members of the Boeing
     mobility, networking and autonomy teams.  Akash Agarwal, Kyle Bae,
     Wayne Benson, Madhuri Madhava Badgandi, Vijayasarathy Rajagopalan,



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     Bhargava Raman Sai Prakash, Katie Tran and Eric Yeh are especially
     acknowledged for their work on the AERO implementation.  Chuck
     Klabunde is honored for his support and guidance, and we mourn his
     untimely loss.

     This work was inspired by the support and encouragement of countless
     outstanding colleagues, managers and program directors over the span
     of many decades.  Beginning in the late 1980s,' the Digital Equipment
     Corporation (DEC) Ultrix Engineering and DECnet Architects groups
     identified early issues with fragmentation and bridging links with
     diverse MTUs.  In the early 1990s, engagements at DEC Project Sequoia
     at UC Berkeley and the DEC Western Research Lab in Palo Alto included
     investigations into large-scale networked filesystems, ATM vs
     Internet and network security proxys.  In the mid-1990s to early
     2000s employment at the NASA Ames Research Center (Sterling Software)
     and SRI International supported early investigations of IPv6, ONR UAV
     Communications and the IETF.  An employment at Nokia where important
     IETF documents were published gave way to a present-day engagement
     with The Boeing Company.  The work matured at Boeing through major
     programs including Future Combat Systems, Advanced Airplane Program,
     DTN for the International Space Station, Mobility Vision Lab, CAST,
     Caravan, Airplane Internet of Things, the NASA UAS/CNS program, the
     FAA/ICAO ATN/IPS program and many others.  An attempt to name all who
     gave support and encouragement would double the current document size
     and result in many unintentional omissions - but to all a humble
     thanks.

     Earlier works on NBMA tunneling approaches are found in
     [RFC2529][RFC5214][RFC5569].

     Many of the constructs presented in this second edition of AERO are
     based on the author's earlier works, including:

     *  Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) [RFC5214]

     *  The Subnetwork Encapsulation and Adaptation Layer (SEAL) [RFC5320]

     *  Virtual Enterprise Traversal (VET) [RFC5558]

     *  Routing and Addressing in Networks with Global Enterprise
        Recursion (RANGER) [RFC5720][RFC6139]

     *  The Internet Routing Overlay Network (IRON) [RFC6179]

     *  AERO, First Edition [RFC6706]






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     Note that these works cite numerous earlier efforts that are not
     included here due to space limitations.  The authors of those earlier
     works are acknowledged for their insights.

     This work is aligned with the NASA Safe Autonomous Systems Operation
     (SASO) program under NASA contract number NNA16BD84C.

     This work is aligned with the FAA as per the SE2025 contract number
     DTFAWA-15-D-00030.

     This work is aligned with the Boeing Commercial Airplanes (BCA)
     Airplane Internet of Things (AIoT) and autonomy programs.

     This work is aligned with the Boeing Information Technology (BIT)
     MobileNet program.

     Honoring life, liberty and the pursuit of happiness.

  9.  References

  9.1.  Normative References

     [I-D.templin-intarea-omni]
                Templin, F., "Transmission of IP Packets over Overlay
                Multilink Network (OMNI) Interfaces", Work in Progress,
                Internet-Draft, draft-templin-intarea-omni-66, 12 February
                2024, <https://datatracker.ietf.org/doc/html/draft-
                templin-intarea-omni-66>.

     [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
                DOI 10.17487/RFC0791, September 1981,
                <https://www.rfc-editor.org/info/rfc791>.

     [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
                RFC 792, DOI 10.17487/RFC0792, September 1981,
                <https://www.rfc-editor.org/info/rfc792>.

     [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119,
                DOI 10.17487/RFC2119, March 1997,
                <https://www.rfc-editor.org/info/rfc2119>.

     [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
                Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
                <https://www.rfc-editor.org/info/rfc4193>.






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     [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
                Border Gateway Protocol 4 (BGP-4)", RFC 4271,
                DOI 10.17487/RFC4271, January 2006,
                <https://www.rfc-editor.org/info/rfc4271>.

     [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
                Architecture", RFC 4291, DOI 10.17487/RFC4291, February
                2006, <https://www.rfc-editor.org/info/rfc4291>.

     [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
                Control Message Protocol (ICMPv6) for the Internet
                Protocol Version 6 (IPv6) Specification", STD 89,
                RFC 4443, DOI 10.17487/RFC4443, March 2006,
                <https://www.rfc-editor.org/info/rfc4443>.

     [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
                "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
                DOI 10.17487/RFC4861, September 2007,
                <https://www.rfc-editor.org/info/rfc4861>.

     [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
                Address Autoconfiguration", RFC 4862,
                DOI 10.17487/RFC4862, September 2007,
                <https://www.rfc-editor.org/info/rfc4862>.

     [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                May 2017, <https://www.rfc-editor.org/info/rfc8174>.

     [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
                (IPv6) Specification", STD 86, RFC 8200,
                DOI 10.17487/RFC8200, July 2017,
                <https://www.rfc-editor.org/info/rfc8200>.

     [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
                Richardson, M., Jiang, S., Lemon, T., and T. Winters,
                "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
                RFC 8415, DOI 10.17487/RFC8415, November 2018,
                <https://www.rfc-editor.org/info/rfc8415>.

     [RFC9268]  Hinden, R. and G. Fairhurst, "IPv6 Minimum Path MTU Hop-
                by-Hop Option", RFC 9268, DOI 10.17487/RFC9268, August
                2022, <https://www.rfc-editor.org/info/rfc9268>.

  9.2.  Informative References

     [BGP]      Huston, G., "BGP in 2015, http://potaroo.net", January
                2016.



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     [EUI]      "IEEE Guidelines for Use of Extended Unique Identifier
                (EUI), Organizationally Unique Identifier (OUI), and
                Company ID, https://standards.ieee.org/wp-
                content/uploads/import/documents/tutorials/eui.pdf", 3
                August 2017.

     [I-D.ietf-6man-comp-rtg-hdr]
                Bonica, R., Kamite, Y., Alston, A., Henriques, D., and L.
                Jalil, "The IPv6 Compact Routing Header (CRH)", Work in
                Progress, Internet-Draft, draft-ietf-6man-comp-rtg-hdr-03,
                18 January 2024, <https://datatracker.ietf.org/doc/html/
                draft-ietf-6man-comp-rtg-hdr-03>.

     [I-D.ietf-intarea-tunnels]
                Touch, J. D. and M. Townsley, "IP Tunnels in the Internet
                Architecture", Work in Progress, Internet-Draft, draft-
                ietf-intarea-tunnels-13, 26 March 2023,
                <https://datatracker.ietf.org/doc/html/draft-ietf-intarea-
                tunnels-13>.

     [I-D.ietf-ipwave-vehicular-networking]
                Jeong, J. P., "IPv6 Wireless Access in Vehicular
                Environments (IPWAVE): Problem Statement and Use Cases",
                Work in Progress, Internet-Draft, draft-ietf-ipwave-
                vehicular-networking-30, 24 October 2022,
                <https://datatracker.ietf.org/doc/html/draft-ietf-ipwave-
                vehicular-networking-30>.

     [I-D.ietf-rtgwg-atn-bgp]
                Templin, F., Saccone, G., Dawra, G., Lindem, A., and V.
                Moreno, "A Simple BGP-based Mobile Routing System for the
                Aeronautical Telecommunications Network", Work in
                Progress, Internet-Draft, draft-ietf-rtgwg-atn-bgp-25, 23
                October 2023, <https://datatracker.ietf.org/doc/html/
                draft-ietf-rtgwg-atn-bgp-25>.

     [I-D.perkins-manet-aodvv2]
                Perkins, C. E., Ratliff, S., Dowdell, J., Steenbrink, L.,
                and V. Pritchard, "Ad Hoc On-demand Distance Vector
                Version 2 (AODVv2) Routing", Work in Progress, Internet-
                Draft, draft-perkins-manet-aodvv2-03, 28 February 2019,
                <https://datatracker.ietf.org/doc/html/draft-perkins-
                manet-aodvv2-03>.








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     [I-D.templin-6man-parcels2]
                Templin, F. L., "IPv6 Parcels and Advanced Jumbos (AJs)",
                Work in Progress, Internet-Draft, draft-templin-6man-
                parcels2-00, 16 February 2024,
                <https://datatracker.ietf.org/api/v1/doc/document/draft-
                templin-6man-parcels2/>.

     [I-D.templin-intarea-parcels2]
                Templin, F., "IPv4 Parcels and Advanced Jumbos (AJs)",
                Work in Progress, Internet-Draft, draft-templin-intarea-
                parcels2-00, 15 February 2024,
                <https://datatracker.ietf.org/doc/html/draft-templin-
                intarea-parcels2-00>.

     [IEN48]    Cerf, V., "The Catenet Model For Internetworking,
                https://www.rfc-editor.org/ien/ien48.txt", July 1978.

     [IEN48-2]  Cerf, V., "The Catenet Model For Internetworking (with
                figures), http://www.postel.org/ien/pdf/ien048.pdf", July
                1978.

     [RFC1035]  Mockapetris, P., "Domain names - implementation and
                specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
                November 1987, <https://www.rfc-editor.org/info/rfc1035>.

     [RFC1256]  Deering, S., Ed., "ICMP Router Discovery Messages",
                RFC 1256, DOI 10.17487/RFC1256, September 1991,
                <https://www.rfc-editor.org/info/rfc1256>.

     [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
                RFC 1812, DOI 10.17487/RFC1812, June 1995,
                <https://www.rfc-editor.org/info/rfc1812>.

     [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
                J., and E. Lear, "Address Allocation for Private
                Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
                February 1996, <https://www.rfc-editor.org/info/rfc1918>.

     [RFC2236]  Fenner, W., "Internet Group Management Protocol, Version
                2", RFC 2236, DOI 10.17487/RFC2236, November 1997,
                <https://www.rfc-editor.org/info/rfc2236>.

     [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
                Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
                <https://www.rfc-editor.org/info/rfc2464>.






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     [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
                Domains without Explicit Tunnels", RFC 2529,
                DOI 10.17487/RFC2529, March 1999,
                <https://www.rfc-editor.org/info/rfc2529>.

     [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
                Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
                DOI 10.17487/RFC3810, June 2004,
                <https://www.rfc-editor.org/info/rfc3810>.

     [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
                Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
                December 2005, <https://www.rfc-editor.org/info/rfc4301>.

     [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
                Network Address Translations (NATs)", RFC 4380,
                DOI 10.17487/RFC4380, February 2006,
                <https://www.rfc-editor.org/info/rfc4380>.

     [RFC4389]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
                Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April
                2006, <https://www.rfc-editor.org/info/rfc4389>.

     [RFC4511]  Sermersheim, J., Ed., "Lightweight Directory Access
                Protocol (LDAP): The Protocol", RFC 4511,
                DOI 10.17487/RFC4511, June 2006,
                <https://www.rfc-editor.org/info/rfc4511>.

     [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
                "Considerations for Internet Group Management Protocol
                (IGMP) and Multicast Listener Discovery (MLD) Snooping
                Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
                <https://www.rfc-editor.org/info/rfc4541>.

     [RFC4605]  Fenner, B., He, H., Haberman, B., and H. Sandick,
                "Internet Group Management Protocol (IGMP) / Multicast
                Listener Discovery (MLD)-Based Multicast Forwarding
                ("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605,
                August 2006, <https://www.rfc-editor.org/info/rfc4605>.

     [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
                "Bidirectional Protocol Independent Multicast (BIDIR-
                PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
                <https://www.rfc-editor.org/info/rfc5015>.







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     [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
                Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
                DOI 10.17487/RFC5214, March 2008,
                <https://www.rfc-editor.org/info/rfc5214>.

     [RFC5320]  Templin, F., Ed., "The Subnetwork Encapsulation and
                Adaptation Layer (SEAL)", RFC 5320, DOI 10.17487/RFC5320,
                February 2010, <https://www.rfc-editor.org/info/rfc5320>.

     [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
                for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
                <https://www.rfc-editor.org/info/rfc5340>.

     [RFC5522]  Eddy, W., Ivancic, W., and T. Davis, "Network Mobility
                Route Optimization Requirements for Operational Use in
                Aeronautics and Space Exploration Mobile Networks",
                RFC 5522, DOI 10.17487/RFC5522, October 2009,
                <https://www.rfc-editor.org/info/rfc5522>.

     [RFC5558]  Templin, F., Ed., "Virtual Enterprise Traversal (VET)",
                RFC 5558, DOI 10.17487/RFC5558, February 2010,
                <https://www.rfc-editor.org/info/rfc5558>.

     [RFC5569]  Despres, R., "IPv6 Rapid Deployment on IPv4
                Infrastructures (6rd)", RFC 5569, DOI 10.17487/RFC5569,
                January 2010, <https://www.rfc-editor.org/info/rfc5569>.

     [RFC5614]  Ogier, R. and P. Spagnolo, "Mobile Ad Hoc Network (MANET)
                Extension of OSPF Using Connected Dominating Set (CDS)
                Flooding", RFC 5614, DOI 10.17487/RFC5614, August 2009,
                <https://www.rfc-editor.org/info/rfc5614>.

     [RFC5720]  Templin, F., "Routing and Addressing in Networks with
                Global Enterprise Recursion (RANGER)", RFC 5720,
                DOI 10.17487/RFC5720, February 2010,
                <https://www.rfc-editor.org/info/rfc5720>.

     [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
                (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
                <https://www.rfc-editor.org/info/rfc5880>.

     [RFC6081]  Thaler, D., "Teredo Extensions", RFC 6081,
                DOI 10.17487/RFC6081, January 2011,
                <https://www.rfc-editor.org/info/rfc6081>.







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     [RFC6106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
                "IPv6 Router Advertisement Options for DNS Configuration",
                RFC 6106, DOI 10.17487/RFC6106, November 2010,
                <https://www.rfc-editor.org/info/rfc6106>.

     [RFC6139]  Russert, S., Ed., Fleischman, E., Ed., and F. Templin,
                Ed., "Routing and Addressing in Networks with Global
                Enterprise Recursion (RANGER) Scenarios", RFC 6139,
                DOI 10.17487/RFC6139, February 2011,
                <https://www.rfc-editor.org/info/rfc6139>.

     [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
                NAT64: Network Address and Protocol Translation from IPv6
                Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
                April 2011, <https://www.rfc-editor.org/info/rfc6146>.

     [RFC6179]  Templin, F., Ed., "The Internet Routing Overlay Network
                (IRON)", RFC 6179, DOI 10.17487/RFC6179, March 2011,
                <https://www.rfc-editor.org/info/rfc6179>.

     [RFC6221]  Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A.
                Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221,
                DOI 10.17487/RFC6221, May 2011,
                <https://www.rfc-editor.org/info/rfc6221>.

     [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
                Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
                January 2012, <https://www.rfc-editor.org/info/rfc6347>.

     [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
                for Equal Cost Multipath Routing and Link Aggregation in
                Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
                <https://www.rfc-editor.org/info/rfc6438>.

     [RFC6621]  Macker, J., Ed., "Simplified Multicast Forwarding",
                RFC 6621, DOI 10.17487/RFC6621, May 2012,
                <https://www.rfc-editor.org/info/rfc6621>.

     [RFC6706]  Templin, F., Ed., "Asymmetric Extended Route Optimization
                (AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012,
                <https://www.rfc-editor.org/info/rfc6706>.

     [RFC7181]  Clausen, T., Dearlove, C., Jacquet, P., and U. Herberg,
                "The Optimized Link State Routing Protocol Version 2",
                RFC 7181, DOI 10.17487/RFC7181, April 2014,
                <https://www.rfc-editor.org/info/rfc7181>.





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     [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
                Kivinen, "Internet Key Exchange Protocol Version 2
                (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
                2014, <https://www.rfc-editor.org/info/rfc7296>.

     [RFC7333]  Chan, H., Ed., Liu, D., Seite, P., Yokota, H., and J.
                Korhonen, "Requirements for Distributed Mobility
                Management", RFC 7333, DOI 10.17487/RFC7333, August 2014,
                <https://www.rfc-editor.org/info/rfc7333>.

     [RFC7401]  Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
                Henderson, "Host Identity Protocol Version 2 (HIPv2)",
                RFC 7401, DOI 10.17487/RFC7401, April 2015,
                <https://www.rfc-editor.org/info/rfc7401>.

     [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
                Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
                Multicast - Sparse Mode (PIM-SM): Protocol Specification
                (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
                2016, <https://www.rfc-editor.org/info/rfc7761>.

     [RFC8175]  Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B.
                Berry, "Dynamic Link Exchange Protocol (DLEP)", RFC 8175,
                DOI 10.17487/RFC8175, June 2017,
                <https://www.rfc-editor.org/info/rfc8175>.

     [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
                Decraene, B., Litkowski, S., and R. Shakir, "Segment
                Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
                July 2018, <https://www.rfc-editor.org/info/rfc8402>.

     [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
                Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
                <https://www.rfc-editor.org/info/rfc8446>.

     [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
                Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
                (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
                <https://www.rfc-editor.org/info/rfc8754>.

     [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
                Multiplexed and Secure Transport", RFC 9000,
                DOI 10.17487/RFC9000, May 2021,
                <https://www.rfc-editor.org/info/rfc9000>.

     [RFC9001]  Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
                QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
                <https://www.rfc-editor.org/info/rfc9001>.



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     [RFC9002]  Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
                and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
                May 2021, <https://www.rfc-editor.org/info/rfc9002>.

  Appendix A.  Non-Normative Considerations

     AERO can be applied to a multitude of Internetworking scenarios, with
     each having its own adaptations.  The following considerations are
     provided as non-normative guidance:

  A.1.  Implementation Strategies for Route Optimization

     Address resolution and route optimization as discussed in
     Section 4.13 results in the creation of NCEs.  The NCE state is set
     to REACHABLE for at most ReachableTime seconds.  In order to refresh
     the NCE lifetime before the ReachableTime timer expires, the
     specification requires implementations to issue a new NS/NA(AR)
     exchange to reset ReachableTime while data messages are still
     flowing.  However, the decision of when to initiate a new NS/NA(AR)
     exchange and to perpetuate the process is left as an implementation
     detail.

     One possible strategy may be to monitor the NCE watching for data
     messages for (ReachableTime - 5) seconds.  If any data messages have
     been sent to the neighbor within this timeframe, then send an NS(AR)
     to receive a new NA(AR).  If no data messages have been sent, wait
     for 5 additional seconds and send an immediate NS(AR) if any data
     packets are sent within this "expiration pending" 5 second window.
     If no additional data messages are sent within the 5 second window,
     reset the NCE state to STALE.

     The monitoring of the neighbor data traffic therefore becomes an
     ongoing process during the NCE lifetime.  If the NCE expires, future
     data messages will trigger a new NS/NA(AR) exchange while the
     messages themselves may be delivered over longer paths until route
     optimization state is re-established.

  A.2.  Implicit Mobility Management

     OMNI interface neighbors MAY provide a configuration option that
     allows them to perform implicit mobility management in which no IPv6
     ND messaging is used.  In that case, the Client only transmits
     carrier packets over a single interface at a time, and the neighbor
     always observes carrier packets arriving from the Client from the
     same L2 source address.






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     If the Client's underlay interface address changes (either due to a
     readdressing of the original interface or switching to a new
     interface) the neighbor immediately updates the NCE for the Client
     and begins accepting and sending carrier packets according to the
     Client's new address.  This implicit mobility method applies to use
     cases such as cellphones with both WiFi and Cellular interfaces where
     only one of the interfaces is active at a given time, and the Client
     automatically switches over to the backup interface if the primary
     interface fails.

  A.3.  Direct Underlying Interfaces

     When a Client's OMNI interface is configured over a Direct interface,
     the neighbor at the other end of the Direct link can receive original
     IP packets/parcels without any encapsulation.  In that case, the
     Client sends packets/parcels over the Direct link according to
     traffic selectors.  If the Direct interface is selected, then the
     Client's packets/parcels are transmitted directly to the peer without
     traversing an ANET/INET.  If other interfaces are selected, then the
     Client's packets/parcels are transmitted via a different interface,
     which may result in the inclusion of Proxy/Servers and Gateways in
     the communications path.  Direct interfaces must be tested
     periodically for reachability, e.g., via NUD.

  A.4.  AERO Critical Infrastructure Considerations

     AERO Gateways can be either Commercial off-the Shelf (COTS) standard
     IP routers or virtual machines in the cloud.  Gateways must be
     provisioned, supported and managed by the INET administrative
     authority, and connected to the Gateways of other INETs via inter-
     domain peerings.  Cost for purchasing, configuring and managing
     Gateways is nominal even for very large OMNI links.

     AERO INET Proxy/Servers can be standard dedicated server platforms,
     but most often will be deployed as virtual machines in the cloud.
     The only requirements for INET Proxy/Servers are that they can run
     the AERO/OMNI code and have at least one network interface connection
     to the INET.  INET Proxy/Servers must be provisioned, supported and
     managed by the INET administrative authority.  Cost for purchasing,
     configuring and managing cloud Proxy/Servers is nominal especially
     for virtual machines.

     AERO ANET Proxy/Servers are most often standard dedicated server
     platforms with one underlay interface connected to the ANET and a
     second interface connected to an INET.  As with INET Proxy/Servers,
     the only requirements are that they can run the AERO/OMNI code and
     have at least one interface connection to the INET.  ANET Proxy/
     Servers must be provisioned, supported and managed by the ANET



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     administrative authority.  Cost for purchasing, configuring and
     managing Proxys is nominal, and borne by the ANET administrative
     authority.

     AERO Relays are simply Proxy/Servers connected to INETs and/or ENETs
     that provide forwarding services for non-MNP destinations.  The Relay
     connects to the OMNI link and engages in eBGP peering with one or
     more Gateways as a stub AS.  The Relay then injects its MNPs and/or
     non-MNP prefixes into the BGP routing system, and provisions the
     prefixes to its downstream-attached networks.  The Relay can perform
     ARS/ARR services the same as for any Proxy/Server, and can route
     between the MNP and non-MNP address spaces.

  A.5.  AERO Server Failure Implications

     AERO Proxy/Servers do not present a single point of failure in the
     architecture since all Proxy/Servers on the link provide identical
     services and loss of a Proxy/Server does not imply immediate and/or
     comprehensive communication failures.  Proxy/Server failure can be
     quickly detected and conveyed by Bidirectional Forward Detection
     (BFD) and/or proactive NUD allowing Clients to migrate to new Proxy/
     Servers.

     If a Proxy/Server fails, peer carrier packet forwarding to Clients
     will continue by virtue of the neighbor cache entries that have
     already been established through address resolution and route
     optimization.  If a Client also experiences mobility events at
     roughly the same time the Proxy/Server fails, uNA messages may be
     lost but neighbor cache entries in the DEPARTED state will ensure
     that carrier packet forwarding to the Client's new locations will
     continue for up to DepartTime seconds.

     If a Client is left without a Proxy/Server for a considerable length
     of time (e.g., greater than ReachableTime seconds) then existing
     neighbor cache entries will eventually expire and both ongoing and
     new communications will fail.  The original source will continue to
     retransmit until the Client has established a new Proxy/Server
     relationship, after which time communications can continue .

     Therefore, links that provide many Proxy/Servers with high
     availability profiles are responsive to loss of individual
     infrastructure elements, since Clients can quickly establish new
     Proxy/Server relationships in event of failures.








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  A.6.  AERO Client / Server Architecture

     The AERO architectural model is client / server in the control plane,
     with route optimization in the data plane.  The same as for common
     Internet services, the AERO Client discovers the addresses of AERO
     Proxy/Servers and connects to one or more of them.  The AERO service
     is analogous to common Internet services such as google.com,
     yahoo.com, cnn.com, etc.  However, there is only one AERO service for
     the link and all Proxy/Servers provide identical services.

     Common Internet services provide differing strategies for advertising
     server addresses to clients.  The strategy is conveyed through the
     DNS resource records returned in response to name resolution queries.
     As of January 2020 Internet-based 'nslookup' services were used to
     determine the following:

     *  When a client resolves the domainname "google.com", the DNS always
        returns one A record (i.e., an IPv4 address) and one AAAA record
        (i.e., an IPv6 address).  The client receives the same addresses
        each time it resolves the domainname via the same DNS resolver,
        but may receive different addresses when it resolves the
        domainname via different DNS resolvers.  But, in each case,
        exactly one A and one AAAA record are returned.

     *  When a client resolves the domainname "ietf.org", the DNS always
        returns one A record and one AAAA record with the same addresses
        regardless of which DNS resolver is used.

     *  When a client resolves the domainname "yahoo.com", the DNS always
        returns a list of 4 A records and 4 AAAA records.  Each time the
        client resolves the domainname via the same DNS resolver, the same
        list of addresses are returned but in randomized order (i.e.,
        consistent with a DNS round-robin strategy).  But, interestingly,
        the same addresses are returned (albeit in randomized order) when
        the domainname is resolved via different DNS resolvers.

     *  When a client resolves the domainname "amazon.com", the DNS always
        returns a list of 3 A records and no AAAA records.  As with
        "yahoo.com", the same three A records are returned from any
        worldwide Internet connection point in randomized order.

     The above example strategies show differing approaches to Internet
     resilience and service distribution offered by major Internet
     services.  The Google approach exposes only a single IPv4 and a
     single IPv6 address to clients.  Clients can then select whichever IP
     protocol version offers the best response, but will always use the
     same IP address according to the current Internet connection point.
     This means that the IP address offered by the network must lead to a



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     highly-available server and/or service distribution point.  In other
     words, resilience is predicated on high availability within the
     network and with no client-initiated failovers expected (i.e., it is
     all-or-nothing from the client's perspective).  However, Google does
     provide for worldwide distributed service distribution by virtue of
     the fact that each Internet connection point responds with a
     different IPv6 and IPv4 address.  The IETF approach is like google
     (all-or-nothing from the client's perspective), but provides only a
     single IPv4 or IPv6 address on a worldwide basis.  This means that
     the addresses must be made highly-available at the network level with
     no client failover possibility, and if there is any worldwide service
     distribution it would need to be conducted by a network element that
     is reached via the IP address acting as a service distribution point.

     In contrast to the Google and IETF philosophies, Yahoo and Amazon
     both provide clients with a (short) list of IP addresses with Yahoo
     providing both IP protocol versions and Amazon as IPv4-only.  The
     order of the list is randomized with each name service query
     response, with the effect of round-robin load balancing for service
     distribution.  With a short list of addresses, there is still
     expectation that the network will implement high availability for
     each address but in case any single address fails the client can
     switch over to using a different address.  The balance then becomes
     one of function in the network vs function in the end system.

     The same implications observed for common highly-available services
     in the Internet apply also to the AERO client/server architecture.
     When an AERO Client connects to one or more ANETs, it discovers one
     or more AERO Proxy/Server addresses through the mechanisms discussed
     in earlier sections.  Each Proxy/Server address presumably leads to a
     fault-tolerant clustering arrangement such as supported by Linux-HA,
     Extended Virtual Synchrony or Paxos.  Such an arrangement has
     precedence in common Internet service deployments in lightweight
     virtual machines without requiring expensive hardware deployment.
     Similarly, common Internet service deployments set service IP
     addresses on service distribution points that may relay requests to
     many different servers.














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     For AERO, the expectation is that a combination of the Google/IETF
     and Yahoo/Amazon philosophies would be employed.  The AERO Client
     connects to different ANET access points and can receive 1-2 Proxy/
     Server ULAs at each point.  It then selects one AERO Proxy/Server
     address, and engages in RS/RA exchanges with the same Proxy/Server
     from all ANET connections.  The Client remains with this Proxy/Server
     unless or until the Proxy/Server fails, in which case it can switch
     over to an alternate Proxy/Server.  The Client can likewise switch
     over to a different Proxy/Server at any time if there is some reason
     for it to do so.  So, the AERO expectation is for a balance of
     function in the network and end system, with fault tolerance and
     resilience at both levels.

  Appendix B.  Change Log

     << RFC Editor - remove prior to publication >>

     Changes from earlier versions:

     *  Submit for review.

  Author's Address

     Fred L. Templin (editor)
     Boeing Research & Technology
     P.O. Box 3707
     Seattle, WA 98124
     United States of America
     Email: fltemplin@acm.org






















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name: draft-tu-nmrg-blockchain-trusted-protocol
rev: 04
title: A Blockchain Trusted Protocol for Intelligent Communication Network
time: 2025-04-10T07:05:02Z
group: none
group_uri: /api/v1/group/group/1027/
expires: 2025-04-10T07:47:00Z
states: ["/api/v1/doc/state/2/", "/api/v1/doc/state/150/"]
categories: []
tags: []
fetched_at: 2026-03-04T00:00:06.360337+00:00
source: ietf
abstract:
  This document defines a blockchain-based trusted protocol for sixth-
    generation (6G) intelligent communication network.

full_text:
  Network Management Research Group                                J. Yan
  Internet-Draft                                                  H. Zhou
  Intended status: Informational                                  Y. Yang                  
  Expires: April 7, 2025                                       H. Song
                                                                    Z. Tu
                                              Beijing Jiaotong University                                                                 					                                                                                                      
                                                            October 7, 2024
  Document: draft-tu-nmrg-blockchain-trusted-protocol-04.txt

        A Blockchain Trusted Protocol for Intelligent Communication
                                   Network

  Abstract

    This document defines a blockchain-based trusted protocol for sixth-
    generation (6G) intelligent communication network.

  Status of this Memo

    This Internet-Draft is submitted in full conformance with the
    provisions of BCP 78 and BCP 79.

    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF). Note that other groups may also distribute working
    documents as Internet-Drafts. The list of current Internet-Drafts is
    at https://datatracker.ietf.org/drafts/current/.

    Internet-Drafts are draft documents valid for a maximum of six months
    and may be updated, replaced, or obsoleted by other documents at any
    time. It is inappropriate to use Internet-Drafts as reference
    material or to cite them other than as "work in progress."

    This Internet-Draft will expire on April 7, 2025.

  Copyright Notice

    Copyright (c) 2024 IETF Trust and the persons identified as the
    document authors. All rights reserved.

    This document is subject to BCP 78 and the IETF Trust's Legal
    Provisions Relating to IETF Documents (https://trustee.ietf.org/
    license-info) in effect on the date of publication of this document.
    Please review these documents carefully, as they describe your rights
    and restrictions with respect to this document. Code Components


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                        Blockchain Trusted Protocol          October   2024

    extracted from this document must include Revised BSD License text as
    described in Section 4.e of the Trust Legal Provisions and are
    provided without warranty as described in the Revised BSD License.

  Table of Contents

     1. Introduction...................................................2
     2. Terminology....................................................3
     3. Blockchain Trusted Protocol Architecture.......................3
     4. Blockchain Trusted Authentication Protocol.....................5
     5. Blockchain Trusted Access Control Protocol.....................6
     6. Blockchain Trusted Locator/Identifier Separation Protocol......8
     7. Blockchain Trusted Feedback Control Protocol..................10
     8. IANA Considerations...........................................12
     9. Security Considerations.......................................12
     10. References...................................................12
     Acknowledgments..................................................13
     Author's Addresses...............................................13

  1. Introduction

     The sixth-generation (6G) network promotes the interconnection of
     everything and put forward higher requirements for network security.
     The existing network architecture mainly focuses on the end-to-end
     communication process and lacks the dynamic feedback to the user
     behavior. Based on the current network architecture, 6G intelligent
     communication network introduces collaborative processing unit and
     intelligent agent unit at the access gateway, and uses environment,
     media, and security knowledge base to realize dynamic control and
     closed-loop feedback of user behavior [ICN20]. Based on the 6G
     intelligent communication network architecture, this document
     proposes a blockchain-based trusted protocol for user behavior
     control. By constructing the trust link of "user identity-
     communication behavior-user reputation-security control", the
     designed trusted protocol forms the dynamic feedback and global
     control ability of the whole-process user behavior after users access
     the network.

     This document designs a blockchain trusted protocol in 6G intelligent
     communication network, including trusted authentication protocol,
     trusted access control protocol, trusted locator/identifier
     separation protocol and trusted feedback control protocol [BTP22].




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                        Blockchain Trusted Protocol          October   2024

  2. Terminology

     The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
     "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
     "OPTIONAL" in this document are to be interpreted as described in BCP
     14 [RFC2119] [RFC8174] when, and only when, they appear in all
     capitals, as shown here.

  3. Blockchain Trusted Protocol Architecture

     The blockchain trusted protocol is deployed at the access gateways in
     the form of smart contracts. The smart contract in the blockchain is
     used to store the user's identity, access behavior, identification
     and other information, and security control the user's behavior
     together with the collaborative processing unit.

     Figure 1 shows the architecture of the blockchain trusted protocol,
     and the components in the architecture will be introduced separately
     next.

     o User Equipment (UE): We use UE to represent all users and devices
       in the 6G intelligent communication network, such as users,
       intelligent devices, IoT devices, etc. When a UE accesses the
       network or want to obtain the network resources, it needs to send
       a corresponding request to the access gateway. Only when the
       initiated request is approved, the UE can obtain the corresponding
       permission to access the network and resources.

     o Access Gateway (AG): The AG is responsible for receiving and
       forwarding requests sent by UE. Multiple different AGs run the
       same consensus algorithm to form a blockchain network. Besides,
       the collaborative processing module is deployed in the AG to
       response and control the user behavior.

     o Collaborative Processing Module (CPM): The CPM responds, forwards,
       and processes the requests initiated by UE. In the process of user
       behavior control, CPM is the executor of security control of UE,
       which is responsible for executing and forwarding the control
       results generated by smart contracts.

     o Smart Contract (SC): The SC is deployed in the blockchain network
       built by the AGs running the same consensus algorithm. In the
       blockchain trusted protocol architecture, the designed trusted
       protocol consists of four sub-protocols. And each sub-protocol is
       deployed in the blockchain network in the form of smart contract
       (identity authentication contract, access control contract,

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                         Blockchain Trusted Protocol       October   2024

        locator/identifier separation contract, and feedback control
        contract) to security manage and control the user behavior.

    +------------------------------------------------------------------+
    |                             SMART CONTRACT                       |
    | +--------------+ +-----------+ +------------------+ +----------+ |
    | |IDENTITY      | |ACCESS     | |LOCATOR/IDENTIFIER| |FEEDBACK  | |
    | |AUTHENTICATION| |CONTROL    | |SEPARATION        | |CONTROL   | |
    | |CONTRACT      | |CONTRACT   | |CONTRACT          | |CONTRACT  | |
    | +-------^------+ +-----^-----+ +--------^---------+ +-----^----+ |
    |         |              |                |                 |      |
    +------------------------------------------------------------------+
              |              |                |                 |
              +----^---------+----------------+--------^--------+
                   |                                   |
    +-----------------------------+      +-----------------------------+
    |              |              |      |             |               |
    |  +-----------v------------+ |      | +-----------v------------+  |
    |  |COLLABORATIVE PROCESSING| |      | |COLLABORATIVE PROCESSING|  |
    |  |          UNIT          | |......| |          UNIT          |  |
    |  +------^---------^-------+ |      | +--------^--------^------+  |
    |ACCESS   |         |         |      |          |        |ACCESS   |
    |GATEWAY-1|         |         |      |          |        |GATEWAY-2|
    +-----------------------------+      +-----------------------------+
              |         |                           |        |
    +---------v--+   +--v---------+      +----------v--+  +--v---------+
    |USER        |   |USER        |      |USER         |  |USER        |
    |EQUIPMENT-1 |   |EQUIPMENT-2 |......|EQUIPMENT-n-1|  |EQUIPMENT-n |
    +------------+   +------------+      +-------------+  +------------+
            Figure 1 The architecture of blockchain trusted protocol

    o Identity Authentication Contract (IAC): The IAC is used to
      authenticate the UE identity and ensure that the identity is
      trusted. In the process of identity authentication, IAC not only
      respond to the authentication request initiated by UE and generate
      the corresponding authentication vectors, but also stores the
      authentication behavior in the blockchain to ensure the
      traceability of the user's authentication behavior.

    o Access Control Contract (ACC): The ACC generates the corresponding
      access control policy according to the access request initiated by
      the UE to ensure the action of UE is trusted. Similarly, the ACC
      records the access behavior of UE. On the one hand, it is used to
      evaluate the user reputation of the feedback control module, and
      on the other hand, it guarantees the traceability of access
      behavior of UE.




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                        Blockchain Trusted Protocol          October   2024

    o Locator/Identifier Separation Contract (LISC): The LISP protocol
      is used in the 6G intelligent communication network to isolate the
      access network form the backbone network, which can improve the
      security capabilities of the network by separating the identity
      and location. Therefore, in the trusted protocol
      architecture, we use the LISC to store the mapping relationship
      between the end-identifier (EID) and the routing-locator (RLOC).

    o Feedback Control Contract (FCC): The FCC is used to evaluate the
      reputation value of the UE. This contract evaluates the UE by
      obtaining various historical behavior data (such as authentication
      behavior, access control behavior, etc.), and develops different
      levels of security control policies based on the calculated
      reputation value.

     In the following section, we describe the four trusted sub-protocols
     in detail.

  4. Blockchain Trusted Authentication Protocol

     In the blockchain trusted authentication protocol, the CPM forwards
     and processes the identity authentication requests of UE, while the
     IAC stores the authentication credentials and generates the user
     authentication vector.

     Figure 2 shows the blockchain trusted authentication protocol, and
     the trusted authentication protocol can be described as the following
     steps.

     STEP 1: UE sends the UE Authentication Request (UAR) to the CPM, the
     UAR contains the identity of UE U_id and UE identification
     information I_ua.

     STEP 2: CPM invokes the interface of IAC UEAuth() to generate
     authentication vector. The input of UEAuth() contains U_id and I_ua.

     STEP 3: IAC returns the generated authentication vector (AV) to CPM.

     STEP 4: UE, CPM, and IAC interact with each other to authenticate UE
     according to the generated authentication vector.

     STEP 5: IAC stores the Identity Authentication Behavior B_ia of UE in
     the blockchain, the B_ia includes the UE identity U_id, identity
     authentication result R_ia, authentication time T_ia.


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     +---+            +---+                 +---+                    +---+
     |UE |            |CPM|                 |IAC|                    |FCC|
     +-+-+            +-+-+                 +-+-+                    +-+-+
       |1.UAR(U_id,I_ua)|                     |                        |
       +---------------->                     |                        |
       |                |2.UEAuth(U_id,I_ua)  |                        |
       |                +--------------------->                        |
       |                |3.Return AV          |                        |
       |                <---------------------+                        |
       |4.UE-CPM-IAC    |                     |                        |
       | Authentication interaction           |                        |
       <----------------+--------------------->                        |
       |                |         +-----------+----------+             |
       |                |         |5.Store               |             |
       |                |         | B_ia={U_id,R_ia,T_ia}|             |
       |                |         +-----------+----------+             |
       |                |6.Return R_ia        |                        |
       |                <---------------------+                        |
       |                |                     |7.UpdateRepIA(U_id,R_ia)|
       |                |                     +------------------------>
       |                |8.Return R_dc        |                        |
       |9.Return        <----------------------------------------------+
       | R_ia or R_dc   |                     |                        |
       <----------------+                     |                        |
       |                |                     |                        |
       +                +                     +                        +
            Figure 2 The blockchain trusted authentication protocol

     STEP 6: IAC returns the authentication result R_ia to CPM.

     STEP 7: The IAC invokes the UpdateRepIA() function of FCC to update
     the identity authentication reputation value of UE. The UpdateRepIA()
     contains U_id and R_ia of UE.

     STEP 8: If the UE is authenticated successfully, go to STEP 9.
     Otherwise, the FCC calculated the authentication reputation,
     generates and rerturns the Dynamic Control Result R_dc to CPM based
     on the evaluated reputation value.

     STEP 9: CPM forwards the R_ia or the R_dc to the UE.

  5. Blockchain Trusted Access Control Protocol

    The trusted access control protocol is used to evaluate UE access
    control behavior. In the trusted access control protocol, the CPM is
    used to forward the access control requests initiated by UE, while


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    the ACC generates the access policy and stores the user access
    control behavior.

    Figure 3 represents the blockchain trusted access control protocol.

    +---+            +---+             +---+        +---+            +---+
    |UE |            |CPM|             |IAC|        |ACC|            |FCC|
    +-+-+            +-+-+             +-+-+        +-+-+            +-+-+
      |1.ACR(U_id,     |                 |            |                |
      | AI_ue,AI_nr)   |                 |            |                |
      +---------------->                 |            |                |
      |                |2.QueryIAR(U_id) |            |                |
      |                +----------------->            |                |
      |                |3.Return R_ia    |            |                |
      |                <-----------------+            |                |
      |                |4.UEAccCtr(U_id,AI_ue,AI_nr)  |                |
      |                +------------------------------>                |
      |                |                 |    +-------+-------+        |
      |                |                 |    |5.Generate R_ac|        |
      |                |                 |    +---------------+        |
      |                |                 |  +---------+-------------+  |
      |                |                 |  |6.Store                |  |
      |                |                 |  | B_ac={U_id, R_ac,T_ac}|  |
      |                |                 |  +---------+-------------+  |
      |                |7.Return R_ac    |            |                |
      |                <------------------------------+8.UpdateRepAC   |
      |                |                 |            | (U_id,R_ac)    |
      |                |                 |            +---------------->
      |                |9.Return R_dc    |            |                |
      |10.Return       <-----------------------------------------------+
      |   R_ac or R_dc |                 |            |                |
      <----------------+                 |            |                |
      |                |                 |            |                |
      +                +                 +            +                +
           Figure 3 The blockchain trusted access control protocol

    STEP 1: UE sends the Access Control Request (ACR) to CPM, ACR
    contains the UE identity U_id, access control information of UE AI_ue,
    and the access control information of the network resource AI_nr.

    STEP 2: The CPM calls the QueryIAR() function of IAC to query the
    authentication result of UE, the input of function QueryIAR()
    contains U_id.

    STEP 3: The IAC queries the authentication result of UE stored in the
    blockchain and returns the identity authentication result R_ia to the



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    CPM. If the UE is illegal, the access control result R_ac is set to 0,
    and the next step is STEP 6. Otherwise, go to STEP 4.

    STEP 4: If the identity of UE is trusted, the CPM invokes the
    UEAccCtr() function of ACC to generate the access control policy of
    UE. The input of UEAccCtr() contains U_id, AI_ue, and AI_nr.

    STEP 5: The ACC generates the access control result R_ac according to
    the AI_ue and AI_nr.

    STEP 6: Based on the generated R_ac, the ACC stores the access
    control behavior B_ac of UE in the blockchain. B_ac contains the U_id,
    R_ac, and access control time T_ac.

    STEP 7: AAC returns the access control result R_ac to CPM.

    STEP 8: The ACC invokes the UpdateRepAC() function of FCC to update
    the access control reputation value of UE. The UpdateRepAC() contains
    U_id and R_ac of UE.

    STEP 9: If the access action of UE is authorized, go to STEP 10.
    Otherwise, the FCC calculated the access control reputation,
    generates and rerturns the Dynamic Control Result R_dc to CPM based
    on the evaluated access control reputation.

    STEP 10: CPM forwards the R_ac or the R_dc to the UE.

  6. Blockchain Trusted Locator/Identifier Separation Protocol

    After the UE completes the authentication and access control process,
    the EID and RLOC mapping transformation process also needs to be
    performed to improve the network security.

    The blockchain trusted locator/identifier separation protocol
    implements the mapping and conversion of EID and RLOC. In the
    blockchain trusted locator/identifier separation protocol, the CPM
    invokes the interface of the LISC to query the mapping relationship
    between EID and RLOC, and the LISC stores and records the pairing
    information of EID and RLOC to realize the separation mapping 
    between user identity and the network location.

    Figure 4 shows the blockchain trusted locator/identifier separation
    protocol, and it can be described as the follows.

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    STEP 1: UE-1 obtains the EID address (EID2) of the UE-n through the
    domain name systems or other methods, fills in each header and
    payload information, and constructs the data packet.

    +------+       +--------+          +----+   +--------+        +------+
    | UE-1 |       |AG-1/CPM|          |LISC|   |AG-2/CPM|        | UE-2 |
    |(EID1)|       |        |          |    |   |        |        |(EID2)|
    +--+---+       +---+----+          +-+--+   +---+----+        +--+---+
       |1.Obtain EID address (EID2)      |          |                |
       <------------------------------------------------------------->
       |2.Send data    |                 |          |                |
       | packet (EID1  |                 |          |                |
       | |EID2|Data)   |                 |          |                |
       +--------------->                 |          |                |
       |     +---------+---------+       |          |                |
       |     |3.Get EID1 and EID2|       |          |                |
       |     +---------+---------+       |          |                |
       |               |4.QueryRLOC      |          |                |
       |               | (EID1,EID2)     |          |                |
       |               +----------------->          |                |
       |               |5.Return         |          |                |
       |               | RLOC address    |          |                |
       |               | (RLOC1,RLOC2)   |          |                |
       |               <-----------------+          |                |
       |     +---------+----------+      |          |                |
       |     |6.Encapsulate packet|      |          |                |
       |     | (RLOC1|RLOC2|EID1  |      |          |                |
       |     | |EID2|Data)        |      |          |                |
       |     +---------+----------+      |          |                |
       |               |7.Forward new packet        |                |
       |               +-------------------------->                  |
       |               |                 | +--------+---------+      |
       |               |                 | |8.Decapsulate     |      |
       |               |                 | | packet           |      |
       |               |                 | | (EID1|EID2|Data) |      |
       |               |                 | +--------+---------+      |
       |               |                 |          |9.Forward packet|
       |               |                 |          |(EID1|EID2|Data)|
       |               |                 |          +---------------->
       |               |                 |          |                |
       +               +                 +          +                +
        Figure 4 The blockchain trusted locator/identifier separation
                                     protocol

   STEP 2: UE-1 sends the data packet, the data packets that need to
   pass through the core network will be directly forwarded to the AG-1. 
   The source and destination address in the packets are set to EID1 and
   EID2, respectively.

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   STEP 3: After receiving the packet, AG-1 obtains the IP address
   information in the packets, including the source address (EID1) and
   destination address (EID2).

   STEP 4: The CPM invokes the QueryRLOC() function of LISC to query the
   RLOC address of EID1 and EID2. The input of QueryRLOC() contains EID1
   and EID2.

   STEP 5: The LISC queries the mapping relationship between EID and
   RLOC registered in the blockchain, and returns the RLOC addresses
   (RLOC1 and RLOC2) corresponding to EID1 and EID2 to CPM respectively.

   STEP 6: After successfully obtaining the RLOC addresses corresponding
   to the two EIDs, AG-1 encapsulates the new IP header before the
   original packet. In the new IP header, the source address is set to
   RLOC1 and the destination address is set to RLOC2.

   STEP 7: Then, AG-1 searches the core routing table and forwards new
   data packets to the core network.

   STEP 8: After the data packet is forwarded through the core network
   to the AG-2 of the network egress router where the destination node
   is located, the CPM in AG-2 decapsulates the packet to obtain the
   original packet. In the original packet, the source address is EID1,
   and the destination address is EID2.

   STEP 9: By searching the local routing table, AG-2 forwards the
   original packet to UE-2.

  7. Blockchain Trusted Feedback Control Protocol

   The blockchain trusted feedback control protocol is used to security
   control the behavior of UE. In the blockchain trusted feedback
   control protocol, the CPM receives the information of potential
   malicious UE detected or inferred by external modules (such as
   malicious traffic detection units, security knowledge bases), and
   calls the FCC interface to perform user historical reputation
   analysis. The FCC contract calculates the reputation value of
   potential malicious UEs, and formulates fine-grained security
   management and control strategies according to the calculated
   reputation value.





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   Figure 5 is the flowchart of the blockchain trusted feedback control
   protocol. It can be expressed in detail as the follows.

   +---------------+       +---+          +---+              +---+ +---+
   |External Module|       |CPM|          |FCC|              |IAC| |ACC|
   +-------+-------+       +-+-+          +-+-+              +-+-+ +-+-+
           |1.MI_ue=         |              |                  |     |
           |{U_id1,U_id2,...}|              |                  |     |
           +----------------->2.GetPolicy   |                  |     |
           |                 | (MI_ue)      |                  |     |
           |                 +-------------->                  |     |
           |                 |              |3.QueryHBIA(MI_ue)|     |
           |                 |              <------------------+     |
           |                 |              |4.QueryHBAC(MI_ue)|     |
           |                 |              <------------------------+
           |                 |  +-----------+-----------+      |     |
           |                 |  |5.Calculated reputation|      |     |
           |                 |  +-----------+-----------+      |     |
           |                 |      +-------+-------+          |     |
           |                 |      |6.Generate P_sc|          |     |
           |                 |      +-------+-------+          |     |
           |                 |7.Return P_sc |                  |     |
           |                 <--------------+                  |     |
           |       +---------+---------+    |                  |     |
           |       |8.Perform different|    |                  |     |
           |       | levels of control |    |                  |     |
           |       +---------+---------+    |                  |     |
           |                 |              |                  |     |
           +                 +              +                  +     +
          Figure 5 The blockchain trusted feedback control protocol

   STEP 1: CPM continuously receives the potentially malicious UE
   information MI_ue sent by the external module. MI_ue is a set of
   identities of potential malicious UE, which can be expressed as
   follows. MI_ue = {U_id1, U_id2, ...}. U_id1 and U_id2 represents
   different UE identities.

   STEP 2: After receiving potentially malicious UE information MI_ue,
   CPM invokes the GetPolicy() function of the FCC to obtain the fine-
   grained security control policies of the potential malicious UEs. The
   input of GetPolicy() is MI_ue.

   STEP 3/4: FCC calls the interface with other smart contract (IAC, ACC)
   to obtain the historical behavior of potentially malicious UE, and
   conducts the reputation value according to the UE historical behavior.
   The interface to IAC is QueryHBIA(), and to ACC is QueryHBAC(). The



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   input of QueryHBIA() and QueryHBAC() is the identity information of
   the potentially malicious user MI_ue.

   STEP 5: FCC calculated the reputation value of the potentially
   malicious UE based on the obtained historical behavior.

   STEP 6: Then, FCC generates the fine-grained security control
   policies P_sc based on the calculated reputation values.

   STEP 7: FCC returns the P_sc to CPM.

   STEP 8: CPM performs different levels of control on malicious UE
   according to the received P_sc, such as invalidation of identity
   authentication information, revocation of access control permissions,
   etc., to achieve fine-grained real-time blocking of malicious attacks
   at the access gateway of the network.

  8. IANA Considerations

   This document has no IANA actions.

  9. Security Considerations

   The blockchain system needs to maintain the overall security of the
   system at the levels of data content, consensus algorithms, smart
   contracts, and peer-to-peer networks through technologies such as
   cryptography and network security. Possible security problems in
   cryptography include improper key management, vulnerabilities in
   cryptographic algorithms or components. In addition, the blockchain
   trusted protocol is deployed at the access gateway of the network, so
   the identity of the access gateway needs to be trusted.

  10. References

    10.1 Normative References

    [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI
              10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

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                      Blockchain Trusted Protocol           October   2024

    [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

  10.2 Informative References

    [ICN20] Jiang, C., Ge, N., Kuang, L., "AI-enabled next-generation
            communication networks: Intelligent agent and AI router,
            IEEE Wireless Communications", September 2020,
            <https://ieeexplore.ieee.org/abstract/document/9210134>.

    [BTP22] Tu, Z., Zhou, H., Li, K., Song, H., Yang, Y., "A Blockchain-
            Enabled Trusted Protocol based on Whole-Process User
            Behavior in 6G Network, Security and Communication
            Networks", September 2022,
            <https://doi.org/10.1155/1970/8188977>.

  Acknowledgments

    TBC

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                      Blockchain Trusted Protocol           October   2024

  Author's Addresses

    Jingfu Yan
    Beijing Jiaotong University
    Beijing
    Phone: <86-18810753358>
    Email: 22110030@bjtu.edu.cn

    Huachun Zhou
    Beijing Jiaotong University
    Beijing
    Phone: <86-13718168186>
    Email: hchzhou@bjtu.edu.cn

    Yuzheng Yang
    Beijing Jiaotong University
    Beijing
    Phone: <86-15802201359>
    Email: 21120151@bjtu.edu.cn

    Zhe Tu
    Beijing Jiaotong University
    Beijing
    Phone: <86-13146050755>
    Email: zhe_tu@bjtu.edu.cn

    Haoxiang Song
    Beijing Jiaotong University
    Beijing
    Phone: <86-13161229322>
    Email: 20120099@bjtu.edu.cn


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name: draft-zhao-anima-automatic-congestion-relief
rev: 00
title: Automatic Network Congestion Relief
time: 2025-09-04T07:09:42Z
group: none
group_uri: /api/v1/group/group/1027/
expires: 2025-09-04T09:42:28Z
states: ["/api/v1/doc/state/2/", "/api/v1/doc/state/150/"]
categories: []
tags: []
fetched_at: 2026-03-04T00:00:06.365341+00:00
source: ietf
abstract:
  This document introduces an automatic congestion relief mechanism
     based on intelligent traffic analysis and dynamic regulation.  In the
     event of congestion caused by fiber optic failures, it can respond
     intelligently and self-heal in real time, ensuring the stable
     operation of the network.

full_text:
  anima                                                       J. Zhao, Ed.
  Internet-Draft                                             S. Zhang, Ed.
  Intended status: Standards Track                            China Unicom
  Expires: 4 September 2025                                   3 March 2025


                    Automatic Network Congestion Relief
              draft-zhao-anima-automatic-congestion-relief-00

  Abstract

     This document introduces an automatic congestion relief mechanism
     based on intelligent traffic analysis and dynamic regulation.  In the
     event of congestion caused by fiber optic failures, it can respond
     intelligently and self-heal in real time, ensuring the stable
     operation of the network.

  Status of This Memo

     This Internet-Draft is submitted in full conformance with the
     provisions of BCP 78 and BCP 79.

     Internet-Drafts are working documents of the Internet Engineering
     Task Force (IETF).  Note that other groups may also distribute
     working documents as Internet-Drafts.  The list of current Internet-
     Drafts is at https://datatracker.ietf.org/drafts/current/.

     Internet-Drafts are draft documents valid for a maximum of six months
     and may be updated, replaced, or obsoleted by other documents at any
     time.  It is inappropriate to use Internet-Drafts as reference
     material or to cite them other than as "work in progress."

     This Internet-Draft will expire on 4 September 2025.

  Copyright Notice

     Copyright (c) 2025 IETF Trust and the persons identified as the
     document authors.  All rights reserved.

     This document is subject to BCP 78 and the IETF Trust's Legal
     Provisions Relating to IETF Documents (https://trustee.ietf.org/
     license-info) in effect on the date of publication of this document.
     Please review these documents carefully, as they describe your rights
     and restrictions with respect to this document.  Code Components
     extracted from this document must include Revised BSD License text as
     described in Section 4.e of the Trust Legal Provisions and are
     provided without warranty as described in the Revised BSD License.




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  Table of Contents

     1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     2.  Automatic Network Congestion Relief . . . . . . . . . . . . .   2
       2.1.  Step1: Traffic Modeling . . . . . . . . . . . . . . . . .   3
       2.2.  Step2: Traffic Monitoring . . . . . . . . . . . . . . . .   3
         2.2.1.  BGP-LS Utilized Bandwidth TLV . . . . . . . . . . . .   4
       2.3.  Step3: Intelligent Policy Generation  . . . . . . . . . .   4
       2.4.  Step4: Policy Propagation . . . . . . . . . . . . . . . .   4
       2.5.  Step5: Policy Reversion . . . . . . . . . . . . . . . . .   5
     3.  usecase . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.  Pre-deployment Conditions . . . . . . . . . . . . . . . .   5
       3.2.  Implementation of Automated Mechanism . . . . . . . . . .   5
     4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     6.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
     Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

  1.  Introduction

     Nowadays, fiber optic failures occur frequently, leading to network
     congestion and becoming a common pain point for operators.  These
     issues necessitate dedicated staff to perform daily traffic
     inspections and manually adjust configurations on an hourly basis,
     which significantly increases the difficulty of network maintenance.

     This draft introduces an automatic congestion relief mechanism based
     on intelligent traffic analysis and auto-regulation.  In the event of
     congestion caused by fiber optic failures, it can intelligently
     respond to congestion and initiate real-time self-healing processes,
     solving the network congestion and maintenance challenges faced by
     operators due to fiber optic failures, and ensuring the stable
     operation of the network.

  2.  Automatic Network Congestion Relief

     This second-level congestion relief mechanism is automated through
     the intelligent module within the device.  Leveraging intelligent
     traffic analysis, it precisely calculates the volume of traffic
     requiring redistribution.  Subsequently, it redirects this traffic to
     paired devices via inter-device protocol announcements and the
     automatic adjustment of routing priorities.









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     +-+-+-+-+-+-+-+-+-+-+       +-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Traffic Modeling  |------>| Traffic Monitoring  |---->| Intelligent policy generation |
     +-+-+-+-+-+-+-+-+-+-+       +-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                                           |
                                                                           |
                                 +-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                 |   Policy Reversion  |<----|      Policy Regulation      |
                                 +-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                Figure 1: Mechanism Framework Description

  2.1.  Step1: Traffic Modeling

     The forwarding chip of the device performs real-time traffic sorting
     using full-flow data and identifies the top N traffic flows.

     The intelligent component of the AI chip subscribes to the BGP RIB-
     out (Routing Information Base-outbound) and employs intelligent flow
     recognition algorithms to perform AI-based traffic modeling.  This
     modeling approach, considering factors such as historical traffic
     patterns and flow behavior, provides a solid basis for subsequent
     traffic detection and regulation.

     The intelligent flow feature statistics cover multiple dimensions,
     arranged in a logical order from macroscopic to microscopic traffic
     characteristics, including flow rate, packet length, the proportion
     of TCP/UDP traffic, the proportion of fragmented packets, and the
     proportion of SYN packets.

  2.2.  Step2: Traffic Monitoring

     Through the extension of the BGP-LS protocol, the inter-domain link
     bandwidth and load changes are obtained by the device in real-time.

     When the link bandwidth exceeds the set congestion threshold, the
     situation where the link bandwidth exceeds the threshold is reported
     quickly.

     The interface statistics are collected at a second-level time
     interval.










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  2.2.1.  BGP-LS Utilized Bandwidth TLV

     The device uses the Utilized Bandwidth to aggregate the inter-domain
     BGP EPE link bandwidth and bandwidth utilization rate.  The BGP-LS
     Utilized Bandwidth TLV reuses the Maximum Link Bandwidth TLV (Type
     1089) [RFC5305] This TLV is used to describe the bandwidth and
     bandwidth utilization of inter-domain BGP Egress Peer Engineering
     (EPE) links.  The format of the BGP-LS Utilized Bandwidth TLV is as
     follows.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |               Type            |             Length            |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                     Utilized Bandwidth                        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2: BGP-LS Utilized Bandwidth KEY TLV

  2.3.  Step3: Intelligent Policy Generation

     When its utilization exceeds the predefined congestion threshold, the
     device launches the intelligent module.  The device intelligently
     identifies the traffic that needs to be adjusted and generates
     policies based on traffic analysis.

     One policy is that, considering the traffic characteristics such as
     flow rate and type, the device can make a more accurate calculation.
     Since the routing prefixes sent to the one network domain (rib-out)
     are the same, it is only necessary to change the attributes after
     finding the TOP N routing prefixes to be adjusted, and lower the
     priority of the faulty plane.

  2.4.  Step4: Policy Propagation

     The device announces the automatic adjustment mechanism of routing
     priorities through the BGP RPD protocol and then automatically
     allocates the calculated traffic to the lightly-loaded plane.  The
     end-to-end process is completed within seconds, effectively
     alleviating the congestion on the original plane.  It can precisely
     control neighbors and paths without affecting the existing routing
     policies in the network.








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  2.5.  Step5: Policy Reversion

     After the interrupted link recovers, the optimization policies will
     be gradually withdrawn.  With the revocation of the policies, the
     network traffic will progressively return to the load-sharing state
     before the failure.

  3.  usecase

      ----------------------------------------
     |  --------                   --------   |
     | |   C2   |    Network2     |    C1  |  |
     |  --------                   --------   |
      ------|--------------------------|------
            |                          |
            |                          x
            | 100GB x 8                x 100GB x 8
            |                          |
     -------|--------------------------|------
     | +-------                   --------   |
     | |  CR2   |    Network1     |  CR1   | |
     |  --------                   --------  |
      ----------------------------------------

                 Figure 3: Intelligent Decision-Making usecase

  3.1.  Pre-deployment Conditions

     Establish topology mapping in Network 1 between CR1 and CR2, and
     between CR1/CR2 and C1/C2 in Network 2, clarifying the connection
     relationships.  Set up BGP-LS peering between CR1 and CR2 to exchange
     topology and bandwidth information.  Enable BGP EPE functionality via
     EBGP peering between CR1 and C1, and between CR2 and C2, to obtain
     link state and bandwidth-related information and generate BGP-LS LINK
     routes.  Activate the BGP RPD neighbor function on CR1 and CR2 to
     receive optimized routing policies.

  3.2.  Implementation of Automated Mechanism

     *  Step1：AI Traffic Modeling

        CR1 and CR2 perform real-time and automatic TOP N traffic modeling
        on the link using the built-in automated algorithms.  Without
        human intervention, they are capable of accurately grasping the
        traffic conditions of the link.  The system automatically monitors
        that the total bandwidth of the traffic channels between CR1-C1
        and CR2-C2 is 100 x 8GB, and the current traffic on both paths is
        600GB.



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     *  Step2：Traffic Monitoring

        If the CR1 device detects that a total of five links between
        CR1-C1 have failed, leaving only three links, the system
        automatically determines that congestion will occur on the CR1-C1
        link.

     *  Step3：Intelligent Policy Generation

        As the primary adjustment device, the intelligent module of CR1
        automatically generates optimization policies based on the
        established TOPN traffic model and the real-time collected prefix
        information.  The entire process does not require human
        intervention, which greatly shortens the time from failure
        discovery to policy formulation, and enables timely response to
        network emergencies to ensure the stable operation of the network.

     *  Step4: Policy Propagation

        After CR1 generates the optimization policies, it automatically
        propagates them to C1.  Upon receiving the policies, C1
        automatically guides the remote routers to adjust their routing
        paths.

        The system intelligently identifies and then automatically diverts
        the two services with the highest priority from the CR1-C1 link to
        the CR2-C2 link (through C2) to alleviate the congestion on the
        CR1-C1 link.  After the policy adjustment, the system
        automatically monitors that the traffic on the CR1-C1 link is
        reduced to 300G, and the traffic on the CR2-C2 link is increased
        to 800G.  The automated policy propagation and traffic diversion
        process is efficient and accurate, effectively improving the
        utilization efficiency of network resources and quickly
        alleviating the link congestion problem.

     *  Step5: Policy Reversion

        When the failed links between CR1-C1 are restored, the system
        automatically detects the link status change and gradually
        withdraws the relevant optimization policies.  Upon the automatic
        revocation of the policies, the network traffic automatically and
        gradually returns to the load-sharing state before the fault.

        This automated mechanism ensures that the network can quickly
        return to normal operation after the fault is eliminated, reducing
        the cost of human intervention and improving the self-healing
        ability of the network.




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  4.  Security Considerations

     TBD.

  5.  IANA Considerations

     TBD.

  6.  Normative References

     [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
                Engineering", RFC 5305, DOI 10.17487/RFC5305, October
                2008, <https://www.rfc-editor.org/info/rfc5305>.

  Authors' Addresses

     Jing Zhao (editor)
     China Unicom
     Beijing
     China
     Email: zhaoj501@chinaunicom.cn


     Shuai Zhang (editor)
     China Unicom
     Beijing
     China
     Email: zhangs633@chinaunicom.cn























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