[newjersey]

With no knowledge of how signal's servers actually implement this, I would like to ask you... how long should the central server hold on to the (encrypted) message that Alice sends to Bob if Bob never becomes available again?

[xorcist]

I haven't thought about what's reasonable (an hour? a week?) but it still shouldn't be dropped but bounced back to the sender with an undelivered message. At last that's how email solved it some thirty years ago.

(But perhaps a new protocol could take the opportunity to improve things further and offer on-line delivery status notifications throughout the process.)

No matter what, messages should never be silently dropped.

[newman314]

Best effort, deliver at least once?

At least that how it seems iMessage seems to work. I have no idea how long iMessage holds on the queue but it seems no more than a few days.

Not sure what kind of attack vector a public TTL would constitute.