说明：  一种基于MP4文件的视频流关键帧索引播放方法，根据MP4格式特点结合rtsp sdp网络传播加入关键帧信息并实现从关键帧处点播
(MP4 file based on key frame index video stream playback method, according to MP4 format for network communication features combined with rtsp sdp adding key frame information and demand from a key frame at the)