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Jadhav, Ed. 3 Internet-Draft November 30, 2020 4 Intended status: Standards Track 5 Expires: June 3, 2021 7 RPL Storing Root-ACK 8 draft-jadhav-roll-storing-rootack-02 10 Abstract 12 This document explains problems with DAO-ACK handling in RPL Storing 13 MOP and provides updates to RFC6550 to solve those problems. 15 Status of This Memo 17 This Internet-Draft is submitted in full conformance with the 18 provisions of BCP 78 and BCP 79. 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF). Note that other groups may also distribute 22 working documents as Internet-Drafts. The list of current Internet- 23 Drafts is at https://datatracker.ietf.org/drafts/current/. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 This Internet-Draft will expire on June 3, 2021. 32 Copyright Notice 34 Copyright (c) 2020 IETF Trust and the persons identified as the 35 document authors. All rights reserved. 37 This document is subject to BCP 78 and the IETF Trust's Legal 38 Provisions Relating to IETF Documents 39 (https://trustee.ietf.org/license-info) in effect on the date of 40 publication of this document. Please review these documents 41 carefully, as they describe your rights and restrictions with respect 42 to this document. Code Components extracted from this document must 43 include Simplified BSD License text as described in Section 4.e of 44 the Trust Legal Provisions and are provided without warranty as 45 described in the Simplified BSD License. 47 Table of Contents 49 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 50 1.1. Requirements Language and Terminology . . . . . . . . . . 3 51 2. Problems with DAO-ACK in Storing MOP . . . . . . . . . . . . 3 52 2.1. End to End Path Establishment Indication . . . . . . . . 4 53 2.2. Target node is unaware if it needs to retry the DAO . . . 5 54 2.3. RPL node acting as router for RULs . . . . . . . . . . . 6 55 3. Requirements for Root-ACK handling in Storing MOP . . . . . . 6 56 4. Root-ACK from Root . . . . . . . . . . . . . . . . . . . . . 6 57 4.1. Transit Information Option update in DAO message . . . . 6 58 4.2. Root sends Root-ACK addressed to Target . . . . . . . . . 7 59 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 60 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 61 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 62 7.1. Normative References . . . . . . . . . . . . . . . . . . 8 63 7.2. Informative References . . . . . . . . . . . . . . . . . 8 64 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 66 1. Introduction 68 RPL [RFC6550] specifies a proactive distance-vector routing scheme 69 designed for LLNs (Low Power and Lossy Networks). RPL enables the 70 network to be formed as a DODAG and supports storing mode and non- 71 storing mode of operations. Non-storing mode allows reduced memory 72 resource usage on the nodes by allowing non-BR nodes to operate 73 without managing a routing table and involves use of source routing 74 by the Root to direct the traffic along a specific path. In storing 75 mode of operation the routing happens on hop-by-hop basis and 76 intermediate routers need to maintain routing tables. 78 DAO messaging helps to install downstream routing paths in the DODAG. 79 DAOs are generated on hop-by-hop basis. DAO may contain multiple RPL 80 Control Options. The Target Option identifies the address prefix for 81 which the route has to be installed and the corresponding Transit 82 Information Option identifies the parameters (such as lifetime, 83 freshness-counter, etc) for the target. The DAO base object contains 84 the 'K' flag indicating that a DAO-ACK is sought by the sender. The 85 DAO, DAO-ACK progresses on hop-by-hop basis all the way till Root. 86 In non-storing MOP, the DAO from the target node is directly 87 addressed to the Root and the Root responds with a DAO-ACK indicating 88 path establishment status. However, in storing MOP, the DAO-ACK is 89 immediately sent by the upstream parent. Thus in case of storing 90 MOP, the target node cannot rely on DAO-ACK as an indication that the 91 end to end (from the target node to Root) path has been established. 93 This draft highlights various issues with RPL DAO-ACK handling in 94 Storing MOP. Section 4 of [I-D.ietf-roll-rpl-observations] provides 95 more context to the problem statement. The draft provides 96 requirements to solve the issues and provides an updates to RFC6550 97 based on these requirements. 99 1.1. Requirements Language and Terminology 101 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 102 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 103 document are to be interpreted as described in RFC 2119 [RFC2119]. 105 MOP: Mode of Operation 107 NS-MOP: RPL Non-Storing Mode of Operation 109 S-MOP: RPL Storing Mode of Operation 111 Root-ACK: The Root-ACK syntax is same as DAO-ACK except that the 112 Root-ACK is addressed directly to the peer who owns the target 113 prefix. DAO-ACK in contrast is always sent using link-local IPv6 114 address in storing MOP. 116 DelayDAO: Section 9.5 of RFC6550 introduces a delay before the DAO 117 transmission is initiated. 119 TIO: (Transit Information Option) Section 6.7.8 of RFC6550. TIO is 120 an option usually carried in DAO message and augments control 121 information for the advertised Target. 123 RUL: (RPL Unaware Leaf) [I-D.ietf-roll-unaware-leaves] 125 This document uses terminology described in [RFC6550]. 127 2. Problems with DAO-ACK in Storing MOP 129 Consider the following topology for the subsequent description: 131 (Root) 132 | 133 | 134 | 135 (A) 136 / \ 137 / \ 138 / \ 139 (B) -(C) 140 | / | 141 | / | 142 | / | 143 (D)- (E) 144 \ ; 145 \ ; 146 \ ; 147 (F) 148 / \ 149 / \ 150 / \ 151 (G) (H) 153 Figure 1: Sample topology 155 2.1. End to End Path Establishment Indication 157 Nodes need to know whether the end to end path till the Root has been 158 established before they can initiate application traffic. In case of 159 NS-MOP, the DAO is addressed to the Root from the Target node and the 160 Root sends DAO-ACK directly addressed back to the target node. Thus 161 in case of NS-MOP, the node can make use of this DAO-ACK as an 162 indication whether the necessary routes have been installed. 163 However, in case of Storing MOP, the DAO/DAO-ACK signaling happens at 164 every hop. 166 Non-Storing MOP 168 | D ======== B ======== A ======== (Root) 169 | ---------------DAO------------> 170 | <-----------DAO-ACK------------ 171 | 172 V 173 time 175 Figure 2: NS-MOP DAO/DAO-ACK handling 177 Storing MOP 179 | D ======== B ======== A ======== (Root) 180 | ---DAO---> 181 | <-DAO-ACK- 182 | ---DAO---> 183 | <-DAO-ACK- 184 | ---DAO---> 185 | <-DAO-ACK- 186 V 187 time 189 Figure 3: Storing MOP DAO/DAO-ACK handling 191 Note that in Storing-MOP, the DAO/DAO-ACK signaling happens on hop- 192 by-hop basis and a DelayDAO timer is used before intermediate 6LRs 193 generate the DAO. This would mean that the DAO reaching the Root may 194 take several seconds. The target node should not generate the 195 application traffic unless the end to end path is established. 197 Consider Figure 1, when node D sends a DAO, the node B receives the 198 DAO and instantly sends back DAO-ACK. Node B then subsequently 199 generates the DAO with Target as Node D and sends it to node A. The 200 DAO with Target as Node D may take time (since the DAO is scheduled 201 with DelayDAO timer by every node) to finally reach the Root at which 202 point the end to end path is established. There is no way for node D 203 to know when the end to end path is established. This information is 204 needed for node D to initiate its application traffic. Initiating 205 application traffic prior to this might almost certainly lead to 206 application packet retries causing congestion in the network. 208 2.2. Target node is unaware if it needs to retry the DAO 210 It is possible that the intermediate 6LR goes down while attempting 211 to generate DAO on behalf of the target node. In this case, the 212 target node has no way of knowing to retry the DAO, in which case the 213 route installation may not happen until the target node's DAO 214 lifetime expires. 216 Consider Figure 1, assume that node A was generating DAO with Target 217 node D and sending it to Root. Node A reboots before attempting to 218 send DAO to Root. Node A has already sent DAO-ACK downstream to node 219 B. In this case, the target node D is not aware that sending DAO has 220 failed somewhere upstream. Note that as per RFC6550 upstream DAO is 221 scheduled based on DelayDAO but DAO-ACK is sent instantaneously on 222 DAO reception from downstream node. 224 2.3. RPL node acting as router for RULs 226 An RPL node may act as a router for RPL unware leaves as described in 227 [I-D.ietf-roll-unaware-leaves]. Ideally an RPL node should start 228 accepting RULs solicitation only after making sure that it has 229 established itself in the network first. In Storing-MOP, there is no 230 way to ascertain this. 232 3. Requirements for Root-ACK handling in Storing MOP 234 Following are the requirements: 236 Indicate end to end path establishment The Target node must know 237 when to initiate the application traffic based on end to end path 238 establishment. 240 Handle multiple targets in DAOs A DAO message may contain multiple 241 Target Options. The Root-ACK mechanism must handle multiple 242 targets in DAO. 244 Handle DAOs with address prefix RPL DAO Target Option may contain an 245 address prefix i.e., not the full address. 247 Provide suitable way for target node to retry The Target node must 248 have a way to know and retry the DAO in case the DAO transmission 249 fails enroute. 251 Backward compatible with current DAO-ACK The current per hop DAO-ACK 252 must function as it is. Legacy nodes should be able to operate 253 without any changes. 255 4. Root-ACK from Root 257 The draft defines a way for the RPL Root to send the Root-ACK back 258 directly addressed to the Target node. The Target node can receive 259 the Root-ACK directly thus getting an indication that the end to end 260 path till the Root has been successfully established. The Root-ACK 261 uses the same syntax and message code as DAO-ACK. The only 262 difference is that the Root-ACK is directly addressed to the Target 263 node who owns the advertised prefix in the Target Option. 265 4.1. Transit Information Option update in DAO message 267 The Target node indicates that it wishes to receive Root-ACK directly 268 from Root by setting the newly defined 'K' flag in Transit 269 Information Option. 271 0 1 2 3 272 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 1 273 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 274 | Type = 0x06 | Option Length |E|I|K| Flags | Path Control | 275 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 276 | Path Sequence | Path Lifetime | 277 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 279 Figure 4: Updated Transit Information Option (New K flag added) 281 The K flag indicates that the Root of the RPLInstance MUST send a 282 Root-ACK directly to the target node. 284 4.2. Root sends Root-ACK addressed to Target 286 On receiving a DAO with Transit Information Option with 'K' flag set, 287 the Root MUST respond with a Root-ACK immediately to the address 288 extracted from the corresponding Target Option. 290 The Root-ACK MUST contain the Transit Information Option with 291 parameters copied from the DAO's Transit Information Option based on 292 which this Root-ACK was generated. The PathSequence in the Transit 293 Information Option helps the Target node to identify for which DAO it 294 generated it has received the Root-ACK. The DAOSequence in the base 295 Root-ACK(DAO-ACK) base object is ignored by the Target node. 297 5. IANA Considerations 299 IANA is requested to allocate bit 2 from the Transit Information 300 Option Flags registry for the 'K' flag (Section 4.1). 302 6. Security Considerations 304 This node introduces a new flag in response to which the Root of the 305 DODAG would send a Root-ACK which serves as an indication for the 306 target node that the end to end route/path is established. The Root- 307 ACK indication eventually would be used by the end node for 308 application layer processing such as initiating the application 309 traffic. A malicious node could generate the Root-ACK pre-maturely 310 i.e, before the end-to-end path is established and cause the 311 application to do some processing pre-maturely. However, the 312 application layer would always account for application layer failures 313 and thus shouldn't result in any security issues. This could result 314 in more control overhead which is currently the case where nodes do 315 not support this specification. 317 A malicious 6LR or 6LN could set the 'K' flag indicating the Root to 318 send a Root-ACK. The Root would generate a Root-ACK for the 319 indicated target. The Root need not keep any additional state for 320 handling the 'K' flag. 322 This document assumes that the security mechanisms as defined in 323 [RFC6550] are followed, which means that all the nodes are part of 324 the RPL network because they have the required credentials. A non- 325 secure RPL network needs to take into consideration the risks 326 highlighted in this section as well as those highlighted in 327 [RFC6550]. 329 7. References 331 7.1. Normative References 333 [I-D.ietf-roll-unaware-leaves] 334 Thubert, P. and M. Richardson, "Routing for RPL Leaves", 335 draft-ietf-roll-unaware-leaves-23 (work in progress), 336 November 2020. 338 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 339 Requirement Levels", BCP 14, RFC 2119, 340 DOI 10.17487/RFC2119, March 1997, 341 . 343 [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., 344 Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, 345 JP., and R. Alexander, "RPL: IPv6 Routing Protocol for 346 Low-Power and Lossy Networks", RFC 6550, 347 DOI 10.17487/RFC6550, March 2012, 348 . 350 7.2. Informative References 352 [I-D.ietf-roll-rpl-observations] 353 Jadhav, R., Sahoo, R., and Y. Wu, "RPL Observations", 354 draft-ietf-roll-rpl-observations-04 (work in progress), 355 May 2020. 357 Author's Address 359 Rahul Arvind Jadhav (editor) 360 Marathahalli 361 Bangalore, Karnataka 560037 362 India 364 Email: rahul.ietf@gmail.com