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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: July 2001 Enke Chen 4 Network Working Group Redback Networks, Inc. 6 BGP support for four-octet AS number space 8 draft-ietf-idr-as4bytes-00.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 as a two- 34 octets field. This document describes extensions to BGP to carry the 35 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 [BGP-CAP] 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 Similarly, this document defines a new aggregator attribute called 78 NEW_AGGREGATOR, which is optional transitive. The NEW_AGGREGATOR 79 attribute has the same semantics as the AGGREGATOR attribute, except 80 that it carries 4-octets AS numbers. The Type Code for this attribute 81 is . 83 Currently assigned 2-octets Autonomous System numbers are converted 84 into 4-octets Autonomous System numbers by setting the high-order 2 85 octets of the 4-octets field to zero. Such a 4-octets AS number is 86 said to be mappable to a 2-octets AS number. 88 To represent 4-octets AS numbers (which are not mapped from 2-octets) 89 as 2-octets AS numbers in the AS path information encoded with 90 2-octets AS numbers, this document reserves a 2-octets AS number. The 91 value of this AS number is . Lets denote this special AS number 92 as AS_TRANS for ease of description in the rest of this 93 specification. This AS number is also placed in the My Autonomous 94 System field of the OPEN message originated by a NEW BGP speaker if 95 the speaker doesn't have a (globally unique) 2-octets AS number. 97 4. Operations 99 4.1. Interaction between NEW BGP speakers 101 A BGP speaker that supports 4-octets Autonomous System numbers may 102 advertise this to its peers using the BGP Capability Advertisements. 103 A BGP speaker that advertises such capability to a particular peer, 104 and receives from that peer the advertisement of such capability MUST 105 encode Autonomous System numbers as 4-octets entities in both the 106 AS_PATH and the AGGREGATOR attributes in the updates it sends to the 107 peer, and MUST assume that these attributes in the updates received 108 from the peer encode Autonomous System numbers as 4-octets entities. 110 The new attributes, NEW_AS_PATH and NEW_AGGREGATOR should not be 111 carried in the updates between NEW BGP peers. A NEW BGP speaker that 112 receives an UPDATE message from a NEW BGP speaker, with the 113 NEW_AS_PATH attribute carried in the UPDATE message must ignore the 114 attribute. The same applies to the NEW_AGGREGATOR attribute. 116 4.2. Interaction between NEW and OLD BGP speaker 118 4.2.1. BGP Peering 120 Note that peering between a NEW BGP speaker and an OLD one is 121 possible only if the NEW BGP speaker has a 2-octets AS number. 122 However, this document doesn't assume that an Autonomous System with 123 NEW speakers has to have a globally unique 2-octets AS number - 124 AS_TRANS could be used instead (even if multiple Autonomous System 125 would use it). 127 4.2.2. Generating Updates 129 When communicating with an OLD BGP speaker, a NEW speaker MUST send 130 the AS path information in the AS_PATH attribute encoded with 131 2-octets AS numbers. The NEW speaker also MUST send the AS path 132 information in the NEW_AS_PATH attribute (encoded with 4-octets AS 133 numbers), except for the case where the entire AS path information is 134 composed of 2-octets AS numbers only. In this case the NEW speaker 135 should not send the NEW_AS_PATH attribute. 137 In the AS_PATH attribute encoded with 2-octets AS numbers, non- 138 mappable 4-octets AS numbers are represented by the well known 139 2-octets AS number, AS_TRANS. This will preserve the path length 140 property of the AS path information; and will also help in updating 141 the AS path information received on a NEW BGP speaker from an OLD 142 speaker, as explained in the next section. 144 The NEW speaker constructs the NEW_AS_PATH attribute from the AS_PATH 145 attribute by taking the attribute length and attribute value from the 146 AS_PATH attribute and placing them into the attribute length and 147 attribute value of the NEW_AS_PATH attribute. The NEW_AS_PATH 148 attribute will be carried across a series of OLD BGP speakers without 149 modification and will help preserve the truely 4-octets AS numbers in 150 the AS path information. 152 Similarly, if the NEW speaker has to send the AGGREGATOR attribute, 153 and if the aggregating Autonomous System's AS number is truely 154 4-octets, the speaker constructs the NEW_AGGREGATOR attributes by 155 taking the attribute length and attribute value from the AGGREGATOR 156 attribute and placing them into the attribute length and attribute 157 value of the NEW_AGGREGATOR attribute, and sets the AS number field 158 in the existing AGGREGATOR attribute to the reserved AS number, 159 AS_TRANS. Note that if the AS number is 2-octets only, then the 160 NEW_AGGREGATE attribute should not be sent. 162 4.2.3. Processing Received Updates 164 When a NEW BGP speaker receives an update from an OLD one, it should 165 be prepared to receive the NEW_AS_PATH attribute along with the 166 existing AS_PATH attribute. If NEW_AS_PATH attribute is also 167 received, both the attributes will be used to construct the exact AS 168 path information, and therefore the information carried by both the 169 attributes will be considered for AS path loop detection. 171 Note that a route may have traversed a series of autonomous systems 172 with 2-octets AS numbers and OLD BGP speakers only. In that case, if 173 the route carries a NEW_AS_PATH attribute, this attribute may not 174 have been updated since the route left the last NEW BGP speaker. The 175 trailing AS path information (representing autonomous systems with 176 2-octets AS numbers and OLD BGP speakers only) is contained only in 177 the current AS_PATH attribute (encoded in the leading part of the 178 AS_PATH attribute). This AS path information should be prepended to 179 the NEW_AS_PATH attribute to construct the exact AS path information. 181 Similarly, a NEW BGP speaker should be prepared to receive the 182 NEW_AGGREGATOR attribute from an OLD BGP speaker. In that case, the 183 AGGREGATOR attribute is ignored and the NEW_AGGREGATOR contains the 184 exact information about the aggregating node. 186 4.3. Interaction between OLD BGP speakers 188 In all other cases the speaker MUST encode Autonomous System numbers 189 as 2-octets entities in both the AS_PATH and the AGGREGATOR attribute 190 in the updates it sends to the peer, and MUST assume that these 191 attributes in the updates received from the peer encoded Autonomous 192 System numbers as 2-octets entities. 194 5. Handling BGP Communities 196 As specified in [RFC1997], when the high-order two-octets of the 197 community attribute is neither 0x0000 nor 0xffff, these two octets 198 encode the Autonomous System number. Quite clearly this would not 199 work for routers that use 4-octets Autonomous System numbers. Such 200 routers should use the Extended Communities [EXT-COMM] attribute 201 instead. 203 6. Transition 205 The scheme described in this document allows a gradual transition 206 from 2-octets AS numbers to 4-octets AS numbers. One can upgrade one 207 Autonomous system or one router at a time. 209 To simplify transition this document assumes that an Autonomous 210 System could start using 4-octets AS number only after all the BGP 211 speakers within that Autonomous System have been upgraded to support 212 4-octets AS numbers. 214 An OLD BGP speaker should not use AS_TRANS as its Autonomous System 215 number. 217 In an environment where an Autonomous System that has OLD BGP 218 speakers peers with two or more Autonomous Systems that have NEW BGP 219 speakers and use AS_TRANS (rather than having a globally unique AS 220 number), use of Multi-Exit Discriminators by the Autonomous System 221 with the OLD speakers may result in a situation where Multi-Exit 222 Discriminator will influence route selection among the routes that 223 were received from different neighboring Autonomous Systems. 225 Under certain conditions it may not be possible to reconstruct the 226 entire AS path information from the AS_PATH and the NEW_AS_PATH 227 attributes of a route. This occurs when two or more routes that carry 228 the NEW_AS_PATH attribute are aggregated by an OLD BGP speaker, and 229 the NEW_AS_PATH attribute of at least one of these routes carries at 230 least one 4-octets AS number (as oppose to a 2-octets AS number that 231 is encoded in 4 octets). When such aggregation results in creating a 232 route that is less specific than any of the component routes, (route 233 whose NLRI covers NLRI of all the component routes), loss of the AS 234 path information does not create a risk of a routing loop. In all 235 other cases loss of the AS path information does create a risk of a 236 routing loop. 238 7. Security Considerations 240 Security issues are not discussed in this document. 242 8. Acknowledgments 244 The authors would like to thank Yakov Rekhter and Chaitanya 245 Kodeboyina for the numerous discussions which went into the making of 246 this draft. 248 9. References 250 [BGP-CAP] Chandra, R., and Scudder, J., "Capabilities Advertisement 251 with BGP-4", RFC 2842, May 2000. 253 [EXT-COM] Ramachandra, S., Tappan, D., and Rekter Y. "BGP Extended 254 Communities Attribute", draft-ramachandra-bgp-ext-communities-02.txt 256 [RFC1997] Chandra, R., Traina, P. and T. Li, "BGP Communities 257 Attribute", RFC 1997, August 1996. 259 10. Author Information 261 Quaizar Vohra 262 Juniper Networks 263 1194 N.Mathilda Ave 264 Sunnyvale, CA 94089 265 e-mail: qv@juniper.net 267 Enke Chen 268 Redback Networks, Inc. 269 350 Holger Way 270 San Jose, CA 95134 271 e-mail: enke@redback.com