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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720,                        May 1, 2019
           rfc6179, rfc6706 (if
           approved)
Intended status: Standards Track
Expires: November 2, 2019


             Asymmetric Extended Route Optimization (AERO)
                  draft-templin-intarea-6706bis-12.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 to manage the routing system.
   Dynamic 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 November 2, 2019.

Copyright Notice

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




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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   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 . . . . . . . . . . . . . . . . . . .  13
     3.4.  AERO Addresses  . . . . . . . . . . . . . . . . . . . . .  15
     3.5.  Spanning Partitioned AERO Networks (SPAN) . . . . . . . .  16
     3.6.  AERO Interface Characteristics  . . . . . . . . . . . . .  20
     3.7.  AERO Interface Initialization . . . . . . . . . . . . . .  24
       3.7.1.  AERO Relay Behavior . . . . . . . . . . . . . . . . .  24
       3.7.2.  AERO Server Behavior  . . . . . . . . . . . . . . . .  24
       3.7.3.  AERO Gateway Behavior . . . . . . . . . . . . . . . .  25
       3.7.4.  AERO Proxy Behavior . . . . . . . . . . . . . . . . .  25
       3.7.5.  AERO Client Behavior  . . . . . . . . . . . . . . . .  25
     3.8.  AERO Interface Neighbor Cache Maintenance . . . . . . . .  26
     3.9.  AERO Interface Forwarding Algorithm . . . . . . . . . . .  28
       3.9.1.  Client Forwarding Algorithm . . . . . . . . . . . . .  29
       3.9.2.  Proxy Forwarding Algorithm  . . . . . . . . . . . . .  29
       3.9.3.  Server Forwarding Algorithm . . . . . . . . . . . . .  30
       3.9.4.  Gateway Forwarding Algorithm  . . . . . . . . . . . .  31
       3.9.5.  Relay Forwarding Algorithm  . . . . . . . . . . . . .  31
     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 . . . . . . . . . . . . . . . .  39
       3.15.3.  AERO Server Behavior . . . . . . . . . . . . . . . .  41
     3.16. The AERO Proxy  . . . . . . . . . . . . . . . . . . . . .  43
     3.17. AERO Route Optimization . . . . . . . . . . . . . . . . .  45
       3.17.1.  Route Optimization Initiation  . . . . . . . . . . .  46
       3.17.2.  Relaying the NS  . . . . . . . . . . . . . . . . . .  46



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       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  . . . . . . . . . . . . .  50
       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.  Implicit Mobility Management . . . . . . . . . . . .  52
       3.19.7.  Moving to a New Server . . . . . . . . . . . . . . .  52
     3.20. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  53
   4.  Direct Underlying Interfaces  . . . . . . . . . . . . . . . .  54
   5.  AERO Clients on the Open Internetwork . . . . . . . . . . . .  54
   6.  Operation over Multiple AERO Links  . . . . . . . . . . . . .  54
   7.  Operation on AERO Links with /64 ASPs . . . . . . . . . . . .  55
   8.  AERO Adaptations for SEcure Neighbor Discovery (SEND) . . . .  56
   9.  AERO Critical Infrastructure Considerations . . . . . . . . .  56
   10. DNS Considerations  . . . . . . . . . . . . . . . . . . . . .  57
   11. Transition Considerations . . . . . . . . . . . . . . . . . .  57
   12. Implementation Status . . . . . . . . . . . . . . . . . . . .  58
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  58
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  58
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  60
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  61
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  61
     16.2.  Informative References . . . . . . . . . . . . . . . . .  62
   Appendix A.  AERO Alternate Encapsulations  . . . . . . . . . . .  69
   Appendix B.  S/TLLAO Extensions for Special-Purpose Links . . . .  70
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  72
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  76

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,
   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.

   AERO links provide a cloud-based service where mobile nodes may use
   any Mobility Anchor Point (MAP) and fixed nodes may use any Gateway



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   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
   route optimization once an initial session has been established.
   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.

   The AERO service comprises Clients, Proxies, 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) [RFC4861]
   protocol 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 Relays are interconnected in a secured private BGP overlay
   routing instance known as the "SPAN".  The SPAN provides a (virtual)
   layer 2 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, the AERO cloud is included within the
   multicast distribution tree unless and until it is optimized out by
   use of AERO Direct Routing.  In all other multicast scenarios there
   are no AERO dependencies.

   AERO is applicable to a wide variety of 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].  AERO is also applicable to
   aeronautical networking for both manned and unmanned aircraft where




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   the aircraft is treated as a mobile node that can connect an Internet
   of Things (IoT).  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.  When there is no
      administrative boundary established between an ANET and the
      outside Internetwork, the ANET and Internetwork are one and the
      same.

   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.

   Internetwork (INET)
      a connected IP network topology with a coherent routing and
      addressing plan and that provides an Internetworking backbone
      service for ANETs.  INETs also provide an underlay service over



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      which the AERO virtual link is configured.  Example INETs include
      corporate enterprise networks, aviation networks, and the public
      Internet itself.

   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 derived from an MSP and delegated to an AERO Client
      or Gateway.

   Fixed Node (FN)
      a node on an INET link serviced by an AERO Gateway.

   Mobile Node (MN)
      an AERO Client and all of its downstream-attached networks.

   Mobile Router (MR)



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

   Correspondent Node (CN)
      a MN or FN that is reachable over the AERO link

   AERO node
      a node that is connected to 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")
      a combined AERO Server/Client that also provides forwarding
      services between CNs on the AERO link and FNs on INET links.  AERO
      Gateways are provisioned with MNPs used for numbering nodes and
      networks on downstream-attached INET interfaces (i.e., the same as
      for an AERO Client) and run a dynamic routing protocol to discover
      any native INET prefixes.  In both cases, the Gateway advertises
      the MSP(s) to FNs in downstream-attached INET networks, and
      distributes all of its associated MNPs and native INET prefixes
      via BGP peerings with Relays (i.e., the same as for an AERO
      Server).

   AERO Relay ("Relay")
      an INET node that provides both layer-3 routing and layer-2
      bridging services (as well as a security trust anchor) for nodes
      on an AERO link.  As a router, the Relay forwards data packets
      using standard IP forwarding.  As a bridge, the Relay securely
      forwards control messages between other AERO nodes.  AERO Relays
      peer with Servers and other Relays to discover the full set of
      MNPs

   AERO Proxy ("Proxy")



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      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)
      a means for bridging disjoint INETs as segments (or, partitions)
      of a unified AERO link, i.e., the same as for a bridged campus
      LAN.  The SPAN is a mid-layer 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)




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      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.

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

   Mobile Router (MR)
      a router on an AERO Client that provides routing services between
      the Client's EUNs and the AERO interface.

   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 Client that initiates route
      optimization.  The ROS may be one of the Client's Servers, Proxies
      or in some cases even the Client itself.

   Route Optimization Responder (ROR)
      a Server of the target Client to which a route optimization
      request is directed.  The ROR (acting as a MAP) returns the most
      current information about the target Client's underlying interface
      connections.

   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 DHCPv6
   client/server/relay [RFC8415].

   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 terms Mobility Anchor Point (MAP), Mobile Router (MR) and
   Distributed Mobility Management (DMM) are used in the same sense as
   standard Internetworking terminology.



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   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.

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

                              .-(::::::::)
                           .-(::::::::::::)-.
                          (:::: Internet ::::)
                           `-(::::::::::::)-'
                              `-(::::::)-'
                                   |
       +--------------+   +--------+-------+   +--------------+
       |AERO Server S1|   | AERO Relay R1  |   |AERO Server S2|
       |  Nbr: C1, R1 |   | Nbr: S1, S2, P1|   |  Nbr: C2, R1 |
       |  default->R1 |   |(X1->S1; X2->S2)|   |  default->R1 |
       |    X1->C1    |   |      MSP M1    |   |    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 Layer 3 service configured over one or
      more underlying INETs which may be managed by different




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      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*, acts as a default
      router for its associated Servers and Proxies (S1, S2, P1), and
      connects the AERO link to the external Internet.  AERO Relays also
      use the SPAN service to bridge disjoint segments (i.e., INETs) of
      a partitioned AERO link.

   o  AERO Servers S1 and S2 associate 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 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
   Proxies.

3.2.  AERO Node Types

   AERO Relays provide both layer-3 routing and layer-2 bridging
   services (as well as a security trust anchor) for nodes on an AERO
   link.  As a router, the Relay forwards data packets using standard IP
   forwarding.  As a bridge, the Relay securely forwards control
   messages between Proxies and Servers both within the same INET and
   between disjoint INETs.  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 forward packets between neighbors
   connected to the same AERO link and also forward packets between the
   AERO link and the outside world.  Relays present the AERO link as a
   set of one or more Mobility Service Prefixes (MSPs).  Relays maintain
   neighbor cache entries for Servers and Proxies, and maintain an IP
   forwarding table entry for each Mobile Network Prefix (MNP).





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   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
   becomes an MNP taken from an MSP.  Servers forward packets between
   AERO interface neighbors, and maintain neighbor cache entries for
   Relays.  They also maintain both neighbor cache entries and IP
   forwarding table entries for each of their associated Clients, and
   track each Client's mobility profiles.

   AERO Clients act as requesting Mobile Routers (MRs) to 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.  Each IPv6 Client receives at least a /64 IPv6
   MNP, and may receive even shorter prefixes.  Similarly, each IPv4
   Client receives at least a /32 IPv4 MNP (i.e., a singleton IPv4
   address), and may receive even shorter prefixes.  Clients maintain an
   AERO interface neighbor cache entry for each of their associated
   Servers as well as for each of their correspondent Clients.  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 Proxies provide a conduit for AERO Clients in ANETs to associate
   with AERO Servers in external INETs.  The Client sends all of its
   control plane messages to the Server via the Proxy, which intercepts
   them before they leave the ANET.  The Proxy forwards the Client's
   control and data plane messages to and from the Client's current
   Server(s).  The Proxy may also discover a better route toward a
   target destination via AERO route optimization, in which case future
   outbound data packets would be forwarded via the more direct route.
   Proxies maintain AERO interface neighbor cache entries for Relays,
   i.e., the same as Servers.  The Proxy function is specified in
   Section 3.16.

   AERO Gateways are combined Client/Servers that also provide
   forwarding services between correspondent nodes (CNs) on the AERO
   interface and fixed nodes (FNs) on INET interfaces.  AERO Gateways
   are provisioned with MNPs used for numbering nodes and networks on
   downstream-attached INET interfaces (i.e., the same as for an AERO
   Client) and may also run a dynamic routing protocol to discover any
   native INET prefixes.  In both cases, the Gateway advertises the
   MSP(s) to correspondent nodes in downstream-attached INET networks,
   and distributes all of its associated MNPs and native INET prefixes
   via BGP peerings with Relays.



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   AERO Relays, Servers, Proxies 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.  Relays
   advertise only a small and unchanging set of MSPs to the outside
   world instead of the full dynamically changing set of MNPs.

   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 Proxies
   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.

   Each Server maintains a working set of associated MNPs and native
   INET prefixes, and dynamically announces new prefixes and withdraws
   departed prefixes in its eBGP updates to Relays.  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 MSP into the INET and forward packets
   between INET interfaces and the AERO interface.

   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.  Relays do not send
   eBGP updates for MNPs to Servers, but instead only originate a
   default 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.





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   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
   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].







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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. ...

      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




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

   Client AERO addresses can be statelessly transformed into an IPv6
   Subnet Router Anycast address and vice-versa.  For example, for the
   AERO address fe80::2001:db8:2000:3000 the corresponding Subnet Router
   Anycast address is 2001:db8:2000:3000::. In the same way, for the
   IPv6 Subnet Router Anycast address 2001:db8:1:2:: the corresponding
   AERO address is fe80::2001:db8:1:2.  In other words, the low-order 64
   bits of an AERO address can be used as the high-order 64 bits of a
   Subnet Router Anycast address, and vice-versa.

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 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



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   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 nodes in the
   same segment, and not with nodes in other segments.  The only means
   for joining the segments therefore is through inter-domain peerings
   between AERO Relays acting as bridges.

   The same as for traditional campus LANs, multiple AERO link segments
   can be joined into a single unified link via a 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:

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

                            Figure 2: The SPAN




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   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 preferred 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::/116, a second could assign fd00::1000/116, a
   third could assign fd00::2000/116, 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 AERO 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 fd00::/116,
   the SPAN address formed from the AERO address fe80::1 is simply
   fd00::1.  (As with AERO addresses, the values ::0 and ::ffff:ffff are
   reserved and not available for general assignment.)

   An "INET address" is an address of a node's interface connection to
   an INET segment.  Each Relay, Server and Proxy connected to the same
   segment maintains a static mapping of AERO/SPAN addresses to INET
   addresses for all fixed infrastructure elements in that segment.  For
   example, if a Server has AERO/SPAN addresses fe80::1/fd00::1 and INET
   address 192.0.2.100, then all other Relays, Servers and Proxys in
   that segment keep a static mapping for those addresses.  In that way,
   any of the AERO/SPAN/INET addresses can be derived from a static
   lookup without the need for protocol messaging.

   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 (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



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   These forwarding table entries are permanent and never change, since
   they correspond to fixed infrastructure elements in their respective
   segments.  This point is of critical importance, since it 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:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |          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.



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   A packet is said to be "forwarded/sent into the SPAN" when it is
   encapsulated as described above then forwarded to a neighboring
   Relay.  This terminology appears throughout the remainder of the
   document.

   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.

   NB: With reference to Figure 3, 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
   subnet router anycast address corresponding 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 subnet
   router anycast address is simply 2001:db8:1000:2000::.)

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:

















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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 = 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).

   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 is not
      being used for PD or neighbor prefix discovery.  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 the SLLAO of an RS message
      sent for the purpose of departing from a Server; otherwise, set to



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      '0'.  If the message contains multiple SLLAOs, only the R value in
      the SLLAO with S set to 1 is consulted and the values in other
      SLLAOs are ignored.  The Server places the corresponding neighbor
      cache entry in the DEPARTED state and releases the corresponding
      PD, then 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'.  If the message
      contains an S/TLLAO with Interface ID 0xffff, the node places the
      corresponding neighbor cache entry in the DEPARTED state.  If the
      message contains multiple S/TLLAOs the D value in each S/TLLAO is
      consulted.

   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 AERO 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 addresses used
      by the AERO node when it sends encapsulated packets over the
      specified ANET/INET interface (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 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



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      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 AERO 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.

   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 AERO 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 AERO 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.

   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.




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   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.  The S bit 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.

3.7.  AERO Interface Initialization

3.7.1.  AERO Relay Behavior

   When a Relay enables an AERO interface, it first assigns an
   administratively-provisioned AERO address (e.g., fe80::1) and its
   companion SPAN address (e.g., fd00::1), where each address MUST be
   unique among all AERO nodes on the link.  The Relay also configures a
   neighbor cache entry for Servers, Gateways and Proxys on the local
   segment, and maintains a list of INET address mappings for all fixed
   infrastructure elements on the local segment.  The Relay then engages
   in a BGP routing protocol session with Servers/Gateways on the local
   segment and other Relays on the AERO link (see: Section 3.3).  Each
   Relay subsequently maintains an IP forwarding table entry for each
   active MNP covered by its MSP(s) as well as for each SPAN prefix.

3.7.2.  AERO Server Behavior

   When a Server enables an AERO interface, it assigns AERO/SPAN
   addresses and maintains a list of INET address mappings the same as
   for Relays.  The Server further configures a service to facilitate
   ND/PD exchanges with AERO Clients, maintains neighbor cache entries
   for one or more Relays on the link, and manages per-Client neighbor
   cache entries and IP forwarding table entries based on control
   message exchanges.  The Server also engages in a BGP routing protocol
   session with its neighboring Relays via the AERO interface, and also
   engages in a dynamic routing protocol over its INET interfaces (see:
   Section 3.3).

   When the Server receives an NS/RS message on the AERO interface it
   authenticates the message and returns a solicited NA/RA message.
   (When the Server receives an unsolicited NA message, it likewise



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   authenticates the message and processes it locally.)  The Server
   further provides a simple link-layer conduit between AERO interface
   neighbors.  In particular, when a packet sent by a source CN arrives
   on the Server's AERO interface and is destined to a CN belonging to a
   MNP not assigned to one of the Server's INET interfaces, the Server
   forwards the packet from within the AERO interface at the link layer
   without ever disturbing the network layer.

3.7.3.  AERO Gateway Behavior

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

3.7.4.  AERO Proxy Behavior

   When a Proxy enables an AERO interface, it assigns AERO/SPAN
   addresses and maintains a list of INET address mappings the same as
   for Relays, Servers and Gateways.  The Proxy further maintains
   neighbor cache entires for one or more Relays, and maintains per-
   Client neighbor cache entries based on control message exchanges.
   Proxies forward packets between each Client and their associated
   Servers and neighbors.

   When the Proxy receives an RS message from a Client, it creates an
   incomplete neighbor cache entry and sends a proxyed RS message to a
   Server via the SPAN while using its own INET address as the source
   address.  When the Server returns an RA message, the Proxy completes
   the proxy neighbor cache entry based on autoconfiguration information
   in the RA and sends a proxyed RA to the Client while using its own
   ANET address as the source address.  The Client, Server and Proxy
   will then have the necessary state for managing the proxy neighbor
   association.

3.7.5.  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 AERO 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.)  See [I-D.templin-6man-dhcpv6-ndopt] for the
   types of ND/PD parameters that can be included in the RS/RA message
   exchanges.

   After the initial ND/PD message exchange, the Client assigns AERO
   addresses to the AERO interface based on the delegated prefix(es).



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   The Client can then register additional ANET interfaces with the
   Server by sending a simple RS message (i.e., one with no PD
   parameters) over each ANET interface using its base AERO address as
   the source network layer address.  The Server will update its
   neighbor cache entry for the Client and return a simple RA message.

   The Client maintains a neighbor cache entry for each of its Servers
   and each of its active target Clients.  When the Client receives ND
   messages on the AERO interface it updates or creates neighbor cache
   entries, including link-layer address and QoS preferences.

   When there is a NAT on the path, the Client must send periodic
   messages to keep NAT state alive.  If no other periodic messaging
   service is available, the Client can send RS messages to receive RA
   replies from its Server(s).

   A Client may be configured as a co-resident function on the same
   platform as a Server.  In that case, no Client/Server ND messaging is
   required and the Client and Server operate as a single functional
   unit.  The Client function can use its MNP(s) to number downstream-
   attached networks, which may connect very large numbers of nodes.

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 Relays maintain permanent
   neighbor cache entries for their associated Relays, Servers, Gateways
   and Proxys, and AERO Servers and Proxys maintain permanent neighbor
   cache entries for their associated Relays.  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
   ND/PD 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



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   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 (i.e., and not the route
   optimization responder (ROR)) creates an entry.

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



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   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,
   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;



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   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 always encapsulates the message
   in SPAN/INET headers, and forwards the message into the SPAN (i.e.,
   it forwards the message to a Relay).  For data packets, if the
   neighboring node can only be reached via the SPAN (or, if it is not
   yet know that the neighboring node is within the local segment) the
   source node encapsulates packets in a SPAN/INET headers and 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, Proxies, Servers and Relays.  In the
   following discussion, a packet's destination address is said to
   "match" if it is a non-link-local address with a prefix covered by an
   MSP/MNP, or if it is an AERO address that embeds an MNP, or if it is
   the same as an administratively-provisioned 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" underlying ANET interfaces in the entry for packet
   forwarding.  If there is no asymmetric neighbor cache entry, the
   Client instead forwards the packets to a Server.

   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 (if necessary) 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.

   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 network-layer destination.  If there is a match, the



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   Proxy uses one or more "reachable" neighbor interfaces in the entry
   for packet forwarding.  Otherwise, the Proxy uses the SPAN/INET
   address in a permanent neighbor cache entry for a Relay (selected
   through longest-prefix match) as the encapsulation addresses and
   forwards the packet into the SPAN.

   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 may return 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 Forwarding Algorithm

   When an IP packet enters a Server's AERO interface from either the
   network or link-layer, it decapsulates the packet (if the packet
   arrived from the link-layer) then processes the packet according to
   the network-layer destination address as follows:

   o  if the destination matches one of the Server's own addresses the
      Server forwards it to the network layer for local delivery.

   o  else, 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 continues
      to forward packets 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" Interface ID than the one the packet
      arrived on.  If there are no "reachable" Interface IDs, the Server
      must drop the packet.

   o  else, if the destination matches an asymmetric neighbor cache
      entry for a target Client, the Server forwards the packet
      according to the cached link-layer information.

   o  else, the Server uses the SPAN/INET address in a permanent
      neighbor cache entry for a Relay (selected through longest-prefix
      match) as the encapsulation addresses.






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3.9.4.  Gateway Forwarding Algorithm

   Gateways perform the same forwarding procedures as for Servers, but
   also forward packets between the AERO interface and any downstream-
   attached INET interfaces.  In particular, if the destination address
   of a packet that arrives on an AERO interfaces matches a prefix
   associated with a downstream-attached INET interface, the Gateway
   forwards the packet to the next hop via the INET interface.
   Conversely, the Gateway forwards packets that arrive on an INET
   interface to the next hop via the AERO interface or another INET
   interface according to longest prefix match.

3.9.5.  Relay Forwarding Algorithm

   Relays forward packets the same as any IP router.  When the Relay
   receives an encapsulated packet via the AERO link, it removes the
   INET header and searches for a forwarding table entry that matches
   the destination address in the SPAN header.  When the Relay receives
   an unencapsulated packet from a node outside the AERO link, it
   searches for a forwarding table entry that matches the IP destination
   address.  The Relay then processes the packet as follows:

   o  if the destination does not match an MSP or the SSP, or if the
      destination matches one of the Relay's own addresses, the Relay
      submits the packet for either IP forwarding or local delivery.

   o  else, if the destination matches an MNP/SPP entry in the IP
      forwarding table the Relay encapsulates the packet in an INET
      header and forwards it to the neighbor.

   o  else, the Relay drops the packet and returns an ICMP Destination
      Unreachable message subject to rate limiting (see: Section 3.14).

   As for any IP router, the Relay decrements the TTL/Hop Count when it
   forwards the packet.

3.10.  AERO Interface Encapsulation and Re-encapsulation

   AERO interfaces encapsulate packets in ANET/INET headers 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.

   The AERO interface encapsulates the packet in an ANET/INET header per
   the Generic UDP Encapsulation (GUE) procedures in
   [I-D.ietf-intarea-gue][I-D.ietf-intarea-gue-extensions], or through



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   an alternate encapsulation format (e.g., see: Appendix A, [RFC2784],
   [RFC8086], [RFC4301], etc.).

   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.

   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.

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.  Decapsulation is per the
   procedures specified for the appropriate encapsulation format.

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 and Servers accept encapsulated packets with a link-
      layer source address that matches a permanent neighbor cache
      entry.




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   o  AERO Servers accept authentic encapsulated ND messages from
      Clients (either directly or via a Proxy), and create or update a
      symmetric neighbor cache entry for the Client based on the
      specific message type.

   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 Proxies 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
   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.




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   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
   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.

   The network layer proceeds as follow 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 AERO interface 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



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   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: [RFC2764] and
   [I-D.ietf-intarea-gue-extensions]) before appending the outer
   encapsulation headers to each fragment.  Since the encapsulation
   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




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   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):

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~
        |        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



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      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
      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 3.19.7.

   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 Relay or Server 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



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   address and writes one of its non link-local addresses as the source
   address.

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 to
   be used for Prefix Delegation (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.







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3.15.2.  AERO Client Behavior

   AERO Clients can discover the INET and AERO addresses of AERO 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-
   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 the S bit 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



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   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.

   The Client then registers additional ANET interfaces with the Server
   by sending additional RS messages including SLLAOs via other ANET
   interfaces after the initial RS/RA exchange.  The Client sends the RS
   messages to the Server's AERO address but omits PD parameters since
   the initial RS/RA exchange has already established PD state.

   The Client examines the X and N bits in the first SLLAO of each RA
   message it receives.  If the X bit value is 1 the Client infers that
   there is a Proxy on the path, and if the N bit value 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 with PD parameters 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 specific to the selected
   ANET interface as the first SLLAO 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 the D bit
   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 the R bit set to 1 in the first SLLAO.



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   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.

3.15.3.  AERO Server Behavior

   AERO Servers act as IPv6 routers and support a PD service for
   Clients.  AERO 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 an AERO Server receives a prospective Client's RS message with
   PD parameters 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



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   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.

   After the initial RS/RA exchange, the AERO 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 an unsolicited RA
   message with PD reconfigure parameters to cause the Client to
   renegotiate its PDs, and may issue an unsolicited RA message with
   Router Lifetime set to 0 if it can no longer service this Client.
   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.





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   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
   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 encapsulated 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 flag in the
      SLLAO to 1 and changes the address in the SLLAO to its own SPAN
      address.  The Proxy finally re-encapsulates the RS message in a
      SPAN header using its own SPAN address as the source address and
      the SPAN address of the Server as the destination address, 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 route information in the message as a
      mapping from the Client's MNPs to the Client's ANET address, and



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      sets the neighbor cache entry state to REACHABLE.  The Proxy then
      changes the Link Layer Address in the SLLAO to its own ANET
      address, re-encapsulates the RA message in an ANET header, sets
      the X flag in the SLLAO to 1, sets the N flag in the SLLAO to 1 if
      the Client is behind a NAT, and forwards the message to the
      Client.

   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.



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   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
   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 this arrangement, the only control messages sent by the Client are
   unencapsulated RS messages with its AERO address as the source
   address and the AERO address of the Server as the destination
   address.  The Client will also receive unencapsulated RA messages
   from the Server via both the Proxy and access router.

   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 INET interfaces serviced by a Gateway, the Gateway is the ROS.

   The route optimization procedure is conducted between the ROS and a
   Route Optimization Responder (ROR) in the same manner as for IPv6 ND
   Address Resolution, and using the same NS/NA messaging.  The ROR is
   the Server (MAP) for MN targets, or the Gateway for FN targets.  The
   procedures are specified in the following sections.







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3.17.1.  Route Optimization Initiation

   While the data packets are flowing from the source CN toward a target
   CN, the ROS also sends 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.

   The ROS then encapsulates the message in a SPAN header with source
   set to its own SPAN address and destination set to the inner packet
   destination, then sends the message 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 outer IP header and determines that the ROR is
   the next hop by consulting its standard IP 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 encapsulate the message in an INET header when
   it delivers the message 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 is the
   aggregation point for the target CN; if not, it drops the NS message.
   Otherwise, if the target CN is serviced by a Client in the DEPARTED
   state the ROR changes the NS message SPAN destination address to the
   address of the Client's new Server, re-encapsulates the message in
   the appropriate SPAN/INET headers and forwards the message to new
   Server.  If the target CN is serviced by a Client in the REACHABLE
   state the ROR adds the AERO source address to the target Client's
   Report List with time set to ReportTime.

   For both Servers and Gateways, the ROR next 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 its own AERO address
   and sets the destination address to the AERO address of the ROS.  The
   NA message includes the Nonce value received in the NS, the current
   Timestamp, and a first TLLAO with Interface ID set to 0xffff, with
   all P(i) values set to "low", with Prefix Length set to the prefix



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   length of the target Client's MNP and with Link Layer Address set to
   the ROR's SPAN address.

   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 are the SPAN address of the ROR.  For
   Proxyed interfaces, the TLLAO Link Layer Addresses are the SPAN
   addresses of the target Client's Proxies, 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 sends 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 IP 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 IP router.
   The final-hop Relay in the SPAN will encapsulate the message in an
   INET header when it delivers the message 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 target Client or Gateway 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 one of the target's CNs via the addresses found in the
   cached link-layer information.  The route optimization is shared by
   all sources that send packets to the target node 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 one of the target's CNs 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.  The ROS then updates



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   the asymmetric neighbor cache entry to refresh ReachableTime, while
   (for target Clients) 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 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 required 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) as
   described in [RFC4861].  NUD is performed either reactively in
   response to persistent link-layer errors (see Section 3.14) or
   proactively to confirm bi-directional 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 one or more target link-layer
   addresses, the ROS SHOULD proactively test the direct path to each
   address by sending an initial NS message to elicit a solicited NA
   response.  While testing the path, the ROS can optionally continue
   sending packets via its default router, maintain a small queue of
   packets until target reachability is confirmed, or (optimistically)
   allow packets to flow directly to the target.

   AERO nodes may have multiple link-layer addresses for the target
   neighbor.  In that case, NUD SHOULD be performed over each address



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   individually, and the source node should only consider the neighbor
   unreachable if NUD fails over multiple underlying interface paths.

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

   Proxies 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.






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3.19.1.  Mobility Update Messaging

   RORs (i.e., Servers 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 an ROR 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 ROR also includes a TLLAO with Interface ID 0xffff
   with Link Layer address set to the ROR's SPAN address, and includes
   additional TLLAOs for all of the target Client's Interface IDs with
   Link Layer Address set to the corresponding SPAN addresses.  The ROR
   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 ROR 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 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 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
   Server.  If the encapsulation source is in the Report List, the
   Server also sends an unsolicited NA message via the SPAN (subject to
   rate limiting) with a TLLAO with Interface ID 0xffff and with D set
   to 1.  The ROS will then realize that it needs to set its asymmetric



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   neighbor cache entry state for the target to DEPARTED, and SHOULD re-
   initiate route optimization after a short delay.

   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 Servers.  If the encapsulation
   source 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 source will then realize that it needs to
   mark its neighbor cache entry 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.

   When a Client receives packets with destination addresses that do not
   match one of its MNPs, it drops the packets silently.

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
   Proxy sends 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, S, Port Number and Link-Layer Address are
   set to 0.  If the RS message is not sent for the purpose of asserting
   a PD, the Prefix Length is 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.

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.  If the RS message is not sent
   for the purpose of asserting a PD, the Prefix Length is set to 0.

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 the D flag set to 1.





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   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 for the sending interface as the first SLLAO and include SLLAOs
   for any ANET interfaces being removed from service as additional
   SLLAOs.

3.19.6.  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
   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.

3.19.7.  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, with R set to 1 in the first SLLAO and
   with PD parameters 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 does not receive an RA reply after
   MAX_RTR_SOLICITATIONS attempts over multiple underlying 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
   D 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, and sets a timer to DepartTime seconds.  The Server then
   returns an RA message to the Client with Router Lifetime set to 0.
   After DepartTime seconds expires, the Server deletes the symmetric
   neighbor cache entry.




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   When the Client receives the RA message with Router Lifetime set to
   0, it still must inform each of its remaining Proxies 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 R set to 1 in the first SLLAO but with
   no PD parameters.  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 serves as an IGMPv2 (IPv4) [RFC2236] or MLDv2 (IPv6)
   proxy [RFC3810][RFC4605] for its EUNs and/or hosted applications.
   The Client forwards IGMPv2/MLDv2 messages over any of its ANET
   interfaces for which group membership is required.  The IGMP/MLDv2
   messages may be further forwarded by a first-hop ANET access router
   acting as an IGMPv2/MLDv2-snooping switch [RFC4541], then ultimately
   delivered to an AERO Proxy/Server acting as a Protocol Independent
   Multicast - Sparse-Mode (PIM-SM) router [RFC7761].  AERO Gateways act
   as PIM-SM routers the same as AERO Proxys/Servers, except that no
   IGMPv2/MLDv2 proxying/snooping are necessary on the Gateway's
   attached EUNs.

   When an AERO Proxy/Server/Gateway "X" acting as a PIM-SM router
   receives a Source-Specific Multicast (SSM) "Join" message for source
   "S" and group "G" (i.e., (S,G)), it forwards the message to a Relay
   via the SPAN.  The SPAN then forwards the message to AERO Server "Y"
   which forwards the message to Proxy "Z" that services "S" (note that
   when "Y" is a Gateway there is no need for Proxy "Z".)  Since the
   Relays in the SPAN do not examine Layer 3 control messages, this
   means that the (reverse) multicast tree path is simply from "S" to
   "Z" to "Y" to "X" with no other Layer 3 multicast-aware routers in
   the path.  If "Z", "Y" and "X" are located on the same SPAN segment,
   the multicast data traffic between them can be sent via simple INET
   encapsulation and need not go over the SPAN.  If any of "Z", "Y" and
   X" are located in different SPAN segments, however, SPAN
   encapsulation is necessary.

   When an AERO Proxy/Server/Gateway "X" acting as a PIM-SM router
   receives an Any Source Multicast (ASM) "Join" message for source "*"
   and group "G" (i.e., (*,G)), it forwards the message toward the



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   Rendezvous Point "RP" for group "G" the same as if "RP" was the
   source "S".  The (reverse) multicast tree path is therefore
   established in the same way as above.

   After the (reverse) multicast tree path has been established via AERO
   Proxy "Z", the AERO Client "C" that hosts "S" may move to a different
   Proxy "Z2".  In that case, the Client's multicast Server "Y" sends a
   PIM-SM "Join" to the new Proxy "Z2" for each multicast group "G",
   then sends a PIM-SM "Prune" to the old Proxy "Z".

   After the (reverse) multicast tree path has been established, the
   AERO Client "C" may move to a different Server "Y2".  In that case,
   the old Server "Y" must transfer its multicast tree state for all of
   Client "C"'s multicast sources to the new Server "Y2", and "Y2" must
   issue PIM-SM "Join" messages for each of Client "C"'s new Proxys.

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 Proxies, 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

   An AERO Client can connect to mutliple AERO links the same as for any
   Layer 2 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 Proxies, thereby
   providing redundancy in case of failures.  Each AERO link would



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   service a distinct MSP such that the Client would receive multiple
   MNP delegations - one for each link.

   The Relays, Servers and Proxies 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 distinct 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 definied 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.

7.  Operation on AERO Links with /64 ASPs

   IPv6 AERO links typically have MSPs that cover 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).

   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"



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   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-
   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.





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   AERO Proxies 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.  Proxies must be provisioned,
   supported and managed by the ANET administrative authority.  Cost for
   purchasing, configuring and managing Proxies is nominal, and borne by
   the ANET administrative authority.

   AERO combined Client/Servers can be any dedicated server or COTS
   router platform with one network interface connected to the INET and
   a second interface connected to a downstream attached network.  The
   Client/Server joins the SPAN over the INET interface and engages in
   eBGP peering with one or more Relays as a stub AS.  The Client/Server
   then injects its MNP into the BGP routing system, and provisions the
   MNP to its downstream-attached networks.  No Client/Server ND
   messaging is necessary, and the Client/Server can perform ROS and ROR
   services the same as for any Server.  The combined Client/Server
   construct is useful for connecting large fixed networks to the AERO
   link.

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.

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 MNP prefixes to eventually reach all nodes (both mobile
   and fixed) in all segments.  This can be accomplished by




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   incrementally deploying AERO Gateways on each INET segment, with each
   Gateway distributing its MNPs to its downstream-attached INET links.

   This gives rise to the opportunity to eventually distribute MNP-based
   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
   protocol version and addressing scheme that is compatible with the
   AERO link MNP-based addressing scheme, the NET segment and AERO link
   can be joined by standard routers.

12.  Implementation Status

   An AERO implementation based on OpenVPN (https://openvpn.net/) was
   announced on the v6ops mailing list on January 10, 2018.  The latest
   version is available at: http://linkupnetworks.net/aero/AERO-OpenVPN-
   2.0.tgz.

   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 version is available at: http://linkupnetworks.net/aero/aero-
   4.0.0.tgz.

   A survey of public domain and commercial SEND implementations is
   available at https://www.ietf.org/mail-archive/web/its/current/
   msg02758.html.

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.

14.  Security Considerations

   AERO link security considerations include considerations for both the
   data plane and the control plane.

   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



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   TLS/SSL [RFC8446], DTLS [RFC6347] 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 TLS/SSL,
   DTLS, IPsec [RFC4301], etc.  AERO Proxies 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/Gateways and Proxies SHOULD pre-configure security
   associations for one or more Relays on their SPAN segments (e.g.,
   using pre-placed keys) and use symmetric network and/or transport
   layer security services such as IPsec, TLS/SSL or DTLS to secure ND
   messages.  The AERO Relays of all SPAN segments in turn SHOULD pre-
   configure security associations for 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 an
   enclave 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/Gateways 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/
   Gateways 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 enclaves.  AERO Servers/Gateways and Proxies
   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.





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   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.

   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.




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   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

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>.



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   [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>.

   [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.







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   [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-01 (work in progress), January 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.

   [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.







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   [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>.

   [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>.






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   [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>.

   [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>.





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   [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>.

   [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>.



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   [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>.

   [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>.

   [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>.

   [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>.





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   [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>.

   [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>.

   [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>.






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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].

   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:








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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        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
   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.



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   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.

        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







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Appendix C.  Change Log

   << RFC Editor - remove prior to publication >>

   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

   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.



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   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

   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





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   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.

   o  Security considerations updates.

   o  General text clarifications and cleanup.

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




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   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.

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








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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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