tc - show / manipulate traffic control settings
tc [ OPTIONS ] qdisc [ add | change | replace |
link | delete ] dev DEV [ parent qdisc-id | root
] [ handle qdisc-id ] [ ingress_block
BLOCK_INDEX ] [ egress_block BLOCK_INDEX ] qdisc [
qdisc specific parameters ]
tc [ OPTIONS ] class [ add | change | replace |
delete | show ] dev DEV parent qdisc-id [
classid class-id ] qdisc [ qdisc specific parameters ]
tc [ OPTIONS ] filter [ add | change | replace |
delete | get ] dev DEV [ parent qdisc-id | root
] [ handle filter-id ] protocol protocol
prio priority filtertype [ filtertype specific parameters ]
flowid flow-id
tc [ OPTIONS ] filter [ add | change | replace |
delete | get ] block BLOCK_INDEX [ handle filter-id
] protocol protocol prio priority filtertype
[ filtertype specific parameters ] flowid flow-id
tc [ OPTIONS ] chain [ add | delete | get ]
dev DEV [ parent qdisc-id | root ]
filtertype [ filtertype specific parameters ]
tc [ OPTIONS ] chain [ add | delete | get ]
block BLOCK_INDEX filtertype [ filtertype specific parameters
]
tc [ OPTIONS ] [ FORMAT ] qdisc { show |
list } [ dev DEV ] [ root | ingress | handle
QHANDLE | parent CLASSID ] [ invisible ]
tc [ OPTIONS ] [ FORMAT ] class show
dev DEV
tc [ OPTIONS ] filter show dev DEV
tc [ OPTIONS ] filter show block
BLOCK_INDEX
tc [ OPTIONS ] chain show dev DEV
tc [ OPTIONS ] chain show block
BLOCK_INDEX
tc [ OPTIONS ] monitor [ file FILENAME
]
OPTIONS := { [ -force ] -b[atch] [
filename ] | [ -n[etns] name ] | [
-N[umeric] ] | [ -nm |
-nam[es] ] | [ { -cf |
-c[onf] } [ filename ] ] [ -t[imestamp]
] | [ -t[short] | [ -o[neline] ] }
FORMAT := { -s[tatistics] |
-d[etails] | -r[aw] | -i[ec] |
-g[raph] | -j[json] | -p[retty] |
-col[or] }
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.
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.
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.
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.
The available filters are:
- basic
- Filter packets based on an ematch expression. See tc-ematch(8) for
details.
- bpf
- Filter packets using (e)BPF, see tc-bpf(8) for details.
- cgroup
- Filter packets based on the control group of their process. See
tc-cgroup(8) for details.
- flow, flower
- Flow-based classifiers, filtering packets based on their flow (identified
by selectable keys). See tc-flow(8) and tc-flower(8) for
details.
- fw
- Filter based on fwmark. Directly maps fwmark value to traffic class. See
tc-fw(8).
- route
- Filter packets based on routing table. See tc-route(8) for
details.
- rsvp
- Match Resource Reservation Protocol (RSVP) packets.
- tcindex
- Filter packets based on traffic control index. See
tc-tcindex(8).
- u32
- Generic filtering on arbitrary packet data, assisted by syntax to abstract
common operations. See tc-u32(8) for details.
- matchall
- Traffic control filter that matches every packet. See
tc-matchall(8) for details.
Qdiscs may invoke user-configured actions when certain interesting
events take place in the qdisc. Each qevent can either be unused, or can
have a block attached to it. To this block are then attached filters using
the "tc block BLOCK_IDX" syntax. The block is executed when the
qevent associated with the attachment point takes place. For example, packet
could be dropped, or delayed, etc., depending on the qdisc and the qevent in
question.
For example:
tc qdisc add dev eth0 root handle 1: red limit 500K avpkt
1K \
qevent early_drop block 10
tc filter add block 10 matchall action mirred egress
mirror dev eth1
The classless qdiscs are:
- choke
- CHOKe (CHOose and Keep for responsive flows, CHOose and Kill for
unresponsive flows) is a classless qdisc designed to both identify and
penalize flows that monopolize the queue. CHOKe is a variation of RED, and
the configuration is similar to RED.
- codel
- CoDel (pronounced "coddle") is an adaptive "no-knobs"
active queue management algorithm (AQM) scheme that was developed to
address the shortcomings of RED and its variants.
- [p|b]fifo
- Simplest usable qdisc, pure First In, First Out behaviour. Limited in
packets or in bytes.
- fq
- Fair Queue Scheduler realises TCP pacing and scales to millions of
concurrent flows per qdisc.
- fq_codel
- Fair Queuing Controlled Delay is queuing discipline that combines Fair
Queuing with the CoDel AQM scheme. FQ_Codel uses a stochastic model to
classify incoming packets into different flows and is used to provide a
fair share of the bandwidth to all the flows using the queue. Each such
flow is managed by the CoDel queuing discipline. Reordering within a flow
is avoided since Codel internally uses a FIFO queue.
- fq_pie
- FQ-PIE (Flow Queuing with Proportional Integral controller Enhanced) is a
queuing discipline that combines Flow Queuing with the PIE AQM scheme.
FQ-PIE uses a Jenkins hash function to classify incoming packets into
different flows and is used to provide a fair share of the bandwidth to
all the flows using the qdisc. Each such flow is managed by the PIE
algorithm.
- gred
- Generalized Random Early Detection combines multiple RED queues in order
to achieve multiple drop priorities. This is required to realize Assured
Forwarding (RFC 2597).
- hhf
- Heavy-Hitter Filter differentiates between small flows and the opposite,
heavy-hitters. The goal is to catch the heavy-hitters and move them to a
separate queue with less priority so that bulk traffic does not affect the
latency of critical traffic.
- ingress
- This is a special qdisc as it applies to incoming traffic on an interface,
allowing for it to be filtered and policed.
- mqprio
- The Multiqueue Priority Qdisc is a simple queuing discipline that allows
mapping traffic flows to hardware queue ranges using priorities and a
configurable priority to traffic class mapping. A traffic class in this
context is a set of contiguous qdisc classes which map 1:1 to a set of
hardware exposed queues.
- multiq
- Multiqueue is a qdisc optimized for devices with multiple Tx queues. It
has been added for hardware that wishes to avoid head-of-line blocking. It
will cycle though the bands and verify that the hardware queue associated
with the band is not stopped prior to dequeuing a packet.
- netem
- Network Emulator is an enhancement of the Linux traffic control facilities
that allow one to add delay, packet loss, duplication and more other
characteristics to packets outgoing from a selected network
interface.
- 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.
- pie
- Proportional Integral controller-Enhanced (PIE) is a control theoretic
active queue management scheme. It is based on the proportional integral
controller but aims to control delay.
- red
- Random Early Detection simulates physical congestion by randomly dropping
packets when nearing configured bandwidth allocation. Well suited to very
large bandwidth applications.
- rr
- Round-Robin qdisc with support for multiqueue network devices. Removed
from Linux since kernel version 2.6.27.
- sfb
- Stochastic Fair Blue is a classless qdisc to manage congestion based on
packet loss and link utilization history while trying to prevent
non-responsive flows (i.e. flows that do not react to congestion marking
or dropped packets) from impacting performance of responsive flows. Unlike
RED, where the marking probability has to be configured, BLUE tries to
determine the ideal marking probability automatically.
- 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.
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.
The classful qdiscs are:
- ATM
- Map flows to virtual circuits of an underlying asynchronous transfer mode
device.
- 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.
- DRR
- The Deficit Round Robin Scheduler is a more flexible replacement for
Stochastic Fairness Queuing. Unlike SFQ, there are no built-in queues --
you need to add classes and then set up filters to classify packets
accordingly. This can be useful e.g. for using RED qdiscs with different
settings for particular traffic. There is no default class -- if a packet
cannot be classified, it is dropped.
- DSMARK
- Classify packets based on TOS field, change TOS field of packets based on
classification.
- ETS
- The ETS qdisc is a queuing discipline that merges functionality of PRIO
and DRR qdiscs in one scheduler. ETS makes it easy to configure a set of
strict and bandwidth-sharing bands to implement the transmission selection
described in 802.1Qaz.
- HFSC
- Hierarchical Fair Service Curve guarantees precise bandwidth and delay
allocation for leaf classes and allocates excess bandwidth fairly. Unlike
HTB, it makes use of packet dropping to achieve low delays which
interactive sessions benefit from.
- 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.
- QFQ
- Quick Fair Queueing is an O(1) scheduler that provides near-optimal
guarantees, and is the first to achieve that goal with a constant cost
also with respect to the number of groups and the packet length. The QFQ
algorithm has no loops, and uses very simple instructions and data
structures that lend themselves very well to a hardware
implementation.
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 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.
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 - major:minor. Both major and
minor are hexadecimal numbers and 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.
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 either a unit (both SI and IEC units supported), or a
float followed by a '%' character to specify the rate as a percentage of
the device's speed (e.g. 5%, 99.5%). Warning: specifying the rate as a
percentage means a fraction of the current speed; if the speed changes,
the value will not be recalculated.
- 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:
- 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.
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.
- get
- Displays a single filter given the interface DEV, qdisc-id,
priority, protocol and filter-id.
- show
- Displays all filters attached to the given interface. A valid parent ID
must be passed.
- link
- Only available for qdiscs and performs a replace where the node must exist
already.
The tc utility can monitor events generated
by the kernel such as adding/deleting qdiscs, filters or actions, or
modifying existing ones.
The following command is available
for monitor :
- file
- If the file option is given, the tc does not listen to kernel
events, but opens the given file and dumps its contents. The file has to
be in binary format and contain netlink messages.
- -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.
- -o, -oneline
- output each record on a single line, replacing line feeds with the
'\' character. This is convenient when you want to count records
with wc(1) or to grep(1) the output.
- -n, -net, -netns
<NETNS>
- switches tc to the specified network namespace NETNS.
Actually it just simplifies executing of:
ip netns exec NETNS tc [ OPTIONS ]
OBJECT { COMMAND | help }
to
tc -n[etns] NETNS [ OPTIONS ]
OBJECT { COMMAND | help }
- -N, -Numeric
- Print the number of protocol, scope, dsfield, etc directly instead of
converting it to human readable name.
- -cf, -conf
<FILENAME>
- specifies path to the config file. This option is used in conjunction with
other options (e.g. -nm).
- -t,
-timestamp
- When tc monitor runs, print timestamp before the
event message in format:
Timestamp: <Day> <Month> <DD> <hh:mm:ss>
<YYYY> <usecs> usec
- -ts,
-tshort
- When tc monitor runs, prints short timestamp before
the event message in format:
[<YYYY>-<MM>-<DD>T<hh:mm:ss>.<ms>]
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
- for u32 filter, decode offset and mask values to equivalent filter
commands based on TCP/IP. In JSON output, add whitespace to improve
readability.
- -iec
- print rates in IEC units (ie. 1K = 1024).
- -g, -graph
- shows classes as ASCII graph. Prints generic stats info under each class
if -s option was specified. Classes can be filtered only by
dev option.
- -c[color][={always|auto|never}
- Configure color output. If parameter is omitted or always, color
output is enabled regardless of stdout state. If parameter is auto,
stdout is checked to be a terminal before enabling color output. If
parameter is never, color output is disabled. If specified multiple
times, the last one takes precedence. This flag is ignored if -json
is also given.
- -j, -json
- Display results in JSON format.
- -nm, -name
- resolve class name from /etc/iproute2/tc_cls file or from file
specified by -cf option. This file is just a mapping of
classid to class name:
# Here is comment
1:40 voip # Here is another comment
1:50 web
1:60 ftp
1:2 home
tc will not fail if -nm was specified
without -cf option but /etc/iproute2/tc_cls file does not exist,
which makes it possible to pass -nm option for creating tc
alias.
- -br, -brief
- Print only essential data needed to identify the filter and action
(handle, cookie, etc.) and stats. This option is currently only supported
by tc filter show and tc actions ls commands.
tc -g class show dev eth0
Shows classes as ASCII graph on eth0 interface.
tc -g -s class show dev eth0
Shows classes as ASCII graph with stats info under each
class.
tc was written by Alexey N. Kuznetsov and added in Linux
2.2.
tc-basic(8), tc-bfifo(8), tc-bpf(8),
tc-cake(8), tc-cbq(8), tc-cgroup(8),
tc-choke(8), tc-codel(8), tc-drr(8),
tc-ematch(8), tc-ets(8), tc-flow(8),
tc-flower(8), tc-fq(8), tc-fq_codel(8),
tc-fq_pie(8), tc-fw(8), tc-hfsc(7), tc-hfsc(8),
tc-htb(8), tc-mqprio(8), tc-pfifo(8),
tc-pfifo_fast(8), tc-pie(8), tc-red(8),
tc-route(8), tc-sfb(8), tc-sfq(8), tc-stab(8),
tc-tbf(8), tc-tcindex(8), tc-u32(8),
User documentation at http://lartc.org/, but please direct bugreports
and patches to: <netdev@vger.kernel.org>
Manpage maintained by bert hubert (ahu@ds9a.nl)