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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Quaizar Vohra 3 Internet Draft Juniper Networks 4 Expiration Date: June 2003 Enke Chen 5 Network Working Group Redback Networks, Inc. 7 BGP support for four-octet AS number space 9 draft-ietf-idr-as4bytes-06.txt 11 1. Status of this Memo 13 This document is an Internet-Draft and is in full conformance with 14 all provisions of Section 10 of RFC2026. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as Internet- 19 Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as ``work in progress.'' 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 2. Abstract 34 Currently the Autonomous System number is encoded in BGP [BGP] as a 35 two-octets field. This document describes extensions to BGP to carry 36 the Autonomous System number as a four-octets field. 38 3. Protocol Extensions 40 For the purpose of this document lets define a BGP speaker which 41 doesn't support the new 4-octets AS number extensions as an OLD BGP 42 speaker, and a BGP speaker which supports the new 4-octets AS number 43 extensions as a NEW BGP speaker. 45 BGP carries the Autonomous System number in the My Autonomous System 46 field of the OPEN message, in the AS_PATH attribute of the UPDATE 47 message, and in the AGGREGATOR attribute of the UPDATE message. BGP 48 also carries the Autonomous System number in the BGP Communities 49 attribute. 51 A NEW BGP speaker uses BGP Capability Advertisements [RFC2842] to 52 advertise to its neighbors (either internal or external) that it 53 supports 4-octets AS number extensions, as specified in this 54 document. 56 The Capability that is used by a BGP speaker to convey to its BGP 57 peer the 4-octets Autonomous System number capability, also carries 58 the 4-octets Autonomous System number of the speaker in the 59 Capability Value field of the Capability Optional Parameter. The 60 Capability Length field of the Capability is set to 4. 62 NEW BGP speakers carry AS path information expressed in terms of 63 4-octets Autonomous Systems numbers by using the existing AS_PATH 64 attribute, except that each AS number in this attribute is encoded 65 not as a 2-octets, but as a 4-octets entity. The same applies to the 66 AGGREGATOR attribute - NEW BGP speakers use the same attribute, 67 except that the AS carried in this attribute is encoded as a 4-octets 68 entity. 70 To preserve AS path information with 4-octets AS numbers across OLD 71 BGP speakers, this document defines a new AS path attribute, called 72 NEW_AS_PATH. This attribute is optional transitive and contains the 73 AS path encoded with 4-octets AS numbers. The NEW_AS_PATH attribute 74 has the same semantics as the AS_PATH attribute, except that it is 75 optional transitive, and it carries 4-octets AS numbers. 77 To prevent the possible propagation of confederation path segments 78 outside of a confederation, the path segment types AS_CONFED_SEQUENCE 79 and AS_CONFED_SET [RFC3065] are declared invalid for the NEW_AS_PATH 80 attribute. 82 Similarly, this document defines a new aggregator attribute called 83 NEW_AGGREGATOR, which is optional transitive. The NEW_AGGREGATOR 84 attribute has the same semantics as the AGGREGATOR attribute, except 85 that it carries 4-octets AS numbers. 87 Currently assigned 2-octets Autonomous System numbers are converted 88 into 4-octets Autonomous System numbers by setting the high-order 2 89 octets of the 4-octets field to zero. Such a 4-octets AS number is 90 said to be mappable to a 2-octets AS number. 92 To represent 4-octets AS numbers (which are not mapped from 2-octets) 93 as 2-octets AS numbers in the AS path information encoded with 94 2-octets AS numbers, this document reserves a 2-octets AS number. 95 Lets denote this special AS number as AS_TRANS for ease of 96 description in the rest of this specification. This AS number is also 97 placed in the My Autonomous System field of the OPEN message 98 originated by a NEW BGP speaker if the speaker doesn't have a 99 (globally unique) 2-octets AS number. 101 4. Operations 103 4.1. Interaction between NEW BGP speakers 105 A BGP speaker that supports 4-octets Autonomous System numbers may 106 advertise this to its peers using the BGP Capability Advertisements. 107 A BGP speaker that advertises such capability to a particular peer, 108 and receives from that peer the advertisement of such capability MUST 109 encode Autonomous System numbers as 4-octets entities in both the 110 AS_PATH and the AGGREGATOR attributes in the updates it sends to the 111 peer, and MUST assume that these attributes in the updates received 112 from the peer encode Autonomous System numbers as 4-octets entities. 114 The new attributes, NEW_AS_PATH and NEW_AGGREGATOR should not be 115 carried in the UPDATE messages between NEW BGP peers. A NEW BGP 116 speaker that receives the NEW_AS_PATH and NEW_AGGREGATOR path 117 attributes in an UPDATE message from a NEW BGP speaker should discard 118 these path attributes and continue processing the UPDATE message. 120 4.2. Interaction between NEW and OLD BGP speaker 122 4.2.1. BGP Peering 124 Note that peering between a NEW BGP speaker and an OLD one is 125 possible only if the NEW BGP speaker has a 2-octets AS number. 126 However, this document doesn't assume that an Autonomous System with 127 NEW speakers has to have a globally unique 2-octets AS number - 128 AS_TRANS could be used instead (even if multiple Autonomous System 129 would use it). 131 4.2.2. Generating Updates 133 When communicating with an OLD BGP speaker, a NEW speaker MUST send 134 the AS path information in the AS_PATH attribute encoded with 135 2-octets AS numbers. The NEW speaker also MUST send the AS path 136 information in the NEW_AS_PATH attribute (encoded with 4-octets AS 137 numbers), except for the case where the entire AS path information is 138 composed of 2-octets AS numbers only. In this case the NEW speaker 139 should not send the NEW_AS_PATH attribute. 141 In the AS_PATH attribute encoded with 2-octets AS numbers, non- 142 mappable 4-octets AS numbers are represented by the well known 143 2-octets AS number, AS_TRANS. This will preserve the path length 144 property of the AS path information; and will also help in updating 145 the AS path information received on a NEW BGP speaker from an OLD 146 speaker, as explained in the next section. 148 The NEW speaker constructs the NEW_AS_PATH attribute from the 149 information carried in the AS_PATH attribute. In the case where the 150 AS_PATH attribute contains either AS_CONFED_SEQUENCE or AS_CONFED_SET 151 path segments, the NEW speaker, when constructing the NEW_AS_PATH 152 attribute from the AS_PATH attribute, must exclude such path 153 segments. The NEW_AS_PATH attribute will be carried across a series 154 of OLD BGP speakers without modification and will help preserve the 155 truely 4-octets AS numbers in the AS path information. 157 Similarly, if the NEW speaker has to send the AGGREGATOR attribute, 158 and if the aggregating Autonomous System's AS number is truely 159 4-octets, the speaker constructs the NEW_AGGREGATOR attributes by 160 taking the attribute length and attribute value from the AGGREGATOR 161 attribute and placing them into the attribute length and attribute 162 value of the NEW_AGGREGATOR attribute, and sets the AS number field 163 in the existing AGGREGATOR attribute to the reserved AS number, 164 AS_TRANS. Note that if the AS number is 2-octets only, then the 165 NEW_AGGREGATE attribute should not be sent. 167 4.2.3. Processing Received Updates 169 When a NEW BGP speaker receives an update from an OLD one, it should 170 be prepared to receive the NEW_AS_PATH attribute along with the 171 existing AS_PATH attribute. If NEW_AS_PATH attribute is also 172 received, both the attributes will be used to construct the exact AS 173 path information, and therefore the information carried by both the 174 attributes will be considered for AS path loop detection. 176 Note that a route may have traversed a series of autonomous systems 177 with 2-octets AS numbers and OLD BGP speakers only. In that case, if 178 the route carries a NEW_AS_PATH attribute, this attribute may not 179 have been updated since the route left the last NEW BGP speaker. The 180 trailing AS path information (representing autonomous systems with 181 2-octets AS numbers and OLD BGP speakers only) is contained only in 182 the current AS_PATH attribute (encoded in the leading part of the 183 AS_PATH attribute). This AS path information should be prepended to 184 the NEW_AS_PATH attribute to construct the exact AS path information. 186 Similarly, a NEW BGP speaker should be prepared to receive the 187 NEW_AGGREGATOR attribute from an OLD BGP speaker. In that case, the 188 AGGREGATOR attribute is ignored and the NEW_AGGREGATOR contains the 189 exact information about the aggregating node. 191 4.3. Interaction between OLD BGP speakers 193 In all other cases the speaker MUST encode Autonomous System numbers 194 as 2-octets entities in both the AS_PATH and the AGGREGATOR attribute 195 in the updates it sends to the peer, and MUST assume that these 196 attributes in the updates received from the peer encoded Autonomous 197 System numbers as 2-octets entities. 199 5. Handling BGP Communities 201 As specified in [RFC1997], when the high-order two-octets of the 202 community attribute is neither 0x0000 nor 0xffff, these two octets 203 encode the Autonomous System number. Quite clearly this would not 204 work for routers that use 4-octets Autonomous System numbers. Such 205 routers should use the Extended Communities [EXT-COMM] attribute 206 instead. 208 6. Transition 210 The scheme described in this document allows a gradual transition 211 from 2-octets AS numbers to 4-octets AS numbers. One can upgrade one 212 Autonomous system or one router at a time. 214 To simplify transition this document assumes that an Autonomous 215 System could start using 4-octets AS number only after all the BGP 216 speakers within that Autonomous System have been upgraded to support 217 4-octets AS numbers. 219 An OLD BGP speaker should not use AS_TRANS as its Autonomous System 220 number. 222 A non-mappable 4-octets AS number can not be used as a "Member AS 223 Number" of a BGP Confederation until all the BGP speakers within the 224 Confederation transitioned to support 4-octets AS numbers. 226 In an environment where an Autonomous System that has OLD BGP 227 speakers peers with two or more Autonomous Systems that have NEW BGP 228 speakers and use AS_TRANS (rather than having a globally unique AS 229 number), use of Multi-Exit Discriminators by the Autonomous System 230 with the OLD speakers may result in a situation where Multi-Exit 231 Discriminator will influence route selection among the routes that 232 were received from different neighboring Autonomous Systems. 234 Under certain conditions it may not be possible to reconstruct the 235 entire AS path information from the AS_PATH and the NEW_AS_PATH 236 attributes of a route. This occurs when two or more routes that carry 237 the NEW_AS_PATH attribute are aggregated by an OLD BGP speaker, and 238 the NEW_AS_PATH attribute of at least one of these routes carries at 239 least one 4-octets AS number (as oppose to a 2-octets AS number that 240 is encoded in 4 octets). When such aggregation results in creating a 241 route that is less specific than any of the component routes, (route 242 whose NLRI covers NLRI of all the component routes), loss of the AS 243 path information does not create a risk of a routing loop. In all 244 other cases loss of the AS path information does create a risk of a 245 routing loop. 247 7. IANA Consideration 249 This document uses a BGP Capability code to indicate that a BGP 250 speaker supports the 4-octets AS numbers. The Capability code has 251 been assigned by IANA per RFC 2842. 253 In addition, this document introduces two new BGP optional transitive 254 attributes. The first is the NEW_AS_PATH attribute, which preserves 255 the AS path information with 4-octet AS numbers across old BGP 256 speakers. The second is the NEW_AGGREGATOR attribute, which is 257 similar in use to the current AGGREGATOR attribute but it carries 258 4-octet AS numbers. The Type Codes for these attributes has been 259 assigned by IANA. 261 Finally, this document introduces a reserved 2-octets AS number - 262 AS_TRANS. The AS number for AS_TRANS has been assigned by the IANA. 264 8. Security Considerations 266 Security issues are not discussed in this document. 268 9. Acknowledgments 270 The authors would like to thank Yakov Rekhter, Chaitanya Kodeboyina, 271 and Jeffrey Haas for the numerous discussions which went into the 272 making of this draft. 274 10. References 276 [BGP] Rekhter, Y., Li, T., "Border Gateway Protocol 4", draft-ietf- 277 idr-bgp4-12.txt 279 [EXT-COM] Ramachandra, S., Tappan, D., and Rekter Y. "BGP Extended 280 Communities Attribute", draft-ramachandra-bgp-ext-communities-02.txt 282 [RFC1997] Chandra, R., Traina, P. and T. Li, "BGP Communities 283 Attribute", RFC 1997, August 1996. 285 [RFC2842] Chandra, R., and Scudder, J., "Capabilities Advertisement 286 with BGP-4", RFC 2842, May 2000. 288 [RFC3065] Traina, P., McPherson, D., Scudder, J., "Autonomous System 289 Confederations for BGP", RFC3065, February 2001. 291 11. Author Information 293 Quaizar Vohra 294 Juniper Networks 295 1194 N.Mathilda Ave 296 Sunnyvale, CA 94089 297 e-mail: qv@juniper.net 299 Enke Chen 300 Redback Networks, Inc. 301 350 Holger Way 302 San Jose, CA 95134 303 e-mail: enke@redback.com