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Networking Fundamentals · Intermediate

Congestion Control

TCP algorithms that adjust the sending rate based on signs of network congestion such as packet loss or delay.

Definition

TCP maintains a congestion window that grows during slow start and congestion avoidance and shrinks when loss or ECN marks indicate congestion. Common algorithms include Reno, CUBIC (the Linux and Windows default) and BBR. UDP has no built-in congestion control, so applications or QUIC must provide it.

Example

A file transfer that speeds up gradually after starting is TCP slow start at work.

Exam tip

TCP reacts to loss by slowing down; UDP does not, which is why UDP floods are effective.

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