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 ] [ ingress_block BLOCK_IN-
DEX ] [ egress_block BLOCK_INDEX ] qdisc [ qdisc specific parameters ]
tc [ OPTIONS ] class [ add | change | replace | delete | show ] 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 ] filter [ add | change | replace | delete | get ] block
BLOCK_INDEX [ handle filter-id ] protocol protocol prio priority filter-
type [ filtertype specific parameters ] flowid flow-id
tc [ OPTIONS ] chain [ add | delete | get ] dev DEV [ parent qdisc-id |
root ] filtertype [ filtertype specific parameters ]
tc [ OPTIONS ] chain [ add | delete | get ] block BLOCK_INDEX filtertype
[ filtertype specific parameters ]
tc [ OPTIONS ] [ FORMAT ] qdisc { show | list } [ dev DEV ] [ root |
ingress | handle QHANDLE | parent CLASSID ] [ invisible ]
tc [ OPTIONS ] [ FORMAT ] class show dev DEV
tc [ OPTIONS ] filter show dev DEV
tc [ OPTIONS ] filter show block BLOCK_INDEX
tc [ OPTIONS ] chain show dev DEV
tc [ OPTIONS ] chain show block BLOCK_INDEX
tc [ OPTIONS ] monitor [ file FILENAME ]
OPTIONS := { [ -force ] -b[atch] [ filename ] | [ -n[etns] name ] | [
-N[umeric] ] | [ -nm | -nam[es] ] | [ { -cf | -c[onf] } [ filename ] ] [
-t[imestamp] ] | [ -t[short] | [ -o[neline] ] | [ -echo ] }
FORMAT := { -s[tatistics] | -d[etails] | -r[aw] | -i[ec] | -g[raph] |
-j[json] | -p[retty] | -col[or] }
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 net-
work behaviour. Shaping occurs on egress.
SCHEDULING
By scheduling the transmission of packets it is possible to im-
prove interactivity for traffic that needs it while still guaran-
teeing bandwidth to bulk transfers. Reordering is also called
prioritizing, and happens only on egress.
POLICING
Whereas shaping deals with transmission of traffic, policing per-
tains 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 interface.
Immediately afterwards, the kernel tries to get as many packets as pos-
sible 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 pri-
oritize 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 de-
tails.
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.
QEVENTS
Qdiscs may invoke user-configured actions when certain interesting
events take place in the qdisc. Each qevent can either be unused, or can
have a block attached to it. To this block are then attached filters us-
ing the "tc block BLOCK_IDX" syntax. The block is executed when the
qevent associated with the attachment point takes place. For example,
packet could be dropped, or delayed, etc., depending on the qdisc and
the qevent in question.
For example:
tc qdisc add dev eth0 root handle 1: red limit 500K avpkt 1K \
qevent early_drop block 10
tc filter add block 10 matchall action mirred egress mirror dev
eth1
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 ad-
dress 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 combines
Fair Queuing with the CoDel AQM scheme. FQ_Codel uses a stochas-
tic 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 in-
ternally uses a FIFO queue.
fq_pie FQ-PIE (Flow Queuing with Proportional Integral controller En-
hanced) is a queuing discipline that combines Flow Queuing with
the PIE AQM scheme. FQ-PIE uses a Jenkins hash function to clas-
sify incoming packets into different flows and is used to provide
a fair share of the bandwidth to all the flows using the qdisc.
Each such flow is managed by the PIE algorithm.
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 pri-
orities 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 one to add delay, packet loss, duplication
and more other characteristics to packets outgoing from a se-
lected 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 the-
oretic active queue management scheme. It is based on the propor-
tional 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.
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 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.
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.
ETS The ETS qdisc is a queuing discipline that merges functionality
of PRIO and DRR qdiscs in one scheduler. ETS makes it easy to
configure a set of strict and bandwidth-sharing bands to imple-
ment the transmission selection described in 802.1Qaz.
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 num-
ber 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 con-
figuration, Type Of Service bits are honored by default.
QFQ Quick Fair Queueing is an O(1) scheduler that provides near-opti-
mal guarantees, and is the first to achieve that goal with a con-
stant cost also with respect to the number of groups and the
packet length. The QFQ algorithm has no loops, and uses very sim-
ple 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 (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 ipta-
bles.
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 fil-
ters 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 lim-
ited to 16 bits. There are two special values: root is signified by ma-
jor and minor of all ones, and unspecified is all zeros.
QDISCS A qdisc, which potentially can have children, gets assigned a ma-
jor 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 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 fol-
lowed 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 di-
rectly to the root of a device. When creating a qdisc or a fil-
ter, 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 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, 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.
MONITOR
The tc utility can monitor events generated by the kernel such as
adding/deleting qdiscs, filters or actions, or modifying existing ones.
The following command is available for monitor :
file If the file option is given, the tc does not listen to kernel
events, but opens the given file and dumps its contents. The file
has to be in binary format and contain netlink messages.
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.
-o, -oneline
output each record on a single line, replacing line feeds with
the '\' character. This is convenient when you want to count
records with wc(1) or to grep(1) the output.
-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 }
-N, -Numeric
Print the number of protocol, scope, dsfield, etc directly in-
stead of converting it to human readable name.
-cf, -conf <FILENAME>
specifies path to the config file. This option is used in con-
junction with other options (e.g. -nm).
-t, -timestamp
When tc monitor runs, print timestamp before the event message in
format:
Timestamp: <Day> <Month> <DD> <hh:mm:ss> <YYYY> <usecs> usec
-ts, -tshort
When tc monitor runs, prints short timestamp before the event
message in format:
[<YYYY>-<MM>-<DD>T<hh:mm:ss>.<ms>]
-echo Request the kernel to send the applied configuration back.
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
for u32 filter, decode offset and mask values to equivalent fil-
ter commands based on TCP/IP. In JSON output, add whitespace to
improve readability.
-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.
-c[color][={always|auto|never}
Configure color output. If parameter is omitted or always, color
output is enabled regardless of stdout state. If parameter is
auto, stdout is checked to be a terminal before enabling color
output. If parameter is never, color output is disabled. If spec-
ified multiple times, the last one takes precedence. This flag is
ignored if -json is also given.
-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 possible
to pass -nm option for creating tc alias.
-br, -brief
Print only essential data needed to identify the filter and ac-
tion (handle, cookie, etc.) and stats. This option is currently
only supported by tc filter show and tc actions ls commands.
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-cake(8), tc-cgroup(8), tc-
choke(8), tc-codel(8), tc-drr(8), tc-ematch(8), tc-ets(8), tc-flow(8),
tc-flower(8), tc-fq(8), tc-fq_codel(8), tc-fq_pie(8), tc-fw(8), tc-
gact(8), tc-hfsc(7), tc-hfsc(8), tc-htb(8), tc-mqprio(8), tc-pfifo(8),
tc-pfifo_fast(8), tc-pie(8), tc-red(8), tc-route(8), tc-sfb(8), tc-
sfq(8), tc-stab(8), tc-tbf(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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