The uTorrent program follows the BitTorrent protocol for file-sharing networks. File sharing means copying files from other private individuals over the Internet. The protocol uses a peer-to-peer architecture, which means there is not a client requesting a resource, nor a server holding resources. Each member of the network is simultaneously client and server, sending files and receiving files. This makes them "peers." The flags displayed in the uTorrent interface relate to the status of connections to these peers.
A file available for download is called a torrent. A copy of a torrent is called a seed and the owner of a partial copy is called a peer. The word "peer" has two different meanings in the uTorrent system. In the context of the status flags, peer does not refer to owners of partial copies uTorrent is trying to get; it means all network members with whom the program is currently in contact, either to fetch a part of a file or to donate one.
uTorrent begins searching for a torrent when the user loads in a torrent metafile. This file contains details of the file and the location of a tracker. The tracker lists the current owners of copies of that torrent and their IP addresses. The uTorrent program works through the list, contacting each to request a block of the torrent. In the meantime, other uTorrent programs in the world contact the requesting program to ask for blocks of torrents it has active. Thus, each torrent program running on different computers throughout the world is both making and receiving requests for files simultaneously. The "Peers" subsection of a uTorrent's detail panel lists current activity for all contact with other computers. The "Flags" column summarises the outcome of the contact.
Each flag is represented by a letter and each line contains several flags. The most common of there are "u"/"U," "d"/"D," "I, "H," "X," "e"/ "E" and "P." The first flag mean uploading (u/U) or downloading (d/D); a capital letter means the contact was successful and the transfer is taking place. A lower-case letter means the owner of the file was too busy to send it. "I" means an incoming connection has been established but no file transfer has yet occurred. The uTorrent system has two other methods for finding peers, other than through the tracker. One method is called DHT, and the "H" flag shows this is how the current peer was located. A peer located through Peer Exchange, where a peer appears on a peer list sent by another peer, is indicated by the "X" flag. The "e" flag shows that the peer is using Protocol Encryption for handshaking and "E" means it uses Protocol Encryption for all traffic. The "P" flag indicates communication is being made using the uTP protocol, which is a lightweight version of the BitTorrent protocol.
The uTorrent user manual lists a total of 17 different flags. Users of the uTorrent program should refer to the help system for a full description of all flags.