Add TCULink and make it the default for --switch user

For some time, UserSwitch has suffered from abysmal performance due
to rx and tx offload settings. In the latest Ubuntu 16.04/Linux 4.4
kernel, it simply doesn't work with the default settings, since
packets with bad TCP checksums are generated at one end, passed through
the user switch, and dropped at the other. It doesn't seem to be
something we can fix in UserSwitch itself although it may be possible
to do something in the C code of the Stanford reference switch and
CPqD switch.

bin/mn

@@ -30,7 +30,7 @@ from mininet.node import ( Host, CPULimitedHost, Controller, OVSController,
                            UserSwitch, OVSSwitch, OVSBridge,
                            IVSSwitch )
 from mininet.nodelib import LinuxBridge
-from mininet.link import Link, TCLink, OVSLink
+from mininet.link import Link, TCLink, TCULink, OVSLink
 from mininet.topo import ( SingleSwitchTopo, LinearTopo,
                            SingleSwitchReversedTopo, MinimalTopo )
 from mininet.topolib import TreeTopo, TorusTopo
@@ -73,18 +73,18 @@ HOSTS = { 'proc': Host,
           'cfs': specialClass( CPULimitedHost, defaults=dict( sched='cfs' ) ) }
 
 CONTROLLERDEF = 'default'
-
 CONTROLLERS = { 'ref': Controller,
                 'ovsc': OVSController,
                 'nox': NOX,
                 'remote': RemoteController,
                 'ryu': Ryu,
-                'default': DefaultController,  # Note: replaced below
+                'default': DefaultController,  # Note: overridden below
                 'none': NullController }
 
 LINKDEF = 'default'
-LINKS = { 'default': Link,
+LINKS = { 'default': Link,  # Note: overridden below
           'tc': TCLink,
+          'tcu': TCULink,
           'ovs': OVSLink }
 
 # TESTS dict can contain functions and/or Mininet() method names
@@ -341,11 +341,19 @@ class MininetRunner( object ):
                                      "for switch %s" %
                                      opts.switch )
 
+
+
         topo = buildTopo( TOPOS, opts.topo )
         switch = customClass( SWITCHES, opts.switch )
         host = customClass( HOSTS, opts.host )
         controller = [ customClass( CONTROLLERS, c )
                        for c in opts.controller ]
+
+        if opts.switch == 'user' and opts.link == 'default':
+            debug( '*** Using TCULink with UserSwitch\n' )
+            # Use link configured correctly for UserSwitch
+            opts.link = 'tcu'
+
         link = customClass( LINKS, opts.link )
 
         if self.validate: