nsport and Encapsulation Reference in IEEE 1588-2008 UDP over IPv4 Annex D UDP over IPv6 Annex E IEEE 802.3/Ethernet Annex F DeviceNET (from www.odva.org) Annex G ControlNET Annex H IEC 61158 Type 10 (PROFINET) Annex I At layer 3, UDP/IP can carry PTP messages as either unicast or multicast or even some mixture of the two depending on the profile and the use case; generally, these encapsulations are informally re- ferred to as PTPoIP (PTP over Internet protocol). Event messages use UDP port number 319 and general messages use 320, which makes it quite easy to discern between the two in an application tool like Wireshark. Although PTP profiles for other industries commonly use L3 multicast, the telecom profiles that use IP as a transport (G.8265.1 and G.8275.2) are unicast only and mandate support for IPv4, whereas IPv6 is optional to implement. 7.3.1 One-Step Versus Two-Step Clocks The preceding section showed the master port sending the Sync message plus the t1 timestamp and the optional Follow_Up message. If the designer of the PTP node feels that the accuracy of the timestamp cannot be assured when it is being sent inside the Sync message, then the implementa- tion allows the timestamp to be included in the Follow U sent a short time later. To indicate that this has been done, PTP sets a flag in the Sync message (twoStepFlag) to indicate that the correct timestamp is coming in the later Follow_U message. So, the options are send the Sync with the t1 timestamp or send the Sync without the t1 and gener- ate the Follow_Up including it. A PTP clock that sends just the Sync message is known as a one- step clock, whereas one that uses a subsequent Follow_Up is known as a two-step clock. Whichever approach is implemented is completely at the discretion of the master port. The down- stream slave is required to accept either approach without problem. The authors have run across implementations (especially some radio devices) that cannot accept PTP messages from a two-step master, which is clearly a faulty implementatio