PRIO(8)				     Linux			       PRIO(8)

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>

iproute2		       16 December 2001			       PRIO(8)