Which Version of TCP are you using?

So often in Wireshark videos and classes we spend a lot of time on TCP behavior. But what I am about to discuss is hardly ever brought up.

The answer to version is simple right? Version 4.

OK, kind of right, but overlooking too much. The truth is that there are multiple “versions” of TCP, and TCP does not have a version number in its header:

OK, so what are these different – let’s call them implementations – of TCP?

In a simple answer, the different “versions” refer to different congestion control algorithms! Here they are:

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