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Re: Suggestion: Add Cache-Control extensibility to Vary header

From: Roy T. Fielding <fielding@kiwi.ICS.UCI.EDU>
Date: Thu, 27 Mar 1997 18:40:46 -0800
To: http-wg@cuckoo.hpl.hp.com
Message-Id: <9703271840.aa09704@paris.ics.uci.edu>
These are actually PEP-related criticisms, but they fit within the
context of Henrik's post.

>By adding the Cache-Control header extensibility mechanism (14.9.6) to the
>Vary header, I believe that we make caches much more robust to extended
>content (or feature) negotiation than by having the current definition of
>Vary.

Vary works well primarily because it is very simple.  PEP's use of a
single header field name "Protocol" to represent multiple, unrelated
control options doesn't work with this simple model.  However, I think
the solution is also simple: fix PEP so that it works better within
the design of HTTP, rather than the other way around.

>[...] if we instead have two extensions where one is optional
>and the other required like this:
>
>	GET /a-document HTTP/1.1
>	Host: a.host
>	Protocol: {http://some.org/a-required-extension {str "req"}},
>		   {http://some.org/an-optional-extension {str "opt"}}
>
>	HTTP/1.1 200 OK
>	Protocol: {http://some.org/a-required-extension {str "req"}},
>		   {http://some.org/an-optional-extension {str "opt"}}
>	Vary: Protocol
>	Content-Type: text/plain
>
>then the cache would only be able to serve the request if both extensions
>are available in the request even though the one is optional.
>
>I believe this in many cases is too strict as it will effectively
>disable caching of all optional extensions.

Yep, no disagreement there.  However, I will note that adding all those
bytes to a typical request is an incredibly bad idea, for the same reason
that adding huge amounts of Accept* stuff to typical requests is a bad idea.
But that is a separate issue.

>I therefore suggest that we define the Vary header to have the same
>extension modifiers to existing header values just like the cache-control
>header. This means that a PEP-aware proxy cache will be able to understand
>something like this
>
>	HTTP/1.1 200 OK
>	Protocol: {http://some.org/a-required-extension {str "req"}},
>		   {http://some.org/an-optional-extension {str "opt"}}
>	Vary: Protocol, "http://some.org/a-required-extension"
>	Content-Type: text/plain
>
>and serve the cached entity in response to a request like this:
>
>	GET /a-document HTTP/1.1
>	Host: a.host
>	Protocol: {http://some.org/a-required-extension {str "req"}
>
>Note that this doesn't _force_ the proxy to serve the request. This is a
>property of the extension itself: The proxy may not be allowed to reply as
>an intermediary depending on the contractual agreement between the proxy
>and the origin server.

That is asking a great deal from a proxy -- it would not only have to store
all of the client's Protocol requests, but on every request it would need
to find the Protocol field, parse it into entries, and then find the
entry associated with {http://some.org/a-required-extension {str "req"}}.
I suppose it would also need to check for any other end-to-end
{str "req"} options and not serve the cached entity if found.

If you want to negotiate based on the contents of individual field values,
then you will need to exchange a negotiation algorithm in the response
along the lines of

   Use-Only-If: Protocol contains
                {http://some.org/a-required-extension {str "req"}}

though I would recommend using an established notation for describing
the algorithm rather than an off-the-cuff example like this.

Another alternative is to ditch the current definitions of Protocol and
C-Protocol and do the following:

   1) Reintroduce the Mandatory field from two years ago, consisting
      of a list of header fields with semantics {str "req"}.

   2) Define a standard value for field self-definition, such that
      any new field whose field-value begins with

          {defined "http://some.org/whatever"}

      means that this field is defined according to the semantics
      of "http://some.org/whatever" (which we'll assume to be a
      URI with the equivalent naming power of a URN).  Any field without
      that value would have the definition given by the HTTP-version
      of the message.

   3) Add a field-name to the beginning of each Protocol-Info entry.

This would also get rid of the unclean use of a URI as the first
part of a bag.  Originally, the bag notation was intended to follow
the BRIO notation, which required that the first element in a list
be a symbol (token) for ease of representation.

.....Roy
Received on Thursday, 27 March 1997 18:52:38 EST

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