[ns] How to set random starting places within a trace file

[Ken Pounds <hemin99@chinaren.com>]

I want to use same trace file for several traffic generators.
According to NsDoc, to avoid synchronization of the traffic generated, I need
to set random starting. But how to set, there is no explicit example.

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set tfile [new Tracefile]
$tfile filename example-trace

set t1 [new Application/Traffic/Trace]
$t1 attach-tracefile $tfile
set t2 [new Application/Traffic/Trace]
$t2 attach-tracefile $tfile
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Who can tell me how to set random starting.

Thanks