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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc6706 (if approved)                           July 21, 2014
Intended status: Standards Track
Expires: January 22, 2015


               Transmission of IP Packets over AERO Links
                     draft-templin-aerolink-30.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
   redirection services for route optimization.  AERO provides an IPv6
   link-local address format known as the AERO address that supports
   operation of the IPv6 Neighbor Discovery (ND) protocol and links IPv6
   ND to IP forwarding.  Admission control and provisioning are
   supported by the Dynamic Host Configuration Protocol for IPv6
   (DHCPv6), and node mobility is naturally supported through dynamic
   neighbor cache updates.  Although IPv6 ND messaging is used in the
   control plane, both IPv4 and IPv6 are supported in the data plane.

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 http://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 January 22, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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 . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Asymmetric Extended Route Optimization (AERO) . . . . . . . .   5
     3.1.  AERO Link Reference Model . . . . . . . . . . . . . . . .   5
     3.2.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  AERO Addresses  . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  AERO Interface Characteristics  . . . . . . . . . . . . .   9
       3.4.1.  Coordination of Multiple Underlying Interfaces  . . .  11
     3.5.  AERO Interface Neighbor Cache Maintenace  . . . . . . . .  12
     3.6.  AERO Interface Sending Algorithm  . . . . . . . . . . . .  13
     3.7.  AERO Interface Encapsulation, Re-encapsulation and
           Decapsulation . . . . . . . . . . . . . . . . . . . . . .  15
     3.8.  AERO Interface Data Origin Authentication . . . . . . . .  16
     3.9.  AERO Interface MTU Considerations . . . . . . . . . . . .  16
     3.10. AERO Router Discovery, Prefix Delegation and Address
           Configuration . . . . . . . . . . . . . . . . . . . . . .  18
       3.10.1.  AERO Client Behavior . . . . . . . . . . . . . . . .  18
       3.10.2.  AERO Server Behavior . . . . . . . . . . . . . . . .  20
       3.10.3.  DHCPv6 Server Behavior . . . . . . . . . . . . . . .  21
     3.11. AERO Relay/Server Routing System  . . . . . . . . . . . .  22
     3.12. AERO Redirection  . . . . . . . . . . . . . . . . . . . .  23
       3.12.1.  Reference Operational Scenario . . . . . . . . . . .  23
       3.12.2.  Concept of Operations  . . . . . . . . . . . . . . .  25
       3.12.3.  Message Format . . . . . . . . . . . . . . . . . . .  25
       3.12.4.  Sending Predirects . . . . . . . . . . . . . . . . .  26
       3.12.5.  Re-encapsulating and Relaying Predirects . . . . . .  27
       3.12.6.  Processing Predirects and Sending Redirects  . . . .  28
       3.12.7.  Re-encapsulating and Relaying Redirects  . . . . . .  30
       3.12.8.  Processing Redirects . . . . . . . . . . . . . . . .  30
       3.12.9.  Server-Oriented Redirection  . . . . . . . . . . . .  31
     3.13. Neighbor Unreachability Detection (NUD) . . . . . . . . .  31
     3.14. Mobility Management . . . . . . . . . . . . . . . . . . .  32
       3.14.1.  Announcing Link-Layer Address Changes  . . . . . . .  32
       3.14.2.  Moving to a New Server . . . . . . . . . . . . . . .  34
     3.15. Encapsulation Protocol Version Considerations . . . . . .  34
     3.16. Multicast Considerations  . . . . . . . . . . . . . . . .  35



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     3.17. Operation on AERO Links Without DHCPv6 Services . . . . .  35
     3.18. Operation on Server-less AERO Links . . . . . . . . . . .  35
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  35
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  36
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  36
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  37
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  38
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  40

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 to neighboring nodes over either IPv6 or
   IPv4 networks, i.e., AERO views the IPv6 and IPv4 networks as
   equivalent links for tunneling.  Nodes attached to AERO links can
   exchange packets via trusted intermediate routers that provide
   forwarding services to reach off-link destinations and redirection
   services for route optimization that addresses the requirements
   outlined in [RFC5522].

   AERO provides an IPv6 link-local address format known as the AERO
   address that supports operation of the IPv6 Neighbor Discovery (ND)
   [RFC4861] protocol and links IPv6 ND to IP forwarding.  Admission
   control and provisioning are supported by the Dynamic Host
   Configuration Protocol for IPv6 (DHCPv6) [RFC3315], and node mobility
   is naturally supported through dynamic neighbor cache updates.
   Although IPv6 ND message signalling is used in the control plane,
   either of IPv4 and IPv6 can be used in the data plane.  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:

   AERO link
      a Non-Broadcast, Multiple Access (NBMA) tunnel virtual overlay
      configured over a node's attached IPv6 and/or IPv4 networks.  All
      nodes on the AERO link appear as single-hop neighbors from the
      perspective of the virtual overlay.

   AERO interface
      a node's attachment to an AERO link.

   AERO address



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      an IPv6 link-local address constructed as specified in Section 3.2
      and applied to a Client's AERO interface.

   AERO node
      a node that is connected to an AERO link and that participates in
      IPv6 ND over the link.

   AERO Client ("Client")
      a node that applies an AERO address to an AERO interface and
      receives an IP prefix delegation.

   AERO Server ("Server")
      a node that configures an AERO interface to provide default
      forwarding services for AERO Clients.  The Server applies the IPv6
      link-local subnet router anycast address (fe80::) to the AERO
      interface and also applies an administratively assigned IPv6 link-
      local unicast address used for operation of the IPv6 ND protocol.

   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
      applies an administratively assigned IPv6 link-local unicast
      address to the AERO interface the same as for a Server.

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

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

   underlying network
      a connected IPv6 or IPv4 network routing region over which the
      tunnel virtual overlay is configured.

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

   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.  Link-layer
      addresses are used as the encapsulation header source and
      destination addresses.




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   network layer address
      the source or destination address of the 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.

   AERO Service Prefix (ASP)
      an IP prefix associated with the AERO link and from which AERO
      Client Prefixes (ACPs) are derived (for example, the IPv6 ACP
      2001:db8:1:2::/64 is derived from the IPv6 ASP 2001:db8::/32).

   AERO Client Prefix (ACP)
      a more-specific IP prefix taken from an ASP and delegated to a
      Client.

   Throughout the document, the simple terms "Client", "Server" and
   "Relay" refer to "AERO Client", "AERO Server" and "AERO Relay",
   respectively.  Capitalization is used to distinguish these terms from
   DHCPv6 client/server/relay.  This is an important distinction to
   avoid ambiguity, e.g., an AERO Server also acts as a DHCPv6 relay, an
   AERO Relay may also act as a DHCPv6 server, etc.

   Throughout the document, it is said that an address is "applied" to
   an AERO interface since the address need not always be "assigned" to
   the interface in the traditional sense.  However, the address must at
   least be bound to the interface in some fashion for operation of the
   IPv6 ND protocol.

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

   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].

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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                             .-(::::::::)
                          .-(:::: IP ::::)-.       +-----------+
                         (:: Internetwork ::)      |  DHCPv6   |
                          `-(::::::::::::)-'       | Server X  |
                             `-(::::::)-'          +-----------+
                                  |
       +--------------+    +------+-------+    +--------------+
       |AERO Server S1|    | AERO Relay R |    |AERO Server S2|
       | (default->R) |    |(C->S1; D->S2)|    | (default->R) |
       |    Nbr: A    |    +-------+------+    |    Nbr: B    |
       +-------+------+            |           +------+-------+
               |                   |                  |
       X---+---+-------------------+------------------+---+---X
           |                  AERO Link                   |
     +-----+--------+                            +--------+-----+
     |AERO Client A |                            |AERO Client B |
     | default->S1  |                            | default->S2  |
     +--------------+                            +--------------+
           .-.                                         .-.
        ,-(  _)-.                                   ,-(  _)-.
     .-(_   IP  )-.                              .-(_   IP  )-.
    (__    EUN      )                           (__    EUN      )
       `-(______)-'                                `-(______)-'
            |                                           |
        +--------+                                  +--------+
        | Host C |                                  | Host D |
        +--------+                                  +--------+

                    Figure 1: AERO Link Reference Model

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

   o  DHCPv6 Server X is the prefix delegation authority for the
      delegation of ACPs taken from the AERO link's ASPs

   o  Relay R associates with Servers S1 and S2, and connects the link
      to the rest of the IP Internetwork

   o  Servers S1 and S2 associate with Relay R and also act as default
      routers for their associated Clients A and B

   o  Clients A and B associate with Servers S1 and S2, respectively and
      also act as default routers for their associated EUNs

   o  Hosts C and D attach to the EUNs served by Clients A and B,
      respectively




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   In this model, there may be many additional Relays, Servers and
   Clients.  Each Sever peers with each Relay in a dynamic routing
   protocol session to advertise its list of associated Clients.  Each
   Relay advertises the ASPs for the AERO link into the native IP
   Internetwork and serves as a gateway between the AERO link and the
   Internetwork.  Clients may associate with only a single Server or
   with multiple Server, e.g., for fault tolerance and/or load
   balancing.

3.2.  AERO Node Types

   The DHCPv6 server is authoritative for the management of the AERO
   link's AERO Service Prefixes (ASPs).  The DHCPv6 server is therefore
   critical infrastructure for the AERO link, but need not otherwise
   participate as an AERO node.  AERO Servers communicate with the
   DHCPv6 server either via the AERO link itself or via a different IPv6
   link.

   AERO Relays relay packets between nodes connected to the same AERO
   link and also forward packets between the AERO link and the native
   Internetwork.  The relaying process entails re-encapsulation of IP
   packets that were received from a first AERO node and are to be
   forwarded without modification to a second AERO node.  AERO Relays
   present the AERO link to the native Internetwork as a set of one or
   more ASPs.

   AERO Servers provide default routing services to AERO Clients.  AERO
   Servers configure a DHCPv6 relay function and facilitate Prefix
   Delegation (PD) exchanges between AERO Clients and the DHCPv6 server.
   Each delegated prefix becomes an AERO Client Prefix (ACP) taken from
   an ASP.

   AERO Clients act as requesting routers to receive ACPs through DHCPv6
   PD exchanges via AERO Servers over the AERO link.  (Each Client MAY
   associate with a single Server or with multiple Servers.)  Each IPv6
   AERO Client receives at least a /64 IPv6 ACP, and may receive even
   shorter prefixes.  Similarly, each IPv4 AERO Client receives at least
   a /32 IPv4 ACP (i.e., a singleton IPv4 address), and may receive even
   shorter prefixes.

   AERO Clients that act as routers sub-delegate portions of their ACPs
   to links on EUNs.  End system applications on AERO Clients that act
   as routers bind to EUN interfaces (i.e., and not the AERO interface).

   AERO Clients that act as ordinary hosts assign one or more IP
   addresses from their ACPs to the AERO interface but DO NOT assign the
   ACP itself to the AERO interface.  Instead, the Client assigns the
   ACP to a "black hole" route so that unused portions of the prefix are



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   nullified.  End system applications on AERO Clients that act as hosts
   bind directly to the AERO interface.

3.3.  AERO Addresses

   An AERO address is an IPv6 link-local address with an embedded ACP
   and applied to a Client's AERO interface.  The AERO address is formed
   as follows:

      fe80::[ACP]

   For IPv6, the AERO address begins with the prefix fe80::/64 and
   includes in its interface identifier the base prefix taken from the
   Client's IPv6 ACP.  The base prefix is determined by masking the ACP
   with the prefix length.  For example, if the AERO Client receives the
   IPv6 ACP:

      2001:db8:1000:2000::/56

   it constructs its AERO address as:

      fe80::2001:db8:1000:2000

   For IPv4, the AERO address is formed as an IPv4-mapped IPv6 address
   [RFC4291] that includes the base prefix taken from the Client's IPv4
   ACP.  For example, if the AERO Client receives the IPv4 ACP:

      192.0.2.32/28

   it constructs its AERO address as:

      fe80::FFFF:192.0.2.32

   The AERO address remains stable as the Client moves between
   topological locations, i.e., even if its link-layer addresses change.

   NOTE: In some cases, prospective neighbors may not have a priori
   knowledge of the Client's ACP length and may therefore send initial
   IPv6 ND messages with an AERO destination address that matches the
   ACP but does not correspond to the base prefix.  In that case, the
   Client MUST accept the address as equivalent to the base address, but
   then use the base address as the source address of any IPv6 ND
   message replies.  For example, if the Client receives the IPv6 ACP
   2001:db8:1000:2000::/56 then subsequently receives an IPv6 ND message
   with destination address fe80::2001:db8:1000:2001, it accepts the
   message but uses fe80::2001:db8:1000:2000 as the source address of
   any IPv6 ND replies.




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3.4.  AERO Interface Characteristics

   AERO interfaces use IP-in-IPv6 encapsulation [RFC2473] to exchange
   tunneled packets with AERO neighbors attached to an underlying IPv6
   network, and use IP-in-IPv4 encapsulation [RFC2003][RFC4213] to
   exchange tunneled packets with AERO neighbors attached to an
   underlying IPv4 network.  AERO interfaces can also coordinate secured
   tunnel types such as IPsec [RFC4301] or TLS [RFC5246].  When Network
   Address Translator (NAT) traversal and/or filtering middlebox
   traversal may be necessary, a UDP header is further inserted
   immediately above the IP encapsulation header.

   AERO interfaces maintain a neighbor cache, and AERO Clients and
   Servers use an adaptation of standard unicast IPv6 ND messaging.
   AERO interfaces use unicast Neighbor Solicitation (NS), Neighbor
   Advertisement (NA), Router Solicitation (RS) and Router Advertisement
   (RA) messages the same as for any IPv6 link.  AERO interfaces use two
   redirection message types -- the first known as a Predirect message
   and the second being the standard Redirect message (see Section 3.9).
   AERO links further use link-local-only addressing; hence, AERO nodes
   ignore any Prefix Information Options (PIOs) they may receive in RA
   messages.

   AERO interface Redirect, Predirect and unsolicited NA messages
   include Target Link-Layer Address Options (TLLAOs) formatted as shown
   in Figure 2:

        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 = 2   |   Length = 3  |           Reserved            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Link ID    |   Preference  |     UDP Port Number (or 0)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +--                                                           --+
       |                                                               |
       +--                        IP Address                         --+
       |                                                               |
       +--                                                           --+
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      Figure 2: AERO Target Link-Layer Address Option (TLLAO) Format

   In this format, Link ID is an integer value between 0 and 255
   corresponding to an underlying interface of the target node, and



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   Preference is an integer value between 0 and 255 indicating the
   node's preference for this underlying interface, with 0 being highest
   preference and 255 being lowest.  UDP Port Number and IP Address are
   set to the addresses used by the target node when it sends
   encapsulated packets over the underlying interface.  When no UDP
   encapsulation is used, 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 [RFC4291].

   When a Relay enables an AERO interface, it applies an
   administratively assigned link-local address fe80::ID to the
   interface for communicating with Servers on the link.  Each fe80::ID
   address MUST be unique among all Relays and Servers on the link, and
   MUST NOT collide with any potential AERO addresses, e.g., the
   addresses could be assigned as fe80::1, fe80::2, fe80::3, etc.  The
   Relay also maintains an IP forwarding table entry for each Client-
   Server association and maintains a neighbor cache entry for each
   Server on the link.  Relays do not require the use of IPv6 ND
   messaging for reachability determination since Relays and Servers
   engage in a dynamic routing protocol over the AERO interface.  At a
   minimum, however, Relays respond to NS messages by returning an NA.

   When a Server enables an AERO interface, it applies the address
   fe80:: to the interface as a link-local Subnet Router Anycast
   address, and also applies an administratively assigned link-local
   address fe80::ID to support the operation of the IPv6 ND protocol and
   to communicate with Relays on the link.  The Server maintains a
   neighbor cache entry for each Relay on the link, and also creates
   per-Client neighbor cache entries whenever it discovers a new Client.
   At a minimum, when the Server receives an NS/RS messages on the AERO
   interface it returns an NA/RA message.  When the Server receives an
   NS/NA, it also update timers in existing neighbor cache entries but
   does not create new neighbor cache entries nor update cached link-
   layer addresses.  Servers also engage in a dynamic routing protocol
   with all Relays on the link.  Finally, the Server provides a simple
   conduit between Clients and Relays, or between Clients and other
   Clients.  Therefore, packets enter the Server's AERO interface from
   the link layer and are forwarded back out the link layer without ever
   leaving the AERO interface and therefore without ever disturbing the
   network layer.

   When a Client enables an AERO interface, it invokes prefix delegation
   to receive an ACP.  Next, it applies the corresponding AERO address
   to the AERO interface, i.e., the prefix delegation bootstraps the
   provisioning of a unique link-local address.  The Client maintains a
   neighbor cache entry for each of its Servers and each of its active
   peer Clients.  When the Client receives Redirect/Predirect messages
   on the AERO interface it updates or creates neighbor cache entries,



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   including link-layer address information.  Unsolicited NA messages
   update the cached link-layer address for the neighbor Client (e.g.,
   following a link-layer address change due to node mobility) but do
   not create new neighbor cache entries.  RA messages as well as NS/NA
   messages used for Neighbor Unreachability Detection (NUD) update
   timers in existing neighbor cache entires but do not update link-
   layer addresses nor create new neighbor cache entries.  Redirect,
   Predirect and unsolicited 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.  Predirect, NS and RS messages
   SHOULD include a Nonce option (see Section 5.3 of [RFC3971]) that
   recipients echo back in corresponding responses.  Finally, the Client
   need not maintain any IP forwarding table entries for neighboring
   Clients.  Instead, it can set a single "route-to-interface" default
   route in the IP forwarding table pointing to the AERO interface, and
   all forwarding decisions can be made within the AERO interface based
   on neighbor cache entries.

3.4.1.  Coordination of Multiple Underlying Interfaces

   AERO interfaces may be configured over multiple underlying
   interfaces.  For example, common mobile handheld devices have both
   wireless local area network ("WLAN") and cellular wireless links.
   These links are 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, terrestrial, air-to-air
   directional, etc.) with diverse performance and cost properties.

   If a Client's multiple underlying interfaces are used "one at a time"
   (i.e., all other interfaces are in standby mode while one interface
   is active), then Redirect, Predirect and unsolicited NA messages
   include only a single TLLAO with Link ID set to a constant value
   (e.g., 0).

   If the Client has multiple active underlying interfaces, then from
   the perspective of IPv6 ND it would appear to have a single link-
   local address with multiple link-layer addresses.  In that case,
   Redirect, Predirect and unsolicited NA messages MAY include multiple
   TLLAOs -- each with a different Link ID that corresponds to a
   specific underlying interface of the Client.  Further details on
   coordination of multiple active underlying interfaces are outside the
   scope of this specification.








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3.5.  AERO Interface Neighbor Cache Maintenace

   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].  Neighbor cache
   entries are created and maintained as follows:

   AERO Relays maintain a permanent neighbor cache entry for each Server
   on the link, and AERO Servers maintain a permanent neighbor cache
   entry for each Relay on the link.  AERO Clients maintain a neighbor
   cache entry for each of their associated Servers, and AERO Servers
   maintain a neighbor cache for each of their associated Clients with a
   lifetime based on the DHCPv6 lease lifetime.  AERO Clients maintain
   neighbor cache entries for each of their active correspondent Clients
   with lifetimes based on IPv6 ND messaging constants.

   When an AERO Server relays a DHCPv6 Reply message to an AERO Client,
   it creates or updates a neighbor cache entry for the Client based on
   the AERO address corresponding to the Client's ACP as the network-
   layer address and with the Client's encapsulation IP address and UDP
   port number as the link-layer address.  The Server also records the
   ACP's lease lifetime and prefix length in the neighbor cache entry.

   When an AERO Client receives a DHCPv6 Reply message from an AERO
   Server, it creates or updates a neighbor cache entry for the Server
   based on the Reply message link-local source address as the network-
   layer address, the lease lifetime as the neighbor cache entry
   lifetime, and the encapsulation IP source address and UDP source port
   number as the link-layer address.

   When an AERO Client receives a valid Predirect message it creates or
   updates a neighbor cache entry for the Predirect target network-layer
   and link-layer addresses plus prefix length.  The node then sets an
   "AcceptTime" variable for the neighbor and uses this value to
   determine whether packets received from the predirected neighbor can
   be accepted.

   When an AERO Client receives a valid Redirect message it creates or
   updates a neighbor cache entry for the Redirect target network-layer
   and link-layer addresses plus prefix length.  The node then sets a
   "ForwardTime" variable for the neighbor and uses this value to
   determine whether packets can be sent directly to the redirected
   neighbor.  The node also maintains a "Retry" variable to limit the
   number of keepalives sent when a neighbor may have gone unreachable.

   When an AERO Client receives a valid NS message corresponding to a
   neighbor cache entry for another Client, it (re)sets AcceptTime for
   the neighbor to ACCEPT_TIME.



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   When an AERO Client receives a valid solicited NA message
   corresponding to a neighbor cache entry for another Client, it
   (re)sets ForwardTime for the neighbor to FORWARD_TIME and sets Retry
   to MAX_RETRY.  (When an AERO Client receives a valid unsolicited NA
   message, it updates the neighbor's link-layer address but DOES NOT
   reset ForwardTime or Retries.)

   It is RECOMMENDED that FORWARD_TIME be set to the default constant
   value 30 seconds to match the default REACHABLE_TIME value specified
   for IPv6 ND [RFC4861].

   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
   redirection procedure can converge before AcceptTime decrements below
   FORWARD_TIME.

   It is RECOMMENDED that MAX_RETRY be set to 3 the same as described
   for IPv6 ND address resolution in Section 7.3.3 of [RFC4861].

   Different values for FORWARD_TIME, ACCEPT_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 redirection
   procedure to converge.

   For AERO Client<->Server neighbor cache entries, AcceptTime and
   ForwardTime are set based on the DHCPv6 lease lifetime and may be
   modified based on the Router Lifetime advertised in the Server's RA
   messages.

3.6.  AERO Interface Sending Algorithm

   When an IP packet enters a Client's AERO interface from the network
   layer, the Client searches its neighbor cache for an entry with an
   AERO address that matches the packet's destination address.  If there
   is a match, the Client uses the link-layer address in the neighbor
   cache entry as the link-layer address for encapsulation then admits
   the packet into the tunnel.  If there is no match, the Client instead
   uses the link-layer address of a neighboring Server as the link-layer
   address for encapsulation.  (Note that the Client caches the ASPs for
   the AERO link and can thus search the neighbor cache only for
   destination addresses that are covered by an ASP.)

   When an IP packet enters a Server's AERO interface from the link
   layer, the Server searches for a neighbor cache match the same as for
   a Client.  If there is a match, the Server uses the link-layer



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   address in the neighbor cache entry as the link-layer address for re-
   encapsulation.  If there is no match, the Server instead uses the
   link-layer address of a neighboring Relay as the link-layer address
   for encapsulation.  Servers also relay Predirect, Redirect and
   unsolicited Neighbor Advertisement messages received from a Client
   and with an AERO destination address.  If the AERO destination
   address is the address of a neighbor, the Server changes the link-
   layer source address to its own address, changes the link-layer
   destination address to the address of the neighbor and forwards the
   message to the neighbor.  If the AERO destination address is not a
   neighbor, the Server instead forwards the message to a Relay.  When
   an AERO Relay forwards either a data packet or an IPv6 ND message to
   an AERO Server, the Server MUST NOT forward the packet back to the
   same or a different Relay.

   When an IP packet enters a Relay's AERO interface from the network
   layer, the Relay searches its IP forwarding table for an entry that
   is covered by an ASP and also matches the destination.  If there is a
   match, the Relay uses the link-layer address in the neighbor cache
   entry for the next-hop Server as the link-layer address for
   encapsulation.  When an IP packet enters a Relay's AERO interface
   from the link-layer, if the destination is not covered by an ASP the
   Relay forwards the packet to another IP link as indicated by the IP
   forwarding table.  If the destination is covered by an ASP, and there
   is a more-specific forwarding table entry that matches the
   destination, the Relay uses the link-layer address in the neighbor
   cache entry for the next-hop Server as the link-layer address for
   encapsulation.  If there is no more-specific entry, the Relay instead
   drops the packet.  Relays also relay Predirect, Redirect and
   unsolicited Neighbor Advertisement messages by searching for an IP
   forwarding table entry that matches the message's AERO destination
   address.  If there is a match, the Relay proxies the packet in the
   same manner as described for Servers above; otherwise, the Relay
   drops the packet.  When an AERO Server forwards either a data packet
   or an IPv6 ND message to an AERO Relay, the Relay MUST NOT forward
   the packet back to the same Server.

   Note that in the above this tunnel exit determination is often based
   on consulting the neighbor cache instead of the IP forwarding table.
   IP forwarding is therefore linked to IPv6 ND via the AERO address.

   When an AERO node forwards a packet back out the same AERO interface
   the packet arrived on, the node MUST NOT decrement the network layer
   TTL/Hop-count.







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3.7.  AERO Interface Encapsulation, Re-encapsulation and Decapsulation

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

   AERO interfaces encapsulate packets per the specifications in
   [RFC2003][RFC2473][RFC4213][RFC4301][RFC5246] (etc.) except that the
   interface copies the "TTL/Hop Limit", "Type of Service/Traffic Class"
   and "Congestion Experienced" values in the packet's IP header into
   the corresponding fields in the encapsulation header.  For packets
   undergoing re-encapsulation, the AERO interface instead copies the
   "TTL/Hop Limit", "Type of Service/Traffic Class" and "Congestion
   Experienced" values in the original encapsulation header into the
   corresponding fields in the new encapsulation header (i.e., the
   values are transferred between encapsulation headers and *not* copied
   from the encapsulated packet's network-layer header).

   When AERO UDP encapsulation is used, the AERO interface encapsulates
   the packet per the specifications in [RFC2003][RFC2473][RFC4213]
   except that it inserts a UDP header between the encapsulation header
   and the packet's IP header.  The AERO interface sets the UDP source
   port to a constant value that it will use in each successive packet
   it sends, sets the UDP checksum field to zero (see:
   [RFC6935][RFC6936]) and sets the UDP length field to the length of
   the IP packet plus 8 bytes for the UDP header itself.  For packets
   sent via a Server, the AERO interface sets the UDP destination port
   to 8060 (i.e., the IANA-registered port number for AERO) when AERO-
   only encapsulation is used.  For packets sent to a neighboring
   Client, the AERO interface sets the UDP destination port to the port
   value stored in the neighbor cache entry for this neighbor.

   The AERO interface next sets the IP protocol number in the
   encapsulation header to the appropriate value for the first protocol
   layer within the encapsulation (e.g., IPv4, IPv6, UDP, IPsec, etc.).
   When IPv6 is used as the encapsulation protocol, the interface then
   sets the flow label value in the encapsulation header the same as
   described in [RFC6438].  When IPv4 is used as the encapsulation
   protocol, the AERO interface sets the DF bit as discussed in
   Section 3.7.

   AERO interfaces decapsulate packets destined either to the node
   itself or to a destination reached via an interface other than the
   AERO interface the packet was received on.  When AERO UDP
   encapsulation is used (i.e., when a UDP header with destination port
   8060 is present) the interface examines the first octet of the
   encapsulated packet.  If the most significant four bits of the first



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   octet encode the value '0110' (i.e., the version number value for
   IPv6) or the value '0100' (i.e., the version number value for IPv4),
   the packet is accepted and the encapsulating UDP header is discarded;
   otherwise, the packet is discarded.

   Further decapsulation then proceeds according to the appropriate
   tunnel type [RFC2003][RFC2473][RFC4213][RFC4301][RFC5246] (etc.).

3.8.  AERO Interface Data Origin Authentication

   AERO nodes employ simple data origin authentication procedures for
   encapsulated packets they receive from other nodes.  In particular,
   AERO Clients accept encapsulated packets with a link-layer source
   address belonging to one of their current AERO Servers, and AERO
   Clients and Servers accept encapsulated packets with a link-layer
   source address that is correct for the network-layer source address.

   The AERO node considers the link-layer source address correct for the
   network-layer source address if there is an AERO interface neighbor
   cache entry with an AERO address that matches the packet's network-
   layer source address prefix, with a link-layer address that matches
   the packet's link-layer source address, and AcceptTime is non-zero.

   An AERO Server also accepts packets with a link-layer source address
   that matches one of its associated Relays, and an AERO Relay accepts
   packets with a source address that matches one of its associated
   Servers.

   Note that this simple data origin authentication only applies to
   environments in which link-layer addresses cannot be spoofed.
   Additional security mitigations may be necessary in other
   environments.

3.9.  AERO Interface MTU Considerations

   The AERO link Maximum Transmission Unit (MTU) is 64KB minus the
   encapsulation overhead for IPv4 as the link-layer [RFC0791] and 4GB
   minus the encapsulation overhead for IPv6 as the link layer
   [RFC2675].  This is the most that IPv4 and IPv6 (respectively) can
   convey within the constraints of protocol constants, but actual sizes
   available for tunneling will frequently be much smaller.

   The base tunneling specifications for IPv4 and IPv6 typically set a
   static MTU on the tunnel interface to 1500 bytes minus the
   encapsulation overhead or smaller still if the tunnel is likely to
   incur additional encapsulations on the path.  This can result in path
   MTU related black holes when packets that are too large to be
   accommodated over the AERO link are dropped, but the resulting ICMP



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   Packet Too Big (PTB) messages are lost on the return path.  As a
   result, AERO nodes use the following MTU mitigations to accommodate
   larger packets.

   AERO nodes set their AERO interface MTU to the larger of the
   underlying interface MTU minus the encapsulation overhead, and 1500
   bytes.  (If there are multiple underlying interfaces, the node sets
   the AERO interface MTU according to the largest underlying interface
   MTU, or 64KB /4G minus the encapsulation overhead if the largest MTU
   cannot be determined.)  AERO nodes optionally cache other per-
   neighbor MTU values in the underlying IP path MTU discovery cache
   initialized to the underlying interface MTU.

   AERO nodes admit packets that are no larger than 1280 bytes minus the
   encapsulation overhead (*) as well as packets that are larger than
   1500 bytes into the tunnel without fragmentation, i.e., as long as
   they are no larger than the AERO interface MTU before encapsulation
   and also no larger than the cached per-neighbor MTU following
   encapsulation.  For IPv4, the node sets the "Don't Fragment" (DF) bit
   to 0 for packets no larger than 1280 bytes minus the encapsulation
   overhead (*) and sets the DF bit to 1 for packets larger than 1500
   bytes.  If a large packet is lost in the path, the node may
   optionally cache the MTU reported in the resulting PTB message or may
   ignore the message, e.g., if there is a possibility that the message
   is spurious.

   For packets destined to an AERO node that are larger than 1280 bytes
   minus the encapsulation overhead (*) but no larger than 1500 bytes,
   the node uses IP fragmentation to fragment the encapsulated packet
   into two pieces (where the first fragment contains 1024 bytes of the
   original IP packet) then admits the fragments into the tunnel.  If
   the link-layer protocol is IPv4, the node admits each fragment into
   the tunnel with DF set to 0 and subject to rate limiting to avoid
   reassembly errors [RFC4963][RFC6864].  For both IPv4 and IPv6, the
   node also sends a 1500 byte probe message (**) to the neighbor,
   subject to rate limiting.

   To construct a probe, the node prepares an NS message with a Nonce
   option plus trailing padding octets added to a length of 1500 bytes
   without including the length of the padding in the IPv6 Payload
   Length field.  The node then encapsulates the NS in the encapsulation
   headers (while including the length of the padding in the
   encapsulation header length fields), sets DF to 1 (for IPv4) and
   sends the padded NS message to the neighbor.  If the neighbor returns
   an NA message with a correct Nonce value, the node may then send
   whole packets within this size range and (for IPv4) relax the rate
   limiting requirement.  (Note that the trailing padding SHOULD NOT be
   included within the Nonce option itself but rather as padding beyond



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   the last option in the NS message; otherwise, the (large) Nonce
   option would be echoed back in the solicited NA message and may be
   lost at a link with a small MTU along the reverse path.)

   AERO nodes MUST be capable of reassembling packets up to 1500 bytes
   plus the encapsulation overhead length.  It is therefore RECOMMENDED
   that AERO nodes be capable of reassembling at least 2KB.

   (*) Note that if it is known without probing that the minimum Path
   MTU to an AERO node is MINMTU bytes (where 1280 < MINMTU < 1500) then
   MINMTU can be used instead of 1280 in the fragmentation threshold
   considerations listed above.

   (**) It is RECOMMENDED that no probes smaller than 1500 bytes be used
   for MTU probing purposes, since smaller probes may be fragmented if
   there is a nested tunnel somewhere on the path to the neighbor.
   Probe sizes larger than 1500 bytes MAY be used, but may be
   unnecessary since original sources are expected to implement
   [RFC4821] when sending large packets.

3.10.  AERO Router Discovery, Prefix Delegation and Address
       Configuration

3.10.1.  AERO Client Behavior

   AERO Clients discover the link-layer addresses of AERO Servers via
   static configuration, or through an automated means such as DNS name
   resolution.  In the absence of other information, the Client resolves
   the Fully-Qualified Domain Name (FQDN) "linkupnetworks.[domainname]"
   where "[domainname]" is the connection-specific DNS suffix for the
   Client's underlying network connection.  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 an ACP through DHCPv6 PD [RFC3315][RFC3633][RFC6355] using
   'fe80::ffff:ffff:ffff:ffff' as the IPv6 source address,
   'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address
   and the link-layer address of the Server as the link-layer
   destination address.  The Client includes a DHCPv6 Unique Identifier
   (DUID) in the Client Identifier option of its DHCPv6 messages (as
   well as a DHCPv6 authentication option if necessary) to identify
   itself to the DHCPv6 server.  The Client also includes a DHCPv6
   Client Link Layer Address Option (CLLAO) [RFC6939] with the link-
   layer address format shown in Figure 2 with Link ID followed by
   Preference followed by the values 0 for both the UDP Port Number and
   IP Address.The Client sets the CLLAO 'option-length' field to 22 (2
   plus the length of the link-layer address) and sets the 'link-layer



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   type' field to TBD1 (see: IANA Considerations).  If the Client is
   pre-provisioned with an ACP associated with the AERO service, it MAY
   also include the ACP in its DHCPv6 PD Request to indicate its
   preferred ACP to the DHCPv6 server.  The Client then sends the
   encapsulated DHCPv6 request via an underlying interface.

   When the Client receives its ACP and the set of ASPs via a Reply from
   the DHCPv6 server, i.e., via an AERO Server acting as a DHCPv6 relay,
   it creates a neighbor cache entry with the Server's link-local
   address (i.e., fe80::ID) as the network-layer address and the
   Server's encapsulation address as the link-layer address.

   The Client then applies the AERO address to the AERO interface and
   sub-delegates the ACP to nodes and links within its attached EUNs
   (the AERO address thereafter remains stable as the Client moves).
   The Client also assigns a default IP route to the AERO interface as a
   route-to-interface, i.e., with no explicit next-hop.

   The Client subsequently renews its ACP delegation by performing
   DHCPv6 Renew/Reply exchanges with its AERO address as the IPv6 source
   address, 'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination
   address, the link-layer address of a Server as the link-layer
   destination address and the same DUID and authentication information.
   If the Client wishes to associate with multiple Servers, it can
   perform DHCPv6 Renew/Reply exchanges via each of the Servers.

   The Client then sends an RS message to each of its associated Servers
   to receive an RA message with a Router Lifetime and any other link-
   specific parameters.  The Client uses the Router Lifetime to set the
   lifetime for the neighbor cache entry for this Server.  The Client
   further ignores any RS messages it might receive, since only Servers
   may process RS messages.

   The Client then sends periodic RS messages to obtain new RA messages,
   and further initiates a new DHCPv6 Renew/Reply exchange before the
   Router Lifetime expires.  The Client can also forward IP packets
   destined to networks beyond its local EUNs via a Server as a default
   router.

   Since the Client's AERO address is configured from the unique ACP
   delegation it receives, there is no need for Duplicate Address
   Detection (DAD) on AERO links.  Other nodes maliciously attempting to
   hijack an authorized Client's AERO address will be denied access to
   the network by the DHCPv6 server due to an unacceptable link-layer
   address and/or security parameters (see: Security Considerations).

   AERO Clients ignore the IP address and UDP port number in any S/TLLAO
   options in ND messages they receive directly from another AERO



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   Client, but examine the Link ID and Preference values to match the
   message with the correct link-layer address information.

   When a source Client forwards a packet to a prospective destination
   Client (i.e., one for which the packet's destination address is
   covered by an ASP), the source Client initiates an AERO route
   optimization procedure as specified in Section 3.12.

3.10.2.  AERO Server Behavior

   AERO Servers configure a DHCPv6 relay function on their AERO links.
   AERO Servers arrange to add their encapsulation layer IP addresses
   (i.e., their link-layer addresses) to the DNS resource records for
   the FQDN "linkupnetworks.[domainname]" before entering service.
   Here, "linkupnetworks" is a constant text string, and "[domainname]"
   is the connection-specific DNS suffix for this underlying network
   connection.

   When an AERO Server relays a prospective Client's DHCPv6 PD messages
   to the DHCPv6 server, it wraps each message in a "Relay-forward"
   message per [RFC3315] and includes a DHCPv6 Interface Identifier
   option that encodes a value that identifies the AERO link to the
   DHCPv6 server.  Without creating internal state, the Server then
   modifies the Client's link-layer address in the CLLAO [RFC6939] by
   writing the client's UDP Port number and IP adddress in the
   corresponding fields of the option.  The Server finally includes a
   DHCPv6 Echo Request Option (ERO) [RFC4994] that encodes the option
   code for the CLLAO in a 'requested-option-code-n' field, then relays
   the message to the DHCPv6 server.  The CLLAO information will
   therefore subsequently be echoed back in the DHCPv6 server's "Relay-
   reply" message.

   When the DHCPv6 server issues the ASPs and ACP prefix delegation in a
   "Relay-reply" message via the AERO Server (acting as a DHCPv6 relay),
   the Server obtains the Client's link-layer address from the echoed
   CLLAO option and also obtains the Client's delegated ACP and lease
   lifetime from the message.  The Server then creates a neighbor cache
   entry for the Client's AERO address with the Client's link-layer
   address as the link-layer address and with lifetime set to no more
   than the lease lifetime.  The Server finally injects the ACP as an IP
   route into the inter-Server/Relay routing system (see: Section 3.11)
   then relays the DHCPv6 message to the Client while using fe80::ID as
   the IPv6 source address, the link-local address found in the "peer
   address" field of the Relay-reply message as the IPv6 destination
   address, and the Client's link-layer address as the destination link-
   layer address.





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   Servers respond to NS/RS messages from Clients on their AERO
   interfaces by returning an NA/RA message.  When the Server returns an
   RA message, it sets Router Lifetime to the neighbor cache entry
   lifetime but does not include any Prefix Information Options (PIOs)
   since the AERO link is link-local-only.  The server decrements the
   neighbor cache entry lifetime according to the system clock.

   Servers ignore any RA messages they may receive from a Client, but
   they MAY examine RA messages received from other Servers for
   consistency verification purposes.

3.10.3.  DHCPv6 Server Behavior

   The DHCPv6 server observes both the base DHCPv6 specification
   [RFC3315] and the DHCPv6 PD specification [RFC3633].  The DHCPv6
   server further MUST honor the DHCPv6 Echo Request Option (ERO) and
   Client Link-Layer Address Option (CLLAO) as discussed in
   Section 3.10.1.

   The DHCPv6 server also includes a DHCPv6 Vendor-Specific Information
   Option with 'enterprise-number' set to "TBD2" (see: IANA
   Considerations).  The option is formatted as shown in[RFC3315] and
   with the AERO enterprise-specific format shown in Figure 3:




























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        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      OPTION_VENDOR_OPTS       |           option-len          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   enterprise-number ("TBD2")                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Reserved                    | Prefix Length |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                             ASP (1)                           +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Reserved                    | Prefix Length |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                             ASP (2)                           +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Reserved                    | Prefix Length |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                             ASP (3)                           +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                             (etc.)                            .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 3: AERO Vendor-Specific Information Option

   Per Figure 3, the option includes one or more ASP.  The Prefix Length
   field must contain a value between 0 - 64 and the ASP field contains
   the leading 64 bits of the ASP.  When the Client receives an AERO
   Vendor-Specific Information Option it accepts the option and caches
   each ASP that observes the format specified above.  If the Client
   cannot parse the ASPs, it ignores the option.

3.11.  AERO Relay/Server Routing System

   Relays require full topology information of all Client/Server
   associations, while individual Servers only require partial topology
   information, i.e., they only need to know the ACPs associated with
   their current set of associated Clients.  This is accomplished
   through the use of an internal instance of the Border Gateway
   Protocol (BGP) [RFC4271] coordinated between Servers and Relays.
   This internal BGP instance does not interact with the public Internet
   BGP instance; therefore, the AERO link is presented to the IP




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   Internetwork as a small set of ASPs as opposed to the full set of
   individual ACPs.

   In a reference BGP arrangement, each AERO Server is configured as an
   Autonomous System Border Router (ASBR) for a stub Autonomous System
   (AS) (possibly using a private AS Number (ASN) [RFC1930]), and each
   Server further peers with each Relay but does not peer with other
   Servers.  Similarly, Relays need not peer with each other, since they
   will receive all updates from all Servers and will therefore have a
   consistent view of the AERO link ACP delegations.

   Each Server maintains a working set of associated Clients, and
   dynamically announces new ACPs and withdraws departed ACPs in its BGP
   updates to Relays.  Relays do not send BGP updates to Servers,
   however, such that the BGP route reporting is unidirectional from the
   Servers to the Relays.

   The Relays therefore discover the full topology of the AERO link in
   terms of the working set of ACPs associated with each Server, while
   the Servers only discover the ACPs of their associated Clients.
   Since Clients are expected to remain associated with their current
   set of Servers for extended timeframes, the amount of BGP control
   messaging between Servers and Relays should be minimal.  However, BGP
   peers SHOULD dampen any route oscillations caused by impatient
   Clients that repeatedly associate and disassociate with Servers.

3.12.  AERO Redirection

3.12.1.  Reference Operational Scenario

   Figure 4 depicts the AERO redirection 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 ('R'), two AERO Servers ('S1', 'S2'),
   two AERO Clients ('A', 'B') and two ordinary IPv6 hosts ('C', 'D'):
















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            +--------------+  +--------------+  +--------------+
            |   Server S1  |  |    Relay R   |  |   Server S2  |
            |    Nbr: A    |  |(C->S1; D->S2)|  |    Nbr: B    |
            +--------------+  +--------------+  +--------------+
                fe80::2            fe80::1           fe80::3
                 L2(S1)             L2(R)             L2(S2)
                   |                  |                 |
       X-----+-----+------------------+-----------------+----+----X
             |       AERO Link                               |
            L2(A)                                          L2(B)
     fe80::2001:db8:0:0                              fe80::2001:db8:1:0
     +--------------+                                 +--------------+
     | AERO Client A|                                 | AERO Client B|
     | (default->S1)|                                 | (default->S2)|
     +--------------+                                 +--------------+
     2001:DB8:0::/48                                  2001:DB8:1::/48
             |                                                |
            .-.                                              .-.
         ,-(  _)-.   2001:db8:0::1      2001:db8:1::1     ,-(  _)-.
      .-(_  IP   )-.   +---------+      +---------+    .-(_  IP   )-.
    (__    EUN      )--|  Host C |      |  Host D |--(__    EUN      )
       `-(______)-'    +---------+      +---------+     `-(______)-'

               Figure 4: AERO Reference Operational Scenario

   In Figure 4, Relay ('R') applies the address fe80::1 to its AERO
   interface with link-layer address L2(R), Server ('S1') applies the
   address fe80::2 with link-layer address L2(S1),and Server ('S2')
   applies 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 ('A') receives the ACP 2001:db8:0::/48 in a DHCPv6 PD
   exchange via AERO Server ('S1') then assigns the address
   fe80::2001:db8:0:0 to its AERO interface with link-layer address
   L2(A).  Client ('A') 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 ('C') connects to the EUN, and configures the
   address 2001:db8:0::1.

   AERO Client ('B') receives the ACP 2001:db8:1::/48 in a DHCPv6 PD
   exchange via AERO Server ('S2') then assigns the address
   fe80::2001:db8:1:0 to its AERO interface with link-layer address
   L2(B).  Client ('B') configures a default route and neighbor cache
   entry via the AERO interface with next-hop address fe80::3 and link-
   layer address L2(S2), then sub-delegates the ACP to its attached




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   EUNs.  IPv6 host ('D') connects to the EUN, and configures the
   address 2001:db8:1::1.

3.12.2.  Concept of Operations

   Again, with reference to Figure 4, when source host ('C') sends a
   packet to destination host ('D'), the packet is first forwarded over
   the source host's attached EUN to Client ('A').  Client ('A') then
   forwards the packet via its AERO interface to Server ('S1') and also
   sends a Predirect message toward Client ('B') via Server ('S1').
   Server ('S1') then re-encapsulates and forwards both the packet and
   the Predirect message out the same AERO interface toward Client ('B')
   via Relay ('R').

   When Relay ('R') receives the packet and Predirect message, it
   consults its forwarding table to discover Server ('S2') as the next
   hop toward Client ('B').  Relay ('R') then forwards both messages to
   Server ('S2'), which then forwards them to Client ('B').

   After Client ('B') receives the Predirect message, it process the
   message and returns a Redirect message toward Client ('A') via Server
   ('S2').  During the process, Client ('B') also creates or updates a
   neighbor cache entry for Client ('A').

   When Server ('S2') receives the Redirect message, it re-encapsulates
   the message and forwards it on to Relay ('R'), which forwards the
   message on to Server ('S1') which forwards the message on to Client
   ('A').  After Client ('A') receives the Redirect message, it
   processes the message and creates or updates a neighbor cache entry
   for Client ('C').

   Following the above Predirect/Redirect message exchange, forwarding
   of packets from Client ('A') to Client ('B') without involving any
   intermediate nodes is enabled.  The mechanisms that support this
   exchange are specified in the following sections.

3.12.3.  Message Format

   AERO Redirect/Predirect messages use the same format as for ICMPv6
   Redirect messages depicted in Section 4.5 of [RFC4861], but also
   include a new "Prefix Length" field taken from the low-order 8 bits
   of the Redirect message Reserved field.  (For IPv6, valid values for
   the Prefix Length field are 0 through 64; for IPv4, valid values are
   0 through 32.)  The Redirect/Predirect messages are formatted as
   shown in Figure 5:






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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 (=137)  |  Code (=0/1)  |          Checksum             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Reserved                    | Prefix Length |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                       Target Address                          +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                     Destination Address                       +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Options ...
       +-+-+-+-+-+-+-+-+-+-+-+-

             Figure 5: AERO Redirect/Predirect Message Format

3.12.4.  Sending Predirects

   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 a Predirect message forward toward the destination
   Client via the Server.

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

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

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




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

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

   o  the Type is set to 137.

   o  the Code is set to 1 to indicate "Predirect".

   o  the Prefix Length is set to the length of the prefix to be applied
      to the Target Address.

   o  the Target Address is set to fe80::2001:db8:0:0 (i.e., the AERO
      address of Client ('A')).

   o  the Destination Address is set to the source address of the
      originating packet that triggered the Predirection event.  (If the
      originating packet is an IPv4 packet, the address is constructed
      in IPv4-compatible IPv6 address format).

   o  the message includes a TLLAO with Link ID and Preference set to
      appropriate values for Client ('A')'s underlying interface, and
      with UDP Port Number and IP Address set to 0'.

   o  the message SHOULD include a Timestamp option.

   o  the message includes a Redirected Header Option (RHO) that
      contains the originating packet truncated to ensure that at least
      the network-layer header is included but the size of the message
      does not exceed 1280 bytes.

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

3.12.5.  Re-encapsulating and Relaying Predirects

   When Server ('S1') receives a Predirect message from Client ('A'), it
   first verifies that the requested redirection is authorized.  If the
   redirection is not permitted, Server ('S1') discards the message.
   Otherwise, Server ('S1') validates the message according to the
   ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861],
   except that the Predirect has Code=1.  Server ('S1') also verifies
   that Client ('A') is authorized to use the Prefix Length in the
   Predirect when applied to the AERO address in the network-layer



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   source address by searching for the AERO address in the neighbor
   cache.  If validation fails, Server ('S1') discards the Predirect;
   otherwise, it copies the correct UDP Port number and IP Address for
   Client ('A') into the (previously empty) TLLAO.

   Server ('S1') then examines the network-layer destination address of
   the Predirect to determine the next hop toward Client ('B') by
   searching for the AERO address in the neighbor cache.  Since Client
   ('B') is not one of its neighbors, Server ('S1') re-encapsulates the
   Predirect and relays it via Relay ('R') by changing the link-layer
   source address of the message to 'L2(S1)' and changing the link-layer
   destination address to 'L2(R)'.  Server ('S1') finally forwards the
   re-encapsulated message to Relay ('R') without decrementing the
   network-layer TTL/Hop Limit field.

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

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

3.12.6.  Processing Predirects and Sending Redirects

   When Client ('B') receives the Predirect message, it accepts the
   Predirect only if the message has a link-layer source address of one
   of its Servers (e.g., L2(S2)).  Client ('B') further accepts the
   message only if it is willing to serve as a redirection target.
   Next, Client ('B') validates the message according to the ICMPv6
   Redirect message validation rules in Section 8.1 of [RFC4861], except
   that it accepts the message even though Code=1 and even though the
   network-layer source address is not that of it's current first-hop
   router.

   In the reference operational scenario, when Client ('B') receives a
   valid Predirect message, it either creates or updates a neighbor
   cache entry that stores the Target Address of the message as the
   network-layer address of Client ('A') , stores the link-layer address
   found in the TLLAO as the link-layer address(es) of Client ('A') and
   stores the Prefix Length as the length to be applied to the network-




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   layer address for forwarding purposes.  Client ('B') then sets
   AcceptTime for the neighbor cache entry to ACCEPT_TIME.

   After processing the message, Client ('B') prepares a Redirect
   message response as follows:

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

   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 AERO address of Client ('B')).

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

   o  the Type is set to 137.

   o  the Code is set to 0 to indicate "Redirect".

   o  the Prefix Length is set to the length of the prefix to be applied
      to the Target Address.

   o  the Target Address is set to fe80::2001:db8:1:0 (i.e., the AERO
      address of Client ('B')).

   o  the Destination Address is set to the destination address of the
      originating packet that triggered the Redirection event.  (If the
      originating packet is an IPv4 packet, the address is constructed
      in IPv4-compatible IPv6 address format).

   o  the message includes a TLLAO with Link ID and Preference set to
      appropriate values for Client ('B')'s underlying interface, and
      with UDP Port Number and IP Address set to '0'.

   o  the message SHOULD include a Timestamp option.

   o  the message includes as much of the RHO copied from the
      corresponding AERO Predirect message as possible such that at
      least the network-layer header is included but the size of the
      message does not exceed 1280 bytes.

   After Client ('B') prepares the Redirect message, it sends the
   message to Server ('S2').





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3.12.7.  Re-encapsulating and Relaying Redirects

   When Server ('S2') receives a Redirect message from Client ('B'), it
   first verifies that the requested redirection is authorized.  If the
   redirection is not permitted, Server ('S2') discards the message.
   Otherwise, Server ('S2') validates the message according to the
   ICMPv6 Redirect message validation rules in Section 8.1 of [RFC4861].
   Server ('S2') also verifies that Client ('B') is authorized to use
   the Prefix Length in the Redirect when applied to the AERO address in
   the network-layer source address by searching for the AERO address in
   the neighbor cache.  If validation fails, Server ('S2') discards the
   Predirect; otherwise, it copies the correct UDP Port number and IP
   Address for Client ('B') into the (previously empty) TLLAO.

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

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

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

3.12.8.  Processing Redirects

   When Client ('A') receives the Redirect message, it accepts the
   message only if it has a link-layer source address of one of its
   Servers (e.g., ''L2(S1)').  Next, Client ('A') validates the message
   according to the ICMPv6 Redirect message validation rules in
   Section 8.1 of [RFC4861], except that it accepts the message even
   though the network-layer source address is not that of it's current
   first-hop router.  Following validation, Client ('A') then processes
   the message as follows.



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   In the reference operational scenario, when Client ('A') receives the
   Redirect message, it either creates or updates a neighbor cache entry
   that stores the Target Address of the message as the network-layer
   address of Client ('B'), stores the link-layer address found in the
   TLLAO as the link-layer address of Client ('B') and stores the Prefix
   Length as the length to be applied to the network-layer address for
   forwarding purposes.  Client ('A') then sets ForwardTime for the
   neighbor cache entry to FORWARD_TIME.

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

3.12.9.  Server-Oriented Redirection

   In some environments, the Server nearest the destination Client may
   need to serve as the redirection target, e.g., if direct Client-to-
   Client communications are not possible.  In that case, the Server
   prepares the Redirect message the same as if it were the destination
   Client (see: Section 3.9.6), except that it writes its own link-layer
   address in the TLLAO option.  The Server must then maintain a
   neighbor cache entry for the redirected source Client.

3.13.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform NUD by sending unicast NS messages to elicit
   solicited NA messages from neighbors the same as described in
   [RFC4861].  When an AERO node sends an NS/NA message, it MUST use its
   AERO address as the IPv6 source address and the link-local address of
   the neighbor as the IPv6 destination address.  When an AERO node
   receives an NS message or a solicited NA message, it accepts the
   message if it has a neighbor cache entry for the neighbor; otherwise,
   it ignores the message.

   When a source Client is redirected to a target Client it SHOULD test
   the direct path by sending an initial NS message to elicit a
   solicited NA response.  While testing the path, the source Client can
   optionally continue sending packets via the Server, maintain a small
   queue of packets until target reachability is confirmed, or
   (optimistically) allow packets to flow directly to the target.  The
   source Client SHOULD thereafter continue to test the direct path to




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   the target Client (see Section 7.3 of [RFC4861]) periodically in
   order to keep neighbor cache entries alive.

   In particular, while the source Client is actively sending packets to
   the target Client it SHOULD also send NS messages separated by
   RETRANS_TIMER milliseconds in order to receive solicited NA messages.
   If the source Client is unable to elicit a solicited NA response from
   the target Client after MAX_RETRY attempts, it SHOULD set ForwardTime
   to 0 and resume sending packets via the Server which may or may not
   result in a new redirection event.  Otherwise, the source Client
   considers the path usable and SHOULD thereafter process any link-
   layer errors as a hint that the direct path to the target Client has
   either failed or has become intermittent.

   When a target Client receives an NS message from a source Client, it
   resets AcceptTime to ACCEPT_TIME if a neighbor cache entry exists;
   otherwise, it discards the NS message.

   When a source Client receives a solicited NA message from a target
   Client, it resets ForwardTime to FORWARD_TIME if a neighbor cache
   entry exists; otherwise, it discards the NA message.

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

3.14.  Mobility Management

3.14.1.  Announcing Link-Layer Address Changes

   When a Client needs to change its link-layer address, e.g., due to a
   mobility event, it performs an immediate DHCPv6 Renew/Reply via each
   of its Servers using the new link-layer address as the source.  The
   DHCPv6 Server will re-authenticate the Client and (assuming
   authentication succeeds) the DHCPv6 Renew/Reply exchange will update
   each Server's neighbor cache.

   Next, the Client sends unsolicited NA messages to each of its active
   neighbors using the same procedures as specified in Section 7.2.6 of
   [RFC4861], except that it sends the messages as unicast to each
   neighbor via a Server instead of multicast.  In this process, the
   Client should send no more than MAX_NEIGHBOR_ADVERTISEMENT messages
   separated by no less than RETRANS_TIMER seconds to each neighbor.




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   With reference to Figure 4, Client ('B') sends unicast unsolicited NA
   messages to Client ('A') via Server ('S2') as follows:

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

   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 AERO address of Client ('B')).

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

   o  the Type is set to 136.

   o  the Code is set to 0.

   o  the Solicited flag is set to 0.

   o  the Override flag is set to 1.

   o  the Target Address is set to fe80::2001:db8:1:0 (i.e., the AERO
      address of Client ('B')).

   o  the message includes a TLLAO with Link ID and Preference set to
      appropriate values for Client ('B')'s underlying interface, and
      with UDP Port Number and IP Address set to '0'.

   o  the message SHOULD include a Timestamp option.

   When Server ('S1') receives the NA message, it relays the message in
   the same way as described for relaying Redirect messages in
   Section 3.12.7.  In particular, Server ('S1') copies the correct UDP
   port number and IP address into the TLLAO, changes the link-layer
   source address to its own address, changes the link-layer destination
   address to the address of Relay ('R'), then forwards the NA message
   via the relaying chain the same as for a Redirect.

   When Client ('A') receives the NA message, it accepts the message
   only if it already has a neighbor cache entry for Client ('B') then
   updates the link-layer address for Client ('B') based on the address
   in the TLLAO.  However, Client ('A') MUST NOT update ForwardTime
   since Client ('B') will not have updated AcceptTime.

   Note that these unsolicited NA messages are unacknowledged; hence,
   Client ('B') has no way of knowing whether Client ('A') has received



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   them.  If the messages are somehow lost, however, Client ('A') will
   soon learn of the mobility event via the NUD procedures specified in
   Section 3.13.

3.14.2.  Moving to a New Server

   When a Client associates with a new Server, it issues a new DHCPv6
   Renew message via the new Server as the DHCPv6 relay.  The new Server
   then relays the message to the DHCPv6 server and processes the
   resulting exchange.  After the Client receives the resulting DHCPv6
   Reply message, it sends an RS message to the new Server to receive a
   new RA message.

   When a Client disassociates with an existing Server, it sends a
   "terminating RS" message to the old Server.  The terminating RS
   message is prepared exactly the same as for an ordinary RS message,
   except that the Code field contains the value '1'.  When the old
   Server receives the terminating RS message, it withdraws the IP route
   from the routing system and deletes the neighbor cache entry for the
   Client.  The old Server then returns an RA message with default
   router lifetime set to 0 which the Client can use to verify that the
   termination signal has been processed.  The client then deletes both
   the default route and the neighbor cache entry for the old Server.
   The old Server SHOULD impose a small delay before deleting the
   neighbor cache entry so that any packets already in the system can
   still be delivered to the Client.

   Clients SHOULD NOT move rapidly between Servers in order to avoid
   causing unpredictable oscillations in the Server/Relay routing
   system.  Such oscillations could result in intermittent reachability
   for the Client itself, while causing little harm to the network due
   to routing protocol dampening.  Examples of when a Client may change
   to a different Server include a Server that has gone unreachable,
   topological movements of significant distance, etc.

3.15.  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 redirection
   messages and continue to send packets via the Server.








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3.16.  Multicast Considerations

   When the underlying network does not support multicast, AERO nodes
   map IPv6 link-scoped multicast addresses (including
   'All_DHCP_Relay_Agents_and_Servers') to the link-layer address of a
   Server.

   When the underlying network supports multicast, AERO nodes use the
   multicast address mapping specification found in [RFC2529] for IPv4
   underlying networks and use a direct multicast mapping for IPv6
   underlying networks.  (In the latter case, "direct multicast mapping"
   means that if the IPv6 multicast destination address of the
   encapsulated packet is "M", then the IPv6 multicast destination
   address of the encapsulating header is also "M".)

3.17.  Operation on AERO Links Without DHCPv6 Services

   When the AERO link does 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.

3.18.  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 Predirect/Redirect 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 prefix delegation
   authority through some means outside the scope of this document.

4.  Implementation Status

   An application-layer implementation is in progress.








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5.  IANA Considerations

   The IANA is instructed to assign a new 2-octet Hardware Type number
   "TBD1" for AERO in the "arp-parameters" registry per Section 2 of
   [RFC5494].  The number is assigned from the 2-octet Unassigned range
   with Hardware Type "AERO" and with this document as the reference.

   The IANA is further instructed to assign a 4-octet Enterprise Number
   "TBD2" for AERO in the "enterprise-numbers" registry per [RFC3315].

6.  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 trust anchor.  AERO also uses DHCPv6
   authentication for Client authentication and network admission
   control.

   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 that
   is often sufficient.  In other instances, additional securing
   mechanisms such as Secure Neighbor Discovery (SeND) [RFC3971], IPsec
   [RFC4301] or TLS [RFC5246] may be necessary.

   AERO Clients MUST ensure that their connectivity is not used by
   unauthorized nodes on 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 unauthorized nodes via a NAT function.)

   On some AERO links, establishment and maintenance of a direct path
   between neighbors requires secured coordination such as through the
   Internet Key Exchange (IKEv2) protocol [RFC5996] to establish a
   security association.

7.  Acknowledgements

   Discussions both on IETF lists and in private exchanges helped shape
   some of the concepts in this work.  Individuals who contributed
   insights include Mikael Abrahamsson, Fred Baker, Stewart Bryant,
   Brian Carpenter, Wojciech Dec, Brian Haberman, Joel Halpern, Sascha
   Hlusiak, Lee Howard, Joe Touch and Bernie Volz.  Members of the IESG



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   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.

   This work has further been encouraged and supported by Boeing
   colleagues including Keith Bartley, Dave Bernhardt, Cam Brodie,
   Balaguruna Chidambaram, Wen Fang, Anthony Gregory, Jeff Holland, Ed
   King, Gen MacLean, Kent Shuey, Brian Skeen, Mike Slane, Julie Wulff,
   Yueli Yang, and other members of the BR&T and BIT mobile networking
   teams.

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

8.  References

8.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.





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   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

   [RFC6434]  Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
              Requirements", RFC 6434, December 2011.

8.2.  Informative References

   [RFC0879]  Postel, J., "TCP maximum segment size and related topics",
              RFC 879, November 1983.

   [RFC1930]  Hawkinson, J. and T. Bates, "Guidelines for creation,
              selection, and registration of an Autonomous System (AS)",
              BCP 6, RFC 1930, March 1996.

   [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
              Domains without Explicit Tunnels", RFC 2529, March 1999.

   [RFC2675]  Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
              RFC 2675, August 1999.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

   [RFC4963]  Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
              Errors at High Data Rates", RFC 4963, July 2007.






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   [RFC4994]  Zeng, S., Volz, B., Kinnear, K., and J. Brzozowski,
              "DHCPv6 Relay Agent Echo Request Option", RFC 4994,
              September 2007.

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              March 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5494]  Arkko, J. and C. Pignataro, "IANA Allocation Guidelines
              for the Address Resolution Protocol (ARP)", RFC 5494,
              April 2009.

   [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, October 2009.

   [RFC5569]  Despres, R., "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd)", RFC 5569, January 2010.

   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
              5996, September 2010.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

   [RFC6204]  Singh, H., Beebee, W., Donley, C., Stark, B., and O.
              Troan, "Basic Requirements for IPv6 Customer Edge
              Routers", RFC 6204, April 2011.

   [RFC6355]  Narten, T. and J. Johnson, "Definition of the UUID-Based
              DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355, August
              2011.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, November 2011.

   [RFC6691]  Borman, D., "TCP Options and Maximum Segment Size (MSS)",
              RFC 6691, July 2012.

   [RFC6706]  Templin, F., "Asymmetric Extended Route Optimization
              (AERO)", RFC 6706, August 2012.



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   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, February 2013.

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935, April 2013.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, April 2013.

   [RFC6939]  Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer
              Address Option in DHCPv6", RFC 6939, May 2013.

   [RFC6980]  Gont, F., "Security Implications of IPv6 Fragmentation
              with IPv6 Neighbor Discovery", RFC 6980, August 2013.

   [RFC7078]  Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing
              Address Selection Policy Using DHCPv6", RFC 7078, January
              2014.

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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