Current download rates decide which peers to unchoke. We use a 20-second average to decide this. Because of the use of TCP (slow-start) rapidly choking and unchoking is bad. Thus, this is calculated every 10 seconds.
What happens if all peers uploading to another peer decide to choke it? We then have to find new peers, but the optimistic unchoking mechanism only checks one unused connection every 30 seconds. To help the download rate recover more, BitTorrent has snubbing.
When we download a torrent, we get a hash of that torrent. To download the torrent without a tracker, we need to find other peers also downloading the torrent. To do this, we need to use a distributed hash table.
Or, on public trackers, downloading torrents made by trusted groups is preferable. After all, would you prefer to download Ubuntu from the Ubuntu team, or the user xxx-HACKER-ELITE-GHOST-PROTOCOL-xxx? 2b1af7f3a8