Random Early Detection Case Study

Random Early Detection (RED) [21] has the potential to overcome the problems discovered in Drop Tail. RED is a congestion avoidance algorithm in the network routers/switches proposed by Floyd and Jacobson [13]. Random Early Detect, also called Random Early Drop (RED) was designed with the objectives to
• minimize packet loss and queuing delay.
• congestion avoidance.
• avoid global synchronization of sources.
• maintain high link utilization.
• remove biases against bursty sources.
• bound on average queue length to limit delay.
The basic idea behind RED queue management is by continually calculating the average queue length (size) of the queue by Exponentially Weighted Moving Average (EWMA) which it uses to detect congestion and compare
The parameter q is used from the equation (3.3).The minimum threshold minth and maximum threshold maxth are determined by the average queue size. The packet-marking probability pb varies from 0 to maxp and it is measured by using the equation (2.6) 3.6
The final packet-marking probability pa is calculated by using the equation (3.7) 3.7
The above equations can be diagrammatically shown as RED modes in figure.3.7.
No Drop: when the average queue is size between 0 and the minimum threshold. Random Drop: when the average queue size is between the minimum and maximum threshold. Random Drop should prevent congestion (prevent tail drops).
Full Drop (Drop Tail): When the average queue size is above the maximum
The iSLIP is an iterative three phase algorithm works in round robin fashion. The working principle of the iSLIP is already explained in section 1.4 with an example. The second part of an IQ switch is VOQ. Initially, a fixed-sized VOQ is used to provide space for incoming packets of upto K number of packets. The size of K is considered as 100 packets. At the outset the VOQ is handled by RED method. In investigating RED queue it is essential to know two parameters namely the actual queue size and average queue size at any instant of time. Therefore the actual queue size q and average queue size avg determines the behavior of RED. Then, the RED algorithm probabilistically accepts or drops packets at the ports of an IQ switch based on these parameters. For each arriving packet average queue length avg is computed by EWMA using the equation (3.5). For calculating avg it is necessary to know the value of q which is taken from the equation (3.3).Based on the output of avg the RED algorithm compare it two thresholds namely minimum threshold-minth and maximum threshold-minth.. If the buffer is full, the packet is dropped or otherwise if avg is greater than maxth, then the packet arrived is discarded. The other case if avg is less than the maxth, then each arriving packet is accepted. However, if avg lies between minth and maxth then the packet is dropped with