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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SPRING Quan Xiong 3 Internet-Draft Greg Mirsky 4 Intended status: Standards Track ZTE Corporation 5 Expires: April 19, 2020 Weiqiang Cheng 6 China Mobile 7 October 17, 2019 9 The Use of Path Segment in SR Inter-domain Scenarios 10 draft-xiong-spring-path-segment-sr-inter-domain-01 12 Abstract 14 This document discusses the inter-domain scenarios for SR-MPLS 15 networks and proposes the solution with the use of path segments. 17 Status of This Memo 19 This Internet-Draft is submitted in full conformance with the 20 provisions of BCP 78 and BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF). Note that other groups may also distribute 24 working documents as Internet-Drafts. The list of current Internet- 25 Drafts is at https://datatracker.ietf.org/drafts/current/. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 This Internet-Draft will expire on April 19, 2020. 34 Copyright Notice 36 Copyright (c) 2019 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents 41 (https://trustee.ietf.org/license-info) in effect on the date of 42 publication of this document. Please review these documents 43 carefully, as they describe your rights and restrictions with respect 44 to this document. Code Components extracted from this document must 45 include Simplified BSD License text as described in Section 4.e of 46 the Trust Legal Provisions and are provided without warranty as 47 described in the Simplified BSD License. 49 Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 54 2. Conventions used in this document . . . . . . . . . . . . . . 3 55 2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 56 2.2. Requirements Language . . . . . . . . . . . . . . . . . . 4 57 3. Path Segment for SR-MPLS Inter-domain . . . . . . . . . . . . 4 58 3.1. Inter-domain Path Segment . . . . . . . . . . . . . . . . 4 59 3.2. End-to-end Path Segment . . . . . . . . . . . . . . . . . 4 60 4. SR-MPLS Inter-domain Scenarios . . . . . . . . . . . . . . . 5 61 4.1. Stitching Inter-domain with i-Path . . . . . . . . . . . 5 62 4.2. Nesting Inter-domain with e-Path . . . . . . . . . . . . 6 63 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 64 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 65 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 66 8. Normative References . . . . . . . . . . . . . . . . . . . . 8 67 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 69 1. Introduction 71 Segment Routing (SR) leverages the source routing paradigm. A node 72 steers a packet through an SR Policy instantiated as an ordered list 73 of instructions called "segments". A segment can represent any 74 instruction, topological or service based. A segment can have a 75 semantic local to an SR node or global within an SR domain. SR 76 supports per-flow explicit routing while maintaining per-flow state 77 only at the ingress nodes of the SR domain. Segment Routing can be 78 instantiated on MPLS data plane which is referred to as SR-MPLS 79 [I-D.ietf-spring-segment-routing-mpls]. SR-MPLS leverages the MPLS 80 label stack to construct the SR path. 82 [I-D.ietf-spring-mpls-path-segment] defines a path segment identifier 83 to support bidirectional path correlation for transport network. In 84 the multi-domain scenarios, the SR bidirectional end-to-end tunnel 85 MAY be established with the use of path segments. The SR-MPLS inter- 86 domain models include the stitching and nesting inter-domain models. 87 Path segment MAY be used to indicate the inter-domain path or the 88 end-to-end path and correlate the inter-domain paths or end-to-end 89 unidirectional paths to achieve the path monitoring. 91 As defined in [RFC8402], the headend of an SR Policy binds a Binding 92 Segment ID(BSID) to its policy. The BSID could be bound to a SID 93 List or selected path and used to stitch the service across multiple 94 domains. For example, as discussed in Section 3 95 [I-D.ietf-spring-mpls-path-segment], the BSID can be used to identify 96 a sub-path and stitched them to an end-to-end SR path in the nesting 97 model. The BSID and path segment can be combined to achieve the 98 inter-domain path monitoring. But the solution is not appropriate 100 Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 102 for the stitching model. The policy MUST be instantiated before the 103 end-to-end service and it can not deploy domains incrementally. 104 Moreover, all of the BSIDs MUST be pushed onto the label stack at the 105 headend but not all of them are popped at an edge nodes. The edge 106 node pops one BSID and bound it to a SID List. That can not meet the 107 independence requirement in the stitching model especially when the 108 domains belong to different operators. 110 This document discusses the inter-domain scenarios for SR-MPLS 111 networks and proposes the solution with the use of path segments for 112 end-to-end bidirectional SR path. 114 2. Conventions used in this document 116 2.1. Terminology 118 ABR: Area Border Routers. Routers used to connect two IGP areas 119 (areas in OSPF or levels in IS-IS). 121 A->B SID list: The SID List from SR node A to SR node B. 123 AS: Autonomous System. An Autonomous System is composed by one or 124 more IGP areas. 126 ASBR: Autonomous System Border Router. A router used to connect 127 together ASes of the same or different service providers via one or 128 more inter-AS links. 130 BSID: Binding Segment ID. 132 Domains:Autonomous System (AS) or IGP Area. An Autonomous System is 133 composed by one or more IGP areas. 135 e-Path: End-to-end Path Segment. 137 s-Path: Sub-path Path Segment. 139 Inter-Area: Two IGP areas interconnects with an ABR in an AS. 141 Inter-AS: Two ASes interconnects with an ASBR. 143 IGP: Interior Gateway Protocol. 145 i-Path/i-PSID: Inter-domain Path Segment. 147 SR: Segment Routing. 149 SR-MPLS: Segment Routing with MPLS data plane. 151 Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 153 2.2. Requirements Language 155 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 156 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 157 "OPTIONAL" in this document are to be interpreted as described in BCP 158 14 [RFC2119] [RFC8174] when, and only when, they appear in all 159 capitals, as shown here. 161 3. Path Segment for SR-MPLS Inter-domain 163 3.1. Inter-domain Path Segment 165 In the stitching inter-domain model, the end-to-end SR path is split 166 into multiple segments. And each segment can be identified by an 167 inter-domain path segment (i-Path or i-PSID). The correlation of 168 path segments can stitch the inter-domain paths and bind 169 unidirectional paths. The i-Paths are valid in the corresponding 170 domain and the border nodes maintain the forwarding entries of that 171 i-Path segment, which binding with the next i-Path and SID list. At 172 the headend node, the i-Path can correlate the inter-domain path of 173 reverse direction and bind the two unidirectional paths. The border 174 nodes should install the following MPLS data entries for path 175 segments: 177 incoming label: i-Path 178 outgoing label: the SID list of the next domain or link + next i-Path 180 Taking Figure 1 as an example, the border node X installs the MPLS 181 data entries: 183 incoming label: i-Path(A->X) 184 outgoing label: X->Y SID list + i-Path(X->Y) 186 The i-Path can be a locally unique label and assigned from the 187 Segment Routing Local Block (SRLB). It is required that the 188 controller(e.g., PCE) assigns the label to ensure the ingress and the 189 egress node can recognize it and it also can be assigned from egress 190 node of each domain. PCEP based i-Path allocation and procedure is 191 defined in [I-D.xiong-pce-stateful-pce-sr-inter-domain]. 193 3.2. End-to-end Path Segment 195 The nesting inter-domain model is described in 196 [I-D.ietf-spring-mpls-path-segment], an end-to-end path segment, also 197 referred to as e-Path, is used to indicate the end-to-end path, and 198 an s-Path is used to indicate the intra-domain path. The e-Path is 199 encapsulated at the ingress nodes and decapsulated at the egress 200 nodes. The transit nodes, even the border nodes of domains, are not 202 Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 204 aware of the e-Path segment. The s-Path can be used as stitching 205 label to correlate the two domains. The use of the binding SID 206 [RFC8402] is also recommended to reduce the size of label stack 207 section 4.2. 209 The e-Path can be a globally unique or local label. If the e-Path is 210 globally unique, it MUST be assigned from the SRGB block of each 211 domain. If the e-Path is a local label, it is required that the 212 controller(e.g., PCE) or a super controller (e.g., hierarchical PCE) 213 assigns the label to ensure the ingress(A) and the egress node(Z) can 214 recognize it and there is no SID collision in the ingress and egress 215 domains. 217 4. SR-MPLS Inter-domain Scenarios 219 The domains of the networks may be IGP Areas or ASes and the inter- 220 domain scenario may be inter-Area or inter-AS. The multiple SR-MPLS 221 domains may be interconnected with a ABR within areas or inter-link 222 between ASes. This document takes IGP Areas domains for example. 223 SR-MPLS domains can be deployed as Figure 1 shown. 225 + + + 226 + + + + + + 227 + + + + + + 228 + + + + + + 229 A SR-MPLS X SR-MPLS Y SR-MPLS Z 230 + IGP 1 + + IGP 2 + + IGP 3 + 231 + + + + + + 232 + + + + + + 233 + + + 235 Figure 1: SR-MPLS and MPLS-TP interworking Scenario 237 Two SR-MPLS inter-domain models are discussed in this document 238 including the stitching and nesting inter-domain model which are 239 described in Section 4.1 and Section 4.2 respectively. 241 4.1. Stitching Inter-domain with i-Path 243 The Figure 1 displays the border node inter-domain scenario. SR node 244 X and SR node Y are the border nodes of two different domains. The 245 i-Paths from A->X, X->Y, and Y->Z are used for the inter-domain path 246 segment. The ingress SR node A encapsulates the data packet with 247 i-Path (A->X) and A->X SID list. The data packet is forwarded to SR 248 node X according to the A->X SID list. Node X pushes the i-Path 249 (X->Y) and X->Y SID list based on the above mentioned forwarding 250 entry. The data packet is forwarded to node Y and then to the SR 252 Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 254 node Z based on the same forwarding procedure. In node Z, the i-Path 255 (Y->Z) can be mapped to the path from Z to Y of reverse direction and 256 correlates the two unidirectional paths. The packet transmission of 257 the reverse direction is the same with the forwarding direction with 258 different i-Paths. The stitching of path segments can achieve the 259 inter-domain stitching and path monitoring 261 .................. ................. .................... 262 . . . . . . 263 +-----+ +-----+ +-----+ +-----+ 264 | A | | X | | Y | | Z | 265 +-----+ +-----+ +-----+ +-----+ 266 . SR Domain 1 . . SR Domain 2 . . SR Domain 3 . 267 .................. ................. .................... 269 |<------------------>|<------------------>|<--------------->| 270 i-Path(A->X) i-Path(X->Y) i-Path(Y->Z) 272 Node A Node X Node Y Node Z 273 +-------------+ +-------------+ +-------------+ 274 |A->X SID list| |X->Y SID list| |Y->Z SID list| 275 +-------------+ +-------------+ +-------------+ +--------------+ 276 |i-Path(A->X) |---->|i-Path(X->Y) |---->|i-Path(Y->Z) |--->| Payload | 277 +-------------+ +-------------+ +-------------+ +--------------+ 278 | Payload | | Payload | | Payload | 279 +-------------+ +-------------+ +-------------+ 281 Figure 2: Stitching Border Node Inter-Domain Scenario 283 4.2. Nesting Inter-domain with e-Path 285 Figure 3 shows the SR-MPLS nesting inter-domain scenario. The 286 e-Path(A->Z) is used to indicate the end-to-end path. The s-Path is 287 used to identify the domain's sub-path. The e-Path, s-Path and SR 288 list are pushed by the ingress node. The e-Path is used to correlate 289 the two unidirectional SR paths to an SR bidirectional path. The 290 s-Path can be used as stitching label to correlate the two inter- 291 domain sub-paths. 293 The use of the binding SID [RFC8402] is also recommended to replace 294 the SR list of each domain. As shown in Figure 3, the B-SID(X->Y) is 295 used to replace the X->Y SID list. Ingress node A pushes 296 e-Path(A->Z), B-SID(Y->Z), B-SID(X-Y), s-Path(A->X) and A->X SID list 297 in turn. When the packet is received at node X, the s-Path(A-X) and 298 X->Y SID list are popped, and the new s-Path(X->Y) is pushed. Also, 300 Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 302 X->Y SID list replaces B-SID(X->Y) to indicate that packet to be 303 forwarded from node X to node Y. The data packet reaches the SR node 304 Z according to the same forwarding procedure. In SR node Z, the 305 e-Path (A->Z) is used to correlate the two unidirectional end-to-end 306 paths. 308 .................. ................. .................... 309 . . . . . . 310 +-----+ +-----+ +-----+ +-----+ 311 | A | | X | | Y | | Z | 312 +-----+ +-----+ +-----+ +-----+ 313 . SR Domain 1 . . SR Domain 2 . . SR Domain 3 . 314 .................. ................. .................... 316 |<------------------>|<------------------>|<--------------->| 317 s-Path(A->X) s-Path(X->Y) s-Path(Y->Z) 318 |<--------------------------------------------------------->| 319 e-Path(A->Z) 320 Node A 321 +-------------+ 322 |A->X SID list| Node X 323 +-------------+ +-------------+ 324 |s-Path(A->X) | |X->Y SID list| Node Y 325 +-------------+ +-------------+ +-------------+ 326 |B-SID(X->Y) | --> |s-Path(X->Y) | |Y->Z SID list| 327 +-------------+ +-------------+ +-------------+ 328 |B-SID(Y->Z) | |B-SID(Y->Z) | --> |s-Path(Y->Z) | Node Z 329 +-------------+ +-------------+ +-------------+ +-------------+ 330 |e-Path(A->Z) | |e-Path(A->Z) | |e-Path(A->Z) | --> |e-Path(A->Z) | 331 +-------------+ +-------------+ +-------------+ +-------------+ 332 | Payload | | Payload | | Payload | | Payload | 333 +-------------+ +-------------+ +-------------+ +-------------+ 335 Figure 3: Nesting Inter-Domain Scenario 337 5. Security Considerations 339 TBA 341 6. Acknowledgements 343 TBA 345 Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 347 7. IANA Considerations 349 TBA 351 8. Normative References 353 [I-D.ietf-spring-mpls-path-segment] 354 Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler, 355 "Path Segment in MPLS Based Segment Routing Network", 356 draft-ietf-spring-mpls-path-segment-01 (work in progress), 357 September 2019. 359 [I-D.ietf-spring-segment-routing-mpls] 360 Bashandy, A., Filsfils, C., Previdi, S., Decraene, B., 361 Litkowski, S., and R. Shakir, "Segment Routing with MPLS 362 data plane", draft-ietf-spring-segment-routing-mpls-22 363 (work in progress), May 2019. 365 [I-D.xiong-pce-stateful-pce-sr-inter-domain] 366 Xiong, Q., hu, f., Mirsky, G., and W. Cheng, "Stateful PCE 367 for SR-MPLS Inter-domain", draft-xiong-pce-stateful-pce- 368 sr-inter-domain-01 (work in progress), July 2019. 370 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 371 Requirement Levels", BCP 14, RFC 2119, 372 DOI 10.17487/RFC2119, March 1997, 373 . 375 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 376 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 377 May 2017, . 379 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 380 Decraene, B., Litkowski, S., and R. Shakir, "Segment 381 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 382 July 2018, . 384 Authors' Addresses 386 Quan Xiong 387 ZTE Corporation 388 No.6 Huashi Park Rd 389 Wuhan, Hubei 430223 390 China 392 Phone: +86 27 83531060 393 Email: xiong.quan@zte.com.cn 395 Internet-DrafThe Use of Path Segment in SR Inter-domain Sce October 2019 397 Greg Mirsky 398 ZTE Corporation 399 USA 401 Email: gregimirsky@gmail.com 403 Weiqiang Cheng 404 China Mobile 405 Beijing 406 China 408 Email: chengweiqiang@chinamobile.com