tc-prio

PRIO(8)                              Linux                             PRIO(8)

NAME
       PRIO - Priority qdisc

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

DESCRIPTION
       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
       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  pack-
       ets 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.

CLASSIFICATION
       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  di-
              rectly 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.

QDISC PARAMETERS
       bands  Number of bands. If changed from the default of 3, priomap  must
              be updated as well.

       priomap
              The priomap maps the priority of a packet to a class. The prior-
              ity 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. Tcp-
              dump -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 Reli-
              ability, 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)
              FTP
                      Control          1000           (minimize delay)
                      Data             0100           (maximize throughput)

              TFTP                     1000           (minimize delay)

              SMTP
                      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)

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

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

BUGS
       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
       Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>,  J  Hadi  Salim  <hadi@cy-
       berus.ca>. This manpage maintained by bert hubert <ahu@ds9a.nl>

iproute2                       16 December 2001                        PRIO(8)