A particular problem for companies attempting to port Windows games to the Macintosh is licensing middleware. Middleware is off-the-shelf software that handles certain aspects of games, making it easier for game creators to develop games in return for paying the middleware developer a licensing fee. However, since the license the Mac porting house obtains from the game creator does not normally include rights to use the middleware as well, the Mac porting company must either license the middleware separately or attempt to find an alternative. Examples of middleware include the Havok physics engine and the GameSpy internet-based multiplayer gaming client.