Onion-like routing. It takes multiple hops to get to a destination. Each hop can only decrypt the next destination to send the packet to (i.e. peeling off a layer of the onion).
Would that keep lawyers from just taking the last ip they get for their frivolous law suits? That way I could get a letter for something I actually didn't download
I mean, you can be sued for anything, but it will get thrown out. Like, I guess the MPAA could offer a movie for download, then try to sue the first hop they upload a chunk to, but that really doesn't make any sense (because they offered it for download in the first place). Furthermore, the first hop(s) aren't the people that are using the file, and they can't even read it. If people could successfully sue nodes, then ISPs and postal services could be sued for anything that passes through their networks.
By how the protocol is structured, it's impossible for the address a downloader sees to know what the packet they forward actually contains, so they're just taking the role of an ISP. Also, they don't know the original source IP.