Re: Iperf max b/w

If we use regular MTU 1500B on GbE and running TCP with timestamp
option (Linux default?), the maximum TCP throughput will be 941
Mbps. It's the line rate of 1 Gbps.

I checked iperf source code and found that the number iperf gives us
is a rate at application layer, i.e. hidden overhead excluded from the
number will be TCP header and options, IP header and options, and
ethernet gap, preamble, header, and trailer. In my case, these are 32
(TCP), 20 (IP) and 38 (ethernet) bytes long.

With the same calculation, UDP will not go over 957 Mbps with MTU
1500B on GbE. If you got those numbers, I think your equipment can be
considered well tuned under that condition.


>> From: Cameron Harr <charr --at-->
>> To: iperf-users --at--
>> Cc: 
>> Subject: Iperf max b/w
>> Date: Fri, 10 Oct 2003 08:17:34 -0600
>> Message-ID: <3F86BF7E.6000501 --at-->
> Hello,
> I seem to have reached some type of maximum bandwidth with iperf 1.7.0. 
> At the default TCP window size, I would generally get a b/w in the 
> 920's. As I stepped up the window size, I started getting a bit higher 
> readings until everything seemed to max out at 941 Mb/s. Now, with a 
> window of 256K and larger, I get 941 Mb/s every time -- almost like it 
> is getting cut off at that point. I guess it could also be the switch 
> (Foundry FI1500), but does anyone know of a possible limit here to 
> iperf? Thanks,
> -Cameron

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