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

[STUART VENTERS]

Jack,

It looks like the changes from NTPv4 draft-8 to draft-9 are editorial except for a couple of new paragraphs.

The editorial changes mostly clean up how references are cited.  (Nice job, it's much better.)

The new paragraphs are as follows:

On page 23 the following was added:
   If NTP has access to the physical layer, then the timestamps are
   associated with the beginning of the symbol after the start of frame.
   Otherwise, implementations should attempt to associate the timestamp
   to the earliest accessible point in the frame.


On page 25, the following was added:
   The Receive Timestamp and the Transmit Timestamp (set by the server)
   are undefined when in a KoD packet and MUST NOT be relied upon to
   have valid values and MUST be discarded.


As an implementer, I'm ok with everything except the new page 23 paragraph.


I'm working on transporting time to the edge of an access network for one-way packet delay measurements.  The server is back in the cloud with 1G interfaces.  The client is at the end of a T1 link.  The goals are modest, to make 1mS accurate measurements.  After adding up all the tolerances, in order to get enough accuracy, I'm going to have to violate the page 23 paragraph.


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.


Regards,

Stuart Venters