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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (January 2003) is 7771 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) == Unused Reference: '3' is defined on line 666, but no explicit reference was found in the text == Unused Reference: '5' is defined on line 491, but no explicit reference was found in the text ** Obsolete normative reference: RFC 2370 (ref. '2') (Obsoleted by RFC 5250) Summary: 5 errors (**), 0 flaws (~~), 4 warnings (==), 2 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Moy (Sycamore Networks) 3 Internet Draft Padma Pillay-Esnault (Juniper Networks) 4 Expiration Date: August 2003 Acee Lindem, Editor (Redback Networks) 5 File name: draft-ietf-ospf-hitless-restart-06.txt January 2003 7 Graceful OSPF Restart 8 draft-ietf-ospf-hitless-restart-06.txt 10 Status of this Memo 12 This document is an Internet-Draft and is in full conformance with 13 all provisions of Section 10 of RFC2026. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that 17 other groups may also distribute working documents as Internet- 18 Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six 21 months and may be updated, replaced, or obsoleted by other documents 22 at any time. It is inappropriate to use Internet- Drafts as 23 reference material or to cite them other than as "work in progress." 25 The list of current Internet-Drafts can be accessed at 26 http://www.ietf.org/ietf/1id-abstracts.txt 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 Abstract 33 This memo documents an enhancement to the OSPF routing protocol, 34 whereby an OSPF router can stay on the forwarding path even as its 35 OSPF software is restarted. This is called "graceful restart" or 36 "non-stop forwarding". A restarting router may not be capable of 37 adjusting its forwarding in a timely manner when the network 38 topology changes. In order to avoid the possible resulting routing 39 loops the procedure in this memo automatically reverts to a normal 40 OSPF restart when such a topology change is detected, or when one or 41 more of the restarting router's neighbors do not support the 42 enhancements in this memo. Proper network operation during a 43 graceful restart makes assumptions upon the operating environment 44 of the restarting router; these assumptions are also documented. 46 Table of Contents 48 1 Overview ............................................... 2 49 1.1 Acknowledgments ........................................ 3 50 2 Operation of restarting router ......................... 3 51 2.1 Entering graceful restart .............................. 4 52 2.2 When to exit graceful restart .......................... 5 53 2.3 Actions on exiting graceful restart .................... 6 54 3 Operation of helper neighbor ........................... 6 55 3.1 Entering helper mode ................................... 7 56 3.2 Exiting helper mode .................................... 8 57 4 Backward compatibility ................................. 9 58 5 Unplanned outages ...................................... 9 59 6 Interaction with Traffic Engineering .................. 10 60 7 Possible Future Work .................................. 10 61 References ............................................ 10 62 A Grace-LSA format ...................................... 11 63 B Configurable Parameters ............................... 13 64 C Change log ............................................ 14 65 Security Considerations ............................... 15 66 Authors' Addresses .................................... 15 68 1. Overview 70 Today many Internet routers implement a separation of control and 71 forwarding functions. Certain processors are dedicated to control 72 and management tasks such as OSPF routing, while other processors 73 perform the data forwarding tasks. This separation creates the 74 possibility of maintaining a router's data forwarding capability 75 while the router's control software is restarted/reloaded. We call 76 such a possibility "graceful restart" or "non-stop forwarding". 78 The problem that the OSPF protocol presents to graceful restart is 79 that, under normal operation, OSPF intentionally routes around a 80 restarting router while it rebuilds its link-state database. OSPF 81 avoids the restarting router to minimize the possibility of routing 82 loops and/or black holes caused by lack of database synchronization. 83 Avoidance is accomplished by having the router's neighbors reissue 84 their LSAs, omitting links to the restarting router. 86 However, if (a) the network topology remains stable and (b) the 87 restarting router is able to keep its forwarding table(s) across the 88 restart, it would be safe to keep the restarting router on the 89 forwarding path. This memo documents an enhancement to OSPF that 90 makes such graceful restart possible, and one that automatically 91 reverts back to a standard OSPF restart for safety when network 92 topology changes are detected. 94 In a nutshell, the OSPF enhancements for graceful restart are as 95 follows. The router attempting a graceful restart originates 96 link-local Opaque-LSAs, herein called Grace-LSAs, announcing the 97 intention to perform a graceful restart, and asking for a "grace 98 period". During the grace period its neighbors continue to announce 99 the restarting router in their LSAs as if it were fully adjacent 100 (i.e., OSPF neighbor state Full), but only if the network topology 101 remains static (i.e, the contents of the LSAs in the link-state 102 database having LS types 1-5,7 remain unchanged; periodic refreshes 103 are allowed). 105 There are two roles being played by OSPF routers during graceful 106 restart. First there is the router that is being restarted. The 107 operation of this router during graceful restart, including how the 108 router enters and leaves graceful restart, is the subject of Section 109 2. Then there are the router's neighbors, which must cooperate in 110 order for the restart to be graceful. During graceful restart we say 111 that the neighbors are executing in "helper mode". Section 3 covers 112 the responsibilities of a router executing in helper mode, including 113 entering and leaving helper mode. 115 1.1. Acknowledgments 117 The authors wish to thank John Drake, Vishwas Manral, 118 Kent Wong, and Don Goodspeed for their helpful comments. 120 2. Operation of restarting router 122 After the router restarts/reloads, it must change its OSPF 123 processing somewhat until it re-establishes full adjacencies with 124 all its previously fully-adjacent neighbors. This time period, 125 between the restart/reload and the reestablishment of adjacencies, 126 is called "graceful restart". During graceful restart: 128 (1) The restarting router does not originate LSAs with LS types 129 1-5,7. Instead, the restarting router wants the other routers 130 in the OSPF domain to calculate routes using the LSAs that it 131 had originated prior to its restart. During this time, the 132 restarting router does not modify or flush received self- 133 originated LSAs, (see Section 13.4 of [1]) but instead 134 accepts them as valid. In particular, the grace-LSAs that the 135 restarting router had originated before the restart are left 136 in place. Received self-originated LSAs will be dealt with 137 when the router exits graceful restart (see Section 2.3). 139 (2) The restarting router runs its OSPF routing calculations, as 140 specified in Section 16 of [1]. This is necessary to 141 return any OSPF virtual links to operation. However, the 142 restarting router does *not* install OSPF routes into the 143 system's forwarding table(s), instead relying on the 144 forwarding entries that it had installed prior to the 145 restart. 147 (3) If the restarting router determines that it was Designated 148 Router on a given segment immediately prior to the restart, 149 it elects itself as Designated Router again. The restarting 150 router knows that it was Designated Router if, while the 151 associated interface is in Waiting state, an Hello packet is 152 received from a neighbor listing the router as Designated 153 Router. 155 Otherwise, the restarting router operates the same as any other OSPF 156 router. It discovers neighbors using OSPF's Hello protocol, elects 157 Designated and Backup Designated Routers, performs the Database 158 Exchange procedure to initially synchronize link-state databases 159 with its neighbors, and maintains this synchronization through 160 flooding. 162 The processes of entering graceful restart, and of exiting graceful 163 restart (either successfully or not) are covered in the following 164 sections. 166 2.1. Entering graceful restart 168 The router (call it Router X) is informed of the desire for its 169 graceful restart when an appropriate command is issued by the 170 network operator. The network operator may also specify the 171 length of the grace period, or the necessary grace period may be 172 calculated by the router's OSPF software. In order to avoid 173 the restarting router's LSAs from aging out, the grace period 174 should not exceed LSRefreshTime (1800 second) [1]. 176 In preparation for the graceful restart, Router X must perform 177 the following actions before its software is restarted/reloaded. 178 Note that common OSPF shutdown procedures are *not* performed, 179 since we want the other OSPF routers to act as if Router X 180 remains in continuous service. For example, Router X does not 181 flush its locally originated LSAs, since we want them to remain 182 in other routers' link-state databases throughout the restart 183 period. 185 (1) Router X must ensure that its forwarding table(s) is/are 186 up-to-date and will remain in place across the restart. 188 (2) The router may need to preserve the cryptographic 189 sequence numbers being used on each interface in 190 non-volatile storage. An alternative is to use the 191 router's clock for cryptographic sequence number 192 generation and ensure the clock is preserved across 193 restarts (either on the same or redundant route 194 processors). If neither of these can be guarenteed, it 195 can take up to RouterDeadInterval seconds after the 196 restart before adjacencies can be reestablished and this 197 would force the grace period to be lengthened greatly. 199 Router X then originates the grace-LSAs. These are link-local 200 Opaque-LSAs (see Appendix A). Their LS Age field is set to 0, 201 and the requested grace period (in seconds) is inserted into the 202 body of the grace-LSA. The precise contents of the grace-LSA are 203 described in Appendix A. 205 A grace-LSA is originated for each of the router's OSPF 206 interfaces. If Router X wants to ensure that its neighbors 207 receive the grace-LSAs, it should retransmit the grace-LSAs 208 until they are acknowledged (i.e, perform standard OSPF reliable 209 flooding of the grace-LSAs). If one or more fully adjacent 210 neighbors do not receive grace-LSAs, they will more than likely 211 cause premature termination of the graceful restart procedure 212 (see Section 4). 214 After the grace-LSAs have been sent, the router should store the 215 fact that it is performing graceful restart along with the 216 length of the requested grace period in non-volatile storage. 217 (Note to implementors: It may be easiest to simply store the 218 absolute time of the end of the grace period). The OSPF 219 software should then be restarted/reloaded, and when the 220 reloaded software starts executing the graceful restart 221 modifications in Section 2 above are followed. (Note that prior 222 to the restart, the router does not know whether its neighbors 223 are going to cooperate as "helpers"; the mere reception of 224 grace-LSAs does not imply acceptance of helper 225 responsibilities. This memo assumes that the router would want 226 to restart anyway, even if the restart is not going to be 227 graceful). 229 2.2. When to exit graceful restart 231 A Router X exits graceful restart when any of the following 232 occurs: 234 (1) Router X has reestablished all its adjacencies. Router X 235 can determine this by examining the router-LSAs that it 236 had last originated before the restart (called the "pre- 237 restart router-LSA"), and, on those segments where the 238 router is Designated Router, the pre-restart network- 239 LSAs. These LSAs will have been received from the helping 240 neighbors, and need not have been stored in non-volatile 241 storage across the restart. All previous adjacencies will 242 be listed as type-1 and type 2 links in the router-LSA, 243 and as neighbors in the body of the network-LSA. 245 (2) Router X receives an LSA that is inconsistent with its 246 pre-restart router-LSA. For example, X receives a router- 247 LSA originated by router Y that does not contain a link 248 to X, even though X's pre-start router-LSA did contain a 249 link to Y. This indicates that either a) Y does not 250 support graceful restart, b) Y never received the grace- 251 LSA or c) Y has terminated its helper mode for some 252 reason (Section 3.2). 254 (3) The grace period expires. 256 2.3. Actions on exiting graceful restart 258 On exiting "graceful restart", the reloaded router reverts back 259 to completely normal OSPF operation, reoriginating LSAs based on 260 the router's current state and updating its forwarding table(s) 261 based on the current contents of the link-state database. In 262 particular, the following actions should be performed when 263 exiting, either successfully or unsuccessfully, graceful 264 restart. 266 (1) The router should reoriginate its router-LSAs for all 267 attached areas, to make sure they have the correct 268 contents. 270 (2) The router should reoriginate network-LSAs on all 271 segments where it is Designated Router. 273 (3) The router reruns its OSPF routing calculations (Section 274 16 of [1]), this time installing the results into the 275 system forwarding table, and originating summary-LSAs, 276 Type-7 LSAs and AS-external-LSAs as necessary. 278 (4) Any remnant entries in the system forwarding table that 279 were installed before the restart, but that are no longer 280 valid, should be removed. 282 (5) Any received self-originated LSAs that are no longer 283 valid should be flushed. 285 (6) Any grace-LSAs that the router had originated should be 286 flushed. 288 3. Operation of helper neighbor 290 The helper relationship is per network segment. As a "helper 291 neighbor" on a segment S for a restarting router X, router Y has 292 several duties. It monitors the network for topology changes, and as 293 long as there are none, continues to its advertise its LSAs as if X 294 had remained in continuous OSPF operation. This means that Y's LSAs 295 continue to list an adjacency to X over network segment S, 296 regardless of the adjacency's current synchronization state. This 297 logic affects the contents of both router-LSAs and network-LSAs, and 298 also depends on the type of network segment S (see Sections 12.4.1.1 299 through 12.4.1.5 and Section 12.4.2 of [1]). When helping over a 300 virtual link, the helper must also continue to set bit V in its 301 router-LSA for the virtual link's transit area (Section 12.4.1 of 302 [1]). 304 Also, if X was the Designated Router on network segment S when the 305 helping relationship began, Y maintains X as Designated router until 306 the helping relationship is terminated. 308 3.1. Entering helper mode 310 When a router Y receives a grace-LSA from router X, it enters 311 helper mode for X, on the associated network segment, as long as 312 all the following checks pass: 314 (1) Y currently has a full adjacency with X (neighbor state 315 Full) over the associated network segment. On broadcast, 316 NBMA and Point-to-MultiPoint segments, the neighbor 317 relationship with X is identified by the IP interface 318 address in the body of the grace-LSA (see Appendix A). On 319 all other segment types X is identified by the grace- 320 LSA's Advertising Router field. 322 (2) There have been no changes in content to the link-state 323 database (LS types 1-5,7) since router X restarted. This 324 is determined as follows. Router Y examines the link- 325 state retransmission list for X over the associated 326 network segment. If there are any LSAs with LS types 327 1-5,7 on the list, then they all must be periodic 328 refreshes. If there are instead LSAs on the list whose 329 contents have changed (see Section 3.3 of [8]), Y must 330 refuse to enter helper mode. 332 (3) The grace period has not yet expired. This means that the 333 LS age of the grace-LSA is less than the grace period 334 specified in the body of the grace-LSA (Appendix A). 336 (4) Local policy allows Y to act as the helper for X. 337 Examples of configured policies might be a) never act as 338 helper, b) never allow the grace period to exceed a Time 339 T, c) only help on software reloads/upgrades, or d) never 340 act as a helper for certain specific routers (specified 341 by OSPF Router ID). 343 There is one exception to the above requirements. If Y was 344 already helping X on the associated network segment, the new 345 grace-LSA should be accepted and the grace period should be 346 updated accordingly. 348 Note that Router Y may be helping X on some network segments, 349 and not on others. However, that circumstance will probably lead 350 to the premature termination of X's graceful restart, as Y will 351 not continue to advertise adjacencies on the segments where it 352 is not helping (see Section 2.2). 354 Alternately, Router Y may choose to enter enter helper mode 355 when a grace LSA is received and the above checks pass for all 356 adjacencies with Router X. This implemenation alternative 357 of aggregating the adjacencies with respect to helper mode is 358 compatible with implementations considering each adjacency 359 independently. 361 A single router is allowed to simultaneously serve as a helper 362 for multiple restarting neighbors. 364 3.2. Exiting helper mode 366 Router Y ceases to perform the helper function for its neighbor 367 Router X on a given segment when one of the following events 368 occurs. 370 (1) The grace-LSA originated by X on the segment is flushed. 371 This is the successful termination of graceful restart. 373 (2) The grace-LSA's grace period expires. 375 (3) A change in link-state database contents indicates a 376 network topology change, which forces termination of a 377 graceful restart. Specifically, if router Y installs a 378 new LSA in its database with LS types 1-5,7 and having 379 the following two properties, it should cease helping X. 380 The two properties of the LSA are a) the contents of the 381 LSA have changed; this includes LSAs with no previous 382 link-state database instance and the flushing of LSAs 383 from the database, but excludes periodic LSA refreshes 384 (see Section 3.3 of [8]), and b) the LSA would have 385 been flooded to X, had Y and X been fully adjacent. As an 386 example of the second property, if Y installs a changed 387 AS-external-LSA, it should not terminate a helping 388 relationship with a neighbor belonging to a stub area, as 389 that neighbor would not see the AS-external-LSA in any 390 case. An implementation MAY provide a configuration 391 option to disable link-state database options from 392 terminating graceful restart. Such an option will, 393 however, increase the risk of routing loops and 394 black holes. 396 When Router Y exits helper mode for X on a given network 397 segment, it reoriginates its LSAs based on the current state of 398 its adjacency to Router X over the segment. In detail, Y takes 399 the following actions: (a) Y recalculates the Designated Router 400 for the segment, (b) Y reoriginates its router-LSA for the 401 segment's OSPF area, (c) if Y is Designated Router for the 402 segment, it reoriginates the network-LSA for the segment and (d) 403 if the segment was a virtual link, Y reoriginates its router-LSA 404 for the virtual link's transit area. 406 If Router Y aggregated adjacencies with Router X when 407 entering helper mode (as described in section 3.1), it must also 408 exit helper mode for all adjacencies with Router X when any one 409 of the exit events occurs for of adjacency with Router X. 411 4. Backward compatibility 413 Backward-compatibility with unmodified OSPF routers is an automatic 414 consequence of the functionality documented above. If one or more 415 neighbors of a router requesting graceful restart are unmodified, or 416 if they do not received the grace-LSA, the graceful restart converts 417 to a normal OSPF restart. 419 The unmodified routers will start routing around the restarted 420 router X as it performs initial database synchronization, by 421 reissuing their LSAs with links to X omitted. These LSAs will be 422 interpreted by helper neighbors as a topology change, and by X as an 423 LSA inconsistency, in either case reverting to normal OSPF 424 operation. 426 5. Unplanned outages 428 The graceful restart mechanisms in this memo can be used for 429 unplanned outages. (Examples of unplanned outages include the crash 430 of a router's control software, an unexpected switchover to a 431 redundant control processor, etc). However, implementors and network 432 operators should note that attempting graceful restart from an 433 unplanned outage may not be a good idea, owing to the router's 434 inability to properly prepare for the restart (see Section 2.1). In 435 particular, it seems unlikely that a router could guarantee the 436 sanity of its forwarding table(s) across an unplanned restart. In 437 any event, implementors providing the option to recover gracefully 438 from unplanned outages must allow a network operator to turn the 439 option off. 441 In contrast to the procedure for planned restart/reloads that was 442 described in Section 2.1, a router attempting graceful restart after 443 an unplanned outage must originate grace-LSAs *after* its control 444 software resumes operation. The following points must be observed 445 during this grace-LSA origination. 447 o The grace-LSAs must be originated and sent *before* the 448 restarted router sends any OSPF Hello Packets. On broadcast 449 networks, this LSA must be flooded to the AllSPFRouters 450 multicast address (224.0.0.5) since the restarting router is 451 not aware of its previous DR state. 453 o The grace-LSAs are encapsulated in Link State Update Packets and 454 sent out all interfaces, even though the restarted router has no 455 adjacencies and no knowledge of previous adjacencies. 457 o To improve the probability that grace-LSAs be delivered, an 458 implementation may send them a number of times (see for example 459 the Robustness Variable in [8]). 461 o The restart reason in the grace-LSAs must be set to unknown(0). 462 This enables the neighbors to decide whether they want to help 463 the router through an unplanned restart. 465 6. Interaction with Traffic Engineering 467 The operation of the Traffic Engineering Extensions to OSPF [4] 468 during OSPF Graceful Restart is specified in [6]. 470 7. Possible Future Work 472 Devise a less conservative algorithm for graceful restart 473 helper termination that provides a comparable level of 474 black hole and routing loop avoidance. 476 Normative References 478 [1] Moy, J., "OSPF Version 2", RFC 2328, April 1998. 480 [2] Coltun, R., "The OSPF Opaque LSA Option", RFC 2370, July 481 1998. 483 Informative References 485 [3] Murphy, S., M. Badger and B. Wellington, "OSPF with Digital 486 Signatures", RFC 2154, June 1997. 488 [4] Katz, D., D. Yeung and K. Kompella, "Traffic Engineering 489 Extensions to OSPF", work in progress. 491 [5] Coltun, R., V. Fuller and P. Murphy, "The OSPF NSSA Option", 492 work in progress. 494 [6] Kompella, K., et. al., "Routing Extensions in Support of 495 Generalized MPLS", work in progress. 497 [7] Moy, J., "Extending OSPF to Support Demand Circuits", RFC 498 1793, April 1995. 500 [8] Fenner, W., "Internet Group Membership Protocol, Version 2", 501 RFC 2236, November 1997. 503 A. Grace-LSA format 505 The grace-LSA is a link-local scoped Opaque-LSA [2] having Opaque 506 Type of 3 and Opaque ID equal to 0. Grace-LSAs are originated by a 507 router that wishes to execute a graceful restart of its OSPF 508 software. A grace-LSA requests that the router's neighbors aid it in 509 its graceful restart by continuing to advertise the router as fully 510 adjacent during a specified grace period. 512 Each grace-LSA has LS age field set to 0 when the LSA is first 513 originated; the current value of LS age then indicates how long ago 514 the restarting router made its request. The body of the LSA is TLV- 515 encoded. The TLV-encoded information includes the length of the 516 grace period, the reason for the graceful restart and, when the 517 grace-LSA is associated with a broadcast, NBMA or Point-to- 518 MultiPoint network segment, the IP interface address of the 519 restarting router. 521 0 1 2 3 522 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 523 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 524 | LS age | Options | 9 | 525 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 526 | 3 | 0 | 527 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 528 | Advertising Router | 529 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 530 | LS sequence number | 531 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 532 | LS checksum | length | 533 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 534 | | 535 +- TLVs -+ 536 | ... | 538 The format of the TLVs within the body of a grace-LSA is the same as 539 the TLV format used by the Traffic Engineering Extensions to OSPF 540 [4]. The TLV header consists of a 16-bit Type field and a 16-bit 541 length field, and is followed by zero or more bytes of value. The 542 length field indicates the length of the value portion in bytes. The 543 value portion is padded to four-octet alignment, but the padding is 544 not included in the length field. For example, a one byte value 545 would have the length field set to 1, and three bytes of padding 546 would be added to the end of the value portion of the TLV. 548 The following is the list of TLVs that can appear in the body of a 549 grace-LSA. 551 o Grace Period (Type=1, length=4). The number of seconds that the 552 router's neighbors should continue to advertise the router as 553 fully adjacent, regardless of the the state of database 554 synchronization between the router and its neighbors. Since this 555 time period began when grace-LSA's LS age was equal to 0, the 556 grace period terminates when either a) the LS age of the grace- 557 LSA exceeds the value of Grace Period or b) the grace-LSA is 558 flushed. See Section 3.2 for other conditions which terminate 559 the grace period. This TLV must always appear in a grace-LSA. 561 o Graceful restart reason (Type=2, length=1). Encodes the reason 562 for the router restart, as one of the following: 0 (unknown), 1 563 (software restart), 2 (software reload/upgrade) or 3 (switch to 564 redundant control processor). This TLV must always appear in a 565 grace-LSA. 567 o IP interface address (Type=3, length=4). The router's IP 568 interface address on the subnet associated with the grace-LSA. 569 Required on broadcast, NBMA and Point-to-MultiPoint segments, 570 where the helper uses the IP interface address to identify the 571 restarting router (see Section 3.1). 573 DoNotAge is never set in a grace-LSA, even if the grace-LSA is 574 flooded over a demand circuit [7]. This is because the grace-LSA's 575 LS age field is used to calculate the extent of the grace period. 577 Grace-LSAs have link-local scope because they only need to be seen 578 by the router's direct neighbors. 580 B. Configurable Parameters 582 OSPF graceful restart parameters are suggested below. Section 583 B.1 contains a minimum subset of parameters that should be 584 supported. B.2 includes some additional configuration parameters 585 an implementation may choose to support. 587 B.1 Global Parameters (Minimum subset) 589 RestartSupport 591 The router's level of support for OSPF graceful restart. 592 Allowable values are none, planned restart only, and 593 planned/unplanned. 595 RestartInterval 597 The graceful restart interval in seconds. The range is from 598 1 to 1800 seconds with a suggested default of 120 seconds. 600 B.2 Global Parameters (Optional) 602 RestartHelperSupport 604 The router's support for acting as an OSPF restart helper. 605 Allowable values are none, planned restart only, and 606 planned/unplanned. 608 RestartHelperStrictLSAChecking 610 Indicates whether or not an OSPF restart helper should 611 terminate graceful restart when there is a change to an LSA 612 that would be flooded to the restarting router or when there 613 is a changed LSA on the restarting router's retransmission list 614 when graceful restart is initiated. The suggested default is 615 enabled. 617 C. Change Log (To be removed prior to publication) 619 Changes from 02 to 03 version: 621 1. Add Padma Pillay-Esnault and Acee Lindem as authors to help 622 finish up the draft. 624 Changes from 03 to 04 version: 626 1. Add change log (Appendix B). 627 2. Document that the grace period is restricted to 628 LSRefreshTime (Section 2.1). 629 3. Document an alternative to saving cryptographic sequence 630 numbers in non-volatile storage (Section 2.1). 631 4. Document that an implementation may aggregate multiple 632 adjacencies with a restarting router when entering or exiting 633 helper mode (Section 3.1 and 3.2). 634 5. Document that an implementation may disable graceful 635 restart helper termination when the link-state database 636 changes (Section 3.2). 637 6. In the case of an unplanned restart, document that 638 grace LSAs should be flooded to AllSPFRouters on 639 broadcast networks (Section 5). 640 7. Remove MOSPF from future work. Add Vishwas's suggested 641 technique for less conservative helper mode termination as 642 possible future work (Section 7). 643 8. Change references and citations to meet prevailing IETF 644 standards. 646 Changes from 04 to 05 version: 648 1. Add Appendix B (Configurable Parameters) and make the Change 649 Log Appendix C. 651 Changes from 05 to 06 version: 653 1. Change from "hitless restart" to "graceful restart" to be 654 consistent with other protocol documents. 656 Security Considerations 658 One of the ways to attack a link-state protocol such as OSPF is to 659 inject false LSAs into, or corrupt existing LSAs in, the link-state 660 database. Injecting a false grace-LSA would allow an attacker to 661 spoof a router that, in reality, has been withdrawn from service. 662 The standard way to prevent such corruption of the link-state 663 database is to secure OSPF protocol exchanges using the 664 Cryptographic authentication specified in [1]. An even stronger 665 way of securing link-state database contents has been proposed in 666 [3]. 668 Authors' Addresses 670 J. Moy 671 Sycamore Networks, Inc. 672 150 Apollo Drive 673 Chelmsford, MA 01824 674 Phone: (978) 367-2505 675 Fax: (978) 256-4203 676 email: jmoy@sycamorenet.com 678 Padma Pillay-Esnault 679 Juniper Networks 680 1194 N, Mathilda Avenue 681 Sunnyvale, CA 94089-1206 682 Email: padma@juniper.net 684 Acee Lindem 685 Redback Networks 686 102 Carric Bend Court 687 Cary, NC 27519 688 Email: acee@redback.com