tc



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 [-s | -d ] qdisc show [ dev DEV ]

       tc [-s | -d ] class show dev DEV

       tc filter show dev DEV



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. See tc-filters(8).

       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, for which see tc-filters(8).


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.




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), tc-filters(8)



AUTHOR

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




iproute2                       16 December 2001                          TC(8)

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