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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) ** Obsolete normative reference: RFC 3107 (Obsoleted by RFC 8277) Summary: 2 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 A. Retana 4 Intended Status: Standards Track E. Chen 5 Expiration Date: February 2010 Cisco Systems 6 J. Scudder 7 Juniper Networks 8 August 3, 2009 10 Advertisement of Multiple Paths in BGP 12 draft-ietf-idr-add-paths-02.txt 14 Status of this Memo 16 This Internet-Draft is submitted to IETF in full conformance with the 17 provisions of BCP 78 and BCP 79. 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/1id-abstracts.html 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html 35 This Internet-Draft will expire on February 4, 2010. 37 Abstract 39 In this document we propose a BGP extension that allows the 40 advertisement of multiple paths for the same address prefix without 41 the new paths implicitly replacing any previous ones. The essence of 42 the extension is that each path is identified by a path identifier in 43 addition to the address prefix. 45 1. Introduction 47 The BGP specification [RFC4271] defines an "Update-Send Process" to 48 advertise the routes chosen by the Decision Process to other BGP 49 speakers. No provisions are made to allow the advertisement of 50 multiple paths for the same address prefix, or Network Layer 51 Reachability Information (NLRI). In fact, a route with the same NLRI 52 as a previously advertised route implicitly replaces the previous 53 advertisement. 55 In this document we propose a BGP extension that allows the 56 advertisement of multiple paths for the same address prefix without 57 the new paths implicitly replacing any previous ones. The essence of 58 the extension is that each path is identified by a path identifier in 59 addition to the address prefix. 61 1.1. Specification of Requirements 63 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 64 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 65 document are to be interpreted as described in [RFC2119]. 67 2. How to Identify a Path 69 As defined in [RFC4271], a path refers to the information reported in 70 the path attribute field of an UPDATE message. As the procedures 71 specified in [RFC4271] allow only the advertisement of one path for a 72 particular address prefix, a path for an address prefix from a BGP 73 peer can be keyed on the address prefix. 75 In order for a BGP speaker to advertise multiple paths for the same 76 address prefix, a new identifier (termed "Path Identifier" hereafter) 77 needs to be introduced so that a particular path for an address 78 prefix can be identified by the combination of the address prefix and 79 the Path Identifier. 81 The assignment of the Path Identifier for a path by a BGP speaker is 82 purely a local matter. However, the Path Identifier MUST be assigned 83 in such a way that the BGP speaker is able to use the (prefix, path 84 identifier) to uniquely identify a path advertised to a neighbor. A 85 BGP speaker that re-advertises a route MUST generate its own Path 86 Identifier to be associated with the re-advertised route. A BGP 87 speaker that receives a route SHOULD NOT assume that the identifier 88 carries any particular semantics; it SHOULD be treated as an opaque 89 value. 91 3. Extended NLRI Encodings 93 In order to carry the Path Identifier in an UPDATE message, the 94 existing NLRI encodings are extended by prepending the Path 95 Identifier field, which is of four-octets. 97 For example, the NLRI encodings specified in [RFC4271, RFC4760] are 98 extended as the following: 100 +--------------------------------+ 101 | Path Identifier (4 octets) | 102 +--------------------------------+ 103 | Length (1 octet) | 104 +--------------------------------+ 105 | Prefix (variable) | 106 +--------------------------------+ 108 and the NLRI encoding specified in [RFC3107] is extended as the 109 following: 111 +--------------------------------+ 112 | Path Identifier (4 octets) | 113 +--------------------------------+ 114 | Length (1 octet) | 115 +--------------------------------+ 116 | Label (3 octets) | 117 +--------------------------------+ 118 | ... | 119 +--------------------------------+ 120 | Prefix (variable) | 121 +--------------------------------+ 123 The usage of the extended NLRI encodings is specified in the 124 Operation section. 126 4. ADD-PATH Capability 128 The ADD-PATH Capability is a new BGP capability [RFC5492]. The 129 Capability Code for this capability is specified in the IANA 130 Considerations section of this document. The Capability Length field 131 of this capability is variable. The Capability Value field consists 132 of one or more of the following tuples: 134 +------------------------------------------------+ 135 | Address Family Identifier (2 octets) | 136 +------------------------------------------------+ 137 | Subsequent Address Family Identifier (1 octet) | 138 +------------------------------------------------+ 139 | Send/Receive (1 octet) | 140 +------------------------------------------------+ 142 The meaning and use of the fields are as follows: 144 Address Family Identifier (AFI): 146 This field is the same as the one used in [RFC4760]. 148 Subsequent Address Family Identifier (SAFI): 150 This field is the same as the one used in [RFC4760]. 152 Send/Receive: 154 This field indicates whether the sender is (a) willing to 155 receive multiple paths from its peer (value 1), (b) would 156 like to send multiple paths to its peer (value 2), or (c) 157 both (value 3) for the . 159 5. Operation 161 The Path Identifier specified in the previous section can be used to 162 advertise multiple paths for the same address prefix without 163 subsequent advertisements replacing the previous ones. Apart from 164 the fact that this is now possible, the route advertisement rules of 165 [RFC4271] are not changed. In particular, a new advertisement for a 166 given address prefix and a given path identifier replaces a previous 167 advertisement for the given address prefix and the given path 168 identifier. 170 A BGP speaker that is willing to receive multiple paths from its 171 peer, or would like to send multiple paths to its peer, SHOULD 172 advertise the ADD-PATH Capability to the peer using BGP Capabilities 173 advertisement [RFC5492]. 175 A BGP speaker MUST follow the existing procedures in generating an 176 UPDATE message for a particular to a peer unless the BGP 177 speaker advertises the ADD-PATH Capability to the peer indicating its 178 desire to send multiple paths for the , and also receives 179 the ADD-PATH Capability from the peer indicating its willingness to 180 receive multiple paths for the , in which case the speaker 181 MUST generate a route update for the based on the 182 combination of the address prefix and the Path Identifier, and use 183 the extended NLRI encodings specified in this document. The peer 184 SHALL act accordingly in processing an UPDATE message related to a 185 particular . 187 As the Path Identifiers are locally assigned, and may or may not be 188 persistent across a control plane restart of a BGP speaker, an 189 implementation SHOULD take special care so that the underlying 190 forwarding plane of a "Receiving Speaker" as described in [RFC4724] 191 is not affected during the graceful restart of a BGP session. 193 6. Applications 195 The BGP extension specified in this document can be used by a BGP 196 speaker to advertise multiple paths in certain applications. The 197 availability of the additional paths can help reduce or eliminate 198 persistent route oscillations [RFC3345]. It can also help with 199 optimal routing and routing convergence in a network. The 200 applications are detailed in separate documents. 202 7. Deployment Considerations 204 The extension proposed in this document provides a mechanism for a 205 BGP speaker to advertise multiple paths over a BGP session. Care 206 needs to be taken in its deployment to ensure consistent routing and 207 forwarding in a network, the details of which will be described in 208 separate application documents. 210 8. IANA Considerations 212 IANA has assigned capability number for the ADD-PATH Capability 213 described in this document. This registration is in the BGP 214 Capability Codes registry. 216 9. Security Considerations 218 This document introduces no new security concerns to BGP or other 219 specifications referenced in this document. 221 10. Acknowledgments 223 We would like to thank David Cook and Naiming Shen for their 224 contributions to the design and development of the extension. 226 Many people have made valuable comments and suggestions, including 227 Rex Fernando, Eugene Kim, Danny McPherson, Dave Meyer, Pradosh 228 Mohapatra, Keyur Patel, Robert Raszuk, Eric Rosen, Srihari Sangli, 229 Dan Tappan, and Mark Turner. 231 11. References 233 11.1. Normative References 235 [RFC4271] Rekhter, Y., T. Li, and S. Hares, "A Border Gateway 236 Protocol 4 (BGP-4)," RFC 4271, January 2006. 238 [RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement 239 with BGP-4", RFC 5492, February 2009. 241 [RFC4760] Bates, T., Chandra, R., Rekhter, Y., and D. Katz, 242 "Multiprotocol Extensions for BGP-4", RFC 4760, January 2007. 244 [RFC3107] Rekhter, R. and E. Rosen, "Carrying Label Information in 245 BGP-4," RFC 3107, May 2001. 247 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 248 Requirement Levels," RFC 2119, BCP 14, March 1997. 250 [RFC4724] Sangli, S., E. Chen, R. Fernando, J. Scudder, and Y. 251 Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724, January 252 2007. 254 11.2. Informative References 256 [RFC3345] McPherson, D., V. Gill, D. Walton, and A. Retana, "Border 257 Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC 258 3345, August 2002. 260 12. Authors' Addresses 262 Daniel Walton 263 Cisco Systems, Inc. 264 7025 Kit Creek Rd. 265 Research Triangle Park, NC 27709 267 Email: dwalton@cisco.com 269 Alvaro Retana 270 Cisco Systems, Inc. 271 7025 Kit Creek Rd. 272 Research Triangle Park, NC 27709 274 Email: aretana@cisco.com 276 Enke Chen 277 Cisco Systems, Inc. 278 170 W. Tasman Dr. 279 San Jose, CA 95134 281 Email: enkechen@cisco.com 283 John Scudder 284 Juniper Networks 286 Email: jgs@juniper.net 288 13. Copyright Notice 290 Copyright (c) 2009 IETF Trust and the persons identified as the 291 document authors. All rights reserved. 293 This document is subject to BCP 78 and the IETF Trust's Legal 294 Provisions Relating to IETF Documents in effect on the date of 295 publication of this document (http://trustee.ietf.org/license-info). 296 Please review these documents carefully, as they describe your rights 297 and restrictions with respect to this document.