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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720,                       May 10, 2018
           rfc6179, rfc6706 (if
           approved)
Intended status: Standards Track
Expires: November 11, 2018


             Asymmetric Extended Route Optimization (AERO)
                     draft-templin-aerolink-82.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.
   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).  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 November 11, 2018.





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Copyright Notice

   Copyright (c) 2018 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) . . . . . . . .   7
     3.1.  AERO Link Reference Model . . . . . . . . . . . . . . . .   7
     3.2.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  AERO Routing System . . . . . . . . . . . . . . . . . . .  10
     3.4.  AERO Interface Link-local Addresses . . . . . . . . . . .  11
     3.5.  AERO Interface Characteristics  . . . . . . . . . . . . .  13
     3.6.  AERO Interface Initialization . . . . . . . . . . . . . .  16
       3.6.1.  AERO Relay Behavior . . . . . . . . . . . . . . . . .  16
       3.6.2.  AERO Server Behavior  . . . . . . . . . . . . . . . .  16
       3.6.3.  AERO Client Behavior  . . . . . . . . . . . . . . . .  17
       3.6.4.  AERO Proxy Behavior . . . . . . . . . . . . . . . . .  17
     3.7.  AERO Interface Neighbor Cache Maintenance . . . . . . . .  18
     3.8.  AERO Interface Forwarding Algorithm . . . . . . . . . . .  19
       3.8.1.  Client Forwarding Algorithm . . . . . . . . . . . . .  20
       3.8.2.  Proxy Forwarding Algorithm  . . . . . . . . . . . . .  21
       3.8.3.  Server Forwarding Algorithm . . . . . . . . . . . . .  21
       3.8.4.  Relay Forwarding Algorithm  . . . . . . . . . . . . .  22
       3.8.5.  Processing Return Packets . . . . . . . . . . . . . .  22
     3.9.  AERO Interface Encapsulation and Re-encapsulation . . . .  23
     3.10. AERO Interface Decapsulation  . . . . . . . . . . . . . .  24
     3.11. AERO Interface Data Origin Authentication . . . . . . . .  24
     3.12. AERO Interface Packet Size Issues . . . . . . . . . . . .  25
     3.13. AERO Interface Error Handling . . . . . . . . . . . . . .  27
     3.14. AERO Router Discovery, Prefix Delegation and
           Autoconfiguration . . . . . . . . . . . . . . . . . . . .  30
       3.14.1.  AERO ND/PD Service Model . . . . . . . . . . . . . .  30
       3.14.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  31
       3.14.3.  AERO Server Behavior . . . . . . . . . . . . . . . .  33
     3.15. AERO Interface Route Optimization . . . . . . . . . . . .  35



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       3.15.1.  Reference Operational Scenario . . . . . . . . . . .  35
       3.15.2.  Concept of Operations  . . . . . . . . . . . . . . .  37
       3.15.3.  Sending NS Messages  . . . . . . . . . . . . . . . .  37
       3.15.4.  Re-encapsulating and Relaying the NS . . . . . . . .  38
       3.15.5.  Processing NSs and Sending NAs . . . . . . . . . . .  39
       3.15.6.  Re-encapsulating and Relaying NAs  . . . . . . . . .  40
       3.15.7.  Processing NAs . . . . . . . . . . . . . . . . . . .  41
       3.15.8.  Server and Proxy Extended Route Optimization . . . .  41
     3.16. Neighbor Unreachability Detection (NUD) . . . . . . . . .  43
     3.17. Mobility Management and Quality of Service (QoS)  . . . .  44
       3.17.1.  Forwarding Packets on Behalf of Departed Clients . .  44
       3.17.2.  Announcing Link-Layer Address and QoS Preference
                Changes  . . . . . . . . . . . . . . . . . . . . . .  44
       3.17.3.  Bringing New Links Into Service  . . . . . . . . . .  45
       3.17.4.  Removing Existing Links from Service . . . . . . . .  45
       3.17.5.  Implicit Mobility Management . . . . . . . . . . . .  45
       3.17.6.  Moving to a New Server . . . . . . . . . . . . . . .  46
     3.18. Multicast Considerations  . . . . . . . . . . . . . . . .  46
   4.  The AERO Proxy  . . . . . . . . . . . . . . . . . . . . . . .  47
   5.  Direct Underlying Interfaces  . . . . . . . . . . . . . . . .  49
   6.  Operation on AERO Links with /64 ASPs . . . . . . . . . . . .  50
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  50
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  51
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  51
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  52
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  53
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  53
     11.2.  Informative References . . . . . . . . . . . . . . . . .  55
   Appendix A.  AERO Alternate Encapsulations  . . . . . . . . . . .  60
   Appendix B.  When to Insert an Encapsulation Fragment Header  . .  62
   Appendix C.  Autoconfiguration for Constrained Platforms  . . . .  62
   Appendix D.  Operational Deployment Alternatives  . . . . . . . .  63
     D.1.  Operation on AERO Links Without DHCPv6 Services . . . . .  63
     D.2.  Operation on Server-less AERO Links . . . . . . . . . . .  64
     D.3.  Operation on Client-less AERO Links . . . . . . . . . . .  64
     D.4.  Manually-Configured AERO Tunnels  . . . . . . . . . . . .  64
     D.5.  Encapsulation Avoidance on Relay-Server Dedicated Links .  65
     D.6.  Encapsulation Protocol Version Considerations . . . . . .  65
     D.7.  Extending AERO Links Through Security Gateways  . . . . .  65
   Appendix E.  Change Log . . . . . . . . . . . . . . . . . . . . .  67
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  68

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



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   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
   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)
   [RFC3315][RFC3633].

   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
   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 remainder of this document presents the AERO specification.

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 [RFC3315] [RFC3633],
      however other services (e.g., alternate ND options, network
      management, static configuration, etc.) are also possible.



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   (native) Internetwork
      a connected IPv6 or IPv4 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.

   AERO link
      a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay
      configured over an underlying Internetwork.  All 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 a
      tunnel virtual overlay 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 IP PDs from one or more AERO Servers.
      Following PD, the Client assigns an 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 for AERO Clients.  The Server assigns an
      administratively-provisioned IPv6 link-local 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 configures an AERO interface to relay IP packets
      between nodes on the same AERO link and/or forward IP packets
      between the AERO link and the native Internetwork.  The Relay



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      assigns an administratively-provisioned IPv6 link-local address to
      the AERO interface the same as for a Server.  An AERO Relay can
      also act as an AERO Server.

   AERO Proxy ("Proxy")
      a node that provides proxying services for Clients that cannot
      associate directly with Servers, e.g., when the Client is located
      in a secured internal enclave and the Server is located in 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




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      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.

   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 and must be no longer than 64 for IPv6 or 32 for IPv4.

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

   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 [RFC3315].

   The terminology of DHCPv6 [RFC3315][RFC3633] 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 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.

   o  AERO Servers S1 and S2 associate with Relay R1 and also act as
      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.

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



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   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 may be many additional Relays, Servers, Clients and Proxies.

3.2.  AERO Node Types

   AERO Relays provide default forwarding services to AERO Servers.
   Each Relay also peers with Servers and other Relays in a dynamic
   routing protocol instance to discover 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 AERO interface neighbor cache entries for Servers,
   and maintain an IP forwarding table entry for each AERO Client Prefix
   (ACP).  AERO Relays can also be configured to act as AERO Servers.

   AERO Servers provide default forwarding services to 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 AERO interface
   neighbor cache entries for Relays.  They also maintain both neighbor
   cache entries and IP forwarding table entries for each of their
   associated Clients.  AERO Servers can also be configured to act as
   AERO Relays.

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

   AERO Proxies provide a conduit for AERO Clients connected to secured
   enclaves to associate with AERO link Servers.  The Proxy can either
   be explicit or transparent.  In the explicit case, the Client sends
   all of its control plane messages addressed to the Server to the
   link-layer address of the Proxy.  In the transparent case, the Client
   sends all of its control plane messages to the Server's link-layer
   address and the Proxy intercepts them before they leave the secured



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   enclave.  In both cases, 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 more direct route toward a target
   destination via AERO route optimization, in which case future
   outbound data packets would be forwarded via the more direct route.
   The Proxy function is specified in Section 4.

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 AERO 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.  All Relays 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.

   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
   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.  At the time of
   this writing, the global public Internet BGP routing system manages
   more than 500K routes with linear growth and no signs of router



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   resource exhaustion [BGP].  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.

   Note that 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.templin-atn-bgp].

3.4.  AERO Interface Link-local Addresses

   AERO interface link-local address types include administratively-
   provisioned addresses and AERO addresses.

   Administratively-provisioned addresses are allocated from the range
   fe80::/96 and assigned to a Server or Relay's AERO interface.



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   Administratively-provisioned addresses MUST be managed for uniqueness
   by the administrative authority for the AERO link.  The address
   fe80:: is reserved as the IPv6 link-local subnet router anycast
   address, and the address fe80::ffff:ffff is reserved as the "prefix-
   solicitation" address used by Clients to bootstrap AERO address
   autoconfiguration.  These reserved addresses are therefore not
   available for general assignment.

   An AERO address is an IPv6 link-local address with an embedded prefix
   based on an ACP and associated with a Client's AERO interface.  AERO
   addresses remain stable as the Client moves between topological
   locations, i.e., even if its link-layer addresses change.

   For IPv6, 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, AERO addresses are based on an IPv4-mapped IPv6 address
   [RFC4291] 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

   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



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      ... etc. ...

      fe80:FFFF:192.0.2.31

   When the Server delegates ACPs to the Client, both the Server and
   Client use the lowest-numbered AERO address from the first ACP
   delegation 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 short prefix lengths), 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.

3.5.  AERO Interface Characteristics

   AERO interfaces use encapsulation (see: Section 3.9) to exchange
   packets with neighbors attached to the AERO link.

   AERO interfaces maintain a neighbor cache for tracking per-neighbor
   state the same as for any interface.  AERO interfaces use ND messages
   including Neighbor Solicitation (NS), Neighbor Advertisement (NA),
   Router Solicitation (RS), Router Advertisement (RA) and Redirect for
   neighbor cache management.  AERO interfaces use RS/RA messages with
   an embedded PD message (e.g., see: [I-D.templin-6man-dhcpv6-ndopt]).
   AERO interfaces include routing information in ND messages to support
   route optimization.

   AERO interface ND messages include one or more Source/Target Link-
   Layer Address Options (S/TLLAOs) formatted as shown in Figure 2:














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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 = 5  |X|         Reserved            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Interface ID         |        UDP Port Number        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                          IP 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 2: 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  X (proXy) is set to '1' in an S/TLLAO if the address corresponds
      to a Proxy; otherwise, X is set to '0'.

   o  Reserved 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.  The value 255 is reserved
      for Server-based route optimization (see: Section 3.15.8).

   o  UDP Port Number and IP 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



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      encapsulation, UDP Port Number is set to '0'.  When the
      encapsulation 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 64 Preference values 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  NAT'ed interfaces connect to a closed network that is separated
      from the open Internetwork by a Network Address Translator (NAT).
      The NAT does not participate in any AERO control message
      signaling, but the AERO Server can issue AERO control messages on
      behalf of the Client.

   o  VPN'ed interfaces use security encapsulation over the Internetwork
      to a Virtual Private Network (VPN) gateway that also acts as an
      AERO Server.  As with NAT'ed links, the AERO Server can issue
      control messages on behalf of the Client.

   o  Proxy'ed interfaces connect to a closed network that is separated
      from the open Internetwork by an AERO Proxy.  Unlike NAT'ed and
      VPN'ed interfaces, the AERO Proxy (rather than the Server) can
      issue control message on behalf of the Client.

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

   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



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   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 NAT or Proxy 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 sets both UDP Port Number
   and IP Address to 0 to indicate that the addresses are unspecified.

   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.

3.6.  AERO Interface Initialization

3.6.1.  AERO Relay Behavior

   When a Relay enables an AERO interface, it first assigns an
   administratively-provisioned link-local address fe80::ID to the
   interface.  Each fe80::ID address MUST be unique among all AERO nodes
   on the link.  The Relay then engages in a dynamic routing protocol
   session with one or more Servers and all 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), and maintains neighbor cache
   entries for all Servers on the link.  Relays exchange NS/NA messages
   with AERO link neighbors the same as for any AERO node.  However,
   Neighbor Unreachability Detection (NUD) (see: Section 3.16) is
   optional since the dynamic routing protocol already provides
   reachability confirmation.

3.6.2.  AERO Server Behavior

   When a Server enables an AERO interface, it assigns an
   administratively-provisioned link-local address fe80::ID the same as
   for Relays.  The Server further configures a service to facilitate PD
   exchanges with AERO Clients.  The Server maintains neighbor cache
   entries for one or more Relays on the link, and manages per-Client



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   neighbor cache entries and IP forwarding table entries based on
   control message exchanges.  Each Server also engages in a dynamic
   routing protocol with their neighboring Relays (see: Section 3.3).

   When the Server receives an NS/RS message from a Client on the AERO
   interface it authenticates the message and returns an NA/RA message.
   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 driver at the link layer without ever disturbing the
   network layer.

3.6.3.  AERO Client Behavior

   When a Client enables an AERO interface, it sends RS messages with PD
   "Solicit" options over an underlying interface using the prefix-
   solicitation address as the source network layer address and all-
   routers [RFC4861] as the destination network layer address to obtain
   ACPs from one or more AERO Servers.  Each Server processes the
   message and returns an RA message with a PD "Reply" option with the
   Server's link-layer address as the source and the base AERO address
   as the destination network layer addresses.  In this way, the ND/PD
   control messages securely perform all autoconfiguration operations in
   a single request/response exchange.

   After the initial ND/PD message exchange, the Client can register
   additional underlying interfaces with the Server by sending an RS
   message over each underlying interface using its base AERO address as
   the source network layer address and without including a PD option.
   The Server will update its neighbor cache entry for the Client and
   return an RA message.

   The Client maintains a neighbor cache entry for each of its Servers
   and each of its active correspondent 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.

3.6.4.  AERO Proxy Behavior

   When a Proxy enables an AERO interface, it maintains per-Client proxy
   neighbor cache entries based on control message exchanges.  Proxies
   forward packets between their associated Clients and the Clients'
   associated Servers.

   When the Proxy receives an RS message from a Client in the secured
   enclave, it creates an incomplete proxy neighbor cache entry and



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   forwards the message to a Server selected by the Client 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 forwards
   the 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 proxyed neighbor association.

3.7.  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, the same as for any IPv6 interface [RFC4861].  AERO interface
   neighbor cache entries are said to be one of "permanent", "static",
   "proxy" or "dynamic".

   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 Servers on the link, and AERO Servers
   maintain permanent neighbor cache entries for Relays.  Each entry
   maintains the mapping between the neighbor's fe80::ID network-layer
   address and corresponding link-layer address.

   Static neighbor cache entries are created and maintained through ND/
   PD exchanges as specified in Section 3.14, and remain in place for
   durations bounded by ND/PD lifetimes.  AERO Servers maintain static
   neighbor cache entries for each of their associated Clients, and AERO
   Clients maintain static neighbor cache entries for each of their
   associated Servers.

   Proxy neighbor cache entries are created and maintained by AERO
   Proxies by gleaning information from 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, and include pointers to the Client's current set
   of Servers.

   Dynamic neighbor cache entries are created or updated based on
   receipt of route optimization messages as specified in Section 3.15,
   and are garbage-collected when keepalive timers expire.  AERO nodes
   maintain dynamic neighbor cache entries for each of their active
   correspondents with lifetimes based on ND messaging constants.

   When a target AERO node receives a valid NS message with an AERO
   source address, it returns an NA message and also creates or updates
   a dynamic neighbor cache entry for the source network-layer and link-
   layer addresses.  The node then sets an "AcceptTime" variable in the



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   neighbor cache entry to ACCEPT_TIME seconds and uses this value to
   determine whether packets received from the correspondent can be
   accepted.  The node resets AcceptTime when it receives a new ND
   message, and otherwise decrements AcceptTime while no ND messages
   have been received.  It is RECOMMENDED that ACCEPT_TIME be set to the
   default constant value 40 seconds to allow a 10 second window so that
   the AERO route optimization procedure can converge before AcceptTime
   decrements below FORWARD_TIME (see below).

   When a source AERO node receives a valid NA message with an AERO
   source address that matches its NS message, it creates or updates a
   dynamic neighbor cache entry for the target network-layer and link-
   layer addresses.  The node then sets a "ForwardTime" variable in the
   neighbor cache entry to FORWARD_TIME seconds and uses this value to
   determine whether packets can be forwarded directly to the
   correspondent, i.e., instead of via a default route.  The node resets
   ForwardTime when it receives a new NA, and otherwise decrements
   ForwardTime while no further NA messages have been received.  It is
   RECOMMENDED that FORWARD_TIME be set to the default constant value 30
   seconds to match the default REACHABLE_TIME value specified in
   [RFC4861].

   The node also sets a "MaxRetry" variable to MAX_RETRY to limit the
   number of keepalives sent when a correspondent may have gone
   unreachable.  It is RECOMMENDED that MAX_RETRY be set to 3 the same
   as described for address resolution in Section 7.3.3 of [RFC4861].

   Different values for ACCEPT_TIME, FORWARD_TIME and MAX_RETRY MAY be
   administratively set, if necessary, to better match the AERO link's
   performance characteristics; however, if different values are chosen,
   all nodes on the link MUST consistently configure the same values.
   Most importantly, ACCEPT_TIME SHOULD be set to a value that is
   sufficiently longer than FORWARD_TIME to allow the AERO route
   optimization procedure to converge.

   When there may be a NAT between the Client and the Server, or if the
   path from the Client to the Server should be tested for reachability,
   the Client can send periodic RS messages to the Server without a PD
   option to receive RA replies.  The RS/RA messaging will keep NAT
   state alive and test Server reachability without disturbing the PD
   service.

3.8.  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



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   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 3.5).  The AERO node uses each packet's
   DSCP value 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.  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 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 link-local address.

3.8.1.  Client Forwarding Algorithm

   When an IP packet enters a Client's AERO interface from the network
   layer the Client searches for a dynamic 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.
   Otherwise, the Client uses the link-layer address in a static
   neighbor cache entry for a Server as the encapsulation address
   (noting that there may be a Proxy on the path to the real Server).

   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 3.13).




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3.8.2.  Proxy Forwarding Algorithm

   When the Proxy receives a packet from a Client within the secured
   enclave, the Proxy searches for a dynamic neighbor cache entry that
   matches the destination.  If there is a match, the Proxy 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.
   Otherwise, the Proxy uses the link-layer address for one of the
   Client's Servers as the encapsulation address.

   When the Proxy receives a packet from an AERO interface neighbor, it
   searches for a proxy neighbor cache entry for a Client within the
   secured enclave that matches the destination.  If there is a match,
   the Proxy forwards the packet to the Client.  Otherwise, the Proxy
   returns the packet to the neighbor, i.e., by reversing the source and
   destination link-layer addresses.

3.8.3.  Server Forwarding Algorithm

   When an IP packet enters a Server's AERO interface from the network
   layer, the Server searches for a static neighbor cache entry for a
   Client that matches the destination.  If there is a match, the Server
   uses one or more link-layer addresses in the entry as the link-layer
   addresses for encapsulation and admits the packet into the AERO link.
   Otherwise, 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.

   When an IP packet enters a Server's AERO interface from the link
   layer, the Server 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 decapsulates the packet and forwards it to the network
      layer for local delivery.

   o  else, if the destination matches a static neighbor cache entry for
      a Client the Server first determines whether the neighbor is the
      same as the one it received the packet from.  If so, the Server
      drops the packet silently to avoid looping; otherwise, the Server
      uses the neighbor's link-layer address(es) as the destination for
      encapsulation and re-admits the packet into the AERO link.

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





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3.8.4.  Relay Forwarding Algorithm

   When an IP packet enters a Relay's AERO interface from the network
   layer, the Relay searches its IP forwarding table for an ACP entry
   that matches the destination and otherwise searches for a neighbor
   cache entry that matches the destination (e.g., for administratively-
   provisioned link-local addresses).  If there is a match, the Relay
   uses the link-layer address in the corresponding neighbor cache entry
   as the link-layer address for encapsulation and forwards the packet
   into the AERO link.  Otherwise, the Relay drops the packet and (for
   non-link-local addresses) returns a network-layer ICMP Destination
   Unreachable message subject to rate limiting (see: Section 3.13).

   When an IP packet enters a Relay's AERO interface from the link-
   layer, the Relay processes the packet as follows:

   o  if the destination does not match an ASP, or if the destination
      matches one of the Relay's own addresses, the Relay decapsulates
      the packet and forwards it to the network layer where it will be
      subject to either IP forwarding or local delivery.

   o  else, if the destination matches an ACP entry in the IP forwarding
      table, or if the destination matches the link-local address in a
      permanent neighbor cache entry, 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 uses the neighbor's link-layer address as the
      destination for encapsulation and re-admits the packet into the
      AERO link.

   o  else, the Relay drops the packet and (for non-link-local
      addresses) returns an ICMP Destination Unreachable message subject
      to rate limiting (see: Section 3.13).

3.8.5.  Processing Return Packets

   When an AERO node receives a return packet such as generated by an
   AERO Proxy (see Section 3.8.2), it proceeds according to the AERO
   link trust basis.  Namely, the return packets have the same trust
   profile as for link-layer Destination Unreachable messages.  If the
   node has sufficient trust basis to accept link-layer Destination
   Unreachable messages, it can then process the return packet as
   described in the following paragraph.  Otherwise, the node SHOULD
   drop the packet and treat it as an indication that a path may be
   failing, and MAY use NUD to test the path for reachability.

   If the node has sufficient trust basis to accept return packets, it
   searches for a dynamic neighbor cache entry that matches the



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   destination.  If there is a match, the neighbor marks the
   corresponding link-layer address as "unreachable", selects the next-
   highest priority "reachable" link-layer address in the entry as the
   link-layer address for encapsulation then (re)admits the packet into
   the AERO link.  If there are no "reachable" link-layer addresses, the
   neighbor instead sets FowardTime in the dynamic neighbor cache entry
   to 0.  If the source address corresponds to one of the neighbor's own
   addresses, the neighbor also forwards the packet to the corresponding
   Server; otherwise, it drops the packet.

3.9.  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 (see: Appendix A).  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 3.12.

   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
   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.



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   Clients normally use the IP address of the underlying interface as
   the encapsulation source address.  If the underlying interface does
   not have an IP address, however, the Client uses an IP address taken
   from an ACP as the encapsulation source address (assuming the node
   has some way of injecting the ACP into the underlying network routing
   system).  For IPv6 addresses, the Client normally uses the ACP Subnet
   Router Anycast address [RFC4291].

3.10.  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.

3.11.  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, and create or update a static neighbor cache entry for
      the Client based on the specific message type.

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

   o  AERO Clients and Servers accept encapsulated packets if there is a
      dynamic neighbor cache entry with an AERO address that matches the
      packet's network-layer source address, with a link-layer address
      that matches the packet's link-layer source address, and with a
      non-zero AcceptTime.

   o  AERO Proxies accept encapsulated packets if there is a proxy
      neighbor cache entry that matches the packet's network-layer
      destination address (i.e., the address of the Client) and link-
      layer source address (i.e., the address of one of the Client's
      Servers).  When the proxy is configured to accept packets
      originating from any address in the open Internetwork however
      (e.g., from another Proxy), it omits the source address check.





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   Note that this simple data origin authentication is effective in
   environments in which link-layer addresses cannot be spoofed.  In
   other environments, each AERO message must 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 environments where
   end systems use end-to-end security, however, it may be sufficient to
   require signatures only for ND and ICMP control plane messages and
   omit signatures for data plane messages.

3.12.  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][RFC1981].  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 a 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]
   (note that common IPv6 over IPv4 tunnels already assume a larger MRU
   than the IPv4 minimum).



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   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
   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/MSU values 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].  For
   AERO links over both IPv4 and IPv6, studies have also shown that IP
   fragments are dropped unconditionally over some network paths [I-
   D.taylor-v6ops-fragdrop].  In environments where IP fragmentation



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

3.13.  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 3.12.)

   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 3 (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 3: 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 dynamic 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 ForwardTime for the corresponding dynamic 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 static neighbor Servers, the Client SHOULD
      math 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 send a PD "Release" message
      to the old Server as specified in Section 3.17.6.

   o  When an AERO Server receives persistent link-layer Destination
      Unreachable messages in response to encapsulated packets that it
      sends to one of its static neighbor Clients, the Server SHOULD
      mark the path as unusable and use another path.  If it receives
      Destination Unreachable messages on multiple paths, the Server
      should take no further actions unless it receives a 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 first writes the network-layer
   source address of the original packet as the destination address of
   the message and determines the next hop to the destination.  If the
   next hop is reached via the AERO interface, the Relay uses the IPv6
   address "::" or the IPv4 address "0.0.0.0" as the source address of
   the message, then encapsulates the message and forwards it to the
   next hop within the AERO interface.  Otherwise, the Relay uses one of
   its non link-local addresses as the source address of the message and
   forwards it via a link outside the AERO interface.

   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 of the message and determines the next hop to the



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   destination.  If the next hop is reached via the AERO interface, the
   node uses the IPv6 address "::" or the IPv4 address "0.0.0.0" as the
   source address of the message, then encapsulates the message and
   forwards it to the next hop within the AERO interface.  Otherwise,
   the node uses one of its non link-local addresses as the source
   address of the message and forwards it via a link outside the AERO
   interface.

   When an AERO node receives any network-layer error message via the
   AERO interface, it examines the network-layer destination address.
   If the next hop toward the destination is via the AERO interface, the
   node re-encapsulates and forwards the message to the next hop within
   the AERO interface.  Otherwise, if the network-layer source address
   is the IPv6 address "::" or the IPv4 address "0.0.0.0", the node
   writes one of its non link-local addresses as the source address,
   recalculates the IP and/or ICMP checksums then forwards the message
   via a link outside the AERO interface.

3.14.  AERO Router Discovery, Prefix Delegation and Autoconfiguration

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

3.14.1.  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.  The RS/RA
   messaging is conducted in the same fashion as specified in [RFC5214].

   AERO Clients and Servers include PD messages as options in the RS/RA
   messages they exchange (see: [I-D.templin-6man-dhcpv6-ndopt]).



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   Client-initiated PD options are included in RS messages, and Server-
   initiated PD options are included in RA messages.  The unified ND/PD
   messages are exchanged between Client and Server according to the
   prefix management schedule determined by the PD service.  The unified
   messages can be protected using SEcure Neighbor Discovery (SEND)
   [RFC3971].

   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 explicit PD options.  Instead, the RS/RA messages 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
   recipient can infer that the sender holds the prefix delegation
   "2001:db8:1:2::/64".

   The following sections specify the Client and Server behavior.

3.14.2.  AERO Client Behavior

   AERO Clients discover the link-layer addresses of AERO Servers 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-Qualfied 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 through a combined ND/PD message exchange.  The Client
   includes a PD "Solicit" message as an ND option in an RS message with
   the prefix-solicitation address as the IPv6 source address, all-
   routers multicast as the IPv6 destination address, 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's underlying interface does not have an IP
   address, the Client can use the ACP Subnet Router Anycast address as
   the link-layer source address.)

   The Client next includes a "Client Identifier" and an "IA_PD" (i.e.,
   prefix request) code in the PD "Solicit" message.  If the Client is
   pre-provisioned with ACPs associated with the AERO service, it MAY
   also include the ACPs in the "IA_PD" option to indicate its
   preferences to the Server.  The Client finally includes any
   additional PD codes (e.g., "Rapid Commit").



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   The Client next includes one or more SLLAOs in the RS message
   formatted as described in Section 3.5 to register its link-layer
   address(es) 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 MUST have assurance that there will be no
   NATs or Proxies on the paths to the Server via those interfaces.
   (Otherwise, the Client can register additional link-layer addresses
   with the Server by sending subsequent NS/RS messages via different
   underlying interfaces after the initial RS/RA exchange).

   The Client then sends the RS message to the AERO Server and waits for
   an RA message reply (see Section 3.14.3) while retrying MAX_RETRY
   times until an RA is received.  If no RA is received, or if it
   receives an RA with Router Lifetime set to 0 and/or a "Reply" with no
   ACPs, the Client SHOULD discontinue autoconfiguration attempts
   through this Server and try another Server.  Otherwise, the Client
   processes the ACPs in the embedded "Reply" message.

   Next, the Client creates a static neighbor cache entry with the
   Server's link-local address as the network-layer address and the
   Server's encapsulation source address as the link-layer address.  The
   Client then autoconfigures AERO addresses for each of the delegated
   ACPs and assigns them to the AERO interface.

   The Client next examines the P bit in the RA message flags field
   [RFC5175].  If the P bit value was 1, the Client assumes that there
   is a NAT or Proxy on the path to the Server via the interface over
   which it sent the RS message.  In that case, the Client sets UDP Port
   Number and IP Address to 0 in the S/TLLAOs of any subsequent ND
   messages it sends to the Server over that link.

   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/MSU values in the MTU options 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.

   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].  The Client subsequently
   maintains its ACP delegations through each of its Servers by sending
   RS "Renew", "Rebind", and/or "Release" messages.  The Server will in
   turn send RA "Reply" messages.

   After the Client registers its Interface IDs and their associated
   UDP/IP addresses and 'P(i)' values, it may wish to change one or more



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   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 unsolicited NA message to send over any
   available underlying interface.  The source and target address of the
   NA message are set to the Client's AERO address, and the destination
   address is set to all-nodes multicast.  The NA MUST include a TLLAO
   specific to the selected available underlying interface as the first
   TLLAO and MAY include any additional TLLAOs specific to other
   underlying interfaces.  The Client includes fresh 'P(i)' values in
   each TLLAO to update the Server's neighbor cache entry.  If the
   Client wishes to update 'P(i)' values without updating the link-layer
   address, it sets the UDP Port Number and IP Address fields to 0.  If
   the Client wishes to disable the interface, it sets all 'P(i)' values
   to '0' ("disabled").

   If the Client wishes to discontinue use of a Server it issues an RS
   "Release" message.  When the Server processes the message, it
   releases the ACP, deletes its neighbor cache entry for the Client,
   withdraws the IP route from the routing system and returns an RA
   "Reply".

3.14.3.  AERO Server Behavior

   AERO Servers act as IPv6 routers and support a PD service on their
   AERO links.  AERO Servers arrange to add their encapsulation layer IP
   addresses (i.e., their link-layer addresses) to a static map of
   Server addresses for the link and/or the DNS resource records for the
   FQDN "linkupnetworks.[domainname]" before entering service.

   When an AERO Server receives a prospective Client's RS "Solicit"
   message on its AERO interface, and the Server is too busy, it SHOULD
   return an immediate RA "Reply" message with no ACPs and with Router
   Lifetime set to 0.  Otherwise, the Server authenticates the RS
   message and processes the embedded "Solicit" option.  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).

   Next, the Server prepares an RA "Reply" message that includes the
   delegated ACPs.  For IPv4 ACPs, the ACP is in IPv4-mapped IPv6
   address format and with prefix length set as specified in
   Section 3.4.  The Server then prepares an RA "Reply" message using
   its link-local address (i.e., fe80::ID) as the network-layer source
   address, the Client's base AERO address from the first ACP 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



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   the RS message as the destination link-layer address.  The Server
   next sets the P flag in the RA message flags field [RFC5175] 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 or
   Proxy on the path; otherwise it sets P to 0.  The Server then
   includes one or more RIOs that encode the ASPs for the AERO link.
   The Server also includes two MTU options - the first MTU option
   includes the MTU for the link and the second MTU option includes the
   MSU for the link (see Section 3.12).  The Server finally sends the RA
   "Reply" message to the Client.

   The Server next creates a static 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 option indexed by the
   Interface ID and including the UDP port number, IP address and P(i)
   values.  For the first SLLAO in the list, however, the Server records
   the actual encapsulation source UDP and IP addresses instead of those
   that appear in the SLLAO in case there was a NAT or Proxy in the
   path.

   After the initial RS/RA exchange, the AERO Server maintains the
   neighbor cache entry for the Client until the PD lifetimes expire.
   If the Client issues an RS "Renew", the Server extends the PD
   lifetimes.  If the Client issues an RS "Release", or if the Client
   does not issue a "Renew" before the lifetime expires, the Server
   deletes the neighbor cache entry for the Client and withdraws the IP
   routes from the AERO routing system.  The Server processes these and
   any other Client PD messages, and returns an RA "Reply".  The Server
   may also issue an unsolicited RA "Reconfigure" message to inform the
   Client that it needs to renegotiate its PDs.

3.14.3.1.  Lightweight DHCPv6 Relay Agent (LDRA)

   When DHCPv6 is used as the PD service, 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 driver 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 option and wraps it in IPv6/UDP headers.  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




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   the UDP fields to values that will be understood by the DHCPv6
   server.

   The LDRA then wraps the message in a 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 SLLAO options 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
   delivers the DHCPv6 message to be wrapped into 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.

3.15.  AERO Interface Route Optimization

   When a source Client forwards packets to a prospective correspondent
   Client within the same AERO link domain (i.e., one for which the
   packet's destination address is covered by an ASP), the source Client
   MAY initiate an AERO link route optimization procedure on behalf of
   any of its native underlying interfaces.  The procedure is based on
   an exchange of IPv6 ND messages using a chain of AERO Servers and
   Relays as a trust basis.

   Although the Client is responsible for initiating route optimization,
   the Server is the policy enforcement point that determines whether
   route optimization is permitted.  For example, on some AERO links
   route optimization would allow traffic to circumvent critical
   network-based traffic inspection points.  In those cases, the Server
   can simply discard any route optimization messages instead of
   forwarding them.

   The following sections specify the AERO link route optimization
   procedure.

3.15.1.  Reference Operational Scenario

   Figure 4 depicts the AERO link route optimization reference
   operational scenario, using IPv6 addressing as the example (while not
   shown, a corresponding example for IPv4 addressing can be easily
   constructed).  The figure shows an AERO Relay ('R1'), two AERO




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   Servers ('S1', 'S2'), two AERO Clients ('C1', 'C2') and two ordinary
   IPv6 hosts ('H1', 'H2'):

            +--------------+  +--------------+  +--------------+
            |   Server S1  |  |    Relay R1  |  |   Server S2  |
            +--------------+  +--------------+  +--------------+
                fe80::2            fe80::1           fe80::3
                 L2(S1)             L2(R1)            L2(S2)
                   |                  |                 |
       X-----+-----+------------------+-----------------+----+----X
             |       AERO Link                               |
           L2(C1)                                          L2(C2)
    fe80::2001:db8:0:0                               fe80::2001:db8:1:0
     +--------------+                                 +--------------+
     |AERO Client C1|                                 |AERO Client C2|
     +--------------+                                 +--------------+
     2001:DB8:0::/48                                  2001:DB8:1::/48
             |                                                |
            .-.                                              .-.
         ,-(  _)-.   2001:db8:0::1      2001:db8:1::1     ,-(  _)-.
      .-(_  IP   )-.   +-------+          +-------+    .-(_  IP   )-.
    (__    EUN      )--|Host H1|          |Host H2|--(__    EUN      )
       `-(______)-'    +-------+          +-------+     `-(______)-'

               Figure 4: AERO Reference Operational Scenario

   In Figure 4, Relay ('R1') assigns the administratively-provisioned
   link-local address fe80::1 to its AERO interface with link-layer
   address L2(R1), Server ('S1') assigns the address fe80::2 with link-
   layer address L2(S1),and Server ('S2') assigns the address fe80::3
   with link-layer address L2(S2).  Servers ('S1') and ('S2') next
   arrange to add their link-layer addresses to a published list of
   valid Servers for the AERO link.

   AERO Client ('C1') receives the ACP 2001:db8:0::/48 in an ND/PD
   exchange via AERO Server ('S1') then assigns the address
   fe80::2001:db8:0:0 to its AERO interface with link-layer address
   L2(C1).  Client ('C1') configures a default route and neighbor cache
   entry via the AERO interface with next-hop address fe80::2 and link-
   layer address L2(S1), then sub-delegates the ACP to its attached
   EUNs.  IPv6 host ('H1') connects to the EUN, and configures the
   address 2001:db8:0::1.

   AERO Client ('C2') receives the ACP 2001:db8:1::/48 in an ND/PD
   exchange via AERO Server ('S2') then assigns the address
   fe80::2001:db8:1:0 to its AERO interface with link-layer address
   L2(C2).  Client ('C2') configures a default route and neighbor cache
   entry via the AERO interface with next-hop address fe80::3 and link-



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   layer address L2(S2), then sub-delegates the ACP to its attached
   EUNs.  IPv6 host ('H2') connects to the EUN, and configures the
   address 2001:db8:1::1.

3.15.2.  Concept of Operations

   Again, with reference to Figure 4, when source host ('H1') sends a
   packet to destination host ('H2'), the packet is first forwarded over
   the source host's attached EUN to Client ('C1').  Client ('C1') then
   forwards the packet via its AERO interface to Server ('S1') and also
   sends an NS message toward Client ('C2') via Server ('S1').

   Server ('S1') then re-encapsulates and forwards both the packet and
   the NS message out the same AERO interface toward Client ('C2') via
   Relay ('R1').  When Relay ('R1') receives the packet and NS message,
   it consults its forwarding table to discover Server ('S2') as the
   next hop toward Client ('C2').  Relay ('R1') then forwards both the
   packet and the NS message to Server ('S2'), which then forwards them
   to Client ('C2').

   After Client ('C2') receives the NS message, it process the message
   and creates or updates a dynamic neighbor cache entry for Client
   ('C1'), then sends the NA response to the link-layer address of
   Server ('S2').  When Server ('S2') receives the NA message it re-
   encapsulates the message and forwards it on to Relay ('R1'), which
   re-encapsulates and forwards the message on to Server ('S1') which
   re-encapsulates and forwards the message on to Client ('C1').

   After Client ('C1') receives the NA message, it processes the message
   and creates or updates a dynamic neighbor cache entry for Client
   ('C2').  Thereafter, forwarding of packets from Client ('C1') to
   Client ('C2') without involving any intermediate nodes is enabled.
   The mechanisms that support this exchange are specified in the
   following sections.

3.15.3.  Sending NS Messages

   When a Client forwards a packet with a source address from one of its
   ACPs toward a destination address covered by an ASP (i.e., toward
   another AERO Client connected to the same AERO link), the source
   Client MAY send an NS message forward toward the destination Client
   via the Server.

   In the reference operational scenario, when Client ('C1') forwards a
   packet toward Client ('C2'), it MAY also send an NS message forward
   toward Client ('C2'), subject to rate limiting (see Section 8.2 of
   [RFC4861]).  Client ('C1') prepares the NS message as follows:




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   o  the link-layer source address is set to 'L2(C1)' (i.e., the link-
      layer address of Client ('C1')).

   o  the link-layer destination address is set to 'L2(S1)' (i.e., the
      link-layer address of Server ('S1')).

   o  the network-layer source address is set to fe80::2001:db8:0:0
      (i.e., the base AERO address of Client ('C1')).

   o  the network-layer destination address is set to the AERO address
      corresponding to the destination address of Client ('C2').

   o  the Type is set to 135.

   o  the Target Address is set to the destination address of the packet
      that triggered route optimization.

   o  the message includes one or more SLLAOs set to appropriate values
      for Client ('C1')'s native underlying interfaces.

   o  the message includes one or more RIOs that include Client ('C1')'s
      ACPs [I-D.templin-6man-rio-redirect].

   o  the message SHOULD include a Timestamp option and a Nonce option.

   Note that the act of sending NS messages is cited as "MAY", since
   Client ('C1') may have advanced knowledge that the direct path to
   Client ('C2') would be unusable or otherwise undesirable.  If the
   direct path later becomes unusable after the initial route
   optimization, Client ('C1') simply allows packets to again flow
   through Server ('S1').

3.15.4.  Re-encapsulating and Relaying the NS

   When Server ('S1') receives an NS message from Client ('C1'), it
   first verifies that the SLLAOs in the NS are a proper subset of the
   link-layer addresses in Client ('C1')'s neighbor cache entry.  If the
   Client's SLLAOs are not acceptable, Server ('S1') discards the
   message.  Otherwise, Server ('S1') verifies that Client ('C1') is
   authorized to use the ACPs encoded in the RIOs of the NS and discards
   the NS if verification fails.

   Server ('S1') then examines the network-layer destination address of
   the NS to determine the next hop toward Client ('C2') by searching
   for the AERO address in the neighbor cache.  Since Client ('C2') is
   not one of its neighbors, Server ('S1') re-encapsulates the NS and
   relays it via Relay ('R1') by changing the link-layer source address
   of the message to 'L2(S1)' and changing the link-layer destination



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   address to 'L2(R1)'.  Server ('S1') finally forwards the re-
   encapsulated message to Relay ('R1') without decrementing the
   network-layer TTL/Hop Limit field.

   When Relay ('R1') receives the NS message from Server ('S1') it
   determines that Server ('S2') is the next hop toward Client ('C2') by
   consulting its forwarding table.  Relay ('R1') then re-encapsulates
   the NS while changing the link-layer source address to 'L2(R1)' and
   changing the link-layer destination address to 'L2(S2)'.  Relay
   ('R1') then relays the NS via Server ('S2').

   When Server ('S2') receives the NS message from Relay ('R1') it
   determines that Client ('C2') is a neighbor by consulting its
   neighbor cache.  Server ('S2') then re-encapsulates the NS while
   changing the link-layer source address to 'L2(S2)' and changing the
   link-layer destination address to 'L2(C2)'.  Server ('S2') then
   forwards the message to Client ('C2').

3.15.5.  Processing NSs and Sending NAs

   When Client ('C2') receives the NS message, it accepts the NS only if
   the message has a link-layer source address of one of its Servers
   (e.g., L2(S2)).  Client ('C2') further accepts the message only if it
   is willing to serve as a route optimization target.

   In the reference operational scenario, when Client ('C2') receives a
   valid NS message, it either creates or updates a dynamic neighbor
   cache entry that stores the source address of the message as the
   network-layer address of Client ('C1') , stores the link-layer
   addresses found in the SLLAOs as the link-layer addresses of Client
   ('C1'), and stores the ACPs encoded in the RIOs of the NS as the ACPs
   for Client ('C1').  Client ('C2') then sets AcceptTime for the
   neighbor cache entry to ACCEPT_TIME.

   After processing the message, Client ('C2') prepares an NA message
   response as follows:

   o  the link-layer source address is set to 'L2(C2)' (i.e., the link-
      layer address of Client ('C2')).

   o  the link-layer destination address is set to 'L2(S2)' (i.e., the
      link-layer address of Server ('S2')).

   o  the network-layer source address is set to fe80::2001:db8:1:0
      (i.e., the base AERO address of Client ('C2')).

   o  the network-layer destination address is set to fe80::2001:db8:0:0
      (i.e., the base AERO address of Client ('C1')).



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   o  the Type is set to 136.

   o  The Target Address is set to the Target Address field in the NS
      message.

   o  the message includes one or more TLLAOs set to appropriate values
      for Client ('C2')'s native underlying interfaces.

   o  the message includes one or more RIOs that include Client ('C2')'s
      ACPs [I-D.templin-6man-rio-redirect].

   o  the message SHOULD include a Timestamp option and MUST echo the
      Nonce option received in the NS (i.e., if a Nonce option is
      included).

   Client ('C2') then sends the NA message to Server ('S2').

3.15.6.  Re-encapsulating and Relaying NAs

   When Server ('S2') receives an NA message from Client ('C2'), it
   first verifies that the TLLAOs in the NA are a proper subset of the
   Interface IDs in Client ('C2')'s neighbor cache entry.  If the
   Client's TLLAOs are not acceptable, Server ('S2') discards the
   message.  Otherwise, Server ('S2') verifies that Client ('C2') is
   authorized to use the ACPs encoded in the RIOs of the NA message.  If
   validation fails, Server ('S2') discards the NA.

   Server ('S2') then examines the network-layer destination address of
   the NA to determine the next hop toward Client ('C1') by searching
   for the AERO address in the neighbor cache.  Since Client ('C1') is
   not a neighbor, Server ('S2') re-encapsulates the NA and relays it
   via Relay ('R1') by changing the link-layer source address of the
   message to 'L2(S2)' and changing the link-layer destination address
   to 'L2(R1)'.  Server ('S2') finally forwards the re-encapsulated
   message to Relay ('R1') without decrementing the network-layer TTL/
   Hop Limit field.

   When Relay ('R1') receives the NA message from Server ('S2') it
   determines that Server ('S1') is the next hop toward Client ('C1') by
   consulting its forwarding table.  Relay ('R1') then re-encapsulates
   the NA while changing the link-layer source address to 'L2(R1)' and
   changing the link-layer destination address to 'L2(S1)'.  Relay
   ('R1') then relays the NA via Server ('S1').

   When Server ('S1') receives the NA message from Relay ('R1') it
   determines that Client ('C1') is a neighbor by consulting its
   neighbor cache.  Server ('S1') then re-encapsulates the NA while
   changing the link-layer source address to 'L2(S1)' and changing the



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   link-layer destination address to 'L2(C1)'.  Server ('S1') then
   forwards the message to Client ('C1').

3.15.7.  Processing NAs

   When Client ('C1') receives the NA message, it first verifies the
   Nonce value matches the value that it included in its NS message (if
   any).  If the Nonce values match, Client ('C1') then processes the
   message as follows.

   In the reference operational scenario, when Client ('C1') receives
   the NA message, it either creates or updates a dynamic neighbor cache
   entry that stores the source address of the message as the network-
   layer address of Client ('C2'), stores the link-layer addresses found
   in the TLLAOs as the link-layer addresses of Client ('C2') and stores
   the ACPs encoded in the RIOs of the NA as the ACPs for Client ('C2').
   Client ('C1') then sets ForwardTime for the neighbor cache entry to
   FORWARD_TIME.

   Now, Client ('C1') has a neighbor cache entry with a valid
   ForwardTime value, while Client ('C2') has a neighbor cache entry
   with a valid AcceptTime value.  Thereafter, Client ('C1') may forward
   ordinary network-layer data packets directly to Client ('C2') without
   involving any intermediate nodes, and Client ('C2') can verify that
   the packets came from an acceptable source.  (In order for Client
   ('C2') to forward packets to Client ('C1'), a corresponding NS/NA
   message exchange is required in the reverse direction; hence, the
   mechanism is asymmetric.)

3.15.8.  Server and Proxy Extended Route Optimization

   Route optimization may be initiated by the source Client by sending
   NS messages with SLLAOs corresponding to its native underlying
   interfaces.  Route optimization for the source Client's other
   interfaces may be initiated by Servers and/or Proxies.  Each node
   initiates route optimization by sending NS messages with SLLAOs only
   for those underlying interfaces they are authoritative for.  Each
   node MUST consistently use the same Interface ID values to denote the
   same interfaces.  The Interface IDs are established and maintained by
   the source Client's RS/RA exchanges.

   The target Client's Server serves as a route optimization target if
   some or all of the target Client's underlying interfaces connect via
   NATs, Proxies and/or VPNs.  In that case, when the source sends an NS
   message the target Server both forwards the NS toward a native
   underlying interface of the target Client (if any) and prepares an NA
   response the same as if it were the target Client (see:
   Section 3.15.5).  (This means that the source may receive two



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   separate NA messages - one from the target Server and one from the
   target Client.  The source must accept the union of the information
   from both messages.)

   For non-native underlying interfaces, the target Server includes a
   first TLLAO option in the NA with Interface ID set to 255 and
   includes any additional TLLAOs corresponding to the Client's NATed,
   Proxyed and/or VPNed underlying interfaces.  The Server writes its
   own link-layer address in TLLAOs corresponding to NATed and VPNed
   underlying interfaces, and writes the link-layer address of the Proxy
   in TLLAOs corresponding to Proxyed underlying interfaces (while also
   setting the X flag).  The Interface ID and QoS Preference values in
   the TLLAOs are those supplied by the Client during the initial RS/RA
   exchange and updated by any ensuing unsolicited NA messages.  The
   target Server must then maintain a dynamic neighbor cache entry for
   the Client, but MUST NOT send BGP updates for Clients discovered
   through dynamic route optimization.

   Thereafter, if the target Client moves to a new Server, the old
   Server sends unsolicited NA messages with no TLLAOs (subject to rate
   limiting) back to the source in response to data packets received
   from a correspondent node while forwarding the packets themselves to
   a Relay.  The Relay will then either forward the packets to the new
   Server if the target Client has moved, or drop the packets if the
   target Client is no longer in the network.  The source then allows
   future packets destined to the target Client to again flow through
   its own Server (or Relay).  Note however that the old Server retains
   the neighbor cache entry with its associated AcceptTime since there
   may be many packets in flight.  AcceptTime will then eventually
   decrement to 0 once the correspondent node processes and acts on the
   unsolicited NAs.

   When the target Client (or Proxy) sends unsolicited NA messages to
   the target Server to update link-layer address and/or QoS
   preferences, the target Server repeats the messages to any of its
   dynamic neighbors while using its own link-layer and link-local
   addresses as the source addresses.  In this way, the target Server
   acts as a link-scoped multicast repeater on behalf of the target
   Client (or Proxy).

   (Note that instead of serving as the route optimization target for
   Proxy interfaces, the target Server could instead forward the
   source's NS messages and allow the Proxies to return NA messages,
   i.e., the same as for Clients on native interfaces.  That would mean
   that the source could receive multiple NA messages from multiple
   Proxies and, if some or all NA messages are lost, the source would
   not be able to determine the full picture of the Client's Proxy
   affiliations.  If this alternate architecture is deemed appropriate



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   in some use cases, then the AERO Proxies could be employed to serve
   as route optimization targets instead of depending on the Servers to
   do so.)

3.16.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform Neighbor Unreachability Detection (NUD) by sending
   NS messages to elicit solicited NA messages from neighbors the same
   as described in [RFC4861].  NUD is performed either reactively in
   response to persistent link-layer errors (see Section 3.13) or
   proactively to update neighbor cache entry timers and/or link-layer
   address information.

   When an AERO node sends an NS/NA message, it uses one of its link-
   local addresses as the IPv6 source address and a link-local address
   of the neighbor as the IPv6 destination address.  When route
   optimization directs a source AERO node to a target AERO node, the
   source node SHOULD proactively test the direct path 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.

   While data packets are still flowing, the source node thereafter
   periodically tests the direct path to the target node (see
   Section 7.3 of [RFC4861]) in order to keep dynamic neighbor cache
   entries alive.  When the target node receives a valid NS message, it
   resets AcceptTime to ACCEPT_TIME and updates its cached link-layer
   addresses (if necessary).  When the source node receives a solicited
   NA message, it resets ForwardTime to FORWARD_TIME and updates its
   cached link-layer addresses (if necessary).  If the source node is
   unable to elicit a solicited NA response from the target node after
   MaxRetry attempts, it SHOULD set ForwardTime to 0.  Otherwise, the
   source node considers the path usable and SHOULD thereafter process
   any link-layer errors as an indication that the direct path to the
   target node has either failed or has become intermittent.

   When ForwardTime for a dynamic neighbor cache entry expires, the
   source node resumes sending any subsequent packets via a Server (or
   Relay) and may (eventually) attempt to re-initiate the AERO route
   optimization process.  When AcceptTime for a dynamic neighbor cache
   entry expires, the target node discards any subsequent packets
   received directly from the source node.  When both ForwardTime and
   AcceptTime for a dynamic neighbor cache entry expire, the node
   deletes the neighbor cache entry.





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

3.17.  Mobility Management and Quality of Service (QoS)

   AERO is an example of a Distributed Mobility Management (DMM)
   service.  Each AERO 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 service for
   all Clients.  AERO Clients coordinate with their regional Servers via
   RS/RA exchanges to maintain the DMM profile, and the AERO routing
   system tracks the current AERO Client/Server peering relationships.

   Mobility management for AERO interfaces is accommodated by sending
   unsolicited NA messages the same as for announcing link-layer address
   changes for any interface that implements IPv6 ND [RFC4861].  When a
   node sends an unsolicited NA message, it sets the IPv6 source to its
   own link-local address, sets the IPv6 destination address to all-
   nodes multicast, sets the link-layer source address to its own
   address and sets the link-layer destination address to either a
   multicast address or the unicast link-layer address of a neighbor.
   If the unsolicited NA message must be received by multiple neighbors,
   the node sends multiple copies of the NA using a different unicast
   link-layer destination address for each neighbor.  Mobility
   management considerations are specified in the following sections.

3.17.1.  Forwarding Packets on Behalf of Departed Clients

   When a Server receives packets with destination addresses that do not
   match one of its static neighbor cache Clients, it forwards the
   packets to a Relay and also returns an unsolicited NA message to the
   sender with no TLLAOs.  The packets will be delivered to the target
   Client's new location, and the sender will realize that it needs to
   deprecate its routing information that associated the target with
   this Server.

3.17.2.  Announcing Link-Layer Address and QoS Preference Changes

   When a Client needs to change its link-layer addresses, e.g., due to
   a mobility event, it sends unsolicited NAs to its neighbors using the
   new link-layer address as the source address and with TLLAOs that
   include the new Client UDP Port Number, IP Address and P(i) values.
   If the Client sends the NA solely for the purpose of updating QoS
   preferences without updating the link-layer address, the Client sets
   the UDP Port Number and IP Address to 0.




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   The Client MAY send up to MaxRetry unsolicited NA messages in
   parallel with sending actual data packets in case one or more NAs are
   lost.  If all NAs are lost, the neighbor will eventually invoke NUD
   by sending NS messages that include SLLAOs.

3.17.3.  Bringing New Links Into Service

   When a Client needs to bring new underlying interfaces into service
   (e.g., when it activates a new data link), it sends unsolicited NAs
   to its neighbors using the new link-layer address as the source
   address and with TLLAOs that include the new Client link-layer
   information.

3.17.4.  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
   unsolicited NAs to its neighbors with TLLAOs with all P(i) values set
   to 0.

   If the Client needs to send the unsolicited NAs over an underlying
   interface other than the one being removed from service, it MUST
   include a current TLLAO for the sending interface as the first TLLAO
   and include TLLAOs for any underlying interface being removed from
   service as additional TLLAOs.

3.17.5.  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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3.17.6.  Moving to a New Server

   When a Client associates with a new Server, it performs the Client
   procedures specified in Section 3.14.2.

   When a Client disassociates with an existing Server, it sends an RS
   "Release" message via a new Server with its base AERO address as the
   network-layer source address and the (administratively-provisioned)
   link-local address of the old Server as the network-layer destination
   address.  The new Server then caches the Client's AERO address and
   "Release" message parameters (e.g., "transaction ID") and writes its
   own administratively-provisioned link-local address as the network-
   layer source address.  The new Server then forwards the message to a
   Relay, which forwards the message to the old Server.

   When the old Server receives the "Release", it releases the Client's
   ACP prefix delegations and routes.  The old Server then deletes the
   Client's neighbor cache entry so that any in-flight packets will be
   forwarded via a Relay to the new Server, which will forward them to
   the Client.  The old Server finally returns a "Reply" message via a
   Relay to the new Server, which will decapsulate the "Reply" message
   and forward it as an RA "Reply" to the Client.

   When the new Server forwards the "Reply" message, the Client can
   delete both the default route and the neighbor cache entry for the
   old Server.  (Note that since messages may be lost in the network the
   Client SHOULD retry until it gets an RA "Reply" indicating that the
   RS "Release" was successful.  If the Client does not receive a
   "Reply" after MaxRetry attempts, the old Server may have failed and
   the Client should discontinue its "Release" attempts.)

   Finally, Clients SHOULD NOT move rapidly between Servers in order to
   avoid causing excessive oscillations in the AERO routing system.
   Such oscillations could result in intermittent reachability for the
   Client itself, while causing little harm to the network.  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, etc.

3.18.  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.  The AERO Proxy

   In some deployments, AERO Clients may be located in secured enclaves
   (e.g., a corporate enterprise network, a radio access network, etc.)
   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 AERO Proxy is located at the secured enclave perimeter and
   listens for RS messages originating from or RA messages destined to
   AERO Clients located within the enclave.  The Proxy acts on these
   control messages as follows:

   o  when the Proxy receives an RS message from a Client within the
      secured enclave, it first authenticates the message then creates a
      proxy neighbor cache entry for the Client in the INCOMPLETE State
      and caches the Client and Server link-layer address along with any
      identifying information including PD "transaction IDs", "Client
      Identifiers", etc. and/or ND Nonce values.  The Proxy then re-
      encapsulates the message and forwards it to the Server indicated
      by the destination link-layer address in the packet while
      substituting its own external address as the source link-layer
      address.

   o  when the Proxy receives an RA message from the Server, it matches
      the message with the (INCOMPLETE) 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 message and
      forwards it to the Client.  At the same time, the Proxy sends an
      unsolicited NA message including a TLLAO with the X flag set back
      to the Server to assert that it is indeed a Proxy as opposed to an
      ordinary NAT.  (In environments where spoofing is a threat, the
      Proxy signs the NA using SEND.)

   After the initial RS/RA handshake, the Proxy can send unsolicited NA
   messages to the Client's Server(s) to update Server neighbor cache
   entries on behalf of the Client.  (For example, the Proxy can send NA
   messages with a TLLAO with UDP Port Number and IP Address set to 0
   and with valid P(i) values to update the Server(s) with the Client's



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   new QoS preferences for that link).  The Proxy also forwards any
   unsolicited NA messages originating from the Client to the Client's
   Server(s) (e.g. if the Client needs to announce new QoS preferences
   on its own behalf), and forwards any data packets originating from
   the Client to the Client's primary Server.

   At the same time, for data packets originating from a Client within
   the enclave with destination addresses that match an ASP, the Proxy
   can initiate route optimization by sending an NS message via the
   Server to solicit an NA message from a target node on the path to the
   destination Client the same as discussed in Section 3.15.  The target
   must deliver the NA message directly to the Proxy, i.e., instead of
   relaying through the backward chain of Relays and Servers, since the
   backward chain could deliver the NA to a different Proxy besides the
   one that produced the NS.  For this reason, the Proxy prepares an NS
   message as specified in Section 3.15.3, but with its own link-layer
   address as the link-layer source address and with a single SLLAO
   containing its link-layer address and with the X flag set to indicate
   that direct delivery is required.

   When the target receives the NS message, it creates a dynamic
   neighbor cache entry in the ACCEPT state and returns an NA message
   directly to the Proxy.  When the target is a Client, it includes
   TLLAOs in the NA message with link-layer addresses corresponding to
   its native underling interfaces.  When the target is a Server, it
   includes a first TLLAO in the NA message with Interface ID set to 255
   and with its own link-layer address information, and also includes
   additional TLLAOs corresponding to the destination Client's Proxyed,
   NATed or VPNed underlying interfaces.  (For NATed or VPNed underlying
   interfaces the server writes its own link-layer address in the TLLAO,
   and for Proxyed interfaces it writes the link-layer address of the
   Proxy.)  When the source Proxy receives the NA message, it creates a
   dynamic neighbor cache entry in the FORWARD state that associates the
   TLLAOs of the NA message as the next-hop toward the routes advertised
   in the NA RIOs.

   When a source Proxy sends route optimization NS messages toward the
   target, it can include RIOs to assert specific routes, and the target
   will only accept packets from the source Proxy with matching source
   addresses.  If the source Proxy wishes to assert a "wildcard" route,
   it includes an RIO in the NS message with Prefix and Prefix Length
   set to 0.  In that case, the target will either accept or ignore the
   NS based on its configured trust policy.  If the target accepts the
   NS, it will accept all packets originating from the source Proxy
   regardless of their source address.

   After the initial NS/NA exchange, the target may need to update the
   neighbor cache entries for any source Proxies for which it holds a



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   dynamic neighbor cache entry in the ACCEPT state.  The target
   therefore sends unsolicited NA messages to announce any link layer
   changes.  As a result:

   o  the source Proxy may receive unsolicited NA messages with TLLAOs
      with new UDP Port Number, IP Address and/or QoS preferences from
      the target.  In that case, the Proxy updates its neighbor cache
      entry and forwards future outbound packets based on the new link
      layer information.

   o  the source Proxy may receive reflected packets destined to the
      link-layer address of a departed Client.  In that case, the Proxy
      proceeds as discussed in Section 3.8.5.

   o  the source Proxy may receive link-layer Destination Unreachable
      messages in response to data packets it sends to one of the target
      link-layer addresses.  In that case, the Proxy processes the link-
      layer error messages as an indication that the path may be failing
      and proceeds as discussed in Section 3.13.

   After the NS/NA exchange, while data packets are still flowing the
   source Proxy sends additional NS messages to the target using the
   address in the target's first TLLAO as the destination.  The NS
   message will update the target's AcceptTime timer, and the resulting
   NA reply will update the source Proxy's ForwardTime timer in their
   respective neighbor cache entries.

   If at some later time the target Client departs from its secured
   enclave, the Proxy sends unsolicited NAs to the Client's Servers to
   announce the departure.

5.  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
   AERO Proxies, Servers and Relays in the communications path.  Direct
   underlying interfaces must be tested periodically for reachability,
   e.g., via NUD, via periodic unsolicited NAs, etc.





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6.  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
   such as a loopback.  In this arrangement, the Client conducts route
   optimization in the same sense as discussed in Section 3.15.

   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.

7.  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-
   1.0.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-
   3.0.3a.tgz.







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8.  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.

9.  Security Considerations

   AERO link security considerations are the same as for standard IPv6
   Neighbor Discovery [RFC4861] except that AERO improves on some
   aspects.  In particular, AERO uses a trust basis between Clients and
   Servers, where the Clients only engage in the AERO mechanism when it
   is facilitated by a trusted Server.

   NS and NA messages SHOULD include a Timestamp option (see Section 5.3
   of [RFC3971]) that other AERO nodes can use to verify the message
   time of origin.  NS and RS messages SHOULD include a Nonce option
   (see Section 5.3 of [RFC3971]) that recipients echo back in
   corresponding responses.  In cases where spoofing cannot be mitigated
   through other means, however, all AERO IPv6 ND messages should employ
   SEND [RFC3971], which also protects the PD information embedded in
   RS/RA message options.

   AERO links must be protected against link-layer address spoofing
   attacks in which an attacker on the link pretends to be a trusted
   neighbor.  Links that provide link-layer securing mechanisms (e.g.,
   IEEE 802.1X WLANs) and links that provide physical security (e.g.,
   enterprise network wired LANs) provide a first line of defense,
   however AERO nodes SHOULD also use securing services such as SEND for
   Client authentication and network admission control.  Following
   authenticated Client admission and prefix delegation procedures, AERO
   nodes MUST ensure that the source of data packets corresponds to the
   node to which the prefixes were delegated.

   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.)





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   AERO Clients, Servers and Relays on the open Internet are susceptible
   to the same attack profiles as for any Internet nodes.  For this
   reason, IP security SHOULD be used when AERO is employed over
   unmanaged/unsecured links using securing mechanisms such as IPsec
   [RFC4301], IKE [RFC5996] and/or TLS [RFC5246].  In some environments,
   however, the use of end-to-end security from Clients to correspondent
   nodes (i.e., other Clients and/or Internet nodes) could obviate the
   need for IP security between AERO Clients, Servers and Relays.

   AERO Servers and Relays present targets for traffic amplification 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 can institute rate limits that protect Clients from receiving
   packet floods that could DoS low data rate links.

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

10.  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, Sri Gundavelli, Brian
   Haberman, Bernhard Haindl, Joel Halpern, Tom Herbert, Sascha Hlusiak,
   Lee Howard, Andre Kostur, Ted Lemon, Andy Malis, Satoru Matsushima,
   Tomek Mrugalski, Alexandru Petrescu, Behcet Saikaya, Michal Skorepa,
   Joe Touch, Bernie Volz, Ryuji Wakikawa, 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
   Danilov, Wen Fang, Anthony Gregory, Jeff Holland, Ed King, Gene
   MacLean III, Rob Muszkiewicz, Sean O'Sullivan, Kent Shuey, Brian



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   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.  Wayne Benson, Kyle Bae 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 Research and Technology (BR&T)
   autonomous systems networking program.

11.  References

11.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.





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

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

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

   [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>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <https://www.rfc-editor.org/info/rfc3315>.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              DOI 10.17487/RFC3633, December 2003,
              <https://www.rfc-editor.org/info/rfc3633>.

   [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>.

   [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>.




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   [RFC5175]  Haberman, B., Ed. and R. Hinden, "IPv6 Router
              Advertisement Flags Option", RFC 5175,
              DOI 10.17487/RFC5175, March 2008,
              <https://www.rfc-editor.org/info/rfc5175>.

   [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>.

11.2.  Informative References

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

   [I-D.ietf-intarea-gue]
              Herbert, T., Yong, L., and O. Zia, "Generic UDP
              Encapsulation", draft-ietf-intarea-gue-05 (work in
              progress), December 2017.

   [I-D.ietf-intarea-gue-extensions]
              Herbert, T., Yong, L., and F. Templin, "Extensions for
              Generic UDP Encapsulation", draft-ietf-intarea-gue-
              extensions-04 (work in progress), March 2018.

   [I-D.ietf-intarea-tunnels]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-08 (work in
              progress), January 2018.

   [I-D.templin-6man-dhcpv6-ndopt]
              Templin, F., "IPv6 Neighbor Discovery Extensions for
              Prefix Delegation", draft-templin-6man-dhcpv6-ndopt-04
              (work in progress), March 2018.

   [I-D.templin-6man-rio-redirect]
              Templin, F. and j. woodyatt, "Route Information Options in
              IPv6 Neighbor Discovery", draft-templin-6man-rio-
              redirect-06 (work in progress), May 2018.

   [I-D.templin-atn-bgp]
              Templin, F., Saccone, G., Dawra, G., and A. Lindem, "A
              Simple BGP-based Mobile Routing System for the
              Aeronautical Telecommunications Network", draft-templin-
              atn-bgp-06 (work in progress), March 2018.






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   [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.

   [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 Models", draft-
              templin-v6ops-pdhost-19 (work in progress), March 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>.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
              1996, <https://www.rfc-editor.org/info/rfc1981>.

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



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   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [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>.

   [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>.






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   [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>.

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

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

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

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

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

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

   [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>.

   [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>.



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   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

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

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

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

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

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

   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5996, DOI 10.17487/RFC5996, September 2010,
              <https://www.rfc-editor.org/info/rfc5996>.

   [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>.

   [RFC6422]  Lemon, T. and Q. Wu, "Relay-Supplied DHCP Options",
              RFC 6422, DOI 10.17487/RFC6422, December 2011,
              <https://www.rfc-editor.org/info/rfc6422>.






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   [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>.

   [TUNTAP]   Wikipedia, W., "http://en.wikipedia.org/wiki/TUN/TAP",
              October 2014.

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 and SSL/TLS.  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.  When to Insert an Encapsulation Fragment Header

   An encapsulation fragment header is inserted when the AERO tunnel
   ingress needs to apply fragmentation to accommodate packets that must
   be delivered without loss due to a size restriction.  Fragmentation
   is performed on the inner packet while encapsulating each inner
   packet fragment in outer IP and encapsulation layer headers that
   differ only in the fragment header fields.

   The fragment header can also be inserted in order to include a
   coherent Identification value with each packet, e.g., to aid in
   Duplicate Packet Detection (DPD).  In this way, network nodes can
   cache the Identification values of recently-seen packets and use the
   cached values to determine whether a newly-arrived packet is in fact
   a duplicate.  The Identification value within each packet could
   further provide a rough indicator of packet reordering, e.g., in
   cases when the tunnel egress wishes to discard packets that are
   grossly out of order.

   In some use cases, there may be operational assurance that no
   fragmentation of any kind will be necessary, or that only occasional
   large control messages will require fragmentation.  In that case, the
   encapsulation fragment header can be omitted and ordinary
   fragmentation of the outer IP protocol version can be applied when
   necessary.

Appendix C.  Autoconfiguration for Constrained Platforms

   On some platforms (e.g., popular cell phone operating systems), the
   act of assigning a default IPv6 route and/or assigning an address to
   an interface may not be permitted from a user application due to



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   security policy.  Typically, those platforms include a TUN/TAP
   interface [TUNTAP] that acts as a point-to-point conduit between user
   applications and the AERO interface.  In that case, the Client can
   instead generate a "synthesized RA" message.  The message conforms to
   [RFC4861] and is prepared as follows:

   o  the IPv6 source address is the Client's AERO address

   o  the IPv6 destination address is all-nodes multicast

   o  the Router Lifetime is set to a time that is no longer than the
      ACP DHCPv6 lifetime

   o  the message does not include a Source Link Layer Address Option
      (SLLAO)

   o  the message includes a Prefix Information Option (PIO) with a /64
      prefix taken from the ACP as the prefix for autoconfiguration

   The Client then sends the synthesized RA message via the TUN/TAP
   interface, where the operating system kernel will interpret it as
   though it were generated by an actual router.  The operating system
   will then install a default route and use StateLess Address
   AutoConfiguration (SLAAC) to configure an IPv6 address on the TUN/TAP
   interface.  Methods for similarly installing an IPv4 default route
   and IPv4 address on the TUN/TAP interface are based on synthesized
   DHCPv4 messages [RFC2131].

Appendix D.  Operational Deployment Alternatives

   AERO can be used in many different variations based on the specific
   use case.  The following sections discuss variations that adhere to
   the AERO principles while allowing selective application of AERO
   components.

D.1.  Operation on AERO Links Without DHCPv6 Services

   When Servers on the AERO link do not provide DHCPv6 services,
   operation can still be accommodated through administrative
   configuration of ACPs on AERO Clients.  In that case, administrative
   configurations of AERO interface neighbor cache entries on both the
   Server and Client are also necessary.  However, this may interfere
   with the ability for Clients to dynamically change to new Servers,
   and can expose the AERO link to misconfigurations unless the
   administrative configurations are carefully coordinated.






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D.2.  Operation on Server-less AERO Links

   In some AERO link scenarios, there may be no Servers on the link and/
   or no need for Clients to use a Server as an intermediary trust
   anchor.  In that case, each Client acts as a Server unto itself to
   establish neighbor cache entries by performing direct Client-to-
   Client IPv6 ND message exchanges, and some other form of trust basis
   must be applied so that each Client can verify that the prospective
   neighbor is authorized to use its claimed ACP.

   When there is no Server on the link, Clients must arrange to receive
   ACPs and publish them via a secure alternate PD authority through
   some means outside the scope of this document.

D.3.  Operation on Client-less AERO Links

   In some environments, the AERO service may be useful for mobile nodes
   that do not implement the AERO Client function and do not perform
   encapsulation.  For example, if the mobile node has a way of
   injecting its ACP into the access subnetwork routing system an AERO
   Server connected to the same access network can accept the ACP prefix
   injection as an indication that a new mobile node has come onto the
   subnetwork.  The Server can then inject the ACP into the BGP routing
   system the same as if an AERO Client/Server DHCPv6 PD exchange had
   occurred.  If the mobile node subsequently withdraws the ACP from the
   access network routing system, the Server can then withdraw the ACP
   from the BGP routing system.

   In this arrangement, AERO Servers and Relays are used in exactly the
   same ways as for environments where DHCPv6 Client/Server exchanges
   are supported.  However, the access subnetwork routing systems must
   be capable of accommodating rapid ACP injections and withdrawals from
   mobile nodes with the understanding that the information must be
   propagated to all routers in the system.  Operational experience has
   shown that this kind of routing system "churn" can lead to overall
   instability and routing system inconsistency.

D.4.  Manually-Configured AERO Tunnels

   In addition to the dynamic neighbor discovery procedures for AERO
   link neighbors described above, AERO encapsulation can be applied to
   manually-configured tunnels.  In that case, the tunnel endpoints use
   an administratively-provisioned link-local address and exchange NS/NA
   messages the same as for dynamically-established tunnels.







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D.5.  Encapsulation Avoidance on Relay-Server Dedicated Links

   In some environments, AERO Servers and Relays may be connected by
   dedicated point-to-point links, e.g., high speed fiberoptic leased
   lines.  In that case, the Servers and Relays can participate in the
   AERO link the same as specified above but can avoid encapsulation
   over the dedicated links.  In that case, however, the links would be
   dedicated for AERO and could not be multiplexed for both AERO and
   non-AERO communications.

D.6.  Encapsulation Protocol Version Considerations

   A source Client may connect only to an IPvX underlying network, while
   the target Client connects only to an IPvY underlying network.  In
   that case, the target and source Clients have no means for reaching
   each other directly (since they connect to underlying networks of
   different IP protocol versions) and so must ignore any route
   optimization messages and continue to send packets via their Servers.

D.7.  Extending AERO Links Through Security Gateways

   When an enterprise mobile node moves from a campus LAN connection to
   a public Internet link, it must re-enter the enterprise via a
   security gateway that has both a physical interface connection to the
   Internet and a physical interface connection to the enterprise
   internetwork.  This most often entails the establishment of a Virtual
   Private Network (VPN) link over the public Internet from the mobile
   node to the security gateway.  During this process, the mobile node
   supplies the security gateway with its public Internet address as the
   link-layer address for the VPN.  The mobile node then acts as an AERO
   Client to negotiate with the security gateway to obtain its ACP.

   In order to satisfy this need, the security gateway also operates as
   an AERO Server with support for AERO Client proxying.  In particular,
   when a mobile node (i.e., the Client) connects via the security
   gateway (i.e., the Server), the Server provides the Client with an
   ACP in a DHCPv6 PD exchange the same as if it were attached to an
   enterprise campus access link.  The Server then replaces the Client's
   link-layer source address with the Server's enterprise-facing link-
   layer address in all AERO messages the Client sends toward neighbors
   on the AERO link.  The AERO messages are then delivered to other
   nodes on the AERO link as if they were originated by the security
   gateway instead of by the AERO Client.  In the reverse direction, the
   AERO messages sourced by nodes within the enterprise network can be
   forwarded to the security gateway, which then replaces the link-layer
   destination address with the Client's link-layer address and replaces
   the link-layer source address with its own (Internet-facing) link-
   layer address.



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   After receiving the ACP, the Client can send IP packets that use an
   address taken from the ACP as the network layer source address, the
   Client's link-layer address as the link-layer source address, and the
   Server's Internet-facing link-layer address as the link-layer
   destination address.  The Server will then rewrite the link-layer
   source address with the Server's own enterprise-facing link-layer
   address and rewrite the link-layer destination address with the
   target AERO node's link-layer address, and the packets will enter the
   enterprise network as though they were sourced from a node located
   within the enterprise.  In the reverse direction, when a packet
   sourced by a node within the enterprise network uses a destination
   address from the Client's ACP, the packet will be delivered to the
   security gateway which then rewrites the link-layer destination
   address to the Client's link-layer address and rewrites the link-
   layer source address to the Server's Internet-facing link-layer
   address.  The Server then delivers the packet across the VPN to the
   AERO Client.  In this way, the AERO virtual link is essentially
   extended *through* the security gateway to the point at which the VPN
   link and AERO link are effectively grafted together by the link-layer
   address rewriting performed by the security gateway.  All AERO
   messaging services (including route optimization and mobility
   signaling) are therefore extended to the Client.

   In order to support this virtual link grafting, the security gateway
   (acting as an AERO Server) must keep static neighbor cache entries
   for all of its associated Clients located on the public Internet.
   The neighbor cache entry is keyed by the AERO Client's AERO address
   the same as if the Client were located within the enterprise
   internetwork.  The neighbor cache is then managed in all ways as
   though the Client were an ordinary AERO Client.  This includes the
   AERO IPv6 ND messaging signaling for Route Optimization and Neighbor
   Unreachability Detection.

   Note that the main difference between a security gateway acting as an
   AERO Server and an enterprise-internal AERO Server is that the
   security gateway has at least one enterprise-internal physical
   interface and at least one public Internet physical interface.
   Conversely, the enterprise-internal AERO Server has only enterprise-
   internal physical interfaces.  For this reason security gateway
   proxying is needed to ensure that the public Internet link-layer
   addressing space is kept separate from the enterprise-internal link-
   layer addressing space.  This is afforded through a natural extension
   of the security association caching already performed for each VPN
   client by the security gateway.







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Appendix E.  Change Log

   Changes from -81 to -82:

   o  Make DHCPv6 the default (but not exclusive) PD service

   o  Support operation with no PD services nor ND Route Information
      Options

   o  Updates to AERO Proxy function

   Changes from -80 to -81:

   o  Updates to Server and Proxy Extended Route Optimization

   o  Updates to AERO Proxy section

   o  Cleanups and clarifications

   Changes from -79 to -80:

   o  Substantial updates to AERO Proxy function

   o  Removed 'V' bit from SLLAO and replaced with 'X' bit

   o  Added concept of Direct, Proxyed, NATed, VPNed and Native
      underlying interfaces

   o  Adjusted route optimization text according to underrlying
      interface types

   Changes from -78 to -79:

   o  Neighbors now set UDP Port Number and IP Address in S/TLLAOs to 0
      if the node is behind a NAT or otherwise does not wish to update
      its link-layer address for this underlying interface

   o  Introduced "proxy" as a new neighbor cache entry type

   o  updated GUE references

   o  multipath considerations for error message handling and NUD

   Changes from -77 to -78:

   o  Added "V" bit to SLLAO flags field for NS messages.  V=1 indicates
      that the NA response must go through the reverse chain of Servers
      and Relays



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   o  Now including DHCPv6 PD messages as IPv6 ND message options

   o  Clarified the use of the "P" bit in the RA flags field

   o  Use of SEND to protect the combined DHCPv6/IPv6ND messages

   o  Proxy now treats a Client's Servers as the default routers (i.e.,
      instead of using a Relay as the default).

   Changes from -76 to -77:

   o  Now using IPv6 ND NS/NA messaging for route optimization (no
      longer using Predirect/Redirect)

   o  Now using combined IPv6 ND/DHCPv6 messaging so autoconfiguration
      can be conducted in a single message exchange

   o  Introduced the AERO Proxy construct.  Critical for applications
      such as ATN/IPS

   Changes from -75 to -76:

   o  Bumped version number ahead of expiration deadline

   Changes from -74 to -75:

   o  Bumped version number ahead of expiration deadline

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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