Network Working Group T. Zhou Internet-Draft J. Xie Updates: 8754 (if approved) Huawei Technologies Intended status: Standards Track 11 July 2022 Expires: 12 January 2023 Define the Value 255 in Last Entry field of Segment Routing Header draft-zhou-spring-srh-le-change-00 Abstract 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 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 12 January 2023. Copyright Notice Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved. Zhou & Xie Expires 12 January 2023 [Page 1] Internet-Draft Change the Meaning of Value 255 of SRH S July 2022 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 and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terms and Abbreviations . . . . . . . . . . . . . . . . . . . 2 3. The Problem and the Proposal . . . . . . . . . . . . . . . . 2 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 3 6. Normative References . . . . . . . . . . . . . . . . . . . . 3 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 3 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. * SRH: Segment Routing Header * LE: Last Entry * SL: Segment Left 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. Zhou & Xie Expires 12 January 2023 [Page 2] Internet-Draft Change the Meaning of Value 255 of SRH S July 2022 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 TBD. 6. 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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017, . [RFC8754] 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, March 2020, . Authors' Addresses Tianran Zhou Huawei Technologies Email: zhoutianran@huawei.com Zhou & Xie Expires 12 January 2023 [Page 3] Internet-Draft Change the Meaning of Value 255 of SRH S July 2022 Jingrong Xie Huawei Technologies Email: xiejingrong@huawei.com Zhou & Xie Expires 12 January 2023 [Page 4]