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       tc - show / manipulate traffic control settings


       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] }


       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
              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.

              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.

              Where 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
       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.


       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.


       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

       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.
              Limited 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
       configured 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
              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 packets available. To
              facilitate 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 though 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.

       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 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.


       All parameters accept a floating point number, possibly followed  by  a

       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

       mb or m

       mbit   Megabits

       kbit   Kilobits

       b or a bare number

       Lengths of time can be specified in:

       s, sec or secs
              Whole seconds

       ms, msec or msecs

       us, usec, usecs or a bare number


       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

              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
              commands based on TCP/IP.

       -iec   print rates in IEC units (ie. 1K = 1024).


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


       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, but please direct bugreports
       and patches to: <>


       Manpage maintained by bert hubert (