]> Path Segment in MPLS Based Segment Routing Network 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 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.
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 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) , bidirectional path , or Performance Measurement (PM) , 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.
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 SRGB: SR Global 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.
Path Segment A Path Segment Identifier(PSID) is a single label that is assigned from the Segment Routing Local Block (SRLB) or Segment Routing Global Block (SRGB) 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 , 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 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 . 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 :
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.
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.
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.
Nesting of Path Segments Binding SID (BSID) 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. 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)| +------------+ +------------+ +------------+ | 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)| +------------+ +------------+ +------------+ +------------+ ]]>
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.
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 SDN controller using the Path Segments of the SR paths.
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 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 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.
IANA Considerations This document does not require any IANA actions.
References Normative References Segment Routing Architecture 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 with the MPLS Data Plane 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). Key words for use in RFCs to Indicate Requirement Levels 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. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words 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 Generalized Multi-Protocol Label Switching (GMPLS) Recovery Functional Specification 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] MPLS Generic Associated Channel 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. Requirements of an MPLS Transport Profile 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] The Use of Entropy Labels in MPLS Forwarding 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] Path Computation Element Communication Protocol (PCEP) Extensions for Segment Routing 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. Active and Passive Metrics and Methods (with Hybrid Types In-Between) 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.
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. 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