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 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.
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 pertains 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.
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
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.
Some qdiscs can contain classes, which contain further qdiscs - traffic may
then be enqueued in any of the inner qdiscs, which are within the
When the kernel tries to dequeue a packet from such a
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
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
qdiscs - they are not masters of what happens.
The classless qdiscs are:
Simplest usable qdisc, pure First In, First Out behaviour. Limited 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.
Random Early Detection simulates physical congestion by randomly dropping
packets when nearing configured bandwidth allocation. Well suited to very
large bandwidth applications.
Stochastic Fairness Queueing reorders queued traffic so each 'session'
gets to send a packet in turn.
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
To remove, issue
tc qdisc del dev
qdisc is the automatic default in the absence of a configured qdisc.
The classful qdiscs are:
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.
The Hierarchy Token Bucket implements a rich linksharing hierarchy 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.
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
Qdiscs which allow dynamic addition of classes can have zero or more
subclasses to which traffic may be enqueued.
Furthermore, each class contains a
which by default has
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
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.
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 details, however.
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.
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
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
Filters have a three part ID, which is only needed when using a hashed
All parameters accept a floating point number, possibly followed by a unit.
Bandwidths or rates can be specified in:
Kilobytes per second
Megabytes per second
Kilobits per second
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 a qdisc, class or filter to a node. For all entities, a
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
parameter. A class is named with the
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.
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,
cannot move a node.
Performs a nearly atomic remove/add on an existing node id. If the node does not exist yet
it is created.
Only available for qdiscs and performs a replace where the node
must exist already.
The show command has additional formatting options:
-s, -stats, -statistics
output more statistics about packet usage.
output more detailed information about rates and cell sizes.
output raw hex values for handles.
decode filter offset and mask values to equivalent filter commands based on TCP/IP.
print rates in IEC units (ie. 1K = 1024).
was written by Alexey N. Kuznetsov and added in Linux 2.2.