Congestion and Flow-Control in the Network Layer

An additional problem to be addressed by the Network Layer occurs if too many packets are 'dumped' into some part of the subnet - network performance will degrade sharply.

Worse still, when performance degrades, timeouts and re-transmission of packets will increase.

Also, if all available buffer space in each router is exhausted, then incoming packets will be discarded (what??!!), causing further re-transmissions.

Congestion is both a problem that a node's Network Layer must avoid (not introduce), and must address (if other nodes' Network Layers cause it).

• Congestion control is concerned with ensuring that the subnet can carry the offered traffic - a global issue concerning all hosts and routers working together.

• Flow control is concerned with end-to-end control (possibly multiple hops apart).

The sender must not swamp the receiver, and typically involves direct feedback from the receiver to the sender to 'slow down'.