Manual: tc-netem
NETEM(8) Linux NETEM(8)
NAME
netem - Network Emulator
SYNOPSIS
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
DESCRIPTION
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.
OPTIONS
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 correlaā
tion. 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 imā
mediately.
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 artifiā
cial 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 bandā
width 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.
LIMITATIONS
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 necā
essary. 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 deā
lays.
EXAMPLES
# 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 Ā±
10ms.
# tc qdisc change dev eth0 root netem delay 100ms 10ms 25%
This causes the added delay of 100ms Ā± 10ms and the next packet delay value will be biased by 25% on the most reā
cent 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 Ā± 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 preā
vious 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, ā¦) 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.
SOURCES
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
SEE ALSO
tc(8)
AUTHOR
Netem was written by Stephen Hemminger at Linux foundation and was inspired by NISTnet.
Original manpage was created by Fabio Ludovici and Hagen Paul Pfeifer .
iproute2 25 November 2011 NETEM(8)