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Parameters for transfer-codings?

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Tue, 18 Nov 97 17:16:00 PST
Message-Id: <9711190116.AA01262@acetes.pa.dec.com>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
I expect to get flamed for this message, but I sort of promised someone
that I would make the attempt.

Some of the potentially-useful transfer-coding algorithms are
inherently parameterizable.  For example, "gzip" has 9 different
compression levels (trading off compression ratio for speed).
"compress" has a range of possible settings (I'm not quite sure how
many).  In both cases, once the sender has chosen a setting, the
recipient should be able to decompress the result, but HTTP provides no
mechanism for the client to suggest the preferred setting.

Somewhat more exotic, but hardly without practical merit, is the
possibility of using pre-agreed compression dictionaries to further
reduce the size of the transferred compressed form.  The idea is that
it should take fewer bytes to send the name of the appropriate
dictionary than to transmit its contents.  However, in this case, the
sender needs a way to name the dictionary used in the message.

One of my friends in the data compression community has been beating me
up about this for a while, and I've fought him off by saying "well,
it's too late to change Accept-Encoding because that would break too
many existing implementations."  But I don't think the same excuse
applies for Accept-Transfer/Transfer-Encoding, so I am stuck with
making the following proposal :-).

Suppose, therefore, that we change this syntax in section 3.6
(Transfer Codings) from

                   transfer-coding   = "chunked" | transfer-extension
                   transfer-extension      = token

to

                   transfer-coding   = "chunked" | transfer-extension
                   transfer-extension  = token | token token-params
		   
		   token-params = *( ";" token-param-name [ "=" 
                                    token-param-value ] )

		   token-param-name = token
		   token-param-value = token | quoted-string

If I got the BNF right (don't count on it!) then this would lead
to examples like

	Accept-Transfer: gzip_p;level="3", compress_p;bits="9-11"

and

	Transfer-Encoding: fooflate;dict="http://dicts.net/37"

The specific definition of the parameter names and values would be
part of the specification for a given tranfer-coding (which is
why my examples do not use the already-defined transfer-codings).

By construction, the BNF does not allow parameters to be used
with "chunked", and I do not believe that any other transfer-coding
values are used in current implementations ... so this should
be compatible with existing implementations.

Also, this BNF leads to the possibility of

	Accept-Transfer: gzip_p;level="3";q=0.5, compress_p;bits="9-11"

because of the qvalues allowed with Accept-Transfer, but I don't
think the parsing problem is too difficult ... although we might
want to say

	The token-param-name MUST NOT be "q".

-Jeff
Received on Tuesday, 18 November 1997 17:22:27 EST

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