Subject: Computer Communication and Telecom Network

Topic: Reliable and Unreliable Transport-layer protocols

Difficulty: Medium

1. Congestion in a network occurs when the load on the network i.e. the number of packets sent on a network is greater than the network capacity i.e. the number of packets that can be handled.
2. Congestion control refers to the mechanism and techniques that are involved to keep the lad below the congestion eves. Congestion occurs because routers and switches have buffers and queues that are meant to hold packets before and after processing.

3. Factors that are responsible for causing congestion includes:

   o Slower processor

   o Bursty traffic

   o Memory insufficiency

4. Packet arrival rate greater than outgoing link capacity. To avoid congesting, the subnet must prevent additional packets from entering the congested region until those already present can be processed as the congested routers can discard queued packets to make rooms for those that are arriving.

5. Congestion control algorithm can be classified broadly into two categories namely open loop and closed loop.
6. In closed loop congestion, the mechanism is involved to remove the congestion after it has occurred. The various methods involved are:

1. In open loop congestion control, the policies are used to prevent the congestion before it happens and is handled either by the source or by the destination. The various methods involved are:

1. The different open loop policies can be defined and described as under:

   o Retransmission policy:

   Under this policy, the sender re-transmits a packet if it sees that the packet it has sent is lost or corrupted. But retransmission of the packets in general will increase the congestion in the network.

   However, building an excellent retransmission policy can be used to prevent congestion. The retransmission policy and retransmission timers need to be designed which in turn optimizes efficiency and at the same time controls congestion.

   o Window policy: Selective reject window is the method that is used to control congestion under window policy. This method is preferred over G-back N method as duplication frames at the cost of increase complexity at the receiver end.

   o Acknowledgment policy: Acknowledgment at the receiver end effects congestion. If the receiver does not acknowledge every packet it receives it may slow down the sender and help prevent congestion.

   Sending fewer acknowledgements can reduce the load on the network however this can be implemented as

   • Receiver may send acknowledgment if it has a packet to be sent.

   • Receiver may send acknowledgment when a timer expires.

   • Receiver may as decide to acknowledge any N packets at a time.

   o Discarding policy: Less sensitive packets are discarded, when the congestion may be felt to be likely to happen. This helps to improve congestion and at the same time doesn’t harm integrity of the transmission.

   o Admission policy: This act as quality of service mechanism and prevent congestion in virtual circuit networks. Resource requirements are checked before admitting the packets to the networks and the virtual circuit congestion connections can be denied if there is congestion in the network.

2. Leaky Bucket and Token Bucket algorithms are few of the algorithms that control network congestion by traffic shaping mechanism and controlling the amount and rate of traffic that is being sent to the network.