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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720,                   January 27, 2020
           rfc6179, rfc6706 (if
           approved)
Intended status: Standards Track
Expires: July 30, 2020


             Asymmetric Extended Route Optimization (AERO)
                    draft-templin-intarea-6706bis-20

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/registration services are employed for network
   admission and to manage the routing system.  Multilink operation,
   mobility management, quality of service (QoS) signaling and route
   optimization are naturally supported through dynamic neighbor cache
   updates.  Standard IP multicasting services are also supported.  AERO
   is a widely-applicable mobile internetworking service 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
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   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 July 30, 2020.








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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Asymmetric Extended Route Optimization (AERO) . . . . . . . .  10
     3.1.  AERO Link Reference Model . . . . . . . . . . . . . . . .  10
     3.2.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .  12
     3.3.  AERO Routing System . . . . . . . . . . . . . . . . . . .  13
       3.3.1.  IPv4 Compatibility Routing  . . . . . . . . . . . . .  15
     3.4.  AERO Addresses  . . . . . . . . . . . . . . . . . . . . .  15
     3.5.  Spanning Partitioned AERO Networks (SPAN) . . . . . . . .  17
       3.5.1.  SPAN Compatibility Addressing . . . . . . . . . . . .  21
     3.6.  AERO Interface Characteristics  . . . . . . . . . . . . .  21
     3.7.  AERO Interface Initialization . . . . . . . . . . . . . .  25
       3.7.1.  AERO Server/Gateway Behavior  . . . . . . . . . . . .  25
       3.7.2.  AERO Proxy Behavior . . . . . . . . . . . . . . . . .  26
       3.7.3.  AERO Client Behavior  . . . . . . . . . . . . . . . .  26
       3.7.4.  AERO Relay Behavior . . . . . . . . . . . . . . . . .  26
     3.8.  AERO Interface Neighbor Cache Maintenance . . . . . . . .  26
     3.9.  AERO Interface Encapsulation and Re-encapsulation . . . .  28
     3.10. AERO Interface Decapsulation  . . . . . . . . . . . . . .  29
     3.11. AERO Interface Data Origin Authentication . . . . . . . .  30
     3.12. AERO Interface Forwarding Algorithm . . . . . . . . . . .  30
       3.12.1.  Client Forwarding Algorithm  . . . . . . . . . . . .  31
       3.12.2.  Proxy Forwarding Algorithm . . . . . . . . . . . . .  31
       3.12.3.  Server/Gateway Forwarding Algorithm  . . . . . . . .  32
       3.12.4.  Relay Forwarding Algorithm . . . . . . . . . . . . .  34
     3.13. AERO Interface MTU and Fragmentation  . . . . . . . . . .  34
       3.13.1.  AERO MTU Requirements  . . . . . . . . . . . . . . .  37
     3.14. AERO Interface Error Handling . . . . . . . . . . . . . .  37
     3.15. AERO Router Discovery, Prefix Delegation and
           Autoconfiguration . . . . . . . . . . . . . . . . . . . .  40
       3.15.1.  AERO ND/PD Service Model . . . . . . . . . . . . . .  40



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       3.15.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  41
       3.15.3.  AERO Server Behavior . . . . . . . . . . . . . . . .  43
     3.16. The AERO Proxy  . . . . . . . . . . . . . . . . . . . . .  45
       3.16.1.  Detecting and Responding to Server Failures  . . . .  48
     3.17. AERO Route Optimization . . . . . . . . . . . . . . . . .  48
       3.17.1.  Route Optimization Initiation  . . . . . . . . . . .  49
       3.17.2.  Relaying the NS  . . . . . . . . . . . . . . . . . .  49
       3.17.3.  Processing the NS and Sending the NA . . . . . . . .  49
       3.17.4.  Relaying the NA  . . . . . . . . . . . . . . . . . .  50
       3.17.5.  Processing the NA  . . . . . . . . . . . . . . . . .  50
       3.17.6.  Route Optimization Maintenance . . . . . . . . . . .  51
     3.18. Neighbor Unreachability Detection (NUD) . . . . . . . . .  52
     3.19. Mobility Management and Quality of Service (QoS)  . . . .  52
       3.19.1.  Mobility Update Messaging  . . . . . . . . . . . . .  53
       3.19.2.  Announcing Link-Layer Address and/or QoS Preference
                Changes  . . . . . . . . . . . . . . . . . . . . . .  54
       3.19.3.  Bringing New Links Into Service  . . . . . . . . . .  54
       3.19.4.  Removing Existing Links from Service . . . . . . . .  54
       3.19.5.  Moving to a New Server . . . . . . . . . . . . . . .  55
     3.20. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  56
       3.20.1.  Source-Specific Multicast (SSM)  . . . . . . . . . .  56
       3.20.2.  Any-Source Multicast (ASM) . . . . . . . . . . . . .  58
       3.20.3.  Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . .  58
     3.21. Operation over Multiple AERO Links (VLANs)  . . . . . . .  59
     3.22. DNS Considerations  . . . . . . . . . . . . . . . . . . .  60
     3.23. Transition Considerations . . . . . . . . . . . . . . . .  60
     3.24. Detecting and Reacting to Server and Relay Failures . . .  61
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  61
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  61
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  62
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  63
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  65
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  65
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  66
   Appendix A.  AERO Alternate Encapsulations  . . . . . . . . . . .  73
   Appendix B.  Non-Normative Considerations . . . . . . . . . . . .  75
     B.1.  Implementation Strategies for Route Optimization  . . . .  75
     B.2.  Implicit Mobility Management  . . . . . . . . . . . . . .  76
     B.3.  Direct Underlying Interfaces  . . . . . . . . . . . . . .  76
     B.4.  AERO Clients on the Open Internetwork . . . . . . . . . .  76
     B.5.  Operation on AERO Links with /64 ASPs . . . . . . . . . .  77
     B.6.  AERO Adaptations for SEcure Neighbor Discovery (SEND) . .  77
     B.7.  AERO Critical Infrastructure Considerations . . . . . . .  78
     B.8.  AERO Server Failure Implications  . . . . . . . . . . . .  79
     B.9.  AERO Client / Server Architecture . . . . . . . . . . . .  79
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  81
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  87




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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 a virtual overlay configured over one or more
   underlying Internetworks, and nodes on the link can exchange IP
   packets via tunneling.  Multilink operation allows for increased
   reliability, bandwidth optimization and traffic path diversity.

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

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

   AERO Relays are interconnected in a secured private BGP overlay
   routing instance known as the "SPAN".  The SPAN provides a hybrid
   routing/bridging service to join the underlying Internetworks of
   multiple disjoint administrative domains into a single unified AERO
   link.  Each AERO link instance is characterized by the set of
   Mobility Service Prefixes (MSPs) common to all mobile nodes.  The
   link extends 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.




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

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

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

2.  Terminology

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

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

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




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   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 a transit backbone service for
      ANET end systems.  INETs also provide an underlay service over
      which the AERO virtual link is configured.  Example INETs include
      corporate enterprise networks, aviation networks, and the public
      Internet itself.  When there is no administrative boundary between
      an ANET and the INET, the ANET and INET are one and the same.

   INET Partition
      frequently, INETs such as large corporate enterprise networks are
      sub-divided internally into separate isolated partitions.  Each
      partition is fully connected internally but disconnected from
      other partitions, and there is no requirement that separate
      partitions maintain consistent Internet Protocol and/or addressing
      plans.  (An INET partition is the same as a SPAN segment discussed
      below.)

   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 links may be configured over multiple
      underlying SPAN segments (see below).

   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].

   underlying interface



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      an ANET or INET interface over which an AERO interface is
      configured.

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

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

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

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

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

   AERO Client ("Client")
      an AERO node that connects to one or more ANETs and requests MNP
      PDs from AERO Servers.  The Client assigns a Client AERO address
      to the AERO interface for use in ND exchanges with other AERO
      nodes and forwards packets to correspondents according to AERO
      interface neighbor cache state.

   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 its AERO interface to support the operation of the ND/
      PD services, and advertises all of its associated MNPs via BGP
      peerings with Relays.

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




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

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

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

   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 SPAN segment.

   SPAN Address
      a global or unique local IPv6 address taken from a SPAN Partition
      Prefix and constructed as specified in Section 3.5.  SPAN
      addresses are statelessly derived from AERO addresses, and vice-
      versa.

   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.



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

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

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

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

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

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

   Route Optimization Source (ROS)
      the AERO node nearest the source that initiates route
      optimization.  The ROS may be a Server or Proxy acting on behalf
      of the source Client.

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

   MAP List
      a geographically and/or topologically referenced list of AERO
      addresses of all MAPs within the same AERO link.  There is a
      single MAP list for the entire AERO link.

   ROS List




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      a list of AERO/SPAN-to-INET address mappings of all ROSes within
      the same SPAN segment.  There is a distinct ROS list for each
      segment.

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

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

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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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



















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

                    Figure 1: AERO Link Reference Model

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

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

   o  AERO Relay R1 aggregates Mobility Service Prefix (MSP) M1,
      discovers Mobile Network Prefixes (MNPs) X* and advertises the MSP
      via BGP peerings over secured tunnels to Servers (S1, S2).  Relays
      use the SPAN service to bridge disjoint segments of a partitioned
      AERO link.

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





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

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

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

3.2.  AERO Node Types

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

   AERO Servers provide default forwarding services and a Mobility
   Anchor Point (MAP) for AERO Client Mobile Nodes (MNs).  Each Server
   also peers with Relays in a dynamic routing protocol instance to
   advertise its list of associated MNPs (see Section 3.3).  Servers
   facilitate PD exchanges with Clients, where each delegated prefix
   becomes an MNP taken from an MSP.  Servers forward packets between
   AERO interface neighbors and track each Client's mobility profiles.

   AERO Clients register their MNPs through PD exchanges with AERO
   Servers over the AERO link, and distribute the MNPs to nodes on EUNs.
   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




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   single functional unit and without the need for any Client/Server
   control messaging.

   AERO Proxys provide a conduit for ANET AERO Clients to associate with
   AERO Servers in external INETs.  Client and Servers exchange control
   plane messages via the Proxy acting as a bridge between the ANET/INET
   boundary.  The Proxy forwards data packets to and from Clients
   according to forwarding information in the neighbor cache.  The Proxy
   function is specified in Section 3.16.

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

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

3.3.  AERO Routing System

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

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

   Relays advertise the AERO link's MSPs and any non-MNP routes to each
   of their Servers.  This means that any aggregated non-MNPs (including
   "default") are advertised to all Servers.  Each Relay configures a
   black-hole route for each of its MSPs.  By black-holing the MSPs, the
   Relay will maintain forwarding table entries only for the MNPs that
   are currently active, and packets destined to all other MNPs will
   correctly incur Destination Unreachable messages due to the black-



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   hole route.  In this way, Servers have only partial topology
   knowledge (i.e., they know only about the MNPs of their directly
   associated Clients) and they forward all other packets to Relays
   which have full topology knowledge.

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

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

   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
   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



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   support small scale initial deployments followed by dynamic
   deployment of additional Clients, Servers and Relays without
   disturbing the already-deployed base.

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

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

3.3.1.  IPv4 Compatibility Routing

   For IPv6 MNPs, the AERO routing system includes ordinary IPv6 routes.
   For IPv4 MNPs, the AERO routing system includes IPv6 routes based on
   an IPv4-embedded IPv6 address format discussed in Section 3.5.1.

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



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   For IPv4, Client AERO addresses are based on an IPv4-mapped IPv6
   address [RFC4291] 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 (also written as
      0:0:0:0:0:FFFF:c000:0210/124)

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

      fe80::FFFF:192.0.2.16

      fe80::FFFF:192.0.2.17

      fe80::FFFF:192.0.2.18

      ... etc. ...

      fe80:FFFF:192.0.2.31

   Relay, Server and Proxy AERO addresses are allocated from the range
   fe80::/96, and MUST be managed for uniqueness.  The lower 32 bits of
   the AERO address includes a unique integer value between 1 and 0xfffe
   (e.g., fe80::1, fe80::2, fe80::3, etc.) as assigned by the
   administrative authority for the link.  If the link spans multiple
   SPAN segments, the AERO addresses are assigned to each segment in 1x1
   correspondence with SPAN addresses (see: Section 3.5).  The address
   fe80:: is the IPv6 link-local Subnet Router Anycast address, and the
   address fe80::ffff:ffff is reserved as the unspecified AERO address.

   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.




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

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

3.5.  Spanning Partitioned AERO Networks (SPAN)

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

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

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








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

                            Figure 2: The SPAN

   To support the SPAN, AERO links require a reserved /64 IPv6 "SPAN
   Service Prefix (SSP)".  Although any routable IPv6 prefix can be
   used, a Unique Local Address (ULA) prefix (e.g., fd00::/64) [RFC4389]
   is recommended since border routers are commonly configured to
   prevent packets with ULAs from being injected into the AERO link by
   an external IPv6 node and from leaking out of the AERO link to the
   outside world.

   Each segment in the SPAN assigns a unique sub-prefix of SSP::/96
   termed a "SPAN Partition Prefix (SPP)".  For example, a first segment
   could assign fd00::1000/116, a second could assign fd00::2000/116, a
   third could assign fd00::3000/116, etc.  The administrative
   authorities for each segment must therefore coordinate to assure
   mutually-exclusive SPP assignments, but internal provisioning of the
   SPP is an independent local consideration for each administrative
   authority.

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



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   AERO address with the SPP.  For example, if the SPP is
   fd00::1000/116, the SPAN address formed from the AERO address
   fe80::1001 is simply fd00::1001.

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

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

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

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

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

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

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

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






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

                       Figure 3: SPAN Encapsulation

   In this format, the inner IP header and packet body are the original
   IP packet, the SPAN header is an IPv6 header prepared according to
   [RFC2473], and the INET header is prepared according to Section 3.9.
   A packet is said to be "forwarded/sent into the SPAN" when it is
   encapsulated as described above then forwarded via a secured tunnel
   to a neighboring Relay.

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

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






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3.5.1.  SPAN Compatibility Addressing

   For IPv4 MNPs, Servers injects a "SPAN Compatibility Prefix (SCP)"
   that embeds the MNP into the BGP routing system.  The SCP begins with
   the upper 64 bits of the SSP, followed by the constant string
   "0000:FFFF" followed by the IPv4 MNP.  For example, if the SSP is
   fd00::/64 and the MNP is 192.0.2.0/24 then the SCP is
   fd00::FFFF:192.0.2.0/120.

   This allows for encapsulation of IPv4 packets in IPv6 headers with
   "SPAN Compatibility Addresses (SCAs)".  In this example, the SCA
   corresponding to the SCP is simply fd00::FFFF:192.0.2.0, and can be
   used as the SPAN destination address for packets forwarded via the
   SPAN.  This allows for forwarding of initial IPv4 packets over IPv6
   SPAN routes, followed by route optimization for direct
   communications.

3.6.  AERO Interface Characteristics

   AERO interfaces are virtual interfaces configured over one or more
   underlying interfaces classified as follows:

   o  Native interfaces have global IP addresses that are reachable from
      any INET correspondent.  All Server and Relay interfaces are
      native interfaces, as are INET-facing interfaces of Proxys.

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

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

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






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   o  Direct interfaces connect a 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.

   AERO interfaces use encapsulation (see: Section 3.9) to exchange
   packets with AERO link neighbors over Native, NATed or VPNed
   interfaces.  AERO interfaces do not use encapsulation over Proxyed
   and Direct underlying interfaces.

   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 interfaces send ND messages with an Overlay Multilink Network
   Interface (OMNI) option formatted as specified in
   [I-D.templin-atn-aero-interface].  The OMNI option includes prefix
   registration information and "ifIndex-tuples" containing link quality
   information for the AERO interface's underlying interfaces.

   When encapsulation is used, AERO interface ND messages MAY also
   include an AERO Source/Target Link-Layer Address Option (S/TLLAO)
   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    |   ifIndex[1]  |V| Reserved[1] |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                     Link Layer Address [1]                    ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Port Number [1]       |   ifIndex[2]  |V| Reserved[2] |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                     Link Layer Address [2]                    ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Port Number [2]       |                               ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
       ~                                                               ~
       ~                              ...                              ~
       ~                                                               ~
       ~                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                               |   ifIndex[N]  |V| Reserved[N] |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                     Link Layer Address [N]                    ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Port Number [N]       |     Trailing zero padding     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               Trailing zero padding (if necessary)            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   In this format, Type and Length are set the same as specified for S/
   TLLAOs in [RFC4861], with trailing zero padding octets added as
   necessary to produce an integral number of 8 octet blocks.  The S/
   TLLAO includes N ifIndex-tuples in correspondence to ifIndex-tuples
   that appear in the OMNI option.  Each ifIndex-tuple includes the
   folllowing information:

   o  ifIndex[i] - the same value as in the corresponding ifIndex-tuple
      included in the OMNI option.

   o  V[i] - a bit that identifies the IP protocol version of the
      address found in the Link Layer Address [i] field.  The bit is set
      to 0 for IPv4 or 1 for IPv6.

   o  Reserved[i] - MUST encode the value 0 on transmission, and ignored
      on reception.






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   o  Link Layer Address [i] - the IPv4 or IPv6 address used as the
      encapsulation source address.  The field is 4 bytes in length for
      IPv4 or 16 bytes in length for IPv6.

   o  Port Number [i] - the upper layer protocol port number used as the
      encapsulation source port, or 0 when no upper layer protocol
      encapsulation is used.  The field is 2 bytes in length.

   If present, the first S/TLLAO ifIndex-tuple MUST correspond to the
   first OMNI option ifIndex-tuple, and any additional S/TLLAO ifIndex-
   tuples MUST correspond to a proper subset of the remaining OMNI
   option ifIndex-tuples.

   A Client's AERO interface may be configured over multiple underlying
   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.

   If a Client's multiple underlying interfaces are used "one at a time"
   (i.e., all other interfaces are in standby mode while one interface
   is active), then ND message OMNI options include only a single
   ifIndex-tuple and set to a constant value.  In that case, the Client
   would appear to have a single interface but with a dynamically
   changing link-layer address.

   If the Client has multiple active underlying interfaces, then from
   the perspective of ND it would appear to have multiple link-layer
   addresses.  In that case, ND message OMNI options MAY include
   multiple ifIndex-tuples - each with a value that corresponds to a
   specific interface.  The OMNI option MUST include a first ifIndex-
   tuple that corresponds to the interface over which the ND message is
   sent.  Every ND message need not include all OMNI and/or S/TLLAO
   ifIndex-tuples; for any ifIndex-tuple not included, the neighbor
   considers the status as unchanged.

   Relay, Server and Proxy AERO interfaces may be configured over one or
   more secured tunnel interfaces.  The AERO interface configures both
   an AERO address and its corresponding SPAN address, while the
   underlying secured tunnel interfaces are either unnumbered or
   configure the same SPAN address.  The AERO interface encapsulates
   each IP packet in a SPAN header and presents the packet to the
   underlying secured tunnel interface.  For Relays that do not
   configure an AERO interface, the secured tunnel interfaces themselves
   are exposed to the IP layer with each interface configuring the



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   Relay's SPAN address.  Routing protocols such as BGP therefore run
   directly over the Relay's secured tunnel interfaces.  For nodes that
   configure an AERO interface, routing protocols such as BGP run over
   the AERO interface but do not employ SPAN encapsulation.  Instead,
   the AERO interface presents the routing protocol messages directly to
   the underlying secured tunnels without applying encapsulation and
   while using the SPAN address as the source address.  This distinction
   must be honored consistently according to each node's configuration
   so that the IP forwarding table will associate discovered IP routes
   with the correct interface.

3.7.  AERO Interface Initialization

   AERO Servers, Proxys and Clients configure AERO interfaces as their
   point of attachment to the AERO link.  AERO nodes assign the MSPs for
   the link to their AERO interfaces (i.e., as a "route-to-interface")
   to ensure that packets with destination addresses covered by an MNP
   not explicitly assigned to a non-AERO interface are directed to the
   AERO interface.

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

3.7.1.  AERO Server/Gateway Behavior

   When a Server enables an AERO interface, it assigns AERO/SPAN
   addresses and configures permanent neighbor cache entries for
   neighbors in the same SPAN segment by consulting the ROS list for the
   segment.  The Server also configures secured tunnels with one or more
   neighboring Relays and engages in a BGP routing protocol session with
   each Relay.

   The AERO interface provides a single interface abstraction to the IP
   layer, but internally comprises multiple secured tunnels as well as
   an NBMA nexus for sending encapsulated data packets to AERO interface
   neighbors.  The Server further configures a service to facilitate ND/
   PD exchanges with AERO Clients and manages per-Client neighbor cache
   entries and IP forwarding table entries based on control message
   exchanges.

   Gateways are simply Servers that run a dynamic routing protocol
   between the AERO interface and INET/EUN interfaces (see:
   Section 3.3).  The Gateway provisions MNPs to networks on the INET/
   EUN interfaces (i.e., the same as a Client would do) and advertises
   the MSP(s) for the AERO link over the INET/EUN interfaces.  The
   Gateway further provides an attachment point of the AERO link to the
   non-MNP-based global topology.




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3.7.2.  AERO Proxy Behavior

   When a Proxy enables an AERO interface, it assigns AERO/SPAN
   addresses and configures permanent neighbor cache entries the same as
   for Servers.  The Proxy also configures secured tunnels with one or
   more neighboring Relays and maintains per-Client neighbor cache
   entries based on control message exchanges.

3.7.3.  AERO Client Behavior

   When a Client enables an AERO interface, it sends an RS message with
   ND/PD parameters over an ANET interface to a Server in the MAP list,
   which returns an RA message with corresponding PD parameters.  (The
   RS/RA messages may pass through a Proxy in the case of a Client's
   Proxyed interface.)

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

3.7.4.  AERO Relay Behavior

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

3.8.  AERO Interface Neighbor Cache Maintenance

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

   Permanent neighbor cache entries are created through explicit
   administrative action; they have no timeout values and remain in
   place until explicitly deleted.  AERO Servers and Proxys maintain
   permanent neighbor cache entries for all other Servers and Proxys
   within the same SPAN segment.  Each entry maintains the mapping
   between the neighbor's network-layer AERO address and corresponding
   INET address.  The list of all permanent neighbor cache entries for
   the SPAN segment is maintained in the segment's ROS list.



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   Symmetric neighbor cache entries are created and maintained through
   RS/RA exchanges as specified in Section 3.15, and remain in place for
   durations bounded by ND/PD lifetimes.  AERO Servers maintain
   symmetric neighbor cache entries for each of their associated
   Clients, and AERO Clients maintain symmetric neighbor cache entries
   for each of their associated Servers.  The list of all Servers on the
   AERO link is maintained in the link's MAP list.

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

   Proxy neighbor cache entries are created and maintained by AERO
   Proxys when they process Client/Server ND/PD exchanges, and remain in
   place for durations bounded by ND/PD lifetimes.  AERO Proxys maintain
   proxy neighbor cache entries for each of their associated Clients.
   Proxy neighbor cache entries track the Client state and the address
   of the Client's associated Server.

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

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



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   seconds to allow a 10 second window so that route optimization can
   converge before ReportTime decrements below REACHABLETIME.

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

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

   AERO interfaces encapsulate packets according to whether they are
   entering the AERO interface from the network layer or if they are
   being re-admitted into the same AERO link they arrived on.  This
   latter form of encapsulation is known as "re-encapsulation".

   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



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   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.

   AERO interfaces configured over INET underlying interfaces
   encapsulate each packet in a SPAN header, then encapsulate the
   resulting SPAN packet in an INET header according to the next hop
   determined in the forwarding algorithm in Section 3.12.  If the next
   hop is reached via a secured tunnel, the AERO interface uses an INET
   encapsulation format specific to the secured tunnel type (see:
   Section 6).  If the next hop is reached via an unsecured underlying
   interface, the AERO interface instead uses Generic UDP Encapsulation
   (GUE) [I-D.ietf-intarea-gue] or an alternate minimal encapsulation
   format Appendix A.

   When GUE encapsulation is used, the AERO interface next sets the UDP
   source port to a constant value that it will use in each successive
   packet it sends, and sets the UDP length field to the length of the
   SPAN 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.

   Client AERO interfaces can avoid encapsulation over Direct underlying
   interface and Proxyed underlying interfaces for which the first-hop
   access router is AERO-aware.

   AERO interfaces observes the packet sizing and fragmentation
   considerations found in Section 3.13.

3.10.  AERO Interface Decapsulation

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





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3.11.  AERO Interface Data Origin Authentication

   AERO nodes employ simple data origin authentication procedures.  In
   particular:

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

   o  AERO Servers and Proxys accept encapsulated data packets and NS
      messages used for Neighbor Unreachability Detection (NUD) received
      from a source found in the ROS list.

   o  AERO Proxys and Clients accept packets that originate from within
      the same secured ANET.

   o  AERO Clients and Gateways accept packets from downstream network
      correspondents based on ingress filtering.

   AERO nodes silently drop any packets that do not satisfy the above
   data origin authentication procedures.  Further security
   considerations are discussed Section 6.

3.12.  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).  All
   packets entering a node's AERO interface first undergo data origin
   authentication as discussed in Section 3.11.  Packets that satisfy
   data origin authentication are processed further, while all others
   are dropped silently.

   Packets that enter the AERO interface from the network layer are
   forwarded 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 interfaces and/or
   neighbor cache entries for neighbors with multiple ifIndex-tuple
   registrations (see Section 3.6).  The AERO interface uses each
   packet's DSCP value (and/or port number) to select an outgoing
   underlying interface based on the node's own QoS preferences, and
   also to select a destination link-layer address based on the
   neighbor's underlying interface with the highest preference.  AERO




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   implementations SHOULD allow for QoS preference values to be modified
   at runtime through network management.

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

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

3.12.1.  Client Forwarding Algorithm

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

   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).

3.12.2.  Proxy Forwarding Algorithm

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

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




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   o  if the destination matches an asymmetric neighbor cache entry, the
      Proxy uses one or more "reachable" neighbor interfaces in the
      entry for packet forwarding via encapsulation.  If the neighbor
      interface is in the same SPAN segment, the Proxy forwards the
      packet directly to the neighbor; otherwise, it forwards the packet
      to a Relay.

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

   When the Proxy receives an encapsulated data packet from an INET
   neighbor or from a secured tunnel, it accepts the packet only if data
   origin authentication succeeds and the SPAN destination address is
   its own address.  If the packet is a SPAN fragment, the Proxy then
   adds the fragment to the reassembly buffer and returns if the
   reassembly is still incomplete.  Otherwise, the Proxy reassembles the
   packet (if necessary) and continues processing.

   Next, the Proxy 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 decapsulates and forwards the packet
   to the Client.  If the neighbor cache entry is in the DEPARTED state,
   the Proxy instead re-encapsulates the message and forwards it to a
   Relay.  If there is no neighbor cache entry, the Proxy instead
   discards the packet.

3.12.3.  Server/Gateway Forwarding Algorithm

   For control messages destined to a target Client's AERO address that
   are received from a secured tunnel, the Server (acting as a MAP)
   intercepts the message and sends an appropriate response on behalf of
   the Client.  (For example, the Server sends an NA message reply in
   response to an NS message directed to one of its associated Clients.)
   If the Client's neighbor cache entry is in the DEPARTED state,
   however, the Server instead forwards the packet to the Client's new
   Server as discussed in Section 3.19.

   When the Server receives an encapsulated data packet from an INET
   neighbor or from a secured tunnel, it accepts the packet only if data
   origin authentication succeeds.  If the SPAN destination address is
   its own address, the Server reassembles if necessary and discards the
   SPAN header (if the reassembly is incomplete, the Server instead adds
   the fragment to the reassembly buffer and returns).  The Server then
   continues processing as follows:



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   o  if the destination matches a symmetric neighbor cache entry in the
      REACHABLE state the Server prepares the packet for forwarding to
      the destination Client.  If the current header is a SPAN header,
      the Server reassembles if necessary and discards the SPAN header
      (if the reassembly is incomplete, the Server instead adds the
      fragment to the reassembly buffer and returns).  The Server then
      forwards the packet according to the cached link-layer
      information, while using SPAN encapsulation for the Client's
      Proxyed/Native interfaces, simple INET encapsulation for NATed/
      VPNed interfaces, or no encapsulation for Direct interfaces.

   o  else, if the destination matches a symmetric neighbor cache entry
      in the DEPARETED state the Server encapsulates the packet in a new
      SPAN header and forwards it to the Client's new Server (noting
      that the encapsulation may result in the addition of a second SPAN
      header).  The Server uses its own SPAN address as the source and
      the SPAN address of the new Server as the destination.

   o  else, if the destination matches an asymmetric neighbor cache
      entry, the Server uses one or more "reachable" neighbor interfaces
      in the entry for packet forwarding via the local INET if the
      neighbor is in the same SPAN segment or via a Relay otherwise.

   o  else, if the destination is an AERO address that is not assigned
      on the AERO interface the Server drops the packet.

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

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

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

   o  else, the Server encapsulates the packet and forwards it to a
      Relay.  For administratively-assigned AERO address destinations,
      the Server uses the SPAN address corresponding to the destination
      as the SPAN destination address.  For Client AERO address



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      destinations, the Server uses the Subnet Router Anycast address
      corresponding to the destination as the SPAN destination address.
      For all others, the Server uses the packet's destination IP
      address as the SPAN destination address.

3.12.4.  Relay Forwarding Algorithm

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

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

   o  else, if the destination matches a forwarding table entry the
      Relay forwards the packet via a secured tunnel to the next hop.
      If the destination matches an MSP without matching an MNP,
      however, the Relay instead drops the packet and returns an ICMP
      Destination Unreachable message subject to rate limiting (see:
      Section 3.14).

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

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

3.13.  AERO Interface MTU and Fragmentation

   The AERO interface is the node's attachment to the AERO link.  For
   AERO link neighbor underlying interface paths that do not require
   encapsulation, the AERO interface sends unencapsulated IP packets.
   For other paths, the AERO interface acts as a tunnel ingress when it
   sends packets to the neighbor and as a tunnel egress when it receives
   packets from the neighbor.

   AERO interfaces configure an MTU the same as for any IP interface,
   however the MTU does not reflect the physical size of any links in
   the path but rather determines the maximum size for reassembly.  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)



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   message returned to support the process known as IP Path MTU
   Discovery (PMTUD) [RFC1191][RFC8201].  However, PTB messages may be
   crafted for malicious purposes 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 three 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.  Finally, link-layer fragmentation (aka link adaptation)
   occurs at a layer below IP and is coordinated between underlying data
   link endpoints.

   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
   1280 bytes unless there is operational assurance that a larger size
   can traverse the link along all paths.

   All AERO interfaces on the link 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
   interfaces MUST configure an MRU large enough to reassemble packets
   up to (MTU+ENCAPS) bytes in length; nodes that cannot configure a
   large-enough MRU MUST NOT enable an AERO interface.  For example, for
   an MTU of 1500 bytes an appropriate MRU might be 2KB.





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   The network layer proceeds as follows when it forwards an IP packet
   to the AERO interface.  For each IPv4 packet that is larger than the
   AERO interface MTU and with DF 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 AERO interface, if the neighbor is
   reached via an underlying interface that does not require
   encapsulation the AERO interface proceeds according to the underlying
   interface MTU.  If the packet is no larger than the underlying
   interface MTU, the AERO interface presents the packet to the
   underlying interface.  Otherwise, for IPv4 packets with DF set to 0
   the AERO interface uses IPv4 fragmentation to break the packet into
   fragments no larger than the underlying interface MTU.  For other
   packets, the AERO interface either performs link adaptation or drops
   the packet and returns a PTB message to the original source.  (If the
   original source corresponds to a local application, the PTB would
   appear to have originated from a router on the path when in fact it
   was locally generated from within the AERO interface.)  In the same
   way, when a packet that has been admitted into the AERO link reaches
   a target neighbor but is too large to be delivered over the final-hop
   underlying interface, the target either performs link adaptation or
   drops the packet and returns a PTB.  Link adaptation is preferred in
   both cases when possible to avoid packet loss.

   For underlying interfaces that require encapsulation, the AERO
   interface (acting as a tunnel ingress) instead encapsulates the
   packet and performs path MTU procedures according to the specific
   encapsulation format.  For INET interfaces, the ingress encapsulates
   the packet in a SPAN header.  If the SPAN packet is larger than the
   MSU, the ingress source fragments the SPAN 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 encapsulates each SPAN packet/fragment
   in an INET header and admits them into the tunnel.  For IPv4, the
   ingress sets the DF bit to 0 in the INET 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.

   By fragmenting at the SPAN layer instead of lower layers, standard
   IPv6 fragmentation and reassembly [RFC8200] ensures that IPv4 issues
   such as data corruption due to reassembly misassociations will not



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   occur [RFC6864][RFC4963].  The ingress sends each fragment with
   minimal delay so that individual fragments are unlikely to be
   diverted to different destinations due to routing fluctuations.

   Since the SPAN header and IPv6 fragment extension header reduces the
   room available for packet data, but the original source has no way to
   control its insertion, the ingress MUST include their lengths in
   ENCAPS even for packets in which the header is absent.

3.13.1.  AERO MTU Requirements

   In light of the above considerations, AERO interfaces SHOULD
   configure an MTU of 9180 bytes.  This means that the AERO interface
   MUST be capable of reassembling original IP packets up to 9180 bytes
   in length.  When an IP packet is admitted into an AERO interface, the
   interface encapsulates the packet using SPAN encapsulation and
   fragments the encapsulated packet into fragments that are no larger
   than 1280 bytes.  The fragments will be reassembled by the tunnel
   egress that services the final destination.

   AERO Clients behind Proxys MAY configure an MTU smaller than 9180
   (but no smaller than IP minimum link MTU).  If Clients configure a
   diversity of MTUs (e.g., 1280, 1500, 4KB, 8KB, etc.) then neighbors
   on the link would appear to have multiple MTUs.  IPv6 Path MTU
   Discovery [RFC8201] accounts for this possibility since MTU discovery
   must be performed even between nodes that appear to be connected to
   the same link.

   Applications that cannot tolerate loss in the network due to MTU
   restrictions should restrict themselves to sending packets no larger
   than the IP minimum link MTU, i.e., even if the current path MTU
   would appear to support a larger size.  This is due to the fact that
   routing changes could cause the path to traverse links with smaller
   MTUs at any given point in time.

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.





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

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

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





























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

         Figure 5: AERO Interface Link-Layer Error Message Format

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

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

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

   o  When an AERO node receives persistent link-layer Destination
      Unreachable messages in response to encapsulated packets that it
      sends to one of its asymmetric neighbor correspondents, the node
      should process the message as an indication that a path may be
      failing, and optionally initiate NUD over that path.  If it
      receives Destination Unreachable messages over multiple paths, the
      node should allow future packets destined to the correspondent to
      flow through a default route and re-initiate route optimization.



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   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 3.19.5.

   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.

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

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

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

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,



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   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.  Clients receive the same service
   regardless of the Servers they select.

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

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

   The following sections specify the Client and Server behavior.

3.15.2.  AERO Client Behavior

   AERO Clients discover the addresses of Servers in the same manner
   described in [RFC5214].  Discovery methods include 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].  Alternatively, the Client can
   discover Server addresses through a layer 2 data link login exchange,
   or through a unicast RA response to a multicast/anycast RS as
   described below.  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 AERO link
   (e.g., "example.com").

   To associate with a Server, the Client acts as a requesting router to
   request MNPs.  The Client prepares an RS message with PD parameters
   and includes a Nonce and Timestamp option if the Client needs to
   correlate RA replies.  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) or subnet routers anycast (fe80::) as the network-layer



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   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 OMNI option in the RS message to register
   its link-layer information with the Server.  The first ifIndex-tuple
   MUST correspond to the underlying interface over which the Client
   will send the RS message.  The Client MAY include additional ifIndex-
   tuples specific to other underlying interfaces.  When encapsulation
   is used, the Client also includes an SLLAO with a single ifIndex-
   tuple corresponding to the first OMNI option ifIndex-tuple, then
   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 directly via Direct
   interfaces, via INET encapsulation for NATed interfaces, via a VPN
   for VPNed interfaces, via a Proxy for proxyed interfaces or via a
   Relay for native interfaces) and waits for an RA message reply (see
   Section 3.15.3).  The Client retries 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 Server's
   encapsulation and/or link-layer addresses as the link-layer address.
   The Client records the RA Router Lifetime field value in the neighbor
   cache entry as the time for which the Server has committed to
   maintaining the MNP in the routing system.  The Client then
   autoconfigures AERO addresses for each of the delegated MNPs and
   assigns them to the AERO interface.  The Client also caches any MSPs
   included in Route Information Options (RIOs) [RFC4191] as MSPs to
   associate with the AERO link, and assigns the MTU value in the MTU
   option to its AERO interface while configuring an appropriate MRU.

   The Client then registers additional underlying interfaces with the
   Server by sending RS messages via each additional interface.  The RS
   messages include the same parameters as for the initial RS/RA
   exchange, but with destination address set to the Server's AERO
   address and with the initial OMNI option ifIndex-tuple corresponding
   to the underlying interface.

   The Client examines the P bit in the RA message flags field.  If the
   P bit is set to 1, this indicates that the Server received an RS
   message with an SLLAO in which the first ifIndex-tuple addressing



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   information did not match the information in the encapsulation
   headers.

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

   After the Client registers its underlying interfaces, it may wish to
   change one or more registrations, e.g., if an 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
   underlying interface.  The RS includes an OMNI option with a first
   ifIndex-tuple specific to the selected interface, and MAY include any
   additional ifIndex-tuples specific to other underlying interfaces.
   The Client includes fresh ifIndex-tuple values to update the Server's
   neighbor cache entry.  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 discontinue use of a Server it issues an RS
   message over any underlying interface with an OMNI option with R set
   to 0.  When the Server processes the message, it releases the MNP,
   sets the symmetric neighbor cache entry state for the Client to
   DEPARTED and returns an RA reply with Router Lifetime set to 0.
   After a short delay (e.g., 2 seconds), the Server withdraws the MNP
   from the routing system.

3.15.3.  AERO Server Behavior

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

   When a Server receives a prospective Client's RS message on its AERO
   interface, it SHOULD return an immediate RA reply with Router
   Lifetime set to 0 if it is currently too busy or otherwise unable to
   service the Client.  Otherwise, the Server authenticates the RS
   message and processes the PD parameters.  The Server first determines
   the correct MNPs to delegate to the Client by searching the Client
   database.  When the Server delegates the MNPs, it also creates an IP



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   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 an IPv6 forwarding table entry for each MNP.  For IPv4, the
   Server creates 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 ifIndex-tuple in the RS OMNI option.  The Server
   also records the actual INET header source address and port number in
   the neighbor cache entry.  If an SLLAO option was present, the Server
   also compares the SLLAO address information for the first ifIndex-
   tuple with the INET header information and sets the P bit in the
   flags field of the RA message if the information was different.

   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 OMNI
   option with an ifIndex-tuple with ifIndex set to 0.  The Server then
   includes one or more RIOs that encode the MSPs for the AERO link,
   plus an MTU option for the link MTU (see Section 3.13).  The Server
   finally forwards the message to the Client using SPAN/INET, INET, or
   NULL encapsulation as necessary.

   After the initial RS/RA exchange, the Server maintains the symmetric
   neighbor cache entry for the Client.  If the Client (or Proxy) issues
   additional NS/RS messages, the Server resets ReachableTime.  If the
   Client (or Proxy) issues an RS with PD release indication (e.g., by
   including an OMNI option with a release indication), or if the Client
   becomes unreachable, the Server sets the Client's symmetric neighbor
   cache entry to the DEPARTED state.  After a short delay (e.g., 2
   seconds), the Server withdraws the MNP from the routing system.

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








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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 OMNI
   option and SLLAO into the Interface-Id option, then forwards the
   message to the DHCPv6 server.

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

3.16.  The AERO Proxy

   Clients may connect to ANETs that require a perimeter security
   gateway to enable communications to Servers in outside INETs.  In
   that case, the ANET can employ an AERO Proxy.  The Proxy is located
   at the ANET/INET border and listens for RS messages originating from
   or RA messages destined to ANET Clients.  The Proxy acts on these
   control messages as follows:

   o  when the Proxy receives an RS message from a new ANET Client, it
      first authenticates the message then examines the 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 or subnet routers



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      anycast, the Proxy selects a "nearby" Server that is likely to be
      a good candidate to serve the Client and replaces the 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 inserts an SLLAO in the RS message with a single ifIndex-
      tuple matching the first ifIndex-tuple in the OMNI option and with
      the Link Layer Address and Port Number fields set to 0.  The Proxy
      finally encapsulates the (proxyed) RS message in a SPAN header
      with destination set to the Server's SPAN address then forwards
      the message into 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 back to the Proxy
      via the SPAN.

   o  when the Proxy receives the RA message, it matches the message
      with the RS that created the proxy neighbor cache entry.  The
      Proxy then caches the PD route information as a mapping from the
      Client's MNPs to the Client's ANET address, and sets the neighbor
      cache entry state to REACHABLE.  The Proxy then forwards the
      (proxyed) 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.  The Proxy instead 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 send RS or
   unsolicited NA messages to update the Server's symmetric neighbor
   cache entries on behalf of the Client and/or to convey QoS updates.
   If the Server ceases to send solicited RA responses, the Proxy marks
   the Server as unreachable and sends an unsolicited RA with Router
   Lifetime set to zero to inform Clients that this Server is no longer
   able to provide service.  Although the Proxy engages in 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.

   If the Client becomes unreachable, the Proxy sets the neighbor cache
   entry state to DEPARTED and retains the entry for DEPARTTIME seconds.
   While the state is DEPARTED, the Proxy forwards any packets destined



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   to the Client to a Relay.  The Relay in turn forwards the packets to
   the Client's current Server.  When DepartTime expires, the Proxy
   deletes the neighbor cache entry and discards any further packets
   destined to this (now forgotten) Client.

   When a neighbor cache entry transitions to DEPARTED, some of the
   fragments of a multiple fragment packet may have already arrived at
   the Proxy while others are en route to the Client's new location.
   However, no special attention in the reassembly algorithm is
   necessary when re-routed packets are simply treated as loss.  Since
   the fragments of a multiple-fragment packet are sent in minimal
   inter-packet delay bursts, such occasions will be rare.

   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.

   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 or subnet router anycast.  The Client includes
   an OMNI option formatted as specified in
   [I-D.templin-atn-aero-interface].

   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 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.






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3.16.1.  Detecting and Responding to Server Failures

   In environments where fast recovery from Server failure is required,
   Proxys SHOULD use proactive Neighbor Unreachability Detection (NUD)
   to track Server reachability in a similar fashion as for
   Bidirectional Forwarding Detection (BFD) [RFC5880].  Proxys can then
   quickly detect and react to failures so that cached information is
   re-established through alternate paths.  The NUD control messaging is
   carried only over well-connected ground domain networks (i.e., and
   not low-end aeronautical radio links) and can therefore be tuned for
   rapid response.

   Proxys perform proactive NUD with Servers for which there are
   currently active ANET Clients by sending continuous NS messages in
   rapid succession, e.g., one message per second.  The Proxy sends the
   NS message via the SPAN with the Proxy's AERO address as the source
   and the AERO address of the Server as the destination.  If the Server
   fails (i.e., if the Proxy ceases to receive solicited NA messages),
   the Proxy can quickly inform Clients by sending RA messages on the
   ANET interface.  The Proxy sends RA messages with source address set
   to the Server's address, destination address set to all-nodes
   multicast, and Router Lifetime set to 0.

   The Proxy SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA messages
   separated by small delays [RFC4861].  Any Clients on the ANET that
   have been using the (now defunct) Server will receive the RA messages
   and associate with a new Server.

3.17.  AERO Route Optimization

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

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

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

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

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

   The route optimization procedure is conducted between the ROS and the
   target Server/Gateway acting as a Route Optimization Responder (ROR)
   in the same manner as for IPv6 ND Address Resolution and using the



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   same NS/NA messaging.  The target may either be a MNP Client serviced
   by a Server, or a non-MNP correspondent reachable via a Gateway.

   The procedures are specified in the following sections.

3.17.1.  Route Optimization Initiation

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

   When the ROS sends an NS, it includes the AERO address of the ROS as
   the source address (e.g., fe80::1) and the AERO address corresponding
   to the data packet's destination address as the destination address
   (e.g., if the destination address is 2001:db8:1:2::1 then the
   corresponding AERO address is fe80::2001:db8:1:2).  The NS message
   includes an OMNI option with a single ifIndex-tuple with ifIndex set
   to 0.  The message includes a Nonce and Timestamp option if the ROS
   needs to correlate NA replies.

   The ROS then encapsulates the NS message in a SPAN header with source
   set to its own SPAN address and destination set to the data packet's
   destination address, then sends 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 NS message from the ROS, it discards the
   INET header and determines that the ROR is the next hop by consulting
   its standard IPv6 forwarding table for the SPAN header destination
   address.  The Relay then forwards the SPAN message toward the ROR the
   same as for any IPv6 router.  The final-hop Relay in the SPAN will
   deliver the message via a secured tunnel to the ROR.

3.17.3.  Processing the NS and Sending the NA

   When the ROR receives the NS message, it examines the AERO
   destination address to determine whether it has a neighbor cache
   entry and/or route that matches the target; if not, it drops the NS
   message.  Next, if the target belongs to an MNP Client neighbor in
   the DEPARTED state the ROR changes the NS message SPAN destination
   address to the SPAN address of the Client's new Server, forwards the
   message into the SPAN and returns from processing.

   If the target belongs to an MNP Client neighbor in the REACHABLE
   state, the ROR instead adds the AERO source address to the target
   Client's Report List with time set to ReportTime.  If the target
   belongs to a non-MNP route, the ROR continues processing without



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   adding an entry to the Report List.  The ROR then prepares a
   solicited NA message to send back to the ROS but does not create a
   neighbor cache entry.  The ROR sets the NA source address to the
   destination AERO address of the NS, and includes the Nonce value
   received in the NS plus the current Timestamp.

   If the target belongs to an MNP Client, the ROR then includes an OMNI
   option with prefix information set according to the MNP prefix
   length; otherwise, it sets it to the maximum of the non-MNP prefix
   length and 64.  (Note that a /64 limit is imposed to avoid causing
   the ROS to set short prefixes (e.g., "default") that would match
   destinations for which the routing system includes more-specific
   prefixes.)

   The ROR next includes a first ifIndex-tuple in the OMNI option with
   with ifIndex set to 0.  If the target belongs to an MNP Client, the
   ROR next includes additional ifIndex-tuples in the OMNI option for
   the target Client's underlying interfaces with current information
   for each interface

   The ROR then includes a TLLAO option with ifIndex-tuples in one-to-
   one correspondence with the tuples that appear in the OMNI option.
   For NATed, VPNed and Direct interfaces, the link layer addresses are
   the SPAN address of the ROR.  For Proxyed and native interfaces, the
   link-layer addresses are the SPAN addresses of the Proxys and the
   Client's native interfaces.

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

3.17.4.  Relaying the NA

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

3.17.5.  Processing the NA

   When the ROS receives the solicited NA message, it caches the source
   SPAN address then discards the INET and SPAN headers.  The ROS next
   verifies the Nonce and Timestamp values (if present), then creates an
   asymmetric neighbor cache entry for the ROR and caches all
   information found in the solicited NA OMNI and TLLAO options.  The



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   ROS finally sets the asymmetric neighbor cache entry lifetime to
   ReachableTime seconds.

3.17.6.  Route Optimization Maintenance

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

   While new data packets destined to the target are flowing through the
   ROS, it sends additional NS messages to the ROR before ReachableTime
   expires to receive a fresh solicited NA message the same as described
   in the previous sections.  (Route optimization refreshment strategies
   are an implementation matter, with a non-normative example given in
   Appendix B.1).  The ROS uses the cached SPAN address of the ROR as
   the NS SPAN destination address, and sends up to MAX_UNICAST_SOLICIT
   NS messages separated by 1 second until an NA is received.  If no NA
   is received, the ROS assumes the current ROR has become unreachable
   and deletes the neighbor cache entry.  Subsequent data packets will
   trigger a new route optimization per Section 3.17.1 to discover a new
   ROR while initial data packets travel over a suboptimal route.

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

   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 ROS simply discards the unsolicited NA.
   Cases in which unsolicited NA messages are generated are specified in
   Section 3.19.

   In this arrangement, the ROS holds an asymmetric neighbor cache entry
   for the ROR, but the ROR does not hold an asymmetric neighbor cache
   entry for the ROS.  The route optimization neighbor relationship is
   therefore asymmetric and unidirectional.  If the target node also has
   packets to send back to the source node, then a separate route
   optimization procedure is performed in the reverse direction.  But,



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   there is no requirement that the forward and reverse paths be
   symmetric.

3.18.  Neighbor Unreachability Detection (NUD)

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

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

   When a ROS sends an NS message used for NUD, it uses its AERO
   addresses as the IPv6 source address and the AERO address
   corresponding to a target link-layer address as the destination.  For
   each target link-layer address, the source node encapsulates the NS
   message in SPAN/INET headers with its own SPAN address as the source
   and the SPAN address of the target as the destination, If the target
   is located within the same SPAN segment, the source sets the INET
   address of the target as the destination; otherwise, it sets the INET
   address of a Relay as the destination.  The source then forwards the
   message into the SPAN.

   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 3.12.

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

3.19.  Mobility Management and Quality of Service (QoS)

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



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   maintain the DMM profile, and the AERO routing system tracks all
   current Client/Server peering relationships.

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

   Mobility management considerations are specified in the following
   sections.

3.19.1.  Mobility Update Messaging

   Servers acting as MAPs accommodate Client mobility and/or QoS change
   events by sending unsolicited NA messages to each ROS in the target
   Client's Report List.  When a MAP sends an unsolicited NA message, it
   sets the IPv6 source address to the Client's AERO address and sets
   the IPv6 destination address to all-nodes multicast (ff02::1).  The
   MAP also includes an OMNI option with a first ifIndex-tuple with
   ifIndex set to 0, and with additional ifIndex-tuples for the target
   Client's remaining interfaces.  The MAP then includes a TLLAO with
   corresponding ifIndex-tuples, with the link layer address of the
   first tuple set to the MAP's SPAN address and with link layer
   addresses of the remaining tuples set to the corresponding target
   SPAN addresses.  The MAP finally encapsulates the message in a SPAN
   header with source set to its own SPAN address and destination set to
   the SPAN address of the ROS, then sends the message to a Relay.

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

   When the 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 OMNI option and TLLAO information to
   update 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.




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   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
   according to its stale neighbor cache information.  When
   ReachableTime is close to expiring, the ROS will re-initiate route
   optimization and receive fresh state information.

   In addition to sending unsolicited NA messages to the current set of
   ROSs for the Client, the MAP also sends unsolicited NAs to the former
   link-layer address for any ifIndex-tuple for which the link-layer
   address has changed.  The NA messages update Proxys or Servers that
   cannot easily detect (e.g., without active probing) when a formerly-
   active Client has departed.

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

   When a Client needs to change its ANET addresses and/or QoS
   preferences (e.g., due to a mobility event), either the Client or its
   Proxys send RS messages to the Server via the SPAN with an OMNI
   option and SLLAO that include an ifIndex-tuple with the new link
   quality and address information.

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

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

3.19.3.  Bringing New Links Into Service

   When a Client needs to bring new underlying interfaces into service
   (e.g., when it activates a new data link), it sends an RS message to
   the Server via the underlying interface with an OMNI option with
   appropriate link quality values and with an SLLAO (if necessary) with
   link-layer address information for the new link..

3.19.4.  Removing Existing Links from Service

   When a Client needs to remove existing underlying interfaces from
   service (e.g., when it de-activates an existing data link), it sends
   an RS message to its Server with an OMNI option with appropriate link
   quality values.

   If the Client needs to send RS messages over an underlying interface
   other than the one being removed from service, it MUST include an
   ifIndex-tuple for the sending interface as the first tuple and



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   include additional ifIndex-tuples with appropriate link quality
   values for any underlying interfaces being removed from service.

3.19.5.  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 also includes a
   notification identifier in the RS message OMNI option per
   [I-D.templin-atn-aero-interface] if it wants the new Server to notify
   the old Server.

   When the new Server receives the Client's RS message, it responds by
   returning an RA as specified in Section 3.15.3.  If the Client's RS
   includes a notification identifier, the new Server also prepares an
   RS to send to the old Server.  The RS message includes the Client's
   AERO address as the source address, the old Server's AERO address as
   the destination address, and an OMNI option and SLLAO with an
   ifIndex-tuple with ifIndex set to 0.  The OMNI option includes a
   release indication, and the SLLAO includes the link-layer address of
   the new Server.  The new Server retries up to MAX_RTR_SOLICITATIONS
   attempts until an RA is received.  (Note that the Client can
   alternatively send RS messages with a release indication to the old
   Server on its own behalf, however, this additional Client messaging
   may be undesirable in some environments.)

   When the old Server processes the RS, it 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
   DepartTime to DEPARTTIME seconds.  The old Server then returns an
   immediate RA message with Router Lifetime set to 0.  After a short
   delay (e.g., 2 seconds) the old Server withdraws the Client's MNP
   from the routing system.  After DepartTime expires, the old Server
   deletes the symmetric neighbor cache entry.

   The old Server also sends unsolicited NA messages to all ROSs in the
   Client's Report List with an OMNI option and TLLAO with a single
   ifIndex-tuple with ifIndex set to 0 and with the link-layer address
   of the new Server.  When the ROS receives the NA, it caches the
   address of the new Server in the existing asymmetric neighbor cache
   entry and marks the entry as STALE.  Subsequent data packets will
   then flow according to any existing cached link-layer information and
   trigger a new NS/NA exchange via the new 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




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   significant distance, movement to a new geographic region, movement
   to a new SPAN segment, etc.

   When a Client moves to a new Server, some of the fragments of a
   multiple fragment packet may have already arrived at the old Server
   while others are en route to the new Server.  However, no special
   attention in the reassembly algorithm is necessary when re-routed
   fragments are simply treated as loss.  Since the fragments of a
   multiple-fragment packet are sent with minimal inter-packet delay,
   such occasions will be rare.

3.20.  Multicast

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

3.20.1.  Source-Specific Multicast (SSM)

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

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



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   with the SPAN addresses corresponding to any ifIndex-tuples that are
   currently servicing S.

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

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

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

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



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   same as if it were receiving a fresh Join message from a node on a
   downstream link.

3.20.2.  Any-Source Multicast (ASM)

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

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

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

3.20.3.  Bi-Directional PIM (BIDIR-PIM)

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








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3.21.  Operation over Multiple AERO Links (VLANs)

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

   The Relays, Servers and Proxys on each AERO link can assign AERO and
   SPAN addresses that use the same or different numberings from those
   on other links.  Since the links are mutually independent there is no
   requirement for avoiding inter-link address duplication, e.g., the
   same AERO address such as fe80::1000 could be used to number distinct
   nodes that connect to different AERO 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 (e.g., IEEE 802.1Q) and/or through
   assignment of distinct sets of MSPs on each link.  This gives rise to
   the opportunity for supporting multiple redundant networked paths,
   where each VLAN is distinguished by a different label (e.g., colors
   such as Red, Green, Blue, etc.).  In particular, the Client can tag
   its RS messages with the appropriate label to cause the network to
   select the desired VLAN.

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

   In a first alternative, if each AERO link services different MSPs,
   then the Client can receive a distinct MNP from each of the links.
   IP routing will therefore assure that the correct Red/Green/Blue/etc.
   network is used for both outbound and inbound traffic.  This can be
   accomplished using existing technologies and approaches, and without
   requiring any special supporting code in correspondent nodes or
   Relays.

   In a second alternative, if each AERO link services the same MSP(s)
   then each link could assign a distinct "AERO Link Anycast" address
   that is configured by all Relays on the link.  Correspondent nodes
   then include a "type 4" routing header with the Anycast address for
   the AERO link as the IPv6 destination and with the address of the
   target encoded as the "next segment" in the routing header
   [RFC8402][I-D.ietf-6man-segment-routing-header].  Standard IP routing
   will then direct the packet to the nearest Relay for the correct AERO



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   link, which will replace the destination address with the target
   address then forward the packet to the target.

3.22.  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 can
   return 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.

3.23.  Transition Considerations

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

   This gives rise to the opportunity to eventually distribute native IP
   addresses to all nodes, and to present a unified AERO link view
   (bridged by the SPAN) even if the INET partitions 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 partition is transitioned to a
   native IP protocol version and addressing scheme that is compatible
   with the AERO link MNP-based addressing scheme, the partition and
   AERO link can be joined by Gateways.

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





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3.24.  Detecting and Reacting to Server and Relay Failures

   In environments where rapid failure recovery is required, Servers and
   Relays SHOULD use Bidirectional Forwarding Detection (BFD) [RFC5880].
   Nodes that use BFD can quickly detect and react to failures so that
   cached information is re-established through alternate nodes.  BFD
   control messaging is carried only over well-connected ground domain
   networks (i.e., and not low-end radio links) and can therefore be
   tuned for rapid response.

   Servers and Relays maintain BFD sessions in parallel with their BGP
   peerings.  If a Server or Relay fails, BGP peers will quickly re-
   establish routes through alternate paths the same as for common BGP
   deployments.

   Proxys SHOULD use proactive NUD for Servers for which there are
   currently active ANET Clients in a manner that parallels BFD, i.e.,
   by sending unicast NS messages in rapid succession to receive
   solicited NA messages.  If a Server fails, the Proxy will cease to
   receive NA messages and can quickly inform Clients of the outage by
   sending RA messages on the ANET interface.

   The Proxy sends RA messages with source address set to the Server's
   address, destination address set to all-nodes multicast, and Router
   Lifetime set to 0.  The Proxy SHOULD send
   MAX_FINAL_RTR_ADVERTISEMENTS RA messages separated by small delays
   [RFC4861].  Any Clients on the ANET interface that have been using
   the (now defunct) Server will receive the RA messages and associate
   with a new Server.

4.  Implementation Status

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

5.  IANA Considerations

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

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

   No further IANA actions are required.



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6.  Security Considerations

   AERO Relays configure secured tunnels with AERO Servers and Proxys
   within their local SPAN segments.  Applicable secured tunnel
   alternatives include IPsec [RFC4301], TLS/SSL [RFC8446], DTLS
   [RFC6347], etc.  The AERO Relays of all SPAN segments in turn
   configure secured tunnels for their neighboring AERO Relays across
   the SPAN.  Therefore, packets that traverse the SPAN between any pair
   of AERO link neighbors are already secured.

   AERO Servers, Gateways and Proxys targeted by a route optimization
   may also receive packets directly from the INET partitions instead of
   via the SPAN.  For INET partitions that apply effective ingress
   filtering to defeat source address spoofing, the simple data origin
   authentication procedures in Section 3.11 can be applied.  This
   implies that the ROS list must be maintained consistently by all
   route optimization targets within the same INET partition, and that
   the ROS list must be securely managed by the partition administrative
   authority.

   For INET partitions that cannot apply effective ingress filtering,
   the two options for securing communications include 1) disable route
   optimization so that all traffic is conveyed over secured tunnels via
   the SPAN, or 2) enable on-demand secure tunnel creation between INET
   partition neighbors.  Option 1) would result in longer routes than
   necessary and traffic concentration on critical infrastructure
   elements.  Option 2) could be coordinated by establishing a secured
   tunnel on-demand instead of performing an NS/NA exchange in the route
   optimization procedures.  Procedures for establishing on-demand
   secured tunnels are out of scope.

   AERO Clients that connect to secured enclaves need not apply security
   to their ND messages, since the messages will be intercepted by a
   perimeter Proxy that applies security on its outward-facing
   interface.  AERO Clients located outside of secured enclaves SHOULD
   use symmetric network and/or transport layer security services, but
   when there are many prospective neighbors with dynamically changing
   connectivity an asymmetric security service such as SEND may be
   needed (see: Appendix B.6).

   Application endpoints SHOULD use application-layer security services
   such as TLS/SSL, DTLS or SSH [RFC4251] to assure the same level of
   protection as for critical secured Internet services.  AERO Clients
   that require host-based VPN services SHOULD use symmetric network
   and/or transport layer security services such as IPsec, TLS/SSL,
   DTLS, etc.  AERO Proxys and Servers can also provide a network-based
   VPN service on behalf of the Client, e.g., if the Client is located




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   within a secured enclave and cannot establish a VPN on its own
   behalf.

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

   AERO Gateways must implement ingress filtering to avoid a spoofing
   attack in which spurious SPAN messages are injected into an AERO link
   from an outside attacker.  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 and ROS lists 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 in a similar
   fashion as in [RFC5214] (e.g., through layer 2 data link login
   messaging, secure upload of a static file, DNS lookups, etc.).  The
   ROS list can be conveyed to Servers and Proxys through administrative
   action, secured file distribution, 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.

7.  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,



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   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, Don Dillenburg, Joe Dudkowski, Wen Fang, Samad Farooqui,
   Anthony Gregory, Jeff Holland, Seth Jahne, Brian Jaury, Greg
   Kimberly, Ed King, Madhuri Madhava Badgandi, Laurel Matthew, Gene
   MacLean III, Rob Muszkiewicz, Vijay Rajagopalan, Sean O'Sullivan,
   Greg Saccone, Rod Santiago, Kent Shuey, Brian Skeen, Mike Slane,
   Carrie Spiker, Katie Tran, Brendan Williams, Julie Wulff, Yueli Yang,
   Eric Yeh and other members of the BR&T and BIT mobile networking
   teams.  Kyle Bae, Wayne Benson, Katie Tran and Eric Yeh are
   especially acknowledged for implementing the AERO functions as
   extensions to the public domain OpenVPN distribution.

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

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

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

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

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

   o  AERO, First Edition [RFC6706]

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

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



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

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

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

8.  References

8.1.  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>.

   [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>.

   [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>.



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

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.

8.2.  Informative References

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

   [I-D.ietf-6man-segment-routing-header]
              Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", draft-ietf-6man-segment-routing-header-26 (work in
              progress), October 2019.

   [I-D.ietf-dmm-distributed-mobility-anchoring]
              Chan, A., Wei, X., Lee, J., Jeon, S., and C. Bernardos,
              "Distributed Mobility Anchoring", draft-ietf-dmm-
              distributed-mobility-anchoring-14 (work in progress),
              November 2019.

   [I-D.ietf-intarea-gue]
              Herbert, T., Yong, L., and O. Zia, "Generic UDP
              Encapsulation", draft-ietf-intarea-gue-09 (work in
              progress), October 2019.





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   [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-10 (work in
              progress), September 2019.

   [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-05 (work in progress), January 2020.

   [I-D.templin-6man-dhcpv6-ndopt]
              Templin, F., "A Unified Stateful/Stateless Configuration
              Service for IPv6", draft-templin-6man-dhcpv6-ndopt-09
              (work in progress), January 2020.

   [I-D.templin-atn-aero-interface]
              Templin, F. and T. Whyman, "Transmission of IPv6 Packets
              over Overlay Multilink Network (OMNI) Interfaces", draft-
              templin-atn-aero-interface-12 (work in progress), January
              2020.

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

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

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

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







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   [I-D.templin-v6ops-pdhost]
              Templin, F., "IPv6 Prefix Delegation and Multi-Addressing
              Models", draft-templin-v6ops-pdhost-25 (work in progress),
              January 2020.

   [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>.

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

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

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





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   [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>.

   [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>.






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   [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>.

   [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>.







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

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

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

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

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

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

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

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

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

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






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

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

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

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

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

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

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

   [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>.





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

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

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

Appendix A.  AERO Alternate Encapsulations

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

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

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
















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

        Minimal Encapsulation in IPv4      Minimal Encapsulation in IPv6


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

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

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


                 Figure 7: Minimal Encapsulation Using GRE

   Alternate encapsulation may be preferred in environments where GUE
   encapsulation would add unnecessary overhead.  For example, certain
   low-bandwidth wireless data links may benefit from a reduced
   encapsulation overhead.





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   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.  Non-Normative Considerations

   AERO can be applied to a multitude of Internetworking scenarios, with
   each having its own adaptations.  The following considerations are
   provided as non-normative guidance:

B.1.  Implementation Strategies for Route Optimization

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

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

   The monitoring of the neighbor data packet traffic therefore becomes
   an asymmetric ongoing process during the neighbor cache entry
   lifetime.  If the neighbor cache entry expires, future data packets



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   will trigger a new NS/NA exchange while the packets themselves are
   delivered over a longer path until route optimization state is re-
   established.

B.2.  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 underlying 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 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.

B.3.  Direct Underlying Interfaces

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

B.4.  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 SPAN segment 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 6).





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B.5.  Operation on AERO Links with /64 ASPs

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

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

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

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

B.6.  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 6).  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




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   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.

B.7.  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.

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

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





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B.8.  AERO Server Failure Implications

   AERO Servers may appear as a single point of failure in the
   architecture, but such is not the case since all Servers on the link
   provide identical services and loss of a Server does not imply
   immediate and/or comprehensive communication failures.  Although
   Clients typically associate with a single Server at a time, Server
   failure is quickly detected and conveyed by Bidirectional Forward
   Detection (BFD) and/or proactive NUD allowing Clients to migrate to
   new Servers.

   If a Server fails, ongoing packet forwarding to Clients will continue
   by virtue of the asymmetric neighbor cache entries that have already
   been established in route optimization sources (ROSs).  If a Client
   also experiences mobility events at roughly the same time the Server
   fails, unsolicited NA messages may be lost but proxy neighbor cache
   entries in the DEPARTED state will ensure that packet forwarding to
   the Client's new locations will continue for up to DEPARTTIME
   seconds.

   If a Client is left without a Server for an extended timeframe (e.g.,
   greater than REACHABLETIIME seconds) then existing asymmetric
   neighbor cache entries will eventually expire and both ongoing and
   new communications will fail.  The original source will continue to
   retransmit until the Client has established a new Server
   relationship, after which time continuous communications will resume.

   Therefore, providing many Servers on the link with high availability
   profiles provides resilience against loss of individual Servers and
   assurance that Clients can establish new Server relationships quickly
   in event of a Server failure.

B.9.  AERO Client / Server Architecture

   The AERO architectural model is client / server in the control plane,
   with route optimization in the data plane.  The same as for common
   Internet services, the AERO Client discovers the addresses of AERO
   Servers and selects one Server to connect to.  The AERO service is
   analogous to common Internet services such as google.com, yahoo.com,
   cnn.com, etc.  However, there is only one AERO service for the link
   and all Servers provide identical services.

   Common Internet services provide differing strategies for advertising
   server addresses to clients.  The strategy is conveyed through the
   DNS resource records returned in response to name resolution queries.
   As of January 2020 Internet-based 'nslookup' services were used to
   determine the following:




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   o  When a client resolves the domainname "google.com", the DNS always
      returns one A record (i.e., an IPv4 address) and one AAAA record
      (i.e., an IPv6 address).  The client receives the same addresses
      each time it resolves the domainname via the same DNS resolver,
      but may receive different addresses when it resolves the
      domainname via different DNS resolvers.  But, in each case,
      exactly one A and one AAAA record are returned.

   o  When a client resolves the domainname "ietf.org", the DNS always
      returns one A record and one AAAA record with the same addresses
      regardless of which DNS resolver is used.

   o  When a client resolves the domainname "yahoo.com", the DNS always
      returns a list of 4 A records and 4 AAAA records.  Each time the
      client resolves the domainname via the same DNS resolver, the same
      list of addresses are returned but in randomized order (i.e.,
      consistent with a DNS round-robin strategy).  But, interestingly,
      the same addresses are returned (albeit in randomized order) when
      the domainname is resolved via different DNS resolvers.

   o  When a client resolves the domainname "amazon.com", the DNS always
      returns a list of 3 A records and no AAAA records.  As with
      "yahoo.com", the same three A records are returned from any
      worldwide Internet connection point in randomized order.

   The above example strategies show differing approaches to Internet
   resilience and service distribution offered by major Internet
   services.  The Google approach exposes only a single IPv4 and a
   single IPv6 address to clients.  Clients can then select whichever IP
   protocol version offers the best response, but will always use the
   same IP address according to the current Internet connection point.
   This means that the IP address offered by the network must lead to a
   highly-available server and/or service distribution point.  In other
   words, resilience is predicated on high availability within the
   network and with no client-initiated failovers expected (i.e., it is
   all-or-nothing from the client's perspective).  However, Google does
   provide for worldwide distributed service distribution by virtue of
   the fact that each Internet connection point responds with a
   different IPv6 and IPv4 address.  The IETF approach is like google
   (all-or-nothing from the client's perspective), but provides only a
   single IPv4 or IPv6 address on a worldwide basis.  This means that
   the addresses must be made highly-available at the network level with
   no client failover possibility, and if there is any worldwide service
   distribution it would need to be conducted by a network element that
   is reached via the IP address acting as a service distribution point.

   In contrast to the Google and IETF philosophies, Yahoo and Amazon
   both provide clients with a (short) list of IP addresses with Yahoo



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   providing both IP protocol versions and Amazon as IPv4-only.  The
   order of the list is randomized with each name service query
   response, with the effect of round-robin load balancing for service
   distribution.  With a short list of addresses, there is still
   expectation that the network will implement high availability for
   each address but in case any single address fails the client can
   switch over to using a different address.  The balance then becomes
   one of function in the network vs function in the end system.

   The same implications observed for common highly-available services
   in the Internet apply also to the AERO client/server architecture.
   When an AERO Client connects to one or more ANETs, it discovers one
   or more AERO Server addresses through the mechanisms discussed in
   earlier sections.  Each Server address presumably leads to a fault-
   tolerant clustering arrangement such as supported by Linux-HA,
   Extended Virtual Synchrony or Paxos.  Such an arrangement has
   precedence in common Internet service deployments in lightweight
   virtual machines without requiring expensive hardware deployment.
   Similarly, common Internet service deployments set service IP
   addresses on service distribution points that may relay requests to
   many different servers.

   For AERO, the expectation is that a combination of the Google/IETF
   and Yahoo/Amazon philosophies would be employed.  The AERO Client
   connects to different ANET access points and can receive 1-2 Server
   AERO addresses at each point.  It then selects one AERO Server
   address, and engages in RS/RA exchanges with the same Server from all
   ANET connections.  The Client remains with this Server unless or
   until the Server fails, in which case it can switch over to an
   alternate Server.  The Client can likewise switch over to a different
   Server at any time if there is some reason for it to do so.  So, the
   AERO expectation is for a balance of function in the network and end
   system, with fault tolerance and resilience at both levels.

Appendix C.  Change Log

   << RFC Editor - remove prior to publication >>

   Changes from draft-templin-intarea-6706bis-19 to draft-templin-
   intrea-6706bis-20:

   o  Included new route optimization source and destination addressing
      strategy.  Now, route optimization maintenance uses the address of
      the existing Server instead of the data packet destination address
      so that less pressure is placed on the BGP routing system
      convergence time and Server constancy is supported.





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   o  Included new method for releasing from old MSE without requiring
      Client messaging.

   o  Included references to new OMNI interface spec (including the OMNI
      option).

   o  New appendix on AERO Client/Server architecture.

   Changes from draft-templin-intarea-6706bis-18 to draft-templin-
   intrea-6706bis-19:

   o  Changed Proxy/Server keepalives to use "proactive NUD" in a manner
      tha paralles BFD

   Changes from draft-templin-intarea-6706bis-17 to draft-templin-
   intrea-6706bis-18:

   o  Discuss how AERO option is used in relation to S/TLLAOs

   o  New text on Bidirectional Forwarding Detection (BFD)

   o  Cleaned up usage (and non-usage) of unsolicited NAs

   o  New appendix on Server failures

   Changes from draft-templin-intarea-6706bis-15 to draft-templin-
   intrea-6706bis-17:

   o  S/TLLAO now includes multiple link-layer addresses within a single
      option instead of requiring multiple options

   o  New unsolicited NA message to inform the old link that a Client
      has moved to a new link

   Changes from draft-templin-intarea-6706bis-14 to draft-templin-
   intrea-6706bis-15:

   o  MTU and fragmentation

   o  New details in movement to new Server

   Changes from draft-templin-intarea-6706bis-13 to draft-templin-
   intrea-6706bis-14:

   o  Security based on secured tunnels, ingress filtering, MAP list and
      ROS list





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

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

   o  Removed extraneous text in Section 3.7

   o  Added new detail to the forwarding algorithm in Section 3.9

   o  Clarified use of fragmentation

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

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

   o  Built out multicast section in detail.

   o  New Appendix on implementation considerations for route
      optimization.

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

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

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

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

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

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

   o  Changed Interface ID for Servers from 255 to 0xffff

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

   o  New Section on Multicast

   o  New Section on AERO Clients in the open Internetwork

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



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   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.

   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:



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   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

   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.




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   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:

   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.



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   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.

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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