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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: August 31, 2020 Futurewei Technologies 6 K. Talaulikar 7 Cisco Systems 8 G. Mirsky 9 ZTE Corp. 10 N. Triantafillis 11 Amazon Web Services 12 February 28, 2020 14 Signaling MSD (Maximum SID Depth) using Border Gateway Protocol - Link 15 State 16 draft-ietf-idr-bgp-ls-segment-routing-msd-11 18 Abstract 20 This document defines a way for a Border Gateway Protocol - Link 21 State (BGP-LS) speaker to advertise multiple types of supported 22 Maximum SID 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 August 31, 2020. 45 Copyright Notice 47 Copyright (c) 2020 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. Procedures for Defining and Using Node and Link MSD 70 Advertisements . . . . . . . . . . . . . . . . . . . . . . . 6 71 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 72 7. Manageability Considerations . . . . . . . . . . . . . . . . 7 73 8. Security Considerations . . . . . . . . . . . . . . . . . . . 8 74 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8 75 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 76 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 77 11.1. Normative References . . . . . . . . . . . . . . . . . . 8 78 11.2. Informative References . . . . . . . . . . . . . . . . . 9 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 81 1. Introduction 83 When Segment Routing (SR) [RFC8402] paths are computed by a 84 centralized controller, it is critical that the controller learn the 85 Maximum SID Depth (MSD) that can be imposed at each node/link on a 86 given SR path. This ensures that the Segment Identifier (SID) stack 87 depth of a computed path doesn't exceed the number of SIDs the node 88 is capable of imposing. 90 [RFC8664] defines how to signal MSD in the Path Computation Element 91 Protocol (PCEP). The OSPF and IS-IS extensions for signaling of MSD 92 are defined in [RFC8476] and [RFC8491] respectively. 94 However, if PCEP is not supported/configured on the head-end of a SR 95 tunnel or a Binding-SID anchor node, and controller does not 96 participate in IGP routing, it has no way of learning the MSD of 97 nodes and links. BGP-LS [RFC7752] defines a way to expose topology 98 and associated attributes and capabilities of the nodes in that 99 topology to a centralized controller. 101 This document defines extensions to BGP-LS to advertise one or more 102 types of MSDs at node and/or link granularity. Other types of MSD 103 are known to be useful. For example, [I-D.ietf-ospf-mpls-elc] and 104 [I-D.ietf-isis-mpls-elc] define Readable Label Depth Capability 105 (RLDC) that is used by a head-end to insert an Entropy Label (EL) at 106 a depth that can be read by transit nodes. 108 In the future, it is expected that new MSD-Types will be defined to 109 signal additional capabilities, e.g., ELs, SIDs that can be imposed 110 through recirculation, or SIDs associated with another data plane 111 such as IPv6. MSD advertisements MAY be useful even if SR itself is 112 not enabled. For example, in a non-SR MPLS network, MSD defines the 113 maximum label depth. 115 1.1. Conventions used in this document 117 1.1.1. Terminology 119 BGP-LS: Distribution of Link-State and TE Information using Border 120 Gateway Protocol 122 MSD: Maximum SID Depth 124 PCC: Path Computation Client 126 PCE: Path Computation Element 128 PCEP: Path Computation Element Protocol 130 SID: Segment Identifier 132 SR: Segment routing 134 Label Imposition: Imposition is the act of modifying and/or adding 135 labels to the outgoing label stack associated with a packet. This 136 includes: 138 o replacing the label at the top of the label stack with a new 139 label. 141 o pushing one or more new labels onto the label stack. The number 142 of labels imposed is then the sum of the number of labels that are 143 replaced and the number of labels that are pushed. See [RFC3031] 144 for further details. 146 1.1.2. Requirements Language 148 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 149 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 150 "OPTIONAL" in this document are to be interpreted as described in BCP 151 14 [RFC2119] [RFC8174] when, and only when, they appear in all 152 capitals, as shown here . 154 2. Advertisement of MSD via BGP-LS 156 This document describes extensions that enable BGP-LS speakers to 157 signal the MSD capabilities (described in [RFC8491] ) of nodes and 158 their links in a network to a BGP-LS consumer of network topology 159 such as a centralized controller. The centralized controller can 160 leverage this information in computation of SR paths and their 161 instantiation on network nodes based on their MSD capabilities. When 162 a BGP-LS speaker is originating the topology learnt via link-state 163 routing protocols like OSPF or IS-IS, the MSD information for the 164 nodes and their links is sourced from the underlying extensions as 165 defined in [RFC8476] and [RFC8491] respectively. 167 The BGP-LS speaker may also advertise the MSD information for the 168 local node and its links when not running any link-state IGP protocol 169 e.g. when running BGP as the only routing protocol. The Protocol-ID 170 field should be set to BGP since the link and node attributes have 171 BGP based identifiers. Deployment model for such case would be: a 172 limited number (meeting resiliecy requirements) of BGP-LS speakers 173 exposing the topology to the controller, full-mesh/RouteReflectors 174 for iBGP(Internal Border Gateway Protocol) or regular eBGP(External 175 Border Gateway Protocol) connectivity between nodes in the topology. 177 The extensions introduced in this document allow for advertisement of 178 different MSD-Types. This document does not define these MSD-Types 179 but leverages the definition, guidelines and the code-point registry 180 specified in [RFC8491]. This enables sharing of MSD-Types that may 181 be defined in the future by the IGPs in BGP-LS. 183 3. Node MSD TLV 185 Node MSD is encoded in a new Node Attribute TLV [RFC7752] using the 186 following format: 188 0 1 2 3 189 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 1 190 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 191 | Type | Length | 192 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 193 | MSD-Type | MSD-Value | MSD-Type... | MSD-Value... | 194 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 196 Figure 1: Node MSD TLV Format 198 Where: 200 o Type: 266 202 o Length: variable (multiple of 2); represents the total length of 203 the value field in octets. 205 o Value : consists of one or more pairs of a 1-octet MSD-Type and 206 1-octet MSD-Value. 208 * MSD-Type : one of the values defined in the IANA registry 209 titled "IGP MSD-Types" under the "Interior Gateway Protocol 210 (IGP) Parameters" registry created by [RFC8491]. 212 * MSD-Value : a number in the range of 0-255. For all MSD-Types, 213 0 represents the lack of ability to impose an MSD stack of any 214 depth; any other value represents that of the node. This value 215 MUST represent the lowest value supported by any link 216 configured for use by the advertising protocol instance. 218 4. Link MSD TLV 220 Link MSD is encoded in a new Link Attribute TLV [RFC7752] using the 221 following format: 223 0 1 2 3 224 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 1 225 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 226 | Type | Length | 227 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 228 | MSD-Type | MSD-Value | MSD-Type... | MSD-Value... | 229 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 231 Figure 2: Link MSD TLV Format 233 Where: 235 o Type: 267 236 o Length: variable (multiple of 2); represents the total length of 237 the value field in octets. 239 o Value : consists of one or more pairs of a 1-octet MSD-Type and 240 1-octet MSD-Value. 242 * MSD-Type : one of the values defined in the IANA registry 243 titled "IGP MSD-Types" under the "Interior Gateway Protocol 244 (IGP) Parameters" registry created by [RFC8491]. 246 * MSD-Value : a number in the range of 0-255. For all MSD-Types, 247 0 represents the lack of ability to impose an MSD stack of any 248 depth; any other value represents that of the link when used as 249 an outgoing interface. 251 5. Procedures for Defining and Using Node and Link MSD Advertisements 253 When Link MSD is present for a given MSD-type, the value of the Link 254 MSD MUST take precedence over the Node MSD. When a Link MSD-type is 255 not signaled but the Node MSD-type is, then the Node MSD-type value 256 MUST be considered as the MSD value for that link. 258 In order to increase flooding efficiency, it is RECOMMENDED that 259 routers with homogenous link MSD values advertise just the Node MSD 260 value. 262 The meaning of the absence of both Node and Link MSD advertisements 263 for a given MSD-type is specific to the MSD-type. Generally it can 264 only be inferred that the advertising node does not support 265 advertisement of that MSD-type. However, in some cases the lack of 266 advertisement might imply that the functionality associated with the 267 MSD-type is not supported. The correct interpretation MUST be 268 specified when an MSD-type is defined in [RFC8491]. 270 6. IANA Considerations 272 This document requests assigning code-points from the registry "BGP- 273 LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute 274 TLVs" based on table below. Early allocation for these code-points 275 have been done by IANA. 277 +------------+-----------------+---------------------------+ 278 | Code Point | Description | IS-IS TLV/Sub-TLV | 279 +------------+-----------------+---------------------------+ 280 | 266 | Node MSD | 242/23 | 281 | 267 | Link MSD | (22,23,25,141,222,223)/15 | 282 +------------+-----------------+---------------------------+ 284 7. Manageability Considerations 286 The new protocol extensions introduced in this document augment the 287 existing IGP topology information that is distributed via [RFC7752]. 288 Procedures and protocol extensions defined in this document do not 289 affect the BGP protocol operations and management other than as 290 discussed in the Manageability Considerations section of [RFC7752]. 291 Specifically, the malformed attribute tests for syntactic checks in 292 the Fault Management section of [RFC7752] now encompass the new BGP- 293 LS Attribute TLVs defined in this document. The semantic or content 294 checking for the TLVs specified in this document and their 295 association with the BGP-LS NLRI types or their BGP-LS Attribute is 296 left to the consumer of the BGP-LS information (e.g. an application 297 or a controller) and not the BGP protocol. 299 A consumer of the BGP-LS information retrieves this information over 300 a BGP-LS session (refer Section 1 and 2 of [RFC7752]). The handling 301 of semantic or content errors by the consumer would be dictated by 302 the nature of its application usage and hence is beyond the scope of 303 this document. 305 This document only introduces new Attribute TLVs and any syntactic 306 error in them would result in the BGP-LS Attribute being discarded 307 with an error log. The MSD information introduced in BGP-LS by this 308 specification, may be used by BGP-LS consumer applications like a SR 309 path computation engine (PCE) to learn the SR SID-stack handling 310 capabilities of the nodes in the topology. This can enable the SR 311 PCE to perform path computations taking into consideration the size 312 of SID Stack that the specific headend node may be able to impose. 313 Errors in the encoding or decoding of the MSD information may result 314 in the unavailability of such information to the SR PCE or incorrect 315 information being made available to it. This may result in the 316 headend node not being able to instantiate the desired SR path in its 317 forwarding and provide the SR based optimization functionality. The 318 handling of such errors by applications like SR PCE may be 319 implementation specific and out of scope of this document. 321 The extensions specified in this document, do not specify any new 322 configuration or monitoring aspects in BGP or BGP-LS. The 323 specification of BGP models BGP and BGP-LS models is an ongoing work 324 based on the [I-D.ietf-idr-bgp-model]. The management of the MSD 325 features within an ietf segment-routing stack is also an ongoing work 326 based on the [I-D.ietf-spring-sr-yang]. Management of the segment 327 routing in IGPs is ongoing work for ISIS [I-D.ietf-isis-sr-yang] , 328 and OSPF [I-D.ietf-ospf-sr-yang]. 330 8. Security Considerations 332 The advertisement of an incorrect MSD value may have negative 333 consequences. If the value is smaller than supported, path 334 computation may fail to compute a viable path. If the value is 335 larger than supported, an attempt to instantiate a path that can't be 336 supported by the head-end (the node performing the SID imposition) 337 may occur. The presence of this information may also inform an 338 attacker of how to induce any of the aforementioned conditions. 340 The document does not introduce additional security issues beyond 341 discussed in [RFC7752], [RFC8476] and [RFC8491]. However, [RFC7752] 342 is being revised in [I-D.ietf-idr-rfc7752bis] to provide additional 343 clarification in several portions of the specification after 344 receiving feedback from implementers. One of the places that is 345 being clarified is the error handling and security. It is expected 346 that after [I-D.ietf-idr-rfc7752bis] is released that implementers 347 will update all BGP-LS base implementations improving the error 348 handling for protocol work (including this document) that depend on 349 this function. 351 9. Contributors 353 Siva Sivabalan 354 Cisco Systems Inc. 355 Canada 357 Email: msiva@cisco.com 359 10. Acknowledgements 361 We like to thank Acee Lindem, Stephane Litkowski and Bruno Decraene 362 for their reviews and valuable comments. 364 11. References 366 11.1. Normative References 368 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 369 Requirement Levels", BCP 14, RFC 2119, 370 DOI 10.17487/RFC2119, March 1997, 371 . 373 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 374 S. Ray, "North-Bound Distribution of Link-State and 375 Traffic Engineering (TE) Information Using BGP", RFC 7752, 376 DOI 10.17487/RFC7752, March 2016, 377 . 379 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 380 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 381 May 2017, . 383 [RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak, 384 "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, 385 DOI 10.17487/RFC8476, December 2018, 386 . 388 [RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, 389 "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, 390 DOI 10.17487/RFC8491, November 2018, 391 . 393 11.2. Informative References 395 [I-D.ietf-idr-bgp-model] 396 Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP 397 YANG Model for Service Provider Networks", draft-ietf-idr- 398 bgp-model-07 (work in progress), October 2019. 400 [I-D.ietf-idr-rfc7752bis] 401 Talaulikar, K., "Distribution of Link-State and Traffic 402 Engineering Information Using BGP", draft-ietf-idr- 403 rfc7752bis-02 (work in progress), November 2019. 405 [I-D.ietf-isis-mpls-elc] 406 Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., 407 and M. Bocci, "Signaling Entropy Label Capability and 408 Entropy Readable Label Depth Using IS-IS", draft-ietf- 409 isis-mpls-elc-10 (work in progress), October 2019. 411 [I-D.ietf-isis-sr-yang] 412 Litkowski, S., Qu, Y., Sarkar, P., Chen, I., and J. 413 Tantsura, "YANG Data Model for IS-IS Segment Routing", 414 draft-ietf-isis-sr-yang-07 (work in progress), January 415 2020. 417 [I-D.ietf-ospf-mpls-elc] 418 Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., 419 and M. Bocci, "Signaling Entropy Label Capability and 420 Entropy Readable Label-stack Depth Using OSPF", draft- 421 ietf-ospf-mpls-elc-12 (work in progress), October 2019. 423 [I-D.ietf-ospf-sr-yang] 424 Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem, 425 "YANG Data Model for OSPF SR (Segment Routing) Protocol", 426 draft-ietf-ospf-sr-yang-11 (work in progress), February 427 2020. 429 [I-D.ietf-spring-sr-yang] 430 Litkowski, S., Qu, Y., Lindem, A., Sarkar, P., and J. 431 Tantsura, "YANG Data Model for Segment Routing", draft- 432 ietf-spring-sr-yang-15 (work in progress), January 2020. 434 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 435 Label Switching Architecture", RFC 3031, 436 DOI 10.17487/RFC3031, January 2001, 437 . 439 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 440 Decraene, B., Litkowski, S., and R. Shakir, "Segment 441 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 442 July 2018, . 444 [RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., 445 and J. Hardwick, "Path Computation Element Communication 446 Protocol (PCEP) Extensions for Segment Routing", RFC 8664, 447 DOI 10.17487/RFC8664, December 2019, 448 . 450 Authors' Addresses 452 Jeff Tantsura 453 Apstra, Inc. 455 Email: jefftant.ietf@gmail.com 457 Uma Chunduri 458 Futurewei Technologies 460 Email: umac.ietf@gmail.com 462 Ketan Talaulikar 463 Cisco Systems 465 Email: ketant@cisco.com 466 Greg Mirsky 467 ZTE Corp. 469 Email: gregimirsky@gmail.com 471 Nikos Triantafillis 472 Amazon Web Services 474 Email: nikost@amazon.com