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Jadhav, Ed. 3 Internet-Draft Huawei 4 Intended status: Standards Track June 5, 2020 5 Expires: December 7, 2020 7 RPL Storing Root-ACK 8 draft-jadhav-roll-storing-rootack-01 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 December 7, 2020. 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 . . . . . . . . . . . . . . . . . . . . . . . . . 7 62 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 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. The draft provides requirements to solve the issues and 95 provides an updates to RFC6550 based on these requirements. 97 1.1. Requirements Language and Terminology 99 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 100 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 101 document are to be interpreted as described in RFC 2119 [RFC2119]. 103 MOP: Mode of Operation 105 NS-MOP: RPL Non-Storing Mode of Operation 107 S-MOP: RPL Storing Mode of Operation 109 Root-ACK: The Root-ACK syntax is same as DAO-ACK except that the 110 Root-ACK is addressed directly to the peer who owns the target 111 prefix. DAO-ACK in contrast is always sent using link-local IPv6 112 address in storing MOP. 114 DelayDAO: Section 9.5 of RFC6550 introduces a delay before the DAO 115 transmission is initiated. 117 TIO: (Transit Information Option) Section 6.7.8 of RFC6550. TIO is 118 an option usually carried in DAO message and augments control 119 information for the advertised Target. 121 RUL: (RPL Unaware Leaf) [I-D.ietf-roll-unaware-leaves] 123 This document uses terminology described in [RFC6550]. 125 2. Problems with DAO-ACK in Storing MOP 127 Consider the following topology for the subsequent description: 129 (Root) 130 | 131 | 132 | 133 (A) 134 / \ 135 / \ 136 / \ 137 (B) -(C) 138 | / | 139 | / | 140 | / | 141 (D)- (E) 142 \ ; 143 \ ; 144 \ ; 145 (F) 146 / \ 147 / \ 148 / \ 149 (G) (H) 151 Figure 1: Sample topology 153 2.1. End to End Path Establishment Indication 155 Nodes need to know whether the end to end path till the Root has been 156 established before they can initiate application traffic. In case of 157 NS-MOP, the DAO is addressed to the Root from the Target node and the 158 Root sends DAO-ACK directly addressed back to the target node. Thus 159 in case of NS-MOP, the node can make use of this DAO-ACK as an 160 indication whether the necessary routes have been installed. 161 However, in case of Storing MOP, the DAO/DAO-ACK signaling happens at 162 every hop. 164 Non-Storing MOP 166 | D ======== B ======== A ======== (Root) 167 | ---------------DAO------------> 168 | <-----------DAO-ACK------------ 169 | 170 V 171 time 173 Figure 2: NS-MOP DAO/DAO-ACK handling 175 Storing MOP 177 | D ======== B ======== A ======== (Root) 178 | ---DAO---> 179 | <-DAO-ACK- 180 | ---DAO---> 181 | <-DAO-ACK- 182 | ---DAO---> 183 | <-DAO-ACK- 184 V 185 time 187 Figure 3: Storing MOP DAO/DAO-ACK handling 189 Note that in Storing-MOP, the DAO/DAO-ACK signaling happens on hop- 190 by-hop basis and a DelayDAO timer is used before intermediate 6LRs 191 generate the DAO. This would mean that the DAO reaching the Root may 192 take several seconds. The target node should not generate the 193 application traffic unless the end to end path is established. 195 Consider Figure 1, when node D sends a DAO, the node B receives the 196 DAO and instantly sends back DAO-ACK. Node B then subsequently 197 generates the DAO with Target as Node D and sends it to node A. The 198 DAO with Target as Node D may take time (since the DAO is scheduled 199 with DelayDAO timer by every node) to finally reach the Root at which 200 point the end to end path is established. There is no way for node D 201 to know when the end to end path is established. This information is 202 needed for node D to initiate its application traffic. Initiating 203 application traffic prior to this might almost certainly lead to 204 application packet retries causing congestion in the network. 206 2.2. Target node is unaware if it needs to retry the DAO 208 It is possible that the intermediate 6LR goes down while attempting 209 to generate DAO on behalf of the target node. In this case, the 210 target node has no way of knowing to retry the DAO, in which case the 211 route installation may not happen until the target node's DAO 212 lifetime expires. 214 Consider Figure 1, assume that node A was generating DAO with Target 215 node D and sending it to Root. Node A reboots before attempting to 216 send DAO to Root. Node A has already sent DAO-ACK downstream to node 217 B. In this case, the target node D is not aware that sending DAO has 218 failed somewhere upstream. Note that as per RFC6550 upstream DAO is 219 scheduled based on DelayDAO but DAO-ACK is sent instantaneously on 220 DAO reception from downstream node. 222 2.3. RPL node acting as router for RULs 224 An RPL node may act as a router for RPL unware leaves as described in 225 [I-D.ietf-roll-unaware-leaves]. Ideally an RPL node should start 226 accepting RULs solicitation only after making sure that it has 227 established itself in the network first. In Storing-MOP, there is no 228 way to ascertain this. 230 3. Requirements for Root-ACK handling in Storing MOP 232 Following are the requirements: 234 Indicate end to end path establishment The Target node must know 235 when to initiate the application traffic based on end to end path 236 establishment. 238 Handle multiple targets in DAOs A DAO message may contain multiple 239 Target Options. The Root-ACK mechanism must handle multiple 240 targets in DAO. 242 Handle DAOs with address prefix RPL DAO Target Option may contain an 243 address prefix i.e., not the full address. 245 Provide suitable way for target node to retry The Target node must 246 have a way to know and retry the DAO in case the DAO transmission 247 fails enroute. 249 Backward compatible with current DAO-ACK The current per hop DAO-ACK 250 must function as it is. Legacy nodes should be able to operate 251 without any changes. 253 4. Root-ACK from Root 255 The draft defines a way for the RPL Root to send the Root-ACK back 256 directly addressed to the Target node. The Target node can receive 257 the Root-ACK directly thus getting an indication that the end to end 258 path till the Root has been successfully established. The Root-ACK 259 uses the same syntax and message code as DAO-ACK. The only 260 difference is that the Root-ACK is directly addressed to the Target 261 node who owns the advertised prefix in the Target Option. 263 4.1. Transit Information Option update in DAO message 265 The Target node indicates that it wishes to receive Root-ACK directly 266 from Root by setting the newly defined 'K' flag in Transit 267 Information Option. 269 0 1 2 3 270 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 1 271 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 272 | Type = 0x06 | Option Length |E|I|K| Flags | Path Control | 273 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 274 | Path Sequence | Path Lifetime | 275 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 277 Figure 4: Updated Transit Information Option (New K flag added) 279 The K flag indicates that the Root of the RPLInstance MUST send a 280 Root-ACK directly to the target node. 282 4.2. Root sends Root-ACK addressed to Target 284 On receiving a DAO with Transit Information Option with 'K' flag set, 285 the Root MUST respond with a Root-ACK immediately to the address 286 extracted from the corresponding Target Option. 288 The Root-ACK MUST contain the Transit Information Option with 289 parameters copied from the DAO's Transit Information Option based on 290 which this Root-ACK was generated. The PathSequence in the Transit 291 Information Option helps the Target node to identify for which DAO it 292 generated it has received the Root-ACK. The DAOSequence in the base 293 Root-ACK(DAO-ACK) base object is ignored by the Target node. 295 5. IANA Considerations 297 IANA is requested to allocate bit 2 from the Transit Information 298 Option Flags registry for the 'K' flag (Section 4.1). 300 6. Security Considerations 302 7. References 304 7.1. Normative References 306 [I-D.ietf-roll-unaware-leaves] 307 Thubert, P. and M. Richardson, "Routing for RPL Leaves", 308 draft-ietf-roll-unaware-leaves-15 (work in progress), 309 April 2020. 311 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 312 Requirement Levels", BCP 14, RFC 2119, 313 DOI 10.17487/RFC2119, March 1997, 314 . 316 [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., 317 Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, 318 JP., and R. Alexander, "RPL: IPv6 Routing Protocol for 319 Low-Power and Lossy Networks", RFC 6550, 320 DOI 10.17487/RFC6550, March 2012, 321 . 323 7.2. Informative References 325 [Perlman83] 326 Perlman, R., "Fault-Tolerant Broadcast of Routing 327 Information", North-Holland Computer Networks, Vol.7, 328 December 1983. 330 Author's Address 332 Rahul Arvind Jadhav (editor) 333 Huawei 334 Whitefield, 335 Bangalore, Karnataka 560037 336 India 338 Email: rahul.ietf@gmail.com