I prefer it when the IP is killed as soon as possible so that I can see
when the connection is lost (ssh sessions get killed etc.)
Another reason for killing as soon as possible is the last-ack problem.
When the other end goes away suddenly TCP often gets into last-ack state.
This means it'll retransmit a FIN until it times out or the other end
answers. Each such retransmitted FIN triggers a new dialin, which can
get quite costly when you don't have flat rate (like still most of Europe).
With your approach (waiting until the new IP is known) it would cost
at least another dialin in this case.
When you have flatrate your way may be better of course, so a final
user space solution could switch it via a pppd flag.
[I agree that the user space way is better than my kernel hacks]
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