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 ] [ -OK ] -b[atch] [ filename ] | [ -n[etns] name ] }
FORMAT := { -s[tatistics] | -d[etails] | -r[aw] | -p[retty] | -i[ec] }
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 filters.
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.
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 ver‐
dict. 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 Flow-based classifier, filtering packets based on their flow (identified by selectable keys). See tc-
flow(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 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. 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 discipline. Reordering within a flow is avoided since Codel internally uses a FIFO
mqprio The Multiqueue Priority Qdisc is a simple queuing discipline that allows mapping traffic flows to hard‐
ware 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 hard‐
ware 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 config‐
ured 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 utiliza‐
tion 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 auto‐
matically.
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 configured qdisc.
CLASSFUL QDISCS
The classful qdiscs are:
ATM Map flows to virtual circuits of an underlying asynchronous transfer mode device.
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 con‐
forming to existing practices. HTB facilitates guaranteeing bandwidth 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 them‐
selves 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 chil‐
dren.
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 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 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.
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 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
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 indi‐
cate 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 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 han‐
dle 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 cre‐
ated.
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 termina‐
tion of tc.
to
tc -n[etns] NETNS [ OPTIONS ] OBJECT { COMMAND | help }
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).
HISTORY
tc was written by Alexey N. Kuznetsov and added in Linux 2.2.
SEE ALSO
tc-basic(8), tc-bfifo(8), tc-cbq(8), tc-cgroup(8), tc-choke(8), tc-codel(8), tc-drr(8), tc-ematch(8), tc-
flow(8), tc-fq(8), tc-fq_codel(8), tc-fw(8), tc-hfsc(7), tc-hfsc(8), tc-htb(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: <[email protected]>
AUTHOR
Manpage maintained by bert hubert ([email protected])
iproute2 16 December 2001 TC(8)
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