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Versions: 00 01 02 draft-ietf-behave-v6v4-xlate

behave                                                        X. Li, Ed.
Internet-Draft                                               C. Bao, Ed.
Intended status: Standards Track       CERNET Center/Tsinghua University
Expires: August 25, 2009                                   F. Baker, Ed.
                                                           Cisco Systems
                                                       February 21, 2009


                     IP/ICMP Translation Algorithm
                 draft-baker-behave-v4v6-translation-02

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 25, 2009.

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.






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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 with neither state nor configuration 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.

   Significant issues exist in the stateless and stateful modes that are
   not addressed in this document, related to the address assignment and
   the maintenance of the translation tables, respectively.  This
   document confines itself to the actual translation.

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.

   2.  Moving the address format to the framework document, to
       coordinate with other drafts on the topic.

   3.  Description of both stateful and stateless operation.

   4.  Some changes in ICMP.

   5.  Updating references.






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Requirements

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].


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.  IPv4-embedded IPv6 addresses and IPv4-related IPv6
           addresses  . . . . . . . . . . . . . . . . . . . . . . . .  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  . . . . . . 13
     2.5.  Transport-layer Header Translation . . . . . . . . . . . . 13
     2.6.  Knowing when to Translate  . . . . . . . . . . . . . . . . 14
   3.  Translating from IPv6 to IPv4  . . . . . . . . . . . . . . . . 14
     3.1.  Translating IPv6 Headers into IPv4 Headers . . . . . . . . 15
     3.2.  Translating ICMPv6 Headers into ICMPv4 Headers . . . . . . 17
     3.3.  Translating ICMPv6 Error Messages into ICMPv4  . . . . . . 18
     3.4.  Transport-layer Header Translation . . . . . . . . . . . . 19
     3.5.  Knowing when to Translate  . . . . . . . . . . . . . . . . 19
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
















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1.  Introduction and Motivation

   An understanding of the framework presented in [FRAMEWORK] is
   presumed in this document.  With that remark...

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

   The SIIT [RFC2765] is designed for the case for small networks (e.g.,
   a single subnet) and for a site which has IPv6-only hosts in a dual
   IPv4/IPv6 network.  This use assumes a mechanism for the IPv6 nodes
   to acquire a temporary address from the pool of IPv4 addresses.
   However, SIIT is not to be useful in the case when the IPv6 nodes to
   acquire temporary IPv4 addresses from a "distant" SIIT box operated
   by a different administration, or require that the IPv6 routing
   contain routes for IPv6-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 deletion problem, it is
   desirable that a single IPv4 address needs to be shared via transport
   port multiplexing technique for different IPv6 nodes when they
   communicate with other IPv4 hosts.

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

   The detailed discussion of the transition scenarios is presented in
   [FRAMEWORK], the technical specifications of the translation
   algorithm itself is illustrated in this document.

1.1.  Translation Model

   This document specifies the traslation algorithm that is one of the
   components descrbed in [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.  The other network either routes IPv6 but not IPv4,
   or contains some hosts that only implement IPv6.  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
   [RFC2765].

   The translation algorithm can be used no only in a subnet or small
   networks, but can also be used in the autonomous system scope.

   The translating function as 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 [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 over wide-
   areas in the Internet and will not be translated by the IP/ICMP



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   translator [Miller].

   The considerations of The IPSec [RFC4301] [RFC4302] [RFC4303]
   functionality discussed in [RFC2765] are applicable in this case as
   well.

   IPv4 multicast addresses [RFC3171] can not be mapped to IPv6
   multicast addresses [RFC3307] based on the unicast mapping rule.
   However, special rule of the address translation can be created for
   the multicast packet translation algorithm and 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.  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 in either domain and
   hence are forced to have their communications translated.

   In the stateless mode, one system has an IPv4 address and one has an
   address of the form specified in [FRAMEWORK], which is explicitly
   mapped to an IPv4 address.  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.

   In the stateful mode, the system with the IPv4 address will be
   represented by that same address type, but the IPv6 system may use
   any [RFC4291] address except one in that range.  In this case, a
   translation table is required.

1.4.  IPv4-embedded IPv6 addresses and IPv4-related IPv6 addresses

   In SIIT [RFC2765] an IPv6 node should send an IPv6 packet where the
   destination address is the IPv4-mapped address and the source address
   is the node's temporarily assigned IPv4-translated address.  If the
   node does not have a temporarily assigned IPv4-translated address it
   should acquire one.  Different from the SIIT model, as describled in
   [FRAMEWORK] the new forms of the IPv6 addresses are introduced.

   The IPv4-embedded IPv6 addresses are the IPv6 addresses which have
   unique relationship to specific IPv4 addresses.  This relationship is
   self described by embedding IPv4 address in the IPv6 address.  The
   IPv4-embedded IPv6 addresses are used for both the statesless and the
   stateful modes.

   The IPv4-related IPv6 addresses are the IPv6 addresses which have



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   unique relationship to specific IPv4 addresses.  This relationship is
   maintained as the states (mapping table between IPv4 address/
   transport_port and IPv6 address/transport_port) in the IP/ICMP
   translator.  The states are session initiated.  The IPv4-related IPv6
   addresses are used fo the stateful mode only.


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 consist of check sums which
   include the IP header, the recalculation and updating of the ICMP
   header and the transport-layer headers MUST be performed.  This is
   different from [RFC2765], since [RFC2765] uses special prefix
   (0::ffff:0:a:b:c:d) to avoid the recalculation of the transport-layer
   header checksum.  The data portion of the packet are 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.




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   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
   might send back ICMP "packet too big" messages to the sender.  When
   these ICMP errors are sent by the IPv6 routers they will pass through
   a translator which 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 perform path MTU discovery 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 packet since IPv6 guarantees that 1280
   byte packets never need to be fragmented.  Also, when the IPv4 sender
   does not perform path MTU discovery 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 that the low-order 16 bits of the fragment
   identification is carried end-end to ensure 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 packet is not a fragment (i.e., the MF
   flag is not set and the Fragment Offset is zero) then there is no
   need to 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 and
      Precedence field (all 8 bits are copied).  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.

   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:  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 or ICMPv6 "ttl exceeded" error.

   Source Address:  The source address is derived from the IPv4 source
      address to form IPv4-embedded IPv6 address as specified in
      [FRAMEWORK].

   Destination Address:  In stateless mode, which is to say that if the
      IPv4 destination address is within the range of the stateless
      translation prefix described in Section 1.3, the destination
      address is derived from the IPv4 destination address to form IPv4-
      embedded IPv6 address in [FRAMEWORK] [I-D.baker-behave-ivi].

      In stateful mode, which is to say that if the IPv4 destination
      address is not within the range of the stateless translation
      prefix described in Section 1.3, the IPv6 address (IPv4-related
      IPv6 address) and transport layer destination port corresponding
      to the IPv4 destination address and destination port are derived
      from the database reflecting current session state in the
      translator [I-D.bagnulo-behave-nat64].

   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



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   sender.

   If there is 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:  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

   If a UDP packet has a zero UDP checksum then a valid checksum must be
   calculated in order to translate the packet.  A stateless translator
   can not do this for fragmented packets but [Miller] indicates that
   fragmented UDP packets with a zero checksum appear to only be used
   for malicious purposes.  Thus this is not believed to be a noticeable
   limitation.

   When a 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.

   When a translator receives an unfragmented UDP IPv4 packet and the
   checksum field is zero the translator MUST 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.




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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
         ICMPv4 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.  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 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.  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).

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

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






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         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.  The
            Pointer needs to be updated to point to the corresponding
            field in the translated include IP header.

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.  Transport-layer Header Translation

   For the IPv6 addresses described in [FRAMEWORK], the recalculation
   and updating of the transport-layer headers MUST be performed.







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2.6.  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 should 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 consist of check sums which
   include the IP header, the recalculation and updating of the ICMP
   header and the transport-layer headers MUST be performed.  This is
   different from [RFC2765], since [RFC2765] uses special prefix
   (0::ffff:0:a:b:c:d) to avoid the recalculation of the transport-layer
   header checksum.  The data portion of the packet are 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 effect the translation.



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   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 an translator since the IPv6 node
   might receive ICMP "packet too big" messages originated by an IPv4
   router that report an MTU less than 1280.  However, [RFC2460]
   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 sender since the presence of an IPv6
   Fragment header is interpreted that is it OK 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 can not 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 content 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.




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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 or ICMPv6 "ttl exceeded" error.

   Protocol:  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 stateless translation
      prefix described in Section 1.3, the source address is derived
      from the IPv4-embedded IPv6 address as specified in [FRAMEWORK]
      [I-D.baker-behave-ivi].

      In stateful mode, which is to say that if the IPv6 source address
      is not within the range of the stateless translation prefix
      described in Section 1.3, the IPv4 source address and transport
      layer source port corresponding to the IPv6 source address (IPv4-
      related IPv6 address) and source port are derived from the
      database reflecting current session state in the translator as
      described in [I-D.bagnulo-behave-nat64].

   Destination Address:  IPv6 packets that are translated have an IPv4-
      mapped destination address.  Thus the address is derived from the
      IPv6 address as specified in [FRAMEWORK].

   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
   attempt to translate them.  However, the Total Length field and the
   Protocol field would have to be 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



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   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:  Next Header value 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 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 ICMPv6 messages are:

   ICMPv6 informational messages:

      Echo Request and Echo Reply (Type 128 and 129)  Adjust the type to
         0 and 8, 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.





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      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.

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

      Parameter Problem (Type 4)  If the Code is 1 translate this to an
         ICMPv4 protocol unreachable (Type 3, Code 2).  Otherwise set
         the Type to 12 and the Code to zero.  The Pointer needs to be
         updated to point to the corresponding field in the translated
         include IP header.

      Unknown error messages  Silently drop.

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



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   the Total Length field in the outer IPv4 header might need to be
   updated.


              +-------------+                 +-------------+
              |    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.

3.4.  Transport-layer Header Translation

   For the IPv6 addresses described in [FRAMEWORK], the recalculation
   and updating of the transport-layer headers MUST be performed.

3.5.  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 should forward it
   without translating it.  When an IP/ICMP translator receives an IPv6
   datagram addressed to a destination towards the IPv4 domain, the
   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



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   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 which are
   used to make the packets reach the translator.

   As the Authentication Header [RFC4302] is specified to include the
   IPv4 Identification field and the translating function not being able
   to always preserve the Identification field, it is not possible for
   an IPv6 endpoint to compute 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, Ed Jankiewicz, Fred
   Baker, Hiroshi Miyata, Iljitsch van Beijnum, John Schnizlein, Kevin
   Yin, Magnus Westerlund, Marcelo Bagnulo Braun, Margaret Wasserman,
   Masahito Endo, Phil Roberts, Philip Matthews, Remi Denis-Courmont,
   Remi Despres, and Xing Li.


7.  References

7.1.  Normative References

   [FRAMEWORK]
              Baker, F., "Framework for IPv4/IPv6 Translation - baker-
              behave-v4v6-framework", October 2008.

   [I-D.bagnulo-behave-nat64]
              Bagnulo, M., Matthews, P., and I. Beijnum, "NAT64: Network



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              Address and Protocol Translation from IPv6 Clients to IPv4
              Servers", draft-bagnulo-behave-nat64-02 (work in
              progress), November 2008.

   [I-D.baker-behave-ivi]
              Li, X., Bao, C., Baker, F., and K. Yin, "IVI Update to
              SIIT and NAT-PT", draft-baker-behave-ivi-01 (work in
              progress), September 2008.

   [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.

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

7.2.  Informative References

   [I-D.petithuguenin-behave-stun-pmtud]
              Petit-Huguenin, M., "Path MTU Discovery Using Session
              Traversal Utilities for NAT (STUN)",
              draft-petithuguenin-behave-stun-pmtud-02 (work in
              progress), November 2008.



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

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, August 1989.

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

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

   [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.

   [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.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [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.

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

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




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   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

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

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


Authors' Addresses

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

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


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

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


   Fred Baker (editor)
   Cisco Systems
   Santa Barbara, California  93117
   USA

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











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