PRIO(8)                (16 December 2001)                 PRIO(8)

          PRIO - Priority qdisc

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

          The PRIO qdisc is a simple classful queueing discipline that
          contains an arbitrary number of classes of differing prior-
          ity. 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.

          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, substi-
          tute 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

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     PRIO(8)                (16 December 2001)                 PRIO(8)

               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.

               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

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     PRIO(8)                (16 December 2001)                 PRIO(8)

               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 inter-
               prets the TOS bits, by showing to which Priority they
               are mapped.

               The last column shows the result of the default pri-
               omap. 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)

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     PRIO(8)                (16 December 2001)                 PRIO(8)

               NNTP                     0001           (minimize monetary cost)

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

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

          Large amounts of traffic in the lower bands can cause star-
          vation 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.

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

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