Re: [LARTC] Is this possible?

From: gypsy <gypsy@iswest.com>
To: lartc@vger.kernel.org
Subject: Re: [LARTC] Is this possible?
Date: Fri, 24 Feb 2006 03:49:10 +0000	[thread overview]
Message-ID: <43FE8236.372C591@iswest.com> (raw)
In-Reply-To: <marc-lartc-99477220206399@msgid-missing>

Russell Stuart wrote:
> 
> On Thu, 2006-02-23 at 10:23 +0100, Andreas Klauer wrote:
> > On Thu, Feb 23, 2006 at 06:38:09PM +1000, Russell Stuart wrote:
> > > For example, lets say we have a 1000kbit link, and two
> > > classes sharing that link:
> > >
> > >   - Voip - ie high prio real time, and
> > >   - Web - background traffic.
> >
> > Have you measured this link, i.e. when there is no activity
> > and you start some Voip sessions, do they get a constant
> > downstream of 1000kbit?
> >
> > It may very well be that you have to measure the real throughput
> > and then go a little lower (since you have to be the bottleneck),
> > however having to throw 30% of bandwidth away sounds a bit too
> > harsh to me.
> 
> The setup I gave was purely hypothetical.  300kbit
> headroom sounds way to high to me as well - any
> advice others may have on this would be appreciated.
> 
> > Another way of indirect headroom would be to hard limit the Web class,
> > i.e. give the Web class a lower ceil than the other classes. This way,
> > there is bandwidth that the Web class can't use no matter what, even
> > if the link is completely empty.
> 
> That is the right answer - it would achieve what I want.
> In hindsight it seems so obvious I don't know why I
> didn't think of it myself.
> 
> Thanks for taking the time to answer my query.

Two more things.  HTTP is a bursty protocol, so you need to think about
the burst and cburst parameters you give it.  If you want to squash TCP
fast start, use a low burst which will backlog and eventually drop the
excessive packets.  On the other hand, my experience is that a slow
started connection never increases its flow rate much even though the
spec says it should.  And you can get better precision from HTB by
setting HYSTERYSIS (did I just misspell that?), thus dequeueing a single
packet rather than a pair.  I don't recommend that, but you should know
about it.  On many ATM links it is a godsend.

In terms of headroom, I find that 85 % of real capacity always works, so
I start with that and push up until something breaks.  YMMV.
--
gypsy
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