tc-prio(8) — Linux manual page


PRIO(8)                           Linux                          PRIO(8)

NAME         top

       PRIO - Priority qdisc

SYNOPSIS         top

       tc qdisc ... dev dev ( parent classid | root) [ handle major: ]
       prio [ bands bands ] [ priomap band band band...  ] [ estimator
       interval timeconstant ]

DESCRIPTION         top

       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 down traffic.

ALGORITHM         top

       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 parameter.


       Three methods are available to PRIO to determine in which band a
       packet will be enqueued.

       From userspace
              A process with sufficient privileges can encode the
              destination class directly with SO_PRIORITY, see

       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

              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

              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)

                      Errors           0000
                      Requests         0000 (mostly)
                      Responses        <same as request> (mostly)

CLASSES         top

       PRIO classes cannot be configured further - they are
       automatically created when the PRIO qdisc is attached. Each class
       however can contain yet a further qdisc.

BUGS         top

       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.

AUTHORS         top

       Alexey N. Kuznetsov, <>,  J Hadi Salim
       <>. This manpage maintained by bert hubert

COLOPHON         top

       This page is part of the iproute2 (utilities for controlling
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iproute2                    16 December 2001                     PRIO(8)

Pages that refer to this page: tc-ets(8)tc-pfifo_fast(8)tc-skbprio(8)