[e2e] TCP Performance with Traffic Policing

Detlef Bosau detlef.bosau at web.de
Thu Aug 18 08:57:10 PDT 2011

On 08/17/2011 08:57 PM, Barry Constantine wrote:
> I understand, but the client iperf “window” setting (-w 64KB) is 
> really the send buffer so only 64KB was attempted to be in-flight (not 
> 4MB).
> Are we in sync?

Not quite.

With regards to the BDP, we discussed this matter off list and my fault 
was to mistake the dimension of your BDP. I read 62 MByte, instead of 62 
So, window clamping etc. should be no problem in your case.

However, iperf is an application and thus does neither control the 
sender's socket buffer* nor the amount of data being in flight.

Particularly, from some "two liners" I dropped down myself, I'm 
obviously too stupid to use TCP_NODELAY correctly :-( In other words: I 
cannot turn the Nagle Algorithm on or off respectively.

By turning of the Nagle Algorithm, you _do_ control the amount of data 
being in flight.

O.k., when iperf does a close() after sending/writing its whole buffer, 
you have some control about the amount of data in flight as well.

* I just had a look at the iperf mangpage. It's stated there, -w would 
control the sender socket's buffersize. However, I do not yet know a 
generic, i.e. particularly being portable, way to do so using the socket 
API. So besides an OS specific behaviour, you might encounter quite 
subtle issues by using the one or the other socket option, which will 
not behave exactly the same way on all OSes and OS releases.


Detlef Bosau
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