Question regarding the way RTP payloading is done in rtph264pay and rtpmp4vpay plugins

[Paul d'AUBIGNY]

Hi guys,

When comparing rtph264pay and rtpmp4vpay plugins, I noticed a difference in
the way RTP packets are filled.
In the case of H264, one RTP packet seems to contain maximum one entire NAL
unit (can be splitted if MTU too small). In the case of rtpmp4vpay, it
fills the RTP packet with eventually several VOP frames as soon as mtu is
not exceeded or a non VOP frame is received.
Therefore, with rtph264pay one RTP packet = one frame maximum, with
rtpmp4vpay one RTP packet = GOP frames maximum.

Why this difference of behavior? I'm wondering this because in case of mpeg
version 4, it can lead to delayed and/or laggy output if it gathers several
frames in one RTP packet and of course in the case of live streaming. Of
course, we can still play with the max-ptime property of the payloader so
that it limits the maximum duration of the packet data, thus we can limit
the number of frame per RTP packet. But by default the behaviors are
different.

To illustrate what I'm saying, here are 2 examples to see the delay and
laggy output in case of MPEG4 (in case of H264 both output streams might be
delayed due to the encoding time but both output streams should be
synchronized):


   - Case of MPEG4:


gst-launch-1.0 udpsrc port=2222 ! application/x-rtp, media=video,
payload=96, clock-rate=90000, encoding-name=MP4V-ES ! decodebin !
xvimagesink sync=false

gst-launch-1.0 v4l2src ! video/x-raw, format=YUY2, width=640, height=480,
interlace-mode=progressive, pixel-aspect-ratio=1/1, framerate=30/1 !
videoconvert ! videorate ! avenc_mpeg4 ! tee name=t  t. ! queue !
rtpmp4vpay config-interval=1 mtu=65507 ! udpsink host=127.0.0.1 port=2222
 t. ! queue ! decodebin ! autovideosink



   - Case of H264:


gst-launch-1.0 udpsrc port=2222 ! application/x-rtp, media=video,
payload=96, clock-rate=90000, encoding-name=H264 ! decodebin ! xvimagesink
sync=false

gst-launch-1.0 v4l2src ! video/x-raw, format=YUY2, width=640, height=480,
interlace-mode=progressive, pixel-aspect-ratio=1/1, framerate=30/1 !
videoconvert ! videorate ! x264enc ! tee name=t  t. ! queue ! rtph264pay
config-interval=1 mtu=65507 ! udpsink host=127.0.0.1 port=2222  t. ! queue
! decodebin ! autovideosink


*Note*: For each case, play the two pipelines at the same time. I put a
high MTU to demonstrate what I'm saying but that could occur even with the
default MTU if the frame size is small enough.


Cheers,


Paul HENRYS
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