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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def protocol(self):
 """The iperf3 instance protocol valid protocols are 'tcp' and 'udp' :rtype: str """

        proto_id = self.lib.iperf_get_test_protocol_id(self._test)

        if proto_id == SOCK_STREAM:
            self._protocol = 'tcp'
        elif proto_id == SOCK_DGRAM:
            self._protocol = 'udp'

        return self._protocol