Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Intended status: Informational                             June 15, 2021
Expires: December 17, 2021


             Asymmetric Extended Route Optimization (AERO)
                       draft-templin-6man-aero-18

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 end
   user devices and many other applications.

Status of This Memo

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

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

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   This Internet-Draft will expire on December 17, 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
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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  . . . . . . . . . . . . .  33
     3.4.  OMNI Interface Initialization . . . . . . . . . . . . . .  35
       3.4.1.  AERO Proxy/Server and Relay Behavior  . . . . . . . .  35
       3.4.2.  AERO Client Behavior  . . . . . . . . . . . . . . . .  36
       3.4.3.  AERO Bridge Behavior  . . . . . . . . . . . . . . . .  36
     3.5.  OMNI Interface Neighbor Cache Maintenance . . . . . . . .  36
       3.5.1.  OMNI ND Messages  . . . . . . . . . . . . . . . . . .  38
       3.5.2.  OMNI Neighbor Advertisement Message Flags . . . . . .  40
       3.5.3.  OMNI Neighbor Window Synchronization  . . . . . . . .  41
     3.6.  OMNI Interface Encapsulation and Re-encapsulation . . . .  41
     3.7.  OMNI Interface Decapsulation  . . . . . . . . . . . . . .  42
     3.8.  OMNI Interface Data Origin Authentication . . . . . . . .  42
     3.9.  OMNI Interface MTU  . . . . . . . . . . . . . . . . . . .  43
     3.10. OMNI Interface Forwarding Algorithm . . . . . . . . . . .  43
       3.10.1.  Client Forwarding Algorithm  . . . . . . . . . . . .  45
       3.10.2.  Proxy/Server and Relay Forwarding Algorithm  . . . .  46
       3.10.3.  Bridge Forwarding Algorithm  . . . . . . . . . . . .  49
     3.11. OMNI Interface Error Handling . . . . . . . . . . . . . .  51
     3.12. AERO Router Discovery, Prefix Delegation and
           Autoconfiguration . . . . . . . . . . . . . . . . . . . .  53
       3.12.1.  AERO Service Model . . . . . . . . . . . . . . . . .  54
       3.12.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  54
       3.12.3.  AERO Proxy/Server Behavior . . . . . . . . . . . . .  56
     3.13. The AERO Proxy Function . . . . . . . . . . . . . . . . .  59
       3.13.1.  Detecting and Responding to Proxy/Server Failures  .  62
       3.13.2.  Point-to-Multipoint Proxy/Server Coordination  . . .  63



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     3.14. AERO Route Optimization . . . . . . . . . . . . . . . . .  64
       3.14.1.  Route Optimization Initiation  . . . . . . . . . . .  65
       3.14.2.  Relaying the NS(AR) *NET Packet(s) . . . . . . . . .  66
       3.14.3.  Processing the NS(AR) and Sending the NA(AR) . . . .  66
       3.14.4.  Relaying the NA(AR)  . . . . . . . . . . . . . . . .  67
       3.14.5.  Processing the NA(AR)  . . . . . . . . . . . . . . .  67
       3.14.6.  Forwarding Packets to Route Optimized Targets  . . .  68
     3.15. Neighbor Unreachability Detection (NUD) . . . . . . . . .  72
     3.16. Mobility Management and Quality of Service (QoS)  . . . .  73
       3.16.1.  Mobility Update Messaging  . . . . . . . . . . . . .  74
       3.16.2.  Announcing Link-Layer Address and/or QoS Preference
                Changes  . . . . . . . . . . . . . . . . . . . . . .  75
       3.16.3.  Bringing New Links Into Service  . . . . . . . . . .  76
       3.16.4.  Deactivating Existing Links  . . . . . . . . . . . .  76
       3.16.5.  Moving Between Proxy/Servers . . . . . . . . . . . .  76
     3.17. Multicast . . . . . . . . . . . . . . . . . . . . . . . .  77
       3.17.1.  Source-Specific Multicast (SSM)  . . . . . . . . . .  78
       3.17.2.  Any-Source Multicast (ASM) . . . . . . . . . . . . .  79
       3.17.3.  Bi-Directional PIM (BIDIR-PIM) . . . . . . . . . . .  80
     3.18. Operation over Multiple OMNI Links  . . . . . . . . . . .  80
     3.19. DNS Considerations  . . . . . . . . . . . . . . . . . . .  81
     3.20. Transition/Coexistence Considerations . . . . . . . . . .  81
     3.21. Detecting and Reacting to Proxy/Server and Bridge
           Failures  . . . . . . . . . . . . . . . . . . . . . . . .  82
     3.22. AERO Clients on the Open Internet . . . . . . . . . . . .  82
     3.23. Time-Varying MNPs . . . . . . . . . . . . . . . . . . . .  84
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  85
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  85
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  86
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  88
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  90
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  90
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  91
   Appendix A.  Non-Normative Considerations . . . . . . . . . . . .  98
     A.1.  Implementation Strategies for Route Optimization  . . . .  98
     A.2.  Implicit Mobility Management  . . . . . . . . . . . . . .  98
     A.3.  Direct Underlying Interfaces  . . . . . . . . . . . . . .  99
     A.4.  AERO Critical Infrastructure Considerations . . . . . . .  99
     A.5.  AERO Server Failure Implications  . . . . . . . . . . . . 100
     A.6.  AERO Client / Server Architecture . . . . . . . . . . . . 100
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . . 102
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 105

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
   multilink operation for increased reliability and path optimization
   while providing fragmentation and reassembly services to support
   Maximum Transmission Unit (MTU) diversity.  In terms of precedence,
   readers may appreciate reading this 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.  Mobile node Clients discover shortest paths to OMNI link
   neighbors through AERO route optimization.  Both unicast and
   multicast communications are supported, and Clients may efficiently
   move between locations while maintaining continuous communications
   with correspondents and without changing their IP Address.

   AERO Bridges peer with Proxy/Servers in a secured private BGP overlay
   routing instance to 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
   the underlying Internetwork to (mobile or fixed) nodes on the OMNI
   link.  To the underlying Internetwork, the Relay is the source of a



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

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

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

   2.  Multinet - the ability to span the OMNI link over a segment
       routing topology with multiple diverse 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 network topology with a coherent IP routing and addressing plan
      and that provides a transit backbone service for its connected end
      systems.  INETs also provide an underlay service over which the
      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 intermediate nodes in a spanning tree the same



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      as a bridged campus LAN.  AERO nodes on the OMNI link appear as
      single-hop neighbors at the network layer even though they may be
      separated by multiple underlying *NET hops; AERO nodes 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 service that subjects original IP packets
      admitted into the interface to mid-layer IPv6 header encapsulation
      followed by fragmentation and reassembly.  The OAL is also
      responsible for generating MTU-related control messages as
      necessary, and for providing addressing context for spanning
      multiple segments of a bridged OMNI link.

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

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

   OAL fragment
      a portion of an OAL packet following fragmentation but prior to
      *NET encapsulation, or following *NET 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 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 networks and links.  AERO Relays configure an OMNI
      interface and assign an ADM-LLA the same as Proxy/Servers, and
      also run a dynamic routing protocol to discover any non-MNP IP GUA
      routes in service on its connected downstream network links.  The
      Relay advertises the MSP(s) to its downstream networks, and
      redistributes downstream network routes into the OMNI link routing
      system the same as for Proxy/Servers.




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   AERO Bridge ("Bridge")
      a BGP hub autonomous system node that also 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 an OAL intermediate node while decrementing the OAL
      IPv6 header Hop Limit but without decrementing the network layer
      IP TTL/Hop Limit.  Bridges peer with Proxy/Servers and other
      Bridges over an OMNI link secured spanning tree to discover the
      full set of MNPs for the link as well as any non-MNP IP GUA routes
      reached 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



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      interface.  The Client/Relay sees each EUN as a "downstream"
      network, and sees the OMNI interface as the point of attachment to
      the "upstream" network.

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

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

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

   Route Optimization responder (ROR)
      the AERO node that responds to route optimization requests on
      behalf of the target.  The ROR may be 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.





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

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

3.  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].  The OMNI interface and OAL provide a
   virtual bridging service, since the inner IP TTL/Hop Limit is not
   decremented.  Each Bridge also peers with Proxy/Servers and other
   Bridges in a dynamic routing protocol instance to provide a
   Distributed Mobility Management (DMM) service for the list of active
   MNPs (see Section 3.2.3).  Bridges 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.




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   AERO Proxy/Servers in distributed SRT segments provide default
   forwarding and mobility/multilink services for AERO Client mobile
   nodes.  Each Proxy/Server also peers with Bridges in a dynamic
   routing protocol instance to advertise its list of associated MNPs
   (see Section 3.2.3).  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;
   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 often traverse an SRT spanning tree that connects multiple
   distinct *NET segments that may be managed under different
   administrative authorities (e.g., as for worldwide aviation service
   providers such as ARINC, SITA, Inmarsat, etc.).  Individual *NETs may
   also be partitioned internally, in which case each internal partition
   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 disjoint segments often have no common physical
   link connections.  Therefore, nodes can only be assured of exchanging
   carrier packets directly with correspondents in the same segment, and
   not with those in other segments.  The only means for joining the
   segments therefore is through inter-domain peerings between AERO
   Bridges.

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















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

            Figure 2: OMNI Link Segment Routing Topology (SRT)

   Bridge, Proxy/Server and Relay OMNI interfaces are configured over
   both secured tunnels and open INET underlying interfaces within their
   respective SRT segments.  Within each segment, Bridges configure
   "hub-and-spokes" BGP peerings with Proxy/Server/Relays as "spokes".
   Adjacent SRT segments are joined by Bridge-to-Bridge peerings to
   collectively form a spanning tree over the entire SRT.  The "secured"
   spanning tree supports authentication and integrity for critical
   control plane messages.  The "unsecured" spanning tree conveys
   ordinary carrier packets without security codes and that must be
   treated by destinations according to data origin authentication
   procedures.  AERO nodes can employ route optimization to cause
   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 Border Gateway Protocol
   (BGP) [RFC4271] service coordinated between Bridges and Proxy/
   Servers.  The service supports carrier packet forwarding at a layer
   below IP, and does not interact with 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 in
   a "hub" AS, which peer with the Proxy/Servers within that segment as
   "spoke" ASes.  All Bridges within the same segment are members of the
   same hub AS, and use iBGP to maintain a consistent view of all active
   routes currently in service.  The Bridges of different segments peer
   with one another using eBGP.

   Bridges maintain forwarding table entries only for the MNP-ULAs
   corresponding to MNP and non-MNP routes that are currently active,
   while carrier packets destined to all other MNP-ULAs are dropped with



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   a Destination Unreachable message returned due to the black-hole
   route.  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 known as the "SRT prefix".  For example, a first segment
   could assign [ULA*]::1000/116, a second could assign
   [ULA*]::2000/116, a third could assign [ULA*]::3000/116, etc.  Within
   each segment, each Proxy/Server configures an ADM-ULA within the
   segment's SRT prefix, e.g., the Proxy/Servers within [ULA*]::2000/116
   could assign the ADM-ULAs [ULA*]::2011/116, [ULA*]::2026/116,
   [ULA*]::2003/116, etc.

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

   ADM-ULA prefixes are statically represented in Bridge forwarding
   tables.  Bridges join multiple SRT segments into a unified OMNI link
   over multiple diverse network administrative domains.  They support a
   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'



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   and 'F') service the MNPs 2001:db8:1000:2000::/56,
   2001:db8:3000:4000::/56 and 2001:db8:5000:6000::/56 then the routing
   system would include:

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

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

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

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

3.2.4.  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 OAL header extension.  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 16 - 31)       |                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               ~
      ~                   Link Layer Address (L2ADDR)                 ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Null Padding (if necessary)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                       Destination Trailer                     ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                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.



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   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).

   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 that informs the LHS Proxy/Server of a
      specific Client underlying interface 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 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 remainder includes an LHS and L2ADDR for IPv4.  If
         FMT-Type is set, the remainder instead includes two null
         padding octets followed by a Destination Trailer.

   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




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      from concatenating [ULA*]::/96 with the 32 bit LHS value (for
      example, the value 16 corresponds to the prefix length 112).

   o  LHS - a 4-octet field 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 is non-zero, SRT and LHS together determine
      the ADM-ULA of the LHS Proxy/Server over the spanning tree.  When
      SRT is 0, LHS instead encodes a Peer Index value meaningful to the
      LHS Bridge.

   o  Link Layer Address (L2ADDR) - an IP encapsulation address
      consulted only when Segments Left is 1.  The L2ADDR IP version is
      determined by the ORH length, since L2ADDR 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 local OMNI link segment.
      If direct INET encapsulation is not indicated, L2ADDR is instead
      set to all-zeros and the SRT/LHS fields are consulted for LHS
      forwarding.

   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 Trailer - a trailing 8-octet field present only when
      indicated by the ORH length and FMT-Type (see below) and encodes
      the 64-bit ULA suffix for the target.

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

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

   o  ORH-1 - The preamble Hdr Ext Len is set to 1.  If FMT Type is
      clear, the remainder includes an LHS and L2ADDR for IPv4.  If FMT
      Type is set, the remainder includes two null padding octets
      followed by Destination Trailer, and all other fields are omitted.



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

   o  ORH-3 - The preamble Hdr Ext Len is set to 3.  LHS and L2ADDR for
      IPv6 are included followed by four null padding octets, and
      Destination Trailer is omitted.

   o  ORH-4 - The preamble Hdr Ext Len is set to 4.  LHS and L2ADDR for
      IPv6 are included followed by four null padding octets followed by
      Destination Trailer.

   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 Trailer with a
   ULA suffix only when necessary (in this case, the ULA suffix is
   2001:db8:1234:5678).  Next, to direct the packet to an LHS Bridge,
   the source prepares an ORH with Segments Left set to 1 and with 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 direct the packet to the LHS Proxy/Server
   (either directly in the local segment or in a remote segment via the
   spanning tree), the OAL source instead includes an ORH (Type 0 or 1)
   with Segments Left set to 0, 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
   the Client and a Destination Trailer with the target ULA suffix must
   be included when necessary; otherwise the Identification must be
   within the window for the LHS Proxy/Server and no Destination Trailer
   is needed.  (Note that if no actual fragmentation is required the OAL
   source still prepares the packet as an "atomic" fragment that



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   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
   encapsulated in an OAL 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 *NET header is prepared as
   discussed in Section 3.6.  The OAL source transmits each such carrier
   packet into the SRT spanning tree, where they are forwarded over
   possibly multiple OAL intermediate nodes until they arrive at the OAL
   destination.

   The SRT forwarding system therefore contains both Client MNP-ULA
   prefix information that may change dynamically due to regional node



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   mobility and per-segment ADM-ULA prefix information that rarely if
   ever changes.  The SRT forwarding system therefore supports link-
   layer bridging through carrier packet forwarding over the spanning
   tree according to link-layer information 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-0/1 during forwarding.  While this
   may yield encapsulation overhead savings in some cases, the AERO node
   can instead simply decrement Segments Left and leave the original ORH
   in place.  The LHS Proxy/Server or destination Client that processes
   the ORH will receive the same information in both cases.

   Note: When the OAL source sets a carrier packet 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
   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.



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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
   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



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   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.)  After segment routing has established spanning tree soft
   state, OAL nodes can employ header compression for most carrier
   packets sent to peers in the same SRT segment to greatly reduce
   encapsulation overhead.

   When a Route Optimization Source (ROS) has an original IP packet to
   send to a target located in the same SRT segment, it places the
   packet on a short queue and initiates window synchronization.  The
   ROS prepares an NS(WIN) (see: Section 3.5.1 and Section 3.14.6 with
   its own LLA as the source.  If FMT-Forward is clear, the ROS sets the
   destination to the ADM-LLA of the LHS Proxy/Server; otherwise, it
   sets the destination to the MNP-LLA of the target Client.  The ROS
   then include an OMNI Interface Attributes option for the underlying
   interface of the source Client.  (The ROS must ensure that the
   NS(WIN) is no larger than the maximum payload size for OAL fragments,
   since multi-fragment control message authentication and integrity
   cannot be assured for all targets.)

   If the ROS is also the source Client, it then performs OAL
   encapsulation with an ORH with Segments Left set to 1 with
   FMT/SRT/LHS/L2ADDR information for the NS(WIN) target while setting
   the OAL source to its own MNP-ULA.  The Client then sets the OAL
   destination to the ADM-ULA of an FHS Proxy/Server and includes an
   authentication signature if necessary then forwards the "atomic
   fragment" carrier packet to the FHS Proxy/Server.  The FHS Proxy/
   Server verifies the signature if necessary, then rewrites the
   Interface Attributes with FMT/SRT/LHS/L2ADDR information for its own
   INET interface, transforms the ORH into an ORH-0/1, sets the OAL
   destination address to the ADM-ULA of the LHS Proxy/Server then
   forwards the carrier packet into the secured spanning tree.  (If the
   ROS is also the FHS Proxy/Server, it instead prepares the NS(WIN)
   itself using its own ADM-LLA as the source and forwards the same as
   above.)

   When the LHS Proxy/Server receives the NS(WIN) in a carrier packet
   with its own ADM-LLA as the destination, it caches the Interface
   Attributes and window syncrhonization information and returns an



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   NA(WIN) (with the LLAs from the NS(WIN) reversed) to the ADM-ULA of
   the FHS Proxy/Server via the secured spanning tree (where the NA(WIN)
   again must fit within a single atomic fragment carrier packet).  When
   the LHS Proxy/Server receives an NS(WIN) with an MNP-LLA destination,
   it instead locates the NCE corresponding to the MNP-LLA and creates a
   Peer List entry for the NS(WIN) source LLA NCE while also caching the
   OMNI Interface Attributes information and assigning a Peer Index for
   this (NCE, Peer List entry)-tuple.  The LHS Proxy/Server then
   includes an ORH with SRT set to 0, with LHS set to this Peer Index,
   and with L2ADDR set to its INET address.  The LHS Proxy/Server then
   includes an authentication signature if necessary, changes the OAL
   destination to the target MNP-ULA, performs INET encapsulation and
   forwards the carrier packet to the target Client.

   When the target Client receives the (carrier packet / NS(WIN)), it
   verifies the signature then creates or updates an NCE for the NS(WIN)
   source LLA while caching window synchronization information and the
   (Peer Index, L2ADDR) for the LHS Proxy/Server (and possibly also the
   LHS Bridge).  The ORH includes the (Peer Index, L2ADDR) information
   for the LHS Proxy/Server, while the OMNI Interface Attributes
   includes the (Peer Index, L2ADDR) for the LHS Bridge when SRT is 0.
   When the target Client forwards future carrier packets with
   compressed headers to the ROS, it must use the correct (Peer Index,
   L2ADDR) information for the next hop (i..e, the Bridge vs the Proxy/
   Server).  The target Client then prepares an NA(WIN) (with the LLAs
   from the NS(WIN) reversed), with Interface Attributes for its
   underlying interface and with an authentication signature.  The
   Client then sets the OAL source to its own MNP-ULA and OAL
   destination to the ADM-ULA of the LHS Proxy/Server, includes an OMNI
   Interface Attributes option with FMT/SRT/LHS/L2ADDR information for
   its own underlying interface as well as an ORH with FMT/SRT/LHS/
   L2ADDR information copied from the NS(WIN) OMNI Interface Attributes
   then forwards the message to the LHS Proxy/Server.  The LHS Proxy/
   Server rewrites the OMNI Interface Attributes FMT/SRT/LHS/L2ADDR
   information, changes the ORH to an ORH-0/1, sets the OAL destination
   to the ADM-ULA of the FHS Proxy/Server, then forwards the carrier
   packet into the secured spanning tree.

   When the FHS Proxy/Server receives an NA(WIN) with its own ADM-LLA as
   the destination, it processes the message locally.  If the NA(WIN)
   destination is an MNP-LLA, the FHS Proxy/Server instead forwards the
   message to the ROS Client.  In the process, the FHS Proxy/Server
   locates the NCE for the MNP-LLA and creates a Peer List entry for the
   NA(WIN) source LLA while also caching the OMNI Interface Attributes
   and assigning a Peer Index for this (NCE, Peer List entry)-tuple the
   same as the LHS Proxy/Server.  The FHS Proxy/Server then includes an
   ORH with SRT set to 0, LHS set to the new Peer Index, and L2ADDR set
   to its own INET address, then incudes an authentication signature if



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   necessary, changes the OAL destination to the MNP-ULA and forwards
   the message to the ROS.  When either the ROS Client or FHS Proxy/
   Server processes the NA(WIN), it creates or updates an NCE for the
   NA(WIN) source LLA while caching window synchronization information,
   (Peer Index, L2ADDR) for the Proxy Server (and possibly also the
   Bridge) and OMNI Interface Attributes.  The ROS then prepares an
   NA(WIN) acknowledgement (with the LLAs from the original NA(WIN)
   reversed) and forwards it back to the target to complete the third
   leg of the window syncrhonization handshake.  (Procedures for
   preparing and forwarding the NA(WIN) acknowledgement are identical to
   those for the initial NA(WIN) above except with the LHS Proxy/Server
   as the destination.)

   Following window synchronzation, either peer can refresh window
   synchronization parameters and/or send other carrier packets
   requiring security at any time using the same secured procedures
   described above.  The peers can also begin exchanging ordinary
   carrier packets with Identification values within their respective
   send/receive windows using INET encapsulation and OAL header
   compression when possible (see below).  The source peer sets the OAL
   destination to the ADM-ULA of the target Proxy/Server if FMT-Forward
   is clear; otherwise, it sets the OAL destination to the MNP-ULA of
   the target Client to cause the target Proxy/Server to forward without
   reassembling.  The source peer also includes an ORH-0/1 with omIndex
   set to select a specific target Client underlying interface;
   otherwise, it sets omIndex to 0 (or omits the ORH entirely) when no
   preference is selected.  The source peer finally encapsulates the
   carrier packets in INET headers with destination addresses set
   according to the target Proxy/Server L2ADDR information.  (If the
   target peer is the Client itself and FMT-Mode is set, the source peer
   can instead forward initial packets via the Proxy/Server then begin
   forwarding directly according to the target Client L2ADDR information
   discovered through NAT traversal procedures.)

   When an ROS has an original IP packet to send to a route optimization
   target located in a remote SRT segment, it places the packet on a
   short queue and initiates wndow synchronization the same as above.
   The ROS then prepares an NS(WIN) and performs OAL encapsulation with
   an ORH with Segments Left set to 1 while setting the OAL source to
   its own ULA and including an ORH with Segments Left set to 1 and with
   FMT/SRT/LHS/L2ADDR information appropriate for the target.  If the
   ROS is a Client, it sets the OAL destination to the ADM-ULA of a FHS
   Proxy/Server and includes an authentication signature if necessary
   then forwards the carrier packet to the FHS Proxy/Server.  The FHS
   Proxy/Server verifies the signature if necessary, then rewrites the
   Interface Attributes with FMT/SRT/LHS/L2ADDR information for its own
   INET interface and sets the OAL destination address to the Subnet
   Router Anycast address for the local SRT segment.  The FHS Proxy/



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   Server then forwards the carrier packet into the secured spanning
   tree which conveys it to the nearest FHS Bridge.

   Bridges forward carrier packets not explicitly addressed to
   themselves via the spanning tree.  For carrier packets that arrived
   via the secured spanning tree, the Bridge forwards to the next hop
   also via the secured spanning tree.  For other carrier packets, the
   Bridge forwards to the next hop via the unsecured spanning tree.
   When an FHS Bridge receives a carrier packet over the secured
   spanning tree destined to its ADM-ULA or the FHS Subnet Router
   Anycast address, it processes the packet locally.  If the packet
   contains an NS(WIN), the FHS Bridge creates or updates a NCE for the
   NS(WIN) source LLA and caches the NS(WIN) destination LLA and window
   synchronization information in the Peer List for this NCE while also
   selecting a unique Peer Index value for this new (NCE, Peer List
   entry)-tuple.  The FHS Bridge next caches the omIndex and
   FMT/SRT/LHS/L2ADDR information found in the OMNI Interface Attributes
   as FHS forwarding infomation for the NCE, and caches the omIndex and
   FMT/SRT/LHS/L2ADDR information found in the ORH as LHS forwarding
   information for the Peer List entry.  The FHS Bridge then rewrites
   the Interface Attributes SRT/LHS to its own ADM-ULA then re-
   encapsulates the NS(WIN) with OAL destination address set to the LHS
   Subnet Router Anycast address (found in the ORH SRT/LHS) and forwards
   the carrier packet into the secured spanning tree.

   When an LHS Bridge receives a carrier packet over the secured
   spanning tree destined to its ADM-ULA or the LHS Subnet Router
   Anycast address, it processes the packet locally.  If the packet
   contains an NS(WIN), the LHS Bridge creates or updates a NCE for the
   NS(WIN) destination LLA and caches the NS(WIN) source LLA and window
   synchronization information in the Peer List for this NCE while also
   selecting a unique Peer Index value for this new (NCE, Peer List
   entry)-tuple.  The LHS Bridge also caches the OMNI option Inteface
   Attributes parameters as FHS forwarding information for this Peer
   List entry, and caches the ORH parameters as LHS forwarding
   information for this NCE.  THe LHS Bridge then rewrites the Interface
   Attributes SRT/LHS to its own ADM-ULA then re-encapsulates the
   NS(WIN) with OAL destination address set to the ADM-ULA of the LHS
   Proxy/Server and forwards the carrrier packet into the secured
   spanning tree.

   When the LHS Proxy/Server receives the carrier packet via the secured
   spanning tree, it proceeds the same as for the local segment case
   above.  Specifically, if the NS(WIN) destination is its own ADM-LLA,
   the LHS Proxy/Server prepares an NA(WIN)) and forwards the resulting
   carrier packet to the ADM-ULA of the LHS Bridge (found in the NS(WIN)
   Interface Attributes) via the secured spanning tree.  If the NS(WIN)
   destination is the MNP-LLA of a local Client, the LHS Proxy/Server



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   instead re-encapsulates and forwards the resulting carrier packet to
   the local Client.  The local Client processes the NS(WIN) and returns
   an NA(WIN) the same as describe above, including the necessary OMNI
   Interface Attributes and ORH.  The LHS Proxy/Server in turn proxies
   the NA(WIN) Interface Attributes, re-encapsulates, and forwards the
   resulting carrier packet to the ADM-ULA of the LHS Bridge.

   Following the initial NS(WIN), both Bridges have established state
   and are prepared to process subsequent NA(WIN) messages in a
   symmetric fashion.  When either Bridge receives an NA(WIN) over the
   secured spanning tree destined to its ADM-ULA, it locates the NCE for
   the NA(WIN) source LLA then locates the Peer List entry for the
   NA(WIN) destination LLA.  The Bridge then forwards the NA(WIN) over
   the secured spanning tree to the ADM-ULA of the peer Bridge found in
   the forwarding information cached in the Peer List entry.  The peer
   Bridge finally forwards the NA(WIN) to the ADM-ULA of the local
   segment Proxy/Server over the secured spanning tree while rewriting
   OMNI Interface Attributes with the value 0 in SRT, the Peer Index in
   LHS and its own INET information in L2ADDR.  In the process, both
   Bridges also update their window synchronization state based on the
   NA(WIN) information.  When the NA(WIN) message finally arrives at the
   ROS, the ROS caches the Peer Index and L2ADDR information for this
   Bridge in the corresponding (NCE, Peer List entry)-tuple and responds
   by returning an NA(WIN) acknolwedgement the same as described above
   to complete the three way window synchronization handshake.

   Following window synchronization, each peer forwards ordinary carrier
   packets directly via their respective FHS Bridges instead of via an
   FHS Proxy/Server.  The source peer employs OAL header compression as
   specified in [I-D.templin-6man-omni] by including a minimal header
   that encodes the Peer Index published in the FHS Bridge NA(WIN)
   Interface Attributes.  When the FHS Bridge receives the carrier
   packets, it locates the NCE corresponding to the Peer Index.  The FHS
   Bridge next examines the Identification and processes the carrier
   packet further only if the Identification is within the receive
   window for this target peer (otherwise, it drops the carrier packet).
   The FHS Bridge next reconstructs the full OAL header based on the
   (NCE, Peer List Entry) information, sets the OAL source to the ULA of
   the source peer, sets the OAL destination to the ADM-ULA of the LHS
   Bridge and includes an ORH with Destination Trailer set to the target
   peer ULA suffix.  The FHS Bridge then forwards the carrier packet via
   the unsecured spanning tree.

   When the LHS Bridge receives the carrier packet addressed to its ADM-
   ULA, it locates the (NCE, Peer List entry)-tuple corresponding to the
   ORH Destination Trailer ULA suffix and OAL source, respectively.  The
   LHS Bridge next examines FMT-Mode to determine whether the target
   peer can accept packets directly (i.e., following any NAT traversal



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   procedures necessary) while bypassing the LHS Proxy/Server.  If the
   taget peer can be reached directly and NAT traversal has converged,
   the LHS Bridge then employs header compression while including Peer
   Index for the (NCE, Peer List entry)-tuple, encapsulates the carrier
   packet according to the NATed L2ADDR information then forwards the
   carrier packet directly to the target peer.  If the target peer
   cannot be reached directly (or if NAT traversal has not yet
   converged), the LHS Bridge instead performs header compression while
   including Peer Index, encapsulates the packet according to the cached
   L2ADDR information for the LHS Proxy/Server then forwards the carrier
   packet directly to the LHS Proxy/Server.

   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, the LHS Proxy/Server verifies that the Identification is
   within its receive window for this source peer then proceeds
   according to FMT-Forward and omIndex, while including an
   authentication signature if necessary.  If FMT-Forward is set, the
   LHS Proxy/Server changes the OAL destination to the MNP-ULA of the
   target Client found in the ORH Destination Trailer, 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.

   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 verifies that the Identification is
   within the receive window for this source peer then 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 an LHS Proxy/Server receives carrier packets with compressed
   headers, it first locates the (NCE, Peer List entry)-tuple
   corresponding to the Peer Index and LHS Bridge INET address and
   verifies that the Identification is acceptable.  If the (NCE, Peer
   List entry)-tuple indicates that the target peer is the LHS Proxy/
   Server, it reconstitutes the OAL headers, reassembles, decapsulates
   then forwards the original IP packet (while re-encapsulating and re-
   fragmenting if necessary).  If the (NCE Peer List entry)-tuple
   indicates that the target peer is a local Client, the LHS Proxy/
   Server instead forwards the packet to the INET destination address of
   the local Client while writing its own INET address as the source and
   its own Peer Index in the compressed header.



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   When a target Client receives carrier packets with OAL destination
   set to is MNP-ULA, it first verifies that the Identification is
   acceptable then reassembles to obtain the OAL packet then
   decapsulates and delivers the original IP packet to upper layers.
   When a target Client receives carrier packets with a compressed
   header, it examines the INET source address.  If the source was the
   Bridge, the Client locates the (NCE, Peer List entry)-tuple based on
   the Bridge INET address and Peer Index; if the source was a Proxy/
   Server, the Client instead locates the (NCE, Peer List entry)-tuple
   based on the Proxy/Server INET address and Peer Index.  The Client
   next verifies that the Identification is acceptable, then
   reconstructs the OAL header, reassembles to obtain the OAL packet,
   then decapsulates and delivers the IP packet to upper layers.

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.

   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.





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   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.

   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



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   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
   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



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   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 NCE is indexed by the
   LLA of the neighbor, while the OAL encapsulation ULA determines the
   context for Identification verification.  Clients and Proxy/Servers
   maintain NCEs through RS/RA exchanges, and also maintain NCEs for any
   active correspondent peers through NS/NA exchanges.

   Bridges also maintain NCEs for Clients within their local segments
   based on NS/NA(WIN) route optimization.  When a Bridge creates/
   updates a NCE for a local segment Client based on NS/NA(WIN) route
   optimization, it also creates/updates a "Peer List" entry for the NCE
   containing the ULA, receive window information and FMT/SRT/LHS/L2ADDR
   information for the source of each NS/NA(WIN) message destined to
   this local segment Client.  The Bridge also assigns a unique "Peer



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   Index" value for each (NCE, Peer List entry)-tuple.  The Bridge
   maintains a separate ReachableTime timer for each NCE Peer List entry
   individually.  When ReachableTimer for an NCE Peer List entry
   expires, the Bridge deletes the Peer List entry and frees the
   associated Peer Index.  When ReachableTime for all of an NCE's Peer
   List entries expire, the Bridge deletes the NCE.  Proxy/Servers also
   create/update a Peer List entry for the NCE of a local Client when
   they forward an NA(WIN) that was not processed by a Bridge.  Proxy/
   Servers maintain the Peer List and Peer Index values the same as for
   Bridges, except that they do not delete the NCE when all Peer List
   entries expire.

   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



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   solicited NA messages arrive.  It is RECOMMENDED that REACHABLE_TIME
   be set to the default constant value 30 seconds as specified in
   [RFC4861].

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

   Different values for 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 an 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 segment routing, link-layer address and
   traffic selector information for the neighbor's underlying
   interfaces.  This information is stored in the neighbor cache and



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   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.
   When the SRT is 0, LHS instead includes a Peer Index value meaningful
   to the node identified by FMT and L2ADDR.

   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 current,
      consistent and authentic 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



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      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
      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



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      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 NS/NA(WIN) message
   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 NS/NA(WN)
   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.

   When Bridges and LHS Proxy/Servers forward and NS/NA(WIN) exchange
   between a ROS and a local segment Client, they also cache receive
   window information in a Peer List entry maintained by the local
   segment Client NCE.  This allows FHS Bridges and Proxy/Servers to
   verify that carrier packets produced by the local Client include
   Identifications within the ROS' receive window before forwarding.

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.  The OAL encapsulates original IP packets
   to form OAL packets subject to fragmentation, then encapsulates the
   resulting OAL fragments 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.





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   When an FHS Bridge or Proxy/Server re-encapsulates a carrier packet
   received from a Client with 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.

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.





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3.9.  OMNI Interface MTU

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

   When the network layer presents an original IP packet to the OMNI
   interface, the OAL source encapsulates and fragments the original IP
   packet if necessary.  When the network layer presents the OMNI
   interface with multiple original IP packets bound to the same OAL
   destination, the OAL source can concatenate them together into a
   single OAL super-packet as discussed in [I-D.templin-6man-omni].  The
   OAL source then fragments the OAL packet if necessary according to
   the minimum/path MPS such that the OAL headers appear in each
   fragment while the original IP packet header appears only in the
   first fragment.  The OAL source then encapsulates each OAL fragment
   in *NET headers for transmission as carrier packets over an
   underlying interface connected to either a physical link (such as
   Ethernet, WiFi and the like) or a virtual link such as an Internet or
   higher-layer tunnel (see the definition of link in [RFC8200]).

   Note: 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 FHS Bridge or 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 an LHS Bridge or
   Proxy/Server (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



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   authentication as discussed in Section 3.8.  Those that satisfy data
   origin authentication are processed further, while all others are
   dropped silently.

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

   OMNI interfaces may have multiple underlying interfaces and/or
   neighbor cache entries for neighbors with multiple underlying
   interfaces (see Section 3.3).  The OAL uses Interface Attributes
   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 segment
   routing and/or link-layer destination addresses 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).





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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
   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



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   network layer.  If the OAL destination does not match, the Client
   drops the original IP packet and MAY return a network-layer ICMP
   Destination Unreachable message subject to rate limiting (see:
   Section 3.11).

   Note: When an LHS Bridge or Proxy/Server forwards an NA(WIN), it
   overwrites its own link-layer address and a Peer Index value in the
   Interface Attribute option.  When the local Client updates the NCE
   for this ROS, it caches the link-layer address and Peer Index
   information as the FHS encapsulation values to use when sending
   return carrier packets to the ROS.

   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.)

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 packet is an NA(WIN) message for a local Client NCE, the
      Proxy/Server examines the Interface Attributes information and
      rewrites the fields if the NA(WIN) was not already processed by a
      (local segment) Bridge as discussed in Section 3.2.7.



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   o  else, 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 via INET
      encapsulation.  If the neighbor is behind a NAT, this FHS Proxy/
      Server instead forwards initial carrier packets via a Bridge (or
      more directly via an LHS Proxy/Server) 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
   original IP packet then acts as a Relay to present it to the network
   layer where it will be delivered according to standard IP forwarding.

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



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   packet and forwards it to a Bridge which will eventually deliver it
   to the new Proxy/Server.

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

   Note: Proxy/Servers may receive carrier packets with ORHs that
   include additional forwarding information.  Proxy/Servers use the
   forwarding information to determine the correct NCE and underlying
   interface for forwarding to the target Client, then remove the 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
   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.



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

   Note: Proxy/Servers forward secure control plane carrier packets via
   the SRT secured spanning tree and forward other carrier packets via
   the unsecured spanning tree.  When a Proxy/Server receives a carrier
   packet from the secured spanning tree, it considers the message as
   authentic without having to verify upper layer authentication
   signatures.  When a Proxy/Server receives a carrier packet from the
   unsecured spanning tree, it 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 critical IPv6 ND control messages or
   routing protocol control messages via the secured spanning tree, and
   may convey other carrier packets via the unsecured spanning tree.
   When the Bridge receives a carrier packet, it removes the outer *NET
   header and searches for a forwarding table 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 processes the carrier
      packet locally before forwarding.  If the carrier packet contains
      an NS/NA(WIN) message, the Bridge creates/updates an NCE for the
      destination Client and rewrites the Interface Attributes as
      discussed in Section 3.2.7.  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 Trailer into the OAL destination.  The Bridge then



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      removes the ORH and forwards the packet using encapsulation based
      on the NATed L2ADDR information.  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.  (Note: for
      IPv6 ND messages that require security, the Bridge must forward
      the message to the LHS Proxy/Server over the secured spanning tree
      instead of directly to the Client itself even if NAT traversal has
      converged.)

   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
   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



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

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










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

         Figure 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
      message as an indication that a path may be failing, and
      optionally initiate NUD over that path.  If it receives
      Destination Unreachable messages over multiple paths, the node
      should allow future carrier packets destined to the correspondent




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

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

   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.

3.12.  AERO Router Discovery, Prefix Delegation and Autoconfiguration

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





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3.12.1.  AERO Service Model

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

   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
   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



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   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.
   The Client can use its MNP-LLA as the network-layer source address
   and include an OMNI option with prefix registration information.  If
   the Client does not yet have an MNP-LLA, it instead sets the network-
   layer source address to unspecified (::) and includes prefix
   delegation parameters in the OMNI option (see:
   [I-D.templin-6man-omni]).

   The Client next includes an authentication sub-option if necessary,
   Interface Attributes corresponding to the underlying interface over
   which it will send the RS message, and optionally any additional
   Interface Attributes corresponding 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 as an SRT local
   segment reference point for 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,



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   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.

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




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   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,
   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 if
   necessary, then decapsulates and authenticates the RS message before
   processing the prefix delegation/registration parameters.  The Proxy/
   Server then 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
   resulting in a BGP update (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
   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



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   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.

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.





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   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.

3.13.  The AERO Proxy Function

   OMNI link Clients register 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 FHS Proxy/Server for each interface is directly connected, for
   Clients on ANET underlying interfaces the Proxy/Server is located on
   the ANET/INET boundary, and for Clients on INET underlying interfaces
   the FHS Proxy/Server is located somewhere in the connected
   Internetwork.  When FHS Proxy/Server "A" processes a Client



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   registration, it must also forward a proxyed registration to any
   Proxy/Servers "B", "C", "D", etc. for the Client's other underlying
   interfaces, which it perceives as LHS Proxy/Servers (i.e., and not
   FHS) from its own reference point.  Proxy/Servers 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 if necessary 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 MS-Register/Release sub-options 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 writes its own FMT/SRT/LHS/L2ADDR information over the
      first Interface Attributes supplied by the Client (i.e., by
      inserting the FMT/SRT/LHS/L2ADDR fields if not already present, or
      increasing/decreasing the L2ADDR field size if the IP version of
      the Client underlying interface 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,
      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
      Attributes sub-option with omIndex 0 and with FMT/SRT/LHS/L2ADDR



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      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 "B", "C", "D" as a FHS
      Proxy/Server while providing MS-Register//Release information for
      the others as LHS Proxy/Servers.

   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 over the Client's underlying
   interface with destination set to (link-local) All-Nodes multicast
   and with Router Lifetime set to zero to inform Clients that another
   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,
   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



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   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: When a Proxy/Server receives a secured Client NS message, it
   performs the same proxying procedures as for described for RS
   messages above.  The proxying procedures for NS/NA message exchanges
   is specified in Section 3.14.

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"
   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 NS/NA(NUD) control messaging is carried
   only over well-connected ground domain networks (i.e., and not low-
   end aeronautical radio links) and can therefore be tuned for rapid
   response.




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   Proxy/Server "A" performs continuous NS/NA(NUD) exchanges with peer
   Proxy/Server "B" for which there are currently active Clients in
   rapid succession, e.g., one exchange per second.  Proxy/Server "A"
   sends the NS(NUD) 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, and Proxy/Server "B" responds with an NA(NUD).  When
   Proxy/Server "A" is also sending RS messages to the peer Proxy/Server
   "B" on behalf of 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(NUD) 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 unsolicited RA
   messages

   Proxy/Server "A" sends unsolicited RA messages 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 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 FHS
   Proxy/Server "A" to coordinate with multiple LHS 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/MS-Release
   OMNI sub-options.

   When FHS Proxy/Server "A" receives the RS and processes the OMNI
   option, it sends a separate RS to each MS-Register/MS-Release LHS
   Proxy/Server "B", "C", "D", etc.  When FHS Proxy/Server "A" receives
   an LHS Proxy/Server RA, it can optionally return an immediate
   "singleton" RA to the Client or record the LHS Proxy/Server's MSID
   for inclusion in a pending "aggregate" RA message.  FHS Proxy/Server
   "A" can then return aggregate RA messages to the Client including
   multiple LHS Proxy/Server MSIDs in order to conserve bandwidth.  Each
   RA includes a proper subset of the LHS Proxy/Server MSIDs from the
   original RS message, and FHS Proxy/Server "A" must ensure that the
   message contents of each RA are consistent with the information
   received from the (aggregated) LHS Proxy/Servers.

   Clients can thereafter employ efficient point-to-multipoint LHS
   Proxy/Server coordination under the assistance of FHS Proxy/Server



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   "A" to reduce the number of messages sent over the *NET.  Clients can
   further include MS-Release sub-options in IPv6 ND messages to request
   FHS Proxy/Server "A" to release from former LHS Proxy/Servers via the
   procedures discussed in Section 3.16.5.

   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
   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 request sent by
   the ROS to the nearest available ROR, which returns a route
   optimization reply.  The exact ROR selected is unimportant as long as
   the information returned is current, consistent and authentic.  The
   ROS is responsible for periodically refreshing the route
   optimization, and the ROR is responsible for quickly informing the
   ROS of any changes.

   The procedures are specified in the following sections.






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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
   authentication sub-option if necessary, an Interface Attributes sub-
   option 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 not exchange other packet
   types with the ROR.)  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 must instead forward the resulting
   carrier packets to the ADM-ULA of one of its current Proxy/Servers.
   The Proxy/Server reassembles if necessary, then verifies the NS(AR)
   authentication signature and writes its FMT/SRT/LHS/L2ADDR
   information into the Interface Attributes sub-option the same as
   described for RS messages in Section 3.13.  The Proxy/Server then re-
   encapsulates the NS(AR) with the OAL source set to its own ADM-ULA
   and OAL destination set to the ULA corresponding to the target,




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   fragments if necessary and forwards 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 notes whether the NS (AR)
      arrived from the secured spanning tree then acts as an ROR to
      provide route optimization information on behalf of the Client.
      If the message arrived via the secured spanning tree the ROR need
      not perform further authentication; otherwise, it must verify the
      message authentication signature before accepting.

   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




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   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 includes an OMNI option
   with Preflen set to the prefix length associated with the NA(AR)
   source address, with S/T-omIndex set to the value that appeared in
   the NS(AR) and with Interface Attributes sub-options 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.

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



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   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 configues the ADM-LLA
   corresponding to the new ADM-ULA, then caches 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 while it still
   has sustained 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 and includes an ORH with Segments Left set to 1 and with
   FMT/SRT/LHSL2ADDR addressing information for the LHS Proxy/Server.
   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, includes an Interface Attributes sub-option for its
   underlying interface and, then forwards the NS(WIN) to its FHS Proxy/
   Server which must reassemble, verify the authentication signature and



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   rewrite the Interface Attributes FMT/SRT/LHS/L2ADDR information 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 Subnet Router
   Anycast address for the LHS as the OAL destination.

   When an LHS Bridge that configures the Subnet Router Anycast address
   receives the NS(WIN) carrier packets over the secured spanning tree,
   it creates/updates a NCE for the NS(WIN) destination and creates a
   Peer List entry and Peer Index as discussed in Section 3.2.7.  The
   Bridge then re-encapsulates, re-fragments and forwards the NS(WIN)
   carrier packets into the secured spanning tree with the ADM-ULA of
   the LHS Proxy/Server as the OAL destination.

   When the LHS Proxy/Server receives the NS(WIN) carrier packets 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 Subnet Router Anycast address for the NS(WIN)
   OAL source.  The LHS Proxy/Server then fragments if necessary,
   includes an ORH with FMT/SRT/LHS/L2ADDR found in the NS(WIN) OMNI
   option, then forwards the resulting carrier packets into the secured
   spanning tree which will deliver them to a FHS Bridge on the ROS
   Proxy/Server segment.

   If the NS(WIN) destination is the MNP-LLA of the target Client, the
   LHS Proxy/Server instead removes the ORH, 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 Client over the underlying interface identified
   by the ORH omIndex (or, over any underlying interface if omIndex is
   0).  When the target Client 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 Interface Attributes
   sub-option and 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 with FMT/SRT/LHS/L2ADDR information
   copied from the NS(WIN) Interface Attributes.  The target Client then
   sets the ORH omIndex to the S/T-omIndex value found in the NS(WIN)
   OMNI option, then forwards the NA(WIN) message to the LHS Proxy/
   Server.



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   When the LHS Proxy/Server receives the message, it reassembles if
   necessary, verifies the authentication signature and rewrites the
   Interface Attributes FMT/SRT/LHS/L2ADDR information if necessary,
   then re-encapsulates using its own ADM-ULA as the source and the ADM-
   ULA of the LHS Bridge as the destination.  The LHS Proxy/Server then
   re-fragments and forwards the NA(WIN) carrier packets into the
   secured spanning tree which will direct them to the LHS Bridge.

   When the LHS Bridge receives the NA(WIN) carrier packets, it
   reassembles if necessary then locates the NCE for the local segment
   Client along with the Peer List information it had previously cached
   for this ROS.  The LHS Bridge then re-encapsulates using its own ADM-
   ULA as the source and the Subnet Router Anycast for the FHS segment
   as the destination.  The LHS Bridge then re-fragments and forwards
   the NA(WIN) carrier packets into the secured spanning tree which will
   direct them to the FHS segment.

   When an FHS Bridge that configures the Subnet Router Anycast address
   receives the NA(WIN) carrier packets, it creates/updates a NCE for
   the NA(WIN) destination, creates a Peer List entry and Peer Index and
   rewrites the Interface Attributes FMT/SRT/LHS/L2ADDR information as
   discussed in Section 3.2.7.  The Bridge then re-encapsulates, re-
   fragments and forwards the NA(WIN) carrier packets into the secured
   spanning tree with the ADM-ULA of the FHS Proxy/Server as the OAL
   destination while leaveing the ADM-ULA of the LHS Bridge as the OAL
   source.

   When the FHS Proxy/Server receives the NA(WIN), it reassembles if
   necessary then updates the NCE for the NA(WIN) target based on the
   message contents if the Proxy/Server itself is the ROS.  If the
   NA(WIN) source was the ADM-LLA of the LHS Proxy/Server, the ROS must
   also create and maintain a NCE for the LHS Proxy/Server which it must
   regard as a companion to the existing target Client NCE.  (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 then re-encapsulates and re-fragments if
   necessary while changing the OAL destination to the MNP-ULA of the
   ROS Client.  The FHS Proxy/Server then forwards the NA(WIN) to the
   ROS Client over the underlying interface identified by the ORH
   omIndex which then updates its own NCE based on the target Client
   MNP-LLA or LHS Proxy/Server ADM-LLA.

   The ROS (whether the FHS Proxy/Server or the Client itself) finally
   arranges to return an acknowledgement if requested by the NA(WIN) by



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   sending an NA(WIN) acknowledgement in the reverse path over the
   secured spanning tree with its own ULA as the source and the ADM-ULA
   of the LHS Bridge as the destination.  When the NA(WIN)
   acknowledgement arrives, the LHS Bridge updates the destination
   Client NCE, rewrites the Interface Attributes and securely forwards
   the NA(WIN) toward the destination.

   After window synchronization state has been established, the ROS and
   target Client 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.  The ROS sends
   carrier packets to the FHS Bridge discovered through the NS/NA(WIN)
   exhange which verifies the Identification is in window for the target
   Client.  The FHS Bridge then forwards the carrier packets over the
   unsecured spanning tree to the LHS Bridge, which forwards them via
   LHS encapsulation to the LHS Proxy/Server or directly to the target
   Client itself.  The target Client in turn sends packets to the ROS in
   the reverse direction while forwarding through the Bridges to
   minimize Proxy/Server load whenever possible.

   The above forwarding procedures apply to the case where the selected
   target interface SRT/LHS codes indicate that the interface is located
   in a remote OMNI link segment.  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 L2ADDR
   encapsulation information following the initial NS/NA(WIN) secured
   spanning tree exchange (or even directly 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 if
   necessary to inform the Proxy/Server as to whether it must reassemble
   and/or the correct target Client omIndex 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.

   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(WIN) 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



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   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 either 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
   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 Trailers (if present)
   are set according to the LLA destination suffix (i.e., and not the
   ULA/GUA destination).  The ROS sets the NS(NUD) OMNI header S/



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   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/L2ADDR 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.

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.




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   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
   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.  If the destination was its own ADM-



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   ULA, the ROS Proxy/Server 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
   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 former Proxy/Server
   associated with the underlying interface for which the link-layer
   address has changed.  These uNA messages update former Proxy/Servers
   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 former 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 former 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 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.

   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.





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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 message to an FHS Proxy/Server with an OMNI option
   with appropriate Interface Attribute values for the deactivated link
   - in particular, the link quality value 0 assures that neighbors will
   cease to use the link.

   If the Client needs to send RS 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 new RS
   messages to an FHS Proxy/Server 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.
   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 INET 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



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   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 it
   includes an Interface Attributes sub-option for the new Proxy/Server
   and with S/T-omIndex set to 0.  The old Proxy/Server then changes the
   Client's NCE state to DEPARTED, sets the FMT/SRT/LHS/L2ADDR
   information for the Client to point to the new Proxy/Server, and
   resets DepartTime.  The old Proxy/Server then returns an RA message
   with zero Router Lifetime 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
   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 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.  The new Proxy/Server will then process the NS(AR) and
   return an NA(AR) response.

   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.

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.



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   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
   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



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   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.

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



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   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
   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 (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 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




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   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.

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



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   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.

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 its IP address and UDP port number in the Interface
   Attributes sub-option corresponding to the underlying interface



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   (otherwise it may omit the FMT/SRT/LHS/L2ADDR fields).  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 Interface Attributes sub-option includes 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 an "INET" address meaning
   that the Client is likely to accept direct messages 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 a "mapped" address 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



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   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.

   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 Proxy/Server forwarding even for these
   other NAT variations.

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



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   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.

   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.



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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).

   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,



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   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.

   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.




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

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

   This work was inspired by the support and encouragement of countless
   outstanding colleagues, managers and program directors over the span
   of many decades.  Beginning in the late 1980s,' the Digital Equipment



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   Corporation (DEC) Ultrix Engineering and DECnet Architects groups
   identified early issues with fragmentation and bridging links with
   diverse MTUs.  In the early 1990s, engagements at DEC Project Sequoia
   at UC Berkeley and the DEC Western Research Lab in Palo Alto included
   investigations into large-scale networked filesystems, ATM vs
   Internet and network security 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, ONR UAV
   Communications and the IETF.  An employment at Nokia where important
   IETF documents were published gave way to a present-day engagement
   with The Boeing Company.  The work matured at Boeing through major
   programs including Future Combat Systems, Advanced Airplane Program,
   DTN for the International Space Station, Mobility Vision Lab, CAST,
   Caravan, the NASA UAS/CNS program, the FAA/ICAO ATN/IPS program and
   many others.  An attempt to name all who gave support and
   encouragement would double the current document size and result in
   many unintentional omissions - but to all a humble thanks.

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

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

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

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

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

   o  AERO, First Edition [RFC6706]

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

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

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

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




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   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>.

   [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>.






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   [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>.

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

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

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

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

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

8.2.  Informative References

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




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

   [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.




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   [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.

   [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>.






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   [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>.

   [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>.








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   [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>.

   [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>.




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   [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>.

   [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>.






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   [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>.

   [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>.



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   [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
   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



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   and begins accepting and sending packets according to the Client's
   new address.  This implicit mobility method applies to use cases such
   as cellphones with both WiFi and Cellular interfaces where only one
   of the interfaces is active at a given time, and the Client
   automatically switches over to the backup interface if the primary
   interface fails.

A.3.  Direct Underlying Interfaces

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

A.4.  AERO Critical Infrastructure Considerations

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

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

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



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

A.5.  AERO Server Failure Implications

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

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

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

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

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.



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

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

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

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

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

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



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   no client failover possibility, and if there is any worldwide service
   distribution it would need to be conducted by a network element that
   is reached via the IP address acting as a service distribution point.

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

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

   For AERO, the expectation is that a combination of the Google/IETF
   and Yahoo/Amazon philosophies would be employed.  The AERO Client
   connects to different ANET access points and can receive 1-2 Proxy/
   Server ADM-LLAs at each point.  It then selects one AERO Proxy/Server
   address, and engages in RS/RA exchanges with the same Proxy/Server
   from all ANET connections.  The Client remains with this Proxy/Server
   unless or until the Proxy/Server fails, in which case it can switch
   over to an alternate Proxy/Server.  The Client can likewise switch
   over to a different Proxy/Server at any time if there is some reason
   for it to do so.  So, the AERO expectation is for a balance of
   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-17 to draft-templin-6man-aero-
   18:




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   o  Interim version with extensive new text - cleanup planned for next
      release.

   Changes from draft-templin-6man-aero-16 to draft-templin-6man-aero-
   17:

   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-15 to draft-templin-6man-aero-
   16:

   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-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).

   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:




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   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.

   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.



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   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.

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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