I think we've been having similar thoughts. The way I'm imagining it, the applications can each provide a function matching some standardized interface to the

I think we've been having similar thoughts. The way I'm imagining it, the applications can each provide a function matching some standardized interface to the network layer. So every time two nodes are near enough to synchronize, they loop through all of the applications they have in common and run the functions for those applications. Maybe some are maintaining a web of trust, others are verifying the content in some way before they decide to store it, who knows--the network layer doesn't care, it just has some node-operator-configured resources to allocate to that app: 1GB of storage and 15 seconds of sync time every 5 minutes or somesuch. It would be up to the apps to write their sync functions such that old or irrelevant data gets forgotten, new or venerated data gets propagated, etc. Also they'd have to figure out how to make it worth everyone's while to actually dedicate those resources. You probably wouldn't end up with millions of apps like this--it would only make sense if many people around you were also using the same app--but even just two or three useful ones would probably make it worthwhile to build the lower layers. E-mail strikes me as one of the more difficult ones to do because it's sort of fundamentally point-to-point, but I could imagine a sync function which gossips about which other nodes that node typically comes across such that whenever two nodes meet in an elevator or at a stoplight or whatever there's a sort of gradient indicating which of the two is more likely to carry a message to its destination.