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Automatic Extended Route Optimization (AERO)
draft-templin-6man-aero-49

The information below is for an old version of the document.
Document Type This is an older version of an Internet-Draft whose latest revision is Active
Author Fred Templin
Last updated 2022-06-13
Replaces draft-templin-intarea-6706bis
Stream Independent Submission
Formats
Stream ISE state In ISE Review
Consensus boilerplate Unknown
Document shepherd Eliot Lear
IESG IESG state I-D Exists
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Responsible AD (None)
Send notices to rfc-ise@rfc-editor.org
draft-templin-6man-aero-49
Network Working Group                                 F. L. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                              13 June 2022
Expires: 15 December 2022

              Automatic Extended Route Optimization (AERO)
                       draft-templin-6man-aero-49

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 unique-local address format
   for IPv6 Neighbor Discovery (IPv6 ND) messaging over the OMNI virtual
   link.  Router discovery and neighbor coordination are employed for
   network admission and to manage the OMNI link 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,
   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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on 15 December 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Automatic Extended Route Optimization (AERO)  . . . . . . . .  16
     3.1.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .  16
     3.2.  The AERO Service over OMNI Links  . . . . . . . . . . . .  17
       3.2.1.  AERO/OMNI Reference Model . . . . . . . . . . . . . .  17
       3.2.2.  Addressing and Node Identification  . . . . . . . . .  21
       3.2.3.  AERO Routing System . . . . . . . . . . . . . . . . .  22
       3.2.4.  Segment Routing Topologies (SRTs) . . . . . . . . . .  24
       3.2.5.  Segment Routing For OMNI Link Selection . . . . . . .  25
     3.3.  OMNI Interface Characteristics  . . . . . . . . . . . . .  25
     3.4.  OMNI Interface Initialization . . . . . . . . . . . . . .  28
       3.4.1.  AERO Proxy/Server and Relay Behavior  . . . . . . . .  28
       3.4.2.  AERO Client Behavior  . . . . . . . . . . . . . . . .  29
       3.4.3.  AERO Host Behavior  . . . . . . . . . . . . . . . . .  29
       3.4.4.  AERO Gateway Behavior . . . . . . . . . . . . . . . .  30
     3.5.  OMNI Interface Neighbor Cache Maintenance . . . . . . . .  30
       3.5.1.  OMNI ND Messages  . . . . . . . . . . . . . . . . . .  32
       3.5.2.  OMNI Neighbor Advertisement Message Flags . . . . . .  34
       3.5.3.  OMNI Neighbor Window Synchronization  . . . . . . . .  34
     3.6.  OMNI Interface Encapsulation and Fragmentation  . . . . .  35
     3.7.  OMNI Interface Decapsulation  . . . . . . . . . . . . . .  37
     3.8.  OMNI Interface Data Origin Authentication . . . . . . . .  38
     3.9.  OMNI Interface MTU  . . . . . . . . . . . . . . . . . . .  39
     3.10. OMNI Interface Forwarding Algorithm . . . . . . . . . . .  39
       3.10.1.  Host Forwarding Algorithm  . . . . . . . . . . . . .  41
       3.10.2.  Client Forwarding Algorithm  . . . . . . . . . . . .  41
       3.10.3.  Proxy/Server and Relay Forwarding Algorithm  . . . .  42
       3.10.4.  Gateway Forwarding Algorithm . . . . . . . . . . . .  45
     3.11. OMNI Interface Error Handling . . . . . . . . . . . . . .  46
     3.12. AERO Mobility Service Coordination  . . . . . . . . . . .  49
       3.12.1.  AERO Service Model . . . . . . . . . . . . . . . . .  49
       3.12.2.  AERO Host and Client Behavior  . . . . . . . . . . .  50
       3.12.3.  AERO Proxy/Server Behavior . . . . . . . . . . . . .  52
     3.13. AERO Address Resolution, Multilink Forwarding and Route
            Optimization . . . . . . . . . . . . . . . . . . . . . .  58
       3.13.1.  Multilink Address Resolution . . . . . . . . . . . .  59

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       3.13.2.  Multilink Forwarding . . . . . . . . . . . . . . . .  64
       3.13.3.  Rapid Commit Route Optimization  . . . . . . . . . .  76
       3.13.4.  Client/Gateway Route Optimization  . . . . . . . . .  77
       3.13.5.  Client/Client Route Optimization . . . . . . . . . .  79
       3.13.6.  Client-to-Client OMNI Link Extension . . . . . . . .  80
       3.13.7.  Intra-ANET/ENET Route Optimization for AERO Peers  .  81
     3.14. Neighbor Unreachability Detection (NUD) . . . . . . . . .  81
     3.15. Mobility Management and Quality of Service (QoS)  . . . .  83
       3.15.1.  Mobility Update Messaging  . . . . . . . . . . . . .  84
       3.15.2.  Announcing Link-Layer Information Changes  . . . . .  85
       3.15.3.  Bringing New Links Into Service  . . . . . . . . . .  85
       3.15.4.  Deactivating Existing Links  . . . . . . . . . . . .  85
       3.15.5.  Moving Between Proxy/Servers . . . . . . . . . . . .  86
     3.16. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  87
       3.16.1.  Source-Specific Multicast (SSM)  . . . . . . . . . .  87
       3.16.2.  Any-Source Multicast (ASM) . . . . . . . . . . . . .  88
       3.16.3.  Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . .  89
     3.17. Operation over Multiple OMNI Links  . . . . . . . . . . .  89
     3.18. DNS Considerations  . . . . . . . . . . . . . . . . . . .  90
     3.19. Transition/Coexistence Considerations . . . . . . . . . .  90
     3.20. Proxy/Server-Gateway Bidirectional Forwarding
            Detection  . . . . . . . . . . . . . . . . . . . . . . .  91
     3.21. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . .  91
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  92
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  92
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  92
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  95
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  96
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  96
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  98
   Appendix A.  Non-Normative Considerations . . . . . . . . . . . . 106
     A.1.  Implementation Strategies for Route Optimization  . . . . 106
     A.2.  Implicit Mobility Management  . . . . . . . . . . . . . . 106
     A.3.  Direct Underlying Interfaces  . . . . . . . . . . . . . . 107
     A.4.  AERO Critical Infrastructure Considerations . . . . . . . 107
     A.5.  AERO Server Failure Implications  . . . . . . . . . . . . 108
     A.6.  AERO Client / Server Architecture . . . . . . . . . . . . 109
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . . 111
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 111

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

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   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 manifested by IPv6 encapsulation and
   configured over a network-of-networks concatenation of underlay
   Internetworks.  Nodes on the link can exchange original IP packets as
   single-hop neighbors on the link - both IP protocol versions (IPv4
   and IPv6) are supported.  The OMNI Adaptation Layer (OAL) supports
   multilink operation for increased reliability and path optimization
   while providing fragmentation and reassembly services to support
   improved performance and Maximum Transmission Unit (MTU) diversity.
   This specification provides a mobility service architecture companion
   to the OMNI specification.

   The AERO service connects Hosts and Clients as OMNI link neighbors
   via Proxy/Servers and Relays as intermediate nodes as necessary; AERO
   further provides Gateways that interconnect diverse Internetworks as
   OMNI link segments through OAL forwarding at a layer below IP.  Each
   node's OMNI interface uses an IPv6 unique-local address format that
   supports operation of the IPv6 Neighbor Discovery (IPv6 ND) protocol
   [RFC4861].  A Client's OMNI interface can be configured over multiple
   underlay 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 using stable IP Addresses not subject to dynamic
   fluctuations.

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   AERO Gateways peer with Proxy/Servers in a secured private BGP
   overlay routing instance to establish a Segment Routing Topology
   (SRT) spanning tree over the underlay Internetworks of one or more
   disjoint administrative domains concatenated 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 underlay
   Internetworks to (mobile or fixed) nodes on the OMNI link.  From the
   perspective of underlay Internetworks, each Relay appears as 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 through a service known
   as "route optimization" is therefore provided.

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

   AERO 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 underlay data links as a single logical unit (i.e., the
       OMNI interface) to achieve the required communications
       performance and reliability objectives.

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   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, other mobile
       Clients, 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 underlay 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
       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] in conjunction with the
      OMNI 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

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      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.

   L2
      The Data Link layer in the OSI network model.  Also known as
      "layer-2", "link-layer", "sub-IP layer", etc.

   L3
      The Network layer in the OSI network model.  Also known as "layer-
      3", "IP layer", etc.

   Adaptation layer
      A mid-layer that adapts L3 to a diverse collection of L2 underlay
      interfaces and their encapsulations.  (No layer number is
      assigned, since numbering was an artifact of the legacy reference
      model that need not carry forward in the modern architecture.)
      The adaptation layer sees the upper layer as "L3" and sees all
      lower layer encapsulations as "L2 encapsulations", which may
      include UDP, IP and true link-layer (e.g., Ethernet, etc.)
      headers.

   Access Network (ANET)
      a connected network region (e.g., an aviation radio access
      network, satellite service provider network, cellular operator
      network, WiFi network, etc.) that joins Clients to the Mobility
      Service.  Physical and/or data link level security is assumed, and
      sometimes referred to as "protected spectrum".  Private enterprise
      networks and ground domain aviation service networks may provide
      multiple secured IP hops between the Client's point of connection
      and the nearest Proxy/Server.

   Internetwork (INET)
      a connected network region with a coherent IP addressing plan that
      provides transit forwarding services between ANETs and AERO/OMNI
      nodes that coordinate with the Mobility Service over unprotected
      media.  No physical and/or data link level security is assumed,
      therefore security must be applied by upper layers.  The global
      public Internet itself is an example.

   End-user Network (ENET)
      a simple or complex "downstream" network that travels with the
      Client as a single logical unit.  The ENET could be as simple as a
      single link connecting a single Host, or as complex as a large
      network with many links, routers, bridges and Hosts.  The ENET
      could also provide an "upstream" link in a recursively-descending
      chain of additional Clients and ENETs.  In this way, an ENET of an
      upstream Client is seen as the ANET of a downstream Client.

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   {A,I,E}NET interface
      a node's attachment to a link in an {A,I,E}NET.

   underlay network/interface
      an ANET/INET/ENET network/interface over which an OMNI interface
      is configured.  The OMNI interface is seen as a network layer (L3)
      interface by the IP layer, and the OMNI adaptation layer sees the
      underlay interface as a data link layer (L2) interface.  The
      underlay interface either connects directly to the physical
      communications media or coordinates with another node where the
      physical media is hosted.

   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 underlay network 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 underlay network 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.

   OMNI Adaptation Layer (OAL)
      an OMNI interface sublayer service that encapsulates original IP
      packets admitted into the interface in an IPv6 header and/or
      subjects them to fragmentation and reassembly.  The OAL is also
      responsible for generating MTU-related control messages as
      necessary, and for providing addressing context for spanning
      multiple segments of an extended OMNI link.

   OMNI Interface
      a node's attachment to an OMNI link (i.e., the same as defined in
      [I-D.templin-6man-omni]).  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].

   (network) partition
      frequently, underlay networks 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 partition is seen as a separate OMNI link segment as
      discussed throughout this document.)

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   L2 encapsulation
      the OAL encapsulation of a packet in an outer header or headers
      that can be routed within the scope of the local {A,I,E}NET
      underlay network partition.  Common L2 encapsulation combinations
      include UDP/IP/Ethernet, etc.

   L2 address(es)
      the addresses that appear in the OAL L2 encapsulations for an
      underlay interface.

   INADDR
      the UDP/IP addresses that appear in an L2 address.

   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 an OAL IPv6 header before
      OAL fragmentation, or following OAL reassembly.

   OAL fragment
      a portion of an OAL packet following fragmentation but prior to L2
      encapsulation, or following L2 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 L2 encapsulation or prior
      to L2 decapsulation.  OAL sources and destinations exchange
      carrier packets over underlay interfaces, and may be separated by
      one or more OAL intermediate nodes.  OAL intermediate nodes re-
      encapsulate carrier packets during forwarding by removing the L2
      headers of the previous hop underlay network and replacing them
      with new L2 headers for the next hop underlay 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 L2 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 L2 headers of carrier packets received from a previous hop,
      then re-encapsulates the carrier packets in new L2 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.

   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.

   Interface Identifier (IID)
      the least significant 64 bits of an IPv6 address, as specified in
      the IPv6 addressing architecture [RFC4291].

   Link Local Address (LLA)
      an IPv6 address beginning with fe80::/64 per the IPv6 addressing
      architecture [RFC4291] and with either a 64-bit MNP (LLA-MNP) or a
      56-bit random value (LLA-RND) encoded in the IID as specified in
      [I-D.templin-6man-omni].

   Unique Local Address (ULA)
      an IPv6 address beginning with fd00::/8 followed by a 40-bit
      Global ID followed by a 16-bit Subnet ID per [RFC4193] and with
      either a 64-bit MNP (ULA-MNP) or a 56-bit random value (ULA-RND)
      encoded in the IID as specified in [I-D.templin-6man-omni].  (Note
      that [RFC4193] specifies a second form of ULAs based on the prefix
      fc00::/8, which are referred to as "ULA-C" throughout this
      document to distinguish them from the ULAs defined here.)

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   Temporary Local Address (TLA)
      a ULA beginning with fd00::/16 followed by a 48-bit randomly-
      initialized value followed by an MNP-based (TLA-MNP) or random
      (TLA-RND) IID as specified in [I-D.templin-6man-omni].  Clients
      use TLAs to bootstrap autoconfiguration in the presence of OMNI
      link infrastructure or for sustained communications in the absence
      of infrastructure.  (Note that in some environments Clients can
      instead use a (Hierarchical) Host Identity Tag ((H)HIT) instead of
      a TLA - see: [I-D.templin-6man-omni].)

   eXtended Local Address (XLA)
      a ULA beginning with fd00::/64 followed by an MNP-based (XLA-MNP)
      or random (XLA-RND) IID as specified in [I-D.templin-6man-omni].
      An XLA can be used to supply a stable address for IPv6 ND
      messaging, a routing table entry for the OMNI link routing system,
      etc.  (Note that XLAs can also be statelessly formed from LLAs
      (and vice-versa) simply by inverting prefix bits 7 and 8.)

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

   AERO Host ("Host")
      an AERO node that configures an OMNI interface over an ENET
      underlying interface serviced by an upstream Client.  The Host
      does not assign an LLA or ULA to the OMNI interface, but instead
      assigns the address taken from the ENET underlying interface.  (As
      an implementation matter, the Host may instead configure the "OMNI
      interface" as a virtual sublayer of the underlay interface
      itself.)  When an AERO host forwards an original IP packet to
      another AERO node on the same ENET, it uses simple IP-in-IP
      encapsulation without including an OAL encapsulation header.  The
      Host is therefore an OMNI link termination endpoint.

   AERO Client ("Client")
      an AERO node that configures an OMNI interface over one or more
      underlay interfaces and requests MNP delegation/registration
      service from AERO Proxy/Servers.  The Client assigns an XLA-MNP
      (as well as Proxy/Server-specific ULA-MNPs) 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.  The Client coordinates with Proxy/Servers
      and/or other Clients over upstream ANET/INET interfaces and may
      also provide Proxy/Server services for Hosts and/or other Clients
      over downstream ENET interfaces.

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   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 a ULA-RND 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 AERO Gateways.

   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 assign a ULA-RND to an OMNI interface and
      maintain BGP peerings with Gateways the same as Proxy/Servers.
      Relays also run a dynamic routing protocol to discover any non-MNP
      IP GUA routes in service on other links/networks, advertise OMNI
      link MSP(s) to other links/networks, and redistribute routes
      discovered on other links/networks into the OMNI link BGP routing
      system.  (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 Gateway ("Gateway")
      a BGP hub autonomous system node that also provides OAL forwarding
      services for nodes on an OMNI link.  Gateways forward 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.  Gateways peer
      with Proxy/Servers and other Gateways to form an IPv6-based OAL
      spanning tree over all OMNI link segments and to discover the set
      of all MNP and non-MNP prefixes in service.  Gateways process
      carrier packets received over the secured spanning tree that are
      addressed to themselves, while forwarding all other carrier
      packets to the next hop also via the secured spanning tree.
      Gateways 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) Client
      a Client that initiates communications with a target peer by
      sending an NS message to establish reverse-path multilink
      forwarding state in OMNI link intermediate nodes on the path to
      the target.  Note that in some arrangements the Client's (FHS)
      Proxy/Server (and not the Client itself) initiates the NS.

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   Last-Hop Segment (LHS) Client
      a Client that responds to a communications request from a source
      peer's NS by returning an NA response to establish forward-path
      multilink forwarding state in OMNI link intermediate nodes on the
      path to the source.  Note that in some arrangements the Client's
      (LHS) Proxy/Server (and not the Client itself) returns the NA.

   First-Hop Segment (FHS) Proxy/Server
      a Proxy/Server for an FHS Client's underlay 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.

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

   Hub Proxy/Server
      a single Proxy/Server selected by a Client that injects the
      Client's XLA-MNP into the BGP routing system and provides a
      designated router service for all of the Client's underlay
      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 as a Hub), 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).

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

   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.

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   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.

   Address Resolution Source (ARS)
      the node nearest the original source that initiates OMNI link
      address resolution.  The ARS may be a Proxy/Server or Relay for
      the source, or may be the source Client itself.  The ARS is often
      (but not always) also the same node that becomes the FHS source
      during route optimization.

   Address Resolution Target (ART)
      the node toward which address resolution is directed.  The ART may
      be a Relay or the target Client itself.  The ART is often (but not
      always) also the same node that becomes the LHS target during
      route optimization.

   Address Resolution Responder (ARR)
      the node that responds to address resolution requests on behalf of
      the ART.  The ARR may be a Relay, the ART itself, or the ART's
      current Hub Proxy/Server.  Note that a Hub Proxy/Server can assume
      the ARR role even if it is located on a different SRT segment than
      the ART.

   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 Gateways that tracks all Proxy/Server-to-Client associations.

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

   AERO Forwarding Information Base (AFIB)
      A forwarding table on each OAL source, destination and
      intermediate node that includes AERO Forwarding Vectors (AFV) with
      both multilink forwarding instructions and context for
      reconstructing compressed headers for specific communicating peer
      underlay interface pairs.  The AFIB also supports route
      optimization where one or more OAL intermediate nodes in the path
      can be "skipped" to reduce path stretch and decrease load on
      critical infrastructure elements.

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   AERO Forwarding Vector (AFV)
      An AFIB entry that includes soft state for each underlay interface
      pairwise communication session between peer OAL nodes.  AFVs are
      identified by both a forward and reverse path AFV Index (AFVI).
      OAL nodes establish reverse path AFVIs when they forward an IPv6
      ND unicast Neighbor Solicitation (NS) message and establish
      forward path AFVIs when they forward the solicited IPv6 ND unicast
      Neighbor Advertisement (NA) response.

   AERO Forwarding Vector Index (AFVI)
      A locally-unique 4 octet value automatically generated by an OAL
      node when it creates an AFV.  OAL intermediate nodes assign two
      distinct AFVIs (called "A" and "B") to each AFV, with "A"
      representing the forward path and "B" representing the reverse
      path.  Meanwhile, the OAL source assigns a single "B" AFVI, and
      the OAL destination assigns a single "A" AFVI.  Each OAL node
      advertises its "A" AFVI to previous hop nodes on the reverse path
      toward the source and advertises its "B" AFVI to next hop nodes on
      the forward path toward the destination.

   AERO Forwarding Parameters (AFP)
      An OMNI option sub-option that appears in IPv6 ND NS/NA messages
      and includes all parameters necessary for establishing AFV state
      in OAL nodes in the path (see: [I-D.templin-6man-omni]).

   Throughout the document, the simple terms "Host", "Client", "Proxy/
   Server", "Gateway" and "Relay" refer to "AERO/OMNI Host", "AERO/OMNI
   Client", "AERO/OMNI Proxy/Server", "AERO/OMNI Gateway" and "AERO/OMNI
   Relay", respectively.  Capitalization is used to distinguish these
   terms from other common Internetworking uses in which they appear
   without capitalization, and implies that the node in question
   configures and uses an OMNI interface.

   The terminology of IPv6 ND [RFC4861], DHCPv6 [RFC8415] and OMNI
   [I-D.templin-6man-omni] (including the names of node variables,
   messages and protocol constants) is used throughout this document.
   The terms "All-Routers multicast", "All-Nodes multicast", "Solicited-
   Node multicast" and "Subnet-Router anycast" are defined in [RFC4291].
   Also, the term "IP" is used to generically refer to either Internet
   Protocol version, i.e., IPv4 [RFC0791] or IPv6 [RFC8200].

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

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3.  Automatic Extended Route Optimization (AERO)

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

3.1.  AERO Node Types

   AERO Hosts configure an OMNI interface over an underlay interface
   connected to a Client's ENET and coordinate with both other AERO
   Hosts and Clients over the ENET.  As an implementation matter, the
   Host either assigns the same (MNP-based) IP address from the underlay
   interface to the OMNI interface, or configures the "OMNI interface"
   as a virtual sublayer of the underlay interface itself.  AERO Hosts
   treat the ENET as an ANET, and treat the upstream Client for the ENET
   as a Proxy/Server.  AERO Hosts are seen as OMNI link termination
   endpoints.

   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 underlay 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 ENETs (which
   may connect AERO Hosts and other Clients).  AERO Clients provide
   Proxy/Server-like services for Hosts and other Clients on downstream-
   attached ENETs.

   AERO Gateways, 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 and Hosts.  AERO Gateways (together with
   Proxy/Servers) provide the secured backbone supporting infrastructure
   for a Segment Routing Topology (SRT) spanning tree for the OMNI link.

   AERO Gateways 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 Gateway also peers with Proxy/Servers and other
   Gateways 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).  Gateways assign one or more Mobility
   Service Prefixes (MSPs) to the OMNI link and configure secured
   tunnels with Proxy/Servers, Relays and other Gateways; they further
   maintain forwarding table entries for each MNP or non-MNP prefix in
   service on the OMNI link.

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   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 Gateways 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 and Hosts 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 Gateway.  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.

   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 Gateway 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:

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                         +-----------------+
                         | AERO Gateway G1 |
                         | Nbr: S1, S2, P1 |
                         |(X1->S1; X2->S2) |
                         |      MSP M1     |
                         +--------+--------+
       +--------------+           |            +--------------+
       |  AERO P/S S1 |           |            |  AERO P/S S2 |
       |  Nbr: C1, G1 |           |            |  Nbr: C2, G1 |
       |  default->G1 |           |            |  default->G1 |
       |    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   )-.   |  AERO |     |  AERO |    .-(_  IP   )-.
   (__    ENET     )--|Host H1|     |Host H2|--(__    ENET     )
      `-(______)-'    +-------+     +-------+     `-(______)-'

                    Figure 1: AERO/OMNI Reference Model

   In this model:

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

   *  AERO Gateway G1 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).
      Gateways provide the backbone for an SRT spanning tree for the
      OMNI link.

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   *  AERO Proxy/Servers S1 and S2 configure secured tunnels with
      Gateway G1 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.)

   *  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 ENETs.

   *  AERO Hosts H1 and H2 attach to the ENETs served by Clients C1 and
      C2, respectively.

   An OMNI link configured over a single underlay network appears as a
   single unified link with a consistent addressing plan; all nodes on
   the link can exchange carrier packets via simple L2 encapsulation
   (i.e., following any necessary NAT traversal) since the underlay is
   connected.  In common practice, however, OMNI links are often
   configured over an SRT spanning tree that bridges multiple distinct
   underlay network segments managed under different administrative
   authorities (e.g., as for worldwide aviation service providers such
   as ARINC, SITA, Inmarsat, etc.).  Individual underlay networks 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
   Gateways.

   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:

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

             Figure 2: OMNI Link Segment Routing Topology (SRT)

   Gateway, Proxy/Server and Relay OMNI interfaces are configured over
   both secured tunnels and open INET underlay interfaces within their
   respective SRT segments.  Within each segment, Gateways configure
   "hub-and-spokes" BGP peerings with Proxy/Servers and Relays as
   "spokes".  Adjacent SRT segments are joined by Gateway-to-Gateway
   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
   carrier packets to take more direct paths between OMNI link neighbors
   without having to follow strict spanning tree paths.

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   The AERO Multinet service concatenates SRT segments to form larger
   networks through Gateway-to-Gateway peerings as originally described
   in the "Catenet Model for Internetworking" [IEN48]; especially
   Figure 2 follows directly from the illustrations in [IEN48-2].  The
   original Catenet vision proposed a "network-of-networks"
   concatenation of independent and diverse Internetwork "segments" to
   form a much larger network supporting end-to-end services.

   The original Catenet vision first articulated in the 1970's was
   distorted through the evolution of the Internet in later decades,
   since a critical element was missing from the architecture.  As a
   result, the Internet evolved as a single, large public routing and
   addressing domain interconnecting private domains (i.e., instead of a
   true network-of-networks) which has impeded flexibility and inhibited
   end-to-end services.  With the advent of the adaptation layer
   established by the AERO/OMNI services, however, the original Catenet
   "network-of-networks" vision is now made possible.

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 to the OMNI interface in a latent
   state and do not employ LLAs for any operational purposes (instead,
   nodes assign LLAs solely to satisfy the requirements of [RFC4861]).
   AERO Clients configure LLAs constructed from MNPs (i.e., "LLA-MNPs")
   while AERO infrastructure nodes construct LLAs based on 56-bit random
   values ("LLA-RNDs") per [I-D.templin-6man-omni].  Non-MNP routes are
   also represented the same as for MNPs, 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 Global ID to form the prefix
   {ULA}::/48, then include a 16-bit Subnet ID '*' to form the prefix
   {ULA*}::/64 [RFC4291].  The AERO node then uses the prefix
   {ULA*}::/64 to form "ULA-MNPs" or "ULA-RNDs" as specified in
   [I-D.templin-6man-omni] to support OAL addressing.  (The prefix
   {ULA*}::/64 appearing alone and with no suffix represents "default"
   for that prefix.)

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   AERO Clients also use Temporary Local Addresses (TLAs) and eXtended
   Local Addresses (XLAs) constructed per [I-D.templin-6man-omni], where
   TLAs are distinguished from ordinary ULAs based on the prefix
   fd00::/16 and XLAs are distinguished from ULAs/TLAs based on the
   prefix fd00::/64.  Clients use TLA-RNDs only in initial control
   message exchanges until a stable MNP is assigned, but may sometimes
   also use them for sustained communications within a local routing
   region.  AERO nodes use XLA-MNPs to provide forwarding information
   for the global routing table as well as IPv6 ND message addressing
   information.

   AERO MSPs, MNPs and non-MNP routes are typically based on Global
   Unicast Addresses (GUAs), but in some cases may be based on IPv4
   private addresses [RFC1918] or IPv6 ULA-C's [RFC4193].  A GUA block
   is also reserved for OMNI link anycast purposes.  See
   [I-D.templin-6man-omni] for a full specification of LLAs, ULAs, TLAs,
   XLAs, GUAs and anycast addresses 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 Gateways and Proxy/
   Servers (Relays also engage in the routing system as simplified
   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 connected 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
   with one or more Gateways but does not peer with other Proxy/Servers.
   Each SRT segment in the OMNI link must include one or more Gateways
   in a "hub" AS, which peer with the Proxy/Servers within that segment
   as "spoke" ASes.  All Gateways 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 Gateways of different
   segments peer with one another using eBGP.

   Gateways maintain forwarding table entries only for ULA prefixes for
   infrastructure elements and XLA-MNPs corresponding to MNP and non-MNP
   routes that are currently active; Gateways also maintain black-hole
   routes for the OMNI link MSPs so that carrier packets destined to
   non-existent more-specific routes are dropped with a Destination

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   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 Gateways
   which have full topology knowledge.

   Each OMNI link segment assigns a unique sub-prefix of {ULA}::/48
   known as the "SRT prefix".  For example, a first segment could assign
   {ULA}:1000::/56, a second could assign {ULA}:2000::/56, a third could
   assign {ULA}:3000::/56, etc.  Within each segment, each Proxy/Server
   configures a ULA-RND within the segment's SRT prefix with a 56-bit
   random value in the interface identifier as specified in
   [I-D.templin-6man-omni].

   The administrative authorities for each segment must therefore
   coordinate to assure mutually-exclusive ULA prefix assignments, but
   internal provisioning of ULAs is an independent local consideration
   for each administrative authority.  For each ULA prefix, the
   Gateway(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::/64 is simply
   {ULA}:1023::/64.

   ULA prefixes are statically represented in Gateway forwarding tables.
   Gateways 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 ULA prefixes either via standard BGP routing or
   static routes.  For example, if three Gateways ('A', 'B' and 'C')
   from different segments serviced {ULA}:1000::/60, {ULA}:2000::/56 and
   {ULA}:3000::/56 respectively, then the forwarding tables in each
   Gateway appear as follows:

   A:  {ULA}:1000::/56->local, {ULA}:2000::/56->B, {ULA}:3000::/56->C

   B:  {ULA}:1000::/56->A, {ULA}:2000::/56->local, {ULA}:3000::/56->C

   C:  {ULA}:1000::/56->A, {ULA}:2000::/56->B, {ULA}:3000::/56->local

   These forwarding table entries rarely change, since they correspond
   to fixed infrastructure elements in their respective segments.

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   MNP (and non-MNP) routes are instead dynamically advertised in the
   AERO routing system by Proxy/Servers and Relays that provide service
   for their corresponding MNPs.  The routes are advertised as XLA-MNP
   prefixes, i.e., as fd00::{MNP} (see: [I-D.templin-6man-omni]).  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:  fd00::2001:db8:1000:2000/120

   E:  fd00::2001:db8:3000:4000/120

   F:  fd00::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 distinct {ULA}::/48 prefixes in an OMNI link domain identify
   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 the 40-bit ULA Global
   ID field 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 Gateways and Proxy/Servers of each independent SRT engage in BGP
   peerings to form a spanning tree with the Gateways in non-leaf nodes
   and the Proxy/Servers in leaf nodes.  The spanning tree is configured
   over both secured and unsecured underlay network paths.  The secured
   spanning tree is used to convey secured control messages between
   Proxy/Servers and Gateways, while the unsecured spanning tree
   forwards data messages and/or unsecured control messages.

   Each SRT segment is identified by a unique ULA prefix used by all
   Proxy/Servers and Gateways 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.

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   Note: The distinct ULA prefixes in an OMNI link domain can be carried
   either in a common BGP routing protocol instance for all OMNI links
   or in distinct BGP routing protocol instances for different OMNI
   links.  In some SBM environments, such separation may be necessary to
   ensure that distinct OMNI links do not include any common
   infrastructure elements as single points of failure.  In other
   environments, carrying the ULAs of multiple OMNI links within a
   common routing system may be acceptable.

3.2.5.  Segment Routing For OMNI Link Selection

   Original IPv6 sources 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
   underlay interfaces classified as follows:

   *  ANET interfaces connect to a protected and secured ANET that is
      separated from the open INET by Proxy/Servers.  The ANET interface
      may be either on the same L2 link segment as a Proxy/Server, or
      separated from a Proxy/Server by multiple IP hops.  (Note that
      NATs may appear internally within an ANET and may require NAT
      traversal on the path to the Proxy/Server the same as for the INET
      case.)

   *  INET interfaces connect to an INET either natively or through one
      or several IPv4 Network Address Translators (NATs).  Native INET
      interfaces have global IP addresses that are reachable from any
      INET correspondent.  NATed INET interfaces typically have private
      IP addresses and connect to a private network behind one or more
      NATs with the outermost NAT providing INET access.

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   *  ENET interfaces connect a Client's downstream-attached networks,
      where the Client provides forwarding services for ENET Host and
      Client communications to remote peers.  An ENET can be as simple
      as a small stub network that travels with a mobile Client (e.g.,
      an Internet-of-Things) to as complex as a large private enterprise
      network that the Client connects to a larger ANET or INET.

   *  VPNed interfaces use security encapsulation over an underlay
      network to a Client or Proxy/Server acting as a Virtual Private
      Network (VPN) gateway.  Other than the link-layer encapsulation
      format, VPNed interfaces behave the same as for Direct interfaces.

   *  Direct (aka "point-to-point") interfaces connect directly to a
      Client or Proxy/Server without crossing any networked paths.  An
      example is a line-of-sight link between a remote pilot and an
      unmanned aircraft.

   OMNI interfaces use OAL encapsulation and fragmentation as discussed
   in Section 3.6.  OMNI interfaces use L2 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 use link-layer encapsulation only (i.e., and no other
   L2 encapsulations) over Direct underlay interfaces or ANET interfaces
   when the Client and FHS Proxy/Server are known to be on the same
   underlay 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), Neighbor Advertisement (NA) and
   Redirect for neighbor cache management.  In environments where
   spoofing may be a threat, OMNI neighbors should invoke 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 AERO
   Forwarding Parameters (AFPs) containing link information parameters
   for the OMNI interface's underlay interfaces and any other per-
   neighbor information.

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   A Host's OMNI interface is configured over an underlay interface
   connected to an ENET provided by an upstream Client.  From the Host's
   perspective, the ENET appears as an ANET and the upstream Client
   appears as a Proxy/Server.  The Host does not provide OMNI
   intermediate node services and is therefore a logical termination
   point for the OMNI link.

   A Client's OMNI interface may be configured over multiple ANET/INET
   underlay 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.

   If a Client's multiple ANET/INET underlay 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 AFP sub-options with the same underlay interface index.
   In that case, the Client would appear to have a single underlay
   interface but with a dynamically changing link-layer address.

   If the Client has multiple active ANET/INET underlay 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 AFP sub-options with different
   underlay interface indexes.

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

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   Gateway and Proxy/Server OMNI interfaces are configured over underlay
   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 a ULA-RND and acts
   as an OAL source to encapsulate original IP packets, then fragments
   the resulting OAL packets and presents the resulting carrier packets
   over the secured or unsecured underlay paths.  Note that Gateway and
   Proxy/Server end-to-end transport protocol sessions used by the BGP
   run directly over the OMNI interface and use ULA-RND source and
   destination addresses.  The ULA-RND addresses that appear in BGP
   protocol session original IP packets may therefore be the same as
   those that appear in the OAL IPv6 encapsulation header.

3.4.  OMNI Interface Initialization

   AERO Proxy/Servers, Clients and Hosts 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
   Hosts and Gateways are discussed in the following sections.

3.4.1.  AERO Proxy/Server and Relay Behavior

   When a Proxy/Server enables an OMNI interface, it assigns a ULA-RND
   appropriate for the given OMNI link SRT segment.  The Proxy/Server
   also configures secured underlay interface tunnels and engages in BGP
   routing protocol sessions over the OMNI interface with one or more
   neighboring Gateways.

   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 underlay 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/ENET
   interfaces (see: Section 3.2.3).  The Relay provisions MNPs to
   networks on the INET/ENET interfaces (i.e., the same as a Client
   would do) and advertises the MSP(s) for the OMNI link over the INET/
   ENET interfaces.  The Relay further provides an attachment point of
   the OMNI link to a non-MNP-based global topology.

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3.4.2.  AERO Client Behavior

   When a Client enables an OMNI interface, it assigns either an XLA-MNP
   or a TLA and sends OMNI-encapsulated RS messages over its ANET/INET
   underlay 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 TLA in its initial RS messages, it may discover ULA-
   MNPs in the corresponding RAs that it receives from FHS Proxy/Servers
   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 TLAs for Client-to-
   Client communications at least until it encounters an infrastructure
   element that can delegate MNPs.)

   A Client can further extend the OMNI link over its (downstream) ENET
   interfaces where it provides a first-hop router for Hosts and other
   AERO Clients connected to the ENET.  A downstream Client that
   connects via the ENET serviced by an upstream Client can in turn
   service further downstream ENETs that connect other Hosts and
   Clients.  This OMNI link extension can be applied recursively over a
   "chain" of ENET Clients.

3.4.3.  AERO Host Behavior

   When a Host enables an OMNI interface, it assigns an address taken
   from the ENET underlay interface which may itself be a GUA delegated
   by the upstream Client.  The Host does not assign a link-local
   address to the OMNI interface, since no autoconfiguration is
   necessary on that interface.  (As an implementation matter, the Host
   could instead configure the "OMNI interface" as a virtual sublayer of
   the ENET underlay interface itself.)

   The Host sends OMNI-encapsulated RS messages over its ENET underlay
   interface to the upstream Client, which returns encapsulated RAs and
   provides routing services in the same fashion that Proxy/Servers
   provides services for Clients.  Hosts represent the leaf end systems
   in recursively-nested chain of concatenated ENETs, i.e., they
   represent terminating endpoints for the OMNI link.

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3.4.4.  AERO Gateway Behavior

   AERO Gateways configure an OMNI interface and assign a ULA-RND and
   corresponding Subnet Router Anycast address for each OMNI link SRT
   segment they connect to.  Gateways configure underlay interface
   secured tunnels with Proxy/Servers in the same SRT segment and other
   Gateways in the same (or an adjacent) SRT segment.  Gateways then
   engage in a BGP routing protocol session with neighbors over the
   secured spanning tree (see: Section 3.2.3).

3.5.  OMNI Interface Neighbor Cache Maintenance

   Each Client, Proxy/Server and Gateway 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 network layer address of the neighbor and
   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.

   Hosts also maintain NCEs for Clients and other Hosts through the
   exchange of RS/RA, NS/NA or Redirect messages.  Each NCE is indexed
   by the address assigned to the Host ENET interface, which is the same
   address used for OMNI L2 encapsulation (i.e., without the insertion
   of an OAL header).  This encapsulation format identifies the NCE as a
   Host-based entry where the Host is a leaf end system in the
   recursively extended OMNI link.

   Gateways also maintain NCEs for Clients within their local segments
   based on NS/NA route optimization messaging (see: Section 3.13.4).
   When a Gateway creates/updates a NCE for a local segment Client based
   on NS/NA route optimization, it also maintains AFVI 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.

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   Clients determine the service profiles for their FHS and Hub Proxy/
   Servers by setting the N/A/U flags in a Neighbor Coordination sub-
   option of 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 OMNI Neighbor Coordination sub-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 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 underlay Interface Attributes, while FHS Proxy/
   Servers acting in "proxy" mode have knowledge of only the individual
   Client underlay 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.

   When an Address Resolution Source (ARS) sends an NS(AR) message
   toward an Address Resolution Target (ART) Client/Relay, the OMNI link
   routing system directs the NS(AR) to a Hub Proxy/Server for the ART.
   The Hub then either acts as an Address Resolution Responder (ARR) on
   behalf of the ART or forwards the NS(AR) to the ART which acts as an
   ARR on its own behalf.  The ARR returns an NA(AR) response to the
   ARS, which creates or updates a NCE for the ART while caching L3 and
   L2 addressing information.  The ARS then (re)sets ReachableTime for
   the NCE to REACHABLE_TIME seconds and performs unicast NS/NA
   exchanges over specific underlay interface pairs to determine paths
   for forwarding carrier packets directly to the ART.  The ARS
   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].

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   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.

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].  OMNI interfaces use ULAs instead of LLAs as
   IPv6 ND message source and destination addresses.  This allows
   multiple different OMNI links to be joined into a single link at some
   future time without requiring a global renumbering event.

   For each IPv6 ND message, the OMNI interface includes one or more
   OMNI options (and any other ND message options) then completely
   populates 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.  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 underlay 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.

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   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 OMNI options and S/TLLAOs appear, the former
   pertains to encapsulation addresses while the latter pertains to the
   native L2 address format of the underlay 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 a ULA-RND while using unicast or anycast OAL/L2
   addresses.  AERO Proxy/Servers respond by returning unicast RA
   messages.  During the RS/RA exchange, AERO Clients and Proxy/Servers
   include state synchronization parameters to establish Identification
   windows and other state.

   AERO Hosts and Clients on ENET underlay networks send RS messages to
   the link-scoped All-Routers multicast address, a ULA-RND of a remote
   Hub Proxy/Server or the ULA-MNP of an upstream Client while using
   unicast or anycast OAL/L2 addresses.  The upstream AERO Client
   responds by returning a unicast RA message.

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

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

   *  NS/NA(NUD) messages are used determine target reachability and
      optionally to also establish AFIB state.  The source sends an
      NS(NUD) to the unicast address of the target while optionally
      including an OMNI AERO Forwarding Parameters (AFP) sub-option
      naming a specific underlay interface pair, and the target returns
      a unicast NA(NUD) that echoes the AFP (if present).  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 used to establish window synchronization and/or AFIB
      state must be secured.

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   *  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.

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

   Additionally, nodes may set 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:

   *  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.

   *  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: The O ("Override") flag is set to 0 for solicited NAs returned
      by a Proxy/Server ARR 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 XLA-MNPs must be
      uniquely assigned to Clients to support correct IPv6 ND protocol
      operation, however, no role is currently seen for assigning the
      same XLA-MNP 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 by including OMNI Neighbor Coordination
   sub-options in RS/RA or NS/NA message exchanges to maintain send/
   receive window state in their respective neighbor cache entries as

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   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 to produce an OAL packet.  For example, an original IP packet
   with source address 2001:db8:1:2::1 and destination address
   2001:db8:1234:5678::1 might cause the OAL encapsulation header to
   include source address {XLA*}::2001:db8:1:2 (i.e., an XLA-MNP) and
   destination address {ULA*}::0012:3456:789a:bcde (i.e., a ULA-RND).

   Following encapsulation, the OAL source then calculates and includes
   a 2-octet OAL checksum, then fragments the OAL packet while including
   an identical Identification value for each fragment that must be
   within the window for the neighbor.  This fragmentation causes the
   OAL checksum to appear as the final 2 octets of the final fragment,
   i.e., as a "trailer".

   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 containing one or more AERO Forwarding Vector Indices
   (AFVIs) if necessary as discussed in Section 3.13.  The OAL source
   can instead invoke OAL header compression by replacing the OAL IPv6
   header, CRH-32 and Fragment Header with an OAL Compressed Header
   (OCH).

   The OAL source finally encapsulates each resulting OAL fragment in L2
   headers to form an OAL carrier packet, with source address set to its
   own L2 address (e.g., 192.0.2.100) and destination set to the L2
   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:

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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |           L2 Headers          |
        |       src = 192.0.2.100       |
        |        dst = 192.0.2.1        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        OAL IPv6 Header        |
        |  src = {XLA*}::2001:db8:1:2   |
        |dst={ULA*}::0012:3456:789a:bcde|
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      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     |
        |     (final-fragment only)     |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 3: Carrier Packet Format

   Note: that carrier packets exchanged by Hosts on ENETs do not include
   the OAL IPv6 or CRH-32 headers, i.e., the OAL encapsulation is NULL
   and only the Fragment Header and L2 encapsulations are included.

   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 L2
   headers are 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 XLA-MNP prefix
   information that may change occasionally due to regional node
   mobility, as well as XLA-MNP prefix information for Relay non-MNPs
   and per-segment ULA prefix information that rarely changes.  OMNI
   link Gateways and Proxy/Servers use the information to establish and

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   maintain a forwarding plane 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 an AERO Forwarding
   Information Base (AFIB) that caches AERO Forwarding Vectors (AFVs)
   which can 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 AFIB 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 removes the L2 encapsulation
   headers and reassembles if necessary to obtain the OAL packet.  The
   OMNI interface then decrements the OAL IPv6 header Hop Limit and
   discards the packet if the Hop Limit reaches 0.  Otherwise, the OMNI
   interface updates the OAL addresses if necessary, recalculates the
   OAL checksum, re-fragments and re-encapsulates each fragment in new
   L2 encapsulation headers appropriate for next segment forwarding.

   When an FHS Gateway forwards a carrier packet to an LHS Gateway over
   the unsecured spanning tree, it reconstructs the OAL header based on
   AFV state, inserts a CRH-32 immediately following the OAL header and
   adjusts the OAL payload length and destination address field.  The
   FHS Gateway includes a single AFVI in the CRH-32 that will be
   meaningful to the LHS Gateway, then forwards the carrier packet over
   the unsecured spanning tree.  When the LHS Gateway receives the
   carrier packet, it locates the AFV for the next hop based on the
   CRH-32 AFVI 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

   When an OAL node receives carrier packets with OAL headers addressed
   to another node, it discards the L2 headers and includes new L2
   headers appropriate for the next hop in the forwarding path to the
   OAL destination.  The node then forwards these new carrier packets
   into the next hop underlay interface.

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   When an OAL node receives carrier packets with OAL headers addressed
   to itself, it discards the L2 headers, verifies the Identification,
   reassembles to obtain the original OAL packet (or super-packet - see:
   [I-D.templin-6man-omni]) then finally verifies the OAL checksum.
   Next, if the original IP packet (or super-packet) is destined either
   to itself or to a destination reached via an interface other than the
   OMNI interface, the OAL node discards the OAL encapsulation and
   forwards the original IP packet(s) to the network layer.

   If the original IP packet (or super-packet) is destined to another
   node reached by the OMNI interface, the OAL node instead changes the
   OAL source to its own address, changes the OAL destination to the ULA
   of the next-hop node over the OMNI interface, recalculates the OAL
   checksum, refragments if necessary, includes new L2 headers
   appropriate for the next hop, then forwards these new carrier packets
   into the next hop underlay interface.

3.8.  OMNI Interface Data Origin Authentication

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

   *  AERO Gateways and Proxy/Servers accept carrier packets received
      from the secured spanning tree.

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

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

   *  AERO Hosts, Clients, Relays, Proxy/Servers and Gateways verify
      carrier packet L2 encapsulation addresses according to
      [I-D.templin-6man-omni].

   *  AERO nodes accept carrier packets addressed to themselves with
      Identification values within the current window for the OAL source
      neighbor 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.

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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 underlay links with diverse MTUs while
   observing both a minimum and per-path Maximum Payload Size (MPS).
   The functions of the OAL and OMNI interface MTU/MRU/MPS
   considerations are specified in [I-D.templin-6man-omni].  (Note that
   the OMNI interface accommodates an assured MTU of 65535 octets due to
   the use of fragmentation, and can optionally expose larger MTUs to
   upper layers for best-effort Jumbogram services.)

   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 as a single OAL
   super-packet as discussed in [I-D.templin-6man-omni] before applying
   fragmentation.  The OAL source then encapsulates each OAL fragment in
   L2 headers for transmission as carrier packets over an underlay
   interface connected to either a physical link (e.g., Ethernet, WiFi,
   Cellular, etc.) 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 Gateway in
   the path (see: Section 3.10.4), 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 underlay interfaces.  (If routing indicates that
   the original IP packet should instead be forwarded back to the

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   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 underlay interfaces and/or neighbor
   cache entries for neighbors with multiple underlay interfaces (see
   Section 3.3).  The OAL uses Interface Attributes and/or Traffic
   Selectors (e.g., port numbers, flow specifications, etc.) to select
   an outbound underlay interface for each OAL packet and also to select
   segment routing and/or link-layer destination addresses based on the
   neighbor's underlay 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 underlay interface
   pair to avoid complicating factors such as delay variance and
   reordering.)  AERO nodes keep track of which underlay interfaces are
   currently "reachable" or "unreachable", and only use "reachable"
   interfaces for forwarding purposes.

   The Subnet ID value in ULAs is used only for subnet coordination
   within a local OMNI link segment.  When a node forwards a packet with
   a ULA with a foreign Global and/or Subnet ID value it forwards the
   packet based solely on the OMNI link routing information.  For this
   reason, OMNI link routing and forwarding table entries always include
   both ULAs with their associated prefix lengths and XLA-MNPs which
   encode an MNP while leaving the Global and Subnet ID values set to 0.

   The following sections discuss the OMNI interface forwarding
   algorithms for Hosts, Clients, Proxy/Servers and Gateways.  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 a ULA-MNP).

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3.10.1.  Host Forwarding Algorithm

   When an original IP packet enters a Host's OMNI interface from the
   network layer the Host searches for a NCE that matches the
   destination.  If there is a matching NCE, the Host performs L2
   encapsulation, fragments the encapsulated packet if necessary and
   forwards the packets into the ENET addressed to the L2 address of the
   neighbor.

   After sending a packet, the Host may receive a Redirect message from
   its upstream Client to inform it of another AERO node on the same
   ENET that would provide a better first hop.  The Host authenticates
   the Redirect message, then updates its neighbor cache accordingly.

3.10.2.  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 on an ANET/INET interface
   (i.e., an upstream interface), the Client selects one or more
   "reachable" neighbor interfaces in the entry for forwarding purposes.
   Otherwise, the Client invokes address resolution and multilink
   forwarding procedures per Section 3.13.  If there is a matching NCE
   on an ENET interface (i.e., a downstream interface), the Client
   instead performs OAL and/or L2 encapsulation and forwards the packet
   to the downstream Host or Client.

   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 matches a NCE for a Host or
   Client on an ENET interface, the Client instead forwards the carrier
   packet to the Host/Client.  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).

   When a Client forwards a carrier packet from an ENET Host to a
   neighbor connected to the same ENET, it also returns a Redirect
   message to inform the source that it can reach the neighbor directly
   as an ENET peer.

   Note: Clients and their FHS Proxy/Server (and other Client) peers can
   exchange original IP packets over ANET underlay 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,

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   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.)

   Note: The forwarding table entries established in peer Clients of a
   multihop forwarding region are based on ULA-MNPs and/or TLAs used to
   seed the multihop routing protocols.  When ULA-MNPs are used, the ULA
   /64 prefix provides topological relevance for the multihop forwarding
   region, while the 64-bit Interface Identifier encodes the Client MNP.
   Therefore, Clients can forward atomic fragments with compressed OAL
   headers that do not include ULA or AFVI information by examining the
   MNP-based addresses in the actual IP packet header.  In other words,
   each forwarding table entry contains two pieces of forwarding
   information - the ULA information in the prefix and the MNP
   information in the interface identifier.

3.10.3.  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 examines the IP destination address to determine if it
   matches the ULA of a neighboring Gateway.  If so, the Proxy/Server
   performs OAL encapsulation and forwards the resulting carrier packets
   to the neighboring Gateway over a secured tunnel.  If the destination
   is not the ULA of a neighboring Gateway, the Proxy/Server instead
   assumes the Relay role and fowards the original IP packet in the same
   manner as a Client.

   When the Proxy/Server receives carrier packets on underlay interfaces
   with OAL destination set to its own ULA, it discards the L2 headers
   and 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 ARS to
   initiate address resolution as specified in Section 3.13.

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   When the Proxy/Server receives a carrier packet with OAL destination
   set to a ULA-MNP 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
   ULA-MNP.  If there is no route, the Proxy/Server forwards the carrier
   packet to the next OAL hop; 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 a ULA-MNP 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
   ULA of the new Proxy/Server, then re-encapsulates the carrier packet
   and forwards it to a Gateway which will eventually deliver it to the
   new Proxy/Server.  If the neighbor cache state for the Client 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
   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 from multiple Proxy/Servers, but
   that can still be reassembled.

   Proxy/Servers process carrier packets with OAL destinations that do
   not match their 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 a ULA-MNP of one of their Clients received over the
   secured spanning tree.)  Proxy/Servers process carrier packets with
   their ULA as the destination by first examining the packet for a
   CRH-32 header or an OCH header.  In that case, the Proxy/Server
   examines the next AFVI in the carrier packet to locate the AFV entry
   in the AFIB 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 AFVI value for the
   next hop found either in the CRH-32 header or in the node's own AFIB.
   Proxy/Servers must verify that the L2 addresses of carrier packets
   not received from the secured spanning tree are "trusted" before
   forwarding according to an AFV (otherwise, the carrier packet must be
   dropped).

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   Note: Proxy/Servers may receive carrier packets addressed to their
   own ULA with CRH-32s that include additional forwarding information.
   Proxy/Servers use the forwarding information to determine the correct
   NCE and underlay 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 underlay 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.
   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 ULA from any Client, the Proxy/Server reassembles if
   necessary then performs ARS 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 ARS on its own behalf and
   thereby "override" the Proxy/Server's ARS 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

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   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].

3.10.4.  Gateway Forwarding Algorithm

   When a Gateway 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 Gateway
   examines the IP destination address to determine if it matches the
   ULA of a neighboring Gateway or Proxy/Server.  If so, the Gateway
   performs OAL encapsulation and forwards the resulting carrier packets
   to the neighboring Gateway or Proxy/Server over a secured tunnel.

   Gateways forward spanning tree carrier packets while decrementing the
   OAL header Hop Count but not the original IP header Hop Count/TTL.
   Gateways 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
   secured/unsecured spanning tree or via more direct paths according to
   AFIB information.  When the Gateway receives a carrier packet, it
   removes the L2 headers and searches for an AFIB entry that matches an
   AFVI or an IP forwarding table entry that matches the OAL destination
   address.

   Gateways process carrier packets with OAL destinations that do not
   match their 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.  Gateways process carrier packets with their ULA or the
   SRT Subnet Router Anycast address as the OAL destination by first
   examining the packet for a full OAL header with a CRH-32 extension or
   an OCH header.  In that case, the Gateway examines the next AFVI in
   the carrier packet to locate the AFV entry in the AFIB for next hop
   forwarding (i.e., without examining IP addresses).  The Gateway then
   reassembles if necessary, verifies the OAL checksum, and processes
   the OAL packet further.

   If the enclosed original IP packet is addressed to itself, the
   Gateway discards the OAL header and forwards the original IP packet
   to the network layer, Otherwise, the Gateway changes the OAL source

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   address to its own address and OAL destination address according to
   the AFVI value for the next hop found either in the CRH-32 header or
   in the node's own AFIB.  The Gateway then re-fragments while
   including an appropriate Identification and includes new L2 headers
   appropriate for the next hop and forwards the carrier packets to the
   next hop.  If the Gateway has a NCE for the target Client with an
   entry for the target underlay interface and current L2 addresses, the
   Gateway instead forwards directly to the target Client while using
   the final hop AFVI instead of the next hop (see: Section 3.13.4).

   Gateways forward carrier packets received from a first segment via
   the secured spanning tree to the next segment also via the secured
   spanning tree.  Gateways forward carrier packets received from a
   first segment via the unsecured spanning tree to the next segment
   also via the unsecured spanning tree.  Gateways 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 underlay interface to be used
   for transmission (i.e., a secured tunnel or an open INET interface).

   As for Proxy/Servers, Gateways must verify that the L2 addresses of
   carrier packets not received from the secured spanning tree are
   "trusted" before forwarding according to an AFV (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.

   A link-layer error indication is an ICMP error message generated by a
   router in an underlay network 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.)

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   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:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~
        |    IP Header of link layer    |
        |         error message         |
        ~                               ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |          ICMP Header          |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
        ~                               ~   P
        |   carrier packet L2 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:

   *  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].

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   *  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.

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

   *  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.

   *  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 underlay path as unusable and use another underlay
      path.

   *  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 Gateway receives a carrier packet for which the network-
   layer destination address is covered by an MSP assigned to a black-
   hole route, the Gateway 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).

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   AERO nodes include ICMPv6 error messages intended for an 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.

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 ANET/INET underlay interfaces with a
   FHS Proxy/Server.  Each FHS Proxy/Server locally services one or more
   of the Client's underlay 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 underlay 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 ULA 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 ULA 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 ULA of
   another Proxy/Server to serve as a Hub.)

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   Hosts and Clients on ENET interfaces associate with an upstream
   Client on the ENET the same as a Client would associate with an ANET
   Proxy/Server.  In particular, the Host/Client sends an RS message via
   the ENET which directs the message to the upstream Client.  The
   upstream Client returns an RA message.  In this way, the downstream
   nodes see the ENET as an ANET and see the upstream Client as a Proxy/
   Server for that ANET.

   AERO Hosts, 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 XLA-MNP as the source
   address of an RS message and with an OMNI option with valid prefix
   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 a TLA 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 Host, 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 Host and Client Behavior

   AERO Hosts and 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 Host/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

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   described below.  In the absence of other information, the Host/
   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 name resolution returns a set of resource
   records with Proxy/Server address information.

   The Host/Client then performs RS/RA exchanges over each of its
   underlay 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 Host/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 Host/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 Host/Client registers its underlay 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 Host/Client prepares an RS message to send over any
   available underlay 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 underlay
   interface.  When the Host/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 Host/Client wishes to discontinue use of a Hub Proxy/Server it
   issues an RS message over any underlay interface with an OMNI option
   with a prefix release indication (i.e., by setting the OMNI Neighbor
   Coordination sub-option Preflen to 0).  When the Hub Proxy/Server
   processes the message, it releases the MNP, sets the NCE state for
   the Host/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 Host/Client associates with a new FHS/Hub
   Proxy/Server it can include an OMNI "Proxy/Server Departure" sub-
   option in RS messages with the ULAs of the Old FHS/Hub Proxy/
   Servers.)

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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 ULAs 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 underlay 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 XLA-MNP prefix parameters and/or the DHCPv6 OMNI sub-
   option.  When the Hub Proxy/Server returns the MNPs, it also creates
   an XLA-MNP forwarding table entry for the MNP resulting in a BGP
   update (see: Section 3.2.3).  The Hub Proxy/Server then returns an RA
   to the Client with destination set to the source of the RS (if an FHS
   Proxy/Server on the return path proxys the RA, it changes the
   destination to the Client's ULA-MNP).

   After the initial RS/RA exchange, the Hub Proxy/Server maintains a
   ReachableTime timer for each of the Client's underlay interfaces
   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 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
   underlay interface, the Hub Proxy/Server marks the interface as DOWN.
   If ReachableTime expires before any new RS is received on any
   individual underlay 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 XLA-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

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   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 secured 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 XLA-MNP 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.

3.12.3.1.  Additional Proxy/Server Considerations

   AERO Clients register with FHS Proxy/Servers for each underlay
   interface.  Each of the Client's FHS Proxy/Servers must inform a
   single Hub Proxy/Server of the Client's underlay interface(s) that it
   services.  For Clients on Direct and VPNed underlay interfaces, the
   FHS Proxy/Server for each interface is directly connected, for
   Clients on ANET underlay interfaces the FHS Proxy/Server is located
   on the ANET/INET boundary, and for Clients on INET underlay
   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:

   *  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 {UAL,XLA}-MNP in the RS source address for
      this Client neighbor and authenticates the message.  If no NCE was
      found, Proxy/Server "B" instead creates one in the STALE state and

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      caches the Client-supplied Interface Attributes, Origin Indication
      and OMNI Neighbor Coordination sub-option window synchronization
      parameters as well as the Client's observed L2 addresses (noting
      that they may differ from the Origin addresses if there were NATs
      on the path).  Proxy/Server "B" then examines the RS destination
      address.  If the destination address is the ULA 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 ULA and destination set to Proxy/Server B's 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.

   *  when Hub Proxy/Server "A" receives the RS, it assume the Hub role,
      delegates an MNP for the Client if necessary, and creates/updates
      a NCE indexed by the Client's XLA-MNP 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 ULA and destination set to the
      source of the RS message, then encapsulates the RA in an OAL
      header with source set to its own ULA and destination set to the
      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.

   *  when FHS Proxy/Server "B" reassembles the RA, it locates the
      Client NCE based on the RA destination.  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
      the sub-option.  If the RA message delegates a new XLA-MNP, Proxy/
      Server "B" then resets the RA destination to the corresponding
      ULA-MNP for this interface.  Proxy/Server "B" then re-encapsulates
      the message with OAL source set to its own ULA and OAL destination
      set to ULA that appeared in the Client's RS message OAL source,
      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 L2 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.

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   *  The Client repeats this process over each of its additional
      underlay 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.  The Client creates/
      updates NCEs for each such FHS Proxy/Server as well as the Hub
      Proxy/Server in the process.

   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 Gateway using OAL encapsulation with its own
   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
   underlay 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 XLA-MNP as the
   source in order to differentiate them from the IPv6 ND messages sent
   by a FHS Proxy/Server.

   If the Client becomes unreachable over all underlay 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 Gateway via OAL encapsulation.  When DepartTime
   expires, the Hub Proxy/Server deletes the NCE, withdraws the XLA-MNP
   route and discards any further carrier packets destined to the former
   Client.

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   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 underlay 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 XLA-MNP (or to a TLA), and
   with destination address set to the ULA 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 an underlay 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.

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

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   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
   ULA as the source and the ULA 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 ULA of the new
   Hub).

3.12.3.3.  DHCPv6-Based Prefix Registration

   When a Client is not pre-provisioned with an MNP, 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 a TLA 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

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   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 creates an
   XLA based on the delegated MNP adds an XLA-MNP route to the routing
   system.  The Hub Proxy/Server then sends an RA back to the Client
   either directly or via an FHS Proxy/Server acting as a proxy.  The
   Proxy/Server that returns the RA directly to the Client sets the
   (newly-created) ULA-MNP as the destination address and with the
   DHCPv6-PD Reply message and OMNI Neighbor Coordination sub-option
   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 {ULA,XLA}-MNP 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].

   Note: when the Hub Proxy/Server forwards an RA to the Client via a
   different node acting as a FHS Proxy/Server, the Hub sets the RA
   destination to the same address that appeared in the RS source.  The
   FHS Proxy/Server then subsequently sets the RA destination to the
   ULA-MNP when it forwards the Proxyed version of the RA to the Client
   - see [I-D.templin-6man-omni] for further details.

3.13.  AERO Address Resolution, Multilink Forwarding and Route
       Optimization

   AERO nodes invoke address resolution, multilink forwarding and route
   optimization when they need to forward initial packets to new target
   neighbors over ANET/INET interfaces and for ongoing multilink
   forwarding for current target neighbors.  Address resolution is based
   on an IPv6 ND NS/NA(AR) messaging exchange between an Address
   Resolution Source (ARS) and the target neighbor as the Address
   Resolution Target (ART).  Either the ART itself or the ART's current
   Hub Proxy/Server serves as the Address Resolution Responder (ARR).

   Address resolution is initiated by the first eligible ARS closest to
   the original source as follows:

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

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

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   *  For Clients on INET interfaces, the Client itself is the ARS.

   *  For correspondent nodes on INET/ENET interfaces serviced by a
      Relay, the Relay is the ARS.

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

   *  For peers within the same ANET/ENET, route optimization is through
      receipt of Redirect messages.

   The AERO routing system directs an address resolution request sent by
   the ARS to the ARR.  The ARR then returns an address resolution reply
   which must include information that is current, consistent and
   authentic.  The ARS is responsible for periodically refreshing the
   address resolution, and the ARR is responsible for quickly informing
   the ARS of any changes.  Following address resolution, the ARS and
   ART perform continuous unicast multilink route optimization exchanges
   to maintain optimal forwarding profiles.

   The address resolution and 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 ARS checks for a NCE with an XLA-MNP that
   matches the target destination.  If there is a NCE in the REACHABLE
   state, the ARS 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 ARS creates one in the INCOMPLETE
   state.

   The ARS next prepares an NS message for Address Resolution (NS(AR))
   to send toward an ART 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:

   *  the ULA of the ARS as the source address.

   *  the XLA 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 fd00::2001:db8:1:2.

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   *  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 an OMNI Neighbor
   Coordination sub-option with Preflen set to the prefix length
   associated with the NS(AR) source.  The ARS also includes Interface
   Attributes and Traffic Selectors for all of the source Client's
   underlay 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 ULA and
   OAL destination set to the XLA-MNP corresponding to the ART and with
   window synchronization parameters.  The ARS then inserts a fragment
   header, calculates the OAL checksum performs fragmentation and L2
   encapsulation, then sends the resulting carrier packets into the SRT
   secured spanning tree without decrementing the network-layer TTL/Hop
   Limit field.

   When the ARS is a Client, it must instead use the ULA of one of its
   FHS Proxy/Servers as the OAL destination.  The ARS Client then
   fragments, performs L2 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 ULA, changes the OAL destination to the XLA-MNP
   corresponding to the ART, selects an appropriate Identification,
   calculates the OAL checksum, then re-fragments and forwards the
   resulting carrier packets into the secured spanning tree on behalf of
   the Client.

   Note: both the ART Client/Relay and its Hub Proxy/Server include
   current and accurate information for the ART's multilink Interface
   Attributes profile.  The Hub Proxy/Server can be trusted to provide
   an authoritative ARR response on behalf of the ART should the need
   arise.  While the ART has no such trust basis, any attempt by the ART
   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 ART
   itself.  Therefore, the ART's asserted Interface Attributes need not
   be validated by the Hub Proxy/Server.

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3.13.1.1.  Relaying the NS(AR)

   When the Gateway receives carrier packets containing the NS(AR), it
   discards the L2 headers and determines the next hop by consulting its
   standard IPv6 forwarding table for the OAL header destination
   address.  The Gateway then decrements the OAL header Hop-Limit, then
   includes new L2 headers 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 Gateway will
   deliver the carrier packets via the secured spanning tree to the Hub
   Proxy/Server (or Relay) that services the ART.

3.13.1.2.  Processing and Responding to the NS(AR)

   When the Hub Proxy/Server for the ART receives the NS(AR) secured
   carrier packets with the XLA-MNP of the ART as the OAL destination,
   it discards the L2 headers, reassembles, verifies the OAL checksum
   then either forwards the NS(AR) to the ART or processes it locally if
   it is acting as a Relay/IP router or the ART's designated ARR.  The
   Hub Proxy/Server processes the message as follows:

   *  if the NS(AR) target matches a Client NCE in the DEPARTED state,
      the (old) Hub Proxy/Server resets the OAL destination address to
      the ULA of the Client's new Hub Proxy/Server.  The old Hub Proxy/
      Server then recalculates the OAL checksum, re-fragments, includes
      appropriate L2 headers then forwards the resulting carrier packets
      over the secured spanning tree.

   *  if the Hub Proxy/Server is the ART's designated ARR, it prepares
      to return an NA(AR) as discussed below; otherwise it continues
      processing.

   *  If the NS(AR) target matches a Client NCE in the REACHABLE state,
      the Hub Proxy/Server notes whether the NS(AR) arrived from the
      secured spanning tree.  If the message arrived via the secured
      spanning tree the Hub Proxy/Server verifies the NS checksum;
      otherwise, it must verify the message authentication signature.
      Next, the Hub Proxy/Server determines the underlay interface for
      the ART and proceeds as follows:

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      -  If the Hub Proxy/Server is not the FHS Proxy/Server on the
         underlay interface used to convey the NS(AR) to the ART, it
         recalculates the NS(AR) checksum, changes the OAL source to its
         own ULA and changes the OAL destination to the ULA of the FHS
         Proxy/Server for this ART interface.  The Hub Proxy/Server next
         includes a suitable Identification value, recalculates the OAL
         checksum, re-fragments if necessary, includes appropriate L2
         headers and forwards the resulting carrier packets over the
         secured spanning tree.

      -  When the FHS Proxy/Server receives the carrier packets, it
         reassembles and verifies the OAL and NS(AR) checksums, then
         changes the OAL source to its own ULA and OAL destination to
         the ULA-MNP of the ART.  The FHS Proxy/Server next includes a
         suitable Identification value, recalculates the OAL checksum,
         re-fragments if necessary, includes appropriate L2 headers and
         forwards the resulting carrier packets over the underlay
         interface to the ART.

      -  If the Hub Proxy/Server is also the FHS Proxy/Server on the
         underlay interface used to convey the NS(AR) to the ART, it
         instead proceeds exactly as for the FHS Proxy/Server
         instructions above.

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

   The ARR then creates a NCE for the NS(AR) 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 ARS with the
   source address set to the ART's XLA, the destination address set to
   the NS(AR) ULA source address and the Target Address set to the same
   value that appeared in the NS(AR) Target Address.  The ARR includes
   an OMNI option with OMNI Neighbor Coordination sub-option Preflen set
   to the prefix length associated with the NA(AR) source address.

   The ARR then includes Interface Attributes and Traffic Selector sub-
   options for all of the ART's underlay interfaces with current
   information for each interface.  The ARR also includes either an
   authentication signature or an NA message checksum as necessary.  The
   ARR next 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 ARR then submits the NA(AR) for OAL
   encapsulation with source set to its own ULA and destination set to
   the ULA that appeared in the NS(AR) OAL source, selects an

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   appropriate Identification, and includes window synchronization
   parameters and authentication signature or checksum, calculates the
   OAL checksum, fragments, then includes appropriate L2 headers and
   forwards the resulting (L2-encapsulated) carrier packets.

   When the FHS Proxy/Server receives the carrier packets sent by an ART
   acting as an ARR on its own behalf, it reassembles if necessary,
   verifies the authentication signature or checksum and includes a new
   authentication signature or checksum.  The FHS Proxy/Server then
   changes the OAL source address to its own ULA, changes the OAL
   destination to the ULA corresponding to the NA(AR) destination,
   includes an appropriate Identification, recalculates the NA and OAL
   checksums and fragments if necessary.  The FHS Proxy/Server finally
   includes appropriate L2 headers and forwards the carrier packets into
   the secured spanning tree.

   Note: If the Hub Proxy/Server is acting as the target's ARR but not
   as a Relay/IP router, it prepares the NA(AR) with the R flag set to 0
   but without setting the SYN/ACK flags in the OMNI Neighbor
   Coordination sub-option window synchronization parameters.  This
   informs the ARS that it must initiate multilink route optimization to
   synchronize with the target either directly or via an LHS Proxy/
   Server (see: Section 3.13.2).  In all other ways, the Hub Proxy/
   Server prepares and returns the NA(AR) the same as for the FHS Proxy/
   Server case above.

3.13.1.3.  Relaying the NA(AR)

   When the Gateway receives NA(AR) carrier packets, it discards the L2
   headers and determines the next hop by consulting its standard IPv6
   forwarding table for the OAL header destination address.  The Gateway
   then decrements the OAL header Hop-Limit, re-encapsulates the carrier
   packets with new L2 headers and forwards them via the SRT secured
   spanning tree where they may traverse multiple OMNI link segments.
   The final-hop Gateway will deliver the carrier packets via the
   secured spanning tree to a Proxy/Server for the ARS.

3.13.1.4.  Processing the NA(AR)

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

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   When the ARS 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 ULA as the OAL source and the ULA-MNP of
   the Client as the OAL destination when it forwards the NA(AR).

3.13.2.  Multilink Forwarding

   Following address resolution, the ARS and ART can assert multilink
   forwarding paths through underlay interface pairs serviced by the
   same source/destination ULAs by sending unicast NS/NA messages with
   OMNI AERO Forwarding Parameter (AFP) and/or Neighbor Coordination
   sub-options.  The unicast NS/NA messages establish multilink
   forwarding state in OAL intermediate nodes in the path between the
   ARS and ART.  Note that either the ARS or ART can independently
   initiate multilink forwarding by sending unicast NS messages on
   behalf of specific underlay interface pairs.

   Nodes that configure OMNI interfaces include an additional forwarding
   table termed the AERO Forwarding Information Base (AFIB) that
   supports carrier packet forwarding based on OMNI neighbor underlay
   interface pairs.  The AFIB contains AERO Forwarding Vectors (AFVs)
   identified by locally-unique 4-octet values known as AFV Indexes
   (AFVIs).  The AFVs cache uncompressed OAL header information as well
   as the previous/next-hop addressing and AFVI information.

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

   When an OAL node initiates or forwards an NS with Job code '00', it
   creates an AFV, records the NS source and destination ULAs then
   generates and assigns a locally-unique "B" AFVI (while also caching
   the "B" values for all previous OAL hops on the path from the FHS OAL
   source).  When the OAL node receives future carrier packets that
   include "B", it can unambiguously locate the correct AFV and
   determine directionality without examining addresses.  When the AFV
   is indexed by its "B" AFVI, it returns the ULAs in (dst,src) order

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   the opposite of how they appeared in the OAL header of the original
   NS to support full header reconstruction for reverse-path forwarding.
   (If the NS message included a nested OAL encapsulation, the ULAs of
   both OAL headers are returned.)

   When an OAL node initiates or forwards a solicited NA with Job code
   '01', it uses the "B" AFVI to locate the AFV created by the NS then
   generates and assigns a locally-unique "A" AFVI (while also caching
   the "A" values for all previous OAL hops on the path from the LHS OAL
   source).  When the OAL node receives future carrier packets that
   include "A", it can unambiguously locate the correct AFV and
   determine directionality without examining addresses.  When the AFV
   is indexed by its "A" AFVI, it returns the ULAs in (src,dst) order
   the same as they appeared in the OAL header of the original NS to
   support full header reconstruction for forward-path forwarding.  (If
   the NS message included a nested OAL encapsulation, the ULAs of both
   OAL headers are returned.)

   OAL nodes generate random non-zero 32-bit values as candidate AFVIs
   which must first be tested for local uniqueness.  If a candidate AFVI
   s already in use, the OAL node repeats the random generation process
   until it obtains a unique non-zero value.  Since the number of AFVs
   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.  Since the uniqueness property is node-local only, an AFVI
   locally generated by a first OAL node must not be tested for
   uniqueness by other OAL nodes.

   OAL nodes cache AFVs for up to ReachableTime seconds following their
   initial creation.  If the node processes another NS or NA message
   specific to an AFV, it resets ReachableTime to REACHABLE_TIME
   seconds, i.e., the same as for NCEs.  If ReachableTime expires, the
   node deletes the AFV and frees its associated AFVIs so they can be
   reused for future AFVs.

   The following sections provide the detailed specifications of these
   NS/NA exchanges for all nodes along the forward and reverse paths.

3.13.2.1.  FHS Client-Proxy/Server NS Forwarding

   When an FHS OAL source has an original IP packet to send to an LHS
   OAL target following multilink address resolution, it first selects a
   source and target underlay interface pair.  The FHS source uses its
   cached information for the target underlay interface as LHS
   information then prepares an NS message with an AFP sub-option with
   Job code '00' and with the NS source set to its own ULA or XLA and
   the NS destination set to the ULA or XLA of the LHS target.  The FHS
   source then sets window synchronization information in the OMNI

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   Neighbor Coordination sub-option and creates a NCE for the selected
   target ULA or XLA in the INCOMPLETE state.  The FHS source next
   creates an AFV then generates and assigns a locally-unique "B" AFVI
   to the AFV while also including it as the first "B" entry in the AFP
   AFVI List.  The FHS source then includes any FHS/LHS addressing
   information it knows locally in the AFP sub-option, i.e., based on
   information discovered through address resolution.

   If the FHS source is the FHS Proxy/Server, it then 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 ULA 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 AFP Tunnel Window
   Synchronization fields.  The FHS Proxy/Server then performs OAL
   encapsulation while setting the OAL source to its own ULA and setting
   the OAL destination to the FHS Subnet Router Anycast ULA determined
   by applying the FHS SRT prefix length to its ULA.  The FHS Proxy/
   Server then selects an appropriate Identification value, calculates
   the OAL checksum, fragments if necessary, encapsulates in appropriate
   L2 headers then forwards the carrier packets into the secured
   spanning tree which will deliver them to a Gateway interface that
   assigns the FHS Subnet Router Anycast ULA.

   If the FHS source is the FHS Client, it instead includes an
   authentication signature if necessary.  If FHS FMT-Forward is set and
   LHS FMT-Forward is clear, the FHS Client creates/updates a NCE for
   the ULA of the LHS Proxy/Server as above and includes Tunnel Window
   Synchronization parameters.  The FHS Client then performs OAL
   encapsulation, sets the OAL source to its own ULA-MNP and sets the
   OAL destination to the ULA of the FHS Proxy/Server.  The FHS Client
   finally selects an appropriate Identification value for the FHS
   Proxy/Server, calculates the OAL checksum, fragments if necessary,
   encapsulates in appropriate L2 headers then forwards the carrier
   packets to the FHS Proxy/Server.

   When the FHS Proxy/Server receives the carrier packets, it verifies
   the Identification then reassembles to obtain the NS, verifies the
   OAL checksum and verifies the NS authentication signature or
   checksum.  The FHS Proxy/Server then creates an AFV (i.e., the same
   as the FHS Client had done) while caching the AFP AFVI List "B" entry
   along with the FHS Client addressing information as previous hop
   information for this AFV.  The FHS Proxy/Server next generates a new
   locally-unique "B" AFVI, then both assigns it as the AFV index and
   writes it as the next "B" entry in the AFP AFVI 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

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   whether to create a NCE for the LHS Proxy/Server ULA and include
   Tunnel Window Synchronization parameters the same as above.  The FHS
   Proxy/Server then calculates the NS checksum and sets the OAL source
   address to its own ULA and destination address to the FHS Subnet
   Router Anycast ULA.  The FHS Proxy/Server finally includes an
   Identification appropriate for the secured spanning tree, calculates
   the OAL checksum and re-fragments if necessary.  The FHS Proxy/Server
   finally includes appropriate L2 headers and forwards the carrier
   packets into the secured spanning tree.

3.13.2.2.  Gateway NS Forwarding

   Gateways 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 Gateway receives a carrier packet over the secured spanning
   tree addressed to its ULA or the FHS Subnet Router Anycast ULA, it
   instead reassembles to obtain the NS then verifies the OAL and NS
   checksums.  The FHS Gateway next creates an AFV (i.e., the same as
   the FHS Proxy/Server had done) while caching the AFP FHS Client and
   Proxy/Server addressing information and corresponding AFVI List "B"
   list values in the AFV to enable future reverse path forwarding to
   this FHS Client.  The FHS Gateway then generates a locally-unique "B"
   AFVI for the AFV and also writes it as the next "B" entry in the NS
   AFP AFVI List.

   The FHS Gateway then examines the SRT prefixes corresponding to both
   FHS and LHS.  If the FHS Gateway has a local interface connection to
   both the FHS and LHS (whether they are the same or different
   segments), the FHS/LHS Gateway caches the NS AFP LHS information in
   the AFV, writes its LHS ULA and INADDR into the NS AFP LHS fields,
   then sets its LHS ULA as the OAL source and the ULA of the LHS Proxy/
   Server as the OAL destination.  If the FHS and LHS prefixes are
   different, the FHS Gateway instead sets its FHS ULA as the OAL source
   and the LHS Subnet Router Anycast ULA as the OAL destination.  The
   FHS Gateway then selects an appropriate Identification, recalculates
   the NS and OAL checksums, re-fragments if necessary, then finally
   includes appropriate L2 headers and forwards the carrier packets into
   the secured spanning tree.

   When the FHS and LHS Gateways are different, the LHS Gateway will
   receive carrier packets over the secured spanning tree from the FHS
   Gateway, noting there may be many intermediate Gateways in the path
   between FHS and LHS which will simply forward the carrier packets
   without further processing.  The LHS Gateway then reassembles to
   obtain the NS, verifies the OAL and NS checksums then creates an AFV
   (i.e., the same as the FHS Gateway had done) while caching the AFP

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   "B" AFVIs and addressing information of previous OAL forwarding hops.
   In particular, the LHS Gateway caches the ULA of the FHS Gateway as
   the spanning tree address for the previous-hop, caches the LHS
   information then generates a locally-unique "B" AFVI for the AFV.
   The LHS Gateway then writes its own LHS ULA and INADDR into the AFP
   sub-option while also writing "B" as the next entry in the AFP AFVI
   List.  The LHS Gateway then sets its own ULA as the OAL source and
   the ULA of the LHS Proxy/Server as the OAL destination, selects an
   appropriate Identification, recalculates the NS and OAL checksums,
   re-fragments if necessary, then finally includes appropriate L2
   headers and forwards the carrier packets into the secured spanning
   tree.

3.13.2.3.  LHS Proxy/Server-Client NS Receipt and NA Forwarding

   When the LHS Proxy/Server receives the carrier packets from the
   secured spanning tree, it reassembles to obtain the NS, verifies the
   OAL and NS checksums 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
   AFV and caches the AFP "B" AFVIs and addressing information of
   previous OAL forwarding hops the same as for the prior hop.  If the
   NS destination is the XLA of the LHS Client, the LHS Proxy/Server
   also generates a locally-unique "B" AFVI and assigns it both to the
   AFVI and as the next "B" entry in the AFVI 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 ULA of the FHS Proxy/
   Server (if necessary) and sets the state to STALE, then caches any
   Tunnel Window Synchronization parameters.

   If the NS destination matches its own ULA, the LHS Proxy/Server next
   prepares to return a solicited NA with Job code '01'.  If the NS
   source was the XLA of the FHS Client, the LHS Proxy/Server first
   creates or updates an NCE for the XLA with state set to STALE.  The
   LHS Proxy/Server next caches the NS OMNI Neighbor Coordination sub-
   option window synchronization parameters and AFP information
   (including the AFVI List) in the NCE corresponding to the ULA source.
   When the LHS Proxy/Server forwards future carrier packets based on
   the NCE, it can populate forwarding information in a CRH-32 routing
   header to enable forwarding based on the cached AFVI List "B" entries
   instead of ULA addresses.

   The LHS Proxy/Server then creates an NA with Job code '01' while
   copying the NS AFP sub-option into the NA.  The LHS Proxy/Server then
   generates a locally-unique "A" AFVI and both assigns it to the AFV
   and includes it as the first "A" entry in the AFP sub-option AFVI
   List (see: [I-D.templin-6man-omni] for details on AFVI List A/B
   processing).  The LHS Proxy/Server then includes end-to-end window

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   synchronization parameters in the OMNI Neighbor Coordination sub-
   option (if necessary) and also tunnel window synchronization
   parameters in the AFP sub-option (if necessary).  The LHS Proxy/
   Server then encapsulates the NA with the OAL source set to its own
   ULA and the OAL destination set to the ULA of the LHS Gateway.  The
   LHS Proxy/Server then selects an appropriate Identification value,
   calculates the NA and OAL checksums, fragments if necessary then
   finally includes appropriate L2 headers and forwards the carrier
   packets into the secured spanning tree.

   If the NS destination was the XLA 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 ULA and changes the OAL destination to the ULA-MNP
   of the LHS Client.  The LHS Proxy/Server then selects an appropriate
   Identification value, calculates the OAL checksum, fragments if
   necessary then finally includes appropriate L2 headers and forwards
   the carrier packets to the LHS Client.  When the LHS Client receives
   the carrier packets, it verifies the Identification and reassembles
   to obtain the NS then verifies the OAL checksum and NS authentication
   signature or checksum.  The LHS Client then creates a NCE for the NS
   ULA source address in the STALE state and examines the AFP sub-
   option.  If LHS FMT-Forward is set, FHS FMT-Forward is clear and the
   NS source was an XLA, the Client also creates a NCE for the ULA of
   the FHS Proxy/Server in the STALE state and caches any Tunnel Window
   Synchronization parameters.  The Client then caches the NS OMNI
   Neighbor Coordination and AFP sub-options in the NCE corresponding to
   the NS ULA source, then creates an AFV, caches the addressing
   information and "B" entries of the previous OAL hops then finally
   generates and assigns a locally-unique "A" AFVI the same as for
   previous hops.

   The LHS Client then prepares an NA using exactly the same procedures
   as for the LHS Proxy/Server above, except that it uses its XLA as the
   NA source and the NS source as the NA destination.  The LHS Client
   also includes an authentication signature if necessary (otherwise
   calculates the checksum), then encapsulates the NA with OAL source
   set to its own ULA-MNP and OAL destination set to the ULA of the LHS
   Proxy/Server.  The LHS Client finally includes an appropriate
   Identification, calculates the OAL checksum, fragments if necessary
   then includes appropriate L2 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 to
   obtain the NA, verifies the OAL checksum and NA authentication
   signature or checksum, then uses the current AFP AFVI List "B" entry
   to locate the AFV.  The LHS Proxy/Server then caches the addressing
   and "A" information for the LHS Client in the AFV, then generates a
   locally-unique "A" AFVI and both assigns it to the AFV and writes it

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   as the next AFP AFVI 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
   calculates the NA checksum, sets the OAL source to its own ULA and
   destination to the ULA of the LHS Gateway, includes an appropriate
   Identification, calculates the OAL checksum, re-fragments if
   necessary then finally includes appropriate L2 headers and forwards
   the carrier packets into the secured spanning tree.

3.13.2.4.  Gateway NA Forwarding

   When the LHS Gateway receives the carrier packets containing the NA
   message, it reassembles and verifies the OAL and NA checksums then
   uses the current NA AFP AFVI List "B" entry to locate the AFV.  The
   LHS Gateway then caches the AFP addressing and AFVI List "A"
   information for the previous hops in the AFV, then generates a
   locally-unique "A" AFVI and both assigns it to the AFV and writes it
   as the next AFP AFVI List "A" entry.  The LHS Gateway then
   recalculates the NA checksum.  If the LHS Gateway is connected
   directly to both the FHS and LHS segments (whether the segments are
   the same or different), the LHS Gateway will have already cached the
   FHS/LHS information based on the original NS; the LHS Gateway then
   sets the OAL source to its FHS ULA and OAL destination to the ULA of
   the FHS Proxy/Server.  Otherwise, the LHS Gateway sets the OAL source
   to its LHS ULA and OAL destination to the ULA of the FHS Gateway.
   The LHS Gateway then selects an appropriate Identification,
   recalculates the OAL checksum, re-fragments if necessary, includes
   appropriate L2 headers and finally forwards the carrier packets into
   the secured spanning tree.

   When the FHS and LHS Gateways are different, the FHS Gateway will
   receive carrier packets containing the NA message from the LHS
   Gateway over the secured spanning tree, where there may have been
   many intermediate Gateway forwarding hops.  The FHS Gateway then
   reassembles to obtain the NA, verifies the OAL and NA checksums and
   locates the AFV based on the current AFP AFVI List "B" entry.  The
   FHS Gateway then caches the addressing and "A" information for the
   previous hops in the AFV and generates a locally-unique "A" AFVI.
   The FHS Gateway then assigns the new "A" value to the AFV, records
   "A" in the AFP AFVI List then writes its FHS ULA and INADDR into the
   AFP FHS Gateway fields.  The FHS Gateway then recalculates the NA
   checksum, sets its FHS ULA as the OAL source and sets the ULA of the
   FHS Proxy/Server as the OAL destination.  The FHS Gateway then
   selects an appropriate Identification value, recalculates the OAL
   checksum, re-fragments if necessary, includes appropriate L2 headers
   and finally forwards the carrier packets into the secured spanning
   tree.

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3.13.2.5.  FHS Proxy/Server-Client NA Receipt

   When the FHS Proxy/Server receives the carrier packets from the
   secured spanning tree, it reassembles to obtain the NA, verifies the
   OAL and NA checksums then locates the AFV based on the current AFP
   AFVI List "B" entry.  The FHS Proxy/Server then caches the AFP
   addressing and "A" information for the previous hops.  If the NA
   destination matches its own ULA, the FHS Proxy/Server then examines
   LHS FMT.  If FMT-Forward is clear, the FHS Proxy/Server locates the
   NCE for the ULA of the LHS Proxy/Server and sets the state to
   REACHABLE then caches any Tunnel Window Synchronization parameters.
   If the NA source is the XLA of the LHS Client, the FHS Proxy/Server
   then locates the LHS Client NCE and sets the state to REACHABLE then
   caches the OMNI Neighbor Coordination window synchronization
   parameters and prepares to return an acknowledgement, if necessary.

   If the NA destination is the XLA of the FHS Client, the FHS Proxy/
   Server also searches for and updates the NCE for the ULA of the LHS
   Proxy/Server if necessary the same as above.  The FHS Proxy/Server
   then generates a locally-unique "A" AFVI and assigns it both to the
   AFV and as the next AFP AFVI 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 ULA and sets the OAL
   destination to the ULA-MNP of the FHS Client.  The FHS Proxy/Server
   then selects an appropriate Identification value, recalculates the
   OAL checksum, re-fragments if necessary, includes appropriate L2
   headers and finally forwards the carrier packets to the FHS Client.

   When the FHS Client receives the carrier packets, it verifies the
   Identification, reassembles to obtain the NA, verifies the OAL
   checksum and NA authentication signature or checksum, then locates
   the AFV based on the current AFP AFVI List "B" entry.  The FHS Client
   then caches the previous hop addressing and "A" information the same
   as for prior hops.  The FHS Client then examines LHS FMT.  If FMT-
   Forward is clear, the FHS Client locates the NCE for the ULA of the
   LHS Proxy/Server and sets the state to REACHABLE then caches any
   Tunnel Window Synchronization parameters.  If the NA source is the
   XLA of the LHS Client, the FHS Client then locates the LHS Client NCE
   and sets the state to REACHABLE then caches the OMNI Neighbor
   Coordination window synchronization parameters and prepares to return
   an unsolicited NA (uNA) acknowledgement, if necessary.

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3.13.2.6.  Returning Window Acknowledgements

   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 AFP 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 uNA
   source to its own ULA or XLA, sets the uNA destination to the ULA or
   XLA of the LHS node then includes Tunnel Window Synchronization
   parameters if necessary.  The FHS node next sets the AFP AFVI 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 ULA as the
   OAL source.  If the FHS node is the Client, it next sets the ULA of
   the FHS Proxy/Server as the OAL destination, includes an
   authentication signature or checksum, selects an appropriate
   Identification value, calculates the OAL checksum, fragments if
   necessary, includes appropriate L2 headers and finally forwards the
   carrier packets to the FHS Proxy/Server.  The FHS Proxy/Server then
   verifies the Identification, reassembles if necessary, verifies the
   OAL checksum and uNA authentication signature or checksum, then uses
   the current AFVI List "A" entry to locate the AFV.  The FHS Proxy/
   Server then writes its "B" AFVI as the next AFP AFVI 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 with Job code '00'.

   The FHS Proxy/Server recalculates the uNA checksum then sets its own
   ULA as the OAL source and the ULA of the FHS Gateway as the OAL
   destination, The FHS Proxy/Server finally selects an appropriate
   Identification, recalculates the OAL checksum, includes appropriate
   L2 headers and finally forwards the carrier packets into the secured
   spanning tree.  When the FHS Gateway receives the carrier packets, it
   reassembles to obtain the uNA, verifies the OAL and uNA checksums
   then uses the current AFVI List "A" entry to locate the AFV.  The FHS
   Gateway then writes its "B" AFVI as the next AFP AFVI List "B" entry,
   then sets the OAL source to its own ULA.  If the FHS Gateway is also
   the LHS Gateway, it sets the OAL destination to the ULA of the LHS
   Proxy/Server; otherwise it sets the OAL destination to the ULA of the
   LHS Gateway.  The FHS Gateway recalculates the uNA checksum then
   selects an appropriate Identification, recalculates the OAL checksum,
   re-fragments if necessary, includes appropriate L2 headers and
   finally forwards the carrier packets into the secured spanning tree.
   If an LHS Gateway receives the carrier packets, it processes them
   exactly the same as the FHS Gateway had done while re-setting the OAL
   destination to the ULA of the LHS Proxy/Server.

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   When the LHS Proxy/Server receives the carrier packets, it
   reassembles to obtain the uNA message then verifies the OAL and uNA
   checksums.  The LHS Proxy/Server then locates the AFV based on the
   current AFP AFVI List "A" entry then determines whether it is a
   tunnel ingress the same as for the original NS.  If so, the LHS
   Proxy/Server updates the NCE for the tunnel far-end based on the
   Tunnel Window Synchronization parameters.  If the uNA destination
   matches its own ULA, the LHS Proxy/Server next updates the NCE for
   the source ULA based on the OMNI Neighbor Coordination sub-option
   window synchronization parameters and MAY compare the AFVI List to
   the version it had cached in the AFV based on the original NS.

   If the uNA destination is the XLA of the LHS Client, the LHS Proxy/
   Server instead writes its "B" AFVI as the next AFP AFVI List "B"
   entry and includes an authentication signature or checksum.  The LHS
   Proxy/Server then writes its own ULA as the OAL source and the ULA-
   MNP of the Client as the OAL destination, then selects an appropriate
   Identification and recalculates the OAL checksum.  The LHS Proxy/
   Server finally re-fragments if necessary, includes appropriate L2
   headers and forwards the resulting carrier packets to the LHS Client.
   When the LHS Client receives the carrier packets, it verifies the
   Identification, reassembles to obtain the uNA, verifies the OAL
   checksum and uNA authentication signature or checksum then processes
   the message exactly the same as for the LHS Proxy/Server case above.

3.13.2.7.  OAL End System Exchanges Following Synchronization

   Following the initial NS/NA exchange with AFP sub-options, OAL end
   systems and tunnel endpoints can begin exchanging ordinary carrier
   packets that include "A/B" AFVIs and with Identification values
   within their respective send/receive windows without requiring
   security signatures and/or secured spanning tree traversal.  OAL end
   systems and intermediate nodes can also consult their AFIBs when they
   receive carrier packets that include "A/B" AFVIs to unambiguously
   locate the correct AFV and can use any discovered "A/B" values of
   other OAL nodes to forward carrier packets to nodes that configure
   the corresponding AFVIs.  OAL end systems must then perform
   continuous NS/NA exchanges to update window state, register new
   interface pairs for optimized multilink forwarding, confirm
   reachability and/or refresh AFIB cache state in the path before
   ReachableTime expires.

   While the OAL end systems continue to actively exchange packets, they
   are jointly responsible for updating cache state and per-interface
   reachability before expiration.  Window synchronization state is
   shared by all underlay interfaces in the FHS peer's NCE that use the
   same destination ULA so that a single NS/NA exchange applies for all
   interfaces regardless of the specific interface used to conduct the

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   exchange.  However, the window synchronization exchange only confirms
   target Client reachability over the specific underlay interface pair.
   Reachability for other underlay interfaces that share the same window
   synchronization state must be determined individually using
   additional NS/NA messages.

   To update AFIB state in the path, the FHS node that sent the original
   NS message with AFP Job code '00' can send additional NS messages
   with Job code '10' (Follow "A"; Record "B").  The message will be
   processed by all intermediate OAL nodes which will refresh AFV timers
   and forward the NS onward toward the LHS node that returned the
   original NA message.  When the LHS node receives the NS, it returns
   an NA message with AFP Job code '11' (Follow "B"; Record "A").

   At the same time, the LHS node that received the original NS message
   with Job code '00' can send additional NS messages with Job code '11'
   in order to cause the FHS node to return an NA message with AFP Job
   code '10'.  The process can therefore be coordinated asynchronously
   with the FHS/LHS nodes initiating an NS/NA exchange independently of
   one another.  The exchanges will succeed as long as the AFIB state in
   the path remains active.  Note that all intermediate node processing
   of Job code '10' and '11' NS/NA messages is conducted the same as for
   the initial NS/NA exchange according to the detailed specifications
   above.

   OAL sources can also begin including CRH-32s in carrier packets with
   a list of "A/B" AFVIs that OAL intermediate nodes can use for
   shortest-path carrier packet forwarding based on AFVIs instead of
   spanning tree addresses.  OAL sources and intermediate nodes can
   instead forward carrier packets with OAL compressed headers termed
   "OCH" (see: [I-D.templin-6man-omni]) that include only a single "A/B"
   AFVI meaningful to the next hop, since all OAL nodes in the path up
   to (and sometimes including) the OAL destination have already
   established AFVs.  Note that when an FHS OAL source receives a
   solicited NA with Job code '01', the AFP sub-option will contain an
   AFVI 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 AFP AFVI List "A" entries last-to-first when it
   populates a CRH-32, or must select the correct "A" entry to include
   in an OCH header based on the intended OAL intermediate node or
   destination.

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   When a Gateway receives unsecured carrier packets destined to a local
   SRT segment Client that has asserted direct reachability, the Gateway
   performs direct carrier packet forwarding while bypassing the local
   Proxy/Server based on the Client's advertised AFVIs 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
   Gateway instead forwards carrier packets directly to the local
   segment Proxy/Server.

   When a Proxy/Server receives carrier packets destined to a local SRT
   segment Client or forwards carrier packets received from a local
   segment Client, it first locates the correct AFV.  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 AFV if
   directed by AFVI and/or OAL addressing.  Otherwise it locates an AFV
   established through an NS/NA exchange between the Client and the
   remote SRT segment 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 a ULA-MNP source and
   CRH-32 routing header, or an OCH header with an AFVI that matches an
   AFV, the ingress encapsulates the carrier packet in a new full OAL
   header or an OCH header containing the next hop AFVI 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.

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   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 fit within 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 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 underlay 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 AFVIs 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 Gateway (following the establishment of INADDR information) while
   bypassing the Proxy/Server following route optimization (see:
   Section 3.13.4).  When a target Client receives carrier packets that
   match a local AFV, 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 AFVIs discovered as above (see: Section 3.13.5).
   The FHS Client will have cached the "A" AFVI for the LHS Client,
   which will have cached the "B" AFVI for the FHS Client.

3.13.3.  Rapid Commit Route Optimization

   When the LHS peer 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 LHS peer
   prepares a unicast NS message with an OMNI option with window
   synchronization information responsive to the NS(AR), with an AFP
   sub-option selected for a specific underlay interface pair and with
   Interface Attributes for all of the LHS peer's other underlay
   interfaces.  The LHS peer can also include ordinary IP packets as OAL
   super-packet extensions to the NS message if it has immediate data to
   send to the FHS peer.  The LHS peer then returns the NS to the FHS
   peer the same as for the NA(AR) case.

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   When the NS message traverses the return path to the FHS peer, 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 FHS peer, 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 FHS peer then completes the route optimization by
   returning a responsive NA.

   Note: The LHS peer 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/Gateway Route Optimization

   Following multilink route optimization for specific underlay
   interface pairs, FHS/LHS Clients located on open INETs can invoke
   Client/Gateway route optimization to improve performance and reduce
   load and congestion on their respective Proxy/Servers.  To initiate
   Client/Gateway route optimization, the Client prepares an NS message
   with its own XLA address as the source and the ULA of its Gateway as
   the destination while creating a NCE for the Gateway 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
   underlay interface as well as an authentication signature but does
   not include window synchronization parameters.  The Client then
   performs OAL encapsulation with its own ULA-MNP as the source and the
   ULA of the Gateway as the destination while including a randomly-
   chosen Identification value, then performs L2 encapsulation on the
   atomic fragment and sends the resulting carrier packet directly to
   the Gateway.

   When the Gateway receives the carrier packet, it verifies the
   authentication signature then creates a NCE for the Client.  The
   Gateway then caches the L2 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
   underlay interface as "trusted".  The Gateway then prepares an NA
   reply with its own ULA as the source and the XLA of the Client as the
   destination where the NA again must be no larger than the minimum
   MPS.

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   The Gateway then echoes the Client's Interface Attributes, includes
   an Origin Indication with the Client's observed L2 addresses and
   includes an authentication signature.  The Gateway then performs OAL
   encapsulation with its own ULA as the source and the ULA-MNP of the
   Client as the destination while using the same Identification value
   that appeared in the NS, then performs L2 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
   L2 source address information as the Gateway target address via this
   underlay interface while marking the interface as "trusted".  The
   Client also caches the Origin Indication L2 address information as
   its own (external) source address for this underlay interface.

   After the Client and Gateway have established NCEs as well as
   "trusted" status for a particular underlay 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 Gateway 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 underlay interface pair status becomes "untrusted".

   Thereafter, when the Client forwards a carrier packet with an AFVI
   toward the Gateway as the next hop, the Client uses the AFVI for the
   Gateway (discovered during multilink route optimization) instead of
   the AFVI for its Proxy/Server; the Gateway will accept the packet
   from the Client if and only if the AFVI matches the correct AFV and
   the underlay interface status is trusted.  (The same is true in the
   reverse direction when the Gateway sends carrier packets directly to
   the Client.)

   Note that the Client and Gateway each maintain a single NCE, but that
   the NCE may aggregate multiple underlay interface pairs.  Each
   underlay interface pair may use differing source and target L2
   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 Gateways 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 ULA of the Gateway, and the Proxy/Server and Gateway could relay

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   the messages over the secured spanning tree.  However, this would
   still require the Client to send additional messages toward the L2
   address of the Gateway to populate NAT state; hence the savings in
   complexity for Gateways would result in increased message overhead
   for Clients.

3.13.5.  Client/Client Route Optimization

   When the FHS/LHS 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
   Gateway) and can begin forwarding packets directly via NAT traversal
   while avoiding any Proxy/Server and/or Gateway hops.

   When the FHS/LHS Clients on the same SRT segment perform the initial
   NS/NA exchange to establish AFIB state, they also include an Origin
   Indication (i.e., in addition to an AFP sub-option) with the mapped
   addresses discovered during the RS/RA exchanges with their respective
   Proxy/Servers.  After the AFV paths have been established, both
   Clients can begin sending packets via strict AFV 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 L2 address for the target Client
   which primes its local chain of NATs for reception of future packets
   from that L2 address (see: [RFC4380] and [I-D.templin-6man-omni]).
   The source Client then prepares an NS message with its own XLA as the
   source, with the XLA 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 ULA-MNP
   as the source, with the ULA-MNP of the target Client as the
   destination and with an in-window Identification for the target.  The
   source Client then fragments and encapsulates in L2 headers addressed
   to its Proxy/Server then forwards the resulting carrier packets to
   the Proxy/Server.

   When the Proxy/Server receives the carrier packets, it re-
   encapsulates and forwards them as unsecured carrier packets according
   to AFIB 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 XLA as the source, with
   the XLA 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 ULA-MNP as the
   source, with the ULA-MNP of the source Client as the destination and

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   with an in-window Identification for the source Client.  The target
   Client then fragments and encapsulates in L2 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) underlay 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 Gateways, 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
   underlay 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.

3.13.6.  Client-to-Client OMNI Link Extension

   Clients may be recursively nested within the ENETs of other Clients.
   When a Client is the downstream-attached ENET neighbor of an upstream
   Client, it still supports the route optimization functions discussed
   above by maintaining an AFIB and assigning AFVI values.  When the
   Client processes an IPv6 ND NS/NA message that includes an AFP sub-
   option, it writes its AFVI information as the first/last AFVI list
   entry the same as for the single Client case discussed above.

   The Client then forwards the NS/NA message to the next Client in the
   extended OMNI link toward the FHS/LHS Proxy/Server, which records the
   AFVI value then overwrites the AFVI list entry with its own AFVI
   value.  This process iteratively continues until the Client that will
   forward the NS/NA message to the FHS/LHS Proxy/Server is reached, at

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   which point the NS/NA AFVI list entries are populated by the
   intermediate nodes on the path to the LHS/FHS the same as discussed
   above.

   In this way, each Client in the extended OMNI link discovers the A/B
   AFVIs of the next/previous Client without intruding into the AFP sub-
   option AFVI list.  Therefore the list can remain fixed at 5 entries
   even though the Client-to-Client OMNI link extension can be
   arbitrarily long.  Therefore, route optimization is not possible
   between consecutive Client members of the extended OMNI link but
   becomes possible at the Internetworking border that separates the FHS
   and LHS elements.

3.13.7.  Intra-ANET/ENET Route Optimization for AERO Peers

   When a Client forwards a packet from a Host or another Client
   connected to one of its downstream ENETs to a peer within the same
   downstream ENET, the Client returns an IPv6 ND Redirect message to
   inform the source that that target can be reached directly.  The
   contents of the Redirect message are the same as specified in
   [RFC4861].

   In the same fashion, when a Proxy/Server forwards a packet from a
   Host or Client connected to one of its downstream ANETs to a peer
   within the same downstream ANET, the Proxy/Server returns an IPv6 ND
   Redirect message.

   All other route optimization functions are conducted per the NS/NA
   messaging discussed in the previous sections.

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 XLAs/ULAs of
   the parties involved in the NUD test.  When only reachability

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   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 route optimization directs a source FHS peer to a target LHS
   peer with one or more link-layer addresses, either node may invoke
   multilink forwarding state initialization to establish authentic
   intermediate node state between specific underlay 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 underlay interface of
   the target itself.  While testing a target underlay 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 either the ULA or XLA of
   the source and either the ULA of the LHS Proxy/Server or the XLA 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 underlay
   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
   target if it is in the local segment or directly to a Gateway in the
   local segment.

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   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 addresses 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 underlay
   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
   Gateway in the local segment.

   When the source receives the NA(NUD), it marks the target underlay
   interface tested as "trusted".  Note that underlay interface states
   are maintained independently of the overall NCE REACHABLE state, and
   that a single NCE may have multiple target underlay 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 underlay
   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
   a proxy to perform some IPv6 ND exchanges on the Client's behalf
   without consuming bandwidth on the Client underlay interface.

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   Mobility management considerations are specified in the following
   sections.

3.15.1.  Mobility Update Messaging

   Hub Proxy/Servers (and/or the mobile Clients themselves) accommodate
   mobility and/or multilink change events by sending secured uNA
   messages to each of the Client's active neighbors.  When a node sends
   a uNA message to each specific neighbor on behalf of a mobile Client,
   it sets the IPv6 source address to its own ULA or XLA, sets the
   destination address to the neighbor's ULA or XLA and sets the Target
   Address to the mobile Client's XLA.  The uNA also includes an OMNI
   option with OMNI Neighbor Coordination sub-option Preflen set to the
   prefix length associated with the mobile Client's XLA, includes
   Interface Attributes and Traffic Selectors for the mobile Client's
   underlay interfaces and includes an authentication signature if
   necessary.  The node 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 either the specific
   neighbor's ULA or the FHS Proxy/Server's ULA.  The uNA message will
   then follow the secured spanning tree and arrive at the specific
   neighbor.

   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 node 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 node 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 FHS/LHS Proxy/Server receives a uNA message prepared as
   above, if the uNA destination was its own ULA 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 XLA of the FHS/LHS
   Client, the Proxy/Server instead changes the OAL source to its own
   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 underlay Interface Attributes and/
   or Traffic Selectors for one or more underlay interfaces (e.g., due
   to a mobility event), the Client sends RS messages to its Hub Proxy/
   Server (via first-hop FHS Proxy/Servers if necessary).  Each 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 underlay
   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 underlay 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 underlay
   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 underlay 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 underlay interface
   other than the one being deactivated, it MUST include Interface
   Attributes with appropriate link quality values for any underlay
   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 underlay interface, neighbors
   that receive the ensuing uNA messages need not purge all references
   for the underlay interface from their neighbor cache entries.  The
   Client may reactivate or reuse the underlay 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 underlay interfaces with the new Hub while
   including the old Hub's ULA in the "Old Hub Proxy/Server ULA" 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 ULA is non-zero and different from its own).  The uNA
   has the XLA of the Client as the source and the ULA of the old hub as
   the destination and with OMNI Neighbor Coordination sub-option
   Preflen set to 0.  The FHS Proxy/Server encapsulates the uNA in an
   OAL header with the ULA of the new Hub as the source and the 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 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 ULA-MNP to the new Hub Proxy/Server's 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 underlay interface at any time such that a
   Client RS/RA exchange over the underlay 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 a ULA for the "Old FHS Proxy/
   Server ULA", and the new FHS Proxy/Server will issue a uNA using the
   same procedures as outlined for the Hub above while using its own ULA
   as the source address.  This can often result in successful delivery
   of packets that would otherwise be lost due to the mobility event.

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   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 ENETs 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 underlay 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 ARS to send NS(AR) messages to an
   ARR for the multicast source.  Clients on INET and ANET underlay
   interfaces without native multicast services instead send NS(AR)
   messages as an ARS to cause their FHS Proxy/Server forward the
   message to an ARR.  When the ARR prepares 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 ARS "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 ULA or XLA
   as the source address, the solicited node multicast address
   corresponding to S as the destination and the XLA of S as the target
   address.  X then 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

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   delivers it to ARR "Y" that services S.  The resulting NA(AR) will
   return an OMNI option with Interface Attributes for any underlay
   interfaces that are currently servicing S.

   When X processes the NA(AR) it selects one or more underlay
   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 XLA of X as the next hop in
   the reverse path.  Since Gateways 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 XLA for source S and sends new Join messages
   in NS/NA exchanges addressed to the new target Client underlay
   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 ARS 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
   address resolution 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 ULA or XLA as the source address
   and the ULA or XLA for R as the destination address, then
   encapsulates the NS message in an OAL header with its own ULA as the
   source and the ULA of R's Proxy/Server as the destination then sends
   the message into the secured spanning tree.

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   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 Gateways and Proxy/Servers,
   thereby providing redundancy in case of failures.

   Each OMNI link could utilize the same or different ANET connections.
   The links can be distinguished at the link-layer via the SRT prefix
   in a similar fashion as for Virtual Local Area Network (VLAN) tagging
   (e.g., IEEE 802.1Q) and/or through assignment of distinct sets of
   MSPs on each link.  This gives rise to the opportunity for supporting
   multiple redundant networked paths (see: Section 3.2.4).

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   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 Gateways.

   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 Gateways 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
   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 Gateways on each INET partition, with each Gateway
   distributing its MNPs and/or discovering non-MNP IP GUA prefixes on

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   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 Gateways.

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

3.20.  Proxy/Server-Gateway Bidirectional Forwarding Detection

   In environments where rapid failure recovery is required, Proxy/
   Servers and Gateways 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 Gateways maintain BFD sessions in parallel with
   their BGP peerings.  If a Proxy/Server or Gateway fails, BGP peers
   will quickly re-establish routes through alternate paths the same as
   for common BGP deployments.

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.

   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

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   its XLA) 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 Gateways configure underlay interface 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
   Gateways of all OMNI link segments in turn configure underlay
   interface secured tunnels with neighboring AERO Gateways for other
   OMNI link segments 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.  (Note that
   this inter-segment Gateway arrangement mirrors the "half-gateway"
   model discussed in the original Catenet proposal.)

   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

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   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 Gateways 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
   origin authentication procedures, as well as connecting Proxy/Servers
   and Gateways 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

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   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
   ENETs 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, OAL 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, Eliot
   Lear, 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 Akash Agarwal, 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, Satish Raghavendran, Vijay Rajagopalan, Greg Saccone,
   Bhargava Raman Sai Prakash, Rod Santiago, Madhanmohan Savadamuthu,
   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.  Akash Agarwal, Kyle Bae, Wayne Benson, Madhuri Madhava
   Badgandi, Vijayasarathy Rajagopalan, Bhargava Raman Sai Prakash,
   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

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   Communications and the IETF.  An employment at Nokia where important
   IETF documents were published gave way to a present-day engagement
   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:

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

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

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

   *  AERO, First Edition [RFC6706]

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

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

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

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

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

8.  References

8.1.  Normative References

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   [I-D.templin-6man-omni]
              Templin, F. L., "Transmission of IP Packets over Overlay
              Multilink Network (OMNI) Interfaces", Work in Progress,
              Internet-Draft, draft-templin-6man-omni-61, 25 April 2022,
              <https://www.ietf.org/archive/id/draft-templin-6man-omni-
              61.txt>.

   [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.

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   [I-D.bonica-6man-comp-rtg-hdr]
              Bonica, R., Kamite, Y., Alston, A., Henriques, D., and L.
              Jalil, "The IPv6 Compact Routing Header (CRH)", Work in
              Progress, Internet-Draft, draft-bonica-6man-comp-rtg-hdr-
              28, 18 May 2022, <https://www.ietf.org/archive/id/draft-
              bonica-6man-comp-rtg-hdr-28.txt>.

   [I-D.bonica-6man-crh-helper-opt]
              Li, X., Bao, C., Ruan, E., and R. Bonica, "Compressed
              Routing Header (CRH) Helper Option", Work in Progress,
              Internet-Draft, draft-bonica-6man-crh-helper-opt-04, 11
              October 2021, <https://www.ietf.org/archive/id/draft-
              bonica-6man-crh-helper-opt-04.txt>.

   [I-D.ietf-intarea-frag-fragile]
              Bonica, R., Baker, F., Huston, G., Hinden, R. M., Troan,
              O., and F. Gont, "IP Fragmentation Considered Fragile",
              Work in Progress, Internet-Draft, draft-ietf-intarea-frag-
              fragile-17, 30 September 2019,
              <https://www.ietf.org/archive/id/draft-ietf-intarea-frag-
              fragile-17.txt>.

   [I-D.ietf-intarea-tunnels]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-intarea-tunnels-10, 12 September 2019,
              <https://www.ietf.org/archive/id/draft-ietf-intarea-
              tunnels-10.txt>.

   [I-D.ietf-ipwave-vehicular-networking]
              Jeong, J. P., "IPv6 Wireless Access in Vehicular
              Environments (IPWAVE): Problem Statement and Use Cases",
              Work in Progress, Internet-Draft, draft-ietf-ipwave-
              vehicular-networking-29, 19 May 2022,
              <https://www.ietf.org/archive/id/draft-ietf-ipwave-
              vehicular-networking-29.txt>.

   [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", Work in
              Progress, Internet-Draft, draft-ietf-rtgwg-atn-bgp-17, 19
              April 2022, <https://www.ietf.org/archive/id/draft-ietf-
              rtgwg-atn-bgp-17.txt>.

   [I-D.templin-6man-dhcpv6-ndopt]
              Templin, F. L., "A Unified Stateful/Stateless
              Configuration Service for IPv6", Work in Progress,

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              Internet-Draft, draft-templin-6man-dhcpv6-ndopt-11, 1
              January 2021, <https://www.ietf.org/archive/id/draft-
              templin-6man-dhcpv6-ndopt-11.txt>.

   [I-D.templin-intarea-seal]
              Templin, F. L., "The Subnetwork Encapsulation and
              Adaptation Layer (SEAL)", Work in Progress, Internet-
              Draft, draft-templin-intarea-seal-68, 3 January 2014,
              <https://www.ietf.org/archive/id/draft-templin-intarea-
              seal-68.txt>.

   [I-D.templin-intarea-vet]
              Templin, F. L., "Virtual Enterprise Traversal (VET)", Work
              in Progress, Internet-Draft, draft-templin-intarea-vet-40,
              3 May 2013, <https://www.ietf.org/archive/id/draft-
              templin-intarea-vet-40.txt>.

   [I-D.templin-ipwave-uam-its]
              Templin, F. L., "Urban Air Mobility Implications for
              Intelligent Transportation Systems", Work in Progress,
              Internet-Draft, draft-templin-ipwave-uam-its-04, 4 January
              2021, <https://www.ietf.org/archive/id/draft-templin-
              ipwave-uam-its-04.txt>.

   [I-D.templin-ironbis]
              Templin, F. L., "The Interior Routing Overlay Network
              (IRON)", Work in Progress, Internet-Draft, draft-templin-
              ironbis-16, 28 March 2014,
              <https://www.ietf.org/archive/id/draft-templin-ironbis-
              16.txt>.

   [I-D.templin-v6ops-pdhost]
              Templin, F. L., "IPv6 Prefix Delegation and Multi-
              Addressing Models", Work in Progress, Internet-Draft,
              draft-templin-v6ops-pdhost-27, 1 January 2021,
              <https://www.ietf.org/archive/id/draft-templin-v6ops-
              pdhost-27.txt>.

   [IEN48]    Cerf, V., "The Catenet Model For Internetworking,
              https://www.rfc-editor.org/ien/ien48.txt", July 1978.

   [IEN48-2]  Cerf, V., "The Catenet Model For Internetworking (with
              figures), http://www.postel.org/ien/pdf/ien048.pdf", July
              1978.

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

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   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,
              <https://www.rfc-editor.org/info/rfc1812>.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <https://www.rfc-editor.org/info/rfc1918>.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              DOI 10.17487/RFC2003, October 1996,
              <https://www.rfc-editor.org/info/rfc2003>.

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

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

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

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

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

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

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

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   [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>.

   [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>.

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   [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>.

   [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>.

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   [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>.

   [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>.

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   [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>.

   [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.

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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

   Address resolution and 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
   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.

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   If the Client's underlay 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 Gateways in the communications path.  Direct
   interfaces must be tested periodically for reachability, e.g., via
   NUD.

A.4.  AERO Critical Infrastructure Considerations

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

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   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 ENETs
   that provide forwarding services for non-MNP destinations.  The Relay
   connects to the OMNI link and engages in eBGP peering with one or
   more Gateways 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
   ARS/ARR 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 through address resolution and route optimization.
   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:

   *  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.

   *  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.

   *  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.

   *  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.

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   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 ULAs at each point.  It then selects one AERO Proxy/Server
   address, and engages in RS/RA exchanges with the same Proxy/Server
   from all ANET connections.  The Client remains with this Proxy/Server
   unless or until the Proxy/Server fails, in which case it can switch
   over to an alternate Proxy/Server.  The Client can likewise switch
   over to a different Proxy/Server at any time if there is some reason
   for it to do so.  So, the AERO expectation is for a balance of
   function in the network and end system, with fault tolerance and
   resilience at both levels.

Appendix B.  Change Log

   << RFC Editor - remove prior to publication >>

   Changes from earlier versions:

   *  Submit for RFC publication.

Author's Address

   Fred L. Templin (editor)
   Boeing Research & Technology
   P.O. Box 3707
   Seattle, WA 98124
   United States of America
   Email: fltemplin@acm.org

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