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


       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 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)

                      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

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iproute2                      16 December 2001                       PRIO(8)