TC(8) Linux TC(8)
NAME
tc - show / manipulate traffic control settings
SYNOPSIS
tc qdisc [ add | change | replace | link ] dev DEV [ par-
ent 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 filter-
type [ filtertype specific parameters ] flowid flow-id
tc [-s | -d ] qdisc show [ dev DEV ]
tc [-s | -d ] class show dev DEV
tc filter show dev DEV
DESCRIPTION
Tc is used to configure Traffic Control in the Linux ker-
nel. 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
Where 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 elemen-
tary 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 process-
ing 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 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 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.
CLASSLESS QDISCS
The classless qdiscs are:
[p|b]fifo
Simplest usable qdisc, pure First In, First Out
behaviour. Limited in packets or in bytes.
pfifo_fast
Standard qdisc for 'Advanced Router' enabled ker-
nels. 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 conges-
tion by randomly dropping packets when nearing con-
figured bandwidth allocation. Well suited to very
large bandwidth applications.
sfq Stochastic Fairness Queueing reorders queued traf-
fic so each 'session' gets to send a packet in
turn.
tbf The Token Bucket Filter is suited for slowing traf-
fic 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:
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 calculations 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 hierarchy of classes with an emphasis
on conforming to existing practices. HTB facili-
tates 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 facil-
itate configuration, Type Of Service bits are hon-
ored by default.
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 though 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 avail-
able, 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 ipt-
ables. See tc-filters(8).
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 spe-
cific 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, sepa-
rated by a colon.
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 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, for which see
tc-filters(8).
UNITS
All parameters accept a floating point number, possibly
followed by a unit.
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
Kilobytes
mb or m
Megabytes
mbit Megabits
kbit Kilobits
b or a bare number
Bytes.
Lengths of time can be specified in:
s, sec or secs
Whole seconds
ms, msec or msecs
Milliseconds
us, usec, usecs or a bare number
Microseconds.
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 pass-
ing 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 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 par-
ent. 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.
HISTORY
tc was written by Alexey N. Kuznetsov and added in Linux
2.2.
SEE ALSO
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), tc-filters(8)
AUTHOR
Manpage maintained by bert hubert (ahu@ds9a.nl)
iproute2 16 December 2001 TC(8)http://yesican.chsoft.biz/lartc/tc.html
