idnits 2.17.1 draft-ietf-lsr-isis-rfc5306bis-07.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (September 18, 2019) is 1682 days in the past. Is this intentional? 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: 0 errors (**), 0 flaws (~~), 1 warning (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IS-IS for IP Internets L. Ginsberg 3 Internet-Draft P. Wells 4 Obsoletes: 5306 (if approved) Cisco Systems, Inc. 5 Intended status: Standards Track September 18, 2019 6 Expires: March 21, 2020 8 Restart Signaling for IS-IS 9 draft-ietf-lsr-isis-rfc5306bis-07 11 Abstract 13 This document describes a mechanism for a restarting router to signal 14 to its neighbors that it is restarting, allowing them to reestablish 15 their adjacencies without cycling through the down state, while still 16 correctly initiating database synchronization. 18 This document additionally describes a mechanism for a router to 19 signal its neighbors that it is preparing to initiate a restart while 20 maintaining forwarding plane state. This allows the neighbors to 21 maintain their adjacencies until the router has restarted, but also 22 allows the neighbors to bring the adjacencies down in the event of 23 other topology changes. 25 This document additionally describes a mechanism for a restarting 26 router to determine when it has achieved Link State Protocol Data 27 Unit (LSP) database synchronization with its neighbors and a 28 mechanism to optimize LSP database synchronization, while minimizing 29 transient routing disruption when a router starts. 31 This document obsoletes RFC 5306. 33 Requirements Language 35 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 36 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 37 "OPTIONAL" in this document are to be interpreted as described in BCP 38 14 [RFC2119] [RFC8174] when, and only when, they appear in all 39 capitals, as shown here. 41 Status of This Memo 43 This Internet-Draft is submitted in full conformance with the 44 provisions of BCP 78 and BCP 79. 46 Internet-Drafts are working documents of the Internet Engineering 47 Task Force (IETF). Note that other groups may also distribute 48 working documents as Internet-Drafts. The list of current Internet- 49 Drafts is at https://datatracker.ietf.org/drafts/current/. 51 Internet-Drafts are draft documents valid for a maximum of six months 52 and may be updated, replaced, or obsoleted by other documents at any 53 time. It is inappropriate to use Internet-Drafts as reference 54 material or to cite them other than as "work in progress." 56 This Internet-Draft will expire on March 21, 2020. 58 Copyright Notice 60 Copyright (c) 2019 IETF Trust and the persons identified as the 61 document authors. All rights reserved. 63 This document is subject to BCP 78 and the IETF Trust's Legal 64 Provisions Relating to IETF Documents 65 (https://trustee.ietf.org/license-info) in effect on the date of 66 publication of this document. Please review these documents 67 carefully, as they describe your rights and restrictions with respect 68 to this document. Code Components extracted from this document must 69 include Simplified BSD License text as described in Section 4.e of 70 the Trust Legal Provisions and are provided without warranty as 71 described in the Simplified BSD License. 73 Table of Contents 75 1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3 76 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 77 3. Approach . . . . . . . . . . . . . . . . . . . . . . . . . . 4 78 3.1. Timers . . . . . . . . . . . . . . . . . . . . . . . . . 4 79 3.2. Restart TLV . . . . . . . . . . . . . . . . . . . . . . . 5 80 3.2.1. Use of RR and RA Bits . . . . . . . . . . . . . . . . 7 81 3.2.2. Use of the SA Bit . . . . . . . . . . . . . . . . . . 8 82 3.2.3. Use of PR and PA Bits . . . . . . . . . . . . . . . . 9 83 3.3. Adjacency (Re)Acquisition . . . . . . . . . . . . . . . . 11 84 3.3.1. Adjacency Reacquisition during Restart . . . . . . . 11 85 3.3.2. Adjacency Acquisition during Start . . . . . . . . . 13 86 3.3.3. Multiple Levels . . . . . . . . . . . . . . . . . . . 15 87 3.4. Database Synchronization . . . . . . . . . . . . . . . . 15 88 3.4.1. LSP Generation and Flooding and SPF Computation . . . 16 89 4. State Tables . . . . . . . . . . . . . . . . . . . . . . . . 19 90 4.1. Running Router . . . . . . . . . . . . . . . . . . . . . 19 91 4.2. Restarting Router . . . . . . . . . . . . . . . . . . . . 20 92 4.3. Starting Router . . . . . . . . . . . . . . . . . . . . . 22 93 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 94 6. Security Considerations . . . . . . . . . . . . . . . . . . . 23 95 7. Manageability Considerations . . . . . . . . . . . . . . . . 24 96 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24 97 9. Normative References . . . . . . . . . . . . . . . . . . . . 24 98 Appendix A. Summary of Changes from RFC 5306 . . . . . . . . . . 25 99 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 101 1. Overview 103 The Intermediate System to Intermediate System (IS-IS) routing 104 protocol [RFC1195] [ISO10589] is a link state intra-domain routing 105 protocol. Normally, when an IS-IS router is restarted, temporary 106 disruption of routing occurs due to events in both the restarting 107 router and the neighbors of the restarting router. 109 The router that has been restarted computes its own routes before 110 achieving database synchronization with its neighbors. The results 111 of this computation are likely to be non-convergent with the routes 112 computed by other routers in the area/domain. 114 Neighbors of the restarting router detect the restart event and cycle 115 their adjacencies with the restarting router through the down state. 116 The cycling of the adjacency state causes the neighbors to regenerate 117 their LSPs describing the adjacency concerned. This in turn causes a 118 temporary disruption of routes passing through the restarting router. 120 In certain scenarios, the temporary disruption of the routes is 121 highly undesirable. This document describes mechanisms to avoid or 122 minimize the disruption due to both of these causes. 124 When an adjacency is reinitialized as a result of a neighbor 125 restarting, a router does three things: 127 1. It causes its own LSP(s) to be regenerated, thus triggering SPF 128 runs throughout the area (or in the case of Level 2, throughout 129 the domain). 131 2. It sets SRMflags on its own LSP database on the adjacency 132 concerned. 134 3. In the case of a Point-to-Point link, it transmits a complete set 135 of Complete Sequence Number PDUs (CSNPs), over the adjacency. 137 In the case of a restarting router process, the first of these is 138 highly undesirable, but the second is essential in order to ensure 139 synchronization of the LSP database. 141 The third action above minimizes the number of LSPs that must be 142 exchanged and, if made reliable, provides a means of determining when 143 the LSP databases of the neighboring routers have been synchronized. 145 This is desirable whether or not the router is being restarted (so 146 that the overload bit can be cleared in the router's own LSP, for 147 example). 149 This document describes a mechanism for a restarting router to signal 150 to its neighbors that it is restarting. The mechanism further allows 151 the neighbors to reestablish their adjacencies with the restarting 152 router without cycling through the down state, while still correctly 153 initiating database synchronization. 155 This document additionally describes a mechanism for a restarting 156 router to determine when it has achieved LSP database synchronization 157 with its neighbors and a mechanism to optimize LSP database 158 synchronization and minimize transient routing disruption when a 159 router starts. 161 It is assumed that the three-way handshake [RFC5303] is being used on 162 Point-to-Point circuits. 164 2. Conventions Used in This Document 166 If the control and forwarding functions in a router can be maintained 167 independently, it is possible for the forwarding function state to be 168 maintained across a resumption of control function operations. This 169 functionality is assumed when the terms "restart/restarting" are used 170 in this document. 172 The terms "start/starting" are used to refer to a router in which the 173 control function has either commenced operations for the first time 174 or has resumed operations, but the forwarding functions have not been 175 maintained in a prior state. 177 The terms "(re)start/(re)starting" are used when the text is 178 applicable to both a "starting" and a "restarting" router. 180 The terms "normal IIH" or "IIH normal" refer to IS-IS Hellos (IIHs) 181 in which the Restart TLV (defined later in this document) has no 182 flags set. 184 3. Approach 186 3.1. Timers 188 Three additional timers, T1, T2, and T3, are required to support the 189 mechanisms defined in this document. Timers T1 and T2 are used both 190 by a restarting router and a starting router. Timer T3 is used only 191 by a restarting router. 193 NOTE: These timers are NOT applicable to a router which is preparing 194 to do a planned restart. 196 An instance of the timer T1 is maintained per interface, and 197 indicates the time after which an unacknowledged (re)start attempt 198 will be repeated. A typical value is 3 seconds. 200 An instance of the timer T2 is maintained for each LSP database 201 (LSPDB) present in the system. For example, for a Level 1/2 system, 202 there will be an instance of the timer T2 for Level 1 and an instance 203 for Level 2. This is the maximum time that the system will wait for 204 LSPDB synchronization. A typical value is 60 seconds. 206 A single instance of the timer T3 is maintained for the entire 207 system. It indicates the time after which the router will declare 208 that it has failed to achieve database synchronization (by setting 209 the overload bit in its own LSP). This is initialized to 65535 210 seconds, but is set to the minimum of the remaining times of received 211 IIHs containing a restart TLV with the Restart Acknowledgement (RA) 212 set and an indication that the neighbor has an adjacency in the "UP" 213 state to the restarting router. (See Section 3.2.1a.) 215 3.2. Restart TLV 217 A new TLV is defined to be included in IIH PDUs. The presence of 218 this TLV indicates that the sender supports the functionality defined 219 in this document. The TLV includes flags that are used to convey 220 information during a (re)start. All IIHs transmitted by a router 221 that supports this capability MUST include this TLV. 223 Type 211 225 Length: Number of octets in the Value field (1 to (3 + ID Length)) 227 Value 229 No. of octets 230 +-----------------------+ 231 | Flags | 1 232 +-----------------------+ 233 | Remaining Time | 2 234 +-----------------------+ 235 | Restarting Neighbor ID| ID Length 236 +-----------------------+ 238 Flags (1 octet) 240 0 1 2 3 4 5 6 7 242 +--+--+--+--+--+--+--+--+ 243 |Reserved|PA|PR|SA|RA|RR| 244 +--+--+--+--+--+--+--+--+ 246 RR - Restart Request 247 RA - Restart Acknowledgement 248 SA - Suppress adjacency advertisement 249 PR - Restart is planned 250 PA - Planned restart acknowledgement 252 Remaining Time (2 octets) 254 Remaining holding time (in seconds). 256 Required when the RA, PR, or PA bit is set. Otherwise 257 this field SHOULD be omitted when sent and 258 MUST be ignored when received. 260 Restarting Neighbor System ID (ID Length octets) 262 The System ID of the neighbor to which an RA/PA refers. 264 Required when the RA or PA bit is set. Otherwise 265 this field SHOULD be omitted when sent and 266 MUST be ignored when received. 268 Note: Very early draft versions of the restart functionality 269 did not include the Restarting Neighbor System ID in the TLV. 270 RFC 5306 allowed for the possibility of interoperating with 271 legacy implementations by stating that a router that 272 is expecting an RA on a LAN circuit should assume that the 273 acknowledgement is directed at the local system if the TLV 274 is received with RA set and Restarting Neighbor System ID 275 is not present. It is an implementation choice whether to 276 continue to accept (on a LAN) a TLV with RA set and 277 Restarting Neighbor System ID absent. Note that the omission 278 of the Restarting Neighbor System ID only introduces ambiguity 279 in the case where there are multiple systems on a LAN 280 simultaneously performing restart. 282 The functionality associated with each of the defined flags (as 283 described in the following sections) is mutually exclusive with any 284 of the other flags. Therefore, it is expected that at most one flag 285 will be set in a TLV. When transmitting a TLV multiple flags MUST 286 NOT be set. Received TLVs which have multiple flags set MUST be 287 ignored. 289 3.2.1. Use of RR and RA Bits 291 The RR bit is used by a (re)starting router to signal to its 292 neighbors that a (re)start is in progress, that an existing adjacency 293 SHOULD be maintained even under circumstances when the normal 294 operation of the adjacency state machine would require the adjacency 295 to be reinitialized, to request a set of CSNPs, and to request 296 setting of the SRMflags. 298 The RA bit is sent by the neighbor of a (re)starting router to 299 acknowledge the receipt of a restart TLV with the RR bit set. 301 When the neighbor of a (re)starting router receives an IIH with the 302 restart TLV having the RR bit set, if there exists on this interface 303 an adjacency in state "UP" with the same System ID, and in the case 304 of a LAN circuit, with the same source LAN address, then, 305 irrespective of the other contents of the "Intermediate System 306 Neighbors" option (LAN circuits) or the "Point-to-Point Three-Way 307 Adjacency" option (Point-to-Point circuits): 309 a. the state of the adjacency is not changed. If this is the first 310 IIH with the RR bit set that this system has received associated 311 with this adjacency, then the adjacency is marked as being in 312 "Restart mode" and the adjacency holding time is refreshed -- 313 otherwise, the holding time is not refreshed. The "remaining 314 time" transmitted according to (b) below MUST reflect the actual 315 time after which the adjacency will now expire. Receipt of an 316 IIH with the RR bit reset will clear the "Restart mode" state. 317 This procedure allows the restarting router to cause the neighbor 318 to maintain the adjacency long enough for restart to successfully 319 complete, while also preventing repetitive restarts from 320 maintaining an adjacency indefinitely. Whether or not an 321 adjacency is marked as being in "Restart mode" has no effect on 322 adjacency state transitions. 324 b. immediately (i.e., without waiting for any currently running 325 timer interval to expire, but with a small random delay of a few 326 tens of milliseconds on LANs to avoid "storms") transmit over the 327 corresponding interface an IIH including the restart TLV with the 328 RR bit clear and the RA bit set, in the case of Point-to-Point 329 adjacencies having updated the "Point-to-Point Three-Way 330 Adjacency" option to reflect any new values received from the 331 (re)starting router. (This allows a restarting router to quickly 332 acquire the correct information to place in its hellos.) The 333 "Remaining Time" MUST be set to the current time (in seconds) 334 before the holding timer on this adjacency is due to expire. If 335 the corresponding interface is a LAN interface, then the 336 Restarting Neighbor System ID SHOULD be set to the System ID of 337 the router from which the IIH with the RR bit set was received. 338 This is required to correctly associate the acknowledgement and 339 holding time in the case where multiple systems on a LAN restart 340 at approximately the same time. This IIH SHOULD be transmitted 341 before any LSPs or SNPs are transmitted as a result of the 342 receipt of the original IIH. 344 c. if the corresponding interface is a Point-to-Point interface, or 345 if the receiving router has the highest LnRouterPriority (with 346 the highest source MAC (Media Access Control) address breaking 347 ties) among those routers to which the receiving router has an 348 adjacency in state "UP" on this interface whose IIHs contain the 349 restart TLV, excluding adjacencies to all routers which are 350 considered in "Restart mode" (note the actual DIS is NOT changed 351 by this process), initiate the transmission over the 352 corresponding interface of a complete set of CSNPs, and set 353 SRMflags on the corresponding interface for all LSPs in the local 354 LSP database. 356 Otherwise (i.e., if there was no adjacency in the "UP" state to the 357 System ID in question), process the IIH as normal by reinitializing 358 the adjacency and setting the RA bit in the returned IIH. 360 3.2.2. Use of the SA Bit 362 The SA bit is used by a starting router to request that its neighbor 363 suppress advertisement of the adjacency to the starting router in the 364 neighbor's LSPs. 366 A router that is starting has no maintained forwarding function 367 state. This may or may not be the first time the router has started. 368 If this is not the first time the router has started, copies of LSPs 369 generated by this router in its previous incarnation may exist in the 370 LSP databases of other routers in the network. These copies are 371 likely to appear "newer" than LSPs initially generated by the 372 starting router due to the reinitialization of LSP fragment sequence 373 numbers by the starting router. This may cause temporary blackholes 374 to occur until the normal operation of the update process causes the 375 starting router to regenerate and flood copies of its own LSPs with 376 higher sequence numbers. The temporary blackholes can be avoided if 377 the starting router's neighbors suppress advertising an adjacency to 378 the starting router until the starting router has been able to 379 propagate newer versions of LSPs generated by previous incarnations. 381 When a router receives an IIH with the restart TLV having the SA bit 382 set, if there exists on this interface an adjacency in state "UP" 383 with the same System ID, and in the case of a LAN circuit, with the 384 same source LAN address, then the router MUST suppress advertisement 385 of the adjacency to the neighbor in its own LSPs. Until an IIH with 386 the SA bit clear has been received, the neighbor advertisement MUST 387 continue to be suppressed. If the adjacency transitions to the "UP" 388 state, the new adjacency MUST NOT be advertised until an IIH with the 389 SA bit clear has been received. 391 Note that a router that suppresses advertisement of an adjacency MUST 392 NOT use this adjacency when performing its SPF calculation. In 393 particular, if an implementation follows the example guidelines 394 presented in [ISO10589], Annex C.2.5, Step 0:b) "pre-load TENT with 395 the local adjacency database", the suppressed adjacency MUST NOT be 396 loaded into TENT. 398 3.2.3. Use of PR and PA Bits 400 The PR bit is used by a router which is planning to initiate a 401 restart to signal to its neighbors that it will be restarting. The 402 router sending an IIH with PR bit set SHOULD set the "remaining time" 403 to a value greater than the expected control plane restart time. The 404 PR bit SHOULD remain set in IIHs until the restart is initiated. 406 The PA bit is sent by the neighbor of a router planning to restart to 407 acknowledge receipt of a restart TLV with the PR bit set. 409 When the neighbor of a router planning a restart receives an IIH with 410 the restart TLV having the PR bit set, if there exists on this 411 interface an adjacency in state "UP" with the same System ID, and in 412 the case of a LAN circuit, with the same source LAN address, then: 414 a. if this is the first IIH with the PR bit set that this system has 415 received associated with this adjacency, then the adjacency is 416 marked as being in "Planned Restart state" and the adjacency 417 holding time is refreshed -- otherwise, the holding time is not 418 refreshed. The holding time SHOULD be set to the "remaining 419 time" specified in the received IIH with PR set. The "remaining 420 time" transmitted according to (b) below MUST reflect the actual 421 time after which the adjacency will now expire. Receipt of an 422 IIH with the PR bit reset will clear the "Planned Restart state" 423 and cause the receiving router to set the adjacency hold time to 424 the locally configured value. This procedure allows the router 425 planning a restart to cause the neighbor to maintain the 426 adjacency long enough for restart to successfully complete. 427 Whether or not an adjacency is marked as being in "Planned 428 Restart state" has no effect on adjacency state transitions. 430 b. immediately (i.e., without waiting for any currently running 431 timer interval to expire, but with a small random delay of a few 432 tens of milliseconds on LANs to avoid "storms") transmit over the 433 corresponding interface an IIH including the restart TLV with the 434 PR bit clear and the PA bit set. The "Remaining Time" MUST be 435 set to the current time (in seconds) before the holding timer on 436 this adjacency is due to expire. If the corresponding interface 437 is a LAN interface, then the Restarting Neighbor System ID SHOULD 438 be set to the System ID of the router from which the IIH with the 439 PR bit set was received. This is required to correctly associate 440 the acknowledgement and holding time in the case where multiple 441 systems on a LAN are planning a restart at approximately the same 442 time. 444 NOTE: Receipt of an IIH with PA bit set indicates to the router 445 planning a restart that the neighbor is aware of the planned restart 446 and - in the absence of topology changes as described below - will 447 maintain the adjacency for the "remaining time" included in the IIH 448 with PA set. 450 By definition, a restarting router maintains forwarding state across 451 the control plane restart (see Section 2). But while a control plane 452 restart is in progress it is expected that the restarting router will 453 be unable to respond to topology changes. It is therefore useful to 454 signal a planned restart so that the neighbors of the restarting 455 router can determine whether it is safe to maintain the adjacency if 456 other topology changes occur prior to the completion of the restart. 457 Signalling a planned restart in the absence of maintained forwarding 458 plane state is likely to lead to significant traffic loss and MUST 459 NOT be done. 461 Neighbors of the router which has signaled planned restart SHOULD 462 maintain the adjacency in a planned restart state until it receives 463 an IIH with the RR bit set, receives an IIH with both PR and RR bits 464 clear, or the adjacency holding time expires - whichever occurs 465 first. 467 While the adjacency is in planned restart state some or all of the 468 following actions MAY be taken: 470 a. if additional topology changes occur, the adjacency which is in 471 planned restart state MAY be brought down even though the hold 472 time has not yet expired. Given that the neighbor which has 473 signaled a planned restart is not expected to update its 474 forwarding plane in response to signalling of the topology 475 changes (since it is restarting) traffic which transits that node 476 is at risk of being improperly forwarded. On a LAN circuit, if 477 the router in planned restart state is the DIS at any supported 478 level, the adjacency(ies) SHOULD be brought down whenever any LSP 479 update is either generated or received, so as to trigger a new 480 DIS election. Failure to do so will compromise the reliability 481 of the Update Process on that circuit. What other criteria are 482 used to determine what topology changes will trigger bringing the 483 adjacency down is a local implementation decision. 485 b. if a BFD [RFC5880] session to the neighbor which signals a 486 planned restart is in the UP state and subsequently goes DOWN, 487 the event MAY be ignored since it is possible this is an expected 488 side effect of the restart. Use of the Control Plane Independent 489 state as signalled in BFD control packets SHOULD be considered in 490 the decision to ignore a BFD Session DOWN event. 492 c. on a Point-to-Point circuit, transmission of LSPs, CSNPs, and 493 PSNPs MAY be suppressed. It is expected that the PDUs will not 494 be received. 496 Use of the PR bit provides a means to safely support restart periods 497 which are significantly longer than standard holdtimes. 499 3.3. Adjacency (Re)Acquisition 501 Adjacency (re)acquisition is the first step in (re)initialization. 502 Restarting and starting routers will make use of the RR bit in the 503 restart TLV, though each will use it at different stages of the 504 (re)start procedure. 506 3.3.1. Adjacency Reacquisition during Restart 508 The restarting router explicitly notifies its neighbor that the 509 adjacency is being reacquired, and hence that it SHOULD NOT 510 reinitialize the adjacency. This is achieved by setting the RR bit 511 in the restart TLV. When the neighbor of a restarting router 512 receives an IIH with the restart TLV having the RR bit set, if there 513 exists on this interface an adjacency in state "UP" with the same 514 System ID, and in the case of a LAN circuit, with the same source LAN 515 address, then the procedures described in Section 3.2.1 are followed. 517 A router that does not support the restart capability will ignore the 518 restart TLV and reinitialize the adjacency as normal, returning an 519 IIH without the restart TLV. 521 On restarting, a router initializes the timer T3, starts the timer T2 522 for each LSPDB, and for each interface (and in the case of a LAN 523 circuit, for each level) starts the timer T1 and transmits an IIH 524 containing the restart TLV with the RR bit set. 526 On a Point-to-Point circuit, the restarting router SHOULD set the 527 "Adjacency Three-Way State" to "Init", because the receipt of the 528 acknowledging IIH (with RA set) MUST cause the adjacency to enter the 529 "UP" state immediately. 531 On a LAN circuit, the LAN-ID assigned to the circuit SHOULD be the 532 same as that used prior to the restart. In particular, for any 533 circuits for which the restarting router was previously DIS, the use 534 of a different LAN-ID would necessitate the generation of a new set 535 of pseudonode LSPs, and corresponding changes in all the LSPs 536 referencing them from other routers on the LAN. By preserving the 537 LAN-ID across the restart, this churn can be prevented. To enable a 538 restarting router to learn the LAN-ID used prior to restart, the LAN- 539 ID specified in an IIH with RR set MUST be ignored. 541 Transmission of "normal IIHs" is inhibited until the conditions 542 described below are met (in order to avoid causing an unnecessary 543 adjacency initialization). Upon expiry of the timer T1, it is 544 restarted and the IIH is retransmitted as above. 546 When a restarting router receives an IIH a local adjacency is 547 established as usual, and if the IIH contains a restart TLV with the 548 RA bit set (and on LAN circuits with a Restart Neighbor System ID 549 that matches that of the local system), the receipt of the 550 acknowledgement over that interface is noted. When the RA bit is set 551 and the state of the remote adjacency is "UP", then the timer T3 is 552 set to the minimum of its current value and the value of the 553 "Remaining Time" field in the received IIH. 555 On a Point-to-Point link, receipt of an IIH not containing the 556 restart TLV is also treated as an acknowledgement, since it indicates 557 that the neighbor is not restart capable. However, since no CSNP is 558 guaranteed to be received over this interface, the timer T1 is 559 cancelled immediately without waiting for a complete set of CSNPs. 560 Synchronization may therefore be deemed complete even though there 561 are some LSPs which are held (only) by this neighbor (see 562 Section 3.4). In this case, we also want to be certain that the 563 neighbor will reinitialize the adjacency in order to guarantee that 564 the SRMflags have been set on its database, thus ensuring eventual 565 LSPDB synchronization. This is guaranteed to happen except in the 566 case where the Adjacency Three-Way State in the received IIH is "UP" 567 and the Neighbor Extended Local Circuit ID matches the extended local 568 circuit ID assigned by the restarting router. In this case, the 569 restarting router MUST force the adjacency to reinitialize by setting 570 the local Adjacency Three-Way State to "DOWN" and sending a normal 571 IIH. 573 In the case of a LAN interface, receipt of an IIH not containing the 574 restart TLV is unremarkable since synchronization can still occur so 575 long as at least one of the non-restarting neighboring routers on the 576 LAN supports restart. Therefore, T1 continues to run in this case. 577 If none of the neighbors on the LAN are restart capable, T1 will 578 eventually expire after the locally defined number of retries. 580 In the case of a Point-to-Point circuit, the "LocalCircuitID" and 581 "Extended Local Circuit ID" information contained in the IIH can be 582 used immediately to generate an IIH containing the correct three-way 583 handshake information. The presence of "Neighbor Extended Local 584 Circuit ID" information that does not match the value currently in 585 use by the local system is ignored (since the IIH may have been 586 transmitted before the neighbor had received the new value from the 587 restarting router), but the adjacency remains in the initializing 588 state until the correct information is received. 590 In the case of a LAN circuit, the source neighbor information (e.g., 591 SNPAAddress) is recorded and used for adjacency establishment and 592 maintenance as normal. 594 When BOTH a complete set of CSNPs (for each active level, in the case 595 of a Point-to-Point circuit) and an acknowledgement have been 596 received over the interface, the timer T1 is cancelled. 598 Once the timer T1 has been cancelled, subsequent IIHs are transmitted 599 according to the normal algorithms, but including the restart TLV 600 with both RR and RA clear. 602 If a LAN contains a mixture of systems, only some of which support 603 the new algorithm, database synchronization is still guaranteed, but 604 the "old" systems will have reinitialized their adjacencies. 606 If an interface is active, but does not have any neighboring router 607 reachable over that interface, the timer T1 would never be cancelled, 608 and according to Section 3.4.1.1, the SPF would never be run. 609 Therefore, timer T1 is cancelled after some predetermined number of 610 expirations (which MAY be 1). 612 3.3.2. Adjacency Acquisition during Start 614 The starting router wants to ensure that in the event that a 615 neighboring router has an adjacency to the starting router in the 616 "UP" state (from a previous incarnation of the starting router), this 617 adjacency is reinitialized. The starting router also wants 618 neighboring routers to suppress advertisement of an adjacency to the 619 starting router until LSP database synchronization is achieved. This 620 is achieved by sending IIHs with the RR bit clear and the SA bit set 621 in the restart TLV. The RR bit remains clear and the SA bit remains 622 set in subsequent transmissions of IIHs until the adjacency has 623 reached the "UP" state and the initial T1 timer interval (see below) 624 has expired. 626 Receipt of an IIH with the RR bit clear will result in the 627 neighboring router utilizing normal operation of the adjacency state 628 machine. This will ensure that any old adjacency on the neighboring 629 router will be reinitialized. 631 Upon receipt of an IIH with the SA bit set, the behavior described in 632 Section 3.2.2 is followed. 634 Upon starting, a router starts timer T2 for each LSPDB. 636 For each interface (and in the case of a LAN circuit, for each 637 level), when an adjacency reaches the "UP" state, the starting router 638 starts a timer T1 and transmits an IIH containing the restart TLV 639 with the RR bit clear and SA bit set. Upon expiry of the timer T1, 640 it is restarted and the IIH is retransmitted with both RR and SA bits 641 set (only the RR bit has changed state from earlier IIHs). 643 Upon receipt of an IIH with the RR bit set (regardless of whether or 644 not the SA bit is set), the behavior described in Section 3.2.1 is 645 followed. 647 When an IIH is received by the starting router and the IIH contains a 648 restart TLV with the RA bit set (and on LAN circuits with a Restart 649 Neighbor System ID that matches that of the local system), the 650 receipt of the acknowledgement over that interface is noted. 652 On a Point-to-Point link, receipt of an IIH not containing the 653 restart TLV is also treated as an acknowledgement, since it indicates 654 that the neighbor is not restart capable. Since the neighbor will 655 have reinitialized the adjacency, this guarantees that SRMflags have 656 been set on its database, thus ensuring eventual LSPDB 657 synchronization. However, since no CSNP is guaranteed to be received 658 over this interface, the timer T1 is cancelled immediately without 659 waiting for a complete set of CSNPs. Synchronization may therefore 660 be deemed complete even though there are some LSPs that are held 661 (only) by this neighbor (see Section 3.4). 663 In the case of a LAN interface, receipt of an IIH not containing the 664 restart TLV is unremarkable since synchronization can still occur so 665 long as at least one of the non-restarting neighboring routers on the 666 LAN supports restart. Therefore, T1 continues to run in this case. 667 If none of the neighbors on the LAN are restart capable, T1 will 668 eventually expire after the locally defined number of retries. The 669 usual operation of the update process will ensure that 670 synchronization is eventually achieved. 672 When BOTH a complete set of CSNPs (for each active level, in the case 673 of a Point-to-Point circuit) and an acknowledgement have been 674 received over the interface, the timer T1 is cancelled. Subsequent 675 IIHs sent by the starting router have the RR and RA bits clear and 676 the SA bit set in the restart TLV. 678 Timer T1 is cancelled after some predetermined number of expirations 679 (which MAY be 1). 681 When the T2 timer(s) are cancelled or expire, transmission of "normal 682 IIHs" will begin. 684 3.3.3. Multiple Levels 686 A router that is operating as both a Level 1 and a Level 2 router on 687 a particular interface MUST perform the above operations for each 688 level. 690 On a LAN interface, it MUST send and receive both Level 1 and Level 2 691 IIHs and perform the CSNP synchronizations independently for each 692 level. 694 On a Point-to-Point interface, only a single IIH (indicating support 695 for both levels) is required, but it MUST perform the CSNP 696 synchronizations independently for each level. 698 3.4. Database Synchronization 700 When a router is started or restarted, it can expect to receive a 701 complete set of CSNPs over each interface. The arrival of the 702 CSNP(s) is now guaranteed, since an IIH with the RR bit set will be 703 retransmitted until the CSNP(s) are correctly received. 705 The CSNPs describe the set of LSPs that are currently held by each 706 neighbor. Synchronization will be complete when all these LSPs have 707 been received. 709 When (re)starting, a router starts an instance of timer T2 for each 710 LSPDB as described in Section 3.3.1 or Section 3.3.2. In addition to 711 normal processing of the CSNPs, the set of LSPIDs contained in the 712 first complete set of CSNPs received over each interface is recorded, 713 together with their remaining lifetime. In the case of a LAN 714 interface, a complete set of CSNPs MUST consist of CSNPs received 715 from neighbors that are not restarting. If there are multiple 716 interfaces on the (re)starting router, the recorded set of LSPIDs is 717 the union of those received over each interface. LSPs with a 718 remaining lifetime of zero are NOT so recorded. 720 As LSPs are received (by the normal operation of the update process) 721 over any interface, the corresponding LSPID entry is removed (it is 722 also removed if an LSP arrives before the CSNP containing the 723 reference). When an LSPID has been held in the list for its 724 indicated remaining lifetime, it is removed from the list. When the 725 list of LSPIDs is empty and the timer T1 has been cancelled for all 726 the interfaces that have an adjacency at this level, the timer T2 is 727 cancelled. 729 At this point, the local database is guaranteed to contain all the 730 LSP(s) (either the same sequence number or a more recent sequence 731 number) that were present in the neighbors' databases at the time of 732 (re)starting. LSPs that arrived in a neighbor's database after the 733 time of (re)starting may or may not be present, but the normal 734 operation of the update process will guarantee that they will 735 eventually be received. At this point, the local database is deemed 736 to be "synchronized". 738 Since LSPs mentioned in the CSNP(s) with a zero remaining lifetime 739 are not recorded, and those with a short remaining lifetime are 740 deleted from the list when the lifetime expires, cancellation of the 741 timer T2 will not be prevented by waiting for an LSP that will never 742 arrive. 744 3.4.1. LSP Generation and Flooding and SPF Computation 746 The operation of a router starting, as opposed to restarting, is 747 somewhat different. These two cases are dealt with separately below. 749 3.4.1.1. Restarting 751 In order to avoid causing unnecessary routing churn in other routers, 752 it is highly desirable that the router's own LSPs generated by the 753 restarting system are the same as those previously present in the 754 network (assuming no other changes have taken place). It is 755 important therefore not to regenerate and flood the LSPs until all 756 the adjacencies have been re-established and any information required 757 for propagation into the local LSPs is fully available. Ideally, the 758 information is loaded into the LSPs in a deterministic way, such that 759 the same information occurs in the same place in the same LSP (and 760 hence the LSPs are identical to their previous versions). If this 761 can be achieved, the new versions may not even cause SPF to be run in 762 other systems. However, provided the same information is included in 763 the set of LSPs (albeit in a different order, and possibly different 764 LSPs), the result of running the SPF will be the same and will not 765 cause churn to the forwarding tables. 767 In the case of a restarting router, none of the router's own LSPs are 768 transmitted, nor are the router's own forwarding tables updated while 769 the timer T3 is running. 771 Redistribution of inter-level information MUST be regenerated before 772 this router's LSP is flooded to other nodes. Therefore, the Level-n 773 non-pseudonode LSP(s) MUST NOT be flooded until the other level's T2 774 timer has expired and its SPF has been run. This ensures that any 775 inter-level information that is to be propagated can be included in 776 the Level-n LSP(s). 778 During this period, if one of the router's own (including 779 pseudonodes) LSPs is received, which the local router does not 780 currently have in its own database, it is NOT purged. Under normal 781 operation, such an LSP would be purged, since the LSP clearly should 782 not be present in the global LSP database. However, in the present 783 circumstances, this would be highly undesirable, because it could 784 cause premature removal of a router's own LSP -- and hence churn in 785 remote routers. Even if the local system has one or more of the 786 router's own LSPs (which it has generated, but not yet transmitted), 787 it is still not valid to compare the received LSP against this set, 788 since it may be that as a result of propagation between Level 1 and 789 Level 2 (or vice versa), a further router's own LSP will need to be 790 generated when the LSP databases have synchronized. 792 During this period, a restarting router SHOULD send CSNPs as it 793 normally would. Information about the router's own LSPs MAY be 794 included, but if it is included it MUST be based on LSPs that have 795 been received, not on versions that have been generated (but not yet 796 transmitted). This restriction is necessary to prevent premature 797 removal of an LSP from the global LSP database. 799 When the timer T2 expires or is cancelled indicating that 800 synchronization for that level is complete, the SPF for that level is 801 run in order to derive any information that is required to be 802 propagated to another level, but the forwarding tables are not yet 803 updated. 805 Once the other level's SPF has run and any inter-level propagation 806 has been resolved, the router's own LSPs can be generated and 807 flooded. Any own LSPs that were previously ignored, but that are not 808 part of the current set of own LSPs (including pseudonodes), MUST 809 then be purged. Note that it is possible that a Designated Router 810 change may have taken place, and consequently the router SHOULD purge 811 those pseudonode LSPs that it previously owned, but that are now no 812 longer part of its set of pseudonode LSPs. 814 When all the T2 timers have expired or been cancelled, the timer T3 815 is cancelled and the local forwarding tables are updated. 817 If the timer T3 expires before all the T2 timers have expired or been 818 cancelled, this indicates that the synchronization process is taking 819 longer than the minimum holding time of the neighbors. The router's 820 own LSP(s) for levels that have not yet completed their first SPF 821 computation are then flooded with the overload bit set to indicate 822 that the router's LSPDB is not yet synchronized (and therefore other 823 routers MUST NOT compute routes through this router). Normal 824 operation of the update process resumes, and the local forwarding 825 tables are updated. In order to prevent the neighbor's adjacencies 826 from expiring, IIHs with the normal interface value for the holding 827 time are transmitted over all interfaces with neither RR nor RA set 828 in the restart TLV. This will cause the neighbors to refresh their 829 adjacencies. The router's own LSP(s) will continue to have the 830 overload bit set until timer T2 has expired or been cancelled. 832 3.4.1.2. Starting 834 In the case of a starting router, as soon as each adjacency is 835 established, and before any CSNP exchanges, the router's own zeroth 836 LSP is transmitted with the overload bit set. This prevents other 837 routers from computing routes through the router until it has 838 reliably acquired the complete set of LSPs. The overload bit remains 839 set in subsequent transmissions of the zeroth LSP (such as will occur 840 if a previous copy of the router's own zeroth LSP is still present in 841 the network) while any timer T2 is running. 843 When all the T2 timers have been cancelled, the router's own LSP(s) 844 MAY be regenerated with the overload bit clear (assuming the router 845 is not in fact overloaded, and there is no other reason, such as 846 incomplete BGP convergence, to keep the overload bit set) and flooded 847 as normal. 849 Other LSPs owned by this router (including pseudonodes) are generated 850 and flooded as normal, irrespective of the timer T2. The SPF is also 851 run as normal and the Routing Information Base (RIB) and Forwarding 852 Information Base (FIB) updated as routes become available. 854 To avoid the possible formation of temporary blackholes, the starting 855 router sets the SA bit in the restart TLV (as described in 856 Section 3.3.2) in all IIHs that it sends. 858 When all T2 timers have been cancelled, the starting router MUST 859 transmit IIHs with the SA bit clear. 861 4. State Tables 863 This section presents state tables that summarize the behaviors 864 described in this document. Other behaviors, in particular adjacency 865 state transitions and LSP database update operation, are NOT included 866 in the state tables except where this document modifies the behaviors 867 described in [ISO10589] and [RFC5303]. 869 The states named in the columns of the tables below are a mixture of 870 states that are specific to a single adjacency (ADJ suppressed, ADJ 871 Seen RA, ADJ Seen CSNP) and states that are indicative of the state 872 of the protocol instance (Running, Restarting, Starting, SPF Wait). 874 Three state tables are presented from the point of view of a running 875 router, a restarting router, and a starting router. 877 4.1. Running Router 878 Event | Running | ADJ suppressed 879 ============================================================== 880 RX PR | Set Planned Restart | 881 | state. | 882 | Update hold time 883 | Send PA | 884 -------------+----------------------+------------------------- 885 RX PR clr | Clear Planned | 886 and RR clr | Restart State | 887 | Restore holdtime to | 888 | local value | 889 -------------+----------------------+------------------------- 890 RX PA | Proceed with planned | 891 | restart | 892 -------------+----------------------+------------------------- 893 RX RR | Maintain ADJ State | 894 | Send RA | 895 | Set SRM,send CSNP | 896 | (Note 1) | 897 | Update Hold Time, | 898 | set Restart Mode | 899 | (Note 2) | 900 -------------+----------------------+------------------------- 901 RX RR clr | Clr Restart mode | 902 -------------+----------------------+------------------------- 903 RX SA | Suppress IS neighbor | 904 | TLV in LSP(s) | 905 | Goto ADJ Suppressed | 906 -------------+----------------------+------------------------- 907 RX SA clr | |Unsuppress IS neighbor 908 | | TLV in LSP(s) 909 | |Goto Running 910 ============================================================== 912 Note 1: CSNPs are sent by routers in accordance with Section 3.2.1c 914 Note 2: If Restart Mode clear 916 4.2. Restarting Router 918 Event | Restarting | ADJ Seen | ADJ Seen | SPF Wait 919 | | RA | CSNP | 920 =================================================================== 921 Restart | Send PR | | | 922 planned | | | | 923 ------------+--------------------+-----------+-----------+------------ 924 Planned | Send PR clr | | | 925 restart | | | | 926 canceled | | | | 927 ------------+--------------------+-----------+-----------+------------ 928 Router | Send IIH/RR | | | 929 restarts | ADJ Init | | | 930 | Start T1,T2,T3 | | | 931 ------------+--------------------+-----------+-----------+------------ 932 RX RR | Send RA | | | 933 ------------+--------------------+-----------+-----------+------------ 934 RX RA | Adjust T3 | | Cancel T1 | 935 | Goto ADJ Seen RA | | Adjust T3 | 936 ----------- +--------------------+-----------+-----------+------------ 937 RX CSNP set| Goto ADJ Seen CSNP | Cancel T1 | | 938 ------------+--------------------+-----------+-----------+------------ 939 RX IIH w/o | Cancel T1 (Point- | | | 940 Restart TLV| to-point only) | | | 941 ------------+--------------------+-----------+-----------+------------ 942 T1 expires | Send IIH/RR |Send IIH/RR|Send IIH/RR| 943 | Restart T1 | Restart T1| Restart T1| 944 ------------+--------------------+-----------+-----------+------------ 945 T1 expires | Send IIH/ | Send IIH/ | Send IIH/ | 946 nth time | normal | normal | normal | 947 ------------+--------------------+-----------+-----------+------------ 948 T2 expires | Trigger SPF | | | 949 | Goto SPF Wait | | | 950 ------------+--------------------+-----------+-----------+------------ 951 T3 expires | Set overload bit | | | 952 | Flood local LSPs | | | 953 | Update fwd plane | | | 954 ------------+--------------------+-----------+-----------+------------ 955 LSP DB Sync| Cancel T2, and T3 | | | 956 | Trigger SPF | | | 957 | Goto SPF wait | | | 958 ------------+--------------------+-----------+-----------+------------ 959 All SPF | | | | Clear 960 done | | | | overload bit 961 | | | | Update fwd 962 | | | | plane 963 | | | | Flood local 964 | | | | LSPs 965 | | | | Goto Running 966 ====================================================================== 968 4.3. Starting Router 970 Event | Starting | ADJ Seen RA| ADJ Seen CSNP 971 ============================================================= 972 Router | Send IIH/SA | | 973 starts | Start T1,T2 | | 974 -------------+-------------------+------------+--------------- 975 RX RR | Send RA | | 976 -------------+-------------------+------------+--------------- 977 RX RA | Goto ADJ Seen RA | | Cancel T1 978 -------------+-------------------+------------+--------------- 979 RX CSNP Set | Goto ADJ Seen CSNP| Cancel T1 | 980 -------------+-------------------+------------+--------------- 981 RX IIH w | Cancel T1 | | 982 no Restart | (Point-to-Point | | 983 TLV | only) | | 984 -------------+-------------------+------------+--------------- 985 ADJ UP | Start T1 | | 986 | Send local LSPs | | 987 | with overload bit| | 988 | set | | 989 -------------+-------------------+------------+--------------- 990 T1 expires | Send IIH/RR |Send IIH/RR | Send IIH/RR 991 | and SA | and SA | and SA 992 | Restart T1 |Restart T1 | Restart T1 993 -------------+-------------------+------------+--------------- 994 T1 expires | Send IIH/SA |Send IIH/SA | Send IIH/SA 995 nth time | | | 996 -------------+-------------------+------------+--------------- 997 T2 expires | Clear overload bit| | 998 | Send IIH normal | | 999 | Goto Running | | 1000 -------------+-------------------+------------+--------------- 1001 LSP DB Sync | Cancel T2 | | 1002 | Clear overload bit| | 1003 | Send IIH normal | | 1004 ============================================================== 1006 5. IANA Considerations 1008 This document defines the following IS-IS TLV that is listed in the 1009 IS-IS TLV codepoint registry: 1011 Type Description IIH LSP SNP Purge 1012 ---- ------------------------------ --- --- --- ----- 1013 211 Restart TLV y n n n 1015 IANA is requested to update the entry in registry to point to this 1016 document. 1018 6. Security Considerations 1020 Any new security issues raised by the procedures in this document 1021 depend upon the ability of an attacker to inject a false but 1022 apparently valid IIH, the ease/difficulty of which has not been 1023 altered. 1025 If the RR bit is set in a false IIH, neighbors who receive such an 1026 IIH will continue to maintain an existing adjacency in the "UP" state 1027 and may (re)send a complete set of CSNPs. While the latter action is 1028 wasteful, neither action causes any disruption in correct protocol 1029 operation. 1031 If the RA bit is set in a false IIH, a (re)starting router that 1032 receives such an IIH may falsely believe that there is a neighbor on 1033 the corresponding interface that supports the procedures described in 1034 this document. In the absence of receipt of a complete set of CSNPs 1035 on that interface, this could delay the completion of (re)start 1036 procedures by requiring the timer T1 to time out the locally defined 1037 maximum number of retries. This behavior is the same as would occur 1038 on a LAN where none of the (re)starting router's neighbors support 1039 the procedures in this document and is covered in Sections 3.3.1 and 1040 3.3.2. 1042 If the SA bit is set in a false IIH, this could cause suppression of 1043 the advertisement of an IS neighbor, which could either continue for 1044 an indefinite period or occur intermittently with the result being a 1045 possible loss of reachability to some destinations in the network 1046 and/or increased frequency of LSP flooding and SPF calculation. 1048 If the PR bit is set in a false IIH, neighbors who receive such an 1049 IIH could modify the holding time of an existing adjacency 1050 inappropriately. In the event of topology changes, the neighbor 1051 might also choose to bring the adjacency down in the false belief 1052 that the forwarding plane of the router identified as the source of 1053 the false IIH is not currently processing announced topology changes. 1055 If the PA bit is set in a false IIH, a router that receives such an 1056 IIH may falsely believe that the neighbor on the corresponding 1057 interface supports the planned restart procedures defined in this 1058 document. If such a router is planning to restart it might then 1059 proceed to initiate a restart in the false expectation that the 1060 neighbor has updated its holding time as requested. This may result 1061 in the neighbor bringing down the adjacency while the receiving 1062 router is restarting, causing unnecessary disruption to forwarding. 1064 The possibility of IS-IS PDU spoofing can be reduced by the use of 1065 authentication as described in [RFC1195] and [ISO10589], and 1066 especially the use of cryptographic authentication as described in 1067 [RFC5304] and [RFC5310]. 1069 7. Manageability Considerations 1071 These extensions that have been designed, developed, and deployed for 1072 many years do not have any new impact on management and operation of 1073 the IS-IS protocol via this standardization process. 1075 8. Acknowledgements 1077 For RFC 5306 the authors acknowledged contributions made by Jeff 1078 Parker, Radia Perlman, Mark Schaefer, Naiming Shen, Nischal Sheth, 1079 Russ White, and Rena Yang. 1081 The authors of this updated version acknowledge the contribution of 1082 Mike Shand, co-auther of RFC 5306. 1084 9. Normative References 1086 [ISO10589] 1087 International Organization for Standardization, 1088 "Intermediate system to Intermediate system intra-domain 1089 routeing information exchange protocol for use in 1090 conjunction with the protocol for providing the 1091 connectionless-mode Network Service (ISO 8473)", ISO/ 1092 IEC 10589:2002, Second Edition, Nov 2002. 1094 [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and 1095 dual environments", RFC 1195, DOI 10.17487/RFC1195, 1096 December 1990, . 1098 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1099 Requirement Levels", BCP 14, RFC 2119, 1100 DOI 10.17487/RFC2119, March 1997, 1101 . 1103 [RFC5303] Katz, D., Saluja, R., and D. Eastlake 3rd, "Three-Way 1104 Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303, 1105 DOI 10.17487/RFC5303, October 2008, 1106 . 1108 [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic 1109 Authentication", RFC 5304, DOI 10.17487/RFC5304, October 1110 2008, . 1112 [RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., 1113 and M. Fanto, "IS-IS Generic Cryptographic 1114 Authentication", RFC 5310, DOI 10.17487/RFC5310, February 1115 2009, . 1117 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 1118 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 1119 . 1121 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1122 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1123 May 2017, . 1125 Appendix A. Summary of Changes from RFC 5306 1127 This document extends RFC 5306 by introducing support for signalling 1128 the neighbors of a restarting router that a planned restart is about 1129 to occur. This allows the neighbors to be aware of the state of the 1130 restarting router so that appropriate action may be taken if other 1131 topology changes occur while the planned restart is in progress. 1132 Since the forwarding plane of the restarting router is maintained 1133 based upon the pre-restart state of the network, additional topology 1134 changes introduce the possibility that traffic may be lost if paths 1135 via the restarting router continue to be used while the restart is in 1136 progress. 1138 In support of this new functionality two new flags have been 1139 introduced: 1141 PR - Restart is planned 1142 PA - Planned restart acknowledgement 1144 No changes to the post restart exchange between the restarting router 1145 and its neighbors have been introduced. 1147 Authors' Addresses 1149 Les Ginsberg 1150 Cisco Systems, Inc. 1152 Email: ginsberg@cisco.com 1154 Paul Wells 1155 Cisco Systems, Inc. 1157 Email: pauwells@cisco.com