Path Segment in MPLS Based Segment Routing Network

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Table of Contents

1. Introduction

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, like end-to-end 1+1 path protection (Live-Live case) Section 7, bidirectional path Section 6, or Performance Measurement (PM) Section 5, the ability to implement path identification on the egress node is a pre-requisite.¶

Therefore, this document introduces a new segment type that is referred to as the 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.¶

2. Path Segment

A Path Segment Identifier(PSID) is a single label that is assigned from the Segment Routing Local Block (SRLB) [RFC8402] or Segment Routing Global Block (SRGB) [RFC8402] or dynamic MPLS label pool of the egress node of an SR path. Whether a PSID is allocated from the SRLB, SRGB, or a dynamic range depends on specific use cases. If the PSID is only used by the egress node to identify an SR path, the SRLB, SRGB or dynamic MPLS label pool can be used. Three use cases are introduced in Section 5, 6, and 7 of this document.¶

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, all the Segment lists in a Candidate path can use a single PSID, and all the Segment Lists in an SR policy can share the same PSID, if customers would like to aggregate the data among the Segment Lists. How to use the PSID to Segment Lists depends on the requirements of the use cases.¶

When a PSID is used, the PSID MUST 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. Otherwise, the PSID may be processed by an intermediate node, which may cause error in forwarding because of mis-matching if the PSID is allocated from a SRLB.¶

The value of the TTL field in the MPLS label stack entry containing the PSID can be set to any value including 0, or the same value as the TTL of the last label stack entry for the last segment in the SR path. If the Path Segment is the bottom label, the S bit MUST be set.¶

A PSID can be used in the case of Penultimate Hop Popping (PHP), where some labels are be popped off at the penultimate hop of an SR path. As per regular MPLS processing, the label below (including the PSID in this case) will not be popped by the penultimate node.¶

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.¶

In some deployments, service labels may be added after the Path Segment label in the MPLS label stack. In this case, the egress node MUST be capable of processing more than one label. The additional processing required, 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, it 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. This includes the PSID.¶

The label stack with Path Segment is shown in Figure 1:¶

            +--------------------+
            |       ...          |
            +--------------------+
            |      Label 1       |
            +--------------------+
            |      Label 2       |
            +--------------------+
            |       ...          |
            +--------------------+
            |      Label n       |
            +--------------------+
            |        PSID        |
            +--------------------+
            ~       Payload      ~
            +--------------------+

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.¶

There may be multiple paths (or sub-path(s)) carried in the label stack, for each path (or sub-path), there may be a corresponding Path Segment carried. A use case can be found in Section 4.¶

3. PSID Allocation and Distribution

There are some ways to assign and distribute the PSID. The PSID can be configured locally or allocated by a centralized controller or by other means, this is out of the scope of this document. If an egress cannot support the use of the PSID, it MUST reject the attempt to configure the label.¶

4. Nesting of Path Segments

Binding SID (BSID) [RFC8402] can be used for SID list compression. With BSID, an end-to-end SR path 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) 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)|
 +------------+  +------------+  +------------+  +------------+

5. Path Segment for Performance Measurement

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.¶

6. 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 [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.¶

7. 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 [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.¶

8. Security Considerations

A Path Segment in SR-MPLS is a label similar to other labels/Segment, such as a VPN label or a Prefix SID, 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.¶

A PSID is allocated by an egress node and distributed to an ingress. The distribution is performed within an SR trusted domain. However, the mechanism of distributing a PSID is out of the scope of this document, and its security consideration will be described in other documents.¶

9. IANA Considerations

This document does not require any IANA actions.¶

Acknowledgements

The authors would like to thank Adrian Farrel, Stewart Bryant, Shuangping Zhan, Alexander Vainshtein, Andrew G. Malis, Ketan Talaulikar, Shraddha Hegde, and Loa Andersson for their review, suggestions and comments 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.¶

Authors' Addresses