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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 XXX 2 XXX WARNING: old character encoding and/or character set 3 XXX 5 Network Working Group D. Walton 6 Internet Draft E. Chen 7 Intended Status: Standards Track Cisco Systems 8 Expiration Date: December 18, 2012 A. Retana 9 Hewlett-Packard Co. 10 J. Scudder 11 Juniper Networks 12 June 17, 2012 14 Advertisement of Multiple Paths in BGP 16 draft-ietf-idr-add-paths-07.txt 18 Status of this Memo 20 This Internet-Draft is submitted to IETF in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF), its areas, and its working groups. Note that 25 other groups may also distribute working documents as Internet- 26 Drafts. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 The list of current Internet-Drafts can be accessed at 34 http://www.ietf.org/1id-abstracts.html 36 The list of Internet-Draft Shadow Directories can be accessed at 37 http://www.ietf.org/shadow.html 39 This Internet-Draft will expire on December 18, 2012. 41 Copyright Notice 43 Copyright (c) 2012 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Abstract 58 In this document we propose a BGP extension that allows the 59 advertisement of multiple paths for the same address prefix without 60 the new paths implicitly replacing any previous ones. The essence of 61 the extension is that each path is identified by a path identifier in 62 addition to the address prefix. 64 1. Introduction 66 The BGP specification [RFC4271] defines an "Update-Send Process" to 67 advertise the routes chosen by the Decision Process to other BGP 68 speakers. No provisions are made to allow the advertisement of 69 multiple paths for the same address prefix, or Network Layer 70 Reachability Information (NLRI). In fact, a route with the same NLRI 71 as a previously advertised route implicitly replaces the previous 72 advertisement. 74 In this document we propose a BGP extension that allows the 75 advertisement of multiple paths for the same address prefix without 76 the new paths implicitly replacing any previous ones. The essence of 77 the extension is that each path is identified by a path identifier in 78 addition to the address prefix. 80 1.1. Specification of Requirements 82 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 83 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 84 document are to be interpreted as described in [RFC2119]. 86 2. How to Identify a Path 88 As defined in [RFC4271], a path refers to the information reported in 89 the path attribute field of an UPDATE message. As the procedures 90 specified in [RFC4271] allow only the advertisement of one path for a 91 particular address prefix, a path for an address prefix from a BGP 92 peer can be keyed on the address prefix. 94 In order for a BGP speaker to advertise multiple paths for the same 95 address prefix, a new identifier (termed "Path Identifier" hereafter) 96 needs to be introduced so that a particular path for an address 97 prefix can be identified by the combination of the address prefix and 98 the Path Identifier. 100 The assignment of the Path Identifier for a path by a BGP speaker is 101 purely a local matter. However, the Path Identifier MUST be assigned 102 in such a way that the BGP speaker is able to use the (prefix, path 103 identifier) to uniquely identify a path advertised to a neighbor. A 104 BGP speaker that re-advertises a route MUST generate its own Path 105 Identifier to be associated with the re-advertised route. A BGP 106 speaker that receives a route SHOULD NOT assume that the identifier 107 carries any particular semantics; it SHOULD be treated as an opaque 108 value. 110 3. Extended NLRI Encodings 112 In order to carry the Path Identifier in an UPDATE message, the 113 existing NLRI encodings are extended by prepending the Path 114 Identifier field, which is of four-octets. 116 For example, the NLRI encodings specified in [RFC4271, RFC4760] are 117 extended as the following: 119 +--------------------------------+ 120 | Path Identifier (4 octets) | 121 +--------------------------------+ 122 | Length (1 octet) | 123 +--------------------------------+ 124 | Prefix (variable) | 125 +--------------------------------+ 127 and the NLRI encoding specified in [RFC3107] is extended as the 128 following: 130 +--------------------------------+ 131 | Path Identifier (4 octets) | 132 +--------------------------------+ 133 | Length (1 octet) | 134 +--------------------------------+ 135 | Label (3 octets) | 136 +--------------------------------+ 137 | ... | 138 +--------------------------------+ 139 | Prefix (variable) | 140 +--------------------------------+ 142 The usage of the extended NLRI encodings is specified in the 143 Operation section. 145 4. ADD-PATH Capability 147 The ADD-PATH Capability is a new BGP capability [RFC5492]. The 148 Capability Code for this capability is specified in the IANA 149 Considerations section of this document. The Capability Length field 150 of this capability is variable. The Capability Value field consists 151 of one or more of the following tuples: 153 +------------------------------------------------+ 154 | Address Family Identifier (2 octets) | 155 +------------------------------------------------+ 156 | Subsequent Address Family Identifier (1 octet) | 157 +------------------------------------------------+ 158 | Send/Receive (1 octet) | 159 +------------------------------------------------+ 161 The meaning and use of the fields are as follows: 163 Address Family Identifier (AFI): 165 This field is the same as the one used in [RFC4760]. 167 Subsequent Address Family Identifier (SAFI): 169 This field is the same as the one used in [RFC4760]. 171 Send/Receive: 173 This field indicates whether the sender is (a) willing to 174 receive multiple paths from its peer (value 1), (b) would 175 like to send multiple paths to its peer (value 2), or (c) 176 both (value 3) for the . 178 5. Operation 180 The Path Identifier specified in the previous section can be used to 181 advertise multiple paths for the same address prefix without 182 subsequent advertisements replacing the previous ones. Apart from 183 the fact that this is now possible, the route advertisement rules of 184 [RFC4271] are not changed. In particular, a new advertisement for a 185 given address prefix and a given path identifier replaces a previous 186 advertisement for the given address prefix and the given path 187 identifier. 189 A BGP speaker that is willing to receive multiple paths from its 190 peer, or would like to send multiple paths to its peer, SHOULD 191 advertise the ADD-PATH Capability to the peer using BGP Capabilities 192 advertisement [RFC5492]. 194 A BGP speaker MUST follow the existing procedures in generating an 195 UPDATE message for a particular to a peer unless the BGP 196 speaker advertises the ADD-PATH Capability to the peer indicating its 197 desire to send multiple paths for the , and also receives 198 the ADD-PATH Capability from the peer indicating its willingness to 199 receive multiple paths for the , in which case the speaker 200 MUST generate a route update for the based on the 201 combination of the address prefix and the Path Identifier, and use 202 the extended NLRI encodings specified in this document. The peer 203 SHALL act accordingly in processing an UPDATE message related to a 204 particular . 206 A BGP speaker SHOULD include the bestpath when more than one path are 207 advertised to a neighbor unless the bestpath is a path received from 208 that neighbor. 210 When deployed as a provider edge router or a peering router that 211 interacts with external neighbors, a BGP speaker usually advertises 212 at most one path to the internal neighbors in a network. In the case 213 the speaker is configured to advertise multiple paths to the internal 214 neighbors, it should include the Edge_Discriminator attribute defined 215 in [FAST-CONV] in order to make the route selection consistent inside 216 the network. 218 As the Path Identifiers are locally assigned, and may or may not be 219 persistent across a control plane restart of a BGP speaker, an 220 implementation SHOULD take special care so that the underlying 221 forwarding plane of a "Receiving Speaker" as described in [RFC4724] 222 is not affected during the graceful restart of a BGP session. 224 6. Applications 226 The BGP extension specified in this document can be used by a BGP 227 speaker to advertise multiple paths in certain applications. The 228 availability of the additional paths can help reduce or eliminate 229 persistent route oscillations [RFC3345]. It can also help with 230 optimal routing and routing convergence in a network. The 231 applications are detailed in separate documents. 233 7. Deployment Considerations 235 The extension proposed in this document provides a mechanism for a 236 BGP speaker to advertise multiple paths over a BGP session. Care 237 needs to be taken in its deployment to ensure consistent routing and 238 forwarding in a network, the details of which will be described in 239 separate application documents. 241 8. IANA Considerations 243 IANA has assigned capability number 69 for the ADD-PATH Capability 244 described in this document. This registration is in the BGP 245 Capability Codes registry. 247 9. Security Considerations 249 This document introduces no new security concerns to BGP or other 250 specifications referenced in this document. 252 10. Acknowledgments 254 We would like to thank David Cook and Naiming Shen for their 255 contributions to the design and development of the extension. 257 Many people have made valuable comments and suggestions, including 258 Rex Fernando, Eugene Kim, Danny McPherson, Dave Meyer, Pradosh 259 Mohapatra, Keyur Patel, Robert Raszuk, Eric Rosen, Srihari Sangli, 260 Dan Tappan, and Mark Turner. 262 11. References 264 11.1. Normative References 266 [RFC4271] Rekhter, Y., T. Li, and S. Hares, "A Border Gateway 267 Protocol 4 (BGP-4)," RFC 4271, January 2006. 269 [RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement 270 with BGP-4", RFC 5492, February 2009. 272 [RFC4760] Bates, T., Chandra, R., Rekhter, Y., and D. Katz, 273 "Multiprotocol Extensions for BGP-4", RFC 4760, January 2007. 275 [RFC3107] Rekhter, R. and E. Rosen, "Carrying Label Information in 276 BGP-4," RFC 3107, May 2001. 278 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 279 Requirement Levels," RFC 2119, BCP 14, March 1997. 281 [RFC4724] Sangli, S., E. Chen, R. Fernando, J. Scudder, and Y. 282 Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724, January 283 2007. 285 [FAST-CONV] Mohapatra, P., R. Fernando, C. Filsfils, R. Raszuk, "Fast 286 Connectivity Restoration Using BGP Add-path", Work in Progress, March 287 2011. 289 11.2. Informative References 291 [RFC3345] McPherson, D., V. Gill, D. Walton, and A. Retana, "Border 292 Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC 293 3345, August 2002. 295 12. Authors' Addresses 297 Daniel Walton 298 Cisco Systems, Inc. 299 7025 Kit Creek Rd. 300 Research Triangle Park, NC 27709 302 Email: dwalton@cisco.com 304 Alvaro Retana 305 Hewlett-Packard Co. 307 2610 Wycliff Road 308 Raleigh, NC 27607 310 Email: alvaro.retana@hp.com 312 Enke Chen 313 Cisco Systems, Inc. 314 170 W. Tasman Dr. 315 San Jose, CA 95134 317 Email: enkechen@cisco.com 319 John Scudder 320 Juniper Networks 322 Email: jgs@juniper.net