[ns] increasing the simulation time in NS2:please help

[Student Ns]

Hi all,
Do you know please how to make the simulation time in NS2 longer?
when
I do :  set val(stop)        1680.0 , I have no errors, but when I want
to increase the simulation time for example : set val(stop)       
1700.0, I have this error
TcpAgent: negative RTO!  (-6442450944.000000).
Can you tell me please how to fix this error?
thank you very much.