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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Standards Track                               A. Whyman
Expires: February 22, 2020               MWA Ltd c/o Inmarsat Global Ltd
                                                         August 21, 2019


   Transmission of IPv6 Packets over Aeronautical ("aero") Interfaces
                draft-templin-atn-aero-interface-06.txt

Abstract

   Mobile nodes (e.g., aircraft of various configurations) communicate
   with networked correspondents over multiple access network data links
   and configure mobile routers to connect their on-board networks.
   Mobile nodes connect to access networks using either the classic or
   mobility service-enabled link model.  This document specifies the
   transmission of IPv6 packets over aeronautical ("aero") 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 February 22, 2020.

Copyright Notice

   Copyright (c) 2019 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
   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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Aeronautical ("aero") Interface Model . . . . . . . . . . . .   4
   5.  Maximum Transmission Unit . . . . . . . . . . . . . . . . . .   6
   6.  Frame Format  . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Link-Local Addresses  . . . . . . . . . . . . . . . . . . . .   6
   8.  Address Mapping - Unicast . . . . . . . . . . . . . . . . . .   8
   9.  Address Mapping - Multicast . . . . . . . . . . . . . . . . .  11
   10. Address Mapping for IPv6 Neighbor Discovery Messages  . . . .  11
   11. Conceptual Sending Algorithm  . . . . . . . . . . . . . . . .  12
     11.1.  Multiple Aero Interfaces . . . . . . . . . . . . . . . .  12
   12. Router Discovery and Prefix Registration  . . . . . . . . . .  13
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     16.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Aero Registration Option Extensions for Special-
                Purpose Links  . . . . . . . . . . . . . . . . . . .  19
   Appendix B.  Prefix Length Considerations . . . . . . . . . . . .  20
   Appendix C.  VDL Mode 2 Considerations  . . . . . . . . . . . . .  20
   Appendix D.  RS/RA Messaging as the Single Standard API . . . . .  22
   Appendix E.  Change Log . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   Mobile Nodes (MNs) such as aircraft of various configurations may
   have multiple data links for communicating with networked
   correspondents.  These data links often have differing performance,
   cost and availability characteristics that can change dynamically
   according to mobility patterns, flight phases, proximity to
   infrastructure, etc.

   Each MN receives an IPv6 Mobile Network Prefix (MNP) that can be used
   by on-board networks independently of the access network data links
   selected for data transport.  The MN performs router discovery the
   same as for customer edge routers [RFC7084], and acts as a mobile
   router on behalf of its on-board networks.  The MN connects to access
   networks using either the classic [RFC4861] or Mobility Service (MS)-
   enabled link model.



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   In the classic model, all IPv6 Neighbor Discovery (IPv6 ND) messaging
   is directly over native access network interfaces managed according
   to the weak end system model.  The MN discovers neighbors on the link
   through link-scoped multicast and/or unicast transmissions that map
   to their corresponding link layer addresses per standard address
   resolution / mapping procedures.  The MN then coordinates with
   mobility agents located in the larger Internetwork beyond the first-
   hop access links by employing an on-board mobility function.  This
   arrangement requires the MN to engage in active mobility messaging on
   its own behalf and with no assistance from the access network.

   In the MS-enabled model, a virtual interface (termed the "aero
   interface") is configured as a thin layer over the underlying access
   network interfaces.  The aero 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 access network interfaces appear as link layer
   communication channels in the architecture.  The aero interface
   connects to a virtual overlay cloud service known as the "aero link".

   Each aero link has one or more associated Mobility Service Prefixes
   (MSPs) that identify the link.  An MSP is an aggregated IPv6 prefix
   from which aero link MNPs are derived.  If the MN connects to
   multiple aero links, then it configures a separate aero interface for
   each link.

   The aero interface interacts with the ground-domain MS through IPv6
   ND control message exchanges [RFC4861].  The MS tracks MN movements
   and represents their MNPs in a global routing or mapping system.

   The aero interface provides a traffic engineering nexus for guiding
   inbound and outbound traffic to the correct underlying interface(s).
   The IPv6 layer sees the aero interface as a point of connection to
   the aero link; if there are multiple aero links (i.e., multiple
   MS's), the IPv6 layer will see multiple aero interfaces.

   This document specifies the transmission of IPv6 packets [RFC8200]
   and MN/MS control messaging over aeronautical ("aero") interfaces in
   the MS-enabled link model, and also includes all necessary details
   for MN operation in the classic link model.

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.

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



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   Access Network (ANET)
      a data link service network (e.g., an aviation radio access
      network, satellite service provider network, cellular operator
      network, etc.) protected by physical and/or link layer security.
      Each ANET connects to outside Internetworks via border security
      devices such as proxys, firewalls, packet filtering gateways, etc.

   ANET interface
      a node'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 mobile nodes and
      INET correspondents.  Examples include private enterprise
      networks, aviation networks and the global public Internet itself.

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

   aero link
      a virtual overlay cloud service configured over one or more INETs
      and their connected ANETs.  An aero link may comprise multiple
      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.

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

   aero address
      an IPv6 link-local address constructed as specified in Section 7,
      and assigned to an aero interface.

3.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  Lower case
   uses of these words are not to be interpreted as carrying RFC2119
   significance.

4.  Aeronautical ("aero") Interface Model

   An aero interface is a MN virtual interface configured over one or
   more ANET interfaces, which may be physical (e.g., an aeronautical
   radio link) or virtual (e.g., an Internet or higher-layer "tunnel").
   The MN coordinates with the MS through IPv6 ND message exchanges.



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   The aero interface architectural layering model is the same as in
   [RFC7847], and augmented as shown in Figure 1.  The IPv6 layer
   therefore sees the aero interface as a single network layer interface
   with multiple underlying ANET interfaces that appear as link layer
   communication channels in the architecture.

                                     +----------------------------+
                                     |          TCP/UDP           |
              Session-to-IP    +---->|                            |
              Address Binding  |     +----------------------------+
                               +---->|            IPv6            |
              IP Address       +---->|                            |
              Binding          |     +----------------------------+
                               +---->|       aero Interface       |
              Logical-to-      +---->|       (aero address)       |
              Physical         |     +----------------------------+
              Interface        +---->|  L2  |  L2  |       |  L2  |
              Binding                |(IF#1)|(IF#2)| ..... |(IF#n)|
                                     +------+------+       +------+
                                     |  L1  |  L1  |       |  L1  |
                                     |      |      |       |      |
                                     +------+------+       +------+

           Figure 1: Aero Interface Architectural Layering Model

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

   o  since aero interface link-local addresses are uniquely derived
      from an MNP (see: Section 7, no Duplicate Address Detection (DAD)
      messaging is necessary over the aero interface.

   o  ANET interfaces can remain unnumbered in environments where
      communications are coordinated entirely over the aero interface.

   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.

   o  exposing a single virtual interface abstraction to the IPv6 layer
      allows for traffic engineering (including QoS based link
      selection, packet replication, load balancing, etc.) at the link



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      layer while still permitting queuing at the IPv6 layer based on,
      e.g., traffic class, flow label, etc.

   o  the IPv6 layer sees the aero interface as a point of connection to
      the aero link; if there are multiple aero links (i.e., multiple
      MS's), the IPv6 layer will see multiple aero interfaces.

   Other opportunities are discussed in [RFC7847].

5.  Maximum Transmission Unit

   All IPv6 interfaces MUST configure an MTU of at least 1280 bytes
   [RFC8200], while the aero interface configures an MTU based on the
   largest MTU among all underlying ANET interfaces.

   The aero interface returns internally-generated IPv6 Path MTU
   Discovery (PMTUD) Packet Too Big (PTB) messages [RFC8201] for packets
   admitted into the aero interface that are too large for the outbound
   underlying ANET interface.  Similarly, the aero interface performs
   PMTUD even if the destination appears to be on the same link since a
   proxy on the path could return a PTB message.  PMTUD therefore
   ensures that the aero interface MTU is adaptive and reflects the
   current path used for a given data flow.

   Applications that cannot tolerate loss due to MTU restrictions should
   refrain from sending packets larger than 1280 bytes, since dynamic
   path changes can reduce the path MTU at any time.

6.  Frame Format

   The aero interface transmits IPv6 packets according to the native
   frame format of each underlying ANET 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 exactly as
   specified in [RFC2473], etc.

7.  Link-Local Addresses

   A MN "aero address" is an IPv6 link-local address with an interface
   identifier based on its assigned MNP.  MN aero addresses begin with
   the prefix fe80::/64 followed by a 64-bit prefix taken from the MNP
   (see: Appendix B).  For example, for the MNP:

      2001:db8:1000:2000::/56

   the corresponding aero addresses are:



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      fe80::2001:db8:1000:2000

      fe80::2001:db8:1000:2001

      fe80::2001:db8:1000:2002

      ... etc. ...

      fe80::2001:db8:1000:20ff

   When the MN configures aero addresses from its MNP, it assigns them
   to each ANET interface (and also to the aero interface in the MS-
   enabled model).  The lowest-numbered aero address serves as the
   "base" address (for example, for the MNP 2001:db8:1000:2000::/56 the
   base aero address is fe80::2001:db8:1000:2000).  The MN uses the base
   aero address for IPv6 ND messaging, but accepts packets destined to
   all aero addresses equally (i.e., the same as for any multi-addressed
   IPv6 interface).

   In the MS-enabled link model, MS endpoint (MSE) aero addresses are
   allocated from the range fe80::/96, and MUST be managed for
   uniqueness by the collective aero link administrative authorities.
   The lower 32 bits of the address includes a unique integer value,
   e.g., fe80::1, fe80::2, fe80::3, etc.  The address fe80::ffff:ffff is
   reserved and the address fe80:: is the IPv6 link-local Subnet Router
   Anycast address [RFC4291]; hence, these values are not available for
   general assignment.

   In the classic link model, ANET link devices number their interfaces
   from the range fe80::/96 the same as above except that these
   addresses need not be managed for uniqueness outside of the local
   ANET link.  It is therefore possible that different ANET links could
   reuse numbers from the fe80::/96 space since the addresses are link-
   scope only.

   In a mixed model, both the classic and MS-enabled numbering schemes
   can be used without conflict within the same ANET, as the two
   services would be conducted as ships in the night.  A mix of MNs
   operating according to classic and MS-enabled models could then
   operate within the same ANETs without interference.

   Since MN aero addresses are guaranteed unique by the nature of the
   unique MNP delegation, aero interfaces set the autoconfiguration
   variable DupAddrDetectTransmits to 0 [RFC4862].







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8.  Address Mapping - Unicast

   Aero interfaces maintain a neighbor cache for tracking per-neighbor
   state the same as for any IPv6 interface and use the link-local
   address format specified in Section 7.  IPv6 Neighbor Discovery (ND)
   [RFC4861] messages on aero interfaces use the native Source/Target
   Link-Layer Address Option (S/TLLAO) formats of the underlying ANET
   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 aero interface would therefore appear to have multiple link layer
   connections, and may include information for multiple ANET interfaces
   in a single message exchange.

   Aero interfaces use a new IPv6 ND options called the "Aero
   Registration (AR)" option.  MNs that wish to invoke the MS include
   the AR option in Router Solicitation (RS) and/or unsolicited Neighbor
   Advertisement (uNA) messages to request registration/deregistration,
   and the MS includes the AR option in Router Advertisement (RA)
   messages to acknowledge the MN's registration/deregistration.

   AR options in a MN's RS/uNA messages are formatted as shown in
   Figure 2:

























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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 |R|  Reserved   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             Nonce                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           ifIndex [1]         |P00|P01|P02|P03|P04|P05|P06|P07|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P08|P09|P10|P11|P12|P13|P14|P15|P16|P17|P18|P19|P20|P21|P22|P23|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P24|P25|P26|P27|P28|P29|P30|P31|P32|P33|P34|P35|P36|P37|P38|P39|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P40|P41|P42|P43|P44|P45|P46|P47|P48|P49|P50|P51|P52|P53|P54|P55|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P56|P57|P58|P59|P60|P61|P62|P63|           ifIndex [2]         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ...
       ...                             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ...                             |           ifIndex [N]         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                Trailing zero padding (0 - 6 octets)           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 2: Aero Registration (AR) Option Format in RS/uNA Messages

   In this format:

   o  Type is set to TBD.






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   o  Length is set to the number of 8 octet blocks in the option (with
      trailing zero padding added if necessary to produce an integral
      number of 8 octet blocks).

   o  Prefix Length is set to the length of the MNP embedded in the MN's
      aero address.

   o  R (the "Register" bit) is set to '1' to sustain the MNP
      registration or set to '0' to request de-registration.

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

   o  Nonce is set to a (pseudo)-random 32-bit value selected by the MN,
      and used to correlate received confirmations.

   o  A list of N (ifIndex[i], P[i])-tuples are included as follows:

      *  ifIndex[i] [RFC2863] is set to a 16-bit integer value
         corresponding to a specific underlying ANET interface.  The
         first ifIndex MUST correspond to the ANET interface over which
         the message is sent.  Once the MN has assigned an ifIndex to an
         ANET interface, the assignment MUST remain unchanged until the
         MN disables the interface.  MNs MUST number each ifIndex with a
         value between '1' and '0xffff'.

      *  P[i] is a per-ifIndex set of Preferences that correspond to the
         64 Differentiated Service Code Point (DSCP) values [RFC2474]
         pertaining to the ANET interface.  Each (P00 - P63) field is
         set to the value '0' ("disabled"), '1' ("low"), '2' ("medium")
         or '3' ("high") to indicate a QoS preference level for ANET
         interface selection purposes.

   AR options in the MS RA replies are 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 = 2  | Prefix Length |R|  Reserved   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             Nonce                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Prefix Lifetime                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            Reserved                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 3: Aero Registration (AR) Option Format in RA messages




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   In this format:

   o  Type is set to TBD.

   o  Length is set to the constant value '2' (i.e., 2 units of 8
      octets).

   o  Prefix Length is set to the length included in the AR option of
      the RS message that triggered the RA response.

   o  R is set to '1' to confirm registration or set to '0' to release/
      decline registration.

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

   o  Nonce echoes the 32 bit value received in the AR option of the
      corresponding RS message.

   o  Prefix Lifetime is set to the time in seconds that the MSE will
      maintain the Prefix registration.

9.  Address Mapping - Multicast

   The multicast address mapping of the native underlying ANET 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 link layer address of the router as the link layer
   address for all multicast packets.

10.  Address Mapping for IPv6 Neighbor Discovery Messages

   Per [RFC4861], IPv6 ND messages may be sent to either a multicast or
   unicast link-scoped IPv6 destination address.  For aero interfaces in
   the MS-enabled model, however, IPv6 ND messaging must be coordinated
   between the MN and MS only without invoking other nodes on the ANET.

   For this reason, ANET links maintain one or more unicast link-layer
   address ("MSADDR") for the purpose of supporting MN/MS IPv6 ND
   messaging.  For Ethernet-compatible ANETs, this specification
   reserves one Ethernet unicast address 00-00-5E-00-52-14.  For non-
   Ethernet statically-addressed ANETs, MSADDR is reserved per the
   assigned numbers authority for the ANET addressing space.  On still
   other links, one or more MSADDR is discovered through dynamic link-
   layer beacons received from ANET access routers.

   MNs operating according to the MS-enabled model map all IPv6 ND
   messages they send (i.e., both multicast and unicast) to an MSADDR
   instead of to an ordinary unicast or multicast link-layer address.



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   In this way, all of the MN's IPv6 ND messages will be received by MS
   devices that are configured to accept packets destined to MSADDR
   (i.e., a point-to-point neighbor model).  Note that multiple MS
   devices on the link could be configured to accept packets destined to
   MSADDR, e.g., as a basis for virtual router redundancy.

   Therefore, ANET access routers MUST accept and process packets
   destined to MSADDR, while all other devices MUST NOT process packets
   destined to MSADDR.  This model has a well-established operational
   experience in Proxy Mobile IPv6 (PMIP) [RFC5213][RFC6543].

11.  Conceptual Sending Algorithm

   The MN's IPv6 layer selects the outbound aero interface according to
   standard IPv6 requirements.  The aero interface maintains default
   routes and neighbor cache entries for MSEs, and may also include
   additional neighbor cache entries created through other means (e.g.,
   Address Resolution, static configuration, etc.).

   When the MN sends an NS message for Address Resolution, the aero
   interface forwards the message to an MSE (see: Section 12) which acts
   as a link-layer forwarding agent according to the NBMA link model.
   The resulting NA message will provide link-layer address information
   for the neighbor.  When Neighbor Unreachability Detection is used,
   the NS/NA exchange confirms reachability the same as for any IPv6
   interface.

   After a packet enters the aero interface, an outbound ANET interface
   is selected based on traffic engineering information such as DSCP,
   application port number, cost, performance, etc.  Aero interface
   traffic engineering could also be configured to perform replication
   across multiple ANET interfaces for increased reliability at the
   expense of packet duplication.

   When a target neighbor has multiple link-layer addresses (each with a
   different traffic engineering profile), the aero interface selects
   ANET interfaces and neighbor link-layer addresses according to both
   its own outbound preferences and the inbound preferences of the
   target neighbor.

11.1.  Multiple Aero Interfaces

   MNs may associate with multiple MS instances concurrently.  Each MS
   instance represents a distinct aero link distinguished by its
   associated MSPs.  The MN configures a separate aero interface for
   each link so that multiple interfaces (e.g., aero0, aero1, aero2,
   etc.) are exposed to the IPv6 layer.




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   Depending on local policy and configuration, an MN may choose between
   alternative active aero interfaces using a packet's DSCP, routing
   information or static configuration.  In particular, the MN can add
   the MSPs received in Prefix Information Options (PIOs) [RFC4861]
   [RFC8028] as guidance for aero interface selection based on per-
   packet source addresses.

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

   Multiple distinct aero 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

   ANET access routers accept IPv6 ND messages destined to the link-
   local Subnet Router Anycast Address (fe80::), all-routers multicast
   and any unicast link-local IPv6 addresses they are configured to
   listen to.  ANET access routers that support the classic link model
   configure link-local addresses that are guaranteed not to conflict
   with MN link-local addresses as discussed in Section 7.  ANET access
   routers that support the MS-enabled model configure the link-layer
   address MSADDR (see: Section 10) and act as proxies for all MSEs from
   the range fe80::1 through fe80::ffff:fffe.

   MNs that support the classic model perform ordinary RS/RA exchanges
   over each ANET the same as for ordinary IPv6 links.  ANET access
   routers send RAs with an IPv6 link-local source address from the
   range fe80::1 through fe80::ffff:fffe that is guaranteed not to
   conflict with the MN's aero address nor the address of any other
   routers on the link.  The RA messages include normal configuration
   options including prefix information, MTU, etc.  The MNs are then
   responsible for coordinating their ANET interfaces on their own
   behalf and for coordinating with any INET-based mobility agents.  No
   further support from the ANET is needed.

   MNs that support the MS-enabled model interface with the MS by
   sending RS messages with AR options.  For each ANET interface, the MN
   sends initial RS messages with AR options with link-layer address set
   to MSADDR and with network-layer address set to either a specific MSE
   address or to all-routers multicast.  The ANET access router receives
   the RS messages and contacts the corresponding MSE (when the
   destination is all-routers multicast, the access router itself
   selects an MSE).  When the MSE responds, the ANET access router



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   returns RA messages with AR options and with any information for the
   link that would normally be delivered in a solicited RA message.

   Note that some ANET access routers that listen on MSADDR may not be
   configured to recognize and/or process AR options.  Those access
   routers must still obey the requirements of [RFC4861] that state:

      "Future versions of this protocol may define new option types.
      Receivers MUST silently ignore any options they do not recognize
      and continue processing the message."

   In that case, the access router processes the RS message and returns
   an RA message according to the classic link model including any
   configuration options but without including an AR option.  Upon
   receiving the RA message, the MN must manage this ANET interface
   according to the classic link model and must not configure it as an
   underlying interface of the aero interface.

   MNs configure aero interfaces that observe the properties discussed
   in the previous section.  The aero 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 aero 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 aero interface also transitions to UP.  When all
   underlying interfaces transition to DOWN, the aero interface also
   transitions to DOWN.

   When an aero interface transitions to UP, the MN sends initial RS
   messages to register its MNP and an initial set of underlying ANET
   interfaces that are also UP.  The MN sends additional RS messages to
   refresh lifetimes and to register/deregister underlying ANET
   interfaces as they transition to UP or DOWN.

   MS-enabled ANET access routers coordinate with the MSE and send RA
   messages with configuration information in response to a MN's RS
   messages.  The RA includes a Router Lifetime value and PIOs with (A;
   L=0) that include MSPs for the link.  The configuration information
   may also include Route Information Options (RIO) options [RFC4191]
   with more-specific routes, and an MTU option that specifies the
   maximum acceptable packet size for the link.  The ANET access router
   sends immediate unicast RA responses without delay; therefore, the
   'MAX_RA_DELAY_TIME' and 'MIN_DELAY_BETWEEN_RAS' constants for
   multicast RAs do not apply.  The ANET access router MAY send periodic
   and/or event-driven unsolicited RA messages, but is not required to
   do so for unicast advertisements [RFC4861].




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   The MN sends RS messages from within the aero interface while using
   an UP underlying ANET interface as the outbound interface.  Each RS
   message is formatted as though it originated from the IPv6 layer, but
   the process is coordinated wholly from within the aero interface and
   is therefore opaque to the IPv6 layer.  The MN sends initial RS
   messages over an UP underlying interface with its aero address as the
   source and the address of an MSE as the destination.  The RS messages
   include AR options with a valid Prefix Length as well as ifIndex and
   P(i) values appropriate for underlying ANET interfaces.  The MS-
   enabled ANET access router processes RS message and forwards the
   information in the AR option to the MSE.

   When the MSE processes the AR information, if the prefix registration
   was accepted the MSE injects the MNP into the routing/mapping system
   then caches the new Prefix Length, MNP, ifIndex and P(i) values.  The
   MSE then returns a non-zero Prefix Lifetime if the prefix assertion
   was acceptable; otherwise, with a zero Prefix Lifetime.  The ANET
   access router then returns an RA message with an AR option to the MN.

   When the MN receives the RA message, it creates a default route with
   next hop address set to the MSE found in the RA source address and
   with link-layer address set to MSADDR.  The ANET access router will
   then forward packets acting as a proxy between the MN and the actual
   MSE.

   The MN then manages its underlying ANET interfaces according to their
   states as follows:

   o  When an underlying ANET interface transitions to UP, the MN sends
      an RS over the ANET interface with an AR option.  The AR option
      contains a first ifIndex-tuple with values appropriate for this
      ANET interface, and may contain additional ifIndex-tuples
      appropriate for other ANET interfaces.

   o  When an underlying ANET interface transitions to DOWN, the MN
      sends an RS/uNA message over any UP ANET interface with an AR
      option containing an ifIndex-tuple for the DOWN ANET interface
      with all P(i) values set to '0'.  The MN sends an RS when an
      acknowledgement is required, or an uNA when reliability is not
      thought to be a concern (e.g., if redundant transmissions are sent
      on multiple ANET interfaces).

   o  When a MN wishes to release from the current MSE, it sends an RS
      message over any UP ANET interface with an AR option with R set to
      0.  The corresponding MSE then withdraws the MNP from the routing/
      mapping system and returns an RA message with an AR option with
      Prefix Lifetime set to 0.




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   o  When all of a MNs underlying interfaces have transitioned to DOWN,
      the MSE withdraws the MNP the same as if it had received a message
      with an AR option with R set to 0.

   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 multiple
   UP ANET interfaces, the MN declares this MSE unreachable and tries a
   different MSE.

   The IPv6 layer sees the aero interface as an ordinary IPv6 interface.
   Therefore, when the IPv6 layer sends an RS message the aero interface
   returns an internally-generated RA message as though the message
   originated from an IPv6 router.  The internally-generated RA message
   contains configuration information (such as Router Lifetime, MTU,
   etc.) that is consistent with the information received from the RAs
   generated by the MS.

   Whether the aero 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 independently
   of any IPv6 layer messaging.

13.  IANA Considerations

   The IANA is instructed to allocate an official Type number from the
   IPv6 Neighbor Discovery Option Formats registry for the Aero
   Registration (AR) option.  Implementations set Type to 253 as an
   interim value [RFC4727].

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

14.  Security Considerations

   Security considerations are the same as defined for the specific
   access network interface types, and readers are referred to the
   appropriate interface specifications.

   IPv6 and IPv6 ND security considerations also apply, and are
   specified in the normative references.







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15.  Acknowledgements

   This document was prepared per the consensus decision at the 8th
   Conference of the International Civil Aviation Organization (ICAO)
   Working Group-I Mobility Subgroup on March 22, 2019.  Attendees and
   contributors included: Guray Acar, Danny Bharj, Francois D'Humieres,
   Pavel Drasil, Nikos Fistas, Giovanni Garofolo, Vaughn Maiolla, Tom
   McParland, Victor Moreno, Madhu Niraula, Brent Phillips, Liviu
   Popescu, Jacky Pouzet, Aloke Roy, Greg Saccone, Robert Segers,
   Stephane Tamalet, Fred Templin, Bela Varkonyi, Tony Whyman, and
   Dongsong Zeng.

   The following individuals are acknowledged for their useful comments:
   Pavel Drasil, Zdenek Jaron, Michael Matyas, Madhu Niraula, Greg
   Saccone.

   .

16.  References

16.1.  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>.

   [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>.

   [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>.






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   [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>.

   [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>.

   [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>.

16.2.  Informative References

   [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>.

   [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>.

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







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   [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>.

   [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>.

Appendix A.  Aero Registration Option Extensions for Special-Purpose
             Links

   Adaptation of the aero interface to the Aeronautical
   Telecommunications Network with Internet Protocol Services (ATN/IPS)
   includes link selection preferences based on transport port numbers
   in addition to the existing DSCP-based preferences.  ATN/IPS nodes
   maintain a map of transport port numbers to 64 possible preference
   fields, e.g., TCP port 22 maps to preference field 8, TCP port 443
   maps to preference field 20, UDP port 8060 maps to preference field
   34, etc.  The extended aero registration option format for ATN/IPS is
   shown in Figure 4, where the 'Q(i)' fields provide link preferences
   for the corresponding transport port number.








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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 |R|   Reserved  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                            Nonce                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          ifIndex [1]          |P00|P01|P02|P03|P04|P05|P06|P07|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P08|P09|P10|P11|P12|P13|P14|P15|P16|P17|P18|P19|P20|P21|P22|P23|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P24|P25|P26|P27|P28|P29|P30|P31|P32|P33|P34|P35|P36|P37|P38|P39|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P40|P41|P42|P43|P44|P45|P46|P47|P48|P49|P50|P51|P52|P53|P54|P55|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P56|P57|P58|P59|P60|P61|P62|P63|Q00|Q01|Q02|Q03|Q04|Q05|Q06|Q07|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q08|Q09|Q10|Q11|Q12|Q13|Q14|Q15|Q16|Q17|Q18|Q19|Q20|Q21|Q22|Q23|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q24|Q25|Q26|Q27|Q28|Q29|Q30|Q31|Q32|Q33|Q34|Q35|Q36|Q37|Q38|Q39|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q40|Q41|Q42|Q43|Q44|Q45|Q46|Q47|Q48|Q49|Q50|Q51|Q52|Q53|Q54|Q55|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q56|Q57|Q58|Q59|Q60|Q61|Q62|Q63| ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 4: ATN/IPS Extended Aero Option Format

Appendix B.  Prefix Length Considerations

   The IPv6 addressing architecture [RFC4291] reserves the prefix ::/8;
   this assures that MNPs will not begin with ::32 so that MN and MS
   aero addresses cannot overlap.  Additionally, this specification
   currently observes the 64-bit boundary in IPv6 addresses [RFC7421].

   MN aero addresses insert the most-significant 64 MNP bits into the
   least-significant 64 bits of the prefix fe80::/64, however [RFC4291]
   defines the link-local prefix as fe80::/10 meaning "fe80" followed by
   54 unused bits followed by the least-significant 64 bits of the
   address.  Future versions of this specification may adapt the 54
   unused bits for extended coding of MNP prefixes of /65 or longer (up
   to /118).

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



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   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
   speaking different protocols.  It is therefore highly desirable that
   all ground stations speak a common language of RS/RA as specified in
   this document.

   An aircraft that encounters a beaconing ground station can elect to
   solicit either the classic or MS-enabled link models discussed in
   Section 12.  If the aircraft employs the classic link model, it sends
   an RS message with no AR option.  The ground station will return an
   RA message with no AR option along with any configuration options for
   the link (e.g., prefix information, MTU, etc.).  If the aircraft
   wishes to engage the MS-enabled model, it instead sends an RS message
   with an AR option.

   Upon receipt of an RS message with an AR option, the ground station
   proceeds according to the MS-enabled model if it is configured to do
   so.  If the ground station does not recognize the AR option (or, if
   it is not configured for MS-enabled operation) it instead ignores the
   option and processes the rest of the RS message per [RFC4861].

   This flexibility notwithstanding, VDLM2 ground stations must be
   consistent in terms of the service they offer.  In particular, while
   it is permissible for a ground station to simultaneously offer
   different link models to different aircraft, it should not switch
   between the classic and MS-enabled operating models for ongoing RS/RA
   exchanges with the same aircraft.








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Appendix D.  RS/RA Messaging as the Single Standard API

   At the ICAO Working Group I Mobility Subgroup meeting in London, July
   8-12, 2019 an assertion was made that the MS-enabled link model must
   employ a different message type besides RS/RA.  This was based on the
   pretext that including a new IPv6 ND option in an RS message would
   cause routers that do not recognize the option to do "strange
   things".  However, [RFC4861] assures that no standards-compliant
   router would have trouble processing an RS with unrecognized options
   due to the following Section 4.1 requirement:

      "Future versions of this protocol may define new option types.
      Receivers MUST silently ignore any options they do not recognize
      and continue processing the message."

   Indeed, this same normative requirement appeared in both RFC2461 and
   RFC1970 (the predecessors of RFC2460) dating back to August 1996.
   Therefore, any router that refused to continue processing an RS
   message after encountering a properly-formed but unrecognized option
   would not be standards-compliant and should not be used in any
   production network capacity.

   Assuming for a moment however that a new message type were used to
   invoke the MS-enabled link model, this would require two message
   exchanges between the MN and ANET access router - a first exchange
   with RS/RA with no new IPv6 ND options, and a second exchange with
   the new message type.  However, on links such as VDLM2 (see:
   Appendix C) the addition of a second message exchange would impart
   unacceptable delay in closing the link and unacceptable extraneous
   message overhead that impacts link capacity for all.  This clearly
   indicates a need to include all link establishment signaling in a
   single message exchange and not multiple.

   We therefore see strong motivation for including a new IPv6 ND option
   in RS/RA messages instead of creating a new message type, and proof
   that doing so will not harm standards-compliant access routers.
   Routers that recognize the options can at their discretion either
   honor or ignore them, while assuring that the MN will be provided
   with either its first choice or second choice link model.  However,
   service providers that do not offer MN customers their first choice
   may risk losing business to others that do.

Appendix E.  Change Log

   << RFC Editor - remove prior to publication >>

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



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   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:

   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





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   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.

   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)
   Boeing Research & Technology
   P.O. Box 3707
   Seattle, WA  98124
   USA

   Email: fltemplin@acm.org









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

   Email: tony.whyman@mccallumwhyman.com












































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