在Coursera SDN开放课程中，编程作业要用Mininet来完成。这里对Mininet做一个简单的介绍。
什么是Mininet
Mininet是由一些虚拟的终端节点（end-hosts）、交换机、路由器连接而成的一个网络仿真器，它采用轻量级的虚拟化技术使得系统可以和真实网络相媲美。
Mininet可以很方便地创建一个支持SDN的网络：host就像真实的电脑一样工作，可以使用ssh登录，启动应用程序，程序可以向以太网端口发送数据包，数据包会被交换机、路由器接收并处理。有了这个网络，就可以灵活地为网络添加新的功能并进行相关测试，然后轻松部署到真实的硬件环境中。
Mininet的特性
可以简单、迅速地创建一个支持用户自定义的网络拓扑，缩短开发测试周期
可以运行真实的程序，在Linux上运行的程序基本上可以都可以在Mininet上运行，如Wireshark
Mininet支持Openflow，在Mininet上运行的代码可以轻松移植到支持OpenFlow的硬件设备上
Mininet可以在自己的电脑，或服务器，或虚拟机，或者云（例如Amazon EC2）上运行
Mininet提供python API，简单易用
Mininet是开源项目，源代码在这里：https://github.com/mininet
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Mininet安装
使用VirtualBox安装Mininet虚拟机：http://mininet.org/download/
使用Mininet创建一个网络
以Coursera SDN Week3 programming assignment为例，创建一个及其简单的数据中心网络。
Data center networks typically have a tree-like topology. End-hosts connect to top-of-rack switches, which form the leaves (edges) of the tree; one or more core switches form the root; and one or more layers of aggregation switches form the middle of the tree. In a basic tree topology, each switch (except the core switch) has a single parent switch. Additional switches and links may be added to construct more complex tree topologies (e.g., fat tree) in an effort to improve fault tolerance or increase inter-rack bandwidth.
In this assignment, your task is to create a simple tree topology. You will assume each level i.e., core, aggregation, edge and host to be composed of a single layer of switches/hosts with a configurable fanout value (k) looks like:
代码：
# CustomTopo.py ''' Coursera: - Software Defined Networking (SDN) course -- Module 3 Programming AssignmentProfessor: Nick Feamster Teaching Assistant: Muhammad Shahbaz '''from mininet.topo import Topo from mininet.net import Mininet from mininet.node import CPULimitedHost from mininet.link import TCLink from mininet.util import irange,dumpNodeConnections from mininet.log import setLogLevelclass CustomTopo(Topo):"Simple Data Center Topology""linkopts - (1:c1, 2:aggregation, 3: edge) parameters""fanout - number of child switch per parent switch"def __init__(self, linkopts1, linkopts2, linkopts3, fanout=2, **opts):# Initialize topology and default optionsTopo.__init__(self, **opts)# Add your logic here ...self.fanout = fanoutcore = self.addSwitch('c1')for i in irange(1, fanout):aggregation = self.addSwitch('a%s' %i)self.addLink(core, aggregation, **linkopts1)for j in irange(1, fanout):edge = self.addSwitch('e%s' %(fanout*(i-1)+j))self.addLink(aggregation, edge, **linkopts2)for k in irange(1, fanout):host = self.addHost('h%s' %((fanout*(fanout*(i-1)+j-1))+k))self.addLink(edge, host, **linkopts3)topos = { 'custom': ( lambda: CustomTopo() ) }def simpleTest():"Create and test a simple network"linkopts1 = dict(bw=10, delay='3ms', use_htb=True)linkopts2 = dict(bw=8, delay='4ms', loss=1, max_queue_size=900, )linkopts3 = dict(bw=6, delay='5ms', loss=1, max_queue_size=800)topo = CustomTopo(linkopts1, linkopts2, linkopts3, fanout=2)net = Mininet(topo, host=CPULimitedHost, link=TCLink)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 informationsetLogLevel('info')simpleTest()
在mininet虚拟机上执行下面操作即可创建自定义的网络拓扑。函数simpleTest()创建网络并进行了简单的ping测试，从屏幕输出可以看到创建的过程。
mininet@mininet-vm:~/mininet$ sudo python CustomTopo.py
更多资料
1. Mininet: http://mininet.org/
2. Mininet wiki: https://github.com/mininet/mininet/wiki