tc-netem (8) - Linux Manuals
tc-netem: Network Emulator
NAME
NetEm - Network Emulator
SYNOPSIS
tc qdisc ... dev DEVICE ] add netem OPTIONSOPTIONS := [ LIMIT ] [ DELAY ] [ LOSS ] [ CORRUPT ] [ DUPLICATION ] [ REORDERING ] [ RATE ] [ SLOT ]
LIMIT := limit packets
DELAY :=
delay
TIME [ JITTER [ CORRELATION ]]]
LOSS :=
loss {
random
PERCENT [ CORRELATION ] |
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 ] |
maximum number of packets the qdisc may hold queued at a time.
reorder
25% 50%
gap
5
reorder
25% 50%
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 artificial delay.
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.
tc qdisc add dev eth0 root netem rate 5kbit 20 100 5
state
p13 [ p31 [ p32 [ p23 [ p14]]]] |
gemodel
p [ r [ 1-h [ 1-k ]]] }
[ ecn ]
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 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
kernel.
netem OPTIONS
netem has the following options:
limit packets
delay
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.
distribution
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.
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 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.
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 probability 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).
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.
in this second example 25% of packets are sent immediately (with correlation 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
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.
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 distribution similar to
distribution
for
delay
option. Slot delays can be specified 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.
LIMITATIONS
The main known limitation of Netem are related to timer granularity, since
Linux is not a real-time operating system.
EXAMPLES
SOURCES
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>