Internet-Draft icmp-eh-len July 2025
Bonica, et al. Expires 8 January 2026 [Page]
Workgroup:
INTAREA Group
Internet-Draft:
draft-ietf-intarea-icmp-exten-hdr-len-02
Updates:
4884 (if approved)
Published:
Intended Status:
Standards Track
Expires:
Authors:
R. Bonica
HPE
X. He
China Telecom
X. Min
ZTE Corporation
T. Mizrahi
Huawei

ICMP Extension Header Length Field

Abstract

The ICMP Extension Structure 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 defines 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.

This document UPDATES RFC 4884.

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 working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

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."

This Internet-Draft will expire on 8 January 2026.

Table of Contents

1. Introduction

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 defines 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.

New implementations SHOULD always include the length field, even though it is not needed when the ICMP message ends with an ICMP Extension Structure.

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

Figure 1 depicts the ICMP Extension Header as updated by this document.

   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.

Reserved (Rsvd): 4 bits

Length: 8 bits

Checksum: 16 bits

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.

4. Backwards Compatibility

Legacy implementations set the length field to 0 as per section 7 of [RFC4884]. When the length field is set to 0, it conveys no information and cannot be used to parse the ICMP packet.

In these cases, one of the following statements MUST be true:

Legacy implementation do not recognize messages that rely on the ICMP Extension Header length field. This is because when the document was published, the IETF had not yet standardized any messages that rely on ICMP Extension Header length field.

An ICMP implementation MUST be capable of processing the ICMP Extension Header length field before recognizing any message that relies on it.

5. IANA Considerations

This document requires no IANA actions.

6. Security Considerations

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

7. Acknowledgements

Thanks to Tom Herbert, Erik Vynke and Michael Welzl for their review and helpful suggestion.

8. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC4884]
Bonica, R., Gan, D., Tappan, D., and C. Pignataro, "Extended ICMP to Support Multi-Part Messages", RFC 4884, DOI 10.17487/RFC4884, , <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, , <https://www.rfc-editor.org/rfc/rfc8174>.

Authors' Addresses

Ron Bonica
HPE
United States of America
Xiaoming He
China Telecom
China
Xiao Min
ZTE Corporation
China
Tal Mizrahi
Huawei
Israel