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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 4, 2017 B. Decraene 6 Orange 7 T. Przygienda 8 Juniper 9 H. Gredler 10 Private Contributer 11 August 03, 2016 13 IS-IS Minimum Remaining Lifetime 14 draft-ietf-isis-remaining-lifetime-02.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 4, 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 Each Link State PDU (LSP) includes a Remaining Lifetime field. This 82 field is set by the originator based on local configuration and then 83 decremented by all systems once the entry is stored in their Link 84 State PDU Database (LSPDB) consistent with the passing of time. This 85 allows all Intermediate Systems (ISs) to age out the LSP at 86 approximately the same time. 88 Each LSP also has a checksum field to allow receiving systems to 89 detect errors which may have occurred during transmission. As the 90 Remaining Lifetime field changes as it is flooded and as the checksum 91 field MUST NOT be altered by receiving ISs the Remaining Lifetime is 92 deliberately excluded from the checksum calculation. In cases where 93 cryptographic authentication is included in an LSP ([RFC5304] or 94 [RFC5310]) the Remaining Lifetime field is also excluded from the 95 hash calculation. If the Remaining Lifetime field gets corrupted 96 during flooding this corruption is therefore undetectable. The 97 consequences of such corruption depend upon how the Remaining 98 Lifetime is altered. 100 In cases where the Remaining Lifetime becomes larger than the 101 originator intended the impact is benign. As the originator is 102 responsible for refreshing the LSP before it ages out a new version 103 of the LSP will be generated before the LSP ages out - so no harm is 104 done. 106 In cases where the Remaining Lifetime field becomes smaller than the 107 originator intended the LSP may age out prematurely (i.e. before the 108 originator refreshes the LSP). This has two negative consequences: 110 1. The LSP will be purged by an IS when the Remaining Lifetime 111 expires. This will cause a temporary loss of reachability to 112 destinations impacted by the content of that LSP. 114 2. Unnecessary LSP flooding will occur as a result of the premature 115 purge and subsequent regeneration/flooding of a new version of 116 the LSP by the originator 118 If the corrupted Remaining Lifetime is only modestly shorter than the 119 lifetime assigned by the originator the negative impacts are also 120 modest. If, however, the corrupted Remaining Lifetime becomes very 121 small, then the negative impacts can become significant - especially 122 in cases where the cause of the corruption is persistent so that the 123 cycle repeats itself frequently. 125 A backwards compatible solution to this problem is defined in the 126 following sections. 128 2. Solution 130 As discussed in the previous section, the problematic case is when 131 Remaining Lifetime is corrupted and becomes much smaller than it 132 should be. The goal of the solution is then to prevent premature 133 purging. 135 Under normal circumstances updates to an LSP - including purging if 136 appropriate - are the responsibility of the originator of the LSP. 137 There is a maximum time between generations of a given LSP. Once 138 this time has expired it is the responsibility of the originator to 139 refresh the LSP (i.e. issue a new version with higher sequence 140 number) even if the contents of the LSP have not changed. [ISO10589] 141 specifies that maximumLSPGenerationInterval MUST be sufficiently less 142 than the maximum lifetime of an LSP so that the new version can be 143 flooded network wide before the old version ages out on any IS. 145 There are two cases where a system other than the originator of an 146 LSP is allowed to purge an LSP: 148 1. The LSP ages out. This should only occur if the originating IS 149 is no longer reachable and therefore is unable to update the LSP 151 2. There is a Designated Intermediate System (DIS) change on a LAN. 152 The pseudo-node LSPs generated by the previous DIS are no longer 153 required and MAY be purged by the new DIS. 155 In both of these cases purging is not necessary for correct operation 156 of the protocol. It is provided as an optimization to remove stale 157 entries from the LSPDB. 159 In cases where the Remaining Lifetime in a received LSP has been 160 corrupted and is smaller than the remaining lifetime at the 161 originating node when the RemainingLifetime expires on the receiving 162 node it can appear as if the originating IS has failed to regenerate 163 the LSP (case #1 above) when in fact the LSP still has significant 164 lifetime remaining. To prevent this from having a negative impact a 165 modest change to the storage of "new" LSPs in the LSPDB is specified. 167 [ISO10589] Section 7.3.16 defines the rules to determine whether a 168 received LSP is older, the same, or newer than the copy of the same 169 LSP in the receiver's LSPDB. The key elements are: 171 o Higher sequence numbers are newer 173 o If sequence numbers are the same, an LSP with zero 174 RemainingLifetime (a purged LSP) is newer than the same LSP w non- 175 zero RemainingLifetime 177 o If both the received and local copy of the LSP have non-zero 178 RemainingLifetime they are considered the same even if the 179 RemainingLifetimes differ 181 [ISO10589] Section 7.3.15.1.e(1) defines the actions to take on 182 receipt of an LSP generated by another IS which is newer than the 183 local copy and has a non-zero RemainingLifetime. An additional 184 action is added: 186 vi. If the RemainingLifetime of the new LSP is less than MaxAge it 187 is set to MaxAge 188 This additional action insures that no matter what value of Remaining 189 Lifetime is received a system other than the originator of an LSP 190 will never purge the LSP until the LSP has existed in the database 191 for at least MaxAge. 193 It is important to note that no change is proposed for handling the 194 receipt of purged LSPs. The rules specified in [ISO10589] 195 Section 7.3.15.1b still apply i.e., an LSP received with zero 196 RemainingLifetime is still considered newer than a matching LSP with 197 non-zero RemainingLifetime. Therefore the changes proposed here will 198 not result in LSPDB inconsistency among routers in the newtork. 200 3. Deployment Considerations 202 This section discusses some possible deployment issues for this 203 protocol extension. 205 3.1. Inconsistent Values for MaxAge 207 [ISO10589] defines MaxAge (the maximum value for Remaining Lifetime 208 in an LSP) as an architectural constant of 20 minutes (1200 seconds). 209 However, in practice, implementations have supported allowing this 210 value to be configurable. The common intent of a configurable value 211 is to support longer lifetimes than the default - thus reducing the 212 periodic regeneration of LSPs in the absence of topology changes. 213 See a discussion of this point in [RFC3719]. It is therefore 214 possible for the value of MaxAge on the IS which originates an LSP to 215 be higher or lower than the value of MaxAge on the ISs which receive 216 the LSP. 218 If the value of MaxAge of the IS which originated the LSP is smaller 219 than the value of MaxAge of the receiver of an LSP, then setting the 220 RemainingLifetime of the received LSP to the local value of MaxAge 221 will insure that it is not purged prematurely. However, if the value 222 of MaxAge on the receiver is less than that of the originator then it 223 is still possible when using the extension defined in the previous 224 section to have an LSP purged prematurely. Implementors of this 225 extension MAY wish to protect against this case by making the value 226 to which RemainingLifetime is set under the conditions described in 227 the previous section configurable. If that is done the configured 228 value MUST be greater than or equal to the locally configured value 229 of MaxAge. 231 3.2. Reporting Corrupted Lifetime 233 Reporting reception of an LSP with a possible corrupt 234 RemainingLifetime field can be useful in identifying a problem in the 235 network. In order to minimize the reports of false positives the 236 following algorithm SHOULD be used in determining whether the 237 RemainingLifetime in the received LSP is possibly corrupt: 239 o The LSP has passed all acceptance tests as specified in [ISO10589] 240 Section 7.3.15.1 242 o The LSP is newer than the copy in the local LSPDB (including the 243 case of not being present in the LSPDB) 245 o RemainingLifetime in the received LSP is less than ZeroAgeLifetime 247 o The adjacency to the neighbor from which the LSP is received has 248 been up for a minimum of ZeroAgeLifetime 250 In such a case an IS SHOULD generate a CorruptRemainingLifetime 251 event. 253 Note that it is not possible to guarantee that all cases of corrupt 254 RemainingLifetime will be detected using the above algorithm. It is 255 also not possible to guarantee that all CorruptRemainingLifetime 256 events reported using the above algorithm are valid. As a diagnostic 257 aid an implementation MAY wish to retain the value of 258 RemainingLifetime received when the LSP was added to the LSPDB. 260 3.3. Impact of Delayed LSP Purging 262 The extensions defined in this document may result in retaining an 263 LSP longer than its original lifetime. In order for this to occur 264 the scheduled refresh of the LSP by the originator of the LSP must 265 fail to occur - which implies the originator is no longer reachable. 266 In such a case its neighbors will update their own LSPs reporting the 267 loss of connectivity to the originator. LSPs from a node which is 268 unreachable (failure of the two-way-connectivity check) MUST NOT be 269 used. Note this behavior applies to ALL information in the set of 270 LSPs from such a node. 272 Retention of stale LSPs therefore has no negative side effects other 273 than requiring additional memory for the LSPDB. In networks where a 274 combination of pathological behaviors (LSP corruption, frequent 275 resetting of nodes in the network) is seen this could lead to a large 276 number of stale LSPs being retained - but such networks are already 277 compromised. 279 4. IANA Considerations 281 None. 283 5. Security Considerations 285 The ability to introduce corrupt LSPs is not altered by the rules 286 defined in this document. Use of authentication as defined in 287 [RFC5304] and [RFC5310] prevents such LSPs from being intentionally 288 introduced. A "man-in-the-middle" attack which modifies an existing 289 LSP by changing the Remaining Lifetime to a small value can cause 290 premature purges even in the presence of cryptographic 291 authentication. The mechanisms defined in this document prevent such 292 an attack from being effective. 294 6. Acknowledgements 296 The problem statement in [LIFE-PROB] motivated this work. 298 7. Contributors 300 The following people gave a substantial conrtibution to the content 301 of this document and should be considered as co-authors: 303 Stefano Previdi 304 Cisco Systems 306 Email: sprevidi@cisco.com 308 8. References 310 8.1. Normative References 312 [ISO10589] 313 International Organization for Standardization, 314 "Intermediate system to Intermediate system intra-domain 315 routeing information exchange protocol for use in 316 conjunction with the protocol for providing the 317 connectionless-mode Network Service (ISO 8473)", ISO/ 318 IEC 10589:2002, Second Edition, Nov 2002. 320 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 321 Requirement Levels", BCP 14, RFC 2119, 322 DOI 10.17487/RFC2119, March 1997, 323 . 325 [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic 326 Authentication", RFC 5304, DOI 10.17487/RFC5304, October 327 2008, . 329 [RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., 330 and M. Fanto, "IS-IS Generic Cryptographic 331 Authentication", RFC 5310, DOI 10.17487/RFC5310, February 332 2009, . 334 8.2. Informational References 336 [LIFE-PROB] 337 "IS-IS LSP lifetime corruption - Problem Statement, draft- 338 decraene-isis-lsp-lifetime-problem-statement-02(work in 339 progress)", July 2016. 341 [RFC3719] Parker, J., Ed., "Recommendations for Interoperable 342 Networks using Intermediate System to Intermediate System 343 (IS-IS)", RFC 3719, DOI 10.17487/RFC3719, February 2004, 344 . 346 Authors' Addresses 348 Les Ginsberg 349 Cisco Systems 350 510 McCarthy Blvd. 351 Milpitas, CA 95035 352 USA 354 Email: ginsberg@cisco.com 356 Paul Wells 357 Cisco Systems 358 170 W Tasman Dr 359 San Jose, Ca 95035 360 USA 362 Email: pauwells@cisco.com 364 Bruno Decraene 365 Orange 366 38 rue du General Leclerc 367 Issy Moulineaux cedex 9 92794 368 France 370 Email: bruno.decraene@orange.com 371 Tony Przygienda 372 Juniper 373 1137 Innovation Way 374 Sunnyvale, Ca 94089 375 USA 377 Email: prz@juniper.net 379 Hannes Gredler 380 Private Contributer 382 Email: hannes@gredler.at