7. Queue Management and Packet Scheduling

Queues represent locations where packets may be held (or dropped). Packet scheduling refers to the decision process used to choose which packets should be serviced or dropped. Buffer management refers to any particular discipline used to regulate the occupancy of a particular queue. At present, support is included for drop-tail (FIFO) queueing, RED buffer management, CBQ (including a priority and round-robin scheduler), and variants of Fair Queueing including, Fair Queueing (FQ), Stochastic Fair Queueing (SFQ), and Deficit Round-Robin (DRR). In the common case where a delay element is downstream from a queue, the queue may be blocked until it is re-enabled by its downstream neighbor. This is the mechanism by which transmission delay is simulated. In addition, queues may be forcibly blocked or unblocked at arbitrary times by their neighbors (which is used to implement multi-queue aggregate queues with inter-queue flow control). Packet drops are implemented in such a way that queues contain a ``drop destination''; that is, an object that receives all packets dropped by a queue. This can be useful to (for example) keep statistics on dropped packets.