[gentoo-user] Low TCP RWIN value

[gentoo-user] Low TCP RWIN value

[Mick]

Hi All,

Reading another posters comments (you learn all sort of interesing stuff
here ;-) I checked my internet connection using the SpeedGuide.net TCP/IP
Analyzer and discovered that my TCP Receive Window is somewhat smaller than
anticipated, or than what a M$Windoze box which is running on the same LAN
(using the same router) shows:

Windoze box:
Default TCP Receive Window (RWIN) = 65535

Gentoo box:
Default TCP Receive Window (RWIN) = 5840

The whole section in the TCP/IP Analyser shows:
=============================================
Default TCP Receive Window (RWIN) = 5840 
RWIN Scaling (RFC1323) = 2 bits (scale factor of 4)
Unscaled TCP Receive Window = 1460 

For optimum performance, consider changing RWIN to a multiple of MSS.
Other values for RWIN that might work well with your current MTU/MSS: 
510784 (MSS x 46 * scale factor of 8)
255392 (MSS x 46 * scale factor of 4)
127696 (MSS x 46 * scale factor of 2)
 63848 (MSS x 46)
=============================================

My MSS=1388.

I have set my router to use an MTU=1428 (following recommendations of my
ISP: http://support.metronet.co.uk/adsl/tips/optimising.html )

Is there anything I need to do with respect to setting a larger RWIN?  I
don't remember ever tweaking my Gentoo TCP/IP settings.  If it is advisable
to increase the RWIN buffer, how do I do this in Gentoo?
-- 
Regards,
Mick

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Re: [gentoo-user] Low TCP RWIN value

[Daniel Drake]

Mick wrote:
> Reading another posters comments (you learn all sort of interesing stuff
> here ;-) I checked my internet connection using the SpeedGuide.net TCP/IP
> Analyzer and discovered that my TCP Receive Window is somewhat smaller than
> anticipated, or than what a M$Windoze box which is running on the same LAN
> (using the same router) shows:
> 
> Windoze box:
> Default TCP Receive Window (RWIN) = 65535
> 
> Gentoo box:
> Default TCP Receive Window (RWIN) = 5840

On Linux, RWIN starts low for every connection and then rises very 
rapidly as subsequent data transfers happen.

You have an element of control over this in /proc/sys/net, but if I were 
you I'd just stick with the defaults and be happy with the knowledge 
that Linux's implementation of windowing is more dynamic, and the 
initial low value is nothing to go by :)

Daniel

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Re: [gentoo-user] Low TCP RWIN value

[Ow Mun Heng]

On Fri, 2006-02-10 at 18:23 +0000, Daniel Drake wrote:
> that Linux's implementation of windowing is more dynamic, and the 
> initial low value is nothing to go by :)

I presume that you're referring to the TCP Receive window. I do believe
the docs which I've read is outdated (circa 2003 - ADSL Bandwidth
Management) which states that Linux TCP window isn't implemented.

$cat /proc/sys/net/ipv4/tcp_adv_win_scale 
2

How can I see the Rwin size for each connection? or track it for that
matter? 


-- 
Ow Mun Heng
Gentoo/Linux on DELL D600 1.4Ghz 1.5GB RAM
98% Microsoft(tm) Free!! 
Neuromancer 11:55:31 up 1 day, 11 min, 6 users, load average: 0.37,
1.06, 1.19 


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