Add RTOMax settings

allowing the user to cap the retranmission timeout, this change adds
RTOMax to SettingEngine and pass it on when creating an sctp Client

sctptransport.go

@@ -58,6 +58,8 @@ type SCTPTransport struct {
 
 	api *API
 	log logging.LeveledLogger
+	// maximum retransmission timeout
+	rtoMax float64
 }
 
 // NewSCTPTransport creates a new SCTPTransport.
@@ -105,11 +107,12 @@ func (r *SCTPTransport) Start(SCTPCapabilities) error {
 	if dtlsTransport == nil || dtlsTransport.conn == nil {
 		return errSCTPTransportDTLS
 	}
-
+	rtoMax := float64(r.api.settingEngine.sctp.rtoMax) / float64(time.Millisecond)
 	sctpAssociation, err := sctp.Client(sctp.Config{
 		NetConn:              dtlsTransport.conn,
 		MaxReceiveBufferSize: r.api.settingEngine.sctp.maxReceiveBufferSize,
 		LoggerFactory:        r.api.settingEngine.LoggerFactory,
+		RTOMax:               rtoMax,
 	})
 	if err != nil {
 		return err

settingengine.go

@@ -76,6 +76,7 @@ type SettingEngine struct {
 	}
 	sctp struct {
 		maxReceiveBufferSize uint32
+		rtoMax               time.Duration
 	}
 	sdpMediaLevelFingerprints                 bool
 	answeringDTLSRole                         DTLSRole
@@ -441,3 +442,9 @@ func (e *SettingEngine) SetSCTPMaxReceiveBufferSize(maxReceiveBufferSize uint32)
 func (e *SettingEngine) SetDTLSCustomerCipherSuites(customCipherSuites func() []dtls.CipherSuite) {
 	e.dtls.customCipherSuites = customCipherSuites
 }
+
+// SetSCTPRTOMax sets the maximum retransmission timeout.
+// Leave this 0 for the default timeout.
+func (e *SettingEngine) SetSCTPRTOMax(rtoMax time.Duration) {
+	e.sctp.rtoMax = rtoMax
+}