How to Calculate TCP throughput for long distance WAN links

How to Calculate TCP throughput for long distance WAN links.

Formula to Calculate TCP throughput

 

TCP-Window-Size-in-bits / Latency-in-seconds = Bits-per-second-throughput

 

So lets work through a simple example. I have a 1Gig Ethernet link from Chicago to New York with a round trip latency of 30 milliseconds. If I try to transfer a large file from a server in Chicago to a server in New York using FTP, what is the best throughput I can expect?

 

 

First lets convert the TCP window size from bytes to bits.  In this case we are using the standard 64KB TCP window size of a Windows machine.

 

64KB = 65536 Bytes.   65536 * 8 = 524288 bits

 

Next, lets take the TCP window in bits and divide it by the round trip latency of our link in seconds.  So if our latency is 30 milliseconds we will use 0.030 in our calculation.

 

524288 bits / 0.030 seconds = 17476266 bits per second throughput = 17.4 Mbps maximum possible throughput

Formula to calculate the optimal TCP window size:

 

Bandwidth-in-bits-per-second * Round-trip-latency-in-seconds = TCP window size in bits / 8 = TCP window size in bytes

 

So in our example of a 1GE link between Chicago and New York with 30 milliseconds round trip latency we would work the numbers like this…

 

1,000,000,000 bps * 0.030 seconds = 30,000,000 bits / 8 = 3,750,000 Bytes

 

Therefore if we configured our servers for a 3750KB TCP Window size our FTP connection would be able to fill the pipe and achieve 1Gbps throughput.

 

 

Reduce latency?  How is that possible?  Unless you can figure out how to overcome the speed of light there is nothing you can do to reduce the real latency between sites.  One option is, again, placing a WAN accelerator at each end that locally acknowledges the TCP segments to the local server, thereby fooling the servers into seeing very low LAN like latency for the TCP data transfers.  Because the local server is seeing very fast local acknowledgments, rather than waiting for the far end server to acknowledge, is the very reason why we do not need to adjust the TCP window size on the servers.

In this example the perfect WAN accelerator would be the Cisco 7371 WAAS Appliance, as it is rated for 1GE of optimized throughput.

WAAS stands for:  Wide Area Application Services

Formula to calculate Maximum Latency for a desired throughput

You might want to achieve 10 Gbps FTP throughput between two servers using standard 64KB TCP window sizes.  What is the maximum latency you can have between these two servers to achieve 10 Gbps?

TCP-window-size-bits / Desired-throughput-in-bits-per-second = Maximum RTT Latency

524288 bits / 10,000,000,000 bits per second = 52.4 microseconds

 

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