Re: Ideal Response Time

From: Alex Rousskov (rousskov@ircache.net)
Date: Thu Dec 16 1999 - 16:07:09 MST


On Thu, 16 Dec 1999 Alan Smithee wrote:

> With Dummynet configured for bake-off ISP environment, what kind
> of mean response time should I see? Should it still be around 1.50
> sec as it was with PolyMix-1?

Alan,

        The formula for PolyMix-2 and WAN simulation is no longer that
simple and obvious because
        (a) there is packet loss on the network and
        (b) packet delays affect the importance of persistent
            connections (TCP handshake sequence is now appropriately
            delayed!).

Without taking (a) and (b) into account the "ideal" response time is
probably

        miss: 75 + 0 + 55 + 2500 + 55 + 0 + 75 = 2760 msec
         hit: 75 + 0 + 75 = 150 msec

     average: 0.45*2760 + 0.55*150 = 1325 msec

Where "0" is an ideal proxy-introduced delay, and "2500" is mean server
side think time. Again, the first request on a connection will
experience an extra delay of at leat one packet round trip time (RTT).
You probably want to double check my calculations.

Alex.

        # ISP client
        ipfw pipe 1 config delay 75ms plr 0.001
        ipfw pipe 2 config delay 75ms plr 0.001

        # ISP server
        ipfw pipe 1 config delay 55ms plr 0.0005
        ipfw pipe 2 config delay 55ms plr 0.0005



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