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Date:      Fri, 5 Sep 2003 14:42:45 -0700
From:      Yuchung Cheng <ycheng@cs.ucsd.edu>
To:        Luigi Rizzo <rizzo@icir.org>
Cc:        ipfw@freebsd.org
Subject:   Re: dummynet and modem pipes
Message-ID:  <20030905144245.A472@cs.ucsd.edu>
In-Reply-To: <20030905112459.A78583@xorpc.icir.org>; from rizzo@icir.org on Fri, Sep 05, 2003 at 11:24:59AM -0700
References:  <20030905095038.D28924@cs.ucsd.edu> <20030905112459.A78583@xorpc.icir.org>

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On 09-05-2003, Luigi Rizzo wrote:
> basically there is no way to accurately simulate modem compression.
> As a rule of thumb, for bulk traffic you can think that the main
> effect is the bandwidth changes (the exact ratio depends on the
> type of traffic), whereas for interactive apps (e.g. rpc and the
> like) more than compression what affects you is the additional
> delay that the modem causes to decide that it is time to build&send
> a new packet.
> 
> also you'd need to take care of ip compression, not just what
> happens in the modem itself
> 
i assume you mean ip header compression over ppp. the simulation traffic
is mainly short web flows (~ 10-20K), so i think header compression does
not matter much (40 bytes vs payload 500 or 1500 bytes) compared to 
modem compression. for html, the compress rate can be as high as 3 based 
on real measurements. it that true? thanks.

-yuchung



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