Transmission of IP Packets over Overlay Multilink Network (OMNI) Interfaces
draft-templin-6man-omni-00

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Authors Fred Templin  , Tony Whyman 
Last updated 2021-03-24
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Network Working Group                                    F. Templin, Ed.
Internet-Draft                                        The Boeing Company
Updates: rfc1191, rfc4443, rfc7526,                            A. Whyman
         rfc8201 (if approved)           MWA Ltd c/o Inmarsat Global Ltd
Intended status: Standards Track                          March 24, 2021
Expires: September 25, 2021

    Transmission of IP Packets over Overlay Multilink Network (OMNI)
                               Interfaces
                       draft-templin-6man-omni-00

Abstract

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

Status of This Memo

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   This Internet-Draft will expire on September 25, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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   (https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Overlay Multilink Network (OMNI) Interface Model  . . . . . .  11
   5.  OMNI Interface Maximum Transmission Unit (MTU)  . . . . . . .  17
   6.  The OMNI Adaptation Layer (OAL) . . . . . . . . . . . . . . .  18
     6.1.  OAL Source Encapsulation and Fragmentation  . . . . . . .  18
     6.2.  OAL *NET Encapsulation and Re-Encapsulation . . . . . . .  23
     6.3.  OAL Destination Decapsulation and Reassembly  . . . . . .  24
     6.4.  OAL Header Compression  . . . . . . . . . . . . . . . . .  25
     6.5.  OAL Fragment Identification Window Maintenance  . . . . .  28
     6.6.  OAL Fragment Retransmission . . . . . . . . . . . . . . .  29
     6.7.  OAL MTU Feedback Messaging  . . . . . . . . . . . . . . .  30
     6.8.  OAL Requirements  . . . . . . . . . . . . . . . . . . . .  32
     6.9.  OAL Fragmentation Security Implications . . . . . . . . .  33
     6.10. OAL Super-Packets . . . . . . . . . . . . . . . . . . . .  34
   7.  Frame Format  . . . . . . . . . . . . . . . . . . . . . . . .  36
   8.  Link-Local Addresses (LLAs) . . . . . . . . . . . . . . . . .  36
   9.  Unique-Local Addresses (ULAs) . . . . . . . . . . . . . . . .  38
   10. Global Unicast Addresses (GUAs) . . . . . . . . . . . . . . .  39
   11. Node Identification . . . . . . . . . . . . . . . . . . . . .  40
   12. Address Mapping - Unicast . . . . . . . . . . . . . . . . . .  40
     12.1.  Sub-Options  . . . . . . . . . . . . . . . . . . . . . .  42
       12.1.1.  Pad1 . . . . . . . . . . . . . . . . . . . . . . . .  44
       12.1.2.  PadN . . . . . . . . . . . . . . . . . . . . . . . .  45
       12.1.3.  Interface Attributes (Type 1)  . . . . . . . . . . .  45
       12.1.4.  Interface Attributes (Type 2)  . . . . . . . . . . .  47
       12.1.5.  Traffic Selector . . . . . . . . . . . . . . . . . .  51
       12.1.6.  MS-Register  . . . . . . . . . . . . . . . . . . . .  51
       12.1.7.  MS-Release . . . . . . . . . . . . . . . . . . . . .  52
       12.1.8.  Geo Coordinates  . . . . . . . . . . . . . . . . . .  53
       12.1.9.  Dynamic Host Configuration Protocol for IPv6
                (DHCPv6) Message . . . . . . . . . . . . . . . . . .  53
       12.1.10. Host Identity Protocol (HIP) Message . . . . . . . .  54
       12.1.11. Reassembly Limit . . . . . . . . . . . . . . . . . .  55
       12.1.12. Fragmentation Report . . . . . . . . . . . . . . . .  57
       12.1.13. Node Identification  . . . . . . . . . . . . . . . .  58
       12.1.14. Sub-Type Extension . . . . . . . . . . . . . . . . .  60

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   13. Address Mapping - Multicast . . . . . . . . . . . . . . . . .  63
   14. Multilink Conceptual Sending Algorithm  . . . . . . . . . . .  63
     14.1.  Multiple OMNI Interfaces . . . . . . . . . . . . . . . .  64
     14.2.  MN<->AR Traffic Loop Prevention  . . . . . . . . . . . .  64
   15. Router Discovery and Prefix Registration  . . . . . . . . . .  65
     15.1.  Router Discovery in IP Multihop and IPv4-Only Networks .  69
     15.2.  MS-Register and MS-Release List Processing . . . . . . .  71
     15.3.  DHCPv6-based Prefix Registration . . . . . . . . . . . .  73
   16. Secure Redirection  . . . . . . . . . . . . . . . . . . . . .  74
   17. AR and MSE Resilience . . . . . . . . . . . . . . . . . . . .  74
   18. Detecting and Responding to MSE Failures  . . . . . . . . . .  75
   19. Transition Considerations . . . . . . . . . . . . . . . . . .  75
   20. OMNI Interfaces on Open Internetworks . . . . . . . . . . . .  76
   21. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . . . .  78
   22. (H)HITs and Temporary ULAs  . . . . . . . . . . . . . . . . .  79
   23. Address Selection . . . . . . . . . . . . . . . . . . . . . .  80
   24. Error Messages  . . . . . . . . . . . . . . . . . . . . . . .  80
   25. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  80
     25.1.  "IEEE 802 Numbers" Registry  . . . . . . . . . . . . . .  80
     25.2.  "IPv6 Neighbor Discovery Option Formats" Registry  . . .  80
     25.3.  "Ethernet Numbers" Registry  . . . . . . . . . . . . . .  81
     25.4.  "ICMPv6 Code Fields: Type 2 - Packet Too Big" Registry .  81
     25.5.  "OMNI Option Sub-Type Values" (New Registry) . . . . . .  81
     25.6.  "OMNI Node Identification ID-Type Values" (New Registry)  82
     25.7.  "OMNI Option Sub-Type Extension Values" (New Registry) .  82
     25.8.  "OMNI RFC4380 UDP/IP Header Option" (New Registry) . . .  83
     25.9.  "OMNI RFC6081 UDP/IP Trailer Option" (New Registry)  . .  83
     25.10. Additional Considerations  . . . . . . . . . . . . . . .  84
   26. Security Considerations . . . . . . . . . . . . . . . . . . .  84
   27. Implementation Status . . . . . . . . . . . . . . . . . . . .  85
   28. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  86
   29. References  . . . . . . . . . . . . . . . . . . . . . . . . .  87
     29.1.  Normative References . . . . . . . . . . . . . . . . . .  87
     29.2.  Informative References . . . . . . . . . . . . . . . . .  89
   Appendix A.  Interface Attribute Preferences Bitmap Encoding  . .  96
   Appendix B.  VDL Mode 2 Considerations  . . . . . . . . . . . . .  98
   Appendix C.  MN / AR Isolation Through L2 Address Mapping . . . .  99
   Appendix D.  Change Log . . . . . . . . . . . . . . . . . . . . .  99
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  99

1.  Introduction

   Mobile Nodes (MNs) (e.g., aircraft of various configurations,
   terrestrial vehicles, seagoing vessels, enterprise wireless devices,
   pedestrians with cellphones, etc.) often have multiple interface
   connections to wireless and/or wired-line data links used for
   communicating with networked correspondents.  These data links may
   have diverse performance, cost and availability properties that can

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   change dynamically according to mobility patterns, flight phases,
   proximity to infrastructure, etc.  MNs coordinate their data links in
   a discipline known as "multilink", in which a single virtual
   interface is configured over the node's underlying interface
   connections to the data links.

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

   Each MN receives a Mobile Network Prefix (MNP) for numbering
   downstream-attached End User Networks (EUNs) independently of the
   access network data links selected for data transport.  The MN
   performs router discovery over the OMNI interface (i.e., similar to
   IPv6 customer edge routers [RFC7084]) and acts as a mobile router on
   behalf of its EUNs.  The router discovery process is iterated over
   each of the OMNI interface's underlying interfaces in order to
   register per-link parameters (see Section 15).

   The OMNI interface provides a multilink nexus for exchanging inbound
   and outbound traffic via the correct underlying interface(s).  The IP
   layer sees the OMNI interface as a point of connection to the OMNI
   link.  Each OMNI link has one or more associated Mobility Service
   Prefixes (MSPs), which are typically IP Global Unicast Address (GUA)
   prefixes from which MNPs are derived.  If there are multiple OMNI
   links, the IPv6 layer will see multiple OMNI interfaces.

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

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   The OMNI interface interacts with a network-based Mobility Service
   (MS) through IPv6 Neighbor Discovery (ND) control message exchanges
   [RFC4861].  The MS provides Mobility Service Endpoints (MSEs) that
   track MN movements and represent their MNPs in a global routing or
   mapping system.

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

   This document specifies the transmission of IP packets and MN/MS
   control messages over OMNI interfaces.  The OMNI interface supports
   either IP protocol version (i.e., IPv4 [RFC0791] or IPv6 [RFC8200])
   as the network layer in the data plane, while using IPv6 ND messaging
   as the control plane independently of the data plane IP protocol(s).
   The OAL operates as a sublayer between L3 and L2 based on IPv6
   encapsulation [RFC2473] as discussed in the following sections.  OMNI
   interfaces enable Multilink, Mobility, Multihop, Multicast and MTU
   services (i.e., the "five M's"), with provisions for both Vehicle-to-
   Infrastructure (V2I) communications and Vehicle-to-Vehicle (V2V)
   communications outside the context of infrastructure.

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.

   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).

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   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:

   Mobile Node (MN)
      an end system with a mobile router having multiple distinct
      upstream data link connections that are grouped together in one or
      more logical units.  The MN's data link connection parameters can
      change over time due to, e.g., node mobility, link quality, etc.
      The MN further connects a downstream-attached End User Network
      (EUN).  The term MN used here is distinct from uses in other
      documents, and does not imply a particular mobility protocol.

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

   Mobility Service (MS)
      a mobile routing service that tracks MN movements and ensures that
      MNs remain continuously reachable even across mobility events.
      Specific MS details are out of scope for this document.

   Mobility Service Endpoint (MSE)
      an entity in the MS (either singular or aggregate) that
      coordinates the mobility events of one or more MN.

   Mobility Service Prefix (MSP)
      an aggregated IP Global Unicast Address (GUA) prefix (e.g.,
      2001:db8::/32, 192.0.2.0/24, etc.) assigned to the OMNI link and
      from which more-specific Mobile Network Prefixes (MNPs) are
      delegated.  OMNI link administrators typically obtain MSPs from an
      Internet address registry, however private-use prefixes can
      alternatively be used subject to certain limitations (see:
      Section 10).  OMNI links that connect to the global Internet
      advertise their MSPs to their interdomain routing peers.

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

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      and non-MNP routes are treated exactly the same by the OMNI
      routing system.

   Access Network (ANET)
      a data link service network (e.g., an aviation radio access
      network, satellite service provider network, cellular operator
      network, WiFi network, etc.) that connects MNs.  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 MN's point of connection and the nearest Access
      Router.

   Access Router (AR)
      a router in the ANET for connecting MNs to correspondents in
      outside Internetworks.  The AR may be located on the same physical
      link as the MN, or may be located multiple IP hops away.  In the
      latter case, the MN uses encapsulation to communicate with the AR
      as though it were on the same physical link.

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

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

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

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

   OMNI link
      a Non-Broadcast, Multiple Access (NBMA) virtual overlay configured
      over one or more INETs and their connected ANETs.  An OMNI link
      can comprise multiple INET segments joined by bridges the same as
      for any link; the addressing plans in each segment may be mutually
      exclusive and managed by different administrative entities.

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

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      in an OMNI domain, a separate OMNI interface is configured for
      each link.

   OMNI Adaptation Layer (OAL)
      an OMNI interface sublayer service whereby original IP packets
      admitted into the interface are wrapped in an IPv6 header and
      subject to fragmentation and reassembly.  The OAL is also
      responsible for generating MTU-related control messages as
      necessary, and for providing addressing context for spanning
      multiple segments of a bridged OMNI link.

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

   OAL packet
      an original IP packet encapsulated in OAL headers and trailers
      before OAL fragmentation, or following OAL reassembly.

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

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

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

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

   OAL destination

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      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 *NET headers of carrier packets received on a first segment,
      then re-encapsulates the carrier packets in new *NET headers and
      forwards them into the next segment.

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

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

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

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

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

   Multilink
      an OMNI interface's manner of managing diverse underlying
      interface connections to data links as a single logical unit.  The
      OMNI interface provides a single unified interface to upper
      layers, while underlying interface selections are performed on a
      per-packet basis considering factors such as DSCP, flow label,
      application policy, signal quality, cost, etc.  Multilinking
      decisions are coordinated in both the outbound (i.e.  MN to
      correspondent) and inbound (i.e., correspondent to MN) directions.

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

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

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

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

   Mobility Service Identification (MSID)
      Each MSE and AR is assigned a unique 32-bit Identification (MSID)
      (see: Section 8).  IDs are assigned according to MS-specific
      guidelines (e.g., see: [I-D.templin-intarea-6706bis]).

   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 underlying interface(s) for packet
      transmission and reception within a single OMNI interface.

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

3.  Requirements

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

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

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4.  Overlay Multilink Network (OMNI) Interface Model

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

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

           Figure 1: OMNI Interface Architectural Layering Model

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

   o  INET interfaces connect to an INET either natively or through one
      or several IPv4 Network Address Translators (NATs).  Native INET
      interfaces have global IP addresses that are reachable from any
      INET correspondent.  NATed INET interfaces typically have private
      IP addresses and connect to a private network behind one or more
      NATs that provide INET access.

   o  ANET interfaces connect to a protected ANET that is separated from
      the open INET by an AR acting as a proxy.  The ANET interface may
      be either on the same L2 link segment as the AR, or separated from
      the AR by multiple IP hops.

   o  VPNed interfaces use security encapsulation over a *NET to a
      Virtual Private Network (VPN) gateway.  Other than the link-layer

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      encapsulation format, VPNed interfaces behave the same as for
      Direct interfaces.

   o  Direct (aka "point-to-point") interfaces connect directly to a
      peer without crossing any *NET 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 *NET headers to create OAL carrier packets for transmission over
   underlying interfaces (L2/L1).  The target OMNI interface receives
   the carrier packets from underlying interfaces (L1/L2) and discards
   the *NET headers.  If the resulting OAL packets/fragments are
   addressed to itself, the OMNI interface acts as an "OAL destination"
   and performs reassembly if necessary, discards the OAL encapsulation,
   and delivers the original IP packet to the network layer (L3).  If
   the OAL fragments are addressed to another node, the OMNI interface
   instead acts as an "OAL intermediate node" by re-encapsulating in new
   *NET headers and forwarding the new carrier packets over an
   underlying interface without reassembling or discarding the OAL
   encapsulation.  The OAL source and OAL destination are seen as
   "neighbors" on the OMNI link, while OAL intermediate nodes are seen
   as "bridges".

   The OMNI interface can send/receive original IP packets to/from
   underlying interfaces while including/omitting various encapsulations
   including OAL, UDP, IP and L2.  The network layer can also access the
   underlying interfaces directly while bypassing the OMNI interface
   entirely when necessary.  This architectural flexibility may be
   beneficial for underlying interfaces (e.g., some aviation data links)
   for which encapsulation overhead may be a primary consideration.
   OMNI interfaces that send original IP packets directly over
   underlying interfaces without invoking the OAL can only reach peers
   located on the same OMNI link segment.  However, an ANET proxy that
   receives the original IP packet can forward it further by performing
   OAL encapsulation with source set to its own address and destination
   set to the OAL destination corresponding to the final destination
   (i.e., even if the OAL destination is on a different OMNI link
   segment).

   Original IP packets sent directly over underlying interfaces are
   subject to the same path MTU related issues as for any
   Internetworking path, and do not include per-packet identifications
   that can be used for data origin verification and/or link-layer
   retransmissions.  Original IP packets presented directly to an
   underlying interface that exceed the underlying network path MTU are

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

      +------------------------------------------------------------+
      |                      Network Layer                         |
      +--+---------------------------------------------------------+
         |                     OMNI Interface                      |
         +--------------------------+------------------------------+
                                    |      OAL Encaps/Decaps       |
                                    +------------------------------+
                                    |        OAL Frag/Reass        |
                       +------------+---------------+--------------+
                       | UDP Encaps/Decaps/Compress |
                  +----+---+------------+--------+--+  +--------+
                  | IP E/D |            | IP E/D |     | IP E/D |
              +---+------+-+----+    +--+---+----+     +----+---+--+
              |L2 E/D|   |L2 E/D|    |L2 E/D|               |L2 E/D|
      +-------+------+---+------+----+------+---------------+------+
      |                   Underlying Interfaces                    |
      +------------------------------------------------------------+

                     Figure 2: OMNI Interface Layering

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

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

   o  since Temporary ULAs are statistically unique, they can be used
      without DAD, e.g. for MN-to-MN communications until an MNP-LLA is
      obtained.

   o  underlying interfaces on the same L2 link segment as an AR do not
      require any L3 addresses (i.e., not even link-local) in

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      environments where communications are coordinated entirely over
      the OMNI interface.

   o  as underlying interface properties change (e.g., link quality,
      cost, availability, etc.), any active interface can be used to
      update the profiles of multiple additional interfaces in a single
      message.  This allows for timely adaptation and service continuity
      under dynamically changing conditions.

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

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

   o  the OMNI interface allows inter-INET traversal when nodes located
      in different INETs need to communicate with one another.  This
      mode of operation would not be possible via direct communications
      over the underlying interfaces themselves.

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

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

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

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

   Other opportunities are discussed in [RFC7847].  Note that even when
   the OMNI virtual interface is present, applications can still access
   underlying interfaces either through the network protocol stack using
   an Internet socket or directly using a raw socket.  This allows for
   intra-network (or point-to-point) communications without invoking the
   OMNI interface and/or OAL.  For example, when an IPv6 OMNI interface
   is configured over an underlying IPv4 interface, applications can
   still invoke IPv4 intra-network communications as long as the

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   communicating endpoints are not subject to mobility dynamics.
   However, the opportunities discussed above are not realized when the
   architectural layering is bypassed in this way.

   Figure 3 depicts the architectural model for a MN with an attached
   EUN connecting to the MS via multiple independent *NETs.  When an
   underlying interface becomes active, the MN's OMNI interface sends
   IPv6 ND messages without encapsulation if the first-hop Access Router
   (AR) is on the same underlying link; otherwise, the interface uses
   IP-in-IP encapsulation.  The IPv6 ND messages traverse the ground
   domain *NETs until they reach an AR (AR#1, AR#2, ..., AR#n), which
   then coordinates with an INET Mobility Service Endpoint (MSE#1,
   MSE#2, ..., MSE#m) and returns an IPv6 ND message response to the MN.
   The Hop Limit in IPv6 ND messages is not decremented due to
   encapsulation; hence, the OMNI interface appears to be attached to an
   ordinary link.

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                           +--------------+        (:::)-.
                           |      MN      |<-->.-(::EUN:::)
                           +--------------+      `-(::::)-'
                           |OMNI interface|
                           +----+----+----+
                  +--------|IF#1|IF#2|IF#n|------ +
                 /         +----+----+----+        \
                /                 |                 \
               /                  |                  \
              v                   v                   v
           (:::)-.              (:::)-.              (:::)-.
      .-(::*NET:::)        .-(::*NET:::)        .-(::*NET:::)
        `-(::::)-'           `-(::::)-'           `-(::::)-'
          +----+               +----+               +----+
    ...   |AR#1|  ..........   |AR#2|   .........   |AR#n|  ...
   .      +-|--+               +-|--+               +-|--+     .
   .        |                    |                    |
   .        v                    v                    v        .
   .             <-----  INET Encapsulation ----->             .
   .                                                           .
   .      +-----+               (:::)-.                        .
   .      |MSE#2|           .-(::::::::)          +-----+      .
   .      +-----+       .-(:::   INET  :::)-.     |MSE#m|      .
   .                  (:::::    Routing  ::::)    +-----+      .
   .                     `-(::: System :::)-'                  .
   .  +-----+                `-(:::::::-'                      .
   .  |MSE#1|          +-----+               +-----+           .
   .  +-----+          |MSE#3|               |MSE#4|           .
   .                   +-----+               +-----+           .
   .                                                           .
   .                                                           .
   .       <----- Worldwide Connected Internetwork ---->       .
    ...........................................................

              Figure 3: MN/MS Coordination via Multiple *NETs

   After the initial IPv6 ND message exchange, the MN (and/or any nodes
   on its attached EUNs) can send and receive original IP packets over
   the OMNI interface.  OMNI interface multilink services will forward
   the packets via ARs in the correct underlying *NETs.  The AR
   encapsulates the packets according to the capabilities provided by
   the MS and forwards them to the next hop within the worldwide
   connected Internetwork via optimal routes.

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5.  OMNI Interface Maximum Transmission Unit (MTU)

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

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

   IPv4 underlying interfaces are REQUIRED to configure a minimum MTU of
   68 bytes [RFC0791] and a minimum MRU of 576 bytes [RFC0791][RFC1122].
   Therefore, when the Don't Fragment (DF) bit in the IPv4 header is set
   to 0 the minimum IPv4 path MTU is 576 bytes since routers on the path
   support network fragmentation and the destination is required to
   reassemble at least that much.  The OMNI interface therefore MUST set
   DF to 0 in the IPv4 encapsulation headers of carrier packets that are
   no larger than 576 bytes, and SHOULD set DF to 1 in larger carrier
   packets.  (Note: even if the encapsulation source has a way to
   determine that the encapsulation destination configures an MRU larger
   than 576 bytes, it should not assume a larger minimum IPv4 path MTU
   without careful consideration of the issues discussed in
   Section 6.9.)

   The OMNI interface configures an MTU and MRU of 9180 bytes [RFC2492];
   the size is therefore not a reflection of the underlying interface or
   *NET path MTUs, but rather determines the largest original IP packet
   the OAL (and/or underlying interface) can forward or reassemble.  For
   each OAL destination (i.e., for each OMNI link neighbor), the OAL
   source may discover "hard" or "soft" Reassembly Limit values smaller
   than the MRU based on receipt of IPv6 ND messages with OMNI
   Reassembly Limit sub-options (see: Section 12.1.11).  The OMNI
   interface employs the OAL as an encapsulation sublayer service to
   transform original IP packets into OAL packets/fragments, and the OAL

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   in turn uses *NET encapsulation to forward carrier packets over the
   underlying interfaces (see: Section 6).

6.  The OMNI Adaptation Layer (OAL)

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

   These carrier packets travel over one or more underlying networks
   bridged by OAL intermediate nodes, which re-encapsulate by removing
   the *NET headers of the first underlying network and appending *NET
   headers appropriate for the next underlying network in succession.
   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 *NET headers and reassembles the resulting OAL fragments into an
   OAL packet as described in Section 6.3.  The OAL destination then
   decapsulates the OAL packet to obtain the original IP packet, which
   it then delivers to the network layer.

   Detailed operations of the OAL are discussed in the following
   sections.

6.1.  OAL Source Encapsulation and Fragmentation

   When the network layer forwards an original IP packet into the OMNI
   interface, the OAL source inserts an IPv6 encapsulation header but
   does not decrement the Hop Limit/TTL of the original IP packet since
   encapsulation occurs at a layer below IP forwarding [RFC2473].  The
   OAL source copies the "Type of Service/Traffic Class" [RFC2983],
   "Flow Label"[RFC6438] (for IPv6) and "Congestion Experienced"
   [RFC3168] values in the original packet's IP header into the
   corresponding fields in the OAL header.  The OAL source finally sets
   the OAL header IPv6 Hop Limit to a small value (e.g., 16) large
   enough to allow forwarding over a small number of OMNI link segments
   and sets the Payload Length to the length of the original IP packet.

   The OAL next selects source and destination addresses for the IPv6
   header of the resulting OAL packet.  MN OMNI interfaces set the OAL
   IPv6 header source address to a Unique Local Address (ULA) based on

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   the Mobile Network Prefix (MNP-ULA), while AR and MSE OMNI interfaces
   set the source address to an Administrative ULA (ADM-ULA) (see:
   Section 9).  When a MN 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).

   When the OAL source forwards an original IP packet toward a final
   destination via an ANET underlying interface, it sets the OAL IPv6
   header source address to its own ULA and sets the destination to
   either the Administrative ULA (ADM-ULA) of the ANET peer or the
   Mobile Network Prefix ULA (MNP-ULA) corresponding to the final
   destination (see below).  The OAL source then fragments the OAL
   packet if necessary, encapsulates the OAL fragments in any ANET
   headers and sends the resulting carrier packets to the ANET peer
   which either reassembles before forwarding if the OAL destination is
   its own ULA or forwards the fragments toward the true OAL destination
   without first reassembling otherwise.

   When the OAL source forwards an original IP packet toward a final
   destination via an INET underlying interface, it sets the OAL IPv6
   header source address to its own ULA and sets the destination to the
   ULA of an OAL destination node on the final *NET segment.  The OAL
   source then fragments the OAL packet if necessary, encapsulates the
   OAL fragments in any *NET headers and sends the resulting carrier
   packets toward the OAL destination on the final segment OMNI node
   which reassembles before forwarding the original IP packets toward
   the final destination.

   Following OAL IPv6 encapsulation and address selection, the OAL
   source next appends a 2 octet trailing Checksum (initialized to 0) at
   the end of the original IP packet while incrementing the OAL header
   IPv6 Payload Length field to reflect the addition of the trailer.
   The format of the resulting OAL packet following encapsulation is
   shown in Figure 4:

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

                 Figure 4: OAL Packet Before Fragmentation

   The OAL source next selects a 32-bit Identification value for the
   packet, beginning with an unpredictable value for the initial OAL
   packet per [RFC7739] and monotonically incrementing for each
   successive OAL packet until a new initial value is chosen.

   The OAL source then calculates the 2's complement (mod 256)
   Fletcher's checksum [CKSUM][RFC2328][RFC0905] over the entire OAL

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   packet beginning with a pseudo-header of the IPv6 header similar to
   that found in Section 8.1 of [RFC8200].  The OAL IPv6 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 IPv6 Pseudo-Header

   The OAL source then inserts a single OMNI Routing Header (ORH) if
   necessary (see: [I-D.templin-intarea-6706bis]) while incrementing
   Payload Length to reflect the addition of the ORH (note that the late
   addition of the ORH is not covered by the trailing checksum).

   The OAL source next fragments the OAL packet if necessary while
   assuming the IPv4 minimum path MTU (i.e., 576 bytes) as the worst
   case for OAL fragmentation regardless of the underlying interface IP
   protocol version since IPv6/IPv4 protocol translation and/or IPv6-in-
   IPv4 encapsulation may occur in any *NET path.  By always assuming
   the IPv4 minimum even for IPv6 underlying interfaces, the OAL source
   may produce smaller fragments with additional encapsulation overhead
   but will always interoperate and never run the risk of loss due to an
   MTU restriction or due to presenting an underlying interface with a
   carrier packet that exceeds its MRU.  Additionally, the OAL path
   could traverse multiple *NET "segments" with intermediate OAL
   forwarding nodes performing re-encapsulation where the *NET
   encapsulation of the previous segment is replaced by the *NET

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   encapsulation of the next segment which may be based on a different
   IP protocol version and/or encapsulation sizes.

   The OAL source therefore assumes a default minimum path MTU of 576
   bytes at each *NET segment for the purpose of generating OAL
   fragments for *NET encapsulation and transmission as carrier packets.
   In the worst case, each successive *NET segment may re-encapsulate
   with either a 20 byte IPv4 or 40 byte IPv6 header, an 8 byte UDP
   header and in some cases an IP security encapsulation (40 bytes
   maximum assumed).  Any *NET segment may also insert a maximum-length
   (40 byte) ORH as an extension to the existing 40 byte OAL IPv6 header
   plus 8 byte Fragment Header if an ORH was not already present.
   Assuming therefore an absolute worst case of (40 + 40 + 8) = 88 bytes
   for *NET encapsulation plus (40 + 40 + 8) = 88 bytes for OAL
   encapsulation leaves (576 - 88 - 88) = 400 bytes to accommodate a
   portion of the original IP packet/fragment.  The OAL source therefore
   sets a minimum Maximum Payload Size (MPS) of 400 bytes as the basis
   for the minimum-sized OAL fragment that can be assured of traversing
   all segments without loss due to an MTU/MRU restriction.  The Maximum
   Fragment Size (MFS) for OAL fragmentation is therefore determined by
   the MPS plus the size of the OAL encapsulation headers.  (Note that
   the OAL source includes the 2 octet trailer as part of the payload
   during fragmentation, and the OAL destination regards it as ordinary
   payload until reassembly and checksum verification are complete.)

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

   The OAL source can also actively probe individual OAL destinations to
   discover larger path MPS values using packetization layer probes per
   [RFC4821][RFC8899], but care must be taken to avoid setting static
   values for dynamically changing paths leading to black holes.  The
   probe involves sending an OAL packet larger than the current path MPS
   and receiving a small acknowledgement message in response (with the
   possible receipt of link-layer error message in case the probe was
   lost).  For this purpose, the OAL source can send an NS message with
   one or more OMNI options with large PadN sub-options (see:

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   Section 12) in order to receive a small NA response from the OAL
   destination.  While observing the minimum MPS will always result in
   robust and secure behavior, the OAL source should optimize path MPS
   values when more efficient utilization may result in better
   performance (e.g. for wireless aviation data links).

   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 of equal length at
   least as large as the minimum MPS, while the final fragment MAY be of
   different length.  The OAL source also converts all original IP
   packets no larger than the current MPS into "atomic fragments" by
   including a Fragment Header with Fragment Offset and More Fragments
   both set to 0.  The OAL source finally encapsulates the fragments in
   *NET headers to form carrier packets and forwards them over an
   underlying interface, while retaining the fragments and their ordinal
   positions (i.e., as Frag #0, Frag #1, Frag #2, etc.) for a timeout
   period in case link-layer retransmission is requested.  The formats
   of OAL fragments and carrier packets are shown in Figure 6.

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        +----------+--+-------------+
        |  OAL Hdr |FH|   Frag #0   |
        +----------+--+-------------+
            +----------+--+-------------+
            |  OAL Hdr |FH|   Frag #1   |
            +----------+--+-------------+
                +----------+--+-------------+
                |  OAL Hdr |FH|   Frag #2   |
                +----------+--+-------------+
                                  ....
                    +----------+--+-------------+----+
                    |  OAL Hdr |FH| Frag #(N-1) |Csum|
                    +----------+--+-------------+----+
        a) OAL fragments after fragmentation
           (FH = Fragment Header; Csum appears only in final fragment)

        +--------+--+-----+-----+-----+-----+-----+----+
        |OAL Hdr |FH|      Original IP packet     |Csum|
        +--------+--+-----+-----+-----+-----+-----+----+
        b) An OAL atomic fragment with FH but no fragmentation.

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

                Figure 6: OAL Fragments and Carrier Packets

6.2.  OAL *NET Encapsulation and Re-Encapsulation

   During *NET encapsulation, OAL sources first encapsulate each OAL
   fragment in a UDP header as the first *NET encapsulation sublayer if
   NAT traversal, packet filtering middlebox traversal and/or OAL header
   compression are necessary.  The OAL then optionally appends
   additional encapsulation sublayer headers, then presents the *NET
   packet to an underlying interface.  This layering can be seen in
   Figure 2.

   When a UDP header is included, the OAL source next sets the UDP
   source port to a constant value that it will use in each successive
   carrier packet it sends to the next OAL hop.  For packets sent to an
   MSE, the OAL source sets the UDP destination port to 8060, i.e., the
   IANA-registered port number for AERO.  For packets sent to a MN peer,
   the source sets the UDP destination port to the cached port value for
   this peer.  The OAL source then sets the UDP length to the total
   length of the OAL fragment in correspondence with the OAL header

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   Payload Length (i.e., the UDP length and IPv6 Payload Length must
   agree).  The OAL source finally sets the UDP checksum to 0
   [RFC6935][RFC6936] since the only fields not already covered by the
   OAL checksum or underlying *NET CRCs are the Fragment Header fields,
   and any corruption in those fields will be garbage collected by the
   reassembly algorithm.  The UDP encapsulation header is often used in
   association with IP encapsulation, but may also be used between
   neighbors on a shared physical link with a true L2 header format such
   as for transmission over IEEE 802 Ethernet links.  This document
   therefore requests a new Ether Type code assignment TBD1 in the IANA
   'ieee-802-numbers' registry for direct User Datagram Protocol (UDP)
   encapsulation over IEEE 802 Ethernet links (see: Section 25).

   For *NET encapsulations, the OAL source next copies the "Type of
   Service/Traffic Class" [RFC2983], "Congestion Experienced" [RFC3168]
   and "Flow Label" [RFC6438] (for IPv6) values in the OAL IPv6 header
   into the corresponding fields in the *NET IP header.  For carrier
   packets undergoing re-encapsulation, OAL intermediate nodes instead
   copy these values from the previous hop *NET encapsulation header
   into both the OAL IPv6 header and the next hop *NET encapsulation
   header, i.e., the IP values are transferred between *NET
   encapsulation headers and *not* copied from the OAL header.  During
   re-encapsulation, the intermediate node decrements the OAL IPv6
   header Hop Limit and discards the carrier packet if the value reaches
   0.

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

6.3.  OAL Destination Decapsulation and Reassembly

   When an OMNI interface receives a carrier packet from an underlying
   interface, the OAL destination discards the *NET encapsulation
   headers and examines the OAL header of the enclosed OAL fragment.  If

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   the OAL fragment is addressed to a different node, the OAL
   destination re-encapsulates and forwards as discussed below.  If the
   OAL fragment is addressed to itself, the OAL destination creates or
   updates a checklist for this (Source, Destination, Identification)-
   tuple to track the fragments already received (i.e., by examining the
   Payload Length, Fragment Offset, More Fragments and Identification
   values supplied by the OAL source).  The OAL destination verifies
   that all non-final OAL fragments are of equal length no less than the
   minimum MPS and that no fragments overlap or leave "holes", while
   dropping any non-conforming fragments.  The OAL destination records
   each conforming OAL fragment's ordinal position based on the OAL
   header Payload Length and Fragment Offset values (i.e., as Frag #0,
   Frag #1, Frag #2, etc.) and admits each fragment into the reassembly
   cache.

   When reassembly is complete, the OAL destination removes the ORH if
   present while decrementing Payload Length to reflect the removal of
   the ORH.  The OAL destination next verifies the resulting OAL
   packet's checksum and discards the packet if the checksum is
   incorrect.  If the OAL packet was accepted, the OAL destination then
   removes the OAL header/trailer, then delivers the original IP packet
   to the network layer.  Note that link layers include a CRC-32
   integrity check which provides effective hop-by-hop error detection
   in the underlying network for payload sizes up to the OMNI interface
   MTU [CRC], but that some hops may traverse intermediate layers such
   as tunnels over IPv4 that do not include integrity checks.  The
   trailing Fletcher checksum therefore allows the OAL destination to
   detect OAL packet splicing errors due to reassembly misassociations
   and/or to verify integrity for OAL packets whose fragments may have
   traversed unprotected underlying network hops [CKSUM].  The Fletcher
   algorithm also provides diversity with respect to both lower layer
   CRCs and upper layer Internet checksums as part of a complimentary
   multi-layer integrity assurance architecture.

6.4.  OAL Header Compression

   When the OAL source and destination are on the same *NET segment, no
   ORH is needed and carrier packet header compression is possible.
   When the OAL source and destination exchange initial IPv6 ND messages
   as discussed in the following Sections, each caches the observed *NET
   UDP source port and source IP (or L2) address associated with the OAL
   IPv6 source address found in the full-length OAL IPv6 header.  After
   the initial IPv6 ND message exchange, the OAL source can begin
   applying OAL Header Compression to significantly reduce the
   encapsulation overhead required in each carrier packet.

   When the OAL source determines that header compression state has been
   established (i.e., following the IPv6 ND message exchange), it can

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   begin sending OAL fragments with significant portions of the IPv6
   header and Fragment Header omitted thereby reducing the amount of
   encapsulation overhead.  For OAL first-fragments (including atomic
   fragments), the OMNI Compressed Header - Type 0 (OCH-0) is used and
   formatted as shown in Figure 7:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ *
     |        Source port            |      Destination port         | U
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ D
     |           Length              |          Checksum             | P
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ *
     |Version| Traffic Class |           Flow Label                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Next Header  |   Reserved  |M|     Identification (0 -1)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Identification (2-3)    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+/

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

   In this format, the UDP header appears in its entirety in the first 8
   octets, then followed by the first 4 octets of the IPv6 header with
   the remainder omitted.  (The IPv6 Version field is set to the value 0
   to distinguish this header from a true IP protocol version number and
   from OCH-1 - see below.)  The compressed IPv6 header is then followed
   by a compressed IPv6 Fragment Header with the Fragment Offset field
   and two Reserved bits omitted (since these fields always encode the
   value 0 in first-fragments), and with the More Fragments (M) bit
   relocated to the least significant bit of the first Reserved field.
   The OCH-0 header is then followed by the OAL fragment body, and the
   UDP length field is reduced by 38 octets (i.e., the difference in
   length between full-length IPv6 and Fragment Headers and the length
   of the compressed headers).

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

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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ *
     |        Source port            |      Destination port         | U
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ D
     |           Length              |          Checksum             | P
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ *
     |V|R|M|      Fragment Offset    |      Identification (0-1)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Identification (1-3)    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   In this format, the UDP header appears in its entirety in the first 8
   octets, but all IPv6 header fields except for the most significant
   Version (V) bit are omitted.  (The V bit is set to the value 1 to
   distinguish this header from a true IP protocol version number and
   from OCH-0.)  The V bit is followed by a single Reserved (R) bit and
   the More Fragments (M) bit in a compressed IPv6 Fragment Header with
   the Next Header and first Reserved fields omitted.  The OCH-1 header
   is then followed by the OAL fragment body, and the UDP length field
   is reduced by 42 octets (i.e., the difference in length between full-
   length IPv6 and Fragment Headers and the length of the compressed
   headers).

   When the OAL destination receives a carrier packet with an OCH, it
   first determines the OAL IPv6 source and destination addresses by
   examining the UDP source port and L2 source address, then determines
   the length by examining the UDP length.  The OAL destination then
   examines the (V)ersion field immediately following the UDP header.
   If the (4-bit) Version field encodes the value 0, the OAL destination
   processes the remainder of the header as an OCH-0, then reconstitutes
   the full-sized IPv6 and Fragment Headers and adds this OAL fragment
   to the reassembly buffer if necessary.  If the (1-bit) V bit encodes
   the value 1, the OAL destination instead processes the remainder of
   the header as an OCH-1, then reconstitutes the full-sized IPv6 and
   Fragment Headers and adds this OAL fragment to the reassembly buffer.
   Note that, since the OCH-1 does not include Traffic Class, Flow Label
   or Next Header information, the OAL destination writes the value 0
   into these fields when it reconstitutes the full headers.  These
   values will be correctly populated during reassembly after an OAL
   first fragment with an OCH-0 or uncompressed OAL header arrives.

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6.5.  OAL Fragment Identification Window Maintenance

   As noted above, the OAL source establishes a window of 32-bit
   Identifications beginning with an unpredictable value for the initial
   message [RFC7739] and monotonically incrementing for each successive
   OAL packet until a new initial value is chosen.  The OAL source
   asserts the starting value by including it as the Identification in
   an IPv6 ND NS/RS messages.  When the OAL destination receives the
   IPv6 ND message, it resets the Identification window for this OAL
   source to the new value coded in the message's OAL header and expects
   future OAL fragments received from this OAL source to include
   sequential Identification values (subject to loss and reordering)
   until the neighbor reachable time expires or the OAL source sends a
   new IPv6 ND message.

   For example, if the OAL destination receives an NS/RS message with
   Identification 0x12345678, it resets the window for this OAL source
   to begin with 0x12345678 and examines the Identification values in
   subsequent OAL fragments received from this OAL source.  If the
   Identification values of subsequent OAL fragments fall within the
   window of (0x12345678 + N) the OAL destination accepts the fragment;
   otherwise, it silently drops the fragment (where "N" represents the
   maximum number of fragments expected before the neighbor reachable
   time expires).

   While monitoring the current window, the OAL destination must accept
   new NS/RS Identification values even if outside the current window.
   The new Identification value resets the OAL destination's window
   start, and the window processing continues from this new starting
   point while allowing a period of overlap in case OAL fragments with
   Identification values from a previous window are still in flight.
   Note also that unsolicited NA messages must include Identification
   values within the current window, and therefore do not reset the
   current window.

   This implies that an IPv6 ND message used to reset the Identification
   window should fit within a single OAL fragment (i.e., within current
   MPS constraints), since a fragmented IPv6 ND message with an out-of-
   window Identification value could be part of a DoS attack.  While
   larger IPv6 ND messages (up to the OMNI interface MTU) can certainly
   be subject to OAL fragmentation, their Identification should be
   within the current window maintained by the OAL destination to
   increase the likelihood that they will be accepted.

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6.6.  OAL Fragment Retransmission

   When the OAL source sends carrier packets with OAL fragments to an
   OAL destination, the source caches them for a timeout period in case
   retransmission may be necessary.  (The timeout duration is an
   implementation matter, and may be influenced by factors such as
   packet arrival rates, OAL source/destination round trip times, etc.)
   The OAL destination in turn maintains a checklist for the (Source,
   Destination, Identification)-tuple of each new OAL fragment received
   and notes the ordinal positions of fragments already received (i.e.,
   as Frag #0, Frag #1, Frag #2, etc.).

   If the OAL destination notices some OAL fragments missing after most
   other fragments within the same Identification window have already
   arrived, it may send an IPv6 ND unsolicited Neighbor Advertisement
   (uNA) message to the OAL source that originated the fragments to
   report loss.  The OAL destination creates a uNA message with an OMNI
   option containing an authentication sub-option to provide
   authentication (if the OAL source is on an open Internetwork)
   followed by a Fragmentation Report sub-option that includes a list of
   (Identification, Bitmap)-tuples for OAL fragments received and
   missing from this OAL source (see: Section 12).  The OAL destination
   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 authenticates the
   message using authentication sub-option (if present) then examines
   the Fragmentation Report.  For each (Source, Destination,
   Identification)-tuple, the OAL source determines whether it still
   holds the original OAL fragments in its cache and retransmits any for
   which the Bitmap indicated a loss event.  For example, if the Bitmap
   indicates that the ordinal OAL fragments Frag #3, Frag #7, Frag #10
   and Frag #13 from the same OAL packet are missing the OAL source
   retransmits these fragments only and no others.

   Note that the goal of this service is to provide a light-weight link-
   layer Automatic Repeat Request (ARQ) capability in the spirit of
   Section 8.1 of [RFC3819].  Rather than provide true end-to-end
   reliability, however, the service provides timely link-layer
   retransmissions that may improve packet delivery ratios and avoid
   some delays inherent in true end-to-end services.

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6.7.  OAL MTU Feedback Messaging

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

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

   An OAL source sends PTB soft error messages by setting the ICMPv4
   header "unused" field or ICMPv6 header Code field to the value 1 if a
   original IP packet was deemed lost (e.g., due to reassembly timeout)
   or to the value 2 otherwise.  The OAL source sets the PTB destination
   address to the original IP packet source, and sets the source address
   to one of its OMNI interface unicast/anycast addresses that is
   routable from the perspective of the original source.  The OAL source
   then sets the MTU field to a value smaller than the original packet
   size but no smaller than 576 for ICMPv4 or 1280 for ICMPv6, writes
   the leading portion of the original IP packet into the "packet in
   error" field, and returns the PTB soft error to the original source.
   When the original source receives the PTB soft error, it temporarily
   reduces the size of the packets it sends the same as for hard errors
   but may seek to increase future packet sizes dynamically while no
   further soft errors are arriving.  (If the original source does not
   recognize the soft error code, it regards the PTB the same as a hard
   error but should heed the retransmission advice given in [RFC8201]
   suggesting retransmission based on normal packetization layer
   retransmission timers.)  This document therefore updates
   [RFC1191][RFC4443] and [RFC8201].  Furthermore, packetization layer

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   probing strategies [RFC4821][RFC8899] must be aware that PTB hard or
   soft errors may arrive at any time, i.e., even following a successful
   probe (this is the same consideration as for an ordinary path
   fluctuation following a successful probe).

   An OAL destination may experience reassembly cache congestion, and
   can return uNA messages to the OAL source that originated the
   fragments (subject to rate limiting) to advertise reduced hard/soft
   Reassembly Limits and/or to report individual reassembly failures.
   The OAL destination creates a uNA message with an OMNI option
   containing an authentication message sub-option (if the OAL source is
   on an open Internetwork) followed optionally by at most one hard and
   one soft Reassembly Limit sub-options with reduced hard/soft values,
   and with one of them optionally including the leading portion an OAL
   first fragment containing the header of an original IP packet whose
   source must be notified (see: Section 12).  The OAL destination
   encapsulates as much of the OAL first fragment (beginning with the
   OAL header) as will fit in the "OAL First Fragment" field of sub-
   option without causing the entire uNA message to exceed the minimum
   MPS, signs the message if an authentication sub-option is included,
   performs OAL encapsulation (with the its own address as the OAL
   source and the source address of the message that prompted the uNA as
   the OAL destination) and sends the message to the OAL source.

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

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   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
   the face of dynamically changing network conditions.  Moreover, since
   PTB soft errors do not indicate a hard limit, original sources that
   receive soft errors can begin sending larger packets without waiting
   for the recommended 10 minutes specified for PTB hard errors
   [RFC1191][RFC8201].  The OMNI interface therefore provides an
   adaptive service that accommodates MTU diversity especially well-
   suited for dynamic multilink environments.

6.8.  OAL Requirements

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

   o  OAL sources MUST NOT send OAL fragments including original IP
      packets larger than the OMNI interface MTU or the OAL destination
      hard Reassembly Limit, i.e., whether or not fragmentation is
      needed.

   o  OAL sources MUST NOT perform OAL fragmentation for original IP
      packets smaller than the minimum MPS minus the trailer size, and
      MUST produce non-final fragments that contain equal-length
      payloads no smaller than the minimum MPS when performing
      fragmentation.

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

   o  OAL intermediate nodes SHOULD and OAL destinations MUST
      unconditionally drop OAL packets/fragments including original IP
      packets larger than the OMNI interface MRU and/or OAL destination
      hard Reassembly Limit, i.e., whether or not reassembly was needed.

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

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

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   o  OAL destination nodes MUST drop any new OAL non-final fragments of
      different length than other non-final fragments that have already
      been received, and MUST drop any new OAL fragments with Offset and
      Payload length that would overlap with other fragments and/or
      leave too-small holes between fragments that have already been
      received.

   Note: Under the minimum MPS, ordinary 1500 byte original IP packets
   would require at most 4 OAL fragments, with each non-final fragment
   containing 400 payload bytes and the final fragment containing 302
   payload bytes (i.e., the final 300 bytes of the original IP packet
   plus the 2 octet trailer).  Likewise, maximum-length 9180 byte
   original IP packets would require at most 23 fragments.  For all
   packet sizes, the likelihood of successful reassembly may improve
   when the OMNI interface sends all fragments of the same fragmented
   OAL packet consecutively over the same underlying interface.
   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 average hardware platforms.  Even so, the OAL
   source could impose an inter-fragment delay while the OAL destination
   is reporting reassembly congestion (see: Section 6.7) and decrease
   the delay when reassembly congestion subsides.

6.9.  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

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       expensive.  The cache should also be indexed based on the arrival
       underlying interface such that congestion experienced over a
       first underlying interface does not cause discard of incomplete
       reassemblies for uncongested underlying interfaces.

   3.  Attacks based on predictable fragment identification values -
       this threat is mitigated by selecting an unpredictable
       Identification value per [RFC7739].  Additionally, inclusion of
       the OAL checksum would make it very difficult for an attacker who
       could somehow predict a fragment identification value to inject
       malicious fragments resulting in undetected reassemblies of bad
       data.

   4.  Evasion of Network Intrusion Detection Systems (NIDS) - this
       threat is mitigated by setting a minimum MPS for OAL
       fragmentation, which defeats all "tiny fragment"-based attacks.

   Additionally, IPv4 fragmentation 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 ID are still alive in the
   network [RFC4963].  However, since the largest carrier packet that
   will be sent via an IPv4 path with DF = 0 is 576 bytes any IPv4
   fragmentation would occur only on links with an IPv4 MTU smaller than
   this size, and [RFC3819] recommendations suggest that such links will
   have low data rates.  Since IPv6 provides a 32-bit Identification
   value, IP ID wraparound at high data rates is not a concern for IPv6
   fragmentation.

   Finally, [RFC6980] documents fragmentation security concerns for
   large IPv6 ND messages.  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
   current MPS for this OAL destination.

6.10.  OAL Super-Packets

   By default, the OAL source includes a 40-byte IPv6 encapsulation
   header for each original IP packet during OAL encapsulation.  The OAL
   source also calculates and appends a 2 octet trailing Fletcher
   checksum then performs fragmentation such that a copy of the 40-byte
   IPv6 header plus an 8-byte IPv6 Fragment Header is included in each
   OAL fragment (when an ORH 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

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   complimentary technique known as "packing" (see:
   [I-D.ietf-intarea-tunnels]) is also supported so that multiple
   original IP packets and/or control messages can be included within a
   single OAL "super-packet".

   When the OAL source has multiple original IP packets to send to the
   same OAL destination with total length no larger than the OAL
   destination MRU, it can concatenate them into a super-packet
   encapsulated in a single OAL header and trailing checksum.  Within
   the OAL super-packet, the IP header of the first original IP packet
   (iHa) followed by its data (iDa) is concatenated immediately
   following the OAL header, then the IP header of the next original
   packet (iHb) followed by its data (iDb) is concatenated immediately
   following the first original packet, etc. with the trailing checksum
   included last.  The OAL super-packet format is transposed from
   [I-D.ietf-intarea-tunnels] and shown in Figure 9:

                   <------- 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 9: OAL Super-Packet Format

   When the OAL source prepares a super-packet, it applies OAL
   fragmentation and *NET encapsulation then sends the carrier packets
   to the OAL destination.  When the OAL destination receives the super-
   packet it reassembles if necessary, verifies and removes the trailing
   checksum, then regards the remaining OAL header Payload Length as the
   sum of the lengths of all payload packets.  The OAL destination then
   selectively extracts each original IP packet (e.g., by setting
   pointers into the super-packet buffer and maintaining a reference
   count, by copying each packet into a separate buffer, etc.) and
   forwards each packet to the network layer.  During extraction, the

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   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.

   Note that OMNI interfaces must take care to avoid processing super-
   packet payload elements that would subvert security.  Specifically,
   if a super-packet contains a mix of data and control payload packets
   (which could include critical security codes), the node MUST NOT
   process the data packets before processing the control packets

7.  Frame Format

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

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

8.  Link-Local Addresses (LLAs)

   OMNI nodes are assigned OMNI interface IPv6 Link-Local Addresses
   (LLAs) through pre-service administrative actions.  "MNP-LLAs" embed
   the MNP assigned to the mobile node, while "ADM-LLAs" include an
   administratively-unique ID that is guaranteed to be unique on the
   link.  LLAs are configured as follows:

   o  IPv6 MNP-LLAs encode the most-significant 64 bits of a 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 is fe80::2001:db8:1000:2000/120.  Non-
      MNP routes are also represented the same as for MNP-LLAs, but
      include a GUA prefix that is not properly covered by the MSP.

   o  IPv4-compatible MNP-LLAs are constructed as fe80::ffff:[IPv4],
      i.e., the interface identifier consists of 16 '0' bits, followed

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      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-Compatible MN OMNI LLA for
      192.0.2.0/24 is fe80::ffff:192.0.2.0/120 (also written as
      fe80::ffff:c000:0200/120).

   o  ADM-LLAs are assigned to ARs and MSEs and MUST be managed for
      uniqueness.  The lower 32 bits of the LLA includes 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].

   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.

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

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

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

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

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

   o  etc.

   Each OMNI interface assigns the Anycast ADM-ULA specific to the OMNI
   link instance.  For example, the OMNI interface connected to instance

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   3 assigns the Anycast address [ULA]:0003::/128.  Routers that
   configure OMNI interfaces advertise the OMNI service prefix (e.g.,
   [ULA]:0003::/64) into the local routing system so that applications
   can direct traffic according to SBM requirements.

   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 ARs and MSEs.  All MNs are also considered to be
   connected to the OMNI link, however OAL encapsulation is omitted
   whenever possible to conserve bandwidth (see: Section 14).

   Each OMNI link can be subdivided into "segments" that often
   correspond to different administrative domains or physical
   partitions.  OMNI nodes can use IPv6 Segment Routing [RFC8402] when
   necessary 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-intarea-6706bis].

   Temporary ULAs are constructed per [RFC8981] based on the prefix
   [ULA]:ffff::/64 and used by MNs when they have no other addresses.
   Temporary ULAs can be used for MN-to-MN communications outside the
   context of any supporting OMNI link infrastructure, and can also be
   used as an initial address while the MN 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.

   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 and MNP addressing under
   certain limiting conditions (see: Section 10).

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 Mobile Nodes (MNs).  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 prefixes 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

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   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 prefixes are assigned by IANA [IPV4-GUA] and/or an
   associated regional assigned numbers authority such that the OMNI
   domain can be interconnected to the global IPv4 Internet without
   causing routing inconsistencies.  An OMNI domain could instead use
   private IPv4 prefixes (e.g., 10.0.0.0/8, etc.)  [RFC3330], however
   this would require IPv4 NAT if the domain were ever connected to the
   global IPv4 Internet.

11.  Node Identification

   OMNI MNs and MSEs that connect over open Internetworks include a
   unique node identification value for themselves in the OMNI options
   of their IPv6 ND messages (see: Section 12.1.13).  One useful
   identification value alternative is the Host Identity Tag (HIT) as
   specified in [RFC7401], while Hierarchical HITs (HHITs)
   [I-D.ietf-drip-rid] may provide a better alternative in certain
   domains such as the Unmanned (Air) Traffic Management (UTM) service
   for Unmanned Air Systems (UAS).  Another alternative 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 MN is truly outside the context of any infrastructure, it may
   have no MNP information at all.  In that case, the MN 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 MN 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 MN connects to ARs over (non-multihop) protected-spectrum
   ANETs, an alternate form of node identification (e.g., MAC address,
   serial number, airframe identification value, VIN, etc.) may be
   sufficient.  The MN can then include OMNI "Node Identification" sub-
   options (see: Section 12.1.13) in IPv6 ND messages should the need to
   transmit identification information over the network arise.

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.
   OMNI interface IPv6 Neighbor Discovery (ND) [RFC4861] messages sent
   over physical underlying interfaces without encapsulation observe the

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   native underlying interface Source/Target Link-Layer Address Option
   (S/TLLAO) format (e.g., for Ethernet the S/TLLAO is specified in
   [RFC2464]).  OMNI interface IPv6 ND messages sent over underlying
   interfaces via encapsulation do not include S/TLLAOs which were
   intended for encoding physical L2 media address formats and not
   encapsulation IP addresses.  Furthermore, S/TLLAOs are not intended
   for encoding additional interface attributes needed for multilink
   coordination.  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.

   MNs such as aircraft typically have many wireless data link types
   (e.g. satellite-based, cellular, terrestrial, air-to-air directional,
   etc.) with diverse performance, cost and availability properties.
   The OMNI interface would therefore appear to have multiple L2
   connections, and may include information for multiple underlying
   interfaces in a single IPv6 ND message exchange.  OMNI interfaces use
   an IPv6 ND option called the OMNI option formatted as shown in
   Figure 10:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Type     |     Length    |    Preflen    |  S/T-omIndex  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                          Sub-Options                          ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 10: OMNI Option Format

   In this format:

   o  Type is set to TBD2.

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

   o  Preflen is an 8 bit field that determines the length of prefix
      associated with an LLA.  Values 0 through 128 specify a valid
      prefix length (all other values are invalid).  For IPv6 ND
      messages sent from a MN to the MS, Preflen applies to the IPv6
      source LLA and provides the length that the MN is requesting or
      asserting to the MS.  For IPv6 ND messages sent from the MS to the
      MN, Preflen applies to the IPv6 destination LLA and indicates the

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      length that the MS is granting to the MN.  For IPv6 ND messages
      sent between MS endpoints, Preflen provides the length associated
      with the source/target MN that is subject of the ND message.

   o  S/T-omIndex is an 8 bit field corresponds to the omIndex value for
      source or target underlying interface used to convey this IPv6 ND
      message.  OMNI interfaces MUST number each distinct underlying
      interface with an omIndex value between '1' and '255' that
      represents a MN-specific 8-bit mapping for the actual ifIndex
      value assigned by network management [RFC2863] (the omIndex value
      '0' is reserved for use by the MS).  For RS and NS messages, S/
      T-omIndex corresponds to the source underlying interface the
      message originated from.  For RA and NA messages, S/T-omIndex
      corresponds to the target underlying interface that the message is
      destined to.  (For NS messages used for Neighbor Unreachability
      Detection (NUD), S/T-omIndex instead identifies the neighbor's
      underlying interface to be used as the target interface to return
      the NA.)

   o  Sub-Options is a Variable-length field, of length such that the
      complete OMNI Option is an integer multiple of 8 octets long.
      Contains one or more Sub-Options, as described in Section 12.1.

   The OMNI option may appear in any IPv6 ND message type; it is
   processed by interfaces that recognize the option and ignored by all
   other interfaces.  If multiple OMNI option instances appear in the
   same IPv6 ND message, the interface processes the Preflen and S/
   T-omIndex fields in the first instance and ignores those fields in
   all other instances.  The interface processes the Sub-Options of all
   OMNI option instances in the same IPv6 ND message in the consecutive
   order in which they appear.

   The OMNI option(s) in each IPv6 ND message may include full or
   partial information for the neighbor.  The union of the information
   in the most recently received OMNI options is therefore retained, and
   the information is aged/removed in conjunction with the corresponding
   neighbor cache entry.

12.1.  Sub-Options

   Each OMNI option includes zero or more Sub-Options.  Each consecutive
   Sub-Option is concatenated immediately after its predecessor.  All
   Sub-Options except Pad1 (see below) are in type-length-value (TLV)
   encoded in the following format:

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         0                   1                   2
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
        | Sub-Type|      Sub-length     | Sub-Option Data ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                       Figure 11: Sub-Option Format

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

        Sub-Option Name             Sub-Type
        Pad1                           0
        PadN                           1
        Interface Attributes (Type 1)  2
        Interface Attributes (Type 2)  3
        Traffic Selector               4
        MS-Register                    5
        MS-Release                     6
        Geo Coordinates                7
        DHCPv6 Message                 8
        HIP Message                    9
        Reassembly Limit              10
        Fragmentation Report          11
        Node Identification           12
        Sub-Type Extension            30

                                 Figure 12

      Sub-Types 13-29 are available for future assignment for major
      protocol functions.  Sub-Type 31 is reserved by IANA.

   o  Sub-Length is an 11-bit field that encodes the length of the Sub-
      Option Data ranging from 0 to 2034 octets.

   o  Sub-Option Data is a block of data with format determined by Sub-
      Type and length determined by Sub-Length.

   During transmission, the OMNI interface codes Sub-Type and Sub-Length
   together in network byte order in 2 consecutive octets, where Sub-
   Option Data may be up to 2034 octets in length.  This allows ample
   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.  If the
   Sub-Options to be coded would cause an OMNI option to exceed 2040
   octets, the OMNI interface codes any remaining Sub-Options in
   additional OMNI option instances in the intended order of processing
   in the same IPv6 ND message.  Implementations must therefore observe

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   size limitations, and must refrain from sending IPv6 ND messages
   larger than the OMNI interface MTU.  If the available OMNI
   information would cause a single IPv6 ND message to exceed the OMNI
   interface MTU, the OMNI interface codes as much as possible in a
   first IPv6 ND message and codes the remainder in additional IPv6 ND
   messages.

   During reception, the OMNI interface processes each OMNI option Sub-
   Option 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 a Sub-Option
   length would cause processing to exceed the OMNI option total length,
   the OMNI interface accepts any Sub-Options already processed and
   ignores the final Sub-Option.  The interface then processes any
   remaining OMNI options in the same fashion to the end of the IPv6 ND
   message.

   Note: large objects that exceed the Sub-Option Data limit of 2034
   octets are not supported under the current specification; if this
   proves to be limiting in practice, future specifications may define
   support for fragmenting large objects across multiple OMNI options
   within the same IPv6 ND message.

   The following Sub-Option types and formats are defined in this
   document:

12.1.1.  Pad1

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

                              Figure 13: Pad1

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

   o  Sub-Type is followed by 3 'x' bits, set to any value on
      transmission (typically all-zeros) and ignored on receipt.  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.

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12.1.2.  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 14: PadN

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

   o  Sub-Length is set to N (from 0 to 2034) that encodes the number of
      padding octets that follow.

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

12.1.3.  Interface Attributes (Type 1)

   The Interface Attributes (Type 1) sub-option provides a basic set of
   attributes for underlying interfaces.  Interface Attributes (Type 1)
   is deprecated throughout the rest of this specification, and
   Interface Attributes (Type 2) (see: Section 12.1.4) are indicated
   wherever the term "Interface Attributes" appears without an
   associated Type designation.

   Nodes SHOULD NOT include Interface Attributes (Type 1) sub-options in
   IPv6 ND messages they send, and MUST ignore any in IPv6 ND messages
   they receive.  If an Interface Attributes (Type 1) is included, it
   must have the following format:

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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Sub-Type=2|   Sub-length=N    |    omIndex    |    omType     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  | Link  | Resvd |P00|P01|P02|P03|P04|P05|P06|P07|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P08|P09|P10|P11|P12|P13|P14|P15|P16|P17|P18|P19|P20|P21|P22|P23|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P24|P25|P26|P27|P28|P29|P30|P31|P32|P33|P34|P35|P36|P37|P38|P39|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P40|P41|P42|P43|P44|P45|P46|P47|P48|P49|P50|P51|P52|P53|P54|P55|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 15: Interface Attributes (Type 1)

   o  Sub-Type is set to 2.  If multiple instances with different
      omIndex values appear in OMNI option of the same message all are
      processed; if multiple instances with the same omIndex value
      appear, the first is processed and all others are ignored

   o  Sub-Length is set to N (from 4 to 2034) that encodes the number of
      Sub-Option Data octets that follow.

   o  omIndex is a 1-octet field containing a value from 0 to 255
      identifying the underlying interface for which the attributes
      apply.

   o  omType is a 1-octet field containing a value from 0 to 255
      corresponding to the underlying interface identified by omIndex.

   o  Provider ID is a 1-octet field containing a value from 0 to 255
      corresponding to the underlying interface identified by omIndex.

   o  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").

   o  Resvd is reserved for future use.  Set to 0 on transmission and
      ignored on reception.

   o  A 16-octet ""Preferences" field immediately follows 'Resvd', with
      values P[00] through P[63] corresponding to the 64 Differentiated
      Service Code Point (DSCP) values [RFC2474].  Each 2-bit P[*] field
      is set to the value '0' ("disabled"), '1' ("low"), '2' ("medium")

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      or '3' ("high") to indicate a QoS preference for underlying
      interface selection purposes.

12.1.4.  Interface Attributes (Type 2)

   The Interface Attributes (Type 2) sub-option provides L2 forwarding
   information for the multilink conceptual sending algorithm discussed
   in Section 14.  The L2 information is used for selecting among
   potentially multiple candidate underlying interfaces that can be used
   to forward carrier packets to the neighbor based on factors such as
   DSCP preferences and link quality.  Interface Attributes (Type 2)
   further includes link-layer address information to be used for either
   OAL encapsulation or direct UDP/IP encapsulation (when OAL
   encapsulation can be avoided).

   Interface Attributes (Type 2) are the sole Interface Attributes
   format in this specification that all OMNI nodes must honor.
   Wherever the term "Interface Attributes" occurs throughout this
   specification without a "Type" designation, the format given below is
   indicated:

        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  |R| API |   SRT   | FMT |   LHS (0 - 7) |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               LHS (bits 8 - 31)               |               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               ~
       ~                                                               ~
       ~                   Link Layer Address (L2ADDR)                 ~
       ~                                                               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Bitmap(0)=0xff|P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P12|P13|P14|P15|P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P28|P29|P30|P31| Bitmap(1)=0xff|P32|P33|P34|P35|P36| ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                 Figure 16: Interface Attributes (Type 2)

   o  Sub-Type is set to 3.  If multiple instances with different
      omIndex values appear in OMNI options of the same message all are
      processed; if multiple instances with the same omIndex value
      appear, the first is processed and all others are ignored.

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   o  Sub-Length is set to N (from 4 to 2034) that encodes the number of
      Sub-Option Data octets that follow.  The 'omIndex', 'omType',
      'Provider ID', 'Link', 'R' and 'API' fields are always present;
      hence, the remainder of the Sub-Option Data is limited to 2030
      octets.

   o  Sub-Option Data contains an "Interface Attributes (Type 2)" option
      encoded as follows:

      *  omIndex is set to an 8-bit integer value corresponding to a
         specific underlying interface the same as specified above for
         the OMNI option S/T-omIndex field.  The OMNI options of a same
         message may include multiple Interface Attributes Sub-Options,
         with each distinct omIndex value pertaining to a different
         underlying interface.  The OMNI option will often include an
         Interface Attributes Sub-Option with the same omIndex value
         that appears in the S/T-omIndex.  In that case, the actual
         encapsulation address of the received IPv6 ND message should be
         compared with the L2ADDR encoded in the Sub-Option (see below);
         if the addresses are different (or, if L2ADDR is absent) the
         presence of a NAT is assumed.

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

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

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

      *  R is reserved for future use.

      *  API - a 3-bit "Address/Preferences/Indexed" code that
         determines the contents of the remainder of the sub-option as
         follows:

         +  When the most significant bit (i.e., "Address") is set to 1,
            the SRT, FMT, LHS and L2ADDR fields are included immediately
            following the API code; else, they are omitted.

         +  When the next most significant bit (i.e., "Preferences") is
            set to 1, a preferences block is included next; else, it is
            omitted.  (Note that if "Address" is set the preferences

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            block immediately follows L2ADDR; else, it immediately
            follows the API code.)

         +  When a preferences block is present and the least
            significant bit (i.e., "Indexed") is set to 0, the block is
            encoded in "Simplex" form as shown in Figure 15; else it is
            encoded in "Indexed" form as discussed below.

      *  When API indicates that an "Address" is included, the following
         fields appear in consecutive order (else, they are omitted):

         +  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.

         +  FMT - a 3-bit "Framework/Mode/Type" code corresponding to
            the included Link Layer Address as follows:

            -  When the most significant bit (i.e., "Framework") is set
               to 1, L2ADDR is the INET encapsulation address for the
               Source/Target Client itself; otherwise L2ADDR is the
               address of the Proxy/Server named in the LHS.

            -  When the next most significant bit (i.e., "Mode") is set
               to 1, the Framework node is (likely) located behind an
               INET Network Address Translator (NAT); otherwise, it is
               on the open INET.

            -  When the least significant bit (i.e., "Type") is set to
               0, L2ADDR includes a UDP Port Number followed by an IPv4
               address; otherwise, it includes a UDP Port Number
               followed by an IPv6 address.

         +  LHS - the 32 bit MSID of the Last Hop Proxy/Server on the
            path to the target.  When SRT and LHS are both set to 0, the
            LHS is considered unspecified in this IPv6 ND message.  When
            SRT is set to 0 and LHS is non-zero, the prefix length is
            set to 128.  SRT and LHS together provide guidance to the
            OMNI interface forwarding algorithm.  Specifically, if SRT/
            LHS is located in the local OMNI link segment then the OMNI
            interface can encapsulate according to FMT/L2ADDR (following
            any necessary NAT traversal messaging); else, it must
            forward according to the OMNI link spanning tree.  See
            [I-D.templin-intarea-6706bis] for further discussion.

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         +  Link Layer Address (L2ADDR) - Formatted according to FMT,
            and identifies the link-layer address (i.e., the
            encapsulation address) of the source/target.  The UDP Port
            Number appears in the first 2 octets and the IP address
            appears in the next 4 octets for IPv4 or 16 octets for IPv6.
            The Port Number and IP address are recorded in network byte
            order, and in ones-compliment "obfuscated" form per
            [RFC4380].  The OMNI interface forwarding algorithm uses
            FMT/L2ADDR to determine the encapsulation address for
            forwarding when SRT/LHS is located in the local OMNI link
            segment.  Note that if the target is behind a NAT, L2ADDR
            will contain the mapped INET address stored in the NAT;
            otherwise, L2ADDR will contain the native INET information
            of the target itself.

      *  When API indicates that "Preferences" are included, a
         preferences block appears as the remainder of the Sub-Option as
         a series of Bitmaps and P[*] values.  In "Simplex" form, the
         index for each singleton Bitmap octet is inferred from its
         sequential position (i.e., 0, 1, 2, ...) as shown in Figure 16.
         In "Indexed" form, each Bitmap is preceded by an Index octet
         that encodes a value "i" = (0 - 255) as the index for its
         companion Bitmap as follows:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
        |   Index=i     |   Bitmap(i)   |P[*] values ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                                 Figure 17

      *  The preferences consist of a first (simplex/indexed) Bitmap
         (i.e., "Bitmap(i)") followed by 0-8 single-octet blocks of
         2-bit P[*] values, followed by a second Bitmap (i), followed by
         0-8 blocks of P[*] values, etc.  Reading from bit 0 to bit 7,
         the bits of each Bitmap(i) that are set to '1'' indicate the
         P[*] blocks from the range P[(i*32)] through P[(i*32) + 31]
         that follow; if any Bitmap(i) bits are '0', then the
         corresponding P[*] block is instead omitted.  For example, if
         Bitmap(0) contains 0xff then the block with P[00]-P[03],
         followed by the block with P[04]-P[07], etc., and ending with
         the block with P[28]-P[31] are included (as shown in
         Figure 15).  The next Bitmap(i) is then consulted with its bits
         indicating which P[*] blocks follow, etc. out to the end of the
         Sub-Option.

      *  Each 2-bit P[*] field is set to the value '0' ("disabled"), '1'
         ("low"), '2' ("medium") or '3' ("high") to indicate a QoS
         preference for underlying interface selection purposes.  Not

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         all P[*] values need to be included in the OMNI option of each
         IPv6 ND message received.  Any P[*] values represented in an
         earlier OMNI option but omitted in the current OMNI option
         remain unchanged.  Any P[*] values not yet represented in any
         OMNI option default to "medium".

      *  The first 16 P[*] blocks correspond to the 64 Differentiated
         Service Code Point (DSCP) values P[00] - P[63] [RFC2474].  Any
         additional P[*] blocks that follow correspond to "pseudo-DSCP"
         traffic classifier values P[64], P[65], P[66], etc.  See
         Appendix A for further discussion and examples.

12.1.5.  Traffic Selector

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=4|     Sub-length=N    |    omIndex    |               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               ~
       ~                                                               ~
       ~                RFC 6088 Format Traffic Selector               ~
       ~                                                               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 18: Traffic Selector

   o  Sub-Type is set to 4.  If multiple instances appear in OMNI
      options of the same message all are processed, i.e., even if the
      same omIndex value appears multiple times.

   o  Sub-Length is set to N (from 1 to 2034) that encodes the number of
      Sub-Option Data octets that follow.

   o  Sub-Option Data contains a 1 octet omIndex encoded exactly as
      specified in Section 12.1.3, followed by an N-1 octet traffic
      selector formatted per [RFC6088] beginning with the "TS Format"
      field.  The largest traffic selector for a given omIndex is
      therefore 2033 octets.

12.1.6.  MS-Register

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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=5|    Sub-length=4n    |      MSID[1] (bits 0 - 15)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     MSID [1] (bits 16 - 32)   |      MSID[2] (bits 0 - 15)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     MSID [2] (bits 16 - 32)   |      MSID[3] (bits 0 - 15)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           ...        ...        ...        ...       ...        ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     MSID [n] (bits 16 - 32)   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 19: MS-Register Sub-option

   o  Sub-Type is set to 5.  If multiple instances appear in OMNI
      options of the same message all are processed.  Only the first
      MAX_MSID values processed (whether in a single instance or
      multiple) are retained and all other MSIDs are ignored.

   o  Sub-Length is set to 4n, with 508 as the maximum value for n.  The
      length of the Sub-Option Data section is therefore limited to 2032
      octets.

   o  A list of n 4 octet MSIDs is included in the following 4n octets.
      The Anycast MSID value '0' in an RS message MS-Register sub-option
      requests the recipient to return the MSID of a nearby MSE in a
      corresponding RA response.

12.1.7.  MS-Release

        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=4n    |      MSID[1] (bits 0 - 15)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     MSID [1] (bits 16 - 32)   |      MSID[2] (bits 0 - 15)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     MSID [2] (bits 16 - 32)   |      MSID[3] (bits 0 - 15)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           ...        ...        ...        ...       ...        ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     MSID [n] (bits 16 - 32)   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 20: MS-Release Sub-option

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   o  Sub-Type is set to 6.  If multiple instances appear in OMNI
      options of the same message all are processed.  Only the first
      MAX_MSID values processed (whether in a single instance or
      multiple) are retained and all other MSIDs are ignored.

   o  Sub-Length is set to 4n, with 508 as the maximum value for n.  The
      length of the Sub-Option Data section is therefore limited to 2032
      octets.

   o  A list of n 4 octet MSIDs is included in the following 4n octets.
      The Anycast MSID value '0' is ignored in MS-Release sub-options,
      i.e., only non-zero values are processed.

12.1.8.  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=7|    Sub-length=N     |      Geo Coordinates
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

                   Figure 21: Geo Coordinates Sub-option

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

   o  Sub-Length is set to N (from 0 to 2034) that encodes the number of
      Sub-Option Data octets that follow.

   o  A set of Geo Coordinates of maximum length 2034 octets.  Format(s)
      to be specified in future documents; should include Latitude/
      Longitude, plus any additional attributes such as altitude,
      heading, speed, etc.

12.1.9.  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 RS messages sent by MNs and RA
   messages returned by MSEs.  ARs that act as proxys to forward RS/RA
   messages between MNs and MSEs also forward DHCPv6 sub-options
   unchanged and do not process DHCPv6 sub-options themselves.  Note
   that DHCPv6 message sub-option integrity is protected by the Checksum
   included in the IPv6 ND message header.

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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=8|    Sub-length=N     |    msg-type   |  id (octet 0) |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   transaction-id (octets 1-2) |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
       |                                                               |
       .                        DHCPv6 options                         .
       .                 (variable number and length)                  .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 22: DHCPv6 Message Sub-option

   o  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.

   o  Sub-Length is set to N (from 4 to 2034) 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 restricted to 2030 octets.

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

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

12.1.10.  Host Identity Protocol (HIP) Message

   The Host Identity Protocol (HIP) Message sub-option may be included
   in the OMNI options of RS messages sent by MNs and RA messages
   returned by ARs.  ARs that act as proxys authenticate and remove HIP
   messages in RS messages they forward from a MN to an MSE.  ARs that
   act as proxys insert and sign HIP messages in the RA messages they
   forward from an MSE to a MN.

   The HIP message sub-option may also be included in any IPv6 ND
   message that may traverse an open Internetwork, i.e., where link-
   layer authentication is not already assured by lower layers.

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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=9|    Sub-length=N     |0| Packet Type |Version| RES.|1|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Checksum             |           Controls            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                Sender's Host Identity Tag (HIT)               |
       |                                                               |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               Receiver's Host Identity Tag (HIT)              |
       |                                                               |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       /                        HIP Parameters                         /
       /                                                               /
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 23: HIP Message Sub-option

   o  Sub-Type is set to 9.  If multiple instances appear in OMNI
      options of the same message the first is processed and all others
      are ignored.

   o  Sub-Length is set to N, i.e., the length of the option in octets
      beginning immediately following the Sub-Length field and extending
      to the end of the HIP parameters.  The length of the entire HIP
      message is therefore restricted to 2034 octets.

   o  The HIP message is coded exactly as specified in 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).  Note that, since the IPv6
      ND message header already includes a Checksum, the HIP message
      Checksum field is set to 0 on transmission and ignored on
      reception.  (The Checksum field is still included to retain the
      [RFC7401] message format.)

12.1.11.  Reassembly Limit

   The Reassembly Limit sub-option may be included in the OMNI options
   of IPv6 ND messages.  The message consists of a 14-bit Reassembly
   Limit value, followed by two flag bits (H, L) optionally followed by

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   an (N-2)-octet leading portion of an OAL First Fragment that
   triggered the message.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S-Type=10|    Sub-length=N     |      Reassembly Limit     |H|L|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            OAL First Fragment (As much of invoking packet     |
       +              as possible without the IPv6 ND message          +
       |                 exceeding the minimum IPv6 MTU)               |
       +                                                               +

                        Figure 24: Reassembly Limit

   o  Sub-Type is set to 10.  If multiple instances appear in OMNI
      options of the same message the first occurring "hard" and "soft"
      Reassembly Limit values are accepted, and any additional
      Reassembly Limit values are ignored.

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

   o  A 14-bit Reassembly Limit follows, and includes a value between
      1500 and 9180.  If any other value is included, the sub-option is
      ignored.  The value indicates the hard or soft limit for original
      IP packets that the source of the message is currently willing to
      reassemble; the source may increase or decrease the hard or soft
      limit at any time through the transmission of new IPv6 ND
      messages.  Until the first IPv6 ND message with a Reassembly Limit
      sub-option arrives, OMNI nodes assume initial default hard/soft
      limits of 9180 bytes (I.e., the OMNI interface MRU).  After IPv6
      ND messages with Reassembly Limit sub-options arrive, the OMNI
      node retains the most recent hard/soft limit values until new IPv6
      ND messages with different values arrive.

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

   o  The 'L' flag is set to 1 if an OAL First Fragment corresponding to
      a reassembly loss event was included; otherwise set to 0.

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

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      First Fragment is consulted regardless of its size, and without
      waiting for additional fragments.)

12.1.12.  Fragmentation Report

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

         0                   1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S-Type=11|   Sub-Length = N    | Identification #1 (bits 0 -15)|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Identification #1 (bits 15-31)|    Bitmap #1 (bits 0 - 15)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Bitmap #1 (bits 16-31)  | Identification #2 (bits 0 -15)|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Identification #2 (bits 15-31)|    Bitmap #2 (bits 0 - 15)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Bitmap #2 (bits 16-31)  | Identification #3 (bits 0 -15)|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Identification #3 (bits 15-31)|    Bitmap #3 (bits 0 - 15)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Bitmap #3 (bits 16-31)  |             ...               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+             ...               +
       |                              ...                              |
       +                              ...                              +

                      Figure 25: Fragmentation Report

   o  Sub-Type is set to 11.  If multiple instances appear in OMNI
      options of the same message all are processed.

   o  Sub-Length is set to N, i.e., the length of the option in octets
      beginning immediately following the Sub-Length field and extending
      to the end of the ICMPv6 error message body.  N must be an
      integral multiple of 8 octets; otherwise, the sub-option is
      ignored.  The length of the entire sub-option should not cause the
      entire IPv6 ND message to exceed the minimum MPS.

   o  Identification (i) includes the IPv6 Identification value found in
      the Fragment Header of a received OAL fragment.  (Only those
      Identification values included represent fragments for which loss
      was unambiguously observed; any Identification values not included

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      correspond to fragments that were either received in their
      entirety or are still in transit.)

   o  Bitmap (i) includes an ordinal checklist of fragments, with each
      bit set to 1 for a fragment received or 0 for a fragment missing.
      For example, for a 20-fragment fragmented OAL packet with ordinal
      fragments #3, #10, #13 and #17 missing and all other fragments
      received, the bitmap would encode:

         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 26

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

12.1.13.  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=12|    Sub-length=N    |     ID-Type    |               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               ~
       ~            Node Identification Value (N-1 octets)             ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 27: Node Identification

   o  Sub-Type is set to 12.  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.  (Note therefore that it is possible for a
      single IPv6 ND message to convey multiple Node Identifications -
      each having a different ID-Type.)

   o  Sub-Length is set to N (from 1 to 2034) 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
      2033 octets.

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

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      *  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 - 252 - Unassigned.

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

      *  255 - reserved by IANA.

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

   When a Node Identification Value is needed for DHCPv6 messaging
   purposes, it is encoded as a DHCP Unique IDentifier (DUID) using the
   "DUID-EN for OMNI" format with enterprise number 45282 (see:
   Section 25) as shown in Figure 28:

        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 28: DUID-EN for OMNI Format

   In this format, the ID-Type and Node Identification Value fields are
   coded exactly as in Figure 27 following the 6 octet DUID-EN header,
   and the entire "DUID-EN for OMNI" is included in a DHCPv6 message per
   [RFC8415].

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12.1.14.  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 29:

        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 29: Sub-Type Extension

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

   o  Sub-Length is set to N (from 1 to 2034) that encodes the number of
      Sub-Option Data octets that follow.  The Extension-Type field is
      always present; hence, the maximum Extension-Type Body length is
      2033 octets.

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

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

   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.  Extension-Type values
   0 and 1 are defined in the following subsections:

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12.1.14.1.  RFC4380 UDP/IP Header 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 30: RFC4380 UDP/IP Header Option (Extension-Type 0)

   o  Sub-Type is set to 30.

   o  Sub-Length is set to N (from 2 to 2034) that encodes the number of
      Sub-Option Data octets that follow.  The Extension-Type and Header
      Type fields are always present; hence, the maximum-length Header
      Option Value is 2032 octets.

   o  Extension-Type is set to 0.  Each instance encodes exactly one
      header option per Section 5.1.1 of [RFC4380], with the leading '0'
      octet omitted and the following octet coded as Header Type.  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.

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

      *  0 - Origin Indication (IPv4) - value coded 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.

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

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12.1.14.2.  RFC6081 UDP/IP Trailer 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 31: RFC6081 UDP/IP Trailer Option (Extension-Type 1)

   o  Sub-Type is set to 30.

   o  Sub-Length is set to N (from 2 to 2034) that encodes the number of
      Sub-Option Data octets that follow.  The Extension-Type and
      Trailer Type fields are always present; hence, the maximum-length
      Trailer Option Value is 2032 octets.

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

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

      *  0 - Unassigned

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

      *  2 - Unassigned

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

      *  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.

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

13.  Address Mapping - Multicast

   The multicast address mapping of the native underlying interface
   applies.  The mobile router on board the MN also serves as an IGMP/
   MLD Proxy for its EUNs and/or hosted applications per [RFC4605] while
   using the L2 address of the AR as the L2 address for all multicast
   packets.

   The MN uses Multicast Listener Discovery (MLDv2) [RFC3810] to
   coordinate with the AR, and *NET L2 elements use MLD snooping
   [RFC4541].

14.  Multilink Conceptual Sending Algorithm

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

   After an original IP packet enters the OMNI interface, one or more
   outbound underlying interfaces are selected based on PBM traffic
   attributes, and one or more neighbor underlying interfaces are
   selected based on the receipt of Interface Attributes sub-options in
   IPv6 ND messages (see: Figure 15).  Underlying interface selection
   for the nodes own local interfaces are based on attributes such as
   DSCP, application port number, cost, performance, message size, etc.
   OMNI interface multilink selections could also be configured to
   perform replication across multiple underlying interfaces for
   increased reliability at the expense of packet duplication.  The set
   of all Interface Attributes received in IPv6 ND messages determines
   the multilink forwarding profile for selecting the neighbor's
   underlying interfaces.

   When the OMNI interface sends an original IP packet over a selected
   outbound underlying interface, the OAL employs encapsulation and
   fragmentation as discussed in Section 5, then performs *NET
   encapsulation as determined by the L2 address information received in
   Interface Attributes.  The OAL also performs encapsulation when the

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   nearest AR is located multiple hops away as discussed in
   Section 15.1.  (Note that the OAL MAY employ packing when multiple
   original IP packets and/or control messages are available for
   forwarding to the same OAL destination.)

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

14.1.  Multiple OMNI Interfaces

   MNs may connect to multiple independent OMNI links concurrently in
   support of SBM.  Each OMNI interface is distinguished by its Anycast
   ULA (e.g., [ULA]:0002::, [ULA]:1000::, [ULA]:7345::, etc.).  The MN
   configures a separate OMNI interface for each link so that multiple
   interfaces (e.g., omni0, omni1, omni2, etc.) are exposed to the IPv6
   layer.  A different Anycast ULA is assigned to each interface, and
   the MN injects the service prefixes for the OMNI link instances into
   the EUN routing system.

   Applications in EUNs can use Segment Routing to select the desired
   OMNI interface based on SBM considerations.  The Anycast ULA is
   written into an original IP packet's IPv6 destination address, and
   the actual destination (along with any additional intermediate hops)
   is written into the Segment Routing Header.  Standard IP routing
   directs the packet to the MN's mobile router entity, and the Anycast
   ULA identifies the OMNI interface to be used for transmission to the
   next hop.  When the MN receives the packet, it replaces the IPv6
   destination address with the next hop found in the routing header and
   transmits the message over the OMNI interface identified by the
   Anycast ULA.

   Multiple distinct OMNI links can therefore be used to support fault
   tolerance, load balancing, reliability, etc.  The architectural model
   is similar to Layer 2 Virtual Local Area Networks (VLANs).

14.2.  MN<->AR Traffic Loop Prevention

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

   If at some later time the MN loses state (e.g., after a reboot), it
   may begin returning packets destined to an MNP address to the AR as
   its default router.  The AR therefore must drop any packets

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   originating from the MN and destined to an address within the MN's
   registered MNP.  To do so, the AR institutes the following check:

   o  if the IP destination address belongs to a neighbor on the same
      OMNI interface, and if the link-layer source address is the same
      as one of the neighbor's link-layer addresses, drop the packet.

15.  Router Discovery and Prefix Registration

   MNs interface with the MS by sending RS messages with OMNI options
   under the assumption that one or more AR on the *NET will process the
   message and respond.  The MN then configures default routes for the
   OMNI interface via the discovered ARs as the next hop.  The manner in
   which the *NET ensures AR coordination is link-specific and outside
   the scope of this document (however, considerations for *NETs that do
   not provide ARs that recognize the OMNI option are discussed in
   Section 20).

   For each underlying interface, the MN sends an RS message with an
   OMNI option to coordinate with MSEs identified by MSID values.
   Example MSID discovery methods are given in [RFC5214] and include
   data link login parameters, name service lookups, static
   configuration, a static "hosts" file, etc.  When the AR receives an
   RS', it selects a nearby MSE (which may be itself) and returns an RA
   with the selected MSID in an MS-Register sub-option.  The AR selects
   only a single nearby MSE while also soliciting the MSEs corresponding
   to any non-zero MSIDs.

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

   When an OMNI interface transitions to UP, the MN sends RS messages to
   register its MNP and an initial set of underlying interfaces that are
   also UP.  The MN sends additional RS messages to refresh lifetimes
   and to register/deregister underlying interfaces as they transition
   to UP or DOWN.  The MN's OMNI interface sends initial RS messages
   over an UP underlying interface with its MNP-LLA as the source (or
   with the unspecified address (::) as the source if it does not yet
   have an MNP-LLA) and with destination set to link-scoped All-Routers
   multicast (ff02::2) [RFC4291].  The OMNI interface includes an OMNI

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   option per Section 12 with a Preflen assertion, Interface Attributes
   appropriate for underlying interfaces, MS-Register/Release sub-
   options containing MSID values, Reassembly Limits, an authentication
   sub-option and with any other necessary OMNI sub-options (e.g., a
   Node Identification sub-option as an identity for the MN).  The OMNI
   interface then sets the S/T-omIndex field to the index of the
   underlying interface over which the RS message is sent.

   The OMNI interface then sends the RS over the underlying interface
   using OAL encapsulation and fragmentation if necessary.  If OAL
   encapsulation is used for RS messages sent over an INET interface,
   the entire RS message must appear within a single carrier packet so
   that it can be authenticated without requiring reassembly.  The OMNI
   interface selects an unpredictable initial Identification value per
   Section 6.5, sets the OAL source address to the ULA corresponding to
   the RS source (Or a Temporary ULA if the RS source is the unspecified
   address (::)) and sets the OAL destination to site-scoped All-Routers
   multicast (ff05::2) then sends the message.

   ARs process IPv6 ND messages with OMNI options and act as an MSE
   themselves and/or as a proxy for other MSEs.  ARs receive RS messages
   and create a neighbor cache entry for the MN, then prepare to act as
   an MSE themselves and/or coordinate with any MSEs named in the
   Register/Release lists in a manner outside the scope of this
   document.  When an MSE processes the OMNI information, it first
   validates the prefix registration information then injects/withdraws
   the MNP in the routing/mapping system and caches/discards the new
   Preflen, MNP and Interface Attributes.  The MSE then informs the AR
   of registration success/failure, and the AR returns an RA message to
   the MN with an OMNI option per Section 12.

   The AR's OMNI interface returns the RA message via the same
   underlying interface of the MN over which the RS was received, and
   with destination address set to the MNP-LLA (i.e., unicast), with
   source address set to its own LLA, and with an OMNI option with S/
   T-omIndex set to the value included in the RS.  The OMNI option also
   includes a Preflen confirmation, Interface Attributes, MS-Register/
   Release and any other necessary OMNI sub-options (e.g., a Node
   Identification sub-option as an identity for the AR).  The RA also
   includes any information for the link, including RA Cur Hop Limit, M
   and O flags, Router Lifetime, Reachable Time and Retrans Timer
   values, and includes any necessary options such as:

   o  PIOs with (A; L=0) that include MSPs for the link [RFC8028].

   o  RIOs [RFC4191] with more-specific routes.

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   o  an MTU option that specifies the maximum acceptable packet size
      for this underlying interface.

   If the RS message arrived as an OAL atomic fragment, the AR prepares
   the RA using OAL encapsulation/fragmentation with the same
   Identification value that appeared in the RS message, with source set
   to the ULA corresponding to the RA source and with destination set to
   the ULA corresponding to the RA destination.  The AR then sends the
   initial RA message to the MN and MAY later send additional periodic
   and/or event-driven unsolicited RA messages per [RFC4861].  In that
   case, the S/T-omIndex field in the OMNI option of the unsolicited RA
   message identifies the target underlying interface of the destination
   MN.

   The AR can combine the information from multiple MSEs into one or
   more "aggregate" RAs sent to the MN in order conserve *NET bandwidth.
   Each aggregate RA includes an OMNI option with MS-Register/Release
   sub-options with the MSEs represented by the aggregate.  If an
   aggregate is sent, the RA message contents must consistently
   represent the combined information advertised by all represented
   MSEs.  Note that since the AR uses its own ADM-LLA as the RA source
   address, the MN determines the addresses of the represented MSEs by
   examining the MS-Register/Release OMNI sub-options.

   When the MN receives the RA message, it creates an OMNI interface
   neighbor cache entry for each MSID that has confirmed MNP
   registration via the L2 address of this AR.  If the MN connects to
   multiple *NETs, it records the additional L2 AR addresses in each
   MSID neighbor cache entry (i.e., as multilink neighbors).  The MN
   then configures a default route via the MSE that returned the RA
   message, and assigns the Subnet Router Anycast address corresponding
   to the MNP (e.g., 2001:db8:1:2::) to the OMNI interface.  The MN then
   manages its underlying interfaces according to their states as
   follows:

   o  When an underlying interface transitions to UP, the MN sends an RS
      over the underlying interface with an OMNI option.  The OMNI
      option contains at least one Interface Attribute sub-option with
      values specific to this underlying interface, and may contain
      additional Interface Attributes specific to other underlying
      interfaces.  The option also includes any MS-Register/Release sub-
      options.

   o  When an underlying interface transitions to DOWN, the MN sends an
      RS or unsolicited NA message over any UP underlying interface with
      an OMNI option containing an Interface Attribute sub-option for
      the DOWN underlying interface with Link set to '0'.  The MN sends
      an RS when an acknowledgement is required, or an unsolicited NA

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      when reliability is not thought to be a concern (e.g., if
      redundant transmissions are sent on multiple underlying
      interfaces).

   o  When the Router Lifetime for a specific AR nears expiration, the
      MN sends an RS over the underlying interface to receive a fresh
      RA.  If no RA is received, the MN can send RS messages to an
      alternate MSID in case the current MSID has failed.  If no RS
      messages are received even after trying to contact alternate
      MSIDs, the MN marks the underlying interface as DOWN.

   o  When a MN wishes to release from one or more current MSIDs, it
      sends an RS or unsolicited NA message over any UP underlying
      interfaces with an OMNI option with a Release MSID.  Each MSID
      then withdraws the MNP from the routing/mapping system and informs
      the AR that the release was successful.

   o  When all of a MNs underlying interfaces have transitioned to DOWN
      (or if the prefix registration lifetime expires), any associated
      MSEs withdraw the MNP the same as if they had received a message
      with a release indication.

   The MN is responsible for retrying each RS exchange up to
   MAX_RTR_SOLICITATIONS times separated by RTR_SOLICITATION_INTERVAL
   seconds until an RA is received.  If no RA is received over an UP
   underlying interface (i.e., even after attempting to contact
   alternate MSEs), the MN declares this underlying interface as DOWN.

   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 that is consistent with the
   information received from the RAs generated by the MS.  Whether the
   OMNI interface IPv6 ND messaging process is initiated from the
   receipt of an RS message from the IPv6 layer is an implementation
   matter.  Some implementations may elect to defer the IPv6 ND
   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 the ordinary RS/RA
   messaging used by the IPv6 layer over the OMNI interface, since they
   are not required to drive the internal RS/RA processing.  (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 MN must send another RS message over this underlying
   interface (e.g., 600 seconds), however that timescale may be

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   significantly longer than the lifetime the MS has committed to retain
   the prefix registration (e.g., REACHABLETIME seconds).  ARs are
   therefore responsible for keeping MS state alive on a shorter
   timescale than the MN is required to do on its own behalf.

   Note: On multicast-capable underlying interfaces, MNs should send
   periodic unsolicited multicast NA messages and ARs 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 an AR acting as a proxy forwards a MN's RS message to
   another node acting as an MSE using UDP/IP encapsulation, it must use
   a distinct UDP source port number for each MN.  This allows the MSE
   to distinguish different MNs behind the same AR at the link-layer,
   whereas the link-layer addresses would otherwise be
   indistinguishable.

   Note: when an AR acting as an MSE returns an RA to an INET Client, it
   includes an OMNI option with an Interface Attributes sub-option with
   omIndex set to 0 and with SRT, FMT, LHS and L2ADDR information for
   its INET interface.  This provides the Client with partition prefix
   context regarding the local OMNI link segment.

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

   On some *NETs, a MN may be located multiple IP hops away from the
   nearest AR.  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.).  These *NETs
   could be either IPv6-enabled or IPv4-only, while IPv4-only *NETs
   could be either multicast-capable or unicast-only (note that for
   IPv4-only *NETs the following procedures apply for both single-hop
   and multihop cases).

   A MN located potentially multiple *NET hops away from the nearest AR
   prepares an RS message with source address set to its MNP-LLA (or to
   the unspecified address (::) if it does not yet have an MNP-LLA), and
   with destination set to link-scoped All-Routers multicast the same as
   discussed above.  The OMNI interface then employs OAL encapsulation
   and fragmentation, and 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 site-scoped All-Routers multicast (ff05::2).  For IPv6-enabled
   *NETs, the MN then encapsulates the message in UDP/IPv6 headers with
   source address set to the underlying interface address (or to the ULA

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   that would be used for OAL encapsulation if the underlying interface
   does not yet have an address) and sets the destination to either a
   unicast or anycast address of an AR.  For IPv4-only *NETs, the MN
   instead encapsulates the RS message in UDP/IPv4 headers with source
   address set to the IPv4 address of the underlying interface and with
   destination address set to either the unicast IPv4 address of an AR
   [RFC5214] or an IPv4 anycast address reserved for OMNI.  The MN then
   sends the encapsulated RS message via the *NET interface, where it
   will be forwarded by zero or more intermediate *NET hops.

   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 MN).  This process
   repeats iteratively until the RS message is received by a penultimate
   *NET hop within single-hop communications range of an AR, which
   forwards the message to the AR.

   When the AR receives the message, it decapsulates the RS (while
   performing OAL reassembly, if necessary) and coordinates with the MS
   the same as for an ordinary link-local RS, since the network layer
   Hop Limit will not have been decremented by the multihop forwarding
   process.  The AR then prepares an RA message with source address set
   to its own ADM-LLA and destination address set to the LLA of the
   original MN.  The AR then performs OAL encapsulation and
   fragmentation, with OAL source set to its own ADM-ULA and destination
   set to the ULA corresponding to the RA source.  The AR then
   encapsulates the message in UDP/IPv4 or UDP/IPv6 headers with source
   address set to its own address and with destination set to the
   encapsulation source of the RS.

   The AR then forwards the message to an *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 MN, which decapsulates the message and
   performs autoconfiguration the same as if it had received the RA
   directly from the AR as an on-link neighbor.

   Note: An alternate approach to multihop forwarding via IPv6
   encapsulation would be for the MN and AR to 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.

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   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.

   Note: An IPv4 anycast address for OMNI in IPv4 networks could be part
   of a new IPv4 /24 prefix allocation, but this may be difficult to
   obtain given IPv4 address exhaustion.  An alternative would be to re-
   purpose the prefix 192.88.99.0 which has been set aside from its
   former use by [RFC7526].

15.2.  MS-Register and MS-Release List Processing

   OMNI links maintain a constant value "MAX_MSID" selected to provide
   MNs with an acceptable level of MSE redundancy while minimizing
   control message amplification.  It is RECOMMENDED that MAX_MSID be
   set to the default value 5; if a different value is chosen, it should
   be set uniformly by all nodes on the OMNI link.

   When a MN sends an RS message with an OMNI option via an underlying
   interface to an AR, the MN must convey its knowledge of its
   currently-associated MSEs.  Initially, the MN will have no associated
   MSEs and should therefore send its initial RS messages to the link-
   scoped All-Routers multicast address.  The AR will then return an RA
   message with source address set to the ADM-LLA of the selected MSE
   (which may be the address of the AR itself).

   As the MN activates additional underlying interfaces, it can
   optionally include an MS-Register sub-option with MSIDs for MSEs
   discovered from previous RS/RA exchanges.  The MN will thus
   eventually begin to learn and manage its currently active set of
   MSEs, and can register with new MSEs or release from former MSEs with
   each successive RS/RA exchange.  As the MN's MSE constituency grows,
   it alone is responsible for including or omitting MSIDs in the MS-
   Register/Release lists it sends in RS messages.  The inclusion or
   omission of MSIDs determines the MN's interface to the MS and defines
   the manner in which MSEs will respond.  The only limiting factor is
   that the MN should include no more than MAX_MSID values in each list
   per each IPv6 ND message, and should avoid duplication of entries in
   each list unless it wants to increase likelihood of control message
   delivery.

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   When an AR receives an RS message sent by a MN with an OMNI option,
   the option will contain zero or more MS-Register and MS-Release sub-
   options containing MSIDs.  After processing the OMNI option, the AR
   will have a list of zero or more MS-Register MSIDs and a list of zero
   or more of MS-Release MSIDs.  The AR then processes the lists as
   follows:

   o  For each list, retain the first MAX_MSID values in the list and
      discard any additional MSIDs (i.e., even if there are duplicates
      within a list).

   o  Next, for each MSID in the MS-Register list, remove all matching
      MSIDs from the MS-Release list.

   o  Next, proceed as follows:

      *  If the AR's own MSID appears in the MS-Register list, send an
         RA message directly back to the MN and send a proxy copy of the
         RS message to each additional MSID in the MS-Register list with
         the MS-Register/Release lists omitted.  Then, send an
         unsolicited NA (uNA) message to each MSID in the MS-Release
         list with the MS-Register/Release lists omitted and with an
         OMNI option with S/T-omIndex set to 0.

      *  Otherwise, send a proxy copy of the RS message to each
         additional MSID in the MS-Register list with the MS-Register
         list omitted.  For the first MSID, include the original MS-
         Release list; for all other MSIDs, omit the MS-Release list.

   Each proxy copy of the RS message will include an OMNI option and OAL
   encapsulation header with the ADM-ULA of the AR as the source and the
   ADM-ULA of the Register MSE as the destination.  When the Register
   MSE receives the proxy RS message, if the message includes an MS-
   Release list the MSE sends a uNA message to each additional MSID in
   the Release list with an OMNI option with S/T-omIndex set to 0.  The
   Register MSE then sends an RA message back to the (Proxy) AR wrapped
   in an OAL encapsulation header with source and destination addresses
   reversed, and with RA destination set to the MNP-LLA of the MN.  When
   the AR receives this RA message, it sends a proxy copy of the RA to
   the MN.

   Each uNA message (whether sent by the first-hop AR or by a Register
   MSE) will include an OMNI option and an OAL encapsulation header with
   the ADM-ULA of the Register MSE as the source and the ADM-ULA of the
   Release MSE as the destination.  The uNA informs the Release MSE that
   its previous relationship with the MN has been released and that the
   source of the uNA message is now registered.  The Release MSE must
   then note that the subject MN of the uNA message is now "departed",

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   and forward any subsequent packets destined to the MN to the Register
   MSE.

   Note that it is not an error for the MS-Register/Release lists to
   include duplicate entries.  If duplicates occur within a list, the AR
   will generate multiple proxy RS and/or uNA messages - one for each
   copy of the duplicate entries.

15.3.  DHCPv6-based Prefix Registration

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

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

   When the MSE 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 MSE
   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 MSE generates a DHCPv6 Solicit message on
   behalf of the MN using an IA_PD option with the prefix length set to
   the OMNI header Preflen value and with a Client Identifier formed
   from the OMNI option Node Identification sub-option; otherwise, the
   MSE uses the DHCPv6 Solicit message contained in the OMNI option.
   The MSE then sends the DHCPv6 message to the DHCPv6 Server, which
   delegates MNPs and returns a DHCPv6 Reply message with PD parameters.
   (If the MSE wishes to defer creation of MN 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 MSE packs any state information needed to return
   an RA to the MN in the Relay-forward Interface ID option so that the
   information will be echoed back in the Relay-reply.)

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   When the MSE receives the DHCPv6 Reply, it adds routes to the routing
   system and creates MNP-LLAs based on the delegated MNPs.  The MSE
   then sends an RA back to the MN 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 MSE includes one of the
   (newly-created) MNP-LLAs as the RA destination address and sets the
   OMNI option Preflen accordingly; otherwise, the MSE includes the RS
   source address as the RA destination address.  The MSE then sets the
   RA source address to its own ADM-LLA then performs OAL encapsulation
   and fragmentation and sends the RA to the MN.  When the MN 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 MN 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.

   Note: After a MN performs a DHCPv6-based prefix registration exchange
   with a first MSE, it would need to repeat the exchange with each
   additional MSE it registers with.  In that case, the MN supplies the
   MNP delegation information received from the first MSE when it
   engages the additional MSEs.

16.  Secure Redirection

   If the *NET link model is multiple access, the AR is responsible for
   assuring that address duplication cannot corrupt the neighbor caches
   of other nodes on the link.  When the MN sends an RS message on a
   multiple access *NET link, the AR verifies that the MN is authorized
   to use the address and returns an RA with a non-zero Router Lifetime
   only if the MN is authorized.

   After verifying MN authorization and returning an RA, the AR MAY
   return IPv6 ND Redirect messages to direct MNs located on the same
   *NET link to exchange packets directly without transiting the AR.  In
   that case, the MNs can exchange packets according to their unicast L2
   addresses discovered from the Redirect message instead of using the
   dogleg path through the AR.  In some *NET links, however, such direct
   communications may be undesirable and continued use of the dogleg
   path through the AR may provide better performance.  In that case,
   the AR can refrain from sending Redirects, and/or MNs can ignore
   them.

17.  AR and MSE Resilience

   *NETs SHOULD deploy ARs in Virtual Router Redundancy Protocol (VRRP)
   [RFC5798] configurations so that service continuity is maintained
   even if one or more ARs fail.  Using VRRP, the MN is unaware which of

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   the (redundant) ARs 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.

   MSEs 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 MSE Failures

   In environments where fast recovery from MSE failure is required, ARs
   SHOULD use proactive Neighbor Unreachability Detection (NUD) in a
   manner that parallels Bidirectional Forwarding Detection (BFD)
   [RFC5880] to track MSE reachability.  ARs can then quickly detect and
   react to failures so that cached information is re-established
   through alternate paths.  Proactive NUD control messaging is carried
   only over well-connected ground domain networks (i.e., and not low-
   end *NET links such as aeronautical radios) and can therefore be
   tuned for rapid response.

   ARs perform proactive NUD for MSEs for which there are currently
   active MNs on the *NET.  If an MSE fails, ARs can quickly inform MNs
   of the outage by sending multicast RA messages on the *NET interface.
   The AR sends RA messages to MNs via the *NET interface with an OMNI
   option with a Release ID for the failed MSE, and with destination
   address set to All-Nodes multicast (ff02::1) [RFC4291].

   The AR SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated
   by small delays [RFC4861].  Any MNs on the *NET interface that have
   been using the (now defunct) MSE will receive the RA messages and
   associate with a new MSE.

19.  Transition Considerations

   When a MN connects to an *NET link for the first time, it sends an RS
   message with an OMNI option.  If the first hop AR recognizes the
   option, it returns an RA with its ADM-LLA as the source, the MNP-LLA
   as the destination and with an OMNI option included.  The MN then
   engages the AR according to the OMNI link model specified above.  If
   the first hop AR 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 MN engages the *NET
   according to the legacy IPv6 link model and without the OMNI
   extensions specified in this document.

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   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 MN sends an RS message on a multiple
   access *NET link with an LLA source address and an OMNI option, ARs
   that recognize the option ensure that the MN is authorized to use the
   address and return an RA with a non-zero Router Lifetime only if the
   MN is authorized.  ARs that do not recognize the option instead
   return an RA that makes no statement about the MN's authorization to
   use the source address.  In that case, the MN 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 MN / AR communications is through L2 address mappings
   as discussed in Appendix C.  This arrangement imparts a (virtual)
   point-to-point link model over the (physical) multiple access link.

20.  OMNI Interfaces on Open Internetworks

   OMNI interfaces configured over IPv6-enabled underlying interfaces on
   an open Internetwork without an OMNI-aware first-hop AR receive RA
   messages that do not include an OMNI option, while OMNI interfaces
   configured over IPv4-only underlying interfaces do not receive any
   (IPv6) RA messages at all (although they may receive IPv4 RA messages
   [RFC1256]).  OMNI interfaces that receive RA messages without an OMNI
   option configure addresses, on-link prefixes, etc. on the underlying
   interface that received the RA according to standard IPv6 ND and
   address resolution conventions [RFC4861] [RFC4862].  OMNI interfaces
   configured over IPv4-only underlying interfaces configure IPv4
   address information on the underlying interfaces using mechanisms
   such as DHCPv4 [RFC2131].

   OMNI interfaces configured over underlying interfaces that connect to
   an open Internetwork can apply security services such as VPNs to
   connect to an MSE, or can establish a direct link to an MSE through
   some other means (see Section 4).  In environments where an explicit
   VPN or direct link may be impractical, OMNI interfaces can instead
   use UDP/IP encapsulation while including authentication signatures in
   IPv6 ND messages.

   OMNI interfaces use UDP service port number 8060 (see: Section 25.10
   and Section 3.6 of [I-D.templin-intarea-6706bis]) according to the
   simple UDP/IP encapsulation format specified in [RFC4380] for both
   IPv4 and IPv6 underlying interfaces.  OMNI interfaces do not include
   the UDP/IP header/trailer extensions specified in [RFC4380][RFC6081],
   but may include them as OMNI sub-options instead when necessary.
   Since the OAL includes an integrity check over the OAL packet, OAL
   sources selectively disable UDP checksums for OAL packets that do not

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   require UDP/IP address integrity, but enable UDP checksums for others
   including non-OAL packets, IPv6 ND messages used to establish link-
   layer addresses, etc.  If the OAL source discovers that packets with
   UDP checksums disabled are being dropped in the path it should enable
   UDP checksums in future packets.  Further considerations for UDP
   encapsulation checksums are found in [RFC6935][RFC6936].

   For "Vehicle-to-Infrastructure (V2I)" coordination, the MN includes
   an authentication sub-option in the OMNI option of IPv6 RS/NS
   messages and the MSE responds with an authentication sub-option in an
   OMNI option of an IPv6 RA/NA message.  HIP security services can be
   applied per [RFC7401] using the IPv6 ND messages as simple "shipping
   containers" to convey the HIP parameters.  Alternatively, a simple
   Hashed Message Authentication Code (HMAC) can be included in the
   manner specified in [RFC4380].  For "Vehicle-to-Vehicle (V2V)"
   coordination, two MNs can coordinate directly with one another with
   HIP "Initiator/Responder" messages coded in OMNI options of IPv6 NS/
   NA messages.  In that case, a four-message HIP exchange (i.e., two
   back-to-back NS/NA exchanges) may be necessary for the two MNs to
   attain mutual authentication.

   After establishing a VPN or preparing for UDP/IP encapsulation, OMNI
   interfaces send control plane messages to interface with the MSE,
   including RS/RA messages used according to Section 15 and NS/NA
   messages used for route optimization and mobility (see:
   [I-D.templin-intarea-6706bis]).  The control plane messages must be
   authenticated while data plane messages are delivered the same as for
   ordinary best-effort traffic with basic source address-based data
   origin verification.  Data plane communications via OMNI interfaces
   that connect over open Internetworks without an explicit VPN should
   therefore employ transport- or higher-layer security to ensure
   integrity and/or confidentiality.

   OMNI interfaces configured over open Internetworks are often located
   behind NATs.  The OMNI interface accommodates NAT traversal using
   UDP/IP encapsulation and the mechanisms discussed in
   [I-D.templin-intarea-6706bis].  To support NAT determination, MSEs
   include an Origin Indication sub-option in RA messages sent in
   response to RS messages received from a Client via UDP/IP
   encapsulation.

   Note: Following the initial HIP Initiator/Responder exchange, OMNI
   interfaces configured over open Internetworks maintain HIP
   associations through the transmission of IPv6 ND messages that
   include OMNI options with HIP "Update" and "Notify" messages.  OMNI
   interfaces use the HIP "Update" message when an acknowledgement is
   required, and use the "Notify" message in unacknowledged isolated
   IPv6 ND messages (e.g., unsolicited NAs).  When HMAC authentication

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   is used instead of HIP, the MN and MSE exchange all IPv6 ND messages
   with HMAC signatures included based on a shared-secret.

   Note: ARs that act as proxys on an open Internetwork authenticate and
   remove authentication OMNI sub-options from IPv6 ND messages they
   forward from a MN, and insert and sign authentication Origin
   Indication sub-options in IPv6 ND messages they forward from the
   network to the MN.  Conversely, ARs that act as proxys forward
   without processing any DHCPv6 information in RS/RA message exchanges
   between MNs and MSEs.  The AR is therefore responsible for MN
   authentication while the MSE is responsible for registering/
   delegating MNPs.  Note also that a simpler arrangement is possible
   when the AR also acts as a MSE itself, i.e., when the proxy and MSE
   functions are combined on a single physical or logical platform
   located somewhere in the Internetwork.

   Note: The [RFC4380] HMAC and/or HIP message [RFC7401] authentication
   sub-options appear in the OMNI option, which may occur anywhere
   within the IPv6 ND message body.  When a node that inserts an
   authentication sub-option generates the authentication signature, it
   calculates the signature over the entire length of the IPv6 ND
   message but with the sub-option authentication field itself set to 0.
   The node then writes the resulting signature into the authentication
   field then continues to prepare the message for transmission.  For
   this reason, if an IPv6 ND message includes multiple authentication
   sub-options, the first sub-option is consulted and any additional
   sub-options are ignored.

21.  Time-Varying MNPs

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

   The prefix delegation services discussed in Section 15.3 allows OMNI
   MNs 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 MN 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 MNs with automated network renumbering
   services, however presents limits for the durations of ongoing
   sessions that would prefer to use a constant address.

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22.  (H)HITs and Temporary ULAs

   MNs 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 MN creates
   an RS message it can set the source to the unspecified address (::)
   and destination to All-Routers multicast.  The IPv6 ND message
   includes an OMNI option with a HIP "Initiator" message sub-option,
   and need not include a Node Identification sub-option since the MN's
   HIT appears in the HIP message.  The MN then encapsulates the message
   in an IPv6 header with the (H)HIT as the source address and with
   destination set to either a unicast or anycast ADM-ULA.  The MN then
   sends the message to the MSE as specified in Section 15.1.

   When the MSE receives the message, it notes that the RS source was
   the unspecified address (::), then examines the RS encapsulation
   source address to determine that the source is a (H)HIT and not a
   Temporary ULA.  The MSE next invokes the DHCPv6 protocol to request
   an MNP prefix delegation while using the HIT 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 MSE next includes an OMNI option with a HIP
   "Responder" message and any DHCPv6 prefix delegation parameters.  The
   MSE then 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 MN.

   MNs can also use (H)HITs and/or Temporary ULAs for direct MN-to-MN
   communications outside the context of any OMNI link supporting
   infrastructure.  When two MNs 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.  MNs can
   also inject their (H)HITs and/or Temporary ULAs into a MANET/VANET
   routing protocol to enable multihop communications.  MNs can further
   exchange IPv6 ND messages (such as NS/NA) using their (H)HITs and/or
   Temporary ULAs as source and destination addresses.  Note that the
   HIP security protocols for establishing secure neighbor relationships
   are based on (H)HITs.  IPv6 ND messages that use Temporary ULAs
   instead use the HMAC authentication service specified in [RFC4380].

   Lastly, when MNs 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 MN sends an RS to a MSE 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

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   MNPs.  This document notes the opportunity, but makes no
   recommendation.

23.  Address Selection

   OMNI MNs use LLAs only for link-scoped communications on the OMNI
   link.  Typically, MNs 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.

   OMNI MNs use MNP-ULAs as source/destination IPv6 addresses in the
   encapsulation headers of OAL packets.  OMNI MNs 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.  OMNI MNs use HITs instead of Temporary ULAs when
   operation outside the context of a specific ULA domain and/or source
   address attestation is necessary.

   OMNI MNs 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.

24.  Error Messages

   An OAL destination or intermediate node may need to return ICMPv6
   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, the OAL
   includes the ICMPv6 error message as an OMNI sub-option in an IPv6 ND
   uNA message.  The OAL also includes a HIP message sub-option if the
   uNA needs to travel over an open Internetwork.

25.  IANA Considerations

   The following IANA actions are requested:

25.1.  "IEEE 802 Numbers" Registry

   The IANA is instructed to allocate an official Ether Type number TBD1
   from the 'ieee-802-numbers' registry for User Datagram Protocol (UDP)
   encapsulation on Ethernet networks.  Guidance is found in [RFC7042].

25.2.  "IPv6 Neighbor Discovery Option Formats" Registry

   The IANA is instructed to allocate an official Type number TBD2 from
   the "IPv6 Neighbor Discovery Option Formats" registry for the OMNI

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   option.  Implementations set Type to 253 as an interim value
   [RFC4727].

25.3.  "Ethernet Numbers" Registry

   The IANA is instructed to allocate one Ethernet unicast address TBD3
   (suggested value '00-52-14') in the 'ethernet-numbers' registry under
   "IANA Unicast 48-bit MAC Addresses" as follows:

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

               Figure 32: IANA Unicast 48-bit MAC Addresses

25.4.  "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.  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 33: ICMPv6 Code Fields: Type 2 - Packet Too Big Values

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

25.5.  "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 (future
   assignments are to be made through Standards Action [RFC8126]):

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      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        Pad1                           [RFCXXXX]
      1        PadN                           [RFCXXXX]
      2        Interface Attributes (Type 1)  [RFCXXXX]
      3        Interface Attributes (Type 2)  [RFCXXXX]
      4        Traffic Selector               [RFCXXXX]
      5        MS-Register                    [RFCXXXX]
      6        MS-Release                     [RFCXXXX]
      7        Geo Coordinates                [RFCXXXX]
      8        DHCPv6 Message                 [RFCXXXX]
      9        HIP Message                    [RFCXXXX]
      10       Reassembly Limit               [RFCXXXX]
      11       Fragmentation Report           [RFCXXXX]
      12       Node Identification            [RFCXXXX]
      13-29    Unassigned
      30       Sub-Type Extension             [RFCXXXX]
      31       Reserved by IANA               [RFCXXXX]

                  Figure 34: OMNI Option Sub-Type Values

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

   The OMNI Node Identification Sub-Option (see: Section 12.1.13)
   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 (future assignments
   are to be made through Expert Review [RFC8126]):

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

            Figure 35: OMNI Node Identification ID-Type Values

25.7.  "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

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   Values".  Initial values are given below (future assignments are to
   be made through Expert Review [RFC8126]):

      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 36: OMNI Option Sub-Type Extension Values

25.8.  "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 (future assignments are to be
   made through Expert Review [RFC8126]):

      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 37: OMNI RFC4380 UDP/IP Header Option

25.9.  "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 (future
   assignments are to be made through Expert Review [RFC8126]):

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

                  Figure 38: OMNI RFC6081 Trailer Option

25.10.  Additional Considerations

   The IANA has assigned the UDP port number "8060" for an earlier
   experimental version of AERO [RFC6706].  This document together with
   [I-D.templin-intarea-6706bis] 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).

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

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

   No further IANA actions are required.

26.  Security Considerations

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

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

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

   Strong network layer security for control plane messages and
   forwarding path integrity for data plane messages between MSEs MUST
   be supported.  In one example, the AERO service
   [I-D.templin-intarea-6706bis] constructs a spanning tree between MSEs
   and secures the links in the spanning tree with network layer
   security mechanisms such as IPsec [RFC4301] or Wireguard.  Control
   plane messages are then constrained to travel only over the secured
   spanning tree paths and are therefore protected from attack or
   eavesdropping.  Since data plane messages can travel over route
   optimized paths that do not strictly follow the spanning tree,
   however, end-to-end transport- or higher-layer security services are
   still required.

   Identity-based key verification infrastructure services such as iPSK
   may be necessary for verifying the identities claimed by MNs.  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 6.9.

27.  Implementation Status

   AERO/OMNI Release-3.0.2 was tagged on October 15, 2020, and is
   undergoing internal testing.  Additional internal releases expected
   within the coming months, with first public release expected end of
   1H2021.

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28.  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:
   Stuart Card, Michael Matyas, Robert Moskowitz, Madhu Niraula, Greg
   Saccone, Stephane Tamalet, Eric Vyncke.  Pavel Drasil, Zdenek Jaron
   and Michal Skorepa are especially recognized for their many helpful
   ideas and suggestions.  Madhuri Madhava Badgandi, Sean Dickson, Don
   Dillenburg, Joe Dudkowski, Vijayasarathy Rajagopalan, Ron Sackman and
   Katherine 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 as
   early as Y2K, with insights from colleagues including Brian
   Carpenter, Ralph Droms, Christian Huitema, Thomas Narten, Dave
   Thaler, Joe Touch, and many others who deserve recognition.

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

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   IETF Path MTU Discovery working group) and other DEC colleagues who
   supported these early investigations are also 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.

29.  References

29.1.  Normative References

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

   [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>.

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   [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>.

   [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>.

   [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>.

   [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>.

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   [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>.

29.2.  Informative References

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

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

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

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

   [I-D.ietf-drip-rid]
              Moskowitz, R., Card, S., Wiethuechter, A., and A. Gurtov,
              "UAS Remote ID", draft-ietf-drip-rid-06 (work in
              progress), December 2020.

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

   [I-D.ietf-ipwave-vehicular-networking]
              Jeong, J., "IPv6 Wireless Access in Vehicular Environments
              (IPWAVE): Problem Statement and Use Cases", draft-ietf-
              ipwave-vehicular-networking-19 (work in progress), July
              2020.

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   [I-D.ietf-tsvwg-udp-options]
              Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
              udp-options-09 (work in progress), November 2020.

   [I-D.templin-6man-dhcpv6-ndopt]
              Templin, F., "A Unified Stateful/Stateless Configuration
              Service for IPv6", draft-templin-6man-dhcpv6-ndopt-11
              (work in progress), January 2021.

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

   [I-D.templin-intarea-6706bis]
              Templin, F., "Asymmetric Extended Route Optimization
              (AERO)", draft-templin-intarea-6706bis-87 (work in
              progress), January 2021.

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

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

   [RFC0905]  "ISO Transport Protocol specification ISO DP 8073",
              RFC 905, DOI 10.17487/RFC0905, April 1984,
              <https://www.rfc-editor.org/info/rfc905>.

   [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>.

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

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   [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>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [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>.

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

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

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

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

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

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

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

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

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

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

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

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

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

   [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>.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   [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>.

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

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

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

   [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>.

   [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>.

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

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

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

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

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

Appendix A.  Interface Attribute Preferences Bitmap Encoding

   Adaptation of the OMNI option Interface Attributes Preferences Bitmap
   encoding to specific Internetworks such as the Aeronautical
   Telecommunications Network with Internet Protocol Services (ATN/IPS)
   may include link selection preferences based on other traffic
   classifiers (e.g., transport port numbers, etc.) in addition to the
   existing DSCP-based preferences.  Nodes on specific Internetworks
   maintain a map of traffic classifiers to additional P[*] preference
   fields beyond the first 64.  For example, TCP port 22 maps to P[67],
   TCP port 443 maps to P[70], UDP port 8060 maps to P[76], etc.

   Implementations use Simplex or Indexed encoding formats for P[*]
   encoding in order to encode a given set of traffic classifiers in the
   most efficient way.  Some use cases may be more efficiently coded
   using Simplex form, while others may be more efficient using Indexed.
   Once a format is selected for preparation of a single Interface
   Attribute the same format must be used for the entire Interface
   Attribute sub-option.  Different sub-options may use different
   formats.

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   The following figures show coding examples for various Simplex and
   Indexed formats:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Sub-Type=3|    Sub-length=N   |    omIndex    |    omType     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  | Link  |R| API | Bitmap(0)=0xff|P00|P01|P02|P03|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|P16|P17|P18|P19|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31| Bitmap(1)=0xff|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Bitmap(2)=0xff|P64|P65|P67|P68| ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

               Figure 39: Example 1: Dense Simplex Encoding

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Sub-Type=3|    Sub-length=N   |    omIndex    |    omType     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  | Link  |R| API | Bitmap(0)=0x00| Bitmap(1)=0x0f|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Bitmap(2)=0x00| Bitmap(3)=0x00| Bitmap(4)=0x00| Bitmap(5)=0x00|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Bitmap(6)=0xf0|192|193|194|195|196|197|198|199|200|201|202|203|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |204|205|206|207| Bitmap(7)=0x00| Bitmap(8)=0x0f|272|273|274|275|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |276|277|278|279|280|281|282|283|284|285|286|287| Bitmap(9)=0x00|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Bitmap(10)=0x00| ...
       +-+-+-+-+-+-+-+-+-+-+-

               Figure 40: Example 2: Sparse Simplex Encoding

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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Sub-Type=3|    Sub-length=N   |    omIndex    |    omType     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  | Link  |R| API |  Index = 0x00 | Bitmap = 0x80 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P00|P01|P02|P03|  Index = 0x01 | Bitmap = 0x01 |P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Index = 0x10 | Bitmap = 0x80 |512|513|514|515|  Index = 0x18 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Bitmap = 0x01 |796|797|798|799| ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                  Figure 41: Example 3: Indexed Encoding

Appendix B.  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].

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Appendix C.  MN / AR Isolation Through L2 Address Mapping

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

   To support MN / AR isolation on some *NET links, ARs can maintain an
   OMNI-specific unicast L2 address ("MSADDR").  For Ethernet-compatible
   *NETs, this specification reserves one Ethernet unicast address TBD3
   (see: Section 25).  For non-Ethernet statically-addressed *NETs,
   MSADDR is reserved per the assigned numbers authority for the *NET
   addressing space.  For still other *NETs, MSADDR may be dynamically
   discovered through other means, e.g., L2 beacons.

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

   Therefore, ARs 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 D.  Change Log

   << RFC Editor - remove prior to publication >>

   Differences from earlier versions to draft-templin-6man-omni-00:

   o  Established working baseline reference.

Authors' Addresses

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

   Email: fltemplin@acm.org

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

   Email: tony.whyman@mccallumwhyman.com

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