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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720,                       May 12, 2020
           rfc6179, rfc6706 (if
           approved)
Intended status: Standards Track
Expires: November 13, 2020


             Asymmetric Extended Route Optimization (AERO)
                    draft-templin-intarea-6706bis-50

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, intelligent transportation systems,
   mobile Virtual Private Networks (VPNs) and many other applications.

Status of This Memo

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

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

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

   This Internet-Draft will expire on November 13, 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
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Asymmetric Extended Route Optimization (AERO) . . . . . . . .  11
     3.1.  AERO Link Reference Model . . . . . . . . . . . . . . . .  11
     3.2.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .  12
     3.3.  AERO Routing System . . . . . . . . . . . . . . . . . . .  13
     3.4.  AERO Addresses  . . . . . . . . . . . . . . . . . . . . .  15
     3.5.  Spanning Partitioned AERO Networks (SPAN) . . . . . . . .  16
       3.5.1.  SPAN Routing Topologies . . . . . . . . . . . . . . .  20
       3.5.2.  Segment Routing Over the SPAN . . . . . . . . . . . .  20
     3.6.  AERO Interface Characteristics  . . . . . . . . . . . . .  21
     3.7.  AERO Interface Initialization . . . . . . . . . . . . . .  26
       3.7.1.  AERO Server/Relay Behavior  . . . . . . . . . . . . .  26
       3.7.2.  AERO Proxy Behavior . . . . . . . . . . . . . . . . .  26
       3.7.3.  AERO Client Behavior  . . . . . . . . . . . . . . . .  26
       3.7.4.  AERO Bridge Behavior  . . . . . . . . . . . . . . . .  27
     3.8.  AERO Interface Neighbor Cache Maintenance . . . . . . . .  27
     3.9.  AERO Interface Encapsulation and Re-encapsulation . . . .  29
     3.10. AERO Interface Decapsulation  . . . . . . . . . . . . . .  30
     3.11. AERO Interface Data Origin Authentication . . . . . . . .  30
     3.12. AERO Interface MTU and Fragmentation  . . . . . . . . . .  31
     3.13. AERO Interface Forwarding Algorithm . . . . . . . . . . .  33
       3.13.1.  Client Forwarding Algorithm  . . . . . . . . . . . .  33
       3.13.2.  Proxy Forwarding Algorithm . . . . . . . . . . . . .  34
       3.13.3.  Server/Relay Forwarding Algorithm  . . . . . . . . .  35
       3.13.4.  Bridge Forwarding Algorithm  . . . . . . . . . . . .  36
     3.14. AERO Interface Error Handling . . . . . . . . . . . . . .  37
     3.15. AERO Router Discovery, Prefix Delegation and
           Autoconfiguration . . . . . . . . . . . . . . . . . . . .  39
       3.15.1.  AERO ND/PD Service Model . . . . . . . . . . . . . .  39
       3.15.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  40



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       3.15.3.  AERO Server Behavior . . . . . . . . . . . . . . . .  42
     3.16. The AERO Proxy  . . . . . . . . . . . . . . . . . . . . .  45
       3.16.1.  Detecting and Responding to Server Failures  . . . .  47
       3.16.2.  Point-to-Multipoint Server Coordination  . . . . . .  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  . . . . . . . . . . . . . . . . . .  51
       3.17.5.  Processing the NA  . . . . . . . . . . . . . . . . .  51
       3.17.6.  Route Optimization Maintenance . . . . . . . . . . .  51
     3.18. Neighbor Unreachability Detection (NUD) . . . . . . . . .  52
     3.19. Mobility Management and Quality of Service (QoS)  . . . .  53
       3.19.1.  Mobility Update Messaging  . . . . . . . . . . . . .  54
       3.19.2.  Announcing Link-Layer Address and/or QoS Preference
                Changes  . . . . . . . . . . . . . . . . . . . . . .  55
       3.19.3.  Bringing New Links Into Service  . . . . . . . . . .  55
       3.19.4.  Removing Existing Links from Service . . . . . . . .  55
       3.19.5.  Moving to a New Server . . . . . . . . . . . . . . .  56
     3.20. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  57
       3.20.1.  Source-Specific Multicast (SSM)  . . . . . . . . . .  57
       3.20.2.  Any-Source Multicast (ASM) . . . . . . . . . . . . .  58
       3.20.3.  Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . .  59
     3.21. Operation over Multiple AERO Links (VLANs)  . . . . . . .  59
     3.22. DNS Considerations  . . . . . . . . . . . . . . . . . . .  60
     3.23. Transition Considerations . . . . . . . . . . . . . . . .  61
     3.24. Detecting and Reacting to Server and Bridge Failures  . .  61
     3.25. AERO Clients on the Open Internet . . . . . . . . . . . .  62
       3.25.1.  Use of SEND and CGA  . . . . . . . . . . . . . . . .  65
     3.26. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . .  66
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  66
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  67
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  67
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  69
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  70
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  70
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  72
   Appendix A.  AERO Alternate Encapsulations  . . . . . . . . . . .  79
   Appendix B.  Non-Normative Considerations . . . . . . . . . . . .  81
     B.1.  Implementation Strategies for Route Optimization  . . . .  81
     B.2.  Implicit Mobility Management  . . . . . . . . . . . . . .  82
     B.3.  Direct Underlying Interfaces  . . . . . . . . . . . . . .  82
     B.4.  Operation on AERO Links with /64 ASPs . . . . . . . . . .  82
     B.5.  AERO Critical Infrastructure Considerations . . . . . . .  83
     B.6.  AERO Server Failure Implications  . . . . . . . . . . . .  84
     B.7.  AERO Client / Server Architecture . . . . . . . . . . . .  84
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  86
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  95



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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 such as intelligent transportation
   systems.  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 Relays that
   are seen as AERO link neighbors as well as Bridges that interconnect
   AERO link segments.  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 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 Relay on the link for efficient communications.  Fixed
   nodes forward packets destined to other AERO nodes to the nearest
   Relay, which forwards them through the cloud.  A mobile node's
   initial packets are forwarded through the Server, 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 Bridges 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 Relay/Server is on the optimal
   route from any correspondent node on the link, and provides a conduit
   between the underlying Internetwork and the SPAN.  To the underlying
   Internetwork, the Relay/Server is the source of a route to the MSP,




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   and hence uplink traffic to the mobile node is naturally routed to
   the nearest Relay/Server.

   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].  It can also be used to
   facilitate vehicular and pedestrian communications services for
   intelligent transportation systems.  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 minimal 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-



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      layer security services such as IEEE 802.1X and physical-layer
      security prevent unauthorized access internally while border
      network-layer security services such as firewalls and proxies
      prevent unauthorized outside access.

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

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

   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.  (Each INET partition is seen as a separate SPAN segment as
      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.

   INET encapsulation
      the encapsulation of a packet in an outer header or headers that
      can be routed within the scope of the local INET partition.

   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).



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   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
      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 Relay.

   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 over one or more underlying interfaces
      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 and mobility/multilink services 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
      Bridges.

   AERO Relay ("Relay")




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      an AERO Server that also provides forwarding services between
      nodes reached via the AERO link and correspondents on other links.
      AERO Relays 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 Relay advertises the MSP(s)
      to its downstream networks, and distributes all of its associated
      MNPs and non-MNP IP routes via BGP peerings with Bridges (i.e.,
      the same as for an AERO Server).

   AERO Bridge ("Bridge")
      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 Bridge forwards packets using standard IP forwarding.  As a
      bridge, the Bridge forwards packets over the AERO link without
      decrementing the IPv6 Hop Limit.  AERO Bridges peer with Servers
      and other Bridges to discover the full set of MNPs for the link as
      well as any non-MNPs that are reachable via Relays.

   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, and
      individual segments are joined by AERO Bridges.  Segment Routing
      [RFC8402][RFC8754] can be used to cause packets to visit selected
      hops on the SPAN.

   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 Client Prefix (SCP)
      a SPAN prefix formed from an AERO Client address.

   SPAN Address




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

   SPAN encapsulation
      the addition of an IPv6 header with SPAN source and destinations
      per [RFC2473].  SPAN encapsulation is often used in conjunction
      with INET encapsulation, or in raw form over ANET interfaces.

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

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

   link-layer address
      an IP address used as an encapsulation header source or
      destination address from the perspective of the AERO interface.
      When an upper layer protocol (e.g., UDP) is used as part of the
      encapsulation, the 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 Relay connects to the rest of the network via the AERO
      interface.  The Client/Relay 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.

   Route Optimization Source (ROS)



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      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 on
      behalf of a target MNP Client, or a Relay for a non-MNP
      destination.

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

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

   Mobility Service (MS)
      the collective set of all Servers, Proxys, Bridges and Relays that
      provide the AERO Service to Clients.

   Mobility Service Endpoint MSE)
      an individual Server, Proxy, Bridge or Relay in the Mobility
      Service.

   Throughout the document, the simple terms "Client", "Server",
   "Bridge", "Proxy" and "Relay" refer to "AERO Client", "AERO Server",
   "AERO Bridge", "AERO Proxy" and "AERO Relay", 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.  The terms "All-Routers multicast", "All-
   Nodes multicast", "Solicited-Node multicast" and "Subnet-Router
   anycast" are defined in [RFC4291] (with Link-Local scope assumed).
   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.





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3.  Asymmetric Extended Route Optimization (AERO)

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

3.1.  AERO Link Reference Model

                          +----------------+
                          | AERO Bridge B1 |
                          | Nbr: S1, S2, P1|
                          |(X1->S1; X2->S2)|
                          |      MSP M1    |
                          +-+---------+--+-+
       +--------------+     | Secured |  |     +--------------+
       |AERO Server S1|     | tunnels |  |     |AERO Server S2|
       |  Nbr: C1, B1 +-----+         |  +-----+  Nbr: C2, B1 |
       |  default->B1 |               |        |  default->B1 |
       |    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 Bridge B1 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).




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      Bridges use the SPAN service to bridge disjoint segments of a
      partitioned AERO link.

   o  AERO Servers S1 and S2 configure secured tunnels with Bridge B1
      and also provide mobility, multilink and default router services
      for their associated Clients C1 and C2.

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

   o  AERO Proxy P1 configures a secured tunnel with Bridge B1 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 Bridges, Servers, Clients and
   Proxys.

3.2.  AERO Node Types

   AERO Bridges provide hybrid routing/bridging services (as well as a
   security trust anchor) for nodes on an AERO link.  Bridges 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 Bridge also peers
   with Servers and other Bridges in a dynamic routing protocol instance
   to provide a Distributed Mobility Management (DMM) service for the
   list of active MNPs (see Section 3.3).  Bridges 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.  Bridges configure
   secured tunnels with Servers, Proxys and other Bridges; 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 and mobility/multilink
   services for AERO Client Mobile Nodes (MNs).  Each Server also peers
   with Bridges 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




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   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
   single functional unit.

   AERO Proxys provide a conduit for ANET Clients to associate with
   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 between Clients and the
   AERO link according to forwarding information in the neighbor cache.
   The Proxy function is specified in Section 3.16.

   AERO Relays are Servers that provide forwarding services between the
   AERO interface and INET/EUN interfaces.  Relays 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 Relay advertises the
   MSP(s) to its connected networks, and distributes all of its
   associated MNPs and non-MNP IP routes via BGP peerings with Bridges.

   AERO Bridges, Servers, Proxys and Relays 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 underlying interfaces with addresses that may change when
   the Client moves to a new network connection point.

3.3.  AERO Routing System

   The AERO routing system comprises a private instance of the Border
   Gateway Protocol (BGP) [RFC4271] that is coordinated between Bridges
   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 Bridges but does
   not peer with other Servers.  Each INET of a multi-segment AERO link
   must include one or more Bridges, which peer with the Servers and
   Proxys within that INET.  All Bridges 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 Bridges of different INETs peer with one another using eBGP.





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   Bridges 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 Bridge configures a
   black-hole route for each of its MSPs.  By black-holing the MSPs, the
   Bridge will maintain forwarding table entries only for the MNPs that
   are currently active, and packets destined to all other MNPs will
   correctly incur Destination Unreachable messages due to the black-
   hole route.  In this way, Servers have only partial topology
   knowledge (i.e., they know only about the MNPs of their directly
   associated Clients) and they forward all other packets to Bridges
   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
   Bridges.  Servers that are configured as Relays also redistribute
   non-MNP routes learned from non-AERO interfaces via their eBGP Bridge
   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 Relays 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 Bridges.  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 Bridge 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 Bridge can carry 1M or more routes, this
   means that at least 1M Clients can be serviced by a single set of
   Bridges.  A means of increasing scaling would be to assign a
   different set of Bridges for each set of MSPs.  In that case, each
   Server still peers with one or more Bridges, but institutes route
   filters so that BGP updates are only sent to the specific set of
   Bridges that aggregate the MSP.  For example, if the MSP for the AERO
   link is 2001:db8::/32, a first set of Bridges could service the MSP
   2001:db8::/40, a second set of Bridges could service
   2001:db8:0100::/40, a third set could service 2001:db8:0200::/40,
   etc.





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   Assuming up to 1K sets of Bridges, 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 Bridges
   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 Bridges as next hops.  This
   arrangement also allows for natural incremental deployment, and can
   support small scale initial deployments followed by dynamic
   deployment of additional Clients, Servers and Bridges without
   disturbing the already-deployed base.

   Server and Bridges 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].

3.4.  AERO Addresses

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

   IPv6 Client AERO addresses encode the Subnet-Router anycast address
   of a MNP (or non-MNP globally routable prefix) within the least-
   significant 112 bits of the IPv6 link-local prefix fe80::/16.  For
   example, for the MNP 2001:db8:1000:2000::/56 the corresponding AERO
   address is fe80:2001:db8:1000:2000::/72.

   IPv4-compatible 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 192.0.2.16/28 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 address 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.



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   Mobility Service (MS) AERO addresses (used by Bridges, Servers,
   Relays and Proxys) 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 0xfeffffff (e.g.,
   fe80::1, fe80::2, fe80::3, etc., fe80::feff:ffff) as assigned by the
   administrative authority for the link.  The address fe80:: is the
   IPv6 link-local Subnet-Router anycast address, and the address range
   fe80::ff00:0000/104 is reserved for future use.

   Finally, the address range fe80::/32 is used as the Teredo service
   prefix for AERO according to the format in Section 4 of [RFC4380]
   (see Section 3.25 for further discussion).

   For a full discussion of the above address format and implications
   for the /64 boundary, see: [I-D.templin-6man-omni-interface].

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 simple
   INET encapsulation, 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 also 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.  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 Bridges.

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

                            Figure 2: The SPAN

   To support the SPAN, AERO links use the Unique Local Address (ULA)
   prefix fd80::/10 [RFC4193] as the SPAN Service Prefix (SSP).  The
   prefix length intentionally matches the IPv6 link-local prefix
   (fe80::/10), and enables a simple stateless translation between AERO
   and SPAN addresses.  Additional SSPs can be employed to identify
   distinct SPAN Routing Topologies (SRTs) (see: Section 3.5.1).

   Each segment in the SPAN assigns a unique sub-prefix of SSP::/96
   termed the "SPAN Partition Prefix (SPP)".  For example, for fd80::/96
   a first segment could assign fd80::1000/116, a second could assign
   fd80::2000/116, a third could assign fd80::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.

   SPAN addresses are formed by simply rewriting the upper 16 bits of
   the corresponding AERO address.  For example:





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   o  the SPAN address formed from the IPv6 Client AERO address
      fe80:2001:db8:1000:2000:: is simply fd80:2001:db8:1000:2000::

   o  the SPAN address formed from the IPv4-compatible Client AERO
      address fe80::ffff:192.0.2.1 is simply fd80::ffff:192.0.2.1

   o  the SPAN address formed from the administrative AERO address
      fe80::1001 is simply fd80::1001.

   AERO Bridges join multiple segments into a unified AERO link over
   multiple diverse administrative domains.  They support a bridging
   function by first establishing forwarding table entries for their
   SPPs either via standard BGP routing or static routes.  For example,
   if three Bridges ('A', 'B' and 'C') from different segments serviced
   the SPPs fd80::1000/116, fd80::2000/116 and fd80::3000/116
   respectively, then the forwarding tables in each Bridge are as
   follows:

   A: fd80::1000/116->local, fd80::2000/116->B, fd80::3000/116->C

   B: fd80::1000/116->A, fd80::2000/116->local, fd80::3000/116->C

   C: fd80::1000/116->A, fd80::2000/116->B, fd80::3000/116->local

   These forwarding table entries are permanent and never change, since
   they correspond to fixed infrastructure elements in their respective
   segments.

   SPAN Client Prefixes (SCPs) are instead dynamically advertised in the
   AERO link routing system by Servers and Relays that provide service
   for their corresponding MNPs.  For example, if three Servers ('D',
   'E' and 'F') service the MNPs 2001:db8:1000:2000::/56,
   2001:db8:3000:4000::/56 and 2001:db8:5000:6000::/56 then the routing
   system would include:

   D: fd80:2001:db8:1000:2000::/72

   E: fd80:2001:db8:3000:4000::/72

   F: fd80:2001:db8:5000:6000::/72

   With the SCPs and SPPs in place in each Bridge's forwarding table,
   control and data packets sent between AERO nodes in different
   segments can therefore be carried over the via encapsulation in a
   mid-layer IPv6 header known as the "SPAN header".  For example, when
   a source AERO node forwards a packet with IPv6 address
   2001:db8:1:2::1 to a target AERO node with IPv6 address
   2001:db8:1000:2000::1, it first encapsulates the packet in a SPAN



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   header with source address set to fd80:2001:db8:1:2:: and destination
   address set to fd80:2001:db8:1000:2000::. Next, it encapsulates the
   resulting SPAN packet 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 Bridge (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:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |          INET Header          |
        |       src = 192.0.2.100       |
        |        dst = 192.0.2.1        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |         SPAN Header           |
        |  src = fd80:2001:db8:1:2::    |
        | dst=fd80:2001:db8:1000:2000:: |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        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 as discussed in
   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 Bridge.

   This gives rise to a routing system that contains both SCP routes
   that may change dynamically due to regional node mobility and SPP
   routes that never change.  The Bridges 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.





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   In normal operations, IPv6 ND messages are conveyed to SPP addresses
   over the SPAN so that specific Proxys, Servers or Relays can be
   addressed without being subject to mobility events.  Conversely, only
   the first few packets destined to SCP addresses traverse the SPAN
   until route optimization can determine a more direct path.

3.5.1.  SPAN Routing Topologies

   In some cases (e.g., when redundant topologies are needed for fault
   tolerance and reliability) it may be beneficial to deploy multiple
   SPAN Routing Topologies (SRTs) that act as independent overlay
   instances.  A communication failure in one instance therefore will
   not affect communications in other instances.

   Each SRT is identified by a distinct value in bits 10-15 of he SSP
   fd80::10, i.e., as fd80::/16, fd81::/16, fd82::/16, etc.  This
   document asserts that up to four SRTs provide a level of safety
   sufficient for critical communications such as civil aviation.  Each
   SRT is designated with a color that identifies a different AERO link
   instance as follows:

   o  Red (default) - corresponds to the SSP fd80::/16

   o  Green - corresponds to the SSP fd81::/16

   o  Blue-1 - corresponds to the SSP fd82::/16

   o  Blue-2 - corresponds to SSP fd83::/16

   o  SSPs fd84::/16 through fdbf::/16 are reserved for future use.

   Each AERO interface assigns a SPAN Anycast address corresponding to
   its SRT prefix.  For example, the SPAN anycast address for the Green
   SRT is simply fd81::. The SPAN anycast address is used for AERO
   interface determination in Safety-Based Multilink (SBM) as discussed
   in [I-D.templin-6man-omni-interface].  Each AERO interface further
   applies Performance-Based Multilink (PBM) internally.

3.5.2.  Segment Routing Over the SPAN

   As discussed in the following sections, Segment Routing is used over
   the SPAN to influence the path of packets destined to Clients on INET
   interfaces without causing all packets to traverse the Client's
   Server.  When a Client, Proxy or Server has a packet to send to a
   target discovered through route optimization located in the same SPAN
   segment, it encapsulates the packet in a SPAN header with the SPAN
   address of the target as the destination address, then uses the
   target's Link Layer Address information for INET encapsulation.



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   When a Client, Proxy or Server has a packet to send to a route
   optimization target located in a different SPAN segment, it
   encapsulates the packet in a SPAN header with the SPAN address of the
   target's Server as the destination.  The node also includes a Segment
   Routing Header (SRH) [RFC8754] with the SPAN address of the target as
   the penultimate address and with the IP encapsulation address of the
   target as the ultimate address.  (When the encapsulation address is
   an IPv6 address and a port number is included, the port number is
   written into the SRH Tag field.)  The node then forwards the packet
   into the SPAN, which will eventually direct it to a Bridge on the
   same segment as the target's Server.

   When a Bridge on the same segment as the target's Server receives a
   SPAN-encapsulated packet destined to the target Server, it looks
   ahead into the Segment Routing List to determine that the penultimate
   destination is set to the target's SPAN address and the ultimate
   destination is set to the target's Link Layer Address.  The Bridge
   then advances the SPAN destination address to the target's SPAN
   address and encapsulates the SPAN packet in an INET header based on
   the target's Link Layer Address, then forwards the packet to the
   target directly while bypassing the target's Server.  In this way,
   the Bridge participates in route optimization to greatly reduce
   traffic load and suboptimal routing through the target's Server.

3.6.  AERO Interface Characteristics

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

   o  INET interfaces connect to an INET either natively or through one
      or several IPv4 Network Address Translators (NATs).  Native INET
      interfaces have global IP addresses that are reachable from any
      INET correspondent.  All Server, Relay and Bridge interfaces are
      native interfaces, as are INET-facing interfaces of Proxys.  NATed
      INET interfaces connect to a private network behind one or more
      NATs that provide INET access.  Clients that are behind a NAT are
      required to send periodic keepalive messages to keep NAT state
      alive when there are no data packets flowing.

   o  Proxyed interfaces connect to an ANET that is separated from the
      open INET by an AERO Proxy.  Proxys can actively issue control
      messages over the INET on behalf of the Client to reduce ANET
      congestion.  Clients connected to Proxyed interfaces receive RAs
      with the P flag set to 1.

   o  VPNed interfaces use security encapsulation over the INET to a
      Virtual Private Network (VPN) server that also acts as an AERO
      Server.  Clients connected to VPNed interfaces receive RAs with



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      the P flag set to 1 the same as for Proxyed interfaces.  Other
      than the link-layer encapsulation format, VPNed interfaces behave
      the same as Direct interfaces.

   o  Direct interfaces connect a Client directly to a Server without
      crossing any ANET/INET paths.  An example is a line-of-sight link
      between a remote pilot and an unmanned aircraft.  The same Client
      considerations apply as for VPNed interfaces above, and the Client
      receives RA messages with the P flag set to 1.

   AERO interfaces use SPAN-layer encapsulation as necessary as
   discussed in Section 3.5.  AERO interfaces use link-layer
   encapsulation (see: Section 3.9) to exchange packets with AERO link
   neighbors over INET 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-6man-omni-interface].  The OMNI option includes prefix
   registration information and "ifIndex-tuples" containing link
   information parameters 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]  | SRT | LHS |FMT|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    Segment Routing List [1]                   ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                     Link Layer Address [1]                    ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Port Number [1]       |   ifIndex[2]  | SRT | LHS |FMT|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    Segment Routing List [2]                   ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                     Link Layer Address [2]                    ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Port Number [2]       |              ....             ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
       ~                              ...                              ~
       ~                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                               |   ifIndex[N]  | SRT | LHS |FMT|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                    Segment Routing List [N]                   ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                     Link Layer Address [N]                    ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Port Number [N]       | 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
   following information:

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

   o  SRT[i] - a 2-bit "SPAN Routing Topology" value (see:
      Section 3.5.1) coded as follows:

      *  000 - Red

      *  001 - Green




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      *  010 - Blue-1

      *  011 - Blue-2

      *  100 - 111 - Reserved

   o  LHS[i] - a 3-bit "LookaHead Segments" value that encodes the
      number (from 0 to 7) of entries in Segment Routing List [i].

   o  FMT[i] - a 2-bit "Format" code.  Determines the format of the Link
      Layer Address [i] field as follows:

      *  00 - Link Layer Address [i] encodes a Teredo-format AERO
         address for a node behind a NAT.

      *  01 - Link Layer Address [i] encodes a Teredo-format AERO
         address for a node on the open INET.

      *  10 - Link Layer Address [i] encodes an IPv6 address for a node
         behind a NAT..

      *  11 - Link Layer Address [i] encodes an IPv6 address for a node
         on the open INET.

   o  Segment Routing List [i] - Includes LHS[i]-many 16 byte SPAN
      addresses corresponding to the Segment IDs (SIDs) that must be
      visited prior to forwarding to Link Layer Address [i].  The
      ultimate SID appears first, followed by the penultimate SID
      second, etc.

   o  Link Layer Address [i] - Included according to FMT[i], and
      identifies the link-layer address of the source/target.  The IP
      address and port number (for IPv4) are recorded in ones-compliment
      "obfuscated" form per [RFC4380].

   o  Port Number [i] - Present only for IPv6 (i.e., when FMT[i] is 10
      or 11).  When present, the field is 2 bytes in length and
      immediately follows Link Layer Address [i], with the value
      recorded in obfuscated form.

   If an S/TLLAO is included, any ifIndex-tuples correspond to a proper
   subset of the OMNI option ifIndex-tuples.  Any S/TLLAO ifIndex-tuple
   having an ifIndex value that does not appear in an OMNI option
   ifindex-tuple is ignored.  If the same ifIndex value appears in
   multiple ifIndex-tuples, the first tuple is processed and the
   remaining tuples are ignored.  Any S/TLLAO ifIndex-tuples can
   therefore be viewed as inter-dependent extensions of their
   corresponding OMNI option ifIndex-tuples, i.e., the OMNI option and



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   S/TLLAO are companions that are interpreted in conjunction with each
   other.

   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 often used "one at a time" with low-cost WLAN
   preferred and highly-available cellular wireless as a standby, but a
   simultaneous-use capability could provide benefits.  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 set to constant values.  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 values that correspond to a
   specific interface.  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.

   Bridge, 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 Bridges that do not
   configure an AERO interface, the secured tunnel interfaces themselves
   are exposed to the IP layer with each interface configuring the
   Bridge's SPAN address.  Routing protocols such as BGP therefore run
   directly over the Bridge'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.




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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 Bridges are discussed in the following sections.

3.7.1.  AERO Server/Relay Behavior

   When a Server enables an AERO interface, it assigns AERO/SPAN
   addresses appropriate for the given SPAN segment.  The Server also
   configures secured tunnels with one or more neighboring Bridges and
   engages in a BGP routing protocol session with each Bridge.

   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.

   Relays are simply Servers that run a dynamic routing protocol to
   redistribute routes between the AERO interface and INET/EUN
   interfaces (see: Section 3.3).  The Relay 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 Relay further provides an attachment point of the AERO link to a
   non-MNP-based global topology.

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 Bridges and maintains per-Client neighbor cache
   entries based on control message exchanges.

3.7.3.  AERO Client Behavior

   When a Client enables an AERO interface, it sends RS messages with
   ND/PD parameters over its underlying interfaces to a Server in the
   MAP list, which returns an RA message with corresponding parameters.



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   (The RS/RA messages may pass through a Proxy in the case of a
   Client's Proxyed interface, or through one or more NATs in the case
   of a Client's INET interface.)

3.7.4.  AERO Bridge Behavior

   AERO Bridges 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 Bridges is therefore
   OPTIONAL, e.g., if an administrative interface is needed.  Bridges
   configure secured tunnels with Servers, Proxys and other Bridges;
   they also configure AERO/SPAN addresses and permanent neighbor cache
   entries the same as Servers.  Bridges engage in a BGP routing
   protocol session with a subset of the Servers on the local SPAN
   segment, and with other Bridges 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 Bridges maintain permanent
   neighbor cache entries for their associated Proxys and Servers (and
   vice-versa).  Each entry maintains the mapping between the neighbor's
   network-layer AERO address and corresponding INET address.

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



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   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(s).

   To the list of neighbor cache entry states in Section 7.3.2 of
   [RFC4861], Proxy and Server 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 "DEPART_TIME"
   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 DEPART_TIME be set to the default constant
   value REACHABLE_TIME plus 10 seconds (40 seconds by default) to allow
   a window for packets in flight to be delivered while stale route
   optimization state may be present.

   When an ROR receives an authentic NS message used for route
   optimization, it searches for a symmetric neighbor cache entry for
   the target Client.  The ROR 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 REPORT_TIME seconds.  The ROR
   resets ReportTime when it receives a new authentic NS message, and
   otherwise decrements ReportTime while no authentic NS messages have
   been received.  It is RECOMMENDED that REPORT_TIME be set to the
   default constant value REACHABLE_TIME plus 10 seconds (40 seconds by
   default) to allow a window for route optimization to converge before
   ReportTime decrements below REACHABLE_TIME.

   When the ROS receives a solicited NA message response to its NS
   message used for route optimization, 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 REACHABLE_TIME 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 REACHABLE_TIME be set to the default constant value
   30 seconds as specified in [RFC4861].

   AERO nodes also use 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



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   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 DEPART_TIME, REPORT_TIME, REACHABLE_TIME,
   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, DEPART_TIME and
   REPORT_TIME SHOULD be set to a value that is sufficiently longer than
   REACHABLE_TIME to avoid packet loss due to stale route optimization
   state.

3.9.  AERO Interface Encapsulation and Re-encapsulation

   In some instances, AERO interfaces insert a mid-layer IPv6 header
   known as the SPAN header as discussed in the following sections.
   After either inserting or omitting the SPAN header, the AERO
   interface inserts an outer encapsulation header as discussed below.

   AERO interfaces avoid outer encapsulation over Direct underlying
   interfaces and Proxyed underlying interfaces for which the first-hop
   access router is AERO-aware.  Other 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 inner packet's IP header into
   the corresponding fields in the SPAN and outer encapsulation
   header(s).

   For packets undergoing re-encapsulation, the AERO interface instead
   copies these values from the original encapsulation header into the
   new encapsulation header, i.e., the values are transferred between
   encapsulation headers and *not* copied from the encapsulated packet's
   network-layer header.  (Note especially that by copying the TTL/Hop
   Limit between encapsulation headers the value will eventually
   decrement to 0 if there is a (temporary) routing loop.)  For IPv4
   encapsulation/re-encapsulation, the AERO interface sets the DF bit as
   discussed in Section 3.12.

   AERO interfaces configured over INET underlying interfaces
   encapsulate packets in INET headers according to the next hop



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   determined in the forwarding algorithm in Section 3.13.  If the next
   hop is reached via a secured tunnel, the AERO interface uses an
   encapsulation format specific to the secured tunnel type (see:
   Section 6).  If the next hop is reached via an unsecured INET
   interface, the AERO interface instead uses UDP/IP encapsulation
   according to the Teredo format specified in [RFC4380] and as extended
   in [RFC6081].

   When Teredo encapsulation is used, the AERO interface next sets the
   UDP source port to a constant value that it will use in each
   successive packet it sends, and sets the UDP length field to the
   length of the encapsulated packet plus 8 bytes for the UDP header
   itself plus the length of any included Teredo extension headers or
   trailers.  The encapsulated packet may be either IPv6 or IPv4, as
   distinguished by the version number found in the first four bits.

   For Teredo-encapsulated packets sent to a Server, Relay or Bridge,
   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 finally includes/omits the UDP checksum according to
   [RFC6935][RFC6936].

   AERO interfaces observe the packet sizing and fragmentation
   considerations found in Section 3.12.

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 SPAN fragments, the AERO interface
   reassembles as discussed in Section 3.12.  Further decapsulation
   steps are performed according to the appropriate encapsulation format
   specification.

3.11.  AERO Interface Data Origin Authentication

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

   o  AERO Bridges, Servers and Proxys accept encapsulated data packets
      and control messages received from secured tunnels via the SPAN.

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




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

   o  AERO Clients, Relays and Servers verify the outer UDP/IP
      encapsulation addresses according to the Teredo specification
      [RFC4380].

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

3.12.  AERO Interface MTU and Fragmentation

   IPv6 underlying interfaces are REQUIRED to configure a minimum
   Maximum Transmission Unit (MTU) of 1280 bytes [RFC8200].  The minimum
   MTU for IPv4 underlying interfaces is only 68 bytes [RFC1122],
   meaning that a packet smaller than the IPv6 MTU may require
   fragmentation when IPv4 encapsulation is used.  Therefore, the Don't
   Fragment (DF) bit in the IPv4 encapsulation header MUST be set to 0.

   The AERO interface configures an MTU of 9180 bytes [RFC2492]; the
   size is therefore not a reflection of the underlying interface MTUs,
   but rather determines the largest packet the AERO interface can
   forward or reassemble.  The AERO interface therefore accommodates IP
   packets up to 9180 bytes while generating IPv6 Path MTU Discovery
   (PMTUD) Packet Too Big (PTB) messages [RFC8201] as necessary (see
   below).

   AERO interfaces employ mid-layer IPv6 encapsulation and
   fragmentation/reassembly per [RFC2473] (aka "SPAN encapsulation") to
   accommodate the 9180 byte MTU.  The AERO interface returns
   internally-generated PTB messages for packets admitted into the
   interface that it deems too large (e.g., according to link
   performance characteristics, reassembly cost, etc.) while either
   dropping or forwarding the packet as necessary.  The AERO interface
   performs PMTUD even if the destination appears to be on the same link
   since intermediate AERO link nodes may return a PTB.  This ensures
   that the path MTU is adaptive and reflects the current path used for
   a given data flow.

   AERO nodes perform SPAN encapsulation and fragmentation/reassembly as
   follows:

   o  When a node's AERO interface sends a packet over a Proxyed, VPNed
      or Direct underlying interface, it sends without SPAN
      encapsulation if the packet is no larger than the underlying
      interface MTU.  Otherwise, it inserts a SPAN header with source
      address set to the node's own SPAN address and destination set to



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      the SPAN address of the link-layer peer Proxy, Server or Client on
      the underlying interface.  The AERO interface then uses IPv6
      fragmentation to break the packet into a minimum number of non-
      overlapping fragments, where the largest fragment size is
      determined by the underlying interface MTU and the smallest
      fragment is no smaller than 640 bytes.  The AERO interface then
      sends the fragments to the link-layer peer, which reassembles
      before forwarding toward the final destination.

   o  When a node's AERO interface sends a packet over an INET
      underlying interface, it sends encapsulated packets no larger than
      1280 bytes without a SPAN header if the destination is reached via
      an INET address within the same SPAN segment.  Otherwise, it
      inserts a SPAN header with source address set to the node's SPAN
      address, destination set to the SPAN address of the next hop AERO
      node toward the final destination and (if necessary) with a SRH
      with the remaining Segment IDs on the path to the final
      destination.  The AERO interface then uses IPv6 fragmentation to
      break the encapsulated packet into a minimum number of non-
      overlapping fragments, where the largest fragment size (including
      both SPAN and INET encapsulation) is 1280 bytes and the smallest
      fragment is no smaller than 640 bytes.  The AERO interface then
      encapsulates the SPAN fragments in INET headers and sends them to
      the SPAN destination, which reassembles before forwarding toward
      the final destination.

   In order to avoid a "tiny fragment" attack, AERO interfaces
   unconditionally drop all SPAN fragments smaller than 640 bytes.  In
   order to set the correct context for reassembly, the AERO interface
   that inserts a SPAN header MUST also be the one that inserts the IPv6
   Fragment Header Identification value.  Although all fragments of the
   same fragmented SPAN packet are typically sent via the same
   underlying interface, this is not strictly required since all
   fragments will arrive at the AERO interface that performs reassembly
   even if they travel over different paths.

   Note that the AERO interface can forward large packets via
   encapsulation and fragmentation while at the same time returning
   advisory PTB messages, e.g., subject to rate limiting.  The receiving
   node that performs reassembly can also send advisory PTB messages if
   reassembly conditions become unfavorable.  The AERO interface can
   therefore continuously forward large packets without loss while
   returning advisory messages recommending a smaller size (but no
   smaller than 1280).  Advisory PTB messages are differentiated from
   PTB messages that report loss by setting the Code field in the ICMPv6
   message header to the value 1.  This document therefore updates
   [RFC4443] and [RFC8201].




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3.13.  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 traffic
   classifiers (e.g., DSCP value, port number, etc.) to select an
   outgoing underlying interface for each packet 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 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 an
   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 Bridges.  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).

3.13.1.  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



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   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).  The Client
   encapsulates the packet in a SPAN header and fragments if necessary
   according to MTU requirements (see: Section 3.12).

   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 reassembles and decapsulates as necessary
   and delivers the inner packet 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).

3.13.2.  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 reassembles and re-fragments if necessary then searches for
   an asymmetric neighbor cache entry that matches the destination and
   forwards as follows:

   o  if the destination matches an asymmetric neighbor cache entry, the
      Proxy uses one or more "reachable" neighbor interfaces in the
      entry for packet forwarding using SPAN encapsulation and including
      a SRH if necessary according to the cached TLLAO information.  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 Bridge.

   o  else, the Proxy uses SPAN encapsulation and forwards the packet to
      a Bridge while using the SPAN address corresponding to the
      packet's destination as the SPAN destination address.

   When the Proxy receives an encapsulated data packet from an INET
   neighbor or from a secured tunnel from a Bridge, it accepts the
   packet only if data origin authentication succeeds and if there is a
   proxy neighbor cache entry that matches the inner destination.  Next,
   the Proxy reassembles the packet (if necessary) and continues
   processing.




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   Next if reassembly is complete and the neighbor cache state is
   REACHABLE, the Proxy returns a PTB if necessary (see: Section 3.12)
   then either drops or forwards the packet to the Client while
   performing SPAN encapsulation and re-fragmentation to the ANET MTU
   size if necessary.  If the neighbor cache entry state is DEPARTED,
   the Proxy instead changes the SPAN destination address to the address
   of the new Server and forwards it to a Bridge while performing re-
   fragmentation to 1280 bytes if necessary.

3.13.3.  Server/Relay Forwarding Algorithm

   For control messages destined to a target Client's AERO address that
   are received from a secured tunnel, the Server 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 continues processing as follows:

   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.  The Server first reassembles (if
      necessary) and forwards the packet (while re-fragmenting if
      necessary) as specified in Section 3.12.

   o  else, if the destination matches a symmetric neighbor cache entry
      in the DEPARETED state the Server re-encapsulates the packet and
      forwards it using the SPAN address of the Client's 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 using SPAN encapsulation
      and Segment Routing if necessary with the final destination set to
      the neighbor's SPAN address 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 Relay) reassembles if necessary,
      decapsulates the packet and releases it to the network layer for



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      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 (such as for in-kernel
      implementations) 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 VPNed or Direct Client, it performs SPAN
   encapsulation and fragmentation if necessary, then 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
      Bridge using its own SPAN address as the source and the SPAN
      address corresponding to the destination as the destination.

3.13.4.  Bridge Forwarding Algorithm

   Bridges forward SPAN-encapsulated packets over secured tunnels the
   same as any IP router.  When the Bridge receives a SPAN-encapsulated
   packet via a secured tunnel, it removes the outer INET header and
   searches for a forwarding table entry that matches the SPAN
   destination address.  The Bridge then processes the packet as
   follows:

   o  if the destination is the SPAN address of a Server located in the
      local SPAN partition, the Bridge checks for a SRH.  If there is a
      SRH with the SPAN address of the final destination as the
      penultimate ID and with a Link Layer Address of the final
      destination as the ultimate ID, the Bridge copies the SPAN address
      of the final destination into the destination address.  If the
      Link Layer Address does not indicate the presence of a NAT, the
      Bridge then forwards the packet directly to the Link Layer Address
      using link-layer (UDP/IP) encapsulation.  Otherwise, the Bridge
      forwards the packet directly to the Server.

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

   o  else, if the destination matches a forwarding table entry the
      Bridge forwards the packet via a secured tunnel to the next hop.
      If the destination matches an MSP without matching an MNP,



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      however, the Bridge instead drops the packet and returns an ICMP
      Destination Unreachable message subject to rate limiting (see:
      Section 3.14).

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

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

3.14.  AERO Interface Error Handling

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

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

   The IP header is followed by an ICMP header that includes an error
   Type, Code and Checksum.  Valid type values include "Destination
   Unreachable", "Time Exceeded" and "Parameter Problem"
   [RFC0792][RFC4443].  (AERO interfaces ignore all link-layer IPv4
   "Fragmentation Needed" and IPv6 "Packet Too Big" messages since they
   only emit packets that are guaranteed to be no larger than the IP
   minimum link MTU as discussed in Section 3.12.)

   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 Bridge 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 Bridge drops the
   packet and returns a network-layer Destination Unreachable message
   subject to rate limiting.  The Bridge 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
   NBMA interfaces, and therefore send unicast RA messages with a short
   Router Lifetime value (e.g., ReachableTime seconds) in response to a
   Client's RS message.  Thereafter, Clients send additional RS messages
   to keep Server state alive.

   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 instead employ prefix
   registration by including its AERO address as the source address of
   an RS message and with an OMNI option with valid prefix registration
   information 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 a similar manner as
   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 the link-scoped All-



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   Routers multicast (ff02::2) or Subnet-Router anycast (fe80::) address
   as the network-layer destination.  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 IPv6 address
   (::/128) 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 Client sets the OMNI
   option prefix registration information according to the MNP, and
   includes an ifIndex-tuple with S set to '1' corresponding 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.  The Client MAY also include an SLLAO
   corresponding to the OMNI option ifIndex-tuple with S set to '1'.

   The Client then sends the RS message (either directly via Direct
   interfaces, via a VPN for VPNed interfaces, via a Proxy for proxyed
   interfaces or via INET encapsulation for INET 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 via this underlying
   interface, and caches the other RA configuration information
   including Cur Hop Limit, M and O flags, Reachable Time and Retrans
   Timer.  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 the underlying interface.

   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.

   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



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   downstream attached "Internet of Things (IoT)".  The Client
   subsequently sends additional RS messages over each underlying
   interface before the Router Lifetime received for that interface
   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 prefix
   registration information specific to its MNP, with an ifIndex-tuple
   specific to the selected underlying interface with S set to '1', and
   with 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 a
   prefix release indication.  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 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 a
   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 SPAN
   Compatibility Prefix (SCP) corresponding to the IPv4 address.




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   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 SPAN/INET addresses in the neighbor cache
   entry.

   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
   sets the Router Lifetime to the time for which it will maintain both
   this underlying interface individually and the symmetric neighbor
   cache entry as a whole.  The Server also sets Cur Hop Limit, M and O
   flags, Reachable Time and Retrans Timer to values appropriate for the
   AERO link.  The Server includes the delegated MNPs, any other PD
   parameters and an OMNI option with no ifIndex-tuples.  The Server
   then includes one or more RIOs that encode the MSPs for the AERO
   link, plus an MTU option (see Section 3.12).  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 a
   ReachableTime timer for each of the Client's underlying interfaces
   individually (and for the Client's symmetric neighbor cache entry
   collectively) set to expire after ReachableTime seconds.  If the
   Client (or Proxy) issues additional RS messages, the Server sends an
   RA response and resets ReachableTime.  If the Server receives an ND
   message with PD release indication it sets the Client's symmetric
   neighbor cache entry to the DEPARTED state and withdraws the MNP from
   the routing system after a short delay (e.g., 2 seconds).  If
   ReachableTime expires before a new RS is received on an individual
   underlying interface, the Server marks the interface as DOWN.  If
   ReachableTime expires before any new RS is received on any individual
   underlying interface, the Server sets the symmetric neighbor cache
   entry state to STALE and sets a 10 second timer.  If the Server has
   not received a new RS or ND message with PD release indication before
   the 10 second timer expires, it deletes the neighbor cache entry and
   withdraws the MNP from the routing system.

   The Server processes any ND/PD messages pertaining to the Client and
   returns an NA/RA reply in response to solicitations.  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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   Note: Clients SHOULD notify former Servers of their departures, but
   Servers are responsible for expiring neighbor cache entries and
   withdrawing routes even if no departure notification is received
   (e.g., if the Client leaves the network unexpectedly).  Servers
   SHOULD therefore set Router Lifetime to ReachableTime seconds in
   solicited RA messages to minimize persistent stale cache information
   in the absence of Client departure notifications.  A short Router
   Lifetime also ensures that proactive Client/Server RS/RA messaging
   will keep any NAT state alive (see above).

   Note: All Servers on an AERO link MUST advertise consistent values in
   the RA Cur Hop Limit, M and O flags, Reachable Time and Retrans Timer
   fields the same as for any link, since unpredictable behavior could
   result if different Servers on the same link advertised different
   values.

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




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   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 deploy perimeter security services
   to facilitate 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-Router 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 link-layer
      addresses along with the OMNI option information and any other
      identifying information including Transaction IDs, Client
      Identifiers, Nonce values, etc.  The Proxy finally encapsulates
      the (proxyed) RS message in a SPAN header with source set to the
      Proxy's SPAN address and destination set to the Server's SPAN
      address then forwards the message into the SPAN.

   o  when the Server receives the RS, 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, it authenticates the message and
      matches it with the proxy neighbor cache entry created by the RS.
      The Proxy then caches the PD route information as a mapping from
      the Client's MNPs to the Client's ANET address, caches the
      Server's advertised Router Lifetime and sets the neighbor cache
      entry state to REACHABLE.  The Proxy then sets the P bit in the RA
      flags field, optionally rewrites the Router Lifetime and forwards
      the (proxyed) message to the Client.  The Proxy finally includes
      an MTU option (if necessary) with an MTU to use for the underlying
      ANET interface.

   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 Bridge using SPAN encapsulation with its own SPAN address
   as the source and the SPAN address corresponding to the Client as the



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   destination.  The Proxy instead forwards any Client data destined to
   an asymmetric neighbor cache target directly to the target according
   to the SPAN/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 sends NS,
   RS and/or unsolicited NA messages to update the Server's symmetric
   neighbor cache entries on behalf of the Client and/or to convey QoS
   updates.  This allows for higher-frequency Proxy-initiated RS/RA
   messaging over well-connected INET infrastructure supplemented by
   lower-frequency Client-initiated RS/RA messaging over constrained
   ANET data links.

   If the Server ceases to send solicited advertisements, the Proxy
   sends unsolicited RAs on the ANET interface with destination set to
   All-Nodes multicast (ff02::1) and with Router Lifetime set to zero to
   inform Clients that the Server has failed.  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.  For this reason,
   the Proxy marks any Client-originated solicitation messages (e.g. by
   inserting a Nonce option) so that it can return the solicited
   advertisement to the Client instead of processing it locally.

   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
   to the Client to a Bridge via SPAN encapsulation with the Client's
   current Server as the destination.  The Bridge 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.

   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 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-6man-omni-interface].



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   The Client then sends the unencapsulated RS message, which will be
   intercepted by the AERO-Aware access router.  The access router then
   encapsulates the RS message in an ANET header with its own address as
   the source address and the address of a Proxy as the destination
   address.  The access router further remembers the address of the
   Proxy so that it can encapsulate future data packets from the Client
   via the same Proxy.  If the access router needs to change to a new
   Proxy, it simply sends another RS message toward the Server via the
   new Proxy on behalf of the Client.

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

3.16.1.  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.  When the
   Proxy is also sending RS messages to the Server on behalf of ANET
   Clients, the resulting RA responses can be considered as equivalent
   hints of forward progress.  This means that the Proxy need not also
   send a periodic NS if it has already sent an RS within the same
   period.  If the Server fails (i.e., if the Proxy ceases to receive
   advertisements), the Proxy can quickly inform Clients by sending
   multicast RA messages on the ANET interface.

   The Proxy sends RA messages on the ANET interface 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 had been using the failed
   Server will receive the RA messages and associate with a new Server.



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3.16.2.  Point-to-Multipoint Server Coordination

   In environments where Client messaging over ANETs is bandwidth-
   limited and/or expensive, Clients can enlist the services of the
   Proxy to coordinate with multiple Servers in a single RS/RA message
   exchange.  The Client can send a single RS message to All-Routers
   multicast that includes the ID's of multiple Servers in MS-Register
   sub-options of the OMNI option.

   When the Proxy receives the RS and processes the OMNI option, it
   performs a separate RS/RA exchange with each MS-Register Server.
   When it has received the RA messages, it creates an "aggregate" RA
   message to return to the Client with an OMNI option with each
   responding Server's ID recorded in an MS-Register sub-option.

   Clients can thereafter employ efficient point-to-multipoint Server
   coordination under the assistance of the Proxy to dramatically reduce
   the number of messages sent over the ANET while enlisting the support
   of multiple Servers for fault tolerance.  Clients can further include
   MS-Release suboptions in RS messages to request the Proxy to release
   from former Servers via the procedures discussed in Section 3.19.5.

   The OMNI interface specification [I-D.templin-6man-omni-interface]
   provides further discussion of the Client/Proxy RS/RA messaging
   involved in point-to-multipoint coordination.

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 and Direct interfaces, the Server is the ROS.

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

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

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

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




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   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
   for Address Resolution (NS(AR)) to receive a solicited NA message
   from the ROR.  When the ROS sends an NS(AR), it includes:

   o  the AERO address of the ROS as the source address.

   o  the data packet's destination as the Target Address.

   o  the Solicited-Node multicast address [RFC4291] formed from the
      lower 24 bits of the data packet's destination as the destination
      address, e.g., for 2001:db8:1:2::10:2000 the NS destination
      address is ff02:0:0:0:0:1:ff10:2000.

   The NS(AR) message includes an OMNI option with no ifIndex-tuples and
   no SLLAO, such that the target will not create a neighbor cache
   entry.

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

3.17.2.  Relaying the NS

   When the Bridge receives the NS(AR) 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 Bridge then forwards the message toward the
   ROR via the SPAN the same as for any IPv6 router.  The final-hop
   Bridge 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(AR) message, it examines the Target
   Address to determine whether it has a neighbor cache entry and/or
   route that matches the target.  If there is no match, the ROR drops
   the message.  Otherwise, the ROR continues processing as follows:

   o  if the target belongs to an MNP Client neighbor in the DEPARTED
      state the ROR changes the NS(AR) message SPAN destination address




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      to the SPAN address of the Client's new Server, forwards the
      message into the SPAN and returns from processing.

   o  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.

   o  If the target belongs to a non-MNP route, the ROR continues
      processing without adding an entry to the Report List.

   The ROR then prepares a solicited NA message to send back to the ROS
   but does not create a neighbor cache entry.  The ROR sets the NA
   source address to the AERO address corresponding to the target, sets
   the Target Address to the target of the solicitation, and sets the
   destination address to the source of the solicitation.

   The ROR then includes an OMNI option with prefix registration length
   set to the length of the MNP if the target is an MNP Client;
   otherwise, set 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.)

   If the target is an MNP Client, the ROR next includes ifIndex-tuples
   in the OMNI option for each of the target Client's underlying
   interfaces with current information for each interface and with the S
   flag set to 0.  The ROR then includes a TLLAO with ifIndex-tuples in
   one-to-one correspondence with the tuples that appear in the OMNI
   option.

   The ROR sets the Link Layer Address and Port Number (if necessary) to
   its own INET address for VPNed and Direct interfaces or to the INET
   address of the Proxy for Proxyed interface, then includes its own
   SPAN address or the SPAN address of the Proxy as the ultimate Segment
   Routing List entry.  For INET interfaces, the ROR instead sets the
   Link Layer Address and Port Number (if necessary) to the Client's
   INET address then sets its own SPAN address in the penultimate
   Segment Routing List entry and sets the target's SPAN address in the
   ultimate Segment Routing List entry.

   The ROR then sets the NA message R flag to 1 (as a router), S flag to
   1 (as a response to a solicitation), and O flag to 0 (as a proxy).
   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(AR) message, then forwards the message into
   the SPAN without decrementing the network-layer TTL/Hop Limit field.





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3.17.4.  Relaying the NA

   When the Bridge 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 Bridge then forwards the SPAN-encapsulated NA message
   toward the ROS the same as for any IPv6 router.  The final-hop Bridge
   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 processes the
   message the same as for standard IPv6 Address Resolution [RFC4861].
   In the process, it caches the source SPAN address then creates an
   asymmetric neighbor cache entry for the ROR and caches all
   information found in the OMNI and TLLAO options.  The ROS finally
   sets the asymmetric neighbor cache entry lifetime to ReachableTime
   seconds.

3.17.6.  Route Optimization Maintenance

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

   While new data packets destined to the target are flowing through the
   ROS, it sends additional NS(AR) 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(AR) SPAN destination address, and
   sends up to MAX_MULTICAST_SOLICIT NS(AR) messages separated by 1
   second until an NA is received.  If no NA is received, the ROS
   assumes that 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




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   deletes the asymmetric neighbor cache entry.  Any 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 (see: Section 3.19).  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 any forward paths are working.  If there is no
   asymmetric neighbor cache entry, the ROS simply discards the
   unsolicited NA.

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

3.18.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform Neighbor Unreachability Detection (NUD) per
   [RFC4861] either reactively in response to persistent link-layer
   errors (see Section 3.14) or proactively to confirm reachability.
   The NUD algorithm is based on periodic control message exchanges.
   The algorithm may further be seeded by ND hints of forward progress,
   but care must be taken to avoid inferring reachability based on
   spoofed information.  For example, authentic IPv6 ND message
   exchanges may be considered as acceptable hints of forward progress,
   while spurious data packets should not be.

   AERO Servers, Proxys and Relays can use standard NS/NA NUD exchanges
   sent over the SPAN to securely test reachability without risk of DoS
   attacks from nodes pretending to be a neighbor; Proxys can further
   perform NUD to securely verify Server reachability on behalf of their
   proxyed Clients.  However, a means for a ROS to test the unsecured
   forward directions of target route optimized paths is also necessary.

   When an ROR directs an ROS to a neighbor with one or more target
   link-layer addresses, the ROS can proactively test each such
   unsecured route optimized path by sending "loopback" NS(NUD)
   messages.  While testing the paths, the ROS can optionally continue
   to send packets via the SPAN, maintain a small queue of packets until
   target reachability is confirmed, or (optimistically) allow packets
   to flow via the route optimized paths.





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   When the ROS sends a loopback NS(NUD) message, it uses its AERO
   address as both the IPv6 source and destination address, and the MNP
   Subnet-Router anycast address as the Target Address.  The ROS
   includes a Nonce and Timestamp option, then encapsulates the message
   in SPAN/INET headers with its own SPAN address as the source and the
   SPAN address of the route optimization target as the destination.
   The ROS then forwards the message to the target (either directly to
   the link layer address of the target if the target is in the same
   SPAN segment, or via a Bridge if the target is in a different SPAN
   segment).

   When the route optimization target receives the NS(NUD) message, it
   notices that the IPv6 destination address is the same as the source
   address.  It then reverses the SPAN source and destination addresses
   and returns the message to the ROS (either directly or via the SPAN).
   The route optimization target does not decrement the NS(NUD) message
   IPv6 Hop-Limit in the process, since the message has not exited the
   SPAN.

   When the ROS receives the NS(NUD) message, it can determine from the
   Nonce, Timestamp and Target Address that the message originated from
   itself and that it transited the forward path.  The ROS need not
   prepare an NA response, since the destination of the response would
   be itself and testing the route optimization path again would be
   redundant.

   The ROS marks route optimization target paths that pass these NUD
   tests as "reachable", and those that do not as "unreachable".  These
   markings inform the AERO interface forwarding algorithm specified in
   Section 3.13.

   Note that to avoid a DoS vector nodes MUST NOT return loopback
   NS(NUD) messages received from an unsecured link-layer source via a
   secured SPAN path.

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 collective entity for all Clients.
   Clients coordinate with their associated Servers via RS/RA exchanges
   to maintain the DMM profile, and the AERO routing system tracks all
   current Client/Server peering relationships.

   Servers provide default routing and mobility/multilink services for
   their dependent Clients.  Clients are responsible for maintaining
   neighbor relationships with their Servers through periodic RS/RA



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   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 also perform some RS/RA exchanges on the Client's behalf.

   Mobility management considerations are specified in the following
   sections.

3.19.1.  Mobility Update Messaging

   Servers accommodate Client mobility/multilink and/or QoS change
   events by sending unsolicited NA (uNA) messages to each ROS in the
   target Client's Report List.  When a Server sends a uNA message, it
   sets the IPv6 source address to the Client's AERO address, sets the
   destination address to All-Nodes multicast and sets the Target
   Address to the Client's Subnet-Router anycast address.  The Server
   also includes an OMNI option with prefix registration information and
   with ifIndex-tuples for the target Client's remaining interfaces with
   S set to 0.  The Server then includes a TLLAO with corresponding
   ifIndex-tuples prepared the same as for the initial route
   optimization event.  The Server sets the NA R flag to 1, the S flag
   to 0 and the O flag to 0, then 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 and sends the message into the SPAN.

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

   When the ROS receives a uNA 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.

   If uNA 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 link-layer address information.



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   In addition to sending uNA messages to the current set of ROSs for
   the Client, the Server also sends uNAs to the former link-layer
   address for any ifIndex-tuple for which the link-layer address has
   changed.  The uNA messages update Proxys 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 underlying interface 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 that includes an ifIndex-tuple with S set to 1 and with
   the new link quality and address information.

   Up to MAX_RTR_SOLICITATIONS 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 uNA 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 that
   includes an ifIndex-tuple with S set to 1 and appropriate link
   quality values and 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 or uNA message to its Server with an OMNI option with
   appropriate link quality values.

   If the Client needs to send RS/uNA messages over an underlying
   interface other than the one being removed from service, it MUST
   include ifIndex-tuples with appropriate link quality values for any
   underlying interfaces being removed from service.








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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 MS-
   Release identifiers in the RS message OMNI option per
   [I-D.templin-6man-omni-interface] if it wants the new Server to
   notify any old Servers from which the Client is departing.

   When the new Server receives the Client's RS message, it returns an
   RA as specified in Section 3.15.3 and sends up to
   MAX_NEIGHBOR_ADVERTIISEMENT uNA messages to any old Servers listed in
   OMNI option MS-Release identifiers.  Each uNA 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 with the
   Register/Release bit set to 0.  The new Server wraps the uNA in a
   SPAN header with its own SPAN address as the source and the old
   Server's SPAN address as the destination, then sends the message into
   the SPAN.

   When an old Server receives the uNA, it changes the Client's neighbor
   cache entry state to DEPARTED, sets the link-layer address of the
   Client to the new Server's SPAN address, and resets DepartTime.
   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 Client's neighbor cache entry.

   The old Server also sends unsolicited NA messages to all ROSs in the
   Client's Report List with an OMNI option with a single ifIndex-tuple
   with ifIndex set to 0 and S set to '1', and with the SPAN address of
   the new Server in a companion TLLAO.  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 for a period of 10
   seconds after which the cache entry is deleted.  While in the STALE
   state, subsequent data packets flow according to any existing cached
   link-layer information and trigger a new NS(AR)/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
   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




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   attention in the reassembly algorithm is necessary when re-routed
   fragments are simply treated as loss.

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 Relays 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/Relay) "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/Relay "Y" that
   services S.  At the same time, if the message was a Join, X sends a
   route-optimization NS message toward each S the same as discussed in
   Section 3.17.  The resulting NAs will return the AERO address for the
   prefix that matches S as the network-layer source address and TLLAOs
   with the SPAN addresses corresponding to any ifIndex-tuples that are
   currently servicing S.

   When Y processes the Join/Prune message, if S located behind any
   INET, Direct, or VPNed 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



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   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 Bridges 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 Bridge.

   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
   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.



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   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.

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 Bridges, Servers and Proxys,
   thereby providing redundancy in case of failures.

   The Bridges, 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



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   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 the SSP in a
   similar fashion as for 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 SRT color (see: Section 3.5.1).  In particular, the
   Client can tag its RS messages with the appropriate label to cause
   the network to select the desired VLAN.

   The Client's IP layer can select the outgoing AERO interface
   appropriate for a given 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
   Bridges.

   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 Bridges on the link.  Correspondent nodes
   can then perform segment routing at the SPAN layer
   [RFC8402][RFC8754].  Segment Routing in the correct SRT will then
   direct the packet over multiple hops 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 Relay NAT64 mapping caches.  In that way, an IPv4
   correspondent node can send packets to the IPv4 address mapping of
   the target MN, and the Relay will translate the IPv4 header and
   destination address into an IPv6 header and IPv6 destination address
   of the MN.





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   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
   Relays on each INET partition, with each Relay 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/multilink 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 Relays.

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

3.24.  Detecting and Reacting to Server and Bridge Failures

   In environments where rapid failure recovery is required, Servers and
   Bridges 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 Bridges maintain BFD sessions in parallel with their BGP
   peerings.  If a Server or Bridge fails, BGP peers will quickly re-
   establish routes through alternate paths the same as for common BGP
   deployments.  Similarly, Proxys maintain BFD sessions with their
   associated Bridges even though they do not establish BGP peerings
   with them.




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   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.  When the Proxy is also sending RS messages on
   behalf of ANET Clients, the RS/RA messaging can be considered as
   equivalent hints of forward progress.  This means that the Proxy need
   not also send a periodic NS if it has already sent an RS within the
   same period.  If a Server fails, the Proxy will cease to receive
   advertisements and can quickly inform Clients of the outage by
   sending multicast RA messages on the ANET interface.

   The Proxy sends multicast 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.

3.25.  AERO Clients on the Open Internet

   AERO Clients that connect to the open Internet via INET interfaces
   can establish a VPN or direct link to securely connect to a Server in
   a "tethered" arrangement with all of the Client's traffic transiting
   the Server.  Alternatively, the Client can associate with an INET
   Server using UDP/IP encapsulation and asymmetric securing services as
   discussed in the following sections.

   When a Client's AERO interface enables an INET underlying interface,
   it first determines whether the interface is likely to be behind a
   NAT.  For IPv4, the Client might assume it is on the open Internet if
   the INET address is not a special-use IPv4 address per [RFC3330].
   Similarly for IPv6, the Client might assume it is on the open
   Internet if the INET address is not a link-local [RFC4291] or unique-
   local [RFC4193] IPv6 address.

   The Client then prepares a UDP/IP-encapsulated RS message with IPv6
   source address set to its AERO address, with IPv6 destination set to
   All-Routers multicast and with an OMNI option with underlying
   interface parameters.  If the Client believes that it is on the open
   Internet, it SHOULD also include an SLLAO set according to the
   address used for INET encapsulation (otherwise, it MAY omit the
   SLLAO).  If the underlying address is IPv4, the Client includes a
   Teredo address [RFC4380] using the prefix fe80::/32 with the Server's
   IPv4 address, and with the IP address and Port Number used for INET
   encapsulation written in obfuscated form and with FMT set to '01'
   (INET) as discussed in Section 3.6.  If the underlying interface
   address is IPv6, the Client instead includes the IPv6 address and



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   Port number in obfuscated form and sets FMT to '11' (INET).  The
   Client finally includes a Teredo Authentication option per [RFC4380]
   to provide message authentication, sets the UDP/IP source to its INET
   address and UDP port, sets the UDP/IP destination to the Server's
   INET address and the AERO service port number (8060), then sends the
   message to the Server.

   When the Server receives the RS, it authenticates the message and
   registers the Client's MNP and INET interface information according
   to the OMNI option parameters.  If the RS message includes an SLLAO,
   the Server compares the encapsulation IP address and UDP port number
   with the (unobfuscated) SLLAO values.  If the values are the same,
   the Server caches the Client's information as "INET" addresses
   meaning that the Client is likely to accept direct messages without
   requiring NAT traversal exchanges.  If the values are different (or,
   if there was no SLLAO) the Server instead caches the Client's
   information as "NAT" addresses meaning that NAT traversal exchanges
   may be necessary.

   The Server then returns an RA message with IPv6 source and
   destination set corresponding to the addresses in the RS, and with a
   Teredo Authentication option.  For IPv4, the Server also includes an
   IPv4 Teredo Origin option per [RFC4380] with the mapped and
   obfuscated IPv4 address and port number observed in the encapsulation
   headers.  For IPv6, the Server instead includes an IPv6 Teredo Origin
   option per Figure 6 with the mapped and obfuscated observed IPv6
   address and port number (note that the value 0x02 in the second octet
   differentiates from other Teredo option types).

      +--------+--------+-----------------+
      |  0x00  | 0x02   | Origin port #   |
      +--------+--------+-----------------+
      ~  Origin IPv6 address              ~
      +-----------------------------------+


                    Figure 6: IPv6 Teredo Origin Option

   When the Client receives the RA message, it compares the mapped IP
   address and port from the Teredo Origin option with its own address.
   If the addresses are the same, the Client assumes the open Internet /
   Cone NAT principle; if the addresses are different, the Client
   instead assumes that further Server qualification procedures are
   necessary to detect the type of NAT and proceeds according to
   standard Teredo procedures.

   After the Client has registered its INET interfaces in such RS/RA
   exchanges it sends periodic RS messages to receive fresh RA messages



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   before the Router Lifetime received on each INET interface expires.
   The Client also maintains default routes via its Servers, i.e., the
   same as described in earlier sections.

   When the Client sends messages to target IP addresses, it also
   invokes route optimization per Section 3.17 using IPv6 ND address
   resolution messaging.  The Client sends the NS(AR) message to the
   Server wrapped in a UDP/IP header with a Teredo Authentication option
   with the NS source address set to the Client's AERO address and
   destination address set to the target solicited node multicast
   address.  The Server authenticates the message and sends a
   corresponding NS(AR) message over the SPAN the same as if it were the
   ROS, but with the SPAN source address set to the Server's SPAN
   address and destination set to the SPAN address of the target.  When
   the ROR receives the NS(AR), it adds the Server's SPAN address and
   Client's AERO address to the target's Report List, and returns an NA
   with OMNI and TLLAO information for the target.  The Server then
   returns a UDP/IP encapsulated NA message with a Teredo Authentication
   option to the Client.

   Following route optimization, for targets in the same SPAN segment if
   the target's TLLAO addresss is on the open INET, the Client forwards
   data packets directly to the target INET address.  If the target's
   TLLAO address is behind a NAT, the Client first establishes NAT state
   for the Link Layer Address using the "bubble" mechanisms specified in
   [RFC6081][RFC4380].  The Client continues to send data packets via
   its Server until NAT state is populated, then begins forwarding
   packets via the direct path through the NAT to the target.  For
   targets in different SPAN segments, the Client inserts a Segment
   Routing header and forwards data packets to the Bridge that returned
   the NA message.

   The ROR may return uNAs via the Server if the target moves, and the
   Server will send corresponding Teredo Authentication-protected uNAs
   to the Client.  The Client can also send "loopback" NS(NUD) messages
   to test forward path reachability even though there is no security
   association between the Client and the target.

   The Client sends Teredo UDP/IP encapsulated IPv6 packets no larger
   than 1280 bytes in one piece.  In order to accommodate larger IPv6
   packets (up to the AERO interface 9180 MTU), the Client inserts a
   SPAN header with source set to its own SPAN address and destination
   set to the SPAN address of the target and uses IPv6 fragmentation
   according to Section 3.12.  The Client then encapsulates each
   fragment in a UDP/IP header and sends the fragments to the next hop.






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3.25.1.  Use of SEND and CGA

   In some environments, use of the Teredo Authentication option alone
   may be sufficient for assuring IPv6 ND message authentication between
   Clients and Servers.  When additional protection is necessary, nodes
   should employ SEcure Neighbor Discovery (SEND) [RFC3971] with
   Cryptographically-Generated Addresses (CGA) [RFC3972].

   When SEND/CGA are used, the Client prepares RS messages with its
   link-local CGA as the IPv6 source and All-Routers as the IPv6
   Destination, includes any SEND options and wraps the message in a
   SPAN header.  The Client sets the SPAN source address to its own SPAN
   address and sets the SPAN destination address to the "All-Routers"
   SPAN address.  The Client then wraps the RS message in UDP/IP headers
   according to the Teredo format and sends the message to the Server.

   When the Server receives the message, it first verifies the Teredo
   Authentication option (if present) then uses the SPAN source address
   to determine the MNP of the Client.  The Server then processes the
   SEND options to authenticate the RS message and prepares an RA
   message response.  The Server prepares the RA with its own link-local
   CGA and the CGA of the Client as the IPv6 source and destination,
   includes any SEND options and wraps the message in a SPAN header.
   The Server sets the SPAN source address to its own SPAN address and
   sets the SPAN destination address to the Client's SPAN address.  The
   Server then wraps the RA message in UDP/IP headers according to the
   Teredo format and sends the message to the Client.  Thereafter, the
   Client/Server send additional RS/RA messages to maintain their
   association and any NAT state.

   The Client and Server also may exchange NS/NA messages using their
   own CGA as the source and with SPAN encapsulation as above.  When a
   Client sends an NS(AR), it sets the IPv6 source to its CGA and sets
   the IPv6 destination to the Solicited-Node Multicast address of the
   target.  The Client then wraps the message in a SPAN header with its
   own SPAN address as the source and the SPAN address of the target as
   the destination and sends it to the Server.  The Server authenticates
   the message, then changes the IPv6 source address to the Client's
   AERO address, removes the SEND options, and sends a corresponding
   NS(AR) into the SPAN.  When the Server receives the corresponding
   SPAN-encapsulated NA, it changes the IPv6 destination address to the
   Client's CGA, inserts SEND options, then wraps the message in UDP/IP
   headers and sends it to the Client.

   When a Client sends a uNA, it sets the IPv6 source address to its own
   CGA and sets the IPv6 destination address to All-Nodes multicast,
   includes SEND options, wraps the message in SPAN and UDP/IP headers
   and sends the message to the Server.  The Server authenticates the



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   message, then changes the IPv6 address to the Client's AERO address,
   removes the SEND options and forwards the message the same as
   discussed in Section 3.19.1.  In the reverse direction, when the
   Server forwards a uNA to the Client, it changes the IPv6 address to
   its own CGA and inserts SEND options then forwards the message to the
   Client.

   When a Client sends an NS(NUD), it sets both the IPv6 source and
   destination address to its own AERO address, wraps the message in a
   SPAN header and UDP/IP headers, then sends the message directly to
   the peer which will loop the message back.  In this case alone, the
   Client does not use the Server as a trust broker for forwarding the
   ND message.

3.26.  Time-Varying MNPs

   In some use cases, it is desirable, beneficial and efficient for the
   Client to receive a constant MNP that travels with the Client
   wherever it moves.  For example, this would allow air traffic
   controllers to easily track aircraft, etc.  In other cases, however
   (e.g., intelligent transportation systems), the MN may be willing to
   sacrifice a modicum of efficiency in order to have time-varying MNPs
   that can be changed every so often to defeat adversarial tracking.

   The DHCPv6-PD service offers a way for Clients that desire time-
   varying MNPs to obtain short-lived prefixes (e.g., on the order of a
   small number of minutes).  In that case, the identity of the Client
   would not be bound to the MNP but rather the Client's identity would
   be bound to the DHCPv6 Device Unique Identifier (DUID) and used as
   the seed for Prefix Delegation.  The Client would then be obligated
   to renumber its internal networks whenever its MNP (and therefore
   also its AERO address) changes.  This should not present a challenge
   for Clients with automated network renumbering services, however
   presents limits for the durations of ongoing sessions that would
   prefer to use a constant address.

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.

   As of 4/1/2020, more recent updated implementations are under
   internal development and testing with plans to release in the near
   future.





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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.

6.  Security Considerations

   AERO Bridges 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], WireGuard, etc.  The AERO Bridges of all SPAN segments in
   turn configure secured tunnels for their neighboring AERO Bridges
   across the SPAN.  Therefore, control messages that traverse the SPAN
   between any pair of AERO link neighbors are already secured.

   AERO Servers, Relays 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.

   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 ANETs need not apply security to
   their ND messages, since the messages will be intercepted by a
   perimeter Proxy that applies security on its INET-facing interface.
   AERO Clients connected to the open INET can use symmetric network
   and/or transport layer security services such as VPNs or can by some
   other means establish a direct link.  When a VPN or direct link may
   be impractical, however, an asymmetric security service such as
   SEcure Neighbor Discovery (SEND) [RFC3971] with Cryptographically
   Generated Addresses (CGAs) [RFC3972] and/or the Teredo Authentication
   option [RFC4380] may be necessary.



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

   AERO Servers and Bridges 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 Bridges 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 Relays must implement ingress filtering to avoid a spoofing
   attack in which spurious SPAN messages are injected into an AERO link
   from an outside attacker.  AERO Clients MUST ensure that their
   connectivity is not used by unauthorized nodes on their EUNs to gain
   access to a protected network, i.e., AERO Clients that act as routers
   MUST NOT provide routing services for unauthorized nodes.  (This
   concern is no different than for ordinary hosts that receive an IP
   address delegation but then "share" the address with other nodes via
   some form of Internet connection sharing such as tethering.)

   The MAP list MUST be well-managed and secured from unauthorized
   tampering, even though the list contains only public information.
   The MAP list can be conveyed to the Client 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.).

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

   Segment routing provides authentication facilities that can be used
   to authenticate the information in the SRH [RFC8754].





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   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,
   Wojciech Dec, Pavel Drasil, Ralph Droms, Adrian Farrel, Nick Green,
   Sri Gundavelli, Brian Haberman, Bernhard Haindl, Joel Halpern, Tom
   Herbert, Sascha Hlusiak, Lee Howard, Zdenek Jaron, 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, John Bush, 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, Sean O'Sullivan, Vijay
   Rajagopalan, Greg Saccone, Rod Santiago, Kent Shuey, Brian Skeen,
   Mike Slane, Carrie Spiker, Katie Tran, Brendan Williams, Amelia
   Wilson, Julie Wulff, Yueli Yang, Eric Yeh and other members of the
   Boeing mobility, networking and autonomy 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]




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

   o  AERO, First Edition [RFC6706]

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

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

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

   This work is aligned with the Boeing 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

   [I-D.templin-6man-omni-interface]
              Templin, F. and T. Whyman, "Transmission of IPv6 Packets
              over Overlay Multilink Network (OMNI) Interfaces", draft-
              templin-6man-omni-interface-21 (work in progress), May
              2020.

   [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>.




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

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

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

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

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              DOI 10.17487/RFC4380, February 2006,
              <https://www.rfc-editor.org/info/rfc4380>.

   [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>.

   [RFC5175]  Haberman, B., Ed. and R. Hinden, "IPv6 Router
              Advertisement Flags Option", RFC 5175,
              DOI 10.17487/RFC5175, March 2008,
              <https://www.rfc-editor.org/info/rfc5175>.

   [RFC6081]  Thaler, D., "Teredo Extensions", RFC 6081,
              DOI 10.17487/RFC6081, January 2011,
              <https://www.rfc-editor.org/info/rfc6081>.





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   [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-15 (work in progress),
              March 2020.

   [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.

   [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.





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

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

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

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

   [I-D.templin-v6ops-pdhost]
              Templin, F., "IPv6 Prefix Delegation and Multi-Addressing
              Models", draft-templin-v6ops-pdhost-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>.



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   [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>.

   [RFC2492]  Armitage, G., Schulter, P., and M. Jork, "IPv6 over ATM
              Networks", RFC 2492, DOI 10.17487/RFC2492, January 1999,
              <https://www.rfc-editor.org/info/rfc2492>.

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

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

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

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

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

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

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




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   [RFC3330]  IANA, "Special-Use IPv4 Addresses", RFC 3330,
              DOI 10.17487/RFC3330, September 2002,
              <https://www.rfc-editor.org/info/rfc3330>.

   [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>.

   [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>.





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   [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>.

   [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>.







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   [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>.

   [RFC5991]  Thaler, D., Krishnan, S., and J. Hoagland, "Teredo
              Security Updates", RFC 5991, DOI 10.17487/RFC5991,
              September 2010, <https://www.rfc-editor.org/info/rfc5991>.

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

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

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

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

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






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

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

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

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

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935,
              DOI 10.17487/RFC6935, April 2013,
              <https://www.rfc-editor.org/info/rfc6935>.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, DOI 10.17487/RFC6936, April 2013,
              <https://www.rfc-editor.org/info/rfc6936>.

   [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>.

   [RFC7421]  Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
              Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
              Boundary in IPv6 Addressing", RFC 7421,
              DOI 10.17487/RFC7421, January 2015,
              <https://www.rfc-editor.org/info/rfc7421>.

   [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>.



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

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

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

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

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

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 7 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 7: Minimal Encapsulation Format using IP-in-IP

   Figure 8 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 8: 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
   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 Bridges in the
   communications path.  Direct interfaces must be tested periodically
   for reachability, e.g., via NUD.

B.4.  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)



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   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.5.  AERO Critical Infrastructure Considerations

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



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

B.6.  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 ReachableTime 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.7.  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,




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   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:

   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



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   (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
   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 >>





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   Changes from draft-templin-intarea-6706bis-48 to draft-templin-
   intrea-6706bis-49:

   o  SPAN Anycast address and SBM/PBM concepts introduced.

   Changes from draft-templin-intarea-6706bis-47 to draft-templin-
   intrea-6706bis-48:

   o  SEND/CGA.

   Changes from draft-templin-intarea-6706bis-46 to draft-templin-
   intrea-6706bis-47:

   o  Major changes to align with Teredo, including changed AERO "Relay"
      to "Bridge", and changed AERO "Gateway" to "Relay".  The term
      "[Rr]elay" now refers to exactly the same thing in both AERO and
      Teredo.

   o  Changed to use Teredo message authentication instead of SEND.

   Changes from draft-templin-intarea-6706bis-42 to draft-templin-
   intrea-6706bis-43:

   o  Segment Routing.

   Changes from draft-templin-intarea-6706bis-39 to draft-templin-
   intrea-6706bis-40:

   o  Teredo.

   Changes from draft-templin-intarea-6706bis-38 to draft-templin-
   intrea-6706bis-39:

   o  Major clarifications and simplifications of SPAN fragmentation/
      reassembly.

   o  Revised AERO address format to support prefix lengths up to 112.

   o  New method for forming SPAN Client Prefixes and population in the
      routing system.

   o  Updates RFC4443 to set a new value in the ICMP PTB Code field.

   Changes from draft-templin-intarea-6706bis-35 to draft-templin-
   intrea-6706bis-36:

   o  Clients in the open Internet secured using SEND/CGA.




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   Changes from draft-templin-intarea-6706bis-32 to draft-templin-
   intrea-6706bis-33:

   o  Updated Proxy discussion with "point-to-multipoint" server
      coordination

   o  Significant updates to Address Resolution and NUD to include
      correct addresses in messages

   o  Differentiate between NS(AR) and NS(NUD) as their addresses and
      use cases differ.

   Changes from draft-templin-intarea-6706bis-30 to draft-templin-
   intrea-6706bis-31:

   o  Added "advisory PTB messages" under FAA SE2025 contract number
      DTFAWA-15-D-00030.

   Changes from draft-templin-intarea-6706bis-29 to draft-templin-
   intrea-6706bis-30:

   o  Deprecate "primary" concept.  Now, RS/RA keepalives are maintained
      over *all* underlying interfaces (i.e., and not just one primary).

   Changes from draft-templin-intarea-6706bis-28 to draft-templin-
   intrea-6706bis-29:

   o  Changed OMNI interface citation to "draft-templin-6man-omni-
      interface"

   o  Changed SPAN Service Prefix to fd80::/10.

   o  Changed S/TLLAO format to include 'S' bit for ifIndex
      corresponding to the underlying interface that is Source of ND
      message.

   o  Updated Path MTU

   Changes from draft-templin-intarea-6706bis-27 to draft-templin-
   intrea-6706bis-28:

   o  MTU and fragmentation.

   Changes from draft-templin-intarea-6706bis-26 to draft-templin-
   intrea-6706bis-27:

   o  MTU and fragmentation.




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   o  SPAN Service Prefix set to fd00::/10

   o  Client SPAN addresses defined.

   Changes from draft-templin-intarea-6706bis-25 to draft-templin-
   intrea-6706bis-26:

   o  MTU and RA configuration information updated.

   Changes from draft-templin-intarea-6706bis-24 to draft-templin-
   intrea-6706bis-25:

   o  Added concept of "primary" to allow for proxyed RS/RA over only
      selected underlying interfaces.

   o  General Cleanup.

   Changes from draft-templin-intarea-6706bis-23 to draft-templin-
   intrea-6706bis-24:

   o  OMNI interface spec now a normative reference.

   o  Use REACHABLE_TIME as the nominal Router Lifetime to return in
      RAs.

   o  General cleanup.

   Changes from draft-templin-intarea-6706bis-22 to draft-templin-
   intrea-6706bis-23:

   o  Choice of using either RS/RA or unsolicited NA for old Server
      notification.

   o  General cleanup.

   Changes from draft-templin-intarea-6706bis-21 to draft-templin-
   intrea-6706bis-22:

   o  Tightened up text on Proxy.

   o  Removed unnecessarily restrictive texts.

   o  General cleanup.

   Changes from draft-templin-intarea-6706bis-20 to draft-templin-
   intrea-6706bis-21:

   o  Clarified relationship between OMNI and S/TLLAO ifIndex-tuples.



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   o  Important text in Section 13.15.3 on Servers timing out Clients
      that have gone silent without sending a departure notification.

   o  New text on RS/RA as "hints of forward progress" for proactive
      NUD.

   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.

   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
      that parallels 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



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   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

   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





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

   o  Changed Interface ID for Servers from 255 to 0xffff

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

   o  New Section on Multicast

   o  New Section on AERO Clients in the open Internetwork

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

   o  New Sections on DNS considerations and Transition considerations

   o

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

   o  Added The SPAN

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

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

   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




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

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

   o  Added network-based VPN under security considerations

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

   o  New section on AERO-Aware Access Router

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

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

   o  Clarifications on solicited vs unsolicited NAs

   o  Clarified use of MAX_NEIGHBOR_ADVERTISEMENT 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.




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

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

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

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

   o  Reworked Security Considerations

   o  Updated references.

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

   o  Added new section on SEND.

   o  Clarifications on "AERO Address" section.

   o  Updated references and added new reference for RFC8086.

   o  Security considerations updates.

   o  General text clarifications and cleanup.

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

   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.





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

   o  Emphasize distributed mobility management

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

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

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

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

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

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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