RFC 2326 - Official RTSP specification, published by the Internet Engineering Task Force (IETF) - http://tools.ietf.org/html/rfc2326
Unofficial IETF MMUSIC WG Page - Information about meetings and documents of the Multiparty Multimedia Session Control WG within the Transport Area of the IETF - http://www.dmn.tzi.org/ietf/mmusic/
SourceForge RTSP Specification Tracker - Official issue/errata list for RTSP, from which the future version of the RTSP specification will be made. - http://sourceforge.net/projects/rtspspec