diff --git a/common/buf/io.go b/common/buf/io.go
index bed9d84f0..0b801449e 100644
--- a/common/buf/io.go
+++ b/common/buf/io.go
@@ -40,13 +40,7 @@ func ReadFullFrom(reader io.Reader, size int32) Supplier {
 	}
 }
 
-// ReadAtLeastFrom create a Supplier to read at least size bytes from the given io.Reader.
-func ReadAtLeastFrom(reader io.Reader, size int) Supplier {
-	return func(b []byte) (int, error) {
-		return io.ReadAtLeast(reader, b, size)
-	}
-}
-
+// WriteAllBytes ensures all bytes are written into the given writer.
 func WriteAllBytes(writer io.Writer, payload []byte) error {
 	for len(payload) > 0 {
 		n, err := writer.Write(payload)
@@ -89,10 +83,3 @@ func NewWriter(writer io.Writer) Writer {
 		Writer: writer,
 	}
 }
-
-// NewSequentialWriter returns a Writer that write Buffers in a MultiBuffer sequentially.
-func NewSequentialWriter(writer io.Writer) Writer {
-	return &SequentialWriter{
-		Writer: writer,
-	}
-}
diff --git a/common/buf/writer.go b/common/buf/writer.go
index 04ca3da42..cb48691bc 100644
--- a/common/buf/writer.go
+++ b/common/buf/writer.go
@@ -171,10 +171,12 @@ func (w *BufferedWriter) Close() error {
 	return common.Close(w.writer)
 }
 
+// SequentialWriter is a Writer that writes MultiBuffer sequentially into the underlying io.Writer.
 type SequentialWriter struct {
 	io.Writer
 }
 
+// WriteMultiBuffer implements Writer.
 func (w *SequentialWriter) WriteMultiBuffer(mb MultiBuffer) error {
 	defer mb.Release()
 
diff --git a/common/buf/writer_test.go b/common/buf/writer_test.go
index 09b01559f..270aabfb8 100644
--- a/common/buf/writer_test.go
+++ b/common/buf/writer_test.go
@@ -73,14 +73,21 @@ func TestDiscardBytesMultiBuffer(t *testing.T) {
 }
 
 func TestWriterInterface(t *testing.T) {
-	assert := With(t)
+	{
+		var writer interface{} = (*BufferToBytesWriter)(nil)
+		switch writer.(type) {
+		case Writer, io.Writer, io.ReaderFrom:
+		default:
+			t.Error("BufferToBytesWriter is not Writer, io.Writer or io.ReaderFrom")
+		}
+	}
 
-	assert((*BufferToBytesWriter)(nil), Implements, (*Writer)(nil))
-	assert((*BufferToBytesWriter)(nil), Implements, (*io.Writer)(nil))
-	assert((*BufferToBytesWriter)(nil), Implements, (*io.ReaderFrom)(nil))
-
-	assert((*BufferedWriter)(nil), Implements, (*Writer)(nil))
-	assert((*BufferedWriter)(nil), Implements, (*io.Writer)(nil))
-	assert((*BufferedWriter)(nil), Implements, (*io.ReaderFrom)(nil))
-	assert((*BufferedWriter)(nil), Implements, (*io.ByteWriter)(nil))
+	{
+		var writer interface{} = (*BufferedWriter)(nil)
+		switch writer.(type) {
+		case Writer, io.Writer, io.ReaderFrom, io.ByteWriter:
+		default:
+			t.Error("BufferedWriter is not Writer, io.Writer, io.ReaderFrom or io.ByteWriter")
+		}
+	}
 }
diff --git a/proxy/dokodemo/dokodemo.go b/proxy/dokodemo/dokodemo.go
index fa4fbfe90..bd71ef0e4 100644
--- a/proxy/dokodemo/dokodemo.go
+++ b/proxy/dokodemo/dokodemo.go
@@ -110,14 +110,14 @@ func (d *DokodemoDoor) Process(ctx context.Context, network net.Network, conn in
 		} else {
 			//if we are in TPROXY mode, use linux's udp forging functionality
 			if !d.config.FollowRedirect {
-				writer = buf.NewSequentialWriter(conn)
+				writer = &buf.SequentialWriter{Writer: conn}
 			} else {
 				srca := net.UDPAddr{IP: dest.Address.IP(), Port: int(dest.Port.Value())}
 				origsend, err := udp.TransmitSocket(&srca, conn.RemoteAddr())
 				if err != nil {
 					return err
 				}
-				writer = buf.NewSequentialWriter(origsend)
+				writer = &buf.SequentialWriter{Writer: origsend}
 			}
 		}
 
diff --git a/proxy/freedom/freedom.go b/proxy/freedom/freedom.go
index fe69381d2..4206d3120 100644
--- a/proxy/freedom/freedom.go
+++ b/proxy/freedom/freedom.go
@@ -128,7 +128,7 @@ func (h *Handler) Process(ctx context.Context, link *core.Link, dialer proxy.Dia
 		if destination.Network == net.Network_TCP {
 			writer = buf.NewWriter(conn)
 		} else {
-			writer = buf.NewSequentialWriter(conn)
+			writer = &buf.SequentialWriter{Writer: conn}
 		}
 		if err := buf.Copy(input, writer, buf.UpdateActivity(timer)); err != nil {
 			return newError("failed to process request").Base(err)
diff --git a/proxy/shadowsocks/client.go b/proxy/shadowsocks/client.go
index 8b1478a28..7c0232d40 100644
--- a/proxy/shadowsocks/client.go
+++ b/proxy/shadowsocks/client.go
@@ -133,10 +133,10 @@ func (c *Client) Process(ctx context.Context, link *core.Link, dialer proxy.Dial
 
 	if request.Command == protocol.RequestCommandUDP {
 
-		writer := buf.NewSequentialWriter(&UDPWriter{
+		writer := &buf.SequentialWriter{Writer: &UDPWriter{
 			Writer:  conn,
 			Request: request,
-		})
+		}}
 
 		requestDone := func() error {
 			defer timer.SetTimeout(sessionPolicy.Timeouts.DownlinkOnly)
diff --git a/proxy/shadowsocks/protocol_test.go b/proxy/shadowsocks/protocol_test.go
index 2a1fd855c..673444dd8 100644
--- a/proxy/shadowsocks/protocol_test.go
+++ b/proxy/shadowsocks/protocol_test.go
@@ -142,7 +142,7 @@ func TestUDPReaderWriter(t *testing.T) {
 		}),
 	}
 	cache := buf.New()
-	writer := buf.NewSequentialWriter(&UDPWriter{
+	writer := &buf.SequentialWriter{Writer: &UDPWriter{
 		Writer: cache,
 		Request: &protocol.RequestHeader{
 			Version: Version,
@@ -151,7 +151,7 @@ func TestUDPReaderWriter(t *testing.T) {
 			User:    user,
 			Option:  RequestOptionOneTimeAuth,
 		},
-	})
+	}}
 
 	reader := &UDPReader{
 		Reader: cache,
diff --git a/proxy/socks/client.go b/proxy/socks/client.go
index 8765b94e7..e837b53c2 100644
--- a/proxy/socks/client.go
+++ b/proxy/socks/client.go
@@ -123,7 +123,7 @@ func (c *Client) Process(ctx context.Context, link *core.Link, dialer proxy.Dial
 		defer udpConn.Close() // nolint: errcheck
 		requestFunc = func() error {
 			defer timer.SetTimeout(p.Timeouts.DownlinkOnly)
-			return buf.Copy(link.Reader, buf.NewSequentialWriter(NewUDPWriter(request, udpConn)), buf.UpdateActivity(timer))
+			return buf.Copy(link.Reader, &buf.SequentialWriter{Writer: NewUDPWriter(request, udpConn)}, buf.UpdateActivity(timer))
 		}
 		responseFunc = func() error {
 			defer timer.SetTimeout(p.Timeouts.UplinkOnly)
diff --git a/proxy/socks/protocol_test.go b/proxy/socks/protocol_test.go
index 325f3c7bf..73f517f75 100644
--- a/proxy/socks/protocol_test.go
+++ b/proxy/socks/protocol_test.go
@@ -20,7 +20,7 @@ func TestUDPEncoding(t *testing.T) {
 		Address: net.IPAddress([]byte{1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4, 5, 6}),
 		Port:    1024,
 	}
-	writer := buf.NewSequentialWriter(NewUDPWriter(request, b))
+	writer := &buf.SequentialWriter{Writer: NewUDPWriter(request, b)}
 
 	content := []byte{'a'}
 	payload := buf.New()