Re: Initializing HTTP headers from HTML documents
Date: Wed, 5 Jan 94 12:59:01 EST
Original-From: hotsand!ellson (John Ellson)
Message-id: <9401051759.AA07042@hotsand.dacsand>
Subject: Re:  Initializing HTTP headers from HTML documents
Content-Length: 1642
> From: Dave_Raggett <>
> I would like to propose a scheme that HTTP servers can use to
> initialize HTTP headers by reading information held at the start of
> HTML/HTML+ documents. This is intended for fields like Expires: which
> are best determined by the document author.
> The generic META element takes the following attributes:
>         NAME    the name of an HTTP header such as Expires
>         VALUE   the value to be passed with the associated header
> e.g.  <META NAME="Expires" VALUE="Tue, 04 Jan 1994 14:13:25 GMT">
> This element is only permitted as part of the document's HEAD
> along with TITLE, ISINDEX and LINK.
> Any comments?
> Dave Raggett


I agree that authors would like to expire their documents at some
point in the future, even if those documents are being served
from a cache site.  But I don't see how a timestamp is sufficiently
expressive of the reasons an author might have to expire a document.
The reasons might not even be known at the time that the document
is written.
Could VALUE be a boolean expression so that we could do something like:
   <META NAME="Expires" VALUE=EXISTSP(<>)>
   <META NAME="Expires" VALUE=GT(DATE, "Tue, 04 Jan 1994 14:13:25 GMT")>
There would need to be a way of preventing caches from cacheing the
result of EXISTSP. 
Also, clients need to do something reasonable if the boolean
expression cannot be evaluated.  Perhaps they could display the page
anyway but with a warning message saying that "the expiry status
of this document is unknown." 
John Ellson
AT&T Bell Labs