[olug] Help w/ my server

Christopher Cashell topher-olug at zyp.org
Tue Jul 24 04:47:04 UTC 2012

On Mon, Jul 23, 2012 at 11:09 PM, Lou Duchez <lou at paprikash.com> wrote:
> Maybe you're the guy to help clear something up for me.  I've experimented
> with tc and I think I can more or less get it to work -- but -- I seem to
> have control over upstream traffic only.  As far as I can tell, my ISP is
> deciding which traffic to send down the pipe to me, and I have no QoS
> control over that.  Does that sound about right?

That is correct.  QoS is pretty much always an outgoing only tool
(unless you have control over the routers on both ends of a
connection).  For incoming packets, by the time they've hit you,
they've already had to travel through the pipe (potentially getting
discarded), so it's too late to do anything very effective.

> I think of QoS as the fine art of deciding whether to drop/delay packets,
> but if you're receiving exactly as many packets as your pipe can carry,
> there's no QoS-ing to do, right?

Generally, if you aren't nearing your throughput capacity, QoS will
have somewhat limited benefit.  It can still be beneficial with
latency sensitive applications like VoIP, as it can help "even out"
the packet rate, reducing jitter.  Also, you don't need to max out a
connection before you see impact, it can happen shy of full.  But QoS
only has a big benefit when you start approaching capacity.

Remember though, many (most?) network protocols, especially TCP based
protocols, will tend to "fill the pipe" whenever possible.  This
results in faster transfers that take a shorter period of time.  There
will always be a bandwidth constraint somewhere; the closer it is to
your machine, the more beneficial QoS will be.

> The other QoS oddity I encounter is, I find that QoS works best with a
> predictable number of connections competing for bandwidth; when the number
> of connections fluctuates heavily, QoS has trouble keeping up with the
> fluctuations.  Does that sound like QoS, or does that sound like a flawed
> attempt at QoS?

I haven't really seen issues like this myself.  My first guess would
be that the QoS configuration might need tweaking.  There are a lot of
different ways to configure QoS (one reason for it's high complexity),
and some work better than others.

One thing to remember, unless you have a guaranteed fixed bandwidth
rate, you will need to do some testing to find out what your max
transfer rates are, and then configure your QoS based on a rate
slightly lower than that.  Otherwise, the QoS policies won't kick in
before packets could get discarded.


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