Re: [lwip-users] [OT] Sync local time to NTP

[address@hidden]

Am 30.12.2022 um 17:10 schrieb Giuseppe Modugno:
> The typical scenario of lwip is an embedded platform. When a wall clock
> is needed and the device is network connected, NTP is usually used to
> retrieve automatically the correct time at startup and stay in sync with
> an accurate reference (in other cases, the user can set a time as he wants).
>
> Consider an embedded device without a battery. At startup there's no
> notion of current time, so we can start from an epoch (1970, 2020 or
> whatever) and consider it the correct time. Another approach is
> considering the time not initialized at all.
>
> lwip has a SNTP client implementation that magically calls a function

Absolutely no magic in there: when a response from a server is received,
it is parsed and this callback is called with the time received.

> SNTP_SET_SYSTEM_TIME_NTP(s, f) when a response from a SNTP server is
> received. Now the question is what to do with s, seconds from 1970 epoch
> (ignore fractional part, just to simplify).

This is one level upwards from lwIP. We provide the protocol
implementations, not your system design.

> One possibility is to have a system counter clocked at 1s that counts
> seconds from 1970. So SNTP_SET_SYSTEM_TIME_NTP() could simply overwrite
> the counter value. However this simple approach brings to a counter that
> isn't monotonic and continuous. I know the best approach is to slowed
> down or accelerate the reference clock of the system counter that tracks
> seconds from epoch.
>
> Do you really implement something similar or are you happy with
> replacing the counter with the new value reported by NTP?

That totally depends on what you desire for your product (what you use
the timestamp for). For example, if you only use it for logging, it
might be enough to add a line to your log that states the received time
and the delta.

Linux, on the other hand has at least 2 clocks, one "monotinic" clock
(e.g. ticks since boot) and one "wall" clock. Timsync daemons may still
vote to add a syslog line if the received SNTP time differs too much
from what they would expect.

Regards,
Simon