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Asymmetric Extended Route Optimization (AERO)
draft-templin-6man-aero-15

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Author Fred Templin
Last updated 2021-06-10 (Latest revision 2021-06-09)
Replaces draft-templin-intarea-6706bis
Replaced by draft-templin-intarea-aero
RFC stream Independent Submission
Formats
Stream ISE state In ISE Review
Consensus boilerplate Unknown
Document shepherd Eliot Lear
IESG IESG state I-D Exists
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Send notices to rfc-ise@rfc-editor.org
draft-templin-6man-aero-15
Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                             June 10, 2021
Expires: December 12, 2021

             Asymmetric Extended Route Optimization (AERO)
                       draft-templin-6man-aero-15

Abstract

   This document specifies an Asymmetric Extended Route Optimization
   (AERO) service for IP internetworking over Overlay Multilink Network
   (OMNI) interfaces.  AERO/OMNI use an IPv6 link-local address format
   that supports operation of the IPv6 Neighbor Discovery (ND) protocol
   and links ND to IP forwarding.  Prefix delegation/registration
   services are employed for network admission and to manage the routing
   system.  Secure multilink operation, mobility management, multicast,
   traffic selector signaling and route optimization are naturally
   supported through dynamic neighbor cache updates.  AERO is a widely-
   applicable mobile internetworking service especially well-suited to
   aviation services, intelligent transportation systems, mobile Virtual
   Private Networks (VPNs) and many other applications.

Status of This Memo

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

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

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

   This Internet-Draft will expire on December 12, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Asymmetric Extended Route Optimization (AERO) . . . . . . . .  13
     3.1.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .  13
     3.2.  The AERO Service over OMNI Links  . . . . . . . . . . . .  14
       3.2.1.  AERO/OMNI Reference Model . . . . . . . . . . . . . .  14
       3.2.2.  Addressing and Node Identification  . . . . . . . . .  17
       3.2.3.  AERO Routing System . . . . . . . . . . . . . . . . .  18
       3.2.4.  OMNI Link Segment Routing . . . . . . . . . . . . . .  20
       3.2.5.  Segment Routing Topologies (SRTs) . . . . . . . . . .  26
       3.2.6.  Segment Routing For OMNI Link Selection . . . . . . .  26
       3.2.7.  Segment Routing Within the OMNI Link  . . . . . . . .  27
     3.3.  OMNI Interface Characteristics  . . . . . . . . . . . . .  29
     3.4.  OMNI Interface Initialization . . . . . . . . . . . . . .  31
       3.4.1.  AERO Proxy/Server and Relay Behavior  . . . . . . . .  31
       3.4.2.  AERO Client Behavior  . . . . . . . . . . . . . . . .  32
       3.4.3.  AERO Bridge Behavior  . . . . . . . . . . . . . . . .  32
     3.5.  OMNI Interface Neighbor Cache Maintenance . . . . . . . .  32
       3.5.1.  OMNI ND Messages  . . . . . . . . . . . . . . . . . .  34
       3.5.2.  OMNI Neighbor Advertisement Message Flags . . . . . .  36
       3.5.3.  OMNI Neighbor Window Synchronization  . . . . . . . .  36
     3.6.  OMNI Interface Encapsulation and Re-encapsulation . . . .  37
     3.7.  OMNI Interface Decapsulation  . . . . . . . . . . . . . .  37
     3.8.  OMNI Interface Data Origin Authentication . . . . . . . .  38
     3.9.  OMNI Interface MTU  . . . . . . . . . . . . . . . . . . .  38
     3.10. OMNI Interface Forwarding Algorithm . . . . . . . . . . .  39
       3.10.1.  Client Forwarding Algorithm  . . . . . . . . . . . .  40
       3.10.2.  Proxy/Server and Relay Forwarding Algorithm  . . . .  42
       3.10.3.  Bridge Forwarding Algorithm  . . . . . . . . . . . .  44
     3.11. OMNI Interface Error Handling . . . . . . . . . . . . . .  46
     3.12. AERO Router Discovery, Prefix Delegation and
           Autoconfiguration . . . . . . . . . . . . . . . . . . . .  49
       3.12.1.  AERO Service Model . . . . . . . . . . . . . . . . .  49
       3.12.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  49
       3.12.3.  AERO Proxy/Server Behavior . . . . . . . . . . . . .  52
     3.13. The AERO Proxy Function . . . . . . . . . . . . . . . . .  55
       3.13.1.  Detecting and Responding to Proxy/Server Failures  .  57
       3.13.2.  Point-to-Multipoint Proxy/Server Coordination  . . .  58

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     3.14. AERO Route Optimization . . . . . . . . . . . . . . . . .  59
       3.14.1.  Route Optimization Initiation  . . . . . . . . . . .  60
       3.14.2.  Relaying the NS(AR) *NET Packet(s) . . . . . . . . .  61
       3.14.3.  Processing the NS(AR) and Sending the NA(AR) . . . .  61
       3.14.4.  Relaying the NA(AR)  . . . . . . . . . . . . . . . .  62
       3.14.5.  Processing the NA(AR)  . . . . . . . . . . . . . . .  63
       3.14.6.  Forwarding Packets to Route Optimized Targets  . . .  63
     3.15. Neighbor Unreachability Detection (NUD) . . . . . . . . .  66
     3.16. Mobility Management and Quality of Service (QoS)  . . . .  68
       3.16.1.  Mobility Update Messaging  . . . . . . . . . . . . .  68
       3.16.2.  Announcing Link-Layer Address and/or QoS Preference
                Changes  . . . . . . . . . . . . . . . . . . . . . .  69
       3.16.3.  Bringing New Links Into Service  . . . . . . . . . .  70
       3.16.4.  Deactivating Existing Links  . . . . . . . . . . . .  70
       3.16.5.  Moving Between Proxy/Servers . . . . . . . . . . . .  70
     3.17. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  72
       3.17.1.  Source-Specific Multicast (SSM)  . . . . . . . . . .  72
       3.17.2.  Any-Source Multicast (ASM) . . . . . . . . . . . . .  74
       3.17.3.  Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . .  74
     3.18. Operation over Multiple OMNI Links  . . . . . . . . . . .  74
     3.19. DNS Considerations  . . . . . . . . . . . . . . . . . . .  75
     3.20. Transition/Coexistence Considerations . . . . . . . . . .  76
     3.21. Detecting and Reacting to Proxy/Server and Bridge
           Failures  . . . . . . . . . . . . . . . . . . . . . . . .  76
     3.22. AERO Clients on the Open Internet . . . . . . . . . . . .  77
     3.23. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . .  79
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  79
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  80
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  80
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  82
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  84
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  84
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  86
   Appendix A.  Non-Normative Considerations . . . . . . . . . . . .  92
     A.1.  Implementation Strategies for Route Optimization  . . . .  92
     A.2.  Implicit Mobility Management  . . . . . . . . . . . . . .  93
     A.3.  Direct Underlying Interfaces  . . . . . . . . . . . . . .  93
     A.4.  AERO Critical Infrastructure Considerations . . . . . . .  94
     A.5.  AERO Server Failure Implications  . . . . . . . . . . . .  94
     A.6.  AERO Client / Server Architecture . . . . . . . . . . . .  95
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  97
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 100

1.  Introduction

   Asymmetric Extended Route Optimization (AERO) fulfills the
   requirements of Distributed Mobility Management (DMM) [RFC7333] and
   route optimization [RFC5522] for aeronautical networking and other

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   network mobility use cases including intelligent transportation
   systems and enterprise mobile device users.  AERO is a secure
   internetworking and mobility management service that employs the
   Overlay Multilink Network Interface (OMNI) [I-D.templin-6man-omni]
   Non-Broadcast, Multiple Access (NBMA) virtual link model.  The OMNI
   link is a virtual overlay configured over one or more underlying
   Internetworks, and nodes on the link can exchange original IP packets
   as single-hop neighbors.  The OMNI Adaptation Layer (OAL) supports
   end system multilink operation for increased reliability, bandwidth
   optimization and traffic path selection while performing
   fragmentation and reassembly to support Internetwork segment routing
   and Maximum Transmission Unit (MTU) diversity.  In terms of
   precedence, readers may appreciate reading the AERO specification
   first to gain an understanding of the overall architecture and
   mobility services then return to the OMNI specification for a deeper
   analysis of the NBMA link model.

   The AERO service comprises Clients, Proxy/Servers and Relays that are
   seen as OMNI link neighbors as well as Bridges that interconnect
   diverse Internetworks as OMNI link segments through OAL forwarding at
   a layer below IP.  Each node's OMNI interface uses an IPv6 link-local
   address format that supports operation of the IPv6 Neighbor Discovery
   (ND) protocol [RFC4861] and links ND to IP forwarding.  A node's OMNI
   interface can be configured over multiple underlying interfaces, and
   therefore appears as a single interface with multiple link-layer
   addresses.  Each link-layer address is subject to change due to
   mobility and/or multilink fluctuations, and link-layer address
   changes are signaled by ND messaging the same as for any IPv6 link.

   AERO provides a secure cloud-based service where mobile node Clients
   may use any Proxy/Server acting as a Mobility Anchor Point (MAP) and
   fixed nodes may use any Relay on the link for efficient
   communications.  Fixed nodes forward original IP packets destined to
   other AERO nodes via the nearest Relay, which forwards them through
   the cloud.  A mobile node's initial packets are forwarded through the
   Proxy/Server, and direct routing is supported through route
   optimization while packets are flowing.  Both unicast and multicast
   communications are supported, and mobile nodes may efficiently move
   between locations while maintaining continuous communications with
   correspondents and without changing their IP Address.

   AERO Bridges peer with Proxy/Servers in a secured private BGP overlay
   routing instance to provide a Segment Routing Topology (SRT) that
   allows the OAL to span the underlying Internetworks of multiple
   disjoint administrative domains as a single unified OMNI link at a
   layer below IP.  Each OMNI link instance is characterized by the set
   of Mobility Service Prefixes (MSPs) common to all mobile nodes.
   Relays provide an optimal route from (fixed) correspondent nodes on

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   the underlying Internetwork to (mobile or fixed) nodes on the OMNI
   link.  To the underlying Internetwork, the Relay is the source of a
   route to the MSP; hence uplink traffic to the mobile node is
   naturally routed to the nearest Relay.  A Relay can be considered as
   a simple case of a Proxy/Server that provides only forwarding and not
   proxying services.

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

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

   AERO was designed as a secure aeronautical internetworking service
   for both manned and unmanned aircraft, where the aircraft is treated
   as a mobile node that can connect an Internet of Things (IoT).  AERO
   is also applicable to a wide variety of other use cases.  For
   example, it can be used to coordinate the links of mobile nodes
   (e.g., cellphones, tablets, laptop computers, etc.) that connect into
   a home enterprise network via public access networks with VPN or non-
   VPN services enabled according to the appropriate security model.
   AERO can also be used to facilitate terrestrial vehicular and urban
   air mobility (as well as pedestrian communication services) for
   future intelligent transportation systems
   [I-D.ietf-ipwave-vehicular-networking][I-D.templin-ipwave-uam-its].
   Other applicable use cases are also in scope.

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

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

   2.  Multinet - the ability to span the OMNI link over a segment
       routing topology with multiple diverse network administrative
       domains while maintaining seamless end-to-end communications
       between mobile Clients and correspondents such as air traffic
       controllers, fleet administrators, 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 underlying interface address change, but without
       disruptions to ongoing communication sessions with peers over the
       OMNI link.

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

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

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

   The following numbered sections present the AERO specification.  The
   appendices at the end of the document are non-normative.

2.  Terminology

   The terminology in the normative references applies; especially, the
   terminology in the OMNI specification [I-D.templin-6man-omni] is used
   extensively throughout.  The following terms are defined within the
   scope of this document:

   IPv6 Neighbor Discovery (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].

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

   Access Network (ANET)
      a node's first-hop data link service network (e.g., a radio access
      network, cellular service provider network, corporate enterprise
      network, etc.) that often provides link-layer security services

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      such as IEEE 802.1X and physical-layer security (e.g., "protected
      spectrum") to prevent unauthorized access internally and with
      border network-layer security services such as firewalls and
      proxys that prevent unauthorized outside access.

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

   Internetwork (INET)
      a connected IP network topology with a coherent routing and
      addressing plan and that provides a transit backbone service for
      ANET end systems.  INETs also provide an underlay service over
      which the AERO virtual link is configured.  Example INETs include
      corporate enterprise networks, aviation networks, and the public
      Internet itself.  When there is no administrative boundary between
      an ANET and the INET, the ANET and INET are one and the same.

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

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

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

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

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

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

   OMNI link
      the same as defined in [I-D.templin-6man-omni], and manifested by
      IPv6 encapsulation [RFC2473].  The OMNI link spans underlying *NET
      segments joined by virtual bridges in a spanning tree the same as

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      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 multiple underlying *NET hops, and can use Segment
      Routing [RFC8402] to cause packets to visit selected waypoints on
      the link.

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

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

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

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

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

   (OAL) atomic fragment
      an OAL packet that does not require fragmentation is always
      encapsulated as an "atomic fragment" and includes a Fragment
      Header with Fragment Offset and More Fragments both set to 0, but
      with a valid Identification value.

   (OAL) carrier packet
      an encapsulated OAL fragment following *NET encapsulation or prior
      to *NET decapsulation.  OAL sources and destinations exchange
      carrier packets over underlying interfaces, and may be separated
      by one or more OAL intermediate nodes.  OAL intermediate nodes re-
      encapsulate carrier packets during forwarding by removing the *NET

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      headers of the previous hop underlying network and replacing them
      with new *NET headers for the next hop underlying network.

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

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

   OAL intermediate node
      an OMNI interface acts as an OAL intermediate node when it removes
      the *NET headers of carrier packets received on a first segment,
      then re-encapsulates the carrier packets in new *NET headers and
      forwards them into the next segment.  OAL intermediate nodes
      decrement the Hop Limit of the OAL IPv6 header during re-
      encapsulation, and discard the packet if the Hop Limit reaches 0.
      OAL intermediate nodes do not decrement the Hop Limit/TTL of the
      original IP packet.

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

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

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

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

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

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

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

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

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

   AERO Proxy/Server ("Proxy/Server")
      a dual-function node that provides a proxying service between AERO
      Clients and external peers on its Client-facing ANET interfaces
      (i.e., in the same fashion as for an enterprise network proxy) as
      well as default forwarding and Mobility Anchor Point (MAP)
      services for coordination with correspondents on its INET-facing
      interfaces.  (Proxy/Servers in the open INET instead configure
      only an INET interface and no ANET interfaces.)  The Proxy/Server
      configures an OMNI interface and assigns an ADM-LLA to support the
      operation of IPv6 ND services, while advertising all of its
      associated MNPs via BGP peerings with Bridges.  Note that the
      Proxy and Server functions can be considered logically separable,
      but since each Proxy/Server must be informed of all of the
      Client's other multilink Proxy/Server affiliations the AERO
      service is best supported when the two functions are coresident on
      the same physical or logical platform.

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

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   AERO Bridge ("Bridge")
      a node that provides OAL forwarding services (as well as a
      security trust anchor) for nodes on an OMNI link.  The Bridge
      forwards carrier packets between OMNI link segments as OAL
      intermediate nodes while decrementing the OAL IPv6 header Hop
      Limit but without decrementing the network layer IP TTL/Hop Limit.
      AERO Bridges peer with Proxy/Servers and other Bridges over
      secured tunnels to discover the full set of MNPs for the link as
      well as any non-MNP IP GUA routes that are reachable via Relays.

   First-Hop Segment (FHS) Proxy/Server
      a Proxy/Server for an underlying interface of the source Client
      that forwards packets sent by the source Client over that
      interface into the segment routing topology.

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

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

   link-layer address
      an IP address used as an encapsulation header source or
      destination address from the perspective of the OMNI interface.
      When an upper layer protocol (e.g., UDP) is used as part of the
      encapsulation, the port number is also considered as part of the
      link-layer address.

   network layer address
      the source or destination address of an original IP packet
      presented to the OMNI interface.

   end user network (EUN)
      an internal virtual or external edge IP network that an AERO
      Client or Relay connects to the rest of the network via the OMNI
      interface.  The Client/Relay sees each EUN as a "downstream"

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      network, and sees the OMNI interface as the point of attachment to
      the "upstream" network.

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

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

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

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

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

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

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

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

   Throughout the document, the simple terms "Client", "Proxy/Server",
   "Bridge" and "Relay" refer to "AERO Client", "AERO Proxy/Server",
   "AERO Bridge" and "AERO Relay", respectively.  Capitalization is used
   to distinguish these terms from other common Internetworking uses in
   which they appear without capitalization.

   The terminology of IPv6 ND [RFC4861] and DHCPv6 [RFC8415] (including
   the names of node variables, messages and protocol constants) is used

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   throughout this document.  The terms "All-Routers multicast", "All-
   Nodes multicast", "Solicited-Node multicast" and "Subnet-Router
   anycast" are defined in [RFC4291].  Also, the term "IP" is used to
   generically refer to either Internet Protocol version, i.e., IPv4
   [RFC0791] or IPv6 [RFC8200].

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

3.  Asymmetric Extended Route Optimization (AERO)

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

3.1.  AERO Node Types

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

   AERO Bridges (together with Proxy/Servers) provide the secured
   backbone supporting infrastructure for a Segment Routing Topology
   (SRT) that spans the OMNI link.  Bridges forward carrier packets both
   within the same SRT segment and between disjoint SRT segments based
   on an IPv6 encapsulation mid-layer known as the OMNI Adaptation Layer
   (OAL) [I-D.templin-6man-omni].  During forwarding, the inner IP layer
   experiences a virtual bridging service since the inner IP TTL/Hop
   Limit is not decremented.  Each Bridge also peers with Proxy/Servers
   and other Bridges in a dynamic routing protocol instance to provide a
   Distributed Mobility Management (DMM) service for the list of active
   MNPs (see Section 3.2.3).  Bridges present the OMNI link as a set of
   one or more Mobility Service Prefixes (MSPs) and configure secured
   tunnels with Proxy/Servers, Relays and other Bridges; they further
   maintain IP forwarding table entries for each MNP and any other
   reachable non-MNP prefixes.

   AERO Proxy/Servers in distributed SRT segments provide default
   forwarding and mobility/multilink services for AERO Client Mobile

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   Nodes (MNs).  Each Proxy/Server also peers with Bridges in a dynamic
   routing protocol instance to advertise its list of associated MNPs
   (see Section 3.2.3).  Proxy/Servers facilitate prefix delegation/
   registration exchanges with Clients, where each delegated prefix
   becomes an MNP taken from an MSP.  Proxy/Servers forward carrier
   packets between OMNI interface neighbors and track each Client's
   mobility profiles.  Proxy/Servers at ANET/INET boundaries provide a
   conduit for ANET Clients to associate with peers reached through
   external INETs.  Proxy/Servers in the open INET support INET Clients
   through authenticated IPv6 ND message exchanges.  Source Clients
   employ First-Hop Segment (FHS) Proxy/Servers to forward packets over
   the SRT to Last-Hop Segment (LHS) Proxy/Servers which finally forward
   to target Clients.

   AERO Relays are Proxy/Servers that provide forwarding services to
   exchange original IP packets between the OMNI interface and INET/EUN
   interfaces.  Relays are provisioned with MNPs the same as for an AERO
   Client, and also run a dynamic routing protocol to discover any non-
   MNP IP routes.  The Relay advertises the MSP(s) to its connected
   networks, and distributes all of its associated MNP and non-MNP
   routes via BGP peerings with Bridges

3.2.  The AERO Service over OMNI Links

3.2.1.  AERO/OMNI Reference Model

   Figure 1 presents the basic OMNI link reference model:

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

                    Figure 1: AERO/OMNI Reference Model

   In this model:

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

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

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

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   o  AERO Clients C1 and C2 associate with Proxy/Servers S1 and S2,
      respectively.  They receive MNP delegations X1 and X2, and also
      act as default routers for their associated physical or internal
      virtual EUNs.  Simple hosts H1 and H2 attach to the EUNs served by
      Clients C1 and C2, respectively.

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

   The addressing plan of each 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 in many cases disjoint segments may not even
   have any common physical link connections.  Therefore, nodes can only
   be assured of exchanging carrier packets directly with correspondents
   in the same segment, and not with those in other segments.  The only
   means for joining the segments therefore is through inter-domain
   peerings between AERO Bridges.

   The same as for traditional campus LANs, the OMNI link SRT spans
   multiple segments that can be joined into a single unified link using
   the OMNI Adaptation Layer (OAL) [I-D.templin-6man-omni] which inserts
   a mid-layer IPv6 encapsulation header that supports inter-segment
   forwarding (i.e., bridging) without decrementing the network-layer
   TTL/Hop Limit of the original IP packet.  An example OMNI link SRT is
   shown in Figure 2:

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

            Figure 2: OMNI Link Segment Routing Topology (SRT)

   Bridges, Proxy/Servers and Relay OMNI interfaces are configured over
   both secured tunnels and open INET underlying interfaces within their
   respective SRT segments.  Within each segment, Bridges configure
   "hub-and-spokes" BGP peerings with Proxy/Server/Relays as "spokes".
   Adjacent SRT segments are joined by Bridge-to-Bridge peerings to
   collectively form a spanning tree over the entire SRT.  The "secured"
   spanning tree supports strong authentication for control plane
   messages.  The "unsecured" spanning tree conveys ordinary carrier
   packets without security codes and that must be treated by
   destinations according to data origin authentication procedures.
   Route optimization can be employed to cause carrier packets to take
   more direct paths between OMNI link neighbors without having to
   follow strict SRT spanning tree paths.

3.2.2.  Addressing and Node Identification

   AERO nodes on OMNI links use the Link-Local Address (LLA) prefix
   fe80::/64 [RFC4291] to assign LLAs used for network-layer addresses
   in link-scoped IPv6 ND and data messages.  AERO Clients use LLAs
   constructed from MNPs (i.e., "MNP-LLAs") while other AERO nodes use

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   LLAs constructed from administrative identification values ("ADM-
   LLAs") as specified in [I-D.templin-6man-omni].  Non-MNP routes are
   also represented the same as for MNP-LLAs, but may include a prefix
   that is not properly covered by the MSP.

   AERO nodes also use the Unique Local Address (ULA) prefix fd00::/8
   followed by a pseudo-random 40-bit OMNI domain identifier to form the
   prefix [ULA]::/48, then include a 16-bit OMNI link identifier '*' to
   form the prefix [ULA*]::/64 [RFC4291].  The AERO node then uses the
   prefix [ULA*]::/64 to form "MNP-ULAs" or "ADM-ULA"s as specified in
   [I-D.templin-6man-omni] to support OAL addressing.  (The prefix
   [ULA*]::/64 appearing alone and with no suffix represents "default".)
   AERO Clients also use Temporary ULAs constructed per
   [I-D.templin-6man-omni], where the addresses are typically used only
   in initial control message exchanges until a stable MNP-LLA/ULA is
   assigned.

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

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

3.2.3.  AERO Routing System

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

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

   Bridges maintain forwarding table entries only for the MNP-ULAs
   corresponding to MNP and non-MNP routes that are currently active,
   and carrier packets destined to all other MNP-ULAs will correctly
   incur Destination Unreachable messages due to the black-hole route.

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   In this way, Proxy/Servers and Relays have only partial topology
   knowledge (i.e., they only maintain routing information for their
   directly associated Clients and non-AERO links) and they forward all
   other carrier packets to Bridges which have full topology knowledge.

   Each OMNI link SRT segment assigns a unique ADM-ULA sub-prefix of
   [ULA*]::/96.  For example, a first segment could assign
   [ULA*]::1000/116, a second could assign [ULA*]::2000/116, a third
   could assign [ULA*]::3000/116, etc.  Within each segment, each Proxy/
   Server configures an ADM-ULA within the segment's prefix, e.g., the
   Proxy/Servers within [ULA*]::2000/116 could assign the ADM-ULAs
   [ULA*]::2011/116, [ULA*]::2026/116, [ULA*]::2003/116, etc.

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

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

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

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

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

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

   MNP ULAs are instead dynamically advertised in the AERO routing
   system by Proxy/Servers and Relays that provide service for their
   corresponding MNPs.  For example, if three Proxy/Servers ('D', 'E'
   and 'F') service the MNPs 2001:db8:1000:2000::/56,
   2001:db8:3000:4000::/56 and 2001:db8:5000:6000::/56 then the routing
   system would include:

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   D: [ULA*]:2001:db8:1000:2000/120

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

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

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

3.2.4.  OMNI Link Segment Routing

   With the Client and SRT segment prefixes in place in Bridge
   forwarding tables, the OMNI interface sends control and data carrier
   packets toward AERO destination nodes located in different OMNI link
   segments over the SRT spanning tree.  The OMNI interface uses the
   OMNI Adaptation Layer (OAL) encapsulation service
   [I-D.templin-6man-omni], and includes an OMNI Routing Header (ORH) as
   an extension to the OAL header.  Each carrier packet includes at most
   one ORH in compressed or uncompressed form, with the uncompressed
   form shown in Figure 3:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Next Header  |  Hdr Ext Len  |  Routing Type | Segments Left |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    omIndex    | FMT |   SRT   |        LHS (bits 0 -15)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        LHS (bits 0 -15)       |                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
      ~                   Link Layer Address (L2ADDR)                 ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Null Padding (if necessary)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                      Destination Suffix                       ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: OMNI Routing Header (ORH) Format

   The ORH includes the following fields, in consecutive order:

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

   o  Hdr Ext Len is the length of the Routing header in 8-octet units
      (not including the first 8 octets).  The field must encode a value
      between 0 and 4 (all other values indicate a parameter problem).

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

   o  Segments Left encodes the value 0 or 1 (all other values indicate
      a parameter problem).

   o  omIndex - a 1-octet field consulted only when Segments Left is 0;
      identifies a specific target Client underlying interface serviced
      by the LHS Proxy-Server when there are multiple alternatives.
      When FMT-Forward is clear, omIndex determines the interface for
      forwarding the ORH packet following reassembly; when FMT-Forward
      is set, omIndex determines the interface for forwarding the raw
      carrier packets without first reassembling.  When omIndex is set
      to 0 (or when no ORH is present), the LHS Proxy/Server selects
      among any of the Client's available underlying interfaces that it
      services locally (i.e., and not those serviced by another Proxy/
      Server).

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

      *  When the most significant bit (i.e., "FMT-Forward") is clear,
         the LHS Proxy/Server must reassemble.  When FMT-Forward is set,
         the LHS Proxy/Server must forward the fragments to the Client
         (while changing the OAL destination address to the MNP-ULA of
         the Client if necessary) without reassembling.

      *  When the next most significant bit (i.e., "FMT-Mode") is clear,
         L2ADDR is the INET address of the LHS Proxy/Server and the
         Client must be reached through the LHS Proxy/Server.  When FMT-
         Mode is set, the Client is eligible for route optimization over
         the open INET where it may be located behind one or more NATs,
         and L2ADDR is either the INET address of the LHS Proxy/Server
         (when FMT-Forward is set) or the native INET address of the
         Client itself (when FMT-Forward is clear).

      *  The least significant bit (i.e., "FMT-Type") is consulted only
         when Hdr Ext Len is 1 and ignored otherwise.  If FMT-Type is
         clear, the remaining 10 ORH octets contain an LHS followed by
         an IPv4 L2ADDR.  If FMT-Type is set, the remainder instead
         contains 2 null padding octets followed by an 8-octet (IPv6)
         Destination Suffix.

   o  SRT - a 5-bit Segment Routing Topology prefix length consulted
      only when Segments Left is 1, and encodes a value that (when added
      to 96) determines the prefix length to apply to the ADM-ULA formed
      from concatenating [ULA*]::/96 with the 32 bit LHS value (for
      example, the value 16 corresponds to the prefix length 112).

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   o  LHS - a 4-octet field present only when indicated by the ORH
      length (see below) and consulted only when Segments Left is 1.
      The field encodes the 32-bit ADM-ULA suffix of an LHS Proxy/Server
      for the target.  When SRT and LHS are both set to 0, the LHS
      Proxy/Server must be reached directly via INET encapsulation
      instead of over the spanning tree.  When SRT is set to 0 and LHS
      is non-zero, the prefix length is set to 128.  SRT and LHS
      determine the ADM-ULA of the LHS Proxy/Server over the spanning
      tree.

   o  Link Layer Address (L2ADDR) - an IP encapsulation address present
      only when indicated by the ORH length (see below) and consulted
      only when Segments Left is 1.  The ORH length also determines the
      L2ADDR IP version since the field will always contain exactly 6
      octets for UDP/IPv4 or 18 octets for UDP/IPv6.  When present,
      provides the link-layer address (i.e., the encapsulation address)
      of the LHS Proxy/Server or the target Client itself.  The UDP Port
      Number appears in the first two octets and the IP address appears
      in the remaining octets.  The Port Number and IP address are
      recorded in network byte order, and in ones-compliment
      "obfuscated" form per [RFC4380].  The OMNI interface forwarding
      algorithm uses L2ADDR as the INET encapsulation address for
      forwarding when SRT/LHS is located in the same OMNI link segment.
      If direct INET encapsulation is not permitted, L2ADDR is instead
      set to all-zeros and the packet must be forwarded to the LHS
      Proxy-Server via the spanning tree.

   o  Null Padding - zero-valued octets added as necessary to pad the
      portion of the ORH included up to this point to an even 8-octet
      boundary.

   o  Destination Suffix - a trailing 8-octet field present only when
      indicated by the ORH length (see below).  When ORH length is 1,
      FMT-Type determines whether the option includes a Destination
      Suffix or an LHS/L2ADDR for IPv4 since there is only enough space
      available for one.  When present, encodes the 64-bit MNP-ULA
      suffix for the target Client.

   The ORH Hdr Ext Len field value also serves as an implicit ORH
   "Type", with 5 distinct Types specified (i.e., ORH-0 through ORH-4).
   All ORH-* Types include the same 6-octet preamble beginning with Next
   Header up to and including omIndex, followed by a Type-specific
   remainder as follows:

   o  ORH-0 - The preamble Hdr Ext Len and Segments Left must both be 0.
      Two null padding octets follow the preamble, and all other fields
      are omitted.

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   o  ORH-1 - The preamble Hdr Ext Len is set to 1.  When FMT-Type is
      clear, the LHS and L2ADDR for IPv4 fields are included and the
      Destination Suffix is omitted.  When FMT-Type is set, the LHS and
      L2ADDR fields are omitted, the Destination Suffix field is
      included and Segments Left must be 0.

   o  ORH-2 - The preamble Hdr Ext Len is set to 2.  The LHS, L2ADDR for
      IPv4 and Destination Suffix fields are all included.

   o  ORH-3 - The preamble Hdr Ext Len is set to 3.  The LHS and L2ADDR
      for IPv6 fields are included and the Destination Suffix field is
      omitted.

   o  ORH-4 - The preamble Hdr Ext Len is set to 4.  The LHS, L2ADDR for
      IPv6 and Destination Suffix fields are all included.

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

   If the neighbor cache information indicates that the target is in a
   different segment, the OAL source next inserts an ORH immediately
   following the OAL header while including Destination Suffix for non-
   first-fragments only when necessary (in this case, the Destination
   Suffix is 2001:db8:1234:5678).  Next, to direct the packet to a
   first-hop Proxy/Server or a Bridge, the source prepares an ORH with
   Segments Left set to 1 and with SRT/LHS/L2ADDR included, then
   overwrites the OAL header destination address with the LHS Subnet
   Router Anycast address (for example, for LHS 3000:4567 with SRT 16,
   the Subnet Router Anycast address is [ULA*]::3000:0000).  To send the
   packet to the LHS Proxy/Server either directly or via the spanning
   tree, the OAL source instead includes an ORH (Type 0 or 1) with
   Segments Left set to 0 and LHS/L2ADDR omitted, and overwrites the OAL
   header destination address with either the LHS Proxy/Server ADM-ULA
   or the MNP-ULA of the Client itself.

   The OAL source then fragments the OAL packet, with each resulting OAL
   fragment including the OAL/ORH headers while only the first fragment
   includes the original IPv6 header.  If FMT-Forward is set, the
   Identification used for fragmentation must be within the window for

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   the Client and a Destination Suffix must be included with each non-
   first-fragment when necessary; otherwise the Identification must be
   within the window for the Client's Proxy/Server and no Destination
   Suffix is needed.  (Note that if no actual fragmentation is required
   the OAL source still prepares the packet as an "atomic" fragment that
   includes a Fragment Header with Offset and More Fragments both set to
   0.)  The OAL source finally encapsulates each resulting OAL fragment
   in an *NET header to form an OAL carrier packet, with source address
   set to its own *NET address (e.g., 192.0.2.100) and destination set
   to the *NET address of the last hop itself or the next hop in the
   spanning tree (e.g., 192.0.2.1).

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

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

                      Figure 4: Carrier Packet Format

   In this format, the original IP header and packet body/fragment are
   from the original IP packet, the OAL header is an IPv6 header
   prepared according to [RFC2473], the ORH is a Routing Header
   extension of the OAL header, the Fragment Header identifies each
   fragment, and the INET header is prepared as discussed in
   Section 3.6.  The OAL source then transmits the resulting carrier

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   packets into the SRT spanning tree, where they are forwarded over
   possibly multiple OAL intermediate nodes until they arrive at the OAL
   destination.

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

   Note: The document recommends that AERO nodes transform ORHs with
   Segments Left set to 1 into ORH-0 or ORH-1 during forwarding.  While
   this may yield encapsulation overhead savings in some cases, the AERO
   node may instead simply set Segments Left to 0 and leave the original
   ORH in place.  The LHS Proxy/Server or target Client that processes
   the ORH will receive the same information in both cases.

   Note: When the OAL source sets a carrier packet OAL destination
   address to a target's MNP-ULA but does not assert a specific target
   underlying interface, it may omit the ORH whether forwarding to the
   LHS Proxy/Server or directly to the target itself.  When the LHS
   Proxy/Server receives a carrier packet with OAL destination set to
   the target MNP-ULA but with no ORH, it forwards over any available
   underlying interface for the target that it services locally.

   Note: When the OAL source and destination are on the same INET
   segment, OAL header compression can be used to significantly reduce
   encapsulation overhead as discussed in [I-D.templin-6man-omni].

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

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   error reports could not be delivered to the original source.  In
   order to address the issues, still more information such as the
   original IPv6 source address could be written into the ORH.  However,
   with the additional information the benefit of the "minimal
   encapsulation" savings quickly diminishes, and becomes overshadowed
   by the implementation and operational irregularities.

3.2.5.  Segment Routing Topologies (SRTs)

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

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

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

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

3.2.6.  Segment Routing For OMNI Link Selection

   Original IPv6 source can direct IPv6 packets to an AERO node by
   including a standard IPv6 Segment Routing Header (SRH) [RFC8754] with
   the anycast ADM-ULA 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

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   original IPv6 packet will traverse when there may be multiple
   alternatives.

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

3.2.7.  Segment Routing Within the OMNI Link

   OAL sources can insert an ORH for Segment Routing within the same
   OMNI link to influence the paths of carrier packets sent to OAL
   destinations in remote SRT segments without requiring all carrier
   packets to traverse strict SRT spanning tree paths.  (OAL sources can
   also insert an ORH in carrier packets sent to OAL destinations in the
   local segment if additional last-hop forwarding information is
   required.)

   When an AERO node's OMNI interface has an original IP packet to send
   to a target discovered through route optimization located in the same
   SRT segment, it acts as an OAL source to perform OAL encapsulation
   and fragmentation.  The node then uses L2ADDR for INET encapsulation
   while including an ORH-0 when sending the resulting carrier packets
   to the ADM-ULA of the LHS Proxy/Server, or optionally omitting the
   ORH-0 when sending to the MNP-ULA of the target Client itself.  When
   the node sends carrier packets with an ORH-0 to the LHS Proxy/Server,
   it sets the OAL destination to the ADM-ULA of the Proxy/Server if the
   Proxy/Server is responsible for reassembly; otherwise, it sets the
   OAL destination to the MNP-ULA of the target Client to cause the
   Proxy/Server to forward without reassembling.  The node also sets
   omIndex to select a specific target Client underlying interface, or
   sets omIndex to 0 when no preference is selected.

   When an AERO node's OMNI interface has an original IP packet to send
   to a route optimization target located in a remote OMNI link segment,
   it acts as an OAL source the same as above but also includes an
   appropriate ORH type with Segments Left set to 1 and with SRT/LHS/
   L2ADDR information while setting the OAL destination to the Subnet
   Router Anycast address for the LHS OMNI link segment.  (The OAL
   source can alternatively include an ORH with Segments Left set to 0
   while setting the OAL destination to the ADM-ULA of the LHS Proxy/
   Server, but this would cause the carrier packets to follow strict
   spanning tree paths.)  The OMNI interface then forwards the resulting
   carrier packets into the spanning tree.

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   When a Bridge receives a carrier packet destined to its Subnet Router
   Anycast address with any ORH type with Segments Left set to 1 and
   with SRT/LHS/L2ADDR values corresponding to the local segment, it
   examines FMT-Mode to determine whether the target Client can accept
   packets directly (i.e., following any NAT traversal procedures
   necessary) while bypassing the LHS Proxy/Server.  If the Client can
   be reached directly and NAT traversal has converged, the Bridge then
   writes the MNP-ULA (found in the inner IPv6 header for first
   fragments or the ORH Destination Suffix for non-first fragments) into
   the OAL destination address, decrements the OAL IPv6 header Hop Limit
   (and discards the packet if Hop Limit reaches 0), removes the ORH,
   re-encapsulates the carrier packet according to L2ADDR then forwards
   the carrier packet directly to the target Client.  If the Client
   cannot be reached directly (or if NAT traversal has not yet
   converged), the Bridge instead transforms the ORH into an ORH-0, re-
   encapsulates the packet according to L2ADDR, changes the OAL
   destination to the ADM-ULA of the LHS Proxy/Server if FMT-Forward is
   clear or the MNP-ULA of the Client if FMT-Forward is set and forwards
   the carrier packet to the LHS Proxy/Server.

   When a Bridge receives a carrier packet destined to its Subnet Router
   Anycast address with any ORH type with Segments Left set to 1 and
   L2ADDR set to 0, the Bridge instead forwards the packet toward the
   LHS Proxy/Server via the spanning tree.  The Bridge changes the OAL
   destination to the ADM-ULA of the LHS Proxy/Server, transforms the
   ORH into an ORH-0 (or an ORH-1 with FMT-Type set and Segments Left
   0), then forwards the packet to the next hop in the spanning tree.
   The Bridge may also elect to forward via the spanning tree as above
   even when it receives a carrier packet with an ORH that includes a
   valid L2ADDR Port Number and IP address, however this may result in a
   longer path than necessary.  If the carrier packet arrived via the
   secured spanning tree, the Bridge forwards to the next hop also via
   the secured spanning tree.  If the carrier packet arrived via the
   unsecured spanning tree, the Bridge forwards to the next hop also via
   the unsecured spanning tree.

   When an LHS Proxy/Server receives carrier packets with any ORH type
   with Segments Left set to 0 and with OAL destination set to its own
   ADM-ULA, it proceeds according to FMT-Forward and omIndex.  If FMT-
   Forward is set, the LHS Proxy/Server changes the OAL destination to
   the MNP-ULA of the target Client found in the IPv6 header for first
   fragments or in the ORH Destination Suffix for non-first-fragments,
   removes the ORH and forwards to the target Client interface
   identified by omIndex.  If FMT-Forward is clear, the LHS Proxy/Server
   instead reassembles then re-encapsulates while refragmenting if
   necessary, removes the ORH and forwards to the target Client
   according to omIndex.

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   When an LHS Proxy/Server receives carrier packets with any ORH type
   with Segments Left set to 0 and with OAL destination set to the MNP-
   ULA of the target Client, it removes the ORH and forwards to the
   target Client according to omIndex.  During forwarding, the LHS
   Proxy/Server must first verify that the omIndex corresponds to a
   target underlying interface that it services locally and must not
   forward to other target underlying interfaces.  If omIndex is 0 (or
   if no ORH is included) the LHS Proxy/Server instead selects among any
   of the target underlying interfaces it services.

   When a target Client receives carrier packets with OAL destination
   set to is MNP-ULA, it reassembles to obtain the OAL packet then
   decapsulates and delivers the original IP packet to upper layers.

   Note: Special handling procedures are employed for the exchange of
   IPv6 ND messages used to establish neighbor cache state as discussed
   in Section 3.14.  The procedures call for hop-by-hop authentication
   and neighbor cache state establishment based on the encapsulation
   ULA, with next-hop determination based on the IPv6 ND message LLA.

3.3.  OMNI Interface Characteristics

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

   o  INET interfaces connect to an INET either natively or through one
      or more NATs.  Native INET interfaces have global IP addresses
      that are reachable from any INET correspondent.  The INET-facing
      interfaces of Proxy/Servers are native interfaces, as are Relay
      and Bridge interfaces.  NATed INET interfaces connect to a private
      network behind one or more NATs that provide INET access.  Clients
      that are behind a NAT are required to send periodic keepalive
      messages to keep NAT state alive when there are no carrier packets
      flowing.

   o  ANET interfaces connect to an ANET that is separated from the open
      INET by an FHS Proxy/Server.  Clients can issue control messages
      over the ANET without including an authentication signature since
      the ANET is secured at the network layer or below.  Proxy/Servers
      can actively issue control messages over the INET on behalf of
      ANET Clients to reduce ANET congestion.

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

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   o  Direct (i.e., single-hop point-to-point) interfaces connect a
      Client directly to an FHS Proxy/Server without crossing any ANET/
      INET paths.  An example is a line-of-sight link between a remote
      pilot and an unmanned aircraft.  The same Client considerations
      apply as for VPNed interfaces.

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

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

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

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

   If a Client's multiple underlying interfaces are used "one at a time"
   (i.e., all other interfaces are in standby mode while one interface
   is active), then successive ND messages all include OMNI option
   Interface Attributes sub-options with the same underlying interface
   index.  In that case, the Client would appear to have a single
   underlying interface but with a dynamically changing link-layer
   address.

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   If the Client has multiple active underlying interfaces, then from
   the perspective of ND it would appear to have multiple link-layer
   addresses.  In that case, ND message OMNI options MAY include
   Interface Attributes sub-options with different underlying interface
   indexes.  Every ND message need not include Interface Attributes for
   all underlying interfaces; for any attributes not included, the
   neighbor considers the status as unchanged.

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

3.4.  OMNI Interface Initialization

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

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

3.4.1.  AERO Proxy/Server and Relay Behavior

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

   The OMNI interface provides a single interface abstraction to the IP
   layer, but internally includes an NBMA nexus for sending carrier
   packets to OMNI interface neighbors over underlying INET interfaces
   and secured tunnels.  The Proxy/Server further configures a service
   to facilitate ND exchanges with AERO Clients and manages per-Client

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   neighbor cache entries and IP forwarding table entries based on
   control message exchanges.

   Relays are simply Proxy/Servers that run a dynamic routing protocol
   to redistribute routes between the OMNI interface and INET/EUN
   interfaces (see: Section 3.2.3).  The Relay provisions MNPs to
   networks on the INET/EUN interfaces (i.e., the same as a Client would
   do) and advertises the MSP(s) for the OMNI link over the INET/EUN
   interfaces.  The Relay further provides an attachment point of the
   OMNI link to a non-MNP-based global topology.

3.4.2.  AERO Client Behavior

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

3.4.3.  AERO Bridge Behavior

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

3.5.  OMNI Interface Neighbor Cache Maintenance

   Each OMNI interface maintains a conceptual neighbor cache that
   includes a Neighbor Cache Entry (NCE) for each of its active
   neighbors on the OMNI link per [RFC4861].  Each route optimization
   source NCE is indexed by the LLA of the neighbor, while the OAL
   encapsulation ULA determines the context for Identification
   verification.  In addition to ordinary neighbor cache entries, proxy
   neighbor cache entries are created and maintained by AERO Proxy/
   Servers when they proxy Client ND message exchanges [RFC4389].  AERO
   Proxy/Servers maintain proxy neighbor cache entries for each of their
   associated Clients.

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   To the list of NCE states in Section 7.3.2 of [RFC4861], Proxy/Server
   OMNI interfaces add an additional state DEPARTED that applies to
   Clients that have recently departed.  The interface sets a
   "DepartTime" variable for the NCE to "DEPART_TIME" seconds.
   DepartTime is decremented unless a new ND message causes the state to
   return to REACHABLE.  While a NCE is in the DEPARTED state, the
   Proxy/Server forwards carrier packets destined to the target Client
   to the Client's new location instead.  When DepartTime decrements to
   0, the NCE is deleted.  It is RECOMMENDED that DEPART_TIME be set to
   the default constant value REACHABLE_TIME plus 10 seconds (40 seconds
   by default) to allow a window for carrier packets in flight to be
   delivered while stale route optimization state may be present.

   Proxy/Servers can act as RORs on behalf of their associated Clients
   according to the Proxy Neighbor Advertisement specification in
   Section 7.2.8 of [RFC4861].  When a Proxy/Server ROR receives an
   authentic NS message used for route optimization, it first searches
   for a NCE for the target Client and accepts the message only if there
   is an entry.  The Proxy/Server then returns a solicited NA message
   while creating or updating a "Report List" entry in the target
   Client's NCE that caches both the LLA and ULA of ROS with a
   "ReportTime" variable set to REPORT_TIME seconds.  The ROR resets
   ReportTime when it receives a new authentic NS message, and otherwise
   decrements ReportTime while no authentic NS messages have been
   received.  It is RECOMMENDED that REPORT_TIME be set to the default
   constant value REACHABLE_TIME plus 10 seconds (40 seconds by default)
   to allow a window for route optimization to converge before
   ReportTime decrements below REACHABLE_TIME.

   When the ROS receives a solicited NA message response to its NS
   message used for route optimization, it creates or updates a NCE for
   the target network-layer and link-layer addresses.  The ROS then
   (re)sets ReachableTime for the NCE to REACHABLE_TIME seconds and
   performs reachability tests over specific underlying interface pairs
   to determine paths for forwarding carrier packets directly to the
   target.  The ROS otherwise decrements ReachableTime while no further
   solicited NA messages arrive.  It is RECOMMENDED that REACHABLE_TIME
   be set to the default constant value 30 seconds as specified in
   [RFC4861].

   AERO nodes also use the value MAX_UNICAST_SOLICIT to limit the number
   of NS messages sent when a correspondent may have gone unreachable,
   the value MAX_RTR_SOLICITATIONS to limit the number of RS messages
   sent without receiving an RA and the value MAX_NEIGHBOR_ADVERTISEMENT
   to limit the number of unsolicited NAs that can be sent based on a
   single event.  It is RECOMMENDED that MAX_UNICAST_SOLICIT,
   MAX_RTR_SOLICITATIONS and MAX_NEIGHBOR_ADVERTISEMENT be set to 3 the
   same as specified in [RFC4861].

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   Different values for DEPART_TIME, REPORT_TIME, REACHABLE_TIME,
   MAX_UNICAST_SOLICIT, MAX_RTR_SOLCITATIONS and
   MAX_NEIGHBOR_ADVERTISEMENT MAY be administratively set; however, if
   different values are chosen, all nodes on the link MUST consistently
   configure the same values.  Most importantly, DEPART_TIME and
   REPORT_TIME SHOULD be set to a value that is sufficiently longer than
   REACHABLE_TIME to avoid packet loss due to stale route optimization
   state.

3.5.1.  OMNI ND Messages

   OMNI interfaces prepare IPv6 ND messages the same as for standard
   IPv6 ND, but also include a new option type termed the OMNI option
   [I-D.templin-6man-omni].  OMNI interfaces prepare IPv6 ND messages
   the same as for standard IPv6 ND, and include one or more OMNI
   options and any other options then completely populate all option
   information.  If the OMNI interface includes an authentication
   signature, it sets the IPv6 ND message Checksum field to 0 and
   calculates the authentication signature over the entire length of the
   message (beginning with a pseudo-header of the IPv6 header) but does
   not then proceed to calculate the IPv6 ND message checksum itself.
   If the OMNI interface forwards the message to a next hop over the
   secured spanning tree path, it omits both the authentication
   signature or checksum since lower layers already ensure
   authentication and integrity.  In all other cases, the OMNI interface
   calculates the standard IPv6 ND message checksum and writes the value
   in the Checksum field.  OMNI interfaces verify authentication and
   integrity of each IPv6 ND message received according to the specific
   check(s) included, and process the message further only following
   verification.

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

   The OMNI option Interface Attributes for each underlying interface
   includes a two-part "Link-Layer Address" consisting of an INET
   encapsulation address determined by the FMT and L2ADDR fields and an
   ADM-ULA determined by the SRT and LHS fields.  Underlying interfaces
   are further selected based on their associated traffic selectors.

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

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

   AERO Clients send RS messages to the All-Routers multicast address
   while using unicast link-layer addresses.  AERO Proxy/Servers respond
   by returning unicast RA messages.  During the RS/RA exchange, AERO
   Clients and Servers include state synchronization parameters to
   establish Identification windows and other state.

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

   o  NS/NA(AR) messages are used for address resolution only.  The ROS
      sends an NS(AR) to the solicited-node multicast address of the
      target, and an ROR in the network with addressing information for
      the target returns a unicast NA(AR).  The NA(AR) contains
      authentic and current target address resolution information, but
      only an implicit third-party assertion of target reachability.
      NS/NA(AR) messages must be secured.

   o  NS/NA(WIN) messages are used for establishing and maintaining
      window synchronization state (and/or any other state such as
      Interface Attributes).  The source sends an NS(WIN) to the unicast
      address of the target, and the target returns a unicast NA(WIN).
      The NS/NA(WIN) exchange synchronizes the sequence number windows
      for Identification values the neighbors will include in subsequent
      carrier packets, and asserts reachability for the target without
      necessarily testing a specific underlying interface pair.  NS/
      NA(WIN) messages must be secured.

   o  NS/NA(NUD) messages are used for determining target reachability.
      The source sends an NS(NUD) to the unicast address of the target
      while naming a specific underlying interface pair, and the target
      returns a unicast NA(NUD).  NS/NA(NUD) messages that use an in-
      window sequence number and do not update any other state need not
      be secured but should include an IPv6 ND message checksum.  NS/
      NA(NUD) messages may also be used in combination with window

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      synchronization (i.e., NUD+WIN), in which case the messages must
      be secured.

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

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

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

3.5.2.  OMNI Neighbor Advertisement Message Flags

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

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

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

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

3.5.3.  OMNI Neighbor Window Synchronization

   In secured environments (e.g., such as between nodes on the same
   secured ANET), OMNI interface neighbors can exchange OAL packets
   using randomly-initialized and monotonically-increasing
   Identification values (modulo 2*32) without window synchronization.
   In environments where spoofing is considered a threat, OMNI interface
   neighbors instead invoke window synchronization in ND message

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   exchanges to maintain send/receive window state in their respective
   neighbor cache entries as specified in [I-D.templin-6man-omni].

   In the asymmetric window synchronization case, the initial ND message
   exchange establishes only the initiator's send window and the
   responder's receive window such that a corresponding exchange would
   be needed to establish the reverse direction.  In the symmetric case,
   the initiator and responder engage in a three-way handshake to
   symmetrically establish the send/receive windows of both parties.

3.6.  OMNI Interface Encapsulation and Re-encapsulation

   The OMNI interface admits original IP packets then acts as an OAL
   source to perform OAL encapsulation and fragmentation as specified in
   [I-D.templin-6man-omni] while including an ORH if necessary as
   specified in Section 3.2.4.  OAL encapsulation produces OAL packets
   subject to fragmentation, with the resulting fragments encapsulated
   in *NET headers as carrier packets.

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

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

3.7.  OMNI Interface Decapsulation

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

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3.8.  OMNI Interface Data Origin Authentication

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

   o  AERO Bridges and Proxy/Servers accept carrier packets received
      from secured underlying interfaces.

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

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

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

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

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

3.9.  OMNI Interface MTU

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

   When the network layer presents an original IP packet to the OMNI
   interface, the OAL source encapsulates and fragments the original IP
   packet if necessary.  When the network layer presents the OMNI
   interface with multiple original IP packets bound to the same OAL
   destination, the OAL source can concatenate them together into a
   single OAL super-packet as discussed in [I-D.templin-6man-omni].  The

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   OAL source then fragments the OAL packet if necessary according to
   the minimum/path MPS such that the OAL headers appear in each
   fragment while the original IP packet header appears only in the
   first fragment.  The OAL source then encapsulates each OAL fragment
   in *NET headers for transmission as carrier packets over an
   underlying interface connected to either a physical link (such as
   Ethernet, WiFi and the like) or a virtual link such as an Internet or
   higher-layer tunnel (see the definition of link in [RFC8200]).

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

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

3.10.  OMNI Interface Forwarding Algorithm

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

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

   OMNI interfaces may have multiple underlying interfaces and/or
   neighbor cache entries for neighbors with multiple underlying
   interfaces (see Section 3.3).  The OAL uses Interface Attributes

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   traffic selectors (e.g., port number, flow specification, etc.) to
   select an outbound underlying interface for each OAL packet based on
   the node's own interface attributes, and also to select a destination
   link-layer address based on the neighbor's underlying interface
   attributes.  AERO implementations SHOULD permit network management to
   dynamically adjust traffic selector values at runtime.

   If an OAL packet matches the traffic selectors of multiple outgoing
   interfaces and/or neighbor interfaces, the OMNI interface replicates
   the packet and sends one copy via each of the (outgoing / neighbor)
   interface pairs; otherwise, it sends a single copy of the OAL packet
   via an interface with the best matching traffic selector.  (While not
   strictly required, the likelihood of successful reassembly may
   improve when the OMNI interface sends all fragments of the same
   fragmented OAL packet consecutively over the same underlying
   interface pair to avoid complicating factors such as delay variance
   and reordering.)  AERO nodes keep track of which underlying
   interfaces are currently "reachable" or "unreachable", and only use
   "reachable" interfaces for forwarding purposes.

   The following sections discuss the OMNI interface forwarding
   algorithms for Clients, Proxy/Servers and Bridges.  In the following
   discussion, an original IP packet's destination address is said to
   "match" if it is the same as a cached address, or if it is covered by
   a cached prefix (which may be encoded in an MNP-LLA).

3.10.1.  Client Forwarding Algorithm

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

   If the neighbor interface selected for forwarding is located on the
   same OMNI link segment and not behind a NAT, the Client forwards the
   carrier packets directly according to the L2ADDR information for the

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   neighbor.  If the neighbor interface is behind a NAT on the same OMNI
   link segment, the Client instead forwards the initial carrier packets
   to the LHS Proxy/Server (while inserting an ORH-0 if necessary) and
   initiates NAT traversal procedures.  If the Client's intended source
   underlying interface is also behind a NAT and located on the same
   OMNI link segment, it sends a "direct bubble" over the interface per
   [RFC6081][RFC4380] to the L2ADDR found in the neighbor cache in order
   to establish state in its own NAT by generating traffic toward the
   neighbor (note that no response to the bubble is expected).

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

   When a carrier packet enters a Client's OMNI interface from the link-
   layer, if the OAL destination matches one of the Client's ULAs the
   Client (acting as an OAL destination) verifies that the
   Identification is in-window for this OAL source, then reassembles and
   decapsulates as necessary and delivers the original IP packet to the
   network layer.  Otherwise, the Client drops the original IP packet
   and MAY return a network-layer ICMP Destination Unreachable message
   subject to rate limiting (see: Section 3.11).

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

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3.10.2.  Proxy/Server and Relay Forwarding Algorithm

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

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

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

   o  else, if the original IP destination matches a non-MNP route in
      the IP forwarding table or an ADM-LLA assigned to the Proxy/
      Server's OMNI interface, the Proxy/Server acting as a Relay
      presents the original IP packet to the network layer for local
      delivery or IP forwarding.

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

   When the Proxy/Server receives a carrier packet with OAL destination
   set to an MNP-ULA that does not match the MSP, it accepts the carrier
   packet only if data origin authentication succeeds and if there is a
   network layer routing table entry for a GUA route that matches the
   MNP-ULA.  If there is no route, the Proxy/Server drops the carrier
   packet; otherwise, it reassembles and decapsulates to obtain the

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   original IP packet and acts as a Relay to present it to the network
   layer where it will be delivered according to standard IP forwarding.

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

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

   Note: Proxy/Servers may receive carrier packets with ORHs that
   include additional forwarding information.  Proxy/Servers use the
   forwarding information to determine the correct interface for
   forwarding to the target Client, then remove the ORH and forward the
   carrier packet.  If the ORH information instead indicates that the
   Proxy/Server is responsible for reassembly, the Proxy/Server
   reassembles first before re-encapsulating (and possibly also re-
   fragmenting) then forwards to the target Client.  For a full
   discussion of cases when the Proxy/Server may receive carrier packets
   with ORHs, see: Section 3.14.6.

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

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   classical network "brittleness".  (Note however that ANET peers can
   send small original IP packets without invoking the OAL, while
   invoking the OAL for larger packets.  This presents the beneficial
   aspects of both small packet efficiency and large packet robustness.)

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

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

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

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

3.10.3.  Bridge Forwarding Algorithm

   Bridges forward carrier packets while decrementing the OAL header Hop
   Count but not the original IP header Hop Count/TTL.  Bridges convey
   carrier packets that encapsulate IPv6 ND control messages or routing
   protocol control messages via the secured spanning tree, and may
   convey carrier packets that encapsulate ordinary data via the
   unsecured spanning tree.  When the Bridge receives a carrier packet,
   it removes the outer *NET header and searches for a forwarding table

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   entry that matches the OAL destination address.  The Bridge then
   processes the packet as follows:

   o  if the carrier packet destination matches its ADM-ULA or the
      corresponding ADM-ULA Subnet Router Anycast address and the OAL
      header is followed by an ORH, the Bridge reassembles if necessary
      then sets aside the ORH and processes the carrier packet locally
      before forwarding.  If the OAL packet contains an NA(NUD) message,
      the Bridge writes FMT/SRT/LHS/L2ADDR information for its own INET
      interface over the OMNI option Interface Attributes sub-option
      supplied by the NA(NUD) message source.  The Bridge next examines
      the ORH, and if FMT-Mode indicates the destination is a Client on
      the open *NET (or, a Client behind a NAT for which NAT traversal
      procedures have already converged) the Bridge writes the MNP-ULA
      formed from the ORH Destination Suffix into the OAL destination.
      The Bridge then removes the ORH and forwards the packet using
      encapsulation based on L2ADDR.  If the LHS Proxy/Server will
      forward to the Client without reassembly, the Bridge writes the
      MNP-ULA into the OAL destination then replaces the ORH with an
      ORH-0 and forwards the carrier packet to the LHS Proxy/Server
      while also invoking NAT traversal procedures if necessary (noting
      that no direct bubbles are necessary since only the target Client
      and not the Bridge is behind a NAT).  If the LHS Proxy/Server must
      perform reassembly before forwarding to the Client, the Bridge
      instead writes the ADM-ULA formed from the ORH SRT/LHS into the
      OAL destination address, replaces the ORH with an ORH-0 and
      forwards the carrier packet to the LHS Proxy/Server.

   o  else, if the carrier packet destination matches its ADM-ULA or the
      corresponding ADM-ULA Subnet Router Anycast address and the OAL
      header is not followed by an ORH with Segments Left set to 1, the
      Bridge submits the packet for reassembly.  When reassembly is
      complete, the Bridge submits the original IP packet to the network
      layer to support local applications such as BGP routing protocol
      sessions.

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

   o  else, the Bridge drops the packet and returns an Destination
      Unreachable as above.

   The Bridge decrements the OAL IPv6 header Hop Limit when it forwards
   the carrier packet and drops the packet if the Hop Limit reaches 0.
   Therefore, only the Hop Limit in the OAL header is decremented and

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   not the TTL/Hop Limit in the original IP packet header.  Bridges do
   not insert OAL/ORH headers themselves; instead, they simply forward
   carrier packets based on their destination addresses while also
   possibly transforming larger ORHs into an ORH-0 (or removing the ORH
   altogether).

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

3.11.  OMNI Interface Error Handling

   When an AERO node admits an original IP packet into the OMNI
   interface, it may receive link-layer or network-layer error
   indications.  The AERO node may also receive OMNI link error
   indications in OAL-encapsulated uNA messages that include
   authentication signatures.

   A link-layer error indication is an ICMP error message generated by a
   router in the INET on the path to the neighbor or by the neighbor
   itself.  The message includes an IP header with the address of the
   node that generated the error as the source address and with the
   link-layer address of the AERO node as the destination address.

   The IP header is followed by an ICMP header that includes an error
   Type, Code and Checksum.  Valid type values include "Destination
   Unreachable", "Time Exceeded" and "Parameter Problem"
   [RFC0792][RFC4443].  (OMNI interfaces ignore link-layer IPv4
   "Fragmentation Needed" and IPv6 "Packet Too Big" messages for carrier
   packets that are no larger than the minimum/path MPS as discussed in
   Section 3.9, however these messages may provide useful hints of probe
   failures during path MPS probing.)

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

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   possible without the length of the ICMP datagram exceeding 576
   bytes".

   The link-layer error message format is shown in Figure 5:

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

         Figure 5: OMNI Interface Link-Layer Error Message Format

   The AERO node rules for processing these link-layer error messages
   are as follows:

   o  When an AERO node receives a link-layer Parameter Problem message,
      it processes the message the same as described as for ordinary
      ICMP errors in the normative references [RFC0792][RFC4443].

   o  When an AERO node receives persistent link-layer Time Exceeded
      messages, the IP ID field may be wrapping before earlier fragments
      awaiting reassembly have been processed.  In that case, the node
      should begin including integrity checks and/or institute rate
      limits for subsequent packets.

   o  When an AERO node receives persistent link-layer Destination
      Unreachable messages in response to carrier packets that it sends
      to one of its neighbor correspondents, the node should process the

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      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.

   o  When an AERO Client receives persistent link-layer Destination
      Unreachable messages in response to carrier packets that it sends
      to one of its neighbor Proxy/Servers, the Client should mark the
      path as unusable and use another path.  If it receives Destination
      Unreachable messages on many or all paths, the Client should
      associate with a new Proxy/Server and release its association with
      the old Proxy/Server as specified in Section 3.16.5.

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

   o  When an AERO Proxy/Server receives link-layer Destination
      Unreachable messages in response to a carrier packet that it sends
      to one of its permanent neighbors, it treats the messages as an
      indication that the path to the neighbor may be failing.  However,
      the dynamic routing protocol should soon reconverge and correct
      the temporary outage.

   When an AERO Bridge receives a carrier packet for which the network-
   layer destination address is covered by an MSP, the Bridge drops the
   packet if there is no more-specific routing information for the
   destination and returns an OMNI interface Destination Unreachable
   message subject to rate limiting.

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

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

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3.12.  AERO Router Discovery, Prefix Delegation and Autoconfiguration

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

3.12.1.  AERO Service Model

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

   AERO Clients and Proxy/Servers use ND messages to maintain neighbor
   cache entries.  AERO Proxy/Servers configure their OMNI interfaces as
   advertising NBMA interfaces, and therefore send unicast RA messages
   with a short Router Lifetime value (e.g., ReachableTime seconds) in
   response to a Client's RS message.  Thereafter, Clients send
   additional RS messages to keep Proxy/Server state alive.

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

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

3.12.2.  AERO Client Behavior

   AERO Clients discover the addresses of candidate FHS Proxy/Servers in
   a similar manner as described in [RFC5214].  Discovery methods
   include static configuration (e.g., from a flat-file map of Proxy/
   Server addresses and locations), or through an automated means such

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   as Domain Name System (DNS) name resolution [RFC1035].
   Alternatively, the Client can discover Proxy/Server addresses through
   a layer 2 data link login exchange, or through a unicast RA response
   to a multicast/anycast RS as described below.  In the absence of
   other information, the Client can resolve the DNS Fully-Qualified
   Domain Name (FQDN) "linkupnetworks.[domainname]" where
   "linkupnetworks" is a constant text string and "[domainname]" is a
   DNS suffix for the OMNI link (e.g., "example.com").

   To associate with a FHS Proxy/Server over an underlying interface,
   the Client acts as a requesting router to request MNPs by preparing
   an RS message with prefix management parameters.  If the Client
   already knows the Proxy/Server's ADM-LLA, it includes the LLA as the
   network-layer destination address; otherwise, the Client includes the
   (link-local) All-Routers multicast as the network-layer destination.
   If the Client already knows its own MNP-LLA, it can use the MNP-LLA
   as the network-layer source address and include an OMNI option with
   prefix registration information.  Otherwise, the Client uses the
   unspecified address (::) as the network-layer source address and
   includes prefix delegation parameters in the OMNI option (see:
   [I-D.templin-6man-omni]).

   The Client next includes Interface Attributes corresponding to the
   underlying interface over which it will send the RS message, and MAY
   include additional Interface Attributes specific to other underlying
   interfaces.  Next, the Client submits the RS for OAL encapsulation
   and fragmentation if necessary with its own MNP-ULA and the Proxy/
   Server's ADM-ULA or (site-scoped) All-Routers multicast as the OAL
   addresses while selecting an Identification value and invoking window
   synchronization as specified in [I-D.templin-6man-omni].

   The Client then sends the RS (either directly via Direct interfaces,
   via a VPN for VPNed interfaces, via an access router for ANET
   interfaces or via INET encapsulation for INET interfaces) then waits
   up to RetransTimer milliseconds for an RA message reply (see
   Section 3.12.3) (retrying up to MAX_RTR_SOLICITATIONS).  If the
   Client receives no RAs, or if it receives an RA with Router Lifetime
   set to 0, the Client SHOULD abandon attempts through the first
   candidate FHS Proxy/Server and try another Proxy/Server.  Otherwise,
   the Client processes the prefix information found in the RA message.

   When the Client processes an RA, it first performs OAL reassembly and
   decapsulation if necessary then creates a NCE with the Proxy/Server's
   ADM-LLA as the network-layer address and the Proxy/Server's
   encapsulation and/or link-layer addresses as the link-layer address.
   The Client then caches the FMT/SRT/LHS/L2ADDR information from the
   Interface Attributes for omIndex 0 included in the RA, and uses these
   values in the Interface Attributes it includes in any future IPv6 ND

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   messages while still associated with this Proxy/Server.  The Client
   next records the RA Router Lifetime field value in the NCE as the
   time for which the Proxy/Server has committed to maintaining the MNP
   in the routing system via this underlying interface, and caches the
   other RA configuration information including Cur Hop Limit, M and O
   flags, Reachable Time and Retrans Timer.  The Client then
   autoconfigures MNP-LLAs for any delegated MNPs and assigns them to
   the OMNI interface.  The Client also caches any MSPs included in
   Route Information Options (RIOs) [RFC4191] as MSPs to associate with
   the OMNI link, and assigns the MTU value in the MTU option to the
   underlying interface.

   The Client then registers its additional underlying interfaces with
   FHS Proxy/Servers for those interfaces discovered by sending RS
   messages via each additional interface as described above.  The RS
   messages include the same parameters as for the initial RS/RA
   exchange, but with destination address set to the Proxy/Server's ADM-
   LLA.  The Client finally sub-delegates the MNPs to its attached EUNs
   and/or the Client's own internal virtual interfaces as described in
   [I-D.templin-v6ops-pdhost] to support the Client's downstream
   attached "Internet of Things (IoT)".  The Client then sends
   additional RS messages over each underlying interface before the
   Router Lifetime received for that interface expires.

   After the Client registers its underlying interfaces, it may wish to
   change one or more registrations, e.g., if an interface changes
   address or becomes unavailable, if traffic selectors change, etc.  To
   do so, the Client prepares an RS message to send over any available
   underlying interface as above.  The RS includes an OMNI option with
   prefix registration/delegation information, with Interface Attributes
   specific to the selected underlying interface, and with any
   additional Interface Attributes specific to other underlying
   interfaces.  When the Client receives the Proxy/Server's RA response,
   it has assurance that the Proxy/Server has been updated with the new
   information.

   If the Client wishes to discontinue use of a Proxy/Server it issues
   an RS message over any underlying interface with an OMNI option with
   a prefix release indication.  When the Proxy/Server processes the
   message, it releases the MNP, sets the NCE state for the Client to
   DEPARTED and returns an RA reply with Router Lifetime set to 0.
   After a short delay (e.g., 2 seconds), the Proxy/Server withdraws the
   MNP from the routing system.

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3.12.3.  AERO Proxy/Server Behavior

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

   When an FHS Proxy/Server receives a prospective Client's RS message
   on its OMNI interface, it SHOULD return an immediate RA reply with
   Router Lifetime set to 0 if it is currently too busy or otherwise
   unable to service the Client.  Otherwise, the Proxy/Server performs
   OAL reassembly and decapsulation if necessary, then authenticates the
   RS message and processes the prefix delegation/registration
   parameters.  The Proxy/Server first determines the correct MNPs to
   provide to the Client by processing the MNP-LLA prefix parameters
   and/or the DHCPv6 OMNI sub-option.  When the Proxy/Server returns the
   MNPs, it also creates a forwarding table entry for the MNP-ULA
   corresponding to each MNP so that the MNPs are propagated into the
   routing system (see: Section 3.2.3).  For IPv6, the Proxy/Server
   creates an IPv6 forwarding table entry for each MNP.  For IPv4, the
   Proxy/Server creates an IPv6 forwarding table entry with the
   IPv4-compatibility MNP-ULA prefix corresponding to the IPv4 address.

   The Proxy/Server next creates a NCE for the Client using the base
   MNP-LLA as the network-layer address.  Next, the Proxy/Server updates
   the NCE by recording the information in each Interface Attributes
   sub-option in the RS OMNI option.  The Proxy/Server also records the
   actual OAL/*NET addresses and RS message window synchronization
   parameters (if any) in the NCE.

   Next, the Proxy/Server prepares an RA message using its ADM-LLA as
   the network-layer source address and the network-layer source address
   of the RS message as the network-layer destination address.  The
   Proxy/Server sets the Router Lifetime to the time for which it will
   maintain both this underlying interface individually and the NCE as a
   whole.  The Proxy/Server also sets Cur Hop Limit, M and O flags,
   Reachable Time and Retrans Timer to values appropriate for the OMNI
   link.  The Proxy/Server includes the MNPs, any other prefix
   management parameters and an OMNI option with an Interface Attributes
   sub-option with omIndex 0 and FMT/SRT/LHS/L2ADDR information for its
   INET interface and an Origin Indication sub-option with the mapped
   and obfuscated Port Number and IP address corresponding to the
   Client's RS encapsulation addresses.  The Proxy/Server then includes
   one or more RIOs that encode the MSPs for the OMNI link, plus an MTU

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   option (see Section 3.9).  The Proxy/Server finally forwards the
   message to the Client using OAL encapsulation/fragmentation if
   necessary while including an acknowledgement if the RS invoked window
   synchronization.

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

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

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

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

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3.12.3.1.  DHCPv6-Based Prefix Registration

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

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

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

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

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

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3.13.  The AERO Proxy Function

   Clients connect to the OMNI link via FHS Proxy/Servers, with one or
   more FHS Proxy/Servers for each underlying interface.  Each of the
   Client's FHS Proxy/Servers must be informed of all of the Client's
   additional underlying interfaces.  For Clients on Direct and VPNed
   underlying interfaces the Proxy/Server "A" for that interface is
   directly connected, for Clients on ANET underlying interfaces Proxy/
   Server "A" is located on the ANET/INET boundary, and for Clients on
   INET underlying interfaces Proxy/Server "A" is located somewhere in
   the connected Internetwork.  When the Client registers with Proxy/
   Server "A", it must also report the registration to any other Proxy/
   Servers for other underlying interfaces "B", "C", "D", etc. for which
   an underlying interface relationship has already been established.
   The Proxy/Server satisfies these requirements as follows:

   o  when FHS Proxy/Server "A" receives a Client RS message, it first
      verifies that the OAL Identification is within the window for the
      NCE that matches the MNP-ULA for this Client neighbor and
      authenticates the message.  (If no NCE was found, Proxy/Server "A
      instead creates one in the STALE state and returns an RA message
      with an authentication signature and any window synchronization
      parameters.)  Proxy/Server "A" then examines the network-layer
      destination address.  If the destination address is the ADM-LLA of
      a different Proxy/Server "B" (or, if the OMNI option included an
      MS-Register sub-option with the ADM-LLAs of one or more different
      "LHS" Proxy/Servers "B", "C", "D", etc.), Proxy/Server "A"
      prepares a separate proxyed version of the RS message with an OAL
      header with source set to its own ADM-ULA and destination set to
      the LHS Proxy/Server's ADM-ULA.  Proxy/Server "A" also overwrites
      the OMNI header Interface Attributes option supplied by the Client
      with its own FMT/SRT/LHS/L2ADDR information (note that the L2ADDR
      field size may need to be increased/decreased if the Client
      underlying interface IP version differs from its own INET
      interface).  Proxy/Server "A" then sets the S/T-omIndex to the
      value for this Client underlying interface, then forwards the
      message into the OMNI link secured spanning tree.

   o  when LHS Proxy/Server "B" receives the RS, it authenticates the
      message then creates or updates a NCE for the Client with FHS
      Proxy/Server "A"'s Interface Attributes as the link-layer address
      information for this S/T-omIndex and caches any window
      synchronization parameters supplied by the Client.  LHS Proxy/
      Server "B" then prepares an RA message with source set to its own
      LLA and destination set to the Client's MNP-LLA, and with any
      window synchronization acknowledgements.  Proxy/Server "B" then
      encapsulates the RA in an OAL header with source set to its own
      ADM-ULA and destination set to the ADM-ULA of Proxy/Server "A,

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      performs fragmentation if necessary, then sends the resulting
      carrier packets into the secured spanning tree.

   o  when Proxy/Server "A" reassembles the RA, it locates the Client
      NCE based on the RA destination LLA.  Proxy/Server "A" then re-
      encapsulates the RA message with OAL source set to its own ADM-ULA
      and OAL destination set to the MNP-ULA of the Client, includes an
      authentication signature if necessary, and includes an Interface
      Attribues sub-option with omIndex 0 and with FMT/SRT/LHS/L2ADDR
      information for its INET interface.  Proxy/Server "A" then
      fragments if necessary and returns the fragments to the Client.

   o  The Client repeats this process over each of its additional
      underlying interfaces while treating each Proxy/Server "B", "C",
      "D" as an FHS while providing MS-Register information for other
      Proxy/Servers as an LHS.

   After the initial RS/RA exchanges each Proxy/Server forwards any of
   the Client's carrier packets with OAL destinations for which there is
   no matching NCE to a Bridge using OAL encapsulation with its own ADM-
   ULA as the source and the destination determined by the ORH supplied
   by the Client.  The Proxy/Server instead forwards any carrier packets
   destined to a neighbor cache target directly to the target according
   to the OAL/link-layer information - the process of establishing
   neighbor cache entries is specified in Section 3.14.

   While the Client is still associated with each Proxy/Server "A", "A"
   can send NS, RS and/or unsolicited NA messages to update the neighbor
   cache entries of other AERO nodes on behalf of the Client and/or to
   convey Interface Attributes updates.  This allows for higher-
   frequency Proxy-initiated RS/RA messaging over well-connected INET
   infrastructure supplemented by lower-frequency Client-initiated RS/RA
   messaging over constrained ANET data links.

   If any Proxy/Server "B", "C", "D" ceases to send solicited RAs,
   Proxy/Server "A" sends unsolicited RAs to the Client with destination
   set to (link-local) All-Nodes multicast and with Router Lifetime set
   to zero to inform Clients that a Proxy/Server has failed.  Although
   Proxy/Server "A" can engage in ND exchanges on behalf of the Client,
   the Client can also send ND messages on its own behalf, e.g., if it
   is in a better position than "A" to convey Interface Attribute
   changes, etc.  The ND messages sent by the Client include the
   Client's MNP-LLA as the source in order to differentiate them from
   the ND messages sent by Proxy/Server "A".

   If the Client becomes unreachable over all underlying interface it
   serves, Proxy/Server "A" sets the NCE state to DEPARTED and retains
   the entry for DepartTime seconds.  While the state is DEPARTED,

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   Proxy/Server "A" forwards any carrier packets destined to the Client
   to a Bridge via OAL/ORH encapsulation.  When DepartTime expires,
   Proxy/Server "A" deletes the NCE and discards any further carrier
   packets destined to the former Client.

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

   If the ANET first-hop access router is on the same underlying link as
   the Client and recognizes the AERO/OMNI protocol, the Client can
   avoid OAL encapsulation for both its control and data messages.  When
   the Client connects to the link, it can send an unencapsulated RS
   message with source address set to its own MNP-LLA (or to a Temporary
   LLA), and with destination address set to the ADM-LLA of the Client's
   selected Proxy/Server or to (link-local) All-Routers multicast.  The
   Client includes an OMNI option formatted as specified in
   [I-D.templin-6man-omni].  The Client then sends the unencapsulated RS
   message, which will be intercepted by the AERO-Aware access router.

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

   Note: When a Proxy/Server alters the IPv6 ND message contents before
   forwarding (e.g., such as altering the OMNI option contents), the
   IPv6 ND message checksum and/or authentication signature are
   invalidated.  If the Proxy/Server forwards the message over the
   secured spanning tree, however, it need not re-calculate the
   checksum/signature since they will not be examined by the next hop.

   Note: The Proxy/Server can apply packing as discussed in
   [I-D.templin-6man-omni] if an opportunity arises to concatenate
   multiple original IP packets destined to the same neighbor.

3.13.1.  Detecting and Responding to Proxy/Server Failures

   In environments where fast recovery from Proxy/Server failure is
   required, Proxy/Server "A" SHOULD use proactive Neighbor
   Unreachability Detection (NUD) to track each peer Proxy/Server "B"

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   reachability in a similar fashion as for Bidirectional Forwarding
   Detection (BFD) [RFC5880].  Proxy/Server "A" can then quickly detect
   and react to failures so that cached information is re-established
   through alternate paths.  The NUD control messaging is carried only
   over well-connected ground domain networks (i.e., and not low-end
   aeronautical radio links) and can therefore be tuned for rapid
   response.

   Proxy/Server "A" performs proactive NUD with peer Proxy/Server "B"
   for which there are currently active Clients by sending continuous NS
   messages in rapid succession, e.g., one message per second.  Proxy/
   Server "A" sends the NS message via the spanning tree with its own
   ADM-LLA as the source and the ADM-LLA of the peer Proxy/Server "B" as
   the destination.  When Proxy/Server "A" is also sending RS messages
   to the peer Proxy/Server "B" on behalf of ANET Clients, the resulting
   RA responses can be considered as equivalent hints of forward
   progress.  This means that Proxy/Server "B" need not also send a
   periodic NS if it has already sent an RS within the same period.  If
   the peer Proxy/Server "B" fails (i.e., if "A" ceases to receive
   advertisements), Proxy/Server "A" can quickly inform Clients by
   sending multicast RA messages on the ANET interface.

   Proxy/Server "A" sends RA messages on the ANET interface with source
   address set to Proxy/Server "B"'s address, destination address set to
   (link-local) All-Nodes multicast, and Router Lifetime set to 0.
   Proxy/Server "A" SHOULD send MAX_FINAL_RTR_ADVERTISEMENTS RA messages
   separated by small delays [RFC4861].  Any Clients on the ANET that
   had been using the failed Proxy/Server "B" will receive the RA
   messages and associate with a new Proxy/Server.

3.13.2.  Point-to-Multipoint Proxy/Server Coordination

   In environments where Client messaging over ANETs is bandwidth-
   limited and/or expensive, Clients can enlist the services of Proxy/
   Server "A" to coordinate with multiple Proxy/Servers "B", "C", "D"
   etc. in a single RS/RA message exchange.  The Client can send a
   single RS message to (link-local) All-Routers multicast that includes
   the ID's of multiple Proxy/Servers in MS-Register sub-options of the
   OMNI option.

   When Proxy/Server "A" receives the RS and processes the OMNI option,
   it sends a separate RS to each MS-Register Proxy/Server ID.  When
   Proxy/Server "A" receives an RA, it can optionally return an
   immediate "singleton" RA to the Client or record the Proxy/Server's
   ID for inclusion in a pending "aggregate" RA message.  Proxy/Server
   "A" can then return aggregate RA messages to the Client including
   multiple Proxy/Server IDs in order to conserve bandwidth.  Each RA
   includes a proper subset of the Proxy/Server IDs from the original RS

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   message, and Proxy/Server "A" must ensure that the message contents
   of each RA are consistent with the information received from the
   (aggregated) additional Proxy/Servers.

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

   When the Client sends an RS with window synchronization parameters
   and with multiple MS-Register Proxy/Server IDs, Proxy/Server "A" may
   receive multiple RAs - each with their own window synchronization
   parameters.  Proxy/Server "A" must then immediately forward these RAs
   to the Client as singletons instead of including them in an
   aggregate, and the Client will use each RA to establish a separate
   NCE and window for each individual Proxy/Server.

   The OMNI interface specification [I-D.templin-6man-omni] provides
   further discussion of the RS/RA messaging involved in point-to-
   multipoint coordination.

3.14.  AERO Route Optimization

   AERO nodes invoke route optimization when they need to forward
   packets to new target destinations.  Route optimization is based on
   IPv6 ND Address Resolution messaging between a Route Optimization
   Source (ROS) and Route Optimization Responder (ROR).  Route
   optimization is initiated by the first eligible ROS closest to the
   source as follows:

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

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

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

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

   The route optimization procedure is conducted between the ROS and an
   LHS Proxy/Server/Relay for the target selected by routing as the ROR.
   In this arrangement, the ROS is always the Client or Proxy/Server/
   Relay nearest the source over the selected source underlying

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   interface, while the ROR is always an LHS Proxy/Server/Relay for the
   target regardless of the target underlying interface.

   The AERO routing system directs a route optimization solicitation
   sent by the ROS to the nearest available ROR, which returns a route
   optimization reply.  The exact ROR selected is unimportant; all that
   matters is that the route optimization information returned must be
   current and authentic.  The ROS is responsible for periodically
   refreshing the route optimization, and the ROR is responsible for
   quickly informing the ROS of any changes.

   The procedures are specified in the following sections.

3.14.1.  Route Optimization Initiation

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

   The ROS next places the original IP packet on a short queue then
   sends an NS message for Address Resolution (NS(AR)) to receive a
   solicited NA(AR) message from an ROR.  The NS(AR) message must be
   sent securely, and includes:

   o  the LLA of the ROS as the source address.

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

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

   The NS(AR) message also includes an OMNI option with an Interface
   Attributes entry for the underlying interface, with S/T-omIndex set
   to the underlying interface index and with Preflen set to the prefix
   length associated with the NS(AR) source.  The ROS then selects an
   Identification value submits the NS(AR) message for OAL encapsulation
   with OAL source set to its own ULA and OAL destination set to the ULA
   corresponding to the target.  (The ROS does not include any window
   synchronization parameters, since it will never exchange other
   carrier packet types directly with the ROR).

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   The ROS then sends the resulting carrier packet(s) into the SRT
   secured spanning tree without decrementing the network-layer TTL/Hop
   Limit field.  (When the ROS is an INET Client, it instead sends the
   resulting carrier packets to the ADM-ULA of one of its current Proxy/
   Servers.  The Proxy/Server reassembles if necessary, verifies the
   NS(AR) signature, then re-encapsulates with the OAL source set to its
   own ADM-ULA and OAL destination set to the ULA corresponding to the
   target.  The Proxy/Server then fragments if necessary and sends the
   resulting carrier packets into the secured spanning tree on behalf of
   the Client.)

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

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

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

   When an LHS Proxy/Server (or Relay) for the target receives the
   secured carrier packet(s), it reassembles if necessary then examines
   the NS(AR) target to determine whether it has a matching NCE and/or
   non-MNP route.  If there is no match, the Proxy/Server drops the
   message.  Otherwise, the LHS Proxy/Server/Relay continues processing
   as follows:

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

   o  If the NS(AR) target matches the MNP-LLA of a Client NCE in the
      REACHABLE state, the Proxy/Server makes note of whether the NS
      (AR) arrived from the secured or unsecured spanning tree then acts
      as an ROR to provide route optimization information on behalf of
      the Client.  (Note that if the message arrived via the secured
      spanning tree the ROR need not perform further authentication, but
      if it arrived over an open INET underlying interface it must
      verify the message authentication signature before accepting.)

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   o  If the NS(AR) target matches one of its non-MNP routes, the Relay
      acts as both an ROR and a route optimization target, since the
      Relay forwards IP packets toward the (fixed network) target at the
      network layer.

   The ROR next checks the target NCE for a Report List entry that
   matches the NS(AR) source LLA/ULA of the ROS.  If there is a Report
   List entry, the ROR refreshes ReportTime for this ROR; otherwise, the
   ROR creates a new entry for the ROS and records both the LLA and ULA.

   The ROR then prepares a (solicited) NA(AR) message to return to the
   ROS with the source address set to its own ADM-LLA, the destination
   address set to the NS(AR) LLA source address and the Target Address
   set to the target Client's MNP-LLA.  The ROR then includes an OMNI
   option with Preflen set to the prefix length associated with the
   NA(AR) source address.  The ROR next includes Interface Attributes in
   the OMNI option for all of the target's underlying interfaces with
   current information for each interface.

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

   The ROR then sets the NA(AR) message R flag to 1 (as a router) and S
   flag to 1 (as a response to a solicitation) and sets the O flag to 0
   (as a proxy) and sets the OMNI header S/T-omIndex to 0.  The ROR
   finally submits the NA(AR) for OAL encapsulation with source set to
   its own ULA and destination set to the same ULA that appeared in the
   NS(AR) OAL source, then performs OAL encapsulation and fragmentation
   using the same Identification value that appeared in the NS(AR) and
   finally forwards the resulting (*NET-encapsulated) carrier packets
   via the secured spanning tree without decrementing the network-layer
   TTL/Hop Limit field.

3.14.4.  Relaying the NA(AR)

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

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3.14.5.  Processing the NA(AR)

   When the ROS receives the NA(AR) message from the ROR, it first
   searches for a NCE that matches the NA(AR) target address.  The ROS
   then processes the message the same as for standard IPv6 Address
   Resolution [RFC4861].  In the process, it caches all OMNI option
   information in the target NCE (including all Interface Attributes),
   and caches the NA(AR) ADM-{LLA,ULA} source addresses as the addresses
   of the ROR.  If the ROS receives additional NA(AR) or uNA messages
   for this target Client with the same ADM-LLA source address but a
   different ADM-ULA source address, it caches the new MSID as the new
   ADM-{LLA,ULA} and deprecates the former ADM-{LLA,ULA}.

   When the ROS is a Client, the solicited NA(AR) message will first be
   delivered via the SRT secured spanning tree to the Proxy/Server that
   forwarded the NS(AR), which reassembles if necessary.  The Proxy/
   Server then forwards the message to the Client while re-encapsulating
   and re-fragmenting if necessary.  If the Client is on a well-managed
   ANET, physical security and protected spectrum ensures security for
   the unmodified NA(AR); if the Client is on the open INET the Proxy/
   Server must instead include an authentication signature (while
   adjusting the OMNI option size, if necessary).  The Proxy/Server uses
   its own ADM-ULA as the OAL source and the MNP-ULA of the Client as
   the OAL destination.

3.14.6.  Forwarding Packets to Route Optimized Targets

   After the ROS receives the route optimization NA(AR) and updates the
   target NCE, it sends additional NS(AR) messages to the ADM-ULA of the
   ROR to refresh the NCE ReachableTime before expiration as long as
   there is continued interest in this target.  While the NCE remains
   REACHABLE, the ROS can forward packets along the best underlying
   interface paths based on the target's Interface Attributes.  The ROS
   selects target underlying interfaces according to traffic selectors
   and/or any other traffic discriminators, however each underlying
   interface selected must first establish window synchronization state
   if necessary.

   To establish window synchronization state, the ROS performs a secured
   unicast NS/NA(WIN) exchange with window synchronization parameters
   according to the Interface Attributes FMT code.  If FMT-Forward is
   set, the ROS prepares an NS(WIN) with its own LLA as the source and
   the MNP-LLA of the target Client as the destination; otherwise, it
   sets the ADM-LLA of the LHS Proxy/Server as the destination.  The ROS
   then encapsulates the NS(WIN) in an OAL header with its own ULA as
   the source.  If the ROS is the Client, it sets the OAL destination to
   the ADM-ULA of its FHS Proxy/Server, includes an authentication
   signature if necessary, and includes an ORH-1 with FMT-Type clear for

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   the first fragment.  The Client sets the ORH Segments Left to 1 and
   includes valid SRT/LHS information for the LHS Proxy/Server with
   L2ADDR set to 0, then forwards the NS(WIN) to its FHS Proxy/Server
   which must reassemble and verify the authentication signature if
   necessary.  The FHS Proxy/Server then re-encapsulates, re-fragments
   and forwards the NS(WIN) carrier packets into the secured spanning
   tree with its own ADM-ULA as the OAL source and the ADM-ULA of the
   LHS Proxy/Server as the OAL destination while replacing the ORH-1
   with an ORH-0.  (If the ROS was the FHS Proxy/Server itself, it
   instead includes an ORH-0, and forwards the carrier packets into the
   secured spanning tree.)

   When an LHS Proxy/Server receives the NS(WIN) it first reassembles if
   necessary.  If the NS(WIN) destination is its own ADM-LLA, the LHS
   Proxy/Server creates an NCE based on the NS(WIN) source LLA, caches
   the window synchronization information, and prepares an NA(WIN) while
   using its own ADM-LLA as the source and the ROS LLA as the
   destination.  The LHS Proxy/Server then encapsulates the NA(WIN) in
   an OAL header with source set to its own ADM-ULA and destination set
   to the NS(WIN) OAL source.  The LHS Proxy/Server then fragments if
   necessary includes an ORH-0 with omIndex set to the S/T-omIndex value
   found in the NS(WIN) OMNI option, then forwards the resulting carrier
   packets into the secured spanning tree which will deliver them to the
   ROS Proxy/Server.

   If the NS(WIN) destination is the MNP-LLA of the target Client, the
   LHS Proxy/Server instead removes the ORH-0, re-encapsulates using the
   same OAL source and the MNP-ULA of the target as the OAL destination
   and includes an authentication signature (while adjusting the OMNI
   option size) if necessary.  The LHS Proxy/Server then forwards the
   NS(WIN) to the target over the underlying interface identified by the
   ORH-0 omIndex (or, over any underlying interface if omIndex is 0).
   When the target receives the NS(WIN), it verifies the authentication
   signature if necessary then creates an NCE for the ROS LLA, caches
   the window synchronization information and prepares an NA(WIN) to
   return to the ROS with its MNP-LLA as the source and the LLA of the
   ROS as the destination, and with an authentication signature if
   necessary.  The target Client then encapsulates the NA(WIN) in an OAL
   header with its own MNP-ULA as the source, the ADM-ULA of the LHS
   Proxy/Server as the destination, and with an ORH-1 with SRT/LHS
   information copied from the ADM-ULA of the FHS Proxy/Server found in
   the NS(WIN) OAL source address.  The target Client then sets the
   ORH-1 omIndex to the S/T-omIndex value found in the NS(WIN) OMNI
   option, then forwards the message to the LHS Proxy/Server.

   When the LHS Proxy/Server receives the message, it reassembles if
   necessary, verifies the authentication signature if necessary then
   re-encapsulates using its own ADM-ULA as the source and the ADM-ULA

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   of the FHS Proxy/Server as the destination The LHS Proxy/Server then
   re-fragments and forwards the NS(WIN) carrier packets into the
   spanning tree while replacing the ORH-1 with an ORH-0.  When the FHS
   Proxy/Server receives the NA(WIN), it reassembles if necessary then
   updates the target NCE based on the message contents if the Proxy/
   Server itself is the ROS.  If the NS(WIN) source was the ADM-LLA of
   the LHS Proxy/Server, the ROS must create and maintain a NCE for the
   LHS Proxy/Server which it must regard as a companion to the existing
   MNP-LLA NCE for the target Client.  (The NCE for the LHS Proxy/Server
   can also be shared by multiple target Client NCEs if the ROS
   communicates with multiple active targets located behind the same LHS
   Proxy/Server.)  If the Client is the ROS, the FHS Proxy/Server
   instead inserts an authentication signature (while adjusting the OMNI
   option size) if necessary, removes the ORH-0 then re-encapsulates and
   re-fragments if necessary while changing the OAL destination to the
   MNP-ULA of the Client.  The FHS Proxy/Server then forwards the
   NA(WIN) to the Client over the underlying interface identified by the
   ORH-0 omIndex which then updates its own NCE based on the target
   Client MNP-LLA or LHS Proxy/Server ADM-LLA.  The ROS (whether the
   Proxy/Server or the Client itself) finally arranges to return an
   acknowledgement if requested by the NA(WIN).

   After window synchronization state has been established, the ROS can
   begin forwarding carrier packets as specified in Section 3.2.7 while
   performing additional NS/NA(WIN) exchanges as above to update window
   state and/or test reachability.  These forwarding procedures apply to
   the case where the selected target interface SRT/LHS codes indicate
   that the interface is located in a foreign OMNI link segment.  In
   that case, the ROS must include ORHs and send the resulting carrier
   packets into the spanning tree.

   If the SRT/LHS codes indicate that the interface is in the local OMNI
   link segment, the ROS can instead forward carrier packets directly to
   the LHS Proxy/Server using the L2ADDR for encapsulation, or even to
   the target Client itself while invoking NAT traversal if necessary.
   When the ROS sends packets directly to the LHS Proxy/Server, it
   includes an ORH-0 if necessary to inform the Proxy/Server as to
   whether it must reassemble and/or the correct target Client interface
   for (re)forwarding.  If the LHS Proxy/Server is required to
   reassemble, the ROS sets the OAL destination to the ADM-ULA of the
   LHS Proxy/Server; otherwise, it sets the OAL destination to the MNP-
   ULA of the target Client itself.  When the ROS sends packets directly
   to the target Client, it need not include an ORH.  The LHS Proxy/
   Server (or target Client) then saves the L2ADDR and full OAL
   addresses in the ROS NCE, and the ROS can begin applying OAL header
   compression in subsequent carrier packets as specified in
   [I-D.templin-6man-omni] since the OAL header is not examined by any
   forwarding nodes in the path.

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   While the ROS continues to actively forward packets to the target
   Client, it is responsible for updating window synchronization state
   and per-interface reachability before expiration.  Window
   synchronization state is shared by all underlying interfaces in the
   ROS' NCE that use the same destination LLA so that a single NS/
   NA(NUD) exchange applies for all interfaces regardless of the
   (single) interface used to conduct the exchange.  However, the window
   synchronization exchange only confirms target Client reachability
   over the specific interface used to conduct the exchange.
   Reachability for other underlying interfaces that share the same
   window synchronization state must be determined individually using
   NS/NA(NUD) messages which need not be secured as long as they use in-
   window Identifications and do not update other state information.

3.15.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform Neighbor Unreachability Detection (NUD) per
   [RFC4861] either reactively in response to persistent link-layer
   errors (see Section 3.11) or proactively to confirm reachability.
   The NUD algorithm is based on periodic control message exchanges and
   may further be seeded by ND hints of forward progress, but care must
   be taken to avoid inferring reachability based on spoofed
   information.  For example, IPv6 ND message exchanges that include
   authentication codes and/or in-window Identifications may be
   considered as acceptable hints of forward progress, while spurious
   random carrier packets should be ignored.

   AERO nodes can perform NS/NA(NUD) exchanges over the OMNI link
   secured spanning tree (i.e. the same as described above for NS/
   NA(WIN)) to test reachability without risk of DoS attacks from nodes
   pretending to be a neighbor.  These NS/NA(NUD) messages use the
   unicast LLAs and ULAs of the parties involved in the NUD test the
   same as for standard IPv6 ND over the secured spanning tree.  When
   only reachability information is required without updating any other
   NCE state, AERO nodes can instead perform NS/NA(NUD) exchanges
   directly between neighbors without employing the secured spanning
   tree as long as they include in-window Identifications and an
   authentication signature or checksum.

   When an ROR directs an ROS to a target neighbor with one or more
   link-layer addresses, the ROS probes each unsecured target underlying
   interface either proactively or on-demand of carrier packets directed
   to the path by multilink forwarding to maintain the interface's state
   as reachable.  Probing is performed through NS(NUD) messages over the
   SRT secured or unsecured spanning tree, or through NS(NUD) messages
   sent directly to an underlying interface of the target itself.  While
   testing a target underlying interface, the ROS can optionally
   continue to forward carrier packets via alternate interfaces and/or

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   maintain a small queue of carrier packets until target reachability
   is confirmed.

   NS(NUD) messages are encapsulated, fragmented and transmitted as
   carrier packets the same as for ordinary original IP data packets,
   however the encapsulated destinations are the LLA of the ROS and
   either the ADM-LLA of the LHS Proxy/Server or the MNP-LLA of the
   target itself.  The ROS encapsulates the NS(NUD) message the same as
   described in Section 3.2.7, however Destination Suffixes (if present)
   are set according to the LLA destination (i.e., and not a ULA/GUA
   destination).  The ROS sets the NS(NUD) OMNI header S/T-omIndex to
   identify the underlying interface used for forwarding (or to 0 if any
   underlying interface can be used).  The ROS also includes an ORH with
   FMT/SRT/LHS/LLADDR information the same as for ordinary data packets,
   but does not include an authentication signature.  The ROS then
   fragments the OAL packet and forwards the resulting carrier packets
   into the unsecured spanning tree or directly to the target (or LHS
   Proxy/Server) if it is in the local segment.

   When the target (or LHS Proxy/Server) receives the NS(NUD) carrier
   packets, it verifies that it has a NCE for this ROS and that the
   Identification is in-window, then submits the carrier packets for
   reassembly.  The node then verifies the authentication signature or
   checksum, then searches for Interface Attributes in its NCE for the
   ROS that match the NS(NUD) S/T-omIndex and uses the FMT/SRT/LHS/
   L2ADDR information to prepare an ORH for the NA(NUD) reply.  The node
   then prepares the NA(NUD) with the source and destination LLAs
   reversed, encapsulates and sets the OAL source and destination, sets
   the NA(NUD) S/T-omIndex to the index of the underlying interface the
   NS(NUD) arrived on and sets the Target Address to the same value
   included in the NS(NUD).  The target next sets the R flag to 1, the S
   flag to 1 and the O flag to 1, then selects an in-window
   Identification for the ROS and performs fragmentation.  The node then
   forwards the carrier packets into the unsecured spanning tree,
   directly to the ROS if it is in the local segment or directly to a
   Bridge in the local segment.

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

   Note also that the exchange of NS/NA(NUD) messages has the useful
   side-benefit of opening holes in NATs that may be useful for NAT
   traversal.

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3.16.  Mobility Management and Quality of Service (QoS)

   AERO is a Distributed Mobility Management (DMM) service.  Each Proxy/
   Server is responsible for only a subset of the Clients on the OMNI
   link, as opposed to a Centralized Mobility Management (CMM) service
   where there is a single network mobility collective entity for all
   Clients.  Clients coordinate with their associated Proxy/Servers via
   RS/RA exchanges to maintain the DMM profile, and the AERO routing
   system tracks all current Client/Proxy/Server peering relationships.

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

   Mobility management messaging is based on the transmission and
   reception of unsolicited Neighbor Advertisement (uNA) messages.  Each
   uNA message sets the IPv6 source address to the LLA of the ROR and
   the destination address to the unicast LLA of the ROS.

   Mobility management considerations are specified in the following
   sections.

3.16.1.  Mobility Update Messaging

   RORs accommodate Client mobility and/or multilink change events by
   sending secured uNA messages to each ROS in the target Client's
   Report List.  When an ROR sends a uNA message, it sets the IPv6
   source address to the its own LLA, sets the destination address to
   the ROS LLA (i.e., an MNP-LLA if the ROS is a Client and an ADM-LLA
   if the ROS is a Proxy/Server) and sets the Target Address to the
   Client's MNP-LLA.  The ROR also includes an OMNI option with Preflen
   set to the prefix length associated with the Client's MNP-LLA, with
   Interface Attributes for the target Client's underlying interfaces
   and with the OMNI header S/T-omIndex set to 0.  The ROR then sets the
   uNA R flag to 1, S flag to 0 and O flag to 1, then encapsulates the
   message in an OAL header with source set to its own ADM-ULA and
   destination set to the ROS ULA (i.e., the ADM-ULA of the ROS Proxy/
   Server) and sends the message into the secured spanning tree.

   As discussed in Section 7.2.6 of [RFC4861], the transmission and
   reception of uNA messages is unreliable but provides a useful
   optimization.  In well-connected Internetworks with robust data links

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   uNA messages will be delivered with high probability, but in any case
   the Proxy/Server can optionally send up to MAX_NEIGHBOR_ADVERTISEMENT
   uNAs to each ROS to increase the likelihood that at least one will be
   received.  Alternatively, the Proxy/Server can set the PNG flag in
   the uNA OMNI option header to request a solicited NA acknowledgement
   as specified in [I-D.templin-6man-omni].

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

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

3.16.2.  Announcing Link-Layer Address and/or QoS Preference Changes

   When a Client needs to change its underlying Interface Attributes
   (e.g., due to a mobility event), the Client requests one of its
   Proxy/Servers to send uNA or RS messages to all of its other Proxy/
   Servers via the secured spanning tree with an OMNI option that
   includes Interface Attributes with the new link quality and address
   information.

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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.

   When the Proxy/Server receives the Client's changes, it sends uNA
   messages to all nodes in the Report List the same as described in the
   previous section.

3.16.3.  Bringing New Links Into Service

   When a Client needs to bring new underlying interfaces into service
   (e.g., when it activates a new data link), it sends an RS message to
   the Proxy/Server via the underlying interface with an OMNI option
   that includes Interface Attributes with appropriate link quality
   values and with link-layer address information for the new link.

3.16.4.  Deactivating Existing Links

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

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

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

3.16.5.  Moving Between Proxy/Servers

   The Client performs the procedures specified in Section 3.12.2 when
   it first associates with a new FHS Proxy/Server or renews its
   association with an existing Proxy/Server.  The Client also includes
   MS-Release identifiers in the RS message OMNI option per
   [I-D.templin-6man-omni] if it wants the new Proxy/Server to notify
   any old Proxy/Servers from which the Client is departing.

   When the new FHS Proxy/Server receives the Client's RS message, it
   returns an RA as specified in Section 3.12.3 and sends RS messages to
   any old Proxy/Servers listed in OMNI option MS-Release identifiers.

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   When the new Proxy/Server sends an RS message, it sets the source to
   the MNP-LLA of the Client and sets the destination to the ADM-LLA of
   the old Proxy/Server.  The new Proxy/Server also includes an OMNI
   option with Preflen set to the prefix length associated with the
   Client's MNP-LLA, with Interface Attributes for its own underlying
   interface, and with the OMNI header S/T-omIndex set to 0.  The new
   Proxy/Server then encapsulates the message in an OAL header with
   source set to its own ADM-ULA and destination set to the ADM-ULA of
   the old Proxy/Server and sends the message into the secured spanning
   tree.

   When an old Proxy/Server receives the RS, it notices that the message
   appears to have originated from the Client's MNP-LLA but that the S/
   T-omIndex is 0.  The old Proxy/Server then changes the Client's NCE
   state to DEPARTED, sets the link-layer address of the Client to the
   new Proxy/Server's ADM-ULA, and resets DepartTime.  The old Proxy/
   Server then returns an RA message via the secured spanning tree by
   reversing the LLA and ULA addresses found in the RS message.  After a
   short delay (e.g., 2 seconds) the old Proxy/Server withdraws the
   Client's MNP from the routing system.  After DepartTime expires, the
   old Proxy/Server deletes the Client's NCE.

   The old Proxy/Server also iteratively sends uNA messages to each ROS
   in the Client's Report List with OAL source address set to the ADM-
   ULA of the new Proxy/Server and OAL destination address set to the
   ULA of the ROS.  When the ROS receives the uNA, it examines the uNA
   Target Address to locate the target Client's NCE and the LLA source
   address to identify the old Proxy/Server.  The ROS then caches the
   MSID found in the ULA source address as the ADM-{LLA/ULA} for the new
   Proxy/Server for this target NCE and marks the entry as STALE.  While
   in the STALE state, the ROS allows new carrier packets to flow
   according to any alternate reachable underlying interfaces and sends
   new NS(AR) messages using its own ULA as the OAL source and the ADM-
   ULA of the new Proxy/Server as the OAL destination address to elicit
   NA(AR) messages that reset the NCE state to REACHABLE.

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

   When a Client moves to a new Proxy/Server, some of the carrier
   packets of a multiple fragment OAL packet may have already arrived at
   the old Proxy/Server while others are en route to the new Proxy/
   Server, however no special attention in the reassembly algorithm is
   necessary since all carrier packets will eventually arrive at the

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   Client which can then reassemble.  However, any carrier packets that
   are somehow lost can often be recovered through retransmissions.

3.17.  Multicast

   The AERO Client provides an IGMP (IPv4) [RFC2236] or MLD (IPv6)
   [RFC3810] proxy service for its EUNs and/or hosted applications
   [RFC4605].  Proxy/Servers act as a Protocol Independent Multicast -
   Sparse-Mode (PIM-SM, or simply "PIM") Designated Router (DR)
   [RFC7761].  AERO Relays also act as PIM routers (i.e., the same as
   AERO Proxys/Servers) on behalf of nodes on INET/EUN networks.

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

   Clients on ANET underlying interfaces without native multicast
   services instead send NS(AR) messages to cause their FHS Proxy/Server
   to act as an ROS and forward the message to an LHS Proxy/Server ROR.
   Clients on INET interfaces act as an ROS on their own behalf and
   forward NS(AR) messages directly to the LHS Proxy/Server ROR (i.e.,
   via the FHS Proxy/Server as a proxy).  When the Client receives an
   NA(AR) response, it initiates PIM protocol messaging according to the
   Source-Specific Multicast (SSM) and Any-Source Multicast (ASM)
   operational modes as discussed in the following sections.

3.17.1.  Source-Specific Multicast (SSM)

   When an ROS "X" (i.e., either a ROS Client or its FHS Proxy Server)
   acting as PIM router receives a Join/Prune message from a node on its
   downstream interfaces containing one or more ((S)ource, (G)roup)
   pairs, it updates its Multicast Routing Information Base (MRIB)
   accordingly.  For each S belonging to a prefix reachable via X's non-
   OMNI interfaces, X then forwards the (S, G) Join/Prune to any PIM
   routers on those interfaces per [RFC7761].

   For each S belonging to a prefix reachable via X's OMNI interface, X
   sends an NS(AR) message (see: Section 3.14) using its own LLA as the
   source address and the LLA of S as the destination address.  X then
   encapsulates the NS(AR) in an OAL header with source address set to
   the ULA of X and destination address set to the solicited node
   multicast address for S, then forwards the message into the secured
   spanning tree, which delivers it to ROR "Y" that services S.  The
   resulting NA(AR) will return the LLA for the prefix that matches S as
   the network-layer source address and with an OMNI option with

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   interface attributes for any underlying interfaces that are currently
   servicing S.

   When X processes the NA(AR) it selects one or more underlying
   interfaces for S and performs an NS/NA(WIN) exchange while including
   a PIM Join/Prune message for each multicast group of interest in the
   OMNI option.  If S is located behind any Proxys "Z"*, each Z* then
   updates its MRIB accordingly and maintains the LLA of X as the next
   hop in the reverse path.  Since the Bridges do not examine network
   layer control messages, 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.

   At some later time, Client C that holds an MNP for source S may
   depart from a first Proxy/Server Z1 and/or connect via a new Proxy/
   Server Z2.  In that case, Y sends a uNA message to X the same as
   specified for unicast mobility in Section 3.16.  When X receives the
   uNA message, it updates its NCE for the LLA for source S and sends
   new Join messages to any new Proxys Z2.  There is no requirement to
   send any Prune messages to old 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 time out since
   no new Joins will arrive.

   After some later time, C may move to a new Proxy/Server Y2 and depart
   from old Sever Y1.  In that case, Y1 sends Join messages for any of
   C's active (S,G) groups to Y2 while including its own LLA as the
   source address.  This causes Y2 to include Y1 in the multicast
   forwarding tree during the interim time that Y1's NCE for C is in the
   DEPARTED state.  At the same time, Y1 sends a uNA message to X with
   an OMNI option with S/T-omIndex set to 0 and a release indication to
   cause X to release its NCE for S.  X then sends a new Join message to
   S via the secured spanning tree and re-initiates route optimization
   the same as if it were receiving a fresh Join message from a node on
   a downstream link.

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3.17.2.  Any-Source Multicast (ASM)

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

   For each source S that sends multicast traffic to group G via R, the
   Proxy/Server Z* for the Client that aggregates S encapsulates the
   original IP packets in PIM Register messages and forwards them to R
   via the secured spanning tree, which may then elect to send a PIM
   Join to Z*. This will result in an (S,G) tree rooted at Z* with R as
   the next hop so that R will begin to receive two copies of the
   original IP packet; one native copy from the (S, G) tree and a second
   copy from the pre-existing (*, G) tree that still uses PIM Register
   encapsulation.  R can then issue a PIM Register-stop message to
   suppress the Register-encapsulated stream.  At some later time, if C
   moves to a new Proxy/Server Z*, it resumes sending original IP
   packets via PIM Register encapsulation via the new Z*.

   At the same time, as multicast listeners discover individual S's for
   a given G, they can initiate an (S,G) Join for each S under the same
   procedures discussed in Section 3.17.1.  Once the (S,G) tree is
   established, the listeners can send (S, G) Prune messages to R so
   that multicast original IP packets for group G sourced by S will only
   be delivered via the (S, G) tree and not from the (*, G) tree rooted
   at R.  All mobility considerations discussed for SSM apply.

3.17.3.  Bi-Directional PIM (BIDIR-PIM)

   Bi-Directional PIM (BIDIR-PIM) [RFC5015] provides an alternate
   approach to ASM that treats the Rendezvous Point (RP) as a Designated
   Forwarder (DF).  Further considerations for BIDIR-PIM are out of
   scope.

3.18.  Operation over Multiple OMNI Links

   An AERO Client can connect to multiple OMNI links the same as for any
   data link service.  In that case, the Client maintains a distinct
   OMNI interface for each link, e.g., 'omni0' for the first link,
   'omni1' for the second, 'omni2' for the third, etc.  Each OMNI link

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   would include its own distinct set of Bridges and Proxy/Servers,
   thereby providing redundancy in case of failures.

   Each OMNI link could utilize the same or different ANET connections.
   The links can be distinguished at the link-layer via the SRT prefix
   in a similar fashion as for Virtual Local Area Network (VLAN) tagging
   (e.g., IEEE 802.1Q) and/or through assignment of distinct sets of
   MSPs on each link.  This gives rise to the opportunity for supporting
   multiple redundant networked paths, with each VLAN distinguished by a
   different SRT "color" (see: Section 3.2.5).

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

   In a first alternative, if each OMNI link services different MSPs,
   then the Client can receive a distinct MNP from each of the links.
   IP routing will therefore assure that the correct OMNI link is used
   for both outbound and inbound traffic.  This can be accomplished
   using existing technologies and approaches, and without requiring any
   special supporting code in correspondent nodes or Bridges.

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

3.19.  DNS Considerations

   AERO Client MNs and INET correspondent nodes consult the Domain Name
   System (DNS) the same as for any Internetworking node.  When
   correspondent nodes and Client MNs use different IP protocol versions
   (e.g., IPv4 correspondents and IPv6 MNs), the INET DNS must maintain
   A records for IPv4 address mappings to MNs which must then be
   populated in Relay NAT64 mapping caches.  In that way, an IPv4
   correspondent node can send original IPv4 packets to the IPv4 address
   mapping of the target MN, and the Relay will translate the IPv4
   header and destination address into an IPv6 header and IPv6
   destination address of the MN.

   When an AERO Client registers with an AERO Proxy/Server, the Proxy/
   Server can return the address(es) of DNS servers in RDNSS options
   [RFC6106].  The DNS server provides the IP addresses of other MNs and
   correspondent nodes in AAAA records for IPv6 or A records for IPv4.

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3.20.  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 Bridges on each INET partition, with each Bridge
   distributing its MNPs and/or discovering non-MNP IP GUA prefixes on
   its INET links.

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

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

3.21.  Detecting and Reacting to Proxy/Server and Bridge Failures

   In environments where rapid failure recovery is required, Proxy/
   Servers and Bridges SHOULD use Bidirectional Forwarding Detection
   (BFD) [RFC5880].  Nodes that use BFD can quickly detect and react to
   failures so that cached information is re-established through
   alternate nodes.  BFD control messaging is carried only over well-
   connected ground domain networks (i.e., and not low-end radio links)
   and can therefore be tuned for rapid response.

   Proxy/Servers and Bridges maintain BFD sessions in parallel with
   their BGP peerings.  If a Proxy/Server or Bridge fails, BGP peers
   will quickly re-establish routes through alternate paths the same as
   for common BGP deployments.  Similarly, Proxys maintain BFD sessions
   with their associated Bridges even though they do not establish BGP
   peerings with them.

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3.22.  AERO Clients on the Open Internet

   AERO Clients that connect to the open Internet via INET interfaces
   can establish a VPN or direct link to securely connect to a FHS
   Proxy/Server in a "tethered" arrangement with all of the Client's
   traffic transiting the Proxy/Server which acts as a router.
   Alternatively, the Client can associate with an INET FHS Proxy/Server
   using UDP/IP encapsulation and control message securing services as
   discussed in the following sections.

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

   The Client then prepares an RS message with IPv6 source address set
   to its MNP-LLA, with IPv6 destination set to (link-local) All-Routers
   multicast and with an OMNI option with underlying interface
   attributes.  If the Client believes that it is on the open Internet,
   it SHOULD include an L2ADDR in the Interface Attributes sub-option
   corresponding to the underlying interface (if the Client believes it
   is behind one or several NATs, it MAY instead set L2ADDR to 0).  If
   the underlying address is IPv4, the Client includes the Port Number
   and IPv4 address written in obfuscated form [RFC4380] as discussed in
   Section 3.3.  If the underlying interface address is IPv6, the Client
   instead includes the Port Number and IPv6 address in obfuscated form.
   The Client finally includes an authentication signature per
   [I-D.templin-6man-omni] to provide message authentication, selects an
   Identification value and window synchronization parameters, and
   submits the RS for OAL encapsulation.  The Client then encapsulates
   the OAL fragment in UDP/IP headers to form a carrier packet, sets the
   UDP/IP source to its INET address and UDP port, sets the UDP/IP
   destination to the FHS Proxy/Server's INET address and the AERO
   service port number (8060), then sends the carrier packet to the
   Proxy/Server.

   When the FHS Proxy/Server receives the RS, it discards the OAL
   encapsulation, authenticates the RS message, creates a NCE and
   registers the Client's MNP, window synchronization state and INET
   interface information according to the OMNI option parameters.  If
   the RS message OMNI option includes Interface Attributes with an
   L2ADDR, the Proxy/Server compares the encapsulation IP address and
   UDP port number with the (unobfuscated) values.  If the values are
   the same, the Proxy/Server caches the Client's information as "INET"
   addresses meaning that the Client is likely to accept direct messages

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   without requiring NAT traversal exchanges.  If the values are
   different (or, if the OMNI option did not include an L2ADDR) the
   Proxy/Server instead caches the Client's information as "mapped"
   addresses meaning that NAT traversal exchanges may be necessary.

   The FHS Proxy/Server then prepares an RA message with IPv6 source and
   destination set corresponding to the addresses in the RS, and with an
   OMNI option with an Origin Indication sub-option per
   [I-D.templin-6man-omni] with the mapped and obfuscated Port Number
   and IP address observed in the encapsulation headers.  The Proxy/
   Server also includes an Interface Attributes sub-option with omIndex
   0 and FMT/SRT/LHS/L2ADDR information appropriate for its INET
   interface, an authentication signature sub-option per
   [I-D.templin-6man-omni] and/or a symmetric window synchronization/
   acknowledgement if necessary.  The Proxy/Server then performs OAL
   encapsulation and fragmentation if necessary and encapsulates each
   fragment in UDP/IP headers with addresses set per the L2ADDR
   information in the NCE for the Client.

   When the Client receives the RA, it authenticates the message then
   process the window synchronization/acknowledgement and compares the
   mapped Port Number and IP address from the Origin Indication sub-
   option with its own address.  If the addresses are the same, the
   Client assumes the open Internet / Cone NAT principle; if the
   addresses are different, the Client instead assumes that further
   qualification procedures are necessary to detect the type of NAT and
   proceeds according to standard procedures [RFC6081][RFC4380].  The
   Client also caches the RA Interface Attributes FMT/SRT/LHS/L2ADDR
   information to discover the Proxy/Server's spanning tree orientation.
   The Client finally arranges to return an explicit/implicit
   acknowledgement, and sends periodic RS messages to receive fresh RA
   messages before the Router Lifetime received on each INET interface
   expires.

   When the Client sends messages to target IP addresses, it also
   invokes route optimization per Section 3.14.  For route optimized
   targets in the same OMNI link segment, if the target's L2ADDR is on
   the open INET, the Client forwards carrier packets directly to the
   target INET address.  If the target is behind a NAT, the Client first
   establishes NAT state for the L2ADDR using the "direct bubble" and
   NUD mechanisms discussed in Section 3.10.1.  The Client continues to
   send carrier packets via its Proxy/Server until NAT state is
   populated, then begins forwarding carrier packets via the direct path
   through the NAT to the target.  For targets in different OMNI link
   segments, the Client uses OAL/ORH encapsulation and forwards carrier
   packets to the Bridge that returned the NA(AR) message.

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   The Client can send original IP packets to route-optimized neighbors
   in the same OMNI link segment no larger than the minimum/path MPS in
   one piece and with OAL encapsulation as atomic fragments.  For larger
   original IP packets, the Client applies OAL encapsulation then
   fragments if necessary according to Section 3.9, with OAL header with
   source set to its own MNP-ULA and destination set to the MNP-ULA of
   the target, and with an in-window Identification value.  The Client
   then encapsulates each resulting carrier packet in UDP/IP *NET
   headers and sends them to the next hop.

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

3.23.  Time-Varying MNPs

   In some use cases, it is desirable, beneficial and efficient for the
   Client to receive a constant MNP that travels with the Client
   wherever it moves.  For example, this would allow air traffic
   controllers to easily track aircraft, etc.  In other cases, however
   (e.g., intelligent transportation systems), the MN may be willing to
   sacrifice a modicum of efficiency in order to have time-varying MNPs
   that can be changed every so often to defeat adversarial tracking.

   The DHCPv6 service offers a way for Clients that desire time-varying
   MNPs to obtain short-lived prefixes (e.g., on the order of a small
   number of minutes).  In that case, the identity of the Client would
   not be bound to the MNP but rather to a Node Identification value
   (see: [I-D.templin-6man-omni]) to be used as the Client ID seed for
   MNP prefix delegation.  The Client would then be obligated to
   renumber its internal networks whenever its MNP (and therefore also
   its MNP-LLA) changes.  This should not present a challenge for
   Clients with automated network renumbering services, however presents
   limits for the durations of ongoing sessions that would prefer to use
   a constant address.

4.  Implementation Status

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

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   AERO Release-3.2 was tagged on March 30, 2021, and is undergoing
   internal testing.  Additional internal releases expected within the
   coming months, with first public release expected end of 1H2021.

   Many AERO/OMNI functions are implemented and undergoing final
   integration.  OAL fragmentation/reassembly buffer management code has
   been cleared for public release and will be presented at the June
   2021 ICAO mobility subgroup meeting.

5.  IANA Considerations

   The IANA is instructed to assign a new type value TBD1 in the IPv6
   Routing Types registry (IANA registration procedure is IETF Review or
   IESG Approval).

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

   No further IANA actions are required.

6.  Security Considerations

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

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

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   For INET partitions that require strong security in the data plane,
   two options for securing communications include 1) disable route
   optimization so that all traffic is conveyed over secured tunnels, or
   2) enable on-demand secure tunnel creation between Client neighbors.
   Option 1) would result in longer routes than necessary and impose
   traffic concentration on critical infrastructure elements.  Option 2)
   could be coordinated between Clients using NS/NA messages with OMNI
   Host Identity Protocol (HIP) "Initiator/Responder" message sub-
   options [RFC7401][I-D.templin-6man-omni] to create a secured tunnel
   on-demand.

   AERO Clients that connect to secured ANETs need not apply security to
   their ND messages, since the messages will be authenticated and
   forwarded by a perimeter Proxy/Server that applies security on its
   INET-facing interface as part of the spanning tree (see above).  AERO
   Clients connected to the open INET can use network and/or transport
   layer security services such as VPNs or can by some other means
   establish a direct link to a Proxy/Server.  When a VPN or direct link
   may be impractical, however, INET Clients and Proxy/Servers SHOULD
   include and verify authentication signatures for their IPv6 ND
   messages as specified in [I-D.templin-6man-omni].

   Application endpoints SHOULD use transport-layer (or higher-layer)
   security services such as TLS/SSL, DTLS or SSH [RFC4251] to assure
   the same level of protection as for critical secured Internet
   services.  AERO Clients that require host-based VPN services SHOULD
   use network and/or transport layer security services such as IPsec,
   TLS/SSL, DTLS, etc.  AERO Proxys and Proxy/Servers can also provide a
   network-based VPN service on behalf of the Client, e.g., if the
   Client is located within a secured enclave and cannot establish a VPN
   on its own behalf.

   AERO Proxy/Servers and Bridges present targets for traffic
   amplification Denial of Service (DoS) attacks.  This concern is no
   different than for widely-deployed VPN security gateways in the
   Internet, where attackers could send spoofed packets to the gateways
   at high data rates.  This can be mitigated through the AERO/OMNI data
   origin authentication procedures, as well as connecting Proxy/Servers
   and Bridges over dedicated links with no connections to the Internet
   and/or when connections to the Internet are only permitted through
   well-managed firewalls.  Traffic amplification DoS attacks can also
   target an AERO Client's low data rate links.  This is a concern not
   only for Clients located on the open Internet but also for Clients in
   secured enclaves.  AERO Proxy/Servers and Proxys can institute rate
   limits that protect Clients from receiving packet floods that could
   DoS low data rate links.

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   AERO Relays must implement ingress filtering to avoid a spoofing
   attack in which spurious messages with ULA addresses are injected
   into an OMNI link from an outside attacker.  AERO Clients MUST ensure
   that their connectivity is not used by unauthorized nodes on their
   EUNs to gain access to a protected network, i.e., AERO Clients that
   act as routers MUST NOT provide routing services for unauthorized
   nodes.  (This concern is no different than for ordinary hosts that
   receive an IP address delegation but then "share" the address with
   other nodes via some form of Internet connection sharing such as
   tethering.)

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

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

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

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

7.  Acknowledgements

   Discussions in the IETF, aviation standards communities and private
   exchanges helped shape some of the concepts in this work.
   Individuals who contributed insights include Mikael Abrahamsson, Mark
   Andrews, Fred Baker, Bob Braden, Stewart Bryant, Scott Burleigh,
   Brian Carpenter, Wojciech Dec, Pavel Drasil, Ralph Droms, Adrian
   Farrel, Nick Green, Sri Gundavelli, Brian Haberman, Bernhard Haindl,
   Joel Halpern, Tom Herbert, Bob Hinden, Sascha Hlusiak, Lee Howard,
   Christian Huitema, Zdenek Jaron, Andre Kostur, Hubert Kuenig, Ted

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   Lemon, Andy Malis, Satoru Matsushima, Tomek Mrugalski, Thomas Narten,
   Madhu Niraula, Alexandru Petrescu, Behcet Saikaya, Michal Skorepa,
   Dave Thaler, Joe Touch, Bernie Volz, Ryuji Wakikawa, Tony Whyman,
   Lloyd Wood and James Woodyatt.  Members of the IESG also provided
   valuable input during their review process that greatly improved the
   document.  Special thanks go to Stewart Bryant, Joel Halpern and
   Brian Haberman for their shepherding guidance during the publication
   of the AERO first edition.

   This work has further been encouraged and supported by Boeing
   colleagues including Kyle Bae, M.  Wayne Benson, Dave Bernhardt, Cam
   Brodie, John Bush, Balaguruna Chidambaram, Irene Chin, Bruce Cornish,
   Claudiu Danilov, Don Dillenburg, Joe Dudkowski, Wen Fang, Samad
   Farooqui, Anthony Gregory, Jeff Holland, Seth Jahne, Brian Jaury,
   Greg Kimberly, Ed King, Madhuri Madhava Badgandi, Laurel Matthew,
   Gene MacLean III, Kyle Mikos, Rob Muszkiewicz, Sean O'Sullivan, Vijay
   Rajagopalan, Greg Saccone, Rod Santiago, Kent Shuey, Brian Skeen,
   Mike Slane, Carrie Spiker, Katie Tran, Brendan Williams, Amelia
   Wilson, Julie Wulff, Yueli Yang, Eric Yeh and other members of the
   Boeing mobility, networking and autonomy teams.  Kyle Bae, Wayne
   Benson, Madhuri Madhava Badgandi, Vijayasarathy Rajagopalan, Katie
   Tran and Eric Yeh are especially acknowledged for their work on the
   AERO implementation.  Chuck Klabunde is honored and remembered for
   his early leadership, and we mourn his untimely loss.

   This work has benefitted from 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, involvement 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 proxies.  In the mid-1990s to early
   2000s employment at the NASA Ames Research Center (Sterling Software)
   and SRI International supported early investigations of IPv6, IETF
   participation and an ONR UAV Communications study.  An employment at
   Nokia where important IETF documents were published gave way to a
   long-term engagement with The Boeing Company to the present day.
   While at Boeing, the work matured through major programs including
   Future Combat Systems, Advanced Airplane Program, DTN for the
   International Space Station, Mobility Vision Lab, CAST, Caravan, the
   NASA Unmanned Air Systems (UAS) Communications, Navigation and
   Surveillance (CNS) program, the FAA/ICAO Aeronautical
   Telecommunications Network (ATN) program and many others.  To begin
   naming all who gave support and encouragement would quickly result in
   a doubling of the current document size and unintentional omissions -
   but to all a humble thanks.

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

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

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

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

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

   o  AERO, First Edition [RFC6706]

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

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

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

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

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

8.  References

8.1.  Normative References

   [I-D.templin-6man-omni]
              Templin, F. L. and T. Whyman, "Transmission of IP Packets
              over Overlay Multilink Network (OMNI) Interfaces", draft-
              templin-6man-omni-03 (work in progress), April 2021.

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

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

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

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

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,
              <https://www.rfc-editor.org/info/rfc3971>.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,
              <https://www.rfc-editor.org/info/rfc3972>.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
              November 2005, <https://www.rfc-editor.org/info/rfc4191>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              DOI 10.17487/RFC4380, February 2006,
              <https://www.rfc-editor.org/info/rfc4380>.

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

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

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

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

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

8.2.  Informative References

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

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

   [I-D.bonica-6man-crh-helper-opt]
              Li, X., Bao, C., Ruan, E., and R. Bonica, "Compressed
              Routing Header (CRH) Helper Option", draft-bonica-6man-
              crh-helper-opt-03 (work in progress), April 2021.

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

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

   [I-D.ietf-ipwave-vehicular-networking]
              (editor), J. (. J., "IPv6 Wireless Access in Vehicular
              Environments (IPWAVE): Problem Statement and Use Cases",
              draft-ietf-ipwave-vehicular-networking-20 (work in
              progress), March 2021.

   [I-D.ietf-rtgwg-atn-bgp]
              Templin, F. L., Saccone, G., Dawra, G., Lindem, A., and V.
              Moreno, "A Simple BGP-based Mobile Routing System for the
              Aeronautical Telecommunications Network", draft-ietf-
              rtgwg-atn-bgp-10 (work in progress), January 2021.

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

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

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

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

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

   [I-D.templin-v6ops-pdhost]
              Templin, F. L., "IPv6 Prefix Delegation and Multi-
              Addressing Models", draft-templin-v6ops-pdhost-27 (work in
              progress), January 2021.

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

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

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

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

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

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

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

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

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

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

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

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,
              <https://www.rfc-editor.org/info/rfc3810>.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   [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>.

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

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

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

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

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

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

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

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

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935,
              DOI 10.17487/RFC6935, April 2013,
              <https://www.rfc-editor.org/info/rfc6935>.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, DOI 10.17487/RFC6936, April 2013,
              <https://www.rfc-editor.org/info/rfc6936>.

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   [RFC7333]  Chan, H., Ed., Liu, D., Seite, P., Yokota, H., and J.
              Korhonen, "Requirements for Distributed Mobility
              Management", RFC 7333, DOI 10.17487/RFC7333, August 2014,
              <https://www.rfc-editor.org/info/rfc7333>.

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <https://www.rfc-editor.org/info/rfc7761>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

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

Appendix A.  Non-Normative Considerations

   AERO can be applied to a multitude of Internetworking scenarios, with
   each having its own adaptations.  The following considerations are
   provided as non-normative guidance:

A.1.  Implementation Strategies for Route Optimization

   Route optimization as discussed in Section 3.14 results in the route
   optimization source (ROS) creating a NCE for the target neighbor.
   The NCE state is set to REACHABLE for at most ReachableTime seconds.
   In order to refresh the NCE lifetime before the ReachableTime timer
   expires, the specification requires implementations to issue a new
   NS/NA(AR) exchange to reset ReachableTime while data packets are
   still flowing.  However, the decision of when to initiate a new NS/
   NA(AR) exchange and to perpetuate the process is left as an
   implementation detail.

   One possible strategy may be to monitor the NCE watching for data
   packets for (ReachableTime - 5) seconds.  If any data packets have

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

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

A.2.  Implicit Mobility Management

   OMNI interface neighbors MAY provide a configuration option that
   allows them to perform implicit mobility management in which no ND
   messaging is used.  In that case, the Client only transmits packets
   over a single interface at a time, and the neighbor always observes
   packets arriving from the Client from the same link-layer source
   address.

   If the Client's underlying interface address changes (either due to a
   readdressing of the original interface or switching to a new
   interface) the neighbor immediately updates the NCE for the Client
   and begins accepting and sending packets according to the Client's
   new address.  This implicit mobility method applies to use cases such
   as cellphones with both WiFi and Cellular interfaces where only one
   of the interfaces is active at a given time, and the Client
   automatically switches over to the backup interface if the primary
   interface fails.

A.3.  Direct Underlying Interfaces

   When a Client's OMNI interface is configured over a Direct interface,
   the neighbor at the other end of the Direct link can receive packets
   without any encapsulation.  In that case, the Client sends packets
   over the Direct link according to traffic selectors.  If the Direct
   interface is selected, then the Client's IP packets are transmitted
   directly to the peer without going through an ANET/INET.  If other
   interfaces are selected, then the Client's IP packets are transmitted
   via a different interface, which may result in the inclusion of
   Proxy/Servers and Bridges in the communications path.  Direct
   interfaces must be tested periodically for reachability, e.g., via
   NUD.

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A.4.  AERO Critical Infrastructure Considerations

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

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

   AERO ANET Proxy/Servers are most often standard dedicated server
   platforms with one underlying interface connected to the ANET and a
   second interface connected to an INET.  As with INET Proxy/Servers,
   the only requirements are that they can run the AERO/OMNI code and
   have at least one interface connection to the INET.  ANET Proxy/
   Servers must be provisioned, supported and managed by the ANET
   administrative authority.  Cost for purchasing, configuring and
   managing Proxys is nominal, and borne by the ANET administrative
   authority.

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

A.5.  AERO Server Failure Implications

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

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   If a Proxy/Server fails, ongoing packet forwarding to Clients will
   continue by virtue of the neighbor cache entries that have already
   been established in route optimization sources (ROSs).  If a Client
   also experiences mobility events at roughly the same time the Proxy/
   Server fails, uNA messages may be lost but neighbor cache entries in
   the DEPARTED state will ensure that packet forwarding to the Client's
   new locations will continue for up to DepartTime seconds.

   If a Client is left without a Proxy/Server for a considerable length
   of time (e.g., greater than ReachableTime seconds) then existing
   neighbor cache entries will eventually expire and both ongoing and
   new communications will fail.  The original source will continue to
   retransmit until the Client has established a new Proxy/Server
   relationship, after which time continuous communications will resume.

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

A.6.  AERO Client / Server Architecture

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

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

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

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

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

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

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

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

   The same implications observed for common highly-available services
   in the Internet apply also to the AERO client/server architecture.
   When an AERO Client connects to one or more ANETs, it discovers one

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

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

Appendix B.  Change Log

   << RFC Editor - remove prior to publication >>

   Changes from draft-templin-6man-aero-14 to draft-templin-6man-aero-
   15:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-13 to draft-templin-6man-aero-
   14:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-12 to draft-templin-6man-aero-
   13:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

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   Changes from draft-templin-6man-aero-11 to draft-templin-6man-aero-
   12:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-10 to draft-templin-6man-aero-
   11:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-09 to draft-templin-6man-aero-
   10:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-08 to draft-templin-6man-aero-
   09:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-07 to draft-templin-6man-aero-
   08:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-06 to draft-templin-6man-aero-
   07:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval (with reference to rfcdiff
      from previous version).

   Changes from draft-templin-6man-aero-05 to draft-templin-6man-aero-
   06:

   o  Final editorial review pass resulting in multiple changes.
      Document now submit for final approval.

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   Changes from draft-templin-6man-aero-04 to draft-templin-6man-aero-
   05:

   o  Changed to use traffic selectors instead of the former multilink
      selection strategy.

   Changes from draft-templin-6man-aero-03 to draft-templin-6man-aero-
   04:

   o  Removed documents from "Obsoletes" list.

   o  Introduced the concept of "secured" and "unsecured" spanning tree.

   o  Additional security considerations.

   o  Additional route optimization considerations.

   Changes from draft-templin-6man-aero-02 to draft-templin-6man-aero-
   03:

   o  Support for extended route optimization from ROR to target over
      target's underlying interfaces.

   Changes from draft-templin-6man-aero-01 to draft-templin-6man-aero-
   02:

   o  Changed reference citations to "draft-templin-6man-omni".

   o  Several important updates to IPv6 ND cache states and route
      optimization message addressing.

   o  Included introductory description of the "6M's".

   o  Updated Multicast specification.

   Changes from draft-templin-6man-aero-00 to draft-templin-6man-aero-
   01:

   o  Changed category to "Informational".

   o  Updated implementation status.

   Changes from earlier versions to draft-templin-6man-aero-00:

   o  Established working baseline reference.

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Author's Address

   Fred L. Templin (editor)
   Boeing Research & Technology
   P.O. Box 3707
   Seattle, WA  98124
   USA

   Email: fltemplin@acm.org

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