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Thubert 7 Cisco 8 July 21, 2019 10 6LoWPAN Fragment Forwarding 11 draft-ietf-6lo-minimal-fragment-03 13 Abstract 15 This document provides a simple method to forwarding 6LoWPAN 16 fragments. When employing adaptation layer fragmentation in 6LoWPAN, 17 it may be beneficial for a forwarder not to have to reassemble each 18 packet in its entirety before forwarding it. This has always been 19 possible with the original fragmentation design of RFC4944. This 20 method reduces the latency and increases end-to-end reliability in 21 route-over forwarding. It is the companion to the virtual Reassembly 22 Buffer which is a pure implementation technique. 24 Status of This Memo 26 This Internet-Draft is submitted in full conformance with the 27 provisions of BCP 78 and BCP 79. 29 Internet-Drafts are working documents of the Internet Engineering 30 Task Force (IETF). Note that other groups may also distribute 31 working documents as Internet-Drafts. The list of current Internet- 32 Drafts is at https://datatracker.ietf.org/drafts/current/. 34 Internet-Drafts are draft documents valid for a maximum of six months 35 and may be updated, replaced, or obsoleted by other documents at any 36 time. It is inappropriate to use Internet-Drafts as reference 37 material or to cite them other than as "work in progress." 39 This Internet-Draft will expire on January 22, 2020. 41 Copyright Notice 43 Copyright (c) 2019 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (https://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 56 Table of Contents 58 1. Overview of 6LoWPAN Fragmentation . . . . . . . . . . . . . . 2 59 2. Limits of Per-Hop Fragmentation and Reassembly . . . . . . . 4 60 2.1. Latency . . . . . . . . . . . . . . . . . . . . . . . . . 4 61 2.2. Memory Management and Reliability . . . . . . . . . . . . 4 62 3. Virtual Reassembly Buffer (VRB) Implementation . . . . . . . 5 63 4. Security Considerations . . . . . . . . . . . . . . . . . . . 6 64 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 65 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6 66 7. Informative References . . . . . . . . . . . . . . . . . . . 6 67 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 69 1. Overview of 6LoWPAN Fragmentation 71 The original 6LoWPAN fragmentation is defined in [RFC4944] and it is 72 implicitly defined for use over a single IP hop though possibly 73 multiple Layer-2 hops in a meshed 6LoWPAN Network. Although 74 [RFC6282] updates [RFC4944], it does not redefine 6LoWPAN 75 fragmentation. 77 We use Figure 1 to illustrate 6LoWPAN fragmentation. We assume node 78 A forwards a packet to node B, possibly as part of a multi-hop route 79 between IPv6 source and destination nodes which are neither A nor B. 81 +---+ +---+ 82 ... ---| A |-------------------->| B |--- ... 83 +---+ +---+ 84 # (frag. 5) 86 123456789 123456789 87 +---------+ +---------+ 88 | # ###| |### # | 89 +---------+ +---------+ 90 outgoing incoming 91 fragmentation reassembly 92 buffer buffer 94 Figure 1: Fragmentation at node A, reassembly at node B. 96 Node A starts by compacting the IPv6 packet using the header 97 compression mechanism defined in [RFC6282]. If the resulting 6LoWPAN 98 packet does not fit into a single link-layer frame, node A's 6LoWPAN 99 sublayer cuts it into multiple 6LoWPAN fragments, which it transmits 100 as separate link-layer frames to node B. Node B's 6LoWPAN sublayer 101 reassembles these fragments, inflates the compressed header fields 102 back to the original IPv6 header, and hands over the full IPv6 packet 103 to its IPv6 layer. 105 In Figure 1, a packet forwarded by node A to node B is cut into nine 106 fragments, numbered 1 to 9. Each fragment is represented by the '#' 107 symbol. Node A has sent fragments 1, 2, 3, 5, 6 to node B. Node B 108 has received fragments 1, 2, 3, 6 from node A. Fragment 5 is still 109 being transmitted at the link layer from node A to node B. 111 Conceptually, a reassembly buffer for 6LoWPAN contains: 113 o a datagram_size, 114 o a datagram_tag, associated to the link-layer sender and receiver 115 addresses to which the datagram_tag is local, 116 o the actual packet data from the fragments received so far, in a 117 form that makes it possible to detect when the whole packet has 118 been received and can be processed or forwarded, 119 o a timer that allows discarding a partially reassembled packet 120 after some timeout. 122 A fragmentation header is added to each fragment; it indicates what 123 portion of the packet that fragment corresponds to. Section 5.3 of 124 [RFC4944] defines the format of the header for the first and 125 subsequent fragments. All fragments are tagged with a 16-bit 126 "datagram_tag", used to identify which packet each fragment belongs 127 to. Each datagram can be uniquely identified by the source and final 128 destination link-layer addresses of the frame that carries it, the 129 fragment size and the datagram_tag. Each fragment can be identified 130 within its datagram by the datagram-offset. 132 Node B's typical behavior, per [RFC4944], is as follows. Upon 133 receiving a fragment from node A with a datagram_tag previously 134 unseen from node A, node B allocates a buffer large enough to hold 135 the entire packet. The length of the packet is indicated in each 136 fragment (the datagram_size field), so node B can allocate the buffer 137 even if the first fragment it receives is not fragment 1. As 138 fragments come in, node B fills the buffer. When all fragments have 139 been received, node B inflates the compressed header fields into an 140 IPv6 header, and hands the resulting IPv6 packet to the IPv6 layer. 142 This behavior typically results in per-hop fragmentation and 143 reassembly. That is, the packet is fully reassembled, then 144 (re)fragmented, at every hop. 146 2. Limits of Per-Hop Fragmentation and Reassembly 148 There are at least 2 limits to doing per-hop fragmentation and 149 reassembly. See [ARTICLE] for detailed simulation results on both 150 limits. 152 2.1. Latency 154 When reassembling, a node needs to wait for all the fragments to be 155 received before being able to generate the IPv6 packet, and possibly 156 forward it to the next hop. This repeats at every hop. 158 This may result in increased end-to-end latency compared to a case 159 where each fragment is forwarded without per-hop reassembly. 161 2.2. Memory Management and Reliability 163 Constrained nodes have limited memory. Assuming 1 kB reassembly 164 buffer, typical nodes only have enough memory for 1-3 reassembly 165 buffers. 167 To illustrate this we use the topology from Figure 2, where nodes A, 168 B, C and D all send packets through node E. We further assume that 169 node E's memory can only hold 3 reassembly buffers. 171 +---+ +---+ 172 ... --->| A |------>| B | 173 +---+ +---+\ 174 \ 175 +---+ +---+ 176 | E |--->| F | ... 177 +---+ +---+ 178 / 179 / 180 +---+ +---+ 181 ... --->| C |------>| D | 182 +---+ +---+ 184 Figure 2: Illustrating the Memory Management Issue. 186 When nodes A, B and C concurrently send fragmented packets, all 3 187 reassembly buffers in node E are occupied. If, at that moment, node 188 D also sends a fragmented packet, node E has no option but to drop 189 one of the packets, lowering end-to-end reliability. 191 3. Virtual Reassembly Buffer (VRB) Implementation 193 Virtual Reassembly Buffer (VRB) is the implementation technique 194 described in [I-D.ietf-lwig-6lowpan-virtual-reassembly] in which a 195 forwarder does not reassemble each packet in its entirety before 196 forwarding it. 198 VRB overcomes the limits listed in Section 2. Nodes do not wait for 199 the last fragment before forwarding, reducing end-to-end latency. 200 Similarly, the memory footprint of VRB is just the VRB table, 201 reducing the packet drop probability significantly. 203 There are, however, limits: 205 Non-zero Packet Drop Probability: The abstract data in a VRB table 206 entry contains at a minimum the MAC address of the predecessor 207 and that of the successor, the datagram_tag used by the 208 predecessor and the local datagram_tag that this node will swap 209 with it. The VRB may need to store a few octets from the last 210 fragment that may not have fit within MTU and that will be 211 prepended to the next fragment. This yields a small footprint 212 that is 2 orders of magnitude smaller compared to needing a 213 1280-byte reassembly buffer for each packet. Yet, the size of 214 the VRB table necessarily remains finite. In the extreme case 215 where a node is required to concurrently forward more packets 216 that it has entries in its VRB table, packets are dropped. 217 No Fragment Recovery: There is no mechanism in VRB for the node that 218 reassembles a packet to request a single missing fragment. 219 Dropping a fragment requires the whole packet to be resent. This 220 causes unnecessary traffic, as fragments are forwarded even when 221 the destination node can never construct the original IPv6 222 packet. 223 No Per-Fragment Routing: All subsequent fragments follow the same 224 sequence of hops from the source to the destination node as the 225 first fragment, because the IP header is required to route the 226 fragment and is only present in the first fragment. A side 227 effect is that the first fragment must always be forwarded first. 229 The severity and occurrence of these limits depends on the link-layer 230 used. Whether these limits are acceptable depends entirely on the 231 requirements the application places on the network. 233 If the limits are present and not acceptable for the application, 234 future specifications may define new protocols to overcome these 235 limits. One example is [I-D.ietf-6lo-fragment-recovery] which 236 defines a protocol which allows fragment recovery. 238 4. Security Considerations 240 An attacker can perform a Denial-of-Service (DoS) attack on a node 241 implementing VRB by generating a large number of bogus "fragment 1" 242 fragments without sending subsequent fragments. This causes the VRB 243 table to fill up. Note that the VRB does not need to remember the 244 full datagram as received so far but only possibly a few octets from 245 the last fragment that could not fit in it. It is expected that an 246 implementation protects itself to keep the number of VRBs within 247 capacity, and that old VRBs are protected by a timer of a reasonable 248 duration for the technology and destroyed upon timeout. 250 Secure joining and the link-layer security that it sets up protects 251 against those attacks from network outsiders. 253 5. IANA Considerations 255 No requests to IANA are made by this document. 257 6. Acknowledgments 259 The authors would like to thank Yasuyuki Tanaka, for his in-depth 260 review of this document. Also many thanks to Georgies Papadopoulos 261 and Dominique Barthel for their own reviews. 263 7. Informative References 265 [ARTICLE] Tanaka, Y., Minet, P., and T. Watteyne, "6LoWPAN Fragment 266 Forwarding", IEEE Communications Standards Magazine , 267 2019. 269 [I-D.ietf-6lo-fragment-recovery] 270 Thubert, P., "6LoWPAN Selective Fragment Recovery", draft- 271 ietf-6lo-fragment-recovery-04 (work in progress), June 272 2019. 274 [I-D.ietf-lwig-6lowpan-virtual-reassembly] 275 Bormann, C. and T. Watteyne, "Virtual reassembly buffers 276 in 6LoWPAN", draft-ietf-lwig-6lowpan-virtual-reassembly-01 277 (work in progress), March 2019. 279 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 280 "Transmission of IPv6 Packets over IEEE 802.15.4 281 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, 282 . 284 [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 285 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, 286 DOI 10.17487/RFC6282, September 2011, 287 . 289 Authors' Addresses 291 Thomas Watteyne (editor) 292 Analog Devices 293 32990 Alvarado-Niles Road, Suite 910 294 Union City, CA 94587 295 USA 297 Email: thomas.watteyne@analog.com 299 Carsten Bormann 300 Universitaet Bremen TZI 301 Postfach 330440 302 Bremen D-28359 303 Germany 305 Email: cabo@tzi.org 307 Pascal Thubert 308 Cisco Systems, Inc 309 Building D 310 45 Allee des Ormes - BP1200 311 MOUGINS - Sophia Antipolis 06254 312 France 314 Email: pthubert@cisco.com