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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-10 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. The BGP-LS speaker 166 may also advertise the MSD information for the local node and its 167 links when not running any link-state IGP protocol e.g. when running 168 BGP as the only routing protocol. The Protocol-ID field should be 169 set to BGP since the link and node attributes have BGP based 170 identifiers. Deployment model for such case would be: a limited 171 number (meeting resiliecy requirements) of BGP-LS speakers exposing 172 the topology to the controller, full-mesh/RouterReflectors for iBGP 173 or regular eBGP connectivity between every node in the topology. 175 The extensions introduced in this document allow for advertisement of 176 different MSD-Types. This document does not define these MSD-Types 177 but leverages the definition, guidelines and the code-point registry 178 specified in [RFC8491]. This enables sharing of MSD-Types that may 179 be defined in the future by the IGPs in BGP-LS. 181 3. Node MSD TLV 183 Node MSD is encoded in a new Node Attribute TLV [RFC7752] using the 184 following format: 186 0 1 2 3 187 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 1 188 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 189 | Type | Length | 190 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 191 | MSD-Type | MSD-Value | MSD-Type... | MSD-Value... | 192 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 194 Figure 1: Node MSD TLV Format 196 Where: 198 o Type: 266 200 o Length: variable (multiple of 2); represents the total length of 201 the value field in octets. 203 o Value : consists of one or more pairs of a 1-octet MSD-Type and 204 1-octet MSD-Value. 206 * MSD-Type : one of the values defined in the IANA registry 207 titled "IGP MSD-Types" under the "Interior Gateway Protocol 208 (IGP) Parameters" registry created by [RFC8491]. 210 * MSD-Value : a number in the range of 0-255. For all MSD-Types, 211 0 represents the lack of ability to impose an MSD stack of any 212 depth; any other value represents that of the node. This value 213 MUST represent the lowest value supported by any link 214 configured for use by the advertising protocol instance. 216 4. Link MSD TLV 218 Link MSD is encoded in a new Link Attribute TLV [RFC7752] using the 219 following format: 221 0 1 2 3 222 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 1 223 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 224 | Type | Length | 225 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 226 | MSD-Type | MSD-Value | MSD-Type... | MSD-Value... | 227 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 229 Figure 2: Link MSD TLV Format 231 Where: 233 o Type: 267 234 o Length: variable (multiple of 2); represents the total length of 235 the value field in octets. 237 o Value : consists of one or more pairs of a 1-octet MSD-Type and 238 1-octet MSD-Value. 240 * MSD-Type : one of the values defined in the IANA registry 241 titled "IGP MSD-Types" under the "Interior Gateway Protocol 242 (IGP) Parameters" registry created by [RFC8491]. 244 * MSD-Value : a number in the range of 0-255. For all MSD-Types, 245 0 represents the lack of ability to impose an MSD stack of any 246 depth; any other value represents that of the link when used as 247 an outgoing interface. 249 5. Procedures for Defining and Using Node and Link MSD Advertisements 251 When Link MSD is present for a given MSD-type, the value of the Link 252 MSD MUST take precedence over the Node MSD. When a Link MSD-type is 253 not signaled but the Node MSD-type is, then the Node MSD-type value 254 MUST be considered as the MSD value for that link. 256 In order to increase flooding efficiency, it is RECOMMENDED that 257 routers with homogenous link MSD values advertise just the Node MSD 258 value. 260 The meaning of the absence of both Node and Link MSD advertisements 261 for a given MSD-type is specific to the MSD-type. Generally it can 262 only be inferred that the advertising node does not support 263 advertisement of that MSD-type. However, in some cases the lack of 264 advertisement might imply that the functionality associated with the 265 MSD-type is not supported. The correct interpretation MUST be 266 specified when an MSD-type is defined in [RFC8491]. 268 6. IANA Considerations 270 This document requests assigning code-points from the registry "BGP- 271 LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute 272 TLVs" based on table below. Early allocation for these code-points 273 have been done by IANA. 275 +------------+-----------------+---------------------------+ 276 | Code Point | Description | IS-IS TLV/Sub-TLV | 277 +------------+-----------------+---------------------------+ 278 | 266 | Node MSD | 242/23 | 279 | 267 | Link MSD | (22,23,25,141,222,223)/15 | 280 +------------+-----------------+---------------------------+ 282 7. Manageability Considerations 284 The new protocol extensions introduced in this document augment the 285 existing IGP topology information that is distributed via [RFC7752]. 286 Procedures and protocol extensions defined in this document do not 287 affect the BGP protocol operations and management other than as 288 discussed in the Manageability Considerations section of [RFC7752]. 289 Specifically, the malformed attribute tests for syntactic checks in 290 the Fault Management section of [RFC7752] now encompass the new BGP- 291 LS Attribute TLVs defined in this document. The semantic or content 292 checking for the TLVs specified in this document and their 293 association with the BGP-LS NLRI types or their BGP-LS Attribute is 294 left to the consumer of the BGP-LS information (e.g. an application 295 or a controller) and not the BGP protocol. 297 A consumer of the BGP-LS information retrieves this information over 298 a BGP-LS session (refer Section 1 and 2 of [RFC7752]). The handling 299 of semantic or content errors by the consumer would be dictated by 300 the nature of its application usage and hence is beyond the scope of 301 this document. 303 This document only introduces new Attribute TLVs and any syntactic 304 error in them would result in the BGP-LS Attribute being discarded 305 with an error log. The MSD information introduced in BGP-LS by this 306 specification, may be used by BGP-LS consumer applications like a SR 307 path computation engine (PCE) to learn the SR SID-stack handling 308 capabilities of the nodes in the topology. This can enable the SR 309 PCE to perform path computations taking into consideration the size 310 of SID Stack that the specific headend node may be able to impose. 311 Errors in the encoding or decoding of the MSD information may result 312 in the unavailability of such information to the SR PCE or incorrect 313 information being made available to it. This may result in the 314 headend node not being able to instantiate the desired SR path in its 315 forwarding and provide the SR based optimization functionality. The 316 handling of such errors by applications like SR PCE may be 317 implementation specific and out of scope of this document. 319 The extensions specified in this document, do not specify any new 320 configuration or monitoring aspects in BGP or BGP-LS. The 321 specification of BGP models BGP and BGP-LS models is an ongoing work 322 based on the [I-D.ietf-idr-bgp-model]. The management of the MSD 323 features within an ietf segment-routing stack is also an ongoing work 324 based on the [I-D.ietf-spring-sr-yang]. Management of the segment 325 routing in IGPs is ongoing work for ISIS [I-D.ietf-isis-sr-yang] , 326 and OSPF [I-D.ietf-ospf-sr-yang]. 328 8. Security Considerations 330 The advertisement of an incorrect MSD value may have negative 331 consequences. If the value is smaller than supported, path 332 computation may fail to compute a viable path. If the value is 333 larger than supported, an attempt to instantiate a path that can't be 334 supported by the head-end (the node performing the SID imposition) 335 may occur. The presence of this information may also inform an 336 attacker of how to induce any of the aforementioned conditions. 338 The document does not introduce additional security issues beyond 339 discussed in [RFC7752], [RFC8476] and [RFC8491]. However, [RFC7752] 340 is being revised in [I-D.ietf-idr-rfc7752bis] to provide additional 341 clarification in several portions of the specification after 342 receiving feedback from implementers. One of the places that is 343 being clarified is the error handling and security. It is expected 344 that after [I-D.ietf-idr-rfc7752bis] is released that implementers 345 will update all BGP-LS base implementations improving the error 346 handling for protocol work (including this document) that depend on 347 this function. 349 9. Contributors 351 Siva Sivabalan 352 Cisco Systems Inc. 353 Canada 355 Email: msiva@cisco.com 357 10. Acknowledgements 359 We like to thank Acee Lindem, Stephane Litkowski and Bruno Decraene 360 for their reviews and valuable comments. 362 11. References 364 11.1. Normative References 366 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 367 Requirement Levels", BCP 14, RFC 2119, 368 DOI 10.17487/RFC2119, March 1997, 369 . 371 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 372 S. Ray, "North-Bound Distribution of Link-State and 373 Traffic Engineering (TE) Information Using BGP", RFC 7752, 374 DOI 10.17487/RFC7752, March 2016, 375 . 377 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 378 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 379 May 2017, . 381 [RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak, 382 "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, 383 DOI 10.17487/RFC8476, December 2018, 384 . 386 [RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, 387 "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, 388 DOI 10.17487/RFC8491, November 2018, 389 . 391 11.2. Informative References 393 [I-D.ietf-idr-bgp-model] 394 Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP 395 YANG Model for Service Provider Networks", draft-ietf-idr- 396 bgp-model-07 (work in progress), October 2019. 398 [I-D.ietf-idr-rfc7752bis] 399 Talaulikar, K., "Distribution of Link-State and Traffic 400 Engineering Information Using BGP", draft-ietf-idr- 401 rfc7752bis-02 (work in progress), November 2019. 403 [I-D.ietf-isis-mpls-elc] 404 Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., 405 and M. Bocci, "Signaling Entropy Label Capability and 406 Entropy Readable Label Depth Using IS-IS", draft-ietf- 407 isis-mpls-elc-10 (work in progress), October 2019. 409 [I-D.ietf-isis-sr-yang] 410 Litkowski, S., Qu, Y., Sarkar, P., Chen, I., and J. 411 Tantsura, "YANG Data Model for IS-IS Segment Routing", 412 draft-ietf-isis-sr-yang-07 (work in progress), January 413 2020. 415 [I-D.ietf-ospf-mpls-elc] 416 Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., 417 and M. Bocci, "Signaling Entropy Label Capability and 418 Entropy Readable Label-stack Depth Using OSPF", draft- 419 ietf-ospf-mpls-elc-12 (work in progress), October 2019. 421 [I-D.ietf-ospf-sr-yang] 422 Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem, 423 "YANG Data Model for OSPF SR (Segment Routing) Protocol", 424 draft-ietf-ospf-sr-yang-11 (work in progress), February 425 2020. 427 [I-D.ietf-spring-sr-yang] 428 Litkowski, S., Qu, Y., Lindem, A., Sarkar, P., and J. 429 Tantsura, "YANG Data Model for Segment Routing", draft- 430 ietf-spring-sr-yang-15 (work in progress), January 2020. 432 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 433 Label Switching Architecture", RFC 3031, 434 DOI 10.17487/RFC3031, January 2001, 435 . 437 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 438 Decraene, B., Litkowski, S., and R. Shakir, "Segment 439 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 440 July 2018, . 442 [RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., 443 and J. Hardwick, "Path Computation Element Communication 444 Protocol (PCEP) Extensions for Segment Routing", RFC 8664, 445 DOI 10.17487/RFC8664, December 2019, 446 . 448 Authors' Addresses 450 Jeff Tantsura 451 Apstra, Inc. 453 Email: jefftant.ietf@gmail.com 455 Uma Chunduri 456 Futurewei Technologies 458 Email: umac.ietf@gmail.com 460 Ketan Talaulikar 461 Cisco Systems 463 Email: ketant@cisco.com 464 Greg Mirsky 465 ZTE Corp. 467 Email: gregimirsky@gmail.com 469 Nikos Triantafillis 470 Amazon Web Services 472 Email: nikost@amazon.com