TC(8) Linux TC(8)
tc - show / manipulate traffic control settings
tc qdisc [ add | change | replace | link ] dev DEV [ parent qdisc-id |
root ] [ handle qdisc-id ] qdisc [ qdisc specific parameters ]
tc class [ add | change | replace ] dev DEV parent qdisc-id [ classid
class-id ] qdisc [ qdisc specific parameters ]
tc filter [ add | change | replace ] dev DEV [ parent qdisc-id | root ]
protocol protocol prio priority filtertype [ filtertype specific param-
eters ] flowid flow-id
tc [-s | -d ] qdisc show [ dev DEV ]
tc [-s | -d ] class show dev DEV
tc filter show dev DEV
Tc is used to configure Traffic Control in the Linux kernel. Traffic
Control consists of the following:
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.
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.
Where shaping deals with transmission of traffic, policing per-
tains to traffic arriving. Policing thus occurs on ingress.
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.
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
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.
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.
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 classless qdiscs are:
Simplest usable qdisc, pure First In, First Out behaviour. Lim-
ited in packets or in bytes.
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.
red Random Early Detection simulates physical congestion by randomly
dropping packets when nearing configured bandwidth allocation.
Well suited to very large bandwidth applications.
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-
The classful qdiscs are:
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.
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-
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
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
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 though another qdisc can be attached in place. This
qdisc may again contain classes, but each class can have only one leaf
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:
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. See tc-filters(8).
Type of Service
Some qdiscs have built in rules for classifying packets based on
the TOS field.
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
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.
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 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 have a three part ID, which is only needed when using a
hashed filter hierarchy, for which see tc-filters(8).
All parameters accept a floating point number, possibly followed by a
Bandwidths or rates can be specified in:
kbps Kilobytes per second
mbps Megabytes per second
kbit Kilobits per second
mbit Megabits per second
bps or a bare number
Bytes per second
Amounts of data can be specified in:
kb or k
mb or m
b or a bare number
Lengths of time can be specified in:
s, sec or secs
ms, msec or msecs
us, usec, usecs or a bare number
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.
remove A qdisc can be removed 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
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.
tc was written by Alexey N. Kuznetsov and added in Linux 2.2.
tc-cbq(8), tc-htb(8), tc-sfq(8), tc-red(8), tc-tbf(8), tc-pfifo(8), tc-
bfifo(8), tc-pfifo_fast(8), http://lartc.org/
Manpage maintained by bert hubert (ahuATds9a.nl)
iproute2 16 December 2001 TC(8)