tc

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 con-
              trol. 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 filters.

QDISCS
       qdisc is short for 'queueing discipline' and it is elementary to under-
       standing traffic control. Whenever the kernel needs to send a packet to
       an interface, it is enqueued to the qdisc configured  for  that  inter-
       face.  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 class-
       ful 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 sub-
       classes, 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-tcin-
              dex(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 iden-
              tify 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 algorithm (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. Lim-
              ited 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 com-
              bines 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 discipline. 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-hitters. 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 ver-
              ify 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  perfor-
              mance of responsive flows.  Unlike RED, where the marking proba-
              bility 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 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  con-
       figured 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  contains shaping elements as well as prioritizing
              capabilities. Shaping is performed using link idle time calcula-
              tions  based  on  average  packet size and underlying link band-
              width. 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 useful 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 classification.

       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 hierar-
              chy  of classes with an emphasis on conforming to existing prac-
              tices. HTB facilitates guaranteeing bandwidth to classes,  while
              also allowing specification of upper limits to inter-class shar-
              ing. It contains shaping elements, based on TBF and can  priori-
              tize 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 facil-
              itate  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 automatically 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 names-
              pace 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.

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  sup-
              ported).

              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 fol-
              lowed 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  respec-
              tively.

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 par-
              ent 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 subclasses and their leaf  qdiscs  are  automati-
              cally 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 execution of the commands, the application  return
              code will be non zero.

       -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 }

              to

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

       -cf, -conf <FILENAME>
              specifies path to the config file. This option is used  in  con-
              juction 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 com-
              mands 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 possi-
              ble 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: <netdev@vger.kernel.org>

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

iproute2                       16 December 2001                          TC(8)
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