pfifo_fast — three-band first in, first out queue
pfifo_fast is the default qdisc of each interface.
Whenever an interface is created, the pfifo_fast qdisc is automatically used as a queue. If another qdisc is attached, it preempts the default pfifo_fast, which automatically returns to function when an existing qdisc is detached.
In this sense this qdisc is magic, and unlike other qdiscs.
The algorithm is very similar to that of the classful tc-prio(8) qdisc. pfifo_fast is like three tc-pfifo(8) queues side by side, where packets can be enqueued in any of the three bands based on their Type of Service bits or assigned priority.
Not all three bands are dequeued simultaneously - as long as lower bands have traffic, higher bands are never dequeued. This can be used to prioritize interactive traffic or penalize 'lowest cost' traffic.
Each band can be txqueuelen packets long, as configured with ifconfig(8) or ip(8). Additional packets coming in are not enqueued but are instead dropped.
See tc-prio(8) for complete details on how TOS bits are translated into bands.
The length of the three bands depends on the interface txqueuelen, as specified with ifconfig(8) or ip(8).
Does not maintain statistics and does not show up in tc qdisc ls. This is because it is the automatic default in the absence of a configured qdisc.