bionic (8) tc-prio.8.gz

Provided by: iproute2_4.15.0-2ubuntu1.3_amd64 bug

NAME

       PRIO - Priority qdisc

SYNOPSIS

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

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 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.

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 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.

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 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)
              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 created 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@cyberus.ca>. This manpage maintained by
       bert hubert <ahu@ds9a.nl>