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tc(8) - phpMan
TC(8) Linux TC(8)
NAME
tc - show / manipulate traffic control settings
SYNOPSIS
tc qdisc [ add | change | replace | link | delete ] dev DEV [ parent qdisc-id | root ] [
handle qdisc-id ] qdisc [ qdisc specific parameters ]
tc class [ add | change | replace | delete ] dev DEV parent qdisc-id [ classid class-id ]
qdisc [ qdisc specific parameters ]
tc filter [ add | change | replace | delete ] dev DEV [ parent qdisc-id | root ] protocol
protocol prio priority filtertype [ filtertype specific parameters ] flowid flow-id
tc [ FORMAT ] qdisc show [ dev DEV ]
tc [ FORMAT ] class show dev DEV
tc filter show dev DEV
tc [ -force ] -b[atch] [ filename ]
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 fil‐
ters.
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 cer‐
tain 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 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 config‐
ured 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 con‐
tains 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 facilitates guaranteeing band‐
width 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 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,
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 iptables.
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 automati‐
cally assigned.
IDs consist of a major number and a minor number, separated by a colon. Both major and
minor number are limited to 16 bits. There are two special values: root is signified by
major and minor of all ones, and unspecified is all zeros.
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 sep‐
arate 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 hier‐
archy.
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
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 followed 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 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 sub‐
classes 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.
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).
-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 execu‐
tion of the commands, the application return code will be non zero.
HISTORY
tc was written by Alexey N. Kuznetsov and added in Linux 2.2.
SEE ALSO
tc-bfifo(8), tc-cbq(8), tc-choke(8), tc-codel(8), tc-drr(8), tc-ematch(8), tc-fq_codel(8),
tc-hfsc(7), tc-hfsc(8), tc-htb(8), tc-mqprio(8), tc-pfifo(8), tc-pfifo_fast(8), tc-red(8),
tc-sfb(8), tc-sfq(8), tc-stab(8), tc-tbf(8),
User documentation at http://lartc.org/, but please direct bugreports and patches to:
<netdev AT vger.org>
AUTHOR
Manpage maintained by bert hubert (ahu AT ds9a.nl)
iproute2 16 December 2001 TC(8)
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