[Http-crcsync] http-sync standard

[Toby Collett]

Hi,
I hadn't seen that RFC, should be very informative.

> How do you imagine that this hash would be used?
I would see this as a relatively rare case, but with careful design in the
encoding you get it for close to free. With the other discussions since I
wrote the first draft, if the hashes include the trailing data in the file,
then we dont actually need to send an extra hash, and so as long as we allow
for byte deltas instead of block in the response then smart servers could
make use of this.

Slashdot is probably a bad example for most of this, as they seem to care
very little about cachability (they mark their pages as private even). But
some sort of news site, if they chose to add the support on the server side
could pre compute the deltas for a set of response pages when new data is
uploaded. Again the rare case, but at a very small cost...

I will get the spec into a code repo somewhere and get another version out
in the next couple of days.

Toby


2009/4/6 <tridge at samba.org>

> Hi Toby,
>
>  > This is transmitted in two forms. One is an SHA1 hash of the complete
> chached
>  > file, and the other is a set of block hashes.
> ...
>  > Content-Hash:
>  > This will be an sha1 hash of the entire cached body, and will allow the
> server
>  > to transmit deltas based on its knowledge of past versions of the page.
>
> How do you imagine that this hash would be used? I don't think it is
> practical to think that servers would keep a record of all the dynamic
> pages they have been served out, and for static pages I think the
> normal cache tag mechanisms already work well.
>
> I think a SHA1 or similar of the servers generated page is really
> worthwhile, but a whole file hash of what the client has in cache
> doesn't gain anything that I can see.
>
> btw, I also wonder if you've seen this:
>
>  http://www.ietf.org/rfc/rfc3229.txt
>
> That is the result of earlier efforts to standardise delta encoding in
> HTTP. I was a little bit involved in that effort, although it
> concentrated on storing old copies on the server, which I wasn't
> interested in.
>
> Still, it should provide some very useful background to the current
> effort.
>
> Cheers, Tridge
>



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