[time-nuts] IEEE 1588 PTP support on raspberry pi 4 compute module

[Wojciech Owczarek]

Hi Bob,

On Wednesday, 21 October 2020, Bob kb8tq <kb8tq at n1k.org> wrote:


> *Networking* hardware is the issue …..
>

Undeniably so, but to what extent?



> If you have fully symmetric delays, then there is no need for 1588. NTP
> will
> do just fine.


Lab results and years of production deployment tell me otherwise. Hardware
timestamps are the key. NTP *with hardware timestamps on both server and
client* will do just fine, yes.


> Unfortunately as even home networking hardware loads up,
> ( = lots of traffic in only one direction) this may not be the case. When
> things
> *do* go asymmetric, then you *do* need 1588 compatible switches.
>
>
Quantify "lots". My argument is that the uncertainty of software-generated
timestamps can outweigh that of a network that isn't loaded up to choking
point e.g. little to no buffering. Home traffic tends to be bursty, with
occasional constant load and yes asymmetric. But your PTP device is not
sitting on your ISP link, neither of them will, and unless your host
running PTP is sitting on a saturated link, it really is not as bad as it
may seem. Even cheap switches these days are non-blocking, so unless you
have lots of multicast, PTP hosts on your network will not be critically
affected by other traffic. Yes, they are store and forward, but it's about
selecting packets in a band with minimum transit latency or, say, mode. The
band has to be wide enough to catch at least one packet per second. With a
rate of say 32 msg/sec chances of catching lucky packets increase. I can't
normally take results out of my work lab unfortunately, but I think I might
just drop a "home" switch in there and put some application traffic through
it for a proper test. I know what difference a PTP transparent clock switch
makes, but even without one, it really is "not that bad". Numbers or it
didn't happen, so I'll ad that to my to-do list.


Cheers,
Wojciech


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Wojciech Owczarek