5.27.1.6 Distribution of Announce messages and best master clock selection

The procedure described in this clause is applicable if DS-TT and NW-TT support operating as a Boundary Clock described in IEEE Std 1588 [126] or as a time-aware system (support of the IEEE 802.1AS [104] PTP profile) and when the PTP instance in 5GS is configured to operate as a time-aware system or as a Boundary Clock. Whether DS-TT/NW-TT support operating as a Boundary Clock or as a time-aware system may be determined as described in clause K.2.1.

The externally-observable behaviour of the Announce message handling by 5GS needs to comply with IEEE Std 802.1AS [104] or IEEE Std 1588 [126], respective to the configured mode of operation.

The DS-TT forwards the received Announce messages to NW-TT over User plane. The NW-TT port forwards the received Announce messages from N6 interface to NW-TT.

The NW-TT maintains the PTP port state for each DS-TT port and NW-TT port. The PTP port states may be determined by NW-TT either via:

• Method a), BMCA procedure.
• Method b), local configuration.

When Method b) is used, the following applies:

• When the PTP GM is external to the 5GS, for one of the NW-TT or DS-TT ports (per each PTP domain) the PTP port state is Follower and for all other NW-TT and DS-TT ports of the same PTP domain the PTP port state is set either to Passive or Leader (depending on implementation).
• When the 5GS is configured as a grandmaster for a (g)PTP domain for the connected networks, all NW-TT ports and DS-TT ports are set to Leader state for that (g)PTP domain.

The local configuration of PTP port states in DS-TT and NW-TT for Method b is described in clause K.2.

When the Method a) is used (PTP port states are determined by BMCA procedure), the NW-TT needs to process the received Announce messages (from NW-TT port(s) and over user plane from the DS-TT(s)) for BMCA procedure, determine port states within the 5GS and maintain the Leader-Follower hierarchy.

The DS-TT maintains the Disabled, Initializing and Faulty (if applicable) state for the PTP ports in DS-TT. While in Disabled, Initializing or Faulty state, the port in the DS-TT discards any (g)PTP messages it may receive from the upstream PTP instance or from the NW-TT via user plane.

When the 5GS Clock is determined as a grandmaster for a (g)PTP domain, the Announce messages are distributed as described in clause 5.27.1.7.

When the grandmaster is external to the 5GS, the NW-TT regenerates the Announce messages based on the Announce messages received from Follower port in NW-TT or DS-TT for the Leader ports in NW-TT and DS-TT(s). The NW-TT/UPF forwards the regenerated Announce messages to the PDU session(s) related to the Leader ports in the DS-TT(s).

NOTE 1: The TSN AF or TSCTSF can use the portDS.portState in the "Time synchronization information for each DS-TT port" element in UMIC to read and get notified for the port state changes for the PTP ports in DS-TT(s) and the portDS.portState in PMIC to read and get notified for the port state changes for the PTP ports in NW-TT. Based on the change of the port states, TSN AF or TSCTSF can determine that an external Grandmaster PTP Instance is found to be used instead of the GM in 5GS, or the GM in 5GS is selected as the Grandmaster PTP Instance and TSN AF or TSCTSF can disable or enable the (g)PTP grandmaster functionality in DS-TT(s), respectively.

NOTE 2: The TSN AF or TSCTSF can use the portDS.portState in PMIC to read and get notified for the port state changes for the PTP ports in DS-TT for the port state Disabled, Initializing or Faulty (if applicable). The TSCTSF or TSN AF can use the portDS.portEnable to indicate to the NW-TT that the DS-TT port is disabled. This avoids unnecessary (g)PTP traffic over User Plane to a DS-TT port in Disabled, Initializing or Faulty state.