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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: August 31, 2010                                   Cisco Systems
                                                       February 27, 2010


                     IP/ICMP Translation Algorithm
                    draft-ietf-behave-v6v4-xlate-10

Abstract

   This document forms a replacement of the Stateless IP/ICMP
   Translation Algorithm (SIIT) described in RFC 2765.  The algorithm
   translates between IPv4 and IPv6 packet headers (including ICMP
   headers).

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 August 31, 2010.

Copyright Notice

   Copyright (c) 2010 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



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   (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.

   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.
































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Table of Contents

   1.  Introduction and Motivation  . . . . . . . . . . . . . . . . .  4
     1.1.  IPv4-IPv6 Translation Model  . . . . . . . . . . . . . . .  4
     1.2.  Applicability and Limitations  . . . . . . . . . . . . . .  5
     1.3.  Stateless vs. Stateful Mode  . . . . . . . . . . . . . . .  5
     1.4.  Path MTU Discovery and Fragmentation . . . . . . . . . . .  6
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Translating from IPv4 to IPv6  . . . . . . . . . . . . . . . .  6
     3.1.  Translating IPv4 Headers into IPv6 Headers . . . . . . . .  8
     3.2.  Translating ICMPv4 Headers into ICMPv6 Headers . . . . . . 10
     3.3.  Translating ICMPv4 Error Messages into ICMPv6  . . . . . . 14
     3.4.  Translator Sending ICMPv4 Error Message  . . . . . . . . . 14
     3.5.  Transport-layer Header Translation . . . . . . . . . . . . 15
     3.6.  Knowing when to Translate  . . . . . . . . . . . . . . . . 15
   4.  Translating from IPv6 to IPv4  . . . . . . . . . . . . . . . . 15
     4.1.  Translating IPv6 Headers into IPv4 Headers . . . . . . . . 17
     4.2.  Translating ICMPv6 Headers into ICMPv4 Headers . . . . . . 19
     4.3.  Translating ICMPv6 Error Messages into ICMPv4  . . . . . . 22
     4.4.  Translator Sending ICMPv6 Error Message  . . . . . . . . . 23
     4.5.  Transport-layer Header Translation . . . . . . . . . . . . 23
     4.6.  Knowing when to Translate  . . . . . . . . . . . . . . . . 24
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
   8.  Appendix: Stateless translation workflow example . . . . . . . 25
     8.1.  H6 establishes communication with H4 . . . . . . . . . . . 26
     8.2.  H4 establishes communication with H6 . . . . . . . . . . . 27
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 28
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 29
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31



















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1.  Introduction and Motivation

   This document is a product of the 2008-2010 effort to define a
   replacement for NAT-PT [RFC2766].  It is directly derivative from
   Erik Nordmark's [RFC2765], which provides stateless translation
   between IPv4 [RFC0791] and IPv6 [RFC2460], and between ICMPv4
   [RFC0792] and ICMPv6 [RFC4443].

   The general IPv4/IPv6 translation framework is now described in
   [I-D.ietf-behave-v6v4-framework].  This document specifies the
   translation algorithms between IPv4 packets and IPv6 packets.  The
   mapping algorithms between IPv4 addresses and IPv6 addresses in the
   packet headers are specified in [I-D.ietf-behave-address-format].

1.1.  IPv4-IPv6 Translation Model

   The translation model is consists of two or more network domains
   connected by one or more IP/ICMP translators (XLATEs) as shown in
   Figure 1.

          --------           --------
        //  IPv4  \\       //  IPv6  \\
       /   Domain   \     /   Domain   \
      /             +-----+      +--+   \
     |              |XLATE|      |S2|    |  Sn: Servers
     | +--+         +-----+      +--+    |  Hn: Clients
     | |S1|         +-----+              |
     | +--+         | DNS |      +--+    |  XLATE: IPv4/IPv6 Translator
      \     +--+    +-----+      |H2|   /   DNS:   DNS64/DNS46
       \    |H1|    /     \      +--+  /
        \\  +--+  //       \\        //
          --------           --------

                   Figure 1: IPv4-IPv6 Translation Model

   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.  A network domain may also consist of a
   single host.  DNS servers include DNS64 and DNS46, while DNS64
   translates A record to AAAA record and DNS46 translates AAAA record
   to A record.







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1.2.  Applicability and Limitations

   This document specifies the translation algorithms itself between
   IPv4 packets and IPv6 packets.  The implementation of IPv4/IPv6
   translation SHOULD also refer to [I-D.ietf-behave-v6v4-framework],
   [I-D.ietf-behave-address-format] and
   [I-D.ietf-behave-v6v4-xlate-stateful], etc.

   As with [RFC2765], 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.

   The issues and algorithms in the translation of datagrams 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 [Miller][Dongjin] and will not be translated by the IP/ICMP
   translator.

   IPv4 multicast addresses [RFC3171] cannot be mapped to IPv6 multicast
   addresses [RFC3307] based on the unicast mapping rule.  However, if a
   multicast address mapping mechanism is defined, the IP/ICMP header
   translation aspect of this document works.

   Translator SHOULD make sure that the packets belonging to the same
   flow leave the translator in the same order in which they arrived.

1.3.  Stateless vs. Stateful Mode

   An 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 (a node or an application) 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 (IPv4-translatable addresses) that can be algorithmically
   mapped to a subset of the service provider's IPv4 addresses.  In
   general, there is no need to concern oneself with translation tables,
   as the IPv4 and IPv6 counterparts are algorithmically related.

   In the stateful mode, a specific IPv6 address range will represent



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   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: path MTU discovery (PMTUD),
   fragmentation, and transport-layer negotiation such as the TCP MSS
   option [RFC0879].  Note that the translator MUST behave as a router,
   i.e. the translator MUST send a "Packet Too Big" error message or
   fragment the packet when the packet size exceeds the MTU of the next
   hop interface.

   "Don't Fragment", ICMP "Packet Too Big", and packet fragmentation are
   discussed in sections 3 and 4 of this document.  The reassembling of
   fragmented packets in the stateful translator is discussed in
   [I-D.ietf-behave-v6v4-xlate-stateful], since it requires state
   maintenance in the translator.


2.  Conventions

   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.  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.  The original IPv4 header on the
   packet is removed and replaced by an IPv6 header.  Since the ICMPv6
   [RFC4443], TCP [RFC0793] and UDP [RFC0768] headers contain checksums
   that cover IP header information, if the address mapping algorithm is
   not checksum-neutral, the ICMPv6 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.





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              +-------------+                 +-------------+
              |    IPv4     |                 |    IPv6     |
              |   Header    |                 |   Header    |
              +-------------+                 +-------------+
              |  Transport  |                 |  Fragment   |
              |   Layer     |      ===>       |   Header    |
              |   Header    |                 | (if needed) |
              +-------------+                 +-------------+
              |             |                 |  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 an IPv4 node performs path MTU discovery (by setting the Don't
   Fragment (DF) bit in the header), 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" messages to the sender.  When the IPv6 routers send these
   ICMPv6 errors they will pass through a translator that will translate
   the ICMPv6 error to a form that the IPv4 sender can understand.  As a
   result, an IPv6 fragment header is only included if the IPv4 packet
   is already fragmented.

   However, when the IPv4 sender does not set the Don't Fragment (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 MTU.  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.

   The rules in section 3.1 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 in section 3.1 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



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   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 ICMPv4 packets require
   special handling in order to translate the content of ICMPv4 error
   messages and also to add the ICMPv6 pseudo-header checksum.

3.1.  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 packet MUST be fragmented so the
   resulting IPv6 packet (with Fragment header added to each fragment)
   will be less than or equal to 1280 bytes.  For example, if the packet
   is fragmented prior to the translation, 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 the total length value of the IPv4 packet plus 20, the
   translator MUST send an ICMPv4 "Fragmentation Needed" error message
   to the 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 equal to zero) then the
   translator SHOULD NOT add a Fragment header to the resulting packet.
   The IPv6 header fields are set as follows:

   Version:  6

   Traffic Class:  By default, copied from IP Type Of Service (TOS)
      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 (TC) 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.





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   Next Header:  For ICMPv4 (1) changed to ICMPv6 (58), otherwise
      protocol field copied from IPv4 header.

   Hop Limit:  The hop limit is derived from the TTL value in the 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 IPv4-converted address derived from the IPv4
      source address per [I-D.ietf-behave-address-format] section 2.1.

      If the translator gets an illegal source address (e.g. 0.0.0.0,
      127.0.0.1, etc.), the translator SHOULD silently drop the packet
      (as discussed in Section 5.3.7 of [RFC1812]).


   Destination Address:  In the stateless mode, which is to say that if
      the IPv4 destination address is within a range of configured IPv4
      stateless translation prefix, the IPv6 destination address is the
      IPv4-translatable address derived from the IPv4 destination
      address per [I-D.ietf-behave-address-format] section 2.1.  A
      workflow example of stateless translation is shown in the Appendix
      of this document.

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


   If any IPv4 options are present in the IPv4 packet, the IPv4 options
   MUST be ignored (i.e., there is no attempt to translate the options)
   and the packet translated normally.  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:






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   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 ICMPv4 (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.

3.2.  Translating ICMPv4 Headers into ICMPv6 Headers

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

   In addition, all ICMPv4 packets need to have the Type value
   translated and, for ICMPv4 error messages, the included IP header
   also needs translation.

   The actions needed to translate various ICMPv4 messages are as
   follows:

   ICMPv4 query messages:

      Echo and Echo Reply (Type 8 and Type 0):  Adjust the Type values
         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.





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      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.  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.

       ICMPv4 error messages:

         Destination Unreachable (Type 3):  For all codes that are not
            explicitly listed below, set the Type field to 1, and adjust
            the ICMP checksum both to take the type change into account
            and to include the ICMPv6 pseudo-header.

            Translate the Code field as follows:

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

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

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

            Code 4 (Fragmentation needed and DF set):  Translate to an
               ICMPv6 Packet Too Big message (Type 2) with Code value
               set to 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_of_IPv6_nexthop,
               (MTU_of_IPv4_nexthop)+20).  Note that if the IPv4 router
               set the MTU field to zero, i.e., the router does not
               implement [RFC1191], then the translator MUST use the
               plateau values specified in [RFC1191] to determine a



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               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 value to 0 (No route
               to destination).  Note that this error is unlikely since
               source routes are not translated.

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

            Code 8:  Set Code value to 0 (No route to destination).

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

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

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

            Code 14 (Host Precedence Violation):  Silently drop.

            Code 15 (Precedence cutoff in effect):  Set Code value 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, and adjust
            the ICMP checksum both to take the type change into account
            and to include the ICMPv6 pseudo-header.  The Code field is
            unchanged.

         Parameter Problem (Type 12):  Set the Type field to 4, and
            adjust the ICMP checksum both to take the type change into
            account and to include the ICMPv6 pseudo-header.  Translate
            the Code field as follows:







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            Code 0 (Pointer indicates the error):  Set the Code value to
               0 (Erroneous header field encountered) and update the
               pointer as defined in Figure 3 (If the Original IPv4
               Pointer Value is not listed or the Translated IPv6
               Pointer Value is listed as "n/a", silently drop the
               packet).

            Code 1 (Missing a required option):  Silently drop

            Code 2 (Bad length):  Set the Code value to 0 (Erroneous
               header field encountered) and update the pointer as
               defined in Figure 3 (If the Original IPv4 Pointer Value
               is not listed or the Translated IPv6 Pointer Value is
               listed as "n/a", silently drop the packet).

            Other Code values:  Silently drop

         Unknown ICMPv4 types:  Silently drop.




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

             Figure 3: Pointer value for translating from IPv4 to IPv6

         ICMP Error Payload:  If the received ICMPv4 packet contains an
            ICMPv4 Extension [RFC4884], the translation of the ICMPv4
            packet will cause the ICMPv6 packet to change length.  When
            this occurs, the ICMPv6 Extension length attribute MUST be
            adjusted accordingly (e.g., longer due to the translation
            from IPv4 to IPv6).  If the ICMPv4 Extension exceeds the
            maximum size of an ICMPv6 message on the outgoing interface,
            the ICMPv4 extension should be simply truncated.  For
            extensions not defined in [RFC4884], the translator passes



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            the extensions as opaque bit strings and those containing
            IPv4 address literals will not have those addresses
            translated to IPv6 address literals; this may cause problems
            with processing of those ICMP extensions.

3.3.  Translating ICMPv4 Error Messages into ICMPv6

   There are some differences between the ICMPv4 and the ICMPv6 error
   message formats as detailed above.  In addition, the ICMP error
   messages contain the packet in error, which needs to be translated
   just like a normal IP packet.  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 4: IPv4-to-IPv6 ICMP Error Translation

   The translation of the inner IP header can be done by invoking the
   function that translated the outer IP headers.  This process SHOULD
   stop at the first embedded header and drop the packet if it contains
   more.

3.4.  Translator Sending ICMPv4 Error Message

   If the IPv4 packet is discarded, then the translator SHOULD be able
   to send back an ICMPv4 error message to the original sender of the
   packet, unless the discarded packet is itself an ICMPv4 message.  The
   ICMPv4 message, if sent, has a Type value of 3 (Destination
   Unreachable) and a Code value of 13 (Communication Administratively
   Prohibited), unless otherwise specified in this document or in
   [I-D.ietf-behave-v6v4-xlate-stateful].  The translator SHOULD allow
   an administrator to configure whether the ICMPv4 error messages are



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   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.

   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.

   For stateful translator, the handling of fragmented UDP IPv4 packets
   with a zero checksum is discussed in
   [I-D.ietf-behave-v6v4-xlate-stateful] section 3.1.

3.6.  Knowing when to Translate

   If the IP/ICMP translator also provides normal forwarding function,
   and the destination IPv4 address is reachable by a more specific
   route without translation, the translator MUST forward it without
   translating it.  Otherwise, when an IP/ICMP translator receives an
   IPv4 datagram addressed to an IPv4 destination representing a host in
   the IPv6 domain, the packet MUST be translated to IPv6.


4.  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
   the received IPv6 packet into an IPv4 header.  The original IPv6
   header on the packet is removed and replaced by an IPv4 header.
   Since the ICMPv6 [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.




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

                    Figure 5: 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
   might receive ICMPv6 "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 ICMPv6 "Packet Too Big"
   message by reducing the path MTU to 1280 and including an IPv6
   fragment header with each packet.  In this case, the translator
   SHOULD set DF to 0 and take the identification value from the IPv6
   fragment header when a fragmentation header with (MF=0; Offset=0) is
   present or set DF to 1 otherwise.  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 drawback with this scheme is that it is not possible to use PMTU
   discovery 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 ICMPv6 "Too Big" errors and cannot adjust
   the size fragments it is sending.



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   On the other hand, a recent study indicates that only 43.46% of IPv6-
   capable web servers include an IPv6 fragmentation header in their
   respond packets after they were sent an ICMPv6 "Packet Too Big"
   message specifying an MTU<1280 bytes [Stasiewicz].  A workaround to
   this problem (ICMPv6 "Packet Too Big" message with MTU<1280) is that
   if there is no fragmentation header in the IPv6 packet, the
   translator SHOULD set DF to 0 for the packets equal to or smaller
   than 1280 bytes and set DF to 1 for packets larger than 1280 bytes.
   In addition, the translator SHOULD take the identification value from
   the IPv6 fragmentation header if present or generate the
   identification value otherwise.  This avoids the introduction of the
   path MTU discovery black hole.  The header translation defined in the
   next section uses this method.

   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 ICMPv6 packets require
   special handling in order to translate the contents of ICMPv6 error
   messages and also to remove the ICMPv6 pseudo-header checksum.

4.1.  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.

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

   Identification:  If the packet size is equal to or smaller than 1280
      bytes, generate the identification value.  If the packet size is
      greater than 1280 bytes, set Identification All zeros.





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   Flags:  The More Fragments (MF) flag is set to zero.  If the packet
      size is equal to or smaller than 1280 bytes, the Don't Fragments
      (DF) flag is set to zero.  If the packet size is greater than 1280
      bytes, the Don't Fragments (DF) flag is set to one.

   Fragment Offset:  All zeros.

   Time to Live:  Time to Live is derived from Hop Limit value in 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 ICMPv4 (1), otherwise Next
      Header field copied from IPv6 header.

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

   Source Address:  In the stateless mode, which is to say that if the
      IPv6 source address is within the range of a configured IPv6
      translation prefix, the IPv4 source address is derived from the
      IPv6 source address per [I-D.ietf-behave-address-format] section
      2.1.  Note that the original IPv6 source address is an IPv4-
      translatable address.  A workflow example of stateless translation
      is shown in Appendix of this document.  If the translator only
      supports stateless mode and if the IPv6 source address is not
      within the range of configured IPv6 prefix(es), the translator
      SHOULD drop the packet and respond with an ICMPv6 Type=1, Code=5
      (Destination Unreachable, Source address failed ingress/egress
      policy).

      In the stateful mode, which is to say that if the IPv6 source
      address is not within the range of any configured 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 Binding Information Bases
      (BIBs) as described in [I-D.ietf-behave-v6v4-xlate-stateful].

      In stateless and stateful modes, if the translator gets an illegal
      source address (e.g. ::1, etc.), the translator SHOULD silently
      drop the packet.

   Destination Address:  The IPv4 destination address is derived from
      the IPv6 destination address of the datagram being translated per
      [I-D.ietf-behave-address-format] section 2.1.  Note that the
      original IPv6 destination address is an IPv4-converted address.



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   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, those IPv6 extension headers MUST be
   ignored (i.e., there is no attempt to translate the extension
   headers) and the packet translated normally.  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 (MF) flag is copied from the M flag in the
      Fragment header.  The Don't Fragments (DF) flag is set to zero
      allowing this packet to be fragmented if required by IPv4 routers.

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

   Protocol:  For ICMPv6 (58) changed to ICMPv4 (1), otherwise Next
      Header field copied from Fragment header.

   If translated packet with DF set to 1 will be larger than the MTU of
   the next-hop interface, then translator MUST drop packet and send
   ICMPv6 "Packet Too Big" (Type 2/Code 0) error message to the IPv6
   host with adjusted MTU in the ICMPv6 message.

4.2.  Translating ICMPv6 Headers into ICMPv4 Headers

   All ICMPv6 messages that are to be translated require that the ICMPv4
   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



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   be IPv4-translatable addresses and there will be no corresponding
   IPv4 addresses represented of this IPv6 address.  In this case, the
   translator can do stateful translation.  A mechanism by which the
   translator can instead do stateless translation is left for future
   work.

   The actions needed to translate various ICMPv6 messages are:

   ICMPv6 informational messages:

      Echo Request and Echo Reply (Type 128 and 129):  Adjust the Type
         values 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, and
         adjust the ICMP checksum both to take the type change into
         account and to exclude the ICMPv6 pseudo-header.  Translate the
         Code field as follows:

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

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

         Code 2 (Beyond scope of source address):  Set Code value to 1
            (Host unreachable).  Note that this error is very unlikely
            since an IPv4-translatable source address is typically
            considered to have global scope.

         Code 3 (Address unreachable):  Set Code value to 1 (Host
            unreachable).







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         Code 4 (Port unreachable):  Set Code value to 3 (Port
            unreachable).

         Other Code values:  Silently drop.

      Packet Too Big (Type 2):  Translate to an ICMPv4 Destination
         Unreachable (Type 3) with Code value equal to 4, and adjust the
         ICMPv4 checksum both to take the type change into account and
         to exclude the ICMPv6 pseudo-header.  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_of_IPv4_nexthop, (MTU_of_IPv6_nexthop)-20)

      Time Exceeded (Type 3):  Set the Type value to 11, and adjust the
         ICMPv4 checksum both to take the type change into account and
         to exclude the ICMPv6 pseudo-header.  The Code field is
         unchanged.

      Parameter Problem (Type 4):  Translate the Type and Code field as
         follows, and adjust the ICMPv4 checksum both to take the type
         change into account and to exclude the ICMPv6 pseudo-header.

         Code 0 (Erroneous header field encountered):  Set Type 12, Code
            0 and update the pointer as defined in Figure 6 (If the
            Original IPv6 Pointer Value is not listed or the Translated
            IPv4 Pointer Value is listed as "n/a", silently drop the
            packet).

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

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

      Unknown error messages:  Silently drop.
















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


            Figure 6: Pointer Value for translating from IPv6 to IPv4

      ICMP Error Payload:  If the received ICMPv6 packet contains an
         ICMPv6 Extension [RFC4884], the translation of the ICMPv6
         packet will cause the ICMPv4 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 may cause
         problems with processing of those ICMP extensions.

4.3.  Translating ICMPv6 Error Messages into ICMPv4

   There are some differences between the ICMPv4 and the ICMPv6 error
   message formats as detailed above.  In addition, the ICMP error
   messages contain the packet in error, which needs to be translated
   just like a normal IP packet.  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 7: IPv6-to-IPv4 ICMP Error Translation

   The translation of the inner IP header can be done by invoking the
   function that translated the outer IP headers.  This process SHOULD
   stop at first embedded header and drop the packet if it contains
   more.  Note that the IPv6 addresses in the IPv6 header may not be
   IPv4-translatable addresses and there will be no corresponding IPv4
   addresses.  In this case, the translator can do stateful translation.
   A mechanism by which the translator can instead do stateless
   translation is left for future work.

4.4.  Translator Sending ICMPv6 Error Message

   If the IPv6 packet is discarded, then the translator SHOULD be able
   to send back an ICMPv6 error message to the original sender of the
   packet, unless the discarded packet is itself an ICMPv6 message.

   If the ICMPv6 error message is being sent because the IPv6 source
   address is not an IPv4-translatable address and the translator is
   stateless, the ICMPv6 message, if sent, has a Type value 1 and Code
   value 5 (Source address failed ingress/egress policy).  In other
   cases, the ICMPv6 message has a Type value of 1 (Destination
   Unreachable) and a Code value of 1 (Communication with destination
   administratively prohibited), unless otherwise specified in this
   document or [I-D.ietf-behave-v6v4-xlate-stateful].  The translator
   SHOULD allow an administrator to configure whether the ICMPv6 error
   messages are sent, rate-limited, or not sent.

4.5.  Transport-layer Header Translation

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



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   performed.

4.6.  Knowing when to Translate

   If the IP/ICMP translator also provides a normal forwarding function,
   and the destination 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 an IPv6 address representing a host in IPv4 domain, the IPv6
   packet MUST be translated to IPv4.


5.  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.


6.  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 deriving an IPv4
   address from an IPv6 address - particularly addresses like broadcast
   or loopback addresses and the non IPv4-translatable IPv6 addresses,
   etc.  The [I-D.ietf-behave-address-format] addresses these issues.

   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



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   receiving packets.


7.  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
   Huang, John Schnizlein, Jouni Korhonen, Kentaro Ebisawa, Kevin Yin,
   Magnus Westerlund, Marcelo Bagnulo Braun, Margaret Wasserman,
   Masahito Endo, Phil Roberts, Philip Matthews, Reinaldo Penno, Remi
   Denis-Courmont, Remi Despres, Senthil Sivakumar, Simon Perreault and
   Zen Cao.


8.  Appendix: Stateless translation workflow example

   A stateless translation workflow example is depicted in the following
   figure.  The document address blocks 2001:DB8::/32 [RFC3849],
   192.0.2.0/24 and 198.51.100.0/24 [RFC5737] are used in this example.


            +--------------+                   +--------------+
            | IPv4 network |                   | IPv6 network |
            |              |     +-------+     |              |
            |   +----+     |-----| XLATE |---- |  +----+      |
            |   | H4 |-----|     +-------+     |--| H6 |      |
            |   +----+     |                   |  +----+      |
            +--------------+                   +--------------+


                                 Figure 8

   A translator (XLATE) connects the IPv6 network to the IPv4 network.
   This XLATE uses the Network Specific Prefix (NSP) 2001:DB8:100::/40
   defined in [I-D.ietf-behave-address-format] to represent IPv4
   addresses in the IPv6 address space (IPv4-converted addresses) and to
   represent IPv6 addresses (IPv4-translatable addresses) in the IPv4
   address space.  In this example, 192.0.2.0/24 is the IPv4 block of
   the corresponding IPv4-translatable addresses.

   Based on the address mapping rule, the IPv6 node H6 has an IPv4-
   translatable IPv6 address 2001:DB8:1C0:2:21:: (address mapping from
   192.0.2.33).  The IPv4 node H4 has IPv4 address 198.51.100.2.

   The IPv6 routing is configured in such a way that the IPv6 packets
   addressed to a destination address in 2001:DB8:100::/40 are routed to



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   the IPv6 interface of the XLATE.

   The IPv4 routing is configured in such a way that the IPv4 packets
   addressed to a destination address in 192.0.2.0/24 are routed to the
   IPv4 interface of the XLATE.

8.1.  H6 establishes communication with H4

   The steps by which H6 establishes communication with H4 are:

   1.  H6 performs the destination address mapping, so the IPv4-
       converted address 2001:DB8:1C6:3364:200:: is formed from
       198.51.100.2 based on the address mapping algorithm
       [I-D.ietf-behave-address-format].

   2.  H6 sends a packet to H4.  The packet is sent from a source
       address 2001:DB8:1C0:2:21:: to a destination address 2001:DB8:
       1C6:3364:200::.

   3.  The packet is routed to the IPv6 interface of the XLATE (since
       IPv6 routing is configured that way).

   4.  The XLATE receives the packet and performs the following actions:

       *  The XLATE translates the IPv6 header into an IPv4 header using
          the IP/ICMP Translation Algorithm defined in this document.

       *  The XLATE includes 192.0.2.33 as source address in the packet
          and 198.51.100.2 as destination address in the packet.  Note
          that 192.0.2.33 and 198.51.100.2 are extracted directly from
          the source IPv6 address 2001:DB8:1C0:2:21:: (IPv4-translatable
          address) and destination IPv6 address 2001:DB8:1C6:3364:200::
          (IPv4-converted address) of the received IPv6 packet that is
          being translated.

   5.  The XLATE sends the translated packet out its IPv4 interface and
       the packet arrives at H4.

   6.  H4 node responds by sending a packet with destination address
       192.0.2.33 and source address 198.51.100.2.

   7.  The packet is routed to the IPv4 interface of the XLATE (since
       IPv4 routing is configured that way).  The XLATE performs the
       following operations:

       *  The XLATE translates the IPv4 header into an IPv6 header using
          the IP/ICMP Translation Algorithm defined in this document.




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       *  The XLATE includes 2001:DB8:1C0:2:21:: as destination address
          in the packet and 2001:DB8:1C6:3364:200:: as source address in
          the packet.  Note that 2001:DB8:1C0:2:21:: and 2001:DB8:1C6:
          3364:200:: are formed directly from the destination IPv4
          address 192.0.2.33 and source IPv4 address 198.51.100.2 of the
          received IPv4 packet that is being translated.

   8.  The translated packet is sent out the IPv6 interface to H6.

   The packet exchange between H6 and H4 continues until the session is
   finished.

8.2.  H4 establishes communication with H6

   The steps by which H4 establishes communication with H6 are:

   1.  H4 performs the destination address mapping, so 192.0.2.33 is
       formed from IPv4-translatable address 2001:DB8:1C0:2:21:: based
       on the address mapping algorithm
       [I-D.ietf-behave-address-format].

   2.  H4 sends a packet to H6.  The packet is sent from a source
       address 198.51.100.2 to a destination address 192.0.2.33.

   3.  The packet is routed to the IPv6 interface of the XLATE (since
       IPv6 routing is configured that way).

   4.  The XLATE receives the packet and performs the following actions:

       *  The XLATE translates the IPv4 header into an IPv6 header using
          the IP/ICMP Translation Algorithm defined in this document.

       *  The XLATE includes 2001:DB8:1C6:3364:200:: as source address
          in the packet and 2001:DB8:1C0:2:21:: as destination address
          in the packet.  Note that 2001:DB8:1C6:3364:200:: (IPv4-
          converted address) and 2001:DB8:1C0:2:21:: (IPv4-translatable
          address) are obtained directly from the source IPv4 address
          198.51.100.2 and destination IPv4 address 192.0.2.33 of the
          received IPv4 packet that is being translated.

   5.  The XLATE sends the translated packet out its IPv6 interface and
       the packet arrives at H6.

   6.  H6 node responds by sending a packet with destination address
       2001:DB8:1C6:3364:200:: and source address 2001:DB8:1C0:2:21::.

   7.  The packet is routed to the IPv6 interface of the XLATE (since
       IPv6 routing is configured that way).  The XLATE performs the



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       following operations:

       *  The XLATE translates the IPv6 header into an IPv4 header using
          the IP/ICMP Translation Algorithm defined in this document.

       *  The XLATE includes 198.51.100.2 as destination address in the
          packet and 192.0.2.33 as source address in the packet.  Note
          that 198.51.100.2 and 192.0.2.33 are formed directly from the
          destination IPv6 address 2001:DB8:1C6:3364:200:: and source
          IPv6 address 2001:DB8:1C0:2:21:: of the received IPv6 packet
          that is being translated.

   8.  The translated packet is sent out the IPv4 interface to H4.

   The packet exchange between H4 and H6 continues until the session
   finished.


9.  References

9.1.  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.

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

   [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers",
              RFC 1812, June 1995.

   [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.



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   [RFC2766]  Tsirtsis, G. and P. Srisuresh, "Network Address
              Translation - Protocol Translation (NAT-PT)", RFC 2766,
              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.

9.2.  Informative References

   [Dongjin]  Lee, D., "Email to the behave mailing list (http://
              www.ietf.org/mail-archive/web/behave/current/
              msg06856.html)", 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-04 (work in progress),
              January 2010.

   [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-06 (work in progress),
              February 2010.

   [I-D.ietf-behave-v6v4-xlate-stateful]
              Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers",
              draft-ietf-behave-v6v4-xlate-stateful-08 (work in
              progress), January 2010.

   [Miller]   Miller, G., "Email to the ngtrans mailing list
              (http://www.mail-archive.com/ipv6@ietf.org/
              msg10159.html)", March 1999.




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   [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
              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.

   [RFC3849]  Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix
              Reserved for Documentation", RFC 3849, July 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.

   [RFC5737]  Arkko, J., Cotton, M., and L. Vegoda, "IPv4 Address Blocks
              Reserved for Documentation", RFC 5737, January 2010.

   [Stasiewicz]
              Stasiewicz, B., "Email to the behave mailing list (http://
              www.ietf.org/mail-archive/web/behave/current/
              msg08093.html)", Feb 2010.





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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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