[ns] a multicast question

[Shuju Wu <swu@mail.sis.pitt.edu>]

HI,

I attached a multicast agent to a node.
After this node receives a data packet, 
this node will multicast an ACK to the whole 
group. But in NAM, when I check the packets, 
the ACK packet is multicast to whole group 
earlier than the data packet, that means 
other nodes receive the ACK packet first, receive 
the data packet after the ACK packet. 

Can anyone tell me why this happends? 
Did I set up something wrong?

Thanks

Shuju