What is TCP congestion avoidance?

When congestion causes routers to drop packets, missing ACKs signal it; TCP reacts by reducing the amount of data sent before an ACK, then gradually increasing again - using algorithms like slow start, congestion avoidance, fast retransmit, and fast recovery.

* Slow start grows the window exponentially, congestion avoidance grows it linearly, and a loss halves it. *

TCP Congestion Avoidance:

When network congestion occurs, routers may discard packets. TCP detects this and reduces its transmission rate.

Congestion Detection:

• Missing acknowledgments indicate possible congestion
• Sender doesn't receive expected ACKs

Congestion Response:

1. Sender reduces the number of bytes sent before requiring an acknowledgment
2. Gradually increases rate as ACKs are received
3. This prevents overwhelming congested network segments

Congestion Control Algorithms:

• Slow Start: Begin with small window, double each RTT
• Congestion Avoidance: Increase window linearly after threshold
• Fast Retransmit: Retransmit after 3 duplicate ACKs
• Fast Recovery: Don't reset to slow start after fast retransmit

Key insight: TCP automatically adjusts its sending rate based on network conditions, balancing throughput and congestion.

Go deeper:

• TCP Congestion Control — animates slow start, AIMD, and how the window backs off on loss.
• Computer Networks: A Systems Approach §6.3 — TCP Congestion Control — the textbook walkthrough of slow start, congestion avoidance, and fast recovery.
• TCP congestion control — Wikipedia overview of the algorithms (Tahoe, Reno, CUBIC) for the carousel.