[ns] Correlating wall-clock time w/ NS time

[Florian-Daniel Otel <otel@ce.chalmers.se>]

Dear all,

We are trying to set up some simulations scenarios where it is
essential that  procedures take place asynchronously in NS
time. For example, when computing routing tables in a link-state
protocol, the receipt of a link state update triggers a re-route
computation. To test the impact of different parameters, it is
esential that this route computation doesn't take place
"automagically" (in 0 NS time) but asynchronously, depending on the
simulation scenario. The method of mapping the wall-clock time to NS
time being done using some ZEN correlation factor. 


So far the only work-around we could find was to profile the relevant
pieces of code, cache the result, and then "post" a re-call in future
e.g.

$ns at [expr $now + $interval] "Do my stuff _again_" 

This will be just a fake re-call that will actually return the result
cached in the previous call path. Of course, there is a pre-defined
overall behaviour when "computation is pending" (e.g. drop all
incoming packets while computing the routing tables). 


Needless to say this is a major PITA and has serious drawbacks
i.e. you can use the same call path only once, code on all affected
call paths must be butchered accordingly, etc. 


I wondwer if anybody succeeded in achieving the same functionality
(maybe i should say "mimicking" it ?)  i.e. succeeded in performing
asynchronous calls in NS time.


Regards,

Florian