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Haas 8 Juniper Networks, Inc. 9 12 September 2021 11 Deprecation of AS_SET and AS_CONFED_SET in BGP 12 draft-ietf-idr-deprecate-as-set-confed-set-06 14 Abstract 16 BCP 172 (i.e., RFC 6472) recommends not using AS_SET and 17 AS_CONFED_SET in the Border Gateway Protocol. This document advances 18 this recommendation to a standards requirement in BGP; it proscribes 19 the use of the AS_SET and AS_CONFED_SET types of path segments in the 20 AS_PATH. This is done to simplify the design and implementation of 21 BGP and to make the semantics of the originator of a route clearer. 22 This will also simplify the design, implementation, and deployment of 23 various BGP security mechanisms. This document (if approved) updates 24 RFC 4271 and RFC 5065 by eliminating AS_SET and AS_CONFED_SET types, 25 and obsoletes RFC 6472. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at https://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on 16 March 2022. 44 Copyright Notice 46 Copyright (c) 2021 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 51 license-info) in effect on the date of publication of this document. 52 Please review these documents carefully, as they describe your rights 53 and restrictions with respect to this document. Code Components 54 extracted from this document must include Simplified BSD License text 55 as described in Section 4.e of the Trust Legal Provisions and are 56 provided without warranty as described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 62 3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 3 63 4. Updates to Existing RFCs . . . . . . . . . . . . . . . . . . 4 64 5. Operational Considerations . . . . . . . . . . . . . . . . . 4 65 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 66 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 67 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 68 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 69 9.1. Normative References . . . . . . . . . . . . . . . . . . 5 70 9.2. Informative References . . . . . . . . . . . . . . . . . 6 71 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 73 1. Introduction 75 BCP 172 [RFC6472] makes a recommendation for not using AS_SET (see 76 [RFC4271]) and AS_CONFED_SET (see [RFC5065]) in the Border Gateway 77 Protocol (BGP). This document advances the BCP recommendation to a 78 standards requirement in BGP; it proscribes the use of the AS_SET and 79 AS_CONFED_SET types of path segments in the AS_PATH. 81 The AS_SET path segment in the AS_PATH attribute (Sections 4.3 and 82 5.1.2 of [RFC4271]) is created by a router that is performing route 83 aggregation and contains an unordered set of Autonomous Systems 84 (ASes) that contributing prefixes in the aggregate have traversed. 85 The AS_CONFED_SET path segment (see [RFC5065]) in the AS_PATH 86 attribute is created by a router that is performing route aggregation 87 and contains an unordered set of Member AS Numbers in the local 88 confederation that contributing prefixes in the aggregate have 89 traversed. It is very similar to an AS_SET but is used within a 90 confederation. 92 By performing aggregation, a router is combining multiple existing 93 routes into a single new route. The aggregation together with the 94 use of AS_SET blurs the semantics of origin AS for the prefix being 95 announced. Therefore, the aggregation with AS_SET (or AS_CONFED_SET) 96 can cause operational issues, such as not being able to authenticate 97 a route origin for the aggregate prefix in new BGP security 98 technologies such as those that take advantage of X.509 extensions 99 for IP addresses and AS identifiers [RFC3779] [RFC6480] [RFC6811] 100 [RFC8205]. This in turn could result in reachability problems for 101 the aggregated prefix and its components (i.e., more specific 102 prefixes). 104 From analysis of past Internet routing data, it is apparent that 105 aggregation that involves AS_SETs is very seldom used in practice on 106 the public Internet [Analysis] and when it is used, it is often used 107 incorrectly -- only a single AS in the AS_SET are by far the most 108 common cases. Also, very often the same AS appears in the 109 AS_SEQUENCE and the AS_SET in the BGP update. The occurrence of 110 reserved AS numbers ([IANA-SP-ASN]) is also somewhat frequent. 111 Because the aggregation involving AS_SETs is very rarely used, the 112 reduction in table size provided by this is extremely small, and any 113 advantage thereof is outweighed by additional complexity in BGP. As 114 noted above, AS_SETs also pose impediments to implementation of new 115 BGP security technologies. 117 2. Requirements Language 119 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 120 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 121 "OPTIONAL" in this document are to be interpreted as described in BCP 122 14 [RFC2119] [RFC8174] when, and only when, they appear in all 123 capitals, as shown here. 125 3. Recommendations 127 BGP speakers conforming to this document (i.e., conformant BGP 128 speakers) MUST NOT locally generate BGP UPDATE messages containing 129 AS_SET or AS_CONFED_SET. Conformant BGP speakers SHOULD NOT send BGP 130 UPDATE messages containing AS_SET or AS_CONFED_SET. Upon receipt of 131 such messages, conformant BGP speakers SHOULD use the "Treat-as- 132 withdraw" error handling behavior as per [RFC7606]. 134 If a network operator wishes to consider BGP UPDATE messages with 135 AS_SET or AS_CONFED_SET (received from an external peer) for path 136 selection, they MAY have a feature (knob) in their BGP speaker to opt 137 to do so on a per peer basis. The operator should understand the 138 full implications of choosing this option. There is no knob 139 concerning locally generated BGP UPDATE messages, i.e., as stated 140 before a conformant BGP speaker must not locally generate BGP UPDATE 141 messages with AS_SET or AS_CONFED_SET. 143 Network operators MUST NOT locally generate any new announcements 144 containing AS_SET or AS_CONFED_SET. If they have announced routes 145 with AS_SET or AS_CONFED_SET in them, then they SHOULD withdraw those 146 routes and re-announce routes for the aggregate or component prefixes 147 (i.e., the more specific routes subsumed by the previously aggregated 148 route) without AS_SET or AS_CONFED_SET in the updates. 150 It is worth noting that new BGP security technologies (such as those 151 that take advantage of X.509 extensions for IP addresses and AS 152 identifiers [RFC3779] [RFC6480] [RFC6811] [RFC8205]) might not 153 support routes with AS_SET or AS_CONFED_SET in them, and may treat 154 routes containing them as infeasible even before the updated BGP in 155 this document is implemented. 157 4. Updates to Existing RFCs 159 This document deprecates the AS_SET (type 1) AS_PATH segment type 160 from [RFC4271]. BGP speakers conforming to this document (i.e., 161 conformant BGP speakers) MUST NOT locally generate BGP UPDATE 162 messages containing AS_SET. Conformant BGP speakers SHOULD NOT send 163 BGP UPDATE messages containing AS_SET. Upon receipt of such 164 messages, conformant BGP speakers SHOULD use the "Treat-as-withdraw" 165 error handling behavior as per [RFC7606]. 167 This document deprecates the AS_CONFED_SET (type 4) AS_PATH segment 168 type from [RFC5065]. Conformant BGP speakers MUST NOT locally 169 generate BGP UPDATE messages containing AS_CONFED_SET. Conformant 170 BGP speakers SHOULD NOT send BGP UPDATE messages containing 171 AS_CONFED_SET. Upon receipt of such messages, conformant BGP 172 speakers SHOULD use the "Treat-as-withdraw" error handling behavior 173 as per [RFC7606]. 175 Wherever mentions of AS_SET or AS_CONFED_SET occur in [RFC4271] and 176 [RFC5065], appropriate modification or elimination of the text must 177 be made in future RFCs that would replace these RFCs, consistent with 178 the deprecation of AS_SET and AS_CONFED_SET. 180 5. Operational Considerations 182 When aggregating prefixes, network operators MUST use brief 183 aggregation. In brief aggregation, the AGGREGATOR attribute is 184 included but the AS_SET or AS_CONFED_SET attribute is not included. 186 When doing the above, operators MUST form the aggregate at the border 187 in the outbound BGP policy and omit any prefixes from the AS that the 188 aggregate is being advertised to. In other words, an aggregate 189 prefix MUST NOT be announced to the contributing ASes. Instead, more 190 specific prefixes (from the aggregate) MUST be announced to each 191 contributing AS, excluding any that were learned from the 192 contributing AS in consideration. For illustration, if p1/24 (from 193 AS1), p2/24 (from AS2), p3/24 (from AS3) and p4/24 (from AS4) are 194 aggregated to p/22, then p/22 will not be announced to AS1, AS2, AS3, 195 or AS4. Instead, as further illustration, p1/24, p2/24 and p4/24 are 196 announced to AS3. Or, possibly q/23 (aggregate of p1/24 and p2/24) 197 and p4/24 are announced to AS3. 199 Operators MUST install egress filters to block data packets when the 200 destination address belongs to an internal prefix. Similarly, any 201 known single-homed customer prefix MUST also be included in the 202 egress filters except on the interface for that customer. This 203 mitigates looping in the data plane when connection to such an 204 internal or customer prefix is lost. This mechanism effectively 205 compensates for the lack of the additional loop detection capability 206 accorded by AS_SETs (if they were allowed). 208 6. Security Considerations 210 This document obsoletes the use of aggregation techniques that create 211 AS_SETs or AS_CONFED_SETs. Obsoleting these path segment types from 212 BGP and removal of the related code from implementations would 213 potentially decrease the attack surface for BGP. Deployments of new 214 BGP security technologies [RFC6480] [RFC6811] [RFC8205] benefit 215 greatly if AS_SET and AS_CONFED_SET are not used in BGP. 217 7. IANA Considerations 219 This document requires no IANA actions. 221 8. Acknowledgements 223 The authors would like to thank John Heasley, Job Snijders, Jared 224 Mauch, Jakob Heitz, Keyur Patel, Douglas Montgomery, Randy Bush, 225 Susan Hares, John Scudder, Curtis Villamizar, Danny McPherson, Chris 226 Morrow, Tom Petch, Ilya Varlashkin, Enke Chen, Tony Li, Florian 227 Weimer, John Leslie, Paul Jakma, Rob Austein, Russ Housley, Sandra 228 Murphy, Steve Bellovin, Steve Kent, Steve Padgett, Alfred Hoenes, and 229 Alvaro Retana for comments and suggestions. 231 9. References 233 9.1. Normative References 235 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 236 Requirement Levels", BCP 14, RFC 2119, 237 DOI 10.17487/RFC2119, March 1997, 238 . 240 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 241 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 242 DOI 10.17487/RFC4271, January 2006, 243 . 245 [RFC5065] Traina, P., McPherson, D., and J. Scudder, "Autonomous 246 System Confederations for BGP", RFC 5065, 247 DOI 10.17487/RFC5065, August 2007, 248 . 250 9.2. Informative References 252 [Analysis] Hannachi, L. and K. Sriram, "Detailed analysis of AS_SETs 253 in BGP updates", NIST Robust Inter-domain Routing Project 254 Website , October 2019, 255 . 258 [IANA-SP-ASN] 259 "Special-Purpose Autonomous System (AS) Numbers", 260 . 263 [RFC3779] Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP 264 Addresses and AS Identifiers", RFC 3779, 265 DOI 10.17487/RFC3779, June 2004, 266 . 268 [RFC6472] Kumari, W. and K. Sriram, "Recommendation for Not Using 269 AS_SET and AS_CONFED_SET in BGP", BCP 172, RFC 6472, 270 DOI 10.17487/RFC6472, December 2011, 271 . 273 [RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support 274 Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480, 275 February 2012, . 277 [RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R. 278 Austein, "BGP Prefix Origin Validation", RFC 6811, 279 DOI 10.17487/RFC6811, January 2013, 280 . 282 [RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K. 283 Patel, "Revised Error Handling for BGP UPDATE Messages", 284 RFC 7606, DOI 10.17487/RFC7606, August 2015, 285 . 287 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 288 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 289 May 2017, . 291 [RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol 292 Specification", RFC 8205, DOI 10.17487/RFC8205, September 293 2017, . 295 Authors' Addresses 297 Warren Kumari 298 Google, Inc. 299 1600 Amphitheatre Parkway 300 Mountain View, CA 94043 301 United States of America 303 Phone: +1 571 748 4373 304 Email: warren@kumari.net 306 Kotikalapudi Sriram 307 USA NIST 308 100 Bureau Drive 309 Gaithersburg, MD 20899 310 United States of America 312 Phone: +1 301 975 3973 313 Email: ksriram@nist.gov 315 Lilia Hannachi 316 USA NIST 317 100 Bureau Drive 318 Gaithersburg, MD 20899 319 United States of America 321 Phone: +1 301 975 3259 322 Email: lilia.hannachi@nist.gov 324 Jeffrey Haas 325 Juniper Networks, Inc. 326 1133 Innovation Way 327 Sunnyvale, CA 94089 328 United States of America 330 Email: jhaas@juniper.net