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-
-
-
-
-
-
-Network Working Group P. Deutsch
-Request for Comments: 1952 Aladdin Enterprises
-Category: Informational May 1996
-
-
- GZIP file format specification version 4.3
-
-Status of This Memo
-
- This memo provides information for the Internet community. This memo
- does not specify an Internet standard of any kind. Distribution of
- this memo is unlimited.
-
-IESG Note:
-
- The IESG takes no position on the validity of any Intellectual
- Property Rights statements contained in this document.
-
-Notices
-
- Copyright (c) 1996 L. Peter Deutsch
-
- Permission is granted to copy and distribute this document for any
- purpose and without charge, including translations into other
- languages and incorporation into compilations, provided that the
- copyright notice and this notice are preserved, and that any
- substantive changes or deletions from the original are clearly
- marked.
-
- A pointer to the latest version of this and related documentation in
- HTML format can be found at the URL
- <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>.
-
-Abstract
-
- This specification defines a lossless compressed data format that is
- compatible with the widely used GZIP utility. The format includes a
- cyclic redundancy check value for detecting data corruption. The
- format presently uses the DEFLATE method of compression but can be
- easily extended to use other compression methods. The format can be
- implemented readily in a manner not covered by patents.
-
-
-
-
-
-
-
-
-
-
-Deutsch Informational [Page 1]
-
-RFC 1952 GZIP File Format Specification May 1996
-
-
-Table of Contents
-
- 1. Introduction ................................................... 2
- 1.1. Purpose ................................................... 2
- 1.2. Intended audience ......................................... 3
- 1.3. Scope ..................................................... 3
- 1.4. Compliance ................................................ 3
- 1.5. Definitions of terms and conventions used ................. 3
- 1.6. Changes from previous versions ............................ 3
- 2. Detailed specification ......................................... 4
- 2.1. Overall conventions ....................................... 4
- 2.2. File format ............................................... 5
- 2.3. Member format ............................................. 5
- 2.3.1. Member header and trailer ........................... 6
- 2.3.1.1. Extra field ................................... 8
- 2.3.1.2. Compliance .................................... 9
- 3. References .................................................. 9
- 4. Security Considerations .................................... 10
- 5. Acknowledgements ........................................... 10
- 6. Author's Address ........................................... 10
- 7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
- 8. Appendix: Sample CRC Code .................................. 11
-
-1. Introduction
-
- 1.1. Purpose
-
- The purpose of this specification is to define a lossless
- compressed data format that:
-
- * Is independent of CPU type, operating system, file system,
- and character set, and hence can be used for interchange;
- * Can compress or decompress a data stream (as opposed to a
- randomly accessible file) to produce another data stream,
- using only an a priori bounded amount of intermediate
- storage, and hence can be used in data communications or
- similar structures such as Unix filters;
- * Compresses data with efficiency comparable to the best
- currently available general-purpose compression methods,
- and in particular considerably better than the "compress"
- program;
- * Can be implemented readily in a manner not covered by
- patents, and hence can be practiced freely;
- * Is compatible with the file format produced by the current
- widely used gzip utility, in that conforming decompressors
- will be able to read data produced by the existing gzip
- compressor.
-
-
-
-
-Deutsch Informational [Page 2]
-
-RFC 1952 GZIP File Format Specification May 1996
-
-
- The data format defined by this specification does not attempt to:
-
- * Provide random access to compressed data;
- * Compress specialized data (e.g., raster graphics) as well as
- the best currently available specialized algorithms.
-
- 1.2. Intended audience
-
- This specification is intended for use by implementors of software
- to compress data into gzip format and/or decompress data from gzip
- format.
-
- The text of the specification assumes a basic background in
- programming at the level of bits and other primitive data
- representations.
-
- 1.3. Scope
-
- The specification specifies a compression method and a file format
- (the latter assuming only that a file can store a sequence of
- arbitrary bytes). It does not specify any particular interface to
- a file system or anything about character sets or encodings
- (except for file names and comments, which are optional).
-
- 1.4. Compliance
-
- Unless otherwise indicated below, a compliant decompressor must be
- able to accept and decompress any file that conforms to all the
- specifications presented here; a compliant compressor must produce
- files that conform to all the specifications presented here. The
- material in the appendices is not part of the specification per se
- and is not relevant to compliance.
-
- 1.5. Definitions of terms and conventions used
-
- byte: 8 bits stored or transmitted as a unit (same as an octet).
- (For this specification, a byte is exactly 8 bits, even on
- machines which store a character on a number of bits different
- from 8.) See below for the numbering of bits within a byte.
-
- 1.6. Changes from previous versions
-
- There have been no technical changes to the gzip format since
- version 4.1 of this specification. In version 4.2, some
- terminology was changed, and the sample CRC code was rewritten for
- clarity and to eliminate the requirement for the caller to do pre-
- and post-conditioning. Version 4.3 is a conversion of the
- specification to RFC style.
-
-
-
-Deutsch Informational [Page 3]
-
-RFC 1952 GZIP File Format Specification May 1996
-
-
-2. Detailed specification
-
- 2.1. Overall conventions
-
- In the diagrams below, a box like this:
-
- +---+
- | | <-- the vertical bars might be missing
- +---+
-
- represents one byte; a box like this:
-
- +==============+
- | |
- +==============+
-
- represents a variable number of bytes.
-
- Bytes stored within a computer do not have a "bit order", since
- they are always treated as a unit. However, a byte considered as
- an integer between 0 and 255 does have a most- and least-
- significant bit, and since we write numbers with the most-
- significant digit on the left, we also write bytes with the most-
- significant bit on the left. In the diagrams below, we number the
- bits of a byte so that bit 0 is the least-significant bit, i.e.,
- the bits are numbered:
-
- +--------+
- |76543210|
- +--------+
-
- This document does not address the issue of the order in which
- bits of a byte are transmitted on a bit-sequential medium, since
- the data format described here is byte- rather than bit-oriented.
-
- Within a computer, a number may occupy multiple bytes. All
- multi-byte numbers in the format described here are stored with
- the least-significant byte first (at the lower memory address).
- For example, the decimal number 520 is stored as:
-
- 0 1
- +--------+--------+
- |00001000|00000010|
- +--------+--------+
- ^ ^
- | |
- | + more significant byte = 2 x 256
- + less significant byte = 8
-
-
-
-Deutsch Informational [Page 4]
-
-RFC 1952 GZIP File Format Specification May 1996
-
-
- 2.2. File format
-
- A gzip file consists of a series of "members" (compressed data
- sets). The format of each member is specified in the following
- section. The members simply appear one after another in the file,
- with no additional information before, between, or after them.
-
- 2.3. Member format
-
- Each member has the following structure:
-
- +---+---+---+---+---+---+---+---+---+---+
- |ID1|ID2|CM |FLG| MTIME |XFL|OS | (more-->)
- +---+---+---+---+---+---+---+---+---+---+
-
- (if FLG.FEXTRA set)
-
- +---+---+=================================+
- | XLEN |...XLEN bytes of "extra field"...| (more-->)
- +---+---+=================================+
-
- (if FLG.FNAME set)
-
- +=========================================+
- |...original file name, zero-terminated...| (more-->)
- +=========================================+
-
- (if FLG.FCOMMENT set)
-
- +===================================+
- |...file comment, zero-terminated...| (more-->)
- +===================================+
-
- (if FLG.FHCRC set)
-
- +---+---+
- | CRC16 |
- +---+---+
-
- +=======================+
- |...compressed blocks...| (more-->)
- +=======================+
-
- 0 1 2 3 4 5 6 7
- +---+---+---+---+---+---+---+---+
- | CRC32 | ISIZE |
- +---+---+---+---+---+---+---+---+
-
-
-
-
-Deutsch Informational [Page 5]
-
-RFC 1952 GZIP File Format Specification May 1996
-
-
- 2.3.1. Member header and trailer
-
- ID1 (IDentification 1)
- ID2 (IDentification 2)
- These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139
- (0x8b, \213), to identify the file as being in gzip format.
-
- CM (Compression Method)
- This identifies the compression method used in the file. CM
- = 0-7 are reserved. CM = 8 denotes the "deflate"
- compression method, which is the one customarily used by
- gzip and which is documented elsewhere.
-
- FLG (FLaGs)
- This flag byte is divided into individual bits as follows:
-
- bit 0 FTEXT
- bit 1 FHCRC
- bit 2 FEXTRA
- bit 3 FNAME
- bit 4 FCOMMENT
- bit 5 reserved
- bit 6 reserved
- bit 7 reserved
-
- If FTEXT is set, the file is probably ASCII text. This is
- an optional indication, which the compressor may set by
- checking a small amount of the input data to see whether any
- non-ASCII characters are present. In case of doubt, FTEXT
- is cleared, indicating binary data. For systems which have
- different file formats for ascii text and binary data, the
- decompressor can use FTEXT to choose the appropriate format.
- We deliberately do not specify the algorithm used to set
- this bit, since a compressor always has the option of
- leaving it cleared and a decompressor always has the option
- of ignoring it and letting some other program handle issues
- of data conversion.
-
- If FHCRC is set, a CRC16 for the gzip header is present,
- immediately before the compressed data. The CRC16 consists
- of the two least significant bytes of the CRC32 for all
- bytes of the gzip header up to and not including the CRC16.
- [The FHCRC bit was never set by versions of gzip up to
- 1.2.4, even though it was documented with a different
- meaning in gzip 1.2.4.]
-
- If FEXTRA is set, optional extra fields are present, as
- described in a following section.
-
-
-
-Deutsch Informational [Page 6]
-
-RFC 1952 GZIP File Format Specification May 1996
-
-
- If FNAME is set, an original file name is present,
- terminated by a zero byte. The name must consist of ISO
- 8859-1 (LATIN-1) characters; on operating systems using
- EBCDIC or any other character set for file names, the name
- must be translated to the ISO LATIN-1 character set. This
- is the original name of the file being compressed, with any
- directory components removed, and, if the file being
- compressed is on a file system with case insensitive names,
- forced to lower case. There is no original file name if the
- data was compressed from a source other than a named file;
- for example, if the source was stdin on a Unix system, there
- is no file name.
-
- If FCOMMENT is set, a zero-terminated file comment is
- present. This comment is not interpreted; it is only
- intended for human consumption. The comment must consist of
- ISO 8859-1 (LATIN-1) characters. Line breaks should be
- denoted by a single line feed character (10 decimal).
-
- Reserved FLG bits must be zero.
-
- MTIME (Modification TIME)
- This gives the most recent modification time of the original
- file being compressed. The time is in Unix format, i.e.,
- seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this
- may cause problems for MS-DOS and other systems that use
- local rather than Universal time.) If the compressed data
- did not come from a file, MTIME is set to the time at which
- compression started. MTIME = 0 means no time stamp is
- available.
-
- XFL (eXtra FLags)
- These flags are available for use by specific compression
- methods. The "deflate" method (CM = 8) sets these flags as
- follows:
-
- XFL = 2 - compressor used maximum compression,
- slowest algorithm
- XFL = 4 - compressor used fastest algorithm
-
- OS (Operating System)
- This identifies the type of file system on which compression
- took place. This may be useful in determining end-of-line
- convention for text files. The currently defined values are
- as follows:
-
-
-
-
-
-
-Deutsch Informational [Page 7]
-
-RFC 1952 GZIP File Format Specification May 1996
-
-
- 0 - FAT filesystem (MS-DOS, OS/2, NT/Win32)
- 1 - Amiga
- 2 - VMS (or OpenVMS)
- 3 - Unix
- 4 - VM/CMS
- 5 - Atari TOS
- 6 - HPFS filesystem (OS/2, NT)
- 7 - Macintosh
- 8 - Z-System
- 9 - CP/M
- 10 - TOPS-20
- 11 - NTFS filesystem (NT)
- 12 - QDOS
- 13 - Acorn RISCOS
- 255 - unknown
-
- XLEN (eXtra LENgth)
- If FLG.FEXTRA is set, this gives the length of the optional
- extra field. See below for details.
-
- CRC32 (CRC-32)
- This contains a Cyclic Redundancy Check value of the
- uncompressed data computed according to CRC-32 algorithm
- used in the ISO 3309 standard and in section 8.1.1.6.2 of
- ITU-T recommendation V.42. (See http://www.iso.ch for
- ordering ISO documents. See gopher://info.itu.ch for an
- online version of ITU-T V.42.)
-
- ISIZE (Input SIZE)
- This contains the size of the original (uncompressed) input
- data modulo 2^32.
-
- 2.3.1.1. Extra field
-
- If the FLG.FEXTRA bit is set, an "extra field" is present in
- the header, with total length XLEN bytes. It consists of a
- series of subfields, each of the form:
-
- +---+---+---+---+==================================+
- |SI1|SI2| LEN |... LEN bytes of subfield data ...|
- +---+---+---+---+==================================+
-
- SI1 and SI2 provide a subfield ID, typically two ASCII letters
- with some mnemonic value. Jean-Loup Gailly
- <gzip@prep.ai.mit.edu> is maintaining a registry of subfield
- IDs; please send him any subfield ID you wish to use. Subfield
- IDs with SI2 = 0 are reserved for future use. The following
- IDs are currently defined:
-
-
-
-Deutsch Informational [Page 8]
-
-RFC 1952 GZIP File Format Specification May 1996
-
-
- SI1 SI2 Data
- ---------- ---------- ----
- 0x41 ('A') 0x70 ('P') Apollo file type information
-
- LEN gives the length of the subfield data, excluding the 4
- initial bytes.
-
- 2.3.1.2. Compliance
-
- A compliant compressor must produce files with correct ID1,
- ID2, CM, CRC32, and ISIZE, but may set all the other fields in
- the fixed-length part of the header to default values (255 for
- OS, 0 for all others). The compressor must set all reserved
- bits to zero.
-
- A compliant decompressor must check ID1, ID2, and CM, and
- provide an error indication if any of these have incorrect
- values. It must examine FEXTRA/XLEN, FNAME, FCOMMENT and FHCRC
- at least so it can skip over the optional fields if they are
- present. It need not examine any other part of the header or
- trailer; in particular, a decompressor may ignore FTEXT and OS
- and always produce binary output, and still be compliant. A
- compliant decompressor must give an error indication if any
- reserved bit is non-zero, since such a bit could indicate the
- presence of a new field that would cause subsequent data to be
- interpreted incorrectly.
-
-3. References
-
- [1] "Information Processing - 8-bit single-byte coded graphic
- character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987).
- The ISO 8859-1 (Latin-1) character set is a superset of 7-bit
- ASCII. Files defining this character set are available as
- iso_8859-1.* in ftp://ftp.uu.net/graphics/png/documents/
-
- [2] ISO 3309
-
- [3] ITU-T recommendation V.42
-
- [4] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",
- available in ftp://ftp.uu.net/pub/archiving/zip/doc/
-
- [5] Gailly, J.-L., GZIP documentation, available as gzip-*.tar in
- ftp://prep.ai.mit.edu/pub/gnu/
-
- [6] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via Table
- Look-Up", Communications of the ACM, 31(8), pp.1008-1013.
-
-
-
-
-Deutsch Informational [Page 9]
-
-RFC 1952 GZIP File Format Specification May 1996
-
-
- [7] Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal,
- pp.118-133.
-
- [8] ftp://ftp.adelaide.edu.au/pub/rocksoft/papers/crc_v3.txt,
- describing the CRC concept.
-
-4. Security Considerations
-
- Any data compression method involves the reduction of redundancy in
- the data. Consequently, any corruption of the data is likely to have
- severe effects and be difficult to correct. Uncompressed text, on
- the other hand, will probably still be readable despite the presence
- of some corrupted bytes.
-
- It is recommended that systems using this data format provide some
- means of validating the integrity of the compressed data, such as by
- setting and checking the CRC-32 check value.
-
-5. Acknowledgements
-
- Trademarks cited in this document are the property of their
- respective owners.
-
- Jean-Loup Gailly designed the gzip format and wrote, with Mark Adler,
- the related software described in this specification. Glenn
- Randers-Pehrson converted this document to RFC and HTML format.
-
-6. Author's Address
-
- L. Peter Deutsch
- Aladdin Enterprises
- 203 Santa Margarita Ave.
- Menlo Park, CA 94025
-
- Phone: (415) 322-0103 (AM only)
- FAX: (415) 322-1734
- EMail: <ghost@aladdin.com>
-
- Questions about the technical content of this specification can be
- sent by email to:
-
- Jean-Loup Gailly <gzip@prep.ai.mit.edu> and
- Mark Adler <madler@alumni.caltech.edu>
-
- Editorial comments on this specification can be sent by email to:
-
- L. Peter Deutsch <ghost@aladdin.com> and
- Glenn Randers-Pehrson <randeg@alumni.rpi.edu>
-
-
-
-Deutsch Informational [Page 10]
-
-RFC 1952 GZIP File Format Specification May 1996
-
-
-7. Appendix: Jean-Loup Gailly's gzip utility
-
- The most widely used implementation of gzip compression, and the
- original documentation on which this specification is based, were
- created by Jean-Loup Gailly <gzip@prep.ai.mit.edu>. Since this
- implementation is a de facto standard, we mention some more of its
- features here. Again, the material in this section is not part of
- the specification per se, and implementations need not follow it to
- be compliant.
-
- When compressing or decompressing a file, gzip preserves the
- protection, ownership, and modification time attributes on the local
- file system, since there is no provision for representing protection
- attributes in the gzip file format itself. Since the file format
- includes a modification time, the gzip decompressor provides a
- command line switch that assigns the modification time from the file,
- rather than the local modification time of the compressed input, to
- the decompressed output.
-
-8. Appendix: Sample CRC Code
-
- The following sample code represents a practical implementation of
- the CRC (Cyclic Redundancy Check). (See also ISO 3309 and ITU-T V.42
- for a formal specification.)
-
- The sample code is in the ANSI C programming language. Non C users
- may find it easier to read with these hints:
-
- & Bitwise AND operator.
- ^ Bitwise exclusive-OR operator.
- >> Bitwise right shift operator. When applied to an
- unsigned quantity, as here, right shift inserts zero
- bit(s) at the left.
- ! Logical NOT operator.
- ++ "n++" increments the variable n.
- 0xNNN 0x introduces a hexadecimal (base 16) constant.
- Suffix L indicates a long value (at least 32 bits).
-
- /* Table of CRCs of all 8-bit messages. */
- unsigned long crc_table[256];
-
- /* Flag: has the table been computed? Initially false. */
- int crc_table_computed = 0;
-
- /* Make the table for a fast CRC. */
- void make_crc_table(void)
- {
- unsigned long c;
-
-
-
-Deutsch Informational [Page 11]
-
-RFC 1952 GZIP File Format Specification May 1996
-
-
- int n, k;
- for (n = 0; n < 256; n++) {
- c = (unsigned long) n;
- for (k = 0; k < 8; k++) {
- if (c & 1) {
- c = 0xedb88320L ^ (c >> 1);
- } else {
- c = c >> 1;
- }
- }
- crc_table[n] = c;
- }
- crc_table_computed = 1;
- }
-
- /*
- Update a running crc with the bytes buf[0..len-1] and return
- the updated crc. The crc should be initialized to zero. Pre- and
- post-conditioning (one's complement) is performed within this
- function so it shouldn't be done by the caller. Usage example:
-
- unsigned long crc = 0L;
-
- while (read_buffer(buffer, length) != EOF) {
- crc = update_crc(crc, buffer, length);
- }
- if (crc != original_crc) error();
- */
- unsigned long update_crc(unsigned long crc,
- unsigned char *buf, int len)
- {
- unsigned long c = crc ^ 0xffffffffL;
- int n;
-
- if (!crc_table_computed)
- make_crc_table();
- for (n = 0; n < len; n++) {
- c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
- }
- return c ^ 0xffffffffL;
- }
-
- /* Return the CRC of the bytes buf[0..len-1]. */
- unsigned long crc(unsigned char *buf, int len)
- {
- return update_crc(0L, buf, len);
- }
-
-
-
-
-Deutsch Informational [Page 12]
-