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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 6lo T. Watteyne, Ed. 3 Internet-Draft Analog Devices 4 Intended status: Informational C. Bormann 5 Expires: December 26, 2019 Universitaet Bremen TZI 6 P. Thubert 7 Cisco 8 June 24, 2019 10 LLN Minimal Fragment Forwarding 11 draft-ietf-6lo-minimal-fragment-02 13 Abstract 15 This document gives an overview of LLN Minimal Fragment Forwarding. 16 When employing adaptation layer fragmentation in 6LoWPAN, it may be 17 beneficial for a forwarder not to have to reassemble each packet in 18 its entirety before forwarding it. This has always been possible 19 with the original fragmentation design of RFC4944. This document is 20 a companion document to [I-D.ietf-lwig-6lowpan-virtual-reassembly], 21 which details the virtual Reassembly Buffer (VRB) implementation 22 technique which reduces the latency and increases end-to-end 23 reliability in route-over forwarding. 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 https://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 December 26, 2019. 42 Copyright Notice 44 Copyright (c) 2019 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 (https://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. Overview of 6LoWPAN Fragmentation . . . . . . . . . . . . . . 2 60 2. Limits of Per-Hop Fragmentation and Reassembly . . . . . . . 4 61 2.1. Latency . . . . . . . . . . . . . . . . . . . . . . . . . 4 62 2.2. Memory Management and Reliability . . . . . . . . . . . . 4 63 3. Virtual Reassembly Buffer (VRB) Implementation . . . . . . . 5 64 4. Security Considerations . . . . . . . . . . . . . . . . . . . 6 65 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 66 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6 67 7. Informative References . . . . . . . . . . . . . . . . . . . 6 68 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 70 1. Overview of 6LoWPAN Fragmentation 72 6LoWPAN fragmentation is defined in [RFC4944]. Although [RFC6282] 73 updates [RFC4944], it does not redefine 6LoWPAN fragmentation. 75 We use Figure 1 to illustrate 6LoWPAN fragmentation. We assume node 76 A forwards a packet to node B, possibly as part of a multi-hop route 77 between IPv6 source and destination nodes which are neither A nor B. 79 +---+ +---+ 80 ... ---| A |-------------------->| B |--- ... 81 +---+ +---+ 82 # (frag. 5) 84 123456789 123456789 85 +---------+ +---------+ 86 | # ###| |### # | 87 +---------+ +---------+ 88 outgoing incoming 89 fragmentation reassembly 90 buffer buffer 92 Figure 1: Fragmentation at node A, reassembly at node B. 94 Node A starts by compacting the IPv6 packet using the header 95 compression mechanism defined in [RFC6282]. If the resulting 6LoWPAN 96 packet does not fit into a single link-layer frame, node A's 6LoWPAN 97 sublayer cuts it into multiple 6LoWPAN fragments, which it transmits 98 as separate link-layer frames to node B. Node B's 6LoWPAN sublayer 99 reassembles these fragments, inflates the compressed header fields 100 back to the original IPv6 header, and hands over the full IPv6 packet 101 to its IPv6 layer. 103 In Figure 1, a packet forwarded by node A to node B is cut into nine 104 fragments, numbered 1 to 9. Each fragment is represented by the '#' 105 symbol. Node A has sent fragments 1, 2, 3, 5, 6 to node B. Node B 106 has received fragments 1, 2, 3, 6 from node A. Fragment 5 is still 107 being transmitted at the link layer from node A to node B. 109 Conceptually, a reassembly buffer for 6LoWPAN contains: 111 o a datagram_size, 112 o a datagram_tag, associated to the link-layer sender and receiver 113 addresses to which the datagram_tag is local, 114 o the actual packet data from the fragments received so far, in a 115 form that makes it possible to detect when the whole packet has 116 been received and can be processed or forwarded, 117 o a timer that allows discarding a partially reassembled packet 118 after some timeout. 120 A fragmentation header is added to each fragment; it indicates what 121 portion of the packet that fragment corresponds to. Section 5.3 of 122 [RFC4944] defines the format of the header for the first and 123 subsequent fragments. All fragments are tagged with a 16-bit 124 "datagram_tag", used to identify which packet each fragment belongs 125 to. Each datagram can be uniquely identified by the source and final 126 destination link-layer addresses of the frame that carries it, the 127 fragment size and the datagram_tag. Each fragment can be identified 128 within its datagram by the datagram-offset. 130 Node B's typical behavior, per [RFC4944], is as follows. Upon 131 receiving a fragment from node A with a datagram_tag previously 132 unseen from node A, node B allocates a buffer large enough to hold 133 the entire packet. The length of the packet is indicated in each 134 fragment (the datagram_size field), so node B can allocate the buffer 135 even if the first fragment it receives is not fragment 1. As 136 fragments come in, node B fills the buffer. When all fragments have 137 been received, node B inflates the compressed header fields into an 138 IPv6 header, and hands the resulting IPv6 packet to the IPv6 layer. 140 This behavior typically results in per-hop fragmentation and 141 reassembly. That is, the packet is fully reassembled, then 142 (re)fragmented, at every hop. 144 2. Limits of Per-Hop Fragmentation and Reassembly 146 There are at least 2 limits to doing per-hop fragmentation and 147 reassembly. See [ARTICLE] for detailed simulation results on both 148 limits. 150 2.1. Latency 152 When reassembling, a node needs to wait for all the fragments to be 153 received before being able to generate the IPv6 packet, and possibly 154 forward it to the next hop. This repeats at every hop. 156 This may result in increased end-to-end latency compared to a case 157 where each fragment is forwarded without per-hop reassembly. 159 2.2. Memory Management and Reliability 161 Constrained nodes have limited memory. Assuming 1 kB reassembly 162 buffer, typical nodes only have enough memory for 1-3 reassembly 163 buffers. 165 To illustrate this we use the topology from Figure 2, where nodes A, 166 B, C and D all send packets through node E. We further assume that 167 node E's memory can only hold 3 reassembly buffers. 169 +---+ +---+ 170 ... --->| A |------>| B | 171 +---+ +---+\ 172 \ 173 +---+ +---+ 174 | E |--->| F | ... 175 +---+ +---+ 176 / 177 / 178 +---+ +---+ 179 ... --->| C |------>| D | 180 +---+ +---+ 182 Figure 2: Illustrating the Memory Management Issue. 184 When nodes A, B and C concurrently send fragmented packets, all 3 185 reassembly buffers in node E are occupied. If, at that moment, node 186 D also sends a fragmented packet, node E has no option but to drop 187 one of the packets, lowering end-to-end reliability. 189 3. Virtual Reassembly Buffer (VRB) Implementation 191 Virtual Reassembly Buffer (VRB) is the implementation technique 192 described in [I-D.ietf-lwig-6lowpan-virtual-reassembly] in which a 193 forwarder does not reassemble each packet in its entirety before 194 forwarding it. 196 VRB overcomes the limits listed in Section 2. Nodes do not wait for 197 the last fragment before forwarding, reducing end-to-end latency. 198 Similarly, the memory footprint of VRB is just the VRB table, 199 reducing the packet drop probability significantly. 201 There are, however, limits: 203 Non-zero Packet Drop Probability: The abstract data in a VRB table 204 entry contains at a minimum the MAC address of the predecessor 205 and that of the successor, the datagram_tag used by the 206 predecessor and the local datagram_tag that this node will swap 207 with it. The VRB may need to store a few octets from the last 208 fragment that may not have fit within MTU and that will be 209 prepended to the next fragment. This yields a small footprint 210 that is 2 orders of magnitude smaller compared to needing a 211 1280-byte reassembly buffer for each packet. Yet, the size of 212 the VRB table necessarily remains finite. In the extreme case 213 where a node is required to concurrently forward more packets 214 that it has entries in its VRB table, packets are dropped. 215 No Fragment Recovery: There is no mechanism in VRB for the node that 216 reassembles a packet to request a single missing fragment. 217 Dropping a fragment requires the whole packet to be resent. This 218 causes unnecessary traffic, as fragments are forwarded even when 219 the destination node can never construct the original IPv6 220 packet. 221 No Per-Fragment Routing: All subsequent fragments follow the same 222 sequence of hops from the source to the destination node as the 223 first fragment, because the IP header is required to route the 224 fragment and is only present in the first fragment. A side 225 effect is that the first fragment must always be forwarded first. 227 The severity and occurrence of these limits depends on the link-layer 228 used. Whether these limits are acceptable depends entirely on the 229 requirements the application places on the network. 231 If the limits are present and not acceptable for the application, 232 future specifications may define new protocols to overcome these 233 limits. One example is [I-D.ietf-6lo-fragment-recovery] which 234 defines a protocol which allows fragment recovery. 236 4. Security Considerations 238 An attacker can perform a Denial-of-Service (DoS) attack on a node 239 implementing VRB by generating a large number of bogus "fragment 1" 240 fragments without sending subsequent fragments. This causes the VRB 241 table to fill up. Note that the VRB does not need to remember the 242 full datagram as received so far but only possibly a few octets from 243 the last fragment that could not fit in it. It is expected that an 244 implementation protects itself to keep the number of VRBs within 245 capacity, and that old VRBs are protected by a timer of a reasonable 246 duration for the technology and destroyed upon timeout. 248 Secure joining and the link-layer security that it sets up protects 249 against those attacks from network outsiders. 251 5. IANA Considerations 253 No requests to IANA are made by this document. 255 6. Acknowledgments 257 The authors would like to thank Yasuyuki Tanaka, for his in-depth 258 review of this document. Also many thanks to Georgies Papadopoulos 259 and Dominique Barthel for their own reviews. 261 7. Informative References 263 [ARTICLE] Tanaka, Y., Minet, P., and T. Watteyne, "6LoWPAN Fragment 264 Forwarding", IEEE Communications Standards Magazine , 265 2019. 267 [BOOK] Shelby, Z. and C. Bormann, "6LoWPAN", John Wiley & Sons, 268 Ltd monograph, DOI 10.1002/9780470686218, November 2009. 270 [I-D.ietf-6lo-fragment-recovery] 271 Thubert, P., "6LoWPAN Selective Fragment Recovery", draft- 272 ietf-6lo-fragment-recovery-04 (work in progress), June 273 2019. 275 [I-D.ietf-lwig-6lowpan-virtual-reassembly] 276 Bormann, C. and T. Watteyne, "Virtual reassembly buffers 277 in 6LoWPAN", draft-ietf-lwig-6lowpan-virtual-reassembly-01 278 (work in progress), March 2019. 280 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 281 "Transmission of IPv6 Packets over IEEE 802.15.4 282 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, 283 . 285 [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 286 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, 287 DOI 10.17487/RFC6282, September 2011, 288 . 290 Authors' Addresses 292 Thomas Watteyne (editor) 293 Analog Devices 294 32990 Alvarado-Niles Road, Suite 910 295 Union City, CA 94587 296 USA 298 Email: thomas.watteyne@analog.com 300 Carsten Bormann 301 Universitaet Bremen TZI 302 Postfach 330440 303 Bremen D-28359 304 Germany 306 Email: cabo@tzi.org 308 Pascal Thubert 309 Cisco Systems, Inc 310 Building D 311 45 Allee des Ormes - BP1200 312 MOUGINS - Sophia Antipolis 06254 313 France 315 Email: pthubert@cisco.com