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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(See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: draft-ietf-roll-rpl has been published as RFC 6550 == Outdated reference: draft-ietf-roll-terminology has been published as RFC 7102 ** Downref: Normative reference to an Informational draft: draft-ietf-roll-terminology (ref. 'I-D.ietf-roll-terminology') == Outdated reference: draft-ietf-roll-of0 has been published as RFC 6552 Summary: 1 error (**), 0 flaws (~~), 4 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 ROLL P. Thubert, Ed. 3 Internet-Draft Cisco Systems 4 Intended status: Standards Track November 16, 2011 5 Expires: May 19, 2012 7 RPL adaptation for asymmetrical links 8 draft-thubert-roll-asymlink-01 10 Abstract 12 The Routing Protocol for Low Power and Lossy Networks defines a 13 generic Distance Vector protocol for Low Power and Lossy Networks, 14 many of which exhibit strongly asymmetrical characteristics. This 15 draft proposes an extension for that optimizes RPL operations whereby 16 upwards and downwards direction-optimized RPL instances are 17 associated. 19 Requirements Language 21 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 22 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 23 document are to be interpreted as described in RFC 2119 [RFC2119]. 25 Status of this Memo 27 This Internet-Draft is submitted in full conformance with the 28 provisions of BCP 78 and BCP 79. 30 Internet-Drafts are working documents of the Internet Engineering 31 Task Force (IETF). Note that other groups may also distribute 32 working documents as Internet-Drafts. The list of current Internet- 33 Drafts is at http://datatracker.ietf.org/drafts/current/. 35 Internet-Drafts are draft documents valid for a maximum of six months 36 and may be updated, replaced, or obsoleted by other documents at any 37 time. It is inappropriate to use Internet-Drafts as reference 38 material or to cite them other than as "work in progress." 40 This Internet-Draft will expire on May 19, 2012. 42 Copyright Notice 44 Copyright (c) 2011 IETF Trust and the persons identified as the 45 document authors. All rights reserved. 47 This document is subject to BCP 78 and the IETF Trust's Legal 48 Provisions Relating to IETF Documents 49 (http://trustee.ietf.org/license-info) in effect on the date of 50 publication of this document. Please review these documents 51 carefully, as they describe your rights and restrictions with respect 52 to this document. Code Components extracted from this document must 53 include Simplified BSD License text as described in Section 4.e of 54 the Trust Legal Provisions and are provided without warranty as 55 described in the Simplified BSD License. 57 Table of Contents 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3 61 3. The asymmetrical link problem . . . . . . . . . . . . . . . . . 4 62 4. Solution Overview . . . . . . . . . . . . . . . . . . . . . . . 4 63 5. Modified DODAG Information Object (DIO) . . . . . . . . . . . . 5 64 6. Operations . . . . . . . . . . . . . . . . . . . . . . . . . . 6 65 7. Backward compatibility . . . . . . . . . . . . . . . . . . . . 7 66 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8 67 9. Security Considerations . . . . . . . . . . . . . . . . . . . . 8 68 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 69 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 70 11.1. Normative References . . . . . . . . . . . . . . . . . . . 8 71 11.2. Informative References . . . . . . . . . . . . . . . . . . 9 72 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9 74 1. Introduction 76 The IETF ROLL Working Group has defined application-specific routing 77 requirements for a Low Power and Lossy Network (LLN) routing 78 protocol, specified in [RFC5548], [RFC5673], [RFC5826], and 79 [RFC5867], many of which explicitly or implicitly refer to links with 80 asymmetrical properties. 82 Upon those requirements, the Routing Protocol for Low Power and Lossy 83 Network [I-D.ietf-roll-rpl] was designed as a platform that can be 84 extended by further specifications or guidances, by adding new 85 metrics, Objective Functions, or additional options. 87 RPL forms Destination Oriented Directed Acyclic Graphs (DODAGs) 88 within instances of the protocol. Each instance is associated with 89 an Objective Function that is designed to solve the problem that is 90 addressed by that instance. 92 In one hand, RPL requires bi-directional links for the control, but 93 on the other, there is no requirement that the properties of a link 94 are the same in both directions. In fact, a perfect symmetry is 95 rarely present in Low Power and Lossy Networks (LLNs), whether links 96 are based on radios or power-line. 98 Some initial implementations require that the quality of both 99 directions of a link is evaluated as very good so that the link can 100 be used for control and data in both directions. This eliminates 101 asymmetrical links that are very good in one direction, but only good 102 enough for scarce activity in the other direction. 104 In practice, a DAG that is built to optimize upwards traffic is 105 generally not congruent with a DAG that is built to optimize 106 downwards traffic. This is why this specification is designed to 107 enable asymmetrical routing DAGs that are bound together to get the 108 maximum benefits of all types of bi-directional links. 110 2. Terminology 112 The terminology used in this document is consistent with and 113 incorporates that described in `Terminology in Low power And Lossy 114 Networks' [I-D.ietf-roll-terminology] and [I-D.ietf-roll-rpl]. 116 The term upwards qualifies a link, a DODAG or an instance that is 117 optimal for sending traffic in the general direction of the root, 118 though may be usable but suboptimal for traffic coming form the 119 direction of the root. The term downwards qualifies the same words 120 for the opposite direction. 122 The term parenting applied to instances refers to the directional 123 association of two instances. The meta graph formed by parented 124 instances must be a DAG. Traffic may be transferred from an instance 125 onto a parent instance under specified circumstances. 127 The draft also uses the following terminology: 129 bi-directional: A link is bi-directional when traffic confirmed 130 possible in both direction, in a fashion that is sufficient to 131 operate RPL control. 133 asymmetric: A link is assymetric if it is bi-directional, but 134 exhibits important differences in link characteristics for both 135 directions. 137 3. The asymmetrical link problem 139 4. Solution Overview 141 With the core RPL specification, [I-D.ietf-roll-rpl] each instance is 142 a separate routing topology, and packets must be forwarded within the 143 same topology / same instance. One direct consequence of that design 144 choice is that a topology must be very good for both upwards and 145 downwards traffic; otherwise, traffic between two nodes in the 146 instance may suffer. 148 RPL is designed to operate on bi-directional links. When the link 149 properties do not widely differ between the upwards and the downwards 150 directions, a single DAG is adequate as the routing topology for both 151 upwards and downwards traffic. In that case, an asymmetrical link, 152 that can only be used for traffic in one direction, can not 153 participate to the routing topology. This results in an unoptimized 154 use of bandwidth and/or a reduction of the possible path diversity. 156 This issue can be addressed with [I-D.ietf-roll-rpl], by constructing 157 two DAGs, one that is used for upwards traffic and one that is used 158 for downwards traffic. This solves the issue for all traffic that 159 transits through the root of the DODAG, whether it is originated in 160 the DODAG going out or is injected into the DODAG from the outside, 161 since in both cases a packet will mostly use the asymmetric links in 162 the appropriate direction. 164 OTOH, with RPL as it is specified, a packet follows the topology that 165 is generated for its instance all the way through a DAG, and 166 transferring a packet from an instance to another is not permitted. 167 As a result, the 2 DAGs solution would penalize Peer to peer traffic 168 that would have to go through the root in order to leave the upward 169 instance and then reenter at the root in order to join the downwards 170 instance. This is not an issue in non-storing mode since the packet 171 has to go through the root to load the routing header downwards 172 anyway, but going through the root stretches the path in storing 173 mode. 175 In order to benefit from both instances and yet avoid extra stretch 176 through the root in storing mode, it is required to extend RPL rules 177 so as to allow traffic to be transferred from one instance to the 178 next before reaching the root on the way up. This document specifies 179 conditions under which 2 instances can be bound together so that a 180 node may transfer traffic from an instance onto another. 182 It can be noted at this point that with [I-D.ietf-roll-rpl], traffic 183 that goes down does not generally go back up again, whereas P2P 184 traffic within a DODAG might go up to a common parent and then down 185 to the destination. In terms of instance relationship, this means 186 that when an upwards and a downwards instances are bound together, 187 traffic from the former may be transferred to the latter, but not the 188 other way around. In other words, there is an order, a parent-child 189 relationship, between the two instances. 191 Additionally, if there is no next-hop for a packet going down within 192 the instance, then with [I-D.ietf-roll-rpl] the packet will be 193 dropped or sent back with an error along the wrong direction of an 194 asymmetric link. In order to avoid those unwanted behaviors, it is 195 tempting though inefficient to lower the constraints that are applied 196 to build the topology. It can be more efficient to actually keep the 197 constraints as they should be, but, instead, enable a less 198 constrained, more spanning, fall-back topology into which traffic can 199 be transferred. 201 For that reason, this solution allows for more complex instance 202 relationships than plain child-parent associations. In order to 203 avoids loops which could be created when transferring packets from 204 one instance to the next, this solution requires that the instances 205 be themselves organized as a Directed Acyclic Graph, and enforce that 206 inter-DAG transfers occur only within that meta DAG. 208 5. Modified DODAG Information Object (DIO) 210 The DODAG Information Object [I-D.ietf-roll-rpl] carries information 211 that allows a node to discover a RPL Instance, learn its 212 configuration parameters, select a DODAG parent set, and maintain the 213 DODAG. This specification defines a new flag bit to indicate that 214 the DAG is directional. 216 0 1 2 3 217 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 1 218 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 219 | RPLInstanceID |Version Number | Rank | 220 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 221 |G|D| MOP | Prf | DTSN | Flags | Reserved | 222 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 223 | | 224 + + 225 | | 226 + DODAGID + 227 | | 228 + + 229 | | 230 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 231 | Option(s)... 232 +-+-+-+-+-+-+-+-+ 234 Figure 1: The DIO Base Object 236 Directional (D): The Directional (D) flag is set to indicate that 237 the instance is intended for directional operation, and reset 238 otherwise. When it is set, a MOP of 0 indicates the upwards 239 direction whereas any other value specified in 240 [I-D.ietf-roll-rpl] indicates downwards. All other values of 241 MOP will be considered downwards unless explicitly specified 242 otherwise. 244 6. Operations 246 This specification allows an organization of Instances as follows: 248 Instances MUST be organized as a Directed Acyclic Graph. This 249 information MUST be commissioned into the devices so they know 250 both which instances they should participate in, and which 251 direction of transfer is allowed between instances. 253 A spanning instance using OF0 [I-D.ietf-roll-of0] MAY be used as 254 root in that instance DAG. 256 This specification defines a new bit in the RPL [I-D.ietf-roll-rpl] 257 DODAG Information Object (DIO) with the Directional (D) flag that 258 indicates a directional operation for a given instance. An 259 implementation that does not support that new bit will not be able to 260 propagate it. 262 In case of a directional operation, 263 The direction is indicated by the MOP field, a MOP of 0 means 264 upwards and otherwise is downwards. 266 Links are still REQUIRED to allow bidirectional operations 268 Only the metrics that correspond to the DAG direction are used for 269 the parent selection. 271 An upward instance SHOULD install routes that lead to the root and 272 beyond - typically the default route. 274 A downwards instance MAY ONLY install more specific routes that 275 are injected by nodes in the DODAG through the DAO process. 277 P2P operations are achieved by associating a child upwards 278 instance with a parent downwards instance. 280 A packet MUST NOT be transferred from a parent instance to a child 281 instance. 283 A packet MAY be transferred from a child instance to its parent 284 instance if and only if the child instance does not provide a 285 route to the destination, or the parent instance provides a more 286 specific route (longer match) to the destination. 288 Transferring from an upwards instance to a downwards instance if 289 generally desirable. Other forms of transfers are generally not 290 desirable. Policies MAY be put in place to ovreride that general 291 guidance. 293 7. Backward compatibility 295 An OF is generally designed to compute a Rank of a directional link 296 in a fashion that is diffent from a bidirectional link, and in 297 particular will not use the same metrics and thusobtain different 298 ranks for a same situation. For that reason, it is important that 299 the OF is aware that an instance is supposed to define a directional 300 DODAG, and it is RECOMMENDED that only devices that support 301 directional DODAGs are allowed in a directional instance. 303 It might happen that for some purposes like higher availability, an 304 implementation that does not support directional links is 305 administratively allowed to join a directional DODAG. In that case, 306 the extension of the DODAG that starts at that device will not be 307 directional, but the instance will still be functional. 309 In that case, it might also happen that a device that supports 310 directional DODAGs per this specification sees candidate neighbors 311 that expose the Directional flag and some others that do not. An OF 312 that supports directional links SHOULD favor directional links over 313 non directional links, in a fashion that is to be specified with the 314 OF. In the case of OF0 [I-D.ietf-roll-of0], the 'D' flag should be 315 accounted for before the computation of item 8 in the "Selection Of 316 The Preferred Parent" section 4.2.1., that is before Ranks and be 317 calculated and compared. 319 8. IANA Considerations 321 This specification requires that a bit in DIO be assigned to indicate 322 directional link operations as specified in section 324 9. Security Considerations 326 Security Considerations for this proposal are to be developed in 327 accordance with recommendations laid out in, for example, 328 [I-D.tsao-roll-security-framework]. 330 10. Acknowledgements 332 The author wishes to recognize Richard Kelsey, JP Vasseur, Tom 333 Phinney, Robert Assimiti, Don Sturek, Thomas Clausen and Yoav Ben- 334 Yehezkel for their various contributions. 336 11. References 338 11.1. Normative References 340 [I-D.ietf-roll-rpl] 341 Winter, T., Thubert, P., Brandt, A., Clausen, T., Hui, J., 342 Kelsey, R., Levis, P., Pister, K., Struik, R., and J. 343 Vasseur, "RPL: IPv6 Routing Protocol for Low power and 344 Lossy Networks", draft-ietf-roll-rpl-19 (work in 345 progress), March 2011. 347 [I-D.ietf-roll-terminology] 348 Vasseur, J., "Terminology in Low power And Lossy 349 Networks", draft-ietf-roll-terminology-06 (work in 350 progress), September 2011. 352 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 353 Requirement Levels", BCP 14, RFC 2119, March 1997. 355 11.2. Informative References 357 [I-D.ietf-roll-of0] 358 Thubert, P., "RPL Objective Function Zero", 359 draft-ietf-roll-of0-20 (work in progress), September 2011. 361 [I-D.tsao-roll-security-framework] 362 Tsao, T., Alexander, R., Daza, V., and A. Lozano, "A 363 Security Framework for Routing over Low Power and Lossy 364 Networks", draft-tsao-roll-security-framework-02 (work in 365 progress), March 2010. 367 [RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel, 368 "Routing Requirements for Urban Low-Power and Lossy 369 Networks", RFC 5548, May 2009. 371 [RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney, 372 "Industrial Routing Requirements in Low-Power and Lossy 373 Networks", RFC 5673, October 2009. 375 [RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation 376 Routing Requirements in Low-Power and Lossy Networks", 377 RFC 5826, April 2010. 379 [RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen, 380 "Building Automation Routing Requirements in Low-Power and 381 Lossy Networks", RFC 5867, June 2010. 383 Author's Address 385 Pascal Thubert (editor) 386 Cisco Systems 387 Village d'Entreprises Green Side 388 400, Avenue de Roumanille 389 Batiment T3 390 Biot - Sophia Antipolis 06410 391 FRANCE 393 Phone: +33 497 23 26 34 394 Email: pthubert@cisco.com