Internet-Draft Change the Meaning of Value 255 of SRH S July 2022
Zhou & Xie Expires 12 January 2023 [Page]
Network Working Group
8754 (if approved)
Intended Status:
Standards Track
T. Zhou
Huawei Technologies
J. Xie
Huawei Technologies

Define the Value 255 in Last Entry field of Segment Routing Header


This document proposes to define the value 255 in Last Entry field in Segment Routing Header (SRH) [RFC8754], to indicate an SRH without any SID left.

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.

Status of This Memo

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This Internet-Draft will expire on 12 January 2023.

Table of Contents

1. Introduction

This document proposes to define the value 255 in Last Entry field in Segment Routing Header (SRH) [RFC8754], to indicate an SRH without any SID present.

2. Terms and Abbreviations

The following abbreviations are used in this document.

3. The Problem and the Proposal

Segment Routing Header (SRH) [RFC8754] is one type of Routing Header (RH) [RFC8200]. A Segment Left field is defined in [RFC8200] as the basic structure of any type of RH. A Last Entry field is defined in [RFC8754] as the structure of SRH. The two fields are both related to the segments, behaving as pointer to one of the segments in the SRH. When the Segment Left field is set to one and the Last Entry field is set to zero, there is one SID present in the SRH. However, with current specification [RFC8754], there is no way to indicate an SRH without any SID present. For example, there is no need for any segment in the Segment List of SRH (the so-called SRv6-BE case), but there is need for HMAC in SRH as the enhanced security mechanism.

This document proposes to use the value 255 of Last Entry field to represent that there is no SID in the Segment List part of the SRH. Accordingly, the Segment Left field MUST be zero in this case.

This has an impact that, the number of Segments allowed in an SRH shrinks, from maximum 256 to maximum 255. Practically this is not a big problem, because the number of segments bigger than 255 is very impossible. Thus, changing the meaning of Last Entry value 255 can support incremental development and deployment.

4. IANA Considerations

There is no relevant IANA codepoint

5. Acknowledgements


6. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, , <>.
Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J., Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header (SRH)", RFC 8754, DOI 10.17487/RFC8754, , <>.

Authors' Addresses

Tianran Zhou
Huawei Technologies
Jingrong Xie
Huawei Technologies