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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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-08) exists of draft-ietf-idr-add-paths-guidelines-07 == Outdated reference: A later version (-04) exists of draft-ietf-idr-route-oscillation-stop-01 Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group D. Walton 3 Internet-Draft Cumulus Networks 4 Intended status: Standards Track A. Retana 5 Expires: June 13, 2016 E. Chen 6 Cisco Systems, Inc. 7 J. Scudder 8 Juniper Networks 9 December 11, 2015 11 Advertisement of Multiple Paths in BGP 12 draft-ietf-idr-add-paths-13 14 Abstract 16 This document defines a BGP extension that allows the advertisement 17 of multiple paths for the same address prefix without the new paths 18 implicitly replacing any previous ones. The essence of the extension 19 is that each path is identified by a path identifier in addition to 20 the address prefix. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on June 13, 2016. 39 Copyright Notice 41 Copyright (c) 2015 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 57 1.1. Specification of Requirements . . . . . . . . . . . . . . 2 58 2. How to Identify a Path . . . . . . . . . . . . . . . . . . . 3 59 3. Extended NLRI Encodings . . . . . . . . . . . . . . . . . . . 3 60 4. ADD-PATH Capability . . . . . . . . . . . . . . . . . . . . . 3 61 5. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 4 62 6. Applications . . . . . . . . . . . . . . . . . . . . . . . . 5 63 7. Deployment Considerations . . . . . . . . . . . . . . . . . . 5 64 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 65 9. Security Considerations . . . . . . . . . . . . . . . . . . . 6 66 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6 67 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 68 11.1. Normative References . . . . . . . . . . . . . . . . . . 6 69 11.2. Informative References . . . . . . . . . . . . . . . . . 7 70 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 72 1. Introduction 74 The BGP specification [RFC4271] defines an Update-Send Process to 75 advertise the routes chosen by the Decision Process to other BGP 76 speakers. No provisions are made to allow the advertisement of 77 multiple paths for the same address prefix, or Network Layer 78 Reachability Information (NLRI). In fact, a route with the same NLRI 79 as a previously advertised route implicitly replaces the previous 80 advertisement. 82 This document defines a BGP extension that allows the advertisement 83 of multiple paths for the same address prefix without the new paths 84 implicitly replacing any previous ones. The essence of the extension 85 is that each path is identified by a path identifier in addition to 86 the address prefix. 88 1.1. Specification of Requirements 90 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 91 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 92 document are to be interpreted as described in [RFC2119]. 94 2. How to Identify a Path 96 As defined in [RFC4271], a path refers to the information reported in 97 the path attribute field of an UPDATE message. As the procedures 98 specified in [RFC4271] allow only the advertisement of one path for a 99 particular address prefix, a path for an address prefix from a BGP 100 peer can be keyed on the address prefix. 102 In order for a BGP speaker to advertise multiple paths for the same 103 address prefix, a new identifier (termed "Path Identifier" hereafter) 104 needs to be introduced so that a particular path for an address 105 prefix can be identified by the combination of the address prefix and 106 the Path Identifier. 108 The assignment of the Path Identifier for a path by a BGP speaker is 109 purely a local matter. However, the Path Identifier MUST be assigned 110 in such a way that the BGP speaker is able to use the (prefix, path 111 identifier) to uniquely identify a path advertised to a neighbor. A 112 BGP speaker that re-advertises a route MUST generate its own Path 113 Identifier to be associated with the re-advertised route. A BGP 114 speaker that receives a route SHOULD NOT assume that the identifier 115 carries any particular semantics; it SHOULD be treated as an opaque 116 value. 118 3. Extended NLRI Encodings 120 In order to carry the Path Identifier in an UPDATE message, the NLRI 121 encoding MUST be extended by prepending the Path Identifier field, 122 which is of four-octets. 124 For example, the NLRI encoding specified in [RFC4271] is extended as 125 the following: 127 +--------------------------------+ 128 | Path Identifier (4 octets) | 129 +--------------------------------+ 130 | Length (1 octet) | 131 +--------------------------------+ 132 | Prefix (variable) | 133 +--------------------------------+ 135 The usage of the extended NLRI encodings is specified in Section 5. 137 4. ADD-PATH Capability 139 The ADD-PATH Capability is a new BGP capability [RFC5492]. The 140 Capability Code for this capability is specified in the IANA 141 Considerations section of this document. The Capability Length field 142 of this capability is variable. The Capability Value field consists 143 of one or more of the following tuples: 145 +------------------------------------------------+ 146 | Address Family Identifier (2 octets) | 147 +------------------------------------------------+ 148 | Subsequent Address Family Identifier (1 octet) | 149 +------------------------------------------------+ 150 | Send/Receive (1 octet) | 151 +------------------------------------------------+ 153 The meaning and use of the fields are as follows: 155 Address Family Identifier (AFI): 157 This field is the same as the one used in [RFC4760]. 159 Subsequent Address Family Identifier (SAFI): 161 This field is the same as the one used in [RFC4760]. 163 Send/Receive: 165 This field indicates whether the sender is (a) able to receive 166 multiple paths from its peer (value 1), (b) able to send 167 multiple paths to its peer (value 2), or (c) both (value 3) for 168 the . 170 If any other value is received, then the capability SHOULD be 171 treated as not understood and ignored [RFC5492]. 173 A BGP speaker that wishes to indicate support for multiple AFI/SAFI 174 MUST do so by including the information in a single instance of the 175 ADD-PATH Capability. 177 5. Operation 179 The Path Identifier specified in the previous section can be used to 180 advertise multiple paths for the same address prefix without 181 subsequent advertisements replacing the previous ones. Apart from 182 the fact that this is now possible, the route advertisement rules of 183 [RFC4271] are not changed. In particular, a new advertisement for a 184 given address prefix and a given path identifier replaces a previous 185 advertisement for the same address prefix and path identifier. If a 186 BGP speaker receives a message to withdraw a prefix with a path 187 identifier not seen before, it SHOULD silently ignore it. 189 For a BGP speaker to be able to send multiple paths to its peer, that 190 BGP speaker MUST advertise the ADD-PATH capability with the Send/ 191 Receive field set to either 2 or 3, and MUST receive from its peer 192 the ADD-PATH capability with the Send/Receive field set to either 1 193 or 3, for the corresponding . 195 A BGP speaker MUST follow the procedures defined in [RFC4271] when 196 generating an UPDATE message for a particular to a peer 197 unless the BGP speaker advertises the ADD-PATH Capability to the peer 198 indicating its ability to send multiple paths for the , 199 and also receives the ADD-PATH Capability from the peer indicating 200 its ability to receive multiple paths for the , in which 201 case the speaker MUST generate a route update for the 202 based on the combination of the address prefix and the Path 203 Identifier, and use the extended NLRI encodings specified in this 204 document. The peer SHALL act accordingly in processing an UPDATE 205 message related to a particular . 207 A BGP speaker SHOULD include the bestpath when more than one path are 208 advertised to a neighbor unless the bestpath is a path received from 209 that neighbor. 211 As the Path Identifiers are locally assigned, and may or may not be 212 persistent across a control plane restart of a BGP speaker, an 213 implementation SHOULD take special care so that the underlying 214 forwarding plane of a "Receiving Speaker" as described in [RFC4724] 215 is not affected during the graceful restart of a BGP session. 217 6. Applications 219 The BGP extension specified in this document can be used by a BGP 220 speaker to advertise multiple paths in certain applications. The 221 availability of the additional paths can help reduce or eliminate 222 persistent route oscillations [RFC3345]. It can also help with 223 optimal routing and routing convergence in a network. The 224 applications are detailed in separate documents. 226 7. Deployment Considerations 228 The extension proposed in this document provides a mechanism for a 229 BGP speaker to advertise multiple paths over a BGP session. Care 230 needs to be taken in its deployment to ensure consistent routing and 231 forwarding in a network, the details of which will be described in 232 separate application documents. 234 When deployed as a provider edge router or a peering router that 235 interacts with external neighbors, a BGP speaker usually advertises 236 at most one path to the internal neighbors in a network. In the case 237 the speaker is configured to advertise multiple paths to the internal 238 neighbors, and additional information is needed for the application, 239 the speaker could use attributes such as the Edge_Discriminator 240 attribute [I-D.pmohapat-idr-fast-conn-restore]. The use of that type 241 of additional information is outside the scope of this document. 243 8. IANA Considerations 245 IANA has assigned capability number 69 for the ADD-PATH Capability 246 described in this document. This registration is in the BGP 247 Capability Codes registry. 249 9. Security Considerations 251 This document defines a BGP extension that allows the advertisement 252 of multiple paths for the same address prefix without the new paths 253 implicitly replacing any previous ones. As a result, multiple paths 254 for a large number of prefixes may be received by a BGP speaker 255 potentially depleting memory resources or even causing network-wide 256 instability. The use of the ADD-PATH Capability is intended to 257 address specific needs related to, for example, eliminating the MED- 258 induced route oscillations in a network 259 [I-D.ietf-idr-route-oscillation-stop]. While the applications for 260 the ADD-PATH Capability are outside the scope of this document, the 261 users are encouraged to examine their behavior and potential impact 262 by studying the best practices described in 263 [I-D.ietf-idr-add-paths-guidelines]. 265 This document introduces no new security concerns in the base 266 operation of BGP [RFC4271]. 268 10. Acknowledgments 270 We would like to thank David Cook and Naiming Shen for their 271 contributions to the design and development of the extension. 273 Many people have made valuable comments and suggestions, including 274 Rex Fernando, Eugene Kim, Danny McPherson, Dave Meyer, Pradosh 275 Mohapatra, Keyur Patel, Robert Raszuk, Eric Rosen, Srihari Sangli, 276 Dan Tappan, Mark Turner, Jeff Haas, Jay Borkenhagen and Mach Chen. 278 11. References 280 11.1. Normative References 282 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 283 Requirement Levels", BCP 14, RFC 2119, 284 DOI 10.17487/RFC2119, March 1997, 285 . 287 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 288 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 289 DOI 10.17487/RFC4271, January 2006, 290 . 292 [RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, 293 "Multiprotocol Extensions for BGP-4", RFC 4760, 294 DOI 10.17487/RFC4760, January 2007, 295 . 297 [RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement 298 with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February 299 2009, . 301 11.2. Informative References 303 [I-D.ietf-idr-add-paths-guidelines] 304 Uttaro, J., Francois, P., Patel, K., Mohapatra, P., Haas, 305 J., Simpson, A., and R. Fragassi, "Best Practices for 306 Advertisement of Multiple Paths in IBGP", draft-ietf-idr- 307 add-paths-guidelines-07 (work in progress), December 2014. 309 [I-D.ietf-idr-route-oscillation-stop] 310 Walton, D., Retana, A., Chen, E., and J. Scudder, "BGP 311 Persistent Route Oscillation Solutions", draft-ietf-idr- 312 route-oscillation-stop-01 (work in progress), October 313 2015. 315 [I-D.pmohapat-idr-fast-conn-restore] 316 Mohapatra, P., Fernando, R., Filsfils, C., and R. Raszuk, 317 "Fast Connectivity Restoration Using BGP Add-path", draft- 318 pmohapat-idr-fast-conn-restore-03 (work in progress), 319 January 2013. 321 [RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana, 322 "Border Gateway Protocol (BGP) Persistent Route 323 Oscillation Condition", RFC 3345, DOI 10.17487/RFC3345, 324 August 2002, . 326 [RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y. 327 Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724, 328 DOI 10.17487/RFC4724, January 2007, 329 . 331 Authors' Addresses 333 Daniel Walton 334 Cumulus Networks 335 185 E. Dana Street 336 Mountain View, CA 94041 337 US 339 Email: dwalton@cumulusnetworks.com 341 Alvaro Retana 342 Cisco Systems, Inc. 343 Kit Creek Rd. 344 Research Triangle Park, NC 27709 345 US 347 Email: aretana@cisco.com 349 Enke Chen 350 Cisco Systems, Inc. 351 170 W. Tasman Dr. 352 San Jose, CA 95134 353 US 355 Email: enkechen@cisco.com 357 John Scudder 358 Juniper Networks 359 1194 N. Mathilda Ave 360 Sunnyvale, CA 94089 361 US 363 Email: jgs@juniper.net