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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group Quaizar Vohra 2 Internet Draft Juniper Networks 3 Expiration Date: August 2001 Enke Chen 4 Network Working Group Redback Networks, Inc. 6 BGP support for four-octet AS number space 8 draft-ietf-idr-as4bytes-01.txt 10 1. Status of this Memo 12 This document is an Internet-Draft and is in full conformance with 13 all provisions of Section 10 of RFC2026. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that 17 other groups may also distribute working documents as Internet- 18 Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as ``work in progress.'' 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/ietf/1id-abstracts.txt 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 2. Abstract 33 Currently the Autonomous System number is encoded in BGP [BGP] as a 34 two-octets field. This document describes extensions to BGP to carry 35 the Autonomous System number as a four-octets field. 37 3. Protocol Extensions 39 For the purpose of this document lets define a BGP speaker which 40 doesn't support the new 4-octets AS number extensions as an OLD BGP 41 speaker, and a BGP speaker which supports the new 4-octets AS number 42 extensions as a NEW BGP speaker. 44 BGP carries the Autonomous System number in the My Autonomous System 45 field of the OPEN message, in the AS_PATH attribute of the UPDATE 46 message, and in the AGGREGATOR attribute of the UPDATE message. BGP 47 also carries the Autonomous System number in the BGP Communities 48 attribute. 50 A NEW BGP speaker uses BGP Capability Advertisements [RFC2842] to 51 advertise to its neighbors (either internal or external) that it 52 supports 4-octets AS number extensions, as specified in this 53 document. The BGP Capability code for this capability is [TBD]. 55 The Capability that is used by a BGP speaker to convey to its BGP 56 peer the 4-octets Autonomous System number capability, also carries 57 the 4-octets Autonomous System number of the speaker in the 58 Capability Value field of the Capability Optional Parameter. The 59 Capability Length field of the Capability is set to 4. 61 NEW BGP speakers carry AS path information expressed in terms of 62 4-octets Autonomous Systems numbers by using the existing AS_PATH 63 attribute, except that each AS number in this attribute is encoded 64 not as a 2-octets, but as a 4-octets entity. The same applies to the 65 AGGREGATOR attribute - NEW BGP speakers use the same attribute, 66 except that the AS carried in this attribute is encoded as a 4-octets 67 entity. 69 To preserve AS path information with 4-octets AS numbers across OLD 70 BGP speakers, this document defines a new AS path attribute, called 71 NEW_AS_PATH. This attribute is optional transitive and contains the 72 AS path encoded with 4-octets AS numbers. The NEW_AS_PATH attribute 73 has the same semantics as the AS_PATH attribute, except that it is 74 optional transitive, and it carries 4-octets AS numbers. The Type 75 Code for this attribute is 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. The Type Code for this attribute 86 is . 88 Currently assigned 2-octets Autonomous System numbers are converted 89 into 4-octets Autonomous System numbers by setting the high-order 2 90 octets of the 4-octets field to zero. Such a 4-octets AS number is 91 said to be mappable to a 2-octets AS number. 93 To represent 4-octets AS numbers (which are not mapped from 2-octets) 94 as 2-octets AS numbers in the AS path information encoded with 95 2-octets AS numbers, this document reserves a 2-octets AS number. The 96 value of this AS number is . Lets denote this special AS number 97 as AS_TRANS for ease of description in the rest of this 98 specification. This AS number is also placed in the My Autonomous 99 System field of the OPEN message originated by a NEW BGP speaker if 100 the speaker doesn't have a (globally unique) 2-octets AS number. 102 4. Operations 104 4.1. Interaction between NEW BGP speakers 106 A BGP speaker that supports 4-octets Autonomous System numbers may 107 advertise this to its peers using the BGP Capability Advertisements. 108 A BGP speaker that advertises such capability to a particular peer, 109 and receives from that peer the advertisement of such capability MUST 110 encode Autonomous System numbers as 4-octets entities in both the 111 AS_PATH and the AGGREGATOR attributes in the updates it sends to the 112 peer, and MUST assume that these attributes in the updates received 113 from the peer encode Autonomous System numbers as 4-octets entities. 115 The new attributes, NEW_AS_PATH and NEW_AGGREGATOR should not be 116 carried in the updates between NEW BGP peers. A NEW BGP speaker that 117 receives an UPDATE message from a NEW BGP speaker, with the 118 NEW_AS_PATH attribute carried in the UPDATE message must ignore the 119 attribute. The same applies to the NEW_AGGREGATOR attribute. 121 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 Except for the case where the AS_PATH attribute contains either 149 AS_CONFED_SEQUENCE or AS_CONFED_SET path segment, the NEW speaker 150 constructs the NEW_AS_PATH attribute from the AS_PATH attribute by 151 taking the attribute length and attribute value from the AS_PATH 152 attribute and placing them into the attribute length and attribute 153 value of the NEW_AS_PATH attribute. In the case where the AS_PATH 154 attribute contains either AS_CONFED_SEQUENCE or AS_CONFED_SET path 155 segments, the NEW speaker, when copying information from the AS_PATH 156 attribute into the NEW_AS_PATH attribute, must exclude such path 157 segments from copying. The NEW_AS_PATH attribute will be carried 158 across a series of OLD BGP speakers without modification and will 159 help preserve the truely 4-octets AS numbers in the AS path 160 information. 162 Similarly, if the NEW speaker has to send the AGGREGATOR attribute, 163 and if the aggregating Autonomous System's AS number is truely 164 4-octets, the speaker constructs the NEW_AGGREGATOR attributes by 165 taking the attribute length and attribute value from the AGGREGATOR 166 attribute and placing them into the attribute length and attribute 167 value of the NEW_AGGREGATOR attribute, and sets the AS number field 168 in the existing AGGREGATOR attribute to the reserved AS number, 169 AS_TRANS. Note that if the AS number is 2-octets only, then the 170 NEW_AGGREGATE attribute should not be sent. 172 4.2.3. Processing Received Updates 174 When a NEW BGP speaker receives an update from an OLD one, it should 175 be prepared to receive the NEW_AS_PATH attribute along with the 176 existing AS_PATH attribute. If NEW_AS_PATH attribute is also 177 received, both the attributes will be used to construct the exact AS 178 path information, and therefore the information carried by both the 179 attributes will be considered for AS path loop detection. 181 Note that a route may have traversed a series of autonomous systems 182 with 2-octets AS numbers and OLD BGP speakers only. In that case, if 183 the route carries a NEW_AS_PATH attribute, this attribute may not 184 have been updated since the route left the last NEW BGP speaker. The 185 trailing AS path information (representing autonomous systems with 186 2-octets AS numbers and OLD BGP speakers only) is contained only in 187 the current AS_PATH attribute (encoded in the leading part of the 188 AS_PATH attribute). This AS path information should be prepended to 189 the NEW_AS_PATH attribute to construct the exact AS path information. 191 Similarly, a NEW BGP speaker should be prepared to receive the 192 NEW_AGGREGATOR attribute from an OLD BGP speaker. In that case, the 193 AGGREGATOR attribute is ignored and the NEW_AGGREGATOR contains the 194 exact information about the aggregating node. 196 4.3. Interaction between OLD BGP speakers 198 In all other cases the speaker MUST encode Autonomous System numbers 199 as 2-octets entities in both the AS_PATH and the AGGREGATOR attribute 200 in the updates it sends to the peer, and MUST assume that these 201 attributes in the updates received from the peer encoded Autonomous 202 System numbers as 2-octets entities. 204 5. Handling BGP Communities 206 As specified in [RFC1997], when the high-order two-octets of the 207 community attribute is neither 0x0000 nor 0xffff, these two octets 208 encode the Autonomous System number. Quite clearly this would not 209 work for routers that use 4-octets Autonomous System numbers. Such 210 routers should use the Extended Communities [EXT-COMM] attribute 211 instead. 213 6. Transition 215 The scheme described in this document allows a gradual transition 216 from 2-octets AS numbers to 4-octets AS numbers. One can upgrade one 217 Autonomous system or one router at a time. 219 To simplify transition this document assumes that an Autonomous 220 System could start using 4-octets AS number only after all the BGP 221 speakers within that Autonomous System have been upgraded to support 222 4-octets AS numbers. 224 An OLD BGP speaker should not use AS_TRANS as its Autonomous System 225 number. 227 A non-mappable 4-octets AS number can not be used as a "Member AS 228 Number" of a BGP Confederation until all the BGP speakers within the 229 Confederation transitioned to support 4-octets AS numbers. 231 In an environment where an Autonomous System that has OLD BGP 232 speakers peers with two or more Autonomous Systems that have NEW BGP 233 speakers and use AS_TRANS (rather than having a globally unique AS 234 number), use of Multi-Exit Discriminators by the Autonomous System 235 with the OLD speakers may result in a situation where Multi-Exit 236 Discriminator will influence route selection among the routes that 237 were received from different neighboring Autonomous Systems. 239 Under certain conditions it may not be possible to reconstruct the 240 entire AS path information from the AS_PATH and the NEW_AS_PATH 241 attributes of a route. This occurs when two or more routes that carry 242 the NEW_AS_PATH attribute are aggregated by an OLD BGP speaker, and 243 the NEW_AS_PATH attribute of at least one of these routes carries at 244 least one 4-octets AS number (as oppose to a 2-octets AS number that 245 is encoded in 4 octets). When such aggregation results in creating a 246 route that is less specific than any of the component routes, (route 247 whose NLRI covers NLRI of all the component routes), loss of the AS 248 path information does not create a risk of a routing loop. In all 249 other cases loss of the AS path information does create a risk of a 250 routing loop. 252 7. Security Considerations 254 Security issues are not discussed in this document. 256 8. Acknowledgments 258 The authors would like to thank Yakov Rekhter and Chaitanya 259 Kodeboyina for the numerous discussions which went into the making of 260 this draft. 262 9. References 264 [BGP] Rekhter, Y., Li, T., "Border Gateway Protocol 4", draft-ietf- 265 idr-bgp4-12.txt 267 [EXT-COM] Ramachandra, S., Tappan, D., and Rekter Y. "BGP Extended 268 Communities Attribute", draft-ramachandra-bgp-ext-communities-02.txt 270 [RFC1997] Chandra, R., Traina, P. and T. Li, "BGP Communities 271 Attribute", RFC 1997, August 1996. 273 [RFC2842] Chandra, R., and Scudder, J., "Capabilities Advertisement 274 with BGP-4", RFC 2842, May 2000. 276 [RFC3065] Traina, P., McPherson, D., Scudder, J., "Autonomous System 277 Confederations for BGP", RFC3065, February 2001. 279 10. Author Information 281 Quaizar Vohra 282 Juniper Networks 283 1194 N.Mathilda Ave 284 Sunnyvale, CA 94089 285 e-mail: qv@juniper.net 287 Enke Chen 288 Redback Networks, Inc. 289 350 Holger Way 290 San Jose, CA 95134 291 e-mail: enke@redback.com