PBFIFO(8) Linux PBFIFO(8)
NAME
pfifo - Packet limited First In, First Out queue
bfifo - Byte limited First In, First Out queue
SYNOPSIS
tc qdisc ... add pfifo [ limit packets ]
tc qdisc ... add bfifo [ limit bytes ]
DESCRIPTION
The pfifo and bfifo qdiscs are unadorned First In, First Out queues. They are the simplest queues possible and
therefore have no overhead. pfifo constrains the queue size as measured in packets. bfifo does so as mea‐
sured in bytes.
Like all non-default qdiscs, they maintain statistics. This might be a reason to prefer pfifo or bfifo over
the default.
ALGORITHM
A list of packets is maintained, when a packet is enqueued it gets inserted at the tail of a list. When a
packet needs to be sent out to the network, it is taken from the head of the list.
If the list is too long, no further packets are allowed on. This is called 'tail drop'.
PARAMETERS
limit Maximum queue size. Specified in bytes for bfifo, in packets for pfifo. For pfifo, defaults to the
interface txqueuelen, as specified with ifconfig(8) or ip(8). The range for this parameter is [0,
UINT32_MAX].
For bfifo, it defaults to the txqueuelen multiplied by the interface MTU. The range for this parameter
is [0, UINT32_MAX] bytes.
Note: The link layer header was considered when counting packets length.
OUTPUT
The output of tc -s qdisc ls contains the limit, either in packets or in bytes, and the number of bytes and
packets actually sent. An unsent and dropped packet only appears between braces and is not counted as 'Sent'.
In this example, the queue length is 100 packets, 45894 bytes were sent over 681 packets. No packets were
dropped, and as the pfifo queue does not slow down packets, there were also no overlimits:
# tc -s qdisc ls dev eth0
qdisc pfifo 8001: dev eth0 limit 100p
Sent 45894 bytes 681 pkts (dropped 0, overlimits 0)
If a backlog occurs, this is displayed as well.
SEE ALSO
tc(8)
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