Support for abs-send-time - closes #1081

[theimowski]

• added a=extmap: http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time to SDP extension headers
• added a=rtcp-fb goog-remb for all media tracks to SDP
• the header extension is added only when remote track supports it
• for now the method can only accept a single extension to be added, but could be made more generic for subsequent extensions
• fixed typo ATTRIBUE in one of the consts

closes #1081

[theimowski]

Thanks! When can we expect a new NuGet version released?

[ChristopheI]

Thanks a lot for this PR.
Could you please share an example to use it ?
Thx

[theimowski]

The abs-send-time extension will apply itself as long as the receiver accepts it.

It will be added to every RTP packet that is send from SIPSorcery sender, in our example it happens when we send Video:

rtcPeerConnection.SendVideo(rtpDuration, sample)

It helps the receiver (in our case web browser) calculate REMB properly, and then on SIPSorcery sender side we can subscribe to REMB events:

  let onReceiveReport: Action<Net.IPEndPoint, SDPMediaTypesEnum, RTCPCompoundPacket> =
    Action<_, _, _>(fun _ _ packet ->
      if not (isNull packet.Feedback) && not (isNull packet.Feedback.Header) then
        let feedback = packet.Feedback
        let header = packet.Feedback.Header

        match header.PacketType, header.PayloadFeedbackMessageType with
        | RTCPReportTypesEnum.PSFB, PSFBFeedbackTypesEnum.AFB ->
          let exponent = feedback.BitrateExp
          let mantissa = feedback.BitrateMantissa
          // This is what REMB tells us the maximum bitrate should be
          let rembMaxBitrateInBitsPerSecond = pown 2u (int exponent) * mantissa
          onREMB.Trigger rembMaxBitrateInBitsPerSecond
        | RTCPReportTypesEnum.PSFB, PSFBFeedbackTypesEnum.PLI -> onPLI.Trigger()
        | _ -> ())

  ...

  rtcPeerConnection.add_OnReceiveReport onReceiveReport