[ns] exchange between two agents on same node?

[nichunsheng@263.net]

hi all,
  I want to simulate a topo like that:

1-----2------3
      |
      |
      4
1 sends pkt to 4. when pkt arrives 2, 2 will do some negotiation with 3,
probably changing the pkt. And then forward pkt to 4.
I want to make two new agents, one for transport and the other for negotiation.
But how will the two agent communicate with each other? 
with one agent,  2 has to delay the pkt forwarding to 4 . After negotiation with 3,
2 will forward pkt to 4, how to do this? 
Or anyone has better method?  
   
 thanks in advance.

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