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.









 
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.








 
RFC 1950       ZLIB Compressed Data Format Specification        May 1996


4. Source code