This presumes humanity is a space fairing or interplanetary civilization.
How would something like the fediverse, internet, cryptocurrency, etc function with major latency? As an example, a signal takes between 5 and 20 minutes to travel from earth to mars. A roundtrip response would take at best 10 minutes and at worst 40 minutes. Now lets say you live on mars and your home lemmy instance is mars.social. You want to see what news people are chatting about on earth and heard that [email protected] is a good community. If you put that into your instance search box on mars.social the absolute best you can hope for is a response in 10 minutes. I assume the request would totally fail anyway due to rtt being set to low and the packets expiring before they ever reached the destination. The internet we all know and love is totally intolerant of high latency. Just ask people who use satellite internet or tor.
Edit: i think, but am not certain, that ipv6 replaced rtt with hop count. If so this may not be an issue as the time it takes would not matter as long as the hop limit was not reached.
I think you’re conflating two different questions here. These questions are really focused on a central question of “How would the internet work with latency measured in minutes, when most systems are configured around latency in milliseconds?” And the answer there is “We’d have to change how some systems work, and others wouldn’t be feasible.” Barring some method of FTL communication (which would be an instant Nobel prize in physics), you’re never going to get real time instant messaging between Earth and Mars, but async methods of communication will work fine, albeit with more latency. But we’re able to exchange digital data across planets now, it’s just that the public internet is built around the assumption that the speed of light is only going to account for ~100ms of lag.
If you take an assumption that high latency digital communication will be feasible relatively soon after we have people living off planet (which is a reasonable assumption), networks like the fediverse will function with a lot of caching, as tikitaki mentioned. You’d never have perfect sync, but the biggest challenge would likely be how much bandwidth is required to keep different caches in sync.
To answer your specific question, you’d probably start an initial cache on planet, then keep it in sync during transit.