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Transmission of IP Packets over Overlay Multilink Network (OMNI) Interfaces
draft-templin-6man-omni-21

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Authors Fred Templin , Tony Whyman
Last updated 2021-06-03 (Latest revision 2021-06-02)
Replaces draft-templin-6man-omni-interface
Replaced by draft-templin-intarea-omni
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Document shepherd Eliot Lear
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Send notices to rfc-ise@rfc-editor.org
draft-templin-6man-omni-21
Network Working Group                                    F. Templin, Ed.
Internet-Draft                                        The Boeing Company
Intended status: Informational                                 A. Whyman
Expires: December 5, 2021                MWA Ltd c/o Inmarsat Global Ltd
                                                            June 3, 2021

    Transmission of IP Packets over Overlay Multilink Network (OMNI)
                               Interfaces
                       draft-templin-6man-omni-21

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

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 5, 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
   (https://trustee.ietf.org/license-info) in effect on the date of

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  12
   4.  Overlay Multilink Network (OMNI) Interface Model  . . . . . .  12
   5.  OMNI Interface Maximum Transmission Unit (MTU)  . . . . . . .  18
   6.  The OMNI Adaptation Layer (OAL) . . . . . . . . . . . . . . .  19
     6.1.  OAL Source Encapsulation and Fragmentation  . . . . . . .  19
     6.2.  OAL *NET Encapsulation and Re-Encapsulation . . . . . . .  24
     6.3.  OAL Destination Decapsulation and Reassembly  . . . . . .  26
     6.4.  OAL Header Compression  . . . . . . . . . . . . . . . . .  26
     6.5.  OAL Identification Window Maintenance . . . . . . . . . .  29
     6.6.  OAL Fragment Retransmission . . . . . . . . . . . . . . .  34
     6.7.  OAL MTU Feedback Messaging  . . . . . . . . . . . . . . .  35
     6.8.  OAL Requirements  . . . . . . . . . . . . . . . . . . . .  37
     6.9.  OAL Fragmentation Security Implications . . . . . . . . .  39
     6.10. OAL Super-Packets . . . . . . . . . . . . . . . . . . . .  40
   7.  Frame Format  . . . . . . . . . . . . . . . . . . . . . . . .  41
   8.  Link-Local Addresses (LLAs) . . . . . . . . . . . . . . . . .  42
   9.  Unique-Local Addresses (ULAs) . . . . . . . . . . . . . . . .  43
   10. Global Unicast Addresses (GUAs) . . . . . . . . . . . . . . .  45
   11. Node Identification . . . . . . . . . . . . . . . . . . . . .  45
   12. Address Mapping - Unicast . . . . . . . . . . . . . . . . . .  46
     12.1.  The OMNI Option  . . . . . . . . . . . . . . . . . . . .  47
     12.2.  OMNI Sub-Options . . . . . . . . . . . . . . . . . . . .  48
       12.2.1.  Pad1 . . . . . . . . . . . . . . . . . . . . . . . .  50
       12.2.2.  PadN . . . . . . . . . . . . . . . . . . . . . . . .  51
       12.2.3.  Interface Attributes (Types 1 through 3) . . . . . .  51
       12.2.4.  Interface Attributes (Type 4)  . . . . . . . . . . .  51
       12.2.5.  MS-Register  . . . . . . . . . . . . . . . . . . . .  55
       12.2.6.  MS-Release . . . . . . . . . . . . . . . . . . . . .  55
       12.2.7.  Geo Coordinates  . . . . . . . . . . . . . . . . . .  56
       12.2.8.  Dynamic Host Configuration Protocol for IPv6
                (DHCPv6) Message . . . . . . . . . . . . . . . . . .  57
       12.2.9.  Host Identity Protocol (HIP) Message . . . . . . . .  58
       12.2.10. PIM-SM Message . . . . . . . . . . . . . . . . . . .  59
       12.2.11. Reassembly Limit . . . . . . . . . . . . . . . . . .  60
       12.2.12. Fragmentation Report . . . . . . . . . . . . . . . .  61
       12.2.13. Node Identification  . . . . . . . . . . . . . . . .  62
       12.2.14. ICMPv6 Error . . . . . . . . . . . . . . . . . . . .  64

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       12.2.15. Sub-Type Extension . . . . . . . . . . . . . . . . .  64
   13. Address Mapping - Multicast . . . . . . . . . . . . . . . . .  67
   14. Multilink Conceptual Sending Algorithm  . . . . . . . . . . .  67
     14.1.  Multiple OMNI Interfaces . . . . . . . . . . . . . . . .  68
     14.2.  MN<->AR Traffic Loop Prevention  . . . . . . . . . . . .  69
   15. Router Discovery and Prefix Registration  . . . . . . . . . .  69
     15.1.  Window Synchronization . . . . . . . . . . . . . . . . .  74
     15.2.  Router Discovery in IP Multihop and IPv4-Only Networks .  74
     15.3.  MS-Register and MS-Release List Processing . . . . . . .  76
     15.4.  DHCPv6-based Prefix Registration . . . . . . . . . . . .  78
   16. Secure Redirection  . . . . . . . . . . . . . . . . . . . . .  79
   17. AR and MSE Resilience . . . . . . . . . . . . . . . . . . . .  80
   18. Detecting and Responding to MSE Failures  . . . . . . . . . .  80
   19. Transition Considerations . . . . . . . . . . . . . . . . . .  81
   20. OMNI Interfaces on Open Internetworks . . . . . . . . . . . .  81
   21. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . . . .  84
   22. (H)HITs and Temporary ULAs  . . . . . . . . . . . . . . . . .  84
   23. Address Selection . . . . . . . . . . . . . . . . . . . . . .  85
   24. Error Messages  . . . . . . . . . . . . . . . . . . . . . . .  85
   25. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  86
     25.1.  "IEEE 802 Numbers" Registry  . . . . . . . . . . . . . .  86
     25.2.  "IPv6 Neighbor Discovery Option Formats" Registry  . . .  86
     25.3.  "Ethernet Numbers" Registry  . . . . . . . . . . . . . .  86
     25.4.  "ICMPv6 Code Fields: Type 2 - Packet Too Big" Registry .  86
     25.5.  "OMNI Option Sub-Type Values" (New Registry) . . . . . .  87
     25.6.  "OMNI Geo Coordinates Type Values" (New Registry)  . . .  87
     25.7.  "OMNI Node Identification ID-Type Values" (New Registry)  88
     25.8.  "OMNI Option Sub-Type Extension Values" (New Registry) .  88
     25.9.  "OMNI RFC4380 UDP/IP Header Option" (New Registry) . . .  89
     25.10. "OMNI RFC6081 UDP/IP Trailer Option" (New Registry)  . .  89
     25.11. Additional Considerations  . . . . . . . . . . . . . . .  89
   26. Security Considerations . . . . . . . . . . . . . . . . . . .  90
   27. Implementation Status . . . . . . . . . . . . . . . . . . . .  91
   28. Document Updates  . . . . . . . . . . . . . . . . . . . . . .  91
   29. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  92
   30. References  . . . . . . . . . . . . . . . . . . . . . . . . .  93
     30.1.  Normative References . . . . . . . . . . . . . . . . . .  93
     30.2.  Informative References . . . . . . . . . . . . . . . . .  95
   Appendix A.  OAL Checksum Algorithm . . . . . . . . . . . . . . . 103
   Appendix B.  VDL Mode 2 Considerations  . . . . . . . . . . . . . 104
   Appendix C.  MN / AR Isolation Through L2 Address Mapping . . . . 105
   Appendix D.  Change Log . . . . . . . . . . . . . . . . . . . . . 105
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 109

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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
   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

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   (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.

   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.

   In addition to many other aspects, OMNI supports the "6M's" of modern
   Internetworking including:

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

   2.  Multinet - the ability to span the OMNI link across multiple
       diverse network administrative segments while maintaining
       seamless end-to-end communications between mobile nodes and
       correspondents such as air traffic controllers, fleet
       administrators, etc.

   3.  Mobility - a mobile node's ability to change network points of
       attachment (e.g., moving between wireless base stations) which
       may result in an underlying interface address change, but without

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       disruptions to ongoing communication sessions with peers over the
       OMNI link.

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

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

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

   This document specifies the transmission of IP packets and 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.

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.  Nodes
   that implement IPv6 ND maintain per-neighbor state in Neighbor Cache
   Entries (NCEs).  Each NCE is indexed by the neighbor's Link-Local
   Address (LLA), which must also match the Unique-Local Address (ULA)
   used for encapsulation.

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

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

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

   Mobile Node (MN)
      an end system with a mobile router that has one or more distinct
      upstream data link connections grouped together into 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
      and non-MNP routes are treated exactly the same by the OMNI
      routing system.

   Access Network (ANET)

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

   OMNI Adaptation Layer (OAL)

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

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

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

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

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

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

   OAL intermediate node

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

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

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

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

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

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

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

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

   Multihop

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      an iterative relaying of IP packets between 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.

   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-6man-aero]).

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

   Performance Based Multilink (PBM)
      A means for selecting 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.

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3.  Requirements

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

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

4.  Overlay Multilink Network (OMNI) Interface Model

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

                                     +----------------------------+
                                     |    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

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      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
      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" capable of multinet concatenation.

   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

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   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
   dropped with an ordinary ICMPv6 Packet Too Big (PTB) message
   returned.  These PTB messages are subject to loss [RFC2923] the same
   as for any non-OMNI IP interface.

   The OMNI interface encapsulation/decapsulation layering possibilities
   are shown in Figure 2 below.  Imaginary vertical lines drawn between
   the Network Layer and Underlying interfaces in the figure denote the
   encapsulation/decapsulation layering combinations possible.  Common
   combinations include NULL (i.e., direct access to underlying
   interfaces with or without using the OMNI interface), 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

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      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
      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.

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   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
   communicating endpoints are not subject to mobility dynamics.

   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 unless it has a way to determine the encapsulation
   destination MRU and has carefully considered 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 12.2.11).  The OMNI
   interface employs the OAL as an encapsulation sublayer service to
   transform original IP packets into OAL packets/fragments, and the OAL
   in turn uses *NET encapsulation to forward carrier packets over the
   underlying interfaces (see: Section 6).

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6.  The OMNI Adaptation Layer (OAL)

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

   These carrier packets travel over one or more underlying networks
   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.
   (This process supports the multinet concatenation capability needed
   for joining multiple diverse networks.)  After re-encapsulation by
   zero or more OAL intermediate nodes, the carrier packets arrive at
   the OAL destination.

   When the OAL destination receives the carrier packets, it discards
   the *NET headers and reassembles the resulting OAL fragments into an
   OAL packet as described in Section 6.3.  The OAL destination then
   decapsulates the OAL packet to obtain the original IP packet, which
   it then delivers to the network layer.

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

6.1.  OAL Source Encapsulation and Fragmentation

   When the network layer forwards an original IP packet into the OMNI
   interface, the OAL source inserts an 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] and
   "Congestion Experienced" [RFC3168] values in the original packet's IP
   header into the corresponding fields in the OAL header, then sets the
   OAL header "Flow Label" as specified in [RFC6438].  The OAL source
   finally sets the OAL header IPv6 Hop Limit to a conservative value
   sufficient to enable loop-free forwarding over multiple concatenated

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   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
   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

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   The OAL source next selects a 32-bit Identification value for the
   packet as specified in Section 6.5 then calculates the checksum per
   the 8-bit Fletcher algorithm specified in Appendix A.  The OAL source
   calculates the checksum over the entire OAL packet beginning with a
   pseudo-header of the IPv6 header similar to that found in Section 8.1
   of [RFC8200] and extending to the end of the (0-initialized) checksum
   trailer.  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

   After calculating the checksum, the OAL source writes the results
   over the (0-initialized) trailing checksum octets.  The OAL source
   then inserts a single OMNI Routing Header (ORH) if necessary (see:
   [I-D.templin-6man-aero]) while incrementing Payload Length to reflect
   the addition of the ORH, where the late addition of the ORH is not
   covered by the checksum.  (Alternatively, the OAL source can defer
   ORH insertion until after fragmentation, then manually insert an
   identical copy of the ORH between the IPv6 header and Fragment Header
   of each fragment while resetting the IPv6 Payload Length and Next
   Header fields accordingly.)

   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-

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   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
   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.

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   The OAL source can also actively probe individual OAL destinations to
   discover larger path MPS values using packetization layer probes in a
   similar fashion as [RFC4821][RFC8899], but care must be taken to
   avoid setting static values for dynamically changing paths leading to
   black holes.  The probe involves sending an OAL packet larger than
   the current path MPS and receiving a small acknowledgement 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: 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).  (If
   so, the OAL source should maintain separate path MPS values for each
   (source, target) underlying interface pair for the same OAL
   destination, since each underlying interface pair may support a
   different path MPS.)

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

   For each fragment produced, the OAL source writes an ordinal number
   for the fragment into the Reserved field in the IPv6 Fragment Header.
   In particular, the OAL source writes the ordinal number '0' for the
   first fragment, '1' for the second fragment, '2' for the third
   fragment, etc. up to and including the final fragment.  Since the
   minMPS is 400 and the MTU is 9180, at most 23 fragments will be
   produced for each OAL packet.

   The OAL source finally encapsulates the fragments in *NET headers to
   form carrier packets and forwards them over an underlying interface,
   while retaining the fragments and their ordinal numbers (i.e., #0,
   #1, #2, etc.) for a link persistence period in case link-layer
   retransmission is requested (see: Section 6.6).  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, the OAL source first encapsulates 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 source then appends any
   additional encapsulation sublayer headers necessary and presents the
   *NET packet to an underlying interface (see: 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
   Payload Length (i.e., the UDP length and IPv6 Payload Length must

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   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 (however, see Section 20 for additional
   considerations).  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 over IP, the OAL source next copies the "Type
   of Service/Traffic Class" [RFC2983] and "Congestion Experienced"
   [RFC3168] values in the OAL IPv6 header into the corresponding fields
   in the *NET IP header, then (for IPv6) sets the *NET IPv6 header
   "Flow Label" as specified in [RFC6438].  The OAL source then sets the
   *NET IP TTL/Hop Limit the same as for any *NET host, i.e., it does
   not copy the Hop Limit value from the OAL header.  For carrier
   packets undergoing OAL intermediate node re-encapsulation, the node
   decrements the OAL IPv6 header Hop Limit and discards the carrier
   packet if the value reaches 0.  The node then copies the "Type of
   Service/Traffic Class" and "Congestion Experienced" values from the
   previous hop *NET encapsulation header into the OAL IPv6 header
   before setting the next hop *NET IP encapsulation header values the
   same as specified for the OAL source above.

   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.

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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
   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 accepts or
   drops the fragment based on the (Source, Destination,
   Identification)-tuple.  The OAL destination next drops all non-final
   OAL fragments smaller than the minimum MPS and all fragments that
   would overlap or leave "holes" smaller than the minimum MPS with
   respect to other fragments already received.  The OAL destination
   records the ordinal number of each accepted fragment of the same OAL
   packet (i.e., as Frag #0, Frag #1, Frag #2, etc.) and admits them
   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 the integrity of OAL packets whose fragments may
   have traversed unprotected underlying network hops [CKSUM].  The
   Fletcher checksum 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,
   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 apply OAL Header Compression for
   subsequent carrier packets to significantly reduce encapsulation
   overhead.

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   When the OAL source determines that header compression state has been
   established (i.e., following an IPv6 ND message exchange), it can
   begin sending OAL fragments with significant portions of the IPv6
   header, Fragment Header and OMNI Routing Header (ORH) omitted.  For
   OAL first-fragments (including atomic fragments), the OAL uses OMNI
   Compressed Header - Type 0 (OCH-0) as shown in Figure 7:

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

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

   In this format, the UDP header appears in its entirety in the first 8
   octets, followed by the first 4 octets of the IPv6 header and with
   the remainder omitted.  The Version field is set to 0 (to distinguish
   OCH-0 from both OCH-1 and a true IP protocol version number) and is
   followed by an uncompressed Traffic Class and Flow Label.  This
   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 (M)ore Fragments bit relocated to the least significant
   bit of the first Reserved field.  The Reserved field must be set to 0
   on transmission and ignored on reception, and the other fields are
   set the same as for uncompressed IPv6 fragmentation.  The compressed
   ORH includes a single omIndex octet that encodes an underlying
   interface index for the target Client (or 0 if the target underlying
   interface is unspecified).  The OCH-0 header is then followed by the
   OAL fragment body, and the UDP length field is reduced by the
   difference in length between the compressed headers and full-length
   (IPv6, Fragment, ORH) headers.  The OCH-0 format applies only for
   first fragments, which are always regarded as ordinal fragment 0 even
   though the OCH-0 does not include an explicit Ordinal field.

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

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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ *
     |Vers=1 | Ordinal |R|M|    Fragment Offset      |     ID (0)    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Identification (1-3)              |    omIndex    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             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 Version are omitted.
   The Version field is set to 1 (to distinguish OCH-1 from both OCH-0
   and a true IP protocol version number) and is followed by a
   compressed IPv6 Fragment Header that includes a 5-bit Ordinal number
   for this fragment and with other unnecessary fields compressed or
   omitted.  (R)eserved is set to 0, and (M)ore Fragments/Fragment
   Offset are copied from the uncompressed fragment header.  The
   compressed ORH includes a single omIndex octet that encodes an
   underlying interface index for the target Client (or 0 if the target
   underlying interface is unspecified).  The OCH-1 header is then
   followed by the OAL fragment body, while the UDP length field is
   reduced by the difference in length between the compressed headers
   and full-length (IPv6, Fragment, ORH) headers.  The OCH-1 format
   applies only for non-first fragments, therefore Ordinal is set to a
   value beginning with 1 for the first non-first fragment and
   monotonically incremented for each successive non-first fragment up
   to and including the final fragment.

   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 Version field immediately following the UDP header.  If
   Version 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 the fragment is acceptable.  If Version 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.  Note that, since OCH-1 does not include Traffic
   Class, Flow Label or Next Header information, the OAL destination
   writes the value 0 into those fields when it reconstitutes the full
   headers.  The values will be correctly populated during reassembly

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   after an OAL first fragment with an OCH-0 or uncompressed OAL header
   arrives.

   When the OAL destination is an LHS Proxy/Server, it examines the
   destination address after re-constituting the OAL header.  If the
   destination address is its own ADM-ULA, the Proxy/Server submits the
   resulting OAL fragment for local reassembly.  Following reassembly,
   the Proxy/Server re-encapsulates the OAL packet (while re-fragmenting
   if necessary) and forwards the packet/fragments to the Client
   underlying interface identified by omIndex.  If the destination
   address is the MNP-ULA of one of its Clients, the Proxy/Server
   instead forwards the OAL fragment via the Client underlying interface
   identified by omIndex.  If the header compression state and/or
   destination address are not recognized, the Proxy/Server instead
   drops the packet.

   When the OAL destination is the Client, it examines the destination
   address after re-constituting the OAL header.  If the destination
   address is its own MNP-ULA, the Client submits the resulting OAL
   fragment for local reassembly.  Otherwise, the Client drops the
   packet.

   Note: OAL header compression does not interfere with checksum
   calculation and verification, which must be applied according to the
   full OAL pseudo-header per Section 6.1 even when compression is used.
   Carrier packets may further include uncompressed headers at any time
   even after header compression state has been established.

6.5.  OAL Identification Window Maintenance

   The OAL encapsulates each original IP packet as an OAL packet then
   performs fragmentation to produce one or more carrier packets with
   the same 32-bit Identification value.  In environments were spoofing
   is not considered a threat, OAL nodes can send OAL packets beginning
   with a random initial Identification value and incremented (modulo
   2**32) for each successive packet.  In other environments, OAL nodes
   should maintain explicit per-neighbor send and receive windows to
   exclude spurious carrier packets that might clutter the reassembly
   cache.  OAL neighbors maintain windows using TCP-like synchronization
   [RFC0793] with Identification sequence numbers beginning with an
   unpredictable initial value [RFC7739] and incremented (modulo 2 *32)
   for each successive OAL packet.

   OAL neighbors exchange IPv6 ND messages with OMNI options that
   include TCP-like information fields to manage streams of OAL packets
   instead of streams of octets.  As a link-layer service, the OAL
   provides low-persistence best-effort retransmission with no
   mitigations for duplication, reordering or deterministic delivery.

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   Since the service model is best-effort and only control message
   sequence numbers are acknowledged, OAL nodes can select unpredictable
   new initial sequence numbers outside of the current window without
   delaying for the Maximum Segment Lifetime (MSL).

   OAL neighbors maintain current and previous window state in IPv6 ND
   neighbor cache entries (NCEs) to support dynamic rollover to a new
   window while still sending OAL packets and accepting carrier packets
   from the previous windows.  Each NCE is indexed by the neighbor's
   LLA, which must also match the ULA used for OAL encapsulation.  OAL
   neighbors synchronize windows through asymmetric and/or symmetric
   IPv6 ND message exchanges.  When an OAL neighbor receives an IPv6 ND
   message with new window information, it resets the previous window
   state based on the current window then resets the current window
   based on new and/or pending information.

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

       Send Sequence Variables (current, previous and pending)

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

       Receive Sequence Variables (current and previous)

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

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

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   When OAL B receives the carrier packets containing the NS/RS SYN, it
   creates a NCE in the STALE state if necessary, resets its RCV
   variables, caches the tentative (send) window size M, and selects a
   (receive) window size N (up to 2^24) to indicate the number of OAL
   packets it is willing to accept under the current RCV.WND.  (The
   RCV.WND should be large enough to minimize control message overhead
   yet small enough to provide an effective filter for spurious carrier
   packets.)  OAL B then prepares a solicited NA/RA message with the ACK
   flag set, with the Acknowledgement Number set to OAL A's next
   sequence number, and with Window set to N.  Since OAL B does not
   assert an ISS of its own, it uses OAL A's IRS as the Identification
   for OAL encapsulation then sends the resulting OAL packet to OAL A.

   When OAL A receives the carrier packets containing the solicited NA/
   RA, it notes that their Identification matches its pending ISS.  OAL
   A then sets the NCE state to REACHABLE and resets its SND variables
   based on the Window size and Acknowledgement Number (which must
   include the sequence number following the pending ISS).  OAL A can
   then begin sending OAL packets to OAL B with Identification values
   within the (new) current SND.WND for up to ReachableTime milliseconds
   or until the NCE is updated by a new IPv6 ND message exchange.  This
   implies that OAL A must send a new NS/RS SYN message before sending
   more than N OAL packets within the current SND.WND, i.e., even if
   ReachableTime is not nearing expiration.

   After OAL B returns the solicited NA/RA, it accepts carrier packets
   received from OAL A within either the current or previous RCV.WND as
   well as any new authentic NS/RS SYN messages received from OAL A even
   if outside the windows.  IPv6 ND messages used for window
   synchronization must therefore fit within a single carrier packet
   (i.e., within current MPS constraints), since the carrier packets of
   fragmented IPv6 ND messages with out-of-window Identification values
   could be part of a DoS attack and should not be admitted into the
   reassembly cache.  OAL B discards all other carrier packets received
   from OAL A with out-of-window Identifications.

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

   o  OAL A prepares an NS/RS SYN message with an unpredictable ISS not
      within the current SND.WND and with Window set to M as a tentative
      receive window size.  OAL A caches the new ISS and Window size as
      pending information, uses the pending ISS as the Identification
      for OAL encapsulation, then sends the resulting OAL packet to OAL
      B and waits up to RetransTimer milliseconds to receive a solicited

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      NA/RA ACK response (retransmitting up to MAX_UNICAST_SOLICIT times
      if necessary).

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

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

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

   Following synchronization, OAL A and OAL B hold updated NCEs and can
   exchange OAL packets with Identifications set to SND.NXT while the
   state remains REACHABLE and there is available window capacity.

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   Either neighbor may at any time send a new NS/RS SYN to assert a new
   ISS.  For example, if OAL A's current SND.WND for OAL B is nearing
   exhaustion and/or ReachableTime is nearing expiration, OAL A
   continues to send OAL packets under the current SND.WND while also
   sending an NS/RS SYN with a new unpredictable ISS.  When OAL B
   receives the NS/RS SYN, it resets its RCV variables and may
   optionally return either an asymmetric NA/RA ACK or a symmetric NA/AR
   SYN/ACK to also assert a new ISS.  While sending IPv6 ND SYNs, both
   neighbors continue to send OAL packets with Identifications set to
   the current SND.NXT then reset the SND variables after an
   acknowledgement is received.

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

   OAL nodes may set the PNG ("ping") flag in IPv6 ND advertisement
   messages when a reachability confirmation is needed.  (OAL nodes
   therefore most often set the PNG flag in (unsolicited) advertisement
   messages and ignore it in solicitation messages.)  When an OAL node
   receives a PNG, it returns a solicited NA ACK with the PNG message
   Identification in the Acknowledgment, but without updating RCV state
   variables.  OAL nodes return unicast solicited NA ACKs even for
   multicast PNG destination addresses, since OMNI link multicast is
   based on unicast emulation.  OAL nodes may also send unsolicited NA
   messages to request selective retransmissions (see: Section 12.2.12).

   OAL nodes that employ the window synchronization procedures described
   above observe the following requirements:

   o  OAL nodes MUST select new unpredictable ISS values that are
      outside of the current SND.WND.

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

   o  OAL nodes that receive NA/RA messages with the PNG and/or SYN flag
      set MUST NOT set the PNG and/or SYN flag in solicited NA
      responses.

   o  OAL nodes that send NA/RA messages with the PNG and/or SYN flag
      set MUST ignore solicited NA responses with the PNG and/or SYN
      flag set.

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   o  OAL nodes MUST send ND messages with authentication signatures
      while using unpredictable Identification values until window
      synchronization is complete.

   When an OAL node sends an RS SYN to the All-Routers multicast
   address, it may receive multiple unicast RA ACK or SYN/ACK replies -
   each with a distinct LLA source address.  The OAL node then creates a
   separate NCE for each distinct neighbor and completes window
   synchronization through independent message exchanges with each
   neighbor.  The fact that all neighbors receive the same ISS in the
   original RS SYN is not a matter for concern, as further window
   synchronization will be conducted on a per-neighbor basis.

   Note: Although the OAL employs TCP-like window synchronization and
   supports solicited NA ACK responses to NA/RA SYNs and PNGs, all other
   aspects of the IPv6 ND protocol (e.g., control message exchanges, NCE
   state management, timers, retransmission limits, etc.) are honored
   exactly per [RFC4861].

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

6.6.  OAL Fragment Retransmission

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

   If the OAL destination notices some fragments missing after most
   other fragments within the same link persistence timeframe have
   already arrived, it may send a uNA message to the OAL source.  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) and one or more Fragmentation
   Report sub-options that include a list of (Identification, Bitmap)-
   tuples for fragments received and missing from this OAL source (see:
   Section 12).  The OAL destination 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

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   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 the 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 corresponding carrier packets in its cache and retransmits
   any for which the Bitmap indicates a loss event.  For example, if the
   Bitmap indicates that ordinal fragments #3, #7, #10 and #13 from the
   same OAL packet are missing the OAL source only retransmits carrier
   packets containing those fragments and no others.  When the OAL
   destination receives the retransmitted carrier packets, it admits the
   enclosed fragments into the reassembly cache and updates its
   checklist.  If some fragments are still missing, the OAL destination
   may repeat the request in a small number of additional uNAs within
   the link persistence timeframe.

   Note that the OAL provides a link-layer low persistence Automatic
   Repeat Request (ARQ) service based on Selective Repeat (SR)
   capability consistent with [RFC3366] and Section 8.1 of [RFC3819].
   The service provides the benefit of timely best-effort link-layer
   retransmissions which may reduce packet loss and avoid some
   unnecessary end-to-end delays.

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

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   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.)

   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 the leading portion of the OAL first fragment (beginning
   with the OAL header) in the "OAL First Fragment" field of sub-option,
   signs the message if an authentication sub-option is included,
   performs OAL encapsulation (with the its own address as the OAL
   source and the source address of the message that prompted the uNA as
   the OAL destination) and sends the message to the OAL source.

   When the OAL source receives the uNA message, it records the new
   hard/soft Reassembly Limit values for this OAL destination if the
   OMNI option includes Reassembly Limit sub-options.  If a hard or soft
   Reassembly Limit sub-option includes an OAL First Fragment, the OAL
   source next sends a corresponding network layer PTB hard or soft

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   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 a PTB soft error (subject to rate
   limiting) with Code set to 2 and MTU set to the current soft limit
   while still forwarding the packet to the OMNI destination.

   Original sources that receive PTB soft errors can dynamically tune
   the size of the original IP packets they to send to produce the best
   possible throughput and latency, with the understanding that these
   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 fragment original IP packets smaller than the
      minimum MPS minus the trailer size, but must instead encapsulate
      them as atomic fragments.

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   o  OAL sources MUST produce non-final fragments with payloads no
      smaller than the minimum MPS during 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 any OAL fragments with offset and length that
      would cause the reassembled packet to exceed the OMNI interface
      MRU and/or OAL destination hard Reassembly Limit.

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

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

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

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

   Note: Certain legacy network hardware of the past millennium was
   unable to accept packet "bursts" resulting from an IP fragmentation
   event - even to the point that the hardware would reset itself when
   presented with a burst.  This does not seem to be a common problem in
   the modern era, where fragmentation and reassembly can be readily
   demonstrated at line rate (e.g., using tools such as 'iperf3') even
   over fast links on ordinary hardware platforms.  Even so, the OAL
   source could impose an inter-fragment delay while the OAL destination
   is reporting reassembly congestion (see: Section 6.7) and decrease
   the delay when reassembly congestion subsides.

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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
       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 - in
       environments where spoofing is possible, this threat is mitigated
       through the use of Identification windows per Section 6.5.  By
       maintaining windows of acceptable Identifications beginning with
       unpredictable values, OAL neighbors can quickly discard spurious
       carrier packets that might otherwise clutter the reassembly
       cache.  The OAL additionally provides an integrity check to
       detect corruption that may be caused by spurious fragments
       received with in-window Identification values.

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

   IPv4 includes a 16-bit Identification (IP ID) field with only 65535
   unique values such that at high data rates the field could wrap and
   apply to new carrier packets while the fragments of old packets using
   the same IP ID are still alive in the network [RFC4963].  Since
   carrier packets sent via an IPv4 path with DF=0 are normally no
   larger than 576 bytes, IPv4 fragmentation is possible only at small-
   MTU links in the path which should support data rates low enough for
   safe reassembly [RFC3819].  (IPv4 carrier packets larger than 576

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   bytes with DF=0 may incur high data rate reassembly errors in the
   path, with the OAL destination checksum providing a last-resort
   integrity verification.)  Since IPv6 provides a 32-bit Identification
   value, IP ID wraparound at high data rates is not a concern for IPv6
   fragmentation.

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

   Unless the path is secured at the network-layer or below (i.e., in
   environments where spoofing is possible), OMNI interfaces MUST NOT
   send ordinary carrier packets with Identification values outside the
   current window and MUST secure IPv6 ND messages used for address
   resolution or window state synchronization.  OAL destinations SHOULD
   therefore discard without reassembling any out-of-window OAL
   fragments received over an unsecured path.

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 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
   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

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   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
   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.

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

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   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
      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.

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   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.

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

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   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
   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-6man-aero].

   Temporary ULAs are constructed per [RFC8981] based on the prefix
   [ULA]:ffff::/64 and used by MNs when they have no other addresses.

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   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/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
   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 12.2.13).  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 an alternative more appropriate for
   certain domains such as the Unmanned (Air) Traffic Management (UTM)
   service for Unmanned Air Systems (UAS).  Another alternative is the

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   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 12.2.13) 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.
   IPv6 Neighbor Discovery (ND) [RFC4861] messages sent over OMNI
   interfaces without encapsulation observe the native underlying
   interface Source/Target Link-Layer Address Option (S/TLLAO) format
   (e.g., for Ethernet the S/TLLAO is specified in [RFC2464]).  IPv6 ND
   messages sent over OMNI interfaces using encapsulation do not include
   S/TLLAOs, but instead include a new option type that encodes
   encapsulation addresses, interface attributes and other OMNI link
   information.  (Note that OMNI interface IPv6 ND messages sent without
   encapsulation may include both OMNI options and S/TLLAOs, but the
   information conveyed in each is mutually exclusive.)  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
   manage their dynamically-changing multilink profiles by including
   OMNI options in IPv6 ND messages as discussed in the following
   subsections.

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12.1.  The OMNI Option

   The first OMNI option appearing in an IPv6 ND message is formatted as
   shown in Figure 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  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Sequence Number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Acknowledgment Number                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S|A|R|O|P|     |                                               |
       |Y|C|S|P|N| Res |                   Window                      |
       |N|K|T|T|G|     |                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                          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
      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 that includes an omIndex value for
      the source or target underlying interface for this IPv6 ND
      message.  MN OMNI interfaces MUST number each distinct underlying

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      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 value '0'
      denotes either an AR's INET interface or "unspecified".

   o  The remaining header fields before the Sub-Options begin are
      modeled from the Transmission Control Protocol (TCP) header
      specified in Section 3.1 of [RFC0793] and include a 32 bit
      Sequence Number followed by a 32 bit Acknowledgement Number
      followed by 8 flags bits followed by a 24-bit Window.  The (SYN,
      ACK, RST) flags are used when TCP-like window synchronization is
      used, while the TCP (URG, PSH, FIN) flags are never used and
      therefore omitted.  The (OPT, PNG) flags are OMNI-specific, and
      the remaining flags are Reserved.  Together, these fields support
      the asymmetric and symmetric OAL window synchronization services
      specified in Section 6.5.

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

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

         0                   1                   2
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
        |     Type      |     Length    | Sub-Options ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The OMNI interface processes the Sub-Options of all OMNI option
   instances received in the same IPv6 ND message in the consecutive
   order in which they appear.  The OMNI option(s) included in each IPv6
   ND message may include full or partial information for the neighbor.
   The union of the information in the most recently received OMNI
   options is therefore retained and aged/removed in conjunction with
   the corresponding NCE.

12.2.  OMNI Sub-Options

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

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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
        Interface Attributes (Type 4)  4
        MS-Register                    5
        MS-Release                     6
        Geo Coordinates                7
        DHCPv6 Message                 8
        HIP Message                    9
        PIM-SM Message                10
        Reassembly Limit              11
        Fragmentation Report          12
        Node Identification           13
        ICMPv6 Error                  14
        Sub-Type Extension            30

                                 Figure 12

      Sub-Types 14-29 are available for future assignment for major
      protocol functions.  Sub-Type 31 is reserved by IANA.

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

   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 2040 octets minus the length of the OMNI
   option header octets preceding the Sub-Options.  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.  The
   OMNI interface codes initial sub-options in a first OMNI option

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

   During reception, the OMNI interface processes the OMNI option Sub-
   Options while skipping over and ignoring any unrecognized sub-
   options.  The OMNI interface processes the Sub-Options of all OMNI
   option instances in the consecutive order in which they appear in the
   IPv6 ND message, beginning with the first instance and continuing
   through any additional instances to the end of the message.  If an
   individual sub-option length would cause processing to exceed the
   OMNI option 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 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:

12.2.1.  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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12.2.2.  PadN

         0                   1                   2
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
        | S-Type=1|    Sub-length=N     | N padding octets ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                              Figure 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 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.

12.2.3.  Interface Attributes (Types 1 through 3)

   Interface Attributes (Type 1) and (Type 2) were defined in
   [I-D.templin-6man-omni-interface] and have been moved to historic
   status.  Their sub-option types (2 and 3) are reserved for future
   use.

   Interface Attributes (Type 3) was never defined; the number was
   skipped to bring (Type 4) into agreement with the corresponding sub-
   option Type value.

12.2.4.  Interface Attributes (Type 4)

   The Interface Attributes (Type 4) 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
   traffic selectors and link quality.  Interface Attributes (Type 4)
   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 4) must be honored by all implementations.
   Throughout the remainder of this specification, when the term
   "Interface Attributes" appears without a "Type" designation the below
   format is indicated:

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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=4|    Sub-length=N     |    omIndex    |   TS Format   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     omType    |  Provider ID  | Link  | Resvd | FMT |   SRT   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              LHS                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                                                               ~
       ~                   Link Layer Address (L2ADDR)                 ~
       ~                                                               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                                                               ~
       ~                RFC 6088 Format Traffic Selector               ~
       ~                                                               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 15: Interface Attributes (Type 4)

   o  Sub-Type is set to 4.  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 Traffic Selectors of all are processed while the
      remaining information is processed only for the first instance and
      ignored in all other instances.

   o  Sub-Length is set to N that encodes the number of Sub-Option Data
      octets that follow.  All fields beginning with omIndex up to and
      including TS Format are always present, while the 'A' and 'T'
      flags determine the remaining Sub-Option Data format.

   o  Sub-Option Data contains an "Interface Attributes (Type 4)" option
      encoded as follows:

      *  omIndex is a 1-octet 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 the presence of a NAT is
         indicated.

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      *  TS Format is a 1-octet field that encodes a Traffic Selector
         version per [RFC6088] when T is 1.  If TS Format encodes the
         value 1, the Traffic Selector includes IPv4 information.  If it
         encodes the value 2, the Traffic Selector includes IPv6
         information.  If it encodes the value 0, the Traffic Selector
         field is omitted.

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

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

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

      *  Resvd is 4-bit field reserved for future use, set to 0 on
         transmit and ignored on receipt.

      *  The following address-related fields appear next in consecutive
         order:

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

            -  When the most significant bit (i.e., "FMT-Forward") is
               clear, the Proxy/Server must reassemble.  When the bit is
               set, the Proxy/Server must forward the fragments to the
               Client (while changing the OAL destination address)
               without reassembling.

            -  When the next most significant bit (i.e., "FMT-Mode") is
               clear, L2ADDR is the address of the Proxy/Server and the
               Client must be reached through the Proxy/Server.  When
               the bit is set, the Client can be reached on the open
               *NET where it may be located behind one or more NATs and
               L2ADDR is either the address of the Proxy/Server (when
               FMT-Forward is set) or the native INET address of the
               Client itself (when FMT-Forward is clear).

            -  The least significant bit (i.e., "FMT-Type") determines
               the IP address version encoded in L2ADDR.  If FMT-Type is
               clear, L2ADDR includes a 4-octet IPv4 address.  If FMT-
               Type is set, L2ADDR includes a 16-octet IPv6 address.

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

         +  LHS - 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-6man-aero] for further discussion.

         +  Link Layer Address (L2ADDR) - identifies the link-layer
            address (i.e., the encapsulation address) of the source/
            target according to FMT.  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.

      *  When TS Format is non-zero, the remainder of the sub-option
         includes a traffic selector formatted per [RFC6088] beginning
         with the "Flags (A-N)" field, and with the Traffic Selector IP
         protocol version coded in the TS Format field.  Note that each
         Interface Attributes sub-option includes at most one IPv4 or
         IPv6 Traffic Selector block.  If a single interface identified
         by omIndex requires traffic selectors for multiple IP protocol
         versions, or if a traffic selector block would exceed the space
         available in a single Interface Attributes sub-option, the
         remaining information is coded in additional sub-options all
         having the same omIndex in 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=4|    Sub-length=N     |    omIndex    |   TS Format   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                                                               ~
       ~                RFC 6088 Format Traffic Selector               ~
       ~                                                               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

12.2.5.  MS-Register

        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 16: 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 n representing the number of MSIDs
      included.

   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.

12.2.6.  MS-Release

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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=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 17: MS-Release Sub-option

   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 n representing the number of MSIDs
      included.

   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.

12.2.7.  Geo Coordinates

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | S-Type=7|    Sub-length=N     |    Geo Type   |Geo Coordinates
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

                   Figure 18: Geo Coordinates Sub-option

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

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

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

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

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

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

   The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) sub-option
   may be included in the OMNI options of 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 messages do not include a Checksum field, but integrity
   is protected by the IPv6 ND message Checksum.

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

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

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

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12.2.9.  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 should be included in any OMNI IPv6 ND
   message that traverses an open Internetwork, i.e., where link-layer
   authentication is not already assured by lower layers.

        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|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Reserved            |           Controls            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                Sender's Host Identity Tag (HIT)               |
       |                                                               |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               Receiver's Host Identity Tag (HIT)              |
       |                                                               |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       /                        HIP Parameters                         /
       /                                                               /
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 20: 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 limited by the remaining available space for
      this OMNI option.

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   o  The HIP message is coded per Section 5 of [RFC7401], except that
      the OMNI "Sub-Type" and "Sub-Length" fields replace the first 2
      octets of the HIP message header (i.e., the Next Header and Header
      Length fields).  Also, since the IPv6 ND message header already
      includes a Checksum the HIP message Checksum field is replaced by
      a Reserved field set to 0 on transmission and ignored on
      reception.

12.2.10.  PIM-SM Message

   The Protocol Independent Multicast - Sparse Mode (PIM-SM) Message
   sub-option may be included in the OMNI options of IPv6 ND messages
   sent by MNs and MSEs.  PIM-SM messages are formatted as specified in
   Section 4.9 of [RFC7761], with the exception that the Checksum field
   is replaced by a Reserved field (set to 0) since the IPv6 ND message
   header already includes a Checksum.  The PIM-SM message sub-option
   format is shown in Figure 21:

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

                  Figure 21: PIM-SM Message Option Format

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

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

   o  The PIM-SM message is coded exactly as specified in Section 4.9 of
      [RFC7761], except that the Checksum field is replaced by a
      Reserved field set to 0 on transmission and ignored on reception.
      The "PIM Ver" field MUST encode the value 2, and the "Type" field
      encodes the PIM message type.  (See Section 4.9 of [RFC7761] for a
      list of PIM-SM message types and formats.)

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12.2.11.  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
   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=11|    Sub-length=N     |      Reassembly Limit       |H|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          OAL First Fragment (As much of invoking packet       |
       +         as possible without causing the IPv6 ND message       +
       |                to exceed the minimum IPv6 MTU)                |
       +                                                               +

                        Figure 22: Reassembly Limit

   o  Sub-Type is set to 11.  If multiple instances appear in OMNI
      options of the same message the first occurring "hard" and "soft"
      Reassembly Limit values are accepted, and any additional
      Reassembly Limit values are ignored.

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

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

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

   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

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      the fragment payload as possible without causing the IPv6 ND
      message to exceed the minimum IPv6 MTU.

12.2.12.  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=12|   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 23: Fragmentation Report

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

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

   o  Bitmap (i) includes an ordinal checklist of fragments, with each
      bit set to 1 for a fragment received or 0 for a fragment missing.
      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 24

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

12.2.13.  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=13|    Sub-length=N    |     ID-Type    |               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               ~
       ~            Node Identification Value (N-1 octets)             ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 25: Node Identification

   o  Sub-Type is set to 13.  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 that encodes the number of Sub-Option Data
      octets that follow.  The ID-Type field is always present; hence,
      the maximum Node Identification Value length is limited by the
      remaining available space in this OMNI option.

   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 used 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 26:

        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 26: DUID-EN for OMNI Format

   In this format, the ID-Type and Node Identification Value fields are
   coded exactly as in Figure 25 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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12.2.14.  ICMPv6 Error

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |S-Type=14|    Sub-length=N    |                                ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-                                ~
       ~                    RFC4443 Error Message Body                 ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 27: ICMPv6 Error

   o  Sub-Type is set to 14.  If multiple instances appear in OMNI
      options of the same IPv6 ND message all are processed

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

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

12.2.15.  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 28:

        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 28: Sub-Type Extension

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

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

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

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

   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:

12.2.15.1.  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 29: RFC4380 UDP/IP Header Option (Extension-Type 0)

   o  Sub-Type is set to 30.

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

   o  Extension-Type is set to 0.  Each instance encodes exactly one
      header option per Section 5.1.1 of [RFC4380], with 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.

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   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.

12.2.15.2.  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 30: RFC6081 UDP/IP Trailer Option (Extension-Type 1)

   o  Sub-Type is set to 30.

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

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

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

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      *  0 - Unassigned

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

      *  2 - Unassigned

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

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

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

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

   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].

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

14.  Multilink Conceptual Sending Algorithm

   The 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

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   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: Section 12.2.4).  Underlying interface
   selection for the node's own local interfaces are based on traffic
   selectors, cost, performance, message size, etc.  Both node-local and
   neighbor underlying interface traffic selectors may also be
   configured to indicate replication 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
   nearest AR is located multiple hops away as discussed in
   Section 15.2.  (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

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   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
   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.

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   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
   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 Identification value (see: 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 NCE 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

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   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.

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

   The AR prepares the RA using OAL encapsulation/fragmentation with an
   Identification value selected per Section 6.5, 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 by sending one
   or more "aggregate" RAs 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.  Note also that
   the AR must return any MSE RA messages that set window
   synchronization flags directly to the MN, i.e., and without including
   them in an aggregate.

   When the MN receives the RA message, it creates an OMNI interface NCE
   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

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   additional L2 AR addresses in each MSID NCE (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
      isolated unsolicited NAs when reliability is not thought to be a
      concern (e.g., if redundant transmissions are sent on multiple
      underlying interfaces), or may instead set the SYN flag in the
      OMNI header to trigger a reliable solicited NA reply.

   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.

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   The IPv6 layer sees the OMNI interface as an ordinary IPv6 interface.
   Therefore, when the IPv6 layer sends an RS message the OMNI interface
   returns an internally-generated RA message as though the message
   originated from an IPv6 router.  The internally-generated RA message
   contains configuration information 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 or independently of 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
   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.

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15.1.  Window Synchronization

   In environments where Identification window synchronization is
   necessary, the RS/RA exchanges discussed above observe the procedures
   specified in Section 6.5.  In the asymmetric case, the initial RS/RA
   exchange establishes only the MN's send window and AR/MSE's receive
   window such that a corresponding NS/NA exchange would be needed in
   the reverse direction.  In the symmetric case, the MN returns an
   explicit/implicit acknowledgement response to the RA to symmetrically
   establish the send/receive windows of both parties.

   The initial RS/RA exchange between a MN and an MSE over a first
   underlying interface must invoke window synchronization, while
   subsequent RS/RA exchanges performed over additional underlying
   interfaces within ReachableTime and with in-window Identification
   values need not also invoke window synchronization.  Following the
   initial exchange, future window (re)synchronizations can occur over
   any underlying interface, i.e., and not necessarily only over the one
   used for the initial exchange.

   When a MN sends an RS SYN that includes an OMNI MS-Register sub-
   option with multiple MSIDs, it may receive multiple RA SYN/ACKs -
   each with their own synchronization parameters.  The resulting
   "multi-three-way" handshake would require the MN to establish
   separate NCE SND/RCV state and return explicit/implicit
   acknowledgements for each responding MSE.

15.2.  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

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   *NETs, the MN then encapsulates the message in UDP/IPv6 headers with
   source address set to the underlying interface address (or to the ULA
   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 a *NET node within communications
   range, which forwards the message according to its routing tables to
   an intermediate node.  The multihop forwarding process within the
   *NET continues repetitively until the message is delivered to the
   original 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

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   considering the infrequent nature and small size of IPv6 ND messages.
   Future documents may consider encapsulation avoidance through
   translation while updating [RFC4861].

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

   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].

15.3.  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: 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.

   Note: the MN is responsible for honoring the window synchronization
   protocol for each responding MSE when it sends a single RS message
   with synchronization parameters and an MS-Register option with
   multiple MSIDs.  Each responding MSE will cache identical RCV state
   information based on the single RS message, then responds with its
   own unique SND parameters.

15.4.  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

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   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.)

   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,

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   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
   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.

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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.

   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

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   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 25.11
   and Section 3.6 of [I-D.templin-6man-aero]) 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 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 MN-to-MSE (e.g., "Vehicle-to-Infrastructure (V2I)") neighbor
   exchanges, the source must include an OMNI option with an
   authentication sub-option in all IPv6 ND messages.  The source can
   apply HIP security services per [RFC7401] using the IPv6 ND message
   OMNI option as a "shipping container" to convey an authentication
   signature in a (unidirectional) HIP "Notify" message.  For MN-to-MN
   (e.g., "Vehicle-to-Vehicle (V2V)") neighbor exchanges, two MNs can
   exchange HIP "Initiator/Responder" messages coded in OMNI options of
   multiple IPv6 NS/NA messages for mutual authentication according to
   the HIP protocol.  (Note: a simple Hashed Message Authentication Code
   (HMAC) such as specified in [RFC4380] can be used as an alternate
   authentication service in some environments.)

   When HIP authentication is used, the IPv6 ND message source should
   include an OMNI option with a HIP message containing a valid
   authentication signature.  When the source prepares the HIP message,
   it includes its own (H)HIT as the Sender's HIT and the neighbor's
   (H)HIT if known as the Receiver's HIT (otherwise 0).  Before
   calculating the HIP signature, the source sets both the ICMPv6
   Checksum field and HIP signature fields to 0.  The source then
   calculates the HIP authentication signature over the full length of
   the IPv6 ND message beginning with the ICMPv6 message header and
   extending over all included IPv6 ND message options including the
   OMNI option itself.  The source next writes the authentication
   signature into the HIP signature field, then calculates the ICMPv6
   message checksum and writes the value into the ICMPv6 Checksum field.

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   After establishing a VPN or preparing for UDP/IP encapsulation, OMNI
   interfaces send RS/RA messages for MN/MSE coordination (see:
   Section 15) and NS/NA messages for route optimization and mobility
   management (see: [I-D.templin-6man-aero]).  These control plane
   messages must be authenticated while data plane messages are
   delivered the same as for ordinary best-effort traffic with source
   address and/or Identification window-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-6man-aero].  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 IPv6 ND message exchange, OMNI interfaces
   configured over open Internetworks maintain neighbor relationships by
   transmitting periodic IPv6 ND messages with OMNI options that include
   HIP "Update" and/or "Notify" messages.  When HMAC authentication is
   used instead of HIP, the MN and MSE exchange all IPv6 ND messages
   with HMAC signatures included based on a shared-secret.

   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.

   Note: OMNI interfaces on open Internetworks should employ the
   Identification window synchronization mechanisms specified in
   Section 6.5 in order to reject spurious carrier packets that might
   otherwise clutter the reassembly cache.  This is especially important
   in environments where carrier packet spoofing is a threat.

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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 15.4 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, but may present limits for the durations of ongoing
   sessions that would prefer to use a constant address.

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 link-scoped All-Routers multicast.  The IPv6 ND
   message includes an OMNI option with a HIP message sub-option, and
   need not include a Node Identification sub-option 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 15.2.

   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
   message sub-option 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.

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   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
   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-like error messages (e.g., Destination Unreachable, Packet Too
   Big, Time Exceeded, etc.)  [RFC4443] to an OAL source.  Since ICMPv6

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   error messages do not themselves include authentication codes, the
   OAL includes the error message as an OMNI ICMPv6 Error sub-option in
   an IPv6 ND uNA message.  The OAL also includes a HIP message sub-
   option if the uNA requires an authentication signature.

25.  IANA Considerations

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

25.1.  "IEEE 802 Numbers" Registry

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

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
   option (registration procedure is RFC required).  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" (registration procedure is Expert
   Review).  The registration should appear as follows:

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

               Figure 31: 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 (registration
   procedure is Standards Action or IESG Approval).  The registry should
   appear as follows:

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

       Figure 32: 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
   (registration procedure is RFC required):

      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        Pad1                           [RFCXXXX]
      1        PadN                           [RFCXXXX]
      2        Interface Attributes (Type 1)  [RFCXXXX]
      3        Interface Attributes (Type 2)  [RFCXXXX]
      4        Interface Attributes (Type 4)  [RFCXXXX]
      5        MS-Register                    [RFCXXXX]
      6        MS-Release                     [RFCXXXX]
      7        Geo Coordinates                [RFCXXXX]
      8        DHCPv6 Message                 [RFCXXXX]
      9        HIP Message                    [RFCXXXX]
      11       PIM-SM Message                 [RFCXXXX]
      11       Reassembly Limit               [RFCXXXX]
      12       Fragmentation Report           [RFCXXXX]
      13       Node Identification            [RFCXXXX]
      14       ICMPv6 Error                   [RFCXXXX]
      15-29    Unassigned
      30       Sub-Type Extension             [RFCXXXX]
      31       Reserved by IANA               [RFCXXXX]

                  Figure 33: OMNI Option Sub-Type Values

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

   The OMNI Geo Coordinates sub-option (see: Section 12.2.7) contains an
   8-bit Type field, for which IANA is instructed to create and maintain
   a new registry entitled "OMNI Geo Coordinates Type Values".  Initial
   values are given below (registration procedure is RFC required):

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      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        NULL                           [RFCXXXX]
      255      Reserved by IANA               [RFCXXXX]

                   Figure 34: OMNI Geo Coordinates Type

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

   The OMNI Node Identification sub-option (see: Section 12.2.13)
   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 (registration
   procedure is RFC required):

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

            Figure 35: OMNI Node Identification ID-Type Values

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

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

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

             Figure 36: OMNI Option Sub-Type Extension Values

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

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

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

               Figure 37: OMNI RFC4380 UDP/IP Header Option

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

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

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

                  Figure 38: OMNI RFC6081 Trailer Option

25.11.  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-6man-aero] reclaims the UDP port number "8060" for
   'aero' as the service port for UDP/IP encapsulation.  (Note that,

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   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
   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-6man-aero] 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

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   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.  Additionally, the OAL Identification value provides
   a first level of data origin authentication that mitigates off-path
   spoofing.

   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.  In environments where spoofing is
   considered a threat, OMNI nodes SHOULD employ Identification window
   synchronization and OAL destinations SHOULD configure an (end-system-
   based) firewall.

27.  Implementation Status

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

28.  Document Updates

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

   o  [RFC1191]

   o  [RFC4443]

   o  [RFC8201]

   o  [RFC7526]

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

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29.  Acknowledgements

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

   The following individuals are acknowledged for their useful comments:
   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.

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

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

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

   This work is aligned with the Boeing Information Technology (BIT)
   Mobility Vision Lab (MVL) program.

30.  References

30.1.  Normative References

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

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

   [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>.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

30.2.  Informative References

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

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

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

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

   [I-D.ietf-drip-rid]
              Moskowitz, R., Card, S. W., Wiethuechter, A., and A.
              Gurtov, "UAS Remote ID", draft-ietf-drip-rid-07 (work in
              progress), January 2021.

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

   [I-D.ietf-ipwave-vehicular-networking]
              (editor), J. (. J., "IPv6 Wireless Access in Vehicular
              Environments (IPWAVE): Problem Statement and Use Cases",
              draft-ietf-ipwave-vehicular-networking-20 (work in
              progress), March 2021.

   [I-D.ietf-tsvwg-udp-options]
              Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
              udp-options-12 (work in progress), May 2021.

   [I-D.templin-6man-aero]
              Templin, F. L., "Automatic Extended Route Optimization
              (AERO)", draft-templin-6man-aero-01 (work in progress),
              April 2021.

   [I-D.templin-6man-dhcpv6-ndopt]
              Templin, F. L., "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. L., "The IPv6 Link-Local Address Type Field",
              draft-templin-6man-lla-type-02 (work in progress),
              November 2020.

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

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

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

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

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

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

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

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

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

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

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

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

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

   [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>.

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   [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>.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   [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>.

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   [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>.

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

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

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

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

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

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

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

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

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

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

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

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

   [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>.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Appendix A.  OAL Checksum Algorithm

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

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

         A := A + D[i]

         B := B + A

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

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

Appendix B.  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 draft-templin-6man-omni-20 to draft-templin-6man-
   omni-21:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Differences from draft-templin-6man-omni-19 to draft-templin-6man-
   omni-20:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

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   Differences from draft-templin-6man-omni-18 to draft-templin-6man-
   omni-19:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Differences from draft-templin-6man-omni-17 to draft-templin-6man-
   omni-18:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Differences from draft-templin-6man-omni-16 to draft-templin-6man-
   omni-17:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Differences from draft-templin-6man-omni-15 to draft-templin-6man-
   omni-16:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval.

   Differences from draft-templin-6man-omni-14 to draft-templin-6man-
   omni-15:

   o  Text restructuring to remove ambiguities, eliminate extraneous
      text and improve readability.

   o  Clarified that the OMNI link model is NBMA and that link-scoped
      multicast is through iterative unicast.

   Differences from draft-templin-6man-omni-13 to draft-templin-6man-
   omni-14:

   o  Brought back the optional two-message exchange feature.

   o  Added TCP RST flag and new (OPT, PNG) flags to the OMNI option
      header.

   o  Require the OAL node that initiates the symmetric connection to
      include its (future) receive window size in the initial SYN.

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   o  Require OAL nodes to select new ISS values that are outside of the
      current SND.WND.

   o  Text clarifications for improved readability.

   Differences from draft-templin-6man-omni-12 to draft-templin-6man-
   omni-13:

   o  Complete revision of OAL Identification Window Maintenance section
      to incorporate well-known protocol conventions and terminology.

   Differences from draft-templin-6man-omni-11 to draft-templin-6man-
   omni-12:

   o  Expanded on details of symmetric window synchronization.

   Differences from draft-templin-6man-omni-10 to draft-templin-6man-
   omni-11:

   o  Included an Ordinal Number field in the Compressed Header format
      for non-final fragments

   o  Clarified that the window coordination protocol is based on the
      IPv6 ND connectionless protocol using TCP constructs, and not
      based on the TCP connection-oriented protocol.

   o  Removed unneeded fields from the OMNI option header.

   Differences from draft-templin-6man-omni-09 to draft-templin-6man-
   omni-10:

   o  Fixed sizing considerations for OMNI option fields.

   o  Updated handling of multiple OMNI options in the same IPv6 ND
      message.  Only the first option includes the header, while all
      other options include only sub-options.

   Differences from draft-templin-6man-omni-08 to draft-templin-6man-
   omni-09:

   o  Included reference to RFC3366 and updated section on Fragment
      Retransmission.

   o  Added "ordinal number" marking in Fragment Header reserved field.

   Differences from draft-templin-6man-omni-07 to draft-templin-6man-
   omni-08:

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   o  Included TCP state variables; window scale

   Differences from draft-templin-6man-omni-06 to draft-templin-6man-
   omni-07:

   o  Moved Interface Attributes, Type 1 and Type 2 to historic status.

   o  Incorporated Traffic Selector into Interface Attributes, Type 4.

   Differences from draft-templin-6man-omni-05 to draft-templin-6man-
   omni-06:

   o  Adopted TCP as an OAL packet-based connection-oriented protocol.

   o  Three-Way handshake for establishing symmetric send/receive
      windows

   o  Window length specified, plus "current" and "previous" windows

   o  New appendix on checksum algorithm, with citations changed

   o  Security architecture considerations.

   o  More details on HIP message signatures.

   o  Require firewalls at OAL destinations.

   o  Removed "equal-length" requirement for OAL non-final fragments.

   Differences from draft-templin-6man-omni-04 to draft-templin-6man-
   omni-05:

   o  Change to S/T-omIndex definition.

   Differences from draft-templin-6man-omni-03 to draft-templin-6man-
   omni-04:

   o  Changed reference citations to "draft-templin-6man-aero".

   o  Included introductory description of the "6M's".

   o  Included new OMNI sub-option for PIM-SM.

   Differences from draft-templin-6man-omni-02 to draft-templin-6man-
   omni-03:

   o  Added citation of RFC8726.

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   Differences from draft-templin-6man-omni-01 to draft-templin-6man-
   omni-02:

   o  Updated IANA registration policies for OMNI registries.

   Differences from draft-templin-6man-omni-00 to draft-templin-6man-
   omni-01:

   o  Changed intended document status to Informational, and removed
      documents from "updates" category.

   o  Updated implementation status.

   o  Minor edits to HIP message specifications.

   o  Clarified OAL and *NET IP header field settings during
      encapsulation and re-encapsulation.

   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

   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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