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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references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 7752 (Obsoleted by RFC 9552) == Outdated reference: A later version (-13) exists of draft-ietf-isis-mpls-elc-08 == Outdated reference: A later version (-15) exists of draft-ietf-ospf-mpls-elc-08 == Outdated reference: A later version (-30) exists of draft-ietf-spring-sr-yang-13 Summary: 1 error (**), 0 flaws (~~), 4 warnings (==), 1 comment (--).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: March 8, 2020 Futurewei Technologies 6 K. Talaulikar 7 Cisco Systems 8 G. Mirsky 9 ZTE Corp. 10 N. Triantafillis 11 Apstra, Inc. 12 September 5, 2019 14 Signaling MSD (Maximum SID Depth) using Border Gateway Protocol Link- 15 State 16 draft-ietf-idr-bgp-ls-segment-routing-msd-06 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 8, 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 . . . . . . . . . . . . . . . . . . . . . . . . 7 73 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 74 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 75 10.1. Normative References . . . . . . . . . . . . . . . . . . 8 76 10.2. Informative References . . . . . . . . . . . . . . . . . 8 77 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 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 of nodes and their links in a network to 157 a BGP-LS consumer of network topology such as a centralized 158 controller. The centralized controller can leverage this information 159 in computation of SR paths and their instantiation on network nodes 160 based on their MSD capabilities. When a BGP-LS speaker is 161 originating the topology learnt via link-state routing protocols like 162 OSPF or IS-IS, the MSD information for the nodes and their links is 163 sourced from the underlying extensions as defined in [RFC8476] and 164 [RFC8491] respectively. The BGP-LS speaker may also advertise the 165 MSD information for the local node and its links when not running any 166 link-state IGP protocol e.g. when running BGP as the only routing 167 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 introduce any new 295 configuration or monitoring aspects in BGP or BGP-LS other than as 296 discussed in [RFC7752]. The manageability aspects of the MSD 297 features are covered by [I-D.ietf-spring-sr-yang]. 299 7. Security Considerations 301 The advertisement of an incorrect MSD value may have negative 302 consequences. If the value is smaller than supported, path 303 computation may fail to compute a viable path. If the value is 304 larger than supported, an attempt to instantiate a path that can't be 305 supported by the head-end (the node performing the SID imposition) 306 may occur. The presence of this information may also inform an 307 attacker of how to induce any of the aforementioned conditions. 309 This document does not introduce security issues beyond those 310 discussed in [RFC7752], [RFC8476] and [RFC8491] 312 8. Contributors 314 Siva Sivabalan 315 Cisco Systems Inc. 316 Canada 318 Email: msiva@cisco.com 320 9. Acknowledgements 322 We like to thank Acee Lindem, Stephane Litkowski and Bruno Decraene 323 for their reviews and valuable comments. 325 10. References 327 10.1. Normative References 329 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 330 Requirement Levels", BCP 14, RFC 2119, 331 DOI 10.17487/RFC2119, March 1997, 332 . 334 [RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and 335 S. Ray, "North-Bound Distribution of Link-State and 336 Traffic Engineering (TE) Information Using BGP", RFC 7752, 337 DOI 10.17487/RFC7752, March 2016, 338 . 340 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 341 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 342 May 2017, . 344 [RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak, 345 "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, 346 DOI 10.17487/RFC8476, December 2018, 347 . 349 [RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, 350 "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, 351 DOI 10.17487/RFC8491, November 2018, 352 . 354 10.2. Informative References 356 [I-D.ietf-isis-mpls-elc] 357 Xu, X., Kini, S., Psenak, P., Filsfils, C., and S. 358 Litkowski, "Signaling Entropy Label Capability and Entropy 359 Readable Label Depth Using IS-IS", draft-ietf-isis-mpls- 360 elc-08 (work in progress), September 2019. 362 [I-D.ietf-ospf-mpls-elc] 363 Xu, X., Kini, S., Psenak, P., Filsfils, C., and S. 364 Litkowski, "Signaling Entropy Label Capability and Entropy 365 Readable Label-stack Depth Using OSPF", draft-ietf-ospf- 366 mpls-elc-08 (work in progress), May 2019. 368 [I-D.ietf-pce-segment-routing] 369 Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., 370 and J. Hardwick, "PCEP Extensions for Segment Routing", 371 draft-ietf-pce-segment-routing-16 (work in progress), 372 March 2019. 374 [I-D.ietf-spring-sr-yang] 375 Litkowski, S., Qu, Y., Lindem, A., Sarkar, P., and J. 376 Tantsura, "YANG Data Model for Segment Routing", draft- 377 ietf-spring-sr-yang-13 (work in progress), July 2019. 379 [RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol 380 Label Switching Architecture", RFC 3031, 381 DOI 10.17487/RFC3031, January 2001, 382 . 384 [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., 385 Decraene, B., Litkowski, S., and R. Shakir, "Segment 386 Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, 387 July 2018, . 389 Authors' Addresses 391 Jeff Tantsura 392 Apstra, Inc. 394 Email: jefftant.ietf@gmail.com 396 Uma Chunduri 397 Futurewei Technologies 399 Email: umac.ietf@gmail.com 401 Ketan Talaulikar 402 Cisco Systems 404 Email: ketant@cisco.com 406 Greg Mirsky 407 ZTE Corp. 409 Email: gregimirsky@gmail.com 410 Nikos Triantafillis 411 Apstra, Inc. 413 Email: nikos@apstra.com