high level questions about using srtpenc/srtpdec

[Andres Gonzalez]

Hi,

First of all, I want to thank you guys for all of your help and for
answering my many (stupid?) questions here. Your answers/comments have
really helped me better understand GStreamer and has enabled me to get some
cool streaming features working in my app.

I currently have a functional traditional RTP/RTCP stream working well. The
pipeline is built using 2 bins, one for all the video elements feed by an
appsrc, and one for all of the audio elements also feed by an appsrc, with
both bins linked to an single rtpbin, then linked to 2 udpsink elements
(outgoing RTP and RTCP) and 1 udpsrc element (incoming RTCP).

I am now trying to add to this basic RTP/RTCP pipeline support for SRTP. I
have finished all of the crypto and key part using openssl so I have the
negotiated key. But I am still confused about the architecture of the
SRTP-enabled RTP pipeline.  

rtpbin has callbacks for encoders/decoder for RTP and RTCP:  e.g.:
request-rtp-encoder. These callbacks return a pointer to a GstElement.

Question: I am assuming the return pointer is the pointer to the srtpenc. 
Is this correct?  The reason I ask what must be a newbie question, is that
when I create the srtpenc element using  gst_element_factory_make() and then
return that reference in the rtpbin request-rtp-encoder callback, I get a
segfault deep within GStreamer code. So I am not sure if I am using the API
correctly (I cannot find any example code for srtpenc/srtpdec and the
current documentation does not expand on its usage nor how to interface them
to the regular rtpbin).

I have looked at the dtlssrtpenc source but it appears to use srtpenc
different from how I thought these should be used. In the dtlssrtpenc
source, it appears they use a funnel element and link both the data feed
(which would be the clear text RTP data in my case??) and the srtpenc
element to the input of the funnel. However, my current understanding is
that the  srtpenc/srtpdec elements are not linked as usual in the pipeline,
but instead these elements are "connected" to the regular rtpbin using the
rtpbin request-xxx callbacks.  So this appears to conflict with how I have
interpreted some of the comments here about srtpenc/srtpdec usage.

Thanks,
-Andres



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