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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IDR Working Group J. Tantsura 3 Internet-Draft Apstra, Inc. 4 Intended status: Standards Track U. Chunduri 5 Expires: March 14, 2020 Futurewei Technologies 6 K. Talaulikar 7 Cisco Systems 8 G. Mirsky 9 ZTE Corp. 10 N. Triantafillis 11 Apstra, Inc. 12 September 11, 2019 14 Signaling MSD (Maximum SID Depth) using Border Gateway Protocol Link- 15 State 16 draft-ietf-idr-bgp-ls-segment-routing-msd-07 18 Abstract 20 This document defines a way for a Border Gateway Protocol Link-State 21 (BGP-LS) speaker to advertise multiple types of supported Maximum SID 22 Depths (MSDs) at node and/or link granularity. 24 Such advertisements allow entities (e.g., centralized controllers) to 25 determine whether a particular Segment Identifier (SID) stack can be 26 supported in a given network. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at https://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on March 14, 2020. 45 Copyright Notice 47 Copyright (c) 2019 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (https://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 63 1.1. Conventions used in this document . . . . . . . . . . . . 3 64 1.1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 3 65 1.1.2. Requirements Language . . . . . . . . . . . . . . . . 4 66 2. Advertisement of MSD via BGP-LS . . . . . . . . . . . . . . . 4 67 3. Node MSD TLV . . . . . . . . . . . . . . . . . . . . . . . . 4 68 4. Link MSD TLV . . . . . . . . . . . . . . . . . . . . . . . . 5 69 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 70 6. Manageability Considerations . . . . . . . . . . . . . . . . 6 71 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 72 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8 73 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 74 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 75 10.1. Normative References . . . . . . . . . . . . . . . . . . 8 76 10.2. Informative References . . . . . . . . . . . . . . . . . 8 77 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 79 1. Introduction 81 When Segment Routing (SR) [RFC8402] paths are computed by a 82 centralized controller, it is critical that the controller learns the 83 Maximum SID Depth (MSD) that can be imposed at each node/link on a 84 given SR path. This ensures that the Segment Identifier (SID) stack 85 depth of a computed path doesn't exceed the number of SIDs the node 86 is capable of imposing. 88 [I-D.ietf-pce-segment-routing] defines how to signal MSD in the Path 89 Computation Element Protocol (PCEP). The OSPF and IS-IS extensions 90 for signaling of MSD are defined in [RFC8476] and [RFC8491] 91 respectively. 93 However, if PCEP is not supported/configured on the head-end of a SR 94 tunnel or a Binding-SID anchor node, and controller does not 95 participate in IGP routing, it has no way of learning the MSD of 96 nodes and links. BGP-LS [RFC7752] defines a way to advertise 97 topology and associated attributes and capabilities of the nodes in 98 that topology to a centralized controller. This document defines 99 extensions to BGP-LS to advertise one or more types of MSDs at node 100 and/or link granularity. 102 Other types of MSD are known to be useful. For example, 103 [I-D.ietf-ospf-mpls-elc] and [I-D.ietf-isis-mpls-elc] define Readable 104 Label Depth Capability (RLDC) that is used by a head-end to insert an 105 Entropy Label (EL) at a depth that can be read by transit nodes. 107 In the future, it is expected that new MSD-Types will be defined to 108 signal additional capabilities, e.g., ELs, SIDs that can be imposed 109 through recirculation, or SIDs associated with another data plane 110 such as IPv6. MSD advertisements MAY be useful even if SR itself is 111 not enabled. For example, in a non-SR MPLS network, MSD defines the 112 maximum label depth. 114 1.1. Conventions used in this document 116 1.1.1. Terminology 118 BGP-LS: Distribution of Link-State and TE Information using Border 119 Gateway Protocol 121 MSD: Maximum SID Depth 123 PCC: Path Computation Client 125 PCE: Path Computation Element 127 PCEP: Path Computation Element Protocol 129 SID: Segment Identifier 131 SR: Segment routing 133 Label Imposition: Imposition is the act of modifying and/or adding 134 labels to the outgoing label stack associated with a packet. This 135 includes: 137 o replacing the label at the top of the label stack with a new 138 label. 140 o pushing one or more new labels onto the label stack The number of 141 labels imposed is then the sum of the number of labels that are 142 replaced and the number of labels that are pushed. See [RFC3031] 143 for further details. 145 1.1.2. Requirements Language 147 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 148 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 149 "OPTIONAL" in this document are to be interpreted as described in BCP 150 14 [RFC2119] [RFC8174] when, and only when, they appear in all 151 capitals, as shown here . 153 2. Advertisement of MSD via BGP-LS 155 This document describes extensions that enable BGP-LS speakers to 156 signal the MSD capabilities (described in [RFC8491] ) of nodes and 157 their links in a network to a BGP-LS consumer of network topology 158 such as a centralized controller. The centralized controller can 159 leverage this information in computation of SR paths and their 160 instantiation on network nodes based on their MSD capabilities. When 161 a BGP-LS speaker is originating the topology learnt via link-state 162 routing protocols like OSPF or IS-IS, the MSD information for the 163 nodes and their links is sourced from the underlying extensions as 164 defined in [RFC8476] and [RFC8491] respectively. The BGP-LS speaker 165 may also advertise the MSD information for the local node and its 166 links when not running any link-state IGP protocol e.g. when running 167 BGP as the only routing protocol. 169 The extensions introduced in this document allow for advertisement of 170 different MSD-Types. This document does not define these MSD-Types 171 but leverages the definition, guidelines and the code-point registry 172 specified in [RFC8491]. This enables sharing of MSD-Types that may 173 be defined in the future by the IGPs in BGP-LS. 175 3. Node MSD TLV 177 Node MSD is encoded in a new Node Attribute TLV [RFC7752] using the 178 following format: 180 0 1 2 3 181 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 182 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 183 | Type | Length | 184 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 185 | MSD-Type | MSD-Value | MSD-Type... | MSD-Value... | 186 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 188 Figure 1: Node MSD TLV Format 190 Where: 192 o Type: 266 194 o Length: variable (multiple of 2); represents the total length of 195 the value field in octets. 197 o Value : consists of one or more pairs of a 1-octet MSD-Type and 198 1-octet MSD-Value. 200 * MSD-Type : one of the values defined in the IANA registry 201 titled "IGP MSD-Types" under the "Interior Gateway Protocol 202 (IGP) Parameters" registry created by [RFC8491]. 204 * MSD-Value : a number in the range of 0-255. For all MSD-Types, 205 0 represents the lack of ability to impose an MSD stack of any 206 depth; any other value represents that of the node. This value 207 MUST represent the lowest value supported by any link 208 configured for use by the advertising protocol instance. 210 4. Link MSD TLV 212 Link MSD is encoded in a new Link Attribute TLV [RFC7752] using the 213 following format: 215 0 1 2 3 216 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 217 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 218 | Type | Length | 219 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 220 | MSD-Type | MSD-Value | MSD-Type... | MSD-Value... | 221 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 223 Figure 2: Link MSD TLV Format 225 Where: 227 o Type: 267 228 o Length: variable (multiple of 2); represents the total length of 229 the value field in octets. 231 o Value : consists of one or more pairs of a 1-octet MSD-Type and 232 1-octet MSD-Value. 234 * MSD-Type : one of the values defined in the IANA registry 235 titled "IGP MSD-Types" under the "Interior Gateway Protocol 236 (IGP) Parameters" registry created by [RFC8491]. 238 * MSD-Value : a number in the range of 0-255. For all MSD-Types, 239 0 represents the lack of ability to impose an MSD stack of any 240 depth; any other value represents that of the link when used as 241 an outgoing interface. 243 5. IANA Considerations 245 This document requests assigning code-points from the registry "BGP- 246 LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute 247 TLVs" based on table below. Early allocation for these code-points 248 have been done by IANA. 250 +------------+-----------------+---------------------------+ 251 | Code Point | Description | IS-IS TLV/Sub-TLV | 252 +------------+-----------------+---------------------------+ 253 | 266 | Node MSD | 242/23 | 254 | 267 | Link MSD | (22,23,25,141,222,223)/15 | 255 +------------+-----------------+---------------------------+ 257 6. Manageability Considerations 259 The new protocol extensions introduced in this document augment the 260 existing IGP topology information that is distributed via [RFC7752]. 261 Procedures and protocol extensions defined in this document do not 262 affect the BGP protocol operations and management other than as 263 discussed in the Manageability Considerations section of [RFC7752]. 264 Specifically, the malformed attribute tests for syntactic checks in 265 the Fault Management section of [RFC7752] now encompass the new BGP- 266 LS Attribute TLVs defined in this document. The semantic or content 267 checking for the TLVs specified in this document and their 268 association with the BGP-LS NLRI types or their BGP-LS Attribute is 269 left to the consumer of the BGP-LS information (e.g. an application 270 or a controller) and not the BGP protocol. 272 A consumer of the BGP-LS information retrieves this information over 273 a BGP-LS session (refer Section 1 and 2 of [RFC7752]). The handling 274 of semantic or content errors by the consumer would be dictated by 275 the nature of its application usage and hence is beyond the scope of 276 this document. 278 This document only introduces new Attribute TLVs and any syntactic 279 error in them would result in the BGP-LS Attribute being discarded 280 with an error log. The MSD information introduced in BGP-LS by this 281 specification, may be used by BGP-LS consumer applications like a SR 282 path computation engine (PCE) to learn the SR SID-stack handling 283 capabilities of the nodes in the topology. This can enable the SR 284 PCE to perform path computations taking into consideration the size 285 of SID Stack that the specific headend node may be able to impose. 286 Errors in the encoding or decoding of the MSD information may result 287 in the unavailability of such information to the SR PCE or incorrect 288 information being made available to it. This may result in the 289 headend node not being able to instantiate the desired SR path in its 290 forwarding and provide the SR based optimization functionality. The 291 handling of such errors by applications like SR PCE may be 292 implementation specific and out of scope of this document. 294 The extensions specified in this document, do not specify any new 295 configuration or monitoring aspects in BGP or BGP-LS. The 296 specification of BGP models BGP and BGP-LS models is an ongoing work 297 based on the [I-D.ietf-idr-bgp-model]. The management of the MSD 298 features within an ietf segment-routing stack is also an ongoing work 299 based on the [I-D.ietf-spring-sr-yang]. Management of the segment 300 routing in IGPs is ongoing work for ISIS [I-D.ietf-isis-sr-yang] , 301 and OSPF [I-D.ietf-ospf-sr-yang]. 303 7. Security Considerations 305 The advertisement of an incorrect MSD value may have negative 306 consequences. If the value is smaller than supported, path 307 computation may fail to compute a viable path. If the value is 308 larger than supported, an attempt to instantiate a path that can't be 309 supported by the head-end (the node performing the SID imposition) 310 may occur. The presence of this information may also inform an 311 attacker of how to induce any of the aforementioned conditions. 313 The document does not introduce additional security issues beyond 314 discussed in [RFC7752], [RFC8476] and [RFC8491]. However, [RFC7752] 315 is being revised in [I-D.ietf-idr-rfc7752bis] to provide additional 316 clarification in several portions of the specification after 317 receiving feedback from implementers. One of the places that is 318 being clarified is the error handling and security. It is expected 319 that after [I-D.ietf-idr-rfc7752bis] is released that implementers 320 will update all BGP-LS base implementations improving the error 321 handling for protocol work (including this document) that depend on 322 this function. 324 8. Contributors 326 Siva Sivabalan 327 Cisco Systems Inc. 328 Canada 330 Email: msiva@cisco.com 332 9. Acknowledgements 334 We like to thank Acee Lindem, Stephane Litkowski and Bruno Decraene 335 for their reviews and valuable comments. 337 10. References 339 10.1. Normative References 341 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 342 Requirement Levels", BCP 14, RFC 2119, 343 DOI 10.17487/RFC2119, March 1997, 344 . 346 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 347 S. Ray, "North-Bound Distribution of Link-State and 348 Traffic Engineering (TE) Information Using BGP", RFC 7752, 349 DOI 10.17487/RFC7752, March 2016, 350 . 352 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 353 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 354 May 2017, . 356 [RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak, 357 "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, 358 DOI 10.17487/RFC8476, December 2018, 359 . 361 [RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, 362 "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, 363 DOI 10.17487/RFC8491, November 2018, 364 . 366 10.2. Informative References 368 [I-D.ietf-idr-bgp-model] 369 Jethanandani, M., Patel, K., and S. Hares, "BGP YANG Model 370 for Service Provider Networks", draft-ietf-idr-bgp- 371 model-06 (work in progress), June 2019. 373 [I-D.ietf-idr-rfc7752bis] 374 Talaulikar, K., Gredler, H., Medved, J., Previdi, S., 375 Farrel, A., and S. Ray, "Distribution of Link-State and 376 Traffic Engineering Information Using BGP", draft-ietf- 377 idr-rfc7752bis-00 (work in progress), September 2019. 379 [I-D.ietf-isis-mpls-elc] 380 Xu, X., Kini, S., Psenak, P., Filsfils, C., and S. 381 Litkowski, "Signaling Entropy Label Capability and Entropy 382 Readable Label Depth Using IS-IS", draft-ietf-isis-mpls- 383 elc-08 (work in progress), September 2019. 385 [I-D.ietf-isis-sr-yang] 386 Litkowski, S., Qu, Y., Sarkar, P., Chen, I., and J. 387 Tantsura, "YANG Data Model for IS-IS Segment Routing", 388 draft-ietf-isis-sr-yang-06 (work in progress), July 2019. 390 [I-D.ietf-ospf-mpls-elc] 391 Xu, X., Kini, S., Psenak, P., Filsfils, C., and S. 392 Litkowski, "Signaling Entropy Label Capability and Entropy 393 Readable Label-stack Depth Using OSPF", draft-ietf-ospf- 394 mpls-elc-09 (work in progress), September 2019. 396 [I-D.ietf-ospf-sr-yang] 397 Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem, 398 "YANG Data Model for OSPF SR (Segment Routing) Protocol", 399 draft-ietf-ospf-sr-yang-10 (work in progress), August 400 2019. 402 [I-D.ietf-pce-segment-routing] 403 Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., 404 and J. Hardwick, "PCEP Extensions for Segment Routing", 405 draft-ietf-pce-segment-routing-16 (work in progress), 406 March 2019. 408 [I-D.ietf-spring-sr-yang] 409 Litkowski, S., Qu, Y., Lindem, A., Sarkar, P., and J. 410 Tantsura, "YANG Data Model for Segment Routing", draft- 411 ietf-spring-sr-yang-13 (work in progress), July 2019. 413 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 414 Label Switching Architecture", RFC 3031, 415 DOI 10.17487/RFC3031, January 2001, 416 . 418 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 419 Decraene, B., Litkowski, S., and R. Shakir, "Segment 420 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 421 July 2018, . 423 Authors' Addresses 425 Jeff Tantsura 426 Apstra, Inc. 428 Email: jefftant.ietf@gmail.com 430 Uma Chunduri 431 Futurewei Technologies 433 Email: umac.ietf@gmail.com 435 Ketan Talaulikar 436 Cisco Systems 438 Email: ketant@cisco.com 440 Greg Mirsky 441 ZTE Corp. 443 Email: gregimirsky@gmail.com 445 Nikos Triantafillis 446 Apstra, Inc. 448 Email: nikos@apstra.com