[time-nuts] Re: Network interface cards that support timestamping

[Poul-Henning Kamp]

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Poul-Henning Kamp via time-nuts writes:

> We used Intel '599 chip based cards in the ELT adaptive optics prototype
> compute cluster, and it has both hardware timestamping of packets and
> a couple of external signals.

I should probably mention a couple of things we learned the hard
way, even though I dont know if this applies to just the '599 chip:

1) The timestamping facility is very obviously a "bolt-on" and as such
   the documentation was severely lacking.

2) The timestamping counter runs at the RX-clock rate.

That means:

   a) You want something 10GE plugged into the port for max resolution.

   b) If you unplug it or it resets or reboots, your timescale jumps.

   c) You timescale is the Xtal at the /far/ end of the cable.

This can probably all be mitigated with a loop-back, but then you cannot
timestamp packets.

3) To timestamp packets, they have to look "sufficiently" like certain PTP
   packets.  This was not documented then, could be now.

Poul-Henning

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