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 [
FORMAT ]
qdisc show [ dev DEV
]
tc [
FORMAT ]
class show dev DEV
tc filter show dev DEV
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
- 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 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 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 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 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 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.
- 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
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.
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.
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).
HISTORY¶
tc was written by Alexey N. Kuznetsov and added in Linux 2.2.
SEE ALSO¶
tc-cbq(8),
tc-choke(8),
tc-drr(8),
tc-htb(8),
tc-hfsc(8),
tc-hfsc(7),
tc-sfb(8),
tc-sfq(8),
tc-red(8),
tc-tbf(8),
tc-pfifo(8),
tc-bfifo(8),
tc-pfifo_fast(8),
tc-stab(8),
User documentation at
http://lartc.org/, but please direct bugreports and
patches to:
<netdev@vger.kernel.org>
AUTHOR¶
Manpage maintained by bert hubert (ahu@ds9a.nl)