As someone who's implemented a P2P feature for an application: UPnP works great... if you have a router that supports it. Many in the wild do not. If yours do

As someone who's implemented a P2P feature for an application: UPnP works great... if you have a router that supports it. Many in the wild do not. If yours doesn't, you have to manually port forward, which, even disregarding the newbie user issue, isn't possible if you're connected to someone else's network. Otherwise, as I assume you know, you can only initiate connections, not receive them, which means you can't talk to anyone with the same limitation - which might not be fatal to your BitTorrent download, albeit limiting your speed, but kinda sucks in something like a one-on-one video chat or game session. AFAIK, all major video chat applications use some variant of STUN, on which ICE is based, along with some proxying mechanism for users who truly can't connect directly to each other, which corresponds to the rest of ICE.