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Proposed solution for issue CLARIFY-NO-CACHE

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Mon, 21 Jul 97 18:58:00 MDT
Message-Id: <9707220158.AA01541@acetes.pa.dec.com>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
The CLARIFY-NO-CACHE issue:
	http://www.w3.org/pub/WWW/Protocols/HTTP/Issues/#CLARIFY-NO-CACHE

boils down to:
	the meaning of no-cache="field-name" in a response probably
	needs to be made more explicit.

To state the issue in somewhat more detail, RFC2068 says this about
the "no-cache" Cache-control directive in a response:

    14.9 Cache-Control
    
		    [...]
	      cache-response-directive =
		    [...]
			      | "no-cache" [ "=" <"> 1#field-name <"> ]
		    [...]
    
    14.9.1 What is Cachable
		    [...]

    no-cache
      Indicates that all or part of the response message MUST NOT be cached
      anywhere. This allows an origin server to prevent caching even by
      caches that have been configured to return stale responses to client
      requests.
    
14.9 also says:

   When a directive appears without any 1#field-name parameter, the
   directive applies to the entire request or response. When such a
   directive appears with a 1#field-name parameter, it applies only to
   the named field or fields, and not to the rest of the request or
   response.  This mechanism supports extensibility; implementations of
   future versions of the HTTP protocol may apply these directives to
   header fields not defined in HTTP/1.1.

However, I think it takes a fair amount of inference to combine these
two paragraphs into a precise definition of what
	Cache-control: no-cache=foo
really means.

PROPOSED SOLUTION:
I replacing (in 14.9.1) the paragraph:

      Indicates that all or part of the response message MUST NOT be cached
      anywhere. This allows an origin server to prevent caching even by
      caches that have been configured to return stale responses to client
      requests.
    
with paragraph:

      If the no-cache directive does not specify a field-name, then a
      cache MUST NOT use the response to satisfy a subsequent request
      without successful revalidation with the origin server.  This
      allows an origin server to prevent caching even by caches that
      have been configured to return stale responses to client
      requests.

      If the no-cache directive does specify one or more field-names,
      then a cache MAY use the response to satisfy a subsequent
      request, subject to any other restrictions on caching; however,
      the specified field-name(s) MUST NOT be sent in the response to a
      subsequent request without successful revalidation with the
      origin server.  This allows an origin server to prevent
      the re-use of certain header fields in a response, while
      still allowing caching of the rest of the response.

-Jeff

P.S.: As I wrote in
	http://www.ics.uci.edu/pub/ietf/http/hypermail/1997q1/0040.html

    I'm not sure that this convoluted definition of no-cache really
    makes life easier for people.  I remember arguing that we should
    be using a wider range of names for cache-control directives, and
    being accused of trying to make the specification "too complex."
    But it's probably too late to change the actual specification
    of "no-cache", although I think it's clear that we need to clarify it.

However, my preference would be to replace the BNF

          cache-response-directive =
                     | "no-cache" [ "=" <"> 1#field-name <"> ]

with
          cache-response-directive =
                     | "no-cache"
                     | "no-cache-field" "=" <"> 1#field-name <"> 

and then do the obvious simplification of the description proposed
above.
Received on Monday, 21 July 1997 19:04:45 EDT

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