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

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. 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).

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" would be 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" would be 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" would be 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 This field identifies the headend of an SR Policy, the same as defined in Section 2.1 of . The headend is a 4-octet IPv4 address or a 16-octet IPv6 address. Color This field associates the SR Policy with an intent or objective (e.g., low latency), the same as defined in Section 2.1 of . The color is an unsigned non-zero 4-octet integer value. Endpoint This field identifies the endpoint of an SR Policy, the same as defined in Section 2.1 of . The endpoint is a 4-octet IPv4 address or a 16-octet IPv6 address.

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 This field identifies the headend of an SR Policy, the same as defined in Section 2.1 of . The headend is a 4-octet IPv4 address or a 16-octet IPv6 address. Color This field associates the SR Policy with an intent or objective (e.g., low latency), the same as defined in Section 2.1 of . The color is an unsigned non-zero 4-octet integer value. Endpoint This field identifies the endpoint of an SR Policy, the same as defined in Section 2.1 of . The endpoint is a 4-octet IPv4 address or a 16-octet IPv6 address. Protocol-Origin This field is associated with the mechanism or protocol used for signaling/provisioning the SR Policy, the same as defined in Section 2.3 of . The protocol-origin of a candidate path is a 1-octet value indicating PCEP, BGP SR Policy, or Via Configuration. The value of protocol-origin is set as specified in Section 2.3 of . Originator This field identifies the node that provisioned or signaled the candidate path on the headend, the same as defined in Section 2.4 of . The originator is a 20-octet numerical value formed by the concatenation of the fields of the tuple <Autonomous System Number (ASN), node-address>, among which ASN is a 4-octet number and node address is a 16-octet value (an IPv6 address or an IPv4 address encoded in the lowest 4 octets). When procotol-origin is respectively indicating Via Configuration, PCEP, or BGP SR Policy, the value of originator is set as specified in Section 2.4 of . Discriminator This field uniquely identifies a candidate path within the context of an SR policy from a specific protocol-origin, the same as defined in Section 2.5 of . The discriminator is a 4-octet value. When protocol-origin is respectively indicating Via Configuration, PCEP, or BGP SR Policy, the value of discriminator is set as specified 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 This field identifies the headend of an SR Policy, the same as defined in Section 2.1 of . The headend is a 4-octet IPv4 address or a 16-octet IPv6 address. Color This field associates the SR Policy with an intent or objective (e.g., low latency), the same as defined in Section 2.1 of . The color is an unsigned non-zero 4-octet integer value. Endpoint This field identifies the endpoint of an SR Policy, the same as defined in Section 2.1 of . The endpoint is a 4-octet IPv4 address or a 16-octet IPv6 address. Protocol-Origin This field is associated with the mechanism or protocol used for signaling/provisioning the SR Policy, the same as defined in Section 2.3 of . The protocol-origin of a candidate path is a 1-octet value indicating PCEP, BGP SR Policy, or Via Configuration. The value of protocol-origin is set as specified in Section 2.3 of . Originator This field identifies the node that provisioned or signaled the candidate path on the headend, the same as defined in Section 2.4 of . The originator is a 20-octet numerical value formed by the concatenation of the fields of the tuple <Autonomous System Number (ASN), node-address>, among which ASN is a 4-octet number and node address is a 16-octet value (an IPv6 address or an IPv4 address encoded in the lowest 4 octets). When procotol-origin is respectively indicating Via Configuration, PCEP, or BGP SR Policy, the value of originator is set as specified in Section 2.4 of . Discriminator This field uniquely identifies a candidate path within the context of an SR policy from a specific protocol-origin, the same as defined in Section 2.5 of . The discriminator is a 4-octet value. When protocol-origin is respectively indicating Via Configuration, PCEP, or BGP SR Policy, the value of discriminator is set as specified 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 same as "Path ID" defined in Section 4.2 of , or "Segment List ID" defined in Section 2.1 of . 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 SHOULD 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 SHOULD 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 end-point 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 { When the Protocol-Origin field in the received SR Candidate Path's PSID sub-TLV is a value indicating PCEP, "PCEP" is the used signaling protocol. And then validate that the PSID matches with the tuple identifying the SR Candidate Path within PCEP { Validate that the signaled headend, color, end-point, originator ASN, originator address and discriminator defined in and , for the PSID, matches with the corresponding fields in the received SR Candidate Path's PSID sub-TLV. } When the Protocol-Origin field in the received SR Candidate Path's PSID sub-TLV is a value indicating BGP SR Policy, "BGP SR Policy" is the used signaling protocol. And then validate that the PSID matches with the tuple identifying the SR Candidate Path within BGP SR Policy { Validate that the signaled headend, policy color, endpoint, ASN, BGP Router-ID and distinguisher defined in and , for the PSID, matches with the corresponding fields in the received SR Candidate Path's PSID sub-TLV. } When the Protocol-Origin field in the received SR Candidate Path's PSID sub-TLV is a value indicating Via Configuration, "Via Configuration" is the used provisioning mechanism. And then validate that the PSID matches with the tuple identifying the SR Candidate Path within Configuration { Validate that the 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 { When the Protocol-Origin field in the received SR Segment List's PSID sub-TLV is a value indicating PCEP, "PCEP" is the used signaling protocol. And then validate that the PSID matches with the tuple identifying the SR Segment List within PCEP { Validate that the signaled headend, color, end-point, originator ASN, originator address and discriminator defined in and , and the signaled Path ID defined in , for the PSID, matches with the corresponding fields in the received SR Segment List's PSID sub-TLV. } When the Protocol-Origin field in the received SR Segment List's PSID sub-TLV is a value indicating BGP SR Policy, "BGP SR Policy" is the used signaling protocol. And then validate that the PSID matches with the tuple identifying the SR Segment List within BGP SR Policy { Validate that the signaled headend, policy color, endpoint, ASN, BGP Router-ID and distinguisher defined in and , and the signaled Segment List ID defined in , for the PSID, matches with the corresponding fields in the received SR Segment List's PSID sub-TLV. } When the Protocol-Origin field in the received SR Segment List's PSID sub-TLV is a value indicating Via Configuration, "Via Configuration" is the used provisioning mechanism. And then validate that the PSID matches with the tuple identifying the SR Segment List within Configuration { Validate that the 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. }

This document defines additional MPLS LSP Ping sub-TLVs and follows the mechanisms defined in . All the security considerations defined in will be applicable for this document and, in addition, 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 and Detao Zhao for their thorough review and very helpful comments. The authors would like to acknowledge Yao Liu for the very helpful f2f discussion.