Path Segment Identifier in MPLS Based Segment Routing Network

1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

1.2. Abbreviations and Terms

MPLS: Multiprotocol Label Switching.¶

SR: Segment Routing.¶

SID: Segment Identifier.¶

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 a path, which contains a sub-set of the nodes and links of the path.¶

PSID: Path Segment Identifier.¶

2.1. Equal-Cost Multipath(ECMP) Considerations

If Entropy Label(EL) is also used on the egress node, as per [RFC6790] 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) [RFC8957], 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. This, in turn, may invalidate certain uses of the PSID, such as performance measurement applications. Therefore, the considerations as per section 5 in [RFC8957] of SFL also apply to PSID in implementation.¶

3.1. PSID for Performance Measurement

As defined in [RFC7799], performance measurement can be classified into Passive, Active, and Hybrid measurement. Since a PSID is encoded in the SR-MPLS Label Stack as shown in Figure 1, existing implementation on the egress node can leverage PSID for measuring packet counts.¶

For Passive performance measurement, path identification at the measuring points is the pre-requisite. PSID 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.¶

Furthermore, PSID 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.¶

• In-situ OAM[RFC9197] for SR-MPLS to identify the SR Path associated with the in-situ data fields in the data packets on the egress node.¶

• In-band Performance Measurement for SR-MPLS to identify the SR Path associated with the collected performance metrics.¶

3.2. PSID for Bidirectional SR Path

In some scenarios, for example, mobile backhaul transport networks, there are requirements to support bidirectional paths[RFC6965], 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 [RFC5654].¶

In the current SR architecture, an SR path is a unidirectional path [RFC8402]. In order to support bidirectional SR paths, a straightforward way is to bind two unidirectional SR paths to a single bidirectional SR path. PSIDs can 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 PSID is out of the scope of this document and has been described in several documents, such as [I-D.ietf-pce-sr-bidir-path] and [I-D.ietf-idr-sr-policy-path-segment].¶

3.3. PSID 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 [RFC4426].¶

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.¶

The detailed solution of using PSID in end-to-end 1+1 path protection is out of the scope of this document.¶

3.4. Nesting of PSIDs

Binding SID (BSID) [RFC8402] 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 subdomains (Access, Aggregation and Core domain) and consists of three sub-paths, one in each subdomain (sub-path (A->B), sub-path (B->C) and sub-path (C->D)).¶

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. Each sub-path is associated with a BSID and an 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.¶

Figure 2 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.¶

         /--------\       /--------\       /--------\
       /            \   /            \   /            \
     A{    Access    }B{  Aggregation }C{     Core     }D
       \            /   \            /   \            /
         \--------/       \--------/       \--------/
       Sub-path(A->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)|
 +------------+  +------------+  +------------+  +------------+

5.1. Huawei Technologies

• Organization: Huawei Technologies.¶

• Implementation: Huawei PTN7900 Series Routers implementation of SR-TP[HW-IMP].¶

• Description: SR-TP is a feature of Huawei PTN7900 series Routers, which uses PSIDs to associate with paths and build up bidirectional paths. Huawei PTN7900 Series Routers with version V100R018C00 and above have commercially implemented the definition of PSID 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 PSID in section 3 are not yet implemented. For control plane, PTN7900 Series Routers support configuring PSID 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¶

5.2. ZTE Corp

• Organization: ZTE Corporation.¶

• Implementation: ZTE's SPN implementation of PSID[ZTE-IMP].¶

• Description: The feature of SR-MPLS PSID 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 PSID 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¶

5.3. New H3C Technologies

• Organization: New H3C Technologies.¶

• Implementation: H3C CR16000, CR19000 series routers implementation of PSID.¶

• 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 PSID 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¶

5.4. Spirent Communications

• Organization: Spirent Communications¶

• Implementation: Spirent Testcenter Product Family implementation of SR-TP test capability[SP-IMP].¶

• Description: Spirent Testcenter product family implements SR-MPLS PSID 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¶

5.5. Fiberhome

• Organization: Fiberhome Corporation.¶

• Implementation: Fiberhome SPN series of products (Citrans 650/690E) implementation of PSID[FH-IMP].¶

• 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 PSID 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 PSID 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¶

5.6. Interoperability Test

[Note to the RFC Editor - remove this section before publication, as well as remove the reference to [RFC7942].¶

The Interoperability test of PSID 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[INTEROP-TEST]. Note that PSID is a key feature of Layer3 in SPN architecture [SPN-L3]. This is reported by Weiqiang Cheng from China Mobile at September, 21, 2023.¶