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Bormann 3 Internet-Draft Universitaet Bremen TZI 4 Intended status: Standards Track June 19, 2014 5 Expires: December 21, 2014 7 6LoWPAN Generic Compression of Headers and Header-like Payloads 8 draft-ietf-6lo-ghc-01 10 Abstract 12 This short specification 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 December 21, 2014. 34 Copyright Notice 36 Copyright (c) 2014 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. Compression Approach . . . . . . . . . . . . . . . . . . 3 54 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 55 1.4. Notation . . . . . . . . . . . . . . . . . . . . . . . . 4 56 2. 6LoWPAN-GHC . . . . . . . . . . . . . . . . . . . . . . . . . 5 57 3. Integrating 6LoWPAN-GHC into 6LoWPAN-HC . . . . . . . . . . . 6 58 3.1. Compressing payloads (UDP and ICMPv6) . . . . . . . . . . 6 59 3.2. Compressing extension headers . . . . . . . . . . . . . . 6 60 3.3. Indicating GHC capability . . . . . . . . . . . . . . . . 7 61 3.4. Using the 6CIO Option . . . . . . . . . . . . . . . . . . 8 62 4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 9 63 5. Security considerations . . . . . . . . . . . . . . . . . . . 10 64 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 65 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 66 7.1. Normative References . . . . . . . . . . . . . . . . . . 11 67 7.2. Informative References . . . . . . . . . . . . . . . . . 11 68 Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 12 69 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22 71 1. Introduction 73 1.1. The Header Compression Coupling Problem 75 6LoWPAN-HC [RFC6282] defines a scheme for header compression in 76 6LoWPAN [RFC4944] packets. As with most header compression schemes, 77 a new specification is needed for every new kind of header that needs 78 to be compressed. In addition, [RFC6282] does not define an 79 extensibility scheme like the ROHC profiles defined in ROHC [RFC3095] 80 [RFC5795]. This leads to the difficult situation that 6LoWPAN-HC 81 tended to be reopened and reexamined each time a new header receives 82 consideration (or an old header is changed and reconsidered) in the 83 6LoWPAN/roll/CoRE cluster of IETF working groups. While [RFC6282] 84 finally got completed, the underlying problem remains unsolved. 86 The purpose of the present contribution is to plug into [RFC6282] as 87 is, using its NHC (next header compression) concept. We add a 88 slightly less efficient, but vastly more general form of compression 89 for headers of any kind and even for header-like payloads such as 90 those exhibited by routing protocols, DHCP, etc. The objective is an 91 extremely simple specification that can be defined on a single page 92 and implemented in a small number of lines of code, as opposed to a 93 general data compression scheme such as that defined in [RFC1951]. 95 1.2. Compression Approach 97 The basic approach of GHC's compression function is to define a 98 bytecode for LZ77-style compression [LZ77]. The bytecode is a series 99 of simple instructions for the decompressor to reconstitute the 100 uncompressed payload. These instructions include: 102 o appending bytes to the reconstituted payload that are literally 103 given with the instruction in the compressed data 105 o appending a given number of zero bytes to the reconstituted 106 payload 108 o appending bytes to the reconstituted payload by copying a 109 contiguous sequence from the payload being reconstituted 110 ("backreferencing") 112 o an ancillary instruction for setting up parameters for the 113 backreferencing instruction in "decompression variables" 115 o a stop code (optional, see Section 3.2) 117 The buffer for the reconstituted payload ("destination buffer") is 118 prefixed by a predefined dictionary that can be used in the 119 backreferencing as if it were a prefix of the payload. This 120 predefined dictionary is built from the IPv6 addresses of the packet 121 being reconstituted, followed by a static component, the "static 122 dictionary". 124 As usual, this specification defines the decompressor operation in 125 detail, but leaves the detailed operation of the compressor open to 126 implementation. The compressor can be implemented as with a 127 classical LZ77 compressor, or it can be a simple protocol encoder 128 that just makes use of known compression opportunities. 130 1.3. Terminology 132 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 133 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 134 document are to be interpreted as described in RFC 2119 [RFC2119]. 136 The term "byte" is used in its now customary sense as a synonym for 137 "octet". 139 1.4. Notation 141 This specification uses a trivial notation for code bytes and the 142 bitfields in them the meaning of which should be mostly obvious. 143 More formally speaking, the meaning of the notation is: 145 Potential values for the code bytes themselves are expressed by 146 templates that represent 8-bit most-significant-bit-first binary 147 numbers (without any special prefix), where 0 stands for 0, 1 for 1, 148 and variable segments in these code byte templates are indicated by 149 sequences of the same letter such as kkkkkkk or ssss, the length of 150 which indicates the length of the variable segment in bits. 152 In the notation of values derived from the code bytes, 0b is used as 153 a prefix for expressing binary numbers in most-significant-bit first 154 notation (akin to the use of 0x for most-significant-digit-first 155 hexadecimal numbers in the C programming language). Where the above- 156 mentioned sequences of letters are then referenced in such a binary 157 number in the text, the intention is that the value from these 158 bitfields in the actual code byte be inserted. 160 Example: The code byte template 162 101nssss 164 stands for a byte that starts (most-significant-bit-first) with the 165 bits 1, 0, and 1, and continues with five variable bits, the first of 166 which is referenced as "n" and the next four are referenced as 167 "ssss". Based on this code byte template, a reference to 169 0b0ssss000 171 means a binary number composed from a zero bit, the four bits that 172 are in the "ssss" field (for 101nssss, the four least significant 173 bits) in the actual byte encountered, kept in the same order, and 174 three more zero bits. 176 2. 6LoWPAN-GHC 178 The format of a GHC-compressed header or payload is a simple 179 bytecode. A compressed header consists of a sequence of pieces, each 180 of which begins with a code byte, which may be followed by zero or 181 more bytes as its argument. Some code bytes cause bytes to be laid 182 out in the destination buffer, some simply modify some decompression 183 variables. 185 At the start of decompressing a header or payload within a L2 packet 186 (= fragment), the decompression variables "sa" and "na" are 187 initialized as zero. 189 The code bytes are defined as follows (Table 1): 191 +----------+---------------------------------------------+----------+ 192 | code | Action | Argument | 193 | byte | | | 194 +----------+---------------------------------------------+----------+ 195 | 0kkkkkkk | Append k = 0b0kkkkkkk bytes of data in the | k bytes | 196 | | bytecode argument (k < 96) | of data | 197 | | | | 198 | 1000nnnn | Append 0b0000nnnn+2 bytes of zeroes | | 199 | | | | 200 | 10010000 | STOP code (end of compressed data, see | | 201 | | Section 3.2) | | 202 | | | | 203 | 101nssss | Set up extended arguments for a | | 204 | | backreference: sa += 0b0ssss000, na += | | 205 | | 0b0000n000 | | 206 | | | | 207 | 11nnnkkk | Backreference: n = na+0b00000nnn+2; s = | | 208 | | 0b00000kkk+sa+n; append n bytes from | | 209 | | previously output bytes, starting s bytes | | 210 | | to the left of the current output pointer; | | 211 | | set sa = 0, na = 0 | | 212 +----------+---------------------------------------------+----------+ 214 Table 1: Bytecodes for generic header compression 216 Note that the following bit combinations are reserved at this time: 217 011xxxxx, and 1001nnnn (where 0b0000nnnn > 0). 219 For the purposes of the backreferences, the expansion buffer is 220 initialized with a predefined dictionary, at the end of which the 221 reconstituted payload begins. This dictionary is composed of the 222 source and destination IPv6 addresses of the packet being 223 reconstituted, followed by a 16-byte static dictionary (Figure 1). 225 These 48 dictionary bytes are therefore available for 226 backreferencing, but not copied into the final reconstituted payload. 228 16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00 230 Figure 1: The 16 bytes of static dictionary (in hex) 232 3. Integrating 6LoWPAN-GHC into 6LoWPAN-HC 234 6LoWPAN-GHC plugs in as an NHC format for 6LoWPAN-HC [RFC6282]. 236 3.1. Compressing payloads (UDP and ICMPv6) 238 GHC is by definition generic and can be applied to different kinds of 239 packets. Many of the examples given in Appendix A are for ICMPv6 240 packets; a single NHC value suffices to define an NHC format for 241 ICMPv6 based on GHC (see below). 243 In addition it is useful to include an NHC format for UDP, as many 244 headerlike payloads (e.g., DHCPv6, DTLS) are carried in UDP. 245 [RFC6282] already defines an NHC format for UDP (11110CPP). GHC uses 246 an analogous NHC byte formatted as shown in Figure 2. The difference 247 to the existing UDP NHC specification is that for 0b11010cpp NHC 248 bytes, the UDP payload is not supplied literally but compressed by 249 6LoWPAN-GHC. 251 0 1 2 3 4 5 6 7 252 +---+---+---+---+---+---+---+---+ 253 | 1 | 1 | 0 | 1 | 0 | C | P | 254 +---+---+---+---+---+---+---+---+ 256 Figure 2: NHC byte for UDP GHC (to be allocated by IANA) 258 To stay in the same general numbering space, we use 0b11011111 as the 259 NHC byte for ICMPv6 GHC (Figure 3). 261 0 1 2 3 4 5 6 7 262 +---+---+---+---+---+---+---+---+ 263 | 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | 264 +---+---+---+---+---+---+---+---+ 266 Figure 3: NHC byte for ICMPv6 GHC (to be allocated by IANA) 268 3.2. Compressing extension headers 270 Compression of specific extension headers is added in a similar way 271 (Figure 4) (however, probably only EID 0 to 3 need to be assigned). 272 As there is no easy way to extract the length field from the GHC- 273 encoded header before decoding, this would make detecting the end of 274 the extension header somewhat complex. The easiest (and most 275 efficient) approach is to completely elide the length field (in the 276 same way NHC already elides the next header field in certain cases) 277 and reconstruct it only on decompression. To serve as a terminator 278 for the extension header, the reserved bytecode 0b10010000 has been 279 assigned as a stop marker. Note that the stop marker is only needed 280 for extension headers, not for the final payloads discussed in the 281 previous subsection, the decompression of which is automatically 282 stopped by the end of the packet. 284 0 1 2 3 4 5 6 7 285 +---+---+---+---+---+---+---+---+ 286 | 1 | 0 | 1 | 1 | EID |NH | 287 +---+---+---+---+---+---+---+---+ 289 Figure 4: NHC byte for extension header GHC 291 3.3. Indicating GHC capability 293 The 6LoWPAN baseline includes just [RFC4944], [RFC6282], [RFC6775] 294 (see [I-D.bormann-6lowpan-roadmap]). To enable the use of GHC 295 towards a neighbor, a 6LoWPAN node needs to know that the neighbor 296 implements it. While this can also simply be administratively 297 required, a transition strategy as well as a way to support mixed 298 networks is required. 300 One way to know a neighbor does implement GHC is receiving a packet 301 from that neighbor with GHC in it ("implicit capability detection"). 302 However, there needs to be a way to bootstrap this, as nobody ever 303 would start sending packets with GHC otherwise. 305 To minimize the impact on [RFC6775], we define an ND option 6LoWPAN 306 Capability Indication (6CIO), as illustrated in Figure 5. 308 0 1 2 3 309 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 1 310 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 311 | Type | Length = 1 |_____________________________|G| 312 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 313 |_______________________________________________________________| 314 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 316 Figure 5: 6LoWPAN Capability Indication Option (6CIO) 318 The G bit indicates whether the node sending the option is GHC 319 capable. 321 Once a node receives either an explicit or an implicit indication of 322 GHC capability from another node, it may send GHC-compressed packets 323 to that node. Where that capability has not been recently confirmed, 324 similar to the way PLPMTUD [RFC4821] finds out about changes in the 325 network, a node SHOULD make use of NUD (neighbor unreachability 326 detection) failures to switch back to basic 6LoWPAN header 327 compression [RFC6282]. 329 3.4. Using the 6CIO Option 331 The 6CIO option will typically only be ever sent in 6LoWPAN-ND RS 332 packets (which cannot itself be GHC compressed unless the host 333 desires to limit itself to talking to GHC capable routers). The 334 resulting 6LoWPAN-ND RA can then already make use of GHC and thus 335 indicate GHC capability implicitly, which in turn allows both nodes 336 to use GHC in the 6LoWPAN-ND NS/NA exchange. 338 6CIO can also be used for future options that need to be negotiated 339 between 6LoWPAN peers; an IANA registry is used to assign the flags. 340 Bits marked by underscores in Figure 5 are unassigned and available 341 for future assignment. They MUST be sent as zero and MUST be ignored 342 on reception until assigned by IANA. Length values larger than 1 343 MUST be accepted by implementations in order to enable future 344 extensions; the additional bits in the option are then deemed 345 unassigned in the same way. For the purposes of the IANA registry, 346 the bits are numbered in most-significant-bit-first order from the 347 16th bit of the option onward, i.e., the G bit is flag number 15. 348 (Additional bits may also be used by a follow-on version of this 349 document if some bit combinations that have been left unassigned here 350 are then used in an upward compatible manner.) 352 Where the use of this option by other specifications is envisioned, 353 the following items have to be kept in mind: 355 o The option can be used in any ND packet. 357 o Specific bits are set in the option to indicate that a capability 358 is present in the sender. (There may be other ways to infer this 359 information, as is the case in this specification.) Bit 360 combinations may be used as desired. The absence of the 361 capability _indication_ is signaled by setting these bits to zero; 362 this does not necessarily mean that the capability is absent. 364 o The intention is not to modify the semantics of the specific ND 365 packet carrying the option, but to provide the general capability 366 indication described above. 368 o Specifications have to be designed such that receivers that do not 369 receive or do not process such a capability indication can still 370 interoperate (presumably without exploiting the indicated 371 capability). 373 o The option is meant to be used sparsely, i.e. once a sender has 374 reason to believe the capability indication has been received, 375 there no longer is a need to continue sending it. 377 4. IANA considerations 379 [This section to be removed/replaced by the RFC Editor.] 381 In the IANA registry for the "LOWPAN_NHC Header Type" (in the "IPv6 382 Low Power Personal Area Network Parameters"), IANA needs to add the 383 assignments in Figure 6. 385 10110IIN: Extension header GHC [RFCthis] 386 11010CPP: UDP GHC [RFCthis] 387 11011111: ICMPv6 GHC [RFCthis] 389 Figure 6: IANA assignments for the NHC byte 391 IANA needs to allocate an ND option number for the 6CIO ND option 392 format in the Registry "IPv6 Neighbor Discovery Option Formats" 393 [RFC4861]. 395 IANA needs to create a registry for "6LoWPAN capability bits" within 396 the "Internet Control Message Protocol version 6 (ICMPv6) 397 Parameters". The bits are assigned by giving their numbers as small 398 non-negative integers as defined in section Section 3.4, preferably 399 in the range 0..47. The policy is "RFC Required" [RFC5226]. The 400 initial content of the registry is as in Figure 7: 402 0..14: unassigned 403 15: GHC capable bit (G bit) [RFCthis] 404 16..47: unassigned 406 Figure 7: IANA assignments for the 6LoWPAN capability bits 408 5. Security considerations 410 The security considerations of [RFC4944] and [RFC6282] apply. As 411 usual in protocols with packet parsing/construction, care must be 412 taken in implementations to avoid buffer overflows and in particular 413 (with respect to the back-referencing) out-of-area references during 414 decompression. 416 One additional consideration is that an attacker may send a forged 417 packet that makes a second node believe a third victim node is GHC- 418 capable. If it is not, this may prevent packets sent by the second 419 node from reaching the third node (at least until robustness features 420 such as those discussed in Section 3.3 kick in). 422 No mitigation is proposed (or known) for this attack, except that a 423 victim node that does implement GHC is not vulnerable. However, with 424 unsecured ND, a number of attacks with similar outcomes are already 425 possible, so there is little incentive to make use of this additional 426 attack. With secured ND, 6CIO is also secured; nodes relying on 427 secured ND therefore should use 6CIO bidirectionally (and limit the 428 implicit capability detection to secured ND packets carrying GHC) 429 instead of basing their neighbor capability assumptions on receiving 430 any kind of unprotected packet. 432 6. Acknowledgements 434 Colin O'Flynn has repeatedly insisted that some form of compression 435 for ICMPv6 and ND packets might be beneficial. He actually wrote his 436 own draft, [I-D.oflynn-6lowpan-icmphc], which compresses better, but 437 addresses basic ICMPv6/ND only and needs a much longer spec (around 438 17 pages of detailed spec, as compared to the single page of core 439 spec here). This motivated the author to try something simple, yet 440 general. Special thanks go to Colin for indicating that he indeed 441 considers his draft superseded by the present one. 443 The examples given are based on pcap files that Colin O'Flynn, Owen 444 Kirby, Olaf Bergmann and others provided. 446 The static dictionary was developed, and the bit allocations 447 validated, based on research by Sebastian Dominik. 449 Erik Nordmark provided input that helped shaping the 6CIO option. 450 Thomas Bjorklund proposed simplifying the predefined dictionary. 452 Yoshihiro Ohba insisted on clarifying the notation used for the 453 definition of the bytecodes and their bitfields. Ulrich Herberg 454 provided some additional review and suggested expanding the 455 introductory material. 457 7. References 459 7.1. Normative References 461 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 462 Requirement Levels", BCP 14, RFC 2119, March 1997. 464 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, 465 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, 466 September 2007. 468 [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, 469 "Transmission of IPv6 Packets over IEEE 802.15.4 470 Networks", RFC 4944, September 2007. 472 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 473 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 474 May 2008. 476 [RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6 477 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, 478 September 2011. 480 [RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann, 481 "Neighbor Discovery Optimization for IPv6 over Low-Power 482 Wireless Personal Area Networks (6LoWPANs)", RFC 6775, 483 November 2012. 485 7.2. Informative References 487 [I-D.bormann-6lowpan-roadmap] 488 Bormann, C., "6LoWPAN Roadmap and Implementation Guide", 489 draft-bormann-6lowpan-roadmap-04 (work in progress), April 490 2013. 492 [I-D.oflynn-6lowpan-icmphc] 493 O'Flynn, C., "ICMPv6/ND Compression for 6LoWPAN Networks", 494 draft-oflynn-6lowpan-icmphc-00 (work in progress), July 495 2010. 497 [LZ77] Ziv, J. and A. Lempel, "A Universal Algorithm for 498 Sequential Data Compression", IEEE Transactions on 499 Information Theory, Vol. 23, No. 3, pp. 337-343, May 1977. 501 [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification 502 version 1.3", RFC 1951, May 1996. 504 [RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., 505 Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, 506 K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., 507 Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header 508 Compression (ROHC): Framework and four profiles: RTP, UDP, 509 ESP, and uncompressed", RFC 3095, July 2001. 511 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU 512 Discovery", RFC 4821, March 2007. 514 [RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust 515 Header Compression (ROHC) Framework", RFC 5795, March 516 2010. 518 Appendix A. Examples 520 This section demonstrates some relatively realistic examples derived 521 from actual PCAP dumps taken at previous interops. 523 Figure 8 shows an RPL DODAG Information Solicitation, a quite short 524 RPL message that obviously cannot be improved much. 526 IP header: 527 60 00 00 00 00 08 3a ff fe 80 00 00 00 00 00 00 528 02 1c da ff fe 00 20 24 ff 02 00 00 00 00 00 00 529 00 00 00 00 00 00 00 1a 530 Payload: 531 9b 00 6b de 00 00 00 00 532 Dictionary: 533 fe 80 00 00 00 00 00 00 02 1c da ff fe 00 20 24 534 ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 1a 535 16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00 536 copy: 04 9b 00 6b de 537 4 nulls: 82 538 Compressed: 539 04 9b 00 6b de 82 540 Was 8 bytes; compressed to 6 bytes, compression factor 1.33 542 Figure 8: A simple RPL example 544 Figure 9 shows an RPL DODAG Information Object, a longer RPL control 545 message that is improved a bit more. Note that the compressed output 546 exposes an inefficiency in the simple-minded compressor used to 547 generate it; this does not devalue the example since constrained 548 nodes are quite likely to make use of simple-minded compressors. 550 IP header: 551 60 00 00 00 00 5c 3a ff fe 80 00 00 00 00 00 00 552 02 1c da ff fe 00 30 23 ff 02 00 00 00 00 00 00 553 00 00 00 00 00 00 00 1a 554 Payload: 555 9b 01 7a 5f 00 f0 01 00 88 00 00 00 20 02 0d b8 556 00 00 00 00 00 00 00 ff fe 00 fa ce 04 0e 00 14 557 09 ff 00 00 01 00 00 00 00 00 00 00 08 1e 80 20 558 ff ff ff ff ff ff ff ff 00 00 00 00 20 02 0d b8 559 00 00 00 00 00 00 00 ff fe 00 fa ce 03 0e 40 00 560 ff ff ff ff 20 02 0d b8 00 00 00 00 561 Dictionary: 562 fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23 563 ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 1a 564 16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00 565 copy: 06 9b 01 7a 5f 00 f0 566 ref(9): 01 00 -> ref 11nnnkkk 0 7: c7 567 copy: 01 88 568 3 nulls: 81 569 copy: 04 20 02 0d b8 570 7 nulls: 85 571 ref(60): ff fe 00 -> ref 101nssss 0 7/11nnnkkk 1 1: a7 c9 572 copy: 08 fa ce 04 0e 00 14 09 ff 573 ref(39): 00 00 01 00 00 -> ref 101nssss 0 4/11nnnkkk 3 2: a4 da 574 5 nulls: 83 575 copy: 06 08 1e 80 20 ff ff 576 ref(2): ff ff -> ref 11nnnkkk 0 0: c0 577 ref(4): ff ff ff ff -> ref 11nnnkkk 2 0: d0 578 4 nulls: 82 579 ref(48): 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 fa ce 580 -> ref 101nssss 1 4/11nnnkkk 6 0: b4 f0 581 copy: 03 03 0e 40 582 ref(9): 00 ff -> ref 11nnnkkk 0 7: c7 583 ref(28): ff ff ff -> ref 101nssss 0 3/11nnnkkk 1 1: a3 c9 584 ref(24): 20 02 0d b8 00 00 00 00 585 -> ref 101nssss 0 2/11nnnkkk 6 0: a2 f0 586 Compressed: 587 06 9b 01 7a 5f 00 f0 c7 01 88 81 04 20 02 0d b8 588 85 a7 c9 08 fa ce 04 0e 00 14 09 ff a4 da 83 06 589 08 1e 80 20 ff ff c0 d0 82 b4 f0 03 03 0e 40 c7 590 a3 c9 a2 f0 591 Was 92 bytes; compressed to 52 bytes, compression factor 1.77 593 Figure 9: A longer RPL example 595 Similarly, Figure 10 shows an RPL DAO message. One of the embedded 596 addresses is copied right out of the pseudo-header, the other one is 597 effectively converted from global to local by providing the prefix 598 FE80 literally, inserting a number of nulls, and copying (some of) 599 the IID part again out of the pseudo-header. Note that a simple 600 implementation would probably emit fewer nulls and copy the entire 601 IID; there are multiple ways to encode this 50-byte payload into 27 602 bytes. 604 IP header: 605 60 00 00 00 00 32 3a ff 20 02 0d b8 00 00 00 00 606 00 00 00 ff fe 00 33 44 20 02 0d b8 00 00 00 00 607 00 00 00 ff fe 00 11 22 608 Payload: 609 9b 02 58 7d 01 80 00 f1 05 12 00 80 20 02 0d b8 610 00 00 00 00 00 00 00 ff fe 00 33 44 06 14 00 80 611 f1 00 fe 80 00 00 00 00 00 00 00 00 00 ff fe 00 612 11 22 613 Dictionary: 614 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 33 44 615 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 11 22 616 16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00 617 copy: 0c 9b 02 58 7d 01 80 00 f1 05 12 00 80 618 ref(60): 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 33 44 619 -> ref 101nssss 1 5/11nnnkkk 6 4: b5 f4 620 copy: 08 06 14 00 80 f1 00 fe 80 621 9 nulls: 87 622 ref(66): ff fe 00 11 22 -> ref 101nssss 0 7/11nnnkkk 3 5: a7 dd 623 Compressed: 624 0c 9b 02 58 7d 01 80 00 f1 05 12 00 80 b5 f4 08 625 06 14 00 80 f1 00 fe 80 87 a7 dd 626 Was 50 bytes; compressed to 27 bytes, compression factor 1.85 628 Figure 10: An RPL DAO message 630 Figure 11 shows the effect of compressing a simple ND neighbor 631 solicitation. 633 IP header: 634 60 00 00 00 00 30 3a ff 20 02 0d b8 00 00 00 00 635 00 00 00 ff fe 00 3b d3 fe 80 00 00 00 00 00 00 636 02 1c da ff fe 00 30 23 637 Payload: 638 87 00 a7 68 00 00 00 00 fe 80 00 00 00 00 00 00 639 02 1c da ff fe 00 30 23 01 01 3b d3 00 00 00 00 640 1f 02 00 00 00 00 00 06 00 1c da ff fe 00 20 24 641 Dictionary: 642 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 3b d3 643 fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23 644 16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00 645 copy: 04 87 00 a7 68 646 4 nulls: 82 647 ref(40): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23 648 -> ref 101nssss 1 3/11nnnkkk 6 0: b3 f0 649 copy: 04 01 01 3b d3 650 4 nulls: 82 651 copy: 02 1f 02 652 5 nulls: 83 653 copy: 02 06 00 654 ref(24): 1c da ff fe 00 -> ref 101nssss 0 2/11nnnkkk 3 3: a2 db 655 copy: 02 20 24 656 Compressed: 657 04 87 00 a7 68 82 b3 f0 04 01 01 3b d3 82 02 1f 658 02 83 02 06 00 a2 db 02 20 24 659 Was 48 bytes; compressed to 26 bytes, compression factor 1.85 661 Figure 11: An ND neighbor solicitation 663 Figure 12 shows the compression of an ND neighbor advertisement. 665 IP header: 666 60 00 00 00 00 30 3a fe fe 80 00 00 00 00 00 00 667 02 1c da ff fe 00 30 23 20 02 0d b8 00 00 00 00 668 00 00 00 ff fe 00 3b d3 669 Payload: 670 88 00 26 6c c0 00 00 00 fe 80 00 00 00 00 00 00 671 02 1c da ff fe 00 30 23 02 01 fa ce 00 00 00 00 672 1f 02 00 00 00 00 00 06 00 1c da ff fe 00 20 24 673 Dictionary: 674 fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23 675 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 3b d3 676 16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00 677 copy: 05 88 00 26 6c c0 678 3 nulls: 81 679 ref(56): fe 80 00 00 00 00 00 00 02 1c da ff fe 00 30 23 680 -> ref 101nssss 1 5/11nnnkkk 6 0: b5 f0 681 copy: 04 02 01 fa ce 682 4 nulls: 82 683 copy: 02 1f 02 684 5 nulls: 83 685 copy: 02 06 00 686 ref(24): 1c da ff fe 00 -> ref 101nssss 0 2/11nnnkkk 3 3: a2 db 687 copy: 02 20 24 688 Compressed: 689 05 88 00 26 6c c0 81 b5 f0 04 02 01 fa ce 82 02 690 1f 02 83 02 06 00 a2 db 02 20 24 691 Was 48 bytes; compressed to 27 bytes, compression factor 1.78 693 Figure 12: An ND neighbor advertisement 695 Figure 13 shows the compression of an ND router solicitation. Note 696 that the relatively good compression is not caused by the many zero 697 bytes in the link-layer address of this particular capture (which are 698 unlikely to occur in practice): 7 of these 8 bytes are copied from 699 the pseudo-header (the 8th byte cannot be copied as the universal/ 700 local bit needs to be inverted). 702 IP header: 703 60 00 00 00 00 18 3a ff fe 80 00 00 00 00 00 00 704 ae de 48 00 00 00 00 01 ff 02 00 00 00 00 00 00 705 00 00 00 00 00 00 00 02 706 Payload: 707 85 00 90 65 00 00 00 00 01 02 ac de 48 00 00 00 708 00 01 00 00 00 00 00 00 709 Dictionary: 710 fe 80 00 00 00 00 00 00 ae de 48 00 00 00 00 01 711 ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 02 712 16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00 713 copy: 04 85 00 90 65 714 ref(11): 00 00 00 00 01 -> ref 11nnnkkk 3 6: de 715 copy: 02 02 ac 716 ref(50): de 48 00 00 00 00 01 717 -> ref 101nssss 0 5/11nnnkkk 5 3: a5 eb 718 6 nulls: 84 719 Compressed: 720 04 85 00 90 65 de 02 02 ac a5 eb 84 721 Was 24 bytes; compressed to 12 bytes, compression factor 2.00 723 Figure 13: An ND router solicitation 725 Figure 14 shows the compression of an ND router advertisement. The 726 indefinite lifetime is compressed to four bytes by backreferencing; 727 this could be improved (at the cost of minor additional decompressor 728 complexity) by including some simple runlength mechanism. 730 IP header: 731 60 00 00 00 00 60 3a ff fe 80 00 00 00 00 00 00 732 10 34 00 ff fe 00 11 22 fe 80 00 00 00 00 00 00 733 ae de 48 00 00 00 00 01 734 Payload: 735 86 00 55 c9 40 00 0f a0 1c 5a 38 17 00 00 07 d0 736 01 01 11 22 00 00 00 00 03 04 40 40 ff ff ff ff 737 ff ff ff ff 00 00 00 00 20 02 0d b8 00 00 00 00 738 00 00 00 00 00 00 00 00 20 02 40 10 00 00 03 e8 739 20 02 0d b8 00 00 00 00 21 03 00 01 00 00 00 00 740 20 02 0d b8 00 00 00 00 00 00 00 ff fe 00 11 22 741 Dictionary: 742 fe 80 00 00 00 00 00 00 10 34 00 ff fe 00 11 22 743 fe 80 00 00 00 00 00 00 ae de 48 00 00 00 00 01 744 16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00 745 copy: 0c 86 00 55 c9 40 00 0f a0 1c 5a 38 17 746 2 nulls: 80 747 copy: 06 07 d0 01 01 11 22 748 4 nulls: 82 749 copy: 06 03 04 40 40 ff ff 750 ref(2): ff ff -> ref 11nnnkkk 0 0: c0 751 ref(4): ff ff ff ff -> ref 11nnnkkk 2 0: d0 752 4 nulls: 82 753 copy: 04 20 02 0d b8 754 12 nulls: 8a 755 copy: 04 20 02 40 10 756 ref(38): 00 00 03 -> ref 101nssss 0 4/11nnnkkk 1 3: a4 cb 757 copy: 01 e8 758 ref(24): 20 02 0d b8 00 00 00 00 759 -> ref 101nssss 0 2/11nnnkkk 6 0: a2 f0 760 copy: 02 21 03 761 ref(84): 00 01 00 00 00 00 762 -> ref 101nssss 0 9/11nnnkkk 4 6: a9 e6 763 ref(40): 20 02 0d b8 00 00 00 00 00 00 00 764 -> ref 101nssss 1 3/11nnnkkk 1 5: b3 cd 765 ref(128): ff fe 00 11 22 766 -> ref 101nssss 0 15/11nnnkkk 3 3: af db 767 Compressed: 768 0c 86 00 55 c9 40 00 0f a0 1c 5a 38 17 80 06 07 769 d0 01 01 11 22 82 06 03 04 40 40 ff ff c0 d0 82 770 04 20 02 0d b8 8a 04 20 02 40 10 a4 cb 01 e8 a2 771 f0 02 21 03 a9 e6 b3 cd af db 772 Was 96 bytes; compressed to 58 bytes, compression factor 1.66 774 Figure 14: An ND router advertisement 776 Figure 15 shows the compression of a DTLS application data packet 777 with a net payload of 13 bytes of cleartext, and 8 bytes of 778 authenticator (note that the IP header is not relevant for this 779 example and has been set to 0). This makes good use of the static 780 dictionary, and is quite effective crunching out the redundancy in 781 the TLS_PSK_WITH_AES_128_CCM_8 header, leading to a net reduction by 782 15 bytes. 784 IP header: 785 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 786 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 787 00 00 00 00 00 00 00 00 788 Payload: 789 17 fe fd 00 01 00 00 00 00 00 01 00 1d 00 01 00 790 00 00 00 00 01 09 b2 0e 82 c1 6e b6 96 c5 1f 36 791 8d 17 61 e2 b5 d4 22 d4 ed 2b 792 Dictionary: 793 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 794 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 795 16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00 796 ref(13): 17 fe fd 00 01 00 00 00 00 00 01 00 797 -> ref 101nssss 1 0/11nnnkkk 2 1: b0 d1 798 copy: 01 1d 799 ref(10): 00 01 00 00 00 00 00 01 -> ref 11nnnkkk 6 2: f2 800 copy: 15 09 b2 0e 82 c1 6e b6 96 c5 1f 36 8d 17 61 e2 801 copy: b5 d4 22 d4 ed 2b 802 Compressed: 803 b0 d1 01 1d f2 15 09 b2 0e 82 c1 6e b6 96 c5 1f 804 36 8d 17 61 e2 b5 d4 22 d4 ed 2b 805 Was 42 bytes; compressed to 27 bytes, compression factor 1.56 807 Figure 15: A DTLS application data packet 809 Figure 16 shows that the compression is slightly worse in a 810 subsequent packet (containing 6 bytes of cleartext and 8 bytes of 811 authenticator, yielding a net compression of 13 bytes). The total 812 overhead does stay at a quite acceptable 8 bytes. 814 IP header: 815 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 816 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 817 00 00 00 00 00 00 00 00 818 Payload: 819 17 fe fd 00 01 00 00 00 00 00 05 00 16 00 01 00 820 00 00 00 00 05 ae a0 15 56 67 92 4d ff 8a 24 e4 821 cb 35 b9 822 Dictionary: 823 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 824 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 825 16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00 826 ref(13): 17 fe fd 00 01 00 00 00 00 00 827 -> ref 101nssss 1 0/11nnnkkk 0 3: b0 c3 828 copy: 03 05 00 16 829 ref(10): 00 01 00 00 00 00 00 05 -> ref 11nnnkkk 6 2: f2 830 copy: 0e ae a0 15 56 67 92 4d ff 8a 24 e4 cb 35 b9 831 Compressed: 832 b0 c3 03 05 00 16 f2 0e ae a0 15 56 67 92 4d ff 833 8a 24 e4 cb 35 b9 834 Was 35 bytes; compressed to 22 bytes, compression factor 1.59 836 Figure 16: Another DTLS application data packet 838 Figure 17 shows the compression of a DTLS handshake message, here a 839 client hello. There is little that can be compressed about the 32 840 bytes of randomness. Still, the net reduction is by 14 bytes. 842 IP header: 843 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 844 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 845 00 00 00 00 00 00 00 00 846 Payload: 847 16 fe fd 00 00 00 00 00 00 00 00 00 36 01 00 00 848 2a 00 00 00 00 00 00 00 2a fe fd 51 52 ed 79 a4 849 20 c9 62 56 11 47 c9 39 ee 6c c0 a4 fe c6 89 2f 850 32 26 9a 16 4e 31 7e 9f 20 92 92 00 00 00 02 c0 851 a8 01 00 852 Dictionary: 853 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 854 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 855 16 fe fd 17 fe fd 00 01 00 00 00 00 00 01 00 00 856 ref(16): 16 fe fd -> ref 101nssss 0 1/11nnnkkk 1 5: a1 cd 857 9 nulls: 87 858 copy: 01 36 859 ref(16): 01 00 00 -> ref 101nssss 0 1/11nnnkkk 1 5: a1 cd 860 copy: 01 2a 861 7 nulls: 85 862 copy: 23 2a fe fd 51 52 ed 79 a4 20 c9 62 56 11 47 c9 863 copy: 39 ee 6c c0 a4 fe c6 89 2f 32 26 9a 16 4e 31 7e 864 copy: 9f 20 92 92 865 3 nulls: 81 866 copy: 05 02 c0 a8 01 00 867 Compressed: 868 a1 cd 87 01 36 a1 cd 01 2a 85 23 2a fe fd 51 52 869 ed 79 a4 20 c9 62 56 11 47 c9 39 ee 6c c0 a4 fe 870 c6 89 2f 32 26 9a 16 4e 31 7e 9f 20 92 92 81 05 871 02 c0 a8 01 00 872 Was 67 bytes; compressed to 53 bytes, compression factor 1.26 874 Figure 17: A DTLS handshake packet (client hello) 876 Author's Address 878 Carsten Bormann 879 Universitaet Bremen TZI 880 Postfach 330440 881 D-28359 Bremen 882 Germany 884 Phone: +49-421-218-63921 885 Email: cabo@tzi.org