AlgoPlus//networking / congestion-control

TCP Congestion Control

Slow start ramps up exponentially; AIMD probes gently and backs off hard on loss.

cwnd

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ssthresh

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Phase

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Legend

Slow start

Congestion avoidance

Loss

AI Tutor Workspace

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In a nutshell

Nothing tells TCP the link speed, so it probes with a congestion window (cwnd) — how much unacknowledged data it allows. Slow start doubles cwnd each round trip until a threshold; then congestion avoidance adds one per round trip. A loss signals congestion, so cwnd is halved. This AIMD loop makes a sawtooth that hovers near capacity.

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Nobody tells TCP the link speed, so it guesses: ramp up fast, then crawl, and the instant a packet drops, halve and back off. The result is a sawtooth that hovers near capacity.

Key terms

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