Some ISP like to kick user offline by a constant time, and pppd's default holdoff time setting is 30 secs.
For example, my ISP will kick me off every 3600 minutes. And when my ISP kick me off, I would experience no Internet for about 32-33 seconds.
So I made this patch to deal with this issue:
https://github.com/vyos/vyos-1x/pull/1899