I have posted a thread asking details about PING programme. Thanks for all who answered and read it. There is much more fun discussing with/talking to others than working alone.But now it's time for some more question on the PING programme :-P It is indicated by Stevens' book: TCP Illustrated that BSD implementation has improved resolution for the timer used in PING programme under it development. Details below: BSD/386 version 0.9.4: ping timer resolution millisecond BSD/386 version 1.0: microsecond. This leads to a improvement in the ping rtt record: from ms to us... But: what about later version of BSD? (I have print the rtt on my Freebsd 6.2 box and it seems have the resolution of us as well) a more important question: does this timer resolution related with the interrupt rate of the CPU? I actually suspect so, but how? Thanks for all. Best wishes, Xiaochuan