我自己在使用SRS服务的Ingest功能时发现在读取一个网络摄像头的RTSP流时一直不成功，
具体分析后发现SRS在调用FFmpeg时出了问题：
/usr/local/ffmpeg/bin/ffmpeg -i rtsp://admin:[email protected]:554/profile1 -vcodec copy -acodec copy -f flv -y rtmp://192.168.50.212/live/test
报错如下：
[rtsp @ 0x55842477d740] UDP timeout, retrying with TCP [rtsp @ 0x55842477d740] method SETUP failed: 404 Unknown Error! [rtsp @ 0x55842477d740] Could not find codec parameters for stream 0 (Video: h264, none): unspecified size Consider increasing the value for the 'analyzeduration' and 'probesize' options Guessed Channel Layout for Input Stream #0.1 : mono Input #0, rtsp, from 'rtsp://admin:[email protected]:554/profile1': Metadata: title : profile1 Duration: N/A, start: 0.000000, bitrate: 64 kb/s Stream #0:0: Video: h264, none, 90k tbr, 90k tbn, 180k tbc Stream #0:1: Audio: pcm_alaw, 8000 Hz, mono, s16, 64 kb/s Output #0, flv, to 'rtmp://192.168.50.212/live/test': Metadata: title : profile1 encoder : Lavf58.27.103 Stream #0:0: Audio: pcm_alaw ([7][0][0][0] / 0x0007), 8000 Hz, mono, s16, 64 kb/s Stream mapping: Stream #0:1 -> #0:0 (copy) Press [q] to stop, [?] for help [flv @ 0x5584247e5a00] Failed to update header with correct duration. [flv @ 0x5584247e5a00] Failed to update header with correct filesize. size= 0kB time=00:00:00.00 bitrate=N/A speed= 0x 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)
在网上搜了一下，发现命令中强制使用TCP链接就不会报错：
/usr/local/ffmpeg/bin/ffmpeg -rtsp_transport tcp -i rtsp://admin:[email protected]:554/profile1 -vcodec copy -acodec copy -f flv -y rtmp://192.168.50.212/live/test
但是搜了半天也没人解释的清楚为什么，暂且找到相关的两篇记录下来：