Controlling congestion in a network can be critical to performance. If a network does not have any congestion control, explain what conditions will cause congestion to occur. Be specific. "too much traffic" is not enough detail. You need to explain your answer in terms of router and link resources.
Try an answer before revealing the guidance below.
Key Concepts
- Output link capacity
- Router buffers
- Queuing delay
- Packet loss
- Wasted retransmissions
Answer Approach
- Look at a single router output link shared by several flows.
- Compare the arrival rate with the link rate.
- Follow what happens to the queue, delay, and finite buffer.
- Consider the effect of retransmissions and of drops downstream.
Full Answer
Answer status: Draft answer (unofficial). Revision notes, not an official marking scheme.
Congestion occurs when the combined arrival rate of packets destined for a router output link exceeds that link’s transmission rate. The excess is queued in the router’s buffer, so queuing delay grows as the arrival rate approaches the link capacity. Buffers are finite, so once they fill, arriving packets are dropped. Senders then retransmit (including unneeded retransmissions after premature timeouts), adding more load, and a packet dropped at a later router wastes the capacity every earlier link spent carrying it — so throughput can collapse.
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