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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720,                 September 07, 2016
           rfc6179, rfc6706 (if
           approved)
Intended status: Standards Track
Expires: March 11, 2017


             Asymmetric Extended Route Optimization (AERO)
                     draft-templin-aerolink-71.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, address/prefix provisioning
   and mobility are supported by the Dynamic Host Configuration Protocol
   for IPv6 (DHCPv6), and route optimization is naturally supported
   through dynamic neighbor cache updates.  Although DHCPv6 and IPv6 ND
   messaging are used in the control plane, both IPv4 and IPv6 are
   supported in the data plane.  AERO is a widely-applicable tunneling
   solution especially well suited to mobile Virtual Private Networks
   (VPNs) and other applications as described in this document.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at 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 11, 2017.





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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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 . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Asymmetric Extended Route Optimization (AERO) . . . . . . . .   6
     3.1.  AERO Link Reference Model . . . . . . . . . . . . . . . .   6
     3.2.  AERO Link Node Types  . . . . . . . . . . . . . . . . . .   8
     3.3.  AERO Addresses  . . . . . . . . . . . . . . . . . . . . .   9
     3.4.  AERO Interface Characteristics  . . . . . . . . . . . . .  10
     3.5.  AERO Link Registration  . . . . . . . . . . . . . . . . .  12
     3.6.  AERO Interface Initialization . . . . . . . . . . . . . .  12
       3.6.1.  AERO Relay Behavior . . . . . . . . . . . . . . . . .  12
       3.6.2.  AERO Server Behavior  . . . . . . . . . . . . . . . .  13
       3.6.3.  AERO Client Behavior  . . . . . . . . . . . . . . . .  13
       3.6.4.  AERO Forwarding Agent Behavior  . . . . . . . . . . .  13
     3.7.  AERO Routing System . . . . . . . . . . . . . . . . . . .  13
     3.8.  AERO Interface Neighbor Cache Maintenace  . . . . . . . .  15
     3.9.  AERO Interface Sending Algorithm  . . . . . . . . . . . .  17
     3.10. AERO Interface Encapsulation and Re-encapsulation . . . .  19
     3.11. AERO Interface Decapsulation  . . . . . . . . . . . . . .  20
     3.12. AERO Interface Data Origin Authentication . . . . . . . .  20
     3.13. AERO Interface Packet Size Issues . . . . . . . . . . . .  20
     3.14. AERO Interface Error Handling . . . . . . . . . . . . . .  22
     3.15. AERO Router Discovery, Prefix Delegation and Address
           Configuration . . . . . . . . . . . . . . . . . . . . . .  25
       3.15.1.  AERO DHCPv6 Service Model  . . . . . . . . . . . . .  25
       3.15.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  26
       3.15.3.  AERO Server Behavior . . . . . . . . . . . . . . . .  29
     3.16. AERO Forwarding Agent Behavior  . . . . . . . . . . . . .  32
     3.17. AERO Link Route Optimization  . . . . . . . . . . . . . .  32
       3.17.1.  Reference Operational Scenario . . . . . . . . . . .  33
       3.17.2.  Concept of Operations  . . . . . . . . . . . . . . .  34
       3.17.3.  Message Format . . . . . . . . . . . . . . . . . . .  35



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       3.17.4.  Sending Predirects . . . . . . . . . . . . . . . . .  35
       3.17.5.  Re-encapsulating and Relaying Predirects . . . . . .  37
       3.17.6.  Processing Predirects and Sending Redirects  . . . .  37
       3.17.7.  Re-encapsulating and Relaying Redirects  . . . . . .  39
       3.17.8.  Processing Redirects . . . . . . . . . . . . . . . .  40
       3.17.9.  Server-Oriented Redirection  . . . . . . . . . . . .  40
       3.17.10. Route Optimization Policy  . . . . . . . . . . . . .  41
       3.17.11. Route Optimization and Multiple ACPs . . . . . . . .  41
     3.18. Neighbor Unreachability Detection (NUD) . . . . . . . . .  41
     3.19. Mobility Management . . . . . . . . . . . . . . . . . . .  42
       3.19.1.  Announcing Link-Layer Address Changes  . . . . . . .  42
       3.19.2.  Bringing New Links Into Service  . . . . . . . . . .  42
       3.19.3.  Removing Existing Links from Service . . . . . . . .  43
       3.19.4.  Implicit Mobility Management . . . . . . . . . . . .  43
       3.19.5.  Moving to a New Server . . . . . . . . . . . . . . .  43
       3.19.6.  Packet Queueing for Mobility . . . . . . . . . . . .  44
     3.20. Proxy AERO  . . . . . . . . . . . . . . . . . . . . . . .  44
     3.21. Extending AERO Links Through Security Gateways  . . . . .  47
     3.22. Extending IPv6 AERO Links to the Internet . . . . . . . .  49
     3.23. Operation on AERO Links Without DHCPv6 Services . . . . .  52
     3.24. Operation on Server-less AERO Links . . . . . . . . . . .  52
     3.25. AERO Operation without DHCPv6 Client/Server Exchanges . .  53
     3.26. Manually-Configured AERO Tunnels  . . . . . . . . . . . .  53
     3.27. Encapsulation Avoidance on Relay-Server Dedicated Links .  53
     3.28. Encapsulation Protocol Version Considerations . . . . . .  54
     3.29. Multicast Considerations  . . . . . . . . . . . . . . . .  54
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  54
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  54
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  55
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  56
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  57
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  57
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  58
   Appendix A.  AERO Alternate Encapsulations  . . . . . . . . . . .  65
   Appendix B.  When to Insert an Encapsulation Fragment Header  . .  66
   Appendix C.  Autoconfiguration for Constrained Platforms  . . . .  67
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  68

1.  Introduction

   This document specifies the operation of IP over tunnel virtual links
   using Asymmetric Extended Route Optimization (AERO).  The AERO link
   can be used for tunneling 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 [RFC5522].



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   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, address/prefix provisioning and mobility are supported by
   the Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315],
   and route optimization is naturally supported through dynamic
   neighbor cache updates.  Although DHCPv6 and IPv6 ND messaging are
   used in the control plane, both IPv4 and IPv6 can be used in the data
   plane.

   AERO is applicable to a wide variety of use cases.  For example, it
   can be used to coordinate the Virtual Private Network (VPN) links of
   mobile devices (e.g., cellphones, tablets, laptop computers, etc.)
   that connect into a home enterprise network via public access
   networks.  AERO can also be applied to aviation applications for both
   manned and unmanned aircraft where the aircraft is treated as a
   mobile host or router that can connect an Internet of Things (IoT).
   Numerous other use cases are also in scope.  The remainder of this
   document presents the AERO specification.

2.  Terminology

   The terminology in the normative references applies; the following
   terms are defined within the scope of this document:

   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 even though they may be
      separated by many underlying network hops.  AERO can also operate
      over native multiple access link types (e.g., Ethernet, WiFi etc.)
      when a tunnel virtual overlay is not needed.

   AERO interface
      a node's attachment to an AERO link.  Since the addresses assigned
      to an AERO interface are obtained from the unique prefix
      delegations it receives, AERO interfaces do not require Duplicate
      Address Detection (DAD) and therefore set the administrative
      variable DupAddrDetectTransmits to zero [RFC4862].

   AERO address
      an IPv6 link-local address constructed as specified in Section 3.3
      and assigned 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.



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   AERO Client ("Client")
      a node that issues DHCPv6 messages to receive IP Prefix
      Delegations (PDs) from one or more AERO Servers.  Following PD,
      the Client assigns an AERO address to the AERO interface for use
      in DHCPv6 and IPv6 ND exchanges with other AERO nodes.

   AERO Server ("Server")
      a node that configures an AERO interface to provide default
      forwarding and DHCPv6 services for AERO Clients.  The Server
      assigns an administratively provisioned IPv6 link-local unicast
      address to support the operation of DHCPv6 and the IPv6 ND
      protocol.  An AERO Server can also act as an AERO Relay.

   AERO Relay ("Relay")
      a node that configures an AERO interface to relay IP packets
      between nodes on the same AERO link and/or forward IP packets
      between the AERO link and the native Internetwork.  The Relay
      assigns an administratively provisioned IPv6 link-local unicast
      address to the AERO interface the same as for a Server.  An AERO
      Relay can also act as an AERO Server.

   AERO Forwarding Agent ("Forwarding Agent")
      a node that performs data plane forwarding services as a companion
      to 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 which the
      tunnel virtual overlay is configured.

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

   link-layer address
      an IP address assigned to an AERO node's underlying interface.
      When UDP encapsulation is used, the UDP port number is also
      considered as part of the link-layer address; otherwise, UDP port
      number is set to the constant value '0'.  Link-layer addresses are
      used as the encapsulation header source and destination addresses.




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

   end user network (EUN)
      an internal virtual or external edge IP network that an AERO
      Client connects to the rest of the network via the AERO interface.

   AERO Service Prefix (ASP)
      an IP prefix associated with the AERO link and from which more-
      specific AERO Client Prefixes (ACPs) are derived.

   AERO Client Prefix (ACP)
      an IP prefix derived from an ASP and delegated to a Client, where
      the ACP prefix length must be no shorter than the ASP prefix
      length and must be no longer than 64 for IPv6 or 32 for IPv4.

   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 [RFC3315].

   The terminology of DHCPv6 [RFC3315] and IPv6 ND [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].  Lower case
   uses of these words are not to be interpreted as carrying RFC2119
   significance.

3.  Asymmetric Extended Route Optimization (AERO)

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

3.1.  AERO Link Reference Model












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                              .-(::::::::)
                           .-(:::: IP ::::)-.
                          (:: Internetwork ::)
                           `-(::::::::::::)-'
                              `-(::::::)-'
                                   |
       +--------------+   +--------+-------+   +--------------+
       |AERO Server S1|   | AERO Relay R1  |   |AERO Server S2|
       |  Nbr: C1; R1 |   |   Nbr: S1; S2  |   |  Nbr: C2; R1 |
       |  default->R1 |   |(P1->S1; P2->S2)|   |  default->R1 |
       |    P1->C1    |   |      ASP A1    |   |    P2->C2    |
       +-------+------+   +--------+-------+   +------+-------+
               |                   |                  |
       X---+---+-------------------+------------------+---+---X
           |                  AERO Link                   |
     +-----+--------+                            +--------+-----+
     |AERO Client C1|                            |AERO Client C2|
     |    Nbr: S1   |                            |   Nbr: S2    |
     | default->S1  |                            | default->S2  |
     |    ACP P1    |                            |    ACP P2    |
     +--------------+                            +--------------+
           .-.                                         .-.
        ,-(  _)-.                                   ,-(  _)-.
     .-(_   IP  )-.                              .-(_   IP  )-.
    (__    EUN      )                           (__    EUN      )
       `-(______)-'                                `-(______)-'
            |                                           |
        +--------+                                  +--------+
        | Host H1|                                  | Host H2|
        +--------+                                  +--------+

                    Figure 1: AERO Link Reference Model

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

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

   o  AERO Servers S1 and S2 associate with Relay R1 and also act as
      default routers for their associated Clients C1 and C2.

   o  AERO Clients C1 and C2 associate with Servers S1 and S2,
      respectively.  They receive AERO Client Prefix (ACP) delegations
      P1 and P2, and also act as default routers for their associated
      physical or internal virtual EUNs.  (Alternatively, clients can
      act as multi-addressed hosts without serving any EUNs).




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   o  Simple hosts H1 and H2 attach to the EUNs served by Clients C1 and
      C2, respectively.

   Each AERO node maintains an AERO interface neighbor cache and an IP
   forwarding table.  For example, AERO Relay R1 in the diagram has
   neighbor cache entries for Servers S1 and S2 as well as IP forwarding
   table entries for the ACPs delegated to Clients C1 and C2.  In common
   operational practice, there may be many additional Relays, Servers
   and Clients.  (Although not shown in the figure, AERO Forwarding
   Agents may also be provided for data plane forwarding offload
   services.)

3.2.  AERO Link Node Types

   AERO Relays provide default forwarding services to AERO Servers.
   Relays forward packets between neighbors connected to the same AERO
   link and also forward packets between the AERO link and the native IP
   Internetwork.  Relays present the AERO link to the native
   Internetwork as a set of one or more AERO Service Prefixes (ASPs) and
   serve as a gateway between the AERO link and the Internetwork.  AERO
   Relays maintain an AERO interface neighbor cache entry for each AERO
   Server, and maintain an IP forwarding table entry for each AERO
   Client Prefix (ACP).  AERO Relays can also be configured to act as
   AERO Servers.

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

   AERO Clients act as requesting routers to receive ACPs through 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, load balancing, etc.  Each IPv6 Client receives at
   least a /64 IPv6 ACP, and may receive even shorter prefixes.
   Similarly, each IPv4 Client receives at least a /32 IPv4 ACP (i.e., a
   singleton IPv4 address), and may receive even shorter prefixes.  AERO
   Clients maintain an AERO interface neighbor cache entry for each of
   their associated Servers as well as for each of their correspondent
   Clients.





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   AERO Forwarding Agents provide data plane forwarding services as
   companions to AERO Servers.  Note that while Servers are required to
   perform both control and data plane operations on their own behalf,
   they may optionally enlist the services of special-purpose Forwarding
   Agents to offload data plane traffic.

3.3.  AERO Addresses

   An AERO address is an IPv6 link-local address with an embedded ACP
   and assigned to a Client's AERO interface.  The AERO address remains
   stable as the Client moves between topological locations, i.e., even
   if its link-layer addresses change.

   For IPv6, the AERO address begins with the prefix fe80::/64 and
   includes in its interface identifier (i.e., the lower 64 bits) 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 based on an IPv4-mapped IPv6 address
   [RFC4291] formed from the ACP and with a Prefix Length of 96 plus the
   ACP prefix length.  For example, for the IPv4 ACP 192.0.2.32/28 the
   IPv4-mapped IPv6 address is:

      0:0:0:0:0:FFFF:192.0.2.32/124

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

      fe80::FFFF:192.0.2.32

   NOTE: In some cases, prospective neighbors may not have advanced
   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.  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 as though it were
   addressed to fe80::2001:db8:1000:2000.





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3.4.  AERO Interface Characteristics

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

   AERO interfaces maintain a neighbor cache, and AERO nodes use both
   DHCPv6 and IPv6 ND control messaging to manage the creation,
   modification and deletion of neighbor cache entries.

   AERO Clients send DHCPv6 Solicit, Rebind, Renew and Release messages
   to AERO Servers, which respond with DHCPv6 Reply messages.  AERO
   nodes use unicast IPv6 ND 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 IPv6 ND redirection message types -- the
   first known as a Predirect message and the second being the standard
   Redirect message (see Section 3.17).

   AERO interface ND messages include one or more Source/Target Link-
   Layer Address Options (S/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     |   Length = 5  |          Reserved1            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Reserved2   | Interface ID  |        UDP Port Number        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                          IP Address                           +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P00|P01|P02|P03|P04|P05|P06|P07|P08|P09|P10|P11|P12|P13|P14|P15|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P16|P17|P18|P19|P20|P21|P22|P23|P24|P25|P26|P27|P28|P29|P30|P31|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P32|P33|P34|P35|P36|P37|P38|P39|P40|P41|P42|P43|P44|P45|P46|P47|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |P48|P49|P50|P51|P52|P53|P54|P55|P56|P57|P58|P59|P60|P61|P62|P63|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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



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   In this format:

   o  Type is set to '1' for SLLAO or '2' for TLLAO the same as for IPv6
      ND.

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

   o  Both Reserved fields are set to the value '0' on transmission and
      ignored on receipt.

   o  Interface ID is set to an integer value between 0 and 255
      corresponding to an underlying interface of the AERO node.

   o  UDP Port Number and IP Address are set to the addresses used by
      the AERO node when it sends encapsulated packets over the
      underlying interface.  When UDP is not used as part of the
      encapsulation, UDP Port Number is set to the value '0'.  When the
      encapsulation IP address family is IPv4, IP Address is formed as
      an IPv4-mapped IPv6 address as specified in Section 3.3.

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

   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, cellular, 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 IPv6 ND messages include only a single S/TLLAO with
   Interface 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 multiple link-
   layer addresses.  In that case, IPv6 ND messages MAY include multiple
   S/TLLAOs -- each with an Interface ID that corresponds to a specific
   underlying interface of the AERO node.





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3.5.  AERO Link Registration

   When an administrative authority first deploys a set of AERO Relays
   and Servers that comprise an AERO link, they also assign a unique
   domain name for the link, e.g., "linkupnetworks.example.com".  Next,
   if administrative policy permits Clients within the domain to serve
   as correspondent nodes for Internet mobile nodes, the administrative
   authority adds a Fully Qualified Domain Name (FQDN) for each of the
   AERO link's ASPs to the Domain Name System (DNS) [RFC1035].  The FQDN
   is based on the suffix "aero.linkupnetworks.net" with a prefix formed
   from the wildcard-terminated reverse mapping of the ASP
   [RFC3596][RFC4592], and resolves to a DNS PTR resource record.  For
   example, for the ASP '2001:db8:1::/48' within the domain name
   "linkupnetworks.example.com", the DNS database contains:

   '*.1.0.0.0.8.b.d.0.1.0.0.2.aero.linkupnetworks.net.  PTR
   linkupnetworks.example.com'

   This DNS registration advertises the AERO link's ASPs to prospective
   correspondent nodes.

3.6.  AERO Interface Initialization

3.6.1.  AERO Relay Behavior

   When a Relay enables an AERO interface, it first assigns an
   administratively provisioned link-local address fe80::ID to the
   interface.  Each fe80::ID address MUST be unique among all AERO nodes
   on the link, and MUST NOT collide with any potential AERO addresses
   nor the special addresses fe80:: and fe80::ffff:ffff:ffff:ffff.  (The
   fe80::ID addresses are typically taken from the available range
   fe80::/96, e.g., as fe80::1, fe80::2, fe80::3, etc.)  The Relay then
   engages in a dynamic routing protocol session with all Servers on the
   link (see: Section 3.7), and advertises its assigned ASPs into the
   native IP Internetwork.

   Each Relay subsequently maintains an IP forwarding table entry for
   each ACP covered by its ASP(s), and maintains a neighbor cache entry
   for each Server on the link.  Relays exchange NS/NA messages with
   AERO link neighbors the same as for any AERO node, however they
   typically do not perform explicit Neighbor Unreachability Detection
   (NUD) (see: Section 3.18) since the dynamic routing protocol already
   provides reachability confirmation.








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3.6.2.  AERO Server Behavior

   When a Server enables an AERO interface, it assigns an
   administratively provisioned link-local address fe80::ID the same as
   for Relays.  The Server further configures a DHCPv6 server function
   to facilitate DHCPv6 PD exchanges with AERO Clients.  The Server
   maintains a neighbor cache entry for each Relay on the link, and
   manages per-ACP neighbor cache entries and IP forwarding table
   entries based on control message exchanges.  Each Server also engages
   in a dynamic routing protocol with each Relay on the link (see:
   Section 3.7).

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

3.6.3.  AERO Client Behavior

   When a Client enables an AERO interface, it uses the special address
   fe80::ffff:ffff:ffff:ffff to obtain one or more ACPs from an AERO
   Server via DHCPv6 PD.  Next, it assigns the corresponding AERO
   address(es) to the AERO interface and creates a neighbor cache entry
   for the Server, i.e., the DHCPv6 PD exchange bootstraps
   autoconfiguration of unique link-local address(es).  The Client
   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.

3.6.4.  AERO Forwarding Agent Behavior

   When a Forwarding Agent enables an AERO interface, it assigns the
   same link-local address(es) as the companion AERO Server.  The
   Forwarding Agent thereafter provides data plane forwarding services
   based solely on the forwarding information assigned to it by the
   companion AERO Server.

3.7.  AERO Routing System

   The AERO routing system is based on a private instance of the Border
   Gateway Protocol (BGP) [RFC4271] that is coordinated between Relays
   and Servers and does not interact with either the public Internet BGP
   routing system or the native IP Internetwork interior routing system.



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   Relays advertise only a small and unchanging set of ASPs to the
   native routing system instead of the full dynamically changing set of
   ACPs.

   In a reference deployment, each AERO Server is configured as an
   Autonomous System Border Router (ASBR) for a stub Autonomous System
   (AS) using an AS Number (ASN) that is unique within the BGP instance,
   and each Server further peers with each Relay but does not peer with
   other Servers.  Similarly, Relays do not peer with each other, since
   they will reliably 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 ACPs, and
   dynamically announces new ACPs and withdraws departed ACPs in its BGP
   updates to Relays.  Clients are expected to remain associated with
   their current Servers for extended timeframes, however Servers SHOULD
   selectively suppress BGP updates for impatient Clients that
   repeatedly associate and disassociate with them in order to dampen
   routing churn.

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

   Scaling properties of the AERO routing system are limited by the
   number of BGP routes that can be carried by Relays.  Assuming O(10^6)
   as a reasonable maximum number of BGP routes, this means that O(10^6)
   Clients can be serviced by a single set of Relays.  A means of
   increasing scaling would be to assign a different set of Relays for
   each set of ASPs.  In that case, each Server still peers with each
   Relay, but the Server institutes route filters so that each set of
   Relays only receives BGP updates for the ASPs they aggregate.  For
   example, if the ASP for the AERO link is 2001:db8::/32, a first set
   of Relays could service the ASP segment 2001:db8::/40, a second set
   of Relays could service 2001:db8:0100::/40, a third set could service
   2001:db8:0200::/40, etc.

   Assuming up to O(10^3) sets of Relays, the AERO routing system can
   then accommodate O(10^9) ACPs with no additional overhead for Servers
   and Relays (for example, it should be possible to service 4 billion
   /64 ACPs taken from a /32 ASP and even more for shorter ASPs).  In
   this way, each set of Relays services a specific set of ASPs that



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   they advertise to the native routing system, and each Server
   configures ASP-specific routes that list the correct set of Relays as
   next hops.  This arrangement also allows for natural incremental
   deployment, and can support small scale initial deployments followed
   by dynamic deployment of additional Clients, Servers and Relays
   without disturbing the already-deployed base.

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

   Finally, Realys can express preferences for ACPs learned from
   multiple Servers by assigning a BGP weight to each Server's peering
   configuration.  In this way Relays can choose the Serevr with the
   highest weight as the preferred path, and then fail over to a Server
   with lower weight in case of ACP withdrawl or Server failure.

3.8.  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
   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.  Each entry
   maintains the mapping between the neighbor's fe80::ID network-layer
   address and corresponding link-layer address.

   Static neighbor cache entries are created through DHCPv6 PD exchanges
   as specified in Section 3.15 and remain in place for durations
   bounded by prefix lifetimes.  AERO Servers maintain static neighbor
   cache entries for the ACPs of each of their associated Clients, and
   AERO Clients maintain a static neighbor cache entry for each of their
   associated Servers.  When an AERO Server sends a Reply message
   response to a Client's Solicit, Rebind or Renew message, it creates
   or updates a static neighbor cache entry based on the Client's DHCP
   Unique Identifier (DUID) as the Client identifier, the AERO
   address(es) corresponding to the Client's ACP(s) as the network-layer



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   address(es), 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(s) as the length to
   apply to the AERO address(es).  When an AERO Client receives a 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 or updated based on
   receipt of a Predirect/Redirect message as specified in Section 3.17,
   and are garbage-collected when keepalive timers expire.  AERO Clients
   maintain dynamic neighbor cache entries for each of their active
   correspondent Client ACPs 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 to ACCEPT_TIME seconds 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 to FORWARD_TIME
   seconds and uses this value to determine whether packets can be sent
   directly to the correspondent.  The Client also sets a "MaxRetry"
   variable to MAX_RETRY to limit the number of keepalives sent when a
   correspondent may have gone unreachable.

   It is RECOMMENDED that 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




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   sufficiently longer than FORWARD_TIME to allow the AERO redirection
   procedure to converge.

   When there may be a Network Address Translator (NAT) between the
   Client and the Server, or if the path from the Client to the Server
   should be tested for reachability, the Client can send periodic RS
   messages to the Server to receive RA replies.  The RS/RA messaging
   will keep NAT state alive and test Server reachability without
   disturbing the DHCPv6 server.

3.9.  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 may
   have 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/hosts and sub-IP layer forwarding nodes.  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
   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 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



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   uses the link-layer address in a permanent neighbor cache entry for a
   Relay selected through longest-prefix-match as the link-layer address
   for encapsulation.

   When an IP packet enters a 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 corresponding
   neighbor cache entry 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 does not match 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 a link-
   local address or a non-link-local address that matches an ASP, and
   there is a more-specific ACP entry in the IP forwarding table, the
   Relay uses the link-layer address in the corresponding neighbor cache
   entry as the link-layer address for encapsulation and re-admits the
   packet into the AERO link.  When an IP packet enters a Relay's AERO
   interface from either the network layer or link-layer, and the
   packet's destination address matches an ASP but there is no more-
   specific ACP entry, the Relay drops the packet and returns an ICMP
   Destination Unreachable message (see: Section 3.14).

   When an AERO Server receives a packet from a Relay via the AERO
   interface, the Server MUST NOT forward the packet back to the same or
   a different Relay.

   When an AERO Relay receives a packet from a Server via the AERO
   interface, the Relay MUST NOT forward the packet back to the same
   Server.

   When an AERO node re-admits a packet into the AERO link without
   involving the network layer, the node MUST NOT decrement the network
   layer TTL/Hop-count.

   When an AERO node forwards a data packet to the primary link-layer
   address of a Server, it may receive Redirect messages with an SLLAO
   that include the link-layer address of an AERO Forwarding Agent.  The
   AERO node SHOULD record the link-layer address in the neighbor cache
   entry for the neighbor and send subsequent data packets via this
   address instead of the Server's primary address (see: Section 3.16).

   AERO nodes may have multiple underlying interfaces and/or neighbor
   cache entries for Clients with multiple Interface ID registrations
   (see Section 3.4).  The AERO node uses the packet's DSCP value to
   select the outgoing underlying interface based on its own Interface
   ID preference values and to select the destination link-layer address



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   based on the neighbor's Interface ID with the highest preference
   value.  If multiple Interface IDs have a preference of "high", the
   AERO node sends one copy of the packet to each of the link-layer
   addresses (i.e., it replicates the packet); otherwise, the node sends
   a single copy of the packet.

3.10.  AERO Interface Encapsulation and Re-encapsulation

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

   The AERO interface encapsulates packets per the Generic UDP
   Encapsulation (GUE) encapsulation procedures in
   [I-D.ietf-nvo3-gue][I-D.herbert-gue-fragmentation], or through an
   alternate encapsulation format (see: Appendix A).  For packets
   entering the AERO link from the IP layer, the AERO interface copies
   the "TTL/Hop Limit", "Type of Service/Traffic Class" [RFC2983], "Flow
   Label"[RFC6438].(for IPv6) and "Congestion Experienced" [RFC3168]
   values in the packet's IP header into the corresponding fields in the
   encapsulation IP header.  For packets undergoing re-encapsulation
   within the AERO link, the AERO interface instead copies the "TTL/Hop
   Limit", "Type of Service/Traffic Class", "Flow Label" and "Congestion
   Experienced" values in the original encapsulation IP header into the
   corresponding fields in the new encapsulation IP header, i.e., the
   values are transferred between encapsulation headers and *not* copied
   from the encapsulated packet's network-layer header.

   When GUE encapsulation is used, the AERO interface next sets the UDP
   source port to a constant value that it will use in each successive
   packet it sends, and sets the UDP length field to the length of the
   encapsulated packet plus 8 bytes for the UDP header itself plus the
   length of the GUE header (or 0 if GUE direct IP encapsulation is
   used).  For packets sent to a Server, the AERO interface sets the UDP
   destination port to 8060, i.e., the IANA-registered port number for
   AERO.  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 then
   either includes or omits the UDP checksum according to the GUE
   specification.

   For IPv4 encapsulation, the AERO interface sets the DF bit as
   discussed in Section 3.13.







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3.11.  AERO Interface Decapsulation

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

3.12.  AERO Interface Data Origin Authentication

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

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

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

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

   o  AERO Clients, Servers and Relays 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 is effective in
   environments in which link-layer addresses cannot be spoofed.  In
   other environments, each AERO message must include a signature that
   the recipient can use to authenticate the message origin.

3.13.  AERO Interface Packet Size Issues

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

   IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of 1280
   bytes [RFC2460].  Although IPv4 specifies a smaller minimum link MTU




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   of 68 bytes [RFC0791], AERO interfaces also observe the IPv6 minimum
   for IPv4 even if the packet may incur fragmentation in the network.

   IPv6 specifies a minimum Maximum Reassembly Unit (MRU) of 1500 bytes
   [RFC2460], while the minimum MRU for IPv4 is only 576 bytes [RFC1122]
   (note that IPv6 over IPv4 tunnels assume a larger MRU than the IPv4
   minimum).

   Original sources expect that IP packets will either be delivered to
   the final destination or a suitable Packet Too Big (PTB) message
   returned.  However, PTB messages may be crafted for malicious
   purposes such as denial of service, or lost in the network [RFC2923]
   resulting in failure of the IP Path MTU Discovery (PMTUD) mechanisms
   [RFC1191][RFC1981].  For these reasons, AERO links employ operational
   procedures that avoid all interactions with PMTUD.

   AERO Servers advertise an MTU that MUST be no smaller than 1280
   bytes, MUST be no larger than the minimum MRU among all nodes on the
   AERO link minus the encapsulation overhead ("ENCAPS"), and SHOULD be
   no smaller than 1500 bytes.  AERO Servers advertise a Maximum
   Fragment Unit (MFU) as the maximum size for the fragments of an
   encapsulated packet that require fragmentation.  The MFU value MUST
   NOT be larger than (MTU+ENCAPS) and MUST NOT be larger than 1280
   bytes unless there is operational assurance that a larger size can
   traverse the link along all paths without fragmentation.

   AERO Clients set the AERO interface MTU/MFU based on the values
   advertised by their Server, and configure an MRU large enough to
   reassemble packets up to (MTU+ENCAPS) bytes.

   All AERO nodes on the link MUST configure the same MTU/MFU values for
   reasons cited in [RFC3819][RFC4861] (in particular, multicast support
   requires a common MTU value among all nodes on the link).

   All AERO nodes on the link MUST configure a minimum MRU of
   (1500+ENCAPS) bytes, and SHOULD be capable of setting a larger MRU
   accoding to the Server's advertised MTU.

   In accordnace with these requirements, the ingress accommodates
   packets of various sizes as follows:

   o  First, for each original IPv4 packet that is larger than the AERO
      interface MTU and with the DF bit set to 0, the ingress uses IPv4
      fragmentation to break the packet into a minimum number of non-
      overlapping fragments where the first fragment is no larger than
      (MFU-ENCAPS) bytes and the remaining fragments are no larger than
      the first.




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   o  Next, for each original IP packet or fragment that is no larger
      than (MFU-ENCAPS) bytes, the ingress encapsulates the packet and
      admits it into the tunnel.  For IPv4 AERO links, the ingress sets
      the Don't Fragment (DF) bit to 0 so that these packets will be
      delivered to the egress even if some fragmentation occurs in the
      network.

   o  For all other original IP packets or fragments, if the packet is
      larger than the AERO interface MTU, the ingress drops the packet
      and returns a PTB message to the original source.  Otherwise, the
      ingress encapsulates the packet and fragments the encapsulated
      packet into a minimum number of non-overlapping fragments where
      the first fragment is no larger than MFU bytes and the remaining
      fragments are no larger than the first.  The ingress then admits
      the fragments into the tunnel, and for IPv4 sets the DF bit to 0
      in the IP encapsulation header.  These fragmented encapsulated
      packets will be delivered to the egress, which reassembles them
      into a whole packet.

   Several factors must be considered when fragmentation of the
   encapsulated packet is needed.  For AERO links over IPv4, the IP ID
   field is only 16 bits in length, meaning that fragmentation at high
   data rates could result in data corruption due to reassembly
   misassociations [RFC6864][RFC4963].  For AERO links over both IPv4
   and IPv6, studies have also shown that IP fragments are dropped
   unconditionally over some network paths [I-D.taylor-v6ops-fragdrop].
   In environments where IP fragmentation issues could result in
   operational problems, the ingress SHOULD employ intermediate-layer
   fragmentation (see: [RFC2764] and [I-D.herbert-gue-fragmentation])
   before appending the outer encapsulation headers to each fragment.

   Since the encapsulation fragment header reduces the room available
   for packet data, but the original source has no way to control its
   insertion, the ingress MUST include the fragment header length in the
   ENCAPS length even for packets in which the header is absent.

3.14.  AERO Interface Error Handling

   When an AERO node admits encapsulated packets into the AERO
   interface, it may receive link-layer (L2) or network-layer (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.




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   The IP header is followed by an ICMP header that includes an error
   Type, Code and Checksum.  Valid type values include "Destination
   Unreachable", "Time Exceeded" and "Parameter Problem"
   [RFC0792][RFC4443].  (AERO interfaces ignore all L2 IPv4
   "Fragmentation Needed" and IPv6 "Packet Too Big" messages since they
   only emit packets that are guaranteed to be no larger than the IP
   minimum link MTU.)

   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:

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~
        |        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



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

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

   o  When an AERO node receives persistent L2 IPv4 Time Exceeded
      messages, the IP ID field may be wrapping before earlier fragments
      have been processed.  In that case, the node SHOULD begin
      including integrity checks and/or institute rate limits for
      subseqent packets.

   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 3.18).  If NUD fails, the Client SHOULD set
      ForwardTime for the corresponding dynamic neighbor cache entry to
      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 for the Client's ACP, withdraw its route for the ACP
      from the AERO routing system and delete the neighbor cache entry
      (see Section 3.18 and Section 3.19).

   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 AERO routing system is likely in a temporary transitional
      state that will soon re-converge.  In case of a prolonged outage,
      however, the AERO routing system will compensate for Relays or
      Servers that have fallen silent.

   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



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   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 an encapsulated packet for which the
   reassembly buffer it too small, it drops the packet and returns an L3
   Packet To Big (PTB) message.  The node first 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 and forwards the message to the next
   hop within the AERO interface.  Otherwise, the node uses one of its
   non link-local addresses as the source address of the L3 PTB 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
   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.

3.15.  AERO Router Discovery, Prefix Delegation and Address
       Configuration

   AERO Router Discovery, Prefix Delegation and Address Configuration
   are coordinated by the DHCPv6 control messaging protocol as discussed
   in the following Sections.

3.15.1.  AERO DHCPv6 Service Model

   Each AERO Server configures a DHCPv6 server function to facilitate PD
   requests from Clients.  Each Server is provisioned with a database of
   ACP-to-Client ID mappings for all Clients enrolled in the AERO
   system, as well as any information necessary to authenticate each



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   Client.  The Client database is maintained by a central
   administrative authority for the AERO link and securely distributed
   to all Servers, e.g., via the Lightweight Directory Access Protocol
   (LDAP) [RFC4511] or a similar distributed database service.

   Therefore, no Server-to-Server DHCPv6 PD state synchronization is
   necessary, and Clients can optionally hold separate PDs for the same
   ACPs from multiple Servers.  In this way, Clients can associate with
   multiple Servers, and can receive new PDs from new Servers before
   deprecating PDs received from existing Servers.  This provides the
   Client with a natural fault-tolerance and/or load balancing profile.

   AERO Clients and Servers exchange configuration information using an
   AERO Vendor-Specific Information Option (AVSIO) formatted as follows:

        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_VENDOR_OPTS       |            option-len         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   enterprise-number = 45282                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                          option-data                          .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 4: AERO Vendor-Specific Information Option (AVSIO)

   In this format, "enterprise-number" is set to 45282 (i.e., the IANA-
   reserved enterprise number for AERO) and "option-length" is set to
   the total length of the option.  A single AVSIO may include one or
   more AERO-specific (sub)options as defined in the following sections.

   AERO Clients MUST include an AVSIO in DHCPv6 Solicit and Rebind
   messages to manage the Server's cached link-layer addresses and
   preferences.  AERO Servers MUST include an AVSIO in DHCPv6 Reply
   messages that correspond to a Client's DHCPv6 message that also
   included an AVSIO option.

   The following sections specify the Client and Server behavior in more
   detail.

3.15.2.  AERO Client Behavior

   AERO Clients discover the link-layer addresses of AERO Servers via
   static configuration (e.g., from a flat-file map of Server addresses
   and locations), or through an automated means such as DNS name



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   resolution.  In the absence of other information, the Client resolves
   the FQDN "linkupnetworks.[domainname]" where "linkupnetworks" is a
   constant text string and "[domainname]" is a DNS suffix for the
   Client's underlying network (e.g., "example.com").  After discovering
   the link-layer addresses, the Client associates with one or more of
   the corresponding Servers.

   To associate with a Server, the Client acts as a requesting router to
   request ACPs through a two-message (i.e., Solicit/Reply) DHCPv6 PD
   exchange [RFC3315][RFC3633].  The Client's includes
   fe80::ffff:ffff:ffff:ffff as the IPv6 source address of the Solicit
   message, 'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination
   address, an underlying interface address of the Client (i.e., the
   link-layer address) as the link-layer source address and the link-
   layer address of the Server as the link-layer destination address.
   The Client also includes a Rapid Commit option, a Client Identifier
   option with the Client's DUID, and an Identity Association for Prefix
   Delegation (IA_PD) option.  If the Client is pre-provisioned with
   ACPs associated with the AERO service, it MAY also include the ACPs
   in the IA_PD to indicate its preferences to the DHCPv6 server.

   The Client also includes an AVSIO option with one or more AERO Client
   Link-Layer Address Options (ACLLAOs) to register its link-layer
   address(es) with the Server.  The first ACLLAO MUST be specific to
   the underlying interface over which the Client will send the Solicit.
   The Client MAY include additonal ACLLAOs specific to other underlying
   interfaces, but if so it MUST have assurance that there will be no
   NATs on the paths to the Server via those interfaces.  (Otherwise,
   the Client MAY issue subsequent Rebind messages after the initial
   Solicit/Reply exchange to register additional link-layer addresses).
   The Server will echo the ACLLAOs in the corresponding Reply message
   as specified in Section 3.15.3.

   The format for the ACLLAO is 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | opt-code = OPTION_ACLLAO (0)  |           option-len          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                 AERO Client Link-Layer Address                .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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





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   In the above format, the Client sets 'opt-code' to 0
   ("OPTION_ACLLAO") and sets 'option-len' to 36 (i.e., the length of
   the option following this field).  The Client then includes an "AERO
   Client Link-Layer Address" in the same format as for S/TLLAOs in
   Figure 2 beginning with the 'Reserved2' field and extending to the
   end of the S/TLLAO.  The Client then sets 'Reserved2', 'Interface
   ID', 'UDP Port Number', 'IP address' and 'P(i)' values for the
   specific underlying interface the same as for S/TLLAO options (see
   Section 3.4).  The Client finally includes any additional DHCPv6
   options (including any necessary authentication options to identify
   itself to the DHCPv6 server), and sends the encapsulated Solicit
   message via the underlying interface corresponding to the Interface
   ID of the first ACLLAO.

   When the Client attempts to perform a DHCPv6 PD exchange with a
   Server that is too busy to service the request, the Client may
   receive an error status code such as "NoPrefixAvail" in the Server's
   Reply [RFC3633] or no Reply at all.  In that case, the Client SHOULD
   discontinue DHCPv6 PD attempts through this Server and try another
   Server.

   When the Client receives a Reply from the AERO Server with an AVSIO
   option and no error status codes, it can compare the UDP Port Number
   and IP Address values in the first ACLLAO with the values the Client
   provided in its request.  If the values are different, the Client can
   infer that there is a NAT on the path to the Server via that
   underlying interface.  If the AVSIO option also includes an ALINFO
   sub-option, the Client also assigns the MTU/MFU values in the ALINFO
   option to its AERO interface, then caches any ASPs included in the
   ALINFO option as ASPs to associate with the AERO link (see
   Section 3.15.3).  This configuration information applies to the AERO
   link as a whole, and all Clients will receive the same information.

   The Client next creates a static neighbor cache entry with the
   Server's link-local address as the network-layer address and the
   Server's encapsulation address as the link-layer address.  Next, the
   Client autoconfigures an AERO address for each of the delegated ACPs,
   assigns the address(es) to the AERO interface and sub-delegates the
   ACPs to its attached EUNs and/or the Client's own internal virtual
   interfaces.  The Client can then configure as many addresses as it
   wants from /64 prefixes taken from the ACPs and assign them to either
   an internal virtual interface ("weak end-system") or to the AERO
   interface itself ("strong end-system") [RFC1122] while black-holing
   the remaining portions of the /64s.  Finally, the Client assigns a
   default IP route to the AERO interface with the link-local address of
   the Server as the next hop and with the PD lifetime as the default
   router lifetime.




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   After the initial Solicit/Reply exchange, the Client SHOULD begin
   using the AERO address as the source address for further DHCPv6
   messaging.  The Client subsequently renews its ACP delegations
   through each of its Servers by sending Renew messages with the link-
   layer address of a Server as the link-layer destination address.  The
   Client MAY subsequently issue Rebind messages with additional ACLLAOs
   if it wishes to register additional Interface IDs and/or update the
   link-layer address information for existing Interface IDs.  In that
   case, the Rebind message MUST be sent over the underlying interface
   corresponding to the first ACLLAO in the message, i.e., the same as
   for Solicits.

   After an AERO Client registers its Interface IDs and their associated
   'P(i)' values with the AERO Server, the Client may wish to change one
   or more Interface ID registrations, e.g., if an underlying interface
   becomes unavailable, if cost profiles change, etc.  To do so, the
   Client prepares a Rebind message to send over any available
   underlying interface.  The Rebind MUST include the ACLLAO specific to
   the selected avaialble underlying interface as the first ACLLAO and
   MAY include any additional ACLLAOs specific to other underlying
   interfaces.  The Client includes fresh 'P(i)' values in each ACLLAO
   to update the Server's neighbor cache entry.  If the Client wishes to
   disable some or all DSCPs for an underlying interface, it includes an
   ACLLAO with 'P(i)' values set to 0 ("disabled").

   If the Client wishes to discontinue use of a Server it issues a
   Release to delete the Server's neighbor cache entry.

3.15.3.  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 a static map of Server
   addresses for the link and/or the DNS resource records for the FQDN
   "linkupnetworks.[domainname]" before entering service.

   When an AERO Server receives a prospective Client's Solicit on its
   AERO interface, and the Server is too busy to service the message, it
   SHOULD return a Reply with status code "NoPrefixAvail" per [RFC3633].
   Otherwise, the Server authenticates the message.  If authentication
   succeeds, the Server determines the correct ACPs to delegate to the
   Client by searching the Client database.

   When the Server delegates the ACPs, it also creates IP forwarding
   table entries so that the AERO BGP-based routing system will
   propagate the ACPs to all Relays that aggregate the corresponding ASP
   (see: Section 3.7).  Next, the Server prepares a Reply message to
   send to the Client while using fe80::ID as the IPv6 source address,



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   the link-local address taken from the Client's Solicit as the IPv6
   destination address, the Server's link-layer address as the source
   link-layer address, and the Client's link-layer address as the
   destination link-layer address.  The Server also includes IA_PD
   options with the delegated ACPs.  For IPv4 ACPs, the prefix included
   in the IA_PD option is in IPv4-mapped IPv6 address format and with
   prefix length set as specified in Section 3.3.  For AERO links where
   a Client may experience a fault that prevents it from issuing a
   Release before departing from the network, Servers should set a short
   prefix lifetime (e.g., 40 seconds) so that stale PD state can be
   flushed out of the network.

   For Replies to Client DHCPv6 messages that include an AVSIO, the
   Server prepares a new AVSIO to include in the Reply.  The Server
   first copies the ACLLAOs in the body of the Client's AVSIO into the
   AVSIO that the Server will supply in the Reply message.  For the
   initial ACLLAO, the Server sets 'UDP Port Number' and 'IP address' to
   the values observed in the outer encapsulating headers of the
   Client's DHCPv6 message, i.e., even if these values are different
   than the ones included by the Client.

   The Server next copies an ALINFO option into the body of the AVSIO
   (i.e., following the ACLLAO options) formatted as shown in Figure 6:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  opt-code = OPTION_ALINFO (1) |           option-len          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Maximum Transmission Unit (MTU)|   Maximum Fragment Unit (MFU) |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Prefix Len #1 |  AERO Service Prefix (ASP) #1 (1 to 8 bytes)  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Prefix Len #2 |  AERO Service Prefix (ASP) #2 (1 to 8 bytes)  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Prefix Len #3 |  AERO Service Prefix (ASP) #3 (1 to 8 bytes)  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                                                               ~
       ~                                                               ~


              Figure 6: AERO Link Information (ALINFO) Option

   In the ALINFO option, the Server sets sets 'opt-code' to 1
   ("OPTION_ALINFO") and sets 'option-len' to the length of the
   remainder of the option.  The Server next sets MTU and MFU according
   to the considerations specified in Section 3.13.  The Server finally
   includes one or more ASPs with 'Prefix Len' set to the ASP prefix



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   length (between 0 and 64), and 'AERO Service Prefix' set to the ASP
   (between 1 and 8 bytes).

   When the Server sends the Reply message, it creates or updates a
   static neighbor cache entry for the Client based on the DUID and AERO
   addresses with lifetime set to no more than the PD lifetimes and
   updates the Client's link-layer addresses according to the ACLLAOs.
   The Server then uses the Client link-layer addresses as the link-
   layer addresses for encapsulation and uses the 'P(i)' values included
   in ACLLAOs as preference levels for each DSCP value.

   After the initial DHCPv6 PD Solicit/Reply exchange, the AERO Server
   maintains the neighbor cache entry for the Client until the PD
   lifetimes expire.  If the Client issues a Rebind, the Server uses any
   included ACLLAOs to update the link-layer information in the Client's
   neighbor cache entry.  If the Client issues a Renew, the Server
   extends the PD lifetimes.  If the Client issues a Release, or if the
   Client does not issue a Renew before the lifetime expires, the Server
   deletes the neighbor cache entry for the Client and withdraws the IP
   routes from the AERO routing system.

3.15.3.1.  Lightweight DHCPv6 Relay Agent (LDRA)

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

   When the LDRA receives a DHCPv6 message from a client, it prepares an
   AVSIO (including any ACLLAO and ALINFO options as described above)
   and copies the option into a DHCPv6 Relay-Supplied Option Option
   (RSOO) [RFC6422].  The LDRA then incorporates the RSOO into the
   Relay-Forward message and forwards the message to the DHCPv6 server.

   When the DHCPv6 server receives the Relay-Forward message, it caches
   the AVSIO included in the RSOO and discards the AVSIO included within
   the Client's message itself.  Next, the server authenticates the
   Client's message and prepares a Reply message if authentication
   succeeds.

   When the DHCPv6 server prepares a Reply message, it then includes the
   relay-supplied AVSIO in the body of the message along with any other
   options, then wraps the message in a Relay-Reply message.  The DHCPv6
   server then delivers the Relay-Reply message to the LDRA, which




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   discards the Relay-Reply wrapper and delivers the DHCPv6 message to
   the Client.

3.16.  AERO Forwarding Agent Behavior

   AERO Servers MAY associate with one or more companion AERO Forwarding
   Agents as platforms for offloading high-speed data plane traffic.
   When an AERO Server receives a Client's Solicit/Renew/Rebind/Release
   message, it services the message then forwards the corresponding
   Reply message to the Forwarding Agent.  When the Forwarding Agent
   receives the Reply message, it creates, updates or deletes a neighbor
   cache entry with the Client's AERO address and link-layer information
   included in the Reply message.  The Forwarding Agent then forwards
   the Reply message back to the AERO Server, which forwards the message
   to the Client.  In this way, Forwarding Agent state is managed in
   conjunction with Server state, with the Client responsible for
   reliability.

   When an AERO Server receives a data packet on an AERO interface with
   a network layer destination address for which it has distributed
   forwarding information to a Forwarding Agent, the Server returns a
   Redirect message to the source neighbor (subject to rate limiting)
   then forwards the data packet as usual.  The Redirect message
   includes a TLLAO with the link-layer address of the Forwarding
   Engine.

   When the source neighbor receives the Redirect message, it SHOULD
   record the link-layer address in the TLLAO as the encapsulation
   addresses to use for sending subsequent data packets.  However, the
   source MUST continue to use the primary link-layer address of the
   Server as the encapsulation address for sending control messages.

3.17.  AERO Link Route Optimization

   When a source Client forwards packets to a prospective correspondent
   Client within the same AERO link domain (i.e., one for which the
   packet's destination address is covered by an ASP), the source Client
   MAY initiate an AERO link route optimization procedure.  It is
   important to note that this procedure is initiated by the Client; if
   the procedure were initiated by the Server, the Server would have no
   way of knowing whether the Client was actually able to contact the
   correspondent over the route-optimized path.

   The procedure is based on an exchange of IPv6 ND messages using a
   chain of AERO Servers and Relays as a trust basis.  This procedure is
   in contrast to the Return Routability procedure required for route
   optimization to a correspondent Client located in the Internet as




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   described in Section 3.22.  The following sections specify the AERO
   link route optimization procedure.

3.17.1.  Reference Operational Scenario

   Figure 7 depicts the AERO link route optimization reference
   operational scenario, using IPv6 addressing as the example (while not
   shown, a corresponding example for IPv4 addressing can be easily
   constructed).  The figure shows an AERO Relay ('R1'), two AERO
   Servers ('S1', 'S2'), two AERO Clients ('C1', 'C2') and two ordinary
   IPv6 hosts ('H1', 'H2'):

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

               Figure 7: AERO Reference Operational Scenario

   In Figure 7, Relay ('R1') assigns the address fe80::1 to its AERO
   interface with link-layer address L2(R1), Server ('S1') assigns the
   address fe80::2 with link-layer address L2(S1),and Server ('S2')
   assigns the address fe80::3 with link-layer address L2(S2).  Servers
   ('S1') and ('S2') next arrange to add their link-layer addresses to a
   published list of valid Servers for the AERO link.

   AERO Client ('C1') receives the ACP 2001:db8:0::/48 in a DHCPv6 PD
   exchange via AERO Server ('S1') then assigns the address
   fe80::2001:db8:0:0 to its AERO interface with link-layer address
   L2(C1).  Client ('C1') configures a default route and neighbor cache
   entry via the AERO interface with next-hop address fe80::2 and link-
   layer address L2(S1), then sub-delegates the ACP to its attached



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   EUNs.  IPv6 host ('H1') connects to the EUN, and configures the
   address 2001:db8:0::1.

   AERO Client ('C2') receives the ACP 2001:db8:1::/48 in a DHCPv6 PD
   exchange via AERO Server ('S2') then assigns the address
   fe80::2001:db8:1:0 to its AERO interface with link-layer address
   L2(C2).  Client ('C2') 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
   EUNs.  IPv6 host ('H2') connects to the EUN, and configures the
   address 2001:db8:1::1.

3.17.2.  Concept of Operations

   Again, with reference to Figure 7, when source host ('H1') sends a
   packet to destination host ('H2'), the packet is first forwarded over
   the source host's attached EUN to Client ('C1').  Client ('C1') then
   forwards the packet via its AERO interface to Server ('S1') and also
   sends a Predirect message toward Client ('C2') via Server ('S1').
   Server ('S1') then re-encapsulates and forwards both the packet and
   the Predirect message out the same AERO interface toward Client
   ('C2') via Relay ('R1').

   When Relay ('R1') receives the packet and Predirect message, it
   consults its forwarding table to discover Server ('S2') as the next
   hop toward Client ('C2').  Relay ('R1') then forwards both the packet
   and the Predirect message to Server ('S2'), which then forwards them
   to Client ('C2').

   After Client ('C2') receives the Predirect message, it process the
   message and returns a Redirect message toward Client ('C1') via
   Server ('S2').  During the process, Client ('C2') also creates or
   updates a dynamic neighbor cache entry for Client ('C1').

   When Server ('S2') receives the Redirect message, it re-encapsulates
   the message and forwards it on to Relay ('R1'), which forwards the
   message on to Server ('S1') which forwards the message on to Client
   ('C1').  After Client ('C1') receives the Redirect message, it
   processes the message and creates or updates a dynamic neighbor cache
   entry for Client ('C2').

   Following the above Predirect/Redirect message exchange, forwarding
   of packets from Client ('C1') to Client ('C2') without involving any
   intermediate nodes is enabled.  The mechanisms that support this
   exchange are specified in the following sections.






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3.17.3.  Message Format

   AERO Redirect/Predirect messages use the same format as for IPv6 ND
   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:

        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

3.17.4.  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 ('C1') forwards a
   packet toward Client ('C2'), it MAY also send a Predirect message



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   forward toward Client ('C2'), subject to rate limiting (see
   Section 8.2 of [RFC4861]).  Client ('C1') prepares the Predirect
   message as follows:

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

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

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

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

   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
      assigned to the Target Address.

   o  the Target Address is set to fe80::2001:db8:0:0 (i.e., the AERO
      address of Client ('C1')).

   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-mapped IPv6 address format).

   o  the message includes one or more TLLAOs set to appropriate values
      for Client ('C1')'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 if necessary 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 ('C1') may have advanced knowledge that the direct path
   to Client ('C2') would be unusable or otherwise undesirable.  If the
   direct path later becomes unusable after the initial route
   optimization, Client ('C1') simply allows packets to again flow
   through Server ('S1').



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3.17.5.  Re-encapsulating and Relaying Predirects

   When Server ('S1') receives a Predirect message from Client ('C1'),
   it first verifies that the TLLAOs in the Predirect are a proper
   subset of the Interface IDs in Client ('C1')'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 Redirect message validation rules in Section 8.1 of [RFC4861],
   except that the Predirect has Code=1.  Server ('S1') also verifies
   that Client ('C1') 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 ('C1')'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 ('C2') by
   searching for the AERO address in the neighbor cache.  Since Client
   ('C2') is not one of its neighbors, Server ('S1') re-encapsulates the
   Predirect and relays it via Relay ('R1') by changing the link-layer
   source address of the message to 'L2(S1)' and changing the link-layer
   destination address to 'L2(R1)'.  Server ('S1') finally forwards the
   re-encapsulated message to Relay ('R1') without decrementing the
   network-layer TTL/Hop Limit field.

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

   When Server ('S2') receives the Predirect message from Relay ('R1')
   it determines that Client ('C2') 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(C2)'.  Server ('S2') then
   forwards the message to Client ('C2').

3.17.6.  Processing Predirects and Sending Redirects

   When Client ('C2') 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 ('C2') further accepts the
   message only if it is willing to serve as a redirection target.
   Next, Client ('C2') validates the message according to the Redirect



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   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 ('C2') 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 ('C1') , stores the link-layer
   addresses found in the TLLAOs as the link-layer addresses of Client
   ('C1') and stores the Prefix Length as the length to be applied to
   the network-layer address for forwarding purposes.  Client ('C2')
   then sets AcceptTime for the neighbor cache entry to ACCEPT_TIME.

   After processing the message, Client ('C2') prepares a Redirect
   message response as follows:

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

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

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

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

   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 ('C2')).

   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-mapped IPv6 address format).

   o  the message includes one or more TLLAOs set to appropriate values
      for Client ('C2')'s underlying interfaces, and with UDP Port
      Number and IP Address set to '0'.





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   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 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 ('C2') prepares the Redirect message, it sends the
   message to Server ('S2').

3.17.7.  Re-encapsulating and Relaying Redirects

   When Server ('S2') receives a Redirect message from Client ('C2'), it
   first verifies that the TLLAOs in the Redirect are a proper subset of
   the Interface IDs in Client ('C2')'s neighbor cache entry.  If the
   Client's TLLAOs are not acceptable, Server ('S2') discards the
   message.  Otherwise, Server ('S2') validates the message according to
   the Redirect message validation rules in Section 8.1 of [RFC4861].
   Server ('S2') also verifies that Client ('C2') 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
   Redirect; otherwise, it copies the correct UDP Port numbers and IP
   Addresses for Client ('C2')'s links into the (previously empty)
   TLLAOs.

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

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

   When Server ('S1') receives the Redirect message from Relay ('R1') it
   determines that Client ('C1') is a neighbor by consulting its
   neighbor cache.  Server ('S1') then re-encapsulates the Redirect



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

3.17.8.  Processing Redirects

   When Client ('C1') 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 ('C1') validates the message
   according to the 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 ('C1') then processes the
   message as follows.

   In the reference operational scenario, when Client ('C1') 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 ('C2'), stores the link-layer
   addresses found in the TLLAOs as the link-layer addresses of Client
   ('C2') and stores the Prefix Length as the length to be applied to
   the network-layer address for forwarding purposes.  Client ('C1')
   then sets ForwardTime for the neighbor cache entry to FORWARD_TIME.

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

3.17.9.  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 3.17.6), except that it writes its own link-layer
   address in the TLLAO option.  The Server must then maintain a dynamic
   neighbor cache entry for the redirected source Client.








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3.17.10.  Route Optimization Policy

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

3.17.11.  Route Optimization and Multiple ACPs

   Clients that receive multiple non-contiguous ACP delegations must
   perform route optimization for each of the individual ACPs based on
   demand of traffic with source addresses taken from those prefixes.
   For example, if Client C1 has already performed route optimization
   for destination ACP X on behalf of its source ACP Y, it must also
   perform route optimization for X on behalf of its source ACP Z.  As a
   result, source route optimization state cannot be shared between non-
   contiguous ACPs and must be managed separately.

3.18.  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 3.14) or
   proactively to test 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
   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 test the
   direct path to the target Client (see Section 7.3 of [RFC4861])
   periodically in order to keep dynamic neighbor cache entries alive.





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   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 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.

3.19.  Mobility Management

3.19.1.  Announcing Link-Layer Address Changes

   When a Client needs to change its link-layer address, e.g., due to a
   mobility event, it issues an immediate Rebind to each of its Servers
   using the new link-layer address as the source address and with an
   ACLLAO that includes the updated client link-layer information.  If
   authentication succeeds, the Server then updates its neighbor cache
   and sends a Reply.  Note that if the Client does not issue a Rebind
   before the PD lifetime expires (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 Predirect messages to each of its
   correspondent Client neighbors using the same procedures as specified
   in Section 3.17.4.  The Client sends the Predirect messages via a
   Server the same as if it was performing the initial route
   optimization procedure with the correspondent.  The Predirect message
   will update the correspondent's link layer address mapping for the
   Client.

3.19.2.  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 issues an immediate
   Rebind to each of its Servers using the new link-layer address as the
   source address and with an ACLLAO that includes the new client link-



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   layer information.  If authentication succeeds, the Server then
   updates its neighbor cache and sends a Reply.  The Client MAY then
   send Predirect messages to each of its correspondent Clients to
   inform them of the new link-layer address as described in
   Section 3.19.1.

3.19.3.  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 issues
   an immediate Rebind to each of its Servers over any available link
   with an ACLLAO that includes P(i) values set to "disabled".  If
   authentication succeeds, the Server then updates its neighbor cache
   and sends a Reply.  The Client SHOULD then send Predirect messages to
   each of its correspondent Clients to inform them of the deprecated
   link-layer address as described in Section 3.19.1.

3.19.4.  Implicit Mobility Management

   AERO Clients and Servers MAY include a configuration knob that allows
   them to perform implicit mobility management in which no DHCPv6
   messaging is used.  In that case, the Client only transmits packets
   over a single interface at a time, and the Server always observes
   packets arriving from the Client from the same link-layer source
   address.

   If the Client's underlying interface address changes (either due to a
   readdressing of the original interface or switching to a new
   interface) the Server immediately updates the neighbor cache entry
   for the Client and begins accepting and sending packets to the
   Client's new link-layer address.  This implicit mobility method
   applies to use cases such as cellphones with both WiFi and Cellular
   interfaces where only one of the interfaces is active at a given
   time, and the Client automatically switches over to the backup
   interface if the primary interface fails.

3.19.5.  Moving to a New Server

   When a Client associates with a new Server, it performs the Client
   procedures specified in Section 3.15.2.

   When a Client disassociates with an existing Server, it sends a
   Release message via a new Server to the unicast link-local network
   layer address of the old Server.  The new Server then writes its own
   link-layer address in the Release message IP source address and
   forwards the message to the old Server.





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   When the old Server receives the Release, it first authenticates the
   message.  Next, it resets the Client's neighbor cache entry lifetime
   to 3 seconds, rewrites the link-layer address in the neighbor cache
   entry to the address of the new Server, then returns a Reply message
   to the Client via the old Server.  When the lifetime expires, the old
   Server withdraws the IP route from the AERO routing system and
   deletes the neighbor cache entry for the Client.  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 since Release/Reply
   messages may be lost in the network the Client SHOULD retry until it
   gets a Reply indicating that the Release was successful.  If the
   Client does not receive a Reply after MAX_RETRY attempts, the old
   Server may have failed and the Client should discontinue its Release
   attempts.)

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

3.19.6.  Packet Queueing for Mobility

   AERO Clients and Servers should maintain a samll queue of packets
   they have recently sent to an AERO neighbor, e.g., a 1 second window.
   If the AERO neighbor moves, the AERO node MAY retransmit the queued
   packets to ensure that they are delviered to the AERO neighbor's new
   location.

   Note that this may have performance implications for asymmetric
   paths.  For example, if the AERO neighbor moves from a 50mbps link to
   a 128kbps link, retransmitting a 1 second window could cause
   significant congestion.  However, any retransmission bursts will
   subside after the 1 second window.

3.20.  Proxy AERO

   Proxy Mobile IPv6 (PMIPv6) [RFC5213][RFC5844][RFC5949] presents a
   network-based localized mobility management scheme for use within an
   access network domain.  It is typically used in WiFi and cellular
   wireless access networks, and allows Mobile Nodes (MNs) to receive
   and retain an IP address that remains stable within the access
   network domain without needing to implement any special mobility
   protocols.  In the PMIPv6 architecture, access network devices known
   as Mobility Access Gateways (MAGs) provide MNs with an access link



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   abstraction and receive prefixes for the MNs from a Local Mobility
   Anchor (LMA).

   In a proxy AERO domain, a proxy AERO Client (acting as a MAG) can
   similarly provide proxy services for MNs that do not participate in
   AERO messaging.  The proxy Client presents an access link abstraction
   to MNs, and performs DHCPv6 PD exchanges over the AERO interface with
   an AERO Server (acting as an LMA) to receive ACPs for address
   provisioning of new MNs that come onto an access link.  This scheme
   assumes that proxy Clients act as fixed (non-mobile) infrastructure
   elements under the same administrative trust basis as for Relays and
   Servers.

   When an MN comes onto an access link within a proxy AERO domain for
   the first time, the proxy Client authenticates the MN and obtains a
   unique identifier that it can use as a DHCPv6 DUID then sends a
   Solicit message to its Server.  When the Server delegates an ACP and
   returns a Reply, the proxy Client creates an AERO address for the MN
   and assigns the ACP to the MN's access link.  The proxy Client then
   configures itself as a default router for the MN and provides address
   autoconfiguration services (e.g., SLAAC, DHCPv6, DHCPv4, etc.) for
   provisioning MN addresses from the ACP over the access link.  Since
   the proxy Client may serve many such MNs simultaneously, it may
   receive multiple ACP delegations and configure multiple AERO
   addresses, i.e., one for each MN.

   When two MNs are associated with the same proxy Client, the Client
   can forward traffic between the MNs without involving a Server since
   it configures the AERO addresses of both MNs and therefore also has
   the necessary routing information.  When two MNs are associated with
   different proxy Clients, the source MN's Client can initiate standard
   AERO link route optimization to discover a direct path to the target
   MN's Client through the exchange of Predirect/Redirect messages.

   When an MN in a proxy AERO domain leaves an access link provided by
   an old proxy Client, the MN issues an access link-specific "leave"
   message that informs the old Client of the link-layer address of a
   new Client on the planned new access link.  This is known as a
   "predictive handover".  When an MN comes onto an access link provided
   by a new proxy Client, the MN issues an access link-specific "join"
   message that informs the new Client of the link-layer address of the
   old Client on the actual old access link.  This is known as a
   "reactive handover".

   Upon receiving a predictive handover indication, the old proxy Client
   sends a Solicit message directly to the new Client and queues any
   arriving data packets addressed to the departed MN.  The Solicit
   message includes the MN's ID as the DUID, the ACP in an IA_PD option,



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   the old Client's address as the link-layer source address and the new
   Client's address as the link-layer destination address.  When the new
   Client receives the Solicit message, it changes the link-layer source
   address to its own address, changes the link-layer destination
   address to the address of its Server, and forwards the message to the
   Server.  At the same time, the new Client creates access link state
   for the ACP in anticipation of the MN's arrival (while queuing any
   data packets until the MN arrives), creates a neighbor cache entry
   for the old Client with AcceptTime set to ACCEPT_TIME, then sends a
   Redirect message back to the old Client.  When the old Client
   receives the Redirect message, it creates a neighbor cache entry for
   the new Client with ForwardTime set to FORWARD_TIME, then forwards
   any queued data packets to the new Client.  At the same time, the old
   Client sends a Release message to its Server.  Finally, the old
   Client sends unsolicited Redirect messages to any of the ACP's
   correspondents with a TLLAO containing the link-layer address of the
   new Client.

   Upon receiving a reactive handover indication, the new proxy Client
   creates access link state for the MN's ACP, sends a Solicit message
   to its Server, and sends a Release message directly to the old
   Client.  The Release message includes the MN's ID as the DUID, the
   ACP in an IA_PD option, the new Client's address as the link-layer
   source address and the old Client's address as the link-layer
   destination address.  When the old Client receives the Release
   message, it changes the link-layer source address to its own address,
   changes the link-layer destination address to the address of its
   Server, and forwards the message to the Server.  At the same time,
   the old Client sends a Predirect message back to the new Client and
   queues any arriving data packets addressed to the departed MN.  When
   the new Client receives the Predirect, it creates a neighbor cache
   entry for the old Client with AcceptTime set to ACCEPT_TIME, then
   sends a Redirect message back to the old Client.  When the old Client
   receives the Redirect message, it creates a neighbor cache entry for
   the new Client with ForwardTime set to FORWARD_TIME, then forwards
   any queued data packets to the new Client.  Finally, the old Client
   sends unsolicited Redirect messages to correspondents the same as for
   the predictive case.

   When a Server processes a Solicit message, it creates a neighbor
   cache entry for this ACP if none currently exists.  If a neighbor
   cache entry already exists, however, the Server changes the link-
   layer address to the address of the new proxy Client (this satisfies
   the case of both the old Client and new Client using the same
   Server).

   When a Server processes a Release message, it resets the neighbor
   cache entry lifetime for this ACP to 3 seconds if the cached link-



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   layer address matches the old proxy Client's address.  Otherwise, the
   Server ignores the Release message (this satisfies the case of both
   the old Client and new Client using the same Server).

   When a correspondent Client receives an unsolicited Redirect message,
   it changes the link-layer address for the ACP's neighbor cache entry
   to the address of the new proxy Client.

   From an architectural perspective, in addition to the use of DHCPv6
   PD and IPv6 ND signaling the AERO approach differs from PMIPv6 in its
   use of the NBMA virtual link model instead of point-to-point tunnels.
   This provides a more agile interface for Client/Server and Client/
   Client coordinations, and also facilitates simple route optimization.
   The AERO routing system is also arranged in such a fashion that
   Clients get the same service from any Server they happen to associate
   with.  This provides a natural fault tolerance and load balancing
   capability such as desired for distributed mobility management.

3.21.  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 a physical interface connection to the
   Internet and a physical interface connection to the enterprise
   internetwork.  This most often entails the establishment of a Virtual
   Private Network (VPN) link over the public Internet from the mobile
   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.



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   After receiving the ACP, the Client can send IP packets that use an
   address taken from the ACP as the network layer source address, the
   Client's link-layer address as the link-layer source address, and the
   Server's Internet-facing link-layer address as the link-layer
   destination address.  The Server will then rewrite the link-layer
   source address with the Server's own enterprise-facing link-layer
   address and rewrite the link-layer destination address with the
   target AERO node's link-layer address, and the packets will enter the
   enterprise network as though they were sourced from a 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.

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

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







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3.22.  Extending IPv6 AERO Links to the Internet

   When an IPv6 host ('H1') with an address from an ACP owned by AERO
   Client ('C1') sends packets to a correspondent IPv6 host ('H2'), the
   packets eventually arrive at the IPv6 router that owns ('H2')s
   prefix.  This IPv6 router may or may not be an AERO Client ('C2')
   either within the same home network as ('C1') or in a different home
   network.

   If Client ('C1') is currently located outside the boundaries of its
   home network, it will connect back into the home network via a
   security gateway acting as an AERO Server.  The packets sent by
   ('H1') via ('C1') will then be forwarded through the security gateway
   then through the home network and finally to ('C2') where they will
   be delivered to ('H2').  This could lead to sub-optimal performance
   when ('C2') could instead be reached via a more direct route without
   involving the security gateway.

   Consider the case when host ('H1') has the IPv6 address
   2001:db8:1::1, and Client ('C1') has the ACP 2001:db8:1::/64 with
   underlying IPv6 Internet address of 2001:db8:1000::1.  Also, host
   ('H2') has the IPv6 address 2001:db8:2::1, and Client ('C2') has the
   ACP 2001:db8:2::/64 with underlying IPv6 address of 2001:db8:2000::1.
   Client ('C1') can determine whether 'C2' is indeed also an AERO
   Client willing to serve as a route optimization correspondent by
   resolving the AAAA records for the DNS FQDN that matches ('H2')s
   prefix, i.e.:

   '0.0.0.0.2.0.0.0.8.b.d.0.1.0.0.2.aero.linkupnetworks.net'

   If ('C2') is indeed a candidate correspondent, the FQDN lookup will
   return a PTR resource record that contains the domain name for the
   AERO link that manages ('C2')s ASP.  Client ('C1') can then attempt
   route optimization using an approach similar to the Return
   Routability procedure specified for Mobile IPv6 (MIPv6) [RFC6275].
   In order to support this process, both Clients MUST intercept and
   decapsulate packets that have a subnet router anycast address
   corresponding to any of the /64 prefixes covered by their respective
   ACPs.

   To initiate the process, Client ('C1') creates a specially-crafted
   encapsulated Predirect message that will be routed through its home
   network then through ('C2')s home network and finally to ('C2')
   itself.  Client ('C1') prepares the initial message in the exchange
   as follows:






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   o  The encapsulating IPv6 header source address is set to
      2001:db8:1:: (i.e., the IPv6 subnet router anycast address for
      ('C1')s ACP)

   o  The encapsulating IPv6 header destination address is set to
      2001:db8:2:: (i.e., the IPv6 subnet router anycast address for
      ('C2')s ACP)

   o  The encapsulating IPv6 header is followed by any additional
      encapsulation headers

   o  The encapsulated IPv6 header source address is set to
      fe80::2001:db8:1:0 (i.e., the AERO address for ('C1'))

   o  The encapsulated IPv6 header destination address is set to
      fe80::2001:db8:2:0 (i.e., the AERO address for ('C2'))

   o  The encapsulated Predirect message includes all of the securing
      information that would occur in a MIPv6 "Home Test Init" message
      (format TBD)

   Client ('C1') then further encapsulates the message in the
   encapsulating headers necessary to convey the packet to the security
   gateway (e.g., through IPsec encapsulation) so that the message now
   appears "double-encapsulated".  ('C1') then sends the message to the
   security gateway, which re-encapsulates and forwards it over the home
   network from where it will eventually reach ('C2').

   At the same time, ('C1') creates and sends a second encapsulated
   Predirect message that will be routed through the IPv6 Internet
   without involving the security gateway.  Client ('C1') prepares the
   message as follows:

   o  The encapsulating IPv6 header source address is set to
      2001:db8:1000:1 (i.e., the Internet IPv6 address of ('C1'))

   o  The encapsulating IPv6 header destination address is set to
      2001:db8:2:: (i.e., the IPv6 subnet router anycast address for
      ('C2')s ACP)

   o  The encapsulating IPv6 header is followed by any additional
      encapsulation headers

   o  The encapsulated IPv6 header source address is set to
      fe80::2001:db8:1:0 (i.e., the AERO address for ('C1'))

   o  The encapsulated IPv6 header destination address is set to
      fe80::2001:db8:2:0 (i.e., the AERO address for ('C2'))



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   o  The encapsulated Predirect message includes all of the securing
      information that would occur in a MIPv6 "Care-of Test Init"
      message (format TBD)

   ('C2') will receive both Predirect messages through its home network
   then return a corresponding Redirect for each of the Predirect
   messages with the source and destination addresses in the inner and
   outer headers reversed.  The first message includes all of the
   securing information that would occur in a MIPv6 "Home Test" message,
   while the second message includes all of the securing information
   that would occur in a MIPv6 "Care-of Test" message (formats TBD).

   When ('C1') receives the Redirect messages, it performs the necessary
   security procedures per the MIPv6 specification.  It then prepares an
   encapsulated NS message that includes the same source and destination
   addresses as for the "Care-of Test Init" Predirect message, and
   includes all of the securing information that would occur in a MIPv6
   "Binding Update" message (format TBD) and sends the message to
   ('C2').

   When ('C2') receives the NS message, if the securing information is
   correct it creates or updates a neighbor cache entry for ('C1') with
   fe80::2001:db8:1:0 as the network-layer address, 2001:db8:1000::1 as
   the link-layer address and with AcceptTime set to ACCEPT_TIME.
   ('C2') then sends an encapsulated NA message back to ('C1') that
   includes the same source and destination addresses as for the "Care-
   of Test" Redirect message, and includes all of the securing
   information that would occur in a MIPv6 "Binding Acknowledgement"
   message (format TBD) and sends the message to ('C1').

   When ('C1') receives the NA message, it creates or updates a neighbor
   cache entry for ('C2') with fe80::2001:db8:2:0 as the network-layer
   address and 2001:db8:2:: as the link-layer address and with
   ForwardTime set to FORWARD_TIME, thus completing the route
   optimization in the forward direction.

   ('C1') subsequently forwards encapsulated packets with outer source
   address 2001:db8:1000::1, with outer destination address
   2001:db8:2::, with inner source address taken from the 2001:db8:1::,
   and with inner destination address taken from 2001:db8:2:: due to the
   fact that it has a securely-established neighbor cache entry with
   non-zero ForwardTime.  ('C2') subsequently accepts any such
   encapsulated packets due to the fact that it has a securely-
   established neighbor cache entry with non-zero AcceptTime.

   In order to keep neighbor cache entries alive, ('C1') periodically
   sends additional NS messages to ('C2') and receives any NA responses.
   If ('C1') moves to a different point of attachment after the initial



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   route optimization, it sends a new secured NS message to ('C2') as
   above to update ('C2')s neighbor cache.

   If ('C2') has packets to send to ('C1'), it performs a corresponding
   route optimization in the opposite direction following the same
   procedures described above.  In the process, the already-established
   unidirectional neighbor cache entries within ('C1') and ('C2') are
   updated to include the now-bidirectional information.  In particular,
   the AcceptTime and ForwardTime variables for both neighbor cache
   entries are updated to non-zero values, and the link-layer address
   for ('C1')s neighbor cache entry for ('C2') is reset to
   2001:db8:2000::1.

   Note that two AERO Clients can use full security protocol messaging
   instead of Return Routability, e.g., if strong authentication and/or
   confidentiality are desired.  In that case, security protocol key
   exchanges such as specified for MOBIKE [RFC4555] would be used to
   establish security associations and neighbor cache entries between
   the AERO clients.  Thereafter, NS/NA messaging can be used to
   maintain neighbor cache entries, test reachability, and to announce
   mobility events.  If reachability testing fails, e.g., if both
   Clients move at roughly the same time, the Clients can tear down the
   security association and neighbor cache entries and again allow
   packets to flow through their home network.

3.23.  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.

3.24.  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.






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

3.25.  AERO Operation without DHCPv6 Client/Server Exchanges

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

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

3.26.  Manually-Configured AERO Tunnels

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

3.27.  Encapsulation Avoidance on Relay-Server Dedicated Links

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







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3.28.  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 their Servers.

3.29.  Multicast Considerations

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

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

4.  Implementation Status

   User-level and kernel-level AERO implementations have been developed
   and are undergoing internal testing within Boeing.

   An initial public release of the AERO source code was announced on
   the intarea mailing list on August 21, 2015, and a pointer to the
   code is available in the list archives.

5.  IANA Considerations

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

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

   No further IANA actions are required.







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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.

   Redirect, Predirect and unsolicited NA messages SHOULD include a
   Timestamp option (see Section 5.3 of [RFC3971]) that other AERO nodes
   can use to verify the message time of origin.  Predirect, NS and RS
   messages SHOULD include a Nonce option (see Section 5.3 of [RFC3971])
   that recipients echo back in corresponding responses.

   AERO links must be protected against link-layer address spoofing
   attacks in which an attacker on the link pretends to be a trusted
   neighbor.  Links that provide link-layer securing mechanisms (e.g.,
   IEEE 802.1X WLANs) and links that provide physical security (e.g.,
   enterprise network wired LANs) provide a first line of defense,
   however AERO nodes SHOULD also use DHCPv6 securing services (e.g.,
   Secure DHCPv6 [I-D.ietf-dhc-sedhcpv6], etc.) for Client
   authentication and network admission control.

   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
   some form of Internet connection sharing.)

   AERO Clients, Servers and Relays on the open Internet are suceptible
   to the same attack profiles as for any Internet nodes.  For this
   reason, IP security MUST be used when AERO is employed over
   unmanaged/unsecured links using securing mechanisms such as IPsec
   [RFC4301], IKE [RFC5996] and/or TLS [RFC5246].

   AERO Servers and Relays present targets for traffic amplification
   Denial of Service (DoS) attacks.  This concern is no different than
   for widely-deployed VPN security gateways in the Internet, where
   attackers could send spoofed packets to the gateways at high data
   rates.  This becomes less of a problem when Relays and Servers are
   connected by dedicated links with no connections to the Internet and/
   or when connections to the Internet asre only permitted through well-
   managed firewalls.

   Traffic amplfication DoS attacks can also target an AERO Client's low
   data rate links.  This is a concern not only for Clients located on



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   the open Internet but also for Clients in protected enclaves.  AERO
   Servers can institute rate limits that protect Clients from receiving
   packet floods that could DoS low data rate links.

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, Adrian
   Farrel, Sri Gundavelli, Brian Haberman, Joel Halpern, Tom Herbert,
   Sascha Hlusiak, Lee Howard, Andre Kostur, Ted Lemon, Andy Malis,
   Satoru Matsushima, Tomek Mrugalski, Alexandru Petrescu, Behcet
   Saikaya, Joe Touch, Bernie Volz, Ryuji Wakikawa and Lloyd Wood.
   Members of the IESG also provided valuable input during their review
   process that greatly improved the document.  Discussions on the v6ops
   list in the December 2015 through January 2016 timeframe further
   helped clarify AERO multi-addressing capabilities.  Special thanks go
   to Stewart Bryant, Joel Halpern and Brian Haberman for their
   shepherding guidance during the publication of the AERO first
   edition.

   This work has further been encouraged and supported by Boeing
   colleagues including M.  Wayne Benson, Dave Bernhardt, Cam Brodie,
   Balaguruna Chidambaram, Irene Chin, Bruce Cornish, Claudiu Danilov,
   Wen Fang, Anthony Gregory, Jeff Holland, Ed King, Gen MacLean, Rob
   Muszkiewicz, Sean O'Sullivan, Kent Shuey, Brian Skeen, Mike Slane,
   Brendan Williams, Julie Wulff, Yueli Yang, and other members of the
   BR&T and BIT mobile networking teams.  Wayne Benson is especially
   acknowledged for his outstanding work in converting the AERO proof-
   of-concept implementation into production-ready code.

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

   Many of the constructs presented in this second edition of AERO are
   based on the author's earlier works, including:

   o  The Internet Routing Overlay Network (IRON)
      [RFC6179][I-D.templin-ironbis]

   o  Virtual Enterprise Traversal (VET)
      [RFC5558][I-D.templin-intarea-vet]

   o  The Subnetwork Encapsulation and Adaptation Layer (SEAL)
      [RFC5320][I-D.templin-intarea-seal]

   o  AERO, First Edition [RFC6706]



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   Note that these works cite numerous earlier efforts that are not also
   cited here due to space limitations.  The authors of those earlier
   works are acknowledged for their insights.

8.  References

8.1.  Normative References

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

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

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

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

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <http://www.rfc-editor.org/info/rfc2473>.

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

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




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

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,
              <http://www.rfc-editor.org/info/rfc3971>.

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

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <http://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <http://www.rfc-editor.org/info/rfc4862>.

   [RFC6434]  Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
              Requirements", RFC 6434, DOI 10.17487/RFC6434, December
              2011, <http://www.rfc-editor.org/info/rfc6434>.

8.2.  Informative References

   [I-D.herbert-gue-fragmentation]
              Herbert, T. and F. Templin, "Fragmentation option for
              Generic UDP Encapsulation", draft-herbert-gue-
              fragmentation-02 (work in progress), October 2015.

   [I-D.ietf-dhc-sedhcpv6]
              Jiang, S., Li, L., Cui, Y., Jinmei, T., Lemon, T., and D.
              Zhang, "Secure DHCPv6", draft-ietf-dhc-sedhcpv6-13 (work
              in progress), July 2016.

   [I-D.ietf-intarea-tunnels]
              Touch, D. and W. Townsley, "IP Tunnels in the Internet
              Architecture", draft-ietf-intarea-tunnels-03 (work in
              progress), July 2016.






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   [I-D.ietf-nvo3-gue]
              Herbert, T., Yong, L., and O. Zia, "Generic UDP
              Encapsulation", draft-ietf-nvo3-gue-04 (work in progress),
              July 2016.

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

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

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

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

   [RFC0879]  Postel, J., "The TCP Maximum Segment Size and Related
              Topics", RFC 879, DOI 10.17487/RFC0879, November 1983,
              <http://www.rfc-editor.org/info/rfc879>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.

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

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

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,
              <http://www.rfc-editor.org/info/rfc1812>.

   [RFC1930]  Hawkinson, J. and T. Bates, "Guidelines for creation,
              selection, and registration of an Autonomous System (AS)",
              BCP 6, RFC 1930, DOI 10.17487/RFC1930, March 1996,
              <http://www.rfc-editor.org/info/rfc1930>.



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

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

   [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
              Domains without Explicit Tunnels", RFC 2529,
              DOI 10.17487/RFC2529, March 1999,
              <http://www.rfc-editor.org/info/rfc2529>.

   [RFC2675]  Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
              RFC 2675, DOI 10.17487/RFC2675, August 1999,
              <http://www.rfc-editor.org/info/rfc2675>.

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

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

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

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

   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, DOI 10.17487/RFC2983, October 2000,
              <http://www.rfc-editor.org/info/rfc2983>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <http://www.rfc-editor.org/info/rfc3168>.

   [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", RFC 3596,
              DOI 10.17487/RFC3596, October 2003,
              <http://www.rfc-editor.org/info/rfc3596>.



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

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

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

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

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

   [RFC4459]  Savola, P., "MTU and Fragmentation Issues with In-the-
              Network Tunneling", RFC 4459, DOI 10.17487/RFC4459, April
              2006, <http://www.rfc-editor.org/info/rfc4459>.

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

   [RFC4555]  Eronen, P., "IKEv2 Mobility and Multihoming Protocol
              (MOBIKE)", RFC 4555, DOI 10.17487/RFC4555, June 2006,
              <http://www.rfc-editor.org/info/rfc4555>.

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
              <http://www.rfc-editor.org/info/rfc4592>.

   [RFC4605]  Fenner, B., He, H., Haberman, B., and H. Sandick,
              "Internet Group Management Protocol (IGMP) / Multicast
              Listener Discovery (MLD)-Based Multicast Forwarding
              ("IGMP/MLD Proxying")", RFC 4605, DOI 10.17487/RFC4605,
              August 2006, <http://www.rfc-editor.org/info/rfc4605>.




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   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <http://www.rfc-editor.org/info/rfc4821>.

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

   [RFC4994]  Zeng, S., Volz, B., Kinnear, K., and J. Brzozowski,
              "DHCPv6 Relay Agent Echo Request Option", RFC 4994,
              DOI 10.17487/RFC4994, September 2007,
              <http://www.rfc-editor.org/info/rfc4994>.

   [RFC5213]  Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
              Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
              RFC 5213, DOI 10.17487/RFC5213, August 2008,
              <http://www.rfc-editor.org/info/rfc5213>.

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,
              DOI 10.17487/RFC5214, March 2008,
              <http://www.rfc-editor.org/info/rfc5214>.

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

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

   [RFC5494]  Arkko, J. and C. Pignataro, "IANA Allocation Guidelines
              for the Address Resolution Protocol (ARP)", RFC 5494,
              DOI 10.17487/RFC5494, April 2009,
              <http://www.rfc-editor.org/info/rfc5494>.

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

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




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

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

   [RFC5844]  Wakikawa, R. and S. Gundavelli, "IPv4 Support for Proxy
              Mobile IPv6", RFC 5844, DOI 10.17487/RFC5844, May 2010,
              <http://www.rfc-editor.org/info/rfc5844>.

   [RFC5949]  Yokota, H., Chowdhury, K., Koodli, R., Patil, B., and F.
              Xia, "Fast Handovers for Proxy Mobile IPv6", RFC 5949,
              DOI 10.17487/RFC5949, September 2010,
              <http://www.rfc-editor.org/info/rfc5949>.

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

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <http://www.rfc-editor.org/info/rfc6146>.

   [RFC6179]  Templin, F., Ed., "The Internet Routing Overlay Network
              (IRON)", RFC 6179, DOI 10.17487/RFC6179, March 2011,
              <http://www.rfc-editor.org/info/rfc6179>.

   [RFC6204]  Singh, H., Beebee, W., Donley, C., Stark, B., and O.
              Troan, Ed., "Basic Requirements for IPv6 Customer Edge
              Routers", RFC 6204, DOI 10.17487/RFC6204, April 2011,
              <http://www.rfc-editor.org/info/rfc6204>.

   [RFC6221]  Miles, D., Ed., Ooghe, S., Dec, W., Krishnan, S., and A.
              Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221,
              DOI 10.17487/RFC6221, May 2011,
              <http://www.rfc-editor.org/info/rfc6221>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <http://www.rfc-editor.org/info/rfc6241>.





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   [RFC6275]  Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
              Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
              2011, <http://www.rfc-editor.org/info/rfc6275>.

   [RFC6355]  Narten, T. and J. Johnson, "Definition of the UUID-Based
              DHCPv6 Unique Identifier (DUID-UUID)", RFC 6355,
              DOI 10.17487/RFC6355, August 2011,
              <http://www.rfc-editor.org/info/rfc6355>.

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

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
              <http://www.rfc-editor.org/info/rfc6438>.

   [RFC6691]  Borman, D., "TCP Options and Maximum Segment Size (MSS)",
              RFC 6691, DOI 10.17487/RFC6691, July 2012,
              <http://www.rfc-editor.org/info/rfc6691>.

   [RFC6706]  Templin, F., Ed., "Asymmetric Extended Route Optimization
              (AERO)", RFC 6706, DOI 10.17487/RFC6706, August 2012,
              <http://www.rfc-editor.org/info/rfc6706>.

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

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935,
              DOI 10.17487/RFC6935, April 2013,
              <http://www.rfc-editor.org/info/rfc6935>.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, DOI 10.17487/RFC6936, April 2013,
              <http://www.rfc-editor.org/info/rfc6936>.

   [RFC6939]  Halwasia, G., Bhandari, S., and W. Dec, "Client Link-Layer
              Address Option in DHCPv6", RFC 6939, DOI 10.17487/RFC6939,
              May 2013, <http://www.rfc-editor.org/info/rfc6939>.

   [RFC6980]  Gont, F., "Security Implications of IPv6 Fragmentation
              with IPv6 Neighbor Discovery", RFC 6980,
              DOI 10.17487/RFC6980, August 2013,
              <http://www.rfc-editor.org/info/rfc6980>.



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   [RFC7078]  Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing
              Address Selection Policy Using DHCPv6", RFC 7078,
              DOI 10.17487/RFC7078, January 2014,
              <http://www.rfc-editor.org/info/rfc7078>.

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

Appendix A.  AERO Alternate Encapsulations

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

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

   Figure 9 shows the AERO IP-in-IP encapsulation format before any
   fragmentation is applied:

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

        Minimal Encapsulation in IPv4      Minimal Encapsulation in IPv6


           Figure 9: Minimal Encapsulation Format using IP-in-IP



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   Figure 10 shows the AERO GRE encapsulation format before any
   fragmentation is applied:

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


                Figure 10: Minimal Encapsulation Using GRE

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

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

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

Appendix B.  When to Insert an Encapsulation Fragment Header

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



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   packet fragment in outer IP and encapsulation layer headers that
   differ only in the fragment header fields.

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

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

Appendix C.  Autoconfiguration for Constrained Platforms

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

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

   o  the IPv6 destination address is all-nodes multicast

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

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

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

   The Client then sends the synthesized RA message via the TUN/TAP
   interface, where the operating system kernel will interpret it as
   though it were generated by an actual router.  The operating system
   will then install a default route and use StateLess Address



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   AutoConfiguration (SLAAC) to configure an IPv6 address on the TUN/TAP
   interface.  Methods for similarly installing an IPv4 default route
   and IPv4 address on the TUN/TAP interface are based on synthesized
   DHCPv4 messages [RFC2131].

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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