Re: Iperf results explanation (-t)
On Wed, Aug 20, 2003 at 03:07:49PM +0200, Binken, Rens wrote:
> Hi,
>
> I am using Iperf to test a satellite link (e.g. high latency). I am running
> the test for 10 sec (-t 10) with the -r option to do a reverse test.
> The following are the results.
>
> ------------------------------------------------------------
> Server listening on TCP port 5001
> TCP window size: 128 KByte (WARNING: requested 128 KByte)
> ------------------------------------------------------------
> ------------------------------------------------------------
> Client connecting to SERVER, TCP port 5001
> TCP window size: 128 KByte (WARNING: requested 128 KByte)
> ------------------------------------------------------------
> [ 4] local CLIENT port 1795 connected with SERVER port 5001
> [ ID] Interval Transfer Bandwidth
> [ 4] 0.0-38.5 sec 10.4 MBytes 2.26 Mbits/sec
> [ 4] local CLIENT port 5001 connected with SERVER port 32771
> [ ID] Interval Transfer Bandwidth
> [ 4] 0.0-10.8 sec 1.24 MBytes 962 Kbits/sec
>
> My question is : Why is the interval upstream 38.5 sec (for a 10 sec test)
> and the downstream interval 10.8 sec? Does this mean that for 28.5/0.8 sec
> Iperf is waiting for ACKs?
Yes, that is probably what is happening. I've seen a situation where an
Iperf will never exit because it is waiting for unacknowledged segments. In
that particular case there was a duplex mismatch that was causing the
problems. (I suppose it may have exited eventually, but the tcpdumps I did
showed mostly the same segments being retransmitted again and again...)
> Does this have to do with Iperf waiting for ACKs, if so what is the exact
> function of -t ? D
Well for TCP it means run for approximately 10 seconds -- since TCP is a
reliable protocol you have a little less say in the exact timing since the
kernel is handling the protocol transactions. That said, Iperf could and
probably should implement a timer that goes off at t + .5t or something
which will force the connection to close and exit.
With UDP -t has a more straighforward meaning, Iperf will run for 10 seconds
and exit. (With some skew due to timers and context switches and such.)
Jon
--
Jon Dugan | Senior Network Engineer, NCSA Network Research
jdugan --at-- ncsa.uiuc.edu | 269 CAB, 605 E Springfield, Champaign, IL 61820
217-244-7715 | http://www.ncsa.uiuc.edu/~jdugan/