Clarifying SRv6 SID List Processing
draft-farrel-6man-sidlist-clarification-01

Network Working Group                                          A. Farrel
Internet-Draft                                        Old Dog Consulting
Intended status: Informational                               S. Krishnan
Expires: 31 August 2025                              Cisco Systems, Inc.
                                                        27 February 2025

                  Clarifying SRv6 SID List Processing
               draft-farrel-6man-sidlist-clarification-01

Abstract

   Segment Routing over IPv6 (SRv6) is the instantiation of Segment
   Routing (SR) on the IPv6 dataplane.  Segments are indicated by
   Segment Identifiers (SIDs).  SRv6 utilizes the Segment Routing Header
   (SRH), an IPv6 extension header, that includes a SID list indicating
   the sequence of segments and any additional processing to be
   performed.

   This document clarifies the processing of SID list entries.  It does
   not change any elements of the SRv6 architecture and makes no
   technical changes to the mechanisms or procedures of SRv6.

Status of This Memo

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   This Internet-Draft will expire on 31 August 2025.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
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   This document is subject to BCP 78 and the IETF Trust's Legal
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   license-info) in effect on the date of publication of this document.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Clarification . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   3
   Normative References  . . . . . . . . . . . . . . . . . . . . . .   3
   Informative References  . . . . . . . . . . . . . . . . . . . . .   3
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   The Segment Routing (SR) architecture is specified in [RFC8402].  SR
   forwards packets along a series of segments, and may perform
   additional segment-specific processing on packets.  Segments are
   indicated by Segment Identifiers (SIDs).

   The mechanisms to achieve Segment Routing for IPv6 (SRv6) include the
   use of the Segment Routing Header (SRH) [RFC8754] an IPv6 extension
   header that includes a SID list indicating the sequence of segments
   and any additional processing to be performed.

   This document clarifies the processing of SID list entries.  It does
   not change any elements of the SRv6 architecture and makes no
   technical changes to the mechanisms or procedures of SRv6.

2.  Clarification

   At the end of a segment, the SRH is processed to determine the next
   segment on the packet's path per Section 4 of [RFC8754].  One
   objective is to determine the value to place in the Destination
   Address field of the IPv6 packet.  To this end, the next entry in the
   SID list in the SRH is processed and mapped to the value to place in
   the Destination Address field.

   The value placed in the 128 bit Destination Address field of an IPv6
   packet header needs to be a routable IPv6 address since that is
   required for forwarding the packet.

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   Note that entries in the SID list do not need to be fully-formed IPv6
   addresses that are copied direct to the Destination Address field of
   the IPv6 packet.  The mapping from SID list entry could be a direct
   copy (the SID list contains a list of IPv6 addresses), or could
   involve a more complex function.

   Examples of such functions are shown in
   [I-D.ietf-spring-srv6-srh-compression] where a REPLACE-CSID
   compressed SID is expanded to be placed in the Destination Address
   field.

3.  Security Considerations

   This document makes no changes to the security properties of SRv6.
   See [I-D.ietf-spring-srv6-security] for more discussion of SRv6
   security.

   Note that the possibility of applying encryption functions to SID
   list members mentioned in Section 2 could offer additional security
   features.

4.  IANA Considerations

   This document makes no requests for IANA action.

Acknowledgments

   Thanks to Eric Vyncke and Erik Kline for inspiring the authors to
   write this document.  Thanks to Bob Hinden and Mohamed Boucadair for
   their reviews and comments that improved this document.

Normative References

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [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,
              <https://www.rfc-editor.org/info/rfc8754>.

Informative References

   [I-D.ietf-spring-srv6-security]
              Buraglio, N., Mizrahi, T., tongtian124, Contreras, L. M.,
              and F. Gont, "Segment Routing IPv6 Security

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              Considerations", Work in Progress, Internet-Draft, draft-
              ietf-spring-srv6-security-02, 26 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              srv6-security-02>.

   [I-D.ietf-spring-srv6-srh-compression]
              Cheng, W., Filsfils, C., Li, Z., Decraene, B., and F.
              Clad, "Compressed SRv6 Segment List Encoding (CSID)", Work
              in Progress, Internet-Draft, draft-ietf-spring-srv6-srh-
              compression-23, 6 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              srv6-srh-compression-23>.

Authors' Addresses

   Adrian Farrel
   Old Dog Consulting
   United Kingdom
   Email: adrian@olddog.co.uk

   Suresh Krishnan
   Cisco Systems, Inc.
   United States of America
   Email: suresh.krishnan@gmail.com

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