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Re: RE-VERSION

From: Klaus Weide <kweide@tezcat.com>
Date: Sun, 10 Aug 1997 08:02:32 -0500 (CDT)
To: "Roy T. Fielding" <fielding@kiwi.ics.uci.edu>
Cc: HTTP Working Group <http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com>
Message-Id: <Pine.SUN.3.95.970810012315.27651B-100000@xochi.tezcat.com>
On Sat, 9 Aug 1997, Roy T. Fielding wrote:

[Josh Cohen <josh@netscape.com>:]
> >To summarize, the draft now indicates that the response version
> >should be the highest version supported by the server, NOT
> >the version of the actual HTTP response.
> 
> To reiterate, it has been that way since 1993.
> 
> >My problem with this is based on the assumption in the draft
> >which dictates that the response version is hop-by-hop.
> 
> It is not an assumption -- it is a requirement of HTTP.
[ -- snip -- ] 
> 
> >Worse, if a 1.0 proxy is in the stream, it will blindly pass
> >the 1.1 response code back to the client, and clearly fails
> >to honor the hop-by-hop nature of the response.
> >(this is the case with squid, netscape proxy, and presumable others)
> >Previously, the 1.0 didnt clearly define what the behaviour
> >should be in this case.
> 
> That simply isn't true unless you have changed the proxy recently.
> 1.0 required that the proxy send it's own version in any forwarded
> request or response.  The CERN, Netscape, and Apache proxies all obeyed
> that restriction last time they were checked.  If Squid managed to scew
> that up even after three RFCs have been published on the subject, then
> they can't be trusted to have implemented anything right.

A quick check with reality.  localhost:8080 is CERN/3.0, localhost:squid
is Squid 1.1.1, none of them "changed recently".

$ telnet localhost 8080
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
HEAD http://www.apache.org/ HTTP/1.0

HTTP/1.1 200 OK
Date: Sun, 10 Aug 1997 06:45:08 GMT
Server: Apache/1.3a2-dev
Cache-Control: max-age=86400
Expires: Mon, 11 Aug 1997 06:45:08 GMT
Connection: close
Content-Type: text/html

Connection closed by foreign host.
$ telnet localhost squid
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
HEAD http://www.apache.org/ HTTP/1.0

HTTP/1.1 200 OK
Date: Sun, 10 Aug 1997 06:47:07 GMT
Server: Apache/1.3a2-dev
Cache-Control: max-age=86400
Expires: Mon, 11 Aug 1997 06:47:07 GMT
Connection: close
Content-Type: text/html

Connection closed by foreign host.

I have also checked, for both proxies, that
- this is not a particularity of the HEAD method (It happens with GET,
  too)
- for GET, if the request is repeated and the response served from the
  proxy's cache, the response is still sent with "HTTP/1.1".

This shows that at least these two 1.0 proxies do not send their own
version in a forwarded (OR cached) RESPONSE.

Now for whether they downgrade the version to their own in a REQUEST -
yes, they both do: (only shown for CERN/3.0)

$ telnet localhost 8080
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
HEAD http://www.apache.org/ HTTP/1.1
Host: www.apache.org

HTTP/1.1 200 OK
Date: Sun, 10 Aug 1997 07:07:25 GMT
Server: Apache/1.3a2-dev
Cache-Control: max-age=86400
Expires: Mon, 11 Aug 1997 07:07:25 GMT
Connection: close
Content-Type: text/html

Connection closed by foreign host.
$

This is what goes over the wire:

        HEAD / HTTP/1.0
        Host: www.apache.org
        User-Agent: Proxy gateway CERN-HTTPD/3.0 libwww/2.17

So both proxies act according to the expectation (aka: requirement) in one
direction but not in the other.

A HTTP 1.0 client accessing a 1.1 origin server through such a proxy
will never improperly receive (for example) a chunked response, since the
server won't send it through that proxy.

The only harm I can see is with a HTTP 1.1 (or higher) client, which could
be misled into believing that it is talking to a 1.1 proxy.  As a result
it might attempt to use a 1.1-only feature in a later request through that
proxy - for example sending a chunked request body in a POST.

> We simply cannot get around this requirement. ANY proxy which fails
> to indicate its own HTTP-version is incapable of supporting ANY
> future enhancements to the protocol, period.  HTTP will be dead.
> Think about it: the only thing that does not get forwarded by a
> valid proxy is the Request-Line/Status-Line.  ANY other indication
> of a version number, e.g. some version number in a header, WILL be
> forwarded by an old proxy.  

And, as shown above, so will version numbers _in responses_, by at least
two 1.0 proxy implementations.

> If we can't rely on HTTP-version, what
> makes you think we can rely on anything else?

In general I think the definition of HTTP-version is fine, as set out in
RFCs 1945, 2068, and 2145.  But the existence of old (pre-1945) proxies
which do not obey RFC 1945
                                     "Since the protocol version
   indicates the protocol capability of the sender, a proxy/gateway must
   never send a message with a version indicator which is greater than
   its native version;"
for response messages should not just be denied.

Higher version clients and servers could work around it by agreeing on a
special connection token for detection, as described below.  Admittedly
it's ugly and wastes bytes - but as far as I can see it would work, under
the stated assumptions.  (It would make all proxies of this kind 
detectable reliably by any >1.0 client if all >1.0 servers would implement
it.  Of course it's already too late for that.)

Assuming that only (some) 1.0 proxies are broken, in the way shown above,
that such proxies still downgrade the REQUEST version correctly, and that
they do NOT understand the Connection header:

   All HTTP/1.N, N > 0 servers send the following IF and only IF the
   request was a HTTP/1.0 request [possible addition: AND it is NOT known
   that the next-hop client is NOT a "broken" 1.0 proxy], AND IF the
   server does not choose to downgrade its response version to HTTP/1.0
   (as allowed by RFC 2145 2.3 last para):

   Connection: If-you-are-gt-1.0-Dont-trust-Version-gt-1.0-for-next-hop

   If a HTTP/X.Y, X.Y > 1.0 client receives this token in a HTTP/1.N, N>0
   response to a request (for it did not downgrade its request version to
   1.0 as allowed by RFC 2145 2.3 third para), it can assume that the
   next-hop server is really a 1.0 proxy.

   And of course in reality the token should be shorter.  Maybe more like

   Version-kludge: my.host.name.com 1.1  (thrown in for debugging)
   Connection: version-kludge, close

This is one idea that would require cooperation (or a protocol change) from
servers not directly involved.  However, since the problem is only with
detecting the correct version of a next-hop proxy, there often will be
other means - out-of-band information or configuration, checking directly
with OPTIONS or TRACE if a proxy appears to be >1.0, or similar.


    Klaus
Received on Sunday, 10 August 1997 06:04:57 EDT

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