idnits 2.17.1 draft-ietf-lsr-isis-rfc5306bis-05.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 16, 2019) is 1708 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 16, 2019 6 Expires: February 17, 2020 8 Restart Signaling for IS-IS 9 draft-ietf-lsr-isis-rfc5306bis-05 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 17, 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 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: Very early draft versions of the restart functionality 267 did not include the Restarting Neighbor System ID in the TLV. 268 RFC 5306 allowed for the possibility of interoperating with 269 legacy implementations by stating that a router that 270 is expecting an RA on a LAN circuit should assume that the 271 acknowledgement is directed at the local system if the TLV 272 is received with RA set and Restarting Neighbor System ID 273 is not present. It is an implementation choice whether to 274 continue to accept (on a LAN) a TLV with RA set and 275 Restarting Neighbor System ID absent. Note that the omission 276 of the Restarting Neighbor System ID only introduces ambiguity 277 in the case where there are multiple systems on a LAN 278 simultaneously performing restart. 280 The functionality associated with each of the defined flags (as 281 described in the following sections) is mutually exclusive with any 282 of the other flags. Therefore, it is expected that at most one flag 283 will be set in a TLV. Received TLVs which have multiple flags set 284 MUST be ignored. 286 3.2.1. Use of RR and RA Bits 288 The RR bit is used by a (re)starting router to signal to its 289 neighbors that a (re)start is in progress, that an existing adjacency 290 SHOULD be maintained even under circumstances when the normal 291 operation of the adjacency state machine would require the adjacency 292 to be reinitialized, to request a set of CSNPs, and to request 293 setting of the SRMflags. 295 The RA bit is sent by the neighbor of a (re)starting router to 296 acknowledge the receipt of a restart TLV with the RR bit set. 298 When the neighbor of a (re)starting router receives an IIH with the 299 restart TLV having the RR bit set, if there exists on this interface 300 an adjacency in state "UP" with the same System ID, and in the case 301 of a LAN circuit, with the same source LAN address, then, 302 irrespective of the other contents of the "Intermediate System 303 Neighbors" option (LAN circuits) or the "Point-to-Point Three-Way 304 Adjacency" option (Point-to-Point circuits): 306 a. the state of the adjacency is not changed. If this is the first 307 IIH with the RR bit set that this system has received associated 308 with this adjacency, then the adjacency is marked as being in 309 "Restart mode" and the adjacency holding time is refreshed -- 310 otherwise, the holding time is not refreshed. The "remaining 311 time" transmitted according to (b) below MUST reflect the actual 312 time after which the adjacency will now expire. Receipt of an 313 IIH with the RR bit reset will clear the "Restart mode" state. 314 This procedure allows the restarting router to cause the neighbor 315 to maintain the adjacency long enough for restart to successfully 316 complete, while also preventing repetitive restarts from 317 maintaining an adjacency indefinitely. Whether or not an 318 adjacency is marked as being in "Restart mode" has no effect on 319 adjacency state transitions. 321 b. immediately (i.e., without waiting for any currently running 322 timer interval to expire, but with a small random delay of a few 323 tens of milliseconds on LANs to avoid "storms") transmit over the 324 corresponding interface an IIH including the restart TLV with the 325 RR bit clear and the RA bit set, in the case of Point-to-Point 326 adjacencies having updated the "Point-to-Point Three-Way 327 Adjacency" option to reflect any new values received from the 328 (re)starting router. (This allows a restarting router to quickly 329 acquire the correct information to place in its hellos.) The 330 "Remaining Time" MUST be set to the current time (in seconds) 331 before the holding timer on this adjacency is due to expire. If 332 the corresponding interface is a LAN interface, then the 333 Restarting Neighbor System ID SHOULD be set to the System ID of 334 the router from which the IIH with the RR bit set was received. 335 This is required to correctly associate the acknowledgement and 336 holding time in the case where multiple systems on a LAN restart 337 at approximately the same time. This IIH SHOULD be transmitted 338 before any LSPs or SNPs are transmitted as a result of the 339 receipt of the original IIH. 341 c. if the corresponding interface is a Point-to-Point interface, or 342 if the receiving router has the highest LnRouterPriority (with 343 the highest source MAC (Media Access Control) address breaking 344 ties) among those routers to which the receiving router has an 345 adjacency in state "UP" on this interface whose IIHs contain the 346 restart TLV, excluding adjacencies to all routers which are 347 considered in "Restart mode" (note the actual DIS is NOT changed 348 by this process), initiate the transmission over the 349 corresponding interface of a complete set of CSNPs, and set 350 SRMflags on the corresponding interface for all LSPs in the local 351 LSP database. 353 Otherwise (i.e., if there was no adjacency in the "UP" state to the 354 System ID in question), process the IIH as normal by reinitializing 355 the adjacency and setting the RA bit in the returned IIH. 357 3.2.2. Use of the SA Bit 359 The SA bit is used by a starting router to request that its neighbor 360 suppress advertisement of the adjacency to the starting router in the 361 neighbor's LSPs. 363 A router that is starting has no maintained forwarding function 364 state. This may or may not be the first time the router has started. 365 If this is not the first time the router has started, copies of LSPs 366 generated by this router in its previous incarnation may exist in the 367 LSP databases of other routers in the network. These copies are 368 likely to appear "newer" than LSPs initially generated by the 369 starting router due to the reinitialization of LSP fragment sequence 370 numbers by the starting router. This may cause temporary blackholes 371 to occur until the normal operation of the update process causes the 372 starting router to regenerate and flood copies of its own LSPs with 373 higher sequence numbers. The temporary blackholes can be avoided if 374 the starting router's neighbors suppress advertising an adjacency to 375 the starting router until the starting router has been able to 376 propagate newer versions of LSPs generated by previous incarnations. 378 When a router receives an IIH with the restart TLV having the SA bit 379 set, if there exists on this interface an adjacency in state "UP" 380 with the same System ID, and in the case of a LAN circuit, with the 381 same source LAN address, then the router MUST suppress advertisement 382 of the adjacency to the neighbor in its own LSPs. Until an IIH with 383 the SA bit clear has been received, the neighbor advertisement MUST 384 continue to be suppressed. If the adjacency transitions to the "UP" 385 state, the new adjacency MUST NOT be advertised until an IIH with the 386 SA bit clear has been received. 388 Note that a router that suppresses advertisement of an adjacency MUST 389 NOT use this adjacency when performing its SPF calculation. In 390 particular, if an implementation follows the example guidelines 391 presented in [ISO10589], Annex C.2.5, Step 0:b) "pre-load TENT with 392 the local adjacency database", the suppressed adjacency MUST NOT be 393 loaded into TENT. 395 3.2.3. Use of PR and PA Bits 397 The PR bit is used by a router which is planning to initiate a 398 restart to signal to its neighbors that it will be restarting. The 399 router sending an IIH with PR bit set SHOULD set the "remaining time" 400 to a value greater than the expected control plane restart time. The 401 PR bit SHOULD remain set in IIHs until the restart is initiated. 403 The PA bit is sent by the neighbor of a router planning to restart to 404 acknowledge receipt of a restart TLV with the PR bit set. 406 When the neighbor of a router planning a restart receives an IIH with 407 the restart TLV having the PR bit set, if there exists on this 408 interface an adjacency in state "UP" with the same System ID, and in 409 the case of a LAN circuit, with the same source LAN address, then: 411 a. if this is the first IIH with the PR bit set that this system has 412 received associated with this adjacency, then the adjacency is 413 marked as being in "Planned Restart state" and the adjacency 414 holding time is refreshed -- otherwise, the holding time is not 415 refreshed. The holding time SHOULD be set to the "remaining 416 time" specified in the received IIH with PR set. The "remaining 417 time" transmitted according to (b) below MUST reflect the actual 418 time after which the adjacency will now expire. Receipt of an 419 IIH with the PR bit reset will clear the "Planned Restart state" 420 and cause the receiving router to set the adjacency hold time to 421 the locally configured value. This procedure allows the router 422 planning a restart to cause the neighbor to maintain the 423 adjacency long enough for restart to successfully complete. 424 Whether or not an adjacency is marked as being in "Planned 425 Restart state" has no effect on adjacency state transitions. 427 b. immediately (i.e., without waiting for any currently running 428 timer interval to expire, but with a small random delay of a few 429 tens of milliseconds on LANs to avoid "storms") transmit over the 430 corresponding interface an IIH including the restart TLV with the 431 PR bit clear and the PA bit set. The "Remaining Time" MUST be 432 set to the current time (in seconds) before the holding timer on 433 this adjacency is due to expire. If the corresponding interface 434 is a LAN interface, then the Restarting Neighbor System ID SHOULD 435 be set to the System ID of the router from which the IIH with the 436 PR bit set was received. This is required to correctly associate 437 the acknowledgement and holding time in the case where multiple 438 systems on a LAN are planning a restart at approximately the same 439 time. 441 NOTE: Receipt of an IIH with PA bit set indicates to the router 442 planning a restart that the neighbor is aware of the planned restart 443 and - in the absence of topology changes as described below - will 444 maintain the adjacency for the "remaining time" included in the IIH 445 with PA set. 447 By definition, a restarting router maintains forwarding state across 448 the control plane restart (see Section 2). But while a control plane 449 restart is in progress it is expected that the restarting router will 450 be unable to respond to topology changes. It is therefore useful to 451 signal a planned restart so that the neighbors of the restarting 452 router can determine whether it is safe to maintain the adjacency if 453 other topology changes occur prior to the completion of the restart. 454 Signalling a planned restart in the absence of maintained forwarding 455 plane state is likely to lead to significant traffic loss and MUST 456 NOT be done. 458 Neighbors of the router which has signaled planned restart SHOULD 459 maintain the adjacency in a planned restart state until it receives 460 an IIH with the RR bit set, receives an IIH with both PR and RR bits 461 clear, or the adjacency holding time expires - whichever occurs 462 first. 464 While the adjacency is in planned restart state some or all of the 465 following actions MAY be taken: 467 a. if additional topology changes occur, the adjacency which is in 468 planned restart state MAY be brought down even though the hold 469 time has not yet expired. Given that the neighbor which has 470 signaled a planned restart is not expected to update its 471 forwarding plane in response to signaling of the topology changes 472 (since it is restarting) traffic which transits that node is at 473 risk of being improperly forwarded. On a LAN circuit, if the 474 router in planned restart state is the DIS at any supported 475 level, the adjacency(ies) SHOULD be brought down whenever any LSP 476 update is either generated or received, so as to trigger a new 477 DIS election. Failure to do so will compromise the reliability 478 of the Update Process on that circuit. What other criteria are 479 used to determine what topology changes will trigger bringing the 480 adjacency down is a local implementation decision. 482 b. if a BFD [RFC5880] session to the neighbor which signals a 483 planned restart is in the UP state and subsequently goes DOWN, 484 the event MAY be ignored since it is possible this is an expected 485 side effect of the restart. Use of the Control Plane Independent 486 state as signalled in BFD control packets SHOULD be considered in 487 the decision to ignore a BFD Session DOWN event. 489 c. on a Point-to-Point circuit, transmission of LSPs, CSNPs, and 490 PSNPs MAY be suppressed. It is expected that the PDUs will not 491 be received. 493 Use of the PR bit provides a means to safely support restart periods 494 which are significantly longer than standard holdtimes. 496 3.3. Adjacency (Re)Acquisition 498 Adjacency (re)acquisition is the first step in (re)initialization. 499 Restarting and starting routers will make use of the RR bit in the 500 restart TLV, though each will use it at different stages of the 501 (re)start procedure. 503 3.3.1. Adjacency Reacquisition during Restart 505 The restarting router explicitly notifies its neighbor that the 506 adjacency is being reacquired, and hence that it SHOULD NOT 507 reinitialize the adjacency. This is achieved by setting the RR bit 508 in the restart TLV. When the neighbor of a restarting router 509 receives an IIH with the restart TLV having the RR bit set, if there 510 exists on this interface an adjacency in state "UP" with the same 511 System ID, and in the case of a LAN circuit, with the same source LAN 512 address, then the procedures described in Section 3.2.1 are followed. 514 A router that does not support the restart capability will ignore the 515 restart TLV and reinitialize the adjacency as normal, returning an 516 IIH without the restart TLV. 518 On restarting, a router initializes the timer T3, starts the timer T2 519 for each LSPDB, and for each interface (and in the case of a LAN 520 circuit, for each level) starts the timer T1 and transmits an IIH 521 containing the restart TLV with the RR bit set. 523 On a Point-to-Point circuit, the restarting router SHOULD set the 524 "Adjacency Three-Way State" to "Init", because the receipt of the 525 acknowledging IIH (with RA set) MUST cause the adjacency to enter the 526 "UP" state immediately. 528 On a LAN circuit, the LAN-ID assigned to the circuit SHOULD be the 529 same as that used prior to the restart. In particular, for any 530 circuits for which the restarting router was previously DIS, the use 531 of a different LAN-ID would necessitate the generation of a new set 532 of pseudonode LSPs, and corresponding changes in all the LSPs 533 referencing them from other routers on the LAN. By preserving the 534 LAN-ID across the restart, this churn can be prevented. To enable a 535 restarting router to learn the LAN-ID used prior to restart, the LAN- 536 ID specified in an IIH with RR set MUST be ignored. 538 Transmission of "normal IIHs" is inhibited until the conditions 539 described below are met (in order to avoid causing an unnecessary 540 adjacency initialization). Upon expiry of the timer T1, it is 541 restarted and the IIH is retransmitted as above. 543 When a restarting router receives an IIH a local adjacency is 544 established as usual, and if the IIH contains a restart TLV with the 545 RA bit set (and on LAN circuits with a Restart Neighbor System ID 546 that matches that of the local system), the receipt of the 547 acknowledgement over that interface is noted. When the RA bit is set 548 and the state of the remote adjacency is "UP", then the timer T3 is 549 set to the minimum of its current value and the value of the 550 "Remaining Time" field in the received IIH. 552 On a Point-to-Point link, receipt of an IIH not containing the 553 restart TLV is also treated as an acknowledgement, since it indicates 554 that the neighbor is not restart capable. However, since no CSNP is 555 guaranteed to be received over this interface, the timer T1 is 556 cancelled immediately without waiting for a complete set of CSNPs. 557 Synchronization may therefore be deemed complete even though there 558 are some LSPs which are held (only) by this neighbor (see 559 Section 3.4). In this case, we also want to be certain that the 560 neighbor will reinitialize the adjacency in order to guarantee that 561 the SRMflags have been set on its database, thus ensuring eventual 562 LSPDB synchronization. This is guaranteed to happen except in the 563 case where the Adjacency Three-Way State in the received IIH is "UP" 564 and the Neighbor Extended Local Circuit ID matches the extended local 565 circuit ID assigned by the restarting router. In this case, the 566 restarting router MUST force the adjacency to reinitialize by setting 567 the local Adjacency Three-Way State to "DOWN" and sending a normal 568 IIH. 570 In the case of a LAN interface, receipt of an IIH not containing the 571 restart TLV is unremarkable since synchronization can still occur so 572 long as at least one of the non-restarting neighboring routers on the 573 LAN supports restart. Therefore, T1 continues to run in this case. 574 If none of the neighbors on the LAN are restart capable, T1 will 575 eventually expire after the locally defined number of retries. 577 In the case of a Point-to-Point circuit, the "LocalCircuitID" and 578 "Extended Local Circuit ID" information contained in the IIH can be 579 used immediately to generate an IIH containing the correct three-way 580 handshake information. The presence of "Neighbor Extended Local 581 Circuit ID" information that does not match the value currently in 582 use by the local system is ignored (since the IIH may have been 583 transmitted before the neighbor had received the new value from the 584 restarting router), but the adjacency remains in the initializing 585 state until the correct information is received. 587 In the case of a LAN circuit, the source neighbor information (e.g., 588 SNPAAddress) is recorded and used for adjacency establishment and 589 maintenance as normal. 591 When BOTH a complete set of CSNPs (for each active level, in the case 592 of a Point-to-Point circuit) and an acknowledgement have been 593 received over the interface, the timer T1 is cancelled. 595 Once the timer T1 has been cancelled, subsequent IIHs are transmitted 596 according to the normal algorithms, but including the restart TLV 597 with both RR and RA clear. 599 If a LAN contains a mixture of systems, only some of which support 600 the new algorithm, database synchronization is still guaranteed, but 601 the "old" systems will have reinitialized their adjacencies. 603 If an interface is active, but does not have any neighboring router 604 reachable over that interface, the timer T1 would never be cancelled, 605 and according to Section 3.4.1.1, the SPF would never be run. 606 Therefore, timer T1 is cancelled after some predetermined number of 607 expirations (which MAY be 1). 609 3.3.2. Adjacency Acquisition during Start 611 The starting router wants to ensure that in the event that a 612 neighboring router has an adjacency to the starting router in the 613 "UP" state (from a previous incarnation of the starting router), this 614 adjacency is reinitialized. The starting router also wants 615 neighboring routers to suppress advertisement of an adjacency to the 616 starting router until LSP database synchronization is achieved. This 617 is achieved by sending IIHs with the RR bit clear and the SA bit set 618 in the restart TLV. The RR bit remains clear and the SA bit remains 619 set in subsequent transmissions of IIHs until the adjacency has 620 reached the "UP" state and the initial T1 timer interval (see below) 621 has expired. 623 Receipt of an IIH with the RR bit clear will result in the 624 neighboring router utilizing normal operation of the adjacency state 625 machine. This will ensure that any old adjacency on the neighboring 626 router will be reinitialized. 628 Upon receipt of an IIH with the SA bit set, the behavior described in 629 Section 3.2.2 is followed. 631 Upon starting, a router starts timer T2 for each LSPDB. 633 For each interface (and in the case of a LAN circuit, for each 634 level), when an adjacency reaches the "UP" state, the starting router 635 starts a timer T1 and transmits an IIH containing the restart TLV 636 with the RR bit clear and SA bit set. Upon expiry of the timer T1, 637 it is restarted and the IIH is retransmitted with both RR and SA bits 638 set (only the RR bit has changed state from earlier IIHs). 640 Upon receipt of an IIH with the RR bit set (regardless of whether or 641 not the SA bit is set), the behavior described in Section 3.2.1 is 642 followed. 644 When an IIH is received by the starting router and the IIH contains a 645 restart TLV with the RA bit set (and on LAN circuits with a Restart 646 Neighbor System ID that matches that of the local system), the 647 receipt of the acknowledgement over that interface is noted. 649 On a Point-to-Point link, receipt of an IIH not containing the 650 restart TLV is also treated as an acknowledgement, since it indicates 651 that the neighbor is not restart capable. Since the neighbor will 652 have reinitialized the adjacency, this guarantees that SRMflags have 653 been set on its database, thus ensuring eventual LSPDB 654 synchronization. However, since no CSNP is guaranteed to be received 655 over this interface, the timer T1 is cancelled immediately without 656 waiting for a complete set of CSNPs. Synchronization may therefore 657 be deemed complete even though there are some LSPs that are held 658 (only) by this neighbor (see Section 3.4). 660 In the case of a LAN interface, receipt of an IIH not containing the 661 restart TLV is unremarkable since synchronization can still occur so 662 long as at least one of the non-restarting neighboring routers on the 663 LAN supports restart. Therefore, T1 continues to run in this case. 664 If none of the neighbors on the LAN are restart capable, T1 will 665 eventually expire after the locally defined number of retries. The 666 usual operation of the update process will ensure that 667 synchronization is eventually achieved. 669 When BOTH a complete set of CSNPs (for each active level, in the case 670 of a Point-to-Point circuit) and an acknowledgement have been 671 received over the interface, the timer T1 is cancelled. Subsequent 672 IIHs sent by the starting router have the RR and RA bits clear and 673 the SA bit set in the restart TLV. 675 Timer T1 is cancelled after some predetermined number of expirations 676 (which MAY be 1). 678 When the T2 timer(s) are cancelled or expire, transmission of "normal 679 IIHs" will begin. 681 3.3.3. Multiple Levels 683 A router that is operating as both a Level 1 and a Level 2 router on 684 a particular interface MUST perform the above operations for each 685 level. 687 On a LAN interface, it MUST send and receive both Level 1 and Level 2 688 IIHs and perform the CSNP synchronizations independently for each 689 level. 691 On a Point-to-Point interface, only a single IIH (indicating support 692 for both levels) is required, but it MUST perform the CSNP 693 synchronizations independently for each level. 695 3.4. Database Synchronization 697 When a router is started or restarted, it can expect to receive a 698 complete set of CSNPs over each interface. The arrival of the 699 CSNP(s) is now guaranteed, since an IIH with the RR bit set will be 700 retransmitted until the CSNP(s) are correctly received. 702 The CSNPs describe the set of LSPs that are currently held by each 703 neighbor. Synchronization will be complete when all these LSPs have 704 been received. 706 When (re)starting, a router starts an instance of timer T2 for each 707 LSPDB as described in Section 3.3.1 or Section 3.3.2. In addition to 708 normal processing of the CSNPs, the set of LSPIDs contained in the 709 first complete set of CSNPs received over each interface is recorded, 710 together with their remaining lifetime. In the case of a LAN 711 interface, a complete set of CSNPs MUST consist of CSNPs received 712 from neighbors that are not restarting. If there are multiple 713 interfaces on the (re)starting router, the recorded set of LSPIDs is 714 the union of those received over each interface. LSPs with a 715 remaining lifetime of zero are NOT so recorded. 717 As LSPs are received (by the normal operation of the update process) 718 over any interface, the corresponding LSPID entry is removed (it is 719 also removed if an LSP arrives before the CSNP containing the 720 reference). When an LSPID has been held in the list for its 721 indicated remaining lifetime, it is removed from the list. When the 722 list of LSPIDs is empty and the timer T1 has been cancelled for all 723 the interfaces that have an adjacency at this level, the timer T2 is 724 cancelled. 726 At this point, the local database is guaranteed to contain all the 727 LSP(s) (either the same sequence number or a more recent sequence 728 number) that were present in the neighbors' databases at the time of 729 (re)starting. LSPs that arrived in a neighbor's database after the 730 time of (re)starting may or may not be present, but the normal 731 operation of the update process will guarantee that they will 732 eventually be received. At this point, the local database is deemed 733 to be "synchronized". 735 Since LSPs mentioned in the CSNP(s) with a zero remaining lifetime 736 are not recorded, and those with a short remaining lifetime are 737 deleted from the list when the lifetime expires, cancellation of the 738 timer T2 will not be prevented by waiting for an LSP that will never 739 arrive. 741 3.4.1. LSP Generation and Flooding and SPF Computation 743 The operation of a router starting, as opposed to restarting, is 744 somewhat different. These two cases are dealt with separately below. 746 3.4.1.1. Restarting 748 In order to avoid causing unnecessary routing churn in other routers, 749 it is highly desirable that the router's own LSPs generated by the 750 restarting system are the same as those previously present in the 751 network (assuming no other changes have taken place). It is 752 important therefore not to regenerate and flood the LSPs until all 753 the adjacencies have been re-established and any information required 754 for propagation into the local LSPs is fully available. Ideally, the 755 information is loaded into the LSPs in a deterministic way, such that 756 the same information occurs in the same place in the same LSP (and 757 hence the LSPs are identical to their previous versions). If this 758 can be achieved, the new versions may not even cause SPF to be run in 759 other systems. However, provided the same information is included in 760 the set of LSPs (albeit in a different order, and possibly different 761 LSPs), the result of running the SPF will be the same and will not 762 cause churn to the forwarding tables. 764 In the case of a restarting router, none of the router's own LSPs are 765 transmitted, nor are the router's own forwarding tables updated while 766 the timer T3 is running. 768 Redistribution of inter-level information MUST be regenerated before 769 this router's LSP is flooded to other nodes. Therefore, the Level-n 770 non-pseudonode LSP(s) MUST NOT be flooded until the other level's T2 771 timer has expired and its SPF has been run. This ensures that any 772 inter-level information that is to be propagated can be included in 773 the Level-n LSP(s). 775 During this period, if one of the router's own (including 776 pseudonodes) LSPs is received, which the local router does not 777 currently have in its own database, it is NOT purged. Under normal 778 operation, such an LSP would be purged, since the LSP clearly should 779 not be present in the global LSP database. However, in the present 780 circumstances, this would be highly undesirable, because it could 781 cause premature removal of a router's own LSP -- and hence churn in 782 remote routers. Even if the local system has one or more of the 783 router's own LSPs (which it has generated, but not yet transmitted), 784 it is still not valid to compare the received LSP against this set, 785 since it may be that as a result of propagation between Level 1 and 786 Level 2 (or vice versa), a further router's own LSP will need to be 787 generated when the LSP databases have synchronized. 789 During this period, a restarting router SHOULD send CSNPs as it 790 normally would. Information about the router's own LSPs MAY be 791 included, but if it is included it MUST be based on LSPs that have 792 been received, not on versions that have been generated (but not yet 793 transmitted). This restriction is necessary to prevent premature 794 removal of an LSP from the global LSP database. 796 When the timer T2 expires or is cancelled indicating that 797 synchronization for that level is complete, the SPF for that level is 798 run in order to derive any information that is required to be 799 propagated to another level, but the forwarding tables are not yet 800 updated. 802 Once the other level's SPF has run and any inter-level propagation 803 has been resolved, the router's own LSPs can be generated and 804 flooded. Any own LSPs that were previously ignored, but that are not 805 part of the current set of own LSPs (including pseudonodes), MUST 806 then be purged. Note that it is possible that a Designated Router 807 change may have taken place, and consequently the router SHOULD purge 808 those pseudonode LSPs that it previously owned, but that are now no 809 longer part of its set of pseudonode LSPs. 811 When all the T2 timers have expired or been cancelled, the timer T3 812 is cancelled and the local forwarding tables are updated. 814 If the timer T3 expires before all the T2 timers have expired or been 815 cancelled, this indicates that the synchronization process is taking 816 longer than the minimum holding time of the neighbors. The router's 817 own LSP(s) for levels that have not yet completed their first SPF 818 computation are then flooded with the overload bit set to indicate 819 that the router's LSPDB is not yet synchronized (and therefore other 820 routers MUST NOT compute routes through this router). Normal 821 operation of the update process resumes, and the local forwarding 822 tables are updated. In order to prevent the neighbor's adjacencies 823 from expiring, IIHs with the normal interface value for the holding 824 time are transmitted over all interfaces with neither RR nor RA set 825 in the restart TLV. This will cause the neighbors to refresh their 826 adjacencies. The router's own LSP(s) will continue to have the 827 overload bit set until timer T2 has expired or been cancelled. 829 3.4.1.2. Starting 831 In the case of a starting router, as soon as each adjacency is 832 established, and before any CSNP exchanges, the router's own zeroth 833 LSP is transmitted with the overload bit set. This prevents other 834 routers from computing routes through the router until it has 835 reliably acquired the complete set of LSPs. The overload bit remains 836 set in subsequent transmissions of the zeroth LSP (such as will occur 837 if a previous copy of the router's own zeroth LSP is still present in 838 the network) while any timer T2 is running. 840 When all the T2 timers have been cancelled, the router's own LSP(s) 841 MAY be regenerated with the overload bit clear (assuming the router 842 is not in fact overloaded, and there is no other reason, such as 843 incomplete BGP convergence, to keep the overload bit set) and flooded 844 as normal. 846 Other LSPs owned by this router (including pseudonodes) are generated 847 and flooded as normal, irrespective of the timer T2. The SPF is also 848 run as normal and the Routing Information Base (RIB) and Forwarding 849 Information Base (FIB) updated as routes become available. 851 To avoid the possible formation of temporary blackholes, the starting 852 router sets the SA bit in the restart TLV (as described in 853 Section 3.3.2) in all IIHs that it sends. 855 When all T2 timers have been cancelled, the starting router MUST 856 transmit IIHs with the SA bit clear. 858 4. State Tables 860 This section presents state tables that summarize the behaviors 861 described in this document. Other behaviors, in particular adjacency 862 state transitions and LSP database update operation, are NOT included 863 in the state tables except where this document modifies the behaviors 864 described in [ISO10589] and [RFC5303]. 866 The states named in the columns of the tables below are a mixture of 867 states that are specific to a single adjacency (ADJ suppressed, ADJ 868 Seen RA, ADJ Seen CSNP) and states that are indicative of the state 869 of the protocol instance (Running, Restarting, Starting, SPF Wait). 871 Three state tables are presented from the point of view of a running 872 router, a restarting router, and a starting router. 874 4.1. Running Router 875 Event | Running | ADJ suppressed 876 ============================================================== 877 RX PR | Set Planned Restart | 878 | state. | 879 | Update hold time 880 | Send PA | 881 -------------+----------------------+------------------------- 882 RX PR clr | Clear Planned | 883 and RR clr | Restart State | 884 | Restore holdtime to | 885 | local value | 886 -------------+----------------------+------------------------- 887 RX PA | Proceed with planned | 888 | restart | 889 -------------+----------------------+------------------------- 890 RX RR | Maintain ADJ State | 891 | Send RA | 892 | Set SRM,send CSNP | 893 | (Note 1) | 894 | Update Hold Time, | 895 | set Restart Mode | 896 | (Note 2) | 897 -------------+----------------------+------------------------- 898 RX RR clr | Clr Restart mode | 899 -------------+----------------------+------------------------- 900 RX SA | Suppress IS neighbor | 901 | TLV in LSP(s) | 902 | Goto ADJ Suppressed | 903 -------------+----------------------+------------------------- 904 RX SA clr | |Unsuppress IS neighbor 905 | | TLV in LSP(s) 906 | |Goto Running 907 ============================================================== 909 Note 1: CSNPs are sent by routers in accordance with Section 3.2.1c 911 Note 2: If Restart Mode clear 913 4.2. Restarting Router 915 Event | Restarting | ADJ Seen | ADJ Seen | SPF Wait 916 | | RA | CSNP | 917 =================================================================== 918 Restart | Send PR | | | 919 planned | | | | 920 ------------+--------------------+-----------+-----------+------------ 921 Planned | Send PR clr | | | 922 restart | | | | 923 canceled | | | | 924 ------------+--------------------+-----------+-----------+------------ 925 Router | Send IIH/RR | | | 926 restarts | ADJ Init | | | 927 | Start T1,T2,T3 | | | 928 ------------+--------------------+-----------+-----------+------------ 929 RX RR | Send RA | | | 930 ------------+--------------------+-----------+-----------+------------ 931 RX RA | Adjust T3 | | Cancel T1 | 932 | Goto ADJ Seen RA | | Adjust T3 | 933 ----------- +--------------------+-----------+-----------+------------ 934 RX CSNP set| Goto ADJ Seen CSNP | Cancel T1 | | 935 ------------+--------------------+-----------+-----------+------------ 936 RX IIH w/o | Cancel T1 (Point- | | | 937 Restart TLV| to-point only) | | | 938 ------------+--------------------+-----------+-----------+------------ 939 T1 expires | Send IIH/RR |Send IIH/RR|Send IIH/RR| 940 | Restart T1 | Restart T1| Restart T1| 941 ------------+--------------------+-----------+-----------+------------ 942 T1 expires | Send IIH/ | Send IIH/ | Send IIH/ | 943 nth time | normal | normal | normal | 944 ------------+--------------------+-----------+-----------+------------ 945 T2 expires | Trigger SPF | | | 946 | Goto SPF Wait | | | 947 ------------+--------------------+-----------+-----------+------------ 948 T3 expires | Set overload bit | | | 949 | Flood local LSPs | | | 950 | Update fwd plane | | | 951 ------------+--------------------+-----------+-----------+------------ 952 LSP DB Sync| Cancel T2, and T3 | | | 953 | Trigger SPF | | | 954 | Goto SPF wait | | | 955 ------------+--------------------+-----------+-----------+------------ 956 All SPF | | | | Clear 957 done | | | | overload bit 958 | | | | Update fwd 959 | | | | plane 960 | | | | Flood local 961 | | | | LSPs 962 | | | | Goto Running 963 ====================================================================== 965 4.3. Starting Router 967 Event | Starting | ADJ Seen RA| ADJ Seen CSNP 968 ============================================================= 969 Router | Send IIH/SA | | 970 starts | Start T1,T2 | | 971 -------------+-------------------+------------+--------------- 972 RX RR | Send RA | | 973 -------------+-------------------+------------+--------------- 974 RX RA | Goto ADJ Seen RA | | Cancel T1 975 -------------+-------------------+------------+--------------- 976 RX CSNP Set | Goto ADJ Seen CSNP| Cancel T1 | 977 -------------+-------------------+------------+--------------- 978 RX IIH w | Cancel T1 | | 979 no Restart | (Point-to-Point | | 980 TLV | only) | | 981 -------------+-------------------+------------+--------------- 982 ADJ UP | Start T1 | | 983 | Send local LSPs | | 984 | with overload bit| | 985 | set | | 986 -------------+-------------------+------------+--------------- 987 T1 expires | Send IIH/RR |Send IIH/RR | Send IIH/RR 988 | and SA | and SA | and SA 989 | Restart T1 |Restart T1 | Restart T1 990 -------------+-------------------+------------+--------------- 991 T1 expires | Send IIH/SA |Send IIH/SA | Send IIH/SA 992 nth time | | | 993 -------------+-------------------+------------+--------------- 994 T2 expires | Clear overload bit| | 995 | Send IIH normal | | 996 | Goto Running | | 997 -------------+-------------------+------------+--------------- 998 LSP DB Sync | Cancel T2 | | 999 | Clear overload bit| | 1000 | Send IIH normal | | 1001 ============================================================== 1003 5. IANA Considerations 1005 This document defines the following IS-IS TLV that is listed in the 1006 IS-IS TLV codepoint registry: 1008 Type Description IIH LSP SNP Purge 1009 ---- ------------------------------ --- --- --- ----- 1010 211 Restart TLV y n n n 1012 IANA is requested to update the entry in registry to point to this 1013 document. 1015 6. Security Considerations 1017 Any new security issues raised by the procedures in this document 1018 depend upon the ability of an attacker to inject a false but 1019 apparently valid IIH, the ease/difficulty of which has not been 1020 altered. 1022 If the RR bit is set in a false IIH, neighbors who receive such an 1023 IIH will continue to maintain an existing adjacency in the "UP" state 1024 and may (re)send a complete set of CSNPs. While the latter action is 1025 wasteful, neither action causes any disruption in correct protocol 1026 operation. 1028 If the RA bit is set in a false IIH, a (re)starting router that 1029 receives such an IIH may falsely believe that there is a neighbor on 1030 the corresponding interface that supports the procedures described in 1031 this document. In the absence of receipt of a complete set of CSNPs 1032 on that interface, this could delay the completion of (re)start 1033 procedures by requiring the timer T1 to time out the locally defined 1034 maximum number of retries. This behavior is the same as would occur 1035 on a LAN where none of the (re)starting router's neighbors support 1036 the procedures in this document and is covered in Sections 3.3.1 and 1037 3.3.2. 1039 If the SA bit is set in a false IIH, this could cause suppression of 1040 the advertisement of an IS neighbor, which could either continue for 1041 an indefinite period or occur intermittently with the result being a 1042 possible loss of reachability to some destinations in the network 1043 and/or increased frequency of LSP flooding and SPF calculation. 1045 If the PR bit is set in a false IIH, neighbors who receive such an 1046 IIH could modify the holding time of an existing adjacency 1047 inappropriately. In the event of topology changes, the neighbor 1048 might also choose to bring the adjacency down in the false belief 1049 that the forwarding plane of the router identified as the source of 1050 the false IIH is not currently processing announced topology changes. 1052 If the PA bit is set in a false IIH, a router that receives such an 1053 IIH may falsely believe that the neighbor on the corresponding 1054 interface supports the planned restart procedures defined in this 1055 document. If such a router is planning to restart it might then 1056 proceed to initiate a restart in the false expectation that the 1057 neighbor has updated its holding time as requested. This may result 1058 in the neighbor bringing down the adjacency while the receiving 1059 router is restarting, causing unnecessary disruption to forwarding. 1061 The possibility of IS-IS PDU spoofing can be reduced by the use of 1062 authentication as described in [RFC1195] and [ISO10589], and 1063 especially the use of cryptographic authentication as described in 1064 [RFC5304] and [RFC5310]. 1066 7. Manageability Considerations 1068 These extensions that have been designed, developed, and deployed for 1069 many years do not have any new impact on management and operation of 1070 the IS-IS protocol via this standardization process. 1072 8. Acknowledgements 1074 For RFC 5306 the authors acknowledged contributions made by Jeff 1075 Parker, Radia Perlman, Mark Schaefer, Naiming Shen, Nischal Sheth, 1076 Russ White, and Rena Yang. 1078 The authors of this updated version acknowledge the contribution of 1079 Mike Shand, co-auther of RFC 5306. 1081 9. Normative References 1083 [ISO10589] 1084 International Organization for Standardization, 1085 "Intermediate system to Intermediate system intra-domain 1086 routeing information exchange protocol for use in 1087 conjunction with the protocol for providing the 1088 connectionless-mode Network Service (ISO 8473)", ISO/ 1089 IEC 10589:2002, Second Edition, Nov 2002. 1091 [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and 1092 dual environments", RFC 1195, DOI 10.17487/RFC1195, 1093 December 1990, . 1095 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1096 Requirement Levels", BCP 14, RFC 2119, 1097 DOI 10.17487/RFC2119, March 1997, 1098 . 1100 [RFC5303] Katz, D., Saluja, R., and D. Eastlake 3rd, "Three-Way 1101 Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303, 1102 DOI 10.17487/RFC5303, October 2008, 1103 . 1105 [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic 1106 Authentication", RFC 5304, DOI 10.17487/RFC5304, October 1107 2008, . 1109 [RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R., 1110 and M. Fanto, "IS-IS Generic Cryptographic 1111 Authentication", RFC 5310, DOI 10.17487/RFC5310, February 1112 2009, . 1114 [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 1115 (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, 1116 . 1118 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1119 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1120 May 2017, . 1122 Appendix A. Summary of Changes from RFC 5306 1124 This document extends RFC 5306 by introducing support for signalling 1125 the neighbors of a restarting router that a planned restart is about 1126 to occur. This allows the neighbors to be aware of the state of the 1127 restarting router so that appropriate action may be taken if other 1128 topology changes occur while the planned restart is in progress. 1129 Since the forwarding plane of the restarting router is maintained 1130 based upon the pre-restart state of the network, additional topology 1131 changes introduce the possibility that traffic may be lost if paths 1132 via the restarting router continue to be used while the restart is in 1133 progress. 1135 In support of this new functionality two new flags have been 1136 introduced: 1138 PR - Restart is planned 1139 PA - Planned restart acknowledgement 1141 No changes to the post restart exchange between the restarting router 1142 and its neighbors have been introduced. 1144 Authors' Addresses 1146 Les Ginsberg 1147 Cisco Systems, Inc. 1149 Email: ginsberg@cisco.com 1151 Paul Wells 1152 Cisco Systems, Inc. 1154 Email: pauwells@cisco.com