Principles of Congestion Control

Too many sources send data too quickly for the network to handle.

1. Packet loss
   : When many packets enter the network at once, router buffers can overflow and packets may be lost. This reduces transmission reliability and communication efficiency.
2. Long delays
   : When network traffic exceeds a router's processing capacity, packets must wait in a queue, causing transmission delays.

• Two senders and two receivers.
• One router with an infinite buffer → No packet loss.
• Output-link capacity: R.
• No retransmissions.
• An unlimited shared output-link buffer.

• In the arrival-rate vs. throughput graph, the output rate reaches its maximum at $R/2$, then remains constant as the link saturates.
• In the arrival-rate vs. delay graph, delay rises sharply as the input rate approaches $R/2$.

• Application-layer input equals application-layer output: $\lambda_{in} = \lambda_{out}$.
• Transport-layer traffic includes retransmissions: $\lambda'_{in} \geq \lambda_{in}$.

• The sender transmits only when the router's buffer has enough space.

• The sender retransmits a packet when the router reports it lost.

• Packets may be dropped at the router when its buffer fills, so a timeout is needed.
• The sender may time out prematurely, send two copies, and have both delivered.

• Four senders and multihop paths.
• Timeouts and retransmissions.
• $\lambda_{in}$: Original data; $\lambda'_{in}$: Retransmitted data.