Re: transmitting packets

[Rick Jones <rick.jones2 () hp com>]

Jan Allman wrote:
> Rick, 

> I'm tring to see how much data I can send over a point to point
> Ethernet link using UDP (i.e. as quickly as possible). I had a repeating
> loop of "sendto" calls. I was using Ethereal to report statistics on the
> throughput I was achieving.
>
> Repeating your test on my machines (between one dual 2.8GHz machine
> and one 3GHz machine)...
>
>   netperf -t UDP_STREAM -H 192.168.0.6 -c -C -- -m 1472
> UDP UNIDIRECTIONAL SEND TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 
> 192.168.0.6 (192.168.0.6) port 0 AF_INET
> Socket Message Elapsed Messages CPU Service
> Size Size Time Okay Errors Throughput Util Demand
> bytes bytes secs # # 10^6bits/sec % SS us/KB
>
> 65535 1472 10.00 811659 0 956.0 15.61 5.410
> 65535 10.00 802851 946.6 49.18 4.201
>
> Is this telling me I am able to get 950Mbps of throughput? (I've
> never used netperf before)

Yes - it is saying that the netperf side (the send_udp_stream() routine) 
believed it was sending at a rate of 956 Mbps, and that the netserver 
size (the recv_udp_stream() routine) a subset of that data at a rate of 
946.6 Mbit/s.  Of the 811659 1472 byte sends netperf made, 802851 of 
them were received by netserver - the rest were lost somewhere between 
the two.

You can use the -f option to change the output units - k, m and g give 
power of ten bits per second; K, M and G give power of two bytes per second.

> When I use Ethereal it calculated an "Avg.
> MBit/sec" of 177.592 (linux kernel 2.4.24, libpcap 0.8.3, ethereal
> 0.10.7). Do you know why this is so much less than the netperf
> reports? Is Ethereal reporting a low rate because it is dropping data
> due to having to maintain its GUI? Is Ethereal displaying a low
> throughput rate due to libpcap limitations for 1472 bytes sized
> frames?

Well, you could check if it was ethereal's GUI by using tcpdump - I'd 
suggest a tcpdump -w <foo>  that you then post-process with ethereal. 
If it then reports closer to what netperf reported, you can ass-u-me it 
was the GUI overhead.  If it still reports what ethereal was reporting, 
you can ass-u-me it related to the libpcap and below not keeping-up.

rick jones
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