diff --git a/sess.go b/sess.go
index 98176e6..b807fec 100644
--- a/sess.go
+++ b/sess.go
@@ -207,7 +207,7 @@ func (s *UDPSession) Write(b []byte) (n int, err error) {
 			}
 		}
 
-		if s.kcp.WaitSnd() < int(s.kcp.snd_wnd) {
+		if s.kcp.WaitSnd() < int(s.kcp.Cwnd()) {
 			n = len(b)
 			max := s.kcp.mss << 8
 			for {