A Simple BGP-based Mobile Routing System for the Aeronautical Telecommunications Network
draft-ietf-rtgwg-atn-bgp-11

Document Type Active Internet-Draft (rtgwg WG)
Authors Fred Templin  , Greg Saccone  , Gaurav Dawra  , Acee Lindem  , Victor Moreno 
Last updated 2021-07-06
Replaces draft-templin-atn-bgp
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Network Working Group                                    F. Templin, Ed.
Internet-Draft                                                G. Saccone
Intended status: Informational              Boeing Research & Technology
Expires: January 7, 2022                                        G. Dawra
                                                                LinkedIn
                                                               A. Lindem
                                                               V. Moreno
                                                     Cisco Systems, Inc.
                                                            July 6, 2021

     A Simple BGP-based Mobile Routing System for the Aeronautical
                       Telecommunications Network
                      draft-ietf-rtgwg-atn-bgp-11

Abstract

   The International Civil Aviation Organization (ICAO) is investigating
   mobile routing solutions for a worldwide Aeronautical
   Telecommunications Network with Internet Protocol Services (ATN/IPS).
   The ATN/IPS will eventually replace existing communication services
   with an IPv6-based service supporting pervasive Air Traffic
   Management (ATM) for Air Traffic Controllers (ATC), Airline
   Operations Controllers (AOC), and all commercial aircraft worldwide.
   This informational document describes a simple and extensible mobile
   routing service based on industry-standard BGP to address the ATN/IPS
   requirements.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 7, 2022.

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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
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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 . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  ATN/IPS Routing System  . . . . . . . . . . . . . . . . . . .   8
   4.  ATN/IPS (Radio) Access Network (ANET) Model . . . . . . . . .  12
   5.  ATN/IPS Route Optimization  . . . . . . . . . . . . . . . . .  14
   6.  BGP Protocol Considerations . . . . . . . . . . . . . . . . .  16
   7.  Stub AS Mobile Routing Services . . . . . . . . . . . . . . .  18
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  18
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  18
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     12.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Appendix A.  BGP Convergence Considerations . . . . . . . . . . .  21
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   The worldwide Air Traffic Management (ATM) system today uses a
   service known as Aeronautical Telecommunications Network based on
   Open Systems Interconnection (ATN/OSI).  The service is used to
   augment controller to pilot voice communications with rudimentary
   short text command and control messages.  The service has seen
   successful deployment in a limited set of worldwide ATM domains.

   The International Civil Aviation Organization (ICAO) is now
   undertaking the development of a next-generation replacement for ATN/
   OSI known as Aeronautical Telecommunications Network with Internet
   Protocol Services (ATN/IPS) [ATN][ATN-IPS].  ATN/IPS will eventually

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   provide an IPv6-based [RFC8200] service supporting pervasive ATM for
   Air Traffic Controllers (ATC), Airline Operations Controllers (AOC),
   and all commercial aircraft worldwide.  As part of the ATN/IPS
   undertaking, a new mobile routing service will be needed.  This
   document presents an approach based on the Border Gateway Protocol
   (BGP) [RFC4271].

   Aircraft communicate via wireless aviation data links that typically
   support much lower data rates than terrestrial wireless and wired-
   line communications.  For example, some Very High Frequency (VHF)-
   based data links only support data rates on the order of 32Kbps and
   an emerging L-Band data link that is expected to play a key role in
   future aeronautical communications only supports rates on the order
   of 1Mbps.  Although satellite data links can provide much higher data
   rates during optimal conditions, like any other aviation data link
   they are subject to errors, delay, disruption, signal intermittence,
   degradation due to atmospheric conditions, etc.  The well-connected
   ground domain ATN/IPS network should therefore treat each safety-of-
   flight critical packet produced by (or destined to) an aircraft as a
   precious commodity and strive for an optimized service that provides
   the highest possible degree of reliability.

   The ATN/IPS is an IPv6-based overlay network configured over one or
   more Internetworking underlays ("INETs") maintained by aeronautical
   network service providers such as ARINC, SITA and Inmarsat.  The
   overlay provides an Overlay Multilink Network Interface (OMNI)
   virtual link layer [I-D.templin-6man-omni] as a Non-Broadcast,
   Multiple Access (NBMA) virtual link that spans the entire ATN/IPS.
   Each aircraft connects to the OMNI link via an OMNI interface
   configured over the aircraft's underlying physical and/or virtual
   access network interfaces.

   Each underlying INET comprises one or more "partitions" where all
   nodes within a partition can exchange packets with all other nodes,
   i.e., the partition is connected internally.  There is no requirement
   that any two INET partitions use the same IP protocol version nor
   have consistent IP addressing plans in comparison with other
   partitions.  Instead, the OMNI link sees each partition as a
   "segment" of a link-layer topology manifested through a (virtual)
   bridging service based on IPv6 encapsulation [RFC2473] known as the
   OMNI Adaptation Layer (OAL)
   [I-D.templin-6man-omni][I-D.templin-6man-aero].

   The IPv6 addressing architecture provides different classes of
   addresses, including Global Unicast Addresses (GUAs), Unique Local
   Addresses (ULAs) and Link-Local Addresses (LLAs) [RFC4291][RFC4193].
   The ATN/IPS receives an IPv6 GUA Mobility Service Prefix (MSP) from
   an Internet assigned numbers authority, and each aircraft will

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   receive a Mobile Network Prefix (MNP) delegation from the MSP that
   accompanies the aircraft wherever it travels.  ATCs and AOCs will
   likewise receive MNPs, but they would typically appear in static (not
   mobile) deployments such as air traffic control towers, airline
   headquarters, etc.

   The OAL uses ULAs in the source and destination addresses of IPv6
   encapsulation headers.  Each ULA includes an MNP in the interface
   identifier ("MNP-ULA") as discussed in [I-D.templin-6man-omni].  Due
   to MNP delegation policies and random MN mobility properties, MNP-
   ULAs are generally not aggregatable in the BGP routing service and
   are represented as many more-specific prefixes instead of a smaller
   number of aggregated prefixes.  In addition, OMNI link service nodes
   configure administratively-assigned ULAs ("ADM-ULA") that are
   statically-assigned and derived from a shorter ADM-ULA prefix
   assigned to their OMNI link partition [I-D.templin-6man-omni].
   Unlike MNP-ULAs, the ADM-ULAs are persistently present and unchanging
   in the routing system.  The BGP routing services therefore perform
   forwarding based on these MNP-ULAs and ADM-ULAs instead of based on
   the GUA MNPs themselves.

   Connexion By Boeing [CBB] was an early aviation mobile routing
   service based on dynamic updates in the global public Internet BGP
   routing system.  Practical experience with the approach has shown
   that frequent injections and withdrawals of prefixes in the Internet
   routing system can result in excessive BGP update messaging, slow
   routing table convergence times, and extended outages when no route
   is available.  This is due to both conservative default BGP protocol
   timing parameters (see Section 6) and the complex peering
   interconnections of BGP routers within the global Internet
   infrastructure.  The situation is further exacerbated by frequent
   aircraft mobility events that each result in BGP updates that must be
   propagated to all BGP routers in the Internet that carry a full
   routing table.

   We therefore consider an approach using a BGP overlay network routing
   system where a private BGP routing protocol instance is maintained
   between ATN/IPS Autonomous System (AS) Border Routers (ASBRs).  The
   private BGP instance does not interact with the native BGP routing
   systems in underlying INETs, and BGP updates are unidirectional from
   "stub" ASBRs (s-ASBRs) to a small set of "core" ASBRs (c-ASBRs) in a
   hub-and-spokes topology.  No extensions to the BGP protocol are
   necessary.  BGP routing is based on the ULAs found in OAL headers,
   i.e., it provides an adaptation layer forwarding service instead of a
   networking layer routing service.

   The s-ASBRs for each stub AS connect to a small number of c-ASBRs via
   dedicated high speed links and/or tunnels across the INET using

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   industry-standard secured encapsulations (e.g., IPsec [RFC4301],
   Wireguard, etc.).  In particular, tunneling must be used when
   neighboring ASBRs are separated by multiple INET hops.

   The s-ASBRs engage in external BGP (eBGP) peerings with their
   respective c-ASBRs, and only maintain routing table entries for the
   MNP-ULAs currently active within the stub AS.  The s-ASBRs send BGP
   updates for MNP-ULA injections or withdrawals to c-ASBRs but do not
   receive any BGP updates from c-ASBRs.  Instead, the s-ASBRs maintain
   default routes with their c-ASBRs as the next hop, and therefore hold
   only partial topology information.

   The c-ASBRs connect to other c-ASBRs within the same partition using
   internal BGP (iBGP) peerings over which they collaboratively maintain
   a full routing table for all active MNP-ULAs currently in service
   within the partition.  Therefore, only the c-ASBRs maintain a full
   BGP routing table and never send any BGP updates to s-ASBRs.  This
   simple routing model therefore greatly reduces the number of BGP
   updates that need to be synchronized among peers, and the number is
   reduced further still when intradomain routing changes within stub
   ASes are processed within the AS instead of being propagated to the
   core.  BGP Route Reflectors (RRs) [RFC4456] can also be used to
   support increased scaling properties.

   When there are multiple INET partitions, the c-ASBRs of each
   partition use eBGP to peer with the c-ASBRs of other partitions so
   that the full set of ULAs for all partitions are known globally among
   all of the c-ASBRs.  Each c/s-ASBR further configures an ADM-ULA
   which is taken from an ADM-ULA prefix assigned to each partition, as
   well as static forwarding table entries for all other OMNI link
   partition prefixes.  Both ADM-ULAs and MNP-ULAs are used by the OAL
   for nested encapsulation where the inner IPv6 packet is encapsulated
   in an IPv6 OAL header with ULA source and destination addresses,
   which is then encapsulated in an IP header specific to the INET
   partition.

   With these intra- and inter-INET BGP peerings in place, a forwarding
   plane spanning tree is established that properly covers the entire
   operating domain.  All nodes in the network can be visited using
   strict spanning tree hops, but in many instances this may result in
   longer paths than are necessary.  The AERO and OMNI services
   [I-D.templin-6man-aero][I-D.templin-6man-omni] provide mechanisms for
   discovering and utilizing (route-optimized) shortcuts that do not
   always follow strict spanning tree paths.

   The remainder of this document discusses the proposed BGP-based ATN/
   IPS mobile routing service.

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2.  Terminology

   The terms Autonomous System (AS) and Autonomous System Border Router
   (ASBR) are the same as defined in [RFC4271].

   The following terms are defined for the purposes of this document:

   Air Traffic Management (ATM)
      The worldwide service for coordinating safe aviation operations.

   Air Traffic Controller (ATC)
      A government agent responsible for coordinating with aircraft
      within a defined operational region via voice and/or data Command
      and Control messaging.

   Airline Operations Controller (AOC)
      An airline agent responsible for tracking and coordinating with
      aircraft within their fleet.

   Aeronautical Telecommunications Network with Internet Protocol
   Services (ATN/IPS)
      A future aviation network for ATCs and AOCs to coordinate with all
      aircraft operating worldwide.  The ATN/IPS will be an IPv6-based
      overlay network service that connects access networks via
      tunneling over one or more Internetworking underlays.

   Internetworking underlay ("INET")
      A wide-area network that supports overlay network tunneling and
      connects Radio Access Networks to the rest of the ATN/IPS.
      Example INET service providers for civil aviation include ARINC,
      SITA and Inmarsat.

   (Radio) Access Network ("ANET")
      An aviation radio data link service provider's network, including
      radio transmitters and receivers as well as supporting ground-
      domain infrastructure needed to convey a customer's data packets
      to outside INETs.  The term ANET is intended in the same spirit as
      for radio-based Internet service provider networks (e.g., cellular
      operators), but can also refer to ground-domain networks that
      connect AOCs and ATCs.

   partition (or "segment")
      A fully-connected internal subnetwork of an INET in which all
      nodes can communicate with all other nodes within the same
      partition using the same IP protocol version and addressing plan.
      Each INET consists of one or more partitions.

   Overlay Multilink Network Interface (OMNI)

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      A virtual layer 2 bridging service that presents an ATN/IPS
      overlay unified link view even though the underlay may consist of
      multiple INET partitions.  The OMNI virtual link is manifested
      through nested encapsulation in which GUA-addressed IPv6 packets
      from the ATN/IPS are first encapsulated in ULA-addressed IPv6
      headers which are then forwarded to the next hop using INET
      encapsulation if necessary.  Forwarding over the OMNI virtual link
      is therefore based on ULAs instead of GUAs.  In this way, packets
      sent from a source can be conveyed over the OMNI virtual link even
      though there may be many underlying INET partitions in the path to
      the destination.

   OMNI Adaptation Layer (OAL)
      A middle layer below the IPv6 layer but above the INET layer that
      applies IPv6-in-IPv6 encapsulation prior to INET encapsulation.
      The IPv6 encapsulation header inserted by the OAL uses ULAs
      instead of GUAs.  Further details on OMNI and the OAL are found in
      [I-D.templin-6man-omni].

   OAL Autonomous System
      A "hub-of-hubs" autonomous system maintained through peerings
      between the core autonomous systems of different OMNI virtual link
      partitions.

   Core Autonomous System Border Router (c-ASBR)
      A BGP router located in the hub of the INET partition hub-and-
      spokes overlay network topology.

   Core Autonomous System
      The "hub" autonomous system maintained by all c-ASBRs within the
      same partition.

   Stub Autonomous System Border Router (s-ASBR)
      A BGP router configured as a spoke in the INET partition hub-and-
      spokes overlay network topology.

   Stub Autonomous System
      A logical grouping that includes all Clients currently associated
      with a given s-ASBR.

   Client
      An ATC, AOC or aircraft that connects to the ATN/IPS as a leaf
      node.  The Client could be a singleton host, or a router that
      connects a mobile or fixed network.

   Proxy
      An ANET/INET border node that acts as a transparent intermediary
      between Clients and s-ASBRs.  From the Client's perspective, the

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      Proxy presents the appearance that the Client is communicating
      directly with the s-ASBR.  From the s-ASBR's perspective, the
      Proxy presents the appearance that the s-ASBR is communicating
      directly with the Client.

   Mobile Network Prefix (MNP)
      An IPv6 prefix that is delegated to any ATN/IPS end system,
      including ATCs, AOCs, and aircraft.

   Mobility Service Prefix (MSP)
      An aggregated prefix assigned to the ATN/IPS by an Internet
      assigned numbers authority, and from which all MNPs are delegated
      (e.g., up to 2**32 IPv6 /56 MNPs could be delegated from a /24
      MSP).

3.  ATN/IPS Routing System

   The ATN/IPS routing system comprises a private BGP instance
   coordinated in an overlay network via tunnels between neighboring
   ASBRs over one or more underlying INETs.  The overlay does not
   interact with the underlying INET BGP routing systems, and only a
   small and unchanging set of MSPs are advertised externally instead of
   the full dynamically changing set of MNPs.

   Within each INET partition, each s-ASBRs connects a stub AS to the
   INET partition core using a distinct stub AS Number (ASN).  Each
   s-ASBR further uses eBGP to peer with one or more c-ASBRs.  All
   c-ASBRs are members of the INET partition core AS, and use a shared
   core ASN.  Unique ASNs are assigned according to the standard 32-bit
   ASN format [RFC4271][RFC6793].  Since the BGP instance does not
   connect with any INET BGP routing systems, the ASNs assigned need not
   be coordinated with IANA and can in fact coincide with values that
   are assigned in other domains.  The only requirement is that ASNs
   must not be duplicated within the ATN/IPS routing system itself.

   The c-ASBRs use iBGP to maintain a synchronized consistent view of
   all active MNP-ULAs currently in service within the INET partition.
   Figure 1 below represents the reference INET partition deployment.
   (Note that the figure shows details for only two s-ASBRs (s-ASBR1 and
   s-ASBR2) due to space constraints, but the other s-ASBRs should be
   understood to have similar Stub AS, MNP and eBGP peering
   arrangements.)  The solution described in this document is flexible
   enough to extend to these topologies.

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     ...........................................................
   .                                                             .
   .               (:::)-.  <- Stub ASes ->  (:::)-.             .
   .   MNPs-> .-(:::::::::)             .-(:::::::::) <-MNPs     .
   .            `-(::::)-'                `-(::::)-'             .
   .             +-------+                +-------+              .
   .             |s-ASBR1+-----+    +-----+s-ASBR2|              .
   .             +--+----+ eBGP \  / eBGP +-----+-+              .
   .                 \           \/            /                 .
   .                  \eBGP      / \          /eBGP              .
   .                   \        /   \        /                   .
   .                    +-------+   +-------+                    .
   .          eBGP+-----+c-ASBR |...|c-ASBR +-----+eBGP          .
   .   +-------+ /      +--+----+   +-----+-+      \ +-------+   .
   .   |s-ASBR +/       iBGP\   (:::)-.  /iBGP      \+s-ASBR |   .
   .   +-------+            .-(::::::::)             +-------+   .
   .       .            .-(::::::::::::::)-.             .       .
   .       .           (::::  Core AS   :::)             .       .
   .   +-------+         `-(:::::::::::::)-'         +-------+   .
   .   |s-ASBR +\      iBGP/`-(:::::::-'\iBGP       /+s-ASBR |   .
   .   +-------+ \      +-+-----+   +----+--+      / +-------+   .
   .          eBGP+-----+c-ASBR |...|c-ASBR +-----+eBGP          .
   .                    +-------+   +-------+                    .
   .                   /                     \                   .
   .                  /eBGP                   \eBGP              .
   .                 /                         \                 .
   .            +---+---+                 +-----+-+              .
   .            |s-ASBR |                 |s-ASBR |              .
   .            +-------+                 +-------+              .
   .                                                             .
   .                                                             .
   .   <----------------- INET Partition  ------------------->   .
    ............................................................

               Figure 1: INET Partition Reference Deployment

   In the reference deployment, each s-ASBR maintains routes for active
   MNP-ULAs that currently belong to its stub AS.  In response to
   "Inter-domain" mobility events, each s-ASBR will dynamically
   announces new MNP-ULAs and withdraws departed MNP-ULAs in its eBGP
   updates to c-ASBRs.  Since ATN/IPS end systems are expected to remain
   within the same stub AS for extended timeframes, however, intra-
   domain mobility events (such as an aircraft handing off between cell
   towers) are handled within the stub AS instead of being propagated as
   inter-domain eBGP updates.

   Each c-ASBR configures a black-hole route for each of its MSPs.  By
   black-holing the MSPs, the c-ASBR will maintain forwarding table

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   entries only for the MNP-ULAs that are currently active, and packets
   destined to all other MNP-ULAs will correctly incur ICMPv6
   Destination Unreachable messages [RFC4443] due to the black hole
   route.  (This is the same behavior as for ordinary BGP routers in the
   Internet when they receive packets for which there is no route
   available.)  The c-ASBRs do not send eBGP updates for MNP-ULAs to
   s-ASBRs, but instead originate a default route.  In this way, s-ASBRs
   have only partial topology knowledge (i.e., they know only about the
   active MNP-ULAs currently within their stub ASes) and they forward
   all other packets to c-ASBRs which have full topology knowledge.

   Each s-ASBR and c-ASBR configures an ADM-ULA that is aggregatable
   within an INET partition, and each partition configures a unique ADM-
   ULA prefix that is permanently announced into the routing system.
   The core ASes of each INET partition are joined together through
   external BGP peerings.  The c-ASBRs of each partition establish
   external peerings with the c-ASBRs of other partitions to form a
   "core-of-cores" OMNI link AS.  The OMNI link AS contains the global
   knowledge of all MNP-ULAs deployed worldwide, and supports ATN/IPS
   overlay communications between nodes located in different INET
   partitions by virtue of OAL encapsulation.  OMNI link nodes can then
   navigate to ASBRs by including an ADM-ULA or directly to an end
   system by including an MNP-ULA in the destination address of an OAL-
   encapsulated packet (see: [I-D.templin-6man-aero]).  Figure 2 shows a
   reference OAL topology.

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                 . . . . . . . . . . . . . . . . . . . . . . . . .
               .                                                   .
               .              .-(::::::::)                          .
               .           .-(::::::::::::)-.   +------+            .
               .          (::: Partition 1 ::)--|c-ASBR|---+        .
               .           `-(::::::::::::)-'   +------+   |        .
               .              `-(::::::)-'                 |        .
               .                                           |        .
               .              .-(::::::::)                 |        .
               .           .-(::::::::::::)-.   +------+   |        .
               .          (::: Partition 2 ::)--|c-ASBR|---+        .
               .           `-(::::::::::::)-'   +------+   |        .
               .              `-(::::::)-'                 |        .
               .                                           |        .
               .              .-(::::::::)                 |        .
               .           .-(::::::::::::)-.   +------+   |        .
               .          (::: Partition 3 ::)--|c-ASBR|---+        .
               .           `-(::::::::::::)-'   +------+   |        .
               .              `-(::::::)-'                 |        .
               .                                           |        .
               .                ..(etc)..                  x        .
               .                                                    .
               .                                                    .
               .    <- ATN/IPS Overlay Bridged by the OAL AS ->     .
                 . . . . . . . . . . . . . .. . . . . . . . . . . .

                Figure 2: Spanning Partitions with the OAL

   Scaling properties of this ATN/IPS routing system are limited by the
   number of BGP routes that can be carried by the c-ASBRs.  A 2015
   study showed that BGP routers in the global public Internet at that
   time carried more than 500K routes with linear growth and no signs of
   router resource exhaustion [BGP].  A more recent network emulation
   study also showed that a single c-ASBR can accommodate at least 1M
   dynamically changing BGP routes even on a lightweight virtual
   machine.  Commercially-available high-performance dedicated router
   hardware can support many millions of routes.

   Therefore, assuming each c-ASBR can carry 1M or more routes, this
   means that at least 1M ATN/IPS end system MNP-ULAs can be serviced by
   a single set of c-ASBRs and that number could be further increased by
   using RRs and/or more powerful routers.  Another means of increasing
   scale would be to assign a different set of c-ASBRs for each set of
   MSPs.  In that case, each s-ASBR still peers with one or more c-ASBRs
   from each set of c-ASBRs, but the s-ASBR institutes route filters so
   that it only sends BGP updates to the specific set of c-ASBRs that
   aggregate the MSP.  In this way, each set of c-ASBRs maintains
   separate routing and forwarding tables so that scaling is distributed

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   across multiple c-ASBR sets instead of concentrated in a single
   c-ASBR set.  For example, a first c-ASBR set could aggregate an MSP
   segment A::/32, a second set could aggregate B::/32, a third could
   aggregate C::/32, etc.  The union of all MSP segments would then
   constitute the collective MSP(s) for the entire ATN/IPS, with
   potential for supporting many millions of mobile networks or more.

   In this way, each set of c-ASBRs services a specific set of MSPs, and
   each s-ASBR configures MSP-specific routes that list the correct set
   of c-ASBRs as next hops.  This design also allows for natural
   incremental deployment, and can support initial medium-scale
   deployments followed by dynamic deployment of additional ATN/IPS
   infrastructure elements without disturbing the already-deployed base.
   For example, a few more c-ASBRs could be added if the MNP service
   demand ever outgrows the initial deployment.  For larger-scale
   applications (such as unmanned air vehicles and terrestrial vehicles)
   even larger scales can be accommodated by adding more c-ASBRs.

4.  ATN/IPS (Radio) Access Network (ANET) Model

   (Radio) Access Networks (ANETs) connect end system Clients such as
   aircraft, ATCs, AOCs etc. to the ATN/IPS routing system.  Clients may
   connect to multiple ANETs at once, for example, when they have both
   satellite and cellular data links activated simultaneously.  Clients
   configure an Overlay Multilink Network (OMNI) Interface
   [I-D.templin-6man-omni] over their underlying ANET interfaces as a
   connection to an NBMA virtual link (manifested by the OAL) that spans
   the entire ATN/IPS.  Clients may further move between ANETs in a
   manner that is perceived as a network layer mobility event.  Clients
   could therefore employ a multilink/mobility routing service such as
   those discussed in Section 7.

   Clients register all of their active data link connections with their
   serving s-ASBRs as discussed in Section 3.  Clients may connect to
   s-ASBRs either directly, or via a Proxy at the ANET/INET boundary.

   Figure 3 shows the ATN/IPS ANET model where Clients connect to ANETs
   via aviation data links.  Clients register their ANET addresses with
   a nearby s-ASBR, where the registration process may be brokered by a
   Proxy at the edge of the ANET.

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                        +-----------------+
                        |     Client      |
         Data Link "A"  +-----------------+  Data Link "B"
                 +----- |  OMNI Interface |--------+
                /       +-----------------+         \
               /                                     \
              /                                       \
           (:::)-.                                   (:::)-.
      .-(:::::::::)<- (Radio) Access Networks ->.-(:::::::::)
        `-(::::)-'                                `-(::::)-'
         +-------+                                +-------+
    ...  | Proxy |  ............................  | Proxy |  ...
   .     +-------+                                +-------+     .
   .         ^^                                      ^^         .
   .         ||                                      ||         .
   .         ||              +--------+              ||         .
   .         ++============> | s-ASBR | <============++         .
   .                         +--------+                         .
   .                              | eBGP                        .
   .                            (:::)-.                         .
   .                        .-(::::::::)                        .
   .                    .-(::: ATN/IPS :::)-.                   .
   .                  (::::: BGP Routing ::::)                  .
   .                     `-(:: System ::::)-'                   .
   .                         `-(:::::::-'                       .
   .                                                            .
   .                                                            .
   .  <------- OMNI virtual link bridged by the OAL -------->   .
    ............................................................

                    Figure 3: ATN/IPS ANET Architecture

   When a Client logs into an ANET it specifies a nearby s-ASBR that it
   has selected to connect to the ATN/IPS.  The login process is
   transparently brokered by a Proxy at the border of the ANET which
   then conveys the connection request to the s-ASBR via tunneling
   across the OMNI virtual link.  Each ANET border Proxy is also equally
   capable of serving in the s-ASBR role so that a first on-link Proxy
   can be selected as the s-ASBR while all others perform the Proxy role
   in a hub-and-spokes arrangement.  An on-link Proxy is selected to
   serve the s-ASBR role when it receives a control message from a
   Client requesting that service.

   A network-based s-ASBR can also be selected when the ANET does not
   provide a Proxy, or when a different ANET Proxy has already been
   selected.  Selection of a network-based s-ASBR could be through an
   address discovered through a first ANET Proxy, through consulting a
   geographically-keyed static host file, through a DNS lookup, through

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   a network query response, etc.  The s-ASBR then registers the address
   of the Proxy as the address for the Client, and the Proxy forwards
   the s-ASBR's reply to the Client.  If the Client connects to multiple
   ANETs, the s-ASBR will register the addresses of all Proxies as
   addresses through which the Client can be reached.

   The s-ASBR represents all of its active Clients as MNP-ULA routes in
   the ATN/IPS BGP routing system.  The s-ASBR's stub AS therefore
   consists of the set of all of its active Clients (i.e., the stub AS
   is a logical construct and not a physical construct).  The s-ASBR
   injects the MNP-ULAs of its active Clients and withdraws the MNP-ULAs
   of its departed Clients via BGP updates to c-ASBRs, which further
   propagate the MNP-ULAs to other c-ASBRs within the OAL AS.  Since
   Clients are expected to remain associated with their current s-ASBR
   for extended periods, the level of MNP-ULA injections and withdrawals
   in the BGP routing system will be on the order of the numbers of
   network joins, leaves and s-ASBR handovers for aircraft operations
   (see: Section 6).  It is important to observe that fine-grained
   events such as Client mobility and Quality of Service (QoS) signaling
   are coordinated only by Proxies and the Client's current s-ASBRs, and
   do not involve other ASBRs in the routing system.  In this way,
   intradomain routing changes within the stub AS are not propagated
   into the rest of the ATN/IPS BGP routing system.

5.  ATN/IPS Route Optimization

   ATN/IPS end systems will frequently need to communicate with
   correspondents associated with other s-ASBRs.  In the BGP peering
   topology discussed in Section 3, this can initially only be
   accommodated by including multiple spanning tree segments in the
   forwarding path.  In many cases, it would be desirable to eliminate
   extraneous spanning tree segments from this "dogleg" route so that
   packets can traverse a minimum number of tunneling hops across the
   OMNI virtual link.  ATN/IPS end systems could therefore employ a
   route optimization service according to the mobility service employed
   (see: Section 7).

   A route optimization example is shown in Figure 4 and Figure 5 below.
   In the first figure, multiple spanning tree segments between Proxys
   and ASBRs are necessary to convey packets between Clients associated
   with different s-ASBRs.  In the second figure, the optimized route
   tunnels packets directly between Proxys without involving the ASBRs.

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         +---------+                             +---------+
         | Client1 |                             | Client2 |
         +---v-----+                             +-----^---+
             *                                         *
             *                                         *
           (:::)-.                                   (:::)-.
      .-(:::::::::)<- (Radio) Access Networks ->.-(:::::::::)
        `-(::::)-'                                `-(::::)-'
         +--------+                              +--------+
    ...  | Proxy1 |  ..........................  | Proxy2 |  ...
   .     +--------+                              +--------+     .
   .             **                               **            .
   .              **                             **             .
   .               **                           **              .
   .           +---------+                  +---------+         .
   .           | s-ASBR1 |                  | s-ASBR2 |         .
   .           +--+------+                  +-----+---+         .
   .                 \  **      Dogleg      **   /              .
   .              eBGP\  **     Route      **   /eBGP           .
   .                   \  **==============**   /                .
   .                   +---------+   +---------+                .
   .                   | c-ASBR1 |   | c-ASBR2 |                .
   .                   +---+-----+   +----+----+                .
   .                       +--------------+                     .
   .                             iBGP                           .
   .                                                            .
   .  <------- OMNI virtual link bridged by the OAL -------->   .
    ............................................................

                Figure 4: Dogleg Route Before Optimization

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         +---------+                             +---------+
         | Client1 |                             | Client2 |
         +---v-----+                             +-----^---+
             *                                         *
             *                                         *
           (:::)-.                                   (:::)-.
      .-(:::::::::) <- (Radio) Access Networks ->.-(:::::::::)
        `-(::::)-'                                `-(::::)-'
         +--------+                              +--------+
    ...  | Proxy1 |  ..........................  | Proxy2 |  ...
   .     +------v-+                              +--^-----+     .
   .             *                                  *           .
   .              *================================*            .
   .                                                            .
   .           +---------+                  +---------+         .
   .           | s-ASBR1 |                  | s-ASBR2 |         .
   .           +--+------+                  +-----+---+         .
   .                 \                           /              .
   .              eBGP\                         /eBGP           .
   .                   \                       /                .
   .                   +---------+   +---------+                .
   .                   | c-ASBR1 |   | c-ASBR2 |                .
   .                   +---+-----+   +----+----+                .
   .                       +--------------+                     .
   .                             iBGP                           .
   .                                                            .
   .  <------- OMNI virtual link bridged by the OAL -------->   .
    ............................................................

                         Figure 5: Optimized Route

6.  BGP Protocol Considerations

   The number of eBGP peering sessions that each c-ASBR must service is
   proportional to the number of s-ASBRs in its local partition.
   Network emulations with lightweight virtual machines have shown that
   a single c-ASBR can service at least 100 eBGP peerings from s-ASBRs
   that each advertise 10K MNP-ULA routes (i.e., 1M total).  It is
   expected that robust c-ASBRs can service many more peerings than this
   - possibly by multiple orders of magnitude.  But even assuming a
   conservative limit, the number of s-ASBRs could be increased by also
   increasing the number of c-ASBRs.  Since c-ASBRs also peer with each
   other using iBGP, however, larger-scale c-ASBR deployments may need
   to employ an adjunct facility such as BGP Route Reflectors
   (RRs)[RFC4456].

   The number of aircraft in operation at a given time worldwide is
   likely to be significantly less than 1M, but we will assume this

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   number for a worst-case analysis.  Assuming a worst-case average 1
   hour flight profile from gate-to-gate with 10 service region
   transitions per flight, the entire system will need to service at
   most 10M BGP updates per hour (2778 updates per second).  This number
   is within the realm of the peak BGP update messaging seen in the
   global public Internet today [BGP2].  Assuming a BGP update message
   size of 100 bytes (800bits), the total amount of BGP control message
   traffic to a single c-ASBR will be less than 2.5Mbps which is a
   nominal rate for modern data links.

   Industry standard BGP routers provide configurable parameters with
   conservative default values.  For example, the default hold time is
   90 seconds, the default keepalive time is 1/3 of the hold time, and
   the default MinRouteAdvertisementinterval is 30 seconds for eBGP
   peers and 5 seconds for iBGP peers (see Section 10 of [RFC4271]).
   For the simple mobile routing system described herein, these
   parameters can be set to more aggressive values to support faster
   neighbor/link failure detection and faster routing protocol
   convergence times.  For example, a hold time of 3 seconds and a
   MinRouteAdvertisementinterval of 0 seconds for both iBGP and eBGP.

   Instead of adjusting BGP default time values, BGP routers can use the
   Bidirectional Forwarding Detection (BFD) protocol [RFC5880] to
   quickly detect link failures that don't result in interface state
   changes, BGP peer failures, and administrative state changes.  BFD is
   important in environments where rapid response to failures is
   required for routing reconvergence and, hence, communications
   continuity.

   Each c-ASBR will be using eBGP both in the ATN/IPS and the INET with
   the ATN/IPS unicast IPv6 routes resolving over INET routes.
   Consequently, c-ASBRs and potentially s-ASBRs will need to support
   separate local ASes for the two BGP routing domains and routing
   policy or assure routes are not propagated between the two BGP
   routing domains.  From a conceptual and operational standpoint, the
   implementation should provide isolation between the two BGP routing
   domains (e.g., separate BGP instances).

   ADM-ULAs and MNP-ULAs begin with fd00::/8 followed by a pseudo-random
   40-bit global ID to form the prefix [ULA]::/48, along with a 16-bit
   OMNI link identifier '*' to form the prefix [ULA*]::/64.  Each
   individual address taken from [ULA*]::/64 includes additional routing
   information in the interface identifier.  For example, for the MNP
   2001:db8:1:0::/56, the resulting MNP-ULA is [ULA*]:2001:db8:1:0/120,
   and for the administrative address 1001:2002/16 the ADM-ULA is
   [ULA*]::1001:2002/112 (see: [I-D.templin-6man-omni] for further
   details).  This gives rise to a BGP routing system that must
   accommodate large numbers of long and non-aggregatable MNP-ULA

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   prefixes as well as moderate numbers of long and semi-aggregatable
   ADM-ULA prefixes.  The system is kept stable and scalable through the
   s-ASBR / c-ASBR hub-and-spokes topology which ensures that mobility-
   related churn is not exposed to the core.

7.  Stub AS Mobile Routing Services

   Stub ASes maintain intradomain routing information for mobile node
   clients, and are responsible for all localized mobility signaling
   without disturbing the BGP routing system.  Clients can enlist the
   services of a candidate mobility service such as Mobile IPv6 (MIPv6)
   [RFC6275], LISP [I-D.ietf-lisp-rfc6830bis] and AERO
   [I-D.templin-6man-aero] according to the service offered by the stub
   AS.  Further details of mobile routing services are out of scope for
   this document.

8.  Implementation Status

   The BGP routing topology described in this document has been modeled
   in realistic network emulations showing that at least 1 million MNP-
   ULAs can be propagated to each c-ASBR even on lightweight virtual
   machines.  No BGP routing protocol extensions need to be adopted.

9.  IANA Considerations

   This document does not introduce any IANA considerations.

10.  Security Considerations

   ATN/IPS ASBRs on the open Internet are susceptible to the same attack
   profiles as for any Internet nodes.  For this reason, ASBRs should
   employ physical security and/or IP securing mechanisms such as IPsec
   [RFC4301], TLS [RFC5246], WireGuard, etc.

   ATN/IPS ASBRs present targets for Distributed Denial of Service
   (DDoS) attacks.  This concern is no different than for any node on
   the open Internet, where attackers could send spoofed packets to the
   node at high data rates.  This can be mitigated by connecting ATN/IPS
   ASBRs over dedicated links with no connections to the Internet and/or
   when ASBR connections to the Internet are only permitted through
   well-managed firewalls.

   ATN/IPS s-ASBRs should institute rate limits to protect low data rate
   aviation data links from receiving DDoS packet floods.

   BGP protocol message exchanges and control message exchanges used for
   route optimization must be secured to ensure the integrity of the
   system-wide routing information base.

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   This document does not include any new specific requirements for
   mitigation of DDoS.

11.  Acknowledgements

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

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

   This work is aligned with the Boeing Commercial Airplanes (BCA)
   Internet of Things (IoT) and autonomy programs.

   This work is aligned with the Boeing Information Technology (BIT)
   MobileNet program.

   The following individuals contributed insights that have improved the
   document: Ahmad Amin, Erik Kline, Hubert Kuenig, Tony Li, Alexandre
   Petrescu, Pascal Thubert, Tony Whyman.

12.  References

12.1.  Normative References

   [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>.

   [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>.

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

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

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

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   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
              <https://www.rfc-editor.org/info/rfc4456>.

   [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>.

   [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>.

12.2.  Informative References

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

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

   [BGP]      Huston, G., "BGP in 2015, http://potaroo.net", January
              2016.

   [BGP2]     Huston, G., "BGP Instability Report,
              http://bgpupdates.potaroo.net/instability/bgpupd.html",
              May 2017.

   [CBB]      Dul, A., "Global IP Network Mobility using Border Gateway
              Protocol (BGP), http://www.quark.net/docs/
              Global_IP_Network_Mobility_using_BGP.pdf", March 2006.

   [I-D.ietf-lisp-rfc6830bis]
              Farinacci, D., Fuller, V., Meyer, D., Lewis, D., and A.
              Cabellos, "The Locator/ID Separation Protocol (LISP)",
              draft-ietf-lisp-rfc6830bis-36 (work in progress), November
              2020.

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

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   [I-D.templin-6man-omni]
              Templin, F. L. and T. Whyman, "Transmission of IP Packets
              over Overlay Multilink Network (OMNI) Interfaces", draft-
              templin-6man-omni-03 (work in progress), April 2021.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              DOI 10.17487/RFC2784, March 2000,
              <https://www.rfc-editor.org/info/rfc2784>.

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

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC6275]  Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
              Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
              2011, <https://www.rfc-editor.org/info/rfc6275>.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,
              <https://www.rfc-editor.org/info/rfc6793>.

Appendix A.  BGP Convergence Considerations

   Experimental evidence has shown that BGP convergence time required
   for when an MNP-ULA is asserted at a new location or withdrawn from
   an old location can be several hundred milliseconds even under
   optimal AS peering arrangements.  This means that packets in flight
   destined to an MNP-ULA route that has recently been changed can be
   (mis)delivered to an old s-ASBR after a Client has moved to a new
   s-ASBR.

   To address this issue, the old s-ASBR can maintain temporary state
   for a "departed" Client that includes an OAL address for the new
   s-ASBR.  The OAL address never changes since ASBRs are fixed
   infrastructure elements that never move.  Hence, packets arriving at
   the old s-ASBR can be forwarded to the new s-ASBR while the BGP
   routing system is still undergoing reconvergence.  Therefore, as long
   as the Client associates with the new s-ASBR before it departs from
   the old s-ASBR (while informing the old s-ASBR of its new location)
   packets in flight during the BGP reconvergence window are
   accommodated without loss.

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Appendix B.  Change Log

   << RFC Editor - remove prior to publication >>

   Changes from -10 to -11:

   o  Introduced notion of the spanning tree

   o  Discussed Proxy/s-ASBR arrangement options.

   Changes from -05 to -06:

   o  OMNI interface introduced

   o  Version and reference update.

   Changes from -04 to -05:

   o  Version and reference update.

   Changes from -03 to -04:

   o  added discussion of Bidirectional Forwarding Detection (BFD).

   Changes from -02 to -03:

   o  added reference to ICAO A/G interface specification.

   Changes from -01 to -02:

   o  introduced the SPAN and the concept of Internetwork partitioning

   o  new terms "ANET" (for (Radio) Access Network) and "INET" (for
      Internetworking underlay)

   o  new appendix on BGP convergence considerations

   Changes from -00 to -01:

   o  incorporated clarifications due to list comments and questions.

   o  new section 7 on Stub AS Mobile Routing Services

   o  updated references, and included new reference for MIPv6 and LISP

   Status as of 08/30/2018:

   o  'draft-templin-atn-bgp' becomes 'draft-ietf-rtgwg-atn-bgp'

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Authors' Addresses

   Fred L. Templin (editor)
   Boeing Research & Technology
   P.O. Box 3707
   Seattle, WA  98124
   USA

   Email: fltemplin@acm.org

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

   Email: gregory.t.saccone@boeing.com

   Gaurav Dawra
   LinkedIn
   USA

   Email: gdawra.ietf@gmail.com

   Acee Lindem
   Cisco Systems, Inc.
   USA

   Email: acee@cisco.com

   Victor Moreno
   Cisco Systems, Inc.
   USA

   Email: vimoreno@cisco.com

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