What is the purpose of transmission control protocol?

Answer 1

TCP (or similar protocols) take care of several generic tasks. For a start, they split a stream of data (e.g., a file) into small pieces, to make them manageable. The pieces are sent individually (in the case of TCP, the IP protocol is used for this), and numbered. The other end can verify that each of the pieces arrived, and request for missing pieces to be resent.

TCP also has the role of controlling network flow. That is, if the communications quality is too low, the speed is reduced; otherwise it may be increased.

TCP also takes care of starting and ending communications session.

Yet another role of TCP is to differentiate different communications between the same pair of computers. This is done via port numbers.