| Internet-Draft | Path Segment in SR-MPLS | September 2023 |
| Cheng, et al. | Expires 24 March 2024 | [Page] |
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 can not determine on which SR path a packet traversed the network from the label stack because the segment identifiers are stripped 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.¶
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Segment Routing (SR) [RFC8402] 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 [RFC8660] through a label stack to construct an SR path.¶
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. So that no label or only the last label (e.g. a service label or an Explicit-Null 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 end-to-end 1+1 path protection (Live-Live case) Section 3.3, bidirectional path Section 3.2, or Performance Measurement (PM) Section 3.1, the ability to implement path identification on the egress node is a pre-requisite.¶
Therefore, this document introduces a new segment type, 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 nodes for path identification hence to support various use-cases including SR path PM, end-to-end 1+1 SR path protection, and bidirectional SR paths correlation. Note that, per-path states will be maintained in the egress node due to the requirements in these use cases, though in normal cases that the per-path states will be maintained in the ingress node only in the SR architecture.¶
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.¶
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.¶
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.¶
A Path Segment is a Local Segment which uniquely identify 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) [RFC8402] of the egress node of an SR path.¶
The term of SR path used in this document is a path described by a Segment-List (SL). 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. How to use the PSID to Segment Lists depends on the requirements of the use cases.¶
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 associated to it, 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 receiving on an intermidate node of the associated path, but it can be the top label in the label stack on the penultimate node after the last forwarding label with Penultimate Hop Popping (PHP) is popped off. Otherwise, the PSID may be processed by an intermediate node, which may cause error in forwarding because of mis-matching.¶
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 [RFC5586], when GAL is used, the ACH appears immediately after the bottom of the label stack.¶
If Entropy Label is also used on this 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.¶
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 [RFC8664]. 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 RFC8491 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 Figure 1:¶
+--------------------+
| ... |
+--------------------+
| Label 1 |
+--------------------+
| Label 2 |
+--------------------+
| ... |
+--------------------+
| Label n |
+--------------------+
| PSID |
+--------------------+
~ Payload ~
+--------------------+
Where:¶
Signaling of the PSID between the egress, ingress and possibly a centralized controller is out of the scope of this document.¶
This section describes use cases which can leveage the Path Segment.¶
As defined in [RFC7799], 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.¶
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 [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. 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.¶
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.¶
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 SDN controller using the Path Segments of the SR paths.¶
Binding SID (BSID) [RFC8402] can be used for SID list compression. With BSID, an end-to-end SR path can be splitted 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) that spans three 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.¶
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)|
+------------+ +------------+ +------------+
| BSID(C->D) | | BSID(C->D) | ~C->D SubPath~
+------------+ +------------+ +------------+ +------------+
|e-PSID(A->D)| |e-PSID(A->D)| |e-PSID(A->D)| |e-PSID(A->D)|
+------------+ +------------+ +------------+ +------------+
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.¶
A Path Segment is used within an SR-MPLS domain [RFC8402] and should not leak outside the domain, therefore no new security threats are introduced comparing to current SR-MPLS. The security consideration of SR-MPLS, such as boundary filtering described in Section 8.1 of [RFC8402] 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.¶
[Note to the RFC Editor - remove this section before publication, as well as remove the reference to [RFC7942].¶
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 [RFC7942]. 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 [RFC7942], "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".¶
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[INTEROP-TEST]. Note that Path Segment is a key feature of Layer3 in SPN architecture [SPN-L3]. This is reported by Weiqiang Cheng from China Mobile at September, 21, 2023.¶
This document does not require any IANA actions.¶
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".¶
The following people have substantially contributed to this document.¶