RE: Dummynet and simulating random delay

From: Luigi Rizzo [mailto:rizzo@xxxxxxxx]

On Wed, Jan 24, 2007 at 06:10:21PM +1100, Peter Jeremy wrote:
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

actually dummynet never does reordering within a single pipe, even
if you change the delay on the fly.

But this said, you should explain "varying but well defined delay",
because if you use TCP or similar as the source, then you
have no control on when the userland write->tcp transmission delay
anyways so the concept is a bit vague and probably not a meaningful
experiment. And even in any common network (from switched
ethernet to wireless to dsl...) you have some variance on the delay,
ranging from a fraction of a millisecond to much larger values,
due to queueing and/or protocol issues (e.g. MAC channel allocation)
and/or switch/router/operating system issues.

I'm trying to simulate a satellite link that has a normal delay of 1
second, but every 20-30 seconds or so the delay shoots up to 3.5
seconds for about 4 seconds and then settles back down to 1 second.
From what you said, I'm thinking that just twiddling the pipe on the
fly will probably work.
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