TCP Throughput Calculations

On a Windows machine, the standard TCP Window Size is 64KB. 64KB converted to bits = 524288

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

Calculate TCP throughput:

Formula:
Window Size / RTT = Throughput

Windows Size in bps (bits per second)
RTT in seconds
Throughput in bps (bits per second)

Lets say you have a 1Gig link between Miami and NY and your latency is 33 milliseconds.

524288 bits / 0.033 = 15,887,515 = 15.8 Mbps MAX Possible Throughput

What this is saying is that if you have two servers, one in Miami and one in NY, and you start a file transfer between the two of them, the most throughput you will get is 15.8Mbps. It doesn’t matter that you have a 1Gig link.

Calculate the Optimal TCP Window Size:

Formula:
Bandwidth * RTT = Windows Size /8 = Window Size

Bandwidth in bps (bits per second)
RTT in seconds
Windows Size in bps (bits per second)
Windows Size in bytes

Now let’s say you want to use the full 1Gig link to do the transfer. What TCP Windows Size will you need?

1,000,000,000 * 0.033 = 30,303,030,303 / 8 = 3,787,878,787 = 3787KB

The TCP Windows Size will have to be set to 3787KB on both servers.

Calculate Maximum Latency for a Desired Throughput:

Formula:
Windows Size / Desired Throughput = MAX RTT

Windows Size in bps (bits per second)
Desired Throughput in bps (bits per second)
RTT Max in seconds

What is the maximum amount of latency to achieve 1Gig data transfer speeds?

524288 / 1,000,000,000 = 0.000524288 seconds = 0.5242 ms

So the maximum amount latency on a 1 Gig pipe is 0.5242 ms.

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