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Re: Suggestion for NEW Issue: Pipelining problems

From: David Morris <dwm@xpasc.com>
Date: Mon, 23 Jul 2007 17:56:57 -0700 (PDT)
cc: HTTP Working Group <ietf-http-wg@w3.org>
Message-ID: <Pine.LNX.4.33.0707231754090.13007-100000@egate.xpasc.com>


Just a footnote ... with content from the likes of cnn, I've measured 150+
objects retrieved to compose a single page. Not all from the same server,
but a significant opportunity for pipelining/persistent connections, etc.

On Mon, 23 Jul 2007, Roy T. Fielding wrote:

>
> On Jul 23, 2007, at 2:02 PM, Jeffrey Mogul wrote:
>
> > Roy writes:
> >
> >     Pipelined requests actually increase congestion because any
> >     messages left unsatisfied have to be sent again on a new
> > connection.
> >
> > Just curious: can you point us to the experimental evidence for this?
>
> No, just ad hoc observation while viewing systems though tcpdump.
>
> > That is, evidence that shows that the effect you described outweighs
> > the congestion that might be avoided when successful pipelining, for
> > example, reduces the burstiness of non-pipelined TCP connections.
>
> It depends on what kind of connections you are talking about.
> If the connection is so heavily used that the persistence remains
> open (either by accident or through special configuration) then
> there is no question that congestion will be reduced on average
> versus multiple TCP connections.  We can just sum the bits for that.
> However, the burstiness of real HTTP traffic is because the
> applications have bursty needs.  I don't see how pipelining can
> change that without being an artificial request profile or an
> unusual application (like Google's spider).
>
> What I've seen in traces is that pipelining does have substantial
> benefits up until the point of reaching an application steady-state
> (for a browser, that means a web page with all inlined resource
> requests complete).  If the client closes the connection at that
> point, instead of waiting for the next request to find out if the
> connection was kept open by the server, then it minimizes its use
> of the network.  Likewise, pipelining has huge benefits for
> specialized HTTP services, such as Subversion (when it uses it)
> and content management systems.
>
> If the connections are used infrequently and are susceptible to
> timeouts occurring while the next request is in transit, then the
> data sent across the network far exceeds any average
> *congestion-control* benefit obtained by avoiding the separate
> connections. A new connection will be required anyway and the
> request message is sent twice (though whether that message hit
> the network or not seems to depend on the TCP implementation).
>
> That should be testable to find the actual trade-off point where
> a client should close the connection, but I have no experimental
> evidence and no time to set up an experiment.  In any case,
> pipelining is still worthwhile for many reasons, particularly
> when we have control over both sides of the connection, so we need
> it defined in HTTP regardless of any single client's request profile.
>
> BTW, do you know of any good (experiment-quality) evaluations
> of HTTP over SCTP?
>
> ....Roy
>
Received on Tuesday, 24 July 2007 00:57:08 GMT

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