'Get port numbers.
src: source switch name
dst: destination switch name
sport: optional source port (otherwise use lowest src port)
returns: tuple (sport, dport), where
sport = port on source switch leading to the destination switch
dport = port on destination switch leading to the source switch
Note that you can also look up ports using linkInfo()'

def port(self, src, dst):

    ports = [(sport, entry[1]) for (sport, entry) in self.ports[src].items() if (entry[0] == dst)]
    return (ports if (len(ports) != 1) else ports[0])
