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: documents="" ftp.uu.net="" graphics="" png="" zdoc-index.html="" zlib="">.</ftp:>

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</gzip@prep.ai.mit.edu>

         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></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</gzip@prep.ai.mit.edu>

   Mark Adler <madler@alumni.caltech.edu></madler@alumni.caltech.edu>

   Editorial comments on this specification can be sent by email to:

   L. Peter Deutsch <ghost@aladdin.com> and</ghost@aladdin.com>

   Glenn Randers-Pehrson <randeg@alumni.rpi.edu></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</gzip@prep.ai.mit.edu>

   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]