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 [ fil-
       tertype 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 filters.


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  how-
       ever 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 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 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  band-
              width 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 configured 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 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 bandwidth 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 pack-
              ets 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 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 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 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 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 subclasses 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: <netdevATvger.org>


AUTHOR
       Manpage maintained by bert hubert (ahuATds9a.nl)




iproute2                                                16 December 2001                                                   TC(8)