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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720,                      April 4, 2019
           rfc6179, rfc6706 (if
           approved)
Intended status: Standards Track
Expires: October 6, 2019


             Asymmetric Extended Route Optimization (AERO)
                  draft-templin-intarea-6706bis-11.txt

Abstract

   This document specifies the operation of IP over tunnel virtual links
   using Asymmetric Extended Route Optimization (AERO).  Nodes attached
   to AERO links can exchange packets via trusted intermediate routers
   that provide forwarding services to reach off-link destinations and
   route optimization services for improved performance.  AERO provides
   an IPv6 link-local address format that supports operation of the IPv6
   Neighbor Discovery (ND) protocol and links ND to IP forwarding.
   Prefix delegation services are employed to manage the routing system.
   Dynamic link selection, mobility management, quality of service (QoS)
   signaling and route optimization are naturally supported through
   dynamic neighbor cache updates.  AERO is a widely-applicable
   tunneling solution especially well-suited to aviation services,
   mobile Virtual Private Networks (VPNs) and other applications as
   described in this document.

Status of This Memo

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

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

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

   This Internet-Draft will expire on October 6, 2019.






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Copyright Notice

   Copyright (c) 2019 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Asymmetric Extended Route Optimization (AERO) . . . . . . . .   8
     3.1.  AERO Link Reference Model . . . . . . . . . . . . . . . .   8
     3.2.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  AERO Routing System . . . . . . . . . . . . . . . . . . .  11
     3.4.  AERO Addresses  . . . . . . . . . . . . . . . . . . . . .  13
     3.5.  Spanning Partitioned AERO Networks (SPAN) . . . . . . . .  15
     3.6.  AERO Interface Characteristics  . . . . . . . . . . . . .  17
     3.7.  AERO Interface Initialization . . . . . . . . . . . . . .  21
       3.7.1.  AERO Relay Behavior . . . . . . . . . . . . . . . . .  21
       3.7.2.  AERO Server Behavior  . . . . . . . . . . . . . . . .  21
       3.7.3.  AERO Proxy Behavior . . . . . . . . . . . . . . . . .  22
       3.7.4.  AERO Client Behavior  . . . . . . . . . . . . . . . .  22
     3.8.  AERO Interface Neighbor Cache Maintenance . . . . . . . .  23
     3.9.  AERO Interface Forwarding Algorithm . . . . . . . . . . .  25
       3.9.1.  Client Forwarding Algorithm . . . . . . . . . . . . .  26
       3.9.2.  Proxy Forwarding Algorithm  . . . . . . . . . . . . .  26
       3.9.3.  Server Forwarding Algorithm . . . . . . . . . . . . .  27
       3.9.4.  Relay Forwarding Algorithm  . . . . . . . . . . . . .  27
     3.10. AERO Interface Encapsulation and Re-encapsulation . . . .  28
     3.11. AERO Interface Decapsulation  . . . . . . . . . . . . . .  29
     3.12. AERO Interface Data Origin Authentication . . . . . . . .  29
     3.13. AERO Interface Packet Size Issues . . . . . . . . . . . .  30
     3.14. AERO Interface Error Handling . . . . . . . . . . . . . .  32
     3.15. AERO Router Discovery, Prefix Delegation and
           Autoconfiguration . . . . . . . . . . . . . . . . . . . .  35
       3.15.1.  AERO ND/PD Service Model . . . . . . . . . . . . . .  35
       3.15.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  36
       3.15.3.  AERO Server Behavior . . . . . . . . . . . . . . . .  38
     3.16. The AERO Proxy  . . . . . . . . . . . . . . . . . . . . .  40



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       3.16.1.  The AERO-Aware Access Router . . . . . . . . . . . .  42
     3.17. AERO Route Optimization . . . . . . . . . . . . . . . . .  43
       3.17.1.  Route Optimization Initiation  . . . . . . . . . . .  43
       3.17.2.  Relaying the NS  . . . . . . . . . . . . . . . . . .  43
       3.17.3.  Processing the NS and Sending the NA . . . . . . . .  44
       3.17.4.  Relaying the NA  . . . . . . . . . . . . . . . . . .  44
       3.17.5.  Processing the NA  . . . . . . . . . . . . . . . . .  44
       3.17.6.  Route Optimization Maintenance . . . . . . . . . . .  45
     3.18. Neighbor Unreachability Detection (NUD) . . . . . . . . .  46
     3.19. Mobility Management and Quality of Service (QoS)  . . . .  46
       3.19.1.  Mobility Update Messaging  . . . . . . . . . . . . .  47
       3.19.2.  Forwarding Packets on Behalf of Departed Clients . .  48
       3.19.3.  Announcing Link-Layer Address and/or QoS Preference
                Changes  . . . . . . . . . . . . . . . . . . . . . .  48
       3.19.4.  Bringing New Links Into Service  . . . . . . . . . .  49
       3.19.5.  Removing Existing Links from Service . . . . . . . .  49
       3.19.6.  Implicit Mobility Management . . . . . . . . . . . .  49
       3.19.7.  Moving to a New Server . . . . . . . . . . . . . . .  50
     3.20. Multicast Considerations  . . . . . . . . . . . . . . . .  50
   4.  Direct Underlying Interfaces  . . . . . . . . . . . . . . . .  51
   5.  Operation on AERO Links with /64 ASPs . . . . . . . . . . . .  51
   6.  AERO Adaptations for SEcure Neighbor Discovery (SEND) . . . .  52
   7.  AERO Critical Infrastructure Considerations . . . . . . . . .  52
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  53
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  53
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  54
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  55
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  56
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  56
     12.2.  Informative References . . . . . . . . . . . . . . . . .  58
   Appendix A.  AERO Alternate Encapsulations  . . . . . . . . . . .  63
   Appendix B.  S/TLLAO Extensions for Special-Purpose Links . . . .  65
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  66
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  70

1.  Introduction

   This document specifies the operation of IP over tunnel virtual links
   using Asymmetric Extended Route Optimization (AERO).  The AERO link
   can be used for tunneling between neighboring nodes over either IPv6
   or IPv4 networks, i.e., AERO views the IPv6 and IPv4 networks as
   equivalent links for tunneling.  Nodes attached to AERO links can
   exchange packets via trusted intermediate routers that provide
   forwarding services to reach off-link destinations and route
   optimization services for improved performance [RFC5522].

   AERO provides an IPv6 link-local address format that supports
   operation of the IPv6 Neighbor Discovery (ND) [RFC4861] protocol and



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   links ND to IP forwarding.  Dynamic link selection, mobility
   management, quality of service (QoS) signaling and route optimization
   are naturally supported through dynamic neighbor cache updates, while
   IPv6 Prefix Delegation (PD) is supported by network services such as
   the Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC8415].

   A node's AERO interface can be configured over multiple underlying
   interfaces.  From the standpoint of ND, AERO interface neighbors
   therefore may appear to have multiple link-layer addresses (i.e., the
   IP addresses assigned to underlying interfaces).  Each link-layer
   address is subject to change due to mobility and/or QoS fluctuations,
   and link-layer address changes are signaled by ND messaging the same
   as for any IPv6 link.

   AERO is applicable to a wide variety of use cases.  For example, it
   can be used to coordinate the Virtual Private Network (VPN) links of
   mobile nodes (e.g., cellphones, tablets, laptop computers, etc.) that
   connect into a home enterprise network via public access networks
   using services such as OpenVPN [OVPN].  AERO is also applicable to
   aviation services for both manned and unmanned aircraft where the
   aircraft is treated as a mobile node that can connect an Internet of
   Things (IoT).  Other applicable use cases are also in scope.

   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; the following
   terms are defined within the scope of this document:

   IPv6 Neighbor Discovery (ND)
      an IPv6 control message service for coordinating neighbor
      relationships between nodes connected to a common link.  The ND
      service used by AERO is specified in [RFC4861].

   IPv6 Prefix Delegation (PD)
      a networking service for delegating IPv6 prefixes to nodes on the
      link.  The nominal PD service is DHCPv6 [RFC8415], however
      alternate services (e.g., based on ND messaging) are also in scope
      [I-D.templin-v6ops-pdhost][I-D.templin-6man-dhcpv6-ndopt].

   (native) Internetwork
      a connected IP network topology over which the AERO link virtual
      overlay is configured and native peer-to-peer communications are
      supported.  Example Internetworks include the global public
      Internet, private enterprise networks, aviation networks, etc.




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   AERO link
      a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay
      configured over an underlying Internetwork.  Nodes on the AERO
      link appear as single-hop neighbors from the perspective of the
      virtual overlay even though they may be separated by many
      underlying Internetwork hops.  The AERO mechanisms can also
      operate over native link types (e.g., Ethernet, WiFi etc.) when
      tunneling is not needed.

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

   AERO address
      an IPv6 link-local address constructed as specified in
      Section 3.4.

   AERO node
      a node that is connected to an AERO link.

   AERO Client ("Client")
      a node that requests PDs from one or more AERO Servers.  Following
      PD, the Client assigns a Client AERO address to the AERO interface
      for use in ND exchanges with other AERO nodes.  A node that acts
      as an AERO Client on one AERO interface can also act as an AERO
      Server on a different AERO interface.

   AERO Server ("Server")
      a node that configures an AERO interface to provide default
      forwarding services and a Mobility Anchor Point (MAP) for AERO
      Clients.  The Server assigns an administratively-provisioned AERO
      address to the AERO interface to support the operation of the ND/
      PD services.  An AERO Server can also act as an AERO Relay.

   AERO Relay ("Relay")
      a node that provides both layer-3 routing and layer-2 bridging
      services (as well as a security trust anchor) for nodes on an AERO
      link.  As a router, the Relay forwards data packets using standard
      IP forwarding.  As a bridge, the Relay securely forwards control
      messages between Proxies and Servers both within the same
      partition and between disjoint partitions.

   AERO Proxy ("Proxy")
      a node that provides proxying services, e.g., when the Client is
      located in a secured internal enclave and the Server is located in



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      the external Internetwork.  The AERO Proxy is a conduit between
      the secured enclave and the external Internetwork in the same
      manner as for common web proxies, and behaves in a similar fashion
      as for ND proxies [RFC4389].

   ingress tunnel endpoint (ITE)
      an AERO interface endpoint that injects encapsulated packets into
      an AERO link.

   egress tunnel endpoint (ETE)
      an AERO interface endpoint that receives encapsulated packets from
      an AERO link.

   underlying network
      the same as defined for Internetwork.

   underlying link
      a link that connects an AERO node to the underlying network.

   underlying interface
      an AERO node's interface point of attachment to an underlying
      link.

   link-layer address
      an IP address assigned to an AERO node's underlying interface.
      When UDP encapsulation is used, the UDP port number is also
      considered as part of the link-layer address.  Packets transmitted
      over an AERO interface use link-layer addresses as encapsulation
      header source and destination addresses.  Destination link-layer
      addresses can be either "reachable" or "unreachable" based on
      dynamically-changing network conditions.

   network layer address
      the source or destination address of an encapsulated IP packet.

   end user network (EUN)
      an internal virtual or external edge IP network that an AERO
      Client connects to the rest of the network via the AERO interface.
      The Client sees each EUN as a "downstream" network and sees the
      AERO interface as its point of attachment to the "upstream"
      network.

   AERO Service Prefix (ASP)
      an IP prefix associated with the AERO link and from which more-
      specific AERO Client Prefixes (ACPs) are derived.  The term ASP is
      equivalent to "Mobility Service Prefix (MSP)" that appears in
      other contexts.




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   AERO Client Prefix (ACP)
      an IP prefix derived from an ASP and delegated to a Client, where
      the ACP prefix length must be no shorter than the ASP prefix
      length.  The term ACP is equivalent to "Mobile Network Prefix
      (MNP)" that appears in other contexts.

   base AERO address
      the lowest-numbered AERO address from the first ACP delegated to
      the Client (see Section 3.4).

   secured enclave
      a private access network (e.g., a corporate enterprise network,
      radio access network, cellular service provider network, etc.)
      with secured links and perimeters.  Link-layer security services
      such as IEEE 802.1X and physical-layer security such as campus
      wired LANs prevent unauthorized access from within the enclave,
      while border network-layer security services such as firewalls and
      proxies prevent unauthorized access from the external
      Internetwork.

   Mobility Anchor Point (MAP)
      an AERO Server that is currently tracking and reporting the
      mobility events of its associated Clients.

   MAP List
      a geographically and/or topologically referenced list of IP
      addresses of Servers for the AERO link.

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

   Route Optimization Source (ROS)
      the AERO node nearest the source Client that initiates route
      optimization.  The ROS may be one of the Client's Servers, Proxies
      or in some cases even the Client itself.

   Route Optimization Responder (ROR)
      a Server of the target Client to which a route optimization
      request is directed.  The ROR (acting as a MAP) returns the most
      current information about the target Client's underlying interface
      connections.

   Spanning Partitioned AERO Networks (SPAN)
      a means for bridging disjoint segments of a partitioned AERO link,
      i.e., the same as for a bridged campus LAN.  The SPAN is an
      underlay encapsulation service in the AERO routing system, and
      provides a unified link view for all partitions.



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   SPAN Service Prefix (SSP)
      a global or unique local /96 IPv6 prefix assigned to the AERO link
      to support SPAN services.

   SPAN Partition Prefix (SPP)
      a sub-prefix of the SPAN Service Prefix uniquely assigned to a
      single partition of the SPAN.

   SPAN Address
      a global or unique local IPv6 address taken from a SPAN Partition
      Prefix.

   Throughout the document, the simple terms "Client", "Server", "Relay"
   and "Proxy" refer to "AERO Client", "AERO Server", "AERO Relay" and
   "AERO Proxy", respectively.  Capitalization is used to distinguish
   these terms from DHCPv6 client/server/relay [RFC8415].

   The terminology of DHCPv6 [RFC8415] and IPv6 ND [RFC4861] (including
   the names of node variables, messages and protocol constants) is used
   throughout this document.  Also, the term "IP" is used to generically
   refer to either Internet Protocol version, i.e., IPv4 [RFC0791] or
   IPv6 [RFC8200].

   The terms Mobility Anchor Point (MAP) and Distributed Mobility
   Management (DMM) are used in the same sense as standard
   Internetworking terminology.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  Lower case
   uses of these words are not to be interpreted as carrying RFC2119
   significance.

3.  Asymmetric Extended Route Optimization (AERO)

   The following sections specify the operation of IP over Asymmetric
   Extended Route Optimization (AERO) links:

3.1.  AERO Link Reference Model












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                              .-(::::::::)
                           .-(::::::::::::)-.
                          (:: Internetwork ::)
                           `-(::::::::::::)-'
                              `-(::::::)-'
                                   |
       +--------------+   +--------+-------+   +--------------+
       |AERO Server S1|   | AERO Relay R1  |   |AERO Server S2|
       |  Nbr: C1, R1 |   |   Nbr: S1, S2  |   |  Nbr: C2, R1 |
       |  default->R1 |   |(X1->S1; X2->S2)|   |  default->R1 |
       |    X1->C1    |   |      ASP A1    |   |    X2->C2    |
       +-------+------+   +--------+-------+   +------+-------+
               |    AERO Link      |                  |
       X---+---+-------------------+-+----------------+---+---X
           |                         |                    |
     +-----+--------+     +----------+------+    +--------+-----+
     |AERO Client C1|     |  AERO Proxy P1  |    |AERO Client C2|
     |    Nbr: S1   |     |(Proxy Nbr Cache)|    |   Nbr: S2    |
     | default->S1  |     +--------+--------+    | default->S2  |
     |    ACP X1    |              |             |    ACP X2    |
     +------+-------+     .--------+------.      +-----+--------+
            |           (- Proxyed Clients -)          |
           .-.            `---------------'           .-.
        ,-(  _)-.                                  ,-(  _)-.
     .-(_  IP   )-.   +-------+     +-------+    .-(_  IP   )-.
   (__    EUN      )--|Host H1|     |Host H2|--(__    EUN      )
      `-(______)-'    +-------+     +-------+     `-(______)-'

                    Figure 1: AERO Link Reference Model

   Figure 1 presents the AERO link reference model.  In this model:

   o  AERO Relay R1 aggregates AERO Service Prefix (ASP) A1, acts as a
      default router for its associated Servers (S1 and S2), and
      connects the AERO link to the rest of the Internetwork.  AERO
      Relays also bridge disjoint segments of a partitioned AERO link.

   o  AERO Servers S1 and S2 associate with Relay R1 and also act as
      Mobility Anchor Points (MAPs) and default routers for their
      associated Clients C1 and C2.

   o  AERO Clients C1 and C2 associate with Servers S1 and S2,
      respectively.  They receive AERO Client Prefix (ACP) 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.





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   o  AERO Proxy P1 provides proxy services for AERO Clients in secured
      enclaves that cannot associate directly with other AERO link
      neighbors.

   Each node on the AERO link maintains an AERO interface neighbor cache
   and an IP forwarding table the same as for any link.  Although the
   figure shows a limited deployment, in common operational practice
   there will normally be many additional Relays, Servers, Clients and
   Proxies.

3.2.  AERO Node Types

   AERO Relays provide both layer-3 routing and layer-2 bridging
   services (as well as a security trust anchor) for nodes on an AERO
   link.  As a router, the Relay forwards data packets using standard IP
   forwarding.  As a bridge, the Relay securely forwards control
   messages between Proxies and Servers both within the same partition
   and between disjoint partitions.  Each Relay also peers with Servers
   and other Relays in a dynamic routing protocol instance to provide a
   Distributed Mobility Management (DMM) service for the list of active
   ACPs (see Section 3.3).  Relays forward packets between neighbors
   connected to the same AERO link and also forward packets between the
   AERO link and the native Internetwork.  Relays present the AERO link
   to the native Internetwork as a set of one or more AERO Service
   Prefixes (ASPs) and serve as a gateway between the AERO link and the
   Internetwork.  Relays maintain neighbor cache entries for Servers and
   Proxies, and maintain an IP forwarding table entry for each AERO
   Client Prefix (ACP).

   AERO Servers provide default forwarding services and a Mobility
   Anchor Point (MAP) for AERO Clients.  Each Server also peers with
   Relays in a dynamic routing protocol instance to advertise its list
   of associated ACPs (see Section 3.3).  Servers facilitate PD
   exchanges with Clients, where each delegated prefix becomes an ACP
   taken from an ASP.  Servers forward packets between AERO interface
   neighbors, and maintain neighbor cache entries for Relays.  They also
   maintain both neighbor cache entries and IP forwarding table entries
   for each of their associated Clients, and track each Client's
   mobility profiles.

   AERO Clients act as requesting routers to receive ACPs through PD
   exchanges with AERO Servers over the AERO link.  Each Client can
   associate with a single Server or with multiple Servers, e.g., for
   fault tolerance, load balancing, etc.  Each IPv6 Client receives at
   least a /64 IPv6 ACP, and may receive even shorter prefixes.
   Similarly, each IPv4 Client receives at least a /32 IPv4 ACP (i.e., a
   singleton IPv4 address), and may receive even shorter prefixes.
   Clients maintain an AERO interface neighbor cache entry for each of



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   their associated Servers as well as for each of their correspondent
   Clients.

   AERO Proxies provide a conduit for AERO Clients in secured enclaves
   to associate with AERO Servers.  The Client sends all of its control
   plane messages to the Server via the Proxy, which intercepts them
   before they leave the secured enclave.  The Proxy forwards the
   Client's control and data plane messages to and from the Client's
   current Server(s).  The Proxy may also discover a better route toward
   a target destination via AERO route optimization, in which case
   future outbound data packets would be forwarded via the more direct
   route.  Proxies maintain AERO interface neighbor cache entries for
   Relays, i.e., the same as Servers.  The Proxy function is specified
   in Section 3.16.

   AERO Relays, Servers and Proxies are critical infrastructure elements
   in fixed (i.e., non-mobile) deployments.  Relays, Servers and Proxies
   must use link-layer addresses that do not change and can be reached
   from any correspondent in the underlying Internetwork (i.e., in the
   same fashion as for popular Internet services).  AERO Clients may be
   mobile, and may not have any public link-layer addresses, e.g., if
   they are located behind NATs or Proxies.

3.3.  AERO Routing System

   The AERO routing system comprises a private instance of the Border
   Gateway Protocol (BGP) [RFC4271] that is coordinated between Relays
   and Servers and does not interact with either the public Internet BGP
   routing system or the native Internetwork routing system.  Relays
   advertise only a small and unchanging set of ASPs to the native
   Internetwork routing system instead of the full dynamically changing
   set of ACPs.

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

   Each Server maintains a working set of associated ACPs, and
   dynamically announces new ACPs and withdraws departed ACPs in its
   eBGP updates to Relays.  Clients are expected to remain associated
   with their current Servers for extended timeframes, however Servers



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   SHOULD selectively suppress updates for impatient Clients that
   repeatedly associate and disassociate with them in order to dampen
   routing churn.

   Each Relay configures a black-hole route for each of its ASPs.  By
   black-holing the ASPs, the Relay will maintain forwarding table
   entries only for the ACPs that are currently active, and packets
   destined to all other ACPs will correctly incur Destination
   Unreachable messages due to the black hole route.  Relays do not send
   eBGP updates for ACPs to Servers, but instead only originate a
   default route.  In this way, Servers have only partial topology
   knowledge (i.e., they know only about the ACPs of their directly
   associated Clients) and they forward all other packets to Relays
   which have full topology knowledge.

   Scaling properties of the AERO routing system are limited by the
   number of BGP routes that can be carried by Relays.  As of 2015, the
   global public Internet BGP routing system manages more than 500K
   routes with linear growth and no signs of router resource exhaustion
   [BGP].  More recent network emulation studies have also shown that a
   single Relay can accommodate at least 1M dynamically changing BGP
   routes even on a lightweight virtual machine, i.e., and without
   requiring high-end dedicated router hardware.

   Therefore, assuming each Relay can carry 1M or more routes, this
   means that at least 1M Clients can be serviced by a single set of
   Relays.  A means of increasing scaling would be to assign a different
   set of Relays for each set of ASPs.  In that case, each Server still
   peers with one or more Relays, but the Server institutes route
   filters so that it only sends BGP updates to the specific set of
   Relays that aggregate the ASP.  For example, if the ASP for the AERO
   link is 2001:db8::/32, a first set of Relays could service the ASP
   segment 2001:db8::/40, a second set of Relays could service
   2001:db8:0100::/40, a third set could service 2001:db8:0200::/40,
   etc.

   Assuming up to 1K sets of Relays, the AERO routing system can then
   accommodate 1B or more ACPs with no additional overhead for Servers
   and Relays (for example, it should be possible to service 1B /64 ACPs
   taken from a /34 ASP and even more for shorter prefixes).  In this
   way, each set of Relays services a specific set of ASPs that they
   advertise to the native Internetwork routing system, and each Server
   configures ASP-specific routes that list the correct set of Relays as
   next hops.  This arrangement also allows for natural incremental
   deployment, and can support small scale initial deployments followed
   by dynamic deployment of additional Clients, Servers and Relays
   without disturbing the already-deployed base.




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   In an alternate routing arrangement, each set of Relays could
   advertise an aggregated ASP for the link into the native Internetwork
   routing system even though each Relay services only smaller segments
   of the ASP.  In that case, a Relay upon receiving a packet with a
   destination address covered by the ASP segment of another Relay can
   simply tunnel the packet to the other Relay.  The tradeoff then is
   the penalty for Relay-to-Relay tunneling compared with reduced
   routing information in the native routing system.

   A full discussion of the BGP-based routing system used by AERO is
   found in [I-D.ietf-rtgwg-atn-bgp].  The system provides for
   Distributed Mobility Management (DMM) per the distributed mobility
   anchoring architecture [I-D.ietf-dmm-distributed-mobility-anchoring].

3.4.  AERO Addresses

   A Client's AERO address is an IPv6 link-local address with an
   interface identifier based on the Client's delegated ACP.  Relay,
   Server and Proxy AERO addresses are assigned from the range fe80::/96
   and include an administratively-provisioned value in the lower 32
   bits.

   For IPv6, Client AERO addresses begin with the prefix fe80::/64 and
   include in the interface identifier (i.e., the lower 64 bits) a
   64-bit prefix taken from one of the Client's IPv6 ACPs.  For example,
   if the AERO Client receives the IPv6 ACP:

      2001:db8:1000:2000::/56

   it constructs its corresponding AERO addresses as:

      fe80::2001:db8:1000:2000

      fe80::2001:db8:1000:2001

      fe80::2001:db8:1000:2002

      ... etc. ...

      fe80::2001:db8:1000:20ff

   For IPv4, Client AERO addresses are based on an IPv4-mapped IPv6
   address formed from an IPv4 ACP and with a Prefix Length of 96 plus
   the ACP prefix length.  For example, for the IPv4 ACP 192.0.2.32/28
   the IPv4-mapped IPv6 ACP is:

      0:0:0:0:0:FFFF:192.0.2.16/124




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   The Client then constructs its AERO addresses with the prefix
   fe80::/64 and with the lower 64 bits of the IPv4-mapped IPv6 address
   in the interface identifier as:

      fe80::FFFF:192.0.2.16

      fe80::FFFF:192.0.2.17

      fe80::FFFF:192.0.2.18

      ... etc. ...

      fe80:FFFF:192.0.2.31

   Relay, Server and Proxy AERO addresses are allocated from the range
   fe80::/96, and MUST be managed for uniqueness.  The lower 32 bits of
   the AERO address includes a unique integer value (e.g., fe80::1,
   fe80::2, fe80::3, etc.) as assigned by the administrative authority
   for the link.  If the link comprises multiple segments, the AERO
   addresses are assigned to each segment in correspondence with the
   SPAN addresses assigned to the segment (see: Section 3.5).  The
   address fe80:: is reserved as the IPv6 link-local Subnet Router
   Anycast address [RFC4291], and the address fe80::ffff:ffff is
   reserved as the unspecified AERO address; hence, these values are not
   available general assignment.

   When the Server delegates ACPs to the Client, the lowest-numbered
   AERO address from the first ACP delegation serves as the "base" AERO
   address (for example, for the ACP 2001:db8:1000:2000::/56 the base
   AERO address is fe80::2001:db8:1000:2000).  The Client then assigns
   the base AERO address to the AERO interface and uses it for the
   purpose of maintaining the neighbor cache entry.  The Server likewise
   uses the AERO address as its index into the neighbor cache for this
   Client.

   If the Client has multiple AERO addresses (i.e., when there are
   multiple ACPs and/or ACPs with prefix lengths shorter than /64), the
   Client originates ND messages using the base AERO address as the
   source address and accepts and responds to ND messages destined to
   any of its AERO addresses as equivalent to the base AERO address.  In
   this way, the Client maintains a single neighbor cache entry that may
   be indexed by multiple AERO addresses.

   AERO addresses that embed an IPv6 prefix can be statelessly
   transformed into an IPv6 Subnet Router Anycast address and vice-
   versa.  For example, for the AERO address fe80::2001:db8:2000:3000
   the corresponding Subnet Router Anycast address is
   2001:db8:2000:3000::.  In the same way, for the IPv6 Subnet Router



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   Anycast address 2001:db8:1:2:: the corresponding AERO address is
   fe80::2001:db8:1:2.  In other words, the low-order 64 bits of an AERO
   address can be used as the high-order 64 bits of a Subnet Router
   Anycast address, and vice-versa.

3.5.  Spanning Partitioned AERO Networks (SPAN)

   In the simplest case, an AERO link configured over a single
   administrative domain (e.g., an enterprise network) appears as a
   single unified link with a consistent underlying network addressing
   plan.  In that case, all nodes on the link can exchange packets
   directly since the underlying network is connected.

   In a more complex case, an AERO link may be partitioned into multiple
   "segments", where each segment is configured over a different
   administrative domain (e.g., as for regional aviation networks).  In
   that case, the underlying network addressing plan of each 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 other segments by network security devices (e.g.,
   firewalls, proxies, packet filtering gateways, etc.), and in many
   cases disjoint segments may not even have any common physical link
   connections at all.  Therefore, the nodes within each segment can
   only be assured of exchanging packets directly with nodes in the same
   segment, and not with nodes in other segments.  The only means for
   joining the segments therefore is through inter-domain peerings
   between segment border routers.

   The same as for traditional campus LANs, multiple AERO link segments
   can be joined into a single unified link via bridging in an underlay
   network termed "The SPAN".  The SPAN performs link-layer packet
   forwarding between segments so that nodes on segment A can exchange
   packets with nodes on segment B via bridging without decrementing the
   network-layer TTL/Hop Limit.  To support the SPAN, AERO links require
   a reserved /96 IPv6 "SPAN Service Prefix (SSP)".  Although any
   routable IPv6 prefix can be reserved, use of a Unique Local Address
   (ULA) prefix (e.g., fd00::/96) [RFC4389] is RECOMMENDED since packets
   with ULAs cannot be injected into the AERO link by an external IPv6
   node and cannot leak out of the AERO link to the outside world.

   Each partition in the SPAN assigns a unique sub-prefix of the SSP,
   i.e., a "SPAN Partition Prefix (SPP)".  For example, a first
   partition could assign fd00::/116, a second partition could assign
   fd00::1000/116, a third could assign fd00::2000/116, etc.  The
   administrative authorities for each partition must therefore
   coordinate to assure mutually-exclusive SPP assignments, but internal
   provisioning of the SPP is a local consideration for each
   administrative authority.



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   A "SPAN address" is an address taken from a SPP and assigned to a
   Relay, Server or Proxy AERO interface.  SPAN addresses are formed by
   simply replacing the upper portion of an administratively-assigned
   AERO address with the SPP.  For example, if the SPP is fd00::/116,
   the SPAN address formed from the AERO address fe80::1 is simply
   fd00::1.  (As with AERO addresses, the values ::0 and ::ffff:ffff are
   reserved and not available for general assignment.)

   AERO Relays serve as bridges to join multiple segments into a unified
   AERO link over multiple diverse administrative domains.  They support
   the bridging function by first exchanging their SPPs via standard BGP
   routing.  For example, if three Relays (Relays 'A', 'B' and 'C') from
   different administrative domains advertised the SPPs fd00::1000/116,
   fd00::2000/116 and fd00::3000/116 respectively, then the forwarding
   tables in each Relay are as follows:

   A: fd00::1000/116->local, fd00::2000/116->B, fd00::3000/116->C

   B: fd00::1000/116->A, fd00::2000/116->local, fd00::3000/116->C

   C: fd00::1000/116->A, fd00::2000/116->B, fd00::3000/116->local

   These forwarding table entries remain in place indefinitely and never
   change, since they correspond to fixed infrastructure elements in
   their respective partitions.  This point is of critical importance,
   since it provides the basis for a link-layer forwarding service that
   cannot be disrupted by routing updates due to node mobility.

   With the SPPs in place in each Relay's forwarding table, control and
   data packets sent between AERO nodes in different partitions can
   therefore be carried over the SPAN via encapsulation.  For example,
   when a node in partition A forwards a packet with IPv6 address
   2001:db8:1:2::1 to a node in partition C with IPv6 address
   2001:db8:1000:2000::1, it first encapsulates the packet in a SPAN
   header with source address from fd00::1000/116 (e.g., fd00::1001) and
   destination address from fd00::3000/116 (e.g., fd00::3001).

   SPAN encapsulation is based on Generic Packet Tunneling in IPv6
   [RFC2473]; the encapsulation format in this example is shown
   inFigure 2:











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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |         Outer Header(s)       |
        |          src = L2(X)          |
        |          dst = L2(Y)          |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |    SPAN Header (RFC2473)      |
        |       src = fd00::1001        |
        |       dst = fd00::3001        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        Inner IP Header        |
        |    src = 2001:db8:1:2::1      |
        |  dst = 2001:db8:1000:2000::1  |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |
        ~                               ~
        ~      Inner Packet Body        ~
        ~                               ~
        |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: SPAN Encapsulation

   In this format, the inner IP header and packet body are the original
   IP packet, the SPAN header is an IPv6 header prepared according to
   [RFC2473], and the outer header is added by the same node ('X') that
   added the SPAN header according to Section 3.10.  The source and
   destination addresses of the outer header are the link-layer
   addresses of nodes 'X' and 'Y' (where 'Y' is a Relay connected to the
   SPAN).

   An (inner) IP packet is said to be "sent into the SPAN" or "sent via
   the SPAN" when it is encapsulated as described above then forwarded
   to a neighboring Relay.  This terminology appears throughout the
   remaining sections of the document.

   This gives rise to a routing system that contains both ACP routes
   that may change dynamically due to regional node mobility and SPAN
   routes that never change.  The Relays can therefore provide link-
   layer bridging by sending packets via the SPAN instead of network-
   layer routing according to ACP routes.  As a result, opportunities
   for packet loss due to node mobility are mitigated.

3.6.  AERO Interface Characteristics

   AERO interfaces use encapsulation (see: Section 3.10) to exchange
   packets with neighbors attached to the AERO link.





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   AERO interfaces maintain a neighbor cache for tracking per-neighbor
   state the same as for any interface.  AERO interfaces use ND messages
   including Router Solicitation (RS), Router Advertisement (RA),
   Neighbor Solicitation (NS) and Neighbor Advertisement (NA) for
   neighbor cache management.

   AERO interface ND messages include one or more Source/Target Link-
   Layer Address Options (S/TLLAOs) formatted as 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Type     |   Length = 5  | Prefix Length |R|D|X|T| Resvd |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Interface ID         |          Port Number          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                       Link Layer Address                      +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 3: AERO Source/Target Link-Layer Address Option (S/TLLAO)
                                  Format

   In this format:

   o  Type is set to '1' for SLLAO or '2' for TLLAO.

   o  Length is set to the constant value '5' (i.e., 5 units of 8
      octets).

   o  Prefix Length is set to the ACP prefix length in an ND message for
      the Client AERO address found in the source (RS), destination (RA)
      or target (NA) address; otherwise set to 0 if the message is not
      being used for PD or neighbor prefix discovery.  If the message
      contains multiple SLLAOs, only the Prefix Length value in the



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      first SLLAO is consulted and the values in other SLLAOs are
      ignored.

   o  R (the "Release" bit) is set to '1' in the SLLAO of an RS message
      sent for the purpose of departing from a Server; otherwise, set to
      '0'.  If the message contains multiple SLLAOs, only the R value in
      the first SLLAO is consulted and the values in other SLLAOs are
      ignored.  The Server places the corresponding neighbor cache entry
      in the DEPARTED state and releases the corresponding PD, then
      returns an RA with Router Lifetime set to '0'.

   o  D (the "Disable" bit) is set to '1' in the S/TLLAOs of an RS/NA
      message for each Interface ID that is to be disabled in the
      neighbor cache entry; otherwise, set to '0'.  If the message
      contains an S/TLLAO with Interface ID 255, the node places the
      corresponding neighbor cache entry in the DEPARTED state.  If the
      message contains multiple S/TLLAOs the D value in each S/TLLAO is
      consulted.

   o  X (the "proXy" bit) is set to '1' in the SLLAO of an RS/RA message
      by the Proxy when there is a Proxy in the path; otherwise, set to
      '0'.  If the message contains multiple SLLAOs, only the X value in
      the first SLLAO is consulted and the values in other SLLAOs are
      ignored.

   o  T (the "Translator" bit) is set to '1' in the SLLAO of an RA
      message by the Server if there is a link-layer address translator
      in the path; otherwise, set to '0'.  If the message contains
      multiple SLLAOs, only the N value in the first SLLAO is consulted
      and the values in other SLLAOs are ignored.

   o  Resvd is set to the value '0' on transmission and ignored on
      receipt.

   o  Interface ID is set to a 16-bit integer value corresponding to an
      underlying interface of the AERO node.  Once the node has assigned
      an Interface ID to an underlying interface, the assignment must
      remain unchanged until the node fully detaches from the AERO link.
      The value '255' is reserved as the AERO Server interface ID, i.e.,
      Servers MUST use Interface ID '255', and Clients MUST number their
      Interface IDs with values in the range of 0-254.

   o  Port Number and Link Layer Address are set to the addresses used
      by the AERO node when it sends encapsulated packets over the
      specified underlying interface (or to '0' when the addresses are
      left unspecified).  When UDP is not used as part of the
      encapsulation, Port Number is set to '0'.  When the encapsulation




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      IP address family is IPv4, IP Address is formed as an IPv4-mapped
      IPv6 address as specified in Section 3.4.

   o  P(i) is a set of Preferences that correspond to the 64
      Differentiated Service Code Point (DSCP) values [RFC2474].  Each
      P(i) is set to the value '0' ("disabled"), '1' ("low"), '2'
      ("medium") or '3' ("high") to indicate a QoS preference level for
      packet forwarding purposes.

   AERO interfaces may be configured over multiple underlying interface
   connections to underlying links.  For example, common mobile handheld
   devices have both wireless local area network ("WLAN") and cellular
   wireless links.  These links are typically used "one at a time" with
   low-cost WLAN preferred and highly-available cellular wireless as a
   standby.  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.

   A Client's underlying interfaces are classified as follows:

   o  Native interfaces connect to the open Internetwork, and have a
      global IP address that is reachable from any open Internetwork
      correspondent.

   o  NATed interfaces connect to a private network behind a Network
      Address Translator (NAT).  The NAT does not participate in any
      AERO control message signaling, but the AERO Server can issue
      control messages on behalf of the Client.  Clients that are behind
      a NAT are required to send periodic keepalive messages to keep NAT
      state alive when there are no data packets flowing.

   o  VPNed interfaces use security encapsulation over the Internetwork
      to a Virtual Private Network (VPN) gateway that also acts as an
      AERO Server.  As with NATed links, the AERO Server can issue
      control messages on behalf of the Client, but the Client need not
      send periodic keepalives in addition to those already used to
      maintain the VPN connection.

   o  Proxyed interfaces connect to a closed network that is separated
      from the open Internetwork by an AERO Proxy.  Unlike NATed and
      VPNed interfaces, the AERO Proxy can also issue control messages
      on behalf of the Client.

   o  Direct interfaces connect the Client directly to a neighbor
      without crossing any networked paths.  An example is a line-of-
      sight link between a remote pilot and an unmanned aircraft.




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   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 ND messages include only a single S/TLLAO with
   Interface ID set to a constant value.  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 messages MAY include multiple S/TLLAOs
   -- each with an Interface ID that corresponds to a specific
   underlying interface of the AERO node.

   When the Client includes an S/TLLAO for an underlying interface for
   which it is aware that there is a Translator on the path to the
   Server, or when a node includes an S/TLLAO solely for the purpose of
   announcing new QoS preferences, the node MAY set both Port Number and
   Link-Layer Address to 0 to indicate that the addresses are
   unspecified at the network layer and must instead be derived from the
   link-layer encapsulation headers.

   When an ND message includes multiple S/TLLAOs, the first S/TLLAO MUST
   correspond to the AERO node's underlying interface used to transmit
   the message.

3.7.  AERO Interface Initialization

3.7.1.  AERO Relay Behavior

   When a Relay enables an AERO interface, it first assigns an
   administratively-provisioned AERO address (e.g., fe80::1) and its
   companion SPAN address (e.g., fd00::1) to the interface, where each
   address MUST be unique among all AERO nodes on the link.  The Relay
   also configures a neighbor cache entry for Servers and Proxys on the
   local segment.  The Relay then engages in a BGP routing protocol
   session with Servers on the local segment and other Relays on the
   link (see: Section 3.3), and advertises its assigned ASPs into the
   native Internetwork.  Each Relay subsequently maintains an IP
   forwarding table entry for each active ACP covered by its ASP(s) as
   well as for each SPAN prefix.

3.7.2.  AERO Server Behavior

   When a Server enables an AERO interface, it assigns AERO and SPAN
   addresses the same as for Relays.  The Server further configures a
   service to facilitate ND/PD exchanges with AERO Clients.  The Server
   maintains neighbor cache entries for one or more Relays on the link,
   and manages per-Client neighbor cache entries and IP forwarding table



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   entries based on control message exchanges.  The Server also engages
   in a BGP routing protocol session with its neighboring Relays (see:
   Section 3.3).

   When the Server receives an NS/RS message on the AERO interface it
   authenticates the message and returns a solicited NA/RA message.
   (When the Server receives an unsolicited NA message, it likewise
   authenticates the message and processes it locally.)  The Server
   further provides a simple link-layer conduit between AERO interface
   neighbors.  In particular, when a packet sent by a source Client
   arrives on the Server's AERO interface and is destined to another
   AERO node, the Server forwards the packet from within the AERO
   interface at the link layer without ever disturbing the network
   layer.

3.7.3.  AERO Proxy Behavior

   When a Proxy enables an AERO interface, it assigns AERO and SPAN
   addresses the same as for Relays and Servers, and maintains neighbor
   cache entires for one or more Relays.  The Proxy further maintains
   per-Client neighbor cache entries based on control message exchanges.
   Proxies forward packets between each Client and their associated
   Servers and neighbors.

   When the Proxy receives an RS message from a Client in the secured
   enclave, it creates an incomplete neighbor cache entry and sends a
   proxyed RS message to a Server via the SPAN while using its own link-
   layer address as the source address.  When the Server returns an RA
   message, the Proxy completes the proxy neighbor cache entry based on
   autoconfiguration information in the RA and sends a proxyed RA to the
   Client while using its own link-layer address as the source address.
   The Client, Server and Proxy will then have the necessary state for
   managing the proxy neighbor association.

3.7.4.  AERO Client Behavior

   When a Client enables an AERO interface, it sends RS messages with
   ND/PD parameters over an underlying interface to one or more AERO
   Servers, which return RA messages with corresponding PD parameters.
   (The RS/RA messages may pass through a Proxy on the path in the case
   of a Client's Proxyed interface.)  See
   [I-D.templin-6man-dhcpv6-ndopt] for the types of ND/PD parameters
   that can be included in the RS/RA message exchanges.

   After the initial ND/PD message exchange, the Client assigns AERO
   addresses to the AERO interface based on the delegated prefix(es).
   The Client can then register additional underlying interfaces with
   the Server by sending a simple RS message (i.e., one with no PD



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   parameters) over each underlying interface using its base AERO
   address as the source network layer address.  The Server will update
   its neighbor cache entry for the Client and return a simple RA
   message.

   The Client maintains a neighbor cache entry for each of its Servers
   and each of its active target Clients.  When the Client receives ND
   messages on the AERO interface it updates or creates neighbor cache
   entries, including link-layer address and QoS preferences.

   When there is a NAT on the path, the Client must send periodic
   messages to keep NAT state alive.  If no other periodic messaging
   service is available, the Client can send RS messages to receive RA
   replies from its Server(s).

3.8.  AERO Interface Neighbor Cache Maintenance

   Each AERO interface maintains a conceptual neighbor cache that
   includes an entry for each neighbor it communicates with on the AERO
   link per [RFC4861].  AERO interface neighbor cache entries are said
   to be one of "permanent", "symmetric", "asymmetric" or "proxy".

   Permanent neighbor cache entries are created through explicit
   administrative action; they have no timeout values and remain in
   place until explicitly deleted.  AERO Relays maintain permanent
   neighbor cache entries for their associated Relays, Servers and
   Proxys, and AERO Servers and Proxys maintain permanent neighbor cache
   entries for their associated Relays.  Each entry maintains the
   mapping between the neighbor's network-layer AERO address and
   corresponding link-layer address.

   Symmetric neighbor cache entries are created and maintained through
   ND/PD exchanges as specified in Section 3.15, and remain in place for
   durations bounded by ND/PD lifetimes.  AERO Servers maintain
   symmetric neighbor cache entries for each of their associated
   Clients, and AERO Clients maintain symmetric neighbor cache entries
   for each of their associated Servers.

   Asymmetric neighbor cache entries are created or updated based on
   route optimization messaging as specified in Section 3.17, and are
   garbage-collected when keepalive timers expire.  AERO route
   optimization sources (ROSs) maintain asymmetric neighbor cache
   entries for each of their active target Clients with lifetimes based
   on ND messaging constants.  Asymmetric neighbor cache entries are
   unidirectional since only the ROS (i.e., and not the route
   optimization responder (ROR)) creates an entry.





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   Proxy neighbor cache entries are created and maintained by AERO
   Proxies when they process Client/Server ND/PD exchanges, and remain
   in place for durations bounded by ND/PD lifetimes.  AERO Proxies
   maintain proxy neighbor cache entries for each of their associated
   Clients.  Proxy neighbor cache entries track the Client state and the
   state of each of the Client's associated Servers.

   To the list of neighbor cache entry states in Section 7.3.2 of
   [RFC4861], AERO interfaces add an additional state DEPARTED that
   applies to symmetric and proxy neighbor cache entries for Clients
   that have recently departed.  The interface sets a "DepartTime"
   variable for the neighbor cache entry to "DEPARTTIME" seconds.
   DepartTime is decremented unless a new ND message causes the state to
   return to REACHABLE.  While a neighbor cache entry is in the DEPARTED
   state, packets destined to the target Client are forwarded to the
   Client's new location instead of being dropped.  When DepartTime
   decrements to 0, the neighbor cache entry is deleted.  It is
   RECOMMENDED that DEPARTTIME be set to the default constant value 40
   seconds to allow for packets in flight to be delivered while stale
   route optimization state may be present.

   When a target AERO Server (acting as a Mobility Anchor Point (MAP))
   receives a valid NS message used for route optimization, it searches
   for a symmetric neighbor cache entry for the target Client.  The
   Server then acts as an ROR and returns a solicited NA message without
   creating a neighbor cache entry for the ROS, but maintains a "Report
   List" for the Client's symmetric neighbor cache entry.  When the ROR
   receives an authentic NS message it adds a Report list entry for the
   ROS and sets a "ReportTime" variable for the entry to REPORTTIME
   seconds.  The ROR resets ReportTime when it receives a new authentic
   NS message, and otherwise decrements ReportTime while no NS messages
   have been received.  It is RECOMMENDED that REPORTTIME be set to the
   default constant value 40 seconds to allow a 10 second window so that
   route optimization can converege before ReportTime decrements below
   REACHABLETIME.

   When the ROS receives a solicited NA message response to its NS
   message, it creates or updates an asymmetric neighbor cache entry for
   the target network-layer and link-layer addresses.  The ROS then
   (re)sets ReachableTime for the neighbor cache entry to REACHABLETIME
   seconds and uses this value to determine whether packets can be
   forwarded directly to the target, i.e., instead of via a default
   route.  The ROS otherwise decrements ReachableTime while no further
   solicited NA messages arrive.  It is RECOMMENDED that REACHABLETIME
   be set to the default constant value 30 seconds as specified in
   [RFC4861].





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   The ROS also uses the value MAX_UNICAST_SOLICIT to limit the number
   of NS keepalives 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 DEPARTTIME, REPORTTIME, REACHABLETIME,
   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, DEPARTTIME and
   REPORTTIME SHOULD be set to a value that is sufficiently longer than
   REACHABLETIME to avoid packet loss due to stale route optimization
   state.

3.9.  AERO Interface Forwarding Algorithm

   IP packets enter a node's AERO interface either from the network
   layer (i.e., from a local application or the IP forwarding system) or
   from the link layer (i.e., from the AERO tunnel virtual link).
   Packets that enter the AERO interface from the network layer are
   encapsulated and forwarded into the AERO link, i.e., they are
   tunneled to an AERO interface neighbor.  Packets that enter the AERO
   interface from the link layer are either re-admitted into the AERO
   link or forwarded to the network layer where they are subject to
   either local delivery or IP forwarding.  In all cases, the AERO
   interface itself MUST NOT decrement the network layer TTL/Hop-count
   since its forwarding actions occur below the network layer.

   AERO interfaces may have multiple underlying interfaces and/or
   neighbor cache entries for neighbors with multiple Interface ID
   registrations (see Section 3.6).  The AERO node uses each packet's
   DSCP value (and/or port number) to select an outgoing underlying
   interface based on the node's own QoS preferences, and also to select
   a destination link-layer address based on the neighbor's underlying
   interface with the highest preference.  AERO implementations SHOULD
   allow for QoS preference values to be modified at runtime through
   network management.

   If multiple outgoing interfaces and/or neighbor interfaces have a
   preference of "high", the AERO node sends one copy of the packet via
   each of the (outgoing / neighbor) interface pairs; otherwise, the
   node sends a single copy of the packet via the interface with the
   highest preference.  AERO nodes keep track of which underlying




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   interfaces are currently "reachable" or "unreachable", and only use
   "reachable" interfaces for forwarding purposes.

   The following sections discuss the AERO interface forwarding
   algorithms for Clients, Proxies, Servers and Relays.  In the
   following discussion, a packet's destination address is said to
   "match" if it is a non-link-local address with a prefix covered by an
   ASP/ACP, or if it is an AERO address that embeds an ACP, or if it is
   the same as an administratively-provisioned AERO address.

3.9.1.  Client Forwarding Algorithm

   When an IP packet enters a Client's AERO interface from the network
   layer the Client searches for an asymmetric neighbor cache entry that
   matches the destination.  If there is a match, the Client uses one or
   more "reachable" link-layer addresses in the entry as the link-layer
   addresses for encapsulation and admits the packet into the AERO link
   (if the link-layer address is a SPAN address, the Client instead
   forwards the packet into the SPAN).  If there is no asymmetric
   neighbor cache entry, the Client instead uses the link-layer address
   in a symmetric neighbor cache entry as the encapsulation address for
   interfaces other than Proxyed interfaces.  For Proxyed interfaces,
   the Client simply forwards the unencapsulated packet to the first-hop
   access router.

   When an IP packet enters a Client's AERO interface from the link-
   layer, if the destination matches one of the Client's ACPs or link-
   local addresses the Client decapsulates the packet and delivers it to
   the network layer.  Otherwise, the Client drops the packet and MAY
   return a network-layer ICMP Destination Unreachable message subject
   to rate limiting (see: Section 3.14).

3.9.2.  Proxy Forwarding Algorithm

   When the Proxy receives a packet from a Client within the secured
   enclave, the Proxy searches for an asymmetric neighbor cache entry
   that matches the network-layer destination.  If there is a match, the
   Proxy uses one or more "reachable" link-layer addresses in the entry
   as the destination link-layer addresses for encapsulation and admits
   the packet into the AERO link (if the link-layer address is a SPAN
   address, the Proxy instead forwards the packet into the SPAN).
   Otherwise, the Proxy uses the link-layer address for one of the
   Client's Servers as the encapsulation address.

   When the Proxy receives an encapsulated data packet from outside of
   the secured enclave, it searches for a proxy neighbor cache entry
   that matches the destination.  If there is a proxy neighbor cache
   entry for the target Client, the Proxy forwards the packet according



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   to the cached link-layer address.  If the proxy neighbor cache entry
   is in the DEPARTED state, the Proxy instead forwards the packet to
   the Client's Server and may return an unsolicited NA message as
   discussed in Section 3.19.  If there is no neighbor cache entry, the
   Proxy discards the packet.

3.9.3.  Server Forwarding Algorithm

   When an IP packet enters a Server's AERO interface from the link-
   layer, it decapsulates the packet and processes it the same as if it
   entered from ethe network layer.  The Server then processes the
   packet according to the network-layer destination address as follows:

   o  if the destination matches one of the Server's own addresses the
      Server forwards it to the network layer for local delivery.

   o  else, if the destination matches a symmetric neighbor cache entry
      the Server first determines whether the packet originated from the
      same Client.  If so, the Server drops the packet silently to avoid
      looping.  Otherwise, the Server uses the neighboring Client's
      link-layer address(es) as the destination for encapsulation,
      (re)encapsulates the packet the packet and forwards the packet to
      the Client.  If the neighbor cache entry is in the DEPARTED state,
      the Server instead continues to forward packets to the Client's
      new Server (either directly of via the SPAN according to the link-
      layer address) as discussed in Section 3.19.

   o  else, if the destination matches an asymmetric neighbor cache
      entry for a target Client, the Server forwards the packet
      according to the link-layer information in the asymmetric neighbor
      cache entry (either directly or via the SPAN according to the
      link-layer address).

   o  else, the Server uses the link-layer address in a permanent
      neighbor cache entry for a Relay (selected through longest-prefix
      match) as the link-layer address for encapsulation.

3.9.4.  Relay Forwarding Algorithm

   Relays forward packets the same as any IP router.  When the Relay
   receives an encapsulated packet from a Server via the AERO link, it
   removes the encapsulation header and searches for a forwarding table
   entry that matches the network layer destination address.  When the
   Relay receives an unencapsulated packet from a node outside the AERO
   link, it performs the same forwarding table lookup.  The Relay then
   processes the packet as follows:





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   o  if the destination does not match an ASP or the SSP, or if the
      destination matches one of the Relay's own addresses, the Relay
      submits the packet for either IP forwarding or local delivery.

   o  else, if the destination matches an ASP/SSP entry in the IP
      forwarding table the Relay first determines whether the neighbor
      is the same as the one it received the packet from.  If so the
      Relay MUST drop the packet silently to avoid looping; otherwise,
      the Relay encapsulates and forwards the packet using the
      neighbor's link-layer address as the destination for
      encapsulation.

   o  else, the Relay drops the packet and returns an ICMP Destination
      Unreachable message subject to rate limiting (see: Section 3.14).

   As for any IP router, the Relay decrements the TTL/Hop Count when it
   forwards the packet.

3.10.  AERO Interface Encapsulation and Re-encapsulation

   AERO interfaces encapsulate IP packets according to whether they are
   entering the AERO interface from the network layer or if they are
   being re-admitted into the same AERO link they arrived on.  This
   latter form of encapsulation is known as "re-encapsulation".

   The AERO interface encapsulates packets per the Generic UDP
   Encapsulation (GUE) procedures in
   [I-D.ietf-intarea-gue][I-D.ietf-intarea-gue-extensions], or through
   an alternate encapsulation format (e.g., see: Appendix A, [RFC2784],
   [RFC8086], [RFC4301], etc.).  For packets entering the AERO interface
   from the network layer, the AERO interface copies the "TTL/Hop
   Limit", "Type of Service/Traffic Class" [RFC2983], "Flow
   Label"[RFC6438] (for IPv6) and "Congestion Experienced" [RFC3168]
   values in the packet's IP header into the corresponding fields in the
   encapsulation IP header.  For packets undergoing re-encapsulation,
   the AERO interface instead copies these values from the original
   encapsulation IP header into the new encapsulation header, i.e., the
   values are transferred between encapsulation headers and *not* copied
   from the encapsulated packet's network-layer header.  (Note
   especially that by copying the TTL/Hop Limit between encapsulation
   headers the value will eventually decrement to 0 if there is a
   (temporary) routing loop.)  For IPv4 encapsulation/re-encapsulation,
   the AERO interface sets the DF bit as discussed in Section 3.13.

   When GUE encapsulation is used, the AERO interface next sets the UDP
   source port to a constant value that it will use in each successive
   packet it sends, and sets the UDP length field to the length of the
   encapsulated packet plus 8 bytes for the UDP header itself plus the



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   length of the GUE header (or 0 if GUE direct IP encapsulation is
   used).  For packets sent to a Server or Relay, the AERO interface
   sets the UDP destination port to 8060, i.e., the IANA-registered port
   number for AERO.  For packets sent to a Client, the AERO interface
   sets the UDP destination port to the port value stored in the
   neighbor cache entry for this Client.  The AERO interface then either
   includes or omits the UDP checksum according to the GUE
   specification.

   When GUE encapsulation is not available, encapsulation between
   Servers and Relays can use standard mechanisms such as Generic
   Routing Encapsulation (GRE) [RFC2784], GRE-in-UDP [RFC8086] and IPSec
   [RFC4301] so that Relays can be standard IP routers with no AERO-
   specific mechanisms.

3.11.  AERO Interface Decapsulation

   AERO interfaces decapsulate packets destined either to the AERO node
   itself or to a destination reached via an interface other than the
   AERO interface the packet was received on.  Decapsulation is per the
   procedures specified for the appropriate encapsulation format.

3.12.  AERO Interface Data Origin Authentication

   AERO nodes employ simple data origin authentication procedures for
   encapsulated packets they receive from other nodes on the AERO link.
   In particular:

   o  AERO Relays and Servers accept encapsulated packets with a link-
      layer source address that matches a permanent neighbor cache
      entry.

   o  AERO Servers accept authentic encapsulated ND messages from
      Clients (either directly or via a Proxy), and create or update a
      symmetric neighbor cache entry for the Client based on the
      specific message type.

   o  AERO Clients and Servers accept encapsulated packets if there is a
      symmetric neighbor cache entry with a link-layer address that
      matches the packet's link-layer source address.

   o  AERO Proxies accept encapsulated packets if there is a proxy
      neighbor cache entry that matches the packet's network-layer
      address.

   Each packet should include a signature that the recipient can use to
   authenticate the message origin, e.g., as for common VPN systems such
   as OpenVPN [OVPN].  In some environments, however, it may be



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   sufficient to require signatures only for ND control plane messages
   (see: Section 10) and omit signatures for data plane messages.

3.13.  AERO Interface Packet Size Issues

   The AERO interface is the node's attachment to the AERO link.  The
   AERO interface acts as a tunnel ingress when it sends a packet to an
   AERO link neighbor and as a tunnel egress when it receives a packet
   from an AERO link neighbor.  AERO interfaces observe the packet
   sizing considerations for tunnels discussed in
   [I-D.ietf-intarea-tunnels] and as specified below.

   The Internet Protocol expects that IP packets will either be
   delivered to the destination or a suitable Packet Too Big (PTB)
   message returned to support the process known as IP Path MTU
   Discovery (PMTUD) [RFC1191][RFC8201].  However, PTB messages may be
   crafted for malicious purposes such as denial of service, or lost in
   the network [RFC2923].  This can be especially problematic for
   tunnels, where a condition known as a PMTUD "black hole" can result.
   For these reasons, AERO interfaces employ operational procedures that
   avoid interactions with PMTUD, including the use of fragmentation
   when necessary.

   AERO interfaces observe two different types of fragmentation.  Source
   fragmentation occurs when the AERO interface (acting as a tunnel
   ingress) fragments the encapsulated packet into multiple fragments
   before admitting each fragment into the tunnel.  Network
   fragmentation occurs when an encapsulated packet admitted into the
   tunnel by the ingress is fragmented by an IPv4 router on the path to
   the egress.  Note that an IPv4 packet that incurs source
   fragmentation may also incur network fragmentation.

   IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of 1280
   bytes [RFC8200].  Although IPv4 specifies a smaller minimum link MTU
   of 68 bytes [RFC0791], AERO interfaces also observe the IPv6 minimum
   for IPv4 even if encapsulated packets may incur network
   fragmentation.

   IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500 bytes
   [RFC8200], while the minimum MRU for IPv4 is only 576 bytes [RFC1122]
   (but, note that many standard IPv6 over IPv4 tunnel types already
   assume a larger MRU than the IPv4 minimum).

   AERO interfaces therefore configure an MTU that MUST NOT be smaller
   than 1280 bytes, MUST NOT be larger than the minimum MRU among all
   nodes on the AERO link minus the encapsulation overhead ("ENCAPS"),
   and SHOULD NOT be smaller than 1500 bytes.  AERO interfaces also
   configure a Maximum Segment Unit (MSU) as the maximum-sized



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   encapsulated packet that the ingress can inject into the tunnel
   without source fragmentation.  The MSU value MUST NOT be larger than
   (MTU+ENCAPS) and MUST NOT be larger than 1280 bytes unless there is
   operational assurance that a larger size can traverse the link along
   all paths.

   All AERO nodes MUST configure the same MTU value for reasons cited in
   [RFC3819][RFC4861]; in particular, multicast support requires a
   common MTU value among all nodes on the link.  All AERO nodes MUST
   configure an MRU large enough to reassemble packets up to
   (MTU+ENCAPS) bytes in length; nodes that cannot configure a large-
   enough MRU MUST NOT enable an AERO interface.

   The network layer proceeds as follow when it presents an IP packet to
   the AERO interface.  For each IPv4 packet that is larger than the
   AERO interface MTU and with the DF bit set to 0, the network layer
   uses IPv4 fragmentation to break the packet into a minimum number of
   non-overlapping fragments where the first fragment is no larger than
   the MTU and the remaining fragments are no larger than the first.
   For all other IP packets, if the packet is larger than the AERO
   interface MTU, the network layer drops the packet and returns a PTB
   message to the original source.  Otherwise, the network layer admits
   each IP packet or fragment into the AERO interface.

   For each IP packet admitted into the AERO interface, the interface
   (acting as a tunnel ingress) encapsulates the packet.  If the
   encapsulated packet is larger than the AERO interface MSU the ingress
   source-fragments the encapsulated packet into a minimum number of
   non-overlapping fragments where the first fragment is no larger than
   the MSU and the remaining fragments are no larger than the first.
   The ingress then admits each encapsulated packet or fragment into the
   tunnel, and for IPv4 sets the DF bit to 0 in the IP encapsulation
   header in case any network fragmentation is necessary.  The
   encapsulated packets will be delivered to the egress, which
   reassembles them into a whole packet if necessary.

   Several factors must be considered when fragmentation is needed.  For
   AERO links over IPv4, the IP ID field is only 16 bits in length,
   meaning that fragmentation at high data rates could result in data
   corruption due to reassembly misassociations [RFC6864][RFC4963].  In
   environments where IP fragmentation issues could result in
   operational problems, the ingress SHOULD employ intermediate-layer
   source fragmentation (see: [RFC2764] and
   [I-D.ietf-intarea-gue-extensions]) before appending the outer
   encapsulation headers to each fragment.  Since the encapsulation
   fragment header reduces the room available for packet data, but the
   original source has no way to control its insertion, the ingress MUST




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   include the fragment header length in the ENCAPS length even for
   packets in which the header is absent.

3.14.  AERO Interface Error Handling

   When an AERO node admits encapsulated packets into the AERO
   interface, it may receive link-layer or network-layer error
   indications.

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

   The IP header is followed by an ICMP header that includes an error
   Type, Code and Checksum.  Valid type values include "Destination
   Unreachable", "Time Exceeded" and "Parameter Problem"
   [RFC0792][RFC4443].  (AERO interfaces ignore all link-layer IPv4
   "Fragmentation Needed" and IPv6 "Packet Too Big" messages since they
   only emit packets that are guaranteed to be no larger than the IP
   minimum link MTU as discussed in Section 3.13.)

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

   The link-layer error message format is shown in Figure 4 (where, "L2"
   and "L3" refer to link-layer and network-layer, respectively):













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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~
        |        L2 IP Header of        |
        |         error message         |
        ~                               ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |         L2 ICMP Header        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---
        ~                               ~   P
        |   IP and other encapsulation  |   a
        | headers of original L3 packet |   c
        ~                               ~   k
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   e
        ~                               ~   t
        |        IP header of           |
        |      original L3 packet       |   i
        ~                               ~   n
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~   e
        |    Upper layer headers and    |   r
        |    leading portion of body    |   r
        |   of the original L3 packet   |   o
        ~                               ~   r
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

         Figure 4: AERO 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 encapsulated packets that it
      sends to one of its asymmetric neighbor correspondents, the node
      SHOULD process the message as an indication that a path may be
      failing, and MAY initiate NUD over that path.  If it receives
      Destination Unreachable messages on many or all paths, the node
      SHOULD set ReachableTime for the corresponding asymmetric neighbor




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      cache entry to 0 and allow future packets destined to the
      correspondent to flow through a default route.

   o  When an AERO Client receives persistent link-layer Destination
      Unreachable messages in response to encapsulated packets that it
      sends to one of its symmetric neighbor 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 Server and release its association
      with the old Server as specified in Section 3.19.7.

   o  When an AERO Server receives persistent link-layer Destination
      Unreachable messages in response to encapsulated packets that it
      sends to one of its symmetric neighbor Clients, the Server SHOULD
      mark the underlying path as unusable and use another underlying
      path.  If it receives Destination Unreachable messages on multiple
      paths, the Server should take no further actions unless it
      receives an explicit ND/PD release message or if the PD lifetime
      expires.  In that case, the Server MUST release the Client's
      delegated ACP, withdraw the ACP from the AERO routing system and
      delete the neighbor cache entry.

   o  When an AERO Relay or Server receives link-layer Destination
      Unreachable messages in response to an encapsulated 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 Relay receives a packet for which the network-layer
   destination address is covered by an ASP, if there is no more-
   specific routing information for the destination the Relay drops the
   packet and returns a network-layer Destination Unreachable message
   subject to rate limiting.  The Relay writes the network-layer source
   address of the original packet as the destination address and uses
   one of its non link-local addresses as the source address of the
   message.

   When an AERO node receives an encapsulated packet for which the
   reassembly buffer it too small, it drops the packet and returns a
   network-layer Packet Too Big (PTB) message.  The node first writes
   the MRU value into the PTB message MTU field, writes the network-
   layer source address of the original packet as the destination
   address and writes one of its non link-local addresses as the source
   address.






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3.15.  AERO Router Discovery, Prefix Delegation and Autoconfiguration

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

3.15.1.  AERO ND/PD Service Model

   Each AERO Server configures a PD service to facilitate Client
   requests.  Each Server is provisioned with a database of ACP-to-
   Client ID mappings for all Clients enrolled in the AERO system, as
   well as any information necessary to authenticate each Client.  The
   Client database is maintained by a central administrative authority
   for the AERO link and securely distributed to all Servers, e.g., via
   the Lightweight Directory Access Protocol (LDAP) [RFC4511], via
   static configuration, etc.  Therefore, no Server-to-Server PD state
   synchronization is necessary, and Clients can optionally hold
   separate PDs for the same ACPs from multiple Servers.  In this way,
   Clients can associate with multiple Servers, and can receive new PDs
   from new Servers before releasing PDs received from existing Servers.
   This provides the Client with a natural fault-tolerance and/or load
   balancing profile.

   AERO Clients and Servers use ND messages to maintain neighbor cache
   entries.  AERO Servers configure their AERO interfaces as advertising
   interfaces, and therefore send unicast RA messages with configuration
   information in response to a Client's RS message.  Thereafter,
   Clients send additional RS messages to the Server's unicast address
   to refresh prefix and/or router lifetimes.

   AERO Clients and Servers include PD parameters in RS/RA messages to
   be used for Prefix Delegation (see [I-D.templin-6man-dhcpv6-ndopt]
   for ND/PD alternatives).  The unified ND/PD messages are exchanged
   between Client and Server according to the prefix management schedule
   required by the PD service.  If the Client knows its ACP in advance,
   it can include its AERO address as the source address of an RS
   message and with an SLLAO with a valid Prefix Length for the ACP.  If
   the Server (and Proxy) accept the Client's ACP assertion, they inject
   the prefix into the routing system and establish the necessary
   neighbor cache state.  If the Client does not know its ACP in
   advance, or if it wishes to engage in an explicit PD exchange, it can
   include ND/PD parameters for an ancillary service such as DHCPv6.

   On Some AERO links, PD arrangements may be through some out-of-band
   service such as network management, static configuration, etc.  In
   those cases, AERO nodes can use simple RS/RA message exchanges with
   no PD options.  In other cases, the RS/RA messages can use AERO
   addresses as a means of representing the delegated prefixes, e.g., if
   a message includes a source address of "fe80::2001:db8:1:2" then the



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   recipient can infer that the sender holds the prefix delegation
   "2001:db8:1:2::/N" (where 'N' is the Prefix Length included in the
   first SLLAO in the message).

   The following sections specify the Client and Server behavior.

3.15.2.  AERO Client Behavior

   AERO Clients can discover the link-layer and AERO addresses of AERO
   Servers in the MAP list via static configuration (e.g., from a flat-
   file map of Server addresses and locations), or through an automated
   means such as Domain Name System (DNS) name resolution [RFC1035].  In
   the absence of other information, the Client resolves the DNS Fully-
   Qualified Domain Name (FQDN) "linkupnetworks.[domainname]" where
   "linkupnetworks" is a constant text string and "[domainname]" is a
   DNS suffix for the Client's underlying interface (e.g.,
   "example.com").  After discovering the link-layer addresses, the
   Client associates with one or more of the corresponding Servers.

   To associate with a Server, the Client acts as a requesting router to
   request ACPs.  The Client prepares an RS message with PD parameters
   (e.g., with an SLLAO with non-zero Prefix Length), the address of the
   Client's underlying interface as the link-layer source address and
   the link-layer address of the Server as the link-layer destination
   address.  If the Client already knows the Server's AERO address, it
   includes the AERO address as the network-layer destination address;
   otherwise, it includes all-routers multicast (ff02::2) as the
   network-layer destination address.  If the Client already knows its
   own AERO address, it uses the AERO address as the network-layer
   source address; otherwise, it uses the unspecified AERO address
   (fe80::ffff:ffff) as the network-layer source address.

   The Client next includes an SLLAO in the RS message formatted as
   described in Section 3.6 to register its link-layer address with the
   Server.  The first SLLAO MUST correspond to the underlying interface
   over which the Client will send the RS message.  The Client MAY
   include additional SLLAOs specific to other underlying interfaces,
   but if so it sets their Port Number and Link Layer Address fields to
   0.

   The Client then sends the RS message (either via a VPN for VPNed
   interfaces, via a Proxy for proxyed interfaces or via the SPAN for
   native interfaces) and waits for an RA message reply (see
   Section 3.15.3) while retrying up to MAX_RTR_SOLICITATIONS times
   until an RA is received.  If the Client receives no RAs, or if it
   receives an RA with Router Lifetime set to 0, the Client SHOULD
   abandon this Server and try another Server.  Otherwise, the Client
   processes the PD information found in the RA message.



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   Next, the Client creates a symmetric neighbor cache entry with the
   Server's AERO address as the network-layer address and the address in
   the first SLLAO as the link-layer address.  The Client records the RA
   Router Lifetime field value in the cache entry as the time for which
   the Server has committed to maintaining the ACP in the routing
   system.  The Client then autoconfigures AERO addresses for each of
   the delegated ACPs and assigns them to the AERO interface.  The
   Client also caches any ASPs included in Route Information Options
   (RIOs) [RFC4191] as ASPs to associate with the AERO link, and assigns
   the MTU value in the MTU option to its AERO interface while
   configuring an appropriate MRU.  This configuration information
   applies to the AERO link as a whole, and all AERO nodes will receive
   the same values.

   The Client then registers additional link-layer addresses with the
   Server by sending additional RS messages including SLLAOs via other
   underlying interfaces after the initial RS/RA exchange.  The Client
   sends the RS messages to the Server's AERO address (discovered in the
   initial RS/RA exchange), but omits PD parameters since the initial
   RS/RA exchange has already established PD state.

   The Client examines the X and N bits in the first SLLAO of each RA
   message it receives.  If the X bit value is '1' the Client infers
   that there is a Proxy on the path via the interface over which it
   sent the RS message, and if the N bit value is '1' the Client infers
   that there is a NAT on the path.  If N is '1', the Client SHOULD set
   Port Number and Link-Layer Address to 0 in the first S/TLLAO of any
   subsequent ND messages it sends to the Server over that link.

   Following autoconfiguration, the Client sub-delegates the ACPs 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
   subsequently maintains its ACP delegations through each of its
   Servers by sending additional RS messages with PD parameters before
   Router Lifetime expires.

   After the Client registers its Interface IDs and their associated
   port numbers, link-layer addresses and 'P(i)' values, it may wish to
   change one or more Interface ID registrations, e.g., if an underlying
   interface changes address or becomes unavailable, if QoS preferences
   change, etc.  To do so, the Client prepares an RS message to send
   over any available underlying interface.  The RS MUST include an
   SLLAO specific to the selected available underlying interface as the
   first SLLAO and MAY include any additional SLLAOs specific to other
   underlying interfaces.  The Client includes fresh 'P(i)' values in
   each SLLAO to update the Server's neighbor cache entry.  If the
   Client wishes to update 'P(i)' values without updating the link-layer



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   address, it sets the Port Number and Link-Layer Address fields to 0.
   If the Client wishes to disable the underlying interface, it sets the
   'D' bit to 1.  When the Client receives the Server's RA response, it
   has assurance that the Server has been updated with the new
   information.

   If the Client wishes to associate with multiple Servers, it repeats
   the same procedures above for each additional Server.  If the Client
   wishes to discontinue use of a Server it issues an RS message over
   any underlying interface with the 'R' bit set to 1 in the first
   SLLAO.  When the Server processes the message, it releases the ACP,
   sets the symmetric neighbor cache entry state for the Client to
   DEPARTED, withdraws the IP route from the routing system and returns
   an RA reply with Router Lifetime set to 0.

3.15.3.  AERO Server Behavior

   AERO Servers act as IPv6 routers and support a PD service for
   Clients.  AERO Servers arrange to add their link-layer and AERO
   address to a static map of Server addresses for the link and/or the
   DNS resource records for the FQDN "linkupnetworks.[domainname]"
   before entering service.  The list of Server addresses should be
   geographically and/or topologically referenced, and forms the MAP
   list for the AERO link.

   When an AERO Server receives a prospective Client's RS message with
   PD parameters on its AERO interface, it SHOULD return an immediate RA
   reply with Router Lifetime set to 0 if it is currently too busy or
   otherwise unable to service the Client.  Otherwise, the Server
   authenticates the RS message and processes the PD parameters.  The
   Server first determines the correct ACPs to delegate to the Client by
   searching the Client database.  When the Server delegates the ACPs,
   it also creates an IP forwarding table entry for each ACP so that the
   AERO BGP-based routing system will propagate the ACPs to the Relays
   that aggregate the corresponding ASP (see: Section 3.3).

   The Server next creates a symmetric neighbor cache entry for the
   Client using the base AERO address as the network-layer address and
   with lifetime set to no more than the smallest PD lifetime.  Next,
   the Server updates the neighbor cache entry link-layer address(es) by
   recording the information in each SLLAO in the RS indexed by the
   Interface ID and including the Port Number, Link Layer Address and
   P(i) values.  For the first SLLAO in the list, however, the Server
   records the actual encapsulation source addresses instead of those
   that appear in the SLLAO in case there was a NAT in the path.  The
   Server also records the value of the X bit to indicate whether there
   is a Proxy on the path.




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   Next, the Server prepares an RA message that includes the delegated
   ACPs, any other PD parameters and an SLLAO with the Server's link-
   layer address and with Interface ID set to 255.  The Server uses its
   AERO address as the network-layer source address, the network-layer
   source address of the RS message as the network-layer destination
   address, the Server's link-layer address as the source link-layer
   address, and the source link-layer address of the RS message as the
   destination link-layer address.  The Server next sets the N flag to 1
   if the source link-layer address in the RS message was different than
   the address in the first SLLAO to indicate that there is a NAT on the
   path.  The Server then includes one or more RIOs that encode the ASPs
   for the AERO link.  The Server also includes an MTU option for the
   MTU for the link (see Section 3.13).  The Server finally sends the RA
   message to the Client via the SPAN.

   After the initial RS/RA exchange, the AERO Server maintains the
   symmetric neighbor cache entry for the Client.  If the Client (or
   Proxy) issues additional NS/RS messages, the Server resets
   ReachableTime.  If the Client (or Proxy) issues an RS with PD release
   parameters (e.g., by including an SLLAO with R set to 1), or if the
   Client becomes unreachable, the Server sets the Client's symmetric
   neighbor cache entry to the DEPARTED state and withdraws the IP
   routes from the AERO routing system.

   The Server processes these and any other Client ND/PD messages, and
   returns an NA/RA reply.  The Server may also issue an unsolicited RA
   message with PD reconfigure parameters to cause the Client to
   renegotiate its PDs, and may issue an unsolicited RA message with
   Router Lifetime set to 0 if it can no longer service this Client.
   Finally, If the symmetric neighbor cache entry is in the DEPARTED
   state, the Server deletes the entry after DepartTime expires.

3.15.3.1.  Lightweight DHCPv6 Relay Agent (LDRA)

   When DHCPv6 is used as the ND/PD service back end, AERO Clients and
   Servers are always on the same link (i.e., the AERO link) from the
   perspective of DHCPv6.  However, in some implementations the DHCPv6
   server and ND function may be located in separate modules.  In that
   case, the Server's AERO interface module can act as a Lightweight
   DHCPv6 Relay Agent (LDRA)[RFC6221] to relay PD messages to and from
   the DHCPv6 server module.

   When the LDRA receives an authentic RS message, it extracts the PD
   message parameters and uses them to construct an IPv6/UDP/DHCPv6
   message.  It sets the IPv6 source address to the source address of
   the RS message, sets the IPv6 destination address to
   'All_DHCP_Relay_Agents_and_Servers' and sets the UDP fields to values
   that will be understood by the DHCPv6 server.



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   The LDRA then wraps the message in a DHCPv6 'Relay-Forward' message
   header and includes an 'Interface-Id' option that includes enough
   information to allow the LDRA to forward the resulting Reply message
   back to the Client (e.g., the Client's link-layer addresses, a
   security association identifier, etc.).  The LDRA also wraps the
   information in all of the SLLAOs from the RS message into the
   Interface-Id option, then forwards the message to the DHCPv6 server.

   When the DHCPv6 server prepares a Reply message, it wraps the message
   in a 'Relay-Reply' message and echoes the Interface-Id option.  The
   DHCPv6 server then delivers the Relay-Reply message to the LDRA,
   which discards the Relay-Reply wrapper and IPv6/UDP headers, then
   uses the DHCPv6 message to construct an RA response to the Client.
   The Server uses the information in the Interface-Id option to prepare
   the RA message and to cache the link-layer addresses taken from the
   SLLAOs echoed in the Interface-Id option.

3.16.  The AERO Proxy

   In some environments, Clients may be located in secured enclaves that
   do not allow direct communications from the Client to a Server in the
   outside Internetwork.  In that case, the secured enclave can employ
   an AERO Proxy.

   The Proxy is located at the secured enclave perimeter and listens for
   encapsulated RS messages originating from or RA messages destined to
   Clients located within the enclave.  The Proxy acts on these control
   messages as follows:

   o  when the Proxy receives an RS message from a new Client within the
      secured enclave, it first authenticates the message then examines
      the RS message network-layer destination address.  If the
      destination address is a Server's AERO address, the Proxy proceeds
      to the next step.  Otherwise, if the destination is all-routers
      multicast the Proxy selects a "nearby" Server that is likely to be
      a good candidate to serve the Client and replaces the RS
      destination address with the AERO address of the Server.
      (Otherwise, the Proxy discards the RS.)  Next, the Proxy creates a
      proxy neighbor cache entry and caches the Client and Server
      addresses along with any identifying information including
      Transaction IDs, Client Identifiers, Nonce values, etc.  The Proxy
      then examines the address in the first SLLAO of the RS message.
      If the address is different than the Client link-layer address,
      the Proxy notes that the Client is behind a NAT.  The Proxy then
      re-encapsulates the RS message using its own external address as
      the source link-layer address, sets the X flag in the first SLLAO
      to '1', changes the address in the first SLLAO to its own external
      address, and forwards the message to the Server via the SPAN.



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   o  when the Server receives the RS message, it authenticates the
      message then creates or updates a symmetric neighbor cache entry
      for the Client with the Proxy's address as the link-layer address.
      The Server then sends an RA message back to the Proxy via the
      SPAN.

   o  when the Proxy receives the RA message, it matches the message
      with the RS that created the proxy neighbor cache entry.  The
      Proxy then caches the route information in the message as a
      mapping from the Client's ACPs to the Client's address within the
      secured enclave, and sets the neighbor cache entry state to
      REACHABLE.  The Proxy then re-encapsulates the RA message using
      its own internal address as the source link-layer address, sets
      the X flag in the first SLLAO to '1', sets the N flag in the first
      SLLAO to '1' if the Client is behind a NAT, and forwards the
      message to the Client.

   After the initial RS/RA exchange, the Proxy forwards any Client data
   packets for which there is no matching asymmetric neighbor cache
   entry to the "eldest" of the Client's Servers, i.e., the first among
   possibly multiple Servers selected by the Client.  If the eldest
   Server becomes unreachable, the Proxy sends future data packets via
   the next-eldest Server, etc.  Finally, the Proxy forwards any Client
   data destined to an asymmetric neighbor cache target directly to the
   target according to the link-layer information - the process of
   establishing asymmetric neighbor cache entries is specified in
   Section 3.17.

   While the Client is still active, the Proxy continues to send NS/RS
   messages to update each Server's symmetric neighbor cache entries on
   behalf of the Client and/or to convey QoS updates.  If the Server
   ceases to send solicited NA/RA responses, the Proxy marks the Server
   as unreachable and sends an unsolicited RA to the Client with Router
   Lifetime set to zero so that the Client knows that this Server is no
   longer able to provide Service.  If the Client becomes unreachable,
   the Proxy sets the neighbor cache entry state to DEPARTED and sends
   an RS message to each Server with an SLLAO with the 'D' bit set to 1
   and with Interface ID set to the Client's interface ID so that the
   Server will de-register this Interface ID.  Although the Proxy
   engages in these 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 the Proxy to convey QoS changes, etc.

   In some subnetworks that employ a Proxy, the Client's ACP can be
   injected into the underlying network routing system.  In that case,
   the Client can send data messages without encapsulation so that the
   native underlying network routing system transports the
   unencapsulated packets to the Proxy.  This can be very beneficial,



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   e.g., if the Client connects to the network via low-end data links
   such as some aviation wireless links.  In that case, however, the
   Client's control messages are still sent encapsulated so as to supply
   the Proxy with the address of the Server and to transport IPv6 ND
   messages without decrementing the hop-count.  In summary, the
   interface becomes one where control messages are encapsulated while
   data messages are either unencapsulated or encapsulated according to
   the specific use case.  This encapsulation avoidance represents a
   form of "header compression", meaning that the MTU should be sized
   based on the size of full encapsulated messages even if most messages
   are sent unencapsulated.

3.16.1.  The AERO-Aware Access Router

   If the Client is aware that its data link interface connects to a
   secured enclave with an AERO-aware Access Router as the first-hop
   router, it can avoid encapsulation for its control messages as well
   as its data messages.  When the Client comes onto the link, it can
   send an unencapsulated RS message with source address set to its AERO
   address and with destination address set to the AERO address of the
   Client's selected Server or to all-routers multicast.

   The Client includes an SLLAO with Interface ID, Prefix Length and
   P(i) information but with Port Number and Link-Layer Address set to
   0.  The Client then sends the unencapsulated RS message, which will
   be intercepted by the on-link AERO-Aware Access Router.  The Access
   Router then encapsulates the RS message in an outer header with its
   own address as the source address and the address of a Proxy as the
   destination address.  The Access Router further remembers the address
   of the Proxy so that it can encapsulate future data packets from the
   Client via the same Proxy.  If the Access Router needs to change to a
   new Proxy, it simply sends another RS message toward the Server via
   the new Proxy on behalf of the Client.

   In this arrangement, the only control messages that would ever be
   sent by the Client are unencapsulated RS messages with its AERO
   address as the source address and the AERO address of the Server as
   the destination address.  The Client will also receive unencapsulated
   RA messages from the Server via both the Proxy and Access Router.

   In some cases, the Access Router and AERO Proxy may be one and the
   same node.  In that case, the node would be located on the same
   physical link as the Client, but its messages exchanges with the
   Server would need to pass through a security gateway at the secured
   enclave ingress/egress.  The method for deploying Access Routers and
   Proxys (i.e. as a single node or multiple nodes) is a subnetwork-
   local administrative consideration.




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3.17.  AERO Route Optimization

   While data packets are flowing from a source Client to a target
   Client that are both holders of ACPs belonging to the same ASP, route
   optimization SHOULD be used to establish the best path(s).  Route
   optimization is initiated by the first eligible Route Optimization
   Source (ROS) closest to the source Client as follows:

   o  For VPNed, NATed and Direct underlying interfaces, the Server is
      the ROS.

   o  For Proxyed underlying interfaces, the Proxy is the ROS.

   o  For native underlying interfaces, the Client itself is the ROS.

   The route optimization procedure is conducted between the ROS and a
   Route Optimization Responder (ROR) in the same manner as for IPv6 ND
   Address Resolution, and using the same NS/NA messaging.  The
   procedures are specified in the following sections.

3.17.1.  Route Optimization Initiation

   While the data packets are flowing from the source Client toward a
   target Client, the ROS also sends an NS message to receive a
   solicited NA message from an ROR acting as a Mobility Anchor Point
   (MAP).

   When the ROS sends an NS, it includes the AERO address of the ROS as
   the source address and the AERO address corresponding to the data
   packet's destination address as the destination address (for example,
   if the destination address is 2001:db8:1:2::1 then the target AERO
   address is fe80::2001:db8:1:2).  The NS message includes no SLLAOs,
   but SHOULD include a Timestamp and Nonce option.

   The ROS then sends the message into the SPAN (but with SPAN
   destination set to the inner packet destination) without decrementing
   the network-layer TTL/Hop Limit field.

3.17.2.  Relaying the NS

   When the Relay receives the (double-encapsulated) NS message from the
   ROS, it discards the outer IP header and determines that the ROR is
   the next hop by consulting its standard IP forwarding table for the
   SPAN header destination address.  The Relay then forwards the SPAN
   message toward the ROR the same as for any IP router.  The final-hop
   Relay in the SPAN will encapsulate the message in an outer IP header
   when it delivers the message to the ROR.




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3.17.3.  Processing the NS and Sending the NA

   When the ROR receives the (double-encapsulated) NS message, it
   discards the outer IP and SPAN headers.  The ROR next examines the
   AERO destination address to determine whether the target Client is
   one of its symmetric neighbors in the REACHABLE state.  If so, the
   ROR adds the AERO source address to the target Client's symmetric
   neighbor cache entry Report list with time set to ReportTime.

   Next, the ROR prepares a solicited NA message to send back to the ROS
   but does not create a neighbor cache entry.  The ROR sets the NA
   source address to its own AERO address and sets the destination
   address to the AERO address of the ROS.  The NA message includes the
   Nonce value received in the NS, the current Timestamp, and a first
   TLLAO with Interface ID set to 255, with all P(i) values set to "low"
   and with "Prefix Length" set to the prefix length of the target
   Client's ACP.  If the ROR and ROS are on the same segment, the ROR
   sets the TLLAO Link Layer address to the ROR's own link-layer
   address; otherwise, set to the ROR's SPAN address.

   If the ROS and ROR are on the same segment, the ROR next includes
   additional TLLAOs for all of the target Client's Interface IDs.  For
   NATed, VPNed and Direct interfaces, the TLLAO addresses are the
   address of the ROR.  For Proxyed interfaces, the TLLAO addresses are
   the addresses of the target Client's Proxies, and for native
   interfaces the TLLAO addresses are the addresses of the target
   Client.

   The ROR then sends the message into the SPAN without decrementing the
   network-layer TTL/Hop Limit field.

3.17.4.  Relaying the NA

   When the Relay receives the (double-encapsulated) NA message from the
   ROR, it discards the outer IP header and determines that the ROS is
   the next hop by consulting its standard IP forwarding table for the
   SPAN header destination address.  The Relay then forwards the SPAN
   message toward the ROS the same as for any IP router.  The final-hop
   Relay in the SPAN will encapsulate the message in an outer IP header
   when it delivers the message to the ROS.

3.17.5.  Processing the NA

   When the ROS receives the (double-encapsulated) solicited NA message,
   it discards the outer IP and SPAN headers.  The ROS next verifies the
   Nonce and Timestamp values, then creates an asymmetric neighbor cache
   entry for the target Client and caches all information found in the




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   solicited NA TLLAOs.  The ROS finally sets the asymmetric neighbor
   cache entry lifetime to ReachableTime seconds.

3.17.6.  Route Optimization Maintenance

   Following route optimization, if the ROS and ROR are on the same SPAN
   segment the ROS forwards future data packets directly to the target
   Client using the cached link-layer information instead of through a
   dogleg route involving unnecessary Servers and/or Relays.  Otherwise,
   the ROS forwards future data packets into the SPAN using the ROS's
   SPAN address as the source address and the ROR's SPAN address as the
   destination address.  In both cases, the route optimization is shared
   by all sources that send packets to the target Client via the ROS,
   i.e., and not just the original source Client.

   While new data packets destined to the target are flowing through the
   ROS, it sends additional NS messages to the ROR before ReachableTime
   expires to receive a fresh solicited NA message the same as described
   in the previous sections.  The ROS then updates the asymmetric
   neighbor cache entry to refresh ReachableTime, while the ROR adds or
   updates the ROS address to the target Client's symmetric neighbor
   cache entry Report list and with time set to ReportTime.  While no
   data packets are flowing, the ROS instead allows ReachableTime for
   the asymmetric neighbor cache entry to expire.  When ReachableTime
   expires, the ROS deletes the asymmetric neighbor cache entry.  Future
   data packets flowing through the ROS will again trigger a new route
   optimization exchange while initial data packets travel over a
   suboptimal route via Servers and/or Relays.

   The ROS may also receive unsolicited NA messages from the ROR at any
   time.  If there is an asymmetric neighbor cache entry for the target,
   the ROS updates the link-layer information but does not update
   ReachableTime since the receipt of an unsolicited NA does not confirm
   that the forward path is still working.  If there is no asymmetric
   neighbor cache entry, the route optimization source simply discards
   the unsolicited NA.  Cases in which unsolicited NA messages are
   generated are specified in Section 3.19.

   In this arrangement, the ROS holds an asymmetric neighbor cache entry
   for the ROR, but the ROR does not hold an asymmetric neighbor cache
   entry for the ROS.  The route optimization neighbor relationship is
   therefore asymmetric and unidirectional.  If the target Client also
   has packets to send back to the source Client, then a separate route
   optimization procedure is required in the reverse direction.  But,
   there is no requirement that the forward and reverse paths be
   symmetric.





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3.18.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform Neighbor Unreachability Detection (NUD) the same
   as described in [RFC4861].  NUD is performed either reactively in
   response to persistent link-layer errors (see Section 3.14) or
   proactively to confirm bi-directional reachability.  The NUD
   algorithm may further be seeded by neighbor discovery hints of
   forward progress, but care must be taken to avoid inferring
   reachability based on spoofed information.

   When an AERO node sends an NS/NA message used for NUD, it uses one of
   its AERO addresses as the IPv6 source address and an AERO address of
   the neighbor as the IPv6 destination address, but does not include S/
   TLLAOs.  When an ROR directs an ROS to one or more target addresses,
   the ROS SHOULD proactively test the direct path to each target
   address by sending an initial NS message to elicit a solicited NA
   response.  While testing the path, the source node can optionally
   continue sending packets via its default router, maintain a small
   queue of packets until target reachability is confirmed, or
   (optimistically) allow packets to flow directly to the target.

   Note that AERO nodes may have multiple underlying interface paths
   toward the target neighbor.  In that case, NUD SHOULD be performed
   over each underlying interface individually and the node should only
   consider the neighbor unreachable if NUD fails over multiple
   underlying interface paths.

   Underlying interface paths that pass NUD tests are marked as
   "reachable", while those that do not are marked as "unreachable".
   These markings inform the AERO interface forwarding algorithm
   specified in Section 3.9.

   Proxies can perform NUD to verify Server reachability on behalf of
   their proxyed Clients so that the Clients need not engage in NUD
   messaging themselves.

3.19.  Mobility Management and Quality of Service (QoS)

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

   Servers provide a Mobility Anchor Point (MAP) for their dependent
   Clients.  Clients are responsible for maintaining neighbor



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   relationships with their Servers through periodic RS/RA exchanges,
   which also serves to confirm neighbor reachability.  When a Client's
   underlying interface address and/or QoS information changes, the
   Client is responsible for updating the Server with this new
   information.  Note that for Proxyed interfaces, however, the Proxy
   can perform the RS/RA exchanges on the Client's behalf.

   Mobility management considerations are specified in the following
   sections.

3.19.1.  Mobility Update Messaging

   RORs (acting as MAPs) accommodate mobility and/or QoS change events
   by sending an unsolicited NA message to each ROS in the target
   Client's Report list.  When an ROR sends an unsolicited NA message,
   it sets the IPv6 source address to the Client's AERO address and sets
   the IPv6 destination address to all-nodes multicast (ff02::1).  The
   ROR also includes a first TLLAO for Interface ID 255 with Link Layer
   address set to the ROR link-layer address if the ROR and ROS are on
   the same segment; otherwise, set to the ROR SPAN address.  If the ROS
   and ROR are on the same segment the ROR next includes additional
   TLLAOs for all of the target Client's Interface IDs.  The ROR then
   finally sends the message into the SPAN.

   As for the hot-swap of interface cards discussed in Section 7.2.6 of
   [RFC4861], the transmission and reception of unsolicited NA messages
   is unreliable but provides a useful optimization.  In well-connected
   Internetworks with robust data links unsolicited NA messages will be
   delivered with high probability, but in any case the ROR can
   optionally send up to MAX_NEIGHBOR_ADVERTISEMENT unsolicited NAs to
   each ROS to increase the likelihood that at least one will be
   received.

   When an ROS receives an unsolicited NA message, it ignores the
   message if there is no existing neighbor cache entry for the Client.
   Otherwise, it uses the included TLLAOs to update the address and QoS
   information in the neighbor cache entry, but does not reset
   ReachableTime since the receipt of an unsolicited NA message from the
   target Server does not provide confirmation that any forward paths to
   the target Client are working.

   If unsolicited NA messages are lost, the ROS may be left with stale
   address and/or QoS information for the Client for up to ReachableTime
   seconds.  During this time, the ROS can continue sending packets to
   the target Client according to its current neighbor cache information
   but may receive persistent unsolicited NA messages as discussed in
   Section 3.19.2.




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3.19.2.  Forwarding Packets on Behalf of Departed Clients

   When a Server receives packets with destination addresses that match
   a symmetric neighbor cache entry in the DEPARTED state, it forwards
   the packets according to the Client's cached link layer address
   information, noting that the information may be stale.  If the
   encapsulation source is in the Report list (i.e., if it is an ROS),
   the Server also sends an unsolicited NA message via the SPAN (subject
   to rate limiting) with a TLLAO with Interface ID 255 and with D set
   to 1.  The ROS will then realize that it needs to set its asymmetric
   neighbor cache entry state for the target to DEPARTED, and SHOULD re-
   initiate route optimization after a short delay.

   When a Proxy receives packets with destination addresses that match a
   proxy neighbor cache entry in the DEPARTED state, it forwards the
   packets to one of the target Client's Servers.  If the encapsulation
   source is neither one of the target Client's Servers nor one of its
   proxy neighbor Clients, the Proxy also returns an unsolicited NA
   message via the SPAN (subject to rate limiting) with a single TLLAO
   with the target Client's Interface ID and with D set to 1.  The
   source will then realize that it needs to mark its neighbor cache
   entry Interface ID for the Proxy as "unreachable", and SHOULD re-
   initiate route optimization while continuing to forward packets
   according to the remaining neighbor cache entry state.

   When a Server receives packets from a symmetric neighbor Client that
   are destined to the same Client, the Server marks the neighbor cache
   entry Interface ID for this path as "unreachable", and forwards the
   packets via a "reachable" Interface ID.  If there are no "reachable"
   Interface IDs, the Server drops the packet.

   When a Client receives packets with destination addresses that do not
   match one of its ACPs, it drops the packets silently.

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

   When a Client needs to change its link-layer addresses and/or QoS
   preferences (e.g., due to a mobility event), either the Client or
   Proxy sends RS messages to its Servers via the SPAN using the new
   link-layer address as the source address and with SLLAOs that include
   the new Client Port Number, Link-Layer Address and P(i) values.  If
   the RS messages are sent solely for the purpose of updating QoS
   preferences without updating the link-layer address, the Port Number
   and Link-Layer Address are set to 0.  If the RS message is not sent
   for the purpose of asserting a PD, the Prefix Length is set to 0.






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

3.19.4.  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 RS messages to
   its Servers using the new link-layer address as the source address
   and with SLLAOs that include the new Client link-layer information.
   If the RS message is not sent for the purpose of asserting a PD, the
   Prefix Length is set to 0.

3.19.5.  Removing Existing Links from Service

   When a Client needs to remove existing underlying interfaces from
   service (e.g., when it de-activates an existing data link), it sends
   RS messages to its Servers with SLLAOs with the D flag set to 1.

   If the Client needs to send RS messages over an underlying interface
   other than the one being removed from service, it MUST include a
   current SLLAO for the sending interface as the first SLLAO and
   include SLLAOs for any underlying interfaces being removed from
   service as additional SLLAOs.

3.19.6.  Implicit Mobility Management

   AERO 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 neighbor cache entry
   for the Client and begins accepting and sending packets to the
   Client's new link-layer 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.








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3.19.7.  Moving to a New Server

   When a Client associates with a new Server, it performs the Client
   procedures specified in Section 3.15.2.  The Client then sends an RS
   message with R set to 1 in the first SLLAO and with PD parameters
   over any working underlying interface to fully release itself from
   the old Server.  The SLLAO also includes the link-layer address of
   the new Server if the new and old Servers are on the same segment;
   otherwise, it includes the SPAN address of the new Server.  If the
   Client does not receive an RA reply after MAX_RTR_SOLICITATIONS
   attempts over multiple underlying interfaces, the old Server may have
   failed and the Client should discontinue its release attempts.

   When the old Server processes the RS, it sends unsolicited NA
   messages with a single TLLAO with Interface ID set to 255 and with D
   set to 1 to all route optimization sources in the Client's Report
   list.  The Server also changes the symmetric neighbor cache entry
   state to DEPARTED, sets the link-layer address of the Client to the
   address found in the RS SLLAO, and sets a timer to DepartTime
   seconds.  The Server then returns an RA message to the Client with
   Router Lifetime set to 0.  After DepartTime seconds expires, the
   Server deletes the symmetric neighbor cache entry.

   When the Client receives the RA message with Router Lifetime set to
   0, it still must inform each of its remaining Proxies that it has
   released the old Server from service.  To do so, it sends an RS over
   each remaining proxyed underlying interface with destination set to
   the old Server's AERO address and with R set to 1 in the first SLLAO
   but with no PD parameters.  The Proxy will mark this Server as
   DEAPARTED and return an immediate RA without first performing an RS/
   RA exchange with the old Server.

   Clients SHOULD NOT move rapidly between 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 Server include a
   Server that has gone unreachable, topological movements of
   significant distance, movement to a new geographic region, movement
   to a new segment, etc.

3.20.  Multicast Considerations

   When the underlying network does not support multicast, AERO Clients
   map link-scoped multicast addresses to the link-layer address of a
   Server, which acts as a multicast forwarding agent.  The AERO Client
   also serves as an IGMP/MLD Proxy for its EUNs and/or hosted
   applications per [RFC4605] while using the link-layer address of the
   Server as the link-layer address for all multicast packets.




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   When the underlying network supports multicast, AERO nodes use the
   multicast address mapping specification found in [RFC2529] for IPv4
   underlying networks and use a TBD site-scoped multicast mapping for
   IPv6 underlying networks.  In that case, border routers must ensure
   that the encapsulated site-scoped multicast packets do not leak
   outside of the site spanned by the AERO link.

4.  Direct Underlying Interfaces

   When a Client's AERO interface is configured over a Direct underlying
   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 the QoS preferences
   associated with its underling interfaces.  If the Direct underlying
   interface has the highest QoS preference, then the Client's IP
   packets are transmitted directly to the peer without going through an
   underlying network.  If other underlying interfaces have higher QoS
   preferences, then the Client's IP packets are transmitted via a
   different underlying interface, which may result in the inclusion of
   Proxies, Servers and Relays in the communications path.  Direct
   underlying interfaces must be tested periodically for reachability,
   e.g., via NUD.

5.  Operation on AERO Links with /64 ASPs

   IPv6 AERO links typically have ASPs that cover many candidate ACPs of
   length /64 or shorter.  However, in some cases it may be desirable to
   use AERO over links that have only a /64 ASP.  This can be
   accommodated by treating all Clients on the AERO link as simple hosts
   that receive /128 prefix delegations.

   In that case, the Client sends an RS message to the Server the same
   as for ordinary AERO links.  The Server responds with an RA message
   that includes one or more /128 prefixes (i.e., singleton addresses)
   that include the /64 ASP prefix along with an interface identifier
   portion to be assigned to the Client.  The Client and Server then
   configure their AERO addresses based on the interface identifier
   portions of the /128s (i.e., the lower 64 bits) and not based on the
   /64 prefix (i.e., the upper 64 bits).

   For example, if the ASP for the host-only IPv6 AERO link is
   2001:db8:1000:2000::/64, each Client will receive one or more /128
   IPv6 prefix delegations such as 2001:db8:1000:2000::1/128,
   2001:db8:1000:2000::2/128, etc.  When the Client receives the prefix
   delegations, it assigns the AERO addresses fe80::1, fe80::2, etc. to
   the AERO interface, and assigns the global IPv6 addresses (i.e., the
   /128s) to either the AERO interface or an internal virtual interface




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   such as a loopback.  In this arrangement, the Client conducts route
   optimization in the same sense as discussed in Section 3.17.

   This specification has applicability for nodes that act as a Client
   on an "upstream" AERO link, but also act as a Server on "downstream"
   AERO links.  More specifically, if the node acts as a Client to
   receive a /64 prefix from the upstream AERO link it can then act as a
   Server to provision /128s to Clients on downstream AERO links.

6.  AERO Adaptations for SEcure Neighbor Discovery (SEND)

   SEcure Neighbor Discovery (SEND) [RFC3971] and Cryptographically
   Generated Addresses (CGAs) [RFC3972] were designed to secure IPv6 ND
   messaging in environments where symmetric network and/or transport-
   layer security services are impractical (see: Section 10).  AERO
   nodes that use SEND/CGA employ the following adaptations.

   When a source AERO node prepares a SEND-protected ND message, it uses
   a link-local CGA as the IPv6 source address and writes the prefix
   embedded in its AERO address (i.e., instead of fe80::/64) in the CGA
   parameters Subnet Prefix field.  When the neighbor receives the ND
   message, it first verifies the message checksum and SEND/CGA
   parameters while using the link-local prefix fe80::/64 (i.e., instead
   of the value in the Subnet Prefix field) to match against the IPv6
   source address of the ND message.

   The neighbor then derives the AERO address of the source by using the
   value in the Subnet Prefix field as the interface identifier of an
   AERO address.  For example, if the Subnet Prefix field contains
   2001:db8:1:2, the neighbor constructs the AERO address as
   fe80::2001:db8:1:2.  The neighbor then caches the AERO address in the
   neighbor cache entry it creates for the source, and uses the AERO
   address as the IPv6 destination address of any ND message replies.

7.  AERO Critical Infrastructure Considerations

   AERO Relays are low-end to midrange Commercial off-the Shelf (COTS)
   standard IP routers with no AERO code.  Relays must be provisioned,
   supported and managed by the AERO Link Service Provider.  Cost for
   purchasing, configuring and managing Relays is nominal even for very
   large AERO links.

   AERO Servers can be standard dedicated server platforms, but most
   often will be deployed as virtual machines in the cloud.  The only
   requirements for Servers are that they can run the AERO user-level
   code and have at least one network interface with a public IP
   address.  As with Relays, Servers must be provisioned, supported and
   managed by the AERO Link Service Provider.  Cost for purchasing,



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   configuring and managing Servers is nominal especially for virtual
   Servers hosted in the cloud.

   AERO Proxies are most often standard dedicated server platforms with
   one network interface connected to the secured enclave and a second
   interface connected to the public Internetwork.  As with Servers, the
   only requirements are that they can run the AERO user-level code and
   have at least one interface with a public IP address.  Proxies must
   be provisioned, supported and managed by the administrative authority
   for the secured enclave.  Cost for purchasing, configuring and
   managing Proxies is nominal, and borne by the secured enclave
   administrative authority.

   AERO Clients are most often mobile nodes, but fixed AERO Clients can
   also be used to attach large non-mobile networks to the AERO link.
   In that case, the AERO Client would be a fixed IPv6 router that would
   appear the same as for any Client, albeit with no mobility signaling
   requirements.

8.  Implementation Status

   An AERO implementation based on OpenVPN (https://openvpn.net/) was
   announced on the v6ops mailing list on January 10, 2018.  The latest
   version is available at: http://linkupnetworks.net/aero/AERO-OpenVPN-
   2.0.tgz.

   An initial public release of the AERO proof-of-concept source code
   was announced on the intarea mailing list on August 21, 2015.  The
   latest version is available at: http://linkupnetworks.net/aero/aero-
   4.0.0.tgz.

   A survey of public domain and commercial SEND implementations is
   available at https://www.ietf.org/mail-archive/web/its/current/
   msg02758.html.

9.  IANA Considerations

   The IANA has assigned a 4-octet Private Enterprise Number "45282" for
   AERO in the "enterprise-numbers" registry.

   The IANA has assigned the UDP port number "8060" for an earlier
   experimental version of AERO [RFC6706].  This document obsoletes
   [RFC6706] and claims the UDP port number "8060" for all future use.

   No further IANA actions are required.






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10.  Security Considerations

   AERO link security considerations include considerations for both the
   data plane and the control plane.

   Data plane security considerations are the same as for ordinary
   Internet communications.  Application endpoints in AERO Clients and
   their EUNs SHOULD use application-layer security services such as
   TLS/SSL [RFC8446], DTLS [RFC6347] 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 symmetric
   network and/or transport layer security services such as TLS/SSL,
   DTLS, IPsec [RFC4301], etc.  AERO Proxies and 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.

   Control plane security considerations are the same as for standard
   IPv6 Neighbor Discovery [RFC4861], except that the MAP list also
   improves security by providing AERO Clients with an authentic list of
   trusted Servers.  As fixed infrastructure elements, AERO Proxies and
   Servers SHOULD pre-configure security associations for one or more
   Relays on their SPAN segments (e.g., using pre-placed keys) and use
   symmetric network and/or transport layer security services such as
   IPsec, TLS/SSL or DTLS to secure ND messages.  The AERO Relays of all
   SPAN segments in turn SHOULD pre-configure security associations for
   their neighboring AERO Relays.  AERO Clients that connect to secured
   enclaves need not apply security to their ND messages, since the
   messages will be intercepted by an enclave perimeter Proxy.  AERO
   Clients located outside of secured enclaves SHOULD use symmetric
   network and/or transport layer security to secure their ND exchanges
   with Servers, but when there are many prospective neighbors with
   dynamically changing connectivity an asymmetric security service such
   as SEND may be needed (see: Section 6).

   AERO Servers and Relays 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 by connecting Relays and Servers 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 Servers and Proxies can institute rate limits that
   protect Clients from receiving packet floods that could DoS low data
   rate links.



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   AERO Relays must implement ingress filtering to avoid a spoofing
   attack in which spurious SPAN messages are injected into an AERO link
   from an outside attacker.  Also, since an AERO link spans one or
   Internetwork segments, restricting access to the link can be achieved
   by having Internetwork border routers implement ingress filtering to
   discard encapsulated packets injected into the link by an outside
   agent.

   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, e.g., through secure
   upload of a static file, through DNS lookups, etc.

   Although public domain and commercial SEND implementations exist,
   concerns regarding the strength of the cryptographic hash algorithm
   have been documented [RFC6273] [RFC4982].

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

11.  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, Brian Carpenter,
   Wojciech Dec, Ralph Droms, Adrian Farrel, Nick Green, Sri Gundavelli,
   Brian Haberman, Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha
   Hlusiak, Lee Howard, Andre Kostur, Hubert Kuenig, Ted Lemon, Andy
   Malis, Satoru Matsushima, Tomek Mrugalski, Madhu Niraula, Alexandru
   Petrescu, Behcet Saikaya, Michal Skorepa, 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, Balaguruna Chidambaram, Irene Chin, Bruce Cornish, Claudiu



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   Danilov, Wen Fang, Anthony Gregory, Jeff Holland, Seth Jahne, Ed
   King, Gene MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Greg
   Saccone, Kent Shuey, Brian Skeen, Mike Slane, Carrie Spiker, Brendan
   Williams, Julie Wulff, Yueli Yang, Eric Yeh and other members of the
   BR&T and BIT mobile networking teams.  Kyle Bae, Wayne Benson and
   Eric Yeh are especially acknowledged for implementing the AERO
   functions as extensions to the public domain OpenVPN distribution.

   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 Information Technology (BIT)
   MobileNet program.

   This work is aligned with the Boeing autonomy program.

12.  References

12.1.  Normative References

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




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

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

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [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>.

   [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>.

   [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>.



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12.2.  Informative References

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

   [I-D.ietf-dmm-distributed-mobility-anchoring]
              Chan, A., Wei, X., Lee, J., Jeon, S., and C. Bernardos,
              "Distributed Mobility Anchoring", draft-ietf-dmm-
              distributed-mobility-anchoring-13 (work in progress),
              March 2019.

   [I-D.ietf-intarea-gue]
              Herbert, T., Yong, L., and O. Zia, "Generic UDP
              Encapsulation", draft-ietf-intarea-gue-07 (work in
              progress), March 2019.

   [I-D.ietf-intarea-gue-extensions]
              Herbert, T., Yong, L., and F. Templin, "Extensions for
              Generic UDP Encapsulation", draft-ietf-intarea-gue-
              extensions-06 (work in progress), March 2019.

   [I-D.ietf-intarea-tunnels]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-09 (work in
              progress), July 2018.

   [I-D.ietf-rtgwg-atn-bgp]
              Templin, F., Saccone, G., Dawra, G., Lindem, A., and V.
              Moreno, "A Simple BGP-based Mobile Routing System for the
              Aeronautical Telecommunications Network", draft-ietf-
              rtgwg-atn-bgp-01 (work in progress), January 2019.

   [I-D.templin-6man-dhcpv6-ndopt]
              Templin, F., "A Unified Stateful/Stateless Configuration
              Service for IPv6", draft-templin-6man-dhcpv6-ndopt-07
              (work in progress), December 2018.

   [I-D.templin-intarea-grefrag]
              Templin, F., "GRE Tunnel Level Fragmentation", draft-
              templin-intarea-grefrag-04 (work in progress), July 2016.

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






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

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

   [I-D.templin-v6ops-pdhost]
              Templin, F., "IPv6 Prefix Delegation and Multi-Addressing
              Models", draft-templin-v6ops-pdhost-23 (work in progress),
              December 2018.

   [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>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

   [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>.

   [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>.

   [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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   [RFC2764]  Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A.
              Malis, "A Framework for IP Based Virtual Private
              Networks", RFC 2764, DOI 10.17487/RFC2764, February 2000,
              <https://www.rfc-editor.org/info/rfc2764>.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              DOI 10.17487/RFC2784, March 2000,
              <https://www.rfc-editor.org/info/rfc2784>.

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, DOI 10.17487/RFC2890, September 2000,
              <https://www.rfc-editor.org/info/rfc2890>.

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery",
              RFC 2923, DOI 10.17487/RFC2923, September 2000,
              <https://www.rfc-editor.org/info/rfc2923>.

   [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>.

   [RFC3819]  Karn, P., Ed., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, DOI 10.17487/RFC3819, July 2004,
              <https://www.rfc-editor.org/info/rfc3819>.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213,
              DOI 10.17487/RFC4213, October 2005,
              <https://www.rfc-editor.org/info/rfc4213>.

   [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>.





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

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

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

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

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

   [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>.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963,
              DOI 10.17487/RFC4963, July 2007,
              <https://www.rfc-editor.org/info/rfc4963>.

   [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>.

   [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>.




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

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

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

   [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>.

   [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>.

   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, DOI 10.17487/RFC6864, February 2013,
              <https://www.rfc-editor.org/info/rfc6864>.

   [RFC8086]  Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
              in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
              March 2017, <https://www.rfc-editor.org/info/rfc8086>.



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   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

   [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>.

Appendix A.  AERO Alternate Encapsulations

   When GUE encapsulation is not needed, AERO can use common
   encapsulations such as IP-in-IP [RFC2003][RFC2473][RFC4213], Generic
   Routing Encapsulation (GRE) [RFC2784][RFC2890] and others.  The
   encapsulation is therefore only differentiated from non-AERO tunnels
   through the application of AERO control messaging and not through,
   e.g., a well-known UDP port number.

   As for GUE encapsulation, alternate AERO encapsulation formats may
   require encapsulation layer fragmentation.  For simple IP-in-IP
   encapsulation, an IPv6 fragment header is inserted directly between
   the inner and outer IP headers when needed, i.e., even if the outer
   header is IPv4.  The IPv6 Fragment Header is identified to the outer
   IP layer by its IP protocol number, and the Next Header field in the
   IPv6 Fragment Header identifies the inner IP header version.  For GRE
   encapsulation, a GRE fragment header is inserted within the GRE
   header [I-D.templin-intarea-grefrag].

   Figure 5 shows the AERO IP-in-IP encapsulation format before any
   fragmentation is applied:





















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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |     Outer IPv4 Header     |      |    Outer IPv6 Header      |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |IPv6 Frag Header (optional)|      |IPv6 Frag Header (optional)|
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      Inner IP Header      |      |       Inner IP Header     |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                           |      |                           |
        ~                           ~      ~                           ~
        ~    Inner Packet Body      ~      ~     Inner Packet Body     ~
        ~                           ~      ~                           ~
        |                           |      |                           |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Minimal Encapsulation in IPv4      Minimal Encapsulation in IPv6


           Figure 5: Minimal Encapsulation Format using IP-in-IP

   Figure 6 shows the AERO GRE encapsulation format before any
   fragmentation is applied:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        Outer IP Header        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |          GRE Header           |
        | (with checksum, key, etc..)   |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        | GRE Fragment Header (optional)|
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |        Inner IP Header        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                               |
        ~                               ~
        ~      Inner Packet Body        ~
        ~                               ~
        |                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure 6: Minimal Encapsulation Using GRE

   Alternate encapsulation may be preferred in environments where GUE
   encapsulation would add unnecessary overhead.  For example, certain
   low-bandwidth wireless data links may benefit from a reduced
   encapsulation overhead.





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   GUE encapsulation can traverse network paths that are inaccessible to
   non-UDP encapsulations, e.g., for crossing Network Address
   Translators (NATs).  More and more, network middleboxes are also
   being configured to discard packets that include anything other than
   a well-known IP protocol such as UDP and TCP.  It may therefore be
   necessary to determine the potential for middlebox filtering before
   enabling alternate encapsulation in a given environment.

   In addition to IP-in-IP, GRE and GUE, AERO can also use security
   encapsulations such as IPsec, TLS/SSL, DTLS, etc.  In that case, AERO
   control messaging and route determination occur before security
   encapsulation is applied for outgoing packets and after security
   decapsulation is applied for incoming packets.

   AERO is especially well suited for use with VPN system encapsulations
   such as OpenVPN [OVPN].

Appendix B.  S/TLLAO Extensions for Special-Purpose Links

   The AERO S/TLLAO format specified in Section 3.6 includes a Length
   value of 5 (i.e., 5 units of 8 octets).  However, special-purpose
   links may extend the basic format to include additional fields and a
   Length value larger than 5.

   For example, adaptation of AERO to the Aeronautical
   Telecommunications Network with Internet Protocol Services (ATN/IPS)
   includes link selection preferences based on transport port numbers
   in addition to the existing DSCP-based preferences.  ATN/IPS nodes
   maintain a map of transport port numbers to 64 possible preference
   fields, e.g., TCP port 22 maps to preference field 8, TCP port 443
   maps to preference field 20, UDP port 8060 maps to preference field
   34, etc.  The extended S/TLLAO format for ATN/IPS is shown in
   Figure 7, where the Length value is 7 and the 'Q(i)' fields provide
   link preferences for the corresponding transport port number.

















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        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Type     |   Length = 7  | Prefix Length |   Reserved    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Interface ID         |          Port Number          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                        Link-Layer Address                     +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q00|Q01|Q02|Q03|Q04|Q05|Q06|Q07|Q08|Q09|Q10|Q11|Q12|Q13|Q14|Q15|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q16|Q17|Q18|Q19|Q20|Q21|Q22|Q23|Q24|Q25|Q26|Q27|Q28|Q29|Q30|Q31|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q32|Q33|Q34|Q35|Q36|Q37|Q38|Q39|Q40|Q41|Q42|Q43|Q44|Q45|Q46|Q47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Q48|Q49|Q50|Q51|Q52|Q53|Q54|Q55|Q56|Q57|Q58|Q59|Q60|Q61|Q62|Q63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 7: ATN/IPS Extended S/TLLAO Format

Appendix C.  Change Log

   << RFC Editor - remove prior to publication >>

   Changes from draft-templin-intarea-6706bis-10 to draft-templin-
   intrea-6706bis-11:

   o  Added The SPAN

   Changes from draft-templin-intarea-6706bis-09 to draft-templin-
   intrea-6706bis-10:

   o  Orphaned packets in flight (e.g., when a neighbor cache entry is
      in the DEPARTED state) are now forwarded at the link layer instead



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      of at the network layer.  Forwarding at the network layer can
      result in routing loops and/or excessive delays of forwarded
      packets while the routing system is still reconverging.

   o  Update route optimization to clarify the unsecured nature of the
      first NS used for route discovery

   o  Many cleanups and clarifications on ND messaging parameters

   Changes from draft-templin-intarea-6706bis-08 to draft-templin-
   intrea-6706bis-09:

   o  Changed PRL to "MAP list"

   o  For neighbor cache entries, changed "static" to "symmetric", and
      "dynamic" to "asymmetric"

   o  Specified Proxy RS/RA exchanges with Servers on behalf of Clients

   o  Added discussion of unsolicited NAs in Section 3.16, and included
      forward reference to Section 3.18

   o  Added discussion of AERO Clients used as critical infrastructure
      elements to connect fixed networks.

   o  Added network-based VPN under security considerations

   Changes from draft-templin-intarea-6706bis-07 to draft-templin-
   intrea-6706bis-08:

   o  New section on AERO-Aware Access Router

   Changes from draft-templin-intarea-6706bis-06 to draft-templin-
   intrea-6706bis-07:

   o  Added "R" bit for release of PDs.  Now have a full RS/RA service
      that can do PD without requiring DHCPv6 messaging over-the-air

   o  Clarifications on solicited vs unsolicited NAs

   o  Clarified use of MAX_NEIGHBOR_ADVERTISEMENTS for the purpose of
      increase reliability

   Changes from draft-templin-intarea-6706bis-05 to draft-templin-
   intrea-6706bis-06:

   o  Major re-work and simplification of Route Optimization function




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   o  Added Distributed Mobility Management (DMM) and Mobility Anchor
      Point (MAP) terminology

   o  New section on "AERO Critical Infrastructure Element
      Considerations" demonstrating low overall cost for the service

   o  minor text revisions and deletions

   o  removed extraneous appendices

   Changes from draft-templin-intarea-6706bis-04 to draft-templin-
   intrea-6706bis-05:

   o  New Appendix E on S/TLLAO Extensions for special-purpose links.
      Discussed ATN/IPS as example.

   o  New sentence in introduction to declare appendices as non-
      normative.

   Changes from draft-templin-intarea-6706bis-03 to draft-templin-
   intrea-6706bis-04:

   o  Added definitions for Potential Router List (PRL) and secure
      enclave

   o  Included text on mapping transport layer port numbers to network
      layer DSCP values

   o  Added reference to DTLS and DMM Distributed Mobility Anchoring
      working group document

   o  Reworked Security Considerations

   o  Updated references.

   Changes from draft-templin-intarea-6706bis-02 to draft-templin-
   intrea-6706bis-03:

   o  Added new section on SEND.

   o  Clarifications on "AERO Address" section.

   o  Updated references and added new reference for RFC8086.

   o  Security considerations updates.

   o  General text clarifications and cleanup.




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   Changes from draft-templin-intarea-6706bis-01 to draft-templin-
   intrea-6706bis-02:

   o  Note on encapsulation avoidance in Section 4.

   Changes from draft-templin-intarea-6706bis-00 to draft-templin-
   intrea-6706bis-01:

   o  Remove DHCPv6 Server Release procedures that leveraged the old way
      Relays used to "route" between Server link-local addresses

   o  Remove all text relating to Relays needing to do any AERO-specific
      operations

   o  Proxy sends RS and receives RA from Server using SEND.  Use CGAs
      as source addresses, and destination address of RA reply is to the
      AERO address corresponding to the Client's ACP.

   o  Proxy uses SEND to protect RS and authenticate RA (Client does not
      use SEND, but rather relies on subnetwork security.  When the
      Proxy receives an RS from the Client, it creates a new RS using
      its own addresses as the source and uses SEND with CGAs to send a
      new RS to the Server.

   o  Emphasize distributed mobility management

   o  AERO address-based RS injection of ACP into underlying routing
      system.

   Changes from draft-templin-aerolink-82 to draft-templin-intarea-
   6706bis-00:

   o  Document use of NUD (NS/NA) for reliable link-layer address
      updates as an alternative to unreliable unsolicited NA.
      Consistent with Section 7.2.6 of RFC4861.

   o  Server adds additional layer of encapsulation between outer and
      inner headers of NS/NA messages for transmission through Relays
      that act as vanilla IPv6 routers.  The messages include the AERO
      Server Subnet Router Anycast address as the source and the Subnet
      Router Anycast address corresponding to the Client's ACP as the
      destination.

   o  Clients use Subnet Router Anycast address as the encapsulation
      source address when the access network does not provide a
      topologically-fixed address.





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Internet-Draft                    AERO                        April 2019


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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