ns 2 agents

Hi to all,

I am trying to simulate a simple call flow in ns 2. Something like the
one attached. My idea is to create a new packet header with one int
value and one agent for each node. Each message in the call flow is
given an ID (1,2,3...). The first agent sends the first packet with
the ID=1 to the second node. The agent on the second node receives the
packet and sends one packet back (ID=2) and one packet to the third
node (ID=3). The procedure continues in the same way.
I created the packet header successfully but i have some problems with
the agents. Has anyone done anything similar? Can anyone help me?

George.