1PRIO(8)                              Linux                             PRIO(8)
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NAME

6       PRIO - Priority qdisc
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SYNOPSIS

9       tc  qdisc ... dev dev ( parent classid | root) [ handle major: ] prio [
10       bands bands ] [ priomap band,band,band...  ] [ estimator interval time‐
11       constant ]
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DESCRIPTION

15       The  PRIO  qdisc is a simple classful queueing discipline that contains
16       an arbitrary number of classes of differing priority. The  classes  are
17       dequeued in numerical descending order of priority. PRIO is a scheduler
18       and never delays packets - it is a work-conserving  qdisc,  though  the
19       qdiscs contained in the classes may not be.
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21       Very useful for lowering latency when there is no need for slowing down
22       traffic.
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ALGORITHM

26       On creation with 'tc qdisc add', a fixed number of  bands  is  created.
27       Each  band is a class, although is not possible to add classes with 'tc
28       qdisc add', the number of bands to be created must instead be specified
29       on the command line attaching PRIO to its root.
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31       When dequeueing, band 0 is tried first and only if it did not deliver a
32       packet does PRIO try band 1, and so onwards. Maximum reliability  pack‐
33       ets should therefore go to band 0, minimum delay to band 1 and the rest
34       to band 2.
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36       As the PRIO qdisc itself will have minor number 0, band 0  is  actually
37       major:1, band 1 is major:2, etc. For major, substitute the major number
38       assigned to the qdisc on 'tc qdisc add' with the handle parameter.
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CLASSIFICATION

42       Three methods are available to PRIO to determine in which band a packet
43       will be enqueued.
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45       From userspace
46              A  process with sufficient privileges can encode the destination
47              class directly with SO_PRIORITY, see tc(7).
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49       with a tc filter
50              A tc filter  attached  to  the  root  qdisc  can  point  traffic
51              directly to a class
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53       with the priomap
54              Based  on the packet priority, which in turn is derived from the
55              Type of Service assigned to the packet.
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57       Only the priomap is specific to this qdisc.
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QDISC PARAMETERS

60       bands  Number of bands. If changed from the default of 3, priomap  must
61              be updated as well.
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63       priomap
64              The priomap maps the priority of a packet to a class. The prior‐
65              ity can either be set directly from  userspace,  or  be  derived
66              from the Type of Service of the packet.
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68              Determines how packet priorities, as assigned by the kernel, map
69              to bands. Mapping occurs based on the TOS octet of  the  packet,
70              which looks like this:
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72              0   1   2   3   4   5   6   7
73              +---+---+---+---+---+---+---+---+
74              |           |               |   |
75              |PRECEDENCE |      TOS      |MBZ|
76              |           |               |   |
77              +---+---+---+---+---+---+---+---+
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79              The four TOS bits (the 'TOS field') are defined as:
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81              Binary Decimal  Meaning
82              -----------------------------------------
83              1000   8         Minimize delay (md)
84              0100   4         Maximize throughput (mt)
85              0010   2         Maximize reliability (mr)
86              0001   1         Minimize monetary cost (mmc)
87              0000   0         Normal Service
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89              As  there  is  1 bit to the right of these four bits, the actual
90              value of the TOS field is double the value of the TOS bits. Tcp‐
91              dump -v -v shows you the value of the entire TOS field, not just
92              the four bits. It is the value you see in the  first  column  of
93              this table:
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95              TOS     Bits  Means                    Linux Priority    Band
96              ------------------------------------------------------------
97              0x0     0     Normal Service           0 Best Effort     1
98              0x2     1     Minimize Monetary Cost   1 Filler          2
99              0x4     2     Maximize Reliability     0 Best Effort     1
100              0x6     3     mmc+mr                   0 Best Effort     1
101              0x8     4     Maximize Throughput      2 Bulk            2
102              0xa     5     mmc+mt                   2 Bulk            2
103              0xc     6     mr+mt                    2 Bulk            2
104              0xe     7     mmc+mr+mt                2 Bulk            2
105              0x10    8     Minimize Delay           6 Interactive     0
106              0x12    9     mmc+md                   6 Interactive     0
107              0x14    10    mr+md                    6 Interactive     0
108              0x16    11    mmc+mr+md                6 Interactive     0
109              0x18    12    mt+md                    4 Int. Bulk       1
110              0x1a    13    mmc+mt+md                4 Int. Bulk       1
111              0x1c    14    mr+mt+md                 4 Int. Bulk       1
112              0x1e    15    mmc+mr+mt+md             4 Int. Bulk       1
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114              The  second  column  contains the value of the relevant four TOS
115              bits, followed by their  translated  meaning.  For  example,  15
116              stands for a packet wanting Minimal Monetary Cost, Maximum Reli‐
117              ability, Maximum Throughput AND Minimum Delay.
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119              The fourth column lists the way the Linux kernel interprets  the
120              TOS bits, by showing to which Priority they are mapped.
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122              The  last column shows the result of the default priomap. On the
123              command line, the default priomap looks like this:
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125                  1, 2, 2, 2, 1, 2, 0, 0 , 1, 1, 1, 1, 1, 1, 1, 1
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127              This means that priority 4, for example,  gets  mapped  to  band
128              number 1.  The priomap also allows you to list higher priorities
129              (> 7) which do not correspond to TOS mappings, but which are set
130              by other means.
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132              This table from RFC 1349 (read it for more details) explains how
133              applications might very well set their TOS bits:
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135              TELNET                   1000           (minimize delay)
136              FTP
137                      Control          1000           (minimize delay)
138                      Data             0100           (maximize throughput)
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140              TFTP                     1000           (minimize delay)
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142              SMTP
143                      Command phase    1000           (minimize delay)
144                      DATA phase       0100           (maximize throughput)
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146              Domain Name Service
147                      UDP Query        1000           (minimize delay)
148                      TCP Query        0000
149                      Zone Transfer    0100           (maximize throughput)
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151              NNTP                     0001           (minimize monetary cost)
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153              ICMP
154                      Errors           0000
155                      Requests         0000 (mostly)
156                      Responses        <same as request> (mostly)
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CLASSES

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

167       Large amounts of traffic in the lower bands  can  cause  starvation  of
168       higher  bands. Can be prevented by attaching a shaper (for example, tc-
169       tbf(8) to these bands to make sure they cannot dominate the link.
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AUTHORS

173       Alexey   N.   Kuznetsov,   <kuznet@ms2.inr.ac.ru>,    J   Hadi    Salim
174       <hadi@cyberus.ca>. This manpage maintained by bert hubert <ahu@ds9a.nl>
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180iproute2                       16 December 2001                        PRIO(8)
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