Compression in browsers
Diwakar Shetty
What are the possible values in "accept-encoding" which can be sent by various browsers with different versions.
e.g: Mozilla1.4a and IE 5.5 support "gzip" and "deflate"
While netscape navigator 4.7 supports only "gzip"
Similarly, what does the various versions of "IE", "Mozilla" and "Navigator" support ?
Thanks in advance for all tips/info
-- Diwakar
Raymond Wan
Hi,
On Wed, 2 Jul 2003, Diwakar Shetty wrote:
> What are the possible values in "accept-encoding" which can be sent by
> various browsers with different versions.
To find out "all possible values", I suggest you check out the W3C
website (http://www.w3c.org/, I believe) and look through the HTTP
standard. If I remember correctly, "gzip" and "deflate" were the only
possible values. The problem is not just with the browsers, of course,
but the web servers and proxies, as well.
If I remember correctly, the current version of HTTP only supports
gzip and deflate; I don't remember a third method.
> Similarly, what does the various versions of "IE", "Mozilla" and
> "Navigator" support ?
And sorry, I don't know specifics about these browsers, though.
Ray
Mark Adler
> What are the possible values in "accept-encoding" which can be sent by various browsers
> with different versions.
RFC 2616, HTTP/1.1, specifies "gzip", "deflate", and "compress" (and
"identity" for no change). Those are respectively the formats
generated by gzip, zlib's deflate() function called to include the
zlib header and trailer, and the Unix compress command. The first two
are defined by RFC's 1952 and 1950. Both of those RFC's reference the
same compressed data format, deflate, defined in RFC 1951. RFC 1950
and 1952 put different wrappers around the compressed data.
The only one that appears to be reliably and correctly implemented in
browsers is "gzip". Unix compress is usually inferior in compression
and has the potential to significantly expand incompressible data,
which is probably why it is not supported by most browsers. "deflate"
seems to have been misinterpreted by some browser implementations to
mean the deflate format (RFC 1951) without the zlib header and trailer
(RFC 1950). So while this is precisely the sort of application that
the zlib format was intended for, confusion about the word "deflate"
has rendered it unreliable. This is despite the fact that RFC 2616
explicitly references RFC 1950 for the "deflate" content encoding.
So you should simply use the "gzip" encoding for HTTP.
mark