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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720,                       May 30, 2019
           rfc6179, rfc6706 (if
           approved)
Intended status: Standards Track
Expires: December 1, 2019


             Asymmetric Extended Route Optimization (AERO)
                  draft-templin-intarea-6706bis-13.txt

Abstract

   This document specifies the operation of IP over tunnel virtual links
   using Asymmetric Extended Route Optimization (AERO).  AERO interfaces
   use an IPv6 link-local address format that supports operation of the
   IPv6 Neighbor Discovery (ND) protocol and links ND to IP forwarding.
   Prefix delegation services are employed for network admission and to
   manage the routing system.  Multilink operation, mobility management,
   quality of service (QoS) signaling and route optimization are
   naturally supported through dynamic neighbor cache updates.  Standard
   IP multicasting services are also supported.  AERO is a widely-
   applicable tunneling solution especially well-suited to aviation
   services, mobile Virtual Private Networks (VPNs) and many other
   applications.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on December 1, 2019.








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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Asymmetric Extended Route Optimization (AERO) . . . . . . . .  10
     3.1.  AERO Link Reference Model . . . . . . . . . . . . . . . .  10
     3.2.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .  11
     3.3.  AERO Routing System . . . . . . . . . . . . . . . . . . .  12
     3.4.  AERO Addresses  . . . . . . . . . . . . . . . . . . . . .  14
     3.5.  Spanning Partitioned AERO Networks (SPAN) . . . . . . . .  16
     3.6.  AERO Interface Characteristics  . . . . . . . . . . . . .  20
     3.7.  AERO Interface Initialization . . . . . . . . . . . . . .  23
       3.7.1.  AERO Server/Gateway Behavior  . . . . . . . . . . . .  24
       3.7.2.  AERO Proxy Behavior . . . . . . . . . . . . . . . . .  24
       3.7.3.  AERO Client Behavior  . . . . . . . . . . . . . . . .  24
       3.7.4.  AERO Relay Behavior . . . . . . . . . . . . . . . . .  25
     3.8.  AERO Interface Neighbor Cache Maintenance . . . . . . . .  25
     3.9.  AERO Interface Forwarding Algorithm . . . . . . . . . . .  27
       3.9.1.  Client Forwarding Algorithm . . . . . . . . . . . . .  28
       3.9.2.  Proxy Forwarding Algorithm  . . . . . . . . . . . . .  28
       3.9.3.  Server/Gateway Forwarding Algorithm . . . . . . . . .  29
       3.9.4.  Relay Forwarding Algorithm  . . . . . . . . . . . . .  30
     3.10. AERO Interface Encapsulation and Re-encapsulation . . . .  31
     3.11. AERO Interface Decapsulation  . . . . . . . . . . . . . .  32
     3.12. AERO Interface Data Origin Authentication . . . . . . . .  32
     3.13. AERO Interface Packet Size Issues . . . . . . . . . . . .  33
     3.14. AERO Interface Error Handling . . . . . . . . . . . . . .  35
     3.15. AERO Router Discovery, Prefix Delegation and
           Autoconfiguration . . . . . . . . . . . . . . . . . . . .  38
       3.15.1.  AERO ND/PD Service Model . . . . . . . . . . . . . .  38
       3.15.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  38
       3.15.3.  AERO Server Behavior . . . . . . . . . . . . . . . .  41
     3.16. The AERO Proxy  . . . . . . . . . . . . . . . . . . . . .  43



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     3.17. AERO Route Optimization . . . . . . . . . . . . . . . . .  45
       3.17.1.  Route Optimization Initiation  . . . . . . . . . . .  45
       3.17.2.  Relaying the NS  . . . . . . . . . . . . . . . . . .  46
       3.17.3.  Processing the NS and Sending the NA . . . . . . . .  46
       3.17.4.  Relaying the NA  . . . . . . . . . . . . . . . . . .  47
       3.17.5.  Processing the NA  . . . . . . . . . . . . . . . . .  47
       3.17.6.  Route Optimization Maintenance . . . . . . . . . . .  47
     3.18. Neighbor Unreachability Detection (NUD) . . . . . . . . .  48
     3.19. Mobility Management and Quality of Service (QoS)  . . . .  49
       3.19.1.  Mobility Update Messaging  . . . . . . . . . . . . .  49
       3.19.2.  Forwarding Packets on Behalf of Departed Clients . .  50
       3.19.3.  Announcing Link-Layer Address and/or QoS Preference
                Changes  . . . . . . . . . . . . . . . . . . . . . .  51
       3.19.4.  Bringing New Links Into Service  . . . . . . . . . .  51
       3.19.5.  Removing Existing Links from Service . . . . . . . .  51
       3.19.6.  Moving to a New Server . . . . . . . . . . . . . . .  51
     3.20. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  52
       3.20.1.  Source-Specific Multicast (SSM)  . . . . . . . . . .  53
       3.20.2.  Any-Source Multicast (ASM) . . . . . . . . . . . . .  54
       3.20.3.  Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . .  55
   4.  Direct Underlying Interfaces  . . . . . . . . . . . . . . . .  55
   5.  AERO Clients on the Open Internetwork . . . . . . . . . . . .  55
   6.  Operation over Multiple AERO Links (VLANs)  . . . . . . . . .  55
   7.  Operation on AERO Links with /64 ASPs . . . . . . . . . . . .  56
   8.  AERO Adaptations for SEcure Neighbor Discovery (SEND) . . . .  57
   9.  AERO Critical Infrastructure Considerations . . . . . . . . .  57
   10. DNS Considerations  . . . . . . . . . . . . . . . . . . . . .  58
   11. Transition Considerations . . . . . . . . . . . . . . . . . .  59
   12. Implementation Status . . . . . . . . . . . . . . . . . . . .  59
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  59
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  60
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  61
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  62
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  63
     16.2.  Informative References . . . . . . . . . . . . . . . . .  64
   Appendix A.  AERO Alternate Encapsulations  . . . . . . . . . . .  70
   Appendix B.  S/TLLAO Extensions for Special-Purpose Links . . . .  72
   Appendix C.  Implicit Mobility Management . . . . . . . . . . . .  73
   Appendix D.  Implementation Strategies for Route Optimization . .  74
   Appendix E.  Change Log . . . . . . . . . . . . . . . . . . . . .  74
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  79

1.  Introduction

   Asymmetric Extended Route Optimization (AERO) fulfills the
   requirements of Distributed Mobility Management (DMM) [RFC7333] and
   route optimization [RFC5522] for aeronautical networking and other
   network mobility use cases.  AERO is based on a Non-Broadcast,



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   Multiple Access (NBMA) virtual link model known as the AERO link.
   The AERO link is configured over one or more underlying
   Internetworks, and nodes on the link can exchange IP packets via
   tunneling.  Multilink operation allows for increased reliability,
   bandwidth optimization and traffic path diversity.

   The AERO service comprises Clients, Proxys, Servers, and Gateways
   that are seen as AERO link neighbors.  Each node's AERO interface
   uses an IPv6 link-local address format (known as the AERO address)
   that supports operation of the IPv6 Neighbor Discovery (ND) protocol
   [RFC4861] and links ND to IP forwarding.  A node's AERO interface can
   be configured over multiple underlying interfaces, and may therefore
   may appear as a single interface with multiple link-layer addresses.
   Each link-layer address is subject to change due to mobility and/or
   QoS fluctuations, and link-layer address changes are signaled by ND
   messaging the same as for any IPv6 link.

   AERO links provide a cloud-based service where mobile nodes may use
   any Server acting as a Mobility Anchor Point (MAP) and fixed nodes
   may use any Gateway on the link for efficient communications.  Fixed
   nodes forward packets destined to other AERO nodes to the nearest
   Gateway, which forwards them through the cloud.  A mobile node's
   initial packets are forwarded through the MAP, while direct routing
   is supported through asymmetric route optimization while data packets
   are flowing.  Both unicast and multicast communications are
   supported, and mobile nodes may efficiently move between locations
   while maintaining continuous communications with correspondents and
   without changing their IP Address.

   AERO Relays are interconnected in a secured private BGP overlay
   routing instance known as the "SPAN".  The SPAN provides a hybrid
   routing/bridging service to join the underlying Internetworks of
   multiple disjoint administrative domains into a single unified AERO
   link.  Each AERO link instance is characterized by the set of
   Mobility Service Prefixes (MSPs) common to all mobile nodes.  The
   link should extend to the point where a Gateway/MAP is on the optimal
   route from any correspondent node on the link, and provides a gateway
   between the underlying Internetwork and the SPAN.  To the underlying
   Internetwork, the Gateway/MAP is the source of a route to its MSP,
   and hence uplink traffic to the mobile node is naturally routed to
   the nearest Gateway/MAP.

   AERO assumes the use of PIM Sparse Mode in support of multicast
   communication.  In support of Source Specific Multicast (SSM) when a
   Mobile Node is the source, AERO route optimization ensures that a
   shortest-path multicast tree is established with provisions for
   mobility and multilink operation.  In all other multicast scenarios
   there are no AERO dependencies.



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   AERO was designed for aeronautical networking for both manned and
   unmanned aircraft, where the aircraft is treated as a mobile node
   that can connect an Internet of Things (IoT).  AERO is also
   applicable to a wide variety of other use cases.  For example, it can
   be used to coordinate the Virtual Private Network (VPN) links of
   mobile nodes (e.g., cellphones, tablets, laptop computers, etc.) that
   connect into a home enterprise network via public access networks
   using services such as OpenVPN [OVPN].  Other applicable use cases
   are also in scope.

   The following numbered sections present the AERO specification.  The
   appendices at the end of the document are non-normative.

2.  Terminology

   The terminology in the normative references applies; the following
   terms are defined within the scope of this document:

   IPv6 Neighbor Discovery (ND)
      an IPv6 control message service for coordinating neighbor
      relationships between nodes connected to a common link.  AERO
      interfaces use the ND service specified in [RFC4861].

   IPv6 Prefix Delegation (PD)
      a networking service for delegating IPv6 prefixes to nodes on the
      link.  The nominal PD service is DHCPv6 [RFC8415], however
      alternate services (e.g., based on ND messaging) are also in scope
      [I-D.templin-v6ops-pdhost][I-D.templin-6man-dhcpv6-ndopt].  Most
      notably, a form of PD known as "Prefix Assertion" can be used if
      the prefix can be represented in the IPv6 source address of an ND
      message.

   Access Network (ANET)
      a node's first-hop data link service network, e.g., a radio access
      network, cellular service provider network, corporate enterprise
      network, or the public Internet itself.  For secured ANETs, link-
      layer security services such as IEEE 802.1X and physical-layer
      security prevent unauthorized access internally while border
      network-layer security services such as firewalls and proxies
      prevent unauthorized outside access.

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

   ANET address
      an IP address assigned to a node's interface connection to an
      ANET.




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   Internetwork (INET)
      a connected IP network topology with a coherent routing and
      addressing plan and that provides an Internetworking backbone
      service.  INETs also provide an underlay service over which the
      AERO virtual link is configured.  Example INETs include corporate
      enterprise networks, aviation networks, and the public Internet
      itself.  When there is no administrative boundary between an ANET
      and the INET, the ANET and INET are one and the same.

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

   INET address
      an IP address assigned to a node's interface connection to an
      INET.

   AERO link
      a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay
      configured over one or more underlying INETs.  Nodes on the AERO
      link appear as single-hop neighbors from the perspective of the
      virtual overlay even though they may be separated by many
      underlying INET hops.

   AERO interface
      a node's attachment to an AERO link.  Since the addresses assigned
      to an AERO interface are managed for uniqueness, AERO interfaces
      do not require Duplicate Address Detection (DAD) and therefore set
      the administrative variable 'DupAddrDetectTransmits' to zero
      [RFC4862].

   AERO address
      an IPv6 link-local address assigned to an AERO interface and
      constructed as specified in Section 3.4.

   base AERO address
      the lowest-numbered AERO address aggregated by the MNP (see
      Section 3.4).

   Mobility Service Prefix (MSP)
      an IP prefix assigned to the AERO link and from which more-
      specific Mobile Network Prefixes (MNPs) are derived.

   Mobile Network Prefix (MNP)
      an IP prefix allocated from an MSP and delegated to an AERO Client
      or Gateway.

   AERO node




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      a node that is connected to an AERO link, or that provides
      services to other nodes on an AERO link.

   AERO Client ("Client")
      an AERO node that connects to one or more ANETs and requests MNP
      PDs from one or more AERO Servers.  Following PD, the Client
      assigns a Client AERO address to the AERO interface for use in ND
      exchanges with other AERO nodes.  A Client can also be deployed on
      the same platform as a Server, and a node that acts as a Client on
      one AERO interface can also act as an AERO Server on a different
      AERO interface.

   AERO Server ("Server")
      an INET node that configures an AERO interface to provide default
      forwarding services and a Mobility Anchor Point (MAP) for AERO
      Clients.  The Server assigns an administratively-provisioned AERO
      address to the AERO interface to support the operation of the ND/
      PD services, and advertises all of its associated MNPs via BGP
      peerings with Relays.

   AERO Gateway ("Gateway")
      an AERO Server that also provides forwarding services between
      nodes reached via the AERO link and correspondents on other links.
      AERO Gateways are provisioned with MNPs (i.e., the same as for an
      AERO Client) and run a dynamic routing protocol to discover any
      non-MNP IP routes.  In both cases, the Gateway advertises the
      MSP(s) over INET interfaces, and distributes all of its associated
      MNPs and non-MNP IP routes via BGP peerings with Relays (i.e., the
      same as for an AERO Server).

   AERO Relay ("Relay")
      a node that provides hybrid routing/bridging services (as well as
      a security trust anchor) for nodes on an AERO link.  As a router,
      the Relay forwards packets using standard IP forwarding.  As a
      bridge, the Relay forwards packets over the AERO link without
      decrementing the IPv6 Hop Limit.  AERO Relays peer with Servers
      and other Relays to discover the full set of MNPs for the link as
      well as any non-MNPs that are reachable via Gateways.

   AERO Proxy ("Proxy")
      a node that provides proxying services between Clients in an ANET
      and Servers in external INETs.  The AERO Proxy is a conduit
      between the ANET and external INETs in the same manner as for
      common web proxies, and behaves in a similar fashion as for ND
      proxies [RFC4389].

   Spanning Partitioned AERO Networks (SPAN)




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      a means for bridging disjoint INETs as segments (or, partitions)
      of a unified AERO link the same as for a bridged campus LAN.  The
      SPAN is a mid-layer IPv6 encapsulation service in the AERO routing
      system that supports a unified AERO link view for all segments.
      Each segment in the SPAN is a distinct INET.

   SPAN Service Prefix (SSP)
      a global or unique local /96 IPv6 prefix assigned to the AERO link
      to support SPAN services.

   SPAN Partition Prefix (SPP)
      a sub-prefix of the SPAN Service Prefix uniquely assigned to a
      single AERO link segment.

   SPAN Address
      a global or unique local IPv6 address taken from a SPAN Partition
      Prefix.

   ingress tunnel endpoint (ITE)
      an AERO interface endpoint that injects encapsulated packets into
      an AERO link.

   egress tunnel endpoint (ETE)
      an AERO interface endpoint that receives encapsulated packets from
      an AERO link.

   link-layer address
      an IP address used as an encapsulation header source or
      destination address from the perspective of the AERO interface.
      When UDP encapsulation is used, the UDP port number is also
      considered as part of the link-layer address.  From the
      perspective of the AERO interface, the link-layer address is
      either an INET address for intra-segment encapsulation or a SPAN
      address for inter-segment encapsulation.

   network layer address
      the source or destination address of an encapsulated IP packet
      presented to the AERO interface.

   end user network (EUN)
      an internal virtual or external edge IP network that an AERO
      Client or Gateway connects to the rest of the network via the AERO
      interface.  The Client/Gateway sees each EUN as a "downstream"
      network, and sees the AERO interface as the point of attachment to
      the "upstream" network.

   Mobile Node (MN)




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      an AERO Client and all of its downstream-attached networks that
      move together as a single unit, i.e., an end system that connects
      an Internet of Things.

   Mobile Router (MR)
      a MN's on-board router that forwards packets between any
      downstream-attached networks and the AERO link.

   Mobility Anchor Point (MAP)
      an AERO Server that is currently tracking and reporting the
      mobility events of its associated Mobile Node Clients.

   MAP List
      a geographically and/or topologically referenced list of IP
      addresses of Servers for the AERO link.

   Distributed Mobility Management (DMM)
      a BGP-based overlay routing service coordinated by Servers and
      Relays that tracks all MAP-to-Client associations.

   Route Optimization Source (ROS)
      the AERO node nearest the source that initiates route
      optimization.  The ROS may be a Server, Proxy or in some cases
      even the Client itself.

   Route Optimization responder (ROR)
      the AERO node nearest the target destination that responds to
      route optimization requests.  The ROR may be a Server acting as a
      MAP on behalf of a target MNP Client, or a Gateway for a non-MNP
      destination.

   Throughout the document, the simple terms "Client", "Server",
   "Relay", "Proxy" and "Gateway" refer to "AERO Client", "AERO Server",
   "AERO Relay", "AERO Proxy" and "AERO Gateway", respectively.
   Capitalization is used to distinguish these terms from other common
   Internetworking uses in which they appear without capitalization.

   The terminology of DHCPv6 [RFC8415] and IPv6 ND [RFC4861] (including
   the names of node variables, messages and protocol constants) is used
   throughout this document.  Also, the term "IP" is used to generically
   refer to either Internet Protocol version, i.e., IPv4 [RFC0791] or
   IPv6 [RFC8200].

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



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3.  Asymmetric Extended Route Optimization (AERO)

   The following sections specify the operation of IP over Asymmetric
   Extended Route Optimization (AERO) links:

3.1.  AERO Link Reference Model

                          +----------------+
                          | AERO Relay R1  |
                          | Nbr: S1, S2, P1|
                          |(X1->S1; X2->S2)|
                          |      MSP M1    |
                          +-+---------+--+-+
       +--------------+     | Secured |  |     +--------------+
       |AERO Server S1|     | tunnels |  |     |AERO Server S2|
       |  Nbr: C1, R1 +-----+         |  +-----+  Nbr: C2, R1 |
       |  default->R1 |               |        |  default->R1 |
       |    X1->C1    |               |        |    X2->C2    |
       +-------+------+               |        +------+-------+
               |       AERO Link      |               |
       X---+---+-------------------+--)---------------+---+---X
           |                       |  |                   |
     +-----+--------+     +--------+--+-----+    +--------+-----+
     |AERO Client C1|     |  AERO Proxy P1  |    |AERO Client C2|
     |    Nbr: S1   |     |(Proxy Nbr Cache)|    |   Nbr: S2    |
     | default->S1  |     +--------+--------+    | default->S2  |
     |    MNP X1    |              |             |    MNP X2    |
     +------+-------+     .--------+------.      +-----+--------+
            |           (- Proxyed Clients -)          |
           .-.            `---------------'           .-.
        ,-(  _)-.                                  ,-(  _)-.
     .-(_  IP   )-.   +-------+     +-------+    .-(_  IP   )-.
   (__    EUN      )--|Host H1|     |Host H2|--(__    EUN      )
      `-(______)-'    +-------+     +-------+     `-(______)-'

                    Figure 1: AERO Link Reference Model

   Figure 1 presents the AERO link reference model.  In this model:

   o  the AERO link is an overlay network service configured over one or
      more underlying INETs which may be managed by different
      administrative authorities and have incompatible protocols and/or
      addressing plans.

   o  AERO Relay R1 aggregates Mobility Service Prefix (MSP) M1,
      discovers Mobile Network Prefixes (MNPs) X* and advertises the MSP
      via BGP peerings over secured tunnels to Servers (S1, S2).  Relays




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      use the SPAN service to bridge disjoint segments (i.e., INETs) of
      a partitioned AERO link.

   o  AERO Servers S1 and S2 configure secured tunnels with Relay R1 and
      also act as Mobility Anchor Points (MAPs) and default routers for
      their associated Clients C1 and C2.

   o  AERO Clients C1 and C2 associate with Servers S1 and S2,
      respectively.  They receive Mobile Network Prefix (MNP)
      delegations X1 and X2, and also act as default routers for their
      associated physical or internal virtual EUNs.  Simple hosts H1 and
      H2 attach to the EUNs served by Clients C1 and C2, respectively.

   o  AERO Proxy P1 configures a secured tunnel with Relay R1 and
      provides proxy services for AERO Clients in secured enclaves that
      cannot associate directly with other AERO link neighbors.

   Each node on the AERO link maintains an AERO interface neighbor cache
   and an IP forwarding table the same as for any link.  Although the
   figure shows a limited deployment, in common operational practice
   there will normally be many additional Relays, Servers, Clients and
   Proxys.

3.2.  AERO Node Types

   AERO Relays provide hybrid routing/bridging services (as well as a
   security trust anchor) for nodes on an AERO link.  Relays use
   standard IPv6 routing to forward packets both within the same INET
   and between disjoint INETs based on a mid-layer IPv6 encapsulation
   known as the SPAN header.  The inner IP layer experiences a virtual
   bridging service since the inner IP TTL/Hop Limit is not decremented
   during forwarding.  Each Relay also peers with Servers and other
   Relays in a dynamic routing protocol instance to provide a
   Distributed Mobility Management (DMM) service for the list of active
   MNPs (see Section 3.3).  Relays present the AERO link as a set of one
   or more Mobility Service Prefixes (MSPs) but as link-layer devices
   need not connect directly to the AERO link themselves unless an
   administrative interface is desired.  Relays configure secured
   tunnels with Servers, Proxys and other Relays; they further maintain
   IP forwarding table entries for each Mobile Network Prefix (MNP) and
   any other reachable non-MNP prefixes.

   AERO Servers provide default forwarding services and a Mobility
   Anchor Point (MAP) for AERO Client Mobile Nodes (MNs).  Each Server
   also peers with Relays in a dynamic routing protocol instance to
   advertise its list of associated MNPs (see Section 3.3).  Servers
   facilitate PD exchanges with Clients, where each delegated prefix




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   becomes an MNP taken from an MSP.  Servers forward packets between
   AERO interface neighbors and track each Client's mobility profiles.

   AERO Clients receive MNPs through PD exchanges with AERO Servers over
   the AERO link, and distribute the MNPs to nodes on EUNs.  Each Client
   can associate with a single Server or with multiple Servers (e.g.,
   for fault tolerance, load balancing, etc).  A Client may also be co-
   resident on the same physical or virtual platform as a Server; in
   that case, the Client and Server behave as a single functional unit
   and without the need for any Client/Server control messaging.

   AERO Proxys provide a conduit for AERO Clients in ANETs to associate
   with AERO Servers in external INETs.  Client and Servers exchange
   control plane messages via the Proxy, which intercepts them before
   they leave the ANET.  The Proxy forwards data packets to and from
   Clients according to forwarding information in the neighbor cache.
   The Proxy function is specified in Section 3.16.

   AERO Gateways are Servers that provide forwarding services between
   the AERO interface and INET/EUN interfaces.  Gateways are provisioned
   with MNPs the same as for an AERO Client, and also run a dynamic
   routing protocol to discover any non-MNP IP routes.  The Gateway
   advertises the MSP(s) to INETs, and distributes all of its associated
   MNPs and non-MNP IP routes via BGP peerings with Relays.

   AERO Relays, Servers, Proxys and Gateways are critical infrastructure
   elements in fixed (i.e., non-mobile) INET deployments and hence have
   permanent and unchanging INET addresses.  AERO Clients are MNs that
   connect via ANET interfaces, i.e., their ANET addresses may change
   when the Client moves to a new ANET connection.

3.3.  AERO Routing System

   The AERO routing system comprises a private instance of the Border
   Gateway Protocol (BGP) [RFC4271] that is coordinated between Relays
   and Servers and does not interact with either the public Internet BGP
   routing system or any underlying INET routing systems.

   In a reference deployment, each Server is configured as an Autonomous
   System Border Router (ASBR) for a stub Autonomous System (AS) using
   an AS Number (ASN) that is unique within the BGP instance, and each
   Server further uses eBGP to peer with one or more Relays but does not
   peer with other Servers.  Each INET of a multi-segment AERO link must
   include one or more Relays, which peer with the Servers and Proxys
   within that INET.  All Relays within the same INET are members of the
   same hub AS using a common ASN, and use iBGP to maintain a consistent
   view of all active MNPs currently in service.  The Relays of
   different INETs peer with one another using eBGP.



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   Relays advertise the AERO link's MSPs and any non-MNP routes to each
   of their Servers.  This means that any aggregated non-MNPs (including
   "default") are advertised to all Servers.  Each Relay configures a
   black-hole route for each of its MSPs.  By black-holing the MSPs, the
   Relay will maintain forwarding table entries only for the MNPs that
   are currently active, and packets destined to all other MNPs will
   correctly incur Destination Unreachable messages due to the black-
   hole route.  In this way, Servers have only partial topology
   knowledge (i.e., they know only about the MNPs of their directly
   associated Clients) and they forward all other packets to Relays
   which have full topology knowledge.

   Servers maintain a working set of associated MNPs, and dynamically
   announce new MNPs and withdraw departed MNPs in eBGP updates to
   Relays.  Servers that are configured as Gateways also redistribute
   non-MNP routes learned from non-AERO interfaces via their eBGP Relay
   peerings.

   Clients are expected to remain associated with their current Servers
   for extended timeframes, however Servers SHOULD selectively suppress
   updates for impatient Clients that repeatedly associate and
   disassociate with them in order to dampen routing churn.  Servers
   that are configured as Gateways advertise the MSPs via INET/EUN
   interfaces, and forward packets between INET/EUN interfaces and the
   AERO interface using standard IP forwarding.

   For IPv6 MNPs, the AERO routing system includes only IPv6 routes.
   For IPv4 MNPs, the AERO routing system includes both IPv4 routes and
   also IPv6 routes based on the IPv4-mapped IPv6 address corresponding
   to the MNP and with prefix length set to 96 plus the length of the
   IPv4 prefix.  (For example, if the IPv4 MNP is 192.0.2.0/24 then the
   IPv4-mapped prefix is 0:0:0:0:0:FFFF:192.0.2.0/120.)

   Scaling properties of the AERO routing system are limited by the
   number of BGP routes that can be carried by Relays.  As of 2015, the
   global public Internet BGP routing system manages more than 500K
   routes with linear growth and no signs of router resource exhaustion
   [BGP].  More recent network emulation studies have also shown that a
   single Relay can accommodate at least 1M dynamically changing BGP
   routes even on a lightweight virtual machine, i.e., and without
   requiring high-end dedicated router hardware.

   Therefore, assuming each Relay can carry 1M or more routes, this
   means that at least 1M Clients can be serviced by a single set of
   Relays.  A means of increasing scaling would be to assign a different
   set of Relays for each set of MSPs.  In that case, each Server still
   peers with one or more Relays, but institutes route filters so that
   BGP updates are only sent to the specific set of Relays that



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   aggregate the MSP.  For example, if the MSP for the AERO link is
   2001:db8::/32, a first set of Relays could service the MSP segment
   2001:db8::/40, a second set of Relays could service
   2001:db8:0100::/40, a third set could service 2001:db8:0200::/40,
   etc.

   Assuming up to 1K sets of Relays, the AERO routing system can then
   accommodate 1B or more MNPs with no additional overhead (for example,
   it should be possible to service 1B /64 MNPs taken from a /34 MSP and
   even more for shorter prefixes).  In this way, each set of Relays
   services a specific set of MSPs that they advertise to the native
   Internetwork routing system, and each Server configures MSP-specific
   routes that list the correct set of Relays as next hops.  This
   arrangement also allows for natural incremental deployment, and can
   support small scale initial deployments followed by dynamic
   deployment of additional Clients, Servers and Relays without
   disturbing the already-deployed base.

   A full discussion of the BGP-based routing system used by AERO is
   found in [I-D.ietf-rtgwg-atn-bgp].  The system provides for
   Distributed Mobility Management (DMM) per the distributed mobility
   anchoring architecture [I-D.ietf-dmm-distributed-mobility-anchoring].

3.4.  AERO Addresses

   A Client's AERO address is an IPv6 link-local address with an
   interface identifier based on the Client's delegated MNP.  Relay,
   Server and Proxy AERO addresses are assigned from the range fe80::/96
   and include an administratively-provisioned value in the lower 32
   bits.

   For IPv6, Client AERO addresses begin with the prefix fe80::/64 and
   include in the interface identifier (i.e., the lower 64 bits) a
   64-bit prefix taken from one of the Client's IPv6 MNPs.  For example,
   if the AERO Client receives the IPv6 MNP:

      2001:db8:1000:2000::/56

   it constructs its corresponding AERO addresses as:

      fe80::2001:db8:1000:2000

      fe80::2001:db8:1000:2001

      fe80::2001:db8:1000:2002

      ... etc. ...




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      fe80::2001:db8:1000:20ff

   For IPv4, Client AERO addresses are based on an IPv4-mapped IPv6
   address formed from an IPv4 MNP and with a Prefix Length of 96 plus
   the MNP prefix length.  For example, for the IPv4 MNP 192.0.2.32/28
   the IPv4-mapped IPv6 MNP is:

      0:0:0:0:0:FFFF:192.0.2.16/124

   The Client then constructs its AERO addresses with the prefix
   fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address
   in the interface identifier as:

      fe80::FFFF:192.0.2.16

      fe80::FFFF:192.0.2.17

      fe80::FFFF:192.0.2.18

      ... etc. ...

      fe80:FFFF:192.0.2.31

   Relay, Server and Proxy AERO addresses are allocated from the range
   fe80::/96, and MUST be managed for uniqueness.  The lower 32 bits of
   the AERO address includes a unique integer value (e.g., fe80::1,
   fe80::2, fe80::3, etc.) as assigned by the administrative authority
   for the link.  If the link spans multiple segments (i.e., multiple
   INETs), the AERO addresses are assigned to each INET in 1x1
   correspondence with SPAN addresses (see: Section 3.5).  The address
   fe80:: is reserved as the IPv6 link-local Subnet Router Anycast
   address [RFC4291], and the address fe80::ffff:ffff is reserved as the
   unspecified AERO address; hence, these values are not available
   general assignment.

   The lowest-numbered AERO address from a Client's MNP delegation
   serves as the "base" AERO address (for example, for the MNP
   2001:db8:1000:2000::/56 the base AERO address is
   fe80::2001:db8:1000:2000).  The Client then assigns the base AERO
   address to the AERO interface and uses it for the purpose of
   maintaining the neighbor cache entry.  The Server likewise uses the
   AERO address as its index into the neighbor cache for this Client.

   If the Client has multiple AERO addresses (i.e., when there are
   multiple MNPs and/or MNPs with prefix lengths shorter than /64), the
   Client originates ND messages using the base AERO address as the
   source address and accepts and responds to ND messages destined to
   any of its AERO addresses as equivalent to the base AERO address.  In



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   this way, the Client maintains a single neighbor cache entry that may
   be indexed by multiple AERO addresses.

   The Client's Subnet Router Anycast address can be statelessly
   determined from its AERO address by simply transposing the AERO
   address into the upper N bits of the Anycast address followed by
   128-N bits of zeros.  For example, for the AERO address
   fe80::2001:db8:1:2 the subnet router anycast address is
   2001:db8:1:2::/64.

   AERO addresses for mobile node Clients embed a MNP as discussed
   above, while AERO addresses for non-MNP destinations are constructed
   in exactly the same way.  A Client AERO address is therefore encodes
   either an MNP if the prefix is reached via the SPAN or a non-MNP if
   the prefix is reached via a Gateway.

3.5.  Spanning Partitioned AERO Networks (SPAN)

   In the simplest case, an AERO link configured over a single INET
   appears as a single unified link with a consistent underlying network
   addressing plan.  In that case, all nodes on the link can exchange
   packets via encapsulation with INET addresses, since the underlying
   INET is connected.  In common practice, however, an AERO link may be
   partitioned into multiple "segments", where each segment is a
   distinct INET potentially managed under a different administrative
   authority (e.g., as for worldwide aviation service providers such as
   ARINC, SITA, Inmarsat, etc.).  Individual INETs may themselves be
   partitioned internally, in which case each internal partition is seen
   as a separate segment.

   The addressing plan of each segment is consistent internally but will
   often bear no relation to the addressing plans of other segments.
   Each segment is also likely to be separated from others by network
   security devices (e.g., firewalls, proxies, packet filtering
   gateways, etc.), and in many cases disjoint segments may not even
   have any common physical link connections at all.  Therefore, nodes
   can only be assured of exchanging packets directly with
   correspondents in the same segment, and not with those in other
   segments.  The only means for joining the segments therefore is
   through inter-domain peerings between AERO Relays.

   The same as for traditional campus LANs, multiple AERO link segments
   can be joined into a single unified link via a virtual bridging
   service termed the "SPAN".  The SPAN performs link-layer packet
   forwarding between segments (i.e., bridging) without decrementing the
   network-layer TTL/Hop Limit.  The SPAN model is depicted in Figure 2:





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                 . . . . . . . . . . . . . . . . . . . . . . .
               .                                               .
               .              .-(::::::::)                     .
               .           .-(::::::::::::)-.   +-+            .
               .          (:::: Segment A :::)--|R|---+        .
               .           `-(::::::::::::)-'   +-+   |        .
               .              `-(::::::)-'            |        .
               .                                      |        .
               .              .-(::::::::)            |        .
               .           .-(::::::::::::)-.   +-+   |        .
               .          (:::: Segment B :::)--|R|---+        .
               .           `-(::::::::::::)-'   +-+   |        .
               .              `-(::::::)-'            |        .
               .                                      |        .
               .              .-(::::::::)            |        .
               .           .-(::::::::::::)-.   +-+   |        .
               .          (:::: Segment C :::)--|R|---+        .
               .           `-(::::::::::::)-'   +-+   |        .
               .              `-(::::::)-'            |        .
               .                                      |        .
               .                ..(etc)..             x        .
               .                                               .
               .                                               .
               .      <- AERO Link Bridged by the SPAN ->      .
                 . . . . . . . . . . . . . .. . . . . . . . .

                            Figure 2: The SPAN

   To support the SPAN, AERO links require a reserved /96 IPv6 "SPAN
   Service Prefix (SSP)".  Although any routable IPv6 prefix can be
   used, a Unique Local Address (ULA) prefix (e.g., fd00::/96) [RFC4389]
   is recommended since border routers are commonly configured to
   prevent packets with ULAs from being injected into the AERO link by
   an external IPv6 node and from leaking out of the AERO link to the
   outside world.

   Each segment in the SPAN assigns a unique sub-prefix of the SSP
   termed a "SPAN Partition Prefix (SPP)".  For example, a first segment
   could assign fd00::1000/116, a second could assign fd00::2000/116, a
   third could assign fd00::3000/116, etc.  The administrative
   authorities for each segment must therefore coordinate to assure
   mutually-exclusive SPP assignments, but internal provisioning of the
   SPP is a local consideration for each administrative authority.

   A "SPAN address" is an address taken from a SPP and assigned to a
   Relay, Server or Proxy interface.  SPAN addresses are formed by
   simply replacing the upper portion of an administratively-assigned
   AERO address with the SPP.  For example, if the SPP is



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   fd00::1000/116, the SPAN address formed from the AERO address
   fe80::1001 is simply fd00::1001.

   An "INET address" is an address of a node's interface connection to
   an INET segment.  AERO/SPAN/INET address mappings are maintained as
   permanent neighbor cache entires as discussed in Section 3.8.

   AERO Relays serve as bridges to join multiple segments into a unified
   AERO link over multiple diverse administrative domains.  They support
   the bridging function by first establishing forwarding table entries
   for their SPPs either via standard BGP routing or static routes.  For
   example, if three Relays ('A', 'B' and 'C') from different segments
   serviced the SPPs fd00::1000/116, fd00::2000/116 and fd00::3000/116
   respectively, then the forwarding tables in each Relay are as
   follows:

   A: fd00::1000/116->local, fd00::2000/116->B, fd00::3000/116->C

   B: fd00::1000/116->A, fd00::2000/116->local, fd00::3000/116->C

   C: fd00::1000/116->A, fd00::2000/116->B, fd00::3000/116->local

   These forwarding table entries are permanent and never change, since
   they correspond to fixed infrastructure elements in their respective
   segments.  This provides the basis for a link-layer forwarding
   service that cannot be disrupted by routing updates due to node
   mobility.

   With the SPPs in place in each Relay's forwarding table, control and
   data packets sent between AERO nodes in different segments can
   therefore be carried over the SPAN via encapsulation.  For example,
   when a source node in segment A forwards a packet with IPv6 address
   2001:db8:1:2::1 to a destination node in segment C with IPv6 address
   2001:db8:1000:2000::1, it first encapsulates the packet in a SPAN
   header with source SPAN address taken from fd00::1000/116 (e.g.,
   fd00::1001) and destination SPAN address taken from fd00::3000/116
   (e.g., fd00::3001).  Next, it encapsulates the SPAN message in an
   INET header with source address set to its own INET address (e.g.,
   192.0.2.100) and destination set to the INET address of a Relay
   (e.g., 192.0.2.1).

   SPAN encapsulation is based on Generic Packet Tunneling in IPv6
   [RFC2473]; the encapsulation format in the above example is shown
   inFigure 3:







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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |          INET Header          |
        |       src = 192.0.2.100       |
        |        dst = 192.0.2.1        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |         SPAN Header           |
        |       src = fd00::1001        |
        |       dst = fd00::3001        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        Inner IP Header        |
        |    src = 2001:db8:1:2::1      |
        |  dst = 2001:db8:1000:2000::1  |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |
        ~                               ~
        ~      Inner Packet Body        ~
        ~                               ~
        |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 3: SPAN Encapsulation

   In this format, the inner IP header and packet body are the original
   IP packet, the SPAN header is an IPv6 header prepared according to
   [RFC2473], and the INET header is prepared according to Section 3.10.
   A packet is said to be "forwarded/sent into the SPAN" when it is
   encapsulated as described above then forwarded via a secured tunnel
   to a neighboring Relay.

   This gives rise to a routing system that contains both MNP routes
   that may change dynamically due to regional node mobility and SPAN
   routes that never change.  The Relays can therefore provide link-
   layer bridging by sending packets into the SPAN instead of network-
   layer routing according to MNP routes.  As a result, opportunities
   for packet loss due to node mobility between different segments are
   mitigated.

   With reference to Figure 3, for a Client's AERO address the SPAN
   address is simply set to the Subnet Router Anycast address.  For non-
   link-local addresses, the destination SPAN address may not be known
   in advance for the first few packets of a flow sent via the SPAN.  In
   that case, the SPAN destination address is set to the original
   packet's destination, and the SPAN routing system will direct the
   packet to the correct SPAN egress node.  (In the above example, the
   SPAN destination address is simply 2001:db8:1000:2000::1.)






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3.6.  AERO Interface Characteristics

   AERO interfaces use encapsulation (see: Section 3.10) to exchange
   packets with neighbors attached to the AERO link.

   AERO interfaces maintain a neighbor cache for tracking per-neighbor
   state the same as for any interface.  AERO interfaces use ND messages
   including Router Solicitation (RS), Router Advertisement (RA),
   Neighbor Solicitation (NS) and Neighbor Advertisement (NA) for
   neighbor cache management.

   AERO interface ND messages include one or more Source/Target Link-
   Layer Address Options (S/TLLAOs) formatted as shown in Figure 4:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Type     |   Length = 5  | Prefix Length |S|R|D|X|N|Resvd|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Interface ID         |          Port Number          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                       Link Layer Address                      +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 4: AERO Source/Target Link-Layer Address Option (S/TLLAO)
                                  Format

   In this format:

   o  Type is set to '1' for SLLAO or '2' for TLLAO.

   o  Length is set to the constant value '5' (i.e., 5 units of 8
      octets).




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   o  Prefix Length is set to the MNP prefix length in an ND message for
      the Client AERO address found in the source (RS), destination (RA)
      or target (NA) address; otherwise set to 0.  If the message
      contains multiple SLLAOs, only the Prefix Length value in the
      SLLAO with S set to 1 is consulted and the values in other SLLAOs
      are ignored.

   o  S (the 'Source' bit) is set to '1' in the S/TLLAO of an ND message
      that corresponds to the ANET/INET interface over which the ND
      message is sent, and set to 0 in all other S/TLLAOs.

   o  R (the "Release" bit) is set to '1' in an S/TLLAO in an RS/NA sent
      for the purpose of departing from a Server; otherwise, set to '0'.
      The recipient places the corresponding neighbor cache entry in the
      DEPARTED state.  For RS message, the recipient then releases the
      corresponding PD and returns an RA with Router Lifetime set to '0'

   o  D (the "Disable" bit) is set to '1' in the S/TLLAOs of an RS/NA
      message for each Interface ID that is to be disabled in the
      neighbor cache entry; otherwise, set to '0'.

   o  X (the "proXy" bit) is set to '1' in the SLLAO of an RS/RA message
      by the Proxy when there is a Proxy in the path; otherwise, set to
      '0'.  If the message contains multiple SLLAOs, only the X value in
      the first SLLAO is consulted and the values in other SLLAOs are
      ignored.

   o  N (the "(Network Address) Translator (NAT)" bit) is set to '1' in
      the SLLAO of an RA message by the Server if there is a translator
      in the path; otherwise, set to '0'.  If the message contains
      multiple SLLAOs, only the N value in the first SLLAO is consulted
      and the values in other SLLAOs are ignored.

   o  Resvd is set to the value '0' on transmission and ignored on
      receipt.

   o  Interface ID is set to a 16-bit integer value corresponding to an
      AERO node's ANET/INET interface.  Once the node has assigned an
      Interface ID to an ANET interface, the assignment must remain
      unchanged until the node fully detaches from the AERO link.  The
      value 0xffff is reserved as the Server's INET Interface ID, i.e.,
      Servers MUST use Interface ID 0xffff, and Clients MUST number
      their ANET Interface IDs with values in the range of 0-0xfffe.

   o  Port Number and Link Layer Address are set to the encapsulation
      addresses required to send packets via the target node (or to '0'
      when the addresses are left unspecified).  When UDP is not used as
      part of the encapsulation, Port Number is set to '0'.  When the



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      encapsulation IP address family is IPv4, IP Address is formed as
      an IPv4-mapped IPv6 address as specified in Section 3.4.

   o  P(i) is a set of Preferences that correspond to the 64
      Differentiated Service Code Point (DSCP) values [RFC2474].  Each
      P(i) is set to the value '0' ("disabled"), '1' ("low"), '2'
      ("medium") or '3' ("high") to indicate a QoS preference level for
      packet forwarding purposes.

   A Client's AERO interface may be configured over multiple ANET
   interface connections.  For example, common mobile handheld devices
   have both wireless local area network ("WLAN") and cellular wireless
   links.  These links are typically used "one at a time" with low-cost
   WLAN preferred and highly-available cellular wireless as a standby.
   In a more complex example, aircraft frequently have many wireless
   data link types (e.g. satellite-based, cellular, terrestrial, air-to-
   air directional, etc.) with diverse performance and cost properties.

   A Client's ANET interfaces are classified as follows:

   o  Native interfaces connect to the open INET, and have a global IP
      address that is reachable from any INET correspondent.

   o  NATed interfaces connect to an ANET behind a Network Address
      Translator (NAT).  The NAT does not participate in any AERO
      control message signaling, but the Server can issue control
      messages on behalf of the Client.  Clients that are behind a NAT
      are required to send periodic keepalive messages to keep NAT state
      alive when there are no data packets flowing.  If no other
      periodic messaging service is available, the Client can send RS
      messages to receive RA replies from its Server(s).

   o  VPNed interfaces use security encapsulation over the ANET to a
      Virtual Private Network (VPN) server that also acts as an AERO
      Server.  As with NATed links, the Server can issue control
      messages on behalf of the Client, but the Client need not send
      periodic keepalives in addition to those already used to maintain
      the VPN connection.

   o  Proxyed interfaces connect to an ANET that is separated from the
      open INET by an AERO Proxy.  Unlike NATed and VPNed interfaces,
      the Proxy can actively issue control messages on behalf of the
      Client.

   o  Direct interfaces connect the Client directly to a neighbor
      without crossing any ANET/INET paths.  An example is a line-of-
      sight link between a remote pilot and an unmanned aircraft.




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   If a Client's multiple ANET interfaces are used "one at a time"
   (i.e., all other interfaces are in standby mode while one interface
   is active), then ND messages include only a single S/TLLAO with
   Interface ID set to a constant value.  In that case, the Client would
   appear to have a single ANET interface but with a dynamically
   changing ANET address.

   If the Client has multiple active ANET interfaces, then from the
   perspective of ND it would appear to have multiple link-layer
   addresses.  In that case, ND messages MAY include multiple S/TLLAOs
   -- each with an Interface ID that corresponds to a specific ANET
   interface.  S must be set to 1 in the S/TLLAO corresponding to the
   AERO node's ANET interface used to transmit the message and set to 0
   in all other S/TLLAOs.

   When the Client includes an S/TLLAO for an ANET interface for which
   it is aware that there is a NAT on the path to the Server, or when a
   node includes an S/TLLAO solely for the purpose of announcing new QoS
   preferences, the node MAY set both Port Number and Link-Layer Address
   to 0 to indicate that the addresses are unspecified at the network
   layer and must instead be derived from the link-layer encapsulation
   headers.

   Relay, Server and Proxy AERO interfaces may be configured over one or
   more secured tunnel interfaces.  The AERO interface configures both
   an AERO address and its corresponding SPAN address, while the
   underlying secured tunnel interfaces also configure the same SPAN
   address.  The AERO interface encapsulates each packet in a SPAN
   header if necessary and presents the packet to the underlying secured
   tunnel interface.  For Relays that do not configure an AERO
   interface, the secured tunnel interfaces themselves are exposed to
   the IP layer with each interface configuring the same SPAN address.
   Routing protocols such as BGP therefore run directly over the secured
   tunnel interfaces.  For nodes that configure an AERO interface,
   routing protocols such as BGP run over the AERO interface but do not
   employ SPAN encapsulation.  Instead, the AERO interface presents the
   routing protocol packets directly to the underlying secured tunnels
   without applying encapsulation and while using the SPAN address as
   the source address.  This distinction must be honored consistently
   according to each node's configuration so that the IP forwarding
   table will associate discovered IP routes with the correct interface.

3.7.  AERO Interface Initialization

   AERO Servers, Proxys and Clients configure AERO interfaces as their
   point of attachment to the AERO link.  AERO nodes assign the MSPs for
   the link to their AERO interfaces (i.e., as a "route-to-interface")
   to ensure that packets with destination addresses covered by an MNP



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   not explicitly assigned to a non-AERO interface are directed to the
   AERO interface.

   AERO interface initialization procedures for Servers, Proxys, Clients
   and Relays are discussed in the following sections.

3.7.1.  AERO Server/Gateway Behavior

   When a Server enables an AERO interface, it assigns AERO/SPAN
   addresses and configures permanent neighbor cache entries for
   neighbors in the same SPAN segment.  The Server also configures
   secured tunnels with one or more neighboring Relays and engages in a
   BGP routing protocol session with each Relay.  The AERO interface
   provides a single interface abstraction to the IP layer, but
   internally comprises multiple secured tunnels as well as an NBMA
   nexus for sending encapsulated data packets to AERO interface
   neighbors.  The Server further configures a service to facilitate ND/
   PD exchanges with AERO Clients and manages per-Client neighbor cache
   entries and IP forwarding table entries based on control message
   exchanges.

   Gateways are simply Servers that run a dynamic routing protocol
   between the AERO interface and INET/EUN interfaces (see:
   Section 3.3).  The Gateway provisions MNPs to networks on the INET/
   EUN interfaces (i.e., the same as a Client would do) and advertises
   the MSP(s) for the AERO link over the INET/EUN interfaces.

3.7.2.  AERO Proxy Behavior

   When a Proxy enables an AERO interface, it assigns AERO/SPAN
   addresses the same as for Servers.  The Proxy further maintains
   permanent neighbor cache entries for neighbors in the same SPAN
   segment, and maintains per-Client neighbor cache entries based on
   control message exchanges.

3.7.3.  AERO Client Behavior

   When a Client enables an AERO interface, it sends RS messages with
   ND/PD parameters over an ANET interface to one or more Servers, which
   return RA messages with corresponding PD parameters.  (The RS/RA
   messages may pass through a Proxy in the case of a Client's Proxyed
   interface.)

   After the initial ND/PD message exchange, the Client assigns AERO
   addresses to the AERO interface based on the delegated prefix(es).
   The Client can then register additional ANET interfaces with the
   Server by sending an RS message over each ANET interface.




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3.7.4.  AERO Relay Behavior

   AERO Relays need not connect directly to the AERO link, since they
   operate as link-layer forwarding devices instead of network layer
   routers.  Configuration of AERO interfaces on Relays is therefore
   OPTIONAL, e.g., if an administrative interface is desired.  Relays
   configure secured tunnels with Servers, Proxys and other Relays; they
   also configure AERO/SPAN addresses and permanent neighbor cache
   entries the same as Servers.  Relays engage in a BGP routing protocol
   session with a subset of the Servers on the local segment, and with
   other Relays on the SPAN (see: Section 3.3).

3.8.  AERO Interface Neighbor Cache Maintenance

   Each AERO interface maintains a conceptual neighbor cache that
   includes an entry for each neighbor it communicates with on the AERO
   link per [RFC4861].  AERO interface neighbor cache entries are said
   to be one of "permanent", "symmetric", "asymmetric" or "proxy".

   Permanent neighbor cache entries are created through explicit
   administrative action; they have no timeout values and remain in
   place until explicitly deleted.  AERO Servers and Proxys maintain
   permanent neighbor cache entries for all other Servers and Proxys
   within the same SPAN segment as well as for their neighboring Relays.
   (Relays also maintain permanent neighbor cache entries for their
   neighbors if they configure an AERO interface.)  Each entry maintains
   the mapping between the neighbor's network-layer AERO address and
   corresponding INET address.

   Symmetric neighbor cache entries are created and maintained through
   RS/RA exchanges as specified in Section 3.15, and remain in place for
   durations bounded by ND/PD lifetimes.  AERO Servers maintain
   symmetric neighbor cache entries for each of their associated
   Clients, and AERO Clients maintain symmetric neighbor cache entries
   for each of their associated Servers.

   Asymmetric neighbor cache entries are created or updated based on
   route optimization messaging as specified in Section 3.17, and are
   garbage-collected when keepalive timers expire.  AERO route
   optimization sources (ROSs) maintain asymmetric neighbor cache
   entries for each of their active target Clients with lifetimes based
   on ND messaging constants.  Asymmetric neighbor cache entries are
   unidirectional since only the ROS and not the target (i.e., the
   Client's MAP) creates an entry.

   Proxy neighbor cache entries are created and maintained by AERO
   Proxys when they process Client/Server ND/PD exchanges, and remain in
   place for durations bounded by ND/PD lifetimes.  AERO Proxys maintain



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   proxy neighbor cache entries for each of their associated Clients.
   Proxy neighbor cache entries track the Client state and the state of
   each of the Client's associated Servers.

   To the list of neighbor cache entry states in Section 7.3.2 of
   [RFC4861], AERO interfaces add an additional state DEPARTED that
   applies to symmetric and proxy neighbor cache entries for Clients
   that have recently departed.  The interface sets a "DepartTime"
   variable for the neighbor cache entry to "DEPARTTIME" seconds.
   DepartTime is decremented unless a new ND message causes the state to
   return to REACHABLE.  While a neighbor cache entry is in the DEPARTED
   state, packets destined to the target Client are forwarded to the
   Client's new location instead of being dropped.  When DepartTime
   decrements to 0, the neighbor cache entry is deleted.  It is
   RECOMMENDED that DEPARTTIME be set to the default constant value 40
   seconds to allow for packets in flight to be delivered while stale
   route optimization state may be present.

   When a target Server (acting as a Mobility Anchor Point (MAP))
   receives a valid NS message used for route optimization, it searches
   for a symmetric neighbor cache entry for the target Client.  The MAP
   then returns a solicited NA message without creating a neighbor cache
   entry for the ROS, but creates a target Client "Report List" entry
   for the ROS and sets a "ReportTime" variable for the entry to
   REPORTTIME seconds.  The MAP resets ReportTime when it receives a new
   authentic NS message, and otherwise decrements ReportTime while no NS
   messages have been received.  It is RECOMMENDED that REPORTTIME be
   set to the default constant value 40 seconds to allow a 10 second
   window so that route optimization can converge before ReportTime
   decrements below REACHABLETIME.

   When the ROS receives a solicited NA message response to its NS
   message, it creates or updates an asymmetric neighbor cache entry for
   the target network-layer and link-layer addresses.  The ROS then
   (re)sets ReachableTime for the neighbor cache entry to REACHABLETIME
   seconds and uses this value to determine whether packets can be
   forwarded directly to the target, i.e., instead of via a default
   route.  The ROS otherwise decrements ReachableTime while no further
   solicited NA messages arrive.  It is RECOMMENDED that REACHABLETIME
   be set to the default constant value 30 seconds as specified in
   [RFC4861].

   The ROS also uses the value MAX_UNICAST_SOLICIT to limit the number
   of NS keepalives sent when a correspondent may have gone unreachable,
   the value MAX_RTR_SOLICITATIONS to limit the number of RS messages
   sent without receiving an RA and the value MAX_NEIGHBOR_ADVERTISEMENT
   to limit the number of unsolicited NAs that can be sent based on a
   single event.  It is RECOMMENDED that MAX_UNICAST_SOLICIT,



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   MAX_RTR_SOLICITATIONS and MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the
   same as specified in [RFC4861].

   Different values for DEPARTTIME, REPORTTIME, REACHABLETIME,
   MAX_UNICAST_SOLICIT, MAX_RTR_SOLCITATIONS and
   MAX_NEIGHBOR_ADVERTISEMENT MAY be administratively set; however, if
   different values are chosen, all nodes on the link MUST consistently
   configure the same values.  Most importantly, DEPARTTIME and
   REPORTTIME SHOULD be set to a value that is sufficiently longer than
   REACHABLETIME to avoid packet loss due to stale route optimization
   state.

3.9.  AERO Interface Forwarding Algorithm

   IP packets enter a node's AERO interface either from the network
   layer (i.e., from a local application or the IP forwarding system) or
   from the link layer (i.e., from an AERO interface neighbor).  Packets
   that enter the AERO interface from the network layer are encapsulated
   and forwarded into the AERO link, i.e., they are tunneled to an AERO
   interface neighbor.  Packets that enter the AERO interface from the
   link layer are either re-admitted into the AERO link or forwarded to
   the network layer where they are subject to either local delivery or
   IP forwarding.  In all cases, the AERO interface itself MUST NOT
   decrement the network layer TTL/Hop-count since its forwarding
   actions occur below the network layer.

   AERO interfaces may have multiple underlying ANET/INET interfaces
   and/or neighbor cache entries for neighbors with multiple Interface
   ID registrations (see Section 3.6).  The AERO interface uses each
   packet's DSCP value (and/or port number) to select an outgoing ANET/
   INET interface based on the node's own QoS preferences, and also to
   select a destination link-layer address based on the neighbor's ANET/
   INET interface with the highest preference.  AERO implementations
   SHOULD allow for QoS preference values to be modified at runtime
   through network management.

   If multiple outgoing interfaces and/or neighbor interfaces have a
   preference of "high", the AERO node replicates the packet and sends
   one copy via each of the (outgoing / neighbor) interface pairs;
   otherwise, the node sends a single copy of the packet via the
   interface with the highest preference.  AERO nodes keep track of
   which ANET/INET interfaces are currently "reachable" or
   "unreachable", and only use "reachable" interfaces for forwarding
   purposes.

   For control messages, the source node encapsulates the message in
   SPAN/INET headers and forwards the message into the SPAN.  For data
   packets, if the neighboring node can only be reached via the SPAN



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   (or, if it is not yet know that the neighboring node is within the
   local segment) the source node forwards them into the SPAN.
   Otherwise, the source node encapsulates packets in only an INET
   header for transmission within the local segment.

   The following sections discuss the AERO interface forwarding
   algorithms for Clients, Proxys, Servers and Relays.  In the following
   discussion, a packet's destination address is said to "match" if it
   is the same as a cached address, or if is covered by a cached prefix
   (which may be encoded in an AERO address).

3.9.1.  Client Forwarding Algorithm

   When an IP packet enters a Client's AERO interface from the network
   layer the Client searches for an asymmetric neighbor cache entry that
   matches the destination.  If there is a match, the Client uses one or
   more "reachable" neighbor interfaces in the entry for packet
   forwarding.  If there is no asymmetric neighbor cache entry, the
   Client instead forwards the packet to a Server (which may be
   intercepted by a Proxy).

   When an IP packet enters a Client's AERO interface from the link-
   layer, if the destination matches one of the Client's MNPs or link-
   local addresses the Client decapsulates the packet and delivers it to
   the network layer.  Otherwise, the Client drops the packet and MAY
   return a network-layer ICMP Destination Unreachable message subject
   to rate limiting (see: Section 3.14).

3.9.2.  Proxy Forwarding Algorithm

   For control messages originating from or destined to a Client, the
   Proxy intercepts the message and updates its proxy neighbor cache
   entry for the Client.  The Proxy then forwards a (proxyed) copy of
   the control message.  (For example, the Proxy forwards a proxied
   version of a Client's Ns/RS message to the target neighbor, and
   forwards a proxied version of the NA/RA reply to the Client.)

   When the Proxy receives a data packet from a Client within the ANET,
   the Proxy searches for an asymmetric neighbor cache entry that
   matches the destination and forwards the packet as follows:

   o  if the destination matches an asymmetric neighbor cache entry, the
      Proxy uses one or more "reachable" neighbor interfaces in the
      entry for packet forwarding via encapsulation.  If the neighbor
      interface is in the same SPAN segment as the Proxy, the Proxy uses
      simple INET encapsulation; otherwise the Proxy forwards the packet
      into the SPAN.




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   o  else, the Proxy forwards the packet into the SPAN while using the
      packet's destination address as the SPAN destination address.  (If
      the destination is an AERO address, the Proxy instead uses the
      corresponding Subnet Router Anycast address for Client AERO
      addresses and the SPAN address for administratively-provisioned
      AERO addresses.)

   When the Proxy receives an encapsulated data packet from the INET, it
   searches for a proxy neighbor cache entry that matches the
   destination.  If there is a proxy neighbor cache entry in the
   REACHABLE state, the Proxy forwards the packet to the Client; if the
   neighbor cache entry is in the DEPARTED state, the Proxy instead
   forwards the packet to the Client's Server and returns an unsolicited
   NA message as discussed in Section 3.19.  If there is no neighbor
   cache entry, the Proxy discards the packet.

3.9.3.  Server/Gateway Forwarding Algorithm

   For control messages destined to a target Client that are received
   from the SPAN, the Server (acting as a MAP) intercepts the message
   and sends an appropriate response on behalf of the Client.  (For
   example, the Server sends an NA message reply via the SPAN in
   response to an NS message directed to one of its associated Clients.)

   For control messages originating from a source Client that are
   received from the SPAN, the Server sends an appropriate response to
   the same Client.  (For example, the Server sends an RA message reply
   via the SPAN in response to an RS message sourced by one of its
   associated Clients.)

   When the Server's AERO interface receives a data packet from the
   link-layer (i.e., from an INET neighbor or from a SPAN secured
   tunnel), it decapsulates and processes the packet according to the
   network-layer destination address as follows:

   o  if the destination matches a symmetric neighbor cache entry the
      Server forwards the packet according to the neighbor cache state
      and link-layer address information.  If the neighbor cache entry
      is in the REACHABLE state, the Server forwards the packet
      according to the cached link-layer information.  If the neighbor
      cache entry is in the DEPARTED state, the Server instead forwards
      the packet to the Client's new Server as discussed in
      Section 3.19.  If the packet is destined to the same Client from
      which it arrived, however, the Server must forward the packet via
      a different "reachable" neighbor interface than the one the packet
      arrived on.  If there are no "reachable" neighbor interfaces, the
      Server drops the packet.




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   o  else, if the destination matches an asymmetric neighbor cache
      entry, the Server uses one or more "reachable" neighbor interfaces
      in the entry for packet forwarding via encapsulation.

   o  else, if the destination is an administrative AERO address that is
      not assigned on the AERO interface the Server forwards the packet
      into the SPAN while using the SPAN address corresponding to the
      destination as the SPAN destination address.  If the packet
      arrived from the SPAN, however, the Server instead drops the
      packet to avoid looping.

   o  else, the Server (acting as a Gateway) releases the packet to the
      network layer for local delivery or IP forwarding.  Based on the
      information in the forwarding table, the network layer may return
      the packet to the same AERO interface in which case further
      processing occurs as below.  (Note that this arrangement
      accommodates common implementations in which the IP forwarding
      table is not accessible from within the AERO interface.  If the
      AERO interface can directly access the IP forwarding table, the
      forwarding table lookup can instead be performed internally from
      within the AERO interface itself.)

   When the Server's AERO interface receives a data packet from the
   network layer, it processes the packet according to the network-layer
   destination address as follows:

   o  if the destination matches a symmetric or asymmetric neighbor
      cache entry the Server processes the packet as above.

   o  else, the Server forwards the packet into the SPAN.  For AERO
      address destinations, the Server uses the SPAN address
      corresponding to the destination as the SPAN destination address;
      for others, the Server uses the packet's destination IP address as
      the SPAN destination address

3.9.4.  Relay Forwarding Algorithm

   Relays forward packets over secured tunnels the same as any IP
   router.  When the Relay receives an encapsulated packet via a secured
   tunnel, it removes the INET header and searches for a forwarding
   table entry that matches the destination address in the next header.
   The Relay then processes the packet as follows:

   o  if the destination matches one of the Relay's own addresses, the
      Relay submits the packet for local delivery.

   o  else, if the destination matches route to an MNP the Relay
      forwards the packet via a secured tunnel to the next hop.  If the



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      destination matches an MSP without matching an MNP, however, the
      Relay drops the packet and returns an ICMP Destination Unreachable
      message subject to rate limiting (see: Section 3.14).

   o  else, if the destination matches a route to a non-MSP the Relay
      forwards the packet via a secured tunnel to the next hop.

   o  else, the Relay drops the packet and returns an ICMP Destination
      Unreachable as above.

   As for any IP router, the Relay decrements the TTL/Hop Limit when it
   forwards the packet.  If the packet is encapsulated in a SPAN header,
   only the Hop Limit in the SPAN header is decremented, and not the
   inner packet header.

3.10.  AERO Interface Encapsulation and Re-encapsulation

   AERO interfaces encapsulate packets according to whether they are
   entering the AERO interface from the network layer or if they are
   being re-admitted into the same AERO link they arrived on.  This
   latter form of encapsulation is known as "re-encapsulation".  Note
   that Clients can avoid encapsulation when the first-hop access router
   is AERO-aware.

   For packets entering the AERO interface from the network layer, the
   AERO interface copies the "TTL/Hop Limit", "Type of Service/Traffic
   Class" [RFC2983], "Flow Label"[RFC6438] (for IPv6) and "Congestion
   Experienced" [RFC3168] values in the packet's IP header into the
   corresponding fields in the encapsulation header(s).

   For packets undergoing re-encapsulation, the AERO interface instead
   copies these values from the original encapsulation header into the
   new encapsulation header, i.e., the values are transferred between
   encapsulation headers and *not* copied from the encapsulated packet's
   network-layer header.  (Note especially that by copying the TTL/Hop
   Limit between encapsulation headers the value will eventually
   decrement to 0 if there is a (temporary) routing loop.)  For IPv4
   encapsulation/re-encapsulation, the AERO interface sets the DF bit as
   discussed in Section 3.13.

   The AERO interface encapsulates the packet according to the next hop
   determined in the forwarding algorithm in Section 3.9.  If the next
   hop is reached via a secured tunnel, the AERO interface encapsulates
   the packet in a SPAN header and uses an INET encapsulation format
   specific to the secured tunnel type (see: Section 14).  If the next
   hop is reached via an unsecured underlying interface, the AERO
   interface instead encapsulates the packet per the Generic UDP
   Encapsulation (GUE) procedures in



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   [I-D.ietf-intarea-gue][I-D.ietf-intarea-gue-extensions], or through
   an alternate encapsulation format (see: Appendix A).

   When GUE encapsulation is used, the AERO interface next sets the UDP
   source port to a constant value that it will use in each successive
   packet it sends, and sets the UDP length field to the length of the
   encapsulated packet plus 8 bytes for the UDP header itself plus the
   length of the GUE header (or 0 if GUE direct IP encapsulation is
   used).  For packets sent to a Server or Relay, the AERO interface
   sets the UDP destination port to 8060, i.e., the IANA-registered port
   number for AERO.  For packets sent to a Client, the AERO interface
   sets the UDP destination port to the port value stored in the
   neighbor cache entry for this Client.  The AERO interface then either
   includes or omits the UDP checksum according to the GUE
   specification.

   As the final aspect of encapsulation, the AERO interface observes the
   packet sizing and fragmentation considerations found in Section 3.13.

3.11.  AERO Interface Decapsulation

   AERO interfaces decapsulate packets destined either to the AERO node
   itself or to a destination reached via an interface other than the
   AERO interface the packet was received on.  When the encapsulated
   packet arrives in multiple fragments, the AERO interface reassembles
   as discussed in Section 3.13.  Further decapsulation steps are
   performed according to the appropriate encapsulation format
   specification.

3.12.  AERO Interface Data Origin Authentication

   AERO nodes employ simple data origin authentication procedures for
   encapsulated packets they receive from other nodes on the AERO link.
   In particular:

   o  AERO Relays, Servers and Proxys accept data packets and control
      messages received from Relays via secured tunnels.

   o  AERO Clients and Servers accept encapsulated packets if there is a
      symmetric neighbor cache entry with a link-layer address that
      matches the packet's link-layer source address.

   o  AERO Proxys accept encapsulated packets if there is a proxy
      neighbor cache entry that matches the packet's network-layer
      address.

   Each packet should include a signature that the recipient can use to
   authenticate the message origin, e.g., as for common VPN systems such



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   as OpenVPN [OVPN].  In some environments, however, it may be
   sufficient to require signatures only for ND control plane messages
   (see: Section 14) and omit signatures for data plane messages.

3.13.  AERO Interface Packet Size Issues

   The AERO interface is the node's attachment to the AERO link.  The
   AERO interface acts as a tunnel ingress when it sends a packet to an
   AERO link neighbor and as a tunnel egress when it receives a packet
   from an AERO link neighbor.  AERO interfaces observe the packet
   sizing considerations for tunnels discussed in
   [I-D.ietf-intarea-tunnels] and as specified below.

   The Internet Protocol expects that IP packets will either be
   delivered to the destination or a suitable Packet Too Big (PTB)
   message returned to support the process known as IP Path MTU
   Discovery (PMTUD) [RFC1191][RFC8201].  However, PTB messages may be
   crafted for malicious purposes such as denial of service, or lost in
   the network [RFC2923].  This can be especially problematic for
   tunnels, where a condition known as a PMTUD "black hole" can result.
   For these reasons, AERO interfaces employ operational procedures that
   avoid interactions with PMTUD, including the use of fragmentation
   when necessary.

   AERO interfaces observe two different types of fragmentation.  Source
   fragmentation occurs when the AERO interface (acting as a tunnel
   ingress) fragments the encapsulated packet into multiple fragments
   before admitting each fragment into the tunnel.  Network
   fragmentation occurs when an encapsulated packet admitted into the
   tunnel by the ingress is fragmented by an IPv4 router on the path to
   the egress.  Note that an IPv4 packet that incurs source
   fragmentation may also incur network fragmentation.

   IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of 1280
   bytes [RFC8200].  Although IPv4 specifies a smaller minimum link MTU
   of 68 bytes [RFC0791], AERO interfaces also observe the IPv6 minimum
   for IPv4 even if encapsulated packets may incur network
   fragmentation.

   IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500 bytes
   [RFC8200], while the minimum MRU for IPv4 is only 576 bytes [RFC1122]
   (but, note that many standard IPv6 over IPv4 tunnel types already
   assume a larger MRU than the IPv4 minimum).

   AERO interfaces therefore configure an MTU that MUST NOT be smaller
   than 1280 bytes, MUST NOT be larger than the minimum MRU among all
   nodes on the AERO link minus the encapsulation overhead ("ENCAPS"),
   and SHOULD NOT be smaller than 1500 bytes.  AERO interfaces also



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   configure a Maximum Segment Unit (MSU) as the maximum-sized
   encapsulated packet that the ingress can inject into the tunnel
   without source fragmentation.  The MSU value MUST NOT be larger than
   (MTU+ENCAPS) and MUST NOT be larger than 1280 bytes unless there is
   operational assurance that a larger size can traverse the link along
   all paths.

   All AERO nodes MUST configure the same MTU value for reasons cited in
   [RFC3819][RFC4861]; in particular, multicast support requires a
   common MTU value among all nodes on the link.  All AERO nodes MUST
   configure an MRU large enough to reassemble packets up to
   (MTU+ENCAPS) bytes in length; nodes that cannot configure a large-
   enough MRU MUST NOT enable an AERO interface.  For example, for an
   MTU of 1500 bytes (or slightly larger) an appropriate MRU might be
   2KB.

   The network layer proceeds as follows when it presents an IP packet
   to the AERO interface.  For each IPv4 packet that is larger than the
   AERO interface MTU and with the DF bit set to 0, the network layer
   uses IPv4 fragmentation to break the packet into a minimum number of
   non-overlapping fragments where the first fragment is no larger than
   the MTU and the remaining fragments are no larger than the first.
   For all other IP packets, if the packet is larger than the AERO
   interface MTU, the network layer drops the packet and returns a PTB
   message to the original source.  Otherwise, the network layer admits
   each IP packet or fragment into the AERO interface.

   For each IP packet admitted into the AERO interface, the interface
   (acting as a tunnel ingress) encapsulates the packet.  If the
   encapsulated packet is larger than the MSU the ingress source-
   fragments the encapsulated packet into a minimum number of non-
   overlapping fragments where the first fragment is no larger than the
   MSU and the remaining fragments are no larger than the first.  The
   ingress then admits each encapsulated packet or fragment into the
   tunnel, and for IPv4 sets the DF bit to 0 in the IP encapsulation
   header in case any network fragmentation is necessary.  The
   encapsulated packets will be delivered to the egress, which
   reassembles them into a whole packet if necessary.

   Several factors must be considered when fragmentation is needed.  For
   AERO links over IPv4, the IP ID field is only 16 bits in length,
   meaning that fragmentation at high data rates could result in data
   corruption due to reassembly misassociations [RFC6864][RFC4963].  In
   environments where IP fragmentation issues could result in
   operational problems, the ingress SHOULD employ intermediate-layer
   source fragmentation (see: [RFC2473] and
   [I-D.ietf-intarea-gue-extensions]) before appending the outer
   encapsulation headers to each fragment.  Since the encapsulation



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   fragment header reduces the room available for packet data, but the
   original source has no way to control its insertion, the ingress MUST
   include the fragment header length in the ENCAPS length even for
   packets in which the header is absent.

3.14.  AERO Interface Error Handling

   When an AERO node admits encapsulated packets into the AERO
   interface, it may receive link-layer or network-layer error
   indications.

   A link-layer error indication is an ICMP error message generated by a
   router in the INET on the path to the neighbor or by the neighbor
   itself.  The message includes an IP header with the address of the
   node that generated the error as the source address and with the
   link-layer address of the AERO node as the destination address.

   The IP header is followed by an ICMP header that includes an error
   Type, Code and Checksum.  Valid type values include "Destination
   Unreachable", "Time Exceeded" and "Parameter Problem"
   [RFC0792][RFC4443].  (AERO interfaces ignore all link-layer IPv4
   "Fragmentation Needed" and IPv6 "Packet Too Big" messages since they
   only emit packets that are guaranteed to be no larger than the IP
   minimum link MTU as discussed in Section 3.13.)

   The ICMP header is followed by the leading portion of the packet that
   generated the error, also known as the "packet-in-error".  For
   ICMPv6, [RFC4443] specifies that the packet-in-error includes: "As
   much of invoking packet as possible without the ICMPv6 packet
   exceeding the minimum IPv6 MTU" (i.e., no more than 1280 bytes).  For
   ICMPv4, [RFC0792] specifies that the packet-in-error includes:
   "Internet Header + 64 bits of Original Data Datagram", however
   [RFC1812] Section 4.3.2.3 updates this specification by stating: "the
   ICMP datagram SHOULD contain as much of the original datagram as
   possible without the length of the ICMP datagram exceeding 576
   bytes".

   The link-layer error message format is shown in Figure 5 (where, "L2"
   and "L3" refer to link-layer and network-layer, respectively):












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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~
        |        L2 IP Header of        |
        |         error message         |
        ~                               ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |         L2 ICMP Header        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
        ~                               ~   P
        |   IP and other encapsulation  |   a
        | headers of original L3 packet |   c
        ~                               ~   k
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   e
        ~                               ~   t
        |        IP header of           |
        |      original L3 packet       |   i
        ~                               ~   n
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~   e
        |    Upper layer headers and    |   r
        |    leading portion of body    |   r
        |   of the original L3 packet   |   o
        ~                               ~   r
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

         Figure 5: AERO Interface Link-Layer Error Message Format

   The AERO node rules for processing these link-layer error messages
   are as follows:

   o  When an AERO node receives a link-layer Parameter Problem message,
      it processes the message the same as described as for ordinary
      ICMP errors in the normative references [RFC0792][RFC4443].

   o  When an AERO node receives persistent link-layer Time Exceeded
      messages, the IP ID field may be wrapping before earlier fragments
      awaiting reassembly have been processed.  In that case, the node
      SHOULD begin including integrity checks and/or institute rate
      limits for subsequent packets.

   o  When an AERO node receives persistent link-layer Destination
      Unreachable messages in response to encapsulated packets that it
      sends to one of its asymmetric neighbor correspondents, the node
      SHOULD process the message as an indication that a path may be
      failing, and MAY initiate NUD over that path.  If it receives
      Destination Unreachable messages on many or all paths, the node
      SHOULD set ReachableTime for the corresponding asymmetric neighbor




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      cache entry to 0 and allow future packets destined to the
      correspondent to flow through a default route.

   o  When an AERO Client receives persistent link-layer Destination
      Unreachable messages in response to encapsulated packets that it
      sends to one of its symmetric neighbor Servers, the Client SHOULD
      mark the path as unusable and use another path.  If it receives
      Destination Unreachable messages on many or all paths, the Client
      SHOULD associate with a new Server and release its association
      with the old Server as specified in Section 3.19.6.

   o  When an AERO Server receives persistent link-layer Destination
      Unreachable messages in response to encapsulated packets that it
      sends to one of its symmetric neighbor Clients, the Server SHOULD
      mark the underlying path as unusable and use another underlying
      path.  If it receives Destination Unreachable messages on multiple
      paths, the Server should take no further actions unless it
      receives an explicit ND/PD release message or if the PD lifetime
      expires.  In that case, the Server MUST release the Client's
      delegated MNP, withdraw the MNP from the AERO routing system and
      delete the neighbor cache entry.

   o  When an AERO Server or Proxy receives link-layer Destination
      Unreachable messages in response to an encapsulated packet that it
      sends to one of its permanent neighbors, it treats the messages as
      an indication that the path to the neighbor may be failing.
      However, the dynamic routing protocol should soon reconverge and
      correct the temporary outage.

   When an AERO Relay receives a packet for which the network-layer
   destination address is covered by an MSP, if there is no more-
   specific routing information for the destination the Relay drops the
   packet and returns a network-layer Destination Unreachable message
   subject to rate limiting.  The Relay writes the network-layer source
   address of the original packet as the destination address and uses
   one of its non link-local addresses as the source address of the
   message.

   When an AERO node receives an encapsulated packet for which the
   reassembly buffer it too small, it drops the packet and returns a
   network-layer Packet Too Big (PTB) message.  The node first writes
   the MRU value into the PTB message MTU field, writes the network-
   layer source address of the original packet as the destination
   address and writes one of its non link-local addresses as the source
   address.






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3.15.  AERO Router Discovery, Prefix Delegation and Autoconfiguration

   AERO Router Discovery, Prefix Delegation and Autoconfiguration are
   coordinated as discussed in the following Sections.

3.15.1.  AERO ND/PD Service Model

   Each AERO Server on the link configures a PD service to facilitate
   Client requests.  Each Server is provisioned with a database of MNP-
   to-Client ID mappings for all Clients enrolled in the AERO service,
   as well as any information necessary to authenticate each Client.
   The Client database is maintained by a central administrative
   authority for the AERO link and securely distributed to all Servers,
   e.g., via the Lightweight Directory Access Protocol (LDAP) [RFC4511],
   via static configuration, etc.  Therefore, no Server-to-Server PD
   state synchronization is necessary, and Clients can optionally hold
   separate PDs for the same MNPs from multiple Servers.  Clients can
   receive new PDs from new Servers before releasing PDs received from
   existing Servers for service continuity.  Clients receive the same
   service regardless of the Servers they select, although selecting
   Servers that are topologically nearby may provide better routing.

   AERO Clients and Servers use ND messages to maintain neighbor cache
   entries.  AERO Servers configure their AERO interfaces as advertising
   interfaces, and therefore send unicast RA messages with configuration
   information in response to a Client's RS message.  Thereafter,
   Clients send additional RS messages to refresh prefix and/or router
   lifetimes.

   AERO Clients and Servers include PD parameters in RS/RA messages (see
   [I-D.templin-6man-dhcpv6-ndopt] for ND/PD alternatives).  The unified
   ND/PD messages are exchanged between Client and Server according to
   the prefix management schedule required by the PD service.  If the
   Client knows its MNP in advance, it can include its AERO address as
   the source address of an RS message and with an SLLAO with a valid
   Prefix Length for the MNP.  If the Server (and Proxy) accept the
   Client's MNP assertion, they inject the prefix into the routing
   system and establish the necessary neighbor cache state.

   The following sections specify the Client and Server behavior.

3.15.2.  AERO Client Behavior

   AERO Clients can discover the INET and AERO addresses of Servers in
   the MAP list via static configuration (e.g., from a flat-file map of
   Server addresses and locations), or through an automated means such
   as Domain Name System (DNS) name resolution [RFC1035].  In the
   absence of other information, the Client can resolve the DNS Fully-



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   Qualified Domain Name (FQDN) "linkupnetworks.[domainname]" where
   "linkupnetworks" is a constant text string and "[domainname]" is a
   DNS suffix for the Client's ANET interface (e.g., "example.com").
   Alternatively, the Client can discover the Server's address through a
   multicast RS as described below.

   To associate with a Server, the Client acts as a requesting router to
   request MNPs.  The Client prepares an RS message with PD parameters
   (e.g., with an SLLAO with non-zero Prefix Length).  If the Client
   already knows the Server's AERO address, it includes the AERO address
   as the network-layer destination address; otherwise, it includes all-
   routers multicast (ff02::2) as the network-layer destination address.
   If the Client already knows its own AERO address, it uses the AERO
   address as the network-layer source address; otherwise, it uses the
   unspecified AERO address (fe80::ffff:ffff) as the network-layer
   source address.

   The Client next includes an SLLAO in the RS message formatted as
   described in Section 3.6 to register its link-layer information with
   the Server.  The SLLAO corresponding to the ANET interface over which
   the Client will send the RS message MUST set S to 1.  The Client MAY
   include additional SLLAOs specific to other underlying interfaces,
   but if so it MUST set their S, Port Number and Link Layer Address
   fields to 0.  If the Client is connected to an ANET for which
   encapsulation is required, the Client finally encapsulates the RS
   message in an ANET header with its own ANET address as the source
   address and the INET address of the Server as the destination.

   The Client then sends the RS message (either via a VPN for VPNed
   interfaces, via a Proxy for proxyed interfaces or via the SPAN for
   native interfaces) and waits for an RA message reply (see
   Section 3.15.3) while retrying up to MAX_RTR_SOLICITATIONS times
   until an RA is received.  If the Client receives no RAs, or if it
   receives an RA with Router Lifetime set to 0, the Client SHOULD
   abandon this Server and try another Server.  Otherwise, the Client
   processes the PD information found in the RA message.

   Next, the Client creates a symmetric neighbor cache entry with the
   Server's AERO address as the network-layer address and the address in
   the first SLLAO as the Server's INET address.  The Client records the
   RA Router Lifetime field value in the neighbor cache entry as the
   time for which the Server has committed to maintaining the MNP in the
   routing system.  The Client then autoconfigures AERO addresses for
   each of the delegated MNPs and assigns them to the AERO interface.
   The Client also caches any MSPs included in Route Information Options
   (RIOs) [RFC4191] as MSPs to associate with the AERO link, and assigns
   the MTU value in the MTU option to its AERO interface while
   configuring an appropriate MRU.



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   The Client then registers additional ANET interfaces with the Server
   by sending RS messages via each additional ANET interface.  The RS
   messages include the same parameters as for the initial RS/RA
   exchange, but with destination address set to the Server's AERO
   address and with an SLLAO specific to the ANET interface.  (The RS
   messages include PD parameters the same as for the initial exchange
   so that the additional ANETs can register the PD information.)

   The Client examines the X and N bits in the SLLAO with S set to 1 in
   each RA message it receives.  If X is 1 the Client infers that there
   is a Proxy on the path, and if N is 1 the Client infers that there is
   a NAT on the path.  If N is 1, the Client SHOULD set Port Number and
   Link-Layer Address to 0 in the first S/TLLAO of any subsequent ND
   messages it sends to the Server over that link.

   Following autoconfiguration, the Client sub-delegates the MNPs to its
   attached EUNs and/or the Client's own internal virtual interfaces as
   described in [I-D.templin-v6ops-pdhost] to support the Client's
   downstream attached "Internet of Things (IoT)".  The Client
   subsequently maintains its MNP delegations through each of its
   Servers by sending additional RS messages before Router Lifetime
   expires.

   After the Client registers its ANET interfaces, it may wish to change
   one or more registrations, e.g., if an ANET interface changes address
   or becomes unavailable, if QoS preferences change, etc.  To do so,
   the Client prepares an RS message to send over any available ANET
   interface.  The RS MUST include an SLLAO with S set to 1 for the
   selected ANET interface and MAY include any additional SLLAOs
   specific to other ANET interfaces.  The Client includes fresh P(i)
   values in each SLLAO to update the Server's neighbor cache entry.  If
   the Client wishes to update only the P(i) values, it sets the Port
   Number and Link-Layer Address fields to 0.  If the Client wishes to
   disable the underlying interface, it sets D to 1.  When the Client
   receives the Server's RA response, it has assurance that the Server
   has been updated with the new information.

   If the Client wishes to associate with multiple Servers, it repeats
   the same procedures above for each additional Server.  If the Client
   wishes to discontinue use of a Server it issues an RS message over
   any underlying interface with an SLLAO with R set to 1 . When the
   Server processes the message, it releases the MNP, sets the symmetric
   neighbor cache entry state for the Client to DEPARTED, withdraws the
   IP route from the routing system and returns an RA reply with Router
   Lifetime set to 0.






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3.15.3.  AERO Server Behavior

   AERO Servers act as IP routers and support a PD service for Clients.
   Servers arrange to add their AERO and INET addresses to a static map
   of Server addresses for the link and/or the DNS resource records for
   the FQDN "linkupnetworks.[domainname]" before entering service.  The
   list of Server addresses should be geographically and/or
   topologically referenced, and forms the MAP list for the AERO link.

   When a Server receives a prospective Client's RS message on its AERO
   interface, it SHOULD return an immediate RA reply with Router
   Lifetime set to 0 if it is currently too busy or otherwise unable to
   service the Client.  Otherwise, the Server authenticates the RS
   message and processes the PD parameters.  The Server first determines
   the correct MNPs to delegate to the Client by searching the Client
   database.  When the Server delegates the MNPs, it also creates an IP
   forwarding table entry for each MNP so that the MNPs are propagated
   into the routing system (see: Section 3.3).  For IPv6, the Server
   creates a single IPv6 forwarding table entry for each MNP.  For IPv4,
   the Server creates both an IPv4 forwarding table entry and an IPv6
   forwarding table entry with the IPv4-mapped IPv6 address
   corresponding to the IPv4 address.

   The Server next creates a symmetric neighbor cache entry for the
   Client using the base AERO address as the network-layer address and
   with lifetime set to no more than the smallest PD lifetime.  Next,
   the Server updates the neighbor cache entry by recording the
   information in each SLLAO in the RS indexed by the Interface ID and
   including the Port Number, Link Layer Address and P(i) values.  For
   the SLLAO with S set to 1, however, the Server records the actual
   INET header source addresses instead of those that appear in the
   SLLAO in case there was a NAT in the path.  The Server also records
   the value of the X bit to indicate whether there is a Proxy on the
   path.

   Next, the Server prepares an RA message using its AERO address as the
   network-layer source address and the network-layer source address of
   the RS message as the network-layer destination address.  The Server
   includes the delegated MNPs, any other PD parameters and an SLLAO
   with the Link Layer Address set to the Server's SPAN address and with
   Interface ID set to 0xffff.  The Server then includes one or more
   RIOs that encode the MSPs for the AERO link, plus an MTU option for
   the link MTU (see Section 3.13).  The Server finally encapsulates the
   message in a SPAN header with source address set to its own SPAN
   address and destination address set to the Client's (or Proxy's) SPAN
   address, then forwards the message into the SPAN.





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   After the initial RS/RA exchange, the Server maintains the symmetric
   neighbor cache entry for the Client.  If the Client (or Proxy) issues
   additional NS/RS messages, the Server resets ReachableTime.  If the
   Client (or Proxy) issues an RS with PD release parameters (e.g., by
   including an SLLAO with R set to 1), or if the Client becomes
   unreachable, the Server sets the Client's symmetric neighbor cache
   entry to the DEPARTED state and withdraws the IP routes from the AERO
   routing system.

   The Server processes these and any other Client ND/PD messages, and
   returns an NA/RA reply.  The Server may also issue unsolicited RA
   messages, e.g., with PD reconfigure parameters to cause the Client to
   renegotiate its PDs, with Router Lifetime set to 0 if it can no
   longer service this Client, etc.  Finally, If the symmetric neighbor
   cache entry is in the DEPARTED state, the Server deletes the entry
   after DepartTime expires.

3.15.3.1.  Lightweight DHCPv6 Relay Agent (LDRA)

   When DHCPv6 is used as the ND/PD service back end, AERO Clients and
   Servers are always on the same link (i.e., the AERO link) from the
   perspective of DHCPv6.  However, in some implementations the DHCPv6
   server and ND function may be located in separate modules.  In that
   case, the Server's AERO interface module can act as a Lightweight
   DHCPv6 Relay Agent (LDRA)[RFC6221] to relay PD messages to and from
   the DHCPv6 server module.

   When the LDRA receives an authentic RS message, it extracts the PD
   message parameters and uses them to construct an IPv6/UDP/DHCPv6
   message.  It sets the IPv6 source address to the source address of
   the RS message, sets the IPv6 destination address to
   'All_DHCP_Relay_Agents_and_Servers' and sets the UDP fields to values
   that will be understood by the DHCPv6 server.

   The LDRA then wraps the message in a DHCPv6 'Relay-Forward' message
   header and includes an 'Interface-Id' option that includes enough
   information to allow the LDRA to forward the resulting Reply message
   back to the Client (e.g., the Client's link-layer addresses, a
   security association identifier, etc.).  The LDRA also wraps the
   information in all of the SLLAOs from the RS message into the
   Interface-Id option, then forwards the message to the DHCPv6 server.

   When the DHCPv6 server prepares a Reply message, it wraps the message
   in a 'Relay-Reply' message and echoes the Interface-Id option.  The
   DHCPv6 server then delivers the Relay-Reply message to the LDRA,
   which discards the Relay-Reply wrapper and IPv6/UDP headers, then
   uses the DHCPv6 message to construct an RA response to the Client.
   The Server uses the information in the Interface-Id option to prepare



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   the RA message and to cache the link-layer addresses taken from the
   SLLAOs echoed in the Interface-Id option.

3.16.  The AERO Proxy

   Clients may connect to ANETs that do not support direct
   communications to Servers in outside INETs.  In that case, the ANET
   can employ an AERO Proxy.  The Proxy is located at the ANET/INET
   border and listens for RS messages originating from or RA messages
   destined to ANET Clients.  The Proxy acts on these control messages
   as follows:

   o  when the Proxy receives an RS message from a new ANET Client, it
      first authenticates the message then examines the RS message
      network-layer destination address.  If the destination address is
      a Server's AERO address, the Proxy proceeds to the next step.
      Otherwise, if the destination is all-routers multicast the Proxy
      selects a "nearby" Server that is likely to be a good candidate to
      serve the Client and replaces the RS destination address with the
      Server's AERO address.  Next, the Proxy creates a proxy neighbor
      cache entry and caches the Client and Server addresses along with
      any identifying information including Transaction IDs, Client
      Identifiers, Nonce values, etc.  The Proxy then examines the
      address in the RS message SLLAO with S set to 1.  If the address
      is different than the Client's ANET address, the Proxy notes that
      the Client is behind a NAT.  The Proxy then sets the X to 1 and
      changes the Link Layer Address to its own SPAN address.  The Proxy
      finally encapsulates the RS message in SPAN/INET headers using the
      SPAN address of the Server as the SPAN destination address and the
      INET address of a Relay as the INET destination address.  The
      Proxy then forwards the message to the Server via the SPAN.

   o  when the Server receives the RS message, it authenticates the
      message then creates or updates a symmetric neighbor cache entry
      for the Client with the Proxy's SPAN address as the link-layer
      address.  The Server then sends an RA message with a single SLLAO
      back to the Proxy via the SPAN.

   o  when the Proxy receives the RA message, it matches the message
      with the RS that created the proxy neighbor cache entry.  The
      Proxy then caches the PD route information as a mapping from the
      Client's MNPs to the Client's ANET address, and sets the neighbor
      cache entry state to REACHABLE.  The Proxy then changes the SLLAO
      Link Layer Address to its own ANET address, sets X to 1, sets N to
      1 if the Client is behind a NAT, then re-encapsulates the RA
      message in an ANET header and forwards it to the Client.





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   After the initial RS/RA exchange, the Proxy forwards any Client data
   packets for which there is no matching asymmetric neighbor cache
   entry to a Relay via the SPAN.  Finally, the Proxy forwards any
   Client data destined to an asymmetric neighbor cache target directly
   to the target according to the link-layer information - the process
   of establishing asymmetric neighbor cache entries is specified in
   Section 3.17.

   While the Client is still attached to the ANET, the Proxy continues
   to send NS/RS messages to update each Server's symmetric neighbor
   cache entries on behalf of the Client and/or to convey QoS updates.
   If the Server ceases to send solicited NA/RA responses, the Proxy
   marks the Server as unreachable and sends an unsolicited RA with
   Router Lifetime set to zero to inform the Client that this Server is
   no longer able to provide Service.  If the Client becomes
   unreachable, the Proxy sets the neighbor cache entry state to
   DEPARTED and sends an RS message to each Server with an SLLAO with D
   set to 1 and with Interface ID set to the Client's interface ID so
   that the Server will de-register this Interface ID.  Although the
   Proxy engages in these ND exchanges on behalf of the Client, the
   Client can also send ND messages on its own behalf, e.g., if it is in
   a better position than the Proxy to convey QoS changes, etc.

   In some ANETs that employ a Proxy, the Client's MNP can be injected
   into the ANET routing system.  In that case, the Client can send data
   messages without encapsulation so that the ANET native routing system
   transports the unencapsulated packets to the Proxy.  This can be very
   beneficial, e.g., if the Client connects to the ANET via low-end data
   links such as some aviation wireless links.  This encapsulation
   avoidance represents a form of "header compression", meaning that the
   MTU should be sized based on the size of full encapsulated messages
   even if most messages are sent unencapsulated.

   If the first-hop ANET access router is AERO-aware, the Client can
   avoid encapsulation for both its control and data messages.  When the
   Client connects to the link, it can send an unencapsulated RS message
   with source address set to its AERO address and with destination
   address set to the AERO address of the Client's selected Server or to
   all-routers multicast.  The Client includes an SLLAO with Interface
   ID, Prefix Length and P(i) information but with Port Number and Link-
   Layer Address set to 0.

   The Client then sends the unencapsulated RS message, which will be
   intercepted by the AERO-Aware access router.  The access router then
   encapsulates the RS message in an ANET header with its own address as
   the source address and the address of a Proxy as the destination
   address.  The access router further remembers the address of the
   Proxy so that it can encapsulate future data packets from the Client



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   via the same Proxy.  If the access router needs to change to a new
   Proxy, it simply sends another RS message toward the Server via the
   new Proxy on behalf of the Client.

   In some cases, the access router and Proxy may be one and the same
   node.  In that case, the node would be located on the same physical
   link as the Client, but its message exchanges with the Server would
   need to pass through a security gateway at the ANET/INET border.  The
   method for deploying access routers and Proxys (i.e. as a single node
   or multiple nodes) is an ANET-local administrative consideration.

3.17.  AERO Route Optimization

   While data packets are flowing between a source and target node,
   route optimization SHOULD be used.  Route optimization is initiated
   by the first eligible Route Optimization Source (ROS) closest to the
   source as follows:

   o  For Clients on VPNed, NATed and Direct interfaces, the Server is
      the ROS.

   o  For Clients on Proxyed interfaces, the Proxy is the ROS.

   o  For Clients on native interfaces, the Client itself is the ROS.

   o  For correspondent nodes on INET/EUN interfaces serviced by a
      Gateway, the Gateway is the ROS.

   The route optimization procedure is conducted between the ROS and the
   target Server/Gateway acting as a Route Optimization Responder (ROR)
   in the same manner as for IPv6 ND Address Resolution and using the
   same NS/NA messaging.  The target may either be a MNP Client serviced
   by a MAP Server, or a non-MNP correspondent reachable via a Gateway.

   The procedures are specified in the following sections.

3.17.1.  Route Optimization Initiation

   While data packets are flowing from the source node toward a target
   node, the ROS performs address resolution by sending an NS message to
   receive a solicited NA message from the ROR.

   When the ROS sends an NS, it includes the AERO address of the ROS as
   the source address (e.g., fe80::1) and the AERO address corresponding
   to the data packet's destination address as the destination address
   (e.g., if the destination address is 2001:db8:1:2::1 then the
   corresponding AERO address is fe80::2001:db8:1:2).  The NS message
   includes no SLLAOs, but SHOULD include a Timestamp and Nonce option.



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   The ROS then encapsulates the NS message in a SPAN header with source
   set to its own SPAN address and destination set to the data packet's
   destination address, then sends it into the SPAN without decrementing
   the network-layer TTL/Hop Limit field.

3.17.2.  Relaying the NS

   When the Relay receives the (double-encapsulated) NS message from the
   ROS, it discards the INET header and determines that the ROR is the
   next hop by consulting its standard IPv6 forwarding table for the
   SPAN header destination address.  The Relay then forwards the SPAN
   message toward the ROR the same as for any IP router.  The final-hop
   Relay in the SPAN will deliver the message via a secured tunnel to
   the ROR.

3.17.3.  Processing the NS and Sending the NA

   When the ROR receives the (double-encapsulated) NS message, it
   examines the AERO destination address to determine whether it has a
   route that matches the target; if not, it drops the NS message and
   returns from processing.  Next, if the target belongs to an MNP
   Client in the DEPARTED state the ROR (acting as a MAP) changes the NS
   message SPAN destination address to the address of the Client's new
   MAP, forwards the message into the SPAN and returns from processing.
   If the target belongs to an MNP Client in the REACHABLE state, the
   ROR instead adds the AERO source address to the target Client's
   Report List with time set to ReportTime.  If the target belongs to a
   non-MNP, the ROR continues processing without adding an entry to the
   Report List.

   The ROR then prepares a solicited NA message to send back to the ROS
   but does not create a neighbor cache entry.  The ROR sets the NA
   source address to the destination AERO address of the NS, and
   includes the Nonce value received in the NS plus the current
   Timestamp.  The ROR next includes a TLLAO with Interface ID set to
   0xffff, with S set to 1, with all P(i) values set to "low", and with
   Link Layer Address set to the ROR's SPAN address.  If the target
   belongs to an MNP Client, the ROR sets the Prefix Length to the MNP
   prefix length; otherwise, it sets Prefix Length to the maximum of the
   non-MNP prefix length and 64.  (Note that a /64 limit is imposed to
   avoid causing the ROS to set short prefixes (e.g., "default") that
   would match destinations for which the routing system includes more-
   specific prefixes.  Note also that prefix lengths longer than /64 are
   out of scope for this specification.)

   If the target belongs to an MNP Client, the ROR next includes
   additional TLLAOs for all of the target Client's Interface IDs.  For
   NATed, VPNed and Direct interfaces, the TLLAO Link Layer Addresses



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   are the SPAN address of the ROR.  For Proxyed interfaces, the TLLAO
   Link Layer Addresses are the SPAN addresses of the target Client's
   Proxys, and for native interfaces the TLLAO Link Layer Addresses are
   the SPAN addresses of the target Client.

   The ROR finally encapsulates the NA message in a SPAN header with
   source set to its own SPAN address and destination set to the source
   SPAN address of the NS message, then forwards the message into the
   SPAN without decrementing the network-layer TTL/Hop Limit field.

3.17.4.  Relaying the NA

   When the Relay receives the (double-encapsulated) NA message from the
   ROR, it discards the INET header and determines that the ROS is the
   next hop by consulting its standard IPv6 forwarding table for the
   SPAN header destination address.  The Relay then forwards the SPAN-
   encapsulated NA message toward the ROS the same as for any IPv6
   router.  The final-hop Relay in the SPAN will deliver the message via
   a secured tunnel to the ROS.

3.17.5.  Processing the NA

   When the ROS receives the (double-encapsulated) solicited NA message,
   it discards the INET and SPAN headers.  The ROS next verifies the
   Nonce and Timestamp values, then creates an asymmetric neighbor cache
   entry for the ROR and caches all information found in the solicited
   NA TLLAOs.  The ROS finally sets the asymmetric neighbor cache entry
   lifetime to ReachableTime seconds.

3.17.6.  Route Optimization Maintenance

   Following route optimization, the ROS forwards future data packets
   destined to the target via the addresses found in the cached link-
   layer information.  The route optimization is shared by all sources
   that send packets to the target via the ROS, i.e., and not just the
   source on behalf of which the route optimization was initiated.

   While new data packets destined to the target are flowing through the
   ROS, it sends additional NS messages to the ROR before ReachableTime
   expires to receive a fresh solicited NA message the same as described
   in the previous sections.  (Route optimization refreshment strategies
   are an implementation matter, with a non-normative example given in
   Appendix D).

   The ROS then updates the asymmetric neighbor cache entry to refresh
   ReachableTime, while (for MNP destinations) the ROR adds or updates
   the ROS address to the target Client's Report List and with time set
   to ReportTime.  While no data packets are flowing, the ROS instead



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   allows ReachableTime for the asymmetric neighbor cache entry to
   expire.  When ReachableTime expires, the ROS deletes the asymmetric
   neighbor cache entry.  Future data packets flowing through the ROS
   will again trigger a new route optimization exchange while initial
   data packets travel over a suboptimal route via Servers and/or
   Relays.

   The ROS may also receive unsolicited NA messages from the ROR at any
   time.  If there is an asymmetric neighbor cache entry for the target,
   the ROS updates the link-layer information but does not update
   ReachableTime since the receipt of an unsolicited NA does not confirm
   that the forward path is still working.  If there is no asymmetric
   neighbor cache entry, the route optimization source simply discards
   the unsolicited NA.  Cases in which unsolicited NA messages are
   generated are specified in Section 3.19.

   In this arrangement, the ROS holds an asymmetric neighbor cache entry
   for the ROR, but the ROR does not hold an asymmetric neighbor cache
   entry for the ROS.  The route optimization neighbor relationship is
   therefore asymmetric and unidirectional.  If the target node also has
   packets to send back to the source node, then a separate route
   optimization procedure is performed in the reverse direction.  But,
   there is no requirement that the forward and reverse paths be
   symmetric.

3.18.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform Neighbor Unreachability Detection (NUD) per
   [RFC4861].  NUD is performed either reactively in response to
   persistent link-layer errors (see Section 3.14) or proactively to
   confirm reachability.  The NUD algorithm may further be seeded by ND
   hints of forward progress, but care must be taken to avoid inferring
   reachability based on spoofed information.

   When an ROR directs an ROS to a neighbor with one or more target
   link-layer addresses, the ROS can proactively test each direct path
   by sending an initial NS message to elicit a solicited NA response.
   While testing the paths, the ROS can optionally continue sending
   packets via the SPAN, maintain a small queue of packets until target
   reachability is confirmed, or (optimistically) allow packets to flow
   via the direct paths.  In any case, the ROS should only consider the
   neighbor unreachable if NUD fails over multiple target link-layer
   address paths.

   When a ROS sends an NS message used for NUD, it uses its AERO
   addresses as the IPv6 source address and the AERO address
   corresponding to a target link-layer address as the destination.  For
   each target link-layer address, if the address is not located within



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   the same AERO link segment the source node encapsulates the NS
   message in a SPAN header with its own SPAN address as the source and
   the SPAN address of the target as the destination, then forwards the
   message into the SPAN.  If the target address is located within the
   same segment, however, the source node omits the SPAN header and
   encapsulates the message in an INET header with its own INET address
   as the source and the INET address of the target as the destination,
   then sends the message directly to the target.

   Paths that pass NUD tests are marked as "reachable", while those that
   do not are marked as "unreachable".  These markings inform the AERO
   interface forwarding algorithm specified in Section 3.9.

   Proxys can perform NUD to verify Server reachability on behalf of
   their proxyed Clients so that the Clients need not engage in NUD
   messaging themselves.

3.19.  Mobility Management and Quality of Service (QoS)

   AERO is a Distributed Mobility Management (DMM) service.  Each Server
   is responsible for only a subset of the Clients on the AERO link, as
   opposed to a Centralized Mobility Management (CMM) service where
   there is a single network mobility service for all Clients.  Clients
   coordinate with their associated Servers via RS/RA exchanges to
   maintain the DMM profile, and the AERO routing system tracks all
   current Client/Server peering relationships.

   Servers provide a Mobility Anchor Point (MAP) for their dependent
   Clients.  Clients are responsible for maintaining neighbor
   relationships with their Servers through periodic RS/RA exchanges,
   which also serves to confirm neighbor reachability.  When a Client's
   underlying interface address and/or QoS information changes, the
   Client is responsible for updating the Server with this new
   information.  Note that for Proxyed interfaces, however, the Proxy
   can perform the RS/RA exchanges on the Client's behalf.

   Mobility management considerations are specified in the following
   sections.

3.19.1.  Mobility Update Messaging

   RORs acting as MAPs accommodate mobility and/or QoS change events by
   sending an unsolicited NA message to each ROS in the target Client's
   Report List.  When a MAP sends an unsolicited NA message, it sets the
   IPv6 source address to the Client's AERO address and sets the IPv6
   destination address to all-nodes multicast (ff02::1).  The MAP also
   includes a TLLAO with Interface ID 0xffff, S set to 1 and Link Layer
   address set to the MAP's SPAN address, and includes additional TLLAOs



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   for all of the target Client's Interface IDs with Link Layer
   Addresses set to the corresponding SPAN addresses.  The MAP finally
   encapsulates the message in a SPAN header with source set to its own
   SPAN address and destination set to the SPAN address of the ROS, then
   sends the message into the SPAN.

   As for the hot-swap of interface cards discussed in Section 7.2.6 of
   [RFC4861], the transmission and reception of unsolicited NA messages
   is unreliable but provides a useful optimization.  In well-connected
   Internetworks with robust data links unsolicited NA messages will be
   delivered with high probability, but in any case the MAP can
   optionally send up to MAX_NEIGHBOR_ADVERTISEMENT unsolicited NAs to
   each ROS to increase the likelihood that at least one will be
   received.

   When an ROS receives an unsolicited NA message, it ignores the
   message if there is no existing neighbor cache entry for the Client.
   Otherwise, it uses the included TLLAOs to update the Link Layer
   Address and QoS information in the neighbor cache entry, but does not
   reset ReachableTime since the receipt of an unsolicited NA message
   from the target Server does not provide confirmation that any forward
   paths to the target Client are working.

   If unsolicited NA messages are lost, the ROS may be left with stale
   address and/or QoS information for the Client for up to ReachableTime
   seconds.  During this time, the ROS can continue sending packets to
   the target Client according to its current neighbor cache information
   but may receive persistent unsolicited NA messages as discussed in
   Section 3.19.2.

3.19.2.  Forwarding Packets on Behalf of Departed Clients

   When a Server acting as a MAP receives packets with destination
   addresses that match a symmetric neighbor cache entry in the DEPARTED
   state, it forwards the packets to the SPAN address corresponding to
   the Client's new MAP.  If the ROS is in the Report List, the old MAP
   also sends an unsolicited NA message via the SPAN (subject to rate
   limiting) with a TLLAO with Interface ID 0xffff and with R set to 1.
   When the ROS receives the NA, it SHOULD delete the asymmetric
   neighbor cache entry and re-initiate route optimization.

   When a Proxy receives packets with destination addresses that match a
   proxy neighbor cache entry in the DEPARTED state, it forwards the
   packets to one of the target Client's MAPs.  If the ROS is not one of
   its proxy neighbor Clients, the Proxy also returns an unsolicited NA
   message via the SPAN (subject to rate limiting) with a single TLLAO
   with the target Client's Interface ID and with D set to 1.  The ROS
   will then realize that it needs to mark its neighbor cache entry



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   Interface ID for the Proxy as "unreachable", and SHOULD re-initiate
   route optimization while continuing to forward packets according to
   the remaining neighbor cache entry state.

3.19.3.  Announcing Link-Layer Address and/or QoS Preference Changes

   When a Client needs to change its ANET addresses and/or QoS
   preferences (e.g., due to a mobility event), either the Client or its
   Proxys send RS messages to its Servers via the SPAN with SLLAOs that
   include the new Client Port Number, Link Layer Address and P(i)
   values.  If the RS messages are sent solely for the purpose of
   updating QoS preferences, Port Number and Link-Layer Address are set
   to 0.

   Up to MAX_RTR_SOLICITATION RS messages MAY be sent in parallel with
   sending actual data packets in case one or more RAs are lost.  If all
   RAs are lost, the Client SHOULD re-associate with a new Server.

   When the Server receives the Client's changes, it sends unsolicited
   NA messages to all nodes in the Report List the same as described in
   the previous section.

3.19.4.  Bringing New Links Into Service

   When a Client needs to bring new ANET interfaces into service (e.g.,
   when it activates a new data link), it sends RS messages to its
   Servers via the ANET interface with SLLAOs that include the new
   Client Link Layer Address information.

3.19.5.  Removing Existing Links from Service

   When a Client needs to remove existing ANET interfaces from service
   (e.g., when it de-activates an existing data link), it sends RS
   messages to its Servers with SLLAOs with D set to 1.

   If the Client needs to send RS messages over an ANET interface other
   than the one being removed from service, it MUST include a current
   SLLAO with S set to 1 for the sending interface and include
   additional SLLAOs for any ANET interfaces being removed from service.

3.19.6.  Moving to a New Server

   When a Client associates with a new Server, it performs the Client
   procedures specified in Section 3.15.2.  The Client then sends an RS
   message over any working ANET interface with destination set to the
   old Server's AERO address and with an SLLAO with R set to 1 to fully
   release itself from the old Server.  The SLLAO also includes the SPAN
   address of the new Server in the Link Layer Address.  If the Client



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   does not receive an RA reply after MAX_RTR_SOLICITATIONS attempts
   over multiple ANET interfaces, the old Server may have failed and the
   Client should discontinue its release attempts.

   When the old Server processes the RS, it sends unsolicited NA
   messages with a single TLLAO with Interface ID set to 0xffff and with
   R and S set to 1 to all ROSs in the Client's Report List.  The Server
   also changes the symmetric neighbor cache entry state to DEPARTED,
   sets the link-layer address of the Client to the address found in the
   RS SLLAO (i.e., the SPAN address of the new Server), and sets a timer
   to DepartTime seconds.  The old Server then returns an RA message to
   the Client with Router Lifetime set to 0.  After DepartTime seconds
   expires, the old Server deletes the symmetric neighbor cache entry.

   When the Client receives the RA message with Router Lifetime set to
   0, it still must inform each of its remaining Proxys that it has
   released the old Server from service.  To do so, it sends an RS over
   each remaining proxyed ANET interface with destination set to the old
   Server's AERO address and with no SLLAO.  The Proxy will mark this
   Server as DEAPARTED and return an immediate RA without first
   performing an RS/RA exchange with the old Server.

   Clients SHOULD NOT move rapidly between Servers in order to avoid
   causing excessive oscillations in the AERO routing system.  Examples
   of when a Client might wish to change to a different Server include a
   Server that has gone unreachable, topological movements of
   significant distance, movement to a new geographic region, movement
   to a new segment, etc.

3.20.  Multicast

   The AERO Client provides an IGMP (IPv4) [RFC2236] or MLD (IPv6)
   [RFC3810] proxy service for its EUNs and/or hosted applications
   [RFC4605].  The Client forwards IGMP/MLD messages over any of its
   ANET interfaces for which group membership is required.  The IGMP/MLD
   messages may be further forwarded by a first-hop ANET access router
   acting as an IGMP/MLD-snooping switch [RFC4541], then ultimately
   delivered to an AERO Proxy/Server acting as a Protocol Independent
   Multicast - Sparse-Mode (PIM-SM, or simply "PIM") Designated Router
   (DR) [RFC7761].  AERO Gateways also act as PIM routers (i.e., the
   same as AERO Proxys/Servers) on behalf of nodes on INET/EUN networks.
   The behaviors identified in the following sections correspond to
   Source-Specific Multicast (SSM) and Any-Source Multicast (ASM)
   operational modes.







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3.20.1.  Source-Specific Multicast (SSM)

   When an ROS (i.e., an AERO Proxy/Server/Gateway) "X" acting as PIM
   router receives a Join/Prune message from a node on its downstream
   interfaces containing one or more ((S)ource, (G)roup) pairs, it
   updates its Multicast Routing Information Base (MRIB) accordingly.
   For each S belonging to a prefix reachable via X's non-AERO
   interfaces, X then forwards the (S, G) Join/Prune to any PIM routers
   on those interfaces per [RFC7761].

   For each S belonging to a prefix reachable via X's AERO interface, X
   originates a separate copy of the Join/Prune for each (S,G) in the
   message using its own AERO address as the source address and ALL-PIM-
   ROUTERS as the destination address.  X then encapsulates each message
   in a SPAN header with source address set to the SPAN address of X and
   destination address set to S then forwards the message into the SPAN.
   The SPAN in turn forwards the message to AERO Server/Gateway "Y" that
   services S.  At the same time, if the message was a Join, X sends a
   route-optimization NS message toward each S the same as discussed in
   Section 3.17.  The resulting NAs will return the AERO address for the
   prefix that matches S as the network-layer source address and TLLAOs
   with the SPAN addresses corresponding to any Interface IDs that are
   currently servicing S.

   When Y processes the Join/Prune message, if S located behind any
   Native, Direct, VPNed or NATed interfaces Y acts as a PIM router and
   updates its MRIB to list X as the next hop in the reverse path.  If S
   is located behind any Proxys "Z"*, Y also forwards the message to
   each Z* over the SPAN while continuing to use the AERO address of X
   as the source address.  Each Z* then updates its MRIB accordingly and
   maintains the AERO address of X as the next hop in the reverse path.
   Since the Relays in the SPAN do not examine network layer control
   messages, this means that the (reverse) multicast tree path is simply
   from each Z* (and/or Y) to X with no other multicast-aware routers in
   the path.  If any Z* (and/or Y) is located on the same SPAN segment
   as X, the multicast data traffic sent to X can use simple INET
   encapsulation and need not go over the SPAN.

   Following the intial Join/Prune and NS/NA messaging, X maintains an
   asymmetric neighbor cache entry for each S the same as if X was
   sending unicast data traffic to S.  In particular, X performs
   additional NS/NA exchanges to keep the neighbor cache entry alive for
   up to t_periodic seconds [RFC7761].  If no new Joins are received
   within t_periodic seconds, X allows the neighbor cache entry to
   expire.  Finally, if X receives any additional Join/Prune messages
   for (S,G) it forwards the messages to each Y and Z* in the neighbor
   cache entry over the SPAN.




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   At some later time, Client C that holds an MNP for source S may
   depart from a first Proxy Z1 and/or connect via a new Proxy Z2.  In
   that case, Y sends an unsolicited NA message to X the same as
   specified for unicast mobility in Section 3.19.  When X receives the
   unsolicited NA message, it updates its asymmetric neighbor cache
   entry for the AERO address for source S and sends new Join messages
   to any new Proxys Z2.  There is no requirement to send any Prune
   messages to old Proxys Z1 since source S will no longer source any
   multicast data traffic via Z1.  Instead, the multicast state for
   (S,G) in Proxy Z1 will soon time out since no new Joins will arrive.

   After some later time, C may move to a new Server Y2 and depart from
   old Sever Y1.  In that case, Y1 sends Join messages for any of C's
   active (S,G) groups to Y2 while including its own AERO address as the
   source address.  This causes Y2 to include Y1 in the multicast
   forwarding tree during the interim time that Y1's symmetric neighbor
   cache entry for C is in the DEPARTED state.  At the same time, Y1
   sends an unsolicited NA message to X with an Interface ID 0xffff and
   R set to 1 to cause X to release its asymmetric neighbor cache entry.
   X then sends a new Join message to S via the SPAN and re-initiates
   route optimization the same as if it were receiving a fresh Join
   message from a node on a downstream link.

3.20.2.  Any-Source Multicast (ASM)

   When an ROS X acting as a PIM router receives a Join/Prune from a
   node on its downstream interfaces containing one or more (*,G) pairs,
   it updates its Multicast Routing Information Base (MRIB) accordingly.
   X then forwards a copy of the message to the Rendezvous Point (RP) R
   for each G over the SPAN.  X uses its own AERO address as the source
   address and ALL-PIM-ROUTERS as the destination address, then
   encapsulates each message in a SPAN header with source address set to
   the SPAN address of X and destination address set to R, then sends
   the message into the SPAN.  At the same time, if the message was a
   Join X initiates NS/NA route optimization the same as for the SSM
   case discussed in Section 3.20.1.

   For each source S that sends multicast traffic to group G via R, the
   Proxy/Server Z* for the Client that aggregates S encapsulates the
   packets in PIM Register messages and forwards them to R via the SPAN.
   R may then elect to send a PIM Join to Z* over the SPAN.  This will
   result in an (S,G) tree rooted at Z* with R as the next hop so that R
   will begin to receive two copies of the packet; one native copy from
   the (S, G) tree and a second copy from the pre-existing (*, G) tree
   that still uses PIM Register encapsulation.  R can then issue a PIM
   Register-stop message to suppress the Register-encapsulated stream.
   At some later time, if C moves to a new Proxy/Server Z*, it resumes
   sending packets via PIM Register encapsulation via the new Z*.



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   At the same time, as multicast listeners discover individual S's for
   a given G, they can initiate an (S,G) Join for each S under the same
   procedures discussed in Section 3.20.1.  Once the (S,G) tree is
   established, the listeners can send (S, G) Prune messages to R so
   that multicast packets for group G sourced by S will only be
   delivered via the (S, G) tree and not from the (*, G) tree rooted at
   R.  All mobility considerations discussed for SSM apply.

3.20.3.  Bi-Directional PIM (BIDIR-PIM)

   Bi-Directional PIM (BIDIR-PIM) [RFC5015] provides an alternate
   approach to ASM that treats the Rendezvous Point (RP) as a Designated
   Forwarder (DF).  Further considerations for BIDIR-PIM are out of
   scope.

4.  Direct Underlying Interfaces

   When a Client's AERO interface is configured over a Direct interface,
   the neighbor at the other end of the Direct link can receive packets
   without any encapsulation.  In that case, the Client sends packets
   over the Direct link according to QoS preferences.  If the Direct
   interface has the highest QoS preference, then the Client's IP
   packets are transmitted directly to the peer without going through an
   ANET/INET.  If other interfaces have higher QoS preferences, then the
   Client's IP packets are transmitted via a different interface, which
   may result in the inclusion of Proxys, Servers and Relays in the
   communications path.  Direct interfaces must be tested periodically
   for reachability, e.g., via NUD.

5.  AERO Clients on the Open Internetwork

   AERO Clients that connect to the open Internetwork via either a
   native or NATed interface can establish a VPN to securely connect to
   a Server.  Alternatively, the Client can exchange ND messages
   directly with other AERO nodes on the same Internetwork using INET
   encapsulation only and without joining the SPAN.  In that case,
   however, the Client must apply asymmetric security for ND messages to
   ensure routing and neighbor cache integrity (see: Section 14).

6.  Operation over Multiple AERO Links (VLANs)

   An AERO Client can connect to multiple AERO links the same as for any
   data link service.  In that case, the Client maintains a distinct
   AERO interface for each link, e.g., 'aero0' for the first link,
   'aero1' for the second, 'aero2' for the third, etc.  Each AERO link
   would include its own distinct set of Relays, Servers and Proxys,
   thereby providing redundancy in case of failures.




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   The Relays, Servers and Proxys on each AERO link can assign AERO and
   SPAN addresses that use the same or different numberings from those
   on other links.  Since the links are mutually independent there is no
   requirement for avoiding inter-link address duplication, e.g., the
   same AERO address such as fe80::1000 could be used to number distinct
   nodes that connect to different links.

   Each AERO link could utilize the same or different ANET connections.
   The links can be distinguished at the link-layer via Virtual Local
   Area Network (VLAN) tagging the same as defined in IEEE 802.1Q.  This
   gives rise to the opportunity for supporting multiple redundant
   networked paths, where each VLAN is distinguished by a different
   label (e.g., colors such as Red, Green, Blue, etc.).  In particular,
   the Client can tag its RS messages with the appropriate label to
   cause the network to select the desired VLAN.

   Clients that connect to multiple AERO interfaces can select the
   outgoing interface appropriate for a given Red/Blue/Green/etc.
   traffic profile while (in the reverse direction) correspondent nodes
   must have some way of steering their packets destined to a target via
   the correct AERO link.

   This can be accomplished by introducing an "AERO Link Anycast"
   address that is configured by all Relays connected to the same AERO
   link.  Correspondent nodes then include a "type 4" routing header
   with the Anycast address for the AERO link as the IPv6 destination
   and with the address of the target encoded as the "next segment" in
   the routing header [RFC8402][I-D.ietf-6man-segment-routing-header].
   Standard IP routing will then direct the packet to the nearest Relay
   for the correct AERO link, which will replace the destination address
   with the target address then forward the packet to the target.

7.  Operation on AERO Links with /64 ASPs

   IPv6 AERO links typically have MSPs that aggregate many candidate
   MNPs of length /64 or shorter.  However, in some cases it may be
   desirable to use AERO over links that have only a /64 MSP.  This can
   be accommodated by treating all Clients on the AERO link as simple
   hosts that receive /128 prefix delegations.

   In that case, the Client sends an RS message to the Server the same
   as for ordinary AERO links.  The Server responds with an RA message
   that includes one or more /128 prefixes (i.e., singleton addresses)
   that include the /64 MSP prefix along with an interface identifier
   portion to be assigned to the Client.  The Client and Server then
   configure their AERO addresses based on the interface identifier
   portions of the /128s (i.e., the lower 64 bits) and not based on the
   /64 prefix (i.e., the upper 64 bits).



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   For example, if the MSP for the host-only IPv6 AERO link is
   2001:db8:1000:2000::/64, each Client will receive one or more /128
   IPv6 prefix delegations such as 2001:db8:1000:2000::1/128,
   2001:db8:1000:2000::2/128, etc.  When the Client receives the prefix
   delegations, it assigns the AERO addresses fe80::1, fe80::2, etc. to
   the AERO interface, and assigns the global IPv6 addresses (i.e., the
   /128s) to either the AERO interface or an internal virtual interface
   such as a loopback.  In this arrangement, the Client conducts route
   optimization in the same sense as discussed in Section 3.17.

   This specification has applicability for nodes that act as a Client
   on an "upstream" AERO link, but also act as a Server on "downstream"
   AERO links.  More specifically, if the node acts as a Client to
   receive a /64 prefix from the upstream AERO link it can then act as a
   Server to provision /128s to Clients on downstream AERO links.

8.  AERO Adaptations for SEcure Neighbor Discovery (SEND)

   SEcure Neighbor Discovery (SEND) [RFC3971] and Cryptographically
   Generated Addresses (CGAs) [RFC3972] were designed to secure IPv6 ND
   messaging in environments where symmetric network and/or transport-
   layer security services are impractical (see: Section 14).  AERO
   nodes that use SEND/CGA employ the following adaptations.

   When a source AERO node prepares a SEND-protected ND message, it uses
   a link-local CGA as the IPv6 source address and writes the prefix
   embedded in its AERO address (i.e., instead of fe80::/64) in the CGA
   parameters Subnet Prefix field.  When the neighbor receives the ND
   message, it first verifies the message checksum and SEND/CGA
   parameters while using the link-local prefix fe80::/64 (i.e., instead
   of the value in the Subnet Prefix field) to match against the IPv6
   source address of the ND message.

   The neighbor then derives the AERO address of the source by using the
   value in the Subnet Prefix field as the interface identifier of an
   AERO address.  For example, if the Subnet Prefix field contains
   2001:db8:1:2, the neighbor constructs the AERO address as
   fe80::2001:db8:1:2.  The neighbor then caches the AERO address in the
   neighbor cache entry it creates for the source, and uses the AERO
   address as the IPv6 destination address of any ND message replies.

9.  AERO Critical Infrastructure Considerations

   AERO Relays can be either Commercial off-the Shelf (COTS) standard IP
   routers or virtual machines in the cloud.  Relays must be
   provisioned, supported and managed by the INET administrative
   authority, and connected to the Relays of other INETs via inter-




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   domain peerings.  Cost for purchasing, configuring and managing
   Relays is nominal even for very large AERO links.

   AERO Servers can be standard dedicated server platforms, but most
   often will be deployed as virtual machines in the cloud.  The only
   requirements for Servers are that they can run the AERO user-level
   code and have at least one network interface connection to the INET.
   As with Relays, Servers must be provisioned, supported and managed by
   the INET administrative authority.  Cost for purchasing, configuring
   and managing Servers is nominal especially for virtual Servers hosted
   in the cloud.

   AERO Proxys are most often standard dedicated server platforms with
   one network interface connected to the ANET and a second interface
   connected to an INET.  As with Servers, the only requirements are
   that they can run the AERO user-level code and have at least one
   interface connection to the INET.  Proxys must be provisioned,
   supported and managed by the ANET administrative authority.  Cost for
   purchasing, configuring and managing Proxys is nominal, and borne by
   the ANET administrative authority.

   AERO Gateways can be any dedicated server or COTS router platform
   connected to INETs and/or EUNs.  The Gateway joins the SPAN and
   engages in eBGP peering with one or more Relays as a stub AS.  The
   Gateway then injects its MNPs and/or non-MNP prefixes into the BGP
   routing system, and provisions the prefixes to its downstream-
   attached networks.  The Gateway can perform ROS and MAP services the
   same as for any Server, and can route between the MNP and non-MNP
   address spaces.

10.  DNS Considerations

   AERO Client MNs and INET correspondent nodes consult the Domain Name
   System (DNS) the same as for any Internetworking node.  When
   correspondent nodes and Client MNs use different IP protocol versions
   (e.g., IPv4 correspondents and IPv6 MNs), the INET DNS must maintain
   A records for IPv4 address mappings to MNs which must then be
   populated in Gateway NAT64 mapping caches.  In that way, an IPv4
   correspondent node can send packets to the IPv4 address mapping of
   the target MN, and the Gateway will translate the IPv4 header and
   destination address into an IPv6 header and IPv6 destination address
   of the MN.

   When an AERO Client registers with an AERO Server, the Server returns
   the address(es) of DNS servers in RDNSS options [RFC6106].  The DNS
   server provides the IP addresses of other MNs and correspondent nodes
   in AAAA records for IPv6 or A records for IPv4.




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11.  Transition Considerations

   The SPAN ensures that dissimilar INET segments can be joined into a
   single unified AERO link, even though the INET segments themselves
   may have differing protocol versions and/or incompatible addressing
   plans.  However, a commonality can be achieved by incrementally
   distributing globally routable (i.e., native) IP prefixes to
   eventually reach all nodes (both mobile and fixed) in all segments.
   This can be accomplished by incrementally deploying AERO Gateways on
   each INET segment, with each Gateway distributing its MNPs and/or
   discovering non-MNP prefixes on its INET links.

   This gives rise to the opportunity to eventually distribute native IP
   addresses to all nodes, and to present a unified AERO link view
   (bridged by the SPAN) even if the INET segments remain in their
   current protocol and addressing plans.  In that way, the AERO link
   can serve the dual purpose of providing a mobility service and a
   transition service.  Or, if an INET segment is transitioned to a
   native IP protocol version and addressing scheme that is compatible
   with the AERO link MNP-based addressing scheme, the INET segment and
   AERO link can be joined by IP standard routers.

   Gateways that connect INETs/EUNs with dissimilar IP protocol versions
   must employ a network address and protocol translation function such
   as NAT64[RFC6146].

12.  Implementation Status

   An AERO implementation based on OpenVPN (https://openvpn.net/) was
   announced on the v6ops mailing list on January 10, 2018 and an
   initial public release of the AERO proof-of-concept source code was
   announced on the intarea mailing list on August 21, 2015.  The latest
   versions are available at: http://linkupnetworks.net/aero.

13.  IANA Considerations

   The IANA has assigned a 4-octet Private Enterprise Number "45282" for
   AERO in the "enterprise-numbers" registry.

   The IANA has assigned the UDP port number "8060" for an earlier
   experimental version of AERO [RFC6706].  This document obsoletes
   [RFC6706] and claims the UDP port number "8060" for all future use.

   No further IANA actions are required.







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14.  Security Considerations

   AERO Relays configure secured tunnels with AERO Servers and Proxys
   within their local SPAN segments independent of the AERO link.
   Secured tunnel encapsulation alternatives include IPsec [RFC4301],
   TLS/SSL [RFC8446], DTLS [RFC6347], etc.  The AERO Relays of all SPAN
   segments in turn configure secured tunnels for their neighboring AERO
   Relays across the SPAN.  Therefore, the bridging service provided by
   Relays is secured, and security considerations for the exchange of
   data plane and control plane messages between AERO link neighbors are
   discussed in the following paragraphs.

   Data plane security considerations are the same as for ordinary
   Internet communications.  Application endpoints in AERO Clients and
   their EUNs SHOULD use application-layer security services such as
   TLS/SSL, DTLS or SSH [RFC4251] to assure the same level of protection
   as for critical secured Internet services.  AERO Clients that require
   host-based VPN services SHOULD use symmetric network and/or transport
   layer security services such as IPsec, TLS/SSL, DTLS, etc.  AERO
   Proxys and Servers can also provide a network-based VPN service on
   behalf of the Client, e.g., if the Client is located within a secured
   enclave and cannot establish a VPN on its own behalf.

   Control plane security considerations are the same as for standard
   IPv6 Neighbor Discovery [RFC4861].  As fixed infrastructure elements,
   AERO Servers and Proxys configure secured tunnels with one or more
   Relays on their SPAN segments using symmetric network and/or
   transport layer security services such as IPsec, TLS/SSL or DTLS.
   The AERO Relays of all SPAN segments in turn configure secured
   tunnels with their neighboring AERO Relays.  AERO Clients that
   connect to secured enclaves need not apply security to their ND
   messages, since the messages will be intercepted by a perimeter
   Proxy.  AERO Clients located outside of secured enclaves SHOULD use
   symmetric network and/or transport layer security to secure their ND
   exchanges with Servers, but when there are many prospective neighbors
   with dynamically changing connectivity an asymmetric security service
   such as SEND may be needed (see: Section 8).

   AERO Servers and Relays present targets for traffic amplification
   Denial of Service (DoS) attacks.  This concern is no different than
   for widely-deployed VPN security gateways in the Internet, where
   attackers could send spoofed packets to the gateways at high data
   rates.  This can be mitigated by connecting Servers and Relays over
   dedicated links with no connections to the Internet and/or when
   connections to the Internet are only permitted through well-managed
   firewalls.  Traffic amplification DoS attacks can also target an AERO
   Client's low data rate links.  This is a concern not only for Clients
   located on the open Internet but also for Clients in secured



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   enclaves.  AERO Servers and Proxys can institute rate limits that
   protect Clients from receiving packet floods that could DoS low data
   rate links.

   AERO Relays must implement ingress filtering to avoid a spoofing
   attack in which spurious SPAN messages are injected into an AERO link
   from an outside attacker.  Restricting access to the link can be
   achieved by having Internetwork border routers implement ingress
   filtering to discard encapsulated packets injected into the link by
   an outside agent.

   AERO Clients MUST ensure that their connectivity is not used by
   unauthorized nodes on their EUNs to gain access to a protected
   network, i.e., AERO Clients that act as routers MUST NOT provide
   routing services for unauthorized nodes.  (This concern is no
   different than for ordinary hosts that receive an IP address
   delegation but then "share" the address with other nodes via some
   form of Internet connection sharing such as tethering.)

   The MAP list MUST be well-managed and secured from unauthorized
   tampering, even though the list contains only public information.
   The MAP list can be conveyed to the Client, e.g., through secure
   upload of a static file, through DNS lookups, etc.

   Although public domain and commercial SEND implementations exist,
   concerns regarding the strength of the cryptographic hash algorithm
   have been documented [RFC6273] [RFC4982].

   Security considerations for accepting link-layer ICMP messages and
   reflected packets are discussed throughout the document.

15.  Acknowledgements

   Discussions in the IETF, aviation standards communities and private
   exchanges helped shape some of the concepts in this work.
   Individuals who contributed insights include Mikael Abrahamsson, Mark
   Andrews, Fred Baker, Bob Braden, Stewart Bryant, Brian Carpenter,
   Wojciech Dec, Ralph Droms, Adrian Farrel, Nick Green, Sri Gundavelli,
   Brian Haberman, Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha
   Hlusiak, Lee Howard, Andre Kostur, Hubert Kuenig, Ted Lemon, Andy
   Malis, Satoru Matsushima, Tomek Mrugalski, Madhu Niraula, Alexandru
   Petrescu, Behcet Saikaya, Michal Skorepa, Joe Touch, Bernie Volz,
   Ryuji Wakikawa, Tony Whyman, Lloyd Wood and James Woodyatt.  Members
   of the IESG also provided valuable input during their review process
   that greatly improved the document.  Special thanks go to Stewart
   Bryant, Joel Halpern and Brian Haberman for their shepherding
   guidance during the publication of the AERO first edition.




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   This work has further been encouraged and supported by Boeing
   colleagues including Kyle Bae, M.  Wayne Benson, Dave Bernhardt, Cam
   Brodie, Balaguruna Chidambaram, Irene Chin, Bruce Cornish, Claudiu
   Danilov, Wen Fang, Anthony Gregory, Jeff Holland, Seth Jahne, Ed
   King, Gene MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Greg
   Saccone, Kent Shuey, Brian Skeen, Mike Slane, Carrie Spiker, Brendan
   Williams, Julie Wulff, Yueli Yang, Eric Yeh and other members of the
   BR&T and BIT mobile networking teams.  Kyle Bae, Wayne Benson and
   Eric Yeh are especially acknowledged for implementing the AERO
   functions as extensions to the public domain OpenVPN distribution.

   Earlier works on NBMA tunneling approaches are found in
   [RFC2529][RFC5214][RFC5569].

   Many of the constructs presented in this second edition of AERO are
   based on the author's earlier works, including:

   o  The Internet Routing Overlay Network (IRON)
      [RFC6179][I-D.templin-ironbis]

   o  Virtual Enterprise Traversal (VET)
      [RFC5558][I-D.templin-intarea-vet]

   o  The Subnetwork Encapsulation and Adaptation Layer (SEAL)
      [RFC5320][I-D.templin-intarea-seal]

   o  AERO, First Edition [RFC6706]

   Note that these works cite numerous earlier efforts that are not also
   cited here due to space limitations.  The authors of those earlier
   works are acknowledged for their insights.

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

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

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

   This work is aligned with the Boeing autonomy program.

16.  References







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16.1.  Normative References

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

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.

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

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

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

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

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

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

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

   [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>.



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

16.2.  Informative References

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

   [I-D.ietf-6man-segment-routing-header]
              Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
              Matsushima, S., and d. daniel.voyer@bell.ca, "IPv6 Segment
              Routing Header (SRH)", draft-ietf-6man-segment-routing-
              header-19 (work in progress), May 2019.

   [I-D.ietf-dmm-distributed-mobility-anchoring]
              Chan, A., Wei, X., Lee, J., Jeon, S., and C. Bernardos,
              "Distributed Mobility Anchoring", draft-ietf-dmm-
              distributed-mobility-anchoring-13 (work in progress),
              March 2019.

   [I-D.ietf-intarea-gue]
              Herbert, T., Yong, L., and O. Zia, "Generic UDP
              Encapsulation", draft-ietf-intarea-gue-07 (work in
              progress), March 2019.

   [I-D.ietf-intarea-gue-extensions]
              Herbert, T., Yong, L., and F. Templin, "Extensions for
              Generic UDP Encapsulation", draft-ietf-intarea-gue-
              extensions-06 (work in progress), March 2019.

   [I-D.ietf-intarea-tunnels]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-09 (work in
              progress), July 2018.

   [I-D.ietf-rtgwg-atn-bgp]
              Templin, F., Saccone, G., Dawra, G., Lindem, A., and V.
              Moreno, "A Simple BGP-based Mobile Routing System for the
              Aeronautical Telecommunications Network", draft-ietf-
              rtgwg-atn-bgp-02 (work in progress), May 2019.

   [I-D.templin-6man-dhcpv6-ndopt]
              Templin, F., "A Unified Stateful/Stateless Configuration
              Service for IPv6", draft-templin-6man-dhcpv6-ndopt-07
              (work in progress), December 2018.



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   [I-D.templin-intarea-grefrag]
              Templin, F., "GRE Tunnel Level Fragmentation", draft-
              templin-intarea-grefrag-04 (work in progress), July 2016.

   [I-D.templin-intarea-seal]
              Templin, F., "The Subnetwork Encapsulation and Adaptation
              Layer (SEAL)", draft-templin-intarea-seal-68 (work in
              progress), January 2014.

   [I-D.templin-intarea-vet]
              Templin, F., "Virtual Enterprise Traversal (VET)", draft-
              templin-intarea-vet-40 (work in progress), May 2013.

   [I-D.templin-ironbis]
              Templin, F., "The Interior Routing Overlay Network
              (IRON)", draft-templin-ironbis-16 (work in progress),
              March 2014.

   [I-D.templin-v6ops-pdhost]
              Templin, F., "IPv6 Prefix Delegation and Multi-Addressing
              Models", draft-templin-v6ops-pdhost-23 (work in progress),
              December 2018.

   [OVPN]     OpenVPN, O., "http://openvpn.net", October 2016.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

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

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

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,
              <https://www.rfc-editor.org/info/rfc1812>.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              DOI 10.17487/RFC2003, October 1996,
              <https://www.rfc-editor.org/info/rfc2003>.






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   [RFC2236]  Fenner, W., "Internet Group Management Protocol, Version
              2", RFC 2236, DOI 10.17487/RFC2236, November 1997,
              <https://www.rfc-editor.org/info/rfc2236>.

   [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>.

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

   [RFC2764]  Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A.
              Malis, "A Framework for IP Based Virtual Private
              Networks", RFC 2764, DOI 10.17487/RFC2764, February 2000,
              <https://www.rfc-editor.org/info/rfc2764>.

   [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>.

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, DOI 10.17487/RFC2890, September 2000,
              <https://www.rfc-editor.org/info/rfc2890>.

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, DOI 10.17487/RFC2923, September 2000,
              <https://www.rfc-editor.org/info/rfc2923>.

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,
              <https://www.rfc-editor.org/info/rfc2983>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,
              <https://www.rfc-editor.org/info/rfc3810>.







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   [RFC3819]  Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, DOI 10.17487/RFC3819, July 2004,
              <https://www.rfc-editor.org/info/rfc3819>.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213,
              DOI 10.17487/RFC4213, October 2005,
              <https://www.rfc-editor.org/info/rfc4213>.

   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
              January 2006, <https://www.rfc-editor.org/info/rfc4251>.

   [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>.

   [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>.

   [RFC4389]  Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
              Proxies (ND Proxy)", RFC 4389, DOI 10.17487/RFC4389, April
              2006, <https://www.rfc-editor.org/info/rfc4389>.

   [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>.

   [RFC4511]  Sermersheim, J., Ed., "Lightweight Directory Access
              Protocol (LDAP): The Protocol", RFC 4511,
              DOI 10.17487/RFC4511, June 2006,
              <https://www.rfc-editor.org/info/rfc4511>.

   [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
              "Considerations for Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
              <https://www.rfc-editor.org/info/rfc4541>.



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

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
              <https://www.rfc-editor.org/info/rfc4607>.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963,
              DOI 10.17487/RFC4963, July 2007,
              <https://www.rfc-editor.org/info/rfc4963>.

   [RFC4982]  Bagnulo, M. and J. Arkko, "Support for Multiple Hash
              Algorithms in Cryptographically Generated Addresses
              (CGAs)", RFC 4982, DOI 10.17487/RFC4982, July 2007,
              <https://www.rfc-editor.org/info/rfc4982>.

   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
              <https://www.rfc-editor.org/info/rfc5015>.

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              DOI 10.17487/RFC5214, March 2008,
              <https://www.rfc-editor.org/info/rfc5214>.

   [RFC5320]  Templin, F., Ed., "The Subnetwork Encapsulation and
              Adaptation Layer (SEAL)", RFC 5320, DOI 10.17487/RFC5320,
              February 2010, <https://www.rfc-editor.org/info/rfc5320>.

   [RFC5522]  Eddy, W., Ivancic, W., and T. Davis, "Network Mobility
              Route Optimization Requirements for Operational Use in
              Aeronautics and Space Exploration Mobile Networks",
              RFC 5522, DOI 10.17487/RFC5522, October 2009,
              <https://www.rfc-editor.org/info/rfc5522>.

   [RFC5558]  Templin, F., Ed., "Virtual Enterprise Traversal (VET)",
              RFC 5558, DOI 10.17487/RFC5558, February 2010,
              <https://www.rfc-editor.org/info/rfc5558>.

   [RFC5569]  Despres, R., "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd)", RFC 5569, DOI 10.17487/RFC5569,
              January 2010, <https://www.rfc-editor.org/info/rfc5569>.




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   [RFC6106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 6106, DOI 10.17487/RFC6106, November 2010,
              <https://www.rfc-editor.org/info/rfc6106>.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC6179]  Templin, F., Ed., "The Internet Routing Overlay Network
              (IRON)", RFC 6179, DOI 10.17487/RFC6179, March 2011,
              <https://www.rfc-editor.org/info/rfc6179>.

   [RFC6221]  Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A.
              Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221,
              DOI 10.17487/RFC6221, May 2011,
              <https://www.rfc-editor.org/info/rfc6221>.

   [RFC6273]  Kukec, A., Krishnan, S., and S. Jiang, "The Secure
              Neighbor Discovery (SEND) Hash Threat Analysis", RFC 6273,
              DOI 10.17487/RFC6273, June 2011,
              <https://www.rfc-editor.org/info/rfc6273>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
              <https://www.rfc-editor.org/info/rfc6438>.

   [RFC6706]  Templin, F., Ed., "Asymmetric Extended Route Optimization
              (AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012,
              <https://www.rfc-editor.org/info/rfc6706>.

   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, DOI 10.17487/RFC6864, February 2013,
              <https://www.rfc-editor.org/info/rfc6864>.

   [RFC7269]  Chen, G., Cao, Z., Xie, C., and D. Binet, "NAT64
              Deployment Options and Experience", RFC 7269,
              DOI 10.17487/RFC7269, June 2014,
              <https://www.rfc-editor.org/info/rfc7269>.






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   [RFC7333]  Chan, H., Ed., Liu, D., Seite, P., Yokota, H., and J.
              Korhonen, "Requirements for Distributed Mobility
              Management", RFC 7333, DOI 10.17487/RFC7333, August 2014,
              <https://www.rfc-editor.org/info/rfc7333>.

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <https://www.rfc-editor.org/info/rfc7761>.

   [RFC8086]  Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
              in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
              March 2017, <https://www.rfc-editor.org/info/rfc8086>.

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

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

Appendix A.  AERO Alternate Encapsulations

   When GUE encapsulation is not needed, AERO can use common
   encapsulations such as IP-in-IP [RFC2003][RFC2473][RFC4213], Generic
   Routing Encapsulation (GRE) [RFC2784][RFC2890] and others.  The
   encapsulation is therefore only differentiated from non-AERO tunnels
   through the application of AERO control messaging and not through,
   e.g., a well-known UDP port number.

   As for GUE encapsulation, alternate AERO encapsulation formats may
   require encapsulation layer fragmentation.  For simple IP-in-IP
   encapsulation, an IPv6 fragment header is inserted directly between
   the inner and outer IP headers when needed, i.e., even if the outer
   header is IPv4.  The IPv6 Fragment Header is identified to the outer
   IP layer by its IP protocol number, and the Next Header field in the
   IPv6 Fragment Header identifies the inner IP header version.  For GRE
   encapsulation, a GRE fragment header is inserted within the GRE
   header [I-D.templin-intarea-grefrag].




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   Figure 6 shows the AERO IP-in-IP encapsulation format before any
   fragmentation is applied:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |     Outer IPv4 Header     |      |    Outer IPv6 Header      |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |IPv6 Frag Header (optional)|      |IPv6 Frag Header (optional)|
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      Inner IP Header      |      |       Inner IP Header     |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                           |      |                           |
        ~                           ~      ~                           ~
        ~    Inner Packet Body      ~      ~     Inner Packet Body     ~
        ~                           ~      ~                           ~
        |                           |      |                           |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Minimal Encapsulation in IPv4      Minimal Encapsulation in IPv6


           Figure 6: Minimal Encapsulation Format using IP-in-IP

   Figure 7 shows the AERO GRE encapsulation format before any
   fragmentation is applied:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        Outer IP Header        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |          GRE Header           |
        | (with checksum, key, etc..)   |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        | GRE Fragment Header (optional)|
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        Inner IP Header        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |
        ~                               ~
        ~      Inner Packet Body        ~
        ~                               ~
        |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure 7: Minimal Encapsulation Using GRE

   Alternate encapsulation may be preferred in environments where GUE
   encapsulation would add unnecessary overhead.  For example, certain




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   low-bandwidth wireless data links may benefit from a reduced
   encapsulation overhead.

   GUE encapsulation can traverse network paths that are inaccessible to
   non-UDP encapsulations, e.g., for crossing Network Address
   Translators (NATs).  More and more, network middleboxes are also
   being configured to discard packets that include anything other than
   a well-known IP protocol such as UDP and TCP.  It may therefore be
   necessary to determine the potential for middlebox filtering before
   enabling alternate encapsulation in a given environment.

   In addition to IP-in-IP, GRE and GUE, AERO can also use security
   encapsulations such as IPsec, TLS/SSL, DTLS, etc.  In that case, AERO
   control messaging and route determination occur before security
   encapsulation is applied for outgoing packets and after security
   decapsulation is applied for incoming packets.

   AERO is especially well suited for use with VPN system encapsulations
   such as OpenVPN [OVPN].

Appendix B.  S/TLLAO Extensions for Special-Purpose Links

   The AERO S/TLLAO format specified in Section 3.6 includes a Length
   value of 5 (i.e., 5 units of 8 octets).  However, special-purpose
   links may extend the basic format to include additional fields and a
   Length value larger than 5.

   For example, adaptation of AERO to the Aeronautical
   Telecommunications Network with Internet Protocol Services (ATN/IPS)
   includes link selection preferences based on transport port numbers
   in addition to the existing DSCP-based preferences.  ATN/IPS nodes
   maintain a map of transport port numbers to 64 possible preference
   fields, e.g., TCP port 22 maps to preference field 8, TCP port 443
   maps to preference field 20, UDP port 8060 maps to preference field
   34, etc.  The extended S/TLLAO format for ATN/IPS is shown in
   Figure 8, where the Length value is 7 and the 'Q(i)' fields provide
   link preferences for the corresponding transport port number.














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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Type     |   Length = 7  | Prefix Length |   Reserved    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Interface ID         |          Port Number          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                        Link-Layer Address                     +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q00|Q01|Q02|Q03|Q04|Q05|Q06|Q07|Q08|Q09|Q10|Q11|Q12|Q13|Q14|Q15|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q16|Q17|Q18|Q19|Q20|Q21|Q22|Q23|Q24|Q25|Q26|Q27|Q28|Q29|Q30|Q31|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q32|Q33|Q34|Q35|Q36|Q37|Q38|Q39|Q40|Q41|Q42|Q43|Q44|Q45|Q46|Q47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q48|Q49|Q50|Q51|Q52|Q53|Q54|Q55|Q56|Q57|Q58|Q59|Q60|Q61|Q62|Q63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 8: ATN/IPS Extended S/TLLAO Format

Appendix C.  Implicit Mobility Management

   AERO interface neighbors MAY provide a configuration option that
   allows them to perform implicit mobility management in which no ND
   messaging is used.  In that case, the Client only transmits packets
   over a single interface at a time, and the neighbor always observes
   packets arriving from the Client from the same link-layer source
   address.

   If the Client's ANET interface address changes (either due to a
   readdressing of the original interface or switching to a new
   interface) the neighbor immediately updates the neighbor cache entry
   for the Client and begins accepting and sending packets according to
   the Client's new ANET address.  This implicit mobility method applies



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   to use cases such as cellphones with both WiFi and Cellular
   interfaces where only one of the interfaces is active at a given
   time, and the Client automatically switches over to the backup
   interface if the primary interface fails.

Appendix D.  Implementation Strategies for Route Optimization

   Route optimization as discussed in Section 3.17 results in the route
   optimization source (ROS) creating an asymmetric neighbor cache entry
   for the target neighbor.  The neighbor cache entry is maintained for
   at most REACHABLE_TIME seconds and then deleted unless updated.  In
   order to refresh the neighbor cache entry lifetime before the
   ReachableTime timer expires, the specification requires
   implementations to issue a new NS/NA exchange to reset ReachableTime
   to REACHABLE_TIME seconds while data packets are still flowing.
   However, the decision of when to initiate a new NS/NA exchange and to
   perpetuate the process is left as an implementation detail.

   One possible strategy may be to monitor the neighbor cache entry
   watching for data packets for (REACHABLE_TIME - 5) seconds.  If any
   data packets have been sent to the neighbor within this timeframe,
   then send an NS to receive a new NA.  If no data packets have been
   sent, wait for 5 additional seconds and send an immediate NS if any
   data packets are sent within this "expiration pending" 5 second
   window.  If no additional data packets are sent within the 5 second
   window, delete the neighbor cache entry.

   The monitoring of the neighbor data packet traffic therefore becomes
   an asymmetric ongoing process during the neighbor cache entry
   lifetime.  If the neighbor cache entry expires, future data packets
   will trigger a new NS/NA exchange while the packets themselves are
   delivered over a longer path until route optimization state is re-
   established.

Appendix E.  Change Log

   << RFC Editor - remove prior to publication >>

   Changes from draft-templin-intarea-6706bis-12 to draft-templin-
   intrea-6706bis-13:

   o  New paragraph in Section 3.6 on AERO interface layering over
      secured tunnels

   o  Removed extraneous text in Section 3.7

   o  Added new detail to the forwarding algorithm in Section 3.9




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   o  Clarified use of fragmentation

   o  Route optimization now supported for both MNP and non-MNP-based
      prefixes

   o  Relays are now seen as link-layer elements in the architecture.

   o  Built out multicast section in detail.

   o  New Appendix on implementation considerations for route
      optimization.

   Changes from draft-templin-intarea-6706bis-11 to draft-templin-
   intrea-6706bis-12:

   o  Introduced Gateways as a new AERO element for connecting
      Correspondent Nodes on INET links

   o  Introduced terms "Access Network (ANET)" and "Internetwork (INET)"

   o  Changed "ASP" to "MSP", and "ACP" to "MNP"

   o  New figure on the relation of Segments to the SPAN and AERO link

   o  New "S" bit in S/TLLAO to indicate the "Source" S/TLLAO as opposed
      to additional S/TLLAOs

   o  Changed Interface ID for Servers from 255 to 0xffff

   o  Significant updates to Route Optimization, NUD, and Mobility
      Management

   o  New Section on Multicast

   o  New Section on AERO Clients in the open Internetwork

   o  New Section on Operation over multiple AERO links (VLANs over the
      SPAN)

   o  New Sections on DNS considerations and Transition considerations

   o

   Changes from draft-templin-intarea-6706bis-10 to draft-templin-
   intrea-6706bis-11:

   o  Added The SPAN




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   Changes from draft-templin-intarea-6706bis-09 to draft-templin-
   intrea-6706bis-10:

   o  Orphaned packets in flight (e.g., when a neighbor cache entry is
      in the DEPARTED state) are now forwarded at the link layer instead
      of at the network layer.  Forwarding at the network layer can
      result in routing loops and/or excessive delays of forwarded
      packets while the routing system is still reconverging.

   o  Update route optimization to clarify the unsecured nature of the
      first NS used for route discovery

   o  Many cleanups and clarifications on ND messaging parameters

   Changes from draft-templin-intarea-6706bis-08 to draft-templin-
   intrea-6706bis-09:

   o  Changed PRL to "MAP list"

   o  For neighbor cache entries, changed "static" to "symmetric", and
      "dynamic" to "asymmetric"

   o  Specified Proxy RS/RA exchanges with Servers on behalf of Clients

   o  Added discussion of unsolicited NAs in Section 3.16, and included
      forward reference to Section 3.18

   o  Added discussion of AERO Clients used as critical infrastructure
      elements to connect fixed networks.

   o  Added network-based VPN under security considerations

   Changes from draft-templin-intarea-6706bis-07 to draft-templin-
   intrea-6706bis-08:

   o  New section on AERO-Aware Access Router

   Changes from draft-templin-intarea-6706bis-06 to draft-templin-
   intrea-6706bis-07:

   o  Added "R" bit for release of PDs.  Now have a full RS/RA service
      that can do PD without requiring DHCPv6 messaging over-the-air

   o  Clarifications on solicited vs unsolicited NAs

   o  Clarified use of MAX_NEIGHBOR_ADVERTISEMENTS for the purpose of
      increase reliability




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   Changes from draft-templin-intarea-6706bis-05 to draft-templin-
   intrea-6706bis-06:

   o  Major re-work and simplification of Route Optimization function

   o  Added Distributed Mobility Management (DMM) and Mobility Anchor
      Point (MAP) terminology

   o  New section on "AERO Critical Infrastructure Element
      Considerations" demonstrating low overall cost for the service

   o  minor text revisions and deletions

   o  removed extraneous appendices

   Changes from draft-templin-intarea-6706bis-04 to draft-templin-
   intrea-6706bis-05:

   o  New Appendix E on S/TLLAO Extensions for special-purpose links.
      Discussed ATN/IPS as example.

   o  New sentence in introduction to declare appendices as non-
      normative.

   Changes from draft-templin-intarea-6706bis-03 to draft-templin-
   intrea-6706bis-04:

   o  Added definitions for Potential Router List (PRL) and secure
      enclave

   o  Included text on mapping transport layer port numbers to network
      layer DSCP values

   o  Added reference to DTLS and DMM Distributed Mobility Anchoring
      working group document

   o  Reworked Security Considerations

   o  Updated references.

   Changes from draft-templin-intarea-6706bis-02 to draft-templin-
   intrea-6706bis-03:

   o  Added new section on SEND.

   o  Clarifications on "AERO Address" section.

   o  Updated references and added new reference for RFC8086.



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   o  Security considerations updates.

   o  General text clarifications and cleanup.

   Changes from draft-templin-intarea-6706bis-01 to draft-templin-
   intrea-6706bis-02:

   o  Note on encapsulation avoidance in Section 4.

   Changes from draft-templin-intarea-6706bis-00 to draft-templin-
   intrea-6706bis-01:

   o  Remove DHCPv6 Server Release procedures that leveraged the old way
      Relays used to "route" between Server link-local addresses

   o  Remove all text relating to Relays needing to do any AERO-specific
      operations

   o  Proxy sends RS and receives RA from Server using SEND.  Use CGAs
      as source addresses, and destination address of RA reply is to the
      AERO address corresponding to the Client's ACP.

   o  Proxy uses SEND to protect RS and authenticate RA (Client does not
      use SEND, but rather relies on subnetwork security.  When the
      Proxy receives an RS from the Client, it creates a new RS using
      its own addresses as the source and uses SEND with CGAs to send a
      new RS to the Server.

   o  Emphasize distributed mobility management

   o  AERO address-based RS injection of ACP into underlying routing
      system.

   Changes from draft-templin-aerolink-82 to draft-templin-intarea-
   6706bis-00:

   o  Document use of NUD (NS/NA) for reliable link-layer address
      updates as an alternative to unreliable unsolicited NA.
      Consistent with Section 7.2.6 of RFC4861.

   o  Server adds additional layer of encapsulation between outer and
      inner headers of NS/NA messages for transmission through Relays
      that act as vanilla IPv6 routers.  The messages include the AERO
      Server Subnet Router Anycast address as the source and the Subnet
      Router Anycast address corresponding to the Client's ACP as the
      destination.





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   o  Clients use Subnet Router Anycast address as the encapsulation
      source address when the access network does not provide a
      topologically-fixed address.

Author's Address

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

   Email: fltemplin@acm.org






































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