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Checking references for intended status: Informational ---------------------------------------------------------------------------- No issues found here. Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force C. Gundogan, Ed. 3 Internet-Draft HAW Hamburg 4 Intended status: Informational D. Barthel 5 Expires: May 20, 2020 Orange 6 E. Baccelli 7 INRIA 8 November 21, 2019 10 DIS Modifications 11 draft-ietf-roll-dis-modifications-01 13 Abstract 15 This document augments RFC6550 with DIS flags and options that 16 allow a RPL node to better control how neighbor RPL routers respond 17 to its solicitation for DIOs. 19 Status of This Memo 21 This Internet-Draft is submitted in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Internet-Drafts are working documents of the Internet Engineering 25 Task Force (IETF). Note that other groups may also distribute 26 working documents as Internet-Drafts. The list of current Internet- 27 Drafts is at http://datatracker.ietf.org/drafts/current/. 29 Internet-Drafts are draft documents valid for a maximum of six months 30 and may be updated, replaced, or obsoleted by other documents at any 31 time. It is inappropriate to use Internet-Drafts as reference 32 material or to cite them other than as "work in progress." 34 This Internet-Draft will expire on September 14, 2017. 36 Copyright Notice 38 Copyright (c) 2017 IETF Trust and the persons identified as the 39 document authors. All rights reserved. 41 This document is subject to BCP 78 and the IETF Trust's Legal 42 Provisions Relating to IETF Documents 43 (http://trustee.ietf.org/license-info) in effect on the date of 44 publication of this document. Please review these documents 45 carefully, as they describe your rights and restrictions with respect 46 to this document. Code Components extracted from this document must 47 include Simplified BSD License text as described in Section 4.e of 48 the Trust Legal Provisions and are provided without warranty as 49 described in the Simplified BSD License. 51 Table of Contents 53 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 54 1.1. RFC6550 refresher . . . . . . . . . . . . . . . . . . . . 2 55 1.2. Undesirable effects . . . . . . . . . . . . . . . . . . . 3 56 1.3. Desired improvments . . . . . . . . . . . . . . . . . . . 4 57 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 3. DIS Base Object flags . . . . . . . . . . . . . . . . . . . . 5 59 4. DIS Options . . . . . . . . . . . . . . . . . . . . . . . . . 6 60 4.1. Metric Container . . . . . . . . . . . . . . . . . . . . 6 61 4.2. Response Spreading . . . . . . . . . . . . . . . . . . . 6 62 4.3. DIO Option Request . . . . . . . . . . . . . . . . . . . 7 63 5. Full behavior illustration . . . . . . . . . . . . . . . . . 8 64 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 65 6.1. DIS Flags . . . . . . . . . . . . . . . . . . . . . . . . 9 66 6.2. RPL Control Message Options . . . . . . . . . . . . . . . 10 67 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 68 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 69 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 70 9.1. Normative References . . . . . . . . . . . . . . . . . . 10 71 9.2. Informative References . . . . . . . . . . . . . . . . . 11 72 Appendix A. Applications . . . . . . . . . . . . . . . . . . . . 11 73 A.1. A Leaf Node Joining a DAG . . . . . . . . . . . . . . . . 11 74 A.2. Identifying A Defunct DAG . . . . . . . . . . . . . . . . 12 75 A.3. Explicit and Implicit DIO Option Requests . . . . . . . . 14 76 Appendix B. Experimental data . . . . . . . . . . . . . . . . . 15 77 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 79 1. Introduction 81 This document augments [RFC6550], the RPL routing protocol 82 specification. 84 1.1. RFC6550 refresher 86 Per [RFC6550], a RPL node can send a DODAG Information Solicitation 87 (DIS) message to solicit DODAG Information Object (DIO) messages from 88 neighbor RPL routers. 90 A DIS can be multicast to all the routers in range or it can be 91 unicast to a specific neighbor router. 93 A DIS may carry a Solicited Information option that specifies the 94 predicates of the DAG(s) the soliciting node is interested in. In 95 the absence of such Solicited Information option, the soliciting node 96 is deemed interested in receiving DIOs for all the DAGs known by the 97 solicited router(s). 99 [RFC6550] requires a router to treat the receipt of a multicast DIS 100 as an inconsistency and hence reset its Trickle timers for the 101 matching DAGs. As a result of the general Trickle timer mechanism, 102 future DIOs will be sent at a higher rate. See [RFC6206] for the 103 specification of Trickle timers and the definition of 104 "inconsistency". 106 [RFC6550] requires a router that receives a unicast DIS to respond by 107 unicasting a DIO for each matching DAG and to not reset the 108 associated Trickle timer. Such a DIO generated in response to a 109 unicast DIS must contain a Configuration option. 111 This description is summarized in Table 1. 113 +----------------------------+----------------------+---------------+ 114 | | Unicast DIS | Multicast DIS | 115 +----------------------------+----------------------+---------------+ 116 | no option present | unicast DIO, don't | do reset | 117 | | reset Trickle timer | Trickle timer | 118 | -------------------------- | -------------------- | ------------- | 119 | Solicited Information | do nothing | do nothing | 120 | option present, not | | | 121 | matching | | | 122 | -------------------------- | -------------------- | ------------- | 123 | Solicited Information | unicast DIO, don't | do reset | 124 | option present, matching | reset Trickle timer | Trickle timer | 125 +----------------------------+----------------------+---------------+ 127 Table 1: Router behavior on receiving a DIS, as per RFC6550 129 More precisely, Table 1 describes the behavior of routers for each 130 DAG they belong to. In the general case where multiple RPL instances 131 co-exist in a network, routers will maintain a Trickle timer for the 132 one DAG of each RPL instance they belong to, and nodes may send a DIS 133 with multiple Solicited Information options pertaining to different 134 DAGs or instances. In this more general case, routers will respond 135 for each individual DAG/instance they belong to as per Table 1. 137 1.2. Undesirable effects 139 Now, consider a RPL leaf node that desires to join a certain DAG. 140 This node can either wait for its neighbor RPL routers to voluntarily 141 transmit DIOs or it can proactively solicit DIOs using a DIS message. 142 Voluntary DIO transmissions may happen after a very long time if the 143 network is stable and the Trickle timer intervals have reached large 144 values. Thus, proactively seeking DIOs using a DIS may be the only 145 reasonable option. Since the node does not know which neighbor 146 routers belong to the DAG, it must solicit the DIOs using a multicast 147 DIS (with predicates of the desired DAG specified inside a Solicited 148 Information option). On receiving this DIS, the neighbor routers 149 that belong to the desired DAG will reset their Trickle timers and 150 quickly transmit their DIOs. The downside of resetting Trickle 151 timers is that the routers will keep transmitting frequent DIOs for a 152 considerable duration until the Trickle timers again reach long 153 intervals. These DIO transmissions are unnecessary, consume precious 154 energy and may contribute to congestion in the network. 156 There are other scenarios where resetting of Trickle timer following 157 the receipt of a multicast DIS is not appropriate. For example, 158 consider a RPL router that desires to free up memory by deleting 159 state for the defunct DAGs it belongs to. Identifying a defunct DAG 160 may require the node to solicit DIOs from its DAG parents using a 161 multicast DIS. 163 Certain scenarios may require a RPL router to solicit a DIO from a 164 parent by using a unicast DIS. The parent is forced to include a 165 Configuration option within the unicast DIO, although the requesting 166 node might still have this information locally available. Since the 167 information within the Configuration option is described as generally 168 static and unchanging throughout the DODAG, it inflates the unicast 169 DIO unnecessarily by 16 bytes for each request. 171 1.3. Desired improvments 173 To deal with the situations described above, there is a need in the 174 industry for DIS flags and options that allow a RPL node to control 175 how neighbor RPL routers respond to its solicitation for DIOs, for 176 example by expressing: 178 o the routing constraints that routers should meet to be allowed to 179 respond, thereby lowering the number of responders 181 o whether the responding routers should reset their Trickle timers 182 or not, thereby limiting the cumulated number of transmitted DIOs 184 o whether the responding routers should respond with a unicast DIO 185 instead of a multicast one, thereby lowering the overhearing cost 186 in the network 188 o whether the responding routers should omit DIO options that were 189 not requested explicitely and thus reducing the amount of traffic 190 and giving full control over the options of the solicited DIO 192 o the time interval over which the responding routers should 193 schedule their DIO transmissions, thereby lowering the occurence 194 of collisions. 196 2. Terminology 198 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 199 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 200 "OPTIONAL" in this document are to be interpreted as described in 201 [RFC2119]. 203 Additionally, this document uses terminology from [RFC6550]. 204 Specifically, the term RPL node refers to a RPL router or a RPL host 205 as defined in [RFC6550]. 207 3. DIS Base Object flags 209 This document defines three new flags inside the DIS base object: 211 o the "No Inconsistency" (N) flag: On receiving a multicast DIS with 212 the N flag set, a RPL router MUST NOT reset the Trickle timers for 213 the matching DAGs. In addition, it MUST take specific action, 214 which is to respond by explicitely sending a DIO. This DIO MUST 215 include a Configuration option. This behavior augments [RFC6550], 216 which had provision for such flag. Since this specific, one-shot 217 DIO is not a consequence of the general Trickle timer mechanism, 218 it will be sent right away if no Response Spreading option is 219 present or it will be scheduled according to the Response 220 Spreading option if one is present in the DIS (see Section 4.2). 222 o the "DIO Type" (T) flag: In case the N flag is set, this T flag 223 specifies what type of DIO is sent in response. It MUST be a 224 unicast DIO if this flag is set and it MUST be a multicast DIO if 225 this flag is reset. 227 o the "DIO Option Request" (R) flag: On receiving a DIS with the R 228 flag set, the receiver MUST include all options that were 229 requested by the DIS containing one or multiple DIO Option Request 230 options. A responding RPL router MUST NOT include DIO options 231 that were not explicitely requested. Note that this behaviour 232 contradicts with [RFC6550] for the case of including a 233 Configuration option in all DIOs requested by a unicast DIS. 235 When a unicast DIS is transmitted, both its N and T flags SHOULD be 236 0, which are the default values per [RFC6550]. On receiving a 237 unicast DIS, the N and T flags MUST be ignored and treated as 00. 238 When the R flag is unset, then a RPL router may include or omit DIO 239 options like specified in [RFC6550]. A rpl router responding to a 240 DIS with the R flag set MUST only include all requested DIO options 241 in the solicited DIO. 243 The modified DIS base object is shown in Figure 1. 245 0 1 2 3 246 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 1 247 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 248 |N|T|R| Flags | Reserved | Option(s)... 249 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 251 Figure 1: Modified DIS Base Object 253 4. DIS Options 255 4.1. Metric Container 257 In order to lower the number of routers that will respond to a DIS, 258 this document allows routing constraints to be carried by a DIS. 259 Only the router(s) that satisfy these constraints is (are) allowed to 260 respond to the DIS. 262 These routing constraints are described using a Metric Container 263 option contained in the DIS. Metric Containers are defined in 264 [RFC6550] and [RFC6551]. Metric Containers options were previously 265 only allowed in DIOs. This document augments [RFC6550] by allowing 266 the inclusion of a Metric Container option inside a DIS as well. 268 A RPL router that receives a DIS with a Metric Container option MUST 269 ignore any Metric object in it, and MUST evaluate the "mandatory" 270 Constraint objects in it by comparing the constraint value to the 271 value of the corresponding routing metric that the router maintains 272 for the matching DAG(s). These routing metric values MUST satisfy 273 all the mandatory constraints in order for the router to consider the 274 solicitation successful for the matching DAG(s). This augments the 275 behavior already present in [RFC6550] with the Solicited Information 276 option. 278 This option can be used in both unicast and multicast DIS. 280 4.2. Response Spreading 281 0 1 2 282 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 283 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 284 | Type = 0x0B | Length | Spread. Inter.| 285 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 287 Figure 2: The Response Spreading option 289 Even with the use of the Solicited Information and the Section 4.1 290 options, a multicast DIS may still lead to a large number of RPL 291 routers taking immediate action and responding with DIOs. Concurrent 292 transmissions by multiple routers are not desirable since they may 293 lead to poor channel utilization or even to packet loss. Unicast 294 DIOs may be able to avail of link-level retransmissions. However, 295 multicast DIOs usually have no such protection, since they commonly 296 make use of link layer broadcast. To avoid such problems, this 297 document specifies an optional DIO response spreading mechanism. 299 This document defines a new RPL control message option called 300 Response Spreading option, shown in Figure 2, with a recommended Type 301 value 0x0B (to be confirmed by IANA). A RPL router that explicitely 302 responds with a specific, one-shot DIO to a DIS that includes a 303 Response Spreading option, MUST wait for a time uniformly chosen in 304 the interval [O..2^SpreadingInterval], expressed in ms, before 305 attempting to transmit its DIO. If the DIS does not include a 306 Response Spreading option, the node is free to transmit the DIO as it 307 otherwise would. 309 A Response Spreading option MAY be included inside a unicast DIS 310 message, but there is no benefit in doing so. 312 Multiple Response Spreading options SHOULD NOT be used inside a same 313 DIS message. 315 This mechanism MUST NOT affect the Trickle timer mechanism. 317 4.3. DIO Option Request 319 0 1 2 320 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 321 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 322 | Type = 0x0C | Length | Req. Opt. Type| 323 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 325 Figure 3: The DIO Option Request option 327 If a unicast DIS is used to request a DIO, then [RFC6550] mandates 328 that a Configuration option MUST be included in this DIO. The 329 Configuration option contains generally static information that stays 330 unmodified throughout the DAG. For scenarios where a RPL node is 331 already part of a DAG and hence is holding the information that is 332 propagated with the Configuration option, an inclusion of such leads 333 to an unnecesary inflation of 16 bytes for each solicited DIO. 335 As per [RFC6550], no process is defined to trigger the inclusion of 336 other DIO options in a solicited DIO. 338 This document defines a new RPL control message option called DIO 339 Option Request option, shown in Figure 3, with a recommended Type 340 value of 0x0C (to be confirmed by IANA). This new option allows full 341 control over the options of the solicited DIO. The target of a 342 unicast or multicast DIS with the R flag set and with one or more DIO 343 Option Request options included, MUST include these requested options 344 in the solicited DIO. For a DIS with the R flag unset, a RPL router 345 behaves like described in [RFC6550] with regard to DIO options. 347 5. Full behavior illustration 349 Figure 4 and Figure 5 illustrate the normative behavior described in 350 Section 3 and Section 4.1. 352 +--------------------+---------------------- 353 | Unicast DIS | Multicast DIS 354 +--------------------+--------------------+- 355 | | N=0 | 356 +--------------------+--------------------+--------------------+- 357 | | unicast DIO, | | 358 | no option present | don't | do | 359 | | reset Trickle timer| reset Trickle timer| 360 +--------------------+--------------------+--------------------+- 361 | Solicited Informa- | | | 362 | tion/Metric Contai-| do nothing | do nothing | 363 | ner option present,| | | 364 | not matching. | | | 365 +--------------------+--------------------+--------------------+- 366 | Solicited Informa- | unicast DIO, | | 367 | tion/Metric Contai-| don't | do | 368 | ner option present,| reset Trickle timer| reset Trickle timer| 369 | matching. | | | 370 +--------------------+--------------------+--------------------+- 372 Figure 4: Overall DIS behavior, part 1 374 Notice that Figure 4 is indeed identical to Table 1 when Metric 375 Container options are not used in DIS. 377 -------------------------------------------+ 378 Multicast DIS | 379 ----------------------+--------------------+ 380 | N=1, T=0 | N=1, T=1 | 381 -+--------------------+--------------------+ 382 | multicast DIO, | unicast DIO, | 383 | don't | don't | 384 | reset Trickle timer| reset Trickle timer| 385 -+--------------------+--------------------+ 386 | | | 387 | do nothing | do nothing | 388 | | | 389 | | | 390 -+--------------------+--------------------+ 391 | multicast DIO, | unicast DIO, | 392 | don't | don't | 393 | reset Trickle timer| reset Trickle timer| 394 | | | 395 -+--------------------+--------------------+ 397 Figure 5: Overall DIS behavior, part 2 399 For the sake of completeness, let's remind here that a specific, one- 400 shot DIO generated in response to a DIS with the R flag unset MUST 401 contain a Configuration option. If the R flag is set, then this DIO 402 contains only explicitely requested DIO options. This DIO's 403 transmission is delayed according to the Delay Spreading option of 404 the DIS, if one such option is present. 406 6. IANA Considerations 408 6.1. DIS Flags 410 IANA is requested to allocate bits 0, 1 and 2 of the DIS Flag Field 411 to become the "No Inconsistency", "DIO Type", and "DIO Option 412 Request" bits, the functionality of which is described in Section 3 413 of this document. 415 +-------+--------------------+---------------+ 416 | Value | Meaning | Reference | 417 +-------+--------------------+---------------+ 418 | 0 | No Inconsistency | This document | 419 | 1 | DIO Type | This document | 420 | 2 | DIO Option Request | This document | 421 +-------+--------------------+---------------+ 423 6.2. RPL Control Message Options 425 IANA is requested to allocate a new code point in the "RPL Control 426 Message Options" registry for the "Response Spreading" option and the 427 "Dio Option Request" option, the behavior of which are described in 428 Section 4.2 and Section 4.3, respectively. 430 +-------+--------------------+---------------+ 431 | Value | Meaning | Reference | 432 +-------+--------------------+---------------+ 433 | 0x0B | Response Spreading | This document | 434 | 0x0C | DIO Option Request | This document | 435 +-------+--------------------+---------------+ 437 RPL Control Message Options 439 7. Security Considerations 441 TBA 443 8. Acknowledgements 445 A lot of text in this document originates from now-expired [I- 446 D.goyal-roll-dis-modifications] co-authored with M. Goyal. The 447 requirements and solutions also draw from now-expired [I-D.dejean- 448 roll-selective-dis] co-authored with N. Dejean. Their contribution 449 is deeply acknowledged. 451 We also thank (TBA) for their useful feedback and discussion. 453 9. References 455 9.1. Normative References 457 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 458 Requirement Levels", BCP 14, RFC 2119, 459 DOI 10.17487/RFC2119, March 1997, 460 . 462 [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., 463 Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, 464 JP., and R. Alexander, "RPL: IPv6 Routing Protocol for 465 Low-Power and Lossy Networks", RFC 6550, 466 DOI 10.17487/RFC6550, March 2012, 467 . 469 [RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N., 470 and D. Barthel, "Routing Metrics Used for Path Calculation 471 in Low-Power and Lossy Networks", RFC 6551, 472 DOI 10.17487/RFC6551, March 2012, 473 . 475 9.2. Informative References 477 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 478 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 479 DOI 10.17487/RFC4861, September 2007, 480 . 482 [RFC5184] Teraoka, F., Gogo, K., Mitsuya, K., Shibui, R., and K. 483 Mitani, "Unified Layer 2 (L2) Abstractions for Layer 3 484 (L3)-Driven Fast Handover", RFC 5184, 485 DOI 10.17487/RFC5184, May 2008, 486 . 488 [RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection 489 (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, 490 DOI 10.17487/RFC5881, June 2010, 491 . 493 [RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko, 494 "The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206, 495 March 2011, . 497 Appendix A. Applications 499 This section details two example mechanisms that use the DIS flags 500 and options defined in this document. The first mechanism describes 501 how a leaf node may join a desired DAG in an energy efficient manner. 502 The second mechanism details how a node may identify defunct DAGs for 503 which it still maintains state. 505 A.1. A Leaf Node Joining a DAG 507 A new leaf node that joins an established LLN runs an iterative 508 algorithm in which it requests (using multicast DIS) DIOs from 509 routers belonging to the desired DAG. 511 The DIS message has the "No Inconsistency" flag set to prevent 512 resetting of Trickle timer in responding routers, thereby keeping the 513 aggregated number of transmissions low. It also has the "DIO Type" 514 flag set to make responding routers send unicast DIOs back, thereby 515 not triggering full reception in nearby nodes that have state-of-the- 516 art radio receivers with hardware-based address filtering. 518 The DIS message can include a Response Spreading option prescribing a 519 suitable spreading interval based on the expected density of nearby 520 routers and on the expected Layer 2 technology. 522 The DIS will likely include a Metric Container listing the routing 523 constraints that the responding routers must satisfy in order to be 524 allowed to respond. 526 At each iteration, the node multicasts such a DIS and waits for 527 forthcoming DIOs. After a time equal to the spreading interval, the 528 node considers the current iteration to be unsuccessful. The node 529 consequently relaxes the routing constraints somewhat and proceeds to 530 the next iteration. 532 The cycle repeats until the node receives one or more DIOs or until 533 it has relaxed the constraints to the lowest acceptable values. 535 This algorithm has been proven in the field to be extremely energy- 536 efficient, especially when routers have a wide communication range. 538 A.2. Identifying A Defunct DAG 540 A RPL node may remove a neighbor from its parent set for a DAG for a 541 number of reasons: 543 o The neighbor is no longer reachable, as determined using a 544 mechanism such as Neighbor Unreachanility Detection (NUD) 545 [RFC4861], Bidirectional Forwarding Detection (BFD) [RFC5881] or 546 L2 triggers [RFC5184]; or 548 o The neighbor advertises an infinite rank in the DAG; or 550 o Keeping the neighbor as a parent would required the node to 551 increase its rank beyond L + DAGMaxRankIncrease, where L is the 552 minimum rank the node has had in this DAG; or 554 o The neighbor advertises membership in a different DAG within the 555 same RPL Instance, where a different DAG is recognised by a 556 different DODAGID or a different DODAGVersionNumber. 558 Even if the conditions listed above exist, a RPL node may fail to 559 remove a neighbor from its parent set because: 561 o The node may fail to receive the neighbor's DIOs advertising an 562 increased rank or the neighbor's membership in a different DAG; 564 o The node may not check, and hence may not detect, the neighbor's 565 unreachability for a long time. For example, the node may not 566 have any data to send to this neighbor and hence may not encounter 567 any event (such as failure to send data to this neighbor) that 568 would trigger a check for the neighbor's reachability. 570 In such cases, a node would continue to consider itself attached to a 571 DAG even if all its parents in the DAG are unreachable or have moved 572 to different DAGs. Such a DAG can be characterized as being defunct 573 from the node's perspective. If the node maintains state about a 574 large number of defunct DAGs, such state may prevent a considerable 575 portion of the total memory in the node from being available for more 576 useful purposes. 578 To alleviate the problem described above, a RPL node may invoke the 579 following procedure to identify a defunct DAG and delete the state it 580 maintains for this DAG. Note that, given the proactive nature of RPL 581 protocol, the lack of data traffic using a DAG can not be considered 582 a reliable indication of the DAG's defunction. Further, the Trickle 583 timer based control of DIO transmissions means the possibility of an 584 indefinite delay in the receipt of a new DIO from a functional DAG 585 parent. Hence, the mechanism described here is based on the use of a 586 DIS message to solicit DIOs about a DAG suspected of defunction. 587 Further, a multicast DIS is used so as to avoid the need to query 588 each parent individually and also to discover other neighbor routers 589 that may serve as the node's new parents in the DAG. 591 When a RPL node has not received a DIO from any of its parents in a 592 DAG for more than a locally configured time duration: 594 o The node generates a multicast DIS message with: 596 * the "No Inconsistency" flag set so that the responding routers 597 do not reset their Trickle timers. 599 * the "DIO Type" flag not set so that the responding routers send 600 multicast DIOs and other nodes in the vicinity do not need to 601 invoke this procedure. 603 * a Solicited Information option to identify the DAG in question. 604 This option must have the I and D flags set and the 605 RPLInstanceID/DODAGID fields must be set to values identifying 606 the DAG. The V flag inside the Solicited Information option 607 should not be set so as to allow the neighbors to send DIOs 608 advertising the latest version of the DAG. 610 * a Response Spreading option specifying a suitable time interval 611 over which the DIO responses may arrive. 613 o After sending the DIS, the node waits for the duration specified 614 inside the Response Spreading option to receive the DIOs generated 615 by its neighbors. At the conclusion of the wait duration: 617 * If the node has received one or more DIOs advertising newer 618 version(s) of the DAG, it joins the latest version of the DAG, 619 selects a new parent set among the neighbors advertising the 620 latest DAG version and marks the DAG status as functional. 622 * Otherwise, if the node has not received a DIO advertising the 623 current version of the DAG from a neighbor in the parent set, 624 it removes that neighbor from the parent set. As a result, if 625 the node has no parent left in the DAG, it marks the DAG as 626 defunct and schedule the deletion of the state it has 627 maintained for the DAG after a locally configured "hold" 628 duration. (This is because, as per RPL specification, when a 629 node no longer has any parents left in a DAG, it is still 630 required to remember the DAG's identity (RPLInstanceID, 631 DODAGID, DODAGVersionNumber), the lowest rank (L) it has had in 632 this DAG and the DAGMaxRankIncrease value for the DAG for a 633 certain time interval to ensure that the node does not join an 634 earlier version of the DAG and does not rejoin the current 635 version of the DAG at a rank higher than L + 636 DAGMaxRankIncrease.) 638 A.3. Explicit and Implicit DIO Option Requests 640 Certain information from a DIO is only needed occasionally or for 641 specific events: 643 o A Configuration option contains information that is static and 644 stays mostly unchanged during the lifetime of the DODAG. Thus, 645 the Configuration option is important when joining the DODAG, but 646 inflates the DIO unnecessarily thereafter. 648 o A Prefix Information option is also useful when joining a DODAG to 649 perform address autoconfiguration and propagating the prefixes. 650 After that, it is only relevant when changes occure that would 651 affect the prefix information (new prefixes, lifetime updates, 652 ...). 654 o More DIO options may be added in the future that have similar 655 properties as mentioned above. 657 The Configuration and Prefix Information options may be omitted from 658 the trickle timer based DIOs, leading to less bytes for each DIO. 659 Once a RPL router decides to join a DODAG, it may solicit a DIO by 660 sending a DIS with the R flag set and two "DIO Option Request" 661 options included. One for the Configuration option, the other for 662 the Prefix Information option. Upon receiving these options in the 663 next DIO, the RPL router can successfully finish the joining process. 665 Appendix B. Experimental data 667 The effectiveness of these flags and options has been measured on 668 real industrial hardware. 670 Data to be added 672 Authors' Addresses 674 Cenk Gundogan (editor) 675 HAW Hamburg 677 Email: cenk.guendogan@haw-hamburg.de 679 Dominique Barthel 680 Orange 681 28 Chemin Du Vieux Chene, BP 98 682 Meylan 38243 683 France 685 Email: dominique.barthel@orange.com 687 Emmanuel Baccelli 688 INRIA 690 Phone: +33-169-335-511 691 Email: Emmanuel.Baccelli@inria.fr 692 URI: http://www.emmanuelbaccelli.org/