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Versions: (draft-despres-softwire-6a44) 00 01 02 RFC 6751

Internet Engineering Task Force                          R. Despres, Ed.
Internet-Draft                                                 RD-IPtech
Intended status: Experimental                               B. Carpenter
Expires: October 25, 2012                              Univ. of Auckland
                                                                 D. Wing
                                                                   Cisco
                                                                S. Jiang
                                                                  Huawei
                                                          April 23, 2012


                  Native IPv6 Behind NAT44 CPEs (6a44)
                         draft-despres-6a44-01

Abstract

   In customer sites having IPv4-only CPEs, Teredo provides a last
   resort IPv6 connectivity [RFC4380] [RFC5991] [RFC6081].  However,
   because it is designed to work without involvement of Internet
   service providers, it has significant limitations (connectivity
   between IPv6 native addresses and Teredo addresses is uncertain;
   connectivity between Teredo addresses fails for some combinations of
   NAT types). 6a44 is a complementary solution that, being base on ISP
   cooperation, avoids these limitations. 6a44 uses specific prefixes
   assigned by local ISPs (rather than the anycast address used by
   Teredo, an evolution similar to that from 6to4 to 6rd).  The
   specification is complete enough for actual deployment, including
   with independently written codes.

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 25, 2012.

Copyright Notice




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   Copyright (c) 2012 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  4
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Design Goals, Requirements, and Model of Operation . . . . . .  6
     4.1.  Hypotheses about NAT Behavior  . . . . . . . . . . . . . .  6
     4.2.  Native IPv6 Connectivity for unmanaged Hosts behind
           NAT44's  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.3.  Operational Requirements . . . . . . . . . . . . . . . . .  7
     4.4.  Model of Operation . . . . . . . . . . . . . . . . . . . .  8
   5.  6a44 Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Specification of Clients and Relays  . . . . . . . . . . . . . 13
     6.1.  Packet Formats . . . . . . . . . . . . . . . . . . . . . . 13
     6.2.  IPv6 Packet Encapsulations . . . . . . . . . . . . . . . . 13
     6.3.  6a44 Bubbles . . . . . . . . . . . . . . . . . . . . . . . 13
     6.4.  Maximum Transmission Units . . . . . . . . . . . . . . . . 15
     6.5.  6a44 Client Specification  . . . . . . . . . . . . . . . . 16
       6.5.1.  Tunnel Maintenance . . . . . . . . . . . . . . . . . . 16
       6.5.2.  Client Transmission  . . . . . . . . . . . . . . . . . 18
       6.5.3.  Client Reception . . . . . . . . . . . . . . . . . . . 20
     6.6.  6a44 Relay Specification . . . . . . . . . . . . . . . . . 22
       6.6.1.  Relay Reception in IPv6  . . . . . . . . . . . . . . . 22
       6.6.2.  Relay Reception in IPv4  . . . . . . . . . . . . . . . 23
     6.7.  Implementation of Automatic Sunset . . . . . . . . . . . . 25
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   8.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 28
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 29
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 29
     10.2. Informative References . . . . . . . . . . . . . . . . . . 29
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31





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1.  Introduction

   Although most customer premise equipments (CPE's) should soon be
   dual-stack capable, a large installed base of IPv4-only CPE's is
   likely to remain for several years.  Their operation is based on IPv4
   NAT's (NAT44's).  Also, due to the IPv4 address shortage, more and
   more Internet service providers (ISP's), and more and more mobile
   operators, will assign private IPv4 addresses of [RFC1918] to their
   customers (the [NAT444] model).  For a rapid and extensive use of
   IPv6 [RFC2460], there is therefore a need for IPv6 connectivity
   behind NAT44's, including those of the [NAT444] model.

   At the moment, there are two tunneling techniques specified for IPv6
   connectivity behind NAT44's:

   o  Configured tunnels.  They involve tunnel brokers with which users
      must register [RFC3053].  Well-known examples include deployments
      of the Hexago tool, and the SixXs collaboration, which are
      suitable for IPv6 early trials.  However, this approach is not
      adequate for mass deployment: it imposes that, even if two hosts
      are in the same customer site, IPv6 packets between them must
      transit via tunnel servers, which may be far away.

   o  Automatic Teredo tunnels [RFC4380] [RFC5991].  Teredo is specified
      as a last resort solution which, due to its objective to work
      without local ISP involvement, has the following limitations:

      *  Connectivity between IPv6 native addresses and Teredo addresses
         is uncertain.  (As explained in [RFC4380] section 8.3, this
         connectivity depends on paths being available from all IPv6
         native addresses to some Teredo Relays.  ISP's lack sufficient
         motivations to ensure it).

      *  Between two Teredo addresses, IPv6 connectivity fails for some
         combinations of NAT44 types([RFC6081] section 3).

      *  According to [RFC4380] section 5.2, each Teredo host has to be
         configured with the IPv4 address of a Teredo server (a
         constraint that can however be avoided in some
         implementations).

   6a44 is designed to avoid Teredo limitations where ISP's can
   participate to the solution.  The approach for this is similar to
   that which permitted 6rd [RFC5569] [RFC5969] to avoid limitations of
   6to4 [RFC3056] [RFC3068]: at the beginning of IPv6 addresses, the
   Teredo well-known prefix is replaced by network specific prefixes
   assigned by local ISP's.




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   This document is organized as follows: terms used in the document are
   defined in Section 3; design goals and model of operation are
   presented in Section 4; Section 5 describes the format of 6a44 IPv6
   addresses; Section 6 specifies in details behaviors of 6a44 clients
   and 6a44 relays; security and IANA considerations are respectively
   covered in Section 7 and Section 8.

   The specification is expected to be complete enough for running codes
   to be independently written and the solution to be incrementally
   deployed and used.  Its intended status is Experimental rather than
   Standard to reflect uncertainty as to which major Internet players
   may be willing to support it.


2.  Requirements Language

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


3.  Definitions

   The following definitions are used in this document:

   MAJOR NEW DEFINITIONS

   "6a44 ISP network":  An IPv4-capable ISP network that supports at
      least one 6a44 relay.  Additional conditions are that it assigns
      individual IPv4 addresses to its customer sites (global or
      private), that it supports ingress filtering [RFC2827], and that
      its path MTU's are at least 1308 octets.

   "6a44 relay":  A node that supports the 6a44 relay function defined
      in this document, and that has interfaces to an IPv6-capable
      upstream network and to an an IPv4-capable downstream network.

   "6a44 client":  A host that supports the 6a44 client function defined
      in this document, and has no other mean than 6a44 to have a IPv6
      native address.

   "6a44 tunnel":  A tunnel established and maintained between a 6a44
      client and 6a44 relays of its ISP network.

   "6a44 bubble":  A UDP/IPv4 packet sent from a 6a44 client to the
      6a44-relay address, or conversely, and having a UDP payload that
      cannot be confused with an IPv6 packet.  In the client to relay
      direction, it is a request for a response bubble.  In the relay to



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      client direction, it conveys the up-to-date IPv6 prefix of the
      client.

   SECONDARY NEW DEFINITIONS
   (for reference, can be skipped by readers familiar with usual
   terminology)

   "6a44 service":  The service offered by a 6a44 ISP network to its
      6a44 clients.

   "6a44-client IPv6 address":  The IPv6 address of a 6a44 client.  It
      is composed of the client IPv6 prefix, received from a 6a44 relay,
      followed by the client local IPv4 address.

   "6a44-client IPv6 prefix":  For a 6a44 client, the IPv6 prefix (/96)
      composed of the IPv6 prefix of the local 6a44-network (/48)
      followed by the UDP/IPv4 mapped address of the client (32 + 16
      bits).

   "6a44-client UDP/IPv4 mapped address":  For a 6a44 client, the
      external UDP/IPv4 address that, in the CPE NAT44 of the site, is
      that of its 6a44 tunnel.

   "6a44-client UDP/IPv4 local address":  For a 6a44 client, the
      combination of its local IPv4 address and the 6a44 port.

   "6a44 port":  The UDP port used for 6a44 (see Section 8).

   "6a44-relay UDP/IPv4 address":  The UDP/IPv4 address composed of the
      6a44-relay anycast address and the 6a44 port.

   "6a44-relay anycast address":  The well-known IPv4 anycast address of
      6a44 relays (see Section 8).

   "6a44-network IPv6 prefix":  An IPv6 /48 prefix assigned by an ISP to
      a 6a44 network.

   USUAL DEFINITIONS

   "Upstream direction":  For a network border node, the direction
      toward the Internet core.

   "Downstream direction":  For a network border node, the direction
      toward end-user nodes (opposite to the upstream direction).







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   "IPv4 private address":  An address that starts with one of the three
      prefixes of [RFC1918] (10/8, 172.16/12, or 192.168/16).

   "IPv6 native address":  An IPv6 global unicast address that starts
      with an aggregetable prefix assigned to an ISP.

   "UDP/IPv4 address":  The combination of an IPv4 address and a UDP
      port.

   "UDP/IPv4 packet":  A UDP datagram contained in an IPv4 packet.

   "IPv6/UDP/IPv4 packet":  An IPv6 packet contained in a UDP/IPv4
      packet.


4.  Design Goals, Requirements, and Model of Operation

4.1.  Hypotheses about NAT Behavior

   Although 6a44 will not work with all possible NAT44 behaviors, 6a44
   is designed to work with endpoint-dependent NAT44 mappings as well as
   with endpoint-independent mappings, including if there are dynamic
   changes from one mode to the other.

   The only assumption is that, after a mapping has been established in
   the NAT44, it is maintained as long as it is re-used at least once,
   in each direction, every 30 seconds.

   NOTE: 30 seconds is the value used for the same mapping-maintenance
   purpose in Teredo [RFC4380], and in SIP [RFC5626].

4.2.  Native IPv6 Connectivity for unmanaged Hosts behind NAT44's

   The objective remains that, as soon as possible, CPEs and ISPs
   support IPv6 native prefixes. 6a44 is therefore designed only as a
   temporary solution for hosts to obtain IPv6 native addresses in sites
   whose CPEs are not IPv6-capable yet.

   As noted in Section 1, IPv6 native addresses obtainable with
   configured tunnels have important limitations.  However, compared to
   6a44 addresses, they have the advantage of remaining unchanged in
   case of NAT44 reset. 6a44 remains therefore the last resort solution
   for IPv6 native addresses in unmanaged hosts of IPv4-only-CPE sites,
   while configured tunnels may still be preferred for some managed
   hosts if reported limitations of configured tunnels are consciously
   found acceptable.  Their scopes being different, the two solutions
   can usefully coexist.




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   Note that Teredo remains a last resort solution for hosts to have
   IPv6 addresses where IPv6 native addresses cannot be available (and
   where Teredo limitations are consciously found acceptable).

4.3.  Operational Requirements

   Operational requirements of 6a44 include the following:

   "Robust IPv6 connectivity":  A node having a 6a44 address must have
      paths across the Internet to and from all IPv6 native addresses
      that are not subject to voluntary firewall filtering.

   "Intra-site path efficiency":  Packets exchanged between 6a44 clients
      that are behind the same CPE NAT44 must not have to traverse it.
      If these clients have IPv4 connectivity using their private IPv4
      addresses, they must also have IPv6 connectivity using their 6a44
      addresses.

   "Plug-and-play operation of 6a44 clients":  In order to obtain a 6a44
      address from its local ISP, a 6a44 client must need no parameter
      configuration.

   "Scalability of ISP functions":  For the solution to be easily
      scalable, ISP-supported functions have to be completely stateless.

   "Anti-spoofing Protection":  Where address anti-spoofing is ensured
      in IPv4 with ingress filtering of [RFC2827] [RFC3704], IPv6
      addresses must benefit from the same degree of anti-spoofing
      protection.

   "Overall Design simplicity":  As Antoine de Saint-Exupery said in
      [The Tool], "it seems that perfection is attained not when there
      is nothing more to add, but when there is nothing more to remove".

   "Incremental deployability":  Hosts and ISP networks must be able to
      become 6a44 capable independently of each other.  IPv6 must be
      operational where both are available, and there must be no
      perceptible effect where they are not both available.













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4.4.  Model of Operation


                             (A) GLOBAL-IPv4 ISP NETWORK
                                   +------------------+
        6a44 customer network(s)   |GLOBAL IPv4       |       Upstream
               +-----------+    ---| MTU >= 1308      +---  IPv4 network
            ---| Private   |       | ingress filtering|   (<== no route
    +----+     |  IPv4  +-----+    | IPv6 optional    |  to 6a44 relays)
    |    |-----|        |NAT44|----+                  |
    +----+     |        +-----+    |      +-------------+
     6a44   ---|MTU >= 1308|       |    --+6a44 relay(s)|--- Upstream
   client(s)   |   no      |    ---|      +-------------+  IPv6 network
               |native IPv6|       |                  |
               +-----------+       +------------------+


                             (B) PRIVATE-IPv4 ISP NETWORK
                                   +------------------+
                                   |PRIVATE IPv4      |
                                   | as above         |
                                ---|                  |
                                   |     +--------------+
                                   |   --+ ISP NAT44(s) |--- Upstream
                  as above     ----+     +--------------+   IPv4 network
                                   |                  |
                                   |     +--------------+
                                ---|   --+6a44 relay(s) |--- Upstream
                                   |     +--------------+   IPv6 network
                                   |                  |
                                   +------------------+


                       6a44 APPLICABILITY SCENARIOS

                                 Figure 1

   The operation of 6a44 involves two types of nodes: 6a44 clients and
   6a44 relays.  Figure 1 shows the two applicability scenarios:

   o  In the first one, IPv4 addresses assigned to customer sites are
      global IPv4.

   o  In the second one, they are private IPv4 addresses ([NAT444] model
      where ISPs operate one or several NAT44's, also called carrier-
      grade NATs, or CGN's).





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   In both configurations, the ISP network may also assign IPv6 prefixes
   to customer sites:

   o  If customer sites are only assigned IPv4 addresses, 6a44 applies
      both to sites whose CPE's are not IPv4-only capable and to sites
      whose CPE's are dual-stack capable.

   o  If customer sites are assigned both IPv4 addresses and IPv6
      prefixes, 6a44 only applies to sites whose CPE's areIPv4-only
      capable.



           CUSTOMER         +-------------------------+
             SITES          |      ISP NETWORK        |
          +---------+       +----------------+        |
          |         |       |6a44 ISP NETWORK|        |   GLOBAL
          |         |       |                |        |  INTERNET
   HOSTS  |      IPv6/UDP/IPv4         +---------+    |             HOST
    +-+   |      +-----+    |          |   6a44  |    |     IPv6    +-+
    |H|---|--.---|NAT44|----|----------.---------.----|--- - - - ---|D|
    +-+   |   \  +-----+    |         /| relay(s)|\   |             +-+
    +-+   |   /     |       |        ' +---------+ '  |
    |A|---|--'      |       |        |       |     |  |
    +-+ IPv6/IPv4   |       |        |       |     |  |
          +---------+       |        |       |     |  |
                            |        |       |     |  |
          +---------+       |        |       |     |  |
          |      IPv6/UDP/IPv4       .       |     |  |
    +-+   |      +-----+    |       /        |     |  |
    |B|---|------|NAT44|----|------'         |     |  |
    +-+   |      +-----+    |                |     |  |
          |         |       +----------------+     |  |
          +---------+       |                      .  |
    +-+                     |                     /   |
    |C|---- - - - - - - ----|--------------------'    |
    +-+           IPv6      |                         |
                            +-------------------------+

   IPv6 PATHS H-A:   A is a 6a44 client in the same site
              H-B:   B is a 6a44 client in another site of the same ISP
              H-C:   C is IPv6 of the same ISP, other than 6a44
              H-D:   D is IPv6 of another ISP

              IPv6 PATHS BETWEEN 6a44 HOSTS AND REMOTE HOSTS

                                 Figure 2




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   Figure 2 illustrates paths of IPv6 packets in between a 6a44 client H
   and various possible locations of remote hosts (A in the same site, B
   in another 6a44 site of the same ISP, C in a non-6a44 IPv6 site of
   the same ISP, D in an IPv6 site of another ISP).  Between 6a44
   clients of a same site, IPv6 packets are encapsulated in IPv4
   packets.  Those Between 6a44 clients and 6a44 relays are encapsulated
   in UDP/IPv4 packets.

   6a44 operates as follows (details in Section 6):

   1.   A 6a44 client starts operation by sending a 6a44 bubble to the
        6a44-relay UDP/IPv4 address.

   2.   When a 6a44 relay receives a bubble from one of its 6a44
        clients, it returns to this client a bubble containing the IPv6
        prefix of this client.

   3.   When a 6a44 client receives a bubble from a 6a44 relay, it
        updates (or confirms) its 6a44 address.  It is an update if the
        client has no IPv6 address yet or if, due to a CPE reset, this
        address has changed.  After receiving a bubble, a client is
        ready to start, or to continue, IPv6 operation.

   4.   When a 6a44 client having a 6a44 address has an IPv6 packet to
        send whose destination IS in the same customer site, it
        encapsulates it in an IPv4 packet whose destination is found in
        the IPv6 destination address.  It then sends the resulting IPv6/
        IPv4 packet.

   5.   When a 6a44 client receives a valid IPv6/IPv4 packet from a 6a44
        client of the same site, it decapsulates the IPv6 packet and
        submits it to further IPv6 processing.

   6.   When a 6a44 client having a 6a44 address has an IPv6 packet to
        send whose destination IS NOT in the same the same customer
        site, it encapsulates the packet in a UDP/IPv4 packet whose
        destination is UDP/IPv4 address of 6a44 relays.  It then sends
        the IPv6/UDP/IPv4 packet.

   7.   When a 6a44 relay receives via its IPv4 interface a valid IPv6/
        UDP/IPv4 packet whose destination IS one of its 6a44 clients, it
        forwards the contained IPv6 packet in a modified IPv6/UDP/IPv4
        packet.  The UDP/IPv4 destination of this packet is found in the
        IPv6 destination address.

   8.   When a 6a44 client receives a valid IPv6/UDP/IPv4 packet from a
        6a44 relay, it decapsulates the IPv6 packet and submits it to
        further IPv6 processing.



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   9.   When a 6a44 relay receives via its IPv4 interface a valid IPv6/
        UDP/IPv4 packet whose IPv6 destination IS NOT one of its 6a44
        clients, it decapsulates the IPv6 packet and sends it via its
        IPv6 interface.

   10.  When a 6a44 relay receives via its IPv6 interface a valid IPv6
        packet whose destination is one of its 6a44 clients, it
        encapsulates the packet in a UDP/IPv4 packet whose destination
        is the UDP/IPv4 address found in the IPv6 destination address.
        It then sends the resulting IPv6/UDP/IPv4 packet via its IPv4
        interface.

   11.  To maintain the NAT44 mapping of its 6a44 tunnel, and to quickly
        detect the need to change its 6a44 address in case of NAT44
        reset, a 6a44 client sends from time to time a bubble to the
        6a44 relay address (see Section 6.5.1) .

   12.  When a 6a44 relay receives via its IPv4 interface an IPv6/UDP/
        IPv4 packet whose IPv6 and UDP/IPv4 source addresses are not
        consistent, it discards the invalid packet, and returns a bubble
        to the UDP/IPv4 source address.  (This permits the 6a44 client
        at this address to update its IPv6 address).


5.  6a44 Addresses

   The 6a44 IPv6 address an ISP assigns to a host must contain all
   pieces of information needed to reach it from other IPv6 addresses.
   These piecesa are, as illustrated in Figure 3:

   o  the 6a44-network IPv6 prefix D (a /48 the ISP has assigned to its
      6a44 relays);

   o  the customer-site IPv4 address N (either global IPv4 or, if the
      ISP uses a [NAT444] model, private IPv4);

   o  the mapped port Z of the 6a44 tunnel (i.e. the external port
      assigned by the NAT44 to the tunnel that the client maintains
      between its UDP/IPv4 local address A:W and the 6a44-relay UDP/IPv4
      address B:W).

   o  the client local IPv4 address A (i.e. the private IPv4 address
      assigned to the client in its customer site; it is needed for
      intra-site IPv6 connectivity).







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                  Customer network       ISP network
                  +--------------+       +------------------+
       Client     |IPv4         CPE      |IPv4              |
       +----+     |           +-----+    |        +----------+
       | ^  |-----|           |NAT44|----+        |6a44 relay|---- IPv6
       +-|-^+     |           +-----+    |        +----------+^
         | |      |          ^   |   ^   |         ^        | |
         | |      +----------|---+   |   +---------|--------+ |
         | |                 |   ^   |             |          |
         | |             >0/0|   |   |N/32<        |          |
         | |                     |                 |          |
         | |                  Mapping              |          |
         | |                <a:w>-<N:Z> (*)        |          |
         | |                                       |          |
         | |A:W<                               >B:W|          |
         |                                                    |
    IPv6 |D.N.Z.A/128<                                        |D/48<

    (*) With NAT44(s) between client and CPE, a:w may differ from A:W


    |0                    47|48           79|80   95|96          127|
    +-------+-------+-------+-------+-------+-------+-------+-------+
    |      6a44-network     | Customer-site |Tunnel |  6a44-client  |
    |      IPv6 prefix      |  IPv4 address |mapped |  local IPv4   |
    |           (D)         |      (N)      |port(Z)|    address (A)|
    +-------+-------+-------+-------+-------+-------+-------+-------+
                                  6a44-client
                            <-- UDP/IPv4 address -->
    <------------ 6a44-client IPv6 prefix --------->
    <---------------- 6a44-client IPv6 address --------------------->


                         HOST-ADDRESS CONSTRUCTION

                                 Figure 3

   NOTE: 6a44 addresses are not guaranteed to comply with the rule of
   [RFC4291] according to which bits 64-127 of aggregetable unicast
   addresses have to be the Modified-EUI-64 IID format.  However, these
   bits of 6a44 addresses are interpreted only where 6a44 addresses are
   processed, i.e. in 6a44 relays and clients.  No operational problem
   is therefore foreseen.  Because it is a purely transitional tool, it
   shouldn't prevent any longer-term "development of future technology
   that can take advantage of interface identifiers with universal
   scope" once the need for the transitional tool has gone away (the
   purpose of this format expressed in [RFC4291]).




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6.  Specification of Clients and Relays

6.1.  Packet Formats

6.2.  IPv6 Packet Encapsulations

   For NAT44 traversal, an IPv6 packet transmitted from a 6a44 client to
   a 6a44 relay or conversely is encapsulated in a UDP/IP packet whose
   source and destinations addresses are those of the two endpoints (A:W
   and B:W in notations of Figure 3).  The IPv4 packet is that of a
   complete datagram (its more-fragment bit is set to 0, its offset is
   set to 0, and its datagram identification may be set to 0).  The UDP
   checksum is set to 0 (there is no need for an additional layer of
   checksum protection).  The length of the IPv6 packet SHOULD NOT
   exceed 1280 octets (see Section 6.4).



        Octets: |0         |20 |28                 |68            |
                +----------+---+-------------------+-------//-----+
                |   IPv4   |UDP|    IPv6 header    | IPv6 payload |
                +----------+---+-------------------+-------//-----+

   An IPv6 packet transmitted from a 6a44 client to another 6a44 client
   of the same site is encapsulated in an IPv4 packet whose source and
   destination addresses are the private-IPv4 addresses of the two
   hosts.  The IPv4 packet is that of a complete datagram (its more-
   fragment bit is set to 0, its offset is set to 0, and its datagram
   identification may be set to 0).  The size of the IPv6 packet SHOULD
   NOT exceed 1280 octets for off-link destinations, and MUST NOT exceed
   the link MTU minus 20 octets for on-link destinations (see
   Section 6.4).


          Octets:  |0         |20                 |60            |
                   +----------+-------------------+-------//-----+
                   |   IPv4   |    IPv6 header    | IPv6 payload |
                   +----------+-------------------+-------//-----+

6.3.  6a44 Bubbles

   A Bubble is a UDP/IPv4 packet whose UDP payload comprises a "6a44-
   client IPv6 prefix" field and a "Bubble ID" field, and whose UDP
   checksum is set to 0.







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                      "6a44-client IPv6 prefix" field
         . from a 6a44 client = 0
         . from a 6a44 relay = 6a44-client IPv6 prefix
                                    |
         Octets:  |0         |20 |28| |40 |48
                  +----------+---+--|-+---+
                  |   IPv4   |UDP|  . | . |
                  +----------+---+----+-|-+
                                        |
                                   "Bubble ID" field
          . from a 6a44 client: a client-selected value
          . from a 6a44 relay:
             - in a response bubble, copy of the received bubble ID
             - in an error signaling bubble, 0

                            6a44 BUBBLE FORMAT

                                 Figure 4

   In a bubble from a 6a44 client to a 6a44 relay, the "6a44-client IPv6
   prefix" field is only reserved space for the response.  It is set to
   0.  In a bubble from a 6a44 relay to a 6a44 client, it contains the
   IPv6 prefix of the client.

   In a bubble from a 6a44 client to a 6a44 relay, the "Bubble ID" field
   contains a randomly chosen value, renewed in circumstances defined in
   Section 6.5.1.  In a bubble from a 6a44 relay to a 6a44 client: if
   the bubble is a response to a bubble received from the client, the
   field contains the value found in the received bubble; if the bubble
   is it is a reaction to a received IPv6/UDP/IPv4 packet whose IPv6 and
   UDP/IPv4 sources are inconsistent, the field is set to 0.  The
   purpose of this field is a protection against 6a44-relay spoofing
   attacks (see Section 7).

   In order to preserve forward compatibility with any extension of
   bubble formats, should one prove useful in the future, 6a44 clients
   and 6a44 relays MUST accept to receive bubbles whose UDP payloads
   lengths are longer than 20 octets (up to that of an IPv6-packet
   header since, as detailed in Section 6.6.2 and Section 6.5.3, bubbles
   are recognized by their lengths being shorter than that of tunneled
   IPv6 packets).










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6.4.  Maximum Transmission Units

   Reassembly of a fragmented IPv4 datagram necessitates to remember its
   identifier from reception of the first fragment to reception the last
   one, and necessitates a timeout protection against packet losses.  If
   such an IP-layer stateful processing would be necessary for 6a44, it
   would make it more complex than needed, would introduce a
   vulnerability to denial of service attacks, and would impose that all
   fragments of a fragmented IPv4 datagram go to the same relay.  This
   last point would be a constraint on how load balancing may be
   performed between multiple 6a44 relays, and would therefore be
   detrimental to scalability.

   For 6a44 processing to remain completely stateless, IPv4 packets
   containing encapsulated IPv6 packets must never be fragmented.  For
   this:

   o  In customer sites, 6a44 clients MUST have IPv4 link MTU's that
      support encapsulated IPv6 packets of lengths up to 1280 octets,
      i.e., for IPv6/UDP/IPv4 packets that traverse the CPE, link MTU's
      of at least 1280+20+8=1308 octets.  (This condition is in general
      satisfied).

   o  For the same reason, 6a44 ISP networks must have IPv4 path MTU's
      of at least 1308 octets.  (This condition is in general
      satisfied).

   o  6a44 clients SHOULD limit the size of IPv6 packets they transmit
      to off-link destinations to 1280 octets.  They MUST also limit the
      size of IPv6 packets they transmit to on-link destinations to the
      link MTU minus the 20 octets of an IPv4 encapsulation header.

   o  6a44 relays SHOULD set their IPv6 MTU to 1280.  (If a relay
      receives an IPv6 packets longer than this MTU via its IPv6
      upstream interface, it MUST return ICMPv6 Packet Too Big message).
      Typical ISP networks have a path MTU's that would permit IPv6
      MTU's of 6a44 to be longer than 1280, but taking 1280 octets is a
      precaution that guarantees against problems with customer sites
      that may have internal path MTU's smaller than those supported by
      their ISP networks.











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6.5.  6a44 Client Specification

6.5.1.  Tunnel Maintenance

   For a 6a44-client IPv6 address to remain valid, the port mapping of
   the 6a44 tunnel MUST be maintained in the CPE NAT44.

                        Initialization
                      ________v________
                     /                 \
                     | "6a44 disabled" |------------<-----------------+
                     \_________________/                              ^
                              v no v6-add AND v4-add                  ^
     +--------->--------------v                                       ^
     ^         +--------------v--------------+                        ^
     ^         |   Reset the attempt count   |                        ^
     ^         |   Renew the bubble ID       |                        ^
     ^         +--------------+--------------+                        ^
     ^    +----->-------------v                                       ^
     ^    ^    +--------------v--------------+                        ^
     ^    ^    |          Send a bubble      |                        ^
     ^    ^    +--------------v--------------+                        ^
     ^    ^           ________v________                               ^
     ^    ^ Timer T1 /                 \ 4 attempts without answer    ^
     ^    +----<-----|  "Bubble sent"  |-------->----------------+    ^
     ^    (1 to 1.5s)\_________________/                         v    ^
     ^                        v        \ v6-add OR no v4-add     v    ^
     ^        Bubble received v         +-----------------------------+
     ^                        v-----------------<-----------+    v    ^
     ^               _________v_________                    ^    v    ^
     ^     Timer T2 /                   \Bubble received    ^    v    ^
     +----------<---| "Bubble Received" |-------->----------+    v    ^
     ^  (30s - 4*T1)\___________________/                        v    ^
     ^                                  \ v6-add OR no v4-add    v    ^
     ^                                   +------->--------------------+
     ^                                                           v    ^
     ^                        +----------------------------------+    ^
     ^                 _______v________                               ^
     ^       Timer T3 /                 \ v6-add OR no v4-add         ^
     +-----------<----| "No 6a44 relay" |----->-----------------------+
             (30 min) \_________________/

                       TUNNEL MAINTENANCE ALGORITHM

                                 Figure 5






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   For this, the 6a44 client SHOULD apply the equivalent of the
   following TM-x rules, illustrated in Figure 5:

   TM-1  At initialization, a timer value T1 is randomly chosen in the
         recommended range 1 to 1.5 seconds, and the "6a44 disabled"
         state is entered.  (Randomness of this value is a precaution to
         avoid that, if many hosts happen to be re-initialized at the
         same time, the bubble traffic resulting from the following
         rules be synchronized.)

   TM-2  In the "6a44-disabled" state, if it appears that the the
         interface has no IPv6 native address BUT has a private IPv4
         address, then: the Attempt count (a local variable) is set to
         1; a new Bubble ID (another local variable) is randomly chosen;
         a bubble is sent with this bubble ID; the "Bubble sent" state
         is entered with the timer set to T1.

   TM-3  In the "Bubble sent" state, if the timer expires AND the
         Attempt count is less than 4, then: the Attempt count is
         increased by 1; a new bubble is sent with the current Bubble
         ID; the "bubble sent" state is re-entered with the timer reset
         to T1.

   TM-4  In the "Bubble sent" state, if a bubble is received, then: the
         6a44-client IPv6 address is set to the received 6a44-client
         IPv6 prefix followed by the host local IPv4 address; the
         "Bubble received" state is entered with the timer set to T2
         whose recommended value is 30 seconds minus 4 times T1.

   TM-5  In the "Bubble sent" state, if timer T1 expires AND the Attempt
         count is equal to 4, then: the "No 6a44 relay" state is entered
         with the timer set to T3 whose recommended value is 30 minutes.

   TM-6  In the "Bubble sent" state, OR the "Bubble received" state, OR
         the "No 6a44 relay" state, if a IPv6 native address is obtained
         by some other mean, OR if the private IPv4 address of the host
         is no longer valid, then: the timer is disarmed; the "6a44
         disabled" state is entered.

   TM-7  In the "Bubble received" state, if the timer T2 expires, then:
         the Attempt count is reset to 1; a new Bubble ID is randomly
         chosen; a bubble is sent with this bubble ID; the "Bubble sent"
         state is entered with the timer set to T1.

   TM-8  In the "Bubble received" state, if a bubble is received, then:
         the timer is reset to T2.  (NOTE: Since a bubble is received by
         a 6a44 client either in response to a bubble it has sent or in
         reaction to a packet it has sent with inconsistent IPv6 and



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         UDP/IPv4 source addresses, receiving a bubble is a sign that
         the tunnel mapping reported in the received bubble prefix has
         recently been used in BOTH directions, a condition required by
         some NAT44s to maintain their mappings).

   TM-9  In the "no 6a44 relay" state, if the timer expires, then: the
         Attempt count is reset to 1; a new Bubble ID is randomly
         chosen; a bubble is sent with this bubble ID; the "Bubble sent"
         state is entered with the timer set to T1.

6.5.2.  Client Transmission

   An 6a44 client transmits packets according to the following CT-x
   rules.  In figures which illustrate these rules, symbols of Section 5
   are re-used; packets are represented as a succession of significant
   fields separated by commas, with sources preceding destinations as
   usual; != means different from.

   CT-1  BUBBLE SENT BY A 6a44 CLIENT


                  (IPv4, A, B, UDP[::/96, <current Bubble ID>])
                                            |
                    +-------+--------+      |
                    |       |  6a44  |      |
                    |       | client +------>---------- >B:W
                    |       |function|A:W<     UDP/IPv4
                    +-------+--------+
                           Host

         Bubbles are transmitted from time to time.  Conditions of their
         transmission are specified specified in Section 6.5.1, and
         their format is specified in Section 6.3.


















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   CT-2  IPv6/IPv4 PACKET SENT TO A HOST OF THE SAME SITE


            [IPv6, <D.N.Z.A>, <D.N..A2>,...]
                  |
                  | (IPv4, A, A2, IP-in-IP[encapsulated packet])
                  |                  |
             +----|--+--------+      |
             |    |  |  6a44  |      |
             |  -->--+ client +------>------ >A2
             |  IPv6 |function|<A       IPv4
             +-------+--------+
                     Host

         If an IPv6 packet is submitted for transmission with ALL the
         following conditions satisfied, the 6a44 client MUST
         encapsulate the IPv6 packet in an IPv4 packet whose protocol is
         set to IP in IP (protocol = 41), and whose IPv4 destination is
         copied from the last 32 bits of the IPv6 destination: (1) the
         IPv6 source address is the 6a44-client IPv6 address; (2) the
         IPv6 destination is a 6a44 address of the same site (it has the
         same 80 bits as the 6a44-client IPv6 address); (3) either the
         IPv6 packet does not exceed 1280 octets, or it is longer but it
         does not exceed the IPv4 link MTU minus 20 octets and the IPv4
         destination address starts with the IPv4 link prefix.

   CT-3  IPv6/UDP/IPv4 PACKET TO A HOST OF ANOTHER SITE


           [IPv6, <D.N.Z.A>, X != <D.N...>, ...]
                 |
                 | (IPv4, B, A, UDP(W, W, [encapsulated packet])
                 |                  |
            +----|--+--------+      |
            |    |  |  6a44  |      |
            |  -->--+ client +------>---------- >B:W
            |  IPv6 |function|A:W<     UDP/IPv4
            +-------+--------+
                    Host

         If an IPv6 packet is submitted for transmission and ALL the
         following conditions are satisfied, the IPv6 packet MUST be
         encapsulated in a UDP/IPv4 packet whose destination is the
         6a44-relay anycast address, and whose source and destination
         ports are both the 6a44 port: (1) the source address is the
         local 6a44-client IPV6 address; (2) The destination is not a
         6a44 address of the same site (its first 80 bits differ from
         those of the 6a44-client IPv6 address); (3) The IPv6 packet



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         does not exceed 1280 octets.

   CT-4  IPv6 PACKET THAT DOESN'T CONCERN 6a44

         If an IPv6 packet is submitted to the 6a44 client function for
         transmission with an IPv6 source address that is not the 6a44-
         client IPv6 address, the packet does not concern 6a44.  It MUST
         be left for any other IPv6 transmission function that may apply
         (the source address can be a link-local address or a ULA of
         [RFC4193]).

6.5.3.  Client Reception

   Upon reception of an IPv4 packet, a 6a44 client applies the following
   CR-x rules:

   CR-1  BUBBLE RECEIVED FROM A 6a44 RELAY


                (IPv4, B, A, UDP(w, w, [<D.N.Z>, <current bubble ID>])
                                    |
            +-------+--------+      |
            |       |  6a44  |      |
            |       | client +------<---------- <B:W
            |       |        |A:W<     UDP/IPv4
            +-------+--------+
                   Host
              (updates D.N.Z)

         If ALL the the following conditions are satisfied (i.e. the
         packet is a 6a44 bubble from a 6a44 relay), the 6a44-client
         IPv6 address MUST be updated using the received IPv6 prefix
         D.N.Z: (1) the IPv4 packet contains a complete UDP datagram
         (protocol = 17, offset = 0, more-fragment bit = 0); (2) Both
         ports of the UDP datagram are the 6a44 port, and the payload
         length is enough to contain a 6a44-client IPv6 prefix and a
         Bubble ID but shorter than an IPv6-packet header(protocol = 17,
         UDP payload length = at least 20 octets and less than 40
         octets); the received Bubble ID matches the current value of
         the Bubble-ID local variable.











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   CR-2  IPv6/IPv6 PACKET FROM A HOST OF THE SAME SITE

             (IPv4, A2, A, IP-in-IP, [IPv6, <D.N..A2>, <D.N.Z.A>, ...])
                                  |
         [decapsulated packet]    |
               |                  |
          +----|--+--------+      |
          |    |  |  6a44  |      |
          |  --<--+ client +------<------ <A2
          |  IPv6 |        |A<       IPv4
          +-------+--------+
                  Host

         If ALL the following conditions are satisfied (i.e. the packet
         comes from a 6a44 client of the same site), the 6a44 client
         MUST decapsulate the inner packet and treat it as a received
         IPv6 packet: (1) the IPv4 packet contains a complete UDP
         datagram (protocol = 17, offset = 0, more-fragment bit = 0);
         (2) both ports of the UDP datagram are the 6a44 port, and the
         UDP payload is an IPv6 packet (UDP length of at least 40
         octets, version = 6); (3) the IPv6 source address is one of the
         same site (the first 80 bits match those of the 6a44-client
         IPv6 address; (4) its last 32 bits are equal to the IPv4 source
         address; (5) the IPv6 destination address is the 6a44-client
         IPv6 address.

   CR-3  IPv6/UDP/IPv4 PACKET FROM A HOST OF ANOTHER SITE


                    (IPv4, B, A, UDP(W, W, [IPv6, X, <D.N.Z.A>,...])
                                       |
              [decapsulated packet]    |
                    |                  |
               +----|--+--------+      |
               |    |  |  6a44  |      |
               |  --<--+ client +------<---------- <B:W
               |  IPv6 |        |A:W<     UDP/IPv4
               +-------+--------+
                       Host

         If ALL the following conditions are satisfied (i.e. the packet
         has been relayed by a 6a44 relay), the 6a44 client MUST
         decapsulate the inner packet and treat it as a received IPv6
         packet: (1) the IPv4 packet contains a complete UDP datagram
         (protocol = 17, offset = 0, more-fragment bit = 0); (2) the UDP
         payload is an IPv6 packet (length of at least 40 octets,
         version = 6); (3) the UDP/IPv4 source address is the 6a44-relay
         UDP/IPv4 address; (4) the IPv6 destination address is the 6a44-



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         client IPv6 address.

   CR-4  RECEIVED IPv4 PACKET OTHER THAN 6a44

         If ANY of the following conditions is verified, the received
         IPv4 packet does not concern 6a44 and MUST therefore be left
         for any other IPv4 reception function that may apply: (1) the
         IPv4 payload is neither UDP nor IPv6 (protocol = neither 17 nor
         41, or protocol = 41 and IP version in the payload is not = 6);
         (2) the IPv4 packet is an IP-datagram fragment other than the
         first one (offset > 0); (3) the IPv4 packet contains the first
         or unique fragment of a UDP datagram (protocol = 17, offset =
         0), with neither port equal to the 6a44 port.

6.6.  6a44 Relay Specification

6.6.1.  Relay Reception in IPv6

   Upon reception of a packet via its IPv6 interface with a destination
   address starting with the 6a44-network IPv6 prefix, a 6a44 relay MUST
   apply the following RR6-x rules:

   RR6-1  VALID IPv6 PACKET FROM OUTSIDE THE 6a44 ISP NETWORK


    [IPv6, (X != <D...> AND != <Teredo(IPv4=B)>), <D.<N != B>.Z...>,...]
                                      |
    (IPv4, B, N, UDP(W, Z,            |
          [encapsulated packet]))     |
            |                         |
            |        +--------+       |
            |   >B:W |  6a44  |D/48<  |
    N:Z< ---<--------| relay  |-------<---- D.N.Z...<
         IPv4        |        |        IPv6
                     +--------+


          If ALL the following conditions are satisfied, the IPv6 packet
          MUST be encapsulated in an UDP/IPv4 packet whose UDP/IPv4
          destination is copied from bits 48 to 95 of the IPv6
          destination address: (1) the IPv6 source address is not that
          of a 6a44 client of the ISP (it does not start with the 6a44-
          network IPv6 prefix); (2) the IPv6 source address is not a
          Teredo address whose embedded UDP/IPv4 address is the 6a44-
          relay anycast address; (3) the customer-site IPv4 address
          embedded in the 6a44 destination address is not the 6a44-relay
          anycast address; (4) the packet has at most 1280 octets.




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   RR6-2  INVALID IPv6 PACKET FROM OUTSIDE THE 6a44 ISP NETWORK

          If ANY of the following conditions is satisfied, the IPv6
          packet MUST be discarded : (1) the packet has more than 1280
          octets (in this case, an ICMP Packet Too Big error message
          MUST be returned to the source); (2) the customer-site IPv4
          address embedded in the IPv6 destination address is the 6a44-
          relay anycast address; (3) the IPv6 source address is a Teredo
          address whose embedded IPv4 address is the 6a44-relay anycast
          address.

6.6.2.  Relay Reception in IPv4

   Upon reception via its IPv4 downstream interface of an IPv4 packet
   that contains a complete IP datagram (fragment offset = 0 and more-
   fragment bit = 0), and that contain a UDP datagram whose UDP/IPv4
   destination is the 6a44-relay UDP/IPv4 address, a 6a44 relay MUST
   apply the following rules:

   RR4-1  BUBBLE FROM 6a44 CLIENT


               (IPv4, N, B, UDP(Z, W, [::/96, bubble ID]))
                                     |
                              IPv4   |    +--------+
                         >B:W ------->----|        |
                                      >B:W|  6a44  |
                                          |  relay |
                         N:Z< -------<----|        |
                              IPv4   |    +--------+
                                     |
                                     |
                (IPv4, B, N, UDP(B, W, [<D.N.Z>, bubble ID]))

          If the following condition is satisfied, the 6a 44 relay MUST
          return to the source a bubble derived from the received one by
          permuting its UDP/IPv4 source and destination, and by putting
          in its 6a44-client-IPv6-prefix field the received UDP/IPv4
          source address: the UDP payload is a bubble, i.e has at least
          20 octets and less than 40 octets











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   RR4-2  IPv6 PACKET FROM A 6a44 CLIENT TO ANOTHER 6a44 CLIENT

     (IPv4, N1, B, UDP(Z1, W, [IPv6, <D.N1.Z1...>, <D.N2.Z2...>, ...]))
                   |
            IPv4   |    +--------+
      >B:W  ------->----|        |
                    >B:W|  6a44  |
                        | relay  |
                        |        |
     N2.Z2< -------<----|        |
            IPv4   |    +--------+
                   |        6a44 Relay
                   |
     (IPv4, B, N2, UDP(B, Z2, [encapsulated packet]))

          If ALL the following conditions are satisfied, the 6a44 relay
          MUST return back via its downstream IPv4 interface an IPv6/
          UDP/IPv4 packet containing the same encapsulated packet,
          having its UDP/IPv4 destination set to the UDP/IPv4 address
          found in the 6a44 destination address, and having its UDP/IPv4
          source set to the 6a44-relay UDP/IPv4 address: (1) the IPv4
          packet contains a complete UDP datagram (protocol = 17, offset
          = 0, more-fragment bit = 0); (2) the UDP payload is an IPv6
          packet (length of at least 40 octets, version = 6); (3) the
          IPv6 source address starts with the 6a44-network IPv6 prefix
          followed by the UDP/IPv4 source address of the received
          packet; (3) the IPv6 destination address starts with the 6a44-
          network IPv6 prefix.

   RR4-3  IPv6 PACKET FROM A 6a44 CLIENT TO A NON-6a44-CLIENT

        (IPv4, N, B, UDP(Z, W, [IPv6, <D.N.Z...>,
                  |    (X != <D...> AND != <Teredo(IPv4=B)), ...]))
                  |
                  |                      [decapsulated packet]
                  |                           |
                  |          +--------+       |
                  |   B:W/48>|  6a44  |<D/48  |
         >B:W  --->----------| relay  |------->---- >
               IPv4          |        |        IPv6
                             +--------+

          If ALL the following conditions are satisfied, the 6a44 relay
          MUST decapsulate the IPv6 packet and forward it via the IPv6
          interface: (1) the IPv4 packet contains a complete UDP
          datagram (protocol = 17, offset = 0, more-fragment bit = 0);
          (2) the UDP payload is an IPv6 packet (length of at least 40
          octets, version = 6); (3) the IPv6 source address starts with



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          the 6a44-network IPv6 prefix, followed by the UDP/IPv4 source
          address of the received packet; (4) the IPv6 destination
          address does not start with the 6a44-network IPv6 prefix and
          is not a Teredo address whose embedded IPv4 address is the
          6a44-relay anycast address.

   RR4-4  INVALID IPv6/UDP/IPv4 PACKET

          If ANY other case, the 6a44 relay MUST discard the packet.

6.7.  Implementation of Automatic Sunset

   6a44 is designed as an interim transition mechanism, not to be used
   any longer than strictly necessary.  Its sole purpose is to
   accelerate availability of IPv6 native addresses where, for any
   reason, CPE's cannot quickly be replaced, or where, for any reason,
   ISP networks cannot quickly support dual-stack routing or 6rd.

   A 6a44-capable ISP can first have an increase of its 6a44 traffic, as
   more and more hosts behind IPv4-only CPEs support the 6a44 client
   function.  But it should later have a decrease of this traffic as
   more and more CPE's operate in dual stack.

   When this traffic becomes sufficiently negligible, it may, after due
   prior notice, discontinue 6a44-relay operation.  This terminates its
   sunset procedure.

   In a host that obtains a IPv6 native address by some other mean than
   6a44, the effect of having the 6a44 function in its protocol stack is
   inexistent.  OS providers may therefore keep this function in their
   code for many years.  When it becomes clear that the number of users
   of this unction has become negligible they can delete it from later
   releases.  This terminates their sunset procedure.


7.  Security Considerations

   Incoming reachability:

      Hosts that acquire 6a44 addresses become reachable from the
      Internet in IPv6 while they remain unreachable in IPv4 at their
      private IPv4 addresses.

      For ordinary use, this should not introduce a perceptible new
      security risk for two reasons: (1) hosts can, without IPv6, use
      NAT44 hole-punching techniques such as ICE of [RFC5245]) to
      receive incoming connections; (2) modern operating systems that
      support IPv6 have by default their own protections against



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      incoming connections.

      If nevertheless 6a44 reachability across an ordinary NAT44 has to
      be barred, this can be done by configuring its port-forwarding
      function with the 6a44 port bound to any internal address that is
      not assigned to any host.  Thus, no bubble from a 6a44 relay can
      reach any 6a44-capable host, and this is sufficient to prevent
      hosts from using 6a44.

      For more sophisticated uses with managed firewalls, default
      configuration are in general such that packets that are not
      explicitly authorized are discarded.  Thus, 6a44 can be used only
      if the 6a44 port is consciously opened to incoming traffic.

   Subscriber authentication:

      Any authentication that applies to an IPv4 address extends its
      effect to 6a44 addresses that are derived from it.

   Host-address spoofing:

      With ingress filtering required in 6a44 ISP networks, and with
      address checks of Section 6, no new IPv6 address-spoofing
      vulnerability is introduced by 6a44.

   Address-and-port scanning:

      To mitigate the (limited) risk of a malicious user trying to scan
      address-and-port IPv4 couples to reach a host, Teredo addresses
      contain 12 random bits [RFC5991]. 6a44 addresses have no random
      bits but contain local IPv4 addresses of clients.  Since possible
      values of these addresses are not deterministically known from
      outside customer sites, and are in ranges that can be configured
      in typical NAT44s, some protection against address and port
      scanning is thus achieved.  This protection may be less effective
      than that achieved with random bits, but is in any case better for
      6a44 IPv6 addresses than for IPv4 addresses alone.

   Denial-of-service:

      Provided 6a44 relays are provisioned with enough processing power,
      which is facilitated by their being completely stateless, 6a44
      introduces no denial of service vulnerabilities of its own.

   Routing-loops:

      A risk of routing-loop attacks has been identified in
      [draft-ietf-v6ops-tunnel-loops].  Without precaution, it applies



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      to some combinations of automatic-tunnel mechanisms such as 6to4,
      ISATAP, 6rd and Teredo.  This risk does not exist with 6a44 for
      the following reasons:

      1.  When an packet enters a 6a44 relay via its IPv6 interface:

          +  An IPv6/UDP/IPv4 packet cannot be sent to another 6a44
             relay because its IPv4 destination would have to be 6a44-
             relay IPv4 address.  This is prevented by rule RR6-1 of
             Section 6.6.1.

          +  If an IPv6/UDP/IPv4 packet is sent to the address of a 6to4
             relay, 6rd relay, or ISATAP relay, it will be discarded
             there because these relays don't accept UDP/IPv4 packets.

          +  If an IPv6/UDP/IPv4 packet is sent to a Teredo relay, it
             will be discarded there because: (1) Teredo relays check
             that the IPv4 addresses that is embedded in the IPv6 source
             address of a received IPv6/IPv4 packet does match the IPv4
             source address of the encapsulating packet (section 5.4.2
             of [RFC4380]); (2) encapsulating packets sent by 6a44
             relays have the 6a44-relay anycast address as IPv4 source
             address; (3) a 6a44 relay forwards a received IPv6 packet
             as an IPv6/UDP/IPv4 packets only if its IPv6 source address
             is not a Teredo address whose embedded IPv4 address is the
             6a44-relay IPv4 address.

      2.  When a packet enters a 6a44 relay via its IPv4 interface:

          +  The received packet cannot come from another 6a44 relay (as
             just explained, 6rd relays do not send IPv6/UDP/IPv4
             packets to other 6a44-relays).

          +  If the IPv4 packet comes a 6to4 relay, a 6rd relay, or an
             ISATAP relay, its IPv6 encapsulated packet cannot be
             forwarded (the received packet is IPv6/IPv4 instead of
             being IPv6/UDP/IPv4, as required by rules RR4-2 and RR4-3
             of Section 6.6.2).

          +  If the received packet is an IPv6/UDP/IPv4 packet coming
             from a Teredo relay, this packet cannot have been sent to
             the Teredo relay by a 6a44 relay ((1) in order to reach the
             6a44 relay, the IPv6 destination of the IPv6 encapsulated
             packet must be a Teredo address whose embedded IPv4 address
             is the 6a44-relay anycast address (section 5.4.1 of
             [RFC4380]); (2) a 6a44 relay does not forward via its IPv6
             interface an IPv6 packet whose destination is a Teredo
             address whose embedded IPv4 address is the 6a44-relay



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             anycast address (rule RR4-3 of Section 6.6.2).

   6a44-relay spoofing:

      In a 6a44 network, no node can spoof a 6a44 relay because ingress
      filtering prevents any 6a44-relay anycast address to be spoofed.

      In a network that does not support ingress filtering (and
      therefore is not a 6a44 network):

      *  6a44 packets sent by 6a44-capable hosts are discarded in the
         IPv4 backbone because their IPv4 destination, the 6a44-relay
         anycast address, does not start with any ISP assigned prefix.

      *  If an attacker tries to send to a 6a44-capable host a faked
         relay-to-client bubble, the probability that it would be
         accepted by its destination is negligible.  It would require
         that all the following conditions be simultaneously satisfied:

         +  The UDP/IPv4 destination set by the attacker must reach a
            NAT44 node in which it is the external mapping of a 6a44
            tunnel established by a 6a44-capable host.

         +  This host must be in the "Bubble sent" state, the only one
            in which it listens to bubbles when its ISP is not 6a44
            capable.  This state is taken only for a few seconds every
            30 minutes (rule TM-5 of Section 6.5.1).

         +  This host accepts the bubble only if its bubble ID has the
            right value, an extremely unlikely possibility with a 64-
            bits randomly chosen Bubble ID (see Section 6.3).

         If, despite its negligible probability, a host would accept a
         faked bubble, the effect would be that a 6a44-capable host
         believes, for about 30 seconds, that it has an IPv6 address.
         All IPv6 packets it would then send with this address as source
         would not reach any destination (no relay will receive them,
         and and no host of the same site will accept them).  The
         consequence would therefore not impair security.


8.  IANA considerations

   IANA is solicited:

      to assign 192.88.99.2 as the 6a44 IPv4 anycast address;





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      to assign a well known UDP port as the 6a44 well known port.
      Proposed value is the currently unused 1009.

   The choice of 192.88.99.2 as 6a44 IPv4 anycast address doesn't
   conflict with any existing IETF specification because:

   o  It starts with the 6to4 prefix 192.88.99.0/24 of [RFC3068].

   o  It differs from the only currently assigned address that starts
      with this prefix (the anycast address of 6to4 relays 192.88.99.1
      of [RFC3068]).

   This choice is made to permit implementations of 6a44 relays both in
   physical nodes that are independent from any 6to4 relay or, if found
   more optimum, in nodes in which 6to4 relays and 6a44 relays are
   collocated.


9.  Acknowledgments

   This specification, whose origin is a convergence effort based on two
   independent proposals, [6rd+] and [SAMPLE], has benefited from
   various suggestions.  Comments have been received during this
   process, in particular from Dave Thaler, Fred Templin, Ole Troan,
   Olivier Vautrin, Pascal Thubert, Washam Fan, and Yu Lee. Authors wish
   to thank them, and all others, for their useful contributions.
   Special recognition is due to Dave Thaler whose detailed review led
   to a few useful modifications.


10.  References

10.1.  Normative References

   [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.

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

10.2.  Informative References

   [6rd+]     Despres, R., "Rapid Deployment of Native IPv6 Behind IPv4
              NATs (6rd+) - draft-despres-softwire-6rdplus-00",
              July 2010.



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   [NAT444]   Yamaguchi, J., Shirasaki, Y., Miyakawa, S., Nakagawa, A.,
              and H. Ashida, "NAT444 addressing models -
              draft-shirasaki-nat444-isp-shared-addr-03 - Work in
              progress", March 2010.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

   [RFC3053]  Durand, A., Fasano, P., Guardini, I., and D. Lento, "IPv6
              Tunnel Broker", RFC 3053, January 2001.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3068]  Huitema, C., "An Anycast Prefix for 6to4 Relay Routers",
              RFC 3068, June 2001.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, March 2004.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              February 2006.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

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

   [RFC5626]  Jennings, C., Mahy, R., and F. Audet, "Managing Client-
              Initiated Connections in the Session Initiation Protocol
              (SIP)", RFC 5626, October 2009.

   [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd) -- Protocol Specification",
              RFC 5969, August 2010.




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   [RFC5991]  Thaler, D., Krishnan, S., and J. Hoagland, "Teredo
              Security Updates", RFC 5991, September 2010.

   [RFC6081]  Thaler, D., "Teredo Extensions", RFC 6081, January 2011.

   [SAMPLE]   Carpenter, B. and S. Jiang, "Legacy NAT Traversal for
              IPv6: Simple Address Mapping for Premises - Legacy
              Equipment (SAMPLE) - draft-carpenter-softwire-sample-00",
              July 2010.

   [The Tool]
              Saint-Exupery, A de., "Wind, sand and Stars, Chap. III -
              The tool", 1939.

   [draft-ietf-v6ops-tunnel-loops]
              Nakibly, G. and F. Templin, "Routing Loop Attack using
              IPv6 Automatic Tunnels: Problem Statement and Proposed
              Mitigations (Work in progress)".


Authors' Addresses

   Remi Despres (editor)
   RD-IPtech
   3 rue du President Wilson
   Levallois,
   France

   Email: despres.remi@laposte.net


   Brian Carpenter
   University of Auckland
   Department of Computer Science
   PB 92019
   Auckland,   1142
   New Zealand

   Email: brian.e.carpenter@gmail.com












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   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134
   USA

   Email: dwing@cisco.com


   Sheng Jiang
   Huawei Technologies Co., Ltd
   Q14, Huawei Campus - No.156 Beiqing Road
   SHai-Dian District, Beijing,   100095
   P.R. China

   Email: shengjiang@huawei.com



































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