[ns] Where is the interface between Agent and Node?

[Wang Ivan]

I traced the C++ source code for Agents and found that packets sent by an
agent is directly passed to the target agent ( target_->recv(p,h) ) rather
than to the lower layer. For real TCP/IP, packets should be passed down to
the physical layer, sent to the target, and then passed up to the upper
layer. I wonder how ns2 work in this part.