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 time-
       constant ]


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	 pack-
       ets 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 prior-
	      ity 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. Tcp-
	      dump -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 Reli-
	      ability, 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 cre-
       ated  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)