I'm a bit confused. In my world, there is the physical layer which is L1 - for instance SONET. Then there is the link layer (L2), which is say PPP or Ethernet or ATM or such. Then is the network layer (L3) which is usually IP. TCP and UDP and RTP and so on are all L4 - the transport layer.

I'm also not sure why Justin thinks that TCP is on top of UDP? They're not stacked like that, though I guess you could picture TCP as adding functionality to UDP, but there are so many differences I'm not sure that's really useful.