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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references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 7752 (Obsoleted by RFC 9552) == Outdated reference: A later version (-17) exists of draft-ietf-idr-bgp-model-08 == Outdated reference: A later version (-13) exists of draft-ietf-isis-mpls-elc-11 == Outdated reference: A later version (-15) exists of draft-ietf-ospf-mpls-elc-12 Summary: 2 errors (**), 0 flaws (~~), 4 warnings (==), 1 comment (--).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 1, 2020 Futurewei Technologies 6 K. Talaulikar 7 Cisco Systems 8 G. Mirsky 9 ZTE Corp. 10 N. Triantafillis 11 Amazon Web Services 12 March 30, 2020 14 Signaling MSD (Maximum SID Depth) using Border Gateway Protocol - Link 15 State 16 draft-ietf-idr-bgp-ls-segment-routing-msd-16 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 1, 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 . . . . . . . . . . . . . . . . . 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 MSD: Maximum SID Depth - the number of SIDs supported by a node or a 120 link on a node 122 PCC: Path Computation Client 124 PCE: Path Computation Element 126 PCEP: Path Computation Element Protocol 128 SID: Segment Identifier as defined in [RFC8402] 130 SR: Segment Routing 132 Label Imposition: Imposition is the act of modifying and/or adding 133 labels to the outgoing label stack associated with a packet. This 134 includes: 136 o replacing the label at the top of the label stack with a new 137 label. 139 o pushing one or more new labels onto the label stack. 141 o The number of labels imposed is then the sum of the number of 142 labels that are replaced and the number of labels that are pushed. 143 See [RFC3031] 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 ([RFC8491] ) of nodes and their links in 157 a network to a BGP-LS consumer of network topology such as a 158 centralized controller. The centralized controller can leverage this 159 information in computation of SR paths based on their MSD 160 capabilities. When a BGP-LS speaker is originating the topology 161 learnt via link-state routing protocols such as OSPF or IS-IS, the 162 MSD information for the nodes and their links is sourced from the 163 underlying extensions as defined in [RFC8476] and [RFC8491] 164 respectively. 166 The extensions introduced in this document allow for advertisement of 167 different MSD-Types, which are defined elsewhere and were introduced 168 in [RFC8491]. This enables sharing of MSD-Types that may be defined 169 in the future by the IGPs in BGP-LS. 171 3. Node MSD TLV 173 The Node MSD ([RFC8476] [RFC8491]) is encoded in a new Node Attribute 174 TLV [RFC7752] to carry the provisioned SID depth of the router 175 identified by the corresponding Router-ID. Node MSD is the smallest 176 MSD supported by the node on the set of interfaces configured for 177 use. MSD values may be learned via a hardware API or may be 178 provisioned. The following format is used: 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 "IGP MSD-Types" 201 registry defined in [RFC8491]. 203 * MSD-Value : a number in the range of 0-255. For all MSD-Types, 204 0 represents the lack of ability to impose an MSD stack of any 205 depth; any other value represents that of the node. This value 206 MUST represent the lowest value supported by any link 207 configured for use by the advertising protocol instance. 209 4. Link MSD TLV 211 The Link MSD ([RFC8476] [RFC8491]) is defined to carry the MSD of the 212 interface associated with the link. It is encoded in a new Link 213 Attribute TLV [RFC7752] using the 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: 267 229 o Length: variable (multiple of 2); represents the total length of 230 the value field in octets. 232 o Value : consists of one or more pairs of a 1-octet MSD-Type and 233 1-octet MSD-Value. 235 * MSD-Type : MSD-Type : one of the values defined in the "IGP 236 MSD-Types" registry defined in [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. Procedures for Defining and Using Node and Link MSD Advertisements 245 When Link MSD is present for a given MSD-type, the value of the Link 246 MSD MUST take precedence over the Node MSD. When a Link MSD-type is 247 not signaled but the Node MSD-type is, then the Node MSD-type value 248 MUST be considered as the MSD value for that link. 250 In order to increase flooding efficiency, it is RECOMMENDED that 251 routers with homogenous link MSD values advertise just the Node MSD 252 value. 254 The meaning of the absence of both Node and Link MSD advertisements 255 for a given MSD-type is specific to the MSD-type. Generally it can 256 only be inferred that the advertising node does not support 257 advertisement of that MSD-type. However, in some cases the lack of 258 advertisement might imply that the functionality associated with the 259 MSD-type is not supported. The correct interpretation MUST be 260 specified when an MSD-type is defined in [RFC8491]. 262 6. IANA Considerations 264 This document requests assigning code-points from the registry "BGP- 265 LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute 266 TLVs" based on table below. Early allocation for these code-points 267 have been done by IANA. 269 +------------+-----------------+---------------------------+-------------------+ 270 | Code Point | Description | IS-IS TLV/Sub-TLV | Reference | 271 +------------+-----------------+---------------------------+-------------------+ 272 | 266 | Node MSD | 242/23 | This document | 273 | 267 | Link MSD | (22,23,25,141,222,223)/15 | This document | 274 +------------+-----------------+---------------------------+-------------------+ 276 7. Manageability Considerations 278 The new protocol extensions introduced in this document augment the 279 existing IGP topology information that is distributed via [RFC7752]. 280 Procedures and protocol extensions defined in this document do not 281 affect the BGP protocol operations and management other than as 282 discussed in the Manageability Considerations section of [RFC7752]. 283 Specifically, the malformed attribute tests for syntactic checks in 284 the Fault Management section of [RFC7752] now encompass the new BGP- 285 LS Attribute TLVs defined in this document. The semantic or content 286 checking for the TLVs specified in this document and their 287 association with the BGP-LS NLRI types or their BGP-LS Attribute is 288 left to the consumer of the BGP-LS information (e.g. an application 289 or a controller) and not the BGP protocol. 291 A consumer of the BGP-LS information retrieves this information over 292 a BGP-LS session (refer Section 1 and 2 of [RFC7752]). 294 This document only introduces new Attribute TLVs and any syntactic 295 error in them would result in the BGP-LS Attribute being discarded 296 [RFC7752]. The MSD information introduced in BGP-LS by this 297 specification, may be used by BGP-LS consumer applications like a SR 298 path computation engine (PCE) to learn the SR SID stack handling 299 capabilities of the nodes in the topology. This can enable the SR 300 PCE to perform path computations taking into consideration the size 301 of SID stack that the specific head-end node may be able to impose. 302 Errors in the encoding or decoding of the MSD information may result 303 in the unavailability of such information to the SR PCE or incorrect 304 information being made available to it. This may result in the head- 305 end node not being able to instantiate the desired SR path in its 306 forwarding and provide the SR based optimization functionality. The 307 handling of such errors by applications like SR PCE may be 308 implementation specific and out of scope of this document. 310 The extensions specified in this document, do not specify any new 311 configuration or monitoring aspects in BGP or BGP-LS. The 312 specification of BGP models is an ongoing work based on the 313 [I-D.ietf-idr-bgp-model]. 315 8. Security Considerations 317 The advertisement of an incorrect MSD value may have negative 318 consequences. If the value is smaller than supported, path 319 computation may fail to compute a viable path. If the value is 320 larger than supported, an attempt to instantiate a path that can't be 321 supported by the head-end (the node performing the SID imposition) 322 may occur. The presence of this information may also inform an 323 attacker of how to induce any of the aforementioned conditions. 325 The procedures and protocol extensions defined in this document do 326 not affect the BGP security model. See the "Security Considerations" 327 section of [RFC4271] for a discussion of BGP security. Also, refer 328 to [RFC4272] and [RFC6952] for analyses of security issues for BGP. 329 Security considerations for acquiring and distributing BGP-LS 330 information are discussed in [RFC7752]. The TLVs introduced in this 331 document are used to propagate the MSD IGP extensions defined in 332 [RFC8476] [RFC8491]. It is assumed that the IGP instances 333 originating these TLVs will support all the required security (as 334 described in [RFC8476] [RFC8491]) in order to prevent any security 335 issues when propagating the TLVs into BGP-LS. The advertisement of 336 the node and link attribute information defined in this document 337 presents no additional risk beyond that associated with the existing 338 node and link attribute information already supported in [RFC7752]. 340 9. Contributors 342 Siva Sivabalan 343 Cisco Systems Inc. 344 Canada 346 Email: msiva@cisco.com 348 10. Acknowledgements 350 We like to thank Acee Lindem, Stephane Litkowski, Bruno Decraene and 351 Alvaro Retana for their reviews and valuable comments. 353 11. References 355 11.1. Normative References 357 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 358 Requirement Levels", BCP 14, RFC 2119, 359 DOI 10.17487/RFC2119, March 1997, 360 . 362 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 363 S. Ray, "North-Bound Distribution of Link-State and 364 Traffic Engineering (TE) Information Using BGP", RFC 7752, 365 DOI 10.17487/RFC7752, March 2016, 366 . 368 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 369 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 370 May 2017, . 372 [RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak, 373 "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, 374 DOI 10.17487/RFC8476, December 2018, 375 . 377 [RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, 378 "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, 379 DOI 10.17487/RFC8491, November 2018, 380 . 382 11.2. Informative References 384 [I-D.ietf-idr-bgp-model] 385 Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP 386 YANG Model for Service Provider Networks", draft-ietf-idr- 387 bgp-model-08 (work in progress), February 2020. 389 [I-D.ietf-isis-mpls-elc] 390 Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., 391 and M. Bocci, "Signaling Entropy Label Capability and 392 Entropy Readable Label Depth Using IS-IS", draft-ietf- 393 isis-mpls-elc-11 (work in progress), March 2020. 395 [I-D.ietf-ospf-mpls-elc] 396 Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., 397 and M. Bocci, "Signaling Entropy Label Capability and 398 Entropy Readable Label-stack Depth Using OSPF", draft- 399 ietf-ospf-mpls-elc-12 (work in progress), October 2019. 401 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 402 Label Switching Architecture", RFC 3031, 403 DOI 10.17487/RFC3031, January 2001, 404 . 406 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 407 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 408 DOI 10.17487/RFC4271, January 2006, 409 . 411 [RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis", 412 RFC 4272, DOI 10.17487/RFC4272, January 2006, 413 . 415 [RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of 416 BGP, LDP, PCEP, and MSDP Issues According to the Keying 417 and Authentication for Routing Protocols (KARP) Design 418 Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013, 419 . 421 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 422 Decraene, B., Litkowski, S., and R. Shakir, "Segment 423 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 424 July 2018, . 426 [RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., 427 and J. Hardwick, "Path Computation Element Communication 428 Protocol (PCEP) Extensions for Segment Routing", RFC 8664, 429 DOI 10.17487/RFC8664, December 2019, 430 . 432 Authors' Addresses 434 Jeff Tantsura 435 Apstra, Inc. 437 Email: jefftant.ietf@gmail.com 439 Uma Chunduri 440 Futurewei Technologies 442 Email: umac.ietf@gmail.com 444 Ketan Talaulikar 445 Cisco Systems 447 Email: ketant@cisco.com 449 Greg Mirsky 450 ZTE Corp. 452 Email: gregimirsky@gmail.com 454 Nikos Triantafillis 455 Amazon Web Services 457 Email: nikost@amazon.com