The PRIO qdisc is a simple classful queueing discipline that contains
an arbitrary number of classes of differing priority. The classes are
dequeued in numerical descending order of priority. PRIO is a
scheduler and never delays packets - it is a work-conserving qdisc,
though the qdiscs contained in the classes may not be.
Very useful for lowering latency when there is no need for slowing
On creation with 'tc qdisc add', a fixed number of bands is created.
Each band is a class, although is not possible to add classes with
'tc qdisc add', the number of bands to be created must instead be
specified on the command line attaching PRIO to its root.
When dequeueing, band 0 is tried first and only if it did not deliver
a packet does PRIO try band 1, and so onwards. Maximum reliability
packets should therefore go to band 0, minimum delay to band 1 and
the rest to band 2.
As the PRIO qdisc itself will have minor number 0, band 0 is actually
major:1, band 1 is major:2, etc. For major, substitute the major
number assigned to the qdisc on 'tc qdisc add' with the handle
Three methods are available to PRIO to determine in which band a
packet will be enqueued.
A process with sufficient privileges can encode the
destination class directly with SO_PRIORITY, see socket(7).
with a tc filter
A tc filter attached to the root qdisc can point traffic
directly to a class
with the priomap
Based on the packet priority, which in turn is derived from
the Type of Service assigned to the packet.
Only the priomap is specific to this qdisc.
bands Number of bands. If changed from the default of 3, priomap
must be updated as well.
The priomap maps the priority of a packet to a class. The
priority can either be set directly from userspace, or be
derived from the Type of Service of the packet.
Determines how packet priorities, as assigned by the kernel,
map to bands. Mapping occurs based on the TOS octet of the
packet, which looks like this:
0 1 2 3 4 5 6 7
| | | |
|PRECEDENCE | TOS |MBZ|
| | | |
The four TOS bits (the 'TOS field') are defined as:
Binary Decimal Meaning
1000 8 Minimize delay (md)
0100 4 Maximize throughput (mt)
0010 2 Maximize reliability (mr)
0001 1 Minimize monetary cost (mmc)
0000 0 Normal Service
As there is 1 bit to the right of these four bits, the actual
value of the TOS field is double the value of the TOS bits.
Tcpdump -v -v shows you the value of the entire TOS field, not
just the four bits. It is the value you see in the first
column of this table:
TOS Bits Means Linux Priority Band
0x0 0 Normal Service 0 Best Effort 1
0x2 1 Minimize Monetary Cost 0 Best Effort 1
0x4 2 Maximize Reliability 0 Best Effort 1
0x6 3 mmc+mr 0 Best Effort 1
0x8 4 Maximize Throughput 2 Bulk 2
0xa 5 mmc+mt 2 Bulk 2
0xc 6 mr+mt 2 Bulk 2
0xe 7 mmc+mr+mt 2 Bulk 2
0x10 8 Minimize Delay 6 Interactive 0
0x12 9 mmc+md 6 Interactive 0
0x14 10 mr+md 6 Interactive 0
0x16 11 mmc+mr+md 6 Interactive 0
0x18 12 mt+md 4 Int. Bulk 1
0x1a 13 mmc+mt+md 4 Int. Bulk 1
0x1c 14 mr+mt+md 4 Int. Bulk 1
0x1e 15 mmc+mr+mt+md 4 Int. Bulk 1
The second column contains the value of the relevant four TOS
bits, followed by their translated meaning. For example, 15
stands for a packet wanting Minimal Monetary Cost, Maximum
Reliability, Maximum Throughput AND Minimum Delay.
The fourth column lists the way the Linux kernel interprets
the TOS bits, by showing to which Priority they are mapped.
The last column shows the result of the default priomap. On
the command line, the default priomap looks like this:
1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
This means that priority 4, for example, gets mapped to band
number 1. The priomap also allows you to list higher
priorities (> 7) which do not correspond to TOS mappings, but
which are set by other means.
This table from RFC 1349 (read it for more details) explains
how applications might very well set their TOS bits:
TELNET 1000 (minimize delay)
Control 1000 (minimize delay)
Data 0100 (maximize throughput)
TFTP 1000 (minimize delay)
Command phase 1000 (minimize delay)
DATA phase 0100 (maximize throughput)
Domain Name Service
UDP Query 1000 (minimize delay)
TCP Query 0000
Zone Transfer 0100 (maximize throughput)
NNTP 0001 (minimize monetary cost)
Requests 0000 (mostly)
Responses <same as request> (mostly)
Large amounts of traffic in the lower bands can cause starvation of
higher bands. Can be prevented by attaching a shaper (for example,
tc-tbf(8) to these bands to make sure they cannot dominate the link.
This page is part of the iproute2 (utilities for controlling TCP/IP
networking and traffic) project. Information about the project can
be found at
If you have a bug report for this manual page, send it to
email@example.com, firstname.lastname@example.org. This page was obtained
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iproute2 16 December 2001 PRIO(8)