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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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-04) exists of draft-ietf-idr-route-oscillation-stop-02 Summary: 0 errors (**), 0 flaws (~~), 2 warnings (==), 1 comment (--).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 D. Walton 3 Internet-Draft Cumulus Networks 4 Intended status: Standards Track A. Retana 5 Expires: November 1, 2016 E. Chen 6 Cisco Systems, Inc. 7 J. Scudder 8 Juniper Networks 9 April 30, 2016 11 Advertisement of Multiple Paths in BGP 12 draft-ietf-idr-add-paths-14 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 November 1, 2016. 39 Copyright Notice 41 Copyright (c) 2016 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. Deployment Considerations . . . . . . . . . . . . . . . . . . 5 63 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 64 8. Security Considerations . . . . . . . . . . . . . . . . . . . 6 65 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6 66 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 67 10.1. Normative References . . . . . . . . . . . . . . . . . . 6 68 10.2. Informative References . . . . . . . . . . . . . . . . . 7 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 71 1. Introduction 73 The BGP specification [RFC4271] defines an Update-Send Process to 74 advertise the routes chosen by the Decision Process to other BGP 75 speakers. No provisions are made to allow the advertisement of 76 multiple paths for the same address prefix, or Network Layer 77 Reachability Information (NLRI). In fact, a route with the same NLRI 78 as a previously advertised route implicitly replaces the previous 79 advertisement. 81 This document defines a BGP extension that allows the advertisement 82 of multiple paths for the same address prefix without the new paths 83 implicitly replacing any previous ones. The essence of the extension 84 is that each path is identified by a path identifier in addition to 85 the address prefix. 87 The availability of the additional paths can help reduce or eliminate 88 persistent route oscillations [RFC3345]. It can also help with 89 optimal routing and routing convergence in a network by providing 90 potential alternate or backup paths, respectively. 92 1.1. 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. How to Identify a Path 100 As defined in [RFC4271], a path refers to the information reported in 101 the path attribute field of an UPDATE message. As the procedures 102 specified in [RFC4271] allow only the advertisement of one path for a 103 particular address prefix, a path for an address prefix from a BGP 104 peer can be keyed on the address prefix. 106 In order for a BGP speaker to advertise multiple paths for the same 107 address prefix, a new identifier (termed "Path Identifier" hereafter) 108 needs to be introduced so that a particular path for an address 109 prefix can be identified by the combination of the address prefix and 110 the Path Identifier. 112 The assignment of the Path Identifier for a path by a BGP speaker is 113 purely a local matter. However, the Path Identifier MUST be assigned 114 in such a way that the BGP speaker is able to use the (prefix, path 115 identifier) to uniquely identify a path advertised to a neighbor. A 116 BGP speaker that re-advertises a route MUST generate its own Path 117 Identifier to be associated with the re-advertised route. A BGP 118 speaker that receives a route should not assume that the identifier 119 carries any particular semantics. 121 3. Extended NLRI Encodings 123 In order to carry the Path Identifier in an UPDATE message, the NLRI 124 encoding MUST be extended by prepending the Path Identifier field, 125 which is of four-octets. 127 For example, the NLRI encoding specified in [RFC4271] is extended as 128 the following: 130 +--------------------------------+ 131 | Path Identifier (4 octets) | 132 +--------------------------------+ 133 | Length (1 octet) | 134 +--------------------------------+ 135 | Prefix (variable) | 136 +--------------------------------+ 138 The usage of the extended NLRI encodings is specified in Section 5. 140 4. ADD-PATH Capability 142 The ADD-PATH Capability is a new BGP capability [RFC5492], with 143 Capability Code 69. The Capability Length field of this capability 144 is variable. The Capability Value field consists of one or more of 145 the following tuples: 147 +------------------------------------------------+ 148 | Address Family Identifier (2 octets) | 149 +------------------------------------------------+ 150 | Subsequent Address Family Identifier (1 octet) | 151 +------------------------------------------------+ 152 | Send/Receive (1 octet) | 153 +------------------------------------------------+ 155 The meaning and use of the fields are as follows: 157 Address Family Identifier (AFI): 159 This field is the same as the one used in [RFC4760]. 161 Subsequent Address Family Identifier (SAFI): 163 This field is the same as the one used in [RFC4760]. 165 Send/Receive: 167 This field indicates whether the sender is (a) able to receive 168 multiple paths from its peer (value 1), (b) able to send 169 multiple paths to its peer (value 2), or (c) both (value 3) for 170 the . 172 If any other value is received, then the capability SHOULD be 173 treated as not understood and ignored [RFC5492]. 175 A BGP speaker that wishes to indicate support for multiple AFI/SAFI 176 MUST do so by including the information in a single instance of the 177 ADD-PATH Capability. 179 5. Operation 181 The Path Identifier specified in Section 3 can be used to advertise 182 multiple paths for the same address prefix without subsequent 183 advertisements replacing the previous ones. Apart from the fact that 184 this is now possible, the route advertisement rules of [RFC4271] are 185 not changed. In particular, a new advertisement for a given address 186 prefix and a given path identifier replaces a previous advertisement 187 for the same address prefix and path identifier. If a BGP speaker 188 receives a message to withdraw a prefix with a path identifier not 189 seen before, it SHOULD silently ignore it. 191 For a BGP speaker to be able to send multiple paths to its peer, that 192 BGP speaker MUST advertise the ADD-PATH capability with the Send/ 193 Receive field set to either 2 or 3, and MUST receive from its peer 194 the ADD-PATH capability with the Send/Receive field set to either 1 195 or 3, for the corresponding . 197 A BGP speaker MUST follow the procedures defined in [RFC4271] when 198 generating an UPDATE message for a particular to a peer 199 unless the BGP speaker advertises the ADD-PATH Capability to the peer 200 indicating its ability to send multiple paths for the , 201 and also receives the ADD-PATH Capability from the peer indicating 202 its ability to receive multiple paths for the , in which 203 case the speaker MUST generate a route update for the 204 based on the combination of the address prefix and the Path 205 Identifier, and use the extended NLRI encodings specified in this 206 document. The peer SHALL act accordingly in processing an UPDATE 207 message related to a particular . 209 A BGP speaker SHOULD include the bestpath when more than one path are 210 advertised to a neighbor unless the bestpath is a path received from 211 that neighbor. 213 As the Path Identifiers are locally assigned, and may or may not be 214 persistent across a control plane restart of a BGP speaker, an 215 implementation SHOULD take special care so that the underlying 216 forwarding plane of a "Receiving Speaker" as described in [RFC4724] 217 is not affected during the graceful restart of a BGP session. 219 6. Deployment Considerations 221 The extension proposed in this document provides a mechanism for a 222 BGP speaker to advertise multiple paths over a BGP session. Care 223 needs to be taken in its deployment to ensure consistent routing and 224 forwarding in a network [I-D.ietf-idr-add-paths-guidelines]. 226 The only explicit indication that the encoding described in Section 3 227 is in use in a particular BGP session is the exchange of Capabilities 228 Section 4. If the negotiation is successful [RFC5492], then the BGP 229 speakers will be able to process all BGP UPDATES properly, as 230 described in Section 5. However, if, for example, a packet analyzer 231 is used on the wire to examine an active BGP session, it may not be 232 able to properly decode the BGP UPDATES because it lacks prior 233 knowledge of the negotiated Capabilities. 235 When deployed as a provider edge router or a peering router that 236 interacts with external neighbors, a BGP speaker usually advertises 237 at most one path to the internal neighbors in a network. In the case 238 the speaker is configured to advertise multiple paths to the internal 239 neighbors, and additional information is needed for the application, 240 the speaker could use attributes such as the Edge_Discriminator 241 attribute [I-D.pmohapat-idr-fast-conn-restore]. The use of that type 242 of additional information is outside the scope of this document. 244 7. IANA Considerations 246 IANA has assigned capability number 69 for the ADD-PATH Capability 247 described in this document. This registration is in the BGP 248 Capability Codes registry. 250 8. Security Considerations 252 This document defines a BGP extension that allows the advertisement 253 of multiple paths for the same address prefix without the new paths 254 implicitly replacing any previous ones. As a result, multiple paths 255 for a large number of prefixes may be received by a BGP speaker 256 potentially depleting memory resources or even causing network-wide 257 instability. The use of the ADD-PATH Capability is intended to 258 address specific needs related to, for example, eliminating the MED- 259 induced route oscillations in a network 260 [I-D.ietf-idr-route-oscillation-stop]. While describing the 261 applications for the ADD-PATH Capability is outside the scope of this 262 document, users are encouraged to examine their behavior and 263 potential impact by studying the best practices described in 264 [I-D.ietf-idr-add-paths-guidelines]. 266 This document introduces no new security concerns in the base 267 operation of BGP [RFC4271]. 269 9. Acknowledgments 271 We would like to thank David Cook and Naiming Shen for their 272 contributions to the design and development of the extension. 274 Many people have made valuable comments and suggestions, including 275 Rex Fernando, Eugene Kim, Danny McPherson, Dave Meyer, Pradosh 276 Mohapatra, Keyur Patel, Robert Raszuk, Eric Rosen, Srihari Sangli, 277 Dan Tappan, Mark Turner, Jeff Haas, Jay Borkenhagen, Mach Chen, Denis 278 Ovsienko, Carlos Pignataro and Meral Shirazipour. 280 10. References 282 10.1. Normative References 284 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 285 Requirement Levels", BCP 14, RFC 2119, 286 DOI 10.17487/RFC2119, March 1997, 287 . 289 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 290 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 291 DOI 10.17487/RFC4271, January 2006, 292 . 294 [RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, 295 "Multiprotocol Extensions for BGP-4", RFC 4760, 296 DOI 10.17487/RFC4760, January 2007, 297 . 299 [RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement 300 with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February 301 2009, . 303 10.2. Informative References 305 [I-D.ietf-idr-add-paths-guidelines] 306 Uttaro, J., Francois, P., Patel, K., Haas, J., Simpson, 307 A., and R. Fragassi, "Best Practices for Advertisement of 308 Multiple Paths in IBGP", draft-ietf-idr-add-paths- 309 guidelines-08 (work in progress), April 2016. 311 [I-D.ietf-idr-route-oscillation-stop] 312 Walton, D., Retana, A., Chen, E., and J. Scudder, "BGP 313 Persistent Route Oscillation Solutions", draft-ietf-idr- 314 route-oscillation-stop-02 (work in progress), April 2016. 316 [I-D.pmohapat-idr-fast-conn-restore] 317 Mohapatra, P., Fernando, R., Filsfils, C., and R. Raszuk, 318 "Fast Connectivity Restoration Using BGP Add-path", draft- 319 pmohapat-idr-fast-conn-restore-03 (work in progress), 320 January 2013. 322 [RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana, 323 "Border Gateway Protocol (BGP) Persistent Route 324 Oscillation Condition", RFC 3345, DOI 10.17487/RFC3345, 325 August 2002, . 327 [RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y. 328 Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724, 329 DOI 10.17487/RFC4724, January 2007, 330 . 332 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