Network Working Group                                 F. L. Templin, Ed.
Internet-Draft                                        The Boeing Company
Intended status: Informational                                 A. Whyman
Expires: 5 July 2022                     MWA Ltd c/o Inmarsat Global Ltd
                                                          1 January 2022


    Transmission of IP Packets over Overlay Multilink Network (OMNI)
                               Interfaces
                       draft-templin-6man-omni-52

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 virtual interface specification is presented that enables
   mobile nodes to coordinate with a network-based mobility service and/
   or with other mobile node peers.  The virtual interface provides an
   adaptation layer service that also applies for more static
   deployments such as enterprise and home networks.  This document
   specifies the transmission of IP packets over Overlay Multilink
   Network (OMNI) Interfaces.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 5 July 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   4.  Overlay Multilink Network (OMNI) Interface Model  . . . . . .  15
   5.  OMNI Interface Maximum Transmission Unit (MTU)  . . . . . . .  21
     5.1.  Jumbograms  . . . . . . . . . . . . . . . . . . . . . . .  22
     5.2.  IPv6 Parcels  . . . . . . . . . . . . . . . . . . . . . .  23
   6.  The OMNI Adaptation Layer (OAL) . . . . . . . . . . . . . . .  23
     6.1.  OAL Source Encapsulation and Fragmentation  . . . . . . .  24
     6.2.  OAL L2 Encapsulation and Re-Encapsulation . . . . . . . .  29
     6.3.  OAL L2 Decapsulation and Reassembly . . . . . . . . . . .  32
     6.4.  OAL Header Compression  . . . . . . . . . . . . . . . . .  33
     6.5.  OAL-in-OAL Encapsulation  . . . . . . . . . . . . . . . .  36
     6.6.  OAL Identification Window Maintenance . . . . . . . . . .  38
     6.7.  OAL Fragment Retransmission . . . . . . . . . . . . . . .  43
     6.8.  OAL MTU Feedback Messaging  . . . . . . . . . . . . . . .  44
     6.9.  OAL Super-Packets . . . . . . . . . . . . . . . . . . . .  46
     6.10. OAL Bubbles . . . . . . . . . . . . . . . . . . . . . . .  48
     6.11. OAL Requirements  . . . . . . . . . . . . . . . . . . . .  48
     6.12. OAL Fragmentation Security Implications . . . . . . . . .  49
     6.13. OMNI Hosts  . . . . . . . . . . . . . . . . . . . . . . .  51
     6.14. IP Parcels  . . . . . . . . . . . . . . . . . . . . . . .  53
   7.  Frame Format  . . . . . . . . . . . . . . . . . . . . . . . .  57
   8.  Link-Local Addresses (LLAs) . . . . . . . . . . . . . . . . .  57
   9.  Unique-Local Addresses (ULAs) . . . . . . . . . . . . . . . .  59
   10. Global Unicast Addresses (GUAs) . . . . . . . . . . . . . . .  61
   11. Node Identification . . . . . . . . . . . . . . . . . . . . .  62
   12. Address Mapping - Unicast . . . . . . . . . . . . . . . . . .  63
     12.1.  The OMNI Option  . . . . . . . . . . . . . . . . . . . .  63
     12.2.  OMNI Sub-Options . . . . . . . . . . . . . . . . . . . .  64
       12.2.1.  Pad1 . . . . . . . . . . . . . . . . . . . . . . . .  67
       12.2.2.  PadN . . . . . . . . . . . . . . . . . . . . . . . .  67
       12.2.3.  Neighbor Coordination  . . . . . . . . . . . . . . .  68
       12.2.4.  Interface Attributes . . . . . . . . . . . . . . . .  70
       12.2.5.  Multilink Forwarding Parameters  . . . . . . . . . .  73
       12.2.6.  Traffic Selector . . . . . . . . . . . . . . . . . .  78
       12.2.7.  Geo Coordinates  . . . . . . . . . . . . . . . . . .  79



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       12.2.8.  Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
               Message . . . . . . . . . . . . . . . . . . . . . . .  80
       12.2.9.  Host Identity Protocol (HIP) Message . . . . . . . .  81
       12.2.10. PIM-SM Message . . . . . . . . . . . . . . . . . . .  83
       12.2.11. Fragmentation Report (FRAGREP) . . . . . . . . . . .  84
       12.2.12. Node Identification  . . . . . . . . . . . . . . . .  85
       12.2.13. ICMPv6 Error . . . . . . . . . . . . . . . . . . . .  87
       12.2.14. QUIC-TLS Message . . . . . . . . . . . . . . . . . .  88
       12.2.15. Proxy/Server Departure . . . . . . . . . . . . . . .  88
       12.2.16. Sub-Type Extension . . . . . . . . . . . . . . . . .  89
   13. Address Mapping - Multicast . . . . . . . . . . . . . . . . .  92
   14. Multilink Conceptual Sending Algorithm  . . . . . . . . . . .  93
     14.1.  Multiple OMNI Interfaces . . . . . . . . . . . . . . . .  93
     14.2.  Client-Proxy/Server Loop Prevention  . . . . . . . . . .  94
   15. Router Discovery and Prefix Registration  . . . . . . . . . .  94
     15.1.  Window Synchronization . . . . . . . . . . . . . . . . . 101
     15.2.  Router Discovery in IP Multihop and IPv4-Only
            Networks . . . . . . . . . . . . . . . . . . . . . . . . 102
     15.3.  DHCPv6-based Prefix Registration . . . . . . . . . . . . 104
     15.4.  Client-to-Client Chaining  . . . . . . . . . . . . . . . 106
   16. Secure Redirection  . . . . . . . . . . . . . . . . . . . . . 106
   17. Proxy/Server Resilience . . . . . . . . . . . . . . . . . . . 107
   18. Detecting and Responding to Proxy/Server Failures . . . . . . 107
   19. Transition Considerations . . . . . . . . . . . . . . . . . . 108
   20. OMNI Interfaces on Open Internetworks . . . . . . . . . . . . 109
   21. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . . . . 111
   22. (H)HITs and Temporary ULAs  . . . . . . . . . . . . . . . . . 112
   23. Address Selection . . . . . . . . . . . . . . . . . . . . . . 113
   24. Error Messages  . . . . . . . . . . . . . . . . . . . . . . . 113
   25. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 114
     25.1.  "IP Option Numbers" Registry . . . . . . . . . . . . . . 114
     25.2.  "Protocol Numbers" Registry  . . . . . . . . . . . . . . 114
     25.3.  "IEEE 802 Numbers" Registry  . . . . . . . . . . . . . . 114
     25.4.  "TCP Parameters" Registry  . . . . . . . . . . . . . . . 114
     25.5.  "IPv4 Special-Purpose Address" Registry  . . . . . . . . 115
     25.6.  "IPv6 Neighbor Discovery Option Formats" Registry  . . . 115
     25.7.  "Ethernet Numbers" Registry  . . . . . . . . . . . . . . 115
     25.8.  "ICMPv6 Code Fields: Type 2 - Packet Too Big"
             Registry  . . . . . . . . . . . . . . . . . . . . . . . 115
     25.9.  "OMNI Option Sub-Type Values" (New Registry) . . . . . . 116
     25.10. "OMNI Geo Coordinates Type Values" (New Registry)  . . . 116
     25.11. "OMNI Node Identification ID-Type Values" (New
             Registry) . . . . . . . . . . . . . . . . . . . . . . . 117
     25.12. "OMNI Option Sub-Type Extension Values" (New
             Registry) . . . . . . . . . . . . . . . . . . . . . . . 117
     25.13. "OMNI RFC4380 UDP/IP Header Option" (New Registry) . . . 118
     25.14. "OMNI RFC6081 UDP/IP Trailer Option" (New Registry)  . . 118
     25.15. Additional Considerations  . . . . . . . . . . . . . . . 119



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   26. Security Considerations . . . . . . . . . . . . . . . . . . . 119
   27. Implementation Status . . . . . . . . . . . . . . . . . . . . 120
   28. Document Updates  . . . . . . . . . . . . . . . . . . . . . . 120
   29. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 121
   30. References  . . . . . . . . . . . . . . . . . . . . . . . . . 122
     30.1.  Normative References . . . . . . . . . . . . . . . . . . 122
     30.2.  Informative References . . . . . . . . . . . . . . . . . 124
   Appendix A.  OAL Checksum Algorithm . . . . . . . . . . . . . . . 134
   Appendix B.  IPv6 ND Message Authentication and Integrity . . . . 135
   Appendix C.  VDL Mode 2 Considerations  . . . . . . . . . . . . . 136
   Appendix D.  Client-Proxy/Server Isolation Through Link-Layer
           Address Mapping . . . . . . . . . . . . . . . . . . . . . 136
   Appendix E.  Change Log . . . . . . . . . . . . . . . . . . . . . 137
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 137

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 underlay
   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
   underlay network interfaces (which may themselves connect to virtual
   or physical links).  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 underlay 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
   adapted to diverse underlay interfaces with heterogeneous properties.
   The OMNI interface connects to a virtual overlay known as the "OMNI
   link".  The OMNI link spans one or more Internetworks that may
   include private-use infrastructures (e.g., enterprise networks) and/
   or the global public Internet itself.

   Client OMNI interfaces interact with the MS and/or other OMNI nodes
   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



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   configure OMNI interfaces.  Automatic Extended Route Optimization
   (AERO) in particular provides a companion MS compatible with the OMNI
   architecture [I-D.templin-6man-aero].  In terms of precedence, AERO
   may provide first-principle insights as useful context for this
   specification.

   Each OMNI interface provides a multilink nexus for exchanging inbound
   and outbound traffic via selected underlay 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
   (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 over Access Networks (ANETs) and/or open
   Internetworks (INETs).  The Client sub-delegates the MNP to
   downstream-attached End-user Networks (ENETs) independently of the
   underlay interfaces selected for data transport.  The Client acts as
   a fixed or mobile router on behalf of peers on its ENETs, and uses
   OMNI interface control messaging to coordinate with Hosts, Proxy/
   Servers and/or other Clients.  The Client iterates its control
   messaging over each of the OMNI interface's ANET/INET underlay
   interfaces in order to register each interface with the MS (see
   Section 15).  The Client can also support recursive chaining for
   other Clients located in downstream-attached ENETs.

   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
   underlay 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 underlay
   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.
   For example, 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



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   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
       underlay interfaces as a single logical unit (i.e., the OMNI
       interface) to achieve the required communications performance and
       reliability objectives.

   2.  Multinet - the ability to span the OMNI link over a segment
       routing topology with multiple diverse administrative domain
       network segments while maintaining seamless end-to-end
       communications between mobile Clients and correspondents such as
       air traffic controllers, fleet administrators, 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 underlay interface address change, but without
       disruptions to ongoing communication sessions with peers over the
       OMNI link.

   4.  Multicast - the ability to send a single network transmission
       that reaches multiple 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 operation of both IP protocol
   versions (i.e., IPv4 [RFC0791] and IPv6 [RFC8200]) is specified as
   the network layer data plane, while OMNI interfaces use IPv6 ND
   messaging in the control plane independently of the data plane



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   protocol(s).  OMNI interfaces also provide an OAL based on
   encapsulation and fragmentation over heterogeneous underlay
   interfaces as an adaptation sublayer between L3 and L2.  Both OMNI
   and the OAL are specified in detail throughout the remainder of this
   document.

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.

   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:

   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.

   adaptation layer
      A mid-layer that adapts L3 to a diverse collection of L2 underlay
      interfaces and their encapsulations.  No layer number is currently
      assigned to the adaptation layer.  The adaptation layer sees all
      lower layer encapsulations as "L2 encapsulations", which may
      include UDP, IP and true link-layer (e.g., Ethernet) headers.





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

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

   End-user Network (ENET)
      a simple or complex "downstream" network that travels with the
      Client as a single logical unit.  The ENET could be as simple as a
      single link connecting a single Host, or as complex as a large
      network with many links, routers, bridges and Hosts.  The ENET
      could also provide an "upstream" link in a recursively-descending
      chain of additional Clients and ENETs.  In this way, an ENET of an
      upstream Client is seen as the ANET of a downstream Client.

   [A,I,E]NET interface
      a Client's attachment to a link in an [A,I,E]NET.

   *NET
      a "wildcard" term used when a given specification applies equally
      to both ANET/INET cases.  From the Client's perspective, *NET
      interfaces are "upstream" interfaces that connect the Client to
      the Mobility Service, while ENET interfaces are "downstream"
      interfaces that the Client uses to connect downstream ENETs, Hosts
      and/or other Clients.

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






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   OMNI link
      a Non-Broadcast, Multiple Access (NBMA) virtual overlay configured
      over one or more INETs and their connected ANETs/ENETs.  An OMNI
      link may comprise multiple distinct "segments" joined by L2
      forwarding devices 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 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 underlay interfaces.  If there are multiple OMNI links in an
      OMNI domain, a separate OMNI interface is configured for each
      link.  As with any interface, the OMNI interface configures a
      Maximum Transmission Unit (MTU) and a Maximum Reassembly Unit
      (MRU).

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

   Host
      an end user device that configures an OMNI interface over an ENET
      interface serviced by a Client.  (As an implementation matter, the
      Host either assigns the same IP address from the ENET underlay
      interface to the OMNI interface, or configures the "OMNI
      interface" as a virtual sublayer of the underlay interface
      itself.)  The IP addresses assigned to each Host ENET interface
      remain stable even if the Client's upstream *NET interface
      connections change.

   Client
      a network platform/device mobile router that configures one or
      more OMNI interfaces over distinct sets of underlay interfaces
      grouped as logical OMNI link units.  The Client coordinates with
      the Mobility Service via *NET interface upstream networks, and
      provides Proxy/Server services for Hosts and other Clients on ENET
      interface downstream networks.  The Client's *NET interface
      addresses and performance characteristics may change over time
      (e.g., due to node mobility, link quality, etc.) while downstream-
      attached Hosts and other Clients see the ENET as a stable ANET.





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   Proxy/Server
      a segment routing topology edge node that configures an OMNI
      interface and provides Clients with Mobility Services.  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.  Proxy/Servers at ANET boundaries configure both
      an ANET downstream interface and *NET upstream interface, while
      INET-based Proxy/Servers configure only an INET interface.

   First-Hop Segment (FHS) Proxy/Server
      a Proxy/Server connected to the source Client's *NET that forwards
      packets sent by the source into the segment routing topology.  FHS
      Proxy/Servers also act as intermediate forwarding nodes to
      facilitate RS/RA exchanges between Clients and Hub Proxy/Servers.

   Last-Hop Segment (LHS) Proxy/Server
      a Proxy/Server connected to the target Client's *NET that forwards
      packets received from the segment routing topology to the target.

   Hub Proxy/Server
      a single Proxy/Server selected by the Client that provides a
      designated router service for all of the Client's*NET underlay
      networks.  Since all Proxy/Servers provide equivalent services,
      Clients normally select the first FHS Proxy/Server they coordinate
      with to serve as the Hub. However, the Hub can also be any
      available Proxy/Server for the OMNI link, i.e., and not
      necessarily one of the Client's FHS Proxy/Servers.

   Segment Routing Topology (SRT)
      a multinet forwarding region configured over one or more INETs
      between the FHS Proxy/Server and LHS Proxy/Server.  The SRT spans
      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 (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 OMNI link supporting infrastructure nodes.  Specific MS
      details are out of scope for this document, with an example found
      in [I-D.templin-6man-aero].






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   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 also
      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 receive MNPs from Proxy/Servers and sub-delegate
      them to routers, Hosts and other Clients located in ENETs.

   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 an IPv6 header (i.e., the
      OAL header) then submitted for OAL fragmentation and reassembly.

   OAL fragment
      a portion of an OAL packet following fragmentation but prior to
      encapsulation, or following 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 L2 encapsulation or prior
      to L2 decapsulation.  OAL sources and destinations exchange
      carrier packets over underlay interfaces, and may be separated by
      one or more OAL intermediate nodes.  OAL intermediate nodes may
      perform re-encapsulation on carrier packets by removing the L2
      headers of the first hop network and replacing them with new L2
      headers for the next hop network.  (The term "carrier" honors
      agents of the service postulated by [RFC1149] and [RFC6214].)





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   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 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
      an OMNI interface acts as an OAL intermediate node when it removes
      the L2 encapsulation headers of carrier packets received on a
      first segment, then re-encapsulates the carrier packets in new L2
      encapsulation 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
      a Client OMNI interface's manner of managing multiple diverse *NET
      underlay interfaces as a single logical unit.  The OMNI interface
      provides a single unified interface to upper layers, while
      underlay 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 underlay interface pairs.






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   Multinet
      an 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 network segments, multiple diverse Internetworks (such as the
      global public IPv4 and IPv6 Internets) can serve as transit
      segments in an end-to-end L2 forwarding path.  This OAL
      concatenation capability provides benefits such as supporting
      IPv4/IPv6 transition and coexistence, joining multiple diverse
      operator networks into a cooperative single service network, etc.
      See: [I-D.templin-6man-aero] for further information.

   Multihop
      an iterative relaying of IP packets between Client's over an OMNI
      underlay 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.

   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 underlay 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.









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   Multilink Forwarding Information Base (MFIB)
      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 underlay 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 underlay 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.

   IP Jumbogram
      an IPv4 or IPv6 packet with a Jumbo Payload option that includes a
      length greater than the maximum size that can be expressed in the
      16-bit [Total, Payload] Length field (see: Section 5.1).  The
      [Total, Payload] Length field must be set to 0.

   IP Parcel
      a special form of an IP Jumbogram with a non-zero segment length
      in the [Total, Payload] Length field and also with a Jumbo Payload
      option (see: Section 5.2).

   INADDR
      the IP address (and also the UDP port number when UDP is used)
      that appears in encapsulation headers in the data plane and in
      IPv6 ND OMNI option sub-options in the control plane.

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.



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   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
   underlay 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 underlay interfaces that appear as L2
   communication channels in the architecture.

                                     +----------------------------+
                                     |    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 underlay interface provides an L2/L1 abstraction according to
   one of the following models:

   *  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 or on the Proxy/Server
      itself and may require NAT traversal the same as for the INET
      case.)

   *  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



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      INET correspondent.  NATed INET interfaces typically configure
      private IP addresses and connect to a private network behind one
      or more NATs with the outermost NAT providing INET access.

   *  ENET interfaces connect a Client's downstream-attached networks,
      where the Client provides forwarding services for ENET Host and
      Client communications to remote peers.  An ENET may be as simple
      as a small stub network that travels with a mobile Client (e.g.,
      an Internet-of-Things) to as complex as a large private enterprise
      network that the Client connects to a larger ANET or INET.
      Downstream-attached Hosts and Clients see the ENET as an ANET and
      see the (upstream) Client as a Proxy/Server.

   *  VPNed interfaces use security encapsulation over an underlay
      network to a Client or 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.

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

   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 underlay network headers (e.g., UDP/IP, IP-only, Ethernet-only,
   etc.) to create L2-encapsulated "carrier packets" for transmission
   over underlay interfaces.  The target OMNI interface receives the
   carrier packets from underlay interfaces and discards the L2
   encapsulation 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.  If the OAL
   fragments are addressed to another node, the OMNI interface instead
   acts as an "OAL intermediate node" by re-encapsulating the carrier
   packets in new underlay network L2 headers and forwarding them over
   an underlay 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 forward original IP packets over underlay
   interfaces while including/omitting various lower layer
   encapsulations including OAL, UDP, IP and Ethernet (ETH) or other
   link-layer header.  The network layer can also access the underlay



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   interfaces directly while bypassing the OMNI interface entirely when
   necessary.  This architectural flexibility may be beneficial for
   underlay 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 underlay
   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 underlay 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
   underlay interface that exceed the underlay 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
   the Network Layer and Underlay interfaces in the figure denote the
   encapsulation/decapsulation layering combinations possible.  Common
   combinations include IP-only (i.e., direct access to underlay
   interfaces with or without using the OMNI interface), IP/IP, IP/UDP/
   IP, IP/UDP/IP/ETH, IP/OAL/UDP/IP, IP/OAL/UDP/ETH, etc.

    +------------------------------------------------------------+  ^
    |             Network Layer (Original IP packets)            |  |
    +--+---------------------------------------------------------+ L3
       |         OMNI Interface (virtual sublayer nexus)         |  |
       +--------------------------+------------------------------+  -
                                  |      OAL Encaps/Decaps       |  |
                                  +------------------------------+ OAL
                                  |        OAL Frag/Reass        |  |
                     +------------+---------------+--------------+  -
                     | UDP Encaps/Decaps/Compress |                 |
                +----+---+------------+--------+--+  +--------+     |
                | IP E/D |            | IP E/D |     | IP E/D |    L2
           +----+-----+--+----+    +--+----+---+     +---+----+--+  |
           |ETH E/D|  |ETH E/D|    |ETH E/D|             |ETH E/D|  |
    +------+-------+--+-------+----+-------+-------------+-------+  v
    |                    Underlay Interfaces                     |
    +------------------------------------------------------------+



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                     Figure 2: OMNI Interface Layering

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

   *  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.

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

   *  underlay 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.

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

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

   *  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.

   *  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 underlay interfaces
      themselves.

   *  the OAL supports lossless and adaptive path MTU mitigations not
      available for communications directly over the underlay interfaces
      themselves.  The OAL supports "packing" of multiple IP payload
      packets within a single OAL "super-packet" and also supports
      transmission of IP packets and parcels of all sizes up to and
      including Jumbograms.



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

   *  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.

   *  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 underlay 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 OMNI interface is configured over an IPv6 underlay
   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 ENET connecting to the OMNI link via multiple independent
   ANETs/INETs (i.e., *NETs).  The Client's OMNI interface sends IPv6 ND
   solicitation messages over available *NET underlay interfaces using
   any necessary L2 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 advertisement message 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.


















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                           +--------------+
                           |Source Client |
                           +--------------+        (:::)-.
                           |OMNI interface|<-->.-(::ENET::)
                           +----+----+----+      `-(::::)-'
                  +--------|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

   After the initial IPv6 ND message exchange, the source Client (as
   well as any nodes on its attached ENETs) 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 underlay *NETs.  The FHS Proxy/Server then forwards the
   packets over the SRT which delivers them to an LHS Proxy/Server, and
   the LHS Proxy/Server in turn forwards them 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.)





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   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
   *NET underlay 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 underlay interfaces
   as discussed in Section 4, where the interfaces (and their associated
   underlay network paths) may have diverse MTUs.  OMNI interface
   considerations for accommodating original IP packets of various sizes
   are discussed in the following sections.

   IPv6 underlay interfaces are REQUIRED to configure a minimum MTU of
   1280 octets and a minimum MRU of 1500 octets [RFC8200].  Therefore,
   the minimum IPv6 path MTU is 1280 octets 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 octets 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 octets
   and larger still if it knows the destination configures a larger MRU,
   this does not affect the minimum IPv6 path MTU.)

   IPv4 underlay interfaces are REQUIRED to configure a minimum MTU of
   68 octets [RFC0791] and a minimum MRU of 576 octets
   [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 octets
   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



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   headers of carrier packets that are no larger than 576 octets, 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 6.12.

   When the network layer admits an original IP packet into the OMNI
   interface the OAL prepends an IPv6 encapsulation header (see:
   Section 6) where the 16-bit Payload Length field limits the maximum-
   sized original IP packet to (2**16 -1) = 65,535 octets; this is also
   the maximum size that the OAL can accommodate with IPv6
   fragmentation.  The OMNI interface therefore sets both an MTU and MRU
   of 65,535 octets to support assured delivery of original packets no
   larger than this size even if IPv6 fragmentation is required.  The
   OMNI interface then employs the OAL as an encapsulation sublayer
   service to transform original IP packets into OAL packets/fragments,
   and the OAL in turn uses underlay network encapsulation to forward
   carrier packets over underlay interfaces (see: Section 6).

   Note: As for any IP interface, the OMNI interface MAY set a smaller
   MTU (to a minimum of 1280 for IPv6 or 576 for IPv4) but MUST NOT set
   a smaller MRU.

5.1.  Jumbograms

   While the maximum-sized original IP packet that the OAL can
   accommodate using IPv6 fragmentation is 65,535 octets, OMNI
   interfaces can forward still larger IPv6 packets as OAL "atomic
   fragments" through the application of IPv6 Jumbograms [RFC2675].  For
   these larger packets, the OMNI interface performs OAL encapsulation
   by appending an IPv6 header followed by an 8-octet Hop-By-Hop header
   with Jumbo Payload option followed by a Routing Header of no more
   than 40-octets (if necessary) and finally followed by an 8-octet
   Fragment Header.

   Since the Jumbo Payload option includes a 32-bit length field, OMNI
   interfaces can therefore configure a larger IP MTU up to a maximum of
   ((2**32 - 1) - 8 - 40 - 8) = 4,294,967,239 octets.  In that case, the
   OAL will still provide original IP packets no larger than 65,535 with
   an IPv6 fragmentation-based assured delivery service while larger IP
   packets will receive a best-effort delivery service as atomic
   fragments (note that the OAL destination is permitted to accept
   atomic fragments that exceed the OMNI interface MRU).









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   The OAL source forwards jumbo atomic fragments under the assumption
   that upper and lower layers will employ sufficient integrity
   assurance, noting that commonly-used 32-bit CRCs may be inadequate
   for these larger sizes [CRC].  If the packet is dropped along the
   path to the OAL destination, the OAL source must arrange to return a
   PTB "hard error" to the original source Section 6.8.

   This document further specifies an IPv4 Jumbogram service where all
   aspects of the service are conducted in an identical fashion as for
   IPv6 with the exception of the IP header format (including the manner
   in which the Jumbo Payload option is encoded).  This document
   requests IANA to assign a new IPv4 Jumbo Payload option code TBD1
   (see: IANA Considerations).  The format of the IPv4 Jumbo Payload
   option is shown in Figure 4:

   +--------+--------+--------+--------+--------+--------+
   |000(TBD1)00000110|       Jumbo Payload Length        |
   +--------+--------+--------+--------+--------+--------+

                    Figure 4: IPv4 Jumbo Payload Option

5.2.  IPv6 Parcels

   An IP Parcel is a variation of the IP Jumbogram construction
   beginning with an IP header with a non-zero [Total, Payload] Length
   value but with a Jumbo Payload option with a length that may be the
   same as or larger than the length in the IP header.  The differences
   in these lengths determines the size and number of upper layer
   protocol segments within the parcel.

   The IP Parcel format and transmission/reception procedures are
   specified in Section 6.14.  End systems that implement either the
   full OMNI interface (i.e., Clients) or enough of the OAL to process
   parcels (i.e., Hosts) are permitted to exchange parcels with
   consenting peers.

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 underlay interfaces,
   the OMNI Adaptation Layer (OAL) acting as the OAL source applies
   encapsulation to form OAL packets subject to fragmentation producing
   OAL fragments suitable for L2 encapsulation and transmission as
   carrier packets over underlay interfaces as described in Section 6.1.

   These carrier packets travel over one or more underlay networks
   spanned by OAL intermediate nodes in the SRT, which re-encapsulate by
   removing the L2 headers of the first underlay network and appending



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   L2 headers appropriate for the next underlay 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.

   When the OAL destination receives the carrier packets, it discards
   the L2 headers and reassembles the resulting OAL fragments (if
   necessary) into an OAL packet as described in Section 6.3.  The OAL
   destination next decapsulates the OAL packet to obtain the original
   IP packet then delivers the original IP packet 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 SRT Gateways as 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
   underlay 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 creates an "OAL packet" by prepending an
   IPv6 OAL encapsulation header per [RFC2473] 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.












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   The OAL next selects OAL packet source and destination addresses.
   Client OMNI interfaces set the OAL 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.)

   The OAL source next selects a 32-bit OAL packet Identification value
   as specified in Section 6.6.  The OAL then calculates a 2-octet 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], then extending over the entire length of the original IP
   packet.  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 octets) as the worst case for OAL fragmentation regardless of the
   underlay interface IP protocol version since IPv6/IPv4 protocol
   translation and/or IPv6-in-IPv4 encapsulation may occur in any
   underlay network path.  By initially assuming the IPv4 minimum even



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   for IPv6 underlay interfaces, the OAL source may produce smaller
   fragments with additional encapsulation overhead but avoids loss due
   to presenting an underlay 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 L2 encapsulation of the previous segment is
   replaced by the L2 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
   octets at each SRT segment for the purpose of generating OAL
   fragments for L2 encapsulation and transmission as carrier packets.
   Each successive SRT intermediate node may include either a 20 octet
   IPv4 or 40 octet IPv6 header, an 8 octet UDP header and in some cases
   an IP security encapsulation (40 octets maximum assumed) during re-
   encapsulation.  Intermediate nodes at any SRT segment may also insert
   or modify the Routing Header (40 octets maximum) following the 40
   octet OAL IPv6 header and preceding the 8 octet Fragment Header.
   Therefore, assuming a worst case of (40 + 40 + 8) = 88 octets for L2
   encapsulations plus (40 + 40 + 8) = 88 octets for OAL encapsulation
   leaves no less than (576 - 88 - 88) = 400 octets remaining to
   accommodate a portion of the original IP packet/fragment.  The OAL
   source therefore sets a minimum Maximum Payload Size (MPS) of 400
   octets 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.

   The OAL source SHOULD maintain "path MPS" values for individual OAL
   destinations initialized to the minimum MPS and increased to larger
   values if better information is known or discovered.  For example,
   when peers share a common underlay network link or a fixed path with
   a known larger MTU, the OAL source can set path MPS to a larger size
   (i.e., greater than 400 octets) as long as the peer reassembles
   before re-encapsulating and forwarding (while re-fragmenting if
   necessary).  Also, if the OAL source has a way of knowing the maximum
   L2 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 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



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   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) and/or with a
   trailing large NULL packet in a super-packet (see: Section 6.9) 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) underlay interface
   pair for the same OAL destination, since different underlay 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 (or larger than 65,535 octets) into atomic fragments by
   including a Fragment Header with Fragment Offset and More Fragments
   both set to 0.  The OAL source then inserts a Routing Header (if
   necessary) following the IPv6 encapsulation header and before the
   Fragment Header.  If the original IP packet is larger than 65,535,
   the OAL source also inserts a Hop-By-Hop header with Jumbo Payload
   option immediately following the IPv6 encapsulation header and before
   the Routing Header (if necessary), then includes an (atomic) Fragment
   Header.  The header extension order for each fragment therefore
   appears as the OAL IPv6 header followed by Hop-By-Hop header followed
   by Routing Header followed by Fragment Header.

   The OAL source next appends the OAL checksum as the final two octets
   of the final fragment while increasing its (Jumbo) Payload Length by
   2.  If appending the checksum would cause the final fragment to
   exceed the current MPS, the OAL source instead reduces this "former"
   final fragment's Payload Length (PL) by (N*8 + (PL mod 8)) octets,
   where N is an integer that would result in a non-zero reduction but
   without causing the former final fragment to become smaller than the
   minimum MPS.  The OAL source then creates a "new" final fragment by
   copying the OAL IPv6 header and extension headers from the former
   final fragment, then copying the (N*8 + (PL mod 8)) octets from the
   end of the former final fragment immediately following the new final
   fragment extension headers.  The OAL source then sets the former
   final fragment's More Fragments flag to 1, increments the new final
   fragment's fragment offset by the former final fragment's new (PL /
   8) and finally appends the checksum the same as discussed above.




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   Next, the OAL source replaces the 1-octet "Reserved" field and 2-bit
   "Reserved Flags" field in the IPv6 Fragment Header with OMNI-specific
   encodings as shown in:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  |  Parcel ID  |A|      Fragment Offset    |P|S|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Identification                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      a) First fragment


      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  |   Ordinal |R|A|      Fragment Offset    |Res|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Identification                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      a) Non-first fragments

          Figure 6: IPv6 Fragment Header Reserved Fields Redefined

   For the first fragment, the OAL source sets the "(A)RQ" flag then
   sets "Parcel ID", "(P)arcel" and "(S)ub-Parcels" as specified in
   Section 6.14.  For each non-first fragment, the OAL source instead
   sets the "(A)RQ" flag and writes a monotonically-increasing "Ordinal"
   value between 1 and 63.  Specifically, the OAL source writes the
   ordinal number '1' for the first non-first fragment, '2' for the
   second, '3' for the third, etc. up to the final fragment or the
   ordinal value '63', whichever comes first.  (For any additional non-
   first fragments beyond ordinal '63', the OAL source instead writes
   the value '0' in the Ordinal field and clears the "(A)RQ" flag.  The
   first fragment is implicitly always considered ordinal number '0'
   even though the header does not include an explicit Ordinal field.)

   The OAL source finally encapsulates the fragments in L2 headers to
   form carrier packets and forwards them over an underlay interface,
   while retaining the fragments and their ordinal numbers (i.e., #0,
   #1, #2, etc. up to #63) for a brief period to support link-layer
   retransmissions (see: Section 6.7).  OAL fragment and carrier packet
   formats are shown in Figure 7.











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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


        +--------+----------+----------------+
        |L2 Hdrs |OAL Header|     Frag #i    |
        +--------+----------+----------------+
        c) OAL carrier packet after L2 encapsulation

                Figure 7: OAL Fragments and Carrier Packets

   Note: the minimum MPS assumes that any middleboxes (e.g.  IPv4 NATs)
   that connect private networks with path MTUs smaller than 576 octets
   must reassemble any fragmented (outbound) IPv4 carrier packets sent
   by OAL sources before forwarding them to external Internetworks since
   middleboxes that connect OAL destinations often unconditionally drop
   (inbound) IPv4 fragments.  However, when the path MTU in the
   destination private network is small, the OAL destination itself will
   be able to reassemble any IPv4 fragmentation that occurs in the
   inbound path.

6.2.  OAL L2 Encapsulation and Re-Encapsulation

   The OAL source or intermediate node next encapsulates each OAL
   fragment (with either full or compressed headers) in L2 encapsulation
   headers to create a carrier packet.  The OAL source or intermediate
   node (i.e., the L2 source) includes a UDP header as the innermost
   sublayer if NAT traversal and/or packet filtering middlebox traversal
   are required; otherwise, the L2 source includes either a full or
   compressed IP header and/or an actual link-layer header (e.g., such



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   as for Ethernet-compatible links).  The L2 source then appends any
   additional encapsulation sublayer headers necessary and presents the
   resulting carrier packet to an underlay interface, where the underlay
   network conveys it to a next-hop OAL intermediate node or destination
   (i.e., the L2 destination).

   The L2 source encapsulates the OAL information immediately following
   the innermost L2 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 uncompressed
   IPv6 header (plus extensions) followed by upper layer protocol
   headers and data.  If the first four bits encode the value '4', an
   uncompressed IPv4 header (plus extensions) followed by upper layer
   protocol headers and data follows.  Otherwise, the first four bits
   include a "Type" value, and the OAL information appears in an
   alternate format as specified in Section 6.4 (Types '0' through '3'
   are currently specified while 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 L2 encapsulation header for OAL
   packets as follows:

   *  For UDP encapsulation, the L2 source sets the UDP source port to
      8060 (i.e., the port number reserved for AERO/OMNI).  When the L2
      destination is a Proxy/Server or Gateway, the L2 source sets the
      UDP destination port to 8060; otherwise, the L2 source sets the
      UDP destination port to its cached port number value for the peer.
      The L2 source finally sets the UDP Length the same as specified in
      [RFC0768].  (If the OAL packet includes an IP Jumbogram, the L2
      source instead sets the UDP length to 0 and includes a Jumbo
      Payload option in the L2 IP header.)

   *  For IP encapsulation, the L2 source sets the IP [Protocol, Next
      Header] to TBD2 (see: IANA Considerations).  For IPv4, the L2
      source next sets the [Total, Payload] Length the same as specified
      in [RFC0791] or [RFC8200].  (If the OAL packet includes a
      Jumbogram, the L2 source instead includes a Jumbo Payload option
      and sets the Jumbo Payload length according to the [Total,
      Payload] Length of the OAL information.)











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   *  For direct encapsulations over Ethernet-compatible links, the
      EtherType is set to TBD3 (see: IANA Considerations).  Since the
      Ethernet header does not include a length field, for the OMNI
      EtherType the Ethernet header is followed by a four-octet Payload
      Length field followed immediately by the encapsulated OAL
      information.  The Payload Length field encodes the length in
      octets (in network byte order) of the OAL information exclusive of
      the lengths of the Ethernet header and trailer.

   When an L2 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 L2
   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 L2 destination.  If the L2 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.

   When an L2 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 packets 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 integrity checks.
   See: [RFC6935][RFC6936] for further discussion.

   For L2 encapsulations over IP, when the L2 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 L2 IP header, then (for IPv6) set the
   L2 IPv6 header "Flow Label" as specified in [RFC6438].  The L2 source
   then sets the L2 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 L2 source decrements the OAL
   header Hop Limit and discards the carrier packet if the value reaches



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   0.  The L2 source then copies the "Type of Service/Traffic Class" and
   "Congestion Experienced" values from the previous hop L2
   encapsulation header into the OAL header, then finally sets the
   source and destination IP addresses the same as above.

   Following L2 encapsulation/re-encapsulation, the L2 source forwards
   the resulting carrier packets over one or more underlay interfaces.
   The underlay 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 underlay interface) to the node hosting the physical media.  The
   OMNI interface may also apply encapsulation at the underlay 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 underlay interface must monitor the node hosting the physical
   media (e.g., through periodic keepalives) so that it can convey
   up/down/status information to the OMNI interface.

6.3.  OAL L2 Decapsulation and Reassembly

   When an OMNI interface receives a carrier packet from an underlay
   interface, it discards the L2 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.


















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   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
   accepted fragments of the same OAL packet that include an Ordinal
   number (i.e., Ordinals 0 through 63), but admits all accepted
   fragments into the reassembly cache after first removing any
   extension headers except for the fragment header itself.  When the
   OAL destination receives the final fragment (i.e., the one with More
   Fragments set to 0), it caches the trailing checksum and reduces the
   Payload Length by 2.  When reassembly is complete, the OAL
   destination verifies the OAL packet checksum and discards the packet
   if the checksum is incorrect.  If the OAL packet was accepted, the
   OAL destination finally removes the OAL headers 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 9180 octets [CRC], but other paths
   may traverse links (such as tunnels over IPv4) that do not include
   adequate integrity protection.  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
   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.











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   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 should 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 L2 encapsulation (i.e., UDP, IP or L2) as
   discussed in Section 6.2.  Four OAL compressed header types (Types
   '0', '1', '2' and '3') are currently specified below.

   For OAL first-fragments (including atomic fragments), the OAL node
   uses OMNI Compressed Header - Type 0/1 (OCH-0/1) 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  | Traffic Class |           Flow Label                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  | Hop Limit |  Parcel ID  |P|S|M|     ID (0)    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Identification (1-3)            |    MFVI (0)   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    MFVI (1-3)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 8: OMNI Compressed Header - Types 0/1 (OCH-0/1)

   The format begins with a 4-bit Type field set to either 0 or 1, 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 field is then
   followed by a compressed Fragment Header consisting of a 7-bit Parcel
   ID, (P)arcel flag, (S)ub-parcels flag, (M)ore Fragments flag and the
   4-octet Identification and with all other fields omitted.  (The
   Parcel ID, P and S flags are set as specified in Section 6.14; the



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   (A)RQ flag is implicitly always set and hence not included in the
   compressed header.)  If Type is 0, the compressed Fragment Header is
   then followed by a 4-octet Multilink Forwarding Vector Index (MFVI);
   otherwise, the MFVI field is omitted.

   The OAL first fragment body is then included immediately following
   the OCH-0/1 header, and the L2 header length field is reduced by the
   difference in length between the compressed headers and full-length
   IPv6 and Fragment headers.  The OAL destination will then be able to
   determine the Payload Length by examining the L2 header length field.
   The OCH-0/1 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 2/3 (OCH-2/3) as
   shown in Figure 9:

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

            Figure 9: OMNI Compressed Header - Type 2/3 (OCH-1)

   The format begins with a Type field set to either 2 or 3 and the IPv6
   header is omitted entirely.  The Type field is then followed by a
   compressed IPv6 Fragment Header with a 6-bit Ordinal number field,
   and with ((M)ore Fragments/Fragment Offset/Identification) copied
   from the uncompressed fragment header.  If Type is 2, the compressed
   Fragment Header is then followed by a 4-octet MFVI; otherwise, the
   MFVI field is omitted.  (As with OCH-0/1, the (A)RQ flag is
   implicitly always set and hence not included in the compressed
   header.)

   The OAL non-first fragment body is then included immediately
   following the OCH-2/3 header, and the L2 header length field is
   reduced by the difference in length between the compressed headers
   and full-length IPv6 and Fragment headers.  The OAL destination will
   then be able to determine the Payload Length by examining the L2
   header length field.  The OCH-2/3 format applies for non-first
   fragments only; therefore, the OAL source sets Ordinal to a
   monotonically increasing value beginning with 1 for the first non-



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   first fragment, 2 for the second non-first fragment, etc., up to and
   including the final fragment.  If more than 63 non-first fragments
   are included, these additional fragments instead set Ordinal to 0.

   When an OAL destination or intermediate node receives a carrier
   packet, it determines the length of the encapsulated OAL information
   by examining the length field of the innermost L2 header, verifies
   that the innermost next header field indicates OMNI (see:
   Section 6.2), then examines the first four bits immediately following
   the innermost header.  If the bits contain the value 4 or 6, the OAL
   node processes the remainder as an uncompressed OAL/IP header.  If
   the bits contain a value 0-3, the OAL node instead processes the
   remainder of the header as an OCH-0/1 or OCH-2/3 as specified above.

   For carrier packets with OCH 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-2/3 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-0/1 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.

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 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 L2 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 L2 headers from the
   previous hop and encapsulates the original compressed/uncompressed
   OAL header within a second compressed/uncompressed OAL header 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 L2 encapsulation headers with destinations appropriate
   for the next hop on the path to the egress tunnel endpoint.  The
   encapsulation appears as shown in Figure 10:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   L2 headers (previous hop)   |   |     L2 headers (next hop)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Original OAL/OCH-0/2 Hdr   |   | Encapsulation OAL/OCH-0/2 Hdr |
   |        Id=0x12345678          |   |         Id=0x98765432         |
   |       MFVI=0x2376a7b5         |   |        MFVI=0xacdebf12        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |   |    Original OAL/OCH-1/3 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 10: 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-0/2 header).  The inner OAL header should omit the
   CRH-32 header or use OCH-1/3, 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 extension sub-option 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



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   OAL A's next sequence number, and with Window set to N.  Since OAL B
   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:

   *  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).

   *  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).





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

   *  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.











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   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].

   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:

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

   *  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.

   *  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.

   *  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.

   *  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].






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   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) underlay interface pairs that
   exchange carrier packets.  Each interface pair represents a distinct
   underlay network path, and the set of paths traversed may be highly
   diverse when multiple interface pairs are used.  OMNI intermediate
   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 (FRAGREP) sub-options that include a
   list of (Identification, Bitmap)-tuples for fragments received and
   missing from this OAL source (see: Section 12 and
   [I-D.templin-6man-fragrep]).  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 FRAGREP.  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



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   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-to-medium 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
   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 and also in [I-D.templin-6man-fragrep].

   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.  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, number of fragments needed, 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
   the packet was dropped or 2 if the packet was forwarded successfully.
   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



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   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 first fragment 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 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) that include OMNI encapsulated
   PTB messages with code 1 or 2.  The OAL destination creates a uNA
   message with an OMNI option containing an authentication message sub-
   option if necessary followed optionally by a ICMPv6 Error sub-option
   that encodes a PTB message with a reduced value and with 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 PTB "packet in error"
   field, 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 sends a
   corresponding network layer PTB soft error to the original source to
   recommend a smaller size.  The OAL source crafts the PTB by
   extracting the leading portion of the original IP packet from the
   OMNI encapsulated PTB message (i.e., not including the OAL header)
   and writes it in the "packet in error" field of a network layer 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.

   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
   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



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   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 resume 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 Super-Packets

   By default, the OAL source includes a 40-octet IPv6 encapsulation
   header for each original IP packet during OAL encapsulation.  The OAL
   source also calculates then performs fragmentation such that a copy
   of the 40-octet IPv6 header plus an 8-octet 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.  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 a trailing checksum field included in the final
   fragment.  The OAL super-packet format is transposed from
   [I-D.ietf-intarea-tunnels] and shown in Figure 11:

















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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 11: OAL Super-Packet Format

   When the OAL source prepares a super-packet, it applies OAL
   fragmentation, includes a trailing checksum in the final fragment,
   applies L2 encapsulation to each fragment then sends the resulting
   carrier packets to the OAL destination.  When the OAL destination
   receives the super-packet it sets aside the trailing checksum,
   reassembles if necessary, then verifies the checksum while regarding
   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 ICMPv6 message checksum
   as the first original IP packet (i.e., iHa/iDa), it calculates the
   authentication signature or checksum over the remainder of super-
   packet.  Security and integrity for forwarding initial protocol data
   packets in conjunction with IPv6 ND messages used to establish NCE
   state are therefore supported.  (A common use case entails a path MS
   probe beginning with a signed IPv6 ND message followed by a NULL IPv6
   packet with a suitably large (Jumbo) Payload Length but with Next
   Header set to 59 for "No Next Header".)



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6.10.  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 L2 headers destined to either the
   mapped address of the OAL destination's first-hop ingress NAT or the
   L2 address of the OAL destination itself.  When the OAL source sends
   the resulting carrier packet, any egress NATs in the path toward the
   L2 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].

6.11.  OAL Requirements

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

   *  OAL sources MUST forward original IP packets either larger than
      the OMNI interface MRU or smaller than the minimum MPS minus the
      trailing checksum size as atomic fragments (i.e., and not as
      multiple fragments).

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

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

   *  OAL destinations MUST drop any new OAL fragments with offset and
      length that would overlap with other fragments and/or leave holes
      smaller than the minimum MPS between fragments that have already
      been received.






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   Note: Under the minimum MPS, ordinary 1500 octet original IP packets
   would require at most 4 OAL fragments, with each non-final fragment
   containing 400 payload octets and the final fragment containing 302
   payload octets (i.e., the final 300 octets of the original IP packet
   plus the 2 octet trailing checksum).  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 underlay interface pair instead of spread
   across multiple underlay 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, while the
   OAL destination is reporting reassembly congestion (see: Section 6.8)
   the OAL source could impose "pacing" by inserting an inter-fragment
   delay and increasing or decreasing the delay according to congestion
   indications.

6.12.  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
       underlay interface such that congestion experienced over a first
       underlay interface does not cause discard of incomplete
       reassemblies for uncongested underlay interfaces.






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




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6.13.  OMNI Hosts

   OMNI Hosts are end systems that configure OMNI interfaces over ENET
   underlay interfaces (i.e., either as a separate virtual interface or
   as a sublayer of the ENET interface itself).  Each ENET is connected
   to the rest of the OMNI link by a Client that receives an MNP
   delegation.  Clients delegate MNP addresses and/or sub-prefixes to
   ENET nodes (i.e., Hosts, other Clients, routers and non-OMNI hosts)
   using standard mechanisms such as DHCP [RFC8415][RFC2131] and IPv6
   Stateless Address AutoConfiguration (SLAAC) [RFC4862].  Clients
   forward packets between their ENET Hosts and peers on external
   networks acting as routers and/or OAL intermediate nodes.

   OMNI Hosts coordinate with Clients and/or other Hosts connected to
   the same ENET using IP-encapsulated IPv6 ND messages.  The IP
   encapsulation headers and ND messages both use the MNP-based
   addresses assigned to ENET underlay interfaces as source and
   destination addresses (i.e., instead of ULAs or LLAs).  For IPv4
   MNPs, the ND messages use IPv4-Mapped IPv6 addresses [RFC4291] in
   place of the IPv4 addresses.

   Hosts discover Clients by sending encapsulated RS messages using an
   OMNI link IP anycast address (or the unicast address of the Client)
   as the RS L2 encapsulation destination as specified in Section 15.
   The Client configures the IPv4 and/or IPv6 anycast addresses for the
   OMNI link on its ENET interface and advertises the address(es) into
   the ENET routing system.  The Client then responds to the
   encapsulated RS messages by sending an encapsulated RA message that
   uses its ENET unicast address as the source.  (To differentiate
   itself from an INET border Proxy/Server, the Client sets the RA
   message OMNI Interface Attributes sub-option SRT and MSID fields to 0
   for the Host's interface index.  When the RS message includes an L2
   anycast destination address, the Client also includes an Interface
   Attributes sub-option for interface index 0 to inform the Host of its
   L2 unicast address - see: Section 15 for full details on the RS and
   RA message contents.)















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   Hosts coordinate with peer Hosts on the same ENET by sending
   encapsulated NS messages to receive an NA reply.  (Hosts determine
   whether a peer is on the same ENET by matching the peer's IP address
   with the MNP (sub)-prefix for the ENET advertised in the Client's RA
   message [RFC8028].)  Each ENET peer then creates a NCE and
   synchronizes Identification windows the same as for OMNI link
   neighbors, and the Host can then engage in OMNI link transactions
   with the Client and/or other ENET Hosts.  By coordinating with the
   Client in this way, the Host treats the Client as if it were an ANET
   Proxy/Server, and the Client provides the same services that a Proxy/
   Server would provide.  By coordinating with other Hosts, the peer
   hosts can exchange large IP packets or parcels over the ENET using
   IPv6 fragmentation if necessary.

   When a Host prepares an IP packet or parcel, it uses the IP address
   of its native ENET interface as the source and the IP address of the
   (remote) peer as the destination.  The Host next performs parcel
   segmentation if necessary (see: Section 6.14) then encapsulates the
   packet/parcel in an IP header of the version supported by the ENET
   while setting the source to the same address and destination to
   either the same address if the peer is on the local ENET, or to the
   IP address of the Client otherwise.  The Host can then proceed to
   exchange packets/parcels with the destination, either directly or via
   the Client as an intermediate node.

   The encapsulation procedures are coordinated per Section 6.1, except
   that the IP encapsulation header version matches the native ENET IP
   protocol version and uses IPv6 GUA or public/private IPv4 addresses
   instead of ULAs or LLAs.  The Host sets the encapsulation IP header
   [Protocol, Next Header] field to TBD2 to indicate that this is an OAL
   encapsulation and not an ordinary IP-in-IP encapsulation.  When the
   inner header is IPv4-based, the Host next translates the
   encapsulation header into an IPv6 header with IPv4-Mapped addresses
   while setting the [IPv6 Traffic Class, Payload Length, Next Header,
   Hop Limit] according to the IPv4 [Type of Service, Total Length,
   Protocol, TTL fields], respectively, while setting Flow Label to 0.
   The Host then calculates an OAL checksum, writes the value as the
   final two octets of the encapsulated packet then applies IPv6
   fragmentation to the encapsulated packet to produce IPv6 fragments no
   smaller than the MPS the same as described in Section 6.1.  If the
   original encapsulation IP header was IPv4, the Host next translates
   the IPv6 encapsulation headers back to IPv4 headers with Protocol
   value set to 44 since the immediately next header is the IPv6
   Fragment Header.  The Host finally sends the IP encapsulated
   fragments to the ENET peer.






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   When the ENET peer receives IP encapsulated fragments, for IPv4 it
   first translates the encapsulation headers back to IPv6 headers with
   IPv4-Mapped addresses the same as above.  The peer then reassembles
   and verifies the OAL checksum.  If the checksum is correct, the peer
   next removes the encapsulation headers and applies parcel reassembly
   if necessary.  The peer then either delivers the encapsulated packet/
   parcel to upper layers if the peer is the destination or forwards the
   packet/parcel toward the final destination if the peer is a Client
   acting as an intermediate node.

   Hosts and Clients that initiate OMNI-based packet/parcel transactions
   with TCP peers not on the local ANET/ENET must negotiate the
   capability upon connection establishment by including the OMNI
   option.  This two-byte option may be sent in a SYN by a TCP that has
   been extended to include the OMNI/OAL functions.  It MUST NOT be sent
   on non-SYN segments.

   The TCP option includes a new TCP Option Kind Number TBD4 (see: IANA
   Considerations) and uses the following format:

             TCP OMNI Option:

             Kind: TBD4

             +---------+---------+
             |Kind=TBD4| Length=2|
             +---------+---------+

                         Figure 12: TCP OMNI Option

   A Host that includes the OMNI option need not implement the full OMNI
   interface abstraction but MUST implement enough of the OAL to be
   capable of fragmenting and reassembling maximum-length encapsulated
   IP packets/parcels and sub-parcels as discussed above and in the
   following section.

6.14.  IP Parcels

   IP parcels are formed by an OMNI Host or Client upper layer protocol
   entity (identified by the "5-tuple" source IP address/port number,
   destination IP address/port number and protocol number) when it
   produces a protocol data unit containing the concatenation of up to
   64 upper layer protocol segments.  All non-final segments MUST be
   equal in length while the final segment MUST NOT be larger but MAY be
   smaller.  Each non-final segment MUST be no larger than 65535 minus
   the length of the IP header plus extensions, minus the length of the
   OAL encapsulation header and trailer.  The upper layer protocol then
   presents the buffer and non-final segment size to the IP layer which



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   appends a single IP header (plus any extension headers) before
   presenting the parcel to the OMNI Interface.

   For IPv4, the IP layer prepares the parcel by appending an IPv4
   header with a Jumbo Payload option (see: Section 5.1) where "Jumbo
   Payload Length" is a 32-bit unsigned integer value (in network byte
   order) set to the lengths of the IPv4 header plus all concatenated
   segments.  The IP layer next sets the IPv4 header DF bit to 1, then
   sets the IPv4 header Total Length field to the length of the IPv4
   header plus the length of the first segment only.

   For IPv6, the IP layer forms a parcel by appending an IPv6 header
   with a Jumbo Payload option the same as for IPv4 above where "Jumbo
   Payload Length" is set to the lengths of the IPv6 Hop-by-Hop Options
   header and any other extension headers present plus all concatenated
   segments.  The IP layer next sets the IPv6 header Payload Length
   field to the lengths of the IPv6 Hop-by-Hop Options header and any
   other extension headers present plus the length of the first segment
   only.  (Note: the IP layer can form true IPv4/IPv6 jumbograms (as
   opposed to parcels) by instead setting the [Total, Payload] Length
   field to 0.)

   An IP parcel therefore has the following structure:

   +--------+--------+--------+--------+
   |                                   |
   ~        Segment J (K octets)       ~
   |                                   |
   +--------+--------+--------+--------+
   ~                                   ~
   ~                                   ~
   +--------+--------+--------+--------+
   |                                   |
   ~        Segment 3 (L octets)       ~
   |                                   |
   +--------+--------+--------+--------+
   |                                   |
   ~        Segment 2 (L octets)       ~
   |                                   |
   +--------+--------+--------+--------+
   |                                   |
   ~        Segment 1 (L octets)       ~
   |                                   |
   +--------+--------+--------+--------+
   |     IP Header Plus Extensions     |
   ~    [Total, Payload] Length = M    ~
   |      Jumbo Payload Length = N     |
   +--------+--------+--------+--------+



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                    Figure 13: OMNI Interface IP Parcels

   where J is the total number of segments (between 1 and 64), L is the
   length of each non-final segment which MUST NOT be larger than 65535
   (minus headers as above) and K is the length of the final segment
   which MUST NOT be larger than L.  The values M and N are then set to
   the length of the IP header plus extensions for IPv4 or to the length
   of the extensions only for IPv6, then further calculated as follows:

      M = M + ((J-1) ? L : K)

      N = N + (((J-1) * L) + K)

   Note: a "singleton" parcel is one that includes only the IP header
   plus extensions with a single segment of length K, while a "null"
   parcel is a singleton with K=0, i.e., a parcel consisting of only the
   IP header plus extensions with no octets beyond.

   When the IP layer forwards a parcel, the OMNI interface invokes the
   OAL which forwards it to either a Client as an intermediate node or
   the final destination itself.  The OAL source first assigns a
   monotonically- incrementing (modulo 127) "Parcel ID" and subdivides
   the parcel into sub-parcels no larger than the maximum of the path
   MTU to the next hop or 64KB (minus the length of encapsulation
   headers).  The OAL source determines the number of segments of length
   L that can fit into each sub-parcel under these size constraints,
   e.g. if the OAL source determines that a sub-parcel can contain 3
   segments of length L, it creates sub-parcels with the first
   containing segments 1-3, the second containing segments 4-6, etc. and
   with the final containing any remaining segments.  The OAL source
   then appends an identical IP header plus extensions to each sub-
   parcel while resetting M and N in each according to the above
   equations with J set to 3 and K set to L for each non-final sub-
   parcel and with J set to the remaining number of segments for the
   final sub-parcel.

   The OAL source next performs encapsulation on each sub-parcel with
   destination set to the next hop address.  If the next hop is reached
   via an ANET/INET interface, the OAL source inserts an OAL header the
   same as discussed in Section 6.1 and sets the destination to the
   MNP_ULA of the target Client.  If the next hop is reached via an ENET
   interface, the OAL source instead inserts an IP header of the
   appropriate protocol version for the underlay ENET (i.e., even if the
   encapsulation header is IPv4) and sets the destination to the ENET IP
   address of the next hop.  The OAL source inserts the encapsulation
   header even if no actual fragmentation is needed and/or even if the
   Jumbo Payload option is present.




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   The OAL source next assigns an Identification number that is
   monotonically-incremented for each consecutive sub-parcel, calculates
   and appends the OAL checksum, then performs IPv6 fragmentation over
   the sub-parcel if necessary to create fragments small enough to
   traverse the path to the next hop.  (If the encapsulation header is
   IPv4, the OAL source first translates the encapsulation header into
   an IPv6 header with IPv4-Mapped IPv6 addresses before performing the
   fragmentation/reassembly operation while inserting an IPv6 Fragment
   Header.)  The OAL source then writes the "Parcel ID" and sets/clears
   the "(P)arcel" and "(More) (S)ub-Parcels" bits in the Fragment Header
   of the first fragment (see: Figure 6).  (The OAL source sets P to 1
   for a parcel or to 0 for a non-parcel.  When P is 1, the OAL next
   sets S to 1 for non-final sub-parcels or to 0 if the sub-parcel
   contains the final segment.)  The OAL source then forwards each IP
   encapsulated packet/fragment to the next hop (i.e., after first
   translating the IPv6 encapsulation header back to IPv4 if necessary).

   When the next hop receives the encapsulated IP fragments or whole
   packets, it acts as an OAL destination and reassembles if necessary
   (i.e., after first translating the IPv4 encapsulation header to IPv6
   if necessary).  If the P flag in the first fragment is 0, the OAL
   destination then processes the reassembled entity as an ordinary IP
   packet; otherwise it continues processing as a sub-parcel.  If the
   OAL destination is not the final destination, it retains the sub-
   parcels along with their Parcel ID and Identification values for a
   brief time in hopes of re-combining with peer sub-parcels of the same
   original parcel identified by the 4-tuple consisting of the IP
   encapsulation source and destination, Identification and Parcel ID.
   The OAL destination re-combines peers by concatenating the segments
   included in sub-parcels with the same Parcel ID and with
   Identification values within 64 of one another to create a larger
   sub-parcel possibly even as large as the entire original parcel.
   Order of concatenation is not important, with the exception that the
   final sub-parcel (i.e., the one with S set to 0) must occur as the
   final concatenation before transmission.  The OAL destination then
   appends a common IP header plus extensions to each re-combined sub-
   parcel while resetting M and N in each according to the above
   equations with J, K and L set accordingly.













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   When the current OAL destination is an intermediate node, it next
   becomes an OAL source to forward the re-combined (sub-)parcel(s) to
   the next hop toward the final destination using encapsulation/
   translation the same as specified above.  (Each such intermediate
   node MUST ensure that the S flag remains set to 0 in the sub-parcel
   that contains the final segment.)  When the parcel or sub-parcels
   arrive at the final OAL destination, it re-combines them into the
   largest possible (sub)-parcels while honoring the S flag then
   delivers them to upper layers which act on the enclosed 5-tuple
   information supplied by the original source.

   Note: while the final destination may be tempted to re-combine the
   sub-parcels of multiple different parcels with identical upper layer
   protocol 5-tuples and with non-final segments of identical length,
   this process could become complicated when the different parcels each
   have final segments of diverse lengths.  Since this could possibly
   defeat any perceived performance advantages, the decision of whether
   and how to perform inter-parcel concatenation is an implementation
   matter.

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 L2 encapsulations and finally engages
   the native frame format of the underlay 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 L2 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 underlay
   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
   Proxy/Servers assign "ADM-LLAs" that include an administrative ID
   guaranteed to be unique on the link.  LLAs are configured as follows:






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   *  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.

   *  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.)

   *  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:

   *  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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   *  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).

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

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

   *  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 ANET/ENET could instead use
   private IPv4 prefixes (e.g., 10.0.0.0/8, etc.)  [RFC3330], however
   this would require IPv4 NAT at the INET-to-ANET/ENET boundary.  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 TBD5 (see: IANA
   Considerations), and IPv4 routers that configure OMNI interfaces
   advertise the prefix TBD5/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:TBD5[32]:MNP[64]:Link_ID[16]

   where TBD5[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:TBD5[32]:2001:db8:0:0:Link_ID[16], the OMNI
   IPv6 anycast address for MSP 192.0.2.0/24 is
   2002:TBD5[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:TBD5[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 underlay
   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) 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 underlay 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.

12.1.  The OMNI Option

   OMNI options appear in IPv6 ND messages formatted as shown in
   Figure 14:



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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Type     |     Length    |         Sub-Options           ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 14: OMNI Option Format

   In this format:

   *  Type is set to TBD6 (see: IANA Considerations).

   *  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.  If multiple OMNI option instances appear in the
      same IPv6 ND message, the union of the contents of all OMNI
      options is accepted unless otherwise qualified for specific sub-
      options below.

   *  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.  The OMNI interface processes all OMNI option
   instances received in the same IPv6 ND message in the consecutive
   order in which they 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 an OMNI-specific 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 ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-




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                        Figure 15: Sub-Option Format

   *  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
           Neighbor Coordination          2
           Interface Attributes           3
           Multilink Forwarding Params    4
           Traffic Selector               5
           Geo Coordinates                6
           DHCPv6 Message                 7
           HIP Message                    8
           PIM-SM Message                 9
           Fragmentation Report          10
           Node Identification           11
           ICMPv6 Error                  12
           QUIC-TLS Message              13
           Proxy/Server Departure        14
           Sub-Type Extension            30

                                  Figure 16

      Sub-Types 15-29 are available for future assignment for major
      protocol functions, while Sub-Type 30 supports scalable extension
      to include other functions.  Sub-Type 31 is reserved by IANA.

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

   *  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.




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   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 12.2.9), it MUST appear as the first sub-option of the
   first OMNI option which must appear immediately following the IPv6 ND
   message header (all other authentication sub-options 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 the first sub-option is not
   authentication, the OMNI interface 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
   underlay interface that will transmit the RS (see: Section 12.2.4)
   immediately following the authentication and header extension sub-
   options 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 underlay interface that will
   transmit the NS (see: Section 12.2.5).









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   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 17: Pad1

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

   *  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.

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 18: PadN

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

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





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   *  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.  Neighbor Coordination

   IPv6 ND messages used for Prefix Length assertion, service
   coordination and/or Window Synchronization include a Neighbor
   Coordination sub-option.  If a Neighbor Coordination sub-option is
   included, it must appear immediately after the authentication sub-
   option if present; otherwise, as the first (non-padding) sub-option
   of the first OMNI option.  If multiple Neighbor Coordination sub-
   options are included (whether in a single OMNI option or multiple),
   only the first is processed and all others are ignored.

   The Neighbor Coordination sub-option is formatted as follows:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=2|    Sub-length=14    |    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|     |                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 19: Neighbor Coordination

   *  Sub-Type is set to 2.

   *  Sub-Length is set to 14.

   *  The first two octets of Sub-Option Data contains a 1-octet Prefix
      Length followed by a 1-octet flags field interpreted as follows:





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      -  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.

      -  The N/A/U flags 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 flag 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
         flag 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 flag 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
         underlay interfaces change.  Proxy/Servers function according
         to the N/A/U flag settings received in the most recent RS
         message to support dynamic Client updates.  In all IPv6 ND
         messages, the remaining 5 flag bits are set to 0 on
         transmission and ignored on reception.

   *  The remainder of Sub-Option Data contains 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 size modeled
      from the Transmission Control Protocol (TCP) header specified in
      Section 3.1 of [RFC0793].  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.







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12.2.4.  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 underlay 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 underlay 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
   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 12.2.5).

   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 underlay 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 underlay 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 L2 address, it MUST include an Interface Attributes sub-
   option with omIndex '0' that encodes a unicast L2 address immediately
   after the Interface Attributes sub-option for the Client's underlay
   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:





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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=3|    Sub-length=N     |    omIndex    |     omType    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  |  Link | Resvd | FMT |   SRT   |               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               ~
       ~                  LHS Proxy/Server MSID/INADDR                 ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 20: Interface Attributes

   *  Sub-Type is set to 3.

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

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

      -  omIndex is a 1-octet value corresponding to a specific underlay
         interface.  Client OMNI interfaces MUST number each distinct
         underlay 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
         underlay 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:






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         o  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 underlay 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.

         o  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
            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 underlay 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



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         since port number 8060 is used when the L2 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].

12.2.5.  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 Multilink
   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 underlay interface used to transmit the
   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 21:













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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=4|    Sub-length=N     |      Job      |   A   |   B   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~        Multilink Forwarding Vector Index (MFVI) List          ~
       ~               (16 consecutive 4-octet MFVIs)                  ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~           Tunnel Window Synchronization Parameters            ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |FHS Cli omIndex|     omType    |  Provider ID  |  Link | Resvd |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | FMT |   SRT   |                                               ~
       +-+-+-+-+-+-+-+-+                                               ~
       ~                 FHS Proxy/Server MSID/INADDR                  ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                   FHS Gateway MSID/INADDR                     ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |LHS Cli omIndex|     omType    |  Provider ID  |  Link | Resvd |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | FMT |   SRT   |                                               ~
       +-+-+-+-+-+-+-+-+                                               ~
       ~                 LHS Proxy/Server MSID/INADDR                  ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    LHS Gateway MSID/INADDR                    ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 21: Multilink Forwarding Parameters

   *  Sub-Type is set to 4.  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.

   *  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 "0" and "1" (see
      below).

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

      -  Job and A/B are 1-octet fields that determine per-hop
         processing of the MFVI List, where A is a 4-bit count of the
         number of "A" MVFI List entries and B is a 4-bit count of the
         number of "B" MVFI List entries (valid A/B values are 0-15).
         Job is a 1-octet code interpreted as follows:




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         o  0 - "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).

         o  1 - "Follow B; Build A" - the LHS source sets this code in a
            solicited NA response to a solicitation with Job code "0"
            (any other messages that include this code MUST be dropped).
            The LHS source first copies the MFVI List and B value from
            the code "0" 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
            node and beginning with an entry for the final intermediate
            node (i.e., the list is populated in the reverse direction).

         o  2 - "Follow A; Record B" - the FHS node that sent the
            original code "0" solicitation and received the
            corresponding code "1" 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 "1" 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.





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         o  3 - "Follow B; Record A" - the LHS node that received the
            original code "0" solicitation and sent the corresponding
            code "1" 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 "0"
            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 and 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 "0"
         solicitations and receive code "1" advertisements discover only
         "A" information, while LHS sources that receive code "0"
         solicitations and return code "1" advertisements discover only
         "B" information.  FHS/LHS intermediate nodes can instead
         examine A, B and the MFVI List to determine the number of
         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 16 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 and A/B
         codes (see above).









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      -  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 "0" and "1" 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:

         o  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
            underlay interface.  These fields are populated based on
            information discovered in Interface Attributes sub-options
            included in earlier RS/RA and/or NS/NA exchanges.

         o  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 underlay
            interface (i.e., the same as described in Section 12.2.4),
            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 12.2.4.

         o  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 L2 encapsulation includes a UDP header.

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









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12.2.6.  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 underlay interfaces.  Traffic Selectors for
   some or all of a target Client's underlay 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=5|    Sub-length=N     |    omIndex    |   TS Format   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                                                               ~
       ~                RFC 6088 Format Traffic Selector               ~
       ~                                                               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 22: Traffic Selector

   *  Sub-Type is set to 5.  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.

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

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

      -  omIndex is a 1-octet value corresponding to a specific underlay
         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.








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      -  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.

12.2.7.  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=6|    Sub-length=N     |    Geo Type   |Geo Coordinates
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

                  Figure 23: Geo Coordinates Sub-option

   *  Sub-Type is set to 6.  If multiple instances appear in OMNI
      options of the same message all are processed.

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

   *  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.

   *  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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12.2.8.  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=7|    Sub-length=N     |    msg-type   |  id (octet 0) |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   transaction-id (octets 1-2) |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
       |                                                               |
       .                        DHCPv6 options                         .
       .                 (variable number and length)                  .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 24: DHCPv6 Message Sub-option

   *  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.

   *  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.

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

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











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12.2.9.  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
   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 as the first OMNI 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 underlay 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.  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.











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   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
   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 octets 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=8|    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 25: HIP Message Sub-option





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   *  Sub-Type is set to 8.  If multiple instances appear in OMNI
      options of the same message the first is processed and all others
      are ignored.

   *  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.

   *  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
      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.

12.2.10.  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 26:

        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     |PIM Ver| Type  |   Reserved    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       /                         PIM-SM Message                        /
       /                                                               /
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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                 Figure 26: PIM-SM Message Option Format

   *  Sub-Type is set to 9.  If multiple instances appear in OMNI
      options of the same message all are processed.

   *  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.

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

12.2.11.  Fragmentation Report (FRAGREP)

   Fragmentation Report (FRAGREP) sub-options may be included in the
   OMNI options of uNA messages sent from an OAL destination to an OAL
   source.  The message consists of (N / 12)-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 - 64)                    |
       +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               | Identification #2 (bits 0-15) |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Identification #2 (bits 15-31)|                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                    Bitmap #2 (bits 0 - 64)                    |
       +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                              ...                              |
       +                              ...                              +

                Figure 27: Fragmentation Report (FRAGREP)



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

   *  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
      12 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.

   *  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.)

   *  Bitmap (i) includes an ordinal checklist of fragments, with each
      bit set to 1 for a fragment received or 0 for a fragment missing.
      (Up to 64 ordinal numbered fragments for each packet may be
      recoded.)  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 28

      (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 29: Node Identification





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   *  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.)

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

   *  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.

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






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   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 30:

        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 30: 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    |     Type      |     Code      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                         Message Body                          +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 31: ICMPv6 Error

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

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

   *  Sub-Option Data includes a one octet Type followed by a one octet
      Code followed by an (N-2)-octet Message Body encoded exactly as
      per Section 2.1 of [RFC4443].  OMNI interfaces include as much of
      the ICMPv6 error message body in the sub-option as possible
      without causing the entire IPv6 ND message to exceed the minimum
      IPv6 MTU.  While all ICMPv6 error message types are supported, OAL
      destinations in particular may include ICMPv6 PTB messages in uNA



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      messages to provide MTU feedback information via the OAL source
      (see: Section 6.8).  Note: ICMPv6 informational messages must not
      be included and must be ignored if received.

12.2.14.  QUIC-TLS 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=13|    Sub-length=N    |                                ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-                                ~
       ~                         QUIC-TLS Message                      ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 32: QUIC-TLS Message

   *  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.

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

   *  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:










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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=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 33: Proxy/Server Departure

   *  Sub-Type is set to 14.

   *  Sub-Length is set to 8.

   *  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 34:

        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 34: Sub-Type Extension

   *  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.




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   *  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.

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

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

   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 35: RFC4380 Header Extension Option (Extension-Type 0)

   *  Sub-Type is set to 30.

   *  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.

   *  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.





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   *  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].

      -  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.

   *  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 36: RFC6081 Trailer Extension Option (Extension-Type 1)

   *  Sub-Type is set to 30.

   *  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.

   *  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.

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



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      -  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].

      -  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.

   *  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 underlay interface
   applies.  The Client mobile router also serves as an IGMP/MLD Proxy
   for its ENETs and/or hosted applications per [RFC4605].

   The Client uses Multicast Listener Discovery (MLDv2) [RFC3810] to
   coordinate with Proxy/Servers, and underlay network 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).











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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 ENET 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 underlay interfaces based on PBM factors (e.g.,
   traffic attributes, cost, performance, message size, etc.) and one or
   more target underlay interfaces for the neighbor based on Interface
   Attributes received in IPv6 ND messages (see: Section 12.2.4).
   Multilink forwarding may also direct packet replication across
   multiple underlay 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 underlay
   interfaces.

   When the OMNI interface sends an original IP packet over a selected
   source underlay interface, it first employs OAL encapsulation and
   fragmentation as discussed in Section 5, then performs L2
   encapsulation as directed by the appropriate MFV.  The OMNI interface
   also performs L2 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 underlay 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 ENET routing system.  Multiple distinct OMNI links
   can therefore be used to support fault tolerance, load balancing,
   reliability, etc.




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   Applications in ENETs 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.

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.




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   For each underlay 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 L2 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
   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 underlay
   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
   underlay interfaces.  When a first underlay interface transitions to
   UP, the OMNI interface also transitions to UP.  When all underlay
   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 underlay interfaces that
   are also UP.  The Client sends additional RS messages to refresh
   lifetimes and to register/deregister underlay interfaces as they
   transition to UP or DOWN.  The Client's OMNI interface sends initial
   RS messages over an UP underlay 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 an OMNI
   header extension with Preflen assertion, N/A/U flags, an Interface
   Attributes sub-option for the underlay 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 underlay 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



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   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 L2 encapsulation is used, the
   Client includes the discovered FHS Proxy/Server INADDR or an anycast
   address as the L2 destination then forwards the resulting carrier
   packet(s) into the underlay network.

   When an FHS Proxy/Server receives the carrier packets containing an
   RS it sets aside the L2 headers, verifies the Identifications and
   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 L2 (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 L2 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 L2 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 L2 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



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   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 L2
   headers with source and destination address information reversed from
   the RS L2 information and returns the resulting carrier packets to
   the Client over the same underlay 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 L2/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 L2 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 L2 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 L2 headers and sends the
   resulting carrier packets into the secured spanning tree.





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   When the FHS Proxy/Server receives the carrier packets it discards
   the L2 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
   L2 IP address if necessary (see above), an Origin Indication sub-
   option with the Client's cached INADDR and an authentication sub-
   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 L2 headers with addresses taken from the Client's NCE and
   returns the resulting carrier packets via the same underlay interface
   over which the RS was received.

   When the Client receives the carrier packets, it discards the L2
   headers, reassembles and removes the OAL header 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 underlay interfaces, it creates
   additional FHS Proxy/Server NCEs as necessary when it receives RAs
   over those interfaces (noting that multiple of the Client's underlay
   interfaces may be serviced by the same FHS Proxy/Server).  For each
   underlay 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 underlay network-local unicast address of
   the FHS Proxy//Server via that underlay interface.)  The Client then
   compares the Origin Indication INADDR information with its own
   underlay 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.








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   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 underlay interfaces according to
   their states as follows:

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

   *  When an underlay interface transitions to DOWN, the Client sends
      unsolicited NA messages over any UP underlay interface with an
      OMNI option containing Interface Attributes sub-options for the
      DOWN underlay interface with Link set to '0'.  The Client sends
      isolated unsolicited NAs when reliability is not thought to be a
      concern (e.g., if redundant transmissions are sent on multiple
      underlay interfaces), or may instead set the PNG flag in the OMNI
      header to trigger a uNA reply.

   *  When the Router Lifetime for the Hub Proxy/Server nears
      expiration, the Client sends an RS over any underlay interface to
      receive a fresh RA from the Hub. If no RA messages are received
      over a first underlay interface (i.e., after retrying), the Client
      marks the underlay interface as DOWN and should attempt to contact
      the Hub Proxy/Server via a different underlay interface.  If the
      Hub Proxy/Server is unresponsive over additional underlay
      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.

   *  When all of a Client's underlay 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
   underlay interface (i.e., even after attempting to contact alternate
   Proxy/Servers), the Client declares this underlay 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].





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   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
   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
   underlay 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 underlay 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
   underlay interfaces, Window Synchronization will initially be
   repeated for the RS/RA exchange over each underlay 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



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   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 L2
   destination address, the FHS Proxy/Server returns the solicited RA
   using the same anycast address as the L2 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 L2
   destination and the FHS Proxy/Server returns the solicited RA using
   the same unicast address as the L2 source.  This will ensure that RS/
   RA exchanges are not impeded by any NATs on the path while 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.



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   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-
   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 underlay interface does not configure
   an IPv6 GUA the Client forwards the message without further
   encapsulation.  Otherwise, the Client encapsulates the message in



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   UDP/IPv6 L2 headers, sets the source to the underlay 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 L2 headers, sets the source to the underlay interface IPv4
   address and sets the destination to the OMNI IPv4 anycast address.
   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 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 L2 encapsulation headers (if necessary)
   with (src, dst) set to the (dst, src) of the RS L2 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.



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   Note: When the RS message includes anycast OAL and/or L2
   encapsulation destinations, the FHS Proxy/Server must use the same
   anycast addresses as the OAL and/or L2 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 6.10.)  After the Client
   begins using unicast OAL/L2 encapsulation addresses in this way, the
   FHS Proxy/Server should also begin using the same unicast addresses
   in the reverse direction.

   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 the OMNI extension header 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



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   "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 extension header
   Preflen value and with a Client Identifier formed from the OMNI
   option Node Identification sub-option; otherwise, the Hub Proxy/
   Server uses the 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 extension header 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 L2 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.




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15.4.  Client-to-Client Chaining

   Clients can provide an OMNI link ingress point for other nodes on
   their (downstream) ENETs that also act as Clients.  When Client A has
   already coordinated with an (upstream) ANET/INET Proxy/Server, Client
   B on an ENET serviced by Client A can send OAL-encapsulated RS
   messages with addresses set the same as specified in Section 15.2.
   When Client A receives the RS message, it infers from the OAL
   encapsulation that Client B is seeking to establish itself as a
   Client instead of just a simple ENET Host.

   Client A then returns an RA message the same as a Proxy/Server would
   do as specified in Section 15.2.  The MNP delegation in the RA
   message must be a sub-MNP from the MNP delegated to Client A.  For
   example, if Client A receives the MNP 2001:db8:1000::/48 it can
   advertise a sub-delegation such as 2001:db8:1000:2000::/56 to Client
   B.  Client B in turn distributes the prefix 2001:db8:1000:2000::/56
   to its own ENET(s), where there may be a further prospective Client C
   that would in turn request chaining services via Client B.

   To support this Client-to-Client chaining, Clients send IPv6 ND
   messages addressed to the OMNI link anycast address via their ANET/
   INET (i.e., upstream) interfaces, but advertise the OMNI link anycast
   address into their ENET (i.e., downstream) networks where there may
   be further prospective Clients wishing to join the chain.  The ENET
   of the upstream Client is therefore seen as an ANET by downstream
   Clients, and the upstream Client is seen as a Proxy/Server by
   downstream Clients.

16.  Secure Redirection

   If the underlay network 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 underlay network, 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.













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   After verifying Client authorization and returning an RA, the Proxy/
   Server MAY return IPv6 ND Redirect messages to direct Clients located
   on the same underlay network 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.  In some underlay networks, 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 links such as aeronautical radios) and can therefore be tuned for
   rapid response.









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   FHS Proxy/Servers perform proactive NUD for Hub Proxy/Servers for
   which there are currently active Clients.  If a Hub Proxy/Server
   fails, the FHS Proxy/Server can quickly inform Clients of the outage
   by sending multicast RA messages.  The FHS Proxy/Server sends RA
   messages to Clients 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 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 link-
   layer address mappings as discussed in Appendix D.  This arrangement
   imparts a (virtual) point-to-point link model over the (physical)
   multiple access link.







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20.  OMNI Interfaces on Open Internetworks

   Client OMNI interfaces configured over IPv6-enabled underlay
   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 underlay 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 underlay interface
   that received the RA according to standard IPv6 ND and address
   resolution conventions [RFC4861] [RFC4862].  Client OMNI interfaces
   configured over IPv4-only underlay interfaces configure IPv4 address
   information on the underlay interfaces using mechanisms such as
   DHCPv4 [RFC2131].

   Client OMNI interfaces configured over underlay 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 L2 encapsulation headers for
   transmission over open Internetworks with UDP service port number
   8060 (see: Section 25.15 and Section 3.6 of [I-D.templin-6man-aero])
   for both IPv4 and IPv6 underlay interfaces.  The OMNI interface
   submits original IP packets for OAL encapsulation, then encapsulates
   the resulting OAL fragments in UDP/IP L2 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.)



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   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
   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.)  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 perform route optimization to
   coordinate with other INET nodes that can provide forwarding services
   (or preferably coordinate directly with peer Clients directly)
   instead of burdening the Proxy/Server.  Procedures for coordinating
   with peer Clients and discovering INET nodes that can provide better
   forwarding services are discussed in [I-D.templin-6man-aero].




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   Clients that attempt to contact peers over INET underlay 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 underlay interfaces maintain neighbor
   relationships by transmitting periodic IPv6 ND messages with OMNI
   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 underlay interfaces should
   employ the Identification window synchronization mechanisms specified
   in Section 6.6 in order to exclude 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.











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   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.
   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.



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   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.

   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 L2 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/L2 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/L2 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.



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25.  IANA Considerations

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

25.1.  "IP Option Numbers" Registry

   The IANA is instructed to allocate a new IP option code in the 'ip
   option numbers' registry for the "JUMBO - IPv4 Jumbo Payload" option.
   The Copy and Class fields must both be set to 0, and the Number and
   Value fields must both be set to TBD1 (registration procedures are
   not defined).

25.2.  "Protocol Numbers" Registry

   The IANA is instructed to allocate an Internet Protocol number TBD2
   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).

25.3.  "IEEE 802 Numbers" Registry

   During final publication stages, the IESG will be requested to
   procure an IEEE EtherType value TBD3 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 TBD3 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).

25.4.  "TCP Parameters" Registry

   The IANA is instructed to allocate a new TCP Option Kind Number in
   the 'tcp-parameters' registry for the "OMNI" option.  The Kind must
   be set to TBD4 (to be assigned by IANA) and the Length must be set to
   2.  Guidance is found in [RFC2780] (registration procedure is
   Standards Action or IESG Approval).











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25.5.  "IPv4 Special-Purpose Address" Registry

   The IANA is instructed to assign TBD5/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
   TBD5/N address/prefix.

25.6.  "IPv6 Neighbor Discovery Option Formats" Registry

   The IANA is instructed to allocate an official Type number TBD6 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].

25.7.  "Ethernet Numbers" Registry

   The IANA is instructed to allocate one Ethernet unicast address TBD7
   (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 37: IANA Unicast 48-bit MAC Addresses

25.8.  "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 38: ICMPv6 Code Fields: Type 2 - Packet Too Big Values






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

25.9.  "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        Neighbor Coordination          [RFCXXXX]
      3        Interface Attributes           [RFCXXXX]
      4        Multilink Forwarding Params    [RFCXXXX]
      5        Traffic Selector               [RFCXXXX]
      6        Geo Coordinates                [RFCXXXX]
      7        DHCPv6 Message                 [RFCXXXX]
      8        HIP Message                    [RFCXXXX]
      9        PIM-SM Message                 [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 39: OMNI Option Sub-Type Values

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

   The OMNI Geo Coordinates sub-option (see: Section 12.2.7) 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]




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                    Figure 40: OMNI Geo Coordinates Type

25.11.  "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 41: OMNI Node Identification ID-Type Values

25.12.  "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):

      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 42: OMNI Option Sub-Type Extension Values










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25.13.  "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 43: OMNI RFC4380 UDP/IP Header Option

25.14.  "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):

      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 44: OMNI RFC6081 Trailer Option










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25.15.  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
   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.)

   OMNI interfaces configured over secured ANET/ENET interfaces inherit
   the physical and/or link-layer security properties (i.e., "protected
   spectrum") of the connected networks.  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], [RFC4380] or [RFC9000] 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.




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   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 6.12.  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.

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:

   *  [RFC1191]

   *  [RFC2675]



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   *  [RFC4443]

   *  [RFC7526]

   *  [RFC8200]

   *  [RFC8201]

   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.  Akash Agarwal, Madhuri Madhava Badgandi, Sean Dickson,
   Don Dillenburg, Joe Dudkowski, Vijayasarathy Rajagopalan, Ron
   Sackman, Bhargava Raman Sai Prakash and Katherine 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.



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   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.

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>.







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   [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>.

   [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>.



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   [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>.

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/", 3
              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)", 10 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.





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   [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)", Work in Progress,
              Internet-Draft, draft-ietf-drip-rid-15, 8 December 2021,
              <https://www.ietf.org/archive/id/draft-ietf-drip-rid-
              15.txt>.

   [I-D.ietf-intarea-tunnels]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-intarea-tunnels-10, 12 September 2019,
              <https://www.ietf.org/archive/id/draft-ietf-intarea-
              tunnels-10.txt>.

   [I-D.ietf-ipwave-vehicular-networking]
              (editor), J. (. J., "IPv6 Wireless Access in Vehicular
              Environments (IPWAVE): Problem Statement and Use Cases",
              Work in Progress, Internet-Draft, draft-ietf-ipwave-
              vehicular-networking-24, 9 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ipwave-
              vehicular-networking-24.txt>.

   [I-D.ietf-tsvwg-udp-options]
              Touch, J., "Transport Options for UDP", Work in Progress,
              Internet-Draft, draft-ietf-tsvwg-udp-options-13, 19 June
              2021, <https://www.ietf.org/archive/id/draft-ietf-tsvwg-
              udp-options-13.txt>.

   [I-D.templin-6man-aero]
              Templin, F. L., "Automatic Extended Route Optimization
              (AERO)", Work in Progress, Internet-Draft, draft-templin-
              6man-aero-37, 15 November 2021,
              <https://www.ietf.org/archive/id/draft-templin-6man-aero-
              37.txt>.

   [I-D.templin-6man-fragrep]
              Templin, F. L., "IPv6 Fragment Retransmission and Path MTU
              Discovery Soft Errors", Work in Progress, Internet-Draft,
              draft-templin-6man-fragrep-03, 16 December 2021,
              <https://www.ietf.org/archive/id/draft-templin-6man-
              fragrep-03.txt>.





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   [I-D.templin-6man-lla-type]
              Templin, F. L., "The IPv6 Link-Local Address Type Field",
              Work in Progress, Internet-Draft, draft-templin-6man-lla-
              type-02, 23 November 2020,
              <https://www.ietf.org/archive/id/draft-templin-6man-lla-
              type-02.txt>.

   [I-D.templin-6man-omni-interface]
              Templin, F. L. and T. Whyman, "Transmission of IP Packets
              over Overlay Multilink Network (OMNI) Interfaces", Work in
              Progress, Internet-Draft, draft-templin-6man-omni-
              interface-99, 24 March 2021,
              <https://www.ietf.org/archive/id/draft-templin-6man-omni-
              interface-99.txt>.

   [I-D.templin-intarea-parcels]
              Templin, F. L., "IP Parcels", Work in Progress, Internet-
              Draft, draft-templin-intarea-parcels-06, 22 December 2021,
              <https://www.ietf.org/archive/id/draft-templin-intarea-
              parcels-06.txt>.

   [IPV4-GUA] Postel, J., "IPv4 Address Space Registry,
              https://www.iana.org/assignments/ipv4-address-space/ipv4-
              address-space.xhtml", 14 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", 14
              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>.





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   [RFC1149]  Waitzman, D., "Standard for the transmission of IP
              datagrams on avian carriers", RFC 1149,
              DOI 10.17487/RFC1149, April 1990,
              <https://www.rfc-editor.org/info/rfc1149>.

   [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>.

   [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>.

   [RFC2675]  Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
              RFC 2675, DOI 10.17487/RFC2675, August 1999,
              <https://www.rfc-editor.org/info/rfc2675>.






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   [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
              Values In the Internet Protocol and Related Headers",
              BCP 37, RFC 2780, DOI 10.17487/RFC2780, March 2000,
              <https://www.rfc-editor.org/info/rfc2780>.

   [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>.

   [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>.







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   [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>.

   [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>.



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   [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>.

   [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>.






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   [RFC6214]  Carpenter, B. and R. Hinden, "Adaptation of RFC 1149 for
              IPv6", RFC 6214, DOI 10.17487/RFC6214, April 2011,
              <https://www.rfc-editor.org/info/rfc6214>.

   [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>.

   [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>.




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   [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>.

   [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>.








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   [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>.

   [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., Tüxen, M., Rüngeler, I., and T.
              Völker, "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>.






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   [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:

      "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]."









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   To calculate the OAL checksum, the above algorithm is applied over
   the N-octet concatenation of the OAL pseudo-header and the
   encapsulated IP packet or packets.  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(s) as data octets D[41] through 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 underlay
   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 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.

   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.  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 all 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.




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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
   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 Link-Layer 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 underlay network.  This
   implies that Client-Proxy/Server control messaging should be isolated
   and not overheard by other nodes on the link.










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   To support Client-Proxy/Server isolation on some links, Proxy/Servers
   can maintain an OMNI-specific unicast link-layer address ("MSADDR").
   For Ethernet-compatible links, this specification reserves one
   Ethernet unicast address TBD7 (see: IANA Considerations).  For non-
   Ethernet statically-addressed links MSADDR is reserved per the
   assigned numbers authority for the link-layer addressing space.  For
   still other links, MSADDR may be dynamically discovered through other
   means, e.g., link-layer 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 link-layer 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:

   *  Submit for RFC publication.

Authors' Addresses

   Fred L. Templin (editor)
   The Boeing Company
   P.O. Box 3707
   Seattle, WA 98124
   United States of America

   Email: fltemplin@acm.org


   Tony Whyman
   MWA Ltd c/o Inmarsat Global Ltd
   99 City Road
   London

   Email: tony.whyman@mccallumwhyman.com




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