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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

            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]
Posted by: Guest on April-19-2021


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


   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

   A pointer to the latest version of this and related documentation in
   HTML format can be found at the URL


   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]
Posted by: Guest on April-19-2021


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

      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

   [2] Thomas Boutell, "PNG (Portable Network Graphics) specification",
       available in

   [3] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",
       available in

   [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:// ITU-T X.244 is also the same as ISO 8073.

Deutsch & Gailly             Informational                      [Page 7]
Posted by: Guest on April-19-2021

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