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Troubleshooting VoIP with sniffers

How to troubleshoot by examining the raw data of sniffers.

Although there are a lot of fancy tools on the market that will tell you everything you want to know about your network performance, sometimes it's better for you to look at the raw data. Understanding what's really happening can help you build better networks and make it easier for you to troubleshoot, even though the manual process is much more cumbersome than the reporting from automated tools.

The easiest way to look at the raw data is with a protocol analyzer, often called a sniffer. For completeness, you'll want to insert your sniffer on both ends of a call, but assuming you have a WAN between your VoIP endpoints, you'll want to pay most attention to the packets that have already crossed the WAN, as this is where they're most likely to see degradation.

To configure your protocol analyzer, you'll want to set a filter for RTP traffic, and then make sure two fields in particular are displayed. First is "delta-time". This is the arrival time at the sniffer between subsequent packets. By default, a lot of sniffers will display the absolute or relative times, which are based on the clock and the start of the trace, respectively. The second field is the RTP Sequence Number. The VoIP sending endpoint increments this number with each packet sent, so the receiving endpoint knows if any packets are out of order. You will often find this value displayed by default inside the "summary" field, but if it's not there, you'll want to add it.

Once you've captured a stream of RTP packets, you can quickly scan down the Sequence numbers and determine if your network is sending any packets out of order. Then scan down the delta-time column and you can see what your jitter really looks like. If you see the same delta-time for every packet, your jitter is zero! If the delta-times are dramatically different, you probably want to rethink your QoS strategy.

Once you get familiar with this raw data, it will be easier for you to interpret the reports from your automated tools.

Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.

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