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Re: A proposal for Shared Dictionary Compression over HTTP

From: Jon Butler <jkbutler@google.com>
Date: Wed, 10 Sep 2008 13:17:37 -0400
Message-ID: <af9c7e730809101017q341f76c9l75294662f374ec44@mail.gmail.com>
To: "Yves Lafon" <ylafon@w3.org>
Cc: "Wei-Hsin Lee" <weihsinl@google.com>, ietf-http-wg@w3.org

An interesting difference between content and transfer codings is that
transfer codings are allowed to have attributes.  Were sdch to use the
Transfer-Encoding header, this might allow more flexibility in the
actual compression scheme used, e.g., Transfer-Encoding: sdch;c=vcdiff
vs Transfer-Encoding: sdch;c=gzip.  The client would indicate what
compression schemes were allowed in the TE header, e.g. TE:
sdch;c="vcdiff,gzip".  In that scenario, I expect we would need to
specify some default compression scheme to be used if none is


On Wed, Sep 10, 2008 at 12:31 PM, Jon Butler <jkbutler@google.com> wrote:
> Yves--
> You raise an interesting point.  SDCH was designed to be applied to
> the message body before gzip (since cross-payload redundancy is much
> harder to detect after gzipping the payloads).  One of the differences
> between the two sets of headers is that transfer encodings must be
> applied after and removed before content encodings, since transfer
> encodings are properties of the message and content encodings are a
> property of the entity inside the message.  So, we have a choice:
> either we indicate both SDCH and gzip in the Content-Encodings, or
> both in the Transfer-Encoding header.  Since the prior art for gzip is
> to indicate it in the Content-Encoding header (a holdover from the
> HTTP/1.0 standard as I understand), we proposed putting sdch there as
> well.
> From my reading of the standard, it would be more in keeping with the
> HTTP/1.1 standard to put both encodings (gzip and sdch) in the
> TE/Transfer-Encoding headers, but it is not clear that it would be
> more practical.
> We'd be happy to hear others' opinions on this.
> Jonathan
> On Wed, Sep 10, 2008 at 4:17 AM, Yves Lafon <ylafon@w3.org> wrote:
>> On Mon, 8 Sep 2008, Wei-Hsin Lee wrote:
>>> Hi,
>>> Over the last few weeks we've been experimenting with a way to get better
>>> compression for HTTP streams using a dictionary-based compression scheme,
>>> where a user agent obtains a site-specific dictionary that then allows
>>> pages
>>> on the site that have many common elements to be transmitted much more
>>> quickly.
>> One question, why using Accept-Encoding/Content-Encoding instead of
>> TE/Transfer-Encoding ?
>> Cheers,
>> --
>> Baroula que barouleras, au tiéu toujou t'entourneras.
>>        ~~Yves
Received on Wednesday, 10 September 2008 17:18:27 UTC

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