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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) No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Link State Routing K. Talaulikar 3 Internet-Draft P. Psenak 4 Intended status: Standards Track Cisco Systems, Inc. 5 Expires: July 9, 2020 H. Johnston 6 AT&T Labs 7 January 6, 2020 9 OSPF Reverse Metric 10 draft-ietf-lsr-ospf-reverse-metric-00 12 Abstract 14 This document specifies the extensions to OSPF that enables a router 15 to signal to its neighbor the metric that the neighbor should use 16 towards itself using link-local advertisement between them. The 17 signalling of this reverse metric, to be used on link(s) towards 18 itself, allows a router to influence the amount of traffic flowing 19 towards itself and in certain use-cases enables routers to maintain 20 symmetric metric on both sides of a link between them. 22 Requirements Language 24 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 25 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 26 "OPTIONAL" in this document are to be interpreted as described in BCP 27 14 [RFC2119] [RFC8174] when, and only when, they appear in all 28 capitals, as shown here. 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at https://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on July 9, 2020. 47 Copyright Notice 49 Copyright (c) 2020 IETF Trust and the persons identified as the 50 document authors. All rights reserved. 52 This document is subject to BCP 78 and the IETF Trust's Legal 53 Provisions Relating to IETF Documents 54 (https://trustee.ietf.org/license-info) in effect on the date of 55 publication of this document. Please review these documents 56 carefully, as they describe your rights and restrictions with respect 57 to this document. Code Components extracted from this document must 58 include Simplified BSD License text as described in Section 4.e of 59 the Trust Legal Provisions and are provided without warranty as 60 described in the Simplified BSD License. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 65 2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 3 66 2.1. Symmetrical Metric Based on Reference Bandwidth . . . . . 3 67 2.2. Adaptive Metric Signaling . . . . . . . . . . . . . . . . 4 68 3. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 5 69 4. LLS Reverse Metric TLV . . . . . . . . . . . . . . . . . . . 6 70 5. LLS Reverse TE Metric TLV . . . . . . . . . . . . . . . . . . 6 71 6. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 7 72 7. Backward Compatibility . . . . . . . . . . . . . . . . . . . 9 73 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 74 9. Security Considerations . . . . . . . . . . . . . . . . . . . 9 75 10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10 76 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 77 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 78 12.1. Normative References . . . . . . . . . . . . . . . . . . 10 79 12.2. Informative References . . . . . . . . . . . . . . . . . 11 80 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 82 1. Introduction 84 Routers running the Open Shortest Path First (OSPFv2) [RFC2328] and 85 OSPFv3 [RFC5340] routing protocols originate a Router-LSA (Link State 86 Advertisement) that describes all its links to its neighbors and 87 includes a metric which indicates its "cost" of reaching the neighbor 88 over that link. Consider two routers R1 and R2 that are connected 89 via a link. The metric for this link in direction R1->R2 is 90 configured on R1 and in the direction R2->R1 is configured on R2. 91 Thus the configuration on R1 influences the traffic that it forwards 92 towards R2 but does not influence the traffic that it may receive 93 from R2 on that same link. 95 This document describes certain use-cases where it is desirable for a 96 router to be able to signal what we call as the "reverse metric" (RM) 97 to its neighbor to adjust the routing metric on the inbound 98 direction. When R1 signals its reverse metric on its link to R2, 99 then R2 advertises this value as its metric to R1 in its Router-LSA 100 instead of its locally configured value. Once this information is 101 part of the topology then all other routers do their computation 102 using this value which results in the desired change in traffic 103 distribution that R1 wanted to achieve towards itself over the link 104 from R2. 106 This document proposes an extension to OSPF link-local signaling 107 (LLS) [RFC5613] for signalling the OSPF reverse metric using the LLS 108 Reverse Metric TLV in Section 4, the reverse Traffic Engineering (TE) 109 metric [RFC3630] using the LLS Reverse TE Metric TLV in Section 5 and 110 describes the related procedures in section Section 6. 112 2. Use Cases 114 This section describes certain use-cases that OSPF reverse metric 115 helps to address. The usage of OSPF reverse metric need not be 116 limited to these cases and is intended to be a generic mechanism. 118 2.1. Symmetrical Metric Based on Reference Bandwidth 120 Certain OSPF implementations and deployments deduce the metric of 121 links based on their bandwidth using a reference bandwidth. The OSPF 122 MIB [RFC4750] has ospfReferenceBandwidth that is used by entries in 123 the ospfIfMetricTable. This mechanism is leveraged in deployments 124 where the link metrics get lowered or increased as bandwidth capacity 125 is removed or added e.g. consider layer-2 links bundled as a layer-3 126 interface on which OSPF is enabled. In the situations where these 127 layer-2 links are directly connected to the two routers, the link and 128 bandwidth availability is detected and updated on both sides. This 129 allows for schemes where the metric is maintained to be symmetric in 130 both directions based on the bandwidth. 132 Now consider variation of the same deployment where the links between 133 routers are not directly connected and instead are provisioned over a 134 layer-2 network consisting of switches or other mechanisms for a 135 layer-2 emulation. In such scenarios, as show in Figure 1, the 136 router on one side of the link would not detect when the neighboring 137 router has lost one of its layer-2 link and has reduced capacity to 138 its layer-2 switch. Note that the number of links and their 139 capacities on the router R0 may not be the same as those on R1, R2 140 and R3. The left hand side diagram shows the actual physical 141 topology in terms of the layer-2 links while the right hand side 142 diagram shows the logical layer-3 link topology between the routers. 144 +--------+ 145 | R0 | 146 | Router | 147 +--------+ +--------+ 148 (a) Physical ^ ^ ^ (b) Layer-3 | R0 | 149 Topology | | | Topology +--------+ 150 v v v ^ ^ ^ 151 +----------------+ | | | 152 | Layer 2 Switch | | | | 153 | (Aggregation) | +---+ | +---+ 154 +----------------+ | | | 155 ^^ ^ ^ ^ ^ ^ v | v 156 || | | | | | +------+ | +------+ 157 +----+| | | | | | | R1 | | | R3 | 158 | +---+ | | | | +----+ +------+ | +------+ 159 v v v v v v v v 160 +--------+ +--------+ +--------+ +--------+ 161 | R1 | | R2 | | R3 | | R2 | 162 | Router | | Router | | Router | +--------+ 163 +-- -----+ +--------+ +--------+ 165 Figure 1: Routers Interconnected over Layer-2 Network 167 In such a scenario, the amount of traffic that can be forwarded in 168 bidirectional manner between say R0 and R1 is dictated by the lower 169 of the link capacity of R0 and R1 to the layer-2 transport network. 170 In this scenario, when one of the link from R1 to the switch goes 171 down, it would increase its link metric to R0 from say 20 to 40. 172 However, similarly R0 also needs to increase its link metric to R1 as 173 well from 20 to 40 as otherwise, the traffic will hit congestion and 174 get dropped. 176 When R1 has the ability to signal the OSPF reverse metric of 40 177 towards itself to R0, then R0 can also update its metric without any 178 manual intervention to ensure the correct traffic distribution. 179 Consider if some destinations were reachable from R0 via R1 180 previously and this automatic metric adjustment now makes some of 181 those destinations reachable from R0 via R3. This allows some 182 traffic load on the link R0 to R1 to now flow via R3 to these 183 destinations. 185 2.2. Adaptive Metric Signaling 187 Now consider another deployment scenario where, as show in Figure 2, 188 two routers AGGR1 and AGGR2 are connected to a bunch of routers R1 189 thru RN that are dual homed to them and aggregating the traffic from 190 them towards a core network. At some point T, AGGR1 loses some of 191 its capacity towards the core or is facing some congestion issue 192 towards the core and it needs to reduce the traffic going through it 193 and perhaps redirect some of that load via AGGR2 which is not facing 194 a similar issue. Altering its own metric towards Rx routers would 195 influence the traffic flowing through it in the direction from core 196 to the Rx but not the other way around as desired. 198 Core Network 199 ^ ^ 200 | | 201 V v 202 +----------+ +----------+ 203 | AGGR1 | | AGGR2 | 204 +----------+ +----------+ 205 ^ ^ ^ ^ 206 | | | | 207 | +-----------+ | 208 | | | | 209 | +--------+ | | 210 v v v v 211 +-----------+ +-----------+ 212 | R1 | | RN | 213 | Router | ... | Router | 214 +-----------+ +-----------+ 216 Figure 2: Adaptive Metric for Dual Gateways 218 In such a scenario, the AGGR1 router could signal an incremental 219 value of OSPF reverse metric towards some or all of the Rx routers. 220 When the Rx routers apply this signaled reverse metric offset value 221 to the original metric on their links towards AGGR1 then the path via 222 AGGR2 becomes a better path causing traffic towards the core getting 223 diverted away from it. Note that the reverse metric mechanism allows 224 such adaptive metric changes to be applied on the AGGR1 as opposed to 225 being provisioning statically on the possibly large number of Rx 226 routers. 228 3. Solution 230 To address the use-cases described earlier and to allow an OSPF 231 router to indicate its reverse metric for a specific point-to-point 232 or point-to-multipoint link to its neighbor, this document proposes 233 to extend OSPF link-local signaling to advertise the Reverse Metric 234 TLV in OSPF Hello packets. This ensures that the RM signaling is 235 scoped ONLY to each specific link individually. The router continues 236 to include the Reverse Metric TLV in its Hello packets on the link as 237 long as it needs its neighbor to use that metric value towards 238 itself. Further details of the procedures involve are specified in 239 Section 6. 241 The RM signaling specified in this document is not required for 242 broadcast or non-broadcast-multi-access (NBMA) links since the same 243 objective is achieved there using the OSPF Two-Part Metric mechanism 244 [RFC8042]. 246 4. LLS Reverse Metric TLV 248 The Reverse Metric TLV is a new LLS TLV. It has following format: 250 0 1 2 3 251 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 1 252 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 253 | Type | Length | 254 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 255 | MTID | Flags |O|H| Reverse Metric | 256 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 258 where: 260 Type: TBD, suggested value 19 262 Length: 4 octet 264 MTID : the multi-topology identifier of the link ([RFC4915]) 266 Flags: 1 octet, following are defined currently and the rest MUST 267 be set to 0 and ignored on reception. 269 * H (0x1) : Indicates that neighbor should use value only if 270 higher than its current metric value in use 272 * O (0x2) : Indicates that the reverse metric value provided is 273 an offset that is to be added to the original metric 275 Reverse Metric: 2 octets, the value or offset of reverse metric to 276 be used 278 5. LLS Reverse TE Metric TLV 280 The Reverse TE Metric TLV is a new LLS TLV. It has following format: 282 0 1 2 3 283 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 1 284 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 285 | Type | Length | 286 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 287 | Flags |O|H| RESERVED | 288 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 289 | Reverse TE Metric | 290 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 292 where: 294 Type: TBD, suggested value 20 296 Length: 4 octet 298 Flags: 1 octet, following are defined currently and the rest MUST 299 be set to 0 and ignored on reception. 301 * H (0x1) : Indicates that neighbor should use value only if 302 higher than its current TE metric value in use 304 * O (0x2) : Indicates that the reverse TE metric value provided 305 is an offset that is to be added to the original TE metric 307 RESERVED: 24-bit field. SHOULD be set to 0 on transmission and 308 MUST be ignored on receipt. 310 Reverse TE Metric: 4 octets, the value or offset of reverse 311 traffic engineering metric to be used 313 6. Procedures 315 When a router needs to signal a RM value that its neigbhor(s) should 316 use towards itself, it includes the Reverse Metric TLV in the LLS 317 block of its hello messages sent on the link and continues to include 318 this TLV for as long as it needs it's neighbor to use this value. 319 The mechanisms used to determine the value to be used for the RM is 320 specific to the implementation and use-case and is outside the scope 321 of this document. e.g. in the use-case related to symmetric metric 322 described in Section 2.1, the RM value may be derived based on the 323 router's link's bandwidth with respect to the reference bandwidth. 325 A router receiving a hello packet from its neighbor that contains the 326 Reverse Metric TLV on its link SHOULD use the RM value to derive the 327 metric for the link in its Router-LSA to the advertising router. 329 When the O flag is set, the value in the TLV needs to be added to the 330 existing original metric provisioned on the link to derive the new 331 metric value to be used. When the O flag is clear, the value in the 332 TLV should be directly used as the metric to be used. When H flag is 333 set and O flag is clear, this is done only when the RM value signaled 334 is higher than the provisioned metric value being used already. This 335 mechanism applies only for point-to-point, point-to-multipoint and 336 hybrid broadcast point-to-multipoint ( [RFC6845]) links. For 337 broadcast and NBMA links the OSPF Two-Part Metric mechanism [RFC8042] 338 should be used in similar use-cases. 340 Implementations SHOULD provide a configuration option to enable the 341 signaling of RM from a router to its neighbors and MAY provide a 342 configuration option to disable the acceptance of the RM from its 343 neighbors. 345 A router stops including the Reverse Metric TLV in its hello messages 346 when it needs its neighbors to go back to using their own provisioned 347 metric values. When that happens, a router which had modified its 348 metric in response to receiving a Reverse Metric TLV from its 349 neighbor should revert back to using its original provisioned metric 350 value. 352 In certain scenarios, it is possible that two or more routers start 353 the RM signaling on the same link. This could create collision 354 scenarios. The following rules MUST be adopted by routers to ensure 355 that there is no instability in the network due to churn in their 356 metric due to signaling of RM: 358 o The RM value that is signaled by a router to its neighbor MUST NOT 359 be derived from the reverse metric being signaled by any of its 360 neighbor on any of its links. 362 o The RM value that is signaled by a router MUST NOT be derived from 363 its own metric which has been modified on account of a RM signaled 364 from any of its neighbors on any of its links. RM signaling from 365 other routers can affect the router's own metric advertised in its 366 Router-LSA. When deriving the RM values that a router signals to 367 its neighbors, it should use its "original" local metric values 368 not influenced by any RM signaling. 370 Based on these rules, a router MUST never start or stop or change its 371 RM metric signaling based on the RM metric signaling initiated by 372 some other router. Based on the local configuration policy, each 373 router would end up accepting the RM value signaled by its neighbor 374 and there would be no churn of metrics on the link or the network on 375 account of RM signaling. 377 In certain use-case as described in Section 2.1 when symmetrical 378 metrics are desired, the RM signaling can be enabled on routers on 379 either ends of a link. In other use-cases as described in 380 Section 2.2 RM signaling may need to be enabled on only router at one 381 end of a link. 383 When using multi-topology routing with OSPF [RFC4915] a router MAY 384 include multiple instances of the Reverse Metric TLV in the LLS block 385 of its hello message - one for each of the topology for which it 386 desires to signal the reserve metric for. 388 In certain scenarios, the OSPF router may also require the 389 modification of the TE metric being advertised by its neighbor router 390 towards itself in the inbound direction. The Reverse TE Metric TLV, 391 using similar procedures as described above, MAY be used to signal 392 the reverse TE metric by a router. The neighbor SHOULD use the 393 reverse TE metric value to derive the TE metric to be used in the TE 394 Metric sub-TLV of the Link TLV in its TE Opaque LSA [RFC3630]. 396 7. Backward Compatibility 398 The signaling specified in this document happens at a link-local 399 level between routers on that link. A router which does not support 400 this specification would ignore the Reverse Metric and Reverse TE 401 Metric LLS TLVs and take no actions to updates its metric in the 402 other LSAs. As a result, the behavior would be the same as before 403 this specification. Therefore, there are no backward compatibility 404 related issues or considerations that need to be taken care of when 405 implementing this specification. 407 8. IANA Considerations 409 This specification updates Link Local Signalling TLV Identifiers 410 registry. 412 Following values are requested for allocation: 414 o TBD (Suggested value 19) - Reverse Metric TLV 416 o TBD (Suggested value 20) - Reverse TE Metric TLV 418 9. Security Considerations 420 The security considerations for "OSPF Link-Local Signaling" [RFC5613] 421 also apply to the extension described in this document. The usage of 422 the reverse metric TLVs is to alter the metrics used by routers on 423 the link and influence the flow and routing of traffic over the 424 network. Hence, modification of the Reverse Metric and Reverse TE 425 Metric TLVs may result in misrouting of traffic. If authentication 426 is being used in the OSPF routing domain [RFC5709][RFC7474], then the 427 Cryptographic Authentication TLV [RFC5613] SHOULD also be used to 428 protect the contents of the LLS block. 430 Receiving a malformed LLS Reverse Metric or Reverse TE Metric TLVs 431 MUST NOT result in a hard router or OSPF process failure. The 432 reception of malformed LLS TLVs or sub-TLVs SHOULD be logged, but 433 such logging MUST be rate- limited to prevent denial-of-service (DoS) 434 attacks. 436 10. Contributors 438 Thanks to Jay Karthik for his contributions on the use-cases related 439 to symmetric metric and the review of the solution. 441 11. Acknowledgements 443 12. References 445 12.1. Normative References 447 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 448 Requirement Levels", BCP 14, RFC 2119, 449 DOI 10.17487/RFC2119, March 1997, 450 . 452 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, 453 DOI 10.17487/RFC2328, April 1998, 454 . 456 [RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering 457 (TE) Extensions to OSPF Version 2", RFC 3630, 458 DOI 10.17487/RFC3630, September 2003, 459 . 461 [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF 462 for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, 463 . 465 [RFC5613] Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D. 466 Yeung, "OSPF Link-Local Signaling", RFC 5613, 467 DOI 10.17487/RFC5613, August 2009, 468 . 470 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 471 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 472 May 2017, . 474 12.2. Informative References 476 [RFC4750] Joyal, D., Ed., Galecki, P., Ed., Giacalone, S., Ed., 477 Coltun, R., and F. Baker, "OSPF Version 2 Management 478 Information Base", RFC 4750, DOI 10.17487/RFC4750, 479 December 2006, . 481 [RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. 482 Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", 483 RFC 4915, DOI 10.17487/RFC4915, June 2007, 484 . 486 [RFC5709] Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M., 487 Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic 488 Authentication", RFC 5709, DOI 10.17487/RFC5709, October 489 2009, . 491 [RFC6845] Sheth, N., Wang, L., and J. Zhang, "OSPF Hybrid Broadcast 492 and Point-to-Multipoint Interface Type", RFC 6845, 493 DOI 10.17487/RFC6845, January 2013, 494 . 496 [RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed., 497 "Security Extension for OSPFv2 When Using Manual Key 498 Management", RFC 7474, DOI 10.17487/RFC7474, April 2015, 499 . 501 [RFC8042] Zhang, Z., Wang, L., and A. Lindem, "OSPF Two-Part 502 Metric", RFC 8042, DOI 10.17487/RFC8042, December 2016, 503 . 505 Authors' Addresses 507 Ketan Talaulikar 508 Cisco Systems, Inc. 509 India 511 Email: ketant@cisco.com 513 Peter Psenak 514 Cisco Systems, Inc. 515 Apollo Business Center 516 Mlynske nivy 43 517 Bratislava 821 09 518 Slovakia 520 Email: ppsenak@cisco.com 521 Hugh Johnston 522 AT&T Labs 523 USA 525 Email: hugh_johnston@labs.att.com