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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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-15) exists of draft-ietf-idr-add-paths-13 -- Obsolete informational reference (is this intentional?): RFC 1863 (Obsoleted by RFC 4223) Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IDR Working Group E. Jasinska 3 Internet-Draft BigWave IT 4 Intended status: Standards Track N. Hilliard 5 Expires: October 31, 2016 INEX 6 R. Raszuk 7 Bloomberg LP 8 N. Bakker 9 Akamai Technologies B.V. 10 April 29, 2016 12 Internet Exchange BGP Route Server 13 draft-ietf-idr-ix-bgp-route-server-10 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 external 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 October 31, 2016. 43 Copyright Notice 45 Copyright (c) 2016 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 . . . . . . . . . . . . . . 5 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 . . . . . . . . . . . . . . 7 72 2.3.2.1. Multiple Route Server RIBs . . . . . . . . . . . 7 73 2.3.2.2. Advertising Multiple Paths . . . . . . . . . . . 7 74 2.3.3. Implementation Suggestions . . . . . . . . . . . . . 8 75 3. Security Considerations . . . . . . . . . . . . . . . . . . . 8 76 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 77 5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 78 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 79 6.1. Normative References . . . . . . . . . . . . . . . . . . 9 80 6.2. Informative References . . . . . . . . . . . . . . . . . 10 81 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 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 external BGP sessions between exchange participants 93 were previously the most common means of exchanging reachability 94 information, the overhead associated with dense interconnection can 95 cause substantial operational scaling problems for partipants of 96 larger Internet exchange points. 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 external 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. Route server functionality is not mandatory in BGP 115 implementations.`` 117 The term "route server" is often in a different context used to 118 describe a BGP node whose purpose is to accept BGP feeds from 119 multiple clients for the purpose of operational analysis and 120 troubleshooting. A system of this form may alternatively be known as 121 a "route collector" or a "route-views server". This document uses 122 the term "route server" exclusively to describe multilateral peering 123 brokerage systems. 125 1.1. Notational Conventions 127 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 128 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 129 "OPTIONAL" in this document are to be interpreted as described in 130 [RFC2119]. 132 2. Technical Considerations for Route Server Implementations 134 2.1. Client UPDATE Messages 136 A route server MUST accept all UPDATE messages received from each of 137 its clients for inclusion in its Adj-RIB-In. These UPDATE messages 138 MAY be omitted from the route server's Loc-RIB or Loc-RIBs, due to 139 filters configured for the purposes of implementing routing policy. 140 The route server SHOULD perform one or more BGP Decision Processes to 141 select routes for subsequent advertisement to its clients, taking 142 into account possible configuration to provide multiple NLRI paths to 143 a particular client as described in Section 2.3.2.2 or multiple Loc- 144 RIBs as described in Section 2.3.2.1. The route server SHOULD 145 forward UPDATE messages from its Loc-RIB or Loc-RIBs to its clients 146 as determined by local policy. 148 2.2. Attribute Transparency 150 As a route server primarily performs a brokering service, 151 modification of attributes could cause route server clients to alter 152 their BGP Decision Process for received prefix reachability 153 information, thereby changing the intended routing policies of 154 exchange participants. Therefore, contrary to what is specified in 155 section 5. of [RFC4271], route servers SHOULD NOT by default (unless 156 explicitly configured) update well-known BGP attributes received from 157 route server clients before redistributing them to their other route 158 server clients. Optional recognized and unrecognized BGP attributes, 159 whether transitive or non-transitive, SHOULD NOT be updated by the 160 route server (unless enforced by local IXP operator configuration) 161 and SHOULD be passed on to other route 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 BGP Decision Process, the 182 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. This 184 differs from the behaviour specified in section 5.1.2 of [RFC4271], 185 which requires that the BGP speaker prepends its own AS number as the 186 last element of the AS_PATH segment. 188 In contrast to what is recommended in section 6.3 of [RFC4271], route 189 server clients need to be able to accept UPDATE messages where the 190 leftmost AS in the AS_PATH attribute is not equal to the AS number of 191 the route server that sent the UPDATE message. If the route server 192 client BGP system has implemented a check for this, the BGP 193 implementation MUST allow this check to be disabled and SHOULD allow 194 the check to be disabled on a per-peer basis. 196 2.2.3. MULTI_EXIT_DISC Attribute 198 MULTI_EXIT_DISC is an optional non-transitive attribute intended to 199 be used on external (inter-AS) links to discriminate among multiple 200 exit or entry points to the same neighboring AS. Contrary to section 201 5.1.4 of [RFC4271], if applied to an NLRI UPDATE sent to a route 202 server, this attribute SHOULD be propagated to other route server 203 clients and the route server SHOULD NOT modify its value. 205 2.2.4. Communities Attributes 207 The BGP COMMUNITIES ([RFC1997]) and Extended Communities ([RFC4360]) 208 attributes are attributes intended for labeling information carried 209 in BGP UPDATE messages. Transitive as well as non-transitive 210 Communities attributes applied to an NLRI UPDATE sent to a route 211 server SHOULD NOT be modified, processed or removed, except as 212 defined by local policy. If a Communities Attribute is intended for 213 processing by the route server itself, as determined by local policy, 214 it MAY be modified or removed. 216 2.3. Per-Client Policy Control in Multilateral Interconnection 218 While IXP participants often use route servers with the intention of 219 interconnecting with as many other route server participants as 220 possible, there are circumstances where control of path distribution 221 on a per-client basis is important to ensure that desired 222 interconnection policies are met. 224 The control of path distribution on a per-client basis can lead to a 225 path being hidden from the route server client. We refer to this as 226 "path hiding". 228 Neither Section 2.3 nor its subsections form part of the normative 229 specification of this document, but are included for information 230 purposes only. 232 2.3.1. Path Hiding on a Route Server 233 ___ ___ 234 / \ / \ 235 ..| AS1 |..| AS2 |.. 236 : \___/ \___/ : 237 : \ / | : 238 : \ / | : 239 : IXP \/ | : 240 : /\ | : 241 : / \ | : 242 : ___/____\_|_ : 243 : / \ / \ : 244 ..| AS3 |..| AS4 |.. 245 \___/ \___/ 247 Figure 1: Per-Client Policy Controlled Interconnection at an IXP 249 Using the example in Figure 1, AS1 does not directly exchange prefix 250 information with either AS2 or AS3 at the IXP, but only interconnects 251 with AS4. 253 In the traditional bilateral interconnection model, per-client policy 254 control to a third party exchange participant is accomplished either 255 by not engaging in a bilateral interconnection with that participant 256 or else by implementing outbound filtering on the BGP session towards 257 that participant. However, in a multilateral interconnection 258 environment, only the route server can perform outbound filtering in 259 the direction of the route server client; route server clients depend 260 on the route server to perform their outbound filtering for them. 262 Assuming the [RFC4271] BGP Decision Process is used, when the same 263 prefix is advertised to a route server from multiple route server 264 clients, the route server will select a single path for propagation 265 to all connected clients. If, however, the route server has been 266 configured to filter the calculated best path from reaching a 267 particular route server client, then that client will not receive a 268 path for that prefix, although alternate paths received by the route 269 server might have been policy compliant for that client. This 270 phenomenon is referred to as "path hiding". 272 For example, in Figure 1, if the same prefix were sent to the route 273 server via AS2 and AS4, and the route via AS2 was preferred according 274 to the BGP Decision Process on the route server, but AS2's policy 275 prevented the route server from sending the path to AS1, then AS1 276 would never receive a path to this prefix, even though the route 277 server had previously received a valid alternative path via AS4. 278 This happens because the BGP Decision Process is performed only once 279 on the route server for all clients. 281 Path hiding will only occur on route servers which employ per-client 282 policy control; if an IXP operator deploys a route server without 283 implementing a per-client routing policy control system, then path 284 hiding does not occur as all paths are considered equally valid from 285 the point of view of the route server. 287 2.3.2. Mitigation of Path Hiding 289 There are several approaches which can be taken to mitigate against 290 path hiding. 292 2.3.2.1. Multiple Route Server RIBs 294 The most portable method to allow for per-client policy control 295 without the occurrence of path hiding, is by using a route server BGP 296 implementation which performs the per-client best path calculation 297 for each set of paths to a prefix, which results after the route 298 server's client policies have been taken into consideration. This 299 can be implemented by using per-client Loc-RIBs, with path filtering 300 implemented between the Adj-RIB-In and the per-client Loc-RIB. 301 Implementations can optimize this by maintaining paths not subject to 302 filtering policies in a global Loc-RIB, with per-client Loc-RIBs 303 stored as deltas. 305 This implementation is highly portable, as it makes no assumptions 306 about the feature capabilities of the route server clients. 308 2.3.2.2. Advertising Multiple Paths 310 The path distribution model described above assumes standard BGP 311 session encoding where the route server sends a single path to its 312 client for any given prefix. This path is selected using the BGP 313 path selection decision process described in [RFC4271]. If, however, 314 it were possible for the route server to send more than a single path 315 to a route server client, then route server clients would no longer 316 depend on receiving a single path to a particular prefix; 317 consequently, the path hiding problem described in Section 2.3.1 318 would disappear. 320 We present two methods which describe how such increased path 321 diversity could be implemented. 323 2.3.2.2.1. Diverse BGP Path Approach 325 The Diverse BGP Path proposal as defined in [RFC6774] is a simple way 326 to distribute multiple prefix paths from a route server to a route 327 server client by using a separate BGP session from the route server 328 to a client for each different path. 330 The number of paths which may be distributed to a client is 331 constrained by the number of BGP sessions which the server and the 332 client are willing to establish with each other. The distributed 333 paths may be established from the global BGP Loc-RIB on the route 334 server in addition to any per-client Loc-RIB. As there may be more 335 potential paths to a given prefix than configured BGP sessions, this 336 method is not guaranteed to eliminate the path hiding problem in all 337 situations. Furthermore, this method may significantly increase the 338 number of BGP sessions handled by the route server, which may 339 negatively impact its performance. 341 2.3.2.2.2. BGP ADD-PATH Approach 343 The [I-D.ietf-idr-add-paths] Internet draft proposes a different 344 approach to multiple path propagation, by allowing a BGP speaker to 345 forward multiple paths for the same prefix on a single BGP session. 346 As [RFC4271] specifies that a BGP listener must implement an implicit 347 withdraw when it receives an UPDATE message for a prefix which 348 already exists in its Adj-RIB-In, this approach requires explicit 349 support for the feature both on the route server and on its clients. 351 If the ADD-PATH capability is negotiated bidirectionally between the 352 route server and a route server client, and the route server client 353 propagates multiple paths for the same prefix to the route server, 354 then this could potentially cause the propagation of inactive, 355 invalid or suboptimal paths to the route server, thereby causing loss 356 of reachability to other route server clients. For this reason, ADD- 357 PATH implementations on a route server should enforce send-only mode 358 with the route server clients, which would result in negotiating 359 receive-only mode from the client to the route server. 361 2.3.3. Implementation Suggestions 363 Route server implementation authors may wish to consider one of the 364 methods described in Section 2.3.2 to allow per-client route server 365 policy control without "path hiding". 367 3. Security Considerations 369 The path hiding problem outlined in section Section 2.3.1 can be used 370 in certain circumstances to proactively block third party path 371 announcements from other route server clients. Route server 372 operators should be aware that security issues may arise unless steps 373 are taken to mitigate against path hiding. 375 The AS_PATH check described in Section 2.2.2 is normally enabled in 376 order to check for malformed AS paths. If this check is disabled, 377 the route server client loses the ability to check incoming UPDATE 378 messages for certain categories of problems. This could potentially 379 cause corrupted BGP UPDATE messages to be propagated where they might 380 not be propagated if the check were enabled. Regardless of any 381 problems relating to malformed UPDATE messages, this check is also 382 used to detect BGP loops, so removing the check could potentially 383 cause routing loops to be formed. Consequently, this check SHOULD 384 NOT be disabled by IXP participants unless it is needed to establish 385 BGP sessions with a route server, and if possible should only be 386 disabled for peers which are route servers. 388 Route server operators should carefully consider the security 389 practices discussed in [RFC7454], "BGP Operations and Security". 391 4. IANA Considerations 393 The new set of mechanisms for route servers does not require any new 394 allocations from IANA. 396 5. Acknowledgments 398 The authors would like to thank Ryan Bickhart, Steven Bakker, Martin 399 Pels, Chris Hall, Aleksi Suhonen, Bruno Decraene, Pierre Francois and 400 Eduardo Ascenco Reis for their valuable input. 402 In addition, the authors would like to acknowledge the developers of 403 BIRD, OpenBGPD, Quagga and IOS whose BGP implementations include 404 route server capabilities which are compliant with this document. 406 Route server functionality was described in 1995 in [RFC1863] and 407 modern route server implementations are based on concepts developed 408 in the 1990s by the Routing Arbiter Project and the Route Server Next 409 Generation Project, managed by ISI and Merit. Although the original 410 RSNG code is no longer in use at any IXPs, the IXP community owes a 411 debt of gratitude to the many people who were involved in route 412 server development in the 1990s. Note that [RFC1863] was made 413 historical by [RFC4223]. 415 6. References 417 6.1. Normative References 419 [RFC1997] Chandra, R., Traina, P., and T. Li, "BGP Communities 420 Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996, 421 . 423 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 424 Requirement Levels", BCP 14, RFC 2119, 425 DOI 10.17487/RFC2119, March 1997, 426 . 428 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 429 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 430 DOI 10.17487/RFC4271, January 2006, 431 . 433 [RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended 434 Communities Attribute", RFC 4360, DOI 10.17487/RFC4360, 435 February 2006, . 437 6.2. Informative References 439 [I-D.ietf-idr-add-paths] 440 Walton, D., Retana, A., Chen, E., and J. Scudder, 441 "Advertisement of Multiple Paths in BGP", draft-ietf-idr- 442 add-paths-13 (work in progress), December 2015. 444 [RFC1863] Haskin, D., "A BGP/IDRP Route Server alternative to a full 445 mesh routing", RFC 1863, DOI 10.17487/RFC1863, October 446 1995, . 448 [RFC4223] Savola, P., "Reclassification of RFC 1863 to Historic", 449 RFC 4223, DOI 10.17487/RFC4223, October 2005, 450 . 452 [RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route 453 Reflection: An Alternative to Full Mesh Internal BGP 454 (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006, 455 . 457 [RFC6774] Raszuk, R., Ed., Fernando, R., Patel, K., McPherson, D., 458 and K. Kumaki, "Distribution of Diverse BGP Paths", 459 RFC 6774, DOI 10.17487/RFC6774, November 2012, 460 . 462 [RFC7454] Durand, J., Pepelnjak, I., and G. Doering, "BGP Operations 463 and Security", BCP 194, RFC 7454, DOI 10.17487/RFC7454, 464 February 2015, . 466 Authors' Addresses 467 Elisa Jasinska 468 BigWave IT 469 ul. Skawinska 27/7 470 Krakow, MP 31-066 471 Poland 473 Email: elisa@bigwaveit.org 475 Nick Hilliard 476 INEX 477 4027 Kingswood Road 478 Dublin 24 479 IE 481 Email: nick@inex.ie 483 Robert Raszuk 484 Bloomberg LP 485 731 Lexington Ave 486 New York City, NY 10022 487 USA 489 Email: robert@raszuk.net 491 Niels Bakker 492 Akamai Technologies B.V. 493 Kingsfordweg 151 494 Amsterdam 1043 GR 495 NL 497 Email: nbakker@akamai.com