[ns] What is the reason for "RTR CBK" Drops in cmu-trace?

[Mayur]

Dear all,

What is theoretical interpretation of the event "RTR CBK" in cmu-trace. 
I found that it is abbreviated for "DROP_RTR_MAC_CALLBACK" in 
cmu-trace.h. It is certainly a Router layer event, but *what is the 
meaning of CALLBACK here?* *What is the exact reason of the DROPping of 
the packet in such trace lines?* One example trace line is shown below:

 *D* 1844.506605289 _4_ *RTR  CBK* 322333 tcp 1060 [13a 5 4 800] ------- 
[4:0 5:0 30 5] [67665 0] 0 0*

regards,
Mayur