Multitrace analysis can be the most interesting, rewarding and unfortunately, most frustrating exercise an analyst will face.
Before we get to the packet analysis, setting up your tools for simultaneous capturing can be a feat in itself.
The time issue is the most critical when using 2 devices since the time is used to calculate the delay, jitter or latency. Some people are fine with syncing both devices to a common ntp server.
Then there’s the “how the #!!$!@#!!” do I physically capture . This is where you have to be familiar with the problem, the network you are working on and what equipment is available to you. If you are lucky enough to be able to change the speed and duplex to 100 half duplex a good old hub fits the bill. Other than the mirror/span command, a tap is also very helpful. Trust me every one of these suggestions comes with their own caveats. You may have to try different tools for different scenarios.
For example, if I am doing a simple pc bootup/login baseline, I am interested in things like total data transferred, which IP’s I am talking to, protocols used, errors, etc. In this case speed and duplex is not important and I can go with a hub. But if I was troubleshooting why something is taking too long, like a backup or replication, changing the speed and duplex would not be a good idea.
If you are lucky enough and can capture from one device, the time accuracy issue goes away and life does get a bit easier. But now you have 2 different captures in the same trace, Yikes!!!!
Not to mention that different network interfaces have different latency or behaviors. I remember trying a usb to 10/100 ethernet adapter to capture packets and quickly realized that this adapter added 30 ms to every packet. Again, if I was troubleshooting latency, this won’t do.
Lastly, if you’re fortunate enough, you might even have an application that takes multiple trace files and calculates all sorts of stuff out for you (hmm.. next article?).
In this example I use Wireshark, my laptops WiFi and Ethernet ports to capture my packet traversing a residential home router. I show some tips and tricks along the way and hope this will help you out.