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Bormann 3 Internet-Draft Universitaet Bremen TZI 4 Intended status: Standards Track October 18, 2013 5 Expires: April 21, 2014 7 6LoWPAN Generic Compression of Headers and Header-like Payloads 8 draft-bormann-6lo-ghc-00 10 Abstract 12 This short I-D provides a simple addition to 6LoWPAN Header 13 Compression that enables the compression of generic headers and 14 header-like payloads, without a need to define a new header 15 compression scheme for each new such header or header-like payload. 17 Status of This Memo 19 This Internet-Draft is submitted in full conformance with the 20 provisions of BCP 78 and BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF). Note that other groups may also distribute 24 working documents as Internet-Drafts. The list of current Internet- 25 Drafts is at http://datatracker.ietf.org/drafts/current/. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 This Internet-Draft will expire on April 21, 2014. 34 Copyright Notice 36 Copyright (c) 2013 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents 41 (http://trustee.ietf.org/license-info) in effect on the date of 42 publication of this document. Please review these documents 43 carefully, as they describe your rights and restrictions with respect 44 to this document. Code Components extracted from this document must 45 include Simplified BSD License text as described in Section 4.e of 46 the Trust Legal Provisions and are provided without warranty as 47 described in the Simplified BSD License. 49 Table of Contents 51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 52 1.1. The Header Compression Coupling Problem . . . . . . . . . 2 53 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2 54 1.3. Notation . . . . . . . . . . . . . . . . . . . . . . . . 3 55 2. 6LoWPAN-GHC . . . . . . . . . . . . . . . . . . . . . . . . . 4 56 3. Integrating 6LoWPAN-GHC into 6LoWPAN-HC . . . . . . . . . . . 5 57 3.1. Compressing payloads (UDP and ICMPv6) . . . . . . . . . . 5 58 3.2. Compressing extension headers . . . . . . . . . . . . . . 5 59 3.3. Indicating GHC capability . . . . . . . . . . . . . . . . 6 60 3.4. Using the 6CIO Option . . . . . . . . . . . . . . . . . . 7 61 4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 8 62 5. Security considerations . . . . . . . . . . . . . . . . . . . 9 63 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 64 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 65 7.1. Normative References . . . . . . . . . . . . . . . . . . 10 66 7.2. Informative References . . . . . . . . . . . . . . . . . 10 67 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 11 68 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21 70 1. Introduction 72 1.1. The Header Compression Coupling Problem 74 6LoWPAN-HC [RFC6282] defines a scheme for header compression in 75 6LoWPAN [RFC4944] packets. As with most header compression schemes, 76 a new specification is needed for every new kind of header that needs 77 to be compressed. In addition, [RFC6282] does not define an 78 extensibility scheme like the ROHC profiles defined in ROHC [RFC3095] 79 [RFC5795]. This leads to the difficult situation that 6LoWPAN-HC 80 tended to be reopened and reexamined each time a new header receives 81 consideration (or an old header is changed and reconsidered) in the 82 6LoWPAN/roll/CoRE cluster of IETF working groups. While [RFC6282] 83 finally got completed, the underlying problem remains unsolved. 85 The purpose of the present contribution is to plug into [RFC6282] as 86 is, using its NHC (next header compression) concept. We add a 87 slightly less efficient, but vastly more general form of compression 88 for headers of any kind and even for header-like payloads such as 89 those exhibited by routing protocols, DHCP, etc. The objective is an 90 extremely simple specification that can be defined on a single page 91 and implemented in a small number of lines of code, as opposed to a 92 general data compression scheme such as that defined in [RFC1951]. 94 1.2. Terminology 95 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 96 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 97 document are to be interpreted as described in RFC 2119 [RFC2119]. 99 The term "byte" is used in its now customary sense as a synonym for 100 "octet". 102 1.3. Notation 104 This specification uses a trivial notation for code bytes and the 105 bitfields in them the meaning of which should be mostly obvious. 106 More formally speaking, the meaning of the notation is: 108 Potential values for the code bytes themselves are expressed by 109 templates that represent 8-bit most-significant-bit-first binary 110 numbers (without any special prefix), where 0 stands for 0, 1 for 1, 111 and variable segments in these code byte templates are indicated by 112 sequences of the same letter such as kkkkkkk or ssss, the length of 113 which indicates the length of the variable segment in bits. 115 In the notation of values derived from the code bytes, 0b is used as 116 a prefix for expressing binary numbers in most-significant-bit first 117 notation (akin to the use of 0x for most-significant-digit-first 118 hexadecimal numbers in the C programming language). Where the above- 119 mentioned sequences of letters are then referenced in such a binary 120 number in the text, the intention is that the value from these 121 bitfields in the actual code byte be inserted. 123 Example: The code byte template 125 101nssss 127 stands for a byte that starts (most-significant-bit-first) with the 128 bits 1, 0, and 1, and continues with five variable bits, the first of 129 which is referenced as "n" and the next four are referenced as 130 "ssss". Based on this code byte template, a reference to 132 0b0ssss000 134 means a binary number composed from a zero bit, the four bits that 135 are in the "ssss" field (for 101nssss, the four least significant 136 bits) in the actual byte encountered, kept in the same order, and 137 three more zero bits. 139 2. 6LoWPAN-GHC 141 The format of a GHC-compressed header or payload is a simple 142 bytecode. A compressed header consists of a sequence of pieces, each 143 of which begins with a code byte, which may be followed by zero or 144 more bytes as its argument. Some code bytes cause bytes to be laid 145 out in the destination buffer, some simply modify some decompression 146 variables. 148 At the start of decompressing a header or payload within a L2 packet 149 (= fragment), variables "sa" and "na" are initialized as zero. 151 The code bytes are defined as follows (Table 1): 153 +------------+------------------------------------------+-----------+ 154 | code byte | Action | Argument | 155 +------------+------------------------------------------+-----------+ 156 | 0kkkkkkk | Append k = 0b0kkkkkkk bytes of data in | k bytes | 157 | | the bytecode argument (k < 96) | of data | 158 | | | | 159 | 1000nnnn | Append 0b0000nnnn+2 bytes of zeroes | | 160 | | | | 161 | 10010000 | STOP code (end of compressed data, see | | 162 | | Section 3.2) | | 163 | | | | 164 | 101nssss | Set up extended arguments for a | | 165 | | backreference: sa += 0b0ssss000, na += | | 166 | | 0b0000n000 | | 167 | | | | 168 | 11nnnkkk | Backreference: n = na+0b00000nnn+2; s = | | 169 | | 0b00000kkk+sa+n; append n bytes from | | 170 | | previously output bytes, starting s | | 171 | | bytes to the left of the current output | | 172 | | pointer; set sa = 0, na = 0 | | 173 +------------+------------------------------------------+-----------+ 175 Table 1: Bytecodes for generic header compression 177 Note that the following bit combinations are reserved at this time: 178 011xxxxx, and 1001nnnn (where 0b0000nnnn > 0). 180 For the purposes of the backreferences, the expansion buffer is 181 initialized with a predefined dictionary, at the end of which the 182 target buffer begins. This dictionary is composed of the pseudo- 183 header for the current packet as defined in [RFC2460], followed by a 184 16-byte static dictionary (Figure 1). These dictionary bytes are 185 therefore available for backreferencing, but not copied into the 186 final result. 188 16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00 190 Figure 1: The 16 bytes of static dictionary (in hex) 192 3. Integrating 6LoWPAN-GHC into 6LoWPAN-HC 194 6LoWPAN-GHC plugs in as an NHC format for 6LoWPAN-HC [RFC6282]. 196 3.1. Compressing payloads (UDP and ICMPv6) 198 GHC is by definition generic and can be applied to different kinds of 199 packets. Many of the examples given in Appendix A are for ICMPv6 200 packets; a single NHC value suffices to define an NHC format for 201 ICMPv6 based on GHC (see below). 203 In addition it is useful to include an NHC format for UDP, as many 204 headerlike payloads (e.g., DHCPv6, DTLS) are carried in UDP. 205 [RFC6282] already defines an NHC format for UDP (11110CPP). GHC uses 206 an analogous NHC byte formatted as shown in Figure 2. The difference 207 to the existing UDP NHC specification is that for 0b11010cpp NHC 208 bytes, the UDP payload is not supplied literally but compressed by 209 6LoWPAN-GHC. 211 0 1 2 3 4 5 6 7 212 +---+---+---+---+---+---+---+---+ 213 | 1 | 1 | 0 | 1 | 0 | C | P | 214 +---+---+---+---+---+---+---+---+ 216 Figure 2: NHC byte for UDP GHC (to be allocated by IANA) 218 To stay in the same general numbering space, we use 0b11011111 as the 219 NHC byte for ICMPv6 GHC (Figure 3). 221 0 1 2 3 4 5 6 7 222 +---+---+---+---+---+---+---+---+ 223 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 224 +---+---+---+---+---+---+---+---+ 226 Figure 3: NHC byte for ICMPv6 GHC (to be allocated by IANA) 228 3.2. Compressing extension headers 230 Compression of specific extension headers is added in a similar way 231 (Figure 4) (however, probably only EID 0 to 3 need to be assigned). 232 As there is no easy way to extract the length field from the GHC- 233 encoded header before decoding, this would make detecting the end of 234 the extension header somewhat complex. The easiest (and most 235 efficient) approach is to completely elide the length field (in the 236 same way NHC already elides the next header field in certain cases) 237 and reconstruct it only on decompression. To serve as a terminator 238 for the extension header, the reserved bytecode 0b10010000 has been 239 assigned as a stop marker. Note that the stop marker is only needed 240 for extension headers, not for the final payloads discussed in the 241 previous subsection, the decompression of which is automatically 242 stopped by the end of the packet. 244 0 1 2 3 4 5 6 7 245 +---+---+---+---+---+---+---+---+ 246 | 1 | 0 | 1 | 1 | EID |NH | 247 +---+---+---+---+---+---+---+---+ 249 Figure 4: NHC byte for extension header GHC 251 3.3. Indicating GHC capability 253 The 6LoWPAN baseline includes just [RFC4944], [RFC6282], [RFC6775] 254 (see [I-D.bormann-6lowpan-roadmap]). To enable the use of GHC 255 towards a neighbor, a 6LoWPAN node needs to know that the neighbor 256 implements it. While this can also simply be administratively 257 required, a transition strategy as well as a way to support mixed 258 networks is required. 260 One way to know a neighbor does implement GHC is receiving a packet 261 from that neighbor with GHC in it ("implicit capability detection"). 262 However, there needs to be a way to bootstrap this, as nobody ever 263 would start sending packets with GHC otherwise. 265 To minimize the impact on [RFC6775], we define an ND option 6LoWPAN 266 Capability Indication (6CIO), as illustrated in Figure 5. 268 0 1 2 3 269 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 1 270 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 271 | Type | Length = 1 |_____________________________|G| 272 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 273 |_______________________________________________________________| 274 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 276 Figure 5: 6LoWPAN Capability Indication Option (6CIO) 278 The G bit indicates whether the node sending the option is GHC 279 capable. 281 Once a node receives either an explicit or an implicit indication of 282 GHC capability from another node, it may send GHC-compressed packets 283 to that node. Where that capability has not been recently confirmed, 284 similar to the way PLPMTUD [RFC4821] finds out about changes in the 285 network, a node SHOULD make use of NUD (neighbor unreachability 286 detection) failures to switch back to basic 6LoWPAN header 287 compression [RFC6282]. 289 3.4. Using the 6CIO Option 291 The 6CIO option will typically only be ever sent in 6LoWPAN-ND RS 292 packets (which cannot itself be GHC compressed unless the host 293 desires to limit itself to talking to GHC capable routers). The 294 resulting 6LoWPAN-ND RA can then already make use of GHC and thus 295 indicate GHC capability implicitly, which in turn allows both nodes 296 to use GHC in the 6LoWPAN-ND NS/NA exchange. 298 6CIO can also be used for future options that need to be negotiated 299 between 6LoWPAN peers; an IANA registry is used to assign the flags. 300 Bits marked by underscores in Figure 5 are unassigned and available 301 for future assignment. They MUST be sent as zero and MUST be ignored 302 on reception until assigned by IANA. Length values larger than 1 303 MUST be accepted by implementations in order to enable future 304 extensions; the additional bits in the option are then deemed 305 unassigned in the same way. For the purposes of the IANA registry, 306 the bits are numbered in most-significant-bit-first order from the 307 16th bit of the option onward, i.e., the G bit is flag number 15. 308 (Additional bits may also be used by a follow-on version of this 309 document if some bit combinations that have been left unassigned here 310 are then used in an upward compatible manner.) 312 Where the use of this option by other specifications is envisioned, 313 the following items have to be kept in mind: 315 o The option can be used in any ND packet. 317 o Specific bits are set in the option to indicate that a capability 318 is present in the sender. (There may be other ways to infer this 319 information, as is the case in this specification.) Bit 320 combinations may be used as desired. The absence of the 321 capability _indication_ is signaled by setting these bits to zero; 322 this does not necessarily mean that the capability is absent. 324 o The intention is not to modify the semantics of the specific ND 325 packet carrying the option, but to provide the general capability 326 indication described above. 328 o Specifications have to be designed such that receivers that do not 329 receive or do not process such a capability indication can still 330 interoperate (presumably without exploiting the indicated 331 capability). 333 o The option is meant to be used sparsely, i.e. once a sender has 334 reason to believe the capability indication has been received, 335 there no longer is a need to continue sending it. 337 4. IANA considerations 339 [This section to be removed/replaced by the RFC Editor.] 341 In the IANA registry for the "LOWPAN_NHC Header Type" (in the "IPv6 342 Low Power Personal Area Network Parameters"), IANA needs to add the 343 assignments in Figure 6. 345 10110IIN: Extension header GHC [RFCthis] 346 11010CPP: UDP GHC [RFCthis] 347 11011111: ICMPv6 GHC [RFCthis] 349 Figure 6: IANA assignments for the NHC byte 351 IANA needs to allocate an ND option number for the 6CIO ND option 352 format in the Registry "IPv6 Neighbor Discovery Option Formats" 353 [RFC4861]. 355 IANA needs to create a registry for "6LoWPAN capability bits" within 356 the "Internet Control Message Protocol version 6 (ICMPv6) 357 Parameters". The bits are assigned by giving their numbers as small 358 non-negative integers as defined in section Section 3.4, preferably 359 in the range 0..47. The policy is "RFC Required" [RFC5226]. The 360 initial content of the registry is as in Figure 7: 362 0..14: unassigned 363 15: GHC capable bit (G bit) [RFCthis] 364 16..47: unassigned 366 Figure 7 368 5. Security considerations 370 The security considerations of [RFC4944] and [RFC6282] apply. As 371 usual in protocols with packet parsing/construction, care must be 372 taken in implementations to avoid buffer overflows and in particular 373 (with respect to the back-referencing) out-of-area references during 374 decompression. 376 One additional consideration is that an attacker may send a forged 377 packet that makes a second node believe a third victim node is GHC- 378 capable. If it is not, this may prevent packets sent by the second 379 node from reaching the third node (at least until robustness features 380 such as those discussed in Section 3.3 kick in). 382 No mitigation is proposed (or known) for this attack, except that a 383 victim node that does implement GHC is not vulnerable. However, with 384 unsecured ND, a number of attacks with similar outcomes are already 385 possible, so there is little incentive to make use of this additional 386 attack. With secured ND, 6CIO is also secured; nodes relying on 387 secured ND therefore should use 6CIO bidirectionally (and limit the 388 implicit capability detection to secured ND packets carrying GHC) 389 instead of basing their neighbor capability assumptions on receiving 390 any kind of unprotected packet. 392 6. Acknowledgements 394 Colin O'Flynn has repeatedly insisted that some form of compression 395 for ICMPv6 and ND packets might be beneficial. He actually wrote his 396 own draft, [I-D.oflynn-6lowpan-icmphc], which compresses better, but 397 addresses basic ICMPv6/ND only and needs a much longer spec (around 398 17 pages of detailed spec, as compared to the single page of core 399 spec here). This motivated the author to try something simple, yet 400 general. Special thanks go to Colin for indicating that he indeed 401 considers his draft superseded by the present one. 403 The examples given are based on pcap files that Colin O'Flynn, Owen 404 Kirby, Olaf Bergmann and others provided. 406 The static dictionary was developed, and the bit allocations 407 validated, based on research by Sebastian Dominik. 409 Erik Nordmark provided input that helped shaping the 6CIO option. 411 Yoshihiro Ohba insisted on clarifying the notation used for the 412 definition of the bytecodes and their bitfields. 414 7. References 416 7.1. Normative References 418 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 419 Requirement Levels", BCP 14, RFC 2119, March 1997. 421 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 422 (IPv6) Specification", RFC 2460, December 1998. 424 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 425 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 426 September 2007. 428 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 429 "Transmission of IPv6 Packets over IEEE 802.15.4 430 Networks", RFC 4944, September 2007. 432 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 433 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 434 May 2008. 436 [RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6 437 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, 438 September 2011. 440 [RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann, 441 "Neighbor Discovery Optimization for IPv6 over Low-Power 442 Wireless Personal Area Networks (6LoWPANs)", RFC 6775, 443 November 2012. 445 7.2. Informative References 447 [I-D.bormann-6lowpan-roadmap] 448 Bormann, C., "6LoWPAN Roadmap and Implementation Guide", 449 draft-bormann-6lowpan-roadmap-04 (work in progress), April 450 2013. 452 [I-D.oflynn-6lowpan-icmphc] 453 O'Flynn, C., "ICMPv6/ND Compression for 6LoWPAN Networks", 454 draft-oflynn-6lowpan-icmphc-00 (work in progress), July 455 2010. 457 [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification 458 version 1.3", RFC 1951, May 1996. 460 [RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., 461 Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, 462 K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., 463 Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header 464 Compression (ROHC): Framework and four profiles: RTP, UDP, 465 ESP, and uncompressed", RFC 3095, July 2001. 467 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 468 Discovery", RFC 4821, March 2007. 470 [RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust 471 Header Compression (ROHC) Framework", RFC 5795, March 472 2010. 474 Appendix A. Examples 476 This section demonstrates some relatively realistic examples derived 477 from actual PCAP dumps taken at previous interops. 479 Figure 8 shows an RPL DODAG Information Solicitation, a quite short 480 RPL message that obviously cannot be improved much. 482 IP header: 483 60 00 00 00 00 08 3a ff fe 80 00 00 00 00 00 00 484 02 1c da ff fe 00 20 24 ff 02 00 00 00 00 00 00 485 00 00 00 00 00 00 00 1a 486 Payload: 487 9b 00 6b de 00 00 00 00 488 Dictionary: 489 fe 80 00 00 00 00 00 00 02 1c da ff fe 00 20 24 490 ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 1a 491 00 00 00 08 00 00 00 3a 16 fe fd 17 fe fd 00 01 492 00 00 00 00 00 01 00 00 493 copy: 04 9b 00 6b de 494 4 nulls: 82 495 Compressed: 496 04 9b 00 6b de 82 497 Was 8 bytes; compressed to 6 bytes, compression factor 1.33 499 Figure 8: A simple RPL example 501 Figure 9 shows an RPL DODAG Information Object, a longer RPL control 502 message that is improved a bit more. Note that the compressed output 503 exposes an inefficiency in the simple-minded compressor used to 504 generate it; this does not devalue the example since constrained 505 nodes are quite likely to make use of simple-minded compressors. 507 IP header: 508 60 00 00 00 00 5c 3a ff fe 80 00 00 00 00 00 00 509 02 1c da ff fe 00 30 23 ff 02 00 00 00 00 00 00 510 00 00 00 00 00 00 00 1a 511 Payload: 512 9b 01 7a 5f 00 f0 01 00 88 00 00 00 20 02 0d b8 513 00 00 00 00 00 00 00 ff fe 00 fa ce 04 0e 00 14 514 09 ff 00 00 01 00 00 00 00 00 00 00 08 1e 80 20 515 ff ff ff ff ff ff ff ff 00 00 00 00 20 02 0d b8 516 00 00 00 00 00 00 00 ff fe 00 fa ce 03 0e 40 00 517 ff ff ff ff 20 02 0d b8 00 00 00 00 518 Dictionary: 519 fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23 520 ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 1a 521 00 00 00 5c 00 00 00 3a 16 fe fd 17 fe fd 00 01 522 00 00 00 00 00 01 00 00 523 copy: 06 9b 01 7a 5f 00 f0 524 ref(9): 01 00 -> ref 11nnnkkk 0 7: c7 525 copy: 01 88 526 3 nulls: 81 527 copy: 04 20 02 0d b8 528 7 nulls: 85 529 ref(68): ff fe 00 -> ref 101nssss 0 8/11nnnkkk 1 1: a8 c9 530 copy: 08 fa ce 04 0e 00 14 09 ff 531 ref(39): 00 00 01 00 00 -> ref 101nssss 0 4/11nnnkkk 3 2: a4 da 532 5 nulls: 83 533 copy: 06 08 1e 80 20 ff ff 534 ref(2): ff ff -> ref 11nnnkkk 0 0: c0 535 ref(4): ff ff ff ff -> ref 11nnnkkk 2 0: d0 536 4 nulls: 82 537 ref(48): 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 fa ce 538 -> ref 101nssss 1 4/11nnnkkk 6 0: b4 f0 539 copy: 03 03 0e 40 540 ref(9): 00 ff -> ref 11nnnkkk 0 7: c7 541 ref(28): ff ff ff -> ref 101nssss 0 3/11nnnkkk 1 1: a3 c9 542 ref(24): 20 02 0d b8 00 00 00 00 543 -> ref 101nssss 0 2/11nnnkkk 6 0: a2 f0 544 Compressed: 545 06 9b 01 7a 5f 00 f0 c7 01 88 81 04 20 02 0d b8 546 85 a8 c9 08 fa ce 04 0e 00 14 09 ff a4 da 83 06 547 08 1e 80 20 ff ff c0 d0 82 b4 f0 03 03 0e 40 c7 548 a3 c9 a2 f0 549 Was 92 bytes; compressed to 52 bytes, compression factor 1.77 551 Figure 9: A longer RPL example 553 Similarly, Figure 10 shows an RPL DAO message. One of the embedded 554 addresses is copied right out of the pseudo-header, the other one is 555 effectively converted from global to local by providing the prefix 556 FE80 literally, inserting a number of nulls, and copying (some of) 557 the IID part again out of the pseudo-header. Note that a simple 558 implementation would probably emit fewer nulls and copy the entire 559 IID; there are multiple ways to encode this 50-byte payload into 27 560 bytes. 562 IP header: 563 60 00 00 00 00 32 3a ff 20 02 0d b8 00 00 00 00 564 00 00 00 ff fe 00 33 44 20 02 0d b8 00 00 00 00 565 00 00 00 ff fe 00 11 22 566 Payload: 567 9b 02 58 7d 01 80 00 f1 05 12 00 80 20 02 0d b8 568 00 00 00 00 00 00 00 ff fe 00 33 44 06 14 00 80 569 f1 00 fe 80 00 00 00 00 00 00 00 00 00 ff fe 00 570 11 22 571 Dictionary: 572 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 33 44 573 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 11 22 574 00 00 00 32 00 00 00 3a 16 fe fd 17 fe fd 00 01 575 00 00 00 00 00 01 00 00 576 copy: 0c 9b 02 58 7d 01 80 00 f1 05 12 00 80 577 ref(68): 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 33 44 578 -> ref 101nssss 1 6/11nnnkkk 6 4: b6 f4 579 copy: 08 06 14 00 80 f1 00 fe 80 580 9 nulls: 87 581 ref(74): ff fe 00 11 22 -> ref 101nssss 0 8/11nnnkkk 3 5: a8 dd 582 Compressed: 583 0c 9b 02 58 7d 01 80 00 f1 05 12 00 80 b6 f4 08 584 06 14 00 80 f1 00 fe 80 87 a8 dd 585 Was 50 bytes; compressed to 27 bytes, compression factor 1.85 587 Figure 10: An RPL DAO message 589 Figure 11 shows the effect of compressing a simple ND neighbor 590 solicitation. 592 IP header: 593 60 00 00 00 00 30 3a ff 20 02 0d b8 00 00 00 00 594 00 00 00 ff fe 00 3b d3 fe 80 00 00 00 00 00 00 595 02 1c da ff fe 00 30 23 596 Payload: 597 87 00 a7 68 00 00 00 00 fe 80 00 00 00 00 00 00 598 02 1c da ff fe 00 30 23 01 01 3b d3 00 00 00 00 599 1f 02 00 00 00 00 00 06 00 1c da ff fe 00 20 24 600 Dictionary: 601 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 3b d3 602 fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23 603 00 00 00 30 00 00 00 3a 16 fe fd 17 fe fd 00 01 604 00 00 00 00 00 01 00 00 605 copy: 04 87 00 a7 68 606 4 nulls: 82 607 ref(48): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23 608 -> ref 101nssss 1 4/11nnnkkk 6 0: b4 f0 609 copy: 04 01 01 3b d3 610 4 nulls: 82 611 copy: 02 1f 02 612 5 nulls: 83 613 copy: 02 06 00 614 ref(24): 1c da ff fe 00 -> ref 101nssss 0 2/11nnnkkk 3 3: a2 db 615 copy: 02 20 24 616 Compressed: 617 04 87 00 a7 68 82 b4 f0 04 01 01 3b d3 82 02 1f 618 02 83 02 06 00 a2 db 02 20 24 619 Was 48 bytes; compressed to 26 bytes, compression factor 1.85 621 Figure 11: An ND neighbor solicitation 623 Figure 12 shows the compression of an ND neighbor advertisement. 625 IP header: 626 60 00 00 00 00 30 3a fe fe 80 00 00 00 00 00 00 627 02 1c da ff fe 00 30 23 20 02 0d b8 00 00 00 00 628 00 00 00 ff fe 00 3b d3 629 Payload: 630 88 00 26 6c c0 00 00 00 fe 80 00 00 00 00 00 00 631 02 1c da ff fe 00 30 23 02 01 fa ce 00 00 00 00 632 1f 02 00 00 00 00 00 06 00 1c da ff fe 00 20 24 633 Dictionary: 634 fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23 635 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 3b d3 636 00 00 00 30 00 00 00 3a 16 fe fd 17 fe fd 00 01 637 00 00 00 00 00 01 00 00 638 copy: 05 88 00 26 6c c0 639 3 nulls: 81 640 ref(64): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23 641 -> ref 101nssss 1 6/11nnnkkk 6 0: b6 f0 642 copy: 04 02 01 fa ce 643 4 nulls: 82 644 copy: 02 1f 02 645 5 nulls: 83 646 copy: 02 06 00 647 ref(24): 1c da ff fe 00 -> ref 101nssss 0 2/11nnnkkk 3 3: a2 db 648 copy: 02 20 24 649 Compressed: 650 05 88 00 26 6c c0 81 b6 f0 04 02 01 fa ce 82 02 651 1f 02 83 02 06 00 a2 db 02 20 24 652 Was 48 bytes; compressed to 27 bytes, compression factor 1.78 654 Figure 12: An ND neighbor advertisement 656 Figure 13 shows the compression of an ND router solicitation. Note 657 that the relatively good compression is not caused by the many zero 658 bytes in the link-layer address of this particular capture (which are 659 unlikely to occur in practice): 7 of these 8 bytes are copied from 660 the pseudo-header (the 8th byte cannot be copied as the universal/ 661 local bit needs to be inverted). 663 IP header: 664 60 00 00 00 00 18 3a ff fe 80 00 00 00 00 00 00 665 ae de 48 00 00 00 00 01 ff 02 00 00 00 00 00 00 666 00 00 00 00 00 00 00 02 667 Payload: 668 85 00 90 65 00 00 00 00 01 02 ac de 48 00 00 00 669 00 01 00 00 00 00 00 00 670 Dictionary: 671 fe 80 00 00 00 00 00 00 ae de 48 00 00 00 00 01 672 ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 02 673 00 00 00 18 00 00 00 3a 16 fe fd 17 fe fd 00 01 674 00 00 00 00 00 01 00 00 675 copy: 04 85 00 90 65 676 ref(11): 00 00 00 00 01 -> ref 11nnnkkk 3 6: de 677 copy: 02 02 ac 678 ref(58): de 48 00 00 00 00 01 679 -> ref 101nssss 0 6/11nnnkkk 5 3: a6 eb 680 6 nulls: 84 681 Compressed: 682 04 85 00 90 65 de 02 02 ac a6 eb 84 683 Was 24 bytes; compressed to 12 bytes, compression factor 2.00 685 Figure 13: An ND router solicitation 687 Figure 14 shows the compression of an ND router advertisement. The 688 indefinite lifetime is compressed to four bytes by backreferencing; 689 this could be improved (at the cost of minor additional decompressor 690 complexity) by including some simple runlength mechanism. 692 IP header: 693 60 00 00 00 00 60 3a ff fe 80 00 00 00 00 00 00 694 10 34 00 ff fe 00 11 22 fe 80 00 00 00 00 00 00 695 ae de 48 00 00 00 00 01 696 Payload: 697 86 00 55 c9 40 00 0f a0 1c 5a 38 17 00 00 07 d0 698 01 01 11 22 00 00 00 00 03 04 40 40 ff ff ff ff 699 ff ff ff ff 00 00 00 00 20 02 0d b8 00 00 00 00 700 00 00 00 00 00 00 00 00 20 02 40 10 00 00 03 e8 701 20 02 0d b8 00 00 00 00 21 03 00 01 00 00 00 00 702 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 11 22 703 Dictionary: 704 fe 80 00 00 00 00 00 00 10 34 00 ff fe 00 11 22 705 fe 80 00 00 00 00 00 00 ae de 48 00 00 00 00 01 706 00 00 00 60 00 00 00 3a 16 fe fd 17 fe fd 00 01 707 00 00 00 00 00 01 00 00 708 copy: 0c 86 00 55 c9 40 00 0f a0 1c 5a 38 17 709 2 nulls: 80 710 copy: 06 07 d0 01 01 11 22 711 4 nulls: 82 712 copy: 06 03 04 40 40 ff ff 713 ref(2): ff ff -> ref 11nnnkkk 0 0: c0 714 ref(4): ff ff ff ff -> ref 11nnnkkk 2 0: d0 715 4 nulls: 82 716 copy: 04 20 02 0d b8 717 12 nulls: 8a 718 copy: 04 20 02 40 10 719 ref(38): 00 00 03 -> ref 101nssss 0 4/11nnnkkk 1 3: a4 cb 720 copy: 01 e8 721 ref(24): 20 02 0d b8 00 00 00 00 722 -> ref 101nssss 0 2/11nnnkkk 6 0: a2 f0 723 copy: 02 21 03 724 ref(84): 00 01 00 00 00 00 725 -> ref 101nssss 0 9/11nnnkkk 4 6: a9 e6 726 ref(40): 20 02 0d b8 00 00 00 00 00 00 00 727 -> ref 101nssss 1 3/11nnnkkk 1 5: b3 cd 728 ref(136): ff fe 00 11 22 729 -> ref 101nssss 0 15/101nssss 0 1/11nnnkkk 3 3: af a1 db 730 Compressed: 731 0c 86 00 55 c9 40 00 0f a0 1c 5a 38 17 80 06 07 732 d0 01 01 11 22 82 06 03 04 40 40 ff ff c0 d0 82 733 04 20 02 0d b8 8a 04 20 02 40 10 a4 cb 01 e8 a2 734 f0 02 21 03 a9 e6 b3 cd af a1 db 735 Was 96 bytes; compressed to 59 bytes, compression factor 1.63 737 Figure 14: An ND router advertisement 739 Figure 15 shows the compression of a DTLS application data packet 740 with a net payload of 13 bytes of cleartext, and 8 bytes of 741 authenticator (note that the IP header is not relevant for this 742 example and has been set to 0). This makes good use of the static 743 dictionary, and is quite effective crunching out the redundancy in 744 the TLS_PSK_WITH_AES_128_CCM_8 header, leading to a net reduction by 745 15 bytes. 747 IP header: 748 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 749 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 750 00 00 00 00 00 00 00 00 751 Payload: 752 17 fe fd 00 01 00 00 00 00 00 01 00 1d 00 01 00 753 00 00 00 00 01 09 b2 0e 82 c1 6e b6 96 c5 1f 36 754 8d 17 61 e2 b5 d4 22 d4 ed 2b 755 Dictionary: 756 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 757 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 758 00 00 00 2a 00 00 00 00 16 fe fd 17 fe fd 00 01 759 00 00 00 00 00 01 00 00 760 ref(13): 17 fe fd 00 01 00 00 00 00 00 01 00 761 -> ref 101nssss 1 0/11nnnkkk 2 1: b0 d1 762 copy: 01 1d 763 ref(10): 00 01 00 00 00 00 00 01 -> ref 11nnnkkk 6 2: f2 764 copy: 15 09 b2 0e 82 c1 6e b6 96 c5 1f 36 8d 17 61 e2 765 copy: b5 d4 22 d4 ed 2b 766 Compressed: 767 b0 d1 01 1d f2 15 09 b2 0e 82 c1 6e b6 96 c5 1f 768 36 8d 17 61 e2 b5 d4 22 d4 ed 2b 769 Was 42 bytes; compressed to 27 bytes, compression factor 1.56 771 Figure 15: A DTLS application data packet 773 Figure 16 shows that the compression is slightly worse in a 774 subsequent packet (containing 6 bytes of cleartext and 8 bytes of 775 authenticator, yielding a net compression of 13 bytes). The total 776 overhead does stay at a quite acceptable 8 bytes. 778 IP header: 779 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 780 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 781 00 00 00 00 00 00 00 00 782 Payload: 783 17 fe fd 00 01 00 00 00 00 00 05 00 16 00 01 00 784 00 00 00 00 05 ae a0 15 56 67 92 4d ff 8a 24 e4 785 cb 35 b9 786 Dictionary: 787 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 788 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 789 00 00 00 23 00 00 00 00 16 fe fd 17 fe fd 00 01 790 00 00 00 00 00 01 00 00 791 ref(13): 17 fe fd 00 01 00 00 00 00 00 792 -> ref 101nssss 1 0/11nnnkkk 0 3: b0 c3 793 copy: 03 05 00 16 794 ref(10): 00 01 00 00 00 00 00 05 -> ref 11nnnkkk 6 2: f2 795 copy: 0e ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 796 Compressed: 797 b0 c3 03 05 00 16 f2 0e ae a0 15 56 67 92 4d ff 798 8a 24 e4 cb 35 b9 799 Was 35 bytes; compressed to 22 bytes, compression factor 1.59 801 Figure 16: Another DTLS application data packet 803 Figure 17 shows the compression of a DTLS handshake message, here a 804 client hello. There is little that can be compressed about the 32 805 bytes of randomness. Still, the net reduction is by 14 bytes. 807 IP header: 808 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 809 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 810 00 00 00 00 00 00 00 00 811 Payload: 812 16 fe fd 00 00 00 00 00 00 00 00 00 36 01 00 00 813 2a 00 00 00 00 00 00 00 2a fe fd 51 52 ed 79 a4 814 20 c9 62 56 11 47 c9 39 ee 6c c0 a4 fe c6 89 2f 815 32 26 9a 16 4e 31 7e 9f 20 92 92 00 00 00 02 c0 816 a8 01 00 817 Dictionary: 818 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 819 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 820 00 00 00 43 00 00 00 00 16 fe fd 17 fe fd 00 01 821 00 00 00 00 00 01 00 00 822 ref(16): 16 fe fd -> ref 101nssss 0 1/11nnnkkk 1 5: a1 cd 823 9 nulls: 87 824 copy: 01 36 825 ref(16): 01 00 00 -> ref 101nssss 0 1/11nnnkkk 1 5: a1 cd 826 copy: 01 2a 827 7 nulls: 85 828 copy: 23 2a fe fd 51 52 ed 79 a4 20 c9 62 56 11 47 c9 829 copy: 39 ee 6c c0 a4 fe c6 89 2f 32 26 9a 16 4e 31 7e 830 copy: 9f 20 92 92 831 3 nulls: 81 832 copy: 05 02 c0 a8 01 00 833 Compressed: 834 a1 cd 87 01 36 a1 cd 01 2a 85 23 2a fe fd 51 52 835 ed 79 a4 20 c9 62 56 11 47 c9 39 ee 6c c0 a4 fe 836 c6 89 2f 32 26 9a 16 4e 31 7e 9f 20 92 92 81 05 837 02 c0 a8 01 00 838 Was 67 bytes; compressed to 53 bytes, compression factor 1.26 840 Figure 17: A DTLS handshake packet (client hello) 842 Author's Address 844 Carsten Bormann 845 Universitaet Bremen TZI 846 Postfach 330440 847 D-28359 Bremen 848 Germany 850 Phone: +49-421-218-63921 851 Email: cabo@tzi.org