tc-netem(8) — Linux manual page


NETEM(8)                          Linux                         NETEM(8)

NAME         top

       netem - Network Emulator

SYNOPSIS         top

       tc qdisc ... dev DEVICE ] add netem OPTIONS

       ] [ 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 ]]




       SLOT := slot { MIN_DELAY [ MAX_DELAY ] |
                      distribution { uniform | normal | pareto |
       paretonormal | FILE } DELAY JITTER }
                    [ packets PACKETS ] [ bytes BYTES ]

       SEED := seed VALUE

DESCRIPTION         top

       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         top

       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.

                     Use an equally weighted distribution of packet

              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.

                     This is a mix of pareto and normal distribution
                     which has properties of both Bell curve and long

       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:

                            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

              Delays packets based on packet size to emulate a fixed
              link speed.  Optional parameters:

                     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.

                     simulate link layer schemes like ATM.

                     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.

LIMITATIONS         top

       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.

EXAMPLES         top

       # 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

       # 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

       # 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

       The reorder form uses a percentage of the packets to get
       # 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
       # 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 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 as being
           priority 3.

SOURCES         top

        1. Hemminger S. , "Network Emulation with NetEm", Open Source
           Development Lab, April 2005 

        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 ⟨

SEE ALSO         top


AUTHOR         top

       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 <>.

COLOPHON         top

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iproute2                    25 November 2011                    NETEM(8)

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