Where congestion forms
Congestion happens when more traffic arrives at a router than its outgoing link can carry. The excess packets pile up in a queue in the router's buffer. As the queue grows, packets wait longer, adding queuing delay. If the buffer fills completely, new packets are dropped, which senders treat as a signal to slow down.
How systems respond
- Senders run congestion control that reduces their rate when they detect loss or delay.
- Routers may use active queue management to drop or mark packets before the buffer is full.
- Explicit congestion notification lets routers mark instead of drop, signaling without loss.
- Fair queuing tries to stop one heavy flow from starving the others sharing the link.
Congestion is self correcting in principle: drops tell senders to back off, the queue drains, and the link recovers. The challenge is tuning buffers and control so the system finds a good operating point. Too aggressive a backoff wastes capacity, while too slow a response lets queues grow and delay everyone. Good queue management aims for high utilization with short queues so latency stays low even under load.
Key idea
Congestion occurs when arrivals exceed a link's rate, building queues that add delay and eventually drop packets, and the network relies on congestion control plus queue management to back senders off and keep latency low.