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Automatic Extended Route Optimization (AERO)
draft-templin-intarea-6706bis-97

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Document Type
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Author Fred Templin
Last updated 2021-03-22
Replaces draft-templin-aerolink
Replaced by draft-templin-6man-aero, draft-templin-6man-aero
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draft-templin-intarea-6706bis-97
Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720,                     March 22, 2021
           rfc6179, rfc6706 (if
           approved)
Intended status: Standards Track
Expires: September 23, 2021

              Automatic Extended Route Optimization (AERO)
                    draft-templin-intarea-6706bis-97

Abstract

   This document specifies an Automatic Extended Route Optimization
   (AERO) service for IP internetworking over Overlay Multilink Network
   (OMNI) interfaces.  AERO/OMNI 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, multicast, quality
   of service (QoS) signaling and route optimization are naturally
   supported through dynamic neighbor cache updates.  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 September 23, 2021.

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Copyright Notice

   Copyright (c) 2021 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.  Automatic Extended Route Optimization (AERO)  . . . . . . . .  12
     3.1.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .  12
     3.2.  The AERO Service over OMNI Links  . . . . . . . . . . . .  13
       3.2.1.  AERO/OMNI Reference Model . . . . . . . . . . . . . .  13
       3.2.2.  Addressing and Node Identification  . . . . . . . . .  15
       3.2.3.  AERO Routing System . . . . . . . . . . . . . . . . .  16
       3.2.4.  OMNI Link Segment Routing . . . . . . . . . . . . . .  18
       3.2.5.  Segment Routing Topologies (SRTs) . . . . . . . . . .  22
       3.2.6.  Segment Routing For OMNI Link Selection . . . . . . .  23
       3.2.7.  Segment Routing Within the OMNI Link  . . . . . . . .  23
     3.3.  OMNI Interface Characteristics  . . . . . . . . . . . . .  24
     3.4.  OMNI Interface Initialization . . . . . . . . . . . . . .  26
       3.4.1.  AERO Proxy/Server and Relay Behavior  . . . . . . . .  26
       3.4.2.  AERO Client Behavior  . . . . . . . . . . . . . . . .  26
       3.4.3.  AERO Bridge Behavior  . . . . . . . . . . . . . . . .  27
     3.5.  OMNI Interface Neighbor Cache Maintenance . . . . . . . .  27
       3.5.1.  OMNI Neighbor Interface Attributes  . . . . . . . . .  28
       3.5.2.  OMNI Neighbor Advertisement Message Flags . . . . . .  29
     3.6.  OMNI Interface Encapsulation and Re-encapsulation . . . .  29
     3.7.  OMNI Interface Decapsulation  . . . . . . . . . . . . . .  30
     3.8.  OMNI Interface Data Origin Authentication . . . . . . . .  30
     3.9.  OMNI Interface MTU  . . . . . . . . . . . . . . . . . . .  31
     3.10. OMNI Interface Forwarding Algorithm . . . . . . . . . . .  32
       3.10.1.  Client Forwarding Algorithm  . . . . . . . . . . . .  33
       3.10.2.  Proxy/Server Forwarding Algorithm  . . . . . . . . .  34
       3.10.3.  Bridge Forwarding Algorithm  . . . . . . . . . . . .  36
     3.11. OMNI Interface Error Handling . . . . . . . . . . . . . .  37
     3.12. AERO Router Discovery, Prefix Delegation and
           Autoconfiguration . . . . . . . . . . . . . . . . . . . .  39

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       3.12.1.  AERO Service Model . . . . . . . . . . . . . . . . .  40
       3.12.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  40
       3.12.3.  AERO Proxy/Server Behavior . . . . . . . . . . . . .  42
     3.13. The AERO Proxy  . . . . . . . . . . . . . . . . . . . . .  45
       3.13.1.  Combined Proxy/Servers . . . . . . . . . . . . . . .  48
       3.13.2.  Detecting and Responding to Proxy/Server Failures  .  48
       3.13.3.  Point-to-Multipoint Server Coordination  . . . . . .  49
     3.14. AERO Address Resolution . . . . . . . . . . . . . . . . .  49
       3.14.1.  Route Optimization Initiation  . . . . . . . . . . .  50
       3.14.2.  Relaying the NS(AR) *NET Packet(s) . . . . . . . . .  51
       3.14.3.  Processing the NS(AR) and Sending the NA(AR) . . . .  51
       3.14.4.  Relaying the NA(AR)  . . . . . . . . . . . . . . . .  53
       3.14.5.  Processing the NA(AR)  . . . . . . . . . . . . . . .  53
       3.14.6.  Route Optimization Maintenance . . . . . . . . . . .  53
     3.15. Neighbor Unreachability Detection (NUD) . . . . . . . . .  55
     3.16. Mobility Management and Quality of Service (QoS)  . . . .  56
       3.16.1.  Mobility Update Messaging  . . . . . . . . . . . . .  57
       3.16.2.  Announcing Link-Layer Address and/or QoS Preference
                Changes  . . . . . . . . . . . . . . . . . . . . . .  58
       3.16.3.  Bringing New Links Into Service  . . . . . . . . . .  58
       3.16.4.  Deactivating Existing Links  . . . . . . . . . . . .  59
       3.16.5.  Moving Between Servers . . . . . . . . . . . . . . .  59
     3.17. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  60
       3.17.1.  Source-Specific Multicast (SSM)  . . . . . . . . . .  61
       3.17.2.  Any-Source Multicast (ASM) . . . . . . . . . . . . .  62
       3.17.3.  Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . .  63
     3.18. Operation over Multiple OMNI Links  . . . . . . . . . . .  63
     3.19. DNS Considerations  . . . . . . . . . . . . . . . . . . .  64
     3.20. Transition Considerations . . . . . . . . . . . . . . . .  64
     3.21. Detecting and Reacting to Server and Bridge Failures  . .  65
     3.22. AERO Clients on the Open Internet . . . . . . . . . . . .  65
     3.23. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . .  69
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  69
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  70
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  70
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  72
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  73
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  73
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  75
   Appendix A.  Non-Normative Considerations . . . . . . . . . . . .  81
     A.1.  Implementation Strategies for Route Optimization  . . . .  81
     A.2.  Implicit Mobility Management  . . . . . . . . . . . . . .  82
     A.3.  Direct Underlying Interfaces  . . . . . . . . . . . . . .  82
     A.4.  AERO Critical Infrastructure Considerations . . . . . . .  82
     A.5.  AERO Server Failure Implications  . . . . . . . . . . . .  83
     A.6.  AERO Client / Server Architecture . . . . . . . . . . . .  84
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  86
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  87

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1.  Introduction

   Automatic 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 an internetworking and mobility management service
   that employs the Overlay Multilink Network Interface (OMNI)
   [I-D.templin-6man-omni-interface] Non-Broadcast, Multiple Access
   (NBMA) virtual link model.  The OMNI link is a virtual overlay
   configured over one or more underlying Internetworks, and nodes on
   the link can exchange original IP packets as single-hop neighbors via
   encapsulation and fragmentation.  The OMNI Adaptation Layer (OAL)
   supports multilink operation for increased reliability, bandwidth
   optimization and traffic path selection while performing
   fragmentation and reassembly to accommodate Maximum Transmission Unit
   (MTU) diversity.

   The AERO service comprises Clients, Proxy/Servers and Relays that are
   seen as OMNI link neighbors as well as Bridges that interconnect OMNI
   link segments through OAL forwarding at a layer below IP.  Each
   node's OMNI interface uses an IPv6 link-local address format that
   supports operation of the IPv6 Neighbor Discovery (ND) protocol
   [RFC4861] and links ND to IP forwarding.  A node's OMNI interface can
   be configured over multiple underlying interfaces, and therefore
   appears 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 provides a cloud-based service where mobile node Clients may use
   any Proxy/Server acting as a Mobility Anchor Point (MAP) and fixed
   nodes may use any Relay on the link for efficient communications.
   Fixed nodes forward original IP packets destined to other AERO nodes
   via the nearest Relay, which forwards them through the cloud.  A
   mobile node's initial packets are forwarded through the Proxy/Server,
   and direct routing is supported through automatic extended route
   optimization while 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 to provide an OAL routing/bridging service that
   joins the underlying Internetworks of multiple disjoint
   administrative domains into a single unified OMNI link at a layer
   below IP.  Each OMNI link instance is characterized by the set of
   Mobility Service Prefixes (MSPs) common to all mobile nodes.  The

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   link extends to the point where a Relay is on the optimal route from
   any correspondent node on the link, and provides a conduit between
   the underlying Internetwork and the OMNI link.  To the underlying
   Internetwork, the Relay is the source of a route to the MSP, and
   hence uplink traffic to the mobile node is naturally routed to the
   nearest Relay.

   AERO can be used with OMNI links that span private-use Internetworks
   and/or public Internetworks such as the global Internet.  In the
   latter case, some end systems may be located behind global Internet
   Network Address Translators (NATs).  A means for robust traversal of
   NATs while avoiding "triangle routing" is therefore provided.

   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 links of mobile nodes (e.g., cellphones,
   tablets, laptop computers, etc.) that connect into a home enterprise
   network via public access networks using tunneling software such as
   OpenVPN [OVPN] with VPN or non-VPN services enabled according to the
   appropriate security model.  AERO can also be used to facilitate
   terrestrial vehicular and urban air mobility (as well as pedestrian
   communication services) for future intelligent transportation systems
   [I-D.ietf-ipwave-vehicular-networking][I-D.templin-ipwave-uam-its].
   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; especially, the
   terminology in the OMNI specification
   [I-D.templin-6man-omni-interface] is used extensively throughout.
   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 uses
      the ND service specified in [RFC4861].

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   IPv6 Prefix Delegation
      a networking service for delegating IPv6 prefixes to nodes on the
      link.  The nominal service is DHCPv6 [RFC8415], however alternate
      services (e.g., based on ND messaging) are also in scope.  Most
      notably, a minimal form of prefix delegation 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, etc.) that often provides link-layer security services
      such as IEEE 802.1X and physical-layer security (e.g., "protected
      spectrum") to prevent unauthorized access internally and with
      border network-layer security services such as firewalls and
      proxys that prevent unauthorized outside access.

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

   *NET
      a "wildcard" term referring to either ANET or INET when it is not
      necessary to draw a distinction between the two.

   *NET interface
      a node's attachment to a link in a *NET.

   *NET Partition
      frequently, *NETs 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 *NET partition is seen as a separate OMNI link
      segment as discussed below.)

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

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

   OMNI link
      the same as defined in [I-D.templin-6man-omni-interface], and
      manifested by IPv6 encapsulation [RFC2473].  The OMNI link spans
      underlying INET segments joined by virtual bridges in a spanning
      tree the same as a bridged campus LAN.  AERO nodes on the OMNI
      link appear as single-hop neighbors even though they may be
      separated by multiple underlying INET hops, and can use Segment
      Routing [RFC8402] to cause packets to visit selected waypoints on
      the link.

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

   OMNI Adaptation Layer (OAL)
      an OMNI interface process whereby original IP packets admitted
      into the interface are wrapped in a mid-layer IPv6 header and
      subject to fragmentation and reassembly.  The OAL is also
      responsible for generating MTU-related control messages as
      necessary, and for providing addressing context for spanning
      multiple segments of a bridged OMNI link.

   original IP packet
      a whole IP packet or fragment admitted into the OMNI interface by
      the network layer prior to OAL encapsulation and fragmentation, or
      an IP packet delivered to the network layer by the OMNI interface
      following OAL decapsulation and reassembly.

   OAL packet
      an original IP packet encapsulated in OAL headers and trailers
      before OAL fragmentation, or following OAL reassembly.

   OAL fragment
      a portion of an OAL packet following fragmentation but prior to
      *NET encapsulation, or following *NET encapsulation but prior to
      OAL reassembly.

   (OAL) carrier packet

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      an encapsulated OAL fragment following *NET encapsulation or prior
      to *NET decapsulation.  OAL sources and destinations exchange
      carrier packets over underlying interfaces, and may be separated
      by one or more OAL intermediate nodes.  OAL intermediate nodes may
      perform re-encapsulation on carrier packets by removing the *NET
      headers of the first hop network and replacing them with new *NET
      headers for the next hop network.

   OAL source
      an OMNI interface acts as an OAL source when it encapsulates
      original IP packets to form OAL packets, then performs OAL
      fragmentation and *NET encapsulation to create carrier packets.

   OAL destination
      an OMNI interface acts as an OAL destination when it decapsulates
      carrier packets, then performs OAL reassembly and decapsulation to
      derive the original IP packet.

   OAL intermediate node
      an OMNI interface acts as an OAL intermediate node when it removes
      the *NET headers of carrier packets received on a first segment,
      then re-encapsulates the carrier packets in new *NET headers and
      forwards them into the next segment.

   underlying interface
      a *NET interface over which an OMNI interface is configured.

   Mobility Service Prefix (MSP)
      an aggregated IP Global Unicast Address (GUA) prefix (e.g.,
      2001:db8::/32, 192.0.2.0/24, etc.) assigned to the OMNI link and
      from which more-specific Mobile Network Prefixes (MNPs) are
      delegated.  OMNI link administrators typically obtain MSPs from an
      Internet address registry, however private-use prefixes can
      alternatively be used subject to certain limitations (see:
      [I-D.templin-6man-omni-interface]).  OMNI links that connect to
      the global Internet advertise their MSPs to their interdomain
      routing peers.

   Mobile Network Prefix (MNP)
      a longer IP prefix delegated from an MSP (e.g.,
      2001:db8:1000:2000::/56, 192.0.2.8/30, etc.) and delegated to an
      AERO Client or Relay.

   Mobile Network Prefix Link Local Address (MNP-LLA)
      an IPv6 Link Local Address that embeds the most significant 64
      bits of an MNP in the lower 64 bits of fe80::/64, as specified in
      [I-D.templin-6man-omni-interface].

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   Mobile Network Prefix Unique Local Address (MNP-ULA)
      an IPv6 Unique-Local Address derived from an MNP-LLA.

   Administrative Link Local Address (ADM-LLA)
      an IPv6 Link Local Address that embeds a 32-bit administratively-
      assigned identification value in the lower 32 bits of fe80::/96,
      as specified in [I-D.templin-6man-omni-interface].

   Administrative Unique Local Address (ADM-ULA)
      an IPv6 Unique-Local Address derived from an ADM-LLA.

   AERO node
      a node that is connected to an OMNI link and participates in the
      AERO internetworking and mobility service.

   AERO Client ("Client")
      an AERO node that connects over one or more underlying interfaces
      and requests MNP delegation/registration service from AERO Proxy/
      Servers.  The Client assigns an MNP-LLA to the OMNI interface for
      use in ND exchanges with other AERO nodes and forwards original IP
      packets to correspondents according to OMNI interface neighbor
      cache state.

   AERO Proxy/Server ("Proxy/Server")
      a dual-function node that provides a proxying service between AERO
      Clients and external peers on its Client-facing ANET interfaces
      (i.e., in the same fashion as for an enterprise network proxy) as
      well as default forwarding and Mobility Anchor Point (MAP)
      services for AERO Clients on its INET-facing interfaces.  The
      Proxy/Server configures an OMNI interface and assigns an ADM-LLA
      to support the operation of IPv6 ND services, while advertising
      all of its associated MNPs via BGP peerings with Bridges.  Note
      that the Proxy and Server functions can be considered logically
      separable, but since each Proxy/Server must be informed of all of
      the Client's other multilink Proxy/Server affiliations the AERO
      service is best supported when the two functions are coresident on
      the same physical or logical platform.

   AERO Relay ("Relay")
      a Proxy/Server that provides forwarding services between nodes
      reached via the OMNI link and correspondents on connected
      downstream links.  AERO Relays configure an OMNI interface and
      assign an ADM-LLA the same as Proxy/Servers.  AERO Relays also run
      a dynamic routing protocol to discover any non-MNP IP GUA routes
      in service on its connected downstream network links.  In both
      cases, the Relay advertises the MSP(s) to its downstream networks,
      and distributes all of its associated non-MNP IP GUA routes via
      BGP peerings with Bridges (i.e., the same as for Proxy/Servers).

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   AERO Bridge ("Bridge")
      a node that provides hybrid routing/bridging services (as well as
      a security trust anchor) for nodes on an OMNI link.  The Bridge
      forwards carrier packets between OMNI link segments as OAL
      intermediate nodes without decrementing the network layer IP TTL/
      Hop Limit.  AERO Bridges peer with Proxy/Servers and other Bridges
      to discover the full set of MNPs for the link as well as any non-
      MNP IP GUA routes that are reachable via Relays.

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

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

   link-layer address
      an IP address used as an encapsulation header source or
      destination address from the perspective of the OMNI 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.

   network layer address
      the source or destination address of an original IP packet
      presented to the OMNI 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 OMNI
      interface.  The Client/Relay sees each EUN as a "downstream"
      network, and sees the OMNI 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 original IP packets between
      any downstream-attached networks and the OMNI link.  The MR is the
      MN entity that hosts the AERO Client.

   Route Optimization Source (ROS)

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      the AERO node nearest the source that initiates route
      optimization.  The ROS may be a Proxy/Server acting on behalf of
      the source Client, or may be the Client itself.

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

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

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

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

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

   Throughout the document, the simple terms "Client", "Proxy/Server",
   "Bridge" and "Relay" refer to "AERO Client", "AERO Proxy/Server",
   "AERO Bridge" 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].  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.  Automatic Extended Route Optimization (AERO)

   The following sections specify the operation of IP over OMNI links
   using the AERO service:

3.1.  AERO Node Types

   AERO Clients are Mobile Nodes (MNs) that configure OMNI interfaces
   over underlying interfaces with addresses that may change when the
   Client moves to a new network connection point.  AERO Clients
   register their Mobile Network Prefixes (MNPs) with the AERO service,
   and distribute the MNPs to nodes on EUNs.  AERO Bridges, Proxy/
   Servers and Relays are critical infrastructure elements in fixed
   (i.e., non-mobile) INET deployments and hence have permanent and
   unchanging INET addresses.  Together, they constitute the AERO
   service which provides an OMNI link virtual overlay for connecting
   AERO Clients.

   AERO Bridges provide hybrid routing/bridging services (as well as a
   security trust anchor) for nodes on an OMNI link.  Bridges use
   standard IPv6 routing to forward carrier packets both within the same
   *NET partition and between disjoint *NET partitions based on an IPv6
   encapsulation mid-layer known as the OMNI Adaptation Layer (OAL)
   [I-D.templin-6man-omni-interface].  During forwarding, the inner IP
   layer experiences a virtual bridging service since the inner IP TTL/
   Hop Limit is not decremented.  Each Bridge also peers with Proxy/
   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 3.2.3).  Bridges present the OMNI link as
   a set of one or more Mobility Service Prefixes (MSPs) and configure
   secured tunnels with Proxy/Servers, Relays and other Bridges; they
   further maintain IP forwarding table entries for each MNP and any
   other reachable non-MNP prefixes.

   AERO Proxy/Servers in distributed INET locations provide default
   forwarding and mobility/multilink services for AERO Client Mobile
   Nodes (MNs).  Each Proxy/Server also peers with Bridges in a dynamic
   routing protocol instance to advertise its list of associated MNPs
   (see Section 3.2.3).  Proxy/Servers facilitate prefix delegation/
   registration exchanges with Clients, where each delegated prefix
   becomes an MNP taken from an MSP.  Proxy/Servers forward carrier
   packets between OMNI interface neighbors and track each Client's
   mobility profiles.  Proxy/Servers provide a conduit for ANET Clients
   to associate with additional Proxy/Servers in external INETs.  The
   Proxy forwards original IP packets between Clients and the OMNI link
   according to forwarding information in the neighbor cache.

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   AERO Relays are Proxy/Servers that provide forwarding services to
   exchange original IP packets between the OMNI 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
   routes via BGP peerings with Bridges

3.2.  The AERO Service over OMNI Links

3.2.1.  AERO/OMNI Reference Model

   Figure 1 presents the basic OMNI link reference model:

                          +----------------+
                          | AERO Bridge B1 |
                          | Nbr: S1, S2, P1|
                          |(X1->S1; X2->S2)|
                          |      MSP M1    |
                          +-+------------+-+
       +--------------+     |  Secured   |     +--------------+
       |  AERO P/S S1 |     |  tunnels   |     |  AERO P/S S2 |
       |  Nbr: C1, B1 +-----+            +-----+  Nbr: C2, B1 |
       |  default->B1 |                        |  default->B1 |
       |    X1->C1    |                        |    X2->C2    |
       +-------+------+                        +------+-------+
               |       OMNI link                      |
       X===+===+======================================+===+===X
           |                                              |
     +-----+--------+                            +--------+-----+
     |AERO Client C1|                            |AERO Client C2|
     |    Nbr: S1   |                            |   Nbr: S2    |
     | default->S1  |                            | default->S2  |
     |    MNP X1    |                            |    MNP X2    |
     +------+-------+                            +-----+--------+
            |                                          |
           .-.                                        .-.
        ,-(  _)-.                                  ,-(  _)-.
     .-(_  IP   )-.   +-------+     +-------+    .-(_  IP   )-.
   (__    EUN      )--|Host H1|     |Host H2|--(__    EUN      )
      `-(______)-'    +-------+     +-------+     `-(______)-'

                    Figure 1: AERO/OMNI Reference Model

   In this model:

   o  the OMNI link is an overlay network service configured over one or
      more underlying *NET partitions which may be managed by different

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      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 Proxy/Servers (S1, S2).
      Bridges connect the disjoint segments of a partitioned OMNI link.

   o  AERO Proxy/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 Proxy/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.

   An OMNI link configured over a single *NET appears as a single
   unified link with a consistent underlying network addressing plan.
   In that case, all nodes on the link can exchange carrier packets via
   simple *NET encapsulation, since the underlying *NET is connected.
   In common practice, however, an OMNI link may be partitioned into
   multiple "segments", where each segment is a distinct *NET
   potentially managed under a different administrative authority (e.g.,
   as for worldwide aviation service providers such as ARINC, SITA,
   Inmarsat, etc.).  Individual *NETs 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, proxys, 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 carrier 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 OMNI link segments
   can be joined into a single unified link via a virtual bridging
   service using the OMNI Adaptation Layer (OAL)
   [I-D.templin-6man-omni-interface] which inserts a mid-layer IPv6
   encapsulation header that supports inter-segment forwarding (i.e.,
   bridging) without decrementing the network-layer TTL/Hop Limit.  This
   bridging of OMNI link segments is shown in Figure 2:

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                 . . . . . . . . . . . . . . . . . . . . . . .
               .                                               .
               .              .-(::::::::)                     .
               .           .-(::::::::::::)-.   +-+            .
               .          (:::: Segment A :::)--|B|---+        .
               .           `-(::::::::::::)-'   +-+   |        .
               .              `-(::::::)-'            |        .
               .                                      |        .
               .              .-(::::::::)            |        .
               .           .-(::::::::::::)-.   +-+   |        .
               .          (:::: Segment B :::)--|B|---+        .
               .           `-(::::::::::::)-'   +-+   |        .
               .              `-(::::::)-'            |        .
               .                                      |        .
               .              .-(::::::::)            |        .
               .           .-(::::::::::::)-.   +-+   |        .
               .          (:::: Segment C :::)--|B|---+        .
               .           `-(::::::::::::)-'   +-+   |        .
               .              `-(::::::)-'            |        .
               .                                      |        .
               .                ..(etc)..             x        .
               .                                               .
               .                                               .
               .    <- OMNI link Bridged by encapsulation ->   .
                 . . . . . . . . . . . . . .. . . . . . . . .

                   Figure 2: Bridging OMNI Link Segments

   Bridges, Proxy/Servers and Relays connect via secured INET tunnels
   over their respective segments in a spanning tree topology rooted at
   the Bridges.  The secured spanning tree supports strong
   authentication for IPv6 ND control messages and may also be used to
   convey the initial carrier packets in a flow.  Route optimization can
   then be employed to cause carrier packets to take more direct paths
   between OMNI link neighbors without having to strictly follow the
   spanning tree.

3.2.2.  Addressing and Node Identification

   AERO nodes on OMNI links use the Link-Local Address (LLA) prefix
   fe80::/64 [RFC4291] to assign LLAs used for network-layer addresses
   in link-scoped IPv6 ND and data messages.  AERO Clients use LLAs
   constructed from MNPs (i.e., "MNP-LLAs") while other AERO nodes use
   LLAs constructed from administrative identification values ("ADM-
   LLAs") as specified in [I-D.templin-6man-omni-interface].

   AERO nodes also use the Unique Local Address (ULA) prefix fd00::/8
   followed by a pseudo-random 40-bit OMNI domain identifier to form the

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   prefix [ULA]::/48, then include a 16-bit OMNI link identifier '*' to
   form the prefix [ULA*]::/64 [RFC4291].  The AERO node then uses the
   prefix [ULA*]::/64 to form "MNP-ULAs" or "ADM-ULA"s as specified in
   [I-D.templin-6man-omni-interface] to support OAL addressing.  AERO
   Clients also use Temporary ULAs constructed per
   [I-D.templin-6man-omni-interface], where the addresses are typically
   used only in initial control message exchanges until a stable MNP-
   LLA/ULA is assigned.

   AERO MSPs, MNPs and non-MNP routes are typically based on Global
   Unicast Addresses (GUAs), but in some cases may be based on private-
   use addresses.  See [I-D.templin-6man-omni-interface] for a full
   specification of LLAs, ULAs and GUAs used by AERO nodes on OMNI
   links.

   Finally, AERO Clients and Proxy/Servers configure node identification
   values as specified in [I-D.templin-6man-omni-interface].

3.2.3.  AERO Routing System

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

   In a reference deployment, each Proxy/Server is configured as an
   Autonomous System Border Router (ASBR) for a stub Autonomous System
   (AS) using a 32-bit AS Number (ASN) [RFC4271] that is unique within
   the BGP instance, and each Proxy/Server further uses eBGP to peer
   with one or more Bridges but does not peer with other Proxy/Servers.
   Each *NET of a multi-segment OMNI link must include one or more
   Bridges, which peer with the Proxy/Servers within that *NET.  All
   Bridges within the same *NET are members of the same hub AS, and use
   iBGP to maintain a consistent view of all active routes currently in
   service.  The Bridges of different *NETs peer with one another using
   eBGP.

   Bridges maintain forwarding table entries only for the ULAs
   corresponding to MNP and non-MNP routes that are currently active,
   and carrier packets destined to all other ULAs will correctly incur
   Destination Unreachable messages due to the black-hole route.  In
   this way, Proxy/Servers and Relays have only partial topology
   knowledge (i.e., they know only about the routes their directly
   associated Clients and non-AERO links) and they forward all other
   carrier packets to Bridges which have full topology knowledge.

   Each OMNI link segment assigns a unique ADM-ULA sub-prefix of
   [ULA*]::/96.  For example, a first segment could assign

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   [ULA*]::1000/116, a second could assign [ULA*]::2000/116, a third
   could assign [ULA*]::3000/116, etc.  Within each segment, each Proxy/
   Server configures an ADM-ULA within the segment's prefix, e.g., the
   Proxy/Servers within [ULA*]::2000/116 could assign the ADM-ULAs
   [ULA*]::2011/116, [ULA*]::2026/116, [ULA*]::2003/116, etc.

   The administrative authorities for each segment must therefore
   coordinate to assure mutually-exclusive ADM-ULA prefix assignments,
   but internal provisioning of ADM-ULAs an independent local
   consideration for each administrative authority.  For each ADM-ULA
   prefix, the Bridge(s) that connect that segment assign the all-zero's
   address of the prefix as a Subnet Router Anycast address.  For
   example, the Subnet Router Anycast address for [ULA*]::1023/116 is
   simply [ULA*]::1000.

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

   A: [ULA*]::1000/116->local, [ULA*]::2000/116->B, [ULA*]::3000/116->C

   B: [ULA*]::1000/116->A, [ULA*]::2000/116->local, [ULA*]::3000/116->C

   C: [ULA*]::1000/116->A, [ULA*]::2000/116->B, [ULA*]::3000/116->local

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

   MNP ULAs are instead dynamically advertised in the AERO routing
   system by Proxy/Servers and Relays that provide service for their
   corresponding MNPs.  For example, if three Proxy/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: [ULA*]:2001:db8:1000:2000/120

   E: [ULA*]:2001:db8:3000:4000/120

   F: [ULA*]:2001:db8:5000:6000/120

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   A full discussion of the BGP-based routing system used by AERO is
   found in [I-D.ietf-rtgwg-atn-bgp].

3.2.4.  OMNI Link Segment Routing

   With the Client and partition prefixes in place in Bridge forwarding
   tables, the OMNI interface sends control and data messages toward
   AERO destination nodes located in different OMNI link segments over
   the spanning tree.  The OMNI interface uses the OMNI Adaptation Layer
   (OAL) encapsulation service [I-D.templin-6man-omni-interface], and
   includes an OMNI Routing Header (ORH) as an extension to the OAL
   header if final segment forwarding information is available, e.g., in
   the neighbor cache.  (For nodes located in the same OMNI link
   segment, or when no final segment forwarding information is
   available, the ORH may be omitted.)  The ORH is formatted as shown in
   Figure 3:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  |  Hdr Ext Len  |  Routing Type |   SRT   | FMT |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Last Hop Segment-id (LHS)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                   Link Layer Address (L2ADDR)                 ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                Destination Suffix (if necessary)              ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                  Null Padding (if necessary)                  ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: OMNI Routing Header (ORH) Format

   In this format:

   o  Next Header identifies the type of header immediately following
      the ORH.

   o  Hdr Ext Len is the length of the Routing header in 8-octet units
      (not including the first 8 octets), with trailing padding added if
      necessary to produce an integral number of 8-octet units.

   o  Routing Type is set to TBD1 (see IANA Considerations).

   o  Segments Left is omitted, and replaced by a 5-bit SRT and 3-bit
      FMT field.

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   o  SRT - a 5-bit Segment Routing Topology prefix length value that
      (when added to 96) determines the prefix length to apply to the
      ADM-ULA formed from concatenating [ULA*]::/96 with the 32 bit LHS
      value that follows (for example, the value 16 corresponds to the
      prefix length 112).

   o  FMT - a 3-bit "Framework/Mode/Type" code corresponding to the
      included Link Layer Address as follows:

      *  When the most significant bit (i.e., "Framework") is set to 1,
         L2ADDR is the *NET encapsulation address for the target Client
         itself; otherwise L2ADDR is the address of the Proxy/Server
         named in the LHS.

      *  When the next most significant bit (i.e., "Mode") is set to 1,
         the Framework node is (likely) located behind a *NET Network
         Address Translator (NAT); otherwise, it is on the open *NET.

      *  When the least significant bit (i.e., "Type") is set to 0,
         L2ADDR includes a UDP Port Number followed by an IPv4 address;
         otherwise, it includes a UDP Port Number followed by an IPv6
         address.

   o  LHS - the 32 bit ID of a node in the Last Hop Segment that
      services the target.  When SRT and LHS are both set to 0, the LHS
      is considered unspecified.  When SRT is set to 0 and LHS is non-
      zero, the prefix length is set to 128.  SRT and LHS provide
      guidance to the OMNI interface forwarding algorithm.
      Specifically, if SRT/LHS is located in the local OMNI link
      segment, the OAL source can omit the ORH and (following any
      necessary NAT traversal messaging) send directly to the OAL
      destination according to FMT/L2ADDR.  Otherwise, it includes the
      ORH and forwards according to the OMNI link spanning tree.

   o  Link Layer Address (L2ADDR) - Formatted according to FMT, and
      identifies the link-layer address (i.e., the encapsulation
      address) of the target.  The UDP Port Number appears in the first
      two octets and the IP address appears in the next 4 octets for
      IPv4 or 16 octets for IPv6.  The Port Number and IP address are
      recorded in network byte order, and in ones-compliment
      "obfuscated" form per [RFC4380].  The OMNI interface forwarding
      algorithm uses FMT/L2ADDR to determine the *NET encapsulation
      address for local forwarding when SRT/LHS is located in the same
      OMNI link segment.  Note that if the target is behind a NAT,
      L2ADDR will contain the mapped *NET address stored in the NAT;
      otherwise, L2ADDR will contain the native *NET information of the
      target itself.

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   o  Destination Suffix is a 64-bit field included only for OAL non-
      first-fragments.  Present only when Hdr Ext Len indicates that at
      least 8 bytes follow L2ADDR.  When present, encodes the 64-bit
      MNP-ULA suffix for the target Client.

   o  Null Padding contains zero-valued octets as necessary to pad the
      ORH to an integral number of 8-octet units.

   AERO neighbors use OAL encapsulation and fragmentation to exchange
   OAL packets as specified in [I-D.templin-6man-omni-interface].  When
   an AERO node's OMNI interface (acting as an OAL source) uses OAL
   encapsulation for an original IP packet with source address
   2001:db8:1:2::1 and destination address 2001:db8:1234:5678::1, it
   sets the OAL header source address to its own ULA (e.g.,
   [ULA*]::2001:db8:1:2), sets the destination address to the MNP-ULA
   corresponding to the IP destination address (e.g.,
   [ULA*]::2001:db8:1234:5678), selects an Identification and appends an
   OAL checksum.

   If the neighbor cache information indicates that the target is in a
   different segment, the OAL source next inserts an ORH immediately
   following the OAL header while including the correct SRT, FMT, LHS,
   L2ADDR and Destination Suffix if fragmentation if needed (in this
   case, the Destination Suffix is 2001:db8:1234:5678).  Next, the OAL
   source overwrites the OAL header destination address with the LHS
   Subnet Router Anycast address (for example, for LHS 3000:4567 with
   SRT 16, the Subnet Router Anycast address is [ULA*]::3000:0000).
   (Note: if the ADM-ULA of the last-hop Proxy/Server is known but the
   SRT, FMT, LHS and L2ADDR are not (yet) known, the OAL source instead
   overwrites the OAL header destination address with the ADM-ULA.)

   The OAL source then fragments the OAL packet, with each resulting OAL
   fragment including the OAL/ORH headers while only the first fragment
   includes the original IPv6 header.  (Note that the packet is prepared
   as an "atomic" OAL fragment even if no actual fragmentation was
   required.)  The OAL source finally encapsulates each resulting OAL
   fragment in an *NET header to form an OAL carrier packet, with source
   address set to its own *NET address (e.g., 192.0.2.100) and
   destination set to the *NET address of a Bridge (e.g., 192.0.2.1).

   The carrier packet encapsulation format in the above example is shown
   in Figure 4:

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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |          *NET Header          |
        |       src = 192.0.2.100       |
        |        dst = 192.0.2.1        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        OAL IPv6 Header        |
        |  src = [ULA*]::2001:db8:1:2   |
        |    dst= [ULA*]::3000:0000     |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |       ORH (if necessary)      |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      OAL Fragment Header      |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |       Original IP Header      |
        |     (first-fragment only)     |
        |    src = 2001:db8:1:2::1      |
        |  dst = 2001:db8:1234:5678::1  |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |
        ~                               ~
        ~ Original Packet Body/Fragment ~
        ~                               ~
        |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 4: Carrier Packet Format

   In this format, the original IP header and packet body/fragment are
   from the original IP packet, the OAL header is an IPv6 header
   prepared according to [RFC2473], the ORH is a Routing Header
   extension of the OAL header the Fragment Header identifies each
   fragment, and the INET header is prepared as discussed in
   Section 3.6.  When the OAL source transmits the resulting carrier
   packets, they are forwarded over possibly multiple OMNI link spanning
   tree hops until they arrive at the OAL destination.

   This gives rise to a routing system that contains both Client MNP-ULA
   routes that may change dynamically due to regional node mobility and
   per-partition ADM-ULA routes that rarely if ever change.  The Bridges
   can therefore provide link-layer bridging by sending carrier packets
   over the spanning tree instead of network-layer routing according to
   MNP routes.  As a result, opportunities for loss due to node mobility
   between different segments are mitigated.

   In normal operations, IPv6 ND messages are conveyed over secured
   paths between OMNI link neighbors so that specific Proxy/Servers or
   Relays can be addressed without being subject to mobility events.
   Conversely, only the first few carrier packets destined to Clients

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   need to traverse secured paths until route optimization can determine
   a more direct path.

   Note: When the OAL source and destination are on the same INET
   segment, the ORH is not needed and OAL header compression can be used
   to significantly reduce encapsulation overhead
   [I-D.templin-6man-omni-interface].

   Note: When the OAL source has multiple original IP packets to send to
   the same OAL destination, it can perform "packing" to generate a
   "super-packet" [I-D.templin-6man-omni-interface].  In that case, the
   OAL/ORH super-packet may include up to N original IP packets as long
   as the total length of the super-packet does not exceed the OMNI
   interface MTU.

   Note: Use of an IPv6 "minimal encapsulation" format (i.e., an IPv6
   variant of [RFC2004]) based on extensions to the ORH was considered
   and abandoned.  In the approach, the ORH would be inserted as an
   extension header to the original IPv6 packet header.  The IPv6
   destination address would then be written into the ORH, and the ULA
   corresponding to the destination would be overwritten in the IPv6
   destination address.  This would seemingly convey enough forwarding
   information so that OAL encapsulation could be avoided.  However,
   this "minimal encapsulation" IPv6 packet would then have a non-ULA
   source address and ULA destination address, an incorrect value in
   upper layer protocol checksums, and a Hop Limit that is decremented
   within the spanning tree when it should not be.  The insertion and
   removal of the ORH would also entail rewriting the Payload Length and
   Next Header fields - again, invalidating upper layer checksums.
   These irregularities would result in implementation challenges and
   the potential for operational issues, e.g., since actionable ICMPv6
   error reports could not be delivered to the original source.  In
   order to address the issues, still more information such as the
   original IPv6 source address could be written into the ORH.  However,
   with the additional information the benefit of the "minimal
   encapsulation" savings quickly diminishes, and becomes overshadowed
   by the implementation and operational irregularities.

3.2.5.  Segment Routing Topologies (SRTs)

   The 64-bit sub-prefixes of [ULA]::/48 identify up to 2^16 distinct
   Segment Routing Topologies (SRTs).  Each SRT is a mutually-exclusive
   OMNI link overlay instance using a distinct set of ULAs, and emulates
   a Virtual LAN (VLAN) service for the OMNI link.  In some cases (e.g.,
   when redundant topologies are needed for fault tolerance and
   reliability) it may be beneficial to deploy multiple SRTs that act as
   independent overlay instances.  A communication failure in one
   instance therefore will not affect communications in other instances.

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   Each SRT is identified by a distinct value in bits 48-63 of
   [ULA]::/48, i.e., as [ULA0]::/64, [ULA1]::/64, [ULA2]::/64, etc.
   Each OMNI interface is identified by a unique interface name (e.g.,
   omni0, omni1, omni2, etc.) and assigns an anycast ADM-ULA
   corresponding to its SRT prefix length.  The anycast ADM-ULA is used
   for OMNI interface determination in Safety-Based Multilink (SBM) as
   discussed in [I-D.templin-6man-omni-interface].  Each OMNI interface
   further applies Performance-Based Multilink (PBM) internally.

3.2.6.  Segment Routing For OMNI Link Selection

   An original IPv6 source can direct an IPv6 packet to an AERO node by
   including a standard IPv6 Segment Routing Header (SRH) [RFC8754] with
   the anycast ADM-ULA for the selected SRT as either the IPv6
   destination or as an intermediate hop within the SRH.  This allows
   the original source to determine the specific OMNI link topology an
   original IPv6 packet will traverse when there may be multiple
   alternatives.

   When the AERO node processes the SRH and forwards the original IPv6
   packet to the correct OMNI interface, the OMNI interface writes the
   next IPv6 address from the SRH into the IPv6 destination address and
   decrements Segments Left.  If decrementing would cause Segments Left
   to become 0, the OMNI interface deletes the SRH before forwarding.
   This form of Segment Routing supports Safety-Based Multilink (SBM).

3.2.7.  Segment Routing Within the OMNI Link

   OAL sources can insert an ORH for Segment Routing within the OMNI
   link to influence the paths of OAL packets sent to OAL destinations
   in remote segments without requiring all carrier packets to traverse
   strict spanning tree paths.

   When an AERO node's OMNI interface has an original IP packet to send
   to a target discovered through route optimization located in the same
   OMNI link segment, it acts as an OAL source to perform OAL
   encapsulation and fragmentation.  The node then uses the target's
   Link Layer Address (L2ADDR) information for *NET encapsulation.

   When an AERO node's OMNI interface has an original IP packet to send
   to a route optimization target located in a remote OMNI link segment,
   it acts as an OAL source the same as above but also includes an ORH
   while setting the OAL destination to the Subnet Router Anycast
   address for the final OMNI link segment, then forwards the resulting
   carrier packets to a Bridge.

   When a Bridge receives a carrier packet destined to its Subnet Router
   Anycast address with an ORH with SRT/LHS values corresponding to the

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   local segment, it examines the L2ADDR according to FMT and removes
   the ORH from the carrier packet.  The Bridge then writes the MNP-ULA
   corresponding to the ORH Destination Suffix into the OAL destination
   address, re-encapsulates the carrier packet according to L2ADDR and
   forwards the carrier packet either to the LHS Proxy/Server or
   directly to the target Client itself.  In this way, the Bridge
   participates in route optimization to reduce traffic load and
   suboptimal routing through strict spanning tree paths.

3.3.  OMNI Interface Characteristics

   OMNI 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 NATs.  Native INET interfaces have global IP addresses
      that are reachable from any INET correspondent.  The INET-facing
      interfaces of Proxy/Servers are native interfaces, as are Relay
      and Bridge interfaces.  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 carrier packets
      flowing.

   o  ANET interfaces connect to an ANET that is separated from the open
      INET by a Proxy/Server.  Proxy/Servers can actively issue control
      messages over the INET on behalf of the Client to reduce ANET
      congestion.

   o  VPNed interfaces use security encapsulation over the INET to a
      Virtual Private Network (VPN) server that also acts as a Proxy/
      Server.  Other than the link-layer encapsulation format, VPNed
      interfaces behave the same as Direct interfaces.

   o  Direct (i.e., single-hop point-to-point) interfaces connect a
      Client directly to a Proxy/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.

   OMNI interfaces use OAL encapsulation and fragmentation as discussed
   in Section 3.2.4.  OMNI interfaces use *NET encapsulation (see:
   Section 3.6) to exchange carrier packets with OMNI link neighbors
   over INET or VPNed interfaces as well as over ANET interfaces for
   which the Client and Proxy/Server may be multiple IP hops away.  OMNI
   interfaces do not use link-layer encapsulation over Direct underlying
   interfaces or ANET interfaces when the Client and Proxy/Server are
   known to be on the same underlying link.

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

   OMNI interfaces send ND messages with an OMNI option formatted as
   specified in [I-D.templin-6man-omni-interface].  The OMNI option
   includes prefix registration information and Interface Attributes
   containing link information parameters for the OMNI interface's
   underlying interfaces.  Each OMNI option may include multiple
   Interface Attributes sub-options, each identified by an ifIndex
   value.

   A Client's OMNI 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
   Interface Attributes sub-option 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 Interface Attributes sub-options - each with values that
   correspond to a specific interface.  Every ND message need not
   include Interface Attributes for all underlying interfaces; for any
   attributes not included, the neighbor considers the status as
   unchanged.

   Bridge and Proxy/Server OMNI interfaces may be configured over one or
   more secured tunnel interfaces.  The OMNI interface configures both
   an ADM-LLA and its corresponding ADM-ULA, while the underlying
   secured tunnel interfaces are either unnumbered or configure the same
   ULA.  The OMNI interface acting as an OAL source encapsulates and
   fragments each original IP packet, then and presents the resulting
   carrier packets to the underlying secured tunnel interface.  Routing
   protocols such as BGP that run over the OMNI interface do not employ

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   OAL encapsulation, but rather present the routing protocol messages
   directly to the underlying secured tunnels while using the ULA as the
   source address.  This distinction must be honored consistently
   according to each node's configuration so that the IP forwarding
   table will associate discovered IP routes with the correct interface.

3.4.  OMNI Interface Initialization

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

   OMNI interface initialization procedures for Proxy/Servers, Clients
   and Bridges are discussed in the following sections.

3.4.1.  AERO Proxy/Server and Relay Behavior

   When a Proxy/Server enables an OMNI interface, it assigns an
   ADM-{LLA,ULA} appropriate for the given OMNI link segment.  The
   Proxy/Server also configures secured tunnels with one or more
   neighboring Bridges and engages in a BGP routing protocol session
   with each Bridge.

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

   Relays are simply Proxy/Servers that run a dynamic routing protocol
   to redistribute routes between the OMNI interface and INET/EUN
   interfaces (see: Section 3.2.3).  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 OMNI link over the INET/EUN
   interfaces.  The Relay further provides an attachment point of the
   OMNI link to a non-MNP-based global topology.

3.4.2.  AERO Client Behavior

   When a Client enables an OMNI interface, it assigns either an
   MNP-{LLA, ULA} or a Temporary ULA and sends RS messages with ND
   parameters over its underlying interfaces to a Proxy/Server, which
   returns an RA message with corresponding parameters.  The RS/RA

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   messages may pass through one or more NATs in the case of a Client's
   INET interface.  (Note: if the Client used a Temporary ULA in its
   initial RS message, it will discover an MNP-{LLA, ULA} in the
   corresponding RA that it receives from the Proxy/Server and begin
   using these new addresses.  If the Client is operating outside the
   context of AERO infrastructure such as in a Mobile Ad-hoc Network
   (MANET), however, it may continue using Temporary ULAs for Client-to-
   Client communications until it encounters an infrastructure element
   that can provide an MNP.)

3.4.3.  AERO Bridge Behavior

   AERO Bridges configure an OMNI interface and assign the ADM-ULA
   Subnet Router Anycast address for each OMNI link segment they connect
   to.  Bridges configure secured tunnels with Proxy/Servers and other
   Bridges, and engage in a BGP routing protocol session with neighbors
   on the spanning tree (see: Section 3.2.3).

3.5.  OMNI Interface Neighbor Cache Maintenance

   Each OMNI interface maintains a conceptual neighbor cache that
   includes an entry for each neighbor it communicates with on the OMNI
   link per [RFC4861].  In addition to ordinary neighbor cache entries,
   Proxy neighbor cache entries are created and maintained by AERO
   Proxy/Servers when they proxy Client ND message exchanges [RFC4389].
   AERO Proxy/Servers maintain proxy neighbor cache entries for each of
   their associated Clients.

   To the list of neighbor cache entry states in Section 7.3.2 of
   [RFC4861], Proxy/Server OMNI interfaces add an additional state
   DEPARTED that applies to 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, carrier 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 carrier packets in flight to be
   delivered while stale route optimization state may be present.

   Proxy/Servers can act as RORs on behalf of disadvantaged Clients
   according to the Proxy Neighbor Advertisement specification in
   Section 7.2.8 of [RFC4861].  (Well-connected Clients can act as an
   ROR on their own behalf, and perform normal IPv6 ND operations the
   same as for any node.)  When a Proxy/Server acting as an ROR receives
   an authentic NS message used for route optimization, it searches for

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   a neighbor cache entry for the target Client.  The ROR then returns a
   solicited NA message while creating a neighbor cache entry for the
   ROS, and also 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 a 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
   carrier 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
   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.

3.5.1.  OMNI Neighbor Interface Attributes

   OMNI interface IPv6 ND messages include OMNI options
   [I-D.templin-6man-omni-interface] with Interface Attributes that
   provide Link-Layer Address and QoS Preference information for the
   neighbor's underlying interfaces.  This information is stored in the
   neighbor cache and provides the basis for the forwarding algorithm

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   specified in Section 3.10.  The information is cumulative and
   reflects the union of the OMNI information from the most recent ND
   messages received from the neighbor; it is therefore not required
   that each ND message contain all neighbor information.

   The OMNI option Interface Attributes for each underlying interface
   includes a two-part "Link-Layer Address" consisting of a simple IP
   encapsulation address determined by the FMT and L2ADDR fields and an
   ADM-ULA determined by the SRT and LHS fields.  Underlying interfaces
   are further selected based on their associated preference values
   "high", "medium", "low" or "disabled".

   Note: the OMNI option is distinct from any Source/Target Link-Layer
   Address Options (S/TLLAOs) that may appear in an ND message according
   to the appropriate IPv6 over specific link layer specification (e.g.,
   [RFC2464]).  If both an OMNI option and S/TLLAO appear, the former
   pertains to encapsulation addresses while the latter pertains to the
   native L2 address format of the underlying media.

3.5.2.  OMNI Neighbor Advertisement Message Flags

   As discussed in Section 4.4 of [RFC4861] NA messages include three
   flag bits R, S and O.  OMNI interface NA messages treat the flags as
   follows:

   o  R: The R ("Router") flag is set to 1 in the NA messages sent by
      all AERO/OMNI node types.  Simple hosts that would set R to 0 do
      not occur on the OMNI link itself, but may occur on the downstream
      links of Clients and Relays.

   o  S: The S ("Solicited") flag is set exactly as specified in
      Section 4.4. of [RFC4861], i.e., it is set to 1 for Solicited NAs
      and set to 0 for Unsolicited NAs (both unicast and multicast).

   o  O: The O ("Override") flag is set to 0 for solicited proxy NAs and
      set to 1 for all other solicited and unsolicited NAs.  For further
      study is whether solicited NAs for anycast targets apply for OMNI
      links.  Since MNP-LLAs must be uniquely assigned to Clients to
      support correct ND protocol operation, however, no role is
      currently seen for assigning the same MNP-LLA to multiple Clients.

3.6.  OMNI Interface Encapsulation and Re-encapsulation

   The OMNI interface admits original IP packets then (acting as an OAL
   source) performs OAL encapsulation and fragmentation as specified in
   [I-D.templin-6man-omni-interface] while including an ORH if necessary
   as specified in Section 3.2.4.  OAL encapsulation produces OAL
   packets, while OAL fragmentation turns them into OAL fragments which

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   are then encapsulated in *NET headers as carrier packets.  For
   carrier packets undergoing re-encapsulation at an intermediate node,
   the OMNI interface removes the *NET encapsulation headers from the
   first segment and re-encapsulates the packet in new *NET
   encapsulation headers for the next segment.

   When a Proxy/Server or Relay re-encapsulates a carrier packet
   received from a Client that includes an OAL but no ORH, it inserts an
   ORH immediately following the OAL header and adjusts the OAL payload
   length and destination address field.  The inserted ORH will be
   removed by the final-hop Bridge, but its insertion and removal will
   not interfere with reassembly at the final destination.  For this
   reason, Clients must reserve 40 bytes for a maximum-length ORH when
   they perform OAL encapsulation (see: Section 3.9).

3.7.  OMNI Interface Decapsulation

   OMNI interfaces (acting as OAL destinations) decapsulate and
   reassemble OAL packets into original IP packets destined either to
   the AERO node itself or to a destination reached via an interface
   other than the OMNI interface the original IP packet was received on.
   When carrier packets containing OAL fragments arrive, the OMNI
   interface reassembles as discussed in Section 3.9.

3.8.  OMNI Interface Data Origin Authentication

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

   o  AERO Bridges and Proxy/Servers accept carrier packets (including
      either data or control messages) received from the (secured)
      spanning tree.

   o  AERO Proxy/Servers and Clients accept carrier packets and original
      IP packets that originate from within the same secured ANET.

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

   o  AERO Clients, Relays and Proxy/Servers verify carrier packet UDP/
      IP encapsulation addresses according to [RFC4380].

   o  AERO Clients (as well as Proxy/Servers and Relays when acting as
      OAL destinations) accept OAL packets/fragments with Identification
      values within the current window for the OAL source.

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

3.9.  OMNI Interface MTU

   The OMNI interface observes the link nature of tunnels, including the
   Maximum Transmission Unit (MTU), Maximum Reassembly Unit (MRU) and
   the role of fragmentation and reassembly [I-D.ietf-intarea-tunnels].
   The OMNI interface employs an OMNI Adaptation Layer (OAL) that
   accommodates multiple underlying links with diverse MTUs while
   observing both a minimum and per-path Maximum Payload Size (MPS).
   The functions of the OAL and the OMNI interface MTU/MRU/MPS are
   specified in Section 5 of [I-D.templin-6man-omni-interface] with MTU/
   MRU both set to the constant value 9180 bytes, with minimum MPS set
   to 400 bytes, and with path MPS set to a potentially larger value
   depending on the underlying path.

   When the network layer presents an original IP packet to the OMNI
   interface, the OAL source encapsulates and fragments the original IP
   packet if necessary.  When the network layer presents the OMNI
   interface with multiple original IP packets bound to the same OAL
   destination, the OAL source can concatenate them together into a
   single OAL super-packet as discussed in
   [I-D.templin-6man-omni-interface].  The OAL source then fragments the
   OAL packet if necessary according to the minimum/path MPS such that
   the OAL headers appear in each fragment while the original IP packet
   header appears only in the first fragment.  The OAL source then
   encapsulates each OAL fragment in *NET headers for transmission as
   carrier packets over an underlying interface connected to either a
   physical link such as Ethernet, WiFi and the like or a virtual link
   such as an Internet or higher-layer tunnel (see the definition of
   link in [RFC8200]).

   Note: A Client that does not (yet) have neighbor cache state for a
   target may omit the ORH in carrier packets with the understanding
   that a Proxy/Server may insert an ORH on its behalf.  For this
   reason, Clients reserve 40 bytes for the largest possible ORH in
   their OAL fragment size calculations.

   Note: Although the ORH may be removed by a Bridge on the path (see:
   Section 3.10.3), this does not interfere with the destination's
   ability to reassemble.  This is due to the fact that the ORH is not
   included in the fragmentable part; therefore, its removal does not
   invalidate the offset values in any fragment headers.

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3.10.  OMNI Interface Forwarding Algorithm

   Original IP packets enter a node's OMNI interface either from the
   network layer (i.e., from a local application or the IP forwarding
   system) while carrier packets enter from the link layer (i.e., from
   an OMNI interface neighbor).  All original IP packets and carrier
   packets entering a node's OMNI interface first undergo data origin
   authentication as discussed in Section 3.8.  Those that satisfy data
   origin authentication are processed further, while all others are
   dropped silently.

   Original IP packets that enter the OMNI interface from the network
   layer are forwarded to an OMNI interface neighbor using OAL
   encapsulation and fragmentation to produce carrier packets for
   transmission over underlying interfaces.  (If routing indicates that
   the original IP packet should instead be forwarded back to the
   network layer, the packet is dropped to avoid looping).  Carrier
   packets that enter the OMNI interface from the link layer are either
   re-encapsulated and re-admitted into the OMNI link, or reassembled
   and forwarded to the network layer where they are subject to either
   local delivery or IP forwarding.  In all cases, the OAL MUST NOT
   decrement the network layer TTL/Hop-count since its forwarding
   actions occur below the network layer.

   OMNI interfaces may have multiple underlying interfaces and/or
   neighbor cache entries for neighbors with multiple underlying
   interfaces (see Section 3.3).  The OAL uses interface attributes and/
   or traffic classifiers (e.g., DSCP value, port number, flow
   specification, etc.) to select an outgoing underlying interface for
   each OAL 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 OAL packet and
   sends one copy via each of the (outgoing / neighbor) interface pairs;
   otherwise, the node sends a single copy of the OAL packet via an
   interface with the highest preference.  (While not strictly required,
   successful delivery may be more likely when all OAL fragments of the
   same OAL packet are sent over the same underlying interface.)  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 OMNI interface forwarding
   algorithms for Clients, Proxy/Servers and Bridges.  In the following

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   discussion, an original IP 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 MNP-LLA).

3.10.1.  Client Forwarding Algorithm

   When an original IP packet enters a Client's OMNI interface from the
   network layer the Client searches for a neighbor cache entry that
   matches the destination.  If there is a match, the Client selects one
   or more "reachable" neighbor interfaces in the entry for forwarding
   purposes.  If there is no neighbor cache entry, the Client instead
   forwards the original IP packet toward a Proxy/Server.  The Client
   (acting as an OAL source) performs OAL encapsulation and sets the OAL
   destination address to the MNP-ULA if there is a matching neighbor
   cache entry; otherwise, it sets the OAL destination to the ADM-ULA of
   the Proxy/Server.  If the Client has multiple original IP packets to
   send to the same neighbor, it can concatenate them in a single super-
   packet [I-D.templin-6man-omni-interface].  The OAL source then
   performs fragmentation to create OAL fragments (see: Section 3.9),
   appends any *NET encapsulation, and sends the resulting carrier
   packets over underlying interfaces to the neighbor acting as an OAL
   destination.

   If the neighbor interface selected for forwarding is located on the
   same OMNI link segment and not behind a NAT, the Client forwards the
   carrier packets directly according to the L2ADDR information for the
   neighbor.  If the neighbor interface is behind a NAT on the same OMNI
   link segment, the Client instead forwards the initial carrier packets
   to its Proxy/Server and initiates NAT traversal procedures.  If the
   Client's intended source underlying interface is also behind a NAT
   and located on the same OMNI link segment, it sends a "direct bubble"
   over the interface per [RFC6081][RFC4380] to the L2ADDR found in the
   neighbor cache in order to establish state in its own NAT by
   generating traffic toward the neighbor (note that no response to the
   bubble is expected).

   The Client next sends an NS(NUD) message toward the MNP-ULA of the
   neighbor via its Proxy/Server as discussed in Section 3.15.  If the
   Client receives an NA(NUD) from the neighbor over the underlying
   interface, it marks the neighbor interface as "trusted" and sends
   future carrier packets directly to the L2ADDR information for the
   neighbor instead of indirectly via the Proxy/Server.  The Client must
   honor the neighbor cache maintenance procedure by sending additional
   direct bubbles and/or NS/NA(NUD) messages as discussed in
   [RFC6081][RFC4380] in order to keep NAT state alive as long as
   carrier packets are still flowing.

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   When an carrier packet enters a Client's OMNI interface from the
   link-layer, if the OAL destination matches one of the Client's MNPs
   or LLAs the Client (acting as an OAL destination) reassembles and
   decapsulates as necessary and delivers the original IP packet to the
   network layer.  Otherwise, the Client drops the original IP packet
   and MAY return a network-layer ICMP Destination Unreachable message
   subject to rate limiting (see: Section 3.11).

3.10.2.  Proxy/Server Forwarding Algorithm

   For IPv6 ND control messages originating from or destined to a
   Client, the Proxy/Server intercepts the message and updates its
   neighbor cache entry for the Client.  If acting in Proxy-only mode,
   it then forwards a (proxyed) copy of the control message to one or
   more neighbors.  (For example, the Proxy forwards a proxyed version
   of a Client's NS/RS message to the neighbor, and forwards a proxyed
   version of the neighbor's NA/RA reply to the Client.)

   When the Proxy/Server receives an original IP packet from the network
   layer, it drops the packet if routing indicates that it should be
   forwarded back to the network layer to avoid looping.  Otherwise, the
   Proxy/Server regards the original IP packet the same as if it had
   arrived as carrier packets with OAL destination set to its own ADM-
   ULA.  When the Proxy/Server receives carrier packets with OAL
   destination set to its own ADM-ULA, it performs OAL reassembly if
   necessary to obtain the original IP packet.

   The Proxy/Server next searches for a neighbor cache entry that
   matches the original IP destination and proceeds as follows:

   o  if the original IP packet destination matches a neighbor cache
      entry, the Proxy/Sever uses one or more "reachable" neighbor
      interfaces in the entry for packet forwarding using OAL
      encapsulation and fragmentation according to the cached link-layer
      address information.  If the neighbor interface is in a different
      OMNI link segment, the Proxy/Server forwards the resulting carrier
      packets to a Bridge; otherwise, it forwards the carrier packets
      directly to the neighbor.  If the neighbor is behind a NAT, the
      Proxy/Server instead forwards initial carrier packets via a Bridge
      while sending an NS(NUD) to the neighbor.  When the Proxy/Server
      receives the NA(NUD), it can begin forwarding carrier packets
      directly to the neighbor the same as discussed in Section 3.10.1
      while sending additional NS(NUD) messages as necessary to maintain
      NAT state.  Note that no direct bubbles are necessary since the
      Proxy/Server is by definition not located behind a NAT.

   o  else, if the original IP destination matches a non-MNP route in
      the IP forwarding table or an ADM-LLA assigned to the Proxy/

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      Server's OMNI interface, the Proxy/Server presents the original IP
      packet to the network layer for local delivery or IP forwarding.

   o  else, the Proxy/Server initiates address resolution as discussed
      in Section 3.14, while retaining initial original IP packets in a
      small queue awaiting address resolution completion.

   When the Proxy/Server receives a carrier packet with OAL destination
   set to a non-MNP ULA, it accepts the carrier packet only if data
   origin authentication succeeds and if there is a network layer
   routing table for a GUA route that matches the non-MNP ULA.  If there
   is no route, the Proxy/Server drops the carrier packet; otherwise, it
   reassembles and decapsulates to obtain the original IP packet and
   presents it to the network layer where it will be delivered according
   to standard IP forwarding.

   When the Proxy/Server receives a carrier packet with OAL destination
   set to an MNP-ULA, it accepts the carrier packet only if data origin
   authentication succeeds and if there is a neighbor cache entry that
   matches the OAL destination.  If the neighbor cache entry state is
   DEPARTED, the Proxy/Server inserts an ORH that encodes the MNP-ULA
   destination suffix and changes the OAL destination address to the
   ADM-ULA of the new Proxy/Server, then re-encapsulates the carrier
   packet and forwards it to a Bridge which will eventually deliver it
   to the new Proxy/Server.

   If the neighbor cache state is REACHABLE, the Proxy/Server can
   instead either reassemble first and then re-encapsulate/re-fragment
   before forwarding to the Client or forward the raw fragments on to
   the Client which then must reassemble.  In the former case, the
   Proxy/Server can re-fragment to a size that better matches the link
   MTU for the Client, which may be important for low-end links with
   large MTUs.  In the latter case, the Client may receive fragments
   that are smaller than its link MTU but can still be reassembled; this
   case may provide an important performance benefit to Proxy/Servers by
   permitting them to avoid excessive reassembly and re-fragmentation
   overhead.  In either case, the Proxy/Server can return a PTB if
   necessary (see: [I-D.templin-6man-omni-interface]) when it receives a
   carrier packet containing an OAL first fragment.

   Note: If the Proxy/Server has multiple original IP packets to send to
   the same neighbor, it can concatenate them in a single OAL super-
   packet [I-D.templin-6man-omni-interface].

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3.10.3.  Bridge Forwarding Algorithm

   Bridges forward OAL fragments over secured tunnels the same as any
   IPv6 router.  When the Bridge receives a carrier packet or an
   original IP packet via a secured tunnel, it removes the outer *NET
   header and searches for a forwarding table entry that matches the OAL
   destination address.  The Bridge then processes the packet as
   follows:

   o  if the packet is a carrier packet with a destination that matches
      its ADM-ULA Subnet Router Anycast address the Bridge processes the
      carrier packet locally before forwarding.  The Bridge drops the
      carrier packet if it does not include an ORH; otherwise, for
      NA(NUD) messages the Bridge replaces the OMNI option Interface
      Attributes sub-option with information for its own interface while
      retaining the ifIndex value supplied by the NA(NUD) message
      source.  The Bridge next examines the ORH FMT code.  If the code
      indicates the destination is a Client on the open *NET (or, a
      Client behind a NAT for which NAT traversal procedures have
      already converged) the Bridge removes the ORH then writes the MNP-
      ULA formed from the ORH Destination Suffix into the OAL
      destination.  The Bridge then re-encapsulates the carrier packet
      and forwards it to the ORH L2ADDR.  For all other destination
      cases, the Bridge instead writes the ADM-ULA formed from the ORH
      SRT/LHS into the OAL destination address and forwards the carrier
      packet to the ADM-ULA Proxy/Server while invoking NAT traversal
      procedures the same as for Proxy/Servers if necessary, noting that
      no direct bubbles are necessary since only the target Client and
      not the Bridge is behind a NAT.

   o  else, if the packet is a carrier packet with a destination that
      matches a forwarding table entry the Bridge forwards the carrier
      packet via a secured tunnel to the next hop.  If the destination
      matches an MSP without matching an MNP, however, the Bridge
      instead drops the packet and returns an ICMP Destination
      Unreachable message subject to rate limiting (see: Section 3.11).

   o  else, if the packet is an original IP packet with a destination
      that matches one of the Bridge's own addresses, the Bridge submits
      the original IP packet for local delivery to support local
      applications such as routing protocols.

   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 carrier packet.  Therefore, when an OAL header is
   present only the Hop Limit in the OAL header is decremented and not

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   the TTL/Hop Limit in the original IP packet header.  Bridges do not
   insert OAL/ORH headers themselves; instead, they act as IPv6 routers
   and forward carrier packets based on their destination addresses.

3.11.  OMNI Interface Error Handling

   When an AERO node admits an original IP packet into the OMNI
   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].  (OMNI interfaces ignore all link-layer IPv4
   "Fragmentation Needed" and IPv6 "Packet Too Big" messages since they
   only emit carrier packets that are guaranteed to be no larger than
   the IP minimum link MTU as discussed in Section 3.9.)

   The ICMP header is followed by the leading portion of the original IP
   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 IP packet |   c
        ~                               ~   k
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   e
        ~                               ~   t
        |        IP header of           |
        |      original IP packet       |   i
        ~                               ~   n
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~   e
        |    Upper layer headers and    |   r
        |    leading portion of body    |   r
        |   of the original IP packet   |   o
        ~                               ~   r
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

         Figure 5: OMNI 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 carrier packets that it sends
      to one of its 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 carrier packets destined to the correspondent

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      to flow through a default route and re-initiate route
      optimization.

   o  When an AERO Client receives persistent link-layer Destination
      Unreachable messages in response to carrier packets that it sends
      to one of its neighbor Proxy/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 Proxy/Server and release its association with
      the old Proxy/Server as specified in Section 3.16.5.

   o  When an AERO Proxy/Server receives persistent link-layer
      Destination Unreachable messages in response to carrier packets
      that it sends to one of its neighbor Clients, the Proxy/Server
      should mark the underlying path as unusable and use another
      underlying path.

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

3.12.  AERO Router Discovery, Prefix Delegation and Autoconfiguration

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

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3.12.1.  AERO Service Model

   Each AERO Proxy/Server on the OMNI link is configured to facilitate
   Client prefix delegation/registration requests.  Each Proxy/Server is
   provisioned with a database of MNP-to-Client ID mappings for all
   Clients enrolled in the AERO service, as well as any information
   necessary to authenticate each Client.  The Client database is
   maintained by a central administrative authority for the OMNI link
   and securely distributed to all Proxy/Servers, e.g., via the
   Lightweight Directory Access Protocol (LDAP) [RFC4511], via static
   configuration, etc.  Clients receive the same service regardless of
   the Proxy/Servers they select.

   AERO Clients and Proxy/Servers use ND messages to maintain neighbor
   cache entries.  AERO Proxy/Servers configure their OMNI 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 Proxy/Server state alive.

   AERO Clients and Proxy/Servers include prefix delegation and/or
   registration parameters in RS/RA messages (see
   [I-D.templin-6man-omni-interface]).  The ND messages are exchanged
   between Client and Proxy/Server according to the prefix management
   schedule required by the service.  If the Client knows its MNP in
   advance, it can employ prefix registration by including its MNP-LLA
   as the source address of an RS message and with an OMNI option with
   valid prefix registration information for the MNP.  If the Proxy/
   Server accepts the Client's MNP assertion, it injects the MNP into
   the routing system and establishes the necessary neighbor cache
   state.  If the Client does not have a pre-assigned MNP, it can
   instead employ prefix delegation by including the unspecified address
   (::) as the source address of an RS message and with an OMNI option
   with prefix delegation parameters to request an MNP.

   The following sections specify the Client and Proxy/Server behavior.

3.12.2.  AERO Client Behavior

   AERO Clients discover the addresses of Proxy/Servers in a similar
   manner as described in [RFC5214].  Discovery methods include static
   configuration (e.g., from a flat-file map of Proxy/Server addresses
   and locations), or through an automated means such as Domain Name
   System (DNS) name resolution [RFC1035].  Alternatively, the Client
   can discover Proxy/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)

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   "linkupnetworks.[domainname]" where "linkupnetworks" is a constant
   text string and "[domainname]" is a DNS suffix for the OMNI link
   (e.g., "example.com").

   To associate with a Proxy/Server, the Client acts as a requesting
   router to request MNPs.  The Client prepares an RS message with
   prefix management parameters and includes a Nonce and Timestamp
   option if the Client needs to correlate RA replies.  If the Client
   already knows the Proxy/Server's ADM-LLA, it includes the LLA as the
   network-layer destination address; otherwise, the Client includes the
   (link-local) All-Routers multicast as the network-layer destination.
   If the Client already knows its own MNP-LLA, it can use the MNP-LLA
   as the network-layer source address and include an OMNI option with
   prefix registration information.  Otherwise, the Client uses the
   unspecified address (::) as the network-layer source address and
   includes prefix delegation parameters in the OMNI option (see:
   [I-D.templin-6man-omni-interface]).  The Client includes Interface
   Attributes corresponding to the underlying interface over which it
   will send the RS message, and MAY include additional Interface
   Attributes specific to other underlying interfaces.

   The Client then sends the RS message (either directly via Direct
   interfaces, via a VPN for VPNed interfaces, via a Proxy for ANET
   interfaces or via INET encapsulation for INET interfaces) while using
   OAL encapsulation/fragmentation, then waits for an RA message reply
   (see Section 3.12.3).  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 Proxy/Server and try another Proxy/Server.  Otherwise,
   the Client processes the prefix information found in the RA message.

   When the Client processes an RA, it first performs OAL reassembly and
   decapsulation then creates a neighbor cache entry with the Proxy/
   Server's ADM-LLA as the network-layer address and the Proxy/Server's
   encapsulation and/or link-layer addresses as the link-layer address.
   The Client next records the RA Router Lifetime field value in the
   neighbor cache entry as the time for which the Proxy/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 MNP-LLAs for any
   delegated MNPs and assigns them to the OMNI interface.  The Client
   also caches any MSPs included in Route Information Options (RIOs)
   [RFC4191] as MSPs to associate with the OMNI link, and assigns the
   MTU value in the MTU option to the underlying interface.

   The Client then registers additional underlying interfaces with the
   Proxy/Server by sending RS messages via each additional interface as

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   described above.  The RS messages include the same parameters as for
   the initial RS/RA exchange, but with destination address set to the
   Proxy/Server's ADM-LLA.  The Client finally sub-delegates the MNPs to
   its attached EUNs and/or the Client's own internal virtual interfaces
   as described in [I-D.templin-v6ops-pdhost] to support the Client's
   downstream attached "Internet of Things (IoT)".  The Client then
   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 as above.  The RS includes an OMNI option with
   prefix registration/delegation information, with Interface Attributes
   specific to the selected underlying interface, and with any
   additional Interface Attributes specific to other underlying
   interfaces.  When the Client receives the Proxy/Server's RA response,
   it has assurance that the Proxy/Server has been updated with the new
   information.

   If the Client wishes to discontinue use of a Proxy/Server it issues
   an RS message over any underlying interface with an OMNI option with
   a prefix release indication.  When the Proxy/Server processes the
   message, it releases the MNP, sets the 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 Proxy/Server
   withdraws the MNP from the routing system.

3.12.3.  AERO Proxy/Server Behavior

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

   When a Proxy/Server receives a prospective Client's RS message on its
   OMNI 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 Proxy/Server performs OAL
   reassembly and decapsulation, then authenticates the RS message and
   processes the prefix delegation/registration parameters.  The Proxy/
   Server first determines the correct MNPs to provide to the Client by
   processing the MNP-LLA prefix parameters and/or the DHCPv6 OMNI sub-
   option.  When the Proxy/Server returns the MNPs, it also creates a

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   forwarding table entry for the MNP-ULA corresponding to each MNP so
   that the MNPs are propagated into the routing system (see:
   Section 3.2.3).  For IPv6, the Proxy/Server creates an IPv6
   forwarding table entry for each MNP.  For IPv4, the Proxy/Server
   creates an IPv6 forwarding table entry with the IPv4-compatibility
   MNP-ULA prefix corresponding to the IPv4 address.

   The Proxy/Server next creates a neighbor cache entry for the Client
   using the base MNP-LLA as the network-layer address and with lifetime
   set to no more than the smallest prefix lifetime.  Next, the Proxy/
   Server updates the neighbor cache entry by recording the information
   in each Interface Attributes sub-option in the RS OMNI option.  The
   Proxy/Server also records the actual OAL/INET addresses in the
   neighbor cache entry.

   Next, the Proxy/Server prepares an RA message using its ADM-LLA as
   the network-layer source address and the network-layer source address
   of the RS message as the network-layer destination address.  The
   Proxy/Server sets the Router Lifetime to the time for which it will
   maintain both this underlying interface individually and the neighbor
   cache entry as a whole.  The Proxy/Server also sets Cur Hop Limit, M
   and O flags, Reachable Time and Retrans Timer to values appropriate
   for the OMNI link.  The Proxy/Server includes the MNPs, any other
   prefix management parameters and an OMNI option with no Interface
   Attributes.  The Proxy/Server then includes one or more RIOs that
   encode the MSPs for the OMNI link, plus an MTU option (see
   Section 3.9).  The Proxy/Server finally forwards the message to the
   Client using OAL encapsulation/fragmentation as necessary.

   After the initial RS/RA exchange, the Proxy/Server maintains a
   ReachableTime timer for each of the Client's underlying interfaces
   individually (and for the Client's neighbor cache entry collectively)
   set to expire after ReachableTime seconds.  If the Client (or Proxy)
   issues additional RS messages, the Proxy/Server sends an RA response
   and resets ReachableTime.  If the Proxy/Server receives an ND message
   with a prefix release indication it sets the Client's 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 Proxy/Server marks the interface as DOWN.  If
   ReachableTime expires before any new RS is received on any individual
   underlying interface, the Proxy/Server sets the neighbor cache entry
   state to STALE and sets a 10 second timer.  If the Proxy/Server has
   not received a new RS or ND message with a prefix release indication
   before the 10 second timer expires, it deletes the neighbor cache
   entry and withdraws the MNP from the routing system.

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

   Note: Clients SHOULD notify former Proxy/Servers of their departures,
   but Proxy/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).  Proxy/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 RS/RA messaging between Clients
   and Proxy/Servers will keep any NAT state alive (see above).

   Note: All Proxy/Servers on an OMNI 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 Proxy/Servers on the same link
   advertised different values.

3.12.3.1.  DHCPv6-Based Prefix Registration

   When a Client is not pre-provisioned with an MNP-LLA, it will need
   for the Proxy/Server to select one or more MNPs on its behalf and set
   up the correct state in the AERO routing service.  (A Client with a
   pre-provisioned MNP may also request the Proxy/Server to select
   additional MNPs.)  The DHCPv6 service [RFC8415] is used to support
   this requirement.

   When a Client needs to have the Proxy/Server select MNPs, it sends an
   RS message with source address set to the unspecified address (::)
   and with an OMNI option that includes a DHCPv6 message sub-option
   with DHCPv6 Prefix Delegation (DHCPv6-PD) parameters.  When the
   Proxy/Server receives the RS message, it extracts the DHCPv6-PD
   message from the OMNI option.

   The Proxy/Server then acts as a "Proxy DHCPv6 Client" in a message
   exchange with the locally-resident DHCPv6 server, which delegates
   MNPs and returns a DHCPv6-PD Reply message.  (If the Proxy/Server
   wishes to defer creation of MN state until the DHCPv6-PD Reply is
   received, it can instead act as a Lightweight DHCPv6 Relay Agent per
   [RFC6221] by encapsulating the DHCPv6-PD message in a Relay-forward/
   reply exchange with Relay Message and Interface ID options.)

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   When the Proxy/Server receives the DHCPv6-PD Reply, it adds a route
   to the routing system and creates an MNP-LLA based on the delegated
   MNP.  The Proxy/Server then sends an RA back to the Client with the
   (newly-created) MNP-LLA as the destination address and with the
   DHCPv6-PD Reply message coded in the OMNI option.  When the Client
   receives the RA, it creates a default route, assigns the Subnet
   Router Anycast address and sets its MNP-LLA based on the delegated
   MNP.

   Note: See [I-D.templin-6man-omni-interface] for an MNP delegation
   alternative in which the Client can optionally avoid including a
   DHCPv6 message sub-option.  Namely, when the Client requests a single
   MNP it can set the RS source to the unspecified address (::) and
   include a Node Identification sub-option and Preflen in the OMNI
   option (but with no DHCPv6 message sub-option).  When the Proxy/
   Server receives the RS message, it forwards a self-generated DHCPv6
   Solicit message to the DHCPv6 server on behalf of the Client.  When
   the Proxy/Server receives the DHCPv6 Reply, it prepares an RA message
   with an OMNI option with Preflen information (but with no DHCPv6
   message sub-option), then places the (newly-created) MNP-LLA in the
   RA destination address and returns the message to the Client.

3.13.  The AERO Proxy

   Clients may connect to protected-spectrum ANETs that employ physical
   and/or link-layer security services to facilitate communications to
   Proxy/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 Proxy/
      Server's ADM-LLA, the Proxy proceeds to the next step.  Otherwise,
      if the destination is (link-local) All-Routers multicast, the
      Proxy selects a "nearby" Proxy/Server that is likely to be a good
      candidate to serve the Client and replaces the destination address
      with the Proxy/Server's ADM-LLA.  Next, the Proxy creates a proxy
      neighbor cache entry and caches the Client and Proxy/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 an OAL header with source set to the
      Proxy's ADM-ULA and destination set to the Proxy/Server's ADM-ULA.
      The Proxy also includes an OMNI header with an Interface
      Attributes option that includes its own INET address plus a unique
      Port Number for this Client, then forwards the message into the

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      OMNI link spanning tree.  (Note: including a unique Port Number
      allows the Proxy/Server to distinguish different Clients located
      behind the same Proxy at the link-layer, whereas the link-layer
      addresses would otherwise be indistinguishable)

   o  when the Proxy/Server receives the RS, it authenticates the
      message then creates or updates a neighbor cache entry for the
      Client with the Proxy's ADM-ULA, INET address and Port Number as
      the link-layer address information.  The Proxy/Server then sends
      an RA message back to the Proxy via the spanning tree.

   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 prefix information as a mapping from the
      Client's MNPs to the Client's link-layer address, caches the
      Proxy/Server's advertised Router Lifetime and sets the neighbor
      cache entry state to REACHABLE.  The Proxy then 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 of the
   Client's carrier packets for which there is no matching neighbor
   cache entry to a Bridge using OAL encapsulation with its own ADM-ULA
   as the source and the MNP-ULA corresponding to the Client as the
   destination.  The Proxy instead forwards any carrier packets destined
   to a neighbor cache target directly to the target according to the
   OAL/link-layer information - the process of establishing neighbor
   cache entries is specified in Section 3.14.

   While the Client is still attached to the ANET, the Proxy sends NS,
   RS and/or unsolicited NA messages to update the Proxy/Server's
   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 Proxy/Server ceases to send solicited advertisements, the
   Proxy sends unsolicited RAs on the ANET interface with destination
   set to (link-local) All-Nodes multicast and with Router Lifetime set
   to zero to inform Clients that the Proxy/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

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   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 carrier packets
   destined to the Client to a Bridge via OAL/ORH encapsulation.  When
   DepartTime expires, the Proxy deletes the neighbor cache entry and
   discards any further carrier 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
   original IP packets without the OAL so that the ANET routing system
   transports the original IP 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 on the same underlying link
   and recognizes the AERO/OMNI protocol, the Client can avoid OAL
   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 own MNP-LLA (or to a Temporary LLA),
   and with destination address set to the ADM-LLA of the Client's
   selected Proxy/Server or to (link-local) All-Routers multicast.  The
   Client includes an OMNI option formatted as specified in
   [I-D.templin-6man-omni-interface].

   The Client then sends the unencapsulated RS message, which will be
   intercepted by the AERO-Aware access router.  The access router then
   performs OAL encapsulation on the RS message with its own OAL/*NET
   addresses as the source and the OAL/*NET addresses of a Proxy as the
   destination.  The access router further remembers the address of the
   Proxy so that it can encapsulate future original IP 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 Proxy/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 Proxy/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.

   Note: The Proxy can apply packing as discussed in
   [I-D.templin-6man-omni-interface] if an opportunity arises to

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   concatenate multiple original IP packets that will be destined to the
   same neighbor.

3.13.1.  Combined Proxy/Servers

   Clients may need to connect directly to Proxy/Servers via INET,
   Direct and VPNed interfaces (i.e., non-ANET interfaces).  If the
   Client's underlying interfaces all connect via the same INET
   partition, then it can connect to a single controlling Proxy/Server
   via all interfaces.

   If some Client interfaces connect via different INET partitions,
   however, the Client still selects a set of controlling Proxy/Servers
   and sends RS messages via their directly-connected Proxy/Servers
   while using the ADM-LLA of the controlling Proxy/Server as the
   destination.

   When a Proxy/Server receives an RS with destination set to the ADM-
   LLA of a controlling Proxy/Server, it acts as a Proxy to forward the
   message to the controlling Proxy/Server while forwarding the
   corresponding RA reply to the Client.

3.13.2.  Detecting and Responding to Proxy/Server Failures

   In environments where fast recovery from Proxy/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 Proxy/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 spanning tree with the Proxy's ADM-LLA as the
   source and the ADM-LLA of the Proxy/Server as the destination.  When
   the Proxy is also sending RS messages to the Proxy/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 Proxy/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.

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   The Proxy sends RA messages on the ANET interface with source address
   set to the Proxy/Server's address, destination address set to (link-
   local) 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 Proxy/Server will receive the RA messages and associate
   with a new Proxy/Server.

3.13.3.  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 Proxy/Servers in a single RS/RA
   message exchange.  The Client can send a single RS message to (link-
   local) All-Routers multicast that includes the ID's of multiple
   Proxy/Servers in MS-Register sub-options of the OMNI option.

   When the Proxy receives the RS and processes the OMNI option, it
   sends a separate RS to each MS-Register Proxy/Server ID.  When the
   Proxy receives an RA, it can optionally return an immediate
   "singleton" RA to the Client or record the Proxy/Server's ID for
   inclusion in a pending "aggregate" RA message.  The Proxy can then
   return aggregate RA messages to the Client including multiple Proxy/
   Server IDs in order to conserve bandwidth.  Each RA includes a proper
   subset of the Proxy/Server IDs from the original RS message, and the
   Proxy must ensure that the message contents of each RA are consistent
   with the information received from the (aggregated) Proxy/Servers.

   Clients can thereafter employ efficient point-to-multipoint Proxy/
   Server coordination under the assistance of the Proxy to reduce the
   number of messages sent over the ANET while enlisting the support of
   multiple Proxy/Servers for fault tolerance.  Clients can further
   include MS-Release sub-options in IPv6 ND messages to request the
   Proxy to release from former Proxy/Servers via the procedures
   discussed in Section 3.16.5.

   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.

3.14.  AERO Address Resolution

   While carrier 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.

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   o  For Clients on ANET 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
   with the target Proxy/Server/Relay or the target Client itself 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 Proxy/Server, or a
   non-MNP correspondent reachable via a Relay.

   The procedures are specified in the following sections.

3.14.1.  Route Optimization Initiation

   When an original IP packet from a source node destined to a target
   node arrives, the ROS checks for a neighbor cache entry for the LLA
   that matches the target destination.  If there is a neighbor cache
   entry in the REACHABLE state, the ROS forwards the packets according
   to the cached state and returns from processing.  Otherwise, if there
   is already a neighbor cache entry in the STALE state the ROS
   continues, and if there or no neighbor cache entry the ROS creates
   one in the INCOMPLETE state.

   The ROS next places the original IP packet on a short queue then
   sends an NS message for Address Resolution (NS(AR)) to receive a
   solicited NA(AR) message from a ROR.  The NS(AR) message must be no
   larger than the OAL minimum MPS so that its entire contents will fit
   in an OAL first fragment (i.e., as an "atomic fragment").  The ROS
   prepares an NS(AR) that includes:

   o  the LLA of the ROS as the source address.

   o  the LLA corresponding to the original IP packet's destination as
      the Target Address, e.g., for 2001:db8:1:2::10:2000 the LLA is
      fe80::2001:db8:1:2.

   o  the Solicited-Node multicast address [RFC4291] formed from the
      lower 24 bits of the original IP 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 also includes an OMNI option with an Interface
   Attributes entry for the sending interface, and with Prefix Length
   set to the length associated with the ROS's LLA.  The ROS then

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   submits the NS(AR) message for OAL encapsulation and fragmentation,
   with OAL source set to its own ULA and OAL destination set to the ULA
   corresponding to the target, and with an unpredictable initial
   Identification value selected according to [RFC7739].  The source
   caches the initial Identification value in the (newly-created)
   neighbor cache entry as the starting sequence number for the "send"
   window for future carrier packets sent to this target.

   The ROS then sends the resulting carrier packet into the spanning
   tree without decrementing the network-layer TTL/Hop Limit field.
   (When the ROS is a Client, it instead must first sign the NS(AR)
   message and send the resulting carrier packet to one of its current
   Proxy/Servers which then verifies the NS(AR) signature and forwards
   the carrier packet into the spanning tree on behalf of the Client.)

3.14.2.  Relaying the NS(AR) *NET Packet(s)

   When the Bridge receives the carrier packet from the ROS, it discards
   the *NET headers and determines the next hop by consulting its
   standard IPv6 forwarding table for the OAL header destination
   address.  The Bridge then re-encapsulates and forwards the carrier
   packet via the spanning tree the same as for any IPv6 router, where
   it may traverse multiple OMNI link segments.  The final-hop Bridge in
   the spanning tree will deliver the carrier packet via a secured
   tunnel to a Proxy/Server or Relay that services the target.

3.14.3.  Processing the NS(AR) and Sending the NA(AR)

   When the Proxy/Server/Relay receives the carrier packet, it examines
   the enclosed atomic OAL fragment to determine that it contains an
   NS(AR) then examines the NS(AR) target to determine whether it has a
   matching neighbor cache entry and/or non-MNP route.  If there is no
   match, the Proxy/Server/Relay drops the message.  Otherwise, the ROR
   continues processing as follows:

   o  if the NS(AR) target matches a Client neighbor cache entry in the
      DEPARTED state, the Proxy/Server inserts an ORH with destination
      prefix set to the lower 64 bits of the Client's MNP-ULA and sets
      the destination address to the ADM-ULA of the Client's new Proxy/
      Server.  The (old) Proxy/Server then re-encapsulates the carrier
      packet, forwards it into the spanning tree and returns from
      processing.

   o  If the NS(AR) target matches a Client neighbor cache entry in the
      REACHABLE state, the Proxy/Server proceeds according to whether
      the Client requires Proxy services.  For "disadvantaged" Clients
      (e.g., those on low-end links) and Clients with complex multilink
      arrangements the Proxy/Server nominates itself as the ROR;

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      otherwise, the Proxy/Server re-encapsulates the carrier packet and
      forwards it to the target Client which will act as an ROR on its
      own behalf.

   o  If the NS(AR) target matches one of its non-MNP routes, the Relay
      acts the ROR.

   The ROR next checks for a neighbor cache entry that matches the
   NS(AR) source LLA.  If there is a neighbor cache entry in the
   REACHABLE state, the ROR accepts the NS(AR) only if the OAL
   Identification value is within the "accept" window for this NS(AR)
   source LLA or if the NS(AR) was forwarded by the (secured) spanning
   tree.  Otherwise, the ROR creates a neighbor cache entry for the
   NS(AR) source LLA if necessary with state set to STALE.  If the ROR
   is a Proxy/Server, it next adds a Report List entry to the target
   Client neighbor cache entry for the NS(AR) source LLA while caching
   the OAL Identification value in the (newly-created) neighbor cache
   entry as the starting sequence number for the "accept" window for
   future carrier packets received from this LLA source.

   The ROR then prepares a (solicited) NA(AR) message to send back to
   the ROS using the same Identification value received in the NS(AR)
   (unlike the NS(AR), the NA(AR) need not fit in a single OAL
   fragment).  The ROR sets the NA(AR) source address to its own LLA,
   sets the destination address to the NS(AR) LLA source address and
   sets the Target Address to the same value that was in the NS(AR).
   The ROR then includes an OMNI option with Prefix Length set to the
   length associated with the MNP-LLA.

   If the NS(AR) target was an MNP Client, the ROR next includes
   Interface Attributes in the OMNI option for each of the target
   Client's underlying interfaces with current information for each
   interface.  If the ROR is a Proxy/Server/Relay, it then sets the S/
   T-ifIndex field in the OMNI header set to 0.  If the ROR is the
   Client itself, it instead sets S/T-ifIndex to the index of the
   underlying interface that will send the NS(AR).

   For each Interface Attributes sub-option, the ROR sets the L2ADDR
   according to the Proxy/Server's INET address for VPNed or Direct
   interfaces, to the INET address of the Proxy for Proxyed interfaces
   or to the Client's INET address for INET interfaces.  The ROR then
   includes the lower 32 bits of the Proxy/Server's ADM-ULA as the LHS,
   encodes the ADM-ULA prefix length code in the SRT field and sets the
   FMT code accordingly as specified in Section 3.3.

   The ROR then sets the NA(AR) message R flag to 1 (as a router) and S
   flag to 1 (as a response to a solicitation).  If the ROR is the
   Client itself, it sets the O flag to 1; if the ROR is the Proxy/

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   Server, it instead sets the O flag to 0 (as a proxy).  The ROR
   finally submits the NA(AR) for OAL encapsulation with source set to
   its own ULA and destination set to the source ULA of the NS(AR)
   message, then performs OAL fragmentation using the same
   Identification value that appeared in the NS(AR) and forwards the
   resulting (*NET-encapsulated) carrier packets into the spanning tree
   without decrementing the network-layer TTL/Hop Limit field.

3.14.4.  Relaying the NA(AR)

   When the Bridge receives the carrier packets from the ROR, it
   discards the *NET header and determines the next hop by consulting
   its standard IPv6 forwarding table for the OAL header destination
   address.  The Bridge then re-encapsulates and forwards the carrier
   packets via the spanning tree the same as for any IPv6 router, where
   it may traverse multiple OMNI link segments.  The final-hop Bridge in
   the spanning tree will deliver the carrier packet via a secured
   tunnel to a Proxy/Server for the ROS.

3.14.5.  Processing the NA(AR)

   When the ROS receives the NA(AR) message, it first searches for a
   neighbor cache entry that matches the NA(AR) target address.  If
   there is an entry in the INCOMPLETE or STALE state, the ROS matches
   the OAL Identification value with the value it had included in the
   corresponding NS(AR).  If the values match, the ROS processes the
   message the same as for standard IPv6 Address Resolution [RFC4861].
   In the process, it caches the target LLA and all information found in
   the OMNI option in the neighbor cache entry for the target.  The ROS
   finally sets the neighbor cache entry state to REACHABLE and sets its
   lifetime to ReachableTime seconds.  (When the ROS is a Client, the
   solicited NA(AR) message will first be delivered via the spanning
   tree to one of its current Proxy/Servers, which then securely
   forwards the message to the Client.)

3.14.6.  Route Optimization Maintenance

   Following route optimization, the ROS forwards future carrier packets
   with user data destined to the target via the addresses found in the
   cached link-layer information.  The route optimization is shared by
   all sources that send original IP packets to the target via the ROS,
   i.e., and not just the source on behalf of which the route
   optimization was initiated.  Note that route optimization is
   performed only for original IP packets that contain user data, and
   not for those that contain other IPv6 ND control messages.

   While the ROS continues to forward additional original IP packets
   destined to the target, it sends additional NS(AR) messages to the

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   ROR before ReachableTime expires to receive a fresh NA(AR) 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 A.1).  The ROS may supply a new
   unpredictable OAL Identification value if it wishes to reset the
   neighbor's "accept" Identification window.  If the ROS is a Client,
   it must sign the NS(AR) message so that the Proxy/Server can
   authenticate.

   The ROS uses the cached ULA of the ROR (i.e., either the ADM-ULA of
   the Proxy/Server or the MNP-ULA of the Client itself) as the NS(AR)
   OAL destination address, and sends up to MAX_MULTICAST_SOLICIT NS(AR)
   messages separated by 1 second until an NA(AR) is received.  If no
   NA(AR) is received, the ROS assumes that the current ROR has become
   unreachable and deletes the target neighbor cache entry.  Subsequent
   original IP packets will trigger a new route optimization event (see:
   Section 3.14.1).

   If an NA(AR) is received, the ROS then updates the neighbor cache
   entry to refresh ReachableTime, while (for MNP destinations) the ROR
   adds or updates the ROS address to the target's Report List and with
   time set to ReportTime.  While no carrier packets are flowing, the
   ROS instead allows ReachableTime for the neighbor cache entry to
   expire.  When ReachableTime expires, the ROS deletes the neighbor
   cache entry.  Any future carrier 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 3.16).  If there is a 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 neighbor
   cache entry, the ROS simply discards the unsolicited NA.

   In this arrangement, the ROS holds a neighbor cache entry with only a
   "send" Identification window value for the target, while the ROR
   holds a neighbor cache entry with only an "accept" Identification
   window value for the ROS.  The route optimization neighbor
   relationship is therefore asymmetric and unidirectional.  If the
   target node also has carrier 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.

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3.15.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform Neighbor Unreachability Detection (NUD) per
   [RFC4861] either reactively in response to persistent link-layer
   errors (see Section 3.11) or proactively to confirm reachability and/
   or establish NAT state.  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 random packets should not be.

   AERO nodes can use standard NS/NA exchanges sent over the OMNI link
   spanning tree to securely test reachability without risk of DoS
   attacks from nodes pretending to be a neighbor (these NS/NA(NUD)
   messages use the unicast LLAs and ULAs of the two parties involved in
   the NUD test the same as for standard IPv6 ND, and both messages flow
   over the spanning tree).  Proxys can further perform NUD to securely
   verify Proxy/Server reachability on behalf of their proxyed Clients.
   However, a means for an ROS to test the unsecured target route
   optimized paths is also necessary.

   When an ROR directs an ROS to a target neighbor with one or more
   link-layer addresses, the ROS can proactively test each such
   unsecured route optimized path through secured NS(NUD) messages over
   the spanning tree that invoke an unsecured NA(NUD) reply that travels
   over the route optimized path.  (The NS(NUD) messages must therefore
   include Nonce and Timestamp options that will be echoed in the
   unsecured NA(NUD) replies.)  While testing the paths, the ROS can
   optionally continue to send carrier packets via the spanning tree,
   maintain a small queue of carrier packets until target reachability
   is confirmed, or (optimistically) allow carrier packets to flow via
   the route optimized paths.

   When the ROS sends an NS(NUD) message, it sets the IPv6 source to its
   own address, sets the destination to the MNP-LLA of the target, and
   sets the target's MNP Subnet-Router anycast address as the Target
   Address.  The ROS also includes an OMNI option with a single
   Interface Attributes sub-option with the SRT, FMT, LHS and L2ADDR
   information for its own underlying interface it wishes to test, but
   sets the S/T-ifIndex field to the index for target's underlying
   interface to be tested.  The ROS includes a Nonce and Timestamp
   option, then encapsulates the message in OAL/INET headers with its
   own ULA as the source and the ULA of the target as the destination.
   The ROS then forwards the NS(NUD) message toward the target via a
   Proxy/Server or Bridge.

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   When the target receives the NS(NUD) message, it creates an NA(NUD)
   by reversing the OAL and IPv6 addresses and including an Interface
   Attributes sub-option with attributes for its own interface
   identified by the NS(NUD) S/T-ifIndex.  The target sets the NA(NUD)
   S/T-ifIndex to the index of the ROS, sets the Target Address to the
   same value that was in the NS(NUD), and returns the message using its
   own underlying interface identified by S/T-ifIndex and destined to
   the ROS's interface identified by the original Interface Attributes
   sub-option.

   When the ROS receives the NA(NUD) message, it can determine from the
   Nonce, Timestamp and Target Address that the message matched its
   NS(NUD) and that it transited the direct path from the ROR using the
   selected underlying interface pair.  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 OMNI interface
   forwarding algorithm specified in Section 3.10.

   Note: If the target determines that the OMNI option Interface
   Attributes in the NS(NUD) is located in a different OMNI link segment
   than its own interface named in the S/T-ifIndex, it instead returns
   the NA(NUD) via the spanning tree while including an ORH and setting
   the OAL destination address to the Subnet Router Anycast address used
   by Bridges on the ROS segment.  When a Bridge on the ROS segment
   receives the NA(NUD), it replaces the Interface Attributes with
   information for its own interface while using the ifIndex value
   specific to the target.

3.16.  Mobility Management and Quality of Service (QoS)

   AERO is a Distributed Mobility Management (DMM) service.  Each Proxy/
   Server is responsible for only a subset of the Clients on the OMNI
   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 Proxy/Servers via
   RS/RA exchanges to maintain the DMM profile, and the AERO routing
   system tracks all current Client/Proxy/Server peering relationships.

   Proxy/Servers provide default routing and mobility/multilink services
   for their dependent Clients.  Clients are responsible for maintaining
   neighbor relationships with their Proxy/Servers through periodic RS/
   RA exchanges, which also serves to confirm neighbor reachability.
   When a Client's underlying interface address and/or QoS information
   changes, the Client is responsible for updating the Proxy/Server with
   this new information.  Note that when there is a Proxy in the path,
   the Proxy can also perform some RS/RA exchanges on the Client's
   behalf.

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   Mobility management messaging is based on the transmission and
   reception of unsolicited Neighbor Advertisement (uNA) messages.  Each
   uNA message sets the IPv6 destination address to (link-local) All-
   Nodes multicast to convey a general update of Interface Attributes to
   (possibly) multiple recipients, or to a specific unicast LLA to
   announce a departure event to a specific recipient.  Implementations
   must therefore examine the destination address to determine the
   nature of the mobility event (i.e., update vs departure).

   Mobility management considerations are specified in the following
   sections.

3.16.1.  Mobility Update Messaging

   Proxy/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 Proxy/Server sends a uNA
   message, it sets the IPv6 source address to the Client's MNP-LLA,
   sets the destination address to (link-local) All-Nodes multicast and
   sets the Target Address to the Client's Subnet-Router anycast
   address.  The Proxy/Server also includes an OMNI option with Prefix
   Length set to the length associated with the Client's MNP-LLA, with
   Interface Attributes for the target Client's underlying interfaces
   and with the OMNI header S/T-ifIndex set to 0.  The Proxy/Server then
   sets the NA R flag to 1, the S flag to 0 and the O flag to 1, then
   encapsulates the message in an OAL header with source set to its own
   ADM-ULA and destination set to the ULA of the ROS and sends the
   message into the spanning tree.

   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 Proxy/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 prepared as above, it ignores the
   message if there is no existing neighbor cache entry for the Client.
   Otherwise, it uses the included OMNI option information to update the
   neighbor cache entry, but does not reset ReachableTime since the
   receipt of an unsolicited NA message from the target Proxy/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 carrier packets

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   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.

   In addition to sending uNA messages to the current set of ROSs for
   the Client, the Proxy/Server also sends uNAs to the MNP-ULA
   associated with the link-layer address for any underlying interface
   for which the link-layer address has changed.  These uNA messages
   update an old Proxy/Server that cannot easily detect (e.g., without
   active probing) when a formerly-active Client has departed.  When the
   Proxy/Server sends the uNA, it sets the IPv6 source address to the
   Client's MNP-LLA, sets the destination address to the old Proxy/
   Server's ADM-LLA, and sets the Target Address to the Client's Subnet-
   Router anycast address.  The Proxy/Server also includes an OMNI
   option with Prefix Length set to the length associated with the
   Client's MNP-LLA, with Interface Attributes for the changed
   underlying interface, and with the OMNI header S/T-ifIndex set to 0.
   The Proxy/Server then sets the NA R flag to 1, the S flag to 0 and
   the O flag to 1, then encapsulates the message in an OAL header with
   source set to its own ADM-ULA and destination set to the ADM-ULA of
   the old Proxy/Server and sends the message into the spanning tree.

3.16.2.  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 Proxy/Server via the spanning
   tree with an OMNI option that includes Interface attributes with the
   new link quality and address information.

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

   When the Proxy/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.

3.16.3.  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 Proxy/Server via the underlying interface with an OMNI option
   that includes Interface Attributes with appropriate link quality
   values and with link-layer address information for the new link.

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3.16.4.  Deactivating Existing Links

   When a Client needs to deactivate an existing underlying interface,
   it sends an RS or uNA message to its Proxy/Server with an OMNI option
   with appropriate Interface Attribute values - in particular, the link
   quality value 0 assures that neighbors will cease to use the link.

   If the Client needs to send RS/uNA messages over an underlying
   interface other than the one being deactivated, it MUST include
   Interface Attributes with appropriate link quality values for any
   underlying interfaces being deactivated.

   Note that when a Client deactivates an underlying interface,
   neighbors that have received the RS/uNA messages need not purge all
   references for the underlying interface from their neighbor cache
   entries.  The Client may reactivate or reuse the underlying interface
   and/or its ifIndex at a later point in time, when it will send RS/uNA
   messages with fresh Interface Attributes to update any neighbors.

3.16.5.  Moving Between Servers

   The Client performs the procedures specified in Section 3.12.2 when
   it first associates with a new Proxy/Server or renews its association
   with an existing Proxy/Server.  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 Proxy/Server to
   notify any old Proxy/Servers from which the Client is departing.

   When the new Proxy/Server receives the Client's RS message, it
   returns an RA as specified in Section 3.12.3 and sends up to
   MAX_NEIGHBOR_ADVERTIISEMENT uNA messages to any old Proxy/Servers
   listed in OMNI option MS-Release identifiers.  When the new Proxy/
   Server sends a uNA message, it sets the IPv6 source address to the
   Client's MNP-LLA, sets the destination address to the old Proxy/
   Server's ADM-LLA, and sets the Target Address to the Client's Subnet-
   Router anycast address.  The new Proxy/Server also includes an OMNI
   option with Prefix Length set to the length associated with the
   Client's MNP-LLA, with Interface Attributes for its own underlying
   interface, and with the OMNI header S/T-ifIndex set to 0.  The new
   Proxy/Server then sets the NA R flag to 1, the S flag to 0 and the O
   flag to 1, then encapsulates the message in an OAL header with source
   set to its own ADM-ULA and destination set to the ADM-ULA of the old
   Proxy/Server and sends the message into the spanning tree.

   When an old Proxy/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 Proxy/Server's ADM-ULA, and resets
   DepartTime.  After a short delay (e.g., 2 seconds) the old Proxy/

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   Server withdraws the Client's MNP from the routing system.  After
   DepartTime expires, the old Proxy/Server deletes the Client's
   neighbor cache entry.

   The old Proxy/Server also iteratively forwards a copy of the uNA
   message to each ROS in the Client's Report List by changing the OAL
   destination address to the ULA of the ROS while leaving all other
   fields of the message unmodified.  When the ROS receives the uNA, it
   examines the Target address to determine the correct neighbor cache
   entry and verifies that the IPv6 destination address matches the old
   Proxy/Server.  The ROS then caches the IPv6 source address as the new
   Proxy/Server for the existing neighbor cache entry and marks the
   entry as STALE.  While in the STALE state, the ROS allows new carrier
   packets to flow according to any existing cached link-layer
   information and sends new NS(AR) messages using its own ULA as the
   OAL source and the ADM-ULA of the new Proxy/Server as the OAL
   destination address to elicit NA messages that reset the neighbor
   cache entry state to REACHABLE.  If no new NA message is received for
   10 seconds while in the STALE state, the ROS deletes the neighbor
   cache entry.

   Clients SHOULD NOT move rapidly between Proxy/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 Proxy/
   Server include a Proxy/Server that has gone unreachable, topological
   movements of significant distance, movement to a new geographic
   region, movement to a new OMNI link segment, etc.

   When a Client moves to a new Proxy/Server, some of the fragments of a
   multiple fragment OAL packet may have already arrived at the old
   Proxy/Server while others are en route to the new Proxy/Server,
   however no special attention in the reassembly algorithm is necessary
   since all fragments will eventually be delivered to the Client which
   can then reassemble.

3.17.  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

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   sections correspond to Source-Specific Multicast (SSM) and Any-Source
   Multicast (ASM) operational modes.

3.17.1.  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-OMNI
   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 OMNI interface, X
   originates a separate copy of the Join/Prune for each (S,G) in the
   message using its own LLA as the source address and ALL-PIM-ROUTERS
   as the destination address.  X then encapsulates each message in an
   OAL header with source address set to the ULA of X and destination
   address set to S then forwards the message into the spanning tree,
   which delivers it to AERO Proxy/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 3.14.  The resulting NAs will return the LLA for the prefix
   that matches S as the network-layer source address and with an OMNI
   option with the ULA corresponding to any underlying interfaces 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 spanning tree while continuing to use the LLA of X as the
   source address.  Each Z* then updates its MRIB accordingly and
   maintains the LLA of X as the next hop in the reverse path.  Since
   the Bridges 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 OMNI link segment as X, the
   multicast data traffic sent to X directly using OAL/INET
   encapsulation instead of via a Bridge.

   Following the initial Join/Prune and NS/NA messaging, X maintains a
   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

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   messages to each Y and Z* in the neighbor cache entry over the
   spanning tree.

   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 3.16.  When X receives the
   unsolicited NA message, it updates its neighbor cache entry for the
   LLA 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 Proxy/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 LLA as the
   source address.  This causes Y2 to include Y1 in the multicast
   forwarding tree during the interim time that Y1's 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 with S/T-ifIndex in
   the header set to 0 and a release indication to cause X to release
   its neighbor cache entry.  X then sends a new Join message to S via
   the spanning tree and re-initiates route optimization the same as if
   it were receiving a fresh Join message from a node on a downstream
   link.

3.17.2.  Any-Source Multicast (ASM)

   When an ROS X acting as a PIM router receives a Join/Prune from a
   node on its downstream interfaces containing one or more (*,G) pairs,
   it updates its Multicast Routing Information Base (MRIB) accordingly.
   X then forwards a copy of the message to the Rendezvous Point (RP) R
   for each G over the spanning tree.  X uses its own LLA as the source
   address and ALL-PIM-ROUTERS as the destination address, then
   encapsulates each message in an OAL header with source address set to
   the ULA of X and destination address set to R, then sends the message
   into the spanning tree.  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 3.17.1.

   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
   original IP packets in PIM Register messages and forwards them to R
   via the spanning tree, which may then elect to send a PIM Join to Z*.
   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 original IP packet;
   one native copy from the (S, G) tree and a second copy from the pre-

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   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 original IP 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 3.17.1.  Once the (S,G) tree is
   established, the listeners can send (S, G) Prune messages to R so
   that multicast original IP 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.

3.17.3.  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.

3.18.  Operation over Multiple OMNI Links

   An AERO Client can connect to multiple OMNI links the same as for any
   data link service.  In that case, the Client maintains a distinct
   OMNI interface for each link, e.g., 'omni0' for the first link,
   'omni1' for the second, 'omni2' for the third, etc.  Each OMNI link
   would include its own distinct set of Bridges and Proxy/Servers,
   thereby providing redundancy in case of failures.

   Each OMNI link could utilize the same or different ANET connections.
   The links can be distinguished at the link-layer via the SRT prefix
   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, with each VLAN distinguished by a
   different SRT "color" (see: Section 3.2.5).

   The Client's IP layer can select the outgoing OMNI interface
   appropriate for a given traffic profile while (in the reverse
   direction) correspondent nodes must have some way of steering their
   original IP packets destined to a target via the correct OMNI link.

   In a first alternative, if each OMNI 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 OMNI link is used
   for both outbound and inbound traffic.  This can be accomplished

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   using existing technologies and approaches, and without requiring any
   special supporting code in correspondent nodes or Bridges.

   In a second alternative, if each OMNI link services the same MSP(s)
   then each link could assign a distinct "OMNI link Anycast" address
   that is configured by all Bridges on the link.  Correspondent nodes
   can then perform Segment Routing to select the correct SRT, which
   will then direct the original IP packet over multiple hops to the
   target.

3.19.  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 original IPv4 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.

   When an AERO Client registers with an AERO Proxy/Server, the Proxy/
   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.

3.20.  Transition Considerations

   OAL encapsulation ensures that dissimilar INET partitions can be
   joined into a single unified OMNI 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
   OMNI link 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 IP GUA 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 OMNI link view even
   if the INET partitions remain in their current protocol and
   addressing plans.  In that way, the OMNI 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

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   OMNI link MNP-based addressing scheme, the partition and OMNI 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].

3.21.  Detecting and Reacting to Server and Bridge Failures

   In environments where rapid failure recovery is required, Proxy/
   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.

   Proxy/Servers and Bridges maintain BFD sessions in parallel with
   their BGP peerings.  If a Proxy/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.

   Proxys SHOULD use proactive NUD for Proxy/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 Proxy/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
   Proxy/Server's address, destination address set to (link-local) 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) Proxy/Server will receive the RA messages and
   associate with a new Proxy/Server.

3.22.  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 Proxy/
   Server in a "tethered" arrangement with all of the Client's traffic

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   transiting the Proxy/Server.  Alternatively, the Client can associate
   with an INET Proxy/Server using UDP/IP encapsulation and control
   message securing services as discussed in the following sections.

   When a Client's OMNI interface enables an INET underlying interface,
   it first determines whether the interface is likely to be behind a
   NAT.  For IPv4, the Client assumes it is on the open Internet if the
   INET address is not a special-use IPv4 address per [RFC3330].
   Similarly for IPv6, the Client assumes it is on the open Internet if
   the INET address is a Global Unicast Address (GUA) [RFC4291].
   Otherwise, the Client assumes it may be behind one or several NATs.

   The Client then prepares an RS message with IPv6 source address set
   to its MNP-LLA, with IPv6 destination set to (link-local) All-Routers
   multicast and with an OMNI option with underlying interface
   attributes.  If the Client believes that it is on the open Internet,
   it SHOULD include an L2ADDR in the Interface Attributes sub-option
   corresponding to the underlying interface; otherwise, it MAY omit
   L2ADDR.  If the underlying address is IPv4, the Client includes the
   Port Number and IPv4 address written in obfuscated form [RFC4380] as
   discussed in Section 3.3.  If the underlying interface address is
   IPv6, the Client instead includes the Port Number and IPv6 address in
   obfuscated form.  The Client finally includes a HIP "Initiator"
   message sub-option in the OMNI option
   [I-D.templin-6man-omni-interface] to provide message authentication
   and performs OAL encapsulation and fragmentation if necessary.  The
   Client then encapsulates each fragment in UDP/IP headers, sets the
   UDP/IP source to its INET address and UDP port, sets the UDP/IP
   destination to the Proxy/Server's INET address and the AERO service
   port number (8060), then sends the (whole or fragmented) message to
   the Proxy/Server.

   When the Proxy/Server receives the RS, it performs OAL reassembly if
   necessary then authenticates the message and registers the Client's
   MNP and INET interface information according to the OMNI option
   parameters.  If the RS message OMNI option includes Interface
   Attributes with an L2ADDR, the Proxy/Server compares the
   encapsulation IP address and UDP port number with the (unobfuscated)
   values.  If the values are the same, the Proxy/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 the OMNI option did
   not include an L2ADDR) the Proxy/Server instead caches the Client's
   information as "mapped" addresses meaning that NAT traversal
   exchanges may be necessary.

   The Proxy/Server then prepares an RA message with IPv6 source and
   destination set corresponding to the addresses in the RS, and with an

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   OMNI option with an Origin Indication sub-option per
   [I-D.templin-6man-omni-interface] with the mapped and obfuscated Port
   Number and IP address observed in the encapsulation headers.  The
   Proxy/Server also includes a HIP "Responder" message sub-option per
   [I-D.templin-6man-omni-interface] that contains an acknowledgement of
   the update sent by the Client.  The Proxy/Server then performs OAL
   encapsulation and fragmentation if necessary, and encapsulates each
   fragment in UDP/IP headers with addresses set per the L2ADDR
   information in the neighbor cache entry for the Client.

   When the Client receives the RA message, it performs OAL reassembly
   if necessary, authenticates the HIP "Responder" message, then
   compares the mapped Port Number and IP address from the Origin
   Indication sub-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
   qualification procedures are necessary to detect the type of NAT and
   proceeds according to standard procedures [RFC6081][RFC4380].

   After the Client has registered its INET interfaces in such RS/RA
   exchanges it sends periodic RS messages to receive fresh RA messages
   before the Router Lifetime received on each INET interface expires.
   The Client also maintains default routes via its Proxy/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 3.14 using IPv6 ND address
   resolution messaging.  The Client sends the NS(AR) message to the
   Proxy/Server with an OMNI option with a HIP "Update/Sequence" message
   sub-option.  The Client sets the NS source address to the Client's
   MNP-LLA and destination address to the target solicited node
   multicast address.  The Client wraps the NS message in an OAL header
   with source address set to its own MNP-ULA and destination address
   set to the Proxy/Server's ADM-ULA.  The Client then performs OAL
   fragmentation if necessary, wraps each fragment in a UDP/IP header
   and sends it to the Proxy/Server.

   When the Proxy/Server receives the OAL-encapsulated NS, it performs
   OAL reassembly if necessary.  The Proxy/Server then authenticates the
   message by processing the HIP message sub-option and sends a
   corresponding NS(AR) message over the spanning tree the same as if it
   were the ROS, but with the OAL source address set to the Proxy/
   Server's ADM-ULA, with destination set to the MNP-ULA of the target,
   and with an OMNI option that includes no sub-options.  When the ROR
   receives the NS(AR), it adds the Proxy/Server's ADM-ULA and Client's
   MNP-LLA to the target's Report List, and returns an NA(AR) with OMNI
   option information for the target including all of the target's
   Interface Attributes.  The ROR sets the NA(AR) source address to the

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   target's MNP-LLA and sets the destination address to the Client's
   MNP-LLA, then sets the OAL source address to the ADM-ULA of the ROR
   and the destination to the ADM-ULA of the Proxy/Server.  When the
   Proxy/Server receives the NA(AR) message, it rewrites the OAL source
   address to its own ADM-ULA and the destination address to the MNP-ULA
   of the Client, then includes a HIP "Update/Acknowledge" message sub-
   option in the OMNI option, performs OAL fragmentation if necessary,
   wraps each fragment in UDP/IP headers, and sends it to the Client.

   Following route optimization for targets in the same OMNI link
   segment, if the target's L2ADDR is on the open INET, the Client
   forwards carrier packets directly to the target INET address.  If the
   target is behind a NAT, the Client first establishes NAT state for
   the L2ADDR using the "direct bubble" and NUD mechanisms discussed in
   Section 3.10.1.  The Client continues to send carrier packets via its
   Proxy/Server until NAT state is populated, then begins forwarding
   carrier packets via the direct path through the NAT to the target.
   For targets in different OMNI link segments, the Client uses OAL/ORH
   encapsulation and forwards carrier packets to the Bridge that
   returned the NA message.

   The ROR may return uNAs via the Proxy/Server if the target moves, and
   the Proxy/Server will send corresponding uNAs to the Client with a
   HIP "Notify" authentication message.  The Client can also send NUD
   messages to test forward path reachability even though there is no
   security association between the Client and the target.

   The Client can send original IP packets to route-optimized neighbors
   in the same OMNI link segment no larger than the path MPS in one
   piece and without OAL encapsulation and fragmentation, but with *NET
   encapsulation.  For larger original IP packets, the Client applies
   OAL encapsulation and fragmentation if necessary according to
   Section 3.9, with OAL header with source set to its own MNP-ULA and
   destination set to the MNP-ULA of the target.  The Client then
   encapsulates each original IP packet or OAL fragment in UDP/IP *NET
   headers and sends them to the next hop.

   Note: The NAT traversal procedures specified in this document are
   applicable for Cone, Address-Restricted and Port-Restricted NATs
   only.  While future updates to this document may specify procedures
   for other NAT variations (e.g., hairpinning and various forms of
   Symmetric NATs), it should be noted that continuous communications
   are always possible through forwarding via a Proxy/Server even if NAT
   traversal is not employed.

   Note: Following the initial HIP Initiator/Responder exchange, AERO
   Clients with OMNI interfaces configured over the open Internet
   maintain HIP associations through the transmission of IPv6 ND

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   messages that include OMNI options with HIP "Update" and "Notify"
   messages.  OMNI interfaces use the HIP "Update" message when an
   acknowledgement is required, and use the "Notify" message in
   unacknowledged isolated IPv6 ND messages (e.g., unsolicited NAs).

   Note: Proxy/Servers on the open Internet that act as Proxys
   authenticate and remove OMNI option HIP message sub-options from RSes
   they forward from the MN to another Proxy/Server, and insert and sign
   HIP message and Origin Indication sub-options in RAs they forward
   from another Proxy/Server to the MN.  Conversely, Proxy/Servers that
   act as Proxys forward without processing any MNP registration/
   delegation information in RS/RA message exchanges between MNs and
   other Proxy/Servers.  The Proxy/Server acting as a Proxy is therefore
   responsible for MN authentication, while the other Proxy/Servers are
   responsible for registering/delegating MNPs (noting that the same
   node can act as both Proxy and Proxy/Server).

3.23.  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 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 to a Node Identification value
   (see: [I-D.templin-6man-omni-interface]) to be used as the Client ID
   seed for MNP prefix delegation.  The Client would then be obligated
   to renumber its internal networks whenever its MNP (and therefore
   also its MNP-LLA) 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 early 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.

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   AERO Release-3.0.2 was tagged on October 15, 2020, and is undergoing
   internal testing.  Additional internal releases expected within the
   coming months, with first public release expected end of 1H2021.

5.  IANA Considerations

   The IANA is instructed to assign a new type value TBD1 in the IPv6
   Routing Types registry.

   The IANA has assigned the UDP port number "8060" for an earlier
   experimental first version of AERO [RFC6706].  This document
   obsoletes [RFC6706], and together with
   [I-D.templin-6man-omni-interface] reclaims the UDP port number "8060"
   for 'aero' as the service port for UDP/IP encapsulation.  (Note that,
   although [RFC6706] was not widely implemented or deployed, any
   messages coded to that specification can be easily distinguished and
   ignored since they use the invalid ICMPv6 message type number '0'.)
   This document makes no request of IANA, since
   [I-D.templin-6man-omni-interface] already provides instructions.

   No further IANA actions are required.

6.  Security Considerations

   AERO Bridges configure secured tunnels with AERO Proxy/Servers,
   Relays and Proxys within their local OMNI link segments.  Applicable
   secured tunnel alternatives include IPsec [RFC4301], TLS/SSL
   [RFC8446], DTLS [RFC6347], WireGuard [WG], etc.  The AERO Bridges of
   all OMNI link segments in turn configure secured tunnels for their
   neighboring AERO Bridges in a spanning tree topology.  Therefore,
   control messages exchanged between any pair of OMNI link neighbors on
   the spanning tree are already secured.

   AERO nodes acting as Route Optimization Responders (RORs) may also
   receive packets directly from arbitrary nodes in INET partitions
   instead of via the spanning tree.  For INET partitions that apply
   effective ingress filtering to defeat source address spoofing, the
   simple data origin authentication procedures in Section 3.8 can be
   applied.

   For INET partitions that require strong security in the data plane,
   two options for securing communications include 1) disable route
   optimization so that all traffic is conveyed over secured tunnels, 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

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   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 as
   part of the spanning tree (see above).  AERO Clients connected to the
   open INET can use 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, the
   authentication services specified in [RFC7401] and/or [RFC4380]
   should be applied.

   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 network and/or
   transport layer security services such as IPsec, TLS/SSL, DTLS, etc.
   AERO Proxys and Proxy/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 Proxy/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 Proxy/
   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 Proxy/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 messages with ULA addresses are injected
   into an OMNI 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.)

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   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.).

   SRH authentication facilities are specified in [RFC8754].

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

   Security considerations for IPv6 fragmentation and reassembly are
   discussed in [I-D.templin-6man-omni-interface].

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.

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

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

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

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

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

   o  AERO, First Edition [RFC6706]

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

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

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

   This work is aligned with the Boeing 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 IP Packets
              over Overlay Multilink Network (OMNI) Interfaces", draft-
              templin-6man-omni-interface-69 (work in progress), January
              2021.

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

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   [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>.

   [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>.

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

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   [RFC7401]  Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
              Henderson, "Host Identity Protocol Version 2 (HIPv2)",
              RFC 7401, DOI 10.17487/RFC7401, April 2015,
              <https://www.rfc-editor.org/info/rfc7401>.

   [RFC7739]  Gont, F., "Security Implications of Predictable Fragment
              Identification Values", RFC 7739, DOI 10.17487/RFC7739,
              February 2016, <https://www.rfc-editor.org/info/rfc7739>.

   [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.bonica-6man-comp-rtg-hdr]
              Bonica, R., Kamite, Y., Alston, A., Henriques, D., and L.
              Jalil, "The IPv6 Compact Routing Header (CRH)", draft-
              bonica-6man-comp-rtg-hdr-24 (work in progress), January
              2021.

   [I-D.bonica-6man-crh-helper-opt]
              Li, X., Bao, C., Ruan, E., and R. Bonica, "Compressed
              Routing Header (CRH) Helper Option", draft-bonica-6man-
              crh-helper-opt-02 (work in progress), October 2020.

   [I-D.ietf-intarea-frag-fragile]
              Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile", draft-
              ietf-intarea-frag-fragile-17 (work in progress), September
              2019.

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   [I-D.ietf-intarea-tunnels]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-10 (work in
              progress), September 2019.

   [I-D.ietf-ipwave-vehicular-networking]
              Jeong, J., "IPv6 Wireless Access in Vehicular Environments
              (IPWAVE): Problem Statement and Use Cases", draft-ietf-
              ipwave-vehicular-networking-19 (work in progress), July
              2020.

   [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-10 (work in progress), January 2021.

   [I-D.templin-6man-dhcpv6-ndopt]
              Templin, F., "A Unified Stateful/Stateless Configuration
              Service for IPv6", draft-templin-6man-dhcpv6-ndopt-11
              (work in progress), January 2021.

   [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-ipwave-uam-its]
              Templin, F., "Urban Air Mobility Implications for
              Intelligent Transportation Systems", draft-templin-ipwave-
              uam-its-04 (work in progress), January 2021.

   [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-27 (work in progress),
              January 2021.

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

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   [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>.

   [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>.

   [RFC2004]  Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
              DOI 10.17487/RFC2004, October 1996,
              <https://www.rfc-editor.org/info/rfc2004>.

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

   [RFC2464]  Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
              <https://www.rfc-editor.org/info/rfc2464>.

   [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>.

   [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>.

   [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>.

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   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

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

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

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

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

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

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

   [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>.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   [RFC6355]  Narten, T. and J. Johnson, "Definition of the UUID-Based
              DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355,
              DOI 10.17487/RFC6355, August 2011,
              <https://www.rfc-editor.org/info/rfc6355>.

   [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>.

   [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>.

   [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>.

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   [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>.

   [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>.

   [WG]       Wireguard, "Wireguard, https://www.wireguard.com", August
              2020.

Appendix A.  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:

A.1.  Implementation Strategies for Route Optimization

   Route optimization as discussed in Section 3.14 results in the route
   optimization source (ROS) creating a 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

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   window.  If no additional data packets are sent within the 5 second
   window, delete the neighbor cache entry.

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

A.2.  Implicit Mobility Management

   OMNI 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.

A.3.  Direct Underlying Interfaces

   When a Client's OMNI 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, Proxy/Servers and Bridges in
   the communications path.  Direct interfaces must be tested
   periodically for reachability, e.g., via NUD.

A.4.  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

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   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 OMNI links.

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

   AERO Relays can be any dedicated server or COTS router platform
   connected to INETs and/or EUNs.  The Relay connects to the OMNI link
   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 Proxy/Server, and can route between the MNP and non-MNP
   address spaces.

A.5.  AERO Server Failure Implications

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

   If a Proxy/Server fails, ongoing packet forwarding to Clients will
   continue by virtue of the 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 Proxy/
   Server fails, unsolicited NA messages may be lost but proxy neighbor
   cache entries in the DEPARTED state will ensure that packet

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   forwarding to the Client's new locations will continue for up to
   DepartTime seconds.

   If a Client is left without a Proxy/Server for an extended timeframe
   (e.g., greater than ReachableTime seconds) then existing 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 Proxy/Server
   relationship, after which time continuous communications will resume.

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

A.6.  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
   Proxy/Servers and selects one Proxy/Server to connect to.  The AERO
   service is analogous to common Internet services such as google.com,
   yahoo.com, cnn.com, etc.  However, there is only one AERO service for
   the link and all Proxy/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,

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      the same addresses are returned (albeit in randomized order) when
      the domainname is resolved via different DNS resolvers.

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

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

   In contrast to the Google and IETF philosophies, Yahoo and Amazon
   both provide clients with a (short) list of IP addresses with Yahoo
   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 Proxy/Server addresses through the mechanisms discussed
   in earlier sections.  Each Proxy/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

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   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 Proxy/
   Server ADM-LLAs at each point.  It then selects one AERO Proxy/Server
   address, and engages in RS/RA exchanges with the same Proxy/Server
   from all ANET connections.  The Client remains with this Proxy/Server
   unless or until the Proxy/Server fails, in which case it can switch
   over to an alternate Proxy/Server.  The Client can likewise switch
   over to a different Proxy/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 B.  Change Log

   << RFC Editor - remove prior to publication >>

   Changes from draft-templin-intarea-6706bis-61 to draft-templin-
   intrea-6706bis-62:

   o  New sub-section on OMNI Neighbor Interface Attributes

   Changes from draft-templin-intarea-6706bis-59 to draft-templin-
   intrea-6706bis-60:

   o  Removed all references to S/TLLAO - all Interface Attributes are
      now maintained completely in the OMNI option.

   Changes from draft-templin-intarea-6706bis-58 to draft-templin-
   intrea-6706bis-59:

   o  The term "Relay" used in older draft versions is now "Bridge".
      "Relay" now refers to what was formally called: "Gateway".

   o  Fine-grained cleanup of Forwarding Algorithm; IPv6 ND message
      addressing; OMNI Prefix Lengths, etc.

   Changes from draft-templin-intarea-6706bis-54 to draft-templin-
   intrea-6706bis-55:

   o  Updates on Segment Routing and S/TLLAO contents.

   o  Various editorials and addressing cleanups.

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   Changes from draft-templin-intarea-6706bis-52 to draft-templin-
   intrea-6706bis-53:

   o  Normative reference to the OMNI spec, and remove portions that are
      already specified in OMNI.

   o  Renamed "AERO interface/link" to "OMIN interface/link" throughout
      the document.

   o  Truncated obsolete back section matter.

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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