Clarifying SRv6 SID List Processing

Introduction The Segment Routing (SR) architecture is specified in . 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) 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.

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 . 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. 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 where a REPLACE-CSID compressed SID is expanded to be placed in the Destination Address field.

Security Considerations This document makes no changes to the security properties of SRv6. See for more discussion of SRv6 security. Note that the possibility of applying encryption functions to SID list members mentioned in could offer additional security features.

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.