This library is basically SANS-IO except for calls to Instant::now. I was wondering what the appetite is for changing that?

• APIs like encapsulate and decapsulate should likely take an now: Instant parameter to accurately reflect timestamps of last received packets.
• update_timers should also take a now: Instant parameter.

update_timers can save the provided now timestamp as local state and all other functions can compute durations etc based on that timestamp. Thus, accuracy of that depends entirely on the user in how often they call update_timers.

We might want to consider renaming update_timers to something like handle_timeout or update_now.