tc <root
TC(8)                                         Linux                                         TC(8)

NAME
       tc - show / manipulate traffic control settings

SYNOPSIS
       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
       flow-id

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

       tc [ OPTIONS ] [ FORMAT ] class show dev DEV

       tc [ OPTIONS ] filter show dev DEV

        OPTIONS  := { [ -force ] -b[atch] [ filename ] | [ -n[etns] name ] | [ -nm | -nam[es] ] |
       [ { -cf | -c[onf] } [ filename ] ] }

        FORMAT := { -s[tatistics] | -d[etails] | -r[aw] | -p[retty] | -i[ec] | -g[raph] }

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
              Whereas  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 fil‐
       ters.

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  cer‐
       tain 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.

       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
              details.

       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.

       tcindex
              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.

CLASSLESS QDISCS
       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
              variants.

       [p|b]fifo
              Simplest usable qdisc, pure First In, First Out behaviour. Limited in packets or in
              bytes.

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

       fq_codel
              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
              queue.

       gred   Generalized Random Early Detection combines multiple RED queues in order to achieve
              multiple drop priorities. This is  required  to  realize  Assured  Forwarding  (RFC
              2597).

       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.

       ingress
              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.

       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.

       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
              applications.

       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.

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:

       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‐
              cation.

       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.

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,
       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.

       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 automati‐
       cally assigned.

       IDs consist of a major number and a minor number, separated  by  a  colon  -  major:minor.
       Both  major  and  minor  are hexadecimal numbers and 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
              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
              Filters have a three part ID, which is only needed when using a hashed filter hier‐
              archy.

PARAMETERS
       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
              unit:

              s, sec or secs
                     Whole seconds

              ms, msec or msecs
                     Milliseconds

              us, usec, usecs or a bare number
                     Microseconds.

              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
                     Bytes.

              kbit   Kilobits

              kb or k
                     Kilobytes

              mbit   Megabits

              mb or m
                     Megabytes

              gbit   Gigabits

              gb or g
                     Gigabytes

              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.

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.

       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.

       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.

OPTIONS
       -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.

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

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

              to

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

       -cf, -conf 
              specifies path to the config file. This option is used  in  conjuction  with  other
              options (e.g.  -nm).

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).

       -g, -graph
              shows  classes  as  ASCII  graph.  Prints generic stats info under each class if -s
              option was specified. Classes can be filtered only by dev option.

       -nm, -name
              resolve class name from /etc/iproute2/tc_cls file or from  file  specified  by  -cf
              option. This file is just a mapping of classid to class name:

                 # Here is comment
                 1:40   voip # Here is another comment
                 1:50   web
                 1:60   ftp
                 1:2    home

              tc  will  not fail if -nm was specified without -cf option but /etc/iproute2/tc_cls
              file does not exist, which makes it possible to pass -nm  option  for  creating  tc
              alias.

EXAMPLES
       tc -g class show dev eth0
           Shows classes as ASCII graph on eth0 interface.

       tc -g -s class show dev eth0
           Shows classes as ASCII graph with stats info under each class.

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

SEE ALSO
       tc-basic(8),  tc-bfifo(8),  tc-bpf(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-mqprio(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 http://lartc.org/, but please  direct  bugreports  and  patches  to:
       

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

iproute2                                 16 December 2001                                   TC(8)

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