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Introduction to Mininet

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Brian O'Connor edited this page Oct 28, 2013 · 75 revisions

by Bob Lantz, Nikhil Handigol, Brandon Heller, and Vimal Jeyakumar

This document is meant to give you a brief sense of what Mininet is and how it works, including a basic introduction to Mininet's Python API, the core of Mininet's functionality that you will usually want to use to create networks and run experiments.

What is Mininet?

Mininet is a network emulator. It runs a collection of end-hosts, switches, routers, and links on a single Linux kernel. It uses lightweight virtualization to make a single system look like a complete network, running the same kernel, system, and user code. A Mininet host behaves just like a real machine; you can ssh into it (if you start up sshd and bridge the network to your host) and run arbitrary programs. The programs you run can send packets through what seems like a real Ethernet interface, with a given link speed and delay. Packets get processed by what looks like a real Ethernet switch, router, or middlebox, with a given amount of queueing. When two programs, like an iperf client and server, communicate through Mininet, the measured performance should match that of two (slower) native machines.

In short, Mininet's virtual hosts, switches, links, and controllers are the real thing – they are just created using software rather than hardware – and for the most part their behavior is similar to discrete hardware elements. It is usually possible to create a Mininet network that resembles a hardware network, or a hardware network that resembles a Mininet network, and to run the same binary code and applications on either platform.

Why is Mininet cool?

  1. It's fast - starting up a simple network takes just a few seconds. This means that your run-edit-debug loop can be very quick.

  2. You can create custom topologies: a single switch, larger Internet-like topologies, the Stanford backbone, a data center, or anything else.

  3. You can run real programs: anything that runs on Linux is available for you to run, from web servers to TCP window monitoring tools to Wireshark.

  4. You can customize packet forwarding: Mininet's switches are programmable using the OpenFlow protocol. Custom Software-Defined Network designs that run in Mininet can easily be transferred to hardware OpenFlow switches for line-rate packet forwarding.

  5. You can run Mininet on your laptop, on a server, in a VM, on a native Linux box (Mininet is included with Ubuntu 12.10+!), or in the cloud (e.g. Amazon EC2.)

  6. You can share and replicate results: anyone with a computer can run your code once you've packaged it up.

  7. You can use it easily: you can create and run Mininet experiments by writing simple (or complex if necessary) Python scripts.

  8. Mininet is an open source project, so you are encouraged to examine its source code on https://github.com/mininet, modify it, fix bugs, file issues/feature requests, and submit patches/pull requests.

  9. Mininet is under active development. So, if it sucks, doesn't make sense, or doesn't work for some reason, please let us know on mininet-discuss and the Mininet user and developer community can try to explain it, fix it, or help you fix it. :-) If you find bugs, you are encouraged to submit patches to fix them, or at least to submit an issue on github including a reproducible test case.

What are Mininet's limitations?

Although we think Mininet is great, it does have some limitations. For example,

  • Running on a single system is convenient, but it imposes resource limits: if your server has 3 GHz of CPU and can switch about 3 Gbps of simulated traffic, those resources will need to be balanced and shared among your virtual hosts and switches.

  • Mininet uses a single Linux kernel for all virtual hosts; this means that you can't run software that depends on BSD, Windows, or other operating system kernels. (Although you can attach VMs to Mininet.)

  • Mininet won't write your OpenFlow controller for you; if you need custom routing or switching behavior, you will need to find or develop a controller with the features you require.

  • Currently Mininet doesn't do NAT out of the box. This means that your virtual hosts will be isolated from your LAN by default; this is usually a good thing, but means your hosts can't talk directly to the Internet unless you provide a means for them to do so. (Note the nat.py example demonstrates how to configure Mininet hosts for external connectivity, and the hwintf.py examples demonstrates how to add a physical interface to Mininet.)

  • Currently all Mininet hosts share the host file system and PID space; this means that you may have to be careful if you are running daemons that require configuration in /etc, and you need to be careful that you don't kill the wrong processes by mistake. (Note the bind.py example demonstrates how to have per-host private directories.)

  • Unlike a simulator, Mininet doesn't have a strong notion of virtual time; this means that timing measurements will be based on real time, and that faster-than-real-time results (e.g. 100 Gbps networks) cannot easily be emulated.

An aside on performance: The main thing you have to keep in mind for network- limited experiments is that you will probably need to use slower links, for example 10 or 100 Mb/sec rather than 10 Gb/sec, due to the fact that packets are forwarded by a collection of software switches (e.g. Open vSwitch) that share CPU and memory resources and usually have lower performance than dedicated switching hardware. For CPU-limited experiments, you will also need to make sure that you carefully limit the CPU bandwidth of your Mininet hosts. If you mainly care about functional correctness, you can run Mininet without specific bandwidth limits - this is the quick and easy way to run Mininet, and it also provides the highest performance at the expense of timing accuracy under load.

With a few possible exceptions, most of these limitations are not intrinsic to Mininet; eliminating them is simply a matter of code, and you are encouraged to contribute any enhancements you may develop!

Working with Mininet

The following sections describe several features of Mininet (and its Python API) which you may find useful while completing the assignments.

Creating Topologies

Mininet supports parametrized topologies. With a few lines of Python code, you can create a flexible topology which can be configured based on the parameters you pass into it, and reused for multiple experiments.

For example, here is a simple network topology (based on mininet/topo.py:SingleSwitchTopo) which consists of a specified number of hosts (h1 through hN) connected to a single switch (s1):

	#!/usr/bin/python
	
	from mininet.topo import Topo
	from mininet.net import Mininet
	from mininet.util import dumpNodeConnections
	from mininet.log import setLogLevel
	
	class SingleSwitchTopo(Topo):
	    "Single switch connected to n hosts."
	    def __init__(self, n=2, **opts):
	        # Initialize topology and default options
	        Topo.__init__(self, **opts)
	        switch = self.addSwitch('s1')
	        # Python's range(N) generates 0..N-1
	        for h in range(n):
	            host = self.addHost('h%s' % (h + 1))
	            self.addLink(host, switch)
	
	def simpleTest():
	    "Create and test a simple network"
	    topo = SingleSwitchTopo(n=4)
	    net = Mininet(topo)
	    net.start()
	    print "Dumping host connections"
	    dumpNodeConnections(net.hosts)
	    print "Testing network connectivity"
	    net.pingAll()
	    net.stop()
	
	if __name__ == '__main__':
	    # Tell mininet to print useful information
	    setLogLevel('info')
	    simpleTest()

Important classes, methods, functions and variables in the above code include:

Topo: the base class for Mininet topologies

addSwitch(): adds a switch to a topology and returns the switch name

addHost(): adds a host to a topology and returns the host name

addLink(): adds a bidirectional link to a topology (and returns a link key, but this is not important). Links in Mininet are bidirectional unless noted otherwise.

Mininet: main class to create and manage a network

start(): starts your network

pingAll(): tests connectivity by trying to have all nodes ping each other

stop(): stops your network

net.hosts: all the hosts in a network

dumpNodeConnections(): dumps connections to/from a set of nodes.

setLogLevel( 'info' | 'debug' | 'output' ): set Mininet's default output level; 'info' is recommended as it provides useful information.

Additional example code may be found in mininet/examples.

Setting Performance Parameters

In addition to basic behavioral networking, Mininet provides performance limiting and isolation features, through the CPULimitedHost and TCLink classes.

There are multiple ways that these classes may be used, but one simple way is to specify them as the default host and link classes/constructors to Mininet(), and then to specify the appropriate parameters in the topology. (You could also specify custom classes in the topology itself, or create custom node and link constructors and/or subclasses.)

	#!/usr/bin/python
	
	from mininet.topo import Topo
	from mininet.net import Mininet
	from mininet.node import CPULimitedHost
	from mininet.link import TCLink
	from mininet.util import dumpNodeConnections
	from mininet.log import setLogLevel
	
	class SingleSwitchTopo(Topo):
	    "Single switch connected to n hosts."
	    def __init__(self, n=2, **opts):
	        Topo.__init__(self, **opts)
	        switch = self.addSwitch('s1')
	        for h in range(n):
	            # Each host gets 50%/n of system CPU
	            host = self.addHost('h%s' % (h + 1),
		           cpu=.5/n)
	            # 10 Mbps, 5ms delay, 10% loss, 1000 packet queue
	            self.addLink(host, switch,
		           bw=10, delay='5ms', loss=10, max_queue_size=1000, use_htb=True)
	
	def perfTest():
	    "Create network and run simple performance test"
	    topo = SingleSwitchTopo(n=4)
	    net = Mininet(topo=topo, 
	                  host=CPULimitedHost, link=TCLink)
	    net.start()
	    print "Dumping host connections"
	    dumpNodeConnections(net.hosts)
	    print "Testing network connectivity"
	    net.pingAll()
	    print "Testing bandwidth between h1 and h4"
	    h1, h4 = net.get('h1', 'h4')
	    net.iperf((h1, h4))
	    net.stop()
	
	if __name__ == '__main__':
	    setLogLevel('info')
	    perfTest()

Important methods and parameters:

self.addHost(name, cpu=f):

This allows you to specify a fraction of overall system CPU resources which will be allocated to the virtual host.

self.addLink( node1, node2, bw=10, delay='5ms', max_queue_size=1000, loss=10, use_htb=True): adds a bidirectional link with bandwidth, delay and loss characteristics, with a maximum queue size of 1000 packets using the Hierarchical Token Bucket rate limiter and netem delay/loss emulator. The parameter bw is expressed as a number in Mb/s; delay is expressed as a string with units in place (e.g. '5ms', '100us', '1s'); loss is expressed as a percentage (between 0 and 100); and max_queue_size is expressed in packets.

You may find it useful to create a Python dictionary to make it easy to pass the same parameters into multiple method calls, for example:

linkopts = dict(bw=10, delay='5ms', loss=10, max_queue_size=1000, use_htb=True)
# alternately: linkopts = {'bw':10, 'delay':'5ms', 'loss':10,
# max_queue_size=1000, 'use_htb':True}
self.addLink(node1, node2, **linkopts)

This same technique (**dict) is useful for passing options to Matplotlib and other libraries.

net.get(): retrieves a node (host or switch) object by name. This is important if you want to send a command to a host (e.g. using host.cmd()) and get its output.

Note: In the current master branch of Mininet, you can simply use braces (e.g. net['h1']) to retrieve a given node by name.

Running Programs in Hosts

One of the most important things you will need to do in your experiments is run programs in hosts, so that you can run more tests than the simple pingAll() and iperf() tests which are provided by Mininet itself.

Each Mininet host is essentially a bash shell process attached to one or more network interfaces, so the easiest way to interact with it is to send input to the shell using the cmd() method.

To run a command in a host and get the output, use the cmd() method.

	h1 = net.get('h1')	
	result = h1.cmd('ifconfig')
	print result

In many cases, you will wish to run a command in the background for a while, stop the command, and save its output to a file:

	from time import sleep
	...
	print "Starting test..."
	h1.cmd('while true; do date; sleep 1; done > /tmp/date.out &')
	sleep(10)
	print "Stopping test"
	h1.cmd('kill %while')
	print "Reading output"
	f = open('/tmp/date.out')
	lineno = 1
	for line in f.readlines():
	    print "%d: %s" % ( lineno, line.strip() )
	    lineno += 1
	f.close()

Note that we used the shell's output redirection feature to send output to /tmp/date.out, the background execution feature & of the shell to run the command in the background, and job control kill %while to shut down the program that is running in the background. Unfortunately, if you leave jobs running in the background they are not guaranteed to stop if Mininet exits (either intentionally or due to an error), so you will need to make sure that you stop all of your jobs cleanly. You may wish to use the ps command periodically to make sure that no zombie jobs are mindlessly marching onward and slowing down your EC2 instance.

(Note: Python strings may be delimited using either single or double quotes. The example above uses double quotes in print statements and single quotes for function arguments, but you can do whatever you like. Python's print statement exists for convenience and possibly to help BASIC programmers feel more at home.)

Having a shell process at your disposal makes it easy to perform other tasks as well. For example, you can find out the PID of a background command using

	pid = int( h1.cmd('echo $!') )

Then you can wait for a particular process to finish execution by using wait, e.g.:

	h1.cmd('wait', pid)

Note that this will only work for UNIX commands and not for commands (e.g. while, cd) which are built into the bash shell itself (and don't have a separate pid!) Note also that each of these commands executed in the foreground (no &) rather than the background (&) since we want to get the output.

[Editorial: UNIX's (and bash's) abbreviations and special characters ($, !, &) date back to the days of blazingly fast 300 bit per second "networks" and Teletype™ terminals that printed on paper as you typed. Linux stuck with them for compatibility and because humans still type slowly. bash sometimes provides more verbose equivalents, if you wish to use them.]

In addition to using the shell's wait mechanism, Mininet itself allows you to start a foreground command using sendCmd() and then wait for it to complete at some later time using waitOutput():

	for h in hosts:
	    h.sendCmd('sleep 20')
	…
	results = {}
	for h in hosts:
	    results[h.name] = h.waitOutput()

If you are sending output to a file, you may wish to monitor that file's contents interactively while your test is running. The examples/multipoll.py example provides a function monitorFiles() which implements one possible mechanism for monitoring multiple output files. This simplifies the implementation of a test which interactively monitors output from multiple hosts:

	def monitorTest( N=3, seconds=3 ):
	    "Run pings and monitor multiple hosts"
	    topo = SingleSwitchTopo( N )
	    net = Mininet( topo )
	    net.start()
	    hosts = net.hosts
	    print "Starting test..."
	    server = hosts[ 0 ]
	    outfiles, errfiles = {}, {}
	    for h in hosts:
	        # Create and/or erase output files
	        outfiles[ h ] = '/tmp/%s.out' % h.name
	        errfiles[ h ] = '/tmp/%s.err' % h.name
	        h.cmd( 'echo >', outfiles[ h ] )
	        h.cmd( 'echo >', errfiles[ h ] )
	        # Start pings
	        h.cmdPrint('ping', server.IP(),
	                   '>', outfiles[ h ],
	                   '2>', errfiles[ h ],
	                   '&' )
	    print "Monitoring output for", seconds, "seconds"
	    for h, line in monitorFiles( outfiles, seconds, timeoutms=500 ):
	        if h:
	            print '%s: %s' % ( h.name, line )
	    for h in hosts:
	        h.cmd('kill %ping')
	    net.stop()

You may wish to run multipoll.py and look at its output.

Another example, examples/multiping.py demonstrates a different (perhaps simpler but less flexible) approach to monitoring standard output from a host, using the Node.monitor() method, so you may wish to look at it as well.

New: popen()/pexec() interface

In addition to the shell-based mechanism of cmd()/sendCmd(), Mininet now also supports a pipe-based interface which returns standard Python Popen() objects (see Python's subprocess module for details.) This mechanism is newer and not as well-tested as the cmd() mechanism, but you may find it convenient for running multiple processes in the background and monitoring their output. A pmonitor() function is provided to make monitoring multiple Popen() objects even easier.

Warning: in Mininet 2.0.0, pmonitor() may return a number of blank lines after receiving EOFs. This should now be fixed in the master branch.

The code in examples/popenpoll.py implements the functionality similar to what is described above, using the popen() interface and pmonitor() helper function:

	def pmonitorTest( N=3, seconds=10 ):
		"Run pings and monitor multiple hosts using pmonitor"
		topo = SingleSwitchTopo( N )
		net = Mininet( topo )
		net.start()
		hosts = net.hosts
		print "Starting test..."
		server = hosts[ 0 ]
		popens = {}
		for h in hosts:
	    	    popens[ h ] = h.popen('ping', server.IP() )
		print "Monitoring output for", seconds, "seconds"
		endTime = time() + seconds
		for h, line in pmonitor( popens, timeoutms=500 ):
	    	    if h:
	        	   print '%s: %s' % ( h.name, line ),
	    	    if time() >= endTime:
	        	   for p in popens.values():
	            	 p.send_signal( SIGINT )
		net.stop()

Note this implementation is slightly different since it pulls the time management out of the helper function, but this enables pmonitor() to catch ping's output after it is interrupted.

Of course, you do not have to use pmonitor() - you can use Popen.communicate() (as long as you don't have too many file descriptors) or select.poll() or any other mechanism that works.

If you find bugs in the popen() interface, please let us know.

Important: Shared Filesystem!

One think to keep in mind is that by default Mininet hosts share the root filesystem of the underlying server. Usually this is a very good thing, since it is a huge pain (and slow) to create a separate filesystem for each Mininet host (which you can do if you like and then chroot into it!)

Sharing the root filesystem also means that you almost never need to copy data between Mininet hosts because it's already there.

One side-effect of this however is that hosts share the Mininet server's /etc directory. This means that if you require specific configuration for a program (e.g. httpd) then you may need to create different configuration files for each Mininet host and specify them as startup options to the program that you are running.

Another side-effect is that you can have file collisions if you try to create the same file in the same directory on multiple hosts.

In the future we may provide a mechanism to automatically create a file structure for each Mininet host that shares most of the underlying filesystem but allows for custom directories (e.g. /etc) as desired.

Host Configuration Methods

Mininet hosts provide a number of convenience methods for network configuration:

  1. IP(): Return IP address of a host or specific interface.
  2. MAC(): Return MAC address of a host or specific interface.
  3. setARP(): Add a static ARP entry to a host's ARP cache.
  4. setIP(): Set the IP address for a host or specific interface.
  5. setMAC(): Set the MAC address for a host or specific interface

For example:

    print "Host", h1.name, "has IP address", h1.IP(), "and MAC address", h1.MAC()

In each case, if you do not provide a specific interface (e.g. h1-eth0 or an interface object) the method will use the host's default interface. The above functions are defined in mininet/node.py.

Naming in Mininet

In order to use Mininet effectively, it is important to understand its naming scheme for hosts, switches and interfaces. Usually, hosts are called h1..hN and switches are called s1..sN. We recommend that you follow this convention or a similar one. For clarity, interfaces belonging to a node are named beginning with the node's name, for example h1-eth0 is host h1's default interface, and s1-eth1 is switch s1's first data port. Host interfaces are only visible from within the host itself, but switch data ports are visible in the "root" namespace (you can see them by typing ip link show in another window while Mininet is running.) As a result, it's easy to examine switch interfaces but slightly trickier to examine host interfaces, since you must tell the host to do so (typically using host.cmd().)

CLI

Mininet includes a command-line interface (CLI) which may be invoked on a network, and provides a variety of useful commands, as well as the ability to display xterm windows and to run commands on individual nodes in your network. You can invoke the CLI on a network by passing the network object into the CLI() constructor:

	from mininet.topo import SingleSwitchTopo
	from mininet.net import Mininet
	from mininet.cli import CLI
	
	net = Mininet(SingleSwitchTopo(2))
	net.start()
	CLI(net)
	net.stop()

Starting up the CLI can be useful for debugging your network, as it allows you to view the network topology (with the net command), test connectivity (with the pingall command), and send commands to individual hosts.

*** Starting CLI:
mininet> net
c0
s1 lo:  s1-eth1:h1-eth0 s1-eth2:h2-eth0
h1 h1-eth0:s1-eth1
h2 h2-eth0:s1-eth2
mininet> pingall
*** Ping: testing ping reachability
h1 -> h2
h2 -> h1
*** Results: 0% dropped (0/2 lost)
mininet> h1 ip link show
746: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
	link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
749: h1-eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
	link/ether d6:13:2d:6f:98:95 brd ff:ff:ff:ff:ff:ff

Starting up the CLI can be useful for debugging your network, as it allows you to view the network topology (with the net command), test connectivity (with the pingall command), and send commands to individual hosts.

Additional Examples

Additional examples of Mininet scripts may be found in mininet/examples. You may find some of them to be useful and others (e.g. consoles.py, miniedit.py) to be entertaining demonstrations.

Note: The examples are intended as instructional material to be read and understood, not as complete, out-of-the-box solutions to whatever problem you may have. You may be able to use some of the code with modification, but it's important to be able to examine and understand the code.

Mininet API Documentation

Mininet includes Python documentation strings for each module and API call. These may be accessed from Python's regular help() mechanism. For example,

	python
	>>> from mininet.node import Host
	>>> help(Host.IP)
	Help on method IP in module mininet.node:
	
	IP(self, intf=None) unbound mininet.node.Host method
		Return IP address of a node or specific interface.

This same documentation is also available on the Mininet web site at http://mininet.github.com/api.

You may wish to generate the HTML (and PDF) documentation yourself using doxypy:

sudo apt-get install doxypy
cd ~/mininet
make doc
cd doc
python -m SimpleHTTPServer

At this point, you can point a web browser to port 8000 of the host that Mininet is running on and browse the documentation for each of Mininet's classes.

Measuring Performance

These are recommended, though you’re free to use any tool you’re familiar with.

  1. Bandwidth (bwm-ng, ethstats)
  2. Latency (use ping)
  3. Queues (use tc included in monitor.py)
  4. TCP CWND statistics (tcp_probe, maybe we should add it to monitor.py)
  5. CPU usage (global: top, or per-container cpuacct)

OpenFlow and Custom Routing

One of Mininet's most powerful and useful features is that it uses Software Defined Networking. Using the OpenFlow (http://openflow.org) protocol and related tools, you can program switches to do almost anything you want with the packets that enter them. OpenFlow makes emulators like Mininet much more useful, since network system designs, including custom packet forwarding with OpenFlow, can easily be transferred to hardware OpenFlow switches for line-rate operation. A tutorial for creating a simple learning switch using Mininet and OpenFlow may be found at:

http://www.openflow.org/wk/index.php/OpenFlow_Tutorial

OpenFlow Controllers

If you run the mn command without specifying a controller, it will use the the ovsc controller,ovs-controller, by default. This is equivalent to

$ sudo mn --controller ovsc

This controller implements a simple Ethernet learning switch, and supports up to 16 individual switches.

If you invoke the Mininet() constructor in your script without specifying a controller class, by default it will use the Controller() class to create an instance of the Stanford/OpenFlow reference controller, controller. Like ovs-controller, it turns your switches into simple learning switches, but if you have installed controller using Mininet's install.sh -f script, the patched version of controller should support a large number of switches (up to 4096 in theory, but you'll probably max out your computing resources much earlier.) You can also select the reference controller for mn by specifying --controller ref.

If you want to use your own controller, you can easily create a custom subclass of Controller() and pass it into Mininet. An example can be seen in mininet.controller.NOX(), which invokes NOX classic with a set of modules passed in as options.

Here's a simple example of creating and using a custom POX Controller subclass:

#!/usr/bin/python                                                                                      
                                                                                                       
from mininet.net import Mininet                                                                        
from mininet.node import Controller                                                                    
from mininet.topo import SingleSwitchTopo                                                              
from mininet.log import setLogLevel                                                                    
                                                                                                       
import os                                                                                              
                                                                                                       
class POXBridge( Controller ):                                                                         
    "Custom Controller class to invoke POX forwarding.l2_learning"                                     
    def start( self ):                                                                                 
        "Start POX learning switch"                                                                    
        self.pox = '%s/pox/pox.py' % os.environ[ 'HOME' ]                                              
        self.cmd( self.pox, 'forwarding.l2_learning &' )                                               
    def stop( self ):                                                                                  
        "Stop POX"                                                                                     
        self.cmd( 'kill %' + self.pox )                                                                
                                                                                                       
controllers = { 'poxbridge': POXBridge }                                                               
                                                                                                       
if __name__ == '__main__':                                                                             
    setLogLevel( 'info' )                                                                              
    net = Mininet( topo=SingleSwitchTopo( 2 ), controller=POXBridge )                                  
    net.start()                                                                                        
    net.pingAll()                                                                                      
    net.stop()

Note that the above script is written so that it also can be used as a custom argument to mn for use with different topologies and tests as well as the Mininet CLI:

$ sudo mn --custom poxbridge.py --controller poxbridge --topo tree,2,2 --test pingall -v output
*** Ping: testing ping reachability
h1 -> h2 h3 h4 
h2 -> h1 h3 h4 
h3 -> h1 h2 h4 
h4 -> h1 h2 h3 
*** Results: 0% dropped (0/12 lost)

If you look at the implementation of the NOX class in mininet/node.py, you will notice that it can actually accept options to allow different modules to be started depending on what arguments are passed into it, from the constructor or the mn command line.

External OpenFlow Controllers

Custom Controller() subclasses are the most convenient method for automatically starting and shutting down your controller. It's easy to create start() and stop() methods so that Mininet will automatically start and stop your controller as needed.

(For more information, check out this blog post.)

However, you may find it useful to connect Mininet to an existing controller that is already running somewhere else, for example somewhere on your LAN, in another VM, or on your laptop.

The RemoteController class acts as a proxy for a controller which may be running anywhere on the control network, but which must be started up and shut down manually or by some other mechanism outside of Mininet's direct control.

You can use RemoteController from Mininet:

Mininet( topo=topo, controller=lambda name: RemoteController( name, ip='127.0.0.1' ) )

You can also create multiple controllers and create a custom Switch() subclass which connects to different controllers as desired:

c0 = Controller( 'c0' )  # local controller
c1 = RemoteController( 'c1', ip='127.0.0.2' )  # external controller
cmap = { 's1': c0, 's2': c1, 's3': c1 }

class MultiSwitch( OVSSwitch ):
    "Custom Switch() subclass that connects to different controllers"
    def start( self, controllers ):
        return OVSSwitch.start( self, [ cmap[ self.name ] ] )

You can also specify an external controller from the mn command line:

$ sudo mn --controller remote,ip=192.168.51.101

Multipath Routing

It's important to remember that Ethernet bridges (also known as learning switches) will flood packets that miss in their MAC tables. They will also flood broadcasts like ARP and DHCP requests. This means that if your network has loops or multiple paths in it, it will not work with the default ovs-controller and controller controllers, nor NOX's pyswitch, nor POX's l2_learning, which all act as learning switches/Ethernet bridges.

In spite of the obviousness of this issue, it has become a Frequently Asked Question.

If you are building a fat-tree like topology, you may wish to take a look at RipLPOX, a basic datacenter controller implemented using POX. You may be able to use it as a starting point for your own custom multipath routing.

You may also wish to implement a custom Controller() subclass to invoke RipLPOX for convenience.

(Or if you're truly daring/insane, you could even try importing Mininet and POX or RipLPOX into a single Python script! But you probably don't want to do that.)

Updating Mininet

If we need to make changes or additions to Mininet to fix bugs or other problems, and you have installed Mininet from source, you may wish to update your copy of Mininet. This is easily done using:

cd ~/mininet
sudo make develop # this only needs to be done once
git fetch
git pull --rebase

Learning Python

Mininet is written in Python and allows Python-based user scripts to interface with it. Fortunately Python is one of the easiest computer languages to understand, learn, and use, due to its (mostly) readable syntax, similarity to other object-oriented languages, and many useful libraries. Once you reconcile yourself to its quirks (significant indentation, mandatory use of self, runtime error checking, etc.) you may appreciate it as a very quick (if sometimes dirty) way to crank out useful scripts and code.

There are a vast variety of free Python tutorials available on the internet, from books to complete courses. The Python documentation at http://docs.python.org will also quickly become your friend if it isn't already.

In addition to helping to locate Python tutorials, Google seems to work astoundingly well for finding answers to Python questions, many of which are answered on http://stackoverflow.com.

Useful Background for Using Mininet

(Random list, no particular order.)

  1. Object oriented programming (creating classes, objects from classes, etc.)

  2. Importing Python modules

    http://docs.python.org/tutorial/modules.html

  3. Invoking system utilities from Python

    http://docs.python.org/library/subprocess.html http://docs.python.org/library/os.html] https://github.com/amoffat/pbs

  4. Parsing output files in your own format

    http://docs.python.org/library/re.html

  5. Passing command line arguments to your script

    http://docs.python.org/dev/library/argparse.html

  6. matplotlib for plotting graphs

    http://matplotlib.sourceforge.net

Useful Python Resources

  1. The Python documentation is a good place to start:

    http://docs.python.org/tutorial/index.html

  2. Beginner’s guide:

    http://www.python.org/about/gettingstarted

How Does It Really Work? (optional: for the curious)

The magic behind Mininet’s illusion is a set of features built into Linux that allow a single system to be split into a bunch of smaller “containers”, each with a fixed share of the processing power, combined with virtual link code that allows links with accurate delays and speeds.

Internally, Mininet employs lightweight virtualization features in the Linux kernel, including process groups, CPU bandwidth isolation, and network namespaces, and combines them with link schedulers and virtual Ethernet links. These features yield a system that starts faster and scales to more hosts than emulators which use full virtual machines.

A Mininet network consists of the following components:

  1. Isolated Hosts. An emulated host in Mininet is a group of user-level processes moved into a network namespace - a container for network state. Network namespaces provide process groups with exclusive ownership of interfaces, ports, and routing tables (such as ARP and IP). For example, two web servers in two network namespaces can coexist on one system, both listening to private eth0 interfaces on port 80. Mininet uses CPU Bandwidth Limiting to limit the fraction of a CPU available to each process group.

  2. Emulated Links. The data rate of each link is enforced by Linux Traffic Control (tc), which has a number of packet schedulers to shape traffic to a configured rate. Each emulated host has its own virtual Ethernet interface(s) (created and installed with ip link add/set). A virtual Ethernet (or veth) pair, acts like a wire connecting two virtual interfaces, or virtual switch ports; packets sent through one interface are delivered to the other, and each interface appears as a fully functional Ethernet port to all system and application software.

  3. Emulated Switches. Mininet typically uses the default Linux bridge or Open vSwitch running in kernel mode to switch packets across interfaces. Switches and routers can run in the kernel (for speed) or in user space (so we can modify them easily).

For more details, see the Mininet website and our Mininet Publications.

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