tc - phpMan

TC(8) Linux TC(8)

NAME
tc - show / manipulate traffic control settings

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 ] block BLOCK_INDEX [ 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

tc [ OPTIONS ] filter show block BLOCK_INDEX

OPTIONS := { [ -force ] [ -OK ] -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-tcindex(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 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.

get Displays a single filter given the interface DEV, qdisc-id, priority, 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 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 }

-cf, -conf <FILENAME>
specifies path to the config file. This option is used in conjunction 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:
<netdev AT vger.org>

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

iproute2 16 December 2001 TC(8)

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