Local Peer Discovery

The Local Peer Discovery protocol, specified as BEP-14,^[1] is an extension to the BitTorrent file-distribution system. It is designed to support the discovery of local BitTorrent peers, aiming to minimize the traffic through the Internet service provider's (ISP) channel and maximize use of higher-bandwidth local area networks (LANs).

Local Peer Discovery is implemented^[2] with HTTP-like messages on User Datagram Protocol (UDP) multicast group 239.192.152.143:6771 (IPv4) or ff15::efc0:988f (IPv6)^[1] which are administratively scoped multicast addresses. It's similar to Simple Service Discovery Protocol but sends BT-SEARCH instead of M-SEARCH:

BT-SEARCH * HTTP/1.1\r\n
Host: <host>\r\n
Port: <port>\r\n
Infohash: <ihash>\r\n
cookie: <cookie (optional)>\r\n
\r\n
\r\n

Since implementation is simple, Local Peer Discovery is implemented in several clients (μTorrent,^[3] BitTorrent/Mainline,^[4] MonoTorrent,^[5] libtorrent^[6] and its derivatives, Transmission,^[7] aria2^[8]). An alternative multicast peer discovery protocol based on ZeroConf is published as BEP 26 Zeroconf Peer Advertising and Discovery, but is not widely adopted since it is considered too complex^[9] in comparison.