freebsd stable - Jan 2007 - Dummynet and simulating random delay

I have a project that requires me to simulate a link with varying but
well defined delay.  The link is guarenteed to deliver packets in
order, so I wish to maintain that behavior with Dummynet.

My first thought was to create three or four different queues with
different delays and use the probability rule to dump them into the
queue, but that gets packets out of order and doesn't work.  

My next thought is to write a script that reconfigures the pipe
randomly every few hundred milliseconds or so during the test, but I'm
not sure what that means for packets that are already in it.  Does it
mean bursts of data when the delay is turned down (which would actually
be realistic in this scenario), or is there a danger of out of order
packets?  Are there any dummynet experts out there that can tell me
exactly how it will behave when delays are changed while there are
still packets in the buckets?  Thanks.

On Tue, 2007-Jan-23 14:22:54 -0500, Andresen, Jason R.
wrote:
>I have a project that requires me to simulate a link with varying but
>well defined delay.  The link is guarenteed to deliver packets in
>order, so I wish to maintain that behavior with Dummynet.
I don't think dummynet can do this in its current form.  Based on
a quick look at the source, the packet delivery time is set to the
current time plus the current delay when the packet arrives (see
sys/netinet/ip_dummynet.c:move_pkt().  This means that changing
the delay will not affect existing queued packets.

Changing dummynet to allow variable delays whilst maintaining packet
order is not immediately trivial.  The only option I can see would be
to change the dummynet curr_time (net.inet.ip.dummynet.curr_time)
to increment at variable rate instead of one count per tick.  You
would probably need to do some fiddling to provide anything better
than very coarse delay variation.

-- 
Peter Jeremy
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