Hi, I'd like to clarify my comment during the meeting.
According to RFC1612 Eqn 3 (incremental checksum update): HC' = ~(C + (-m) + m') = ~(~HC + ~m + m') Where HC' is the updated checksum, HC is the current (message) checksum, m' in the new value that is going to be written instead of the value m in the message. If I know that value of m is zero, I can calculate the new checksum using the old checksum and the new value alone. That is, if the NTP/OWAMP/TWAMP CPU sets the transmit timestamp to zero (i.e. m = 0), the FPGA or switch that measures the transmit timestamp once the first byte of the message is sent on the wire, can update the UDP checksum itself incrementally and doesn't need the extension. The extension is need only if you have to read the timestamp which appears later in the packet in order to update the checksum. This is very different from 'real' intermediate devices (i.e. transparent clocks) used by PTP. Best, Ron
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