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TC(8)                                         Linux                                         TC(8)

       tc - show / manipulate traffic control settings

       tc  qdisc  [ add | change | replace | link | delete ] dev DEV [ parent qdisc-id | root ] [
       handle qdisc-id ] qdisc [ qdisc specific parameters ]

       tc class [ add | change | replace | delete ] dev DEV parent qdisc-id [ classid class-id  ]
       qdisc [ qdisc specific parameters ]

       tc  filter [ add | change | replace | delete ] 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 ] -b[atch] [ filename ]

        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.

              Whereas  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 fil‐

       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

       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  cer‐
       tain 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 qdisc.

       It is important to notice that filters reside within qdiscs - they are not masters of what

       The classless qdiscs are:

              Simplest usable qdisc, pure First In, First Out behaviour. Limited in packets or in

              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

       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  config‐
              ured 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  con‐
              tains  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 band‐
              width  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.

       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,
       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.

              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  automati‐
       cally 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  signified  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 residing under a qdisc share their qdisc major number, but each have a sep‐
              arate 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 hier‐

       The  following  parameters  are widely used in TC. For other parameters, see the man pages
       for individual qdiscs.

       RATES  Bandwidths or rates.  These parameters accept a  floating  point  number,  possibly
              followed by a unit (both SI and IEC units supported).

              bit or a bare number
                     Bits per second

              kbit   Kilobits per second

              mbit   Megabits per second

              gbit   Gigabits per second

              tbit   Terabits per second

              bps    Bytes per second

              kbps   Kilobytes per second

              mbps   Megabytes per second

              gbps   Gigabytes per second

              tbps   Terabytes per second

              To  specify  in  IEC  units, replace the SI prefix (k-, m-, g-, t-) with IEC prefix
              (ki-, mi-, gi- and ti-) respectively.

              TC store rates as a 32-bit unsigned integer in bps internally, so we can specify  a
              max rate of 4294967295 bps.

       TIMES  Length of time. Can be specified as a floating point number followed by an optional

              s, sec or secs
                     Whole seconds

              ms, msec or msecs

              us, usec, usecs or a bare number

              TC defined its own time unit (equal to  microsecond)  and  stores  time  values  as
              32-bit unsigned integer, thus we can specify a max time value of 4294967295 usecs.

       SIZES  Amounts  of  data.  Can  be  specified  as  a  floating point number followed by an
              optional unit:

              b or a bare number

              kbit   Kilobits

              kb or k

              mbit   Megabits

              mb or m

              gbit   Gigabits

              gb or g

              TC stores sizes internally as 32-bit unsigned integer in byte, so we can specify  a
              max size of 4294967295 bytes.

       VALUES Other  values  without  a  unit.   These  parameters  are interpreted as decimal by
              default, but you can indicate TC to interpret them  as  octal  and  hexadecimal  by
              adding a '0' or '0x' prefix respectively.

       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.

       delete A qdisc can be deleted by specifying its handle, which may also be 'root'. All sub‐
              classes  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.

              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).

       -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 execu‐
              tion of the commands, the application return code will be non zero.

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

       tc-bfifo(8), tc-cbq(8), tc-choke(8), tc-codel(8), tc-drr(8), tc-ematch(8), tc-fq_codel(8),
       tc-hfsc(7), tc-hfsc(8), tc-htb(8), tc-mqprio(8), tc-pfifo(8), tc-pfifo_fast(8), tc-red(8),
       tc-sfb(8), tc-sfq(8), tc-stab(8), tc-tbf(8),
       User  documentation  at  http://lartc.org/,  but  please direct bugreports and patches to:
       <netdev AT vger.org>

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

iproute2                                 16 December 2001                                   TC(8)

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