NePA TesT: Community Network Emulator

"""

    Mininet: A simple networking testbed for OpenFlow/SDN!

author: Bob Lantz (rlantz@cs.stanford.edu)

author: Brandon Heller (brandonh@stanford.edu)

Mininet creates scalable OpenFlow test networks by using

process-based virtualization and network namespaces.

Simulated hosts are created as processes in separate network

namespaces. This allows a complete OpenFlow network to be simulated on

top of a single Linux kernel.

Each host has:

A virtual console (pipes to a shell)

A virtual interfaces (half of a veth pair)

A parent shell (and possibly some child processes) in a namespace

Hosts have a network interface which is configured via ifconfig/ip

link/etc.

This version supports both the kernel and user space datapaths

from the OpenFlow reference implementation (openflowswitch.org)

as well as OpenVSwitch (openvswitch.org.)

In kernel datapath mode, the controller and switches are simply

processes in the root namespace.

Kernel OpenFlow datapaths are instantiated using dpctl(8), and are

attached to the one side of a veth pair; the other side resides in the

host namespace. In this mode, switch processes can simply connect to the

controller via the loopback interface.

In user datapath mode, the controller and switches can be full-service

nodes that live in their own network namespaces and have management

interfaces and IP addresses on a control network (e.g. 192.168.123.1,

currently routed although it could be bridged.)

In addition to a management interface, user mode switches also have

several switch interfaces, halves of veth pairs whose other halves

reside in the host nodes that the switches are connected to.

Consistent, straightforward naming is important in order to easily

identify hosts, switches and controllers, both from the CLI and

from program code. Interfaces are named to make it easy to identify

which interfaces belong to which node.

The basic naming scheme is as follows:

    Host nodes are named h1-hN

    Switch nodes are named s1-sN

    Controller nodes are named c0-cN

    Interfaces are named {nodename}-eth0 .. {nodename}-ethN

Note: If the network topology is created using mininet.topo, then

node numbers are unique among hosts and switches (e.g. we have

h1..hN and SN..SN+M) and also correspond to their default IP addresses

of 10.x.y.z/8 where x.y.z is the base-256 representation of N for

hN. This mapping allows easy determination of a node's IP

address from its name, e.g. h1 -> 10.0.0.1, h257 -> 10.0.1.1.

Note also that 10.0.0.1 can often be written as 10.1 for short, e.g.

"ping 10.1" is equivalent to "ping 10.0.0.1".

Currently we wrap the entire network in a 'mininet' object, which

constructs a simulated network based on a network topology created

using a topology object (e.g. LinearTopo) from mininet.topo or

mininet.topolib, and a Controller which the switches will connect

to. Several configuration options are provided for functions such as

automatically setting MAC addresses, populating the ARP table, or

even running a set of terminals to allow direct interaction with nodes.

After the network is created, it can be started using start(), and a

variety of useful tasks maybe performed, including basic connectivity

and bandwidth tests and running the mininet CLI.

Once the network is up and running, test code can easily get access

to host and switch objects which can then be used for arbitrary

experiments, typically involving running a series of commands on the

hosts.

After all desired tests or activities have been completed, the stop()

method may be called to shut down the network.

"""

import os

import re

import select

import signal

from time import sleep

from itertools import chain

from mininet.cli import CLI

from mininet.log import info, error, debug, output

from mininet.node import Host, OVSKernelSwitch, Controller

from mininet.link import Link, Intf

from mininet.util import quietRun, fixLimits, numCores, ensureRoot

from mininet.util import macColonHex, ipStr, ipParse, netParse, ipAdd

from mininet.term import cleanUpScreens, makeTerms

# Mininet version: should be consistent with README and LICENSE

VERSION = "2.1.0+"

class Mininet( object ):

    "Network emulation with hosts spawned in network namespaces."

    def __init__( self, topo=None, switch=OVSKernelSwitch, host=Host,

                  controller=Controller, link=Link, intf=Intf,

                  build=True, xterms=False, cleanup=False, ipBase='10.0.0.0/8',

                  inNamespace=False,

                  autoSetMacs=False, autoStaticArp=False, autoPinCpus=False,

                  listenPort=None ):

        """Create Mininet object.

           topo: Topo (topology) object or None

           switch: default Switch class

           host: default Host class/constructor

           controller: default Controller class/constructor

           link: default Link class/constructor

           intf: default Intf class/constructor

           ipBase: base IP address for hosts,

           build: build now from topo?

           xterms: if build now, spawn xterms?

           cleanup: if build now, cleanup before creating?

           inNamespace: spawn switches and controller in net namespaces?

           autoSetMacs: set MAC addrs automatically like IP addresses?

           autoStaticArp: set all-pairs static MAC addrs?

           autoPinCpus: pin hosts to (real) cores (requires CPULimitedHost)?

           listenPort: base listening port to open; will be incremented for

               each additional switch in the net if inNamespace=False"""

        self.topo = topo

        self.switch = switch

        self.host = host

        self.controller = controller

        self.link = link

        self.intf = intf

        self.ipBase = ipBase

        self.ipBaseNum, self.prefixLen = netParse( self.ipBase )

        self.nextIP = 1  # start for address allocation

        self.inNamespace = inNamespace

        self.xterms = xterms

        self.cleanup = cleanup

        self.autoSetMacs = autoSetMacs

        self.autoStaticArp = autoStaticArp

        self.autoPinCpus = autoPinCpus

        self.numCores = numCores()

        self.nextCore = 0  # next core for pinning hosts to CPUs

        self.listenPort = listenPort

        self.hosts = []

        self.switches = []

        self.controllers = []

        self.nameToNode = {}  # name to Node (Host/Switch) objects

        self.terms = []  # list of spawned xterm processes

        Mininet.init()  # Initialize Mininet if necessary

        self.built = False

        if topo and build:

            self.build()

    def addHost( self, name, cls=None, **params ):

        """Add host.

           name: name of host to add

           cls: custom host class/constructor (optional)

           params: parameters for host

           returns: added host"""

        # Default IP and MAC addresses

        defaults = { 'ip': ipAdd( self.nextIP,

                                  ipBaseNum=self.ipBaseNum,

                                  prefixLen=self.prefixLen ) +

                                  '/%s' % self.prefixLen }

        if self.autoSetMacs:

            defaults[ 'mac'] = macColonHex( self.nextIP )

        if self.autoPinCpus:

            defaults[ 'cores' ] = self.nextCore

            self.nextCore = ( self.nextCore + 1 ) % self.numCores

        self.nextIP += 1

        defaults.update( params )

        if not cls:

            cls = self.host

        h = cls( name, **defaults )

        self.hosts.append( h )

        self.nameToNode[ name ] = h

        return h

    def addSwitch( self, name, cls=None, **params ):

        """Add switch.

           name: name of switch to add

           cls: custom switch class/constructor (optional)

           returns: added switch

           side effect: increments listenPort ivar ."""

        defaults = { 'listenPort': self.listenPort,

                     'inNamespace': self.inNamespace }

        defaults.update( params )

        if not cls:

            cls = self.switch

        sw = cls( name, **defaults )

        if not self.inNamespace and self.listenPort:

            self.listenPort += 1

        self.switches.append( sw )

        self.nameToNode[ name ] = sw

        return sw

    def addController( self, name='c0', controller=None, **params ):

        """Add controller.

           controller: Controller class"""

        # Get controller class

        if not controller:

            controller = self.controller

        # Construct new controller if one is not given

        if isinstance(name, Controller):

            controller_new = name

            # Pylint thinks controller is a str()

            # pylint: disable=E1103

            name = controller_new.name

            # pylint: enable=E1103

        else:

            controller_new = controller( name, **params )

        # Add new controller to net

        if controller_new:  # allow controller-less setups

            self.controllers.append( controller_new )

            self.nameToNode[ name ] = controller_new

        return controller_new

    # BL: We now have four ways to look up nodes

    # This may (should?) be cleaned up in the future.

    def getNodeByName( self, *args ):

        "Return node(s) with given name(s)"

        if len( args ) == 1:

            return self.nameToNode[ args[ 0 ] ]

        return [ self.nameToNode[ n ] for n in args ]

    def get( self, *args ):

        "Convenience alias for getNodeByName"

        return self.getNodeByName( *args )

    # Even more convenient syntax for node lookup and iteration

    def __getitem__( self, key ):

        """net [ name ] operator: Return node(s) with given name(s)"""

        return self.nameToNode[ key ]

    def __iter__( self ):

        "return iterator over node names"

        for node in chain( self.hosts, self.switches, self.controllers ):

            yield node.name

    def __len__( self ):

        "returns number of nodes in net"

        return ( len( self.hosts ) + len( self.switches ) +

                 len( self.controllers ) )

    def __contains__( self, item ):

        "returns True if net contains named node"

        return item in self.nameToNode

    def keys( self ):

        "return a list of all node names or net's keys"

        return list( self )

    def values( self ):

        "return a list of all nodes or net's values"

        return [ self[name] for name in self ]

    def items( self ):

        "return (key,value) tuple list for every node in net"

        return zip( self.keys(), self.values() )

    def addLink( self, node1, node2, port1=None, port2=None,

                 cls=None, **params ):

        """"Add a link from node1 to node2

            node1: source node

            node2: dest node

            port1: source port

            port2: dest port

            returns: link object"""

        defaults = { 'port1': port1,

                     'port2': port2,

                     'intf': self.intf }

        defaults.update( params )

        if not cls:

            cls = self.link

        return cls( node1, node2, **defaults )

    def configHosts( self ):

        "Configure a set of hosts."

        for host in self.hosts:

            info( host.name + ' ' )

            intf = host.defaultIntf()

            if intf:

                host.configDefault()

            else:

                # Don't configure nonexistent intf

                host.configDefault( ip=None, mac=None )

            # You're low priority, dude!

            # BL: do we want to do this here or not?

            # May not make sense if we have CPU lmiting...

            # quietRun( 'renice +18 -p ' + repr( host.pid ) )

            # This may not be the right place to do this, but

            # it needs to be done somewhere.

            host.cmd( 'ifconfig lo up' )

        info( '\n' )

    def buildFromTopo( self, topo=None ):

        """Build mininet from a topology object

           At the end of this function, everything should be connected

           and up."""

        # Possibly we should clean up here and/or validate

        # the topo

        if self.cleanup:

            pass

        info( '*** Creating network\n' )

        if not self.controllers and self.controller:

            # Add a default controller

            info( '*** Adding controller\n' )

            classes = self.controller

            if type( classes ) is not list:

                classes = [ classes ]

            for i, cls in enumerate( classes ):

                self.addController( 'c%d' % i, cls )

        info( '*** Adding hosts:\n' )

        for hostName in topo.hosts():

            self.addHost( hostName, **topo.nodeInfo( hostName ) )

            info( hostName + ' ' )

        info( '\n*** Adding switches:\n' )

        for switchName in topo.switches():

            self.addSwitch( switchName, **topo.nodeInfo( switchName) )

            info( switchName + ' ' )

        info( '\n*** Adding links:\n' )

        for srcName, dstName in topo.links(sort=True):

            src, dst = self.nameToNode[ srcName ], self.nameToNode[ dstName ]

            params = topo.linkInfo( srcName, dstName )

            srcPort, dstPort = topo.port( srcName, dstName )

            self.addLink( src, dst, srcPort, dstPort, **params )

            info( '(%s, %s) ' % ( src.name, dst.name ) )

        info( '\n' )

    def configureControlNetwork( self ):

        "Control net config hook: override in subclass"

        raise Exception( 'configureControlNetwork: '

                         'should be overriden in subclass', self )

    def build( self ):

        "Build mininet."

        if self.topo:

            self.buildFromTopo( self.topo )

        if self.inNamespace:

            self.configureControlNetwork()

        info( '*** Configuring hosts\n' )

        self.configHosts()

        if self.xterms:

            self.startTerms()

        if self.autoStaticArp:

            self.staticArp()

        self.built = True

    def startTerms( self ):

        "Start a terminal for each node."

        if 'DISPLAY' not in os.environ:

            error( "Error starting terms: Cannot connect to display\n" )

            return

        info( "*** Running terms on %s\n" % os.environ[ 'DISPLAY' ] )

        cleanUpScreens()

        self.terms += makeTerms( self.controllers, 'controller' )

        self.terms += makeTerms( self.switches, 'switch' )

        self.terms += makeTerms( self.hosts, 'host' )

    def stopXterms( self ):

        "Kill each xterm."

        for term in self.terms:

            os.kill( term.pid, signal.SIGKILL )

        cleanUpScreens()

    def staticArp( self ):

        "Add all-pairs ARP entries to remove the need to handle broadcast."

        for src in self.hosts:

            for dst in self.hosts:

                if src != dst:

                    src.setARP( ip=dst.IP(), mac=dst.MAC() )

    def start( self ):

        "Start controller and switches."

        if not self.built:

            self.build()

        info( '*** Starting controller\n' )

        for controller in self.controllers:

            controller.start()

        info( '*** Starting %s switches\n' % len( self.switches ) )

        for switch in self.switches:

            info( switch.name + ' ')

            switch.start( self.controllers )

        info( '\n' )

    def stop( self ):

        "Stop the controller(s), switches and hosts"

        if self.terms:

            info( '*** Stopping %i terms\n' % len( self.terms ) )

            self.stopXterms()

        info( '*** Stopping %i switches\n' % len( self.switches ) )

        for switch in self.switches:

            info( switch.name + ' ' )

            switch.stop()

        info( '\n' )

        info( '*** Stopping %i hosts\n' % len( self.hosts ) )

        for host in self.hosts:

            info( host.name + ' ' )

            host.terminate()

        info( '\n' )

        info( '*** Stopping %i controllers\n' % len( self.controllers ) )

        for controller in self.controllers:

            info( controller.name + ' ' )

            controller.stop()

        info( '\n*** Done\n' )

    def run( self, test, *args, **kwargs ):

        "Perform a complete start/test/stop cycle."

        self.start()

        info( '*** Running test\n' )

        result = test( *args, **kwargs )

        self.stop()

        return result

    def monitor( self, hosts=None, timeoutms=-1 ):

        """Monitor a set of hosts (or all hosts by default),

           and return their output, a line at a time.

           hosts: (optional) set of hosts to monitor

           timeoutms: (optional) timeout value in ms

           returns: iterator which returns host, line"""

        if hosts is None:

            hosts = self.hosts

        poller = select.poll()

        Node = hosts[ 0 ]  # so we can call class method fdToNode

        for host in hosts:

            poller.register( host.stdout )

        while True:

            ready = poller.poll( timeoutms )

            for fd, event in ready:

                host = Node.fdToNode( fd )

                if event & select.POLLIN:

                    line = host.readline()

                    if line is not None:

                        yield host, line

            # Return if non-blocking

            if not ready and timeoutms >= 0:

                yield None, None

    # XXX These test methods should be moved out of this class.

    # Probably we should create a tests.py for them

    @staticmethod

    def _parsePing( pingOutput ):

        "Parse ping output and return packets sent, received."

        # Check for downed link

        if 'connect: Network is unreachable' in pingOutput:

            return 1, 0

        r = r'(\d+) packets transmitted, (\d+) received'

        m = re.search( r, pingOutput )

        if m is None:

            error( '*** Error: could not parse ping output: %s\n' %

                   pingOutput )

            return 1, 0

        sent, received = int( m.group( 1 ) ), int( m.group( 2 ) )

        return sent, received

    def ping( self, hosts=None, timeout=None ):

        """Ping between all specified hosts.

           hosts: list of hosts

           timeout: time to wait for a response, as string

           returns: ploss packet loss percentage"""

        # should we check if running?

        packets = 0

        lost = 0

        ploss = None

        if not hosts:

            hosts = self.hosts

            output( '*** Ping: testing ping reachability\n' )

        for node in hosts:

            output( '%s -> ' % node.name )

            for dest in hosts:

                if node != dest:

                    opts = ''

                    if timeout:

                        opts = '-W %s' % timeout

                    result = node.cmd( 'ping -c1 %s %s' % (opts, dest.IP()) )

                    sent, received = self._parsePing( result )

                    packets += sent

                    if received > sent:

                        error( '*** Error: received too many packets' )

                        error( '%s' % result )

                        node.cmdPrint( 'route' )

                        exit( 1 )

                    lost += sent - received

                    output( ( '%s ' % dest.name ) if received else 'X ' )

            output( '\n' )

        if packets > 0:

            ploss = 100 * lost / packets

            received = packets - lost

            output( "*** Results: %i%% dropped (%d/%d received)\n" %

                    ( ploss, received, packets ) )

        else:

            ploss = 0

            output( "*** Warning: No packets sent\n" )

        return ploss

    @staticmethod

    def _parsePingFull( pingOutput ):

        "Parse ping output and return all data."

        errorTuple = (1, 0, 0, 0, 0, 0)

        # Check for downed link

        r = r'[uU]nreachable'

        m = re.search( r, pingOutput )

        if m is not None:

            return errorTuple

        r = r'(\d+) packets transmitted, (\d+) received'

        m = re.search( r, pingOutput )

        if m is None:

            error( '*** Error: could not parse ping output: %s\n' %

                   pingOutput )

            return errorTuple

        sent, received = int( m.group( 1 ) ), int( m.group( 2 ) )

        r = r'rtt min/avg/max/mdev = '

        r += r'(\d+\.\d+)/(\d+\.\d+)/(\d+\.\d+)/(\d+\.\d+) ms'

        m = re.search( r, pingOutput )

        if m is None:

            error( '*** Error: could not parse ping output: %s\n' %

                   pingOutput )

            return errorTuple

        rttmin = float( m.group( 1 ) )

        rttavg = float( m.group( 2 ) )

        rttmax = float( m.group( 3 ) )

        rttdev = float( m.group( 4 ) )

        return sent, received, rttmin, rttavg, rttmax, rttdev

    def pingFull( self, hosts=None, timeout=None ):

        """Ping between all specified hosts and return all data.

           hosts: list of hosts

           timeout: time to wait for a response, as string

           returns: all ping data; see function body."""

        # should we check if running?

        # Each value is a tuple: (src, dsd, [all ping outputs])

        all_outputs = []

        if not hosts:

            hosts = self.hosts

            output( '*** Ping: testing ping reachability\n' )

        for node in hosts:

            output( '%s -> ' % node.name )

            for dest in hosts:

                if node != dest:

                    opts = ''

                    if timeout:

                        opts = '-W %s' % timeout

                    result = node.cmd( 'ping -c1 %s %s' % (opts, dest.IP()) )

                    outputs = self._parsePingFull( result )

                    sent, received, rttmin, rttavg, rttmax, rttdev = outputs

                    all_outputs.append( (node, dest, outputs) )

                    output( ( '%s ' % dest.name ) if received else 'X ' )

            output( '\n' )

        output( "*** Results: \n" )

        for outputs in all_outputs:

            src, dest, ping_outputs = outputs

            sent, received, rttmin, rttavg, rttmax, rttdev = ping_outputs

            output( " %s->%s: %s/%s, " % (src, dest, sent, received ) )

            output( "rtt min/avg/max/mdev %0.3f/%0.3f/%0.3f/%0.3f ms\n" %

                    (rttmin, rttavg, rttmax, rttdev) )

        return all_outputs

    def pingAll( self ):

        """Ping between all hosts.

           returns: ploss packet loss percentage"""

        return self.ping()

    def pingPair( self ):

        """Ping between first two hosts, useful for testing.

           returns: ploss packet loss percentage"""

        hosts = [ self.hosts[ 0 ], self.hosts[ 1 ] ]

        return self.ping( hosts=hosts )

    def pingAllFull( self ):

        """Ping between all hosts.

           returns: ploss packet loss percentage"""

        return self.pingFull()

    def pingPairFull( self ):

        """Ping between first two hosts, useful for testing.

           returns: ploss packet loss percentage"""

        hosts = [ self.hosts[ 0 ], self.hosts[ 1 ] ]

        return self.pingFull( hosts=hosts )

    @staticmethod

    def _parseIperf( iperfOutput ):

        """Parse iperf output and return bandwidth.

           iperfOutput: string

           returns: result string"""

        r = r'([\d\.]+ \w+/sec)'

        m = re.findall( r, iperfOutput )

        if m:

            return m[-1]

        else:

            # was: raise Exception(...)

            error( 'could not parse iperf output: ' + iperfOutput )

            return ''

    # XXX This should be cleaned up

    def iperf( self, hosts=None, l4Type='TCP', udpBw='10M' ):

        """Run iperf between two hosts.

           hosts: list of hosts; if None, uses opposite hosts

           l4Type: string, one of [ TCP, UDP ]

           returns: results two-element array of server and client speeds"""

        if not quietRun( 'which telnet' ):

            error( 'Cannot find telnet in $PATH - required for iperf test' )

            return

        if not hosts:

            hosts = [ self.hosts[ 0 ], self.hosts[ -1 ] ]

        else:

            assert len( hosts ) == 2

        client, server = hosts

        output( '*** Iperf: testing ' + l4Type + ' bandwidth between ' )

        output( "%s and %s\n" % ( client.name, server.name ) )

        server.cmd( 'killall -9 iperf' )

        iperfArgs = 'iperf '

        bwArgs = ''

        if l4Type == 'UDP':

            iperfArgs += '-u '

            bwArgs = '-b ' + udpBw + ' '

        elif l4Type != 'TCP':

            raise Exception( 'Unexpected l4 type: %s' % l4Type )

        server.sendCmd( iperfArgs + '-s', printPid=True )

        servout = ''

        while server.lastPid is None:

            servout += server.monitor()

        if l4Type == 'TCP':

            while 'Connected' not in client.cmd(

                    'sh -c "echo A | telnet -e A %s 5001"' % server.IP()):

                output('waiting for iperf to start up...')

                sleep(.5)

        cliout = client.cmd( iperfArgs + '-t 5 -c ' + server.IP() + ' ' +

                             bwArgs )

        debug( 'Client output: %s\n' % cliout )

        server.sendInt()

        servout += server.waitOutput()

        debug( 'Server output: %s\n' % servout )

        result = [ self._parseIperf( servout ), self._parseIperf( cliout ) ]

        if l4Type == 'UDP':

            result.insert( 0, udpBw )

        output( '*** Results: %s\n' % result )

        return result

    def runCpuLimitTest( self, cpu, duration=5 ):

        """run CPU limit test with 'while true' processes.

        cpu: desired CPU fraction of each host

        duration: test duration in seconds

        returns a single list of measured CPU fractions as floats.

        """

        pct = cpu * 100

        info('*** Testing CPU %.0f%% bandwidth limit\n' % pct)

        hosts = self.hosts

        for h in hosts:

            h.cmd( 'while true; do a=1; done &' )

        pids = [h.cmd( 'echo $!' ).strip() for h in hosts]

        pids_str = ",".join(["%s" % pid for pid in pids])

        cmd = 'ps -p %s -o pid,%%cpu,args' % pids_str

        # It's a shame that this is what pylint prefers

        outputs = []

        for _ in range( duration ):

            sleep( 1 )

            outputs.append( quietRun( cmd ).strip() )

        for h in hosts:

            h.cmd( 'kill %1' )

        cpu_fractions = []

        for test_output in outputs:

            # Split by line.  Ignore first line, which looks like this:

            # PID %CPU COMMAND\n

            for line in test_output.split('\n')[1:]:

                r = r'\d+\s*(\d+\.\d+)'

                m = re.search( r, line )

                if m is None:

                    error( '*** Error: could not extract CPU fraction: %s\n' %

                           line )

                    return None

                cpu_fractions.append( float( m.group( 1 ) ) )

        output( '*** Results: %s\n' % cpu_fractions )

        return cpu_fractions

    # BL: I think this can be rewritten now that we have

    # a real link class.

    def configLinkStatus( self, src, dst, status ):

        """Change status of src <-> dst links.

           src: node name

           dst: node name

           status: string {up, down}"""

        if src not in self.nameToNode:

            error( 'src not in network: %s\n' % src )

        elif dst not in self.nameToNode:

            error( 'dst not in network: %s\n' % dst )

        else:

            if type( src ) is str:

                src = self.nameToNode[ src ]

            if type( dst ) is str:

                dst = self.nameToNode[ dst ]

            connections = src.connectionsTo( dst )

            if len( connections ) == 0:

                error( 'src and dst not connected: %s %s\n' % ( src, dst) )

            for srcIntf, dstIntf in connections:

                result = srcIntf.ifconfig( status )

                if result:

                    error( 'link src status change failed: %s\n' % result )

                result = dstIntf.ifconfig( status )

                if result:

                    error( 'link dst status change failed: %s\n' % result )

    def interact( self ):

        "Start network and run our simple CLI."

        self.start()

        result = CLI( self )

        self.stop()

        return result

    inited = False

    @classmethod

    def init( cls ):

        "Initialize Mininet"

        if cls.inited:

            return

        ensureRoot()

        fixLimits()

        cls.inited = True

class MininetWithControlNet( Mininet ):

    """Control network support:

       Create an explicit control network. Currently this is only

       used/usable with the user datapath.

       Notes:

       1. If the controller and switches are in the same (e.g. root)

          namespace, they can just use the loopback connection.

       2. If we can get unix domain sockets to work, we can use them

          instead of an explicit control network.

       3. Instead of routing, we could bridge or use 'in-band' control.

       4. Even if we dispense with this in general, it could still be

          useful for people who wish to simulate a separate control

          network (since real networks may need one!)

       5. Basically nobody ever used this code, so it has been moved

          into its own class.

       6. Ultimately we may wish to extend this to allow us to create a

          control network which every node's control interface is