Congestion control in network layer occurs when a node or link carries data beyond its limit. This often leads to the queuing of packets—and in the worst case, loss of packets—as well as a decrease in the network's Quality of Service (QoS).

Congestion Control is a mechanism that controls the entry of data packets into the network, enabling a better use of a shared network infrastructure and avoiding congestive collapse.

Transmission Control Protocol (TCP) uses a congestion control algorithm that includes various aspects of an additive increase/multiplicative decrease (AIMD) scheme, along with other schemes including slow start and a congestion window (CWND), to achieve congestion avoidance.

A router needs to decide when to signal congestion, and if it de- cides to, which packet to mark or drop to signal congestion. These two decisions are made by the buffer managememt algorithm at the router. A good buffer man- agement scheme provides good incentives for end-to-end congestion control to be implemented.