Symptoms
The WANScaler UI is accessed with an Internet browser application. An issue was recently found in Microsoft Windows Vista, which might have intermittent problems connecting to the WANScaler UI.
When the issue occurs, Internet browsers in Windows Vista either timeout or report the UI Web page cannot not be displayed.
You may need to restart the WANScaler to restore the UI access. This issue does not depend on the type of Internet browser being used; it only occurs with Windows Vista. No such issues occur in Windows XP or earlier Windows versions.
Cause
Microsoft has added a new networking feature in Windows Vista called Receive Window Auto-Tuning, which enhances TCP in Windows Vista to receive data more efficiently than Windows XP. However, this feature confuses the WANScaler TCP buffer because WANScaler uses similar features in the WAN side for communications between WANScaler peers.
Receive Window Auto-Tuning is enabled by default in Windows Vista, but not in Windows XP or earlier Windows versions.
Workaround
Use Windows XP to access the WANScaler UI.
Resolution
If you must use Windows Vista, disable Receive Window Auto-Tuning before accessing the WANScaler UI.
The following commands can disable, enable, or show the Receive Window Auto-Tuning setting in Windows Vista. No restart in Windows Vista is required for the commands to take effect.
To disable Receive Window Auto-Tuning, issue the following command:
netsh interface tcp set global autotunninglevel=disabled
To enable Receive Window Auto-Tuning, issue the following command:
netsh interface tcp set global autotunninglevel=normal
To show the TCP enhancement setting, issue the following command:
netsh interface tcp show global
More Information
Receive Window Auto-Tuning is a TCP enhancement feature in Vista based on window scaling, which is described in RFC1323. This feature increases the TCP receive window, which traditionally has a maximum size of 64 kilobytes (KB). By increasing the TCP receive window with a multiplier factor in window scaling, the TCP sender can send more than 64 KB of data in one chunk before waiting for an acknowledgement from the receiver. This feature has a trade-off in the reliability of TCP flow control.
Higher transfer rates can threaten TCP reliability because the probability for the TCP sequence number to be wrapped around and re-used increases. With the increasing use of Gigabit Ethernet in LANs, the chance is even greater than before. Gigabit Ethernet has about 17 seconds of TCP wraparound time, which is too small for a larger receive window as explained in RFC1323. If the sequence number is accidentally duplicated due to transmission surging and latency, the TCP connection may lock up.
TCP window scaling does not happen if only one side of the TCP peers can perform window scaling. The WANScaler is capable of window scaling, as is Windows Vista. In fact, the WANScaler will respond to window scaling in all of its Ethernet interfaces if window scaling is received. When Windows Vista accesses the WANScaler Web UI, it negotiates a higher receive window size during the TCP 3-way handshake process. This is achieved by sending out a TCP-SYN packet with window scaling = 2 in the TCP header, telling its TCP peer that it can increase its advertised receive window size. If the TCP peer responds with window scaling in the TCP header, Windows Vista assume it can run Receive Window Auto-Tuning.
The screen shots below show the TCP 3-way handshake process between Windows Vista and the WANScaler during the Web UI session establishment.
In the following screen shot, Windows Vista sends a TCP-SYN packet to the WANScaler with window scaling=2. Windows Vista increases its TCP receiver buffer to 32768 (multiplied by 4) from the advertised window size of 8192 after the WANScaler responds with the window scaling option:
In the following screen shot, the WANScaler responds with a TCP SYN-ACK packet to Windows Vista with window scaling=7. This packet tells Windows Vista that the WANScaler can receive up to 8 megabytes (MB) of data in one chunk (128 x 64 KB):
In conclusion, Windows Vista systems typically send much more data into the LAN than Windows XP systems, which can result in network traffic spiking. This is not necessarily a problem in itself, but it does increase the importance of LAN traffic Quality of Service (QoS) and TCP stack tuning. As several devices usually share LANs, traffic spiking may make the LAN temporarily congested, which may increase the chance of retransmissions or timeouts in WANScaler UI sessions.
