Label Switched Path Ping for Segment Routing Path Segment Identifier with MPLS Data Plane

Label Switched Path Ping for Segment Routing Path Segment Identifier with MPLS Data Plane ZTE Corp.

Nanjing China +86 18061680168 xiao.min2@zte.com.cn

ZTE Corp.

Nanjing China peng.shaofu@zte.com.cn

China Mobile

Beijing China gongliyan@chinamobile.com

Cisco Systems, Inc.

Canada rgandhi@cisco.com

Blue Fern Consulting

United States of America carlos@bluefern.consulting

RTG mpls Segment Routing (SR) leverages source routing to steer packets through an ordered list of instructions called "segments". SR can be instantiated over the MPLS data plane. Path Segment Identifiers (PSIDs) are used to identify and correlate bidirectional or end-to-end paths in SR networks. This document defines procedures (i.e., six new Target Forwarding Equivalence Class (FEC) Stack sub-TLVs) for the use of LSP Ping to support connectivity verification and fault isolation for SR paths that include PSIDs. The mechanisms described enable the validation and tracing of SR paths with Path SIDs in MPLS networks, complementing existing SR-MPLS Operations, Administration, and Maintenance (OAM) capabilities.

Introduction A Path Segment is a local segment that uniquely identifies an SR path on the egress node. A Path Segment Identifier (PSID) is a single label that is assigned from the SR Local Block (SRLB) of the egress node of an SR path. 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 pops the PSID. The procedure for LSP Ping as defined in is also applicable to PSID; this document appends the existing step 4a with a new step 4b specific to PSID. Concretely, LSP Ping can be used to check the correct operation of a PSID and verify the PSID against the control plane. Checking correct operation means that an initiator can use LSP Ping to check whether a PSID reached the intended node and got processed by that node correctly. Moreover, verifying a PSID against the control plane means that the initiator can use LSP Ping to verify the SR path context (segment-list, candidate path, or SR Policy) associated with the PSID as signaled or provisioned at the egress node. To that end, this document specifies six new Target Forwarding Equivalence Class (FEC) Stack sub-TLVs for such PSID checks. LSP Traceroute is left out of this document because transit nodes are not involved in PSID processing.

Conventions

Requirements Language 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.

Terminology This document uses the terminology defined in , , , and ; readers are expected to be familiar with the terms in those documents. This document introduces the following additional term:

Segment-List-ID: The Segment-List-ID field is a 4-octet identifier that uniquely identifies a segment list within the context of the candidate path of an SR Policy. Although not defined in , the Segment-List-ID is the same identifier as the one that can be signaled through control plane protocols including Border Gateway Protocol (BGP) (, Path Computation Element Communication Protocol (PCEP) (), and Border Gateway Protocol - Link State (BGP-LS) ().

Path Segment ID Sub-TLVs Analogous to what's defined in and , six 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 six new sub-TLVs follow the TLV's structure defined in . Sub-TLVs for PSID Checks

Sub-Type	Sub-TLV Name
49	SR Policy Associated PSID - IPv4
50	SR Candidate Path Associated PSID - IPv4
51	SR Segment List Associated PSID - IPv4
52	SR Policy Associated PSID - IPv6
53	SR Candidate Path Associated PSID - IPv6
54	SR Segment List Associated PSID - IPv6

As specified in , a PSID is used to identify the following:

a single segment list, some segment lists, or all segment lists in a candidate path of an SR Policy,
some segment lists across multiple candidate paths of an SR Policy, or
all segment lists in all candidate paths of an SR Policy.

Therefore, six different Target FEC Stack sub-TLVs need to be defined for PSID. The ordered list of selection rules for the six Target FEC Stack sub-TLVs are defined as follows:

When a PSID is used to identify all segment lists in an SR Policy, the Target FEC Stack sub-TLV of the type "SR Policy Associated PSID" (for IPv4 or IPv6) MUST be used for PSID checks.
When a PSID is used to identify all segment lists in an SR Candidate Path, the Target FEC Stack sub-TLV of the type "SR Candidate Path Associated PSID" (for IPv4 or IPv6) MUST be used for PSID checks.
When a PSID is used to identify a single segment list, the Target FEC Stack sub-TLV of the type "SR Segment List Associated PSID" (for IPv4 or IPv6) MUST be used for PSID checks.
When a PSID is used to identify some segment lists in a candidate path or an SR Policy, the Target FEC Stack sub-TLV of the type "SR Segment List Associated PSID" (for IPv4 or IPv6) MUST be used for PSID checks. In this case, multiple LSP Ping messages MUST be sent, and one Target FEC Stack sub-TLV of the type "SR Segment List Associated PSID" (for IPv4 or IPv6) MUST be carried in each LSP Ping message.

These six new Target FEC Stack sub-TLVs are not expected to be present in the same message. If more than one of these sub-TLVs are present in a message, only the first sub-TLV will be processed, per the validation rules in .

SR Policy Associated PSID - IPv4 Sub-TLV The SR Policy Associated PSID - IPv4 sub-TLV is defined as follows:

SR Policy Associated PSID - IPv4 Sub-TLV Format

Type (length: 2 octets): The Type field identifies the sub-TLV as an SR Policy Associated PSID - IPv4 sub-TLV. The value is set to 49.
Length (length: 2 octets): The Length field indicates the length of the sub-TLV in octets, excluding the first 4 octets (Type and Length fields). The value MUST be set to 12.
Headend (length: 4 octets): The Headend field encodes the headend IPv4 address of the SR Policy. This field is defined in .
Color (length: 4 octets): The Color field identifies the color (i.e., policy identifier) of the SR Policy and is encoded as defined in .
Endpoint (length: 4 octets): The Endpoint field encodes the endpoint IPv4 address of the SR Policy. This field is defined in .

SR Candidate Path Associated PSID - IPv4 Sub-TLV The SR Candidate Path Associated PSID - IPv4 sub-TLV is defined as follows:

SR Candidate Path Associated PSID - IPv4 Sub-TLV Format

Type (length: 2 octets): The Type field identifies the sub-TLV as an SR Candidate Path Associated PSID - IPv4 sub-TLV. The value is set to 50.
Length (length: 2 octets): The Length field indicates the length of the sub-TLV in octets, excluding the first 4 octets (Type and Length fields). The value MUST be set to 40.
Headend (length: 4 octets): The Headend field encodes the headend IPv4 address of the SR Candidate Path. This field is defined in .
Color (length: 4 octets): The Color field identifies the policy color and is defined in .
Endpoint (length: 4 octets): The Endpoint field encodes the endpoint IPv4 address of the SR Candidate Path. This field is defined in .
Protocol-Origin (length: 1 octet): The Protocol-Origin field indicates the protocol that originated the SR Candidate Path. It is defined in and takes values from the IANA registry . If an unsupported value is used, validation at the responder MUST fail.
Reserved (length: 3 octets): The Reserved field is reserved for future use. It MUST be set to zero when sent and MUST be ignored upon receipt.
Originator (length: 20 octets): The Originator field identifies the originator of the SR Candidate Path and is encoded as defined in .
Discriminator (length: 4 octets): The Discriminator field uniquely identifies the SR Candidate Path within the context of the Headend, Color, and Endpoint fields. This field is defined in .

SR Segment List Associated PSID - IPv4 Sub-TLV The SR Segment List Associated PSID - IPv4 sub-TLV is used to identify a specific segment list within the context of a candidate path of an SR Policy. The format of this sub-TLV is shown in .

SR Segment List Associated PSID - IPv4 Sub-TLV Format

Type (length: 2 octets): The Type field identifies the sub-TLV as an SR Segment List Associated PSID - IPv4 sub-TLV. The value is set to 51.
Length (length: 2 octets): The Length field indicates the length of the sub-TLV in octets, excluding the first 4 octets (Type and Length fields). The value MUST be set to 44.
Headend (length: 4 octets): The Headend field encodes the headend IPv4 address of the SR Policy. This field is defined in .
Color (length: 4 octets): The Color field identifies the color of the SR Policy and is encoded as specified in .
Endpoint (length: 4 octets): The Endpoint field specifies the endpoint IPv4 address of the SR Policy, as defined in .
Protocol-Origin (length: 1 octet): The Protocol-Origin field indicates the protocol that originated the SR Candidate Path. It is defined in and takes values from the IANA registry . If an unsupported value is used, validation at the responder MUST fail.
Reserved (length: 3 octets): The Reserved field is reserved for future use. It MUST be set to zero when transmitted and MUST be ignored upon receipt.
Originator (length: 20 octets): The Originator field identifies the originator of the SR Candidate Path and is defined in .
Discriminator (length: 4 octets): The Discriminator field uniquely identifies the SR Candidate Path within the context of the Headend, Color, and Endpoint fields. This field is defined in .
Segment-List-ID (length: 4 octets): The Segment-List-ID field is a 4-octet identifier that uniquely identifies a segment list within the context of the candidate path of an SR Policy. This field is defined in .

SR Policy Associated PSID - IPv6 Sub-TLV The SR Policy Associated PSID - IPv6 sub-TLV is defined as follows:

SR Policy Associated PSID - IPv6 Sub-TLV Format

Type (length: 2 octets): The Type field identifies the sub-TLV as an SR Policy Associated PSID - IPv6 sub-TLV. The value is set to 52.
Length (length: 2 octets): The Length field indicates the length of the sub-TLV in octets, excluding the first 4 octets (Type and Length fields). The value MUST be set to 36.
Headend (length: 16 octets): The Headend field encodes the headend IPv6 address of the SR Policy. This field is defined in .
Color (length: 4 octets): The Color field identifies the color (i.e., policy identifier) of the SR Policy and is encoded as defined in .
Endpoint (length: 16 octets): The Endpoint field encodes the endpoint IPv6 address of the SR Policy. This field is defined in .

SR Candidate Path Associated PSID - IPv6 Sub-TLV The SR Candidate Path Associated PSID - IPv6 sub-TLV is defined as follows:

SR Candidate Path Associated PSID - IPv6 Sub-TLV Format

Type (length: 2 octets): The Type field identifies the sub-TLV as an SR Candidate Path Associated PSID - IPv6 sub-TLV. The value is set to 53.
Length (length: 2 octets): The Length field indicates the length of the sub-TLV in octets, excluding the first 4 octets (Type and Length fields). The value MUST be set to 64.
Headend (length: 16 octets): The Headend field encodes the headend IPv6 address of the SR Candidate Path. This field is defined in .
Color (length: 4 octets): The Color field identifies the policy color and is defined in .
Endpoint (length: 16 octets): The Endpoint field encodes the endpoint IPv6 address of the SR Candidate Path. This field is defined in .
Protocol-Origin (length: 1 octet): The Protocol-Origin field indicates the protocol that originated the SR Candidate Path. It is defined in and takes values from the IANA registry . If an unsupported value is used, validation at the responder MUST fail.
Reserved (length: 3 octets): The Reserved field is reserved for future use. It MUST be set to zero when sent and MUST be ignored upon receipt.
Originator (length: 20 octets): The Originator field identifies the originator of the SR Candidate Path and is encoded as defined in .
Discriminator (length: 4 octets): The Discriminator field uniquely identifies the SR Candidate Path within the context of the Headend, Color, and Endpoint fields. This field is defined in .

SR Segment List Associated PSID - IPv6 Sub-TLV The SR Segment List Associated PSID - IPv6 sub-TLV is used to identify a specific segment list within the context of a candidate path of an SR Policy. The format of this sub-TLV is shown in .

SR Segment List Associated PSID - IPv6 Sub-TLV Format

Type (length: 2 octets): The Type field identifies the sub-TLV as an SR Segment List Associated PSID - IPv6 sub-TLV. The value is set to 54.
Length (length: 2 octets): The Length field indicates the length of the sub-TLV in octets, excluding the first 4 octets (Type and Length fields). The value MUST be set to 68.
Headend (length: 16 octets): The Headend field encodes the headend IPv6 address of the SR Policy. This field is defined in .
Color (length: 4 octets): The Color field identifies the color of the SR Policy and is encoded as specified in .
Endpoint (length: 16 octets): The Endpoint field specifies the endpoint IPv6 address of the SR Policy, as defined in .
Protocol-Origin (length: 1 octet): The Protocol-Origin field indicates the protocol that originated the SR Candidate Path. It is defined in and takes values from the IANA registry . If an unsupported value is used, validation at the responder MUST fail.
Reserved (length: 3 octets): The Reserved field is reserved for future use. It MUST be set to zero when transmitted and MUST be ignored upon receipt.
Originator (length: 20 octets): The Originator field identifies the originator of the SR Candidate Path and is defined in .
Discriminator (length: 4 octets): The Discriminator field uniquely identifies the SR Candidate Path within the context of the Headend, Color, and Endpoint fields. This field is defined in .
Segment-List-ID (length: 4 octets): The Segment-List-ID field is a 4-octet identifier that uniquely identifies a segment list within the context of the candidate path of an SR Policy. This field is defined in .

PSID FEC Validation The MPLS LSP Ping procedures may be initiated by the headend of the SR path or a centralized topology-aware data plane monitoring system as described in . For the PSID, the responder nodes that receive an echo request and send an echo reply MUST be the endpoint of the SR path. When an endpoint receives the LSP echo request packet with the top FEC being the PSID, it MUST perform validity checks on the content of the PSID Target FEC Stack sub-TLV. If a malformed Target FEC Stack sub-TLV is received, then a return code of 1, "Malformed echo request received" as defined in MUST be sent. The section below is appended to step 4a of .

PSID FEC Validation Rules " if any below conditions fail (the notation 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 match with the corresponding fields in the received SR Policy Associated PSID - IPv4 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 ". Set the FEC-Status to 1 and return. } Else, if the Label-stack-depth is 1 and the Target FEC Stack sub-TLV at FEC-stack-depth is 50 (SR Candidate Path Associated PSID - IPv4 sub-TLV), { Set the Best-return-code to 10 "Mapping for this FEC is not the given label at stack-depth " 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 match with the corresponding fields in the received SR Candidate Path Associated PSID - IPv4 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 ". Set the FEC-Status to 1 and return. } Else, if the Label-stack-depth is 1 and the Target FEC Stack sub-TLV at FEC-stack-depth is 51 (SR Segment List Associated PSID - IPv4 sub-TLV), { Set the Best-return-code to 10 "Mapping for this FEC is not the given label at stack-depth " 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 match with the corresponding fields in the received SR Segment List Associated PSID - IPv4 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 ". Set the FEC-Status to 1 and return. } Else, if the Label-stack-depth is 1 and the Target FEC Stack sub-TLV at FEC-stack-depth is 52 (SR Policy Associated PSID - IPv6 sub-TLV), { Set the Best-return-code to 10 "Mapping for this FEC is not the given label at stack-depth " if any below conditions fail - 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 match with the corresponding fields in the received SR Policy Associated PSID - IPv6 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 ". Set the FEC-Status to 1 and return. } Else, if the Label-stack-depth is 1 and the Target FEC Stack sub-TLV at FEC-stack-depth is 53 (SR Candidate Path Associated PSID - IPv6 sub-TLV), { Set the Best-return-code to 10 "Mapping for this FEC is not the given label at stack-depth " 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 match with the corresponding fields in the received SR Candidate Path Associated PSID - IPv6 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 ". Set the FEC-Status to 1 and return. } Else, if the Label-stack-depth is 1 and the Target FEC Stack sub-TLV at FEC-stack-depth is 54 (SR Segment List Associated PSID - IPv6 sub-TLV), { Set the Best-return-code to 10 "Mapping for this FEC is not the given label at stack-depth " 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 match with the corresponding fields in the received SR Segment List Associated PSID - IPv6 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 ". Set the FEC-Status to 1 and return. }]]> When any of the following is carried in a 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.

SR Policy Associated PSID - IPv4 sub-TLV,
SR Candidate Path Associated PSID - IPv4 sub-TLV,
SR Segment List Associated PSID - IPv4 sub-TLV,
SR Policy Associated PSID - IPv6 sub-TLV,
SR Candidate Path Associated PSID - IPv6 sub-TLV, or
SR Segment List Associated PSID - IPv6 sub-TLV

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.

Security Considerations This document defines additional MPLS LSP Ping sub-TLVs and follows the mechanisms defined in . All the security considerations defined in 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 Considerations IANA has assigned six Target FEC Stack sub-TLVs from the "Sub-TLVs for TLV Types 1, 16, and 21" registry within the "TLVs" registry of the "Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry group. The Standards Action range that requires an error message to be returned if the sub-TLV is not recognized (range 0-16383) has been used. Sub-TLVs for TLV Types 1, 16, and 21 Registry

Sub-Type	Sub-TLV Name	Reference
49	SR Policy Associated PSID - IPv4	of RFC 9884
50	SR Candidate Path Associated PSID - IPv4	of RFC 9884
51	SR Segment List Associated PSID - IPv4	of RFC 9884
52	SR Policy Associated PSID - IPv6	of RFC 9884
53	SR Candidate Path Associated PSID - IPv6	of RFC 9884
54	SR Segment List Associated PSID - IPv6	of RFC 9884

References Normative References SR Policy Protocol Origin IANA Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters IANA Informative References Advertisement of Segment Routing Policies Using BGP - Link State Individual Cisco Systems Huawei Technologies RtBrick Inc. Nvidia

Acknowledgements The authors would like to acknowledge , , , , , , , , , , , , , and for their thorough review and very helpful comments. The authors would like to acknowledge and for the very helpful face to face discussion.