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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720,                       June 2, 2016
           rfc6179, rfc6706 (if
           approved)
Intended status: Standards Track
Expires: December 4, 2016


             Asymmetric Extended Route Optimization (AERO)
                     draft-templin-aerolink-67.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 using standard control messaging exchanges 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 December 4, 2016.





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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  . . . . . . . . . . . . . . . . .  11
     3.6.  AERO Interface Initialization . . . . . . . . . . . . . .  12
       3.6.1.  AERO Relay Behavior . . . . . . . . . . . . . . . . .  12
       3.6.2.  AERO Server Behavior  . . . . . . . . . . . . . . . .  12
       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  . . . . . . . . . . . .  16
     3.10. AERO Interface Encapsulation and Re-encapsulation . . . .  18
     3.11. AERO Interface Decapsulation  . . . . . . . . . . . . . .  19
     3.12. AERO Interface Data Origin Authentication . . . . . . . .  19
     3.13. AERO Interface MTU and Fragmentation  . . . . . . . . . .  20
       3.13.1.  Operational Assurance of Sufficient MTU  . . . . . .  20
       3.13.2.  Classical Path MTU Discovery with Reactive
                Fragmentation  . . . . . . . . . . . . . . . . . . .  21
       3.13.3.  Proactive Fragmentation with Expectation of
                Packetization Layer Path MTU Discovery . . . . . . .  21
       3.13.4.  Accommodating Large Control Messages . . . . . . . .  23
       3.13.5.  Integrity  . . . . . . . . . . . . . . . . . . . . .  24
     3.14. AERO Interface Error Handling . . . . . . . . . . . . . .  25
     3.15. AERO Router Discovery, Prefix Delegation and Address
           Configuration . . . . . . . . . . . . . . . . . . . . . .  29
       3.15.1.  AERO DHCPv6 Service Model  . . . . . . . . . . . . .  29



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       3.15.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  30
       3.15.3.  AERO Server Behavior . . . . . . . . . . . . . . . .  33
       3.15.4.  Deleting Link Registrations  . . . . . . . . . . . .  36
     3.16. AERO Forwarding Agent Behavior  . . . . . . . . . . . . .  36
     3.17. AERO Intradomain Route Optimization . . . . . . . . . . .  37
       3.17.1.  Reference Operational Scenario . . . . . . . . . . .  37
       3.17.2.  Concept of Operations  . . . . . . . . . . . . . . .  39
       3.17.3.  Message Format . . . . . . . . . . . . . . . . . . .  39
       3.17.4.  Sending Predirects . . . . . . . . . . . . . . . . .  40
       3.17.5.  Re-encapsulating and Relaying Predirects . . . . . .  41
       3.17.6.  Processing Predirects and Sending Redirects  . . . .  42
       3.17.7.  Re-encapsulating and Relaying Redirects  . . . . . .  44
       3.17.8.  Processing Redirects . . . . . . . . . . . . . . . .  44
       3.17.9.  Server-Oriented Redirection  . . . . . . . . . . . .  45
       3.17.10. Route Optimization Policy  . . . . . . . . . . . . .  45
       3.17.11. Route Optimization and Multiple ACPs . . . . . . . .  46
     3.18. Neighbor Unreachability Detection (NUD) . . . . . . . . .  46
     3.19. Mobility Management . . . . . . . . . . . . . . . . . . .  47
       3.19.1.  Announcing Link-Layer Address Changes  . . . . . . .  47
       3.19.2.  Bringing New Links Into Service  . . . . . . . . . .  47
       3.19.3.  Removing Existing Links from Service . . . . . . . .  47
       3.19.4.  Moving to a New Server . . . . . . . . . . . . . . .  48
     3.20. Proxy AERO  . . . . . . . . . . . . . . . . . . . . . . .  48
     3.21. Extending AERO Links Through Security Gateways  . . . . .  51
     3.22. Extending IPv6 AERO Links to the Internet . . . . . . . .  53
     3.23. Encapsulation Protocol Version Considerations . . . . . .  56
     3.24. Multicast Considerations  . . . . . . . . . . . . . . . .  56
     3.25. Operation on AERO Links Without DHCPv6 Services . . . . .  57
     3.26. Operation on Server-less AERO Links . . . . . . . . . . .  57
     3.27. Manually-Configured AERO Tunnels  . . . . . . . . . . . .  57
     3.28. Intradomain Routing . . . . . . . . . . . . . . . . . . .  57
   4.  Implementation Status . . . . . . . . . . . . . . . . . . . .  57
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  58
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  58
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  59
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  60
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  60
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  62
   Appendix A.  AERO Alternate Encapsulations  . . . . . . . . . . .  68
   Appendix B.  When to Insert an Encapsulation Fragment Header  . .  70
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  70

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



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   equivalent links for tunneling.  Nodes attached to AERO links can
   exchange packets via trusted intermediate routers that provide
   forwarding services to reach off-link destinations and redirection
   services for route optimization [RFC5522].

   AERO provides an IPv6 link-local address format known as the AERO
   address that supports operation of the IPv6 Neighbor Discovery (ND)
   [RFC4861] protocol and links IPv6 ND to IP forwarding.  Admission
   control, 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 a widely-applicable tunneling solution using standard
   control messaging exchanges as described in this document.  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.  Nodes typically have a
      single AERO interface; support for multiple AERO interfaces is
      also possible but out of scope for this document.  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.

   AERO Client ("Client")



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      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.  A typical example is an
      enterprise network, but many other use cases are also in scope.

   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 AERO
      Client Prefixes (ACPs) are derived (for example, the IPv6 ACP
      2001:db8:1:2::/64 is derived from the IPv6 ASP 2001:db8::/32).

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

   Throughout the document, the simple terms "Client", "Server" and
   "Relay" refer to "AERO Client", "AERO Server" and "AERO Relay",
   respectively.  Capitalization is used to distinguish these terms from
   DHCPv6 client/server/relay [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 is
   formed as follows:

      fe80::[ACP]

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

      2001:db8:1000:2000::/56

   it constructs its AERO address as:

      fe80::2001:db8:1000:2000

   For IPv4, the AERO address is formed from the lower 64 bits of an
   IPv4-mapped IPv6 address [RFC4291] that includes the base prefix
   taken from the Client's IPv4 ACP.  For example, if the AERO Client
   receives the IPv4 ACP:

      192.0.2.32/28

   it constructs its AERO address as:

      fe80::FFFF:192.0.2.32

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

   NOTE: In some cases, prospective neighbors may not have 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 PD and IPv6
   ND control messaging.  AERO Clients send DHCPv6 Solicit, Rebind,
   Renew and Release messages to AERO Servers, which respond with DHCPv6
   Reply messages.  These messages result in the creation, modification
   and deletion of neighbor cache entries.

   AERO interfaces 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 links further use link-
   local-only addressing; hence, AERO nodes ignore any Prefix
   Information Options (PIOs) they may receive in RA messages over an
   AERO interface.

   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 = 2   |   Length = 3  |           Reserved            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Link ID    |    NDSCPs     |  DSCP #1  |Prf|  DSCP #2  |Prf|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  DSCP #3  |Prf|  DSCP #4  |Prf| ....
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        UDP Port Number        |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                                                               |
       +                                                               +
       |                          IP Address                           |
       +                                                               +
       |                                                               |
       +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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



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   encodes an integer value between 0 and 64 indicating the number of
   Differentiated Services Code Point (DSCP) octets that follow.  Each
   DSCP octet is a 6-bit integer DSCP value followed by a 2-bit
   Preference ("Prf") value.  Each DSCP value encodes an integer between
   0 and 63 associated with this Link ID, where the value 0 means
   "default" and other values are interpreted as specified in [RFC2474].
   The 'Prf' qualifier for each DSCP value is set to the value 0
   ("deprecated'), 1 ("low"), 2 ("medium"), or 3 ("high") to indicate a
   preference level for packet forwarding purposes.  When a particular
   DSCP value is not specified, its preference level is set to "medium"
   by default.

   UDP Port Number and IP Address are set to the addresses used by the
   target node when it sends encapsulated packets over the underlying
   interface.  When 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 [RFC4291].

   AERO interfaces may be configured over multiple underlying
   interfaces.  For example, common mobile handheld devices have both
   wireless local area network ("WLAN") and cellular wireless links.
   These links are typically used "one at a time" with low-cost WLAN
   preferred and highly-available cellular wireless as a standby.  In a
   more complex example, aircraft frequently have many wireless data
   link types (e.g. satellite-based, terrestrial, air-to-air
   directional, etc.) with diverse performance and cost properties.

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

   If the Client has multiple active underlying interfaces, then from
   the perspective of IPv6 ND it would appear to have multiple link-
   layer addresses.  In that case, Redirect and Predirect messages MAY
   include multiple TLLAOs -- each with a Link ID that corresponds to a
   specific underlying interface of the Client.

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



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   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 ASP prefixes
   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.

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).





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   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.
   Therefore, packets enter the Server's AERO interface from the link
   layer and are forwarded back out the link layer without ever leaving
   the AERO interface and therefore without ever disturbing the network
   layer.

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.
   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.





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   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.

   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
   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.




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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
   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 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, and are garbage-collected if
   not used within a bounded timescale.  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



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   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
   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



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   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
   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).



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   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).

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



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   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.

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.






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3.13.  AERO Interface MTU and Fragmentation

   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 links over IP networks have a maximum link MTU of 64KB minus the
   encapsulation overhead (i.e., 64KB-ENCAPS), since the maximum packet
   size in the base IP specifications is 64KB [RFC0791][RFC2460].  While
   IPv6 jumbograms can be up to 4GB [RFC2675], they are considered
   optional for IPv6 nodes [RFC6434] and therefore out of scope for this
   document.  MTU and fragmentation considerations for tunnels are
   further discussed in [RFC4459].

   The AERO interface can configure either an indefinite MTU (i.e.,
   64KB-ENCAPS) or a smaller fixed MTU that determines the maximum sized
   packet that can be admitted into the AERO interface.  The MTU for
   each AERO interface neighbor is therefore constrained by the minimum
   of the MTU of the AERO interface, the MTU of the underlying interface
   used for tunneling (minus ENCAPS), and the path MTU within the tunnel
   (minus ENCAPS).

   IPv6 specifies a minimum link MTU of 1280 bytes [RFC2460].  This is
   the minimum packet size the AERO interface MUST admit without
   returning an ICMP Packet Too Big (PTB) message.  Although IPv4
   specifies a smaller minimum link MTU of 68 bytes [RFC0791], AERO
   interfaces also observe a 1280 byte minimum for IPv4 even if some
   fragmentation is needed.

   The vast majority of links in the Internet configure an MTU of at
   least 1500 bytes.  Original source hosts have therefore become
   conditioned to expect that IP packets up to 1500 bytes in length will
   either be delivered to the final destination or a suitable 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, the tunnel ingress sends
   encapsulated packets to the tunnel egress subject to the specific
   path considerations as follows:

3.13.1.  Operational Assurance of Sufficient MTU

   When there is operational assurance that all links in the paths that
   the tunnel may traverse are capable of passing packets up to S bytes
   in length, the ingress can admit all packets up to (S-ENCAPS) bytes
   without loss due to path MTU restrictions and without invoking
   fragmentation.  An example is a closed data center where it is known



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   that all links configure an MTU of at least 9KB.  Note that since
   there may be additional encapsulations on the path from the ingress
   to the egress, however, it may not always be sufficient to rely on
   operational assurance.  In that case, the ingress should observe one
   or both of the following approaches.

3.13.2.  Classical Path MTU Discovery with Reactive Fragmentation

   When the original source, ingress and egress are all within the same
   well-managed administrative domain, the ingress admits a packet into
   the tunnel if it is no larger than the current path MTU estimate for
   this egress (initially set to the MTU of the underlying link to be
   used for tunneling minus ENCAPS).  Otherwise, the ingress drops the
   packet and sends a network layer (L3) PTB message back to the
   original source.  Additionally, the ingress SHOULD cache the MTU
   value in any link-layer (L2) PTB messages it receives from a router
   on the path to the egress as a new path MTU estimate.  Thereafter,
   the ingress SHOULD periodically reset the path MTU estimate to the
   MTU of the underlying link minus ENCAPS to detect path MTU increases.

   These procedures apply when the path MTU for this egress is no
   smaller than (1280+ENCAPS) bytes; otherwise, the ingress can either
   declare the egress unreachable or commence fragmentation in a manner
   that parallels the standard behavior specified in [RFC2473].  In that
   case, the ingress encapsulates all packets that are no larger than
   1280 bytes while using encapsulation layer fragmentation if necessary
   as specified in Section 3.13.3.  (For IPv4 packets with DF=0 that are
   larger than 1280 bytes, the ingress instead uses IPv4 fragmentation
   before encapsulation.)

3.13.3.  Proactive Fragmentation with Expectation of Packetization Layer
         Path MTU Discovery

   When the original source, ingress and egress are not all within the
   same well-managed administrative domain, the ingress admits all
   packets up to 1500 bytes in length even if some fragmentation is
   needed, and admits larger packets without fragmentation in case they
   are able to traverse the tunnel in one piece.

   Several factors must be considered when fragmentation is needed.  For
   AERO links over IPv4, the IP ID field is only 16 bits in length,
   meaning that fragmentation at high data rates could result in data
   corruption due to reassembly misassociations [RFC6864][RFC4963] (see:
   Section 3.13.5).  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].  For these reasons,
   when fragmentation is needed the ingress inserts an encapsulation
   layer fragment header and applies tunnel fragmentation in the manner



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   suggested in Section 3.1.7 of [RFC2764] instead of IP fragmentation.
   Since the 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.

   The ingress therefore sends encapsulated packets to the egress
   according to the following algorithm:

   o  For IP packets that are no larger than (1280-ENCAPS) bytes, the
      ingress encapsulates the packet and admits it into the tunnel
      without fragmentation.  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 there is a restricting link in the
      path, i.e., unless lost due to congestion or routing errors.

   o  For IP packets that are larger than (1280-ENCAPS) bytes but no
      larger than 1500 bytes, the ingress encapsulates the packet and
      inserts an encapsulation layer fragment header.  Next, the ingress
      fragments the packet into a minimum number of non-overlapping
      fragments where the first fragment (including ENCAPS) is no larger
      than 1024 bytes and the remaining fragments are no larger than the
      first.  Each fragment consists of corresponding encapsulation
      headers followed by the fragment of the encapsulated packet
      itself.  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.  The egress
      therefore MUST be capable of reassembling packets up to
      (1500+ENCAPS) bytes in length; hence, it is RECOMMENDED that the
      egress be capable of reassembling at least 2KB.

   o  For IPv4 packets that are larger than 1500 bytes and with the DF
      bit set to 0, the ingress uses ordinary IPv4 fragmentation to
      break the unencapsulated packet into a minimum number of non-
      overlapping fragments where the first fragment (including ENCAPS)
      is no larger than 1024 bytes and the remaining fragments are no
      larger than the first.  The ingress then encapsulates each
      fragment (and for IPv4 sets the DF bit to 0) then admits them into
      the tunnel.  These fragments will be delivered to the final
      destination via the egress.

   o  For all other IP packets, if the packet is larger than the current
      path MTU estimate for this egress, the ingress drops the packet
      and returns an L3 PTB message to the original source with MTU set
      to the larger of 1500 bytes or the current path MTU estimate.
      Otherwise, the ingress encapsulates the packet and admits it into
      the tunnel without fragmentation (and for IPv4 sets the DF bit to



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      1).  Since PTB messages may either be lost or contain insufficient
      information, however, it is RECOMMENDED that original sources that
      send unfragmentable IP packets larger than 1500 bytes use
      Packetization Layer Path MTU Discovery (PLPMTUD) [RFC4821].

   A first exception to these procedures occurs when the ingress and
   egress are both within the same well-managed administrative domain.
   In that case, the ingress MAY initially admit all packets into the
   tunnel without fragmentation.  If the ingress subsequently receives
   an L2 PTB message reporting a size smaller than (1500+ENCAPS) it can
   commence fragmentation per the above algorithm.

   A second exception occurs when the original source and ingress are
   both within the same well-managed administrative domain.  In that
   case, if the underlying interface used by the ingress for tunneling
   configures an MTU smaller than (1500+HLEN) bytes, the ingress MAY
   drop packets that are larger than 1280 bytes and larger than the
   underlying interface MTU following encapsulation, and return an L3
   PTB message to the original source.

3.13.4.  Accommodating Large Control Messages

   The tunnel ingress MUST accommodate control messages (i.e., IPv6 ND,
   DHCPv6, etc.) even if the path MTU is insufficient to deliver the
   message without fragmentation.  For control messages that are larger
   than the known or assumed minimum path MTU, the ingress encapsulates
   the packet and inserts an encapsulation layer fragment header.  Next,
   the ingress breaks the packet into a minimum number of non-
   overlapping fragments where the first fragment (including ENCAPS) is
   no larger than 1024 bytes and the remaining fragments are no larger
   than the first.  The ingress then encapsulates each fragment (and for
   IPv4 sets the DF bit to 0) then admits them into the tunnel.

   Control messages that exceed the 2KB minimum reassembly size rarely
   occur in current operational practices, however the egress SHOULD be
   able to reassemble them if they appear in future applications.  This
   means that the egress SHOULD include a configuration knob allowing
   the operator to set a larger reassembly buffer size if large control
   messages become more common in the future.

   The ingress MAY send large control messages without fragmentation if
   there is assurance that large packets can traverse the tunnel without
   fragmentation.








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3.13.5.  Integrity

   When fragmentation is needed, there must be assurance that reassembly
   can be safely conducted without incurring data corruption.  Sources
   of corruption can include implementation errors, memory errors and
   misassociations of fragments from a first datagram with fragments of
   another datagram.  The first two conditions (implementation and
   memory errors) are mitigated by modern systems and implementations
   that have demonstrated integrity through decades of operational
   practice.  The third condition (reassembly misassociations) must be
   accounted for by AERO.

   The fragmentation procedure described in the above algorithms can
   reuse standard IPv6 fragmentation and reassembly code.  Since
   encapsulation layer fragment headers include a 32-bit ID field, there
   would need to be 2^32 packets alive in the network before a second
   packet with a duplicate ID enters the system with the (remote)
   possibility for a reassembly misassociation.  For 1280 byte packets,
   and for a maximum network lifetime value of 60 seconds [RFC2460],
   this means that the ingress would need to produce ~(7 *10^12) bits/
   sec in order for a duplication event to be possible.  This exceeds
   the bandwidth of modern data link technologies at the time of this
   writing, but not necessarily so for future datal links.  Although
   wireless data links commonly used by AERO Clients support vastly
   lower data rates, the aggregate data rates between AERO Servers and
   Relays may be substantial.  However, high speed data links in the
   network core are expected to configure larger MTUs (e.g., 4KB, 8KB or
   even larger) such that unfragmented packets can be used.  Hence, no
   integrity check is included to cover fragmentation and reassembly
   procedures.

   When the ingress sends an IPv4-encapsulated packet with the DF bit
   set to 0 in the above algorithms, there is a chance that the packet
   may be fragmented by an IPv4 router somewhere within the tunnel.
   Since the largest such packet is only 1280 bytes, however, it is very
   likely that the packet will traverse the tunnel without incurring a
   restricting link.  Even when a link within the tunnel configures an
   MTU smaller than 1280 bytes, it is very likely that it does so due to
   limited performance characteristics [RFC3819].  This means that the
   tunnel would not be able to convey fragmented IPv4-encapsulated
   packets fast enough to produce reassembly misassociations, as
   discussed above.  However, AERO must also account for the possibility
   of tunnel paths that traverse a high-speed IPv4 link with a
   degenerate MTU.

   Since the IPv4 header includes only a 16-bit ID field, there would
   only need to be 2^16 packets alive in the network before a second
   packet with a duplicate ID enters the system.  For 1280 byte packets,



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   and for a maximum network lifetime value of 120 seconds[RFC0791],
   this means that the ingress would only need to produce ~(5 *10^6)
   bits/sec in order for a duplication event to be possible - a value
   that is well within range for modern wired and wireless data link
   technologies.

   Therefore, if there is strong operational assurance that no IPv4
   links capable of supporting data rates of 5Mbps or more configure an
   MTU smaller than 1280 the ingress MAY omit an integrity check for the
   IPv4 fragmentation and reassembly procedures; otherwise, the ingress
   SHOULD include an integrity check.  When an upper layer encapsulation
   (e.g., IPsec) already includes an integrity check, the ingress need
   not include an additional check.  Otherwise, the ingress calculates
   the encapsulation layer checksum over the encapsulated packet and
   writes the value into the encapsulation layer checksum header.  The
   egress will then verify the checksum and discard the packet if the
   checksum is incorrect.

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.

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

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




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   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

   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 IPv4 integrity checks (see: Section 3.13.5).

   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



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      (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.

   o  When an AERO node receives an L2 PTB message, it caches the MTU
      field value of the L2 ICMP header then translates the message into
      an L3 PTB message if possible and forwards the message toward the
      original source as described below.  Note that in some instances
      the packet-in-error field of an L2 PTB message may not include
      enough information for translation to an L3 PTB message.  In that
      case, the AERO interface simply discards the L2 PTB message since
      translation of L2 PTB messages to L3 PTB messages can provide a
      useful optimization when possible, but is not critical for sources
      that correctly use PLPMTUD.

   To translate an L2 PTB message to an L3 PTB message, the AERO node
   discards the L2 IP and ICMP headers, and also discards the
   encapsulation headers of the original L3 packet.  Next the node
   encapsulates the included segment of the original L3 packet in an L3
   IP and ICMP header, and sets the ICMP header Type and Code values to
   appropriate values for the L3 IP protocol.  When the AERO node, AERO
   link neighbor and original source are all within the same
   administrative domain, the node writes the maximum of 1280 bytes and
   (L2 MTU - ENCAPS) into the MTU field of the L3 ICMP header.




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   Otherwise, the node translates L2 PTB messages for which (L2 MTU -
   ENCAPS) is no less than 1500 bytes and discards all other L2 PTBs.

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

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

             Figure 4: AERO Interface L3 Error Message Format

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

   When an AERO Relay receives an L3 packet for which the destination
   address is covered by an ASP, if there is no more-specific routing
   information for the destination the Relay drops the packet and
   returns an L3 Destination Unreachable message.  The Relay first
   writes the IP source address of the original L3 packet as the
   destination address of the L3 Destination Unreachable message and
   determines the next hop to the destination.  If the next hop is



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   reached via the AERO interface, the Relay uses the IPv6 address "::"
   or the IPv4 address "0.0.0.0" as the IP source address of the L3
   Destination Unreachable message and forwards the message to the next
   hop within the AERO interface.  Otherwise, the Relay uses one of its
   non link-local addresses as the source address of the L3 Destination
   Unreachable message and forwards the message via a link outside the
   AERO interface.

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

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
   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 delegation state
   synchronization is necessary, and Clients can optionally hold
   separate delegations for the same ACPs from multiple Servers.  In
   this way, Clients can associate with multiple Servers, and can
   receive new delegations from new Servers before deprecating
   delegations received from existing Servers.  This provides the Client
   with a natural fault-tolerance and/or load balancing profile.

   AERO Clients and Servers exchange Client link-layer address
   information using an option format similar to the Client Link Layer
   Address Option (CLLAO) defined in [RFC6939].  Due to practical
   limitations of CLLAO, however, AERO interfaces instead use Vendor-
   Specific Information Options as described in the following sections.




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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
   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 Solicit message includes
   fe80::ffff:ffff:ffff:ffff as the IPv6 source address,
   'All_DHCP_Relay_Agents_and_Servers' as the IPv6 destination address
   and the link-layer address of the Server as the link-layer
   destination address.  The Solicit message also includes a Client
   Identifier option with a 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 SHOULD include an AERO Link-registration Request
   (ALREQ) option in the Solicit message to register one or more links
   with the Server.  The Server will include an AERO Link-registration
   Reply (ALREP) option in the corresponding Reply message as specified
   in Section 3.15.3.  (The Client MAY omit the ALREQ option, in which
   case the Server will still include an ALREP option in its Reply with
   "Link ID" set to 0.)

   The format for the ALREQ option 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      OPTION_VENDOR_OPTS       |         option-len (1)        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   enterprise-number = 45282                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  opt-code = OPTION_ALREQ (0)  |         option-len (2)        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Link ID   |  DSCP #1  |Prf|  DSCP #2  |Prf|   ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

          Figure 5: AERO Link-registration Request (ALREQ) Option




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   In the above format, the Client sets 'option-code' to
   OPTION_VENDOR_OPTS, sets 'option-len (1)' to the length of the option
   following this field, sets 'enterprise-number' to 45282 (see: "IANA
   Considerations"), sets opt-code to the value 0 ("OPTION_ALREQ") and
   sets 'option-len (2)' to the length of the remainder of the option.
   The Client includes appropriate 'Link ID, 'DSCP' and 'Prf' values for
   the underlying interface over which the Solicit message will be
   issued the same as specified for an S/TLLAO Section 3.4.  The Server
   will register each value with the Link ID in the Client's neighbor
   cache entry.  The Client finally includes any necessary
   authentication options to identify itself to the DHCPv6 server, and
   sends the encapsulated Solicit message via the underlying interface
   corresponding to Link ID.  (Note that this implies that the Client
   must send additional Rebind messages with ALREQ options to the server
   following the initial PD exchange using different underlying
   interfaces and their corresponding Link IDs if it wishes to register
   additional link-layer addresses and their associated DSCPs.)

   When the Client receives its ACP via a Reply from the AERO Server, it
   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.  The Client then considers the
   link-layer address of the Server as the primary default encapsulation
   address for forwarding packets for which no more-specific forwarding
   information is available.  The Client further caches any ASPs
   included in the ALREP option as ASPs to apply to the AERO link.

   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.  Alternatively, the Client can 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 one or more default IP routes to the AERO
   interface with the link-local address of a Server as the next hop.

   After AERO address autoconfiguration, 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 and the same options
   that were used in the initial PD request.  Note that if the Client
   does not issue a Renew before the delegations expire (e.g., if the
   Client has been out of touch with the Server for a considerable
   amount of time) it must re-initiate the DHCPv6 PD procedure.




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   Since the addresses assigned to the Client's AERO interface are
   obtained from the unique ACP delegations it receives, there is no
   need for DAD on AERO links.  Other nodes maliciously attempting to
   hijack addresses from an authorized Client's ACPs will be denied
   access to the network by the Server due to an unacceptable link-layer
   address and/or security parameters (see: Security Considerations).

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

3.15.2.1.  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
   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].





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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 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, 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.  Since the 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 prefix delegation state can be flushed out of the network.

   The Server also includes an ALREP option that includes the UDP Port
   Number and IP Address values it observed when it received the ALREQ
   in the Client's original DHCPv6 message (if present) followed by the
   ASP(s) for the AERO link.  The ALREP option is formatted as shown in
   Figure 6:

















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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      OPTION_VENDOR_OPTS       |         option-len (1)        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   enterprise-number = 45282                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  opt-code = OPTION_ALREP (1)  |         option-len (2)        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Link ID    |    Reserved   |         UDP Port Number       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                          IP Address                           +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +              AERO Service Prefix (ASP) #1     +-+-+-+-+-+-+-+-+
       |                                               |  Prefix Len   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +              AERO Service Prefix (ASP) #2     +-+-+-+-+-+-+-+-+
       |                                               |  Prefix Len   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                                                               ~
       ~                                                               ~


           Figure 6: AERO Link-registration Reply (ALREP) Option

   In the ALREP, the Server sets 'option-code' to OPTION_VENDOR_OPTS,
   sets 'option-length (1)' to the length of the option, sets
   'enterprise-number' to 45282 (see: "IANA Considerations"), sets opt-
   code to OPTION_ALREP (1), and sets 'option-len (2)' to the length of
   the remainder of the option.  Next, the Server sets 'Link ID' to the
   same value that appeared in the ALREQ, sets Reserved to 0 and sets
   'UDP Port Number' and 'IP address' to the Client's link-layer
   address.  The Server next includes one or more ASP with the IP prefix
   as it would appear in the interface identifier portion of the
   corresponding AERO address (see: Section 3.3), except that the low-
   order 8 bits of the ASP field encode the prefix length instead of the
   low-order 8 bits of the prefix.  The longest prefix that can
   therefore appear as an ASP is /56 for IPv6 or /24 for IPv4.  (Note
   that if the Client did not include an ALREQ option in its DHCPv6




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   message, the Server MUST still include an ALREP option in the
   corresponding reply with 'Link ID' set to 0.)

   When the Server admits the Reply message into the AERO interface, it
   creates a static neighbor cache entry for the Client based on the
   DUID and AERO addresses with lifetime set to no more than the
   delegation lifetimes and the Client's link-layer address as the link-
   layer address for the Link ID specified in the ALREQ.  The Server
   then uses the Client link-layer address information in the ALREQ
   option as the link-layer address for encapsulation based on the
   (DSCP, Prf) information.

   After the initial DHCPv6 PD exchange, the AERO Server maintains the
   neighbor cache entry for the Client until the delegation lifetimes
   expire.  If the Client issues a Renew, the Server extends the
   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
   ALREP option the same as described above then wraps the option in a
   Relay-Supplied DHCP Option option (RSOO) [RFC6422].  The LDRA then
   incorporates the option 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 ALREP option and authenticates the encapsulated DHCPv6 message.
   The DHCPv6 server subsequently ignores the ALREQ option itself, since
   the relay has already included the ALREP option.

   When the DHCPv6 server prepares a Reply message, it then includes the
   ALREP option 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 discards the
   Relay-Reply wrapper and delivers the DHCPv6 message to the Client.





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3.15.4.  Deleting Link Registrations

   After an AERO Client registers its Link IDs and their associated
   (DSCP,Prf) values with the AERO Server, the Client may wish to delete
   one or more Link registrations, e.g., if an underlying link becomes
   unavailable.  To do so, the Client prepares a Rebind message that
   includes an AERO Link-registration Delete (ALDEL) option and sends
   the Rebind message to the Server over any available underlying link.
   The ALDEL option is formatted as shown in Figure 7:

        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 (1)        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   enterprise-number = 45282                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  opt-code = OPTION_ALDEL (2)  |         option-len (2)        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Link ID #1  |  Link ID #2   |  Link ID #3   |    ...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-


          Figure 7: AERO Link-registration Delete (ALDEL) Option

   In the ALDEL, the Client sets 'option-code' to OPTION_VENDOR_OPTS,
   sets 'option-length (1)' to the length of the option, sets
   'enterprise-number' to 45282 (see: "IANA Considerations"), sets
   optcode to OPTION_ALDEL (2), and sets 'option-len (2)' to the length
   of the remainder of the option.  Next, the Server includes each 'Link
   ID' value that it wishes to delete.

   If the Client wishes to discontinue use of a Server and thereby
   delete all of its Link ID associations, it must issue a Release to
   delete the entire neighbor cache entry, i.e., instead of issuing a
   Rebind with one or more ALDEL options.

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



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   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 Intradomain 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 intra-domain AERO 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
   described in Section 3.22.  The following sections specify the AERO
   intradomain route optimization procedure.

3.17.1.  Reference Operational Scenario

   Figure 8 depicts the AERO intradomain 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'):







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            +--------------+  +--------------+  +--------------+
            |   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 8: AERO Reference Operational Scenario

   In Figure 8, 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
   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.




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3.17.2.  Concept of Operations

   Again, with reference to Figure 8, 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.

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 9:








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

             Figure 9: 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
   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')).




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   o  the network-layer source address is set to fe80::2001:db8:0:0
      (i.e., the AERO address of Client ('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-compatible IPv6 address format).

   o  the message includes one or more TLLAOs with Link ID and DSCPs 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').

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 Link 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



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   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
   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



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   ('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-compatible IPv6 address format).

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

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

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





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   After Client ('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 Link 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
   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



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   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.

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.






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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.

   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



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   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
   ALREQ that includes the correct Link ID and DSCP values.  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 prefix delegation 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' 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 ALREQ that includes the new Link ID and
   DSCP values.  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 ALDEL that includes the deprecated Link ID.  If
   authentication succeeds, the Server then updates its neighbor cache



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   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.  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.

   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 MUST 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.

3.20.  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



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   as Mobility Access Gateways (MAGs) provide MNs with an access link
   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 prefix 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 intradomain 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



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   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,
   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).




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



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   the link-layer source address with its own (Internet-facing) link-
   layer address.

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

   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.  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.

3.24.  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




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   that the encapsulated site-scoped multicast packets do not leak
   outside of the site spanned by the AERO link.

3.25.  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.

3.26.  Operation on Server-less AERO Links

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

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

3.27.  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.

3.28.  Intradomain Routing

   After a tunnel neighbor relationship has been established, neighbors
   can use a traditional dynamic routing protocol over the tunnel to
   exchange routing information without having to inject the routes into
   the AERO routing system.

4.  Implementation Status

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




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   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.

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.  Unless there is some other means
   of authenticating the Client's identity (e.g., link-layer security),
   AERO nodes SHOULD also use DHCPv6 securing services (e.g., DHCPv6
   authentication, Secure DHCPv6 [I-D.ietf-dhc-sedhcpv6], etc.) for
   Client authentication and network admission control.  In particular,
   Clients SHOULD include authenticating information on each
   Solicit/Rebind/Release message they send, but omit authenticating
   information on Renew messages.  Renew messages are exempt due to the
   fact that the Renew must already be checked for having a correct
   link-layer address and does not update any link-layer addresses.
   Therefore, asking the Server to also authenticate the Renew message
   would be unnecessary and could result in excessive processing
   overhead.

   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 that
   is often sufficient.  In other instances, additional securing




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   mechanisms such as Secure Neighbor Discovery (SeND) [RFC3971], IPsec
   [RFC4301] or TLS [RFC5246] may be necessary.

   AERO Clients MUST ensure that their connectivity is not used by
   unauthorized nodes on their EUNs to gain access to a protected
   network, i.e., AERO Clients that act as routers MUST NOT provide
   routing services for unauthorized nodes.  (This concern is no
   different than for ordinary hosts that receive an IP address
   delegation but then "share" the address with unauthorized nodes via
   some form of Internet connection sharing.)

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

   An AERO Client's link-layer address could be rewritten by a link-
   layer switching element on the path from the Client to the Server and
   not detected by the DHCPv6 security mechanism.  However, such a
   condition would only be a matter of concern on unmanaged/unsecured
   links where the link-layer switching elements themselves present a
   man-in-the-middle attack threat.  For this reason, IP security MUST
   be used when AERO is employed over unmanaged/unsecured 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



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   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]

   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>.







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   [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>.

   [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>.



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   [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-sedhcpv6-12 (work
              in progress), April 2016.

   [I-D.ietf-nvo3-gue]
              Herbert, T., Yong, L., and O. Zia, "Generic UDP
              Encapsulation", draft-ietf-nvo3-gue-02 (work in progress),
              December 2015.

   [I-D.templin-intarea-grefrag]
              Templin, F., "GRE Tunnel Fragmentation", draft-templin-
              intarea-grefrag-02 (work in progress), January 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>.





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   [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>.

   [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>.



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   [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>.

   [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>.





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   [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>.

   [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>.




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   [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>.

   [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>.





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   [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>.

   [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>.



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   [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>.

   [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 10 shows the AERO IP-in-IP encapsulation format before any
   fragmentation is applied:



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

        Minimal Encapsulation in IPv4      Minimal Encapsulation in IPv6


          Figure 10: Minimal Encapsulation Format using IP-in-IP

   Figure 11 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 11: Minimal Encapsulation Using GRE

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





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

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

Appendix B.  When to Insert an Encapsulation Fragment Header

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

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

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

Author's Address










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   Fred L. Templin (editor)
   Boeing Research & Technology
   P.O. Box 3707
   Seattle, WA  98124
   USA

   Email: fltemplin@acm.org












































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