The operational mode of the egress timestamping function is determined by the settings on the TX_PTP_1588OP_IN port. The information contained within the command port indicates one of the following:
• No operation: the frame is not a PTP frame and no timestamp action should be taken.
• Two-step operation requires a tag value (user-sequence ID) which is provided as part of the control input; the frame should be timestamped, and the timestamp is made available to the client logic along with the provided tag value for the frame. The additional MAC transmitter ports provide this function.
• 1-step operation requirements:
° For the ToD timer and timestamp format, a timestamp offset value is provided as part of the command port; the frame should be timestamped, and the timestamp should be inserted into the frame at the provided offset (number of bytes) into the frame.
° For the Transparent clock mode, a Correction Field offset value is provided as part of the command port; the frame should be timestamped, and the captured 64-bit Timestamp is summed with the existing Correction Field contained within the frame and the summed result is overwritten into original Correction Field of the frame.
For a 1-step operation following the frame modification, the cyclic redundancy check (CRC) value of the frame should also be updated/recalculated. For UDP IPv4 and IPv6 PTP formatted frames, the checksum value in the header of the frame needs to be updated/recalculated.
• For 1-step UDP frame types, the UDP checksum is updated in accordance with IETF RFC 1624.
° If using the Ordinary Clock mode, in order for this update function to work correctly, the original checksum value for the frame sent for transmission should be calculated using a zero value for the timestamp data. This particular restriction does not apply when using the Transparent clock mode.
° If using the Transparent clock mode, a different restriction does apply; the separation between the UDP Checksum field and the Correction Field within the 1588 PTP frame header is a fixed interval of bytes, supporting the 1588 PTP frame definition. This is a requirement to minimize the latency through the MAC since both the checksum and the correction field must both be fully contained in the MAC pipeline in order for the checksum to be correctly updated. This particular restriction does not apply to the Ordinary Clock mode because the original timestamp data is calculated as a zero value; consequently the checksum and timestamp position can be independently located within the frame.