[e2e] opening multiple TCP connections getting popular

Mike Fisk mfisk at lanl.gov
Fri Aug 31 14:43:59 PDT 2007

On Fri, 31 Aug 2007, Agarwal, Anil wrote:

> 1. What are the default and maximum receive and transmit window size
> values of Linux 2.6.x, Windows and Vista? If they are less than 64
> kbytes, then enabling window scaling does not accomplish much, except
> for applications that explicitly configure large maximum window sizes
> both at the transmitter and the receiver.

As of 2.6.7 and 2.4.27, Linux uses receive-size auto-tuning of buffer 
sizes based on what we implemented in 2001[1].  The receive window starts 
small (but with a wscale based on the max receive window setting) and is 
essentially kept at twice the current measured delay-bandwidth product, in 
order to not constrain the sender's cwnd growth.

For Linux 2.6.19, the default receive window max is 1M, so the requested 
wscale is 5. The inital window advertisement is 64620.  If you change the 
max to 2^27, then the initial advertisement is a wscale of 12, but the 
initial window advertisement is still 64240.

Route caching of previous connecton windows can limit the inital receive 
window to smaller values.

[1] "Dynamic Right-Sizing in TCP", M. Fisk, W. Feng, Proceedings of LACSI 
Symposium 2001, October 2001. 

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