[rtmpdump] Compatibility with librtmp?

[Andrew McClure]

Hi. We are developing an RTMP application and are having trouble getting it
to interoperate with the librtmp (as used in FFmpeg).


Our application acts like a proxy. In our test configuration, we have a port
1936 which accepts RTMP publishing, and a different port 1935 which accepts
RTMP playing. Packets received from the publisher connected to port 1936 are
forwarded to the player connected on port 1935 (with some minimal munging).
We have two simple flash applets (written in Flex, using the standard
Video/Camera/NetConnection objects) which connect to these ports; one
streams video from a webcam, the other plays the video thus streamed.


When we try to play video originating in Flex, forwarded through our proxy
and played back in Flex, it works. The video appears on the screen.


However then we try something different. We compiled a version of ffmpeg
(svn r26326) with librtmp (from rtmpdump svn r555). (If there is a better
way to test publish with librtmp than this, I would be curious to know.) We
try using ffmpeg to publish to our port 1936:


./ffmpeg -loglevel debug -i ~/Downloads/20051210-w50s.flv -re -ac 1 -acodec
copy -vcodec flv -f copy rtmp://localhost:1936/


And then play back the video by connecting the Flex applet to port 1935. We
are having a problem where we are able to handshake, understand ffmpeg's
publish commands and forward the packets through to the Flex applet as
appropriate... *but* the Flex applet does not display video or audio. We get
a black windowpane and silence.


When we forward the ffmpeg streams, we appear to be doing (or failing to do)
something to the RTMP packets such that the Flex player no longer
understands the RTMP stream. But of course this problem only occurs with
streams generated through ffmpeg. So it appears this is due to some slight
difference between the RTMP protocol as implemented by ffmpeg and Flex-- we
are compatible with the Flex publisher's notion of RTMP but not ffmpeg's.
What I am trying to figure out:


- What might this difference in RTMP implementation be, which is causing
ffmpeg to apparently not be compatible with our RTMP proxy?


- Is there a way to get ffmpeg (or another librtmp tool) to act like a
"server", such that our Flex video object could connect directly to ffmpeg,
issue a play request, and start receiving a video? I ask this in case the
problem is not in fact that we are damaging ffmpeg's stream, but rather that
ffmpeg just is not generating data Flex knows how to work with in the first
place. (I find this "ffserver" tool, however this does not look like what I
am looking for; the docs seem to imply it does RTP/RTSP only, and the html
doc also says "It can also stream from files, though that is currently
broken.")


One possible source of concern is that when the player connects on 1935, it
does not see the beginning of the stream-- it comes in after the stream has
been waiting a few seconds. However once connected we do wait until we see a
keyframe to begin streaming, and we forward a cached copy of the metadata
packet at that time.


We also did try forcing ffmpeg to publish in codecs I know Flex understands,
but it does not help:


./ffmpeg -loglevel debug -i ~/Downloads/20051210-w50s.flv -re -ac 1 -acodec
nellymoser -vcodec flv -f flv rtmp://localhost:1936/


Do you have any suggestions? What are the requirements our application must
follow to interoperate with ffmpeg/librtmp? To our understanding we are
compatible with the published RTMP spec but the spec is ambiguous on many
points.


Note, I was unsure the most appropriate place to ask these questions so I
also sent this post to the ffmpeg-user list. Thanks.
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