Final answer:
TCP performs retransmission when it detects the loss of a segment, which is determined through the acknowledgment mechanism and the retransmission timeout.
Step-by-step explanation:
In TCP, the acknowledgment mechanism plays a crucial role in determining when to perform a retransmission. When a sender transmits data, it expects to receive acknowledgment (ACK) from the receiver. If the sender doesn't receive an ACK within a certain time period, it assumes that the segment was lost in transit and initiates a retransmission. This timeout period is dynamically determined based on the network conditions and past experiences.
The retransmission timeout (RTO) is calculated using various factors, including the round-trip time (RTT) and the deviation of RTT. The RTO is adjusted dynamically to adapt to changes in network conditions. For example, if the sender receives an ACK quickly, indicating a short RTT, it may reduce the RTO. On the other hand, if there is a delay in receiving an ACK, it may increase the RTO to account for potential network congestion or packet loss.
Additionally, TCP uses a selective acknowledgment (SACK) mechanism to identify specific segments that need to be retransmitted, improving efficiency by retransmitting only the necessary data rather than the entire window.
In summary, TCP relies on the acknowledgment mechanism and dynamically adjusted retransmission timeout to detect and respond to lost segments, ensuring reliable data transmission over the network.