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Re: Proxies and incorrect Content-Length

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Thu, 23 Mar 2000 15:48:24 -0800
Message-Id: <200003232348.PAA03864@wera.pa.dec.com>
To: Miles Sabin <msabin@cromwellmedia.co.uk>
Cc: 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.

We decided, when editing the HTTP/1.1 spec, NOT to specify
the behavior of the protocol in every possible error condition,
because there are so many possible error cases (since there
are so many different headers).

Instead, I think one should fall back on one or both of two
very basic principles, when looking for guidance about how
to make an implementation decision (and when you are sure
that the spec itself provides no guidance!):

(1) The Robustness Principle, as explained in RFC791 (the
IPv4 specification), section 3.2:
  The implementation of a protocol must be robust.  Each implementation
  must expect to interoperate with others created by different
  individuals.  While the goal of this specification is to be explicit
  about the protocol there is the possibility of differing
  interpretations.  In general, an implementation must be conservative
  in its sending behavior, and liberal in its receiving behavior.  That
  is, it must be careful to send well-formed datagrams, but must accept
  any datagram that it can interpret (e.g., not object to technical
  errors where the meaning is still clear).

To apply this to an HTTP proxy, I would focus on the "be liberal
in its receiving behavior" - i.e., don't drop responses because
they appear to be illegal.  In fact, I don't think we have ever
defined a status code that a proxy could use to say "I dropped
the response you were expecting because it seemed illegal to me",
and it seems like a bug to silently drop (or truncate) responses.

(2) The (less formally stated) rule for HTTP proxies:
	It's always allowed to act like a tunnel.

RFC2616 says
					tunnels are used when the
   communication needs to pass through an intermediary (such as a
   firewall) even when the intermediary cannot understand the contents
   of the messages.

which arguably applies in this case.

However, there is a catch: if the proxy has modified the client's
original request (e.g., to convert a simple request into a Range
request), then it has already blown its chance to act like a
true tunnel.  I'm not sure what to do in this case.
You suggest, as one option:

  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.

This seems like the most straightforward "act like a tunnel"
approach.  As you point out, it's no worse than if the proxy
weren't involved in the first place, which is really the
implication of the Robustness Principle ("first, do no harm").

Received on Thursday, 23 March 2000 23:49:51 UTC

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