SPRING Quan Xiong Internet-Draft Greg Mirsky Intended status: Standards Track ZTE Corporation Expires: April 19, 2020 Weiqiang Cheng China Mobile October 17, 2019 The Use of Path Segment in SR Inter-domain Scenarios draft-xiong-spring-path-segment-sr-inter-domain-01 Abstract This document discusses the inter-domain scenarios for SR-MPLS networks and proposes the solution 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 April 19, 2020. Copyright Notice Copyright (c) 2019 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. Quan Xiong, et al. Expires April 19, 2020 [Page 1] Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions used in this document . . . . . . . . . . . . . . 3 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. Requirements Language . . . . . . . . . . . . . . . . . . 4 3. Path Segment for SR-MPLS Inter-domain . . . . . . . . . . . . 4 3.1. Inter-domain Path Segment . . . . . . . . . . . . . . . . 4 3.2. End-to-end Path Segment . . . . . . . . . . . . . . . . . 4 4. SR-MPLS Inter-domain Scenarios . . . . . . . . . . . . . . . 5 4.1. Stitching Inter-domain with i-Path . . . . . . . . . . . 5 4.2. Nesting Inter-domain with e-Path . . . . . . . . . . . . 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. [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 tunnel MAY be established with the use of path segments. The SR-MPLS inter- domain models include the stitching and nesting inter-domain models. Path segment MAY be used to indicate the inter-domain path or the end-to-end path and correlate the inter-domain paths or end-to-end unidirectional paths to achieve the path monitoring. As defined in [RFC8402], the headend of an SR Policy binds a Binding Segment ID(BSID) to its policy. The BSID could be bound to a SID List or selected path and used to stitch the service across multiple domains. For example, as discussed in Section 3 [I-D.ietf-spring-mpls-path-segment], the BSID can be used to identify a sub-path and stitched them to an end-to-end SR path in the nesting model. The BSID and path segment can be combined to achieve the inter-domain path monitoring. But the solution is not appropriate Quan Xiong, et al. Expires April 19, 2020 [Page 2] Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 for the stitching model. The policy MUST be instantiated before the end-to-end service and it can not deploy domains incrementally. Moreover, all of the BSIDs MUST be pushed onto the label stack at the headend but not all of them are popped at an edge nodes. The edge node pops one BSID and bound it to a SID List. That can not meet the independence requirement in the stitching model especially when the domains belong to different operators. This document discusses the inter-domain scenarios for SR-MPLS networks and proposes the solution with the use of path segments for end-to-end bidirectional SR path. 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. BSID: 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. s-Path: Sub-path 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. i-Path/i-PSID: Inter-domain Path Segment. SR: Segment Routing. SR-MPLS: Segment Routing with MPLS data plane. Quan Xiong, et al. Expires April 19, 2020 [Page 3] Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 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. 3. Path Segment for SR-MPLS Inter-domain 3.1. Inter-domain Path Segment In the stitching inter-domain model, the end-to-end SR path is split into multiple segments. And each segment can be identified by an inter-domain path segment (i-Path or i-PSID). The correlation of path segments can stitch the inter-domain paths and bind unidirectional paths. The i-Paths are valid in the corresponding domain and the border nodes maintain the forwarding entries of that i-Path segment, which binding with the next i-Path and SID list. At the headend node, the i-Path can correlate the inter-domain path of reverse direction and bind the two unidirectional paths. The border nodes should install the following MPLS data entries for path segments: incoming label: i-Path outgoing label: the SID list of the next domain or link + next i-Path Taking Figure 1 as an example, the border node X installs the MPLS data entries: incoming label: i-Path(A->X) outgoing label: X->Y SID list + i-Path(X->Y) The i-Path 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 i-Path allocation and procedure is defined in [I-D.xiong-pce-stateful-pce-sr-inter-domain]. 3.2. End-to-end Path Segment The nesting inter-domain model is described in [I-D.ietf-spring-mpls-path-segment], an end-to-end path segment, also referred to as e-Path, is used to indicate the end-to-end path, and an s-Path is used to indicate the intra-domain path. The e-Path is encapsulated at the ingress nodes and decapsulated at the egress nodes. The transit nodes, even the border nodes of domains, are not Quan Xiong, et al. Expires April 19, 2020 [Page 4] Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 aware of the e-Path segment. The s-Path can be used as stitching label to correlate the two domains. The use of the binding SID [RFC8402] is also recommended to reduce the size of label stack section 4.2. The e-Path can be a globally unique or local label. If the e-Path is globally unique, it MUST be assigned from the SRGB block of each domain. If the e-Path 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. 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 and MPLS-TP interworking Scenario Two SR-MPLS inter-domain models are discussed in this document including the stitching and nesting inter-domain model which are described in Section 4.1 and Section 4.2 respectively. 4.1. Stitching Inter-domain with i-Path 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 i-Paths 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 i-Path (A->X) 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 i-Path (X->Y) 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 Quan Xiong, et al. Expires April 19, 2020 [Page 5] Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 node Z based on the same forwarding procedure. In node Z, the i-Path (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 i-Paths. The stitching of path segments can achieve the inter-domain stitching and path monitoring .................. ................. .................... . . . . . . +-----+ +-----+ +-----+ +-----+ | A | | X | | Y | | Z | +-----+ +-----+ +-----+ +-----+ . SR Domain 1 . . SR Domain 2 . . SR Domain 3 . .................. ................. .................... |<------------------>|<------------------>|<--------------->| i-Path(A->X) i-Path(X->Y) i-Path(Y->Z) Node A Node X Node Y Node Z +-------------+ +-------------+ +-------------+ |A->X SID list| |X->Y SID list| |Y->Z SID list| +-------------+ +-------------+ +-------------+ +--------------+ |i-Path(A->X) |---->|i-Path(X->Y) |---->|i-Path(Y->Z) |--->| Payload | +-------------+ +-------------+ +-------------+ +--------------+ | Payload | | Payload | | Payload | +-------------+ +-------------+ +-------------+ Figure 2: Stitching Border Node Inter-Domain Scenario 4.2. Nesting Inter-domain with e-Path 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 s-Path is used to identify the domain's sub-path. The e-Path, s-Path and SR list are pushed by the ingress node. The e-Path is used to correlate the two unidirectional SR paths to an SR bidirectional path. The s-Path can be used as stitching label to correlate the two inter- domain sub-paths. The use of the binding SID [RFC8402] 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 e-Path(A->Z), B-SID(Y->Z), B-SID(X-Y), s-Path(A->X) and A->X SID list in turn. When the packet is received at node X, the s-Path(A-X) and X->Y SID list are popped, and the new s-Path(X->Y) is pushed. Also, Quan Xiong, et al. Expires April 19, 2020 [Page 6] Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 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 according to the same forwarding procedure. In SR node Z, the e-Path (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 . .................. ................. .................... |<------------------>|<------------------>|<--------------->| s-Path(A->X) s-Path(X->Y) s-Path(Y->Z) |<--------------------------------------------------------->| e-Path(A->Z) Node A +-------------+ |A->X SID list| Node X +-------------+ +-------------+ |s-Path(A->X) | |X->Y SID list| Node Y +-------------+ +-------------+ +-------------+ |B-SID(X->Y) | --> |s-Path(X->Y) | |Y->Z SID list| +-------------+ +-------------+ +-------------+ |B-SID(Y->Z) | |B-SID(Y->Z) | --> |s-Path(Y->Z) | Node Z +-------------+ +-------------+ +-------------+ +-------------+ |e-Path(A->Z) | |e-Path(A->Z) | |e-Path(A->Z) | --> |e-Path(A->Z) | +-------------+ +-------------+ +-------------+ +-------------+ | Payload | | Payload | | Payload | | Payload | +-------------+ +-------------+ +-------------+ +-------------+ Figure 3: Nesting Inter-Domain Scenario 5. Security Considerations TBA 6. Acknowledgements TBA Quan Xiong, et al. Expires April 19, 2020 [Page 7] Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 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-01 (work in progress), September 2019. [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., hu, f., Mirsky, G., and W. Cheng, "Stateful PCE for SR-MPLS Inter-domain", draft-xiong-pce-stateful-pce- sr-inter-domain-01 (work in progress), July 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 Quan Xiong, et al. Expires April 19, 2020 [Page 8] Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 Greg Mirsky ZTE Corporation USA Email: gregimirsky@gmail.com Weiqiang Cheng China Mobile Beijing China Email: chengweiqiang@chinamobile.com Quan Xiong, et al. Expires April 19, 2020 [Page 9]