表示基本的一条链路，最基本的链路在 mininet 中其实就是一对 veth 接口对。

  1. def __init__( self, node1, node2, port1=None, port2=None,
  2.                   intfName1=None, intfName2=None,
  3.                   intf=Intf, cls1=None, cls2=None, params1=None,
  4.                   params2=None ):
  5.         """Create veth link to another node, making two new interfaces.
  6.            node1: first node
  7.            node2: second node
  8.            port1: node1 port number (optional)
  9.            port2: node2 port number (optional)
  10.            intf: default interface class/constructor
  11.            cls1, cls2: optional interface-specific constructors
  12.            intfName1: node1 interface name (optional)
  13.            intfName2: node2  interface name (optional)
  14.            params1: parameters for interface 1
  15.            params2: parameters for interface 2"""
  16.         # This is a bit awkward; it seems that having everything in
  17.         # params would be more orthogonal, but being able to specify
  18.         # in-line arguments is more convenient!
  19.         if port1 is None:
  20.             port1 = node1.newPort()
  21.         if port2 is None:
  22.             port2 = node2.newPort()
  23.         if not intfName1:
  24.             intfName1 = self.intfName( node1, port1 )
  25.         if not intfName2:
  26.             intfName2 = self.intfName( node2, port2 )
  28.         self.makeIntfPair( intfName1, intfName2 )
  30.         if not cls1:
  31.             cls1 = intf
  32.         if not cls2:
  33.             cls2 = intf
  34.         if not params1:
  35.             params1 = {}
  36.         if not params2:
  37.             params2 = {}
  39.         intf1 = cls1( name=intfName1, node=node1, port=port1,
  40.                       link=self, **params1  )
  41.         intf2 = cls2( name=intfName2, node=node2, port=port2,
  42.                       link=self, **params2 )
  44.         # All we are is dust in the wind, and our two interfaces
  45.         self.intf1, self.intf2 = intf1, intf2

创建链路时，需要在两个节点上分别生成两个端口，利用节点和端口，获取对应的两个网络接口的名称，例如 s1-eth0 和 h1-eth0，然后调用 makeIntfPair() 方法，最终调用 util.py 中的 makeIntfPair() 方法，调用系统中的 ip link 命令来创造一对 veth pair。

  1. def makeIntfPair( intf1, intf2 ):
  2.     """Make a veth pair connecting intf1 and intf2.
  3.        intf1: string, interface
  4.        intf2: string, interface
  5.        returns: success boolean"""
  6.     # Delete any old interfaces with the same names
  7.     quietRun( 'ip link del ' + intf1 )
  8.     quietRun( 'ip link del ' + intf2 )
  9.     # Create new pair
  10.     cmd = 'ip link add name ' + intf1 + ' type veth peer name ' + intf2
  11.     return quietRun( cmd )