| 1 | |
| 2 | |
| 3 | |
| 4 | |
| 5 | |
| 6 | |
| 7 | Network Working Group P. Deutsch |
| 8 | Request for Comments: 1950 Aladdin Enterprises |
| 9 | Category: Informational J-L. Gailly |
| 10 | Info-ZIP |
| 11 | May 1996 |
| 12 | |
| 13 | |
| 14 | ZLIB Compressed Data Format Specification version 3.3 |
| 15 | |
| 16 | Status of This Memo |
| 17 | |
| 18 | This memo provides information for the Internet community. This memo |
| 19 | does not specify an Internet standard of any kind. Distribution of |
| 20 | this memo is unlimited. |
| 21 | |
| 22 | IESG Note: |
| 23 | |
| 24 | The IESG takes no position on the validity of any Intellectual |
| 25 | Property Rights statements contained in this document. |
| 26 | |
| 27 | Notices |
| 28 | |
| 29 | Copyright (c) 1996 L. Peter Deutsch and Jean-Loup Gailly |
| 30 | |
| 31 | Permission is granted to copy and distribute this document for any |
| 32 | purpose and without charge, including translations into other |
| 33 | languages and incorporation into compilations, provided that the |
| 34 | copyright notice and this notice are preserved, and that any |
| 35 | substantive changes or deletions from the original are clearly |
| 36 | marked. |
| 37 | |
| 38 | A pointer to the latest version of this and related documentation in |
| 39 | HTML format can be found at the URL |
| 40 | <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>. |
| 41 | |
| 42 | Abstract |
| 43 | |
| 44 | This specification defines a lossless compressed data format. The |
| 45 | data can be produced or consumed, even for an arbitrarily long |
| 46 | sequentially presented input data stream, using only an a priori |
| 47 | bounded amount of intermediate storage. The format presently uses |
| 48 | the DEFLATE compression method but can be easily extended to use |
| 49 | other compression methods. It can be implemented readily in a manner |
| 50 | not covered by patents. This specification also defines the ADLER-32 |
| 51 | checksum (an extension and improvement of the Fletcher checksum), |
| 52 | used for detection of data corruption, and provides an algorithm for |
| 53 | computing it. |
| 54 | |
| 55 | |
| 56 | |
| 57 | |
| 58 | Deutsch & Gailly Informational [Page 1] |
| 59 | \f |
| 60 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
| 61 | |
| 62 | |
| 63 | Table of Contents |
| 64 | |
| 65 | 1. Introduction ................................................... 2 |
| 66 | 1.1. Purpose ................................................... 2 |
| 67 | 1.2. Intended audience ......................................... 3 |
| 68 | 1.3. Scope ..................................................... 3 |
| 69 | 1.4. Compliance ................................................ 3 |
| 70 | 1.5. Definitions of terms and conventions used ................ 3 |
| 71 | 1.6. Changes from previous versions ............................ 3 |
| 72 | 2. Detailed specification ......................................... 3 |
| 73 | 2.1. Overall conventions ....................................... 3 |
| 74 | 2.2. Data format ............................................... 4 |
| 75 | 2.3. Compliance ................................................ 7 |
| 76 | 3. References ..................................................... 7 |
| 77 | 4. Source code .................................................... 8 |
| 78 | 5. Security Considerations ........................................ 8 |
| 79 | 6. Acknowledgements ............................................... 8 |
| 80 | 7. Authors' Addresses ............................................. 8 |
| 81 | 8. Appendix: Rationale ............................................ 9 |
| 82 | 9. Appendix: Sample code ..........................................10 |
| 83 | |
| 84 | 1. Introduction |
| 85 | |
| 86 | 1.1. Purpose |
| 87 | |
| 88 | The purpose of this specification is to define a lossless |
| 89 | compressed data format that: |
| 90 | |
| 91 | * Is independent of CPU type, operating system, file system, |
| 92 | and character set, and hence can be used for interchange; |
| 93 | |
| 94 | * Can be produced or consumed, even for an arbitrarily long |
| 95 | sequentially presented input data stream, using only an a |
| 96 | priori bounded amount of intermediate storage, and hence can |
| 97 | be used in data communications or similar structures such as |
| 98 | Unix filters; |
| 99 | |
| 100 | * Can use a number of different compression methods; |
| 101 | |
| 102 | * Can be implemented readily in a manner not covered by |
| 103 | patents, and hence can be practiced freely. |
| 104 | |
| 105 | The data format defined by this specification does not attempt to |
| 106 | allow random access to compressed data. |
| 107 | |
| 108 | |
| 109 | |
| 110 | |
| 111 | |
| 112 | |
| 113 | |
| 114 | Deutsch & Gailly Informational [Page 2] |
| 115 | \f |
| 116 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
| 117 | |
| 118 | |
| 119 | 1.2. Intended audience |
| 120 | |
| 121 | This specification is intended for use by implementors of software |
| 122 | to compress data into zlib format and/or decompress data from zlib |
| 123 | format. |
| 124 | |
| 125 | The text of the specification assumes a basic background in |
| 126 | programming at the level of bits and other primitive data |
| 127 | representations. |
| 128 | |
| 129 | 1.3. Scope |
| 130 | |
| 131 | The specification specifies a compressed data format that can be |
| 132 | used for in-memory compression of a sequence of arbitrary bytes. |
| 133 | |
| 134 | 1.4. Compliance |
| 135 | |
| 136 | Unless otherwise indicated below, a compliant decompressor must be |
| 137 | able to accept and decompress any data set that conforms to all |
| 138 | the specifications presented here; a compliant compressor must |
| 139 | produce data sets that conform to all the specifications presented |
| 140 | here. |
| 141 | |
| 142 | 1.5. Definitions of terms and conventions used |
| 143 | |
| 144 | byte: 8 bits stored or transmitted as a unit (same as an octet). |
| 145 | (For this specification, a byte is exactly 8 bits, even on |
| 146 | machines which store a character on a number of bits different |
| 147 | from 8.) See below, for the numbering of bits within a byte. |
| 148 | |
| 149 | 1.6. Changes from previous versions |
| 150 | |
| 151 | Version 3.1 was the first public release of this specification. |
| 152 | In version 3.2, some terminology was changed and the Adler-32 |
| 153 | sample code was rewritten for clarity. In version 3.3, the |
| 154 | support for a preset dictionary was introduced, and the |
| 155 | specification was converted to RFC style. |
| 156 | |
| 157 | 2. Detailed specification |
| 158 | |
| 159 | 2.1. Overall conventions |
| 160 | |
| 161 | In the diagrams below, a box like this: |
| 162 | |
| 163 | +---+ |
| 164 | | | <-- the vertical bars might be missing |
| 165 | +---+ |
| 166 | |
| 167 | |
| 168 | |
| 169 | |
| 170 | Deutsch & Gailly Informational [Page 3] |
| 171 | \f |
| 172 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
| 173 | |
| 174 | |
| 175 | represents one byte; a box like this: |
| 176 | |
| 177 | +==============+ |
| 178 | | | |
| 179 | +==============+ |
| 180 | |
| 181 | represents a variable number of bytes. |
| 182 | |
| 183 | Bytes stored within a computer do not have a "bit order", since |
| 184 | they are always treated as a unit. However, a byte considered as |
| 185 | an integer between 0 and 255 does have a most- and least- |
| 186 | significant bit, and since we write numbers with the most- |
| 187 | significant digit on the left, we also write bytes with the most- |
| 188 | significant bit on the left. In the diagrams below, we number the |
| 189 | bits of a byte so that bit 0 is the least-significant bit, i.e., |
| 190 | the bits are numbered: |
| 191 | |
| 192 | +--------+ |
| 193 | |76543210| |
| 194 | +--------+ |
| 195 | |
| 196 | Within a computer, a number may occupy multiple bytes. All |
| 197 | multi-byte numbers in the format described here are stored with |
| 198 | the MOST-significant byte first (at the lower memory address). |
| 199 | For example, the decimal number 520 is stored as: |
| 200 | |
| 201 | 0 1 |
| 202 | +--------+--------+ |
| 203 | |00000010|00001000| |
| 204 | +--------+--------+ |
| 205 | ^ ^ |
| 206 | | | |
| 207 | | + less significant byte = 8 |
| 208 | + more significant byte = 2 x 256 |
| 209 | |
| 210 | 2.2. Data format |
| 211 | |
| 212 | A zlib stream has the following structure: |
| 213 | |
| 214 | 0 1 |
| 215 | +---+---+ |
| 216 | |CMF|FLG| (more-->) |
| 217 | +---+---+ |
| 218 | |
| 219 | |
| 220 | |
| 221 | |
| 222 | |
| 223 | |
| 224 | |
| 225 | |
| 226 | Deutsch & Gailly Informational [Page 4] |
| 227 | \f |
| 228 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
| 229 | |
| 230 | |
| 231 | (if FLG.FDICT set) |
| 232 | |
| 233 | 0 1 2 3 |
| 234 | +---+---+---+---+ |
| 235 | | DICTID | (more-->) |
| 236 | +---+---+---+---+ |
| 237 | |
| 238 | +=====================+---+---+---+---+ |
| 239 | |...compressed data...| ADLER32 | |
| 240 | +=====================+---+---+---+---+ |
| 241 | |
| 242 | Any data which may appear after ADLER32 are not part of the zlib |
| 243 | stream. |
| 244 | |
| 245 | CMF (Compression Method and flags) |
| 246 | This byte is divided into a 4-bit compression method and a 4- |
| 247 | bit information field depending on the compression method. |
| 248 | |
| 249 | bits 0 to 3 CM Compression method |
| 250 | bits 4 to 7 CINFO Compression info |
| 251 | |
| 252 | CM (Compression method) |
| 253 | This identifies the compression method used in the file. CM = 8 |
| 254 | denotes the "deflate" compression method with a window size up |
| 255 | to 32K. This is the method used by gzip and PNG (see |
| 256 | references [1] and [2] in Chapter 3, below, for the reference |
| 257 | documents). CM = 15 is reserved. It might be used in a future |
| 258 | version of this specification to indicate the presence of an |
| 259 | extra field before the compressed data. |
| 260 | |
| 261 | CINFO (Compression info) |
| 262 | For CM = 8, CINFO is the base-2 logarithm of the LZ77 window |
| 263 | size, minus eight (CINFO=7 indicates a 32K window size). Values |
| 264 | of CINFO above 7 are not allowed in this version of the |
| 265 | specification. CINFO is not defined in this specification for |
| 266 | CM not equal to 8. |
| 267 | |
| 268 | FLG (FLaGs) |
| 269 | This flag byte is divided as follows: |
| 270 | |
| 271 | bits 0 to 4 FCHECK (check bits for CMF and FLG) |
| 272 | bit 5 FDICT (preset dictionary) |
| 273 | bits 6 to 7 FLEVEL (compression level) |
| 274 | |
| 275 | The FCHECK value must be such that CMF and FLG, when viewed as |
| 276 | a 16-bit unsigned integer stored in MSB order (CMF*256 + FLG), |
| 277 | is a multiple of 31. |
| 278 | |
| 279 | |
| 280 | |
| 281 | |
| 282 | Deutsch & Gailly Informational [Page 5] |
| 283 | \f |
| 284 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
| 285 | |
| 286 | |
| 287 | FDICT (Preset dictionary) |
| 288 | If FDICT is set, a DICT dictionary identifier is present |
| 289 | immediately after the FLG byte. The dictionary is a sequence of |
| 290 | bytes which are initially fed to the compressor without |
| 291 | producing any compressed output. DICT is the Adler-32 checksum |
| 292 | of this sequence of bytes (see the definition of ADLER32 |
| 293 | below). The decompressor can use this identifier to determine |
| 294 | which dictionary has been used by the compressor. |
| 295 | |
| 296 | FLEVEL (Compression level) |
| 297 | These flags are available for use by specific compression |
| 298 | methods. The "deflate" method (CM = 8) sets these flags as |
| 299 | follows: |
| 300 | |
| 301 | 0 - compressor used fastest algorithm |
| 302 | 1 - compressor used fast algorithm |
| 303 | 2 - compressor used default algorithm |
| 304 | 3 - compressor used maximum compression, slowest algorithm |
| 305 | |
| 306 | The information in FLEVEL is not needed for decompression; it |
| 307 | is there to indicate if recompression might be worthwhile. |
| 308 | |
| 309 | compressed data |
| 310 | For compression method 8, the compressed data is stored in the |
| 311 | deflate compressed data format as described in the document |
| 312 | "DEFLATE Compressed Data Format Specification" by L. Peter |
| 313 | Deutsch. (See reference [3] in Chapter 3, below) |
| 314 | |
| 315 | Other compressed data formats are not specified in this version |
| 316 | of the zlib specification. |
| 317 | |
| 318 | ADLER32 (Adler-32 checksum) |
| 319 | This contains a checksum value of the uncompressed data |
| 320 | (excluding any dictionary data) computed according to Adler-32 |
| 321 | algorithm. This algorithm is a 32-bit extension and improvement |
| 322 | of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073 |
| 323 | standard. See references [4] and [5] in Chapter 3, below) |
| 324 | |
| 325 | Adler-32 is composed of two sums accumulated per byte: s1 is |
| 326 | the sum of all bytes, s2 is the sum of all s1 values. Both sums |
| 327 | are done modulo 65521. s1 is initialized to 1, s2 to zero. The |
| 328 | Adler-32 checksum is stored as s2*65536 + s1 in most- |
| 329 | significant-byte first (network) order. |
| 330 | |
| 331 | |
| 332 | |
| 333 | |
| 334 | |
| 335 | |
| 336 | |
| 337 | |
| 338 | Deutsch & Gailly Informational [Page 6] |
| 339 | \f |
| 340 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
| 341 | |
| 342 | |
| 343 | 2.3. Compliance |
| 344 | |
| 345 | A compliant compressor must produce streams with correct CMF, FLG |
| 346 | and ADLER32, but need not support preset dictionaries. When the |
| 347 | zlib data format is used as part of another standard data format, |
| 348 | the compressor may use only preset dictionaries that are specified |
| 349 | by this other data format. If this other format does not use the |
| 350 | preset dictionary feature, the compressor must not set the FDICT |
| 351 | flag. |
| 352 | |
| 353 | A compliant decompressor must check CMF, FLG, and ADLER32, and |
| 354 | provide an error indication if any of these have incorrect values. |
| 355 | A compliant decompressor must give an error indication if CM is |
| 356 | not one of the values defined in this specification (only the |
| 357 | value 8 is permitted in this version), since another value could |
| 358 | indicate the presence of new features that would cause subsequent |
| 359 | data to be interpreted incorrectly. A compliant decompressor must |
| 360 | give an error indication if FDICT is set and DICTID is not the |
| 361 | identifier of a known preset dictionary. A decompressor may |
| 362 | ignore FLEVEL and still be compliant. When the zlib data format |
| 363 | is being used as a part of another standard format, a compliant |
| 364 | decompressor must support all the preset dictionaries specified by |
| 365 | the other format. When the other format does not use the preset |
| 366 | dictionary feature, a compliant decompressor must reject any |
| 367 | stream in which the FDICT flag is set. |
| 368 | |
| 369 | 3. References |
| 370 | |
| 371 | [1] Deutsch, L.P.,"GZIP Compressed Data Format Specification", |
| 372 | available in ftp://ftp.uu.net/pub/archiving/zip/doc/ |
| 373 | |
| 374 | [2] Thomas Boutell, "PNG (Portable Network Graphics) specification", |
| 375 | available in ftp://ftp.uu.net/graphics/png/documents/ |
| 376 | |
| 377 | [3] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification", |
| 378 | available in ftp://ftp.uu.net/pub/archiving/zip/doc/ |
| 379 | |
| 380 | [4] Fletcher, J. G., "An Arithmetic Checksum for Serial |
| 381 | Transmissions," IEEE Transactions on Communications, Vol. COM-30, |
| 382 | No. 1, January 1982, pp. 247-252. |
| 383 | |
| 384 | [5] ITU-T Recommendation X.224, Annex D, "Checksum Algorithms," |
| 385 | November, 1993, pp. 144, 145. (Available from |
| 386 | gopher://info.itu.ch). ITU-T X.244 is also the same as ISO 8073. |
| 387 | |
| 388 | |
| 389 | |
| 390 | |
| 391 | |
| 392 | |
| 393 | |
| 394 | Deutsch & Gailly Informational [Page 7] |
| 395 | \f |
| 396 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
| 397 | |
| 398 | |
| 399 | 4. Source code |
| 400 | |
| 401 | Source code for a C language implementation of a "zlib" compliant |
| 402 | library is available at ftp://ftp.uu.net/pub/archiving/zip/zlib/. |
| 403 | |
| 404 | 5. Security Considerations |
| 405 | |
| 406 | A decoder that fails to check the ADLER32 checksum value may be |
| 407 | subject to undetected data corruption. |
| 408 | |
| 409 | 6. Acknowledgements |
| 410 | |
| 411 | Trademarks cited in this document are the property of their |
| 412 | respective owners. |
| 413 | |
| 414 | Jean-Loup Gailly and Mark Adler designed the zlib format and wrote |
| 415 | the related software described in this specification. Glenn |
| 416 | Randers-Pehrson converted this document to RFC and HTML format. |
| 417 | |
| 418 | 7. Authors' Addresses |
| 419 | |
| 420 | L. Peter Deutsch |
| 421 | Aladdin Enterprises |
| 422 | 203 Santa Margarita Ave. |
| 423 | Menlo Park, CA 94025 |
| 424 | |
| 425 | Phone: (415) 322-0103 (AM only) |
| 426 | FAX: (415) 322-1734 |
| 427 | EMail: <ghost@aladdin.com> |
| 428 | |
| 429 | |
| 430 | Jean-Loup Gailly |
| 431 | |
| 432 | EMail: <gzip@prep.ai.mit.edu> |
| 433 | |
| 434 | Questions about the technical content of this specification can be |
| 435 | sent by email to |
| 436 | |
| 437 | Jean-Loup Gailly <gzip@prep.ai.mit.edu> and |
| 438 | Mark Adler <madler@alumni.caltech.edu> |
| 439 | |
| 440 | Editorial comments on this specification can be sent by email to |
| 441 | |
| 442 | L. Peter Deutsch <ghost@aladdin.com> and |
| 443 | Glenn Randers-Pehrson <randeg@alumni.rpi.edu> |
| 444 | |
| 445 | |
| 446 | |
| 447 | |
| 448 | |
| 449 | |
| 450 | Deutsch & Gailly Informational [Page 8] |
| 451 | \f |
| 452 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
| 453 | |
| 454 | |
| 455 | 8. Appendix: Rationale |
| 456 | |
| 457 | 8.1. Preset dictionaries |
| 458 | |
| 459 | A preset dictionary is specially useful to compress short input |
| 460 | sequences. The compressor can take advantage of the dictionary |
| 461 | context to encode the input in a more compact manner. The |
| 462 | decompressor can be initialized with the appropriate context by |
| 463 | virtually decompressing a compressed version of the dictionary |
| 464 | without producing any output. However for certain compression |
| 465 | algorithms such as the deflate algorithm this operation can be |
| 466 | achieved without actually performing any decompression. |
| 467 | |
| 468 | The compressor and the decompressor must use exactly the same |
| 469 | dictionary. The dictionary may be fixed or may be chosen among a |
| 470 | certain number of predefined dictionaries, according to the kind |
| 471 | of input data. The decompressor can determine which dictionary has |
| 472 | been chosen by the compressor by checking the dictionary |
| 473 | identifier. This document does not specify the contents of |
| 474 | predefined dictionaries, since the optimal dictionaries are |
| 475 | application specific. Standard data formats using this feature of |
| 476 | the zlib specification must precisely define the allowed |
| 477 | dictionaries. |
| 478 | |
| 479 | 8.2. The Adler-32 algorithm |
| 480 | |
| 481 | The Adler-32 algorithm is much faster than the CRC32 algorithm yet |
| 482 | still provides an extremely low probability of undetected errors. |
| 483 | |
| 484 | The modulo on unsigned long accumulators can be delayed for 5552 |
| 485 | bytes, so the modulo operation time is negligible. If the bytes |
| 486 | are a, b, c, the second sum is 3a + 2b + c + 3, and so is position |
| 487 | and order sensitive, unlike the first sum, which is just a |
| 488 | checksum. That 65521 is prime is important to avoid a possible |
| 489 | large class of two-byte errors that leave the check unchanged. |
| 490 | (The Fletcher checksum uses 255, which is not prime and which also |
| 491 | makes the Fletcher check insensitive to single byte changes 0 <-> |
| 492 | 255.) |
| 493 | |
| 494 | The sum s1 is initialized to 1 instead of zero to make the length |
| 495 | of the sequence part of s2, so that the length does not have to be |
| 496 | checked separately. (Any sequence of zeroes has a Fletcher |
| 497 | checksum of zero.) |
| 498 | |
| 499 | |
| 500 | |
| 501 | |
| 502 | |
| 503 | |
| 504 | |
| 505 | |
| 506 | Deutsch & Gailly Informational [Page 9] |
| 507 | \f |
| 508 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
| 509 | |
| 510 | |
| 511 | 9. Appendix: Sample code |
| 512 | |
| 513 | The following C code computes the Adler-32 checksum of a data buffer. |
| 514 | It is written for clarity, not for speed. The sample code is in the |
| 515 | ANSI C programming language. Non C users may find it easier to read |
| 516 | with these hints: |
| 517 | |
| 518 | & Bitwise AND operator. |
| 519 | >> Bitwise right shift operator. When applied to an |
| 520 | unsigned quantity, as here, right shift inserts zero bit(s) |
| 521 | at the left. |
| 522 | << Bitwise left shift operator. Left shift inserts zero |
| 523 | bit(s) at the right. |
| 524 | ++ "n++" increments the variable n. |
| 525 | % modulo operator: a % b is the remainder of a divided by b. |
| 526 | |
| 527 | #define BASE 65521 /* largest prime smaller than 65536 */ |
| 528 | |
| 529 | /* |
| 530 | Update a running Adler-32 checksum with the bytes buf[0..len-1] |
| 531 | and return the updated checksum. The Adler-32 checksum should be |
| 532 | initialized to 1. |
| 533 | |
| 534 | Usage example: |
| 535 | |
| 536 | unsigned long adler = 1L; |
| 537 | |
| 538 | while (read_buffer(buffer, length) != EOF) { |
| 539 | adler = update_adler32(adler, buffer, length); |
| 540 | } |
| 541 | if (adler != original_adler) error(); |
| 542 | */ |
| 543 | unsigned long update_adler32(unsigned long adler, |
| 544 | unsigned char *buf, int len) |
| 545 | { |
| 546 | unsigned long s1 = adler & 0xffff; |
| 547 | unsigned long s2 = (adler >> 16) & 0xffff; |
| 548 | int n; |
| 549 | |
| 550 | for (n = 0; n < len; n++) { |
| 551 | s1 = (s1 + buf[n]) % BASE; |
| 552 | s2 = (s2 + s1) % BASE; |
| 553 | } |
| 554 | return (s2 << 16) + s1; |
| 555 | } |
| 556 | |
| 557 | /* Return the adler32 of the bytes buf[0..len-1] */ |
| 558 | |
| 559 | |
| 560 | |
| 561 | |
| 562 | Deutsch & Gailly Informational [Page 10] |
| 563 | \f |
| 564 | RFC 1950 ZLIB Compressed Data Format Specification May 1996 |
| 565 | |
| 566 | |
| 567 | unsigned long adler32(unsigned char *buf, int len) |
| 568 | { |
| 569 | return update_adler32(1L, buf, len); |
| 570 | } |
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| 618 | Deutsch & Gailly Informational [Page 11] |
| 619 | \f |