• "Anatomy" of torrent-session

"Anatomy" of torrent-session
Many users don't how torrent-client communicates with tracker. Let's observe one concrete session. "Communication" of client and tracker is realized with simple HTTP-requests using method GET. The first command usually looks like this:
GET /announce.php?passkey=xxx&info_hash=c%97%91%C5jG%951%BE%C7M%F9%BFa%03%F2%2C%ED%EE%0F& peer_id=S588-----gqQ8TqDeqaY&port=55500&uploaded=0&downloaded=0&left=753690875&event=started
Let's see, from which parts it consists:
• passkey - unique value used to identify you to tracker;
• info_hash - unique value (control sum, hash), used to identify torrent;
• peer_id - unique value used to identify used torrent-client (as example, s588 - Shad0w's client v5.8.8, after it comes random string);
• port - port number, which is used by client;
• uploaded=0 - uploaded information amount;
• downloaded=0 - downloaded information amount;
• left=753690875 - left information amount;
• event=started - system message to inform tracker, that client is starting downloading.
Client's IP-address in this command is optional, so it wasn't used. Tracker automatically makes the conclusion about neccessity of using IP-address and binding it to the specific user_id. The downloaded torrent will appear in section "Downloading now" of your profile as active.
During downloading client will periodically send tracker requests like:
GET /announce.php?passkey=xxx&info_hash=c%97%91%C5jG%951%BE%C7M%F9%BFa%03%F2%2C%ED%EE%0F& peer_id=S588-----gqQ8TqDeqaY&port=55500&uploaded=67960832&downloaded=40828928& left=715417851&numwant=0
As you can see from request string, that client has uploaded 68 MB and downloaded 40. Using the command "numwant" client reports tracker how many peers it needs (currently - 0). Received this request from client, tracker automatically updates information about downloaded and uploaded data for current torrent, and in general statistics. All these intermediate HTTP-requests are sent to the tracker with certain periodicity, which is determined from client's and tracker's settings. You can even send manual update request to the tracker (in particular, uTorrent has special command "Update tracker").
At the of session client sends tracker command like:
GET /announce.php?passkey=xxx&info_hash=c%97%91%C5jG%951%BE%C7M%F9%BFa%03%F2%2C%ED%EE%0F& peer_id=S588-----gqQ8TqDeqaY&port=55500&uploaded=754384896&downloaded=754215163 &left=0&numwant=0&event=completed
When download is finished client sends system message, like "event=completed". After this information about torrent will be deleted from section "Downloading now" of your profile. If in some cases (like tracker is overloaded or disabled, unstable client version is used, tracker connection error or other technical problems on your computer, etc) the last request haven't reached the server, torrent, probably, will "hang" in your profile as active until tracker will delete "dead" connections during cleanup or you will do it manually.
We draw your attention to importance of right way of shutting down the client. Before closing the programm it is reuqired to stop all active torrents with commands like "Stop" or "Cancel" (tracker will receive system message like "event=stopped"). Using commands "Pause"/"Continue" to stop active torrents, and simply closing the programm is highly undesirable due to opportunity of appearence of "hanging" torrents and statistics errors (because system messages may not be sent during programm shutdown).