diff --git a/config.go b/config.go
index b70a1cb..b12da3c 100644
--- a/config.go
+++ b/config.go
@@ -16,6 +16,7 @@ type Config struct {
 	CheckTimeoutInterval         int32 // in millisecond
 	CheckBytesReadInterval       int32 // in millisecond
 	SendBytesReadThreshold       int32 // in millisecond
+	Verbose                      bool
 }
 
 var DefaultConfig = Config{
@@ -32,6 +33,7 @@ var DefaultConfig = Config{
 	CheckTimeoutInterval:         50,
 	CheckBytesReadInterval:       100,
 	SendBytesReadThreshold:       200,
+	Verbose:                      false,
 }
 
 func MergeConfig(conf *Config) (*Config, error) {
diff --git a/connection.go b/connection.go
index 0cee326..36f6e43 100644
--- a/connection.go
+++ b/connection.go
@@ -159,7 +159,9 @@ func (conn *Connection) tx() error {
 			if err == ErrConnClosed {
 				return err
 			}
-			log.Println(err)
+			if conn.session.config.Verbose {
+				log.Println(err)
+			}
 
 			// reduce window size
 			conn.Lock()
@@ -234,17 +236,24 @@ func (conn *Connection) sendAck() error {
 			BytesRead:   conn.session.GetBytesRead(),
 		})
 		if err != nil {
-			log.Println(err)
+			if conn.session.config.Verbose {
+				log.Println(err)
+			}
 			time.Sleep(time.Second)
 			continue
 		}
 
 		err = conn.session.sendWith(conn.localClientID, conn.remoteClientID, buf, conn.retransmissionTimeout)
 		if err != nil {
+			if conn.session.IsClosed() {
+				return ErrSessionClosed
+			}
 			if err == ErrConnClosed {
 				return err
 			}
-			log.Println(err)
+			if conn.session.config.Verbose {
+				log.Println(err)
+			}
 			time.Sleep(time.Second)
 			continue
 		}
diff --git a/session.go b/session.go
index 31fa23a..721d4c6 100644
--- a/session.go
+++ b/session.go
@@ -342,7 +342,9 @@ func (session *Session) startFlush() error {
 			if session.context.Err() != nil {
 				return session.context.Err()
 			}
-			log.Println(err)
+			if session.config.Verbose {
+				log.Println(err)
+			}
 			continue
 		}
 	}
@@ -370,7 +372,9 @@ func (session *Session) startCheckBytesRead() error {
 			BytesRead: bytesRead,
 		})
 		if err != nil {
-			log.Println(err)
+			if session.config.Verbose {
+				log.Println(err)
+			}
 			time.Sleep(time.Second)
 			continue
 		}
@@ -379,7 +383,9 @@ func (session *Session) startCheckBytesRead() error {
 		for _, connection := range session.connections {
 			err = session.sendWith(connection.localClientID, connection.remoteClientID, buf, connection.RetransmissionTimeout())
 			if err != nil {
-				log.Println(err)
+				if session.config.Verbose {
+					log.Println(err)
+				}
 				time.Sleep(time.Second)
 				continue
 			}
@@ -889,7 +895,9 @@ func (session *Session) Close() error {
 		if session.config.Linger != 0 {
 			err := session.flushSendBuffer()
 			if err != nil {
-				log.Println(err)
+				if session.config.Verbose {
+					log.Println(err)
+				}
 			}
 
 			func() {
@@ -911,7 +919,9 @@ func (session *Session) Close() error {
 
 		err := session.sendClosePacket()
 		if err != nil {
-			log.Println(err)
+			if session.config.Verbose {
+				log.Println(err)
+			}
 		}
 
 		session.cancel()