TCP Performance

Nick Olsen nick at flhsi.com
Tue Aug 27 17:11:52 UTC 2013


I do indeed have stats for "TX Pause Frames" And they do increment. 
However, Our router is ignoring them since it doesn't support flow 
control.
I guess my next question would be. In the scenario where we insert a switch 
between the radio and the router that does support flow control. Are we not 
only moving where the overflow is going to occur? Will we not see the 
router still burst traffic at line rate toward the switch, Which then 
buffer overflows sending to the radio on account of it receiving pause 
frames?

Nick Olsen
Network Operations (855) FLSPEED  x106

----------------------------------------
From: "Tim Warnock" <timoid at timoid.org>
Sent: Tuesday, August 27, 2013 1:08 PM
To: "Blake Dunlap" <ikiris at gmail.com>, "nick at flhsi.com" <nick at flhsi.com>
Cc: "nanog at nanog.org" <nanog at nanog.org>
Subject: RE: TCP Performance

> Regardless, your problem looks like either tail drops or packet loss, 
which
> you showed originally. The task is to find out where this is occurring, 
and
> which of the two it is. If you want to confirm what is going on, there 
are
> some great bandwidth calculators on the internet which will show you 
what
> bandwidth you can get with a given ms delay and % packet loss.
> 
> As far as flow control, its really outside the scope. If you ever need 
flow
> control, there is usually a specific reason like FCoE, and if not, it's
> generally better to just fix the backplane congestion issue if you can,
> than ever worry about using FC. The problem with FC isn't node to node, 
its
> when you have node to node to node with additional devices, it isn't 
smart
> enough to discriminate, and can crater your network 3 devices over when 
it
> would be much better to just lose a few packets.
> 
> -Blake

In my experience - if you're traversing licenced microwave links as 
indicated flow control will definitely need to be ON.

Check the radio modem stats to confirm but - if you're seeing lots of drops 
there you're overflowing the buffers on the radio modem.




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