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Network Working Group                                    F. Templin, Ed.
Internet-Draft                                        The Boeing Company
Updates: rfc1191, rfc4443, rfc8201 (if                         A. Whyman
         approved)                       MWA Ltd c/o Inmarsat Global Ltd
Intended status: Standards Track                        October 22, 2020
Expires: April 25, 2021


    Transmission of IP Packets over Overlay Multilink Network (OMNI)
                               Interfaces
                  draft-templin-6man-omni-interface-50

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 therefore needed
   for coordination with the network-based mobility service.  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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

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

Copyright Notice

   Copyright (c) 2020 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Overlay Multilink Network (OMNI) Interface Model  . . . . . .   8
   5.  The OMNI Adaptation Layer (OAL) . . . . . . . . . . . . . . .  13
     5.1.  Fragmentation Security Implications . . . . . . . . . . .  17
   6.  Frame Format  . . . . . . . . . . . . . . . . . . . . . . . .  18
   7.  Link-Local Addresses (LLAs) . . . . . . . . . . . . . . . . .  18
   8.  Domain-Local Addresses (DLAs) . . . . . . . . . . . . . . . .  20
   9.  Address Mapping - Unicast . . . . . . . . . . . . . . . . . .  21
     9.1.  Sub-Options . . . . . . . . . . . . . . . . . . . . . . .  23
       9.1.1.  Pad1  . . . . . . . . . . . . . . . . . . . . . . . .  24
       9.1.2.  PadN  . . . . . . . . . . . . . . . . . . . . . . . .  24
       9.1.3.  Interface Attributes  . . . . . . . . . . . . . . . .  24
       9.1.4.  Traffic Selector  . . . . . . . . . . . . . . . . . .  28
       9.1.5.  MS-Register . . . . . . . . . . . . . . . . . . . . .  29
       9.1.6.  MS-Release  . . . . . . . . . . . . . . . . . . . . .  29
       9.1.7.  Network Access Identifier (NAI) . . . . . . . . . . .  30
       9.1.8.  Geo Coordinates . . . . . . . . . . . . . . . . . . .  31
       9.1.9.  DHCP Unique Identifier (DUID) . . . . . . . . . . . .  31
   10. Address Mapping - Multicast . . . . . . . . . . . . . . . . .  32
   11. Multilink Conceptual Sending Algorithm  . . . . . . . . . . .  32
     11.1.  Multiple OMNI Interfaces . . . . . . . . . . . . . . . .  33
     11.2.  MN<->AR Traffic Loop Prevention  . . . . . . . . . . . .  33
   12. Router Discovery and Prefix Registration  . . . . . . . . . .  34
     12.1.  Router Discovery in IP Multihop and IPv4-Only Access
            Networks . . . . . . . . . . . . . . . . . . . . . . . .  37
     12.2.  MS-Register and MS-Release List Processing . . . . . . .  39
     12.3.  DHCPv6-based Prefix Registration . . . . . . . . . . . .  41
   13. Secure Redirection  . . . . . . . . . . . . . . . . . . . . .  41
   14. AR and MSE Resilience . . . . . . . . . . . . . . . . . . . .  42
   15. Detecting and Responding to MSE Failures  . . . . . . . . . .  42
   16. Transition Considerations . . . . . . . . . . . . . . . . . .  43
   17. OMNI Interfaces on the Open Internet  . . . . . . . . . . . .  43
   18. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . . . .  44
   19. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  45
   20. Security Considerations . . . . . . . . . . . . . . . . . . .  46
   21. Implementation Status . . . . . . . . . . . . . . . . . . . .  47
   22. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  47



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   23. References  . . . . . . . . . . . . . . . . . . . . . . . . .  48
     23.1.  Normative References . . . . . . . . . . . . . . . . . .  48
     23.2.  Informative References . . . . . . . . . . . . . . . . .  50
   Appendix A.  Interface Attribute Preferences Bitmap Encoding  . .  55
   Appendix B.  VDL Mode 2 Considerations  . . . . . . . . . . . . .  56
   Appendix C.  MN / AR Isolation Through L2 Address Mapping . . . .  57
   Appendix D.  Change Log . . . . . . . . . . . . . . . . . . . . .  58
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  60

1.  Introduction

   Mobile Nodes (MNs) (e.g., aircraft of various configurations,
   terrestrial vehicles, seagoing vessels, enterprise wireless devices,
   etc.) often have multiple data links 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 underlying
   data links.

   The MN configures a virtual interface (termed the "Overlay Multilink
   Network (OMNI) interface") as a thin layer over the underlying Access
   Network (ANET) 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 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 12).

   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) from which OMNI link MNPs are derived.  If there are
   multiple OMNI links, the IPv6 layer will see multiple OMNI
   interfaces.



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   MNs may connect to multiple distinct OMNI links by configuring
   multiple OMNI interfaces, e.g., omni0, omni1, omni2, etc.  Each OMNI
   interface is configured over a set of underlying interfaces and
   provides a nexus for Safety-Based Multilink (SBM) operation.  The IP
   layer selects an OMNI interface based on SBM routing considerations,
   then the selected interface 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 are currenty under active consideration.  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].
   The IETF IP Wireless Access in Vehicular Environments (ipwave)
   working group has further included problem statement and use case
   analysis for OMNI in a document now in AD evaluation for RFC
   publication [I-D.ietf-ipwave-vehicular-networking].  Still other
   communities of interest include AEEC, RTCA Special Committee 228 (SC-
   228) and NASA programs that examine commercial aviation, Urban Air
   Mobility (UAM) and Unmanned Air Systems (UAS).  Pedestrians with
   handheld devices represent another large class of potential OMNI
   users.

   This document specifies the transmission of IP packets and MN/MS
   control messages over OMNI interfaces.  The OMNI interface supports
   either IP protocol version (i.e., IPv4 [RFC0791] or IPv6 [RFC8200])
   as the network layer in the data plane, while using IPv6 ND messaging
   as the control plane independently of the data plane IP protocol(s).
   The OMNI Adaptation Layer (OAL) which operates as a mid-layer between
   L3 and L2 is based on IP-in-IPv6 encapsulation per [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.



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

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

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

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

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

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

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

   Mobility Service Prefix (MSP)
      an aggregated IP prefix (e.g., 2001:db8::/32, 192.0.2.0/24, etc.)
      advertised to the rest of the Internetwork by the MS, and from
      which more-specific Mobile Network Prefixes (MNPs) are derived.

   Mobile Network Prefix (MNP)
      a longer IP prefix taken 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.

   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 between the MN and ANET are assumed.

   Access Router (AR)
      a first-hop router in the ANET for connecting MNs to
      correspondents in outside Internetworks.

   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 for ANET MNs and INET
      correspondents.  Examples include private enterprise networks,
      ground domain aviation service networks and the global public
      Internet itself.

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

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

   OMNI domain
      a set of affiliated OMNI links that collectively provide services
      under a common (set of) Mobility Service Prefixes (MSPs).

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

   OMNI Adaptation Layer (OAL)
      an OMNI interface process whereby packets admitted into the
      interface are wrapped in a mid-layer IPv6 header and fragmented/
      reassembled if necessary to support the OMNI link Maximum
      Transmission Unit (MTU).  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.




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   OMNI Link-Local Address (LLA)
      a link local IPv6 address per [RFC4291] constructed as specified
      in Section 7.

   OMNI Domain-Local Address (DLA)
      an IPv6 address from the prefix [DLA]::/10 constructed as
      specified in Section 8.  OMNI DLAs are statelessly derived from
      OMNI LLAs, and vice-versa.

   OMNI Option
      an IPv6 Neighbor Discovery option providing multilink parameters
      for the OMNI interface as specified in Section 9.

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

   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
      an ANET/INET 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.

   Mobility Service Identification (MSID)
      Each MSE and AR is assigned a unique 32-bit Identification (MSID)
      as specified in Section 7.

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

   Performance Based Multilink (PBM)




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

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.

   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.

   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 MN virtual interface configured over one or
   more underlying interfaces, which may be physical (e.g., an
   aeronautical radio link) or virtual (e.g., an Internet or higher-
   layer "tunnel").  The MN receives a MNP from the MS, and coordinates
   with the MS through IPv6 ND message exchanges.  The MN uses the MNP
   to construct a unique OMNI LLA through the algorithmic derivation
   specified in Section 7 and assigns the LLA to the OMNI interface.

   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 with
   multiple underlying interfaces that appear as L2 communication
   channels in the architecture.















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

           Figure 1: OMNI Interface Architectural Layering Model

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

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

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

   o  VPNed interfaces use security encapsulation over an INET/ANET 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 ANET/INET paths.  An example is a line-
      of-sight link between a remote pilot and an unmanned aircraft.

   The OMNI virtual interface model gives rise to a number of
   opportunities:






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   o  since OMNI LLAs are uniquely derived from an MNP, no Duplicate
      Address Detection (DAD) or Multicast Listener Discovery (MLD)
      messaging is necessary.

   o  ANET 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.  (An alternative would be to also assign the
      same OMNI LLA to all ANET interfaces.)

   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 OMNI Adaptation Layer (OAL) within the OMNI interface supports
      lossless and adaptive path MTU mitigations not available for
      communications directly over the underlying interfaces themselves.

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

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

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



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   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.
   However, the opportunities discussed above are not available when the
   architectural layering is bypassed in this way.

   Figure 2 depicts the architectural model for a MN connecting to the
   MS via multiple independent ANETs.  When an underlying interface
   becomes active, the MN's OMNI interface sends native (i.e.,
   unencapsulated) IPv6 ND messages via the underlying interface.  IPv6
   ND messages traverse the ground domain ANETs until they reach an
   Access Router (AR#1, AR#2, .., AR#n).  The AR then coordinates with a
   Mobility Service Endpoint (MSE#1, MSE#2, ..., MSE#m) in the INET and
   returns an IPv6 ND message response to the MN.  IPv6 ND messages
   traverse the ANET at layer 2; hence, the Hop Limit is not
   decremented.


































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

              Figure 2: MN/MS Coordination via Multiple ANETs

   After the initial IPv6 ND message exchange, the MN can send and
   receive unencapsulated IP data packets over the OMNI interface.  OMNI
   interface multilink services will forward the packets via ARs in the
   correct underlying ANETs.  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.

   OMNI links span one or more underlying Internetwork via the OMNI
   Adaptation Layer (OAL) which is based on a mid-layer overlay
   encapsulation using [RFC2473].  Each OMNI link corresponds to a
   different overlay (differentiated by an address codepoint) which may
   be carried over a completely separate underlying topology.  Each MN



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   can facilitate SBM by connecting to multiple OMNI links using a
   distinct OMNI interface for each link.

5.  The OMNI Adaptation Layer (OAL)

   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 that may have diverse MTUs.  OMNI interfaces accommodate
   MTU diversity through the use of the OMNI Adaptation Layer (OAL) as
   discussed in this section.

   IPv6 underlying interfaces are REQUIRED to configure a minimum MTU of
   1280 bytes and a minimum MRU of 1500 bytes [RFC8200], meaning that
   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 packets of at least 1280 bytes without generating an
   IPv6 Path MTU Discovery (PMTUD) Packet Too Big (PTB) message
   [RFC8201].  (Note: the source can apply "source fragmentation" for
   locally-generated IPv6 packets up to 1500 bytes and larger still if
   it if has a way to determine that the destination configures a larger
   MRU, but 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 DF bit in the IPv4 encapsulation
   headers of packets sent over IPv4 underlying interfaces therefore
   MUST be set to 0.  (Note: even if the encapsulation source has a way
   to determine that the encapsulation destination configures an MRU
   larger than 576 bytes, it should not assume a larger minimum IPv4
   path MTU without careful consideration of the issues discussed in
   Section 5.1.)

   The OMNI interface configures both an MTU and MRU of 9180 bytes
   [RFC2492]; the size is therefore not a reflection of the underlying
   interface MTUs, but rather determines the largest packet the OMNI
   interface can forward or reassemble.  The OMNI interface uses the
   OMNI Adaptation Layer (OAL) to admit packets from the network layer
   that are no larger than the OMNI interface MTU while generating
   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




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   as "PTBs", and introduces a distinction between PTB "hard" and "soft"
   errors as discussed below.

   For IPv4 packets with DF=0, the network layer performs IPv4
   fragmentation if necessary then admits the packets/fragments into the
   OMNI interface; these fragments will be reassembled by the final
   destination.  For IPv4 packets with DF=1 and IPv6 packets, the
   network layer admits the packet if it is no larger than the OMNI
   interface MTU; otherwise, it drops the packet and returns a PTB hard
   error message to the source.

   For each admitted IP packet/fragment, the OMNI interface internally
   employs the OAL when necessary by inserting a mid-layer IPv6 header
   between the inner IP packet/fragment and any outer IP encapsulation
   headers per [RFC2473] (but without decrementing the inner IP Hop
   Limit/TTL since the insertion occurs at a layer below IP forwarding).
   The OAL then calculates the 32-bit CRC over the entire mid-layer
   packet and writes the value in a trailing 4-octet field at the end of
   the packet.  Next, the OAL fragments this mid-layer IPv6 packet,
   forwards the fragments (using outer IP encapsulation if necessary),
   and returns an internally-generated PTB soft error message (subject
   to rate limiting) if it deems the 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 OAL operates with respect to both the minimum IPv6 and IPv4 path
   MTUs as follows:

   o  When an OMNI interface sends a packet toward a final destination
      via an ANET peer, it sends without OAL encapsulation if the packet
      (including any outer-layer ANET encapsulations) is no larger than
      the underlying interface MTU for on-link ANET peers or the minimum
      ANET path MTU for peers separated by multiple IP hops.  Otherwise,
      the OAL inserts an IPv6 header per [RFC2473] with source address
      set to the node's own OMNI Domain-Local Address (DLA) (see:
      Section 8) and destination set to the OMNI DLA of the ANET peer.
      The OAL then calculates and appends the trailing 32-bit CRC, then
      uses IPv6 fragmentation to break the packet into a minimum number
      of non-overlapping fragments where the largest fragment size
      (including both the OMNI and any outer-layer ANET encapsulations)
      is determined by the underlying interface MTU for on-link ANET
      peers or the minimum ANET path MTU for peers separated by multiple
      IP hops.  The OAL then encapsulates the fragments in any ANET
      headers and sends them to the ANET peer, which reassembles before
      forwarding toward the final destination.





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   o  When an OMNI interface sends a packet toward a final destination
      via an INET interface, it sends packets (including any outer-layer
      INET encapsulations) no larger than the minimum INET path MTU
      without OAL encapsulation if the destination is reached via an
      INET address within the same OMNI link segment.  Otherwise, the
      OAL inserts an IPv6 header per [RFC2473] with source address set
      to the node's OMNI DLA, destination set to the DLA of the next hop
      OMNI node toward the final destination and (if necessary) with a
      Segment Routing Header with the remaining Segment IDs on the path
      to the final destination.  The OAL then calculates and appends the
      trailing 32-bit CRC, then uses IPv6 fragmentation to break the
      packet into a minimum number of non-overlapping fragments where
      the largest fragment size (including both the OMNI and outer-layer
      INET encapsulations) is the minimum INET path MTU, and the
      smallest fragment size is no smaller than 256 bytes (i.e.,
      slightly less than half the minimum IPv4 path MTU).  The OAL then
      encapsulates the fragments in any INET headers and sends them to
      the OMNI link neighbor, which reassembles before forwarding toward
      the final destination.

   The OAL unconditionally drops all OAL fragments received from an INET
   peer that are smaller than 256 bytes (note that no minimum fragment
   size is specified for ANET peers since the underlying ANET is secured
   against tiny fragment attacks).  In order to set the correct context
   for reassembly, the OAL of the OMNI interface that inserts the IPv6
   header MUST also be the one that inserts the IPv6 Fragment Header
   Identification value.  While not strictly required, sending all
   fragments of the same fragmented OAL packet consecutively over the
   same underlying interface with minimal inter-fragment delay may
   increase the likelihood of successful reassembly.

   Ordinary PTB messages with ICMPv4 header "unused" field or ICMPv6
   header Code field value 0 are hard errors that always indicate that a
   packet has been dropped due to a real MTU restriction.  However, the
   OAL can also forward large packets via encapsulation and
   fragmentation while at the same time returning PTB soft error
   messages (subject to rate limiting) indicating that a forwarded
   packet was uncomfortably large.  The OMNI interface can therefore
   continuously forward large packets without loss while returning PTB
   soft error messages recommending a smaller size.  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.

   The OAL sets the ICMPv4 header "unused" field or ICMPv6 header Code
   field to the value 1 in PTB soft error messages.  The OAL sets the
   PTB destination address to the source address of the original packet,
   and sets the source address to the MNP Subnet Router Anycast address
   of the MN (i.e., whether the MN was the source or target of the



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   original packet).  When the original source receives the PTB, it
   reduces its path MTU estimate the same as for hard errors but does
   not regard the message as a loss indication.  (If the original source
   does not recognize the soft error code, it regards the PTB the same
   as a hard error but should heed the retransmission advice given in
   [RFC8201] suggesting retransmission based on normal packetization
   layer retransmission timers.)  This document therefore updates
   [RFC1191][RFC4443] and [RFC8201].  Furthermore, implementations of
   [RFC4821] must be aware that PTB hard or soft errors may arrive at
   any time even if after a successful MTU probe (this is the same
   consideration as for an ordinary path fluctuation following a
   successful probe).

   In summary, the OAL 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 loss,
   original sources that receive soft errors can quickly scan for path
   MTU increases without waiting for the minimum 10 minutes specified
   for loss-oriented PTB hard errors [RFC1191][RFC8201].  The OAL
   therefore provides a lossless and adaptive service that accommodates
   MTU diversity especially well-suited for dynamic multilink
   environments.

   Note: In network paths where IPv6/IPv4 protocol translation or IPv6-
   in-IPv4 encapsulation may be prevalent, it may be prudent for the OAL
   to always assume the IPv4 minimum path MTU (i.e., 576 bytes)
   regardless of the underlying interface IP protocol version.  Always
   assuming the IPv4 minimum path MTU even for IPv6 underlying
   interfaces may produce more fragments and additional header overhead,
   but will always interoperate and never run the risk of presenting an
   IPv4 node with a packet that exceeds its MRU.

   Note: An OMNI interface that reassembles OAL fragments may experience
   congestion-oriented loss in its reassembly cache and can optionally
   send PTB soft errors to the original source and/or ICMP "Time
   Exceeded" messages to the source of the OAL fragments.  In
   environments where the messages may contribute to unacceptable
   additional congestion, however, the OMNI interface can simply regard
   the loss as an ordinary unreported congestion event for which the
   original source will eventually compensate.

   Note: When the network layer forwards an IPv4 packet/fragment with
   DF=0 into the OMNI interface, the interface can optionally perform
   (further) IPv4 fragmentation before invoking the OAL so that the
   fragments will be reassembled by the final destination.  When the
   network layer performs IPv6 fragmentation for locally-generated IPv6
   packets, the OMNI interface typically invokes the OAL without first
   applying (further) IPv6 fragmentation; the network layer should



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   therefore fragment to the minimum IPv6 path MTU (or smaller still) to
   push the reassembly burden to the final destination and avoid
   receiving PTB soft errors from the OMNI interface.  Aside from these
   non-normative guidelines, the manner in which any IP fragmentation is
   invoked prior to OAL encapsulation/fragmentation is an implementation
   matter.

   Note: Inclusion of the 32-bit CRC prior to fragmentation assumes that
   the receiving OAL will discard any packets with incorrect CRC values
   following reassembly.  The 32-bit CRC is sufficient to detect
   reassembly misassociations for packet sizes up to the OMNI interface
   MTU 9180 but may not be sufficient to detect errors for larger sizes
   [CRC].

   Note: Some underlying interface types (e.g., VPNs) may already
   provide their own robust fragmentation and reassembly services even
   without OAL encapsulation.  In those cases, the OAL can invoke the
   inherent underlying interface schemes instead while employing PTB
   soft errors in the same fashion as described above.  Other underlying
   interface properties such as header/message compression can also be
   harnessed in a similar fashion.

   Note: Applications can dynamically tune the size of the 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.

5.1.  Fragmentation Security Implications

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

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

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



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

   3.  Attacks based on predictable fragment identification values -
       this threat is mitigated by selecting a suitably random ID value
       per [RFC7739].

   4.  Evasion of Network Intrusion Detection Systems (NIDS) - this
       threat is mitigated by disallowing "tiny fragments" per the OAL
       fragmentation procedures specified above.

   Additionally, IPv4 fragmentation includes a 16-bit Identification (IP
   ID) field with only 65535 unique values such that at high data rates
   the field could wrap and apply to new packets while the fragments of
   old packets using the same ID are still alive in the network
   [RFC4963].  However, since the largest OAL fragment that will be sent
   via an IPv4 INET path is 576 bytes any IPv4 fragmentation would occur
   only on links with an IPv4 MTU smaller than this size, and [RFC3819]
   recommendations suggest that such links will have low data rates.
   Since IPv6 provides a 32-bit Identification value, IP ID wraparound
   at high data rates is not a concern for IPv6 fragmentation.

6.  Frame Format

   The OMNI interface transmits IPv6 packets according to the native
   frame format of each underlying interface.  For example, for
   Ethernet-compatible interfaces the frame format is specified in
   [RFC2464], for aeronautical radio interfaces the frame format is
   specified in standards such as ICAO Doc 9776 (VDL Mode 2 Technical
   Manual), for tunnels over IPv6 the frame format is specified in
   [RFC2473], etc.

7.  Link-Local Addresses (LLAs)

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

   o  IPv6 MN OMNI LLAs encode the most-significant 112 bits of a MNP
      within the least-significant 112 bits of the IPv6 link-local
      prefix fe80::/16.  The Prefix Length is determined by adding 16 to
      the length of the embedded MNP.  For example, for the MNP
      2001:db8:1000:2000::/56 the corresponding MN OMNI LLA is
      fe80:2001:db8:1000:2000::/72.  This specification updates the IPv6
      link-local address format specified in Section 2.5.6 of [RFC4291]
      by defining a use for bits 11 - 63.



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   o  IPv4-compatible MN OMNI LLAs are constructed as fe80::ffff:[IPv4],
      i.e., the most significant 16 bits of the prefix fe80::/16,
      followed by 64 '0' bits, followed by 16 '1' bits, followed by a
      32bit IPv4 address/prefix.  The Prefix Length is determined by
      adding 96 to the length of the embedded IPv4 address/prefix.  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  MS OMNI LLAs are assigned to ARs and MSEs from the range
      fe80::/96, 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::feff:ffff.  The MS OMNI LLA Prefix Length is
      determined by adding 96 to the MSID prefix length.  For example,
      if the MSID '0x10002000' prefix length is 16 then the MS OMNI LLA
      Prefix Length is set to 112 and the LLA is written as
      fe80::1000:2000/112.  Finally, the MSID 0x00000000 is the
      "Anycast" MSID and corresponds to the link-local Subnet-Router
      anycast address (fe80::) [RFC4291]; the MSID range 0xff000000
      through 0xffffffff is reserved for future use.

   o  Temporary OMNI LLAs are configured from the range
      fe80:0:0:ffff::/64 with the lower 64 bits set according to the
      specification in [I-D.ietf-6man-rfc4941bis].  Temporary OMNI LLAs
      are used for the short-term purpose of procuring an actual MN OMNI
      LLA upon startup or (re)connecting to the network.

   o  Cryptographically-Generated Address (CGA) OMNI LLAs are configured
      from the range fe80:0:0:fffe::/64 with the lower 64 bits set
      according to the specification in [RFC3972].  CGAs are used for
      operation of the Secure Neighbor Discovery (SEND) protocol.

   o  the remaining /64 sub-prefixes of fe80:0:0::/48 (other than
      fe80::/64) are available for future assignment to other LLA types

   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 above OMNI LLA constructs.

   Since MN OMNI LLAs are based on the distribution of administratively
   assured unique MNPs, and since MS OMNI LLAs are guaranteed unique
   through administrative assignment, OMNI interfaces set the
   autoconfiguration variable DupAddrDetectTransmits to 0 [RFC4862].
   Temporary OMNI LLAs employ optimistic DAD principles [RFC4429] since
   they are probabilistically unique and their use is short-duration in
   nature.




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8.  Domain-Local Addresses (DLAs)

   OMNI links use IPv6 Domain-Local Addresses (i.e., "OMNI DLAs") as the
   source and destination addresses in OAL IPv6 encapsulation headers.
   This document uses the prefix [DLA]::/10 for mapping OMNI LLAs to
   routable OMNI DLAs (where the actual prefix assignment for [DLA]::/10
   is TBD).

   Each OMNI link instance is identified by bits 10-15 of the OMNI
   service prefix [DLA]::/10.  For example, OMNI DLAs associated with
   instance 0 are configured from the prefix [DLA0]::/16, instance 1
   from [DLA1]::/16, instance 2 from [DLA2]::/16, etc.  OMNI DLAs and
   their associated prefix lengths are configured in one-to-one
   correspondence with OMNI LLAs through stateless prefix translation.
   For example, for OMNI link instance [DLA0]::/16:

   o  the OMNI DLA corresponding to fe80:2001:db8:1:2::/80 is simply
      [DLA0]:2001:db8:1:2::/80

   o  the OMNI DLA corresponding to fe80::ffff:192.0.2.0/120 is simply
      [DLA0]::ffff:192.0.2.0/120

   o  the OMNI DLA corresponding to fe80::1000/112 is simply
      [DLA0]::1000/112

   o  the OMNI DLA corresponding to fe80::/128 is simply [DLA0]::/128.

   o  the OMNI DLA corresponding to fe80:0:0:ffff:[Temporary]/64 is
      simply [DLA0]:0:0:ffff:[Temporary]/64.

   o  etc.

   Each OMNI interface assigns the Anycast OMNI DLA specific to the OMNI
   link instance, e.g., the OMNI interface connected to instance 3
   assigns the Anycast OMNI DLA [DLA3]:. Routers that configure OMNI
   interfaces advertise the OMNI service prefix (e.g., [DLA3]::/16) into
   the local routing system so that applications can direct traffic
   according to SBM requirements.

   The OMNI DLA presents an IPv6 address format that is routable within
   the OMNI domain 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 over ANET links when possible to conserve
   bandwidth (see: Section 11).




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   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 packet forwarding to destinations
   located in other OMNI link segments.  A full discussion of Segment
   Routing over the OMNI link appears in [I-D.templin-intarea-6706bis].

   NOTE: Since DLAs are only routable within the scope of an OMNI
   domain, [DLA]::/10 need not be based on an IPv6 Globally Unique
   Address (GUA) prefix.  The IPv6 Unique Local Address (ULA) prefix
   fc00::/7 [RFC4193] could provide an acceptable alternative provided
   the exact [DLA]::/10 sub-prefix could be annexed for use by this
   document.  Re-purposing the (deprecated) IPv6 "site-local" address
   block fec0::/10 would seem to be a natural alternative since both
   fe80::/10 and fec0::/10 have identical prefix lengths and differ only
   in a single bit setting; they can also be unambiguously allocated in
   one-to-one correspondence with one another.  This alternative would
   also make good use of an otherwise-wasted address range that has been
   "parked" since the 2004 deprecation.  However, re-purposing the block
   would require an IETF standards action acknowledging this document as
   obsoleting [RFC3879] and updating [RFC4291].

9.  Address Mapping - Unicast

   OMNI interfaces maintain a neighbor cache for tracking per-neighbor
   state and use the link-local address format specified in Section 7.
   OMNI interface IPv6 Neighbor Discovery (ND) [RFC4861] messages sent
   over physical underlying 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]).  OMNI interface IPv6 ND messages sent over underlying
   interfaces via encapsulation do not include S/TLLAOs which were
   intended for encoding physical L2 media address formats and not
   encapsulation IP addresses.  Furthermore S/TLLAOs are not intended
   for encoding additional interface attributes.  Hence, this document
   does not define an S/TLLAO format but instead defines a new option
   type termed the "OMNI option" designed for these purposes.

   MNs such as aircraft typically have many wireless data link types
   (e.g. satellite-based, cellular, terrestrial, air-to-air directional,
   etc.) with diverse performance, cost and availability properties.
   The OMNI interface would therefore appear to have multiple L2
   connections, and may include information for multiple underlying
   interfaces in a single IPv6 ND message exchange.  OMNI interfaces use
   an IPv6 ND option called the OMNI option formatted as shown in
   Figure 3:





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

                       Figure 3: OMNI Option Format

   In this format:

   o  Type is set to TBD.  If multiple OMNI option instances appear in
      the same IPv6 ND message, the first instance is processed and all
      other instances are ignored.

   o  Length is set to the number of 8 octet blocks in the option.

   o  Prefix Length is determines the length of prefix to be applied to
      an OMNI MN LLA/DLA.  For IPv6 ND messages sent from a MN to the
      MS, Prefix Length is the length that the MN is requesting or
      asserting to the MS.  For IPv6 ND messages sent from the MS to the
      MN, Prefix Length indicates the length that the MS is granting to
      the MN.  For IPv6 ND messages sent between MS endpoints, Prefix
      Length indicates the length associated with the target MN that is
      subject of the ND message.

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

   o  Sub-Options is a Variable-length field, of length such that the
      complete OMNI Option is an integer multiple of 8 octets long.
      Contains one or more Sub-Options, as described in Section 9.1.








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9.1.  Sub-Options

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

         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 4: Sub-Option Format

   o  Sub-Type is a 1-octet field that encodes the Sub-Option type.
      Sub-Options defined in this document are:

        Option Name               Sub-Type
        Pad1                           0
        PadN                           1
        Interface Attributes           2
        Traffic Selector               3
        MS-Register                    4
        MS-Release                     5
        Network Access Identifier      6
        Geo Coordinates                7
        DHCP Unique Identifier (DUID)  8

                                 Figure 5

      Sub-Types 253 and 254 are reserved for experimentation, as
      recommended in [RFC3692].

   o  Sub-Length is a 1-octet field that encodes the length of the Sub-
      Option Data (i.e., ranging from 0 to 255 octets).

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

   During processing, unrecognized Sub-Options are ignored and the next
   Sub-Option processed until the end of the OMNI option is reached.

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






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9.1.1.  Pad1

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

                              Figure 6: Pad1

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

   o  No Sub-Length or Sub-Option Data follows (i.e., the "Sub-Option"
      consists of a single zero octet).

9.1.2.  PadN

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

                              Figure 7: PadN

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

   o  Sub-Length is set to N (from 0 to 255) being the number of padding
      octets that follow.

   o  Sub-Option Data consists of N zero-valued octets.

9.1.3.  Interface Attributes

   The Interface Attributes sub-option provides L2 forwarding
   information for the multilink conceptual sending algorithm discussed
   in Section 11.  The L2 information is used for selecting among
   potentially multiple candidate underlying interfaces that can be used
   to forward packets to the neighbor based on factors such as DSCP
   preferences and link quality.  Interface Attributes further include
   link-layer address information to be used for either OAL
   encapsulation or direct UDP/IP encapsulation (when OAL encapsulation
   can be avoided).  The Interface Attributes format and contents are
   given in Figure 8 below:





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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Sub-Type=2  | Sub-length=N  |    ifIndex    |    ifType     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  | Link  |R| APS |   SRT   | FMT |   LHS (0 - 7) |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               LHS (bits 8 - 31)               |               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               ~
       ~                                                               ~
       ~                   Link Layer Address (L2ADDR)                 ~
       ~                                                               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Bitmap(0)=0xff|P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P12|P13|P14|P15|P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P28|P29|P30|P31| Bitmap(1)=0xff|P32|P33|P34|P35|P36| ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                      Figure 8: Interface Attributes

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

   o  Sub-Length is set to N (from 4 to 255) that encodes the number of
      Sub-Option Data octets that follow.

   o  Sub-Option Data contains an "Interface Attribute" option encoded
      as follows (note that the first four octets must be present):

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

      *  ifType is set to an 8-bit integer value corresponding to the
         underlying interface identified by ifIndex.  The value



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         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 ifIndex.

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

      *  R is reserved for future use.

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

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

         +  When the next most significant bit (i.e., "Preferences") is
            set to 1, a preferences block is included next; else, it is
            omitted.  (Note that if "Address" is set the preferences
            block immediately follows L2ADDR; else, it immediately
            follows the APS code.)

         +  When a preferences block is present and the least
            significant bit (i.e., "Simplex") is set to 1, the block is
            encoded in "Simplex" form as shown in Figure 8; else it is
            encoded in "Indexed" form as discussed below.

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

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

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

            -  When the most significant bit (i.e., "Framework") is set
               to 0, L2ADDR is the INET encapsulation address of a
               Proxy/Server; otherwise, it is the address for the
               Source/Target itself



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            -  When the next most significant bit (i.e., "Mode") is set
               to 0, the Source/Target L2ADDR is on the open INET;
               otherwise, it is (likely) located behind a Network
               Address Translator (NAT).

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

         +  LHS - the 32 bit MSID of the Last Hop Server/Proxy 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 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; else, it
            must forward according to the OMNI link spanning tree.  See
            [I-D.templin-intarea-6706bis] for further discussion.

         +  Link Layer Address (L2ADDR) - Formatted according to FMT,
            and identifies the link-layer address (i.e., the
            encapsulation address) of the source/target.  The UDP Port
            Number appears in the first two 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 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 APS indicates that "Preferences" are included, a
         preferences block appears as the remainder of the Sub-Option as
         a series of Bitmaps and P[*] values.  In "Simplex" form, the
         index for each singleton Bitmap octet is inferred from its
         sequential position (i.e., 0, 1, 2, ...) as shown in Figure 8.
         In "Indexed" form, each Bitmap is preceded by an Index octet
         that encodes a value "i" = (0 - 255) as the index for its
         companion Bitmap as follows:

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

                                 Figure 9





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

      *  Each 2-bit P[*] field is set to the value '0' ("disabled"), '1'
         ("low"), '2' ("medium") or '3' ("high") to indicate a QoS
         preference for underlying interface selection purposes.  Not
         all P[*] values need to be included in the OMNI option of each
         IPv6 ND message received.  Any P[*] values represented in an
         earlier OMNI option but omitted in the current OMNI option
         remain unchanged.  Any P[*] values not yet represented in any
         OMNI option default to "medium".

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

9.1.4.  Traffic Selector

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

                        Figure 10: Traffic Selector

   o  Sub-Type is set to 3.  If multiple instances appear in the same
      OMNI option all are processed, i.e., even if the same ifIndex
      value appears multiple times.




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

   o  Sub-Option Data contains a 1-octet ifIndex encoded exactly as
      specified in Section 9.1.3, followed by an N-1 octet traffic
      selector formatted per [RFC6088] beginning with the "TS Format"
      field.  The largest traffic selector for a given ifIndex is
      therefore 254 octets.

9.1.5.  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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Sub-Type=4  | 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 11: MS-Register Sub-option

   o  Sub-Type is set to 4.  If multiple instances appear in the same
      OMNI option 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.

   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.

9.1.6.  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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Sub-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 12: MS-Release Sub-option

   o  Sub-Type is set to 5.  If multiple instances appear in the same
      IPv6 OMNI option 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.

   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.

9.1.7.  Network Access Identifier (NAI)

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Sub-Type=6  | Sub-length=N  |Network Access Identifier (NAI)
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...

           Figure 13: Network Access Identifier (NAI) Sub-option

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

   o  Sub-Length is set to N.

   o  A Network Access Identifier (NAI) up to 255 octets in length is
      coded per [RFC7542].







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9.1.8.  Geo Coordinates

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

                   Figure 14: Geo Coordinates Sub-option

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

   o  Sub-Length is set to N.

   o  A set of Geo Coordinates up to 255 octets in length (format TBD).
      Includes Latitude/Longitude at a minimum; may also include
      additional attributes such as altitude, heading, speed, etc.).

9.1.9.  DHCP Unique Identifier (DUID)

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Sub-Type=8  | Sub-length=N  |           DUID-Type           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .             type-specific DUID body (variable length)         .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 15: DHCP Unique Identifier (DUID) Sub-option

   o  Sub-Type is set to 8.  If multiple instances appear in the same
      OMNI option the first is processed and all others are ignored.

   o  Sub-Length is set to N (i.e., the length of the option beginning
      with the DUID-Type and continuing to the end of the type-specific
      body).

   o  DUID-Type is a two-octet field coded in network byte order that
      determines the format and contents of the type-specific body
      according to Section 11 of [RFC8415].  DUID-Type 4 in particular
      corresponds to the Universally Unique Identifier (UUID) [RFC6355]
      which will occur in common operational practice.






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   o  A type-specific DUID body up to 253 octets in length follows,
      formatted according to DUID-type.  For example, for type 4 the
      body consists of a 128-bit UUID selected according to [RFC6355].

10.  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 ANET L2 elements use MLD snooping
   [RFC4541].

11.  Multilink Conceptual Sending Algorithm

   The MN's IPv6 layer selects the outbound OMNI interface according to
   SBM considerations when forwarding data packets from local or EUN
   applications to external correspondents.  Each OMNI interface
   maintains a neighbor cache the same as for any IPv6 interface, but
   with additional state for multilink coordination.  Each OMNI
   interface maintains default routes via ARs discovered as discussed in
   Section 12, and may configure more-specific routes discovered through
   means outside the scope of this specification.

   After a packet enters the OMNI interface, one or more outbound
   underlying interfaces are selected based on PBM traffic attributes,
   and one or more neighbor underlying interfaces are selected based on
   the receipt of Interface Attributes sub-options in IPv6 ND messages
   (see: Figure 8).  Underlying interface selection for the nodes own
   local interfaces are based on attributes such as DSCP, application
   port number, cost, performance, message size, etc.  OMNI interface
   multilink selections could also be configured to perform replication
   across multiple underlying interfaces for increased reliability at
   the expense of packet duplication.  The set of all Interface
   Attributes received in IPv6 ND messages determine the multilink
   forwarding profile for selecting the neighbor's underlying
   interfaces.

   When the OMNI interface sends a packet over a selected outbound
   underlying interface, the OAL includes or omits a mid-layer
   encapsulation header as necessary as discussed in Section 5 and 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 12.1.




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

11.1.  Multiple OMNI Interfaces

   MNs may connect to multiple independent OMNI links concurrently in
   support of SBM.  Each OMNI interface is distinguished by its Anycast
   OMNI DLA (e.g., fec0::, fec1::, fec2::).  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 OMNI DLA 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 OMNI DLA is
   written into the IPv6 destination address, and the actual destination
   (along with any additional intermediate hops) is written into the
   Segment Routing Header.  Standard IP routing directs the packets to
   the MN's mobile router entity, and the Anycast OMNI DLA identifies
   the OMNI interface to be used for transmission to the next hop.  When
   the MN receives the message, 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 OMNI DLA.

   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).

11.2.  MN<->AR Traffic Loop Prevention

   After an AR has registered an MNP for a MN (see: Section 12), 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.




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12.  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 ANET 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 ANET ensures AR coordination is link-specific and outside
   the scope of this document (however, considerations for ANETs that do
   not provide ARs that recognize the OMNI option are discussed in
   Section 17).

   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.  The MN can also send an
   RS with an MS-Register suboption that includes the Anycast MSID value
   '0', i.e., instead of or in addition to any non-zero MSIDs.  When the
   AR receives an RS with a MSID '0', it selects a nearby MSE (which may
   be itself) and returns an RA with the selected MSID in an MS-Register
   suboption.  The AR selects only a single wildcard MSE (i.e., even if
   the RS MS-Register suboption included multiple '0' MSIDs) while also
   soliciting the MSEs corresponding to any non-zero MSIDs.

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

   When an OMNI interface transitions to UP, the MN sends RS messages to
   register its MNP and an initial set of underlying interfaces that are
   also UP.  The MN sends additional RS messages to refresh lifetimes
   and to register/deregister underlying interfaces as they transition
   to UP or DOWN.  The MN sends initial RS messages over an UP
   underlying interface with its OMNI LLA as the source and with
   destination set to All-Routers multicast (ff02::2) [RFC4291].  The RS
   messages include an OMNI option per Section 9 with valid prefix
   registration information, Interface Attributes appropriate for
   underlying interfaces, MS-Register/Release sub-options containing
   MSID values, and with any other necessary OMNI sub-options.  The S/
   T-ifIndex field is set to the index of the underlying interface over
   which the RS message is sent.



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   ARs process IPv6 ND messages with OMNI options and act as an MSE
   themselves and/or as a proxy for other MSEs.  ARs receive RS messages
   and create a neighbor cache entry for the MN, then 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 Prefix Length, MNP and Interface Attributes.  The MSE then
   informs the AR of registration success/failure, and the AR returns an
   RA message to the MN with an OMNI option per Section 9.

   The AR 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 MN OMNI LLA (i.e., unicast), with source address set to
   its own OMNI LLA, and with an OMNI option with S/T-ifIndex set to the
   value included in the RS.  The OMNI option also includes valid prefix
   registration information, Interface Attributes, MS-Register/Release
   and any other necessary OMNI sub-options.  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 ANET interface.

   The AR MAY also send periodic and/or event-driven unsolicited RA
   messages per [RFC4861].  In that case, the S/T-ifIndex field in the
   OMNI header of the unsolicited RA message identifies the target
   underlying interface of the destination MN.

   The AR can combine the information from multiple MSEs into one or
   more "aggregate" RAs sent to the MN in order conserve ANET 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 OMNI LLA as the RA source
   address, the MN determines the addresses of the represented MSEs by
   examining the MS-Register/Release OMNI sub-options.

   When the MN receives the RA message, it creates an OMNI interface
   neighbor cache entry for each MSID that has confirmed MNP
   registration via the L2 address of this AR.  If the MN connects to
   multiple ANETs, it records the additional L2 AR addresses in each



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   MSID neighbor cache entry (i.e., as multilink neighbors).  The MN
   then configures a default route via the MSE that returned the RA
   message, and assigns the Subnet Router Anycast address corresponding
   to the MNP (e.g., 2001:db8:1:2::) to the omni interface.  The MN then
   manages its underlying interfaces according to their states as
   follows:

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

   o  When an underlying interface transitions to DOWN, the MN sends an
      RS or unsolicited NA message over any UP underlying interface with
      an OMNI option containing an Interface Attribute sub-option for
      the DOWN underlying interface with Link set to '0'.  The MN sends
      an RS when an acknowledgement is required, or an unsolicited NA
      when reliability is not thought to be a concern (e.g., if
      redundant transmissions are sent on multiple underlying
      interfaces).

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

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

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

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




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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 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.

12.1.  Router Discovery in IP Multihop and IPv4-Only Access Networks

   On some ANET types a MN may be located multiple IP hops away from the
   nearest AR.  Forwarding through IP multihop ANETs is conducted
   through the application of a routing protocol (e.g., a Mobile Ad-hoc
   Network (MANET) routing protocol over omni-directional wireless
   interfaces, an inter-domain routing protocol in an enterprise
   network, etc.).  These ANETs could be either IPv6-enabled or
   IPv4-only, while IPv4-only ANETs could be either multicast-capable or



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   unicast-only (note that for IPv4-only ANETs the following procedures
   apply for both single-hop and multihop cases).

   A MN located potentially multiple ANET hops away from the nearest AR
   prepares an RS message with source address set to either its OMNI MN
   LLA or an OMNI Temporary LLA, and with destination set to link-scoped
   All-Routers multicast the same as discussed above.  For IPv6-enabled
   ANETs, the MN then encapsulates the message in an IPv6 header with
   source address set to the DLA corresponding to the LLA source address
   and with destination set to either a unicast or anycast DLA address
   (e.g., [DLA]::).  For IPv4-only ANETs, the MN instead encapsulates
   the RS message in an IPv4 header with source address set to the
   node's own IPv4 address 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 ANET interface, where it will be forwarded by zero or
   more intermediate ANET hops.

   When an intermediate ANET 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
   ANET 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 and
   coordinates with the MS the same as for an ordinary link-local RS,
   since the inner 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 LLA and destination address set to the
   LLA of the original MN, then encapsulates the message in an IPv4/IPv6
   header with source address set to its own IPv4/DLA address and with
   destination set to the encapsulation source of the RS.

   The AR then forwards the message to an ANET node within
   communications range, which forwards the message according to its
   routing tables to an intermediate node.  The multihop forwarding
   process within the ANET 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 to statelessly translate the IPv6 LLAs into
   DLAs and forward the 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



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   Hop Limit less than 255.  This document therefore advocates
   encapsulation since the overhead is nominal considering the
   infrequent nature and small size of IPv6 ND messages.  Future
   documents may consider encapsulation avoidance through translation
   while updating [RFC4861].

   Note: An alternate approach to multihop forwarding via IPv4
   encapsulation would be to employ IPv6/IPv4 protocol translation.
   However, for IPv6 ND messages the OMNI LLA addresses 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].

12.2.  MS-Register and MS-Release List Processing

   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 include an MS-Register sub-option with the
   single MSID value 0 which requests the AR to select and assign an
   MSE.  The AR will then return an RA message with source address set
   to the OMNI LLA containing the MSE of the selected MSE.

   As the MN activates additional underlying interfaces, it can
   optionally include an MS-Register sub-option with MSID value 0, or
   with non-zero 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.

   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



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   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 according to whether the AR's own MSID or the value
      0 appears in the MS-Register list as follows:

      *  If yes, 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 a uNA message to each MSID in the MS-Release list
         with the MS-Register/Release lists omitted and with an OMNI
         header with S/T-ifIndex set to 0.

      *  If no, 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
   encapsulation header with the DLA of the AR as the source and the DLA
   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.  The Register MSE then sends an RA message back to the
   (Proxy) AR wrapped in an OMNI encapsulation header with source and
   destination addresses reversed, and with RA destination set to the
   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 send by the first-hop AR or by a Register
   MSE) will include an OMNI option and an encapsulation header with the
   DLA of the Register MSE as the source and the DLA of the Release ME
   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",
   and forward any subsequent packets destined to the MN to the Register
   MSE.

   Note that it is not an error for the MS-Register/Release lists to
   include duplicate entries.  If duplicates occur within a list, the AR



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   will generate multiple proxy RS and/or uNA messages - one for each
   copy of the duplicate entries.

12.3.  DHCPv6-based Prefix Registration

   When a MN is not pre-provisioned with an OMNI LLA containing a MNP,
   it will need for the AR to select an MNP on its behalf and set up the
   correct routing state within the MS.  The DHCPv6 service [RFC8415] is
   used to support this requirement.

   When an MN without a pre-provisioned OMNI MN LLA needs to have the AR
   select an MNP, it sends an RS message with source address set to an
   OMNI Temporary LLA.  The RS message includes an OMNI option with a
   DUID suboption that contains a unique DUID value for the MN, and a
   Prefix Length value corresponding to the length of MNP it wishes to
   receive.

   When the AR received the RS message, it notes that the source was a
   Temporary LLA and derives the DUID and Prefix Length from the OMNI
   option.  The AR (acting as a "proxy DHCP client") then crafts a well-
   formed DHCPv6 Solicit message with Prefix Delegation (PD) parameters,
   and forwards the message to a locally-resident DHCPv6 server.  The
   DHCPv6 server then delegates an MNP and returns a DHCPv6 Reply
   message with PD parameters.  (Note: the Solicit message should
   contain enough identifying information for the MN so that the AR can
   correlate the server's Reply with the original Solicit.)

   When the AR receives the DHCPv6 Reply, it adds a route to the routing
   system and creates an OMNI MN LLA based on the delegated MNP.  The AR
   then sends an RA back to the MN with the (newly-created) OMNI MN LLA
   as the destination address and with Prefix Length set in the OMNI
   option.  When the MN receives the RA, it creates a default route,
   assigns the Subnet Router Anycast addresss and sets its OMNI LLA
   based on the delegated MNP.

13.  Secure Redirection

   If the ANET 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 ANET 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
   ANET link to exchange packets directly without transiting the AR.  In
   that case, the MNs can exchange packets according to their unicast L2



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   addresses discovered from the Redirect message instead of using the
   dogleg path through the AR.  In some ANET links, however, such direct
   communications may be undesirable and continued use of the dogleg
   path through the AR may provide better performance.  In that case,
   the AR can refrain from sending Redirects, and/or MNs can ignore
   them.

14.  AR and MSE Resilience

   ANETs 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.

15.  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 ANET 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 ANET.  If an MSE fails, ARs can quickly inform MNs
   of the outage by sending multicast RA messages on the ANET interface.
   The AR sends RA messages to MNs via the ANET 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 ANET interface that have
   been using the (now defunct) MSE will receive the RA messages and
   associate with a new MSE.




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16.  Transition Considerations

   When a MN connects to an ANET 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 MS OMNI LLA as the source, the MN
   OMNI 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 ANET according to the legacy IPv6 link model and without the OMNI
   extensions specified in this document.

   If the ANET 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 ANET link with an OMNI 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 ANET 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.

17.  OMNI Interfaces on the Open Internet

   OMNI interfaces configured over IPv6-enabled underlying interfaces on
   the open Internet 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.  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
   the open Internet can apply security services such as VPNs to connect
   to an MSE or establish a direct link to an MSE through some other
   means (see Section 4).  In environments where an explicit VPN or



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   direct link may be impractical, OMNI interfaces can instead use UDP/
   IP encapsulation per [RFC6081][RFC4380].  (SEcure Neighbor Discovery
   (SEND) and Cryptographically Generated Addresses (CGA)
   [RFC3971][RFC3972] or other protocol-specific security services can
   also be used if additional authentication is necessary.)

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

   When SEND/CGA are used over an open Internet underlying interfaces,
   each OMNI node configures an OMNI CGA LLA for use as the source
   address of IPv6 ND messages.  The node then employs OAL encapsulation
   and sets the IPv6 source address of the OAL header to the DLA
   corresponding to its OMNI LLA.  Any Prefix Length values in the IPv6
   ND message OMNI option then apply to the DLA found in the OMNI
   header, i.e., and not to the CGA found in the IPv6 ND message source
   address.

   OMNI interfaces in the open Internet are often located behind Network
   Address Translators (NATs).  The OMNI interface accommodates NAT
   traversal using UDP/IP encapsulation and the mechanisms discussed in
   [RFC6081][RFC4380][I-D.templin-intarea-6706bis].

18.  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 12.3 allows OMNI
   MNs that desire time-varying MNPs to obtain short-lived prefixes to
   use an OMNI Temporary LLA as the source address of an RS message with
   an OMNI option with a DHCPv6 Device Unique IDentifier (DUID)
   [RFC8415] sub-option.  The MN would then be obligated to renumber its
   internal networks whenever its MNP (and therefore also its OMNI



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   address) changes.  This should not present a challenge for MNs with
   automated network renumbering services, however presents limits for
   the durations of ongoing sessions that would prefer to use a constant
   address.

19.  IANA Considerations

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

   The IANA is instructed to assign a new Code value "1" in the "ICMPv6
   Code Fields: Type 2 - Packet Too Big" registry.  The registry should
   read as follows:

      Code      Name                         Reference
      ---       ----                         ---------
      0         Diagnostic Packet Too Big    [RFC4443]
      1         Advisory Packet Too Big      [RFCXXXX]

                  Figure 16: OMNI Option Sub-Type Values

   The IANA is instructed to allocate one Ethernet unicast address TBD2
   (suggest 00-00-5E-00-52-14 [RFC5214]) in the registry "IANA Ethernet
   Address Block - Unicast Use".

   The OMNI option also defines an 8-bit Sub-Type field, for which IANA
   is instructed to create and maintain a new registry entitled "OMNI
   option Sub-Type values".  Initial values for the OMNI option Sub-Type
   values registry are given below; future assignments are to be made
   through Expert Review [RFC8126].



















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      Value    Sub-Type name                  Reference
      -----    -------------                  ----------
      0        Pad1                           [RFCXXXX]
      1        PadN                           [RFCXXXX]
      2        Interface Attributes           [RFCXXXX]
      3        Traffic Selector               [RFCXXXX]
      4        MS-Register                    [RFCXXXX]
      5        MS-Release                     [RFCXXXX]
      6        Network Access Identifier      [RFCXXXX]
      7        Geo Coordinates                [RFCXXXX]
      8        DHCP Unique Identifier (DUID)  [RFCXXXX]
      9-252    Unassigned
      253-254  Experimental                   [RFCXXXX]
      255      Reserved                       [RFCXXXX]

                  Figure 17: OMNI Option Sub-Type Values

20.  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.

   OMNI interfaces configured over secured ANET interfaces inherit the
   physical and/or link-layer security properties of the connected
   ANETs.  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, an asymmetric security service such as SEcure
   Neighbor Discovery (SEND) [RFC3971] with Cryptographically Generated
   Addresses (CGAs) [RFC3972], the authentication option specified in
   [RFC4380] or other protocol control message security mechanisms may
   be necessary.  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.

   The Mobility Service MUST provide strong network layer security for
   control plane messages and forwarding path integrity for data plane
   messages.  In one example, the AERO service
   [I-D.templin-intarea-6706bis] constructs a spanning tree between
   mobility service elements and secures the links in the spanning tree
   with network layer security mechanisms such as IPsec [RFC4301] or
   Wireguard.  Control plane messages are then constrained to travel
   only over the secured spanning tree paths and are therefore protected
   from attack or eavesdropping.  Since data plane messages can travel
   over route optimized paths that do not strictly follow the spanning




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   tree, however, end-to-end transport- or higher-layer security
   services are still required.

   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 5.1.

21.  Implementation Status

   Draft -29 is implemented in the recently tagged AERO/OMNI 3.0.0
   internal release, and Draft -30 is now tagged as the AERO/OMNI 3.0.1.
   Newer specification versions will be tagged in upcoming releases.
   First public release expected before the end of 2020.

22.  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:
   Michael Matyas, Madhu Niraula, Michael Richardson, Greg Saccone,
   Stephane Tamalet, Eric Vyncke.  Pavel Drasil, Zdenek Jaron and Michal
   Skorepa are recognized for their many helpful ideas and suggestions.
   Madhuri Madhava Badgandi, Katherine Tran, and Vijayasarathy
   Rajagopalan are acknowledged for their hard work on the
   implementation and insights that led to improvements to 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.

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




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

23.  References

23.1.  Normative References

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

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

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

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

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,
              <https://www.rfc-editor.org/info/rfc3972>.

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

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

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

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



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

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

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

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

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

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

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

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

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







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23.2.  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.

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

   [I-D.ietf-6man-rfc4941bis]
              Gont, F., Krishnan, S., Narten, T., and R. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", draft-ietf-6man-rfc4941bis-11
              (work in progress), September 2020.

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

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

   [I-D.templin-6man-dhcpv6-ndopt]
              Templin, F., "A Unified Stateful/Stateless Configuration
              Service for IPv6", draft-templin-6man-dhcpv6-ndopt-10
              (work in progress), June 2020.

   [I-D.templin-intarea-6706bis]
              Templin, F., "Asymmetric Extended Route Optimization
              (AERO)", draft-templin-intarea-6706bis-66 (work in
              progress), October 2020.

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

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





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

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

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

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

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

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

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

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

   [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>.



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   [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>.

   [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>.

   [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>.





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   [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>.

   [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>.

   [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>.

   [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>.






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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>.

   [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>.

   [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>.

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








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Appendix A.  Interface Attribute Preferences Bitmap Encoding

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

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

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

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Sub-Type=2  |  Sub-length=N |    ifIndex    |    ifType     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  | Link  |R| APS | Bitmap(0)=0xff|P00|P01|P02|P03|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|P16|P17|P18|P19|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31| Bitmap(1)=0xff|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Bitmap(2)=0xff|P64|P65|P67|P68| ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

               Figure 18: Example 1: Dense Simplex Encoding








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

               Figure 19: Example 2: Sparse Simplex Encoding

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Sub-Type=2  |  Sub-length=N |    ifIndex    |    ifType     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  | Link  |R| APS |  Index = 0x00 | Bitmap = 0x80 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P00|P01|P02|P03|  Index = 0x01 | Bitmap = 0x01 |P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Index = 0x10 | Bitmap = 0x80 |512|513|514|515|  Index = 0x18 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Bitmap = 0x01 |796|797|798|799| ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                  Figure 20: Example 3: Indexed Encoding

Appendix B.  VDL Mode 2 Considerations

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





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

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

   Note that links of this nature may benefit from compression
   techniques that reduce the bandwidth necessary for conveying the same
   amount of data.  The IETF lpwan working group is considering possible
   alternatives: [https://datatracker.ietf.org/wg/lpwan/documents].

Appendix 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 ANET.  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 ANET links, ARs can maintain an
   OMNI-specific unicast L2 address ("MSADDR").  For Ethernet-compatible
   ANETs, this specification reserves one Ethernet unicast address TBD2
   (see: Section 19).  For non-Ethernet statically-addressed ANETs,
   MSADDR is reserved per the assigned numbers authority for the ANET
   addressing space.  For still other ANETs, 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.




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   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-interface-35 to draft-
   templin-6man-omni-interface-36:

   o  Major clarifications on aspects such as "hard/soft" PTB error
      messages

   o  Made generic so that either IP protocol version (IPv4 or IPv6) can
      be used in the data plane.

   Differences from draft-templin-6man-omni-interface-31 to draft-
   templin-6man-omni-interface-32:

   o  MTU

   o  Support for multi-hop ANETS such as ISATAP.

   Differences from draft-templin-6man-omni-interface-29 to draft-
   templin-6man-omni-interface-30:

   o  Moved link-layer addressing information into the OMNI option on a
      per-ifIndex basis

   o  Renamed "ifIndex-tuple" to "Interface Attributes"

   Differences from draft-templin-6man-omni-interface-27 to draft-
   templin-6man-omni-interface-28:

   o  Updates based on implementation expereince.

   Differences from draft-templin-6man-omni-interface-25 to draft-
   templin-6man-omni-interface-26:

   o  Further clarification on "aggregate" RA messages.

   o  Expanded Security Considerations to discuss expectations for
      security in the Mobility Service.

   Differences from draft-templin-6man-omni-interface-20 to draft-
   templin-6man-omni-interface-21:



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   o  Safety-Based Multilink (SBM) and Performance-Based Multilink
      (PBM).

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

   o  SEND/CGA.

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

   o  Teredo

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

   o  Prefix length discussions removed.

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

   o  Teredo

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

   o  Major simplifications and clarifications on MTU and fragmentation.

   o  Document now updates RFC4443 and RFC8201.

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

   o  Removed /64 assumption, resulting in new OMNI address format.

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

   o  OMNI MNs in the open Internet

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

   o  Brought back L2 MSADDR mapping text for MN / AR isolation based on
      L2 addressing.

   o  Expanded "Transition Considerations".




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

   o  Brought back OMNI option "R" flag, and discussed its use.

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

   o  Transition considerations, and overhaul of RS/RA addressing with
      the inclusion of MSE addresses within the OMNI option instead of
      as RS/RA addresses (developed under FAA SE2025 contract number
      DTFAWA-15-D-00030).

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

   o  Added "advisory PTB messages" under FAA SE2025 contract number
      DTFAWA-15-D-00030.

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

   o  Removed "Primary" flag and supporting text.

   o  Clarified that "Router Lifetime" applies to each ANET interface
      independently, and that the union of all ANET interface Router
      Lifetimes determines MSE lifetime.

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

   o  "All-MSEs" OMNI LLA defined.  Also reserved fe80::ff00:0000/104
      for future use (most likely as "pseudo-multicast").

   o  Non-normative discussion of alternate OMNI LLA construction form
      made possible if the 64-bit assumption were relaxed.

   First draft version (draft-templin-atn-aero-interface-00):

   o  Draft based on consensus decision of ICAO Working Group I Mobility
      Subgroup March 22, 2019.

Authors' Addresses








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

   Email: fltemplin@acm.org


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

   Email: tony.whyman@mccallumwhyman.com



































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