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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 consists of the following:
SHAPING
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.
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 per-
tains to traffic arriving. Policing thus occurs on ingress.
DROPPING
Traffic exceeding a set bandwidth may also be dropped forth-
with, 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 - traf-
fic 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 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.
CLASSLESS QDISCS
The classless qdiscs are:
[p|b]fifo
Simplest usable qdisc, pure First In, First Out behaviour. Lim-
ited 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 ran-
domly dropping packets when nearing configured bandwidth allo-
cation. 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-
figured 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 calcu-
lations 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 hier-
archy of classes with an emphasis on conforming 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.
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 num-
ber 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 speci-
fied 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
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 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
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@ds9a.nl)
iproute2 16 December 2001 TC(8)
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