Label Switched Path (LSP) Ping for Segment Routing (SR) Path Segment Identifier with MPLS Data Plane

Path Segment is a type of Segment Routing (SR) segment, and a Path Segment Identifier (PSID) is used to identify an SR path. PSID in MPLS-based segment routing networks is defined in . As specified in , PSID is a single label inserted by the ingress node of the SR path, and then processed by the egress node of the SR path. The PSID is placed within the MPLS label stack as a label immediately following the last label of the SR path. The egress node MUST pop the PSID. This document provides Target Forwarding Equivalence Class (FEC) Stack TLV and sub-TLV definitions for PSID. Procedures for LSP Ping as defined in and are applicable to PSID as well, which can be used to check correct operation of the PSID and to verify the PSID against the control plane. Note that LSP Traceroute is left out of this document because the transit node is not involved in PSID processing.

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.

This document uses the terminology defined in , , and , readers are expected to be familiar with those terms.

Analogous to what's defined in Section 5 of and Section 4 of , three new sub-TLVs are defined for the Target FEC Stack TLV (Type 1), the Reverse-Path Target FEC Stack TLV (Type 16), and the Reply Path TLV (Type 21). Note that the structures of the three new sub-TLVs follow the TLV's structure defined in Section 3 of .

As specified in Section 2 of , a PSID is used to identify a segment list, some or all segment lists in a Candidate path or an SR policy, so three different Target FEC Stack sub-TLVs need to be defined for PSID. When a PSID is used to identify an SR Policy, the Target FEC Stack sub-TLV of the type "SR Policy's PSID" is used to validate the control plane to forwarding plane synchronization for this PSID; When a PSID is used to identify an SR Candidate Path, the Target FEC Stack sub-TLV of the type "SR Candidate Path's PSID" is used to validate the control plane to forwarding plane synchronization for this PSID; When a PSID is used to identify a Segment List, the Target FEC Stack sub-TLV of the type "SR Segment List's PSID" is used to validate the control plane to forwarding plane synchronization for this PSID. Note that the three new Target FEC Stack sub-TLVs are mutual exclusive and they wouldn't be present in one message simultaneously.

The format of SR Policy's PSID sub-TLV is specified as below:

Type This field is set to the value (TBD1) which indicates that it's an SR Policy's PSID sub-TLV. Length This field is set to the length of the sub-TLV's Value field in octets. If Headend and Endpoint fields are in IPv4 address format which is 4 octets long, it MUST be set to 12; If Headend and Endpoint fields are in IPv6 address format which is 16 octets long, it MUST be set to 36. Headend The same as defined in Section 2.1 of . Color The same as defined in Section 2.1 of . Endpoint The same as defined in Section 2.1 of .

The format of SR Candidate Path's PSID sub-TLV is specified as below:

Type This field is set to the value (TBD2) which indicates that it's an SR Candidate Path's PSID sub-TLV. Length This field is set to the length of the sub-TLV's Value field in octets. If Headend and Endpoint fields are in IPv4 address format which is 4 octets long, it MUST be set to 40; If Headend and Endpoint fields are in IPv6 address format which is 16 octets long, it MUST be set to 64. Headend The same as defined in Section 2.1 of . Color The same as defined in Section 2.1 of . Endpoint The same as defined in Section 2.1 of . Protocol-Origin The same as defined in Section 2.3 of . The value of this field follows the IANA registry requested in Section 8.4 of . Originator The same as defined in Section 2.4 of . Discriminator The same as defined in Section 2.5 of .

The format of SR Segment List's PSID sub-TLV is specified as below:

Type This field is set to the value (TBD3) which indicates that it's an SR Segment List's PSID sub-TLV. Length This field is set to the length of the sub-TLV's Value field in octets. If Headend and Endpoint fields are in IPv4 address format which is 4 octets long, it MUST be set to 44; If Headend and Endpoint fields are in IPv6 address format which is 16 octets long, it MUST be set to 68. Headend The same as defined in Section 2.1 of . Color The same as defined in Section 2.1 of . Endpoint The same as defined in Section 2.1 of . Protocol-Origin The same as defined in Section 2.3 of . The value of this field follows the IANA registry requested in Section 8.4 of . Originator The same as defined in Section 2.4 of . Discriminator The same as defined in Section 2.5 of . Segment-List-ID This field identifies an SR path within the context of a candidate path of an SR Policy. The Segment-List-ID is a 4-octet identifier of a segment list.

The MPLS LSP Ping procedures may be initiated by the headend of the Segment Routing path or a centralized topology-aware data plane monitoring system as described in . For the PSID, the responder nodes that receive echo request and send echo reply MUST be the endpoint of the Segment Routing path. When an endpoint receives the LSP echo request packet with top FEC being the PSID, it MUST perform validity checks on the content of the PSID FEC Stack sub-TLV. The basic length check should be performed on the received FEC.

If a malformed FEC Stack sub-TLV is received, then a return code of 1, "Malformed echo request received" as defined in MUST be sent. The below section augments Section 7.4 of . 4a. Segment Routing PSID Validation: If the Label-stack-depth is 0 and the Target FEC Stack sub-TLV at FEC-stack-depth is TBD1 (SR Policy's PSID sub-TLV), { Set the Best-return-code to 10, "Mapping for this FEC is not the given label at stack-depth <RSC>" if any below conditions fail (the notation <RSC> refers to the Return Subcode): Validate that the PSID is signaled or provisioned for the SR Policy { Validate that the signaled or provisioned headend, color, and endpoint, for the PSID, matches with the corresponding fields in the received SR Policy's PSID sub-TLV. } } If all the above validations have passed, set the return code to 3 "Replying router is an egress for the FEC at stack-depth <RSC>". Set FEC-Status to 1 and return. } Else, if the Label-stack-depth is 0 and the Target FEC Stack sub-TLV at FEC-stack-depth is TBD2 (SR Candidate Path's PSID sub-TLV), { Set the Best-return-code to 10, "Mapping for this FEC is not the given label at stack-depth <RSC>" if any below conditions fail: Validate that the PSID is signaled or provisioned for the SR Candidate Path { Validate that the signaled or provisioned headend, color, endpoint, originator, and discriminator, for the PSID, matches with the corresponding fields in the received SR Candidate Path's PSID sub-TLV. } } If all the above validations have passed, set the return code to 3 "Replying router is an egress for the FEC at stack-depth <RSC>". Set FEC-Status to 1 and return. } Else, if the Label-stack-depth is 0 and the Target FEC Stack sub-TLV at FEC-stack-depth is TBD3 (SR Segment List's PSID sub-TLV), { Set the Best-return-code to 10, "Mapping for this FEC is not the given label at stack-depth <RSC>" if any below conditions fail: Validate that the PSID is signaled or provisioned for the SR Segment List { Validate that the signaled or provisioned headend, color, endpoint, originator, discriminator, and segment-list-id, for the PSID, matches with the corresponding fields in the received SR Segment List's PSID sub-TLV. } } If all the above validations have passed, set the return code to 3 "Replying router is an egress for the FEC at stack-depth <RSC>". Set FEC-Status to 1 and return. } When a sub-TLV defined in this document is carried in Reverse-Path Target FEC Stack TLV (Type 16) or Reply Path TLV (Type 21), it MUST be sent by an endpoint in an echo reply. The headend MUST perform validity checks as described above without setting the return code. If any of the validations fail, then the headend MUST drop the echo reply and SHOULD log and/or report an error.

This document defines additional MPLS LSP Ping sub-TLVs and follows the mechanisms defined in . All the security considerations defined in Section 5 of apply to this document. The MPLS LSP Ping sub-TLVs defined in this document do not impose any additional security challenges to be considered.

IANA is requested to assign three new sub-TLVs from the "Sub-TLVs for TLV Types 1, 16, and 21" sub-registry in the "TLVs" registry of the "Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" name space. The Standards Action range that requires an error message to be returned if the sub-TLV is not recognized (range 0-16383) should be used.

The authors would like to acknowledge Loa Andersson, Detao Zhao, Ben Niven-Jenkins, Greg Mirsky, Ketan Talaulikar, James Guichard, Jon Geater, Gorry Fairhurst, and Bing Liu for their thorough review and very helpful comments. The authors would like to acknowledge Yao Liu and Quan Xiong for the very helpful f2f discussion.