Clarification of Segment ID Sub-TLV Length for RFC 8287

Introduction defines the extensions to MPLS LSP Ping and Traceroute for Segment Routing IGP-Prefix and IGP-Adjacency Segment Identifiers (SIDs) with the MPLS data plane. proposes three Target FEC Stack sub-TLVs. While RFC 8287 defines the format and procedure to handle those sub-TLVs, it does not sufficiently clarify how the length of the Segment ID sub-TLVs should be computed to be included in the Length field of the sub-TLVs, which may result in interoperability issues. This document updates by clarifying the length of each Segment ID sub-TLVs defined in .

Terminology This document uses the terminology defined in , , and ; readers are expected to be familiar with the terms as used in those documents.

Requirements Notation 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 when, and only when, they appear in all capitals, as shown here.

Length Field Clarification for Segment ID Sub-TLVs defines three different Segment ID sub-TLVs that can be included in the Target FEC Stack TLV defined in . The length of each sub-TLV MUST be calculated as defined in this section. The TLV representations defined in Sections , , and of are updated to clarify the length calculations, as shown in Sections , , and , respectively. The updated TLV representations contain explicitly defined lengths.

IPv4 IGP-Prefix Segment ID Sub-TLV The sub-TLV length for the IPv4 IGP-Prefix Segment ID MUST be set to 8, as shown in the TLV format below:

IPv6 IGP-Prefix Segment ID Sub-TLV The sub-TLV length for the IPv6 IGP-Prefix Segment ID MUST be set to 20, as shown in the TLV format below:

IGP-Adjacency Segment ID Sub-TLV The sub-TLV length for the IGP-Adjacency Segment ID varies depending on the Adjacency Type and Protocol. In any of the allowed combinations of Adjacency Type and Protocol, the sub-TLV length MUST be calculated by including 2 octets of the Reserved field. lists the length for different combinations of Adj. Type and Protocol. IGP-Adjacency SID Length Computation

Protocol	Length for Adj. Type
Protocol	Parallel	IPv4	IPv6	Unnumbered
OSPF	20	20	44	20
ISIS	24	24	48	24
Any	20	20	44	20

For example, when the Adj. Type is set to Parallel Adjacency and the Protocol is set to 0, the sub-TLV will be as below:

IANA Considerations IANA has listed this document as an additional reference for the following entries in the "Sub-TLVs for TLV Types 1, 16, and 21" registry: Sub-TLVs for TLV Types 1, 16, and 21 (Updated Entries)

Sub-Type	Sub-TLV Name	Reference
34	IPv4 IGP-Prefix Segment ID	; RFC 8690
35	IPv6 IGP-Prefix Segment ID	; RFC 8690
36	IGP-Adjacency Segment ID	; RFC 8690

Security Considerations This document updates and does not introduce any additional security considerations.

Normative References

Acknowledgements The authors would like to thank Michael Gorokhovsky and Manohar Doppalapudi for investigating the interoperability issue during European Advanced Network Test Center (EANTC) testing.

Contributors The following individual contributed to this document: Zafar Ali, Cisco Systems, Inc.