ffmpeg保存rtp流

[xiaodong60606]

./ffmpeg -i rtsp://192.168.1.100:855 -vcodec copy out.avi
这个命令哪里错了？
为什么不能保存呢。
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)