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Network Working Group                                    F. Templin, Ed.
Internet-Draft                              Boeing Research & Technology
Obsoletes: rfc6706 (if approved)                           April 2, 2014
Intended status: Standards Track
Expires: October 4, 2014


              Transmission of IPv6 Packets over AERO Links
                     draft-templin-aerolink-12.txt

Abstract

   This document specifies the operation of IPv6 over tunnel virtual
   Non-Broadcast, Multiple Access (NBMA) links using Asymmetric Extended
   Route Optimization (AERO).  Nodes attached to AERO links can exchange
   packets via trusted intermediate routers on the link that provide
   forwarding services to reach off-link destinations and/or redirection
   services to inform the node of an on-link neighbor that is closer to
   the final destination.  Operation of the IPv6 Neighbor Discovery (ND)
   protocol over AERO links is based on an IPv6 link local address
   format known as the AERO address.

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 October 4, 2014.

Copyright Notice

   Copyright (c) 2014 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



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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Asymmetric Extended Route Optimization (AERO)  . . . . . . . .  6
     3.1.  AERO Node Types  . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  AERO Interface Characteristics . . . . . . . . . . . . . .  7
     3.3.  AERO Addresses . . . . . . . . . . . . . . . . . . . . . .  9
     3.4.  AERO Interface Data Origin Authentication  . . . . . . . .  9
     3.5.  AERO Interface Conceptual Data Structures and Protocol
           Constants  . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.6.  AERO Interface MTU Considerations  . . . . . . . . . . . . 10
     3.7.  AERO Interface Encapsulation, Re-encapsulation and
           Decapsulation  . . . . . . . . . . . . . . . . . . . . . . 12
     3.8.  AERO Reference Operational Scenario  . . . . . . . . . . . 13
     3.9.  AERO Router Discovery and Prefix Delegation  . . . . . . . 15
       3.9.1.  AERO Client Behavior . . . . . . . . . . . . . . . . . 15
       3.9.2.  AERO Server Behavior . . . . . . . . . . . . . . . . . 16
     3.10. AERO Neighbor Solicitation and Advertisement . . . . . . . 16
     3.11. AERO Redirection . . . . . . . . . . . . . . . . . . . . . 18
       3.11.1. Classical Redirection Approaches . . . . . . . . . . . 18
       3.11.2. AERO Redirection Concept of Operations . . . . . . . . 19
       3.11.3. AERO Redirection Message Format  . . . . . . . . . . . 19
       3.11.4. Sending Predirects . . . . . . . . . . . . . . . . . . 20
       3.11.5. Processing Predirects and Sending Redirects  . . . . . 21
       3.11.6. Re-encapsulating and Relaying Redirects  . . . . . . . 22
       3.11.7. Processing Redirects . . . . . . . . . . . . . . . . . 23
     3.12. Neighbor Reachability Maintenance  . . . . . . . . . . . . 23
     3.13. Mobility and Link-Layer Address Change Considerations  . . 24
     3.14. Underlying Protocol Version Considerations . . . . . . . . 25
     3.15. Multicast Considerations . . . . . . . . . . . . . . . . . 25
     3.16. Operation on Server-less AERO Links  . . . . . . . . . . . 25
     3.17. Other Considerations . . . . . . . . . . . . . . . . . . . 25
   4.  Implementation Status  . . . . . . . . . . . . . . . . . . . . 26
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 27
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 28
   Appendix A.  AERO Server and Relay Interworking  . . . . . . . . . 29



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   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 31


















































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1.  Introduction

   This document specifies the operation of IPv6 over tunnel virtual
   Non-Broadcast, Multiple Access (NBMA) links using Asymmetric Extended
   Route Optimization (AERO).  Nodes attached to AERO links can exchange
   packets via trusted intermediate routers on the link that provide
   forwarding services to reach off-link destinations and/or redirection
   services to inform the node of an on-link neighbor that is closer to
   the final destination.

   Nodes on AERO links use an IPv6 link-local address format known as
   the AERO Address.  This address type has properties that statelessly
   link IPv6 Neighbor Discovery (ND) to IPv6 routing.  The AERO link can
   be used for tunneling to neighboring nodes on either IPv6 or IPv4
   networks, i.e., AERO views the IPv6 and IPv4 networks as equivalent
   links for tunneling.  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 IPv6.

   AERO interface
      a node's attachment to an AERO link.  The AERO interface Maximum
      Transmission Unit (MTU) is less than or equal to the AERO link
      MTU.

   AERO address
      an IPv6 link-local address assigned to an AERO interface and
      constructed as specified in Section 3.6.

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

   AERO Client ("client")
      a node that configures either a host interface or a router
      interface on an AERO link.





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   AERO Server ("server")
      a node that configures a router interface on an AERO link over
      which it can provide default forwarding and redirection services
      for other AERO nodes.

   AERO Relay ("relay")
      a node that relays IPv6 packets between Servers on the same AERO
      link, and/or that forwards IPv6 packets between the AERO link and
      the IPv6 Internet.  An AERO Relay may or may not also be
      configured as an AERO Server.

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

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

   underlying network
      a connected IPv6 or IPv4 network routing region over which AERO
      nodes tunnel IPv6 packets.

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

   underlying address
      an IPv6 or IPv4 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 underlying address.  Underlying
      addresses are used as the source and destination addresses of the
      AERO encapsulation header.

   link-layer address
      the same as defined for "underlying address" above.

   network layer address
      an IPv6 address used as the source or destination address of the
      inner IPv6 packet header.

   end user network (EUN)
      an IPv6 network attached to a downstream interface of an AERO
      Client (where the AERO interface is seen as the upstream
      interface).

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this



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   document are to be interpreted as described in [RFC2119].


3.  Asymmetric Extended Route Optimization (AERO)

   The following sections specify the operation of IPv6 over Asymmetric
   Extended Route Optimization (AERO) links:

3.1.  AERO Node Types

   AERO Relays relay packets between nodes connected to the same AERO
   link and also forward packets between the AERO link and the native
   IPv6 network.  The relaying process entails re-encapsulation of IPv6
   packets that were received from a first AERO node and are to be
   forwarded without modification to a second AERO node.

   AERO Servers configure their AERO interfaces as router interfaces,
   and provide default routing services to AERO Clients.  AERO Servers
   configure a DHCPv6 Relay or Server function and facilitate DHCPv6
   Prefix Delegation (PD) exchanges.  An AERO Server may also act as an
   AERO Relay.

   AERO Clients act as requesting routers to receive IPv6 prefixes
   through a DHCPv6 PD exchange via an AERO Server over the AERO link.
   Each AERO Client receives at least a /64 prefix delegation, and may
   receive even shorter prefixes.

   AERO Clients that act as routers configure their AERO interfaces as
   router interfaces, i.e., even if the AERO Client otherwise displays
   the outward characteristics of an ordinary host (for example, the
   Client may internally configure both an AERO interface and (internal
   virtual) End User Network (EUN) interfaces).  AERO Clients that act
   as routers sub-delegate portions of their received prefix delegations
   to links on EUNs.

   AERO Clients that act as ordinary hosts configure their AERO
   interfaces as host interfaces and assign one or more IPv6 addresses
   taken from their received prefix delegations to the AERO interface
   but DO NOT assign the delegated prefix itself to the AERO interface.
   Instead, the host assigns the delegated prefix to a "black hole"
   route so that unused portions of the prefix are nullified.

   End system applications on AERO hosts bind directly to the AERO
   interface, while applications on AERO routers (or IPv6 hosts served
   by an AERO router) bind to EUN interfaces.






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3.2.  AERO Interface Characteristics

   AERO interfaces use IPv6-in-IPv6 encapsulation [RFC2473] to exchange
   tunneled packets with AERO neighbors attached to an underlying IPv6
   network, and use IPv6-in-IPv4 encapsulation [RFC4213] to exchange
   tunneled packets with AERO neighbors attached to an underlying IPv4
   network.  AERO interfaces can also use IPsec encapsulation [RFC4301]
   (either IPv6-in-IPsec-in-IPv6 or IPv6-in-IPsec-in-IPv4) in
   environments where strong authentication and confidentiality are
   required.  When NAT traversal and/or filtering middlebox traversal is
   necessary, a UDP header is further inserted between the outer IP
   encapsulation header and the inner packet.

   Servers assign the address 'fe80::0' to their AERO interface; this
   provides a link-local address handle for Clients to insert into a
   neighbor cache entry for their current Server.  Clients initially
   assign no address to their AERO interface, but use 'fe80::1' as the
   IPv6 link-local address in the DHCPv6 PD exchanges used to derive an
   AERO address.  After the Client receives a prefix delegation, it
   assigns the corresponding AERO address to the AERO interface.

   AERO interfaces maintain a neighbor cache and use a variation of
   standard unicast IPv6 ND messaging.  AERO interfaces use Neighbor
   Solicitation (NS), Neighbor Advertisement (NA) and Redirect messages
   the same as for any IPv6 link.  They do not use Router Solicitation
   (RS) and Router Advertisement (RA) messages for several reasons.
   First, default router discovery is supported through other means more
   appropriate for AERO links as described below.  Second, discovery of
   more-specific routes is through the receipt of Redirect messages.
   Finally, AERO nodes obtain their delegated IPv6 prefixes using DHCPv6
   PD; hence, there is no need for RA-based prefix discovery.

   AERO Neighbor Solicitation (NS) and Neighbor Advertisement (NA)
   messages do not include Source/Target Link Layer Address Options
   (S/TLLAO).  Instead, AERO nodes determine the link-layer addresses of
   neighbors by examining the encapsulation IP source address and UDP
   port number (when UDP encapsulation is used) of any NS/NA messages
   they receive and ignore any S/TLLAOs included in these messages.
   This is vital to the operation of AERO links for which neighbors are
   separated by Network Address Translators (NATs) - either IPv4 or
   IPv6.

   AERO Redirect messages include a TLLAO the same as for any IPv6 link.
   The TLLAO includes the link-layer address of the target node,
   including both the IP address and the UDP source port number used by
   the target when it sends UDP-encapsulated packets over the AERO
   interface (the TLLAO instead encodes the value 0 when the target does
   not use UDP encapsulation).  TLLAOs for target nodes that use an IPv6



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   underlying address include the full 16 bytes of the IPv6 address as
   shown in Figure 1, while TLLAOs for target nodes that use an IPv4
   underlying address include only the 4 bytes of the IPv4 address as
   shown in Figure 2.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Type = 2   |   Length = 3  |     UDP Source Port (or 0)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Reserved                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +--                                                           --+
       |                                                               |
       +--                       IPv6 Address                        --+
       |                                                               |
       +--                                                           --+
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                   Figure 1: AERO TLLAO Format for IPv6

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Type = 2   |   Length = 1  |     UDP Source Port (or 0)    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         IPv4 Address                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                   Figure 2: AERO TLLAO  Format for IPv4

   Finally, AERO interface NS/NA messages only update existing neighbor
   cache entires and do not create new neighbor cache entries, whereas
   Redirect messages both update and create neighbor cache entries.
   This represents a departure from the normal operation of IPv6 ND over
   common link types, but is consistent with the spirit of IPv6 over
   NBMA links as discussed in [RFC4861].  Note however that this
   restriction may be relaxed and/or redefined on AERO links that
   participate in a fully distributed mobility management model
   coordinated in a manner outside the scope of this document.







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3.3.  AERO Addresses

   An AERO address is an IPv6 link-local address assigned to an AERO
   interface and with an IPv6 prefix embedded within the interface
   identifier.  The AERO address is formatted as:

      fe80::[IPv6 prefix]

   Each AERO Client configures an AERO address based on the delegated
   prefix it has received from the DHCPv6 server.  The address begins
   with the prefix fe80::/64 and includes in its interface identifier
   the base /64 prefix taken from the Client's delegated IPv6 prefix.
   The base prefix is determined by masking the delegated prefix with
   the prefix length.  For example, if an AERO Client has received the
   prefix delegation:

      2001:db8:1000:2000::/56

   it would construct its AERO address as:

      fe80::2001:db8:1000:2000

   The AERO address remains stable as the Client moves between
   topological locations, i.e., even if its underlying address changes.

3.4.  AERO Interface Data Origin Authentication

   Nodes on AERO interfaces use a simple data origin authentication for
   encapsulated packets they receive from other nodes.  In particular,
   AERO Clients accept encapsulated packets with a link-layer source
   address belonging to their current AERO Server.  AERO nodes also
   accept encapsulated packets with a link-layer source address that is
   correct for the network-layer source address.

   The AERO node considers the link-layer source address correct for the
   network-layer source address if there is an IPv6 route that matches
   the network-layer source address as well as a neighbor cache entry
   corresponding to the next hop that includes the link-layer address.
   An exception is that NS, NA and Redirect messages may include a
   different link-layer address than the one currently in the neighbor
   cache, and the new link-layer address updates the neighbor cache
   entry.

3.5.  AERO Interface Conceptual Data Structures and Protocol Constants

   Each AERO node maintains a per-AERO interface conceptual neighbor
   cache that includes an entry for each neighbor it communicates with
   on the AERO link, the same as for any IPv6 interface (see [RFC4861]).



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   Neighbor cache entries are either static or dynamic.  Static neighbor
   cache entries (including a Client's neighbor cache entry for a Server
   or a Server's neighbor cache entry for a Client) do not have timeout
   values and are retained until explicitly deleted.  Dynamic neighbor
   cache entries are created and maintained by the AERO redirection
   procedures describe in the following sections.

   When an AERO node receives a valid Predirect message (See Section
   3.11.5) it creates or updates a dynamic neighbor cache entry for the
   Predirect target L3 and L2 addresses, and also creates an IPv6 route
   for the Predirected (source) prefix.  The node then sets an ACCEPT
   timer and uses this timer to validate any messages received from the
   Predirected neighbor.

   When an AERO node receives a valid Redirect message (see Section
   3.11.7) it creates or updates a dynamic neighbor cache entry for the
   Redirect target L3 and L2 addresses, and also creates an IPv6 route
   for the Redirected (destination) prefix.  The node then sets a
   FORWARD timer and uses this timer to determine whether packets can be
   sent directly to the Redirected neighbor.  The node also maintains a
   constant value MAX_RETRY to limit the number of keepalives sent when
   a neighbor has gone unreachable.

   It is RECOMMENDED that FORWARD_TIME be set to the default constant
   value 30 seconds to match the default REACHABLE_TIME value specified
   for IPv6 neighbor discovery [RFC4861].

   It is RECOMMENDED that ACCEPT_TIME be set to the default constant
   value 40 seconds to allow a 10 second window so that the AERO
   redirection procedure can converge before the ACCEPT_TIME timer
   decrements below FORWARD_TIME.

   It is RECOMMENDED that MAX_RETRY be set to 3 the same as described
   for IPv6 neighbor discovery address resolution in Section 7.3.3 of
   [RFC4861].

   Different values for FORWARD_TIME, ACCEPT_TIME, and MAX_RETRY MAY be
   administratively set, if necessary, to better match the AERO link's
   performance characteristics; however, if different values are chosen,
   all nodes on the link MUST consistently configure the same values.
   ACCEPT_TIME SHOULD further be set to a value that is sufficiently
   longer than FORWARD_TIME to allow the AERO redirection procedure to
   converge.

3.6.  AERO Interface MTU Considerations

   The AERO link Maximum Transmission Unit (MTU) is 64KB minus the
   encapsulation overhead for IPv4 [RFC0791] and 4GB minus the



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   encapsulation overhead for IPv6 [RFC2675].  This is the most that
   IPv4 and IPv6 (respectively) can convey within the constraints of
   protocol constants, but actual sizes available for tunneling will
   frequently be much smaller.

   The base tunneling specifications for IPv4 and IPv6 typically set a
   static MTU on the tunnel interface to 1500 bytes minus the
   encapsulation overhead or smaller still if the tunnel is likely to
   incur additional encapsulations such as IPsec on the path.  This can
   result in path MTU related black holes when packets that are too
   large to be accommodated over the AERO link are dropped, but the
   resulting ICMP Packet Too Big (PTB) messages are lost on the return
   path.  As a result, AERO nodes use the following MTU mitigations to
   accommodate larger packets.

   AERO nodes set their AERO interface MTU to the larger of 1500 bytes
   and the underlying interface MTU minus the encapsulation overhead.
   AERO nodes optionally cache other per-neighbor MTU values in the
   underlying IP path MTU discovery cache initialized to the underlying
   interface MTU.

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

   For packets destined to an AERO node that are larger than 1280 bytes
   minus the encapsulation overhead (*) but no larger than 1500 bytes,
   the node uses outer IP fragmentation to fragment the packet into two
   pieces (where the first fragment contains 1024 bytes of the
   fragmented inner packet) then admits the fragments into the tunnel.
   If the outer protocol is IPv4, the node admits the packet into the
   tunnel with DF set to 0 and subject to rate limiting to avoid
   reassembly errors [RFC4963][RFC6864].  For both IPv4 and IPv6, the
   node also sends a 1500 byte probe message (**) to the neighbor,
   subject to rate limiting.  To construct a probe, the node prepares an
   ICMPv6 Neighbor Solicitation (NS) message with trailing padding
   octets added to a length of 1500 bytes but does not include the
   length of the padding in the IPv6 Payload Length field.  The node
   then encapsulates the NS in the outer encapsulation headers (while



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   including the length of the padding in the outer length fields), sets
   DF to 1 (for IPv4) and sends the padded NS message to the neighbor.
   If the neighbor returns an NA message, the node may then send whole
   packets within this size range and (for IPv4) relax the rate limiting
   requirement.

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

   (*) Note that if it is known that the minimum Path MTU to an AERO
   node is MINMTU bytes (where 1280 < MINMTU < 1500) then MINMTU can be
   used instead of 1280 in the fragmentation threshold considerations
   listed above.

   (**) It is RECOMMENDED that no probes smaller than 1500 bytes be used
   for MTU probing purposes, since smaller probes may be fragmented if
   there is a nested tunnel somewhere on the path to the neighbor.

3.7.  AERO Interface Encapsulation, Re-encapsulation and Decapsulation

   AERO interfaces encapsulate IPv6 packets according to whether they
   are entering the AERO interface for the first time or if they are
   being forwarded out the same AERO interface that they arrived on.
   This latter form of encapsulation is known as "re-encapsulation".

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

   When UDP encapsulation is used, the AERO interface inserts a UDP
   header between the inner packet and outer IP header.  If the outer
   header is IPv6 and is followed by an IPv6 Fragment header, the AERO
   interface inserts the UDP header between the outer IPv6 header and
   IPv6 Fragment header.  The AERO interface sets the UDP source port to
   a constant value that it will use in each successive packet it sends,
   sets the UDP checksum field to zero (see: [RFC6935][RFC6936]) and
   sets the UDP length field to the length of the inner packet plus 8
   bytes for the UDP header itself.  For packets sent via a Server, the
   AERO interface sets the UDP destination port to 8060 (i.e., the IANA-
   registerd port number for AERO).  For packets sent to a neighboring



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   Client, the AERO interface sets the UDP destination port to the port
   value stored in the neighbor cache entry for this neighbor.

   The AERO interface next sets the outer IP protocol number to the
   appropriate value for the first protocol layer within the
   encapsulation (e.g., IPv6, IPv6 Fragment Header, UDP, etc.).  When
   IPv6 is used as the outer IP protocol, the ITE then sets the flow
   label value in the outer IPv6 header the same as described in
   [RFC6438].  When IPv4 is used as the outer IP protocol, the AERO
   interface sets the DF bit as discussed in Section 3.2.

   AERO interfaces decapsulate packets destined either to the node
   itself or to a destination reached via an interface other than the
   receiving AERO interface per the specifications in , [RFC2473],
   [RFC4213].  When the encapsulated packet includes a UDP header, the
   AERO interface examines the first octet of data following the UDP
   header to determine the inner header type.  If the most significant
   four bits of the first octet encode the value '0110', the inner
   header is an IPv6 header.  Otherwise, the interface considers the
   first octet as an IP protocol type that encodes the value '44' for
   IPv6 fragment header, the value '50' for Encapsulating Security
   Payload, the value '51' for Authentication Header etc.  (If the first
   octet encodes the value '0', the interface instead discards the
   packet, since this is the value reserved for experimentation under ,
   [RFC6706]).  During the decapsulation, the AERO interface records the
   UDP source port in the neighbor cache entry for this neighbor then
   discards the UDP header.

3.8.  AERO Reference Operational Scenario

   Figure 3 depicts the AERO reference operational scenario.  The figure
   shows an AERO Server('A'), two AERO Clients ('B', 'D') and three
   ordinary IPv6 hosts ('C', 'E', 'F'):


















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                    .-(::::::::)
                 .-(::: IPv6 :::)-.   +-------------+
                (:::: Internet ::::)--|    Host F   |
                 `-(::::::::::::)-'   +-------------+
                    `-(::::::)-'       2001:db8:3::1
                         |
                  +--------------+
                  | AERO Server A|
                  | (C->B; E->D) |
                  +--------------+
                      fe80::0
                       L2(A)
                         |
       X-----+-----------+-----------+--------X
             |       AERO Link       |
            L2(B)                  L2(D)
     fe80::2001:db8:0:0      fe80::2001:db8:1:0         .-.
     +--------------+         +--------------+       ,-(  _)-.
     | AERO Client B|         | AERO Client D|    .-(_ IPv6  )-.
     | (default->A) |         | (default->A) |--(__    EUN      )
     +--------------+         +--------------+     `-(______)-'
     2001:DB8:0::/48           2001:DB8:1::/48           |
             |                                     2001:db8:1::1
            .-.                                   +-------------+
         ,-(  _)-.      2001:db8:0::1             |    Host E   |
      .-(_ IPv6  )-.   +-------------+            +-------------+
    (__    EUN      )--|    Host C   |
       `-(______)-'    +-------------+

               Figure 3: AERO Reference Operational Scenario

   In Figure 3, AERO Server ('A') connects to the AERO link and connects
   to the IPv6 Internet, either directly or via an AERO Relay (not
   shown).  Server ('A') assigns the address fe80::0 to its AERO
   interface with link-layer address L2(A).  Server ('A') next arranges
   to add L2(A) to a published list of valid Servers for the AERO link.

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

   AERO Client ('D') registers the IPv6 prefix 2001:db8:1::/48 in a
   DHCPv6 PD exchange via Server ('A') then assigns the address fe80::
   2001:db8:1:0 to its AERO interface with link-layer address L2(D).



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   Client ('D') configures a default route via the AERO interface with
   next-hop address fe80::0 and link-layer address L2(A), then sub-
   delegates the prefix 2001:db8:1::/48 to its attached EUNs.  IPv6 host
   ('E') connects to the EUN, and configures the address 2001:db8:1::1.

   Finally, IPv6 host ('F') connects to an IPv6 network outside of the
   AERO link domain.  Host ('F') configures its IPv6 interface in a
   manner specific to its attached IPv6 link, and assigns the address
   2001:db8:3::1 to its IPv6 link interface.

3.9.  AERO Router Discovery and Prefix Delegation

3.9.1.  AERO Client Behavior

   AERO Clients observe the IPv6 router requirements defined in
   [RFC6434].  AERO Clients first discover the link-layer address of an
   AERO Server via static configuration, or through an automated means
   such as DNS name resolution.  In the absence of other information,
   the Client resolves the Fully-Qualified Domain Name (FQDN)
   "linkupnetworks.domainname", where "domainname" is the DNS domain
   appropriate for the Client's attached underlying network.  The Client
   then creates a static neighbor cache entry with fe80::0 as the
   network-layer address and the discovered address as the link-layer
   address.  The Client further creates a static default IPv6 route with
   fe80::0 as the next hop address.

   Next, the Client acts as a requesting router to register its IPv6
   prefix through DHCPv6 PD [RFC3633] via the Server using fe80::1 as
   the IPv6 source address and fe80::0 as the IPv6 destination address.
   The Client further includes a DHCPv6 Unique Identifier (DUID) based
   on a Universally Unique Identifier (UUID) (also known as DUID-UUID)
   as described in [RFC6355].

   After the Client registers its prefixes, it assigns the link-local
   AERO address taken from its delegated prefix to the AERO interface
   (see: Section 3.3) and sub-delegates the prefix to nodes and links
   within its attached EUNs (the AERO link-local address thereafter
   remains stable as the Client moves).

   The Client sends periodic NS messages to the Server to obtain new NAs
   in order to refresh any network state.  The Client can also forward
   IPv6 packets destined to networks beyond its local EUNs via the
   Server as an IPv6 default router.  The Server may in turn return a
   Redirect message informing the Client of a neighbor on the AERO link
   that is topologically closer to the final destination as specified in
   Section 3.11.





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3.9.2.  AERO Server Behavior

   AERO Servers observe the IPv6 router requirements defined in
   [RFC6434].  They further configure a DHCPv6 relay/server function on
   their AERO links.  When the Server facilitates a DHCPv6 PD exchange,
   it creates a temporary cache entry referenced by the DHCPv6 request's
   DUID-UUID.  After the PD request is satisfied, the Server creates a
   static neighbor cache entry for the Client's AERO address (see:
   Section 3.3) and a static IPv6 forwarding table entry that lists the
   Client's AERO address as the next hop toward the delegated IPv6
   prefix .  The Server also injects the Client's prefix into the
   routing system as described in IRON [IRON].

   Servers respond to NS messages from Clients on their AERO interfaces
   by returning an NA message.  When the Server receives an NS message,
   it updates the neighbor cache entry using the network layer source
   address as the neighbor's network layer address and using the link-
   layer source address of the NS message as the neighbor's link-layer
   address.

   When the Server forwards a packet via the same AERO interface on
   which it arrived, it initiates an AERO route optimization procedure
   as specified in Section 3.11.

3.10.  AERO Neighbor Solicitation and Advertisement

   Each AERO node uses its delegated prefix to create an AERO address
   (see: Section 3.3).  It can then send NS messages to elicit NA
   messages from other AERO nodes.  When the AERO node sends NS/NA
   messages, however, it must also include the length of the prefix
   corresponding to the AERO address.  AERO NS/NA messages therefore
   include an 8-bit "Prefix Length" field take from the low-order 8 bits
   of the Reserved field as shown in Figure 4 and 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 (=135)  |     Code      |          Checksum             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Reserved                      | Prefix Length |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                       Target Address                          +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Options ...
       +-+-+-+-+-+-+-+-+-+-+-+-

         Figure 4: AERO Neighbor Solicitation (NS) Message Format

        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 (=136)  |     Code      |          Checksum             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | R|S|O|               Reserved                 | Prefix Length |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                       Target Address                          +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Options ...
       +-+-+-+-+-+-+-+-+-+-+-+-

         Figure 5: AERO Neighbor Advertisement (NA) Message Format

   When an AERO node sends an NS/NA message, it MUST use its AERO
   address as the IPv6 source address and the AERO address of the
   neighbor as the IPv6 destination address.  It MUST also include its
   AERO address prefix length in the Prefix Length field.

   When an AERO node receives an NS/NA message, it accepts the message
   if the Prefix Length applied to the source address is correct for the
   neighbor; otherwise, it ignores the message.



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3.11.  AERO Redirection

   Section 3.8 describes the AERO reference operational scenario.  We
   now discuss the operation and protocol details of AERO Redirection
   with respect to this reference scenario.

3.11.1.  Classical Redirection Approaches

   With reference to Figure 3, when the IPv6 source host ('C') sends a
   packet to an IPv6 destination host ('E'), the packet is first
   forwarded via the EUN to AERO Client ('B').  Client ('B') then
   forwards the packet over its AERO interface to AERO Server ('A'),
   which then re-encapsulates and forwards the packet to AERO Client
   ('D'), where the packet is finally forwarded to the IPv6 destination
   host ('E').  When Server ('A') re-encapsulates and forwards the
   packet back out on its advertising AERO interface, it must arrange to
   redirect Client ('B') toward Client ('D') as a better next-hop node
   on the AERO link that is closer to the final destination.  However,
   this redirection process applied to AERO interfaces must be more
   carefully orchestrated than on ordinary links since the parties may
   be separated by potentially many underlying network routing hops.

   Consider a first alternative in which Server ('A') informs Client
   ('B') only and does not inform Client ('D') (i.e., "classical
   redirection").  In that case, Client ('D') has no way of knowing that
   Client ('B') is authorized to forward packets from the claimed source
   address, and it may simply elect to drop the packets.  Also, Client
   ('B') has no way of knowing whether Client ('D') is performing some
   form of source address filtering that would reject packets arriving
   from a node other than a trusted default router, nor whether Client
   ('D') is even reachable via a direct path that does not involve
   Server ('A').

   Consider a second alternative in which Server ('A') informs both
   Client ('B') and Client ('D') separately, via independent redirection
   control messages (i.e., "augmented redirection").  In that case, if
   Client ('B') receives the redirection control message but Client
   ('D') does not, subsequent packets sent by Client ('B') could be
   dropped due to filtering since Client ('D') would not have a route to
   verify the claimed source address.  Also, if Client ('D') receives
   the redirection control message but Client ('B') does not, subsequent
   packets sent in the reverse direction by Client ('D') would be lost.

   Since both of these alternatives have shortcomings, a new redirection
   technique (i.e., "AERO redirection") is needed.






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3.11.2.  AERO Redirection Concept of Operations

   Again, with reference to Figure 3, when source host ('C') sends a
   packet to destination host ('E'), the packet is first forwarded over
   the source host's attached EUN to Client ('B'), which then forwards
   the packet via its AERO interface to Server ('A').

   Server ('A') then re-encapsulates and forwards the packet out the
   same AERO interface toward Client ('D') and also sends an AERO
   "Predirect" message forward to Client ('D') as specified in
   Section 3.11.4.  The Predirect message includes Client ('B')'s
   network- and link-layer addresses as well as information that Client
   ('D') can use to determine the IPv6 prefix used by Client ('B') .
   After Client ('D') receives the Predirect message, it process the
   message and returns an AERO Redirect message destined for Client
   ('B') via Server ('A') as specified in Section 3.11.5.  During the
   process, Client ('D') also creates or updates a dynamic neighbor
   cache entry for Client ('B'), and creates an IPv6 route for Client
   ('B')'s IPv6 prefix.

   When Server ('A') receives the Redirect message, it re-encapsulates
   the message and forwards it on to Client ('B') as specified in
   Section 3.11.6.  The message includes Client ('D')'s network- and
   link-layer addresses as well as information that Client ('B') can use
   to determine the IPv6 prefix used by Client ('D').  After Client
   ('B') receives the Redirect message, it processes the message as
   specified in Section 3.11.7.  During the process, Client ('B') also
   creates or updates a dynamic neighbor cache entry for Client ('D'),
   and creates an IPv6 route for Client ('D')'s IPv6 prefix.

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

3.11.3.  AERO Redirection Message Format

   AERO Redirect/Predirect messages use the same format as for ICMPv6
   Redirect messages depicted in Section 4.5 of [RFC4861], but also
   include a new "Prefix Length" field taken from the low-order 8 bits
   of the Redirect message Reserved field.  The Redirect/Predirect
   messages are formatted as shown in Figure 6:









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

             Figure 6: AERO Redirect/Predirect Message Format

3.11.4.  Sending Predirects

   When an AERO Server forwards a packet out the same AERO interface
   that it arrived on, the Server sends a Predirect message forward
   toward the AERO Client nearest the destination instead of sending a
   Redirect message back to AERO Client nearest the source.

   In the reference operational scenario, when Server ('A') forwards a
   packet sent by Client ('B') toward Client ('D'), it also sends a
   Predirect message forward toward Client ('D'), subject to rate
   limiting (see Section 8.2 of [RFC4861]).  Server ('A') prepares the
   Predirect message as follows:

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

   o  the link-layer destination address is set to 'L2(D)' (i.e., the
      underlying address of Client ('D')).





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   o  the network-layer source address is set to fe80::0 (i.e., the
      link-local address of Server ('A')).

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

   o  the Type is set to 137.

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

   o  the Prefix Length is set to the length of the prefix to be applied
      to Target address.

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

   o  the Destination Address is set to the IPv6 source address of the
      packet that triggered the Predirection event.

   o  the message includes a TLLAO set to 'L2(B)' (i.e., the underlying
      address of Client ('B')).

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

   Server ('A') then sends the message forward to Client ('D').

3.11.5.  Processing Predirects and Sending Redirects

   When Client ('D') receives a Predirect message, it accepts the
   message only if it has a link-layer source address of the Server,
   i.e.  'L2(A)'.  Client ('D') further accepts the message only if it
   is willing to serve as a redirection target.  Next, Client ('D')
   validates the message according to the ICMPv6 Redirect message
   validation rules in Section 8.1 of [RFC4861].

   In the reference operational scenario, when the Client ('D') 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 ('B') and stores the link-layer
   address found in the TLLAO as the link-layer address of Client ('B').
   Client ('D') then applies the Prefix Length to the Interface
   Identifier portion of the Target Address and records the resulting
   IPv6 prefix in its IPv6 forwarding table.

   After processing the message, Client ('D') prepares a Redirect



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   message response as follows:

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

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

   o  the network-layer source address is set to 'L3(D)' (i.e., the AERO
      address of Client ('D')).

   o  the network-layer destination address is set to 'L3(B)' (i.e., the
      AERO address of Client ('B')).

   o  the Type is set to 137.

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

   o  the Prefix Length is set to the length of the prefix to be applied
      to the Target and Destination address.

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

   o  the Destination Address is set to the IPv6 destination address of
      the packet that triggered the Redirection event.

   o  the message includes a TLLAO set to 'L2(D)' (i.e., the underlying
      address of Client ('D')).

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

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

3.11.6.  Re-encapsulating and Relaying Redirects

   When Server ('A') receives a Redirect message, it accepts the message
   only if it has a neighbor cache entry that associates the message's
   link-layer source address with the network-layer source address.
   Next, Server ('A') validates the message according to the ICMPv6
   Redirect message validation rules in Section 8.1 of [RFC4861].
   Following validation, Server ('A') re-encapsulates the Redirect then
   relays the re-encapsulated Redirect on to Client ('B') as follows.




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   In the reference operational scenario, Server ('A') receives the
   Redirect message from Client ('D') and prepares to re-encapsulate and
   forward the message to Client ('B').  Server ('A') first verifies
   that Client ('D') is authorized to use the Prefix Length in the
   Redirect message when applied to the AERO address in the network-
   layer source of the Redirect message, and discards the message if
   verification fails.  Otherwise, Server ('A') re-encapsulates the
   message by changing the link-layer source address of the message to
   'L2(A)', changing the network-layer source address of the message to
   fe80::0, and changing the link-layer destination address to 'L2(B)' .
   Server ('A') finally relays the re-encapsulated message to the
   ingress node ('B') without decrementing the network-layer IPv6 header
   Hop Limit field.

   While not shown in Figure 3, AERO Relays relay Redirect and Predirect
   messages in exactly this same fashion described above.  See Figure 7
   in Appendix A for an extension of the reference operational scenario
   that includes Relays.

3.11.7.  Processing Redirects

   When Client ('B') receives the Redirect message, it accepts the
   message only if it has a link-layer source address of the Server,
   i.e.  'L2(A)'.  Next, Client ('B') validates the message according to
   the ICMPv6 Redirect message validation rules in Section 8.1 of
   [RFC4861].  Following validation, Client ('B') then processes the
   message as follows.

   In the reference operational scenario, when Client ('B') receives the
   Redirect message, it either creates or updates a dynamic neighbor
   cache entry that stores the Target Address of the message as the
   network-layer address of Client ('D') and stores the link-layer
   address found in the TLLAO as the link-layer address of Client ('D').
   Client ('B') then applies the Prefix Length to the Interface
   Identifier portion of the Target Address and records the resulting
   IPv6 prefix in its IPv6 forwarding table.

   Now, Client ('B') has an IPv6 forwarding table entry for
   Client('D')'s prefix, and Client ('D') has an IPv6 forwarding table
   entry for Client ('B')'s prefix.  Thereafter, the clients may
   exchange ordinary network-layer data packets directly without
   forwarding through Server ('A').

3.12.  Neighbor Reachability Maintenance

   When a source Client is redirected to a target Client it MUST test
   the direct path to the target by sending an initial NS message to
   elicit a solicited NA response.  While testing the path, the source



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   Client SHOULD continue sending packets via the Server until target
   Client reachability has been confirmed.  The source Client MUST
   thereafter continue to test the direct path to the target Client
   using Neighbor Unreachability Detection (NUD) (see Section 7.3 of
   [RFC4861]) in order to keep dynamic neighbor cache entries alive.  In
   particular, the source Client sends NS messages to the target Client
   subject to rate limiting in order to receive solicited NA messages.
   If at any time the direct path appears to be failing, the source
   Client can resume sending packets via the Server which may or may not
   result in a new redirection event.

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

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

   When both the FORWARD_TIME and ACCEPT_TIME timers on a dynamic
   neighbor cache entry expire, the Client deletes both the neighbor
   cache entry and the corresponding IPv6 route.

   If the source Client is unable to elicit an NA response from the
   target Client after MAX_RETRY attempts, it SHOULD consider the direct
   path unusable for forwarding purposes.  Otherwise, the source Client
   may continue to send packets directly to the target Client and SHOULD
   thereafter process any link-layer errors as a hint that the direct
   path to the target Client has either failed or has become
   intermittent.

3.13.  Mobility and Link-Layer Address Change Considerations

   When a Client needs to change its link-layer address (e.g., due to a
   mobility event, due to a change in underlying network interface,
   etc.), it sends an immediate NS message forward to any of its
   correspondents (including the Server and any other Clients) which
   then discover the new link-layer address.

   If two Clients change their link-layer addresses simultaneously, the
   NS/NA messages may be lost.  In that case, the Clients SHOULD delete
   their respective dynamic neighbor cache entries and IPv6 routes, and
   allow packets to again flow through their Server(s) which MAY result
   in a new AERO redirection exchange.

   When a Client needs to change to a new Server, it changes the link-
   layer address of the neighbor cache entry for fe80::0 to the address
   of the new Server and performs a DHCPv6 PD exchange via the new



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   Server.  After the prefix delegation is satisfied, the Client then
   sends a terminating NS message (format TBD) to the old Server, which
   in turn deletes its neighbor cache entry and IPv6 route for the
   Client and withdraws the IPv6 route from the routing system.

3.14.  Underlying 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 source Client has no means for reaching the target
   directly (since they connect to underlying networks of different IP
   protocol versions) and so must ignore any Redirects and continue to
   send packets via the Server.

3.15.  Multicast Considerations

   When the underlying network does not support multicast, AERO nodes
   map IPv6 link-scoped multicast addresses (including
   "All_DHCP_Relay_Agents_and_Servers") to the underlying IP address of
   the AERO Server.

   When the underlying network supports multicast, AERO nodes use the
   multicast address mapping specification found in [RFC2529] for IPv4
   underlying networks and use a direct multicast mapping for IPv6
   underlying networks.  (In the latter case, "direct multicast mapping"
   means that if the IPv6 multicast destination address of the inner
   packet is "M", then the IPv6 multicast destination address of the
   encapsulating header is also "M".)

3.16.  Operation on Server-less AERO Links

   In some AERO link scenarios, there may be no Server on the link
   and/or no need for Clients to use a Server as an intermediary trust
   anchor.  In that case, Clients can establish dynamic neighbor cache
   entries and IPv6 routes by performing direct Client-to-Client
   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 prefix.

   When there is no Server on the link, Clients must arrange to receive
   prefix delegations and publish the delegations via a secure prefix
   discovery service through some means outside the scope of this
   document.

3.17.  Other Considerations

   IPv6 hosts serviced by an AERO Client can reach IPv4-only services
   via a NAT64 gateway [RFC6146] within the IPv6 network.



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   AERO nodes can use the Default Address Selection Policy with DHCPv6
   option [RFC7078] the same as on any IPv6 link.

   All other (non-multicast) functions that operate over ordinary IPv6
   links operate in the same fashion over AERO links.


4.  Implementation Status

   An early implementation is available at:
   http://linkupnetworks.com/aero/aerov2-0.1.tgz.


5.  IANA Considerations

   This document uses the UDP Service Port Number 8060 reserved by IANA
   for AERO in [RFC6706].  Therefore, there are no new IANA actions
   required for this document.


6.  Security Considerations

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

   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.,
   WiFi networks) and links that provide physical security (e.g.,
   enterprise network LANs) provide a first line of defense that is
   often sufficient.  In other instances, securing mechanisms such as
   Secure Neighbor Discovery (SeND) [RFC3971] or IPsec [RFC4301] may be
   necessary.

   AERO Clients MUST ensure that their connectivity is not used by
   unauthorized nodes to gain access to a protected network.  (This
   concern is no different than for ordinary hosts that receive an IP
   address delegation but then "share" the address with unauthorized
   nodes via an IPv6/IPv6 NAT function.)

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





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7.  Acknowledgements

   Discussions both on the v6ops list and in private exchanges helped
   shape some of the concepts in this work.  Individuals who contributed
   insights include Mikael Abrahamsson, Fred Baker, Stewart Bryant,
   Brian Carpenter, Brian Haberman, Joel Halpern, Sascha Hlusiak, Lee
   Howard and Joe Touch.  Members of the IESG also provided valuable
   input during their review process that greatly improved the document.
   Special thanks go to Stewart Bryant, Joel Halpern and Brian Haberman
   for their shepherding guidance.

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

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


8.  References

8.1.  Normative References

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

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

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

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic



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              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862, September 2007.

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

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

8.2.  Informative References

   [IRON]     Templin, F., "The Internet Routing Overlay Network
              (IRON)", Work in Progress, June 2012.

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

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

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

   [RFC3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

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

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

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

   [RFC5214]  Templin, F., Gleeson, T., and D. Thaler, "Intra-Site
              Automatic Tunnel Addressing Protocol (ISATAP)", RFC 5214,



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              March 2008.

   [RFC5569]  Despres, R., "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd)", RFC 5569, January 2010.

   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5996, September 2010.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

   [RFC6204]  Singh, H., Beebee, W., Donley, C., Stark, B., and O.
              Troan, "Basic Requirements for IPv6 Customer Edge
              Routers", RFC 6204, April 2011.

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

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

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

   [RFC6864]  Touch, J., "Updated Specification of the IPv4 ID Field",
              RFC 6864, February 2013.

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935, April 2013.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, April 2013.

   [RFC6980]  Gont, F., "Security Implications of IPv6 Fragmentation
              with IPv6 Neighbor Discovery", RFC 6980, August 2013.

   [RFC7078]  Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing
              Address Selection Policy Using DHCPv6", RFC 7078,
              January 2014.


Appendix A.  AERO Server and Relay Interworking

   Figure 3 depicts a reference AERO operational scenario with a single



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   Server on the AERO link.  In order to support scaling to larger
   numbers of nodes, the AERO link can deploy multiple Servers and
   Relays, e.g., as shown in Figure 7.

                             .-(::::::::)
                          .-(::: IPv6 :::)-.
                         (:: Internetwork ::)
                          `-(::::::::::::)-'
                             `-(::::::)-'
                                  |
       +--------------+    +------+-------+    +--------------+
       |AERO Server C |    | AERO Relay D |    |AERO Server E |
       | (default->D) |    | (A->C; G->E) |    | (default->D) |
       |    (A->B)    |    +-------+------+    |    (G->F)    |
       +-------+------+            |           +------+-------+
               |                   |                  |
       X---+---+-------------------+------------------+---+---X
           |                  AERO Link                   |
     +-----+--------+                            +--------+-----+
     |AERO Client B |                            |AERO Client F |
     | (default->C) |                            | (default->E) |
     +--------------+                            +--------------+
           .-.                                         .-.
        ,-(  _)-.                                   ,-(  _)-.
     .-(_ IPv6  )-.                              .-(_ IPv6  )-.
    (__    EUN      )                           (__    EUN      )
       `-(______)-'                                `-(______)-'
            |                                           |
        +--------+                                  +--------+
        | Host A |                                  | Host G |
        +--------+                                  +--------+

                 Figure 7: AERO Server/Relay Interworking

   In this example, AERO Client ('B') associates with AERO Server ('C'),
   while AERO Client ('F') associates with AERO Server ('E').
   Furthermore, AERO Servers ('C') and ('E') do not associate with each
   other directly, but rather have an association with AERO Relay ('D')
   (i.e., a router that has full topology information concerning its
   associated Servers and their Clients).  Relay ('D') connects to the
   AERO link, and also connects to the native IPv6 Internetwork.

   When host ('A') sends a packet toward destination host ('G'), IPv6
   forwarding directs the packet through the EUN to Client ('B'), which
   forwards the packet to Server ('C') in absence of more-specific
   forwarding information.  Server ('C') forwards the packet, and it
   also generates an AERO Predirect message that is then forwarded
   through Relay ('D') to Server ('E').  When Server ('E') receives the



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   message, it forwards the message to Client ('F').

   After processing the AERO Predirect message, Client ('F') sends an
   AERO Redirect message to Server ('E').  Server ('E'), in turn,
   forwards the message through Relay ('D') to Server ('C').  When
   Server ('C') receives the message, it forwards the message to Client
   ('B') informing it that host 'G's EUN can be reached via Client
   ('F'), thus completing the AERO redirection.

   The network layer routing information shared between Servers and
   Relays must be carefully coordinated in a manner outside the scope of
   this document.  In particular, Relays require full topology
   information, while individual Servers only require partial topology
   information (i.e., they only need to know the EUN prefixes associated
   with their current set of Clients).  See [IRON] for an architectural
   discussion of routing coordination between Relays and Servers.


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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