Re: UDP Connection tuning questions regarding the -b and -l options
Jim Mozley wrote:
> Jim Mozley wrote:
> > So I need to script a wrapper to set the correct -b option given a -l
> > value so I allow for the overhead, assuming I want a constant wire rate.
>
> From a previous post
> http://archive.ncsa.uiuc.edu/lists/iperf-users/jan03/msg00023.html I see
> that "There is no way to really before hand compute the amount of
> headers a piece of information will get covered with."
>
> So setting -b for different lengths of buffer ( -l option ) is a trial
> and error process and not something that can be calculated :-(. This
> would explain why my calculations and observable results are different.
>
> I'm guessing different operating systems and network card combinations
> for the Iperf testers will behave differently in this regard.
Actually you are quoting my post ever so slightly out of context. It is
true that there is no _programatical_ way to before hand compute the
amount of headers, i.e. Iperf will never be able to do it. However a
user (or more likely network admin) can "easily" compute this
information before hand. The caveat being if you are sending data over the
Internet in general then you will not be able to calculate it. If you know
exactly the path that every packet will take and what those network types
are then you can calculate the header size. For instance if I have 2 GigE
(or 10 or 100 Mb/s ethernet link) cards connected over a crossover cable
then I know that the headers (and footers) are ethernet, IP, UDP. Even if
I threw in a ethernet switch I would still get the same headers. However
if I connected two machines directly through a ATM link I would get a
different set of headers. Since Iperf can not detect these link types or
any along a multi-hop path it can not provide the functionality you are
looking for. You should be able to provide it yourself given you know your
setup.
Kevin