FFmpeg RTSP流通过UDP传输问题

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:admin123@192.168.50.187:554/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:  Unknown Error!
[rtsp @ 0x55842477d740] Could not find codec parameters for stream  (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 #, rtsp, from 'rtsp://admin:admin123@192.168.50.187:554/profile1':
  Metadata:
    title           : profile1
  Duration: N/A, start: 0.000000, bitrate:  kb/s
    Stream #:: Video: h264, none, 90k tbr, 90k tbn, 180k tbc
    Stream #:: Audio: pcm_alaw,  Hz, mono, s16,  kb/s
Output #, flv, to 'rtmp://192.168.50.212/live/test':
  Metadata:
    title           : profile1
    encoder         : Lavf58.27.103
    Stream #:: Audio: pcm_alaw ([][][][] / 0x0007),  Hz, mono, s16,  kb/s
Stream mapping:
  Stream #: -> #: (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 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:admin123@192.168.50.187:554/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：

https://github.com/ZoneMinder/ZoneMinder/issues/811

http://mamicode.com/info-detail-1871429.html