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Network Working Group                                    F. Templin, Ed.
Internet-Draft                                                G. Saccone
Intended status: Informational              Boeing Research & Technology
Expires: September 4, 2018                                      G. Dawra
                                                               A. Lindem
                                                     Cisco Systems, Inc.
                                                          March 03, 2018

     A Simple BGP-based Mobile Routing System for the Aeronautical
                       Telecommunications Network


   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

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
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   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 September 4, 2018.

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Copyright Notice

   Copyright (c) 2018 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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  ATN/IPS Routing System  . . . . . . . . . . . . . . . . . . .   6
   4.  ATN/IPS Multilink and Mobility Service  . . . . . . . . . . .   9
   5.  ATN/IPS Route Optimization  . . . . . . . . . . . . . . . . .  10
   6.  BGP Protocol Considerations . . . . . . . . . . . . . . . . .  13
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  14
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     11.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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 rudimenatary
   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/IPS will eventually provide an
   IPv6-based 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

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   new mobile routing service will be needed.  This document presents a
   candidate approach based on the Border Gateway Protocol (BGP)

   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 Traffic Engineering
   service that provides the highest possible degree of reliability.

   The ATN/IPS is an IPv6-based [RFC8200] overlay network that assumes a
   worldwide connected Internetworking underlay for carrying tunneled
   ATM communications.  The Internetworking underlay could be manifested
   as a private collection of long-haul backbone links (e.g.,
   fiberoptics, copper, SATCOM, etc.) interconnected by high-performance
   networking gear such as bridges, switches, and routers.  Such a
   private network would need to connect all ATN/IPS participants
   worldwide, and could therefore present a considerable cost for a
   large-scale deployment of new infrastructure.  Alternatively, the
   ATN/IPS could be deployed as a secured overlay over the existing
   global public Internet.  For example, ATN/IPS nodes could be deployed
   as part of an SD-WAN or an MPLS-WAN that rides over the public
   Internet via secured tunnels.

   The ATN/IPS further assumes that each aircraft will receive an IPv6
   Mobile Network Prefix (MNP) that accompanies the aircraft wherever it
   travels.  ATCs and AOCs will likewise receive IPv6 prefixes, but they
   would typically appear in static (not mobile) deployments such as air
   traffic control towers, airline headquarters, etc.  Throughout the
   rest of this document, we therefore use the term "MNP" when
   discussing an IPv6 prefix that is delegated to any ATN/IPS end
   system, including ATCs, AOCs, and aircraft.  We also use the term
   Mobility Service Prefix (MSP) to refer to 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 /64
   MNPs could be delegated from the MSP 2001:db8::/32).

   Connexion By Boeing [CBB] was an early aviation mobile routing
   service based on dynamic updates in the global public Internet BGP

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   routing system.  Practical experience with the approach has shown
   that frequent injections and withdrawals of MNPs 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
   system in the connected Internetworking underlay, and BGP updates are
   unidirectional from "stub" ASBRs (s-ASBRs) to a very small set of
   "core" ASBRs (c-ASBRs) in a hub-and-spokes topology.  The Asymmetric
   Extended Route Optimization (AERO) architecture
   [I-D.templin-aerolink] is used to support mobility and route
   optimization services, where the BGP s-ASBRs are one and the same as
   AERO Servers and the BGP c-ASBRs are one and the same as AERO Relays.
   No extensions to the BGP protocol are necessary.

   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 Internetworking
   underlay using industry-standard encapsulations (e.g., Generic
   Routing Encapsulation (GRE) [RFC2784], IPsec [RFC4301], etc.).  The
   s-ASBRs engage in external BGP (eBGP) peerings with their respective
   c-ASBRs, and only maintain routing table entries for the MNPs
   currently active within the stub AS.  The s-ASBRs send BGP updates
   for MNP 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 using iBGP peerings over which
   they collaboratively maintain a full routing table for all active
   MNPs currently in service.  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 localized mobility events within stub ASes
   (i.e., "intra-domain" mobility events) 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.

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

2.  Terminology

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

   The terms "AERO Client", "AERO Proxy", "AERO Server", and "AERO
   Relay" are the same as defined in [I-D.templin-aerolink].

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

   Air Traffic Managemnet (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 an Internetworking underlay.

   Internetworking underlay  A connected wide-area network that supports
      overlay network tunneling and connects Radio Access Networks to
      the rest of the ATN/IPS.

   Radio Access Network (RAN)
      An aviation radio data link service provider's network, including
      radio transmitters and receivers as well as suppporting ground-
      domain infrastructure needed to convey a customer's data packets
      to the outside world.  The term RAN is intended in the same spirit
      as for cellular operator networks and other radio-based Internet
      service provider networks.  For simplicity, we also use the term
      RAN to refer to ground-domain networks that connect AOCs and ATCs
      without any aviation radio communications.

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   Core Autonomous System Border Router (c-ASBR)  A BGP router located
      in the hub of a hub-and-spokes overlay network topology.  Each
      c-ASBR is also an AERO Relay.

   Stub Autonomous System Border Router (s-ASBR)  A BGP router
      configured as a spoke in a hub-and-spokes overlay network
      topology.  Each s-ASBR is also an AERO Server.

   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 network.

   Proxy  A node at the edge of a RAN that acts as a proxy go-between
      between Clients and Servers.

   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 /64 MNPs could be
      delegated from the MSP 2001:db8::/32).

3.  ATN/IPS Routing System

   The proposed ATN/IPS routing system comprises a private BGP instance
   coordinated between ASBRs in an overlay network via tunnels over the
   Internetworking underlay (where the tunnels between neighboring ASBRs
   are set up as part of the BGP peering configuration.)  The overlay
   does not interact with the native BGP routing system in the connected
   undelying Internetwork, and each c-ASBR advertises only a small and
   unchanging set of MSPs into the Internetworking underlay routing
   system instead of the full dynamically changing set of MNPs.  (For
   example, when the Internetworking underlay is the global public
   Internet the c-ASBRs advertise the MSPs in the public BGP Internet
   routing system.)  The routing system is discussed in detail in

   In a reference deployment, one or more s-ASBRs connect each stub AS
   to the overlay using a shared 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 same core AS, and use a shared core ASN.  Since the
   private BGP instance is separate from the global public Internet BGP
   routing system, the ASBRs can use either a private ASN per [RFC6996]
   or simply use public ASNs noting that the ASNs may overlap with those
   already assigned in the Internet.  (A third alternative would be to
   procure globally-unique public ASNs, but cost and maintenance
   requirements must be conisdered.)

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   The c-ASBRs use iBGP to maintain a synchronized consistent view of
   all active MNPs currently in service.  Figure 1 below represents the
   reference 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 and MNP
   arrangements.)  The solution described in this document is flexible
   enough to extend to these topologies.

   .                                                             .
   .               (:::)-.  <- Stub ASes ->  (:::)-.             .
   .   MNPs-> .-(:::::::::)             .-(:::::::::) <-MNPs     .
   .            `-(::::)-'                `-(::::)-'             .
   .             +-------+                +-------+              .
   .             |s-ASBR1|                |s-ASBR2|              .
   .             +--+----+                +-----+-+              .
   .                 \                         /                 .
   .                  \eBGP                   /eBGP              .
   .                   \                     /                   .
   .                    +-------+   +-------+                    .
   .          eBGP+-----+c-ASBR1|   +c-ASBR2+-----+eBGP          .
   .   +-------+ /      +--+----+   +-----+-+      \ +-------+   .
   .   |s-ASBRn+/       iBGP\   (:::)-.  /iBGP      \+s-ASBR3|   .
   .   +-------+            .-(::::::::)             +-------+   .
   .       .            .-(::::::::::::::)-.                     .
   .       .           (::::  Core AS   :::)                     .
   .   +-------+         `-(:::::::::::::)-'         +-------+   .
   .   |s-ASBR7+\      iBGP/`-(:::::::-'\iBGP       /+s-ASBR4|   .
   .   +-------+ \      +-+-----+   +----+--+      / +-------+   .
   .          eBGP+-----+c-ASBRn|   |c-ASBR3+-----+eBGP          .
   .                    +-------+   +-------+                    .
   .                   /                     \                   .
   .                  /eBGP                   \eBGP              .
   .                 /                         \                 .
   .            +---+---+                 +-----+-+              .
   .            |s-ASBR6|                 |s-ASBR5|              .
   .            +-------+                 +-------+              .
   .                                                             .
   .                                                             .
   .   <------------ Internetworking Underlay  -------------->   .

                      Figure 1: Reference Deployment

   In the reference deployment, each s-ASBR maintains routes for active
   MNPs that currently belong to its stub AS.  In response to "Inter-
   domain" mobility events, each S-ASBR will dynamically announces new
   MNPs and withdraws departed MNPs in its eBGP updates to c-ASBRs.

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   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

   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
   entries only for the MNPs that are currently active, and packets
   destined to all other MNPs 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 MNPs 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 MNPs
   currently within their stub ASes) and they forward all other packets
   to c-ASBRs which have full topology knowledge.

   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 MNPs can be serviced by a
   single set of c-ASBRs and that number could be furhter increased by
   using RRs.  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.  For
   example, if the MSP for the ATN/IPS deployment is 2001:db8::/32, a
   first set of c-ASBRs could service the MSP segment 2001:db8::/40, a
   second set could service 2001:db8:0100::/40, a third set could
   service 2001:db8:0200::/40, etc.

   In this way, each set of c-ASBRs services a specific set of MSPs that
   they inject into the Internetworking underlay native routing system,
   and each s-ASBR configures MSP-specific routes that list the correct
   set of c-ASBRs as next hops.  This BGP routing design also allows for
   natural incremental deployment, and can support initial small-scale

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   deployments followed by dynamic deployment of additional ATN/IPS
   infrastructure elements without disturbing the already-deployed base.

4.  ATN/IPS Multilink and Mobility Service

   ATN/IPS end system multilink and mobility services are based on the
   AERO architecture [I-D.templin-aerolink], where end systems connect
   to aviation data link service provider Radio Access Networks (RANs).
   ATN/IPS end systems such as aircraft act as AERO Clients and may
   connect to multiple RANs at once, for example, when they have both a
   satellite link and an L-Band link activated simultaneously.  Clients
   register all of their active data link connections with one or more
   AERO Servers which also act as s-ASBRs as discussed in Section 3.
   Clients may connect to Servers either directly, or via an AERO Proxy
   at the edge of the RAN.  The Proxy function corresponds to the manner
   in which web proxies communicate with web servers on behalf of
   clients in secured domains such as corporate enterprise networks.

   Figure 2 shows the ATN/IPS multilink and mobility model where Clients
   connect to RANs via aviation data links.  Clients register their RAN
   addresses with a nearby Server, where the registration process may be
   brokered by a Proxy at the edge of the RAN.

            Data Link "A"     +--------+  Data Link "B"
                 +----------- | Client |-----------+
                /             +--------+            \
               /                                     \
              /                                       \
           (:::)-.                                   (:::)-.
      .-(:::::::::) <- Radio Access Networks -> .-(:::::::::)
        `-(::::)-'                                `-(::::)-'
         +-------+                                +-------+
    ...  | Proxy |  ............................  | Proxy |  ...
   .     +-------+                                +-------+     .
   .                                                            .
   .                                                            .
   .                        +--------+               (:::)-.    .
   .                        | Server | eBGP <-> .-(:::::::::)   .
   .                        |(s-ASBR)|            `-(::::)-'    .
   .                        +--------+     ATN/IPS BGP Overlay  .
   .                                                            .
   .                                                            .
   .   <------------- Internetworking Underlay ------------->   .

           Figure 2: ATN/IPS Multilink and Mobility Architecture

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   In this model, when a Client logs into a RAN it specifies a nearby
   Server (s-ASBR) that it has selected to connect to the ATN/IPS.  The
   login process is brokered by a Proxy at the border of the RAN, which
   then conveys the connection request to the Server via tunneling
   across the Internetworking underlay.  The Server then registers the
   address of the Proxy as the address for the Client, and the Proxy
   forwards the Server's reply to the Client.  If the Client connects to
   multiple RANs, the Server will register the addresses of all Proxies
   along with their Quality of Service (QoS) preferences as addresses
   through which the Client can be reached.

   Once the Client has registered its data link addresses with the
   Server via one or more Proxies, the Proxies can signal fine-grained
   events like QoS changes to the Server on behalf of the Clients.  For
   example, if a data link signal is fading, the Proxy can inform the
   Server without involvement of the Client.  Moreover, if the RAN
   supports intra-domain route injection, the Client can avoid
   encapsulation and send and receive all of its packets unencapsulated
   since the RAN will natively route them to and from the Proxy.  The
   Proxy will then tunnel the packets to and from the Server across the
   Internetworking underlay so that the Client need not incur any over-
   the-air encapsulation on performance-constrained aviation data links.

   The Server represents all of its active Clients as MNP routes in the
   ATN/IPS BGP routing system.  The Server's stub AS therefore consists
   of the set of all of its active Clients.  The Server injects the MNPs
   of its active Clients and withdraws the MNPs of its departed Clients
   via BGP updates to c-ASBRs.  Since Clients are expected to remain
   associated with their current Servers for extended periods, the level
   of MNP injections and withdrawals in the BGP routing system will be
   on the order of the numbers of network joins, leaves and Server
   handovers for aircraft operations (see: Section 6).  It is important
   to observe that fine-grained events such as Client mobility and QoS
   signaling are coordinated only by Proxies and Servers, and do not
   involve other ASBRs in the routing system.  In this way, localized
   events are not propagated into the global 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 ASBR peering
   topology discussed in Section 3, this can initially only be
   accommodated by including multiple ASBRs-to-ASBR tunnel segments in
   the forwarding path.  In many cases, it would be desirable to
   eliminate extraneous ASBR tunnel segments from this "dogleg" route so
   that packets can traverse a minimum number of tunneling hops across
   the Internetworking underlay using the AERO route optimization
   service [I-D.templin-aerolink].

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   A route optimization example is shown in Figure 3 and Figure 4 below.
   In the first figure, packets sent from Client1 to Client2 are
   transmitted across the source RAN to Proxy1 without encapsulation.
   Proxy1 then tunnels the packets to Server 1 (s-ASBR1), which tunnels
   them to Relay 1 (c-ASBR1), which tunnels them to Relay2 (c-ASBR2),
   which tunnels them to Server2 (s-ASBR2), which finally tunnels them
   to Proxy2.  In the second figure, the optimized route tunnels packets
   directly from Proxy1 to Proxy2 without involving the ASBRs.

         +---------+                             +---------+
         | Client1 |                             | Client2 |
         +---v-----+                             +-----^---+
             *                                         *
             *                                         *
           (:::)-.                                   (:::)-.
      .-(:::::::::)  <- Radio Access Networks ->  .-(:::::::::)
        `-(::::)-'                                `-(::::)-'
         +--------+                              +--------+
    ...  | Proxy1 |  ..........................  | Proxy2 |  ...
   .     +--------+                              +--------+     .
   .             **                               **            .
   .              **                             **             .
   .               **                           **              .
   .           +---------+                  +---------+         .
   .           | Server1 |                  | Server2 |         .
   .           |(s-ASBR1)|                  |(s-ASBR2)|         .
   .           +--+------+                  +-----+---+         .
   .                 \  **      Dogleg      **   /              .
   .              eBGP\  **     Route      **   /eBGP           .
   .                   \  **==============**   /                .
   .                   +---------+   +---------+                .
   .                   | Relay1  |   | Relay2  |                .
   .                   |(c-ASBR1)|   |(c-ASBR2)|                .
   .                   +---+-----+   +----+----+                .
   .                       +--------------+                     .
   .                             iBGP                           .
   .                                                            .
   .   <------------- Internetworking Underlay ------------->   .

                Figure 3: Dogleg Route Before Optimization

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         +---------+                             +---------+
         | Client1 |                             | Client2 |
         +---v-----+                             +-----^---+
             *                                         *
             *                                         *
           (:::)-.                                   (:::)-.
      .-(:::::::::)  <- Radio Access Networks -> .-(:::::::::)
        `-(::::)-'                                `-(::::)-'
         +--------+                              +--------+
    ...  | Proxy1 |  ..........................  | Proxy2 |  ...
   .     +------v-+                              +--^-----+     .
   .             *                                  *           .
   .              *================================*            .
   .                                                            .
   .           +---------+                  +---------+         .
   .           | Server1 |                  | Server2 |         .
   .           |(s-ASBR1)|                  |(s-ASBR2)|         .
   .           +--+------+                  +-----+---+         .
   .                 \                           /              .
   .              eBGP\                         /eBGP           .
   .                   \                       /                .
   .                   +---------+   +---------+                .
   .                   | Relay1  |   | Relay2  |                .
   .                   |(c-ASBR1)|   |(c-ASBR2)|                .
   .                   +---+-----+   +----+----+                .
   .                       +--------------+                     .
   .                             iBGP                           .
   .                                                            .
   .   <------------- Internetworking Underlay ------------->   .

                         Figure 4: Optimized Route

   The route optimization is accommodated by control message signaling
   between the Proxies and ASBRs.  When the Proxy nearest the source
   sends a route optimization request, the request is forwarded toward
   the Server and nearest the destination.  If the request is authentic,
   the destination Server provides the source Proxy with the address of
   the destination Proxy so that unnecessary tunnel segments are
   eliminated and direct Proxy-to-Proxy tunneling is enabled.  At the
   same time, the destination Server keeps track of the source Proxies
   it has sent route optimization messages to so it can quickly update
   them if network mobility or Quality of Service (QoS) conditions

   Note that route optimization can fail if Proxy1 cannot tunnel packets
   directly to Proxy2 due to some form of blockage in the
   Internetworking underlay such as filtering middle-boxes.  It is also

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   necessary for Proxy1 to detect and adjust to failure of Proxy2
   through receipt of a Server's IPv6 Neighbor Advertisement message
   and/or Neighbor Unreachability Detection (NUD) [RFC4861].  Note also
   that the Servers still maintain state so they can echo link QoS
   update messages coming from the RANs to inform correspondents of QoS
   changes (e.g., a link signal strength fading, a data link connection
   loss, etc.).

   Finally, each s-ASBR always has a default route and can therefore
   always send packets via the dogleg route through a c-ASBR even if a
   route optimized path has been established.  The direct paths between
   s-ASBRs and c-ASBRs are tunnels are maintained by BGP peering session
   keepalives such that, if a link or an ASBR goes down, BGP will detect
   the failure and readjust the routing tables.  However, ASBRs and the
   links that interconnect them are expected to be secured as highly-
   available and fault tolerant critical infrastructure such that
   peering session failures should be extremely rare.

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 the system.  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 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
   number for a worst-case analysis.  Assuming a worst-case average 1
   hour flight profile from gate-to-gate with 10 Server 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

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   peers and 5 seconds for iBGP peers (see Section 10 of [RFC4271]).
   For the simple mobile routing system described herein, these
   parameters can and should 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.

   C-ASBRs will be using EBGP both in the ATN/IPS and the
   Internetworking Underlay with the ATN/IPS unicast IPv6 routes
   resolving over Internetworking Underlay 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).

7.  Implementation Status

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

8.  IANA Considerations

   This document does not introduce any IANA considerations.

9.  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], 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.

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

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10.  Acknowledgements

   This work is aligned with the FAA as per the SE2025 contract number

   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 Information Technology (BIT)
   MobileNet program.

11.  References

11.1.  Normative References

   [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,

   [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,

   [RFC4451]  McPherson, D. and V. Gill, "BGP MULTI_EXIT_DISC (MED)
              Considerations", RFC 4451, DOI 10.17487/RFC4451, March
              2006, <https://www.rfc-editor.org/info/rfc4451>.

   [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,

   [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,

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

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11.2.  Informative References

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

   [BGP2]     Huston, G., "BGP Instability Report,
              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.

              Fuller, V., Lewis, D., Ermagan, V., Jain, A., and A.
              Smirnov, "LISP Delegated Database Tree", draft-ietf-lisp-
              ddt-09 (work in progress), January 2017.

              Templin, F., "Asymmetric Extended Route Optimization
              (AERO)", draft-templin-aerolink-81 (work in progress),
              February 2018.

   [ICAO]     ICAO, I., "http://www.icao.int/Pages/default.aspx",
              February 2017.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              DOI 10.17487/RFC2784, March 2000,

   [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,

   [RFC6836]  Fuller, V., Farinacci, D., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol Alternative Logical
              Topology (LISP+ALT)", RFC 6836, DOI 10.17487/RFC6836,
              January 2013, <https://www.rfc-editor.org/info/rfc6836>.

   [RFC6996]  Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", BCP 6, RFC 6996, DOI 10.17487/RFC6996, July
              2013, <https://www.rfc-editor.org/info/rfc6996>.

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Authors' Addresses

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

   Email: fltemplin@acm.org

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

   Email: gregory.t.saccone@boeing.com

   Gaurav Dawra

   Email: gdawra.ietf@gmail.com

   Acee Lindem
   Cisco Systems, Inc.

   Email: acee@cisco.com

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