Initialize source and dest port if possible

if association is initialized with a net.Conn take the ports from there

association.go

@@ -101,6 +101,16 @@ const (
 	acceptChSize = 16
 )
 
+func getPort(addr net.Addr) uint16 {
+	switch addr := addr.(type) {
+	case *net.UDPAddr:
+		return uint16(addr.Port)
+	case *net.TCPAddr:
+		return uint16(addr.Port)
+	}
+	return 0
+}
+
 func getAssociationStateString(a uint32) string {
 	switch a {
 	case closed:
@@ -337,6 +347,10 @@ func createAssociation(config Config) *Association {
 		log:                     config.LoggerFactory.NewLogger("sctp"),
 	}
 
+	if config.NetConn != nil {
+		a.sourcePort = getPort(config.NetConn.LocalAddr())
+		a.destinationPort = getPort(config.NetConn.RemoteAddr())
+	}
 	a.name = fmt.Sprintf("%p", a)
 
 	// RFC 4690 Sec 7.2.1
@@ -399,8 +413,12 @@ func (a *Association) sendInit() error {
 
 	outbound := &packet{}
 	outbound.verificationTag = a.peerVerificationTag
-	a.sourcePort = defaultSCTPSrcDstPort
-	a.destinationPort = defaultSCTPSrcDstPort
+	if a.sourcePort == 0 {
+		a.sourcePort = defaultSCTPSrcDstPort
+	}
+	if a.destinationPort == 0 {
+		a.destinationPort = defaultSCTPSrcDstPort
+	}
 	outbound.sourcePort = a.sourcePort
 	outbound.destinationPort = a.destinationPort