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Automatic Extended Route Optimization (AERO)
draft-templin-6man-aero3-04

Document Type Active Internet-Draft (individual)
Author Fred Templin
Last updated 2024-05-22
Replaces draft-templin-intarea-aero2
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draft-templin-6man-aero3-04
Network Working Group                                 F. L. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Standards Track                             22 May 2024
Expires: 23 November 2024

              Automatic Extended Route Optimization (AERO)
                      draft-templin-6man-aero3-04

Abstract

   This document specifies an Automatic Extended Route Optimization
   (AERO) service for IP internetworking over Overlay Multilink Network
   (OMNI) interfaces.  AERO/OMNI uses IPv6 Neighbor Discovery (IPv6 ND)
   for control plane 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 path selection, multinet traversal, mobility
   management, multicast forwarding, multihop operation and route
   optimization are naturally supported through dynamic neighbor cache
   updates on a per flow basis.  Both Provider-Aggregated (PA) and
   Provider-Independent (PI) addressing services are supported.  AERO is
   a widely-applicable service especially well-suited for air/land/sea/
   space mobility applications including aviation, intelligent
   transportation systems, mobile end user devices, space exploration
   and many others.

Status of This Memo

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

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

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

   This Internet-Draft will expire on 23 November 2024.

Copyright Notice

   Copyright (c) 2024 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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  19
   4.  Automatic Extended Route Optimization (AERO)  . . . . . . . .  20
     4.1.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .  20
     4.2.  The AERO Service over OMNI Links  . . . . . . . . . . . .  21
       4.2.1.  AERO/OMNI Reference Model . . . . . . . . . . . . . .  21
       4.2.2.  Addressing and Node Identification  . . . . . . . . .  25
       4.2.3.  AERO Routing System . . . . . . . . . . . . . . . . .  26
       4.2.4.  Segment Routing Topologies (SRTs) . . . . . . . . . .  28
       4.2.5.  Segment Routing For OMNI Link Selection . . . . . . .  29
     4.3.  OMNI Interface Characteristics  . . . . . . . . . . . . .  29
     4.4.  OMNI Interface Initialization . . . . . . . . . . . . . .  32
       4.4.1.  AERO Gateway Behavior . . . . . . . . . . . . . . . .  32
       4.4.2.  AERO Proxy/Server and Relay Behavior  . . . . . . . .  32
       4.4.3.  AERO Client Behavior  . . . . . . . . . . . . . . . .  33
       4.4.4.  AERO Host Behavior  . . . . . . . . . . . . . . . . .  33
     4.5.  OMNI Interface Neighbor Cache Maintenance . . . . . . . .  34
       4.5.1.  OMNI ND Messages  . . . . . . . . . . . . . . . . . .  37
       4.5.2.  OMNI Neighbor Advertisement Message Flags . . . . . .  39
       4.5.3.  OMNI Neighbor Window Synchronization  . . . . . . . .  40
     4.6.  OMNI Interface Encapsulation and Fragmentation  . . . . .  40
     4.7.  OMNI Interface Decapsulation  . . . . . . . . . . . . . .  43
     4.8.  OMNI Interface Data Origin Authentication . . . . . . . .  43
     4.9.  OMNI Interface MTU  . . . . . . . . . . . . . . . . . . .  44
     4.10. OMNI Interface Forwarding Algorithm . . . . . . . . . . .  44
       4.10.1.  Host Forwarding Algorithm  . . . . . . . . . . . . .  46
       4.10.2.  Client Forwarding Algorithm  . . . . . . . . . . . .  47
       4.10.3.  Proxy/Server and Relay Forwarding Algorithm  . . . .  48
       4.10.4.  Gateway Forwarding Algorithm . . . . . . . . . . . .  50
     4.11. OMNI Interface Error Handling . . . . . . . . . . . . . .  52
     4.12. AERO Mobility Service Coordination  . . . . . . . . . . .  55
       4.12.1.  AERO Service Model . . . . . . . . . . . . . . . . .  55
       4.12.2.  AERO Host and Client Behavior  . . . . . . . . . . .  57
       4.12.3.  AERO Proxy/Server Behavior . . . . . . . . . . . . .  57
     4.13. AERO Address Resolution, Multilink Forwarding and Route
            Optimization . . . . . . . . . . . . . . . . . . . . . .  63

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       4.13.1.  Multilink Address Resolution . . . . . . . . . . . .  65
       4.13.2.  Multilink Forwarding . . . . . . . . . . . . . . . .  70
       4.13.3.  Mobile Ad-hoc Network (MANET) Forwarding . . . . . .  79
       4.13.4.  Client/Client Route Optimization . . . . . . . . . .  80
       4.13.5.  Intra-ANET/ENET Route Optimization for AERO Peers  .  82
     4.14. Neighbor Unreachability Detection (NUD) . . . . . . . . .  82
     4.15. Mobility Management and Quality of Service (QoS)  . . . .  84
       4.15.1.  Mobility Update Messaging  . . . . . . . . . . . . .  85
       4.15.2.  Announcing Link-Layer Information Changes  . . . . .  86
       4.15.3.  Bringing New Links Into Service  . . . . . . . . . .  86
       4.15.4.  Deactivating Existing Links  . . . . . . . . . . . .  86
       4.15.5.  Moving Between Proxy/Servers . . . . . . . . . . . .  87
       4.15.6.  Accommodating Path Changes . . . . . . . . . . . . .  88
     4.16. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  89
       4.16.1.  Source-Specific Multicast (SSM)  . . . . . . . . . .  90
       4.16.2.  Any-Source Multicast (ASM) . . . . . . . . . . . . .  91
       4.16.3.  Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . .  92
     4.17. Operation over Multiple OMNI Links  . . . . . . . . . . .  92
     4.18. DNS Considerations  . . . . . . . . . . . . . . . . . . .  93
     4.19. Transition/Coexistence Considerations . . . . . . . . . .  93
     4.20. Proxy/Server-Gateway Bidirectional Forwarding
            Detection  . . . . . . . . . . . . . . . . . . . . . . .  94
     4.21. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . .  94
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .  94
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  95
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  95
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  98
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 100
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . 100
     9.2.  Informative References  . . . . . . . . . . . . . . . . . 102
   Appendix A.  Non-Normative Considerations . . . . . . . . . . . . 108
     A.1.  Implementation Strategies for Route Optimization  . . . . 108
     A.2.  Implicit Mobility Management  . . . . . . . . . . . . . . 109
     A.3.  Direct Underlying Interfaces  . . . . . . . . . . . . . . 109
     A.4.  AERO Critical Infrastructure Considerations . . . . . . . 110
     A.5.  AERO Server Failure Implications  . . . . . . . . . . . . 110
     A.6.  AERO Client / Server Architecture . . . . . . . . . . . . 111
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . . 113
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 113

1.  Introduction

   Automatic Extended Route Optimization (AERO) fulfills the
   requirements of Distributed Mobility Management (DMM) [RFC7333] and
   route optimization [RFC5522] for air/land/sea/space mobility
   applications including aeronautical networking intelligent
   transportation systems, home network users, enterprise mobile device
   users, space exploration and many others.  AERO is a secure

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   internetworking and mobility management service that employs the
   Overlay Multilink Network Interface (OMNI) [I-D.templin-6man-omni3]
   Non-Broadcast, Multiple Access (NBMA) virtual link model.

   The OMNI link is an adaptation layer virtual overlay manifested by
   IPv6 encapsulation over a network-of-networks concatenation of
   underlay Internetworks.  Nodes on the link can exchange original IP
   packets or parcels
   [I-D.templin-6man-parcels2][I-D.templin-intarea-parcels2] as single-
   hop neighbors; 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 end systems
   via Proxy/Servers and Relays as intermediate systems as necessary;
   AERO further employs Gateways that interconnect diverse Internetworks
   as OMNI link segments through OAL forwarding at a layer below IP.
   Each node's OMNI interface supports operation of IPv6 Neighbor
   Discovery (IPv6 ND) [RFC4861] as the mobility service control message
   protocol.  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
   use Proxy/Servers acting as proxys and/or designated routers while
   correspondent nodes on foreign networks may use any Relay on the link
   for efficient communications.  Foreign network correspondent nodes
   forward original IP packets/parcels 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.

   AERO supports both Provider-Aggregated (PA) and Provider-Independent
   (PI)addressing.  Correspondent nodes on foreign networks configure PA
   addresses from Foreign Network Prefixes (FNPs) advertised by Relays.
   AERO Clients instead obtain stable PA addresses from Stable Network
   Prefixes (SNPs) assigned to and managed by First Hop Segment (FHS)
   Proxy/Servers.  Mobile node Clients can also register Mobile Network
   Prefixes (MNPs) with Mobility Anchor Point (MAP) Proxy/Servers to
   support PI mobile networking.

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   AERO Clients receive SNP (PA) addresses and optionally also MNP (PI)
   prefix delegations through control message exchanges with Proxy/
   Servers over their local networks.  Proxy/Servers provide anchor
   points for both local network PA operation and global mobility.  By
   linking mobile PI prefixes with stable PA addresses, the AERO service
   supports the best aspects of PA/PI working together.

   AERO Gateways peer with Proxy/Servers in a secured private BGP
   overlay routing instance to establish a Segment Routing Topology
   (SRT) virtual 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 a set of
   Mobility Service Prefixes (MSPs) common to all mobile nodes.  Relays
   provide an optimal route from correspondent nodes on foreign links/
   networks to mobile or fixed nodes on the local 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 IPv4 and IPv6
   Internets.  In both cases, Clients may be located behind Network
   Address Translators (NATs) on the path to their associated Proxy/
   Servers and/or peers.  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.

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   AERO provides a secure aeronautical internetworking service for both
   manned and unmanned aircraft, where the aircraft is treated as a
   mobile node (MN) that can connect airborne Internet of Things (IoT)
   sub-networks.  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 Virtual Private Network (VPN) or open Internetwork services
   enabled according to the appropriate security model.  AERO also
   supports terrestrial vehicular, urban air mobility and mobile
   pedestrian communication services for intelligent transportation
   systems [RFC9365].  Other applicable use cases including home and
   small office networks, enterprise networks and many others represent
   additional large classes of potential AERO/OMNI users.

   Along with OMNI, AERO provides secured optimal routing support for
   the "6 M'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.

   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.

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   6.  (Performance) Maximization - the ability to exchange large
       packets/parcels between peers without loss due to a link size
       restriction, and to adaptively adjust packet/parcel sizes to
       maintain the best performance profile 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
   OMNI specification terminology [I-D.templin-6man-omni3] and the IPv6
   Neighbor Discovery (IPv6 ND) [RFC4861] node variables, protocol
   constants and message types (including Router Solicitation (RS),
   Router Advertisement (RS), Neighbor Solicitation (NS), Neighbor
   Advertisement (NA), unsolicited NA (uNA) and Redirect) are cited
   extensively throughout.

   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 both
   configures an OMNI interface and engages the OMNI Adaptation Layer.

   The terms "All-Routers multicast", "All-Nodes multicast", "Solicited-
   Node multicast" and "Subnet-Router anycast" are defined in [RFC4291].

   The term "IP" refers generically to either Internet Protocol version
   (IPv4 [RFC0791] or IPv6 [RFC8200]) for specification elements that
   apply equally to both.

   The terms "application layer (L5 and higher)", "transport layer
   (L4)", "network layer (L3)", "(data) link layer (L2)" and "physical
   layer (L1)" are used consistently with common Internetworking
   terminology, with the understanding that reliable delivery protocol
   users of UDP are considered as transport layer elements.  The OMNI
   specification further defines an "adaptation layer" positioned below
   the network layer but above the link layer, which may include
   physical links and Internet- or higher-layer tunnels.  A (network)
   interface is a node's attachment to a link (via L2), and an OMNI
   interface is therefore a node's attachment to an OMNI link (via the
   adaptation layer).

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   The terms "IP jumbogram", "advanced jumbo (AJ)" and "IP parcel" refer
   to special packet formats that enable a new link model for the
   Internet as discussed in [I-D.templin-6man-parcels2]
   [I-D.templin-intarea-parcels2].

   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-omni3].

   IPv6 Prefix Delegation (IPv6 PD)
      a networking service for delegating IPv6 prefixes to nodes on the
      link.  AERO nodes apply the IPv6 PD service provided by DHCPv6
      [RFC8415] in conjunction with OMNI interface IPv6 ND.

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

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

   Adaptation layer
      An encapsulation 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 network layer as
      "L3" and sees all link 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, corporate enterprise network, satellite service provider
      network, cellular operator network, residential WiFi network,
      etc.) that connects Clients to the Mobility Service over the OMNI
      lin.  Physical and/or data link level security is assumed and
      sometimes referred to as "protected spectrum" for wireless
      domains.  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.

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   Mobile Ad-hoc NETwork (MANET)
      a connected network region that shares the same properties as an
      ANET except that links often have undetermined connectivity
      properties, physical and/or data link layer security cannot always
      be assumed and adaptation layer multihop forwarding between
      Clients acting as MANET routers is often necessary.  MANETs use
      IPv6 Unique Local Addressing (ULAs) [RFC4193] internally to
      support multihop packet forwarding between neighboring node as an
      adaptation layer forwarding and addressing service.

   Internetwork (INET)
      a connected network region with a coherent IP addressing plan that
      provides transit forwarding services between (M)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 the network and/or
      higher layers.  The global public Internet itself is an example.

   End-user Network (ENET)
      a simple or complex "downstream" network tethered to a Client as a
      single logical unit that travels together.  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 end user
      devices.  The ENET provides an "upstream" link for arbitrarily
      many low-, medium- or high-end devices dependent on the Client for
      their upstream connectivity, i.e., as Internet of Things (IoT)
      entities.  The ENET can also support a recursively-descending
      chain of additional Clients such that the ENET of an upstream
      Client is seen as the ANET of a downstream Client.

   *NET
      a "wildcard" term used when a given specification applies equally
      to all MANET/ANET/INET cases.  From the Client's perspective, *NET
      interfaces are "upstream" interfaces that connect the Client to
      the Mobility Service, while ENET interfaces are "downstream"
      interfaces that the Client uses to connect downstream ENETs, Hosts
      and/or other Clients.  Local communications between correspondents
      within the same *NET can often be conducted based on IPv6 Unique
      Local Addresses (ULAs) at a layer below IP but without requiring
      adaptation layer encapsulation.

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   underlay network/interface
      a *NET or 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 or
      virtual communications media or coordinates with another node that
      hosts the media.

   MANET Interface
      a node's underlay interface connection to an ANET with
      indeterminant neighborhood properties over which adaptation layer
      multihop relaying may be necessary.  All MANET interfaces used by
      AERO/OMNI are IPv6 interfaces and therefore must configure a
      Maximum Transmission Unit (MTU) at least as large as the IPv6
      minimum MTU (1280 octets) even if link-layer fragmentation is
      needed.

   OMNI link
      the same as defined in [I-D.templin-6man-omni3].  The OMNI link
      employs IPv6 encapsulation to traverse intermediate systems 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/parcels to visit selected waypoints within the
      same OMNI link.

   OMNI link segment
      a Proxy/Server and all of its constituent Clients within any
      attached *NETs is considered as a leaf OMNI link segment, with
      each leaf interconnected via links and "bridge" nodes in
      intermediate OMNI link segments.  When the *NETs of multiple leaf
      segments overlap (e.g., due to network mobility), they can combine
      to form larger *NETs with no changes to Client-to-Proxy/Server
      relationships.  The OMNI link consists of the concatenation of all
      OMNI link leaf and intermediate segments as a loop-free spanning
      tree.

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

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   OMNI Adaptation Layer (OAL)
      an OMNI interface sublayer service that encapsulates original IP
      packets/parcels 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.

   (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
      (multi-)segment as discussed throughout this document.)

   (OMNI) L2 encapsulation
      the OMNI protocol encapsulation of OAL packets/fragments in an
      outer header or headers to form carrier packets that can be routed
      within the scope of the local *NET or ENET underlay network
      partition.  Common L2 encapsulation combinations include UDP/IP/
      Ethernet, etc. using a port/protocol/type number for OMNI.

   L2 address (L2ADDR)
      an address that appears in the L2 encapsulation for an underlay
      interface and also in IPv6 ND message OMNI options.  L2ADDR can be
      either an IP address for IP encapsulations or an IEEE EUI address
      [EUI] for direct data link encapsulation.  (When UDP/IP
      encapsulation is used, the UDP port number is considered an
      ancillary extension of the IP L2ADDR.)

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

   OAL packet
      an original IP packet/parcel encapsulated in an OAL IPv6 header
      with an IPv6 Extended Fragment Header extension that includes an
      8-octet (64-bit) OAL Identification value.  Each OAL packet is
      then subject to OAL fragmentation and reassembly.

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   OAL fragment
      a portion of an OAL packet following fragmentation but prior to L2
      encapsulation/fragmentation, or following L2 reassembly/
      decapsulation but prior to OAL reassembly.

   (OAL) atomic fragment
      an OAL packet that can be forwarded without fragmentation, but
      still includes an IPv6 Extended Fragment Header with an 8-octet
      (64-bit) OAL Identification value and with Index and More
      Fragments both set to 0.  (Note that control message atomic
      fragments also omit the Extended Fragment Header over secured
      spanning tree links.)

   (L2) carrier packet
      an encapsulated OAL packet/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 systems.  OAL intermediate systems
      re-encapsulate OAL packets/fragments during forwarding by removing
      the L2 headers of carrier packets from a previous hop underlay
      network and replacing them with new L2 headers for the next hop
      underlay network.  Carrier packets may themselves be subject to
      fragmentation and reassembly in L2 underlay networks at a layer
      below the OAL.  Carrier packets sent over unsecured paths use OMNI
      protocol L2 encapsulations, while those sent over the secured
      paths use L2 security encapsulations such as IPsec [RFC4301], etc.

   OAL source
      an OMNI interface acts as an OAL source when it encapsulates
      original IP packets/parcels to form OAL packets, then performs OAL
      fragmentation and L2 encapsulation to create carrier packets.
      Every OAL source is also an OAL end system.

   OAL destination
      an OMNI interface acts as an OAL destination when it decapsulates
      carrier packets, then performs OAL reassembly/decapsulation to
      restore the original IP packet/parcel.  Every OAL destination is
      also an OAL end system.

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   OAL intermediate system
      an OMNI interface acts as an OAL intermediate system when it
      performs L2 reassembly/decapsulation for carrier packets received
      from a previous hop, then performs L2 encapsulation/fragmentation
      on the enclosed OAL packets/fragments and forwards these new
      carrier packets to the next hop.  OAL intermediate systems
      decrement the OAL Hop Limit during forwarding, and discard the OAL
      packet/fragment if the Hop Limit reaches 0.  OAL intermediate
      systems do not decrement the TTL/Hop Limit of the original IP
      packet/parcel.

   Mobility Service Prefix (MSP)
      an aggregated IP Global Unicast Address (GUA) prefix (e.g.,
      2001:db8::/32, 2002:192.0.2.0::/40, etc.) assigned to the OMNI
      link and from which more-specific Mobile and Stable Network
      Prefixes (MNPs/SNPs) are delegated, where IPv4 MSPs are
      represented as "6to4 prefixes" per [RFC3056].  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-omni3]).
      OMNI links that connect to the global Internet advertise their
      MSPs to interdomain routing peers.

   Mobile Network Prefix (MNP)
      a longer IP prefix derived from an MSP (e.g.,
      2001:db8:1000:2000::/56, 2002:192.0.2.8::/48, etc.) and delegated
      to an AERO Client.

   Stable Network Prefix (SNP)
      a global and unique-local IP prefix pair assigned to one or more
      Proxy/Servers that connect local *NET Client groups to the rest of
      the OMNI link.  Clients request address delegations from the SNP
      that can be used to support PA communications.  Clients
      communicate internally within (M)ANETs and INET groups using IPv6
      Unique Local Addresses (ULAs) [RFC4193] assigned in 1x1
      correspondence to SNP Globally Unique Addresses (GUAs) [RFC4291]
      made visible to external peers through IP network address/prefix
      translation [RFC6145][RFC6146][RFC6147]
      [I-D.bctb-6man-rfc6296-bis].

   Foreign Network Prefix (FNP)
      a global IP prefix not covered by a MSP and assigned to a link or
      network outside of the AERO/OMNI domain.  Relays advertise any of
      their associated FNPs into the AERO/OMNI routing system and
      forward packets between MNP/SNP mobile or fixed nodes on the OMNI
      link and FNP correspondent nodes on other links.

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   Subnet Router Anycast (SRA) Address
      An IPv6 address taken from an FNP/MNP/SNP in which the remainder
      of the address beyond the final bit of the prefix is set to the
      value "all-zeros".  For example, the SRA for 2001:db8:1::/48 is
      simply 2001:db8:1:: (i.e., with the 80 least significant bits set
      to 0).  For IPv4, the IPv6 SRA corresponding to the IPv4 prefix
      192.0.2.0/24 is 2002:192.0.2.0::/40 per [RFC3056].

   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].

   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].  (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.)

   Globally Unique Address (GUA)
      a globally unique IPv6 address per the IPv6 addressing
      architecture [RFC4291] or a globally unique IPv4 address that is
      not reserved for a special-purpose per [RFC6890].

   Hierarchical Host Identity Tag (HHIT)
      a 128-bit IPv6 address according to [RFC9374].  Each Client
      assigns a unique HHIT used to bootstrap autoconfiguration in the
      presence of OMNI link infrastructure or for sustained
      communications in the absence of infrastructure.  When a Client
      receives a PA SNP GUA/ULA delegation from a Proxy/Server or a PI
      prefix delegation from a MAP, it can begin using PA/PI addresses
      instead of its HHIT for Internetworking communications.

   Provider Independent (PI) Address
      a global unicast IP address allocated from an MNP delegated to a
      Client via a MAP Proxy/Server is considered Provider-Independent
      (Proxy/Server-Independent) or "PI".

   Provider Aggregated (PA) Address
      a global unicast IP address delegated to a Client from a SNP
      assigned to a FHS Proxy/Server is considered Provider-Aggregated
      (Proxy/Server-Aggregated) or "PA".

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

   (AERO) Host
      an AERO node that configures an OMNI interface over an ENET
      underlay 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 underlay interface.  When
      an AERO host forwards an original IP packet/parcel to another AERO
      node on the same ENET, it uses simple IP-in-L2 OMNI encapsulation
      without including an OAL encapsulation header.  The Host is
      therefore an OMNI link termination endpoint.  (Note: as an
      implementation matter, the Host may instead configure the "OMNI
      interface" as a virtual sublayer of the underlay interface
      itself.)

   (AERO) Client
      an AERO node that configures an OMNI interface over one or more
      underlay interfaces and requests SNP address and/or MNP prefix
      delegations from AERO Proxy/Servers.  The Client assigns a HHIT
      (as well as Proxy/Server-specific ULAs) to the OMNI interface for
      use in IPv6 ND exchanges with other AERO nodes and forwards
      original IP packets/parcels 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.

   (AERO) Proxy/Server
      an AERO node that provides a proxying service between AERO Clients
      and external peers on its Client-facing (M)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 (M)ANET
      interfaces.)  The Proxy/Server configures an OMNI interface and
      maintains BGP peerings with Gateways to provide a local anchor
      point for its stable and/or mobile Clients.  All Proxy/Servers
      configure a Stable Network Prefix (SNP) and manage 1x1 mappings of
      internal Unique Local Addresses (ULAs) and external Globally
      Unique Addresses (GUAs) according to Network Prefix Translation
      for IPv6 (NPTv6) [I-D.bctb-6man-rfc6296-bis].

   (AERO) Relay
      an AERO Proxy/Server that provides forwarding services between
      nodes reached via the OMNI link and correspondents on foreign
      links/networks.  AERO Relays maintain BGP peerings with Gateways

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      the same as Proxy/Servers.  Relays also run a dynamic routing
      protocol to discover any Foreign Network Prefix (FNP) routes in
      service on other links/networks, advertise OMNI link MSP(s) to
      other links/networks, and redistribute FNPs 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 Internets can also be configured to advertise "default"
      routes into the OMNI link BGP routing system.)

   (AERO) Gateway
      a BGP hub autonomous system node that also provides OAL forwarding
      services for nodes on an OMNI link.  Gateways forward OAL packets/
      fragments between OMNI link segments as OAL intermediate systems
      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 FNP/MNP/SNP prefixes in service.  Gateways process OAL
      packets/fragments received over the secured spanning tree that are
      addressed to themselves, while forwarding all other OAL packets/
      fragments to the next hop also via the secured spanning tree.
      Gateways forward OAL packets/fragments 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.  It is important to note that all Gateways are
      also Proxy/Servers, but only those Proxy/Servers configured as
      intermediate nodes in the spanning tree are considered Gateways.

   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 systems 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.

   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 systems 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.

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

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

   Mobility Anchor Point (MAP) Proxy/Server
      a single Proxy/Server selected by a Client that injects the
      Client's 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 MAP role (as all FHS Proxy/Servers are
      equally capable candidates to serve as a MAP), however the Client
      can also select any available Proxy/Server for the OMNI link (as
      there is no requirement that the MAP must also be one of the
      Client's FHS Proxy/Servers).  This flexible arrangement supports a
      fully distributed mobility management service.

   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 communicating peer nodes.  The SRT maintains a
      spanning tree established through BGP peerings between Gateways
      and Proxy/Servers.  Each SRT leaf 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 and its
      connected IoT sub-networks.

   Mobile Router (MR)
      a MN's on-board router that forwards original IP packets/parcels
      between any downstream-attached networks and the OMNI link.  The
      MR is the MN entity that hosts the AERO Client.

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   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 MAP Proxy/Server.  Note that a MAP Proxy/Server can assume
      the ARR role even if it is located on a different SRT segment than
      the ART.  The MAP Proxy/Server assumes the ARR role only when it
      receives an RS message from the ART with the 'ARR' flag set (see:
      [I-D.templin-6man-omni3]).

   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 system 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 systems in the
      path can be "skipped" to reduce path stretch and decrease load on
      critical infrastructure elements.

   AERO Forwarding Vector (AFV)
      An AFIB entry that includes soft state for each underlay interface
      pairwise communication session between peer neighbors.  AFVs are

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      identified by an AFV Index (AFVI) paired with the previous hop L2
      address, with the pair established based on an IPv6 NS/NA or RS/RA
      exchanges.  The AFV also caches underlay interface Identification
      sequence number parameters to support carrier packet filtering.

   AERO Forwarding Vector Index (AFVI)
      A 2-octet or 4-octet integer value supplied by a first hop OAL
      node when it requests a next hop OAL node to create an AFV.  (The
      AFVI is always processed as a 4-octet value, but may be
      transmitted as only the 2 least significant octets when the 2 most
      significant octets are 0.)  The next hop OAL node caches the AFVI
      and L2 address supplied by the previous hop as header compression/
      decompression state for future OAL packets with compressed
      headers.  The first hop OAL node must ensure that the AFVI values
      it assigns to the next hop via a specific underlay interface are
      distinct and reused only after their useful lifetimes expire.  The
      special AFVI value 0 means that no AFVI is assigned.

   flow
      A sequence of packets sent from a particular source to a
      particular unicast, anycast, or multicast destination that a node
      desires to label as a flow.  The 3-tuple of the Flow Label, Source
      Address and Destination Address fields enable efficient IPv6 flow
      classification.  The IPv6 Flow Label Specification is observed per
      [RFC6437] [RFC6438].

3.  Requirements

   OMNI interfaces limit the size of their IPv6 ND control plane
   messages (plus any original IP packet/parcel attachments) to the
   minimum IPv6 link MTU minus overhead for adaptation and link layer
   encapsulation.  If there are sufficient OMNI parameters and/or IP
   packet/parcel attachments that would exceed this size, the OMNI
   interface forwards the information as multiple smaller IPv6 ND
   messages and the recipient accepts the union of all information
   received.  This allows the messages to travel without loss due to a
   size restriction over secured control plane paths that include IPsec
   tunnels [RFC4301], secured direct point-to-point links and/or
   unsecured paths that require an authentication signature.

   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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4.  Automatic Extended Route Optimization (AERO)

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

4.1.  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 receive Provider-Aggregated (PA) SNP
   addresses from their Proxy/Servers.  AERO Clients that obtain
   Provider-Independent (PI) Mobile Network Prefixes (MNPs) register
   them 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) *NET boundary (or standalone
   INET) deployments and hence have permanent and unchanging INET
   addresses.  Together, they provide access to the AERO service OMNI
   link virtual overlay for connecting AERO Clients and Hosts.  AERO
   Gateways (together with Proxy/Servers and Relays) provide the secured
   backbone supporting infrastructure for a Segment Routing Topology
   (SRT) spanning tree for the OMNI link.

   AERO Gateways are Proxy/Servers deployed as OMNI link intermediate
   systems that forward 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).  The OMNI interface
   and OAL provide an adaptation layer forwarding service that the
   network layer perceives as L2 bridging, since the inner IP TTL/Hop
   Limit is not decremented.  Each Gateway peers with Proxy/Servers,
   Relays and other Gateways in a dynamic routing protocol instance as a
   Distributed Mobility Management (DMM) service for the list of active
   MNPs (see: Section 4.2.3).  Gateways assign one or more Mobility

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   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 FNP/MNP/SNP prefix in
   service on the OMNI link.

   AERO Proxy/Servers distributed across one or more SRT segments
   provide default forwarding and mobility/multilink services for AERO
   Client mobile nodes.  Each Proxy/Server acts as either an OMNI link
   intermediate system or end system according to the service model
   selected by each Client.  Proxy/Servers also peer with Gateways in an
   adaptation layer dynamic routing protocol instance to advertise its
   list of associated MNPs (see Section 4.2.3).  MAP Proxy/Servers
   provide prefix delegation 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 *NET
   boundaries provide a primary forwarding service for (M)ANET Client/
   Host communications with peers in external INETs, while Proxy/Servers
   in open INETs provide an authentication service for IPv6 ND messages
   but should be used only as a last resort data plane forwarding
   service when a Client cannot forward directly to an INET 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/parcels between the OMNI link and fixed
   or mobile nodes on other links/networks.  Relays run a dynamic
   routing protocol to discover any FNP prefixes in service on foreign
   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 FNPs via OMNI link Gateway BGP peerings.

4.2.  The AERO Service over OMNI Links

4.2.1.  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 diverse
      administrative domains using 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 other Gateways in the SRT
      (not shown).  Together, the set of all 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
      FNP 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) 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/parcel.  An
   example OMNI link SRT is shown in Figure 2:

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    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
  .                                                                           .
 .                                                                             .
 .     .-(::::::::)                .-(::::::::)               .-(::::::::)     .
 .  .-(::::::::::::)-.   +-+    .-(::::::::::::)-.   +-+   .-(::::::::::::)-.  .
 . (::::    FHS    :::)--|G|--(::: Intermediate ::)--|G|--(::::    LHS    :::) .
 .  `-(::::::::::::)-'   +-+    `-(::Segments::)-'   +-+   `-(::::::::::::)-'  .
 .     `-(::::::)-'                `-(::::::)-'               `-(::::::)-'     .
 .           |                                                      |          .
 .         +---+                                                  +---+        .
 .         |P/S|                                                  |P/S|        .
 .         +---+                                                  +---+        .
 .           |                                                      |          .
 .     .-(::::::::)                                          .-(::::::::)      .
 .  .-(: First Hop :)-.  +-------+             +-------+   .-(: Last Hop :)-.  .
 . (::::  Access  ::::)--| Source|             | Target|--(::::  Access  ::::) .
 .  `-(:: Network ::)-'  | Client|             | Client|     (:: Network ::)-' .
 .     `-(::::::)-'      +-------+             +-------+      `-(::::::)-'     .
 .                                                                             .
 .                                                                             .
 .         <--  Segment Routing Topology (SRT) Spanned by OMNI Link -->        .
   .                                                                          .
     . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

          Figure 2: OMNI Link Segment Routing Topology (SRT)

   In the Segment Routing Topology, a source Client connects via a first
   hop access network served by a First Hop Segment (FHS) Proxy/Server.
   The FHS Proxy/Server then forwards to an FHS Gateway which connects
   to an arbitrarily complex set of Intermediate Segments.  Adjacent
   intermediate Segments are joined by intermediate Gateways (not shown)
   that serve as adaptation layer IPv6 routers, with the final segment
   connected by a Last Hop Segment (LHS) Gateway.  The LHS Gateway then
   forwards to an LHS Proxy/Server which in turn connects to the last
   hop access network where the target Client resides.

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   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 (and any trailing data plane message
   extensions).  The "unsecured spanning tree" conveys ordinary carrier
   packets without security codes and that must be examined 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.

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

   The Catenet concept first articulated in the 1970's was distorted
   through the evolution of the Internet in the decades that followed,
   since the adaptation layer was a critical element missing from the
   architecture.  As a result, while the Internet has been successful
   beyond measure it has evolved as a monolithic public routing and
   addressing service interconnecting private domains instead of a true
   network-of-networks which has impeded flexibility and inhibited end-
   to-end services.  The adaptation layer manifested by AERO and OMNI
   now provides the means to address these limitations as well as the
   other "6 Ms of Modern Internetworking" according to the original
   Catenet network-of-networks vision.

4.2.2.  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 to satisfy
   the requirements of [RFC4861].  AERO node LLAs only need to be unique
   on the local OMNI link segment, however, since there is no way to
   coordinate duplicate address detection between disjoint OMNI link
   segments.  Therefore, OMNI interface intermediate systems should not
   forward packets with LLA source and/or destination addresses.

   AERO Clients 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

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   {ULA*}::/64 [RFC4291].  AERO Proxy/Servers assign ULAs to Clients as
   *NET internal addresses in 1x1 correspondence with Globally-Unique
   Addresses (GUAs) as *NET external addresses according to NPTv6
   [I-D.bctb-6man-rfc6296-bis].

   AERO MSPs, FNPs, MNPs and SNPs 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].  See
   [I-D.templin-6man-omni3] for a full specification of LLAs, ULAs, GUAs
   and anycast addresses used by AERO nodes on OMNI links.

   Finally, AERO Clients configure HHITs as specified in
   [I-D.templin-6man-omni3] to bootstrap the process of receiving ULA
   delegations from Proxy/Servers.

4.2.3.  AERO Routing System

   The AERO routing system comprises a private Border Gateway Protocol
   (BGP) [RFC4271] service coordinated between Gateways as interior
   nodes and Proxy/Servers and Relays as leaf nodes of a spanning tree.
   The service supports OAL packet/fragment 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 for the SNP prefixes
   assigned to Proxy/Serves and the set of all FNP/MNP routes that are
   currently active; Gateways also maintain black-hole routes for the
   OMNI link MSPs so that OAL packets/fragments destined to non-existent
   more-specific routes are flushed from the routing system.  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 foreign links) and they forward all other OAL
   packets/fragments to Gateways which have full topology knowledge.

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   Each OMNI link segment assigns a unique sub-prefix of the MSP known
   as the "SRT prefix".  For example, a first segment could assign
   2001:db8::/48, a second could assign 2001:db8:1::/48, a third could
   assign 2001:db8:2::/48, etc.  Within each segment, each Proxy/Server
   and Gateway configures an SNP within the segment's SRT prefix, e.g.,
   the SNPs 2001:db8::/64, 2001:db8:0:1::/64 2001:db8:0:2::/64 all
   belong to the SRT prefix 2001:db8::/48.

   The administrative authorities for each segment must therefore
   coordinate to assure mutually-exclusive SNP assignments, but internal
   provisioning of SNPs is an independent local consideration for each
   administrative authority.  For each SRT prefix, the Gateway(s) that
   connect that segment assign the all-zero's address of the prefix as a
   Subnet Router Anycast (SRA) address.  For example, the SRA address
   for 2001:db8::/48 is simply 2001:db8::. All Proxy/Servers also assign
   the SRA address taken from their uniquely-assigned more-specific SNP,
   e.g., the SRA address for the SNP 2001:db8:0:1::/64 is simply
   2001:db8:0:1::.

   SRT prefixes (and their SNP sub-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 SRT prefixes either
   via standard BGP routing or static routes.  For example, if three
   Gateways ('A', 'B' and 'C') from different segments serviced
   2001:db8::/48, 2001:db8:1::/48 and 2001:db8:2::/48 respectively, then
   the forwarding tables in each gateway appear as follows:

   A:  2001:db8::/48->local, 2001:db8:1::/48->B, 2001:db8:2::/48->C

   B:  2001:db8::/48->A, 2001:db8:1::/48->local, 2001:db8:2::/48->C

   C:  2001:db8::/48->A, 2000:db8:1::/48->B, 2001:db8:2::/48->local

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

   FNP and MNP routes are instead dynamically advertised in the AERO
   routing system by Proxy/Servers and Relays that provide anchor points
   for their corresponding prefixes.  For example, if three Proxy/
   Servers ('D', 'E' and 'F') service the MNPs 2001:db8:1000:1::64/,
   2001:db8:1000:2::/64 and 2001:db8:1000:2::/48 then the routing system
   would include:

   D:  2001:db8:1000:1::/64

   E:  2001:db8:1000:2::/64

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   F:  2001:db8:1000:3::/64

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

4.2.4.  Segment Routing Topologies (SRTs)

   The distinct GUA 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 GUAs, 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 GUA prefix and assigns an IPv6
   Subnet Router Anycast (SRA) address used for OMNI interface
   determination in Safety-Based Multilink (SBM) as discussed in
   [I-D.templin-6man-omni3].  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 (and
   sometimes data message extensions) between Proxy/Servers and
   Gateways, while the unsecured spanning tree forwards bulk data
   messages and/or unsecured control messages.

   Each SRT segment is identified by a unique GUA 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.

   Note: The distinct GUA 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 GUAs of multiple OMNI links within a
   common routing system may be acceptable.

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4.2.5.  Segment Routing For OMNI Link Selection

   Original IPv6 sources can direct IPv6 packets/parcels to an AERO node
   by including a standard IPv6 Segment Routing Header (SRH) [RFC8754]
   with the IPv6 SRA 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/parcel will traverse when there may be multiple
   alternatives.

   When an AERO node processes the SRH and forwards the original IPv6
   packet/parcel 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).

4.3.  OMNI Interface Characteristics

   OMNI interfaces are virtual interfaces configured over one or more
   underlay interfaces classified as follows:

   *  (M)ANET interfaces connect to a protected and secured ANET or an
      open MANET that connects to an INETs via Proxy/Servers.  The
      (M)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 a (M)ANET and
      may require NAT traversal on the path to the Proxy/Server the same
      as for the INET case.)  MANETs are special cases of ANETs in which
      adaptation layer multihop forwarding may be necessary, and
      protected secured underlay links cannot always be assumed.

   *  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
      correspondent on the same INET.  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.

   *  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 IoT sub-network that travels with a mobile Client to as
      complex as a large private enterprise network that the Client
      connects to a larger ANET or INET.

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   *  VPN interfaces use security encapsulations (e.g.  IPsec tunnels)
      over underlay networks to connect Clients, Proxy/Servers and/or
      Gateways.  VPN interfaces provide security services at lower
      layers of the architecture (L2/L1) the same as for Direct point-
      to-point interfaces.

   *  Direct point-to-point interfaces securely connect Clients, Proxy/
      Servers and/or Gateways over physical or virtual media that does
      not transit any open Internetwork paths.  Examples include a line-
      of-sight link between a remote pilot and an unmanned aircraft, a
      fiberoptic link between Gateways, etc.

   OMNI interfaces use OAL encapsulation and fragmentation as discussed
   in Section 4.6.  OMNI interfaces use L2 encapsulation (see:
   Section 4.6) to exchange carrier packets with OMNI link neighbors
   over INET interfaces and IPsec tunnels as well as over ANET
   interfaces for which the Client and neighbor 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
   (M)ANET interfaces when the Client and neighbor are known to be on
   the same underlay link.

   OMNI interfaces maintain an adaptation layer neighbor cache for
   tracking per-neighbor state.  OMNI interfaces use IPv6 ND messages
   including Router Solicitation (RS), Router Advertisement (RA),
   Neighbor Solicitation (NS), Neighbor Advertisement (NA), unsolicited
   Neighbor Advertisement (uNA) and Redirect to manage the neighbor
   cache.  OMNI neighbors invoke per-flow OAL Identification window
   synchronization in their IPv6 ND message exchanges to enable source
   address verification, header compression and robust fragmentation/
   reassembly.

   OMNI interfaces send IPv6 ND messages with an OMNI option formatted
   as specified in [I-D.templin-6man-omni3].  The OMNI option includes
   prefix registration information, Interface Attributes and/or
   Multilink Vectors containing link information parameters for the OMNI
   interface's underlay interfaces (as well as any other per-neighbor
   information).  The presence of the OMNI option identifies each IPv6
   ND message as an adaptation layer (i.e., and not a network layer)
   control message.

   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 system services and is therefore a logical termination
   point for the OMNI link.

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   A Client's OMNI interface may be configured over multiple *NET
   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 *NET 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 Interface Attributes, Traffic Selector and/or Multilink
   Vector sub-options with the same underlay interface ifIndex.  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 *NET 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 sub-options with different underlay interface ifIndexes.

   Proxy/Servers on the open Internet include only a single INET
   underlay interface.  INET Clients therefore discover only the L2ADDR
   information for the Proxy/Server's INET interface.  Proxy/Servers on
   a (M)ANET/INET boundary include both (M)ANET and INET underlay
   interfaces.  (M)ANET Clients therefore must discover both the (M)ANET
   and INET L2ADDR information for their Proxy/Servers.

   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 data plane messages.  The OMNI interface configures a GUA
   and acts as an OAL source to encapsulate original IP packets/parcels,
   then fragments the resulting OAL packets, performs L2 encapsulation/
   fragmentation and sends 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 SNP GUA SRA source and
   destination addresses.  The GUA addresses that appear in the original
   IP packets/parcels of a BGP protocol session may therefore be the
   same as those that appear in the OAL IPv6 encapsulation header.

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4.4.  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/parcels with
   destination addresses covered by an MNP not explicitly associated
   with another interface are directed to an OMNI interface.

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

4.4.1.  AERO Gateway Behavior

   AERO Gateways configure an OMNI interface and assign an SNP with
   corresponding SRA GUA for their OMNI link SRT segments.  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 4.2.3).

4.4.2.  AERO Proxy/Server and Relay Behavior

   When a Proxy/Server enables an OMNI interface, it assigns an SNP GUA/
   ULA prefix pair.  The Proxy/Server then configures an SRA GUA
   appropriate for the given OMNI link SRT segment externally and
   configures an SRA ULA appropriate for the locally attached *NET
   internally.  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
   network layer, but internally serves as an NBMA nexus for exchanging
   carrier packets with other OMNI nodes over underlay interfaces and/or
   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 foreign
   networks/links (see: Section 4.2.3).  The Relay provisions MNPs and
   advertises the MSP(s) for the OMNI link over its foreign network
   interface attachments.  The Relay further provides an OMNI link
   attachment point for FNP-based topologies.

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4.4.3.  AERO Client Behavior

   When a Client enables an OMNI interface, it assigns a HHIT and sends
   OMNI-encapsulated RS messages over its *NET underlay interfaces to an
   FHS Proxy/Server, which allocates an SNP ULA/GUA address pair and
   optionally coordinates with a MAP Proxy/Server that delegates one or
   more MNPs.  The MAP/FHS Proxy/Servers then return an RA message to
   the Client.  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 HHIT in its initial RS messages, it will discover ULAs
   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 HHITs for Client-to-Client
   communications either indefinitely or at least until it encounters an
   infrastructure element that can delegate SNP ULA/GUA pairs and/or
   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.

4.4.4.  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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4.5.  OMNI Interface Neighbor Cache Maintenance

   Each Client and Proxy/Server OMNI interface maintains a network layer
   conceptual neighbor cache per [RFC1256] or [RFC4861] the same as for
   any IP interface.  The OMNI interface network layer neighbor cache is
   maintained through static and/or dynamic neighbor cache entry
   configurations.

   Each OMNI interface also maintains a separate internal adaptation
   layer conceptual neighbor cache that includes a Neighbor Cache Entry
   (NCE) for each of its active OAL neighbors per [RFC4861].  IPv6 ND
   messages that update the adaptation layer neighbor cache include ULA
   addresses as well as one or more OMNI options.  Throughout this
   document, the terms "neighbor cache" and "NCE" refer to this
   adaptation layer neighbor cache unless otherwise specified.

   Each OMNI interface NCE is indexed by the IPv6 address of a neighbor
   found in the ND message IPv6 header and determines the context for
   Identification verification.  Clients and Proxy/Servers maintain NCEs
   through dynamic RS/RA message exchanges, and also maintain NCEs for
   any active correspondent peers through dynamic NS/NA message
   exchanges.

   Hosts maintain NCEs for Clients and other Hosts through the exchange
   of RS/RA, NS/NA or Redirect messages.  Each NCE is indexed by the IP
   address assigned to the Host ENET interface, which is the same
   address used for 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.

   Clients establish NCEs for their associated FHS and MAP Proxy/Servers
   through the exchange of RS/RA messages.  When a Client and Proxy/
   Server establish NCEs, they set a ReachableTime timer to
   REACHABLE_TIME seconds.  Clients determine the service profiles for
   their FHS and MAP Proxy/Servers by setting the NUD/ARR/RPT flags in
   RS messages and also by setting/clearing the FMT-Forward and FMT-Mode
   flags in the Interface Attributes sub-option.  When the NUD/ARR/RPT
   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.

   Clients can therefore set the NUD/ARR/RPT flags in RS messages they
   send to select their Proxy/Server service profiles.  If the NUD flag
   is set, the FHS Proxy/Server that forwards the RS message assumes the

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   role of responding to NS messages and maintains peer NCEs associated
   with the NCE for this Client.  If the ARR flag is set, the MAP Proxy/
   Server that processes the RS message assumes the role of responding
   to NS for Address Resolution (NS(AR)) messages on behalf of this
   Client NCE.  If the RPT flag is set, the MAP Proxy/Server that
   processes the RS message becomes responsible for maintaining a
   "Report List" for each Client NCE for the source addresses of NS(AR)
   messages it forwards or responds to on behalf of this Client.

   When a Client sets the RPT flag, the MAP Proxy/Server maintains
   Report List entries based on a ReportTime timer initialized to
   REACHABLE_TIME seconds upon receipt of an NS(AR) and decremented once
   per second while no additional NS(AR)s arrive.  The MAP Proxy/Server
   then sends uNA Mobility Management (MM) messages to each Report List
   entry when it receives a Client mobility update indication (e.g.,
   through receipt of an RS with updated Interface Attributes and/or
   Traffic Selectors).  When a Report List entry ReportTime timer
   expires, the MAP Proxy/Server deletes the entry.  When a Client NCE
   timer expires, the MAP Proxy/Server deletes the NCE along with its
   associated Report List.

   Clients can also set/clear the FMT-Forward and FMT-Mode flags in the
   Interface Attributes sub-option of each RS message to express their
   desired service profile from each FHS Proxy/Server for a specific
   underlay interface.  The FHS Proxy/Server will consider the Client's
   preferences and either accept or override by setting/clearing the
   flags in the corresponding RA message reply.  Implications for these
   bit settings are discussed in [I-D.templin-6man-omni3].

   Both the Client and its MAP Proxy/Server have full knowledge of the
   Client's current underlay Interface Attributes and Traffic Selectors,
   while FHS Proxy/Servers acting in "proxy" mode have knowledge of only
   the individual Client underlay interfaces they service.  Clients
   determine their FHS and MAP Proxy/Server service models by setting
   the NUD/ARR/RPT flags in the RS messages they send as discussed
   above.

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   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 MAP Proxy/Server for the ART.
   The MAP 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 NS/NA multilink
   forwarding exchanges over specific underlay interface pairs to
   determine paths for sending carrier packets directly to the ART.  The
   ARS otherwise decrements ReachableTime while no further solicited NA
   messages arrive.

   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 OAL packets/fragments destined to the target Client to the
   Client's new FHS/MAP Proxy/Server instead.

   It is RECOMMENDED that REACHABLE_TIME be set to the default constant
   value 30 seconds as specified in [RFC4861].  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 ReachableTime or
   DepartTime decrement to 0, the NCE is deleted.

   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 uNAs 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.

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4.5.1.  OMNI ND Messages

   OMNI interfaces use IPv6 ND messages as the secured control plane
   messaging service for all adaptation layer neighbor coordination
   exchanges.  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-omni3].  OMNI interfaces use ULAs/GUAs
   instead of LLAs as adaptation layer IPv6 ND message source and
   destination addresses.  This allows multiple different OMNI links/
   segments to be joined into a single link/segment 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 sub-option information.  If the OMNI interface includes
   an authentication sub-option, it first writes the value 0 into the
   authentication signature field then calculates the signature
   beginning with the first IPv6 ND message octet following the header
   Checksum field and continuing over the entire length of the message.
   The OMNI interface next writes the authentication signature value
   into the appropriate OMNI authentication option field, then
   calculates the IPv6 ND message checksum per [RFC4443] beginning with
   a pseudo-header of the IPv6 header and writes the value into the
   Checksum field.  The IPv6 ND message checksum therefore provides
   integrity assurance for the message, while the authentication
   signature covers the entire packet or super-packet.  OMNI interfaces
   verify integrity and authentication of each message received, and
   process the message further only following successful 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
   both the neighbor cache and AERO Forwarding Information Base (AFIB)
   as basis for the forwarding algorithm specified in Section 4.10.  The
   information is cumulative and reflects the union of the OMNI
   information from the most recent IPv6 ND messages received from the
   neighbor.

   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 the adaptation layer to underlay interface address
   mappings while the latter pertains to the native L2 address format of
   the underlay media.

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   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
   solicitation message includes a Timestamp option, the recipient must
   also include a Timestamp option in its advertisement reply.  All
   unsolicited advertisement and redirect messages should include a
   Timestamp option.

   AERO Clients send RS messages to the link-scoped All-Routers
   multicast address or the ULA/GUA (SRA) address of a Proxy/Server
   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 GUA (SRA) address of
   a remote MAP Proxy/Server or the MNP (SRA) address 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.  The ARS sends
      an NS(AR) to the unicast 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.

   *  Other NS/NA message exchanges are used to determine target
      reachability (NS/NA(NUD)), establish/maintain Route Optimization
      state (NS/NA(RO)) or establish/maintain multilink forwarding state
      (NS/NA(MF)).  The source sends an NS to the unicast address of the
      target while optionally including an OMNI Multilink Vector sub-
      option naming a specific underlay interface pair, and the target
      returns a responsive NA.  NS/NA messages that use an in-window
      sequence number and do not update any other state need not include
      an authentication signature but must include an IPv6 ND message
      checksum.  NS/NA messages used to establish or update NCE and/or
      AFIB state must be secured.

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   *  Unsolicited NA (uNA) messages are used to signal addressing and/or
      other mobility management (uNA(MM)) neighbor state changes (e.g.,
      address changes due to mobility, signal degradation, traffic
      selector updates, etc.). uNA messages can also be also used to
      acknowledge receipt of other IPv6 ND messages (uNA(ACK)) as well
      as to securely convey ICMP error information (uNA(ERR)). 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.

   AERO and OMNI together support an added reliability feature not
   available in ordinary IPv6 ND messaging.  In particular, nodes can
   set the OMNI Neighbor Coordination SNR flag or Window Synchronization
   SYN/TST flag in unicast non-solicitation IPv6 ND messages (including
   RA, NA and Redirect) to request a synchronous (but "unsolicited")
   uNA(ACK) acknowledgement response (see: [I-D.templin-6man-omni3]).

   The node that processes an SNR/SYN/TST message prepares the response
   the same as for an ordinary uNA as specified in [RFC4861], including
   the setting of the R/S/O flags as discussed below.  The node sets the
   uNA(ACK) Target Address to the unicast destination and uNA(ACK)
   destination address to the unicast source of the original message.

   The node then sets the uNA(ACK) source address to its own address and
   includes any necessary OMNI sub-options but MUST NOT itself set the
   SNR/SYN/TST flags.  If the SNR/SYN/TST message included a Nonce and/
   or Timestamp option, the node includes matching Nonce/Timestamp
   options in the uNA(ACK) response.  The node finally returns the uNA
   message to the source of the SNR/SYN/TST message.

4.5.2.  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 forwarding nodes on the OMNI link.  (AERO Hosts are by
      definition the only non-forwarding nodes on the OMNI link and
      therefore set the R flag to 0.)

   *  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).

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   *  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.

4.5.3.  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 AERO control messages without
   including Identification values.  In environments where spoofing is
   considered a threat, OMNI interface neighbors instead invoke
   Identification window synchronization by including OMNI Multilink
   Vector sub-options in RS/RA and NS/NA message exchanges to maintain
   send/receive window state in their respective neighbor caches as well
   as in AFIB entries of all OAL intermediate nodes in the forward and
   reverse paths.

   In common arrangements, OAL Identification window synchronization is
   necessary for Client to Client, Client to Proxy/Server or Proxy/
   Server to Proxy/Server message exchanges conducted over unsecured
   Internetwork paths.  Conversely, Proxy/Server to Proxy/Server, Proxy/
   Server to Gateway and Gateway to Gateway message exchanges carried
   over the secured spanning tree do not require window synchronization.

   OAL end system and intermediate nodes verify Identification values of
   OAL packets that traverse the unsecured spanning tree according to
   their populated AFIB state.  This allows each OAL node to exclude
   spurious packets injected into the OMNI link from an off-path
   adversary.

4.6.  OMNI Interface Encapsulation and Fragmentation

   When the network layer forwards an original IP packet/parcel 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-omni3] which encapsulates the packet/parcel in an
   IPv6 header to produce an OAL packet with ULA/GUA addresses taken
   from a SNP assigned by a Proxy/Server.

   Following encapsulation, the OAL source then fragments the OAL packet
   while including an identical Identification value for each fragment
   that must be within the window for the flow over the interface pair
   selected for the neighbor.  The OAL source includes any necessary OAL
   IPv6 extension headers including an identical Compressed Routing
   Header (CRH) [I-D.ietf-6man-comp-rtg-hdr] with each fragment
   containing an AERO Forwarding Vector Index (AFVI) as discussed in
   Section 4.13.  (The OAL source includes AFVIs no larger than 65535 as

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   2-octet values and includes larger AFVIs as 4-octet values when it
   prepares the CRH.)  The OAL source can instead invoke OAL header
   compression by replacing the full OAL IPv6 header (OFH), CRH and
   Extended Fragment Header with an OAL Compressed Header (OCH) (see:
   [I-D.templin-6man-omni3]).

   For messages that will traverse unsecured paths, the OAL source
   finally performs L2 encapsulation/fragmentation on each resulting OAL
   fragment to form a 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 system or destination (e.g.,
   192.0.2.1).  The carrier packet encapsulation format in the above
   example is shown in Figure 3:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |           L2 Headers          |
        ~       src = 192.0.2.100       ~
        |        dst = 192.0.2.1        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~   L2 IPv6 Extension Headers   ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        OAL IPv6 Header        |
        ~        src={ULA,GUA}-1        ~
        |        dst={ULA,GUA}-2        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~   OAL IPv6 Extension Headers  ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |       Original IP Header      |
        ~     (first-fragment only)     ~
        ~        src={ULA,GUA}-3        ~
        |        dst={ULA,GUA}-4        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |
        ~                               ~
        ~ Original Packet Body/Fragment ~
        ~                               ~
        |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 3: Carrier Packet Format

   (Note that carrier packets exchanged by ENET Hosts do not include the
   OAL IPv6 or CRH headers, i.e., the OAL encapsulation is NULL and only
   the L2 encapsulations including any L2 IPv6 extension headers are
   included.)

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   In this format, the OAL source encapsulates the original IP header
   and packet/parcel body/fragment in an OAL IPv6 header, the CRH is a
   Routing Header extension of the OAL header, the Extended Fragment
   Header identifies each fragment, and the L2 headers are prepared as
   discussed in [I-D.templin-6man-omni3].  The OAL source sends each
   such carrier packet into the SRT unsecured spanning tree, where they
   may be forwarded over multiple OAL intermediate systems until they
   arrive at the OAL destination.  These carrier packets may themselves
   be subject to L2 fragmentation and reassembly along the path.

   The OMNI link control plane service distributes Client MNP prefix
   information that may change occasionally due to regional node
   mobility, as well as more static information for Relay FNPs and per-
   segment SNPs that rarely change.  OMNI link Gateways and Proxy/
   Servers use the information to establish and maintain a forwarding
   plane spanning tree that connects all nodes on the link.  The
   spanning tree supports a 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 both carrier packet Identification context and 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 entries established through route
   optimization to greatly improve performance and reduce load on
   critical infrastructure elements.

   For OAL packets/fragments undergoing L2 re-encapsulation at an OAL
   intermediate system, the OMNI interface performs L2 reassembly/
   decapsulation followed by Identification verification and OAL
   reassembly only if the OAL packet/fragment is addressed to itself.
   The OMNI interface then decrements the OAL IPv6 header Hop Limit and
   discards the packet/fragment if the Hop Limit reaches 0.  Otherwise,
   the OMNI interface updates the OAL addresses if necessary, includes
   an appropriate Identification, performs OAL fragmentation then for
   each OAL fragment performs L2 encapsulation/fragmentation to produce
   carrier packets appropriate for next segment forwarding.

   OAL packets/fragments that travel over secured spanning tree hops do
   not include OMNI L2 encapsulations.  They are instead admitted into
   secured links such as IPsec tunnels or direct links where they may be
   subject to L2 security encapsulations as secured carrier packets.
   (Note that OMNI protocol L2 encapsulations could be used above the L2
   security services, but this could result in excessive encapsulation
   in some instances.)

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4.7.  OMNI Interface Decapsulation

   When an OAL node receives OAL packets/fragments 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 (after first performing any necessary L2 fragmentation or
   reassembly).  The node then sends these new carrier packets into the
   next hop underlay interface.

   When an OAL node receives OAL packets/fragments addressed to itself,
   it performs L2 reassembly/decapsulation, verifies the Identification,
   then performs OAL reassembly/decapsulation to obtain the original OAL
   packet or super-packet (see: [I-D.templin-6man-omni3]).  Next, if the
   enclosed original IP packet(s)/parcel(s) are 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)/parcel(s) to the network layer.

   If the original IP packet(s)/parcel(s) are 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 address
   of the next-hop node over the OMNI interface, decrements the Hop
   Limit, then performs L2 encapsulation/fragmentation and forwards
   these new carrier packets into the next hop underlay interface.

   Further OMNI link decapsulation details are specified in
   [I-D.templin-6man-omni3].  Further OMNI link forwarding procedures
   are specified in Section 4.10.

4.8.  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/parcels that originate from within the same secured
      ANET.

   *  AERO Clients and Relays accept original IP packets/parcels 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-omni3].

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   *  OAL end systems and intermediate systems forward/accept OAL
      packets/fragments with Identification values within the current
      window for the OAL source neighbor for a specific underlay
      interface pair and drop any packets with out-of-window
      Identification values.

   AERO nodes silently drop any packets/parcels that do not satisfy the
   above data origin authentication procedures.  Further security
   considerations are discussed in Section 7.

4.9.  OMNI Interface MTU

   The OMNI interface observes the link nature of tunnels, including the
   Maximum Transmission Unit (MTU), Effective MTU to Receive (EMTU_R)
   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.  OMNI interface packet sizing considerations are
   specified in [I-D.templin-6man-omni3], where the OMNI interface MTU
   can essentially be considered "unlimited".

   When the network layer presents an original IP packet/parcel to the
   OMNI interface, the OAL source encapsulates and fragments the packet/
   parcel if necessary.  When the network layer presents the OMNI
   interface with multiple original IP packets/parcels addressed to the
   same IPv6 flow, the OAL source can concatenate them as a single OAL
   super-packet as discussed in [I-D.templin-6man-omni3] before applying
   fragmentation.  The OAL source then submits each OAL fragment for L2
   encapsulation/fragmentation 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.

4.10.  OMNI Interface Forwarding Algorithm

   Original IP packets/parcels 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/
   parcels and carrier packets entering a node's OMNI interface first
   undergo data origin authentication as discussed in Section 4.8.
   Those that satisfy data origin authentication are processed further,
   while all others are dropped silently.

   Original IP packets/parcels 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 forwarding state

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   indicates that the original IP packet/parcel should instead be
   forwarded back to the network layer, the packet/parcel 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
   link layer, or reassembled and forwarded to the network layer where
   they are subject to either local delivery or IP forwarding.

   When the network layer of a router forwards an original IP packet/
   parcel into the OMNI interface, it decrements the TTL/Hop Limit
   following standard IP router conventions.  Once inside the OMNI
   interface, however, the OAL does not further decrement the original
   IP packet/parcel TTL/Hop Limit since its adaptation layer forwarding
   actions occur below the network layer.  The original IP packet/
   parcel's TTL/Hop Limit will therefore be the same when it exits the
   destination OMNI interface as when it first entered the source OMNI
   interface.

   When an OAL intermediate system receives a carrier packet, it
   performs L2 reassembly/decapsulation to obtain the enclosed OAL
   packet/fragment.  When the intermediate system forwards an OAL
   packet/fragment not addressed to itself, it decrements the OAL Hop
   Limit without decrementing the network layer IP TTL/Hop Limit.  If
   decrementing would cause the OAL Hop Limit to become 0, the OAL
   intermediate system drops the OAL packet/fragment.  This ensures that
   original IP packet(s)/parcel(s) cannot enter an endless loop.

   OMNI interfaces may have multiple underlay interfaces and/or neighbor
   cache entries for neighbors with multiple underlay interfaces (see
   Section 4.3).  The OAL uses Interface Attributes and/or Traffic
   Selectors 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 target underlay
   interfaces.  AERO implementations SHOULD permit network management to
   dynamically adjust Traffic Selector values at runtime.

   If an OAL packet/fragment matches the Interface Attributes and/or
   Traffic Selectors of multiple outgoing interfaces and/or neighbor
   interfaces, the OMNI interface replicates the packet and sends a
   separate copy via each of the (outgoing / neighbor) interface pairs;
   otherwise, it sends a single copy via an interface with the best
   matching attributes/selectors.  (While not strictly required, the
   likelihood of successful reassembly may improve when the OMNI
   interface sends all fragments of the same fragmented OAL packet/
   fragment 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.

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   In addition to standard forwarding based on Interface Attributes and/
   or Traffic Selectors, nodes may employ a policy engine that would
   provide further guidance to the forwarding algorithm.  For example
   the policy engine may suggest a load balancing profile over multiple
   underlay interface pairs, with portions of a traffic flow spread
   between multiple paths according to Equal Cost MultiPath or Link
   Aggregation Groups (LAGs) [RFC6438] (note that Interface Attributes
   include an underlay interface group identifier).  Other policies may
   suggest the use of paths with the least cost, best performance, etc.
   This document therefore specifies mechanisms without mandating any
   particular policies.

   All Clients, Proxy/Servers and Gateways serve as OAL intermediate
   nodes for the purpose of forwarding OAL packets/fragments that
   include a CRH or OCH with non-zero AFVI over the unsecured spanning
   tree based on AFIB entries.  When an OAL intermediate node forwards
   an OAL packet/fragment with an L2 source address and AFVI that
   matches an AFV, the node first verifies that the Identification is in
   sequence.  The OAL intermediate node then rewrites the packet's AFVI
   with a value that will be recognized by the next OAL hop and forwards
   the packet.  (For OAL packets/fragments that do not include a non-
   zero AFVI, the OAL intermediate node instead forwards based on
   matching the OAL IPv6 destination address with a standard IPv6
   forwarding table entry.)  The chain of OAL source, intermediate and
   destination nodes may therefore traverse many Clients, Proxy/Servers
   and Gateways on the path.

   The following sections discuss the OMNI interface-specific forwarding
   algorithms for Hosts, Clients, Proxy/Servers and Gateways.  In the
   following discussion, an original IP packet/parcel's destination
   address is said to "match" if it is the same as a cached address, or
   if it is covered by a cached FNP/SNP/MNP.

4.10.1.  Host Forwarding Algorithm

   When an original IP packet/parcel 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 OMNI L2
   encapsulation/fragmentation as discussed in [I-D.templin-6man-omni3]
   then forwards the resulting carrier packets into the ENET addressed
   to the L2 address of the neighbor.  If there is no match, the host
   instead sends the carrier packets to its upstream Client.

   After sending carrier packets, the Host may receive an OAL 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.

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4.10.2.  Client Forwarding Algorithm

   When an original IP packet/parcel 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 for a neighbor reached via a
   *NET interface (i.e., an upstream interface), the Client selects one
   or more "reachable" neighbor interfaces in the entry for forwarding
   purposes.  Otherwise, the Client performs OAL encapsulation and
   fragmentation if necessary, forwards the resulting OAL packet/
   fragments to an FHS Proxy/Server, then either invokes address
   resolution and multilink forwarding procedures per Section 4.13 or
   allows the FHS Proxy/Server to invoke these procedures on its behalf.
   If there is a matching NCE for a neighbor reached via an ENET
   interface (i.e., a downstream interface), the Client instead forwards
   the original IP packet/parcel to the downstream Host or Client using
   L2 encapsulation and fragmentation if necessary.

   When a carrier packet enters a Client's OMNI interface from the link
   layer, the Client performs L2 reassembly/decapsulation if necessary
   to obtain the OAL packet/fragment then examines the OAL destination
   (i.e., after locating the correct AFV if the OAL packet header is
   OCH).  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 the matching AFV, then reassembles/
   decapsulates as necessary and delivers the original IP packet/parcel
   to the network layer.  If the OAL destination matches a NCE for a
   dependent peer Client on an ENET interface, the Client instead
   forwards the OAL packet/fragment to the peer while decrementing the
   OAL Hop Limit.  If the OAL destination matches a NCE for a Host on an
   ENET interface, the Client instead reassembles then forwards the
   original IP packet/parcel to the Host while using L2 encapsulation/
   fragmentation (i.e., without invoking the OAL) if necessary.  If the
   OAL destination does not match, the Client drops the original IP
   packet/parcel and MAY return a network layer ICMP Destination
   Unreachable message subject to rate limiting (see: Section 4.11).

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

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   Note: The forwarding table entries established in peer Clients of a
   MANET multihop forwarding region are based on ULAs and/or HHITs used
   to seed the multihop routing protocols.  When ULAs are used, the
   subnet ID in the ULA /64 prefix provides topological relevance for
   the multihop forwarding region, while the 64-bit Interface Identifier
   encodes the 1x1 mapping of the MANET-internal ULA to the MANET-
   external GUA maintained by the Proxy/Server that configures the GUA/
   ULA SNP.

   Note: Clients within MANETs support Client-to-Client multihop
   forwarding when necessary to reach destinations or FHS Proxy/Servers
   that may be multiple OAL hops away.  In this way, forwarding Clients
   act as OAL intermediate nodes and forward using OCH compression based
   on AFV state that is indexed by the AFVIs included in each OAL
   packet/fragment.  ULA-based communications are sufficient for Client-
   to-Client communications within a MANET, while packets that enter or
   exit the MANET via a FHS Proxy/Server may be subject to NPTv6
   [I-D.bctb-6man-rfc6296-bis].

4.10.3.  Proxy/Server and Relay Forwarding Algorithm

   When the network layer admits an original IP packet/parcel into a
   Proxy/Server's OMNI interface, the OAL drops the packet/parcel to
   avoid looping if forwarding state indicates that it should be
   forwarded back to the network layer.  Otherwise, the OAL examines the
   IP destination address to determine if it matches the SNP SRA GUA of
   a neighboring Gateway found in the OMNI interface's network layer
   neighbor cache.  If so, the Proxy/Server performs OAL encapsulation
   and fragmentation then performs L2 encapsulation/fragmentation and
   forwards the resulting carrier packets to the Gateway over a secured
   link (e.g., an IPsec tunnel, Direct link, etc.) to support control
   plane functions such as the operation of the BGP routing protocol.
   If the destination matches an FNP/MNP associated with a (foreign)
   Proxy/Server or Client, the (local) Proxy/Server instead assumes the
   Relay role and forwards the original IP packet/parcel in a similar
   manner as for Clients.  Specifically, if there is a matching NCE the
   Proxy/Server selects one or more "reachable" neighbor interfaces in
   the entry for forwarding purposes; otherwise, the Proxy/Server
   performs OAL encapsulation/fragmentation followed by L2
   encapsulation/fragmentation and forwards the resulting carrier
   packets while invoking address resolution and multilink forwarding
   procedures per Section 4.13.

   When the Proxy/Server receives/reassembles carrier packets on
   underlay interfaces that contain OAL packets/fragments with both a
   source and destination OAL address that correspond to the same
   Client's delegated MNP or SNP GUA, the Proxy/Server drops the carrier
   packets regardless of their OMNI link point of origin.  The Proxy/

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   Server also drops original IP packets/parcels received on underlay
   interfaces either directly from a (M)ANET Client or following
   reassembly of carrier packets received from a *NET Client if the
   original IP destination corresponds to the same Client's delegated
   MNP or SNP GUA.  Proxy/Servers also drop carrier packets that contain
   OAL packets/fragments with foreign OAL destinations that do not match
   the SNP/MNP GUA associated with one of their local *NET Clients.
   These checks are essential to prevent forwarding inconsistencies from
   accidentally or intentionally establishing endless loops that could
   congest nodes and/or *NET links.

   Proxy/Servers process carrier packets that contain OAL packets/
   fragments with OCH headers or with destinations that match their SNP
   SRA ULA/GUA and also include a CRH header that encodes AFVI
   information.  The Proxy/Server examines the L2 source address and
   AFVI to locate the corresponding AFV entry in the AFIB.  The Proxy/
   Server then forwards them according to the AFV state while
   decrementing the OAL packet/fragment Hop Limit.

   For OAL packets/fragments with destinations that match their SNP SRA
   ULA/GUA but do not include a CRH/OCH with a non-zero AFVI, the Proxy/
   Server instead performs L2 reassembly/decapsulation, verifies the
   Identification and performs OAL reassembly to obtain the original IP
   packet/parcel.  For data packets/parcels addressed to its own GUA
   that arrived via the secured spanning tree, the Proxy/Server delivers
   the original IP packet/parcel to the network layer to support secured
   BGP routing protocol control messaging.  For data packets/parcels
   originating from one of its dependent Clients, the Proxy/Server
   instead performs OAL encapsulation/fragmentation followed by L2
   encapsulation/fragmentation and sends the resulting carrier packets
   while invoking address resolution and multilink forwarding procedures
   per Section 4.13.  For IPv6 ND control messages, the Proxy/Server
   instead authenticates the message and processes it as specified in
   later sections of this document while updating neighbor cache and/or
   AFIB state accordingly.

   When the Proxy/Server receives a carrier packet that contains an OAL
   packet/fragment with OAL destination set to an SNP ULA or MNP GUA 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 SNP SRA GUA of the new Proxy/Server,
   decrements the OAL Hop Limit, then performs L2 encapsulation/
   fragmentation and forwards the resulting carrier packets into the
   spanning tree which will eventually deliver them to the new Proxy/
   Server.  If the neighbor cache state for the Client is REACHABLE and
   the Proxy/Server is a MAP responsible for serving as the Client's
   address resolution responder and/or default router, it verifies the

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   Identification then submits the OAL packet/fragment for reassembly.
   The Proxy/Server then decapsulates and processes the resulting IPv6
   ND message or original IP packet/parcel accordingly.  Otherwise, the
   Proxy/Server decrements the OAL Hop Limit, performs L2 encapsulation/
   fragmentation and sends the carrier packets to the Client which must
   then perform data origin verification and reassembly.  (In the latter
   case, the Client may receive fragments of the same original IP
   packet/parcel from different Proxy/Servers but this will not
   interfere with reassembly.)

   When the Proxy/Server receives a carrier packet that contains an OAL
   packet/fragment with OAL destination set to a FNP address that does
   not match the MSP, it accepts the carrier packet only if data origin
   authentication succeeds and if there is a network layer forwarding
   table entry for the FNP.  The Proxy/Server then performs L2
   reassembly/decapsulation, verifies the Identification, performs OAL
   reassembly/decapsulation to obtain the original IP packet/parcel,
   then presents it to the network layer (as a Relay) where it will be
   delivered according to standard IP forwarding.

   When a Proxy/Server receives a carrier packet from the secured
   spanning tree, it considers the message as authentic without having
   to verify network or higher layer authentication signatures.

   If the Proxy/Server has multiple original IP packets/parcels to send
   to the same neighbor, it can concatenate them as a single OAL super-
   packet [I-D.templin-6man-omni3].

4.10.4.  Gateway Forwarding Algorithm

   When the network layer admits an original IP packet/parcel into the
   Gateway's OMNI interface, the OAL 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 SNP SRA GUA of a neighboring
   Gateway or Proxy/Server by examining the OMNI interface's network
   layer neighbor cache.  If so, the Gateway performs OAL encapsulation/
   fragmentation followed by L2 encapsulation/fragmentation and forwards
   the resulting carrier packets to the neighboring Gateway or Proxy/
   Server over a secured link (e.g., an IPsec tunnel, etc.) to support
   the operation of control plane functions (including the BGP routing
   protocol) between OAL neighbors.

   Gateways forward OAL packets/fragments reassembled from spanning tree
   carrier packets while decrementing the OAL Hop Limit but not the
   original IP header TTL/Hop Limit.  Gateways send carrier packets that
   contain OAL packets/fragments with critical IPv6 ND control messages
   or BGP routing protocol control messages via the SRT secured spanning

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   tree, and may send 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 reassembles/
   decapsulates to obtain the OAL packet/fragment then searches for an
   AFIB entry that matches the OAL header AFVI or an IP forwarding table
   entry that matches the OAL destination address.

   Gateways process carrier packets that contain OAL packets/fragments
   with OAL destinations that do not match their SNP/SRT SRA GUA in the
   same manner as for traditional IP forwarding within the OAL, i.e.,
   they forward packets not explicitly addressed to themselves.
   Gateways locally process OAL packets/fragments with OCH headers or
   full OAL headers with their SNP/SRT SRA GUA as the OAL destination.
   If the OAL packet/fragment contains an OCH or a full OAL header with
   a CRH extension, the Gateway examines the AFVI to locate the AFV
   entry in the AFIB for next hop forwarding.  If an AFV is found, the
   Gateway uses the next hop AFVI to forward the OAL packet/fragment to
   the next hop while decrementing the OAL Hop Limit but without
   reassembling.  When the Gateway forwards the OAL packet/fragment, it
   rewrites the OCH/CRH AFVI with the value it will represent to the
   next OAL hop.

   If the OAL packet/fragment includes a full OAL header addressed to
   itself but does not include an AFVI, the Gateway instead reassembles
   if necessary and processes the OAL packet further.  The Gateway first
   determines whether the OAL packet includes an NS/NA message then
   processes the message according to the multilink forwarding
   procedures discussed in Section 4.13.  If the carrier packets arrived
   over the secured spanning tree and the enclosed OAL packets/fragments
   are addressed to its SNP/SRT SRA GUA, the Gateway instead reassembles
   then discards the OAL header and forwards the original IP packet/
   parcel to the network layer to support secured BGP routing protocol
   control messaging.  The Gateway instead drops all other OAL packets.

   Gateways forward OAL packets/fragments received in carrier packets
   that arrived from a first segment via the secured spanning tree to
   the next segment also via the secured spanning tree.  Gateways
   forward OAL packets/fragments received in carrier packets that
   arrived from a first segment via the unsecured spanning tree to the
   next segment also via the unsecured spanning tree.  Gateways
   configure a single IPv6 routing table that determines the 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 (e.g., an IPsec tunnel or an open INET
   interface).

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   As for Proxy/Servers, Gateways must verify that the L2 source
   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).

4.11.  OMNI Interface Error Handling

   When an AERO node admits an original IP packet/parcel into the OMNI
   interface, it may receive link and/or network layer error
   indications.  The AERO node may also receive OMNI link error
   indications in OAL-encapsulated uNA(ERR) 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", "Parameter Problem" etc.
   [RFC0792][RFC4443].

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

   The link layer error message format is shown in Figure 4:

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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |
        ~    IP Header of link layer    ~
        ~         error message         ~
        |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~          ICMP Header          ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
        |                               |   P
        ~   carrier packet L2 and OAL   ~   a
        ~     encapsulation headers     ~   c
        |                               |   k
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   e
        |                               |   t
        ~original IP packet/parcel hdrs ~
        ~    (first-fragment only)      ~   i
        |                               |   n
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |   e
        ~    Portion of the body of     ~   r
        ~ the original IP packet/parcel ~   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].

   *  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 carrier packets.

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   *  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 4.15.5.

   *  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 re-converge 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 carrier 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 OAL reassembly
   is currently congested, it returns an OMNI interface Packet Too Big
   (PTB) message as discussed in [I-D.templin-6man-omni3] (note that the
   PTB messages could indicate either "hard" or "soft" errors).

   AERO nodes include ICMPv6 error messages intended for an OAL source
   as sub-options in the OMNI option of secured uNA(ERR) messages.  When
   the OAL source receives the uNA(ERR) 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.

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4.12.  AERO Mobility Service Coordination

   AERO nodes observes the Router Discovery and Prefix Registration
   specifications found in [I-D.templin-6man-omni3].  AERO nodes further
   coordinate their autoconfiguration actions with the mobility service
   as discussed in the following sections.

4.12.1.  AERO Service Model

   Each AERO Proxy/Server on the OMNI link is configured to respond to
   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.  (Note: an OMNI link can instead
   delegate non-correlated MNPs to Clients instead of maintaining such a
   database.  In that case, each Client may be delegated a different MNP
   each time it registers with the OMNI domain and may need to renumber
   its downstream-attached ENETs.)

   Clients associate each of their *NET underlay interfaces with FHS
   Proxy/Servers.  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 MAP Proxy/Server (the Client may
   instead select a "third-party" MAP 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 MAP Proxy/Server and Client without assuming the
   MAP role functions themselves.

   Each Client typically associates with a single MAP Proxy/Server,
   while all other Proxy/Servers are candidates for providing the MAP
   role for other Clients.  An FHS Proxy/Server assumes the MAP role
   when it receives an RS message with its own SNP SRA GUA/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 SNP
   SRA GUA 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 SNP SRA GUA of a better candidate Proxy/Server to serve as a
   MAP.)

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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.  Specifically, the Host/Client sends an RS message via
   the ENET which directs the message to the upstream Client.  The
   upstream Client then responds to the RS message by returning an RA.
   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 adaptation layer NCEs.  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 FHS/MAP Proxy/Servers include SNP ULA/GUA address
   delegation (and optionally also MNP prefix delegation) DHCPv6
   parameters in RS/RA messages.  The IPv6 ND messages are exchanged
   between the Client and any FHS Proxy/Servers acting as proxys for the
   MAP Proxy/Server as specified in [I-D.templin-6man-omni3] according
   to the address/prefix management schedule required by the service.
   If the Client knows its MNP in advance, it can include the MNP in its
   DHCPv6 prefix delegation request.  If the MAP Proxy/Server accepts
   the Client's MNP assertion (or if it delegates a new MNP for the
   Client), it injects the MNP into the routing system and establishes
   the necessary neighbor cache state.

   AERO Clients and their FHS Proxy/Servers on MANETs and open INETs
   must establish and maintain Identification synchronization windows in
   their RS/RA exchanges.  The window synchronization provides a well-
   managed Identification value that the Client and Proxy/Server can use
   for validating IPv6 ND messages with authentication signatures.

   All Host, Client and Proxy/Server behaviors for the exchange of RS/RA
   messages are conducted according to the Router Discovery and Prefix
   Registration specifications found in Section 15 of
   [I-D.templin-6man-omni3].  The following 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.

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4.12.2.  AERO Host and Client Behavior

   AERO Hosts and Clients discover the addresses of candidate FHS Proxy/
   Servers as specified in Section 15 of [I-D.templin-6man-omni3].  The
   Host/Client then performs RS/RA exchanges over each of its underlay
   interfaces to associate with an FHS Proxy/Server for each interface
   and a single MAP Proxy/Server if necessary.  The Host/Client sends
   each RS (either directly via Direct interfaces, via an IPsec tunnel
   for VPN interfaces, via an access router for (M)ANET interfaces or
   via INET encapsulation for INET interfaces) and waits up to
   RetransTimer milliseconds for an RA message reply (see
   Section 4.12.3) 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 MAP Proxy/Server's RA
   response, it has assurance that both the MAP and FHS Proxy/Servers
   have been updated with the new information.

   If the Host/Client wishes to discontinue use of a MAP Proxy/Server it
   issues an RS message over any underlay interface with an OMNI Proxy/
   Server Departure sub-option that encodes the (old) MAP Proxy/Server's
   SNP SRA GUA.  When the MAP Proxy/Server processes the message, it
   releases any MNPs, 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 MAP Proxy/Server withdraws the MNP from
   the routing system.  (Alternatively, when the Host/Client associates
   with a new FHS/MAP Proxy/Server it can include an OMNI "Proxy/Server
   Departure" sub-option in RS messages with the SNA SRA GUAs of the Old
   FHS/MAP Proxy/Servers.)

4.12.3.  AERO Proxy/Server Behavior

   AERO Proxy/Servers act as both IP routers and IPv6 ND proxys, and
   support address and prefix delegation services for Clients.  When a
   FHS/MAP Proxy/Server receives a prospective Client's secured 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 and performs DHCPv6

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   address delegation for SNP ULA/GUA pairs while returning the ULA/GUA
   prefixes per [RFC8028] as specified in Section 15 of
   [I-D.templin-6man-omni3].  If the RS message also contains DHCPv6
   prefix delegation parameters the FHS Proxy/Server processes the
   prefix delegations locally as a MAP or forwards a proxyed version of
   the RS to another candidate MAP Proxy/Server.

   When the MAP Proxy/Server processes the RS, it determines the correct
   MNPs for the Client by processing OMNI DHCPv6 sub-option(s).  When
   the MAP Proxy/Server returns the MNPs, it also creates forwarding
   table entries for the MNP resulting in BGP updates (see:
   Section 4.2.3).  The MAP Proxy/Server then returns an RA to the
   Client via the FHS Proxy/server as specified in Section 15 of
   [I-D.templin-6man-omni3].

   After the initial RS/RA exchange, the MAP 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 MAP Proxy/Server sends an RA
   response and resets ReachableTime.  If the MAP 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 MAP Proxy/Server marks the interface as DOWN.
   If ReachableTime expires before any new RS is received on any
   individual underlay interface, the MAP Proxy/Server sets the NCE
   state to STALE and sets a 10 second timer.  If the MAP Proxy/Server
   has not received a new RS or uNA(MM) message with a prefix release
   indication before the 10 second timer expires, it deletes the NCE and
   withdraws the MNP from the routing system.

   The MAP 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 MAP Proxy/Server may also issue unsolicited
   RA messages, e.g., with reconfigure parameters to cause the Client to
   renegotiate its prefix delegation/registrations, with Router Lifetime
   set to 0 if it can no longer service this Client, etc.  The MAP
   Proxy/Server may also receive carrier packets via the secured
   spanning tree that contain initial data sent while route optimization
   is in progress.  The MAP Proxy/Server reassembles the enclosed OAL
   packets/fragments, then re-encapsulates/re-fragments and sends the
   carrier packets to the target Client via an FHS Proxy/Server if
   necessary.  Finally, If the NCE is in the DEPARTED state, the old MAP
   Proxy/Server forwards any OAL packets/fragments it receives from the
   secured spanning tree and destined to the Client to the new MAP
   Proxy/Server, then deletes the entry after DepartTime expires.

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   Note: Clients SHOULD arrange to notify former MAP Proxy/Servers of
   their departures, but MAP Proxy/Servers are responsible for expiring
   neighbor cache entries and withdrawing MNP routes even if no
   departure notification is received (e.g., if the Client leaves the
   network unexpectedly).  MAP 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.

4.12.3.1.  Additional Proxy/Server Considerations

   AERO Clients register with FHS Proxy/Servers for each underlay
   interface.  Each of the Client's FHS Proxy/Servers in turn inform a
   single MAP Proxy/Server of the Client's underlay interface(s) that it
   services.  For Clients on Direct and VPN/IPsec underlay interfaces,
   the FHS Proxy/Server for each interface is directly connected, for
   Clients on (M)ANET underlay interfaces the FHS Proxy/Server is
   located on the (M)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 MAP role or forward a proxyed
   registration to another Proxy/Server "A" acting as the MAP.  Proxy/
   Servers satisfy these requirements as follows:

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   *  when FHS Proxy/Server "B" receives a Client RS message, it first
      verifies that the OAL Identification is within the window for the
      flow AFV associated with the NCE for this Client and authenticates
      the message.  If no NCE was found, Proxy/Server "B" instead
      creates one in the STALE state and caches the Client-supplied
      Interface Attributes, Origin Indication and Multilink Vector sub-
      option parameters as well as the Client's observed L2 address
      (noting that it may differ from the Origin address if there were
      NATs on the path).  Proxy/Server "B" then examines the RS
      destination address.  If the destination address is the SNP SRA
      GUA 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 SNP SRA GUA and destination set to Proxy/
      Server A's SNP SRA GUA.  Proxy/Server "B" also writes its own L2
      address information over the Interface Attributes sub-option L2
      information supplied by the Client, omits or zeros the Origin
      Indication sub-option, omits the Multilink Vector sub-option then
      forwards the message into the OMNI link secured spanning tree.

   *  when MAP Proxy/Server "A" receives the RS, it assumes the MAP
      role, delegates MNPs for the Client if necessary, and creates/
      updates a NCE indexed by the Client's MNP SRA GUA(s) with FHS
      Proxy/Server "B"'s Interface Attributes as the link layer address
      information for this FHS ifIndex.  MAP Proxy/Server "A" then
      prepares an RA message with source set to its own SNP SRA GUA,
      destination set to the Client's SNP GUA, and with OMNI option
      DHCPv6 sub-options with the prefix delegation results.  MAP Proxy/
      Server "A" then encapsulates the RA in an OAL header with source
      set to its own SNP SRA GUA and destination set to the SNP SRA GUA
      of FHS Proxy/Server "B", then finally 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 with non zero
      old FHS/MAP Proxy/Server SNP GUAs that do not match its own GUA,
      FHS Proxy/Server "B" first sends a uNA(MM) to the old FHS/MAP
      Proxy/Servers named in the sub-option.  Proxy/Server "B" then re-
      inserts the cached Multilink Vector sub-option for this Client
      while updating the window synchronization parameters.  If the RA
      message delegates a new SNP ULA/GUA pair, Proxy/Server "B" then
      resets the RA destination to the corresponding Client SNP ULA for
      this interface.

   *  Proxy/Server "B" then re-encapsulates the message with OAL source
      set to its own ULA and OAL destination set to the address that
      appeared in the Client's RS message OAL source, with an

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      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] then fragments the RA if necessary and returns
      the fragments to the Client.

   *  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 MAP "A" and the Client.  The Client creates/
      updates NCEs for each such FHS Proxy/Server as well as the MAP
      Proxy/Server in the process.

   After the initial RS/RA exchanges each FHS Proxy/Server forwards any
   of the Client's carrier packets that contain OAL packets/fragments
   with destinations for which there is no matching NCE to a Gateway
   using OAL encapsulation with its own SNP SRA GUA as the source and
   with destination determined by the Client.  The Proxy/Server instead
   forwards any OAL packets/fragments destined to a neighbor cache
   target directly to the target according to the OAL or link layer
   information - the process of establishing neighbor cache entries is
   specified in Section 4.13.

   While the Client is still associated with FHS Proxy/Servers "B", "C",
   "D", "E", etc., each FHS Proxy/Server can send NS, RS and/or uNA
   messages to update the NCEs of other AERO nodes on behalf of the
   Client based on changes in Interface Attributes, Traffic Selectors,
   Multilink Vectors, 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 (M)ANET data links.

   If the MAP Proxy/Server "A" ceases to send solicited RAs, FHS Proxy/
   Servers "B", "C", "D", "E", etc. can send unsolicited RAs over to the
   Client with destination set to (link-local) All-Nodes multicast and
   with Router Lifetime set to zero to announce the MAP Proxy/Server
   failure.  Although Proxy/Servers "B", "C", "D", "E", etc. can engage
   in IPv6 ND exchanges on behalf of the Client, the Client can also
   send IPv6 ND messages on its own behalf, e.g., if it is in a better
   position to convey state changes.  The IPv6 ND messages sent by the
   Client include the Client's MNP SRA GUA as the source in order to
   differentiate them from the IPv6 ND messages sent by a FHS Proxy/
   Server.

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   If the Client becomes unreachable over all underlay interfaces it
   serves, the MAP Proxy/Server sets the NCE state to DEPARTED and
   retains the entry for DepartTime seconds.  While the state is
   DEPARTED, the MAP Proxy/Server forwards any OAL packets/fragments
   destined to the Client to a new MAP Proxy/Server if known; otherwise,
   it discards the OAL packets/fragments.  When DepartTime expires, the
   MAP Proxy/Server deletes the NCE, withdraws the MNP route and
   discards any further carrier packets that contain OAL packets/
   fragments destined to the former Client.

   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 and authentication signature values
   are invalidated and must be re-calculated.

4.12.3.2.  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 MAP Proxy/Server reachability
   in a fashion that parallels 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 control messaging is carried only over
   well-connected ground domain networks (i.e., and not low-end
   aeronautical radio links) and can therefore be tuned for rapid
   response.

   FHS Proxy/Servers can perform continuous NS/NA exchanges with the MAP
   Proxy/Server, e.g., one exchange per N seconds.  The FHS Proxy/Server
   sends the NS message via the spanning tree with its own SNP SRA GUA
   as the source and the SNP SRA GUA of the MAP Proxy/Server as the
   destination, and the MAP Proxy/Server responds with an NA.  When the
   FHS Proxy/Server also sends RS messages to a MAP 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 if it has already sent an RS
   within the same period.  If the MAP 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

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   The FHS Proxy/Server sends unsolicited RA messages with source
   address set to the MAP 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 MAP Proxy/Server will receive the RA messages
   and select a different Proxy/Server to assume the MAP role (i.e., by
   sending an RS with destination set to the SNP SRA GUA of the new
   MAP).

4.13.  AERO Address Resolution, Multilink Forwarding and Route
       Optimization

   AERO nodes invoke address resolution, multilink forwarding and route
   optimization when they need to forward the initial original IP
   packets/parcel of a new flow to new neighbors over (M)ANET/INET
   interfaces and for ongoing multilink forwarding coordination with
   existing neighbors.

   Possible source and destination addresses for original IP packets
   that traverse a local (M)MANET/INET and/or the rest of the OMNI link
   include addresses taken from an FNP or MNP, or the SNP ULA/GUA
   assigned to a Client.  (No other IP address types should appear in
   original packets forwarded over the OMNI link.)  The flow is then
   identified by the 3-tuple consisting of the IPv6 flow label along
   with the source and destination address.

   Address resolution is based on an IPv6 ND NS/NA(AR) messaging
   exchange between an Address Resolution Source (ARS) as the NS(AR)
   source and the target neighbor as the Address Resolution Target
   (ART).  Either the original source or the original source's current
   FHS Proxy/Server serves as the ARS.  Either the ART itself or the
   ART's current FHS/MAP Proxy/Server (or Relay) serves as the Address
   Resolution Responder (ARR), i.e., the NA(AR) source.

   If the original IP packet uses an FNP/MNP address as the source, the
   NS(AR) source and NA(AR) destination are set to the corresponding
   FNP/MNP Subnet Router Anycast (SRA) address.  If the original IP
   packet uses an SNP GUA as the source, the NS(AR) source and NA(AR)
   destination are set to the SNP GUA.  The original IP packet
   destination address appears in the NS(AR) destination as well as the
   Target Address of each NS/NA(AR).  The NA(AR) source is set to the
   ARR's SNP GUA.

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

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

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

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

   *  For FNP correspondent nodes on foreign links/networks serviced by
      a Relay, the Relay is the ARS.

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

   *  For peers within the same (M)ANET/ENET, address resolution and
      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 complete, current, consistent
   and authentic.  Both the ARS and ARR are then jointly responsible for
   periodically refreshing the address resolution, and for quickly
   informing each other of any changes.  Following address resolution,
   the ARS and ART perform subsequent multilink forwarding and route
   optimization exchanges to maintain optimal forwarding profiles for
   each distinct flow.

   During address resolution, multilink forwarding and/or route
   optimization an NS/NA message source may attach a small number of
   original IP packets/parcels associated with the message exchange as
   super-packet extensions per [I-D.templin-6man-omni3].  The
   authentication signatures and/or lower-layer security features
   employed at the OAL source and each OAL intermediate system will
   provide authorization and integrity services for both the NS/NA
   messages and their IP packet/parcel attachments.  The final OAL
   intermediate system in the path will then securely forward the NS/NA
   message IP packet/parcel attachments to the target.

   The source can attach original IP packets/parcels to the subject NS/
   NA message, but this may cause the message size to exceed the IPv6
   minimum MTU and/or result in sub-optimal forwarding for the IP
   packet/parcel attachments.  In that case, the source can instead
   create small uNA "pilot" messages used to transport attachments as
   "passengers" over shortest paths determined by routing.  The OAL
   source can attach as many IP packets/parcels as will fit without
   causing the OAL packet to exceed the minimum OAL Fragment Size (OFS)
   using the "super-packet" construct discussed in
   [I-D.templin-6man-omni3].

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   If an original IP packet/parcel is larger than the minimum OFS, the
   OAL source performs OAL encapsulation and fragmentation using an
   Extended Fragment Header with a randomly-chosen 8-octet
   Identification (see: [I-D.templin-6man-omni3]) then attaches each
   fragment to a different pilot uNA.  The source then sets the uNA
   source to its own SNP GUA and destination to the address of the final
   destination.  The uNA source next calculates an authentication
   signature over the length of the entire message (including the uNA
   and any attachments) if necessary and forwards each message to the
   next hop subject to rate limiting (see: [RFC4443]).

   When the target Proxy/Server or Client receives a pilot uNA, it
   removes all passenger attachment packets, performs OAL reassembly and
   decapsulation if necessary, then delivers the original IP packet(s)
   to the destination.  This service supports assured (but sub-optimal)
   short-term delivery of protocol data while neighbor coordination is
   in progress without creating network state.

   The address resolution, multilink forwarding and route optimization
   procedures are specified in the following sections.

4.13.1.  Multilink Address Resolution

   When one or more original IP packets/parcels for a flow 3-tuple are
   forwarded into an OMNI interface, the ARS checks for a NCE with an
   FNP/MNP SRA prefix or SNP GUA 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 then prepares an NS message for Address Resolution
   (NS(AR)) to send toward an ART while (optionally) attaching the
   original IP packet(s)/parcel(s) to the end of the NS(AR) as an OAL
   super-packet (see above).  The resulting NS(AR) message must be sent
   securely and includes source, destination and target addresses as
   discussed above.  If the source address is an MNP SRA address, the
   NS(AR) message also includes Route Information Options (RIOs)
   [RFC4191] for any of the source Client's MNPs.

   The ARS then includes an OMNI option with an authentication sub-
   option (if necessary), Interface Attributes and/or Traffic Selectors
   for all of the source Client's underlay interfaces.  The ARS then
   calculates and includes an authentication signature (if necessary)
   followed by the checksum, then submits the NS(AR) message for OAL
   encapsulation.

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   When the ARS is a FHS Proxy/Server, it sets the OAL source to its own
   SNP GUA and sets the OAL destination according to the Client's RS
   message "RPT" flag (see: [I-D.templin-6man-omni3]).  If the "RPT"
   flag was set, the ARS sets the OAL destination to the SNP SRA GUA of
   its MAP Proxy/Server which maintains a Report List; otherwise, the
   ARS sets the OAL destination to the FNP/MNP SRA GUA or SNP GUA
   corresponding to the ART.  The ARS then performs L2 encapsulation/
   fragmentation and 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 its own SNP ULA as the
   OAL source and the SNP SRA ULA of the interface-specific FHS Proxy/
   Server as the OAL destination.  If the Client is in a MANET or an
   open INET, it next calculates and includes an authentication
   signature and includes an OAL IPv6 Extended Fragment Header with
   Identification set to an in-window value for this FHS Proxy/Server.
   The ARS Client then performs L2 encapsulation/fragmentation and
   forwards the carrier packets to the FHS Proxy/Server.

   The FHS Proxy/Server then performs L2 reassembly/decapsulation,
   verifies the Identification, verifies the NS(AR) checksum/
   authentication signature and confirms that the Client's claimed MNP
   RIOs and SNP ULA source address are correct.  The FHS Proxy/Server
   then changes the OAL source to its own SNP SRA GUA and changes the
   OAL destination to the SNP SRA GUA of the MAP Proxy/Server or FNP/MNP
   SRA GUA or SNP GUA corresponding to the ART as specified above.  The
   FHS Proxy/Server next removes the OAL IPv6 Extended Fragment Header,
   performs L2 encapsulation/fragmentation and sends the resulting
   carrier packets into the secured spanning tree on behalf of the
   Client.

   Note: both the source and target Client/Relay and their MAP Proxy/
   Servers include current and accurate information for their multilink
   Interface Attributes profile.  The MAP Proxy/Servers can be trusted
   to provide an authoritative ARR response and/or mobility update
   message on behalf of the source/target should the need arise.  While
   the source or target itself has no such trust basis, any attempt 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 source/target
   itself.  The source/target's asserted Interface Attributes therefore
   do not need to be validated by the MAP Proxy/Server.

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4.13.1.1.  ARS MAP Proxy/Server NS(AR) Processing

   If the ARS Client's MAP Proxy/Server maintains a Report List, the
   carrier packets containing the NS(AR) will first arrive at the MAP
   due to the OAL destination address supplied by the ARS (see above).
   This source MAP then performs L2 reassembly/decapsulation and records
   the NS(AR) Target Address in the Report List for this source Client.
   The MAP then leaves the OAL source address unchanged, but changes the
   OAL destination address to the FNP/MNP SRA GUA or SNP GUA
   corresponding to the ART.  The MAP then decrements the OAL header Hop
   Limit, performs L2 encapsulation/fragmentation and sends the
   resulting carrier packets into the secured spanning tree.

4.13.1.2.  Relaying the NS(AR)

   When a Gateway receives carrier packets containing the NS(AR), it
   performs L2 reassembly/decapsulation and determines the next hop by
   consulting its standard IPv6 forwarding table for the OAL header
   destination address.  The Gateway next decrements the OAL header Hop
   Limit, performs L2 encapsulation/fragmentation and sends the carrier
   packet(s) via the secured spanning tree the same as for any IPv6
   router where they may traverse multiple intermediate OMNI link
   segments interconnected by Gateways.  The final Gateway will deliver
   the carrier packets via the secured spanning tree to the LHS/MAP
   Proxy/Server (or Relay) that services the ART.

4.13.1.3.  NS(AR) Processing at the ARR/ART

   When the LHS/MAP Proxy/Server (or Relay) of the ART receives the
   NS(AR) secured carrier packets with the FNP/MNP SRA GUA or SNP GUA of
   the ART as the OAL destination, it performs L2 reassembly/
   decapsulation then either forwards the NS(AR) to the ART or processes
   it locally if it is acting as the ART's designated ARR.  The LHS/MAP
   Proxy/Server (or Relay) processes the message as follows:

   *  if the NS(AR) target matches a Client NCE in the DEPARTED state,
      the (old) MAP Proxy/Server resets the OAL destination address to
      the SNP SRA GUA of the Client's new MAP Proxy/Server.  The old MAP
      Proxy/Server then decrements the OAL header Hop Limit, performs L2
      encapsulation/fragmentation and forwards the resulting carrier
      packets over the secured spanning tree.

   *  If the NS(AR) target matches a Client NCE in the REACHABLE state,
      the LHS/MAP Proxy/Server (or Relay) notes whether the NS(AR)
      arrived from the secured spanning tree.  If the message arrived
      via the secured spanning tree the LHS/MAP Proxy/Server (or Relay)
      verifies the NS(AR) checksum only; otherwise, it must also verify
      the message authentication signature.

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   *  If the LHS/MAP Proxy/Server maintains a Report List for the ART,
      it next records the NS(AR) source address in the Report List for
      this ART.  If the MAP Proxy/Server is the ART's designated ARR, it
      forwards any original IP packet(s)/parcel(s) attached to the
      NS(AR) super-packet to the ART and prepares to return an NA(AR) as
      discussed below; otherwise, the LHS/MAP Proxy/Server determines
      the underlay interface for the ART and proceeds as follows:

      -  If the LHS/MAP Proxy/Server is also the ART's FHS Proxy/Server
         on the underlay interface used to convey the NS(AR) to the ART,
         it includes an OAL IPv6 Extended Fragment Header with an in-
         window Identification for the ART Client and authentication
         signature if necessary then recalculates the NS(AR) checksum.
         The Proxy/Server then changes the OAL source to its own SNP SRA
         ULA and OAL destination to the ULA of the ART, decrements the
         OAL Hop Limit, performs L2 encapsulation/fragmentation and
         forwards the resulting carrier packets over the underlay
         interface to the ART.

      -  If the MAP Proxy/Server is not the LHS Proxy/Server on the
         underlay interface used to convey the NS(AR) to the ART, it
         instead changes the OAL source to its own SNP SRA GUA and
         changes the OAL destination to the SNP SRA GUA of the LHS
         Proxy/Server for this ART interface.  The MAP Proxy/Server next
         decrements the OAL Hop Limit, performs L2 encapsulation/
         fragmentation and forwards the resulting carrier packets over
         the secured spanning tree.

      -  When the LHS Proxy/Server receives the carrier packets, it
         performs L2 reassembly/decapsulation, verifies the NS(AR)
         checksum, then forwards to the ART while changing the OAL
         addresses to ULAs and including an authentication signature and
         IPv6 Extended Fragment Header if necessary while recalculating
         the checksum the same as described above.

   *  If the NS(AR) target matches one of its FNP routes, the MAP Proxy/
      Server serves as both a Relay and an ARR, since the Relay forwards
      original IP packets/parcels toward foreign target nodes at the
      network layer.

   *  Note: when the target's MAP Proxy/Server acts as the ARR, it
      detaches any original IP packets attached to the NS(AR) and
      attaches them to a uNA message addressed to the ART.  The MAP
      Proxy/Server then forwards the resulting super-packet into the
      secured spanning tree or includes an authentication signature if
      the MAP is also a FHS Proxy/Server of the ART.

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   If the ARR is a Relay or the ART itself, it first creates or updates
   a NCE for the NS(AR) source address while caching all RIOs, Interface
   Attributes and Traffic Selector information.  Next, the ARR prepares
   a solicited NA(AR) message to return to the ARS with the IPv6 source,
   destination and target addresses set as described above.

   The ARR then includes RIOs for all of the ART's MNPs plus Interface
   Attributes and Traffic Selector sub-options for all of the ART's
   underlay interfaces with current information for each interface.  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 finally includes an authentication signature and IPv6
   Extended Fragment Header if necessary, calculates the NA(AR) message
   checksum, then submits the NA(AR) for encapsulation with OAL source
   set to its own ULA and destination set to the ULA that appeared in
   the NS(AR) OAL source.  The ARR then performs L2 encapsulation/
   fragmentation, and forwards the resulting carrier packets.

   When the ART's FHS Proxy/Server receives carrier packets sent by an
   ART acting as an ARR on its own behalf, it performs L2 reassembly and
   decapsulation then verifies the OAL Identification and NA (AR)
   message checksum/authentication signature.  The Proxy/Server then
   verifies that any RIO information is acceptable, changes the OAL
   source address to its own SNP SRA GUA and changes the OAL destination
   to the FNP/MNP SRA GUA or SNP GUA corresponding to the NA(AR)
   destination.  The Proxy/Server next decrements the OAL Hop Limit,
   removes the OAL Extended Fragment Header, then performs L2
   encapsulation/fragmentation and forwards the resulting carrier
   packets into the secured spanning tree.

4.13.1.4.  Relaying the NA(AR)

   When a Gateway receives NA(AR) carrier packets, it performs L2
   reassembly/decapsulation 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,
   performs L2 encapsulation/fragmentation and forwards the resulting
   carrier packets via the SRT secured spanning tree where they may
   traverse multiple intermediate OMNI link segments interconnected by
   other Gateways.  The final-hop Gateway will deliver the carrier
   packets via the secured spanning tree to a Proxy/Server for the ARS.

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4.13.1.5.  Processing the NA(AR) at the ARS

   When the ARS receives NA(AR) carrier packets, it performs L2
   reassembly/decapsulation then 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 RIO and OMNI option information in the NCE for the ART
   (including Interface Attributes, Traffic Selectors, etc.), and caches
   the NA(AR) source address plus any RIO/MNP SRA GUAs as ART addresses.

   When the ARS is a Client, the SRT secured spanning tree will first
   deliver the solicited NA(AR) message to the Client's FHS Proxy/
   Server, which rewrites the OAL header addresses, includes an OAL
   Extended Fragment Header with an in-window Identification for this
   Client, 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 or Identification; if the Client is in a MANET or in the
   open INET the Proxy/Server must instead include an Identification and
   authentication signature (while adjusting the OMNI option size, if
   necessary).  The Proxy/Server uses its own SNP SRA ULA as the OAL
   source and the SNP ULA of the Client as the OAL destination when it
   forwards the NA(AR).  The Proxy/Server then decrements the OAL Hop
   Limit, performs L2 encapsulation/fragmentation and forwards the
   resulting carrier packets over the underlay interface to the Client.

4.13.1.6.  Reliability

   After the ARS transmits the first NS(AR), it should wait up to
   RETRANS_TIMER seconds to receive a responsive NA(AR).  The ARS can
   then retransmit the NS(AR) up to MAX_UNICAST_SOLICIT times before
   giving up.

4.13.2.  Multilink Forwarding

   Following address resolution, the ARS and ART (i.e., the end system
   Clients or their respective Proxy/Servers) can assert per-flow
   multilink forwarding paths through underlay interface pairs serviced
   by the same source/destination ULAs by sending NS/NA messages for
   Multilink Forwarding (MF) with OMNI Multilink Vector sub-options.
   The NS/NA(MF) messages establish per-flow multilink forwarding and
   header compression state in OAL intermediate systems in the path
   between the ARS and ART.  Note that either the ARS or ART can
   independently initiate multilink forwarding by sending NS(MF)
   messages on behalf of specific flows over underlay interface pairs.

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   If the original IP packet uses an FNP/MNP GUA as the source, the
   source Client or Proxy/Server uses the same FNP/MNP GUA as the NS(MF)
   source.  If the original IP packet uses an SNP GUA as the source, the
   source Client or Proxy/Server uses the same SNP GUA as the NS(MF)
   source.  In both cases, the source Client or Proxy/Server uses the
   original IP packet destination address as the destination address and
   uses the SNP GUA associated with the destination as the Target
   Address of the NS(MF).  The source then includes the original IP
   packet flow label as the final element of the flow 3-tuple.  When the
   target Client or Proxy/Server returns an NA(MF), it sets the
   destination to the same address that appeared in the NS(MF) source,
   sets the Target Address to the same address that appeared in the
   NS(MF) target and sets the source address to the SNP GUA of the
   target Client.

   When an OAL source asserts a multilink forwarding path through the
   transmission of an NS(MF) message, it includes an IPv6 Minimum Path
   MTU Hop-by-Hop Option for the (adaptation layer) IPv6 header per
   [RFC9268].  Each OAL intermediate node along the path then updates
   the minimum MTU per the specification.  When the OAL destination
   responds with an NA(MF) message, it returns an IPv6 Minimum Path MTU
   Option based on the one it received in the NS(MF) message per
   [RFC9268].  This allows the OAL source to discover any OAL Fragment
   Size (OFS) limitations for this OAL destination (see:
   [I-D.templin-6man-omni3]).  For this reason, OAL nodes that connect
   SRT segments MUST implement [RFC9268].

   The multilink forwarding profile provides support for redundant paths
   that each OAL node can harness to its best advantage.  For example,
   OAL nodes can use traffic selectors to guide the dispersal of
   different traffic types over available multilink paths, while other
   factors such as metrics, cost, provider, etc. can also provide useful
   decision points.  OAL nodes can also employ multilink forwarding for
   fault tolerance by sending redundant data over multiple paths
   simultaneously, or for load balancing where the individual packets of
   a single traffic flow are spread across multiple independent paths.
   OAL nodes that engage in multilink forwarding therefore must
   incorporate a policy engine that selects both inbound and outbound
   multilink paths for a given traffic profile at a given point in time.
   This specification therefore provides multilink forwarding mechanisms
   without mandating any specific multilink policy.

   All Client, Proxy/Server and Gateway nodes that configure OMNI
   interfaces and engage in multilink coordination include an additional
   forwarding table termed the AERO Forwarding Information Base (AFIB)
   that supports OAL packet/fragment forwarding based on original IP
   packet flows over specific OMNI neighbor interface pairs.  The AFIB
   contains per-flow AERO Forwarding Vectors (AFVs) identified by the L2

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   address of the previous OAL hop plus a value known as the AFV Index
   (AFVI).  The AFVs cache uncompressed OAL header information to
   support forwarding of packets with compressed headers as well as
   previous/next-hop addressing and AFVI information.  The AFVs also
   cache window synchronization state (i.e., the starting sequence
   number and window size) for each specific flow.  Using the window
   synchronization state, simple Identification-based data origin
   authentication is enabled at each OAL source, intermediate system and
   target node.

   Client and Proxy/Server OMNI interfaces manage end system AFIB
   entries in conjunction with their internal Neighbor Cache, where the
   NCEs link to (possibly) multiple AFVs with one per flow over a
   specific FHS/LHS interface ifIndex pair.  When OMNI interface peers
   need to coordinate, they locate a NCE for the peer (established
   through address resolution) then use the NCE as a nexus that
   aggregates potentially many AVFs which cache AFVIs to be used for
   multilink forwarding on a per-flow basis.  Gateway OMNI interfaces
   and the OMNI interfaces of Clients or Proxy/Servers acting as OAL
   intermediate nodes manage transit AFIB entries independently of their
   internal Neighbor Caches.  These transit AFVs are indexed by the L2
   address and AFVI supplied by the previous hop.

   OAL source, intermediate system and target nodes create AFVs/AFVIs
   when they process an NS/NA(MF) initiator or responsive message with
   an OMNI Multilink Vector sub-option (see: [I-D.templin-6man-omni3]).
   The OAL source of the initiating NS/NA(MF) (which is also the OAL
   destination of the responsive NA(MF)) is considered to reside in the
   "First Hop Segment (FHS)", while the OAL destination of the NS/NA(MF)
   (which is also the OAL source of the responsive NA(MF)) is considered
   to reside in the "Last Hop Segment (LHS)".

   The FHS and LHS roles are determined on a per-flow and per-interface-
   pair basis.  After address resolution, either peer is equally capable
   of initiating multilink forwarding on behalf of a specific flow.  The
   peer that sends the initiating NS/NA(MF) message with Multilink
   Vector for a specific pair becomes the FHS peer while the one that
   returns the responsive NA(MF) becomes the LHS peer for that (flow,
   interface pair) only.  It is therefore commonplace that peers may
   assume the FHS role for some flows while assuming the LHS role for
   others, i.e., even though each peer maintains only a single NCE.

   When an OAL node sends/forwards an initiating NS/NA(MF) or responsive
   NA(MF) with a Multilink Vector sub-option, it creates an AFV, caches
   the Identification window information, caches the NS/NA(MF) and OAL
   IPv6 headers, records the previous hop L2 address and AFVI, then
   generates a new next hop AFVI.  The next hop AFVI should be selected
   within the range [1 - (2**16-1)] unless all values within that range

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   are already in active use for this next hop; otherwise, the AFVI must
   be selected within the range [2**16 - (2**32-1)].  (Note that the
   AFVI value 0 is used to indicate the NULL AFV, i.e., one that always
   matches but includes NULL header compression or forwarding
   information.)  When the OAL node forwards future OAL packets/
   fragments that include the previous hop L2 address and AFVI, it can
   unambiguously locate the correct AFV and use the cached forwarding
   information to forward to the next OAL hop.

   OAL nodes cache AFVs for up to ReachableTime seconds following their
   initial creation.  If the node processes another NS/NA(MF) 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.

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

4.13.2.1.  FHS Client-Proxy/Server NS(MF) Forwarding

   When an FHS OAL source has an original IP packet/parcel to send
   toward an LHS OAL target, it first performs address resolution
   resulting in the creation of a NCE for the SNP GUA of the target then
   selects a source and target underlay interface pair.  The FHS source
   then uses its cached information for the target interface as LHS
   information then prepares an NS(MF) message with a Multilink Vector
   sub-option while setting the NS(MF) source, target and destination
   addresses as specified above.

   The FHS source next creates an AFV then generates and assigns an AFVI
   for the flow over this interface pair; the AFVI must be unique for
   its communications to this next OAL hop.  The FHS source then
   includes the AFVI and window synchronization parameters in the
   Multilink Vector sub-option and prepares the NS(MF) message for
   transmission while also caching the window synchronization parameters
   in the local AFV.

   If the FHS source is the FHS Proxy/Server, it performs OAL
   encapsulation while setting the OAL source to the Client's SNP GUA
   and setting the OAL destination to the SNP GUA determined through
   address resolution for the target.  The FHS Proxy/Server then
   performs L2 encapsulation/fragmentation then forwards the resulting
   carrier packets into the secured spanning tree which will deliver
   them to an FHS Gateway.

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   If the FHS source is the FHS Client, it instead includes an
   authentication signature and OAL Extended Fragment Header with an in-
   window Identification for its FHS Proxy/Server if necessary.  If FMT-
   Forward and FMT-Mode are both set, the Client sets the Multilink
   Vector LHS ifIndex to the ifIndex of the target; otherwise, it sets
   the ifIndex to 0 to allow the FHS Proxy/Server to select the target
   ifIndex.  The FHS Client then calculates the NS(MF) message checksum,
   performs OAL encapsulation and sets the OAL source to its own SNP ULA
   and sets the OAL destination to the SNP SRA ULA of the FHS Proxy/
   Server.  The FHS Client finally performs L2 encapsulation/
   fragmentation and forwards the resulting carrier packets to the FHS
   Proxy/Server.

   When the FHS Proxy/Server receives the carrier packets, it performs
   L2 reassembly/decapsulation, verifies the Identification, and
   verifies the NS(MF) checksum and authentication signature.  The FHS
   Proxy/Server then creates an AFV (i.e., the same as the FHS Client
   had done) while caching the FHS Client addressing, AFVI and window
   synchronization information as previous hop information for this AFV.
   The FHS Proxy/Server next generates a new unique AFVI to forward to
   the next OAL hop, then both caches the AFVI in the AFV and writes it
   into the Multilink Vector sub-option, i.e., while over-writing the
   value supplied by the FHS Client.  The FHS Proxy/Server then
   calculates the NS(MF) checksum and sets the OAL source address to the
   Client's SNP GUA and destination address to the SNP GUA corresponding
   to the NS(MF) Target Address.  The FHS Proxy/Server finally
   decrements the OAL Hop Limit, removes the OAL Extended Fragment
   Header, performs L2 encapsulation/fragmentation and forwards the
   resulting carrier packets into the secured spanning tree.

4.13.2.2.  FHS/intermediate/LHS Gateway NS(MF) Forwarding

   Gateways in the spanning tree forward OAL packets/fragments 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 an OAL packet/fragment
   over the secured spanning tree, it performs L2 reassembly/
   decapsulation then verifies the NS(MF) checksum.  The FHS Gateway
   next creates an AFV and caches the previous hop Multilink Vector
   information, i.e.  the same as the FHS Proxy/Server had done.  The
   FHS Gateway then generates a locally-unique AFVI for the next hop and
   both caches the value in the AFV and copies it into the Multilink
   Vector sub-option.

   The FHS Gateway then examines the SRT prefixes corresponding to both
   the 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

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   segments), the FHS/LHS Gateway caches the NS(MF) Multilink Vector
   information in the AFV, and writes a new locally-unique AFVI for the
   next hop into the AFV and Multilink Vector.  The FHS Gateway then
   decrements the OAL Hop Limit, recalculates the NS(MF) checksum,
   performs L2 encapsulation/fragmentation and forwards the resulting
   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 update their transit AFVs in the same
   fashion while selecting new locally-unique AFVIs for the next hop and
   caching/updating Multilink Vector information.  The LHS Gateway then
   performs L2 reassembly/decapsulation, verifies the Identification,
   verifies the NS(MF) checksum then creates an AFV (i.e., the same as
   all previous hop Gateways had done) while caching the Multilink
   Vector information from the previous hop and creating a new AFVI for
   the next hop.  The LHS Gateway then decrements the OAL Hop Limit,
   recalculates the NS(MF) checksum, performs L2 encapsulation/
   fragmentation and forwards the resulting carrier packets into the
   secured spanning tree.

4.13.2.3.  LHS Proxy/Server-Client NS/NA(MF) Receipt/Forwarding

   When the LHS Proxy/Server receives the carrier packets from the
   secured spanning tree, it performs L2 reassembly/decapsulation,
   verifies the NS(MF) checksum then creates an AFV and caches the
   previous hop Multilink Vector and addressing information.

   If the NS(MF) destination matches the SNP GUA of the target and the
   LHS Proxy/Server is configured to respond on the target's behalf,
   (i.e., if FMT-Forward is set) it next prepares to return a responsive
   NA(MF).  The LHS Proxy/Server next creates or updates an NCE for the
   SRA corresponding to the NS(MF) source address (if necessary) with
   state set to STALE.

   The LHS Proxy/Server then creates an NA(MF) while copying the NS(MF)
   Multilink Vector sub-option into the NA(MF) and including responsive
   window synchronization information.  The LHS Proxy/Server then
   encapsulates the NA(MF) with OAL source set to the NS(MF) OAL
   destination and with OAL destination set to the NS(MF) OAL source.
   The LHS Proxy/Server then calculates the NA(MF) checksum, performs L2
   encapsulation/fragmentation and forwards the resulting carrier
   packets into the secured spanning tree.

   The LHS Proxy/Server then creates a locally-unique AFVI for the
   Client and both caches it in the newly-created AFV and writes it into
   the Multilink Vector.  If FMT-Forward is clear and FMT-Mode is set,

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   the LHS Proxy/Server next resets the Multilink Vector FHS ifIndex to
   0 and changes the OAL source to its own SNP SRA ULA.  The LHS Proxy/
   Server next includes an authentication signature in the NS(MF) if
   necessary, then recalculates the NS(MF) checksum, changes the OAL
   source to its own SNP SRA ULA and changes the OAL destination to the
   SNP ULA of the LHS Client.  The LHS Proxy/Server then decrements the
   OAL Hop Limit, includes an OAL Extended Fragment Header with an
   appropriate Identification value if necessary, performs L2
   encapsulation/fragmentation and forwards the resulting carrier
   packets to the LHS Client.

   When the LHS Client receives the carrier packets, it performs L2
   reassembly/decapsulation, verifies the Identification, then verifies
   the NS(MF) checksum/authentication signature.  The LHS Client then
   creates a NCE for the NS(MF) source address (if necessary) in the
   STALE state and caches the NS(MF) Multilink Vector information in a
   new AFV associated with the NCE corresponding to the NS(MF) source.
   If the LHS Client will request reverse path state establishment, it
   finally generates and assigns a locally-unique AFVI for a flow to be
   forwarded to the previous hop, which it caches in the new AFV.

   The LHS Client then prepares an NA(MF) using the same procedures as
   for the LHS Proxy/Server above (while including responsive window
   synchronization information and the new AFVI in the Multilink Vector
   sub-option).  The LHS Client includes an authentication signature if
   necessary, calculates the NA(MF) message checksum, then encapsulates
   the NA(MF) with OAL source set to its own SNP ULA and OAL destination
   set to the SNP SRA ULA of the LHS Proxy/Server.  The LHS Client
   finally includes an appropriate Identification if necessary, performs
   L2 encapsulation/fragmentation and forwards the resulting carrier
   packets to the LHS Proxy/Server.

4.13.2.4.  Reverse Path Forwarding of the NA(MF)

   When the LHS Proxy/Server receives the carrier packets, it performs
   L2 reassembly/decapsulation, verifies the Identification and verifies
   the NA(MF) checksum/authentication signature.  If the reverse path
   state must be established (i.e., if the SYN flag is set in the NA(MF)
   Multilink Vector sub-option) the LHS Proxy/Server then creates an AFV
   that caches the Client's AFVI and other Multilink Vector information
   in the same manner as the FHS Proxy/Server had done.  The LHS Proxy/
   Server then removes the OAL Extended Fragment Header and forwards the
   NA(MF) over the reverse path toward the initiating FHS node, where it
   may traverse many intermediate Gateways.

   Each Gateway along the reverse path processes the Multilink Vector
   information in the NA(MF) message in the same way that the Gateways
   in the forward path had processed the NS(MF).  In the end, the NA(MF)

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   will arrive at the initiating FHS node and AFV state will be
   established in all end and intermediate systems in both the forward
   and reverse paths.

   The forward and reverse paths between the FHS initiator and LHS
   responder may be different and may therefore have different OAL path
   MTUs.  If the LHS responder included a SYN directive in the Multilink
   Vector sub-option, it will also require a responsive ACK.  For this
   reason, the FHS initiator returns a secured uNA(MF) that includes an
   IPv6 Minimum Path MTU Option based on the one it received in the
   NA(MF) message per [RFC9268].  This provides the LHS responder with
   both a responsive ACK and a measurement of the LHS->FHS OAL path MTU.

   Note: NS/NA(MF) exchanges often establish state only on the forward
   path from the FHS to the LHS, i.e., they are often unidirectional.
   Only in cases where the LHS also has packets to return to the FHS
   will bidirectional state establishment occur in both the forward and
   reverse directions.

4.13.2.5.  OAL End System Exchanges Following Synchronization

   Following the initial NS/NA (plus optional uNA(MF)) exchange OAL end
   systems can begin exchanging ordinary carrier packets for
   synchronized flows that include AFVIs and with Identification values
   within their respective send windows without requiring security
   signatures and/or secured spanning tree traversal.  OAL end and
   intermediate systems can also consult their AFIBs when they receive
   carrier packets that contain OAL packets/fragments with AFVIs to
   unambiguously locate the correct AFV and can use the AFV state to
   forward OAL packets/fragments to the next hop.  OAL end systems must
   then perform continuous NS/NA(MF) exchanges to update window state,
   register new flows 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 OAL packets,
   they are jointly responsible for updating cache state and per-
   interface reachability before expiration.  Window synchronization
   state is performed on a per-flow basis and tracked in the AFVs which
   are also linked to the appropriate NCE.  However, the window
   synchronization exchange only confirms target Client reachability
   over the specific underlay interface pair.  Reachability for other
   underlay interfaces that share the must be determined individually
   using additional NS/NA(MF) messages that include Multilink Vectors.

   OAL sources can the begin including CRHs in OAL packets/fragments
   with an AFVI that OAL intermediate systems can use for shortest-path
   forwarding based on AFVIs instead of spanning tree OAL IPv6

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   addresses.  OAL sources and intermediate systems can instead forward
   OAL packets/fragments with OCH/OFH headers that include an AFVI since
   all OAL nodes in the path up to (and sometimes including) the OAL
   destination have already established AFVs.

   When a Proxy/Server receives OAL packets/fragments destined to a
   local SRT segment Client or forwards OAL packets/fragments received
   from a local segment Client, it first locates the correct AFV.  If
   the OAL packet/fragment includes a secured IPv6 ND message, the
   Proxy/Server uses the Client's NCE established through RS/RA
   exchanges to re-encapsulate/re-fragment while sending outbound
   secured carrier packets via the secured spanning tree and sending
   inbound secured carrier packets while including an authentication
   signature/checksum.  For ordinary OAL packets/fragments, 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(MF) exchange
   between the Client and the remote SRT segment peer, and forwards the
   OAL packet/fragments without first reassembling/decapsulating.

   When a source Client forwards OAL packets/fragments it can employ
   header compression according to the AFVs established through an NS/
   NA(MF) exchange with a remote or local peer.  When a target Client
   receives carrier packets that contain OAL packets/fragments that
   match a local AFV, the Client first verifies the Identification then
   decompresses the headers if necessary, reassembles to obtain the OAL
   packet then decapsulates and delivers the original IP packet/parcel
   to the network layer.

   When synchronized peer Clients in the same SRT segment with FMT-
   Forward and FMT-Mode set discover each other's NATed L2ADDR
   addresses, they can exchange carrier packets that contain OAL
   packets/fragments directly with header compression using AFVIs
   discovered as above (see: Section 4.13.4).

   When the FHS Client or FHS Proxy/Server sends an NS(MF) for the
   purpose of establishing multilink forwarding state, it should wait up
   to RETRANS_TIMER seconds to receive a responsive NA(MF).  The FHS
   node can then retransmit the NS(MF) up to MAX_UNICAST_SOLICIT times
   before giving up.  Note that each successive attempt establishes new
   AFV state in the OAL intermediate systems, but that any abandoned
   stale AFV state will be quickly reclaimed.

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4.13.3.  Mobile Ad-hoc Network (MANET) Forwarding

   Clients with OMNI interfaces configured over underlay interfaces with
   indeterminant neighborhood properties may be connected to a Mobile
   Ad-hoc NETwork (MANET).  Each MANET may be either completely outside
   of the range of any OMNI link Proxy/Servers or may require multihop
   traversal between Clients acting as MANET routers to reach Proxy/
   Servers that connect to the rest of the OMNI link.  The former class
   of MANETs must operate in isolation solely based on the HHIT unique
   IPv6 addresses they configure locally.  The latter class allows MANET
   routers to extend infrastructure-based addressing information
   including MNPs over multiple OMNI link hops as discussed in the OMNI
   specification.

   MANET Clients configure their OMNI interfaces over one or more MANET
   interfaces where multihop forwarding may be necessary.  Routing
   protocols suitable for use over MANET interfaces include OSPFv3
   [RFC5340] with MANET Designated Router (OSPF-MDR) extensions
   [RFC5614], OLSR [RFC7181], AODV [I-D.perkins-manet-aodvv2] and
   others.  Other services specific to MANET link-local and/or site-
   local operations (including SMF [RFC6621], DLEP [RFC8175] and others)
   are also considered in-scope.  These services strive for optimal use
   of available radio bandwidth and power consumption in their control
   message transmissions, but efficient data plane operation is also
   essential.

   Clients must therefore reduce overhead through minimal encapsulation
   and effective header compression whenever possible.  For this reason,
   when the MANET routing protocol discovers a new route the Client
   configures a lesser-preferred forwarding table entry over the
   corresponding MANET interface and a more-preferred forwarding table
   entry over the OMNI interface.  This will cause the network layer to
   direct outbound packets to the OMNI interface, which can apply header
   compression and underlay MANET interface selection.

   Proxy/Servers that connect a MANET to the rest of the OMNI link act
   as regular Proxy/Servers for exchanges with external INETs, but act
   as Clients over their MANET interfaces.  Each such Proxy/Server
   therefore has at least two underlay interfaces, including an INET
   interface and a MANET interface.  The Proxy/Server therefore services
   the MANET as if it were an ordinary Client but presents itself as a
   Proxy/Server to external facing INETs.

   The process for a multihop Client to establish multilink forwarding
   and header compression AFV state in the MANET is conducted in the
   same fashion as described above and using the same NS/NA(MF) message
   exchanges.  Each intermediate MANET node in the path configures AFV
   state and assigns AFVIs in the same fashion as for intermediate

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   Gateways in the secured spanning tree except that the NS/NA(MF)
   messages requires an authentication signature (unless neighboring
   MANET nodes configure IPsec tunnels) and an Identification that is
   within the window for its serving Proxy/Server if the destination is
   outside of the local MANET.  The NS/NA(MF) messages extend from the
   initiating FHS MANET Client, then across any intermediate FHS MANET
   Clients to the FHS Proxy/Server, then across the secured SRT spanning
   tree, then to the LHS Proxy/Server, then finally across any
   intermediate LHS MANET Clients to the responding LHS Client.  The LHS
   responder then returns an NA(MF) along the reverse path.  In all
   other ways, the NS/NA(MF) exchanges are the same as discussed in
   Section 4.13.2.

   Following the NS/NA(MF) exchanges, each MANET router in the forward
   (and optionally also reverse) paths in both the FHS and LHS MANETs
   will have established AFVs containing multilink forwarding and header
   compression state for the flow.  The AFVs determine AFVI-based
   forwarding based on the OCH header contents, and each MANET router
   only forwards packets with in-window Identification values for the
   flow.  MANET routers maintain AFVs for up to ReachableTime seconds
   unless they are refreshed by an NS/NA(MF) TST/ACK exchange.  New
   window synchronization exchanges must also be performed periodically
   to avoid window exhaustion and/or spoofing based on predictable
   Identifications.

   Note: while the MANET routing protocol runs directly over the node's
   MANET interfaces to discover routing information, the node configures
   lesser-preferred forwarding table entries over the MANET interface
   and corresponding more-preferred forwarding table entries over the
   OMNI interface.  This causes the network layer to forward outbound
   packets via the OMNI interface which applies encapsulation,
   fragmentation and/or header compression as necessary before
   forwarding over the underlying MANET interface.  The OMNI protocol
   designator in the UDP port, IP protocol or Ethernet EtherType field
   will then cause the packets to visit the OMNI interface of each
   successive next-hop MANET node.

4.13.4.  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 any shared segment
   Gateways) and can begin sending carrier packets directly via NAT
   traversal while avoiding any Proxy/Server and/or Gateway hops.

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   When the FHS/LHS Clients on the same SRT segment perform initial NS/
   NA(MF) exchanges to establish AFIB state, they first examine the FMT-
   Forward and FMT-Mode settings to determine whether direct-path
   forwarding is even possible for one or both Clients (direct-path
   forwarding is only possible for one or both when FMT-Forward and FMT-
   Mode are both set).  The NS/NA(MF) messages then include an Origin
   Indication (i.e., in addition to a Multilink Vector 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 carrier 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 the local chain of NATs for reception of future carrier
   packets from that L2 address (see: [RFC4380] and
   [I-D.templin-6man-omni3]).  The source Client then prepares an NS(RO)
   message with its own MNP SRA GUA or SNP GUA as the source, with the
   MNP SRA GUA or SNP GUA of the target as the destination and with an
   OMNI option with an Interface Attributes sub-option.  The source
   Client then encapsulates the NS(RO) in an OAL header with its own SNP
   ULA as the source, with the SNP ULA of the target Client as the
   destination and with an in-window Identification for the target.  The
   source Client then performs L2 encapsulation/fragmentation with L2
   headers addressed to its Proxy/Server then sends the resulting
   carrier packets to the Proxy/Server.

   When the Proxy/Server receives the carrier packets, it re-
   encapsulates and sends them as unsecured carrier packets according to
   AFIB state where they will eventually arrive at the target Client
   which can perform L2 reassembly/decapsulation.  Following reassembly,
   the target Client prepares an NA(RO) message with its own MNP SRA GUA
   or SNP GUA as the source, with the MNP SRA GUA or SNP GUA 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(RO) in an OAL header with its own SNP ULA as the source, with
   the SNP ULA of the source Client as the destination and with an in-
   window Identification for the source Client.  The target Client then
   performs L2 encapsulation/fragmentation then forwards the resulting
   carrier packets directly to the source Client.

   Following the initial NS/NA(RO) exchange, both Clients mark their
   respective (source, target) underlay interface pairs as "trusted" for
   no more than ReachableTime seconds.  The Clients can then begin
   exchanging ordinary data packets as OCH encapsulated carrier packets.
   While the Clients continue to exchange packets via the direct path
   avoiding all Proxy/Servers and Gateways, they should perform

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   additional NS/NA(RO) 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: 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: each communicating pair of Clients may need to maintain NAT
   state for peer to peer communications via multiple underlay interface
   pairs and/or multiple flows.  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: the source and target Client exchange Origin information during
   the secured NS/NA(RO) multilink route optimization exchange.  This
   allows for subsequent NS/NA(RO) 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.

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

   When a Client forwards an OAL packet (or an original IP packet/
   parcel) 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], and should also
   include any RIOs with MNP information corresponding to the target.

   In the same fashion, when a Proxy/Server forwards an OAL packet (or
   original IP packet/parcel) from a Host or Client connected to one of
   its downstream *NETs to a peer within the same downstream *NET, the
   Proxy/Server returns an IPv6 ND Redirect message.

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

4.14.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform Neighbor Unreachability Detection (NUD) per
   [RFC4861] either reactively in response to persistent link layer
   errors (see Section 4.11) 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

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   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 GUAs/ULAs of
   the parties involved in the NUD test.  When only reachability
   information is required without updating any other NCE state, AERO
   nodes can instead perform NS/NA(NUD) exchanges directly between
   neighbors without employing the secured spanning tree as long as they
   include in-window Identifications and an authentication signature/
   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 system 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 for a
   given flow, the source can optionally continue to forward OAL
   packets/fragments via alternate interfaces or maintain a small queue
   of carrier packets until target reachability is confirmed.

   NS(NUD) messages are encapsulated, fragmented and transmitted as
   carrier packets the same as for ordinary original IP data packets/
   parcels.  The source encapsulates the NS(NUD) message the same as
   described in Section 4.13.2 and includes an Interface Attributes sub-
   option with ifIndex set to identify its underlay interface used for
   forwarding.  The source then includes an in-window Identification,
   performs L2 encapsulation/fragmentation then forwards the resulting
   carrier packets into the unsecured spanning tree directly to the
   target if it is in the local segment.

   When the target receives the NS(NUD) carrier packets, it performs L2
   reassembly/decapsulation, verifies that it has a NCE for this source
   and that the Identification is in-window then performs OAL
   reassembly.  The target next verifies the NS(NUD) checksum/
   authentication signature, then searches for Interface Attributes in
   its NCE for the source that match the NS 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

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   NA(NUD) to identify the ifIndex 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 includes an in-window Identification for
   the source.  The node then performs L2 encapsulation/fragmentation
   and forwards the resulting carrier packets into the unsecured
   spanning tree directly to the source if it is 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.

   Communicating peer nodes can also use NUD to test forward path AFV
   state for individual flows.  To test flow state, the OAL source can
   send an NS message with a Multilink Vector OMNI sub-option with the
   TST flag set and with a current Sequence Number and next hop AFVI for
   the flow.  If the OAL destination receives the NS message, it returns
   a responsive NA with Multilink Vector with the ACK flag set and with
   the OAL source's Sequence Number in the Acknowledgement field.  If
   the OAL source receives the NA, it considers the flow state as
   viable; otherwise, it should issue a new NS/NA(MF) exchange to
   establish new flow state.

4.15.  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 MAP Proxy/Servers
   via RS/RA exchanges to maintain the DMM profile, and the AERO routing
   system tracks all current Client/Proxy/Server peering relationships.

   MAP 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 MAP 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 MAP 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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   Note: when a Client's underlay interface address changes, the Client
   and/or its (former) FHS Proxy/Server for this interface must
   invalidate any AFVs based on the (changed) interface.  Future data
   packet forwarding will then trigger a new multilink forwarding NS/
   NA(MF) exchange to re-seed new AFVs in the path.

   Mobility management considerations are specified in the following
   sections.

4.15.1.  Mobility Update Messaging

   Mobile Clients (and/or their MAP Proxy/Servers) accommodate mobility
   and/or multilink change events by sending secured uNA Mobility
   Management (MM) messages to each active neighbor.  When a node sends
   a uNA(MM) message to each specific neighbor on behalf of a mobile
   Client, it sets the IPv6 source address to its own MNP SRA GUA or SNP
   ULA/GUA, sets the destination and target address to the neighbor's
   SNP ULA/GUA or one of the mobile Client's MNP SRA GUAs.  The uNA(MM)
   also includes an OMNI option with OMNI Interface Attributes and
   Traffic Selector sub-options for the mobile Client's underlay
   interfaces and includes an authentication signature if necessary.
   The node then sets the uNA(MM) 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 SNP GUA/ULA and destination set to either the specific
   neighbor's SNP ULA/GUA or the FHS Proxy/Server's SNP SRA ULA.
   Following OAL and L2 encapsulation/fragmentation, the carrier packets
   containing the uNA(MM) 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(MM) messages is unreliable but provides a useful
   optimization.  In well-connected Internetworks with robust data links
   uNA(MM) messages will be delivered with high probability, but in any
   case the node can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT
   uNA(MM)s to each neighbor to increase the likelihood that at least
   one will be received.  Alternatively, the node can set the SNR flag
   in the uNA(MM) OMNI option header to request a uNA(ACK) response
   (see: Section 4.5.1).

   When the FHS/LHS Proxy/Server receives a secured uNA(MM) message
   prepared as above, if the uNA(MM) destination was its own SNP SRA 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(MM) message does not provide confirmation that any
   forward paths to the target Client are working.  If the destination
   was the SNP GUA of the FHS/LHS Client, the Proxy/Server instead
   changes the OAL source to its own SNP SRA GUA/ULA, includes an
   authentication signature if necessary, and includes an in-window

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   Identification for this Client.  Finally, if the uNA(MM) message SNR
   flag was set, the node that processes the uNA(MM) also returns a
   uNA(ACK) response (see: Section 4.5.1).

4.15.2.  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 MAP Proxy/
   Server (via first-hop FHS Proxy/Servers if necessary).  Each RS
   includes an OMNI option with Interface Attributes and/or Traffic
   Selector sub-options for the ifIndex in question.

   Note that the first FHS Proxy/Server may change due to the underlay
   interface change.  If the Client RS includes an OMNI Proxy/Server
   Departure sub-option for the former FHS Proxy/Server, the new FHS
   Proxy/Server can send a departure indication (see Section 4.15.5);
   otherwise, any stale state in the former FHS Proxy/Server will simply
   expire after ReachableTime expires with no effect on the MAP 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(MM)
   messages to all neighbors the same as described in the previous
   section.

4.15.3.  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 MAP 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 interface parameters and with
   link layer address information for the new link.  The Client then
   again sends uNA(MM) messages to all neighbors the same as described
   above.

4.15.4.  Deactivating Existing Links

   When a Client needs to deactivate an existing underlay interface, it
   sends a uNA(MM) message toward the MAP Proxy/Server via an FHS Proxy/
   Server with an OMNI option with appropriate Interface Attributes
   values for the deactivated link.

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   If the Client needs to send uNA(MM) messages over an underlay
   interface other than the one being deactivated, it MUST include
   Interface Attributes for any underlay interfaces being deactivated.
   The Client then again sends uNA(MM) messages to all neighbors the
   same as described above.

   Note that when a Client deactivates an underlay interface, neighbors
   that receive the ensuing uNA(MM) 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 ifIndex 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.  The manner in which the Client dynamically
   manages its local ifIndex to interface mappings is a local decision,
   but should not be done in a manner that could cause state
   inconsistencies in the network.

4.15.5.  Moving Between Proxy/Servers

   The Client performs the procedures specified in Section 4.12.2 when
   it first associates with a new MAP Proxy/Server or renews its
   association with an existing MAP Proxy/Server.

   When a Client associates with a new MAP Proxy/Server, it sends RS
   messages to register its underlay interfaces with the new MAP while
   including the old MAP's GUA in the "Old MAP Proxy/Server GUA" field
   of a Proxy/Server Departure OMNI sub-option.  When the new MAP Proxy/
   Server returns the RA message via the FHS Proxy/Server (acting as a
   proxy), the FHS Proxy/Server sends a uNA(MM) to the old MAP Proxy/
   Server (i.e., if the GUA is non-zero and different from its own).
   The uNA(MM) has the MNP SRA GUA of the Client as the source and the
   SNP SRA GUA of the old MAP as the destination and with an OMNI Proxy/
   Server Departure sub-option as above.  The FHS Proxy/Server
   encapsulates the uNA(MM) in an OAL header with the SNP SRA GUA of the
   new MAP as the source and the SNP SRA GUA of the old MAP as the
   destination, then performs L2 encapsulation/fragmentation and
   forwards the resulting carrier packets via the secured spanning tree.

   When the old MAP Proxy/Server receives the carrier packets, it
   decapsulates and reassembles if necessary to obtain the uNA(MM) then
   changes the Client's NCE state to DEPARTED, resets DepartTime and
   caches the new MAP Proxy/Server GUA.  After a short delay (e.g., 2
   seconds) the old MAP Proxy/Server withdraws the Client's MNP(s) from
   the routing system.  While in the DEPARTED state, the old MAP Proxy/
   Server forwards any carrier packets received via the secured spanning
   tree destined to the Client's MNP GUAs or SNP GUA to the new MAP
   Proxy/Server's SNP GUA.  When DepartTime expires, the old MAP Proxy/
   Server deletes the Client's NCE.

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   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 GUA for the "Old FHS Proxy/
   Server GUA", and the new FHS Proxy/Server will issue a uNA(MM) using
   the same procedures as outlined for the MAP above while using its own
   SNP SRA GUA as the source address.  This can often result in
   successful delivery of carrier packets that would otherwise be lost
   due to the mobility event.

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

4.15.6.  Accommodating Path Changes

   After AFV state has been established for a flow, all OAL intermediate
   systems in the forward path will have AFVs with header compression
   state and (AFVI, L2ADDR) information for the next hop.  However,
   paths can fluctuate due to factors such as node mobility, routing
   changes, network membership, etc.  If an OAL intermediate system
   forwarding OAL packets with OCH headers detects that the next hop in
   the path has changed, it immediately reverts to sending the packets
   with header compression disabled by including full OFH and IPv6
   Extended Fragment Headers (plus full original IP headers) in future
   packets.

   If the OAL intermediate system receives an OCH1 packet with the Q bit
   set and M bit clear during a path change event, it first decompresses
   the original IP headers of each payload packet in the (packed) OAL
   packet while retaining the packets as attachments to the (full) OAL
   header.  The OAL intermediate system then processes the OAL packet
   further.

   If an OAL packet is larger than the minimum OFS, the OAL intermediate
   system applies OAL fragmentation to produce (sub-)fragments of
   minimal length.  If the original OAL packet/fragment had a fragment
   ordinal value N, the OAL intermediate node writes the same value N
   into each of the (sub-)fragments produced.

   The OAL intermediate node then encapsulates the OAL packet or
   fragments as attachments to OAL-encapsulated uNA messages (i.e., the
   same as for initial packets during a multilink forwarding exchange)

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   but also includes an OMNI Neighbor Control sub-option with the "Path
   Change (PCH)" bit set (see: [I-D.templin-6man-omni3].  The OAL
   intermediate node then applies an authentication signature and
   includes an IPv6 Extended Fragment Header if necessary or admits the
   uNA-encapsulated packet/fragments into the secured spanning tree.
   These (sub-)fragments (along with any other OAL fragments) will not
   be further fragmented by other OAL intermediate nodes on the path and
   will be reassembled by the OAL destination.

   When the OAL destination begins to receive OAL packets with full
   headers and with the PCH bit set, it assumes that the network path
   has changed and begins sending uNA messages to the OAL source.  The
   OAL destination sends the uNA messages subject to rate limiting, and
   includes a Multilink Vector OMNI sub-option with both the ACK and RST
   flags set and with the most recent OAL packet Identification written
   into the Acknowledgment field.

   When the OAL source receives the uNA messages with ACK and RST set,
   it re-initiates multilink forwarding for this flow by issuing a new
   NS/NA(MF) exchange the same as for a new flow as specified in
   Section 4.13.2.  The AFV state in the former path then simply becomes
   stale and is soon purged by the former OAL intermediate nodes.

4.16.  Multicast

   Each Client provides an IGMP (IPv4) [RFC2236] or MLD (IPv6) [RFC3810]
   proxy service for its ENETs and/or hosted applications [RFC4605] and
   acts 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, VPN/
   IPsec or Direct interfaces, and Relays also act as OMNI link PIM
   routers on behalf of nodes on other links/networks.

   Clients on VPN/IPsec, Direct or (M)ANET underlay interfaces for which
   the *NET has deployed native multicast services forward IGMP/MLD
   messages into the *NET.  The IGMP/MLD messages may be further
   forwarded by a first-hop *NET access router acting as an IGMP/MLD-
   snooping switch [RFC4541], then ultimately delivered to a *NET (FHS)
   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 *NET
   underlay interfaces without native multicast services instead send
   NS(AR) messages as an ARS to cause their FHS Proxy/Server to 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.

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4.16.1.  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].  The same as for unicast
   destinations, the 3-tuple of source, destination and flow label
   identifies a flow for multicast group G.

   For each S belonging to a prefix reachable via X's OMNI interface, X
   sends an NS(AR) message (see: Section 4.13) using its own MNP SRA GUA
   or SNP GUA as the source address and the MNP/SNP GUA of S as both the
   destination and target addresses.  X then encapsulates the NS(AR) in
   an OAL header with source address set to its own GUA/ULA and
   destination address set to the GUA/ULA for S, then forwards the
   message into the secured spanning tree which delivers it to ARR "Y"
   that services S.  Y will then return an NA(AR) that includes 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(MF) multilink forwarding
   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 MNP SRA GUA or SNP GUA 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(MF) 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(MF)
   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(MM) message to X the same as specified for
   unicast mobility in Section 4.15.  When X receives the uNA(MM)
   message, it updates its NCE for the XLA for source S and sends new

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   Join messages in NS/NA(MF) 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.

4.16.2.  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(MF) message with its own MNP SRA GUA or SNP GUA as
   the source address and the MNP SRA GUA or SNP GUA for R as the
   destination and target address, then encapsulates the NS(MF) message
   in an OAL header with its own GUA/ULA as the source and the GUA/ULA
   of R's Proxy/Server as the destination then sends the message into
   the secured spanning tree.

   For each source "S" that sends multicast traffic to group G via R,
   Client S* that aggregates S (or its Proxy/Server) encapsulates the
   original IP packets/parcels in PIM Register messages, includes the
   PIM Register messages in the OMNI options of uNA(MM) messages,
   performs OAL encapsulation and fragmentation with Identification
   values within the receive window for Client R* that aggregates R,
   then performs L2 encapsulation/fragmentation and forwards the
   resulting carrier packets.  Client R* may then elect to send a PIM
   Join to S* in the OMNI option of a uNA(MM) 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/parcel; one native copy from the (S, G) tree and a second
   copy from the pre-existing (*, G) tree that still uses uNA(MM) PIM
   Register encapsulation.  R can then issue a uNA(MM) 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/parcels via
   uNA(MM) PIM Register encapsulation via the new Proxy/Server.

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

4.16.3.  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.

4.17.  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/INET link
   layer 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 4.2.4).

   The Client's network layer can select the outbound 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/parcels 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.

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   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/parcel over multiple hops to
   the target.

4.18.  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/parcels 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.

4.19.  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 FNPs on its INET links.

   This gives rise to the opportunity to eventually distribute native IP
   addresses to all nodes, and to present a unified OMNI link view even
   if the INET partitions remain in their current protocol and
   addressing plans.  In that way, the OMNI link can serve the dual
   purpose of providing a mobility/multilink service and a transition/
   coexistence service.  Alternatively, if an INET partition is
   transitioned to a native IP protocol version and addressing scheme
   compatible with the OMNI link MNP-based addressing scheme, the
   partition and OMNI link can be joined by Gateways.

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   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].

4.20.  Proxy/Server-Gateway Bidirectional Forwarding Detection

   In environments where rapid failure recovery is essential, 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 can 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 operational practice.

4.21.  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-omni3]) that can serve as a Client ID seed
   for MNP prefix delegation.  The Client would then be obligated to
   renumber its internal networks whenever its MNP changes.  This should
   not present problems for Clients with automated network renumbering
   services, however it can limit the durations of ongoing sessions that
   would prefer to use a constant address.

5.  Implementation Status

   AERO/OMNI Release-3.2 was tagged on March 30, 2021, and was subject
   to internal testing.  The implementation is not planned for public
   release.

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   A new implementation architecture based on a clean-slate has been
   developed and will incorporate updated aspects of the AERO/OMNI
   specs, with the goal of producing a reference implementation for
   future release.

6.  IANA Considerations

   The IANA has assigned the UDP port number "8060" for an experimental
   first edition of AERO [RFC6706].  This document together with OMNI
   [I-D.templin-6man-omni3] reclaims UDP port number "8060" as the
   service port for AERO/OMNI UDP/IP encapsulation.  This document makes
   no IANA request, since the OMNI specification already provides IANA
   guidance.  (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.

7.  Security Considerations

   AERO Gateways establish security associations with AERO Proxy/Servers
   and Relays within their local OMNI link segments using secured
   tunnels over underlay interfaces.  The AERO Gateways of all OMNI link
   segments in turn configure secured tunnels with neighboring AERO
   Gateways for other OMNI link segments in a secured spanning tree
   topology.  Applicable security services include IPsec [RFC4301] with
   IKEv2 [RFC7296], etc.  (Note that secured direct point-to-point links
   can also be used instead of or in addition to network layer
   security.)  Together, these services are responsible for assuring
   connectionless integrity and data origin authentication with optional
   protection against replays for control messages that traverse the
   secured spanning tree.

   To prevent unauthorized local applications from congesting the
   secured spanning tree, Proxy/Servers and Gateways configure local
   access controls to permit only the BGP protocol service daemon to
   source routing protocol control messages with the ULA assigned to the
   OMNI interface as the source over the secured spanning tree.  This
   could be implemented as a port/address filtering configuration that
   permits only TCP port 179 (as defined in the IANA "Service Names and
   Port Numbers" registry) when using the ULA assigned to the OMNI
   interface.  To prevent malicious Clients from congesting the secured
   spanning tree, Proxy/Servers should also rate-limit the secured IPv6
   ND NS/NA messages they process for the same (source, target) pair,
   e.g., by applying IPv6 ND MAX_UNICAST_SOLICIT;
   MAX_NEIGHBOR_ADVERTISEMENT limits.

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   To prevent spoofing, Proxy/Servers MUST silently discard without
   responding to any unsecured IPv6 ND messages with OMNI sub-options
   that would otherwise affect state.  Also, Proxy/Servers MUST silently
   discard without forwarding any original IP packets/parcels received
   from one of their own Clients (whether directly or following OAL
   reassembly) with a source address that does not match the Client's
   MNP and/or a destination address that does match the Client's MNP.
   Finally, Proxy/Servers MUST silently discard without forwarding any
   carrier packets that include an OAL packet/fragment with source and
   destination that both match the same MNP.

   AERO Clients that connect to secured ANETs need not apply additional
   security to their IPv6 ND messages, since the messages will be
   accepted and forwarded by a perimeter Proxy/Server that applies
   security over its INET-facing interface to the secured spanning tree
   (see above).  AERO Clients that connect to MANETs or open INETs can
   use network and/or transport layer security services such as VPNs
   (e.g., IPsec tunnels) or can by some other means establish a secured
   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 IPv6 ND messages as
   specified in [I-D.templin-6man-omni3].

   End systems SHOULD apply transport or higher layer security services
   such as QUIC-TLS [RFC9000], TLS/SSL [RFC8446], DTLS [RFC6347], etc.
   to provide a level of protection comparable to critical secured
   Internet services.  End systems that require host-based VPN services
   SHOULD use network and/or transport layer security services such as
   IPsec, TLS/SSL, DTLS, etc.  AERO Proxy/Servers and Clients can also
   provide a network-based VPN service on behalf of end systems, e.g.,
   if the end system is located within a secured enclave and cannot
   establish a VPN on its own behalf.

   For INET partitions that require strong network layer security in the
   data plane, two options for securing communications include 1)
   disable route optimization and direct all traffic over the secured
   spanning tree, or 2) enable on-demand secure tunnel establishment
   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-omni3] or QUIC-TLS protocol message sub-options
   [RFC9000][RFC9001] [RFC9002] to establish secured sessions.

   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

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   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
   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 carrier 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 AERO service for MANET and 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 Proxy/Servers and
   potential correspondent peer nodes.  Similarly, each Client must be
   able to determine the public key of each Proxy/Server, e.g. by
   consulting an online database.

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

   Security considerations for IPv6 fragmentation and reassembly are
   discussed in [I-D.templin-6man-omni3].  In environments where
   spoofing is considered a threat, all OAL nodes SHOULD employ
   Identification window synchronization and OAL end systems SHOULD
   configure an (end-system-based) firewall.

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

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8.  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, Sean Dickson, 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,
   Kristina Ross, Greg Saccone, Ron Sackman, 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 for his support and guidance, 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)

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   and SRI International supported early investigations of IPv6, ONR UAV
   Communications and the IETF.  An employment at Nokia where important
   IETF documents were published gave way to a present-day engagement
   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:

   *  Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) [RFC5214]

   *  The Subnetwork Encapsulation and Adaptation Layer (SEAL) [RFC5320]

   *  Virtual Enterprise Traversal (VET) [RFC5558]

   *  Routing and Addressing in Networks with Global Enterprise
      Recursion (RANGER) [RFC5720][RFC6139]

   *  The Internet Routing Overlay Network (IRON) [RFC6179]

   *  AERO, First Edition [RFC6706]

   Note that these works cite numerous earlier efforts that are not
   included 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)
   Airplane Internet of Things (AIoT) and autonomy programs.

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

   Honoring life, liberty and the pursuit of happiness.

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9.  References

9.1.  Normative References

   [I-D.templin-6man-omni3]
              Templin, F., "Transmission of IP Packets over Overlay
              Multilink Network (OMNI) Interfaces", Work in Progress,
              Internet-Draft, draft-templin-6man-omni3-03, 15 April
              2024, <https://datatracker.ietf.org/doc/html/draft-
              templin-6man-omni3-03>.

   [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>.

   [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>.

   [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>.

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

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   [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>.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437,
              DOI 10.17487/RFC6437, November 2011,
              <https://www.rfc-editor.org/info/rfc6437>.

   [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>.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,
              <https://www.rfc-editor.org/info/rfc6890>.

   [RFC8028]  Baker, F. and B. Carpenter, "First-Hop Router Selection by
              Hosts in a Multi-Prefix Network", RFC 8028,
              DOI 10.17487/RFC8028, November 2016,
              <https://www.rfc-editor.org/info/rfc8028>.

   [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>.

   [RFC9268]  Hinden, R. and G. Fairhurst, "IPv6 Minimum Path MTU Hop-
              by-Hop Option", RFC 9268, DOI 10.17487/RFC9268, August
              2022, <https://www.rfc-editor.org/info/rfc9268>.

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9.2.  Informative References

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

   [EUI]      "IEEE Guidelines for Use of Extended Unique Identifier
              (EUI), Organizationally Unique Identifier (OUI), and
              Company ID, https://standards.ieee.org/wp-
              content/uploads/import/documents/tutorials/eui.pdf", 3
              August 2017.

   [I-D.bctb-6man-rfc6296-bis]
              Cullen, M., Baker, F., Trøan, O., and N. Buraglio, "RFC
              6296bis IPv6-to-IPv6 Network Prefix Translation", Work in
              Progress, Internet-Draft, draft-bctb-6man-rfc6296-bis-02,
              26 January 2024, <https://datatracker.ietf.org/doc/html/
              draft-bctb-6man-rfc6296-bis-02>.

   [I-D.ietf-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-ietf-6man-comp-rtg-hdr-06,
              3 May 2024, <https://datatracker.ietf.org/doc/html/draft-
              ietf-6man-comp-rtg-hdr-06>.

   [I-D.ietf-intarea-tunnels]
              Touch, J. D. and M. Townsley, "IP Tunnels in the Internet
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-intarea-tunnels-13, 26 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-intarea-
              tunnels-13>.

   [I-D.ietf-rtgwg-atn-bgp]
              Templin, F., Saccone, G., Dawra, G., Lindem, A., and V.
              Moreno, "A Simple BGP-based Mobile Routing System for the
              Aeronautical Telecommunications Network", Work in
              Progress, Internet-Draft, draft-ietf-rtgwg-atn-bgp-26, 29
              March 2024, <https://datatracker.ietf.org/doc/html/draft-
              ietf-rtgwg-atn-bgp-26>.

   [I-D.perkins-manet-aodvv2]
              Perkins, C. E., Dowdell, J., Steenbrink, L., and V.
              Pritchard, "Ad Hoc On-demand Distance Vector Version 2
              (AODVv2) Routing", Work in Progress, Internet-Draft,
              draft-perkins-manet-aodvv2-04, 3 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-perkins-
              manet-aodvv2-04>.

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   [I-D.templin-6man-parcels2]
              Templin, F., "IPv6 Parcels and Advanced Jumbos (AJs)",
              Work in Progress, Internet-Draft, draft-templin-6man-
              parcels2-03, 12 April 2024,
              <https://datatracker.ietf.org/doc/html/draft-templin-6man-
              parcels2-03>.

   [I-D.templin-intarea-parcels2]
              Templin, F., "IPv4 Parcels and Advanced Jumbos (AJs)",
              Work in Progress, Internet-Draft, draft-templin-intarea-
              parcels2-03, 12 April 2024,
              <https://datatracker.ietf.org/doc/html/draft-templin-
              intarea-parcels2-03>.

   [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.

   [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>.

   [RFC1256]  Deering, S., Ed., "ICMP Router Discovery Messages",
              RFC 1256, DOI 10.17487/RFC1256, September 1991,
              <https://www.rfc-editor.org/info/rfc1256>.

   [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>.

   [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>.

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   [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>.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, DOI 10.17487/RFC3056, February
              2001, <https://www.rfc-editor.org/info/rfc3056>.

   [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>.

   [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>.

   [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>.

   [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>.

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

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

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

   [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>.

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   [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>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [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>.

   [RFC5614]  Ogier, R. and P. Spagnolo, "Mobile Ad Hoc Network (MANET)
              Extension of OSPF Using Connected Dominating Set (CDS)
              Flooding", RFC 5614, DOI 10.17487/RFC5614, August 2009,
              <https://www.rfc-editor.org/info/rfc5614>.

   [RFC5720]  Templin, F., "Routing and Addressing in Networks with
              Global Enterprise Recursion (RANGER)", RFC 5720,
              DOI 10.17487/RFC5720, February 2010,
              <https://www.rfc-editor.org/info/rfc5720>.

   [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>.

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

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

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

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
              <https://www.rfc-editor.org/info/rfc6145>.

   [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>.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              DOI 10.17487/RFC6147, April 2011,
              <https://www.rfc-editor.org/info/rfc6147>.

   [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>.

   [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>.

   [RFC6621]  Macker, J., Ed., "Simplified Multicast Forwarding",
              RFC 6621, DOI 10.17487/RFC6621, May 2012,
              <https://www.rfc-editor.org/info/rfc6621>.

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

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   [RFC7181]  Clausen, T., Dearlove, C., Jacquet, P., and U. Herberg,
              "The Optimized Link State Routing Protocol Version 2",
              RFC 7181, DOI 10.17487/RFC7181, April 2014,
              <https://www.rfc-editor.org/info/rfc7181>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [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>.

   [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>.

   [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>.

   [RFC8175]  Ratliff, S., Jury, S., Satterwhite, D., Taylor, R., and B.
              Berry, "Dynamic Link Exchange Protocol (DLEP)", RFC 8175,
              DOI 10.17487/RFC8175, June 2017,
              <https://www.rfc-editor.org/info/rfc8175>.

   [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>.

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   [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>.

   [RFC9365]  Jeong, J., Ed., "IPv6 Wireless Access in Vehicular
              Environments (IPWAVE): Problem Statement and Use Cases",
              RFC 9365, DOI 10.17487/RFC9365, March 2023,
              <https://www.rfc-editor.org/info/rfc9365>.

   [RFC9374]  Moskowitz, R., Card, S., Wiethuechter, A., and A. Gurtov,
              "DRIP Entity Tag (DET) for Unmanned Aircraft System Remote
              ID (UAS RID)", RFC 9374, DOI 10.17487/RFC9374, March 2023,
              <https://www.rfc-editor.org/info/rfc9374>.

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 4.13 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 messages 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.

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   One possible strategy may be to monitor the NCE watching for data
   messages for (ReachableTime - 5) seconds.  If any data messages have
   been sent to the neighbor within this timeframe, then send an NS(AR)
   to receive a new NA(AR).  If no data messages 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 messages are sent within the 5 second window,
   reset the NCE state to STALE.

   The monitoring of the neighbor data traffic therefore becomes an
   ongoing process during the NCE lifetime.  If the NCE expires, future
   data messages will trigger a new NS/NA(AR) exchange while the
   messages themselves may be delivered over longer paths 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
   carrier packets over a single interface at a time, and the neighbor
   always observes carrier packets arriving from the Client from the
   same L2 source address.

   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 carrier 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 original
   IP packets/parcels without any encapsulation.  In that case, the
   Client sends packets/parcels over the Direct link according to
   traffic selectors.  If the Direct interface is selected, then the
   Client's packets/parcels are transmitted directly to the peer without
   traversing an ANET/INET.  If other interfaces are selected, then the
   Client's packets/parcels 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.

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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
   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 do not present a single point of failure in the
   architecture 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 can be
   quickly detected and conveyed by Bidirectional Forward Detection
   (BFD) and/or proactive NUD allowing Clients to migrate to new Proxy/
   Servers.

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   If a Proxy/Server fails, peer carrier 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(MM) messages may be
   lost but neighbor cache entries in the DEPARTED state will ensure
   that carrier 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 communications can continue .

   Therefore, links that provide many Proxy/Servers with high
   availability profiles are responsive to loss of individual
   infrastructure elements, since Clients can quickly establish new
   Proxy/Server relationships in event of failures.

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.

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   *  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
   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

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   or more AERO Proxy/Server addresses through the mechanisms discussed
   in earlier sections.  Each Proxy/Server address presumably leads to a
   fault-tolerant clustering arrangement such as supported by Linux-HA,
   Extended Virtual Synchrony or Paxos.  Such an arrangement has
   precedence in common Internet service deployments in lightweight
   virtual machines without requiring expensive hardware deployment.
   Similarly, common Internet service deployments set service IP
   addresses on service distribution points that may relay requests to
   many different servers.

   For AERO, the expectation is that a combination of the Google/IETF
   and Yahoo/Amazon philosophies would be employed.  The AERO Client
   connects to different ANET access points and can receive 1-2 Proxy/
   Server 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 review.

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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