QUEUE TYPE

 

Queue Management is defined as the algorithm that manages the length of the packet queues by dropping packets when necessary. 

Passive Queue Management: In Passive Queue Management the packet drop occurs only when the buffer gets full. Ex: Drop Tail.

Active Queue Management: Active Queue Management employs preventive packet drops. It provides an implicit feedback mechanism to notify senders of the onset of congestion. Arriving packets are randomly dropped. Ex: RED.

Droptail: In Droptail, the router accepts and forwards all the packets that arrive as long as its buffer space is available for the incoming packets. If a packet arrives and the queue is full, the incoming packet will be dropped. The sender eventually detects the packet lost and shrinks its sending window. Drop-tail queues have a tendency to penalize bursty flows, and to cause global synchronization between flows.

RED: RED is a type of active queue management technique used for congestion avoidance. RED monitors the average queue size and drops (or marks when used in conjunction with ECN) packets based on statistical probabilities. If the buffer is almost empty, all incoming packets are accepted. As the queue grows, the probability for dropping an incoming packet grows too. When the buffer is full, the probability has reached 1 and all incoming packets are dropped

REM: REM is an active queue management scheme that measures congestion not by performance measure such as loss or delay, but by quantity. REM can achieve high utilization, small queue length, and low buffer overflow probability. Many works have used control theory to provide the stable condition of REM without considering the feedback delay. In case of (Random Exponential Marking) REM, the key idea is to decouple congestion measure from performance measure (loss, queue length or delay).

Fair Queuing In fair queuing every flow gets the bandwidth propositional to its demand. The main goal of fair queuing is to allocate resources fairly to keep separate queue for each flow currently flowing via a router. Every queue gets equal bandwidth when the packets are in same size and non-empty queue follows a round robin fashion like FIFO. But if the packets are in different size the flow of large size packets gets more bandwidth than small size packets and these problems are overcome by weighted fair queuing algorithm, etc. Maintaining a separate queue for each flow requires a gateway or router to map from source to destination address pair for the related queue on a per packet basis. 

Stochastic Fair Queuing Stochastic Fair Queuing uses a hash algorithm to divide the traffic over a limited number of queues. Due to the hashing in SFQ multiple sessions might end up into the same bucket. SFQ changes its hashing algorithm so that any two colliding sessions will only work for a small number of seconds. Stochastic Fair Queuing algorithm is the best algorithm among all algorithms in case of providing satisfactory bandwidth to the legitimate users (TCP and UDP) in network. It is called Stochastic due to the reason that it does not actually assign a queue for every session; it has an algorithm which divides traffic over a restricted number of queues using a hashing algorithm.SFQ assigns a pretty large number of FIFO queues. Stochastic Fair Queuing (SFQ) ensures fair access to network resources and prevents a busty flow from consuming more than its fair share.SFQ has a minimum average loss ratio and maximum throughput compared to RED.