zero flow duration?

[Michael Hornung]

I'm trying to look at average bandwidth utilization per host offering 
services on a segment I'm monitoring.  To do so I'm running the following 
on an argus file:

racluster -r file -M norep -w - -- ip |  \
ra -s daddr bytes dur -- 'dst net (blah)'

In many of the records there is a byte count but the flow duration reads 
"0.000000".  Can you explain in what curcumstances a flow has a duration 
of 0?  My guess is that the given flow did not end within the file I'm 
passing through racluster?

-Mike