DTLS retransmission interval setting

Add SetDTLSRetranmissionInterval setting to SettingEngine.

Add test for SetDTLSRetransmissionInterval

Author: boks1971

dtlstransport.go

@@ -327,6 +327,10 @@ func (t *DTLSTransport) Start(remoteParameters DTLSParameters) error {
 		dtlsConfig.ReplayProtectionWindow = int(*t.api.settingEngine.replayProtection.DTLS)
 	}
 
+	if t.api.settingEngine.dtls.retransmissionInterval != 0 {
+		dtlsConfig.FlightInterval = t.api.settingEngine.dtls.retransmissionInterval
+	}
+
 	// Connect as DTLS Client/Server, function is blocking and we
 	// must not hold the DTLSTransport lock
 	if role == DTLSRoleClient {

settingengine.go

@@ -51,6 +51,9 @@ type SettingEngine struct {
 		SRTP  *uint
 		SRTCP *uint
 	}
+	dtls struct {
+		retransmissionInterval time.Duration
+	}
 	sdpMediaLevelFingerprints                 bool
 	answeringDTLSRole                         DTLSRole
 	disableCertificateFingerprintVerification bool
@@ -296,3 +299,8 @@ func (e *SettingEngine) DisableMediaEngineCopy(isDisabled bool) {
 func (e *SettingEngine) SetReceiveMTU(receiveMTU uint) {
 	e.receiveMTU = receiveMTU
 }
+
+// SetDTLSRetransmissionInterval sets the retranmission interval for DTLS.
+func (e *SettingEngine) SetDTLSRetransmissionInterval(interval time.Duration) {
+	e.dtls.retransmissionInterval = interval
+}

settingengine_test.go

@@ -213,3 +213,23 @@ func TestSettingEngine_SetDisableMediaEngineCopy(t *testing.T) {
 		closePairNow(t, offerer, answerer)
 	})
 }
+
+func TestSetDTLSRetransmissionInterval(t *testing.T) {
+	s := SettingEngine{}
+
+	if s.dtls.retransmissionInterval != 0 {
+		t.Fatalf("SettingEngine defaults aren't as expected.")
+	}
+
+	s.SetDTLSRetransmissionInterval(100 * time.Millisecond)
+	if s.dtls.retransmissionInterval == 0 ||
+		s.dtls.retransmissionInterval != 100*time.Millisecond {
+		t.Errorf("Failed to set DTLS retransmission interval")
+	}
+
+	s.SetDTLSRetransmissionInterval(1 * time.Second)
+	if s.dtls.retransmissionInterval == 0 ||
+		s.dtls.retransmissionInterval != 1*time.Second {
+		t.Errorf("Failed to set DTLS retransmission interval")
+	}
+}