Re: A proposal for Shared Dictionary Compression over HTTP

I should have perhaps used zlib or some other compression format
rather than gzip in my example below.  My intent was to address Mark's
point that a variety of compressors could underly a dictionary-based
protocol like SDCH.


On Wed, Sep 10, 2008 at 1:17 PM, Jon Butler <> wrote:
> 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
> specified.
> Jonathan
> On Wed, Sep 10, 2008 at 12:31 PM, Jon Butler <> 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 <> 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 17:32:38 UTC