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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc6706 (if approved)                           June 30, 2014
Intended status: Standards Track
Expires: January 1, 2015


               Transmission of IP Packets over AERO Links
                     draft-templin-aerolink-29.txt

Abstract

   This document specifies the operation of IP over tunnel virtual Non-
   Broadcast, Multiple Access (NBMA) links using Asymmetric Extended
   Route Optimization (AERO).  Nodes attached to AERO links can exchange
   packets via trusted intermediate routers that provide forwarding
   services to reach off-link destinations and redirection services for
   route optimization.  AERO provides an IPv6 link-local address format
   known as the AERO address that supports operation of the IPv6
   Neighbor Discovery (ND) protocol and links IPv6 ND to IP forwarding.
   Admission control and provisioning are supported by the Dynamic Host
   Configuration Protocol for IPv6 (DHCPv6), and node mobility is
   naturally supported through dynamic neighbor cache updates.  Although
   IPv6 ND messaging is used in the control plane, both IPv4 and IPv6
   are supported in the data plane.

Status of This Memo

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

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

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

   This Internet-Draft will expire on January 1, 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 Node Types . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  AERO Addresses  . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  AERO Interface Characteristics  . . . . . . . . . . . . .   7
       3.3.1.  Coordination of Multiple Underlying Interfaces  . . .   9
     3.4.  AERO Interface Neighbor Cache Maintenace  . . . . . . . .  10
     3.5.  AERO Interface Data Origin Authentication . . . . . . . .  11
     3.6.  AERO Interface MTU Considerations . . . . . . . . . . . .  11
     3.7.  AERO Interface Encapsulation, Re-encapsulation and
           Decapsulation . . . . . . . . . . . . . . . . . . . . . .  13
     3.8.  AERO Router Discovery, Prefix Delegation and Address
           Configuration . . . . . . . . . . . . . . . . . . . . . .  14
       3.8.1.  AERO Server Behavior  . . . . . . . . . . . . . . . .  14
       3.8.2.  AERO Client Behavior  . . . . . . . . . . . . . . . .  16
     3.9.  AERO Redirection  . . . . . . . . . . . . . . . . . . . .  17
       3.9.1.  Reference Operational Scenario  . . . . . . . . . . .  17
       3.9.2.  Concept of Operations . . . . . . . . . . . . . . . .  19
       3.9.3.  Message Format  . . . . . . . . . . . . . . . . . . .  20
       3.9.4.  Sending Predirects  . . . . . . . . . . . . . . . . .  20
       3.9.5.  Re-encapsulating and Relaying Predirects  . . . . . .  22
       3.9.6.  Processing Predirects and Sending Redirects . . . . .  22
       3.9.7.  Re-encapsulating and Relaying Redirects . . . . . . .  24
       3.9.8.  Processing Redirects  . . . . . . . . . . . . . . . .  24
       3.9.9.  AERO Relay Operation  . . . . . . . . . . . . . . . .  25
       3.9.10. Server-Oriented Redirection . . . . . . . . . . . . .  25
     3.10. Neighbor Unreachability Detection (NUD) . . . . . . . . .  26
     3.11. Mobility Management . . . . . . . . . . . . . . . . . . .  27
       3.11.1.  Announcing Link-Layer Address Changes  . . . . . . .  27
       3.11.2.  Moving to a New Server . . . . . . . . . . . . . . .  28
     3.12. Encapsulation Protocol Version Considerations . . . . . .  29
     3.13. Multicast Considerations  . . . . . . . . . . . . . . . .  29
     3.14. Operation on AERO Links Without DHCPv6 Services . . . . .  29
     3.15. Operation on Server-less AERO Links . . . . . . . . . . .  30
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  30



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   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  31
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  32
   Appendix A.  AERO Server and Relay Interworking . . . . . . . . .  34
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

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

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

   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 assigned to a Client's AERO interface.



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

   AERO Client ("Client")
      a node that assigns an AERO address on an AERO interface and
      receives a Client Prefix delegation.

   AERO Server ("Server")
      a node that assigns the IPv6 link-local subnet router anycast
      address (fe80::) and an administratively provisioned IPv6 link-
      local unicast address on an AERO interface over which it can
      provide default forwarding services for AERO Clients.

   AERO Relay ("Relay")
      a node that relays IP packets between Servers on the same AERO
      link, and/or that forwards IP packets between the AERO link and
      the native Internetwork.  An AERO Relay may or may not also be
      configured as an AERO 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 whichthe
      tunnel virtual overrlay is configured.

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

   link-layer address
      an IP address assigned to an AERO node's underlying interface.
      When UDP encapsulation is used, the UDP port number is also
      considered as part of the link-layer address.  Link-layer
      addresses are used as the encapsulation header source and
      destination addresses.

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

   end user network (EUN)




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      an internal virtual or external edge IP network that an AERO
      Client connects to the AERO interface.

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

   Client Prefix (CP)
      an IP prefix taken from an ASP and delegated to a Client.

   Throughout the document, the simple terms "Client", "Server" and
   "Relay" refer to "AERO Client", "AERO Server" and "AERO Relay",
   respectively.  Capitalization is used to distinguish these terms from
   DHCPv6 client/server/relay.  This is an important distinction, since
   an AERO Server may be a DHCPv6 relay, and an AERO Relay may be a
   DHCPv6 server.

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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Asymmetric Extended Route Optimization (AERO)

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

3.1.  AERO Node Types

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

   AERO Servers provide default routing services to AERO Clients.  AERO
   Servers configure a DHCPv6 relay or server function and facilitate
   DHCPv6 Prefix Delegation (PD) exchanges for AERO Clients.  Each
   delegated prefix becomes a Client Prefix (CP) taken from an ASP.  An
   AERO Server may also act as an AERO Relay.




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   AERO Clients act as requesting routers to receive CPs through a
   DHCPv6 PD exchange via AERO Servers over the AERO link.  (Each Client
   MAY associate with multiple Servers, but associating with many
   Servers may result in excessive control message overhead.)  Each IPv6
   AERO Client receives at least a /64 IPv6 CP, and may receive even
   shorter prefixes.  Similarly, each IPv4 AERO Client receives at least
   a /32 IPv4 CP (i.e., a singleton IPv4 address), and may receive even
   shorter prefixes.

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

   AERO Clients that act as ordinary hosts assign one or more IP
   addresses from their CPs to the AERO interface but DO NOT assign the
   CP itself to the AERO interface.  Instead, the Client assigns the CP
   to a "black hole" route so that unused portions of the prefix are
   nullified.  End system applications on AERO Clients that act as hosts
   bind directly to the AERO interface.

3.2.  AERO Addresses

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

      fe80::[Client Prefix (CP)]

   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 CP.  The base prefix is determined by masking the CP
   with the prefix length.  For example, if the AERO Client receives the
   IPv6 CP:

      2001:db8:1000:2000::/56

   it constructs its AERO address as:

      fe80::2001:db8:1000:2000

   For IPv4, the AERO address begins with the prefix fe80::/96 and
   includes in its interface identifier the base prefix taken from the
   Client's IPv4 CP.  For example, if the AERO Client receives the IPv4
   CP:

      192.0.2.32/28

   it constructs its AERO address as:



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      fe80::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 CP length and may therefore send initial
   IPv6 ND messages with an AERO destination address that matches the CP
   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 CP
   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.

3.3.  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 operate over
   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.

   Servers and Relays maintain a set of ASPs from which CPs are
   delegated to Clients.  Relays present the ASPs to the native
   Internetwork as a set of aggregated prefixes in the routing system.
   The ASPs are never deaggregated within the native Internetwork
   routing system; hence, no Client mobility events within the AERO link
   are exposed outside of the AERO link.

   Servers assign the address fe80:: to their AERO interfaces as a link-
   local Subnet Router Anycast address.  Servers and Relays also assign
   a link-local address fe80::ID to support the operation of the IPv6 ND
   protocol and the inter-Server/Relay routing system (see: Appendix A).
   Each fe80::ID address MUST be unique among all Servers and Relays on
   the AERO link, and MUST NOT collide with any potential AERO addresses
   (e.g., the addresses for Servers and Relays on the link could be
   assigned as fe80::1, fe80::2, fe80::3, etc.).  Servers accept IPV6 ND
   messages with either fe80::ID or fe80:: as the IPv6 destination
   address, but MUST use the fe80::ID address as the IPv6 source address
   of any IPv6 ND messages they generate.




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   When a Client does not know the fe80::ID address of a Server, it can
   use fe80:: as a temporary destination address in IPv6 ND messages.
   When a Client enables an AERO interface, it invokes DHCPv6 PD using
   the temporary IPv6 link-local source address
   fe80::ffff:ffff:ffff:ffff to receive one or more CP delegations.
   After the Client receives a CP, it assigns the corresponding AERO
   address to the AERO interface and deprecates the temporary address,
   i.e., the Client invokes DHCPv6 to bootstrap the provisioning of a
   unique link-local address before invoking IPv6 ND.

   AERO interfaces maintain a neighbor cache and 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, Clients ignore any Prefix
   Information Options (PIOs) they may receive in RA messages with the
   'A' and/or 'L' bits set.

   AERO interface Redirect, Predirect and unsolicited NA messages
   include Target Link-Layer Address Options (TLLAOs) formatted as shown
   in Figure 1:

        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 1: 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
   node's preference for this underlying interface, with 0 being highest



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   preference and 255 being lowest.  UDP Port Number and IP Address are
   set to the addresses used by the target node when it sends
   encapsulated packets over the underlying interface.  When no UDP
   encapsulation is used, UDP Port Number is set to 0.  When the
   encapsulation IP address family is IPv4, IP Address is formed as an
   IPv4-compatible IPv6 address [RFC4291], i.e., 96 bits of leading 0's
   followed by a 32-bit IPv4 address

   AERO interface Redirect/Predirect messages can both update and create
   neighbor cache entries, including link-layer address information.
   AERO interface unsolicited NA messages update a neighbor's cached
   link-layer address for the sender, e.g., following a link-layer
   address change due to node mobility.  AERO interface RS/RA messages
   as well as NS/NA messages used for Neighbor Unreachability Detection
   (NUD) update timers in existing neighbor cache entires but do not
   update link-layer addresses nor create new neighbor cache entries.

   AERO interface 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
   interface NS/RS messages SHOULD include a Nonce option (see
   Section 5.3 of [RFC3971]) that recipients echo back in corresponding
   NA/RA responses.

3.3.1.  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 0.

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



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3.4.  AERO Interface Neighbor Cache Maintenace

   Each AERO interface maintains a conceptual neighbor cache that
   includes an entry for each neighbor it communicates with on the AERO
   link, the same as for any IPv6 interface [RFC4861].  Neighbor cache
   entries are created and maintained as follows:

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

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

   When an AERO Client receives a valid Predirect message it creates or
   updates a neighbor cache entry for the Predirect target network-layer
   and link-layer addresses, and also creates an IP forwarding table
   entry for the predirected (source) CP.  The node then sets an
   "AcceptTime" variable for the neighbor and uses this value to
   determine whether packets received from the predirected neighbor can
   be accepted.

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

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

   When an AERO Client receives a valid solicited NA message, it
   (re)sets ForwardTime for the neighbor to FORWARD_TIME.  (When an AERO
   Client receives a valid unsolicited NA message, it updates the
   neighbor's link-layer address but DOES NOT reset ForwardTime.)

   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].



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   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.

3.5.  AERO Interface Data Origin Authentication

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

   The AERO node considers the link-layer source address correct for the
   network-layer source address if there is an IP forwarding table entry
   that matches the network-layer source address as well as a neighbor
   cache entry corresponding to the next hop that includes the link-
   layer address and AcceptTime is non-zero.

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

3.6.  AERO Interface MTU Considerations

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

   The base tunneling specifications for IPv4 and IPv6 typically set a
   static MTU on the tunnel interface to 1500 bytes minus the
   encapsulation overhead or smaller still if the tunnel is likely to



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   incur additional encapsulations on the path.  This can result in path
   MTU related black holes when packets that are too large to be
   accommodated over the AERO link are dropped, but the resulting ICMP
   Packet Too Big (PTB) messages are lost on the return path.  As a
   result, AERO nodes use the following MTU mitigations to accommodate
   larger packets.

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

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

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

   To construct a probe, the node prepares an NS message with a Nonce
   option plus trailing padding octets added to a length of 1500 bytes
   without including the length of the padding in the IPv6 Payload
   Length field.  The node then encapsulates the NS in the encapsulation
   headers (while including the length of the padding in the
   encapsulation header length fields), sets DF to 1 (for IPv4) and
   sends the padded NS message to the neighbor.  If the neighbor returns
   an NA message with a correct Nonce value, the node may then send



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   whole packets within this size range and (for IPv4) relax the rate
   limiting requirement.  (Note that the trailing padding SHOULD NOT be
   included within the Nonce option itself but rather as padding beyond
   the last option in the NS message; otherwise, the (large) Nonce
   option would be echoed back in the solicited NA message and may be
   lost at a link with a small MTU along the reverse path.)

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

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

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

3.7.  AERO Interface Encapsulation, Re-encapsulation and Decapsulation

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

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

   When AERO UDP encapsulation is used, the AERO interface encapsulates
   the packet per the specifications in [RFC2003][RFC2473][RFC4213]
   except that it inserts a UDP header between the encapsulation header
   and the packet's IP header.  The AERO interface sets the UDP source
   port to a constant value that it will use in each successive packet
   it sends, sets the UDP checksum field to zero (see:



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   [RFC6935][RFC6936]) and sets the UDP length field to the length of
   the IP packet plus 8 bytes for the UDP header itself.  For packets
   sent via a Server, the AERO interface sets the UDP destination port
   to 8060 (i.e., the IANA-registered port number for AERO) when AERO-
   only encapsulation is used.  For packets sent to a neighboring
   Client, the AERO interface sets the UDP destination port to the port
   value stored in the neighbor cache entry for this neighbor.

   The AERO interface next sets the IP protocol number in the
   encapsulation header to the appropriate value for the first protocol
   layer within the encapsulation (e.g., IPv4, IPv6, UDP, IPsec, etc.).
   When IPv6 is used as the encapsulation protocol, the interface then
   sets the flow label value in the encapsulation header the same as
   described in [RFC6438].  When IPv4 is used as the encapsulation
   protocol, the AERO interface sets the DF bit as discussed in
   Section 3.6.

   AERO interfaces decapsulate packets destined either to the node
   itself or to a destination reached via an interface other than the
   receiving AERO interface.  When AERO UDP encapsulation is used (i.e.,
   when a UDP header with destination port 8060 is present) the
   interface examines the first octet of the encapsulated packet.  If
   the most significant four bits of the first octet encode the value
   '0110' (i.e., the version number value for IPv6) or the value '0100'
   (i.e., the version number value for IPv4), the packet is accepted and
   the encapsulating UDP header is discarded; otherwise, the packet is
   discarded.

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

3.8.  AERO Router Discovery, Prefix Delegation and Address Configuration

3.8.1.  AERO Server Behavior

   AERO Servers configure a DHCPv6 relay function on their AERO links.
   AERO Servers arrange to add their encapsulation layer IP addresses
   (i.e., their link-layer addresses) to the DNS resource records for
   the FQDN "linkupnetworks.domainname" before entering service.

   When an AERO Server relays a prospective Client's DHCPv6 PD messages
   to the DHCPv6 server, it wraps each message in a "Relay-forward"
   message per [RFC3315] and includes a DHCPv6 Interface Identifier
   option that encodes a value that identifies the AERO link to the
   DHCPv6 server.  Without creating internal state, the Server then
   includes the Client's link-layer address in a DHCPv6 Client Link
   Layer Address Option (CLLAO) [RFC6939] with the link-layer address
   format shown in Figure 1 (i.e., Link ID followed by Preference



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   followed by UDP Port Number followed by IP Address).  The Server sets
   the CLLAO 'option-length' field to 22 (2 plus the length of the link-
   layer address) and sets the 'link-layer type' field to TBD (see: IANA
   Considerations).  The Server finally includes a DHCPv6 Echo Request
   Option (ERO) [RFC4994] that encodes the option code for the CLLAO in
   a 'requested-option-code-n' field, then relays the message to the
   DHCPv6 server.  The CLLAO information will therefore subsequently be
   echoed back in the DHCPv6 server's "Relay-reply" message.

   When the DHCPv6 server issues the CP prefix delegation in a "Relay-
   reply" message via the AERO Server (acting as a DHCPv6 relay), the
   Server obtains the Client's link-layer address from the echoed CLLAO
   option and also obtains the Client's delegated CP from the message.
   The Server then creates a neighbor cache entry for the Client's AERO
   address with the Client's link-layer address as the link-layer
   address for the neighbor cache entry.  The neighbor cache entry is
   created with both AcceptTime and ForwardTime set to REACHABLE_TIME,
   since the Client will continue to send RS messages within
   REACHABLE_TIME seconds as long as it wishes to remain associated with
   this Server.

   The Server also configures an IP forwarding table entry that lists
   the Client's AERO address as the next hop toward the CP with a
   lifetime derived from the DHCPv6 lease lifetime.  The Server finally
   injects the CP as an IP route into the inter-Server/Relay routing
   system (see: Appendix A) then relays the DHCPv6 message to the Client
   while using fe80::ID as the IPv6 source address, the link-local
   address found in the "peer address" field of the Relay-reply message
   as the IPv6 destination address, and the Client's link-layer address
   as the destination link-layer address.

   Servers respond to NS/RS messages from Clients on their AERO
   interfaces by returning an NA/RA message.  When the Server returns an
   RA message, it includes the set of AERO link ASPs in PIOs, and MUST
   set the 'A' and 'L' bits in each PIO to '0' since the ASPs are
   associated with the AERO link but not assigned to the AERO link.
   When the Server receives an NS/RS message from the Client, it resets
   AcceptTime and ForwardTime to REACHABLE_TIME.

   Servers ignore any RA messages they may receive from a Client, but
   they MAY examine RA messages received from other Servers for
   consistency verification purposes.  Servers do not send NS messages
   for the purpose of updating Client neighbor cache timers, since
   Clients are responsible for refreshing neighbor cache state.







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3.8.2.  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 "domainname" is the DNS domain appropriate for the Client's
   attached underlying network.  After discovering the link-layer
   addresses, the Client associates with one or more of the
   corresponding Servers.

   To associate with a Server, the Client acts as a requesting router to
   request a CP through DHCPv6 PD [RFC3315][RFC3633][RFC6355] using
   fe80::ffff:ffff:ffff:ffff as the IPv6 source address (see
   Section 3.3), 'All_DHCP_Relay_Agents_and_Servers' as the IPv6
   destination address and the link-layer address of the Server as the
   link-layer destination address.  The Client includes a DHCPv6 Unique
   Identifier (DUID) in the Client Identifier option of its DHCPv6
   messages (as well as a DHCPv6 authentication option if necessary) to
   identify itself to the DHCPv6 server.  If the Client is pre-
   provisioned with a CP associated with the AERO service, it MAY also
   include the CP in its DHCPv6 PD Request to indicate its preferred CP
   to the DHCPv6 server.  The Client then sends the encapsulated DHCPv6
   request via an underlying interface.

   When the Client receives its CP via a Reply from the DHCPv6 server,
   it creates a neighbor cache entry with the Server's link-local
   address (i.e., fe80::ID) as the network-layer address and the
   Server's encapsulation address as the link-layer addresses.  Next,
   the Client assigns the AERO address derived from the CP to the AERO
   interface and sub-delegates the CP to nodes and links within its
   attached EUNs (the AERO address thereafter remains stable as the
   Client moves).  The Client also sets both AcceptTime and ForwardTime
   for each Server to the constant value REACHABLE_TIME.  The Client
   further renews its CP delegation by performing DHCPv6 Renew/Reply
   exchanges with its AERO address as the IPv6 source address,
   'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address,
   the link-layer address of a Server as the link-layer destination
   address and the same DUID and authentication information.  If the
   Client wishes to associate with multiple Servers, it can perform
   DHCPv6 Renew/Reply exchanges via each of the Servers.

   The Client then sends an RS message to each of its associated Servers
   to receive an RA message with a default router lifetime, any other
   link-specific parameters and one or more PIOs with ASPs.  When the
   Client receives an RA message, it configures or updates a default
   route according to the default router lifetime and caches the list of
   ASPs as the set of service prefixes for the AERO link.  The Client



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   further ignores any RS messages it might receive, since only Servers
   may process RS messages.

   The Client then sends periodic RS messages to each Server before
   AcceptTime and ForwardTime expire to obtain new RA messages.  When
   the Client receives a new RA message, it resets AcceptTime and
   ForwardTime to REACHABLE_TIME.  The Client can also forward IP
   packets destined to networks beyond its local EUNs via a Server as a
   default router.  The Server may in turn return a redirection message
   informing the Client of a neighbor on the AERO link that is
   topologically closer to the final destination (see Section 3.9).

   Since the Client's AERO address is configured from the unique CP
   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 do not include S/TLLAO options in ND messages they send
   directly to another AERO Client (i.e., without involving a Server).
   AERO Clients ignore any S/TLLAO options in ND messages they receive
   directly from another AERO Client.

   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 3.9.

3.9.  AERO Redirection

3.9.1.  Reference Operational Scenario

   Figure 2 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 Server('A'), two AERO Clients ('B', 'C') and
   three ordinary IPv6 hosts ('D', 'E', 'F'):













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                    .-(::::::::)
                 .-(:::: IP ::::)-.   +-------------+
                (:: Internetwork ::)--|    Host F   |
                 `-(::::::::::::)-'   +-------------+
                    `-(::::::)-'       2001:db8:2::1
                         |
                  +--------------+
                  | AERO Server A|
                  | (D->B; E->C) |
                  +--------------+
                      fe80::ID
                       L2(A)
                         |
       X-----+-----------+-----------+--------X
             |       AERO Link       |
            L2(B)                  L2(C)
     fe80::2001:db8:0:0      fe80::2001:db8:1:0         .-.
     +--------------+         +--------------+       ,-(  _)-.
     | AERO Client B|         | AERO Client C|    .-(_  IP   )-.
     | (default->A) |         | (default->A) |--(__    EUN      )
     +--------------+         +--------------+     `-(______)-'
     2001:DB8:0::/48           2001:DB8:1::/48           |
             |                                     2001:db8:1::1
            .-.                                   +-------------+
         ,-(  _)-.      2001:db8:0::1             |    Host E   |
      .-(_  IP   )-.   +-------------+            +-------------+
    (__    EUN      )--|    Host D   |
       `-(______)-'    +-------------+

               Figure 2: AERO Reference Operational Scenario

   In Figure 2, AERO Server ('A') connects to the AERO link and connects
   to the IP Internetwork, either directly or via an AERO Relay (not
   shown).  Server ('A') assigns the address fe80::ID to its AERO
   interface with link-layer address L2(A).  Server ('A') next arranges
   to add L2(A) to a published list of valid Servers for the AERO link
   and also arranges to advertise the AERO link's set of ASPs via its RA
   responses to a Client's RS messages.

   AERO Client ('B') receives the CP 2001:db8:0::/48 in a DHCPv6 PD
   exchange via AERO Server ('A') then assigns the address
   fe80::2001:db8:0:0 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::ID and link-
   layer address L2(A), then sub-delegates the CP to its attached EUNs.
   IPv6 host ('D') connects to the EUN, and configures the address
   2001:db8:0::1.




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   AERO Client ('C') receives the CP 2001:db8:1::/48 in a DHCPv6 PD
   exchange via AERO Server ('A') then assigns the address
   fe80::2001:db8:1:0 to its AERO interface with link-layer address
   L2(C).  Client ('C') configures a default route and neighbor cache
   entry via the AERO interface with next-hop address fe80::ID and link-
   layer address L2(A), then sub-delegates the CP to its attached EUNs.
   IPv6 host ('E') connects to the EUN, and configures the address
   2001:db8:1::1.

   Finally, IPv6 host ('F') connects to a network outside of the AERO
   link domain.  Host ('F') configures its IPv6 interface in a manner
   specific to its attached IPv6 link, and assigns the address
   2001:db8:2::1 to its IPv6 link interface.

3.9.2.  Concept of Operations

   Again, with reference to Figure 2, when source host ('D') sends a
   packet to destination host ('E'), the packet is first forwarded over
   the source host's attached EUN to Client ('B').  Client ('B') then
   forwards the packet via its AERO interface to Server ('A') and also
   sends a Predirect message toward Client ('C') via Server ('A') as
   specified in Section 3.9.4.  Server ('A') then re-encapsulates and
   forwards both the packet and the Predirect message out the same AERO
   interface toward Client ('C').

   After Client ('C') receives the Predirect message, it process the
   message and returns a Redirect message toward Client ('B') via Server
   ('A') as specified in Section 3.9.6.  During the process, Client
   ('C') also creates or updates a neighbor cache entry for Client ('B')
   and creates an IP forwarding table entry for Client ('B')'s CP.

   When Server ('A') receives the Redirect message, it re-encapsulates
   the message and forwards it on to Client ('B') as specified in
   Section 3.9.7.  After Client ('B') receives the Redirect message, it
   processes the message as specified in Section 3.9.8.  During the
   process, Client ('B') also creates or updates a neighbor cache entry
   for Client ('C') and creates an IP forwarding table entry for Client
   ('C')'s CP.

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








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3.9.3.  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 3:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Type (=137)  |  Code (=0/1)  |          Checksum             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Reserved                    | Prefix Length |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                       Target Address                          +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                     Destination Address                       +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Options ...
       +-+-+-+-+-+-+-+-+-+-+-+-

             Figure 3: AERO Redirect/Predirect Message Format

3.9.4.  Sending Predirects

   When a Client forwards a packet with a source address from one of its
   CPs 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 ('B') forwards a
   packet toward Client ('C'), it MAY also send a Predirect message



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   forward toward Client ('C'), 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(B)' (i.e., the link-
      layer address of Client ('B')).

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

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

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

   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 ('B')).

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

   o  the message includes a TLLAO with Link ID and Preference set to
      appropriate values for Client ('B')'s underlying interface, and
      with UDP Port Number and IP Address set to 'L2(B)'.

   o  the message SHOULD include a Timestamp option.

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

   Note that the act of sending Predirect messages is cited as "MAY",
   since Client ('B') must test reachability on the direct path to
   Client ('C') via the NUD procedures in Section 3.10 to determine when
   AERO route optimization is possible.  If the direct path is unusable,
   Client ('B') simply discontinues the AERO route optimization process




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   for Client ('C') and allows packets to again flow through Server
   ('A').

3.9.5.  Re-encapsulating and Relaying Predirects

   When Server ('A') receives a Predirect message from Client ('B'), it
   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 ('A') also verifies that Client ('B')
   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' embedded CP in the IP forwarding table.  If
   validation fails, Server ('A') discards the Predirect; otherwise, it
   copies the correct UDP Port number and IP Address for Client ('B')
   into the (previously empty) TLLAO.

   Server ('A') then examines the network-layer destination address of
   the Predirect to determine the next hop toward Client ('C') by
   searching for the AERO address' embedded CP in the IP routing table.
   If the next hop is reached via the AERO interface, Server ('A') re-
   encapsulates the Predirect and relays it on to Client ('C') by
   changing the link-layer source address of the message to 'L2(A)' and
   changing the link-layer destination address to 'L2(C)'.  Server ('A')
   finally forwards the re-encapsulated message to Client ('C') without
   decrementing the network-layer TTL/Hop Limit field.

3.9.6.  Processing Predirects and Sending Redirects

   When Client ('C') receives the Predirect message, it accepts the
   Predirect only if the message has a link-layer source address of the
   Server, i.e.  'L2(A)'.  Client ('C') further accepts the message only
   if it is willing to serve as a redirection target.  Next, Client
   ('C') 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 ('C') receives a
   valid Predirect message, it either creates or updates a neighbor
   cache entry that stores the Target Address of the message as the
   network-layer address of Client ('B') and stores the link-layer
   address found in the TLLAO as the link-layer address(es) of Client
   ('B').  Client ('C') then sets AcceptTime for the neighbor cache
   entry to ACCEPT_TIME.  Next, Client ('C') applies the Prefix Length
   to the Destination Address and records the resulting CP its IP
   forwarding table.





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

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

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

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

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

   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 ('C')).

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

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

   o  the message SHOULD include a Timestamp option.

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

   After Client ('C') prepares the Redirect message, it sends the
   message to Server ('A').








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3.9.7.  Re-encapsulating and Relaying Redirects

   When Server ('A') receives a Redirect message from Client ('C'), it
   validates the message according to the ICMPv6 Redirect message
   validation rules in Section 8.1 of [RFC4861].  Server ('A') also
   verifies that Client ('C') is authorized to use the Prefix Length in
   the Redirect message when applied to the AERO address in the network-
   layer source of the Redirect message by searching for the AERO
   address' embedded CP in the IP forwarding table.  If validation
   fails, Server ('A') discards the message; otherwise, it copies the
   correct UDP Port number and IP Address for Client ('C') into the
   (previously empty) TLLAO.

   Server ('A') then examines the network-layer destination address of
   the message to determine the next hop toward Client ('B') by
   searching for the AERO address' embedded CP in the IP forwarding
   table.  If the next hop is reached via the AERO interface, Server
   ('A') re-encapsulates the Redirect and relays it on to Client ('B')
   by changing the link-layer source address of the message to 'L2(A)'
   and changing the link-layer destination address to 'L2(B)'.  Server
   ('A') finally forwards the re-encapsulated message to Client ('B')
   without decrementing the network-layer TTL/Hop Limit field.

3.9.8.  Processing Redirects

   When Client ('B') receives the Redirect message, it accepts the
   message only if it has a link-layer source address of the Server,
   i.e.  'L2(A)'.  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 the
   network-layer source address is not that of it's current first-hop
   router.  Following validation, Client ('B') then processes the
   message as follows.

   In the reference operational scenario, when Client ('B') receives the
   Redirect message, it either creates or updates a neighbor cache entry
   that stores the Target Address of the message as the network-layer
   address of Client ('C') and stores the link-layer address found in
   the TLLAO as the link-layer address of Client ('C').  Client ('B')
   then sets ForwardTime for the neighbor cache entry to FORWARD_TIME.
   Next, Client ('B') applies the Prefix Length to the Destination
   Address and records the resulting CP in its IP forwarding table.

   Now, Client ('B') has a forwarding table entry for Client('C')'s CP
   and a neighbor cache entry with a valid ForwardTime value, while
   Client ('C') has an forwarding table entry for Client ('B')'s CP with
   a valid AcceptTime value.  Thereafter, Client ('B') may forward
   ordinary network-layer data packets directly to Client ("C") without



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   involving Server ('A') and Client ('C') can verify that the packets
   came from an acceptable source.  (In order for Client ('C') to
   forward packets to Client ('B'), a corresponding Predirect/Redirect
   message exchange is required in the reverse direction; hence, the
   mechanism is asymmetric.)

3.9.9.  AERO Relay Operation

   The reference operational scenario shown in Figure 2 applies to the
   case when Clients ('B') and ('C') are associated with the same Server
   ('A').

   When the Clients are associated with different Servers, Client (B')'s
   Server ('S1') forwards packets and Predirect messages to a Relay
   ('R') which in turn forwards them to Client ('C')'s Server ('S2').
   Similarly, Client ('C')'s Server ('S2') forwards Redirect messages to
   Relay ('R') which in turn forwards them to Client ('B')'s Server
   ('S1').  In this process:

   o  When Server ('S1') forwards a packet or Predirect message to Relay
      ('R'), it sets the link-layer source address to its own address
      and sets the link-layer destination address to Relay ('R')'s link-
      layer address.

   o  When Relay ('R') forwards a packet or Predirect message to Server
      ('S2'), it sets the link-layer source address to its own address
      and sets the link-layer destination address to Server ('S2')'s
      link-layer address.

   o  When Server ('S2') forwards a Redirect message to Relay ('R'), it
      sets the link-layer source address to its own address and sets the
      link-layer destination address to Relay ('R')'s link-layer
      address.

   o  When Relay ('R') forwards a Redirect message to Server ('S1'), it
      sets the link-layer source address to its own address and sets the
      link-layer destination address to Server ('S1')'s link-layer
      address.

   See Appendix A for further discussion of AERO Server/Relay
   interworking.

3.9.10.  Server-Oriented Redirection

   In some environments, the Server nearest the destination Client may
   need to serve as the redirection target, e.g., if direct Client-to-
   Client communications are not possible.  In that case, the Server
   prepares the Redirect message the same as if it were the destination



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   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.

3.10.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform NUD by sending unicast NS messages to elicit
   solicited NA messages from neighbors the same as described in
   [RFC4861].  When an AERO node sends an NS/NA message, it MUST use its
   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 has a neighbor cache entry for the
   neighbor; otherwise, it ignores the message.

   When a source Client is redirected to a target Client it SHOULD test
   the direct path by sending an initial NS message to elicit a
   solicited NA response.  While testing the path, the source Client can
   optionally continue sending packets via the Server, maintain a small
   queue of packets until target reachability is confirmed, or
   (optimistically) allow packets to flow directly to the target.  The
   source Client SHOULD thereafter continue to test the direct path to
   the target Client (see Section 7.3 of [RFC4861]) periodically in
   order to keep neighbor cache entries alive.

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

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

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

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



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   discards any subsequent packets received directly from the source
   Client.  When both ForwardTime and AcceptTime for a neighbor cache
   entry expire, the Client deletes both the neighbor cache entry and
   the corresponding IP forwarding table entry.

3.11.  Mobility Management

3.11.1.  Announcing Link-Layer Address Changes

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

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

   With reference to Figure 2, Client ('C') sends unicast unsolicited NA
   messages to Client ('B') via Server ('A') as follows:

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

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

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

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

   o  the Type is set to 136.

   o  the Code is set to 0.

   o  the Solicited flag is set to 0.

   o  the Override flag is set to 1.

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



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

   o  the message SHOULD include a Timestamp option.

   When Server ('A') receives the NA message, it relays the message in
   the same way as described for relaying Redirect messages in
   Section 3.9.7.  In particular, Server ('A') copies the correct UDP
   port number and IP address into the TLLAO, changes the link-layer
   source address to its own address, changes the link-layer destination
   address to the address of Client ('B'), then forwards the NA message
   based on an IP forwarding table entry matching the AERO address in
   the network-layer destination address.

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

   Note that these unsolicited NA messages are unacknowledged; hence,
   Client ('C') has no way of knowing whether Client ('B') has received
   them.  If the messages are somehow lost, however, Client ('B') will
   soon learn of the mobility event via the NUD procedures specified in
   Section 3.10.

3.11.2.  Moving to a New Server

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

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



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   forwarding table entries so that any packets already in the system
   can still be delivered to the Client.)

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

3.12.  Encapsulation Protocol Version Considerations

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

3.13.  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".)

3.14.  Operation on AERO Links Without DHCPv6 Services

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






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3.15.  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 and IP forwarding table entries by
   performing direct Client-to-Client Predirect/Redirect exchanges, and
   some other form of trust basis must be applied so that each Client
   can verify that the prospective neighbor is authorized to use its
   claimed CP.

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

4.  Implementation Status

   An application-layer implementation is in progress.

5.  IANA Considerations

   The IANA is instructed to assign a new 2-octet Hardware Type number
   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.

6.  Security Considerations

   AERO link security considerations are the same as for standard IPv6
   Neighbor Discovery [RFC4861] except that AERO improves on some
   aspects.  In particular, AERO uses a trust basis between Clients and
   Servers, where the Clients only engage in the AERO mechanism when it
   is facilitated by a trust anchor.  AERO also uses DHCPv6
   authentication for Client authentication and network admission
   control.

   AERO links must be protected against link-layer address spoofing
   attacks in which an attacker on the link pretends to be a trusted
   neighbor.  Links that provide link-layer securing mechanisms (e.g.,
   IEEE 802.1X WLANs) and links that provide physical security (e.g.,
   enterprise network wired LANs) provide a first line of defense that
   is often sufficient.  In other instances, additional securing
   mechanisms such as Secure Neighbor Discovery (SeND) [RFC3971], IPsec
   [RFC4301] or TLS [RFC5246] may be necessary.

   AERO Clients MUST ensure that their connectivity is not used by
   unauthorized nodes on EUNs to gain access to a protected network,
   i.e., AERO Clients that act as routers MUST NOT provide routing



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   services for unauthorized nodes.  (This concern is no different than
   for ordinary hosts that receive an IP address delegation but then
   "share" the address with unauthorized nodes via a NAT function.)

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

7.  Acknowledgements

   Discussions both on IETF lists and in private exchanges helped shape
   some of the concepts in this work.  Individuals who contributed
   insights include Mikael Abrahamsson, Fred Baker, Stewart Bryant,
   Brian Carpenter, Wojciech Dec, Brian Haberman, Joel Halpern, Sascha
   Hlusiak, Lee Howard, Joe Touch and Bernie Volz.  Members of the IESG
   also provided valuable input during their review process that greatly
   improved the document.  Special thanks go to Stewart Bryant, Joel
   Halpern and Brian Haberman for their shepherding guidance.

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

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

8.  References

8.1.  Normative References

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

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

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

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

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




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   [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.

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

8.2.  Informative References

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

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

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

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

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




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

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

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

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

   [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.





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

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

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

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

   [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.

Appendix A.  AERO Server and Relay Interworking

   Figure 2 depicts a reference AERO operational scenario with a single
   Server on the AERO link.  In order to support scaling to larger
   numbers of nodes, the AERO link can deploy multiple Servers and
   Relays, e.g., as shown in Figure 4.











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                             .-(::::::::)
                          .-(:::: IP ::::)-.
                         (:: Internetwork ::)
                          `-(::::::::::::)-'
                             `-(::::::)-'
                                  |
       +--------------+    +------+-------+    +--------------+
       |AERO Server C |    | AERO Relay D |    |AERO Server E |
       | (default->D) |    | (A->C; G->E) |    | (default->D) |
       |    (A->B)    |    +-------+------+    |    (G->F)    |
       +-------+------+            |           +------+-------+
               |                   |                  |
       X---+---+-------------------+------------------+---+---X
           |                  AERO Link                   |
     +-----+--------+                            +--------+-----+
     |AERO Client B |                            |AERO Client F |
     | (default->C) |                            | (default->E) |
     +--------------+                            +--------------+
           .-.                                         .-.
        ,-(  _)-.                                   ,-(  _)-.
     .-(_   IP  )-.                              .-(_   IP  )-.
    (__    EUN      )                           (__    EUN      )
       `-(______)-'                                `-(______)-'
            |                                           |
        +--------+                                  +--------+
        | Host A |                                  | Host G |
        +--------+                                  +--------+

                 Figure 4: AERO Server/Relay Interworking

   In this example, Client ('B') associates with Server ('C'), while
   Client ('F') associates with Server ('E').  Furthermore, Servers
   ('C') and ('E') do not associate with each other directly, but rather
   have an association with Relay ('D') (i.e., a router that has full
   topology information concerning its associated Servers and their
   Clients).  Relay ('D') connects to the AERO link, and also connects
   to the native IP Internetwork.

   When source host ('A') sends a packet toward destination host ('G'),
   IP forwarding directs the packet through the EUN to Client ('B'),
   which forwards the packet to Server ('C') and also generates a
   Predirect message.  Server ('C') forwards both the packet and
   Predirect through Relay ('D'), which then forwards both the packet
   and Predirect to Server ('E').  When Server ('E') receives the packet
   and Predirect message, it forwards them to Client ('F').

   After processing the Predirect message, Client ('F') sends a Redirect
   message to Server ('E').  Server ('E'), in turn, forwards the



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Internet-Draft                    AERO                         June 2014


   Redirect through Relay ('D') to Server ('C') which forwards the
   Redirect to Client ('B') informing it that host 'G's EUN can be
   reached via Client ('F'), thus completing the AERO redirection.

   The network-layer routing information shared between Servers and
   Relays must be carefully coordinated.  In particular, Relays require
   full topology information, while individual Servers only require
   partial topology information, i.e., they only need to know the set of
   ASPs associated with the AERO link and the CPs associated with their
   current set of associated Clients.  This can be accomplished in a
   number of ways, but a prominent example is 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 CPs.

   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.  Each Server maintains a working set of associated Clients,
   and dynamically announces new CPs and withdraws departed CPs in its
   BGP updates.  The Relays therefore discover the full topology of the
   AERO link in terms of the working set of Clients associated with each
   Server.  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, Servers SHOULD dampen any route oscillations caused by
   impatient Clients that repeatedly associate and disassociate with the
   Server.

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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