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Friday, July 24, 2009

One Key Sign of QoS Problems

There are some trace files that SCREAM at you! If you stand too closely you can feel spit hitting your face!

In the "Top 10 Reasons Your Network is Slow" online course (course abstract), we examine one of the causes of slow network performance. We look at a trace file of traffic that has passed through a router set up with QoS. You may not be aware how obvious QoS issues can be when analyzing traffic - feed a nice steady stream through that puppy and catch the traffic on the other side to see how it performed its duties.

Look for an EKG Pattern
In a datastream that is 'steady' - as in the video streaming example shown in the picture, we look for an "EKG pattern" in data coming through the router. This pattern is seen when data is held in the queue temporarily and then released (causing the sudden jump in the IO). As you can see in the image above, we can also spot packets that are droped by the queue. (Make sure you take a trace on the other side of the router to compare the IO graphs - you want to be certain a steady stream of data is traveling towards the QoS device and any alteration in the IO pattern has not already occurred.)

Get the Trace File
Go ahead - try checking it out yourself. Open up mcaststream-queued2.pcap in Wireshark. Select Statistics > IO Graph.

What? It's not screaming at you? Aha! That is because the X axis is too large - you are looking at ants from space! Change the X axis value to 0.01 seconds.

Do you see it? Right around 1.10 seconds into the trace - the EKG pattern! If users are not complaining about performance then dont' sweat it. Keep an eye on times when the line drops and doesn't jump up above the average point - those are dropped packets.

I'll be teaching the "Top 10 Reasons Your Network is Slow" on July 30th - it's a fun class to teach (although last time I was demonstrating the process of jamming a wireless network and nearly killed my own seminar hosting connection - duh). Register here.

Enjoy the trace! See you online!