8. What is meant by the term queuing discipline?

Queuing discipline refers to the methods used by a router to determine which packets are queued or dropped from the router queue.  Queuing disciplines discussed in the text include first-in-first-out (FIFO) and Random Early Detection (RED).  These two disciplines are considered within the policies collectively known as Active Queue Management (AQM).

First-In-First-Out is just that.  The first packet arriving at an output port is forwarded as it arrives.  Other arriving packets are queued then forwarded according to when they arrived at the queue.

Under Random Early Detection (RED) a weighted average is kept for the length of the output queue.  If the average queue length is less than the minimum threshold set by the router, when the packet arrives, the packet is allowed to enter the queue.  If the packet arrives and the length of the queue is greater than the maximum threshold, then the packet is dropped.  For packets arriving when the queue length is between the minimum and maximum queue lengths the packet is marked or dropped based on probability based marking protocols, which are not discussed in the text (Kurose and Ross, 322)

Queuing Discipline in Linux - http://qos.ittc.ukans.edu/howto/node8.html

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