FFMPEG Demuxer-Muxer unable to operate with all input information: -r and -copy_unknown not working

[Javier Delgado del Cerro]

I am using FFMPEG as a demuxer-muxer to receive a MPEG source with audio and video streams, separate the streams and introduce the least possible delay. I have all the information about the input, including the container, codecs, audio and video rate, and audio channels.

I've been researching the different options that FFMPEG provides, and I've found the following ones which could help me:

• -codec:a and -codec:v to provide the information about the input's audio and video codecs.
• -f to provide the information about the used container.
• -r:v to provide the information about the video framerate. (I've also tried with -framerate, but it seems that is only supported by a few demuxers, and mpeg is not one of them).
• -probesize to avoid spending time probing the input, as I can provide the inpromation.
• -copy_unknown to simply copy the streams instead of failing if it can't be recognized.

With all this, I'm using the following command:
./ffmpeg -hide_banner -loglevel debug -codec:v h264 -codec:a pcm_alaw -flags low_delay -probesize 32 -analyzeduration 0 -r:v 15 -f mpeg -i udp://127.0.0.1:41071?listen -y -map 0:a:0? -acodec copy -copy_unknown -ar 8000 -payload_type 8 -f rtp udp://127.0.0.1:33605?pkt_size=1200 -map 0:v:0? -vcodec copy -copy_unknown -payload_type 96 -f rtp udp://127.0.0.1:48527?pkt_size=1200

Which outputs:

Splitting the commandline.
Reading option '-hide_banner' ... matched as option 'hide_banner' (do not show program banner) with argument '1'.
Reading option '-loglevel' ... matched as option 'loglevel' (set logging level) with argument 'debug'.
Reading option '-codec:v' ... matched as option 'codec' (codec name) with argument 'h264'.
Reading option '-codec:a' ... matched as option 'codec' (codec name) with argument 'pcm_alaw'.
Reading option '-flags' ... matched as AVOption 'flags' with argument 'low_delay'.
Reading option '-probesize' ... matched as AVOption 'probesize' with argument '32'.
Reading option '-analyzeduration' ... matched as AVOption 'analyzeduration' with argument '0'.
Reading option '-r:v' ... matched as option 'r' (set frame rate (Hz value, fraction or abbreviation)) with argument '15'.
Reading option '-f' ... matched as option 'f' (force format) with argument 'mpeg'.
Reading option '-i' ... matched as input url with argument 'udp://127.0.0.1:41071?listen'.
Reading option '-y' ... matched as option 'y' (overwrite output files) with argument '1'.
Reading option '-map' ... matched as option 'map' (set input stream mapping) with argument '0:a:0?'.
Reading option '-acodec' ... matched as option 'acodec' (force audio codec ('copy' to copy stream)) with argument 'copy'.
Reading option '-copy_unknown' ... matched as option 'copy_unknown' (Copy unknown stream types) with argument '1'.
Reading option '-ar' ... matched as option 'ar' (set audio sampling rate (in Hz)) with argument '8000'.
Reading option '-payload_type' ... matched as AVOption 'payload_type' with argument '8'.
Reading option '-f' ... matched as option 'f' (force format) with argument 'rtp'.
Reading option 'udp://127.0.0.1:33605?pkt_size=1200' ... matched as output url.
Reading option '-map' ... matched as option 'map' (set input stream mapping) with argument '0:v:0?'.
Reading option '-vcodec' ... matched as option 'vcodec' (force video codec ('copy' to copy stream)) with argument 'copy'.
Reading option '-copy_unknown' ... matched as option 'copy_unknown' (Copy unknown stream types) with argument '1'.
Reading option '-payload_type' ... matched as AVOption 'payload_type' with argument '96'.
Reading option '-f' ... matched as option 'f' (force format) with argument 'rtp'.
Reading option 'udp://127.0.0.1:48527?pkt_size=1200' ... matched as output url.
Finished splitting the commandline.
Parsing a group of options: global .
Applying option hide_banner (do not show program banner) with argument 1.
Applying option loglevel (set logging level) with argument debug.
Applying option y (overwrite output files) with argument 1.
Applying option copy_unknown (Copy unknown stream types) with argument 1.
    Last message repeated 1 times
Successfully parsed a group of options.
Parsing a group of options: input url udp://127.0.0.1:41071?listen.
Applying option codec:v (codec name) with argument h264.
Applying option codec:a (codec name) with argument pcm_alaw.
Applying option r:v (set frame rate (Hz value, fraction or abbreviation)) with argument 15.
Applying option f (force format) with argument mpeg.
Successfully parsed a group of options.
Opening an input file: udp://127.0.0.1:41071?listen.
[mpeg @ 0x55a769cc3d40] Opening 'udp://127.0.0.1:41071?listen' for reading
[udp @ 0x55a769cc4b00] No default whitelist set
[udp @ 0x55a769cc4b00] end receive buffer size reported is 425984
[mpeg @ 0x55a769cc3d40] Before avformat_find_stream_info() pos: 6 bytes read:40 seeks:0 nb_streams:0
[mpeg @ 0x55a769cc3d40] probing stream 1 pp:2500
[mpeg @ 0x55a769cc3d40] probed stream 1
[mpeg @ 0x55a769cc3d40] parser not found for codec pcm_alaw, packets or times may be invalid.
[mpeg @ 0x55a769cc3d40] Probe buffer size limit of 32 bytes reached
[mpeg @ 0x55a769cc3d40] Stream #0: not enough frames to estimate rate; consider increasing probesize
[mpeg @ 0x55a769cc3d40] Could not find codec parameters for stream 0 (Video: h264, 1 reference frame, none): unspecified size
Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (32) options
[mpeg @ 0x55a769cc3d40] After avformat_find_stream_info() pos: 87804 bytes read:88476 seeks:0 frames:1
Input #0, mpeg, from 'udp://127.0.0.1:41071?listen':
  Duration: N/A, start: 50436.772978, bitrate: 64 kb/s
  Stream #0:0[0x1e0], 0, 1/90000: Video: h264, 1 reference frame, none, 90k tbr, 90k tbn
  Stream #0:1[0x1c0], 1, 1/90000: Audio: pcm_alaw, 8000 Hz, mono, s16, 64 kb/s
Successfully opened the file.
Parsing a group of options: output url udp://127.0.0.1:33605?pkt_size=1200.
Applying option map (set input stream mapping) with argument 0:a:0?.
Applying option acodec (force audio codec ('copy' to copy stream)) with argument copy.
Applying option ar (set audio sampling rate (in Hz)) with argument 8000.
Applying option f (force format) with argument rtp.
Successfully parsed a group of options.
Opening an output file: udp://127.0.0.1:33605?pkt_size=1200.
[udp @ 0x55a769cf6500] No default whitelist set
Successfully opened the file.
Parsing a group of options: output url udp://127.0.0.1:48527?pkt_size=1200.
Applying option map (set input stream mapping) with argument 0:v:0?.
Applying option vcodec (force video codec ('copy' to copy stream)) with argument copy.
Applying option f (force format) with argument rtp.
Successfully parsed a group of options.
Opening an output file: udp://127.0.0.1:48527?pkt_size=1200.
[udp @ 0x55a769d08fc0] No default whitelist set
Successfully opened the file.
Output #0, rtp, to 'udp://127.0.0.1:33605?pkt_size=1200':
  Metadata:
    encoder         : Lavf59.16.100
  Stream #0:0, 0, 1/8000: Audio: pcm_alaw, 8000 Hz, mono, s16, 64 kb/s
[rtp @ 0x55a769d06f40] dimensions not set
Could not write header for output file #1 (incorrect codec parameters ?): Invalid argument
Error initializing output stream 1:0 -- 
Stream mapping:
  Stream #0:1 -> #0:0 (copy)
  Stream #0:0 -> #1:0 (copy)
    Last message repeated 1 times
[AVIOContext @ 0x55a769d06d40] Statistics: 0 bytes written, 0 seeks, 0 writeouts
[AVIOContext @ 0x55a769d19800] Statistics: 0 bytes written, 0 seeks, 0 writeouts
[AVIOContext @ 0x55a769cd4fc0] Statistics: 88476 bytes read, 0 seeks

Expected behavior:
FFMPEG should be able to operate with all input information and avoid using a large probesize, resulting in the least possible latency. FFMPEG should not crush, as the -copy_unknown option is specified (I've also tried specifying it on the input, but the result is the same)

Actual behavior:
FFMPEG is unable to operate without specifying a large probesize, leading to increased latency and in some cases, failure. The -copy_unknown and -re:v 15 options are ignored.