NetEm is an enhancement of the Linux traffic control facilities that
allow to add delay, packet loss, duplication and more other
characteristics 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
netem has the following options:
limits the effect of selected options to the indicated number of next
adds the chosen delay to the packets outgoing to chosen network
interface. The optional parameters allows to introduce a delay
variation and a correlation. Delay and jitter values are expressed
in ms while correlation is percentage.
allow the user to choose the delay distribution. If not specified,
the default distribution is Normal. Additional parameters allow to
consider situations in which network has variable delays depending on
traffic flows concurring on the same path, that causes several delay
peaks and a tail.
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.
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 parameters allows 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.
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 probability in the good state.
can be used optionally to mark packets instead of dropping them. A
loss model has to be used for this to be enabled.
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.
using this option the chosen percent of packets is duplicated before
queuing them. It is also possible to add a correlation through the
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 packets 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
correlation of 50%) while the others are delayed by 10 ms.
delay packets based on packet size and is a replacement for TBF.
Rate can be specified in common units (e.g. 100kbit). Optional
PACKETOVERHEAD (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 payload 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
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:
1. Hemminger S. , "Network Emulation with NetEm", Open Source
Development Lab, April 2005 (http://devresources.linux-
2. Netem page from Linux foundation,
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
This page is part of the iproute2 (utilities for controlling TCP/IP
networking and traffic) project. Information about the project can
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iproute2 25 November 2011 NETEM(8)