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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc5320, rfc5558, rfc5720,                   October 13, 2016
           rfc6179, rfc6706 (if
           approved)
Intended status: Standards Track
Expires: April 16, 2017


             Asymmetric Extended Route Optimization (AERO)
                     draft-templin-aerolink-73.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 that supports operation of the IPv6
   Neighbor Discovery (ND) protocol and links IPv6 ND to IP forwarding.
   Admission control and address/prefix provisioning are supported by
   the Dynamic Host Configuration Protocol for IPv6 (DHCPv6), while
   mobility management and route optimization are naturally supported
   through dynamic neighbor cache updates.  Although DHCPv6 and IPv6 ND
   messaging are used in the control plane, both IPv4 and IPv6 are
   supported in the data plane.  AERO is a widely-applicable tunneling
   solution especially well suited to mobile Virtual Private Networks
   (VPNs) and other applications as described in this document.

Status of This Memo

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

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

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

   This Internet-Draft will expire on April 16, 2017.





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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Asymmetric Extended Route Optimization (AERO) . . . . . . . .   7
     3.1.  AERO Link Reference Model . . . . . . . . . . . . . . . .   7
     3.2.  AERO Node Types . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  AERO Routing System . . . . . . . . . . . . . . . . . . .   9
     3.4.  AERO Interface Link-local Addresses . . . . . . . . . . .  10
     3.5.  AERO Interface Characteristics  . . . . . . . . . . . . .  12
     3.6.  AERO Interface Initialization . . . . . . . . . . . . . .  14
       3.6.1.  AERO Relay Behavior . . . . . . . . . . . . . . . . .  14
       3.6.2.  AERO Server Behavior  . . . . . . . . . . . . . . . .  15
       3.6.3.  AERO Client Behavior  . . . . . . . . . . . . . . . .  15
     3.7.  AERO Interface Neighbor Cache Maintenace  . . . . . . . .  15
     3.8.  AERO Interface Forwarding Algorithm . . . . . . . . . . .  17
       3.8.1.  Client Fowarding Algorithm  . . . . . . . . . . . . .  18
       3.8.2.  Server Fowarding Algorithm  . . . . . . . . . . . . .  18
       3.8.3.  Relay Fowarding Algorithm . . . . . . . . . . . . . .  19
     3.9.  AERO Interface Encapsulation and Re-encapsulation . . . .  19
     3.10. AERO Interface Decapsulation  . . . . . . . . . . . . . .  20
     3.11. AERO Interface Data Origin Authentication . . . . . . . .  20
     3.12. AERO Interface Packet Size Issues . . . . . . . . . . . .  21
     3.13. AERO Interface Error Handling . . . . . . . . . . . . . .  23
     3.14. AERO Router Discovery, Prefix Delegation and
           Autoconfiguration . . . . . . . . . . . . . . . . . . . .  26
       3.14.1.  AERO DHCPv6 and IPv6 ND Service Model  . . . . . . .  26
       3.14.2.  AERO Client Behavior . . . . . . . . . . . . . . . .  27
       3.14.3.  AERO Server Behavior . . . . . . . . . . . . . . . .  29
     3.15. AERO Interface Route Optimization . . . . . . . . . . . .  31
       3.15.1.  Reference Operational Scenario . . . . . . . . . . .  31
       3.15.2.  Concept of Operations  . . . . . . . . . . . . . . .  33
       3.15.3.  Message Format . . . . . . . . . . . . . . . . . . .  33



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       3.15.4.  Sending Predirects . . . . . . . . . . . . . . . . .  34
       3.15.5.  Re-encapsulating and Relaying Predirects . . . . . .  35
       3.15.6.  Processing Predirects and Sending Redirects  . . . .  36
       3.15.7.  Re-encapsulating and Relaying Redirects  . . . . . .  38
       3.15.8.  Processing Redirects . . . . . . . . . . . . . . . .  38
       3.15.9.  Server-to-Client and Client-to-Server Redirection  .  39
       3.15.10. Server-to-Server Redirection . . . . . . . . . . . .  40
     3.16. Neighbor Unreachability Detection (NUD) . . . . . . . . .  40
     3.17. Mobility Management . . . . . . . . . . . . . . . . . . .  41
       3.17.1.  Announcing Link-Layer Address Changes  . . . . . . .  42
       3.17.2.  Bringing New Links Into Service  . . . . . . . . . .  42
       3.17.3.  Removing Existing Links from Service . . . . . . . .  42
       3.17.4.  Implicit Mobility Management . . . . . . . . . . . .  42
       3.17.5.  Moving to a New Server . . . . . . . . . . . . . . .  43
       3.17.6.  Packet Queueing for Mobility . . . . . . . . . . . .  44
       3.17.7.  Alternate Mobility Security Model  . . . . . . . . .  44
     3.18. Multicast Considerations  . . . . . . . . . . . . . . . .  44
   4.  AERO Variations . . . . . . . . . . . . . . . . . . . . . . .  45
     4.1.  Operation on Host-Only IPv6 AERO Links  . . . . . . . . .  45
     4.2.  Operation on AERO Links Without DHCPv6 Services . . . . .  46
     4.3.  Operation on Server-less AERO Links . . . . . . . . . . .  46
     4.4.  Operation on Client-less AERO Links . . . . . . . . . . .  46
     4.5.  Manually-Configured AERO Tunnels  . . . . . . . . . . . .  47
     4.6.  Encapsulation Avoidance on Relay-Server Dedicated Links .  47
     4.7.  Encapsulation Protocol Version Considerations . . . . . .  47
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .  47
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  47
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  48
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  49
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  50
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  52
   Appendix A.  AERO Alternate Encapsulations  . . . . . . . . . . .  59
   Appendix B.  When to Insert an Encapsulation Fragment Header  . .  60
   Appendix C.  Autoconfiguration for Constrained Platforms  . . . .  61
   Appendix D.  Extending AERO Links Through Security Gateways . . .  62
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  63

1.  Introduction

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



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   AERO provides an IPv6 link-local address format that supports
   operation of the IPv6 Neighbor Discovery (ND) [RFC4861] protocol and
   links IPv6 ND to IP forwarding.  Admission control and address/prefix
   provisioning are supported by the Dynamic Host Configuration Protocol
   for IPv6 (DHCPv6) [RFC3315], while mobility management and route
   optimization are 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.

   A node's AERO interface can be configured over multiple underlying
   interfaces.  From the standpoint of IPv6 ND, AERO interface neighbors
   therefore may appear to have multiple link-layer addresses.  Each
   link-layer address is subject to change due to mobility, and link-
   layer address changes are signaled by IPv6 ND messaging the same as
   for any IPv6 link.

   AERO is applicable to a wide variety of use cases.  For example, it
   can be used to coordinate the Virtual Private Network (VPN) links of
   mobile nodes (e.g., cellphones, tablets, laptop computers, etc.) that
   connect into a home enterprise network via public access networks
   using services such as OpenVPN [OVPN].  AERO is also applicable to
   aviation applications for both manned and unmanned aircraft where the
   aircraft is treated as a mobile node that can connect an Internet of
   Things (IoT).  Numerous other use cases are also in scope.

   The AERO mobile VPN capability and Border Gateway Protocol (BGP)-
   based core routing system can further be employed either in
   conjunction or separately according to the specific use case (see
   Section 4).  This allows for correct fitting of the (modular) AERO
   components to match the specific application.  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.  The AERO mechanisms
      can also operate over native link types (e.g., Ethernet, WiFi
      etc.) when a tunnel virtual overlay is not needed.

   AERO interface




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      a node's attachment to an AERO link.  Since the addresses assigned
      to an AERO interface are managed for uniqueness, 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 3.4.

   AERO node
      a node that is connected to an AERO link.

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

   AERO Server ("Server")
      a node that configures an AERO interface to provide default
      forwarding services for AERO Clients.  The Server assigns an
      administratively provisioned IPv6 link-local unicast address to
      the AERO interface 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.

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

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

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




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

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

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

   end user network (EUN)
      an internal virtual or external edge IP network that an AERO
      Client connects to the rest of the network via the AERO interface.
      The Client sees each EUN as a "downstream" network and sees the
      AERO interface as its point of attachment to the "upstream"
      network.

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

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

   base AERO address
      the lowest-numbered AERO address from the first ACP delegated to
      the Client (see Section 3.4).

   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



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   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

                              .-(::::::::)
                           .-(:::: 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      )--|Host H1|     |Host H2|--(__    EUN      )
      `-(______)-'    +-------+     +-------+     `-(______)-'

                    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.



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   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).

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

   Each node on the AERO link maintains an AERO interface neighbor cache
   and an IP forwarding table the same as for any link.  In common
   operational practice, there may be many additional Relays, Servers
   and Clients.

3.2.  AERO Node Types

   AERO Relays provide default forwarding services to AERO Servers.
   Each Relay also peers with each Server in a dynamic routing protocol
   instance to discover the Server's list of associated ACPs (see
   Section 3.3).  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.  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 (see Section 3.3).
   Servers configure a DHCPv6 server function and act as delegating
   routers 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 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 can
   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.



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   Clients maintain an AERO interface neighbor cache entry for each of
   their associated Servers as well as for each of their correspondent
   Clients.

3.3.  AERO Routing System

   The AERO routing system comprises 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 all
   Relays will therefore have a consistent view of the AERO link ACP
   delegations.

   Each Server maintains a working set of associated ACPs, and
   dynamically announces new ACPs and withdraws departed ACPs in its BGP
   updates to Relays.  Clients are expected to remain associated with
   their current Servers for extended timeframes, however Servers SHOULD
   selectively suppress BGP updates for impatient Clients that
   repeatedly associate and disassociate with them in order to dampen
   routing churn.

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

   Scaling properties of the AERO routing system are limited by the
   number of BGP routes that can be carried by Relays.  At the time of
   this writing, the global public Internet BGP routing system manages
   well in excess of 500K routes with linear growth and no signs of
   router resource exhaustion [BGP].  Therefore, assuming each BGP
   router can eventually carry 1M or more routes, this means that at



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   least 1M 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 it only
   sends BGP updates to the specific set of Relays that aggregate the
   ASP.  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 1K sets of Relays, the AERO routing system can then
   accommodate 1B ACPs with no additional overhead for Servers and
   Relays (for example, it should be possible to service 1B /64 ACPs
   taken from a /34 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 an aggregated ASP for the link into the native
   routing system even though each Relay services only smaller segments
   of the ASP.  In that case, a Relay upon receiving a packet with a
   destination address covered by the ASP segment of another Relay can
   simply tunnel the packet to the correct Relay.  The tradeoff then is
   the penalty for Relay-to-Relay tunneling compared with reduced
   routing information in the native routing system.

   Finally, Relays may have multiple Routing Information Base (RIB)
   entries for a single ACP advertised by multiple Servers, but will
   place only one entry in the Forwarding Information Base (FIB).
   Servers can assign a BGP weight to their peering configurations so
   that Relays can determine preferences for ACPs learned from multiple
   Servers.  In this way, Relays can choose the Server with the highest
   weight and insert the corresponding RIB route into the FIB.  The
   Relay can then fail over to a Server with lower weight in case of ACP
   withdrawal or Server failure.

3.4.  AERO Interface Link-local Addresses

   AERO interface link-local address types include administratively-
   provisioned addresses and AERO addresses.

   Administratively-provisioned addresses are allocated from the range
   fe80::/96 and assigned to a Server or Relay's AERO interface.



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   Administratively-provisioned addresses MUST be managed for uniqueness
   by the administrative authority for the AERO link.  (Note that fe80::
   is the IPv6 link-local subnet router anycast address, and
   fe80::ffff:ffff is the address used by Clients to bootstrap AERO
   address autoconfiguration.  These special addresses are therefore not
   available for administrative provisioning.)

   An AERO address is an IPv6 link-local address with an embedded prefix
   based on an ACP and associated with a Client's AERO interface.  AERO
   addresses remain stable as the Client moves between topological
   locations, i.e., even if its link-layer addresses change.

   For IPv6, AERO addresses begin with the prefix fe80::/64 and include
   in the interface identifier (i.e., the lower 64 bits) a 64-bit prefix
   taken from one of the Client's IPv6 ACPs.  For example, if the AERO
   Client receives the IPv6 ACP:

      2001:db8:1000:2000::/56

   it constructs its corresponding AERO addresses as:

      fe80::2001:db8:1000:2000

      fe80::2001:db8:1000:2001

      fe80::2001:db8:1000:2002

      ... etc. ...

      fe80::2001:db8:1000:20ff

   For IPv4, AERO addresses are based on an IPv4-mapped IPv6 address
   [RFC4291] formed from an IPv4 ACP and with a Prefix Length of 96 plus
   the ACP prefix length.  For example, for the IPv4 ACP 192.0.2.32/28
   the IPv4-mapped IPv6 ACP is:

      0:0:0:0:0:FFFF:192.0.2.16/124

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

      fe80::FFFF:192.0.2.16

      fe80::FFFF:192.0.2.17

      fe80::FFFF:192.0.2.18




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      ... etc. ...

      fe80:FFFF:192.0.2.31

   When the Server delegates ACPs to the Client, both the Server and
   Client use the lowest-numbered AERO address from the first ACP
   delegation as the "base" AERO address.  (For example, for the ACP
   2001:db8:1000:2000::/56 the base address is 2001:db8:1000:2000.)  The
   Client then assigns the base AERO address to the AERO interface and
   uses it for the purpose of maintaining the neighbor cache entry.  If
   the Client has multiple AERO addresses (i.e., when there are multiple
   ACPs and/or ACPs with short prefix lengths), the Client originates
   IPv6 ND messages using the base AERO address as the source address
   and accepts and responds to IPv6 ND messages destined to any of its
   AERO addresses as equivalent to the base AERO address.  In this way,
   the Client maintains a single neighbor cache entry that may include
   multiple AERO addresses.

3.5.  AERO Interface Characteristics

   AERO interfaces use encapsulation (see: Section 3.9) to exchange
   packets with neighbors attached to the AERO link.

   AERO interfaces maintain a neighbor cache, and use both DHCPv6 and
   IPv6 ND control messaging to manage the creation, modification and
   deletion of neighbor cache entries.  AERO interfaces use standard
   DHCPv6 messaging for prefix delegation, admission control and
   neighbor cache entry management.  AERO interfaces use unicast IPv6 ND
   Neighbor Solicitation (NS), Neighbor Advertisement (NA), Router
   Solicitation (RS) and Router Advertisement (RA) messages for neighbor
   cache management 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 3.15).

   AERO interface ND messages include one or more Source/Target Link-
   Layer Address Options (S/TLLAOs) formatted as shown in Figure 2:














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

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

   In this format:

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

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

   o  Reserved is set to the value '0' on transmission and ignored on
      receipt.

   o  Interface ID is set to an integer value between 0 and 65535
      corresponding to an underlying interface of the AERO node.

   o  UDP Port Number and IP Address are set to the addresses used by
      the AERO node when it sends encapsulated packets over the
      underlying interface.  When UDP is not used as part of the
      encapsulation, UDP Port Number is set to the value '0'.  When the
      encapsulation IP address family is IPv4, IP Address is formed as
      an IPv4-mapped IPv6 address as specified in Section 3.4.




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

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

   If a Client's multiple underlying interfaces are used "one at a time"
   (i.e., all other interfaces are in standby mode while one interface
   is active), then IPv6 ND messages include only a single S/TLLAO with
   Interface ID set to a constant value.  In that case, the Client would
   appear to have a single underlying interface but with a dynamically
   changing link-layer address.

   If the Client has multiple active underlying interfaces, then from
   the perspective of IPv6 ND it would appear to have multiple link-
   layer addresses.  In that case, IPv6 ND messages MAY include multiple
   S/TLLAOs -- each with an Interface ID that corresponds to a specific
   underlying interface of the AERO node.

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 is taken from the range fe80::/96 but excluding the
   special addresses fe80:: and fe80::ffff:ffff.  The Relay then engages
   in a dynamic routing protocol session with all Servers on the link
   (see: Section 3.3), and advertises its assigned ASPs into the native
   IP Internetwork.

   Each Relay subsequently maintains an IP forwarding table entry for
   each active 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 3.16) since the dynamic routing protocol already
   provides reachability confirmation.



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3.6.2.  AERO Server Behavior

   When a Server enables an AERO interface, it assigns an
   administratively-provisioned link-local address fe80::ID the same as
   for Relays.  The Server further configures a DHCPv6 server function
   to facilitate DHCPv6 PD exchanges with AERO Clients.  The Server
   maintains a neighbor cache entry for each Relay on the link, and
   manages per-Client 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 3.3).

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

3.6.3.  AERO Client Behavior

   When a Client enables an AERO interface, it uses the special
   administratively-provisioned link-local address fe80::ffff:ffff as
   the source network-layer address in DHCPv6 PD messages to obtain one
   or more ACPs from an AERO Server.  Next, the Client assigns the base
   AERO address to the AERO interface and sends an RS to the Server to
   receive an RA.  In this way, the DHCPv6 PD exchange securely
   bootstraps autoconfiguration of unique link-local address(es) while
   the RS/RA exchange establishes link-layer addresses and
   autoconfigures AERO link parameters.  The Client maintains a neighbor
   cache entry for each of its Servers and each of its active
   correspondent Clients.  When the Client receives IPv6 ND messages on
   the AERO interface it updates or creates neighbor cache entries,
   including link-layer address information.

3.7.  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



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   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 and maintained through
   DHCPv6 PD and IPv6 ND exchanges as specified in Section 3.14, and
   remain in place for durations bounded by prefix delegation 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 delegates prefixes via DHCPv6 PD, it creates a static
   neighbor cache entry for the Client using the Client's base AERO
   address as the network-layer address and associates all of the
   Client's other AERO addresses with the neighbor cache entry.  When
   the Client receives the prefix delegation, it creates a static
   neighbor cache entry for the Server based on the DHCPv6 Reply message
   link-local source address as the network-layer address and the
   encapsulation IP source address and UDP source port number as the
   link-layer address.  The Client then sends an RS message to inform
   the Server of its link-layer addresses and to solicit an RA.  When
   the Server returns an RA message, the Client uses the
   autoconfiguration information in the RA message to configure AERO
   interface parameters.

   Dynamic neighbor cache entries are created or updated based on
   receipt of Predirect/Redirect messages as specified in Section 3.15,
   and are garbage-collected when keepalive timers expire.  AERO Clients
   maintain dynamic neighbor cache entries for each of their active
   correspondent Clients with lifetimes based on IPv6 ND messaging
   constants.

   When an AERO Client receives a valid Predirect message it creates or
   updates a dynamic neighbor cache entry for the Predirect target
   network-layer and link-layer addresses.  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.  The node resets AcceptTime
   when it receives a new Predirect, and otherwise decrements AcceptTime
   while no Predirects have been received.  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 (see below).

   When an AERO Client receives a valid Redirect message it creates or
   updates a dynamic neighbor cache entry for the Redirect target
   network-layer and link-layer addresses.  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



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   directly to the correspondent.  The node resets ForwardTime when it
   receives a new Redirect, and otherwise decrements ForwardTime while
   no Redirects have been received.  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].

   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 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 ACCEPT_TIME, FORWARD_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.

3.8.  AERO Interface Forwarding 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 forwarded 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 forwarded to the network layer where they are subject to
   either local delivery or IP forwarding.  In all cases, the AERO
   interface itself MUST NOT decrement the network layer TTL/Hop-count
   since its forwarding actions occur below the network layer.

   AERO interfaces may have multiple underlying interfaces and/or
   neighbor cache entries for neighbors with multiple Interface ID
   registrations (see Section 3.5).  The AERO node uses each packet's
   DSCP value to select an outgoing underlying interface based on the
   node's own preference values, and also to select a destination link-
   layer address based on the neighbor's underlying interface with the
   highest preference value.  If multiple outgoing interfaces and/or



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   neighbor interfaces have a preference of "high", the AERO node sends
   one copy of the packet via each of the (outgoing / neighbor)
   interface pairs; otherwise, the node sends a single copy of the
   packet.

   The following sections discuss the AERO interface forwarding
   algorithms for Clients, Servers and Relays.  In the following
   discussion, a packet's destination address is said to "match" if it
   is a non-link-local address with a prefix covered by an ASP/ACP, or
   if it is an AERO address that embeds an ACP, or if it is the same as
   an administratively-provisioned link-local address.

3.8.1.  Client Fowarding Algorithm

   When an IP packet enters a Client's AERO interface from the network
   layer the Client searches for a neighbor cache entry that matches the
   destination.  If there is a match, the Client uses one or more link-
   layer addresses in the entry as the link-layer addresses for
   encapsulation and admits the packet into the AERO link.  Otherwise,
   the Client uses the link-layer address in a static neighbor cache
   entry for a Server as the encapsulation address.

   When an IP packet enters a Client's AERO interface from the link-
   layer, if the destination matches one of the Client's ACPs or link-
   local addresses the Client decapsulates the packet and delivers it to
   the network layer.  Otherwise, the Client drops the packet silently.

3.8.2.  Server Fowarding Algorithm

   When an IP packet enters a Server's AERO interface from the network
   layer, the Server searches for a static or dynamic neighbor cache
   entry that matches the destination.  If there is a match, the Server
   uses one or more link-layer addresses in the entry as the link-layer
   addresses for encapsulation and admits the packet into the AERO link.
   Otherwise, the Server 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 Server's AERO interface from the link
   layer, the Server processes the packet as follows:

   o  if the destination matches one of the Server's own addresses the
      Server decapsulates the packet and forwards it to the network
      layer for local delivery.

   o  else, if the destination matches a static or dynamic neighbor
      cache entry the Server first determines whether the neighbor is
      the same as the one it received the packet from.  If so, the



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      Server MUST drop the packet silently to avoid looping; otherwise,
      the Server uses the neighbor's link-layer address(es) as the
      destination for encapsulation and re-admits the packet into the
      AERO link.

   o  else, the Server 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.

3.8.3.  Relay Fowarding Algorithm

   When an IP packet enters a Relay's AERO interface from the network
   layer, the Relay searches its IP forwarding table for an ACP entry
   that matches the destination and otherwise searches for a neighbor
   cache entry that 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 forwards the
   packet into the AERO link.  Otherwise, the Relay drops the packet and
   (for non-link-local addresses) returns an ICMP Destination
   Unreachable message subject to rate limiting (see: Section 3.13).

   When an IP packet enters a Relay's AERO interface from the link-
   layer, the Relay processes the packet as follows:

   o  if the destination does not match an ASP, or if the destination
      matches one of the Relay's own addresses, the Relay decapsulates
      the packet and forwards it to the network layer where it will be
      subject to either local delivery or IP forwarding.

   o  else, if the destination matches an ACP entry in the IP forwarding
      table, or if the destination matches the link-local address in a
      permanent neighbor cache entry, the Relay first determines whether
      the neighbor is the same as the one it received the packet from.
      If so the Relay MUST drop the packet silently to avoid looping;
      otherwise, the Relay uses the neighbor's link-layer address as the
      destination for encapsulation and re-admits the packet into the
      AERO link.

   o  else, the Relay drops the packet and (for non-link-local
      addresses) returns an ICMP Destination Unreachable message subject
      to rate limiting (see: Section 3.13).

3.9.  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".



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   The AERO interface encapsulates packets per the Generic UDP
   Encapsulation (GUE) 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 interface from the network 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, the AERO interface instead copies these
   values from the original encapsulation IP header into the new
   encapsulation header, i.e., the values are transferred between
   encapsulation headers and *not* copied from the encapsulated packet's
   network-layer header.  (Note especially that by copying the TTL/Hop
   Limit between encapsulation headers the value will eventually
   decrement to 0 if there is a (temporary) routing loop.)  For IPv4
   encapsulation/re-encapsulation, the AERO interface sets the DF bit as
   discussed in Section 3.12.

   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 or Relay, the AERO interface
   sets the UDP destination port to 8060, i.e., the IANA-registered port
   number for AERO.  For packets sent to a Client, the AERO interface
   sets the UDP destination port to the port value stored in the
   neighbor cache entry for this Client.  The AERO interface then either
   includes or omits the UDP checksum according to the GUE
   specification.

3.10.  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.

3.11.  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.



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   o  AERO Servers accept authentic encapsulated DHCPv6 and IPv6 ND
      messages from Clients, and create or update a static neighbor
      cache entry for the Client based on the specific 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 and Servers accept encapsulated packets if there is a
      dynamic neighbor cache entry with an AERO address that matches the
      packet's network-layer source address, with a link-layer address
      that matches the packet's link-layer source address, and with a
      non-zero AcceptTime.

   Note that this simple data origin authentication 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, e.g., as
   for common VPN systems such as OpenVPN [OVPN].  In environments where
   end systems use end-to-end security, however, it may be sufficient to
   require signatures only for AERO DHCPv6, IPv6 ND and ICMP control
   plane messages and omit signatures for data plane messages.

3.12.  AERO Interface Packet Size Issues

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

   The Internet Protocol expects that IP packets will either be
   delivered to the destination or a suitable Packet Too Big (PTB)
   message returned to support the process known as IP Path MTU
   Discovery (PMTUD) [RFC1191][RFC1981].  However, PTB messages may be
   crafted for malicious purposes such as denial of service, or lost in
   the network [RFC2923].  This can be especially problematic for
   tunnels, where a condition known as a PMTUD "black hole" can result.
   For these reasons, AERO interfaces employ operational procedures that
   avoid interactions with PMTUD, including the use of fragmentation
   when necessary.

   AERO interfaces observe two different types of fragmentation.  Source
   fragmentation occurs when the AERO interface (acting as a tunnel
   ingress) fragments the encapsulated packet into multiple fragments
   before admitting each fragment into the tunnel.  Network
   fragmentation occurs when an encapsulated packet admitted into the



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   tunnel by the ingress is fragmented by an IPv4 router on the path to
   the egress.  Note that a packet that incurs source fragmentation may
   also incur network fragmentation.

   IPv6 specifies a minimum link Maximum Transmission Unit (MTU) of 1280
   bytes [RFC2460].  Although IPv4 specifies a smaller minimum link MTU
   of 68 bytes [RFC0791], AERO interfaces also observe the IPv6 minimum
   for IPv4 even if encapsulated packets may incur network
   fragmentation.

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

   AERO interfaces therefore configure an MTU that MUST NOT be smaller
   than 1280 bytes, MUST NOT be larger than the minimum MRU among all
   nodes on the AERO link minus the encapsulation overhead ("ENCAPS"),
   and SHOULD NOT be smaller than 1500 bytes.  AERO interfaces also
   configure a Maximum Segment Unit (MSU) as the maximum-sized
   encapsulated packet that the ingress can inject into the tunnel
   without source fragmentation.  The MSU value MUST NOT be larger than
   (MTU+ENCAPS) and MUST NOT be larger than 1280 bytes unless there is
   operational assurance that a larger size can traverse the link along
   all paths.

   All AERO nodes MUST configure the same MTU/MSU values for reasons
   cited in [RFC3819][RFC4861]; in particular, multicast support
   requires a common MTU value among all nodes on the link.  All AERO
   nodes MUST configure an MRU large enough to reassemble packets up to
   (MTU+ENCAPS) bytes in length; nodes that cannot configure a large-
   enough MRU MUST NOT enable an AERO interface.

   The network layer proceeds as follow when it presents an IP packet to
   the AERO interface.  For each IPv4 packet that is larger than the
   AERO interface MTU and with the DF bit set to 0, the network layer
   uses IPv4 fragmentation to break the packet into a minimum number of
   non-overlapping fragments where the first fragment is no larger than
   the MTU and the remaining fragments are no larger than the first.
   For all other IP packets, if the packet is larger than the AERO
   interface MTU, the network layer drops the packet and returns a PTB
   message to the original source.  Otherwise, the network layer admits
   each IP packet or fragment into the AERO interface.

   For each IP packet admitted into the AERO interface, the interface
   (acting as a tunnel ingress) encapsulates the packet.  If the
   encapsulated packet is larger than the AERO interface MSU the ingress
   source-fragments the encapsulated packet into a minimum number of



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   non-overlapping fragments where the first fragment is no larger than
   the MSU and the remaining fragments are no larger than the first.
   The ingress then admits each encapsulated packet or fragment into the
   tunnel, and for IPv4 sets the DF bit to 0 in the IP encapsulation
   header in case any network fragmentation is necessary.  The
   encapsulated packets will be delivered to the egress, which
   reassembles them into a whole packet if necessary.

   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].  For
   AERO links over both IPv4 and IPv6, studies have also shown that IP
   fragments are dropped unconditionally over some network paths [I-
   D.taylor-v6ops-fragdrop].  In environments where IP fragmentation
   issues could result in operational problems, the ingress SHOULD
   employ intermediate-layer source fragmentation (see: [RFC2764] and
   [I-D.herbert-gue-fragmentation]) before appending the outer
   encapsulation headers to each fragment.  Since the encapsulation
   fragment header reduces the room available for packet data, but the
   original source has no way to control its insertion, the ingress MUST
   include the fragment header length in the ENCAPS length even for
   packets in which the header is absent.

3.13.  AERO Interface Error Handling

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

   A link-layer 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.  Valid type values include "Destination
   Unreachable", "Time Exceeded" and "Parameter Problem"
   [RFC0792][RFC4443].  (AERO interfaces ignore all link-layer IPv4
   "Fragmentation Needed" and IPv6 "Packet Too Big" messages since they
   only emit packets that are guaranteed to be no larger than the IP
   minimum link MTU as discussed in Section 3.12.)

   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



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

   The link-layer error message format is shown in Figure 3 (where, "L2"
   and "L3" refer to link-layer and network-layer, respectively):

        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        ~                               ~
        |        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 Link-Layer Error Message Format

   The AERO node rules for processing these link-layer error messages
   are as follows:

   o  When an AERO node receives a link-layer 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 link-layer Time Exceeded
      messages, the IP ID field may be wrapping before earlier fragments
      awaiting reassembly have been processed.  In that case, the node



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      SHOULD begin including integrity checks and/or institute rate
      limits for subsequent packets.

   o  When an AERO node receives persistent link-layer Destination
      Unreachable messages in response to encapsulated packets that it
      sends to one of its dynamic neighbor correspondents, the node
      SHOULD test the path to the correspondent using Neighbor
      Unreachability Detection (NUD) (see Section 3.16).  If NUD fails,
      the node SHOULD set ForwardTime for the corresponding dynamic
      neighbor cache entry to 0 and allow future packets destined to the
      correspondent to flow through a default route.

   o  When an AERO Client receives persistent link-layer Destination
      Unreachable messages in response to encapsulated 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 associate with a new Server and send a DHCPv6 Release
      message to the old Server as specified in Section 3.17.5.

   o  When an AERO Server receives persistent link-layer Destination
      Unreachable messages in response to encapsulated 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 3.16 and Section 3.17).

   o  When an AERO Relay or Server receives link-layer Destination
      Unreachable messages in response to an encapsulated packet that it
      sends to one of its permanent neighbors, it treats the messages as
      an indication that the path to the neighbor may be failing.
      However, neighbor reachability will be determined by the dynamic
      routing protocol.

   When an AERO Relay receives a packet for which the network-layer
   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 a network-layer Destination Unreachable message
   subject to rate limiting.  The Relay first writes the network-layer
   source address of the original packet as the destination address of
   the message and determines the next hop to the destination.  If the
   next hop is reached via the AERO interface, the Relay uses the IPv6
   address "::" or the IPv4 address "0.0.0.0" as the source address of
   the message, then encapsulates the message and forwards it 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 message and
   forwards it via a link outside the AERO interface.




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   When an AERO node receives an encapsulated packet for which the
   reassembly buffer it too small, it drops the packet and returns an
   network-layer Packet Too Big (PTB) message.  The node first writes
   the MRU value into the PTB message MTU field, writes the network-
   layer source address of the original packet as the destination
   address of the 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
   source address of the message, then encapsulates the message and
   forwards it to the next hop within the AERO interface.  Otherwise,
   the node uses one of its non link-local addresses as the source
   address of the message and forwards it via a link outside the AERO
   interface.

   When an AERO node receives any network-layer error message via the
   AERO interface, it examines the network-layer destination address.
   If the next hop toward the destination is via the AERO interface, the
   node re-encapsulates and forwards the message to the next hop within
   the AERO interface.  Otherwise, if the network-layer source address
   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,
   recalculates the IP and/or ICMP checksums then forwards the message
   via a link outside the AERO interface.

3.14.  AERO Router Discovery, Prefix Delegation and Autoconfiguration

   AERO Router Discovery, Prefix Delegation and Autoconfiguration are
   coordinated by the DHCPv6 and IPv6 ND control messaging protocols as
   discussed in the following Sections.

3.14.1.  AERO DHCPv6 and IPv6 ND 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], via static configuration, etc.

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




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   AERO Clients and Servers use unicast IPv6 ND messages to maintain
   neighbor cache entries the same as for any link.  AERO Servers act as
   default routers for AERO Clients, and therefore send unicast RA
   messages with configuration information in response to a Client's RS
   message.

   The following sections specify the Client and Server behavior.

3.14.2.  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 DHCPv6 PD exchange [RFC3315][RFC3633].  The
   Client's DHCPv6 Solicit message includes fe80::ffff:ffff as the IPv6
   source address, 'All_DHCP_Relay_Agents_and_Servers' as the IPv6
   destination address, the address of the Client's underlying interface
   as the link-layer source address and the link-layer address of the
   Server as the link-layer destination address.  The Client also
   includes a Client Identifier option with the Client's DUID, and an
   Identity Association for Prefix Delegation (IA_PD) option.  If the
   Client is pre-provisioned with ACPs associated with the AERO service,
   it MAY also include the ACPs in the IA_PD to indicate its preferences
   to the DHCPv6 server.  The Client finally includes any additional
   DHCPv6 options (including any necessary authentication options to
   identify itself to the DHCPv6 server), and sends the encapsulated
   Solicit message via any available underlying interface.

   When the Client attempts to perform a DHCPv6 PD exchange with a
   Server that is too busy to service the request, the Client may
   receive an error status code such as "NoPrefixAvail" in the Server's
   Reply [RFC3633] or no Reply at all.  In that case, the Client SHOULD
   discontinue DHCPv6 PD attempts through this Server and try another
   Server.  When the Client receives a Reply from the AERO Server 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.  Next, the Client autoconfigures
   AERO addresses for each of the delegated ACPs and assigns the base
   AERO address to the AERO interface.




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   The Client then prepares a unicast RS message to send to the Server
   in order to obtain a solicited RA.  The Client includes its base AERO
   address as the network-layer source address, the Server's link-local
   address as the network-layer destination address, the Client's link-
   layer address as the link-layer source address, and Server's link-
   layer address as the link-layer destination address.  The Client also
   includes one or more SLLAOs formatted as described in Section 3.5 to
   register its link-layer address(es) with the Server.

   The first SLLAO MUST correspond to the underlying interface over
   which the Client will send the RS.  The Client MAY include additional
   SLLAOs specific to other underlying interfaces, but if so it MUST
   have assurance that there will be no NATs on the paths to the Server
   via those interfaces (otherwise, the Client can register additional
   link-layer addresses with the Server by sending subsequent
   unsolicited NA messages after the initial RS/RA exchange).  The
   Server will use the S/TLLAOs to populate its link-layer address
   information for the Client.

   When the Client receives an RA from the AERO Server (see
   Section 3.14.3), it configures a default route with the Server as the
   next hop via the AERO interface.  The Client next examines the Code
   value in the RA message; if Code was 1 the Client can assume there
   was a NAT on the path to the Server.  The Client also caches any ASPs
   included in Prefix Information Options (PIOs) as ASPs to associate
   with the AERO link, and assigns the MTU/MSU values in the MTU options
   to its AERO interface while configuring an appropriate MRU.  This
   configuration information applies to the AERO link as a whole, and
   all AERO nodes will use the same values.

   Following autoconfiguration, the Client sub-delegates the ACPs to its
   attached EUNs and/or the Client's own internal virtual interfaces.
   In the former case, the Client acts as a router for nodes on its
   attached EUNs.  In the latter case, the Client acts as a host and 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.  The Client subsequently renews its ACP delegations through
   each of its Servers by sending DHCPv6 Renew messages.

   After the Client registers its Interface IDs and their associated
   'P(i)' values, it may wish to change one or more Interface ID
   registrations, e.g., if an underlying interface becomes unavailable,
   if cost profiles change, etc.  To do so, the Client prepares an
   unsolicited NA message to send over any available underlying
   interface.  The NA MUST include a S/TLLAO specific to the selected
   available underlying interface as the first S/TLLAO and MAY include



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   any additional S/TLLAOs specific to other underlying interfaces.  The
   Client includes fresh 'P(i)' values in each S/TLLAO to update the
   Server's neighbor cache entry.  If the Client wishes to disable some
   or all DSCPs for an underlying interface, it includes an S/TLLAO with
   'P(i)' values set to 0 ("disabled").

   If the Client wishes to discontinue use of a Server it issues a
   DHCPv6 Release message to both delete the Server's neighbor cache
   entry and release the DHCPv6 PD.

3.14.3.  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.

   Next, the Server prepares a Reply message to send to the Client while
   using fe80::ID as the network-layer source address, the link-local
   address taken from the Client's Solicit as the network-layer
   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 an IA_PD
   option with the delegated ACPs.  For IPv4 ACPs, the prefix included
   in the IA_PD option is in IPv4-mapped IPv6 address format and with
   prefix length set as specified in Section 3.4.

   When the Server sends the Reply message, it creates a static neighbor
   cache entry for the Client using the base AERO address as the
   network-layer address and with lifetime set to no more than the
   smallest PD lifetime.  The Client will subsequently issue an RS
   message with one or more SLLAO options and with the Client's base
   AERO address as the source address.

   When the Server receives the RS message, it first verifies that a
   neighbor cache entry for the Client exists (otherwise, it discards
   the RS).  The Server then updates the neighbor cache entry link-layer
   address(es) by recording the information in each SLLAO option indexed
   by the Interface ID and including the UDP port number, IP address and
   P(i) values.  For the first SLLAO in the list, however, the Server



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   records the actual encapsulation source UDP and IP addresses instead
   of those that appear in the SLLAO in case there was a NAT in the
   path.

   The Server then prepares a unicast RA message to send back to the
   Client using fe80::ID as the network-layer source address, the
   Client's base AERO address as the network-layer destination address,
   the Server's link-layer address as the source link-layer address, and
   the source link-layer address of the RS message as the destination
   link-layer address.  In the RA message, if the actual encapsulation
   addresses in the RS message were the same as those that appeared in
   the first SLLAO (see above), the Server sets the Code field to 0;
   otherwise it sets Code to 1.  The Server then includes one or more
   PIOs that encode the ASPs for the AERO link, and with flags A=0; L=1.
   The Server also includes two MTU options - the first MTU option
   includes the MTU for the link and the second MTU option includes the
   MSU for the link (see Section 3.12).

   When the Server delegates the ACPs, it also creates an IP forwarding
   table entry for each ACP so that the AERO BGP-based routing system
   will propagate the ACPs to all Relays that aggregate the
   corresponding ASP (see: Section 3.3).

   After the initial DHCPv6 PD Solicit/Reply and IPv6 ND RS/RA
   exchanges, the AERO Server maintains the neighbor cache entry for the
   Client until the PD lifetimes expire.  If the Client issues a Renew,
   the Server extends the PD lifetimes.  If the Client issues a Release,
   or if the Client does not issue a Renew before the lifetime expires,
   the Server deletes the neighbor cache entry for the Client and
   withdraws the IP routes from the AERO routing system.

3.14.3.1.  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 addressed to
   either 'All_DHCP_Relay_Agents_and_Servers' or the Server's fe80::ID
   unicast address, it wraps the message in a Relay-Forward message
   header and includes an Interface-ID option that includes enough
   information to allow the LDRA to forward the resulting Reply message
   back to the Client (this information may include the Client's UDP and




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   IP addresses, a security association identifier, etc).  The LDRA then
   forwards the message to the DHCPv6 server.

   When the DHCPv6 server prepares a Reply message, it wraps the message
   in a Relay-Reply message and echoes the Interface-ID option.  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 based on the information in the Interface ID
   option.

3.15.  AERO Interface Route Optimization

   When a source Client forwards packets to a prospective correspondent
   Client within the same AERO link domain (i.e., one for which the
   packet's destination address is covered by an ASP), the source Client
   MAY initiate an AERO link route optimization procedure.  The
   procedure is based on an exchange of IPv6 ND messages using a chain
   of AERO Servers and Relays as a trust basis.

   Although the Client is responsible for initiating route optimization,
   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 simply discard any route optimization messages instead of
   forwarding them.

   The following sections specify the AERO link route optimization
   procedure.

3.15.1.  Reference Operational Scenario

   Figure 4 depicts the AERO link route optimization reference
   operational scenario, using IPv6 addressing as the example (while not
   shown, a corresponding example for IPv4 addressing can be easily
   constructed).  The figure shows an AERO Relay ('R1'), two AERO
   Servers ('S1', 'S2'), two AERO Clients ('C1', 'C2') and two ordinary
   IPv6 hosts ('H1', 'H2'):













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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(C1)                                          L2(C2)
    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 4: AERO Reference Operational Scenario

   In Figure 4, Relay ('R1') assigns the administratively-provisioned
   link-local 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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3.15.2.  Concept of Operations

   Again, with reference to Figure 4, 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.

3.15.3.  Message Format

   AERO Redirect/Predirect messages use the same format as for IPv6 ND
   Redirect messages depicted in Section 4.5 of [RFC4861].  AERO
   Redirect/Predirect messages formats are identical except that
   Redirect messages use Code=0, while Predirect messages use Code=1.
   The Redirect/Predirect message format is shown in Figure 5:










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

             Figure 5: AERO Redirect/Predirect Message Format

3.15.4.  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 base AERO address of Client ('C1')).

   o  the network-layer destination address is set to the AERO address
      corresponding to the destination address of Client ('C2').

   o  the Type is set to 137.

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

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

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

   o  the message includes one or more TLLAOs set to appropriate values
      for Client ('C1')'s underlying interfaces.

   o  the message includes one or more Route Information Options (RIOs)
      [RFC4191] that include Client ('C1')'s ACPs.

   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').

3.15.5.  Re-encapsulating and Relaying Predirects

   When Server ('S1') receives a Predirect message from Client ('C1'),
   it first verifies that the TLLAOs in the Predirect are a proper
   subset of the Interface IDs in Client ('C1')'s neighbor cache entry.
   If the Client's TLLAOs are not acceptable, Server ('S1') discards the
   message.  Otherwise, Server ('S1') validates the message according to
   the Redirect message validation rules in Section 8.1 of [RFC4861],
   except that the Predirect has Code=1.  Server ('S1') also verifies
   that Client ('C1') is authorized to use the ACPs encoded in the RIOs



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   of the Predirect.  If validation fails, Server ('S1') discards the
   Predirect; otherwise, it copies the correct UDP Port number and IP
   Address for Client ('C1')'s underlying link into the first TLLAO in
   case the addresses have been subject to NAT.

   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').

3.15.6.  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
   ('C1'), and stores the ACPs encoded in the RIOs of the Predirect as




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   the ACPs for Client ('C1').  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 base AERO address of Client ('C2')).

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

   o  the Type is set to 137.

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

   o  the Target Address is set to fe80::2001:db8:1:0 (i.e., the base
      AERO address of Client ('C2')).

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

   o  the message includes one or more TLLAOs set to appropriate values
      for Client ('C2')'s underlying interfaces.

   o  the message includes one or more Route Information Options (RIOs)
      that include Client ('C2')'s ACPs.

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

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

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




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3.15.7.  Re-encapsulating and Relaying Redirects

   When Server ('S2') receives a Redirect message from Client ('C2'), it
   first verifies that the TLLAOs in the Redirect are a proper subset of
   the Interface IDs in Client ('C2')'s neighbor cache entry.  If the
   Client's TLLAOs are not acceptable, Server ('S2') discards the
   message.  Otherwise, Server ('S2') validates the message according to
   the Redirect message validation rules in Section 8.1 of [RFC4861].
   Server ('S2') also verifies that Client ('C2') is authorized to use
   the ACPs encoded in the RIOs of the Redirect message.  If validation
   fails, Server ('S2') discards the Redirect; otherwise, it copies the
   correct UDP Port number and IP Address for Client ('C2')'s underlying
   link into the first TLLAO in case the addresses have been subject to
   NAT.

   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').

3.15.8.  Processing Redirects

   When Client ('C1') receives the Redirect message, it accepts the
   message only if it has a link-layer source address of one of its
   Servers (e.g., 'L2(S1)').  Next, Client ('C1') validates the message
   according to the Redirect message validation rules in Section 8.1 of
   [RFC4861], except that it accepts the message even though the
   network-layer source address is not that of it's current first-hop




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   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 ACPs encoded in the RIOs of the Redirect as the
   ACPs for Client ('C2').. 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.)

3.15.9.  Server-to-Client and Client-to-Server Redirection

   In some environments, the Server nearest the target Client may need
   to serve as a proxy redirection target, e.g., if direct Client-to-
   Client communications are not possible.  In that case, when the
   source Client sends a Predirect message the target Server prepares a
   corresponding Redirect the same as if it were the target Client (see:
   Section 3.15.6), 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.

   Similarly, when the source Client must send all packets via its own
   Server and cannot act on a route optimization request, the source
   Server can send a Predirect message toward the target Client.  The
   target Client then prepares a corresponding Redirect message the same
   as for Client-to-Client route optimization and sends the Redirect
   message back to the source Server.

   Thereafter, if a Client moves to a new Server, the old Server sends
   ICMP "Destination Unreachable" messages subject to rate limiting in
   response to data packets received from a correspondent node to report
   that the route optimization ForwardTime should be set to 0.  The
   correspondent Client (or Server) then allows future packets destined
   to the departed Client to again flow through its own Server (or
   Relay).  Note however that the old Server retains the neighbor cache
   entry and does not set AcceptTime to 0 since there may be many



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   packets in flight.  When the old Server receives these packets, it
   forwards them to a Relay which will forward them to the departed
   Client's new Server.  AcceptTime will then eventually decrement to 0
   once the correspondent node processes and acts on the Destination
   Unreachables.

   In any case, a Server MUST NOT send a BGP update to its Relays for
   Clients discovered through dynamic route optimization redirection.
   BGP updates are only to be sent for the Server's working set of
   statically-associated Clients.

3.15.10.  Server-to-Server Redirection

   If neither the source nor target Clients are capable of sending
   packets other than via their own Servers, a Server-to-Server route
   optimization can still be employed.  In that case, the source
   Client's Server can send a Predirect message via a Relay to the AERO
   address of the target Client, and the Relay will forward the message
   to the target Client's Server.  The target Server prepares the
   Redirect message the same as if it were the target Client, except
   that it writes its own link-layer address in the TLLAO option then
   sends a Redirect message back to the source Server via the Relay.
   Both Servers must then maintain a dynamic neighbor cache entry for
   the redirected Clients.

   Thereafter, if a Client moves to a new Server, the old Server sends
   ICMP "Destination Unreachable" messages subject to rate limiting in
   response to data packets forwarded by the correspondent Server to
   report that the route optimization ForwardTime should be set to 0.
   The correspondent Server then allows future packets destined to the
   departed Client to again flow through its own Relay.  Note however
   that the old Server retains the neighbor cache entry and does not set
   AcceptTime to 0 since there may be many packets in flight.  When the
   old Server receives these packets, it forwards them to a Relay which
   will forward them to the departed Client's new Server.  AcceptTime
   will then eventually decrement to 0 once the correspondent node
   processes and acts on the Destination Unreachables.

   In any case, a Server MUST NOT send a BGP update to its Relays for
   Clients discovered through dynamic route optimization redirection.
   BGP updates are only to be sent for the Server's working set of
   statically-associated Clients..

3.16.  Neighbor Unreachability Detection (NUD)

   AERO nodes perform Neighbor Unreachability Detection (NUD) by sending
   unicast NS messages with SLLAOs to elicit solicited NA messages from
   neighbors the same as described in [RFC4861].  NUD is performed



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   either reactively in response to persistent L2 errors (see
   Section 3.13) or proactively to update neighbor cache entry timers
   and/or link-layer address information.

   When an AERO node sends an NS/NA message, it MUST use one of its
   link-local addresses as the IPv6 source address and a 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 AERO node is redirected to a target AERO node 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
   node can optionally continue sending packets via its Server (or
   Relay), maintain a small queue of packets until target reachability
   is confirmed, or (optimistically) allow packets to flow directly to
   the target.

   While data packets are still flowing, the source node thereafter
   periodically tests the direct path to the target node (see
   Section 7.3 of [RFC4861]) in order to keep dynamic neighbor cache
   entries alive.  When the target node receives a valid NS message, it
   resets AcceptTime to ACCEPT_TIME and updates its cached link-layer
   addresses (if necessary).  When the source node receives a solicited
   NA message, it resets ForwardTime to FORWARD_TIME and updates its
   cached link-layer addresses (if necessary).  If the source node is
   unable to elicit a solicited NA response from the target node after
   MaxRetry attempts, it SHOULD set ForwardTime to 0.  Otherwise, the
   source node considers the path usable and SHOULD thereafter process
   any link-layer errors as a hint that the direct path to the target
   node has either failed or has become intermittent.

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

3.17.  Mobility Management








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3.17.1.  Announcing Link-Layer Address Changes

   When a Client needs to change its link-layer addresses, e.g., due to
   a mobility event, it sends unsolicited NAs to its neighbors using the
   new link-layer address as the source address and with TLLAOs that
   include the updated Client link-layer information.

   The Client MAY send up to MaxRetry unsolicited NA messages in
   parallel with sending actual data packets in case one or more NAs are
   lost.  If all NAs are lost, the Client will eventually invoke NUD by
   sending NS messages that include SLLAOs.

3.17.2.  Bringing New Links Into Service

   When a Client needs to bring new underlying interfaces into service
   (e.g., when it activates a new data link), it sends unsolicited NAs
   to its neighbors using the new link-layer address as the source
   address and with TLLAOs that include the new Client link-layer
   information.

3.17.3.  Removing Existing Links from Service

   When a Client needs to remove existing underlying interfaces from
   service (e.g., when it de-activates an existing data link), it sends
   unsolicited NAs to its neighbors with TLLAOs that include P(i) values
   set to "disabled".

   If the Client needs to send the unsolicited NAs over a link other
   than the one being removed from service, it MUST include a TLLAO for
   the sending link as the first TLLAO and include the TLLAO for the
   link being removed from service as an additional TLLAO.

3.17.4.  Implicit Mobility Management

   AERO interface neighbors MAY include a configuration knob that allows
   them to perform implicit mobility management in which no IPv6 ND
   messaging is used.  In that case, the Client only transmits packets
   over a single interface at a time, and the neighbor always observes
   packets arriving from the Client from the same link-layer source
   address.

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



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   time, and the Client automatically switches over to the backup
   interface if the primary interface fails.

3.17.5.  Moving to a New Server

   When a Client associates with a new Server, it performs the Client
   procedures specified in Section 3.14.2.

   When a Client disassociates with an existing Server, it sends a
   DHCPv6 Release message via a new Server with its base AERO address as
   the network-layer source address and the unicast link-local address
   of the old Server as the network-layer destination address.  The new
   Server then encapsulates the Release message in a DHCPv6 Relay-
   Forward message header, writes the Client's source address in the
   'peer-address' field, and writes its own link-local address in the IP
   source address (i.e., the new Server acts as a DHCPv6 relay agent).
   The new Server then forwards the message to an Relay, which forwards
   the message to the old Server based on the network-layer destination
   address.

   When the old Server receives the Release, it first authenticates the
   message then releases the DHCPv6 PDs and deletes the Client's ACP
   routes.  The old Server then deletes the Client's neighbor cache
   entry so that any in-flight packets will be forwarded via a Relay to
   the new Server, which will forward them to the Client.  The old
   Server finally returns a DHCPv6 Relay-Reply message via an Relay to
   the new Server, which will decapsulate the DHCPv6 Reply message and
   forward it to the Client.

   When the new Server forwards the Reply message, the Client can delete
   both the default route and the neighbor cache entry for the old
   Server.  (Note that since Release/Reply messages may be lost in the
   network the Client SHOULD retry until it gets a Reply indicating that
   the Release was successful.  If the Client does not receive a Reply
   after MaxRetry attempts, the old Server may have failed and the
   Client should discontinue its Release attempts.)

   Note that this DHCPv6 relay-chaining approach is necessary to avoid
   failures, e.g., due to temporary routing fluctuations.  In
   particular, the Client should always be able to forward messages via
   its new Server but may not always be able to send messages directly
   to an old Server.  But, the new Server and Old Server should always
   be able to exchange messages with one another.

   Finally, 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



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   when a Client might wish to change to a different Server include a
   Server that has gone unreachable, topological movements of
   significant distance, etc.

3.17.6.  Packet Queueing for Mobility

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

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

3.17.7.  Alternate Mobility Security Model

   In some environments, an AERO node may have no way of authenticating
   any unsolicited NA messages it receives.  In that case, the target
   AERO node SHOULD ignore any unsolicited NA messages it receives, and
   the source AERO node SHOULD inform the target of its new link-layer
   addresses by sending a fresh Predirect message via its Server (or
   Relay).  The target AERO node can then accept the Predirect message
   and update its link-layer addresses based on the Predirect TLLAOs.

3.18.  Multicast Considerations

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

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








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4.  AERO Variations

   AERO can be used in many different variations based on the specific
   use case.  The following sections discuss variations that adhere to
   the AERO principles while allowing selective application of AERO
   components.

4.1.  Operation on Host-Only IPv6 AERO Links

   IPv6 AERO links typically have ASPs that cover many candidate ACPs of
   length /64 or shorter.  However, in some cases it may be desirable to
   use AERO over links that have only a /64 ASP.  This can be
   accommodated by treating all Clients on the AERO link as simple hosts
   that receive /128 prefix delegations.

   In that case, each Client configures an administratively-provisioned
   link-local address instead of an AERO address, i.e., the same as for
   Servers and Relays.  The Client discovers its link-local address by
   including an IA_NA option in its DHCPv6 Solicit message to the
   Server.  The Server responds by returning the Client's
   administratively-provisioned link-local address in the IA_NA option
   plus any IPv6 addresses for the Client in IA_PD options with prefix
   length /128.

   For example, if the ASP for the host-only IPv6 AERO link is
   2001:db8:1000:2000::/64, each Client will receive one or more /128
   IPv6 prefix delegations such as 2001:db8:1000:2000::1/128,
   2001:db8:1000:2000::2/128, etc.  The Client then assigns the /128s to
   the AERO interface as IPv6 addresses, and the Client's applications
   treat the AERO interface as an ordinary host interface.

   In this arrangement, the Client conducts route optimization in the
   same sense as discussed in Section 3.15, except that the Predirect
   message network-layer source address is the Client's
   administratively-assigned link-local address and the network-layer
   destination address is the same as the destination address of the
   packet that triggered the redirection.  All other aspects of AERO
   operation are the same as described in earlier sections.

   This has applicability for nodes that act as a Client on an
   "upstream" AERO link, but also act as a Server on "downstream" AERO
   links.  More specifically, if the node acts as a Client to receive a
   /64 prefix from the upstream AERO link it can then act as a Server to
   provision /128s to Clients on downstream AERO links.

   Note that, due to the nature of the AERO address format, valid IPv6
   ACP lengths are either /64 or shorter, or exactly /128 (i.e., prefix
   lengths between /65 and /127 cannot be accommodated).



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4.2.  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.

4.3.  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 PD authority through
   some means outside the scope of this document.

4.4.  Operation on Client-less AERO Links

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

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




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4.5.  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-provisioned link-local address and exchange NS/NA
   messages the same as for dynamically-established tunnels.

4.6.  Encapsulation Avoidance on Relay-Server Dedicated Links

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

4.7.  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.

5.  Implementation Status

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

   An initial public release of the AERO proof-of-concept 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.

6.  IANA Considerations

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

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

   No further IANA actions are required.





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7.  Security Considerations

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

   Redirect, Predirect and unsolicited NA messages SHOULD include a
   Timestamp option (see Section 5.3 of [RFC3971]) that other AERO nodes
   can use to verify the message time of origin.  Predirect, NS and RS
   messages SHOULD include a Nonce option (see Section 5.3 of [RFC3971])
   that recipients echo back in corresponding responses.  In cases where
   spoofing cannot be mitigated through other means, however, all AERO
   IPv6 ND messages should employ Secure Neighbor Discovery (SeND)
   [RFC3971].

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

   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 other nodes via some
   form of Internet connection sharing.)

   AERO Clients, Servers and Relays on the open Internet are susceptible
   to the same attack profiles as for any Internet nodes.  For this
   reason, IP security SHOULD be used when AERO is employed over
   unmanaged/unsecured links using securing mechanisms such as IPsec
   [RFC4301], IKE [RFC5996] and/or TLS [RFC5246].  In some environments,
   however, the use of end-to-end security from Clients to correspondent
   nodes (i.e., other Clients and/or Internet nodes) could obviate the
   need for IP security between AERO Clients, Servers and Relays.

   AERO Servers and Relays present targets for traffic amplification DoS
   attacks.  This concern is no different than for widely-deployed VPN



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   security gateways in the Internet, where attackers could send spoofed
   packets to the gateways at high data rates.  This can be mitigated by
   connecting Relays and Servers over dedicated links with no
   connections to the Internet and/or when connections to the Internet
   are only permitted through well-managed firewalls.

   Traffic amplification DoS attacks can also target an AERO Client's
   low data rate links.  This is a concern not only for Clients located
   on the open Internet but also for Clients in protected enclaves.
   AERO Servers can institute rate limits that protect Clients from
   receiving packet floods that could DoS low data rate links.

8.  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, Bob
   Braden, 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, Gene MacLean III,
   Rob Muszkiewicz, Sean O'Sullivan, Kent Shuey, Brian Skeen, Mike
   Slane, Carrie Spiker, Brendan Williams, Julie Wulff, Yueli Yang, and
   other members of the BR&T and BIT mobile networking teams.  Wayne
   Benson is especially acknowledged for his outstanding work in
   converting the AERO proof-of-concept implementation into production-
   ready code.

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

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





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   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.

   This work is aligned with the NASA Safe Autonomous Systems Operation
   (SASO) program under NASA contract number NNA16BD84C.

   This work is aligned with the FAA as per the SE2025 contract number
   DTFAWA-15-D-00030.

   This work is aligned with the Boeing Information Technology (BIT)
   MobileNet program.

   This work is aligned with the Boeing Research and Technology (BR&T)
   autonomous systems networking program.

9.  References

9.1.  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>.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
              November 2005, <http://www.rfc-editor.org/info/rfc4191>.

   [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>.




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

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

9.2.  Informative References

   [BGP]      Huston, G., "BGP in 2015, http://potaroo.net", January
              2016.

   [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-sedhcpv6-14 (work
              in progress), October 2016.

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

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

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

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

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





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

   [OVPN]     OpenVPN, O., "http://openvpn.net", October 2016.

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

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

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

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

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

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

   [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>.






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

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

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

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

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

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

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

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

   [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>.





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

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

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

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

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

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

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

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

   [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>.






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

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

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

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

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

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

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

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

   [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>.




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

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

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

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

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

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

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

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

   [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>.





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

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

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

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

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

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

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

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

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

   [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.




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

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

        Minimal Encapsulation in IPv4      Minimal Encapsulation in IPv6


           Figure 6: Minimal Encapsulation Format using IP-in-IP

   Figure 7 shows the AERO GRE encapsulation format before any
   fragmentation is applied:








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


                 Figure 7: Minimal Encapsulation Using GRE

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

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

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

   AERO is especially well suited for use with VPN system encapsulations
   such as OpenVPN [OVPN].

Appendix B.  When to Insert an Encapsulation Fragment Header

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



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

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

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

Appendix C.  Autoconfiguration for Constrained Platforms

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

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

   o  the IPv6 destination address is all-nodes multicast

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

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

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

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



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

Appendix D.  Extending AERO Links Through Security Gateways

   When an enterprise mobile node 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
   node to the security gateway.  During this process, the mobile node
   supplies the security gateway with its public Internet address as the
   link-layer address for the VPN.  The mobile node 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 node (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
   nodes 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 nodes within the enterprise network can be
   forwarded to the security gateway, which then replaces the link-layer
   destination address with the Client's link-layer address and replaces
   the link-layer source address with its own (Internet-facing) link-
   layer address.

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



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   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.

Author's Address

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

   Email: fltemplin@acm.org










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