GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1952

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]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1952.txt · Last modified: 1996/05/28 17:13 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki