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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IDR Working Group E. Jasinska 3 Internet-Draft Microsoft Corporation 4 Intended status: Standards Track N. Hilliard 5 Expires: March 02, 2014 INEX 6 R. Raszuk 7 NTT MCL Inc. 8 N. Bakker 9 AMS-IX B.V. 10 August 29, 2013 12 Internet Exchange Route Server 13 draft-ietf-idr-ix-bgp-route-server-03 15 Abstract 17 This document outlines a specification for multilateral 18 interconnections at Internet exchange points (IXPs). Multilateral 19 interconnection is a method of exchanging routing information between 20 three or more exterior BGP speakers using a single intermediate 21 broker system, referred to as a route server. Route servers are 22 typically used on shared access media networks, such as Internet 23 exchange points (IXPs), to facilitate simplified interconnection 24 between multiple Internet routers. 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at http://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on March 02, 2014. 43 Copyright Notice 45 Copyright (c) 2013 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (http://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction to Multilateral Interconnection . . . . . . . . 2 61 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 62 2. Technical Considerations for Route Server Implementations . . 3 63 2.1. Client UPDATE Messages . . . . . . . . . . . . . . . . . 3 64 2.2. Attribute Transparency . . . . . . . . . . . . . . . . . 4 65 2.2.1. NEXT_HOP Attribute . . . . . . . . . . . . . . . . . 4 66 2.2.2. AS_PATH Attribute . . . . . . . . . . . . . . . . . . 4 67 2.2.3. MULTI_EXIT_DISC Attribute . . . . . . . . . . . . . . 4 68 2.2.4. Communities Attributes . . . . . . . . . . . . . . . 5 69 2.3. Per-Client Policy Control in Multilateral Interconnection 5 70 2.3.1. Path Hiding on a Route Server . . . . . . . . . . . . 5 71 2.3.2. Mitigation of Path Hiding . . . . . . . . . . . . . . 6 72 2.3.2.1. Multiple Route Server RIBs . . . . . . . . . . . 6 73 2.3.2.2. Advertising Multiple Paths . . . . . . . . . . . 7 74 2.3.3. Implementation Recommendations . . . . . . . . . . . 8 75 3. Security Considerations . . . . . . . . . . . . . . . . . . . 8 76 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 77 5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 78 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 79 6.1. Normative References . . . . . . . . . . . . . . . . . . 9 80 6.2. Informative References . . . . . . . . . . . . . . . . . 9 81 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 83 1. Introduction to Multilateral Interconnection 85 Internet exchange points (IXPs) provide IP data interconnection 86 facilities for their participants, typically using shared Layer-2 87 networking media such as Ethernet. The Border Gateway Protocol (BGP) 88 [RFC4271], an inter-Autonomous System routing protocol, is commonly 89 used to facilitate exchange of network reachability information over 90 such media. 92 While bilateral exterior BGP sessions between exchange participants 93 were previously the most common means of exchanging reachability 94 information, the overhead associated with dense interconnection has 95 caused substantial operational scaling problems for Internet exchange 96 point participants. 98 Multilateral interconnection is a method of interconnecting BGP 99 speaking routers using a third party brokering system, commonly 100 referred to as a route server and typically managed by the IXP 101 operator. Each of the multilateral interconnection participants 102 (usually referred to as route server clients) announces network 103 reachability information to the route server using exterior BGP, and 104 the route server in turn forwards this information to each other 105 route server client connected to it, according to its configuration. 106 Although a route server uses BGP to exchange reachability information 107 with each of its clients, it does not forward traffic itself and is 108 therefore not a router. 110 A route server can be viewed as similar in function to an [RFC4456] 111 route reflector, except that it operates using EBGP instead of iBGP. 112 Certain adaptions to [RFC4271] are required to enable an EBGP router 113 to operate as a route server; these are outlined in Section 2 of this 114 document. 116 The term "route server" is often in a different context used to 117 describe a BGP node whose purpose is to accept BGP feeds from 118 multiple clients for the purpose of operational analysis and 119 troubleshooting. A system of this form may alternatively be known as 120 a "route collector" or a "route-views server". This document uses 121 the term "route server" exclusively to describe multilateral peering 122 brokerage systems. 124 1.1. Notational Conventions 126 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 127 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 128 "OPTIONAL" in this document are to be interpreted as described in 129 [RFC2119]. 131 2. Technical Considerations for Route Server Implementations 133 2.1. Client UPDATE Messages 135 A route server MUST accept all UPDATE messages received from each of 136 its clients for inclusion in its Adj-RIB-In. These UPDATE messages 137 MAY be omitted from the route server's Loc-RIB or Loc-RIBs, due to 138 filters configured for the purposes of implementing routing policy. 139 The route server SHOULD perform one or more BGP Decision Processes to 140 select routes for subsequent advertisement to its clients, taking 141 into account possible configuration to provide multiple NLRI paths to 142 a particular client as described in Section 2.3.2.2 or multiple Loc- 143 RIBs as described in Section 2.3.2.1. The route server SHOULD 144 forward UPDATE messages where appropriate from its Loc-RIB or Loc- 145 RIBs to its clients. 147 2.2. Attribute Transparency 149 As a route server primarily performs a brokering service, 150 modification of attributes could cause route server clients to alter 151 their BGP best path selection process for received prefix 152 reachability information, thereby changing the intended routing 153 policies of exchange participants. Therefore, contrary to what is 154 specified in section 5. of [RFC4271], route servers SHOULD NOT by 155 default (unless explicitly configured) update well-known BGP 156 attributes received from route server clients before redistributing 157 them to their other route server clients. Optional recognized and 158 unrecognized BGP attributes, whether transitive or non-transitive, 159 SHOULD NOT be updated by the route server (unless enforced by local 160 IX operator configuration) and SHOULD be passed on to other route 161 server clients. 163 2.2.1. NEXT_HOP Attribute 165 The NEXT_HOP is a well-known mandatory BGP attribute which defines 166 the IP address of the router used as the next hop to the destinations 167 listed in the Network Layer Reachability Information field of the 168 UPDATE message. As the route server does not participate in the 169 actual routing of traffic, the NEXT_HOP attribute MUST be passed 170 unmodified to the route server clients, similar to the "third party" 171 next hop feature described in section 5.1.3. of [RFC4271]. 173 2.2.2. AS_PATH Attribute 175 AS_PATH is a well-known mandatory attribute which identifies the 176 autonomous systems through which routing information carried in the 177 UPDATE message has passed. 179 As a route server does not participate in the process of forwarding 180 data between client routers, and because modification of the AS_PATH 181 attribute could affect route server client best path calculations, 182 the route server SHOULD NOT prepend its own AS number to the AS_PATH 183 segment nor modify the AS_PATH segment in any other way. 185 2.2.3. MULTI_EXIT_DISC Attribute 186 MULTI_EXIT_DISC is an optional non-transitive attribute intended to 187 be used on external (inter-AS) links to discriminate among multiple 188 exit or entry points to the same neighboring AS. Contrary to section 189 5.1.4 of [RFC4271], if applied to an NLRI UPDATE sent to a route 190 server, this attribute SHOULD be propagated to other route server 191 clients and the route server SHOULD NOT modify its value. 193 2.2.4. Communities Attributes 195 The BGP COMMUNITIES ([RFC1997]) and Extended Communities ([RFC4360]) 196 attributes are attributes intended for labeling information carried 197 in BGP UPDATE messages. Transitive as well as non-transitive 198 Communities attributes applied to an NLRI UPDATE sent to a route 199 server SHOULD NOT be modified, processed or removed. However, if 200 such an attribute is intended for processing by the route server 201 itself, it MAY be modified or removed. 203 2.3. Per-Client Policy Control in Multilateral Interconnection 205 While IXP participants often use route servers with the intention of 206 interconnecting with as many other route server participants as 207 possible, there are circumstances where control of path distribution 208 on a per-client basis is important to ensure that desired 209 interconnection policies are met. 211 The control of path distribution on a per-client basis can lead to a 212 path being hidden from the route server client. We refer to this as 213 "path hiding". 215 2.3.1. Path Hiding on a Route Server 217 ___ ___ 218 / \ / \ 219 ..| AS1 |..| AS2 |.. 220 : \___/ \___/ : 221 : \ / | : 222 : \ / | : 223 : IXP \/ | : 224 : /\ | : 225 : / \ | : 226 : ___/____\_|_ : 227 : / \ / \ : 228 ..| AS3 |..| AS4 |.. 229 \___/ \___/ 231 Figure 1: Per-Client Policy Controlled Interconnection at an IXP 233 Using the example in Figure 1, AS1 does not directly exchange prefix 234 information with either AS2 or AS3 at the IXP, but only interconnects 235 with AS4. 237 In the traditional bilateral interconnection model, per-client policy 238 control to a third party exchange participant is accomplished either 239 by not engaging in a bilateral interconnection with that participant 240 or else by implementing outbound filtering on the BGP session towards 241 that participant. However, in a multilateral interconnection 242 environment, only the route server can perform outbound filtering in 243 the direction of the route server client; route server clients depend 244 on the route server to perform their outbound filtering for them. 246 Assuming a traditional best path selection, when the same prefix is 247 advertised to a route server from multiple route server clients, the 248 route server will select a single best path for propagation to all 249 connected clients. If, however, the route server has been configured 250 to filter the calculated best path from reaching a particular route 251 server client, then that client will not receive a path for that 252 prefix, although alternate paths received by the route server might 253 have been policy compliant for that client. This phenomenon is 254 referred to as "path hiding". 256 For example, in Figure 1, if the same prefix were sent to the route 257 server via AS2 and AS4, and the route via AS2 was preferred according 258 to BGP's traditional best path selection, but AS1's policy prevents 259 AS2's path from being accepted, then AS1 would never receive a path 260 to this prefix, even though the route server had previously received 261 a valid alternative path via AS4. This happens because the best path 262 selection is performed only once on the route server for all clients. 264 Path hiding will only occur on route servers which employ per-client 265 policy control; if an IXP operator deploys a route server without 266 implementing a per-client routing policy control system, then path 267 hiding does not occur as all paths are considered equally valid from 268 the point of view of the route server. 270 2.3.2. Mitigation of Path Hiding 272 There are several approaches which can be taken to mitigate against 273 path hiding. 275 2.3.2.1. Multiple Route Server RIBs 277 The most portable method to allow for per-client policy control 278 without the occurrence of path hiding, is by using a route server BGP 279 implementation which performs the per-client best path calculation 280 for each set of paths to a prefix, which results after the route 281 server's client policies have been taken into consideration. This 282 can be implemented by using per-client Loc-RIBs, with path filtering 283 implemented between the Adj-RIB-In and the per-client Loc-RIB. 284 Implementations MAY optimize this by maintaining paths not subject to 285 filtering policies in a global Loc-RIB, with per-client Loc-RIBs 286 stored as deltas. 288 This implementation is highly portable, as it makes no assumptions 289 about the feature capabilities of the route server clients. 291 2.3.2.2. Advertising Multiple Paths 293 The path distribution model described above assumes standard BGP 294 session encoding where the route server sends a single path to its 295 client for any given prefix. This path is selected using the BGP 296 path selection decision process described in [RFC4271]. If, however, 297 it were possible for the route server to send more than a single path 298 to a route server client, then route server clients would no longer 299 depend on receiving a single best path to a particular prefix; 300 consequently, the path hiding problem described in Section 2.3.1 301 would disappear. 303 We present two methods which describe how such increased path 304 diversity could be implemented. 306 2.3.2.2.1. Diverse BGP Path Approach 308 The Diverse BGP Path proposal as defined in 309 [I-D.ietf-grow-diverse-bgp-path-dist] is a simple way to distribute 310 multiple prefix paths from a route server to a route server client by 311 using a separate BGP session from the route server to a client for 312 each different path. 314 The number of paths which may be distributed to a client is 315 constrained by the number of BGP sessions which the server and the 316 client are willing to establish with each other. The distributed 317 paths may be established from the global BGP Loc-RIB on the route 318 server in addition to any per-client Loc-RIB. As there may be more 319 potential paths to a given prefix than configured BGP sessions, this 320 method is not guaranteed to eliminate the path hiding problem in all 321 situations. Furthermore, this method may significantly increase the 322 number of BGP sessions handled by the route server, which may 323 negatively impact its performance. 325 2.3.2.2.2. BGP ADD-PATH Approach 327 The [I-D.ietf-idr-add-paths] Internet draft proposes a different 328 approach to multiple path propagation, by allowing a BGP speaker to 329 forward multiple paths for the same prefix on a single BGP session. 330 As [RFC4271] specifies that a BGP listener must implement an implicit 331 withdraw when it receives an UPDATE message for a prefix which 332 already exists in its Adj-RIB-In, this approach requires explicit 333 support for the feature both on the route server and on its clients. 335 If the ADD-PATH capability is negotiated bidirectionally between the 336 route server and a route server client, and the route server client 337 propagates multiple paths for the same prefix to the route server, 338 then this could potentially cause the propagation of inactive, 339 invalid or suboptimal paths to the route server, thereby causing loss 340 of reachability to other route server clients. For this reason, ADD- 341 PATH implementations on a route server SHOULD enforce send-only mode 342 with the route server clients, which would result in negotiating 343 receive-only mode from the client to the route server. 345 2.3.3. Implementation Recommendations 347 A route server SHOULD implement one of the methods described in 348 Section 2.3.2 to allow per-client routing policy control without 349 "path hiding". 351 3. Security Considerations 353 The path hiding problem outlined in section Section 2.3.1 can be used 354 in certain circumstances to proactively block third party path 355 announcements from other route server clients. Route server 356 operators should be aware that security issues may arise unless steps 357 are taken to mitigate against path hiding. 359 4. IANA Considerations 361 The new set of mechanisms for route servers does not require any new 362 allocations from IANA. 364 5. Acknowledgments 366 The authors would like to thank Ryan Bickhart, Steven Bakker, Martin 367 Pels, Chris Hall, Aleksi Suhonen, Bruno Decraene, Pierre Francois and 368 Eduardo Ascenco Reis for their valuable input. 370 In addition, the authors would like to acknowledge the developers of 371 BIRD, OpenBGPD and Quagga, whose open source BGP implementations 372 include route server capabilities which are compliant with this 373 document. 375 6. References 376 6.1. Normative References 378 [RFC1997] Chandrasekeran, R., Traina, P., and T. Li, "BGP 379 Communities Attribute", RFC 1997, August 1996. 381 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 382 Requirement Levels", BCP 14, RFC 2119, March 1997. 384 [RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway 385 Protocol 4 (BGP-4)", RFC 4271, January 2006. 387 [RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended 388 Communities Attribute", RFC 4360, February 2006. 390 6.2. Informative References 392 [I-D.ietf-grow-diverse-bgp-path-dist] 393 Raszuk, R., Fernando, R., Patel, K., McPherson, D., and K. 394 Kumaki, "Distribution of diverse BGP paths.", draft-ietf- 395 grow-diverse-bgp-path-dist-08 (work in progress), July 396 2012. 398 [I-D.ietf-idr-add-paths] 399 Walton, D., Retana, A., Chen, E., and J. Scudder, 400 "Advertisement of Multiple Paths in BGP", draft-ietf-idr- 401 add-paths-08 (work in progress), December 2012. 403 [RFC1863] Haskin, D., "A BGP/IDRP Route Server alternative to a full 404 mesh routing", RFC 1863, October 1995. 406 [RFC4223] Savola, P., "Reclassification of RFC 1863 to Historic", 407 RFC 4223, October 2005. 409 [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route 410 Reflection: An Alternative to Full Mesh Internal BGP 411 (IBGP)", RFC 4456, April 2006. 413 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 414 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 415 May 2008. 417 Authors' Addresses 418 Elisa Jasinska 419 Microsoft Corporation 420 One Microsoft Way 421 Redmond, WA 98052 422 US 424 Email: ejas@microsoft.com 426 Nick Hilliard 427 INEX 428 4027 Kingswood Road 429 Dublin 24 430 IE 432 Email: nick@inex.ie 434 Robert Raszuk 435 NTT MCL Inc. 436 101 S Ellsworth Avenue Suite 350 437 San Mateo, CA 94401 438 US 440 Email: robert@raszuk.net 442 Niels Bakker 443 AMS-IX B.V. 444 Westeinde 12 445 Amsterdam, NH 1017 ZN 446 NL 448 Email: niels.bakker@ams-ix.net