Users wishing to obtain a copy of a file typically first download a
torrent file that describes the file(s) to be shared, as well as the
URLs of one or more central computers called
trackers that maintain a list of peers currently sharing the file(s) described in the .torrent file. In the original BitTorrent design, peers then depended on this central tracker to find each other and maintain the swarm. Later development of
distributed hash tables (DHTs) meant that partial lists of peers could be held by other computers in the swarm and the load on the central tracker computer could be reduced. PEX allows peers in a swarm to exchange information about the swarm directly without asking (
polling) a tracker computer or a DHT. By doing so, PEX leverages the knowledge of peers that a user is connected to by asking them for the addresses of peers that they are connected to. This is faster and more efficient than relying solely on one tracker and reduces the processing load on the tracker. It also keeps swarms together when the tracker is down. Peer exchange cannot be used on its own to introduce a new peer to a swarm. To make initial contact with a swarm, each peer must either connect to a tracker using a ".torrent" file, or else use a router computer called a
bootstrap node to find a distributed hash table (DHT) which describes a swarm's list of peers. For most BitTorrent users, DHT and PEX will start to work automatically after the user launches a BitTorrent client and opens a .torrent file. A notable exception is "private torrents" which are not freely available; these will commonly disable DHT.
Peer Exchange Conventions Extensions to BitTorrent such as PEX are typically implemented using one of two common extension protocols, AZMP or LTEP. Both types of peer exchange send messages containing a group of peers to be added to the swarm and a group of peers to be removed. It was agreed between the
Azureus and
μTorrent developers that any clients which implement either of the mechanisms above try to obey the following limits when sending PEX messages:
DHT To create a PEX protocol providing a uniformly-distributed peer selection, one could form a small distributed hash table (DHT) local to a torrent. For each desired new peer one would look up a (uniformly) random key, and use the node responsible for the key as a new peer. This is conceptually simple but would require quite some overhead. For "trackerless" torrents, it is not clear if PEX provides any value since the
mainline DHT can distribute load as necessary. Each DHT node acting as a tracker may store only a subset of the peers, but these are
maximal subsets constrained only by DHT node load rather than by a single peer's view. Private torrents commonly disable the DHT, and for this case, PEX might be useful provided the peer obtains enough peers from the tracker. ==Versions==