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 ]
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 ]
DESCRIPTION
NetEm is an enhancement of the Linux traffic control facilities that
allow one to add delay, packet loss, duplication and more other charac-
teristics to packets outgoing from a selected network interface. NetEm
is built using the existing Quality Of Service (QOS) and Differentiated
Services (diffserv) facilities in the Linux kernel.
netem OPTIONS
netem has the following options:
limit packets
maximum number of packets the qdisc may hold queued at a time.
delay
adds the chosen delay to the packets outgoing to chosen network inter-
face. The optional parameters allows one to introduce a delay variation
and a correlation. Delay and jitter values are expressed in ms while
correlation is percentage.
distribution
allow the user to choose the delay distribution. If not specified, the
default distribution is Normal. Additional parameters allow one to con-
sider situations in which network has variable delays depending on
traffic flows concurring on the same path, that causes several delay
peaks and a tail.
loss random
adds an independent loss probability to the packets outgoing from the
chosen network interface. It is also possible to add a correlation, but
this option is now deprecated due to the noticed bad behavior.
loss state
adds packet losses according to the 4-state Markov using the transition
probabilities as input parameters. The parameter p13 is mandatory and
if used alone corresponds to the Bernoulli model. The optional parame-
ters allows one to extend the model to 2-state (p31), 3-state (p23 and
p32) and 4-state (p14). State 1 corresponds to good reception, State 4
to independent losses, State 3 to burst losses and State 2 to good
reception within a burst.
loss gemodel
adds packet losses according to the Gilbert-Elliot loss model or its
special cases (Gilbert, Simple Gilbert and Bernoulli). To use the
Bernoulli model, the only needed parameter is p while the others will
be set to the default values r=1-p, 1-h=1 and 1-k=0. The parameters
needed for the Simple Gilbert model are two (p and r), while three
parameters (p, r, 1-h) are needed for the Gilbert model and four (p, r,
1-h and 1-k) are needed for the Gilbert-Elliot model. As known, p and r
are the transition probabilities between the bad and the good states,
1-h is the loss probability in the bad state and 1-k is the loss proba-
bility in the good state.
ecn
can be used optionally to mark packets instead of dropping them. A loss
model has to be used for this to be enabled.
corrupt
allows the emulation of random noise introducing an error in a random
position for a chosen percent of packets. It is also possible to add a
correlation through the proper parameter.
duplicate
using this option the chosen percent of packets is duplicated before
queuing them. It is also possible to add a correlation through the
proper parameter.
reorder
to use reordering, a delay option must be specified. There are two ways
to use this option (assuming 'delay 10ms' in the options list).
reorder 25% 50% gap 5
in this first example, the first 4 (gap - 1) packets are delayed by
10ms and subsequent packets are sent immediately with a probability of
0.25 (with correlation of 50% ) or delayed with a probability of 0.75.
After a packet is reordered, the process restarts i.e. the next 4 pack-
ets are delayed and subsequent packets are sent immediately or delayed
based on reordering probability. To cause a repeatable pattern where
every 5th packet is reordered reliably, a reorder probability of 100%
can be used.
reorder 25% 50%
in this second example 25% of packets are sent immediately (with corre-
lation of 50%) while the others are delayed by 10 ms.
rate
delay packets based on packet size and is a replacement for TBF. Rate
can be specified in common units (e.g. 100kbit). Optional PACKETOVER-
HEAD (in bytes) specify an per packet overhead and can be negative. A
positive value can be used to simulate additional link layer headers. A
negative value can be used to artificial strip the Ethernet header
(e.g. -14) and/or simulate a link layer header compression scheme. The
third parameter - an unsigned value - specify the cellsize. Cellsize
can be used to simulate link layer schemes. ATM for example has an pay-
load cellsize of 48 bytes and 5 byte per cell header. If a packet is 50
byte then ATM must use two cells: 2 * 48 bytes payload including 2 * 5
byte header, thus consume 106 byte on the wire. The last optional value
CELLOVERHEAD can be used to specify per cell overhead - for our ATM
example 5. CELLOVERHEAD can be negative, but use negative values with
caution.
Note that rate throttling is limited by several factors: the kernel
clock granularity avoid a perfect shaping at a specific level. This
will show up in an artificial packet compression (bursts). Another
influence factor are network adapter buffers which can also add artifi-
cial delay.
slot
defer delivering accumulated packets to within a slot. Each available
slot can be configured with a minimum delay to acquire, and an optional
maximum delay. Alternatively it can be configured with the distribu-
tion similar to distribution for delay option. Slot delays can be spec-
ified in nanoseconds, microseconds, milliseconds or seconds (e.g.
800us). Values for the optional parameters BYTES will limit the number
of bytes delivered per slot, and/or PACKETS will limit the number of
packets delivered per slot.
These slot options can provide a crude approximation of bursty MACs
such as DOCSIS, WiFi, and LTE.
Note that slotting 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.
LIMITATIONS
The main known limitation of Netem are related to timer granularity,
since Linux is not a real-time operating system.
EXAMPLES
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 byte:
SOURCES
1. Hemminger S. , "Network Emulation with NetEm", Open Source Develop-
ment Lab, April 2005 (http://devresources.linux-founda-
tion.org/shemminger/netem/LCA2005_paper.pdf)
2. Netem page from Linux foundation, (https://wiki.linuxfounda-
tion.org/networking/netem)
3. 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://net-
group.uniroma2.it/NetemCLG
SEE ALSO
tc(8), tc-tbf(8)
AUTHOR
Netem was written by Stephen Hemminger at Linux foundation and is based
on NISTnet. This manpage was created by Fabio Ludovici <fabio.ludovici
at yahoo dot it> and Hagen Paul Pfeifer <hagen AT jauu.net>
iproute2 25 November 2011 NETEM(8)
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