Friday, August 6, 2010

TCP Window scaling may cause transfer problems

I noticed this problem first in Italy with an Alice Gate 2 Plus Wi-Fi modem and now I see a similar behaviour in Sweden with a NETGEAR Wireless Cable Voice Gateway modem used by ComHem. This made me think it was a good idea to rewrite in English my original post which was in Italian.

The symptoms (Alice Gate 2 Plus Wi-Fi):

- In GNU Linux this modem/router only works in modem modality, that is, by manually creating a pppoe connection with pppoeconf/pon/poff. If used as a router (automatic connection) web pages are only loaded partially or not loaded at all. Skype works in any case.
- In Windows 7, the router modality only works in combination with MS Internet Explorer (other browsers, such as Firefox and Chrome, show the same problem as in Linux)

The symptoms (NETGEAR Wireless Cable Voice Gateway):

- In GNU Linux the NETGEAR gateway works fine as long as browsing is concerned. When using SSH and many services using SSH, such as SVN, rsync, etc, sometimes the connection stalls and eventually times out
- I do not have a Windows computer in Sweden and could not check if the problem affects Windows as well, but, given the problems in saw in Italy, I would guess it does at least starting with Vista and newer versions.

The problem:

It seems that in both cases the routers do not handle properly the TCP option "window scaling" also called "receive window auto-tuning". This option is implemented in Linux and in Windows starting with Vista, and that is why the problem never occurred with Window XP or older.

The solution:

A solution is to disable window scaling in the TCP options, that is
- in GNU Linux open a terminal and run:
sudo sysctl net.ipv4.tcp_window_scaling=0
To make the option permanent, you have to add the line net.ipv4.tcp_window_scaling=0 in the /etc/sysctl.conf file.
- in Windows, start cmd as administrator and run:
netsh int tcp set global autotuninglevel=disabled

This may make the TCP protocol less efficient, so do this only if you encounter the problems described above.

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