The following is a summary of techniques to maximize TCP WAN throughput.

TCP uses what is called the “congestion window”, or CWND, to determine how many packets can be sent at one time. The larger the congestion window size, the higher the throughput. The TCP “slow start” and “congestion avoidance” algorithms determine the size of the congestion window. The maximum congestion window is related to the amount of buffer space that the kernel allocates for each socket. For each socket, there is a default value for the buffer size, which can be changed by the program using a system library call just before opening the socket. There is also a kernel enforced maximum buffer size. The buffer size can be adjusted for both the send and receive ends of the socket.

To get maximal throughput it is critical to use optimal TCP send and receive socket buffer sizes for the link you are using. If the buffers are too small, the TCP congestion window will never fully open up. If the receiver buffers are too large, TCP flow control breaks and the sender can overrun the receiver, which will cause the TCP window to shut down. This is likely to happen if the sending host is faster than the receiving host. Overly large windows on the sending side is not a big problem as long as you have excess memory.

The optimal buffer size is twice the bandwidth*delay product of the link:

buffer size = 2 * bandwidth * delay

The ping program can be used to get the delay, and tools such as pathrate to get the end-to-end capacity (the bandwidth of the slowest hop in your path). Since ping gives the round trip time (RTT), this formula can be used instead of the previous one:

buffer size = bandwidth * RTT.

For example, if your ping time is 50 ms, and the end-to-end network consists of all 100 BT Ethernet and OC3 (155 Mbps), the TCP buffers should be .05 sec * (100 Mbits / 8 bits) = 625 KBytes. (When in doubt, 10 MB/s is a good first approximation for network bandwidth on ESnet/vBNS/Abilene-like networks).

There are 2 TCP settings you need to know about. The default TCP send and receive buffer size, and the maximum TCP send and receive buffer size. Note that most of today UNIX OS's by default have a maximum TCP buffer size of only 256 KB! For instructions on how to increase the maximum TCP buffer, see the OS specific instructions for setting system defaults. You won't want to increase your default buffer size greater than 128 KB, because this may adversely affect LAN performance. Therefore you need to use the UNIX setsockopt call in your sender and receiver to set the optimal buffer size for the link you are using.