diff --git a/acceptor_windows.go b/acceptor_windows.go
index 13d551f82..25717e59f 100644
--- a/acceptor_windows.go
+++ b/acceptor_windows.go
@@ -69,7 +69,7 @@ func (eng *engine) listen() (err error) {
 			}
 			el := eng.eventLoops.next(tc.RemoteAddr())
 			c := newTCPConn(tc, el)
-			el.ch <- c
+			el.ch <- &openConn{c: c}
 			go func(c *conn, tc net.Conn, el *eventloop) {
 				var buffer [0x10000]byte
 				for {
diff --git a/client_test.go b/client_test.go
index f6b4fd6e8..0a4212f60 100644
--- a/client_test.go
+++ b/client_test.go
@@ -4,9 +4,11 @@
 package gnet
 
 import (
+	"bytes"
 	"io"
 	"math/rand"
 	"net"
+	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -41,6 +43,13 @@ func (ev *clientEvents) OnBoot(e Engine) Action {
 	return None
 }
 
+var pingMsg = []byte("PING\r\n")
+
+func (ev *clientEvents) OnOpen(Conn) (out []byte, action Action) {
+	out = pingMsg
+	return
+}
+
 func (ev *clientEvents) OnClose(Conn, error) Action {
 	if ev.svr != nil {
 		if atomic.AddInt32(&ev.svr.clientActive, -1) == 0 {
@@ -53,7 +62,7 @@ func (ev *clientEvents) OnClose(Conn, error) Action {
 func (ev *clientEvents) OnTraffic(c Conn) (action Action) {
 	handler := c.Context().(*connHandler)
 	if handler.network == "udp" {
-		ev.packetLen = 1024
+		ev.packetLen = datagramLen
 	}
 	buf, err := c.Next(-1)
 	assert.NoError(ev.tester, err)
@@ -190,19 +199,20 @@ func TestServeWithGnetClient(t *testing.T) {
 
 type testClientServer struct {
 	*BuiltinEventEngine
-	client       *Client
-	tester       *testing.T
-	eng          Engine
-	network      string
-	addr         string
-	multicore    bool
-	async        bool
-	nclients     int
-	started      int32
-	connected    int32
-	clientActive int32
-	disconnected int32
-	workerPool   *goPool.Pool
+	client        *Client
+	tester        *testing.T
+	eng           Engine
+	network       string
+	addr          string
+	multicore     bool
+	async         bool
+	nclients      int
+	started       int32
+	connected     int32
+	clientActive  int32
+	disconnected  int32
+	workerPool    *goPool.Pool
+	udpReadHeader int32
 }
 
 func (s *testClientServer) OnBoot(eng Engine) (action Action) {
@@ -211,7 +221,7 @@ func (s *testClientServer) OnBoot(eng Engine) (action Action) {
 }
 
 func (s *testClientServer) OnOpen(c Conn) (out []byte, action Action) {
-	c.SetContext(c)
+	c.SetContext(&sync.Once{})
 	atomic.AddInt32(&s.connected, 1)
 	require.NotNil(s.tester, c.LocalAddr(), "nil local addr")
 	require.NotNil(s.tester, c.RemoteAddr(), "nil remote addr")
@@ -223,7 +233,7 @@ func (s *testClientServer) OnClose(c Conn, err error) (action Action) {
 		logging.Debugf("error occurred on closed, %v\n", err)
 	}
 	if s.network != "udp" {
-		require.Equal(s.tester, c.Context(), c, "invalid context")
+		require.IsType(s.tester, c.Context(), new(sync.Once), "invalid context")
 	}
 
 	atomic.AddInt32(&s.disconnected, 1)
@@ -236,7 +246,25 @@ func (s *testClientServer) OnClose(c Conn, err error) (action Action) {
 	return
 }
 
+func (s *testClientServer) OnShutdown(Engine) {
+	if s.network == "udp" {
+		require.EqualValues(s.tester, int32(s.nclients), atomic.LoadInt32(&s.udpReadHeader))
+	}
+}
+
 func (s *testClientServer) OnTraffic(c Conn) (action Action) {
+	readHeader := func() {
+		ping := make([]byte, len(pingMsg))
+		n, err := io.ReadFull(c, ping)
+		require.NoError(s.tester, err)
+		require.EqualValues(s.tester, len(pingMsg), n)
+		require.Equal(s.tester, string(pingMsg), string(ping), "bad header")
+	}
+	v := c.Context()
+	if v != nil && s.network != "udp" {
+		v.(*sync.Once).Do(readHeader)
+	}
+
 	if s.async {
 		buf := bbPool.Get()
 		_, _ = c.WriteTo(buf)
@@ -247,14 +275,30 @@ func (s *testClientServer) OnTraffic(c Conn) (action Action) {
 			_ = c.OutboundBuffered()
 			_, _ = c.Discard(1)
 		}
+		if s.network == "udp" && bytes.Equal(buf.Bytes(), pingMsg) {
+			atomic.AddInt32(&s.udpReadHeader, 1)
+			buf.Reset()
+		}
 		_ = s.workerPool.Submit(
 			func() {
-				_ = c.AsyncWrite(buf.Bytes(), nil)
+				if buf.Len() > 0 {
+					err := c.AsyncWrite(buf.Bytes(), nil)
+					require.NoError(s.tester, err)
+				}
 			})
 		return
 	}
+
 	buf, _ := c.Next(-1)
-	_, _ = c.Write(buf)
+	if s.network == "udp" && bytes.Equal(buf, pingMsg) {
+		atomic.AddInt32(&s.udpReadHeader, 1)
+		buf = nil
+	}
+	if len(buf) > 0 {
+		n, err := c.Write(buf)
+		require.NoError(s.tester, err)
+		require.EqualValues(s.tester, len(buf), n)
+	}
 	return
 }
 
@@ -343,7 +387,7 @@ func startGnetClient(t *testing.T, cli *Client, network, addr string, multicore,
 	for time.Since(start) < duration {
 		reqData := make([]byte, streamLen)
 		if network == "udp" {
-			reqData = reqData[:1024]
+			reqData = reqData[:datagramLen]
 		}
 		_, err = rand.Read(reqData)
 		require.NoError(t, err)
diff --git a/client_unix.go b/client_unix.go
index fc4470b3f..64890b6c7 100644
--- a/client_unix.go
+++ b/client_unix.go
@@ -228,10 +228,16 @@ func (cli *Client) EnrollContext(c net.Conn, ctx interface{}) (Conn, error) {
 		return nil, errorx.ErrUnsupportedProtocol
 	}
 	gc.SetContext(ctx)
-	err = cli.el.poller.UrgentTrigger(cli.el.register, gc)
+
+	connOpened := make(chan struct{})
+	ccb := &connWithCallback{c: gc, cb: func() {
+		close(connOpened)
+	}}
+	err = cli.el.poller.UrgentTrigger(cli.el.register, ccb)
 	if err != nil {
 		gc.Close()
 		return nil, err
 	}
+	<-connOpened
 	return gc, nil
 }
diff --git a/client_windows.go b/client_windows.go
index b856bf7c3..07d2294b2 100644
--- a/client_windows.go
+++ b/client_windows.go
@@ -147,6 +147,7 @@ func (cli *Client) Enroll(nc net.Conn) (gc Conn, err error) {
 }
 
 func (cli *Client) EnrollContext(nc net.Conn, ctx interface{}) (gc Conn, err error) {
+	connOpened := make(chan struct{})
 	switch v := nc.(type) {
 	case *net.TCPConn:
 		if cli.opts.TCPNoDelay == TCPNoDelay {
@@ -165,7 +166,7 @@ func (cli *Client) EnrollContext(nc net.Conn, ctx interface{}) (gc Conn, err err
 
 		c := newTCPConn(nc, cli.el)
 		c.SetContext(ctx)
-		cli.el.ch <- c
+		cli.el.ch <- &openConn{c: c, cb: func() { close(connOpened) }}
 		go func(c *conn, tc net.Conn, el *eventloop) {
 			var buffer [0x10000]byte
 			for {
@@ -181,7 +182,7 @@ func (cli *Client) EnrollContext(nc net.Conn, ctx interface{}) (gc Conn, err err
 	case *net.UnixConn:
 		c := newTCPConn(nc, cli.el)
 		c.SetContext(ctx)
-		cli.el.ch <- c
+		cli.el.ch <- &openConn{c: c, cb: func() { close(connOpened) }}
 		go func(c *conn, uc net.Conn, el *eventloop) {
 			var buffer [0x10000]byte
 			for {
@@ -204,6 +205,7 @@ func (cli *Client) EnrollContext(nc net.Conn, ctx interface{}) (gc Conn, err err
 		c := newUDPConn(cli.el, nc.LocalAddr(), nc.RemoteAddr())
 		c.SetContext(ctx)
 		c.rawConn = nc
+		cli.el.ch <- &openConn{c: c, isDatagram: true, cb: func() { close(connOpened) }}
 		go func(uc net.Conn, el *eventloop) {
 			var buffer [0x10000]byte
 			for {
@@ -222,5 +224,6 @@ func (cli *Client) EnrollContext(nc net.Conn, ctx interface{}) (gc Conn, err err
 		return nil, errorx.ErrUnsupportedProtocol
 	}
 
+	<-connOpened
 	return
 }
diff --git a/connection_unix.go b/connection_unix.go
index 44cd351d4..ba7cffa77 100644
--- a/connection_unix.go
+++ b/connection_unix.go
@@ -84,6 +84,7 @@ func newUDPConn(fd int, el *eventloop, localAddr net.Addr, sa unix.Sockaddr, con
 }
 
 func (c *conn) release() {
+	c.opened = false
 	c.ctx = nil
 	c.buffer = nil
 	if addr, ok := c.localAddr.(*net.TCPAddr); ok && c.localAddr != c.loop.ln.addr && len(addr.Zone) > 0 {
@@ -102,7 +103,6 @@ func (c *conn) release() {
 	c.remoteAddr = nil
 	c.pollAttachment.FD, c.pollAttachment.Callback = 0, nil
 	if !c.isDatagram {
-		c.opened = false
 		c.peer = nil
 		c.inboundBuffer.Done()
 		c.outboundBuffer.Release()
@@ -110,6 +110,10 @@ func (c *conn) release() {
 }
 
 func (c *conn) open(buf []byte) error {
+	if c.isDatagram && c.peer == nil {
+		return unix.Send(c.fd, buf, 0)
+	}
+
 	n, err := unix.Write(c.fd, buf)
 	if err != nil && err == unix.EAGAIN {
 		_, _ = c.outboundBuffer.Write(buf)
diff --git a/connection_windows.go b/connection_windows.go
index c41a3f94f..61619b5bb 100644
--- a/connection_windows.go
+++ b/connection_windows.go
@@ -42,6 +42,12 @@ type udpConn struct {
 	c *conn
 }
 
+type openConn struct {
+	c          *conn
+	cb         func()
+	isDatagram bool
+}
+
 type conn struct {
 	ctx           interface{}        // user-defined context
 	loop          *eventloop         // owner event-loop
diff --git a/eventloop_unix.go b/eventloop_unix.go
index d434267cd..6cb1cd73c 100644
--- a/eventloop_unix.go
+++ b/eventloop_unix.go
@@ -61,8 +61,19 @@ func (el *eventloop) closeConns() {
 	})
 }
 
+type connWithCallback struct {
+	c  Conn
+	cb func()
+}
+
 func (el *eventloop) register(itf interface{}) error {
-	c := itf.(*conn)
+	c, ok := itf.(*conn)
+	if !ok {
+		ccb := itf.(*connWithCallback)
+		c = ccb.c.(*conn)
+		defer ccb.cb()
+	}
+
 	if err := el.poller.AddRead(&c.pollAttachment); err != nil {
 		_ = unix.Close(c.fd)
 		c.release()
@@ -71,7 +82,7 @@ func (el *eventloop) register(itf interface{}) error {
 
 	el.connections.addConn(c, el.idx)
 
-	if c.isDatagram {
+	if c.isDatagram && c.peer != nil {
 		return nil
 	}
 	return el.open(c)
diff --git a/eventloop_windows.go b/eventloop_windows.go
index 5ae95ca3d..074318c58 100644
--- a/eventloop_windows.go
+++ b/eventloop_windows.go
@@ -67,7 +67,7 @@ func (el *eventloop) run() (err error) {
 			err = v
 		case *netErr:
 			err = el.close(v.c, v.err)
-		case *conn:
+		case *openConn:
 			err = el.open(v)
 		case *tcpConn:
 			unpackTCPConn(v)
@@ -90,9 +90,16 @@ func (el *eventloop) run() (err error) {
 	return nil
 }
 
-func (el *eventloop) open(c *conn) error {
-	el.connections[c] = struct{}{}
-	el.incConn(1)
+func (el *eventloop) open(oc *openConn) error {
+	if oc.cb != nil {
+		defer oc.cb()
+	}
+
+	c := oc.c
+	if !oc.isDatagram {
+		el.connections[c] = struct{}{}
+		el.incConn(1)
+	}
 
 	out, action := el.eventHandler.OnOpen(c)
 	if out != nil {
diff --git a/gnet_test.go b/gnet_test.go
index 36bba8c73..e6150023d 100644
--- a/gnet_test.go
+++ b/gnet_test.go
@@ -25,7 +25,10 @@ import (
 	goPool "github.com/panjf2000/gnet/v2/pkg/pool/goroutine"
 )
 
-var streamLen = 1024 * 1024
+var (
+	datagramLen = 1024
+	streamLen   = 1024 * 1024
+)
 
 func TestServe(t *testing.T) {
 	// start an engine
@@ -415,7 +418,7 @@ func startClient(t *testing.T, network, addr string, multicore, async bool) {
 	for time.Since(start) < duration {
 		reqData := make([]byte, streamLen)
 		if network == "udp" {
-			reqData = reqData[:1024]
+			reqData = reqData[:datagramLen]
 		}
 		_, err = rand.Read(reqData)
 		require.NoError(t, err)