SPRING Q. Xiong Internet-Draft G. Mirsky Intended status: Informational ZTE Corporation Expires: January 14, 2021 W. Cheng China Mobile July 13, 2020 The Use of Path Segment in SR Inter-domain Scenarios draft-xiong-spring-path-segment-sr-inter-domain-02 Abstract This document illustrates the inter-domain scenarios for SR-MPLS networks to support end-to-end bidirectional tunnel across multiple domains with the use of Path Segments. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 14, 2021. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Xiong, et al. Expires January 14, 2021 [Page 1] Internet-DraThe Use of Path Segment in SR Inter-domain Scenar July 2020 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions used in this document . . . . . . . . . . . . . . 3 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. Requirements Language . . . . . . . . . . . . . . . . . . 3 3. End-to-end Path Segment . . . . . . . . . . . . . . . . . . . 4 3.1. S-PSID . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.2. N-PSID . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. SR-MPLS Inter-domain Scenarios . . . . . . . . . . . . . . . 4 4.1. Stitching of Path Segments . . . . . . . . . . . . . . . 5 4.2. Nesting of Path Segments . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. Normative References . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction Segment Routing (SR) leverages the source routing paradigm. A node steers a packet through an SR Policy instantiated as 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 supports per-flow explicit routing while maintaining per-flow state only at the ingress nodes of the SR domain. Segment Routing can be instantiated on MPLS data plane which is referred to as SR-MPLS [I-D.ietf-spring-segment-routing-mpls]. SR-MPLS leverages the MPLS label stack to construct the SR path. As defined in [RFC8402], the headend of an SR Policy binds a Binding Segment ID (B-SID) to its policy. The B-SID could be bound to a SID List or selected path and used to stitch the SR list and the SR Label Switched Paths (LSP) across multiple domains. In some scenarios, for example, a mobile backhaul transport network, it is required to provide end-to-end bidirectional path across SR networks. [I-D.ietf-spring-mpls-path-segment] defines a path segment identifier to support bidirectional path correlation for transport network. In the multi-domain scenarios, the SR bidirectional end-to-end path MAY be established with the use of path segments. Path segment MAY be used to indicate the end-to-end bidirectional path to achieve the path monitoring including nesting of Path Segments or Path SID (N-PSID) and stitching of Path Segments or Path SID (S-PSID). This document illustrates the inter-domain scenarios for SR-MPLS networks to support end-to-end bidirectional tunnel across multiple domains with the use of Path Segments. Xiong, et al. Expires January 14, 2021 [Page 2] Internet-DraThe Use of Path Segment in SR Inter-domain Scenar July 2020 2. Conventions used in this document 2.1. Terminology ABR: Area Border Routers. Routers used to connect two IGP areas (areas in OSPF or levels in IS-IS). A->B SID list: The SID List from SR node A to SR node B. AS: Autonomous System. An Autonomous System is composed by one or more IGP areas. ASBR: Autonomous System Border Router. A router used to connect together ASes of the same or different service providers via one or more inter-AS links. B-SID: Binding Segment ID. Domains:Autonomous System (AS) or IGP Area. An Autonomous System is composed by one or more IGP areas. e-Path: End-to-end Path Segment. Inter-Area: Two IGP areas interconnects with an ABR in an AS. Inter-AS: Two ASes interconnects with an ASBR. IGP: Interior Gateway Protocol. N-PSID: Nesting of Path Segments. S-PSID: Stitching of Path Segments. SR: Segment Routing. SR-MPLS: Segment Routing with MPLS data plane. 2.2. 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. Xiong, et al. Expires January 14, 2021 [Page 3] Internet-DraThe Use of Path Segment in SR Inter-domain Scenar July 2020 3. End-to-end Path Segment 3.1. S-PSID As described in [I-D.ietf-spring-mpls-path-segment], an end-to-end Path Segment, also referred to as e-Path. In the inter-domain scenario, the end-to-end SR path is split into multiple segments. And each segment can be identified by S-PSIDs in stitching model. The correlation of path segments can stitch the inter-domain paths and bind unidirectional paths. The S-PSIDs are valid in the corresponding domain and the border nodes maintain the forwarding entries of that S-PSID. At the headend node, the S-PSID can correlate the inter-domain path of reverse direction and bind the two unidirectional paths. The S-PSID can be a locally unique label and assigned from the Segment Routing Local Block (SRLB). It is required that the controller(e.g., PCE) assigns the label to ensure the ingress and the egress node can recognize it and it also can be assigned from egress node of each domain. PCEP based S-PSID allocation and procedure is defined in [I-D.xiong-pce-stateful-pce-sr-inter-domain]. 3.2. N-PSID As described in [I-D.ietf-spring-mpls-path-segment], an end-to-end Path Segment, also referred to as e-Path. In nesting model, the e-Path is also referred to as N-PSID which is encapsulated at the ingress nodes and decapsulated at the egress nodes. The transit nodes, even the border nodes of domains, are not aware of the N-PSID. The use of the B-SID is also recommended to reduce t he size of label stack section 4.2 and stitch the SR list and the SR LSP. The N-PSID can be used to indicate the end-to-end path and achieve the bidirectional path monitoring. The N-PSID can be a globally unique or local label. If the N-PSID is globally unique, it MUST be assigned from the SRGB block of each domain. If the N-PSID is a local label, it is required that the controller(e.g., PCE) or a super controller (e.g., hierarchical PCE) assigns the label to ensure the ingress(A) and the egress node(Z) can recognize it and there is no SID collision in the ingress and egress domains. 4. SR-MPLS Inter-domain Scenarios The domains of the networks may be IGP Areas or ASes and the inter- domain scenario may be inter-Area or inter-AS. The multiple SR-MPLS domains may be interconnected with a ABR within areas or inter-link between ASes. This document takes IGP Areas domains for example. Xiong, et al. Expires January 14, 2021 [Page 4] Internet-DraThe Use of Path Segment in SR Inter-domain Scenar July 2020 The border link scenarios are in future discussion. SR-MPLS domains can be deployed as Figure 1 shown. + + + + + + + + + + + + + + + + + + + + + A SR-MPLS X SR-MPLS Y SR-MPLS Z + IGP 1 + + IGP 2 + + IGP 3 + + + + + + + + + + + + + + + + Figure 1: SR-MPLS inter-domain Scenario Two SR-MPLS inter-domain models are discussed in this document including using the stitching and nesting of Path Segments which are described in Section 4.1 and Section 4.2 respectively. 4.1. Stitching of Path Segments The Figure 1 displays the border node inter-domain scenario. SR node X and SR node Y are the border nodes of two different domains. The S-PSIDs from A->X, X->Y, and Y->Z are used for the inter-domain path segment. The ingress SR node A encapsulates the data packet with S-PSID (A->X), B-SID(Y->Z), B-SID(X->Y) and A->X SID list. The data packet is forwarded to SR node X according to the A->X SID list. Node X pushes the S-PSID (X->Y), B-SID(Y->Z) and X->Y SID list based on the above mentioned forwarding entry. The data packet is forwarded to node Y and then to the SR node Z b ased on the same forwarding procedure. In node Z, the S-PSID (Y->Z) can be mapped to the path from Z to Y of reverse direction and correlates the two unidirectional paths. The packet transmission of the reverse direction is the same with the forwarding direction with different S-PSID. The stitching of path segments can achieve the inter-domain path monitoring. Xiong, et al. Expires January 14, 2021 [Page 5] Internet-DraThe Use of Path Segment in SR Inter-domain Scenar July 2020 .................. ................. .................... . . . . . . +-----+ +-----+ +-----+ +-----+ | A | | X | | Y | | Z | +-----+ +-----+ +-----+ +-----+ . SR Domain 1 . . SR Domain 2 . . SR Domain 3 . .................. ................. .................... Service Layer: |<----------------------End-to-end Service--------------->| Path Segment: |<-----S-PSID----->o<------S-PSID----->o<-----S-PSID----->| LSP/Tunnel: |<------SR-LSP---->|<-----SR-LSP------>|<-----SR-LSP----->| Node: |<----SID List---->|<-----SID List---->|<----SID List---->| Node A Node X Node Y Node Z +-------------+ |A->X SID list| +-------------+ +-------------+ | B-SID(X->Y) | --->|X->Y SID list| +-------------+ +-------------+ +-------------+ | B-SID(Y->Z) | | B-SID(Y->Z) | --->|Y->Z SID list| +-------------+ +-------------+ +-------------+ +--------------+ |S-PSID(A->X) | |S-PSID(X->Y) | |S-PSID(Y->Z) | -->| Payload | +-------------+ +-------------+ +-------------+ +--------------+ | Payload | | Payload | | Payload | +-------------+ +-------------+ +-------------+ Figure 2: Stitching of Path Segments in Inter-Domain Scenario 4.2. Nesting of Path Segments Figure 3 shows the SR-MPLS nesting inter-domain scenario. The e-Path(A->Z) is used to indicate the end-to-end path. The N-PSID, B-SID and SR list are pushed by the ingress node. The N-PSID is used to correlate the two unidirectional SR paths to an SR bidirectional path. The use of the B-SID is also recommended to replace the SR list of each domain. As shown in Figure 3, the B-SID(X->Y) is used to replace the X->Y SID list. Ingress node A pushes N-PSID(A->Z), B-SID(Y->Z), B-SID(X->Y), and A->X SID list in turn. When the packet is received at node X, the X->Y SID list are popped. Also, X->Y SID list replaces B-SID(X->Y) to indicate that packet to be forwarded from node X to node Y. The data packet reaches the SR node Z Xiong, et al. Expires January 14, 2021 [Page 6] Internet-DraThe Use of Path Segment in SR Inter-domain Scenar July 2020 according to the same forwarding procedure. In SR node Z, the N-PSID (A->Z) is used to correlate the two unidirectional end-to-end paths. .................. ................. .................... . . . . . . +-----+ +-----+ +-----+ +-----+ | A | | X | | Y | | Z | +-----+ +-----+ +-----+ +-----+ . SR Domain 1 . . SR Domain 2 . . SR Domain 3 . .................. ................. .................... Service Layer: |<----------------------End-to-end Service--------------->| Path Segment: |<------------------------N-PSID------------------------->| LSP/Tunnel: |<------SR-LSP---->o<-------SR-LSP----->o<-----SR-LSP---->| Node: |<----SID List---->|<-----SID List----->|<----SID List--->| Node A Node X Node Y Node Z +-------------+ |A->X SID list| +-------------+ +-------------+ |B-SID(X->Y) | --> |X->Y SID list| +-------------+ +-------------+ +-------------+ |B-SID(Y->Z) | |B-SID(Y->Z) | --> |Y->Z SID list| +-------------+ +-------------+ +-------------+ +-------------+ |N-PSID(A->Z) | |N-PSID(A->Z) | |N-PSID(A->Z) | --> | Payload | +-------------+ +-------------+ +-------------+ +-------------+ | Payload | | Payload | | Payload | +-------------+ +-------------+ +-------------+ Figure 3: Nesting of Path Segments in Inter-Domain Scenario 5. Security Considerations TBA 6. Acknowledgements TBA Xiong, et al. Expires January 14, 2021 [Page 7] Internet-DraThe Use of Path Segment in SR Inter-domain Scenar July 2020 7. IANA Considerations TBA 8. Normative References [I-D.ietf-spring-mpls-path-segment] Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler, "Path Segment in MPLS Based Segment Routing Network", draft-ietf-spring-mpls-path-segment-02 (work in progress), February 2020. [I-D.ietf-spring-segment-routing-mpls] Bashandy, A., Filsfils, C., Previdi, S., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing with MPLS data plane", draft-ietf-spring-segment-routing-mpls-22 (work in progress), May 2019. [I-D.xiong-pce-stateful-pce-sr-inter-domain] Xiong, Q., Mirsky, G., hu, f., and W. Cheng, "Stateful PCE for SR-MPLS Inter-domain", draft-xiong-pce-stateful-pce- sr-inter-domain-02 (work in progress), October 2019. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, July 2018, . Authors' Addresses Quan Xiong ZTE Corporation No.6 Huashi Park Rd Wuhan, Hubei 430223 China Phone: +86 27 83531060 Email: xiong.quan@zte.com.cn Xiong, et al. Expires January 14, 2021 [Page 8] Internet-DraThe Use of Path Segment in SR Inter-domain Scenar July 2020 Greg Mirsky ZTE Corporation USA Email: gregimirsky@gmail.com Weiqiang Cheng China Mobile Beijing China Email: chengweiqiang@chinamobile.com Xiong, et al. Expires January 14, 2021 [Page 9]