# tc-prio

## 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.
    


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