Provided by: iproute_20111117-1ubuntu2_amd64 bug

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
       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  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
              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 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
       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-choke(8),  tc-drr(8),  tc-htb(8),  tc-hfsc(8),  tc-hfsc(7),  tc-sfq(8),  tc-
       red(8), tc-tbf(8), tc-pfifo(8), tc-bfifo(8), tc-pfifo_fast(8), tc-stab(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)