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TC(8)                                         Linux                                         TC(8)

       tc - show / manipulate traffic control settings

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

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

       tc [ OPTIONS ] filter [ add | change | replace | delete ] dev DEV [ parent qdisc-id | root
       ] protocol protocol prio priority filtertype [ filtertype  specific  parameters  ]  flowid

       tc [ OPTIONS ] [ FORMAT ] qdisc show [ dev DEV ]

       tc [ OPTIONS ] [ FORMAT ] class show dev DEV

       tc [ OPTIONS ] filter show dev DEV

        OPTIONS := { [ -force ] [ -OK ] -b[atch] [ filename ] | [ -n[etns] name ] }

        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 available filters are:

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

       bpf    Filter packets using (e)BPF, see tc-bpf(8) for details.

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

       flow, flower
              Flow-based classifiers, filtering  packets  based  on  their  flow  (identified  by
              selectable keys). See tc-flow(8) and tc-flower(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.

              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.

              Traffic control filter that matches every packet. See tc-matchall(8) for details.

       The classless qdiscs are:

       choke  CHOKe (CHOose and Keep for responsive  flows,  CHOose  and  Kill  for  unresponsive
              flows)  is  a  classless  qdisc  designed  to both identify and penalize flows that
              monopolize the queue. CHOKe is a variation of RED, and the configuration is similar
              to RED.

       codel  CoDel (pronounced "coddle") is an adaptive "no-knobs" active queue management algo-
              rithm (AQM) scheme that was developed to address the shortcomings of  RED  and  its

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

       fq     Fair Queue Scheduler realises TCP pacing and scales to millions of concurrent flows
              per qdisc.

              Fair Queuing Controlled Delay is queuing discipline that combines Fair Queuing with
              the CoDel AQM scheme. FQ_Codel uses a stochastic model to classify incoming packets
              into  different  flows  and is used to provide a fair share of the bandwidth to all
              the flows using the queue. Each such flow is managed by the  CoDel  queuing  disci-
              pline.  Reordering  within  a  flow  is  avoided since Codel internally uses a FIFO

       gred   Generalized Random Early Detection combines multiple RED queues in order to achieve
              multiple  drop  priorities.  This  is  required  to realize Assured Forwarding (RFC

       hhf    Heavy-Hitter Filter differentiates between small flows and the opposite, heavy-hit-
              ters. The goal is to catch the heavy-hitters and move them to a separate queue with
              less priority so that bulk traffic does not affect the latency of critical traffic.

              This is a special qdisc as it applies to incoming traffic on an interface, allowing
              for it to be filtered and policed.

       mqprio The  Multiqueue  Priority  Qdisc is a simple queuing discipline that allows mapping
              traffic flows to hardware queue ranges using priorities and a configurable priority
              to  traffic  class  mapping. A traffic class in this context is a set of contiguous
              qdisc classes which map 1:1 to a set of hardware exposed queues.

       multiq Multiqueue is a qdisc optimized for devices with multiple Tx queues.  It  has  been
              added  for  hardware  that  wishes  to  avoid head-of-line blocking.  It will cycle
              though the bands and verify that the hardware queue associated with the band is not
              stopped prior to dequeuing a packet.

       netem  Network  Emulator  is  an  enhancement of the Linux traffic control facilities that
              allow to add delay, packet loss, duplication  and  more  other  characteristics  to
              packets outgoing from a selected network interface.

              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.

       pie    Proportional Integral controller-Enhanced (PIE) is a control theoretic active queue
              management scheme. It is based on the proportional integral controller but aims  to
              control delay.

       red    Random  Early  Detection simulates physical congestion by randomly dropping packets
              when nearing configured bandwidth allocation. Well suited to very  large  bandwidth

       rr     Round-Robin  qdisc  with support for multiqueue network devices. Removed from Linux
              since kernel version 2.6.27.

       sfb    Stochastic Fair Blue is a classless qdisc to manage congestion based on packet loss
              and  link  utilization  history  while trying to prevent non-responsive flows (i.e.
              flows that do not react to congestion marking or dropped  packets)  from  impacting
              performance  of responsive flows.  Unlike RED, where the marking probability has to
              be configured, BLUE tries to determine the ideal marking probability automatically.

       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:

       ATM    Map flows to virtual circuits of an underlying asynchronous transfer mode device.

       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.

       DRR    The Deficit Round Robin Scheduler is a more  flexible  replacement  for  Stochastic
              Fairness  Queuing.  Unlike  SFQ,  there  are  no built-in queues -- you need to add
              classes and then set up filters to classify packets accordingly.  This can be  use-
              ful e.g. for using RED qdiscs with different settings for particular traffic. There
              is no default class -- if a packet cannot be classified, it is dropped.

       DSMARK Classify packets based on TOS field, change TOS field of packets based on classifi-

       HFSC   Hierarchical  Fair  Service Curve guarantees precise bandwidth and delay allocation
              for leaf classes and allocates excess bandwidth fairly. Unlike HTB, it makes use of
              packet dropping to achieve low delays which interactive sessions benefit from.

       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.

       QFQ    Quick Fair Queueing is an O(1) scheduler that provides near-optimal guarantees, and
              is the first to achieve that goal with a constant cost also  with  respect  to  the
              number  of  groups  and the packet length. The QFQ algorithm has no loops, and uses
              very simple instructions and data structures that lend themselves very  well  to  a
              hardware implementation.

       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.

       -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.

       -OK    in  batch  mode, print OK and a new line on standard output after each successfully
              interpreted command.

       -n, -net, -netns <NETNS>
              switches tc to the specified network namespace NETNS.  Actually it just  simplifies
              executing of:

              ip netns exec NETNS tc [ OPTIONS ] OBJECT { COMMAND | help }


              tc -n[etns] NETNS [ OPTIONS ] OBJECT { COMMAND | help }

       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-basic(8),  tc-bfifo(8),  tc-cbq(8),  tc-cgroup(8), tc-choke(8), tc-codel(8), tc-drr(8),
       tc-ematch(8), tc-flow(8), tc-flower(8), tc-fq(8),  tc-fq_codel(8),  tc-fw(8),  tc-hfsc(7),
       tc-hfsc(8),  tc-htb(8),  tc-pfifo(8), tc-pfifo_fast(8), tc-red(8), tc-route(8), tc-sfb(8),
       tc-sfq(8), tc-stab(8), tc-tbf(8), tc-tcindex(8), tc-u32(8),
       User documentation at, but please  direct  bugreports  and  patches  to:
       <netdev AT>

       Manpage maintained by bert hubert (ahu AT

iproute2                                 16 December 2001                                   TC(8)

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