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 ] 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 ]]]] DESCRIPTION NetEm is an enhancement of the Linux traffic control facilities that allow to add delay, packet loss, duplica‐ tion 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 limits the effect of selected options to the indicated number of next 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. Addi‐ tional 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 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 pack‐ ets. 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 imme‐ diately 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. 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 neg‐ ative, 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. SOURCES 1. Hemminger S. , "Network Emulation with NetEm", Open Source Development Lab, April 2005 (http://devre‐ sources.linux-foundation.org/shemminger/netem/LCA2005_paper.pdf) 2. Netem page from Linux foundation, (http://www.linuxfoundation.org/en/Net: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 <[email protected]> iproute2 25 November 2011 NETEM(8)