[ns] round trip delay

[Huike LI <huike@ee.mu.oz.au>]

I tried to get the rtt_ or srtt_ of a tcp agent, but it always gives me 1s
no matter I set the link delay as 15ms 25ms etc. Then I noticed it may
because of the timer used in ns, that is its resolution. Then I set the
link delay as 15s then got the correct result rtt_=30s.

Then how can I get the accurate round trip delay?(the easiest way, I won't
set a ping to measure it)

Thanks,
Victor