W3C home > Mailing lists > Public > ietf-http-wg-old@w3.org > September to December 1995

Rethinking content negotiation (Was: rethinking caching)

From: Daniel DuBois <ddubois@rafiki.spyglass.com>
Date: Wed, 20 Dec 1995 14:11:45 -0600
Message-Id: <9512202011.AA06231@rafiki.spyglass.com>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
>>>good first step. For some headers, (accept, for example), you need
>>>more than merely knowing what it depended upon, but also the WAY in
>>>which it depended upon the original data, so that the proxy itself can
>>>decide whether a cached item is appropriate.
>>
>>Absolutely. A Vary: 1#(http-header-name) header returned by the server
>>would help solve this problem.
>
>I like this syntax. The URI header would work, but this is short and
>to the point.

I think this syntax is insufficient.  This is pretty much the information
that was contained in the HTTP/1.0-01 draft (AKA HTTP/1.1 before there was a
1.1) and it was presumably changed because others felt likewise.  Using that
scheme would require that the caching proxy keep the exact header(s) stored
for the specified vary quanity for comparison purposes.  This is a huge
burden on a proxy because it doesn't just have to save headers once, it has
to save headers for each request that doesn't have the exact same paramters
as previous requests.

An example might be useful here, if not longwinded.  This demonstrates the
insufficent-ness of the old URI:, and a Vary: header scheme as well.

   ---Request 1
GET /index HTTP/1.1
Accept: image/gif, image/jpeg, image/helper-app1, image/helper-app2
   ---Response
URI: </index.gif>, </index; vary=language, type>
   ---cache has to store
1./index | T | Accept: image/gif, image/jpeg, image/helper-app1,
image/helper-app2

   ---Request 2
GET /index HTTP/1.1
Accept: image/gif, image/jpeg, image/helper-app1
Accept-Language: fr
   ---Response
URI: </index.gif>, </index; vary=language, type>
   ---cache has to store
1. /index | T | Accept: image/gif, image/jpeg, image/helper-app1,
image/helper-app2
2. /index | T | Accept: image/gif, image/jpeg, image/helper-app1,
image/helper-app2 | L | Accept-Language: fr

   ---Request 3
GET /index HTTP/1.1
Accept: image/gif, image/jpeg, image/helper-app1
   ---Response
URI: </index.gif>, </index>; vary=language, type
   ---cache has to store
1. /index | T | Accept: image/gif, image/jpeg, image/helper-app1,
image/helper-app2
2. /index | T | Accept: image/gif, image/jpeg, image/helper-app1,
image/helper-app2 | L | Accept-Language: fr
3. /index | T | Accept: image/gif, image/jpeg, image/helper-app1

This is totally unworkable.  It's not just going to be one header
permutation combo of charsets/encodings/types/langs per browser either, as
good browsers now let people set some of these parameters to their
preference.  This list will grow forever, and proxies just won't cache
things that vary, or will forget about content negotiation altogether.

The new scheme has the big advantage of including all variants, their
metainformaiton, and the qs's of the variants in the URI header.  This gives
the proxy the information it needs to do the negotiation itself, and is the
only way to assure a trip to the original server would yield the same
result, and that serving from the cache is acceptable.  (I completely ignore
the vary by User-Agent here, but until we invent the psychic proxy, no proxy
can solve for server's varying content outside of the content negotiation
scheme without listing all 2000 user agents that come through its doors.)

I've always been very hesitant about the server's content variant-picking
algorithm being part of the protocol, because I saw that as a server-side
implementation issue, but as time goes on, I become more and more convinced
of the weight of this issue and its impact on the scalabilty of the web.
The new URI header solves this caching problem perfectly, and
single-handedly convinces me of the concept of opening the algorithm to the
protocol.

Now, if I can just 1) convince Roy to default charsets/encodings to .001 on
absent headers, and 2) propose a different sytax than the URL: {} lispism, I
might die happy.  The current one can cause extremely big headers (you think
Accept is bad?) and doesn't parse like existing headers.

How about:

Location: http://www.spyglass.com/index.html
URI: "/index.html"; t=text/html; qs=.9,
     "/index.en.html"; t=text/html; l=en; qs=.9
URI: "/index.jp.txt"; t=text/plain; l=jp-JP; c=isp-2022-jp1; qs=.1

And maybe even <uhg>:
URI "/index.nscp.html"; ua="Mozilla/1.2N (Windows; I; 32bit)"

where the absence of an encoding, language, or charset mean they're
indeterminate for that resource.  I like saving a few bytes on the
"language" & "encoding" etc tags, and on the default to "variant=", and I
like sticking with a format that parses like the Accept headers.  Besides
the current 9/22 draft looks too much like Logic Bags :)  If anyone wishes,
I will gladly write up a BNF.
-----
Dan DuBois, Software Animal             http://www.spyglass.com/~ddubois/
		I absolutely do not speak for Spyglass.
Received on Wednesday, 20 December 1995 12:17:20 EST

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