Create a Distributed File Transfer System like Bittorrent

How will you ensure that a file is hosted on the system reliably, even if one machine goes down?

If a user wants to download a file, how do you locate the machine that holds it?

You probably need some sort of ledger or metadata table, how will you store that?

How will you make sure that a download is completed?

When a machine is added, how do you scale the cluster?

How do you secure the files and ensure no one modifies them?