tech-invite   World Map     

IETF     RFCs     Groups     SIP     ABNFs    |    3GPP     Specs     Glossaries     Architecture     IMS     UICC    |    search     info

RFC 7915

 Errata 
Proposed STD
Pages: 34
Top     in Index     Prev     Next
in Group Index     Prev in Group     No Next: Highest Number in Group     Group: ~icmp

IP/ICMP Translation Algorithm

Part 1 of 2, p. 1 to 15
None       Next RFC Part

Obsoletes:    6145


Top       ToC       Page 1 
Internet Engineering Task Force (IETF)                            C. Bao
Request for Comments: 7915                                         X. Li
Obsoletes: 6145                        CERNET Center/Tsinghua University
Category: Standards Track                                       F. Baker
ISSN: 2070-1721                                            Cisco Systems
                                                             T. Anderson
                                                          Redpill Linpro
                                                                 F. Gont
                                                     Huawei Technologies
                                                               June 2016


                     IP/ICMP Translation Algorithm

Abstract

   This document describes the Stateless IP/ICMP Translation Algorithm
   (SIIT), which translates between IPv4 and IPv6 packet headers
   (including ICMP headers).  This document obsoletes RFC 6145.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7915.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Top       Page 2 
Table of Contents

   1.  Introduction and Motivation . . . . . . . . . . . . . . . . .   3
     1.1.  IPv4-IPv6 Translation Model . . . . . . . . . . . . . . .   3
     1.2.  Applicability and Limitations . . . . . . . . . . . . . .   3
     1.3.  Stateless vs. Stateful Mode . . . . . . . . . . . . . . .   4
     1.4.  Path MTU Discovery and Fragmentation  . . . . . . . . . .   5
   2.  Changes from RFC 6145 . . . . . . . . . . . . . . . . . . . .   5
   3.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Translating from IPv4 to IPv6 . . . . . . . . . . . . . . . .   6
     4.1.  Translating IPv4 Headers into IPv6 Headers  . . . . . . .   7
     4.2.  Translating ICMPv4 Headers into ICMPv6 Headers  . . . . .   9
     4.3.  Translating ICMPv4 Error Messages into ICMPv6 . . . . . .  13
     4.4.  Generation of ICMPv4 Error Message  . . . . . . . . . . .  14
     4.5.  Transport-Layer Header Translation  . . . . . . . . . . .  14
     4.6.  Knowing When to Translate . . . . . . . . . . . . . . . .  15
   5.  Translating from IPv6 to IPv4 . . . . . . . . . . . . . . . .  15
     5.1.  Translating IPv6 Headers into IPv4 Headers  . . . . . . .  17
       5.1.1.  IPv6 Fragment Processing  . . . . . . . . . . . . . .  19
     5.2.  Translating ICMPv6 Headers into ICMPv4 Headers  . . . . .  19
     5.3.  Translating ICMPv6 Error Messages into ICMPv4 . . . . . .  22
     5.4.  Generation of ICMPv6 Error Messages . . . . . . . . . . .  23
     5.5.  Transport-Layer Header Translation  . . . . . . . . . . .  23
     5.6.  Knowing When to Translate . . . . . . . . . . . . . . . .  24
   6.  Mapping of IP Addresses . . . . . . . . . . . . . . . . . . .  24
   7.  Special Considerations for ICMPv6 Packet Too Big  . . . . . .  24
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  25
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  27
   Appendix A.  Stateless Translation Workflow Example . . . . . . .  30
     A.1.  H6 Establishes Communication with H4  . . . . . . . . . .  30
     A.2.  H4 Establishes Communication with H6  . . . . . . . . . .  32
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

Top      ToC       Page 3 
1.  Introduction and Motivation

   This document obsoletes [RFC6145].

   Readers of this document are expected to have read and understood the
   framework described in [RFC6144].  Implementations of this IPv4/IPv6
   translation specification MUST support one or more address mapping
   algorithms, which are defined in Section 6.

1.1.  IPv4-IPv6 Translation Model

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

               ---------          ---------
             //        \\       //         \\
           /             +----+              \
          |              |XLAT|               | XLAT: IP/ICMP
          |   IPv4       +----+   IPv6        |       Translator
          |   Domain     |    |   Domain      |
          |              |    |               |
           \             |    |              /
            \\         //      \\          //
               --------          ---------

                   Figure 1: IPv4-IPv6 Translation Model

   The scenarios of the translation model are discussed in [RFC6144].

1.2.  Applicability and Limitations

   This document specifies the translation algorithms between IPv4
   packets and IPv6 packets.

   As with [RFC6145], the translating function specified in this
   document does not translate any IPv4 options, and it does not
   translate IPv6 extension headers except the Fragment Header.

   The issues and algorithms in the translation of datagrams containing
   TCP segments are described in [RFC5382].

   Fragmented IPv4 UDP packets that do not contain a UDP checksum (i.e.,
   the UDP checksum field is zero) are not of significant use on the
   Internet, and in general will not be translated by the IP/ICMP
   translator (Section 4.5).  However, when the translator is configured
   to forward the packet without a UDP checksum, the fragmented IPv4 UDP
   packets will be translated.

Top      ToC       Page 4 
   Fragmented ICMP/ICMPv6 packets will not be translated by IP/ICMP
   translators.

   The IP/ICMP header translation specified in this document is
   consistent with requirements of multicast IP/ICMP headers.  However,
   IPv4 multicast addresses [RFC5771] cannot be mapped to IPv6 multicast
   addresses [RFC3307] based on the unicast mapping rule [RFC6052].  An
   example of experiments of the multicast address mapping can be found
   in [RFC6219].

1.3.  Stateless vs. Stateful Mode

   An IP/ICMP translator has two possible modes of operation: stateless
   and stateful [RFC6144].  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, or they might have contiguous routing
   connectivity but are interacting via masking addresses (i.e.,
   hairpinning) [RFC4787], and hence are forced to have their
   communications translated.

   In the stateless mode, an IP/ICMP translator will convert IPv4
   addresses to IPv6 and vice versa solely based on the configuration of
   the stateless IP/ICMP translator and information contained within the
   packet being translated.  For example, for the default behavior
   defined in [RFC6052], 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.  Other
   stateless translation algorithms are defined in Section 6.  The
   stateless translator does not keep any dynamic session or binding
   state, thus there is no requirement that the packets in a single
   session or flow traverse a single translator.

   In the stateful mode, a specific IPv6 address range (consisting of
   IPv4-converted IPv6 addresses) will typically represent IPv4 systems.
   The IPv6 nodes may use any IPv6 addresses [RFC4291] except in that
   range.  A stateful IP/ICMP translator continuously maintains a
   dynamic translation table containing bindings between the IPv4 and
   IPv6 addresses, and likely also the Layer-4 identifiers, that are
   used in the translated packets.  The exact address translations of
   any given packet thus become dependent on how packets belonging to
   the same session or flow have been translated.  For this reason,
   stateful translation generally requires that all packets belonging to
   a single flow must traverse the same translator.

Top      ToC       Page 5 
   In order to be able to successfully translate a packet from IPv4 to
   IPv6 or vice versa, the translator must implement an address mapping
   algorithm.  This document does not specify any such algorithms,
   instead these are referenced from Section 6.

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
   Maximum Segment Size (MSS) option [RFC6691].  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 4 and 5 of this document.  The reassembling of
   fragmented packets in the stateful translator is discussed in
   [RFC6146], since it requires state maintenance in the translator.

2.  Changes from RFC 6145

   The changes from RFC 6145 are the following:

   1.  Inserted the notes about IPv6 extension header handling:
       [Err3059], [Err3060], [Err3061], and [Err4090].

   2.  Deprecated the algorithm that generates the IPv6 atomic
       fragments, as a result of the analysis in [ATOMIC] and the
       specification in [IPv6].

   3.  Inserted the notes for stateless source address mapping for
       ICMPv6 packets [RFC6791].

   4.  Supported new address mapping algorithms and moved the discussion
       of these algorithms to Section 6.

3.  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].

Top      ToC       Page 6 
4.  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, and the transport
   checksum is updated as needed, if that transport is supported by the
   translator.  The data portion of the packet is left unchanged.  The
   IP/ICMP translator then forwards the packet based on the IPv6
   destination address.

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

                    Figure 2: IPv4-to-IPv6 Translation

   Path MTU discovery is mandatory in IPv6, but it is optional in IPv4.
   IPv6 routers 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 DF bit, the translator
   MUST ensure that the packet does not exceed the path MTU on the IPv6
   side.  This is done by fragmenting the IPv4 packet (with Fragment
   Headers) so that it fits in 1280-byte IPv6 packets, since that is the

Top      ToC       Page 7 
   minimum IPv6 MTU.  The IPv6 Fragment Header has been shown to cause
   operational difficulties in practice due to limited firewall
   fragmentation support, etc.  In an environment where the network
   owned/operated by the same entity that owns/operates the translator,
   the translator MUST provide a configuration function for the network
   administrator to adjust the threshold of the minimum IPv6 MTU to a
   value that reflects the real value of the minimum IPv6 MTU in the
   network (greater than 1280 bytes).  This will help reduce the chance
   of including the Fragment Header in the packets.

   When the IPv4 sender does not set the DF bit, the translator MUST NOT
   include the Fragment Header for the non-fragmented IPv6 packets.

   The rules in Section 4.1 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.

   Other than the special rules for handling fragments and path MTU
   discovery, the actual translation of the packet header consists of a
   simple translation 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.

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

4.1.  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 a user-defined length (hereinafter referred to as
   "lowest-ipv6-mtu", and which defaults to 1280 bytes), the packet
   SHOULD be fragmented so that the resulting IPv6 packet (with Fragment
   Header added to each fragment) will be less than or equal to lowest-
   ipv6-mtu, For example, if the packet is fragmented prior to the
   translation, the IPv4 packets should 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
   translator MUST provide a configuration function for the network
   administrator to adjust the threshold of the minimum IPv6 MTU to a
   value greater than 1280 bytes if the real value of the minimum IPv6
   MTU in the network is known to the administrator.  The resulting
   fragments are then translated independently using the logic described
   below.

Top      ToC       Page 8 
   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.

   The IPv6 header fields are set as follows:

   Version:  6

   Traffic Class:  By default, copied from the 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
      support an administratively configurable option 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 the IPv4 header, minus the
      size of the IPv4 header and IPv4 options, if present.

   Next Header:  For ICMPv4 (1), it is changed to ICMPv6 (58);
      otherwise, the protocol field MUST be copied from the 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) MUST check for zero and send the ICMPv4 "TTL Exceeded" or
      ICMPv6 "Hop Limit Exceeded" error.

   Source Address:  Mapped to an IPv6 address based on the algorithms
      presented in Section 6.

      If the translator gets an illegal source address (e.g., 0.0.0.0,
      127.0.0.1, etc.), the translator SHOULD silently discard the
      packet (as discussed in Section 5.3.7 of [RFC1812]).  Note when
      translating ICMPv4 Error Messages into ICMPv6, the "illegal"
      source address will be translated for the purpose of trouble
      shooting.

   Destination Address:  Mapped to an IPv6 address based on the
      algorithms presented in Section 6.

Top      ToC       Page 9 
   If any IPv4 options are present in the IPv4 packet, they MUST be
   ignored and the packet translated normally; there is no attempt to
   translate the options.  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 packet is a fragment
   or the DF bit is not set and the packet size is greater than the
   minimum IPv6 MTU in the network set by the translator configuration
   function), the header fields are set as above with the following
   exceptions:

   IPv6 fields:

      Payload Length:  Total length value from the 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), it is changed to ICMPv6 (58);
         otherwise, the protocol field MUST be copied from the 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.

4.2.  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 MUST have the Type translated and,
   for ICMPv4 error messages, the included IP header also MUST be
   translated.

Top      ToC       Page 10 
   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.

      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 Multicast Listener Discovery (MLD)
         messages specified in [RFC2710], [RFC3590], and [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):  Translate the Code as
            described below, set the Type to 1, and adjust the ICMP
            checksum both to take the type/code change into account and
            to include the ICMPv6 pseudo-header.

            Translate the Code as follows:

            Code 0, 1 (Net Unreachable, Host Unreachable):  Set the Code
               to 0 (No route to destination).

Top      ToC       Page 11 
            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 the Code to 4 (Port
               unreachable).

            Code 4 (Fragmentation Needed and DF was Set):  Translate to
               an ICMPv6 Packet Too Big message (Type 2) with Code set
               to 0.  The MTU field MUST be adjusted for the difference
               between the IPv4 and IPv6 header sizes, but MUST NOT be
               set to a value smaller than the minimum IPv6 MTU (1280
               bytes).  That is, it should be set to

               maximum(1280,
                 minimum((MTU value in the Packet Too Big Message) + 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 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,
               but that is larger than or equal to 1280.)

               See also the requirements in Section 7.

            Code 5 (Source Route Failed):  Set the Code to 0 (No route
               to destination).  Note that this error is unlikely since
               source routes are not translated.

            Code 6, 7, 8:  Set the Code to 0 (No route to destination).

            Code 9, 10  (Communication with Destination Host
               Administratively Prohibited): Set the Code to 1
               (Communication with destination administratively
               prohibited).

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

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

            Code 14 (Host Precedence Violation):  Silently drop.

Top      ToC       Page 12 
            Code 15 (Precedence cutoff in effect):  Set the Code to 1
               (Communication with destination administratively
               prohibited).

            Other Code values:  Silently drop.

         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 to 3, and adjust the
            ICMP checksum both to take the type change into account and
            to include the ICMPv6 pseudo-header.  The Code is unchanged.

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

            Translate the Code as follows:

            Code 0 (Pointer indicates the error):  Set the Code 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 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.

Top      ToC       Page 13 
    +--------------------------------+--------------------------------+
    |   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
            the extensions as opaque bit strings, and those containing
            IPv4 address literals will not have their included addresses
            translated to IPv6 address literals; this may cause problems
            with processing of those ICMP extensions.

4.3.  Translating ICMPv4 Error Messages into ICMPv6

   There are some differences between the ICMPv4 and the ICMPv6 error
   message formats as detailed above.  The ICMP error messages
   containing the packet in error MUST be translated just like a normal
   IP packet (except the TTL value of the inner IPv4/IPv6 packet).  If
   the translation of this "packet in error" changes the length of the
   datagram, the Total Length field in the outer IPv6 header MUST be
   updated.

Top      ToC       Page 14 
              +-------------+                 +-------------+
              |    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 MUST
   stop at the first embedded header and drop the packet if it contains
   more embedded headers.

4.4.  Generation of 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 error
   message.  The ICMPv4 message, if sent, has a Type of 3 (Destination
   Unreachable) and a Code of 13 (Communication Administratively
   Prohibited), unless otherwise specified in this document or in
   [RFC6146].  The translator SHOULD allow an administrator to configure
   whether the ICMPv4 error messages are sent, rate-limited, or not
   sent.

4.5.  Transport-Layer Header Translation

   If the address translation algorithm is not checksum neutral (see
   Section 4.1 of [RFC6052]), the recalculation and updating of the
   transport-layer headers that contain pseudo-headers need to be
   performed.  Translators MUST do this for TCP and ICMP packets and for
   UDP packets that contain a UDP checksum (i.e., the UDP checksum field
   is not zero).

   For UDP packets that do not contain a UDP checksum (i.e., the UDP
   checksum field is zero), the translator SHOULD provide a
   configuration function to allow:

Top      ToC       Page 15 
   1.  Dropping the packet and generating a system management event that
       specifies at least the IP addresses and port numbers of the
       packet.

   2.  Calculating an IPv6 checksum and forwarding the packet (which has
       performance implications).

       A stateless translator cannot compute the UDP checksum of
       fragmented packets, so 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 that specifies at least the IP
       addresses and port numbers in the packet.

       For a stateful translator, the handling of fragmented UDP IPv4
       packets with a zero checksum is discussed in [RFC6146],
       Section 3.4.

   Other transport protocols (e.g., the Datagram Congestion Control
   Protocol (DCCP)) are OPTIONAL to support.  In order to ease debugging
   and troubleshooting, translators MUST forward all transport protocols
   as described in the "Next Header" step of Section 4.1.

4.6.  Knowing When to Translate

   If the IP/ICMP translator also provides a 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.



(page 15 continued on part 2)

Next RFC Part