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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) 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 ROLL R. Jadhav, Ed. 3 Internet-Draft Huawei 4 Intended status: Standards Track P. Thubert 5 Expires: January 9, 2020 Cisco 6 R. Sahoo 7 Z. Cao 8 Huawei 9 July 8, 2019 11 Efficient Route Invalidation 12 draft-ietf-roll-efficient-npdao-15 14 Abstract 16 This document explains the problems associated with the current use 17 of NPDAO messaging and also discusses the requirements for an 18 optimized route invalidation messaging scheme. Further a new 19 proactive route invalidation message called as "Destination Cleanup 20 Object" (DCO) is specified which fulfills requirements of an 21 optimized route invalidation messaging. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at https://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on January 9, 2020. 40 Copyright Notice 42 Copyright (c) 2019 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (https://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 58 1.1. Requirements Language and Terminology . . . . . . . . . . 3 59 1.2. Current NPDAO messaging . . . . . . . . . . . . . . . . . 4 60 1.3. Why Is NPDAO Important? . . . . . . . . . . . . . . . . . 5 61 2. Problems with current NPDAO messaging . . . . . . . . . . . . 6 62 2.1. Lost NPDAO due to link break to the previous parent . . . 6 63 2.2. Invalidate Routes of Dependent Nodes . . . . . . . . . . 6 64 2.3. Possible route downtime caused by asynchronous operation 65 of NPDAO and DAO . . . . . . . . . . . . . . . . . . . . 6 66 3. Requirements for the NPDAO Optimization . . . . . . . . . . . 6 67 3.1. Req#1: Remove messaging dependency on link to the 68 previous parent . . . . . . . . . . . . . . . . . . . . . 6 69 3.2. Req#2: Dependent nodes route invalidation on parent 70 switching . . . . . . . . . . . . . . . . . . . . . . . . 7 71 3.3. Req#3: Route invalidation should not impact data traffic 7 72 4. Changes to RPL signaling . . . . . . . . . . . . . . . . . . 7 73 4.1. Change in RPL route invalidation semantics . . . . . . . 7 74 4.2. Transit Information Option changes . . . . . . . . . . . 8 75 4.3. Destination Cleanup Object (DCO) . . . . . . . . . . . . 9 76 4.3.1. Secure DCO . . . . . . . . . . . . . . . . . . . . . 10 77 4.3.2. DCO Options . . . . . . . . . . . . . . . . . . . . . 10 78 4.3.3. Path Sequence number in the DCO . . . . . . . . . . . 10 79 4.3.4. Destination Cleanup Option Acknowledgment (DCO-ACK) . 11 80 4.3.5. Secure DCO-ACK . . . . . . . . . . . . . . . . . . . 12 81 4.4. DCO Base Rules . . . . . . . . . . . . . . . . . . . . . 12 82 4.5. Unsolicited DCO . . . . . . . . . . . . . . . . . . . . . 12 83 4.6. Other considerations . . . . . . . . . . . . . . . . . . 13 84 4.6.1. Dependent Nodes invalidation . . . . . . . . . . . . 13 85 4.6.2. NPDAO and DCO in the same network . . . . . . . . . . 13 86 4.6.3. Considerations for DCO retry . . . . . . . . . . . . 14 87 4.6.4. DCO with multiple preferred parents . . . . . . . . . 15 88 5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16 89 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 90 6.1. New Registry for the Destination Cleanup Object (DCO) 91 Flags . . . . . . . . . . . . . . . . . . . . . . . . . . 16 92 6.2. New Registry for the Destination Cleanup Object 93 Acknowledgment (DCO-ACK) Status field . . . . . . . . . . 17 94 6.3. New Registry for the Destination Cleanup Object (DCO) 95 Acknowledgment Flags . . . . . . . . . . . . . . . . . . 17 96 7. Security Considerations . . . . . . . . . . . . . . . . . . . 18 97 8. Normative References . . . . . . . . . . . . . . . . . . . . 19 98 Appendix A. Example Messaging . . . . . . . . . . . . . . . . . 19 99 A.1. Example DCO Messaging . . . . . . . . . . . . . . . . . . 20 100 A.2. Example DCO Messaging with multiple preferred parents . . 21 101 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 103 1. Introduction 105 RPL [RFC6550] (Routing Protocol for Low power and lossy networks) 106 specifies a proactive distance-vector based routing scheme. RPL has 107 optional messaging in the form of DAO (Destination Advertisement 108 Object) messages, which the 6LBR (6Lo Border Router) and 6LR (6Lo 109 Router) can use to learn a route towards the downstream nodes. In 110 storing mode, DAO messages would result in routing entries being 111 created on all intermediate 6LRs from the node's parent all the way 112 towards the 6LBR. 114 RPL allows the use of No-Path DAO (NPDAO) messaging to invalidate a 115 routing path corresponding to the given target, thus releasing 116 resources utilized on that path. A NPDAO is a DAO message with route 117 lifetime of zero, originates at the target node and always flows 118 upstream towards the 6LBR. This document explains the problems 119 associated with the current use of NPDAO messaging and also discusses 120 the requirements for an optimized route invalidation messaging 121 scheme. Further a new proactive route invalidation message called as 122 "Destination Cleanup Object" (DCO) is specified which fulfills 123 requirements of an optimized route invalidation messaging. 125 The document only caters to the RPL's storing mode of operation 126 (MOP). The non-storing MOP does not require use of NPDAO for route 127 invalidation since routing entries are not maintained on 6LRs. 129 1.1. Requirements Language and Terminology 131 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 132 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 133 "OPTIONAL" in this document are to be interpreted as described in BCP 134 14 [RFC2119] [RFC8174] when, and only when, they appear in all 135 capitals, as shown here. 137 This specification requires readers to be familiar with all the terms 138 and concepts that are discussed in "RPL: IPv6 Routing Protocol for 139 Low-Power and Lossy Networks" [RFC6550]. 141 Low Power and Lossy Networks (LLN): 142 Network in which both the routers and their interconnect are 143 constrained. LLN routers typically operate with constraints on 144 processing power, memory, and energy (batter power). Their 145 interconnects are characterized by high loss rates, low data 146 rates, and instability. 147 6LoWPAN Router (6LR): 148 An intermediate router that is able to send and receive Router 149 Advertisements (RAs) and Router Solicitations (RSs) as well as 150 forward and route IPv6 packets. 151 Directed Acyclic Graph (DAG): 152 A directed graph having the property that all edges are oriented 153 in such a way that no cycles exist. 154 Destination-Oriented DAG (DODAG): 155 A DAG rooted at a single destination, i.e., at a single DAG root 156 with no outgoing edges. 157 6LoWPAN Border Router (6LBR): 158 A border router which is a DODAG root and is the edge node for 159 traffic flowing in and out of the 6LoWPAN network. 160 Destination Advertisement Object (DAO): 161 DAO messaging allows downstream routes to the nodes to be 162 established. 163 DODAG Information Object (DIO): 164 DIO messaging allows upstream routes to the 6LBR to be 165 established. DIO messaging is initiated at the DAO root. 166 Common Ancestor node 167 6LR/6LBR node which is the first common node between two paths of 168 a target node. 169 No-Path DAO (NPDAO): 170 A DAO message which has target with lifetime 0 used for the 171 purpose of route invalidation. 172 Destination Cleanup Object (DCO): 173 A new RPL control message code defined by this document. DCO 174 messaging improves proactive route invalidation in RPL. 175 Regular DAO: 176 A DAO message with non-zero lifetime. Routing adjacencies are 177 created or updated based on this message. 178 Target node: 179 The node switching its parent whose routing adjacencies are 180 updated (created/removed). 182 1.2. Current NPDAO messaging 184 RPL uses NPDAO messaging in the storing mode so that the node 185 changing its routing adjacencies can invalidate the previous route. 186 This is needed so that nodes along the previous path can release any 187 resources (such as the routing entry) they maintain on behalf of 188 target node. 190 For the rest of this document consider the following topology: 192 (6LBR) 193 | 194 | 195 | 196 (A) 197 / \ 198 / \ 199 / \ 200 (G) (H) 201 | | 202 | | 203 | | 204 (B) (C) 205 \ ; 206 \ ; 207 \ ; 208 (D) 209 / \ 210 / \ 211 / \ 212 (E) (F) 214 Figure 1: Sample topology 216 Node (D) is connected via preferred parent (B). (D) has an alternate 217 path via (C) towards the 6LBR. Node (A) is the common ancestor for 218 (D) for paths through (B)-(G) and (C)-(H). When (D) switches from 219 (B) to (C), RPL allows sending NPDAO to (B) and regular DAO to (C). 221 1.3. Why Is NPDAO Important? 223 Nodes in LLNs may be resource constrained. There is limited memory 224 available and routing entry records are one of the primary elements 225 occupying dynamic memory in the nodes. Route invalidation helps 6LR 226 nodes to decide which entries could be discarded to better optimize 227 resource utilization. Thus it becomes necessary to have an efficient 228 route invalidation mechanism. Also note that a single parent switch 229 may result in a "sub-tree" switching from one parent to another. 230 Thus the route invalidation needs to be done on behalf of the sub- 231 tree and not the switching node alone. In the above example, when 232 Node (D) switches parent, the route updates needs to be done for the 233 routing tables entries of (C),(H),(A),(G), and (B) with destination 234 (D),(E) and (F). Without efficient route invalidation, a 6LR may 235 have to hold a lot of stale route entries. 237 2. Problems with current NPDAO messaging 239 2.1. Lost NPDAO due to link break to the previous parent 241 When a node switches its parent, the NPDAO is to be sent to its 242 previous parent and a regular DAO to its new parent. In cases where 243 the node switches its parent because of transient or permanent parent 244 link/node failure then the NPDAO message is bound to fail. 246 2.2. Invalidate Routes of Dependent Nodes 248 RPL does not specify how route invalidation will work for dependent 249 nodes rooted at the switching node, resulting in stale routing 250 entries of the dependent nodes. The only way for 6LR to invalidate 251 the route entries for dependent nodes would be to use route lifetime 252 expiry which could be substantially high for LLNs. 254 In the example topology, when Node (D) switches its parent, Node (D) 255 generates an NPDAO on its behalf. There is no NPDAO generated by the 256 dependent child nodes (E) and (F), through the previous path via (D) 257 to (B) and (G), resulting in stale entries on nodes (B) and (G) for 258 nodes (E) and (F). 260 2.3. Possible route downtime caused by asynchronous operation of NPDAO 261 and DAO 263 A switching node may generate both an NPDAO and DAO via two different 264 paths at almost the same time. There is a possibility that an NPDAO 265 generated may invalidate the previous route and the regular DAO sent 266 via the new path gets lost on the way. This may result in route 267 downtime impacting downward traffic for the switching node. 269 In the example topology, consider Node (D) switches from parent (B) 270 to (C). An NPDAO sent via the previous route may invalidate the 271 previous route whereas there is no way to determine whether the new 272 DAO has successfully updated the route entries on the new path. 274 3. Requirements for the NPDAO Optimization 276 3.1. Req#1: Remove messaging dependency on link to the previous parent 278 When the switching node sends the NPDAO message to the previous 279 parent, it is normal that the link to the previous parent is prone to 280 failure (that's why the node decided to switch). Therefore, it is 281 required that the route invalidation does not depend on the previous 282 link which is prone to failure. The previous link referred here 283 represents the link between the node and its previous parent (from 284 whom the node is now disassociating). 286 3.2. Req#2: Dependent nodes route invalidation on parent switching 288 It should be possible to do route invalidation for dependent nodes 289 rooted at the switching node. 291 3.3. Req#3: Route invalidation should not impact data traffic 293 While sending the NPDAO and DAO messages, it is possible that the 294 NPDAO successfully invalidates the previous path, while the newly 295 sent DAO gets lost (new path not set up successfully). This will 296 result in downstream unreachability to the node switching paths. 297 Therefore, it is desirable that the route invalidation is 298 synchronized with the DAO to avoid the risk of route downtime. 300 4. Changes to RPL signaling 302 4.1. Change in RPL route invalidation semantics 304 As described in Section 1.2, the NPDAO originates at the node 305 changing to a new parent and traverses upstream towards the root. In 306 order to solve the problems as mentioned in Section 2, the document 307 adds a new proactive route invalidation message called "Destination 308 Cleanup Object" (DCO) that originates at a common ancestor node and 309 flows downstream between the new and old path. The common ancestor 310 node generates a DCO in response to the change in the next-hop on 311 receiving a regular DAO with updated Path Sequence for the target. 313 The 6LRs in the path for DCO take action such as route invalidation 314 based on the DCO information and subsequently send another DCO with 315 the same information downstream to the next hop. This operation is 316 similar to how the DAOs are handled on intermediate 6LRs in storing 317 MOP in [RFC6550]. Just like DAO in storing MOP, the DCO is sent 318 using link-local unicast source and destination IPv6 address. Unlike 319 DAO, which always travels upstream, the DCO always travels 320 downstream. 322 In Figure 1, when node D decides to switch the path from B to C, it 323 sends a regular DAO to node C with reachability information 324 containing the address of D as the target and an incremented Path 325 Sequence. Node C will update the routing table based on the 326 reachability information in the DAO and in turn generate another DAO 327 with the same reachability information and forward it to H. Node H 328 also follows the same procedure as Node C and forwards it to node A. 329 When node A receives the regular DAO, it finds that it already has a 330 routing table entry on behalf of the target address of node D. It 331 finds however that the next hop information for reaching node D has 332 changed i.e., node D has decided to change the paths. In this case, 333 Node A which is the common ancestor node for node D along the two 334 paths (previous and new), should generate a DCO which traverses 335 downwards in the network. Node A handles normal DAO forwarding to 336 6LBR as required by [RFC6550]. 338 4.2. Transit Information Option changes 340 Every RPL message is divided into base message fields and additional 341 Options as described in Section 6 of [RFC6550]. The base fields 342 apply to the message as a whole and options are appended to add 343 message/use-case specific attributes. As an example, a DAO message 344 may be attributed by one or more "RPL Target" options which specify 345 the reachability information for the given targets. Similarly, a 346 Transit Information option may be associated with a set of RPL Target 347 options. 349 This document specifies a change in the Transit Information Option to 350 contain the "Invalidate previous route" (I) flag. This I-flag 351 signals the common ancestor node to generate a DCO on behalf of the 352 target node. The I-flag is carried in the Transit Information Option 353 which augments the reachability information for a given set of RPL 354 Target(s). Transit Information Option with I-flag set should be 355 carried in the DAO message when route invalidation is sought for the 356 corresponding target(s). 358 0 1 2 3 359 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 1 360 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 361 | Type = 0x06 | Option Length |E|I| Flags | Path Control | 362 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 363 | Path Sequence | Path Lifetime | 364 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 366 Figure 2: Updated Transit Information Option (New I flag added) 368 I (Invalidate previous route) flag: The 'I' flag is set by the target 369 node to indicate to the common ancestor node that it wishes to 370 invalidate any previous route between the two paths. 372 [RFC6550] allows the parent address to be sent in the Transit 373 Information Option depending on the mode of operation. In case of 374 storing mode of operation the field is usually not needed. In case 375 of DCO, the parent address field MUST NOT be included. 377 The common ancestor node SHOULD generate a DCO message in response to 378 this I-flag when it sees that the routing adjacencies have changed 379 for the target. The I-flag is intended to give the target node 380 control over its own route invalidation, serving as a signal to 381 request DCO generation. 383 4.3. Destination Cleanup Object (DCO) 385 A new ICMPv6 RPL control message code is defined by this 386 specification and is referred to as "Destination Cleanup Object" 387 (DCO), which is used for proactive cleanup of state and routing 388 information held on behalf of the target node by 6LRs. The DCO 389 message always traverses downstream and cleans up route information 390 and other state information associated with the given target. 392 0 1 2 3 393 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 1 394 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 395 | RPLInstanceID |K|D| Flags | Reserved | DCOSequence | 396 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 397 | | 398 + + 399 | | 400 + DODAGID(optional) + 401 | | 402 + + 403 | | 404 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 405 | Option(s)... 406 +-+-+-+-+-+-+-+-+ 408 Figure 3: DCO base object 410 RPLInstanceID: 8-bit field indicating the topology instance 411 associated with the DODAG, as learned from the DIO. 413 K: The 'K' flag indicates that the recipient of DCO message is 414 expected to send a DCO-ACK back. If the DCO-ACK is not received even 415 after setting the 'K' flag, an implementation may retry the DCO at a 416 later time. The number of retries are implementation and deployment 417 dependent and are expected to be kept similar with those used in DAO 418 retries in [RFC6550]. Section 4.6.3 specifies the considerations for 419 DCO retry. A node receiving a DCO message without the 'K' flag set 420 MAY respond with a DCO-ACK, especially to report an error condition. 421 An example error condition could be that the node sending the DCO-ACK 422 does not find the routing entry for the indicated target. When the 423 sender does not set the 'K' flag it is an indication that the sender 424 does not expect a response, and the sender SHOULD NOT retry the DCO. 426 D: The 'D' flag indicates that the DODAGID field is present. This 427 flag MUST be set when a local RPLInstanceID is used. 429 Flags: The 6 bits remaining unused in the Flags field are reserved 430 for future use. These bits MUST be initialized to zero by the sender 431 and MUST be ignored by the receiver. 433 Reserved: 8-bit unused field. The field MUST be initialized to zero 434 by the sender and MUST be ignored by the receiver. 436 DCOSequence: 8-bit field incremented at each unique DCO message from 437 a node and echoed in the DCO-ACK message. The initial DCOSequence 438 can be chosen randomly by the node. Section 4.4 explains the 439 handling of the DCOSequence. 441 DODAGID (optional): 128-bit unsigned integer set by a DODAG root that 442 uniquely identifies a DODAG. This field MUST be present when the 'D' 443 flag is set and MUST NOT be present if 'D' flag is not set. DODAGID 444 is used when a local RPLInstanceID is in use, in order to identify 445 the DODAGID that is associated with the RPLInstanceID. 447 4.3.1. Secure DCO 449 A Secure DCO message follows the format in [RFC6550] Figure 7, where 450 the base message format is the DCO message shown in Figure 3. 452 4.3.2. DCO Options 454 The DCO message MUST carry at least one RPL Target and the Transit 455 Information Option and MAY carry other valid options. This 456 specification allows for the DCO message to carry the following 457 options: 459 0x00 Pad1 460 0x01 PadN 461 0x05 RPL Target 462 0x06 Transit Information 463 0x09 RPL Target Descriptor 465 Section 6.7 of [RFC6550] defines all the above mentioned options. 466 The DCO carries an RPL Target Option and an associated Transit 467 Information Option with a lifetime of 0x00000000 to indicate a loss 468 of reachability to that Target. 470 4.3.3. Path Sequence number in the DCO 472 A DCO message may contain a Path Sequence in the Transit Information 473 Option to identify the freshness of the DCO message. The Path 474 Sequence in the DCO MUST use the same Path Sequence number present in 475 the regular DAO message when the DCO is generated in response to a 476 DAO message. Thus if a DCO is received by a 6LR and subsequently a 477 DAO is received with an old sequence number, then the DAO MUST be 478 ignored. When the DCO is generated in response to a DCO from 479 upstream parent, the Path Sequence MUST be copied from the received 480 DCO. 482 4.3.4. Destination Cleanup Option Acknowledgment (DCO-ACK) 484 The DCO-ACK message SHOULD be sent as a unicast packet by a DCO 485 recipient in response to a unicast DCO message with 'K' flag set. If 486 'K' flag is not set then the receiver of the DCO message MAY send a 487 DCO-ACK, especially to report an error condition. 489 0 1 2 3 490 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 1 491 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 492 | RPLInstanceID |D| Flags | DCOSequence | Status | 493 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 494 | | 495 + + 496 | | 497 + DODAGID(optional) + 498 | | 499 + + 500 | | 501 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 503 Figure 4: DCO-ACK base object 505 RPLInstanceID: 8-bit field indicating the topology instance 506 associated with the DODAG, as learned from the DIO. 508 D: The 'D' flag indicates that the DODAGID field is present. This 509 flag MUST be set when a local RPLInstanceID is used. 511 Flags: 7-bit unused field. The field MUST be initialized to zero by 512 the sender and MUST be ignored by the receiver. 514 DCOSequence: 8-bit field. The DCOSequence in DCO-ACK is copied from 515 the DCOSequence received in the DCO message. 517 Status: Indicates the completion. Status 0 is defined as unqualified 518 acceptance in this specification. Status 1 is defined as "No 519 routing-entry for the Target found". The remaining status values are 520 reserved as rejection codes. 522 DODAGID (optional): 128-bit unsigned integer set by a DODAG root that 523 uniquely identifies a DODAG. This field MUST be present when the 'D' 524 flag is set and MUST NOT be present when 'D' flag is not set. 526 DODAGID is used when a local RPLInstanceID is in use, in order to 527 identify the DODAGID that is associated with the RPLInstanceID. 529 4.3.5. Secure DCO-ACK 531 A Secure DCO-ACK message follows the format in [RFC6550] Figure 7, 532 where the base message format is the DCO-ACK message shown in 533 Figure 4. 535 4.4. DCO Base Rules 537 1. If a node sends a DCO message with newer or different information 538 than the prior DCO message transmission, it MUST increment the 539 DCOSequence field by at least one. A DCO message transmission 540 that is identical to the prior DCO message transmission MAY 541 increment the DCOSequence field. The DCOSequence counter follows 542 the sequence counter operation as defined in Section 7.2 of 543 [RFC6550]. 544 2. The RPLInstanceID and DODAGID fields of a DCO message MUST be the 545 same value as that of the DAO message in response to which the 546 DCO is generated on the common ancestor node. 547 3. A node MAY set the 'K' flag in a unicast DCO message to solicit a 548 unicast DCO-ACK in response in order to confirm the attempt. 549 4. A node receiving a unicast DCO message with the 'K' flag set 550 SHOULD respond with a DCO-ACK. A node receiving a DCO message 551 without the 'K' flag set MAY respond with a DCO-ACK, especially 552 to report an error condition. 553 5. A node receiving a unicast DCO message MUST verify the stored 554 Path Sequence in context to the given target. If the stored Path 555 Sequence is more fresh, newer than the Path Sequence received in 556 the DCO, then the DCO MUST be dropped. 557 6. A node that sets the 'K' flag in a unicast DCO message but does 558 not receive DCO-ACK in response MAY reschedule the DCO message 559 transmission for another attempt, up until an implementation 560 specific number of retries. 561 7. A node receiving a unicast DCO message with its own address in 562 the RPL Target Option MUST strip-off that Target Option. If this 563 Target Option is the only one in the DCO message then the DCO 564 message MUST be dropped. 566 The scope of DCOSequence values is unique to the node which generates 567 it. 569 4.5. Unsolicited DCO 571 A 6LR may generate an unsolicited DCO to unilaterally cleanup the 572 path on behalf of the target entry. The 6LR has all the state 573 information, namely, the Target address and the Path Sequence, 574 required for generating DCO in its routing table. The conditions why 575 6LR may generate an unsolicited DCO are beyond the scope of this 576 document but some possible reasons could be: 578 1. On route expiry of an entry, a 6LR may decide to graciously 579 cleanup the entry by initiating DCO. 580 2. 6LR needs to entertain higher priority entries in case the 581 routing table is full, thus resulting in eviction of an existing 582 routing entry. In this case the eviction can be handled 583 graciously using DCO. 585 Note that if the 6LR initiates a unilateral path cleanup using DCO 586 and if it has the latest state for the target then the DCO would 587 finally reach the target node. Thus the target node would be 588 informed of its invalidation. 590 4.6. Other considerations 592 4.6.1. Dependent Nodes invalidation 594 Current RPL [RFC6550] does not provide a mechanism for route 595 invalidation for dependent nodes. This document allows the dependent 596 nodes invalidation. Dependent nodes will generate their respective 597 DAOs to update their paths, and the previous route invalidation for 598 those nodes should work in the similar manner described for switching 599 node. The dependent node may set the I-flag in the Transit 600 Information Option as part of regular DAO so as to request 601 invalidation of previous route from the common ancestor node. 603 Dependent nodes do not have any indication regarding if any of their 604 parents in turn have decided to switch their parent. Thus for route 605 invalidation the dependent nodes may choose to always set the 'I' 606 flag in all its DAO message's Transit Information Option. Note that 607 setting the I-flag is not counterproductive even if there is no 608 previous route to be invalidated. 610 4.6.2. NPDAO and DCO in the same network 612 The current NPDAO mechanism in [RFC6550] can still be used in the 613 same network where DCO is used. The NPDAO messaging can be used, for 614 example, on route lifetime expiry of the target or when the node 615 simply decides to gracefully terminate the RPL session on graceful 616 node shutdown. Moreover, a deployment can have a mix of nodes 617 supporting the DCO and the existing NPDAO mechanism. It is also 618 possible that the same node supports both the NPDAO and DCO signaling 619 for route invalidation. 621 Section 9.8 of [RFC6550] states, "When a node removes a node from its 622 DAO parent set, it SHOULD send a No-Path DAO message to that removed 623 DAO parent to invalidate the existing router". This document 624 introduces an alternative and more optimized way of route 625 invalidation but it also allows existing NPDAO messaging to work. 626 Thus an implementation has two choices to make when a route 627 invalidation is to be initiated: 629 1. Use NPDAO to invalidate the previous route and send regular DAO 630 on the new path. 631 2. Send regular DAO on the new path with the 'I' flag set in the 632 Transit Information Option such that the common ancestor node 633 initiates the DCO message downstream to invalidate the previous 634 route. 636 This document recommends using option 2 for reasons specified in 637 Section 3 in this document. 639 This document assumes that all the 6LRs in the network support this 640 specification. If there are 6LRs en-route DCO message path which do 641 not support this document, then the route invalidation for 642 corresponding targets may not work or may work partially i.e., only 643 part of the path supporting DCO may be invalidated. Alternatively, a 644 node could generate an NPDAO if it does not receive a DCO with itself 645 as target within specified time limit. The specified time limit is 646 deployment specific and depends upon the maximum depth of the network 647 and per hop average latency. Note that sending NPDAO and DCO for the 648 same operation would not result in unwanted side-effects because the 649 acceptability of NPDAO or DCO depends upon the Path Sequence 650 freshness. 652 4.6.3. Considerations for DCO retry 654 A DCO message could be retried by a sender if it sets the 'K' flag 655 and does not receive a DCO-ACK. The DCO retry time could be 656 dependent on the maximum depth of the network and average per hop 657 latency. This could range from 2 seconds to 120 seconds depending on 658 the deployment. In case the latency limits are not known, an 659 implementation MUST NOT retry more than once in 3 seconds and MUST 660 NOT retry more than 3 times. 662 The number of retries could also be set depending on how critical the 663 route invalidation could be for the deployment and the link layer 664 retry configuration. For networks supporting only MP2P and P2MP 665 flows, such as in AMI and telemetry applications, the 6LRs may not be 666 very keen to invalidate routes, unless they are highly memory- 667 constrained. For home and building automation networks which may 668 have substantial P2P traffic, the 6LRs might be keen to invalidate 669 efficiently because it may additionally impact the forwarding 670 efficiency. 672 4.6.4. DCO with multiple preferred parents 674 [RFC6550] allows a node to select multiple preferred parents for 675 route establishment. Section 9.2.1 of [RFC6550] specifies, "All DAOs 676 generated at the same time for the same Target MUST be sent with the 677 same Path Sequence in the Transit Information". Subsequently when 678 route invalidation has to be initiated, RPL mentions use of NPDAO 679 which can be initiated with an updated Path Sequence to all the 680 parent nodes through which the route is to be invalidated. 682 With DCO, the Target node itself does not initiate the route 683 invalidation and it is left to the common ancestor node. A common 684 ancestor node when it discovers an updated DAO from a new next-hop, 685 it initiates a DCO. With multiple preferred parents, this handling 686 does not change. But in this case it is recommended that an 687 implementation initiates a DCO after a time period (DelayDCO) such 688 that the common ancestor node may receive updated DAOs from all 689 possible next-hops. This will help to reduce DCO control overhead 690 i.e., the common ancestor can wait for updated DAOs from all possible 691 directions before initiating a DCO for route invalidation. After 692 timeout, the DCO needs to be generated for all the next-hops for whom 693 the route invalidation needs to be done. 695 This document recommends using a DelayDCO timer value of 1sec. This 696 value is inspired by the default DelayDAO value of 1sec in [RFC6550]. 697 Here the hypothesis is that the DAOs from all possible parent sets 698 would be received on the common ancestor within this time period. 700 It is still possible that a DCO is generated before all the updated 701 DAOs from all the paths are received. In this case, the ancestor 702 node would start the invalidation procedure for paths from which the 703 updated DAO is not received. The DCO generated in this case would 704 start invalidating the segments along these paths on which the 705 updated DAOs are not received. But once the DAO reaches these 706 segments, the routing state would be updated along these segments and 707 should not lead to any inconsistent routing state. 709 Note that there is no requirement for synchronization between DCO and 710 DAOs. The DelayDCO timer simply ensures that the DCO control 711 overhead can be reduced and is only needed when the network contains 712 nodes using multiple preferred parent. 714 5. Acknowledgments 716 Many thanks to Alvaro Retana, Cenk Gundogan, Simon Duquennoy, 717 Georgios Papadopoulous, Peter Van Der Stok for their review and 718 comments. Alvaro Retana helped shape this document's final version 719 with critical review comments. 721 6. IANA Considerations 723 IANA is requested to allocate new codes for the DCO and DCO-ACK 724 messages from the RPL Control Codes registry. 726 +------+---------------------------------------------+--------------+ 727 | Code | Description | Reference | 728 +------+---------------------------------------------+--------------+ 729 | TBD1 | Destination Cleanup Object | This | 730 | | | document | 731 | TBD2 | Destination Cleanup Object Acknowledgment | This | 732 | | | document | 733 | TBD3 | Secure Destination Cleanup Object | This | 734 | | | document | 735 | TBD4 | Secure Destination Cleanup Object | This | 736 | | Acknowledgment | document | 737 +------+---------------------------------------------+--------------+ 739 IANA is requested to allocate bit 1 from the Transit Information 740 Option Flags registry for the I-flag (Section 4.2) 742 6.1. New Registry for the Destination Cleanup Object (DCO) Flags 744 IANA is requested to create a registry for the 8-bit Destination 745 Cleanup Object (DCO) Flags field. This registry should be located in 746 existing category of "Routing Protocol for Low Power and Lossy 747 Networks (RPL)". 749 New bit numbers may be allocated only by an IETF Review. Each bit is 750 tracked with the following qualities: 752 o Bit number (counting from bit 0 as the most significant bit) 753 o Capability description 754 o Defining RFC 756 The following bits are currently defined: 758 +------------+------------------------------+---------------+ 759 | Bit number | Description | Reference | 760 +------------+------------------------------+---------------+ 761 | 0 | DCO-ACK request (K) | This document | 762 | 1 | DODAGID field is present (D) | This document | 763 +------------+------------------------------+---------------+ 765 DCO Base Flags 767 6.2. New Registry for the Destination Cleanup Object Acknowledgment 768 (DCO-ACK) Status field 770 IANA is requested to create a registry for the 8-bit Destination 771 Cleanup Object Acknowledgment (DCO-ACK) Status field. This registry 772 should be located in existing category of "Routing Protocol for Low 773 Power and Lossy Networks (RPL)". 775 New Status values may be allocated only by an IETF Review. Each 776 value is tracked with the following qualities: 778 o Status Code 779 o Description 780 o Defining RFC 782 The following values are currently defined: 784 +------------+----------------------------------------+-------------+ 785 | Status | Description | Reference | 786 | Code | | | 787 +------------+----------------------------------------+-------------+ 788 | 0 | Unqualified acceptance | This | 789 | | | document | 790 | 1 | No routing-entry for the indicated | This | 791 | | Target found | document | 792 +------------+----------------------------------------+-------------+ 794 DCO-ACK Status Codes 796 6.3. New Registry for the Destination Cleanup Object (DCO) 797 Acknowledgment Flags 799 IANA is requested to create a registry for the 8-bit Destination 800 Cleanup Object (DCO) Acknowledgment Flags field. This registry 801 should be located in existing category of "Routing Protocol for Low 802 Power and Lossy Networks (RPL)". 804 New bit numbers may be allocated only by an IETF Review. Each bit is 805 tracked with the following qualities: 807 o Bit number (counting from bit 0 as the most significant bit) 808 o Capability description 809 o Defining RFC 811 The following bits are currently defined: 813 +------------+------------------------------+---------------+ 814 | Bit number | Description | Reference | 815 +------------+------------------------------+---------------+ 816 | 0 | DODAGID field is present (D) | This document | 817 +------------+------------------------------+---------------+ 819 DCO-ACK Base Flags 821 7. Security Considerations 823 This document introduces the ability for a common ancestor node to 824 invalidate a route on behalf of the target node. The common ancestor 825 node could be directed to do so by the target node using the I-flag 826 in DCO's Transit Information Option. However, the common ancestor 827 node is in a position to unilaterally initiate the route invalidation 828 since it possesses all the required state information, namely, the 829 Target address and the corresponding Path Sequence. Thus a rogue 830 common ancestor node could initiate such an invalidation and impact 831 the traffic to the target node. 833 This document also introduces an I-flag which is set by the target 834 node and used by the ancestor node to initiate a DCO if the ancestor 835 sees an update in the route adjacency. However, this flag could be 836 spoofed by a malicious 6LR in the path and can cause invalidation of 837 an existing active path. Note that invalidation will happen only if 838 the other conditions such as Path Sequence condition is also met. 839 Having said that, such a malicious 6LR may spoof a DAO on behalf of 840 the (sub) child with the I-flag set and can cause route invalidation 841 on behalf of the (sub) child node. Note that, using existing 842 mechanisms offered by [RFC6550], a malicious 6LR might also spoof a 843 DAO with lifetime of zero or otherwise cause denial of service by 844 dropping traffic entirely, so the new mechanism described in this 845 document does not present a substantially increased risk of 846 disruption. 848 This document assumes that the security mechanisms as defined in 849 [RFC6550] are followed, which means that the common ancestor node and 850 all the 6LRs are part of the RPL network because they have the 851 required credentials. A non-secure RPL network needs to take into 852 consideration the risks highlighted in this section as well as those 853 highlighted in [RFC6550]. 855 All RPL messages support a secure version of messages which allows 856 integrity protection using either a MAC or a signature. Optionally, 857 secured RPL messages also have encryption protection for 858 confidentiality. 860 The document adds new messages (DCO, DCO-ACK) which are syntactically 861 similar to existing RPL messages such as DAO, DAO-ACK. Secure 862 versions of DCO and DCO-ACK are added similar to other RPL messages 863 (such as DAO, DAO-ACK). 865 RPL supports three security modes as mentioned in Section 10.1 of 866 [RFC6550]: 868 1. Unsecured: In this mode, it is expected that the RPL control 869 messages are secured by other security mechanisms, such as link- 870 layer security. In this mode, the RPL control messages, 871 including DCO, DCO-ACK, do not have Security sections. Also note 872 that unsecured mode does not imply that all messages are sent 873 without any protection. 874 2. Preinstalled: In this mode, RPL uses secure messages. Thus 875 secure versions of DCO, DCO-ACK MUST be used in this mode. 876 3. Authenticated: In this mode, RPL uses secure messages. Thus 877 secure versions of DCO, DCO-ACK MUST be used in this mode. 879 8. Normative References 881 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 882 Requirement Levels", BCP 14, RFC 2119, 883 DOI 10.17487/RFC2119, March 1997, 884 . 886 [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., 887 Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, 888 JP., and R. Alexander, "RPL: IPv6 Routing Protocol for 889 Low-Power and Lossy Networks", RFC 6550, 890 DOI 10.17487/RFC6550, March 2012, 891 . 893 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 894 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 895 May 2017, . 897 Appendix A. Example Messaging 898 A.1. Example DCO Messaging 900 In Figure 1, node (D) switches its parent from (B) to (C). This 901 example assumes that Node D has already established its own route via 902 Node B-G-A-6LBR using pathseq=x. The example uses DAO and DCO 903 messaging convention and specifies only the required parameters to 904 explain the example namely, the parameter 'tgt', which stands for 905 Target Option and value of this parameter specifies the address of 906 the target node. The parameter 'pathseq', which specifies the Path 907 Sequence value carried in the Transit Information Option. The 908 parameter 'I_flag' specifies the 'I' flag in the Transit Information 909 Option. sequence of actions is as follows: 911 1. Node D switches its parent from node B to node C 912 2. D sends a regular DAO(tgt=D,pathseq=x+1,I_flag=1) in the updated 913 path to C 914 3. C checks for a routing entry on behalf of D, since it cannot find 915 an entry on behalf of D it creates a new routing entry and 916 forwards the reachability information of the target D to H in a 917 DAO(tgt=D,pathseq=x+1,I_flag=1). 918 4. Similar to C, node H checks for a routing entry on behalf of D, 919 cannot find an entry and hence creates a new routing entry and 920 forwards the reachability information of the target D to A in a 921 DAO(tgt=D,pathseq=x+1,I_flag=1). 922 5. Node A receives the DAO(tgt=D,pathseq=x+1,I_flag=1), and checks 923 for a routing entry on behalf of D. It finds a routing entry but 924 checks that the next hop for target D is different (i.e., Node 925 G). Node A checks the I_flag and generates 926 DCO(tgt=D,pathseq=x+1) to previous next hop for target D which is 927 G. Subsequently, Node A updates the routing entry and forwards 928 the reachability information of target D upstream 929 DAO(tgt=D,pathseq=x+1,I_flag=1). 930 6. Node G receives the DCO(tgt=D,pathseq=x+1). It checks if the 931 received path sequence is later than the stored path sequence. 932 If it is later, Node G invalidates the routing entry of target D 933 and forwards the (un)reachability information downstream to B in 934 DCO(tgt=D,pathseq=x+1). 935 7. Similarly, B processes the DCO(tgt=D,pathseq=x+1) by invalidating 936 the routing entry of target D and forwards the (un)reachability 937 information downstream to D. 938 8. D ignores the DCO(tgt=D,pathseq=x+1) since the target is itself. 939 9. The propagation of the DCO will stop at any node where the node 940 does not have an routing information associated with the target. 941 If cached routing information is present and the cached Path 942 Sequence is higher than the value in the DCO, then the DCO is 943 dropped. 945 A.2. Example DCO Messaging with multiple preferred parents 947 (6LBR) 948 | 949 | 950 | 951 (N11) 952 / \ 953 / \ 954 / \ 955 (N21) (N22) 956 / / \ 957 / / \ 958 / / \ 959 (N31) (N32) (N33) 960 : | / 961 : | / 962 : | / 963 (N41) 965 Figure 5: Sample topology 2 967 In Figure 5, node (N41) selects multiple preferred parents (N32) and 968 (N33). The sequence of actions is as follows: 970 1. (N41) sends DAO(tgt=N41,PS=x,I_flag=1) to (N32) and (N33). Here 971 I_flag refers to the Invalidation flag and PS refers to Path 972 Sequence in Transit Information option. 973 2. (N32) sends DAO(tgt=N41,PS=x,I_flag=1) to (N22). (N33) also 974 sends DAO(tgt=N41,PS=x,I_flag=1) to (N22). (N22) learns 975 multiple routes for the same destination (N41) through multiple 976 next-hops. (N22) may receive the DAOs from (N32) and (N33) in 977 any order with the I_flag set. The implementation should use 978 the DelayDCO timer to wait to initiate the DCO. If (N22) 979 receives an updated DAO from all the paths then the DCO need not 980 be initiated in this case. Thus the route table at N22 should 981 contain (Dst,NextHop,PS): { (N41,N32,x), (N41,N33,x) }. 982 3. (N22) sends DAO(tgt=N41,PS=x,I_flag=1) to (N11). 983 4. (N11) sends DAO(tgt=N41,PS=x,I_flag=1) to (6LBR). Thus the 984 complete path is established. 985 5. (N41) decides to change preferred parent set from { N32, N33 } 986 to { N31, N32 }. 987 6. (N41) sends DAO(tgt=N41,PS=x+1,I_flag=1) to (N32). (N41) sends 988 DAO(tgt=N41,PS=x+1,I_flag=1) to (N31). 989 7. (N32) sends DAO(tgt=N41,PS=x+1,I_flag=1) to (N22). (N22) has 990 multiple routes to destination (N41). It sees that a new Path 991 Sequence for Target=N41 is received and thus it waits for pre- 992 determined time period (DelayDCO time period) to invalidate 993 another route {(N41),(N33),x}. After time period, (N22) sends 994 DCO(tgt=N41,PS=x+1) to (N33). Also (N22) sends the regular 995 DAO(tgt=N41,PS=x+1,I_flag=1) to (N11). 996 8. (N33) receives DCO(tgt=N41,PS=x+1). The received Path Sequence 997 is latest and thus it invalidates the entry associated with 998 target (N41). (N33) then sends the DCO(tgt=N41,PS=x+1) to 999 (N41). (N41) sees itself as the target and drops the DCO. 1000 9. From Step 6 above, (N31) receives the 1001 DAO(tgt=N41,PS=x+1,I_flag=1). It creates a routing entry and 1002 sends the DAO(tgt=N41,PS=x+1,I_flag=1) to (N21). Similarly 1003 (N21) receives the DAO and subsequently sends the 1004 DAO(tgt=N41,PS=x+1,I_flag=1) to (N11). 1005 10. (N11) receives DAO(tgt=N41,PS=x+1,I_flag=1) from (N21). It 1006 waits for DelayDCO timer since it has multiple routes to (N41). 1007 (N41) will receive DAO(tgt=N41,PS=x+1,I_flag=1) from (N22) from 1008 Step 7 above. Thus (N11) has received regular 1009 DAO(tgt=N41,PS=x+1,I_flag=1) from all paths and thus does not 1010 initiate DCO. 1011 11. (N11) forwards the DAO(tgt=N41,PS=x+1,I_flag=1) to 6LBR and the 1012 full path is established. 1014 Authors' Addresses 1016 Rahul Arvind Jadhav (editor) 1017 Huawei 1018 Kundalahalli Village, Whitefield, 1019 Bangalore, Karnataka 560037 1020 India 1022 Phone: +91-080-49160700 1023 Email: rahul.ietf@gmail.com 1025 Pascal Thubert 1026 Cisco Systems, Inc 1027 Building D 1028 45 Allee des Ormes - BP1200 1029 MOUGINS - Sophia Antipolis 06254 1030 France 1032 Phone: +33 497 23 26 34 1033 Email: pthubert@cisco.com 1034 Rabi Narayan Sahoo 1035 Huawei 1036 Kundalahalli Village, Whitefield, 1037 Bangalore, Karnataka 560037 1038 India 1040 Phone: +91-080-49160700 1041 Email: rabinarayans@huawei.com 1043 Zhen Cao 1044 Huawei 1045 W Chang'an Ave 1046 Beijing 1047 P.R. China 1049 Email: zhencao.ietf@gmail.com