Attach to a socket and trap all incoming and/or outgoing packet such that each packet requires appoval by the user before it is sent/received. Additional, each packet would be subject to editing by the user before being sent/received.

There are bugs all the way through Mac OS X 10.4.8 pertaining to recalculating the checksums for IP packets.


   [Man in the Middle]

While man in the middle is generally understood to be a malicious threat to a network, it can be incredibly useful for debugging a network issue on a switched network without having to debug the problem directly from one of the machines experiencing the problem.