Automatic Extended Route Optimization (AERO)
draft-templin-6man-aero-35

Document Type Active Internet-Draft (individual)
Author Fred Templin 
Last updated 2021-10-15
Replaces draft-templin-intarea-6706bis
Stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Standards Track                        October 15, 2021
Expires: April 18, 2022

              Automatic Extended Route Optimization (AERO)
                       draft-templin-6man-aero-35

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 (IPv6 ND)
   protocol.  Prefix delegation/registration services are employed for
   network admission and to manage the IP forwarding and routing
   systems.  Secure multilink operation, mobility management, multicast,
   traffic path selection 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 end user devices
   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
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   This Internet-Draft will expire on April 18, 2022.

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

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   (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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Automatic Extended Route Optimization (AERO)  . . . . . . . .  13
     3.1.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .  14
     3.2.  The AERO Service over OMNI Links  . . . . . . . . . . . .  15
       3.2.1.  AERO/OMNI Reference Model . . . . . . . . . . . . . .  15
       3.2.2.  Addressing and Node Identification  . . . . . . . . .  18
       3.2.3.  AERO Routing System . . . . . . . . . . . . . . . . .  18
       3.2.4.  Segment Routing Topologies (SRTs) . . . . . . . . . .  20
       3.2.5.  Segment Routing For OMNI Link Selection . . . . . . .  21
     3.3.  OMNI Interface Characteristics  . . . . . . . . . . . . .  21
     3.4.  OMNI Interface Initialization . . . . . . . . . . . . . .  23
       3.4.1.  AERO Proxy/Server and Relay Behavior  . . . . . . . .  24
       3.4.2.  AERO Client Behavior  . . . . . . . . . . . . . . . .  24
       3.4.3.  AERO Bridge Behavior  . . . . . . . . . . . . . . . .  24
     3.5.  OMNI Interface Neighbor Cache Maintenance . . . . . . . .  25
       3.5.1.  OMNI ND Messages  . . . . . . . . . . . . . . . . . .  27
       3.5.2.  OMNI Neighbor Advertisement Message Flags . . . . . .  28
       3.5.3.  OMNI Neighbor Window Synchronization  . . . . . . . .  29
     3.6.  OMNI Interface Encapsulation and Fragmentation  . . . . .  29
     3.7.  OMNI Interface Decapsulation  . . . . . . . . . . . . . .  31
     3.8.  OMNI Interface Data Origin Authentication . . . . . . . .  32
     3.9.  OMNI Interface MTU  . . . . . . . . . . . . . . . . . . .  32
     3.10. OMNI Interface Forwarding Algorithm . . . . . . . . . . .  33
       3.10.1.  Client Forwarding Algorithm  . . . . . . . . . . . .  34
       3.10.2.  Proxy/Server and Relay Forwarding Algorithm  . . . .  35
       3.10.3.  Bridge Forwarding Algorithm  . . . . . . . . . . . .  37
     3.11. OMNI Interface Error Handling . . . . . . . . . . . . . .  38
     3.12. AERO Mobility Service Coordination  . . . . . . . . . . .  41
       3.12.1.  AERO Service Model . . . . . . . . . . . . . . . . .  42
       3.12.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  43
       3.12.3.  AERO Proxy/Server Behavior . . . . . . . . . . . . .  44
     3.13. AERO Route Optimization . . . . . . . . . . . . . . . . .  50
       3.13.1.  Multilink Address Resolution . . . . . . . . . . . .  51
       3.13.2.  Multilink Route Optimization . . . . . . . . . . . .  55
       3.13.3.  Rapid Commit Route Optimization  . . . . . . . . . .  67
       3.13.4.  Client/Bridge Route Optimization . . . . . . . . . .  67
       3.13.5.  Client/Client Route Optimization . . . . . . . . . .  69

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     3.14. Neighbor Unreachability Detection (NUD) . . . . . . . . .  71
     3.15. Mobility Management and Quality of Service (QoS)  . . . .  72
       3.15.1.  Mobility Update Messaging  . . . . . . . . . . . . .  73
       3.15.2.  Announcing Link-Layer Information Changes  . . . . .  74
       3.15.3.  Bringing New Links Into Service  . . . . . . . . . .  74
       3.15.4.  Deactivating Existing Links  . . . . . . . . . . . .  74
       3.15.5.  Moving Between Proxy/Servers . . . . . . . . . . . .  75
     3.16. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  76
       3.16.1.  Source-Specific Multicast (SSM)  . . . . . . . . . .  76
       3.16.2.  Any-Source Multicast (ASM) . . . . . . . . . . . . .  77
       3.16.3.  Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . .  78
     3.17. Operation over Multiple OMNI Links  . . . . . . . . . . .  78
     3.18. DNS Considerations  . . . . . . . . . . . . . . . . . . .  79
     3.19. Transition/Coexistence Considerations . . . . . . . . . .  79
     3.20. Proxy/Server-Bridge Bidirectional Forwarding Detection  .  80
     3.21. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . .  80
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  81
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  81
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  81
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  84
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  85
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  86
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  87
   Appendix A.  Non-Normative Considerations . . . . . . . . . . . .  94
     A.1.  Implementation Strategies for Route Optimization  . . . .  94
     A.2.  Implicit Mobility Management  . . . . . . . . . . . . . .  94
     A.3.  Direct Underlying Interfaces  . . . . . . . . . . . . . .  95
     A.4.  AERO Critical Infrastructure Considerations . . . . . . .  95
     A.5.  AERO Server Failure Implications  . . . . . . . . . . . .  96
     A.6.  AERO Client / Server Architecture . . . . . . . . . . . .  97
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  99
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  99

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 including intelligent transportation
   systems and enterprise mobile device users.  AERO is a secure
   internetworking and mobility management service that employs the
   Overlay Multilink Network Interface (OMNI) [I-D.templin-6man-omni]
   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.  The OMNI Adaptation Layer (OAL) supports
   multilink operation for increased reliability and path optimization
   while providing fragmentation and reassembly services to support

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   Maximum Transmission Unit (MTU) diversity.  In terms of precedence,
   this specification may provide first-principle insights into a
   representative mobility service architecture as context for
   understanding the OMNI specification.

   The AERO service connects Clients over Proxy/Servers and Relays that
   are seen as OMNI link neighbors, and includes Bridges that
   interconnect diverse Internetworks as 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 (IPv6 ND) protocol [RFC4861].  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 multilink fluctuations, and link-layer address
   changes are signaled by ND messaging the same as for any IPv6 link.

   AERO provides a secure cloud-based service where mobile node Clients
   may use Proxy/Servers acting as proxys and/or designated routers
   while 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.  Mobile node Clients discover shortest paths to OMNI link
   neighbors through AERO route optimization.  Both unicast and
   multicast communications are supported, and Clients may efficiently
   move between locations while maintaining continuous communications
   with correspondents and without changing their IP Address.

   AERO Bridges peer with Proxy/Servers in a secured private BGP overlay
   routing instance to establish a Segment Routing Topology (SRT)
   spanning tree over the underlying Internetworks of one or more
   disjoint administrative domains as a single unified OMNI link.  Each
   OMNI link instance is characterized by the set of Mobility Service
   Prefixes (MSPs) common to all mobile nodes.  Relays provide an
   optimal route from (fixed) correspondent nodes on the underlying
   Internetwork to (mobile or fixed) nodes on the OMNI link.  To the
   underlying Internetwork, the Relay is the source of a route to the
   MSP; hence uplink traffic to mobile nodes 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 both
   cases, Clients may be located behind Network Address Translators
   (NATs) on the path to their associated Proxy/Servers.  A means for
   robust traversal of NATs while avoiding "triangle routing" and
   critical infrastructure traffic concentration is therefore provided.

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

   AERO provides a secure aeronautical internetworking service 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 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.

   Along with OMNI, AERO provides secured optimal routing support for
   the "6M's" of modern Internetworking, including:

   1.  Multilink - a mobile node's ability to coordinate multiple
       diverse underlying data links as a single logical unit (i.e., the
       OMNI interface) to achieve the required communications
       performance and reliability objectives.

   2.  Multinet - the ability to span the OMNI link over a segment
       routing topology with multiple diverse administrative domain
       network segments while maintaining seamless end-to-end
       communications between mobile Clients and correspondents such as
       air traffic controllers, fleet administrators, etc.

   3.  Mobility - a mobile node's ability to change network points of
       attachment (e.g., moving between wireless base stations) which
       may result in an underlying interface address change, but without
       disruptions to ongoing communication sessions with peers over the
       OMNI link.

   4.  Multicast - the ability to send a single network transmission
       that reaches multiple nodes belonging to the same interest group,
       but without disturbing other nodes not subscribed to the interest
       group.

   5.  Multihop - a mobile node vehicle-to-vehicle relaying capability
       useful when multiple forwarding hops between vehicles may be

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       necessary to "reach back" to an infrastructure access point
       connection to the OMNI link.

   6.  MTU assurance - the ability to deliver packets of various robust
       sizes between peers without loss due to a link size restriction,
       and to dynamically adjust packets sizes to achieve the optimal
       performance for each independent traffic flow.

   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] is used
   extensively throughout.  The following terms are defined within the
   scope of this document:

   IPv6 Neighbor Discovery (IPv6 ND)
      a control message service for coordinating neighbor relationships
      between nodes connected to a common link.  AERO uses the IPv6 ND
      messaging service specified in [RFC4861] while including the OMNI
      option extensions specified in [I-D.templin-6man-omni].

   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 IPv6 ND messaging) are also in scope.  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 source address of an IPv6 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 network topology with a coherent IP routing and addressing plan
      and that provides a transit backbone service for its connected end
      systems.  INETs also provide an underlay service over which the

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      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 (a
      technique also known as "network segmentation").  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 throughout this document.)

   *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.

   *NET address
      the IP address (and also UDP port number when UDP is used) that
      appears in *NET encapsulations sent over a node's interface
      connection to a *NET.

   INADDR
      the same as defined for "*NET" address above, with both terms used
      interchangeably throughout the document.

   OMNI link
      the same as defined in [I-D.templin-6man-omni].  The OMNI link
      employs IPv6 encapsulation [RFC2473] to traverse intermediate
      nodes in a spanning tree over underlying *NET segments the same as
      a bridged campus LAN.  AERO nodes on the OMNI link appear as
      single-hop neighbors at the network layer even though they may be
      separated by many underlying *NET hops; AERO nodes can employ
      Segment Routing [RFC8402] to navigate between different OMNI
      links, and/or to cause packets to visit selected waypoints within
      the same OMNI link.

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   OMNI Interface
      a node's attachment to an OMNI link.  Since OMNI interface
      addresses 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 service that subjects original IP packets
      admitted into the interface to mid-layer IPv6 header encapsulation
      followed by 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 decapsulation but prior to
      OAL reassembly.

   (OAL) atomic fragment
      an OAL packet that can be forwarded without fragmentation, but
      still includes a Fragment Header with a valid Identification value
      and with Fragment Offset and More Fragments both set to 0.

   (OAL) carrier packet
      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 re-
      encapsulate carrier packets during forwarding by removing the *NET
      headers of the previous hop underlying network and replacing them
      with new *NET headers for the next hop underlying 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.

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   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 from a previous hop,
      then re-encapsulates the carrier packets in new *NET headers and
      forwards them to the next hop.  OAL intermediate nodes decrement
      the Hop Limit of the OAL IPv6 header during re-encapsulation, and
      discard the packet if the Hop Limit reaches 0.  OAL intermediate
      nodes do not decrement the Hop Limit/TTL of the original IP
      packet.

   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]).  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].

   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].

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

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   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 IPv6 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 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
      designated router services for coordination with correspondents on
      its INET-facing interfaces.  (Proxy/Servers in the open INET
      instead configure only a single INET interface and no ANET
      interfaces.)  The Proxy/Server configures an OMNI interface and
      assigns an ADM-LLA to support the operation of IPv6 ND services,
      while advertising any associated MNPs for which it is acting as a
      hub via BGP peerings with Bridges.

   AERO Relay ("Relay")
      a Proxy/Server that provides forwarding services between nodes
      reached via the OMNI link and correspondents on other links/
      networks.  AERO Relays configure an OMNI interface, assign an ADM-
      LLA and maintain BGP peerings with Bridges the same as Proxy/
      Servers and run a dynamic routing protocol to discover any non-MNP
      IP GUA routes in service on other links/networks.  The Relay
      advertises the MSP(s) to its other links/networks, and
      redistributes routes discovered on other links/networks into the
      OMNI link BGP routing system the same as for Proxy/Servers.
      (Relays that connect to major Internetworks such as the global
      IPv6 or IPv4 Internet can also be configured to advertise
      "default" routes into the OMNI link BGP routing system.)

   AERO Bridge ("Bridge")
      a BGP hub autonomous system node that also provides OAL forwarding
      services for nodes on an OMNI link.  Bridges forwards carrier
      packets between OMNI link segments as OAL intermediate nodes while
      decrementing the OAL IPv6 header Hop Limit but without
      decrementing the network layer IP TTL/Hop Limit.  Bridges peer
      with Proxy/Servers and other Bridges to form a spanning tree over
      all OMNI link segments and to discover the set of all MNP and non-
      MNP prefixes in service.  Bridges process carrier packets received
      over the secured spanning tree that are addressed to themselves,

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      while forwarding all other carrier packets to the next hop also
      via the secured spanning tree.  Bridges forward carrier packets
      received over the unsecured spanning tree to the next hop either
      via the unsecured spanning tree or via direct encapsulation if the
      next hop is on the same OMNI link segment.

   First-Hop Segment (FHS) Proxy/Server
      a Proxy/Server for a source Client's underlying interface that
      forwards the Client's packets into the segment routing topology.
      FHS Proxy/Servers also act as intermediate forwarding nodes to
      facilitate RS/RA exchanges between a Client and its Hub Proxy/
      Server.

   Hub Proxy/Server
      a single Proxy/Server selected by a Client that provides a
      designated router service for all of the Client's underlying
      interfaces.  Clients often select the first FHS Proxy/Server they
      coordinate with to serve in the Hub role (as all FHS Proxy/Servers
      are equally capable candidates to serve in that capacity), however
      the Client can also select any available Proxy/Server for the OMNI
      link (as there is no requirement that the Hub must also be one of
      the Client's FHS Proxy/Servers).

   Last-Hop Segment (LHS) Proxy/Server
      a Proxy/Server for an underlying interface of the target Client
      that forwards packets received from the segment routing topology
      to the target Client over that interface.

   Segment Routing Topology (SRT)
      a multinet OMNI link forwarding region between FHS and LHS Proxy/
      Servers.  FHS/LHS Proxy/Servers and SRT Bridges span the OMNI link
      on behalf of source/target Client pairs.  The SRT maintains a
      spanning tree established through BGP peerings between Bridges and
      Proxy/Servers.  Each SRT segment includes Bridges in a "hub" and
      Proxy/Servers in "spokes", while adjacent segments are
      interconnected by Bridge-Bridge peerings.  The BGP peerings are
      configured over both secured and unsecured underlying network
      paths such that a secured spanning tree is available for critical
      control messages while other messages can use the unsecured
      spanning tree.

   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.

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   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)
      the AERO node nearest the source that initiates route
      optimization.  The ROS may be a FHS Proxy/Server or Relay for the
      source, or may be the source Client itself.

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

   Potential Router List (PRL)
      a geographically and/or topologically referenced list of addresses
      of all Proxy/Servers within the same OMNI link.  Each OMNI link
      has its own PRL.

   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.

   Multilink Forwarding Information Base (MFIB)
      A forwarding table on each AERO/OMNI source, destination and
      intermediate node that includes Multilink Forwarding Vectors (MFV)

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      with both next hop forwarding instructions and context for
      reconstructing compressed headers for specific underlying
      interface pairs used to communicate with peers.

   Multilink Forwarding Vector (MFV)
      An MFIB entry that includes soft state for each underlying
      interface pairwise communication session between peer OMNI nodes.
      MFVs are identified by both a next-hop and previous-hop MFV Index
      (MFVI), with the next-hop established based on an IPv6 ND
      solicitation and the previous hop established based on the
      solicited IPv6 ND advertisement response.

   Multilink Forwarding Vector Index (MVFI)
      A 4 octet value selected by an AERO/OMNI node when it creates an
      MFV, then advertises to either a next-hop or previous-hop.  AERO/
      OMNI intermediate nodes assign two distinct local MFVIs for each
      MFV and advertise one to the next-hop and the other to the
      previous-hop.  AERO/OMNI end systems assign and advertise a single
      MFVI.  AERO/OMNI nodes also discover the remote MFVIs advertised
      by other nodes that indicate a value the remote node is willing to
      accept.

   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 IPv6 ND [RFC4861] and DHCPv6 [RFC8415] (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.

3.  Automatic Extended Route Optimization (AERO)

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

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3.1.  AERO Node Types

   AERO Clients can be deployed as fixed infrastructure nodes close to
   end systems, or as Mobile Nodes (MNs) that can change their network
   attachment points dynamically.  AERO Clients configure OMNI
   interfaces over underlying interfaces with addresses that may change
   due to mobility.  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 (together with Proxy/Servers) provide the secured
   backbone supporting infrastructure for a Segment Routing Topology
   (SRT) spanning tree for the OMNI link.  Bridges forward carrier
   packets both within the same SRT segment and between disjoint SRT
   segments based on an IPv6 encapsulation mid-layer known as the OMNI
   Adaptation Layer (OAL) [I-D.templin-6man-omni].  The OMNI interface
   and OAL provide 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 assign one or more
   Mobility Service Prefixes (MSPs) to the OMNI link and configure
   secured tunnels with Proxy/Servers, Relays and other Bridges; they
   further maintain forwarding table entries for each MNP or non-MNP
   prefix in service on the OMNI link.

   AERO Proxy/Servers distributed across one or more SRT segments
   provide default forwarding and mobility/multilink services for AERO
   Client mobile nodes.  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).  Hub Proxy/Servers provide prefix
   delegation/registration services and track the mobility/multilink
   profiles of each of their associated Clients, where each delegated
   prefix becomes an MNP taken from an MSP.  Proxy/Servers at ANET/INET
   boundaries provide a forwarding service for ANET Clients to
   communicate with peers in external INETs, while Proxy/Servers in the
   open INET provide an authentication service for INET Client IPv6 ND
   messages but only a secondary forwarding service when the Client
   cannot forward directly to a peer or Bridge.  Source Clients securely
   coordinate with target Clients by sending control messages via a
   First-Hop Segment (FHS) Proxy/Server which forwards them over the SRT
   spanning tree to a Last-Hop Segment (LHS) Proxy/Server which finally
   forwards them to the target.

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   AERO Relays are Proxy/Servers that provide forwarding services to
   exchange original IP packets between the OMNI link and nodes on other
   links/networks.  Relays run a dynamic routing protocol to discover
   any non-MNP prefixes in service on other links/networks, and Relays
   that connect to larger Internetworks (such as the Internet) may
   originate default routes.  The Relay redistributes OMNI link MSP(s)
   into other links/networks, and redistributes non-MNP prefixes via
   OMNI link Bridge BGP peerings.

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    |
                          +-------+--------+
       +--------------+           |            +--------------+
       |  AERO P/S S1 |           |            |  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:

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

   o  AERO Bridge B1 aggregates Mobility Service Prefix (MSP) M1,
      discovers Mobile Network Prefixes (MNPs) X* and advertises the MSP
      via BGP peerings over secured tunnels to Proxy/Servers (S1, S2).
      Bridges provide the backbone for an SRT spanning tree for the OMNI
      link.

   o  AERO Proxy/Servers S1 and S2 configure secured tunnels with Bridge
      B1 and also provide mobility, multilink, multicast and default
      router services for the MNPs of their associated Clients C1 and
      C2.  (Proxy/Servers that act as Relays can also advertise non-MNP
      routes for non-mobile correspondent nodes the same as for MNP
      Clients.)

   o  AERO Clients C1 and C2 associate with Proxy/Servers S1 and S2,
      respectively.  They receive 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;
   all nodes on the link can exchange carrier packets via simple *NET
   encapsulation (i.e., following any necessary NAT traversal) since the
   underlying *NET is connected.  In common practice, however, OMNI
   links are often configured over an SRT spanning tree that bridges
   multiple distinct *NET segments managed under different
   administrative authorities (e.g., as for worldwide aviation service
   providers such as ARINC, SITA, Inmarsat, etc.).  Individual *NETs may
   also be partitioned internally, in which case each internal partition
   appears as a separate segment.

   The addressing plan of each SRT 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 disjoint segments often have no 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.

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   The OMNI link spans multiple SRT segments using the OMNI Adaptation
   Layer (OAL) [I-D.templin-6man-omni] to provide the network layer with
   a virtual abstraction similar to a bridged campus LAN.  The OAL is an
   OMNI interface sublayer that inserts a mid-layer IPv6 encapsulation
   header for inter-segment forwarding (i.e., bridging) without
   decrementing the network-layer TTL/Hop Limit of the original IP
   packet.  An example OMNI link SRT is shown in Figure 2:

                 . . . . . . . . . . . . . . . . . . . . . . .
               .                                               .
               .              .-(::::::::)                     .
               .           .-(::::::::::::)-.   +-+            .
               .          (:::: Segment A :::)--|B|---+        .
               .           `-(::::::::::::)-'   +-+   |        .
               .              `-(::::::)-'            |        .
               .                                      |        .
               .              .-(::::::::)            |        .
               .           .-(::::::::::::)-.   +-+   |        .
               .          (:::: Segment B :::)--|B|---+        .
               .           `-(::::::::::::)-'   +-+   |        .
               .              `-(::::::)-'            |        .
               .                                      |        .
               .              .-(::::::::)            |        .
               .           .-(::::::::::::)-.   +-+   |        .
               .          (:::: Segment C :::)--|B|---+        .
               .           `-(::::::::::::)-'   +-+   |        .
               .              `-(::::::)-'            |        .
               .                                      |        .
               .                ..(etc)..             x        .
               .                                               .
               .                                               .
               .    <-    Segment Routing Topology (SRT) ->    .
               .             (Spanned by OMNI Link)            .
                 . . . . . . . . . . . . . .. . . . . . . . .

            Figure 2: OMNI Link Segment Routing Topology (SRT)

   Bridge, Proxy/Server and Relay OMNI interfaces are configured over
   both secured tunnels and open INET underlying interfaces within their
   respective SRT segments.  Within each segment, Bridges configure
   "hub-and-spokes" BGP peerings with Proxy/Server/Relays as "spokes".
   Adjacent SRT segments are joined by Bridge-to-Bridge peerings to
   collectively form a spanning tree over the entire SRT.  The "secured"
   spanning tree supports authentication and integrity for critical
   control plane messages.  The "unsecured" spanning tree conveys
   ordinary carrier packets without security codes and that must be
   treated by destinations according to data origin authentication
   procedures.  AERO nodes can employ route optimization to cause

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   carrier packets to take more direct paths between OMNI link neighbors
   without having to follow strict spanning tree paths.

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 based on 32-bit Mobility Service ID (MSID) values
   ("ADM-LLAs") per [I-D.templin-6man-omni].  Non-MNP routes are also
   represented the same as for MNP-LLAs, but may include a prefix that
   is not properly covered by an MSP.

   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
   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] to support OAL addressing.  (The prefix
   [ULA*]::/64 appearing alone and with no suffix represents "default".)
   AERO Clients also use Temporary ULAs constructed per
   [I-D.templin-6man-omni], 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.  A GUA block is also reserved for OMNI link anycast
   purposes.  See [I-D.templin-6man-omni] 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].

3.2.3.  AERO Routing System

   The AERO routing system comprises a private Border Gateway Protocol
   (BGP) [RFC4271] service coordinated between Bridges and Proxy/
   Servers.  The service supports carrier packet forwarding at a layer
   below IP and does not interact with the public Internet BGP routing
   system, but supports redistribution of information for other links
   and networks discovered by Relays.

   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

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   with one or more Bridges but does not peer with other Proxy/Servers.
   Each SRT segment in the OMNI link must include one or more Bridges in
   a "hub" AS, which peer with the Proxy/Servers within that segment as
   "spoke" ASes.  All Bridges within the same segment 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 segments peer
   with one another using eBGP.

   Bridges maintain forwarding table entries only for the MNP-ULAs
   corresponding to MNP and non-MNP routes that are currently active,
   and also maintain black-hole routes for the OMNI link MSPs so that
   carrier packets destined to non-existent MNP-ULAs are dropped with a
   Destination Unreachable message returned.  In this way, Proxy/Servers
   and Relays have only partial topology knowledge (i.e., they only
   maintain routing information for 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 known as the "SRT prefix".  For example, a first segment
   could assign [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 SRT 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 SRT segments into a unified OMNI link
   over multiple diverse network administrative domains.  They support a
   virtual bridging service 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 appear as follows:

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

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   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 rarely change, since they correspond
   to fixed infrastructure elements in their respective segments.

   MNP (and non-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

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

3.2.4.  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 bridged campus LAN 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.

   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 OMNI IPv6 anycast address
   used for OMNI interface determination in Safety-Based Multilink (SBM)
   as discussed in [I-D.templin-6man-omni].  Each OMNI interface further
   applies Performance-Based Multilink (PBM) internally.

   The Bridges and Proxy/Servers of each independent SRT engage in BGP
   peerings to form a spanning tree with the Bridges in non-leaf nodes
   and the Proxy/Servers in leaf nodes.  The spanning tree is configured
   over both secured and unsecured underlying network paths.  The
   secured spanning tree is used to convey secured control messages

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   between Proxy/Servers and Bridges, while the unsecured spanning tree
   forwards data messages and/or unsecured control messages.

   Each SRT segment is identified by a unique ADM-ULA prefix used by all
   Proxy/Servers and Bridges in the segment.  Each AERO node must
   therefore discover an SRT prefix that correspondents can use to
   determine the correct segment, and must publish the SRT prefix in
   IPv6 ND messages.

3.2.5.  Segment Routing For OMNI Link Selection

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

   When an 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).

3.3.  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 more 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 with the outermost NAT providing
      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 an FHS Proxy/Server.  Clients can issue control messages
      over the ANET without including an authentication signature since
      the ANET is secured at the network layer or below.  Proxy/Servers
      can actively issue control messages over the INET on behalf of
      ANET Clients to reduce ANET congestion.  The same as for INET

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      interfaces, there may be NATs on the path from the Client to its
      FHS Proxy/Server.

   o  VPNed interfaces use security encapsulation over the INET to a
      Virtual Private Network (VPN) server that also acts as an FHS
      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 an FHS 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 3.6.  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 FHS 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 FHS
   Proxy/Server are known to be on the same underlying link.

   OMNI interfaces maintain a neighbor cache for tracking per-neighbor
   state the same as for any interface.  OMNI interfaces use IPv6 ND
   messages including Router Solicitation (RS), Router Advertisement
   (RA), Neighbor Solicitation (NS) and Neighbor Advertisement (NA) for
   neighbor cache management.  In environments where spoofing may be a
   threat, OMNI neighbors should employ OAL Identification window
   synchronization in their IPv6 ND message exchanges.

   OMNI interfaces send IPv6 ND messages with an OMNI option formatted
   as specified in [I-D.templin-6man-omni].  The OMNI option includes
   prefix registration information, Interface Attributes and/or
   Multilink Forwarding Parameters containing link information
   parameters for the OMNI interface's underlying interfaces and any
   other per-neighbor information.

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

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   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 successive IPv6 ND messages all include OMNI option
   Multilink Forwarding Parameters sub-options with the same underlying
   interface index.  In that case, the Client would appear to have a
   single underlying interface but with a dynamically changing link-
   layer address.

   If the Client has multiple active underlying interfaces, then from
   the perspective of IPv6 ND it would appear to have multiple link-
   layer addresses.  In that case, IPv6 ND message OMNI options MAY
   include Interface Attributes and/or Multilink Forwarding Parameters
   sub-options with different underlying interface indexes.

   Proxy/Servers on the open Internet include only a single INET
   underlying interface.  INET Clients therefore discover only the
   INADDR information for the Proxy/Server's INET interface.  Proxy/
   Servers on an ANET/INET boundary include both an ANET and INET
   underlying interface.  ANET Clients therefore must discover both the
   ANET and INET INADDR information for the Proxy/Server.

   Bridge and Proxy/Server OMNI interfaces are configured over
   underlying interfaces that provide both secured tunnels for carrying
   IPv6 ND and BGP protocol control plane messages and open INET access
   for carrying unsecured messages.  The OMNI interface configures both
   an ADM-LLA and its corresponding ADM-ULA, and acts as an OAL source
   to encapsulate and fragment original IP packets while presenting the
   resulting carrier packets over the secured or unsecured underlying
   paths.  Note that Bridge and Proxy/Server end-to-end transport
   protocol sessions used by the BGP are run directly over the OMNI
   interface and use ADM-ULA source and destination addresses.  The OMNI
   interface employs the OAL to encapsulate the original IP packets for
   these sessions as carrier packets (i.e., even though the OAL header
   may use the same ADM-ULAs as the original IP header) and forwards
   them over the secured underlying path.

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 associated with another interface are
   directed to an OMNI interface.

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

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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 SRT segment.  The
   Proxy/Server also configures secured tunnels with one or more
   neighboring Bridges and engages in BGP routing protocol sessions with
   one or more Bridges.

   The OMNI interface provides a single interface abstraction to the IP
   layer, but internally includes an NBMA nexus for sending carrier
   packets to OMNI interface neighbors over underlying INET interfaces
   and secured tunnels.  The Proxy/Server further configures a service
   to facilitate IPv6 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 over its
   underlying interfaces to an FHS Proxy/Server, which coordinates with
   a Hub Proxy/Server that returns an RA message with corresponding
   parameters.  The RS/RA messages may pass through one or more NATs in
   the path between the Client and FHS Proxy/Server.  (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 FHS 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 delegate an MNP.)

3.4.3.  AERO Bridge Behavior

   AERO Bridges configure an OMNI interface and assign an ADM-ULA and
   corresponding Subnet Router Anycast address for each OMNI link SRT
   segment they connect to.  Bridges configure secured tunnels with
   Proxy/Servers in the same SRT segment and other Bridges in the same
   (or an adjacent) SRT segment.  Bridges then engage in a BGP routing

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   protocol session with neighbors over the secured spanning tree (see:
   Section 3.2.3).

3.5.  OMNI Interface Neighbor Cache Maintenance

   Each OMNI interface maintains a conceptual neighbor cache that
   includes a Neighbor Cache Entry (NCE) for each of its active
   neighbors on the OMNI link per [RFC4861].  Each NCE is indexed by the
   LLA of the neighbor, while the OAL encapsulation ULA determines the
   context for Identification verification.  Clients and Proxy/Servers
   maintain NCEs through RS/RA exchanges, and also maintain NCEs for any
   active correspondent peers through NS/NA exchanges.

   Bridges also maintain NCEs for Clients within their local segments
   based on NS/NA route optimization messaging (see: Section 3.13.4).
   When a Bridge creates/updates a NCE for a local segment Client based
   on NS/NA route optimization, it also maintains MVFI and INADDR state
   for messages destined to this local segment Client.

   Proxy/Servers add an additional state DEPARTED to the list of NCE
   states found in Section 7.3.2 of [RFC4861].  When a Client terminates
   its association, the Proxy/Server OMNI interface sets a "DepartTime"
   variable for the NCE to "DEPART_TIME" seconds.  DepartTime is
   decremented unless a new IPv6 ND message causes the state to return
   to REACHABLE.  While a NCE is in the DEPARTED state, the Proxy/Server
   forwards carrier packets destined to the target Client to the
   Client's new FHS/Hub Proxy/Server instead.  It is RECOMMENDED that
   DEPART_TIME be set to the default constant value 10 seconds to accept
   any carrier packets that may be in flight.  When DepartTime
   decrements to 0, the NCE is deleted.

   Clients determine the service profiles for their FHS and Hub Proxy/
   Servers by setting the N/A/U flags in the first OMNI option in RS
   messages.  When the N/A/U flags are clear, Proxy/Servers forward all
   NS/NA messages to the Client, while the Client performs mobility
   update signaling through the transmission of uNA messages to all
   active neighbors following a mobility event.  However, in some
   environments this may result in excessive NS/NA control message
   overhead especially for Clients connected to low-end data links.

   To minimize NS/NA message overhead, Clients can set the N/A/U flags
   in the first OMNI option of RS messages they send.  If the N flag is
   set, the FHS Proxy/Server that forwards the RS message assumes the
   role of responding to NS(NUD) messages and maintains peer NCEs
   associated with the NCE for this Client.  If the A flag is set, the
   Hub Proxy/Server that processes the RS message assumes the role of
   responding to NS(AR) messages on behalf of this Client NCE.  If the U
   flag is set, the Hub Proxy/Server that processes the RS message

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   becomes responsible for maintaining a "Report List" of sources from
   which it has received an NS(AR) for this Client NCE.  The Hub Proxy/
   Server maintains each Report List entry for REPORT_TIME seconds, and
   sends uNA messages to each member of the Report List when it receives
   a Client mobility update indication (e.g., through receipt of an RS
   with updated Interface Attributes, Traffic Selectors, etc.).

   Clients and their Hub Proxy/Servers have full knowledge of the
   Client's current underlying Interface Attributes, while FHS Proxy/
   Servers acting in "proxy" mode have knowledge of only the individual
   Client underlying interfaces they service.  Clients determine their
   FHS and Hub Proxy/Server service models by setting the N/A/U flags in
   the RS messages they send as discussed above.

   Clients act as RORs on their own behalf when they receive an NS(AR)
   from an ROS via their Hub Proxy/Server (Relays instead act as RORs on
   behalf of non-MNP targets specific to other links/networks that the
   Relay services and/or "default").  The ROR returns and NA(AR)
   response to the ROS, which creates or updates a NCE for the target
   network-layer and link-layer addresses.  The ROS then (re)sets
   ReachableTime for the NCE to REACHABLE_TIME seconds and performs
   reachability tests over specific underlying interface pairs to
   determine paths for forwarding carrier packets directly to the
   target.  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 messages 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 the above constants 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.

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3.5.1.  OMNI ND Messages

   OMNI interfaces prepare IPv6 ND messages the same as for standard
   IPv6 ND, but also include a new option type termed the OMNI option
   [I-D.templin-6man-omni].  For each IPv6 ND message, OMNI interfaces
   include one or more OMNI options (and any other ND message options)
   then completely populate all option information.  If the OMNI
   interface includes an authentication signature, it sets the IPv6 ND
   message Checksum field to 0 and calculates the authentication
   signature over the entire length of the OAL packet or super-packet
   (beginning with a pseudo-header of the IPv6 header up to but not
   including the trailing OAL checksum) but does not then proceed to
   calculate the IPv6 ND message checksum itself.  Otherwise, the OMNI
   interface calculates the standard IPv6 ND message checksum over the
   OAL packet or super-packet and writes the value in the Checksum
   field.  OMNI interfaces verify authentication and integrity of each
   IPv6 ND message received according to the specific check(s) included,
   and process the message further only following verification.

   OMNI options include per-neighbor information that provides multilink
   forwarding, link-layer address and traffic selector information for
   the neighbor's underlying interfaces.  This information is stored in
   the neighbor cache and provides the basis for the forwarding
   algorithm specified in Section 3.10.  The information is cumulative
   and reflects the union of the OMNI information from the most recent
   IPv6 ND messages received from the neighbor; it is therefore not
   required that each IPv6 ND message contain all neighbor information.

   The OMNI option is distinct from any Source/Target Link-Layer Address
   Options (S/TLLAOs) that may appear in an IPv6 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

   OMNI interface IPv6 ND messages may also include other IPv6 ND
   options.  In particular, solicitation messages may include a Nonce
   option if required for verification of advertisement replies.  If an
   OMNI IPv6 ND solicitation message includes a Nonce option, the
   advertisement reply must echo the same Nonce.  If an OMNI IPv6 ND
   advertisement message includes a Timestamp option, the recipient
   should check the Timestamp to determine if the message is current.

   AERO Clients send RS messages to the link-scoped All-Routers
   multicast address or an ADM-LLA while using unicast link-layer
   addresses.  AERO Proxy/Servers respond by returning unicast RA
   messages.  During the RS/RA exchange, AERO Clients and Proxy/Servers

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   include state synchronization parameters to establish Identification
   windows and other state.

   AERO nodes use NS/NA messages for the following purposes:

   o  NS/NA(AR) messages are used for address resolution and optionally
      to establish sequence number windows.  The ROS sends an NS(AR) to
      the solicited-node multicast address of the target, and an ROR
      with addressing information for the target returns a unicast
      NA(AR) that contains current, consistent and authentic target
      address resolution information.  NS/NA(AR) messages must be
      secured.

   o  NS/NA(NUD) messages are used to establish multilink forwarding
      state and determine target reachability.  The source sends an
      NS(NUD) to the unicast address of the target while naming a
      specific underlying interface pair, and the target returns a
      unicast NA(NUD).  NS/NA(NUD) messages that use an in-window
      sequence number and do not update any other state need not include
      an authentication signature but instead must include an IPv6 ND
      message checksum.  NS/NA(NUD) messages may also be used to
      establish window synchronization and/or MFIB state, in which case
      the messages must be secured.

   o  Unsolicited NA (uNA) messages are used to signal addressing and/or
      other neighbor state changes (e.g., address changes due to
      mobility, signal degradation, traffic selector updates, etc.). uNA
      messages that update state information must be secured.

   o  NS/NA(DAD) messages are not used in AERO, since Duplicate Address
      Detection is not required.

   Additionally, nodes may sent the OMNI option PNG flag in NA/RA
   messages to receive a uNA response from the neighbor.  The uNA
   response MUST set the ACK flag (without also setting the SYN or PNG
   flags) with the Acknowledgement field set to the Identification used
   in the PNG message.

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.

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   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 uNAs (both unicast and multicast).

   o  O: The O ("Override") flag is set to 0 for solicited NAs returned
      by a Proxy/Server ROR 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 IPv6 ND protocol
      operation, however, no role is currently seen for assigning the
      same MNP-LLA to multiple Clients.

3.5.3.  OMNI Neighbor Window Synchronization

   In secured environments (e.g., between secured spanning tree
   neighbors, between neighbors on the same secured ANET, etc.), OMNI
   interface neighbors can exchange OAL packets using randomly-
   initialized and monotonically-increasing Identification values
   (modulo 2**32) without window synchronization.  In environments where
   spoofing is considered a threat, OMNI interface neighbors instead
   invoke window synchronization in NS/NA message exchanges to maintain
   send/receive window state in their respective neighbor cache entries
   as specified in [I-D.templin-6man-omni].

3.6.  OMNI Interface Encapsulation and Fragmentation

   When the network layer forwards an original IP packet into an OMNI
   interface, the interface locates or creates a Neighbor Cache Entry
   (NCE) that matches the destination.  The OMNI interface then invokes
   the OMNI Adaptation Layer (OAL) as discussed in
   [I-D.templin-6man-omni] which encapsulates the packet in an IPv6
   header, and IPv6 Fragment Header and 2-octet Checksum trailer to
   produce an OAL packet.  This OAL source then calculates the checksum
   and fragments the OAL packet (and invokes OAL header compression when
   appropriate) while including an identical Identification value for
   each fragment that must be within the window for the LHS Proxy/Server
   or the target Client itself.

   The OAL source next includes an identical Compressed Routing Header
   with 32-bit ID fields (CRH-32) [I-D.bonica-6man-comp-rtg-hdr] with
   each fragment if necessary containing one or more Multilink
   Forwarding Vector Indices (MFVIs) as discussed in Section 3.13.  (The
   OAL source can then optionally invoke OAL header compression by
   replacing the OAL IPv6 header and CRH-32 with an OAL Compressed
   Header (OCH), Type 0 or 1.)

   The OAL source finally encapsulates each resulting OAL fragment in
   *NET headers to form an OAL carrier packet, with source address set

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   to its own *NET address (e.g., 192.0.2.100) and destination set to
   the *NET address of the next hop OAL intermediate node or destination
   (e.g., 192.0.2.1).  The carrier packet encapsulation format in the
   above example is shown in Figure 3:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |          *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     |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      CRH-32 (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 ~
        ~                               ~
        |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |     OAL Trailing Checksum     |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 3: Carrier Packet Format

   In this format, the OAL source encapsulates the original IP header
   and packet body/fragment in an OAL IPv6 header prepared according to
   [RFC2473], the CRH-32 is a Routing Header extension of the OAL
   header, the Fragment Header identifies each fragment, and the *NET
   header is prepared as discussed in [I-D.templin-6man-omni].  The OAL
   source transmits each such carrier packet into the SRT spanning tree,
   where they are forwarded over possibly multiple OAL intermediate
   nodes until they arrive at the OAL destination.

   The OMNI link control plane service distributes Client MNP-ULA prefix
   information that may change dynamically due to regional node mobility
   as well as Relay non-MNP-ULA and per-segment ADM-ULA prefix
   information that rarely changes.  OMNI link Bridges and Proxy/Servers
   use the information to establish and maintain a forwarding plane

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   spanning tree that connects all nodes on the link.  The spanning tree
   supports a carrier packet virtual bridging service according to link-
   layer (instead of network-layer) information, but may often include
   longer paths than necessary.

   Each OMNI interface therefore also includes a Multilink Forwarding
   Information Base (MFIB) with Multilink Forwarding Vectors (MFVs) that
   can often provide more direct forwarding "shortcuts" that avoid
   strict spanning tree paths.  As a result, the spanning tree is always
   available but OMNI interfaces can often use the MFIB to greatly
   improve performance and reduce load on critical infrastructure
   elements.

   For carrier packets undergoing re-encapsulation at an OAL
   intermediate node, the OMNI interface decrements the OAL IPv6 header
   Hop Limit and discards the carrier packet if the Hop Limit reaches 0.
   The intermediate node next removes the *NET encapsulation headers
   from the first segment and re-encapsulates the packet in new *NET
   encapsulation headers for the next segment.

   When an FHS Bridge receives a carrier packet with an OCH-0/1 header
   that must be forwarded to an LHS Bridge over the unsecured spanning
   tree, it reconstructs the headers based on MFV state, inserts a
   CRH-32 immediately following the OAL header and adjusts the OAL
   payload length and destination address field.  The FHS Bridge
   includes a single MFVI in the CRH-32 that will be meaningful to the
   LHS Bridge.  When the LHS Bridge receives the carrier packet, it
   locates the MFV for the next hop based on the CRH-32 MFVI then re-
   applies header compression (resulting in the removal of the CRH-32)
   and forwards the carrier packet to the next hop.

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 addressed to itself
   arrive, this OAL destination discards the NET encapsulation headers
   and reassembles to obtain the OAL packet or super-packet (see:
   [I-D.templin-6man-omni]).  The OAL destination then verifies the OAL
   checksum, discards the OAL encapsulations to obtain the original IP
   packet(s) and finally forwards them to either the network layer or a
   next-hop on the OMNI link.

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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 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, Proxy/Servers and Bridges verify carrier
      packet UDP/IP encapsulation addresses according to
      [I-D.templin-6man-omni].

   o  AERO nodes accept carrier packets addressed to themselves with
      Identification values within the current window for the OAL source
      neighbor (when window synchronization is used) and drop any
      carrier packets with out-of-window Identification values.  (AERO
      nodes may forward carrier packets not addressed to themselves
      without verifying the Identification value.)

   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 [I-D.templin-6man-omni] with MTU/MRU both set to the
   constant value 9180 bytes, with minimum MPS set to 400 bytes, and
   with potentially larger per-path MPS values 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

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   single OAL super-packet as discussed in [I-D.templin-6man-omni].  (If
   the super-packet begins with an IPv6 ND message that includes and
   authentication signature, the signature is calculated over the entire
   length of the super-packet up to but not including the trailing
   Checksum.)

   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: Although a CRH-32 may be inserted or removed by a Bridge in the
   path (see: Section 3.10.3), this does not interfere with the
   destination's ability to reassemble since the CRH-32 is not included
   in the fragmentable part and its removal/transformation does not
   invalidate fragment header information.

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 original IP packet 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/

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   or Traffic Selectors (e.g., port number, flow specification, etc.) to
   select an outbound underlying interface for each OAL packet and also
   to select segment routing and/or link-layer destination addresses
   based on the neighbor's underlying interfaces.  AERO implementations
   SHOULD permit network management to dynamically adjust Traffic
   Selector values at runtime.

   If an OAL packet matches the Traffic Selectors of multiple outgoing
   interfaces and/or neighbor interfaces, the OMNI interface replicates
   the packet and sends one copy via each of the (outgoing / neighbor)
   interface pairs; otherwise, it sends a single copy of the OAL packet
   via an interface with the best matching Traffic Selector.  (While not
   strictly required, the likelihood of successful reassembly may
   improve when the OMNI interface sends all fragments of the same
   fragmented OAL packet consecutively over the same underlying
   interface pair to avoid complicating factors such as delay variance
   and reordering.)  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
   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 NCE that matches the
   destination.  If there is a matching NCE, the Client selects one or
   more "reachable" neighbor interfaces in the entry for forwarding
   purposes.  Otherwise, the Client invokes route optimization per
   Section 3.13 and follows the multilink forwarding procedures outlined
   there.

   When a carrier packet enters a Client's OMNI interface from the link-
   layer, if the OAL destination matches one of the Client's ULAs the
   Client (acting as an OAL destination) verifies that the
   Identification is in-window for this OAL source, then reassembles and
   decapsulates as necessary and delivers the original IP packet to the
   network layer.  If the OAL destination does not match, 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).

   Note: Clients and their FHS Proxy/Server (and other Client) peers can
   exchange original IP packets over ANET underlying interfaces without

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   invoking the OAL, since the ANET is secured at the link and physical
   layers.  By forwarding original IP packets without invoking the OAL,
   however, the ANET peers can engage only in classical path MTU
   discovery since the packets are subject to loss and/or corruption due
   to the various per-link MTU limitations that may occur within the
   ANET.  Moreover, the original IP packets do not include either the
   OAL integrity check or per-packet Identification values that can be
   used for data origin authentication and link-layer retransmissions.
   The tradeoff therefore involves an assessment of the per-packet
   encapsulation overhead saved by bypassing the OAL vs. inheritance of
   classical network "brittleness".  (Note however that ANET peers can
   send small original IP packets without invoking the OAL, while
   invoking the OAL for larger packets.  This presents the beneficial
   aspects of both small packet efficiency and large packet robustness,
   with delay variance and reordering as possible side effects.)

3.10.2.  Proxy/Server and Relay Forwarding Algorithm

   When a 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 on underlying
   interfaces with OAL destination set to its own ADM-ULA, it performs
   OAL reassembly if necessary to obtain the original IP packet.  The
   Proxy/Server then supports multilink forwarding procedures as
   specified in Section 3.13.2 and/or acts as an ROS to initiate route
   optimization as specified in Section 3.13.

   When the Proxy/Server receives a carrier packet with OAL destination
   set to an MNP-ULA that does not match the MSP, it accepts the carrier
   packet only if data origin authentication succeeds and if there is a
   network layer routing table entry for a GUA route that matches the
   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 then acts as a Relay to present it to the network
   layer where it will be delivered according to standard IP forwarding.

   When a Proxy/Server receives a carrier packet from one of its Client
   neighbors with OAL destination set to another node, it forwards the
   packets via a matching NCE or via the spanning tree if there is no
   matching entry.  When the Proxy/Server receives a carrier packet with
   OAL destination set to the MNP-ULA of one of its Client neighbors
   established through RS/RA exchanges, it accepts the carrier packet
   only if data origin authentication succeeds.  If the NCE state is
   DEPARTED, the Proxy/Server changes the OAL destination address to the
   ADM-ULA of the new Proxy/Server, then re-encapsulates the carrier

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   packet and forwards it to a Bridge which will eventually deliver it
   to the new Proxy/Server.  If the neighbor cache state for the MNP-ULA
   is REACHABLE, the Proxy/Server forwards the carrier packets to the
   Client which then must reassemble.  (Note that the Proxy/Server does
   not reassemble carrier packets not explicitly addressed to its own
   ADM-ULA, since some of the carrier packets of the same original IP
   packet could be forwarded through a different Proxy/Server.)  In that
   case, the Client may receive fragments that are smaller than its link
   MTU but that can still be reassembled.

   Proxy/Servers process carrier packets with OAL destinations that do
   not match their ADM-ULA in the same manner as for traditional IP
   forwarding within the OAL, i.e., nodes use IP forwarding to forward
   packets not explicitly addressed to themselves.  (Proxy/Servers
   include a special case that accepts and reassembles carrier packets
   destined to the MNP-ULA of one of their Clients received over the
   secured spanning tree.)  Proxy/Servers process carrier packets with
   their ADM-ULA as the destination by first examining the packet for a
   CRH-32 header or an OCH-0/1 header.  In that case, the Proxy/Server
   examines the next MFVI in the carrier packet to locate the MFV entry
   in the MFIB for next hop forwarding (i.e., without examining IP
   addresses).  When the Proxy/Server forwards the carrier packet, it
   changes the destination address according to the MFVI value for the
   next hop found either in the CRH-32 header or in the node's own MFIB.
   Proxy/Servers must verify that the *NET addresses of carrier packets
   not received from the secured spanning tree are "trusted" before
   forwarding according to an MFV (otherwise, the carrier packet must be
   dropped).

   Note: Proxy/Servers may receive carrier packets addressed to their
   own ADM-ULA with CRH-32s that include additional forwarding
   information.  Proxy/Servers use the forwarding information to
   determine the correct NCE and underlying interface for forwarding to
   the target Client, then remove the CRH-32 and forward the carrier
   packet.  If necessary, the Proxy/Server reassembles first before re-
   encapsulating (and possibly also re-fragmenting) then forwards to the
   target Client.

   Note: Clients and their FHS Proxy/Server peers can exchange original
   IP packets over ANET underlying interfaces without invoking the OAL,
   since the ANET is secured at the link and physical layers.  By
   forwarding original IP packets without invoking the OAL, however, the
   Client and Proxy/Server can engage only in classical path MTU
   discovery since the packets are subject to loss and/or corruption due
   to the various per-link MTU limitations that may occur within the
   ANET.  Moreover, the original IP packets do not include either the
   OAL integrity check or per-packet Identification values that can be
   used for data origin authentication and link-layer retransmissions.

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   The tradeoff therefore involves an assessment of the per-packet
   encapsulation overhead saved by bypassing the OAL vs.  inheritance of
   classical network "brittleness".  (Note however that ANET peers can
   send small original IP packets without invoking the OAL, while
   invoking the OAL for larger packets.  This presents the beneficial
   aspects of both small packet efficiency and large packet robustness.)

   Note: When a Proxy/Server receives a (non-OAL) original IP packet
   from an ANET Client, or a carrier packet with OAL destination set to
   its own ADM-ULA from any Client, the Proxy/Server reassembles if
   necessary then performs ROS functions on behalf of the Client.  The
   Client may at some later time begin sending carrier packets to the
   OAL address of the actual target instead of the Proxy/Server, at
   which point it may begin functioning as an ROS on its own behalf and
   thereby "override" the Proxy/Server's ROS role.

   Note; Proxy/Servers drop any original IP packets (received either
   directly from an ANET Client or following reassembly of carrier
   packets received from an ANET/INET Client) with a destination that
   corresponds to the Client's delegated MNP.  Similarly, Proxy/Servers
   drop any carrier packet received with both a source and destination
   that correspond to the Client's delegated MNP regardless of their
   OMNI link point of origin.  These checks are necessary to prevent
   Clients from either accidentally or intentionally establishing
   endless loops that could congest Proxy/Servers and/or ANET/INET
   links.

   Note: Proxy/Servers forward secure control plane carrier packets via
   the SRT secured spanning tree and forward other carrier packets via
   the unsecured spanning tree.  When a Proxy/Server receives a carrier
   packet from the secured spanning tree, it considers the message as
   authentic without having to verify upper layer authentication
   signatures.  When a Proxy/Server receives a carrier packet from the
   unsecured spanning tree, it applies data origin authentication itself
   and/or forwards the unsecured message toward the destination which
   must apply data origin authentication on its own behalf.

   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].

3.10.3.  Bridge Forwarding Algorithm

   Bridges forward spanning tree carrier packets while decrementing the
   OAL header Hop Count but not the original IP header Hop Count/TTL.
   Bridges convey carrier packets that encapsulate critical IPv6 ND
   control messages or routing protocol control messages via the SRT
   secured spanning tree, and may convey other carrier packets via the

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   secured/unsecured spanning tree or via more direct paths according to
   MFIB information.  When the Bridge receives a carrier packet, it
   removes the outer *NET header and searches for an MFIB entry that
   matches an MFVI or an IP forwarding table entry that matches the OAL
   destination address.

   Bridges process carrier packets with OAL destinations that do not
   match their ADM-ULA or the SRT Subnet Router Anycast address in the
   same manner as for traditional IP forwarding within the OAL, i.e.,
   nodes use IP forwarding to forward packets not explicitly addressed
   to themselves.  Bridges process carrier packets with their ADM-ULA or
   the SRT Subnet Router Anycast address as the destination by first
   examining the packet for a full OAL header with a CRH-32 extension or
   an OCH-0/1 header.  In that case, the Bridge examines the next MFVI
   in the carrier packet to locate the MFV entry in the MFIB for next
   hop forwarding (i.e., without examining IP addresses).  When the
   Bridge forwards the carrier packet, it changes the destination
   address according to the MFVI value for the next hop found either in
   the CRH-32 header or in the node's own MFIB.  If the Bridge has a NCE
   for the target Client with an entry for the target underlying
   interface and current *NET addresses, the Bridge instead forwards
   directly to the target Client while using the final hop MFVI instead
   of the next hop (see: Section 3.13.4).

   Bridges forward carrier packets received from a first segment via the
   secured spanning tree to the next segment also via the secured
   spanning tree.  Bridges forward carrier packets received from a first
   segment via the unsecured spanning tree to the next segment also via
   the unsecured spanning tree.  Bridges use a single IPv6 routing table
   that always determines the same next hop for a given OAL destination,
   where the secured/unsecured spanning tree is determined through the
   selection of the underlying interface to be used for transmission
   (i.e., a secured tunnel or an open INET interface).

   As for Proxy/Servers, Bridges must verify that the *NET addresses of
   carrier packets not received from the secured spanning tree are
   "trusted" before forwarding according to an MFV (otherwise, the
   carrier packet must be dropped).

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.  The AERO node may also receive OMNI link error
   indications in OAL-encapsulated uNA messages that include
   authentication signatures.

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   A link-layer error indication is an ICMP error message generated by a
   router in the *NET 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 link-layer IPv4
   "Fragmentation Needed" and IPv6 "Packet Too Big" messages for carrier
   packets that are no larger than the minimum/path MPS as discussed in
   Section 3.9, however these messages may provide useful hints of probe
   failures during path MPS probing.)

   The ICMP header is followed by the leading portion of the carrier
   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 4:

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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~
        |    IP Header of link layer    |
        |         error message         |
        ~                               ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |          ICMP Header          |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
        ~                               ~   P
        |  carrier packet *NET and OAL  |   a
        |     encapsulation headers     |   c
        ~                               ~   k
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   e
        ~                               ~   t
        |  original IP packet headers   |
        |    (first-fragment only)      |   i
        ~                               ~   n
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~   e
        |    Portion of the body of     |   r
        |    the original IP packet     |   r
        |       (all fragments)         |   o
        ~                               ~   r
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

         Figure 4: 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 3.15.5.

   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 assigned to a black-
   hole route, the Bridge drops the packet if there is no more-specific
   routing information for the destination and returns an OMNI interface
   Destination Unreachable message subject to rate limiting.

   When an AERO node receives a carrier packet for which reassembly is
   currently congested, it returns an OMNI interface Packet Too Big
   (PTB) message as discussed in [I-D.templin-6man-omni] (note that the
   PTB messages could indicate either "hard" or "soft" errors).

   AERO nodes include ICMPv6 error messages intended for the OAL source
   as sub-options in the OMNI option of secured uNA messages.  When the
   OAL source receives the uNA message, it can extract the ICMPv6 error
   message enclosed in the OMNI option and either process it locally or
   translate it into a network-layer error to return to the original
   source.

3.12.  AERO Mobility Service Coordination

   AERO nodes observes the Router Discovery and Prefix Registration
   specifications found in Section 15 of [I-D.templin-6man-omni].  AERO
   nodes further coordinate their autoconfiguration actions with the
   mobility service 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.

   Clients associate each of their underlying interfaces with a FHS
   Proxy/Server.  Each FHS Proxy/Server locally services one or more of
   the Client's underlying interfaces, and the Client typically selects
   one among them to serve as the Hub Proxy/Server (the Client may
   instead select a "third-party" Hub Proxy/Server that does not
   directly service any of its underlying interfaces).  All of the
   Client's other FHS Proxy/Servers forward proxyed copies of RS/RA
   messages between the Hub Proxy/Server and Client without assuming the
   Hub role functions themselves.

   Each Client associates with a single Hub Proxy/Server at a time,
   while all other Proxy/Servers are candidates for providing the Hub
   role for other Clients.  An FHS Proxy/Server assumes the Hub role
   when it receives an RS message with its own ADM-LLA or link-scoped
   All-Routers multicast as the destination.  An FHS Proxy/Server
   assumes the proxy role when it receives an RS message with the ADM-
   LLA of another Proxy/Server as the destination.  (An FHS Proxy/Server
   can also assume the proxy role when it receives an RS message
   addressed to link-scoped All-Routers multicast if it can determine
   the ADM-LLA of another Proxy/Server to serve as a Hub.)

   AERO Clients and Proxy/Servers use IPv6 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 Hub Proxy/Servers include prefix delegation and/or
   registration parameters in RS/RA messages.  The IPv6 ND messages are
   exchanged between the Client and Hub Proxy/Server (via any FHS Proxy/
   Servers acting as proxys) 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

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   registration information for the MNP.  If the Hub 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 outlines Client and Proxy/Server behaviors
   based on the Router Discovery and Prefix Registration specifications
   found in Section 15 of [I-D.templin-6man-omni].  These sections
   observe all of the OMNI specifications, and include additional
   specifications of the interactions of Client-Proxy/Server RS/RA
   exchanges with the AERO mobility service.

3.12.2.  AERO Client Behavior

   AERO Clients discover the addresses of candidate Proxy/Servers by
   resolving the Potential Router List (PRL) in a similar manner as
   described in [RFC5214].  Discovery methods include static
   configuration (e.g., 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 an RA response to a multicast/anycast RS as
   described below.  In the absence of other information, the Client can
   resolve the DNS Fully-Qualified Domain Name (FQDN)
   "linkupnetworks.[domainname]" where "linkupnetworks" is a constant
   text string and "[domainname]" is a DNS suffix for the OMNI link
   (e.g., "example.com").

   The Client then performs RS/RA exchanges over each of its underlying
   interfaces to associate with (possibly multiple) FHS Proxy/Serves and
   a single Hub Proxy/Server as specified in Section 15 of
   [I-D.templin-6man-omni].  The Client then sends each RS (either
   directly via Direct interfaces, via a VPN for VPNed interfaces, via
   an access router for ANET interfaces or via INET encapsulation for
   INET interfaces) and waits up to RetransTimer milliseconds for an RA
   message reply (see Section 3.12.3) while retrying up to
   MAX_RTR_SOLICITATIONS if necessary.  If the Client receives no RAs,
   or if it receives an RA with Router Lifetime set to 0, the Client
   SHOULD abandon attempts through the first candidate Proxy/Server and
   try another Proxy/Server.

   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 traffic selectors change, etc.  To
   do so, the Client prepares an RS message to send over any available

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   underlying interface as above.  The RS includes an OMNI option with
   prefix registration/delegation information and with an Interface
   Attributes sub-option specific to the selected underlying interface.
   When the Client receives the Hub Proxy/Server's RA response, it has
   assurance that both the Hub and FHS Proxy/Servers have been updated
   with the new information.

   If the Client wishes to discontinue use of a Hub Proxy/Server it
   issues an RS message over any underlying interface with an OMNI
   option with a prefix release indication (i.e., by setting Preflen to
   0).  When the Hub Proxy/Server processes the message, it releases the
   MNP, sets the NCE 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 Hub Proxy/Server withdraws the MNP from the routing
   system.  (Alternatively, when the Client associates with a new FHS/
   Hub Proxy/Server it can include an OMNI "Proxy/Server Departure" sub-
   option in RS messages with the MSIDs of the Old FHS/Hub Proxy/
   Server.)

3.12.3.  AERO Proxy/Server Behavior

   AERO Proxy/Servers act as both IP routers and IPv6 ND proxys, and
   support a prefix delegation/registration service for Clients.  Proxy/
   Servers arrange to add their ADM-LLAs to the PRL maintained in a
   static map of Proxy/Server addresses for the link, the DNS resource
   records for the FQDN "linkupnetworks.[domainname]", etc.  before
   entering service.  The PRL should be arranged such that Clients can
   discover the addresses of Proxy/Servers that are geographically and/
   or topologically "close" to their underlying network connections.

   When a FHS/Hub Proxy/Server receives a prospective Client's RS
   message, 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, it processes the RS as specified in Section 15
   of [I-D.templin-6man-omni].  When the Hub Proxy/Server receives the
   RS, it 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 Hub Proxy/Server returns the MNPs, it also creates
   a forwarding table entry for the MNP-ULA corresponding to each MNP
   resulting in a BGP update (see: Section 3.2.3).  For IPv6, the Hub
   Proxy/Server creates an IPv6 forwarding table entry for each MNP-ULA.
   For IPv4, the Hub Proxy/Server creates an IPv6 forwarding table entry
   with the IPv4-compatibility MNP-ULA prefix corresponding to the IPv4
   address.  The Hub Proxy/Server then returns an RA to the Client via
   an FHS Proxy/Server if necessary.

   After the initial RS/RA exchange, the Hub Proxy/Server maintains a
   ReachableTime timer for each of the Client's underlying interfaces

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   individually (and for the Client's NCE collectively) set to expire
   after ReachableTime seconds.  If the Client (or an FHS Proxy/Server)
   issues additional RS messages, the Hub Proxy/Server sends an RA
   response and resets ReachableTime.  If the Hub Proxy/Server receives
   an IPv6 ND message with a prefix release indication it sets the
   Client's NCE to the DEPARTED state and withdraws the MNP-ULA route
   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 Hub Proxy/Server marks the interface as
   DOWN.  If ReachableTime expires before any new RS is received on any
   individual underlying interface, the Hub Proxy/Server sets the NCE
   state to STALE and sets a 10 second timer.  If the Hub Proxy/Server
   has not received a new RS or uNA message with a prefix release
   indication before the 10 second timer expires, it deletes the NCE and
   withdraws the MNP from the routing system.

   The Hub Proxy/Server processes any IPv6 ND messages pertaining to the
   Client while forwarding to the Client or responding on the Client's
   behalf as necessary.  The Hub 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.  The Hub
   Proxy/Server may also receive carrier packets via the secured
   spanning tree that contain initial data packets sent while route
   optimization is in progress.  The Hub Proxy/Server reassembles, then
   re-encapsulates/re-fragments and forwards the packets to the target
   Client via an FHS Proxy/Server if necessary.  Finally, If the NCE is
   in the DEPARTED state, the old Hub Proxy/Server forwards any carrier
   packets it receives from the secure spanning tree and destined to the
   Client to the new Hub Proxy/Server, then deletes the entry after
   DepartTime expires.

   Note: Clients SHOULD arrange to notify former Hub Proxy/Servers of
   their departures, but Hub 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).  Hub 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 FHS 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.

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3.12.3.1.  Additional Proxy/Server Considerations

   AERO Clients register with FHS Proxy/Servers for each underlying
   interface.  Each of the Client's FHS Proxy/Servers must inform a
   single Hub Proxy/Server of the Client's underlying interface(s) that
   it services.  For Clients on Direct and VPNed underlying interfaces,
   the FHS Proxy/Server for each interface is directly connected, for
   Clients on ANET underlying interfaces the FHS Proxy/Server is located
   on the ANET/INET boundary, and for Clients on INET underlying
   interfaces the FHS Proxy/Server is located somewhere in the connected
   Internetwork.  When FHS Proxy/Server "B" processes a Client
   registration, it must either assume the Hub role or forward a proxyed
   registration to another Proxy/Server "A" acting as the Hub. Proxy/
   Servers satisfy these requirements as follows:

   o  when FHS Proxy/Server "B" receives a Client RS message, it first
      verifies that the OAL Identification is within the window for the
      NCE that matches the MNP-ULA for this Client neighbor and
      authenticates the message.  If no NCE was found, Proxy/Server "B"
      instead creates one in the STALE state and caches the Client-
      supplied Interface Attributes, Origin Indication and Window
      Synchronization parameters as well as the Client's observed *NET
      addresses (noting that they may differ from the Origin addresses
      if there were NATs on the path).  Proxy/Server "B" then examines
      the network-layer destination address.  If the destination address
      is the ADM-LLA of a different Proxy/Server "A", Proxy/Server "B"
      prepares a separate proxyed version of the RS message with an OAL
      header with source set to its own ADM-ULA and destination set to
      Proxy/Server B's ADM-ULA.  Proxy/Server "B" also writes its own
      information over the Interface Attributes sub-option supplied by
      the Client, omits or zeros the Origin Indication sub-option then
      forwards the message into the OMNI link secured spanning tree.

   o  when Hub Proxy/Server "A" receives the RS, it assume the Hub role
      and creates or updates a NCE for the Client with FHS Proxy/Server
      "B"'s Interface Attributes as the link-layer address information
      for this FHS omIndex.  Hub Proxy/Server "A" then prepares an RA
      message with source set to its own LLA and destination set to the
      Client's MNP-LLA, then encapsulates the RA in an OAL header with
      source set to its own ADM-ULA and destination set to the ADM-ULA
      of FHS Proxy/Server "B".  Hub Proxy/Server "A" then performs
      fragmentation if necessary and sends the resulting carrier packets
      into the secured spanning tree.

   o  when FHS Proxy/Server "B" reassembles the RA, it locates the
      Client NCE based on the RA destination LLA.  If the RA message
      includes an OMNI "Proxy/Server Departure" sub-option, Proxy/Server
      "B" first sends a uNA to the old FHS/Hub Proxy/Servers named in

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      the sub-option.  Proxy/Server "B" then re-encapsulates the RA
      message with OAL source set to its own ADM-ULA and OAL destination
      set to the MNP-ULA of the Client, with an appropriate
      Identification value, with an authentication signature if
      necessary, with the Client's Interface Attributes sub-option
      echoed and with the cached observed *NET addresses written into an
      Origin Indication sub-option.  Proxy/Server "B" sets the P flag in
      the RA flags field to indicate that the message has passed through
      a proxy [RFC4389], includes responsive Window Synchronization
      parameters, then fragments the RA if necessary and returns the
      fragments to the Client.

   o  The Client repeats this process over each of its additional
      underlying interfaces while treating each additional FHS Proxy/
      Server "C", "D", "E", etc. as a proxy to facilitate RS/RA
      exchanges between the Hub and the Client.

   After the initial RS/RA exchanges each FHS Proxy/Server forwards any
   of the Client's carrier packets with OAL destinations for which there
   is no matching NCE to a Bridge using OAL encapsulation with its own
   ADM-ULA as the source and with destination determined by the Client.
   The Proxy/Server 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 3.13.

   While the Client is still associated with FHS Proxy/Servers "B", "C",
   "D", etc., each FHS Proxy/Server can send NS, RS and/or unsolicited
   NA messages to update the neighbor cache entries of other AERO nodes
   on behalf of the Client based on changes in Interface Attributes,
   Traffic Selectors, etc.  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 Hub Proxy/Server "A" ceases to send solicited RAs, FHS Proxy/
   Servers "B", "C", "D" can send unsolicited RAs over the Client's
   underlying interface with destination set to (link-local) All-Nodes
   multicast and with Router Lifetime set to zero to inform Clients that
   the Hub Proxy/Server has failed.  Although FHS Proxy/Servers "B", "C"
   and "D" can engage in IPv6 ND exchanges on behalf of the Client, the
   Client can also send IPv6 ND messages on its own behalf, e.g., if it
   is in a better position to convey state changes.  The IPv6 ND
   messages sent by the Client include the Client's MNP-LLA as the
   source in order to differentiate them from the IPv6 ND messages sent
   by a FHS Proxy/Server.

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   If the Client becomes unreachable over all underlying interface it
   serves, the Hub Proxy/Server sets the NCE state to DEPARTED and
   retains the entry for DepartTime seconds.  While the state is
   DEPARTED, the Hub Proxy/Server forwards any carrier packets destined
   to the Client to a Bridge via OAL encapsulation.  When DepartTime
   expires, the Hub Proxy/Server deletes the NCE and discards any
   further carrier packets destined to the former Client.

   In some ANETs that employ a Proxy/Server, the Client's MNP can be
   injected into the ANET routing system.  In that case, the Client can
   send original IP packets without invoking the OAL so that the ANET
   routing system transports the original IP packets to the Proxy/
   Server.  This can be beneficial, e.g., if the Client connects to the
   ANET via low-end data links such as some aviation wireless links.

   If the ANET first-hop access router is on the same underlying link as
   the Client 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-scoped All-Routers multicast.  The
   Client includes an OMNI option formatted as specified in
   [I-D.templin-6man-omni].  The Client then sends the unencapsulated RS
   message, which will be intercepted by the AERO-aware ANET access
   router.

   The ANET access router then performs OAL encapsulation on the RS
   message and forwards it to a Proxy/Server at the ANET/INET boundary.
   When the access router and Proxy/Server are one and the same node,
   the Proxy/Server would share and underlying link with the Client but
   its message exchanges with outside correspondents 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: When a Proxy/Server alters the IPv6 ND message contents before
   forwarding (e.g., such as altering the OMNI option contents), the
   original IPv6 ND message checksum or authentication signature is
   invalidated, and a new checksum or authentication signature must be
   calculated and included.

   Note: When a Proxy/Server receives a secured Client NS message, it
   performs the same proxying procedures as for described for RS
   messages above.  The proxying procedures for NS/NA message exchanges
   is specified in Section 3.13.

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3.12.3.2.  Detecting and Responding to Proxy/Server Failures

   In environments where fast recovery from Proxy/Server failure is
   required, FHS Proxy/Servers SHOULD use proactive Neighbor
   Unreachability Detection (NUD) to track Hub Proxy/Server reachability
   in a similar fashion as for Bidirectional Forwarding Detection (BFD)
   [RFC5880].  Each FHS Proxy/Server can then quickly detect and react
   to failures so that cached information is re-established through
   alternate paths.  The NS/NA(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.

   FHS Proxy/Servers perform continuous NS/NA(NUD) exchanges with the
   Hub Proxy/Server, e.g., one exchange per second.  The FHS Proxy/
   Server sends the NS(NUD) message via the spanning tree with its own
   ADM-LLA as the source and the ADM-LLA of the Hub Proxy/Server as the
   destination, and the Hub Proxy/Server responds with an NA(NUD).  When
   the FHS Proxy/Server is also sending RS messages to a Hub Proxy/
   Server on behalf of Clients, the resulting RA responses can be
   considered as equivalent hints of forward progress.  This means that
   the FHS Proxy/Server need not also send a periodic NS(NUD) if it has
   already sent an RS within the same period.  If the Hub Proxy/Server
   fails (i.e., if the FHS Proxy/Server ceases to receive
   advertisements), the FHS Proxy/Server can quickly inform Clients by
   sending unsolicited RA messages

   The FHS Proxy/Server sends unsolicited RA messages with source
   address set to the Hub Proxy/Server's address, destination address
   set to (link-local) All-Nodes multicast, and Router Lifetime set to
   0.  The FHS Proxy/Server SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA
   messages separated by small delays [RFC4861].  Any Clients that had
   been using the failed Hub Proxy/Server will receive the RA messages
   and select one of its other FHS Proxy/Servers to assume the Hub role
   (i.e., by sending an RS with destination set to the ADM-LLA of the
   new Hub).

3.12.3.3.  DHCPv6-Based Prefix Registration

   When a Client is not pre-provisioned with an MNP-LLA, it will need
   for the Hub 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 Hub Proxy/Server to select
   additional MNPs.)  The DHCPv6 service [RFC8415] is used to support
   this requirement.

   When a Client needs to have the Hub Proxy/Server select MNPs, it
   sends an RS message with source address set to the unspecified

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   address (::) and with an OMNI option that includes a DHCPv6 message
   sub-option with DHCPv6 Prefix Delegation (DHCPv6-PD) parameters.
   When the Hub Proxy/Server receives the RS message, it extracts the
   DHCPv6-PD message from the OMNI option.

   The Hub 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 Hub
   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.)

   When the Hub 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 Hub 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 and Preflen 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: Further details of the DHCPv6-PD based MNP registration (as
   well as a minimal MNP delegation alternative that avoids including a
   DHCPv6 message sub-option in the RS) are found in
   [I-D.templin-6man-omni].

3.13.  AERO Route Optimization

   AERO nodes invoke route optimization when they need to forward
   initial packets to new target destinations and for ongoing multilink
   forwarding for current destinations.  Route optimization is based on
   IPv6 ND Address Resolution messaging between a Route Optimization
   Source (ROS) and a Relay or the target Client itself (reached via the
   current Hub Proxy/Server) acting as a Route Optimization Responder
   (ROR).  Route optimization is initiated by the first eligible ROS
   closest to the source as follows:

   o  For Clients on VPNed and Direct interfaces, the Client's FHS
      Proxy/Server is the ROS.

   o  For Clients on ANET interfaces, either the Client or the FHS
      Proxy/Server may be the ROS.

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

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

   o  For Clients that engage the Hub Proxy/Server in "mobility anchor"
      mode, the Hub Proxy/Server is the ROS.

   The AERO routing system directs a route optimization request sent by
   the ROS to the ROR, which returns a route optimization reply which
   must include information that is current, consistent and authentic.
   The ROS is responsible for periodically refreshing the route
   optimization, and the ROR is responsible for quickly informing the
   ROS of any changes.  Following address resolution, the ROS and ROR
   perform ongoing multilink route optimizations to maintain optimal
   forwarding profiles.

   The route optimization procedures are specified in the following
   sections.

3.13.1.  Multilink Address Resolution

   When one or more original IP packets from a source node destined to a
   target node arrives, the ROS checks for a NCE with an MNP-LLA that
   matches the target destination.  If there is a NCE in the REACHABLE
   state, the ROS invokes the OAL and forwards the resulting carrier
   packets according to the cached state then returns from processing.
   Otherwise, if there is no NCE the ROS creates one in the INCOMPLETE
   state.

   The ROS next prepares an NS message for Address Resolution (NS(AR))
   to send toward an ROR while including the original IP packet(s) as
   trailing data following the NS(AR) in an OAL super-packet
   [I-D.templin-6man-omni].  The resulting NS(AR) message must be sent
   securely, and includes:

   o  the LLA of the ROS as the source address.

   o  the MNP-LLA corresponding to the original IP packet's destination
      as the Target Address, e.g., for 2001:db8:1:2::10:2000 the Target
      Address 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(AR)
      destination address is ff02:0:0:0:0:1:ff10:2000.

   The NS(AR) message also includes an OMNI option with an
   authentication sub-option if necessary and with Preflen set to the
   prefix length associated with the NS(AR) source.  The ROS also

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   includes Interface Attributes and Traffic Selectors for all of the
   source Client's underlying interfaces, calculates the authentication
   signature or checksum, then selects an Identification value and
   submits the NS(AR) message for OAL encapsulation with OAL source set
   to its own {ADM,MNP}-ULA and OAL destination set to the MNP-ULA
   corresponding to the target and with window synchronization
   parameters.  The ROS then inserts a fragment header, performs
   fragmentation and *NET encapsulation, then sends the resulting
   carrier packets into the SRT secured spanning tree without
   decrementing the network-layer TTL/Hop Limit field.

   When the ROS is a Client, it must instead use the ADM-ULA of one of
   its FHS Proxy/Servers as the destination.  The ROS Client then
   fragments, performs *NET encapsulation and forwards the carrier
   packets to the FHS Proxy/Server.  The FHS Proxy/Server then
   reassembles, verifies the NS(AR) authentication signature or
   checksum, changes the OAL source to its own ADM-ULA, changes the OAL
   destination to the MNP-ULA corresponding to the target, selects an
   appropriate Identification, then re-fragments and forwards the
   resulting carrier packets into the secured spanning tree on behalf of
   the Client.

   Note: both the target Client and its Hub Proxy/Server include current
   and accurate information for the Client's multilink Interface
   Attributes profile.  The Hub Proxy/Server can be trusted to provide
   an authoritative response on behalf of the Client should the need
   arise.  While the Client has no such trust basis, any attempt by the
   Client to mount an attack by providing false Interface Attributes
   information would only result in black-holing of return traffic,
   i.e., the "attack" could only result in denial of service to the
   Client itself.  Therefore, the Client's asserted Interface Attributes
   need not be validated by the Hub Proxy/Server.

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

   When the Bridge receives carrier packets containing the NS(AR), 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 decrements the OAL header Hop-Limit, then
   re-encapsulates and forwards the carrier packet(s) via the secured
   spanning tree the same as for any IPv6 router, where they may
   traverse multiple OMNI link segments.  The final-hop Bridge will
   deliver the carrier packet via the secured spanning tree to the Hub
   Proxy/Server (or Relay) that services the target.

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3.13.1.2.  Processing and Responding to the NS(AR)

   When the Hub Proxy/Server for the target receives the NS(AR) secured
   carrier packets with the MNP-ULA of the target as the OAL
   destination, it reassembles then forwards the message to the target
   Client (while including an authentication signature and encapsulation
   if necessary) or processes the NS(AR) locally if it is acting as a
   Relay/IP router or the Client's designated ROR.  The Hub Proxy/Server
   processes the message as follows:

   o  if the NS(AR) target matches a Client NCE in the DEPARTED state,
      the (old) Hub Proxy/Server re-encapsulates by setting the OAL
      destination address to the ADM-ULA of the Client's new Hub Proxy/
      Server.  The old Hub Proxy/Server then re-fragments and re-
      encapsulates, then forwards the resulting carrier packets over the
      secured spanning tree.

   o  If the NS(AR) target matches the MNP-LLA of a Client NCE in the
      REACHABLE state, the Hub Proxy/Server notes whether the NS(AR)
      arrived from the secured spanning tree then sets the OAL
      destination address to the MNP-ULA of the Client or the ADM-ULA of
      the selected FHS Proxy/Server for the Client.  If the message
      arrived via the secured spanning tree the Hub Proxy/Server
      verifies the checksum; otherwise, it must verify the message
      authentication signature before forwarding.  When the Hub Proxy/
      Server determines the underlying interface for the target Client,
      it then changes the OAL destination to the ADM-ULA of the target
      Client's FHS Proxy/Server, re-fragments and forwards the resulting
      carrier packets into the secured spanning tree.  When the FHS
      Proxy/Server receives the carrier packets, it reassembles and
      verifies the checksum, then includes an authentication signature
      if necessary, changes the OAL source to its own ADM-ULA and
      destination to the MNP-ULA of the target Client, includes an
      Identification value within the current window, then re-fragments
      and forwards the resulting carrier packets to the target Client
      ROR.  (Note that if the Hub and FHS Proxy/Server are one and the
      same the Hub itself will perform the FHS procedures.)

   o  If the NS(AR) target matches one of its non-MNP routes, the Hub
      Proxy/Server serves as both a Relay and a ROR, since the Relay
      forwards IP packets toward the (fixed network) target at the
      network layer.

   The ROR then creates a NCE for the NS(AR) LLA source address if
   necessary, processes the window synchronization parameters, caches
   all Interface Attributes and Traffic Selector information, and
   prepares a (solicited) NA(AR) message to return to the ROS with the
   source address set to its own MNP-LLA, the destination address set to

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   the NS(AR) LLA source address and the Target Address set to the same
   value that appeared in the NS(AR) Target Address.  The ROR includes
   an OMNI option with Preflen set to the prefix length associated with
   the NA(AR) source address.

   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) and sets the O flag to 1
   (as an authoritative responder).  The ROR finally submits the NA(AR)
   for OAL encapsulation with source set to its own ULA and destination
   set to either the ULA corresponding to the NS(AR) source or the ADM-
   ULA of its FHS Proxy/Server, selects an appropriate Identification,
   and includes window synchronization parameters and authentication
   signature or checksum.  The ROR then includes Interface Attributes
   and Traffic Selector sub-options for all of the target's underlying
   interfaces with current information for each interface, fragments and
   encapsulates each fragment in appropriate *NET headers, then forwards
   the resulting (*NET-encapsulated) carrier packets to the FHS Proxy/
   Server.

   When the FHS Proxy/Server receives the carrier packets, it
   reassembles if necessary and verifies the authentication signature or
   checksum.  The FHS Proxy/Server then changes the OAL source address
   to its own ADM-ULA, changes the destination to the {ADM,MNP}-ULA
   corresponding to the NA(AR) LLA destination, includes an appropriate
   Identification, then fragments and forwards the carrier packets into
   the secured spanning tree.

   Note: If the Hub Proxy/Server is acting as the Client's ROR but not
   as a Relay/IP router (i.e., by virtue of receipt of an RS message
   with the A flag set), it prepares the NS(AR) with the R flag set to 0
   but without setting the SYN flag in the window synchronization
   parameters.  This informs the ROS that it must initiate multilink
   route optimization to synchronize with the Client either directly or
   via a FHS Proxy/Server (see: Section 3.13.2).

3.13.1.3.  Relaying the NA(AR)

   When the Bridge receives NA(AR) carrier packets, 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 decrements the OAL header Hop-Limit, re-encapsulates the carrier
   packet and forwards it via the SRT secured spanning tree, where it
   may traverse multiple OMNI link segments.  The final-hop Bridge will
   deliver the carrier packet via the secured spanning tree to a Proxy/
   Server for the ROS.

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3.13.1.4.  Processing the NA(AR)

   When the ROS receives the NA(AR) message, it first searches for a NCE
   that matches the NA(AR) target address.  The ROS then processes the
   message the same as for standard IPv6 Address Resolution [RFC4861].
   In the process, it caches all OMNI option information in the target
   NCE (including all Interface Attributes), and caches the NA(AR) MNP-
   LLA source address as the address of the target Client.

   When the ROS is a Client, the SRT secured spanning tree will first
   deliver the solicited NA(AR) message to the FHS Proxy/Server, which
   re-encapsulates and forwards the message to the Client.  If the
   Client is on a well-managed ANET, physical security and protected
   spectrum ensures security for the NA(AR) without needing an
   additional authentication signature; if the Client is on the open
   INET the Proxy/Server must instead include an authentication
   signature (while adjusting the OMNI option size, if necessary).  The
   Proxy/Server uses its own ADM-ULA as the OAL source and the MNP-ULA
   of the Client as the OAL destination.

3.13.2.  Multilink Route Optimization

   Following address resolution, the ROS and ROR can assert multilink
   paths through underlying interface pairs serviced by the same source/
   destination LLAs by sending unicast NS/NA messages with Multilink
   Forwarding Parameters and window synchronization parameters when
   necessary.  The unicast NS/NA messages establish multilink forwarding
   state in intermediate nodes in the path between the ROS and ROR.

   To support multilink route optimization, OMNI interfaces include an
   additional forwarding table termed the Multilink Forwarding
   Information Base (MFIB) that supports carrier packet forwarding based
   on OMNI neighbor underlying interface pairs.  The MFIB contains
   Multilink Forwarding Vectors (MFVs) indexed by 4-octet values known
   as MFV Indexes (MFVIs).

   OAL source, intermediate and destination nodes create MFVs/MFVIs when
   they process an NS message with a Multilink Forwarding Parameters
   sub-option with Job code "00" (Initialize; Build B) or a solicited NA
   with Job code "01" (Follow B; Build A) (see:
   [I-D.templin-6man-omni]).  The OAL source of the NS (and OAL
   destination of the solicited NA) are considered to reside in the
   "First Hop Segment (FHS)", while the OAL destination of the NS (and
   OAL source of the solicited NA) are considered to reside in the "Last
   Hop Segment (LHS)".

   When an OAL node processes an NS with Job code "00", it creates an
   MFV, records the NS source and destination ULAs and assigns a "B"

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   MFVI.  When the "B" MVFI is referenced, the MVF retains the ULAs in
   (dst,src) order the opposite of how they appeared in the original NS
   to support full header reconstruction.  (If the NS message included a
   nested OAL encapsulation, the ULAs of both OAL headers are retained.)

   When an OAL node processes a solicited NA with Job code "01", it
   locates the MFV created by the NS and assigns an "A" MFVI.  When the
   "A" MFVI is referenced, the MFV retains the ULAs in (src,dst) order
   the same as they appeared in the original NS to support full header
   reconstruction.  (If the NS message included a nested OAL
   encapsulation, the ULAs of both OAL headers are retained.)

   OAL nodes generate random 32-bit values as candidate A/B MFVIs which
   must first be tested for local uniqueness.  If a candidate MFVI s
   already in use (or if the value is 0), the OAL node repeats the
   random generation process until it obtains a unique non-zero value.
   (Since the number of MFVs in service at each OAL node is likely to be
   much smaller than 2**32, the process will generate a unique value
   after a small number of tries; also, an MFVI generated by a first OAL
   node is never tested for uniqueness on other OAL nodes, since the
   uniqueness property is node-local only.)

   OAL nodes maintain A/B MFVIs as follows:

   o  "B1" - a locally-unique MFVI maintained independently by each OAL
      node on the path from the FHS OAL source to the last OAL
      intermediate node before the LHS OAL destination.  The OAL node
      generates and assigns a "B1" MFVI to a newly-created MFV when it
      processes an NS message with Job code "00".  When the OAL node
      receives future carrier packets that include this value, it can
      unambiguously locate the correct MFV and determine directionality
      without examining addresses.

   o  "A1" - a locally unique MFVI maintained independently by each OAL
      node on the path from the LHS OAL source to the last OAL
      intermediate node before the FHS OAL destination.  The OAL node
      generates and assigns an "A1" MFVI to the MVF that configures the
      corresponding "B1" MFVI when it processes a solicited NA message
      with Job code "01".  When the OAL node receives future carrier
      packets that include this value, it can unambiguously locate the
      correct MFV and determine directionality without examining
      addresses.

   o  "A2" - the A1 MFVI of a remote OAL node discovered by an FHS OAL
      source or OAL intermediate node when it processes an NA message
      with Job code "01" that originated from an LHS OAL source.  A2
      values MUST NOT be tested for uniqueness within the OAL node's
      local context.

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   o  "B2" - the B1 MFVI of a remote OAL node discovered by an LHS OAL
      source or OAL intermediate node when it processes an NS message
      with Job code "00" that originated from an FHS OAL source.  B2
      values MUST NOT be tested for uniqueness within the OAL node's
      local context.

   When an FHS OAL source has an original IP packet to send to an LHS
   OAL destination discovered via multilink address resolution, it first
   selects a source and target underlying interface pair.  The OAL
   source uses its cached information for the target underlying
   interface as LHS information then prepares an NS message with an OMNI
   Multilink Forwarding Parameters sub-option with Job code "00" and
   with source set to its own {ADM,MNP}-LLA.  If the LHS FMT-Forward and
   FMT-Mode bits are both clear, the OAL source sets the destination to
   the ADM-LLA of the LHS Proxy/Server; otherwise, it sets the
   destination to the MNP-LLA of the target Client.  The OAL source then
   sets window synchronization information in the OMNI header and
   updates/creates a NCE for the selected destination {ADM,MNP}-LLA in
   the INCOMPLETE state.  The OAL source next creates an MFV based on
   the NS source and destination LLAs, then generates a "B1" MFVI and
   assigns it to the MFV while also including it as the first B entry in
   the MFVI List.  The OAL source then populates the NS Multilink
   Forwarding Parameters based on any FHS/LHS information it knows
   locally.  OAL intermediate nodes on the path to the OAL destination
   may populate additional FHS/LHS information on a hop-by-hop basis.

   If the OAL source is the FHS Proxy/Server, it then performs OAL
   encapsulation/fragmentation while setting the source to its own ADM-
   ULA and setting the destination to the FHS Subnet Router Anycast ULA
   determined by applying the FHS SRT prefix length to its ADM-ULA.  The
   FHS Proxy/Server next examines the LHS FMT code.  If FMT-Forward is
   clear and FMT-Mode is set, the FHS Proxy/Server checks for a NCE for
   the ADM-LLA of the LHS Proxy/Server.  If there is no NCE, the FHS
   Proxy/Server creates one in the INCOMPLETE state.  If a new NCE was
   created (or if the existing NCE requires fresh window
   synchronization), the FHS Proxy/Server then writes window
   synchronization parameters into the OMNI Multilink Forwarding
   Parameters Tunnel Window Synchronization fields.  The FHS Proxy/
   Server then selects an appropriate Identification value and *NET
   headers and forwards the resulting carrier packets into the secured
   spanning tree which will deliver them to a Bridge interface that
   assigns the FHS Subnet Router Anycast ULA.

   If the OAL source is the FHS Client, it instead includes an
   authentication signature if necessary, performs OAL encapsulation,
   sets the source to its own MNP-ULA, sets the destination to
   {ADM,MNP}-ULA of the FHS Proxy/Server and selects an appropriate
   Identification value for the FHS Proxy/Server.  If FHS FMT-Forward is

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   set and LHS FMT-Forward is clear, the FHS Client creates/updates a
   NCE for the ADM-LLA of the LHS Proxy/Server as above and includes
   Tunnel Window Synchronization parameters.  The FHS Client then
   fragments and encapsulates in appropriate *NET headers then forwards
   the carrier packets to the FHS Proxy/Server.  When the FHS Proxy/
   Server receives the carrier packets, it verifies the Identification,
   reassembles/decapsulates to obtain the NS then verifies the
   authentication signature or checksum.  The FHS Proxy/Server then
   creates an MFV (i.e., the same as the FHS Client had done) while
   assigning the current B entry in the MFVI List (i.e., the one
   included by the FHS Client) as the "B2" MFVI for this MVF.  The FHS
   Proxy/Server next generates a new unique "B1" MFVI, then both assigns
   it to the MFV and writes it as the next B entry in the OMNI Multilink
   Forwarding Parameters MFVI List (while also writing any FHS Client
   and Proxy/Server addressing information).  The FHS Proxy/Server then
   checks FHS/LHS FMT-Forward/Mode to determine whether to create a NCE
   for the LHS Proxy/Server ADM-LLA and include Tunnel Window
   Synchronization parameters the same as above.  The FHS Proxy/Server
   then calculates the checksum, re-fragments while setting the OAL
   source address to its own ADM-ULA and destination address to the FHS
   Subnet Router Anycast ULA, and includes an Identification appropriate
   for the secured spanning tree.  The FHS Proxy/Server finally includes
   appropriate *NET headers and forwards the carrier packets into the
   secured spanning tree the same as above.

   Bridges in the spanning tree forward carrier packets not explicitly
   addressed to themselves, while forwarding those that arrived via the
   secured spanning tree to the next hop also via the secured spanning
   tree and forwarding all others via the unsecured spanning tree.  When
   an FHS Bridge receives a carrier packet over the secured spanning
   tree addressed to its ADM-ULA or the FHS Subnet Router Anycast ULA,
   it instead reassembles/decapsulates to obtain the NS then verifies
   the checksum.  The FHS Bridge next creates an MFV (i.e., the same as
   the FHS Proxy/Server had done) while assigning the current B entry in
   the MFVI List as the MFV "B2" index.  The FHS Bridge also caches the
   NS Multilink Forwarding Parameters FHS information in the MFV, and
   also caches the first B entry in the MFVI List as "FHS-Client" when
   FHS FMT-Forward/Mode are both set to enable future direct forwarding
   to this FHS Client.  The FHS Bridge then generates a "B1" MFVI for
   the MFV and also writes it as the next B entry in the NS's MFVI List.

   The FHS Bridge then examines the SRT prefixes corresponding to both
   FHS and LHS.  If the FHS Bridge has a local interface connection to
   both the FHS and LHS (whether they are the same or different
   segments), the FHS/LHS Bridge caches the NS LHS information, writes
   its ADM-ULA suffix and LHS INADDR into the NS OMNI Multilink
   Forwarding Parameters LHS fields, then sets its own ADM-ULA as the
   source and the ADM-ULA of the LHS Proxy/Server as the destination

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   while selecting an appropriate identification.  If the FHS and LHS
   prefixes are different, the FHS Bridge instead sets the LHS Subnet
   Router Anycast ULA as the destination.  The FHS Bridge then
   recalculates the NS checksum, selects an appropriate Identification
   and *NET headers as above then forwards the carrier packets into the
   secured spanning tree.

   When the FHS and LHS Bridges are different, the LHS Bridge will
   receive carrier packets over the secured spanning tree from the FHS
   Bridge.  The LHS Bridge reassembles/decapsulates to obtain the NS
   then verifies the checksum and creates an MFV (i.e., the same as the
   FHS Bridge had done) while assigning the current B entry in the MFVI
   List as the MFV "B2" index.  The LHS Bridge also caches the ADM-ULA
   of the FHS Bridge found in the Multilink Forwarding Parameters as the
   spanning tree address for "B2", caches the NS Multilink Forwarding
   Parameters LHS information then generates a "B1" MFVI for the MFV
   while also writing it as the next B entry in the MFVI List.  The LHS
   Bridge also writes its own ADM-ULA suffix and LHS INADDR into the
   OMNI Multilink Forwarding Parameters.  The LHS Bridge then sets the
   its own ADM-ULA as the source and the ADM-ULA of the LHS Proxy/Server
   as the OAL destination, recalculates the checksum, selects an
   appropriate Identification, then fragments while including
   appropriate *NET headers and forwards the carrier packets into the
   secured spanning tree.

   When the LHS Proxy/Server receives the carrier packets from the
   secured spanning tree, it reassembles/decapsulates to obtain the NS,
   verifies the checksum then verifies that the LHS information supplied
   by the FHS source is consistent with its own cached information.  If
   the information is consistent, the LHS Proxy/Server then creates an
   MFV and assigns the current B entry in the MFVI List as the "B2" MFVI
   the same as for the prior hop.  If the NS destination is the MNP-LLA
   of the target Client, the LHS Proxy/Server also generates a "B1" MFVI
   and assigns it both to the MFVI and as the next B entry in the MFVI
   List.  The LHS Proxy/Server then examines FHS FMT; if FMT-Forward is
   clear and FMT-Mode is set, the LHS Proxy/Server creates a NCE for the
   ADM-LLA of the FHS Proxy/Server (if necessary) and sets the state to
   STALE, then caches any Tunnel Window Synchronization parameters.

   If the NS destination is its own ADM-LLA, the LHS Proxy/Server next
   prepares to return a solicited NA with Job code "01".  If the NS
   source was the MNP-LLA of the FHS Client, the LHS Proxy/Server first
   creates or updates an NCE for the MNP-LLA with state set to STALE.
   The LHS Proxy/Server next caches the NS OMNI header window
   synchronization parameters and Multilink Forwarding Parameters
   information (including the MFVI List) in the NCE corresponding to the
   LLA source.  When the LHS Proxy/Server forwards future carrier
   packets based on the NCE, it can populate reverse-path forwarding

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   information in a CRH-32 routing header to enable forwarding based on
   the cached MFVI List B entries instead of ULA addresses.

   The LHS Proxy/Server then creates an NA with Job code "01" while
   copying the NS OMNI Multilink Forwarding Parameters FHS/LHS
   information into the corresponding fields in the NA.  The LHS Proxy/
   Server then generates an "A1" MFVI and both assigns it to the MFV and
   includes it as the first A entry in NA's MFVI List (see:
   [I-D.templin-6man-omni] for details on MFVI List A/B processing).
   The LHS Proxy/Server then includes end-to-end window synchronization
   parameters in the OMIN header (if necessary) and also tunnel window
   synchronization parameters in the Multilink Forwarding Parameters (if
   necessary).  The LHS Proxy/Server then encapsulates the NA,
   calculates the checksum, sets the source to its own ADM-ULA, sets the
   destination to the ADM-ULA of the LHS Bridge, selects an appropriate
   Identification value and *NET headers then forwards the carrier
   packets into the secured spanning tree.

   If the NS destination was the MNP-LLA of the LHS Client, the LHS
   Proxy/Server instead includes an authentication signature in the NS
   if necessary (otherwise recalculates the checksum), then changes the
   OAL source to its own ADM-ULA and changes the destination to the MNP-
   ULA of the LHS Client.  The LHS Proxy/Server then selects an
   appropriate Identification value, fragments if necessary, includes
   appropriate *NET headers and forwards the carrier packets to the LHS
   Client.  When the LHS Client receives the carrier packets, it
   verifies the Identification and reassembles/decapsulates to obtain
   the NS then verifies the authentication signature or checksum.  The
   LHS Client then creates a NCE for the NS LLA source address in the
   STALE state.  If LHS FMT-Forward is set, FHS FMT-Forward is clear and
   the NS source was an MNP-LLA, the Client also creates a NCE for the
   ADM-LLA of the FHS Proxy/Server in the STALE state and caches any
   Tunnel Window Synchronization parameters.  The Client then caches the
   NS OMNI header window synchronization parameters and Multilink
   Forwarding Parameters in the NCE corresponding to the NS LLA source,
   then creates an MFV and assigns both the current MFVI List B entry as
   "B2" and a locally generated "A1" MFVI the same as for previous hops
   (the LHS Client also includes the "A1" value in the solicited NA -
   see above and below).  The LHS Client also caches the previous MFVI
   List B entry as "LHS-Bridge" since it can include this value when it
   sends future carrier packets directly to the Bridge (following
   appropriate neighbor coordination).

   The LHS Client then prepares an NA using exactly the same procedures
   as for the LHS Proxy/Server above, except that it uses its MNP-LLA as
   the source and the {ADM,MNP}-LLA of the FHS correspondent as the
   destination.  The LHS Client also includes an authentication
   signature if necessary (otherwise calculates the checksum), then

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   encapsulates the NA with OAL source set to its own MNP-ULA and
   destination set to the ADM-ULA of the LHS Proxy/Server, includes an
   appropriate Identification and *NET headers and forwards the carrier
   packets to the LHS Proxy/Server.  When the LHS Proxy/Server receives
   the carrier packets, it verifies the Identifications, reassembles/
   decapsulates to obtain the NA, verifies the authentication signature
   or checksum, then uses the current MVFI List B entry to locate the
   MFV.  The LHS Proxy/Server then writes the current MFVI List A entry
   as the "A2" value for the MVF, generates an "A1" MFVI and both
   assigns it to the MFV and writes it as the next MFVI List A entry.
   The LHS Proxy/Server then examines the FHS/LHS FMT codes to determine
   if it needs to include Tunnel Window Synchronization parameters.  The
   LHS Proxy/Server then recalculates the checksum, re-fragments the NA
   while setting the OAL source to its own ADM-ULA and destination to
   the ADM-ULA of the LHS Bridge, includes an appropriate Identification
   and *NET headers and forwards the carrier packets into the secured
   spanning tree.

   When the LHS Bridge receives the carrier packets, it reassembles/
   decapsulates to obtain the NA while verifying the checksum then uses
   the current MFVI List B entry to locate the MFV.  The LHS Bridge then
   writes the current MFVI List A entry as the MFV "A2" index and
   generates a new "A1" value which it both assigns the MFV and writes
   as the next MFVI List A entry.  (The LHS Bridge also caches the first
   A entry in the MFVI List as "LHS-Client" when LHS FMT-Forward/Mode
   are both set to enable future direct forwarding to this LHS Client.)
   If the LHS Bridge is connected directly to both the FHS and LHS
   segments (whether the segments are the same or different), the FHS/
   LHS Bridge will have already cached the FHS/LHS information based on
   the original NS.  The FHS/LHS Bridge recalculates the checksum then
   re-fragments the NA while setting the OAL source to its own ADM-ULA
   and destination to the ADM-ULA of the FHS Proxy/Server.  If the FHS
   and LHS prefixes are different, the FHS Bridge instead re-fragments
   while setting the destination to the ADM-ULA of the FHS Bridge.  The
   LHS Bridge selects an appropriate Identification and *NET headers
   then forwards the carrier packets into the secured spanning tree.

   When the FHS and LHS Bridges are different, the FHS Bridge will
   receive the carrier packets from the LHS Bridge over the secured
   spanning tree.  The FHS Bridge reassembles/decapsulates to obtain the
   NA while verifying the checksum, then locates the MFV based on the
   current MFVI List B entry.  The FHS Bridge then assigns the current
   MFVI List A entry as the MFV "A2" index and caches the ADM-ULA of the
   LHS Bridge as the spanning tree address for "A2".  The FHS Bridge
   then generates an "A1" MVFI and both assigns it to the MVF and writes
   it as the next MFVI List A entry while also writing its ADM-ULA and
   INADDR in the NA FHS Bridge fields.  The FHS Bridge then recalculates
   the checksum, re-encapsulates/re-fragments with its own ADM-ULA as

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   the source, with the ADM-ULA of the FHS Proxy/Server as the
   destination, then selects an appropriate Identification value and
   *NET headers and forwards the carrier packets into the secured
   spanning tree.

   When the FHS Proxy/Server receives the carrier packets from the
   secured spanning tree, it reassembles/decapsulates to obtain the NA
   while verifying the checksum then locates the MFV based on the
   current MFVI List B entry.  The FHS Proxy/Server then assigns the
   current MFVI List A entry as the "A2" MFVI the same as for the prior
   hop.  If the NA destination is its own ADM-LLA, the FHS Proxy/Server
   then caches the NA Multilink Forwarding Parameters with the MFV and
   examines LHS FMT.  If FMT-Forward is clear, the FHS Proxy/Server
   locates the NCE for the ADM-LLA of the LHS Proxy/Server and sets the
   state to REACHABLE then caches any Tunnel Window Synchronization
   parameters.  If the NA source is the MNP-LLA of the LHS Client, the
   FHS Proxy/Server then locates the LHS Client NCE and sets the state
   to REACHABLE then caches the OMNI header window synchronization
   parameters and prepares to return an NA acknowledgement, if
   necessary.

   If the NA destination is the MNP-LLA of the FHS Client, the FHS
   Proxy/Server also searches for and updates the NCE for the ADM-LLA of
   the LHS Proxy/Server if necessary the same as above.  The FHS Proxy/
   Server then generates an "A1" MFVI and assigns it both to the MFVI
   and as the next MFVI List A entry, then includes an authentication
   signature or checksum in the NA message.  The FHS Proxy/Server then
   sets the OAL source to its own ADM-LA and sets the destination to the
   MNP-ULA of the FHS Client, then selects an appropriate Identification
   value and *NET headers and forwards the carrier packets to the FHS
   Client.

   When the FHS Client receives the carrier packets, it verifies the
   Identification, reassembles/decapsulates to obtain the NA, verifies
   the authentication signature or checksum, then locates the MFV based
   on the current MFVI List B entry.  The FHS Client then assigns the
   current MFVI List A entry as the "A2" MFVI the same as for the prior
   hop.  The FHS Client then caches the NA Multilink Forwarding
   Parameters (including the MFVI List) with the MFV and examines LHS
   FMT.  If FMT-Forward is clear, the FHS Client locates the NCE for the
   ADM-LLA of the LHS Proxy/Server and sets the state to REACHABLE then
   caches any Tunnel Window Synchronization parameters.  If the NA
   source is the MNP-LLA of the LHS Client, the FHS Proxy/Server then
   locates the LHS Client NCE and sets the state to REACHABLE then
   caches the OMNI header window synchronization parameters and prepares
   to return an NA acknowledgement, if necessary.  The FHS Client also
   caches the previous MFVI List A entry as "FHS-Bridge" since it can

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   include this value when it sends future carrier packets directly to
   the Bridge (following appropriate neighbor coordination).

   If either the FHS Client or FHS Proxy/Server needs to return an
   acknowledgement to complete window synchronization, it prepares a uNA
   message with an OMNI Multilink Forwarding Parameters sub-option with
   Job code set to "10" (Follow A; Record B) (note that this step is
   unnecessary when Rapid Commit route optimization is used per
   Section 3.13.3).  The FHS node sets the source to its own {ADM,MNP}-
   LLA, sets the destination to the {ADM,MNP}-LLA of the LHS node then
   includes Tunnel Window Synchronization parameters if necessary.  The
   FHS node next sets the MFVI List to the cached list of A entries
   received in the Job code "01" NA, but need not set any other FHS/LHS
   information.  The FHS node then encapsulates the uNA message in an
   OAL header with its own {ADM,MNP}-ULA as the source.  If the FHS node
   is the Client, it next sets the ADM-ULA of the FHS Proxy/Server as
   the OAL destination, includes an authentication signature or
   checksum, selects an appropriate Identification value and *NET
   headers and forwards the carrier packets to the FHS Proxy/Server.
   The FHS Proxy/Server then verifies the Identification, reassembles/
   decapsulates, verifies the authentication signature or checksum, then
   uses the current MFVI List A entry to locate the MFV.  The FHS Proxy/
   Server then writes its "B1" MFVI as the next MFVI List B entry and
   determines whether it needs to include Tunnel Window Synchronization
   parameters the same as it had done when it forwarded the original NS.

   The FHS Proxy/Server recalculates the uNA checksum then re-fragments
   while setting its own ADM-ULA as the source and the ADM-ULA of the
   FHS Bridge as the destination, then selects an appropriate
   Identification and *NET headers and forwards the carrier packets into
   the secured spanning tree.  When the FHS Bridge receives the carrier
   packets, it reassembles/decapsulates to obtain the uNA while
   verifying the checksum then uses the current MFVI List A entry to
   locate the MFV.  The FHS Bridge then writes its "B1" MFVI as the next
   MFVI List B entry, then re-fragments while setting the OAL source and
   destination.  If the FHS Bridge is also the LHS Bridge, it sets the
   ADM-ULA of the LHS Proxy/Server as the destination; otherwise it sets
   the ADM-ULA of the LHS Bridge.  The FHS Bridge recalculates the
   checksum then selects an appropriate Identification and *NET headers,
   re-fragments/forwards the carrier packets into the secured spanning
   tree.  If an LHS Bridge receives the carrier packets, it processes
   them exactly the same as the FHS Bridge had done while setting the
   carrier packet destination to the ADM-ULA of the LHS Proxy/Server.

   When the LHS Proxy/Server receives the carrier packets, it
   reassembles/decapsulates to obtain the uNA message while verifying
   the checksum.  The LHS Proxy/Server then locates the MFV based on the
   current MFVI List A entry then determines whether it is a tunnel

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   ingress the same as for the original NS.  If it is a tunnel ingress,
   the LHS Proxy/Server updates the NCE for the tunnel far-end based on
   the Tunnel Window Synchronization parameters.  If the uNA destination
   is its own ADM-LLA, the LHS Proxy/Server next updates the NCE for the
   source LLA based on the OMNI header Window Synchronization parameters
   and MAY compare the MVFI List to the version it had cached in the MFV
   based on the original NS.

   If the uNA destination is the MNP-LLA of the LHS Client, the LHS
   Proxy/Server instead writes its "B1" MFV as the next MFVI List B
   entry, includes an authentication signature or checksum, writes its
   own ADM-ULA as the source and the MNP-ULA of the Client as the
   destination then selects an appropriate Identification and *NET
   headers and forwards the resulting carrier packets to the LHS Client.
   When the LHS Client receives the carrier packets, it verifies the
   Identification, reassembles/decapsulates to obtain the uNA, verifies
   the authentication signature or checksum then processes the message
   exactly the same as for the LHS Proxy/Server case above.

   Following the NS/NA exchange with Multilink Forwarding Parameters,
   OAL end systems and tunnel endpoints can begin exchanging ordinary
   carrier packets with Identification values within their respective
   send/receive windows without requiring security signatures and/or
   secured spanning tree traversal.  Either peer can refresh window
   synchronization parameters and/or send other carrier packets
   requiring security at any time using the same secured procedures
   described above.  OAL end systems and intermediate nodes can also use
   their own A1/B1 MFVIs when they receive carrier packets to
   unambiguously locate the correct MFV and determine directionality and
   can use any discovered A2/B2 MFVIs to forward carrier packets to
   other OAL nodes that configure the corresponding A1/B1 MFVIs.  When
   an OAL node uses an MFVI included in a carrier packet to locate an
   MFV, it need not also examine the carrier packet addresses.

   OAL sources can also begin including CRH-32s in carrier packets with
   a list of A/B MFVIs that OAL intermediate nodes can use for shortest-
   path carrier packet forwarding based on MFVIs instead of spanning
   tree addresses.  OAL sources and intermediate nodes can also begin
   forwarding carrier packets with OAL compressed headers termed "OCH-
   0/1" (Type 0 or 1) (see: [I-D.templin-6man-omni]) that include only a
   single A/B MFVI meaningful to the next hop, since all nodes in the
   path up to (and sometimes including) the OAL destination have already
   established MFV forwarding information.  Note that when an FHS OAL
   source receives a solicited NA with Job code "01', the message will
   contain an MFVI List with A entries populated in the reverse order
   needed for populating a CRH-32 routing header.  The FHS OAL source
   must therefore write the MFVI List A entries last-to-first when it
   populates a CRH-32, or must select the correct A entry to include in

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   an OCH-0/1 header based on the intended OAL intermediate node or
   destination.

   When a Bridge receives unsecured carrier packets destined to a local
   segment Client that has asserted direct reachability, the Bridge
   performs direct carrier packet forwarding while bypassing the local
   Proxy/Server based on the Client's advertised MFVIs and discovered
   NATed INADDR information (see: Section 3.13.4).  If the Client cannot
   be reached directly (or if NAT traversal has not yet converged), the
   Bridge instead forwards carrier packets directly to the local Proxy/
   Server.

   When a Proxy/Server receives carrier packets destined to a local
   Client or forwards carrier packets received from a local Client, it
   first locates the correct MFV.  If the carrier packets include a
   secured IPv6 ND message, the Proxy/Server uses the Client's NCE
   established through RS/RA exchanges to re-encapsulate/re-fragment
   while forwarding outbound secured carrier packets via the secured
   spanning tree and forwarding inbound secured carrier packets while
   including an authentication signature or checksum.  For ordinary
   carrier packets, the Proxy/Server uses the same MFV if directed by
   MFVI and/or OAL addressing.  Otherwise it locates an MFV established
   through an NS/NA exchange between the Client and the remote peer, and
   forwards the carrier packets without first reassembling/
   decapsulating.

   When a Proxy/Server or Client configured as a tunnel ingress receives
   a carrier packet with a full OAL header with an MNP-ULA source and
   CRH-32 routing header, or an OCH-0/1 header with an MFVI that matches
   an MFV, the ingress encapsulates the carrier packet in a new full OAL
   header or an OCH-0/1 header containing the next hop MVFI and an
   Identification value appropriate for the end-to-end window and the
   outer header containing an Identification value appropriate for the
   tunnel endpoints.  When a Proxy/Server or Client configured as a
   tunnel egress receives an encapsulated carrier packet, it verifies
   the Identification in the outer header, then discards the outer
   header and forwards the inner carrier packet to the final
   destination.

   When a Proxy/Server with FMT-Forward/Mode set to 0/1 for a source
   Client receives carrier packets from the source Client, it first
   reassembles to obtain the original OAL packet then re-fragments if
   necessary to cause the Client's packets to match the MPS on the path
   from the Proxy/Server as a tunnel ingress to the tunnel egress.  The
   Proxy/Server then performs OAL-in-OAL encapsulation and forwards the
   resulting carrier packets to the tunnel egress.  When a Proxy/Server
   with FMT-Forward/Mode set to 0/1 for a target Client receives carrier
   packets from a tunnel ingress, it first decapsulates to obtain the

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   original fragments then reassembles to obtain the original OAL
   packet.  The Proxy/Server then re-fragments if necessary to cause the
   fragments to match the target Client's underlying interface (Path)
   MTU and forwards the resulting carrier packets to the target Client.

   When a source Client forwards carrier packets it can employ header
   compression according to the MFVIs established through an NS/NA
   exchange with a remote or local peer.  When the source Client
   forwards to a remote peer, it can forward carrier packets to a local
   SRT Bridge (following the establishment of INADDR information) while
   bypassing the Proxy/Server (see: Section 3.13.4).  When a target
   Client receives carrier packets that match a local MFV, the Client
   first verifies the Identification then decompresses the headers if
   necessary, reassembles if necessary to obtain the OAL packet then
   decapsulates and delivers the IP packet to upper layers.

   When synchronized peer Clients in the same SRT segment with FMT-
   Forward and FMT-Mode set discover each other's NATed INADDR
   addresses, they can exchange carrier packets directly with header
   compression using MFVIs discovered as above (see: Section 3.13.5).
   The FHS Client will have cached the A MFVI for the LHS Client, which
   will have cached the B MVFI for the FHS Client.

   After window synchronization state has been established, the ROS and
   ROR can begin forwarding carrier packets while performing additional
   NS/NA exchanges as above to update window state, register new
   interface pairs for optimized multilink forwarding and/or confirm
   reachability.  The ROS sends carrier packets to the FHS Bridge
   discovered through the NS/NA exchange.  The FHS Bridge then forwards
   the carrier packets over the unsecured spanning tree to the LHS
   Bridge, which forwards them via LHS encapsulation to the LHS Proxy/
   Server or directly to the target Client itself.  The target Client in
   turn sends packets to the ROS in the reverse direction while
   forwarding through the Bridges to minimize Proxy/Server load whenever
   possible.

   While the ROS continues to actively forward packets to the target
   Client, it is responsible for updating window synchronization state
   and per-interface reachability before expiration.  Window
   synchronization state is shared by all underlying interfaces in the
   ROS' NCE that use the same destination LLA so that a single NS/NA
   exchange applies for all interfaces regardless of the specific
   interface used to conduct the exchange.  However, the window
   synchronization exchange only confirms target Client reachability
   over the specific underlying interface pair.  Reachability for other
   underlying interfaces that share the same window synchronization
   state must be determined individually using additional NS/NA
   messages.

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3.13.3.  Rapid Commit Route Optimization

   When the ROR receives an NS(AR) with a set of Interface Attributes
   for the source Client, it can perform "rapid commit" by immediately
   invoking multilink route optimization as above instead of returning
   an NA(AR).  In order to perform rapid commit, the ROR prepares a
   unicast NS message with an OMNI option with Window Synchronization
   information responsive to the NS(AR), with a Multilink Forwarding
   Parameters sub-option selected for a specific underlying interface
   pair and with Interface Attributes for all of the ROR's other
   underlying interfaces.  The ROR can also include ordinary IP packets
   as OAL super-packet trailers to the NS message if it has immediate
   data to send to the ROS.  The ROR then returns the NS to the ROS the
   same as for the NA(AR) case.

   When the NS message traverses the return path to the ROR, all
   intermediate nodes in the path establish state exactly the same as
   for an ordinary NS/NA multilink route optimization exchange.  When
   the NS message arrives at the ROS, the Window Synchronization
   parameters confirm that the NS is taking the place of the NA(AR),
   thereby eliminating an extraneous message transmission and associated
   delay.  The ROS then completes the route optimization by returning a
   responsive NA.

   Note: The ROS must accept unicast NS messages with an ACK matching
   the SYN included in the NS(AR) as an equivalent message replacement
   for the NA(AR).  Address resolution and multilink forwarding
   coordination can therefore be coordinated in a single three-way
   handshake connection with minimal messaging and delay (i.e., as
   opposed to a four-message exchange).

3.13.4.  Client/Bridge Route Optimization

   Following multilink route optimization for specific underlying
   interface pairs, ROS/ROR Clients located on open INETs can invoke
   Client/Bridge route optimization to improve performance and reduce
   load and congestion on their respective FHS/LHS Proxy/Servers.  To
   initiate Client/Bridge route optimization, the Client prepares an NS
   message with its own MNP-LLA address as the source and the ADM-LLA of
   its Bridge as the destination while creating a NCE for the Bridge if
   necessary.  The NS message must be no larger than the minimum MPS and
   encapsulated as an atomic fragment.

   The Client then includes an Interface Attributes sub-option for its
   underlying interface as well as an authentication signature but does
   not include Window Synchronization parameters.  The Client then
   performs OAL encapsulation with its own MNP-ULA as the source and the
   ADM-ULA of the Bridge as the destination while including a randomly-

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   chosen Identification value, then performs *NET encapsulation on the
   atomic fragment and sends the resulting carrier packet directly to
   the Bridge.

   When the Bridge receives the carrier packet, it verifies the
   authentication signature then creates a NCE for the Client.  The
   Bridge then caches the *NET encapsulation addresses (which may have
   been altered by one or more NATs on the path) as well as the
   Interface Attributes for this Client omIndex, and marks this Client
   underlying interface as "trusted".  The Bridge then prepares an NA
   reply with its own ADM-LLA as the source and the MNP-LLA of the
   Client as the destination where the NA again must be no larger than
   the minimum MPS.

   The Bridge then echoes the Client's Interface Attributes, includes an
   Origin Indication with the Client's observed *NET addresses and
   includes an authentication signature.  The Bridge then performs OAL
   encapsulation with its own ADM-ULA as the source and the MNP-ULA of
   the Client as the destination while using the same Identification
   value that appeared in the NS, then performs *NET encapsulation on
   the atomic fragment and sends the resulting carrier packet directly
   to the Client.

   When the Client receives the NA reply, it caches the carrier packet
   *NET source address information as the Bridge target address via this
   underlying interface while marking the interface as "trusted".  The
   Client also caches the Origin Indication *NET address information as
   its own (external) source address for this underlying interface.

   After the Client and Bridge have established NCEs as well as
   "trusted" status for a particular underlying interface pair, each
   node can begin forwarding ordinary carrier packets intended for this
   multilink route optimization directly to one another while omitting
   the Proxy/Server from the forwarding path while the status is
   "trusted".  The NS/NA messaging will have established the correct
   state in any NATs in the path so that NAT traversal is naturally
   supported.  The Client and Bridge must maintain a timer that watches
   for activity on the path; if no carrier packets and/or NS/NA messages
   are sent or received over the path before NAT state is likely to have
   expired, the underlying interface pair status becomes "untrusted".

   Thereafter, when the Client forwards a carrier packet with an MFVI
   toward the Bridge as the next hop, the Client uses the MFVI for the
   Bridge (discovered during multilink route optimization) instead of
   the MFVI for its Proxy/Server; the Bridge will accept the packet from
   the Client if and only if the underlying interface status is trusted
   and if the MFVI is correct for the next hop toward the final

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   destination.  (The same is true in the reverse direction when the
   Bridge sends carrier packets directly to the Client.)

   Note that the Client and Bridge each maintain a single NCE, but that
   the NCE may aggregate multiple underlying interface pairs.  Each
   underlying interface pair may use differing source and target *NET
   addresses according to NAT mappings, and the "trusted/untrusted"
   status of each pair must be tested independently.  When no "trusted"
   pairs remain, the NCE is deleted.

   Note that the above method requires Bridges to participate in NS/NA
   message authentication signature application and verification.  In an
   alternate approach, the Client could instead exchange NS/NA messages
   with authentication signatures via its Proxy/Server but addressed to
   the ADM-LLA of the Bridge, and the Proxy/Server and Bridge could
   relay the messages over the secured spanning tree.  However, this
   would still require the Client to send additional messages toward the
   *NET address of the Bridge to populate NAT state; hence the savings
   in complexity for Bridges would result in increased message overhead
   for Clients.

3.13.5.  Client/Client Route Optimization

   When the ROS/ROR Clients are both located on the same SRT segment,
   Client-to-Client route optimization is possible following the
   establishment of any necessary state in NATs in the path.  Both
   Clients will have already established state via their respective
   shared segment Proxy/Servers (and possibly also the shared segment
   Bridge) and can begin forwarding packets directly via NAT traversal
   while avoiding any Proxy/Server and/or Bridge hops.

   When the ROR/ROS Clients on the same SRT segment perform the initial
   NS/NA exchange to establish Multilink Forwarding state, they also
   include an Origin Indication (i.e., in addition to Multilink
   Forwarding Parameters) with the mapped addresses discovered during
   the RS/RA exchanges with their respective Proxy/Servers.  After the
   MFV paths have been established, both Clients can begin sending
   packets via strict MFV paths while establishing a direct path for
   Client-to-Client route optimization.

   To establish the direct path, either Client (acting as the source)
   transmits a bubble to the mapped *NET address for the target Client
   which primes its local chain of NATs for reception of future packets
   from that *NET address (see: [RFC4380] and [I-D.templin-6man-omni]).
   The source Client then prepares an NS message with its own MNP-LLA as
   the source, with the MNP-LLA of the target as the destination and
   with an OMNI option with an Interface Attributes sub-option.  The
   source Client then encapsulates the NS in an OAL header with its own

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   MNP-ULA as the source, with the MNP-ULA of the target Client as the
   destination and with an in-window Identification for the target.  The
   source Client then fragments and encapsulates in *NET headers
   addressed to its FHS Proxy/Server then forwards the resulting carrier
   packets to the Proxy/Server.

   When the FHS Proxy/Server receives the carrier packets, it re-
   encapsulates and forwards them as unsecured carrier packets according
   to MFV state where they will eventually arrive at the target Client
   which can verify that the identifications are within the acceptable
   window and reassemble if necessary.  Following reassembly, the target
   Client prepares an NA message with its own MNP-LLA as the source,
   with the MNP-LLA of the source Client as the destination and with an
   OMNI option with an Interface Attributes sub-option.  The target
   Client then encapsulates the NA in an OAL header with its own MNP-ULA
   as the source, with the MNP-ULA of the source Client as the
   destination and with an in-window Identification for the source
   Client.  The target Client then fragments and encapsulates in *NET
   headers addressed to the source Client's Origin addresses then
   forwards the resulting carrier packets directly to the source Client.

   Following the initial NS/NA exchange, both Clients mark their
   respective (source, target) underlying interface pairs as "trusted"
   for no more than ReachableTime seconds.  While the Clients continue
   to exchange carrier packets via the direct path avoiding all Proxy/
   Servers and Bridges, they should perform additional NS/NA exchanges
   via their local Proxy/Servers to refresh NCE state as well as send
   additional bubbles to the peer's Origin address information if
   necessary to refresh NAT state.

   Note that these procedures are suitable for a widely-deployed but
   basic class of NATs.  Procedures for advanced NAT classes are
   outlined in [RFC6081], which provides mechanisms that can be employed
   equally for AERO using the corresponding sub-options specified by
   OMNI.

   Note also that each communicating pair of Clients may need to
   maintain NAT state for peer to peer communications via multiple
   underlying interface pairs.  It is therefore important that Origin
   Indications are maintained with the correct peer interface and that
   the NCE may cache information for multiple peer interfaces.

   Note that the source and target Client exchange Origin information
   during the secured NS/NA multilink route optimization exchange.  This
   allows for subsequent NS/NA exchanges to proceed using only the
   Identification value as a data origin confirmation.  However, Client-
   to-Client peerings that require stronger security may also include
   authentication signatures for mutual authentication.

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3.14.  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.
   The NUD algorithm is based on periodic control message exchanges and
   may further be seeded by IPv6 ND hints of forward progress, but care
   must be taken to avoid inferring reachability based on spoofed
   information.  For example, IPv6 ND message exchanges that include
   authentication codes and/or in-window Identifications may be
   considered as acceptable hints of forward progress, while spurious
   random carrier packets should be ignored.

   AERO nodes can perform NS/NA(NUD) exchanges over the OMNI link
   secured spanning tree (i.e. the same as described above) to 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 parties involved in the NUD test.  When only reachability
   information is required without updating any other NCE state, AERO
   nodes can instead perform NS/NA(NUD) exchanges directly between
   neighbors without employing the secured spanning tree as long as they
   include in-window Identifications and either an authentication
   signature or checksum.

   After an ROR directs an ROS to a target neighbor with one or more
   link-layer addresses, either node may invoke multilink forwarding
   state initialization to establish authentic intermediate node state
   between specific underlying interface pairs which also tests their
   reachability.  Thereafter, either node acting as the source may
   perform additional reachability probing through NS(NUD) messages over
   the SRT secured or unsecured spanning tree, or through NS(NUD)
   messages sent directly to an underlying interface of the target
   itself.  While testing a target underlying interface, the source can
   optionally continue to forward carrier packets via alternate
   interfaces, maintain a small queue of carrier packets until target
   reachability is confirmed or include them as trailing data with the
   NS(NUD) in an OAL super-packet [I-D.templin-6man-omni].

   NS(NUD) messages are encapsulated, fragmented and transmitted as
   carrier packets the same as for ordinary original IP data packets,
   however the encapsulated destinations are the LLA of the source and
   either the ADM-LLA of the LHS Proxy/Server or the MNP-LLA of the
   target itself.  The source encapsulates the NS(NUD) message the same
   as described in Section 3.13.2 and includes an Interface Attributes
   sub-option with omIndex set to identify its underlying interface used
   for forwarding.  The source then includes an in-window
   Identification, fragments the OAL packet and forwards the resulting
   carrier packets into the unsecured spanning tree, directly to the

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   target if it is in the local segment or directly to a Bridge in the
   local segment.

   When the target receives the NS(NUD) carrier packets, it verifies
   that it has a NCE for this source and that the Identification is in-
   window, then submits the carrier packets for reassembly.  The target
   then verifies the authentication signature or checksum, then searches
   for Interface Attributes in its NCE for the source that match the
   NS(NUD) for the NA(NUD) reply.  The target then prepares the NA(NUD)
   with the source and destination LLAs reversed, encapsulates and sets
   the OAL source and destination, includes an Interface Attributes sub-
   option in the NA(NUD) to identify the omIndex of the underlying
   interface the NS(NUD) arrived on and sets the Target Address to the
   same value included in the NS(NUD).  The target next sets the R flag
   to 1, the S flag to 1 and the O flag to 1, then selects an in-window
   Identification for the source and performs fragmentation.  The node
   then forwards the carrier packets into the unsecured spanning tree,
   directly to the source if it is in the local segment or directly to a
   Bridge in the local segment.

   When the source receives the NA(NUD), it marks the target underlying
   interface tested as "trusted".  Note that underlying interface states
   are maintained independently of the overall NCE REACHABLE state, and
   that a single NCE may have multiple target underlying interfaces in
   various "trusted/untrusted" states while the NCE state as a whole
   remains REACHABLE.

3.15.  Mobility Management and Quality of Service (QoS)

   AERO is a fully Distributed Mobility Management (DMM) service in
   which each Proxy/Server is responsible for only a small subset of the
   Clients on the OMNI link.  This is in contrast to a Centralized
   Mobility Management (CMM) service where there are only one or a few
   network mobility collective entities for large Client populations.
   Clients coordinate with their associated FHS and Hub Proxy/Servers
   via RS/RA exchanges to maintain the DMM profile, and the AERO routing
   system tracks all current Client/Proxy/Server peering relationships.

   Hub Proxy/Servers provide a designated router service for their
   dependent Clients, while FHS Proxy/Servers provide a proxy conduit
   between the Client and both the Hub and OMNI link in general.
   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 attributes change, the Client is responsible for updating
   the Hub Proxy/Server through new RS/RA exchanges using the FHS Proxy/
   Server as a first-hop conduit.  The FHS Proxy/Server can also act as

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   a proxy to perform some IPv6 ND exchanges on the Client's behalf
   without consuming bandwidth on the Client underlying interface.

   Mobility management considerations are specified in the following
   sections.

3.15.1.  Mobility Update Messaging

   RORs and ROSs accommodate Client mobility and/or multilink change
   events by sending secured uNA messages to each active neighbor.  When
   an ROR/ROS sends a uNA message, it sets the IPv6 source address to
   the its own LLA, sets the destination address to the neighbor's
   {ADM,MNP}-LLA and sets the Target Address to the Client's MNP-LLA.
   The ROR/ROS also includes an OMNI option with Preflen set to the
   prefix length associated with the Client's MNP-LLA, includes
   Interface Attributes and Traffic Selectors for the Client's
   underlying interfaces and includes an authentication signature if
   necessary.  The ROR then sets the uNA R flag to 1, S flag to 0 and O
   flag to 1, then encapsulates the message in an OAL header with source
   set to its own ULA and destination set to its FHS Proxy/Server's ADM-
   ULA.  When the FHS Proxy/Server receives the uNA, it reassembles,
   verifies the authentication signature, then changes the destination
   to the ULA corresponding to the LLA destination and forwards the uNA
   into the secured 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 ROR/ROS can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT uNAs
   to each neighbor to increase the likelihood that at least one will be
   received.  Alternatively, the ROR/ROS can set the PNG flag in the uNA
   OMNI option header to request a uNA acknowledgement as specified in
   [I-D.templin-6man-omni].

   When the ROR/ROS Proxy/Server receives a uNA message prepared as
   above, if the uNA destination was its own ADM-LLA the Proxy/Server
   uses the included OMNI option information to update its NCE for the
   target but does not reset ReachableTime since the receipt of a uNA
   message does not provide confirmation that any forward paths to the
   target Client are working.  If the destination was the MNP-LLA of the
   ROR/ROS Client, the Proxy/Server instead changes the OAL source to
   its own ADM-ULA, includes an authentication signature if necessary,
   and includes an in-window Identification for this Client.  Finally,
   if the uNA message PNG flag was set, the node that processes the uNA
   returns a uNA acknowledgement as specified in
   [I-D.templin-6man-omni].

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3.15.2.  Announcing Link-Layer Information Changes

   When a Client needs to change its underlying Interface Attributes
   and/or Traffic Selectors (e.g., due to a mobility event), the Client
   sends an RS message to its Hub Proxy/Server via a first-hop FHS
   Proxy/Server, if necessary.  The RS includes an OMNI option with an
   Interface Attributes sub-option with the omIndex and with new link
   quality and any other information.

   Note that the first FHS Proxy/Server may change due to the underlying
   interface change.  If the Client supplies the address of the former
   FHS Proxy/Server, the new FHS Proxy/Server can send a departure
   indication (see below); otherwise, any stale state in the former FHS
   Proxy/Server will simply expire after ReachableTime expires with no
   effect on the Hub Proxy/Server.

   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.

   After performing the RS/RA exchange, the Client sends uNA messages to
   all neighbors the same as described in the previous section.

3.15.3.  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 Hub Proxy/Server via a FHS Proxy/Server for the underlying
   interface (if necessary) with an OMNI option that includes an
   Interface Attributes sub-option with appropriate link quality values
   and with link-layer address information for the new link.  The Client
   then again sends uNA messages to all neighbors the same as described
   above.

3.15.4.  Deactivating Existing Links

   When a Client needs to deactivate an existing underlying interface,
   it sends a uNA message toward the Hub Proxy/Server via an FHS Proxy/
   Server with an OMNI option with appropriate Interface Attributes
   values for the deactivated link - in particular, the link quality
   value 0 assures that neighbors will cease to use the link.

   If the Client needs to send 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.  The Client then again sends uNA
   messages to all neighbors the same as described above.

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   Note that when a Client deactivates an underlying interface,
   neighbors that receive the ensuing 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 omIndex at a later point in time, when it will send new RS
   messages to an FHS Proxy/Server with fresh interface parameters to
   update any neighbors.

3.15.5.  Moving Between Proxy/Servers

   The Client performs the procedures specified in Section 3.12.2 when
   it first associates with a new Hub Proxy/Server or renews its
   association with an existing Hub Proxy/Server.

   When a Client associates with a new Hub Proxy/Server, it sends RS
   messages to register its underlying interfaces with the new Hub while
   including the 32 least significant bits of the old Hub's ADM-LLA in
   the "Old Hub Proxy/Server MSID" field of a Proxy/Server Departure
   OMNI sub-option.  When the new Hub Proxy/Server returns the RA
   message via the FHS Proxy/Server (acting as a Proxy), the FHS Proxy/
   Server sends a uNA to the old Hub Proxy/Server (i.e., if the MSID is
   non-zero and different from its own).  The uNA has the MNP-LLA of the
   Client as the source and the ADM-LLA of the old hub as the
   destination and with Preflen set to 0.  The FHS Proxy/Server
   encapsulates the uNA in an OAL header with the ADM-ULA of the new Hub
   as the source and the ADM-ULA of the old Hub as the destination, the
   fragments and sends the carrier packets via the secured spanning
   tree.

   When the old Hub Proxy/Server receives the uNA, it changes the
   Client's NCE state to DEPARTED, resets DepartTime and caches the new
   Hub Proxy/Server ADM-ULA.  After a short delay (e.g., 2 seconds) the
   old Hub Proxy/Server withdraws the Client's MNP from the routing
   system.  While in the DEPARTED state, the old Hub Proxy/Server
   forwards any carrier packets received via the secured spanning tree
   destined to the Client's MNP-ULA to the new Hub Proxy/Server's ADM-
   ULA.  After DepartTime expires, the old Hub Proxy/Server deletes the
   Client's NCE.

   Mobility events may also cause a Client to change to a new FHS Proxy/
   Server over a specific underlying interface at any time such that a
   Client RS/RA exchange over the underlying interface will engage the
   new FHS Proxy/Server instead of the old.  The Client can arrange to
   inform the old FHS Proxy/Server of the departure by including a
   Proxy/Server Departure sub-option with an MSID for the "Old FHS
   Proxy/Server MSID", and the new FHS Proxy/Server will issue a uNA
   using the same procedures as outlined for the Hub above while using
   its own ADM-ULA as the source address.  This can often result in

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   successful delivery of packets that would otherwise be lost due to
   the mobility event.

   Clients SHOULD NOT move rapidly between Hub 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 Hub
   Proxy/Server include a Hub 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.

3.16.  Multicast

   Clients provide an IGMP (IPv4) [RFC2236] or MLD (IPv6) [RFC3810]
   proxy service for its EUNs and/or hosted applications [RFC4605] and
   act as a Protocol Independent Multicast - Sparse-Mode (PIM-SM, or
   simply "PIM") Designated Router (DR) [RFC7761] on the OMNI link.
   Proxy/Servers act as OMNI link PIM routers for Clients on ANET, VPNed
   or Direct interfaces, and Relays also act as OMNI link PIM routers on
   behalf of nodes on other links/networks.

   Clients on VPNed, Direct or ANET underlying interfaces for which the
   ANET has deployed native multicast services forward IGMP/MLD messages
   into the ANET.  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 ANET Proxy/Server.  The
   FHS Proxy/Server then acts as an ROS to send NS(AR) messages to an
   ROR for the multicast source.  Clients on INET and ANET underlying
   interfaces without native multicast services instead send NS(AR)
   messages as an ROS to cause their FHS Proxy/Server forward the
   message to an ROR.  When the ROR receives an NA(AR) response, it
   initiates PIM protocol messaging according to the Source-Specific
   Multicast (SSM) and Any-Source Multicast (ASM) operational modes as
   discussed in the following sections.

3.16.1.  Source-Specific Multicast (SSM)

   When an ROS "X" (i.e., either a Client or Proxy/Server) 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
   sends an NS(AR) message (see: Section 3.13) using its own LLA as the
   source address, the solicited node multicast address corresponding to
   S as the destination and the LLA of S as the target address.  X then

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   encapsulates the NS(AR) in an OAL header with source address set to
   its own ULA and destination address set to the ULA for S, then
   forwards the message into the secured spanning tree which delivers it
   to ROR "Y" that services S.  The resulting NA(AR) will return an OMNI
   option with Interface Attributes for any underlying interfaces that
   are currently servicing S.

   When X processes the NA(AR) it selects one or more underlying
   interfaces for S and performs an NS/NA multilink route optimization
   exchange over the secured spanning tree while including a PIM Join/
   Prune message for each multicast group of interest in the OMNI
   option.  If S is located behind any Proxys "Z"*, each Z* then updates
   its MRIB accordingly and maintains the LLA of X as the next hop in
   the reverse path.  Since Bridges forward messages not addressed to
   themselves without examining them, this means that the (reverse)
   multicast tree path is simply from each Z* (and/or S) to X with no
   other multicast-aware routers in the path.

   Following the initial combined Join/Prune and NS/NA messaging, X
   maintains a NCE 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 NCE alive for up to t_periodic seconds [RFC7761].  If no
   new Joins are received within t_periodic seconds, X allows the NCE to
   expire.  Finally, if X receives any additional Join/Prune messages
   for (S,G) it forwards the messages over the secured spanning tree.

   Client C that holds an MNP for source S may later depart from a first
   Proxy/Server Z1 and/or connect via a new Proxy/Server Z2.  In that
   case, Y sends a uNA message to X the same as specified for unicast
   mobility in Section 3.15.  When X receives the uNA message, it
   updates its NCE for the LLA for source S and sends new Join messages
   in NS/NA exchanges addressed to the new target Client underlying
   interface connection for S.  There is no requirement to send any
   Prune messages to old Proxy/Server Z1 since source S will no longer
   source any multicast data traffic via Z1.  Instead, the multicast
   state for (S,G) in Proxy/Server Z1 will soon expire since no new
   Joins will arrive.

3.16.2.  Any-Source Multicast (ASM)

   When an ROS X acting as a PIM router receives Join/Prune messages
   from a node on its downstream interfaces containing one or more (*,G)
   pairs, it updates its Multicast Routing Information Base (MRIB)
   accordingly.  X first performs an NS/NA(AR) exchange to receive route
   optimization information for Rendezvous Point (RP) R for each G.  X
   then includes a copy of each Join/Prune message in the OMNI option of
   an NS message with its own LLA as the source address and the LLA for
   R as the destination address, then encapsulates the NS message in an

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   OAL header with its own ULA as the source and the ADM-ULA of R's
   Proxy/Server as the destination then sends the message into the
   secured spanning tree.

   For each source S that sends multicast traffic to group G via R,
   Client S* that aggregates S (or its Proxy/Server) encapsulates the
   original IP packets in PIM Register messages, includes the PIM
   Register messages in the OMNI options of uNA messages, performs OAL
   encapsulation and fragmentation then forwards the resulting carrier
   packets with Identification values within the receive window for
   Client R* that aggregates R.  Client R* may then elect to send a PIM
   Join to S* in the OMNI option of a uNA over the secured spanning
   tree.  This will result in an (S,G) tree rooted at S* 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-existing (*, G) tree that still uses uNA PIM Register
   encapsulation.  R can then issue a uNA PIM Register-stop message over
   the secured spanning tree to suppress the Register-encapsulated
   stream.  At some later time, if Client S* moves to a new Proxy/
   Server, it resumes sending original IP packets via uNA PIM Register
   encapsulation via the new Proxy/Server.

   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 3.16.1.  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.

3.16.3.  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.

3.17.  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

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   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 (see: Section 3.2.4).

   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 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 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.

3.18.  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.

3.19.  Transition/Coexistence 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

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   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 Bridges on each INET partition, with each Bridge
   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/
   coexistence service.  Or, if an INET partition is transitioned to a
   native IP protocol version and addressing scheme that is compatible
   with the OMNI link MNP-based addressing scheme, the partition and
   OMNI link can be joined by Bridges.

   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].

3.20.  Proxy/Server-Bridge Bidirectional Forwarding Detection

   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.

3.21.  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.

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   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]) 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.

   Many AERO/OMNI functions are implemented and undergoing final
   integration.  OAL fragmentation/reassembly buffer management code has
   been cleared for public release.

5.  IANA Considerations

   The IANA has assigned the UDP port number "8060" for an earlier
   experimental first version of AERO [RFC6706].  This document together
   with [I-D.templin-6man-omni] reclaims UDP port number "8060" as the
   service port for UDP/IP encapsulation.  This document makes no
   request of IANA, since [I-D.templin-6man-omni] already provides
   instructions.  (Note: although [RFC6706] was not widely implemented
   or deployed, it need not be obsoleted since its messages use the
   invalid ICMPv6 message type number '0' which implementations of this
   specification can easily distinguish and ignore.)

   No further IANA actions are required.

6.  Security Considerations

   AERO Bridges configure secured tunnels with AERO Proxy/Servers and
   Relays 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 secured spanning tree topology.  Therefore, control
   messages exchanged between any pair of OMNI link neighbors over the
   secured spanning tree are already protected.

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   To prevent spoofing vectors, Proxy/Servers MUST discard without
   responding to any unsecured NS/NA(AR) messages.  Also, Proxy/Servers
   MUST discard without forwarding any original IP packets received from
   one of their own Clients (whether directly or following OAL
   reassembly) with a source address that does not match the Client's
   MNP and/or a destination address that does match the Client's MNP.
   Finally, Proxy/Servers MUST discard without forwarding any carrier
   packets with an OAL source and destination that both match the same
   MNP.

   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 Client neighbors.
   Option 1) would result in longer routes than necessary and impose
   traffic concentration on critical infrastructure elements.  Option 2)
   could be coordinated between Clients using NS/NA messages with OMNI
   Host Identity Protocol (HIP) "Initiator/Responder" message sub-
   options [RFC7401][I-D.templin-6man-omni] to create a secured tunnel
   on-demand, or to use the QUIC-TLS protocol to establish a secured
   connection [RFC9000][RFC9001][RFC9002].

   AERO Clients that connect to secured ANETs need not apply security to
   their IPv6 ND messages, since the messages will be authenticated and
   forwarded by a perimeter Proxy/Server that applies security on its
   INET-facing interface as part of the secured 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 to a Proxy/Server.  When a VPN or
   direct link may be impractical, however, INET Clients and Proxy/
   Servers SHOULD include and verify authentication signatures for their
   IPv6 ND messages as specified in [I-D.templin-6man-omni].

   Application endpoints SHOULD use transport-layer (or higher-layer)
   security services such as QUIC-TLS, 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 through the AERO/OMNI data

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   origin authentication procedures, as well as 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.)

   The PRL MUST be well-managed and secured from unauthorized tampering,
   even though the list contains only public information.  The PRL can
   be conveyed to the Client in a similar fashion as in [RFC5214] (e.g.,
   through layer 2 data link login messaging, secure upload of a static
   file, DNS lookups, etc.).

   The AERO service for open INET Clients depends on a public key
   distribution service in which Client public keys and identities are
   maintained in a shared database accessible to all open INET Proxy/
   Servers.  Similarly, each Client must be able to determine the public
   key of each Proxy/Server, e.g. by consulting an online database.
   When AERO nodes register their public keys indexed by a unique Host
   Identity Tag (HIT) [RFC7401] in a distributed database such as the
   DNS, and use the HIT as an identity for applying IPv6 ND message
   authentication signatures, a means for determining public key
   attestation is available.

   Security considerations for IPv6 fragmentation and reassembly are
   discussed in [I-D.templin-6man-omni].  In environments where spoofing
   is considered a threat, OMNI nodes SHOULD employ Identification
   window synchronization and OAL destinations SHOULD configure an (end-
   system-based) firewall.

   SRH authentication facilities are specified in [RFC8754].  Security
   considerations for accepting link-layer ICMP messages and reflected
   packets are discussed throughout the document.

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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, Scott Burleigh,
   Brian Carpenter, Wojciech Dec, Pavel Drasil, Ralph Droms, Adrian
   Farrel, Nick Green, Sri Gundavelli, Brian Haberman, Bernhard Haindl,
   Joel Halpern, Tom Herbert, Bob Hinden, Sascha Hlusiak, Lee Howard,
   Christian Huitema, Zdenek Jaron, Andre Kostur, Hubert Kuenig, Ted
   Lemon, Andy Malis, Satoru Matsushima, Tomek Mrugalski, Thomas Narten,
   Madhu Niraula, Alexandru Petrescu, Behcet Saikaya, Michal Skorepa,
   Dave Thaler, 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, Kyle Mikos, 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, Madhuri Madhava Badgandi, Vijayasarathy Rajagopalan, Katie
   Tran and Eric Yeh are especially acknowledged for their work on the
   AERO implementation.  Chuck Klabunde is honored and remembered for
   his early leadership, and we mourn his untimely loss.

   This work was inspired by the support and encouragement of countless
   outstanding colleagues, managers and program directors over the span
   of many decades.  Beginning in the late 1980s,' the Digital Equipment
   Corporation (DEC) Ultrix Engineering and DECnet Architects groups
   identified early issues with fragmentation and bridging links with
   diverse MTUs.  In the early 1990s, engagements at DEC Project Sequoia
   at UC Berkeley and the DEC Western Research Lab in Palo Alto included
   investigations into large-scale networked filesystems, ATM vs
   Internet and network security proxys.  In the mid-1990s to early
   2000s employment at the NASA Ames Research Center (Sterling Software)
   and SRI International supported early investigations of IPv6, ONR UAV
   Communications and the IETF.  An employment at Nokia where important
   IETF documents were published gave way to a present-day engagement

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   with The Boeing Company.  The work matured at Boeing through major
   programs including Future Combat Systems, Advanced Airplane Program,
   DTN for the International Space Station, Mobility Vision Lab, CAST,
   Caravan, Airplane Internet of Things, the NASA UAS/CNS program, the
   FAA/ICAO ATN/IPS program and many others.  An attempt to name all who
   gave support and encouragement would double the current document size
   and result in many unintentional omissions - but to all a humble
   thanks.

   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

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8.1.  Normative References

   [I-D.templin-6man-omni]
              Templin, F. L. and T. Whyman, "Transmission of IP Packets
              over Overlay Multilink Network (OMNI) Interfaces", draft-
              templin-6man-omni-47 (work in progress), September 2021.

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

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

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

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <https://www.rfc-editor.org/info/rfc2473>.

   [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>.

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

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

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

   [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-26 (work in progress), May 2021.

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   [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-04 (work in progress), October 2021.

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

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

   [I-D.ietf-ipwave-vehicular-networking]
              (editor), J. (. J., "IPv6 Wireless Access in Vehicular
              Environments (IPWAVE): Problem Statement and Use Cases",
              draft-ietf-ipwave-vehicular-networking-24 (work in
              progress), October 2021.

   [I-D.ietf-rtgwg-atn-bgp]
              Templin, F. L., 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-11 (work in progress), July 2021.

   [I-D.templin-6man-dhcpv6-ndopt]
              Templin, F. L., "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. L., "The Subnetwork Encapsulation and
              Adaptation Layer (SEAL)", draft-templin-intarea-seal-68
              (work in progress), January 2014.

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

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

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

   [I-D.templin-v6ops-pdhost]
              Templin, F. L., "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.

   [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>.

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   [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>.

   [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>.

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

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

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

   [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>.

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   [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>.

   [RFC6139]  Russert, S., Ed., Fleischman, E., Ed., and F. Templin,
              Ed., "Routing and Addressing in Networks with Global
              Enterprise Recursion (RANGER) Scenarios", RFC 6139,
              DOI 10.17487/RFC6139, February 2011,
              <https://www.rfc-editor.org/info/rfc6139>.

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

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

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

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

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

   [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>.

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   [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>.

   [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>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

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   [RFC9001]  Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
              <https://www.rfc-editor.org/info/rfc9001>.

   [RFC9002]  Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
              May 2021, <https://www.rfc-editor.org/info/rfc9002>.

   [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 3.13 results in the
   creation of NCEs.  The NCE state is set to REACHABLE for at most
   ReachableTime seconds.  In order to refresh the NCE lifetime before
   the ReachableTime timer expires, the specification requires
   implementations to issue a new NS/NA(AR) exchange to reset
   ReachableTime while data packets are still flowing.  However, the
   decision of when to initiate a new NS/NA(AR) exchange and to
   perpetuate the process is left as an implementation detail.

   One possible strategy may be to monitor the NCE 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(AR)
   to receive a new NA(AR).  If no data packets have been sent, wait for
   5 additional seconds and send an immediate NS(AR) if any data packets
   are sent within this "expiration pending" 5 second window.  If no
   additional data packets are sent within the 5 second window, reset
   the NCE state to STALE.

   The monitoring of the neighbor data packet traffic therefore becomes
   an ongoing process during the NCE lifetime.  If the NCE expires,
   future data packets will trigger a new NS/NA(AR) 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 IPv6
   ND messaging is used.  In that case, the Client only transmits

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   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 NCE 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 traffic selectors.  If the Direct
   interface is selected, then the Client's IP packets are transmitted
   directly to the peer without going through an ANET/INET.  If other
   interfaces are selected, then the Client's IP packets are transmitted
   via a different interface, which may result in the inclusion of
   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
   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 INET Proxy/Servers can be standard dedicated server platforms,
   but most often will be deployed as virtual machines in the cloud.
   The only requirements for INET Proxy/Servers are that they can run
   the AERO/OMNI code and have at least one network interface connection
   to the INET.  INET Proxy/Servers must be provisioned, supported and
   managed by the INET administrative authority.  Cost for purchasing,
   configuring and managing cloud Proxy/Servers is nominal especially
   for virtual machines.

   AERO ANET Proxy/Servers are most often standard dedicated server
   platforms with one underlying interface connected to the ANET and a

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   second interface connected to an INET.  As with INET Proxy/Servers,
   the only requirements are that they can run the AERO/OMNI code and
   have at least one interface connection to the INET.  ANET Proxy/
   Servers 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 are simply Proxy/Servers connected to INETs and/or EUNs
   that provide forwarding services for non-MNP destinations.  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.  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, uNA messages may be lost but neighbor cache entries in
   the DEPARTED state will ensure that packet forwarding to the Client's
   new locations will continue for up to DepartTime seconds.

   If a Client is left without a Proxy/Server for a considerable length
   of time (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.

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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 connects to one or more of them.  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,
      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

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   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
   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

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   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 earlier versions to draft-templin-6man-aero-33:

   o  New baseline version with corrections and section reorganizations
      to improve document flow.

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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