A retransmission timer is a timeout mechanism that resends data when an expected acknowledgment does not arrive in time. It is common in network protocols that must cope with packet loss, delay, or reordering, especially over unreliable transports such as UDP.
In cyber security, retransmission logic matters because it is part of the protocol state machine, not just a convenience feature. If the timer fires at the wrong moment, or if the code resumes from stale state, an attacker can sometimes trigger crashes, leaks, or desynchronization bugs. Secure implementations must track which bytes were already sent, which messages were acknowledged, and whether a resend should restart from the beginning or continue from a saved offset. Defenders look for abnormal retransmit loops, handshake failures, and memory-safety issues in timeout paths, since these code paths are often less tested than the main data flow.