Congestion Avoidance in Computer Networks with a Connectionless Network Layer, Part I: Concepts, Goals and Methodology

Congestion is said to occur in the network when the resource demands exceed the capacity and packets are lost due to too much queuing in the network. During congestion, the network throughput may drop to zero and the path delay may become very high. A congestion control scheme helps the network to recover from the congestion state. A congestion avoidance scheme allows a network to operate in the region of low delay and high throughput. Such schemes prevent a network from entering the congested state. Congestion avoidance is a prevention mechanism while congestion control is a recovery mechanism. We compare the concept of congestion avoidance with that of ow control and congestion control. A number of possible alternative for congestion avoidance have been identi ed. From these a few were selected for study. The criteria for selection and goals for these schemes have been described. In particular, we wanted the scheme to be globally e cient, fair, dynamic, convergent, robust, distributed, con guration independent, etc. These goals and the test cases used to verify whether a particular scheme has met the goals have been described. We model the network and the user policies for congestion avoidance as a feedback control system. The key components of a generic congestion avoidance scheme are: congestion detection, congestion feedback, feedback selector, signal lter, decision function, and increase/decrease algorithms. These components have been explained. The congestion avoidance research was done using a combination of analytical modeling and simulation techniques. The features of simulation model used have been described. This is the rst report in a series on congestion avoidance schemes. Other reports in this series describe the application of these ideas leading to the development of speci c congestion avoidance schemes. Version: September 2, 1998