Using Dummy IPv4 Address and Node Identification Extensions for IP/ICMP translators (XLATs)
draft-ietf-v6ops-icmpext-xlat-v6only-source-01

IPv6 operations                                             D. Lamparter
Internet-Draft                                              NetDEF, Inc.
Obsoletes: 6791 (if approved)                                 J. Linkova
Updates: 7915 (if approved)                                       Google
Intended status: Standards Track                         6 November 2025
Expires: 10 May 2026

Using Dummy IPv4 Address and Node Identification Extensions for IP/ICMP
                          translators (XLATs)
             draft-ietf-v6ops-icmpext-xlat-v6only-source-01

Abstract

   This document suggests that when a source IPv6 address of an ICMPv6
   message can not be translated to an IPv4 address, the protocol
   translators use the dummy IPv4 address (192.0.0.8) to translate the
   IPv6 source address, and utilize the ICMP extension for Node
   Identification (draft-ietf-intarea-extended-icmp-nodeid) to carry the
   original IPv6 source address of ICMPv6 messages.  This document
   obsoletes RFC6791, Stateless Source Address Mapping for ICMPv6
   Packets and updates IP/ICMP Translation Algorithm (RFC7915).

About This Document

   This note is to be removed before publishing as an RFC.

   Source, version control history, and issue tracker for this draft can
   be found at https://github.com/eqvinox/icmpext-clat-source.

   (Note the draft was renamed (from "clat" to "xlat") prior to
   submission but changing the repository name on github breaks too many
   things to be worth the effort.)

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on 10 May 2026.

Copyright Notice

   Copyright (c) 2025 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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Translation Behavior  . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Adding Node Identification Extension Object . . . . . . .   5
       4.2.1.  Order of Operations and Translating ICMPv6 Packet Too
               Big . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Previous Work . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Updates to RFC7915  . . . . . . . . . . . . . . . . . . . . .   6
   7.  Applicability Considerations  . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   11. Appendix  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     11.1.  Adding a Node Identification Extension Object: Suggested
            Algorithm  . . . . . . . . . . . . . . . . . . . . . . .   8
       11.1.1.  Adding a New ICMP Extension Structure  . . . . . . .   8
       11.1.2.  Adding a Node Identification Extension Object to an
               Existing ICMP Extension Structure . . . . . . . . . .   9
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     12.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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1.  Introduction

   To allow communication between IPv6-only and IPv4-only devices, IPv4/
   IPv6 translators translate IPv6 and IPv4 packet headers according to
   the IP/ICMP Translation Algorithm defined in [RFC7915].  For example,
   464XLAT ([RFC6877]) defines an architecture for providing IPv4
   connectivity across an IPv6-only network.  The solution contains two
   key elements: provider-side translator (PLAT, usually in a form of
   stateful NAT64, [RFC6146])) and customer-side translator (CLAT).
   CLAT implementations translate private IPv4 addresses to global IPv6
   addresses, and vice versa, as defined in [RFC7915].

   To map IPv4 addresses to IPv6 ones the translators use the
   translation prefix (either a well-known or a network-specific one,
   see [RFC6052]).  The resulting IPv6 addresses can be statelessly
   translated back to IPv4.  However, this is not the case for arbitrary
   global IPv6 addresses; those addresses can only be translated to IPv4
   by stateful translators and only if a corresponding translation rule
   exists.

   One scenario where this is necessary is translating ICMPv6 error
   messages sent by intermediate nodes to the CLAT address in a 464XLAT
   environment.  The most typical example is a diagnostic tool like
   traceroute sending packets to an IPv4 destination from an IPv6-only
   host.  Received ICMPv6 Time Exceeded messages are translated to ICMP
   Time Exceeded messages.  If those packets were originated from an
   IPv4 address and translated to ICMPv6 by the PLAT (NAT64) device,
   then the source address of such a packet can be mapped back to IPv4
   by removing the translation prefix.  However ICMPv6 error messages
   sent by devices located between the IPv6-only host and the NAT64
   device have "native" IPv6 source addresses, which cannot be mapped
   back to IPv4.  Those packets are usually dropped and tools like
   traceroute can not represent IPv6 intermediate hops in any meaningful
   way.  Such behaviour complicates troubleshooting.  It might also be
   confusing for users and increases operational costs, as users report
   packet loss in the network based on traceroute output.

   It should be noted that a similar issue occurs in IPv6 Data Center
   Environments when an ICMPv6 error message is sent to an IPv4-only
   client.  As per Section 4.8 of [RFC7755], ICMPv6 error packets are
   usually dropped by the translator.

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   [I-D.ietf-intarea-extended-icmp-nodeid] defines the Node
   Identification Object which can carry an IP Address Sub-Object,
   containing an IP address.  This document proposes that when an ICMPv6
   error message is translated to an ICMP one, and the IPv6 source
   address cannot be mapped or translated to an IPv4 one, the translator
   uses the dummy IPv4 address as the IPv4 source and appends a Node
   Identification Object with an IP Address Sub-Object, containing the
   IPv6 address of the original ICMPv6 error message.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   Translator: a device performing translation between IPv6 and IPv4
   packet headers according to the IP/ICMP Translation Algorithm defined
   in [RFC7915].  Translators can be stateless ([RFC7915]) or stateful
   ([RFC6146]).

   Translatable IPv6 address: an IPv6 address which matches the NAT64
   prefix known to the translator, or for which the translator has a
   stateful entry, mapping that IPv6 address to an IPv4 one.

   Untranslatable IPv6 address: an IPv6 address which does not match the
   NAT64 prefix(es) configured on the translator, and for which the
   translator does not have a stateful entry, mapping this IPv6 address
   to an IPv4 one.

4.  Translation Behavior

4.1.  Overview

   Whenever a translator translates an ICMPv6 Destination Unreachable,
   ICMPv6 Time Exceeded or ICMPv6 Packet Too Big ([RFC4443]) to the
   corresponding ICMPv4 ([RFC0792]) message, and the IPv6 source address
   in the outermost IPv6 header is untranslatable, the translator SHOULD
   use the dummy IPv4 address (192.0.0.8) as the IPv4 source address for
   the translated packet.

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   To preserve the original IPv6 source address of the packet, the
   translator SHOULD append a Node Identification Object
   ([I-D.ietf-intarea-extended-icmp-nodeid]) with an IP Address Sub-
   Object containing the IPv6 source address of the original ICMPv6
   packet.

   The translator MUST NOT use 192.0.0.8/32 to translate the source IPv6
   address and MUST NOT add the extension if the packet's IPv6 source
   address is translatable.

4.2.  Adding Node Identification Extension Object

   A Node Identification Extension Object SHOULD be added when
   translating:

   *  ICMPv6 Destination Unreachable to ICMPv4 Destination Unreachable

   *  ICMPv6 Time Exceeded to ICMPv4 Time Exceeded.

   *  ICMPv6 Packet Too Big to ICMPv4 Destination Unreachable.

   and the IPv6 source address in the outermost IPv6 header is
   untranslatable.

   When adding the Node Identification Extension Object, the translator
   MUST include the IP Address Sub-Object containing the original IPv6
   source address of the packet.

   This document doesn't prescribe the exact procedure for adding the
   Node Identification Extension Object when translating ICMPv6 messages
   to ICMPv4.  Section 11.1 describes one possible approach, but the
   choice is left to implementors, as long as the following requirements
   are met:

   *  The resulting ICMPv4 message contains the Node Identification
      Extension Object with the IP Address Sub-Object.

   *  The checksum field of the Extension Header (Section 7 of
      [RFC4884]) is updated accordingly.

4.2.1.  Order of Operations and Translating ICMPv6 Packet Too Big

   This specification does not prescribe whether the Node Identification
   Extension Object is added before or after translating an ICMPv6
   message to ICMPv4.  This choice is up to the implementation.  However
   ICMP Extensions can not be added to ICMPv6 Packet Too Big messages
   (see Section 4.6 of [RFC4884]).  Therefore in order to be able to add
   the Node Identification Extension Object and preserve the original

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   untranslatable IPv6 address, the translator needs to add the object
   to the resulting ICMPv4 packet after it's been translated from
   ICMPv6.  The translator MAY choose to not append the Node
   Identification Extension Object when translating an ICMPv6 Packet Too
   Big to an ICMPv4 Destination Unreachable.  Such implementations
   SHOULD still translate ICMPv6 Packet Too Big from untranslatable
   sources using 192.0.0.8 as an IPv4 source address and SHOULD NOT drop
   those packets.

5.  Previous Work

   [RFC6791] proposes using the Interface Information Object and
   Interface IP Address Sub-Object defined in [RFC5837] to preserve
   information about untranslatable IPv6 addresses.  However it should
   be noted that Section 4.2 of [RFC5837] suggests that an IPv4 packet
   MUST only contain an IP Interface Sub-Object representing an IPv4
   address.  Therefore using the mechanism described in [RFC6791]
   requires updating [RFC5837].

   More importantly, Section 3.2 of [RFC6791] recommends using a single
   (or small pool of) public IPv4 address as the source address of the
   translated ICMP message.  Such an approach assumes that the
   translator is configured with at least one public IPv4 address, which
   is often not feasible for CLAT instances running on endpoints like
   mobile phones, laptops etc.

   The solution proposed in this document has a number of benefits:

   *  It does not require public IPv4 addresses configured on the
      translator.

   *  No changes to processing of the Interface Information Object are
      required.

   *  Using a dedicated IPV4 dummy address 192.0.0.8 indicates to the
      user that it's not an actual IPv4 address of the intermediate
      node.

   Therefore this document deprecates [RFC6791].

6.  Updates to RFC7915

   This document makes the following changes to Section 5.1
   ("Translating IPv6 Headers into IPv4 Headers") of [RFC7915]:

   OLD TEXT

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   |  Source Address: Mapped to an IPv4 address based on the algorithms
   |  presented in Section 6.

   NEW TEXT

   |  Source Address: Mapped to an IPv4 address based on the algorithms
   |  presented in Section 6.  When translating ICMPv6 error messages to
   |  ICMPv4 error messages and the valid IPv6 source address in the
   |  outermost IPv6 header can not be mapped to an IPv4 address (the
   |  address does not match the prefix used in algorithmic mapping and
   |  there are no static or stateful entries for that address), the
   |  translator SHOULD follow the recommendations in draft-ietf-v6ops-
   |  icmpext-xlat-v6only-source.

   This document also updates the very last paragraph of Section 5.2 of
   [RFC7915] ("Error payload:") as follows:

   OLD TEXT:

   |  For extensions not defined in [RFC4884], the translator passes the
   |  extensions as opaque bit strings and any IPv6 address literals
   |  contained therein will not be translated to IPv4 address literals;
   |  this may cause problems with processing of those ICMP extensions.

   NEW TEXT:

   |  For extensions not defined in [RFC4884], the translator passes the
   |  extensions as opaque bit strings and any IPv6 address literals
   |  contained therein will not be translated to IPv4 address literals;
   |  this may cause problems with processing of those ICMP extensions.
   |  If the valid IPv6 source address in the outermost IPv6 header of
   |  the ICMPv6 messages cannot be mapped to an IPv4 address (the
   |  address does not match the prefix used in algorithmic mapping and
   |  there are no static or stateful entries for that address), the
   |  translator SHOULD follow the recommendations in draft-equinox-
   |  intarea-icmpext-xlat-source.

7.  Applicability Considerations

   The mechanism described in this document necessitates that the
   translator distinguishes between ICMPv6 packets originating from
   untranslatable addresses requiring translation (triggered by an IPv4
   packet translated to IPv6) and native IPv6 traffic that does not.
   When the translator employs dedicated IPv6 address(es) for IPv4
   translation (e.g., a CLAT instance acquiring dedicated address(es) or
   a dedicated /64), this differentiation is straightforward.

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   However, if the same IPv6 address is used for both IPv4 translation
   and native IPv6 traffic, the translator may require more complex
   techniques to differentiate.  These techniques could include
   maintaining state and/or analyzing the invoking packet header within
   the ICMPv6 message body to determine if the invoking packet was
   translated

8.  Security Considerations

   This document does not introduce new security considerations.

9.  Privacy Considerations

   This document does not introduce any privacy considerations.

10.  IANA Considerations

   This memo includes no request to IANA.

11.  Appendix

11.1.  Adding a Node Identification Extension Object: Suggested
       Algorithm

11.1.1.  Adding a New ICMP Extension Structure

   If the original ICMPv6 message does not contain an ICMP Extension
   Structure (as defined in Section 7 of [RFC4884]), the translator
   SHOULD append a new ICMP Extension Structure to the ICMP message.
   When adding the new Extension Structure, the translator MUST:

   *  Create a new ICMP Extension Structure, containing one Extension
      Header and one Node Identification Extension object.  The Node
      Identification Extension object MUST contain an IP Address Sub-
      Object, carrying the IPv6 source address of the ICMPv6 message
      being translated.

   *  Append that Extension Structure to the ICMP message.

   *  If the resulting packet size exceeds the minimum IPv6 MTU:
      truncate the embedded invoking packet by removing the trailing 28
      octets (to accommodate for 4 octets of the extension header and 24
      octets of the extension object).

   *  Set the length field of the ICMP message to the length of the
      padded "original datagram" field, measured in 32-bit words.

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11.1.2.  Adding a Node Identification Extension Object to an Existing
         ICMP Extension Structure

   If the original ICMPv6 message already contains an ICMP Extension
   Structure, the translator SHOULD append a Node Identification
   Extension object containing the IP Address Sub-Object to that
   structure.  When appending the object, the translator MUST:

   *  Create a Node Identification Extension object containing an IP
      Address Sub-Object.  The IP Address Sub-Object MUST contain the
      original source IPv6 address of the ICMPv6 message being
      translated.

   *  Append a Node Identification Extension object to the Extension
      Structure.

   *  Update the checksum field of the Extension Header accordingly.

   *  If the resulting packet size exceeds the minimum IPv6 MTU:
      truncate the embedded invoking packet by removing the trailing 24
      octets (to accommodate for 24 octets of the extension object) and
      update the length field of the ICMP message

12.  References

12.1.  Normative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC4884]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
              "Extended ICMP to Support Multi-Part Messages", RFC 4884,
              DOI 10.17487/RFC4884, April 2007,
              <https://www.rfc-editor.org/info/rfc4884>.

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   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010,
              <https://www.rfc-editor.org/info/rfc6052>.

   [RFC7915]  Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,
              "IP/ICMP Translation Algorithm", RFC 7915,
              DOI 10.17487/RFC7915, June 2016,
              <https://www.rfc-editor.org/info/rfc7915>.

   [RFC6791]  Li, X., Bao, C., Wing, D., Vaithianathan, R., and G.
              Huston, "Stateless Source Address Mapping for ICMPv6
              Packets", RFC 6791, DOI 10.17487/RFC6791, November 2012,
              <https://www.rfc-editor.org/info/rfc6791>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [I-D.ietf-intarea-extended-icmp-nodeid]
              Fenner, B. and R. Thomas, "Adding Extensions to ICMP
              Errors for Originating Node Identification", Work in
              Progress, Internet-Draft, draft-ietf-intarea-extended-
              icmp-nodeid-04, 19 August 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-intarea-
              extended-icmp-nodeid-04>.

12.2.  Informative References

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <https://www.rfc-editor.org/info/rfc6146>.

   [RFC5837]  Atlas, A., Ed., Bonica, R., Ed., Pignataro, C., Ed., Shen,
              N., and JR. Rivers, "Extending ICMP for Interface and
              Next-Hop Identification", RFC 5837, DOI 10.17487/RFC5837,
              April 2010, <https://www.rfc-editor.org/info/rfc5837>.

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,
              <https://www.rfc-editor.org/info/rfc6877>.

   [RFC7755]  Anderson, T., "SIIT-DC: Stateless IP/ICMP Translation for
              IPv6 Data Center Environments", RFC 7755,
              DOI 10.17487/RFC7755, February 2016,
              <https://www.rfc-editor.org/info/rfc7755>.

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Acknowledgements

   This document is the result of discussions with Thomas Jensen.  The
   authors would like to thank Ondřej Caletka, Lorenzo Colitti, Darren
   Dukes, Bill Fenner, Tobias Fiebig, and Jordi Palet Martínez for their
   feedback, comments and guidance.  The authors would like to
   particularly thank Tore Anderson for pointing out the existence and
   relevance of [RFC6791].

Authors' Addresses

   David 'equinox' Lamparter
   NetDEF, Inc.
   04229 Leipzig
   Germany
   Email: equinox@diac24.net, equinox@opensourcerouting.org

   Jen Linkova
   Google
   1 Darling Island Rd
   Pyrmont NSW 2009
   Australia
   Email: furry13@gmail.com, furry@google.com

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