In a P2P video streaming it is said that "when a client wants to see a video, it contacts it's tracker to find other peers that have a copy of the video"
My question is: What does "a copy of the video" mean? Does it mean that the video is in one of the peer's memory? Or does the tracker keep track of where the video on a peer's local hard drive is, if so, what if the video is moved/deleted?
When downloading a file from a tracker in a P2P system peers announce that they are participating in sharing a particular file or set of files.
The tracker doesn't need to know where that file is on the remote peer, just that it has it.
When a peer contacts the tracker it is told what other peers have access to the file. The peer can then ask other peers "tracker said you have a file I want, what parts do you have?"
From there it's just downloading blocks of files from each other. A peer doesn't need to know the exact location on a remote system, it just works on a file ID and block numbers and it is up to the remote peer to keep track of the file and blocks.
If you move a file out of your download area and/or stop sharing it then your local P2P software can no longer share it because it has no idea where you moved it to.
In bittorrent the tracker(s) only track announcements of peers that say they want to participate in the swarm, that is all. In other words they only track contact information (IP address, port) for peers that say they have some particular content (torrent hash). They generally do not verify such data and only keep some limited statistics self-reported by the peers beyond that.
All the heavy lifting happens between the peers once they have found each other through the tracker.
