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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc6706 (if approved)                      September 12, 2014
Intended status: Standards Track
Expires: March 16, 2015


               Transmission of IP Packets over AERO Links
                     draft-templin-aerolink-35.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 DHCPv6 and 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 March 16, 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 . . . . . . . . . . . . . . . .   6
     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  . . . . . . . .  11
     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 . . . . . . . . . . . .  17
     3.10. AERO Interface Error Handling . . . . . . . . . . . . . .  20
     3.11. AERO Router Discovery, Prefix Delegation and Address
           Configuration . . . . . . . . . . . . . . . . . . . . . .  24
       3.11.1.  AERO DHCPv6 Service Model  . . . . . . . . . . . . .  24
       3.11.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  24
       3.11.3.  AERO Server Behavior . . . . . . . . . . . . . . . .  27
     3.12. AERO Relay/Server Routing System  . . . . . . . . . . . .  29
     3.13. AERO Redirection  . . . . . . . . . . . . . . . . . . . .  29
       3.13.1.  Reference Operational Scenario . . . . . . . . . . .  29
       3.13.2.  Concept of Operations  . . . . . . . . . . . . . . .  31
       3.13.3.  Message Format . . . . . . . . . . . . . . . . . . .  31
       3.13.4.  Sending Predirects . . . . . . . . . . . . . . . . .  32
       3.13.5.  Re-encapsulating and Relaying Predirects . . . . . .  33
       3.13.6.  Processing Predirects and Sending Redirects  . . . .  34
       3.13.7.  Re-encapsulating and Relaying Redirects  . . . . . .  36
       3.13.8.  Processing Redirects . . . . . . . . . . . . . . . .  36
       3.13.9.  Server-Oriented Redirection  . . . . . . . . . . . .  37
     3.14. Neighbor Unreachability Detection (NUD) . . . . . . . . .  37
     3.15. Mobility Management . . . . . . . . . . . . . . . . . . .  39
       3.15.1.  Announcing Link-Layer Address Changes  . . . . . . .  39
       3.15.2.  Bringing New Links Into Service  . . . . . . . . . .  40
       3.15.3.  Removing Existing Links from Service . . . . . . . .  40



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       3.15.4.  Moving to a New Server . . . . . . . . . . . . . . .  41
     3.16. Encapsulation Protocol Version Considerations . . . . . .  41
     3.17. Multicast Considerations  . . . . . . . . . . . . . . . .  42
     3.18. Operation on AERO Links Without DHCPv6 Services . . . . .  42
     3.19. Operation on Server-less AERO Links . . . . . . . . . . .  42
     3.20. Extending AERO Links Through Security Gateways  . . . . .  42
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  44
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  44
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  44
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  45
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  46
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  46
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  47
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  49

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 DHCPv6 and IPv6 ND message signalling is used in the control
   plane, both 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.




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

   AERO address
      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 and DHCPv6 messaging over the link.

   AERO Client ("Client")
      a node that applies an AERO address to an AERO interface and
      receives an IP prefix via a DHCPv6 Prefix Delegation (PD) exchange
      with one or more AERO Servers.

   AERO Server ("Server")
      a node that configures an AERO interface to provide default
      forwarding and DHCPv6 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
      DHCPv6 and 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.  A typical example is an
      enterprise network.

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




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   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.

   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.

   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 to support the
   operation of DHCPv6 and 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:




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3.1.  AERO Link Reference Model

                              .-(::::::::)
                           .-(:::: IP ::::)-.
                          (:: Internetwork ::)
                           `-(::::::::::::)-'
                              `-(::::::)-'
                                   |
       +--------------+    +-------+------+    +--------------+
       |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  Relay R act as a default router for its associated Servers S1 and
      S2, and connects the AERO 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 common operational practice, there may be many additional Relays,
   Servers and Clients.

3.2.  AERO Node Types

   AERO Relays provide default forwarding services to AERO Servers.
   Relays forward packets between Servers connected to the same AERO
   link and also forward packets between the AERO link and the native
   Internetwork.  Relays present the AERO link to the native
   Internetwork as a set of one or more ASPs.  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.  AERO Relays
   maintain an AERO interface neighbor cache entry for each AERO Server,
   and maintain an IP forwarding table entry for each AERO Client.

   AERO Servers provide default forwarding services to AERO Clients.
   Each Server also peers with each Relay in a dynamic routing protocol
   instance to advertise its list of associated Clients.  Servers
   configure a DHCPv6 server function to facilitate Prefix Delegation
   (PD) exchanges with Clients.  Each delegated prefix becomes an AERO
   Client Prefix (ACP) taken from an ASP.  Servers forward packets
   between Clients and Relays, as well as between Clients and other
   Clients associated with the same Server.  AERO Servers maintain an
   AERO interface neighbor cache entry for each AERO Relay.  They also
   maintain both a neighbor cache entry and an IP forwarding table entry
   for each of their associated Clients.

   AERO Clients act as requesting routers to receive ACPs through DHCPv6
   PD exchanges with AERO Servers over the AERO link.  (Each Client MAY
   associate with a single Server or with multiple Servers, e.g., for
   fault tolerance and/or load balancing.)  Each IPv6 Client receives at
   least a /64 IPv6 ACP, and may receive even shorter prefixes.
   Similarly, each IPv4 Client receives at least a /32 IPv4 ACP (i.e., a
   singleton IPv4 address), and may receive even shorter prefixes.  AERO
   Clients maintain an AERO interface neighbor cache entry for each of
   their associated Servers as well as for each of their correspondent
   Clients.

   AERO Clients that act as routers sub-delegate portions of their ACPs
   to links on EUNs.  End system applications on 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
   nullified.  End system applications on Clients that act as hosts bind
   directly to the AERO interface.



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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 from the lower 64 bits of 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 ND messages include one or more 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
   Preference is an integer value between 0 and 255 indicating the



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   node's preference for this underlying interface (with 255 being the
   highest preference, 1 being the lowest, and 0 meaning "link
   disabled").  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 DHCPv6 and the IPv6 ND
   protocol as well as to communicate with Relays on the link.  (The
   Server then accepts IPv6 ND solicitation messages destined to either
   the fe80:: or fe80::ID addresses, but always uses fe80::ID as the
   source address of the corresponding advertisements.)  The Server
   further configures a DHCPv6 server function to facilitate DHCPv6 PD
   exchanges with AERO Clients.  The Server also maintains a neighbor
   cache entry for each Relay on the link, and manages per-Client
   neighbor cache entries and IP forwarding table entries based on
   DHCPv6 exchanges.  When the Server receives an NS/RS message on the
   AERO interface it returns an NA/RA message but does not update the
   neighbor cache.  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 DHCPv6 PD to
   receive an ACP from an AERO Server.  Next, it applies the
   corresponding AERO address to the AERO interface and creates a
   neighbor cache entry for the Server, i.e., the PD exchange bootstraps
   the provisioning of a unique link-local address.  The Client



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   maintains a neighbor cache entry for each of its Servers and each of
   its active correspondent Clients.  When the Client receives Redirect/
   Predirect messages on the AERO interface it updates or creates
   neighbor cache entries, including link-layer address information.
   Unsolicited NA messages update the cached link-layer addresses for
   correspondent Clients (e.g., following a link-layer address change
   due to node mobility) but do not create new neighbor cache entries.
   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.  Finally,
   the Client need not maintain any IP forwarding table entries for its
   Servers or correspondent 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.  On
   systems in which adding a deafult route would violate security
   policy, the default route could instead be installed via a
   "synthesized RA" as discussed in Secction 3.11.2.

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.

   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.

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].  AERO interface




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   neighbor cache entires are said to be one of "permanent", "static" or
   "dynamic".

   Permanent neighbor cache entries are created through explicit
   administrative action; they have no timeout values and remain in
   place until explicitly deleted.  AERO Relays maintain 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.

   Static neighbor cache entries are created though DHCPv6 PD exchanges
   and remain in place for durations bounded by prefix lifetimes.  AERO
   Servers maintain a static neighbor cache entry for each of their
   associated Clients, and AERO Clients maintain a static neighbor cache
   for each of their associated Servers.  When an AERO Server sends a
   DHCPv6 Reply message response to a Client's DHCPv6 Solicit or Renew
   message, it creates or updates a static neighbor cache entry based on
   the Client's AERO address as the network-layer address, the prefix
   lifetime as the neighbor cache entry lifetime, the Client's
   encapsulation IP address and UDP port number as the link-layer
   address and the prefix length as the length to apply to the AERO
   address.  When an AERO Client receives a DHCPv6 Reply message from a
   Server, it creates or updates a static neighbor cache entry based on
   the Reply message link-local source address as the network-layer
   address, the prefix lifetime as the neighbor cache entry lifetime,
   and the encapsulation IP source address and UDP source port number as
   the link-layer address.

   Dynamic neighbor cache entries are created based on receipt of an
   IPv6 ND message, and are garbage-collected if not used within a short
   timescale.  AERO Clients maintain dynamic neighbor cache entries for
   each of their active correspondent Clients with lifetimes based on
   IPv6 ND messaging constants.  When an AERO Client receives a valid
   Predirect message it creates or updates a dynamic neighbor cache
   entry for the Predirect target network-layer and link-layer addresses
   plus prefix length.  The node then sets an "AcceptTime" variable in
   the neighbor cache entry and uses this value to determine whether
   packets received from the correspondent can be accepted.  When an
   AERO Client receives a valid Redirect message it creates or updates a
   dynamic neighbor cache entry for the Redirect target network-layer
   and link-layer addresses plus prefix length.  The Client then sets a
   "ForwardTime" variable in the neighbor cache entry and uses this
   value to determine whether packets can be sent directly to the
   correspondent.  The Client also maintains a "MaxRetry" variable to
   limit the number of keepalives sent when a correspondent may have
   gone unreachable.

   For dynamic neighbor cache entries, when an AERO Client receives a
   valid NS message it (re)sets AcceptTime for the neighbor to



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   ACCEPT_TIME.  When an AERO Client receives a valid solicited NA
   message, it (re)sets ForwardTime for the neighbor to FORWARD_TIME and
   sets MaxRetry to MAX_RETRY.  When an AERO Client receives a valid
   unsolicited NA message, it updates the correspondent's link-layer
   addresses but DOES NOT reset AcceptTime, ForwardTime or MaxRetry.

   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.

3.6.  AERO Interface Sending Algorithm

   IP packets enter a node's AERO interface either from the network
   layer (i.e., from a local application or the IP forwarding system),
   or from the link-layer (i.e., from the AERO tunnel virtual link).
   Packets that enter the AERO interface from the network layer are
   encapsulated and admitted into the AERO link (i.e., they are
   tunnelled to an AERO interface neighbor).  Packets that enter the
   AERO interface from the link layer are either re-admitted into the
   AERO link or delivered to the network layer where they are subject to
   either local delivery or IP forwarding.  Since each AERO node has
   only partial information about neighbors on the link, AERO interfaces
   may forward packets with link-local destination addresses at a layer
   below the network layer.  This means that AERO nodes act as both IP
   routers and link-layer "bridges".  AERO interface sending
   considerations for Clients, Servers and Relays are given below.

   When an IP packet enters a Client's AERO interface from the network
   layer, if the destination is covered by an ASP the Client searches
   for a dynamic neighbor cache entry with a non-zero ForwardTime and an
   AERO address that matches the packet's destination address.  (The
   destination address may be either an address covered by the



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   neighbor's ACP or the (link-local) AERO address itself.)  If there is
   a match, the Client uses a link-layer address in the entry as the
   link-layer address for encapsulation then admits the packet into the
   AERO link.  If there is no match, the Client instead uses the link-
   layer address of a neighboring Server as the link-layer address for
   encapsulation.

   When an IP packet enters a Server's AERO interface from the link
   layer, if the destination is covered by an ASP the Server searches
   for a static neighbor cache entry with an AERO address that matches
   the packet's destination address.  (The destination address may be
   either an address covered by the neighbor's ACP or the AERO address
   itself.)  If there is a match, the Server uses a link-layer address
   in the entry as the link-layer address for encapsulation and re-
   admits the packet into the AERO link.  If there is no match, the
   Server instead uses the link-layer address in any permanent neighbor
   cache entry as the link-layer address for encapsulation.  When a
   Server receives a packet from a Relay, the Server MUST NOT loop 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 and admits the packet into the AERO link.  When an IP
   packet enters a Relay's AERO interface from the link-layer, if the
   destination is not a link-local address and is not covered by an ASP
   the Relay removes the packet from the AERO interface and uses IP
   forwarding to forward the packet to the Internetwork.  If the
   destination address is covered by an ASP, and there is a more-
   specific IP 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 and re-
   admits the packet into the AERO link.  If there is no more-specific
   entry, the Relay instead drops the packet and returns an ICMP
   Destination Unreachable message (see: Section 3.10).  When an Relay
   receives a packet from a Server, the Relay MUST NOT forward the
   packet back to the same Server.

   Note that in the above that the link-layer address for encapsulation
   may be through consulting either the neighbor cache or the IP
   forwarding table.  IP forwarding is therefore linked to IPv6 ND via
   the AERO address.

   When an AERO node re-admits a packet into the AERO link, 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 re-admitted into the same AERO link 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 above tunneling specifications 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
   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 correspondent Client, the AERO interface
   sets the UDP destination port to the port value stored in the
   neighbor cache entry for this correspondent.

   The AERO interface also sets the UDP checksum field to zero (see:
   [RFC6935][RFC6936]) for packets that do not require assurance against
   reassembly errors.  For packets that require reassembly checks (see
   Section 3.9), the AERO interface instead (re)calculates the UDP
   checksum and writes the resulting value in the UDP checksum field.

   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 3.8.





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   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 first verifies the UDP checksum in the
   UDP checksum was non-zero then examines the first octet of the
   encapsulated packet.  The packet is accepted if the most significant
   four bits of the first 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).  Otherwise, the packet is accepted if the
   first octet encodes a valid IP protocol number per the IANA
   "protocol-numbers" registry that matches a supported encapsulation
   type.  Otherwise, the packet is discarded.

   Further decapsulation then proceeds according to the appropriate
   tunnel type per the above specifications.

3.8.  AERO Interface Data Origin Authentication

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

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

   o  AERO Servers accept authentic encapsulated DHCPv6 messages, and
      create or update a static neighbor cache entry for the source
      based on the specific message type.

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

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

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

   Note that this simple data origin authentication only applies to
   environments in which link-layer addresses cannot be spoofed.



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   Additional security mitigations may be necessary in other
   environments.

3.9.  AERO Interface MTU Considerations

   The AERO interface is the node's point of attachment to the AERO
   link.  AERO links over IP networks have a maximum link MTU of 64KB
   minus the encapsulation overhead (i.e., "64KB-ENCAPS"), since the
   maximum packet size in the base IP specifications is 64KB
   [RFC0791][RFC2460].  AERO links over IPv6 networks have a theoretical
   maximum link MTU of 4GB-ENCAPS [RFC2675], however IPv6 Jumbograms are
   considered optional for IPv6 nodes [RFC6434] and therefore out of
   scope for this document.

   The IP layer sees the AERO interface as an ordinary interface that
   configures an MTU that is no larger than the link MTU, i.e., the same
   as for any interface.  Routers MAY set an AERO interface MTU up to
   the maximum link MTU for the specific IP protocol version.  Hosts
   SHOULD set a more conservative AERO interface MTU so that upper layer
   protocols will see an appropriate maximum packet size, for example
   when setting an initial TCP Maximum Segment Size (MSS).  In all
   cases, routers and hosts MUST set an MTU of at least 1500 bytes.

   IPv6 specifies a minimum link MTU of 1280 bytes [RFC2460].  This is
   the minimum packet size an AERO interface MUST be capable of
   forwarding without returning an ICMP Packet Too Big (PTB) message.
   Although IPv4 specifies a smaller minimum link MTU of 68 bytes
   [RFC0791], AERO interfaces also observe a 1280 byte minimum for IPv4.
   Additionally, the vast majority of links in the Internet configure an
   MTU of at least 1500 bytes.  Hosts have therefore become conditioned
   to expect that IP packets up to 1500 bytes in length will either be
   delivered to the final destination or a suitable ICMP Packet Too Big
   (PTB) message returned, however such PTB messages are often lost
   [RFC2923].  Therefore, AERO interfaces MUST pass IP packets of at
   least 1500 bytes even if the encapsulated packet must be fragmented.

   PTB messages may be generated by the IP layer of the AERO node if the
   packet is too large to enter the AERO interface, from within the AERO
   interface itself if the packet is larger than 1500 bytes and also
   larger than the MTU of the underlying interface to be used for
   tunneling minus ENCAPS, or from a router within the tunnel after the
   encapsulated packet has been admitted into the tunnel.  The latter
   condition would result in a Layer-2 (L2) PTB message delivered to the
   AERO interface, while the former two conditions would result in a
   Layer-3 (L3) PTB message delivered to the original source.

   For AERO links over IPv4, the IP ID field is only 16 bits in length,
   meaning that fragmentation at high data rates could result in



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   dangerous reassembly misassociations [RFC6864][RFC4963].  For that
   reason, AERO interfaces that send fragmented IPv4-encapsulated
   packets MUST either institute rate limiting to ensure that the IP ID
   field will not wrap before all earlier fragments have been processed,
   or include an integrity check to detect reassembly errors.

   The AERO interface therefore admits packets into the tunnel (using
   fragmentation as necessary) as follows:

   o  For IP packets that are no larger than (1280-ENCAPS) bytes, the
      AERO interface admits the packet into the tunnel without
      fragmentation.  For IPv4 AERO links, the AERO interface sets the
      Don't Fragment (DF) bit to 0 so that these packets will be
      deterministically delivered even if there is a restricting link in
      the path.  The AERO interface need not perform rate limiting or
      include integrity checks for these packets, since any IPv4 links
      in the path that configure an MTU smaller than 1280 bytes are very
      likely to be slow links [RFC3819].

   o  For IP packets that are larger than (1280-ENCAPS) bytes but no
      larger than 1500 bytes, the AERO interface encapsulates the
      packet.  (For IPv4 AERO links, the AERO interface then sets the DF
      bit to 0 and calculates the UDP checksum for the encapsulated
      packet as an integrity check to account for the potential for
      reassembly misassociations.  If the encapsulation does not include
      a UDP header or other integrity check, the AERO interface instead
      MUST institute rate limiting.)  Next, the AERO interface uses IP
      fragmentation to fragment the encapsulated packet into two
      fragments where the first fragment is no larger than 1024 bytes
      and the other fragment is no larger than the first fragment.  The
      AERO interface then admits both fragments into the tunnel.

   o  For IPv4 packets that are larger than 1500 bytes and with the DF
      bit set to 0, the AERO interface fragments the unencapsulated
      packet into a minimum number of fragments where the first fragment
      is no larger than 1024 bytes and all other fragments are no larger
      than the first fragment.  The AERO interface then encapsulates
      each fragment (and for IPv4 sets the DF bit to 0) and sends each
      fragment to the neighbor.  These encapsulated fragments will be
      deterministically delivered to the final destination.  (The AERO
      interface need not perform rate limiting or include integrity
      checks for these packets since it is not the original source of
      the unencapsulated packet.)

   o  For all other IP packets, if the packet is larger than the AERO
      interface MTU the AERO node drops the packet and returns an L3 PTB
      message with MTU set to the AERO interface MTU; otherwise, the
      node admits the packet into the AERO interface.  Next, if the



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      packet length is larger than the MTU of the underlying interface
      to be used for tunneling minus ENCAPS, the AERO interface drops
      the packet and returns an L3 PTB message with MTU set to the
      larger of 1500 or the underlying interface MTU minus ENCAPS.
      Otherwise, the AERO interface encapsulates the packet and admits
      it into the tunnel without fragmentation (and for IPv4 sets the DF
      bit to 1) and translates any L2 PTB messages it may receive from
      the network into corresponding L3 PTB messages to send to the
      original source as specified in Section 3.10.  Since both L2 and
      L3 PTB messages may be either lost or contain insufficient
      information, however, it is RECOMMENDED that sources that send
      unfragmentable IP packets larger than 1500 bytes use Packetization
      Layer Path MTU Discovery (PLPMTUD) [RFC4821].

   While sending packets according to the above specifications, the AERO
   interface MAY also send 1500 byte probe packets to the tunnel egress
   to determine whether the probes can traverse the tunnel without
   fragmentation.  If the probes succeed, the tunnel ingress can begin
   sending packets that are larger than 1280-ENCAPS bytes but no larger
   than 1500 bytes without fragmentation (and for IPv4 with DF set to
   1).  Since the path MTU within the tunnel may fluctuate due to
   routing changes, the tunnel ingress SHOULD continually send
   additional probes subject to rate limiting in case L2 PTB messages
   are lost.  If the path MTU within the tunnel later becomes
   insufficient, the tunnel ingress must resume fragmentation.

   To construct a probe, the AERO interface 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 padded NS message in
   the encapsulation headers (and for IPv4 sets DF to 1) then sends the
   message to the neighbor.  Note that the trailing padding SHOULD NOT
   be included within the Nonce option itself but rather as padding
   beyond 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.

   In light of the above fragmentation and reassembly recommendations,
   the tunnel egress MUST be capable of reassembling encapsulated
   packets up to 1500+ENCAPS bytes in length.  It is therefore
   RECOMMENDED that the tunnel egress be capable of reassembling at
   least 2KB.  Also, in some environments there may be operational
   assurance that all links within the routing region spanned by the
   tunnel configure sufficiently large MTUs so that fragmentation and
   reassembly can be avoided.  In those cases, specific tunnel
   specifications must explain the circumstances under which the above
   fragmentation and reassembly recommendations need not be applied.




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   Of possible concern is that some network middleboxes hold the
   fragments of a fragmented UDP packet until all fragments have arrived
   before forwarding the fragments to the final destination.  This means
   that the network middlebox must also be able to accommodate
   fragmented UDP packets up to 1500+ENCAPS bytes in length which cannot
   be controlled by the tunnel egress.  However, network middleboxes
   already must be capable of passing fragmented UDP datagrams up to the
   maximum fragmented IP packet size as evidenced through actual
   operational experience (see the thread "PMTUD issue discussion" in
   the IETF v6ops archive dated September 10, 2014).  Hence, there is no
   need for AERO to stipulate a minimum reassembly size for those
   devices.

3.10.  AERO Interface Error Handling

   When an AERO node admits encapsulated packets into the AERO
   interface, it may receive L2 or an L3 error indications.

   An L2 error indication is an ICMP error message generated by a router
   on the path to the neighbor or by the neighbor itself.  The message
   includes an IP header with the address of the node that generated the
   error as the source address and with the link-layer address of the
   AERO node as the destination address.

   The IP header is followed by an ICMP header that includes an error
   Type, Code and Checksum.  For ICMPv6 [RFC4443], the error Types
   include "Destination Unreachable", "Packet Too Big (PTB)", "Time
   Exceeded" and "Parameter Problem".  For ICMPv4 [RFC0792], the error
   Types include "Destination Unreachable", "Fragmentation Needed" (a
   Destination Unreachable Code that is analogous to the ICMPv6 PTB),
   "Time Exceeded" and "Parameter Problem".

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

   The L2 error message format is shown in Figure 3:






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

             Figure 3: AERO Interface L2 Error Message Format

   The AERO node rules for processing these L2 error messages is as
   follows:

   o  When an AERO node receives an L2 "Parameter Problem", it processes
      the message the same as described as for ordinary ICMP errors in
      the normative references [RFC0792][RFC4443].

   o  When an AERO node receives persistent L2 Time Exceeded messages,
      it SHOULD reduce its current rate of admitting fragmented
      encapsulated packets into the tunnel to ensure that the IP ID
      field will not wrap before all earlier fragments have been
      processed.  If the AERO node includes an integrity check vector,
      however, it MAY ignore the messages and continue sending
      fragmented encapsulated packets without rate limiting.

   o  When an AERO Client receives persistent L2 Destination Unreachable
      messages in response to tunneled packets that it sends to one of
      its dynamic neighbor correspondents, the Client SHOULD test the
      path to the correspondent using Neighbor Unreachability Detection
      (NUD) (see Section 3.14).  If NUD fails, the Client SHOULD set
      ForwardTime for the corresponding dynamic neighbor cache entry to



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      0 and allow future packets destined to the correspondent to flow
      through a Server.

   o  When an AERO Client receives persistent L2 Destination Unreachable
      messages in response to tunneled packets that it sends to one of
      its static neighbor Servers, the Client SHOULD test the path to
      the Server using NUD.  If NUD fails, the Client SHOULD delete the
      neighbor cache entry and attempt to associate with a new Server.

   o  When an AERO Server receives persistent L2 Destination Unreachable
      messages in response to tunneled packets that it sends to one of
      its static neighbor Clients, the Server SHOULD test the path to
      the Client using NUD.  If NUD fails, the Server SHOULD cancel the
      DHCPv6 PD lease for the Client's ACP, withdraw its route for the
      ACP from the AERO routing system and delete the neighbor cache
      entry (see Sections 3.11 and 3.12).

   o  When an AERO Relay or Server receives an L2 Destination
      Unreachable message in response to a tunneled packet that it sends
      to one of its permanent neighbors, it discards the message since
      the routing system is likely in a temporary transitional state
      that will soon re-converge.

   o  When an AERO node receives an L2 PTB message, it translates the
      message into an L3 PTB message if possible (*) and forwards the
      message toward the original source as described below.

   To translate an L2 PTB message to an L3 PTB message, the AERO node
   first caches the values in the Type, Code and MTU fields of the L2
   ICMP header.  The node next discards the L2 IP and ICMP headers, and
   also discards the encapsulation headers of the original L3 packet.
   Next the node encapsulates the included segment of the original L3
   packet in an L3 IP and ICMP header.  In the process, the node uses
   the cached L2 Type and Code values to set corresponding values in the
   Type and Code fields of the L3 ICMP header, then writes the maximum
   of 1500 bytes and (L2 MTU - ENCAPS) into MTU field of the L3 ICMP
   header.

   The node next writes the IP source address of the original L3 packet
   as the destination address of the L3 PTB message and determines the
   next hop to the destination.  If the next hop is reached via the AERO
   interface, the node uses the IPv6 address "::" or the IPv4 address
   "0.0.0.0" as the IP source address of the L3 PTB message.  Otherwise,
   the node uses one of its non link-local addresses as the source
   address of the L3 PTB message.  The node finally calculates the ICMP
   checksum over the L3 PTB message and writes the Checksum in the
   corresponding field of the L3 ICMP header.  The L3 PTB message
   therefore is formatted as follows:



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        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~
        |        L3 IP Header of        |
        |         error message         |
        ~                               ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |         L3 ICMP Header        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ---
        ~                               ~   p
        |        IP header of           |   k
        |      original L3 packet       |   t
        ~                               ~
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   i
        ~                               ~   n
        |    Upper layer headers and    |
        |    leading portion of body    |   e
        |   of the original L3 packet   |   r
        ~                               ~   r
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ---

             Figure 4: AERO Interface L3 Error Message Format

   After the node has prepared the L3 PTB message, it either forwards
   the message via a link outside of the AERO interface without
   encapsulation, or encapsulates and forwards the message to the next
   hop via the AERO interface.

   When an AERO Relay receives an L3 packet for which the destination
   address is covered by an ASP, if there is no more-specific routing
   information for the destination the Relay drops the packet and
   returns an L3 Destination Unreachable message.  The Relay first
   writes the IP source address of the original L3 packet as the
   destination address of the L3 Destination Unreachable message and
   determines the next hop to the destination.  If the next hop is
   reached via the AERO interface, the Relay uses the IPv6 address "::"
   or the IPv4 address "0.0.0.0" as the IP source address of the L3
   Destination Unreachable message and forwards the message to the next
   hop within the AERO interface.  Otherwise, the Relay uses one of its
   non link-local addresses as the source address of the L3 Destination
   Unreachable message and forwards the message via a link outside the
   AERO interface.

   When an AERO node receives any L3 error message via the AERO
   interface, it examines the destination address in the L3 IP header of
   the message.  If the next hop toward the destination address of the
   error message is via the AERO interface, the node re-encapsulates and
   forwards the message to the next hop within the AERO interface.
   Otherwise, if the source address in the L3 IP header of the message



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   is the IPv6 address "::" or the IPv4 address "0.0.0.0", the node
   writes one of its non link-local addresses as the source address of
   the L3 message and recalculates the IP and/or ICMP checksums.  The
   node finally forwards the message via a link outside of the AERO
   interface.

   (*) Note that in some instances the packet-in-error field of an L2
   PTB message may not include enough information for translation to an
   L3 PTB message.  In that case, the AERO interface simply discards the
   L2 PTB message.  It can therefore be said that translation of L2 PTB
   messages to L3 PTB messages can provide a useful optimization when
   possible, but is not critical for sources that correctly use PLPMTUD.

3.11.  AERO Router Discovery, Prefix Delegation and Address
       Configuration

3.11.1.  AERO DHCPv6 Service Model

   Each AERO Server configures a DHCPv6 server function to facilitate PD
   requests from Clients.  Each Server is pre-configured with an
   identical list of ACP-to-Client ID mappings for all Clients enrolled
   in the AERO system, as well as any information necessary to
   authenticate Clients.  The configuration information is maintained by
   a central administrative authority for the AERO link and securely
   propagated to all Servers whenever a new Client is enrolled or an
   existing Client is withdrawn.

   With these identical configurations, each Server can function
   independently of all other Servers, including the maintenance of
   active leases.  Therefore, no Server-to-Server DHCPv6 state
   synchronization is necessary, and Clients can optionally hold
   separate leases for the same ACP from multiple Servers.

   In this way, Clients can easily associate with multiple Servers, and
   can receive new leases from new Servers before deprecating leases
   held through old Servers.  This enables a graceful "make-before-
   break" capability.

3.11.2.  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 "linkupnetworks" is a constant text string and "[domainname]"
   is the connection-specific DNS suffix for the Client's underlying
   network connection (e.g., "example.com").  After discovering the




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   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 a DHCPv6 PD exchange[RFC3315][RFC3633] in
   which the Client's Solicit/Request messages use the IPv6
   "unspecified" address (i.e., "::") 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 also includes a Client Identifier
   option with a DHCP Unique Identifier (DUID) plus any necessary
   authentication options to identify itself to the DHCPv6 server, and
   includes a Client Link Layer Address Option (CLLAO) [RFC6939] with
   the format shown in Figure 5:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | OPTION_CLIENT_LINKLAYER_ADDR  |           option-length       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   link-layer type (16 bits)   |    Link ID    |   Preference  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 5: AERO Client Link-Layer Address Option (CLLAO) Format

   The Client sets the CLLAO 'option-length' field to 4 and sets the
   'link-layer type' field to TBD1 (see: IANA Considerations), then
   includes appropriate Link ID and Preference values for the underlying
   interface over which the Solicit/Request will be issued (note that
   these are the same values that would be included in a TLLAO as shown
   in Figure 2).  If the Client is pre-provisioned with an ACP
   associated with the AERO service, it MAY also include the ACP in the
   Solicit/Request message Identity Association (IA) option to indicate
   its preferred ACP to the DHCPv6 server.  The Client then sends the
   encapsulated DHCPv6 request via the underlying interface.

   When the Client receives its ACP and the set of ASPs via a DHCPv6
   Reply from the AERO Server, it creates a static 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 records the lifetime for the ACP in
   the neighbor cache entry and marks the neighbor cache entry as
   "default", i.e., the Client considers the Server as a default router.
   If the Reply message contains a Vendor-Specific Information Option
   (see: Section 3.10.3) the Client also caches each ASP in the option.

   The Client then applies the AERO address to the AERO interface and
   sub-delegates the ACP to nodes and links within its attached EUNs



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   (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 next hop
   will then be determined after a packet has been submitted to the AERO
   interface by inspecting the neighbor cache (see above).

   On some platforms (e.g., on some popular cell phone operating
   systems), the act of assigning a default IPv6 route to the AERO
   interface may not be permitted from a user application.  On those
   platforms, sub-delegation of the ACP may also not be feasible.
   Typically, those platforms include a TUN/TAP interface that acts as a
   point-to-point conduit between user applications and the AERO
   interface.  In that case, the Client can instead generate a
   "synthesized RA" message.  The message conforms to [RFC4861] and is
   prepared as follows:

   o  the IPv6 source address is fe80::

   o  the IPv6 destination address is all-nodes multicast

   o  the Router Lifetime is non-zero, which provides both a default
      router lifetime and an indication of the time at which additional
      synthesized RAs must be sent

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

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

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

   The Client subsequently renews its ACP delegation through each of its
   Servers 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 Client identifier, authentication options and
   CLLAO option as was used in the initial PD request.  Note that if the
   Client does not issue a DHCPv6 Renew before the Server has terminated
   the lease (e.g., if the Client has been out of touch with the Server



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   for a considerable amount of time), the Server's Reply will report
   NoBinding and the Client must re-initiate the DHCPv6 PD procedure.

   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
   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 3.13.

3.11.3.  AERO Server Behavior

   AERO Servers configure a DHCPv6 server 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.

   When an AERO Server receives a prospective Client's DHCPv6 PD
   Solicit/Request message, it first authenticates the message.  If
   authentication succeeds, the Server determines the correct ACP to
   delegate to the Client by matching the Client's DUID within an online
   directory service (e.g., LDAP).  The Server then delegates the ACP
   and creates a static neighbor cache entry for the Client's AERO
   address with lifetime set to no more than the lease lifetime and the
   Client's link-layer address as the link-layer address for the Link ID
   specified in the CLLAO option.  The Server then creates an IP
   forwarding table entry so that the AERO routing system will propagate
   the ACP to all Relays (see: Section 3.12).  Finally, the Server sends
   a DHCPv6 Reply message to the Client while using fe80::ID as the IPv6
   source address, the Client's AERO address as the IPv6 destination
   address, and the Client's link-layer address as the destination link-
   layer address.  The Server also includes a Server Unicast option with
   server-address set to fe80::ID so that all future Client/Server
   transactions will be link-local-only unicast over the AERO link.

   When the Server sends the DHCPv6 Reply message, it also includes a
   DHCPv6 Vendor-Specific Information Option with 'enterprise-number'
   set to "TBD2" (see: IANA Considerations).  The option is formatted as



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   shown in[RFC3315] and with the AERO enterprise-specific format shown
   in Figure 6:

        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 6: AERO Vendor-Specific Information Option

   Per Figure 6, the option includes one or more ASP.  The ASP field
   contains the IP prefix as it would appear in the interface identifier
   portion of the corresponding AERO address (see: Section 3.3).  For
   IPv6, valid values for the Prefix Length field are 0 through 64; for
   IPv4, valid values are 0 through 32.

   After the initial DHCPv6 PD exchange, the AERO Server maintains the
   neighbor cache entry for the Client as long as the lease lifetime
   remains current.  If the Client issues a Renew/Reply exchange, the
   Server extends the lifetime.  If the Client issues a Release/Reply
   exchange, or if the Client does not issue a Renew/Reply within the
   lease lifetime, the Server deletes the neighbor cache entry for the
   Client and withdraws the IP route from the AERO routing system.





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3.12.  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
   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.

3.13.  AERO Redirection

3.13.1.  Reference Operational Scenario

   Figure 7 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 7: AERO Reference Operational Scenario

   In Figure 7, 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 applies 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 applies 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.

3.13.2.  Concept of Operations

   Again, with reference to Figure 7, 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 the packet
   and the Predirect message 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
   dynamic 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 dynamic 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.

3.13.3.  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 8:





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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 8: AERO Redirect/Predirect Message Format

3.13.4.  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 one or more TLLAOs with Link ID and
      Preference set to appropriate values for Client ('A')'s underlying
      interfaces, and with UDP Port Number and IP Address set to 0'.

   o  the message SHOULD include a Timestamp option and a Nonce 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 or otherwise undesirable.  If the
   direct path later becomes unusable after the initial route
   optimization, Client ('A') simply allows packets to again flow
   through Server ('S1').

3.13.5.  Re-encapsulating and Relaying Predirects

   When Server ('S1') receives a Predirect message from Client ('A'), it
   first verifies that the TLLAOs in the Predirect are a proper subset
   of the Link IDs in Client ('A')'s neighbor cache entry.  If the
   Client's TLLAOs are not acceptable, 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



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   verifies that Client ('A') is authorized to use the Prefix Length in
   the Predirect 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 ('S1') discards the Predirect;
   otherwise, it copies the correct UDP Port numbers and IP Addresses
   for Client ('A')'s links into the (previously empty) TLLAOs.

   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').

3.13.6.  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 dynamic
   neighbor cache entry that stores the Target Address of the message as
   the network-layer address of Client ('A') , stores the link-layer
   addresses found in the TLLAOs as the link-layer addresses of Client



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   ('A') and stores the Prefix Length as the length to be applied to the
   network-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 one or more TLLAOs with Link ID and
      Preference set to appropriate values for Client ('B')'s underlying
      interfaces, and with UDP Port Number and IP Address set to '0'.

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

   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.





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

3.13.7.  Re-encapsulating and Relaying Redirects

   When Server ('S2') receives a Redirect message from Client ('B'), it
   first verifies that the TLLAOs in the Redirect are a proper subset of
   the Link IDs in Client ('B')'s neighbor cache entry.  If the Client's
   TLLAOs are not acceptable, 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 numbers and IP
   Addresses for Client ('B')'s links into the (previously empty)
   TLLAOs.

   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').

3.13.8.  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



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   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.

   In the reference operational scenario, when Client ('A') receives the
   Redirect message, it either creates or updates a dynamic neighbor
   cache entry that stores the Target Address of the message as the
   network-layer address of Client ('B'), stores the link-layer
   addresses found in the TLLAOs as the link-layer addresses 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.)

3.13.9.  Server-Oriented Redirection

   In some environments, the Server nearest the target 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.

3.14.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform Neighbor Unreachability Detection (NUD) by sending
   unicast NS messages to elicit solicited NA messages from neighbors
   the same as described in [RFC4861].  NUD is performed either
   reactively in response to persistent L2 errors (see Section 3.10) or
   proactively to refresh existing neighbor cache entries.

   When an AERO node sends an NS/NA message, it MUST use its link-local
   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




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   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
   proactively 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 proactively
   test the direct path to the target Client (see Section 7.3 of
   [RFC4861]) periodically in order to keep dynamic 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 one of its Servers.  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.  If ForwardTime is non-zero,
   the target Client then sends a solicited NA message to the link-layer
   address of the source Client; otherwise, it sends the solicited NA
   message to the link-layer address of one of its Servers.

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








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3.15.  Mobility Management

3.15.1.  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 Rebind/Reply exchange
   via each of its Servers using the new link-layer address as the
   source and with a CLLAO that includes the correct Link ID and
   Preference values.  If authentication succeeds, the Server then
   update its neighbor cache and sends a DHCPv6 Reply.  Note that if the
   Client does not issue a DHCPv6 Rebind before the Server has
   terminated the lease (e.g., if the Client has been out of touch with
   the Server for a considerable amount of time), the Server's Reply
   will report NoBinding and the Client must re-initiate the DHCPv6 PD
   procedure.

   Next, the Client sends unsolicited NA messages to each of its
   correspondent Client 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.

   With reference to Figure 7, 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.





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

   o  the message includes one or more TLLAOs with Link ID and
      Preference set to appropriate values for Client ('B')'s underlying
      interfaces, 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 numbers and IP addresses into the TLLAOs, 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 addresses for Client ('B') based on the
   addresses in the TLLAOs.  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
   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.

3.15.2.  Bringing New Links Into Service

   When a Client needs to bring a new underlying interface into service
   (e.g., when it activates a new data link), it performs an immediate
   Rebind/Reply exchange via each of its Servers using the new link-
   layer address as the source address and with a CLLAO that includes
   the new Link ID and Preference values.  If authentication succeeds,
   the Server then updates its neighbor cache and sends a DHCPv6 Reply.
   The Client MAY then send unsolicited NA messages to each of its
   correspondent Clients to inform them of the new link-layer address as
   described in Section 3.14.1.

3.15.3.  Removing Existing Links from Service

   When a Client needs to remove an existing underlying interface from
   service (e.g., when it de-activates an existing data link), it
   performs an immediate Rebind/Reply exchange via each of its Servers
   over any available link with a CLLAO that includes the deprecated
   Link ID and a Preference value of 0.  If authentication succeeds, the



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   Server then updates its neighbor cache and sends a DHCPv6 Reply.  The
   Client SHOULD then send unsolicited NA messages to each of its
   correspondent Clients to inform them of the deprecated link-layer
   address as described in Section 3.14.1.

3.15.4.  Moving to a New Server

   When a Client associates with a new Server, it performs the Client
   procedures specified in Section 3.10.

   When a Client disassociates with an existing Server, it sends a
   DHCPv6 Release message to the unicast link-local network layer
   address of the old Server.  The Client SHOULD send the message via a
   new Server (i.e., by setting the link-layer destination address to
   the address of the new Server) in case the old Server is unreachable
   at the link layer, e.g., if the old Server is in a different network
   partition.  The new Server will forward the message to a Relay, which
   will in turn forward the message to the old Server.

   When the old Server receives the DHCPv6 Release, it first
   authenticates the message.  If authentication succeeds, the old
   Server withdraws the IP route from the AERO routing system and
   deletes the neighbor cache entry for the Client.  (The old Server MAY
   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.)  The old Server then returns a DHCPv6 Reply message via a
   Relay.  The Client can then use the Reply message to verify that the
   termination signal has been processed, and can delete both the
   default route and the neighbor cache entry for the old Server.  (Note
   that the Server's Reply to the Client's Release message may be lost,
   e.g., if the AERO routing system has not yet converged.  Since the
   Client is responsible for reliability, however, it will retry until
   it gets an indication that the Release was successful.)

   Clients SHOULD NOT move rapidly between Servers in order to avoid
   causing unpredictable oscillations in the AERO 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 might wish to change
   to a different Server include a Server that has gone unreachable,
   topological movements of significant distance, etc.

3.16.  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



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   different IP protocol versions) and so must ignore any redirection
   messages and continue to send packets via the Server.

3.17.  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".)

3.18.  Operation on AERO Links Without DHCPv6 Services

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

3.19.  Operation on Server-less AERO Links

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

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

3.20.  Extending AERO Links Through Security Gateways

   When an enterprise mobile device moves from a campus LAN connection
   to a public Internet link, it must re-enter the enterprise via a
   security gateway that has both an physical interface connection to



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   the Internet and a physical interface connection to the enterprise
   internetwork.  This most often entails the establishment of a Virtual
   Private Network (VPN) link over the public Internet from the mobile
   device to the security gateway.  During this process, the mobile
   device supplies the security gateway with its public Internet address
   as the link-layer address for the VPN.  The mobile device then acts
   as an AERO Client to negotiate with the security gateway to obtain
   its ACP.

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

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



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

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

4.  Implementation Status

   An application-layer implementation is in progress.

5.  IANA Considerations

   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.

   IANA is 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 nodes SHOULD also use DHCPv6
   securing services (e.g., DHCPv6 authentication,
   [I-D.ietf-dhc-sedhcpv6], etc.) for Client authentication and network
   admission control.





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   AERO 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.  AERO Predirect, NS and
   RS messages SHOULD include a Nonce option (see Section 5.3 of
   [RFC3971]) that recipients echo back in corresponding responses.

   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 their EUNs to gain access to a protected
   network, i.e., AERO Clients that act as routers MUST NOT provide
   routing services for unauthorized nodes.  (This concern is no
   different than for ordinary hosts that receive an IP address
   delegation but then "share" the address with 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, Mark Andrews, Fred Baker,
   Stewart Bryant, Brian Carpenter, Wojciech Dec, Ralph Droms, Brian
   Haberman, Joel Halpern, Sascha Hlusiak, Lee Howard, Andre Kostur, Ted
   Lemon, Joe Touch and Bernie Volz.  Members of the IESG also provided
   valuable input during their review process that greatly improved the
   document.  Special thanks go to Stewart Bryant, Joel Halpern and
   Brian Haberman for their shepherding guidance.

   This work has further been encouraged and supported by Boeing
   colleagues including Keith Bartley, Dave Bernhardt, Cam Brodie,
   Balaguruna Chidambaram, Claudiu Danilov, 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.





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   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.

   [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.



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

8.2.  Informative References

   [I-D.ietf-dhc-sedhcpv6]
              Jiang, S., Shen, S., Zhang, D., and T. Jinmei, "Secure
              DHCPv6 with Public Key", draft-ietf-dhc-sedhcpv6-03 (work
              in progress), June 2014.

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

   [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers", RFC
              1812, June 1995.

   [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.

   [RFC2923]  Lahey, K., "TCP Problems with Path MTU Discovery", RFC
              2923, September 2000.

   [RFC3819]  Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
              Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, July 2004.

   [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.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.





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   [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.

   [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.



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   [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.

   [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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