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Proxies and incorrect Content-Length

From: Miles Sabin <msabin@cromwellmedia.co.uk>
Date: Mon, 20 Mar 2000 12:35:14 -0000
Message-ID: <AA4C152BA2F9D211B9DD0008C79F760A95D74C@odin.cromwellmedia.co.uk>
To: http-wg@hplb.hpl.hp.com
I'm looking for a brief rundown on best-practice for how non-
caching, limited-buffering, proxies should handle origin server
responses with incorrect Content-Length headers.

As far as I can make out there are are only two cases where
a proxy will be able to _reliably_ detect an incorrect Content-
Length,

  HTTP1.1 origin server with Connection: close
  HTTP1.0 origin server with no Connection: keep-alive

in both cases a proxy can infer a Content-Length overrun
because it expects the connection to be closed at the end of 
the response entity. Overruns with persistent connections can't
easily be distinguished from a broken subsequent response, and
underruns can't easily be distinguished from a broken 
connection.

Given that overruns are quite common (usually the result of 
broken CGIs/SSIs not accounting for the length of non-static 
data) I'd quite like to be able to forward such responses.
However I don't want to have to buffer the whole response to
recalculate the CL. I can see a couple of possibilities,

  HTTP1.1 downstream client
    Strip off the response Content-Length and forward with
    chunked transfer encoding.

  HTTP1.0 downstream client
    Strip off the response Content-Length and close the
    connection after the response entity.

Unfortunately the second of these effectively precludes the
use of Keep-Alive on all HTTP1.0 responses: because the proxy
won't be able to determine whether or not there's been an
overrun until the origin-server has run over the end, so *all*
responses have to be presumed to be potential overrunners.

Other options avoid that problem, but look troublesome,

  Truncate the reponse entity
    Dangerous for non text/* types; problematic even for those
    (eg. stripped trailing copyright messages).

  Forward any content overrun, then close the connection.
    Problematic for HTTP1.0 Keep-Alive clients which might
    attempt to interpret the overrun as the headers of a
    subsequest response; technically illegal for an HTTP1.1
    proxy. OTOH, the proxy would be forwarding stuff which is
    no more broken than would have been received had the origin
    server been contacted directly.

Opinions?

Cheers,


Miles

-- 
Miles Sabin                       Cromwell Media
Internet Systems Architect        5/6 Glenthorne Mews
+44 (0)20 8817 4030               London, W6 0LJ, England
msabin@cromwellmedia.com          http://www.cromwellmedia.com/
Received on Monday, 20 March 2000 12:36:58 EST

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