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Wednesday, August 28, 2013

What are different policies to prevent congestion at different layers?

- Many times it happens that the demand for the resource is more than what network can offer i.e., its capacity. 
- Too much queuing occurs in the networks leading to a great loss of packets. 
When the network is in the state of congestive collapse, its throughput drops down to zero whereas the path delay increases by a great margin. 
- The network can recover from this state by following a congestion control scheme.
- A congestion avoidance scheme enables the network to operate in an environment where the throughput is high and the delay is low. 
- In other words, these schemes prevent a computer network from falling prey to the vicious clutches of the network congestion problem. 
- Recovery mechanism is implemented through congestion and the prevention mechanism is implemented through congestion avoidance. 
The network and the user policies are modeled for the purpose of congestion avoidance. 
- These act like a feedback control system. 

The following are defined as the key components of a general congestion avoidance scheme:
Ø  Congestion detection
Ø  Congestion feedback
Ø  Feedback selector
Ø  Signal filter
Ø  Decision function
Ø  Increase and decrease algorithms

- The problem of congestion control gets more complex when the network is using a connection-less protocol. 
- Avoiding congestion rather than simply controlling it is the main focus. 
- A congestion avoidance scheme is designed after comparing it with a number of other alternative schemes. 
- During the comparison, the algorithm with the right parameter values is selected. 
For doing so few goals have been set with which there is an associated test for verifying whether it is being met by the scheme or not:
Ø  Efficient: If the network is operating at the “knee” point, then it is said to be working efficiently.
Ø  Responsiveness: There is a continuous variation in the configuration and the traffic of the network. Therefore the point for optimal operation also varies continuously.
Ø Minimum oscillation: Only those schemes are preferred that have smaller oscillation amplitude.
Ø Convergence: The scheme should be such that it should bring the network to a point of stable operation for keeping the workload as well as the network configuration stable. The schemes that are able to satisfy this goal are called convergent schemes and the divergent schemes are rejected.
Ø Fairness: This goal aims at providing a fair share of resources to each independent user.
Ø  Robustness: This goal defines the capability of the scheme to work in any random environment. Therefore the schemes that are capable of working only for the deterministic service times are rejected.
Ø  Simplicity: Schemes are accepted in their most simple version.
Ø Low parameter sensitivity: Sensitivity of a scheme is measured with respect to its various parameter values. The scheme which is found to be too much sensitive to a particular parameter, it is rejected.
Ø Information entropy: This goal is about how the feedback information is used. The goal is to get maximum info with the minimum possible feedback.
Ø Dimensionless parameters: A parameter having the dimensions such as the mass, time and the length is taken as a network configuration or speed function. A parameter that has no dimensions has got more applicability.
Ø Configuration independence: The scheme is accepted only if it has been tested for various different configurations.

Congestion avoidance scheme has two main components:
Ø  Network policies: It consists of the following algorithms: feedback filter, feedback selector and congestion detection.
Ø  User policies: It consists of the following algorithms: increase/ decrease algorithm, decision function and signal filter.
These algorithms decide whether the network feedback has to be implemented via packet header field or as source quench messages.

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