RTCDtlsTransport

The RTCDtlsTransport interface provides access to information about the Datagram Transport Layer Security (DTLS) transport over which a RTCPeerConnection's RTP and RTCP packets are sent and received by its RTCRtpSender and RTCRtpReceiver objects.

A DTLS transport is also used to provide information about SCTP packets transmitted and received by an connection's data channels.

Features of the DTLS transport include the addition of security to the underlying transport; the RTCDtlsTransport interface can be used to obtain information about the underlying transport and the security added to it by the DTLS layer.

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Description

Allocation of DTLS transports

RTCDtlsTransport objects are created when an app calls either setLocalDescription() or setRemoteDescription(). The number of DTLS transports created and how they're used depends on the bundling mode used when creating the RTCPeerConnection.

Whether bundling is used depends on what the other endpoint is able to negotiate. All browsers support bundling, so when both endpoints are browsers, you can rest assured that bundling will be used.

Some non-browser legacy endpoints, however, may not support bundle. To be able to negotiate with such endpoints (or to exclude them entirely), the RTCConfiguration property bundlePolicy may be provided when creating the connection. The bundlePolicy lets you control how to negotiate with these legacy endpoints. The default policy is "balanced", which provides a balance between performance and compatibility.

For example, to create the connection using the highest level of bundling:

const rtcConfig = {
  bundlePolicy: "max-bundle"
};

const pc = new RTCPeerConnection(rtcConfig);

Bundling lets you use one RTCDtlsTransport to carry the data for multiple higher-level transports, such as multiple RTCRtpTransceivers.

When not using BUNDLE

When the connection is created without using BUNDLE, each RTP or RTCP component of each RTCRtpTransceiver has its own RTCDtlsTransport; that is, every RTCRtpSender and RTCRtpReceiver, has its own transport, and all RTCDataChannel objects share a transport dedicated to SCTP.

When using BUNDLE

When the connection is using BUNDLE, each RTCDtlsTransport object represents a group of RTCRtpTransceiver objects. If the connection was created using max-compat mode, each transport is responsible for handling all of the communications for a given type of media (audio, video, or data channel). Thus, a connection that has any number of audio and video channels will always have exactly one DTLS transport for audio and one for video communications.

Because transports are established early in the negotiation process, it's likely that it won't be known until after they're created whether or not the remote peer supports bundling or not. For this reason, you'll sometimes see separate transports created at first, one for each track, then see them get bundled up once it's known that bundling is possible. If your code accesses RTCRtpSenders and/or RTCRtpReceivers directly, you may encounter situations where they're initially separate, then half or more of them get closed and the senders and receivers updated to refer to the appropriate remaining RTCDtlsTransport objects.

Data channels

RTCDataChannels use SCTP to communicate. All of a peer connection's data channels share a single RTCSctpTransport, found in the connection's sctp property.

You can, in turn, identify the RTCDtlsTransport used to securely encapsulate the data channels' SCTP communications by looking at the RTCSctpTransport object's transport property.

Examples

This example presents a function, tallySenders(), which iterates over an RTCPeerConnection's RTCRtpSenders, tallying up how many of them are in various states. The function returns an object containing properties whose values indicate how many of the senders are in each state.

let pc = new RTCPeerConnection({ bundlePolicy: "max-bundle" });

/* ... */

function tallySenders(pc) {
  let results = {
    transportMissing: 0,
    connectionPending: 0,
    connected: 0,
    closed: 0,
    failed: 0,
    unknown: 0
  };

  let senderList = pc.getSenders();
  senderList.forEach(sender => {
    let transport = sender.transport;

    if (!transport) {
      results.transportMissing++;
    } else {
      switch(transport.state) {
        case "new":
        case "connecting":
          results.connectionPending++;
          break;
       case "connected":
          results.connected++;
          break;
       case "closed":
          results.closed++;
          break;
       case "failed":
          results.failed++;
          break;
       default:
          results.unknown++;
          break;
      }
    }
  });
  return results;
}

Note that in this code, the new and connecting states are being treated as a single connectionPending status in the returned object.

Specifications

Browser compatibility

See also

• RTCRtpSender.transport and RTCRtpReceiver.transport
• RTCSctpTransport.transport