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Clarification proposal for Deflate RFC

Roy, all,

Here are recommended changes to rfc1951 "DEFLATE Compressed Data Format
Specification":

Add the following statement at the very end of section 2,
Compressed representation overview.

"There must be no additional data sent beyond that described in section
3,
Detailed specification. Specifically. neither the library header nor the

Adler32 checksum generated by Zlib must be sent."

Add a new section "Zlib interface recommendations"
between the existing section 4. Compression algorithm details, and
section
5. References, as follows:

5. Zlib interface recommendations

The default initialization call to the Zlib implementation of Deflate
produces an unnecessary wrapper consisting of a Zlib specific header and

checksum.  If this publicly available implementation of Deflate is used,

then it should be initialized in a manner similar to the following:

  returnCode = deflateInit2_(
                &stream,
                Z_DEFAULT_COMPRESSION,
                Z_DEFLATED,
                -WINDOW_SIZE,
                DEF_MEM_LEVEL,
                Z_DEFAULT_STRATEGY,
                ZLIB_VERSION,
                sizeof(z_stream));

The absolute value of the WINDOW_SIZE argument controls the size of the
history buffer used by deflate, in powers of 2 bytes (i.e. WINDOW_SIZE =
11
configures a 2048 byte history buffer).  Negating this argument will
prevent Zlib from adding the Zlib header or Adler32 checksum.

Likewise, the initialization of the Zlib decompression implementation
Inflate should be invoked in a manner to the following:

  returnCode = inflateInit2_(
                &stream,
                -WINDOW_SIZE,
                ZLIB_VERSION,
                sizeof(z_stream));

In this case, WINDOW_SIZE should be 15, permitting the maximum history
buffer of 32768.  By setting the argument to -WINDOW_SIZE, or -32768,
Zlib
will be configured not to expect a Zlib header or Adler32 checksum.



--
Bronislav Kavsan
IRE Secure Solutions, Inc.
100 Conifer Hill Drive  Suite 513
Danvers, MA  01923
voice: 978-539-4816
http://www.ire.com