Final answer:
TCP uses window size as the mechanism for flow control, where the receiver dictates the amount of data it can handle before the sender must wait for acknowledgment.
Step-by-step explanation:
The mechanism used by TCP (Transmission Control Protocol) to provide flow control as segments travel from source to destination is the window size. Flow control is a crucial part of TCP's responsibility to ensure that a sender does not overwhelm a receiver with too much data at once. It works as follows: the receiver advertises a window size to the sender, indicating the number of bytes it is willing to accept. The sender can then send only that amount of data before waiting for an acknowledgment and potential update of the window size from the receiver. This process is continuous, with the receiver adjusting the window size based on its current buffer capacity, which provides a dynamic way of managing the flow of data to prevent buffer overflow and ensure reliable data transfer.