tcpdump is not installed by default even on test and dev builds.  To add it to a running system you can use gmerge and the devserver.  You should also be familiar with the process for building packages document in the Developer Guide.  Due to the oddities of gmerge you will need to perform the steps below in exactly in the order specified. This is because gmerge removes files from the install within the chroot environment of your build computer.

You should now be able to run tcpdump to collect traces.  Copy the files back to your development machine and analyze the traces with your favorite tool (tcpdump, WireShark, etc).