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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 7752 (Obsoleted by RFC 9552) == Outdated reference: A later version (-17) exists of draft-ietf-idr-bgp-model-08 == Outdated reference: A later version (-13) exists of draft-ietf-isis-mpls-elc-11 == Outdated reference: A later version (-15) exists of draft-ietf-ospf-mpls-elc-13 Summary: 2 errors (**), 0 flaws (~~), 4 warnings (==), 1 comment (--). 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: October 28, 2020 Futurewei Technologies 6 K. Talaulikar 7 Cisco Systems 8 G. Mirsky 9 ZTE Corp. 10 N. Triantafillis 11 Amazon Web Services 12 April 26, 2020 14 Signaling MSD (Maximum SID Depth) using Border Gateway Protocol - Link 15 State 16 draft-ietf-idr-bgp-ls-segment-routing-msd-17 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 October 28, 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 . . . . . . . . . . . . . . . . 6 73 8. Security Considerations . . . . . . . . . . . . . . . . . . . 7 74 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8 75 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 76 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 77 11.1. Normative References . . . . . . . . . . . . . . . . . . 8 78 11.2. Informative References . . . . . . . . . . . . . . . . . 8 79 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 81 1. Introduction 83 When Segment Routing (SR) [RFC8402] paths are computed by a 84 centralized controller, it is critical that the controller learns 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 the 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 MSD: Maximum SID Depth - the number of SIDs supported by a node or a 120 link on a node 122 PCE: Path Computation Element 124 PCEP: Path Computation Element Protocol 126 SID: Segment Identifier as defined in [RFC8402] 128 SR: Segment Routing 130 Label Imposition: Imposition is the act of modifying and/or adding 131 labels to the outgoing label stack associated with a packet. This 132 includes: 134 o replacing the label at the top of the label stack with a new 135 label. 137 o pushing one or more new labels onto the label stack. 139 o The number of labels imposed is then the sum of the number of 140 labels that are replaced and the number of labels that are pushed. 141 See [RFC3031] for further details. 143 1.1.2. Requirements Language 145 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 146 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 147 "OPTIONAL" in this document are to be interpreted as described in BCP 148 14 [RFC2119] [RFC8174] when, and only when, they appear in all 149 capitals, as shown here . 151 2. Advertisement of MSD via BGP-LS 153 This document describes extensions that enable BGP-LS speakers to 154 signal the MSD capabilities ([RFC8491] ) of nodes and their links in 155 a network to a BGP-LS consumer of network topology such as a 156 centralized controller. The centralized controller can leverage this 157 information in computation of SR paths based on their MSD 158 capabilities. When a BGP-LS speaker is originating the topology 159 learnt via link-state routing protocols such as OSPF or IS-IS, the 160 MSD information for the nodes and their links is sourced from the 161 underlying extensions as defined in [RFC8476] and [RFC8491] 162 respectively. 164 The extensions introduced in this document allow for advertisement of 165 different MSD-Types, which are defined elsewhere and were introduced 166 in [RFC8491]. This enables sharing of MSD-Types that may be defined 167 in the future by the IGPs in BGP-LS. 169 3. Node MSD TLV 171 The Node MSD ([RFC8476] [RFC8491]) is encoded in a new Node Attribute 172 TLV [RFC7752] to carry the provisioned SID depth of the router 173 identified by the corresponding Router-ID. Node MSD is the smallest 174 MSD supported by the node on the set of interfaces configured for 175 use. MSD values may be learned via a hardware API or may be 176 provisioned. The following format is used: 178 0 1 2 3 179 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 1 180 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 181 | Type | Length | 182 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 183 | MSD-Type | MSD-Value | MSD-Type... | MSD-Value... | 184 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 186 Figure 1: Node MSD TLV Format 188 Where: 190 o Type: 266 191 o Length: variable (multiple of 2); represents the total length of 192 the value field in octets. 194 o Value : consists of one or more pairs of a 1-octet MSD-Type and 195 1-octet MSD-Value. 197 * MSD-Type : one of the values defined in the "IGP MSD-Types" 198 registry defined in [RFC8491]. 200 * MSD-Value : a number in the range of 0-255. For all MSD-Types, 201 0 represents the lack of ability to impose an MSD stack of any 202 depth; any other value represents that of the node. This value 203 MUST represent the lowest value supported by any link 204 configured for use by the advertising protocol instance. 206 4. Link MSD TLV 208 The Link MSD ([RFC8476] [RFC8491]) is defined to carry the MSD of the 209 interface associated with the link. It is encoded in a new Link 210 Attribute TLV [RFC7752] using the following format: 212 0 1 2 3 213 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 1 214 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 215 | Type | Length | 216 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 217 | MSD-Type | MSD-Value | MSD-Type... | MSD-Value... | 218 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 220 Figure 2: Link MSD TLV Format 222 Where: 224 o Type: 267 226 o Length: variable (multiple of 2); represents the total length of 227 the value field in octets. 229 o Value : consists of one or more pairs of a 1-octet MSD-Type and 230 1-octet MSD-Value. 232 * MSD-Type : MSD-Type : one of the values defined in the "IGP 233 MSD-Types" registry defined in [RFC8491]. 235 * MSD-Value : a number in the range of 0-255. For all MSD-Types, 236 0 represents the lack of ability to impose an MSD stack of any 237 depth; any other value represents that of the link when used as 238 an outgoing interface. 240 5. Procedures for Defining and Using Node and Link MSD Advertisements 242 When Link MSD is present for a given MSD-type, the value of the Link 243 MSD MUST take precedence over the Node MSD. When a Link MSD-type is 244 not signaled but the Node MSD-type is, then the Node MSD-type value 245 MUST be considered as the MSD value for that link. 247 In order to increase flooding efficiency, it is RECOMMENDED that 248 routers with homogenous link MSD values advertise just the Node MSD 249 value. 251 The meaning of the absence of both Node and Link MSD advertisements 252 for a given MSD-type is specific to the MSD-type. Generally it can 253 only be inferred that the advertising node does not support 254 advertisement of that MSD-type. However, in some cases the lack of 255 advertisement might imply that the functionality associated with the 256 MSD-type is not supported. The correct interpretation MUST be 257 specified when an MSD-type is defined in [RFC8491]. 259 6. IANA Considerations 261 This document requests assigning code-points from the registry "BGP- 262 LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute 263 TLVs" based on table below. Early allocation for these code-points 264 have been done by IANA. 266 +------------+-----------------+---------------------------+-------------------+ 267 | Code Point | Description | IS-IS TLV/Sub-TLV | Reference | 268 +------------+-----------------+---------------------------+-------------------+ 269 | 266 | Node MSD | 242/23 | This document | 270 | 267 | Link MSD | (22,23,25,141,222,223)/15 | This document | 271 +------------+-----------------+---------------------------+-------------------+ 273 7. Manageability Considerations 275 The new protocol extensions introduced in this document augment the 276 existing IGP topology information that is distributed via [RFC7752]. 277 Procedures and protocol extensions defined in this document do not 278 affect the BGP protocol operations and management other than as 279 discussed in the Manageability Considerations section of [RFC7752]. 280 Specifically, the malformed attribute tests for syntactic checks in 281 the Fault Management section of [RFC7752] now encompass the new BGP- 282 LS Attribute TLVs defined in this document. The semantic or content 283 checking for the TLVs specified in this document and their 284 association with the BGP-LS NLRI types or their BGP-LS Attribute is 285 left to the consumer of the BGP-LS information (e.g. an application 286 or a controller) and not the BGP protocol. 288 A consumer of the BGP-LS information retrieves this information over 289 a BGP-LS session (refer Section 1 and 2 of [RFC7752]). 291 This document only introduces new Attribute TLVs and any syntactic 292 error in them would result in the BGP-LS Attribute being discarded 293 [RFC7752]. The MSD information introduced in BGP-LS by this 294 specification, may be used by BGP-LS consumer applications like a SR 295 path computation engine (PCE) to learn the SR SID stack handling 296 capabilities of the nodes in the topology. This can enable the SR 297 PCE to perform path computations taking into consideration the size 298 of SID stack that the specific head-end node may be able to impose. 299 Errors in the encoding or decoding of the MSD information may result 300 in the unavailability of such information to the SR PCE or incorrect 301 information being made available to it. This may result in the head- 302 end node not being able to instantiate the desired SR path in its 303 forwarding and provide the SR based optimization functionality. The 304 handling of such errors by applications like SR PCE may be 305 implementation specific and out of scope of this document. 307 The extensions specified in this document do not specify any new 308 configuration or monitoring aspects in BGP or BGP-LS. The 309 specification of BGP models is an ongoing work based on the 310 [I-D.ietf-idr-bgp-model]. 312 8. Security Considerations 314 The advertisement of an incorrect MSD value may have negative 315 consequences. If the value is smaller than supported, path 316 computation may fail to compute a viable path. If the value is 317 larger than supported, an attempt to instantiate a path that can't be 318 supported by the head-end (the node performing the SID imposition) 319 may occur. The presence of this information may also inform an 320 attacker of how to induce any of the aforementioned conditions. 322 The procedures and protocol extensions defined in this document do 323 not affect the BGP security model. See the "Security Considerations" 324 section of [RFC4271] for a discussion of BGP security. Also, refer 325 to [RFC4272] and [RFC6952] for analyses of security issues for BGP. 326 Security considerations for acquiring and distributing BGP-LS 327 information are discussed in [RFC7752]. The TLVs introduced in this 328 document are used to propagate the MSD IGP extensions defined in 329 [RFC8476] [RFC8491]. It is assumed that the IGP instances 330 originating these TLVs will support all the required security (as 331 described in [RFC8476] [RFC8491]) in order to prevent any security 332 issues when propagating the TLVs into BGP-LS. The advertisement of 333 the node and link attribute information defined in this document 334 presents no additional risk beyond that associated with the existing 335 node and link attribute information already supported in [RFC7752]. 337 9. Contributors 339 Siva Sivabalan 340 Cisco Systems Inc. 341 Canada 343 Email: msiva@cisco.com 345 10. Acknowledgements 347 We like to thank Acee Lindem, Stephane Litkowski, Bruno Decraene and 348 Alvaro Retana for their reviews and valuable comments. 350 11. References 352 11.1. Normative References 354 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 355 Requirement Levels", BCP 14, RFC 2119, 356 DOI 10.17487/RFC2119, March 1997, 357 . 359 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 360 S. Ray, "North-Bound Distribution of Link-State and 361 Traffic Engineering (TE) Information Using BGP", RFC 7752, 362 DOI 10.17487/RFC7752, March 2016, 363 . 365 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 366 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 367 May 2017, . 369 [RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak, 370 "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, 371 DOI 10.17487/RFC8476, December 2018, 372 . 374 [RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, 375 "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, 376 DOI 10.17487/RFC8491, November 2018, 377 . 379 11.2. Informative References 381 [I-D.ietf-idr-bgp-model] 382 Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP 383 YANG Model for Service Provider Networks", draft-ietf-idr- 384 bgp-model-08 (work in progress), February 2020. 386 [I-D.ietf-isis-mpls-elc] 387 Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., 388 and M. Bocci, "Signaling Entropy Label Capability and 389 Entropy Readable Label Depth Using IS-IS", draft-ietf- 390 isis-mpls-elc-11 (work in progress), March 2020. 392 [I-D.ietf-ospf-mpls-elc] 393 Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., 394 and M. Bocci, "Signaling Entropy Label Capability and 395 Entropy Readable Label Depth Using OSPF", draft-ietf-ospf- 396 mpls-elc-13 (work in progress), April 2020. 398 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 399 Label Switching Architecture", RFC 3031, 400 DOI 10.17487/RFC3031, January 2001, 401 . 403 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 404 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 405 DOI 10.17487/RFC4271, January 2006, 406 . 408 [RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", 409 RFC 4272, DOI 10.17487/RFC4272, January 2006, 410 . 412 [RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of 413 BGP, LDP, PCEP, and MSDP Issues According to the Keying 414 and Authentication for Routing Protocols (KARP) Design 415 Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013, 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 [RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., 424 and J. Hardwick, "Path Computation Element Communication 425 Protocol (PCEP) Extensions for Segment Routing", RFC 8664, 426 DOI 10.17487/RFC8664, December 2019, 427 . 429 Authors' Addresses 430 Jeff Tantsura 431 Apstra, Inc. 433 Email: jefftant.ietf@gmail.com 435 Uma Chunduri 436 Futurewei Technologies 438 Email: umac.ietf@gmail.com 440 Ketan Talaulikar 441 Cisco Systems 443 Email: ketant@cisco.com 445 Greg Mirsky 446 ZTE Corp. 448 Email: gregimirsky@gmail.com 450 Nikos Triantafillis 451 Amazon Web Services 453 Email: nikost@amazon.com