Transport Layer - Congestion Control and QoS
The resources being shared in the network include the bandwidth of the links, and the buffers on the routers or switches where packets are queued waiting to be transmitted over these links. Packets contend at a router for the use of a link, with each contending packet placed in a queue waiting its turn to be transmitted over the link. When enough packets are contending for the same link, the queue overflows and packets have to be dropped. It is at this stage that the network is said to be congested. Most networks provide a congestion-control mechanism to deal with just such a situation.