Transmission of IP Packets over Overlay Multilink Network (OMNI) Interfaces
draft-templin-6man-omni-48

Document Type Active Internet-Draft (individual)
Authors Fred Templin  , Tony Whyman 
Last updated 2021-10-15
Replaces draft-templin-6man-omni-interface
Stream Internet Engineering Task Force (IETF)
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Network Working Group                                    F. Templin, Ed.
Internet-Draft                                        The Boeing Company
Intended status: Standards Track                               A. Whyman
Expires: April 18, 2022                  MWA Ltd c/o Inmarsat Global Ltd
                                                        October 15, 2021

    Transmission of IP Packets over Overlay Multilink Network (OMNI)
                               Interfaces
                       draft-templin-6man-omni-48

Abstract

   Mobile network platforms and devices (e.g., aircraft of various
   configurations, terrestrial vehicles, seagoing vessels, enterprise
   wireless devices, pedestrians with cell phones, etc.) communicate
   with networked correspondents over multiple access network data links
   and configure mobile routers to connect end user networks.  A
   multilink interface specification is presented that enables mobile
   nodes to coordinate with a network-based mobility service and/or with
   other mobile node peers.  This document specifies the transmission of
   IP packets over Overlay Multilink Network (OMNI) Interfaces.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 18, 2022.

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   Copyright (c) 2021 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

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   4.  Overlay Multilink Network (OMNI) Interface Model  . . . . . .  13
   5.  OMNI Interface Maximum Transmission Unit (MTU)  . . . . . . .  19
   6.  The OMNI Adaptation Layer (OAL) . . . . . . . . . . . . . . .  20
     6.1.  OAL Source Encapsulation and Fragmentation  . . . . . . .  21
     6.2.  OAL *NET Encapsulation and Re-Encapsulation . . . . . . .  25
     6.3.  OAL *NET Decapsulation and Reassembly . . . . . . . . . .  28
     6.4.  OAL Header Compression  . . . . . . . . . . . . . . . . .  28
     6.5.  OAL-in-OAL Encapsulation  . . . . . . . . . . . . . . . .  31
     6.6.  OAL Identification Window Maintenance . . . . . . . . . .  33
     6.7.  OAL Fragment Retransmission . . . . . . . . . . . . . . .  38
     6.8.  OAL MTU Feedback Messaging  . . . . . . . . . . . . . . .  39
     6.9.  OAL Requirements  . . . . . . . . . . . . . . . . . . . .  41
     6.10. OAL Fragmentation Security Implications . . . . . . . . .  42
     6.11. OAL Super-Packets . . . . . . . . . . . . . . . . . . . .  44
     6.12. OAL Bubbles . . . . . . . . . . . . . . . . . . . . . . .  46
   7.  Frame Format  . . . . . . . . . . . . . . . . . . . . . . . .  46
   8.  Link-Local Addresses (LLAs) . . . . . . . . . . . . . . . . .  46
   9.  Unique-Local Addresses (ULAs) . . . . . . . . . . . . . . . .  48
   10. Global Unicast Addresses (GUAs) . . . . . . . . . . . . . . .  50
   11. Node Identification . . . . . . . . . . . . . . . . . . . . .  51
   12. Address Mapping - Unicast . . . . . . . . . . . . . . . . . .  52
     12.1.  The OMNI Option  . . . . . . . . . . . . . . . . . . . .  53
     12.2.  OMNI Sub-Options . . . . . . . . . . . . . . . . . . . .  55
       12.2.1.  Pad1 . . . . . . . . . . . . . . . . . . . . . . . .  57
       12.2.2.  PadN . . . . . . . . . . . . . . . . . . . . . . . .  58
       12.2.3.  Interface Attributes . . . . . . . . . . . . . . . .  58
       12.2.4.  Multilink Forwarding Parameters  . . . . . . . . . .  61
       12.2.5.  Traffic Selector . . . . . . . . . . . . . . . . . .  66
       12.2.6.  Geo Coordinates  . . . . . . . . . . . . . . . . . .  67
       12.2.7.  Dynamic Host Configuration Protocol for IPv6
                (DHCPv6) Message . . . . . . . . . . . . . . . . . .  68
       12.2.8.  Host Identity Protocol (HIP) Message . . . . . . . .  68
       12.2.9.  PIM-SM Message . . . . . . . . . . . . . . . . . . .  71
       12.2.10. Reassembly Limit . . . . . . . . . . . . . . . . . .  72
       12.2.11. Fragmentation Report . . . . . . . . . . . . . . . .  73
       12.2.12. Node Identification  . . . . . . . . . . . . . . . .  74

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       12.2.13. ICMPv6 Error . . . . . . . . . . . . . . . . . . . .  76
       12.2.14. QUIC-TLS Message . . . . . . . . . . . . . . . . . .  76
       12.2.15. Proxy/Server Departure . . . . . . . . . . . . . . .  77
       12.2.16. Sub-Type Extension . . . . . . . . . . . . . . . . .  78
   13. Address Mapping - Multicast . . . . . . . . . . . . . . . . .  81
   14. Multilink Conceptual Sending Algorithm  . . . . . . . . . . .  81
     14.1.  Multiple OMNI Interfaces . . . . . . . . . . . . . . . .  82
     14.2.  Client-Proxy/Server Loop Prevention  . . . . . . . . . .  83
   15. Router Discovery and Prefix Registration  . . . . . . . . . .  83
     15.1.  Window Synchronization . . . . . . . . . . . . . . . . .  90
     15.2.  Router Discovery in IP Multihop and IPv4-Only Networks .  91
     15.3.  DHCPv6-based Prefix Registration . . . . . . . . . . . .  93
   16. Secure Redirection  . . . . . . . . . . . . . . . . . . . . .  94
   17. Proxy/Server Resilience . . . . . . . . . . . . . . . . . . .  95
   18. Detecting and Responding to Proxy/Server Failures . . . . . .  95
   19. Transition Considerations . . . . . . . . . . . . . . . . . .  96
   20. OMNI Interfaces on Open Internetworks . . . . . . . . . . . .  96
   21. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . . . .  99
   22. (H)HITs and Temporary ULAs  . . . . . . . . . . . . . . . . .  99
   23. Address Selection . . . . . . . . . . . . . . . . . . . . . . 100
   24. Error Messages  . . . . . . . . . . . . . . . . . . . . . . . 101
   25. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 101
     25.1.  "Protocol Numbers" Registry  . . . . . . . . . . . . . . 101
     25.2.  "IEEE 802 Numbers" Registry  . . . . . . . . . . . . . . 102
     25.3.  "IPv4 Special-Purpose Address" Registry  . . . . . . . . 102
     25.4.  "IPv6 Neighbor Discovery Option Formats" Registry  . . . 102
     25.5.  "Ethernet Numbers" Registry  . . . . . . . . . . . . . . 102
     25.6.  "ICMPv6 Code Fields: Type 2 - Packet Too Big" Registry . 103
     25.7.  "OMNI Option Sub-Type Values" (New Registry) . . . . . . 103
     25.8.  "OMNI Geo Coordinates Type Values" (New Registry)  . . . 104
     25.9.  "OMNI Node Identification ID-Type Values" (New Registry) 104
     25.10. "OMNI Option Sub-Type Extension Values" (New Registry) . 104
     25.11. "OMNI RFC4380 UDP/IP Header Option" (New Registry) . . . 105
     25.12. "OMNI RFC6081 UDP/IP Trailer Option" (New Registry)  . . 105
     25.13. Additional Considerations  . . . . . . . . . . . . . . . 106
   26. Security Considerations . . . . . . . . . . . . . . . . . . . 106
   27. Implementation Status . . . . . . . . . . . . . . . . . . . . 108
   28. Document Updates  . . . . . . . . . . . . . . . . . . . . . . 108
   29. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 108
   30. References  . . . . . . . . . . . . . . . . . . . . . . . . . 110
     30.1.  Normative References . . . . . . . . . . . . . . . . . . 110
     30.2.  Informative References . . . . . . . . . . . . . . . . . 112
   Appendix A.  OAL Checksum Algorithm . . . . . . . . . . . . . . . 120
   Appendix B.  IPv6 ND Message Authentication and Integrity . . . . 121
   Appendix C.  VDL Mode 2 Considerations  . . . . . . . . . . . . . 122
   Appendix D.  Client-Proxy/Server Isolation Through L2 Address
                Mapping  . . . . . . . . . . . . . . . . . . . . . . 123
   Appendix E.  Change Log . . . . . . . . . . . . . . . . . . . . . 123

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 123

1.  Introduction

   Mobile network platforms and devices (e.g., aircraft of various
   configurations, terrestrial vehicles, seagoing vessels, enterprise
   wireless devices, pedestrians with cellphones, etc.) configure mobile
   routers with multiple interface connections to wireless and/or wired-
   line data links.  These data links may have diverse performance, cost
   and availability properties that can change dynamically according to
   mobility patterns, flight phases, proximity to infrastructure, etc.
   The mobile router acts as a Client of a network-based Mobility
   Service (MS) by configuring a virtual interface over its underlying
   interface data link connections to support the "6M's of modern
   Internetworking" (see below).

   Each Client configures a virtual interface (termed the "Overlay
   Multilink Network Interface (OMNI)") as a thin layer over its
   underlying interfaces.  The OMNI interface is therefore the only
   interface abstraction exposed to the IP layer and behaves according
   to the Non-Broadcast, Multiple Access (NBMA) interface principle,
   while underlying interfaces appear as link layer communication
   channels in the architecture.  The OMNI interface internally employs
   the "OMNI Adaptation Layer (OAL)" to ensure that original IP packets
   are delivered without loss due to size restrictions.  The OMNI
   interface connects to a virtual overlay service known as the "OMNI
   link".  The OMNI link spans one or more Internetworks that may
   include private-use infrastructures and/or the global public Internet
   itself.

   The Client's OMNI interface interacts with the MS and/or other
   Clients through IPv6 Neighbor Discovery (ND) control message
   exchanges [RFC4861].  The MS consists of a distributed set of service
   nodes known as Proxy/Servers (and other infrastructure elements) that
   also configure OMNI interfaces.  An example MS termed "Automatic
   Extended Route Optimization (AERO)" appears in
   [I-D.templin-6man-aero].  In terms of precedence, the AERO
   specification may provide first-principle insights into a
   representative mobility service architecture as useful context for
   this specification.

   Each OMNI interface provides a multilink nexus for exchanging inbound
   and outbound traffic via selected underlying interface(s).  The IP
   layer sees the OMNI interface as a point of connection to the OMNI
   link.  Each OMNI link has one or more associated Mobility Service
   Prefixes (MSPs), which are typically IP Global Unicast Address (GUA)
   prefixes assigned to the link and from which Mobile Network Prefixes

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   (MNPs) are derived.  If there are multiple OMNI links, the IP layer
   will see multiple OMNI interfaces.

   Each Client receives an MNP through IPv6 ND control message exchanges
   with Proxy/Servers.  The Client uses the MNP for numbering
   downstream-attached End User Networks (EUNs) independently of the
   access network data links selected for data transport.  The Client
   acts as a mobile router on behalf of its EUNs, and uses OMNI
   interface control messaging to coordinate with Proxy/Servers and/or
   other Clients.  The Client iterates its control messaging over each
   of the OMNI interface's underlying interfaces in order to register
   each interface with the MS (see Section 15).

   Clients may connect to multiple distinct OMNI links within the same
   OMNI domain by configuring multiple OMNI interfaces, e.g., omni0,
   omni1, omni2, etc.  Each OMNI interface is configured over a set of
   underlying interfaces and provides a nexus for Safety-Based Multilink
   (SBM) operation.  Each OMNI interface within the same OMNI domain
   configures a common ULA prefix [ULA]::/48, and configures a unique
   16-bit Subnet ID '*' to construct the sub-prefix [ULA*]::/64 (see:
   Section 9).  The IP layer applies SBM routing to select a specific
   OMNI interface, then the selected OMNI interface applies Performance-
   Based Multilink (PBM) internally to select appropriate underlying
   interfaces.  Applications select SBM topologies based on IP layer
   Segment Routing [RFC8402], while each OMNI interface orchestrates PBM
   internally based on OMNI layer Segment Routing.

   OMNI provides a link model suitable for a wide range of use cases.
   In particular, the International Civil Aviation Organization (ICAO)
   Working Group-I Mobility Subgroup is developing a future Aeronautical
   Telecommunications Network with Internet Protocol Services (ATN/IPS)
   and has issued a liaison statement requesting IETF adoption [ATN] in
   support of ICAO Document 9896 [ATN-IPS].  The IETF IP Wireless Access
   in Vehicular Environments (ipwave) working group has further included
   problem statement and use case analysis for OMNI in a document now in
   AD evaluation for RFC publication
   [I-D.ietf-ipwave-vehicular-networking].  Still other communities of
   interest include AEEC, RTCA Special Committee 228 (SC-228) and NASA
   programs that examine commercial aviation, Urban Air Mobility (UAM)
   and Unmanned Air Systems (UAS).  Pedestrians with handheld devices
   represent another large class of potential OMNI users.

   OMNI supports the "6M's of modern Internetworking" including:

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

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   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, etc.

   3.  Mobility - a Client's ability to change network points of
       attachment (e.g., moving between wireless base stations) which
       may result in an underlying 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 Clients belonging to the same interest
       group, but without disturbing other Clients not subscribed to the
       interest group.

   5.  Multihop - a mobile Client 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.  MTU assurance - the ability to deliver packets of various robust
       sizes between peers without loss due to a link size restriction,
       and to dynamically adjust packets sizes to achieve the optimal
       performance for each independent traffic flow.

   This document specifies the transmission of IP packets and control
   messages over OMNI interfaces.  The OMNI interface supports either IP
   protocol version (i.e., IPv4 [RFC0791] or IPv6 [RFC8200]) as the
   network layer data plane, while using IPv6 ND messaging as the
   control plane independently of the data plane IP protocol(s).  The
   OAL operates as a sublayer between L3 and L2 based on IPv6
   encapsulation [RFC2473] as discussed in the following sections.

2.  Terminology

   The terminology in the normative references applies; especially, the
   terms "link" and "interface" are the same as defined in the IPv6
   [RFC8200] and IPv6 Neighbor Discovery (ND) [RFC4861] specifications.
   Additionally, this document assumes the following IPv6 ND message
   types: Router Solicitation (RS), Router Advertisement (RA), Neighbor
   Solicitation (NS), Neighbor Advertisement (NA) and Redirect.  Clients
   and Proxy/Servers that implement IPv6 ND maintain per-neighbor state
   in Neighbor Cache Entries (NCEs).  Each NCE is indexed by the
   neighbor's Link-Local Address (LLA), while the Unique-Local Address
   (ULA) used for encapsulation provides context for Identification
   verification.

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   The Protocol Constants defined in Section 10 of [RFC4861] are used in
   their same format and meaning in this document.  The terms "All-
   Routers multicast", "All-Nodes multicast" and "Subnet-Router anycast"
   are the same as defined in [RFC4291] (with Link-Local scope assumed).

   The term "IP" is used to refer collectively to either Internet
   Protocol version (i.e., IPv4 [RFC0791] or IPv6 [RFC8200]) when a
   specification at the layer in question applies equally to either
   version.

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

   Client
      a network platform/device mobile router that has one or more
      distinct upstream data link connections grouped together into one
      or more logical units.  The Client's data link connection
      parameters can change over time due to, e.g., node mobility, link
      quality, etc.  The Client further connects downstream-attached End
      User Networks (EUNs).

   End User Network (EUN)
      a simple or complex downstream-attached mobile network that
      travels with the Client as a single logical unit.  The IP
      addresses assigned to EUN devices remain stable even if the
      Client's upstream data link connections change.

   Mobility Service (MS)
      a mobile routing service that tracks Client movements and ensures
      that Clients remain continuously reachable even across mobility
      events.  The MS consists of the set of all Proxy/Servers (and any
      other supporting infrastructure nodes) for the OMNI link.
      Specific MS details are out of scope for this document, with an
      example found in [I-D.templin-6man-aero].

   Proxy/Server
      a segment routing topology edge node that provides Clients with a
      multi-purpose interface to the MS.  As a server, the Proxy/Server
      responds directly to some Client IPv6 ND messages.  As a proxy,
      the Proxy/Server forwards other Client IPv6 ND messages to other
      Proxy/Servers and Clients.  As a router, the Proxy/Server provides
      a forwarding service for ordinary data packets that may be
      essential in some environments and a last resort in others.

   First-Hop Segment (FHS) Proxy/Server
      a Proxy/Server reached via an underlying interface of the source
      Client that forwards packets sent by the source Client over that
      interface into the segment routing topology.  FHS Proxy/Servers

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      act as intermediate forwarding nodes to facilitate RS/RA exchanges
      between a Client and its Hub Proxy/Server.

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

   Hub Proxy/Server
      a single Proxy/Server selected by the Client that provides a
      designated router service for all of the Client's underlying
      interfaces.  Clients normally 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 in that capacity),
      however the Hub can also be 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 forwarding region between the FHS Proxy/Server and LHS
      Proxy/Server.  FHS/LHS Proxy/Servers and the SRT span the OMNI
      link on behalf of source/target Client pairs using segment routing
      in a manner outside the scope of this document (see:
      [I-D.templin-6man-aero]).

   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
      Internet address registry, however private-use prefixes can
      alternatively be used subject to certain limitations (see:
      Section 10).  OMNI links that connect to the global Internet
      advertise their MSPs to their interdomain routing peers.

   Mobile Network Prefix (MNP)
      a longer IP prefix delegated from an MSP (e.g.,
      2001:db8:1000:2000::/56, 192.0.2.8/30, etc.) and assigned to a
      Client.  Clients sub-delegate the MNP to devices located in EUNs.
      Note that OMNI link Relay nodes may also service non-MNP routes
      (i.e., GUA prefixes not covered by an MSP) but that these
      correspond to fixed correspondent nodes and not Clients.  Other
      than this distinction, MNP and non-MNP routes are treated exactly
      the same by the OMNI routing system.

   Access Network (ANET)
      a data link service network (e.g., an aviation radio access
      network, satellite service provider network, cellular operator

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      network, WiFi network, etc.) that connects Clients.  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.

   ANET interface
      a Client's attachment to a link in an ANET.

   Internetwork (INET)
      a connected network region with a coherent IP addressing plan that
      provides transit forwarding services between ANETs and nodes that
      connect directly to the open INET via unprotected media.  No
      physical and/or data link level security is assumed, therefore
      security must be applied by upper layers.  The global public
      Internet itself is an example.

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

   *NET
      a "wildcard" term used when a given specification applies equally
      to both ANET and INET cases.

   INADDR
      the IP address (and also the UDP port number when UDP is used)
      that appear in *NET header address fields.  The terms "*NET
      address" and "INADDR" are used interchangeably.

   OMNI link
      a Non-Broadcast, Multiple Access (NBMA) virtual overlay configured
      over one or more INETs and their connected ANETs.  An OMNI link
      may comprise multiple distinct segments joined by bridges the same
      as for any link; the addressing plans in each segment may be
      mutually exclusive and managed by different administrative
      entities.  Proxy/Servers and other MS infrastructure elements
      extend the link to support communications between Clients as
      single-hop neighbors.

   OMNI interface
      a node's attachment to an OMNI link, and configured over one or
      more underlying *NET interfaces.  If there are multiple OMNI links
      in an OMNI domain, a separate OMNI interface is configured for
      each link.

   OMNI Adaptation Layer (OAL)

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      an OMNI interface sublayer service whereby original IP packets
      admitted into the interface are wrapped in an IPv6 header and
      subject to fragmentation and reassembly.  The OAL is also
      responsible for generating MTU-related control messages as
      necessary, and for providing addressing context for OMNI link SRT
      traversal.

   original IP packet
      a whole IP packet 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 decapsulation and reassembly.

   OAL packet
      an original IP packet encapsulated in OAL headers and trailers,
      which is then submitted for OAL fragmentation and reassembly.

   OAL fragment
      a portion of an OAL packet following fragmentation but prior to
      *NET encapsulation, or following *NET encapsulation but prior to
      OAL reassembly.

   (OAL) atomic fragment
      an OAL packet that does not require fragmentation is always
      encapsulated as an "atomic fragment" with a Fragment Header with
      Fragment Offset and More Fragments both set to 0, but with a valid
      Identification value.

   (OAL) carrier packet
      an encapsulated OAL fragment following *NET encapsulation or prior
      to *NET decapsulation.  OAL sources and destinations exchange
      carrier packets over underlying interfaces, and may be separated
      by one or more OAL intermediate nodes.  OAL intermediate nodes may
      perform re-encapsulation on carrier packets by removing the *NET
      headers of the first hop network and replacing them with new *NET
      headers for the next hop network.

   OAL source
      an OMNI interface acts as an OAL source when it encapsulates
      original IP packets to form OAL packets, then performs OAL
      fragmentation and *NET encapsulation to create carrier packets.

   OAL destination
      an OMNI interface acts as an OAL destination when it decapsulates
      carrier packets, then performs OAL reassembly and decapsulation to
      derive the original IP packet.

   OAL intermediate node

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      an OMNI interface acts as an OAL intermediate node when it removes
      the *NET headers of carrier packets received on a first segment,
      then re-encapsulates the carrier packets in new *NET headers and
      forwards them into the next segment.

   OMNI Option
      an IPv6 Neighbor Discovery option providing multilink parameters
      for the OMNI interface as specified in Section 12.

   Mobile Network Prefix Link Local Address (MNP-LLA)
      an IPv6 Link Local Address that embeds the most significant 64
      bits of an MNP in the lower 64 bits of fe80::/64, as specified in
      Section 8.

   Mobile Network Prefix Unique Local Address (MNP-ULA)
      an IPv6 Unique-Local Address derived from an MNP-LLA.

   Administrative Link Local Address (ADM-LLA)
      an IPv6 Link Local Address that embeds a 32-bit administratively-
      assigned identification value in the lower 32 bits of fe80::/96,
      as specified in Section 8.

   Administrative Unique Local Address (ADM-ULA)
      an IPv6 Unique-Local Address derived from an ADM-LLA.

   Multilink
      an OMNI interface's manner of managing diverse underlying
      interface connections to data links as a single logical unit.  The
      OMNI interface provides a single unified interface to upper
      layers, while underlying interface selections are performed on a
      per-packet basis considering traffic selectors such as DSCP, flow
      label, application policy, signal quality, cost, etc.  Multilink
      selections are coordinated in both the outbound and inbound
      directions based on source/target underlying interface pairs.

   Multinet
      an OAL intermediate node's manner of spanning multiple diverse IP
      Internetwork and/or private enterprise network "segments" at the
      OAL layer below IP.  Through intermediate node concatenation of
      SRT bridged network segments, multiple diverse Internetworks (such
      as the global public IPv4 and IPv6 Internets) can serve as transit
      segments in a bridged path for forwarding IP packets end-to-end.
      This bridging capability provide benefits such as supporting IPv4/
      IPv6 transition and coexistence, joining multiple diverse operator
      networks into a cooperative single service network, etc.

   Multihop

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      an iterative relaying of IP packets between Client's over an OMNI
      underlying interface technology (such as omnidirectional wireless)
      without support of fixed infrastructure.  Multihop services entail
      Client-to-Client relaying within a Mobile/Vehicular Ad-hoc Network
      (MANET/VANET) for Vehicle-to-Vehicle (V2V) communications and/or
      for Vehicle-to-Infrastructure (V2I) "range extension" where
      Clients within range of communications infrastructure elements
      provide forwarding services for other Clients.

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

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

   underlying interface
      a *NET interface over which an OMNI interface is configured.  The
      OMNI interface is seen as a L3 interface by the IP layer, and each
      underlying interface is seen as a L2 interface by the OMNI
      interface.  The underlying interface either connects directly to
      the physical communications media or coordinates with another node
      where the physical media is hosted.

   Mobility Service Identification (MSID)
      All MS elements (including Proxy/Servers and other MS nodes)
      assign a unique 32-bit Identification (MSID) (see: Section 8)
      according to MS-specific guidelines (e.g., see:
      [I-D.templin-6man-aero]).

   Safety-Based Multilink (SBM)
      A means for ensuring fault tolerance through redundancy by
      connecting multiple affiliated OMNI interfaces to independent
      routing topologies (i.e., multiple independent OMNI links).

   Performance Based Multilink (PBM)
      A means for selecting one or more underlying interface(s) for
      packet transmission and reception within a single OMNI interface.

   OMNI Domain
      The set of all SBM/PBM OMNI links that collectively provides
      services for a common set of MSPs.  Each OMNI domain consists of a
      set of affiliated OMNI links that all configure the same ::/48 ULA
      prefix with a unique 16-bit Subnet ID as discussed in Section 9.

   Multilink Forwarding Information Base (MFIB)

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      A forwarding table on each OMNI source, destination and
      intermediate node that includes Multilink Forwarding Vectors (MFV)
      with both next hop forwarding instructions and context for
      reconstructing compressed headers for specific underlying
      interface pairs used to communicate with peers.  See:
      [I-D.templin-6man-aero] for further discussion.

   Multilink Forwarding Vector (MFV)
      An MFIB entry that includes soft state for each underlying
      interface pairwise communication session between peers.  MFVs are
      identified by both a next-hop and previous-hop MFV Index (MFVI),
      with the next-hop established based on an IPv6 ND solicitation and
      the previous hop established based on the solicited IPv6 ND
      advertisement response.  See: [I-D.templin-6man-aero] for further
      discussion.

   Multilink Forwarding Vector Index (MVFI)
      A 4 octet value selected by an OMNI node when it creates an MFV,
      then advertised to either a next-hop or previous-hop.  OMNI
      intermediate nodes assign two distinct MFVIs for each MFV and
      advertise one to the next-hop and the other to the previous-hop.
      OMNI end systems assign and advertise a single MFVI.  See:
      [I-D.templin-6man-aero] for further discussion.

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.

   An implementation is not required to internally use the architectural
   constructs described here so long as its external behavior is
   consistent with that described in this document.

4.  Overlay Multilink Network (OMNI) Interface Model

   An OMNI interface is a virtual interface configured over one or more
   underlying interfaces, which may be physical (e.g., an aeronautical
   radio link, etc.) or virtual (e.g., an Internet or higher-layer
   "tunnel").  The OMNI interface architectural layering model is the
   same as in [RFC5558][RFC7847], and augmented as shown in Figure 1.
   The IP layer therefore sees the OMNI interface as a single L3
   interface nexus for multiple underlying interfaces that appear as L2
   communication channels in the architecture.

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                                     +----------------------------+
                                     |    Upper Layer Protocol    |
              Session-to-IP    +---->|                            |
              Address Binding  |     +----------------------------+
                               +---->|           IP (L3)          |
              IP Address       +---->|                            |
              Binding          |     +----------------------------+
                               +---->|       OMNI Interface       |
              Logical-to-      +---->|   (OMNI Adaptation Layer)  |
              Physical         |     +----------------------------+
              Interface        +---->|  L2  |  L2  |       |  L2  |
              Binding                |(IF#1)|(IF#2)| ..... |(IF#n)|
                                     +------+------+       +------+
                                     |  L1  |  L1  |       |  L1  |
                                     |      |      |       |      |
                                     +------+------+       +------+

           Figure 1: OMNI Interface Architectural Layering Model

   Each underlying interface provides an L2/L1 abstraction according to
   one of the following models:

   o  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 any
      INET correspondent.  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.

   o  ANET interfaces connect to a protected and secured ANET that is
      separated from the open INET 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.)

   o  VPNed interfaces use security encapsulation over a *NET to a
      Proxy/Server acting as a Virtual Private Network (VPN) gateway.
      Other than the link-layer encapsulation format, VPNed interfaces
      behave the same as for Direct interfaces.

   o  Direct (aka "point-to-point") interfaces connect directly to a
      peer (i.e., a Proxy/Server or another Client) without crossing any
      *NET paths.  An example is a line-of-sight link between a remote
      pilot and an unmanned aircraft.

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   The OMNI interface forwards original IP packets from the network
   layer (L3) using the OMNI Adaptation Layer (OAL) (see: Section 5) as
   an encapsulation and fragmentation sublayer service.  This "OAL
   source" then further encapsulates the resulting OAL packets/fragments
   in *NET headers to create "carrier packets" for transmission over
   underlying interfaces (L2/L1).  The target OMNI interface receives
   the carrier packets from underlying interfaces (L1/L2) and discards
   the *NET headers.  If the resulting OAL packets/fragments are
   addressed to itself, the OMNI interface acts as an "OAL destination"
   and performs reassembly if necessary, discards the OAL encapsulation,
   and delivers the original IP packet to the network layer (L3).  If
   the OAL fragments are addressed to another node, the OMNI interface
   instead acts as an "OAL intermediate node" by re-encapsulating in new
   *NET headers and forwarding the new carrier packets over an
   underlying interface without reassembling or discarding the OAL
   encapsulation.  The OAL source and OAL destination are seen as
   "neighbors" on the OMNI link, while OAL intermediate nodes provide a
   virtual bridging service that joins the segments of a (multinet)
   Segment Routing Topology (SRT).

   The OMNI interface can send/receive original IP packets to/from
   underlying interfaces while including/omitting various encapsulations
   including OAL, UDP, IP and L2.  The network layer can also access the
   underlying interfaces directly while bypassing the OMNI interface
   entirely when necessary.  This architectural flexibility may be
   beneficial for underlying interfaces (e.g., some aviation data links)
   for which encapsulation overhead may be a primary consideration.
   OMNI interfaces that send original IP packets directly over
   underlying interfaces without invoking the OAL can only reach peers
   located on the same OMNI link segment.  Source Clients can instead
   use the OAL to coordinate with target Clients in the same or
   different OMNI link segments by sending initial carrier packets to a
   First-Hop Segment (FHS) Proxy/Server.  The FHS Proxy/Sever then
   forwards the packets into the SRT spanning tree, which transports
   them to a Last-Hop Segment (LHS) Proxy/Server for the target Client.

   Original IP packets sent directly over underlying interfaces are
   subject to the same path MTU related issues as for any
   Internetworking path, and do not include per-packet identifications
   that can be used for data origin verification and/or link-layer
   retransmissions.  Original IP packets presented directly to an
   underlying interface that exceed the underlying network path MTU are
   dropped with an ordinary ICMPv6 Packet Too Big (PTB) message
   returned.  These PTB messages are subject to loss [RFC2923] the same
   as for any non-OMNI IP interface.

   The OMNI interface encapsulation/decapsulation layering possibilities
   are shown in Figure 2 below.  Imaginary vertical lines drawn between

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   the Network Layer and Underlying interfaces in the figure denote the
   encapsulation/decapsulation layering combinations possible.  Common
   combinations include NULL (i.e., direct access to underlying
   interfaces with or without using the OMNI interface), IP/IP, IP/UDP/
   IP, IP/UDP/IP/L2, IP/OAL/UDP/IP, IP/OAL/UDP/L2, etc.

      +------------------------------------------------------------+
      |             Network Layer (Original IP packets)            |
      +--+---------------------------------------------------------+
         |         OMNI Interface (virtual sublayer nexus)         |
         +--------------------------+------------------------------+
                                    |      OAL Encaps/Decaps       |
                                    +------------------------------+
                                    |        OAL Frag/Reass        |
                       +------------+---------------+--------------+
                       | UDP Encaps/Decaps/Compress |
                  +----+---+------------+--------+--+  +--------+
                  | IP E/D |            | IP E/D |     | IP E/D |
              +---+------+-+----+    +--+---+----+     +----+---+--+
              |L2 E/D|   |L2 E/D|    |L2 E/D|               |L2 E/D|
      +-------+------+---+------+----+------+---------------+------+
      |                   Underlying Interfaces                    |
      +------------------------------------------------------------+

                     Figure 2: OMNI Interface Layering

   The OMNI/OAL model gives rise to a number of opportunities:

   o  Clients receive MNPs from the MS, and coordinate with the MS
      through IPv6 ND message exchanges with Proxy/Servers.  Clients use
      the MNP to construct a unique Link-Local Address (MNP-LLA) through
      the algorithmic derivation specified in Section 8 and assign the
      LLA to the OMNI interface.  Since MNP-LLAs are uniquely derived
      from an MNP, no Duplicate Address Detection (DAD) or Multicast
      Listener Discovery (MLD) messaging is necessary.

   o  since Temporary ULAs are statistically unique, they can be used
      without DAD until an MNP-LLA is obtained.

   o  underlying interfaces on the same L2 link segment as a Proxy/
      Server do not require any L3 addresses (i.e., not even link-local)
      in environments where communications are coordinated entirely over
      the OMNI interface.

   o  as underlying interface properties change (e.g., link quality,
      cost, availability, etc.), any active interface can be used to
      update the profiles of multiple additional interfaces in a single

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      message.  This allows for timely adaptation and service continuity
      under dynamically changing conditions.

   o  coordinating underlying interfaces in this way allows them to be
      represented in a unified MS profile with provisions for mobility
      and multilink operations.

   o  exposing a single virtual interface abstraction to the IPv6 layer
      allows for multilink operation (including QoS based link
      selection, packet replication, load balancing, etc.) at L2 while
      still permitting L3 traffic shaping based on, e.g., DSCP, flow
      label, etc.

   o  the OMNI interface allows multinet traversal over the SRT when
      communications across different administrative domain network
      segments are necessary.  This mode of operation would not be
      possible via direct communications over the underlying interfaces
      themselves.

   o  the OAL supports lossless and adaptive path MTU mitigations not
      available for communications directly over the underlying
      interfaces themselves.  The OAL supports "packing" of multiple IP
      payload packets within a single OAL "super-packet".

   o  the OAL applies per-packet identification values that allow for
      link-layer reliability and data origin authentication.

   o  L3 sees the OMNI interface as a point of connection to the OMNI
      link; if there are multiple OMNI links (i.e., multiple MS's), L3
      will see multiple OMNI interfaces.

   o  Multiple independent OMNI interfaces can be used for increased
      fault tolerance through Safety-Based Multilink (SBM), with
      Performance-Based Multilink (PBM) applied within each interface.

   Note that even when the OMNI virtual interface is present,
   applications can still access underlying interfaces either through
   the network protocol stack using an Internet socket or directly using
   a raw socket.  This allows for intra-network (or point-to-point)
   communications without invoking the OMNI interface and/or OAL.  For
   example, when an IPv6 OMNI interface is configured over an underlying
   IPv4 interface, applications can still invoke IPv4 intra-network
   communications as long as the communicating endpoints are not subject
   to mobility dynamics.

   Figure 3 depicts the architectural model for a source Client with an
   attached EUN connecting to the OMNI link via multiple independent
   *NETs.  The Client's OMNI interface sends IPv6 ND messages over

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   available underlying interfaces to FHS Proxy/Servers using any
   necessary *NET encapsulations.  The IPv6 ND messages traverse the
   *NETs until they reach an FHS Proxy/Server (FHS#1, FHS#2, ...,
   FHS#n), which returns an IPv6 ND message response and/or forwards a
   proxyed version of the message over the SRT to an LHS Proxy/Server
   near the target Client (LHS#1, LHS#2, ..., LHS#m).  The Hop Limit in
   IPv6 ND messages is not decremented due to encapsulation; hence, the
   source and target Client OMNI interfaces appear to be attached to a
   common link.

                           +--------------+        (:::)-.
                           |Source Client |<-->.-(::EUN:::)
                           +--------------+      `-(::::)-'
                           |OMNI interface|
                           +----+----+----+
                  +--------|IF#1|IF#2|IF#n|------ +
                 /         +----+----+----+        \
                /                 |                 \
               /                  |                  \
              v                   v                   v
           (:::)-.              (:::)-.              (:::)-.
      .-(::*NET:::)        .-(::*NET:::)        .-(::*NET:::)
        `-(::::)-'           `-(::::)-'           `-(::::)-'
         +-----+              +-----+              +-----+
    ...  |FHS#1|  .........   |FHS#2|   .........  |FHS#n|  ...
   .     +--|--+              +--|--+              +--|--+     .
   .        |                    |                    |
   .        \                    v                    /        .
   .         \                                       /         .
   .           v                 (:::)-.           v            .
   .                        .-(::::::::)                       .
   .                    .-(::: Segment :::)-.                  .
   .                  (:::::   Routing   ::::)                 .
   .                     `-(:: Topology ::)-'                  .
   .                         `-(:::::::-'                      .
   .                  /          |          \                  .
   .                 /           |           \                 .
   .                v            v            v
   .     +-----+              +-----+              +-----+     .
    ...  |LHS#1|  .........   |LHS#2|   .........  |LHS#m|  ...
         +--|--+              +--|--+              +--|--+
             \                   |                    /
              v                  v                   v
                       <-- Target Clients -->

      Figure 3: Source/Target Client Coordination over the OMNI Link

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   After the initial IPv6 ND message exchange, the source Client (and/or
   any nodes on its attached EUNs) can send packets to the target Client
   over the OMNI interface.  OMNI interface multilink services will
   forward the packets via FHS Proxy/Servers for the correct underlying
   *NETs.  The FHS Proxy/Server then forwards them over the SRT which
   delivers them to an LHS Proxy/Server, and the LHS Proxy/Server in
   turn forwards the packets to the target Client.  (Note that when the
   source and target Client are on the same SRT segment, the FHS and LHS
   Proxy/Servers may be one and the same.)

   Clients select a Hub Proxy/Server (not shown in the figure), which
   will often be one of their FHS Proxy/Servers but could also be any
   Proxy/Server on the OMNI link.  Clients then register all of their
   underlying interfaces with the Hub Proxy/Server via the FHS Proxy/
   Server in a pure proxy role.  The Hub Proxy/Server then provides a
   designated router service for the Client, and the Client can quickly
   migrate to a new Hub Proxy/Server if the first becomes unresponsive.

   Clients therefore use Proxy/Servers as gateways into the SRT to reach
   OMNI link correspondents via a spanning tree established in a manner
   outside the scope of this document.  Proxy/Servers forward critical
   MS control messages via the secured spanning tree and forward other
   messages via the unsecured spanning tree (see Security
   Considerations).  When route optimization is applied as discussed in
   [I-D.templin-6man-aero], Clients can instead forward directly to an
   SRT intermediate node themselves (or directly to correspondents in
   the same SRT segment) to reduce Proxy/Server load.

5.  OMNI Interface Maximum Transmission Unit (MTU)

   The OMNI interface observes the link nature of tunnels, including the
   Maximum Transmission Unit (MTU), Maximum Reassembly Unit (MRU) and
   the role of fragmentation and reassembly [I-D.ietf-intarea-tunnels].
   The OMNI interface is configured over one or more underlying
   interfaces as discussed in Section 4, where the interfaces (and their
   associated *NET paths) may have diverse MTUs.  OMNI interface
   considerations for accommodating original IP packets of various sizes
   are discussed in the following sections.

   IPv6 underlying interfaces are REQUIRED to configure a minimum MTU of
   1280 bytes and a minimum MRU of 1500 bytes [RFC8200].  Therefore, the
   minimum IPv6 path MTU is 1280 bytes since routers on the path are not
   permitted to perform network fragmentation even though the
   destination is required to reassemble more.  The network therefore
   MUST forward original IP packets of at least 1280 bytes without
   generating an IPv6 Path MTU Discovery (PMTUD) Packet Too Big (PTB)
   message [RFC8201].  (While the source can apply "source
   fragmentation" for locally-generated IPv6 packets up to 1500 bytes

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   and larger still if it knows the destination configures a larger MRU,
   this does not affect the minimum IPv6 path MTU.)

   IPv4 underlying interfaces are REQUIRED to configure a minimum MTU of
   68 bytes [RFC0791] and a minimum MRU of 576 bytes [RFC0791][RFC1122].
   Therefore, when the Don't Fragment (DF) bit in the IPv4 header is set
   to 0 the minimum IPv4 path MTU is 576 bytes since routers on the path
   support network fragmentation and the destination is required to
   reassemble at least that much.  The OMNI interface therefore MUST set
   DF to 0 in the IPv4 encapsulation headers of carrier packets that are
   no larger than 576 bytes, and SHOULD set DF to 1 in larger carrier
   packets unless it has a way to determine the encapsulation
   destination MRU and has carefully considered the issues discussed in
   Section 6.10.

   The OMNI interface configures an MTU and MRU of 9180 bytes [RFC2492];
   the size is therefore not a reflection of the underlying interface or
   *NET path MTUs, but rather determines the largest original IP packet
   the OAL (and/or underlying interface) can forward or reassemble.  For
   each OAL destination (i.e., for each OMNI link neighbor), the OAL
   source may discover "hard" or "soft" Reassembly Limit values smaller
   than the MRU based on receipt of IPv6 ND messages with OMNI
   Reassembly Limit sub-options (see: Section 12.2.10).  The OMNI
   interface employs the OAL as an encapsulation sublayer service to
   transform original IP packets into OAL packets/fragments, and the OAL
   in turn uses *NET encapsulation to forward carrier packets over the
   underlying interfaces (see: Section 6).

6.  The OMNI Adaptation Layer (OAL)

   When an OMNI interface forwards an original IP packet from the
   network layer for transmission over one or more underlying
   interfaces, the OMNI Adaptation Layer (OAL) acting as the OAL source
   drops the packet and returns a PTB message if the packet exceeds the
   MRU and/or the hard Reassembly Limit for the intended OAL
   destination.  Otherwise, the OAL source applies encapsulation to form
   OAL packets subject to fragmentation producing OAL fragments suitable
   for *NET encapsulation and transmission as carrier packets over
   underlying interfaces as described in Section 6.1.

   These carrier packets travel over one or more underlying networks
   spanned by OAL intermediate nodes in the SRT, which re-encapsulate by
   removing the *NET headers of the first underlying network and
   appending *NET headers appropriate for the next underlying network in
   succession.  (This process supports the multinet concatenation
   capability needed for joining multiple diverse networks.)  After re-
   encapsulation by zero or more OAL intermediate nodes, the carrier
   packets arrive at the OAL destination.

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   When the OAL destination receives the carrier packets, it discards
   the *NET headers and reassembles the resulting OAL fragments into an
   OAL packet as described in Section 6.3.  The OAL destination then
   decapsulates the OAL packet to obtain the original IP packet, which
   it then delivers to the network layer.  The OAL source may be either
   the source Client or its FHS Proxy/Server, while the OAL destination
   may be either the LHS Proxy/Server or the target Client.  Proxy/
   Servers (and other SRT infrastructure node types such as those
   discussed in [I-D.templin-6man-aero]) may also serve as OAL
   intermediate nodes.

   The OAL presents an OMNI sublayer abstraction similar to ATM
   Adaptation Layer 5 (AAL5).  Unlike AAL5 which performs segmentation
   and reassembly with fixed-length 53 octet cells over ATM networks,
   however, the OAL uses IPv6 encapsulation, fragmentation and
   reassembly with larger variable-length cells over heterogeneous
   underlying networks.  Detailed operations of the OAL are specified in
   the following sections.

6.1.  OAL Source Encapsulation and Fragmentation

   When the network layer forwards an original IP packet into the OMNI
   interface, the OAL source inserts an OAL header consisting of an IPv6
   encapsulation header followed by an IPv6 Fragment Header (see
   [RFC2473] and below) but does not decrement the Hop Limit/TTL of the
   original IP packet since encapsulation occurs at a layer below IP
   forwarding.  The OAL source copies the "Type of Service/Traffic
   Class" [RFC2983] and "Congestion Experienced" [RFC3168] values in the
   original packet's IP header into the corresponding fields in the OAL
   header, then sets the OAL header "Flow Label" as specified in
   [RFC6438].  The OAL source finally sets the OAL header IPv6 Hop Limit
   to a conservative value sufficient to enable loop-free forwarding
   over multiple concatenated OMNI link segments and sets the Payload
   Length to the length of the original IP packet.

   The OAL next selects source and destination addresses for the IPv6
   header of the resulting OAL packet.  Client OMNI interfaces set the
   OAL header source address to a Unique Local Address (ULA) based on
   the Mobile Network Prefix (MNP-ULA), while Proxy/Server OMNI
   interfaces set the source address to an Administrative ULA (ADM-ULA)
   (see: Section 9).  When a Client OMNI interface does not (yet) have
   an MNP-ULA, it can use a Temporary ULA and/or Host Identity Tag (HIT)
   instead (see: Section 22) as OAL addresses.  (In addition to ADM-
   ULAs, Proxy/Servers also process packets with anycast and/or
   multicast OAL addresses.)

   Following OAL encapsulation and address selection, the OAL source
   next appends a 2 octet trailing Checksum field (initialized to 0) at

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   the end of the original IP packet while incrementing the OAL header
   IPv6 Payload Length field to reflect the addition of the trailer.
   The format of the resulting OAL packet following encapsulation is
   shown in Figure 4:

      +----------+-----+-----+-----+-----+-----+-----+----+
      |OAL Header|         Original IP packet        |Csum|
      +----------+-----+-----+-----+-----+-----+-----+----+

                 Figure 4: OAL Packet Before Fragmentation

   The OAL source next selects a 32-bit Identification value for the
   packet to place in the Fragment Header as specified in Section 6.6
   then calculates an OAL checksum using the algorithm specified in
   Appendix A.  The OAL source calculates the checksum over the OAL
   packet beginning with a pseudo-header of the OAL header similar to
   that found in Section 8.1 of [RFC8200], followed by the Original IP
   packet and extending to the end of the (0-initialized) Checksum
   trailer.  The OAL pseudo-header is formed as shown in Figure 5:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                     OAL Source Address                        +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                  OAL Destination Address                      +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       OAL Payload Length      |     zero      |  Next Header  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Identification                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 5: OAL Pseudo-Header

   After calculating the checksum, the OAL source next fragments the OAL
   packet if necessary while assuming the IPv4 minimum path MTU (i.e.,
   576 bytes) as the worst case for OAL fragmentation regardless of the

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   underlying interface IP protocol version since IPv6/IPv4 protocol
   translation and/or IPv6-in-IPv4 encapsulation may occur in any *NET
   path.  By always assuming the IPv4 minimum even for IPv6 underlying
   interfaces, the OAL source may produce smaller fragments with
   additional encapsulation overhead but will always interoperate and
   never run the risk of loss due to an MTU restriction or due to
   presenting an underlying interface with a carrier packet that exceeds
   its MRU.  Additionally, the OAL path could traverse multiple SRT
   segments with intermediate OAL forwarding nodes performing re-
   encapsulation where the *NET encapsulation of the previous segment is
   replaced by the *NET encapsulation of the next segment which may be
   based on a different IP protocol version and/or encapsulation sizes.

   The OAL source therefore assumes a default minimum path MTU of 576
   bytes at each SRT segment for the purpose of generating OAL fragments
   for *NET encapsulation and transmission as carrier packets.  Each
   successive SRT intermediate node may include either a 20 byte IPv4 or
   40 byte IPv6 header, an 8 byte UDP header and in some cases an IP
   security encapsulation (40 bytes maximum assumed) during re-
   encapsulation.  Intermediate nodes at any SRT segment may also insert
   a Routing Header (40 bytes maximum assumed) as an extension to the
   existing 40 byte OAL IPv6 header plus 8 byte Fragment Header.
   Therefore, assuming a worst case of (40 + 40 + 8) = 88 bytes for *NET
   encapsulation plus (40 + 40 + 8) = 88 bytes for OAL encapsulation
   leaves no less than (576 - 88 - 88) = 400 bytes remaining to
   accommodate a portion of the original IP packet/fragment.  The OAL
   source therefore sets a minimum Maximum Payload Size (MPS) of 400
   bytes as the basis for the minimum-sized OAL fragment that can be
   assured of traversing all SRT segments without loss due to an MTU/MRU
   restriction.  The Maximum Fragment Size (MFS) for OAL fragmentation
   is therefore determined by the MPS plus the size of the OAL
   encapsulation headers.  (Note that the OAL source includes the 2
   octet trailer as part of the payload during fragmentation, and the
   OAL destination regards it as ordinary payload until reassembly and
   checksum verification are complete.)

   The OAL source SHOULD maintain "path MPS" values for individual OAL
   destinations initialized to the minimum MPS and increased to larger
   values (up to the OMNI interface MTU) if better information is known
   or discovered.  For example, when *NET peers share a common
   underlying link or a fixed path with a known larger MTU, the OAL
   source can set path MPS to this larger size (i.e., greater than 576
   bytes) as long as the *NET peer reassembles before re-encapsulating
   and forwarding (while re-fragmenting if necessary).  Also, if the OAL
   source has a way of knowing the maximum *NET encapsulation size for
   all SRT segments along the path it may be able to increase path MPS
   to reserve additional room for payload data.  Even when OAL header
   compression is used, the OAL source must include the uncompressed OAL

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   header size in its path MPS calculation since it may need to include
   a full header at any time.

   The OAL source can also optimistically set a larger path MPS and/or
   actively probe individual OAL destinations to discover larger sizes
   using packetization layer probes in a similar fashion as
   [RFC4821][RFC8899], but care must be taken to avoid setting static
   values for dynamically changing paths leading to black holes.  The
   probe involves sending an OAL packet larger than the current path MPS
   and receiving a small acknowledgement response (with the possible
   receipt of link-layer error message when a probe is lost).  For this
   purpose, the OAL source can send an NS message with one or more OMNI
   options with large PadN sub-options (see: Section 12) in order to
   receive a small NA response from the OAL destination.  While
   observing the minimum MPS will always result in robust and secure
   behavior, the OAL source should optimize path MPS values when more
   efficient utilization may result in better performance (e.g. for
   wireless aviation data links).  The OAL source should maintain
   separate path MPS values for each (source, target) underlying
   interface pair for the same OAL destination, since different
   underlying interface pairs may support differing path MPS values.

   When the OAL source performs fragmentation, it SHOULD produce the
   minimum number of non-overlapping fragments under current MPS
   constraints, where each non-final fragment MUST be at least as large
   as the minimum MPS, while the final fragment MAY be smaller.  The OAL
   source also converts all original IP packets no larger than the
   current MPS into "atomic fragments" by including a Fragment Header
   with Fragment Offset and More Fragments both set to 0.

   For each fragment produced, the OAL source writes an ordinal number
   for the fragment into the Reserved field in the IPv6 Fragment Header.
   Specifically, the OAL source writes the ordinal number '0' for the
   first fragment, '1' for the second fragment, '2' for the third
   fragment, etc. up to and including the final fragment.  Since the
   minMPS is 400 and the MTU is 9180, the OAL source will produce at
   most 23 fragments for each OAL packet; the OAL destination therefore
   unconditionally discards any fragments with an ordinal number larger
   than 22.

   The OAL source finally encapsulates the fragments in *NET headers to
   form carrier packets and forwards them over an underlying interface,
   while retaining the fragments and their ordinal numbers (i.e., #0,
   #1, #2, etc.) for a brief period to support link-layer
   retransmissions (see: Section 6.7).  OAL fragment and carrier packet
   formats are shown in Figure 6.

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        +----------+----------------+
        |OAL Header|     Frag #0    |
        +----------+----------------+
            +----------+----------------+
            |OAL Header|     Frag #1    |
            +----------+----------------+
                +----------+----------------+
                |OAL Header|     Frag #2    |
                +----------+----------------+
                                  ....
                    +----------+----------------+----+
                    |OAL Header|   Frag #(N-1)  |Csum|
                    +----------+----------------+----+
        a) OAL fragmentation (Csum in final fragment)

        +----------+-----+-----+-----+-----+-----+----+
        |OAL Header|      Original IP packet     |Csum|
        +----------+-----+-----+-----+-----+-----+----+
        b) An OAL atomic fragment

        +--------+----------+----------------+
        |*NET Hdr|OAL Header|     Frag #i    |
        +--------+----------+----------------+
        c) OAL carrier packet after *NET encapsulation

                Figure 6: OAL Fragments and Carrier Packets

6.2.  OAL *NET Encapsulation and Re-Encapsulation

   The OAL source or intermediate node encapsulates each OAL fragment
   (with either full or compressed headers) in *NET encapsulation
   headers to create a carrier packet.  The OAL source or intermediate
   node (i.e., the *NET source) includes a UDP header as the innermost
   sublayer if NAT traversal and/or packet filtering middlebox traversal
   are required; otherwise, the *NET source includes either a full or
   compressed IP header or a true L2 header (e.g., such as for Ethernet-
   compatible links).  The *NET source then appends any additional
   encapsulation sublayer headers necessary and presents the resulting
   carrier packet to an underlying interface, where the underlying
   network conveys it to a next-hop OAL intermediate node or destination
   (i.e., the *NET destination).

   The *NET source encapsulates the OAL information immediately
   following the *NET innermost sublayer header.  If the first four bits
   of the encapsulated OAL information following the innermost sublayer
   header encode the value '6', the information must include an

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   uncompressed IPv6 header followed by any IPv6 extension headers
   followed by upper layer protocol headers and data.  Otherwise, the
   first four bits include a "Type" value, and the OAL information
   appears in an alternate format as specified in Section 6.4).
   Alternate formats for Types '0' and '1' are currently specified,
   while Type value '4' is permanently reserved and all other values are
   reserved for future use.  Carrier packets that contain an unsupported
   Type value are unconditionally dropped.

   The OAL node prepares the innermost *NET encapsulation header for OAL
   packets as follows:

   o  For UDP, the *NET source sets the UDP source port to 8060 (i.e.,
      the port number reserved for AERO/OMNI).  When the *NET
      destination is a Proxy/Server or Bridge, the *NET source sets the
      UDP destination port to 8060; otherwise, the *NET source sets the
      UDP destination port to its cached port number value for the peer.
      The *NET source finally sets the UDP Length the same as specified
      in [RFC0768].

   o  For IP encapsulation, the IP protocol number or Next Header is set
      to TBD1 as the Internet Protocol number for OMNI (see: IANA
      Considerations).  For IPv4, the *NET source sets the Total Length
      the same as specified in [RFC0791]; for IPv6, the *NET source sets
      the Payload Length the same as specified in [RFC8200].

   o  For encapsulations over Ethernet-compatible L2s, the EtherType is
      set to TBD2 as the EtherType number for OMNI (see: IANA
      Considerations).  Since the Ethernet header does not include a
      length field, for the OMNI EtherType the Ethernet header is
      followed by a two-octet length field followed immediately by the
      encapsulated OAL information.  The length field encodes the length
      in octets (in network byte order) of the information following the
      Ethernet header including the length field, but excluding the
      Ethernet trailer.

   When a *NET source includes a UDP header, it SHOULD calculate and
   include a UDP checksum in carrier packets with full OAL headers to
   prevent mis-delivery, and MAY disable UDP checksums in carrier
   packets with compressed OAL headers (see: Section 6.4).  If the *NET
   source discovers that a path is dropping carrier packets with UDP
   checksums disabled, it should enable UDP checksums in future carrier
   packets sent to the same *NET destination.  If the *NET source
   discovers that a path is dropping carrier packets that do not include
   a UDP header, it should include a UDP header in future carrier
   packets.

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   When a *NET source sends carrier packets with compressed OAL headers
   and with UDP checksums disabled, mis-delivery due to corruption of
   the 4-octet Multilink Forwarding Vector Index (MFVI) is possible but
   unlikely since the corrupted index would somehow have to match valid
   state in the (sparsely-populated) Multilink Forwarding Information
   Based (MFIB).  In the unlikely event that a match occurs, an OAL
   destination may receive a mis-delivered carrier packet but can
   immediately reject packet with an incorrect Identification.  If the
   Identification value is somehow accepted, the OAL destination may
   submit the mis-delivered carrier packet to the reassembly cache where
   it will most likely be rejected due to incorrect reassembly
   parameters.  If a reassembly that includes the mis-delivered carrier
   packets somehow succeeds (or, for atomic fragments) the OAL
   destination will verify the OAL checksum to detect corruption.
   Finally, any spurious data that somehow eludes all prior checks will
   be detected and rejected by end-to-end upper layer security.  See:
   [RFC6935][RFC6936] for further discussion.

   For *NET encapsulations over IP, when the *NET source is also the OAL
   source it next copies the "Type of Service/Traffic Class" [RFC2983]
   and "Congestion Experienced" [RFC3168] values in the OAL header into
   the corresponding fields in the *NET IP header, then (for IPv6) set
   the *NET IPv6 header "Flow Label" as specified in [RFC6438].  The
   *NET source then sets the *NET IP TTL/Hop Limit the same as for any
   host (i.e., it does not copy the Hop Limit value from the OAL header)
   and finally sets the source and destination IP addresses to direct
   the carrier packet to the next hop.  For carrier packets undergoing
   re-encapsulation, the OAL intermediate node *NET source decrements
   the OAL header Hop Limit and discards the carrier packet if the value
   reaches 0.  The *NET source then copies the "Type of Service/Traffic
   Class" and "Congestion Experienced" values from the previous hop *NET
   encapsulation header into the OAL header, then finally sets the
   source and destination IP addresses the same as above.

   Following *NET encapsulation/re-encapsulation, the *NET source sends
   the resulting carrier packets over one or more underlying interfaces.
   The underlying interfaces often connect directly to physical media on
   the local platform (e.g., a laptop computer with WiFi, etc.), but in
   some configurations the physical media may be hosted on a separate
   Local Area Network (LAN) node.  In that case, the OMNI interface can
   establish a Layer-2 VLAN or a point-to-point tunnel (at a layer below
   the underlying interface) to the node hosting the physical media.
   The OMNI interface may also apply encapsulation at the underlying
   interface layer (e.g., as for a tunnel virtual interface) such that
   carrier packets would appear "double-encapsulated" on the LAN; the
   node hosting the physical media in turn removes the LAN encapsulation
   prior to transmission or inserts it following reception.  Finally,
   the underlying interface must monitor the node hosting the physical

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   media (e.g., through periodic keepalives) so that it can convey
   up/down/status information to the OMNI interface.

6.3.  OAL *NET Decapsulation and Reassembly

   When an OMNI interface receives a carrier packet from an underlying
   interface, it discards the *NET encapsulation headers and examines
   the OAL header of the enclosed OAL fragment.  If the OAL fragment is
   addressed to a different node, the OMNI interface (acting as an OAL
   intermediate node) re-encapsulates and forwards as discussed in
   Section 6.2.  If the OAL fragment is addressed to itself, the OMNI
   interface (acting as an OAL destination) accepts or drops the
   fragment based on the (Source, Destination, Identification)-tuple
   and/or integrity checks.

   The OAL destination next drops all non-final OAL fragments smaller
   than the minimum MPS and all fragments that would overlap or leave
   "holes" smaller than the minimum MPS with respect to other fragments
   already received.  The OAL destination updates a checklist of the
   ordinal numbers of each accepted fragment of the same OAL packet
   (i.e., as Frag #0, Frag #1, Frag #2, etc.), then admits the fragments
   into the reassembly cache.  When reassembly is complete, the OAL
   destination next verifies the OAL packet checksum and discards the
   packet if the checksum is incorrect.  If the OAL packet was accepted,
   the OAL destination then removes the OAL header/trailer and delivers
   the original IP packet to the network layer.

   Carrier packets often travel over paths where all links in the path
   include CRC-32 integrity checks for effective hop-by-hop error
   detection for payload sizes up to the OMNI interface MTU [CRC], but
   other paths may traverse links (such as tunnels over IPv4) that do
   not include integrity checks.  The OAL checksum therefore allows OAL
   destinations to detect reassembly misassociation splicing errors and/
   or carrier packet corruption caused by unprotected links [CKSUM].

   The OAL checksum also provides algorithmic diversity with respect to
   both lower layer CRCs and upper layer Internet checksums as part of a
   complimentary multi-layer integrity assurance architecture.  Any
   corruption not detected by lower layer integrity checks is therefore
   very likely to be detected by upper layer integrity checks that use
   diverse algorithms.

6.4.  OAL Header Compression

   OAL sources that send carrier packets with full OAL headers include a
   CRH-32 extension for segment-by-segment forwarding based on a
   Multilink Forwarding Information Base (MFIB) in each OAL intermediate
   node.  OAL source, intermediate and destination nodes can instead

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   establish header compression state through IPv6 ND NS/NA message
   exchanges.  After an initial NS/NA exchange, OAL nodes can apply OAL
   Header Compression to significantly reduce encapsulation overhead.

   Each OAL node establishes MFIB soft state entries known as Multilink
   Forwarding Vectors (MVFs) which support both carrier packet
   forwarding and OAL header compression/decompression.  For OAL
   sources, each MFV is referenced by a single MFV Index (MFVI) that
   provides compression/decompression and forwarding context for the
   next hop.  For OAL destinations, the MFV is referenced by a single
   MFVI that provides context for the previous hop.  For OAL
   intermediate nodes, the MFV is referenced by two MFVIs - one for the
   previous hop and one for the next hop.

   When an OAL node forwards carrier packets to a next hop, it can
   include a full OAL header with a CRH-32 extension containing one or
   more MVFIs.  The OAL node can instead omit significant portions of
   the OAL header (including the CRH-32) while applying OAL header
   compression.  The full or compressed OAL header follows immediately
   after the innermost *NET encapsulation (i.e., UDP, IP or L2) as
   discussed in Section 6.2.  Two OAL compressed header types (Type '0'
   and Type '1') are currently specified below.

   For OAL first-fragments (including atomic fragments), the OAL node
   uses OMNI Compressed Header - Type 0 (OCH-0) as shown in Figure 7:

       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=0 | Traffic Class |           Flow Label                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  | Hop Limit |I|M|      Identification (0-1)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Identification (2-3)    |           MFVI (0-1)          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            MFVI (2-3)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 7: OMNI Compressed Header - Type 0 (OCH-0)

   The format begins with a 4-bit Type field set to 0, and is followed
   by the uncompressed Traffic Class and Flow Label copied from the OAL
   header, followed by a Next Header field set to the protocol number
   for the header immediately following the IPv6 Fragment Header.  The
   Next Header field is then followed by a 6-bit compressed Hop Limit
   field set to the minimum of 63 and the uncompressed OAL IPv6 Hop
   Limit value.  The Hop Limit is then followed by an (I)ndex bit and a
   compressed Fragment Header that includes only the (M)ore Fragments

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   bit and the 4-octet Identification and with all other fields omitted.
   When the I bit is set, the compressed Fragment Header is then
   followed by a 4-octet Multilink Forwarding Vector Index (MFVI);
   otherwise, the MFVI is omitted.

   The OAL fragment body is then included immediately following the
   OCH-0 header, and the *NET header length field is reduced by the
   difference in length between the compressed headers and full-length
   IPv6 and Fragment headers.  The OCH-0 format applies for first
   fragments only, which are always regarded as ordinal fragment 0 even
   though no explicit Ordinal field is included.

   For OAL non-first fragments (i.e., those with non-zero Fragment
   Offsets), the OAL uses OMNI Compressed Header - Type 1 (OCH-1) as
   shown in Figure 8:

       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=1 | Ordinal |I|M|    Fragment Offset      |     Id(0)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Identification (1-3)              |    MFVI(0)    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  MFVI (1-3)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 8: OMNI Compressed Header - Type 1 (OCH-1)

   The format begins with a Type field set to 1 and the IPv6 header is
   omitted entirely.  The Type field is then followed by a compressed
   IPv6 Fragment Header with a 5-bit Ordinal number field, an (I)ndex
   bit, and with ((M)ore Fragments/Fragment Offset/Identification)
   copied from the uncompressed fragment header.  When the I bit is set,
   the compressed Fragment Header is followed by a 4-octet MFVI;
   otherwise the MFVI is omitted (i.e., the same as for OCH-0).

   The OAL fragment body is then included immediately following the
   OCH-1 header, and the *NET header length field is reduced by the
   difference in length between the compressed headers and full-length
   IPv6 and Fragment headers.  The OCH-1 format applies for non-first
   fragments only; therefore, Ordinal is set to a monotonically
   increasing value beginning with 1 for the first non-first fragment, 2
   for the second non-first fragment, etc., up to and including the
   final fragment (with maximum value 22 since there are at most 23
   fragments).

   When an OAL destination or intermediate node receives a carrier
   packet, it determines the length of the encapsulated OAL information

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   by examining the length field of the innermost *NET header, verifies
   that the appropriate *NET header next header field indicates OMNI
   (see: Section 6.2), then examines the first four bits immediately
   following the *NET header.  If the bits contain the value 6, the OAL
   node processes the remainder as an uncompressed OAL header.  If the
   bits contain the value 0 or 1, the OAL node instead processes the
   remainder of the header as an OCH-0 or OCH-1, respectively.

   For carrier packets with OCH-0/1 or full OAL headers addressed to
   itself and with CRH-32 extensions, the OAL node then uses the MFVI to
   locate the cached MFV which determines the next hop.  During
   forwarding, the OAL node changes the MFVI to the cached value for the
   MVF next hop.  If the OAL node is the destination, it instead
   reconstructs the full OAL headers then adds the resulting OAL
   fragment to the reassembly cache if the Identification is acceptable.
   For carrier packets with OCH-1 headers that do not include Traffic
   Class, Flow Label, Next Header or Hop Limit information, the OAL node
   writes the value 0 into those fields when it reconstructs the full
   OAL headers.  The values will be correctly populated during
   reassembly after an OAL first fragment with an OCH-0 or uncompressed
   OAL header arrives.

   Note: OAL header compression does not interfere with checksum
   calculation and verification, which must be applied according to the
   full OAL pseudo-header per Section 6.1 even when compression is used.

   Note: when OAL-in-OAL encapsulation is used, any outer OCH-0/1
   headers or full OAL headers with CRH-32 extensions include an MVFI,
   while the innermost OCH-0/1 header or full OAL header must not
   include an MFVI.

6.5.  OAL-in-OAL Encapsulation

   When an OAL source is unable to forward carrier packets directly to
   an OAL destination without "tunneling" through a pair of OAL
   intermediate nodes, the OAL source must regard the intermediate nodes
   as ingress and egress tunnel endpoints.  This will result in nested
   OAL-in-OAL encapsulation in which the OAL source performs
   fragmentation on the inner OAL packet then forwards the fragments to
   the ingress tunnel endpoint which encapsulates each resulting OAL
   fragment in an additional OAL header/trailer before performing
   fragmentation following encapsulation.

   For example, if the OAL source has an NCE for the OAL destination
   with MFVI 0x2376a7b5 and Identification 0x12345678 and the OAL
   ingress tunnel endpoint has an NCE for the OAL egress tunnel endpoint
   with MFVI 0xacdebf12 and Identification 0x98765432, the OAL source
   prepares the carrier packets using compressed/uncompressed OAL

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   headers that include the MFVI and Identification corresponding to the
   OAL destination and with *NET header information addressed to the
   next hop toward the ingress tunnel endpoint.  When the ingress tunnel
   endpoint receives the carrier packet, it recognizes the current MFVI
   included by the OAL source and determines the correct next hop MFVI.

   The ingress tunnel endpoint then discards the *NET headers from the
   previous hop and encapsulates the original compressed/uncompressed
   OAL header within a second compressed/uncompressed OAL header/trailer
   while including the next-hop MVFI in the outer OAL encapsulation
   header and omitting the MFVI in the inner header.  The ingress tunnel
   endpoint then includes *NET encapsulation headers with destinations
   appropriate for the next hop on the path to the egress tunnel
   endpoint.  The encapsulation appears as shown in Figure 9:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  *NET headers (previous hop)  |   |    *NET headers (next hop)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Original OAL/OCH Hdr      |   |   Encapsulation OAL/OCH Hdr   |
   |        Id=0x12345678          |   |         Id=0x98765432         |
   |       MFVI=0x2376a7b5         |   |        MFVI=0xacdebf12        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |   |      Original OAL/OCH Hdr     |
   |                               |   |         Id=0x12345678         |
   |      Carrier packet data      |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |   |                               |
   |                               |   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |      Carrier packet data      |
   |     Original OAL Checksum     |   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                               |
       Original Carrier packet         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          from OAL source              |     Original OAL Checksum     |
                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                       |   Encapsulation OAL Checksum  |
                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                      Carrier packet following OAL ingress
                                    (re)encapsulation before fragmentation

         Figure 9: Carrier Packet in Carrier Packet Encapsulation

   Note that only a single OAL-in-OAL encapsulation layer is supported,
   and that MFVIs appear only in the outer OAL header (i.e., either
   within a CRH-32 routing header when a full OAL header is used or
   within an OCH-0/1 header).  The inner OAL/OCH header should omit the
   CRH-32 header or set I to 0, respectively.

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   Note that OAL/OCH encapsulation may cause the payloads of OAL packets
   produced by the ingress tunnel endpoint to exceed the minimum MPS by
   a small amount.  If the ingress has assurance that the path to the
   egress will include only links capable of transiting the resulting
   (slightly larger) carrier packets it should forward without further
   fragmentation.  Otherwise, the ingress must perform fragmentation
   following encapsulation to produce two fragments such that the size
   of the first fragment matches the size of the original OAL packet,
   and with the remainder in a second fragment.  The egress tunnel
   endpoint must then reassemble then decapsulate to arrive at the
   original OAL packet which is then subject to further forwarding.

6.6.  OAL Identification Window Maintenance

   The OAL encapsulates each original IP packet as an OAL packet then
   performs fragmentation to produce one or more carrier packets with
   the same 32-bit Identification value.  In environments where spoofing
   is not considered a threat, OMNI interfaces send OAL packets with
   Identifications beginning with an unpredictable Initial Send Sequence
   (ISS) value [RFC7739] monotonically incremented (modulo 2**32) for
   each successive OAL packet sent to either a specific neighbor or to
   any neighbor.  (The OMNI interface may later change to a new
   unpredictable ISS value as long as the Identifications are assured
   unique within a timeframe that would prevent the fragments of a first
   OAL packet from becoming associated with the reassembly of a second
   OAL packet.)  In other environments, OMNI interfaces should maintain
   explicit per-neighbor send and receive windows to detect and exclude
   spurious carrier packets that might clutter the reassembly cache as
   discussed below.

   OMNI interface neighbors use TCP-like synchronization to maintain
   windows with unpredictable ISS values incremented (modulo 2**32) for
   each successive OAL packet and re-negotiate windows often enough to
   maintain an unpredictable profile.  OMNI interface neighbors exchange
   IPv6 ND messages with OMNI options that include TCP-like information
   fields to manage streams of OAL packets instead of streams of octets.
   As a link-layer service, the OAL provides low-persistence best-effort
   retransmission with no mitigations for duplication, reordering or
   deterministic delivery.  Since the service model is best-effort and
   only control message sequence numbers are acknowledged, OAL nodes can
   select unpredictable new initial sequence numbers outside of the
   current window without delaying for the Maximum Segment Lifetime
   (MSL).

   OMNI interface neighbors maintain current and previous window state
   in IPv6 ND neighbor cache entries (NCEs) to support dynamic rollover
   to a new window while still sending OAL packets and accepting carrier
   packets from the previous windows.  Each NCE is indexed by the

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   neighbor's LLA, which must also match the ULA used for OAL
   encapsulation.  OMNI interface neighbors synchronize windows through
   asymmetric and/or symmetric IPv6 ND message exchanges.  When a node
   receives an IPv6 ND message with new window information, it resets
   the previous window state based on the current window then resets the
   current window based on new and/or pending information.

   The IPv6 ND message OMNI option header includes TCP-like information
   fields including Sequence Number, Acknowledgement Number, Window and
   flags (see: Section 12).  OMNI interface neighbors maintain the
   following TCP-like state variables in the NCE:

       Send Sequence Variables (current, previous and pending)

         SND.NXT - send next
         SND.WND - send window
         ISS     - initial send sequence number

       Receive Sequence Variables (current and previous)

         RCV.NXT - receive next
         RCV.WND - receive window
         IRS     - initial receive sequence number

   OMNI interface neighbors "OAL A" and "OAL B" exchange IPv6 ND
   messages per [RFC4861] with OMNI options that include TCP-like
   information fields.  When OAL A synchronizes with OAL B, it maintains
   both a current and previous SND.WND beginning with a new
   unpredictable ISS and monotonically increments SND.NXT for each
   successive OAL packet transmission.  OAL A initiates synchronization
   by including the new ISS in the Sequence Number of an authentic IPv6
   ND message with the SYN flag set and with Window set to M (up to
   2**24) as a tentative receive window size while creating a NCE in the
   INCOMPLETE state if necessary.  OAL A caches the new ISS as pending,
   uses the new ISS as the Identification for OAL encapsulation, then
   sends the resulting OAL packet to OAL B and waits up to RetransTimer
   milliseconds to receive an IPv6 ND message response with the ACK flag
   set (retransmitting up to MAX_UNICAST_SOLICIT times if necessary).

   When OAL B receives the SYN, it creates a NCE in the STALE state if
   necessary, resets its RCV variables, caches the tentative (send)
   window size M, and selects a (receive) window size N (up to 2**24) to
   indicate the number of OAL packets it is willing to accept under the
   current RCV.WND.  (The RCV.WND should be large enough to minimize
   control message overhead yet small enough to provide an effective
   filter for spurious carrier packets.)  OAL B then prepares an IPv6 ND
   message with the ACK flag set, with the Acknowledgement Number set to
   OAL A's next sequence number, and with Window set to N.  Since OAL B

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   does not assert an ISS of its own, it uses the IRS it has cached for
   OAL A as the Identification for OAL encapsulation then sends the ACK
   to OAL A.

   When OAL A receives the ACK, it notes that the Identification in the
   OAL header matches its pending ISS.  OAL A then sets the NCE state to
   REACHABLE and resets its SND variables based on the Window size and
   Acknowledgement Number (which must include the sequence number
   following the pending ISS).  OAL A can then begin sending OAL packets
   to OAL B with Identification values within the (new) current SND.WND
   for up to ReachableTime milliseconds or until the NCE is updated by a
   new IPv6 ND message exchange.  This implies that OAL A must send a
   new SYN before sending more than N OAL packets within the current
   SND.WND, i.e., even if ReachableTime is not nearing expiration.
   After OAL B returns the ACK, it accepts carrier packets received from
   OAL A within either the current or previous RCV.WND as well as any
   new authentic NS/RS SYN messages received from OAL A even if outside
   the windows.

   OMNI interface neighbors can employ asymmetric window synchronization
   as described above using two independent (SYN -> ACK) exchanges
   (i.e., a four-message exchange), or they can employ symmetric window
   synchronization using a modified version of the TCP three-way
   handshake as follows:

   o  OAL A prepares a SYN with an unpredictable ISS not within the
      current SND.WND and with Window set to M as a tentative receive
      window size.  OAL A caches the new ISS and Window size as pending
      information, uses the pending ISS as the Identification for OAL
      encapsulation, then sends the resulting OAL packet to OAL B and
      waits up to RetransTimer milliseconds to receive an ACK response
      (retransmitting up to MAX_UNICAST_SOLICIT times if necessary).

   o  OAL B receives the SYN, then resets its RCV variables based on the
      Sequence Number while caching OAL A's tentative receive Window
      size M and a new unpredictable ISS outside of its current window
      as pending information.  OAL B then prepares a response with
      Sequence Number set to the pending ISS and Acknowledgement Number
      set to OAL A's next sequence number.  OAL B then sets both the SYN
      and ACK flags, sets Window to N and sets the OPT flag according to
      whether an explicit concluding ACK is optional or mandatory.  OAL
      B then uses the pending ISS as the Identification for OAL
      encapsulation, sends the resulting OAL packet to OAL A and waits
      up to RetransTimer milliseconds to receive an acknowledgement
      (retransmitting up to MAX_UNICAST_SOLICIT times if necessary).

   o  OAL A receives the SYN/ACK, then resets its SND variables based on
      the Acknowledgement Number (which must include the sequence number

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      following the pending ISS) and OAL B's advertised Window N.  OAL A
      then resets its RCV variables based on the Sequence Number and
      marks the NCE as REACHABLE.  If the OPT flag is clear, OAL A next
      prepares an immediate solicited NA message with the ACK flag set,
      the Acknowledgement Number set to OAL B's next sequence number,
      with Window set a value that may be the same as or different than
      M, and with the OAL encapsulation Identification to SND.NXT, then
      sends the resulting OAL packet to OAL B.  If the OPT flag is set
      and OAL A has OAL packets queued to send to OAL B, it can
      optionally begin sending their carrier packets under the (new)
      current SND.WND as implicit acknowledgements instead of returning
      an explicit ACK.  In that case, the tentative Window size M
      becomes the current receive window size.

   o  OAL B receives the implicit/explicit acknowledgement(s) then
      resets its SND state based on the pending/advertised values and
      marks the NCE as REACHABLE.  If OAL B receives an explicit
      acknowledgement, it uses the advertised Window size and abandons
      the tentative size.  (Note that OAL B sets the OPT flag in the
      SYN/ACK to assert that it will interpret timely receipt of carrier
      packets within the (new) current window as an implicit
      acknowledgement.  Potential benefits include reduced delays and
      control message overhead, but use case analysis is outside the
      scope of this specification.)

   Following synchronization, OAL A and OAL B hold updated NCEs and can
   exchange OAL packets with Identifications set to SND.NXT while the
   state remains REACHABLE and there is available window capacity.
   Either neighbor may at any time send a new SYN to assert a new ISS.
   For example, if OAL A's current SND.WND for OAL B is nearing
   exhaustion and/or ReachableTime is nearing expiration, OAL A
   continues to send OAL packets under the current SND.WND while also
   sending a SYN with a new unpredictable ISS.  When OAL B receives the
   SYN, it resets its RCV variables and may optionally return either an
   asymmetric ACK or a symmetric SYN/ACK to also assert a new ISS.
   While sending SYNs, both neighbors continue to send OAL packets with
   Identifications set to the current SND.NXT then reset the SND
   variables after an acknowledgement is received.

   While the optimal symmetric exchange is efficient, anomalous
   conditions such as receipt of old duplicate SYNs can cause confusion
   for the algorithm as discussed in Section 3.4 of [RFC0793].  For this
   reason, the OMNI option header includes an RST flag which OAL nodes
   set in solicited NA responses to ACKs received with incorrect
   acknowledgement numbers.  The RST procedures (and subsequent
   synchronization recovery) are conducted exactly as specified in
   [RFC0793].

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   OMNI interfaces may set the PNG ("ping") flag when a reachability
   confirmation outside the context of the IPv6 ND protocol is needed
   (OMNI interfaces therefore most often set the PNG flag in
   advertisement messages and ignore it in solicitation messages).  When
   an OMNI interface receives a PNG, it returns an unsolicited NA (uNA)
   ACK with the PNG message Identification in the Acknowledgment, but
   without updating RCV state variables.  OMNI interfaces return unicast
   uNA ACKs even for multicast PNG destination addresses, since OMNI
   link multicast is based on unicast emulation.

   OMNI interfaces that employ the window synchronization procedures
   described above observe the following requirements:

   o  OMNI interfaces MUST select new unpredictable ISS values that are
      at least a full window outside of the current SND.WND.

   o  OMNI interfaces MUST set the initial SYN message Window field to a
      tentative value to be used only if no concluding NA ACK is sent.

   o  OMNI interfaces that receive advertisements with the PNG and/or
      SYN flag set MUST NOT set the PNG and/or SYN flag in uNA
      responses.

   o  OMNI interfaces that send advertisements with the PNG and/or SYN
      flag set MUST ignore uNA responses with the PNG and/or SYN flag
      set.

   o  OMNI interfaces MUST send IPv6 ND messages used for window
      synchronization securely while using unpredictable initial
      Identification values until synchronization is complete.

   Note: Although OMNI interfaces employ TCP-like window synchronization
   and support uNA ACK responses to SYNs and PNGs, all other aspects of
   the IPv6 ND protocol (e.g., control message exchanges, NCE state
   management, timers, retransmission limits, etc.) are honored exactly
   per [RFC4861].

   Note: Recipients of OAL-encapsulated IPv6 ND messages index the NCE
   based on the ULA source address, which also determines the carrier
   packet Identification window.  However, IPv6 ND messages may contain
   an LLA source address that does not match the ULA source address when
   the recipient acts as a proxy.

   Note: OMNI interface neighbors apply the same send and receive
   windows for all of their (multilink) underlying interface pairs that
   exchange carrier packets.  Each interface pair represents a distinct
   underlying network path, and the set of paths traversed may be highly
   diverse when multiple interface pairs are used.  OMNI intermediate

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   nodes therefore SHOULD NOT cache window synchronization parameters in
   IPv6 ND messages they forward since there is no way to ensure
   network-wide middlebox state consistency.

6.7.  OAL Fragment Retransmission

   When the OAL source sends carrier packets to an OAL destination, it
   should cache recently sent packets in case timely best-effort
   selective retransmission is requested.  The OAL destination in turn
   maintains a checklist for the (Source, Destination, Identification)-
   tuple of recently received carrier packets and notes the ordinal
   numbers of OAL packet fragments already received (i.e., as Frag #0,
   Frag #1, Frag #2, etc.).  The timeframe for maintaining the OAL
   source and destination caches determines the link persistence (see:
   [RFC3366]).

   If the OAL destination notices some fragments missing after most
   other fragments within the same link persistence timeframe have
   already arrived, it may issue an Automatic Repeat Request (ARQ) with
   Selective Repeat (SR) by sending a uNA message to the OAL source.
   The OAL destination creates a uNA message with an OMNI option with
   one or more Fragmentation Report sub-options that include a list of
   (Identification, Bitmap)-tuples for fragments received and missing
   from this OAL source (see: Section 12).  The OAL destination includes
   an authentication signature if necessary, performs OAL encapsulation
   (with the its own address as the OAL source and the source address of
   the message that prompted the uNA as the OAL destination) and sends
   the message to the OAL source.

   When the OAL source receives the uNA message, it authenticates the
   message then examines the Fragmentation Report.  For each (Source,
   Destination, Identification)-tuple, the OAL source determines whether
   it still holds the corresponding carrier packets in its cache and
   retransmits any for which the Bitmap indicates a loss event.  For
   example, if the Bitmap indicates that ordinal fragments #3, #7, #10
   and #13 from the OAL packet with Identification 0x12345678 are
   missing the OAL source only retransmits carrier packets containing
   those fragments.  When the OAL destination receives the retransmitted
   carrier packets, it admits the enclosed fragments into the reassembly
   cache and updates its checklist.  If some fragments are still
   missing, the OAL destination may send a small number of additional
   uNA ARQ/SRs within the link persistence timeframe.

   The OAL therefore provides a link-layer low persistence ARQ/SR
   service consistent with [RFC3366] and Section 8.1 of [RFC3819].  The
   service provides the benefit of timely best-effort link-layer
   retransmissions which may reduce packet loss and avoid some
   unnecessary end-to-end delays.  This best-effort network-based

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   service therefore compliments higher layer end-to-end protocols
   responsible for true reliability.

6.8.  OAL MTU Feedback Messaging

   When the OMNI interface forwards original IP packets from the network
   layer, it invokes the OAL and returns internally-generated ICMPv4
   Fragmentation Needed [RFC1191] or ICMPv6 Path MTU Discovery (PMTUD)
   Packet Too Big (PTB) [RFC8201] messages as necessary.  This document
   refers to both of these ICMPv4/ICMPv6 message types simply as "PTBs",
   and introduces a distinction between PTB "hard" and "soft" errors as
   discussed below.

   Ordinary PTB messages with ICMPv4 header "unused" field or ICMPv6
   header Code field value 0 are hard errors that always indicate that a
   packet has been dropped due to a real MTU restriction.  In
   particular, the OAL source drops the packet and returns a PTB hard
   error if the packet exceeds the OAL destination MRU.  However, the
   OMNI interface can also forward large original IP packets via OAL
   encapsulation and fragmentation while at the same time returning PTB
   soft error messages (subject to rate limiting) if it deems the
   original IP packet too large according to factors such as link
   performance characteristics, reassembly congestion, etc.  This
   ensures that the path MTU is adaptive and reflects the current path
   used for a given data flow.  The OMNI interface can therefore
   continuously forward packets without loss while returning PTB soft
   error messages recommending a smaller size if necessary.  Original
   sources that receive the soft errors in turn reduce the size of the
   packets they send (i.e., the same as for hard errors), but can soon
   resume sending larger packets if the soft errors subside.

   An OAL source sends PTB soft error messages by setting the ICMPv4
   header "unused" field or ICMPv6 header Code field to the value 1 if a
   original IP packet was deemed lost (e.g., due to reassembly timeout)
   or to the value 2 otherwise.  The OAL source sets the PTB destination
   address to the original IP packet source, and sets the source address
   to one of its OMNI interface addresses that is routable from the
   perspective of the original source.  The OAL source then sets the MTU
   field to a value smaller than the original packet size but no smaller
   than 576 for ICMPv4 or 1280 for ICMPv6, writes the leading portion of
   the original IP packet into the "packet in error" field, and returns
   the PTB soft error to the original source.  When the original source
   receives the PTB soft error, it temporarily reduces the size of the
   packets it sends the same as for hard errors but may seek to increase
   future packet sizes dynamically while no further soft errors are
   arriving.  (If the original source does not recognize the soft error
   code, it regards the PTB the same as a hard error but should heed the

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   retransmission advice given in [RFC8201] suggesting retransmission
   based on normal packetization layer retransmission timers.)

   An OAL destination may experience reassembly cache congestion, and
   can return uNA messages to the OAL source that originated the
   fragments (subject to rate limiting) to advertise reduced hard/soft
   Reassembly Limits and/or to report individual reassembly failures.
   The OAL destination creates a uNA message with an OMNI option
   containing an authentication message sub-option if necessary followed
   optionally by at most one hard and one soft Reassembly Limit sub-
   options with reduced hard/soft values, and with one of them
   optionally including the leading portion an OAL first fragment
   containing the header of an original IP packet whose source must be
   notified (see: Section 12).  The OAL destination encapsulates the
   leading portion of the OAL first fragment (beginning with the OAL
   header) in the "OAL First Fragment" field of sub-option, signs the
   message if an authentication sub-option is included, performs OAL
   encapsulation (with the its own address as the OAL source and the
   source address of the message that prompted the uNA as the OAL
   destination) and sends the message to the OAL source.

   When the OAL source receives the uNA message, it records the new
   hard/soft Reassembly Limit values for this OAL destination if the
   OMNI option includes Reassembly Limit sub-options.  If a hard or soft
   Reassembly Limit sub-option includes an OAL First Fragment, the OAL
   source next sends a corresponding network layer PTB hard or soft
   error to the original source to recommend a smaller size.  For hard
   errors, the OAL source sets the PTB Code field to 0.  For soft
   errors, the OAL source sets the PTB Code field to 1 if the L flag in
   the Reassembly Limit sub-option is 1; otherwise, the OAL source sets
   the Code field to 2.  The OAL source crafts the PTB by extracting the
   leading portion of the original IP packet from the OAL First Fragment
   field (i.e., not including the OAL header) and writes it in the
   "packet in error" field of a PTB with destination set to the original
   IP packet source and source set to one of its OMNI interface
   addresses that is routable from the perspective of the original
   source.  For future transmissions, if the original IP packet is
   larger than the hard Reassembly Limit for this OAL destination the
   OAL source drops the packet and returns a PTB hard error with MTU set
   to the hard Reassembly Limit.  If the packet is no larger than the
   current hard Reassembly Limit but larger than the current soft limit,
   the OAL source can also return a PTB soft error (subject to rate
   limiting) with Code set to 2 and MTU set to the current soft limit
   while still forwarding the packet to the OMNI destination.

   Original sources that receive PTB soft errors can dynamically tune
   the size of the original IP packets they to send to produce the best
   possible throughput and latency, with the understanding that these

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   parameters may change over time due to factors such as congestion,
   mobility, network path changes, etc.  The receipt or absence of soft
   errors should be seen as hints of when increasing or decreasing
   packet sizes may be beneficial.  The OMNI interface supports
   continuous transmission and reception of packets of various sizes in
   the face of dynamically changing network conditions.  Moreover, since
   PTB soft errors do not indicate a hard limit, original sources that
   receive soft errors can begin sending larger packets without waiting
   for the recommended 10 minutes specified for PTB hard errors
   [RFC1191][RFC8201].  The OMNI interface therefore provides an
   adaptive service that accommodates MTU diversity especially well-
   suited for dynamic multilink environments.

6.9.  OAL Requirements

   In light of the above, OAL sources, destinations and intermediate
   nodes observe the following normative requirements:

   o  OAL sources MUST NOT use the OAL to forward original IP packets
      larger than the OMNI interface MTU or the OAL destination hard
      Reassembly Limit (i.e., whether as atomic fragments or multiple
      fragments).

   o  OAL sources MUST forward original IP packets smaller than the
      minimum MPS minus the trailer size as atomic fragments (i.e., and
      not as multiple fragments).

   o  OAL sources MUST produce non-final fragments with payloads no
      smaller than the minimum MPS during fragmentation.

   o  OAL sources MUST NOT produce fragments that include any extension
      headers other than a single Fragment Header.

   o  OAL intermediate nodes SHOULD and OAL destinations MUST
      unconditionally drop any OAL fragments with offset and length that
      would cause the reassembled packet to exceed the OMNI interface
      MRU and/or OAL destination hard Reassembly Limit.

   o  OAL intermediate nodes SHOULD and OAL destinations MUST
      unconditionally drop any non-final OAL fragments with payloads
      smaller than the minimum MPS.

   o  OAL intermediate nodes SHOULD and OAL destinations MUST
      unconditionally drop OAL fragments that include any extension
      headers other than a single Fragment Header.

   o  OAL destinations MUST drop any new OAL fragments with Offset and
      Payload length that would overlap with other fragments and/or

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      leave holes smaller than the minimum MPS between fragments that
      have already been received.

   Note: Under the minimum MPS, ordinary 1500 byte original IP packets
   would require at most 4 OAL fragments, with each non-final fragment
   containing 400 payload bytes and the final fragment containing 302
   payload bytes (i.e., the final 300 bytes of the original IP packet
   plus the 2 octet trailer).  Likewise, maximum-length 9180 byte
   original IP packets would require at most 23 fragments.  For all
   packet sizes, the likelihood of successful reassembly may improve
   when the OMNI interface sends all fragments of the same fragmented
   OAL packet consecutively over the same underlying interface pair
   instead of spread across multiple underlying interface pairs.
   Finally, an assured minimum/path MPS allows continuous operation over
   all paths including those that traverse bridged L2 media with
   dissimilar MTUs.

   Note: Certain legacy network hardware of the past millennium was
   unable to accept packet "bursts" resulting from an IP fragmentation
   event - even to the point that the hardware would reset itself when
   presented with a burst.  This does not seem to be a common problem in
   the modern era, where fragmentation and reassembly can be readily
   demonstrated at line rate (e.g., using tools such as 'iperf3') even
   over fast links on ordinary hardware platforms.  Even so, the OAL
   source could impose an inter-fragment delay while the OAL destination
   is reporting reassembly congestion (see: Section 6.8) and decrease
   the delay when reassembly congestion subsides.

6.10.  OAL Fragmentation Security Implications

   As discussed in Section 3.7 of [RFC8900], there are four basic
   threats concerning IPv6 fragmentation; each of which is addressed by
   effective mitigations as follows:

   1.  Overlapping fragment attacks - reassembly of overlapping
       fragments is forbidden by [RFC8200]; therefore, this threat does
       not apply to the OAL.

   2.  Resource exhaustion attacks - this threat is mitigated by
       providing a sufficiently large OAL reassembly cache and
       instituting "fast discard" of incomplete reassemblies that may be
       part of a buffer exhaustion attack.  The reassembly cache should
       be sufficiently large so that a sustained attack does not cause
       excessive loss of good reassemblies but not so large that (timer-
       based) data structure management becomes computationally
       expensive.  The cache should also be indexed based on the arrival
       underlying interface such that congestion experienced over a

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       first underlying interface does not cause discard of incomplete
       reassemblies for uncongested underlying interfaces.

   3.  Attacks based on predictable fragment identification values - in
       environments where spoofing is possible, this threat is mitigated
       through the use of Identification windows beginning with
       unpredictable values per Section 6.6.  By maintaining windows of
       acceptable Identifications, OAL neighbors can quickly discard
       spurious carrier packets that might otherwise clutter the
       reassembly cache.  The OAL additionally provides an integrity
       check to detect corruption that may be caused by spurious
       fragments received with in-window Identification values.

   4.  Evasion of Network Intrusion Detection Systems (NIDS) - since the
       OAL source employs a robust MPS, network-based firewalls can
       inspect and drop OAL fragments containing malicious data thereby
       disabling reassembly by the OAL destination.  However, since OAL
       fragments may take different paths through the network (some of
       which may not employ a firewall) each OAL destination must also
       employ a firewall.

   IPv4 includes a 16-bit Identification (IP ID) field with only 65535
   unique values such that at high data rates the field could wrap and
   apply to new carrier packets while the fragments of old packets using
   the same IP ID are still alive in the network [RFC4963].  Since
   carrier packets sent via an IPv4 path with DF=0 are normally no
   larger than 576 bytes, IPv4 fragmentation is possible only at small-
   MTU links in the path which should support data rates low enough for
   safe reassembly [RFC3819].  (IPv4 carrier packets larger than 576
   bytes with DF=0 may incur high data rate reassembly errors in the
   path, but the OAL checksum provides OAL destination integrity
   assurance.)  Since IPv6 provides a 32-bit Identification value, IP ID
   wraparound at high data rates is not a concern for IPv6
   fragmentation.

   Fragmentation security concerns for large IPv6 ND messages are
   documented in [RFC6980].  These concerns are addressed when the OMNI
   interface employs the OAL instead of directly fragmenting the IPv6 ND
   message itself.  For this reason, OMNI interfaces MUST NOT send IPv6
   ND messages larger than the OMNI interface MTU, and MUST employ OAL
   encapsulation and fragmentation for IPv6 ND messages larger than the
   minimum/path MPS for this OAL destination.

   Unless the path is secured at the network-layer or below (i.e., in
   environments where spoofing is possible), OMNI interfaces MUST NOT
   send ordinary carrier packets with Identification values outside the
   current window and MUST secure IPv6 ND messages used for address
   resolution or window state synchronization.  OAL destinations SHOULD

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   therefore discard without reassembling any out-of-window OAL
   fragments received over an unsecured path.

6.11.  OAL Super-Packets

   By default, the OAL source includes a 40-byte IPv6 encapsulation
   header for each original IP packet during OAL encapsulation.  The OAL
   source also calculates and appends a 2 octet trailing Checksum field
   then performs fragmentation such that a copy of the 40-byte IPv6
   header plus an 8-byte IPv6 Fragment Header is included in each OAL
   fragment (when a Routing Header is added, the OAL encapsulation
   headers become larger still).  However, these encapsulations may
   represent excessive overhead in some environments.  OAL header
   compression can dramatically reduce the amount of encapsulation
   overhead, however a complimentary technique known as "packing" (see:
   [I-D.ietf-intarea-tunnels]) supports encapsulation of multiple
   original IP packets and/or control messages within a single OAL
   "super-packet".

   When the OAL source has multiple original IP packets to send to the
   same OAL destination with total length no larger than the OAL
   destination MRU, it can concatenate them into a super-packet
   encapsulated in a single OAL header and trailing Checksum field.
   Within the OAL super-packet, the IP header of the first original IP
   packet (iHa) followed by its data (iDa) is concatenated immediately
   following the OAL header, then the IP header of the next original
   packet (iHb) followed by its data (iDb) is concatenated immediately
   following the first original packet, etc. with the trailing Checksum
   field included last.  The OAL super-packet format is transposed from
   [I-D.ietf-intarea-tunnels] and shown in Figure 10:

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                   <------- Original IP packets ------->
                   +-----+-----+
                   | iHa | iDa |
                   +-----+-----+
                         |
                         |     +-----+-----+
                         |     | iHb | iDb |
                         |     +-----+-----+
                         |           |
                         |           |     +-----+-----+
                         |           |     | iHc | iDc |
                         |           |     +-----+-----+
                         |           |           |
                         v           v           v
        +----------+-----+-----+-----+-----+-----+-----+----+
        |  OAL Hdr | iHa | iDa | iHb | iDb | iHc | iDc |Csum|
        +----------+-----+-----+-----+-----+-----+-----+----+
        <--- OAL "Super-Packet" with single OAL Hdr/Csum --->

                    Figure 10: OAL Super-Packet Format

   When the OAL source prepares a super-packet, it applies OAL
   fragmentation and *NET encapsulation then sends the resulting carrier
   packets to the OAL destination.  When the OAL destination receives
   the super-packet it reassembles if necessary, verifies the checksum,
   removes the trailing Checksum field, then regards the remaining OAL
   header Payload Length as the sum of the lengths of all payload
   packets.  The OAL destination then selectively extracts each original
   IP packet (e.g., by setting pointers into the super-packet buffer and
   maintaining a reference count, by copying each packet into a separate
   buffer, etc.) and forwards each packet to the network layer.  During
   extraction, the OAL determines the IP protocol version of each
   successive original IP packet 'j' by examining the four most-
   significant bits of iH(j), and determines the length of the packet by
   examining the rest of iH(j) according to the IP protocol version.

   When an OAL source prepares a super-packet that includes an IPv6 ND
   message with an authentication signature or checksum as the first
   original IP packet (i.e., iHa/iDa), it calculates the authentication
   signature or checksum over the remainder of super-packet up to but
   not including the trailing OAL Checksum field.  Security and
   integrity for forwarding initial protocol data packets in conjunction
   with IPv6 ND messages used to establish NCE state are therefore
   supported.

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6.12.  OAL Bubbles

   OAL sources may send NULL OAL packets known as "bubbles" for the
   purpose of establishing Network Address Translator (NAT) state on the
   path to the OAL destination.  The OAL source prepares a bubble by
   crafting an OAL header with appropriate IPv6 source and destination
   ULAs, with the IPv6 Next Header field set to the value 59 ("No Next
   Header" - see [RFC8200]) and with only the trailing OAL Checksum
   field (i.e., and no protocol data) immediately following the IPv6
   header.

   The OAL source includes a random Identification value then
   encapsulates the OAL packet in *NET headers destined to either the
   mapped address of the OAL destination's first-hop ingress NAT or the
   INADDR of the OAL destination itself.  When the OAL source sends the
   resulting carrier packet, any egress NATs in the path toward the *NET
   destination will establish state based on the activity but the bubble
   will be harmlessly discarded by either an ingress NAT on the path to
   the OAL destination or by the OAL destination itself.

   The bubble concept for establishing NAT state originated in [RFC4380]
   and was later updated by [RFC6081].  OAL bubbles may be employed by
   mobility services such as [I-D.templin-6man-aero].

7.  Frame Format

   When the OMNI interface forwards original IP packets from the network
   layer it first invokes the OAL to create OAL packets/fragments if
   necessary, then includes any *NET encapsulations and finally engages
   the native frame format of the underlying interface.  For example,
   for Ethernet-compatible interfaces the frame format is specified in
   [RFC2464], for aeronautical radio interfaces the frame format is
   specified in standards such as ICAO Doc 9776 (VDL Mode 2 Technical
   Manual), for various forms of tunnels the frame format is found in
   the appropriate tunneling specification, etc.

   See Figure 2 for a map of the various *NET layering combinations
   possible.  For any layering combination, the final layer (e.g., UDP,
   IP, Ethernet, etc.) must have an assigned number and frame format
   representation that is compatible with the selected underlying
   interface.

8.  Link-Local Addresses (LLAs)

   OMNI interfaces assign IPv6 Link-Local Addresses (LLAs) through pre-
   service administrative actions.  Clients assign "MNP-LLAs" with
   interface identifiers that embed the Client's unique MNP, while

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   Proxy/Servers assign "ADM-LLAs" that include an administrative ID
   guaranteed to be unique on the link.  LLAs are configured as follows:

   o  IPv6 MNP-LLAs encode the most-significant 64 bits of an MNP within
      the least-significant 64 bits of the IPv6 link-local prefix
      fe80::/64, i.e., in the LLA "interface identifier" portion.  The
      prefix length for the LLA is determined by adding 64 to the MNP
      prefix length.  For example, for the MNP 2001:db8:1000:2000::/56
      the corresponding MNP-LLA prefix is fe80::2001:db8:1000:2000/120.
      (The base MNP-LLA for each "/N" prefix sets the final 128-N bits
      to 0, but all MNP-LLAs that match the prefix are also accepted.)
      Non-MNP routes are also represented the same as for MNP-LLAs, but
      include a GUA prefix that is not properly covered by the MSP.

   o  IPv4-mapped MNP-LLAs are constructed as fe80::ffff:[IPv4], i.e.,
      the interface identifier consists of 16 '0' bits, followed by 16
      '1' bits, followed by a 32bit IPv4 address/prefix.  The prefix
      length for the LLA is determined by adding 96 to the MNP prefix
      length.  For example, the IPv4-mapped MNP-LLA for 192.0.2.0/24 is
      fe80::ffff:192.0.2.0/120, also written as
      fe80::ffff:c000:0200/120.  (The base MNP-LLA for each "/N" prefix
      sets the final 128-N bits to 0, but all MNP-LLAs that match the
      prefix are also accepted.)

   o  ADM-LLAs are assigned to Proxy/Servers (and possibly other SRT
      infrastructure elements) and MUST be managed for uniqueness.  The
      upper 96 bits of the LLA encode the prefix fe80::/96, and the
      lower 32 bits include a unique integer "MSID" value between
      0x00000001 and 0xfeffffff, e.g., as in fe80::1, fe80::2, fe80::3,
      etc., fe80::feffffff.  The ADM-LLA prefix length is determined by
      adding 96 to the MSID prefix length.  For example, if the prefix
      length for MSID 0x10012001 is 16 then the ADM-LLA prefix length is
      set to 112 and the LLA is written as fe80::1001:2001/112.  The
      "zero" address for each ADM-LLA prefix is the Subnet-Router
      anycast address for that prefix [RFC4291]; for example, the
      Subnet-Router anycast address for fe80::1001:2001/112 is simply
      fe80::1001:2000.  The MSID range 0xff000000 through 0xffffffff is
      reserved for future use.

   Since the prefix 0000::/8 is "Reserved by the IETF" [RFC4291], no
   MNPs can be allocated from that block ensuring that there is no
   possibility for overlap between the different MNP- and ADM-LLA
   constructs discussed above.

   Since MNP-LLAs are based on the distribution of administratively
   assured unique MNPs, and since ADM-LLAs are guaranteed unique through
   administrative assignment, OMNI interfaces set the autoconfiguration
   variable DupAddrDetectTransmits to 0 [RFC4862].

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   Note: If future protocol extensions relax the 64-bit boundary in IPv6
   addressing, the additional prefix bits of an MNP could be encoded in
   bits 16 through 63 of the MNP-LLA.  (The most-significant 64 bits
   would therefore still be in bits 64-127, and the remaining bits would
   appear in bits 16 through 48.)  However, the analysis provided in
   [RFC7421] suggests that the 64-bit boundary will remain in the IPv6
   architecture for the foreseeable future.

   Note: Even though this document honors the 64-bit boundary in IPv6
   addressing, it specifies prefix lengths longer than /64 for routing
   purposes.  This effectively extends IPv6 routing determination into
   the interface identifier portion of the IPv6 address, but it does not
   redefine the 64-bit boundary.  Modern routing protocol
   implementations honor IPv6 prefixes of all lengths, up to and
   including /128.

9.  Unique-Local Addresses (ULAs)

   OMNI domains use IPv6 Unique-Local Addresses (ULAs) as the source and
   destination addresses in OAL packet IPv6 encapsulation headers.  ULAs
   are only routable within the scope of a an OMNI domain, and are
   derived from the IPv6 Unique Local Address prefix fc00::/7 followed
   by the L bit set to 1 (i.e., as fd00::/8) followed by a 40-bit
   pseudo-random Global ID to produce the prefix [ULA]::/48, which is
   then followed by a 16-bit Subnet ID then finally followed by a 64 bit
   Interface ID as specified in Section 3 of [RFC4193].  All nodes in
   the same OMNI domain configure the same 40-bit Global ID as the OMNI
   domain identifier.  The statistic uniqueness of the 40-bit pseudo-
   random Global ID allows different OMNI domains to be joined together
   in the future without requiring renumbering.

   Each OMNI link instance is identified by a 16-bit Subnet ID value
   between 0x0000 and 0xfeff in bits 48-63 of [ULA]::/48.  The Subnet ID
   values 0xff00 through 0xfffe are reserved for future use, while
   0xffff denotes the presence of a Temporary ULA (see below).  For
   example, OMNI ULAs associated with instance 0 are configured from the
   prefix [ULA]:0000::/64, instance 1 from [ULA]:0001::/64, instance 2
   from [ULA]:0002::/64, etc.  ULAs and their associated prefix lengths
   are configured in correspondence with LLAs through stateless prefix
   translation where "MNP-ULAs" are assigned in correspondence to MNP-
   LLAs and "ADM-ULAs" are assigned in correspondence to ADM-LLAs.  For
   example, for OMNI link instance [ULA]:1010::/64:

   o  the MNP-ULA corresponding to the MNP-LLA fe80::2001:db8:1:2 with a
      56-bit MNP length is derived by copying the lower 64 bits of the
      LLA into the lower 64 bits of the ULA as
      [ULA]:1010:2001:db8:1:2/120 (where, the ULA prefix length becomes
      64 plus the IPv6 MNP length).

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   o  the MNP-ULA corresponding to fe80::ffff:192.0.2.0 with a 28-bit
      MNP length is derived by simply writing the LLA interface ID into
      the lower 64 bits as [ULA]:1010:0:ffff:192.0.2.0/124 (where, the
      ULA prefix length is 64 plus 32 plus the IPv4 MNP length).

   o  the ADM-ULA corresponding to fe80::1000/112 is simply
      [ULA]:1010::1000/112.

   o  the ADM-ULA corresponding to fe80::/128 is simply
      [ULA]:1010::/128.

   o  etc.

   The ULA presents an IPv6 address format that is routable within the
   OMNI routing system and can be used to convey link-scoped IPv6 ND
   messages across multiple hops using IPv6 encapsulation [RFC2473].
   The OMNI link extends across one or more underling Internetworks to
   include all Proxy/Servers.  All Clients are also considered to be
   connected to the OMNI link, however unnecessary encapsulations are
   omitted whenever possible to conserve bandwidth (see: Section 14).

   Temporary ULAs are constructed per [RFC8981] based on the prefix
   [ULA]:ffff::/64 and used by Clients when they have no other
   addresses.  Temporary ULAs can be used for Client-to-Client
   communications outside the context of any supporting OMNI link
   infrastructure, and can also be used as an initial address while the
   Client is in the process of procuring an MNP.  Temporary ULAs are not
   routable within the OMNI routing system, and are therefore useful
   only for OMNI link "edge" communications.  Temporary ULAs employ
   optimistic DAD principles [RFC4429] since they are probabilistically
   unique.

   Each OMNI link may be subdivided into SRT segments that often
   correspond to different administrative domains or physical
   partitions.  OMNI nodes can use Segment Routing [RFC8402] to support
   efficient forwarding to destinations located in other OMNI link
   segments.  A full discussion of Segment Routing over the OMNI link
   appears in [I-D.templin-6man-aero].

   Note: IPv6 ULAs taken from the prefix fc00::/7 followed by the L bit
   set to 0 (i.e., as fc00::/8) are never used for OMNI OAL addressing,
   however the range could be used for MSP/MNP addressing under certain
   limiting conditions (see: Section 10).

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10.  Global Unicast Addresses (GUAs)

   OMNI domains use IP Global Unicast Address (GUA) prefixes [RFC4291]
   as Mobility Service Prefixes (MSPs) from which Mobile Network
   Prefixes (MNP) are delegated to Clients.  Fixed correspondent node
   networks reachable from the OMNI domain are represented by non-MNP
   GUA prefixes that are not derived from the MSP, but are treated in
   all other ways the same as for MNPs.

   For IPv6, GUA MSPs are assigned by IANA [IPV6-GUA] and/or an
   associated regional assigned numbers authority such that the OMNI
   domain can be interconnected to the global IPv6 Internet without
   causing inconsistencies in the routing system.  An OMNI domain could
   instead use ULAs with the 'L' bit set to 0 (i.e., from the prefix
   fc00::/8)[RFC4193], however this would require IPv6 NAT if the domain
   were ever connected to the global IPv6 Internet.

   For IPv4, GUA MSPs are assigned by IANA [IPV4-GUA] and/or an
   associated regional assigned numbers authority such that the OMNI
   domain can be interconnected to the global IPv4 Internet without
   causing routing inconsistencies.  An OMNI domain could instead use
   private IPv4 prefixes (e.g., 10.0.0.0/8, etc.)  [RFC3330], however
   this would require IPv4 NAT if the domain were ever connected to the
   global IPv4 Internet.  OMNI interfaces advertise IPv4 MSPs into IPv6
   routing systems as IPv4-mapped IPv6 prefixes [RFC4291] (e.g., the
   IPv6 prefix for the IPv4 MSP 192.0.2.0/24 is ::ffff:192.0.2.0/120).

   OMNI interfaces assign the IPv4 anycast address TBD3, and IPv4
   routers that configure OMNI interfaces advertise the prefix TBD3/N
   into the routing system of other networks (see: IANA Considerations).
   OMNI interfaces also configure global IPv6 anycast addresses formed
   according to [RFC3056] as:

   2002:TBD3[32]:MNP[64]:Link_ID[16]

   where TBD3[32] is the 32 bit IPv4 anycast address, MNP[64] encodes an
   MSP zero-padded to 64 bits (if necessary) and Link_ID[16] encodes a
   16 bit value between 0 and 0xfffe that identifies a specific OMNI
   link within an OMNI domain (the Link_ID value 0xffff is an OMNI link
   "anycast" value configured by all OMNI interfaces within the same
   domain).  For example, the OMNI IPv6 anycast address for MSP
   2001:db8::/32 is 2002:TBD3[32]:2001:db8:0:0:Link_ID[16], the OMNI
   IPv6 anycast address for MSP 192.0.2.0/24 is
   2002:TBD3[32]:0000:ffff:c000:0200:Link_ID[16], etc.).

   OMNI interfaces assign OMNI IPv6 anycast addresses, and IPv6 routers
   that configure OMNI interfaces advertise the corresponding prefixes
   into the routing system of other networks.  An OMNI IPv6 anycast

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   prefix is formed the same as for any IPv6 prefix; for example, the
   prefix 2002:TBD3[32]:2001:db8::/80 matches all OMNI IPv6 anycast
   addresses covered by the prefix.  By advertising OMNI IPv6 anycast
   prefixes in this way, OMNI Clients can locate and associate with the
   OMNI domain and/or a specific link within the OMNI domain that
   services the MSP of interest.

   OMNI interfaces use OMNI IPv6 and IPv4 anycast addresses to support
   Service Discovery in the spirit of [RFC7094], i.e., the addresses are
   not intended for use in long-term transport protocol sessions.
   Specific applications for OMNI IPv6 and IPv4 anycast addresses are
   discussed throughout the document as well as in
   [I-D.templin-6man-aero].

11.  Node Identification

   OMNI Clients and Proxy/Servers that connect over open Internetworks
   include a unique node identification value for themselves in the OMNI
   options of their IPv6 ND messages (see: Section 12.2.12).  An example
   identification value alternative is the Host Identity Tag (HIT) as
   specified in [RFC7401], while Hierarchical HITs (HHITs)
   [I-D.ietf-drip-rid] may be more appropriate for certain domains such
   as the Unmanned (Air) Traffic Management (UTM) service for Unmanned
   Air Systems (UAS).  Another example is the Universally Unique
   IDentifier (UUID) [RFC4122] which can be self-generated by a node
   without supporting infrastructure with very low probability of
   collision.

   When a Client is truly outside the context of any infrastructure, it
   may have no MNP information at all.  In that case, the Client can use
   an IPv6 temporary ULA or (H)HIT as an IPv6 source/destination address
   for sustained communications in Vehicle-to-Vehicle (V2V) and
   (multihop) Vehicle-to-Infrastructure (V2I) scenarios.  The Client can
   also propagate the ULA/(H)HIT into the multihop routing tables of
   (collective) Mobile/Vehicular Ad-hoc Networks (MANETs/VANETs) using
   only the vehicles themselves as communications relays.

   When a Client connects via a protected-spectrum ANET, an alternate
   form of node identification (e.g., MAC address, serial number,
   airframe identification value, VIN, etc.) embedded in an LLA/ULA may
   be sufficient.  The Client can then include OMNI "Node
   Identification" sub-options (see: Section 12.2.12) in IPv6 ND
   messages should the need to transmit identification information over
   the network arise.

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12.  Address Mapping - Unicast

   OMNI interfaces maintain a neighbor cache for tracking per-neighbor
   state and use the link-local address format specified in Section 8.
   IPv6 Neighbor Discovery (ND) [RFC4861] messages sent over OMNI
   interfaces without encapsulation observe the native underlying
   interface Source/Target Link-Layer Address Option (S/TLLAO) format
   (e.g., for Ethernet the S/TLLAO is specified in [RFC2464]).  IPv6 ND
   messages sent over OMNI interfaces using encapsulation do not include
   S/TLLAOs, but instead include a new option type that encodes
   encapsulation addresses, interface attributes and other OMNI link
   information.  Hence, this document does not define an S/TLLAO format
   but instead defines a new option type termed the "OMNI option"
   designed for these purposes.  (Note that OMNI interface IPv6 ND
   messages sent without encapsulation may include both OMNI options and
   S/TLLAOs, but the information conveyed in each is mutually
   exclusive.)

   OMNI interfaces prepare IPv6 ND messages that include one or more
   OMNI options (and any other IPv6 ND options) then completely populate
   all option information.  If the OMNI interface includes an
   authentication signature, it sets the IPv6 ND message Checksum field
   to 0 and calculates the authentication signature over the length of
   the entire OAL packet or super-packet (beginning with a pseudo-header
   of the IPv6 ND message IPv6 header up to but not including the
   trailing OAL Checksum field) but does not calculate/include the IPv6
   ND message checksum itself.  Otherwise, the OMNI interface calculates
   the standard IPv6 ND message checksum over the entire OAL packet or
   super-packet and writes the value in the Checksum field noting that
   optimized implementations can verify both the OAL and IPv6 ND message
   checksums in a single pass over the message data.  OMNI interfaces
   verify authentication and/or integrity of each IPv6 ND message
   received according to the specific check(s) included, and process the
   message further only following verification.

   OMNI interface Clients such as aircraft typically have multiple
   wireless data link types (e.g. satellite-based, cellular,
   terrestrial, air-to-air directional, etc.) with diverse performance,
   cost and availability properties.  The OMNI interface would therefore
   appear to have multiple L2 connections, and may include information
   for multiple underlying interfaces in a single IPv6 ND message
   exchange.  OMNI interfaces manage their dynamically-changing
   multilink profiles by including OMNI options in IPv6 ND messages as
   discussed in the following subsections.

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12.1.  The OMNI Option

   The first OMNI option appearing in an IPv6 ND message is formatted as
   shown in Figure 11:

        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    |    Preflen    |N|A|U| Reservd |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Sequence Number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Acknowledgment Number                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S|A|R|O|P|     |                                               |
       |Y|C|S|P|N| Res |                   Window                      |
       |N|K|T|T|G|     |                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                          Sub-Options                          ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 11: OMNI Option Format

   In this format:

   o  Type is set to TBD4 (see: IANA Considerations).

   o  Length is set to the number of 8 octet blocks in the option.  The
      value 0 is invalid, while the values 1 through 255 (i.e., 8
      through 2040 octets, respectively) indicate the total length of
      the OMNI option.

   o  Preflen is an 8 bit field that determines the length of prefix
      associated with an LLA.  Values 0 through 128 specify a valid
      prefix length (if any other value appears the OMNI option must be
      ignored).  For IPv6 ND messages sent from a Client to the MS,
      Preflen applies to the IPv6 source LLA and provides the length
      that the Client is requesting from or asserting to the MS.  For
      IPv6 ND messages sent from the MS to the Client, Preflen applies
      to the IPv6 destination LLA and indicates the length that the MS
      is granting to the Client.  For IPv6 ND messages sent between MS
      endpoints, Preflen provides the length associated with the source/
      target Client MNP that is subject of the ND message.  When an IPv6
      ND RS/RA message sets Preflen to 0, the recipient regards the
      message as a prefix release indication.

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   o  The N/A/U bits are set or cleared in Client RS messages as
      directives to FHS and Hub Proxy/Servers and ignored in all other
      IPv6 ND messages.  When an FHS Proxy/Server forwards or processes
      an RS with the N bit set, it responds directly to NS Neighbor
      Unreachability Detection (NUD) messages by returning NA(NUD)
      replies; otherwise, it forwards NS(NUD) messages to the Client.
      When the Hub Proxy/Server receives an RS with the A bit set, it
      responds directly to NS Address Resolution (AR) messages by
      returning NA(AR) replies; otherwise, it forwards NS(AR) messages
      to the Client.  When the Hub Proxy/Server receives an RS with the
      U bit set, it maintains a Report List of recent NS(AR) message
      sources for this Client and sends uNA messages to all list members
      if any aspects of the Client's underlying interfaces change.
      Proxy/Servers function according to the N/A/U bit settings
      received in the most recent RS message to support dynamic Client
      updates.  In all IPv6 ND messages, the remaining 5 bits contain a
      Reserved field set to 0 on transmission and ignored on reception.

   o  The remaining header fields before "Sub-Options" are modeled from
      the Transmission Control Protocol (TCP) header specified in
      Section 3.1 of [RFC0793] and include a 32 bit Sequence Number
      followed by a 32 bit Acknowledgement Number followed by 8 flags
      bits followed by a 24-bit Window.  The (SYN, ACK, RST) flags are
      used for TCP-like window synchronization, while the TCP (URG, PSH,
      FIN) flags are not used and therefore omitted.  The (OPT, PNG)
      flags are OMNI-specific, and the remaining flags are Reserved.
      Together, these fields support the asymmetric and symmetric OAL
      window synchronization services specified in Section 6.6.

   o  Sub-Options is a Variable-length field padded if necessary such
      that the complete OMNI Option is an integer multiple of 8 octets
      long.  Sub-Options contains zero or more sub-options as specified
      in Section 12.2.

   The OMNI option is included in all OMNI interface IPv6 ND messages;
   the option is processed by receiving interfaces that recognize it and
   otherwise ignored.  If multiple OMNI option instances appear in the
   same IPv6 ND message, only the first option includes the OMNI header
   fields before the Sub-Options while all others are coded as follows:

         0                   1                   2
         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
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
        |     Type      |     Length    | Sub-Options ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The OMNI interface processes all OMNI option instances received in
   the same IPv6 ND message in the consecutive order in which they

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   appear.  The OMNI option(s) included in each IPv6 ND message may
   include full or partial information for the neighbor.  The OMNI
   interface therefore retains the union of the information in the most
   recently received OMNI options in the corresponding NCE.

12.2.  OMNI Sub-Options

   Each OMNI option includes a Sub-Options block containing zero or more
   individual sub-options.  Each consecutive sub-option is concatenated
   immediately following its predecessor.  All sub-options except Pad1
   (see below) are in type-length-value (TLV) format encoded as follows:

         0                   1                   2
         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
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
        | Sub-Type|      Sub-length     | Sub-Option Data ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                       Figure 12: Sub-Option Format

   o  Sub-Type is a 5-bit field that encodes the sub-option type.  Sub-
      option types defined in this document are:

        Sub-Option Name             Sub-Type
        Pad1                           0
        PadN                           1
        Interface Attributes           2
        Multilink Fwding Parameters    3
        Traffic Selector               4
        Geo Coordinates                5
        DHCPv6 Message                 6
        HIP Message                    7
        PIM-SM Message                 8
        Reassembly Limit               9
        Fragmentation Report          10
        Node Identification           11
        ICMPv6 Error                  12
        QUIC-TLS Message              13
        Proxy/Server Departure        14
        Sub-Type Extension            30

                                 Figure 13

      Sub-Types 15-29 are available for future assignment for major
      protocol functions.  Sub-Type 31 is reserved by IANA.

   o  Sub-Length is an 11-bit field that encodes the length of the Sub-
      Option Data in octets.

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   o  Sub-Option Data is a block of data with format determined by Sub-
      Type and length determined by Sub-Length.  Note that each
      individual sub-option may end on an arbitrary octet boundary,
      whereas the OMNI option itself must include padding if necessary
      for 8-octet alignment.

   The OMNI interface codes each sub-option with a 2 octet header that
   includes Sub-Type in the most significant 5 bits followed by Sub-
   Length in the next most significant 11 bits.  Each sub-option encodes
   a maximum Sub-Length value of 2038 octets minus the lengths of the
   OMNI option header and any preceding sub-options.  This allows ample
   Sub-Option Data space for coding large objects (e.g., ASCII strings,
   domain names, protocol messages, security codes, etc.), while a
   single OMNI option is limited to 2040 octets the same as for any IPv6
   ND option.

   The OMNI interface codes initial sub-options in a first OMNI option
   instance and subsequent sub-options in additional instances in the
   same IPv6 ND message in the intended order of processing.  The OMNI
   interface can then code any remaining sub-options in additional IPv6
   ND messages if necessary.  Implementations must observe these size
   limits and refrain from sending IPv6 ND messages larger than the OMNI
   interface MTU.

   The OMNI interface processes all OMNI option Sub-Options received in
   an IPv6 ND message while skipping over and ignoring any unrecognized
   sub-options.  The OMNI interface processes the Sub-Options of all
   OMNI option instances in the consecutive order in which they appear
   in the IPv6 ND message, beginning with the first instance and
   continuing through any additional instances to the end of the
   message.  If an individual sub-option length would cause processing
   to exceed the OMNI option instance and/or IPv6 ND message lengths,
   the OMNI interface accepts any sub-options already processed and
   ignores the remainder of that instance.  The interface then processes
   any remaining OMNI option instances in the same fashion to the end of
   the IPv6 ND message.

   When an OMNI interface includes an authentication sub-option (e.g.,
   see: Section 12.2.8), it MUST appear as the first sub-option of the
   first OMNI option which must appear immediately following the IPv6 ND
   message header.  If the IPv6 ND message includes additional
   authentication sub-options, only the first sub-option is processed
   and all others are ignored.  If the IPv6 ND message is the first
   packet in a combined OAL super-packet, the OMNI interface calculates
   the authentication signature over the entire length of the super-
   packet, i.e., and not just to the end of the IPv6 ND message itself.
   (When no authentication sub-option is included, the OMNI interface

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   instead calculates the IPv6 ND message checksum over the entire
   length of the packet/super-packet.)

   When a Client OMNI interface prepares a secured unicast RS message,
   it includes an Interface Attributes sub-option specific to the
   underlying interface that will transmit the RS (see: Section 12.2.3)
   immediately following the authentication sub-option if present;
   otherwise as the first sub-option of the first OMNI option which must
   appear immediately following the IPv6 ND message header.  When a
   Client OMNI interface prepares a secured unicast NS message, it
   instead includes a Multilink Forwarding Parameters sub-option
   specific to the underlying interface that will transmit the NS (see:
   Section 12.2.4).

   Note: large objects that exceed the maximum Sub-Option Data length
   are not supported under the current specification; if this proves to
   be limiting in practice, future specifications may define support for
   fragmenting large sub-options across multiple OMNI options within the
   same IPv6 ND message (or even across multiple IPv6 ND messages, if
   necessary).

   The following sub-option types and formats are defined in this
   document:

12.2.1.  Pad1

         0
         0 1 2 3 4 5 6 7
        +-+-+-+-+-+-+-+-+
        | S-Type=0|x|x|x|
        +-+-+-+-+-+-+-+-+

                              Figure 14: Pad1

   o  Sub-Type is set to 0.  If multiple instances appear in OMNI
      options of the same message all are processed.

   o  Sub-Type is followed by 3 'x' bits, set to any value on
      transmission (typically all-zeros) and ignored on reception.  Pad1
      therefore consists of 1 octet with the most significant 5 bits set
      to 0, and with no Sub-Length or Sub-Option Data fields following.

   If more than one octet of padding is required, the PadN option,
   described next, should be used, rather than multiple Pad1 options.

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12.2.2.  PadN

         0                   1                   2
         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
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
        | S-Type=1|    Sub-length=N     | N padding octets ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                              Figure 15: PadN

   o  Sub-Type is set to 1.  If multiple instances appear in OMNI
      options of the same message all are processed.

   o  Sub-Length is set to N that encodes the number of padding octets
      that follow.

   o  Sub-Option Data consists of N octets, set to any value on
      transmission (typically all-zeros) and ignored on receipt.

   When a proxy forwards an IPv6 ND message with OMNI options, it can
   employ PadN to cancel any sub-options (other than Pad1) that should
   not be processed by the next hop by simply writing the value '1' over
   the Sub-Type.  When the proxy alters the IPv6 ND message contents in
   this way, any included authentication and integrity checks are
   invalidated.  See: Appendix B for a discussion of IPv6 ND message
   authentication and integrity.

12.2.3.  Interface Attributes

   The Interface Attributes sub-option provides neighbors with
   forwarding information for the multilink conceptual sending algorithm
   discussed in Section 14.  Neighbors use the forwarding information to
   selecting among potentially multiple candidate underlying interfaces
   that can be used to forward carrier packets to the neighbor based on
   factors such as traffic selectors and link quality.  Interface
   Attributes further include link-layer address information to be used
   for either direct INET encapsulation for targets in the local SRT
   segment or spanning tree forwarding for targets in remote SRT
   segments.

   OMNI nodes include Interface Attributes for some/all of a target
   Client's underlying interfaces in NS/NA and uNA messages used to
   publish Client information (see: [I-D.templin-6man-aero]).  At most
   one Interface Attributes sub-option for each distinct omIndex may be
   included; if an NS/NA message includes multiple Interface Attributes
   sub-options for the same omIndex, the first is processed and all
   others are ignored.  OMNI nodes that receive NS/NA messages can use
   all of the included Interface Attributes and/or Traffic Selectors to

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   formulate a map of the prospective target node as well as to seed the
   information to be populated in a Multilink Forwarding Parameters sub-
   option (see: Section 12.2.4).

   OMNI Clients and Proxy/Servers also include Interface Attributes sub-
   options in RS/RA messages used to initialize, discover and populate
   routing and addressing information.  Each RS message MUST contain
   exactly one Interface Attributes sub-option with an omIndex
   corresponding to the Client's underlying interface used to transmit
   the message, and each RA message MUST echo the same Interface
   Attributes sub-option with any (proxyed) information populated by the
   FHS Proxy/Server to provide operational context.

   OMNI Client RS and Proxy/Server RA messages MUST include the
   Interface Attributes sub-option for the Client underlying interface
   in the first OMNI option immediately following an authentication
   message sub-option if present; otherwise, immediately following the
   OMNI header.  When an FHS Proxy/Server receives an RS message
   destined to an anycast *NET address, it MUST include an Interface
   Attributes sub-option with omIndex '0' that encodes a unicast *NET
   INADDR immediately after the Interface Attributes sub-option for the
   Client's underlying interface in the solicited RA response.  Any
   additional Interface Attributes sub-options that appear in RS/RA
   messages are ignored.

   The Interface Attributes sub-options are formatted as shown below:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=2|    Sub-length=N     |    omIndex    |     omType    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  |  Link | Resvd | FMT |   SRT   |               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               ~
       ~                  LHS Proxy/Server MSID/INADDR                 ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 16: Interface Attributes

   o  Sub-Type is set to 2.

   o  Sub-Length is set to N that encodes the number of Sub-Option Data
      octets that follow.

   o  Sub-Option Data contains an "Interface Attributes" option encoded
      as follows:

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      *  omIndex is a 1-octet value corresponding to a specific
         underlying interface.  Client OMNI interfaces MUST number each
         distinct underlying interface with an omIndex value between '1'
         and '255' that represents a Client-specific 8-bit mapping for
         the actual ifIndex value assigned by network management
         [RFC2863], then set omIndex to either a specific omIndex value
         or '0' to denote "unspecified".

      *  omType is set to an 8-bit integer value corresponding to the
         underlying interface identified by omIndex.  The value
         represents an OMNI interface-specific 8-bit mapping for the
         actual IANA ifType value registered in the 'IANAifType-MIB'
         registry [http://www.iana.org].

      *  Provider ID is set to an OMNI interface-specific 8-bit ID value
         for the network service provider associated with this omIndex.

      *  Link encodes a 4-bit link metric.  The value '0' means the link
         is DOWN, and the remaining values mean the link is UP with
         metric ranging from '1' ("lowest") to '15' ("highest").

      *  Resvd is a 4-bit Reserved field set to 0 on transmission and
         ignored on reception.

      *  FMT - a 3-bit "Forward/Mode/Type" code interpreted as follows:

         +  The most significant two bits (i.e., "FMT-Forward" and "FMT-
            Mode") are interpreted in conjunction with one another.
            When FMT-Forward is clear, the LHS Proxy/Server performs OAL
            reassembly and decapsulation to obtain the original IP
            packet before forwarding.  If the FMT-Mode bit is clear, the
            LHS Proxy/Server then forwards the original IP packet at
            layer 3; otherwise, it invokes the OAL to re-encapsulate,
            re-fragment and forwards the resulting carrier packets to
            the Client via the selected underlying interface.  When FMT-
            Forward is set, the LHS Proxy/Server forwards unsecured OAL
            fragments to the Client without reassembling, while
            reassembling secured OAL fragments before re-fragmenting and
            forwarding to the Client.  If FMT-Mode is clear, all carrier
            packets destined to the Client must always be forwarded
            through the LHS Proxy/Server; otherwise the Client is
            eligible for direct forwarding over the open INET where it
            may be located behind one or more NATs.

         +  The least significant bit (i.e., "FMT-Type") determines the
            length of the LHS Proxy/Server INADDR field for NS/NA
            messages; if FMT-Type is clear, INADDR includes a 4-octet
            IPv4 address (otherwise a 16-octet IPv6 address).  For RS/RA

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            messages, the LHS Proxy/Server INADDR field is always
            exactly 16 octets.  If FMT-type is clear, INADDR encodes an
            IPv4-mapped IPv6 address; otherwise an ordinary IPv6
            address.

      *  SRT - a 5-bit Segment Routing Topology prefix length value that
         (when added to 96) determines the prefix length to apply to the
         ULA formed from concatenating [ULA*]::/96 with the 32 bit LHS
         MSID value that follows.  For example, the value 16 corresponds
         to the prefix length 112.

      *  LHS Proxy/Server MSID/INADDR - the first 32 bits includes the
         MSID of the LHS Proxy/Server on the path from a source neighbor
         to the target Client's underlying interface.  When SRT and MSID
         are both set to 0, the LHS Proxy/Server is considered
         unspecified in this IPv6 ND message.  FMT, SRT and LHS together
         provide guidance for the OMNI interface forwarding algorithm.
         Specifically, if SRT/LHS is located in the local OMNI link
         segment, then the source can reach the target Client either
         through its dependent Proxy/Server or through direct
         encapsulation following NAT traversal according to FMT.
         Otherwise, the target Client is located on a different SRT
         segment and the path from the source must employ a combination
         of route optimization and spanning tree hop traversals.  INADDR
         identifies the LHS Proxy/Server's INET-facing interface not
         located behind NATs, therefore no UDP port number is included
         since port number 8060 is used when the *NET encapsulation
         includes a UDP header.  Instead, INADDR includes only a 4-octet
         IPv4 or 16-octet IPv6 address with type and length determined
         by FMT-Type.  The IP address is recorded in network byte order
         in ones-compliment "obfuscated" form per [RFC4380].

12.2.4.  Multilink Forwarding Parameters

   OMNI nodes include the Multilink Forwarding Parameters sub-option in
   NS/NA messages used to coordinate with multilink route optimization
   targets.  If an NS message includes the sub-option, the solicited NA
   response must also include the sub-option.  The OMNI node MUST
   include the sub-option in the first OMNI option immediately following
   an authentication message sub-option.  Otherwise, the OMNI node MUST
   include the sub-option immediately following the OMNI header.  Each
   NS/NA message may contain at most one Multilink Forwarding Parameters
   sub-option; if an NS/NA message contains additional Multlink
   Forwarding Parameters sub-options, the first is processed and all
   others are ignored.

   When an NS/NA message includes the sub-option, the FHS Client omIndex
   MUST correspond to the underlying interface used to transmit the

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   message.  When the NS/NA message also includes Interface Attributes
   sub-options any that include the same FHS/LHS Client omIndex are
   ignored while all others are processed.

   The Multilink Forwarding Parameters sub-option includes the necessary
   state for establishing Multilink Forwarding Vectors (MFVs) in the
   Multilink Forwarding Information Bases (MFIBs) of the OAL source,
   destination and intermediate nodes in the path.  The sub-option also
   records addressing information for FHS/LHS nodes on the path,
   including "INADDRs" which MUST be unicast IP encapsulation addresses
   (i.e., and not anycast/multicast).  The manner for populating
   multilink forwarding information is specified in detail in
   [I-D.templin-6man-aero].

   The Multilink Forwarding Parameters sub-option is formatted as shown
   in Figure 17:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=3|    Sub-length=N     |Job|  A  |  B  |               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               ~
       ~        Multilink Forwarding Vector Index (MFVI) List          ~
       ~               (5 consecutive 4-octet MFVIs)                   ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~           Tunnel Window Synchronization Parameters            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |FHS Cli omIndex|     omType    |  Provider ID  |  Link | Resvd |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | FMT |   SRT   |                                               ~
       +-+-+-+-+-+-+-+-+                                               ~
       ~                 FHS Proxy/Server MSID/INADDR                  ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    FHS Bridge MSID/INADDR                     ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |LHS Cli omIndex|     omType    |  Provider ID  |  Link | Resvd |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | FMT |   SRT   |                                               ~
       +-+-+-+-+-+-+-+-+                                               ~
       ~                 LHS Proxy/Server MSID/INADDR                  ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                     LHS Bridge MSID/INADDR                    ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 17: Multilink Forwarding Parameters

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   o  Sub-Type is set to 3.  If multiple instances appear in the same
      message (i.e., whether in a single OMNI option or multiple) the
      first instance is processed and all others are ignored.

   o  Sub-Length encodes the number of Sub-Option Data octets that
      follow.  The length includes all fields up to and including the
      Tunnel Window Synchronization Parameters for all Job codes, while
      including the remaining fields only for Job codes "00" and "01"
      (see below).

   o  Sub-Option Data contains Multilink Forwarding Parameters as
      follows:

      *  Job/A/B is a 1-octet field that determines per-hop processing
         of the MFVI List, where A is a 3-bit count of the number of "A"
         MVFI List entries and B is a 3-bit count of the number of "B"
         MVFI List entries (valid A/B values are 0-5).  Job is a 2-bit
         code interpreted as follows:

         +  00 - "Initialize; Build B" - the FHS source sets this code
            in an NS used to initialize MFV state (any other messages
            that include this code MUST be dropped).  The FHS source
            first sets A/B to 0, and the FHS source and each
            intermediate node along the path to the LHS destination that
            processes the message creates a new MFV.  Each node that
            processes the message then assigns a unique 4-octet "B" MFVI
            to the MVF and also writes the value into list entry B, then
            increments B.  When the message arrives at the LHS
            destination, B will contain the number of MFVI List "B"
            entries, with the FHS source entry first, followed by
            entries for each consecutive intermediate node and ending
            with an entry for the final intermediate node (i.e., the
            list is populated in the forward direction).

         +  01 - "Follow B; Build A" - the LHS source sets this code in
            a solicited NA response to a solicitation with code "00"
            (any other messages that include this code MUST be dropped).
            The LHS source first copies the MFVI List and B value from
            the code "00" solicitation into these fields and sets A to
            0.  The LHS source and each intermediate node along the path
            to the FHS destination that processes the message then uses
            MFVI List entry B to locate the corresponding MFV.  Each
            node that processes the message then assigns a unique
            4-octet "A" MFVI to the MVF and also writes the value into
            list entry B, then increments A and decrements B.  When the
            message arrives at the FHS destination, A will contain the
            number of MFVI List "A" entries, with the LHS source entry
            last, preceded by entries for each consecutive intermediate

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            node and beginning with an entry for the final intermediate
            node (i.e., the list is populated in the reverse direction).

         +  10 - "Follow A; Record B" - the FHS node that sent the
            original code "00" solicitation and received the
            corresponding code "01" advertisement sets this code in any
            subsequent NS/NA messages sent to the same LHS destination.
            The FHS source copies the MVFI List and A value from the
            code "01" advertisement into these fields and sets B to 0.
            The FHS source and each intermediate node along the path to
            the LHS destination that processes the message then uses the
            "A" MFVI found at list entry B to locate the corresponding
            MFV.  Each node that processes the message then writes the
            MVF's "B" MFVI into list entry B, then decrements A and
            increments B.  When the message arrives at the LHS
            destination, B will contain the number of MFVI List "B"
            entries populated in the forward direction.

         +  11 - "Follow B; Record A" - the LHS node that received the
            original code "00" solicitation and sent the corresponding
            code "01" advertisement sets this code in any subsequent NS/
            NA messages sent to the same FHS destination.  The LHS
            source copies the MVFI List and B values from the code "00"
            solicitation into these fields and sets A to 0.  The LHS
            source and each intermediate node along the path to the FHS
            destination that processes the message then uses the "B"
            MFVI List entry found at list entry B to locate the
            corresponding MFV.  Each node that processes the message
            then writes the MFV's "A" MFVI into list entry B, then
            increments A and decrements B.  When the message arrives at
            the FHS destination, A will contain the number of MFVI List
            "A" entries populated in the reverse direction.

         Job/A/B together determine the per-hop behavior at each FHS/LHS
         source, intermediate node and destination that processes an
         IPv6 ND message.  When a Job code specifies "Initialize", each
         FHS/LHS node that processes the message creates a new MVF.
         When a Job code specifies "Build", each node that processes the
         message assigns a new MFVI.  When a Job code specifies
         "Follow", each node that processes the message uses an A/B MFVI
         List entry to locate an MFV (if the MFV cannot be located, the
         node returns a parameter problem and drops the message).  Using
         this algorithm, FHS sources that send code "00" solicitations
         and receive code "01 advertisements discover only "A"
         information, while LHS sources that receive code "00"
         solicitations and return code "01" advertisements discover only
         "B" information.  FHS/LHS intermediate nodes can instead
         examine A, B and the MFVI List to determine the number of

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         previous hops, the number of remaining hops, and the A/B MFVIs
         associated with the previous/remaining hops.  However, no
         intermediate nodes will discover inappropriate A/B MFVIs for
         their location in the multihop forwarding chain.  See:
         [I-D.templin-6man-aero] for further discussion on A/B MFVI
         processing.

      *  Multilink Forwarding Vector Index (MFVI) List is a 20-octet
         block that contains 5 consecutive 4-octet MFVI entries.  The
         FHS/LHS source and each intermediate node on the path to the
         destination processes the list according to the Job/A/B codes
         (see above).

      *  Tunnel Window Synchronization Parameters is a 12-octet block
         that consists of a 4-octet Sequence Number followed by a
         4-octet Acknowledgement Number followed by a 1-octet Flags
         field followed by a 3-octet Window field (i.e., the same as for
         the OMNI header parameters).  Tunnel endpoints use these
         parameters for simultaneous middlebox window synchronization in
         a single solicitation/advertisement message exchange.

      *  For Job codes "00" and "01" only, two trailing state variable
         blocks are included for First-Hop Segment (FHS) followed by
         Last-Hop Segment (LHS) network elements.  When present, each
         block encodes the following information:

         +  Client omIndex, omType, Provider ID and Resvd/Link are
            1-octet fields (at offset 0 from the beginning of the Sub-
            Option Data) that include link parameters for the Client
            underlying interface.  These fields are populated based on
            information discovered in Interface Attributes sub-options
            included in earlier RS/RA and/or NS/NA exchanges.

         +  FMT/SRT is a 1-octet field with a 5-bit SRT prefix length
            that applies to all elements in the segment.  The FMT-
            Forward/Mode bits determine the characteristics of the
            Proxy/Server relationship for this specific Client
            underlying interface (i.e., the same as described in
            Section 12.2.3), and the FMT-Type bits determine the IP
            address version for all INADDR fields relative to this SRT
            segment.  Unlike the case for Interface Attributes, all
            INADDR fields are always 16 bits in length regardless of the
            IP protocol version (for IPv4, INADDR is encoded as an
            IPv4-mapped IPv6 address [RFC4291]).  The IP address is
            recoded in network byte order, and in ones-compliment
            "obfuscated" form the same as described in Section 12.2.3.

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         +  Proxy/Server MSID/INADDR includes a 4-octet Proxy/Server
            MSID followed by a 16 octet INADDR.  INADDR identifies an
            open INET interface not located behind NATs, therefore no
            UDP port number is included since port number 8060 is used
            when the *NET encapsulation includes a UDP header.

         +  Bridge MSID/INADDR encodes a 4 octet MSID followed by a
            16-octet INADDR exactly as for the Proxy/Server MSID/INADDR.

12.2.5.  Traffic Selector

   When used in conjunction with Interface Attributes and/or Multilink
   Forwarding Parameters information, the Traffic Selector sub-option
   provides forwarding information for the multilink conceptual sending
   algorithm discussed in Section 14.

   IPv6 ND messages include Traffic Selectors for some or all of the
   source/target Client's underlying interfaces.  Traffic Selectors for
   some or all of a target Client's underlying interfaces are also
   included in uNA messages used to publish Client information changes.
   See: [I-D.templin-6man-aero] for more information.

   Traffic Selectors must be honored by all implementations in the
   format shown below:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=4|    Sub-length=N     |    omIndex    |   TS Format   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                                                               ~
       ~                RFC 6088 Format Traffic Selector               ~
       ~                                                               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 18: Traffic Selector

   o  Sub-Type is set to 4.  Each IPv6 ND message may contain zero or
      more Traffic Selectors for each omIndex; when multiple Traffic
      Selectors for the same omIndex appear, all are processed and the
      cumulative information from all is accepted.

   o  Sub-Length is set to N that encodes the number of Sub-Option Data
      octets that follow.

   o  Sub-Option Data contains a "Traffic Selector" encoded as follows:

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      *  omIndex is a 1-octet value corresponding to a specific
         underlying interface the same as specified above for Interface
         Attributes and Multilink Forwarding Parameters above.  The OMNI
         options of a single message may include multiple Traffic
         Selector sub-options; each with the same or different omIndex
         values.

      *  TS Format is a 1-octet field that encodes a Traffic Selector
         version per [RFC6088].  If TS Format encodes the value 1 or 2,
         the Traffic Selector includes IPv4 or IPv6 information,
         respectively.  If TS Format encodes any other value, the sub-
         option is ignored.

      *  The remainder of the sub-option includes a traffic selector
         formatted per [RFC6088] beginning with the "Flags (A-N)" field,
         and with the Traffic Selector IP protocol version coded in the
         TS Format field.  If a single interface identified by omIndex
         requires Traffic Selectors for multiple IP protocol versions,
         or if a Traffic Selector block would exceed the available
         space, the remaining information is coded in additional Traffic
         Selector sub-options that all encode the same omIndex.

12.2.6.  Geo Coordinates

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=5|    Sub-length=N     |    Geo Type   |Geo Coordinates
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

                   Figure 19: Geo Coordinates Sub-option

   o  Sub-Type is set to 5.  If multiple instances appear in OMNI
      options of the same message all are processed.

   o  Sub-Length is set to N that encodes the number of Sub-Option Data
      octets that follow.

   o  Geo Type is a 1 octet field that encodes a type designator that
      determines the format and contents of the Geo Coordinates field
      that follows.  The following types are currently defined:

      *  0 - NULL, i.e., the Geo Coordinates field is zero-length.

   o  A set of Geo Coordinates of length up to the remaining available
      space for this OMNI option.  New formats to be specified in future
      documents and may include attributes such as latitude/longitude,
      altitude, heading, speed, etc.

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12.2.7.  Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Message

   The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) sub-option
   may be included in the OMNI options of Client RS messages and Proxy/
   Server RA messages.  FHS Proxy/Servers that forward RS/RA messages
   between a Client and an LHS Proxy/Server also forward DHCPv6 Sub-
   Options unchanged.  Note that DHCPv6 messages do not include a
   Checksum field since integrity is protected by the IPv6 ND message
   checksum, authentication signature and/or lower-layer authentication
   and integrity checks.

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=6|    Sub-length=N     |    msg-type   |  id (octet 0) |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   transaction-id (octets 1-2) |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
       |                                                               |
       .                        DHCPv6 options                         .
       .                 (variable number and length)                  .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 20: DHCPv6 Message Sub-option

   o  Sub-Type is set to 6.  If multiple instances appear in OMNI
      options of the same message the first is processed and all others
      are ignored.

   o  Sub-Length is set to N that encodes the number of Sub-Option Data
      octets that follow.  The 'msg-type' and 'transaction-id' fields
      are always present; hence, the length of the DHCPv6 options is
      limited by the remaining available space for this OMNI option.

   o  'msg-type' and 'transaction-id' are coded according to Section 8
      of [RFC8415].

   o  A set of DHCPv6 options coded according to Section 21 of [RFC8415]
      follows.

12.2.8.  Host Identity Protocol (HIP) Message

   The Host Identity Protocol (HIP) Message sub-option (when present)
   provides authentication for IPv6 ND messages exchanged between
   Clients and FHS Proxy/Servers over an open Internetwork.  FHS Proxy/
   Servers authenticate the HIP authentication signatures in source
   Client IPv6 ND messages before securely forwarding them to other OMNI

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   nodes.  LHS Proxy/Servers that receive secured IPv6 ND messages from
   other OMNI nodes that do not already include a security sub-option
   insert HIP authentication signatures before forwarding them to the
   target Client.

   OMNI interfaces MUST include the HIP message (when present) as the
   first sub-option of the first OMNI option, which MUST appear
   immediately following the IPv6 ND message header.  OMNI interfaces
   can therefore easily locate the HIP message and verify the
   authentication signature without applying deep inspection.  OMNI
   interfaces that receive IPv6 ND messages without a HIP (or other
   authentication) sub-option instead verify the IPv6 ND message
   checksum.

   OMNI interfaces include the HIP message sub-option when they forward
   IPv6 ND messages that require security over INET underlying
   interfaces, i.e., where authentication and integrity is not already
   assured by lower layers.  The OMNI interface calculates the
   authentication signature over the entire length of the OAL packet (or
   super-packet) beginning with a pseudo-header of the IPv6 ND message
   header and extending over the remainder of the OAL packet up to but
   not including the trailing OAL Checksum field.  OMNI interfaces that
   process OAL packets that contain secured IPv6 ND messages verify the
   signature then either process the rest of the message locally or
   forward a proxyed copy to the next hop.

   When a FHS Client inserts a HIP message sub-option in an NS/NA
   message destined to a target in a remote spanning tree segment, it
   must ensure that the insertion does not cause the message to exceed
   the OMNI interface MTU.  When the remote segment LHS Proxy/Server
   forwards the NS/NA message from the spanning tree to the target
   Client, it inserts a new HIP message sub-option if necessary while
   overwriting or cancelling the (now defunct) HIP message sub-option
   supplied by the FHS Client.

   If the defunct HIP sub-option size was smaller than the space needed
   for the LHS Client HIP message (or, if no defunct HIP sub-option is
   present), the LHS Proxy/Server adjusts the space immediately
   following the OMNI header by copying the preceding portion of the
   IPv6 ND message into buffer headroom free space or copying the
   remainder of the IPv6 ND message into buffer tailroom free space.
   The LHS Proxy/Server then insets the new HIP sub-option immediately
   after the OMNI header and immediately before the next sub-option
   while properly overwriting the defunct sub-option if present.

   If the defunct HIP sub-option size was larger than the space needed
   for the LHS Client HIP message, the LHS Proxy/Server instead
   overwrites the existing sub-option and writes a single Pad1 or PadN

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   sub-option over the next 1-2 octets to cancel the remainder of the
   defunct sub-option.  If the LHS Proxy/Server cannot create sufficient
   space through any means without causing the OMNI option to exceed
   2040 bytes or causing the IPv6 ND message to exceed the OMNI
   interface MTU, it returns a suitable error (see: Section 12.2.13) and
   drops the message.

   The HIP message sub-option is formatted as shown below:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=7|    Sub-length=N     |0| Packet Type |Version| RES.|1|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Reserved            |           Controls            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                Sender's Host Identity Tag (HIT)               |
       |                                                               |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               Receiver's Host Identity Tag (HIT)              |
       |                                                               |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       /                        HIP Parameters                         /
       /                                                               /
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 21: HIP Message Sub-option

   o  Sub-Type is set to 7.  If multiple instances appear in OMNI
      options of the same message the first is processed and all others
      are ignored.

   o  Sub-Length is set to N, i.e., the length of the option in octets
      beginning immediately following the Sub-Length field and extending
      to the end of the HIP parameters.  The length of the entire HIP
      message is therefore limited by the remaining available space for
      this OMNI option.

   o  The HIP message is coded per Section 5 of [RFC7401], except that
      the OMNI "Sub-Type" and "Sub-Length" fields replace the first 2
      octets of the HIP message header (i.e., the Next Header and Header
      Length fields).  Also, since the IPv6 ND message is already

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      protected by the authentication signature and/or lower-layer
      authentication and integrity checks, the HIP message Checksum
      field is replaced by a Reserved field set to 0 on transmission and
      ignored on reception.

   Note: In some environments, maintenance of a Host Identity Tag (HIT)
   namespace may be unnecessary for securely associating an OMNI node
   with an IPv6 address-based identity.  In that case, other types of
   IPv6 addresses (e.g., a Client's MNP-LLA, a Proxy/Server's ADM-LLA,
   etc.) can be used instead of HITs in the authentication signature as
   long as the address can be uniquely associated with the Sender/
   Receiver.

12.2.9.  PIM-SM Message

   The Protocol Independent Multicast - Sparse Mode (PIM-SM) Message
   sub-option may be included in the OMNI options of IPv6 ND messages.
   PIM-SM messages are formatted as specified in Section 4.9 of
   [RFC7761], with the exception that the Checksum field is replaced by
   a Reserved field (set to 0) since the IPv6 ND message is already
   protected by the IPv6 ND message checksum, authentication signature
   and/or lower-layer authentication and integrity checks.  The PIM-SM
   message sub-option format is shown in Figure 22:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=8|    Sub-length=N     |PIM Ver| Type  |   Reserved    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       /                         PIM-SM Message                        /
       /                                                               /
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 22: PIM-SM Message Option Format

   o  Sub-Type is set to 8.  If multiple instances appear in OMNI
      options of the same message all are processed.

   o  Sub-Length is set to N, i.e., the length of the option in octets
      beginning immediately following the Sub-Length field and extending
      to the end of the PIM-SM message.  The length of the entire PIM-SM
      message is therefore limited by the remaining available space for
      this OMNI option.

   o  The PIM-SM message is coded exactly as specified in Section 4.9 of
      [RFC7761], except that the Checksum field is replaced by a

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      Reserved field set to 0 on transmission and ignored on reception.
      The "PIM Ver" field MUST encode the value 2, and the "Type" field
      encodes the PIM message type.  (See Section 4.9 of [RFC7761] for a
      list of PIM-SM message types and formats.)

12.2.10.  Reassembly Limit

   The Reassembly Limit sub-option may be included in the OMNI options
   of IPv6 ND messages.  The message consists of a 15-bit Reassembly
   Limit value, followed by a flag bit (H) optionally followed by an (N-
   2)-octet leading portion of an OAL First Fragment that triggered the
   message.

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=9|    Sub-length=N     |      Reassembly Limit       |H|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          OAL First Fragment (As much of invoking packet       |
       +         as possible without causing the IPv6 ND message       +
       |                to exceed the minimum IPv6 MTU)                |
       +                                                               +

                        Figure 23: Reassembly Limit

   o  Sub-Type is set to 9.  If multiple instances appear in OMNI
      options of the same message the first occurring "hard" and "soft"
      Reassembly Limit values are accepted, and any additional
      Reassembly Limit values are ignored.

   o  Sub-Length is set to 2 if no OAL First Fragment is included, or to
      a value N greater than 2 if an OAL First Fragment is included.

   o  A 15-bit Reassembly Limit follows, and includes a value between
      1500 and 9180.  If any other value is included, the sub-option is
      ignored.  The value indicates the hard or soft limit for original
      IP packets that the source of the message is currently willing to
      reassemble; the source may increase or decrease the hard or soft
      limit at any time through the transmission of new IPv6 ND
      messages.  Until the first IPv6 ND message with a Reassembly Limit
      sub-option arrives, OMNI nodes assume initial default hard/soft
      limits of 9180 (I.e., the OMNI interface MRU).  After IPv6 ND
      messages with Reassembly Limit sub-options arrive, the OMNI node
      retains the most recent hard/soft limit values until new IPv6 ND
      messages with different values arrive.

   o  The 'H' flag is set to 1 if the Reassembly Limit is a "Hard"
      limit, and set to 0 if the Reassembly Limit is a "Soft" limit.

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   o  If N is greater than 2, the remainder of the Reassembly Limit sub-
      option encodes the leading portion of an OAL First Fragment that
      prompted this IPv6 ND message.  The first fragment is included
      beginning with the OAL header, and continuing with as much of the
      fragment payload as possible without causing the IPv6 ND message
      to exceed the minimum IPv6 MTU.

12.2.11.  Fragmentation Report

   The Fragmentation Report may be included in the OMNI options of uNA
   messages sent from an OAL destination to an OAL source.  The message
   consists of (N / 8)-many (Identification, Bitmap)-tuples which
   include the Identification values of OAL fragments received plus a
   Bitmap marking the ordinal positions of individual fragments received
   and fragments missing.

         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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S-Type=10|   Sub-Length = N    | Identification #1 (bits 0 -15)|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Identification #1 (bits 15-31)|    Bitmap #1 (bits 0 - 15)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Bitmap #1 (bits 16-31)  | Identification #2 (bits 0 -15)|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Identification #2 (bits 15-31)|    Bitmap #2 (bits 0 - 15)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Bitmap #2 (bits 16-31)  | Identification #3 (bits 0 -15)|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Identification #3 (bits 15-31)|    Bitmap #3 (bits 0 - 15)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Bitmap #3 (bits 16-31)  |             ...               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+             ...               +
       |                              ...                              |
       +                              ...                              +

                      Figure 24: Fragmentation Report

   o  Sub-Type is set to 10.  If multiple instances appear in OMNI
      options of the same message all are processed.

   o  Sub-Length is set to N, i.e., the length of the option in octets
      beginning immediately following the Sub-Length field and extending
      to the end of the sub-option.  If N is not an integral multiple of
      8 octets, the sub-option is ignored.  The length of the entire
      sub-option should not cause the entire IPv6 ND message to exceed
      the minimum IPv6 MTU.

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   o  Identification (i) includes the IPv6 Identification value found in
      the Fragment Header of a received OAL fragment.  (Only those
      Identification values included represent fragments for which loss
      was unambiguously observed; any Identification values not included
      correspond to fragments that were either received in their
      entirety or may still be in transit.)

   o  Bitmap (i) includes an ordinal checklist of fragments, with each
      bit set to 1 for a fragment received or 0 for a fragment missing.
      (Each OAL packet may consist of at most 23 fragments, therefore
      Bitmap (i) bits 0-22 are consulted while bits 23-31 are reserved
      for future use and ignored.)  For example, for a 20-fragment OAL
      packet with ordinal fragments #3, #10, #13 and #17 missing and all
      other fragments received, Bitmap (i) encodes the following:

         0                   1                   2
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
        |1|1|1|0|1|1|1|1|1|1|0|1|1|0|1|1|1|0|1|1|0|0|0|...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                                 Figure 25

      (Note that loss of an OAL atomic fragment is indicated by a
      Bitmap(i) with all bits set to 0.)

12.2.12.  Node Identification

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S-Type=11|    Sub-length=N    |     ID-Type    |               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               ~
       ~            Node Identification Value (N-1 octets)             ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 26: Node Identification

   o  Sub-Type is set to 11.  If multiple instances appear in OMNI
      options of the same IPv6 ND message the first instance of a
      specific ID-Type is processed and all other instances of the same
      ID-Type are ignored.  (It is therefore possible for a single IPv6
      ND message to convey multiple distinct Node Identifications - each
      with a different ID-Type.)

   o  Sub-Length is set to N that encodes the number of Sub-Option Data
      octets that follow.  The ID-Type field is always present; hence,

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      the maximum Node Identification Value length is limited by the
      remaining available space in this OMNI option.

   o  ID-Type is a 1 octet field that encodes the type of the Node
      Identification Value.  The following ID-Type values are currently
      defined:

      *  0 - Universally Unique IDentifier (UUID) [RFC4122].  Indicates
         that Node Identification Value contains a 16 octet UUID.

      *  1 - Host Identity Tag (HIT) [RFC7401].  Indicates that Node
         Identification Value contains a 16 octet HIT.

      *  2 - Hierarchical HIT (HHIT) [I-D.ietf-drip-rid].  Indicates
         that Node Identification Value contains a 16 octet HHIT.

      *  3 - Network Access Identifier (NAI) [RFC7542].  Indicates that
         Node Identification Value contains an N-1 octet NAI.

      *  4 - Fully-Qualified Domain Name (FQDN) [RFC1035].  Indicates
         that Node Identification Value contains an N-1 octet FQDN.

      *  5 - IPv6 Address.  Indicates that Node Identification contains
         a 16-octet IPv6 address that is not a (H)HIT.  The IPv6 address
         type is determined according to the IPv6 addressing
         architecture [RFC4291].

      *  6 - 252 - Unassigned.

      *  253-254 - Reserved for experimentation, as recommended in
         [RFC3692].

      *  255 - reserved by IANA.

   o  Node Identification Value is an (N - 1) octet field encoded
      according to the appropriate the "ID-Type" reference above.

   OMNI interfaces code Node Identification Values used for DHCPv6
   messaging purposes as a DHCP Unique IDentifier (DUID) using the
   "DUID-EN for OMNI" format with enterprise number 45282 (see:
   Section 25) as shown in Figure 27:

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        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         DUID-Type (2)         |      EN (high bits == 0)      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     EN (low bits = 45282)     |    ID-Type    |               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               |
       .                    Node Identification Value                  .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 27: DUID-EN for OMNI Format

   In this format, the OMNI interface codes the ID-Type and Node
   Identification Value fields from the OMNI sub-option following a 6
   octet DUID-EN header, then includes the entire "DUID-EN for OMNI" in
   a DHCPv6 message per [RFC8415].

12.2.13.  ICMPv6 Error

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S-Type=12|    Sub-length=N    |                                ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-                                ~
       ~                    RFC4443 Error Message Body                 ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 28: ICMPv6 Error

   o  Sub-Type is set to 12.  If multiple instances appear in OMNI
      options of the same IPv6 ND message all are processed.

   o  Sub-Length is set to N that encodes the number of Sub-Option Data
      octets that follow.

   o  RFC4443 Error Message Body is an N-octet field encoding the body
      of an ICMPv6 Error Message per Section 2.1 of [RFC4443] (ICMPv6
      informational messages must not be included and must be ignored if
      received).  OMNI interfaces include as much of the ICMPv6 error
      message body in the sub-option as possible without causing the
      IPv6 ND message to exceed the minimum IPv6 MTU.

12.2.14.  QUIC-TLS Message

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        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S-Type=13|    Sub-length=N    |                                ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-                                ~
       ~                         QUIC-TLS Message                      ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 29: QUIC-TLS Message

   o  Sub-Type is set to 13.  If multiple instances appear in OMNI
      options of the same IPv6 ND message, the first is processed and
      all others are ignored.

   o  Sub-Length is set to N that encodes the number of Sub-Option Data
      octets that follow.

   o  The QUIC-TLS message [RFC9000][RFC9001][RFC9002] encodes the QUIC
      and TLS message parameters necessary to support QUIC connection
      establishment.

   When present, the QUIC-TLS Message sub-option MUST appear immediately
   after the header of the first OMNI option in the IPv6 ND message; if
   the sub-option appears in any other location it MUST be ignored.
   IPv6 ND solicitation and advertisement messages serve as couriers to
   transport the QUIC and TLS parameters necessary to establish a
   secured QUIC connection.

12.2.15.  Proxy/Server Departure

   OMNI Clients include a Proxy/Server Departure sub-option in RS
   messages when they associate with a new FHS and/or Hub Proxy/Server
   and need to send a departure indication to an old FHS and/or Hub
   Proxy/Server.  The Proxy/Server Departure sub-option is formatted as
   shown below:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S-Type=14|    Sub-length=8     |Old FHS Proxy/Server MSID (0-1)|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Old FHS Proxy/Server MSID (2-3)|Old Hub Proxy/Server MSID (0-1)|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Old Hub Proxy/Server MSID (2-3)|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 30: Proxy/Server Departure

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   o  Sub-Type is set to 14.

   o  Sub-Length is set to 8.

   o  Sub-Option Data contains the 4-octet MSID for the "Old FHS Proxy/
      Server" followed by a 4-octet MSID for an "Old Hub Proxy/Server.
      (If the Old FHS/Hub is unspecified, the corresponding MSID instead
      includes the value 0.)

12.2.16.  Sub-Type Extension

   Since the Sub-Type field is only 5 bits in length, future
   specifications of major protocol functions may exhaust the remaining
   Sub-Type values available for assignment.  This document therefore
   defines Sub-Type 30 as an "extension", meaning that the actual Sub-
   Option type is determined by examining a 1 octet "Extension-Type"
   field immediately following the Sub-Length field.  The Sub-Type
   Extension is formatted as shown in Figure 31:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S-Type=30|     Sub-length=N    | Extension-Type|               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               ~
       ~                                                               ~
       ~                       Extension-Type Body                     ~
       ~                                                               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 31: Sub-Type Extension

   o  Sub-Type is set to 30.  If multiple instances appear in OMNI
      options of the same message all are processed, where each
      individual extension defines its own policy for processing
      multiple of that type.

   o  Sub-Length is set to N that encodes the number of Sub-Option Data
      octets that follow.  The Extension-Type field is always present,
      and the maximum Extension-Type Body length is limited by the
      remaining available space in this OMNI option.

   o  Extension-Type contains a 1 octet Sub-Type Extension value between
      0 and 255.

   o  Extension-Type Body contains an N-1 octet block with format
      defined by the given extension specification.

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   Extension-Type values 0 and 1 are defined in the following
   subsections, while Extension-Type values 2 through 252 are available
   for assignment by future specifications which must also define the
   format of the Extension-Type Body and its processing rules.
   Extension-Type values 253 and 254 are reserved for experimentation,
   as recommended in [RFC3692], and value 255 is reserved by IANA.

12.2.16.1.  RFC4380 Header Extension Option

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S-Type=30|      Sub-length=N   |   Ext-Type=0  |   Header Type |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                      Header Option Value                      ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 32: RFC4380 Header Extension Option (Extension-Type 0)

   o  Sub-Type is set to 30.

   o  Sub-Length is set to N that encodes the number of Sub-Option Data
      octets that follow.  The Extension-Type and Header Type fields are
      always present, and the Header Option Value is limited by the
      remaining available space in this OMNI option.

   o  Extension-Type is set to 0.  Each instance encodes exactly one
      header option per Section 5.1.1 of [RFC4380], with Ext-Type and
      Header Type representing the first two octets of the option.  If
      multiple instances of the same Header Type appear in OMNI options
      of the same message the first instance is processed and all others
      are ignored.  If Header Type indicates an Authentication
      Encapsulation (see below), the entire sub-option MUST appear as
      the first sub-option of the first OMNI option, which MUST appear
      immediately following the IPv6 ND message header.

   o  Header Type and Header Option Value are coded exactly as specified
      in Section 5.1.1 of [RFC4380]; the following types are currently
      defined:

      *  0 - Origin Indication (IPv4) - value coded as a UDP port number
         followed by a 4-octet IPv4 address both in "obfuscated" form
         per Section 5.1.1 of [RFC4380].

      *  1 - Authentication Encapsulation - value coded per
         Section 5.1.1 of [RFC4380].

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      *  2 - Origin Indication (IPv6) - value coded per Section 5.1.1 of
         [RFC4380], except that the address is a 16-octet IPv6 address
         instead of a 4-octet IPv4 address.

   o  Header Type values 3 through 252 are available for assignment by
      future specifications, which must also define the format of the
      Header Option Value and its processing rules.  Header Type values
      253 and 254 are reserved for experimentation, as recommended in
      [RFC3692], and value 255 is Reserved by IANA.

12.2.16.2.  RFC6081 Trailer Extension Option

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S-Type=30|      Sub-length=N   |   Ext-Type=1  |  Trailer Type |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                     Trailer Option Value                      ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 33: RFC6081 Trailer Extension Option (Extension-Type 1)

   o  Sub-Type is set to 30.

   o  Sub-Length is set to N that encodes the number of Sub-Option Data
      octets that follow.  The Extension-Type and Trailer Type fields
      are always present, and the maximum-length Trailer Option Value is
      limited by the remaining available space in this OMNI option.

   o  Extension-Type is set to 1.  Each instance encodes exactly one
      trailer option per Section 4 of [RFC6081].  If multiple instances
      of the same Trailer Type appear in OMNI options of the same
      message the first instance is processed and all others ignored.

   o  Trailer Type and Trailer Option Value are coded exactly as
      specified in Section 4 of [RFC6081]; the following Trailer Types
      are currently defined:

      *  0 - Unassigned

      *  1 - Nonce Trailer - value coded per Section 4.2 of [RFC6081].

      *  2 - Unassigned

      *  3 - Alternate Address Trailer (IPv4) - value coded per
         Section 4.3 of [RFC6081].

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      *  4 - Neighbor Discovery Option Trailer - value coded per
         Section 4.4 of [RFC6081].

      *  5 - Random Port Trailer - value coded per Section 4.5 of
         [RFC6081].

      *  6 - Alternate Address Trailer (IPv6) - value coded per
         Section 4.3 of [RFC6081], except that each address is a
         16-octet IPv6 address instead of a 4-octet IPv4 address.

   o  Trailer Type values 7 through 252 are available for assignment by
      future specifications, which must also define the format of the
      Trailer Option Value and its processing rules.  Trailer Type
      values 253 and 254 are reserved for experimentation, as
      recommended in [RFC3692], and value 255 is Reserved by IANA.

13.  Address Mapping - Multicast

   The multicast address mapping of the native underlying interface
   applies.  The Client mobile router also serves as an IGMP/MLD Proxy
   for its EUNs and/or hosted applications per [RFC4605].

   The Client uses Multicast Listener Discovery (MLDv2) [RFC3810] to
   coordinate with Proxy/Servers, and *NET L2 elements use MLD snooping
   [RFC4541].  The Client can also employ multicast routing protocols to
   coordinate with network-based multicast sources as specified in
   [I-D.templin-6man-aero].

   Since the OMNI link model is NBMA, OMNI links support link-scoped
   multicast through iterative unicast transmissions to individual
   multicast group members (i.e., unicast/multicast emulation).

14.  Multilink Conceptual Sending Algorithm

   The Client's IPv6 layer selects the outbound OMNI interface according
   to SBM considerations when forwarding original IP packets from local
   or EUN applications to external correspondents.  Each OMNI interface
   maintains a neighbor cache the same as for any IPv6 interface, but
   includes additional state for multilink coordination.  Each Client
   OMNI interface maintains default routes via Proxy/Servers discovered
   as discussed in Section 15, and may configure more-specific routes
   discovered through means outside the scope of this specification.

   For each original IP packet it forwards, the OMNI interface selects
   one or more source underlying interfaces based on PBM factors (e.g.,
   traffic attributes, cost, performance, message size, etc.) and one or
   more target underlying interfaces for the neighbor based on Interface
   Attributes received in IPv6 ND messages (see: Section 12.2.3).

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   Multilink forwarding may also direct packet replication across
   multiple underlying interface pairs for increased reliability at the
   expense of duplication.  The set of all Interface Attributes and
   Traffic Selectors received in IPv6 ND messages determines the
   multilink forwarding profile for selecting target underlying
   interfaces.

   When the OMNI interface sends an original IP packet over a selected
   source underlying interface, it first employs OAL encapsulation and
   fragmentation as discussed in Section 5, then performs *NET
   encapsulation as directed by the appropriate MFV.  The OMNI interface
   also performs *NET encapsulation (following OAL encapsulation) when
   the nearest Proxy/Server is located multiple hops away as discussed
   in Section 15.2.

   OMNI interface multilink service designers MUST observe the BCP
   guidance in Section 15 [RFC3819] in terms of implications for
   reordering when original IP packets from the same flow may be spread
   across multiple underlying interfaces having diverse properties.

14.1.  Multiple OMNI Interfaces

   Clients may connect to multiple independent OMNI links within the
   same or different OMNI domains to support SBM.  The Client configures
   a separate OMNI interface for each link so that multiple interfaces
   (e.g., omni0, omni1, omni2, etc.) are exposed to the IP layer.  Each
   OMNI interface configures one or more OMNI anycast addresses (see:
   Section 10), and the Client injects the corresponding anycast
   prefixes into the EUN routing system.  Multiple distinct OMNI links
   can therefore be used to support fault tolerance, load balancing,
   reliability, etc.

   Applications in EUNs can use Segment Routing to select the desired
   OMNI interface based on SBM considerations.  The application writes
   an OMNI anycast address into the original IP packet's destination
   address, and writes the actual destination (along with any additional
   intermediate hops) into the Segment Routing Header.  Standard IP
   routing directs the packet to the Client's mobile router entity,
   where the anycast address identifies the correct OMNI interface for
   next hop forwarding.  When the Client receives the packet, it
   replaces the IP destination address with the next hop found in the
   Segment Routing Header and forwards the message via the OMNI
   interface identified by the anycast address.

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14.2.  Client-Proxy/Server Loop Prevention

   After a Proxy/Server has registered an MNP for a Client (see:
   Section 15), the Proxy/Server will forward all packets destined to an
   address within the MNP to the Client.  The Client will under normal
   circumstances then forward the packet to the correct destination
   within its internal networks.

   If at some later time the Client loses state (e.g., after a reboot),
   it may begin returning packets with destinations corresponding to its
   MNP to the Proxy/Server as its default router.  The Proxy/Server
   therefore drops any original IP packets received from the Client with
   a destination address that corresponds to the Client's MNP (i.e.,
   whether LLA, ULA or GUA), and drops any carrier packets with both
   source and destination address corresponding to the same Client's MNP
   regardless of their origin.

15.  Router Discovery and Prefix Registration

   Clients engage the MS by sending RS messages with OMNI options under
   the assumption that one or more Proxy/Server will process the message
   and respond.  The RS message is received by a FHS Proxy/Server, which
   may in turn forward a proxyed copy of the RS to a Hub Proxy/Server
   located on the same or different SRT segment.  The Hub Proxy/Server
   then returns an RA message either directly to the Client or via an
   FHS Proxy/Server acting as a proxy.

   Clients and FHS Proxy/Servers include an authentication signature in
   their RS/RA exchanges when necessary; otherwise, they calculate and
   include a valid IPv6 ND message checksum (see: Section 12 and
   Appendix B).  FHS and Hub Proxy/Server RS/RA message exchanges over
   the SRT secured spanning tree instead always include the checksum and
   omit the authentication signature.  Clients and Proxy/Servers use the
   information included in RS/RA messages to establish NCE state and
   OMNI link autoconfiguration information as discussed in this section.

   For each underlying interface, the Client sends RS messages with OMNI
   options to coordinate with a (potentially different) FHS Proxy/Server
   for each interface and a single Hub Proxy/Server.  All Proxy/Servers
   are identified by their MSIDs and accept carrier packets addressed to
   their anycast/unicast *NET INADDRs; the Hub Proxy/Server may be
   chosen among any of the Client's FHS Proxy/Servers or may be any
   other Proxy/Server for the OMNI link.  Example MSID/INADDR discovery
   methods are given in [RFC5214] and include data link login
   parameters, name service lookups, static configuration, a static
   "hosts" file, etc.  In the absence of other information, the Client
   can resolve the DNS Fully-Qualified Domain Name (FQDN)
   "linkupnetworks.[domainname]" where "linkupnetworks" is a constant

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   text string and "[domainname]" is a DNS suffix for the OMNI link
   (e.g., "example.com").

   Clients configure OMNI interfaces that observe the properties
   discussed in previous sections.  The OMNI interface and its
   underlying interfaces are said to be in either the "UP" or "DOWN"
   state according to administrative actions in conjunction with the
   interface connectivity status.  An OMNI interface transitions to UP
   or DOWN through administrative action and/or through state
   transitions of the underlying interfaces.  When a first underlying
   interface transitions to UP, the OMNI interface also transitions to
   UP.  When all underlying interfaces transition to DOWN, the OMNI
   interface also transitions to DOWN.

   When a Client OMNI interface transitions to UP, it sends RS messages
   to register its MNP and an initial set of underlying interfaces that
   are also UP.  The Client sends additional RS messages to refresh
   lifetimes and to register/deregister underlying interfaces as they
   transition to UP or DOWN.  The Client's OMNI interface sends initial
   RS messages over an UP underlying interface with its MNP-LLA as the
   source (or with the unspecified address (::) as the source if it does
   not yet have an MNP-LLA) and with destination set to link-scoped All-
   Routers multicast or the ADM-LLA of a specific Proxy/Server.  The
   OMNI interface includes an OMNI option per Section 12 with a Preflen
   assertion, N/A/U flags, an Interface Attributes sub-option for the
   underlying interface and with any other necessary OMNI sub-options
   such as authentication, Proxy/Server Departure, Reassembly Limits,
   etc.

   The Client then calculates the authentication signature or checksum
   and prepares to forward the RS over the underlying interface using
   OAL encapsulation and fragmentation if necessary.  If the Client uses
   OAL encapsulation for RS messages sent to an unsynchronized FHS
   Proxy/Server over an INET interface, the entire RS message must fit
   within a single carrier packet (i.e., an atomic fragment) so that the
   FHS Proxy/Server can verify the authentication signature without
   having to reassemble.  The OMNI interface selects an Identification
   value (see: Section 6.6), sets the OAL source address to the ULA
   corresponding to the RS source (or a Temporary ULA or (H)HIT if the
   RS source is the unspecified address (::)), sets the OAL destination
   to an OMNI IPv6 anycast or ADM-ULA unicast address then performs
   fragmentation if necessary.  When *NET encapsulation is used, the
   Client includes the discovered FHS Proxy/Server INADDR or an anycast
   address as the *NET destination then forwards the resulting carrier
   packet(s) into the *NET.

   When an FHS Proxy/Server receives the carrier packets containing an
   RS it sets aside the *NET headers, verifies the Identifications and

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   reassembles if necessary, sets aside the OAL header, then verifies
   the RS authentication signature or checksum.  The FHS Proxy/Server
   then caches the OMNI Window Synchronization parameters, Interface
   Attributes and any Traffic Selector sub-options in a NCE for the
   Client while also caching the *NET (UDP/IP) and OAL (ULA) source and
   destination address information.  The FHS Proxy/Server next caches
   the OMNI option N flag to determine its role in processing NS(NUD)
   messages (see: Section 12.1) then examines the RS destination
   address.  If the destination matches its own ADM-LLA, the FHS Proxy/
   Server assumes the Hub role and acts as the sole entry point for
   injecting the Client's MNP into the MS routing system (i.e., after
   performing any necessary MNP prefix delegation operations) according
   to the RS source address and OMNI option Prefix Length.  The FHS/Hub
   Proxy/Server then caches the OMNI option A/U flags to determine its
   role in processing NS(AR) messages and generating uNA messages (see:
   Section 12.1).

   The FHS/Hub Proxy/Server then prepares to return an RA message
   directly to the Client by first populating the Cur Hop Limit, Flags,
   Router Lifetime, Reachable Time and Retrans Timer fields with values
   appropriate for the OMNI link.  The FHS/Hub Proxy/Server next
   includes as the first RA message option an OMNI option with Window
   Synchronization information, an authentication sub-option if
   necessary and a (proxyed) copy of the Client's original Interface
   Attributes sub-option with its INET-facing interface information
   written in the FMT/SRT and LHS Proxy/Server MSID/INADDR fields.  If
   the RS *NET destination IP address was anycast, the FHS/Hub Proxy/
   Server next includes a second Interface Attributes sub-option with
   omIndex set to '0' and with a unicast *NET IP address for its Client-
   facing interface in the INADDR field.

   The FHS/Hub Proxy/Server next includes an Origin Indication sub-
   option that includes the RS *NET source INADDR information (see:
   Section 12.2.16.1), then includes any other necessary OMNI sub-
   options (either within the same OMNI option or in additional OMNI
   options).  Following the OMNI option(s), the FHS/Hub Proxy/Server
   next includes any other necessary RA options such as PIOs with (A;
   L=0) that include the OMNI link MSPs [RFC8028], RIOs [RFC4191] with
   more-specific routes, Nonce and Timestamp options, etc.  The FHS/Hub
   Proxy/Server then sets the RA source address to its own ADM-LLA and
   destination address to the Client's MNP-LLA, then calculates the
   authentication signature or checksum.  The FHS/Hub Proxy/Server
   finally performs OAL encapsulation with source set to its own ADM-ULA
   and destination set to the OAL source that appeared in the RS, then
   fragments if necessary, encapsulates each fragment in appropriate
   *NET headers with source and destination address information reversed
   from the RS *NET information and returns the resulting carrier

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   packets to the Client over the same underlying interface the RS
   arrived on.

   When an FHS Proxy/Server receives an RS with a valid authentication
   signature or checksum and with destination set to link-scoped All-
   Routers multicast, it can either assume the Hub role the same as
   above or act as a proxy and select the ADM-LLA of another Proxy/
   Server to serve as the Hub. When an FHS Proxy/Server assumes the
   proxy role or receives an RS with destination set to the ADM-LLA of
   another Proxy/Server, it proxys the message.  The FHS Proxy/Server
   caches the Client's Window Synchronization, N flag, Interface
   Attributes and *NET/OAL address information as above then writes its
   own INET-facing FMT/SRT and LHS Proxy/Server MSID/INADDR information
   into the appropriate Interface Attributes sub-option fields.  The FHS
   Proxy/Server then calculates and includes the checksum, performs OAL
   encapsulation with source set to its own ADM-ULA and destination set
   to the ADM-ULA of the Hub Proxy/Server, fragments if necessary,
   encapsulates each fragment in appropriate *NET headers and sends the
   resulting carrier packets into the SRT secured spanning tree.

   When the Hub Proxy/Server receives the carrier packets, it discards
   the *NET headers, reassembles if necessary to obtain the proxyed RS
   (i.e., one with an ADM-ULA source address) then caches any state
   (including the A/U flags, OAL addresses, Interface Attributes
   information and Traffic Selectors) in a NCE for the Client and
   performs any necessary prefix delegation and routing protocol
   injection.  The Hub Proxy/Server then returns an RA that echoes the
   Client's (proxyed) Interface Attributes sub-option and with any RA
   parameters the same as specified above.  The Hub Proxy/Server then
   sets the RA source address to its own ADM-LLA and destination address
   to the Client's MNP-LLA, calculates the checksum then encapsulates
   the RA as an OAL packet with source set to its own ADM-ULA and
   destination set to the ADM-ULA of the FHS Proxy/Server that sent the
   RS.  The Hub Proxy/Server finally fragments if necessary,
   encapsulates each fragment in appropriate *NET headers and sends the
   resulting carrier packets into the secured spanning tree.

   When the FHS Proxy/Server receives the carrier packets it discards
   the *NET headers, reassembles to obtain the RA message, verifies the
   checksum then updates the OMNI interface NCEs for both the Hub and
   Client.  The FHS Proxy/Server then proxies the RA by changing the OAL
   source to its own ADM-ULA and the OAL destination to the MNP-ULA or
   temporary ULA of the Client, then sets the P flag in the RA flags
   field [RFC4389].  The FHS Proxy/Server next includes Window
   Synchronization parameters responsive to those in the Client's RS, an
   Interface Attributes sub-option with omIndex '0' and with its unicast
   *NET IP address if necessary (see above), an Origin Indication sub-
   option with the Client's cached INADDR and an authentication sub-

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   option if necessary.  The FHS Proxy/Server finally selects an
   Identification value per Section 6.6, calculates the authentication
   signature or checksum, fragments if necessary, encapsulates each
   fragment in *NET headers with addresses taken from the Client's NCE
   and returns the resulting carrier packets via the same underlying
   interface over which the RS was received.

   When the Client receives the carrier packets, it discards the *NET
   headers, reassembles and removes the OAL header/trailer to obtain the
   RA message, verifies the authentication signature or checksum, then
   updates the OMNI interface NCEs for both the Hub and FHS Proxy/
   Server.  If the Client has multiple underlying interfaces, it creates
   additional FHS Proxy/Server NCEs as necessary when it receives RAs
   over those interfaces (noting that multiple of the Client's
   underlying interfaces may be serviced by the same FHS Proxy/Server).
   For each underlying interface, the Client caches the (filled-out)
   Interface Attributes for its own omIndex and Origin Indication
   information that it received in an RA message over that interface so
   that it can include them in future NS/NA messages to provide
   neighbors with accurate FMT/SRT/LHS information.  (If the message
   includes an Interface Attributes sub-option with omIndex '0', the
   Client also caches the INADDR as the *NET-local unicast address of
   the FHS Proxy//Server via that underlying interface.)  The Client
   then compares the Origin Indication INADDR information with its own
   underlying interface addresses to determine whether there may be NATs
   on the path to the FHS Proxy/Server; if the INADDR information
   differs, the Client is behind a NAT and must supply the Origin
   information in IPv6 ND message exchanges with prospective neighbors
   on the same SRT segment.  The Client finally configures default
   routes and assigns the OMNI Subnet Router Anycast address
   corresponding to the MNP (e.g., 2001:db8:1:2::) to the OMNI
   interface.

   Following the initial exchange, the FHS Proxy/Server MAY later send
   additional periodic and/or event-driven unsolicited RA messages per
   [RFC4861].  (The unsolicited RAs may be initiated either by the FHS
   Proxy/Server itself or by the Hub via the FHS as a proxy.)  The
   Client then continuously manages its underlying interfaces according
   to their states as follows:

   o  When an underlying interface transitions to UP, the Client sends
      an RS over the underlying interface with an OMNI option with sub-
      options as specified above.

   o  When an underlying interface transitions to DOWN, the Client sends
      unsolicited NA messages over any UP underlying interface with an
      OMNI option containing Interface Attributes sub-options for the
      DOWN underlying interface with Link set to '0'.  The Client sends

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      isolated unsolicited NAs when reliability is not thought to be a
      concern (e.g., if redundant transmissions are sent on multiple
      underlying interfaces), or may instead set the PNG flag in the
      OMNI header to trigger a uNA reply.

   o  When the Router Lifetime for the Hub Proxy/Server nears
      expiration, the Client sends an RS over any underlying interface
      to receive a fresh RA from the Hub. If no RA messages are received
      over a first underlying interface (i.e., after retrying), the
      Client marks the underlying interface as DOWN and should attempt
      to contact the Hub Proxy/Server via a different underlying
      interface.  If the Hub Proxy/Server is unresponsive over
      additional underlying interface, the Client selects a different
      FHS Proxy/Server and sends an RS message with destination set to
      the ADM-LLA of the FHS Proxy/Server which will then assume the Hub
      role.

   o  When all of a Client's underlying interfaces have transitioned to
      DOWN (or if the prefix registration lifetime expires), the Hub
      Proxy/Server withdraws the MNP the same as if it had received a
      message with a release indication.

   The Client is responsible for retrying each RS exchange up to
   MAX_RTR_SOLICITATIONS times separated by RTR_SOLICITATION_INTERVAL
   seconds until an RA is received.  If no RA is received over an UP
   underlying interface (i.e., even after attempting to contact
   alternate Proxy/Servers), the Client declares this underlying
   interface as DOWN.  When changing to a new FHS or Hub Proxy/Server,
   the Client also includes a Proxy/Server Departure OMNI sub-option in
   new RS messages; the (new) FHS Proxy/Server will in turn send uNA
   messages to the old FHS and/or Hub Proxy/Server to announce the
   Client's departure as discussed in [I-D.templin-6man-aero].

   The IPv6 layer sees the OMNI interface as an ordinary IPv6 interface.
   Therefore, when the IPv6 layer sends an RS message the OMNI interface
   returns an internally-generated RA message as though the message
   originated from an IPv6 router.  The internally-generated RA message
   contains configuration information consistent with the information
   received from the RAs generated by the Hub Proxy/Server.  Whether the
   OMNI interface IPv6 ND messaging process is initiated from the
   receipt of an RS message from the IPv6 layer or independently of the
   IPv6 layer is an implementation matter.  Some implementations may
   elect to defer the OMNI interface internal RS/RA messaging process
   until an RS is received from the IPv6 layer, while others may elect
   to initiate the process proactively.  Still other deployments may
   elect to administratively disable IPv6 layer RS/RA messaging over the
   OMNI interface, since the messages are not required to drive the OMNI
   interface internal RS/RA process.  (Note that this same logic applies

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   to IPv4 implementations that employ ICMP-based Router Discovery per
   [RFC1256].)

   Note: The Router Lifetime value in RA messages indicates the time
   before which the Client must send another RS message over this
   underlying interface (e.g., 600 seconds), however that timescale may
   be significantly longer than the lifetime the MS has committed to
   retain the prefix registration (e.g., REACHABLETIME seconds).  Proxy/
   Servers are therefore responsible for keeping MS state alive on a
   shorter timescale than the Client may be required to do on its own
   behalf.

   Note: On certain multicast-capable underlying interfaces, Clients
   should send periodic unsolicited multicast NA messages and Proxy/
   Servers should send periodic unsolicited multicast RA messages as
   "beacons" that can be heard by other nodes on the link.  If a node
   fails to receive a beacon after a timeout value specific to the link,
   it can initiate a unicast exchange to test reachability.

   Note: If a single FHS Proxy/Server services multiple of a Client's
   underlying interfaces, Window Synchronization will initially be
   repeated for the RS/RA exchange over each underlying interface, i.e.,
   until the Client discovers the many-to-one relationship.

   Note: Although the Client's FHS Proxy/Server is a first-hop segment
   node from its own perspective, the Client stores the Proxy/Server's
   FMT/SRT/MSID/INADDR as last-hop segment (LHS) information.  This
   allows both the Client and Hub Proxy/Server to supply the information
   to neighbors that will perceive it as LHS information on the return
   path to the Client.

   Note: The Hub Proxy/Server injects Client MNP-ULAs into the routing
   system by simply creating a route-to-interface forwarding table entry
   for the MNP-ULA via the OMNI interface.  The dynamic routing protocol
   will notice the new entry and advertise the MNP-ULA to its peers.  If
   the Hub receives additional RS messages, it need not re-create the
   MNP-ULA forwarding table entry (nor disturb the dynamic routing
   protocol) if an entry is already present.

   Note: If the Client's initial RS message includes an anycast *NET
   destination address, the FHS Proxy/Server returns the solicited RA
   using the same anycast address as the *NET source while including an
   Interface Attributes sub-option with omIndex '0' and its true unicast
   address in the INADDR.  When the Client sends additional RS messages,
   it includes this FHS Proxy/Server unicast address as the *NET
   destination and the FHS Proxy/Server returns the solicited RA using
   the same unicast address as the *NET source.  This will ensure that
   RS/RA exchanges are not impeded by any NATs on the path while

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   avoiding long-term exposure of messages that use an anycast address
   as the source.

   Note: The Origin Indication sub-option is included only by the FHS
   Proxy/Server and not by the Hub (unless the Hub is also serving as an
   FHS).

   Note: Clients should set the N/A/U flags consistently in successive
   RS messages and only change those settings when an FHS/Hub Proxy/
   Server service profile update is necessary.

15.1.  Window Synchronization

   In environments where Identification window synchronization is
   necessary, the RS/RA exchanges discussed above observe the principles
   specified in Section 6.6.  Window synchronization is conducted
   between the Client and each FHS Proxy/Server used to contact the Hub
   Proxy/Server, i.e., and not between the Client and the Hub.  This is
   due to the fact that the Hub Proxy/Server is responsible only for
   forwarding all control and data messages via the secured spanning
   tree to FHS Proxy/Servers, and is not responsible for forwarding
   messages directly to the Client under a synchronized window.  Also,
   in the reverse direction the FHS Proxy/Servers handle all default
   forwarding actions without forwarding Client-initiated data to the
   Hub.

   When a Client needs to perform window synchronization via a new FHS
   Proxy/Server, it sets the RS source address to its own MNP-LLA and
   destination address to the ADM-LLA of the Hub Proxy/Server, then sets
   the SYN flag and includes an initial Sequence Number for Window
   Synchronization.  The Client then performs OAL encapsulation using
   its own MNP-ULA as the source and the ADM-ULA of the FHS Proxy/Server
   as the destination and includes an Interface Attributes sub-option
   then forwards the resulting carrier packets to the FHS Proxy/Server.
   The FHS Proxy/Server then extracts the RS message and caches the
   Window Synchronization parameters then re-encapsulates with its own
   ADM-ULA as the source and the ADM-ULA of the Hub Proxy/Server as the
   target.

   The FHS Proxy/Server then forwards the resulting carrier packets via
   the secured spanning tree to the Hub Proxy/Server, which updates the
   Client's Interface Attributes and returns a unicast RA message with
   source set to its own ADM-LLA and destination set to the Client's
   MNP-LLA and with the Client's Interface Attributes echoed.  The Hub
   Proxy/Server then performs OAL encapsulation using its own ADM-ULA as
   the source and the ADM-ULA of the FHS Proxy/Server as the
   destination, then forwards the carrier packets via the secured
   spanning tree to the FHS Proxy/Server.  The FHS Proxy/Server then re-

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   encapsulates the message using its own ADM-ULA as the source, the
   MNP-ULA of the Client as the destination and includes responsive
   Window Synchronization information.  The FHS Proxy/Server then
   forwards the message to the Client which updates its window
   synchronization information for the FHS Proxy/Server as necessary.

   Following the initial RS/RA-driven window synchronization, the Client
   can re-assert new windows with specific FHS Proxy/Servers by
   performing NS/NA exchanges between its own MNP-LLA and the ADM-LLAs
   of the FHS Proxy/Servers without having to disturb the Hub.

15.2.  Router Discovery in IP Multihop and IPv4-Only Networks

   On some *NETs, a Client may be located multiple IP hops away from the
   nearest OMNI link Proxy/Server.  Forwarding through IP multihop *NETs
   is conducted through the application of a routing protocol (e.g., a
   MANET/VANET routing protocol over omni-directional wireless
   interfaces, an inter-domain routing protocol in an enterprise
   network, etc.).

   A Client located potentially multiple *NET hops away from the nearest
   Proxy/Server prepares an RS message, sets the source address to its
   MNP-LLA (or to the unspecified address (::) if it does not yet have
   an MNP-LLA), and sets the destination to link-scoped All-Routers
   multicast or a unicast ADM-LLA the same as discussed above.  The OMNI
   interface then employs OAL encapsulation, sets the OAL source address
   to the ULA corresponding to the RS source (or to a Temporary ULA if
   the RS source was the unspecified address (::)) and sets the OAL
   destination to an OMNI IPv6 anycast address based on either a native
   IPv6 or IPv4-mapped IPv6 prefix (see: Section 10).

   For IPv6-enabled *NETs, if the underlying interface does not
   configure an IPv6 GUA the Client forwards the message without further
   encapsulation.  Otherwise, the Client encapsulates the message in
   UDP/IPv6 *NET headers, sets the source to the underlying interface
   GUA and sets the destination to the same OMNI IPv6 anycast address.
   The Client then forwards the message into the IPv6 multihop routing
   system which conveys it to the nearest Proxy/Server that advertises a
   matching OMNI IPv6 anycast prefix.

   For IPv4-only *NETs, the Client encapsulates the RS message in UDP/
   IPv4 *NET headers, sets the source to the underlying interface IPv4
   address and sets the destination to the IPv4 anycast address TBD3
   (see: IANA Considerations).  The Client then forwards the message
   into the IPv4 multihop routing system which conveys it to the nearest
   Proxy/Server that advertises the corresponding IPv4 prefix.  If the
   nearest Proxy/Server is too busy and/or does not configure the
   specified OMNI IPv6 anycast address, it should forward (without

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   Proxying) the OAL-encapsulated RS to another nearby Proxy/Server
   connected to the same IPv4 (multihop) network that configures the
   OMNI IPv6 anycast address.  (In environments where reciprocal RS
   forwarding cannot be supported, the first Proxy/Server should instead
   return an RA based on its own MSP(s).)

   When an intermediate *NET hop that participates in the routing
   protocol receives the encapsulated RS, it forwards the message
   according to its routing tables (note that an intermediate node could
   be a fixed infrastructure element or another MANET/VANET node).  This
   process repeats iteratively until the RS message is received by a
   penultimate *NET hop within single-hop communications range of a
   Proxy/Server, which forwards the message to the Proxy/Server.

   When a Proxy/Server that configures the OMNI IPv6 anycast OAL
   destination receives the message, it decapsulates the RS and assumes
   either the Hub or FHS role (in which case, it forwards the RS to a
   candidate Hub).  The Hub Proxy/Server then prepares an RA message
   with source address set to its own ADM-LLA and destination address
   set to the Client MNP-LLA.  The Hub Proxy/Server then performs OAL
   encapsulation with the RA OAL source/destination set to the RS OAL
   destination/source and forwards the RA to the FHS Proxy/Server or
   directly to the Client.

   When the Hub or FHS Proxy/Server forwards the RA to the Client, it
   encapsulates the message in *NET encapsulation headers (if necessary)
   with (src, dst) set to the (dst,src) of the RS *NET encapsulation
   headers.  The Proxy/Server then forwards the message to a *NET node
   within communications range, which forwards the message according to
   its routing tables to an intermediate node.  The multihop forwarding
   process within the *NET continues repetitively until the message is
   delivered to the original Client, which decapsulates the message and
   performs autoconfiguration the same as if it had received the RA
   directly from a Proxy/Server on the same physical link.

   Note: When the RS message includes anycast OAL and/or *NET
   encapsulation destinations, the FHS Proxy/Server must use the same
   anycast addresses as the OAL and/or *NET encapsulation sources to
   support forwarding of the RA message and any initial data packets
   over any NATs on the path.  When the Client receives the RA, it will
   discover the unicast OAL and/or IPv4 encapsulation addresses and can
   forward future packets using the unicast (instead of anycast)
   addresses to populate NAT state in the forward path.  (If the Client
   does not have immediate data to send to the FHS Proxy/Server, it can
   instead send an OAL "bubble" - see Section 6.12.)  After the Client
   begins using unicast OAL/*NET encapsulation addresses in this way,
   the FHS Proxy/Server should also begin using the same unicast
   addresses in the reverse direction.

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   Note: As an alternate approach to multihop forwarding via IPv6
   encapsulation, the Client and Proxy/Server could statelessly
   translate the IPv6 LLAs into ULAs and forward the RS/RA messages
   without encapsulation.  This would violate the [RFC4861] requirement
   that certain IPv6 ND messages must use link-local addresses and must
   not be accepted if received with Hop Limit less than 255.  This
   document therefore mandates encapsulation since the overhead is
   nominal considering the infrequent nature and small size of IPv6 ND
   messages.  Future documents may consider encapsulation avoidance
   through translation while updating [RFC4861].

   Note: An alternate approach to multihop forwarding via IPv4
   encapsulation would be to employ IPv6/IPv4 protocol translation.
   However, for IPv6 ND messages the LLAs would be truncated due to
   translation and the OMNI Router and Prefix Discovery services would
   not be able to function.  The use of IPv4 encapsulation is therefore
   indicated.

15.3.  DHCPv6-based Prefix Registration

   When a Client is not pre-provisioned with an MNP-LLA (or, when the
   Client requires additional MNP delegations), it requests the MS to
   select MNPs on its behalf and set up the correct routing state.  The
   DHCPv6 service [RFC8415] supports this requirement.

   When a Client requires the MS to select MNPs, it sends an RS message
   with source set to the unspecified address (::) if it has no
   MNP_LLAs.  If the Client requires only a single MNP delegation, it
   can then include a Node Identification sub-option in the OMNI option
   and set Preflen to the length of the desired MNP.  If the Client
   requires multiple MNP delegations and/or more complex DHCPv6
   services, it instead includes a DHCPv6 Message sub-option containing
   a Client Identifier, one or more IA_PD options and a Rapid Commit
   option then sets the 'msg-type' field to "Solicit", and includes a 3
   octet 'transaction-id'.  The Client then sets the RS destination to
   link-scoped All-Routers multicast and sends the message using OAL
   encapsulation and fragmentation if necessary as discussed above.

   When the Hub Proxy/Server receives the RS message, it performs OAL
   reassembly if necessary.  Next, if the RS source is the unspecified
   address (::) and/or the OMNI option includes a DHCPv6 message sub-
   option, the Hub Proxy/Server acts as a "Proxy DHCPv6 Client" in a
   message exchange with the locally-resident DHCPv6 server.  If the RS
   did not contain a DHCPv6 message sub-option, the Hub Proxy/Server
   generates a DHCPv6 Solicit message on behalf of the Client using an
   IA_PD option with the prefix length set to the OMNI header Preflen
   value and with a Client Identifier formed from the OMNI option Node
   Identification sub-option; otherwise, the Hub Proxy/Server uses the

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   DHCPv6 Solicit message contained in the OMNI option.  The Hub Proxy/
   Server then sends the DHCPv6 message to the DHCPv6 Server, which
   delegates MNPs and returns a DHCPv6 Reply message with PD parameters.
   (If the Hub Proxy/Server wishes to defer creation of Client state
   until the DHCPv6 Reply is received, it can instead act as a
   Lightweight DHCPv6 Relay Agent per [RFC6221] by encapsulating the
   DHCPv6 message in a Relay-forward/reply exchange with Relay Message
   and Interface ID options.  In the process, the Hub Proxy/Server packs
   any state information needed to return an RA to the Client in the
   Relay-forward Interface ID option so that the information will be
   echoed back in the Relay-reply.)

   When the Hub Proxy/Server receives the DHCPv6 Reply, it create OMNI
   interface MNP-ULA forwarding table entries (i.e., to prompt the
   dynamic routing protocol) and creates MNP-LLAs based on the delegated
   MNPs.  The Hub Proxy/Server then sends an RA back to the Client with
   the DHCPv6 Reply message included in an OMNI DHCPv6 message sub-
   option if and only if the RS message had included an explicit DHCPv6
   Solicit.  If the RS message source was the unspecified address (::),
   the Hub Proxy/Server includes one of the (newly-created) MNP-LLAs as
   the RA destination address and sets the OMNI option Preflen
   accordingly; otherwise, the Hub Proxy/Server includes the RS source
   address as the RA destination address.  The Hub Proxy/Server then
   sets the RA source address to its own ADM-LLA, performs OAL
   encapsulation and fragmentation, performs *NET encapsulation and
   sends the RA to the Client (i.e., either directly or via an FHS
   Proxy/Server as discussed above).  When the Client receives the RA,
   it reassembles and discards the OAL encapsulation, then creates a
   default route, assigns Subnet Router Anycast addresses and uses the
   RA destination address as its primary MNP-LLA.  The Client will then
   use this primary MNP-LLA as the source address of any IPv6 ND
   messages it sends as long as it retains ownership of the MNP.

16.  Secure Redirection

   If the *NET link model is multiple access, the FHS Proxy/Server is
   responsible for assuring that address duplication cannot corrupt the
   neighbor caches of other nodes on the link.  When the Client sends an
   RS message on a multiple access *NET link, the Proxy/Server verifies
   that the Client is authorized to use the address and responds with an
   RA (or forwards the RS to the Hub) only if the Client is authorized.

   After verifying Client authorization and returning an RA, the Proxy/
   Server MAY return IPv6 ND Redirect messages to direct Clients located
   on the same *NET link to exchange packets directly without transiting
   the Proxy/Server.  In that case, the Clients can exchange packets
   according to their unicast L2 addresses discovered from the Redirect
   message instead of using the dogleg path through the Proxy/Server.

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   In some *NET links, however, such direct communications may be
   undesirable and continued use of the dogleg path through the Proxy/
   Server may provide better performance.  In that case, the Proxy/
   Server can refrain from sending Redirects, and/or Clients can ignore
   them.

17.  Proxy/Server Resilience

   *NETs SHOULD deploy Proxy/Servers in Virtual Router Redundancy
   Protocol (VRRP) [RFC5798] configurations so that service continuity
   is maintained even if one or more Proxy/Servers fail.  Using VRRP,
   the Client is unaware which of the (redundant) FHS Proxy/Servers is
   currently providing service, and any service discontinuity will be
   limited to the failover time supported by VRRP.  Widely deployed
   public domain implementations of VRRP are available.

   Proxy/Servers SHOULD use high availability clustering services so
   that multiple redundant systems can provide coordinated response to
   failures.  As with VRRP, widely deployed public domain
   implementations of high availability clustering services are
   available.  Note that special-purpose and expensive dedicated
   hardware is not necessary, and public domain implementations can be
   used even between lightweight virtual machines in cloud deployments.

18.  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) in a manner that parallels
   Bidirectional Forwarding Detection (BFD) [RFC5880] to track Hub
   Proxy/Server reachability.  FHS Proxy/Servers can then quickly detect
   and react to failures so that cached information is re-established
   through alternate paths.  Proactive NUD control messaging is carried
   only over well-connected ground domain networks (i.e., and not low-
   end *NET links such as aeronautical radios) and can therefore be
   tuned for rapid response.

   FHS Proxy/Servers perform proactive NUD for Hub Proxy/Servers for
   which there are currently active Clients on the *NET.  If a Hub
   Proxy/Server fails, the FHS Proxy/Server can quickly inform Clients
   of the outage by sending multicast RA messages on the *NET interface.
   The FHS Proxy/Server sends RA messages to Clients via the *NET
   interface with source set to the ADM-LLA of the Hub, with destination
   address set to All-Nodes multicast (ff02::1) [RFC4291] and with
   Router Lifetime set to 0.

   The FHS Proxy/Server SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA
   messages separated by small delays [RFC4861].  Any Clients on the

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   *NET interface that have been using the (now defunct) Hub Proxy/
   Server will receive the RA messages.

19.  Transition Considerations

   When a Client connects to an *NET link for the first time, it sends
   an RS message with an OMNI option.  If the first hop router
   recognizes the option, it responds according to the appropriate FHS/
   Hub Proxy/Server role resulting in an RA message with an OMNI option
   returned to the Client.  The Client then engages this FHS Proxy/Sever
   according to the OMNI link model specified above.  If the first hop
   router is a legacy IPv6 router, however, it instead returns an RA
   message with no OMNI option and with a non-OMNI unicast source LLA as
   specified in [RFC4861].  In that case, the Client engages the *NET
   according to the legacy IPv6 link model and without the OMNI
   extensions specified in this document.

   If the *NET link model is multiple access, there must be assurance
   that address duplication cannot corrupt the neighbor caches of other
   nodes on the link.  When the Client sends an RS message on a multiple
   access *NET link with an LLA source address and an OMNI option, first
   hop routers that recognize the OMNI option ensure that the Client is
   authorized to use the address and return an RA with a non-zero Router
   Lifetime only if the Client is authorized.  First hop routers that do
   not recognize the OMNI option instead return an RA that makes no
   statement about the Client's authorization to use the source address.
   In that case, the Client should perform Duplicate Address Detection
   to ensure that it does not interfere with other nodes on the link.

   An alternative approach for multiple access *NET links to ensure
   isolation for Client-Proxy/Server communications is through L2
   address mappings as discussed in Appendix D.  This arrangement
   imparts a (virtual) point-to-point link model over the (physical)
   multiple access link.

20.  OMNI Interfaces on Open Internetworks

   Client OMNI interfaces configured over IPv6-enabled underlying
   interfaces on an open Internetwork without an OMNI-aware first-hop
   router receive IPv6 RA messages with no OMNI options, while OMNI
   interfaces configured over IPv4-only underlying interfaces receive no
   IPv6 RA messages at all (but may receive IPv4 RA messages [RFC1256]).
   Client OMNI interfaces that receive RA messages with OMNI options
   configure addresses, on-link prefixes, etc. on the underlying
   interface that received the RA according to standard IPv6 ND and
   address resolution conventions [RFC4861] [RFC4862].  Client OMNI
   interfaces configured over IPv4-only underlying interfaces configure

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   IPv4 address information on the underlying interfaces using
   mechanisms such as DHCPv4 [RFC2131].

   Client OMNI interfaces configured over underlying interfaces
   connected to open Internetworks can apply security services such as
   VPNs to connect to a Proxy/Server, or can establish a direct link to
   the Proxy/Server through some other means (see Section 4).  In
   environments where an explicit VPN or direct link may be impractical,
   Client OMNI interfaces can instead send IPv6 ND messages with OMNI
   options that include authentication signatures.

   OMNI interfaces use UDP/IP as *NET encapsulation headers for
   transmission over open Internetworks with UDP service port number
   8060 (see: Section 25.13 and Section 3.6 of [I-D.templin-6man-aero])
   for both IPv4 and IPv6 underlying interfaces.  The OMNI interface
   submits original IP packets for OAL encapsulation, then encapsulates
   the resulting OAL fragments in UDP/IP *NET headers to form carrier
   packets.  (The first four bits following the UDP header determine
   whether the OAL headers are uncompressed/compressed as discussed in
   Section 6.4.)  The OMNI interface sets the UDP length to the
   encapsulated OAL fragment length and sets the IP length to an
   appropriate value at least as large as the UDP datagram.

   For Client-Proxy/Server (e.g., "Vehicle-to-Infrastructure (V2I)")
   neighbor exchanges, the source must include an OMNI option with an
   authentication sub-option in all IPv6 ND messages.  The source can
   apply HIP security services per [RFC7401] using the IPv6 ND message
   OMNI option as a "shipping container" to convey an authentication
   signature in a (unidirectional) HIP "Notify" message.  For Client-
   Client (e.g., "Vehicle-to-Vehicle (V2V)") neighbor exchanges, two
   Clients can exchange HIP "Initiator/Responder" messages coded in OMNI
   options of multiple IPv6 NS/NA messages for mutual authentication
   according to the HIP protocol.  (Note: a simple Hashed Message
   Authentication Code (HMAC) such as specified in [RFC4380] or the
   QUIC-TLS connection-oriented service [RFC9000] can be used as an
   alternate authentication service in some environments.)

   When an OMNI interface includes an authentication sub-option, it must
   appear as the first sub-option of the first OMNI option in the IPv6
   ND message which must appear immediately following the IPv6 ND
   message header.  When an OMNI interface prepares a HIP message sub-
   option, it includes its own (H)HIT as the Sender's HIT and the
   neighbor's (H)HIT if known as the Receiver's HIT (otherwise 0).  If
   (H)HITs are not available within the OMNI operational environment,
   the source can instead include other IPv6 address types instead of
   (H)HITs as long as the Sender and Receiver have some way to associate
   information embedded in the IPv6 address with the neighbor; such

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   information could include a node identifier, vehicle identifier, MAC
   address, etc.

   Before calculating the authentication signature, the source includes
   any other necessary sub-options (such as Interface Attributes and
   Origin Indication) and sets both the IPv6 ND message Checksum and
   authentication signature fields to 0.  The source then calculates the
   authentication signature over the full length of the IPv6 ND message
   beginning with a pseudo-header of the IPv6 header (i.e., the same as
   specified in [RFC4443]) and extending over the length of the message.
   (If the IPv6 ND message is part of an OAL super-packet, the source
   instead calculates the authentication signature over the remainder of
   the super-packet up to but not including the trailing OAL Checksum
   field.)  The source next writes the authentication signature into the
   sub-option signature field and forwards the message.

   After establishing a VPN or preparing for UDP/IP encapsulation, OMNI
   interfaces send RS/RA messages for Client-Proxy/Server coordination
   (see: Section 15) and NS/NA messages for route optimization, window
   synchronization and mobility management (see:
   [I-D.templin-6man-aero]).  These control plane messages must be
   authenticated while other control and data plane messages are
   delivered the same as for ordinary best-effort traffic with source
   address and/or Identification window-based data origin verification.
   Upper layer protocol sessions over OMNI interfaces that connect over
   open Internetworks without an explicit VPN should therefore employ
   transport- or higher-layer security to ensure authentication,
   integrity and/or confidentiality.

   Clients should avoid using INET Proxy/Servers as general-purpose
   routers for steady streams of carrier packets that do not require
   authentication.  Clients should instead coordinate with other INET
   nodes that can provide forwarding services instead of burdening the
   Proxy/Server (or preferably coordinate directly with peer Clients
   directly).  Procedures for coordinating with peer Clients and
   discovering INET nodes that can provide better forwarding services
   are discussed in [I-D.templin-6man-aero].

   Clients that attempt to contact peers over INET underlying interfaces
   often encounter NATs in the path.  OMNI interfaces accommodate NAT
   traversal using UDP/IP encapsulation and the mechanisms discussed in
   [I-D.templin-6man-aero].  FHS Proxy/Servers include Origin
   Indications in RA messages to allow Clients to detect the presence of
   NATs.

   Note: Following the initial IPv6 ND message exchange, OMNI interfaces
   configured over INET underlying interfaces maintain neighbor
   relationships by transmitting periodic IPv6 ND messages with OMNI

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   options that include HIP "Update" and/or "Notify" messages.  When
   HMAC authentication is used instead of HIP, the Client and Proxy/
   Server exchange all IPv6 ND messages with HMAC signatures included
   based on a shared-secret.  When QUIC-TLS connections are used, the
   Client and Proxy/Server observe QUIC-TLS conventions
   [RFC9000][RFC9001].

   Note: OMNI interfaces configured over INET underlying interfaces
   should employ the Identification window synchronization mechanisms
   specified in Section 6.6 in order to reject spurious carrier packets
   that might otherwise clutter the reassembly cache.  This is
   especially important in environments where carrier packet spoofing
   and/or corruption is a threat.

   Note: NATs may be present on the path from a Client to its FHS Proxy/
   Server, but never on the path from the FHS Proxy/Server to the Hub
   where only INET and/or spanning tree hops occur.  Therefore, the FHS
   Proxy/Server does not communicate Client origin information to the
   Hub where it would serve no purpose.

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 Client 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 prefix delegation services discussed in Section 15.3 allows
   Clients that desire time-varying MNPs to obtain short-lived prefixes
   to send RS messages with source set to the unspecified address (::)
   and/or with an OMNI option with DHCPv6 Option sub-options.  The
   Client would then be obligated to renumber its internal networks
   whenever its MNP (and therefore also its OMNI address) changes.  This
   should not present a challenge for Clients with automated network
   renumbering services, but may disrupt persistent sessions that would
   prefer to use a constant address.

22.  (H)HITs and Temporary ULAs

   Clients that generate (H)HITs but do not have pre-assigned MNPs can
   request MNP delegations by issuing IPv6 ND messages that use the
   (H)HIT instead of a Temporary ULA.  In particular, when a Client
   creates an RS message it can set the source to the unspecified
   address (::) and destination to link-scoped All-Routers multicast.

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   The IPv6 ND message includes an OMNI option with a HIP message sub-
   option, and need not include a Node Identification sub-option if the
   Client's HIT appears in the HIP message.  The Client then
   encapsulates the message in an IPv6 header with the (H)HIT as the
   source address.  The Client then sends the message as specified in
   Section 15.2.

   When a Proxy/Server receives the RS message, it notes that the source
   was the unspecified address (::), then examines the OAL encapsulation
   source address to determine that the source is a (H)HIT and not a
   Temporary ULA.  The Proxy/Server next invokes the DHCPv6 protocol to
   request an MNP prefix delegation while using the HIT (in the form of
   a DUID) as the Client Identifier, then prepares an RA message with
   source address set to its own ADM-LLA and destination set to the MNP-
   LLA corresponding to the delegated MNP.  The Proxy/Server next
   includes an OMNI option with a HIP message sub-option and any DHCPv6
   prefix delegation parameters.  The Proxy/Server finally encapsulates
   the RA in an IPv6 header with source address set to its own ADM-ULA
   and destination set to the (H)HIT from the RS encapsulation source
   address, then returns the encapsulated RA to the Client.

   Clients can also use (H)HITs and/or Temporary ULAs for direct Client-
   to-Client communications outside the context of any OMNI link
   supporting infrastructure.  When two Clients encounter one another
   they can use their (H)HITs and/or Temporary ULAs as original IPv6
   packet source and destination addresses to support direct
   communications.  Clients can also inject their (H)HITs and/or
   Temporary ULAs into a MANET/VANET routing protocol to enable multihop
   communications.  Clients can further exchange IPv6 ND messages (such
   as NS/NA) using their (H)HITs and/or Temporary ULAs as source and
   destination addresses.

   Lastly, when Clients are within the coverage range of OMNI link
   infrastructure a case could be made for injecting (H)HITs and/or
   Temporary ULAs into the global MS routing system.  For example, when
   the Client sends an RS to an FHS Proxy/Server it could include a
   request to inject the (H)HIT / Temporary ULA into the routing system
   instead of requesting an MNP prefix delegation.  This would
   potentially enable OMNI link-wide communications using only (H)HITs
   or Temporary ULAs, and not MNPs.  This document notes the
   opportunity, but makes no recommendation.

23.  Address Selection

   Clients use LLAs only for link-scoped communications on the OMNI
   link.  Typically, Clients use LLAs as source/destination IPv6
   addresses of IPv6 ND messages, but may also use them for addressing
   ordinary original IP packets exchanged with an OMNI link neighbor.

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   Clients use MNP-ULAs as source/destination IPv6 addresses in the
   encapsulation headers of OAL packets.  Clients use Temporary ULAs for
   OAL addressing when an MNP-ULA is not available, or as source/
   destination IPv6 addresses for communications within a MANET/VANET
   local area.  Clients can also use (H)HITs instead of Temporary ULAs
   when operation outside the context of a specific ULA domain and/or
   source address attestation is necessary.

   Clients use MNP-based GUAs as original IP packet source and
   destination addresses for communications with Internet destinations
   when they are within range of OMNI link supporting infrastructure
   that can inject the MNP into the routing system.

   Clients use anycast GUAs as OAL and/or *NET encapsulation destination
   addresses for RS messages used to discover the nearest FHS Proxy/
   Server.  When the Proxy/Server returns a solicited RA, it must also
   use the same anycast address as the RA OAL/*NET encapsulation source
   in order to successfully traverse any NATs in the path.  The Client
   should then immediately transition to using the FHS Proxy/Server's
   discovered unicast OAL/*NET address as the destination in order to
   minimize dependence on the Proxy/Server's use of an anycast source
   address.

24.  Error Messages

   An OAL destination or intermediate node may need to return
   ICMPv6-like error messages (e.g., Destination Unreachable, Packet Too
   Big, Time Exceeded, etc.)  [RFC4443] to an OAL source.  Since ICMPv6
   error messages do not themselves include authentication codes, OAL
   nodes can return error messages as an OMNI ICMPv6 Error sub-option in
   a secured IPv6 ND uNA message.

25.  IANA Considerations

   The following IANA actions are requested in accordance with [RFC8126]
   and [RFC8726]:

25.1.  "Protocol Numbers" Registry

   The IANA is instructed to allocate an Internet Protocol number TBD1
   from the 'protocol numbers' registry for the Overlay Multilink
   Network Interface (OMNI) protocol.  Guidance is found in [RFC5237]
   (registration procedure is IESG Approval or Standards Action).

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25.2.  "IEEE 802 Numbers" Registry

   During final publication stages, the IESG will be requested to
   procure an IEEE EtherType value TBD2 for OMNI according to the
   statement found at https://www.ietf.org/about/groups/iesg/statements/
   ethertypes/.

   Following this procurement, the IANA is instructed to register the
   value TBD2 in the 'ieee-802-numbers' registry for Overlay Multilink
   Network Interface (OMNI) encapsulation on Ethernet networks.
   Guidance is found in [RFC7042] (registration procedure is Expert
   Review).

25.3.  "IPv4 Special-Purpose Address" Registry

   The IANA is instructed to assign TBD3/N as an "OMNI IPv4 anycast"
   address/prefix in the "IPv4 Special-Purpose Address" registry.  This
   specification recommends assigning the address 192.88.99.100/24 as
   the "OMNI IPv4 anycast" address/prefix, since the former use of the
   address/prefix 192.88.99.1/24 is deprecated by [RFC7526].  In the
   event that conflicts with the former use are deemed irreconcilable,
   the IANA is instructed to work with authors to determine an alternate
   TBD3/N address/prefix.

25.4.  "IPv6 Neighbor Discovery Option Formats" Registry

   The IANA is instructed to allocate an official Type number TBD4 from
   the "IPv6 Neighbor Discovery Option Formats" registry for the OMNI
   option (registration procedure is RFC required).  Implementations set
   Type to 253 as an interim value [RFC4727].

25.5.  "Ethernet Numbers" Registry

   The IANA is instructed to allocate one Ethernet unicast address TBD5
   (suggested value '00-52-14') in the 'ethernet-numbers' registry under
   "IANA Unicast 48-bit MAC Addresses" (registration procedure is Expert
   Review).  The registration should appear as follows:

   Addresses      Usage                                         Reference
   ---------      -----                                         ---------
   00-52-14       Overlay Multilink Network (OMNI) Interface    [RFCXXXX]

               Figure 34: IANA Unicast 48-bit MAC Addresses

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25.6.  "ICMPv6 Code Fields: Type 2 - Packet Too Big" Registry

   The IANA is instructed to assign two new Code values in the "ICMPv6
   Code Fields: Type 2 - Packet Too Big" registry (registration
   procedure is Standards Action or IESG Approval).  The registry should
   appear as follows:

      Code      Name                         Reference
      ---       ----                         ---------
      0         PTB Hard Error               [RFC4443]
      1         PTB Soft Error (loss)        [RFCXXXX]
      2         PTB Soft Error (no loss)     [RFCXXXX]

       Figure 35: ICMPv6 Code Fields: Type 2 - Packet Too Big Values

   (Note: this registry also to be used to define values for setting the
   "unused" field of ICMPv4 "Destination Unreachable - Fragmentation
   Needed" messages.)

25.7.  "OMNI Option Sub-Type Values" (New Registry)

   The OMNI option defines a 5-bit Sub-Type field, for which IANA is
   instructed to create and maintain a new registry entitled "OMNI
   Option Sub-Type Values".  Initial values are given below
   (registration procedure is RFC required):

      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        Pad1                           [RFCXXXX]
      1        PadN                           [RFCXXXX]
      2        Interface Attributes           [RFCXXXX]
      3        Multilink Fwding Parameters    [RFCXXXX]
      4        Traffic Selector               [RFCXXXX]
      5        Geo Coordinates                [RFCXXXX]
      6        DHCPv6 Message                 [RFCXXXX]
      7        HIP Message                    [RFCXXXX]
      8        PIM-SM Message                 [RFCXXXX]
      9        Reassembly Limit               [RFCXXXX]
      10       Fragmentation Report           [RFCXXXX]
      11       Node Identification            [RFCXXXX]
      12       ICMPv6 Error                   [RFCXXXX]
      13       QUIC-TLS Message               [RFCXXXX]
      14       Proxy/Server Departure         [RFCXXXX]
      15-29    Unassigned
      30       Sub-Type Extension             [RFCXXXX]
      31       Reserved by IANA               [RFCXXXX]

                  Figure 36: OMNI Option Sub-Type Values

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25.8.  "OMNI Geo Coordinates Type Values" (New Registry)

   The OMNI Geo Coordinates sub-option (see: Section 12.2.6) contains an
   8-bit Type field, for which IANA is instructed to create and maintain
   a new registry entitled "OMNI Geo Coordinates Type Values".  Initial
   values are given below (registration procedure is RFC required):

      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        NULL                           [RFCXXXX]
      1-252    Unassigned                     [RFCXXXX]
      253-254  Reserved for Experimentation   [RFCXXXX]
      255      Reserved by IANA               [RFCXXXX]

                   Figure 37: OMNI Geo Coordinates Type

25.9.  "OMNI Node Identification ID-Type Values" (New Registry)

   The OMNI Node Identification sub-option (see: Section 12.2.12)
   contains an 8-bit ID-Type field, for which IANA is instructed to
   create and maintain a new registry entitled "OMNI Node Identification
   ID-Type Values".  Initial values are given below (registration
   procedure is RFC required):

      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        UUID                           [RFCXXXX]
      1        HIT                            [RFCXXXX]
      2        HHIT                           [RFCXXXX]
      3        Network Access Identifier      [RFCXXXX]
      4        FQDN                           [RFCXXXX]
      5        IPv6 Address                   [RFCXXXX]
      6-252    Unassigned                     [RFCXXXX]
      253-254  Reserved for Experimentation   [RFCXXXX]
      255      Reserved by IANA               [RFCXXXX]

            Figure 38: OMNI Node Identification ID-Type Values

25.10.  "OMNI Option Sub-Type Extension Values" (New Registry)

   The OMNI option defines an 8-bit Extension-Type field for Sub-Type 30
   (Sub-Type Extension), for which IANA is instructed to create and
   maintain a new registry entitled "OMNI Option Sub-Type Extension
   Values".  Initial values are given below (registration procedure is
   RFC required):

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      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        RFC4380 UDP/IP Header Option   [RFCXXXX]
      1        RFC6081 UDP/IP Trailer Option  [RFCXXXX]
      2-252    Unassigned
      253-254  Reserved for Experimentation   [RFCXXXX]
      255      Reserved by IANA               [RFCXXXX]

             Figure 39: OMNI Option Sub-Type Extension Values

25.11.  "OMNI RFC4380 UDP/IP Header Option" (New Registry)

   The OMNI Sub-Type Extension "RFC4380 UDP/IP Header Option" defines an
   8-bit Header Type field, for which IANA is instructed to create and
   maintain a new registry entitled "OMNI RFC4380 UDP/IP Header Option".
   Initial registry values are given below (registration procedure is
   RFC required):

      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        Origin Indication (IPv4)       [RFC4380]
      1        Authentication Encapsulation   [RFC4380]
      2        Origin Indication (IPv6)       [RFCXXXX]
      3-252    Unassigned
      253-254  Reserved for Experimentation   [RFCXXXX]
      255      Reserved by IANA               [RFCXXXX]

               Figure 40: OMNI RFC4380 UDP/IP Header Option

25.12.  "OMNI RFC6081 UDP/IP Trailer Option" (New Registry)

   The OMNI Sub-Type Extension for "RFC6081 UDP/IP Trailer Option"
   defines an 8-bit Trailer Type field, for which IANA is instructed to
   create and maintain a new registry entitled "OMNI RFC6081 UDP/IP
   Trailer Option".  Initial registry values are given below
   (registration procedure is RFC required):

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      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        Unassigned
      1        Nonce                          [RFC6081]
      2        Unassigned
      3        Alternate Address (IPv4)       [RFC6081]
      4        Neighbor Discovery Option      [RFC6081]
      5        Random Port                    [RFC6081]
      6        Alternate Address (IPv6)       [RFCXXXX]
      7-252    Unassigned
      253-254  Reserved for Experimentation   [RFCXXXX]
      255      Reserved by IANA               [RFCXXXX]

                  Figure 41: OMNI RFC6081 Trailer Option

25.13.  Additional Considerations

   The IANA has assigned the UDP port number "8060" for an earlier
   experimental version of AERO [RFC6706].  This document reclaims the
   UDP port number "8060" for 'aero' as the service port for UDP/IP
   encapsulation.  (Note that, although [RFC6706] was not widely
   implemented or deployed, any messages coded to that specification can
   be easily distinguished and ignored since they use an invalid ICMPv6
   message type number '0'.)  The IANA is therefore instructed to update
   the reference for UDP port number "8060" from "RFC6706" to "RFCXXXX"
   (i.e., this document) while retaining the existing name 'aero'.

   The IANA has assigned a 4 octet Private Enterprise Number (PEN) code
   "45282" in the "enterprise-numbers" registry.  This document is the
   normative reference for using this code in DHCP Unique IDentifiers
   based on Enterprise Numbers ("DUID-EN for OMNI Interfaces") (see:
   Section 11).  The IANA is therefore instructed to change the
   enterprise designation for PEN code "45282" from "LinkUp Networks" to
   "Overlay Multilink Network Interface (OMNI)".

   The IANA has assigned the ifType code "301 - omni - Overlay Multilink
   Network Interface (OMNI)" in accordance with Section 6 of [RFC8892].
   The registration appears under the IANA "Structure of Management
   Information (SMI) Numbers (MIB Module Registrations) - Interface
   Types (ifType)" registry.

   No further IANA actions are required.

26.  Security Considerations

   Security considerations for IPv4 [RFC0791], IPv6 [RFC8200] and IPv6
   Neighbor Discovery [RFC4861] apply.  OMNI interface IPv6 ND messages
   SHOULD include Nonce and Timestamp options [RFC3971] when transaction

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   confirmation and/or time synchronization is needed.  (Note however
   that when OAL encapsulation is used the (echoed) OAL Identification
   value can provide sufficient transaction confirmation.)

   Client OMNI interfaces configured over secured ANET interfaces
   inherit the physical and/or link-layer security properties (i.e.,
   "protected spectrum") of the connected ANETs.  Client OMNI interfaces
   configured over open INET interfaces can use symmetric securing
   services such as VPNs or can by some other means establish a direct
   link.  When a VPN or direct link may be impractical, however, the
   security services specified in [RFC7401] and/or [RFC4380] can be
   employed.  While the OMNI link protects control plane messaging,
   applications must still employ end-to-end transport- or higher-layer
   security services to protect the data plane.

   Strong network layer security for control plane messages and
   forwarding path integrity for data plane messages between Proxy/
   Servers MUST be supported.  In one example, the AERO service
   [I-D.templin-6man-aero] constructs an SRT spanning tree with Proxy/
   Serves as leaf nodes and secures the spanning tree links with network
   layer security mechanisms such as IPsec [RFC4301] or WireGuard.
   Secured control plane messages are then constrained to travel only
   over the secured spanning tree paths and are therefore protected from
   attack or eavesdropping.  Other control and data plane messages can
   travel over route optimized paths that do not strictly follow the
   secured spanning tree, therefore end-to-end sessions should employ
   transport- or higher-layer security services.  Additionally, the OAL
   Identification value can provide a first level of data origin
   authentication to mitigate off-path spoofing in some environments.

   Identity-based key verification infrastructure services such as iPSK
   may be necessary for verifying the identities claimed by Clients.
   This requirement should be harmonized with the manner in which
   (H)HITs are attested in a given operational environment.

   Security considerations for specific access network interface types
   are covered under the corresponding IP-over-(foo) specification
   (e.g., [RFC2464], [RFC2492], etc.).

   Security considerations for IPv6 fragmentation and reassembly are
   discussed in Section 6.10.  In environments where spoofing is
   considered a threat, OMNI nodes SHOULD employ Identification window
   synchronization and OAL destinations SHOULD configure an (end-system-
   based) firewall.

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27.  Implementation Status

   AERO/OMNI Release-3.2 was tagged on March 30, 2021, and is undergoing
   internal testing.  Additional internal releases expected within the
   coming months, with first public release expected end of 1H2021.

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

28.  Document Updates

   This document does not itself update other RFCs, but suggests that
   the following could be updated through future IETF initiatives:

   o  [RFC1191]

   o  [RFC4443]

   o  [RFC8201]

   o  [RFC7526]

   Updates can be through, e.g., standards action, the errata process,
   etc. as appropriate.

29.  Acknowledgements

   The first version of this document was prepared per the consensus
   decision at the 7th Conference of the International Civil Aviation
   Organization (ICAO) Working Group-I Mobility Subgroup on March 22,
   2019.  Consensus to take the document forward to the IETF was reached
   at the 9th Conference of the Mobility Subgroup on November 22, 2019.
   Attendees and contributors included: Guray Acar, Danny Bharj,
   Francois D'Humieres, Pavel Drasil, Nikos Fistas, Giovanni Garofolo,
   Bernhard Haindl, Vaughn Maiolla, Tom McParland, Victor Moreno, Madhu
   Niraula, Brent Phillips, Liviu Popescu, Jacky Pouzet, Aloke Roy, Greg
   Saccone, Robert Segers, Michal Skorepa, Michel Solery, Stephane
   Tamalet, Fred Templin, Jean-Marc Vacher, Bela Varkonyi, Tony Whyman,
   Fryderyk Wrobel and Dongsong Zeng.

   The following individuals are acknowledged for their useful comments:
   Amanda Barber, Stuart Card, Donald Eastlake, Michael Matyas, Robert
   Moskowitz, Madhu Niraula, Greg Saccone, Stephane Tamalet, Eduard
   Vasilenko, Eric Vyncke.  Pavel Drasil, Zdenek Jaron and Michal
   Skorepa are especially recognized for their many helpful ideas and
   suggestions.  Madhuri Madhava Badgandi, Sean Dickson, Don Dillenburg,
   Joe Dudkowski, Vijayasarathy Rajagopalan, Ron Sackman and Katherine

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   Tran are acknowledged for their hard work on the implementation and
   technical insights that led to improvements for the spec.

   Discussions on the IETF 6man and atn mailing lists during the fall of
   2020 suggested additional points to consider.  The authors gratefully
   acknowledge the list members who contributed valuable insights
   through those discussions.  Eric Vyncke and Erik Kline were the
   intarea ADs, while Bob Hinden and Ole Troan were the 6man WG chairs
   at the time the document was developed; they are all gratefully
   acknowledged for their many helpful insights.  Many of the ideas in
   this document have further built on IETF experiences beginning in the
   1990s, with insights from colleagues including Ron Bonica, Brian
   Carpenter, Ralph Droms, Christian Huitema, Thomas Narten, Dave
   Thaler, Joe Touch, and many others who deserve recognition.

   Early observations on IP fragmentation performance implications were
   noted in the 1986 Digital Equipment Corporation (DEC) "qe reset"
   investigation, where fragment bursts from NFS UDP traffic triggered
   hardware resets resulting in communication failures.  Jeff Chase,
   Fred Glover and Chet Juzsczak of the Ultrix Engineering Group led the
   investigation, and determined that setting a smaller NFS mount block
   size reduced the amount of fragmentation and suppressed the resets.
   Early observations on L2 media MTU issues were noted in the 1988 DEC
   FDDI investigation, where Raj Jain, KK Ramakrishnan and Kathy Wilde
   represented architectural considerations for FDDI networking in
   general including FDDI/Ethernet bridging.  Jeff Mogul (who led the
   IETF Path MTU Discovery working group) and other DEC colleagues who
   supported these early investigations are also acknowledged.

   Throughout the 1990's and into the 2000's, many colleagues supported
   and encouraged continuation of the work.  Beginning with the DEC
   Project Sequoia effort at the University of California, Berkeley,
   then moving to the DEC research lab offices in Palo Alto CA, then to
   Sterling Software at the NASA Ames Research Center, then to SRI in
   Menlo Park, CA, then to Nokia in Mountain View, CA and finally to the
   Boeing Company in 2005 the work saw continuous advancement through
   the encouragement of many.  Those who offered their support and
   encouragement are gratefully acknowledged.

   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 Information Technology (BIT)
   Mobility Vision Lab (MVL) program.

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30.  References

30.1.  Normative References

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

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [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>.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,
              <https://www.rfc-editor.org/info/rfc3971>.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
              November 2005, <https://www.rfc-editor.org/info/rfc4191>.

   [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>.

   [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>.

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   [RFC4727]  Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
              ICMPv6, UDP, and TCP Headers", RFC 4727,
              DOI 10.17487/RFC4727, November 2006,
              <https://www.rfc-editor.org/info/rfc4727>.

   [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>.

   [RFC6088]  Tsirtsis, G., Giarreta, G., Soliman, H., and N. Montavont,
              "Traffic Selectors for Flow Bindings", RFC 6088,
              DOI 10.17487/RFC6088, January 2011,
              <https://www.rfc-editor.org/info/rfc6088>.

   [RFC8028]  Baker, F. and B. Carpenter, "First-Hop Router Selection by
              Hosts in a Multi-Prefix Network", RFC 8028,
              DOI 10.17487/RFC8028, November 2016,
              <https://www.rfc-editor.org/info/rfc8028>.

   [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>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

   [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>.

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30.2.  Informative References

   [ATN]      Maiolla, V., "The OMNI Interface - An IPv6 Air/Ground
              Interface for Civil Aviation, IETF Liaison Statement
              #1676, https://datatracker.ietf.org/liaison/1676/", March
              2020.

   [ATN-IPS]  WG-I, ICAO., "ICAO Document 9896 (Manual on the
              Aeronautical Telecommunication Network (ATN) using
              Internet Protocol Suite (IPS) Standards and Protocol),
              Draft Edition 3 (work-in-progress)", December 2020.

   [CKSUM]    Stone, J., Greenwald, M., Partridge, C., and J. Hughes,
              "Performance of Checksums and CRC's Over Real Data, IEEE/
              ACM Transactions on Networking, Vol. 6, No. 5", October
              1998.

   [CRC]      Jain, R., "Error Characteristics of Fiber Distributed Data
              Interface (FDDI), IEEE Transactions on Communications",
              August 1990.

   [I-D.ietf-drip-rid]
              Moskowitz, R., Card, S. W., Wiethuechter, A., and A.
              Gurtov, "DRIP Entity Tag (DET) for Unmanned Aircraft
              System Remote Identification (UAS RID)", draft-ietf-drip-
              rid-10 (work in progress), September 2021.

   [I-D.ietf-intarea-tunnels]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-10 (work in
              progress), September 2019.

   [I-D.ietf-ipwave-vehicular-networking]
              (editor), J. (. J., "IPv6 Wireless Access in Vehicular
              Environments (IPWAVE): Problem Statement and Use Cases",
              draft-ietf-ipwave-vehicular-networking-24 (work in
              progress), October 2021.

   [I-D.ietf-tsvwg-udp-options]
              Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
              udp-options-13 (work in progress), June 2021.

   [I-D.templin-6man-aero]
              Templin, F. L., "Automatic Extended Route Optimization
              (AERO)", draft-templin-6man-aero-34 (work in progress),
              September 2021.

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   [I-D.templin-6man-lla-type]
              Templin, F. L., "The IPv6 Link-Local Address Type Field",
              draft-templin-6man-lla-type-02 (work in progress),
              November 2020.

   [I-D.templin-6man-omni-interface]
              Templin, F. L. and T. Whyman, "Transmission of IP Packets
              over Overlay Multilink Network (OMNI) Interfaces", draft-
              templin-6man-omni-interface-99 (work in progress), March
              2021.

   [IPV4-GUA]
              Postel, J., "IPv4 Address Space Registry,
              https://www.iana.org/assignments/ipv4-address-space/ipv4-
              address-space.xhtml", December 2020.

   [IPV6-GUA]
              Postel, J., "IPv6 Global Unicast Address Assignments,
              https://www.iana.org/assignments/ipv6-unicast-address-
              assignments/ipv6-unicast-address-assignments.xhtml",
              December 2020.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [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>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC1146]  Zweig, J. and C. Partridge, "TCP alternate checksum
              options", RFC 1146, DOI 10.17487/RFC1146, March 1990,
              <https://www.rfc-editor.org/info/rfc1146>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

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

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   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC2225]  Laubach, M. and J. Halpern, "Classical IP and ARP over
              ATM", RFC 2225, DOI 10.17487/RFC2225, April 1998,
              <https://www.rfc-editor.org/info/rfc2225>.

   [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>.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <https://www.rfc-editor.org/info/rfc2473>.

   [RFC2492]  Armitage, G., Schulter, P., and M. Jork, "IPv6 over ATM
              Networks", RFC 2492, DOI 10.17487/RFC2492, January 1999,
              <https://www.rfc-editor.org/info/rfc2492>.

   [RFC2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
              Addresses", RFC 2526, DOI 10.17487/RFC2526, March 1999,
              <https://www.rfc-editor.org/info/rfc2526>.

   [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>.

   [RFC2863]  McCloghrie, K. and F. Kastenholz, "The Interfaces Group
              MIB", RFC 2863, DOI 10.17487/RFC2863, June 2000,
              <https://www.rfc-editor.org/info/rfc2863>.

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, DOI 10.17487/RFC2923, September 2000,
              <https://www.rfc-editor.org/info/rfc2923>.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,
              <https://www.rfc-editor.org/info/rfc2983>.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, DOI 10.17487/RFC3056, February
              2001, <https://www.rfc-editor.org/info/rfc3056>.

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   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3330]  IANA, "Special-Use IPv4 Addresses", RFC 3330,
              DOI 10.17487/RFC3330, September 2002,
              <https://www.rfc-editor.org/info/rfc3330>.

   [RFC3366]  Fairhurst, G. and L. Wood, "Advice to link designers on
              link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
              DOI 10.17487/RFC3366, August 2002,
              <https://www.rfc-editor.org/info/rfc3366>.

   [RFC3692]  Narten, T., "Assigning Experimental and Testing Numbers
              Considered Useful", BCP 82, RFC 3692,
              DOI 10.17487/RFC3692, January 2004,
              <https://www.rfc-editor.org/info/rfc3692>.

   [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>.

   [RFC3819]  Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, DOI 10.17487/RFC3819, July 2004,
              <https://www.rfc-editor.org/info/rfc3819>.

   [RFC3879]  Huitema, C. and B. Carpenter, "Deprecating Site Local
              Addresses", RFC 3879, DOI 10.17487/RFC3879, September
              2004, <https://www.rfc-editor.org/info/rfc3879>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [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>.

   [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>.

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   [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>.

   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
              <https://www.rfc-editor.org/info/rfc4429>.

   [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>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://www.rfc-editor.org/info/rfc4821>.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963,
              DOI 10.17487/RFC4963, July 2007,
              <https://www.rfc-editor.org/info/rfc4963>.

   [RFC5175]  Haberman, B., Ed. and R. Hinden, "IPv6 Router
              Advertisement Flags Option", RFC 5175,
              DOI 10.17487/RFC5175, March 2008,
              <https://www.rfc-editor.org/info/rfc5175>.

   [RFC5213]  Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
              Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
              RFC 5213, DOI 10.17487/RFC5213, August 2008,
              <https://www.rfc-editor.org/info/rfc5213>.

   [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>.

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   [RFC5237]  Arkko, J. and S. Bradner, "IANA Allocation Guidelines for
              the Protocol Field", BCP 37, RFC 5237,
              DOI 10.17487/RFC5237, February 2008,
              <https://www.rfc-editor.org/info/rfc5237>.

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

   [RFC5798]  Nadas, S., Ed., "Virtual Router Redundancy Protocol (VRRP)
              Version 3 for IPv4 and IPv6", RFC 5798,
              DOI 10.17487/RFC5798, March 2010,
              <https://www.rfc-editor.org/info/rfc5798>.

   [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>.

   [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>.

   [RFC6247]  Eggert, L., "Moving the Undeployed TCP Extensions RFC
              1072, RFC 1106, RFC 1110, RFC 1145, RFC 1146, RFC 1379,
              RFC 1644, and RFC 1693 to Historic Status", RFC 6247,
              DOI 10.17487/RFC6247, May 2011,
              <https://www.rfc-editor.org/info/rfc6247>.

   [RFC6355]  Narten, T. and J. Johnson, "Definition of the UUID-Based
              DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355,
              DOI 10.17487/RFC6355, August 2011,
              <https://www.rfc-editor.org/info/rfc6355>.

   [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>.

   [RFC6543]  Gundavelli, S., "Reserved IPv6 Interface Identifier for
              Proxy Mobile IPv6", RFC 6543, DOI 10.17487/RFC6543, May
              2012, <https://www.rfc-editor.org/info/rfc6543>.

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

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935,
              DOI 10.17487/RFC6935, April 2013,
              <https://www.rfc-editor.org/info/rfc6935>.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, DOI 10.17487/RFC6936, April 2013,
              <https://www.rfc-editor.org/info/rfc6936>.

   [RFC6980]  Gont, F., "Security Implications of IPv6 Fragmentation
              with IPv6 Neighbor Discovery", RFC 6980,
              DOI 10.17487/RFC6980, August 2013,
              <https://www.rfc-editor.org/info/rfc6980>.

   [RFC7042]  Eastlake 3rd, D. and J. Abley, "IANA Considerations and
              IETF Protocol and Documentation Usage for IEEE 802
              Parameters", BCP 141, RFC 7042, DOI 10.17487/RFC7042,
              October 2013, <https://www.rfc-editor.org/info/rfc7042>.

   [RFC7084]  Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
              Requirements for IPv6 Customer Edge Routers", RFC 7084,
              DOI 10.17487/RFC7084, November 2013,
              <https://www.rfc-editor.org/info/rfc7084>.

   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,
              <https://www.rfc-editor.org/info/rfc7094>.

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,
              <https://www.rfc-editor.org/info/rfc7323>.

   [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>.

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   [RFC7421]  Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
              Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
              Boundary in IPv6 Addressing", RFC 7421,
              DOI 10.17487/RFC7421, January 2015,
              <https://www.rfc-editor.org/info/rfc7421>.

   [RFC7526]  Troan, O. and B. Carpenter, Ed., "Deprecating the Anycast
              Prefix for 6to4 Relay Routers", BCP 196, RFC 7526,
              DOI 10.17487/RFC7526, May 2015,
              <https://www.rfc-editor.org/info/rfc7526>.

   [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542,
              DOI 10.17487/RFC7542, May 2015,
              <https://www.rfc-editor.org/info/rfc7542>.

   [RFC7739]  Gont, F., "Security Implications of Predictable Fragment
              Identification Values", RFC 7739, DOI 10.17487/RFC7739,
              February 2016, <https://www.rfc-editor.org/info/rfc7739>.

   [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>.

   [RFC7847]  Melia, T., Ed. and S. Gundavelli, Ed., "Logical-Interface
              Support for IP Hosts with Multi-Access Support", RFC 7847,
              DOI 10.17487/RFC7847, May 2016,
              <https://www.rfc-editor.org/info/rfc7847>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [RFC8726]  Farrel, A., "How Requests for IANA Action Will Be Handled
              on the Independent Stream", RFC 8726,
              DOI 10.17487/RFC8726, November 2020,
              <https://www.rfc-editor.org/info/rfc8726>.

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   [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>.

   [RFC8892]  Thaler, D. and D. Romascanu, "Guidelines and Registration
              Procedures for Interface Types and Tunnel Types",
              RFC 8892, DOI 10.17487/RFC8892, August 2020,
              <https://www.rfc-editor.org/info/rfc8892>.

   [RFC8899]  Fairhurst, G., Jones, T., Tuexen, M., Ruengeler, I., and
              T. Voelker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <https://www.rfc-editor.org/info/rfc8899>.

   [RFC8900]  Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile",
              BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
              <https://www.rfc-editor.org/info/rfc8900>.

   [RFC8981]  Gont, F., Krishnan, S., Narten, T., and R. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", RFC 8981,
              DOI 10.17487/RFC8981, February 2021,
              <https://www.rfc-editor.org/info/rfc8981>.

   [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>.

   [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.  OAL Checksum Algorithm

   The OAL Checksum Algorithm adopts the 8-bit Fletcher algorithm
   specified in Appendix I of [RFC1146] as also analyzed in [CKSUM].
   [RFC6247] declared [RFC1146] historic for the reason that the
   algorithms had never seen widespread use with TCP, however this
   document adopts the 8-bit Fletcher algorithm for a different purpose.
   Quoting from Appendix I of [RFC1146], the OAL Checksum Algorithm
   proceeds as follows:

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      "The 8-bit Fletcher Checksum Algorithm is calculated over a
      sequence of data octets (call them D[1] through D[N]) by
      maintaining 2 unsigned 1's-complement 8-bit accumulators A and B
      whose contents are initially zero, and performing the following
      loop where i ranges from 1 to N:

         A := A + D[i]

         B := B + A

      It can be shown that at the end of the loop A will contain the
      8-bit 1's complement sum of all octets in the datagram, and that B
      will contain (N)D[1] + (N-1)D[2] + ... + D[N]."

   To calculate the OAL checksum, the above algorithm is applied over
   the N-octet concatenation of the OAL pseudo-header, the encapsulated
   IP packet and the two-octet trailing checksum field initialized to 0.
   Specifically, the algorithm is first applied over the 40 octets of
   the OAL pseudo-header as data octets D[1] through D[40], then
   continues over the entire length of the original IP packet as data
   octets D[41] through D[N-2] and finally concludes with the two
   trailing 0 octets as data octets D[N-1] and D[N].

Appendix B.  IPv6 ND Message Authentication and Integrity

   OMNI interface IPv6 ND messages are subject to authentication and
   integrity checks at multiple levels.  When an OMNI interface sends an
   IPv6 ND message over an INET interface, it includes an authentication
   sub-option with a valid signature but does not include an IPv6 ND
   message checksum.  The OMNI interface that receives the message
   verifies the OAL checksum as a first-level integrity check, then
   verifies the authentication signature (while ignoring the IPv6 ND
   message checksum) to ensure IPv6 ND message authentication and
   integrity.

   When an OMNI interface sends an IPv6 ND message over an underlying
   interface connected to a secured network, it omits the authentication
   sub-option but instead calculates/includes an IPv6 ND message
   checksum.  The OMNI interface that receives the message applies any
   lower-layer authentication and integrity checks, then verifies both
   the OAL checksum (if present) and the IPv6 ND message checksum.
   (Note that optimized implementations can verify both the OAL and IPv6
   ND message checksums in a single pass over the data.)  When an OMNI
   interface sends IPv6 ND messages to a synchronized neighbor, it
   includes an authentication sub-option only if authentication is
   necessary; otherwise, it calculates/includes the IPv6 ND message
   checksum.

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   When the OMNI interface calculates the authentication signature or
   IPv6 ND message checksum, it performs the calculation beginning with
   a pseudo-header of the IPv6 ND message header and extends over all
   following OAL packet data up to but not including the trailing OAL
   checksum field.  In particular, for OAL super-packets any additional
   original IP packets included beyond the end of the IPv6 ND message
   are simply considered as extensions of the IPv6 ND message for the
   purpose of the calculation.

   OAL destinations discard carrier packets with unacceptable
   Identifications and submit the encapsulated fragments in others for
   reassembly.  The reassembly algorithm rejects any fragments with
   unacceptable sizes, offsets, etc. and reassembles all others.
   Following reassembly, the OAL checksum algorithm provides an
   integrity assurance layer that compliments any integrity checks
   already applied by lower layers as well as a first-pass filter for
   any checks that will be applied later by upper layers.

Appendix C.  VDL Mode 2 Considerations

   ICAO Doc 9776 is the "Technical Manual for VHF Data Link Mode 2"
   (VDLM2) that specifies an essential radio frequency data link service
   for aircraft and ground stations in worldwide civil aviation air
   traffic management.  The VDLM2 link type is "multicast capable"
   [RFC4861], but with considerable differences from common multicast
   links such as Ethernet and IEEE 802.11.

   First, the VDLM2 link data rate is only 31.5Kbps - multiple orders of
   magnitude less than most modern wireless networking gear.  Second,
   due to the low available link bandwidth only VDLM2 ground stations
   (i.e., and not aircraft) are permitted to send broadcasts, and even
   so only as compact layer 2 "beacons".  Third, aircraft employ the
   services of ground stations by performing unicast RS/RA exchanges
   upon receipt of beacons instead of listening for multicast RA
   messages and/or sending multicast RS messages.

   This beacon-oriented unicast RS/RA approach is necessary to conserve
   the already-scarce available link bandwidth.  Moreover, since the
   numbers of beaconing ground stations operating within a given spatial
   range must be kept as sparse as possible, it would not be feasible to
   have different classes of ground stations within the same region
   observing different protocols.  It is therefore highly desirable that
   all ground stations observe a common language of RS/RA as specified
   in this document.

   Note that links of this nature may benefit from compression
   techniques that reduce the bandwidth necessary for conveying the same

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   amount of data.  The IETF lpwan working group is considering possible
   alternatives: [https://datatracker.ietf.org/wg/lpwan/documents].

Appendix D.  Client-Proxy/Server Isolation Through L2 Address Mapping

   Per [RFC4861], IPv6 ND messages may be sent to either a multicast or
   unicast link-scoped IPv6 destination address.  However, IPv6 ND
   messaging should be coordinated between the Client and Proxy/Server
   only without invoking other nodes on the *NET.  This implies that
   Client-Proxy/Server control messaging should be isolated and not
   overheard by other nodes on the link.

   To support Client-Proxy/Server isolation on some *NET links, Proxy/
   Servers can maintain an OMNI-specific unicast L2 address ("MSADDR").
   For Ethernet-compatible *NETs, this specification reserves one
   Ethernet unicast address TBD5 (see: IANA Considerations).  For non-
   Ethernet statically-addressed *NETs, MSADDR is reserved per the
   assigned numbers authority for the *NET addressing space.  For still
   other *NETs, MSADDR may be dynamically discovered through other
   means, e.g., L2 beacons.

   Clients map the L3 addresses of all IPv6 ND messages they send (i.e.,
   both multicast and unicast) to MSADDR instead of to an ordinary
   unicast or multicast L2 address.  In this way, all of the Client's
   IPv6 ND messages will be received by Proxy/Servers that are
   configured to accept packets destined to MSADDR.  Note that multiple
   Proxy/Servers on the link could be configured to accept packets
   destined to MSADDR, e.g., as a basis for supporting redundancy.

   Therefore, Proxy/Servers must accept and process packets destined to
   MSADDR, while all other devices must not process packets destined to
   MSADDR.  This model has well-established operational experience in
   Proxy Mobile IPv6 (PMIP) [RFC5213][RFC6543].

Appendix E.  Change Log

   << RFC Editor - remove prior to publication >>

   Differences from earlier versions to draft-templin-6man-omni-45:

   o  New baseline version with corrections and section reorganizations
      to improve document flow.

Authors' Addresses

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   Fred L. Templin (editor)
   The Boeing Company
   P.O. Box 3707
   Seattle, WA  98124
   USA

   Email: fltemplin@acm.org

   Tony Whyman
   MWA Ltd c/o Inmarsat Global Ltd
   99 City Road
   London  EC1Y 1AX
   England

   Email: tony.whyman@mccallumwhyman.com

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