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
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iproute2 10 January 2002 PFIFO_FAST(8)