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date Mon, 15 Sep 2025 11:43:07 +0200
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7 Network Working Group P. Deutsch
8 Request for Comments: 1952 Aladdin Enterprises
9 Category: Informational May 1996
10
11
12 GZIP file format specification version 4.3
13
14 Status of This Memo
15
16 This memo provides information for the Internet community. This memo
17 does not specify an Internet standard of any kind. Distribution of
18 this memo is unlimited.
19
20 IESG Note:
21
22 The IESG takes no position on the validity of any Intellectual
23 Property Rights statements contained in this document.
24
25 Notices
26
27 Copyright (c) 1996 L. Peter Deutsch
28
29 Permission is granted to copy and distribute this document for any
30 purpose and without charge, including translations into other
31 languages and incorporation into compilations, provided that the
32 copyright notice and this notice are preserved, and that any
33 substantive changes or deletions from the original are clearly
34 marked.
35
36 A pointer to the latest version of this and related documentation in
37 HTML format can be found at the URL
38 <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>.
39
40 Abstract
41
42 This specification defines a lossless compressed data format that is
43 compatible with the widely used GZIP utility. The format includes a
44 cyclic redundancy check value for detecting data corruption. The
45 format presently uses the DEFLATE method of compression but can be
46 easily extended to use other compression methods. The format can be
47 implemented readily in a manner not covered by patents.
48
49
50
51
52
53
54
55
56
57
58 Deutsch Informational [Page 1]
59
60 RFC 1952 GZIP File 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 ......................................... 4
73 2.1. Overall conventions ....................................... 4
74 2.2. File format ............................................... 5
75 2.3. Member format ............................................. 5
76 2.3.1. Member header and trailer ........................... 6
77 2.3.1.1. Extra field ................................... 8
78 2.3.1.2. Compliance .................................... 9
79 3. References .................................................. 9
80 4. Security Considerations .................................... 10
81 5. Acknowledgements ........................................... 10
82 6. Author's Address ........................................... 10
83 7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
84 8. Appendix: Sample CRC Code .................................. 11
85
86 1. Introduction
87
88 1.1. Purpose
89
90 The purpose of this specification is to define a lossless
91 compressed data format that:
92
93 * Is independent of CPU type, operating system, file system,
94 and character set, and hence can be used for interchange;
95 * Can compress or decompress a data stream (as opposed to a
96 randomly accessible file) to produce another data stream,
97 using only an a priori bounded amount of intermediate
98 storage, and hence can be used in data communications or
99 similar structures such as Unix filters;
100 * Compresses data with efficiency comparable to the best
101 currently available general-purpose compression methods,
102 and in particular considerably better than the "compress"
103 program;
104 * Can be implemented readily in a manner not covered by
105 patents, and hence can be practiced freely;
106 * Is compatible with the file format produced by the current
107 widely used gzip utility, in that conforming decompressors
108 will be able to read data produced by the existing gzip
109 compressor.
110
111
112
113
114 Deutsch Informational [Page 2]
115
116 RFC 1952 GZIP File Format Specification May 1996
117
118
119 The data format defined by this specification does not attempt to:
120
121 * Provide random access to compressed data;
122 * Compress specialized data (e.g., raster graphics) as well as
123 the best currently available specialized algorithms.
124
125 1.2. Intended audience
126
127 This specification is intended for use by implementors of software
128 to compress data into gzip format and/or decompress data from gzip
129 format.
130
131 The text of the specification assumes a basic background in
132 programming at the level of bits and other primitive data
133 representations.
134
135 1.3. Scope
136
137 The specification specifies a compression method and a file format
138 (the latter assuming only that a file can store a sequence of
139 arbitrary bytes). It does not specify any particular interface to
140 a file system or anything about character sets or encodings
141 (except for file names and comments, which are optional).
142
143 1.4. Compliance
144
145 Unless otherwise indicated below, a compliant decompressor must be
146 able to accept and decompress any file that conforms to all the
147 specifications presented here; a compliant compressor must produce
148 files that conform to all the specifications presented here. The
149 material in the appendices is not part of the specification per se
150 and is not relevant to compliance.
151
152 1.5. Definitions of terms and conventions used
153
154 byte: 8 bits stored or transmitted as a unit (same as an octet).
155 (For this specification, a byte is exactly 8 bits, even on
156 machines which store a character on a number of bits different
157 from 8.) See below for the numbering of bits within a byte.
158
159 1.6. Changes from previous versions
160
161 There have been no technical changes to the gzip format since
162 version 4.1 of this specification. In version 4.2, some
163 terminology was changed, and the sample CRC code was rewritten for
164 clarity and to eliminate the requirement for the caller to do pre-
165 and post-conditioning. Version 4.3 is a conversion of the
166 specification to RFC style.
167
168
169
170 Deutsch Informational [Page 3]
171
172 RFC 1952 GZIP File Format Specification May 1996
173
174
175 2. Detailed specification
176
177 2.1. Overall conventions
178
179 In the diagrams below, a box like this:
180
181 +---+
182 | | <-- the vertical bars might be missing
183 +---+
184
185 represents one byte; a box like this:
186
187 +==============+
188 | |
189 +==============+
190
191 represents a variable number of bytes.
192
193 Bytes stored within a computer do not have a "bit order", since
194 they are always treated as a unit. However, a byte considered as
195 an integer between 0 and 255 does have a most- and least-
196 significant bit, and since we write numbers with the most-
197 significant digit on the left, we also write bytes with the most-
198 significant bit on the left. In the diagrams below, we number the
199 bits of a byte so that bit 0 is the least-significant bit, i.e.,
200 the bits are numbered:
201
202 +--------+
203 |76543210|
204 +--------+
205
206 This document does not address the issue of the order in which
207 bits of a byte are transmitted on a bit-sequential medium, since
208 the data format described here is byte- rather than bit-oriented.
209
210 Within a computer, a number may occupy multiple bytes. All
211 multi-byte numbers in the format described here are stored with
212 the least-significant byte first (at the lower memory address).
213 For example, the decimal number 520 is stored as:
214
215 0 1
216 +--------+--------+
217 |00001000|00000010|
218 +--------+--------+
219 ^ ^
220 | |
221 | + more significant byte = 2 x 256
222 + less significant byte = 8
223
224
225
226 Deutsch Informational [Page 4]
227
228 RFC 1952 GZIP File Format Specification May 1996
229
230
231 2.2. File format
232
233 A gzip file consists of a series of "members" (compressed data
234 sets). The format of each member is specified in the following
235 section. The members simply appear one after another in the file,
236 with no additional information before, between, or after them.
237
238 2.3. Member format
239
240 Each member has the following structure:
241
242 +---+---+---+---+---+---+---+---+---+---+
243 |ID1|ID2|CM |FLG| MTIME |XFL|OS | (more-->)
244 +---+---+---+---+---+---+---+---+---+---+
245
246 (if FLG.FEXTRA set)
247
248 +---+---+=================================+
249 | XLEN |...XLEN bytes of "extra field"...| (more-->)
250 +---+---+=================================+
251
252 (if FLG.FNAME set)
253
254 +=========================================+
255 |...original file name, zero-terminated...| (more-->)
256 +=========================================+
257
258 (if FLG.FCOMMENT set)
259
260 +===================================+
261 |...file comment, zero-terminated...| (more-->)
262 +===================================+
263
264 (if FLG.FHCRC set)
265
266 +---+---+
267 | CRC16 |
268 +---+---+
269
270 +=======================+
271 |...compressed blocks...| (more-->)
272 +=======================+
273
274 0 1 2 3 4 5 6 7
275 +---+---+---+---+---+---+---+---+
276 | CRC32 | ISIZE |
277 +---+---+---+---+---+---+---+---+
278
279
280
281
282 Deutsch Informational [Page 5]
283
284 RFC 1952 GZIP File Format Specification May 1996
285
286
287 2.3.1. Member header and trailer
288
289 ID1 (IDentification 1)
290 ID2 (IDentification 2)
291 These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139
292 (0x8b, \213), to identify the file as being in gzip format.
293
294 CM (Compression Method)
295 This identifies the compression method used in the file. CM
296 = 0-7 are reserved. CM = 8 denotes the "deflate"
297 compression method, which is the one customarily used by
298 gzip and which is documented elsewhere.
299
300 FLG (FLaGs)
301 This flag byte is divided into individual bits as follows:
302
303 bit 0 FTEXT
304 bit 1 FHCRC
305 bit 2 FEXTRA
306 bit 3 FNAME
307 bit 4 FCOMMENT
308 bit 5 reserved
309 bit 6 reserved
310 bit 7 reserved
311
312 If FTEXT is set, the file is probably ASCII text. This is
313 an optional indication, which the compressor may set by
314 checking a small amount of the input data to see whether any
315 non-ASCII characters are present. In case of doubt, FTEXT
316 is cleared, indicating binary data. For systems which have
317 different file formats for ascii text and binary data, the
318 decompressor can use FTEXT to choose the appropriate format.
319 We deliberately do not specify the algorithm used to set
320 this bit, since a compressor always has the option of
321 leaving it cleared and a decompressor always has the option
322 of ignoring it and letting some other program handle issues
323 of data conversion.
324
325 If FHCRC is set, a CRC16 for the gzip header is present,
326 immediately before the compressed data. The CRC16 consists
327 of the two least significant bytes of the CRC32 for all
328 bytes of the gzip header up to and not including the CRC16.
329 [The FHCRC bit was never set by versions of gzip up to
330 1.2.4, even though it was documented with a different
331 meaning in gzip 1.2.4.]
332
333 If FEXTRA is set, optional extra fields are present, as
334 described in a following section.
335
336
337
338 Deutsch Informational [Page 6]
339
340 RFC 1952 GZIP File Format Specification May 1996
341
342
343 If FNAME is set, an original file name is present,
344 terminated by a zero byte. The name must consist of ISO
345 8859-1 (LATIN-1) characters; on operating systems using
346 EBCDIC or any other character set for file names, the name
347 must be translated to the ISO LATIN-1 character set. This
348 is the original name of the file being compressed, with any
349 directory components removed, and, if the file being
350 compressed is on a file system with case insensitive names,
351 forced to lower case. There is no original file name if the
352 data was compressed from a source other than a named file;
353 for example, if the source was stdin on a Unix system, there
354 is no file name.
355
356 If FCOMMENT is set, a zero-terminated file comment is
357 present. This comment is not interpreted; it is only
358 intended for human consumption. The comment must consist of
359 ISO 8859-1 (LATIN-1) characters. Line breaks should be
360 denoted by a single line feed character (10 decimal).
361
362 Reserved FLG bits must be zero.
363
364 MTIME (Modification TIME)
365 This gives the most recent modification time of the original
366 file being compressed. The time is in Unix format, i.e.,
367 seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this
368 may cause problems for MS-DOS and other systems that use
369 local rather than Universal time.) If the compressed data
370 did not come from a file, MTIME is set to the time at which
371 compression started. MTIME = 0 means no time stamp is
372 available.
373
374 XFL (eXtra FLags)
375 These flags are available for use by specific compression
376 methods. The "deflate" method (CM = 8) sets these flags as
377 follows:
378
379 XFL = 2 - compressor used maximum compression,
380 slowest algorithm
381 XFL = 4 - compressor used fastest algorithm
382
383 OS (Operating System)
384 This identifies the type of file system on which compression
385 took place. This may be useful in determining end-of-line
386 convention for text files. The currently defined values are
387 as follows:
388
389
390
391
392
393
394 Deutsch Informational [Page 7]
395
396 RFC 1952 GZIP File Format Specification May 1996
397
398
399 0 - FAT filesystem (MS-DOS, OS/2, NT/Win32)
400 1 - Amiga
401 2 - VMS (or OpenVMS)
402 3 - Unix
403 4 - VM/CMS
404 5 - Atari TOS
405 6 - HPFS filesystem (OS/2, NT)
406 7 - Macintosh
407 8 - Z-System
408 9 - CP/M
409 10 - TOPS-20
410 11 - NTFS filesystem (NT)
411 12 - QDOS
412 13 - Acorn RISCOS
413 255 - unknown
414
415 XLEN (eXtra LENgth)
416 If FLG.FEXTRA is set, this gives the length of the optional
417 extra field. See below for details.
418
419 CRC32 (CRC-32)
420 This contains a Cyclic Redundancy Check value of the
421 uncompressed data computed according to CRC-32 algorithm
422 used in the ISO 3309 standard and in section 8.1.1.6.2 of
423 ITU-T recommendation V.42. (See http://www.iso.ch for
424 ordering ISO documents. See gopher://info.itu.ch for an
425 online version of ITU-T V.42.)
426
427 ISIZE (Input SIZE)
428 This contains the size of the original (uncompressed) input
429 data modulo 2^32.
430
431 2.3.1.1. Extra field
432
433 If the FLG.FEXTRA bit is set, an "extra field" is present in
434 the header, with total length XLEN bytes. It consists of a
435 series of subfields, each of the form:
436
437 +---+---+---+---+==================================+
438 |SI1|SI2| LEN |... LEN bytes of subfield data ...|
439 +---+---+---+---+==================================+
440
441 SI1 and SI2 provide a subfield ID, typically two ASCII letters
442 with some mnemonic value. Jean-Loup Gailly
443 <gzip@prep.ai.mit.edu> is maintaining a registry of subfield
444 IDs; please send him any subfield ID you wish to use. Subfield
445 IDs with SI2 = 0 are reserved for future use. The following
446 IDs are currently defined:
447
448
449
450 Deutsch Informational [Page 8]
451
452 RFC 1952 GZIP File Format Specification May 1996
453
454
455 SI1 SI2 Data
456 ---------- ---------- ----
457 0x41 ('A') 0x70 ('P') Apollo file type information
458
459 LEN gives the length of the subfield data, excluding the 4
460 initial bytes.
461
462 2.3.1.2. Compliance
463
464 A compliant compressor must produce files with correct ID1,
465 ID2, CM, CRC32, and ISIZE, but may set all the other fields in
466 the fixed-length part of the header to default values (255 for
467 OS, 0 for all others). The compressor must set all reserved
468 bits to zero.
469
470 A compliant decompressor must check ID1, ID2, and CM, and
471 provide an error indication if any of these have incorrect
472 values. It must examine FEXTRA/XLEN, FNAME, FCOMMENT and FHCRC
473 at least so it can skip over the optional fields if they are
474 present. It need not examine any other part of the header or
475 trailer; in particular, a decompressor may ignore FTEXT and OS
476 and always produce binary output, and still be compliant. A
477 compliant decompressor must give an error indication if any
478 reserved bit is non-zero, since such a bit could indicate the
479 presence of a new field that would cause subsequent data to be
480 interpreted incorrectly.
481
482 3. References
483
484 [1] "Information Processing - 8-bit single-byte coded graphic
485 character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987).
486 The ISO 8859-1 (Latin-1) character set is a superset of 7-bit
487 ASCII. Files defining this character set are available as
488 iso_8859-1.* in ftp://ftp.uu.net/graphics/png/documents/
489
490 [2] ISO 3309
491
492 [3] ITU-T recommendation V.42
493
494 [4] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",
495 available in ftp://ftp.uu.net/pub/archiving/zip/doc/
496
497 [5] Gailly, J.-L., GZIP documentation, available as gzip-*.tar in
498 ftp://prep.ai.mit.edu/pub/gnu/
499
500 [6] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via Table
501 Look-Up", Communications of the ACM, 31(8), pp.1008-1013.
502
503
504
505
506 Deutsch Informational [Page 9]
507
508 RFC 1952 GZIP File Format Specification May 1996
509
510
511 [7] Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal,
512 pp.118-133.
513
514 [8] ftp://ftp.adelaide.edu.au/pub/rocksoft/papers/crc_v3.txt,
515 describing the CRC concept.
516
517 4. Security Considerations
518
519 Any data compression method involves the reduction of redundancy in
520 the data. Consequently, any corruption of the data is likely to have
521 severe effects and be difficult to correct. Uncompressed text, on
522 the other hand, will probably still be readable despite the presence
523 of some corrupted bytes.
524
525 It is recommended that systems using this data format provide some
526 means of validating the integrity of the compressed data, such as by
527 setting and checking the CRC-32 check value.
528
529 5. Acknowledgements
530
531 Trademarks cited in this document are the property of their
532 respective owners.
533
534 Jean-Loup Gailly designed the gzip format and wrote, with Mark Adler,
535 the related software described in this specification. Glenn
536 Randers-Pehrson converted this document to RFC and HTML format.
537
538 6. Author's Address
539
540 L. Peter Deutsch
541 Aladdin Enterprises
542 203 Santa Margarita Ave.
543 Menlo Park, CA 94025
544
545 Phone: (415) 322-0103 (AM only)
546 FAX: (415) 322-1734
547 EMail: <ghost@aladdin.com>
548
549 Questions about the technical content of this specification can be
550 sent by email to:
551
552 Jean-Loup Gailly <gzip@prep.ai.mit.edu> and
553 Mark Adler <madler@alumni.caltech.edu>
554
555 Editorial comments on this specification can be sent by email to:
556
557 L. Peter Deutsch <ghost@aladdin.com> and
558 Glenn Randers-Pehrson <randeg@alumni.rpi.edu>
559
560
561
562 Deutsch Informational [Page 10]
563
564 RFC 1952 GZIP File Format Specification May 1996
565
566
567 7. Appendix: Jean-Loup Gailly's gzip utility
568
569 The most widely used implementation of gzip compression, and the
570 original documentation on which this specification is based, were
571 created by Jean-Loup Gailly <gzip@prep.ai.mit.edu>. Since this
572 implementation is a de facto standard, we mention some more of its
573 features here. Again, the material in this section is not part of
574 the specification per se, and implementations need not follow it to
575 be compliant.
576
577 When compressing or decompressing a file, gzip preserves the
578 protection, ownership, and modification time attributes on the local
579 file system, since there is no provision for representing protection
580 attributes in the gzip file format itself. Since the file format
581 includes a modification time, the gzip decompressor provides a
582 command line switch that assigns the modification time from the file,
583 rather than the local modification time of the compressed input, to
584 the decompressed output.
585
586 8. Appendix: Sample CRC Code
587
588 The following sample code represents a practical implementation of
589 the CRC (Cyclic Redundancy Check). (See also ISO 3309 and ITU-T V.42
590 for a formal specification.)
591
592 The sample code is in the ANSI C programming language. Non C users
593 may find it easier to read with these hints:
594
595 & Bitwise AND operator.
596 ^ Bitwise exclusive-OR operator.
597 >> Bitwise right shift operator. When applied to an
598 unsigned quantity, as here, right shift inserts zero
599 bit(s) at the left.
600 ! Logical NOT operator.
601 ++ "n++" increments the variable n.
602 0xNNN 0x introduces a hexadecimal (base 16) constant.
603 Suffix L indicates a long value (at least 32 bits).
604
605 /* Table of CRCs of all 8-bit messages. */
606 unsigned long crc_table[256];
607
608 /* Flag: has the table been computed? Initially false. */
609 int crc_table_computed = 0;
610
611 /* Make the table for a fast CRC. */
612 void make_crc_table(void)
613 {
614 unsigned long c;
615
616
617
618 Deutsch Informational [Page 11]
619
620 RFC 1952 GZIP File Format Specification May 1996
621
622
623 int n, k;
624 for (n = 0; n < 256; n++) {
625 c = (unsigned long) n;
626 for (k = 0; k < 8; k++) {
627 if (c & 1) {
628 c = 0xedb88320L ^ (c >> 1);
629 } else {
630 c = c >> 1;
631 }
632 }
633 crc_table[n] = c;
634 }
635 crc_table_computed = 1;
636 }
637
638 /*
639 Update a running crc with the bytes buf[0..len-1] and return
640 the updated crc. The crc should be initialized to zero. Pre- and
641 post-conditioning (one's complement) is performed within this
642 function so it shouldn't be done by the caller. Usage example:
643
644 unsigned long crc = 0L;
645
646 while (read_buffer(buffer, length) != EOF) {
647 crc = update_crc(crc, buffer, length);
648 }
649 if (crc != original_crc) error();
650 */
651 unsigned long update_crc(unsigned long crc,
652 unsigned char *buf, int len)
653 {
654 unsigned long c = crc ^ 0xffffffffL;
655 int n;
656
657 if (!crc_table_computed)
658 make_crc_table();
659 for (n = 0; n < len; n++) {
660 c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
661 }
662 return c ^ 0xffffffffL;
663 }
664
665 /* Return the CRC of the bytes buf[0..len-1]. */
666 unsigned long crc(unsigned char *buf, int len)
667 {
668 return update_crc(0L, buf, len);
669 }
670
671
672
673
674 Deutsch Informational [Page 12]
675