ICMP Extension Header Length Field

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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            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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 as per [RFC4884].¶

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].¶

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.¶

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:¶

• The ICMP Extension Structure is the final item in the ICMP packet.¶

• The length of the ICMP Extension Structure can be inferred from other fields in the packet.¶

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, March 1997, <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, 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>.

Authors' Addresses