Re: TCP/IP Window Size Problem/Query

Re: TCP/IP Window Size Problem/Query

[Shmulik Hen]

On Monday 11 August 2003 07:22 pm, Hare, Matthew wrote:
> I have two systems running SuSE Linux. System 1 is running SuSE 7.1
> with the 2.2.18 kernel. System 2 is running SuSE 7.3 with the
> 2.4.17 kernel. Both systems are PC based (Pentium III, 512MB RAM).
>
>
> When I open a TCP connection from System 1 I get a reported window
> size (I use Ethereal to check window sizes) of 32k. When I do the
> same with System 2 I get a reported window size of 5840bytes and
> the window size increases as the rate of data being sent via TCP
> increases. This is causing me problems, I need a fixed window size
> of 32k on System 2 so that it matches System 1.
>

We've encountered similar behavior on a server running Red Hat 9 
starting with the ~5k window size and publishing it is willing to 
increase it (forgot the exact name of the flags), problem is the 
clients are all w2k systems (for using Chariot as the performance 
analyzer), and they don't seem to be able to handle that kind of 
window size management, so we're always stuck with the original ~5k 
window size and the performance sucks. When using w2k on the same 
server, everything starts with 64k window, so the performance is 
great.

Isn't there a way to ask the TCP/IP stack not to begin with small 
window size and then try to increase, but rather start with the 
largest windows size and then decrease ?
Using jumbo frames and TSO doesn't seem to help a lot.

-- 
| Shmulik Hen   Advanced Network Services  |
| Israel Design Center, Jerusalem          |
| LAN Access Division, Platform Networking |
| Intel Communications Group, Intel corp.  |

TCP/IP Window Size Problem/Query

[Hare, Matthew]

Forgive this mass mailing out but I am getting desperate for help. My
problem is with reported TCP window sizes.

I have two systems running SuSE Linux. System 1 is running SuSE 7.1 with the
2.2.18 kernel. System 2 is running SuSE 7.3 with the 2.4.17 kernel. Both
systems are PC based (Pentium III, 512MB RAM).

When I open a TCP connection from System 1 I get a reported window size (I
use Ethereal to check window sizes) of 32k. When I do the same with System 2
I get a reported window size of 5840bytes and the window size increases as
the rate of data being sent via TCP increases. This is causing me problems,
I need a fixed window size of 32k on System 2 so that it matches System 1.

I have done a bit of testing to try and work out where this window size is
set. I am by no means a Linux guru so I was working with little knowledge at
the start. The application we use to open the TCP connections is our own
code (C++) so I have hacked that and used some setsockopt() calls to set
SO_SNDBUF. I tried setting this to 32k but this appears to have no effect on
my reported window sizes.

Does anyone know if setsockopt() can be used to change the size of the
window I am seeing? Is SO_SNDBUF the same as the TCP window size that
Ethereal is reporting?

Is there a way to fix the TCP window size in the 2.4.17 kernel? I am
familiar with recompiling the kernel but I've not found any kernel options
that seem to do what I want. Can anyone help me or at least point me in the
right direction?

Thanks for your time.

Mat Hare
SAINTT Developer
AMS UK






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