Blizzard offers an individual client for each download they offer, instead of just a torrent file. When a large update is issued, Blizzard Downloader is downloaded to the WoW directory and it fetches the update file, using TCP ports 37. Early versions were based on the source code of the original BitTorrent client, though it is unclear what the source model is now. It is also used to download the trailers for The Burning Crusade, StarCraft II, and Diablo III. It is used to download the full game installer package, and periodic updates for the game.
It was first implemented with the MMORPG World of Warcraft.
Blizzard Downloader (or Blizzard Background Downloader, Background Downloader, BackgroundDownloader.exe, and others named something-BKGND-downloader) is a combination BitTorrent/HTTP client used for Blizzard Entertainment's software products.
