ICMP Extension Structure Length Field
draft-ietf-intarea-icmp-exten-hdr-len-07

INTAREA Group                                                  R. Bonica
Internet-Draft                                                       HPE
Updates: 4884 (if approved)                                        X. He
Intended status: Standards Track                           China Telecom
Expires: 15 February 2026                                         X. Min
                                                         ZTE Corporation
                                                              T. Mizrahi
                                                                  Huawei
                                                          14 August 2025

                 ICMP Extension Structure Length Field
                draft-ietf-intarea-icmp-exten-hdr-len-07

Abstract

   The ICMP Extension Structure (RFC4884) does not have a length field.
   Therefore, unless the length of the Extension Structure can be
   inferred from other data in the ICMP message, the Extension Structure
   must be the last item in the ICMP message.

   This document updates RFC 4884 to define a length field for the ICMP
   Extension Structure.  When length information is provided, receivers
   can use it to parse ICMP messages.  Specifically, receivers can use
   length information to determine the offset at which the item after
   the ICMP Extension Structure begins.

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 15 February 2026.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  The ICMP Extension Structure  . . . . . . . . . . . . . . . .   3
   4.  Backwards Compatibility . . . . . . . . . . . . . . . . . . .   4
   5.  UPDATES to RFC 4884 . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Length Field  . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  Malformed Extension Headers . . . . . . . . . . . . . . .   5
     5.3.  Padding Requirement . . . . . . . . . . . . . . . . . . .   5
     5.4.  Ignore Reserved Field . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The ICMP Extension Structure [RFC4884] does not have a length field.
   This means it is expected to be the last element of an ICMP message.
   However, there are cases where additional fields need to be inserted
   after the ICMP Extension Structure.

   For example, [I-D.ietf-intarea-rfc8335bis] enhances the PROBE utility
   by adding a new field to ICMP Extended Echo and ICMP Extended Echo
   Reply messages.  To maintain compatibility with existing PROBE
   implementations, this new field is placed after the ICMP Extension
   Structure.

   Because the ICMP Extension Structure does not have a length field,
   [I-D.ietf-intarea-rfc8335bis] requires implementations to determine
   the length of the extension structure from the known message format
   and the assumption that these packets contain only a single ICMP
   Extension Object.

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   This special handling for PROBE packets is not ideal.  For future
   use, a mechanism to explicitly specify the extension structure length
   would be beneficial.

   This document adds a length field to the ICMP Extension Header.  It
   does not define data items that might follow the ICMP Extension
   Structure.

   The specifications of this document apply to all ICMP Extension
   Structures, regardless of whether they appear in ICMPv4 [RFC0792] or
   ICMPv6 [RFC4443] messages.

   This document UPDATES [RFC4884].

2.  Conventions and Definitions

   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
   BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  The ICMP Extension Structure

   An ICMP Extension Structure contains exactly one Extension Header
   followed by one or more objects.  The Extension Header format is
   defined in Section 7 of [RFC4884].  This document modifies the
   Extension Header format by allocating the lower 8 bits of the
   reserved field for a new length field.  Figure 1 depicts the updated
   Extension Header format.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Version|  Rsvd |     Length    |           Checksum            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 1: ICMP Extension Header As Updated By This Document

   Version: 4 bits.

   *  ICMP Extension Header version number.  This is version 2 as per
      [RFC4884].

   Reserved (Rsvd): 4 bits

   *  MUST be set to 0 by the sender and MUST be ignored by the
      receiver.

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   Length: 8 bits

   *  This field represents the length of the ICMP Extension Structure,
      including all options and optional padding, but excluding the ICMP
      Extension Header.  The length is measured in 4-byte words.  Legacy
      implementations set this field to 0 as per section 7 of [RFC4884].
      Therefore, implementation SHOULD NOT drop packets if this field is
      set to 0.

   Checksum: 16 bits

   *  As per [RFC4884], the checksum is the one's complement of the
      one's complement sum of the data structure, with the checksum
      field replaced by zero for the purpose of computing the checksum.
      An all-zero value means that no checksum was transmitted.  See
      Section 5.2 of [RFC4884] for a description of how this field is
      used.

   The ICMP Extension Structure MUST be zero-padded so that it ends on a
   4-byte boundary.  If it does not end on a 4-byte boundary, the
   receiving node will parse the ICMP message incorrectly and may
   discard it.

   The receiver MUST silently discard an ICMP message in the following
   cases:

   *  The length field in the ICMP Extension Header indicates that the
      ICMP Extension Structure is too large to fit in the ICMP message.

   *  The length field in the final ICMP Extension Object indicates that
      the final ICMP Extension Object is too large to fit in the ICMP
      Extension Structure.

   *  The final three bytes of the ICMP Extension Structure are neither
      padding (i.e., zeros) nor part of a well-formed ICMP Extension
      Object.

4.  Backwards Compatibility

   Legacy implementations that do not support the mechanism defined in
   this document set the length field to zero when sending a packet and
   ignore the length field in received ICMP messages.

   Such implementations require one of the following:

   *  The ICMP Extension Structure is final item in the ICMP packet.

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   *  The length of the ICMP Extension Structure can be inferred from
      other fields in the packet (e.g., [I-D.ietf-intarea-rfc8335bis].

   Currently, no mechanisms rely on the ICMP extension structure length
   field.  Should such mechanisms be defined in the future, backward
   compatibility with legacy implementations should be discussed for
   each case.

5.  UPDATES to RFC 4884

5.1.  Length Field

   In Section 7 of [RFC4884], an ICMP Extension Header contains a 12-bit
   reserved field.

   Section 3 of this document allocates the lower 8 bits of that field
   for a new length field.  Figure 1 provides a diagram of the updated
   ICMP Extension header.

5.2.  Malformed Extension Headers

   [RFC4884] offered no advice regarding the processing of malformed
   ICMP Extension Headers.

   Section 3 of this document offers the following advice:

   The receiver MUST silently discard an ICMP message in the following
   cases:

   *  The length field in the ICMP Extension Header indicates that the
      ICMP Extension Structure is too large to fit in the ICMP message.

   *  The length field in the final ICMP Extension Object indicates that
      the final ICMP Extension Object is too large to fit in the ICMP
      Extension Structure.

   *  The final three bytes of the ICMP Extension Structure are neither
      padding (i.e., zeros) nor part of a well-formed ICMP Extension
      Object.

5.3.  Padding Requirement

   In [RFC4884], the ICMP Extension Structure was not required to end on
   a 4-byte boundary.

   Section 3 of this document adds the following requirement:

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   The ICMP Extension Structure MUST be zero-padded so that it ends on a
   4-byte boundary.  If it does not end on a 4-byte boundary, the
   receiving node will parse the ICMP message incorrectly and may
   discard it.

5.4.  Ignore Reserved Field

   [RFC4884] describes the reserved field of the ICMP Extension Header
   as follows:

   Must be set to 0.

   Section 3 of this document describes the reserved field as follows:

   MUST be set to 0 by the sender and MUST be ignored by the receiver.

6.  IANA Considerations

   This document requires no IANA actions.

7.  Security Considerations

   This document introduces no security vulnerabilities.  However, it
   does inherit security considerations from [RFC4884].

8.  Acknowledgements

   Thanks to Tom Herbert, Jen Linkova, Erik Vynke and Michael Welzl for
   their review and helpful suggestions.

9.  References

9.1.  Normative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/rfc/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/rfc/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/rfc/rfc4443>.

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   [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/rfc/rfc4884>.

   [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/rfc/rfc8174>.

9.2.  Informative References

   [I-D.ietf-intarea-rfc8335bis]
              Fenner, B., Bonica, R., Thomas, R., Linkova, J., Lenart,
              C., and M. Boucadair, "PROBE: A Utility for Probing
              Interfaces", Work in Progress, Internet-Draft, draft-ietf-
              intarea-rfc8335bis-01, 21 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-intarea-
              rfc8335bis-01>.

Authors' Addresses

   Ron Bonica
   HPE
   United States of America
   Email: rbonica@juniper.net

   Xiaoming He
   China Telecom
   China
   Email: hexm4@chinatelecom.cn

   Xiao Min
   ZTE Corporation
   China
   Email: xiao.min2@zte.com.cn

   Tal Mizrahi
   Huawei
   Israel
   Email: tal.mizrahi.phd@gmail.com

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