• Open Loop Congestion Control

i. In this method, policies are used to prevent the congestion before it happens.

ii. Congestion control is handled either by the source or by the destination. The various methods used for open loop congestion control are:

1) Retransmission Policy

a. The sender retransmits a packet, if it feels that the packet it has sent is lost or corrupted.

b. However retransmission in general may increase the congestion in the network. But we need to implement good retransmission policy to prevent congestion.

c. The retransmission policy and the retransmission timers need to be designed to optimize efficiency and at the same time prevent the congestion.

2) Window Policy

a. To implement window policy, selective reject window method is used for congestion control.

b. Selective Reject method is preferred over Go-back-n window as in Go-back-n method, when timer for a packet times out, several packets are resent, although some may have arrived safely at the receiver. Thus, this duplication may make congestion worse.

c. Selective reject method sends only the specific lost or damaged packets.

3) Acknowledgement Policy

a. The acknowledgement policy imposed by the receiver may also affect congestion.

b. If the receiver does not acknowledge every packet it receives it may slow down the sender and help prevent congestion.

c. Acknowledgments also add to the traffic load on the network. Thus, by sending fewer acknowledgements we can reduce load on the network.

d. To implement it, several approaches can be used:

• A receiver may send an acknowledgement only if it has a packet to be sent.

• A receiver may send an acknowledgement when a timer expires.

• A receiver may also decide to acknowledge only N packets at a time.

4) Discarding Policy

a. A router may discard less sensitive packets when congestion is likely to happen.

b. Such a discarding policy may prevent congestion and at the same time may not harm the integrity of the transmission.

5) Admission Policy

a. An admission policy, which is a quality-of-service mechanism, can also prevent congestion in virtual circuit networks.

b. Switches in a flow first check the resource requirement of a flow before admitting it to the network.

c. A router can deny establishing a virtual circuit connection if there is congestion in the "network or if there is a possibility of future congestion.