rfc9884v1.txt		rfc9884.txt
	skipping to change at line 13 ¶		skipping to change at line 13 ¶
	Request for Comments: 9884 S. Peng		Request for Comments: 9884 S. Peng
	Category: Standards Track ZTE Corp.		Category: Standards Track ZTE Corp.
	ISSN: 2070-1721 L. Gong		ISSN: 2070-1721 L. Gong
	China Mobile		China Mobile
	R. Gandhi		R. Gandhi
	Cisco Systems, Inc.		Cisco Systems, Inc.
	C. Pignataro		C. Pignataro
	Blue Fern Consulting		Blue Fern Consulting
	October 2025		October 2025
	A Label Switched Path Ping for the Segment Routing Path Segment		Label Switched Path Ping for Segment Routing Path Segment Identifier
	Identifier with an MPLS Data Plane		with MPLS Data Plane
	Abstract		Abstract
	Segment Routing (SR) leverages source routing to steer packets		Segment Routing (SR) leverages source routing to steer packets
	through an ordered list of instructions called "segments". SR can be		through an ordered list of instructions called "segments". SR can be
	instantiated over the MPLS data plane. Path Segment Identifiers		instantiated over the MPLS data plane. Path Segment Identifiers
	(PSIDs) are used to identify and correlate bidirectional or end-to-		(PSIDs) are used to identify and correlate bidirectional or end-to-
	end paths in Segment Routing networks. This document defines		end paths in SR networks. This document defines procedures (i.e.,
	procedures (i.e., six new Target Forwarding Equivalence Class (FEC)		six new Target Forwarding Equivalence Class (FEC) Stack sub-TLVs) for
	Stack sub-TLVs) for the use of LSP Ping to support connectivity		the use of LSP Ping to support connectivity verification and fault
	verification and fault isolation for SR paths that include Path		isolation for SR paths that include PSIDs. The mechanisms described
	Segment Identifiers. The mechanisms described enable the validation		enable the validation and tracing of SR paths with Path SIDs in MPLS
	and tracing of SR paths with Path SIDs in MPLS networks,		networks, complementing existing SR-MPLS Operations, Administration,
	complementing existing SR-MPLS Operations, Administration, and		and Maintenance (OAM) capabilities.
	Maintenance (OAM) capabilities.
	Status of This Memo		Status of This Memo
	This is an Internet Standards Track document.		This is an Internet Standards Track document.
	This document is a product of the Internet Engineering Task Force		This document is a product of the Internet Engineering Task Force
	(IETF). It represents the consensus of the IETF community. It has		(IETF). It represents the consensus of the IETF community. It has
	received public review and has been approved for publication by the		received public review and has been approved for publication by the
	Internet Engineering Steering Group (IESG). Further information on		Internet Engineering Steering Group (IESG). Further information on
	Internet Standards is available in Section 2 of RFC 7841.		Internet Standards is available in Section 2 of RFC 7841.
	skipping to change at line 87 ¶		skipping to change at line 86 ¶
	7. References		7. References
	7.1. Normative References		7.1. Normative References
	7.2. Informative References		7.2. Informative References
	Acknowledgements		Acknowledgements
	Authors' Addresses		Authors' Addresses
	1. Introduction		1. Introduction
	A Path Segment is a local segment [RFC9545] that uniquely identifies		A Path Segment is a local segment [RFC9545] that uniquely identifies
	an SR path on the egress node. A Path Segment Identifier (PSID) is a		an SR path on the egress node. A Path Segment Identifier (PSID) is a
	single label that is assigned from the Segment Routing Local Block		single label that is assigned from the SR Local Block (SRLB)
	(SRLB) [RFC8402] of the egress node of an SR path.		[RFC8402] of the egress node of an SR path.
	As specified in [RFC9545], PSID is a single label inserted by the		As specified in [RFC9545], PSID is a single label inserted by the
	ingress node of the SR path and then processed by the egress node of		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		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		label immediately following the last label of the SR path. The
	egress node pops the PSID.		egress node pops the PSID.
	The procedure for LSP Ping [RFC8029] as defined in Section 7.4 of		The procedure for LSP Ping [RFC8029] as defined in Section 7.4 of
	[RFC8287] is also applicable to PSID; this document appends the		[RFC8287] is also applicable to PSID; this document appends the
	existing step 4a with a new step 4b specific to PSID. Concretely,		existing step 4a with a new step 4b specific to PSID. Concretely,
	skipping to change at line 138 ¶		skipping to change at line 137 ¶
	This document introduces the following additional term:		This document introduces the following additional term:
	Segment-List-ID		Segment-List-ID
	The Segment-List-ID field is a 4-octet identifier that uniquely		The Segment-List-ID field is a 4-octet identifier that uniquely
	identifies a segment list within the context of the candidate path		identifies a segment list within the context of the candidate path
	of an SR Policy. Although not defined in [RFC9256], the Segment-		of an SR Policy. Although not defined in [RFC9256], the Segment-
	List-ID is the same identifier as the one that can be signaled		List-ID is the same identifier as the one that can be signaled
	through control plane protocols including Border Gateway Protocol		through control plane protocols including Border Gateway Protocol
	(BGP) (Section 2.1 of [SR-SEGLIST-ID], Path Computation Element		(BGP) (Section 2.1 of [SR-SEGLIST-ID], Path Computation Element
	Communication Protocol (PCEP) (Section 5.2 of [PCE-MULTIPATH]),		Communication Protocol (PCEP) (Section 4.2 of [PCE-MULTIPATH]),
	and Border Gateway Protocol - Link State (BGP-LS) (Section 5.7.4		and Border Gateway Protocol - Link State (BGP-LS) (Section 5.7.4
	of [RFC9857]).		of [RFC9857]).
	3. Path Segment ID Sub-TLVs		3. Path Segment ID Sub-TLVs
	Analogous to what's defined in Section 5 of [RFC8287] and Section 4		Analogous to what's defined in Section 5 of [RFC8287] and Section 4
	of [RFC9703], six new sub-TLVs are defined for the Target FEC Stack		of [RFC9703], six new sub-TLVs are defined for the Target FEC Stack
	TLV (Type 1), the Reverse-Path Target FEC Stack TLV (Type 16), and		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		the Reply Path TLV (Type 21). Note that the structures of the six
	new sub-TLVs follow the TLV's structure defined in Section 3 of		new sub-TLVs follow the TLV's structure defined in Section 3 of
	skipping to change at line 169 ¶		skipping to change at line 168 ¶
	+----------+------------------------------------------+		+----------+------------------------------------------+
	| 52 | SR Policy Associated PSID - IPv6 |		| 52 | SR Policy Associated PSID - IPv6 |
	+----------+------------------------------------------+		+----------+------------------------------------------+
	| 53 | SR Candidate Path Associated PSID - IPv6 |		| 53 | SR Candidate Path Associated PSID - IPv6 |
	+----------+------------------------------------------+		+----------+------------------------------------------+
	| 54 | SR Segment List Associated PSID - IPv6 |		| 54 | SR Segment List Associated PSID - IPv6 |
	+----------+------------------------------------------+		+----------+------------------------------------------+
	Table 1: Sub-TLVs for PSID Checks		Table 1: Sub-TLVs for PSID Checks
	As specified in Section 2 of [RFC9545], a PSID is used to identify a		As specified in Section 2 of [RFC9545], a PSID is used to identify
	segment list and/or some or all segment lists in a Candidate path or		the following:
	an SR policy, so six different Target FEC Stack sub-TLVs need to be
	defined for PSID. The ordered list of selection rules for the six		* a single segment list, some segment lists, or all segment lists in
	Target FEC Stack sub-TLVs are defined as follows:		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,		* 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		the Target FEC Stack sub-TLV of the type "SR Policy Associated
	PSID" (for IPv4 or IPv6) MUST be used for PSID checks.		PSID" (for IPv4 or IPv6) MUST be used for PSID checks.
	* When a PSID is used to identify all segment lists in an SR		* 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, the Target FEC Stack sub-TLV of the type "SR
	Candidate Path Associated PSID" (for IPv4 or IPv6) MUST be used		Candidate Path Associated PSID" (for IPv4 or IPv6) MUST be used
	for PSID checks.		for PSID checks.
	skipping to change at line 416 ¶		skipping to change at line 424 ¶
	| |		| |
	| Endpoint (16 octets) |		| Endpoint (16 octets) |
	| |		| |
	| |		| |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Figure 4: SR Policy Associated PSID - IPv6 Sub-TLV Format		Figure 4: SR Policy Associated PSID - IPv6 Sub-TLV Format
	Type (length: 2 octets)		Type (length: 2 octets)
	The Type field identifies the sub-TLV as an SR Policy Associated		The Type field identifies the sub-TLV as an SR Policy Associated
	PSID - IPv6 Sub-TLV. The value is set to 52.		PSID - IPv6 sub-TLV. The value is set to 52.
	Length (length: 2 octets)		Length (length: 2 octets)
	The Length field indicates the length of the sub-TLV in octets,		The Length field indicates the length of the sub-TLV in octets,
	excluding the first 4 octets (Type and Length fields). The value		excluding the first 4 octets (Type and Length fields). The value
	MUST be set to 36.		MUST be set to 36.
	Headend (length: 16 octets)		Headend (length: 16 octets)
	The Headend field encodes the headend IPv6 address of the SR		The Headend field encodes the headend IPv6 address of the SR
	Policy. This field is defined in Section 2.1 of [RFC9256].		Policy. This field is defined in Section 2.1 of [RFC9256].
	skipping to change at line 600 ¶		skipping to change at line 608 ¶
	This field is defined in Section 2.5 of [RFC9256].		This field is defined in Section 2.5 of [RFC9256].
	Segment-List-ID (length: 4 octets)		Segment-List-ID (length: 4 octets)
	The Segment-List-ID field is a 4-octet identifier that uniquely		The Segment-List-ID field is a 4-octet identifier that uniquely
	identifies a segment list within the context of the candidate path		identifies a segment list within the context of the candidate path
	of an SR Policy. This field is defined in Section 2.2.		of an SR Policy. This field is defined in Section 2.2.
	4. PSID FEC Validation		4. PSID FEC Validation
	The MPLS LSP Ping procedures may be initiated by the headend of the		The MPLS LSP Ping procedures may be initiated by the headend of the
	Segment Routing path or a centralized topology-aware data plane		SR path or a centralized topology-aware data plane monitoring system
	monitoring system as described in [RFC8403]. For the PSID, the		as described in [RFC8403]. For the PSID, the responder nodes that
	responder nodes that receive an echo request and sends an echo reply		receive an echo request and send an echo reply MUST be the endpoint
	MUST be the endpoint of the SR path.		of the SR path.
	When an endpoint receives the LSP echo request packet with the top		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		FEC being the PSID, it MUST perform validity checks on the content of
	the PSID FEC Stack sub-TLV.		the PSID Target FEC Stack sub-TLV.
	If a malformed FEC Stack sub-TLV is received, then a return code of		If a malformed Target FEC Stack sub-TLV is received, then a return
	1, "Malformed echo request received" as defined in [RFC8029] MUST be		code of 1, "Malformed echo request received" as defined in [RFC8029]
	sent. The section below is appended to step 4a of Section 7.4 of		MUST be sent. The section below is appended to step 4a of
	[RFC8287].		Section 7.4 of [RFC8287].
	4.1. PSID FEC Validation Rules		4.1. PSID FEC Validation Rules
	4b. Segment Routing PSID Validation:		4b. Segment Routing PSID Validation:
	If the Label-stack-depth is 1 and the Target FEC Stack sub-TLV at		If the Label-stack-depth is 1 and the Target FEC Stack sub-TLV at
	FEC-stack-depth is 49 (SR Policy Associated PSID - IPv4 sub-TLV), {		FEC-stack-depth is 49 (SR Policy Associated PSID - IPv4 sub-TLV), {
	Set the Best-return-code to 10 "Mapping for this FEC is not the		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		given label at stack-depth <RSC>" if any below conditions fail
	skipping to change at line 681 ¶		skipping to change at line 689 ¶
	at FEC-stack-depth is 51 (SR Segment List Associated PSID - IPv4		at FEC-stack-depth is 51 (SR Segment List Associated PSID - IPv4
	sub-TLV), {		sub-TLV), {
	Set the Best-return-code to 10 "Mapping for this FEC is not the		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:		given label at stack-depth <RSC>" if any below conditions fail:
	- Validate that the PSID is signaled or provisioned for the SR		- Validate that the PSID is signaled or provisioned for the SR
	Segment List {		Segment List {
	* Validate that the signaled or provisioned headend, color,		* Validate that the signaled or provisioned headend, color,
	endpoint, originator, discriminator, and segment-list-id,		endpoint, originator, discriminator, and segment-list-id
	for the PSID match with the corresponding fields in the		for the PSID match with the corresponding fields in the
	received SR Segment List Associated PSID - IPv4 sub-TLV.		received SR Segment List Associated PSID - IPv4 sub-TLV.
	}		}
	}		}
	If all the above validations have passed, set the return code to 3,		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>".		"Replying router is an egress for the FEC at stack-depth <RSC>".
	Set the FEC-Status to 1 and return.		Set the FEC-Status to 1 and return.
	}		}
	Else, if the Label-stack-depth is 1 and the Target FEC Stack sub-TLV		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		at FEC-stack-depth is 52 (SR Policy Associated PSID - IPv6
	sub-TLV), {		sub-TLV), {
	Set the Best-return-code to 10 "Mapping for this FEC is not the		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		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		- Validate that the PSID is signaled or provisioned for the SR
	Policy {		Policy {
	* Validate that the signaled or provisioned headend, color,		* Validate that the signaled or provisioned headend, color,
	and endpoint for the PSID match with the corresponding		and endpoint for the PSID match with the corresponding
	fields in the received SR Policy Associated PSID - IPv6 sub-		fields in the received SR Policy Associated PSID - IPv6 sub-
	TLV.		TLV.
	}		}
	skipping to change at line 976 ¶		skipping to change at line 983 ¶
	Email: gongliyan@chinamobile.com		Email: gongliyan@chinamobile.com
	Rakesh Gandhi		Rakesh Gandhi
	Cisco Systems, Inc.		Cisco Systems, Inc.
	Canada		Canada
	Email: rgandhi@cisco.com		Email: rgandhi@cisco.com
	Carlos Pignataro		Carlos Pignataro
	Blue Fern Consulting		Blue Fern Consulting
	United States of America		United States of America
	Email: carlos@bluefern.consulting, cpignata@gmail.com		Email: carlos@bluefern.consulting
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