Initialize source port and destination 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:
@@ -335,6 +345,8 @@ func createAssociation(config Config) *Association {
 		silentError:             ErrSilentlyDiscard,
 		stats:                   &associationStats{},
 		log:                     config.LoggerFactory.NewLogger("sctp"),
+		sourcePort:              getPort(config.NetConn.LocalAddr()),
+		destinationPort:         getPort(config.NetConn.RemoteAddr()),
 	}
 
 	a.name = fmt.Sprintf("%p", a)
@@ -399,8 +411,12 @@ 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