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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | LIMITATIONS | EXAMPLES | SOURCES | SEE ALSO | AUTHOR | COLOPHON |
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NETEM(8) Linux NETEM(8)
netem - Network Emulator
tc qdisc ... dev DEVICE ] add netem OPTIONS
OPTIONS := [ LIMIT ] [ DELAY ] [ LOSS ] [ CORRUPT ] [ DUPLICATION
] [ REORDERING ] [ RATE ] [ SLOT ] [ SEED ]
LIMIT := limit packets
DELAY := delay TIME [ JITTER [ CORRELATION ]]]
[ distribution { uniform | normal | pareto | paretonormal }
]
LOSS := loss { random PERCENT [ CORRELATION ] |
state p13 [ p31 [ p32 [ p23 [ p14]]]] |
gemodel p [ r [ 1-h [ 1-k ]]] } [ ecn ]
CORRUPT := corrupt PERCENT [ CORRELATION ]]
DUPLICATION := duplicate PERCENT [ CORRELATION ]]
REORDERING := reorder PERCENT [ CORRELATION ] [ gap DISTANCE ]
RATE := rate RATE [ PACKETOVERHEAD [ CELLSIZE [ CELLOVERHEAD ]]]]
SLOT := slot { MIN_DELAY [ MAX_DELAY ] |
distribution { uniform | normal | pareto |
paretonormal | FILE } DELAY JITTER }
[ packets PACKETS ] [ bytes BYTES ]
SEED := seed VALUE
The netem queue discipline provides Network Emulation
functionality for testing protocols by emulating the properties of
real-world networks.
The queue discipline provides one or more network impairments to
packets such as: delay, loss, duplication, and packet corruption.
limit COUNT
Limits the maximum number of packets the qdisc may hold
when doing delay.
delay TIME [ JITTER [ CORRELATION ]]]
Delays the packets before sending. The optional parameters
allow introducing a delay variation and a correlation.
Delay and jitter values are expressed in milliseconds;
Correlation is set by specifying a percent of how much the
previous delay will impact the current random value.
distribution TYPE
Specifies a pattern for delay distribution.
uniform
Use an equally weighted distribution of packet
delays.
normal Use a Gaussian distribution of delays. Sometimes
called a Bell Curve.
pareto Use a Pareto distribution of packet delays. This is
useful to emulate long-tail distributions.
paretonormal
This is a mix of pareto and normal distribution
which has properties of both Bell curve and long
tail.
loss MODEL
Drop packets based on a loss model. MODEL can be one of
random PERCENT
Each packet loss is independent.
state P13 [ P31 [ P32 [ P23 P14 ]]]
Use a 4-state Markov chain to describe packet loss.
P13 is the packet loss. Optional parameters extend
the model to 2-state P31, 3-state P23, P32 and
4-state P14.
The Markov chain states are:
1 good packet reception (no loss).
2 good reception within a burst.
3 burst losses.
4 independent losses.
gemodel PERCENT [ R [ 1-H [ 1-K ]]]
Use a Gilbert-Elliot (burst loss) model based on:
PERCENT
probability of starting bad (lossy) state.
R probability of exiting bad state.
1-H loss probability in bad state.
1-K loss probability in good state.
ecn Use Explicit Congestion Notification (ECN) to mark packets
instead of dropping them. A loss model has to be used for
this to be enabled.
corrupt PERCENT
modifies the contents of the packet at a random position
based on PERCENT.
duplicate PERCENT
creates a copy of the packet before queuing.
reorder PERCENT
modifies the order of packet in the queue.
gap DISTANCE
sends some packets immediately. The first packets
(DISTANCE - 1) are delayed and the next packet is sent
immediately.
rate RATE [ PACKETOVERHEAD [ CELLSIZE [ CELLOVERHEAD ]]]
Delays packets based on packet size to emulate a fixed link
speed. Optional parameters:
PACKETOVERHEAD
Specify a per packet overhead in bytes. Used to
simulate additional link layer headers. A negative
value can be used to simlate when the Ethernet
header is stripped (e.g. -14) or header compression
is used.
CELLSIZE
simulate link layer schemes like ATM.
CELLOVERHEAD
specify per cell overhead.
Rate throttling impacted by several factors including the kernel
clock granularity. This will show up in an artificial packet
compression (bursts).
slot MIN_DELAY [ MAX_DELAY ]
allows emulating slotted networks. Defer delivering
accumulated packets to within a slot. Each available slot
is configured with a minimum delay to acquire, and an
optional maximum delay.
slot distribution
allows configuring based on distribution similar to
distribution option for packet delays.
These slot options can provide a crude approximation of
bursty MACs such as DOCSIS, WiFi, and LTE.
Slot emulation is limited by several factors: the kernel
clock granularity, as with a rate, and attempts to deliver
many packets within a slot will be smeared by the timer
resolution, and by the underlying native bandwidth also.
It is possible to combine slotting with a rate, in which
case complex behaviors where either the rate, or the slot
limits on bytes or packets per slot, govern the actual
delivered rate.
seed VALUE
Specifies a seed to guide and reproduce the randomly
generated loss or corruption events.
Netem is limited by the timer granularity in the kernel. Rate and
delay maybe impacted by clock interrupts.
Mixing forms of reordering may lead to unexpected results. For
any method of reordering to work, some delay is necessary. If the
delay is less than the inter-packet arrival time then no
reordering will be seen. Due to mechanisms like TSQ (TCP Small
Queues), for TCP performance test results to be realistic netem
must be placed on the ingress of the receiver host.
Combining netem with other qdisc is possible but may not always
work because netem use skb control block to set delays.
# tc qdisc add dev eth0 root netem delay 100ms
Add fixed amount of delay to all packets going out on device
eth0. Each packet will have added delay of 100ms ± 10ms.
# tc qdisc change dev eth0 root netem delay 100ms 10ms 25%
This causes the added delay of 100ms ± 10ms and the next
packet delay value will be biased by 25% on the most recent
delay. This isn't a true statistical correlation, but an
approximation.
# tc qdisc change dev eth0 root netem delay 100ms 20ms distribution normal
This delays packets according to a normal distribution (Bell
curve) over a range of 100ms ± 20ms.
# tc qdisc change dev eth0 root netem loss 0.1%
This causes 1/10th of a percent (i.e 1 out of 1000) packets to
be randomly dropped.
An optional correlation may also be added. This causes the
random number generator to be less random and can be used to
emulate packet burst losses.
# tc qdisc change dev eth0 root netem duplicate 1%
This causes one percent of the packets sent on eth0 to be
duplicated.
# tc qdisc change dev eth0 root netem loss 0.3% 25%
This will cause 0.3% of packets to be lost, and each
successive probability depends is biased by 25% of the
previous one.
There are two different ways to specify reordering. The gap
method uses a fixed sequence and reorders every Nth packet.
# tc qdisc change dev eth0 root netem gap 5 delay 10ms
This causes every 5th (10th, 15th, …) packet to go to be sent
immediately and every other packet to be delayed by 10ms.
This is predictable and useful for base protocol testing like
reassembly.
The reorder form uses a percentage of the packets to get
misordered.
# tc qdisc change dev eth0 root netem delay 10ms reorder 25% 50%
In this example, 25% of packets (with a correlation of 50%) will
get sent immediately, others will be delayed by 10ms.
Packets will also get reordered if jitter is large enough.
# tc qdisc change dev eth0 root netem delay 100ms 75ms
If the first packet gets a random delay of 100ms (100ms base -
0ms jitter) and the second packet is sent 1ms later and gets a
delay of 50ms (100ms base - 50ms jitter); the second packet
will be sent first. This is because the queue discipline
tfifo inside netem, keeps packets in order by time to send.
If you don't want this behavior then replace the internal queue
discipline tfifo with a simple FIFO queue discipline.
# tc qdisc add dev eth0 root handle 1: netem delay 10ms 100ms
# tc qdisc add dev eth0 parent 1:1 pfifo limit 1000
Example of using rate control and cells size.
# tc qdisc add dev eth0 root netem rate 5kbit 20 100 5
Delay all outgoing packets on device eth0 with a rate of
5kbit, a per packet overhead of 20 byte, a cellsize of 100
byte and a per celloverhead of 5 bytes.
It is possible to selectively apply impairment using traffic
classification.
# tc qdisc add dev eth0 root handle 1: prio
# tc qdisc add dev eth0 parent 1:3 handle 30: tbf rate 20kbit buffer 1600 limit 3000
# tc qdisc add dev eth0 parent 30:1 handle 31: netem delay 200ms 10ms distribution normal
# tc filter add dev eth0 protocol ip parent 1:0 prio 3 u32 match ip dst 65.172.181.4/32 flowid 1:3
This example uses a priority queueing discipline; a TBF is
added to do rate control; and a simple netem delay. A filter
classifies all packets going to 65.172.181.4 as being priority
3.
1. Hemminger S. , "Network Emulation with NetEm", Open Source
Development Lab, April 2005
⟨http://devresources.linux-
foundation.org/shemminger/netem/LCA2005_paper.pdf⟩
2. Salsano S., Ludovici F., Ordine A., "Definition of a general
and intuitive loss model for packet networks and its
implementation in the Netem module in the Linux kernel",
available at ⟨http://netgroup.uniroma2.it/NetemCLG⟩
tc(8)
Netem was written by Stephen Hemminger at Linux foundation and was
inspired by NISTnet.
Original manpage was created by Fabio Ludovici <fabio.ludovici at
yahoo dot it> and Hagen Paul Pfeifer <hagen@jauu.net>.
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
netdev@vger.kernel.org, shemminger@osdl.org. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2025-02-02. (At that time, the date of the most recent commit
that was found in the repository was 2025-01-21.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
iproute2 25 November 2011 NETEM(8)
Pages that refer to this page: ovs-vswitchd.conf.db(5)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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