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 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, 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
automatically 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 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.
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 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).
- -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 execution 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@vger.kernel.org>
AUTHOR¶
Manpage maintained by bert hubert (ahu@ds9a.nl)