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Peer exchange or PEX is a way for the computers in BitTorrent file sharing networks to connect with each other. It helps to maintain the group (known as a "swarm") of users (or peers) that are collaborating to share a given file.

In the original BitTorrent design the many peers (users) downloaded a ".torrent" file describing the file(s) to be shared and then depended on a central computer called a tracker to find each other and maintain the swarm. Later development of distributed hash tables (DHT) meant that sub-lists of peers could be held by other computers in the swarm and ease the load on the central tracker computer. Peer exchange allows the members of a swarm to exchange information about the swarm directly without asking (polling) a tracker computer or DHT. This is faster and reduces the load on the tracker.

Peer exchange can not introduce a new peer to a swarm entirely on its own. To make initial contact with the swarm, each peer must either connect to a tracker with a list of computers using the desired ".torrent" file, or use a router computer called a bootstrap node to find a distributed hash table which also contains a list of peers in that swarm, but is not held on any one central computer. For most Bittorrent users DHT and PEX will start to work automatically when they start a torrent. A notable exception is "private torrents" which are not freely available; these will disable DHT.

Recent development of a magnet link system to find other peers on a DHT means that peers can join a swarm without ever downloading a .torrent file.

[edit] Clients supporting peer exchange

Each of these clients implement some version of peer exchange:

• Vuze, formerly Azureus, and clients based on it (The Vuze PEX is only compatible with the Transmission client. PEX with other clients has been implemented into Vuze and into Azureus from 3.0.4.3 onwards)
• BitComet^{[citation needed]}
• Bitflu^[1]
• BitTorrent^{[citation needed]}
• FlashGet^{[citation needed]}
• KTorrent has implemented full µTorrent PEX support as of 2.1 RC1^[2]
• Opera 9.5 - µTorrent PEX support^[3]
• qTorrent - µTorrent PEX support
• µTorrent^[4]
• libtorrent and clients based on it (Deluge^[5], qBittorrent^[6], MooPolice^[7]) compatible with µTorrent
• rTorrent^[8]
• Transmission (compatible with both the μTorrent and Vuze implementations)^[9]
• XTorrent being based on Transmission source code, equally fully supports the Vuze and µTorrent implementations as of version 1.0 (v40)^{[citation needed]}
• aria2 - µTorrent PEX support

[edit] Implementation

[edit] DHT

To create a PEX protocol providing a uniformly distributed peer selection, one could form a small 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 disable the DHT, and for this case, PEX might be useful provided the peer obtains enough peers from the tracker.

[edit] References

[edit] External links

• Description on the official Azureus wiki