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Date:      Fri, 6 Mar 2009 19:59:04 +0100 (CET)
From:      Oliver Fromme <olli@lurza.secnetix.de>
To:        freebsd-ipfw@FreeBSD.ORG, sebastian.mellmann@net.t-labs.tu-berlin.de
Subject:   Re: ipfw (dummynet) adds delay, but not configured to do so
Message-ID:  <200903061859.n26Ix4QA070286@lurza.secnetix.de>
In-Reply-To: <64393.62.206.221.107.1236323210.squirrel@anubis.getmyip.com>

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Sebastian Mellmann wrote:
 > Luigi Rizzo wrote:
 > > The delay Sebastian is seeing comes from the babdnwidth limitation,
 > > which is causing a non-zero "transmission time" which is rounded up.
 > 
 > Let me get this right:
 > 
 > When I configure a pipe with bandwidth limitations, I'll always get some
 > additional delay when a packet passes this pipe?

Yes, of course.  That's expected.

Transmitting a packet through a 10 Mbit link takes longer
than transmitting the same packet through a 100 Mbit link.
Dummynet correctly emulates that behaviour, but it is
limited by the granularity of the kernel clock, which runs
at 1000 Hz by default, so the delays are rounded to 1 ms.

For example, transferring a 1 KB data packet (that's about
10 kbits including headers of the various protocols) will
take about 1 ms on a 10 Mbit link, and 0.1 ms on 100 Mbit.
Voila.

Best regards
   Oliver

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