Well, you can add LSPs (Layered Service Providers) to a networking stack. If placed on the top of the stack, it will intercept all traffic and filter it for whatever you specify and pass the rest down the stack. This is how most VPN clients operate although they tend to call these things "SHIMS" (see www.v-one.com for info on how this works). Also, some older spyware operated this way. New.net is one that comes to mind.

So in fact, you *can* modify a network stack and it will work fine. Tell your teacher to head back to the CHUBB institute for a refresher or if you prefer, I can learn him.