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Linux 2.6 - man page for tc (linux section 8)

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 parameters ] 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 control. 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 pertains 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 fil-
       ters.

QDISCS
       qdisc is short for 'queueing discipline' and it is  elementary  to  understanding  traffic
       control. Whenever the kernel needs to send a packet to an interface, it is enqueued to the
       qdisc configured for that interface. 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 classful qdisc it can come from any of the classes. A
       qdisc may for example prioritize certain kinds of traffic by trying to dequeue  from  cer-
       tain 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 subclasses, 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. Limited 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  config-
	      ured 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 configured qdisc.

CLASSFUL QDISCS
       The classful qdiscs are:

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

       HTB    The  Hierarchy Token Bucket implements a rich linksharing hierarchy of classes with
	      an emphasis on conforming to existing practices. HTB facilitates guaranteeing band-
	      width  to classes, while also allowing specification of upper limits to inter-class
	      sharing. It contains shaping elements, based on TBF and can prioritize 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
	      facilitate 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	automati-
       cally 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 namespace 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 sep-
	      arate 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 hier-
	      archy.

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 parent 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 sub-
	      classes  and  their  leaf  qdiscs are automatically 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 commands 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-drr(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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