The TCP window scale option is an option to increase the TCP receive window size above its maximum value of 65,535 bytes. This TCP option, along with several others, is defined in IETF RFC 1323 which deals with Long-Fat Networks, or LFN.
In fact, the throughput of a communication is limited by two windows: the congestion window and the receive window. The first one tries not to exceed the capacity of the network (congestion control) and the second one tries not to exceed the capacity of the receiver to process data (flow control). The receiver may be overwhelmed by data if for example it is very busy (such as a Web server). Each TCP segment contains the current value of the receive window. If for example a sender receives an ack which acknowledges byte 4000 and specifies a receive window of 10000 (bytes), the sender will not send packets after byte 14000, even if the congestion window allows it.
Contents |
The TCP window scale option is needed for efficient transfer of data when the bandwidth-delay product is greater than 64K. For instance, if a T1 transmission line of 1.5Mbits/second was used over a satellite link with a 513 millisecond round trip time (RTT), the bandwidth-delay product is (1500000 * 0.513) = 769,500 bits or 96,188 bytes. Using a maximum buffer size of 64K only allows the buffer to be filled to (65535 / 96188) = 68% of the theoretical maximum speed of 1.5Mbits/second, or 1.02 Mbit/s.
By using the window scale option, files can be transferred at nearly 1.5Mbit/second utilizing nearly all of the available bandwidth.
This option is also useful when sending large files greater than 64KB over slow networks.
By using the window scale option, the receive window size may be increased up to a maximum value of 1,073,725,440 bytes; almost 1 Gibibyte. This is done by specifying a one byte shift count in the header options field. The true receive window size is left shifted by the value in shift count. A maximum value of 14 may be used for the shift count value.
Because many routers and firewalls do not properly implement TCP Window Scaling, it can cause a user's Internet connection to malfunction intermittently for a few minutes, then appear to start working again for no reason. If "diagnose problem" is selected in Vista, an error message will be displayed "cannot communicate with primary DNS server." [1]
There is also an issue if a firewall doesn't support the TCP extensions. [2]
TCP Window Scaling is implemented in Windows since Windows 2000.[3][4] It is enabled by default in Windows Vista / Server 2008 and newer, but can be turned off manually if required.[5]
Linux kernels (from 2.6.8, August 2004) have enabled TCP Window Scaling by default. It chooses the good value of the option by default. The configuration parameters are found in the /proc filesystem, see pseudo-file /proc/sys/net/ipv4/tcp_window_scaling and its companions /proc/sys/net/ipv4/tcp_rmem and /proc/sys/net/ipv4/tcp_wmem (more information: man tcp, section sysctl).
Scaling can be turned off by issuing the command sysctl -w "net.ipv4.tcp_window_scaling=0" as root. To maintain the changes after a restart, include the line "net.ipv4.tcp_window_scaling=0" in /etc/sysctl.conf.
The default setting for Mac OS X is to have window scaling (and other features related to RFC 1323) enabled.
To verify their status, a user can open /Applications/Utilities/Terminal and check the value of the "net.inet.tcp.rfc1323" variable via the sysctl command:
sysctl net.inet.tcp.rfc1323
A value of 1 (output "net.inet.tcp.rfc1323=1") means scaling is enabled, 0 means "disabled". If enabled it can be turned off by issuing the command:
sudo sysctl -w net.inet.tcp.rfc1323=0
This setting is lost across a system restart, to make it permanent it must be written in the /etc/sysctl.conf configuration file, that can be accomplished via the command:
echo 'net.inet.tcp.rfc1323=0' | sudo tee -a /etc/sysctl.conf