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Dhamija 9 Rakuten 10 March 25, 2022 12 IGP Unreachable Prefix Announcement 13 draft-ppsenak-lsr-igp-ureach-prefix-announce-00 15 Abstract 17 In the presence of summarization, there is a need to signal loss of 18 reachability to an individual prefix covered by the summary in order 19 to enable fast convergence away from paths to the node which owns the 20 prefix which is no longer reachable. This document describes how to 21 use existing protocol mechanisms in IS-IS and OSPF to advertise such 22 prefix reachability loss. 24 Requirements Language 26 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 27 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 28 "OPTIONAL" in this document are to be interpreted as described in BCP 29 14 [RFC2119][RFC8174] when, and only when, they appear in all 30 capitals, as shown here. 32 Status of This Memo 34 This Internet-Draft is submitted in full conformance with the 35 provisions of BCP 78 and BCP 79. 37 Internet-Drafts are working documents of the Internet Engineering 38 Task Force (IETF). Note that other groups may also distribute 39 working documents as Internet-Drafts. The list of current Internet- 40 Drafts is at https://datatracker.ietf.org/drafts/current/. 42 Internet-Drafts are draft documents valid for a maximum of six months 43 and may be updated, replaced, or obsoleted by other documents at any 44 time. It is inappropriate to use Internet-Drafts as reference 45 material or to cite them other than as "work in progress." 47 This Internet-Draft will expire on September 26, 2022. 49 Copyright Notice 51 Copyright (c) 2022 IETF Trust and the persons identified as the 52 document authors. All rights reserved. 54 This document is subject to BCP 78 and the IETF Trust's Legal 55 Provisions Relating to IETF Documents 56 (https://trustee.ietf.org/license-info) in effect on the date of 57 publication of this document. Please review these documents 58 carefully, as they describe your rights and restrictions with respect 59 to this document. Code Components extracted from this document must 60 include Simplified BSD License text as described in Section 4.e of 61 the Trust Legal Provisions and are provided without warranty as 62 described in the Simplified BSD License. 64 Table of Contents 66 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 67 2. Supporting UPA in IS-IS . . . . . . . . . . . . . . . . . . . 3 68 2.1. Advertisement of UPA in IS-IS . . . . . . . . . . . . . . 3 69 2.2. Propagation of UPA in IS-IS . . . . . . . . . . . . . . . 4 70 3. Supporting UPA in OSPF . . . . . . . . . . . . . . . . . . . 4 71 3.1. Advertisement of UPA in OSPF . . . . . . . . . . . . . . 5 72 3.2. Propagation of UPA in OSPF . . . . . . . . . . . . . . . 5 73 4. Deployment Considerations for UPA . . . . . . . . . . . . . . 5 74 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 75 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 76 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 77 8. Normative References . . . . . . . . . . . . . . . . . . . . 6 78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 80 1. Introduction 82 Link-state IGP protocols like IS-IS and OSPF are primarily used to 83 distribute routing information between routers belonging to a single 84 Autonomous System (AS) and to calculate the reachability for IPv4 or 85 IPv6 prefixes advertised by the individual nodes inside the AS. Each 86 node advertises the state of its local adjacencies, connected 87 prefixes, capabilities, etc. The collection of these states from all 88 the routers inside the area form a link-state database (LSDB) that 89 describes the topology of the area and holds additional state 90 information about the prefixes, router capabilities, etc. 92 The growth of networks running a link-state routing protocol results 93 in the addition of more state which leads to scalability and 94 convergence challenges. The organization of networks into levels/ 95 areas and IGP domains helps limit the scope of link-state information 96 within certain boundaries. However, the state related to prefix 97 reachability often requires propagation across a multi-area/ level 98 and/or multi-domain IGP network. Techniques such as summarization 99 have been used traditionally to address the scale challenges 100 associated with advertising prefix state outside of the local area/ 101 domain. However, this results in suppression of the individual 102 prefix state that is useful for triggering fast-convergence 103 mechanisms outside of the IGPs - e.g., BGP PIC Edge [I-D.ietf-rtgwg- 104 bgp-pic]. 106 This document describes how the use of existing protocol mechanisms 107 can support the necessary functionality without the need for any 108 protocol extensions. The functionality being described is called 109 Unreachable Prefix Announcement (UPA). 111 2. Supporting UPA in IS-IS 113 [RFC5305] defines the encoding for advertising IPv4 prefixes using 4 114 octets of metric information. Section 4 specifies: 116 "If a prefix is advertised with a metric larger then MAX_PATH_METRIC 117 (0xFE000000, see paragraph 3.0), this prefix MUST NOT be considered 118 during the normal SPF computation. This allows advertisement of a 119 prefix for purposes other than building the normal IP routing table. 120 " 122 Similarly, [RFC5308] defines the encoding for advertising IPv6 123 prefixes using 4 octets of metric information. Section 2 states: 125 "...if a prefix is advertised with a metric larger than 126 MAX_V6_PATH_METRIC (0xFE000000), this prefix MUST NOT be considered 127 during the normal Shortest Path First (SPF) computation. This will 128 allow advertisement of a prefix for purposes other than building the 129 normal IPv6 routing table." 131 This functionality can be used to advertise a prefix (IPv4 or IPv6) 132 in a manner which indicates that reachability has been lost - and to 133 do so without requiring all nodes in the network to be upgraded to 134 support the functionality. 136 2.1. Advertisement of UPA in IS-IS 138 Existing nodes in a network which receive UPA advertisements will 139 ignore them. This allows flooding of such advertisements to occur 140 without the need to upgrade all nodes in a network. 142 Recognition of the advertisement as UPA is only required on routers 143 which have a use case for this information. Area Border Routers 144 (ABRs), which would be responsible for propagating UPA advertisements 145 into other areas would need to recognize such advertisements. 147 As per the definitions referenced in the preceding section, any 148 prefix advertisement with a metric value greater than 0xFE000000 can 149 be used for purposes other than normal routing calculations. Such an 150 advertisement can be interpreted by the receiver as a UPA. 152 Optionally, an implementation may use local configuration to limit 153 the set of metric values which will be interpreted as UPA. The only 154 restriction is that such values MUST be greater than 0xFE000000. 156 2.2. Propagation of UPA in IS-IS 158 ISIS L1/L2 routers may wish to advertise received UPAs into other 159 areas (upwards and/or downwards). When propagating UPAs the original 160 metric value MUST be preserved. The cost to reach the originator of 161 the received UPA MUST NOT be considered when readvertising the UPA. 163 3. Supporting UPA in OSPF 165 [RFC2328] Appendix B defines the following architectural constant for 166 OSPF: 168 "LSInfinity The metric value indicating that the destination 169 described by an LSA is unreachable. Used in summary-LSAs and AS- 170 external-LSAs as an alternative to premature aging (see 171 Section 14.1). It is defined to be the 24-bit binary value of all 172 ones: 0xffffff." 174 [RFC5340] Appendix B states: 176 "Architectural constants for the OSPF protocol are defined in 177 Appendix B of OSPFV2." 179 indicating that these same constants are applicable to OSPFv3. 181 [RFC2328] section 14.1. also describes the usage of LSInfinity as a 182 way to indicate loss of prefix reachability: 184 "Premature aging can also be used when, for example, one of the 185 router's previously advertised external routes is no longer 186 reachable. In this circumstance, the router can flush its AS- 187 external-LSA from the routing domain via premature aging. This 188 procedure is preferable to the alternative, which is to originate a 189 new LSA for the destination specifying a metric of LSInfinity." 191 3.1. Advertisement of UPA in OSPF 193 Using the existing mechanism already defined in the standards, as 194 described in previous section, an advertisement of the inter-area or 195 external prefix inside OSPF or OSPFv3 LSA that has the age set to 196 value lower than MaxAge and metic set to LSInfinity can be 197 interpreted by the receiver as a UPA. 199 Existing nodes in a network which receive UPA advertisements will 200 propagate it following existing standard procedures defined by OSPF. 202 OSPF Area Border Routers (ABRs), which would be responsible for 203 propagating UPA advertisements into other areas would need to 204 recognize such advertisements. 206 3.2. Propagation of UPA in OSPF 208 OSPF ABRs may wish to advertise received UPAs into other connected 209 areas. When doing so, the original LSInfinity metric value in UPA 210 MUST be preserved. The cost to reach the originator of the received 211 UPA MUST NOT be considered when readvertising the UPA to connected 212 areas. 214 4. Deployment Considerations for UPA 216 The intent of UPA is to provide an event driven signal of the 217 transition of a destination from reachable to unreachable. It is not 218 intended to advertise a persistent state. UPA advertisements should 219 therefore be withdrawn after a modest amount of time, that would 220 provides sufficient time for UPA to be flooded network-wide and acted 221 upon by receiving nodes, but limits the presence of UPA in the 222 network to a short time period. The time the UPA is kept in the 223 network SHOULD also reflect the intended use-case for which the UPA 224 was advertised. 226 As UPA advertisements in ISIS are advertised in existing Link State 227 PDUs (LSPs) and the unit of flooding in IS-IS is an LSP, it is 228 recommended that, when possible, UPAs are advertised in LSPs 229 dedicated to this type of advertisement. This will minimize the 230 number of LSPs which need to be updated when UPAs are advertised and 231 withdrawn. 233 In OSPF and OSPFv3, each inter-area and external prefix is advertised 234 in it's own LSA, so the above optimisation does not apply to OSPF. 236 It is also recommended that implementations limit the number of UPA 237 advertisements which can be originated at a given time. 239 5. IANA Considerations 241 This document makes no requests to IANA. 243 6. Security Considerations 245 The use of UPAs introduces the possibility that an attacker could 246 inject a false, but apparently valid, UPA. However, the risk of this 247 occurring is no greater than the risk today of an attacker injecting 248 any other type of false advertisement . 250 The risks can be reduced by the use of existing security extensions 251 as described in [RFC5304] and [RFC5310] for IS-IS, in [RFC2328][ and 252 [RFC7474] for OSPFv2, and in [RFC5340] and [RFC4552] for OSPFv3. 254 7. Acknowledgements 256 The authors would like to thank Kamran Raza and Michael MacKenzie for 257 their contribution to the overall solution proposed in this document. 259 8. Normative References 261 [ISO10589] 262 International Organization for Standardization, 263 "Intermediate system to Intermediate system intra-domain 264 routeing information exchange protocol for use in 265 conjunction with the protocol for providing the 266 connectionless-mode Network Service (ISO 8473)", Nov 2002. 268 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 269 Requirement Levels", BCP 14, RFC 2119, 270 DOI 10.17487/RFC2119, March 1997, 271 . 273 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, 274 DOI 10.17487/RFC2328, April 1998, 275 . 277 [RFC4552] Gupta, M. and N. Melam, "Authentication/Confidentiality 278 for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006, 279 . 281 [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic 282 Authentication", RFC 5304, DOI 10.17487/RFC5304, October 283 2008, . 285 [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic 286 Engineering", RFC 5305, DOI 10.17487/RFC5305, October 287 2008, . 289 [RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308, 290 DOI 10.17487/RFC5308, October 2008, 291 . 293 [RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., 294 and M. Fanto, "IS-IS Generic Cryptographic 295 Authentication", RFC 5310, DOI 10.17487/RFC5310, February 296 2009, . 298 [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF 299 for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008, 300 . 302 [RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed., 303 "Security Extension for OSPFv2 When Using Manual Key 304 Management", RFC 7474, DOI 10.17487/RFC7474, April 2015, 305 . 307 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 308 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 309 May 2017, . 311 Authors' Addresses 313 Peter Psenak (editor) 314 Cisco Systems 315 Pribinova Street 10 316 Bratislava 81109 317 Slovakia 319 Email: ppsenak@cisco.com 321 Clarence Filsfils 322 Cisco Systems 323 Brussels 324 Belgium 326 Email: cfilsfil@cisco.com 327 Stephane Litkowski 328 Cisco Systems 329 La Rigourdiere 330 Cesson Sevigne 331 France 333 Email: slitkows@cisco.com 335 Daniel Voyer 336 Bell Canada 338 Email: daniel.voyer@bell.ca 340 Amit Dhamija 341 Rakuten 343 Email: amit.dhamija@rakuten.com