Provided by: iproute2_5.15.0-1ubuntu2_amd64 bug


       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 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.


       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 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)


       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.


       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.


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