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

[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 
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).

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?