Grouping the features needed in a networking stack in a bunch of layered modules is a great idea. Unfortunately, you could place several essential features like error recovery, retransmission, and flow control in several different layers, from the data link layer dealing with individual network segments, to the transport layer dealing with reliable end-to-end transmissions.

Where should we put those modules? As always, the correct answer is it depends, in this particular case, on transmission reliability, latency, and bandwidth cost. You’ll find more details in the Retransmissions and Flow Control part of How Networks Really Work webinar.