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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DNA WG J. Choi 3 Internet-Draft Samsung AIT 4 Expires: October 3, 2005 E. Nordmark 5 Sun Microsystems 6 April 2005 8 DNA with unmodified routers: Prefix list based approach 9 draft-ietf-dna-cpl-01.txt 11 Status of this Memo 13 By submitting this Internet-Draft, each author represents that any 14 applicable patent or other IPR claims of which he or she is aware 15 have been or will be disclosed, and any of which he or she becomes 16 aware will be disclosed, in accordance with Section 6 of BCP 79. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet- 21 Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress." 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/ietf/1id-abstracts.txt. 31 The list of Internet-Draft Shadow Directories can be accessed at 32 http://www.ietf.org/shadow.html. 34 This Internet-Draft will expire on October 3, 2005. 36 Copyright Notice 38 Copyright (C) The Internet Society (2005). 40 Abstract 42 Upon establishing a new link-layer connection, a host determines 43 whether a link change has occurred, that is, whether or not it has 44 moved at layer 3 and therefor needs new IP configuration. This draft 45 presents a way to robustly check for link change without assuming any 46 changes to the routers. We choose to uniquely identify each link by 47 the set of prefixes assigned to it. We propose that, at each 48 attached link, the host generates the complete prefix list, that is, 49 a prefix list containing all the valid prefixes on the link, and when 50 it receives a hint that indicates a possible link change, it detects 51 the identity of the currently attached link by consulting the 52 existing prefix list. This memo describes how to generate the 53 complete prefix list and to robustly detect the link identity even in 54 the presence of packet loss. 56 Table of Contents 58 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 59 2. Prefix list based approach . . . . . . . . . . . . . . . . . 4 60 2.1 Approach . . . . . . . . . . . . . . . . . . . . . . . . . 4 61 2.2 Assumptions . . . . . . . . . . . . . . . . . . . . . . . 5 62 2.3 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5 63 3. DNA based on the complete prefix list . . . . . . . . . . . 7 64 3.1 Complete prefix list generation . . . . . . . . . . . . . 7 65 3.2 Erroneous Prefix Lists . . . . . . . . . . . . . . . . . . 8 66 3.3 Link identity detection . . . . . . . . . . . . . . . . . 9 67 3.4 Renumbering . . . . . . . . . . . . . . . . . . . . . . . 10 68 4. Protocol Specification . . . . . . . . . . . . . . . . . . . 12 69 4.1 Conceptual data structures . . . . . . . . . . . . . . . . 12 70 4.2 Merging Candidate Link objects . . . . . . . . . . . . . . 13 71 4.3 Timer handling and Garbage Collection . . . . . . . . . . 13 72 4.4 Receiving link UP notifications . . . . . . . . . . . . . 14 73 4.5 Receiving valid Router Advertisements . . . . . . . . . . 14 74 4.6 Changing the link in Neighbor Discovery . . . . . . . . . 16 75 5. CPL without a 'link UP' notification . . . . . . . . . . . . 18 76 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . 20 77 7. Security Considerations . . . . . . . . . . . . . . . . . . 21 78 8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 22 79 8.1 Example with link UP event notification . . . . . . . . . 22 80 8.2 Example without link UP event notification . . . . . . . . 22 81 9. Protocol Constants . . . . . . . . . . . . . . . . . . . . . 24 82 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25 83 11. Performance Analysis . . . . . . . . . . . . . . . . . . . . 26 84 12. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 28 85 13. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 29 86 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 87 14.1 Normative References . . . . . . . . . . . . . . . . . . 30 88 14.2 Informative References . . . . . . . . . . . . . . . . . 30 89 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 31 90 Intellectual Property and Copyright Statements . . . . . . . 32 92 1. Introduction 94 When a host establishes a new link-layer connection, it may or may 95 not have a valid IP configuration, such as the subnet prefixes or the 96 default router addresses, for the link. Though the host has changed 97 its network attachment point (at layer 2), it may still be at the 98 same link (at layer 3). The term 'link' used in this document is as 99 defined in RFC 2461 [1], which is a layer 3 definition. NOTE that 100 that definition is completely different than the definition of the 101 term 'link' in IEEE 802 standards. 103 Thus the host needs to check for a link change, i.e. it needs to 104 verify whether it is attached to the same or a different link as 105 before [4]. The host can keep current IP configuration if and only 106 if it remains at the same link. 108 A host receives the link information from RA (Router Advertisement) 109 messages. However, as described in 2.2. [4], it's difficult for a 110 host to correctly detect the identity of a link with a single RA. 111 None of the information in an RA can indicate a link change properly. 112 Neither router address nor prefixes will do. 114 It may be better to design a new way to represent the identity of a 115 link, and/or add new pieces of information to RA or RS (Router 116 Solicitation) messages. Several new approaches to properly indicate 117 link change have been considered by the design team - see [10]. 119 However, even if some such new scheme is standardized and 120 implemented, hosts would still need to cope with routers which do not 121 (yet) implement such a scheme. Thus it makes sense to write down the 122 rules for how to robustly detect the link identity without assuming 123 any changes to the routers, which is the purpose of this document. 125 2. Prefix list based approach 127 2.1 Approach 129 Currently there is one thing which can represent the identify of a 130 link, 132 'The set of all the valid and global prefixes assigned to a link.' 134 If a host has the complete list of all the assigned prefixes, it can 135 properly determine whether a link change has occurred. If the host 136 receives an RA containing one or more prefixes and none of the 137 prefixes in it matches the previously known prefixes for the link, 138 then it is assumed to be a new link. 140 This works because each and every valid global prefix on a link must 141 not be used on any other link thus the sets of global prefixes on 142 different links must be disjoint [3]. 144 This is the case even as there is renumbering. During graceful 145 renumbering a prefix would gradually have its (preferred and valid) 146 lifetimes decrement, until the valid lifetime reaches zero. Some 147 point after the valid lifetime has reached zero, the prefix may be 148 reassigned to some different link. Even during 'flash' renumbering, 149 when the prefix isn't allowed to gracefully move through the 150 deprecated state [2], independently of DNA, the prefix needs to be 151 advertised with a zero valid lifetime on the old link before it can 152 be reassigned. Thus we can assume that a prefix with a non-zero 153 valid lifetime can at most be assigned to one link at any given time. 155 For the purposes of determining the prefixes, this specification uses 156 both 'on-link' and 'addrconf' prefixes [1], that is, prefixes that 157 have either the 'on-link' flag set, the 'autonomous address- 158 autoconfiguration' flag set, or both flags set. This is a safe 159 approach since both the set of valid on-link and the set of valid 160 addrconf prefixes must be uniquely assigned to one link. 162 While the approach is conceptually simple, the difficulty lies both 163 in ensuring that the host knows the complete prefix list for a single 164 link, and preventing prefixes from possibly different links to be 165 viewed as the prefixes for a single link. This is challenging for 166 several reasons: A single RA is not required to include all prefixes 167 for the link, RAs might be subject to packet loss, new routers and 168 new prefixes (due to renumbering) might appear at any time on a link, 169 and the host might move to a different link at any time. 171 If the prefix list determination is incorrect, there can be two 172 different types of failures. One is detecting a new link when in 173 fact the host remains attached to the same link. The other is 174 failing to detect when the host attaches to a different link. The 175 former failure is undesirable because it might trigger other 176 protocols, such as Mobile IPv6 [5], to do unneeded signaling, thus it 177 is important to minimize this type of failure. The latter type of 178 failure can lead to long outages when the host is not able to 179 communicate at all, thus these failures must be prevented. 181 2.2 Assumptions 183 In this approach, we assume that an interface of a host can not be 184 attached to multiple links at the same time. Though this kind of 185 multiple attachments is allowed in neither Ethernet nor 802.11b, it 186 may be possible in some Cellular System, especially CDMA. 188 This assumption implies that, should the host use a layer 2 189 technology which can be multiply connected, this needs to be 190 represented to the DNA (and layer 3 on the host in general), as 191 separate (virtual) interfaces, so that the DNA module can associate 192 each received RA message with a particular (virtual) interface. 194 We also assume that when a host changes its attachment point, the DNA 195 module will be notified of the event using some form of 'link UP' 196 event notification, and that the DNA module determine which RAs 197 arrived before the event and which arrived after the event [9]. This 198 assumption places some requirements on the host implementation, but 199 does not place any assumptions on the layer 2 protocol. 201 It is possible to have CPL operate in less robust fashion when the 202 implementation does not provide such a 'link UP' event notification. 203 We mention this possibility in Section 5. 205 2.3 Overview 207 Hints are used to tell a host that a link change might have happened. 208 This hint itself doesn't confirm a link change, but can be used to 209 initiate the appropriate procedures [4]. 211 In order to never view two different links as one it is critical that 212 when the host might have attached to a link, there has to be some 213 form of hint. This hint doesn't imply that a movement to a different 214 link has occurred, but instead, in the absence of such a hint there 215 could not have been an attachment to a different link. 217 If the IP stack is notified by the link layer when a new attachment 218 is established (e.g., when associating to a different access point in 219 802.11), this will serve as such a hint. It helps to reduce the risk 220 that the assignment of an additional prefix to a link will be 221 misinterpreted as being attached to a different link. Note that this 222 hint is merely a local notification and does not require any protocol 223 changes. For instance, in many implementations this would be a 224 notification passed from a link-layer device driver to the IP layer 225 [9]. 227 Once a hint is received the host will start to collect a new set of 228 valid prefixes for the possibly different link, and compare them with 229 the valid prefixes known from before the hint. If there is one or 230 more common prefixes it is safe to assume that the host is attached 231 to the same link, in which case the prefixes learned after the hint 232 can be merged with the prefixes learned before the hint. But if the 233 sets of valid prefixes are disjoint, then at some point in time the 234 host will decide that it is attached to a different link. 236 The process of collecting valid prefixes starts when the host is 237 powered on and first attaches to a link. 239 Since each RA message isn't guaranteed to contain all valid prefixes 240 it is a challenge for a host to attain and retain the complete prefix 241 list, especially when packets can be lost on the link. 243 The host has to rely on approximate knowledge of the prefix list 244 using RS/ RA exchanges. Just as specified in [1], when the host 245 attaches to a potentially new link, it sends an RS message to All- 246 Router multicast address, then waits for the solicited RAs. If there 247 was no packet loss, the host would receive the RAs from all the 248 routers on the link in a few seconds thereby knowing all the valid 249 prefixes on the link. Taking into account packet loss, the host may 250 need to perform RS/ RA exchanges multiple times to corroborate the 251 result. 253 When a hint indicating a possible link change happens, if the host is 254 reasonably sure that its prefix list is complete, it can determine 255 whether it is attached to the same link on the reception of just one 256 RA containing one or more valid prefixes. 258 Otherwise, to make matters certain, the host may need to attempt 259 further procedures. A first step to clarify link identity is to wait 260 for all RAs which would have been sent in response to the RS. A 261 further step is to send multiple RSs (and wait for the resulting 262 RAs). 264 All tracking of the prefix lists must take the valid lifetime of the 265 prefixes into account. The prefix list is maintained separately per 266 network interface. 268 3. DNA based on the complete prefix list 270 We choose to identify a link by the set of valid prefixes that are 271 assigned to the link, and we denote this 'the complete prefix list'. 272 Each link has its unique complete prefix list. We also say that the 273 prefix list is complete if all the prefixes on the link belong to it. 275 In case that a host has the complete prefix list, it can properly 276 determine whether it is attached to the same link or not, when it 277 receives a single RA message after a hint that a link change might 278 have occurred. 280 This section presents a procedure to generate the complete prefix 281 list and a way to detect the link identity based on the existing 282 prefix list even in the presence of packet losses. 284 3.1 Complete prefix list generation 286 To efficiently check for link change, a host always maintains the 287 list of all known prefixes on the link. This procedure of attaining 288 and retaining the complete prefix list is initialized when the host 289 is powered on. 291 The host forms the prefix list at any attachment point, that is, this 292 process starts independently of any movement. Though the procedure 293 may take some time, that doesn't matter unless the host moves very 294 fast. A host can generate the complete prefix list with reasonable 295 certainty if it remains attached to a link sufficiently long. It 296 will take approximately 12 seconds, when it actively perform 3 RS/ RA 297 exchanges. If it passively relies on unsolicited RA messages 298 instead, it may take much more time. 300 First the host sends an RS to All-Router multicast address. Assuming 301 there is no packet loss, every router on the link would receive the 302 RS and usually reply with an RA containing all the prefixes that the 303 router advertises. However, RFC 2461 mandates certain delays for the 304 RA transmissions. 306 After an RS transmission, the host waits for all RAs that would have 307 been triggered by the RS. There is an upper limit on the delay of 308 the RAs. MIN_DELAY_BETWEEN_RAS (3 Sec) + MAX_RA_DELAY_TIME (0.5 Sec) 309 + network propagation delay is the maximum delay between an RS and 310 the resulting RAs [1]. 4 seconds would be a safe number for the host 311 to wait for the resulting RAs. Assuming no packet loss, within 4 312 seconds, the host would receive all the RAs and know all the 313 prefixes. Thus we pick 4 seconds as the value for MAX_RA_WAIT. 315 In case of packet loss, things get more complicated. In the above 316 process, there may be a packet loss that results in the generation of 317 the incomplete prefix list, i.e. the prefix list that misses some 318 prefix on the link. To remedy this deficiency, the host may perform 319 multiple RS/ RA exchanges to collect all the assigned prefixes. 321 After one RS/ RA exchange, to corroborate the completeness of the 322 prefix list, the host may send additional RSs and wait for the 323 resulting RAs. The number of RSs is limited to MAX_RTR_SOLICITATIONS 324 [1]. The host takes the union of the prefixes from all the RAs to 325 generate the prefix list. The more RS/ RA exchange the host 326 performs, the more probable that the resulting prefix list is 327 complete. Section 11 gives the detailed analysis. 329 To ascertain whether its existing prefix list is complete or not, the 330 host can set its own policy. The host may take into consideration 331 the estimated packet loss rate of the link and the number of RAs it 332 received or should have received from each router while it was 333 attached to the link. Per [1] each router should multicast a RA at 334 least every 1800 seconds. Furthermore, [5] defines a Advertisement 335 Interval option, which the host can use to determine how often it 336 should receive RAs. 338 For example, the host may keep track of how many RAs it has received 339 from each router on this attachment point, and if this is 3 or more 340 it assumes that the resulting prefix list is complete. But if this 341 is only 1 or 2, the host doesn't assume the completeness of the 342 prefix list. 344 In general, the higher the error rate, the longer time and more RA 345 transmissions from the routers are needed to assure the completeness 346 of the prefix list. 348 3.2 Erroneous Prefix Lists 350 The host may generate either 1) the incomplete prefix list, i.e. the 351 prefix list does not include all the prefixes that are assigned to 352 the link or 2) the superfluous prefix list, i.e. the prefix list that 353 contains some prefix that is not assigned to the link. 355 It is noted that 1) and 2) is not exclusive. The host may generate 356 the prefix list that excludes some prefix on the link but includes 357 the prefix not assigned to the link. 359 Severe packet losses during prefix list generation may cause the 360 incomplete prefix list. Or the host may have undergone a link change 361 before finishing the procedure of the complete prefix list 362 generation. Later we will deal with the case that the host can't be 363 sure of the completeness of the prefix list. 365 Even if the host falsely assumes that the incomplete prefix list is 366 complete, the effect of that assumption is that the host might later 367 think it has moved to a different link when in fact it has not. 369 In case that a link change happens, even if the host has the 370 incomplete prefix list, it will detect a link change. Hence the 371 incomplete prefix list doesn't cause a connection disruption. But it 372 may cause extra signaling messages, for example Binding Update 373 messages in [5]. 375 The superfluous prefix list presents a more serious problem. 377 Without the assumed 'link UP' event notification from the link-layer, 378 the host can't perceive that it has changed its attachment point, 379 i.e. it has torn down an old link-layer connection and established a 380 new one. We further discuss the issues, should this assumption be 381 removed, in Section 5. 383 With the assumed 'link UP' notification, and the assumption of 384 different concurrent layer 2 connections being represented as 385 different (virtual) interfaces to the DNA module (see Section 2.2) 386 the host will never treat RAs from different links as being part of 387 the same link. Hence it can not create a superfluous prefix list. 389 3.3 Link identity detection 391 When a host receives a hint that indicates a possible link change, it 392 initiates DNA procedure to determine whether it still remains at the 393 same link or not. At this time, the complete prefix list generation 394 may or may not be finished. 396 First, if the host has finished prefix list generation and can be 397 reasonably sure of its completeness, the receipt of a single RA (with 398 at least one valid prefix) is enough to detect the identify of the 399 currently attached link. 401 Assume that, after the hint, the host receives an RA that contains at 402 least one valid prefix. The host compares the valid prefixes in the 403 RA with those in the existing prefix list. If the RA contains a 404 prefix that is also a member of the existing prefix list, the host is 405 still at the same link. Otherwise, if none of the prefixes in that 406 RA matches the previously known prefixes, it is at a different link. 408 If the host is not sure that the prefix list was complete before the 409 hint reception, then the host needs to take several RAs into account 410 after the hint reception, before it can determine that it has moved 411 to a different link. 413 Suppose that before finishing the prefix list generation, the host 414 receives the hint that indicates a possible link change. Then the 415 host can't assume the completeness of the prefix list. 417 The host can then generate another (complete) prefix list for the 418 (potentially new) link, which compensates for the uncertainty of the 419 old prefix list. After the hint, it performs one or more RS/ RA 420 exchanges additionally to collect all the prefixes on the currently 421 attached link. With the resulting prefixes, the host generates the 422 second prefix list. 424 Then the host compares two prefix lists and if the lists are 425 disjoint, i.e. have no prefix in common, it assumes that a link 426 change has occurred. Note that if during this procedure, the host 427 finds a common valid prefix between even one RA and the old prefix 428 list, it can immediately determine that it has not moved to a 429 different link. 431 For example, assume that the host keeps track of how many RAs it has 432 received from each router while attached to a link. If this is 3 or 433 more, the host assumes that it has seen all the prefixes. Suppose 434 that the host has received only one RA from each router, and then it 435 receives a link UP notification that causes it to initiate the DNA 436 procedure. If the first RA does not have a valid prefix which is 437 common with the old prefixes, then the host needs to wait for 438 additional RAs, and perhaps also send additional RSs and wait for the 439 resulting RAs. In case that the lists are disjoint, the host can 440 assume it has moved. 442 In summary, first a host makes the complete prefix list. When a hint 443 occurs, if the host decides that the prefix list is complete, it will 444 check for link change with just one RA (with a prefix). Otherwise, 445 in case that the host can't be so sure, it will perform additional 446 RS/ RA exchanges to corroborate the decision. 448 3.4 Renumbering 450 When the host is sure that the prefix list is complete, a false 451 movement assumption may happen due to renumbering when a new prefix 452 is introduced in RAs at about the same time as the host handles the 453 'link UP' event. We may solve the renumbering problem with minor 454 modification like below. 456 When a router starts advertising a new prefix, for the time being, 457 every time the router advertises a new prefix in an RA, it includes 458 at least one old prefix in the same RA. The old prefix assures that 459 the host doesn't falsely assume a link change because of a new 460 prefix. After a while, hosts will recognize the new prefix as the 461 one assigned to the current link and update its prefix list. 463 In this way, we may provide a fast and robust solution. If a host 464 can make the complete prefix list with certainty, it can check for 465 link change fast. Otherwise, it can fall back on a slow but robust 466 scheme. It is up to the host to decide which scheme to use. 468 4. Protocol Specification 470 This section provides the actual specification for a host 471 implementing this draft. For generality the specification assumes 472 that the host retains multiple (an unbounded set) of prefix lists 473 until the information times out, while an actual implementation would 474 limit the number of sets maintained. 476 This description assumes that the link layer driver provides a 'link 477 UP' notification when the host might have moved to a different link. 479 4.1 Conceptual data structures 481 This section describes a conceptual model of one possible data 482 structure organization that hosts will maintain for the purposes of 483 DNA. The described organization is provided to facilitate the 484 explanation of how this protocol should behave. This document does 485 not mandate that implementations adhere to this model as long as 486 their external behavior is consistent with that described in this 487 document. 489 The basic conceptual data type for the protocol is the Candidate Link 490 object. This is an object which contains all the information learned 491 from RA messages that are known to belong to a single link. These 492 data structures are maintained separately for each interface. In 493 particular, this includes 495 o The valid prefixes learned from the prefix information options, 496 the A/L bits and their valid and preferred lifetimes. 498 o The default routers and their lifetimes. 500 o Any other option content such as the MTU etc. 502 The lifetimes for the prefixes and default routers in the Candidate 503 Link objects should decrement in real time that is, at any point in 504 time they will be the remaining lifetime. An implementation could 505 handle that by recording the 'expire' time for the information, or by 506 periodically decrementing the remaining lifetime. 508 For each interface, the host maintains a notion of its Current 509 Candidate Link (CCL) object. As we will see below, this might 510 actually be different than the prefix list and default router lists 511 maintained by Neighbor Discovery when the host is in the process of 512 determining whether it has attached to a different link or not. 514 In addition, the host maintains previous Candidate Link objects. It 515 is per interface since there are some security issues when merging 516 across interfaces. 518 The previous Candidate Link objects can be found by knowing at least 519 one prefix that is part of the object. 521 The operations on Candidate Link objects is to create a new one, 522 discard one, and merge two of them together. The issues with merging 523 are discussed in the next section. 525 For each interface, the host maintains the last time a valid RA was 526 received (called time_last_RA_received in this document), which 527 actually ignores RAs without prefix options, and the last time a link 528 UP notification was received from the link layer on the host (called 529 time_last_linkUP_received in this document). Together these two 530 conceptual variables serve to identify when a RA containing disjoint 531 prefixes can't be due to being attached to a new link, because there 532 was no link UP notification. 534 For each interface, the host also maintains a counter (called 535 num_RS_RA) which counts how many successful RS/RA exchanges have been 536 performed since the last time the host moved to a different link. By 537 "successful exchange" we mean an RS that resulted in receiving at 538 least one RA (with at least one prefix) within MAX_RA_WAIT seconds. 539 This counter is used to determine when prefix list is considered to 540 be complete. This document considers it to be complete when 541 NUM_RS_RA_COMPLETE (set to 1) number of successful RS/RA exchanges 542 have been performed. 544 4.2 Merging Candidate Link objects 546 When a host has been collecting information about a potentially 547 different link in its Current Candidate Link object, and it discovers 548 that it is in fact the same link as another Candidate Link object, 549 then it needs to merge the information in the two objects to produce 550 a single new object. Since the CCL contains the most recent 551 information, any information contained in it will override the 552 information in the old Candidate Link, for example the remaining 553 lifetimes for the prefixes. When the two objects contain different 554 pieces of information, for instance different prefixes or default 555 routers, the union of these are used in the resulting merged object. 557 4.3 Timer handling and Garbage Collection 559 As stated above, the lifetimes for the prefixes and default routers 560 in each Candidate Link object must be decremented in real time. When 561 a prefix' valid lifetime has expired, the prefix should be removed 562 from its object. Likewise, when a default router lifetime has 563 expired, it should be removed from its object. When a Candidate Link 564 object contains neither any prefixes nor any default routers, the 565 object, including additional information such as MTU, should be 566 discarded. 568 There is nothing to prevent a host from garbage collecting Candidate 569 Link objects before their expire. However, for performance reason a 570 host must be able to retain at least two of them at any given time. 572 It is recommended to put 90 minute upper limit on how long the 573 objects, other than the CCL, should be retained, to make the protocol 574 more robust against flash renumbering and reassignment. 576 4.4 Receiving link UP notifications 578 When the host receives a link UP notification from its link layer, it 579 sets time_last_linkUP_received to the current time. 581 The host also uses this to trigger sending an RS, subject to the rate 582 limitations in [1]. Since there is no natural limit on how 583 frequently the link UP notifications might be generated, we take the 584 conservative approach that even if the host establishes new link 585 layer connectivity very often, under no circumstances should it send 586 Router Solicitations more frequently than RTR_SOLICITATION_INTERVAL. 587 Thus if it handled the most recent link UP notification less than 588 MAX_RA_WAIT seconds ago, it can not immediately send one when it 589 processes a link UP notification. 591 If the RS does not result in the host receiving at least one RA with 592 at least one valid prefix, then the host can retransmit the RS. It 593 is allowed to multicast up to MAX_RTR_SOLICITATIONS [1] RS messages 594 spaced RTR_SOLICITATION_INTERVAL apart. 596 Note that if link-layer notifications are reliable, a host can reset 597 the number of sent Router Solicitations to 0, while still maintaining 598 RTR_SOLICITATION_INTERVAL between RSs. Resetting the count is 599 necessary so that after each link up notification, the host is 600 allowed to send MAX_RTR_SOLICITATIONS to reliably discover the, 601 possibly new, prefix list. 603 4.5 Receiving valid Router Advertisements 605 When a host receives a valid RA message (after the validity checks 606 specified in [1]) it performs the following processing in addition to 607 the processing specified in [1] and [2] 609 If the valid RA does not contain any prefix information options, or 610 all the prefixes have a zero valid lifetime, then no further 611 processing is performed. Note that not even the 612 time_last_RA_received is updated. 614 If time_last_RA_received is more recent than 615 time_last_linkUP_received, then the host could not possibly have 616 moved to a different link. Hence the only action needed for DNA is 617 to update the current Candidate Link object with the information in 618 the RA, and set time_last_RA_received to the current time. No 619 further processing is performed. 621 Otherwise, that is if a linkUP indication has been received more 622 recently than time_last_RA_received, we have the case when the host 623 needs to perform comparisons of the prefix sets in its Candidate Link 624 objects and the prefix set in the RA. In this case, 625 time_last_RA_received is always set to the current time. 627 Should the received RA contain at least one valid prefix which is in 628 the prefix list in the CCL, then the host is still attached to the 629 same link, and just needs to update the CCL with any new information 630 in the RA. 632 Otherwise, if the received RA contains one or more prefixes which are 633 part of a prefix list in some retained Candidate Link object, then 634 the host has most likely moved back to that link. In this case the 635 host may retain the content of the CCL for future matching, but 636 switch the CCL to be that matching object. The, now new, CCL should 637 be updated based on the information in the RA. Then the DNA module 638 informs the Neighbor Discovery module to replace the old information 639 with the information in the new CCL as specified in Section 4.6. 641 It is possible that the above comparison will result in matching 642 multiple Candidate Link objects. For example, if the RA contains the 643 prefixes P1 and P2, and there is one Candidate Link object with P1 644 and P3 and other Candidate Link object with P2 and P4. This should 645 not happen during normal operation, but if links have been renumbered 646 or physically separate links have been made into one link (before the 647 lifetimes in the Candidate Link objects expired), then the host could 648 observe this. One possible action in this case would be for the host 649 to merge all such matching Candidate Link objects together with the 650 information in the receive RA and make this the new CCL. Doing this 651 merging correctly requires that each Candidate Link object contains 652 the time it was last updated by a RA, so that more recent information 653 can override older information. The security issues involved in such 654 merging is the prime motivation for not allowing the Candidate Link 655 objects to be shared between different interfaces. 657 The easy cases of staying on the same link or moving to a previously 658 visited link have been handled above. The harder case is when the 659 first RA after a link UP notification contains prefixes that are new 660 to the host. If the host considers its Current Candidate Link object 661 complete (num_RS_RA is at least NUM_RS_RA_COMPLETE), then a RS where 662 the prefixes are disjoint from those in the CCL, can be assumed to be 663 a link change in accordance with Section 4.6. If the CCL is not 664 considered to be complete, then it isn't obvious whether the host has 665 moved or not, because a new prefix could have been added to the 666 existing link instead of being associated with a different link. In 667 order to distinguish those to cases the host needs to do some extra 668 work. Thus the host needs to create a new Candidate Link object 669 based on the received RA, and make this object the CCL. However, it 670 does not yet treat this as a new link; it is merely a candidate. 671 Thus it MUST NOT perform the actions in Section 4.6 at this point in 672 time. Instead, the host should wait for MAX_RA_WAIT seconds, and all 673 RAs that are received during that time interval are processed as 674 specified above. 676 This processing might result in finding a prefix in common between a 677 Candidate Link object and the CCL, in which case the host knows 678 whether and to which link it has moved. But should the MAX_RA_WAIT 679 seconds expire without any common prefix, then it will conclude that 680 it has moved to a new link and inform the rest of the host of the 681 movement (Section 4.6.) Note that the arrival of a new link UP 682 notification during the MAX_RA_WAIT second timer must prevent the 683 MAX_RA_WAIT second timer from firing. In this case the host might 684 yet again have moved so it is necessary to restart the process of 685 inspecting the RAs. 687 Subject to local policy, and perhaps also the host's knowledge of the 688 packet loss characteristics of the interface or type of L2 689 technology, the host can try harder than just waiting for MAX_RA_WAIT 690 seconds, by sending additional Router Solicitations. It is allowed 691 to multicast up to MAX_RTR_SOLICITATIONS [1] RS messages spaced 692 RTR_SOLICITATION_INTERVAL apart. In the most conservative approach 693 this means a 12 second delay until the host will declare that is has 694 moved to a new link. Just as above, this process should be 695 terminated should a new link UP notification arrive during the 12 696 seconds. 698 4.6 Changing the link in Neighbor Discovery 700 When DNA detects that it has moved to a different link this needs to 701 cause Neighbor Discovery, Address autoconfiguration, and DHCPv6 to 702 take some action. While the full implications are outside of the 703 scope of this document, here is what we know about the impact on 704 Neighbor Discovery. 706 Everything learned from the RAs on the interface should be discarded, 707 such as the default router list and the on-link prefix list. 709 Furthermore, all neighbor cache entries, in particular redirects, 710 need to be discarded. Finally the information in the Current 711 Candidate Link object is used to create a new default router list and 712 on-link prefix list. 714 The list of things are potentially affected by this movement is 715 fairly extensive, since new Neighbor Discovery options are being 716 created. In addition to what is mentioned above, the list includes: 718 o The MTU option defined in [1]. 720 o The Advertisement Interval option defined in [5]. 722 o The Home Agent Information option defined in [5]. 724 o The Route Information option defined in [11]. 726 In addition, when the host determines it has moved it needs to set 727 num_RS_RA to zero. 729 5. CPL without a 'link UP' notification 731 If the host implementation does not provide any link-layer event 732 notifications [9], and in particular, a link UP notification, the 733 host needs additional logic to try to decide whether a received RA 734 applies to the "old" link or a "new" link. 736 In this case there is an increased risk that the host get confused, 737 thus it isn't clear whether this should be part of the 738 recommendation, or whether we should just require that hosts which 739 implement this draft have a 'link UP' notification. 741 As the protocol is specified in Section 4, if there is no 'link UP' 742 notification when the host might have moved, the host would collect 743 the prefixes from multiple links into a single Candidate Link object, 744 and would never detect movement. 746 Here is an example. The host begins to collect the prefixes on a 747 link. But before the prefix list generation is completed, without 748 its knowledge, the host moves to a new link. Unaware that now it is 749 at the different link, the host keeps collecting prefixes from the 750 received RAs to generate the prefix list. This results in the prefix 751 list containing prefixes from two different links. If the host uses 752 this prefix list, it fails to detect a link change. 754 A possible way to prevent this situation for implementations without 755 a link UP notification, is to treat the arrival of a RA with a 756 disjoint set of prefixes as a hint, the same way Section 4 treats the 757 link UP notification as a hint, as specified below. 759 The implications of treating such an RA as a hint, is that such an RA 760 would set 'time_last_linkUP_received' to the current time, create a 761 new Candidate Link object with the information extracted from that 762 RA, and then send an RS as specified in Section 4.4. 764 However, there is still a risk for confusion because the host can not 765 tell from the RAs whether they were solicited by the host. (RFC 2461 766 recommends that solicited RAs be multicast.) The danger is 767 exemplified by this: 769 1. Assume the host has a CCL with prefixes P1 and P2. 771 2. The host changes link layer attachment, but there is no link UP 772 notification. 774 3. The host receives an RA with a disjoint set of prefixes: prefix 775 P3. This causes the host to form a new Candidate Link object 776 with P3 and send an RS. 778 4. The host again changes link layer attachment, and no link UP 779 notification. 781 5. The host receives one of the periodic multicast RAs on the link, 782 which contains prefix P4. It can not tell whether this RA was in 783 response to the RS it send above. The host ends up adding this 784 to the CCL, which now has P3 and P4, even though those prefixes 785 are assigned to different links. 787 There doesn't appear to be a way to solve this problem without 788 changes to the routers and the Router Advertisement messages. 789 However, the probability of this occurring can be limited by limiting 790 the window of exposure. The simplest approach is for the host to 791 assume that any RA received within MAX_RA_WAIT seconds after sending 792 a RS was in response to the RS. Basically this relies on the small 793 probability of both moving again in that MAX_RA_WAIT second interval, 794 and receiving one of the periodic RAs. If the periodic RAs are sent 795 infrequently enough, this might work in practise, but is by no means 796 bullet-proof. 798 6. IANA Considerations 800 No new message formats or services are defined in this document. 802 7. Security Considerations 804 DNA process is intimately related to Neighbor Discovery protocol and 805 its trust model and threats have much in common with the ones 806 presented in RFC 3756 [7]. Nodes connected over wireless interfaces 807 may be particularly susceptible to jamming, monitoring, and packet 808 insertion attacks. Use of [6] to secure Neighbor Discovery are 809 important in achieving reliable detection of network attachment. DNA 810 schemes SHOULD incorporate the solutions developed in IETF SEND WG if 811 available, where assessment indicates such procedures are required. 813 The threats specific to DNA are that an attacker might fool a node to 814 detect attachment to a different link when it is in fact still 815 attached to the same link, and conversely, the attacker might fool a 816 node to not detect attachment to a new link. 818 The first form of attack is not very serious, since at worst it would 819 imply some additional higher-level signaling to register a new 820 (care-of) address. The second form of attack can be more serious, 821 especially if the attacker can prevent a host from detecting a new 822 link. The protocol as specified would require an attacker to be on- 823 link and be authenticated and authorized to send Router 824 Advertisements when Secure Neighbor Discovery [6] is in use. 825 However, even without SEND, an attacker would need to send RAs 826 containing the prefixes to which it wants the host to be unable to 827 detect movement. This can be done for a small number of prefixes, 828 but it isn't possible for the attacker to completely disable DNA for 829 all possible prefixes on other links. 831 8. Examples 833 This section contains some example packet flows showing the operation 834 of prefix based DNA. 836 8.1 Example with link UP event notification 838 Assume the host has seen no link UP notification for a long time and 839 that it has the prefixes P1, P2, and P3 in its prefix list for the 840 interface. 842 The IP layer receives a link UP notification. This hint makes it 843 multicast an RS and start collecting the received prefixes in a new 844 list of prefixes. 846 The host receives an RA containing no prefixes. This has no effect 847 on the algorithm contained in this specification. 849 The host receives an RA containing only the prefix P4. This could be 850 due to being attached to a different link or that there is a new 851 prefix on the existing link which is not announced in RAs together 852 with other prefixes, and a spurious hint. In this example the host 853 decides to wait for another RA before deciding. 855 One second later an RA arrives which contains P1 and P2. As a result 856 the "new" prefix list has P1, P2, and P4 hence is not disjoint from 857 the "old" prefix list with P1, P2, and P3. Thus the host concludes 858 it has not moved to a different link and its prefix list is now P1, 859 P2, P3, and P4. 861 Some time later a new link UP notification is received by the IP 862 layer. Triggers sending a RS. 864 An RA containing P5 and P6 is received by the host. Based on some 865 heuristic (for instance, the number of RAs it received on the old 866 link, or the assumed frequency of prefixes being added to an existing 867 link) this time the host decides that it is on a new link. 869 One second later an RA with prefix P7 is received. Thus the prefix 870 list now contains P5, P6, and P7. 872 8.2 Example without link UP event notification 874 Assume the host has collected the prefixes P1, P2, and P3 in its 875 prefix list for the interface. 877 The host receives an RA containing only prefix P4. The fact that P4 878 is disjoint from the prefix list makes this be treated as a hint. 880 This hint makes the host multicast an RS and start collecting the 881 received prefixes in a new list of prefixes, which is initially set 882 to contain P4. 884 The host receives an RA containing no prefixes. This has no effect 885 on the algorithm contained in this specification. 887 The host receives an RA containing only the prefix P4. This could be 888 due to being attached to a different link or that there is a new 889 prefix on the existing link which is not announced in RAs together 890 with other prefixes. In this example the host decides to wait for 891 another RA before deciding. 893 One second later an RA arrives which contains P1 and P2. As a result 894 the "new" prefix list has P1, P2, and P4 hence is not disjoint from 895 the "old" prefix list with P1, P2, and P3. Thus the host concludes 896 it has not moved to a different link and its prefix list is now P1, 897 P2, P3, and P4. 899 Some time later the host receives an RA containing prefix P7. This 900 is treated as a hint since it is not part of the current set of 901 prefixes. Triggers sending a RS and initializing the new prefix list 902 to P7. 904 An RA containing P5 and P6 is received by the host. This is disjoint 905 with both of the previous prefix lists, thus the host might be 906 attached to a 3rd link after very briefly being attached to the link 907 with prefix P7. The host decides to wait for more RAs. 909 One second later an RA with prefix P7 is received. It still isn't 910 certain whether P5, P6, and P7 are assigned to the same link (and 911 without a link UP notification such uncertainties do exist). 913 A millisecond later an RA with prefixes P6 and P7 is received. Now 914 the host decides that P5,P6, and P7 are assigned to the same link. 916 Four seconds after the RS was sent and no RA containing P1, P2, P3, 917 or P4 has been received the host can conclude with high probability 918 that it is no longer attached to the link which had those prefixes. 920 9. Protocol Constants 922 The following protocol constants are defined in this document. 924 +--------------------+----------------+ 925 | Constant name | Constant value | 926 +--------------------+----------------+ 927 | NUM_RS_RA_COMPLETE | 1 | 928 | | | 929 | MAX_RA_WAIT | 4 seconds | 930 +--------------------+----------------+ 932 Table 1 934 10. Acknowledgements 936 The authors would like to acknowledge the many careful comments from 937 Greg Daley that helped improve the clarity of the document, as well 938 as the review of the DNA WG participants in general. 940 11. Performance Analysis 942 In this section, we compute the probability that a host fails to 943 generate the complete prefix list due to packet loss, and 944 consequently assumes a link change when the host in fact did not move 945 to a different link. 947 Suppose, in a link, there are N routers, R[1], R[2],...., R[N]. 949 Each R[i] advertises the Router Advertisement RA[i] with the prefix 950 P[i]. 952 It is the worst case that each router advertises the different 953 prefix. It is necessary to receive all the RA[i] to generate the 954 complete prefix list. 956 We assume there is a host, H, and when the host sends a Router 957 Solicitation, let P be the probability that it fails to receive a 958 RA[i] because of a RA loss. For the simplicity, we disregard RS 959 losses. 961 So when the sends a Router Solicitation, the probability that it will 962 receive all RA[i] is (1-P)^N. 964 Let's assume the host performs RS/ RA exchange T times, 1,2,..,T. 966 Let S[k] be the set of all RAs which the host H successfully receives 967 at k-th RS/RA exchange. The probability that R[i] belongs to S[k] is 968 (1-P). 970 Let PL[k] be the set of prefixes which are made from S[k], i.e. the 971 set of P[j] such that RA[j] belongs to S[k]. Obviously, the 972 probability that P[i] belongs to PL[k] is also (1-P). 974 Let PL be the union of all PL[k], from k=1 to k=T. PL is the prefix 975 list made from performing RS/ RA exchange T times. 977 1) The probability of the complete prefix list generation 979 First the probability that P[i] belongs to PL is 1-P^T. The 980 probability that the prefix list PL is complete is (1-P^T)^N. 982 For example, assume the error rate is 1 % and there are 3 routers in 983 a link, then, with 2 RS/ RA exchanges, the probability of generating 984 an accurate Complete Prefix List is roughly 99.97 %. 986 At this point, assume that the host H receives a hint that a link 987 change might have happened and consequently initiates the procedure 988 of checking a link change. 990 2) The false DNA probability if the host checks for link change with 991 one RA. 993 Assume one RA, whether solicited or unsolicited arrives. If the host 994 H makes a decision based solely on the RA and the prefix list, the 995 probability that it falsely assume a link change is P^T. 997 For example, given the error rate is 1%, with 2 RS/ RA exchanges, the 998 probability of false movement detection is 1/ 10000. 1000 3) The false DNA probability if the host checks for link change with 1001 additional RS/ RA exchanges. 1003 Instead of depending on the single RA, the host H performs additional 1004 RS/ RA exchange U times, 1,2...U. Then the probability that H falsely 1005 assumes a link change is 1007 [P^T + P^U - P^(T+U)]^N. 1009 For example, given the error rate is 1 % and there are 3 routers in a 1010 link, if the host H performs 2 RS/ RA exchanges before the hint and 1 1011 RS/ RA exchange after one, the probability of false movement 1012 detection is roughly 1/1000000. 1014 In the above formula, the result goes to P^(U*N) as T goes infinity. 1015 The term P^(U*N) results from the probability that the host receives 1016 no RA during U RS/ RA exchange after the hint. To see that it still 1017 remains at the same link, a host needs to receive at least one RA. 1019 We think it is reasonable to assume that the RS will be retransmitted 1020 until at least one RA arrives. If we take a one more assumption that 1021 the host receives at least one RA, the probability will be 1023 [[P^T + P^U - P(T+U)]^N - P^(U*N)]/ [1- P^(U*N)] 1025 The above converges to zero as T approaches infinity. 1027 12. Change Log 1029 The following changes have been made since draft-ietf-dna-cpl-00: 1031 o Many editorial fixes 1033 o Added a count to the CCL to track whether it is likely to be 1034 complete (num_RS_RA) 1036 o Set the default threshold for this count to 1, that is, after a 1037 single RS/RA exchange that resulted in at least one RA being 1038 received with a useful prefix, the prefix list will be considered 1039 to be complete. The value is named NUM_RS_RA_COMPLETE. 1041 o In section 4.5 added some fudge around whether merging when a RA 1042 has prefixes which matches multiple Candidate Link objects. We 1043 need to decide what to specify in this area. 1045 o Clarified section 4.5 that Candidate Link objects can not be 1046 shared between different interfaces. 1048 The following changes have been made since draft-jinchoi-dna-cpl-01: 1050 o Clarified that only prefixes with a non-zero valid lifetime are 1051 considered. 1053 o Added some text about renumbering considerations. 1055 o Limited the retention of old Candidate Link objects to 90 minutes 1056 to avoid problems if there is flash renumbering *and* a prefix is 1057 reassigned to a different link in less than 90 minutes. 1059 o Explicitly made the assumption that the host implementation has a 1060 'link UP' event notification. 1062 o Added missing text in section 4.4 about sending a RS when a link 1063 UP notification is processed. 1065 o Added text in section 4.6 to say that current and future ND 1066 options need to be included in the information that is discarded 1067 when the host declares that is has moved to a different link. 1069 o Made the Candidate Link objects be per interface, since there are 1070 some security issues when they are shared between interfaces that 1071 might be of different trustworthyness. 1073 o Many editorial clarifications. 1075 13. Open Issues 1077 o Should we worry about implementations without 'link Up' 1078 notifications? The technique in Section 5 is far from bullet- 1079 proof. 1081 o Flash renumbering and immediate reassignment may cause a problem. 1082 Assume a prefix is suddenly removed from one link and immediately 1083 reassigned to an another link. A host in first link may not 1084 perceive the prefix removal and mistakenly assume the prefix is 1085 still valid. If the host moves to the second link and check for 1086 link change with the prefix, it will make a false decision. 1088 o The document currently proposes that hosts send one RS (and 1089 retransmit until at least one RA is received) after a hint, and 1090 afterwards wait for 2 additional unsolicited RAs from each router, 1091 before declaring the prefix list complete. With the default 1092 timers in RFC 2461 this will take a long time (60-90 minutes). 1093 Should we instead recommend that the host send 2 or 3 RSs 1094 initially? If so, how frequently? The additional RSs will 1095 increase the total amount of multicast packets on the link - 1096 perhaps significantly - so there are some tradeoffs involved here. 1098 14. References 1100 14.1 Normative References 1102 [1] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery 1103 for IP Version 6 (IPv6)", RFC 2461, December 1998. 1105 [2] Thomson, S. and T. Narten, "IPv6 Stateless Address 1106 Autoconfiguration", RFC 2462, December 1998. 1108 [3] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6) 1109 Addressing Architecture", RFC 3513, April 2003. 1111 [4] Choi, J., "Goals of Detecting Network Attachment in IPv6", 1112 draft-ietf-dna-goals-04 (work in progress), December 2004. 1114 14.2 Informative References 1116 [5] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in 1117 IPv6", RFC 3775, June 2004. 1119 [6] Arkko, J., Kempf, J., Sommerfeld, B., Zill, B., and P. 1120 Nikander, "SEcure Neighbor Discovery (SEND)", 1121 draft-ietf-send-ndopt-06 (work in progress), July 2004. 1123 [7] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor 1124 Discovery (ND) Trust Models and Threats", RFC 3756, May 2004. 1126 [8] Choi, J. and E. Nordmark, "DNA solution framework", 1127 draft-jinchoi-dna-soln-frame-00 (work in progress), July 2004. 1129 [9] Yegin, A., "Link-layer Event Notifications for Detecting 1130 Network Attachments", draft-ietf-dna-link-information-01 (work 1131 in progress), February 2005. 1133 [10] Pentland, B., "An Overview of Approaches to Detecting Network 1134 Attachment in IPv6", draft-dnadt-dna-discussion-00 (work in 1135 progress), February 2005. 1137 [11] Draves, R. and D. Thaler, "Default Router Preferences and More- 1138 Specific Routes", draft-ietf-ipv6-router-selection-07 (work in 1139 progress), January 2005. 1141 Authors' Addresses 1143 JinHyeock Choi 1144 Samsung AIT 1145 Communication & N/W Lab 1146 P.O.Box 111 Suwon 440-600 1147 KOREA 1149 Phone: +82 31 280 9233 1150 Email: jinchoe@samsung.com 1152 Erik Nordmark 1153 Sun Microsystems 1154 17 Network Circle 1155 Menlo Park, CA 94043 1156 USA 1158 Phone: +1 650 786 2921 1159 Email: erik.nordmark@sun.com 1161 Intellectual Property Statement 1163 The IETF takes no position regarding the validity or scope of any 1164 Intellectual Property Rights or other rights that might be claimed to 1165 pertain to the implementation or use of the technology described in 1166 this document or the extent to which any license under such rights 1167 might or might not be available; nor does it represent that it has 1168 made any independent effort to identify any such rights. 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