Label Switched Path (LSP) Ping/Traceroute for Segment Routing (SR) Path Segment Identifiers (SIDs) with MPLS Data Planes

Path Segment is a type of SR segment, which is used to identify an SR path. Path Segment in MPLS based segment routing network is defined in . When Path Segment is used, it's inserted by the ingress node of the SR path, and then processed by the egress node of the SR path. The position of Path Segment Label within the MPLS label stack is immediately following the segment list of the SR path. Note that the Path Segment would not be popped up until it reaches the egress node. This document provides Target Forwarding Equivalence Class (FEC) stack TLV definitions for Path-SIDs. Procedures for LSP Ping and Traceroute as defined in and are applicable to Path-SIDs as well.

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 , two 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 3 of , the Path Segment can be used to identify an SR path (specified by SID list) or an SR candidate path, so two different Target FEC sub-TLVs need to be defined for Path Segment ID. When a Path Segment is used to identify an SR path, then the Target FEC sub-TLV of SR Segment List's Path SID would be used to validate the control plane to forwarding plane synchronization for this Path-SID; When a Path Segment is used to identify an SR candidate path, then the Target FEC sub-TLV of SR Candidate Path's Path SID would be used to validate the control plane to forwarding plane synchronization for this Path-SID.

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

Type This field is set to the value (TBD1) which indicates that it's a SR Candidate Path's Path SID 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, the same as defined in Section 2.1 of . The color is a 4-octet numerical 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 identifies the component or protocol that originates or signals the candidate path for an SR Policy, the same as defined in Section 2.3 of . The protocol-origin is a 1-octet value that follows the recommendation from Table 1 of Section 2.3 of , which specifies value 10 for "PCEP", value 20 for "BGP SR Policy" and value 30 for "Via Configuration". Originator This field identifies the node which provisioned or signaled the candidate path for an SR Policy, 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 <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 "Via Configuration", or "PCEP", or "BGP SR Policy", the values of ASN and node address follow the specification in Section 2.4 of . Discriminator This field uniquely identifies a candidate path within the context of an SR policy, the same as defined in Section 2.5 of . The discriminator is a 4-octet value. When Protocol-Origin is respectively "Via Configuration", or "PCEP", or "BGP SR Policy", the value of discriminator follows the specification in Section 2.5 of .

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

Type This field is set to the value (TBD2) which indicates that it's a SR Segment List's Path SID 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, the same as defined in Section 2.1 of . The color is a 4-octet numerical 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 identifies the component or protocol that originates or signals the candidate path for an SR Policy, the same as defined in Section 2.3 of . The protocol-origin is a 1-octet value that follows the recommendation from Table 1 of Section 2.3 of , which specifies value 10 for "PCEP", value 20 for "BGP SR Policy" and value 30 for "Via Configuration". Originator This field identifies the node which provisioned or signaled the candidate path for an SR Policy, 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 <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 "Via Configuration", or "PCEP", or "BGP SR Policy", the values of ASN and node address follow the specification in Section 2.4 of . Discriminator This field uniquely identifies a candidate path within the context of an SR policy, the same as defined in Section 2.5 of . The discriminator is a 4-octet value. When Protocol-Origin is respectively "Via Configuration", or "PCEP", or "BGP SR Policy", the value of discriminator follows the specification 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 "List Identifier" defined in Section 2.2 of . The segment-list-ID is a 4-octet identifier of the corresponding segment list.

The MPLS LSP Ping/Traceroute 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 Path-SID, 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 Path-SID, it SHOULD perform validity checks on the content of the Path-SID FEC sub-TLV. The basic length check should be performed on the received FEC.

If a malformed FEC sub-TLV is received, then a return code of 1, "Malformed echo request received" as defined in SHOULD be sent. The below section augments the section 7.4 of . 4a. Segment Routing Path-SID Validation: If the Label-stack-depth is 0 and the Target FEC Stack sub-TLV at FEC-stack-depth is TBD1 (SR Candidate Path's Path SID 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 Path Segment ID is signaled or provisioned for the SR Candidate Path { When the Protocol-Origin field in the received SR Candidate Path's Path SID sub-TLV is 10, "PCEP" is used as the signaling protocol. And then validate that the Path Segment ID matches with the tuples identifying the SR Candidate Path within PCEP { Validate that the signaled headend defined in and , for the Path SID, matches with the headend field in the received SR Candidate Path's Path SID sub-TLV. Validate that the signaled color defined in and , for the Path SID, matches with the color field in the received SR Candidate Path's Path SID sub-TLV. Validate that the signaled end-point defined in and , for the Path SID, matches with the endpoint field in the received SR Candidate Path's Path SID sub-TLV. Validate that the signaled both originator ASN and originator address defined in and , for the Path SID, matches with the originator field in the received SR Candidate Path's Path SID sub-TLV. Validate that the signaled discriminator defined in and , for the Path SID, matches with the discriminator field in the received SR Candidate Path's Path SID sub-TLV. } When the Protocol-Origin field in the received SR Candidate Path's Path SID sub-TLV is 20, "BGP SR Policy" is used as the signaling protocol. And then validate that the Path Segment ID matches with the tuples identifying the SR Candidate Path within BGP SR Policy { Validate that the signaled headend defined in and , for the Path SID, matches with the headend field in the received SR Candidate Path's Path SID sub-TLV. Validate that the signaled policy color defined in and , for the Path SID, matches with the color field in the received SR Candidate Path's Path SID sub-TLV. Validate that the signaled endpoint defined in and , for the Path SID, matches with the endpoint field in the received SR Candidate Path's Path SID sub-TLV. Validate that the signaled both ASN and BGP Router-ID defined in and , for the Path SID, matches with the originator field in the received SR Candidate Path's Path SID sub-TLV. Validate that the signaled distinguisher defined in and , for the Path SID, matches with the discriminator field in the received SR Candidate Path's Path SID sub-TLV. } When the Protocol-Origin field in the received SR Candidate Path's Path SID sub-TLV is 30, "Via Configuration" is used. And then validate that the Path Segment ID matches with the tuples identifying the SR Candidate Path within Configuration { Validate that the provisioned headend defined in , for the Path SID, matches with the headend field in the received SR Candidate Path's Path SID sub-TLV. Validate that the provisioned color defined in , for the Path SID, matches with the color field in the received SR Candidate Path's Path SID sub-TLV. Validate that the provisioned endpoint defined in , for the Path SID, matches with the endpoint field in the received SR Candidate Path's Path SID sub-TLV. Validate that the provisioned originator defined in , for the Path SID, matches with the originator field in the received SR Candidate Path's Path SID sub-TLV. Validate that the provisioned discriminator defined in , for the Path SID, matches with the discriminator field in the received SR Candidate Path's Path SID 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 Segment List's Path SID 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 Path Segment ID is signaled or provisioned for the SR Segment List { When the Protocol-Origin field in the received SR Segment List's Path SID sub-TLV is 10, "PCEP" is used as the signaling protocol. And then validate that the Path Segment ID matches with the tuples identifying the SR Segment List within PCEP { Validate that the signaled headend defined in and , for the Path SID, matches with the headend field in the received SR Segment List's Path SID sub-TLV. Validate that the signaled color defined in and , for the Path SID, matches with the color field in the received SR Segment List's Path SID sub-TLV. Validate that the signaled end-point defined in and , for the Path SID, matches with the endpoint field in the received SR Segment List's Path SID sub-TLV. Validate that the signaled both originator ASN and originator address defined in and , for the Path SID, matches with the originator field in the received SR Segment List's Path SID sub-TLV. Validate that the signaled discriminator defined in and , for the Path SID, matches with the discriminator field in the received SR Segment List's Path SID sub-TLV. Validate that the signaled Segment-List-ID by PCEP, for the Path SID, matches with the Segment-List-ID field in the received SR Segment List's Path SID sub-TLV. } When the Protocol-Origin field in the received SR Segment List's Path SID sub-TLV is 20, "BGP SR Policy" is used as the signaling protocol. And then validate that the Path Segment ID matches with the tuples identifying the SR Segment List within BGP SR Policy { Validate that the signaled headend defined in and , for the Path SID, matches with the headend field in the received SR Segment List's Path SID sub-TLV. Validate that the signaled policy color defined in and , for the Path SID, matches with the color field in the received SR Segment List's Path SID sub-TLV. Validate that the signaled endpoint defined in and , for the Path SID, matches with the endpoint field in the received SR Segment List's Path SID sub-TLV. Validate that the signaled both ASN and BGP Router-ID defined in and , for the Path SID, matches with the originator field in the received SR Segment List's Path SID sub-TLV. Validate that the signaled distinguisher defined in and , for the Path SID, matches with the discriminator field in the received SR Segment List's Path SID sub-TLV. Validate that the signaled List Identifier defined in , for the Path SID, matches with the Segment-List-ID field in the received SR Segment List's Path SID sub-TLV. } When the Protocol-Origin field in the received SR Segment List's Path SID sub-TLV is 30, "Via Configuration" is used. And then validate that the Path Segment ID matches with the tuples identifying the SR Segment List within Configuration { Validate that the provisioned headend defined in , for the Path SID, matches with the headend field in the received SR Segment List's Path SID sub-TLV. Validate that the provisioned color defined in , for the Path SID, matches with the color field in the received SR Segment List's Path SID sub-TLV. Validate that the provisioned endpoint defined in , for the Path SID, matches with the endpoint field in the received SR Segment List's Path SID sub-TLV. Validate that the provisioned originator defined in , for the Path SID, matches with the originator field in the received SR Segment List's Path SID sub-TLV. Validate that the provisioned discriminator defined in , for the Path SID, matches with the discriminator field in the received SR Segment List's Path SID sub-TLV. Validate that the provisioned Segment-List-ID through Yang Model, for the Path SID, matches with the Segment-List-ID field in the received SR Segment List's Path SID 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 does not raise any additional security issues beyond those of the specifications referred to in the list of normative references.

IANA is requested to assign two new sub-TLVs from the "sub-TLVs for TLV Types 1, 16, and 21" subregistry of the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry .

The authors would like to acknowledge Zhao Detao for his thorough review and very helpful comments.