diff --git a/src/core/mux-relay.js b/src/core/mux-relay.js
index ab6b754..f5714bf 100644
--- a/src/core/mux-relay.js
+++ b/src/core/mux-relay.js
@@ -15,6 +15,34 @@ import {
   WssInbound, WssOutbound,
 } from '../transports';
 
+class MuxInbound extends EventEmitter {
+
+  constructor(inbound) {
+    super();
+    this._inbound = inbound;
+    this._inbound.on('drain', () => this.emit('drain'));
+  }
+
+  get bufferSize() {
+    return this._inbound.bufferSize;
+  }
+
+}
+
+class MuxOutbound extends EventEmitter {
+
+  constructor(outbound) {
+    super();
+    this._outbound = outbound;
+    this._outbound.on('drain', () => this.emit('drain'));
+  }
+
+  get bufferSize() {
+    return this._outbound.bufferSize;
+  }
+
+}
+
 // .on('_connect')
 // .on('_read')
 // .on('_write')
@@ -82,6 +110,7 @@ export class MuxRelay extends EventEmitter {
 
     const { Inbound } = this._getBounds(transport);
     const inbound = new Inbound({ config, source, conn });
+    inbound.setOutbound(new MuxOutbound(outbound));
     inbound.on('_error', (err) => this.emit('_error', err));
     inbound.on('data', (buffer) => {
       if (!isFirstFrameOut) {
@@ -189,6 +218,7 @@ export class MuxRelay extends EventEmitter {
 
     // outbound
     const outbound = new Outbound({ config, source });
+    outbound.setInbound(new MuxInbound(inbound));
     outbound.on('_error', (err) => this.emit('_error', err));
     outbound.on('data', (buffer) => {
       pipe.feed(PIPE_ENCODE, buffer, { cid });
@@ -228,7 +258,7 @@ export class MuxRelay extends EventEmitter {
         inbound.__tracker.trace(PIPE_DECODE, data.length);
         setImmediate(() => this.emit('_read', data.length));
       } else {
-        logger.error(`[mux-relay] couldn't delivery data frame to cid=${cid}, dump=${dumpHex(data)}`);
+        logger.debug(`[mux-relay] couldn't delivery data frame to cid=${cid}, dump=${dumpHex(data)}`);
       }
     } else {
       const outbound = this._outbounds.get(cid);
diff --git a/src/transports/h2.js b/src/transports/h2.js
index fd4f0a8..8f65067 100644
--- a/src/transports/h2.js
+++ b/src/transports/h2.js
@@ -14,6 +14,7 @@ export class Http2Inbound extends Inbound {
     super(props);
     this._stream = this._conn;
     this._stream.on('data', this.onReceive);
+    this._stream.on('drain', this.onDrain);
     this._stream.on('error', this.onError);
     this._stream.on('close', this.onClose);
     this._stream.on('timeout', this.onTimeout);
@@ -47,6 +48,10 @@ export class Http2Inbound extends Inbound {
     this.emit('data', buffer);
   };
 
+  onDrain = () => {
+    this.emit('drain');
+  };
+
   onTimeout = () => {
     logger.warn(`[${this.name}] [${this.remote}] timeout: no I/O on the connection for ${this._config.timeout / 1e3}s`);
     this.onClose();
@@ -107,6 +112,10 @@ export class Http2Outbound extends Outbound {
     this.emit('data', buffer);
   };
 
+  onDrain = () => {
+    this.emit('drain');
+  };
+
   onTimeout = () => {
     logger.warn(`[${this.name}] [${this.remote}] timeout: no I/O on the connection for ${this._config.timeout / 1e3}s`);
     this.onClose();
@@ -153,6 +162,7 @@ export class Http2Outbound extends Outbound {
           });
           this._stream.on('error', this.onError);
           this._stream.on('data', this.onReceive);
+          this._stream.on('drain', this.onDrain);
           this._stream.on('timeout', this.onTimeout);
           this._stream.on('close', this.onClose);
           this._stream.setTimeout(this._config.timeout);
diff --git a/src/transports/tcp.js b/src/transports/tcp.js
index 17cfa29..4d8be42 100644
--- a/src/transports/tcp.js
+++ b/src/transports/tcp.js
@@ -1,6 +1,6 @@
 import net from 'net';
 import { Inbound, Outbound } from './defs';
-import { DNSCache, SpeedTester, logger } from '../utils';
+import { DNSCache, logger } from '../utils';
 import {
   ACL_PAUSE_RECV,
   ACL_PAUSE_SEND,
@@ -14,8 +14,6 @@ export class TcpInbound extends Inbound {
 
   _socket = null;
 
-  _speeder = new SpeedTester();
-
   _destroyed = false;
 
   constructor(props) {
@@ -57,25 +55,17 @@ export class TcpInbound extends Inbound {
 
   onReceive = (buffer) => {
     this.emit('data', buffer);
-    this._speeder.feed(buffer.length);
     // throttle receiving data to reduce memory grow:
     // https://github.com/blinksocks/blinksocks/issues/60
     // https://nodejs.org/dist/latest/docs/api/net.html#net_socket_buffersize
     const outbound = this.getOutbound();
     if (outbound && outbound.bufferSize > MAX_BUFFERED_SIZE) {
-      const delta = outbound.bufferSize - (MAX_BUFFERED_SIZE >> 1);
-      // this speed is almost equivalent to outbound::write() speed
-      const speed = this._speeder.getSpeed();
-      if (speed < 1) {
-        return;
-      }
-      const sec = (delta / speed).toFixed(3);
-      logger.debug(`[${this.name}] [${this.remote}] recv paused for ${sec}s due to inbound.bufferSize=${outbound.bufferSize} >= ${MAX_BUFFERED_SIZE}`);
+      logger.debug(`[${this.name}] [${this.remote}] recv paused due to inbound.bufferSize=${outbound.bufferSize} >= ${MAX_BUFFERED_SIZE}`);
       this.pause();
-      setTimeout(() => {
+      outbound.once('drain', () => {
         logger.debug(`[${this.name}] [${this.remote}] resume to recv`);
         this.resume();
-      }, sec * 1e3);
+      });
     }
   };
 
@@ -90,13 +80,15 @@ export class TcpInbound extends Inbound {
 
   onHalfClose = () => {
     const outbound = this.getOutbound();
-    outbound && outbound.end && outbound.end();
+    if (outbound && outbound.end) {
+      outbound.end();
+    }
   };
 
   onClose = () => {
     this.close();
     const outbound = this.getOutbound();
-    if (outbound) {
+    if (outbound && outbound.close) {
       outbound.close();
       this.setOutbound(null);
     }
@@ -155,8 +147,6 @@ export class TcpOutbound extends Outbound {
 
   _socket = null;
 
-  _speeder = new SpeedTester();
-
   _destroyed = false;
 
   get name() {
@@ -184,25 +174,17 @@ export class TcpOutbound extends Outbound {
 
   onReceive = (buffer) => {
     this.emit('data', buffer);
-    this._speeder.feed(buffer.length);
     // throttle receiving data to reduce memory grow:
     // https://github.com/blinksocks/blinksocks/issues/60
     // https://nodejs.org/dist/latest/docs/api/net.html#net_socket_buffersize
     const inbound = this.getInbound();
     if (inbound && inbound.bufferSize > MAX_BUFFERED_SIZE) {
-      const delta = inbound.bufferSize - (MAX_BUFFERED_SIZE >> 1);
-      // this speed is almost equivalent to inbound::write() speed
-      const speed = this._speeder.getSpeed();
-      if (speed < 1) {
-        return;
-      }
-      const sec = (delta / speed).toFixed(3);
-      logger.debug(`[${this.name}] [${this.remote}] recv paused for ${sec}s due to inbound.bufferSize=${inbound.bufferSize} >= ${MAX_BUFFERED_SIZE}`);
+      logger.debug(`[${this.name}] [${this.remote}] recv paused due to inbound.bufferSize=${inbound.bufferSize} >= ${MAX_BUFFERED_SIZE}`);
       this.pause();
-      setTimeout(() => {
+      inbound.once('drain', () => {
         logger.debug(`[${this.name}] [${this.remote}] resume to recv`);
         this.resume();
-      }, sec * 1e3);
+      });
     }
   };
 
@@ -217,13 +199,15 @@ export class TcpOutbound extends Outbound {
 
   onHalfClose = () => {
     const inbound = this.getInbound();
-    inbound && inbound.end && inbound.end();
+    if (inbound && inbound.end) {
+      inbound.end();
+    }
   };
 
   onClose = () => {
     this.close();
     const inbound = this.getInbound();
-    if (inbound) {
+    if (inbound && inbound.close) {
       inbound.close();
       this.setInbound(null);
     }
diff --git a/src/transports/ws.js b/src/transports/ws.js
index f927a08..5eef32d 100644
--- a/src/transports/ws.js
+++ b/src/transports/ws.js
@@ -18,6 +18,7 @@ export class WsInbound extends Inbound {
   constructor(props) {
     super(props);
     this._socket = this._conn;
+    this._socket._socket.on('drain', this.onDrain);
     this._socket.on('message', this.onReceive);
     this._socket.on('error', this.onError);
     this._socket.on('close', this.onClose);
@@ -45,6 +46,10 @@ export class WsInbound extends Inbound {
     this.emit('data', buffer);
   };
 
+  onDrain = () => {
+    this.emit('drain');
+  };
+
   onError = (err) => {
     logger.warn(`[${this.name}] [${this.remote}] ${err.message}`);
     this.emit('_error', err);
@@ -100,6 +105,10 @@ export class WsOutbound extends Outbound {
     this.emit('data', buffer);
   };
 
+  onDrain = () => {
+    this.emit('drain');
+  };
+
   onError = (err) => {
     logger.warn(`[${this.name}] [${this.remote}] ${err.message}`);
     this.emit('_error', err);
@@ -136,7 +145,10 @@ export class WsOutbound extends Outbound {
             handshakeTimeout: 1e4, // 10s
             perMessageDeflate: false,
           }));
-          this._socket.on('open', resolve);
+          this._socket.on('open', () => {
+            this._socket._socket.on('drain', this.onDrain);
+            resolve();
+          });
           this._socket.on('message', this.onReceive);
           this._socket.on('error', this.onError);
           this._socket.on('close', this.onClose);