From owner-freebsd-ipfw@FreeBSD.ORG  Fri Mar  6 07:17:26 2009
Return-Path: <owner-freebsd-ipfw@FreeBSD.ORG>
Delivered-To: freebsd-ipfw@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id D5CC4106564A
	for <freebsd-ipfw@freebsd.org>; Fri,  6 Mar 2009 07:17:26 +0000 (UTC)
	(envelope-from luigi@onelab2.iet.unipi.it)
Received: from onelab2.iet.unipi.it (onelab2.iet.unipi.it [131.114.9.129])
	by mx1.freebsd.org (Postfix) with ESMTP id 982668FC18
	for <freebsd-ipfw@freebsd.org>; Fri,  6 Mar 2009 07:17:26 +0000 (UTC)
	(envelope-from luigi@onelab2.iet.unipi.it)
Received: by onelab2.iet.unipi.it (Postfix, from userid 275)
	id D3F5A73098; Fri,  6 Mar 2009 08:22:29 +0100 (CET)
Date: Fri, 6 Mar 2009 08:22:29 +0100
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: Sebastian Mellmann <sebastian.mellmann@net.t-labs.tu-berlin.de>
Message-ID: <20090306072229.GB94585@onelab2.iet.unipi.it>
References: <49AED3B1.1060209@net.t-labs.tu-berlin.de>
	<20090304210017.GA29615@onelab2.iet.unipi.it>
	<20090306153751.D71460@sola.nimnet.asn.au>
	<20090306070011.GA94585@onelab2.iet.unipi.it>
	<64393.62.206.221.107.1236323210.squirrel@anubis.getmyip.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <64393.62.206.221.107.1236323210.squirrel@anubis.getmyip.com>
User-Agent: Mutt/1.4.2.3i
Cc: freebsd-ipfw@freebsd.org
Subject: Re: ipfw (dummynet) adds delay, but not configured to do so
X-BeenThere: freebsd-ipfw@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: IPFW Technical Discussions <freebsd-ipfw.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-ipfw>,
	<mailto:freebsd-ipfw-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-ipfw>
List-Post: <mailto:freebsd-ipfw@freebsd.org>
List-Help: <mailto:freebsd-ipfw-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-ipfw>,
	<mailto:freebsd-ipfw-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Fri, 06 Mar 2009 07:17:27 -0000

On Fri, Mar 06, 2009 at 08:06:50AM +0100, Sebastian Mellmann wrote:
> 
> >> Secondly, apropos Sebastian's experience, should this say "The value
> >> (even if 0) is rounded to the next multiple of the clock tick .." ?
> >> ^^^^^^^^^^^
> >
> > 0 is rounded to 0 so that's not an issue.
> > 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?

"additional" compared to the case of no bandwidth limitations.
But the delay is exactly the effect of bandwidth limitations
and the presence of the queue (and possibly +/- 1 tick of rounding),
so you have to understand what is modeled if you want to account
for it precisely.