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 ]

       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 ]

DESCRIPTION         top

       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 OPTIONS         top

       netem has the following options:

   limit packets
       maximum number of packets the qdisc may hold queued at a time.

       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.

   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.

       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
       proper parameter.

       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
       artificial delay.

       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.

       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         top

       The main known limitation of Netem are related to timer granularity,
       since Linux is not a real-time operating system.

EXAMPLES         top

       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         top

        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

SEE ALSO         top

       tc(8), tc-tbf(8)

AUTHOR         top

       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

COLOPHON         top

       This page is part of the iproute2 (utilities for controlling TCP/IP
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iproute2                      25 November 2011                      NETEM(8)

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