Implement DTLS/SRTP/SCTP restart

Fixes #1636

Author: Antonito

datachannel.go

@@ -69,7 +69,7 @@ func (api *API) NewDataChannel(transport *SCTPTransport, params *DataChannelPara
 		return nil, err
 	}
 
-	err = d.open(transport)
+	err = d.open(transport, false)
 	if err != nil {
 		return nil, err
 	}
@@ -103,14 +103,14 @@ func (api *API) newDataChannel(params *DataChannelParameters, log logging.Levele
 }
 
 // open opens the datachannel over the sctp transport
-func (d *DataChannel) open(sctpTransport *SCTPTransport) error {
+func (d *DataChannel) open(sctpTransport *SCTPTransport, restart bool) error {
 	association := sctpTransport.association()
 	if association == nil {
 		return errSCTPNotEstablished
 	}
 
 	d.mu.Lock()
-	if d.sctpTransport != nil { // already open
+	if d.sctpTransport != nil && !restart { // already open & not restarting
 		d.mu.Unlock()
 		return nil
 	}

dtlstransport.go

@@ -213,6 +213,31 @@ func (t *DTLSTransport) startSRTP() error {
 		return fmt.Errorf("%w: %v", errDtlsKeyExtractionFailed, err)
 	}
 
+	isAlreadyRunning := func() bool {
+		select {
+		case <-t.srtpReady:
+			return true
+		default:
+			return false
+		}
+	}()
+
+	if isAlreadyRunning {
+		if sess, ok := t.srtpSession.Load().(*srtp.SessionSRTP); ok {
+			if updateErr := sess.UpdateContext(srtpConfig); updateErr != nil {
+				return updateErr
+			}
+		}
+
+		if sess, ok := t.srtcpSession.Load().(*srtp.SessionSRTCP); ok {
+			if updateErr := sess.UpdateContext(srtpConfig); updateErr != nil {
+				return updateErr
+			}
+		}
+
+		return nil
+	}
+
 	srtpSession, err := srtp.NewSessionSRTP(t.srtpEndpoint, srtpConfig)
 	if err != nil {
 		return fmt.Errorf("%w: %v", errFailedToStartSRTP, err)

peerconnection.go

@@ -1108,7 +1108,59 @@ func (pc *PeerConnection) SetRemoteDescription(desc SessionDescription) error {
 			pc.ops.Enqueue(func() {
 				pc.startRTP(true, &desc, currentTransceivers)
 			})
+		} else if pc.dtlsTransport.State() != DTLSTransportStateNew {
+			fingerprint, fingerprintHash, fErr := extractFingerprint(desc.parsed)
+			if fErr != nil {
+				return fErr
+			}
+
+			fingerPrintDidChange := true
+
+			for _, fp := range pc.dtlsTransport.remoteParameters.Fingerprints {
+				if fingerprint == fp.Value && fingerprintHash == fp.Algorithm {
+					fingerPrintDidChange = false
+					break
+				}
+			}
+
+			if fingerPrintDidChange {
+				pc.ops.Enqueue(func() {
+					// SCTP uses DTLS, so prevent any use, by locking, while
+					// DTLS is restarting.
+					pc.sctpTransport.lock.Lock()
+					defer pc.sctpTransport.lock.Unlock()
+
+					if dErr := pc.dtlsTransport.Stop(); dErr != nil {
+						pc.log.Warnf("Failed to stop DTLS: %s", dErr)
+					}
+
+					// libwebrtc switches the connection back to `new`.
+					pc.dtlsTransport.lock.Lock()
+					pc.dtlsTransport.onStateChange(DTLSTransportStateNew)
+					pc.dtlsTransport.lock.Unlock()
+
+					// Restart the dtls transport with updated fingerprints
+					err = pc.dtlsTransport.Start(DTLSParameters{
+						Role:         dtlsRoleFromRemoteSDP(desc.parsed),
+						Fingerprints: []DTLSFingerprint{{Algorithm: fingerprintHash, Value: fingerprint}},
+					})
+					pc.updateConnectionState(pc.ICEConnectionState(), pc.dtlsTransport.State())
+					if err != nil {
+						pc.log.Warnf("Failed to restart DTLS: %s", err)
+						return
+					}
+
+					// If SCTP was enabled, restart it with the new DTLS transport.
+					if pc.sctpTransport.isStarted {
+						if dErr := pc.sctpTransport.restart(pc.dtlsTransport.conn); dErr != nil {
+							pc.log.Warnf("Failed to restart SCTP: %s", dErr)
+							return
+						}
+					}
+				})
+			}
 		}
+
 		return nil
 	}
 
@@ -1317,7 +1369,7 @@ func (pc *PeerConnection) startSCTP() {
 	var openedDCCount uint32
 	for _, d := range dataChannels {
 		if d.ReadyState() == DataChannelStateConnecting {
-			err := d.open(pc.sctpTransport)
+			err := d.open(pc.sctpTransport, false)
 			if err != nil {
 				pc.log.Warnf("failed to open data channel: %s", err)
 				continue
@@ -1775,7 +1827,7 @@ func (pc *PeerConnection) CreateDataChannel(label string, options *DataChannelIn
 
 	// If SCTP already connected open all the channels
 	if pc.sctpTransport.State() == SCTPTransportStateConnected {
-		if err = d.open(pc.sctpTransport); err != nil {
+		if err = d.open(pc.sctpTransport, false); err != nil {
 			return nil, err
 		}
 	}