Simulate delayed and dropped packets on Linux

Question

I would like to simulate packet delay and loss for UDP and TCP on Linux to measure the performance of an application   Is there a simple way to do this

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This tutorial on networking physics simulations contains a C   class in the sample code for simulating latency and packet loss in a UDP connection and may be of guidance  See the public latency and packetLoss variables of the Connection class found in the Connection h file of the downloadable source code

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iptables 8  has a statistic match module that can be used to match every nth packet  To drop this packet  just append -j DROP

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One of my colleagues uses tc to do this  Refer to the man page for more information  You can see an example of its usage here

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Haven t tried it myself  but this page has a list of plugin modules that run in Linux  built in iptables IP filtering system  One of the modules is called  nth   and allows you to set up a rule that will drop a configurable rate of the packets  Might be a good place to start  at least

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You can try http   snad ncsl nist gov nistnet  https   www-x antd nist gov nistnet  It s quite old NIST project  last release 2005   but it works for me

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netem leverages functionality already built into Linux and userspace utilities to simulate networks   This is actually what Mark s answer refers to  by a different name   The examples on their homepage already show how you can achieve what you ve asked for      Examples      Emulating wide area network delays      This is the simplest example  it just adds a fixed amount of delay to all packets going out of the local Ethernet     tc qdisc add dev eth0 root netem delay 100ms       Now a simple ping test to host on the local network should show an increase of 100 milliseconds  The delay is limited by the clock resolution of the kernel  Hz   On most 2 4 systems  the system clock runs at 100  Hz which allows delays in increments of 10  ms  On 2 6  the value is a configuration parameter from 1000 to 100  Hz       Later examples just change parameters without reloading the qdisc      Real wide area networks show variability so it is possible to add random variation     tc qdisc change dev eth0 root netem delay 100ms 10ms       This causes the added delay to be 100    10  ms  Network delay variation isn t purely random  so to emulate that there is a correlation value as well     tc qdisc change dev eth0 root netem delay 100ms 10ms 25        This causes the added delay to be 100    10  ms with the next random element depending 25  on the last one  This isn t true statistical correlation  but an approximation       Delay distribution      Typically  the delay in a network is not uniform  It is more common to use a something like a normal distribution to describe the variation in delay  The netem discipline can take a table to specify a non-uniform distribution     tc qdisc change dev eth0 root netem delay 100ms 20ms distribution normal       The actual tables  normal  pareto  paretonormal  are generated as part of the iproute2 compilation and placed in  usr lib tc  so it is possible with some effort to make your own distribution based on experimental data       Packet loss      Random packet loss is specified in the  tc  command in percent  The smallest possible non-zero value is       2-32   0 0000000232     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 loss 0 3  25        This will cause 0 3  of packets to be lost  and each successive probability depends by a quarter on the last one       Probn   0 25    Probn-1   0 75    Random   Note that you should use tc qdisc add if you have no rules for that interface or tc qdisc change if you already have rules for that interface  Attempting to use tc qdisc change on an interface with no rules will give the error RTNETLINK answers  No such file or directory

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An easy to use network fault injection tool is Saboteur  It can simulate         Total network partition   Remote service dead  not listening on the expected port    Delays   Packet loss   -TCP connection timeout  as often happens when two systems are separated by a stateful firewall

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For dropped packets I would simply use iptables and the statistic module   iptables -A INPUT -m statistic --mode random --probability 0 01 -j DROP   Above will drop an incoming packet with a 1  probability  Be careful  anything above about 0 14 and most of you tcp connections will most likely stall completely   Take a look at man iptables and search for  statistic  for more information

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One of the most used tool in the scientific community to that purpose is DummyNet  Once you have installed the ipfw kernel module  in order to introduce 50ms propagation delay between 2 machines simply run these commands     ipfw pipe 1 config delay 50ms   ipfw add 1000 pipe 1 ip from  IP MACHINE 1 to  IP MACHINE 2   In order to also introduce 50  of packet losses you have to run     ipfw pipe 1 config plr 0 5   Here more details

[linux] Simulate delayed and dropped packets on Linux

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