Internet DRAFT - draft-templin-intarea-aero

draft-templin-intarea-aero







Network Working Group                                 F. L. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Standards Track                        16 February 2024
Expires: 19 August 2024


              Automatic Extended Route Optimization (AERO)
                     draft-templin-intarea-aero-67

Abstract

   This document specifies an Automatic Extended Route Optimization
   (AERO) service for IP internetworking over Overlay Multilink Network
   (OMNI) interfaces.  AERO/OMNI use an IPv6 unique-local address format
   for IPv6 Neighbor Discovery (IPv6 ND) messaging over the OMNI virtual
   link.  Router discovery and neighbor coordination are employed for
   network admission and to manage the OMNI link forwarding and routing
   systems.  Secure multilink path selection, multinet traversal,
   mobility management, multicast forwarding, multihop operation and
   route optimization are naturally supported through dynamic neighbor
   cache updates.  AERO is a widely-applicable mobile internetworking
   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.

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   This Internet-Draft will expire on 19 August 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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  17
   4.  Automatic Extended Route Optimization (AERO)  . . . . . . . .  17
     4.1.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .  17
     4.2.  The AERO Service over OMNI Links  . . . . . . . . . . . .  19
       4.2.1.  AERO/OMNI Reference Model . . . . . . . . . . . . . .  19
       4.2.2.  Addressing and Node Identification  . . . . . . . . .  23
       4.2.3.  AERO Routing System . . . . . . . . . . . . . . . . .  24
       4.2.4.  Segment Routing Topologies (SRTs) . . . . . . . . . .  26
       4.2.5.  Segment Routing For OMNI Link Selection . . . . . . .  27
     4.3.  OMNI Interface Characteristics  . . . . . . . . . . . . .  27
     4.4.  OMNI Interface Initialization . . . . . . . . . . . . . .  30
       4.4.1.  AERO Proxy/Server and Relay Behavior  . . . . . . . .  30
       4.4.2.  AERO Client Behavior  . . . . . . . . . . . . . . . .  31
       4.4.3.  AERO Host Behavior  . . . . . . . . . . . . . . . . .  31
       4.4.4.  AERO Gateway Behavior . . . . . . . . . . . . . . . .  32
     4.5.  OMNI Interface Neighbor Cache Maintenance . . . . . . . .  32
       4.5.1.  OMNI ND Messages  . . . . . . . . . . . . . . . . . .  35
       4.5.2.  OMNI Neighbor Advertisement Message Flags . . . . . .  37
       4.5.3.  OMNI Neighbor Window Synchronization  . . . . . . . .  38
     4.6.  OMNI Interface Encapsulation and Fragmentation  . . . . .  38
     4.7.  OMNI Interface Decapsulation  . . . . . . . . . . . . . .  41
     4.8.  OMNI Interface Data Origin Authentication . . . . . . . .  42
     4.9.  OMNI Interface MTU  . . . . . . . . . . . . . . . . . . .  42
     4.10. OMNI Interface Forwarding Algorithm . . . . . . . . . . .  43
       4.10.1.  Host Forwarding Algorithm  . . . . . . . . . . . . .  45
       4.10.2.  Client Forwarding Algorithm  . . . . . . . . . . . .  45
       4.10.3.  Proxy/Server and Relay Forwarding Algorithm  . . . .  47
       4.10.4.  Gateway Forwarding Algorithm . . . . . . . . . . . .  49
     4.11. OMNI Interface Error Handling . . . . . . . . . . . . . .  51
     4.12. AERO Mobility Service Coordination  . . . . . . . . . . .  54
       4.12.1.  AERO Service Model . . . . . . . . . . . . . . . . .  54
       4.12.2.  AERO Host and Client Behavior  . . . . . . . . . . .  55
       4.12.3.  AERO Proxy/Server Behavior . . . . . . . . . . . . .  56
     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 . . . . . .  84
       4.13.4.  Client/Gateway Route Optimization  . . . . . . . . .  87
       4.13.5.  Client/Client Route Optimization . . . . . . . . . .  89
       4.13.6.  Intra-ANET/ENET Route Optimization for AERO Peers  .  91
     4.14. Neighbor Unreachability Detection (NUD) . . . . . . . . .  91
     4.15. Mobility Management and Quality of Service (QoS)  . . . .  93
       4.15.1.  Mobility Update Messaging  . . . . . . . . . . . . .  93
       4.15.2.  Announcing Link-Layer Information Changes  . . . . .  94
       4.15.3.  Bringing New Links Into Service  . . . . . . . . . .  95
       4.15.4.  Deactivating Existing Links  . . . . . . . . . . . .  95
       4.15.5.  Moving Between Proxy/Servers . . . . . . . . . . . .  95
     4.16. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  97
       4.16.1.  Source-Specific Multicast (SSM)  . . . . . . . . . .  97
       4.16.2.  Any-Source Multicast (ASM) . . . . . . . . . . . . .  98
       4.16.3.  Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . .  99
     4.17. Operation over Multiple OMNI Links  . . . . . . . . . . .  99
     4.18. DNS Considerations  . . . . . . . . . . . . . . . . . . . 100
     4.19. Transition/Coexistence Considerations . . . . . . . . . . 100
     4.20. Proxy/Server-Gateway Bidirectional Forwarding
            Detection  . . . . . . . . . . . . . . . . . . . . . . . 101
     4.21. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . . 101
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . . 102
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 102
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . 102
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 105
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 107
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . 107
     9.2.  Informative References  . . . . . . . . . . . . . . . . . 108
   Appendix A.  Non-Normative Considerations . . . . . . . . . . . . 115
     A.1.  Implementation Strategies for Route Optimization  . . . . 115
     A.2.  Implicit Mobility Management  . . . . . . . . . . . . . . 115
     A.3.  Direct Underlying Interfaces  . . . . . . . . . . . . . . 116
     A.4.  AERO Critical Infrastructure Considerations . . . . . . . 116
     A.5.  AERO Server Failure Implications  . . . . . . . . . . . . 117
     A.6.  AERO Client / Server Architecture . . . . . . . . . . . . 118
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . . 120
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 120

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, enterprise mobile device users space
   exploration and many others.  AERO is a secure internetworking and



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   mobility management service that employs the Overlay Multilink
   Network Interface (OMNI) [I-D.templin-intarea-omni] 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 uses an IPv6 unique-local address format
   that supports operation of the IPv6 Neighbor Discovery (IPv6 ND)
   protocol [RFC4861].  A Client's OMNI interface can be configured over
   multiple underlay interfaces, and therefore appears as a single
   interface with multiple link layer addresses.  Each link layer
   address is subject to change due to mobility and/or multilink
   fluctuations, and link layer address changes are signaled by ND
   messaging the same as for any IPv6 link.

   AERO provides a secure cloud-based service where mobile node Clients
   use Proxy/Servers acting as proxys and/or designated routers while
   fixed nodes may use any Relay on the link for efficient
   communications.  Fixed nodes forward original IP packets/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, and Clients may
   efficiently move between locations while maintaining continuous
   communications with correspondents using stable IP Addresses not
   subject to dynamic fluctuations.

   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 (fixed) correspondent nodes on underlay
   Internetworks to (mobile or fixed) nodes on the OMNI link.  From the



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   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.

   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 [I-D.ietf-ipwave-vehicular-networking].  Other applicable use
   cases are also in scope.

   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.



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

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

   5.  Multihop - a mobile node vehicle-to-vehicle relaying capability
       useful when multiple forwarding hops between vehicles may be
       necessary to "reach back" to an infrastructure access point
       connection to the OMNI link.

   6.  (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-intarea-omni] 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.

   OMNI interfaces normally limit the size of their IPv6 ND control
   plane messages to the minimum IPv6 link MTU, but some messages may
   exceed this size if there are sufficient OMNI parameters and/or IP
   packet/parcel attachments.  These larger messages can still travel
   over secured underlying network control plane paths that include
   IPsec tunnels [RFC4301] and/or secured direct point-to-point links
   without loss due to a size restriction by engaging OMNI IPv6
   encapsulation/fragmentation as necessary up to a maximum size of
   65535 octets.

   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



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   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).

   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-intarea-omni].

   IPv6 Prefix Delegation
      a networking service for delegating IPv6 prefixes to nodes on the
      link.  The nominal service is DHCPv6 [RFC8415], however alternate
      services (e.g., based on IPv6 ND messaging) are also in scope.  A
      minimal form of prefix delegation known as "prefix registration"
      can be used if the Client knows its prefix in advance and can
      represent it in the source address of an IPv6 ND message.

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



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

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

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





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

   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 multihop forwarding between Clients acting as MANET
      routers may be necessary.  Proxy/Servers that connect the MANET to
      outside networks act as Clients on their MANET interfaces and act
      as ordinary Proxy/Servers on their ANET/INET interfaces, while
      Clients configure MANET interfaces and provide multihop forwarding
      services for other Clients as necessary.

   MANET Interface
      a node's underlay interface connection to a local network with
      indeterminant neighborhood properties over which 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 lower-layer fragmentation is needed.

   OMNI link
      the same as defined in [I-D.templin-intarea-omni].  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 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.





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

   (network) partition
      frequently, underlay networks such as large corporate enterprise
      networks are sub-divided internally into separate isolated
      partitions (a technique also known as "network segmentation").
      Each partition is fully connected internally but disconnected from
      other partitions, and there is no requirement that separate
      partitions maintain consistent Internet Protocol and/or addressing
      plans.  (Each partition is seen as a separate OMNI link segment as
      discussed throughout this document.)

   (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 ANET/INET/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 and
      fragmentation, or an IP packet 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.

   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.



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   (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 Fragment Offset and
      More Fragments both set to 0.

   (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.

   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, 192.0.2.0/24, etc.) assigned to the OMNI link and
      from which more-specific Mobile Network Prefixes (MNPs) are
      delegated.  OMNI link administrators typically obtain MSPs from an



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      Internet address registry, however private-use prefixes can
      alternatively be used subject to certain limitations (see:
      [I-D.templin-intarea-omni]).  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, 192.0.2.8/30, etc.) and delegated to an
      AERO Client or Relay.

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

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

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

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

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




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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 MNP delegation/registration
      service from AERO Proxy/Servers.  The Client assigns an XLA-MNP
      (as well as Proxy/Server-specific ULA-MNPs) to the OMNI interface
      for use in IPv6 ND exchanges with other AERO nodes and forwards
      original IP packets/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 ANET interfaces (i.e., in
      the same fashion as for an enterprise network proxy) as well as
      designated router services for coordination with correspondents on
      its INET-facing interfaces.  (Proxy/Servers in the open INET
      instead configure only a single INET interface and no ANET
      interfaces.)  The Proxy/Server configures an OMNI interface and
      assigns a ULA-RND to support the operation of IPv6 ND services,
      while advertising any associated MNPs for which it is acting as a
      hub via BGP peerings with AERO Gateways.

   (AERO) Relay
      an AERO Proxy/Server that provides forwarding services between
      nodes reached via the OMNI link and correspondents on other links/
      networks.  AERO Relays assign a ULA-RND to an OMNI interface and
      maintain BGP peerings with Gateways the same as Proxy/Servers.
      Relays also run a dynamic routing protocol to discover any non-MNP
      IP GUA routes in service on other links/networks, advertise OMNI



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      link MSP(s) to other links/networks, and redistribute routes
      discovered on other links/networks into the OMNI link BGP routing
      system.  (Relays that connect to major Internetworks such as the
      global IPv6 or IPv4 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 MNP and non-MNP 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.

   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.

   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 Hub
      Proxy/Server.







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   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.

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

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

   Mobile Node (MN)
      an AERO Client and all of its downstream-attached networks that
      move together as a single unit, i.e., an end system 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.

   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.






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

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

   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 (including addressing and
      Identification information) for each underlay interface pairwise
      communication session between peer OAL nodes.  AFVs are identified
      by both a forward and reverse path AFV Index (AFVI).  OAL nodes
      establish reverse path AFVIs when they forward an IPv6 ND unicast
      NS message and establish forward path AFVIs when they forward the
      solicited IPv6 ND unicast NA response.





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   AERO Forwarding Vector Index (AFVI)
      A locally-unique 2-octet or 4-octet value automatically generated
      by an OAL node when it creates an AFV.  OAL intermediate systems
      assign two distinct 4-octet AFVIs (called "A" and "B") to each
      AFV, with "A" representing the forward path and "B" representing
      the reverse path.  Meanwhile, the OAL source assigns a single "B"
      AFVI, and the OAL destination assigns a single "A" AFVI.  Each OAL
      node advertises its "A" AFVI to previous hop nodes on the reverse
      path toward the source and advertises its "B" AFVI to next hop
      nodes on the forward path toward the destination.  Clients in
      MANETs also assign distinct 2-octet AFVIs (called "C" and "D") to
      support local multihop forwarding.  The same as for the A/B AFVIs,
      the "C" AFVI represents the forward path and the "D" AFVI
      represents the reverse path.  For unidirectional MANET paths, only
      the forward path ("C") AFVI is used.

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

3.  Requirements

   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.

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.






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   AERO Clients can be deployed as fixed infrastructure nodes close to
   end systems, or as Mobile Nodes (MNs) that can change their network
   attachment points dynamically.  AERO Clients configure OMNI
   interfaces over underlay interfaces with addresses that may change
   due to mobility.  AERO Clients register their Mobile Network Prefixes
   (MNPs) with the AERO service, and distribute the MNPs to ENETs (which
   may connect AERO Hosts and other Clients).  AERO Clients provide
   Proxy/Server-like services for Hosts and other Clients on downstream-
   attached ENETs.

   AERO Gateways, Proxy/Servers and Relays are critical infrastructure
   elements in fixed (i.e., non-mobile) ANET/INET 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 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 to provide a Distributed Mobility
   Management (DMM) service for the list of active MNPs (see
   Section 4.2.3).  Gateways assign one or more Mobility Service
   Prefixes (MSPs) to the OMNI link and configure secured tunnels with
   Proxy/Servers, Relays and other Gateways; they further maintain
   forwarding table entries for each MNP or non-MNP prefix in service on
   the OMNI link.

   AERO Proxy/Servers distributed across one or more SRT segments
   provide default forwarding and mobility/multilink services for AERO
   Client mobile nodes.  The Proxy/Server acts as either OMNI link
   intermediate systems or end systems according to the service model
   selected by each Client.  Proxy/Server also peer with Gateways in an
   adaptation layer dynamic routing protocol instance to advertise its
   list of associated MNPs (see Section 4.2.3).  Hub Proxy/Servers
   provide prefix delegation/registration services and track the
   mobility/multilink profiles of each of their associated Clients,
   where each delegated prefix becomes an MNP taken from an MSP.  Proxy/
   Servers at ANET/INET boundaries provide a primary forwarding service
   for 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 a last resort data plane



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   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 non-MNP prefixes in service on other
   links/networks, and Relays that connect to larger Internetworks (such
   as the Internet) may originate default routes.  The Relay
   redistributes OMNI link MSP(s) into other links/networks, and
   redistributes non-MNP prefixes via OMNI link Gateway BGP peerings.

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 Proxy/Servers (S1, S2).
      Gateways provide the backbone for an SRT spanning tree for the
      OMNI link.









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   *  AERO Proxy/Servers S1 and S2 configure secured tunnels with
      Gateway G1 and also provide mobility, multilink, multicast and
      default router services for the MNPs of their associated Clients
      C1 and C2.  (Proxy/Servers that act as Relays can also advertise
      non-MNP routes for non-mobile correspondent nodes the same as for
      MNP Clients.)

   *  AERO Clients C1 and C2 associate with Proxy/Servers S1 and S2,
      respectively.  They receive MNP delegations X1 and X2, and also
      act as default routers for their associated physical or internal
      virtual ENETs.

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

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

   The addressing plan of each SRT segment is consistent internally but
   will often bear no relation to the addressing plans of other
   segments.  Each segment is also likely to be separated from others by
   network security devices (e.g., firewalls, proxys, packet filtering
   gateways, etc.), and disjoint segments often have no common physical
   link connections.  Therefore, nodes can only be assured of exchanging
   carrier packets directly with correspondents in the same segment, and
   not with those in other segments.  The only means for joining the
   segments therefore is through inter-domain peerings between AERO
   Gateways.

   The OMNI link spans multiple SRT segments using the OMNI Adaptation
   Layer (OAL) 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 treated by
   destinations according to data origin authentication procedures.
   AERO nodes can employ route optimization to cause carrier packets to
   take more direct paths between OMNI link neighbors without having to
   follow strict spanning tree paths.

   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 Clients configure LLAs
   constructed from MNPs (i.e., "LLA-MNPs") while AERO infrastructure
   nodes construct LLAs based on 56-bit random values ("LLA-RNDs") per
   [I-D.templin-intarea-omni].  Non-MNP routes are also represented the
   same as for MNPs, but may include a prefix that is not properly
   covered by an MSP.






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   AERO nodes also use the Unique Local Address (ULA) prefix fd00::/8
   followed by a pseudo-random 40-bit Global ID to form the prefix
   {ULA}::/48, then include a 16-bit Subnet ID '*' to form the prefix
   {ULA*}::/64 [RFC4291].  The AERO node then uses the prefix
   {ULA*}::/64 to form "ULA-MNPs" or "ULA-RNDs" as specified in
   [I-D.templin-intarea-omni] to support OAL addressing.  (The prefix
   {ULA*}::/64 appearing alone and with no suffix represents "default"
   for that prefix.)

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

   AERO MSPs, MNPs and non-MNP routes are typically based on Global
   Unicast Addresses (GUAs), but in some cases may be based on IPv4
   private addresses [RFC1918] or IPv6 ULA-C's [RFC4193].  A GUA block
   is also reserved for OMNI link anycast purposes.  See
   [I-D.templin-intarea-omni] for a full specification of LLAs, ULAs,
   TLAs, XLAs, GUAs and anycast addresses used by AERO nodes on OMNI
   links.

   Finally, AERO Clients and Proxy/Servers configure node Identification
   values as specified in [I-D.templin-intarea-omni].

4.2.3.  AERO Routing System

   The AERO routing system comprises a private Border Gateway Protocol
   (BGP) [RFC4271] service coordinated between Gateways and Proxy/
   Servers (Relays also engage in the routing system as simplified
   Proxy/Servers).  The service supports 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



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   as "spoke" ASes.  All Gateways within the same segment are members of
   the same hub AS, and use iBGP to maintain a consistent view of all
   active routes currently in service.  The Gateways of different
   segments peer with one another using eBGP.

   Gateways maintain forwarding table entries only for ULA prefixes for
   infrastructure elements and XLA-MNPs corresponding to MNP and non-MNP
   routes that are currently active; Gateways also maintain black-hole
   routes for the OMNI link MSPs so that 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 non-AERO links) and they
   forward all other OAL packets/fragments to Gateways which have full
   topology knowledge.

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

   The administrative authorities for each segment must therefore
   coordinate to assure mutually-exclusive ULA prefix assignments, but
   internal provisioning of ULAs is an independent local consideration
   for each administrative authority.  For each ULA prefix, the
   Gateway(s) that connect that segment assign the all-zero's address of
   the prefix as a Subnet Router Anycast address.  For example, the
   Subnet Router Anycast address for {ULA}:1023::/64 is simply
   {ULA}:1023::/64.

   ULA prefixes are statically represented in Gateway forwarding tables.
   Gateways join multiple SRT segments into a unified OMNI link over
   multiple diverse network administrative domains.  They support a
   virtual bridging service by first establishing forwarding table
   entries for their ULA prefixes either via standard BGP routing or
   static routes.  For example, if three Gateways ('A', 'B' and 'C')
   from different segments serviced {ULA}:1000::/56, {ULA}:2000::/56 and
   {ULA}:3000::/56 respectively, then the forwarding tables in each
   Gateway appear as follows:

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

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

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



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

   MNP (and non-MNP) routes are instead dynamically advertised in the
   AERO routing system by Proxy/Servers and Relays that provide service
   for their corresponding MNPs.  The routes are advertised as XLA-MNP
   prefixes, i.e., as fd00::{MNP} (see: [I-D.templin-intarea-omni]).
   For example, if three Proxy/Servers ('D', 'E' and 'F') service the
   MNPs 2001:db8:1000:2000::/56, 2001:db8:3000:4000::/56 and
   2001:db8:5000:6000::/56 then the routing system would include:

   D:  fd00::2001:db8:1000:2000/120

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

   F:  fd00::2001:db8:5000:6000/120

   Note: the MNP length found in an OMNI Neighbor Control sub-option
   encodes a Preflen between 1 and 64, but the corresponding XLA-MNP is
   entered into the routing system with length (64 + MNP length).  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 {ULA}::/48 prefixes in an OMNI link domain identify
   distinct Segment Routing Topologies (SRTs).  Each SRT is a mutually-
   exclusive OMNI link overlay instance using a distinct set of ULAs,
   and emulates a bridged campus LAN service for the OMNI link.  In some
   cases (e.g., when redundant topologies are needed for fault tolerance
   and reliability) it may be beneficial to deploy multiple SRTs that
   act as independent overlay instances.  A communication failure in one
   instance therefore will not affect communications in other instances.

   Each SRT is identified by a distinct value in the 40-bit ULA Global
   ID field and assigns an OMNI IPv6 anycast address used for OMNI
   interface determination in Safety-Based Multilink (SBM) as discussed
   in [I-D.templin-intarea-omni].  Each OMNI interface further applies
   Performance-Based Multilink (PBM) internally.

   The Gateways and Proxy/Servers of each independent SRT engage in BGP
   peerings to form a spanning tree with the Gateways in non-leaf nodes
   and the Proxy/Servers in leaf nodes.  The spanning tree is configured
   over both secured and unsecured underlay network paths.  The secured
   spanning tree is used to convey secured control messages (and
   sometimes data message extensions) between Proxy/Servers and
   Gateways, while the unsecured spanning tree forwards bulk data
   messages and/or unsecured control messages.



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

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

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 OMNI IPv6 anycast address for the selected OMNI link as
   either the IPv6 destination or as an intermediate hop within the SRH.
   This allows the original source to determine the specific OMNI link
   SRT an original IPv6 packet/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:

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






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   *  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.

   *  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 FHS Proxy/Server may be multiple
   IP hops away.  OMNI interfaces use link layer encapsulation only
   (i.e., and no other L2 encapsulations) over Direct underlay
   interfaces or ANET interfaces when the Client and FHS Proxy/Server
   are known to be on the same underlay link.

   OMNI interfaces maintain 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.  In environments where spoofing may be a threat, OMNI
   neighbors should invoke OAL Identification window synchronization in
   their IPv6 ND message exchanges.

   OMNI interfaces send IPv6 ND messages with an OMNI option formatted
   as specified in [I-D.templin-intarea-omni].  The OMNI option includes
   prefix registration information, Interface Attributes and/or AERO



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   Forwarding Parameters (AFPs) 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.

   A Client's OMNI interface may be configured over multiple ANET/INET
   underlay interfaces.  For example, common mobile handheld devices
   have both wireless local area network ("WLAN") and cellular wireless
   links.  These links are often used "one at a time" with low-cost WLAN
   preferred and highly-available cellular wireless as a standby, but a
   simultaneous-use capability could provide benefits.  In a more
   complex example, aircraft frequently have many wireless data link
   types (e.g.  satellite-based, cellular, terrestrial, air-to-air
   directional, etc.) with diverse performance and cost properties.

   If a Client's multiple ANET/INET underlay interfaces are used "one at
   a time" (i.e., all other interfaces are in standby mode while one
   interface is active), then successive IPv6 ND messages all include
   OMNI option Interface Attributes, Traffic Selector and/or AFP 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 ANET/INET underlay interfaces, then
   from the perspective of IPv6 ND it would appear to have multiple link
   layer addresses.  In that case, IPv6 ND message OMNI options MAY
   include 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
   an ANET/INET boundary include both an ANET and INET underlay
   interface.  ANET Clients therefore must discover both the 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 ULA-
   RND and acts as an OAL source to encapsulate original IP packets/



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   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 ULA-RND source and
   destination addresses.  The ULA-RND 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.

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

4.4.1.  AERO Proxy/Server and Relay Behavior

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

   The OMNI interface provides a single interface abstraction to the
   network layer, but internally serves as an NBMA nexus for sending
   carrier packets to OMNI interface neighbors 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 INET/ENET
   interfaces (see: Section 4.2.3).  The Relay provisions MNPs to
   networks on the INET/ENET interfaces (i.e., the same as a Client
   would do) and advertises the MSP(s) for the OMNI link over the INET/
   ENET interfaces.  The Relay further provides an OMNI link attachment
   point for non-MNP-based topologies.






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4.4.2.  AERO Client Behavior

   When a Client enables an OMNI interface, it assigns either an XLA-MNP
   or a TLA and sends OMNI-encapsulated RS messages over its ANET/INET
   underlay interfaces to an FHS Proxy/Server, which coordinates with a
   Hub Proxy/Server that returns an RA message with corresponding
   parameters.  The RS/RA messages may pass through one or more NATs in
   the path between the Client and FHS Proxy/Server.  (Note: if the
   Client used a TLA in its initial RS messages, it may discover ULA-
   MNPs in the corresponding RAs that it receives from FHS Proxy/Servers
   and begin using these new addresses.  If the Client is operating
   outside the context of AERO infrastructure such as in a Mobile Ad-hoc
   Network (MANET), however, it may continue using TLAs for Client-to-
   Client communications either indefinitely or at least until it
   encounters an infrastructure element that can delegate MNPs.)

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

4.4.3.  AERO Host Behavior

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

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











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4.4.4.  AERO Gateway Behavior

   AERO Gateways configure an OMNI interface and assign a ULA-RND and
   corresponding Subnet Router Anycast address for each of 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).

4.5.  OMNI Interface Neighbor Cache Maintenance

   Each Client, Proxy/Server and Gateway 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 ULA 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.

   Gateways maintain NCEs for Clients within their local segments based
   on NS/NA route optimization messaging (see: Section 4.13.4).  When a
   Gateway creates/updates a NCE for a local segment Client based on NS/
   NA route optimization, it also maintains AFIB state for messages
   destined to this local segment Client.





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   Clients establish NCEs for their associated FHS and Hub 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 Hub 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
   role of responding to NS messages and maintains peer NCEs associated
   with the NCE for this Client.  If the ARR flag is set, the Hub Proxy/
   Server that processes the RS message assumes the role of responding
   to NS(AR) messages on behalf of this Client NCE.  If the RPT flag is
   set, the Hub 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 on behalf of this
   Client.

   When a Client sets the RPT flag, the Hub 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 Hub Proxy/Server
   then sends uNA 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 Hub Proxy/Server
   deletes the entry.  When a Client NCE timer expires, the Hub 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-intarea-omni].

   Both the Client and its Hub 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



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   the individual Client underlay interfaces they service.  Clients
   determine their FHS and Hub Proxy/Server service models by setting
   the NUD/ARR/RPT flags in the RS messages they send as discussed
   above.

   When an Address Resolution Source (ARS) sends an NS(AR) message
   toward an Address Resolution Target (ART) Client/Relay, the OMNI link
   routing system directs the NS(AR) to a Hub Proxy/Server for the ART.
   The Hub then either acts as an Address Resolution Responder (ARR) on
   behalf of the ART or forwards the NS(AR) to the ART which acts as an
   ARR on its own behalf.  The ARR returns an NA(AR) response to the
   ARS, which creates or updates a NCE for the ART while caching L3 and
   L2 addressing information.  The ARS then (re)sets ReachableTime for
   the NCE to REACHABLE_TIME seconds and performs unicast NS/NA
   exchanges over specific underlay interface pairs to determine paths
   for 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/Hub 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-intarea-omni].  OMNI interfaces use ULAs instead
   of LLAs as adaptation layer IPv6 ND message source and destination
   addresses.  This allows multiple different OMNI links to be joined
   into a single link at some future time without requiring a global
   renumbering event.

   OMNI interfaces normally limit the size of the IPv6 ND messages they
   send to the IPv6 minimum link MTU, but messages that include a
   substantial amount of OMNI parameters and/or IP packet/parcel
   attachments may occasionally exceed that size.  The OMNI interface
   engages IPv6 encapsulation followed by fragmentation to break IPv6 ND
   messages as large as 65535 octets into fragments no larger than 1280
   octets.  Whenever possible, OMNI interfaces should send multiple
   smaller IPv6 ND messages instead of singleton larger messages to
   minimize fragmentation.

   For each IPv6 ND message, the OMNI interface includes one or more
   OMNI options (and any other ND message options) then completely
   populates all option information.  If the OMNI interface includes an
   authentication 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.




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

   OMNI interface IPv6 ND messages may also include other IPv6 ND
   options.  In particular, solicitation messages may include a Nonce
   option if required for verification of advertisement replies.  If an
   OMNI IPv6 ND solicitation message includes a Nonce option, the
   advertisement reply must echo the same Nonce.  If an OMNI IPv6 ND
   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 a ULA-RND while using unicast or anycast OAL/L2
   addresses.  AERO Proxy/Servers respond by returning unicast RA
   messages.  During the RS/RA exchange, AERO Clients and Proxy/Servers
   include state synchronization parameters to establish Identification
   windows and other state.

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

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

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

   *  Ordinary NS/NA messages are used determine target reachability,
      establish and maintain NAT state, and/or establish multilink
      forwarding (i.e., AFIB) state.  The source sends an NS to the
      unicast address of the target while optionally including an OMNI
      AERO Forwarding Parameters (AFP) sub-option naming a specific
      underlay interface pair, and the target returns a unicast NA that



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      includes a responsive AFP if necessary.  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 window
      synchronization and/or AFIB state must be secured.

   *  Unsolicited NA (uNA) messages are used to signal addressing and/or
      other neighbor state changes (e.g., address changes due to
      mobility, signal degradation, traffic selector updates, etc.). uNA
      messages can also be also used to acknowledge receipt of non-
      solicitation IPv6 ND messages (see below).  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 flag in unicast non-solicitation IPv6 ND messages (including RA,
   NA and Redirect) to request a synchronous (but "unsolicited") uNA
   response (see: [I-D.templin-intarea-omni]).

   The node that processes an SNR/SYN 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
   Target Address to the unicast destination and uNA destination address
   to the unicast source of the original message.

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





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

   *  O: The O ("Override") flag is set to 0 for solicited NAs returned
      by a Proxy/Server ARR and set to 1 for all other solicited and
      unsolicited NAs.  For further study is whether solicited NAs for
      anycast targets apply for OMNI links.  Since XLA-MNPs must be
      uniquely assigned to Clients to support correct IPv6 ND protocol
      operation, however, no role is currently seen for assigning the
      same XLA-MNP to multiple Clients.

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 OAL packets that include randomly-
   initialized and monotonically-increasing (extended) Identification
   values (modulo 2**64) without window synchronization.  In
   environments where spoofing is considered a threat, OMNI interface
   neighbors instead invoke window synchronization by including OMNI
   Window Synchronization sub-options in RS/RA or NS/NA message
   exchanges to maintain send/receive window state in their respective
   neighbor cache and AFIB entries as specified in
   [I-D.templin-intarea-omni].

   In common arrangements, OAL Identification window synchronization is
   necessary between Clients and Proxy/Servers or other Clients since
   their message exchanges are typically conducted over unsecured
   Internetworks.  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.

   All OAL nodes must verify Identification values of OAL packets
   addressed to themselves when window synchronization is required; OAL
   Intermediate systems forward OAL packets/fragments not addressed to
   themselves without examining their Identification values.

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-intarea-omni] which encapsulates the packet/parcel in an
   IPv6 header to produce an OAL packet.  For example, an original IP
   packet/parcel with source address 2001:db8:1:2::1 and destination
   address 2001:db8:1234:5678::1 might cause the OAL encapsulation



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   header to include source address {XLA*}::2001:db8:1:2 (i.e., an XLA-
   MNP) and destination address {ULA*}::0012:3456:789a:bcde (i.e., a
   ULA-RND).

   Following encapsulation, the OAL source then fragments the OAL packet
   while including an identical Identification value for each fragment
   that must be within the window for the neighbor.  The OAL source
   includes any necessary OAL IPv6 extension headers including an
   identical Compressed Routing Header with 32-bit ID fields (CRH-32)
   [I-D.ietf-6man-comp-rtg-hdr] with each fragment containing AERO
   Forwarding Vector Indexes (AFVIs) as discussed in Section 4.13.  The
   OAL source can instead invoke OAL header compression by replacing the
   OAL IPv6 header, CRH-32 and Extended Fragment Header with an OAL
   Compressed Header (OCH).

   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 = {XLA*}::2001:db8:1:2   ~
        |dst={ULA*}::0012:3456:789a:bcde|
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~   OAL IPv6 Extension Headers  ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |       Original IP Header      |
        ~     (first-fragment only)     ~
        ~    src = 2001:db8:1:2::1      ~
        |  dst = 2001:db8:1234:5678::1  |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |
        ~                               ~
        ~ Original Packet Body/Fragment ~
        ~                               ~
        |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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                      Figure 3: Carrier Packet Format

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

   In this format, the OAL source encapsulates the original IP header
   and packet/parcel body/fragment in an OAL IPv6 header, the CRH-32 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-intarea-omni].  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 XLA-MNP prefix
   information that may change occasionally due to regional node
   mobility, as well as XLA-MNP prefix information for Relay non-MNPs
   and per-segment ULA prefix information that rarely changes.  OMNI
   link Gateways and Proxy/Servers use the information to establish and
   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.





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   When an FHS Gateway forwards an OAL packet/fragment to an LHS Gateway
   over the unsecured spanning tree, it reconstructs the OAL header
   based on AFV state, inserts a CRH-32 immediately following the OAL
   header and adjusts the OAL payload length and destination address
   field.  The FHS Gateway includes a single AFVI in the CRH-32 that the
   LHS Gateway can use to search its AFIB, then forwards the OAL packet/
   fragment over the unsecured spanning tree.  When the LHS Gateway
   receives the OAL packet/fragment, it locates the AFV for the next hop
   based on the CRH-32 AFVI then re-applies header compression
   (resulting in the removal of the CRH-32) and forwards the OAL packet/
   fragment to the next hop.

   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.)

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-intarea-omni]).  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 ULA 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-intarea-omni].  Further OMNI link forwarding procedures
   are specified in Section 4.10.



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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-intarea-omni].

   *  AERO nodes that invoke window synchronization 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-intarea-omni], where the OMNI interface MTU
   can essentially be considered "unlimited".













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   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 bound to the same
   OAL destination, the OAL source can concatenate them as a single OAL
   super-packet as discussed in [I-D.templin-intarea-omni] 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.

   Note: Although a CRH-32 may be inserted or removed by a Gateway in
   the path (see: Section 4.10.4), this does not interfere with the
   destination's ability to reassemble since the CRH-32 is not included
   in the fragmentable part and its removal/transformation does not
   invalidate fragment header information.

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
   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 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.



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   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.

   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.

   The ULA Subnet ID value is used only for subnet coordination within a
   local OMNI link segment.  When a node forwards an OAL packet/fragment
   addressed to a ULA with a foreign Global and/or Subnet ID value, it
   forwards the OAL packet/fragment based solely on the OMNI link



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   routing information.  For this reason, OMNI link routing and
   forwarding table entries always include both ULA-RNDs with their
   associated prefix lengths and XLA-MNPs which encode an MNP while
   leaving the Global and Subnet ID values set to 0.

   The following sections discuss the OMNI interface-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 prefix (which may be encoded in an
   {ULA,XLA}-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 Section 6.13 of
   [I-D.templin-intarea-omni] 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.

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
   an ANET/INET interface (i.e., an upstream interface), the Client
   selects one or more "reachable" neighbor interfaces in the entry for
   forwarding purposes.  Otherwise, the Client performs OAL
   encapsulation and fragmentation, forwards the resulting OAL packet/
   fragment 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
   encapsulation and fragmentation if necessary.






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   When a carrier packet enters a Client's OMNI interface from the link
   layer, the Client performs L2 reassembly/decapsulation to obtain the
   OAL packet/fragment then examines the OAL destination.  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 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.

   Note: Clients and their FHS Proxy/Server (and other Client) peers can
   exchange original IP packets/parcels over ANET underlay interfaces
   using OMNI L2 encapsulation/fragmentation without invoking the OAL,
   since the ANET is secured at the link and physical layers.  By
   forwarding original IP packets/parcels without invoking the OAL, the
   ANET peers use the same L2 encapsulation/fragmentation procedures as
   specified for Hosts above.

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









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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 ULA 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 is a non-ULA, the 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, the Proxy/Server drops the carrier packets
   regardless of their OMNI link point of origin.  The Proxy/Server also
   drops original IP packets/parcels received on underlay interfaces
   either directly from an ANET Client or following reassembly of
   carrier packets received from an ANET/INET Client if the original IP
   destination corresponds to the same Client's delegated MNP.  Proxy/
   Servers also drop carrier packets that contain OAL packets/fragments
   with foreign OAL destinations that do not match their own ULA, the
   ULA of one of their Clients or a ULA corresponding to one of their
   GUA routes.  These checks are essential to prevent forwarding
   inconsistencies from accidentally or intentionally establishing
   endless loops that could congest nodes and/or ANET/INET links.

   Proxy/Servers process carrier packets that contain OAL packets/
   fragments with OCH headers or with destinations that match their ULA
   and also include a CRH-32 header that encodes AFVI information.  The
   Proxy/Server examines the AFVI to locate the corresponding AFV entry
   in the AFIB.  If the carrier packets were not received from the
   secured spanning tree, the Proxy/Server must then verify that the L2
   addresses are "trusted" according to the AFV.  If the carrier packets
   were trusted, the Proxy/Server then forwards them according to the
   AFV state while decrementing the OAL packet/fragment Hop Limit.



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   For OAL packets/fragments with destinations that match their ULA but
   do not include a CRH-32/OCH, 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 ULA 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 a {ULA,XLA}-MNP of one of
   its Client neighbors established through RS/RA exchanges, it accepts
   the carrier packet only if data origin authentication succeeds.  If
   the NCE state is DEPARTED, the Proxy/Server changes the OAL
   destination address to the ULA of the new Proxy/Server, 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 Hub responsible for serving as the Client's address resolution
   responder and/or default router, it verifies the Identification then
   submits the OAL packet/fragment for reassembly 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 {ULA,XLA}-MNP that does
   not match the MSP, it accepts the carrier packet only if data origin
   authentication succeeds and if there is a network layer forwarding
   table entry for a GUA route that matches the MNP.  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.



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   Clients and their FHS Proxy/Server peers can exchange original IP
   packets/parcels over ANET underlay interfaces using L2 encapsulation
   IPv6 Extended Fragment Headers only and no OAL addressing
   information, since the ANET is secured at the link and physical
   layers.  (For packets that do not require fragmentation, the peers
   can even omit the Extended Fragment Header.)  FHS Proxy/Servers will
   then supply an OAL Full (OFH) or Compressed (OCH) header when they
   forward ANET Client original IP packets/parcels toward final
   destinations located in other networks.

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

   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-intarea-omni].

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 ULA of a neighboring Gateway
   or Proxy/Server by examining the OMNI interface's network layer
   neighbor cache.  If so, the Gateway performs OAL encapsulation and
   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
   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/



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   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 ULA or the SRT Subnet
   Router Anycast address 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 ULA or the
   SRT Subnet Router Anycast address as the OAL destination.  If the OAL
   packet/fragment contains an OCH or a full OAL header with a CRH-32
   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.
   If the Gateway has a NCE for the target Client with an entry for the
   target underlay interface and current L2 addresses, the Gateway
   instead forwards the OAL packet/fragment directly to the target
   Client while using the final hop AFVI instead of the next hop (see:
   Section 4.13.4).

   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 ULA, 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 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 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-intarea-omni] (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 messages.  When the
   OAL source receives the uNA message, it can extract the ICMPv6 error
   message enclosed in the OMNI option and either process it locally or
   translate it into a network layer error to return to the original
   source.



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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-intarea-omni].  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.

   Clients associate each of their ANET/INET 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 Hub Proxy/Server (the Client
   may instead select a "third-party" Hub Proxy/Server that does not
   directly service any of its underlay interfaces).  All of the
   Client's other FHS Proxy/Servers forward proxyed copies of RS/RA
   messages between the Hub Proxy/Server and Client without assuming the
   Hub role functions themselves.

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

   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.



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   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 Hub Proxy/Servers include prefix delegation and/or
   registration parameters in RS/RA messages.  The IPv6 ND messages are
   exchanged between the Client and Hub Proxy/Server (via any FHS Proxy/
   Servers acting as proxys) according to the prefix management schedule
   required by the service.  If the Client knows its MNP in advance, it
   can employ prefix registration by including its XLA-MNP as the source
   address of an RS message and with an OMNI option with valid prefix
   registration information for the MNP.  If the Hub Proxy/Server
   accepts the Client's MNP assertion, it injects the MNP into the
   routing system and establishes the necessary neighbor cache state.
   If the Client does not have a pre-assigned MNP, it can instead employ
   prefix delegation by including a TLA as the source address of an RS
   message and with an OMNI option with prefix delegation parameters to
   request an MNP.

   The following sections outline Host, Client and Proxy/Server
   behaviors based on the Router Discovery and Prefix Registration
   specifications found in Section 15 of [I-D.templin-intarea-omni].
   These sections observe all of the OMNI specifications, and include
   additional specifications of the interactions of Client-Proxy/Server
   RS/RA exchanges with the AERO mobility service.

4.12.2.  AERO Host and Client Behavior

   AERO Hosts and Clients discover the addresses of candidate Proxy/
   Servers by resolving the Potential Router List (PRL) in a similar
   manner as described in [RFC5214].  Discovery methods include static
   configuration (e.g., a flat-file map of Proxy/Server addresses and
   locations), or through an automated means such as Domain Name System
   (DNS) name resolution [RFC1035].  Alternatively, the Host/Client can
   discover Proxy/Server addresses through a data link layer login
   exchange, or through an RA response to a multicast/anycast RS as
   described below.  In the absence of other information, the Host/
   Client can resolve the DNS Fully-Qualified Domain Name (FQDN)
   "linkupnetworks.[domainname]" where "linkupnetworks" is a constant
   text string and "[domainname]" is a DNS suffix for the OMNI link
   (e.g., "example.com").  The name resolution returns a set of resource
   records with Proxy/Server address information.





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   The Host/Client then performs RS/RA exchanges over each of its
   underlay interfaces to associate with (possibly multiple) FHS Proxy/
   Serves and a single Hub Proxy/Server as specified in Section 15 of
   [I-D.templin-intarea-omni].  The Host/Client sends each RS (either
   directly via Direct interfaces, via an IPsec tunnel for VPN
   interfaces, via an access router for ANET interfaces or via INET
   encapsulation for INET interfaces) and waits up to RetransTimer
   milliseconds for an RA message reply (see Section 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 Hub Proxy/Server's RA
   response, it has assurance that both the Hub and FHS Proxy/Servers
   have been updated with the new information.

   If the Host/Client wishes to discontinue use of a Hub Proxy/Server it
   issues an RS message over any underlay interface with an OMNI Proxy/
   Server Departure sub-option that encodes the (old) Hub Proxy/Server's
   ULA.  When the Hub Proxy/Server processes the message, it releases
   the MNP, sets the NCE state for the Host/Client to DEPARTED and
   returns an RA reply with Router Lifetime set to 0.  After a short
   delay (e.g., 2 seconds), the Hub Proxy/Server withdraws the MNP from
   the routing system.  (Alternatively, when the Host/Client associates
   with a new FHS/Hub Proxy/Server it can include an OMNI "Proxy/Server
   Departure" sub-option in RS messages with the ULAs of the Old FHS/Hub
   Proxy/Servers.)

4.12.3.  AERO Proxy/Server Behavior

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





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   When a FHS/Hub 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 as specified in
   Section 15 of [I-D.templin-intarea-omni].  When the Hub Proxy/Server
   receives the RS, it determines the correct MNPs for the Client by
   processing the XLA-MNP prefix parameters and/or the DHCPv6 OMNI sub-
   option.  When the Hub Proxy/Server returns the MNPs, it also creates
   an XLA-MNP forwarding table entry for the MNP resulting in a BGP
   update (see: Section 4.2.3).  The Hub Proxy/Server then returns an RA
   to the Client with destination set to the source of the RS (if an FHS
   Proxy/Server on the return path proxys the RA, it changes the
   destination to the Client's ULA-MNP).

   After the initial RS/RA exchange, the Hub Proxy/Server maintains a
   ReachableTime timer for each of the Client's underlay interfaces
   individually (and for the Client's NCE collectively) set to expire
   after ReachableTime seconds.  If the Client (or an FHS Proxy/Server)
   issues additional RS messages, the Hub Proxy/Server sends an RA
   response and resets ReachableTime.  If the Hub Proxy/Server receives
   an IPv6 ND message with a prefix release indication it sets the
   Client's NCE to the DEPARTED state and withdraws the XLA-MNP route
   from the routing system after a short delay (e.g., 2 seconds).  If
   ReachableTime expires before a new RS is received on an individual
   underlay interface, the Hub Proxy/Server marks the interface as DOWN.
   If ReachableTime expires before any new RS is received on any
   individual underlay interface, the Hub Proxy/Server sets the NCE
   state to STALE and sets a 10 second timer.  If the Hub Proxy/Server
   has not received a new RS or uNA message with a prefix release
   indication before the 10 second timer expires, it deletes the NCE and
   withdraws the XLA-MNP from the routing system.

   The Hub Proxy/Server processes any IPv6 ND messages pertaining to the
   Client while forwarding to the Client or responding on the Client's
   behalf as necessary.  The Hub Proxy/Server may also issue unsolicited
   RA messages, e.g., with reconfigure parameters to cause the Client to
   renegotiate its prefix delegation/registrations, with Router Lifetime
   set to 0 if it can no longer service this Client, etc.  The Hub
   Proxy/Server may also receive carrier packets via the secured
   spanning tree that contain initial data sent while route optimization
   is in progress.  The Hub 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 Hub
   Proxy/Server forwards any OAL packets/fragments it receives from the
   secured spanning tree and destined to the Client to the new Hub
   Proxy/Server, then deletes the entry after DepartTime expires.




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   Note: Clients SHOULD arrange to notify former Hub Proxy/Servers of
   their departures, but Hub Proxy/Servers are responsible for expiring
   neighbor cache entries and withdrawing XLA-MNP routes even if no
   departure notification is received (e.g., if the Client leaves the
   network unexpectedly).  Hub Proxy/Servers SHOULD therefore set Router
   Lifetime to ReachableTime seconds in solicited RA messages to
   minimize persistent stale cache information in the absence of Client
   departure notifications.  A short Router Lifetime also ensures that
   proactive RS/RA messaging between Clients and FHS Proxy/Servers will
   keep any NAT state alive (see above).

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

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 must inform a
   single Hub 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 ANET underlay interfaces the FHS Proxy/Server is located
   on the ANET/INET boundary, and for Clients on INET underlay
   interfaces the FHS Proxy/Server is located somewhere in the connected
   Internetwork.  When FHS Proxy/Server "B" processes a Client
   registration, it must either assume the Hub role or forward a proxyed
   registration to another Proxy/Server "A" acting as the Hub. Proxy/
   Servers satisfy these requirements as follows:




















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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
      NCE that matches the {ULA,XLA}-MNP in the RS source address for
      this Client neighbor and authenticates the message.  If no NCE was
      found, Proxy/Server "B" instead creates one in the STALE state and
      caches the Client-supplied Interface Attributes, Origin Indication
      and OMNI Window Synchronization sub-option parameters as well as
      the Client's observed L2 addresses (noting that they may differ
      from the Origin addresses if there were NATs on the path).  Proxy/
      Server "B" then examines the RS destination address.  If the
      destination address is the ULA of a different Proxy/Server "A",
      Proxy/Server "B" prepares a separate proxyed version of the RS
      message with an OAL header with source set to its own ULA and
      destination set to Proxy/Server A's ULA.  Proxy/Server "B" also
      writes its own information over the Interface Attributes sub-
      option supplied by the Client, omits or zeros the Origin
      Indication sub-option then forwards the message into the OMNI link
      secured spanning tree.

   *  when Hub Proxy/Server "A" receives the RS, it assumes the Hub
      role, delegates an MNP for the Client if necessary according to
      the Preflen in a Neighbor Control sub-option included by the
      Client, and creates/updates a NCE indexed by the Client's XLA-MNP
      with FHS Proxy/Server "B"'s Interface Attributes as the link layer
      address information for this FHS ifIndex.  Hub Proxy/Server "A"
      then prepares an RA message with source set to its own ULA,
      destination set to the source of the RS message, and with a
      Neighbor Control sub-option with Preflen set to the actual MNP
      length it will delegate to the Client.  Hub Proxy/Server "A" then
      encapsulates the RA in an OAL header with source set to its own
      ULA and destination set to the ULA of FHS Proxy/Server "B", 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/Hub Proxy/Server ULAs that do not match its own ULA, FHS
      Proxy/Server "B" first sends a uNA to the old FHS/Hub Proxy/
      Servers named in the sub-option.  If the RA message delegates a
      new XLA-MNP, Proxy/Server "B" then resets the RA destination to
      the corresponding ULA-MNP for this interface.  Proxy/Server "B"
      then re-encapsulates the message with OAL source set to its own
      ULA and OAL destination set to ULA that appeared in the Client's
      RS message OAL source, with an appropriate Identification value,
      with an authentication signature if necessary, with the Client's
      Interface Attributes sub-option echoed and with the cached
      observed L2 addresses written into an Origin Indication sub-



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      option.  Proxy/Server "B" sets the P flag in the RA flags field to
      indicate that the message has passed through a proxy [RFC4389],
      includes responsive window synchronization parameters, then
      fragments the RA if necessary and returns the fragments to the
      Client.

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

   After the initial RS/RA exchanges each FHS Proxy/Server forwards any
   of the Client's carrier packets that contain OAL packets/fragments
   with destinations for which there is no matching NCE to a Gateway
   using OAL encapsulation with its own ULA as the source and with
   destination determined by the Client.  The Proxy/Server instead
   forwards any 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 neighbor cache entries of other AERO nodes on
   behalf of the Client based on changes in Interface Attributes,
   Traffic Selectors, etc.  This allows for higher-frequency Proxy-
   initiated RS/RA messaging over well-connected INET infrastructure
   supplemented by lower-frequency Client-initiated RS/RA messaging over
   constrained ANET data links.

   If the Hub Proxy/Server "A" ceases to send solicited RAs, FHS Proxy/
   Servers "B", "C", "D", "E", etc. can send unsolicited RAs over the
   Client's underlay interface with destination set to (link-local) All-
   Nodes multicast and with Router Lifetime set to zero to inform
   Clients that the Hub Proxy/Server has failed.  Although 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 XLA-MNP
   as the source in order to differentiate them from the IPv6 ND
   messages sent by a FHS Proxy/Server.

   If the Client becomes unreachable over all underlay interfaces it
   serves, the Hub Proxy/Server sets the NCE state to DEPARTED and
   retains the entry for DepartTime seconds.  While the state is
   DEPARTED, the Hub Proxy/Server forwards any OAL packets/fragments



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   destined to the Client to a Gateway via OAL encapsulation.  When
   DepartTime expires, the Hub Proxy/Server deletes the NCE, withdraws
   the XLA-MNP route and discards any further carrier packets that
   contain OAL packets/fragments destined to the former Client.

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

   If the ANET first-hop access router is on the same underlay link as
   the Client and recognizes the AERO/OMNI protocol, the Client can
   avoid OAL encapsulation for both its control and data messages.  When
   the Client connects to the link, it can send an unencapsulated RS
   message with source address set to its own XLA-MNP (or to a TLA), and
   with destination address set to the ULA of the Client's selected
   Proxy/Server or to link-scoped All-Routers multicast.  The Client
   includes an OMNI option formatted as specified in
   [I-D.templin-intarea-omni].  The Client then sends the unencapsulated
   RS message, which will be intercepted by the AERO-aware ANET access
   router.

   The ANET access router then performs OAL encapsulation on the RS
   message and forwards it to a Proxy/Server at the ANET/INET boundary.
   When the access router and Proxy/Server are one and the same node,
   the Proxy/Server would share an underlay link with the Client but its
   message exchanges with outside correspondents would need to pass
   through a security gateway at the ANET/INET border.  The method for
   deploying access routers and Proxys (i.e. as a single node or
   multiple nodes) is an ANET-local administrative consideration.

   Note: When a Proxy/Server alters the IPv6 ND message contents before
   forwarding (e.g., such as altering the OMNI option contents), the
   original IPv6 ND message checksum and authentication signature values
   are invalidated and must be re-calculated.

   Note: When a Proxy/Server receives a secured Client NS message, it
   performs the same proxying procedures as for described for RS
   messages above.  The proxying procedures for NS/NA message exchanges
   is specified in Section 4.13.








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

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

4.12.3.3.  DHCPv6-Based Prefix Registration

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








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

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

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

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

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

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 initial original IP packets/
   parcels to new neighbors over ANET/INET interfaces and for ongoing
   multilink forwarding coordination with existing neighbors.  Address
   resolution is based on an IPv6 ND NS/NA(AR) messaging exchange
   between an Address Resolution Source (ARS) and the target neighbor as



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   the Address Resolution Target (ART).  Either the ART itself or the
   ART's current Hub Proxy/Server serves as the Address Resolution
   Responder (ARR).

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

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

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

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

   *  For peers within the same ANET/ENET, 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 continuous multilink forwarding and route
   optimization exchanges to maintain optimal forwarding profiles.

   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-intarea-omni].  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.  When an OAL source
   or intermediate system forwards a secured NS/NA super-packet, it
   should perform OAL encapsulation followed by fragmentation using a
   fragment size no larger than 1280 octets to ensure that the fragments
   will traverse any possible secured spanning tree paths.  The final
   OAL intermediate system in the path will then securely forward the
   NS/NA message IP packet/parcel attachments to the OAL target.




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   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 from a source node
   destined to a target node arrive, the ARS checks for a NCE with an
   XLA-MNP that matches the target destination.  If there is a NCE in
   the REACHABLE state, the ARS invokes the OAL and sends 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 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:

   *  the ULA of the ARS as the source address.

   *  the XLA corresponding to the original IP packet/parcel's
      destination as the Target Address, e.g., for 2001:db8:1:2::10:2000
      the Target Address is fd00::2001:db8:1:2.

   *  the Solicited-Node multicast address [RFC4291] formed from the
      lower 24 bits of the original IP packet/parcel's destination as
      the destination address, e.g., for 2001:db8:1:2::10:2000 the
      NS(AR) destination address is ff02:0:0:0:0:1:ff10:2000.

   The NS(AR) message also includes an OMNI option with an
   authentication sub-option (if necessary), with Interface Attributes
   and/or Traffic Selectors for all of the source Client's underlay
   interfaces and with a Neighbor Control sub-option with a valid
   Preflen for its claimed MNP.  The ARS then calculates and includes
   the authentication signature (if necessary) followed by the checksum,
   then submits the NS(AR) message for OAL encapsulation.  The ARS sets
   the OAL source to its own ULA and sets the OAL destination according
   to the Client's RS message "RPT" flag (see:
   [I-D.templin-intarea-omni]).  If the "RPT" flag was set, the ARS sets
   the OAL destination to the ULA of its Hub Proxy/Server which
   maintains a Report List; otherwise, the ARS sets the destination to
   the XLA-MNP corresponding to the ART.  The ARS then includes an
   appropriate Identification value, performs OAL fragmentation and L2
   encapsulation/fragmentation, then sends the resulting carrier packets
   into the SRT secured spanning tree without decrementing the network
   layer TTL/Hop Limit field.




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   When the ARS is a Client, it must instead use the ULA of one of its
   FHS Proxy/Servers as the OAL destination.  The ARS Client then
   performs OAL fragmentation followed by L2 encapsulation/fragmentation
   then forwards the carrier packets to the FHS Proxy/Server.  The FHS
   Proxy/Server then performs L2 reassembly/decapsulation, verifies the
   Identification, performs OAL reassembly, verifies the NS(AR)
   checksum/authentication signature and confirms that the Client's
   claimed Neighbor Control Preflen is valid for its ULA-MNP source
   address.  The FHS Proxy/Server then changes the OAL source to its own
   ULA and changes the OAL destination to the ULA of the Hub Proxy/
   Server or XLA-MNP corresponding to the ART as specified above.  The
   FHS Proxy/Server next includes an appropriate Identification,
   performs OAL fragmentation, 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 Hub Proxy/
   Servers include current and accurate information for their multilink
   Interface Attributes profile.  The Hub 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 Hub Proxy/Server.

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

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









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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 XLA
   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 OMNI link segments.  The
   final-hop Gateway will deliver the carrier packets via the secured
   spanning tree to the Hub Proxy/Server (or Relay) that services the
   ART.

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

   When the Hub Proxy/Server of the ART receives the NS(AR) secured
   carrier packets with the XLA-MNP of the ART as the OAL destination,
   it performs L2 reassembly/decapsulation followed by OAL reassembly
   then either forwards the NS(AR) to the ART or processes it locally if
   it is acting as a Relay or as the ART's designated ARR.  The Hub
   Proxy/Server processes the message as follows:

   *  if the NS(AR) target matches a Client NCE in the DEPARTED state,
      the (old) Hub Proxy/Server resets the OAL destination address to
      the ULA of the Client's new Hub Proxy/Server.  The old Hub Proxy/
      Server then decrements the OAL header Hop Limit, performs OAL
      fragmentation followed by 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 Hub Proxy/Server notes whether the NS(AR) arrived from the
      secured spanning tree.  If the message arrived via the secured
      spanning tree the Hub Proxy/Server verifies the NS checksum only;
      otherwise, it must also verify the message authentication
      signature.  If the Hub Proxy/Server maintains a Report List for
      the ART, it next records the NS source address in the Report List
      for this ART.  If the Hub 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 Hub Proxy/Server
      determines the underlay interface for the ART and proceeds as
      follows:

      -  If the Hub Proxy/Server is also the FHS Proxy/Server on the
         underlay interface used to convey the NS(AR) to the ART, it
         includes an authentication signature if necessary then
         recalculates the NS(AR) checksum.  The Hub then changes the OAL



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         source to its own ULA and OAL destination to the ULA-MNP of the
         ART, decrements the OAL Hop Limit, includes an appropriate
         Identification value, performs OAL fragmentation followed by L2
         encapsulation/fragmentation and forwards the resulting carrier
         packets over the underlay interface to the ART.

      -  If the Hub Proxy/Server is not the FHS Proxy/Server on the
         underlay interface used to convey the NS(AR) to the ART, it
         instead recalculates the NS(AR) checksum, changes the OAL
         source to its own ULA and changes the OAL destination to the
         ULA of the FHS Proxy/Server for this ART interface.  The Hub
         Proxy/Server next decrements the OAL Hop Limit, includes an
         appropriate Identification value, performs OAL fragmentation
         followed by L2 encapsulation/fragmentation and forwards the
         resulting carrier packets over the secured spanning tree.

      -  When the FHS Proxy/Server receives the carrier packets, it
         performs L2 reassembly/decapsulation, reassembles the NS(AR)
         and verifies the checksum, then forwards to the ART the same as
         described above.

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

   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 Interface
   Attributes and Traffic Selector information.  Next, the ARR prepares
   a solicited NA(AR) message to return to the ARS with the source
   address set to the ART's XLA, the destination address set to the
   NS(AR) ULA source address and the Target Address set to the same
   value that appeared in the NS(AR) Target Address.

   The ARR then includes Interface Attributes and Traffic Selector sub-
   options for all of the ART's underlay interfaces with current
   information for each interface and includes a Neighbor Control sub-
   option with the Preflen to apply to the ART's MNP.  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 if necessary, calculates the NA message checksum, then
   submits the NA(AR) for OAL encapsulation with source set to its own
   ULA and destination set to the ULA that appeared in the NS(AR) OAL
   source while including an appropriate Identification.  The ARR then
   performs OAL fragmentation followed by L2 encapsulation/
   fragmentation, and forwards the resulting carrier packets.




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   When the ART Proxy/Server receives carrier packets sent by an ART
   acting as an ARR on its own behalf, it performs L2 reassembly and
   decapsulation, verifies the Identification, performs OAL reassembly,
   then verifies the checksum/authentication signature.  The Proxy/
   Server then verifies that the Neighbor Control Preflen is acceptable,
   changes the OAL source address to its own ULA and changes the OAL
   destination to the ULA corresponding to the NA(AR) destination.  The
   Proxy/Server next decrements the OAL Hop Limit, includes an
   appropriate Identification then recalculates the NA checksum.  The
   Proxy/Server finally performs OAL fragmentation followed by 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 OMNI link segments.  The final-hop Gateway will
   deliver the carrier packets via the secured spanning tree to a Proxy/
   Server for the ARS.

4.13.1.5.  Processing the NA(AR) at the ARS

   When the ARS receives NA(AR) carrier packets, it performs L2
   reassembly/decapsulation, verifies the Identification, performs OAL
   reassembly, 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
   OMNI option information in the NCE for the ART (including Interface
   Attributes, Traffic Selectors, etc.), and caches the NA(AR) XLA
   source address as the address of the ART.
















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   When the ARS is a Client, the SRT secured spanning tree will first
   deliver the solicited NA(AR) message to the FHS Proxy/Server, which
   re-adjusts the OAL header and forwards the message to the Client.  If
   the Client is on a well-managed ANET, physical security and protected
   spectrum ensures security for the NA(AR) without needing an
   additional authentication signature; if the Client is on the open
   INET the Proxy/Server must instead include an authentication
   signature (while adjusting the OMNI option size, if necessary).  The
   Proxy/Server uses its own ULA as the OAL source and the ULA-MNP of
   the Client as the OAL destination when it forwards the NA(AR).  The
   Proxy/Server then decrements the OAL Hop Limit, includes an
   appropriate Identification, performs OAL fragmentation followed by 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 (or their respective
   FHS Proxy/Servers) can assert multilink forwarding paths through
   underlay interface pairs serviced by the same source/destination ULAs
   by sending unicast NS/NA messages with OMNI AERO Forwarding Parameter
   (AFP) sub-options.  The unicast NS/NA messages establish multilink
   forwarding 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 unicast NS messages on
   behalf of specific underlay interface pairs.  (Underlay interface
   directionality (i.e., in/out) must also be factored into the paths
   established for multilink forwarding.)

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




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   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.

   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 OMNI neighbor underlay interface pairs.  The AFIB
   contains per-interface-pair AERO Forwarding Vectors (AFVs) identified
   by locally-unique values known as AFV Indexes (AFVIs).  The AFVs
   cache uncompressed OAL header information as well as the previous/
   next-hop addressing and AFVI information.  The AFVs also cache window
   synchronization state for the specific underlay interface pair.
   Using the window synchronization state, simple Identification-based
   data origin authentication is enabled at each OAL source,
   intermediate system and target node.

   OMNI interfaces manage the AFIB in conjunction with their internal
   Neighbor Cache.  OMNI interface NCEs link to (possibly) multiple
   AFVs, with one AVF per underlay interface pair (according to
   directionality).  When OMNI interface peers need to coordinate, they
   locate a NCE for the peer then use the NCE as a nexus that aggregates
   potentially many AVFs.  In particular, the NCE caches the AFVI to be
   used to index the local AFV at the head end of the path.

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






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   The FHS and LHS roles are determined on a per-interface-pair basis.
   After address resolution, either peer is equally capable of
   initiating multilink forwarding on behalf of a specific FHS/LHS
   underlay interface pair.  The peer that sends the initiating NS/NA
   with Job code '00' message for a specific pair becomes the FHS peer
   while the one that returns the responsive NA becomes the LHS peer for
   that pair only.  It is therefore commonplace that peers may assume
   the FHS role for some pairs while assuming the LHS role for other
   pairs, i.e., even though each peer maintains only a single NCE.

   When an OAL node initiates or forwards an NS/NA with Job code '00',
   it creates an AFV, records the NS/NA source and destination ULAs then
   generates and assigns a locally-unique "B" AFVI (while also caching
   the "B" values for all previous OAL hops on the path from the FHS OAL
   source).  When the OAL node receives future OAL packets/fragments
   that include "B", it can unambiguously locate the correct AFV and
   determine directionality without examining addresses.  When the AFV
   is indexed by its "B" AFVI, it returns the ULAs in (dst,src) order
   the opposite of how they appeared in the OAL header of the original
   NS to support full header reconstruction for reverse-path forwarding.
   (If the NS message included a nested OAL encapsulation, the ULAs of
   both OAL headers are returned.)

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

   OAL nodes generate random non-zero 32-bit values as candidate AFVIs
   which must first be tested for local uniqueness.  If a candidate AFVI
   is already in use, the OAL node repeats the random generation process
   until it obtains a unique non-zero value.  Since the number of AFVs
   in service at each OAL node is likely to be much smaller than 2**32,
   the process will generate a unique value after a small number of
   tries.  Since the uniqueness property is node-local only, an AFVI
   locally generated by a first OAL node must not be tested for
   uniqueness by other OAL nodes.






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

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

4.13.2.1.  FHS Client-Proxy/Server NS Forwarding

   When an FHS OAL source has an original IP packet/parcel to send
   toward an LHS OAL target, it first performs multilink address
   resolution resulting in the creation of a NCE for the XLA 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 message with an AFP sub-option
   with Job code '00', includes window synchronization information, then
   sets the NS source to the XLA of the FHS Client and the NS target to
   the XLA of the LHS Client.  The FHS source next creates an AFV then
   generates and assigns a locally-unique "B" AFVI to the AFV while also
   including it as the first "B" entry in the AFP AFVI List.  The FHS
   source then includes any FHS/LHS addressing information it knows
   locally in the AFP sub-option, i.e., based on information discovered
   through address resolution.

   If the FHS source is the FHS Proxy/Server, it then examines the LHS
   FMT-Forward code.  If FMT-Forward is clear the FHS Proxy/Server sets
   the NS destination to the ULA of the LHS Proxy/Server; otherwise, it
   sets the NS destination to the same address as the target.  The FHS
   Proxy/Server then performs OAL encapsulation while setting the OAL
   source to its own ULA and setting the OAL destination to the FHS
   Subnet Router Anycast ULA determined by applying the FHS SRT prefix
   length to its ULA.  The FHS Proxy/Server then includes an appropriate
   Identification value, performs OAL fragmentation followed by L2
   encapsulation/fragmentation then forwards the resulting carrier
   packets into the secured spanning tree which will deliver them to a
   Gateway interface that assigns the FHS Subnet Router Anycast ULA.

   If the FHS source is the FHS Client, it instead includes an
   authentication signature if necessary.  If LHS FMT-Forward is clear,
   the FHS Client sets the NS destination to the ULA of the LHS Proxy/
   Server; otherwise, it sets the NS destination to the same address as
   the target.  The FHS Client then calculates the NS message checksum,
   performs OAL encapsulation, sets the OAL source to its own ULA-MNP
   and sets the OAL destination to the ULA of the FHS Proxy/Server.  The
   FHS Client finally includes an appropriate Identification value for



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   the FHS Proxy/Server, performs OAL fragmentation followed by 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, performs
   OAL reassembly if necessary and verifies the NS checksum or
   authentication signature.  The FHS Proxy/Server then creates an AFV
   (i.e., the same as the FHS Client had done) while caching the AFP "B"
   entry along with the FHS Client addressing information as previous
   hop information for this AFV.  The FHS Proxy/Server next generates a
   new locally-unique "B" AFVI, then assigns it as the AFV index and
   writes it as the next "B" entry in the AFP AFVI List (while also
   writing any FHS Client and Proxy/Server addressing information).  The
   FHS Proxy/Server then calculates the NS checksum and sets the OAL
   source address to its own ULA and destination address to the FHS
   Subnet Router Anycast ULA.  The FHS Proxy/Server finally decrements
   the OAL Hop Limit and includes an Identification appropriate for the
   secured spanning tree.  The FHS Proxy/Server finally performs OAL
   fragmentation followed by L2 encapsulation/fragmentation and forwards
   the resulting carrier packets into the secured spanning tree.

4.13.2.2.  FHS/intermediate/LHS Gateway NS 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 addressed to its ULA or the FHS Subnet
   Router Anycast ULA, it instead performs L2 reassembly/decapsulation,
   verifies the Identification, then finally performs OAL reassembly to
   obtain the NS then verifies the NS checksum.  The FHS Gateway next
   creates an AFV (i.e., the same as the FHS Proxy/Server had done)
   while caching the AFP FHS Client and Proxy/Server addressing
   information, window synchronization information and corresponding
   AFVI List "B" values in the AFV to enable future reverse path
   forwarding to this FHS Client.  The FHS Gateway then generates a
   locally-unique "B" AFVI for the AFV and writes it as the next "B"
   entry in the NS AFP AFVI List.

   The FHS Gateway then examines the SRT prefixes corresponding to both
   FHS and LHS.  If the FHS Gateway has a local interface connection to
   both the FHS and LHS (whether they are the same or different
   segments), the FHS/LHS Gateway caches the NS AFP LHS information in
   the AFV, writes its LHS ULA and L2ADDR into the NS AFP LHS fields,
   then sets its LHS ULA as the OAL source and the ULA of the LHS Proxy/
   Server as the OAL destination.  If the FHS and LHS prefixes are



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   different, the FHS Gateway instead sets its FHS ULA as the OAL source
   and the LHS Subnet Router Anycast ULA as the OAL destination.  The
   FHS Gateway then decrements the OAL Hop Limit, includes an
   appropriate Identification, recalculates the NS checksum, performs
   OAL fragmentation followed by 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 simply forward the enclosed IPv6 OAL
   packets/fragments without further processing.  The LHS Gateway then
   performs L2 reassembly/decapsulation, verifies the Identification,
   performs OAL reassembly to obtain the NS, verifies the NS checksum
   then creates an AFV (i.e., the same as the FHS Gateway had done)
   while caching the AFP "B" AFVIs and addressing information of
   previous OAL forwarding hops along with window synchronization
   information.  In particular, the LHS Gateway caches the ULA of the
   FHS Gateway as the spanning tree address for the previous-hop, caches
   the LHS information then generates a locally-unique "B" AFVI for the
   AFV.  The LHS Gateway then writes its own LHS ULA and L2ADDR into the
   AFP sub-option while also writing "B" as the next entry in the AFP
   AFVI List.  The LHS Gateway then sets its own ULA as the OAL source
   and the ULA of the LHS Proxy/Server as the OAL destination,
   decrements the OAL Hop Limit, includes an appropriate Identification,
   recalculates the NS checksum, performs OAL fragmentation followed by
   L2 encapsulation/fragmentation and forwards the resulting carrier
   packets into the secured spanning tree.

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

   When the LHS Proxy/Server receives the carrier packets from the
   secured spanning tree, it performs L2 reassembly/decapsulation,
   verifies the Identification, performs OAL reassembly, verifies the NS
   checksum then verifies that the LHS information supplied by the FHS
   source is consistent with its own cached information.  If the
   information is consistent, the LHS Proxy/Server then creates an AFV
   and caches the AFP "B" AFVIs and addressing information of previous
   OAL forwarding hops the same as for the prior hop.  The LHS Proxy/
   Server next caches the NS window synchronization parameters in the
   AFV.  If the NS destination is the XLA of the LHS Client, the LHS
   Proxy/Server also generates a locally-unique "B" AFVI and assigns it
   both to the AFV and as the next "B" entry in the NS AFVI List.

   If the NS destination matches its own ULA, the LHS Proxy/Server next
   prepares to return a responsive NA with Job code '01'.  The LHS
   Proxy/Server next creates or updates an NCE for the NS source address



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   (if necessary) with state set to STALE and with an AFVI pointer to
   the new AFV state.  When the LHS Proxy/Server forwards future carrier
   packets based on the cached information, it can populate forwarding
   information in a CRH-32 routing header to enable forwarding based on
   the cached AFVI List "B" entries.

   The LHS Proxy/Server then creates an NA with Job code '01' while
   copying the NS AFP sub-option into the NA and including responsive
   window synchronization information.  The LHS Proxy/Server then
   generates a locally-unique "A" AFVI and both assigns it to the AFV
   and includes it as the first "A" entry in the AFP sub-option AFVI
   List (see: [I-D.templin-intarea-omni] for details on AFVI List A/B
   processing).  The LHS Proxy/Server then encapsulates the NA with OAL
   source set to its own ULA and OAL destination set to the ULA of the
   LHS Gateway.  The LHS Proxy/Server then includes an appropriate
   Identification value, calculates the NA checksum, performs OAL
   fragmentation followed by L2 encapsulation/fragmentation and forwards
   the resulting carrier packets into the secured spanning tree.

   If the NS destination was the XLA of the LHS Client, the LHS Proxy/
   Server includes an authentication signature in the NS if necessary,
   then recalculates the NS checksum, changes the OAL source to its own
   ULA and changes the OAL destination to the ULA-MNP of the LHS Client.
   The LHS Proxy/Server then decrements the OAL Hop Limit, includes an
   appropriate Identification value, performs OAL fragmentation followed
   by 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, performs OAL reassembly then verifies the NS
   checksum/authentication signature.  The LHS Client then creates a NCE
   for the NS ULA source address (if necessary) in the STALE state and
   examines the AFP sub-option.  The Client then caches the NS OMNI AFP
   sub-options in the NCE corresponding to the NS ULA source, then
   creates an AFV, caches the addressing information and "B" entries of
   the previous OAL hops then finally generates and assigns a locally-
   unique "A" AFVI the same as for previous hops.  The Client finally
   caches the new AFVI in the NCE so that future communications can
   locate the correct AFV.

   The LHS Client then prepares an NA using exactly the same procedures
   as for the LHS Proxy/Server above (while including responsive window
   synchronization information), except that it uses its XLA as the NA
   source and the NS source as the NA destination.  The LHS Client also
   includes an authentication signature if necessary, calculates the NA
   message checksum, then encapsulates the NA with OAL source set to its
   own ULA-MNP and OAL destination set to the ULA of the LHS Proxy/
   Server.  The LHS Client finally includes an appropriate
   Identification, performs OAL fragmentation followed by L2



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   encapsulation/fragmentation and forwards the resulting carrier
   packets to the LHS Proxy/Server.  When the LHS Proxy/Server receives
   the carrier packets, it performs L2 reassembly/decapsulation,
   verifies the Identification, performs OAL reassembly, verifies the NA
   checksum/authentication signature, then uses the current AFP AFVI
   List "B" entry to locate the AFV.  The LHS Proxy/Server then caches
   the addressing and "A" information for the LHS Client in the AFV,
   then generates a locally-unique "A" AFVI and both assigns it to the
   AFV and writes it as the next AFP AFVI List "A" entry.  The LHS
   Proxy/Server then calculates the NA checksum, sets the OAL source to
   its own ULA and destination to the ULA of the LHS Gateway, decrements
   the OAL Hop Limit, includes an appropriate Identification, performs
   OAL fragmentation followed by L2 encapsulation/fragmentation and
   forwards the resulting carrier packets into the secured spanning
   tree.

4.13.2.4.  LHS/intermediate/FHS Gateway NA Forwarding

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

   When the FHS and LHS Gateways are different, the FHS Gateway will
   receive carrier packets containing the NA message from the LHS
   Gateway over the secured spanning tree, where there may have been
   many intermediate Gateway forwarding hops.  The FHS Gateway performs
   L2 reassembly/decapsulation, verifies the Identification, performs
   OAL reassembly, verifies the NA checksum and locates the AFV based on
   the current AFP AFVI List "B" entry.  The FHS Gateway then caches the
   addressing and "A" information for the previous hops in the AFV and
   generates a locally-unique "A" AFVI.  The FHS Gateway then assigns
   the new "A" value to the AFV, records "A" in the AFP AFVI List then



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   writes its FHS ULA and L2ADDR into the AFP FHS Gateway fields.  The
   FHS Gateway then recalculates the NA checksum, sets its FHS ULA as
   the OAL source and sets the ULA of the FHS Proxy/Server as the OAL
   destination.  The FHS Gateway then decrements the OAL Hop Limit,
   includes an appropriate Identification value, performs OAL
   fragmentation followed by L2 encapsulation/fragmentation and forwards
   the resulting carrier packets into the secured spanning tree.

4.13.2.5.  FHS Proxy/Server-Client NA Receipt

   When the FHS Proxy/Server receives the carrier packets from the
   secured spanning tree, it performs L2 reassembly/decapsulation,
   verifies the Identification, performs OAL reassembly, verifies the NA
   checksum then locates the AFV based on the current AFP AFVI List "B"
   entry.  The FHS Proxy/Server then caches the AFP addressing and "A"
   information for the previous hops.  If the NA destination matches its
   own ULA, the FHS Proxy/Server locates the NCE for the ULA of the LHS
   Proxy/Server or XLA of the LHS Client and sets the state to
   REACHABLE.  The FHS Proxy/Server then caches the window
   synchronization parameters and prepares to return an acknowledgement,
   if necessary.

   If the NA destination is the XLA of the FHS Client, the FHS Proxy/
   Server instead generates a locally-unique "A" AFVI and assigns it
   both to the AFV and as the next AFP AFVI List "A" entry, then
   includes an authentication signature/checksum in the NA message.  The
   FHS Proxy/Server then sets the OAL source to its own ULA and sets the
   OAL destination to the ULA-MNP of the FHS Client.  The FHS Proxy/
   Server then decrements the OAL Hop Limit, includes an appropriate
   Identification value, performs OAL fragmentation followed by L2
   encapsulation/fragmentation and finally forwards the resulting
   carrier packets to the FHS Client.

   When the FHS Client receives the carrier packets, it performs L2
   reassembly/decapsulation, verifies the Identification, performs OAL
   reassembly, verifies the NA checksum/authentication signature, then
   locates the AFV based on the current AFP AFVI List "B" entry.  The
   FHS Client then caches the previous hop addressing and "A"
   information the same as for prior hops.  The FHS Client then locates
   the NCE for the NS source address and sets the state to REACHABLE,
   then caches the window synchronization parameters and prepares to
   return a uNA acknowledgement, if necessary.









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4.13.2.6.  Returning Window Acknowledgements

   If either the FHS Client or FHS Proxy/Server needs to return an
   acknowledgement to complete window synchronization, it prepares a uNA
   message with an AFP sub-option with Job code set to '10' (Follow A;
   Record B).  The FHS node sets the uNA source to its own ULA or XLA,
   then sets the uNA destination to the ULA or XLA of the LHS node.  The
   FHS node next sets the AFP AFVI List to the cached list of "A"
   entries received in the Job code '01' NA, but need not set any other
   FHS/LHS information.  The FHS node then encapsulates the uNA message
   in an OAL header with its own ULA as the OAL source.  If the FHS node
   is the Client, it next sets the ULA of the FHS Proxy/Server as the
   OAL destination, includes an authentication signature/checksum,
   includes an appropriate Identification value, performs OAL
   fragmentation followed by L2 encapsulation/fragmentation and forwards
   the resulting carrier packets to the FHS Proxy/Server.  The FHS
   Proxy/Server then verifies the Identification, performs OAL
   reassembly, verifies the uNA checksum/authentication signature, then
   uses the current AFVI List "A" entry to locate the AFV.

   The FHS Proxy/Server then writes its "B" AFVI as the next AFP AFVI
   List "B" entry, recalculates the uNA checksum then sets its own ULA
   as the OAL source and the ULA of the FHS Gateway as the OAL
   destination, The FHS Proxy/Server finally decrements the OAL Hop
   Limit, includes an appropriate Identification then finally performs
   OAL fragmentation followed by L2 encapsulation/fragmentation and
   forwards the resulting carrier packets into the secured spanning
   tree.  When the FHS Gateway receives the carrier packets, it performs
   L2 reassembly/decapsulation, verifies the Identification, performs
   OAL reassembly, verifies the uNA checksum then uses the current AFVI
   List "A" entry to locate the AFV.  The FHS Gateway then writes its
   "B" AFVI as the next AFP AFVI List "B" entry, then sets the OAL
   source to its own ULA.  If the FHS Gateway is also the LHS Gateway,
   it sets the OAL destination to the ULA of the LHS Proxy/Server;
   otherwise it sets the OAL destination to the ULA of the LHS Gateway.
   The FHS Gateway recalculates the uNA checksum then decrements the OAL
   Hop Limit, includes an appropriate Identification, performs OAL
   fragmentation followed by L2 encapsulation/fragmentation and forwards
   the resulting carrier packets into the secured spanning tree.  If an
   LHS Gateway receives the carrier packets, it processes them exactly
   the same as the FHS Gateway had done while re-setting the OAL
   destination to the ULA of the LHS Proxy/Server.

   When the LHS Proxy/Server receives the carrier packets, it performs
   L2 reassembly/decapsulation, verifies the Identification, performs
   OAL reassembly, then verifies the uNA checksum.  The LHS Proxy/Server
   then locates the AFV based on the current AFP AFVI List "A" entry.
   If the uNA destination matches its own ULA, the LHS Proxy/Server next



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   updates the NCE/AFV for the source ULA based on the uNA window
   synchronization parameters and MAY compare the AFVI List to the
   version it had cached in the AFV based on the original NS.

   If the uNA destination is the XLA of the LHS Client, the LHS Proxy/
   Server instead writes its "B" AFVI as the next AFP AFVI List "B"
   entry and includes an authentication signature/checksum.  The LHS
   Proxy/Server then writes its own ULA as the OAL source and the ULA-
   MNP of the Client as the OAL destination, then decrements the OAL Hop
   Limit and includes an appropriate Identification.  The LHS Proxy/
   Server finally performs OAL fragmentation followed by 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, performs OAL reassembly, verifies the uNA checksum/
   authentication signature then processes the message exactly the same
   as for the LHS Proxy/Server case above.

   Note: If either the LHS Client or LHS Proxy/Server needs to return an
   acknowledgement to complete window synchronization, it prepares a uNA
   message with an AFP sub-option with Job code set to '11' (Follow B;
   Record A).  All other procedures are exactly the opposite as per the
   FHS case specified above.

4.13.2.7.  OAL End System Exchanges Following Synchronization

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

   While the OAL end systems continue to actively exchange OAL packets,
   they are jointly responsible for updating cache state and per-
   interface reachability before expiration.  Window synchronization
   state is performed on a per-interface-pair 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



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   for other underlay interfaces that share the same window
   synchronization state must be determined individually using
   additional NS/NA messages.

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

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

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










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   When a Gateway receives unsecured carrier packets that contain OAL
   packets/fragments destined to a local SRT segment Client that has
   asserted direct reachability, the Gateway performs direct forwarding
   while bypassing the local Proxy/Server based on the Client's
   advertised AFVIs and discovered NATed L2ADDR information (see:
   Section 4.13.4).  If the Client cannot be reached directly (or if NAT
   traversal has not yet converged), the Gateway instead forwards OAL
   packets/fragments directly to the local segment Proxy/Server.

   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 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 AFVIs established through an NS/
   NA exchange with a remote or local peer.  When the source Client
   forwards to a remote peer, it can forward OAL packets/fragments to a
   local SRT Gateway (following the establishment of L2ADDR information)
   while bypassing the Proxy/Server following route optimization (see:
   Section 4.13.4).  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 4.13.5).  The FHS Client will have
   cached the "A" AFVI for the LHS Client, which will have cached the
   "B" AFVI for the FHS Client.









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   When the FHS Client or FHS Proxy/Server sends an NS for the purpose
   of establishing multilink forwarding state, it should wait up to
   RETRANS_TIMER seconds to receive a responsive NA.  The FHS node can
   then retransmit the NS 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.

4.13.2.8.  Rapid Commit Multilink Forwarding

   Multilink forwarding can often be invoked in conjunction with Address
   Resolution in order to reduce control message overhead and round-trip
   delays.  When an ART acting as an ARR receives an NS(AR) with a set
   of Interface Attributes for the ARS source Client, it can perform
   "rapid commit" by immediately invoking multilink forwarding as above
   at the same time as returning the NA(AR).

   In order to perform rapid commit, the ARR includes an AFP sub-option
   with Job code '00' and a Window Synchronization sub-option as though
   it were initiating a multilink coordination NS/NA exchange as
   specified above.  The ARR then includes any Interface Attributes and/
   or Traffic Selector sub-options as necessary to satisfy the address
   resolution request.  The ARR then returns the NA(AR) to the ARS using
   the same hop-by-hop OAL addressing disciplines as specified above for
   an ordinary multilink NS/NA exchange.  This will cause the NA(AR) to
   visit all FHS/LHS OAL intermediate systems on the path towards the
   ARS.

   When the NA(AR) traverses the return path to the ARS, OAL
   intermediate systems in the path process the NS AFP information
   exactly the same as for an ordinary multilink forwarding exchange as
   specified above, i.e., without examining the remaining NA(AR) message
   contents.  This results in the ARR node now assuming the FHS role and
   the ARS assuming the LHS role from the perspective of multilink
   forwarding coordination.  When the NA(AR) arrives, the ARS processes
   the AFP and window synchronization parameters while also processing
   all other NA(AR) OMNI option information, thereby eliminating an
   extraneous message transmission and associated delay.  The ARS (now
   acting as an LHS peer) then completes the exchange by returning a
   responsive NA with an AFP sub-option with Job code '01'; if no NA
   response is received within RETRANS_TIMER seconds, the ARR can
   retransmit the NA(AR) up to MAX_NEIGHBOR_ADVERTISEMENT times before
   giving up.








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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 ANETs
   coordinated as Mobile Ad-hoc NETworks (MANETs).  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 unique IPv6 addresses they configure locally, including
   TLAs and HHITs.  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.

   When two Clients within the same MANET communicate using IP addresses
   that are advertised in the MANET routing protocol, their OMNI
   interfaces can avoid OAL encapsulation and treat the IP header
   supplied by the network layer as if it were an OAL encapsulation
   header.  This includes the application of OAL fragmentation and
   header compression as discussed in the OMNI specification.










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   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 header compression
   state in the MANET is conducted as a MANET-local aspect of the NS/NA
   multilink forwarding message exchange discussed in Section 4.13.2.
   The process can be used to establish either asymmetric or symmetric
   path header compression state.  In the asymmetric case, the forward
   path from the source Client to the destination Client or a MANET
   border Proxy/Server may be different than the reverse path.  In the
   symmetric case, both the forward and reverse paths traverse the same
   set of MANET routers.

   When the OMNI interface of a MANET source Client sends an NS to
   establish asymmetric path header compression state, it also includes
   a CRH-16 extension header and Window Synchronization parameters.  The
   source Client selects a non-zero 16-bit "C" AFVI that is unique for
   the L2 address of the next MANET forwarding hop for the NS message
   and writes that value into the first SID field of the CRH-16 while
   writing the value 0 into the second SID field.  The source Client
   then caches the full OAL header in an AFV for the destination and
   sends the NS to the next hop.

   When the next MANET forwarding hop's OMNI interface receives the NS,
   it creates an AFV and caches the full OAL header as well as the
   previous hop's "C" AFVI, L2 address and Window Synchronization
   parameters for the forward path.  The OMNI interface then selects its
   own non-zero/unique "C" AFVI and over-writes that value into the
   first SID field of the CRH-16.  Consecutive MANET forwarding hops
   then repetitively forward the NS to their respective next hops, which
   perform the same procedures as above.  The process continues until
   the NS reaches either a final destination within the same MANET or a
   MANET border Proxy/Server that can forward to destinations in other
   networks.

   When the final destination is within the same MANET, the destination
   OMNI interface returns an NA with a CRH-16 and uses the same non-
   zero/unique "C" AVFI discipline described above in the reverse path
   which may travel over a completely different set of MANET routers
   than those in the forward path.  Otherwise, the Proxy/Server that
   receives the NS forwards it to other networks according to the same
   multilink forwarding procedures discussed in Section 4.13.2.  When
   the Proxy/Server eventually receives an NA to return to the original



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   source, the Proxy/Server inserts a CRH-16 (while removing the CRH-32
   if present) and performs the same reverse path forwarding that an
   ordinary MANET destination would perform as described above.  When
   the original source receives the NA, header compression state will
   have been completely populated in both the forward and reverse paths
   and the source and destination nodes can begin sending ordinary
   packets with OCH headers instead of full OAL headers.

   The same procedures that appear above also apply when an NS
   originating from a remote network arrives at a MANET border Proxy/
   Server for a MANET that contains the final destination.  The Proxy/
   Server assumes the source role, inserts a CRH-16 with a non-zero/
   unique "C" AFVI and forwards it to the next MANET forwarding hop
   toward the final destination.  The forwarding process continues
   between successive MANET routers until the final destination receives
   the NS.  The final destination then prepares a responsive NA again
   while inserting a CRH-16 with a non-zero/unique "C" AFVI and returns
   the NA through the MANET toward the same Proxy/Server that forwarded
   the NS.  Note that it is important that the NA message contains the
   OAL address of the same Proxy/Server, since that is the only location
   where state resides to enable the return of the NA message to the
   original source.

   In order to establish symmetric MANET paths, the initiating Client
   can instead send an NS that includes a CRH-16 with a non-zero/unique
   2-octet "D" AFVI written into the second SID field and 0 written into
   the first SID field.  The Client then forwards the NS message to the
   next MANET forwarding hop toward the destination.  When the next
   MANET forwarding hop receives the NS, it creates an AFV and caches
   the (previous hop) "D" AFVI, then overwrites the second CRH-16 SID
   field with a newly-generated (next hop) non-zero/unique "D" AFVI
   value.  Consecutive MANET forwarding hops then repetitively forward
   the NS and create new AFVs in the same fashion until the NS reaches
   either a final destination within the same MANET or a MANET border
   Proxy/Server.

   The destination or Proxy/Server then returns an NA along the reverse
   path with the (previous hop) "D" AFVI in the second CRH-16 SID field,
   and with a newly-generated (next hop) non-zero/unique "C" AFVI in the
   first CRH-16 SID field.  When the previous MANET hop processes the
   NA, it locates the AFV based on the "D" AFVI, caches the "C" AFVI and
   generates a new non-zero/unique "C" AFVI.  The MANET node then
   overwrites the second CRH-16 SID with its cached previous hop "D"
   value and overwrites the first CRH-16 SID with the new "C" AFVI value
   and returns the NA to the previous hop.  The process continues until
   the NA message reaches the original multihop Client that transmitted
   the NS, at which point header compression state is established in
   both the forward and reverse directions of the MANET symmetric path.



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   Following the NS/NA exchanges in both the asymmetric and symmetric
   cases discussed above, each MANET router in the path in both the FHS
   and LHS MANETs will have established AFVs containing header
   compression state.  The AFVs determine AFVI-based forwarding based on
   the OCH header contents, and each MANET router only forwards packet
   with in-window Identification values.  MANET routers maintain AFVs
   for up to ReachableTime seconds unless they are refreshed by either a
   new NS/NA exchange or the transmission of any data packet with a full
   OAL header with an in-window Identification value and a CRH-16
   extension.  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/Gateway Route Optimization

   Following multilink route optimization for specific underlay
   interface pairs, FHS/LHS Clients located on open INETs can invoke
   Client/Gateway route optimization to improve performance and reduce
   load and congestion on their respective Proxy/Servers.  To initiate
   Client/Gateway route optimization, the Client prepares an NS message
   with its own XLA address as the source and the ULA of its Gateway as
   the destination while creating a NCE for the Gateway if necessary.
   The NS message must be encapsulated as an atomic fragment and not
   subject to OAL fragmentation.

   The Client then includes an Interface Attributes sub-option for its
   underlay interface as well as an authentication signature but does
   not include window synchronization parameters.  The Client then
   performs OAL encapsulation with its own ULA-MNP as the source and the
   ULA of the Gateway as the destination while including a randomly-
   chosen Identification value, then performs L2 encapsulation/
   fragmentation on the OAL atomic fragment and forwards the resulting
   carrier packets directly to the Gateway.






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   When the Gateway receives the carrier packets, it performs L2
   decapsulation/reassembly to recover the OAL atomic fragment then
   verifies the NS checksum/authentication signature and creates a NCE
   for the Client.  The Gateway then caches the L2 encapsulation
   addresses (which may have been altered by one or more NATs on the
   path) as well as the Interface Attributes for this Client ifIndex,
   and marks this Client underlay interface as "trusted".  The Gateway
   then prepares an NA reply with its own ULA as the source and the XLA
   of the Client as the destination where the NA again must be an atomic
   fragment.

   The Gateway then echoes the Client's Interface Attributes, includes
   an Origin Indication with the Client's observed L2 addresses and
   includes an authentication signature.  The Gateway then performs OAL
   encapsulation with its own ULA as the source and the ULA-MNP of the
   Client as the destination while using the same Identification value
   that appeared in the NS, then performs L2 encapsulation/fragmentation
   on the OAL atomic fragment and forwards the resulting carrier packets
   directly to the Client.

   When the Client receives the NA reply, it caches the carrier packet
   L2 source address information as the Gateway target address via this
   underlay interface while marking the interface as "trusted".  The
   Client also caches the Origin Indication L2 address information as
   its own (external) source address for this underlay interface.

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

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





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   Note: the Client and Gateway each maintain a single NCE, but the NCE
   may aggregate multiple underlay interface pairs.  Each underlay
   interface pair may use differing source and target L2 addresses
   according to NAT mappings, and the "trusted/untrusted" status of each
   pair must be tested independently.  When no "trusted" pairs remain,
   the NCE is deleted.

   Note: the above method requires Gateways to participate in NS/NA
   message authentication signature application and verification.  In an
   alternate approach, the Client could instead exchange NS/NA messages
   with authentication signatures via its Proxy/Server but addressed to
   the ULA of the Gateway, and the Proxy/Server and Gateway could relay
   the messages over the secured spanning tree.  However, this would
   still require the Client to send additional messages toward the L2
   address of the Gateway to populate NAT state; hence the savings in
   complexity for Gateways would result in increased message overhead
   for Clients.

4.13.5.  Client/Client Route Optimization

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

   When the FHS/LHS Clients on the same SRT segment perform the initial
   NS/NA exchange to establish AFIB state, they 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 1).  The NS/NA messages then include an Origin
   Indication (i.e., in addition to an AFP sub-option) with the mapped
   addresses discovered during the RS/RA exchanges with their respective
   Proxy/Servers.  After the AFV paths have been established, both
   Clients can begin sending 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 its local chain of NATs for reception of future carrier
   packets from that L2 address (see: [RFC4380] and
   [I-D.templin-intarea-omni]).  The source Client then prepares an NS
   message with its own XLA as the source, with the XLA of the target as
   the destination and with an OMNI option with an Interface Attributes
   sub-option.  The source Client then encapsulates the NS in an OAL



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   header with its own ULA-MNP as the source, with the ULA-MNP of the
   target Client as the destination and with an in-window Identification
   for the target.  The source Client then performs OAL fragmentation
   followed by 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, verify the
   Identification and perform OAL reassembly.  Following reassembly, the
   target Client prepares an NA message with its own XLA as the source,
   with the XLA of the source Client as the destination and with an OMNI
   option with an Interface Attributes sub-option.  The target Client
   then encapsulates the NA in an OAL header with its own ULA-MNP as the
   source, with the ULA-MNP of the source Client as the destination and
   with an in-window Identification for the source Client.  The target
   Client then performs OAL fragmentation followed by L2 encapsulation/
   fragmentation then forwards the resulting carrier packets directly to
   the source Client.

   Following the initial NS/NA exchange, both Clients mark their
   respective (source, target) underlay interface pairs as "trusted" for
   no more than ReachableTime seconds.  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
   additional NS/NA exchanges via their local Proxy/Servers to refresh
   NCE state as well as send additional bubbles to the peer's Origin
   address information if necessary to refresh NAT state.

   Note: 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.  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.









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

4.13.6.  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 a Neighbor Control sub-option with the Preflen of the MNP
   found in the Target Address field.

   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 ANETs to a peer within the same downstream ANET, the
   Proxy/Server returns an IPv6 ND Redirect message.

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

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
   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 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 messages use the unicast XLAs/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 exchanges directly between neighbors without employing
   the secured spanning tree as long as they include in-window
   Identifications and an authentication signature/checksum.



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   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
   messages over the SRT secured or unsecured spanning tree, or through
   NS messages sent directly to an underlay interface of the target
   itself.  While testing a target underlay interface, 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 messages are encapsulated, fragmented and transmitted as carrier
   packets the same as for ordinary original IP data packets/parcels,
   however the encapsulated destinations are either the ULA or XLA of
   the source and either the ULA of the LHS Proxy/Server or the XLA of
   the target itself.  The source encapsulates the NS 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 OAL fragmentation followed by L2
   encapsulation/fragmentation then forwards the resulting carrier
   packets into the unsecured spanning tree, either directly to the
   target if it is in the local segment or directly to a Gateway in the
   local segment.

   When the target receives the NS 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 checksum/authentication
   signature, then searches for Interface Attributes in its NCE for the
   source that match the NS for the NA reply.  The target then prepares
   the NA with the source and destination addresses reversed,
   encapsulates and sets the OAL source and destination, includes an
   Interface Attributes sub-option in the NA to identify the ifIndex of
   the underlay interface the NS arrived on and sets the Target Address
   to the same value included in the NS.  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 OAL
   fragmentation followed by L2 encapsulation/fragmentation and forwards
   the resulting carrier packets into the unsecured spanning tree either
   directly to the source if it is in the local segment or directly to a
   Gateway in the local segment.

   When the source receives the NA, it marks the target underlay
   interface tested as "trusted".  Note that underlay interface states
   are maintained independently of the overall NCE REACHABLE state, and



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   that a single NCE may have multiple target underlay interfaces in
   various "trusted/untrusted" states while the NCE state as a whole
   remains REACHABLE.

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

   Hub Proxy/Servers provide a designated router service for their
   dependent Clients, while FHS Proxy/Servers provide a proxy conduit
   between the Client and both the Hub and OMNI link in general.
   Clients are responsible for maintaining neighbor relationships with
   their Proxy/Servers through periodic RS/RA exchanges, which also
   serves to confirm neighbor reachability.  When a Client's underlay
   interface attributes change, the Client is responsible for updating
   the Hub Proxy/Server through new RS/RA exchanges using the FHS Proxy/
   Server as a first-hop conduit.  The FHS Proxy/Server can also act as
   a proxy to perform some IPv6 ND exchanges on the Client's behalf
   without consuming bandwidth on the Client underlay interface.

   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
   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 Hub Proxy/Servers) accommodate mobility
   and/or multilink change events by sending secured uNA messages to
   each active neighbor.  When a node sends a uNA message to each
   specific neighbor on behalf of a mobile Client, it sets the IPv6
   source address to its own ULA or XLA, sets the destination address to
   the neighbor's ULA or XLA and sets the Target Address to the mobile
   Client's XLA.  The uNA also includes an OMNI option with OMNI
   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 R flag to 1, S flag to 0



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   and O flag to 1, then encapsulates the message in an OAL header with
   source set to its own ULA and destination set to either the specific
   neighbor's ULA or the FHS Proxy/Server's ULA.  Following OAL
   fragmentation and L2 encapsulation/fragmentation, the carrier packets
   containing the uNA message will then follow the secured spanning tree
   and arrive at the specific neighbor.

   As discussed in Section 7.2.6 of [RFC4861], the transmission and
   reception of uNA messages is unreliable but provides a useful
   optimization.  In well-connected Internetworks with robust data links
   uNA messages will be delivered with high probability, but in any case
   the node can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT uNAs to
   each neighbor to increase the likelihood that at least one will be
   received.  Alternatively, the node can set the SNR flag in the uNA
   OMNI option header to request a uNA response (see: Section 4.5.1).

   When the FHS/LHS Proxy/Server receives a secured uNA message prepared
   as above, if the uNA destination was its own ULA the Proxy/Server
   uses the included OMNI option information to update its NCE for the
   target but does not reset ReachableTime since the receipt of a uNA
   message does not provide confirmation that any forward paths to the
   target Client are working.  If the destination was the XLA of the
   FHS/LHS Client, the Proxy/Server instead changes the OAL source to
   its own ULA, includes an authentication signature if necessary, and
   includes an in-window Identification for this Client.  Finally, if
   the uNA message SNR flag was set, the node that processes the uNA
   also returns a uNA 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 Hub 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 Hub Proxy/
   Server.







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

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

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

4.15.4.  Deactivating Existing Links

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

   If the Client needs to send uNA 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 messages to all neighbors the same as described
   above.

   Note that when a Client deactivates an underlay interface, neighbors
   that receive the ensuing uNA messages need not purge all references
   for the underlay interface from their neighbor cache entries.  The
   Client may reactivate or reuse the underlay interface and/or its
   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.

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 Hub Proxy/Server or renews its
   association with an existing Hub Proxy/Server.






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

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

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

   Clients SHOULD NOT move rapidly between Hub Proxy/Servers in order to
   avoid causing excessive oscillations in the AERO routing system.
   Examples of when a Client might wish to change to a different Hub
   Proxy/Server include a Hub Proxy/Server that has become unresponsive,
   topological movements of significant distance, movement to a new
   geographic region, movement to a new OMNI link segment, etc.







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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 ANET underlay interfaces for which
   the ANET has deployed native multicast services forward IGMP/MLD
   messages into the ANET.  The IGMP/MLD messages may be further
   forwarded by a first-hop ANET access router acting as an IGMP/MLD-
   snooping switch [RFC4541], then ultimately delivered to an ANET (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 ANET/
   INET 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.

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].

   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 ULA or XLA
   as the source address, the solicited node multicast address
   corresponding to S as the destination and the XLA of S as the target
   address.  X then encapsulates the NS(AR) in an OAL header with source
   address set to its own ULA and destination address set to the ULA for
   S, then forwards the message into the secured spanning tree which
   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 multilink forwarding exchange
   over the secured spanning tree while including a PIM Join/Prune



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   message for each multicast group of interest in the OMNI option.  If
   S is located behind any Proxys "Z"*, each Z* then updates its MRIB
   accordingly and maintains the XLA of X as the next hop in the reverse
   path.  Since Gateways forward messages not addressed to themselves
   without examining them, this means that the (reverse) multicast tree
   path is simply from each Z* (and/or S) to X with no other multicast-
   aware routers in the path.

   Following the initial combined Join/Prune and NS/NA messaging, X
   maintains a NCE for each S the same as if X was sending unicast data
   traffic to S.  In particular, X performs additional NS/NA exchanges
   to keep the NCE alive for up to t_periodic seconds [RFC7761].  If no
   new Joins are received within t_periodic seconds, X allows the NCE to
   expire.  Finally, if X receives any additional Join/Prune messages
   for (S,G) it forwards the messages over the secured spanning tree.

   Client C that holds an MNP for source S may later depart from a first
   Proxy/Server Z1 and/or connect via a new Proxy/Server Z2.  In that
   case, Y sends a uNA message to X the same as specified for unicast
   mobility in Section 4.15.  When X receives the uNA message, it
   updates its NCE for the XLA for source S and sends new Join messages
   in NS/NA exchanges addressed to the new target Client underlay
   interface connection for S.  There is no requirement to send any
   Prune messages to old Proxy/Server Z1 since source S will no longer
   source any multicast data traffic via Z1.  Instead, the multicast
   state for (S,G) in Proxy/Server Z1 will soon expire since no new
   Joins will arrive.

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 message with its own ULA or XLA as the source address
   and the ULA or XLA for R as the destination address, then
   encapsulates the NS message in an OAL header with its own ULA as the
   source and the ULA of R's Proxy/Server as the destination then sends
   the message into the secured spanning tree.

   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 messages, performs
   OAL encapsulation and fragmentation with Identification values within
   the receive window for Client R* that aggregates R, then performs L2



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   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 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 PIM Register encapsulation.
   R can then issue a uNA PIM Register-stop message over the secured
   spanning tree to suppress the Register-encapsulated stream.  At some
   later time, if Client S* moves to a new Proxy/Server, it resumes
   sending original IP packets/parcels via uNA PIM Register
   encapsulation via the new Proxy/Server.

   At the same time, as multicast listeners discover individual S's for
   a given G, they can initiate an (S,G) Join for each S under the same
   procedures discussed in Section 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).






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   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.

   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



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   distributing its MNPs and/or discovering non-MNP IP GUA prefixes on
   its INET links.

   This gives rise to the opportunity to eventually distribute native IP
   addresses to all nodes, and to present a unified OMNI link view even
   if the INET partitions remain in their current protocol and
   addressing plans.  In that way, the OMNI link can serve the dual
   purpose of providing a mobility/multilink service and a transition/
   coexistence service.  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.

   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-intarea-omni]) that can serve as a Client ID seed
   for MNP prefix delegation.  The Client would then be obligated to



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   renumber its internal networks whenever its MNP (and therefore also
   its XLA) 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

   An early AERO implementation based on OpenVPN (https://openvpn.net/)
   was announced on the v6ops mailing list on January 10, 2018 and an
   initial public release of the AERO proof-of-concept source code was
   announced on the intarea mailing list on August 21, 2015.

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

   Implementation of AERO/OMNI functions specified in more recent
   document versions is considered work in progress.

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-intarea-omni] 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.



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   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.

   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 the open INET 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-intarea-omni].

   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.





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   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-intarea-omni] 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
   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 open INET Clients depends on a public key
   distribution service in which Client public keys and identities are
   maintained in a shared database accessible to all open INET Proxy/
   Servers.  Similarly, each Client must be able to determine the public
   key of each Proxy/Server, e.g. by consulting an online database.








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   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-intarea-omni].  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.

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,



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   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)
   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]






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   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.

9.  References

9.1.  Normative References

   [I-D.templin-intarea-omni]
              Templin, F., "Transmission of IP Packets over Overlay
              Multilink Network (OMNI) Interfaces", Work in Progress,
              Internet-Draft, draft-templin-intarea-omni-66, 12 February
              2024, <https://datatracker.ietf.org/doc/html/draft-
              templin-intarea-omni-66>.

   [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>.

   [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>.






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   [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>.

   [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>.

   [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>.

9.2.  Informative References

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



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   [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.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-03,
              18 January 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-6man-comp-rtg-hdr-03>.

   [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-ipwave-vehicular-networking]
              Jeong, J. P., "IPv6 Wireless Access in Vehicular
              Environments (IPWAVE): Problem Statement and Use Cases",
              Work in Progress, Internet-Draft, draft-ietf-ipwave-
              vehicular-networking-30, 24 October 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ipwave-
              vehicular-networking-30>.

   [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-25, 23
              October 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-rtgwg-atn-bgp-25>.

   [I-D.perkins-manet-aodvv2]
              Perkins, C. E., Ratliff, S., 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-aodvv2-03, 28 February 2019,
              <https://datatracker.ietf.org/doc/html/draft-perkins-
              manet-aodvv2-03>.








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   [I-D.templin-6man-parcels2]
              Templin, F. L., "IPv6 Parcels and Advanced Jumbos (AJs)",
              Work in Progress, Internet-Draft, draft-templin-6man-
              parcels2-00, 16 February 2024,
              <https://datatracker.ietf.org/api/v1/doc/document/draft-
              templin-6man-parcels2/>.

   [I-D.templin-intarea-parcels2]
              Templin, F., "IPv4 Parcels and Advanced Jumbos (AJs)",
              Work in Progress, Internet-Draft, draft-templin-intarea-
              parcels2-00, 15 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-templin-
              intarea-parcels2-00>.

   [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>.

   [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>.

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

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

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

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

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

   [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>.

   [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>.





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   [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>.

   [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>.



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   [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>.

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.

   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.






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   If the Client's underlay interface address changes (either due to a
   readdressing of the original interface or switching to a new
   interface) the neighbor immediately updates the NCE for the Client
   and begins accepting and sending 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.

A.4.  AERO Critical Infrastructure Considerations

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

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

   AERO ANET Proxy/Servers are most often standard dedicated server
   platforms with one underlay interface connected to the ANET and a
   second interface connected to an INET.  As with INET Proxy/Servers,
   the only requirements are that they can run the AERO/OMNI code and
   have at least one interface connection to the INET.  ANET Proxy/
   Servers must be provisioned, supported and managed by the ANET



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   administrative authority.  Cost for purchasing, configuring and
   managing Proxys is nominal, and borne by the ANET administrative
   authority.

   AERO Relays are simply Proxy/Servers connected to INETs and/or ENETs
   that provide forwarding services for non-MNP destinations.  The Relay
   connects to the OMNI link and engages in eBGP peering with one or
   more Gateways as a stub AS.  The Relay then injects its MNPs and/or
   non-MNP prefixes into the BGP routing system, and provisions the
   prefixes to its downstream-attached networks.  The Relay can perform
   ARS/ARR services the same as for any Proxy/Server, and can route
   between the MNP and non-MNP address spaces.

A.5.  AERO Server Failure Implications

   AERO Proxy/Servers 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.

   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 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.








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A.6.  AERO Client / Server Architecture

   The AERO architectural model is client / server in the control plane,
   with route optimization in the data plane.  The same as for common
   Internet services, the AERO Client discovers the addresses of AERO
   Proxy/Servers and connects to one or more of them.  The AERO service
   is analogous to common Internet services such as google.com,
   yahoo.com, cnn.com, etc.  However, there is only one AERO service for
   the link and all Proxy/Servers provide identical services.

   Common Internet services provide differing strategies for advertising
   server addresses to clients.  The strategy is conveyed through the
   DNS resource records returned in response to name resolution queries.
   As of January 2020 Internet-based 'nslookup' services were used to
   determine the following:

   *  When a client resolves the domainname "google.com", the DNS always
      returns one A record (i.e., an IPv4 address) and one AAAA record
      (i.e., an IPv6 address).  The client receives the same addresses
      each time it resolves the domainname via the same DNS resolver,
      but may receive different addresses when it resolves the
      domainname via different DNS resolvers.  But, in each case,
      exactly one A and one AAAA record are returned.

   *  When a client resolves the domainname "ietf.org", the DNS always
      returns one A record and one AAAA record with the same addresses
      regardless of which DNS resolver is used.

   *  When a client resolves the domainname "yahoo.com", the DNS always
      returns a list of 4 A records and 4 AAAA records.  Each time the
      client resolves the domainname via the same DNS resolver, the same
      list of addresses are returned but in randomized order (i.e.,
      consistent with a DNS round-robin strategy).  But, interestingly,
      the same addresses are returned (albeit in randomized order) when
      the domainname is resolved via different DNS resolvers.

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

   The above example strategies show differing approaches to Internet
   resilience and service distribution offered by major Internet
   services.  The Google approach exposes only a single IPv4 and a
   single IPv6 address to clients.  Clients can then select whichever IP
   protocol version offers the best response, but will always use the
   same IP address according to the current Internet connection point.
   This means that the IP address offered by the network must lead to a



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   highly-available server and/or service distribution point.  In other
   words, resilience is predicated on high availability within the
   network and with no client-initiated failovers expected (i.e., it is
   all-or-nothing from the client's perspective).  However, Google does
   provide for worldwide distributed service distribution by virtue of
   the fact that each Internet connection point responds with a
   different IPv6 and IPv4 address.  The IETF approach is like google
   (all-or-nothing from the client's perspective), but provides only a
   single IPv4 or IPv6 address on a worldwide basis.  This means that
   the addresses must be made highly-available at the network level with
   no client failover possibility, and if there is any worldwide service
   distribution it would need to be conducted by a network element that
   is reached via the IP address acting as a service distribution point.

   In contrast to the Google and IETF philosophies, Yahoo and Amazon
   both provide clients with a (short) list of IP addresses with Yahoo
   providing both IP protocol versions and Amazon as IPv4-only.  The
   order of the list is randomized with each name service query
   response, with the effect of round-robin load balancing for service
   distribution.  With a short list of addresses, there is still
   expectation that the network will implement high availability for
   each address but in case any single address fails the client can
   switch over to using a different address.  The balance then becomes
   one of function in the network vs function in the end system.

   The same implications observed for common highly-available services
   in the Internet apply also to the AERO client/server architecture.
   When an AERO Client connects to one or more ANETs, it discovers one
   or more AERO Proxy/Server addresses through the mechanisms discussed
   in earlier sections.  Each Proxy/Server address presumably leads to a
   fault-tolerant clustering arrangement such as supported by Linux-HA,
   Extended Virtual Synchrony or Paxos.  Such an arrangement has
   precedence in common Internet service deployments in lightweight
   virtual machines without requiring expensive hardware deployment.
   Similarly, common Internet service deployments set service IP
   addresses on service distribution points that may relay requests to
   many different servers.














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   For AERO, the expectation is that a combination of the Google/IETF
   and Yahoo/Amazon philosophies would be employed.  The AERO Client
   connects to different ANET access points and can receive 1-2 Proxy/
   Server ULAs at each point.  It then selects one AERO Proxy/Server
   address, and engages in RS/RA exchanges with the same Proxy/Server
   from all ANET connections.  The Client remains with this Proxy/Server
   unless or until the Proxy/Server fails, in which case it can switch
   over to an alternate Proxy/Server.  The Client can likewise switch
   over to a different Proxy/Server at any time if there is some reason
   for it to do so.  So, the AERO expectation is for a balance of
   function in the network and end system, with fault tolerance and
   resilience at both levels.

Appendix B.  Change Log

   << RFC Editor - remove prior to publication >>

   Changes from earlier versions:

   *  Submit for 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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