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 | get ] dev DEV
       [  parent qdisc-id | root ] [ handle filter-id ] 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] | -j[json] }

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.

       matchall
              Traffic control  filter  that  matches  every  packet.  See  tc-
              matchall(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  either  a unit (both SI and IEC
              units supported), or a float followed  by  a  '%'  character  to
              specify the rate as a percentage of the device's speed (e.g. 5%,
              99.5%). Warning: specifying the rate as  a  percentage  means  a
              fraction  of  the current speed; if the speed changes, the value
              will not be recalculated.

              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.

       get    Displays a single filter given the interface DEV, qdisc-id, pri-
              ority, protocol and filter-id.

       show   Displays  all  filters  attached to the given interface. A valid
              parent ID must be passed.

       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-
              junction 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.

       -j, -json
              Display results in JSON format.

       -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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