Bursty data transfer with Dummynet

[Ahmed Hamza]

On Tue, Nov 12, 2013 at 8:50 PM, Julian Elischer <julian at freebsd.org> wrote:
> On 11/12/13, 6:35 PM, Ahmed Hamza wrote:
>>
>> Hi All,
>>
>> I'm trying to use Dummynet to test the behaviour of my video streaming
>> application in various network conditions. Dummynet was compiled and
>> installed on an Ubuntu 12.04 box with a 2.6 Linux kernel. I'm
>> experiencing a strange behaviour when I reduce the bandwidth of the
>> link/path.
>>
>> For some reason, instead of having a slow download speed. It seems as
>> if the download is happening in bursts! A portion of the data is
>> downloaded at a high speed, then the data transfer stops for a period
>> of time and then resumes again (and so on). Does anyone have an idea
>> what could be the cause? Or is this even an expected behaviour? If so,
>> why?
>
>
> I can't really speak for dummynet on Linux but the granularity of the queues
> is dependent on the timer granularity of the kernel you have and to some
> extent will rely on the correct integration of dummynet into the timer
> facility of the kernel you are running it in. On freeBSD with a 1kHz 'tick'
> I'd' expect to see packets being release from the queue each mSec or so.
> if you are getting second sized chunks then it probably is a bug. Either
> dummynet is not compatible with that kind of kernel, something else has gone
> wrong.  It COULD also be that you are catching the wrong packets.. I've seen
> similar behaviour when I was accidentally queuing all the acks instead of
> all the data going in the other direction, but I presume you have already
> checked to see what you are queuing.
>

Thanks Julian and Matthew for your replies. To clarify my settings,
below are the outputs from 'ipfw show' and 'ipfw pipe show'. I should
also mention that it seems the by default the Ubuntu kernel is
configured for 250Hz, not 1000Hz.

root at vm1:~/# ipfw pipe 1 config bw 500Kbit/s queue 500Kbyte

root at nsl-vm1:~/# ipfw show
00100  202247  105063701 pipe 1 ip from 192.168.56.4 to any
65535 2245577 2648958386 allow ip from any to any

root at nsl-vm1:~/# ipfw pipe show
00001: 500.000 Kbit/s    0 ms  500 KB 1 queues (1 buckets) droptail
    mask: 0x00 0x00000000/0x0000 -> 0x00000000/0x0000
BKT Prot ___Source IP/port____ ____Dest. IP/port____ Tot_pkt/bytes Pkt/Byte Drp
  0 tcp     192.168.56.1/33547    192.168.56.4/80    238083 134515909
0    0 623