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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 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] }


DESCRIPTION
       Tc  is  used to configure Traffic Control in the Linux kernel. Traffic Control con-
       sists 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 interac-
              tivity for traffic that needs it while still guaranteeing bandwidth to  bulk
              transfers.  Reordering  is  also  called  prioritizing,  and happens only on
              egress.


       POLICING
              Whereas 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 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 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 clas-
       sified. 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 available filters are:

       basic  Filter packets based on an ematch expression. See tc-ematch(8) for  details.

       cgroup Filter packets based on the control group of their process. See tc-cgroup(8)
              for details.

       flow   Flow-based classifier, filtering packets based on their flow (identified  by
              selectable keys). See tc-flow(8) for details.

       fw     Filter based on fwmark. Directly maps fwmark value to traffic class. See tc-
              fw(8).

       route  Filter packets based on routing table. See tc-route(8) for details.

       rsvp   Match Resource Reservation Protocol (RSVP) packets.

       tcindex
              Filter packets based on traffic control index. See tc-index(8).

       u32    Generic filtering on arbitrary packet data, assisted by syntax  to  abstract
              common operations. See tc-u32(8) for details.


CLASSLESS QDISCS
       The classless qdiscs are:

       [p|b]fifo
              Simplest  usable qdisc, pure First In, First Out behaviour. Limited in pack-
              ets 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 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 inter-
              faces.

       HTB    The Hierarchy Token  Bucket  implements  a  rich  linksharing  hierarchy  of
              classes  with  an  emphasis on conforming to existing practices. HTB facili-
              tates 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
              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 multi-
       ple  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 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 auto-
       matically 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 special values: root is sig-
       nified 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 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 fil-
              ter 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).


       -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  suc-
              cessfully interpreted command.


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

SEE ALSO
       tc-basic(8),  tc-bfifo(8),  tc-cbq(8),  tc-cgroup(8),  tc-drr(8), tc-ematch(8), tc-
       flow(8),  tc-fw(8),  tc-htb(8),  tc-pfifo(8),  tc-pfifo_fast(8),   tc-red(8),   tc-
       route(8), tc-sfq(8), tc-tbf(8), tc-tcindex(8), tc-u32(8),
       User  documentation  at http://lartc.org/, but please direct bugreports and patches
       to: <netdev AT vger.org>


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




iproute2                       16 December 2001                          TC(8)

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