When troubleshooting many types of network problems, a protocol analyzer is an invaluable tool. However, for VoIP and IP Telephony, it's often overlooked, as many organizations don't want to spend the money to get a tool that can decode and play back audio from a capture file. But, you may have tools you don't know about.
If you're using Cisco's Unity voice mail server, there are two spiffy utilities tucked inside. As with all the good stuff, Cisco recommends you only use these with TAC assistance. Yeah, whatever...
The first tool is called "capripper." Every protocol analyzer I'm aware of allows you to store captured packets in a .cap format. You can capture various VoIP conversations with the analyzer of your choice, and then feed the .cap files to capripper, which, among other things, will save each RTP conversation it finds in a separate .wav file.
This is really handy for a lot of reasons. First, even if you have a top-shelf protocol analyzer like
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ClearSight (formerly AppDancer) capable of audio play back from the trace file, with capripper, you can give just the audio to interested parties, many of whom may not have a copy of your protocol analyzer. In my experience, many of these "interested parties" are customers or non-technical management who want to be involved in the troubleshooting of their systems. But giving them access to a protocol analyzer is a really bad idea. This way, you're giving them what they want, but not handing them all your network data -- including clear-text passwords, file transfers, etc. -- in a format they may not even understand. Besides, when you want to show how the voice network is performing, giving them actual audio is even better than giving them PowerPoint charts.
Second, capripper makes it much easier to archive the files. You can use this to baseline the network, and compare audio. Storing the RTP streams separately means you can give each a filename, etc.
The second tool that comes with Unity is called "RTP Parser". It's an add-on to Microsoft's NetMon, the packet-capturing and protocol-analyzing utility that is built-in to all the Windows server products since the first NT version. For details on RTP Parser, and on capripper, visit Cisco's website and read up on both of these two freebies.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.
This was first published in December 2003