TCP employs four main congestion control methods to dynamically adjust data transmission rates based on network conditions, preventing congestion and improving bandwidth utilization. Here’s a detailed explanation of each method:
Slow Start
- Working Principle: Slow start is a sender-based congestion control mechanism. Its core idea is to gradually increase the amount of data sent through a 试探性 approach. The sender maintains a congestion window (cwnd), which starts with a relatively small initial value. For each ACK (acknowledgment) packet received, the size of cwnd increases according to specific rules.
- Purpose: To avoid sending excessive data at the beginning, which could cause network congestion. During the slow start phase, the size of cwnd grows exponentially.
Congestion Avoidance
- Working Principle: When cwnd grows to the slow start threshold (ssthresh), TCP transitions from the slow start phase to the congestion avoidance phase. In this phase, the growth rate of cwnd slows down; typically, cwnd increases by 1 for each round-trip time (RTT).
- Purpose: To prevent cwnd from growing too quickly and causing network congestion. During the congestion avoidance phase, the size of cwnd grows linearly.
Fast Retransmit
- Working Principle: When the sender receives three consecutive duplicate ACK packets, it indicates that a segment has been lost. At this point, the sender will immediately retransmit the lost segment without waiting for the retransmission timer to expire.
- Purpose: To reduce data transmission delays caused by timeouts. The fast retransmit algorithm enables the sender to detect segment loss early and perform retransmission promptly.
Fast Recovery
- Working Principle: After the sender executes fast retransmit, it halves the slow start threshold and sets the congestion window to the value of the slow start threshold plus the size of three segments. Then, the sender starts executing the congestion avoidance algorithm instead of re-entering the slow start phase.
- Purpose: To avoid the congestion window from restarting growth from 1 due to timeouts, thereby improving the efficiency of network transmission.
These four congestion control methods allow TCP to dynamically adjust the sending rate based on the real-time network status, ensuring efficient and stable data transmission. If you have any further questions or need more details, feel free to ask.
