A method implemented in a network device coupled to a software-defined networking, SDN, system , wherein the SDN system contains a set of network devices and a SDN controller managing the set of network devices, the method comprising: receiving , at the network device , a packet from the SDN controller to monitor a path for a service between the network device and another network device ; and sending , at the network device , the packet to at least a first and a second port of the network device , wherein a first instance of the packet is transmitted from the first port to the other network device to monitor the path, and wherein a second instance of the packet is transmitted from the second port that loops back the second instance of the packet; characterised in that the second port loops back the packet to the network device at a transmission interval so that the packet is sent to the first port repetitively at the transmission interval; and in that the method further comprises: determining whether or not the packet is received by the network device from the other network device within a timeout interval; and based on the determining , performing one of: discarding the packet upon determination that the packet is received by the network device within the timeout interval; and upon determining that the packet is not received within the timeout interval, notifying the SDN controller , determining the packet is received after the timeout interval, and notifying the SDN controller of the packet being received after the timeout interval. receiving , at the network device , a packet from the SDN controller to monitor a path for a service between the network device and another network device ; and sending , at the network device , the packet to at least a first and a second port of the network device , wherein a first instance of the packet is transmitted from the first port to the other network device to monitor the path, and wherein a second instance of the packet is transmitted from the second port that loops back the second instance of the packet; characterised in that the second port loops back the packet to the network device at a transmission interval so that the packet is sent to the first port repetitively at the transmission interval; and in that the method further comprises: determining whether or not the packet is received by the network device from the other network device within a timeout interval; and based on the determining , performing one of: discarding the packet upon determination that the packet is received by the network device within the timeout interval; and upon determining that the packet is not received within the timeout interval, notifying the SDN controller , determining the packet is received after the timeout interval, and notifying the SDN controller of the packet being received after the timeout interval. discarding the packet upon determination that the packet is received by the network device within the timeout interval; and upon determining that the packet is not received within the timeout interval, notifying the SDN controller , determining the packet is received after the timeout interval, and notifying the SDN controller of the packet being received after the timeout interval. <EOS>