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
FORMAT
:= { -s[tatistics] |
-d[etails] | -r[aw]
| -p[retty] | -i[ec]
}
add an example, a script, a trick and tips
examples
no example yet ...
... Feel free to add your own example above to help other Linux-lovers !
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.
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.
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.
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.
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.
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).
naming
All qdiscs, classes and filters have IDs, which can either be
specified or be automatically assigned.
ID’s 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.
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.
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.
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.
history
tc was
written by Alexey N. Kuznetsov and added in Linux 2.2.
see also
tc-bfifo,
tc-cbq, tc-choke, tc-codel,
tc-drr, tc-ematch,
tc-fq_codel, tc-hfsc, tc-hfsc,
tc-htb, tc-pfifo,
tc-pfifo_fast, tc-red, tc-sfb,
tc-sfq, tc-stab, tc-tbf,
User documentation at http://lartc.org/, but please
direct bugreports and patches to:
<netdev[:at:]vger.kernel[:dot:]org>
author
Manpage
maintained by bert hubert (ahu[:at:]ds9a[:dot:]nl)