[ntpwg] NTPv4 draft-8 to draft-9 changes / alternative paragraph

[STUART VENTERS]

Greg,

(Assuming I guessed right on which common practice you were thinking about...)

Yes, the rx timestamp words were as intended. The reason is as follows:

Placing the rx timestamp at SOP is a common practice in IEEE-1588.  Another common practice is the forwarding behavior of packet switching gear.  It tends to start sending a packet after receiving it.

A consequence of the choice that 1588 made is that it needs on-path to support time transfer to accuracy's better than a packet time for surprisingly simple packet paths.

Since NTP is interoperating with packet switches and not 1588 gear, it seems better to choose a practice compatible with the gear NTP is interoperating with.

Regards,

Stuart

ps. I used to think that SOP was a good compromise too.  Now, with an available, better alternative, it's just a compromise.


>Greg Dowd said:

>I still think the quasi-leading edge (SOF) is a good compromise.  If you
>can only timestamp after you have received a complete packet, I wonder
>if you have the intended definition of access to the physical layer.  If
>you do, common practice is to strike a timestamp on SOF and then make a
>decision to save once you've analyzed the frame. If you have physical
>layer access only at the end (but it's deterministic), it seems like you
>could make an empirical or predefined adjustment to approximate ingress
>time.

>>Stuart Venters said:
>>Here's an alternative paragraph which documents current practice, works
>>on existing packet paths, and leaves wiggle room for future spec efforts
>>with more time for careful study.

>>For interoperatability with existing equipment (switches, routers, and
>>the reference NTP implementation), implementations should use the
>>following guidelines for associating timestamps with physical layer
>>transitions.  Receive timestamps should be at the first point which
>>indicates a complete, good frame has gone by.  Transmit timestamps
>>should be at the first point which indicates that a frame transmission
>>has started.  Accuracy requirements and media specific details are FFS.