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TC(8)				     Linux				 TC(8)

NAME
       tc - show / manipulate traffic control settings

SYNOPSIS
       tc  qdisc [ add | change | replace | link ] dev DEV [ parent qdisc-id |
       root ] [ handle qdisc-id ] qdisc [ qdisc specific parameters ]

       tc class [ add | change | replace ] dev DEV parent qdisc-id  [  classid
       class-id ] qdisc [ qdisc specific parameters ]

       tc filter [ add | change | replace ] dev DEV [ parent qdisc-id | root ]
       protocol protocol prio priority filtertype [ filtertype specific param‐
       eters ] flowid flow-id

       tc [ FORMAT ] qdisc show [ dev DEV ]

       tc [ FORMAT ] class show dev DEV

       tc filter show dev DEV

FORMAT := { -s[tatistics] | -d[etails] | -r[aw] | -p[retty] | i[ec] }

DESCRIPTION
       Tc  is  used  to configure Traffic Control in the Linux kernel. Traffic
       Control consists of the following:

       SHAPING
	      When traffic is shaped, its rate of transmission is  under  con‐
	      trol.  Shaping may be more than lowering the available bandwidth
	      - it is also used to smooth out bursts  in  traffic  for	better
	      network behaviour. Shaping occurs on egress.

       SCHEDULING
	      By  scheduling  the  transmission	 of  packets it is possible to
	      improve interactivity for traffic	 that  needs  it  while	 still
	      guaranteeing  bandwidth  to  bulk	 transfers. Reordering is also
	      called prioritizing, and happens only on egress.

       POLICING
	      Where shaping deals with transmission of traffic, policing  per‐
	      tains to traffic arriving. Policing thus occurs on ingress.

       DROPPING
	      Traffic exceeding a set bandwidth may also be dropped forthwith,
	      both on ingress and on egress.

       Processing of traffic is controlled by three kinds of objects:  qdiscs,
       classes and filters.

QDISCS
       qdisc is short for 'queueing discipline' and it is elementary to under‐
       standing traffic control. Whenever the kernel needs to send a packet to
       an  interface,  it  is enqueued to the qdisc configured for that inter‐
       face. Immediately afterwards, the kernel tries to get as	 many  packets
       as  possible  from  the	qdisc,	for giving them to the network adaptor
       driver.

       A simple QDISC is the 'pfifo' one, which does no processing at all  and
       is a pure First In, First Out queue. It does however store traffic when
       the network interface can't handle it momentarily.

CLASSES
       Some qdiscs can contain classes, which contain further qdiscs - traffic
       may  then  be enqueued in any of the inner qdiscs, which are within the
       classes.	 When the kernel tries to dequeue a packet from such a	class‐
       ful  qdisc it can come from any of the classes. A qdisc may for example
       prioritize certain kinds of traffic by trying to dequeue	 from  certain
       classes before others.

FILTERS
       A  filter  is  used  by	a classful qdisc to determine in which class a
       packet will be enqueued. Whenever traffic arrives at a class with  sub‐
       classes,	 it needs to be classified. Various methods may be employed to
       do so, one of these are the filters. All filters attached to the	 class
       are called, until one of them returns with a verdict. If no verdict was
       made, other criteria may be available. This differs per qdisc.

       It is important to notice that filters reside within qdiscs - they  are
       not masters of what happens.

CLASSLESS QDISCS
       The classless qdiscs are:

       [p|b]fifo
	      Simplest	usable qdisc, pure First In, First Out behaviour. Lim‐
	      ited in packets or in bytes.

       pfifo_fast
	      Standard qdisc for 'Advanced Router' enabled  kernels.  Consists
	      of  a  three-band	 queue	which honors Type of Service flags, as
	      well as the priority that may be assigned to a packet.

       red    Random Early Detection simulates physical congestion by randomly
	      dropping	packets	 when nearing configured bandwidth allocation.
	      Well suited to very large bandwidth applications.

       sfq    Stochastic Fairness Queueing reorders  queued  traffic  so  each
	      'session' gets to send a packet in turn.

       tbf    The  Token Bucket Filter is suited for slowing traffic down to a
	      precisely configured rate. Scales well to large bandwidths.

CONFIGURING CLASSLESS QDISCS
       In the absence  of  classful  qdiscs,  classless	 qdiscs	 can  only  be
       attached at the root of a device. Full syntax:

       tc qdisc add dev DEV root QDISC QDISC-PARAMETERS

       To remove, issue

       tc qdisc del dev DEV root

       The  pfifo_fast qdisc is the automatic default in the absence of a con‐
       figured qdisc.

CLASSFUL QDISCS
       The classful qdiscs are:

       CBQ    Class Based Queueing implements a rich linksharing hierarchy  of
	      classes.	 It  contains shaping elements as well as prioritizing
	      capabilities. Shaping is performed using link idle time calcula‐
	      tions  based  on	average	 packet size and underlying link band‐
	      width. The latter may be ill-defined for some interfaces.

       HTB    The Hierarchy Token Bucket implements a rich linksharing hierar‐
	      chy  of classes with an emphasis on conforming to existing prac‐
	      tices. HTB facilitates guaranteeing bandwidth to classes,	 while
	      also allowing specification of upper limits to inter-class shar‐
	      ing. It contains shaping elements, based on TBF and can  priori‐
	      tize classes.

       PRIO   The  PRIO	 qdisc	is  a non-shaping container for a configurable
	      number of classes which are dequeued in order. This  allows  for
	      easy  prioritization  of	traffic,  where lower classes are only
	      able to send if higher ones have no packets available. To facil‐
	      itate  configuration,  Type  Of  Service	bits  are  honored  by
	      default.

THEORY OF OPERATION
       Classes form a tree, where each class has a single parent.  A class may
       have  multiple  children.  Some	qdiscs	allow  for runtime addition of
       classes (CBQ, HTB) while others (PRIO) are created with a static number
       of children.

       Qdiscs  which  allow  dynamic addition of classes can have zero or more
       subclasses to which traffic may be enqueued.

       Furthermore, each class contains a leaf	qdisc  which  by  default  has
       pfifo  behaviour	 though	 another  qdisc can be attached in place. This
       qdisc may again contain classes, but each class can have only one  leaf
       qdisc.

       When  a	packet	enters a classful qdisc it can be classified to one of
       the classes within. Three criteria  are	available,  although  not  all
       qdiscs will use all three:

       tc filters
	      If  tc filters are attached to a class, they are consulted first
	      for relevant instructions. Filters can match on all fields of  a
	      packet  header,  as  well	 as  on	 the  firewall mark applied by
	      ipchains or iptables.

       Type of Service
	      Some qdiscs have built in rules for classifying packets based on
	      the TOS field.

       skb->priority
	      Userspace	 programs can encode a class-id in the 'skb->priority'
	      field using the SO_PRIORITY option.

       Each node within the tree can have its own  filters  but	 higher	 level
       filters may also point directly to lower classes.

       If  classification  did	not  succeed, packets are enqueued to the leaf
       qdisc attached  to  that	 class.	 Check	qdisc  specific	 manpages  for
       details, however.

NAMING
       All qdiscs, classes and filters have IDs, which can either be specified
       or be automatically assigned.

       IDs consist of a major number and a minor number, separated by a colon.

       QDISCS A qdisc, which potentially can have children,  gets  assigned  a
	      major number, called a 'handle', leaving the minor number names‐
	      pace available for classes. The handle is	 expressed  as	'10:'.
	      It is customary to explicitly assign a handle to qdiscs expected
	      to have children.

       CLASSES
	      Classes residing under a qdisc share their qdisc	major  number,
	      but  each	 have  a separate minor number called a 'classid' that
	      has no relation to their parent classes, only  to	 their	parent
	      qdisc. The same naming custom as for qdiscs applies.

       FILTERS
	      Filters  have a three part ID, which is only needed when using a
	      hashed filter hierarchy.

UNITS
       All parameters accept a floating point number, possibly followed	 by  a
       unit.

       Bandwidths or rates can be specified in:

       kbps   Kilobytes per second

       mbps   Megabytes per second

       kbit   Kilobits per second

       mbit   Megabits per second

       bps or a bare number
	      Bytes per second

       Amounts of data can be specified in:

       kb or k
	      Kilobytes

       mb or m
	      Megabytes

       mbit   Megabits

       kbit   Kilobits

       b or a bare number
	      Bytes.

       Lengths of time can be specified in:

       s, sec or secs
	      Whole seconds

       ms, msec or msecs
	      Milliseconds

       us, usec, usecs or a bare number
	      Microseconds.

TC COMMANDS
       The following commands are available for qdiscs, classes and filter:

       add    Add a qdisc, class or filter to a node. For all entities, a par‐
	      ent must be passed, either by passing its	 ID  or	 by  attaching
	      directly	to  the	 root of a device.  When creating a qdisc or a
	      filter, it can be named with the handle parameter.  A  class  is
	      named with the classid parameter.

       remove A	 qdisc can be removed by specifying its handle, which may also
	      be 'root'. All subclasses and their leaf	qdiscs	are  automati‐
	      cally deleted, as well as any filters attached to them.

       change Some  entities  can be modified 'in place'. Shares the syntax of
	      'add', with the exception that the handle cannot be changed  and
	      neither  can  the	 parent.  In other words, change cannot move a
	      node.

       replace
	      Performs a nearly atomic remove/add on an existing node  id.  If
	      the node does not exist yet it is created.

       link   Only  available for qdiscs and performs a replace where the node
	      must exist already.

FORMAT
       The show command has additional formatting options:

       -s, -stats, -statistics
	      output more statistics about packet usage.

       -d, -details
	      output more detailed information about rates and cell sizes.

       -r, -raw
	      output raw hex values for handles.

       -p, -pretty
	      decode filter offset and mask values to equivalent  filter  com‐
	      mands based on TCP/IP.

       -iec   print rates in IEC units (ie. 1K = 1024).

HISTORY
       tc was written by Alexey N. Kuznetsov and added in Linux 2.2.

SEE ALSO
       tc-cbq(8), tc-htb(8), tc-sfq(8), tc-red(8), tc-tbf(8), tc-pfifo(8), tc-
       bfifo(8), tc-pfifo_fast(8),
       User documentation at http://lartc.org/, but please  direct  bugreports
       and patches to: <netdev@vger.kernel.org>

AUTHOR
       Manpage maintained by bert hubert (ahu@ds9a.nl)

iproute2		       16 December 2001				 TC(8)
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