Congestion and Flow-Control, continued

Both congestion and flow-control have the same aim - to reduce the offered traffic entering the network when the load is already high.

Congestion will be detected through a number of local and global metrics -

• percentage of packets discarded for lack of buffer space,
• average router queue lengths,
• number of packets timing out requiring retransmission, and
• average (or standard deviation of) packet delay.

Open loop control attempts to prevent congestion in the first place (good design), rather than correcting it. Virtual circuit systems will perform open loop control by preallocating buffer space for each virtual circuit (VC).

• New VCs will not be created via an intermediate router is low on memory.
• A new VC may have to be created via another path.

Closed loop control maintains a feedback loop of three stages -

• monitoring of local subnet to detect where congestion occurs,
• passing information to where corrective action may be taken, and
• adjustment of local operation to correct problem.