Network testing and latency

[Paul Lussier]

Dave Johnson <dave-gnhlug at davej.org> writes:

> Note that as mentioned before, limiting bandwidth and introducing
> latency and/or jitter are different things.
>
> If you want to simulate a bandwidth limited link you need to both
> limit bandwidth and queue packets.  If you simply drop and don't queue
> then there is no possibility of latency.
>
> Latency and jitter are side effects due to queuing prior to the
> bandwidth limited hop.  Protocols such as TCP are designed to avoid
> introducing latency when a slow link is in the path.
>
> Anyway, onto the implementation.
>
> Below script limits bandwidth in both directions when forwarding
> through two interfaces.  Note you'll need to setup the appropriate
> interfaces and routes.  Each side has it's own bandwidth and queue
> with a max size in bytes.  This is equivilant to a full-duplex T1
> pipe using a linux box and 2 ethernet interfaces representing the two
> endpoints of the T1.

Dave, does this add jitter as well?  I assume that since you're
queueing, it does to some extent.  Would it also be wise to inject
traffic on the simulated T1 connection by having other hosts
communicating as well?  For example I could have 1 linux system in
between 2 switches, on which were several other systems all
communicating with each other doing things like generating web
requests, copying large files, etc.

It seems this approach would add to the randomness of the connection
to some extent, and increase both the latency and jitter experienced
by the 2 systems we're really trying to test.

Thanks again!


-- 

Seeya,
Paul