Peer exchange (PEX) is a feature of the BitTorrent peer-to-peer protocol which, like trackers and DHT, can be utilized to gather peers. Using peer exchange, an existing peer is used to trade the information required to find and connect to additional peers. While it may improve (local) performance and robustness—e.g. if a tracker is slow or even down—heavy reliance on PEX can lead to the formation of groups of peers who tend to only share information with each other, which may yield slow propagation of data through the network, due to the few peers sending information to those outside the group they are in.
Clients supporting peer exchange
Each of these clients implement an incompatible version of peer exchange:
- Azureus and clients based on it (The Azureus PEX is only compatible with the Transmission client. PEX with other clients has been implemented from Azureus 3.0.4.3 onwards)
- BitComet
- Bitflu
- BitTorrent
- Deluge
- FlashGet
- KTorrent has implemented full µTorrent PEX support as of 2.1 RC1
- Opera 9.5 - µTorrent PEX support
- qTorrent - µTorrent PEX support
- µTorrent
- libtorrent and clients based on it (Deluge, qBittorrent, MooPolice) compatible with µTorrent
- rTorrent
- Transmission (compatible with both the μTorrent and Azureus implementations)
- XTorrent being based on Transmission source code, equally fully supports the Azureus and µTorrent implementations as of version 1.0 (v40)
- aria2 - µTorrent PEX support
Implementation
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.
