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Versions: (draft-baker-behave-v4v6-translation) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 RFC 6145

behave                                                             X. Li
Internet-Draft                                                    C. Bao
Obsoletes: 2765 (if approved)          CERNET Center/Tsinghua University
Intended status: Standards Track                                F. Baker
Expires: June 20, 2010                                     Cisco Systems
                                                       December 17, 2009


                     IP/ICMP Translation Algorithm
                    draft-ietf-behave-v6v4-xlate-05

Abstract

   This document specifies an update to the Stateless IP/ICMP
   Translation Algorithm (SIIT) described in RFC 2765.  The algorithm
   translates between IPv4 and IPv6 packet headers (including ICMP
   headers).

   This specification addresses both a stateless and a stateful mode.
   In the stateless mode, translation information is carried in the
   address itself, permitting both IPv4->IPv6 and IPv6->IPv4 session
   establishment without maintaining state in the IP/ICMP translator.
   In the stateful mode, translation state is maintained between IPv4
   address/transport port tuples and IPv6 address/transport port tuples,
   enabling IPv6 systems to open sessions with IPv4 systems.  The choice
   of operational mode is made by the operator deploying the network and
   is critical to the operation of the applications using it.

Acknowledgement of previous work

   This document is a product of the 2008-2009 effort to define a
   replacement for NAT-PT.  It is an update to and directly derivative
   from Erik Nordmark's [RFC2765], which similarly provides both
   stateless and stateful translation between IPv4 [RFC0791] and IPv6
   [RFC2460], and between ICMPv4 [RFC0792] and ICMPv6 [RFC4443].  The
   original document was a product of the NGTRANS working group.

   The changes in this document reflect five components:

   1.  Redescribing the network model to map to present and projected
       usage [I-D.ietf-behave-v6v4-framework].

   2.  Moving the address format to the address format document
       [I-D.ietf-behave-address-format], to coordinate with other drafts
       on the topic.

   3.  Describing both stateful and stateless operation.




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   4.  Some changes in ICMP.

   5.  Updating references.

Requirements

   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].

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on June 20, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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




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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.


Table of Contents

   1.  Introduction and Motivation  . . . . . . . . . . . . . . . . .  4
     1.1.  Translation Model  . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Applicability and Limitations  . . . . . . . . . . . . . .  5
     1.3.  Stateless vs. Stateful Mode  . . . . . . . . . . . . . . .  6
     1.4.  Path MTU discovery and fragmentation . . . . . . . . . . .  6
   2.  Translating from IPv4 to IPv6  . . . . . . . . . . . . . . . .  7
     2.1.  Translating IPv4 Headers into IPv6 Headers . . . . . . . .  8
     2.2.  Translating UDP over IPv4  . . . . . . . . . . . . . . . . 10
     2.3.  Translating ICMPv4 Headers into ICMPv6 Headers . . . . . . 11
     2.4.  Translating ICMPv4 Error Messages into ICMPv6  . . . . . . 14
     2.5.  Translator sending ICMP error message  . . . . . . . . . . 14
     2.6.  Transport-layer Header Translation . . . . . . . . . . . . 14
     2.7.  Knowing when to Translate  . . . . . . . . . . . . . . . . 15
   3.  Translating from IPv6 to IPv4  . . . . . . . . . . . . . . . . 15
     3.1.  Translating IPv6 Headers into IPv4 Headers . . . . . . . . 16
     3.2.  Translating ICMPv6 Headers into ICMPv4 Headers . . . . . . 18
     3.3.  Translating ICMPv6 Error Messages into ICMPv4  . . . . . . 20
     3.4.  Translator sending ICMPv6 error message  . . . . . . . . . 21
     3.5.  Transport-layer Header Translation . . . . . . . . . . . . 21
     3.6.  Knowing when to Translate  . . . . . . . . . . . . . . . . 21
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   7.  Appendix A: Translating pointer in Parameter Problem . . . . . 23
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 24
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26








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1.  Introduction and Motivation

   An understanding of the framework presented in
   [I-D.ietf-behave-v6v4-framework] is presumed in this document.

   The transition mechanisms specified in [RFC4213] handle the case of
   dual IPv4/IPv6 hosts interoperating with both dual IPv4/IPv6 hosts
   and IPv4-only hosts, which is needed early in the transition to IPv6.
   The dual IPv4/IPv6 hosts are assigned both an IPv4 and one or more
   IPv6 addresses.  The number of available globally unique IPv4
   addresses is becoming smaller and smaller as the Internet grows; we
   expect that there will be a desire to take advantage of the large
   IPv6 address space and not require that every new Internet node have
   a permanently assigned IPv4 address.

   SIIT [RFC2765] is designed for the case of small networks (e.g., a
   single subnet) and for a site that has IPv6-only hosts in a dual
   IPv4/IPv6 network.  This use assumes a mechanism for IPv6 nodes to
   acquire a temporary address from the pool of IPv4 addresses.
   However, SIIT is not useful in the case when the IPv6 nodes need to
   acquire temporary IPv4 addresses from a "distant" SIIT box operated
   by a different administration, or require that the IPv6 Internet
   contain routes for IPv4-mapped addresses (The latter is known to be a
   very bad idea due to the size of the IPv4 routing table that would
   potentially be injected into IPv6 routing in the form of IPv4-mapped
   addresses.)

   In addition, due to the IPv4 address depletion problem, it is
   desirable that a single IPv4 address needs to be shared via transport
   port multiplexing for different IPv6 nodes when they communicate with
   other IPv4 hosts.

   Furthermore, in SIIT [RFC2765], an IPv6-only node that works through
   SIIT translators needs some modifications beyond a normal IPv6-only
   node.  These modifications are not strictly implied in this document,
   since normal IPv6 addresses can be used in the IPv6 end nodes.

   A detailed discussion of translation scenarios is presented in
   [I-D.ietf-behave-v6v4-framework], while the technical specification
   of the translation algorithm itself is covered in this document.

1.1.  Translation Model

   This document specifies the translation algorithm that is one of the
   components described in [I-D.ietf-behave-v6v4-framework] needed to
   make IPv6-only nodes interoperate with IPv4-only nodes as shown in
   Figure 1.




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                --------          --------
              //  IPv4  \\      //  IPv6  \\
             /   Domain   \    /   Domain   \
            /             +----+      +--+   \
           |              |XLAT|      |S2|    |  Sn: Servers
           | +--+         +----+      +--+    |  Hn: Clients
           | |S1|         +----+              |
           | +--+         |DNS |      +--+    |  XLAT: V4/V6 Translator
            \     +--+    +----+      |H2|   /   DNS:  DNS Server
             \    |H1|    /    \      +--+  /
              \\  +--+  //      \\        //
                --------          --------

                        Figure 1: Translation Model

   The translation model consists of two or more network domains
   connected by one or more IP/ICMP translators.  One of those networks
   either routes IPv4 but not IPv6, or contains some hosts that only
   implement IPv4 or have IPv4 only applications (even if the host and
   the network support IPv6).  The other network either routes IPv6 but
   not IPv4, or contains some hosts that only implement IPv6 or has IPv6
   only applications.  Both networks contain clients, servers, and
   peers.

1.2.  Applicability and Limitations

   The use of this translation algorithm assumes that the IPv6 network
   is somehow well-connected i.e., when an IPv6 node wants to
   communicate with another IPv6 node there is an IPv6 path between
   them.  Various tunneling schemes exist that can provide such a path,
   but those mechanisms and their use is outside the scope of this
   document and [RFC2765].

   The translation algorithm can be used not only in a subnet, but can
   also be used in service provider's backbone network.

   The translating function specified in this document does not
   translate any IPv4 options and it does not translate IPv6 routing
   headers, hop-by-hop extension headers, destination options headers or
   source routing headers, same as in [RFC2765].

   The issues and algorithms in the translation of datagram containing
   TCP segments are described in [RFC5382].  The considerations of that
   document are applicable in this case as well.

   Fragmented IPv4 UDP packets that do not contain a UDP checksum (i.e.
   the UDP checksum field is zero) are not of significant use in the
   Internet and will not be translated by the IP/ICMP translator



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   [Miller][Dongjin].

   IPv4 multicast addresses [RFC3171] cannot be mapped to IPv6 multicast
   addresses [RFC3307] based on the unicast mapping rule.  However, a
   special rule for address translation can be created for the multicast
   packet translation algorithm; if that is done, the IP/ICMP header
   translation aspect of this memo works.

1.3.  Stateless vs. Stateful Mode

   The IP/ICMP translator has two possible modes of operation: stateless
   and stateful [I-D.ietf-behave-v6v4-framework].  In both cases, we
   assume that a system that has an IPv4 address but not an IPv6 address
   is communicating with a system that has an IPv6 address but no IPv4
   address, or that the two systems do not have contiguous routing
   connectivity and hence are forced to have their communications
   translated.

   In the stateless mode, a specific IPv6 address range will represent
   IPv4 systems (IPv4-converted addresses), and the IPv6 systems have
   addresses that can be algorithmatically mapped to a subset of the
   service provider's IPv4 addresses (IPv4-translatable addresses).  In
   this mode, there is no need to concern oneself with port translation
   or translation tables, as the IPv4 and IPv6 counterparts are
   algorithmically related.  An implementation example of the stateless
   translation is summarized in [I-D.xli-behave-ivi].

   In the stateful mode, a specific IPv6 address range will represent
   IPv4 systems (IPv4-converted addresses), but the IPv6 systems may use
   any [RFC4291] addresses except in that range.  In this case, a
   translation table is required to bind the IPv6 systems' addresses to
   the IPv4 addresses maintained in the translator.

   The address translation mechanisms for the stateless and the stateful
   translations are defined in [I-D.ietf-behave-address-format].

1.4.  Path MTU discovery and fragmentation

   Due to the different sizes of the IPv4 and IPv6 header, which are 20+
   octets and 40+ octets respectively, handling the maximum packet size
   is critical for the operation of the IPv4/IPv6 translator.  There are
   three mechanisms to handle this issue: the path MTU discovery,
   fragmentation and transport layer negotiation such as the TCP MSS
   option.  Note that the translator MUST behave as a router, i.e. the
   translator MUST send packet too big error message when the packet
   size exceeds the MTU of the next hop interface.

   "Don't Fragment", ICMP "too big", and packet fragmentation are



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   discussed in "Translating from IPv4 to IPv6" and "Translating from
   IPv6 to IPv4" sections of this document.  The reassembling of the
   fragmented packets in the stateful translator is discussed in
   [I-D.ietf-behave-v6v4-xlate-stateful], since it requires the state
   maintenance in the translator.


2.  Translating from IPv4 to IPv6

   When an IP/ICMP translator receives an IPv4 datagram addressed to a
   destination towards the IPv6 domain, it translates the IPv4 header of
   that packet into an IPv6 header.  Since the ICMP [RFC0792][RFC4443],
   TCP [RFC0793] and UDP [RFC0768] headers contain checksums that cover
   IP header information, if the address mapping algorithm is not
   checksum-neutral, the ICMP and transport-layer headers MUST be
   updated.  The data portion of the packet is left unchanged.  The IP/
   ICMP translator then forwards the packet based on the IPv6
   destination address.  The original IPv4 header on the packet is
   removed and replaced by an IPv6 header.

              +-------------+                 +-------------+
              |    IPv4     |                 |    IPv6     |
              |   Header    |                 |   Header    |
              +-------------+                 +-------------+
              |  Transport  |                 |  Fragment   |
              |   Layer     |      ===>       |   Header    |
              |   Header    |                 |(not always) |
              +-------------+                 +-------------+
              |             |                 |  Transport  |
              ~    Data     ~                 |   Layer     |
              |             |                 |   Header    |
              +-------------+                 +-------------+
                                              |             |
                                              ~    Data     ~
                                              |             |
                                              +-------------+

                    Figure 2: IPv4-to-IPv6 Translation

   One of the differences between IPv4 and IPv6, is that in IPv6 path
   MTU discovery is mandatory but it is optional in IPv4.  This implies
   that IPv6 routers will never fragment a packet - only the sender can
   do fragmentation.

   When the IPv4 node performs path MTU discovery (by setting the DF bit
   in the header) the path MTU discovery can operate end-to-end, i.e.,
   across the translator.  In this case either IPv4 or IPv6 routers
   (including the translator) might send back ICMP "packet too big"



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   messages to the sender.  When the IPv6 routers send these ICMP errors
   they will pass through a translator that will translate the ICMP
   error to a form that the IPv4 sender can understand.  In this case,
   an IPv6 fragment header is only included if the IPv4 packet is
   already fragmented.

   However, when the IPv4 sender does not set the DF bit, the translator
   has to ensure that the packet does not exceed the path MTU on the
   IPv6 side.  This is done by fragmenting the IPv4 packet so that it
   fits in 1280 byte IPv6 packets, since that is the minimum IPv6 packet
   size.  Also, when the IPv4 sender does not set the DF bit the
   translator MUST always include an IPv6 fragment header to indicate
   that the sender allows fragmentation.  That is needed should the
   packet pass through an IP/ICMP translator.

   The above rules ensure that when packets are fragmented, either by
   the sender or by IPv4 routers, the low-order 16 bits of the fragment
   identification are carried end-to-end, ensuring that packets are
   correctly reassembled.  In addition, the rules use the presence of an
   IPv6 fragment header to indicate that the sender might not be using
   path MTU discovery, i.e., the packet should not have the DF flag set
   should it later be translated back to IPv4.

   Other than the special rules for handling fragments and path MTU
   discovery, the actual translation of the packet header consists of a
   simple mapping as defined below.  Note that ICMP packets require
   special handling in order to translate the content of ICMP error
   message and also to add the ICMP pseudo-header checksum.

2.1.  Translating IPv4 Headers into IPv6 Headers

   If the DF flag is not set and the IPv4 packet will result in an IPv6
   packet larger than 1280 bytes the IPv4 packet MUST be fragmented
   prior to translating it.  Since IPv4 packets with DF not set will
   always result in a fragment header being added to the packet the IPv4
   packets must be fragmented so that their length, excluding the IPv4
   header, is at most 1232 bytes (1280 minus 40 for the IPv6 header and
   8 for the Fragment header).  The resulting fragments are then
   translated independently using the logic described below.

   If the DF bit is set and the MTU of the next-hop interface is less
   than or equal to the total length value of the IPv4 packet minus 20,
   Send ICMP (packet too big) to IPv4 source address.

   If the DF bit is set and the packet is not a fragment (i.e., the MF
   flag is not set and the Fragment Offset is zero) then the translator
   SHOULD NOT add a fragment header to the packet.  The IPv6 header
   fields are set as follows:



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   Version:  6

   Traffic Class:  By default, copied from IP Type Of Service octet.
      According to [RFC2474] the semantics of the bits are identical in
      IPv4 and IPv6.  However, in some IPv4 environments these fields
      might be used with the old semantics of "Type Of Service and
      Precedence".  An implementation of a translator SHOULD provide the
      ability to ignore the IPv4 "TOS" and always set the IPv6 traffic
      class to zero.  In addition, if the translator is at an
      administrative boundary, the filtering and update considerations
      of [RFC2475] may be applicable.

   Flow Label:  0 (all zero bits)

   Payload Length:  Total length value from IPv4 header, minus the size
      of the IPv4 header and IPv4 options, if present.

   Next Header:  For ICMP (1) changed to ICMPv6 (58), otherwise protocol
      field copied from IPv4 header.

   Hop Limit:  TTL value copied from IPv4 header.  Since the translator
      is a router, as part of forwarding the packet it needs to
      decrement either the IPv4 TTL (before the translation) or the IPv6
      Hop Limit (after the translation).  As part of decrementing the
      TTL or Hop Limit the translator (as any router) needs to check for
      zero and send the ICMPv4 "ttl exceeded" or ICMPv6 "hop limit
      exceeded" error.

   Source Address:  The IPv6 source address is derived from the IPv4
      source address.  Note that the IPv6 source address is the IPv4-
      converted address.

   Destination Address:  In stateless mode, which is to say that if the
      IPv4 destination address is within the range of the IPv4 stateless
      translation prefix, the IPv6 destination address is derived from
      the IPv4 destination address.  Note that the IPv6 destination
      address is the IPv4-translatable address.

      In stateful mode, which is to say that if the IPv4 destination
      address is not within the range of the IPv4 stateless translation
      prefix, the IPv6 address and corresponding transport-layer
      destination port are derived from the database reflecting current
      session state in the translator.  Database maintenance is as
      described in [I-D.ietf-behave-v6v4-xlate-stateful].

      If the destination address is in the multicast range, the
      multicast address mapping method should be applied.




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   If IPv4 options are present in the IPv4 packet, they are ignored
   i.e., there is no attempt to translate them.  However, if an
   unexpired source route option is present then the packet MUST instead
   be discarded, and an ICMPv4 "destination unreachable/source route
   failed" (Type 3/Code 5) error message SHOULD be returned to the
   sender.

   If there is a need to add a fragment header (the DF bit is not set or
   the packet is a fragment) the header fields are set as above with the
   following exceptions:

   IPv6 fields:

      Payload Length:  Total length value from IPv4 header, plus 8 for
         the fragment header, minus the size of the IPv4 header and IPv4
         options, if present.

      Next Header:  Fragment Header (44).

   Fragment header fields:

      Next Header:  For ICMP (1) changed to ICMPv6 (58), otherwise
         protocol field copied from IPv4 header.

      Fragment Offset:  Fragment Offset copied from the IPv4 header.

      M flag  More Fragments bit copied from the IPv4 header.

      Identification  The low-order 16 bits copied from the
         Identification field in the IPv4 header.  The high-order 16
         bits set to zero.

2.2.  Translating UDP over IPv4

   When a translator receives an unfragmented UDP IPv4 packet and the
   checksum field is zero, the translator SHOULD compute the missing UDP
   checksum as part of translating the packet.  Also, the translator
   SHOULD maintain a counter of how many UDP checksums are generated in
   this manner.

   When a stateless translator receives the first fragment of a
   fragmented UDP IPv4 packet and the checksum field is zero, the
   translator SHOULD drop the packet and generate a system management
   event specifying at least the IP addresses and port numbers in the
   packet.  When it receives fragments other than the first it SHOULD
   silently drop the packet, since there is no port information to log.

   The handling of the fragmented UDP IPv4 packets with zero checksum



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   field is discussed in [I-D.ietf-behave-v6v4-xlate-stateful].

2.3.  Translating ICMPv4 Headers into ICMPv6 Headers

   All ICMP messages that are to be translated require that the ICMP
   checksum field be updated as part of the translation since ICMPv6,
   unlike ICMPv4, has a pseudo-header checksum just like UDP and TCP.

   In addition, all ICMP packets need to have the Type value translated
   and, for ICMP error messages, the included IP header also needs
   translation.

   The actions needed to translate various ICMPv4 messages are:

   ICMPv4 query messages:

      Echo and Echo Reply (Type 8 and Type 0)  Adjust the type to 128
         and 129, respectively, and adjust the ICMP checksum both to
         take the type change into account and to include the ICMPv6
         pseudo-header.

      Information Request/Reply (Type 15 and Type 16)  Obsoleted in
         ICMPv6.  Silently drop.

      Timestamp and Timestamp Reply (Type 13 and Type 14)  Obsoleted in
         ICMPv6.  Silently drop.

      Address Mask Request/Reply (Type 17 and Type 18)  Obsoleted in
         ICMPv6.  Silently drop.

      ICMP Router Advertisement (Type 9)  Single hop message.  Silently
         drop.

      ICMP Router Solicitation (Type 10)  Single hop message.  Silently
         drop.

      Unknown ICMPv4 types  Silently drop.

      IGMP messages:  While the MLD messages [RFC2710][RFC3590][RFC3810]
         are the logical IPv6 counterparts for the IPv4 IGMP messages
         all the "normal" IGMP messages are single-hop messages and
         should be silently dropped by the translator
         [I-D.venaas-behave-v4v6mc-framework].  Other IGMP messages
         might be used by multicast routing protocols and, since it
         would be a configuration error to try to have router
         adjacencies across IP/ICMP translators those packets should
         also be silently dropped.




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       ICMPv4 error messages:

         Destination Unreachable (Type 3)  For all codes that are not
            explicitly listed below, set the Type to 1.

            Translate the code field as follows:

            Code 0, 1 (net, host unreachable):  Set Code to 0 (no route
               to destination).

            Code 2 (protocol unreachable):  Translate to an ICMPv6
               Parameter Problem (Type 4, Code 1) and make the Pointer
               point to the IPv6 Next Header field.

            Code 3 (port unreachable):  Set Code to 4 (port
               unreachable).

            Code 4 (fragmentation needed and DF set):  Translate to an
               ICMPv6 Packet Too Big message (Type 2) with code 0.  The
               MTU field needs to be adjusted for the difference between
               the IPv4 and IPv6 header sizes, i.e. minimum(advertised
               MTU+20, MTU_t6, MTU_t4+20).  Note that if the IPv4 router
               did not set the MTU field, i.e., the router does not
               implement [RFC1191], then the translator must use the
               plateau values specified in [RFC1191] to determine a
               likely path MTU and include that path MTU in the ICMPv6
               packet.  (Use the greatest plateau value that is less
               than the returned Total Length field.)

            Code 5 (source route failed):  Set Code to 0 (no route to
               destination).  Note that this error is unlikely since
               source routes are not translated.

            Code 6,7:  Set Code to 0 (no route to destination).

            Code 8:  Set Code to 0 (no route to destination).

            Code 9, 10 (communication with destination host
            administratively prohibited):  Set Code to 1 (communication
               with destination administratively prohibited)

            Code 11, 12:  Set Code to 0 (no route to destination).

            Code 13 (Communication Administratively Prohibited):  Set
               Code to 1 (communication with destination
               administratively prohibited).





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            Code 14 (Host Precedence Violation):  Silently drop.

            Code 15 (Precedence cutoff in effect):  Set Code to 1
               (communication with destination administratively
               prohibited).

         Redirect (Type 5)  Single hop message.  Silently drop.

         Alternative Host Address (Type 6)  Silently drop.

         Source Quench (Type 4)  Obsoleted in ICMPv6.  Silently drop.

         Time Exceeded (Type 11)  Set the Type field to 3.  The Code
            field is unchanged.

         Parameter Problem (Type 12)  Set the Type field to 4.
            Translate the code field as follows:

            Code 0 (Pointer indicates the error):  Set the code to 0
               (erroneous header field encountered) and update the
               pointer as defined in Appendix A.

            Code 1 (Missing a required option):  Silently drop

            Code 2 (Bad length):  Set the code to 0 (erroneous header
               field encountered) and update the pointer as defined in
               Appendix A.

            Other Code values:  Silently drop

         Unknown ICMPv4 types  Silently drop.

         ICMP Error Payload  If the received ICMP packet contains an
            ICMP Extension [RFC4884] the translation of the ICMP packet
            will cause the ICMP packet to change length.  When this
            occurs, the ICMP Extension length attribute MUST be adjusted
            accordingly (e.g., longer due to the translation from IPv4
            to IPv6).  If the ICMP Extension exceeds the maximum size of
            an ICMPv6 message on the outgoing interface, the ICMP
            extension should be simply truncated.  For extensions not
            defined in [RFC4884], the translator passes the extensions
            as opaque bit strings and those containing IPv4 address
            literals will not have those addresses translated to IPv6
            address literals; this is likely to cause problems with
            processing of those ICMP extensions.






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2.4.  Translating ICMPv4 Error Messages into ICMPv6

   There are some differences between the IPv4 and the IPv6 ICMP error
   message formats as detailed above.  In addition, the ICMP error
   messages contain the IP header for the packet in error, which needs
   to be translated just like a normal IP header.  The translation of
   this "packet in error" is likely to change the length of the
   datagram.  Thus the Payload Length field in the outer IPv6 header
   might need to be updated.

              +-------------+                 +-------------+
              |    IPv4     |                 |    IPv6     |
              |   Header    |                 |   Header    |
              +-------------+                 +-------------+
              |   ICMPv4    |                 |   ICMPv6    |
              |   Header    |                 |   Header    |
              +-------------+                 +-------------+
              |    IPv4     |      ===>       |    IPv6     |
              |   Header    |                 |   Header    |
              +-------------+                 +-------------+
              |   Partial   |                 |   Partial   |
              |  Transport  |                 |  Transport  |
              |   Layer     |                 |   Layer     |
              |   Header    |                 |   Header    |
              +-------------+                 +-------------+

               Figure 3: IPv4-to-IPv6 ICMP Error Translation

   The translation of the inner IP header can be done by recursively
   invoking the function that translated the outer IP headers.

2.5.  Translator sending ICMP error message

   If the packet is discarded, then the translator SHOULD be able to
   send back an ICMP message to the original sender of the packet,
   unless the discarded packet is itself an ICMP message.  The ICMP
   message, if sent, has a type of 3 (Destination Unreachable) and a
   code of 13 (Communication Administratively Prohibited).  The
   translator device MUST allow to configure whether the ICMP error
   messages are sent, rate-limited or not sent.

2.6.  Transport-layer Header Translation

   If the address translation algorithm is not checksum neutral, the
   recalculation and updating of the transport-layer headers MUST be
   performed.  UDP/IPv4 datagrams with a checksum of zero MAY be dropped
   and MAY have their checksum calculated for injection into the IPv6
   domain.  This choice SHOULD be under configuration control.



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2.7.  Knowing when to Translate

   If the IP/ICMP translator is implemented in a router providing both
   translation and normal forwarding, and the address is reachable by a
   more specific route without translation, the router MUST forward it
   without translating it.  Otherwise, when an IP/ICMP translator
   receives an IPv4 datagram addressed to a destination towards the IPv6
   domain, the packet will be translated to IPv6.


3.  Translating from IPv6 to IPv4

   When an IP/ICMP translator receives an IPv6 datagram addressed to a
   destination towards the IPv4 domain, it translates the IPv6 header of
   that packet into an IPv4 header.  Since the ICMP [RFC0792][RFC4443],
   TCP [RFC0793] and UDP [RFC0768] headers contain checksums that cover
   the IP header, if the address mapping algorithm is not checksum-
   neutral, the ICMP and transport-layer headers MUST be updated.  The
   data portion of the packet is left unchanged.  The IP/ICMP translator
   then forwards the packet based on the IPv4 destination address.  The
   original IPv6 header on the packet is removed and replaced by an IPv4
   header.

              +-------------+                 +-------------+
              |    IPv6     |                 |    IPv4     |
              |   Header    |                 |   Header    |
              +-------------+                 +-------------+
              |  Fragment   |                 |  Transport  |
              |   Header    |      ===>       |   Layer     |
              |(if present) |                 |   Header    |
              +-------------+                 +-------------+
              |  Transport  |                 |             |
              |   Layer     |                 ~    Data     ~
              |   Header    |                 |             |
              +-------------+                 +-------------+
              |             |
              ~    Data     ~
              |             |
              +-------------+

                    Figure 4: IPv6-to-IPv4 Translation

   There are some differences between IPv6 and IPv4 in the area of
   fragmentation and the minimum link MTU that affect the translation.
   An IPv6 link has to have an MTU of 1280 bytes or greater.  The
   corresponding limit for IPv4 is 68 bytes.  Thus, unless there were
   special measures, it would not be possible to do end-to-end path MTU
   discovery when the path includes a translator since the IPv6 node



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   might receive ICMP "packet too big" messages originated by an IPv4
   router that report an MTU less than 1280.  However, [RFC2460] section
   5 requires that IPv6 nodes handle such an ICMP "packet too big"
   message by reducing the path MTU to 1280 and including an IPv6
   fragment header with each packet.  This allows end-to-end path MTU
   discovery across the translator as long as the path MTU is 1280 bytes
   or greater.  When the path MTU drops below the 1280 limit the IPv6
   sender will originate 1280-byte packets that will be fragmented by
   IPv4 routers along the path after being translated to IPv4.

   The only drawback with this scheme is that it is not possible to use
   PMTU to do optimal UDP fragmentation (as opposed to completely
   avoiding fragmentation) at the sender, since the presence of an IPv6
   fragment header is interpreted that it is okay to fragment the packet
   on the IPv4 side.  Thus if a UDP application wants to send large
   packets independent of the PMTU, the sender will only be able to
   determine the path MTU on the IPv6 side of the translator.  If the
   path MTU on the IPv4 side of the translator is smaller, then the IPv6
   sender will not receive any ICMP "too big" errors and cannot adjust
   the size fragments it is sending.

   Other than the special rules for handling fragments and path MTU
   discovery the actual translation of the packet header consists of a
   simple mapping as defined below.  Note that ICMP packets require
   special handling in order to translate the contents of ICMP error
   message and also to add the ICMP pseudo-header checksum.

3.1.  Translating IPv6 Headers into IPv4 Headers

   If there is no IPv6 Fragment header, the IPv4 header fields are set
   as follows:

   Version:  4

   Internet Header Length:  5 (no IPv4 options)

   Type of Service (TOS) Octet:  By default, copied from the IPv6
      Traffic Class (all 8 bits).  According to [RFC2474] the semantics
      of the bits are identical in IPv4 and IPv6.  However, in some IPv4
      environments, these bits might be used with the old semantics of
      "Type Of Service and Precedence".  An implementation of a
      translator SHOULD provide the ability to ignore the IPv6 traffic
      class and always set the IPv4 TOS Octet to a specified value.  In
      addition, if the translator is at an administrative boundary, the
      filtering and update considerations of [RFC2475] may be
      applicable.





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   Total Length:  Payload length value from IPv6 header, plus the size
      of the IPv4 header.

   Identification:  All zero.

   Flags:  The More Fragments flag is set to zero.  The Don't Fragments
      flag is set to one.

   Fragment Offset:  All zero.

   Time to Live:  Hop Limit value copied from IPv6 header.  Since the
      translator is a router, as part of forwarding the packet it needs
      to decrement either the IPv6 Hop Limit (before the translation) or
      the IPv4 TTL (after the translation).  As part of decrementing the
      TTL or Hop Limit the translator (as any router) needs to check for
      zero and send the ICMPv4 "ttl exceeded" or ICMPv6 "hop limit
      exceeded" error.

   Protocol:  For ICMPv6 (58) changed to ICMP (1), otherwise Next Header
      field copied from IPv6 header.

   Header Checksum:  Computed once the IPv4 header has been created.

   Source Address:  In stateless mode, which is to say that if the IPv6
      source address is within the range of the IPv6 stateless
      translation prefix, the IPv4 source address is derived from the
      IPv6 address.  Note that the original IPv6 source address is the
      IPv4-translatable address.

      In stateful mode, which is to say that if the IPv6 source address
      is not within the range of the IPv6 stateless translation prefix,
      the IPv4 source address and transport layer source port
      corresponding to the IPv4-related IPv6 source address and source
      port are derived from the database reflecting current session
      state in the translator.  Database maintenance is described in
      [I-D.ietf-behave-v6v4-xlate-stateful].

   Destination Address:  The IPv4 destination address is derived from
      the IPv6 destination address of the datagram being translated.
      Note that the original IPv6 destination address is the IPv4-
      converted address.

      If the destination address is in the multicast range, the
      multicast address mapping method should be applied.

   If any of an IPv6 hop-by-hop options header, destination options
   header, or routing header with the Segments Left field equal to zero
   are present in the IPv6 packet, they are ignored i.e., there is no



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   attempt to translate them.  However, the Total Length field and the
   Protocol field is adjusted to "skip" these extension headers.

   If a routing header with a non-zero Segments Left field is present
   then the packet MUST NOT be translated, and an ICMPv6 "parameter
   problem/erroneous header field encountered" (Type 4/Code 0) error
   message, with the Pointer field indicating the first byte of the
   Segments Left field, SHOULD be returned to the sender.

   If the IPv6 packet contains a Fragment header the header fields are
   set as above with the following exceptions:

   Total Length:  Payload length value from IPv6 header, minus 8 for the
      Fragment header, plus the size of the IPv4 header.

   Identification:  Copied from the low-order 16-bits in the
      Identification field in the Fragment header.

   Flags:  The More Fragments flag is copied from the M flag in the
      Fragment header.  The Don't Fragments flag is set to zero allowing
      this packet to be fragmented by IPv4 routers.

   Fragment Offset:  Copied from the Fragment Offset field in the
      Fragment header.

   Protocol:  For ICMPv6 (58) changed to ICMP (1), otherwise Next Header
      field copied from Fragment header.

3.2.  Translating ICMPv6 Headers into ICMPv4 Headers

   All ICMP messages that are to be translated require that the ICMP
   checksum field be updated as part of the translation since ICMPv6
   (unlike ICMPv4) includes a pseudo-header in the checksum just like
   UDP and TCP.

   In addition all ICMP packets need to have the Type value translated
   and, for ICMP error messages, the included IP header also needs
   translation.  Note that the IPv6 addresses in the IPv6 header may not
   be the IPv4-translatable addresses and there will be no corresponding
   IPv4 addresses.  In this case, a special block of the IPv4 address
   can be used to indicate this phenomenon.

   The actions needed to translate various ICMPv6 messages are:

   ICMPv6 informational messages:






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      Echo Request and Echo Reply (Type 128 and 129)  Adjust the type to
         8 and 0, respectively, and adjust the ICMP checksum both to
         take the type change into account and to exclude the ICMPv6
         pseudo-header.

      MLD Multicast Listener Query/Report/Done (Type 130, 131, 132)
         Single hop message.  Silently drop.

      Neighbor Discover messages (Type 133 through 137)  Single hop
         message.  Silently drop.

      Unknown informational messages  Silently drop.

   ICMPv6 error messages:

      Destination Unreachable (Type 1)  Set the Type field to 3.
         Translate the code field as follows:

         Code 0 (no route to destination):  Set Code to 1 (host
            unreachable).

         Code 1 (communication with destination administratively
         prohibited):  Set Code to 10 (communication with destination
            host administratively prohibited).

         Code 2 (beyond scope of source address):  Set Code to 1 (host
            unreachable).  Note that this error is very unlikely since
            the IPv4-translatable source address is considered to have
            global scope.

         Code 3 (address unreachable):  Set Code to 1 (host
            unreachable).

         Code 4 (port unreachable):  Set Code to 3 (port unreachable).

      Packet Too Big (Type 2)  Translate to an ICMPv4 Destination
         Unreachable with code 4.  The MTU field needs to be adjusted
         for the difference between the IPv4 and IPv6 header sizes
         taking into account whether or not the packet in error includes
         a Fragment header, i.e. minimum(advertised MTU-20, MTU_t4,
         MTU_t6-20)

      Time Exceeded (Type 3)  Set the Type to 11.  The Code field is
         unchanged.







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      Parameter Problem (Type 4)  Translate the type and code field as
         follows:

         Code 1 (unrecognized Next Header type encountered):  Translate
            this to an ICMPv4 protocol unreachable (Type 3, Code 2).

         Code 0 (erroneous header field encountered):  Set Type 12, Code
            0 and update the pointer as defined in Appendix A.

         Code 2 (unrecognized IPv6 option encountered):  Silently drop.

      Unknown error messages  Silently drop.

      ICMP Error Payload  If the received ICMPv6 packet contains an ICMP
         Extension [RFC4884], the translation of the ICMPv6 packet will
         cause the ICMPv6 packet to change length.  When this occurs,
         the ICMPv6 Extension length attribute MUST be adjusted
         accordingly (e.g., shorter due to the translation from IPv6 to
         IPv4).  For extensions not defined in [RFC4884], the translator
         passes the extensions as opaque bit strings and those
         containing IPv6 address literals will not have those addresses
         translated to IPv4 address literals; this is likely to cause
         problems with processing of those ICMP extensions.

3.3.  Translating ICMPv6 Error Messages into ICMPv4

   There are some differences between the IPv4 and the IPv6 ICMP error
   message formats as detailed above.  In addition, the ICMP error
   messages contain the IP header for the packet in error, which needs
   to be translated just like a normal IP header.  The translation of
   this "packet in error" is likely to change the length of the datagram
   thus the Total Length field in the outer IPv4 header might need to be
   updated.


















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              +-------------+                 +-------------+
              |    IPv6     |                 |    IPv4     |
              |   Header    |                 |   Header    |
              +-------------+                 +-------------+
              |   ICMPv6    |                 |   ICMPv4    |
              |   Header    |                 |   Header    |
              +-------------+                 +-------------+
              |    IPv6     |      ===>       |    IPv4     |
              |   Header    |                 |   Header    |
              +-------------+                 +-------------+
              |   Partial   |                 |   Partial   |
              |  Transport  |                 |  Transport  |
              |   Layer     |                 |   Layer     |
              |   Header    |                 |   Header    |
              +-------------+                 +-------------+

               Figure 5: IPv6-to-IPv4 ICMP Error Translation

   The translation of the inner IP header can be done by recursively
   invoking the function that translated the outer IP headers.  Note
   that the IPv6 addresses in the IPv6 header may not be the IPv4-
   translatable addresses and there will be no corresponding IPv4
   addresses.  In this case, a special block of the IPv4 address can be
   used to indicate this phenomenon.

3.4.  Translator sending ICMPv6 error message

   If the packet is discarded, then the translator SHOULD be able to
   send back an ICMPv6 message to the original sender of the packet,
   unless the discarded packet is itself an ICMPv6 message.  The ICMPv6
   message, if sent, has a type of 1 (Destination Unreachable) and a
   code of 1 (Communication with destination administratively
   prohibited).  The translator device MUST allow configuring whether
   the ICMPv6 error messages are sent, rate-limited or not sent.

3.5.  Transport-layer Header Translation

   If the address translation algorithm is not checksum neutral, the
   recalculation and updating of the transport-layer headers MUST be
   performed.

3.6.  Knowing when to Translate

   If the IP/ICMP translator is implemented in a router providing both
   translation and normal forwarding, and the address is reachable by a
   more specific route without translation, the router MUST forward it
   without translating it.  When an IP/ICMP translator receives an IPv6
   datagram addressed to a destination towards the IPv4 domain, the



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   packet will be translated to IPv4.


4.  IANA Considerations

   This memo adds no new IANA considerations.

   Note to RFC Editor: This section will have served its purpose if it
   correctly tells IANA that no new assignments or registries are
   required, or if those assignments or registries are created during
   the RFC publication process.  From the author's perspective, it may
   therefore be removed upon publication as an RFC at the RFC Editor's
   discretion.


5.  Security Considerations

   The use of stateless IP/ICMP translators does not introduce any new
   security issues beyond the security issues that are already present
   in the IPv4 and IPv6 protocols and in the routing protocols that are
   used to make the packets reach the translator.

   There are potential issues that might arise by extracting an IPv4
   address from the IPv6 address - particularly addresses like broadcast
   or loopback addresses and the non IPv4-translatable IPv6 addresses,
   etc.  Special action SHOULD be taken to avoid security problems.

   As the Authentication Header [RFC4302] is specified to include the
   IPv4 Identification field and the translating function is not able to
   always preserve the Identification field, it is not possible for an
   IPv6 endpoint to verify the AH on received packets that have been
   translated from IPv4 packets.  Thus AH does not work through a
   translator.

   Packets with ESP can be translated since ESP does not depend on
   header fields prior to the ESP header.  Note that ESP transport mode
   is easier to handle than ESP tunnel mode; in order to use ESP tunnel
   mode, the IPv6 node needs to be able to generate an inner IPv4 header
   when transmitting packets and remove such an IPv4 header when
   receiving packets.


6.  Acknowledgements

   This is under development by a large group of people.  Those who have
   posted to the list during the discussion include Andrew Sullivan,
   Andrew Yourtchenko, Brian Carpenter, Dan Wing, Dave Thaler, Ed
   Jankiewicz, Hiroshi Miyata, Iljitsch van Beijnum, Jari Arkko, Jerry



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   Huang, John Schnizlein, Kentaro Ebisawa, Kevin Yin, Magnus
   Westerlund, Marcelo Bagnulo Braun, Margaret Wasserman, Masahito Endo,
   Phil Roberts, Philip Matthews, Remi Denis-Courmont, Remi Despres,
   Senthil Sivakumar, Simon Perreault and Zen Cao.


7.  Appendix A: Translating pointer in Parameter Problem



    * Translating from IPv4 to IPv6:

     |   Original IPv4 Pointer        | Translated IPv6 Pointer Vaule  |
     +--------------------------------+--------------------------------+
     |  0  | Version/IHL              |  0  | Version/Traffic Class    |
     |  1  | Type Of Service          | 0or1| Traffic Class/Flow Label |
     | 2,3 | Total Length             | 4,5 | Payload Length           |
     | 4,5 | Identification           |  -  |                          |
     |  6  | Flags/Fragment Offset    |  -  |                          |
     |  7  | Fragment Offset          |  -  |                          |
     |  8  | Time to Live             |  7  | Hop Limit                |
     |  9  | Protocol                 |  6  | Next Header              |
     |10,11| Header Checksum          |  -  |                          |
     |12-15| Source Address           | 8-23| Source Address           |
     |16-19| Destination Address      |24-39| Destination Address      |
     +--------------------------------+--------------------------------+

    * Translating from IPv6 to IPv4:

     |   Original IPv6 Pointer        | Translated IPv4 Pointer Vaule  |
     +--------------------------------+--------------------------------+
     |  0  | Version/Traffic Class    | 0or1| Version/IHL, Type Of Ser |
     |  1  | Traffic Class/Flow Label |  1  | Type Of Service          |
     | 2,3 | Flow Label               |  -  |                          |
     | 4,5 | Payload Length           | 2,3 | Total Length             |
     |  6  | Next Header              |  9  | Protocol                 |
     |  7  | Hop Limit                |  8  | Time to Live             |
     | 8-23| Source Address           |12-15| Source Address           |
     |24-39| Destination Address      |16-19| Destination Address      |
     +--------------------------------+--------------------------------+


                                 Figure 6


8.  References





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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.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, 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.

   [RFC2765]  Nordmark, E., "Stateless IP/ICMP Translation Algorithm
              (SIIT)", RFC 2765, February 2000.

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

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4884]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
              "Extended ICMP to Support Multi-Part Messages", RFC 4884,
              April 2007.

   [RFC5382]  Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, October 2008.

8.2.  Informative References

   [Dongjin]  Lee, D., "Email to the behave mailing list", Sept 2009.

   [I-D.ietf-behave-address-format]
              Huitema, C., Bao, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators",
              draft-ietf-behave-address-format-02 (work in progress),
              December 2009.




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   [I-D.ietf-behave-v6v4-framework]
              Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
              IPv4/IPv6 Translation",
              draft-ietf-behave-v6v4-framework-03 (work in progress),
              October 2009.

   [I-D.ietf-behave-v6v4-xlate-stateful]
              Bagnulo, M., Matthews, P., and I. Beijnum, "NAT64: Network
              Address and Protocol Translation from IPv6 Clients to IPv4
              Servers", draft-ietf-behave-v6v4-xlate-stateful-05 (work
              in progress), December 2009.

   [I-D.venaas-behave-v4v6mc-framework]
              Venaas, S., Li, X., and C. Bao, "Framework for IPv4/IPv6
              Multicast Translation",
              draft-venaas-behave-v4v6mc-framework-01 (work in
              progress), October 2009.

   [I-D.xli-behave-ivi]
              Li, X., Bao, C., Chen, M., Zhang, H., and J. Wu, "The
              CERNET IVI Translation Design and Deployment for the IPv4/
              IPv6 Coexistence and Transition", draft-xli-behave-ivi-05
              (work in progress), December 2009.

   [Miller]   Miller, G., "Email to the ngtrans mailing list",
              March 1999.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, December 1998.

   [RFC2710]  Deering, S., Fenner, W., and B. Haberman, "Multicast
              Listener Discovery (MLD) for IPv6", RFC 2710,
              October 1999.

   [RFC3171]  Albanna, Z., Almeroth, K., Meyer, D., and M. Schipper,
              "IANA Guidelines for IPv4 Multicast Address Assignments",
              BCP 51, RFC 3171, August 2001.

   [RFC3307]  Haberman, B., "Allocation Guidelines for IPv6 Multicast



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              Addresses", RFC 3307, August 2002.

   [RFC3590]  Haberman, B., "Source Address Selection for the Multicast
              Listener Discovery (MLD) Protocol", RFC 3590,
              September 2003.

   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

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

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.


Authors' Addresses

   Xing Li
   CERNET Center/Tsinghua University
   Room 225, Main Building, Tsinghua University
   Beijing,   100084
   China

   Phone: +86 10-62785983
   Email: xing@cernet.edu.cn


   Congxiao Bao
   CERNET Center/Tsinghua University
   Room 225, Main Building, Tsinghua University
   Beijing,   100084
   China

   Phone: +86 10-62785983
   Email: congxiao@cernet.edu.cn


   Fred Baker
   Cisco Systems
   Santa Barbara, California  93117
   USA

   Phone: +1-408-526-4257
   Email: fred@cisco.com






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