idnits 2.17.1 draft-ietf-lsr-isis-rfc5306bis-03.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 (August 10, 2019) is 1713 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 August 10, 2019 6 Expires: February 11, 2020 8 Restart Signaling for IS-IS 9 draft-ietf-lsr-isis-rfc5306bis-03 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 February 11, 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 . . . . . . . . . . . . . . . . 6 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 . . . . . . . . . . . . . . . . . . . . . . . . 18 90 4.1. Running Router . . . . . . . . . . . . . . . . . . . . . 19 91 4.2. Restarting Router . . . . . . . . . . . . . . . . . . . . 20 92 4.3. Starting Router . . . . . . . . . . . . . . . . . . . . . 21 93 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22 94 6. Security Considerations . . . . . . . . . . . . . . . . . . . 22 95 7. Manageability Considerations . . . . . . . . . . . . . . . . 23 96 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23 97 9. Normative References . . . . . . . . . . . . . . . . . . . . 23 98 Appendix A. Summary of Changes from RFC 5306 . . . . . . . . . . 24 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 that it is restarting to its neighbors, and allow them to reestablish 151 their adjacencies without cycling through the down state, while still 152 correctly initiating database synchronization. 154 This document additionally describes a mechanism for a restarting 155 router to determine when it has achieved LSP database synchronization 156 with its neighbors and a mechanism to optimize LSP database 157 synchronization and minimize transient routing disruption when a 158 router starts. 160 It is assumed that the three-way handshake [RFC5303] is being used on 161 Point-to-Point circuits. 163 2. Conventions Used in This Document 165 If the control and forwarding functions in a router can be maintained 166 independently, it is possible for the forwarding function state to be 167 maintained across a resumption of control function operations. This 168 functionality is assumed when the terms "restart/restarting" are used 169 in this document. 171 The terms "start/starting" are used to refer to a router in which the 172 control function has either commenced operations for the first time 173 or has resumed operations, but the forwarding functions have not been 174 maintained in a prior state. 176 The terms "(re)start/(re)starting" are used when the text is 177 applicable to both a "starting" and a "restarting" router. 179 The terms "normal IIH" or "IIH normal" refer to IS-IS Hellos (IIHs) 180 in which the Restart TLV (defined later in this document) has no 181 flags set. 183 3. Approach 185 3.1. Timers 187 Three additional timers, T1, T2, and T3, are required to support the 188 behavior of a restarting router defined in this document. 190 NOTE: These timers are NOT applicable to a router which is preparing 191 to do a planned restart. 193 An instance of the timer T1 is maintained per interface, and 194 indicates the time after which an unacknowledged (re)start attempt 195 will be repeated. A typical value is 3 seconds. 197 An instance of the timer T2 is maintained for each LSP database 198 (LSPDB) present in the system, i.e., for a Level 1/2 system, there 199 will be an instance of the timer T2 for Level 1 and an instance for 200 Level 2. This is the maximum time that the system will wait for 201 LSPDB synchronization. A typical value is 60 seconds. 203 A single instance of the timer T3 is maintained for the entire 204 system. It indicates the time after which the router will declare 205 that it has failed to achieve database synchronization (by setting 206 the overload bit in its own LSP). This is initialized to 65535 207 seconds, but is set to the minimum of the remaining times of received 208 IIHs containing a restart TLV with the Restart Acknowledgement (RA) 209 set and an indication that the neighbor has an adjacency in the "UP" 210 state to the restarting router. 212 NOTE: The timer T3 is only used by a restarting router. 214 3.2. Restart TLV 216 A new TLV is defined to be included in IIH PDUs. The presence of 217 this TLV indicates that the sender supports the functionality defined 218 in this document and it carries flags that are used to convey 219 information during a (re)start. All IIHs transmitted by a router 220 that supports this capability MUST include this TLV. 222 Type 211 224 Length: Number of octets in the Value field (1 to (3 + ID Length)) 226 Value 228 No. of octets 229 +-----------------------+ 230 | Flags | 1 231 +-----------------------+ 232 | Remaining Time | 2 233 +-----------------------+ 234 | Restarting Neighbor ID| ID Length 235 +-----------------------+ 237 Flags (1 octet) 239 0 1 2 3 4 5 6 7 240 +--+--+--+--+--+--+--+--+ 241 |Reserved|PA|PR|SA|RA|RR| 242 +--+--+--+--+--+--+--+--+ 244 RR - Restart Request 245 RA - Restart Acknowledgement 246 SA - Suppress adjacency advertisement 247 PR - Restart is planned 248 PA - Planned restart acknowledgement 250 Remaining Time (2 octets) 252 Remaining/recommended holding time (in seconds). 254 Required when the RA, PR, or PA bit is set. Otherwise 255 this field SHOULD be omitted when sent and 256 MUST be ignored when received. 258 Restarting Neighbor System ID (ID Length octets) 260 The System ID of the neighbor to which an RA/PA refers. 262 Required when the RA or PA bit is set. Otherwise 263 this field SHOULD be omitted when sent and 264 MUST be ignored when received. 266 Note: Implementations based on earlier drafts of RFC 5306 267 may not include this field in the TLV when the RA bit is set. 268 In this case, a router that is expecting an RA on a LAN circuit 269 SHOULD assume that the acknowledgement is directed at the local 270 system. 272 The functionality associated with each of the defined flags (as 273 described in the following sections) is mutually exclusive with any 274 of the other flags. Therefore, it is expected that at most one flag 275 will be set in a TLV. Received TLVs which have multiple flags set 276 MUST be ignored. 278 3.2.1. Use of RR and RA Bits 280 The RR bit is used by a (re)starting router to signal to its 281 neighbors that a (re)start is in progress, that an existing adjacency 282 SHOULD be maintained even under circumstances when the normal 283 operation of the adjacency state machine would require the adjacency 284 to be reinitialized, to request a set of CSNPs, and to request 285 setting of the SRMflags. 287 The RA bit is sent by the neighbor of a (re)starting router to 288 acknowledge the receipt of a restart TLV with the RR bit set. 290 When the neighbor of a (re)starting router receives an IIH with the 291 restart TLV having the RR bit set, if there exists on this interface 292 an adjacency in state "UP" with the same System ID, and in the case 293 of a LAN circuit, with the same source LAN address, then, 294 irrespective of the other contents of the "Intermediate System 295 Neighbors" option (LAN circuits) or the "Point-to-Point Three-Way 296 Adjacency" option (Point-to-Point circuits): 298 a. the state of the adjacency is not changed. If this is the first 299 IIH with the RR bit set that this system has received associated 300 with this adjacency, then the adjacency is marked as being in 301 "Restart mode" and the adjacency holding time is refreshed -- 302 otherwise, the holding time is not refreshed. The "remaining 303 time" transmitted according to (b) below MUST reflect the actual 304 time after which the adjacency will now expire. Receipt of an 305 IIH with the RR bit reset will clear the "Restart mode" state. 306 This procedure allows the restarting router to cause the neighbor 307 to maintain the adjacency long enough for restart to successfully 308 complete, while also preventing repetitive restarts from 309 maintaining an adjacency indefinitely. Whether or not an 310 adjacency is marked as being in "Restart mode" has no effect on 311 adjacency state transitions. 313 b. immediately (i.e., without waiting for any currently running 314 timer interval to expire, but with a small random delay of a few 315 tens of milliseconds on LANs to avoid "storms") transmit over the 316 corresponding interface an IIH including the restart TLV with the 317 RR bit clear and the RA bit set, in the case of Point-to-Point 318 adjacencies having updated the "Point-to-Point Three-Way 319 Adjacency" option to reflect any new values received from the 320 (re)starting router. (This allows a restarting router to quickly 321 acquire the correct information to place in its hellos.) The 322 "Remaining Time" MUST be set to the current time (in seconds) 323 before the holding timer on this adjacency is due to expire. If 324 the corresponding interface is a LAN interface, then the 325 Restarting Neighbor System ID SHOULD be set to the System ID of 326 the router from which the IIH with the RR bit set was received. 327 This is required to correctly associate the acknowledgement and 328 holding time in the case where multiple systems on a LAN restart 329 at approximately the same time. This IIH SHOULD be transmitted 330 before any LSPs or SNPs are transmitted as a result of the 331 receipt of the original IIH. 333 c. if the corresponding interface is a Point-to-Point interface, or 334 if the receiving router has the highest LnRouterPriority (with 335 the highest source MAC (Media Access Control) address breaking 336 ties) among those routers to which the receiving router has an 337 adjacency in state "UP" on this interface whose IIHs contain the 338 restart TLV, excluding adjacencies to all routers which are 339 considered in "Restart mode" (note the actual DIS is NOT changed 340 by this process), initiate the transmission over the 341 corresponding interface of a complete set of CSNPs, and set 342 SRMflags on the corresponding interface for all LSPs in the local 343 LSP database. 345 Otherwise (i.e., if there was no adjacency in the "UP" state to the 346 System ID in question), process the IIH as normal by reinitializing 347 the adjacency and setting the RA bit in the returned IIH. 349 3.2.2. Use of the SA Bit 351 The SA bit is used by a starting router to request that its neighbor 352 suppress advertisement of the adjacency to the starting router in the 353 neighbor's LSPs. 355 A router that is starting has no maintained forwarding function 356 state. This may or may not be the first time the router has started. 357 If this is not the first time the router has started, copies of LSPs 358 generated by this router in its previous incarnation may exist in the 359 LSP databases of other routers in the network. These copies are 360 likely to appear "newer" than LSPs initially generated by the 361 starting router due to the reinitialization of LSP fragment sequence 362 numbers by the starting router. This may cause temporary blackholes 363 to occur until the normal operation of the update process causes the 364 starting router to regenerate and flood copies of its own LSPs with 365 higher sequence numbers. The temporary blackholes can be avoided if 366 the starting router's neighbors suppress advertising an adjacency to 367 the starting router until the starting router has been able to 368 propagate newer versions of LSPs generated by previous incarnations. 370 When a router receives an IIH with the restart TLV having the SA bit 371 set, if there exists on this interface an adjacency in state "UP" 372 with the same System ID, and in the case of a LAN circuit, with the 373 same source LAN address, then the router MUST suppress advertisement 374 of the adjacency to the neighbor in its own LSPs. Until an IIH with 375 the SA bit clear has been received, the neighbor advertisement MUST 376 continue to be suppressed. If the adjacency transitions to the "UP" 377 state, the new adjacency MUST NOT be advertised until an IIH with the 378 SA bit clear has been received. 380 Note that a router that suppresses advertisement of an adjacency MUST 381 NOT use this adjacency when performing its SPF calculation. In 382 particular, if an implementation follows the example guidelines 383 presented in [ISO10589], Annex C.2.5, Step 0:b) "pre-load TENT with 384 the local adjacency database", the suppressed adjacency MUST NOT be 385 loaded into TENT. 387 3.2.3. Use of PR and PA Bits 389 The PR bit is used by a router which is planning to initiate a 390 restart to signal to its neighbors that it will be restarting. The 391 router sending an IIH with PR bit set SHOULD set the "remaining time" 392 to a value greater than the expected control plane restart time. The 393 PR bit SHOULD remain set in IIHs until the restart is initiated. 395 The PA bit is sent by the neighbor of a router planning to restart to 396 acknowledge receipt of a restart TLV with the PR bit set. 398 When the neighbor of a router planning a restart receives an IIH with 399 the restart TLV having the PR bit set, if there exists on this 400 interface an adjacency in state "UP" with the same System ID, and in 401 the case of a LAN circuit, with the same source LAN address, then: 403 a. if this is the first IIH with the PR bit set that this system has 404 received associated with this adjacency, then the adjacency is 405 marked as being in "Planned Restart state" and the adjacency 406 holding time is refreshed -- otherwise, the holding time is not 407 refreshed. The holding time SHOULD be set to the "remaining 408 time" specified in the received IIH with PR set. The "remaining 409 time" transmitted according to (b) below MUST reflect the actual 410 time after which the adjacency will now expire. Receipt of an 411 IIH with the PR bit reset will clear the "Planned Restart state" 412 and cause the receiving router to set the adjacency hold time to 413 the locally configured value. This procedure allows the router 414 planning a restart to cause the neighbor to maintain the 415 adjacency long enough for restart to successfully complete. 416 Whether or not an adjacency is marked as being in "Planned 417 Restart state" has no effect on adjacency state transitions. 419 b. immediately (i.e., without waiting for any currently running 420 timer interval to expire, but with a small random delay of a few 421 tens of milliseconds on LANs to avoid "storms") transmit over the 422 corresponding interface an IIH including the restart TLV with the 423 PR bit clear and the PA bit set. The "Remaining Time" MUST be 424 set to the current time (in seconds) before the holding timer on 425 this adjacency is due to expire. If the corresponding interface 426 is a LAN interface, then the Restarting Neighbor System ID SHOULD 427 be set to the System ID of the router from which the IIH with the 428 PR bit set was received. This is required to correctly associate 429 the acknowledgement and holding time in the case where multiple 430 systems on a LAN are planning a restart at approximately the same 431 time. 433 NOTE: Receipt of an IIH with PA bit set indicates to the router 434 planning a restart that the neighbor is aware of the planned restart 435 and - in the absence of topology changes as described below - will 436 maintain the adjacency for the "remaining time" included in the IIH 437 with PA set. 439 While a control plane restart is in progress it is expected that the 440 restarting router will be unable to respond to topology changes. It 441 is therefore useful to signal a planned restart (if the forwarding 442 plane on the restarting router is maintained) so that the neighbors 443 of the restarting router can determine whether it is safe to maintain 444 the adjacency if other topology changes occur prior to the completion 445 of the restart. Signalling a planned restart in the absence of 446 maintained forwarding plane state is likely to lead to significant 447 traffic loss and MUST NOT be done. 449 Neighbors of the router which has signaled planned restart SHOULD 450 maintain the adjacency in a planned restart state until it receives 451 an IIH with the RR bit set, receives an IIH with both PR and RR bits 452 clear, or the adjacency holding time expires - whichever occurs 453 first. 455 While the adjacency is in planned restart state some or all of the 456 following actions MAY be taken: 458 a. If additional topology changes occur, the adjacency which is in 459 planned restart state MAY be brought down even though the hold 460 time has not yet expired. Given that the neighbor which has 461 signaled a planned restart is not expected to update its 462 forwarding plane in response to signaling of the topology changes 463 (since it is restarting) traffic which transits that node is at 464 risk of being improperly forwarded. On a LAN circuit, if the 465 router in planned restart state is the DIS at any supported 466 level, the adjacency(ies) SHOULD be brought down whenever any LSP 467 update is either generated or received, so as to trigger a new 468 DIS election. Failure to do so will compromise the reliability 469 of the Update Process on that circuit. What other criteria are 470 used to determine what topology changes will trigger bringing the 471 adjacency down is a local implementation decision. 473 b. If a BFD [RFC5880] session to the neighbor which signals a 474 planned restart is in the UP state and subsequently goes DOWN, 475 the event MAY be ignored since it is possible this is an expected 476 side effect of the restart. Use of the Control Plane Independent 477 state as signalled in BFD control packets SHOULD be considered in 478 the decision to ignore a BFD Session DOWN event. 480 c. On a Point-to-Point circuit, transmission of LSPs, CSNPs, and 481 PSNPs MAY be suppressed. It is expected that the PDUs will not 482 be received. 484 Use of the PR bit provides a means to safely support restart periods 485 which are significantly longer than standard holdtimes. 487 3.3. Adjacency (Re)Acquisition 489 Adjacency (re)acquisition is the first step in (re)initialization. 490 Restarting and starting routers will make use of the RR bit in the 491 restart TLV, though each will use it at different stages of the 492 (re)start procedure. 494 3.3.1. Adjacency Reacquisition during Restart 496 The restarting router explicitly notifies its neighbor that the 497 adjacency is being reacquired, and hence that it SHOULD NOT 498 reinitialize the adjacency. This is achieved by setting the RR bit 499 in the restart TLV. When the neighbor of a restarting router 500 receives an IIH with the restart TLV having the RR bit set, if there 501 exists on this interface an adjacency in state "UP" with the same 502 System ID, and in the case of a LAN circuit, with the same source LAN 503 address, then the procedures described in Section 3.2.1 are followed. 505 A router that does not support the restart capability will ignore the 506 restart TLV and reinitialize the adjacency as normal, returning an 507 IIH without the restart TLV. 509 On restarting, a router initializes the timer T3, starts the timer T2 510 for each LSPDB, and for each interface (and in the case of a LAN 511 circuit, for each level) starts the timer T1 and transmits an IIH 512 containing the restart TLV with the RR bit set. 514 On a Point-to-Point circuit, the restarting router SHOULD set the 515 "Adjacency Three-Way State" to "Init", because the receipt of the 516 acknowledging IIH (with RA set) MUST cause the adjacency to enter the 517 "UP" state immediately. 519 On a LAN circuit, the LAN-ID assigned to the circuit SHOULD be the 520 same as that used prior to the restart. In particular, for any 521 circuits for which the restarting router was previously DIS, the use 522 of a different LAN-ID would necessitate the generation of a new set 523 of pseudonode LSPs, and corresponding changes in all the LSPs 524 referencing them from other routers on the LAN. By preserving the 525 LAN-ID across the restart, this churn can be prevented. To enable a 526 restarting router to learn the LAN-ID used prior to restart, the LAN- 527 ID specified in an IIH with RR set MUST be ignored. 529 Transmission of "normal IIHs" is inhibited until the conditions 530 described below are met (in order to avoid causing an unnecessary 531 adjacency initialization). Upon expiry of the timer T1, it is 532 restarted and the IIH is retransmitted as above. 534 When a restarting router receives an IIH a local adjacency is 535 established as usual, and if the IIH contains a restart TLV with the 536 RA bit set (and on LAN circuits with a Restart Neighbor System ID 537 that matches that of the local system), the receipt of the 538 acknowledgement over that interface is noted. When the RA bit is set 539 and the state of the remote adjacency is "UP", then the timer T3 is 540 set to the minimum of its current value and the value of the 541 "Remaining Time" field in the received IIH. 543 On a Point-to-Point link, receipt of an IIH not containing the 544 restart TLV is also treated as an acknowledgement, since it indicates 545 that the neighbor is not restart capable. However, since no CSNP is 546 guaranteed to be received over this interface, the timer T1 is 547 cancelled immediately without waiting for a complete set of CSNPs. 548 Synchronization may therefore be deemed complete even though there 549 are some LSPs which are held (only) by this neighbor (see 550 Section 3.4). In this case, we also want to be certain that the 551 neighbor will reinitialize the adjacency in order to guarantee that 552 the SRMflags have been set on its database, thus ensuring eventual 553 LSPDB synchronization. This is guaranteed to happen except in the 554 case where the Adjacency Three-Way State in the received IIH is "UP" 555 and the Neighbor Extended Local Circuit ID matches the extended local 556 circuit ID assigned by the restarting router. In this case, the 557 restarting router MUST force the adjacency to reinitialize by setting 558 the local Adjacency Three-Way State to "DOWN" and sending a normal 559 IIH. 561 In the case of a LAN interface, receipt of an IIH not containing the 562 restart TLV is unremarkable since synchronization can still occur so 563 long as at least one of the non-restarting neighboring routers on the 564 LAN supports restart. Therefore, T1 continues to run in this case. 565 If none of the neighbors on the LAN are restart capable, T1 will 566 eventually expire after the locally defined number of retries. 568 In the case of a Point-to-Point circuit, the "LocalCircuitID" and 569 "Extended Local Circuit ID" information contained in the IIH can be 570 used immediately to generate an IIH containing the correct three-way 571 handshake information. The presence of "Neighbor Extended Local 572 Circuit ID" information that does not match the value currently in 573 use by the local system is ignored (since the IIH may have been 574 transmitted before the neighbor had received the new value from the 575 restarting router), but the adjacency remains in the initializing 576 state until the correct information is received. 578 In the case of a LAN circuit, the source neighbor information (e.g., 579 SNPAAddress) is recorded and used for adjacency establishment and 580 maintenance as normal. 582 When BOTH a complete set of CSNPs (for each active level, in the case 583 of a Point-to-Point circuit) and an acknowledgement have been 584 received over the interface, the timer T1 is cancelled. 586 Once the timer T1 has been cancelled, subsequent IIHs are transmitted 587 according to the normal algorithms, but including the restart TLV 588 with both RR and RA clear. 590 If a LAN contains a mixture of systems, only some of which support 591 the new algorithm, database synchronization is still guaranteed, but 592 the "old" systems will have reinitialized their adjacencies. 594 If an interface is active, but does not have any neighboring router 595 reachable over that interface, the timer T1 would never be cancelled, 596 and according to Section 3.4.1.1, the SPF would never be run. 597 Therefore, timer T1 is cancelled after some predetermined number of 598 expirations (which MAY be 1). 600 3.3.2. Adjacency Acquisition during Start 602 The starting router wants to ensure that in the event that a 603 neighboring router has an adjacency to the starting router in the 604 "UP" state (from a previous incarnation of the starting router), this 605 adjacency is reinitialized. The starting router also wants 606 neighboring routers to suppress advertisement of an adjacency to the 607 starting router until LSP database synchronization is achieved. This 608 is achieved by sending IIHs with the RR bit clear and the SA bit set 609 in the restart TLV. The RR bit remains clear and the SA bit remains 610 set in subsequent transmissions of IIHs until the adjacency has 611 reached the "UP" state and the initial T1 timer interval (see below) 612 has expired. 614 Receipt of an IIH with the RR bit clear will result in the 615 neighboring router utilizing normal operation of the adjacency state 616 machine. This will ensure that any old adjacency on the neighboring 617 router will be reinitialized. 619 Upon receipt of an IIH with the SA bit set, the behavior described in 620 Section 3.2.2 is followed. 622 Upon starting, a router starts timer T2 for each LSPDB. 624 For each interface (and in the case of a LAN circuit, for each 625 level), when an adjacency reaches the "UP" state, the starting router 626 starts a timer T1 and transmits an IIH containing the restart TLV 627 with the RR bit clear and SA bit set. Upon expiry of the timer T1, 628 it is restarted and the IIH is retransmitted with both RR and SA bits 629 set (only the RR bit has changed state from earlier IIHs). 631 Upon receipt of an IIH with the RR bit set (regardless of whether or 632 not the SA bit is set), the behavior described in Section 3.2.1 is 633 followed. 635 When an IIH is received by the starting router and the IIH contains a 636 restart TLV with the RA bit set (and on LAN circuits with a Restart 637 Neighbor System ID that matches that of the local system), the 638 receipt of the acknowledgement over that interface is noted. 640 On a Point-to-Point link, receipt of an IIH not containing the 641 restart TLV is also treated as an acknowledgement, since it indicates 642 that the neighbor is not restart capable. Since the neighbor will 643 have reinitialized the adjacency, this guarantees that SRMflags have 644 been set on its database, thus ensuring eventual LSPDB 645 synchronization. However, since no CSNP is guaranteed to be received 646 over this interface, the timer T1 is cancelled immediately without 647 waiting for a complete set of CSNPs. Synchronization may therefore 648 be deemed complete even though there are some LSPs that are held 649 (only) by this neighbor (see Section 3.4). 651 In the case of a LAN interface, receipt of an IIH not containing the 652 restart TLV is unremarkable since synchronization can still occur so 653 long as at least one of the non-restarting neighboring routers on the 654 LAN supports restart. Therefore, T1 continues to run in this case. 655 If none of the neighbors on the LAN are restart capable, T1 will 656 eventually expire after the locally defined number of retries. The 657 usual operation of the update process will ensure that 658 synchronization is eventually achieved. 660 When BOTH a complete set of CSNPs (for each active level, in the case 661 of a Point-to-Point circuit) and an acknowledgement have been 662 received over the interface, the timer T1 is cancelled. Subsequent 663 IIHs sent by the starting router have the RR and RA bits clear and 664 the SA bit set in the restart TLV. 666 Timer T1 is cancelled after some predetermined number of expirations 667 (which MAY be 1). 669 When the T2 timer(s) are cancelled or expire, transmission of "normal 670 IIHs" will begin. 672 3.3.3. Multiple Levels 674 A router that is operating as both a Level 1 and a Level 2 router on 675 a particular interface MUST perform the above operations for each 676 level. 678 On a LAN interface, it MUST send and receive both Level 1 and Level 2 679 IIHs and perform the CSNP synchronizations independently for each 680 level. 682 On a Point-to-Point interface, only a single IIH (indicating support 683 for both levels) is required, but it MUST perform the CSNP 684 synchronizations independently for each level. 686 3.4. Database Synchronization 688 When a router is started or restarted, it can expect to receive a 689 complete set of CSNPs over each interface. The arrival of the 690 CSNP(s) is now guaranteed, since an IIH with the RR bit set will be 691 retransmitted until the CSNP(s) are correctly received. 693 The CSNPs describe the set of LSPs that are currently held by each 694 neighbor. Synchronization will be complete when all these LSPs have 695 been received. 697 When (re)starting, a router starts an instance of timer T2 for each 698 LSPDB as described in Section 3.3.1 or Section 3.3.2. In addition to 699 normal processing of the CSNPs, the set of LSPIDs contained in the 700 first complete set of CSNPs received over each interface is recorded, 701 together with their remaining lifetime. In the case of a LAN 702 interface, a complete set of CSNPs MUST consist of CSNPs received 703 from neighbors that are not restarting. If there are multiple 704 interfaces on the (re)starting router, the recorded set of LSPIDs is 705 the union of those received over each interface. LSPs with a 706 remaining lifetime of zero are NOT so recorded. 708 As LSPs are received (by the normal operation of the update process) 709 over any interface, the corresponding LSPID entry is removed (it is 710 also removed if an LSP arrives before the CSNP containing the 711 reference). When an LSPID has been held in the list for its 712 indicated remaining lifetime, it is removed from the list. When the 713 list of LSPIDs is empty and the timer T1 has been cancelled for all 714 the interfaces that have an adjacency at this level, the timer T2 is 715 cancelled. 717 At this point, the local database is guaranteed to contain all the 718 LSP(s) (either the same sequence number or a more recent sequence 719 number) that were present in the neighbors' databases at the time of 720 (re)starting. LSPs that arrived in a neighbor's database after the 721 time of (re)starting may or may not be present, but the normal 722 operation of the update process will guarantee that they will 723 eventually be received. At this point, the local database is deemed 724 to be "synchronized". 726 Since LSPs mentioned in the CSNP(s) with a zero remaining lifetime 727 are not recorded, and those with a short remaining lifetime are 728 deleted from the list when the lifetime expires, cancellation of the 729 timer T2 will not be prevented by waiting for an LSP that will never 730 arrive. 732 3.4.1. LSP Generation and Flooding and SPF Computation 734 The operation of a router starting, as opposed to restarting, is 735 somewhat different. These two cases are dealt with separately below. 737 3.4.1.1. Restarting 739 In order to avoid causing unnecessary routing churn in other routers, 740 it is highly desirable that the router's own LSPs generated by the 741 restarting system are the same as those previously present in the 742 network (assuming no other changes have taken place). It is 743 important therefore not to regenerate and flood the LSPs until all 744 the adjacencies have been re-established and any information required 745 for propagation into the local LSPs is fully available. Ideally, the 746 information is loaded into the LSPs in a deterministic way, such that 747 the same information occurs in the same place in the same LSP (and 748 hence the LSPs are identical to their previous versions). If this 749 can be achieved, the new versions may not even cause SPF to be run in 750 other systems. However, provided the same information is included in 751 the set of LSPs (albeit in a different order, and possibly different 752 LSPs), the result of running the SPF will be the same and will not 753 cause churn to the forwarding tables. 755 In the case of a restarting router, none of the router's own LSPs are 756 transmitted, nor are the router's own forwarding tables updated while 757 the timer T3 is running. 759 Redistribution of inter-level information MUST be regenerated before 760 this router's LSP is flooded to other nodes. Therefore, the Level-n 761 non-pseudonode LSP(s) MUST NOT be flooded until the other level's T2 762 timer has expired and its SPF has been run. This ensures that any 763 inter-level information that is to be propagated can be included in 764 the Level-n LSP(s). 766 During this period, if one of the router's own (including 767 pseudonodes) LSPs is received, which the local router does not 768 currently have in its own database, it is NOT purged. Under normal 769 operation, such an LSP would be purged, since the LSP clearly should 770 not be present in the global LSP database. However, in the present 771 circumstances, this would be highly undesirable, because it could 772 cause premature removal of a router's own LSP -- and hence churn in 773 remote routers. Even if the local system has one or more of the 774 router's own LSPs (which it has generated, but not yet transmitted), 775 it is still not valid to compare the received LSP against this set, 776 since it may be that as a result of propagation between Level 1 and 777 Level 2 (or vice versa), a further router's own LSP will need to be 778 generated when the LSP databases have synchronized. 780 During this period, a restarting router SHOULD send CSNPs as it 781 normally would. Information about the router's own LSPs MAY be 782 included, but if it is included it MUST be based on LSPs that have 783 been received, not on versions that have been generated (but not yet 784 transmitted). This restriction is necessary to prevent premature 785 removal of an LSP from the global LSP database. 787 When the timer T2 expires or is cancelled indicating that 788 synchronization for that level is complete, the SPF for that level is 789 run in order to derive any information that is required to be 790 propagated to another level, but the forwarding tables are not yet 791 updated. 793 Once the other level's SPF has run and any inter-level propagation 794 has been resolved, the router's own LSPs can be generated and 795 flooded. Any own LSPs that were previously ignored, but that are not 796 part of the current set of own LSPs (including pseudonodes), MUST 797 then be purged. Note that it is possible that a Designated Router 798 change may have taken place, and consequently the router SHOULD purge 799 those pseudonode LSPs that it previously owned, but that are now no 800 longer part of its set of pseudonode LSPs. 802 When all the T2 timers have expired or been cancelled, the timer T3 803 is cancelled and the local forwarding tables are updated. 805 If the timer T3 expires before all the T2 timers have expired or been 806 cancelled, this indicates that the synchronization process is taking 807 longer than the minimum holding time of the neighbors. The router's 808 own LSP(s) for levels that have not yet completed their first SPF 809 computation are then flooded with the overload bit set to indicate 810 that the router's LSPDB is not yet synchronized (and therefore other 811 routers MUST NOT compute routes through this router). Normal 812 operation of the update process resumes, and the local forwarding 813 tables are updated. In order to prevent the neighbor's adjacencies 814 from expiring, IIHs with the normal interface value for the holding 815 time are transmitted over all interfaces with neither RR nor RA set 816 in the restart TLV. This will cause the neighbors to refresh their 817 adjacencies. The router's own LSP(s) will continue to have the 818 overload bit set until timer T2 has expired or been cancelled. 820 3.4.1.2. Starting 822 In the case of a starting router, as soon as each adjacency is 823 established, and before any CSNP exchanges, the router's own zeroth 824 LSP is transmitted with the overload bit set. This prevents other 825 routers from computing routes through the router until it has 826 reliably acquired the complete set of LSPs. The overload bit remains 827 set in subsequent transmissions of the zeroth LSP (such as will occur 828 if a previous copy of the router's own zeroth LSP is still present in 829 the network) while any timer T2 is running. 831 When all the T2 timers have been cancelled, the router's own LSP(s) 832 MAY be regenerated with the overload bit clear (assuming the router 833 is not in fact overloaded, and there is no other reason, such as 834 incomplete BGP convergence, to keep the overload bit set) and flooded 835 as normal. 837 Other LSPs owned by this router (including pseudonodes) are generated 838 and flooded as normal, irrespective of the timer T2. The SPF is also 839 run as normal and the Routing Information Base (RIB) and Forwarding 840 Information Base (FIB) updated as routes become available. 842 To avoid the possible formation of temporary blackholes, the starting 843 router sets the SA bit in the restart TLV (as described in 844 Section 3.3.2) in all IIHs that it sends. 846 When all T2 timers have been cancelled, the starting router MUST 847 transmit IIHs with the SA bit clear. 849 4. State Tables 851 This section presents state tables that summarize the behaviors 852 described in this document. Other behaviors, in particular adjacency 853 state transitions and LSP database update operation, are NOT included 854 in the state tables except where this document modifies the behaviors 855 described in [ISO10589] and [RFC5303]. 857 The states named in the columns of the tables below are a mixture of 858 states that are specific to a single adjacency (ADJ suppressed, ADJ 859 Seen RA, ADJ Seen CSNP) and states that are indicative of the state 860 of the protocol instance (Running, Restarting, Starting, SPF Wait). 862 Three state tables are presented from the point of view of a running 863 router, a restarting router, and a starting router. 865 4.1. Running Router 867 Event | Running | ADJ suppressed 868 ============================================================== 869 RX PR | Set Planned Restart | 870 | state. | 871 | Update hold time 872 | Send PA | 873 -------------+----------------------+------------------------- 874 RX PR clr | Clear Planned | 875 and RR clr | Restart State | 876 | Restore holdtime to | 877 | local value | 878 -------------+----------------------+------------------------- 879 RX PA | Proceed with planned | 880 | restart | 881 -------------+----------------------+------------------------- 882 RX RR | Maintain ADJ State | 883 | Send RA | 884 | Set SRM,send CSNP | 885 | (Note 1) | 886 | Update Hold Time, | 887 | set Restart Mode | 888 | (Note 2) | 889 -------------+----------------------+------------------------- 890 RX RR clr | Clr Restart mode | 891 -------------+----------------------+------------------------- 892 RX SA | Suppress IS neighbor | 893 | TLV in LSP(s) | 894 | Goto ADJ Suppressed | 895 -------------+----------------------+------------------------- 896 RX SA clr | |Unsuppress IS neighbor 897 | | TLV in LSP(s) 898 | |Goto Running 899 ============================================================== 901 Note 1: CSNPs are sent by routers in accordance with Section 3.2.1c 903 Note 2: If Restart Mode clear 905 4.2. Restarting Router 907 Event | Restarting | ADJ Seen | ADJ Seen | SPF Wait 908 | | RA | CSNP | 909 =================================================================== 910 Restart | Send PR | | | 911 planned | | | | 912 ------------+--------------------+-----------+-----------+------------ 913 Planned | Send PR clr | | | 914 restart | | | | 915 canceled | | | | 916 ------------+--------------------+-----------+-----------+------------ 917 Router | Send IIH/RR | | | 918 restarts | ADJ Init | | | 919 | Start T1,T2,T3 | | | 920 ------------+--------------------+-----------+-----------+------------ 921 RX RR | Send RA | | | 922 ------------+--------------------+-----------+-----------+------------ 923 RX RA | Adjust T3 | | Cancel T1 | 924 | Goto ADJ Seen RA | | Adjust T3 | 925 ----------- +--------------------+-----------+-----------+------------ 926 RX CSNP set| Goto ADJ Seen CSNP | Cancel T1 | | 927 ------------+--------------------+-----------+-----------+------------ 928 RX IIH w/o | Cancel T1 (Point- | | | 929 Restart TLV| to-point only) | | | 930 ------------+--------------------+-----------+-----------+------------ 931 T1 expires | Send IIH/RR |Send IIH/RR|Send IIH/RR| 932 | Restart T1 | Restart T1| Restart T1| 933 ------------+--------------------+-----------+-----------+------------ 934 T1 expires | Send IIH/ | Send IIH/ | Send IIH/ | 935 nth time | normal | normal | normal | 936 ------------+--------------------+-----------+-----------+------------ 937 T2 expires | Trigger SPF | | | 938 | Goto SPF Wait | | | 939 ------------+--------------------+-----------+-----------+------------ 940 T3 expires | Set overload bit | | | 941 | Flood local LSPs | | | 942 | Update fwd plane | | | 943 ------------+--------------------+-----------+-----------+------------ 944 LSP DB Sync| Cancel T2, and T3 | | | 945 | Trigger SPF | | | 946 | Goto SPF wait | | | 947 ------------+--------------------+-----------+-----------+------------ 948 All SPF | | | | Clear 949 done | | | | overload bit 950 | | | | Update fwd 951 | | | | plane 952 | | | | Flood local 953 | | | | LSPs 954 | | | | Goto Running 955 ====================================================================== 957 4.3. Starting Router 959 Event | Starting | ADJ Seen RA| ADJ Seen CSNP 960 ============================================================= 961 Router | Send IIH/SA | | 962 starts | Start T1,T2 | | 963 -------------+-------------------+------------+--------------- 964 RX RR | Send RA | | 965 -------------+-------------------+------------+--------------- 966 RX RA | Goto ADJ Seen RA | | Cancel T1 967 -------------+-------------------+------------+--------------- 968 RX CSNP Set | Goto ADJ Seen CSNP| Cancel T1 | 969 -------------+-------------------+------------+--------------- 970 RX IIH w | Cancel T1 | | 971 no Restart | (Point-to-Point | | 972 TLV | only) | | 973 -------------+-------------------+------------+--------------- 974 ADJ UP | Start T1 | | 975 | Send local LSPs | | 976 | with overload bit| | 977 | set | | 978 -------------+-------------------+------------+--------------- 979 T1 expires | Send IIH/RR |Send IIH/RR | Send IIH/RR 980 | and SA | and SA | and SA 981 | Restart T1 |Restart T1 | Restart T1 982 -------------+-------------------+------------+--------------- 983 T1 expires | Send IIH/SA |Send IIH/SA | Send IIH/SA 984 nth time | | | 985 -------------+-------------------+------------+--------------- 986 T2 expires | Clear overload bit| | 987 | Send IIH normal | | 988 | Goto Running | | 989 -------------+-------------------+------------+--------------- 990 LSP DB Sync | Cancel T2 | | 991 | Clear overload bit| | 992 | Send IIH normal | | 993 ============================================================== 995 5. IANA Considerations 997 This document defines the following IS-IS TLV that is listed in the 998 IS-IS TLV codepoint registry: 1000 Type Description IIH LSP SNP Purge 1001 ---- ------------------------------ --- --- --- ----- 1002 211 Restart TLV y n n n 1004 IANA is requested to update the entry in registry to point to this 1005 document. 1007 6. Security Considerations 1009 Any new security issues raised by the procedures in this document 1010 depend upon the ability of an attacker to inject a false but 1011 apparently valid IIH, the ease/difficulty of which has not been 1012 altered. 1014 If the RR bit is set in a false IIH, neighbors who receive such an 1015 IIH will continue to maintain an existing adjacency in the "UP" state 1016 and may (re)send a complete set of CSNPs. While the latter action is 1017 wasteful, neither action causes any disruption in correct protocol 1018 operation. 1020 If the RA bit is set in a false IIH, a (re)starting router that 1021 receives such an IIH may falsely believe that there is a neighbor on 1022 the corresponding interface that supports the procedures described in 1023 this document. In the absence of receipt of a complete set of CSNPs 1024 on that interface, this could delay the completion of (re)start 1025 procedures by requiring the timer T1 to time out the locally defined 1026 maximum number of retries. This behavior is the same as would occur 1027 on a LAN where none of the (re)starting router's neighbors support 1028 the procedures in this document and is covered in Sections 3.3.1 and 1029 3.3.2. 1031 If the SA bit is set in a false IIH, this could cause suppression of 1032 the advertisement of an IS neighbor, which could either continue for 1033 an indefinite period or occur intermittently with the result being a 1034 possible loss of reachability to some destinations in the network 1035 and/or increased frequency of LSP flooding and SPF calculation. 1037 If the PR bit is set in a false IIH, neighbors who receive such an 1038 IIH could modify the holding time of an existing adjacency 1039 inappropriately. In the event of topology changes, the neighbor 1040 might also choose to bring the adjacency down in the false belief 1041 that the forwarding plane of the router identified as the source of 1042 the false IIH is not currently processing announce topology changes. 1044 If the PA bit is set in a false IIH, a router that receives such an 1045 IIH may falsely believe that the neighbor on the corresponding 1046 interface supports the planned restart procedures defined in this 1047 document. If such a router is planning to restart it might then 1048 proceed to initiate restart in the false expectation that the 1049 neighbor has updated its holding time as requested. This may result 1050 in the neighbor bringing down the adjacency while the receiving 1051 router is restarting, causing in unnecessary disruption to 1052 forwarding. 1054 The possibility of IS-IS PDU spoofing can be reduced by the use of 1055 authentication as described in [RFC1195] and [ISO10589], and 1056 especially the use of cryptographic authentication as described in 1057 [RFC5304] and [RFC5310]. 1059 7. Manageability Considerations 1061 These extensions that have been designed, developed, and deployed for 1062 many years do not have any new impact on management and operation of 1063 the IS-IS protocol via this standardization process. 1065 8. Acknowledgements 1067 For RFC 5306 the authors acknowledged contributions made by Jeff 1068 Parker, Radia Perlman, Mark Schaefer, Naiming Shen, Nischal Sheth, 1069 Russ White, and Rena Yang. 1071 The authors of this updated version acknowledge the contribution of 1072 Mike Shand, co-auther of RFC 5306. 1074 9. Normative References 1076 [ISO10589] 1077 International Organization for Standardization, 1078 "Intermediate system to Intermediate system intra-domain 1079 routeing information exchange protocol for use in 1080 conjunction with the protocol for providing the 1081 connectionless-mode Network Service (ISO 8473)", ISO/ 1082 IEC 10589:2002, Second Edition, Nov 2002. 1084 [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and 1085 dual environments", RFC 1195, DOI 10.17487/RFC1195, 1086 December 1990, . 1088 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1089 Requirement Levels", BCP 14, RFC 2119, 1090 DOI 10.17487/RFC2119, March 1997, 1091 . 1093 [RFC5303] Katz, D., Saluja, R., and D. Eastlake 3rd, "Three-Way 1094 Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303, 1095 DOI 10.17487/RFC5303, October 2008, 1096 . 1098 [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic 1099 Authentication", RFC 5304, DOI 10.17487/RFC5304, October 1100 2008, . 1102 [RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., 1103 and M. Fanto, "IS-IS Generic Cryptographic 1104 Authentication", RFC 5310, DOI 10.17487/RFC5310, February 1105 2009, . 1107 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 1108 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 1109 . 1111 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1112 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1113 May 2017, . 1115 Appendix A. Summary of Changes from RFC 5306 1117 This document extends RFC 5306 by introducing support for signalling 1118 the neighbors of a restarting router that a planned restart is about 1119 to occur. This allows the neighbors to be aware of the state of the 1120 restarting router so that appropriate action may be taken if other 1121 topology changes occur while the planned restart is in progress. 1122 Since the forwarding plane of the restarting router is maintained 1123 based upon the pre-restart state of the network, additional topology 1124 changes introduce the possibility that traffic may be lost if paths 1125 via the restarting router continue to be used while the restart is in 1126 progress. 1128 In support of this new functionality two new flags have been 1129 introduced: 1131 PR - Restart is planned 1132 PA - Planned restart acknowledgement 1134 No changes to the post restart exchange between the restarting router 1135 and its neighbors have been introduced. 1137 Authors' Addresses 1139 Les Ginsberg 1140 Cisco Systems, Inc. 1142 Email: ginsberg@cisco.com 1144 Paul Wells 1145 Cisco Systems, Inc. 1147 Email: pauwells@cisco.com