Computer Networking Basics
Which correctly describes the basic idea of TCP congestion control?
- AThe receiver reports its free buffer space to the sender so that its buffer does not overflow
- BThe sender treats signs such as packet loss as congestion, shrinks its congestion window to send less, and increases it gradually when there is no congestion
- CA congested router tells each sender a numerical rate at which it may send, and the senders comply
Show answer and explanation
Answer: B. The sender treats signs such as packet loss as congestion, shrinks its congestion window to send less, and increases it gradually when there is no congestion
When a router along the path becomes congested, its queue grows, delay increases and packets that overflow are dropped. A TCP sender infers loss from missing ACKs or duplicate ACKs and reduces the amount it sends on its own (the congestion window). At the start of a connection, slow start roughly doubles the window every round trip to find the available bandwidth quickly; after that the window grows gradually and is cut sharply when loss occurs (additive increase, multiplicative decrease). The amount actually sent is limited by the smaller of the congestion window and the receive window. Variants such as CUBIC and BBR differ in how they grow the window and how they judge congestion.
- A. That describes flow control (the receive window).
- B. Correct. The sender infers congestion from loss and similar signs and adjusts the amount it sends.
- C. In TCP congestion control the sender infers congestion from loss or delay and adjusts by itself.