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Re: Editorial note and question on connections

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Fri, 12 Feb 99 16:56:56 PST
Message-Id: <9902130056.AA29669@acetes.pa.dec.com>
To: Howard Melman <howard@silverstream.com>
Cc: HTTP Working Group <http-wg@cuckoo.hpl.hp.com>
    Second, I was looking through this section trying to find
    out if it's legal for proxies to multiplex requests from
    different clients on the same connection to a server.  I
    couldn't find a clear answer in the spec or in the
    archives.  That is given two clients A and B talking to the
    same server S via the same proxy P, what is minimum number
    of connections required.  Clearly there must be a connection
    from A to P and from B to P.  Can the proxy use just one
    connection to S or must it open separate connections for
    each client?
    
Its's legal, but not advisable.  Section 8.1.4 says

               A proxy SHOULD use up to 2*N connections to another
   server or proxy, where N is the number of simultaneously active
   users.

It might have been a good idea to explain the reasons behind
this a bit more carefully.

The problem is "head-of-line blocking."  Consider, in your
scenario, what would happen if A makes a request that generates
a really lengthy response, and then B issues a request.  Because
there is no packet-level multiplexing, B has to wait until
A's response is fully transmitted.

More concretely, suppose that
	A = Administrative Assistant
	B = Big Boss
and A requests a download of, say, the latest MP3 hit just
before B requests, say, a short piece of critical business
information.  (Not likely that these two resources would be
on the same server, I guess, but this is just an example.)

If we ever get a decent QoS infrastructure, this would make
it even more desirable to segregate request streams from
different users onto different TCP connections.

-Jeff
Received on Saturday, 13 February 1999 00:58:17 EST

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