]> China Mobile

chengweiqiang@chinamobile.com

China Mobile

lihan@chinamobile.com

Huawei Technologies Co., Ltd

China c.l@huawei.com

Cisco Systems, Inc.

Canada rgandhi@cisco.com

Broadcom

royi.zigler@broadcom.com

Routing Area SPRING Working Group A Segment Routing (SR) path is identified by an SR segment list. A sub-set of segments from the segment list cannot distinguish one SR path from another as they may be partially congruent. SR path identification is a pre-requisite for various use-cases such as Performance Measurement (PM), and end-to-end 1+1 path protection. In SR for MPLS data plane (SR-MPLS), an Egress node cannot determine on which SR path a packet traversed the network from the label stack because the segment identifiers are removed from the label stack as the packet transits the network. This document defines Path Segment to identify an SR path on the egress node of the path.

Introduction Segment Routing (SR) leverages the source-routing paradigm to steer packets from a source node through a controlled set of instructions, called segments, by prepending the packet with an SR header. In the MPLS data plane SR-MPLS the SR header is instantiated through a label stack. In an SR-MPLS network, when a packet is transmitted along an SR path, the labels in the MPLS label stack will be swapped or popped. The result of this is that no label or only the last label may be left in the MPLS label stack when the packet reaches the egress node. Thus, the egress node cannot use the SR label stack to determine along which SR path the packet came. However, to support various use-cases in SR-MPLS networks, such as Performance Measurement (PM), bidirectional path , and end-to-end 1+1 path protection (Live-Live case) , the ability to implement path identification on the egress node is a pre-requisite. Therefore, this document defines a new segment type, referred to herein as a Path Segment. A Path Segment is defined to uniquely identify an SR path on the egress node of the path. It MAY be used by the egress node for path identification. Note that, per-path state will be maintained in the egress node due to the requirements in the aforementioned use cases, though in normal cases that the per-path state will be maintained in the ingress node only.

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.

Abbreviations and Terms DM: Delay Measurement. LM: Loss Measurement. MPLS: Multiprotocol Label Switching. MSD: Maximum SID Depth. PM: Performance Measurement. PSID: Path Segment ID. SID: Segment ID. SL: Segment List. SR: Segment Routing. SRLB: SR Local Block SR-MPLS: Instantiation of SR on the MPLS data plane. SR path: A SR path is a path described by a Segment-List. Sub-Path: A sub-path is a part of the a path, which contains a sub-set of the nodes and links of the path.

Path Segment A Path Segment is a Local Segment which uniquely identifies 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 (SRLB) of the egress node of an SR path. A PSID is used to identify a Segment List. However, one PSID can be used to identify multiple Segment Lists in some use cases if needed. For example, one single PSID MAY be used to identify some or all Segment lists in a Candidate path or an SR policy, if an operator would like to aggregate these Segment Lists in operation. When a PSID is used, the PSID can be inserted at the ingress node and MUST immediately follow the last label of the SR path, in other words, inserted after the routing segment (adjacency/node/prefix segment) pointing to the egress node of the SR path. Therefore, a PSID will not be the top label in the label stack when received on an intermedate node of the associated path, but it can be the top label in the label stack on the penultimate node. The value of the TTL field in the MPLS label stack entry containing a PSID can be set to any value except 0. If a PSID is the bottom label, the S bit MUST be set. The egress node MUST pop the PSID. The egress node MAY use the PSID for further processing. For example, when performance measurement is enabled on the SR path, it can trigger packet counting or timestamping. The addition of the PSID will require the egress to read and process the PSID label in addition to the regular processing (such as a below MPLS label or the MPLS payload). This additional processing may have an impact on forwarding performance. Generic Associated Label (GAL) MAY be used for Operations, Administration and Maintenance (OAM) in MPLS networks. As per , when GAL is used, the ACH appears immediately after the bottom of the label stack. The SR path computation needs to know the Maximum SID Depth (MSD) that can be imposed at the ingress node of a given SR path . This ensures that the SID stack depth of a computed path does not exceed the number of SIDs the node is capable of imposing. As per the MSD signals the total number of MPLS labels that can be imposed, where the total number of MPLS labels includes the PSID. The label stack with Path Segment is shown in :

Label Stack with Path Segment

Where:

• The Labels 1 to n are the segment label stack used to direct how to steer the packets along the SR path.
• The PSID identifies the SR path in the context of the egress node of the SR path.

Signaling of the PSID between the egress, ingress and possibly a centralized controller is out of the scope of this document.

Equal-Cost Multipath Considerations If Entropy Label(EL) is also used on the egress node, as per the Entropy label Indicator (ELI) and Entropy Label (EL) would be placed before the tunnel label and hence does not interfere with the PSID which is placed below. It is worthy to note that in case of ECMP, with or without the use of EL, the SR packets may be forwarded over multiple paths. In this case, the SID list cannot directly reflect the actual forwarding path and the PSID can only identify the SID list rather than the actual forwarding path. Also, similar to Synonymous Flow Labels(SFL) , the introduction of an PSID to an existing flow may cause that flow to take a different path through the network under conditions of Equal-Cost Multipath (ECMP). This, in turn, may invalidate certain uses of the PSID, such as performance measurement applications. Therefore, the considerations as per section 5 in of SFL also apply to PSID in implementation.

Use cases This section describes use cases which can leveage the Path Segment.

Path Segment for Performance Measurement As defined in , performance measurement can be classified into Passive, Active, and Hybrid measurement. Since Path Segment is encoded in the SR-MPLS Label Stack as shown in Figure 1, existing implementation on the egress node can be leveraged for measuring packet counts using the incoming SID (the PSID). For Passive performance measurement, path identification at the measuring points is the pre-requisite. Path Segment can be used by the measuring points (e.g., the ingress and egress nodes of the SR path or a centralized controller) to correlate the packet counts and timestamps from the ingress and egress nodes for a specific SR path, then packet loss and delay can be calculated for the end-to-end path, respectively. Path Segment can also be used for Active performance measurement for an SR path in SR-MPLS networks for collecting packet counters and timestamps from the egress node using probe messages. Path Segment can also be used for In-situ OAM for SR-MPLS to identify the SR Path associated with the in-situ data fields in the data packets on the egress node. Path Segment can also be used for In-band PM for SR-MPLS to identify the SR Path associated with the collected performance metrics.

Path Segment for Bidirectional SR Path In some scenarios, for example, mobile backhaul transport networks, there are requirements to support bidirectional paths, and the path is normally treated as a single entity. Forward and reverse directions of the path have the same fate, for example, failure in one direction will result in switching traffic at both directions. MPLS supports this by introducing the concepts of co-routed bidirectional LSP and associated bidirectional LSP . In the current SR architecture, an SR path is a unidirectional path . In order to support bidirectional SR paths, a straightforward way is to bind two unidirectional SR paths to a single bidirectional SR path. Path Segments can then be used to identify and correlate the traffic for the two unidirectional SR paths at both ends of the bidirectional path. The mechanism of constructing bidirectional path using path segment is out of the scope of this document and has been described in several documents, such as and .

Path Segment for End-to-end Path Protection For end-to-end 1+1 path protection (i.e., Live-Live case), the egress node of the path needs to know the set of paths that constitute the primary and the secondaries, in order to select the primary path packets for onward transmission, and to discard the packets from the secondaries . To do this in Segment Routing, each SR path needs a path identifier that is unique at the egress node. For SR-MPLS, this can be the Path Segment label allocated by the egress node. There then needs to be a method of binding this SR path identifiers into equivalence groups such that the egress node can determine for example, the set of packets that represent a single primary path. This equivalence group can be instantiated in the network by an controller using the Path Segments of the SR paths.

Nesting of Path Segments Binding SID (BSID) can be used for SID list compression. With BSID, an end-to-end SR path in a trusted domain can be split into several sub-paths, each sub-path is identified by a BSID. Then an end-to-end SR path can be identified by a list of BSIDs, therefore, it can provide better scalability. BSID and PSID can be combined to achieve both sub-path and end-to-end path monitoring. A reference model for such a combination in (Figure 2) shows an end-to-end path (A->D) in a trusted domain that spans three sub-domains (Access, Aggregation and Core domain) and consists of three sub-paths, one in each sub-domain (sub-path (A->B), sub-path (B->C) and sub-path (C->D)). Each sub-path is associated with a BSID and a s-PSID. The SID list of the end-to-end path can be expressed as <BSID1, BSID2, ..., BSIDn, e-PSID>, where the e-PSID is the PSID of the end-to-end path. The SID list of a sub-path can be expressed as <SID1, SID2, ...SIDn, s-PSID>, where the s-PSID is the PSID of the sub-path. shows the details of the label stacks when PSID and BSID are used to support both sub-path and end-to-end path monitoring in a multi-domain scenario.

Nesting of Path Segments B) Sub-path(B->C) Sub-path(C->D) |<--------------->|<-------------->|<-------------->| E2E Path(A->D) |<------------------------------------------------->| +------------+ ~A->B SubPath~ +------------+ +------------+ |s-PSID(A->B)| ~B->C SubPath~ +------------+ +------------+ +------------+ | BSID(B->C) | |s-PSID(B->C)| ~C->D SubPath~ +------------+ +------------+ +------------+ | BSID(C->D) | | BSID(C->D) | |s-PSID(C->D)| +------------+ +------------+ +------------+ +------------+ |e-PSID(A->D)| |e-PSID(A->D)| |e-PSID(A->D)| |e-PSID(A->D)| +------------+ +------------+ +------------+ +------------+ ]]>

Security Considerations A Path Segment in SR-MPLS is a local label similar to other labels/Segment, such as a VPN label, defined in MPLS and SR-MPLS. The data plane processing of a PSID is a local implementation of an ingress node, or an egress node, which follows the same logic of existing MPLS dataplane. As per definition of PSID, only the egress node and the ingress node of the associated path will learn the information of PSID. The intermediate nodes of this path will not learn it. A PSID may be used on an ingress node that is not the ingress of the associated path, if the associated label stack with PSID is part of a deeper label stack which represents a longer path. For example the case described in and the related BSID is not used while the original label stack of sub-path is inserted as a part of the whole label stack. In this case, the PSID must be distributed in a trusted domain under the considerations defined in . A Path Segment is used within an SR-MPLS trusted domain and must not leak outside the domain, therefore no new security threats are introduced comparing to current SR-MPLS. As per , SR domain boundary routers MUST filter any external traffic destined to a label associated with a segment within the trusted domain, this applies to Path Segment as well. Other security considerations of SR-MPLS, described in applies to this document. The distribution of a PSID from an egress nodes to an ingress nodes is performed within an SR trusted domain, and it is out of the scope of this document. The details of the mechanism and related security considerations will be described in other documents.

Implementation Status [Note to the RFC Editor - remove this section before publication, as well as remove the reference to . This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in . The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to , "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".

Huawei Technologies

• Organization: Huawei Technologies.
• Implementation: Huawei PTN7900 Series Routers implementation of SR-TP.
• Description: SR-TP is a feature of Huawei PTN7900 series Routers, which uses Path Segments to associate with paths and build up bidirectional paths. Huawei PTN7900 Series Routers with version V100R018C00 and above have commercially implemented the definition of Path Segment and use cases which is defined in section 2 and Section 3.2 in this document, including all the "MUST" and "SHOULD" clauses, while other use cases for Path Segment in section 3 are not yet implemented. For control plane, PTN7900 Series Routers support configuring Path Segment using NETCONF.
• Maturity Level: Product
• Coverage: Partial, section 2 and use case section 3.2.
• Version: Draft-12
• Licensing: N/A
• Implementation experience: Nothing specific.
• Contact: li.fan@huawei.com
• Last updated: September 14, 2023

ZTE Corp

• Organization: ZTE Corporation.
• Implementation: ZTE's SPN implementation of path segment.
• Description: The feature of SR-MPLS Path Segment has been implemented in ZTE SPN products and follows the definition and mechanism as defined in section 2 and Section 3.2 including all the "MUST" and "SHOULD" clauses while other use cases for Path Segment in section 3 are not yet implemented.
• Maturity Level: Product
• Coverage: Partial,section 2 and use case section 3.2.
• Version: Draft-12
• Licensing: N/A
• Implementation experience: Nothing specific.
• Contact: liu.aihua@zte.com.cn
• Last updated: September 21, 2023

New H3C Technologies

• Organization: New H3C Technologies.
• Implementation: H3C CR16000, CR19000 series routers implementation of Path Segment.
• Description: Section 2 and Section 3.2 including all the "MUST" and "SHOULD" clauses have been implemented in above mentioned New H3C Products(running Version 7.1.086 and above) for testing, while other use cases for Path Segment in section 3 are not yet implemented.
• Maturity Level: Beta
• Coverage: Partial, section 2 and use case section 3.2.
• Version: Draft-12
• Licensing: N/A
• Implementation experience: Nothing specific.
• Contact: linchangwang.04414@h3c.com
• Last updated: September 13, 2023

Spirent Communications

• Organization: Spirent Communications
• Implementation: Spirent Testcenter Product Family implementation of SR-TP test capability.
• Description: Spirent Testcenter product family implements SR-MPLS path segment test capabilities on the versions above Spirent Testcenter 4.85. Spirent Testcenter fully support testing all clauses defined in section 2 and Section 3.1,3.2,3.4 , including all the "MUST" and "SHOULD" clauses, and partially support the test of clauses in section 3.3.
• Maturity Level: Production
• Coverage: fully cover section 2 and use case section 3.1,3.2, 3.4, partially cover section 3.3
• Version: Draft-12
• Licensing: N/A
• Implementation experience: Nothing specific.
• Contact: junqi.zhao@spirent.com
• Last updated: September 21, 2023

Fiberhome

• Organization: Fiberhome Corporation.
• Implementation: Fiberhome SPN series of products (Citrans 650/690E) implementation of path segment.
• Description: SR-TP is a feature of SPN products， which realizes a controllable L3 tunnel, builds the end-to-end L3 deployment business model. The path segment follows the definition and mechanism as defined in section 2 and Section 3.2 including all the "MUST" and "SHOULD" clauses had been implemented, while other use cases for Path Segment in section 3 are not yet implemented.
• Maturity Level: Product
• Coverage: Partial,section 2 and use case section 3.2.
• Version: Draft-12
• Licensing: N/A
• Implementation experience: Nothing specific.
• Contact: zhhan@fiberhome.com
• Last updated: September 21, 2023

Interoperability Test [Note to the RFC Editor - remove this section before publication, as well as remove the reference to . The Interoperability test of path segment had been done among products from several vendors, including Huawei(PTN7900, V100R018C00), ZTE(ZXCTN 6180, Ver 4.00.00), FiberHome(Citrans 650/690E) , Spirent (Chassis: SPT-N4U-220.Test. Module: PX3-QSFP28-12-225A. Version: 4.86) and Nokia in 2018. Note that Path Segment is a key feature of Layer3 in SPN architecture . This is reported by Weiqiang Cheng from China Mobile at September, 21, 2023.

IANA Considerations This document does not require any IANA actions.

References Normative References Segment Routing (SR) leverages the source routing paradigm. A node steers a packet through an ordered list of instructions, called "segments". A segment can represent any instruction, topological or service based. A segment can have a semantic local to an SR node or global within an SR domain. SR provides a mechanism that allows a flow to be restricted to a specific topological path, while maintaining per-flow state only at the ingress node(s) to the SR domain. SR can be directly applied to the MPLS architecture with no change to the forwarding plane. A segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels. The segment to process is on the top of the stack. Upon completion of a segment, the related label is popped from the stack. SR can be applied to the IPv6 architecture, with a new type of routing header. A segment is encoded as an IPv6 address. An ordered list of segments is encoded as an ordered list of IPv6 addresses in the routing header. The active segment is indicated by the Destination Address (DA) of the packet. The next active segment is indicated by a pointer in the new routing header. Segment Routing (SR) leverages the source-routing paradigm. A node steers a packet through a controlled set of instructions, called segments, by prepending the packet with an SR header. In the MPLS data plane, the SR header is instantiated through a label stack. This document specifies the forwarding behavior to allow instantiating SR over the MPLS data plane (SR-MPLS). In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document presents a functional description of the protocol extensions needed to support Generalized Multi-Protocol Label Switching (GMPLS)-based recovery (i.e., protection and restoration). Protocol specific formats and mechanisms will be described in companion documents. [STANDARDS-TRACK] This document generalizes the applicability of the pseudowire (PW) Associated Channel Header (ACH), enabling the realization of a control channel associated to MPLS Label Switched Paths (LSPs) and MPLS Sections in addition to MPLS pseudowires. In order to identify the presence of this Associated Channel Header in the label stack, this document also assigns one of the reserved MPLS label values to the Generic Associated Channel Label (GAL), to be used as a label based exception mechanism. This document specifies the requirements of an MPLS Transport Profile (MPLS-TP). This document is a product of a joint effort of the International Telecommunications Union (ITU) and IETF to include an MPLS Transport Profile within the IETF MPLS and PWE3 architectures to support the capabilities and functionalities of a packet transport network as defined by International Telecommunications Union - Telecommunications Standardization Sector (ITU-T). This work is based on two sources of requirements: MPLS and PWE3 architectures as defined by IETF, and packet transport networks as defined by ITU-T. The requirements expressed in this document are for the behavior of the protocol mechanisms and procedures that constitute building blocks out of which the MPLS Transport Profile is constructed. The requirements are not implementation requirements. [STANDARDS-TRACK] Load balancing is a powerful tool for engineering traffic across a network. This memo suggests ways of improving load balancing across MPLS networks using the concept of "entropy labels". It defines the concept, describes why entropy labels are useful, enumerates properties of entropy labels that allow maximal benefit, and shows how they can be signaled and used for various applications. This document updates RFCs 3031, 3107, 3209, and 5036. [STANDARDS-TRACK] Segment Routing (SR) enables any head-end node to select any path without relying on a hop-by-hop signaling technique (e.g., LDP or RSVP-TE). It depends only on "segments" that are advertised by link-state Interior Gateway Protocols (IGPs). An SR path can be derived from a variety of mechanisms, including an IGP Shortest Path Tree (SPT), an explicit configuration, or a Path Computation Element (PCE). This document specifies extensions to the Path Computation Element Communication Protocol (PCEP) that allow a stateful PCE to compute and initiate Traffic-Engineering (TE) paths, as well as a Path Computation Client (PCC) to request a path subject to certain constraints and optimization criteria in SR networks. This document updates RFC 8408. This memo provides clear definitions for Active and Passive performance assessment. The construction of Metrics and Methods can be described as either "Active" or "Passive". Some methods may use a subset of both Active and Passive attributes, and we refer to these as "Hybrid Methods". This memo also describes multiple dimensions to help evaluate new methods as they emerge. This document describes a simple process that allows authors of Internet-Drafts to record the status of known implementations by including an Implementation Status section. This will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. This process is not mandatory. Authors of Internet-Drafts are encouraged to consider using the process for their documents, and working groups are invited to think about applying the process to all of their protocol specifications. This document obsoletes RFC 6982, advancing it to a Best Current Practice. RFC 8372 ("MPLS Flow Identification Considerations") describes the requirement for introducing flow identities within the MPLS architecture. This document describes a method of accomplishing this by using a technique called "Synonymous Flow Labels" in which labels that mimic the behavior of other labels provide the identification service. These identifiers can be used to trigger per-flow operations on the packet at the receiving label switching router. This document defines a way for an Intermediate System to Intermediate System (IS-IS) router to advertise multiple types of supported Maximum SID Depths (MSDs) at node and/or link granularity. Such advertisements allow entities (e.g., centralized controllers) to determine whether a particular Segment ID (SID) stack can be supported in a given network. This document only defines one type of MSD: Base MPLS Imposition. However, it defines an encoding that can support other MSD types. This document focuses on MSD use in a network that is Segment Routing (SR) enabled, but MSD may also be useful when SR is not enabled. Huawei Technologies Huawei Technologies China Mobile Cisco Systems, Inc. ZTE Corporation The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Clients (PCCs) requests. Segment routing (SR) leverages the source routing and tunneling paradigms. The Stateful PCEP extensions allow stateful control of Segment Routing Traffic Engineering (TE) Paths. Furthermore, PCEP can be used for computing SR TE paths in the network. This document defines PCEP extensions for grouping two unidirectional SR Paths (one in each direction in the network) into a single associated bidirectional SR Path. The mechanisms defined in this document can also be applied using a stateful PCE for both PCE- initiated and PCC-initiated LSPs or when using a stateless PCE. Huawei Technologies Huawei Technologies China Telecom China Mobile Cisco Systems A Segment Routing (SR) policy is a set of candidate SR paths consisting of one or more segment lists with necessary path attributes. For each SR path, it may also have its own path attributes, and Path Segment is one of them. A Path Segment is defined to identify an SR path, which can be used for performance measurement, path correlation, and end-2-end path protection. Path Segment can be also used to correlate two unidirectional SR paths into a bidirectional SR path which is required in some scenarios, for example, mobile backhaul transport network. This document defines extensions to BGP to distribute SR policies carrying Path Segment and bidirectional path information. China Mobile China Mobile

Acknowledgements The authors would like to thank Adrian Farrel, Stewart Bryant, Shuangping Zhan, Alexander Vainshtein, Andrew G. Malis, Ketan Talaulikar, Shraddha Hegde, Xinyue Zhang, Loa Andersson and Bruno Decraene for their review, suggestions, comments and contributions to this document. The authors would like to acknowledge the contribution from Alexander Vainshtein on "Nesting of Path Segments".

Contributors The following people have substantially contributed to this document. Huawei Technologies Co., Ltd

mach.chen@huawei.com

China Mobile

wangleiyj@chinamobile.com

ZTE Corp

liu.aihua@zte.com.cn

ZTE Corp

gregimirsky@gmail.com

Verizon Inc.

gyan.s.mishra@verizon.com