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 -- Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing b. M. Handelsregister: Registergericht Muenchen, HRA 74606, Geschäftsfuehrung: secnetix Verwaltungsgesellsch. mbH, Handelsregister: Registergericht Mün- chen, HRB 125758, Geschäftsführer: Maik Bachmann, Olaf Erb, Ralf Gebhart FreeBSD-Dienstleistungen, -Produkte und mehr: http://www.secnetix.de/bsd "In My Egoistical Opinion, most people's C programs should be indented six feet downward and covered with dirt." -- Blair P. Houghton
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