Network Working Group                                    F. Templin, Ed.
Internet-Draft                                        The Boeing Company
Intended status: Standards Track                               A. Whyman
Expires: October 5, 2020                 MWA Ltd c/o Inmarsat Global Ltd
                                                           April 3, 2020


   Transmission of IPv6 Packets over Overlay Multilink Network (OMNI)
                               Interfaces
                  draft-templin-6man-omni-interface-10

Abstract

   Mobile nodes (e.g., aircraft of various configurations, terrestrial
   vehicles, seagoing vessels, mobile enterprise 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 IPv6 packets over Overlay
   Multilink Network (OMNI) Interfaces.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on October 5, 2020.

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
   publication of this document.  Please review these documents



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Overlay Multilink Network (OMNI) Interface Model  . . . . . .   6
   5.  Maximum Transmission Unit (MTU) and Fragmentation . . . . . .  10
   6.  Frame Format  . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Link-Local Addresses  . . . . . . . . . . . . . . . . . . . .  11
   8.  SPAN Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Address Mapping - Unicast . . . . . . . . . . . . . . . . . .  13
     9.1.  Sub-Options . . . . . . . . . . . . . . . . . . . . . . .  14
       9.1.1.  Pad1  . . . . . . . . . . . . . . . . . . . . . . . .  15
       9.1.2.  PadN  . . . . . . . . . . . . . . . . . . . . . . . .  15
       9.1.3.  ifIndex-tuple (Type 1)  . . . . . . . . . . . . . . .  16
       9.1.4.  ifIndex-tuple (Type 2)  . . . . . . . . . . . . . . .  18
       9.1.5.  MS-Register . . . . . . . . . . . . . . . . . . . . .  18
       9.1.6.  MS-Release  . . . . . . . . . . . . . . . . . . . . .  19
   10. Address Mapping - Multicast . . . . . . . . . . . . . . . . .  19
   11. Conceptual Sending Algorithm  . . . . . . . . . . . . . . . .  19
     11.1.  Multiple OMNI Interfaces . . . . . . . . . . . . . . . .  20
   12. Router Discovery and Prefix Registration  . . . . . . . . . .  20
   13. AR and MSE Resilience . . . . . . . . . . . . . . . . . . . .  23
   14. Detecting and Responding to MSE Failures  . . . . . . . . . .  23
   15. Transition Considerations . . . . . . . . . . . . . . . . . .  24
   16. OMNI Interfaces on the Open Internet  . . . . . . . . . . . .  24
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   18. Security Considerations . . . . . . . . . . . . . . . . . . .  26
   19. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   20. References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     20.1.  Normative References . . . . . . . . . . . . . . . . . .  27
     20.2.  Informative References . . . . . . . . . . . . . . . . .  28
   Appendix A.  Type 1 ifIndex-tuple Traffic Classifier Preference
                Encoding . . . . . . . . . . . . . . . . . . . . . .  30
   Appendix B.  Prefix Length Considerations . . . . . . . . . . . .  32
   Appendix C.  VDL Mode 2 Considerations  . . . . . . . . . . . . .  33
   Appendix D.  MN / AR Isolation Through L2 Address Mapping . . . .  33
   Appendix E.  Change Log . . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39






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1.  Introduction

   Mobile Nodes (MNs) (e.g., aircraft of various configurations,
   terrestrial vehicles, seagoing vessels, mobile enterprise 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 IPv6 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 a worldwide Internetwork 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
   IPv6 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.

   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.

   This document specifies the transmission of IPv6 packets [RFC8200]
   and MN/MS control messaging over OMNI interfaces.



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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.
   Also, 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 defined in [RFC4291] (with Link-Local scope assumed).

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

   Mobile Node (MN)
      an end system with multiple distinct upstream data link
      connections that are managed together as a single logical unit.
      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 IPv6 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 singluar or aggregate) that
      coordinates the mobility events of one or more MN.

   Mobility Service Prefix (MSP)
      an aggregated IPv6 prefix (e.g., 2001:db8::/32) 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 IPv6 prefix taken from an MSP (e.g.,
      2001:db8:1000:2000::/56) 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 virtual overlay configured over one or more INETs and their
      connected ANETs.  An OMNI link can comprise multiple INET segments
      joined by bridges the same as for any link; the addressing plans
      in each segment may be mutually exclusive and managed by different
      administrative entities.

   OMNI interface
      a node's attachment to an OMNI link, and configured over one or
      more underlying ANET/INET interfaces.

   OMNI link local address (LLA)
      an IPv6 link-local address constructed as specified in Section 7,
      and assigned to an OMNI interface.

   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



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      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", "IPv6 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.

   Spanning Partitioned Administrative Networks (SPAN)
      A means for bridging disjoint INET partitions as segments of a
      unified OMNI link the same as for a bridged campus LAN.  The SPAN
      is a mid-layer IPv6 encapsulation service that supports a unified
      AERO link view for all segments.

3.  Requirements

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

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

4.  Overlay Multilink Network (OMNI) Interface Model

   An OMNI interface is a 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.



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   The OMNI interface architectural layering model is the same as in
   [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.

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

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

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

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

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





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   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  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.

   Other opportunities are discussed in [RFC7847].

   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 IPv6 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.








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5.  Maximum Transmission Unit (MTU) and Fragmentation

   All IPv6 interfaces are REQUIRED to configure a minimum Maximum
   Transmission Unit (MTU) of 1280 bytes [RFC8200].  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].

   The OMNI interface configures an MTU 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 employs mid-layer IPv6 encapsulation and
   fragmentation/reassembly per [RFC2473] if necssary to accommodate
   large packets.  The interface returns internally-generated PTB
   messages for packets admitted into the interface that it deems too
   large for outbound underlying interfaces (e.g., according to
   underlying interface performance characteristics, cost, MTU, etc).
   For all other packets, the OMNI interface performs PMTUD even if the
   destination appears to be on the same link since an OMNI link node on
   the path could return a PTB message.  This ensures that the path MTU
   is adaptive and reflects the current path used for a given data flow.

   For underlying interfaces that have sufficiently large MTUs, the MN's
   OMNI interface sends packets according to the ANET interface L2 frame
   format without fragmentation.  For all other cases, the OMNI
   interface encapsulates the packet in a mid-layer IPv6 header with
   source address set to the MN's SPAN address and destination set to
   the SPAN address corresponding to the packet's destination (see:
   Section 8).  The OMNI interface then uses IPv6 fragmentation to break
   the encapsulated packet into a minimum number of non-overlapping
   fragments, where the smallest fragment generated MUST be no smaller
   than 640 bytes.  For ANET interfaces that connect via ARs, the
   largest fragment size is determined by the ANET interface MTU, while
   for other underllying interface types the largest fragment size MUST
   be 1280 bytes.  (Note that the outbound fragments can further be
   spread across multiple underlying interfaces, since they will be
   reassembled by the OMNI interface closest to the final destination.)

   When an AR receives a fragmented or whole packet from the INET
   destined to an ANET MN, it first determines whether to forward or
   drop and return a PTB.  If the AR deems the packet to be of
   acceptable size, it first re-adjusts fragment sizes (if necessary)
   then forwards the packet/fragments to the MN.  If the packet is no
   larger than the ANET MTU, the AR forwards according to the ANET L2
   frame format.  If the packet is larger than the ANET MTU, the AR
   instead uses IPv6 encapsulation and fragmentation as above.  The MN



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   then reassembles and discards the encapsulation header, then forwards
   the whole packet to the final destination.

   In order to avoid a "tiny fragment" attack, AERO nodes
   unconditionally drop all fragments smaller than 640 bytes.  In order
   to set the correct context for reassembly, the AERO node that inserts
   a SPAN header MUST also be the node that inserts the IPv6 Fragment
   Header Identification value.

   Note also that the OMNI interface can forward large packets via
   encapsulation and fragmentation while at the same time returning
   advisory PTB messages, e.g., subject to rate limiting.  The receiving
   node that performs reassembly can also send advisory PTB messages if
   reassembly conditions become unfavorable.  The OMNI interface can
   therefore continuously forward large packets without loss while
   returning advisory messages recommending a smaller size.

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

   OMNI interfaces assign IPv6 Link-Local Addresses (i.e., "OMNI LLAs")
   using the following constructs:

   o  IPv6 MN OMNI LLAs encode the most-significant 64 bits of a MNP
      within the least-significant 64 bits (i.e., the interface ID) of a
      Link-Local IPv6 Unicast Address (see: [RFC4291], Section 2.5.6).
      For example, for the MNP 2001:db8:1000:2000::/56 the corresponding
      LLA is fe80::2001:db8:1000:2000.

   o  IPv4-compatible MN OMNI LLAs are assigned as fe80::ffff:[v4addr],
      i.e., the most significant 10 bits of the prefix fe80::/10,
      followed by 70 '0' bits, followed by 16 '1' bits, followed by a
      32bit IPv4 address.  For example, the IPv4-Compatible MN OMNI LLA
      for 192.0.2.1 is fe80::ffff:192.0.2.1 (also written as
      fe80::ffff:c000:0201).

   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



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      0x00000001 and 0xfeffffff, e.g., as in fe80::1, fe80::2, fe80::3,
      etc., fe80::feff:ffff.  The MSID 0x00000000 corresponds to the
      link-local Subnet-Router anycast address (fe80::) [RFC4291] and
      the MSID 0xffffffff corresponds to the "All-MSEs" address
      (fe80::ffff:ffff).  The MSID range 0xff00000000 through 0xfffffffe
      is reserved for future use.  (Note that distinct OMNI link
      segments can avoid overlap by assigning MS OMNI LLAs from unique
      fe80::/96 sub-prefixes.  For example, a first segment could assign
      from fe80::1000/116, a second from fe80::2000/116, a third from
      fe80::3000/116, etc.)

   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].

8.  SPAN Addresses

   OMNI links employ an overlay network instance called the SPAN
   (Spanning Partitioned Administrative Networks) that supports
   forwarding of link-local messages over a private IPv6 routing
   instance that spans the entire link without decrementing the (link-
   local) Hop Limit.  The OMNI link reserves the Unique Local Address
   (ULA) prefix fd80::/10 [RFC4193] known as the SPAN Service Prefix
   (SSP) and used for mapping OMNI LLAs to IPv6 addresses that are
   routable via the SPAN.

   SPAN addresses are configured in one-to-one correspondence with MN/MS
   OMNI LLAs by simply zeroing bit 7 of the LLA.  For example:

   o  the SPAN address corresponding to fe80::2001:db8:1:2 is simply
      fd80::2001:db8:1:2

   o  the SPAN address corresponding to fe80::ffff:192.0.2.1 is simply
      fd80::ffff:192.0.2.1

   o  the SPAN address corresponding to fe80::1000 is simply fd80::1000

   Note that for MNPs longer than 64 bits (see: Appendix B), the
   resulting OMNI LLA would include non-routable bits when converted to
   a SPAN address.  For this reason, the Subnet-Router anycast address
   for an IPv6 MNP is used as a "pseudo-SPAN" address instead of the
   fd80::/10 expansion (e.g., 2001:db8:1:2:: insteadl of
   fd80::2001:db8:1:2).  This implies that the SPAN routing system needs



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   to maintain multiple types of IPv6 routes (i.e. a mix of ULA and GUA
   routes), which would not be necessary if a mandate naming /64 as the
   longest IPv6 prefix length were issued.

   Further details of the SPAN are out of scope for this document.  A
   full discussion of the SPAN appears in [I-D.templin-intarea-6706bis].

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.
   IPv6 Neighbor Discovery (ND) [RFC4861] messages on MN OMNI interfaces
   observe the native Source/Target Link-Layer Address Option (S/TLLAO)
   formats of the underlying interfaces (e.g., for Ethernet the S/TLLAO
   is specified in [RFC2464]).

   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:

        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 |R|   Reserved  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                          Sub-Options                          ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 3: OMNI Option Format

   In this format:

   o  Type is set to TBD.

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

   o  Prefix Length is set according to the IPv6 source address type.
      For MN OMNI LLAs, the value is set to the length of the embedded
      MNP.  For IPv4-compatible MN OMNI LLAs, the value is set to 96




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      plus the length of the embedded IPv4 prefix.  For MS OMNI LLAs,
      the value is set to 128.

   o  R (the "Register/Release" bit) is set to 1/0 to request the
      message recipient to register/release a MN's MNP.  The OMNI option
      may additionally include MSIDs for the recipient to contact to
      also register/release the MNP.

   o  Reserved is set to the value '0' on transmission and ignored on
      reception.

   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 options, as described in Section 8.1.

9.1.  Sub-Options

   The OMNI option includes zero or more Sub-Options, some of which may
   appear multiple times in the same message.  Each consecutive Sub-
   Option is concatenated immediately after its predecessor.  All Sub-
   Options except Pad1 (see below) are 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-byte field that encodes the Sub-Option type.  Sub-
      Options defined in this document are:

        Option Name            Sub-Type
        Pad1                        0
        PadN                        1
        ifIndex-tuple (Type 1)      2
        ifIndex-tuple (Type 2)      3
        MS-Register                 4
        MS-Release                  5

                                 Figure 5

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





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   o  Sub-Length is a 1-byte field that encodes the length of the Sub-
      Option Data, in bytes

   o  Sub-Option Data is a byte string 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.

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

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.

   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-2 | N-2 padding bytes ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                              Figure 7: PadN

   o  Sub-Type is set to 1.

   o  Sub-Length is set to N-2 being the number of padding bytes that
      follow.

   o  Sub-Option Data consists of N-2 zero-valued octets.








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9.1.3.  ifIndex-tuple (Type 1)

        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=4+N|    ifIndex    |    ifType     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  | Link  |S|I|RSV| 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| ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                     Figure 8: ifIndex-tuple (Type 1)

   o  Sub-Type is set to 2.

   o  Sub-Length is set to 4+N (the number of Sub-Option Data bytes that
      follow).

   o  Sub-Option Data contains an "ifIndex-tuple" (Type 1) encoded as
      follows (note that the first four bytes must be present):

      *  ifIndex is set to an 8-bit integer value corresponding to a
         specific underlying interface.  OMNI options MAY include
         multiple ifIndex-tuples, and MUST number each with an ifIndex
         value between '1' and '255' that represents a MN-specific 8-bit
         mapping for the actual ifIndex value assigned to the underlying
         interface by network management [RFC2863] (the ifIndex value
         '0' is reserved for use by the MS).  Multiple ifIndex-tuples
         with the same ifIndex value MAY appear in the same OMNI option.

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

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




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      *  S is set to '1' if this ifIndex-tuple corresponds to the
         underlying interface that is the source of the ND message.  Set
         to '0' otherwise.

      *  I is set to '0' ("Simplex") if the index for each singleton
         Bitmap byte in the Sub-Option Data is inferred from its
         sequential position (i.e., 0, 1, 2, ...), or set to '1'
         ("Indexed") if each Bitmap is preceded by an Index byte.
         Figure 8 shows the simplex case for I set to '0'.  For I set to
         '1', each Bitmap is instead preceded by an Index byte that
         encodes a value "i" = (0 - 255) as the index for its companion
         Bitmap as follows:

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

                                 Figure 9

      *  RSV is set to the value 0 on transmission and ignored on
         reception.

      *  The remainder of the Sub-Option Data contains N = (0 - 251)
         bytes of traffic classifier preferences consisting of a first
         (indexed) Bitmap (i.e., "Bitmap(i)") followed by 0-8 1-byte
         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 showin 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.  The first 16 P[*] blocks correspond to the 64
         Differentiated Service Code Point (DSCP) values P[00] - P[63]
         [RFC2474].  If additional P[*] blocks follow, their values
         correspond to "pseudo-DSCP" traffic classifier values P[64],
         P[65], P[66], etc.  See Appendix A for further discussion and
         examples.

      *  Each 2-bit P[*] field is set to the value '0' ("disabled"), '1'
         ("low"), '2' ("medium") or '3' ("high") to indicate a QoS
         preference level for underlying interface selection purposes.
         Not all P[*] values need to be included in all OMNI option
         instances of a given ifIndex-tuple.  Any P[*] values



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         represented in an earlier OMNI option but ommitted in the
         current OMNI option remain unchanged.  Any P[*] values not yet
         represented in any OMNI option default to "medium".

9.1.4.  ifIndex-tuple (Type 2)

        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=4+N|    ifIndex    |    ifType     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  | Link  |S|Resvd|                               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
       ~                                                               ~
       ~                RFC 6088 Format Traffic Selector               ~
       ~                                                               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 10: ifIndex-tuple (Type 2)

   o  Sub-Type is set to 3.

   o  Sub-Length is set to 4+N (the number of Sub-Option Data bytes that
      follow).

   o  Sub-Option Data contains an "ifIndex-tuple" (Type 2) encoded as
      follows (note that the first four bytes must be present):

      *  ifIndex, ifType, Provider ID, Link and S are set exactly as for
         Type 1 ifIndex-tuples as specified in Section 9.1.3.

      *  the remainder of the Sub-Option body encodes a variable-length
         traffic selector formatted per [RFC6088], beginning with the
         "TS Format" field.

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=4  |        MSID (bits 0 - 15)     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      MSID (bits 16 - 32)      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 11: MS-Register Sub-option

   o  Sub-Type is set to 4.



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   o  Sub-Length is set to 4.

   o  MSID contains the 32 bit ID of an MSE or AR, in network byte
      order.  OMNI options contain zero or more MS-Register sub-options.

9.1.6.  MS-Release

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Sub-Type=5  | Sub-length=4  |        MSID (bits 0 - 15)     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      MSID (bits 16 - 32)      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 12: MS-Release Sub-option

   o  Sub-Type is set to 5.

   o  Sub-Length is set to 4.

   o  MSIID contains the 32 bit ID of an MS or AR, in network byte
      order.  OMNI options contain zero or more MS-Release sub-options.

10.  Address Mapping - Multicast

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

11.  Conceptual Sending Algorithm

   The MN's IPv6 layer selects the outbound OMNI interface according to
   standard IPv6 requirements when forwarding data packets from local or
   EUN applications to external correspondents.  The OMNI interface
   maintains a neighbor cache the same as for any IPv6 interface, but
   with additional state for multilink coordination.

   After a packet enters the OMNI interface, an outbound underlying
   interface is selected based on multilink parameters 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.





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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 associate with multiple MS instances concurrently.  Each MS
   instance represents a distinct OMNI link distinguished by its
   associated MSPs.  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.

   Depending on local policy and configuration, an MN may choose between
   alternative active OMNI interfaces using a packet's DSCP, routing
   information or static configuration.  Interface selection based on
   per-packet source addresses is also enabled when the MSPs for each
   OMNI interface are known (e.g., discovered through Prefix Information
   Options (PIOs) and/or Route Information Options (RIOs)).

   Each OMNI interface can be configured over the same or different sets
   of underlying interfaces.  Each ANET distinguishes between the
   different OMNI links based on the MSPs represented in per-packet IPv6
   addresses.

   Multiple distinct OMNI links can therefore be used to support fault
   tolerance, load balancing, reliability, etc.  The architectural model
   parallels Layer 2 Virtual Local Area Networks (VLANs), where the MSPs
   serve as (virtual) VLAN tags.

12.  Router Discovery and Prefix Registration

   MNs interface with the MS by sending RS messages with OMNI options
   that include MSIDs.  For each underlying interface, the MN sends an
   RS message with an OMNI option with (R,A) flags, wth MS-Register/
   Release suboptions, and with destination address set to All-Routers
   multicast (ff02::2) [RFC4291].  Example MSID discovery methods are
   given in [RFC5214], including data link login parameters, name
   service lookups, static configuration, etc.  Alternatively, MNs can
   discover indiviual MSIDs by sending an initial RS with MS-Register
   MSID set to 0x00000000, or associate with all MSEs by sending an RS
   with MS-Register MSID set to 0xffffffff.

   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



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   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.  The RS messages include an
   OMNI option per Section 9 with a valid Prefix Length, (R, A) flags,
   ifIndex-tuples appropriate for underlying interfaces and with MS-
   Register/Release sub-options.

   ARs process IPv6 ND messages with OMNI options and act as a proxy for
   MSEs.  ARs receive RS messages and create a neighbor cache entry for
   the MN, then coordinate with any named MSIDs in a manner outside the
   scope of this document.  The AR returns an RA message with
   destination address set to the MN OMNI LLA (i.e., unicast), with
   source address set to its MS OMNI LLA, with the P(roxy) bit set in
   the RA flags [RFC4389], with an OMNI option with (R, A) flags,
   ifIndex tuples and MS-Register/Release sub-options, and with any
   information for the link that would normally be delivered in a
   solicited RA message.  ARs return RA messages with configuration
   information in response to a MN's RS messages.  The AR sets the 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 coordinates with each Register/Release MSID then sends an
   immediate unicast RA response without delay; therefore, the IPv6 ND
   MAX_RA_DELAY_TIME and MIN_DELAY_BETWEEN_RAS constants for multicast
   RAs do not apply.  The AR MAY send periodic and/or event-driven
   unsolicited RA messages according to the standard [RFC4861].

   When the MSE processes the OMNI information, it first validates the
   prefix registration information.  The MSE then injects/withdraws the
   MNP in the routing/mapping system and caches/discards the new Prefix
   Length, MNP and ifIndex-tuples.  The MSE then informs the AR of



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   registration success/failure, and the AR adds the MSE to the list of
   Register/Release MSIDs to return in an RA message OMNI option per
   Section 9.

   When the MN receives the RA message, it creates an OMNI interface
   neighbor cache entry with the AR's address as an L2 address and
   records the MSIDs that have confirmed MNP registration via this AR.
   If the MN connects to multiple ANETs, it establishes additional AR L2
   addresses (i.e., as a Multilink neighbor).  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 with R set to 1.
      The OMNI option contains at least one ifIndex-tuple with values
      specific to this underlying interface, and may contain additional
      ifIndex-tuples specific to this and/or other underlying
      interfaces.  The option also includes any Register/Release MSIDs.

   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 ifIndex-tuple 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 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 a an UP
   underlying interface, 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.

   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 finer-grained
   timescale than the MN is required to do on its own behalf.

13.  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.

14.  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.



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   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 the MN 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.

15.  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, the P(roxy) bit set in the RA flags 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 D.  This arrangement imparts a (virtual)
   point-to-point link model over the (physical) multiple access link.

16.  OMNI Interfaces on the Open Internet

   OMNI interfaces that connect to the open Internet via native and/or
   NATed underlying interfaces can apply symmetric security services
   such as VPNs to establish secured tunnels to MSEs.  In environments



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   where an explicit VPN may be too restrictive, OMNI interfaces can
   instead ensure neighbor cache integrity using SEcure Neighbor
   Discovery (SEND) [RFC3971] and Cryptographically Generated Addresses
   (CGAs) [RFC3972].

   When SEND/CGA are used, the IPv6 ND control plane messages used to
   establish neighbor cache state are authenticated while data plane
   messages are delivered the same as for ordinary best-effort Internet
   traffic.  Instead, data plane communications via OMNI interfaces that
   connect over the open Internet without an explicit VPN must emply
   transport- or higher-layer security to ensure integrity and/or
   confidentiality.

   In addition to secured OMNI interface RS/RA exchanges, SEND/CGA
   supports safe address resolution and neighbor unreachability
   detection as discused in Asymmetric Extended Route Optimization
   (AERO) [I-D.templin-intarea-6706bis].  This allows for efficient
   multilink operations over the open Internet with assured neighbor
   cache integrity.

17.  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 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].

      Value    Sub-Type name              Reference
      -----    -------------              ----------
      0        Pad1                       [RFCXXXX]
      1        PadN                       [RFCXXXX]
      2        ifIndex-tuple (Type 1)     [RFCXXXX]
      3        ifIndex-tuple (Type 2)     [RFCXXXX]
      4        MS-Register                [RFCXXXX]
      5        MS-Release                 [RFCXXXX]
      6-252    Unassigned
      253-254  Experimental               [RFCXXXX]
      255      Reserved                   [RFCXXXX]

                  Figure 13: OMNI Option Sub-Type Values





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   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".

18.  Security Considerations

   Security considerations for IPv6 [RFC8200] and IPv6 Neighbor
   Discovery [RFC4861] apply.  OMNI interface IPv6 ND messages SHOULD
   include Nonce and Timestamp options [RFC3971] when synchronized
   transaction confirmation is needed.

   OMNI interfaces configured over secured underlying ANET interfaces
   inherit the physical and/or link-layer security aspects of the
   connected ANETs.  OMNI interfaces configured over open Internet
   interfaces must use symmetric securing services such as VPNs or
   asymmetric services such as SEND/CGA [RFC3971][RFC3972].

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

19.  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, Greg Saccone, Stephane Tamalet, Eric
   Vyncke.  Pavel Drasil, Zdenek Jaron and Michal Skorepa are recognized
   for their many helpful ideas and suggestions.

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

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





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20.  References

20.1.  Normative References

   [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>.

   [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>.






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   [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>.

20.2.  Informative References

   [I-D.templin-intarea-6706bis]
              Templin, F., "Asymmetric Extended Route Optimization
              (AERO)", draft-templin-intarea-6706bis-37 (work in
              progress), April 2020.

   [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>.




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   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.

   [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>.




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   [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>.

   [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>.

   [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>.

Appendix A.  Type 1 ifIndex-tuple Traffic Classifier Preference Encoding

   Adaptation of the OMNI option Type 1 ifIndex-tuple's traffic
   classifier Bitmap 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 ifIndex-tuple
   the same format must be used for the entire Sub-Option.  Different
   Sub-Options may use different formats.

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




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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=4+N|    ifIndex    |    ifType     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  | Link  |S|0|RSV| 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 14: Example 1: Dense Simplex Encoding

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Sub-Type=2  | Sub-length=4+N|    ifIndex    |    ifType     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  | Link  |S|0|RSV| 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 15: Example 2: Sparse Simplex Encoding










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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Sub-Type=2  | Sub-length=4+N|    ifIndex    |    ifType     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Provider ID  | Link  |S|1|RSV|  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 16: Example 3: Indexed Encoding

Appendix B.  Prefix Length Considerations

   The 64-bit boundary in IPv6 addresses [RFC7421] determines the MN
   OMNI LLA format for encoding the most-significant 64 MNP bits into
   the least-significant 64 bits of the prefix fe80::/64 as discussed in
   Section 7.

   [RFC4291] defines the link-local address format as the most
   significant 10 bits of the prefix fe80::/10, followed by 54 unused
   bits, followed by the least-significant 64 bits of the address.  If
   the 64-bit boundary is relaxed through future standards activity,
   then the 54 unused bits can be employed for extended coding of MNPs
   of length /65 up to /118.

   The extended coding format would continue to encode MNP bits 0-63 in
   bits 64-127 of the OMNI LLA, while including MNP bits 64-117 in bits
   10-63.  For example, the OMNI LLA corresponding to the MNP
   2001:db8:1111:2222:3333:4444:5555::/112 would be
   fe8c:ccd1:1115:5540:2001:db8:1111:2222/128, and would still be a
   valid IPv6 LLA per [RFC4291].  However, a prefix length shorter than
   /128 cannot be applied due to the non-sequential byte ordering.

   Note that if the 64-bit boundary were relaxed an alternate form of
   OMNI LLA construction could be employed by embedding the MNP
   beginning with the most significant bit immediately following bit 10
   of the prefix fe80::/10.  For example, the OMNI LLA corresponding to
   the MNP 2001:db8:1111:2222:3333:4444:5555::/112 would be written as
   fe88:0043:6e04:4448:888c:ccd1:1115:5540/122.  This alternate form may
   provide a more natural coding for the MS along with the ability to
   apply a fully-qualified prefix length.  It has the disadvantages of
   requiring an unweildy 10-bit right-shift of a 16byte address, as well
   as presenting a non-human-readable form.



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Appendix C.  VDL Mode 2 Considerations

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

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

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

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

Appendix D.  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
   coordinations 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 17).  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.



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

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

Appendix E.  Change Log

   << RFC Editor - remove prior to publication >>

   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  Explanded "Transition Considerations".

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

   o  Brought back OMNI option "R" flag, and dicussed 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.



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   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 reserverd 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.

   Differences from draft-templin-atn-aero-interface-21 to draft-
   templin-6man-omni-interface-00:

   o  Minor clarification on Type-2 ifIndex-tuple encoding.

   o  Draft filename change (replaces draft-templin-atn-aero-interface).

   Differences from draft-templin-atn-aero-interface-20 to draft-
   templin-atn-aero-interface-21:

   o  OMNI option format

   o  MTU

   Differences from draft-templin-atn-aero-interface-19 to draft-
   templin-atn-aero-interface-20:

   o  MTU

   Differences from draft-templin-atn-aero-interface-18 to draft-
   templin-atn-aero-interface-19:

   o  MTU

   Differences from draft-templin-atn-aero-interface-17 to draft-
   templin-atn-aero-interface-18:

   o  MTU and RA configuration information updated.





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   Differences from draft-templin-atn-aero-interface-16 to draft-
   templin-atn-aero-interface-17:

   o  New "Primary" flag in OMNI option.

   Differences from draft-templin-atn-aero-interface-15 to draft-
   templin-atn-aero-interface-16:

   o  New note on MSE OMNI LLA uniqueness assurance.

   o  General cleanup.

   Differences from draft-templin-atn-aero-interface-14 to draft-
   templin-atn-aero-interface-15:

   o  General cleanup.

   Differences from draft-templin-atn-aero-interface-13 to draft-
   templin-atn-aero-interface-14:

   o  General cleanup.

   Differences from draft-templin-atn-aero-interface-12 to draft-
   templin-atn-aero-interface-13:

   o  Minor re-work on "Notify-MSE" (changed to Notification ID).

   Differences from draft-templin-atn-aero-interface-11 to draft-
   templin-atn-aero-interface-12:

   o  Removed "Request/Response" OMNI option formats.  Now, there is
      only one OMNI option format that applies to all ND messages.

   o  Added new OMNI option field and supporting text for "Notify-MSE".

   Differences from draft-templin-atn-aero-interface-10 to draft-
   templin-atn-aero-interface-11:

   o  Changed name from "aero" to "OMNI"

   o  Resolved AD review comments from Eric Vyncke (posted to atn list)

   Differences from draft-templin-atn-aero-interface-09 to draft-
   templin-atn-aero-interface-10:

   o  Renamed ARO option to AERO option

   o  Re-worked Section 13 text to discuss proactive NUD.



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   Differences from draft-templin-atn-aero-interface-08 to draft-
   templin-atn-aero-interface-09:

   o  Version and reference update

   Differences from draft-templin-atn-aero-interface-07 to draft-
   templin-atn-aero-interface-08:

   o  Removed "Classic" and "MS-enabled" link model discussion

   o  Added new figure for MN/AR/MSE model.

   o  New Section on "Detecting and responding to MSE failure".

   Differences from draft-templin-atn-aero-interface-06 to draft-
   templin-atn-aero-interface-07:

   o  Removed "nonce" field from AR option format.  Applications that
      require a nonce can include a standard nonce option if they want
      to.

   o  Various editorial cleanups.

   Differences from draft-templin-atn-aero-interface-05 to draft-
   templin-atn-aero-interface-06:

   o  New Appendix C on "VDL Mode 2 Considerations"

   o  New Appendix D on "RS/RA Messaging as a Single Standard API"

   o  Various significant updates in Section 5, 10 and 12.

   Differences from draft-templin-atn-aero-interface-04 to draft-
   templin-atn-aero-interface-05:

   o  Introduced RFC6543 precedent for focusing IPv6 ND messaging to a
      reserved unicast link-layer address

   o  Introduced new IPv6 ND option for Aero Registration

   o  Specification of MN-to-MSE message exchanges via the ANET access
      router as a proxy

   o  IANA Considerations updated to include registration requests and
      set interim RFC4727 option type value.

   Differences from draft-templin-atn-aero-interface-03 to draft-
   templin-atn-aero-interface-04:



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   o  Removed MNP from aero option format - we already have RIOs and
      PIOs, and so do not need another option type to include a Prefix.

   o  Clarified that the RA message response must include an aero option
      to indicate to the MN that the ANET provides a MS.

   o  MTU interactions with link adaptation clarified.

   Differences from draft-templin-atn-aero-interface-02 to draft-
   templin-atn-aero-interface-03:

   o  Sections re-arranged to match RFC4861 structure.

   o  Multiple aero interfaces

   o  Conceptual sending algorithm

   Differences from draft-templin-atn-aero-interface-01 to draft-
   templin-atn-aero-interface-02:

   o  Removed discussion of encapsulation (out of scope)

   o  Simplified MTU section

   o  Changed to use a new IPv6 ND option (the "aero option") instead of
      S/TLLAO

   o  Explained the nature of the interaction between the mobility
      management service and the air interface

   Differences from draft-templin-atn-aero-interface-00 to draft-
   templin-atn-aero-interface-01:

   o  Updates based on list review comments on IETF 'atn' list from
      4/29/2019 through 5/7/2019 (issue tracker established)

   o  added list of opportunities afforded by the single virtual link
      model

   o  added discussion of encapsulation considerations to Section 6

   o  noted that DupAddrDetectTransmits is set to 0

   o  removed discussion of IPv6 ND options for prefix assertions.  The
      aero address already includes the MNP, and there are many good
      reasons for it to continue to do so.  Therefore, also including
      the MNP in an IPv6 ND option would be redundant.




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   o  Significant re-work of "Router Discovery" section.

   o  New Appendix B on Prefix Length considerations

   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

   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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