TCP Congestion Control Explained | Algorithm | MSS | Data in Flight | Phases | Problem

TCP congestion control is a mechanism designed to prevent network congestion and ensure fair sharing of network resources among multiple TCP connections. It dynamically adjusts the sending rate of TCP traffic based on the perceived network conditions.

Here are the key components and techniques used in TCP congestion control:

Slow Start: When a TCP connection is established or after a period of inactivity, the sender starts in a "slow start" phase. Initially, it sends a small number of segments and gradually increases the sending rate exponentially until it reaches a threshold known as the congestion window (cwnd).

Congestion Avoidance: Once the congestion window is reached, TCP switches to a "congestion avoidance" phase. In this phase, the sender increases the sending rate more conservatively by linearly increasing the congestion window instead of exponentially.

Congestion Detection: TCP monitors the network for signs of congestion. It detects congestion by observing packet loss, increased round-trip time (RTT), or receiving explicit congestion signals from network devices.

Fast Retransmit and Fast Recovery: If TCP detects packet loss, it assumes that it is due to congestion. Instead of waiting for a timeout, TCP performs a fast retransmit by resending the missing segment based on duplicate ACKs (acknowledgments). This helps recover from the loss more quickly and resumes the transmission without resetting the congestion window to slow start.

Congestion Window Reduction: Upon detecting congestion, TCP reduces its congestion window size. This reduces the sending rate to alleviate congestion in the network. TCP typically cuts the congestion window in half, which is known as "multiplicative decrease."

Additive Increase: After congestion is resolved, TCP gradually increases the congestion window size again. It adds a small fixed value to the congestion window for every successful acknowledgment received, resulting in a slow but steady increase in the sending rate.

Explicit Congestion Notification (ECN): TCP can also use ECN to receive explicit congestion signals from network devices. The network devices mark packets with an ECN codepoint to indicate congestion. TCP responds to these marks by reducing the sending rate.

By using these congestion control mechanisms, TCP can dynamically adapt to network conditions, prevent congestion collapse, and ensure fair utilization of network resources. It allows TCP connections to coexist and share the available bandwidth while minimizing the impact of congestion on overall network performance.