Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The Yahoo! Messenger Protocol is the underlying network protocol used by the Yahoo! Messenger instant messaging client, for Yahoo!. Yahoo! Instant Messager supports many features beyond just messaging, including off-line messaging, file transfer, chat, conferencing, voice chat, webcams and avatars. The purpose of the YMSG protocol is to provide a language and series of conventions for software communicating with Yahoo!''s Instant Messaging service. In essence YMSG performs the same role for IM as HTTP does for the World Wide Web. Unlike HTTP, however, YMSG is a proprietary standard, aligned only with a single messaging service provider (namely, Yahoo!). Rival messaging services have their own protocols, some based on open standards, others proprietary, each effectively fulfilling the same role with different mechanics.