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rfc:rfc1950

Network Working Group P. Deutsch Request for Comments: 1950 Aladdin Enterprises Category: Informational J-L. Gailly

                                                              Info-ZIP
                                                              May 1996
       ZLIB Compressed Data Format Specification version 3.3

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 and Jean-Loup Gailly
 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.  The
 data can be produced or consumed, even for an arbitrarily long
 sequentially presented input data stream, using only an a priori
 bounded amount of intermediate storage.  The format presently uses
 the DEFLATE compression method but can be easily extended to use
 other compression methods.  It can be implemented readily in a manner
 not covered by patents.  This specification also defines the ADLER-32
 checksum (an extension and improvement of the Fletcher checksum),
 used for detection of data corruption, and provides an algorithm for
 computing it.

Deutsch & Gailly Informational [Page 1] RFC 1950 ZLIB Compressed Data 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 ......................................... 3
    2.1. Overall conventions ....................................... 3
    2.2. Data format ............................................... 4
    2.3. Compliance ................................................ 7
 3. References ..................................................... 7
 4. Source code .................................................... 8
 5. Security Considerations ........................................ 8
 6. Acknowledgements ............................................... 8
 7. Authors' Addresses ............................................. 8
 8. Appendix: Rationale ............................................ 9
 9. Appendix: Sample code ..........................................10

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 be produced or consumed, even for an arbitrarily long

sequentially presented input 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;
  • Can use a number of different compression methods;
  • Can be implemented readily in a manner not covered by

patents, and hence can be practiced freely.

    The data format defined by this specification does not attempt to
    allow random access to compressed data.

Deutsch & Gailly Informational [Page 2] RFC 1950 ZLIB Compressed Data Format Specification May 1996

 1.2. Intended audience
    This specification is intended for use by implementors of software
    to compress data into zlib format and/or decompress data from zlib
    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 compressed data format that can be
    used for in-memory compression of a sequence of arbitrary bytes.
 1.4. Compliance
    Unless otherwise indicated below, a compliant decompressor must be
    able to accept and decompress any data set that conforms to all
    the specifications presented here; a compliant compressor must
    produce data sets that conform to all the specifications presented
    here.
 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
    Version 3.1 was the first public release of this specification.
    In version 3.2, some terminology was changed and the Adler-32
    sample code was rewritten for clarity.  In version 3.3, the
    support for a preset dictionary was introduced, and the
    specification was converted to RFC style.

2. Detailed specification

 2.1. Overall conventions
    In the diagrams below, a box like this:
       +---+
       |   | <-- the vertical bars might be missing
       +---+

Deutsch & Gailly Informational [Page 3] RFC 1950 ZLIB Compressed Data Format Specification May 1996

    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|
       +--------+
    Within a computer, a number may occupy multiple bytes.  All
    multi-byte numbers in the format described here are stored with
    the MOST-significant byte first (at the lower memory address).
    For example, the decimal number 520 is stored as:
           0     1
       +--------+--------+
       |00000010|00001000|
       +--------+--------+
        ^        ^
        |        |
        |        + less significant byte = 8
        + more significant byte = 2 x 256
 2.2. Data format
    A zlib stream has the following structure:
         0   1
       +---+---+
       |CMF|FLG|   (more-->)
       +---+---+

Deutsch & Gailly Informational [Page 4] RFC 1950 ZLIB Compressed Data Format Specification May 1996

    (if FLG.FDICT set)
         0   1   2   3
       +---+---+---+---+
       |     DICTID    |   (more-->)
       +---+---+---+---+
       +=====================+---+---+---+---+
       |...compressed data...|    ADLER32    |
       +=====================+---+---+---+---+
    Any data which may appear after ADLER32 are not part of the zlib
    stream.
    CMF (Compression Method and flags)
       This byte is divided into a 4-bit compression method and a 4-
       bit information field depending on the compression method.
          bits 0 to 3  CM     Compression method
          bits 4 to 7  CINFO  Compression info
    CM (Compression method)
       This identifies the compression method used in the file. CM = 8
       denotes the "deflate" compression method with a window size up
       to 32K.  This is the method used by gzip and PNG (see
       references [1] and [2] in Chapter 3, below, for the reference
       documents).  CM = 15 is reserved.  It might be used in a future
       version of this specification to indicate the presence of an
       extra field before the compressed data.
    CINFO (Compression info)
       For CM = 8, CINFO is the base-2 logarithm of the LZ77 window
       size, minus eight (CINFO=7 indicates a 32K window size). Values
       of CINFO above 7 are not allowed in this version of the
       specification.  CINFO is not defined in this specification for
       CM not equal to 8.
    FLG (FLaGs)
       This flag byte is divided as follows:
          bits 0 to 4  FCHECK  (check bits for CMF and FLG)
          bit  5       FDICT   (preset dictionary)
          bits 6 to 7  FLEVEL  (compression level)
       The FCHECK value must be such that CMF and FLG, when viewed as
       a 16-bit unsigned integer stored in MSB order (CMF*256 + FLG),
       is a multiple of 31.

Deutsch & Gailly Informational [Page 5] RFC 1950 ZLIB Compressed Data Format Specification May 1996

    FDICT (Preset dictionary)
       If FDICT is set, a DICT dictionary identifier is present
       immediately after the FLG byte. The dictionary is a sequence of
       bytes which are initially fed to the compressor without
       producing any compressed output. DICT is the Adler-32 checksum
       of this sequence of bytes (see the definition of ADLER32
       below).  The decompressor can use this identifier to determine
       which dictionary has been used by the compressor.
    FLEVEL (Compression level)
       These flags are available for use by specific compression
       methods.  The "deflate" method (CM = 8) sets these flags as
       follows:
          0 - compressor used fastest algorithm
          1 - compressor used fast algorithm
          2 - compressor used default algorithm
          3 - compressor used maximum compression, slowest algorithm
       The information in FLEVEL is not needed for decompression; it
       is there to indicate if recompression might be worthwhile.
    compressed data
       For compression method 8, the compressed data is stored in the
       deflate compressed data format as described in the document
       "DEFLATE Compressed Data Format Specification" by L. Peter
       Deutsch. (See reference [3] in Chapter 3, below)
       Other compressed data formats are not specified in this version
       of the zlib specification.
    ADLER32 (Adler-32 checksum)
       This contains a checksum value of the uncompressed data
       (excluding any dictionary data) computed according to Adler-32
       algorithm. This algorithm is a 32-bit extension and improvement
       of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073
       standard. See references [4] and [5] in Chapter 3, below)
       Adler-32 is composed of two sums accumulated per byte: s1 is
       the sum of all bytes, s2 is the sum of all s1 values. Both sums
       are done modulo 65521. s1 is initialized to 1, s2 to zero.  The
       Adler-32 checksum is stored as s2*65536 + s1 in most-
       significant-byte first (network) order.

Deutsch & Gailly Informational [Page 6] RFC 1950 ZLIB Compressed Data Format Specification May 1996

 2.3. Compliance
    A compliant compressor must produce streams with correct CMF, FLG
    and ADLER32, but need not support preset dictionaries.  When the
    zlib data format is used as part of another standard data format,
    the compressor may use only preset dictionaries that are specified
    by this other data format.  If this other format does not use the
    preset dictionary feature, the compressor must not set the FDICT
    flag.
    A compliant decompressor must check CMF, FLG, and ADLER32, and
    provide an error indication if any of these have incorrect values.
    A compliant decompressor must give an error indication if CM is
    not one of the values defined in this specification (only the
    value 8 is permitted in this version), since another value could
    indicate the presence of new features that would cause subsequent
    data to be interpreted incorrectly.  A compliant decompressor must
    give an error indication if FDICT is set and DICTID is not the
    identifier of a known preset dictionary.  A decompressor may
    ignore FLEVEL and still be compliant.  When the zlib data format
    is being used as a part of another standard format, a compliant
    decompressor must support all the preset dictionaries specified by
    the other format. When the other format does not use the preset
    dictionary feature, a compliant decompressor must reject any
    stream in which the FDICT flag is set.

3. References

 [1] Deutsch, L.P.,"GZIP Compressed Data Format Specification",
     available in ftp://ftp.uu.net/pub/archiving/zip/doc/
 [2] Thomas Boutell, "PNG (Portable Network Graphics) specification",
     available in ftp://ftp.uu.net/graphics/png/documents/
 [3] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",
     available in ftp://ftp.uu.net/pub/archiving/zip/doc/
 [4] Fletcher, J. G., "An Arithmetic Checksum for Serial
     Transmissions," IEEE Transactions on Communications, Vol. COM-30,
     No. 1, January 1982, pp. 247-252.
 [5] ITU-T Recommendation X.224, Annex D, "Checksum Algorithms,"
     November, 1993, pp. 144, 145. (Available from
     gopher://info.itu.ch). ITU-T X.244 is also the same as ISO 8073.

Deutsch & Gailly Informational [Page 7] RFC 1950 ZLIB Compressed Data Format Specification May 1996

4. Source code

 Source code for a C language implementation of a "zlib" compliant
 library is available at ftp://ftp.uu.net/pub/archiving/zip/zlib/.

5. Security Considerations

 A decoder that fails to check the ADLER32 checksum value may be
 subject to undetected data corruption.

6. Acknowledgements

 Trademarks cited in this document are the property of their
 respective owners.
 Jean-Loup Gailly and Mark Adler designed the zlib format and wrote
 the related software described in this specification.  Glenn
 Randers-Pehrson converted this document to RFC and HTML format.

7. Authors' Addresses

 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>
 Jean-Loup Gailly
 EMail: <gzip@prep.ai.mit.edu>
 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 & Gailly Informational [Page 8] RFC 1950 ZLIB Compressed Data Format Specification May 1996

8. Appendix: Rationale

 8.1. Preset dictionaries
    A preset dictionary is specially useful to compress short input
    sequences. The compressor can take advantage of the dictionary
    context to encode the input in a more compact manner. The
    decompressor can be initialized with the appropriate context by
    virtually decompressing a compressed version of the dictionary
    without producing any output. However for certain compression
    algorithms such as the deflate algorithm this operation can be
    achieved without actually performing any decompression.
    The compressor and the decompressor must use exactly the same
    dictionary. The dictionary may be fixed or may be chosen among a
    certain number of predefined dictionaries, according to the kind
    of input data. The decompressor can determine which dictionary has
    been chosen by the compressor by checking the dictionary
    identifier. This document does not specify the contents of
    predefined dictionaries, since the optimal dictionaries are
    application specific. Standard data formats using this feature of
    the zlib specification must precisely define the allowed
    dictionaries.
 8.2. The Adler-32 algorithm
    The Adler-32 algorithm is much faster than the CRC32 algorithm yet
    still provides an extremely low probability of undetected errors.
    The modulo on unsigned long accumulators can be delayed for 5552
    bytes, so the modulo operation time is negligible.  If the bytes
    are a, b, c, the second sum is 3a + 2b + c + 3, and so is position
    and order sensitive, unlike the first sum, which is just a
    checksum.  That 65521 is prime is important to avoid a possible
    large class of two-byte errors that leave the check unchanged.
    (The Fletcher checksum uses 255, which is not prime and which also
    makes the Fletcher check insensitive to single byte changes 0 <->
    255.)
    The sum s1 is initialized to 1 instead of zero to make the length
    of the sequence part of s2, so that the length does not have to be
    checked separately. (Any sequence of zeroes has a Fletcher
    checksum of zero.)

Deutsch & Gailly Informational [Page 9] RFC 1950 ZLIB Compressed Data Format Specification May 1996

9. Appendix: Sample code

 The following C code computes the Adler-32 checksum of a data buffer.
 It is written for clarity, not for speed.  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 right shift operator. When applied to an
           unsigned quantity, as here, right shift inserts zero bit(s)
           at the left.
    <<     Bitwise left shift operator. Left shift inserts zero
           bit(s) at the right.
    ++     "n++" increments the variable n.
    %      modulo operator: a % b is the remainder of a divided by b.
    #define BASE 65521 /* largest prime smaller than 65536 */
    /*
       Update a running Adler-32 checksum with the bytes buf[0..len-1]
     and return the updated checksum. The Adler-32 checksum should be
     initialized to 1.
     Usage example:
       unsigned long adler = 1L;
       while (read_buffer(buffer, length) != EOF) {
         adler = update_adler32(adler, buffer, length);
       }
       if (adler != original_adler) error();
    */
    unsigned long update_adler32(unsigned long adler,
       unsigned char *buf, int len)
    {
      unsigned long s1 = adler & 0xffff;
      unsigned long s2 = (adler >> 16) & 0xffff;
      int n;
      for (n = 0; n < len; n++) {
        s1 = (s1 + buf[n]) % BASE;
        s2 = (s2 + s1)     % BASE;
      }
      return (s2 << 16) + s1;
    }
    /* Return the adler32 of the bytes buf[0..len-1] */

Deutsch & Gailly Informational [Page 10] RFC 1950 ZLIB Compressed Data Format Specification May 1996

    unsigned long adler32(unsigned char *buf, int len)
    {
      return update_adler32(1L, buf, len);
    }

Deutsch & Gailly Informational [Page 11]

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