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TC(8)				     Linux				 TC(8)

       tc - show / manipulate traffic control settings

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

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

       tc filter [ add | change | replace | delete ] dev DEV [ parent qdisc-id
       |  root	] protocol protocol prio priority filtertype [ filtertype spe‐
       cific parameters ] flowid flow-id

       tc [ FORMAT ] qdisc show [ dev DEV ]

       tc [ FORMAT ] class show dev DEV

       tc filter show dev DEV

       tc [ -force ] [ -OK ] -b[atch] [ filename ]

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

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

	      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.

	      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.

	      Whereas  shaping	deals  with  transmission of traffic, policing
	      pertains to traffic arriving. Policing thus occurs on ingress.

	      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.

       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

       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.

       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.

       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.

       The classless qdiscs are:

	      Simplest usable qdisc, pure First In, First Out behaviour.  Lim‐
	      ited in packets or in bytes.

	      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.

       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.

       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

       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, although another qdisc can be attached in place.  This
       qdisc  may again contain classes, but each class can have only one leaf

       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.

	      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.

       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.
       Both major and minor number are limited to 16 bits. There are two  spe‐
       cial  values:  root  is	signified  by major and minor of all ones, and
       unspecified is all zeros.

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

       The following parameters are widely used in TC. For  other  parameters,
       see the man pages for individual qdiscs.

       RATES  Bandwidths  or  rates.  These parameters accept a floating point
	      number, possibly followed by a unit (both SI and IEC units  sup‐

	      bit or a bare number
		     Bits per second

	      kbit   Kilobits per second

	      mbit   Megabits per second

	      gbit   Gigabits per second

	      tbit   Terabits per second

	      bps    Bytes per second

	      kbps   Kilobytes per second

	      mbps   Megabytes per second

	      gbps   Gigabytes per second

	      tbps   Terabytes per second

	      To  specify in IEC units, replace the SI prefix (k-, m-, g-, t-)
	      with IEC prefix (ki-, mi-, gi- and ti-) respectively.

	      TC store rates as a 32-bit unsigned integer in  bps  internally,
	      so we can specify a max rate of 4294967295 bps.

       TIMES  Length of time. Can be specified as a floating point number fol‐
	      lowed by an optional unit:

	      s, sec or secs
		     Whole seconds

	      ms, msec or msecs

	      us, usec, usecs or a bare number

	      TC defined its own time unit (equal to microsecond)  and	stores
	      time  values  as	32-bit unsigned integer, thus we can specify a
	      max time value of 4294967295 usecs.

       SIZES  Amounts of data. Can be specified as  a  floating	 point	number
	      followed by an optional unit:

	      b or a bare number

	      kbit   Kilobits

	      kb or k

	      mbit   Megabits

	      mb or m

	      gbit   Gigabits

	      gb or g

	      TC  stores  sizes internally as 32-bit unsigned integer in byte,
	      so we can specify a max size of 4294967295 bytes.

       VALUES Other values without a unit.  These parameters  are  interpreted
	      as decimal by default, but you can indicate TC to interpret them
	      as octal and hexadecimal by adding a '0' or '0x' prefix  respec‐

       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.

       delete A	 qdisc can be deleted 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

	      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.

       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).

       -b, -b filename, -batch, -batch filename
	      read  commands  from  provided file or standard input and invoke
	      them.  First failure will cause termination of tc.

       -force don't terminate tc on errors in batch mode.  If there  were  any
	      errors  during execution of the commands, the application return
	      code will be non zero.

       -OK    in batch mode, print OK and a new line on standard output	 after
	      each successfully interpreted command.

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

       tc-bfifo(8),   tc-cbq(8),   tc-choke(8),	 tc-codel(8),  tc-drr(8),  tc-
       ematch(8),  tc-fq_codel(8),  tc-hfsc(7),	 tc-hfsc(8),  tc-htb(8),   tc-
       pfifo(8),   tc-pfifo_fast(8),   tc-red(8),  tc-sfb(8),  tc-sfq(8),  tc-
       stab(8), tc-tbf(8),
       User documentation at, but please  direct  bugreports
       and patches to: <>

       Manpage maintained by bert hubert (

iproute2		       16 December 2001				 TC(8)

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