What are the retransmission rules for TCP?

TCP (Transmission Control Protocol) is a connection-oriented protocol that provides reliable, ordered, and error-checked data stream services during data transmission. The retransmission rules of TCP are one of the key mechanisms for ensuring reliable data transmission. The following are the basic rules and mechanisms for TCP retransmission:

1. Timeout-based Retransmission:

   • When TCP sends a data packet, it initiates a timer (referred to as the retransmission timer) to await an acknowledgment (ACK) from the receiver confirming successful reception. If no ACK is received within the timer's duration, TCP assumes the packet was lost or corrupted and automatically retransmits it.
   • For instance, if a TCP connection has a retransmission timeout (RTO) configured to 200 milliseconds, and no ACK is received within that period after sending a data packet, TCP will retransmit it.

2. Fast Retransmission:

   • Fast retransmission is initiated by receiving duplicate acknowledgments (duplicate ACKs). If the sender receives three consecutive duplicate ACKs for the same packet, it immediately retransmits that packet without waiting for the retransmission timer to expire.
   • For example, if the sender receives three consecutive ACKs for packet 100 but expects an ACK for packet 101, this indicates that packet 101 may have been lost or corrupted, and TCP will immediately retransmit packet 101.

3. Selective Retransmission:

   • In this mechanism, if the receiver only misses certain packets, it does not reset the receive window but instead requests retransmission of only the lost packets. This is implemented by sending Selective Acknowledgment (SACK), enabling the sender to retransmit only the packets that were lost.
   • For instance, if the receiver receives packets 102 and 104 but misses packet 103, it can send SACK to specify the received packets and request retransmission of the lost packet 103.

These retransmission rules and mechanisms allow TCP to maintain reliable data transmission across various network conditions. By adapting to different network delays and packet loss scenarios, TCP efficiently manages the data stream to ensure accurate delivery of information to the destination.