Re: [RFC] CryptoAPI & Compression

[Herbert Xu <herbert@gondor.apana.org.au>]

On Sun, Apr 03, 2005 at 01:45:58PM +0400, Artem B. Bityuckiy wrote:
> 
> Here is a cite from RFC-1951 (page 4):
> 
>    A compressed data set consists of a series of blocks, corresponding
>    to successive blocks of input data.  The block sizes are arbitrary,
>    except that non-compressible blocks are limited to 65,535 bytes.
> 
> Thus,
> 
> 1. 64K is only applied to non-compressible data, in which case zlib just 
> copies it as it is, adding a 1-byte header and a 1-byte EOB marker.

I think the overhead could be higher.  But even if it is 2 bytes
per block, then for 1M of incompressible input the total overhead is

2 * 1048576 / 65536 = 32

bytes.

Also I'm not certain if it will actually create 64K blocks.  It might
be as low as 16K according to the zlib documentation.

> 3. If zlib compressed data (i.e., applied LZ77 & Huffman), blocks may 
> have arbitrary length.

Actually there is a limit on that too but that's not relevant to
this discussion.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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