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.




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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.







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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
        +---+




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     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-->)
        +---+---+








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     (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.




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     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.








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  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.







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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: <[email protected]>


  Jean-Loup Gailly

  EMail: <[email protected]>

  Questions about the technical content of this specification can be
  sent by email to

  Jean-Loup Gailly <[email protected]> and
  Mark Adler <[email protected]>

  Editorial comments on this specification can be sent by email to

  L. Peter Deutsch <[email protected]> and
  Glenn Randers-Pehrson <[email protected]>






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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.)








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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] */




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















































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