Cisco PPP DS-3 limitations - 42.9Mbpbs?

Wed Feb 20 21:24:25 UTC 2002

> On Wed, 20 Feb 2002, Jon Mansey wrote:
>
> > OMG! Arent we missing the point here? What about never running
> links above
> > 60% or so to allow for bursts against the 5 min average, and <shudder>
> > upgrading or adding capacity when we get too little headroom.
>
> > And here we are, nickel and diming over a few MBps near to 45M
> on a DS3...
>
> And why not? Obviously there is a reason why they're not upgrading,
> because there is plenty of traffic to fill up a second or faster circuit
> if packets are being dropped because of congestion. (Which has not been
> confirmed so far.)
>
> There shouldn't be any problems pushing a DS3 well beyond 99% utilization,
> by the way. With an average packet size of 500 bytes and 98 packets in the
> output queue on average, 99% only introduces a 9 ms delay. The extra RTT
> will also slow TCP down, but not in such a brutal way as significant
> numbers of lost packets will. Just use a queue size of 500 or so, and
> enable (W)RED to throttle back TCP when there are large bursts.
>

One problem you have here is how you are getting the utilization statistics.
Since you are
looking at 5 minute averages, chances are real good that your instantaneous
figure is probably
at the full capacity of the DS-3.  As you approach the maximum capacity and
start dropping packets,
your throughput on the line will bounce around near the 45 megs figure but
the "goodput" that the
customer sees will drop dramatically.  You will be sending retransmissions
of the dropped packet,
which then causes less bandwidth to be available for current traffic, which
causes more drops,
which causes more retransmissions and backoffs.  My experience in the real
world is that once
you get over 40 mbps on a DS-3 you need to look at upgrading.  Another thing
I would question
is where the Cisco is counting traffic.  Since it is most likely looking at
the real user traffic
there is more likely some overhead to manage the DS-3 itself and some L2
protocol stuff also.  This
is definitely a factor when running ATM because the router only counts the
input/output traffic after
the ATM overhead has been stripped away unless you are looking at the
controller and not the interface.
Remember the interface is just that, an interface to the line itself.  You
may be able to get more accurate
data by looking at the controller stats because they operate at a lower
level than the interface.

Steven Naslund
Network Engineering Manager
Hosting.com - Chicago