Four Messages, Two Questions: On the Equivalence of the PTP / White Rabbit Exchange and the OAE Four-Message Commit

The delay request-response exchange of IEEE 1588, which White Rabbit inherits unchanged, uses four messages: Sync, Follow_Up, Delay_Req and Delay_Resp. The Open Æthernet (OAE) commit also uses four: prepare, prepare-ack, commit and commit-ack. We show that the two are the same protocol in a precise and limited sense. Strip the two-step Follow_Up, which carries a timestamp and adds no causal loop, and the PTP exchange is exactly the first three messages of the OAE commit, with the same alternation of direction, the same reply relation on the last two messages, and the same knowledge at every step. The protocols then differ in two places only. PTP stops one message short, so its master ends every exchange in the state the OAE custody table calls "message 3 landed, message 4 not landed". And PTP asks of its four timestamps a question they cannot answer: the one-way delay, and hence the offset, is not determined by any exchanged timestamps without a prior on asymmetry. The OAE commit asks only whether the third message landed, a local fact independent of every clock. The same four-message budget buys an estimate in one protocol and an agreement in the other. White Rabbit sharpens the timestamps to picoseconds and supplies the asymmetry prior by calibration; it changes neither the shape of the exchange nor the question asked of it.