RTP video payloads and basic rules

Hi, everyone!

Now I’m investigating implementation of Jicofo and Jitsi Video Bridge modules and implementation of WebRTC signaling.

Could anybody clarify a few things about RTP video payloads and basic signaling rules?

To simplify things, let’s suppose only VP8 video codec is enabled.

  1. I’ve discovered that in case of 3 or more participants VP8 payload is always set to 100 for every participant. VP8 payload is always 96 when I change jicofo/sip-communicator.properties like below:
    org.jitsi.jicofo.VP8_PT=96

  2. From the JitsiMeet client perspective signaling looks like below:
    2.1 JitsiMeet sends focus request to Jicofo:
    2.2 Then JitsiMeet receives from Jicofo ‘session-initiate’ response with codec names, payload, SSRC, etc.
    2.3 JitsiMeet converts this data to SDP offer, generates SDP answer, converts the answer to session ‘session-accept’ XMPP response and send it to Jicofo.

So I made the following conclusions.

  1. On WebRTC signaling phase Jicofo always generates SDP offer and JitsiMeet always generates SDP answer.
    Is this correct?

  2. When Jicofo creates a room and allocates media channels it sets payload value to VP8 codec.
    the default value is 100 or other from the configuration.
    If so, all clients joining the conference always have the same VP8 payload value:
    100 (or from config).
    Is this correct?

Please correct me if I have any misunderstanding.

One more question:

  1. Are asymmetric RTP payloads supported by Jitsi?
    For example: 2 conference participants have VP8 payload=100 but the third participant VP8 payload is 96.
    Is it possible?

Thanks.