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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]
+
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