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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Possible downref: Non-RFC (?) normative reference: ref. 'ISO10589' Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Networking Working Group L. Ginsberg 3 Internet-Draft P. Wells 4 Intended status: Standards Track Cisco Systems 5 Expires: February 9, 2017 B. Decraene 6 Orange 7 T. Przygienda 8 Juniper 9 H. Gredler 10 Private Contributer 11 August 08, 2016 13 IS-IS Minimum Remaining Lifetime 14 draft-ietf-isis-remaining-lifetime-03.txt 16 Abstract 18 Corruption of the Remainining Lifetime Field in a Link State PDU can 19 go undetected. In certain scenarios this may cause or exacerbate 20 flooding storms. It is also a possible denial of service attack 21 vector. This document defines a backwards compatible solution to 22 this problem. 24 Requirements Language 26 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 27 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 28 document are to be interpreted as described in RFC 2119 [RFC2119]. 30 Status of This Memo 32 This Internet-Draft is submitted in full conformance with the 33 provisions of BCP 78 and BCP 79. 35 Internet-Drafts are working documents of the Internet Engineering 36 Task Force (IETF). Note that other groups may also distribute 37 working documents as Internet-Drafts. The list of current Internet- 38 Drafts is at http://datatracker.ietf.org/drafts/current/. 40 Internet-Drafts are draft documents valid for a maximum of six months 41 and may be updated, replaced, or obsoleted by other documents at any 42 time. It is inappropriate to use Internet-Drafts as reference 43 material or to cite them other than as "work in progress." 45 This Internet-Draft will expire on February 9, 2017. 47 Copyright Notice 49 Copyright (c) 2016 IETF Trust and the persons identified as the 50 document authors. All rights reserved. 52 This document is subject to BCP 78 and the IETF Trust's Legal 53 Provisions Relating to IETF Documents 54 (http://trustee.ietf.org/license-info) in effect on the date of 55 publication of this document. Please review these documents 56 carefully, as they describe your rights and restrictions with respect 57 to this document. Code Components extracted from this document must 58 include Simplified BSD License text as described in Section 4.e of 59 the Trust Legal Provisions and are provided without warranty as 60 described in the Simplified BSD License. 62 Table of Contents 64 1. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 2 65 2. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . 3 66 3. Deployment Considerations . . . . . . . . . . . . . . . . . . 5 67 3.1. Inconsistent Values for MaxAge . . . . . . . . . . . . . 5 68 3.2. Reporting Corrupted Lifetime . . . . . . . . . . . . . . 5 69 3.3. Impact of Delayed LSP Purging . . . . . . . . . . . . . . 6 70 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 71 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 72 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 73 7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 7 74 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 75 8.1. Normative References . . . . . . . . . . . . . . . . . . 7 76 8.2. Informational References . . . . . . . . . . . . . . . . 8 77 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 79 1. Problem Statement 81 [ISO10589] defines the format of a Link State PDU (LSP) which 82 includes a Remaining Lifetime field. This field is set by the 83 originator based on local configuration and then decremented by all 84 systems once the entry is stored in their Link State PDU Database 85 (LSPDB) consistent with the passing of time. This allows all 86 Intermediate Systems (ISs) to age out the LSP at approximately the 87 same time. 89 Each LSP also has a checksum field to allow receiving systems to 90 detect errors which may have occurred during transmission. As the 91 Remaining Lifetime field changes as it is flooded and as the checksum 92 field MUST NOT be altered by receiving ISs the Remaining Lifetime is 93 deliberately excluded from the checksum calculation. In cases where 94 cryptographic authentication is included in an LSP ([RFC5304] or 96 [RFC5310]) the Remaining Lifetime field is also excluded from the 97 hash calculation. If the Remaining Lifetime field gets corrupted 98 during flooding this corruption is therefore undetectable. The 99 consequences of such corruption depend upon how the Remaining 100 Lifetime is altered. 102 In cases where the Remaining Lifetime becomes larger than the 103 originator intended the impact is benign. As the originator is 104 responsible for refreshing the LSP before it ages out a new version 105 of the LSP will be generated before the LSP ages out - so no harm is 106 done. 108 In cases where the Remaining Lifetime field becomes smaller than the 109 originator intended the LSP may age out prematurely (i.e. before the 110 originator refreshes the LSP). This has two negative consequences: 112 1. The LSP will be purged by an IS when the Remaining Lifetime 113 expires. This will cause a temporary loss of reachability to 114 destinations impacted by the content of that LSP. 116 2. Unnecessary LSP flooding will occur as a result of the premature 117 purge and subsequent regeneration/flooding of a new version of 118 the LSP by the originator 120 If the corrupted Remaining Lifetime is only modestly shorter than the 121 lifetime assigned by the originator the negative impacts are also 122 modest. If, however, the corrupted Remaining Lifetime becomes very 123 small, then the negative impacts can become significant - especially 124 in cases where the cause of the corruption is persistent so that the 125 cycle repeats itself frequently. 127 A backwards compatible solution to this problem is defined in the 128 following sections. 130 2. Solution 132 As discussed in the previous section, the problematic case is when 133 Remaining Lifetime is corrupted and becomes much smaller than it 134 should be. The goal of the solution is then to prevent premature 135 purging. 137 Under normal circumstances updates to an LSP - including purging if 138 appropriate - are the responsibility of the originator of the LSP. 139 There is a maximum time between generations of a given LSP. Once 140 this time has expired it is the responsibility of the originator to 141 refresh the LSP (i.e. issue a new version with higher sequence 142 number) even if the contents of the LSP have not changed. [ISO10589] 143 specifies that maximumLSPGenerationInterval MUST be sufficiently less 144 than the maximum lifetime of an LSP so that the new version can be 145 flooded network wide before the old version ages out on any IS. 147 There are two cases where a system other than the originator of an 148 LSP is allowed to purge an LSP: 150 1. The LSP ages out. This should only occur if the originating IS 151 is no longer reachable and therefore is unable to update the LSP 153 2. There is a Designated Intermediate System (DIS) change on a LAN. 154 The pseudo-node LSPs generated by the previous DIS are no longer 155 required and MAY be purged by the new DIS. 157 In both of these cases purging is not necessary for correct operation 158 of the protocol. It is provided as an optimization to remove stale 159 entries from the LSPDB. 161 In cases where the Remaining Lifetime in a received LSP has been 162 corrupted and is smaller than the remaining lifetime at the 163 originating node when the RemainingLifetime expires on the receiving 164 node it can appear as if the originating IS has failed to regenerate 165 the LSP (case #1 above) when in fact the LSP still has significant 166 lifetime remaining. To prevent this from having a negative impact a 167 modest change to the storage of "new" LSPs in the LSPDB is specified. 169 [ISO10589] Section 7.3.16 defines the rules to determine whether a 170 received LSP is older, the same, or newer than the copy of the same 171 LSP in the receiver's LSPDB. The key elements are: 173 o Higher sequence numbers are newer 175 o If sequence numbers are the same, an LSP with zero 176 RemainingLifetime (a purged LSP) is newer than the same LSP w non- 177 zero RemainingLifetime 179 o If both the received and local copy of the LSP have non-zero 180 RemainingLifetime they are considered the same even if the 181 RemainingLifetimes differ 183 [ISO10589] Section 7.3.15.1.e(1) defines the actions to take on 184 receipt of an LSP generated by another IS which is newer than the 185 local copy and has a non-zero RemainingLifetime. An additional 186 action is added: 188 vi. If the RemainingLifetime of the new LSP is less than MaxAge it 189 is set to MaxAge 190 This additional action insures that no matter what value of Remaining 191 Lifetime is received a system other than the originator of an LSP 192 will never purge the LSP until the LSP has existed in the database 193 for at least MaxAge. 195 It is important to note that no change is proposed for handling the 196 receipt of purged LSPs. The rules specified in [ISO10589] 197 Section 7.3.15.1b still apply i.e., an LSP received with zero 198 RemainingLifetime is still considered newer than a matching LSP with 199 non-zero RemainingLifetime. Therefore the changes proposed here will 200 not result in LSPDB inconsistency among routers in the newtork. 202 3. Deployment Considerations 204 This section discusses some possible deployment issues for this 205 protocol extension. 207 3.1. Inconsistent Values for MaxAge 209 [ISO10589] defines MaxAge (the maximum value for Remaining Lifetime 210 in an LSP) as an architectural constant of 20 minutes (1200 seconds). 211 However, in practice, implementations have supported allowing this 212 value to be configurable. The common intent of a configurable value 213 is to support longer lifetimes than the default - thus reducing the 214 periodic regeneration of LSPs in the absence of topology changes. 215 See a discussion of this point in [RFC3719]. It is therefore 216 possible for the value of MaxAge on the IS which originates an LSP to 217 be higher or lower than the value of MaxAge on the ISs which receive 218 the LSP. 220 If the value of MaxAge of the IS which originated the LSP is smaller 221 than the value of MaxAge of the receiver of an LSP, then setting the 222 RemainingLifetime of the received LSP to the local value of MaxAge 223 will insure that it is not purged prematurely. However, if the value 224 of MaxAge on the receiver is less than that of the originator then it 225 is still possible when using the extension defined in the previous 226 section to have an LSP purged prematurely. Implementors of this 227 extension MAY wish to protect against this case by making the value 228 to which RemainingLifetime is set under the conditions described in 229 the previous section configurable. If that is done the configured 230 value MUST be greater than or equal to the locally configured value 231 of MaxAge. 233 3.2. Reporting Corrupted Lifetime 235 Reporting reception of an LSP with a possible corrupt 236 RemainingLifetime field can be useful in identifying a problem in the 237 network. In order to minimize the reports of false positives the 238 following algorithm SHOULD be used in determining whether the 239 RemainingLifetime in the received LSP is possibly corrupt: 241 o The LSP has passed all acceptance tests as specified in [ISO10589] 242 Section 7.3.15.1 244 o The LSP is newer than the copy in the local LSPDB (including the 245 case of not being present in the LSPDB) 247 o RemainingLifetime in the received LSP is less than ZeroAgeLifetime 249 o The adjacency to the neighbor from which the LSP is received has 250 been up for a minimum of ZeroAgeLifetime 252 In such a case an IS SHOULD generate a CorruptRemainingLifetime 253 event. 255 Note that it is not possible to guarantee that all cases of corrupt 256 RemainingLifetime will be detected using the above algorithm. It is 257 also not possible to guarantee that all CorruptRemainingLifetime 258 events reported using the above algorithm are valid. As a diagnostic 259 aid an implementation MAY wish to retain the value of 260 RemainingLifetime received when the LSP was added to the LSPDB. 262 3.3. Impact of Delayed LSP Purging 264 The extensions defined in this document may result in retaining an 265 LSP longer than its original lifetime. In order for this to occur 266 the scheduled refresh of the LSP by the originator of the LSP must 267 fail to occur - which implies the originator is no longer reachable. 268 In such a case its neighbors will update their own LSPs reporting the 269 loss of connectivity to the originator. LSPs from a node which is 270 unreachable (failure of the two-way-connectivity check) MUST NOT be 271 used. Note this behavior applies to ALL information in the set of 272 LSPs from such a node. 274 Retention of stale LSPs therefore has no negative side effects other 275 than requiring additional memory for the LSPDB. In networks where a 276 combination of pathological behaviors (LSP corruption, frequent 277 resetting of nodes in the network) is seen this could lead to a large 278 number of stale LSPs being retained - but such networks are already 279 compromised. 281 4. IANA Considerations 283 None. 285 5. Security Considerations 287 The ability to introduce corrupt LSPs is not altered by the rules 288 defined in this document. Use of authentication as defined in 289 [RFC5304] and [RFC5310] prevents such LSPs from being intentionally 290 introduced. A "man-in-the-middle" attack which modifies an existing 291 LSP by changing the Remaining Lifetime to a small value can cause 292 premature purges even in the presence of cryptographic 293 authentication. The mechanisms defined in this document prevent such 294 an attack from being effective. 296 6. Acknowledgements 298 The problem statement in [LIFE-PROB] motivated this work. 300 7. Contributors 302 The following people gave a substantial conrtibution to the content 303 of this document and should be considered as co-authors: 305 Stefano Previdi 306 Cisco Systems 308 Email: sprevidi@cisco.com 310 8. References 312 8.1. Normative References 314 [ISO10589] 315 International Organization for Standardization, 316 "Intermediate system to Intermediate system intra-domain 317 routeing information exchange protocol for use in 318 conjunction with the protocol for providing the 319 connectionless-mode Network Service (ISO 8473)", ISO/ 320 IEC 10589:2002, Second Edition, Nov 2002. 322 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 323 Requirement Levels", BCP 14, RFC 2119, 324 DOI 10.17487/RFC2119, March 1997, 325 . 327 [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic 328 Authentication", RFC 5304, DOI 10.17487/RFC5304, October 329 2008, . 331 [RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., 332 and M. Fanto, "IS-IS Generic Cryptographic 333 Authentication", RFC 5310, DOI 10.17487/RFC5310, February 334 2009, . 336 8.2. Informational References 338 [LIFE-PROB] 339 "IS-IS LSP lifetime corruption - Problem Statement, draft- 340 decraene-isis-lsp-lifetime-problem-statement-02(work in 341 progress)", July 2016. 343 [RFC3719] Parker, J., Ed., "Recommendations for Interoperable 344 Networks using Intermediate System to Intermediate System 345 (IS-IS)", RFC 3719, DOI 10.17487/RFC3719, February 2004, 346 . 348 Authors' Addresses 350 Les Ginsberg 351 Cisco Systems 352 510 McCarthy Blvd. 353 Milpitas, CA 95035 354 USA 356 Email: ginsberg@cisco.com 358 Paul Wells 359 Cisco Systems 360 170 W Tasman Dr 361 San Jose, Ca 95035 362 USA 364 Email: pauwells@cisco.com 366 Bruno Decraene 367 Orange 368 38 rue du General Leclerc 369 Issy Moulineaux cedex 9 92794 370 France 372 Email: bruno.decraene@orange.com 373 Tony Przygienda 374 Juniper 375 1137 Innovation Way 376 Sunnyvale, Ca 94089 377 USA 379 Email: prz@juniper.net 381 Hannes Gredler 382 Private Contributer 384 Email: hannes@gredler.at