man/man8/tc-prio.8

   1 .TH PRIO 8 "16 December 2001" "iproute2" "Linux"
   2 .SH NAME
   3 PRIO \- Priority qdisc
   4 .SH SYNOPSIS
   5 .B tc qdisc ... dev
   6 dev
   7 .B  ( parent
   8 classid
   9 .B | root) [ handle
  10 major:
  11 .B ] prio [ bands
  12 bands
  13 .B ] [ priomap
  14 band band band...
  15 .B ] [ estimator
  16 interval timeconstant
  17 .B ]
  18
  19 .SH DESCRIPTION
  20 The PRIO qdisc is a simple classful queueing discipline that contains
  21 an arbitrary number of classes of differing priority. The classes are
  22 dequeued in numerical descending order of priority. PRIO is a scheduler
  23 and never delays packets - it is a work-conserving qdisc, though the qdiscs
  24 contained in the classes may not be.
  25
  26 Very useful for lowering latency when there is no need for slowing down
  27 traffic.
  28
  29 .SH ALGORITHM
  30 On creation with 'tc qdisc add', a fixed number of bands is created. Each
  31 band is a class, although is not possible to add classes with 'tc qdisc
  32 add', the number of bands to be created must instead be specified on the
  33 command line attaching PRIO to its root.
  34
  35 When dequeueing, band 0 is tried first and only if it did not deliver a
  36 packet does PRIO try band 1, and so onwards. Maximum reliability packets
  37 should therefore go to band 0, minimum delay to band 1 and the rest to band
  38 2.
  39
  40 As the PRIO qdisc itself will have minor number 0, band 0 is actually
  41 major:1, band 1 is major:2, etc. For major, substitute the major number
  42 assigned to the qdisc on 'tc qdisc add' with the
  43 .B handle
  44 parameter.
  45
  46 .SH CLASSIFICATION
  47 Three methods are available to PRIO to determine in which band a packet will
  48 be enqueued.
  49 .TP
  50 From userspace
  51 A process with sufficient privileges can encode the destination class
  52 directly with SO_PRIORITY, see
  53 .BR socket(7).
  54 .TP
  55 with a tc filter
  56 A tc filter attached to the root qdisc can point traffic directly to a class
  57 .TP
  58 with the priomap
  59 Based on the packet priority, which in turn is derived from the Type of
  60 Service assigned to the packet.
  61 .P
  62 Only the priomap is specific to this qdisc.
  63 .SH QDISC PARAMETERS
  64 .TP
  65 bands
  66 Number of bands. If changed from the default of 3,
  67 .B priomap
  68 must be updated as well.
  69 .TP
  70 priomap
  71 The priomap maps the priority of
  72 a packet to a class. The priority can either be set directly from userspace,
  73 or be derived from the Type of Service of the packet.
  74
  75 Determines how packet priorities, as assigned by the kernel, map to
  76 bands. Mapping occurs based on the TOS octet of the packet, which looks like
  77 this:
  78
  79 .nf
  80 0   1   2   3   4   5   6   7
  81 +---+---+---+---+---+---+---+---+
  82 |           |               |   |
  83 |PRECEDENCE |      TOS      |MBZ|
  84 |           |               |   |
  85 +---+---+---+---+---+---+---+---+
  86 .fi
  87
  88 The four TOS bits (the 'TOS field') are defined as:
  89
  90 .nf
  91 Binary Decimal  Meaning
  92 -----------------------------------------
  93 1000   8         Minimize delay (md)
  94 0100   4         Maximize throughput (mt)
  95 0010   2         Maximize reliability (mr)
  96 0001   1         Minimize monetary cost (mmc)
  97 0000   0         Normal Service
  98 .fi
  99
 100 As there is 1 bit to the right of these four bits, the actual value of the
 101 TOS field is double the value of the TOS bits. Tcpdump -v -v shows you the
 102 value of the entire TOS field, not just the four bits. It is the value you
 103 see in the first column of this table:
 104
 105 .nf
 106 TOS     Bits  Means                    Linux Priority    Band
 107 ------------------------------------------------------------
 108 0x0     0     Normal Service           0 Best Effort     1
 109 0x2     1     Minimize Monetary Cost   1 Filler          2
 110 0x4     2     Maximize Reliability     0 Best Effort     1
 111 0x6     3     mmc+mr                   0 Best Effort     1
 112 0x8     4     Maximize Throughput      2 Bulk            2
 113 0xa     5     mmc+mt                   2 Bulk            2
 114 0xc     6     mr+mt                    2 Bulk            2
 115 0xe     7     mmc+mr+mt                2 Bulk            2
 116 0x10    8     Minimize Delay           6 Interactive     0
 117 0x12    9     mmc+md                   6 Interactive     0
 118 0x14    10    mr+md                    6 Interactive     0
 119 0x16    11    mmc+mr+md                6 Interactive     0
 120 0x18    12    mt+md                    4 Int. Bulk       1
 121 0x1a    13    mmc+mt+md                4 Int. Bulk       1
 122 0x1c    14    mr+mt+md                 4 Int. Bulk       1
 123 0x1e    15    mmc+mr+mt+md             4 Int. Bulk       1
 124 .fi
 125
 126 The second column contains the value of the relevant
 127 four TOS bits, followed by their translated meaning. For example, 15 stands
 128 for a packet wanting Minimal Monetary Cost, Maximum Reliability, Maximum
 129 Throughput AND Minimum Delay.
 130
 131 The fourth column lists the way the Linux kernel interprets the TOS bits, by
 132 showing to which Priority they are mapped.
 133
 134 The last column shows the result of the default priomap. On the command line,
 135 the default priomap looks like this:
 136
 137     1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 138
 139 This means that priority 4, for example, gets mapped to band number 1.
 140 The priomap also allows you to list higher priorities (> 7) which do not
 141 correspond to TOS mappings, but which are set by other means.
 142
 143 This table from RFC 1349 (read it for more details) explains how
 144 applications might very well set their TOS bits:
 145
 146 .nf
 147 TELNET                   1000           (minimize delay)
 148 FTP
 149         Control          1000           (minimize delay)
 150         Data             0100           (maximize throughput)
 151
 152 TFTP                     1000           (minimize delay)
 153
 154 SMTP
 155         Command phase    1000           (minimize delay)
 156         DATA phase       0100           (maximize throughput)
 157
 158 Domain Name Service
 159         UDP Query        1000           (minimize delay)
 160         TCP Query        0000
 161         Zone Transfer    0100           (maximize throughput)
 162
 163 NNTP                     0001           (minimize monetary cost)
 164
 165 ICMP
 166         Errors           0000
 167         Requests         0000 (mostly)
 168         Responses        <same as request> (mostly)
 169 .fi
 170
 171
 172 .SH CLASSES
 173 PRIO classes cannot be configured further - they are automatically created
 174 when the PRIO qdisc is attached. Each class however can contain yet a
 175 further qdisc.
 176
 177 .SH BUGS
 178 Large amounts of traffic in the lower bands can cause starvation of higher
 179 bands. Can be prevented by attaching a shaper (for example,
 180 .BR tc-tbf(8)
 181 to these bands to make sure they cannot dominate the link.
 182
 183 .SH AUTHORS
 184 Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>,  J Hadi Salim
 185 <hadi@cyberus.ca>. This manpage maintained by bert hubert <ahu@ds9a.nl>
 186
 187