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TC(8) Linux TC(8)
NAME
tc - show / manipulate traffic control settings
SYNOPSIS
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 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 kernel. Traffic Control con-
sists 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 interac-
tivity 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 traf-
fic 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. 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 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 clas-
sified. 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 pack-
ets or in bytes.
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.
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 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 bandwidth. 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 facilitate configuration, Type Of Service bits are
honored by default.
THEORY OF OPERATION
Classes form a tree, where each class has a single parent. A class may have multi-
ple 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 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. 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 specific manpages for details, however.
NAMING
All qdiscs, classes and filters have IDs, which can either be specified or be auto-
matically 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 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 fil-
ter 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 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 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.
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 AT ds9a.nl)
iproute2 16 December 2001 TC(8)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-11-24 05:03 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)