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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: January 17, 2019 Universitaet Bremen TZI 6 P. Thubert 7 Cisco 8 July 16, 2018 10 LLN Minimal Fragment Forwarding 11 draft-watteyne-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 been always 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 January 17, 2019. 42 Copyright Notice 44 Copyright (c) 2018 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 . . . . . . . . . . . . . . . . . . . 5 65 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 66 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6 67 7. Informative References . . . . . . . . . . . . . . . . . . . 6 68 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 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 header compression 95 defined in [RFC6282]. If the resulting 6LoWPAN packet does not fit 96 into a single link-layer frame, node A's 6LoWPAN sublayer cuts it 97 into multiple 6LoWPAN fragments, which it transmits as separate link- 98 layer frames to node B. Node B's 6LoWPAN sublayer reassembles these 99 fragments, inflates the compressed header fields back to the original 100 IPv6 header, and hands over the full IPv6 packet to its IPv6 layer. 102 In Figure 1, a packet forwarded by node A to node B is cut into nine 103 fragments, numbered 1 to 9. Each fragment is represented by the '#' 104 symbol. Node A has sent fragments 1, 2, 3, 5, 6 to node B. Node B 105 has received fragments 1, 2, 3, 6 from node A. Fragment 5 is still 106 being transmitted at the link layer from node A to node B. 108 A reassembly buffer for 6LoWPAN contains: 110 o datagram_size, 111 o datagram_tag and link-layer sender and receiver addresses (to 112 which the datagram_tag is local), 113 o actual packet data from the fragments received so far, in a form 114 that makes it possible to detect when the whole packet has been 115 received and can be processed or forwarded, 116 o a timer that allows discarding the partial packet after a timeout. 118 A fragmentation header is added to each fragment; it indicates what 119 portion of the packet that fragment corresponds to. Section 5.3 of 120 [RFC4944] defines the format of the header for the first and 121 subsequent fragments. All fragments are tagged with a 16-bit 122 "datagram_tag", used to identify which packet each fragment belongs 123 to. Each fragment can be uniquely identified by the source and 124 destination link-layer addresses of the frame that carries it, and 125 the datagram_tag. The value of the datagram_tag only needs to be 126 locally unique to nodes A and B. 128 Node B's typical behavior, per [RFC4944], is as follows. Upon 129 receiving a fragment from node A with a datagram_tag previously 130 unseen from node A, node B allocates a buffer large enough to hold 131 the entire packet. The length of the packet is indicated in each 132 fragment (the datagram_size field), so node B can allocate the buffer 133 even if the first fragment it receives is not fragment 1. As 134 fragments come in, node B fills the buffer. When all fragments have 135 been received, node B inflates the compressed header fields into an 136 IPv6 header, and hands the resulting IPv6 packet to the IPv6 layer. 138 This behavior typically results in per-hop fragmentation and 139 reassembly. That is, the packet is fully reassembled, then 140 (re)fragmented, at every hop. 142 2. Limits of Per-Hop Fragmentation and Reassembly 144 There are at least 2 limits to doing per-hop fragmentation and 145 reassembly: 147 2.1. Latency 149 When reassembling, a node needs to wait for all the fragments to be 150 received before being able to generate the IPv6 packet, and possibly 151 forward it to the next hop. This repeats at every hop. 153 This may result in increased end-to-end latency compared to the case 154 where each fragment would be forwarded without per-hop reassembly. 156 2.2. Memory Management and Reliability 158 Constrained nodes have limited memory. Assuming 1 kB reassembly 159 buffers, typical nodes only have enough memory for 1-3 reassembly 160 buffers. 162 Assuming the topology from Figure 2, where nodes A, B, C and D all 163 send packets through node E. We further assume that node E's memory 164 can only hold 3 reassembly buffers. 166 +---+ +---+ 167 ... --->| A |------>| B | 168 +---+ +---+\ 169 \ 170 +---+ +---+ 171 | E |--->| F | ... 172 +---+ +---+ 173 / 174 / 175 +---+ +---+ 176 ... --->| C |------>| D | 177 +---+ +---+ 179 Figure 2: Illustrating the Memory Management Issue. 181 When nodes A, B and C concurrently send fragmented packets, all 3 182 reassembly buffers in node E are occupied. If, at that moment, node 183 D also sends a fragmented packet, node E has no option but to drop 184 one of the packets, lowering end-to-end reliability. 186 3. Virtual Reassembly Buffer (VRB) Implementation 188 Virtual Reassembly Buffer (VRB) is the implementation technique 189 described in [I-D.ietf-lwig-6lowpan-virtual-reassembly] in which a 190 forwarder does not reassemble each packet in its entirety before 191 forwarding it. 193 VRB overcomes the limits listed in Section 2. Nodes don't wait for 194 the last fragment before forwarding, reducing end-to-end latency. 195 Similarly, the memory footprint of VRB is just the VRB table, 196 reducing the packet drop probability significantly. 198 There are, however, limits: 200 Non-zero Packet Drop Probability: Each VRB table entry can be 12 B 201 (assuming 16-bit link-layer addresses). This is a footprint 2 202 orders of magnitude smaller compared to needing a 1280-byte 203 reassembly buffer for each packet. Yet, the size of the VRB 204 table necessarily remains finite. In the extreme case where a 205 node is required to concurrently forward more packets that it has 206 entries in its VRB table, packets are dropped. 207 No Fragment Recovery: There is no mechanism in VRB for the node that 208 reassembles a packet to request a single missing fragment. 209 Dropping a fragment requires the whole packet to be resent. This 210 causes unnecessary traffic, as fragments are forwarded even when 211 the destination node can never construct the original IPv6 212 packet. 213 No Per-Fragment Routing: All subsequent fragments follow the same 214 sequence of hops from the source to the destination node as 215 fragment 1. 217 The severity and occurrence of these limits depends on the link-layer 218 used. Whether these limits are acceptable depends entirely on the 219 requirements the application places on the network. 221 If the limits are both present and not accepted by the application, 222 future specifications may define new protocols to overcome these 223 limits. One example is [I-D.thubert-6lo-fragment-recovery] which 224 defines a protocol which allows fragment recovery. 226 4. Security Considerations 228 An attacker can perform a DoS attack on a node implementing VRB by 229 generating a large number of bogus "fragment 1" fragments without 230 sending subsequent fragments. This causes the VRB table to fill up. 232 Secure joining and the link-layer security that it sets up protects 233 against those attacks from network outsiders. 235 5. IANA Considerations 237 No requests to IANA are made by this document. 239 6. Acknowledgments 241 The authors would like to thank Yasuyuki Tanaka for his in-depth 242 review of this document. 244 7. Informative References 246 [BOOK] Shelby, Z. and C. Bormann, "6LoWPAN", John Wiley & Sons, 247 Ltd monograph, DOI 10.1002/9780470686218, November 2009. 249 [I-D.ietf-lwig-6lowpan-virtual-reassembly] 250 Bormann, C. and T. Watteyne, "Virtual reassembly buffers 251 in 6LoWPAN", draft-ietf-lwig-6lowpan-virtual-reassembly-00 252 (work in progress), July 2018. 254 [I-D.thubert-6lo-fragment-recovery] 255 Thubert, P., "6LoWPAN Selective Fragment Recovery", draft- 256 thubert-6lo-fragment-recovery-01 (work in progress), June 257 2018. 259 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 260 "Transmission of IPv6 Packets over IEEE 802.15.4 261 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, 262 . 264 [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 265 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, 266 DOI 10.17487/RFC6282, September 2011, 267 . 269 Authors' Addresses 271 Thomas Watteyne (editor) 272 Analog Devices 273 32990 Alvarado-Niles Road, Suite 910 274 Union City, CA 94587 275 USA 277 Email: thomas.watteyne@analog.com 278 Carsten Bormann 279 Universitaet Bremen TZI 280 Postfach 330440 281 Bremen D-28359 282 Germany 284 Email: cabo@tzi.org 286 Pascal Thubert 287 Cisco Systems, Inc 288 Building D 289 45 Allee des Ormes - BP1200 290 MOUGINS - Sophia Antipolis 06254 291 France 293 Email: pthubert@cisco.com