:Specifications of the RTS signaling protocol

Signaling process

The following figure shows the signaling process.

Signaling process

1. The client sends a request with a Session Description Protocol (SDP) offer.
   1. Create an RTCPeerConnection object on the client, specify whether to receive or send audio and video signals, and then create an SDP offer.

      // Open audio and video, recvonly or sendonly
      { offerToReceiveVideo: true, offerToReceiveAudio: true }

   2. Send a stream pulling request from the client to ApsaraVideo Live by using the HTTPS POST method. The request body is a JSON string. For more information about the request parameters, see the Definition of the RTS signaling protocol section of this topic.
      Note

      • The version parameter specifies the version of the RTS signaling protocol. Set the value to 2.
      • The sdk_version parameter specifies the version of the RTS SDK. You can set the parameter as needed.
   3. Send the constructed request to ApsaraVideo Live based on the signaling URL by using the POST method. Specify the source URL in the JSON-formatted request body.

      POST /app/streamname?auth=xxx HTTP/1.1
      Host: domain
      Connection: keep-alive
      Content-Length: 2205
      Content-Type: application/json

      Note The content of a signaling URL is basically the same as that of a source URL, except the protocol header. The following URLs provide examples:

      • Signaling URL: https://domain/app/streamname?auth=xxx
      • Source URL: artc://domain/app/streamname?auth=xxx
2. The server returns a response with an SDP answer.

   After the server of ApsaraVideo Live verifies the request, the server generates an SDP answer and returns a response that contains the information about the live streaming node to the client. For more information about the response parameters, see the Definition of the RTS signaling protocol section of this topic.

3. The client initiates Interactive Connectivity Establishment (ICE).
   1. After the client receives the response with an SDP answer, specify the session description in the RTCPeerConnection object.

      peerConnection.setRemoteDescription(new RTCSessionDescription(answer.jsep));

   2. Use the RTCPeerConnection object to initiate ICE and Datagram Transport Layer Security (DTLS) encryption. After the signaling channel is established, the client can pull streams from ApsaraVideo Live. This way, you can implement stream pulling and playback based on the standards of WebRTC.
4. The client initiates a disconnection.

   The client sends a DTLS alert message that initiates a disconnection to stop stream ingest or playback.

   

Sample code for the HTML5 player

// Create peer connection and local offer sdp.
peerConnection = new RTCPeerConnection();
peerConnection.onicecandidate = iceCandidateCallback;
peerConnection.ontrack = remoteStreamCallback;
peerConnection.createOffer({ offerToReceiveVideo: true, offerToReceiveAudio: true })
      .then(signaling_pull).catch(errorHandler);


// CDN live post pull stream request.
function signaling_pull(offer_sdp) {
  console.log('local offer sdp', offer_sdp);

  peerConnection.setLocalDescription(offer_sdp).then(function() {
    // Get pull stream url.
    var stream_url = $("#stream_url").val();
    console.log("stream url:" , stream_url);

    // Add sdk and protocol versions.
    var protocol_version = 2;
    var sdk_version = "0.0.1";

    $.ajax({url: stream_url, data: JSON.stringify({
          mode: "live",
          version: protocol_version,
          sdk_version: sdk_version,
          jsep:description,
      }),
      type: "post",
      success:function(result){
          var signal = JSON.parse(result);
          peerConnection.setRemoteDescription(new RTCSessionDescription(signal.jsep)).then(function() {
              console.log("get remote answer sdp: ", signal.jsep.sdp);
          }).catch(errorHandler);
      }});
  }).catch(errorHandler);
}

Definition of the RTS signaling protocol

The RTS signaling protocol establishes a short-lived connection based on HTTPS. The protocol uses messages in the JSON format. This section describes the request, response, and error codes based on the RTS signaling protocol.

Sample request

Request:
{
    "version":2,
    "sdk_version":"0.0.1",
    "mode":"live",
    "pull_streams":[
        {
            "url":"artc://demo.aliyundoc.com/liveApp****/liveStream****",
            "amsid":[
                "rts audio"
            ],
            "vmsid":[
                "rts video"
            ]
        }
    ],
    "jsep":{
        "type":"offer",
        "sdp":"v=0\n\ro=- 6839248142876176651 2 IN IP4 127.0.0.1\n\rs=-\n\r Omitted content"
    }
}

Sample success response

Response:
{
    "trace_id":"2_1591173296_101.227.XX.XX_702080732320_dec327eb6eed0e0b07b349c8a565****",
    "code":200,
    "jsep":{
        "type":"answer",
        "sdp":"v=0\r\no=- 1591173291 2 IN IP4 127.0.0.1\n\r Omitted content"
    }
}

Enhanced SDP negotiation

Messages are exchanged in the SDP format during signaling. SDP negotiation is generally based on RFC 4566. RTS expands more semantics to make the negotiation compatible with the characteristics of the live streaming industry. RTS supports more container formats of videos and audio and more communications protocols. This way, RTS resolves the issue that WebRTC supports only the Opus format for audio and does not support B frames. RTS meets the needs of increasing streaming protocols.

Support audio in the AAC formats

RTS can transmit audio in various Advanced Audio Coding (AAC) formats over Real-Time Messaging Protocol (RTMP). The AAC formats include AAC-LC, HE-AACv1, and HE-AACv2. For more information about AAC formats, see ISO IEC 14496-3.

RTS can transmit audio in AAC formats by using the Low-overhead MPEG-4 Audio Transport Multiplex (LATM) container format. LATM determines whether the encoding information about audio is transmitted in in-band or out-of-band mode based on whether the audio contains the encoding information. In-band transmission sends the encoding information for each audio frame. Out-of-band transmission sends the encoding information only once. The muxconfigPresent parameter in an AudioMuxElement array specifies whether the information in AudioSpecificConfig is transmitted in in-band or out-of-band mode. Therefore, LATM is more flexible than Audio Data Transport Stream (ADTS). If the information in AudioSpecificConfig remains unchanged, the information in StreamMuxConfig can be first transmitted in an SDP message.

During signaling, RTS parses the encoding information during audio stream ingest and returns the parsed information in the negotiation response, as shown in the following code.

m=audio 9 UDP/RTP/AVPF 120 96 
a=rtpmap:120 MP4A-LATM/44100/2

AudioSpecificConfig = 0x1210

AudioSpecificConfig = 2b920800

AudioSpecificConfig = eb8a0800

a=rtpmap:120 MP4A-LATM/44100/2
a=fmtp:120 cpresent=0;profile-level-id=1;object=2;config=400024203fc0

a=rtpmap:120 MP4A-LATM/44100/2 
a=fmtp:120 cpresent=0;profile-level-id=1;object=2;config=4000572410003fc0;SBR-enabled=1

a=rtpmap:120 MP4A-LATM/44100/2 
a=fmtp:120 cpresent=0;object=2;profile-level-id=1;config=4001d71410003fc0;PS-enabled=1;SBR-enabled=1

If SBR-enabled=1 is added in the fmtp attribute of MP4A-LATM, the AAC format is AAC-HE. If SBR-enabled=1 and PS-enabled=1 are added, the AAC format is HE-AACv2. The AAC format is evolved from AAC-LC to HE-AACv2. Therefore, the SBR and PS fields are used to indicate an AAC format. In addition, config=StreamMuxConfig is added in the fmtp attribute. StreamMuxConfig assembles the information in AudioSpecificConfig of the streams to be ingested and contains the parameters that are related to the details of the encoding information. The client can obtain the details as needed.

For more information, see AAC-LC / HE-AACv1 / HE-AACv2 Encoder Parameters.

Support videos in the H.265 format

RTS parses the encoding information of the videos in the H.264 or H.265 format during video stream ingest and returns the information about the videos in the H.264 or H. 265 format in the SDP answer.

a=rtpmap:102 H265/90000

a=rtpmap:122 H265/90000
a=fmtp:122

Support videos with B frames

During signaling, the client can add a field in the SDP offer to specify whether to decode videos with B frames. If the client adds BFrame-enabled = 1 in the fmtp attribute of videos, the client can decode videos with B frames. In this case, RTP timestamp = PTS can be added, which means the client decodes each frame based on the increasing sequence number. If videos with B frames are not supported, RTS can transcode the source video streams to remove B frames.

In addition, RTS can return a composition timestamp (CTS). This allows the client to calculate the decoding timestamp (DTS) based on the following formula: Presentation timestamp (PTS) = DTS + CTS. If an SDP offer contains a=extmap:{$id} uri:webrtc:rtc:rtp-hdrext:video:CompositionTime, RTS adds extension identifier = {$id} to the first Real-time Transport Protocol (RTP) packet of each video frame. The value of the id variable is determined by the SDP offer that is sent by the client. The following figures show the partial content of the SDP offer and the packet capture during stream pulling:

RTS allows the client to determine whether to decode videos with B frames and whether to return CTS information. This ensures general capabilities in communications.

MSID mechanism

For more information about MSID, see The Msid Mechanism.