[ntpwg] NTP interleaved modes

[David L. Mills]

Guys,

I should have mentioned the proof of performance is measured in the 
updated briefing 
http://eecis.udel.edu/~mills/database/brief/precise/precise.html.

Dave

David L. Mills wrote:

> Guys,
>
> NTP symmetric and broadcast interleaved modes as described in a 
> previous message have been implemented and tested in the development 
> version. The best performance shows reliable jitter after the clock 
> filter in the order of 20 us. The white paper at 
> http://www.eecis.udel.edu/~mills/onwire.html and briefing at 
> http://www.ecis.udel.edu/~mills/database/brief/onwire/onwire.ppt have 
> been updated. The documentation on the web has been updated as well.
>
> The interleaved modes coexist with the normal modes just fine. 
> Interleaved broadcast can be operated simultaneously with ordinary 
> broadcasts, so prior versions will not be disturbed. Interleaved 
> clients will discover the new mode and automaticallyh switch. A 
> symmetric peer capable of interleaved mode will revert to ordinary 
> mode if the other peer cannot do interleave. Interleaved mode is 
> activated by a new xleave option in the broadcast and peer commands.
>
> While the new modes reduce the output wait due to kernel and queueing 
> latencies from 16 us to over 1 ms, depeing on NIC hardware and driver, 
> obviously they are designed for hardware and/or driver timestamping. 
> I've been looking at ways to do that. I have a couple of IEEE 1588 
> NICs from Meinberg and have looked over the driver and documentation. 
> It uses the AMD79C973 PCNET chip augmented by a FPGA that on command 
> captures the SOF timestamp. The pcn driver in FreeBSD can uses this 
> chip. The FPGA is treated as a separate read-only device triggered by 
> the chip interrupt.
>
> The original motivation for this project was to use the protocol with 
> the CCSDS Proximity protocol designed for near-space links used on 
> Mars missions and soon on Moon missions. NASA won't let me have a 
> rover for experiment, so the next best thing might be an Ethernet 
> project. I'm somewhat over my head with Unix drivers, but I have had a 
> long look at the FreeBSD drivers and Ethernet interface code. I've 
> even had a serious experience with the AMD chip documentation (Wow, 
> are those chips getting fancy!). The Meingerg driver is for Linux and 
> from previous ex;perience I really don't want to use that system. The 
> FreeBSD pcn driver could easily be hooked to do the same thing as the 
> Meinberg driver. My problem is the FPGA driver, which shouldn't be 
> hard for an experienced driverologist.
>
> So, I'm tossing out the challenge. Anybody want to take on the FPGA 
> driver? Make no mistake, this is not intended as a challenge to 1588, 
> but rather an interesting challenge with goal NTP performance over the 
> wire is as good as the PPS signal, which is in the 2-us range.
>
> Dave
>