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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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-03) exists of draft-pmohapat-idr-fast-conn-restore-02 Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Pradosh Mohapatra 3 Internet Draft Rex Fernando 4 Intended Status: Proposed Standard Eric C. Rosen 5 Expires: May 27, 2013 Cisco Systems, Inc. 7 James Uttaro 8 ATT 10 November 27, 2012 12 The Accumulated IGP Metric Attribute for BGP 14 draft-ietf-idr-aigp-09.txt 16 Abstract 18 Routing protocols that have been designed to run within a single 19 administrative domain ("IGPs") generally do so by assigning a metric 20 to each link, and then choosing as the installed path between two 21 nodes the path for which the total distance (sum of the metric of 22 each link along the path) is minimized. BGP, designed to provide 23 routing over a large number of independent administrative domains 24 ("autonomous systems"), does not make its path selection decisions 25 through the use of a metric. It is generally recognized that any 26 attempt to do so would incur significant scalability problems, as 27 well as inter-administration coordination problems. However, there 28 are deployments in which a single administration runs several 29 contiguous BGP networks. In such cases, it can be desirable, within 30 that single administrative domain, for BGP to select paths based on a 31 metric, just as an IGP would do. The purpose of this document is to 32 provide a specification for doing so. 34 Status of this Memo 36 This Internet-Draft is submitted to IETF in full conformance with the 37 provisions of BCP 78 and BCP 79. 39 Internet-Drafts are working documents of the Internet Engineering 40 Task Force (IETF), its areas, and its working groups. Note that 41 other groups may also distribute working documents as Internet- 42 Drafts. 44 Internet-Drafts are draft documents valid for a maximum of six months 45 and may be updated, replaced, or obsoleted by other documents at any 46 time. It is inappropriate to use Internet-Drafts as reference 47 material or to cite them other than as "work in progress." 49 The list of current Internet-Drafts can be accessed at 50 http://www.ietf.org/ietf/1id-abstracts.txt. 52 The list of Internet-Draft Shadow Directories can be accessed at 53 http://www.ietf.org/shadow.html. 55 Copyright and License Notice 57 Copyright (c) 2012 IETF Trust and the persons identified as the 58 document authors. All rights reserved. 60 This document is subject to BCP 78 and the IETF Trust's Legal 61 Provisions Relating to IETF Documents 62 (http://trustee.ietf.org/license-info) in effect on the date of 63 publication of this document. Please review these documents 64 carefully, as they describe your rights and restrictions with respect 65 to this document. Code Components extracted from this document must 66 include Simplified BSD License text as described in Section 4.e of 67 the Trust Legal Provisions and are provided without warranty as 68 described in the Simplified BSD License. 70 Table of Contents 72 1 Specification of requirements ......................... 3 73 2 Introduction .......................................... 3 74 3 AIGP Attribute ........................................ 5 75 3.1 Applicability Restrictions and Cautions ............... 6 76 3.2 Restrictions on Sending/Receiving ..................... 6 77 3.3 Creating and Modifying the AIGP Attribute ............. 7 78 3.3.1 Originating the AIGP Attribute ........................ 7 79 3.3.2 Modifications by the Originator ....................... 8 80 3.3.3 Modifications by a Non-Originator ..................... 8 81 4 Decision Process ...................................... 10 82 4.1 When a Route has an AIGP Attribute .................... 10 83 4.2 When the Route to the Next Hop has an AIGP attribute .. 11 84 5 Deployment Considerations ............................. 12 85 6 IANA Considerations ................................... 12 86 7 Security Considerations ............................... 12 87 8 Acknowledgments ....................................... 12 88 9 Authors' Addresses .................................... 13 89 10 Normative References .................................. 13 90 11 Informative References ................................ 14 92 1. Specification of requirements 94 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 95 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 96 document are to be interpreted as described in [RFC2119]. 98 2. Introduction 100 There are many routing protocols that have been designed to run 101 within a single administrative domain. These are known collectively 102 as "Interior Gateway Protocols" (IGPs). Typically, each link is 103 assigned a particular "metric" value. The path between two nodes can 104 then be assigned a "distance", which is the sum of the metrics of all 105 the links that belong to that path. An IGP selects the "shortest" 106 (minimal distance) path between any two nodes, perhaps subject to the 107 constraint that if the IGP provides multiple "areas", it may prefer 108 the shortest path within an area to a path that traverses more than 109 one area. Typically the administration of the network has some 110 routing policy which can be approximated by selecting shortest paths 111 in this way. 113 BGP, as distinguished from the IGPs, was designed to run over an 114 arbitrarily large number of administrative domains ("autonomous 115 systems", or "ASes") with limited coordination among the various 116 administrations. BGP does not make its path selection decisions 117 based on a metric; there is no such thing as an "inter-AS metric". 118 There are two fundamental reasons for this: 120 - The distance between two nodes in a common administrative domain 121 may change at any time due to events occurring in that domain. 122 These changes are not propagated around the Internet unless they 123 actually cause the border routers of the domain to select routes 124 with different BGP attributes for some set of address prefixes. 125 This accords with a fundamental principle of scaling, viz., that 126 changes with only local significance must not have global 127 effects. If local changes in distance were always propagated 128 around the Internet, this principle would be violated. 130 - A basic principle of inter-domain routing is that the different 131 administrative domains may have their own policies, which do not 132 have to be revealed to other domains, and which certainly do not 133 have to be agreed to by other domains. Yet the use of inter-AS 134 metric in the Internet would have exactly these effects. 136 There are, however, deployments in which a single administration runs 137 a network which has been sub-divided into multiple, contiguous ASes, 138 each running BGP. There are several reasons why a single 139 administrative domain may be broken into several ASes (which, in this 140 case, are not really "autonomous".) It may be that the existing IGPs 141 do not scale well in the particular environment; it may be that a 142 more generalized topology is desired than could be obtained by use of 143 a single IGP domain; it may be that a more finely grained routing 144 policy is desired than can be supported by an IGP. In such 145 deployments, it can be useful to allow BGP to make its routing 146 decisions based on the IGP metric, so that BGP chooses the "shortest" 147 path between two nodes, even if the nodes are in two different ASes 148 within that same administrative domain. We will refer to the set of 149 ASes in a common administrative domain as an "AIGP Administrative 150 Domain". 152 There are in fact some implementations that already do something like 153 this, using BGP's MULTI_EXIT_DISC (MED) attribute to carry a value 154 based on IGP metrics. However, that doesn't really provide IGP-like 155 "shortest path" routing, as the BGP decision process gives priority 156 to other factors, such as the AS_PATH length. Also, the standard 157 procedures for use of the MED do not ensure that the IGP metric is 158 properly accumulated so that it covers all the links along the path. 160 In this document, we define a new optional, non-transitive BGP 161 attribute, called the "Accumulated IGP Metric Attribute", or "AIGP 162 attribute", and specify the procedures for using it. 164 The specified procedures prevent the AIGP attribute from "leaking 165 out" past an AIGP administrative domain boundary into the Internet. 167 The specified procedures also ensure that the value in the AIGP 168 attribute has been accumulated all along the path from the 169 destination, i.e., that the AIGP attribute does not appear when there 170 are "gaps" along the path where the IGP metric is unknown. 172 3. AIGP Attribute 174 The AIGP Attribute is an optional non-transitive BGP Path Attribute. 175 The attribute type code for the AIGP Attribute is 26. 177 The value field of the AIGP Attribute is defined here to be a set of 178 elements encoded as "Type/Length/Value" (i.e., a set of "TLVs"). 179 Each such TLV is encoded as shown in Figure 1. 181 0 1 2 3 182 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 1 183 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 184 | Type | Length | | 185 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 186 ~ ~ 187 | Value | 188 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+.......................... 190 AIGP TLV 191 Figure 1 193 - Type: A single octet encoding the TLV Type. Only type 1, "AIGP 194 TLV", is defined in this document. 196 - Length: Two octets encoding the length in octets of the TLV, 197 including the type and length fields. The length is encoded as an 198 unsigned binary integer. (Note that the minimum length is 3, 199 indicating that no value field is present.) 201 - A value field containing zero or more octets. 203 This document defines only a single such TLV, the "AIGP TLV". The 204 AIGP TLV is encoded as follows: 206 - Type: 1 208 - Length: 11 210 - Accumulated IGP Metric. 212 The value field of the AIGP TLV is always 8 bytes long. IGP 213 metrics are frequently expressed as 4-octet values, and this 214 ensures that the AIGP attribute can be used to hold the sum of an 215 arbitrary number of 4-octet values. 217 3.1. Applicability Restrictions and Cautions 219 This document only considers the use of the AIGP attribute in 220 networks where each router uses tunneling of some sort to deliver a 221 packet to its BGP next hop. Use of the AIGP attribute in networks 222 that do not use tunneling is outside the scope of this document. 224 If a Route Reflector supports the AIGP attribute, but some of its 225 clients do not, then the routing choices that result may not all 226 reflect the intended routing policy. 228 3.2. Restrictions on Sending/Receiving 230 An implementation that supports the AIGP attribute MUST support a 231 per-session configuration item, AIGP_SESSION, that indicates whether 232 the attribute is enabled or disabled for use on that session. 234 - The default value of AIGP_SESSION, for EBGP sessions, MUST be 235 "disabled". 237 - The default value of AIGP_SESSION, for IBGP and confederation- 238 EBGP sessions, MUST be "enabled." 240 The AIGP attribute MUST NOT be sent on any BGP session for which 241 AIGP_SESSION is disabled. 243 If an AIGP attribute is received on a BGP session for which 244 AIGP_SESSION is disabled, the attribute MUST be treated exactly as if 245 it were an unrecognized non-transitive attribute. That is, "it MUST 246 be quietly ignored and not passed along to other BGP peers" (see 248 [BGP], section 5). 250 3.3. Creating and Modifying the AIGP Attribute 252 3.3.1. Originating the AIGP Attribute 254 An implementation that supports the AIGP attribute MUST support a 255 configuration item, AIGP_ORIGINATE, that enables or disables its 256 creation and attachment to routes. The default value of 257 AIGP_ORIGINATE MUST be "disabled". 259 A BGP speaker MUST NOT add the AIGP attribute to any route whose path 260 leads outside the "AIGP administrative domain" to which the BGP 261 speaker belongs. It may be added only to routes that satisfy one of 262 the following conditions: 264 - The route is a static route that is being redistributed into BGP 266 - The route is an IGP route that is being redistributed into BGP 268 - The route is an IBGP-learned route whose AS_PATH attribute is 269 empty. 271 - The route is an EBGP-learned route whose AS_PATH contains only 272 ASes that are in the same AIGP Administrative Domain as the BGP 273 speaker. 275 A BGP speaker R MUST NOT add the AIGP attribute to any route for 276 which R does not set itself as the next hop. 278 It SHOULD be possible to set AIGP_ORIGINATE to "enabled for the 279 routes of a particular IGP that are redistributed into BGP" (where "a 280 particular IGP" might be "OSPF" or "ISIS"). Other policies 281 determining when and whether to originate an AIGP attribute are also 282 possible, depending on the needs of a particular deployment scenario. 284 When originating an AIGP attribute for a BGP route to address prefix 285 P, the value of the attribute is set according to policy. There are 286 a number of useful policies, some of which are in the following list: 288 - When a BGP speaker R is redistributing into BGP an IGP route to 289 address prefix P, the IGP will have computed a "distance" from R 290 to P. This distance MAY be assigned as the value of AIGP 291 attribute. 293 - A BGP speaker R may be redistributing into BGP a static route to 294 address prefix P, for which a "distance" from R to P has been 295 configured. This distance MAY be assigned as the value of AIGP 296 attribute. 298 - A BGP speaker R may have received and installed a BGP-learned 299 route to prefix P, with next hop N. Or it may be redistributing 300 a static route to P, with next hop N. Then: 302 * If R has an IGP route to N, the IGP-computed distance from R 303 to N MAY be used as the AIGP attribute value of the route to 304 P. 306 * If R has a BGP route to N, and an AIGP attribute value has 307 been computed for that route (see section 3.3.3), that value 308 MAY be used as the AIGP attribute value of the route to P. 310 3.3.2. Modifications by the Originator 312 If BGP speaker R is the originator of the AIGP attribute of prefix P, 313 and at some point the "distance" from R to P changes, R SHOULD issue 314 a new BGP update containing the new value of the AIGP attribute. 315 (Here we use the term "distance" to refer to whatever value the 316 originator assigns to the AIGP attribute, however it is computed; see 317 section 3.3.1.) However, if the difference between the new distance 318 and the distance advertised in the AIGP attribute is less than a 319 configurable threshold, the update MAY be suppressed. 321 3.3.3. Modifications by a Non-Originator 323 Suppose a BGP speaker R1 receives a route with an AIGP attribute 324 whose value is A, and a Next Hop whose value is R2. Suppose also 325 that R1 is about to redistribute that route on a BGP session that is 326 enabled for sending/receiving the attribute. 328 If R1 does not change the Next Hop of the route, then R1 MUST NOT 329 change the AIGP attribute value of the route. 331 If R1 changes the Next Hop of the route from R2 to R1, and if R1's 332 route to R2 is an IGP-learned route, or a static route that does not 333 require recursive next hop resolution, then R1 must increase the 334 value of the AIGP attribute by adding to A the distance from R1 to 335 R2. This distance is either the IGP-computed distance from R1 to R2, 336 or some value determined by policy. However, A MUST be increased by 337 a non-zero amount. 339 Note that if R1 and R2 above are EBGP neighbors, and there is a 340 direct link between them on which no IGP is running, then when R1 341 changes the next hop of a route from R2 to R1, the AIGP metric value 342 MUST be increased by a non-zero amount. The amount of the increase 343 SHOULD be such that it is properly comparable to the IGP metrics. 344 E.g., if the IGP metric is a function of latency, then the amount of 345 the increase should be a function of the latency from R1 to R2. 347 If R1 changes the Next Hop of the route from R2 to R1, and if R1's 348 route to R2 is a BGP-learned route, or a static route that requires 349 recursive next hop resolution, then the AIGP attribute value needs to 350 be increased in several steps, according to the following procedure. 351 (Note that this procedure is ONLY used when recursive next hop 352 resolution is needed.) 354 1. Let Xattr be the new AIGP attribute value. 356 2. Initialize Xattr to A. 358 3. Set the XNH to R2. 360 4. Find the route to XNH. 362 5. If the route to XNH does not require recursive next hop 363 resolution, get the distance D from R1 to XNH. (Note that this 364 condition cannot be satisfied the first time through this 365 procedure.) If D is above a configurable threshold, set the 366 AIGP attribute value to Xattr+D. If D is below a configurable 367 threshold, set the AIGP attribute value to Xattr. In either 368 case, exit this procedure. 370 6. If the route to XNH is a BGP-learned route, and the route does 371 NOT have an AIGP attribute, then exit this procedure and do not 372 pass on any AIGP attribute. 374 7. If the route to XNH is a BGP-learned route, and the route has 375 an AIGP attribute value of Y, then set Xattr=Xattr+Y, and set 376 XNH to the next hop of this route. (The intention here is that 377 Y is the AIGP value of the route as it was received by R1, 378 without having been modified by R1.) 380 8. Go to step 4. 382 The AIGP value of a given route depends on (a) the AIGP values of all 383 the next hops that are recursively resolved during this procedure, 384 and (b) the IGP distance to any next hop that is not recursively 385 resolved. Any change due to (a) in any of these values MUST trigger 386 a new AIGP computation for that route. Whether a change due to (b) 387 triggers a new AIGP computation depends upon whether the change in 388 IGP distance exceeds a configurable threshold. 390 If the AIGP attribute is carried across several ASes, each with its 391 own IGP domain, it is clear that these procedures are unlikely to 392 give a sensible result if the IGPs are different (e.g., some OSPF and 393 some IS-IS), or if the meaning of the metrics is different in the 394 different IGPs (e.g., if the metric represents bandwidth in some IGP 395 domains but represents latency in others). These procedures also are 396 unlikely to give a sensible result if the metric assigned to inter-AS 397 BGP links (on which no IGP is running) or to static routes is not 398 comparable to the IGP metrics. All such cases are outside the scope 399 of the current document. 401 4. Decision Process 403 Support for the AIGP attribute involves several modifications to the 404 tie breaking procedures of the BGP "phase 2" decision described in 405 [BGP], section 9.1.2.2. These modifications are described below in 406 sections 4.1 and 4.2. 408 In some cases, the BGP decision process may install a route without 409 executing any tie breaking procedures. This may happen, e.g., if 410 only one route to a given prefix has the highest degree of preference 411 (as defined in [BGP] section 9.1.1). In this case, the AIGP 412 attribute is not considered. 414 In other cases, some routes may be eliminated before the tie breaking 415 procedures are invoked, e.g., routes with AS-PATH attributes 416 indicating a loop, or routes with unresolvable next hops. In these 417 cases, the AIGP attributes of the eliminated routes are not 418 considered. 420 4.1. When a Route has an AIGP Attribute 422 Assuming that the BGP decision process invokes the tie breaking 423 procedures, the procedures in this section MUST be executed BEFORE 424 any of the tie breaking procedures described in [BGP] section 9.1.2.2 425 are executed. 427 If any routes have an AIGP attribute, remove from consideration all 428 routes that do not have an AIGP attribute. 430 If router R is considering route T, where T has an AIGP attribute, 431 - then R must compute the value A, defined as follows: set A to the 432 sum of (a) T's AIGP attribute value and (b) the IGP distance from 433 R to T's next hop. 435 - remove from consideration all routes that are not tied for the 436 lowest value of A. 438 4.2. When the Route to the Next Hop has an AIGP attribute 440 Suppose that a given router R1 is comparing two BGP-learned routes, 441 such that either: 443 - the two routes have equal AIGP attribute values, or else 445 - neither of the two routes has an AIGP attribute. 447 The BGP decision process as specified in [BGP] makes use, in its tie 448 breaker procedures, of "interior cost", defined as follows: 450 "interior cost of a route is determined by calculating the metric 451 to the NEXT_HOP for the route using the Routing Table." 453 Suppose route T has a next hop of N. We modify the notion of the 454 "interior cost" from node R1 to node N as follows: 456 - Let R2 be the BGP next hop of the route to N, after all recursive 457 resolution of the next hop is done. Let m be the IGP distance 458 (or in the case of a static route, the configured distance) from 459 R1 to R2. 461 - If the installed route to N has an AIGP attribute, set A to the 462 AIGP value of the route to N, computing the AIGP value of the 463 route according to the procedure of section 3.3.3. 465 - If the installed route to N does not have an AIGP value, set A to 466 0. 468 - The "interior cost" of route T is the quantity A+m. 470 5. Deployment Considerations 472 Using the AIGP attribute to achieve a desired routing policy will be 473 more effective if each BGP speaker can use it to choose from among 474 multiple routes. Thus is it highly recommended that the procedures of 475 [BESTEXT] and [ADDPATH] be used in conjunction with the AIGP 476 Attribute. 478 If a Route Reflector does not pass all paths to its clients, then it 479 will tend to pass the paths for which the IGP distance from the Route 480 Reflector itself to the next hop is smallest. This may result in a 481 non-optimal choice by the clients. 483 6. IANA Considerations 485 IANA has assigned the codepoint 26 in the "BGP Path Attributes" 486 registry to the AIGP attribute. 488 IANA shall create a registry for "BGP AIGP Attribute Types". The 489 type field consists of a single octet, with possible values from 0 to 490 255. The allocation policy for this field is to be "Standards Action 491 with Early Allocation". Type 1 should be defined as "AIGP", and 492 should refer to this document. 494 7. Security Considerations 496 The spurious introduction, though error or malfeasance, of an AIGP 497 attribute, could result in the selection of paths other than those 498 desired. 500 Improper configuration on both ends of an EBGP connection could 501 result in an AIGP attribute being passed from one service provider to 502 another. This would likely result in an unsound selection of paths. 504 8. Acknowledgments 506 The authors would like to thank Waqas Alam, Rajiv Asati, Clarence 507 Filsfils, Robert Raszuk, Yakov Rekhter, Eric Rosenberg, Samir Saad, 508 John Scudder, and Shyam Sethuram for their input. 510 9. Authors' Addresses 512 Rex Fernando 513 Cisco Systems, Inc. 514 170 Tasman Drive 515 San Jose, CA 95134 516 Email: rex@cisco.com 518 Pradosh Mohapatra 519 Cisco Systems, Inc. 520 170 Tasman Drive 521 San Jose, CA 95134 522 Email: pmohapat@cisco.com 524 Eric C. Rosen 525 Cisco Systems, Inc. 526 1414 Massachusetts Avenue 527 Boxborough, MA, 01719 528 Email: erosen@cisco.com 530 James Uttaro 531 AT&T 532 200 S. Laurel Avenue 533 Middletown, NJ 07748 534 Email: uttaro@att.com 536 10. Normative References 538 [BGP], "A Border Gateway Protocol 4 (BGP-4)", Y. Rekhter, T. Li, S. 539 Hares, RFC 4271, January 2006. 541 11. Informative References 543 [ADDPATH] "Fast Connectivity Restoration Using BGP Add-Path", P. 544 Mohapatra, R. Fernando, C. Filsfils, R. Raszuk, draft-pmohapat-idr- 545 fast-conn-restore-02.txt, October 2011. 547 [BESTEXT], "Advertisement of the Best External Route in BGP", P. 548 Marques, R. Fernando, E. Chen, P. Mohapatra, H. Gredler, draft-ietf- 549 idr-best-external-05.txt, January 2012. 551 [RFC2119] "Key words for use in RFCs to Indicate Requirement 552 Levels.", S. Bradner, March 1997.