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Thubert 3 Internet-Draft Cisco 4 Intended status: Standards Track February 7, 2008 5 Expires: August 10, 2008 7 LoWPAN simple fragment Recovery 8 draft-thubert-lowpan-simple-fragment-recovery-00 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on August 10, 2008. 35 Copyright Notice 37 Copyright (C) The IETF Trust (2008). 39 Abstract 41 Considering that 6LoWPAN packets can be as large as 2K bytes and that 42 an 802.15.4 frame with security will carry in the order of 80 bytes 43 of effective payload, a packet might end up fragmented into as many 44 as 25 fragments at the 6LoWPAN shim layer. If a single one of those 45 fragments is lost in transmission, all fragments must be resent, 46 further contributing to the congestion that might have caused the 47 initial packet loss. This draft introduces a simple protocol to 48 recover individual fragments between 6LoWPAN endpoints. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 54 3. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 4 55 4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4 56 5. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 57 6. New Dispatch types and headers . . . . . . . . . . . . . . . . 6 58 6.1. Recoverable Fragment Dispatch type and Header . . . . . . 7 59 6.2. Fragment Acknowledgement Dispatch type and Header . . . . 7 60 7. Security Considerations . . . . . . . . . . . . . . . . . . . 8 61 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 62 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 63 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 64 10.1. Normative References . . . . . . . . . . . . . . . . . . . 8 65 10.2. Informative References . . . . . . . . . . . . . . . . . . 8 66 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9 67 Intellectual Property and Copyright Statements . . . . . . . . . . 10 69 1. Introduction 71 Considering that 6LoWPAN packets can be as large as 2K bytes and that 72 a 802.15.4 frame with security will carry in the order of 80 bytes of 73 effective payload, a packet might be fragmented into about 25 74 fragments at the 6LoWPAN shim layer. This level of fragmentation is 75 much higher than that traditionally experienced over the Internet 76 with IPv4 fragments. At the same time, the use of radios increases 77 the probability of transmission loss and Mesh-Under techniques 78 compound that risk over multiple hops. 80 Past experience with fragmentation has shown that missassociated or 81 lost fragments can lead to poor network behaviour and, eventually, 82 trouble at application layer. The reader might start his research 83 from [I-D.mathis-frag-harmful] and follow the references. That 84 experience led to the definition of the Path MTU discovery [RFC1191] 85 protocol that avoids fragmentation over the Internet. 87 An end-to-end fragment recovery mechanism might be a good complement 88 to a hop-by-hop MAC level recovery with a limited number of retries. 89 This draft introduces a simple protocol to recover individual 90 fragments between 6LoWPAN endpoints. Specifically in the case of 91 UDP, valuable additional information can be found in UDP Usage 92 Guidelines for Application Designers [I-D.ietf-tsvwg-udp-guidelines]. 94 2. Terminology 96 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 97 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 98 document are to be interpreted as described in [RFC2119]. 100 Readers are expected to be familiar with all the terms and concepts 101 that are discussed in "IPv6 over Low-Power Wireless Personal Area 102 Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and 103 Goals" [RFC4919] and "Transmission of IPv6 Packets over IEEE 802.15.4 104 Networks" [RFC4944]. 106 ERP 108 Error Recovery Procedure. 110 LoWPAN endpoints 112 The LoWPAN nodes in charge of generating or expanding a 6LoWPAN 113 header from/to a full IPv6 packet. The LoWPAN endpoints are the 114 points where fragmentation and reassembly take place. 116 3. Rationale 118 There are a number of usages for large packets in Wireless Sensor 119 Networks. Such usages may not be the most typical or represent the 120 largest amount of traffic over the LoWPAN; however, the associated 121 functionality can be critical enough to justify extra care for 122 ensuring effective transport of large packets across the LoWPAN. 124 The list of those usages includes: 126 Towards the LoWPAN node: 128 Packages of Commands: A number of commands or a full 129 configuration can by packaged as a single message to ensure 130 consistency and enable atomic execution or complete roll back. 131 Until such commands are fully received and interpreted, the 132 intended operation will not take effect. 134 Firmware update: For example, a new version of the LoWPAN node 135 software is downloaded from a system manager over unicast or 136 multicast services. Such a reflashing operation typically 137 involves updating a large number of similar 6LoWPAN nodes over 138 a relatively short period of time. 140 From the LoWPAN node: 142 Waveform captures: A number of consecutive samples are measured 143 at a high rate for a short time and then transferred from a 144 sensor to a gateway or an edge server as a single large report. 146 Large data packets: Rich data types might require more than one 147 fragment. 149 Uncontrolled firmware download or waveform upload can easily result 150 in a massive increase of the traffic and saturate the network. When 151 a fragment is lost in transmission, all fragments are resent, further 152 contributing to the congestion that caused the initial loss, and 153 potentially leading to congestion collapse. 155 This saturation may lead to excessive radio interference, or random 156 early discard (leaky bucket) in relaying nodes. Additional queueing 157 and memory congestion may result while waiting for a low power next 158 hop to emerge from its sleeping state. 160 4. Requirements 162 This paper proposes a method to recover individual fragments between 163 LoWPAN endpoints. The method is designed to fit the following 164 requirements of a LoWPAN (with or without a Mesh-Under routing 165 protocol): 167 Controlled latency 169 The ERP mechanism must succeed or give up within the time boundary 170 imposed by the recovery process of the Upper Layer Protocols. 172 Minimimum acknowledgement overhead 174 Because the radio is inherently half duplex, an acknowledgement 175 consumes roughly as many resources as the fragment itself. 177 Support for out-of-order fragment delivery 179 A Mesh-Under load balancing mechanism such as the ISA100 Data Link 180 Layer can introduce out-of-sequence packets. The recovery 181 mechanism must account for packets that appear lost but are 182 actually only delayed over a different path. 184 Optional flow control 186 The aggregation of multiple concurrent flows may lead to the 187 saturation of the radio network and congestion collapse. 189 Backward compatibility 191 A node that implements this draft should be able to communicate 192 with a node that implements [RFC4944]. The current draft assumes 193 that compatibility information about the remote LoWPAN endpoint is 194 obtained by external means. 196 5. Overview 198 The fragmentation/reassembly of a packet must complete within an 199 acceptable overall latency, otherwise the packet expires and must be 200 dropped. This latency must be smaller than Upper Layer Protocol 201 retry values, and smaller than expiration period of the information 202 transported. 204 The sender transfers a controlled number of fragments (possibly all 205 of them) and flags the last fragment of a series with an 206 Acknowledgement request. 208 The sender sets a retry timer for the fragment that carries the 209 Acknowledgement request. That fragment is retransmitted individually 210 upon time out. This is repeated until an Acknowledgement comes back 211 or the packet expires. 213 Upon receipt of an Acknowledgement request, the receiver responds 214 with an Acknowledgement containing a bitmap that indicates which 215 fragments were actually received. The bitmap is a 32bit DWORD, which 216 accommodates up to 32 fragments and is sufficient for the 6LoWPAN 217 MTU. For all n in [0..31], bit n is set to 1 in the bitmap to 218 indicate that fragment n was received, otherwise the bit is set to 0. 219 If any fragment immmediately preceding the acknowledgement request is 220 missing, the receiver MAY intentionally delay its response to allow 221 in-transit fragments to arrive. 223 The sender has either one or no Acknowledgement pending. An 224 Acknowledgement that is not expected or does not acknowledge the 225 pending sequence in the bitmap is a duplicate and is ignored. 227 When a valid Acknowledgement is received, the sender resumes sending 228 fragments and the process is repeated until all fragments are 229 acknowledged or the packet expires. 231 Fragments are sent in a round robin fashion: the sender sends all the 232 fragments for a first time before it retries any lost fragment; lost 233 fragments are retried in sequence, oldest first. This mechanism 234 enables the receiver to acknowledge fragments that were delayed in 235 the network before they are actually retried. 237 It is up to the sender to decide how many fragments are (re)sent 238 before an acknowledgement is received, and the sender can adapt that 239 number to the network conditions. This way, the number of 240 outstanding fragments can be used as a flow control mechanism to 241 protect the network. 243 6. New Dispatch types and headers 245 This specification extends "Transmission of IPv6 Packets over IEEE 246 802.15.4 Networks" [RFC4944] with 3 new dispatch types, for 247 Recoverable Fragments (RFRAG) headers with or without Acknowledgement 248 Request, and for the Acknowledgement back. 250 Pattern Header Type 251 +------------+-----------------------------------------------+ 252 | 11 101000 | RFRAG - Recoverable Fragment | 253 | 11 101001 | RFRAG-AR - RFRAG with Acknowledgement Req | 254 | 11 101010 | RFRAG-ACK - RFRAG Acknowledgement | 255 +------------+-----------------------------------------------+ 257 Figure 1: Additional Dispatch Value Bit Patterns 259 In the following sections, the semantics of "datagram_tag," 260 "datagram_offset" and "datagram_size" and the reassembly process are 261 unchanged from [RFC4944] Section 5.3. "Fragmentation Type and 262 Header." 264 6.1. Recoverable Fragment Dispatch type and Header 266 1 2 3 267 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 1 268 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 269 |1 1 1 0 1 0 0 X|datagram_offset| datagram_tag | 270 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 271 |Sequence | datagram_size | 272 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ X set == Ack Requested 274 Figure 2: Recoverable Fragment Dispatch type and Header 276 X bit 278 When set, the sender requires an Acknowledgement from the receiver 280 Sequence 282 The sequence number of the fragment. Fragments are numbered 283 [0..N] where N is in [0..31]. 285 6.2. Fragment Acknowledgement Dispatch type and Header 287 1 2 3 288 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 1 289 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 290 |1 1 1 0 1 0 1 0| datagram_tag | 291 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 292 | Acknowledgement Bitmap | 293 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+ 294 ^ ^ 295 | | bitmap indicating whether 296 | +-----Fragment with sequence 10 was received 297 +-------------------------Fragment with sequence 00 was received 299 Figure 3: Fragment Acknowledgement Dispatch type and Header 301 Acknowledgement Bitmap 303 Each bit in the Bitmap refers to a particular fragment: bit n set 304 indicates that fragment with sequence n was received, for n in 305 [0..31]. All zeroes means that the fragment was dropped because 306 it corresponds to an obsolete datagram_tag. This happens if the 307 packet was already reassembled and passed to the network upper 308 layer, or the packet expired and was dropped. 310 7. Security Considerations 312 The process of recovering fragments does not appear to create any 313 opening for new threat. 315 8. IANA Considerations 317 Need extensions for formats defined in "Transmission of IPv6 Packets 318 over IEEE 802.15.4 Networks" [RFC4944]. ? Is that IANA ?. 320 9. Acknowledgments 322 The author wishes to thank Jay Werb, Christos Polyzois, Soumitri 323 Kolavennu and Harry Courtice for their contribution and review. 325 10. References 327 10.1. Normative References 329 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, 330 November 1990. 332 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 333 Requirement Levels", BCP 14, RFC 2119, March 1997. 335 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 336 "Transmission of IPv6 Packets over IEEE 802.15.4 337 Networks", RFC 4944, September 2007. 339 10.2. Informative References 341 [I-D.ietf-tsvwg-udp-guidelines] 342 Eggert, L. and G. Fairhurst, "UDP Usage Guidelines for 343 Application Designers", draft-ietf-tsvwg-udp-guidelines-04 344 (work in progress), November 2007. 346 [I-D.mathis-frag-harmful] 347 Mathis, M., "Fragmentation Considered Very Harmful", 348 draft-mathis-frag-harmful-00 (work in progress), 349 July 2004. 351 [RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6 352 over Low-Power Wireless Personal Area Networks (6LoWPANs): 353 Overview, Assumptions, Problem Statement, and Goals", 354 RFC 4919, August 2007. 356 Author's Address 358 Pascal Thubert 359 Cisco Systems 360 Village d'Entreprises Green Side 361 400, Avenue de Roumanille 362 Batiment T3 363 Biot - Sophia Antipolis 06410 364 FRANCE 366 Phone: +33 4 97 23 26 34 367 Email: pthubert@cisco.com 369 Full Copyright Statement 371 Copyright (C) The IETF Trust (2008). 373 This document is subject to the rights, licenses and restrictions 374 contained in BCP 78, and except as set forth therein, the authors 375 retain all their rights. 377 This document and the information contained herein are provided on an 378 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 379 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 380 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 381 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 382 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 383 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 385 Intellectual Property 387 The IETF takes no position regarding the validity or scope of any 388 Intellectual Property Rights or other rights that might be claimed to 389 pertain to the implementation or use of the technology described in 390 this document or the extent to which any license under such rights 391 might or might not be available; nor does it represent that it has 392 made any independent effort to identify any such rights. 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