diff --git a/cmd/internal/peer/udp_client_test.go b/cmd/internal/peer/udp_client_test.go
index 95aa4c1..d23750c 100644
--- a/cmd/internal/peer/udp_client_test.go
+++ b/cmd/internal/peer/udp_client_test.go
@@ -4,15 +4,56 @@ import (
 	"net"
 	"os"
 	"path/filepath"
+	"sync"
 	"testing"
 	"time"
 
+	"omnisocketgo/cmd/internal/latencylog"
 	"omnisocketgo/cmd/internal/protocol"
 	"omnisocketgo/cmd/internal/server"
 	"omnisocketgo/cmd/internal/transport"
 )
 
-// TestUDPDialAndSendText 验证 UDP 客户端可以成功连接、注册并发送文本消息。
+type recordingLatencyLogger struct {
+	mu     sync.Mutex
+	events []latencylog.Event
+}
+
+func (l *recordingLatencyLogger) LogEvent(event latencylog.Event) error {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	l.events = append(l.events, event)
+	return nil
+}
+
+func (l *recordingLatencyLogger) Events() []latencylog.Event {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	return append([]latencylog.Event(nil), l.events...)
+}
+
+type recordingUDPTXTimestampDebugLogger struct {
+	mu      sync.Mutex
+	records []transport.TXTimestampDebugRecord
+}
+
+func (l *recordingUDPTXTimestampDebugLogger) LogTXTimestampDebugRecord(record transport.TXTimestampDebugRecord) error {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	l.records = append(l.records, record)
+	return nil
+}
+
+func (l *recordingUDPTXTimestampDebugLogger) Records() []transport.TXTimestampDebugRecord {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	return append([]transport.TXTimestampDebugRecord(nil), l.records...)
+}
+
 func TestUDPDialAndSendText(t *testing.T) {
 	hubAddr := startUDPTestHub(t)
 
@@ -28,15 +69,12 @@ func TestUDPDialAndSendText(t *testing.T) {
 	}
 	defer clientB.Close()
 
-	// 等待注册被处理
 	time.Sleep(50 * time.Millisecond)
 
-	// peer-a 发送文本给 peer-b
 	if err := clientA.SendText("peer-b", "hello from udp"); err != nil {
 		t.Fatalf("SendText() error = %v", err)
 	}
 
-	// peer-b 接收
 	msg := receiveUDPClientMessage(t, clientB)
 	if msg.Type != protocol.MessageTypeText {
 		t.Fatalf("message type = %s, want text", msg.Type)
@@ -46,7 +84,6 @@ func TestUDPDialAndSendText(t *testing.T) {
 	}
 }
 
-// TestUDPClientID 验证 ID() 返回正确的 peer 标识。
 func TestUDPClientID(t *testing.T) {
 	hubAddr := startUDPTestHub(t)
 
@@ -61,7 +98,6 @@ func TestUDPClientID(t *testing.T) {
 	}
 }
 
-// TestUDPClientPersistMessage 验证 UDP 客户端可以将消息持久化到磁盘。
 func TestUDPClientPersistMessage(t *testing.T) {
 	hubAddr := startUDPTestHub(t)
 
@@ -73,7 +109,6 @@ func TestUDPClientPersistMessage(t *testing.T) {
 
 	inboxDir := t.TempDir()
 
-	// 持久化文本消息
 	textMsg := protocol.Message{
 		Type: protocol.MessageTypeText,
 		ID:   1,
@@ -90,7 +125,6 @@ func TestUDPClientPersistMessage(t *testing.T) {
 		t.Fatalf("PersistMessage(text) returned empty path")
 	}
 
-	// 持久化文件消息
 	fileMsg := protocol.Message{
 		Type:     protocol.MessageTypeFile,
 		ID:       2,
@@ -114,7 +148,6 @@ func TestUDPClientPersistMessage(t *testing.T) {
 	}
 }
 
-// TestUDPClientSendFile 验证 UDP 客户端可以发送文件消息。
 func TestUDPClientSendFile(t *testing.T) {
 	hubAddr := startUDPTestHub(t)
 
@@ -149,8 +182,83 @@ func TestUDPClientSendFile(t *testing.T) {
 	}
 }
 
-// startUDPTestHub 创建并启动一个测试用 UDPHub。
+func TestUDPClientBidirectionalLogsAndServerDiagnostics(t *testing.T) {
+	serverLogger := &recordingLatencyLogger{}
+	serverDebugLogger := &recordingUDPTXTimestampDebugLogger{}
+	hubAddr := startUDPTestHubWithOptions(
+		t,
+		server.WithUDPLogger(serverLogger),
+		server.WithUDPTXTimestampDebugLogger(serverDebugLogger),
+	)
+
+	clientALogger := &recordingLatencyLogger{}
+	clientADebugLogger := &recordingUDPTXTimestampDebugLogger{}
+	clientA, err := DialUDP(
+		hubAddr.String(),
+		"peer-a",
+		WithLogger(clientALogger),
+		WithTXTimestampDebugLogger(clientADebugLogger),
+	)
+	if err != nil {
+		t.Fatalf("DialUDP(peer-a) error = %v", err)
+	}
+	defer clientA.Close()
+
+	clientBLogger := &recordingLatencyLogger{}
+	clientBDebugLogger := &recordingUDPTXTimestampDebugLogger{}
+	clientB, err := DialUDP(
+		hubAddr.String(),
+		"peer-b",
+		WithLogger(clientBLogger),
+		WithTXTimestampDebugLogger(clientBDebugLogger),
+	)
+	if err != nil {
+		t.Fatalf("DialUDP(peer-b) error = %v", err)
+	}
+	defer clientB.Close()
+
+	time.Sleep(50 * time.Millisecond)
+
+	if err := clientA.SendText("peer-b", "hello from peer-a"); err != nil {
+		t.Fatalf("clientA.SendText() error = %v", err)
+	}
+	msgFromA := receiveUDPClientMessage(t, clientB)
+	if string(msgFromA.Body) != "hello from peer-a" {
+		t.Fatalf("message body = %q, want %q", string(msgFromA.Body), "hello from peer-a")
+	}
+
+	if err := clientB.SendText("peer-a", "hello from peer-b"); err != nil {
+		t.Fatalf("clientB.SendText() error = %v", err)
+	}
+	msgFromB := receiveUDPClientMessage(t, clientA)
+	if string(msgFromB.Body) != "hello from peer-b" {
+		t.Fatalf("message body = %q, want %q", string(msgFromB.Body), "hello from peer-b")
+	}
+
+	assertPeerEvent(t, clientBLogger.Events(), latencylog.EventBRXSoftware, "peer-a", "peer-b", msgFromA.ID)
+	assertPeerEvent(t, clientBLogger.Events(), latencylog.EventBAppRecv, "peer-a", "peer-b", msgFromA.ID)
+	assertPeerEvent(t, clientALogger.Events(), latencylog.EventBRXSoftware, "peer-b", "peer-a", msgFromB.ID)
+	assertPeerEvent(t, clientALogger.Events(), latencylog.EventBAppRecv, "peer-b", "peer-a", msgFromB.ID)
+
+	if len(clientADebugLogger.Records()) == 0 {
+		t.Fatal("clientA debug records = 0, want at least 1")
+	}
+	if len(clientBDebugLogger.Records()) == 0 {
+		t.Fatal("clientB debug records = 0, want at least 1")
+	}
+	if len(serverLogger.Events()) == 0 {
+		t.Fatal("server latency events = 0, want at least 1")
+	}
+	if len(serverDebugLogger.Records()) == 0 {
+		t.Fatal("server debug records = 0, want at least 1")
+	}
+}
+
 func startUDPTestHub(t *testing.T) *net.UDPAddr {
+	return startUDPTestHubWithOptions(t)
+}
+
+func startUDPTestHubWithOptions(t *testing.T, opts ...server.UDPOption) *net.UDPAddr {
 	t.Helper()
 
 	addr, err := net.ResolveUDPAddr("udp", "127.0.0.1:0")
@@ -163,7 +271,7 @@ func startUDPTestHub(t *testing.T) *net.UDPAddr {
 		t.Fatalf("ListenUDP() error = %v", err)
 	}
 
-	hub, err := server.NewUDPHub(conn)
+	hub, err := server.NewUDPHub(conn, opts...)
 	if err != nil {
 		_ = conn.Close()
 		t.Fatalf("NewUDPHub() error = %v", err)
@@ -180,7 +288,6 @@ func startUDPTestHub(t *testing.T) *net.UDPAddr {
 	return conn.LocalAddr().(*net.UDPAddr)
 }
 
-// receiveUDPClientMessage 从 UDP 客户端接收一条消息，带超时。
 func receiveUDPClientMessage(t *testing.T, client *UDPClient) protocol.Message {
 	t.Helper()
 
@@ -207,5 +314,24 @@ func receiveUDPClientMessage(t *testing.T, client *UDPClient) protocol.Message {
 	}
 }
 
+func assertPeerEvent(t *testing.T, events []latencylog.Event, wantEvent, wantFrom, wantTo string, wantMessageID uint64) {
+	t.Helper()
+
+	for _, event := range events {
+		if event.Event != wantEvent {
+			continue
+		}
+		if event.From != wantFrom || event.To != wantTo || event.MessageID != wantMessageID {
+			continue
+		}
+		if event.TsUnixNano <= 0 {
+			t.Fatalf("event %s timestamp must be positive: %+v", wantEvent, event)
+		}
+		return
+	}
+
+	t.Fatalf("missing event %s from %s to %s for message %d in %+v", wantEvent, wantFrom, wantTo, wantMessageID, events)
+}
+
 // Ensure transport package is used (needed for WithTXTimestampDebugLogger option).
 var _ transport.TXTimestampDebugLogger = nil
diff --git a/cmd/internal/server/udp_hub.go b/cmd/internal/server/udp_hub.go
index 4275337..680b94d 100644
--- a/cmd/internal/server/udp_hub.go
+++ b/cmd/internal/server/udp_hub.go
@@ -21,16 +21,22 @@ func WithUDPLogger(logger latencylog.Logger) UDPOption {
 	}
 }
 
+// WithUDPTXTimestampDebugLogger 为 UDP hub 注入 TX errqueue 调试日志器。
+func WithUDPTXTimestampDebugLogger(logger transport.TXTimestampDebugLogger) UDPOption {
+	return func(hub *UDPHub) {
+		hub.txTimestampDebugLogger = logger
+	}
+}
+
 // UDPHub 管理通过 UDP 注册的 peer，并负责在它们之间转发消息。
-// 与 TCP Hub 不同，UDPHub 使用单个 net.UDPConn 与所有 peer 通信，
-// 通过维护 peerID -> UDPAddr 映射表来寻址。
 type UDPHub struct {
 	mu    sync.RWMutex
-	peers map[string]*net.UDPAddr // peerID -> 对端 UDP 地址
-	addrs map[string]string       // addr.String() -> peerID，用于反查
+	peers map[string]*net.UDPAddr
+	addrs map[string]string
 
-	conn   *transport.UDPConn
-	logger latencylog.Logger
+	conn                   *transport.UDPConn
+	logger                 latencylog.Logger
+	txTimestampDebugLogger transport.TXTimestampDebugLogger
 }
 
 // NewUDPHub 创建一个新的 UDP 连接中心。
@@ -53,28 +59,27 @@ func NewUDPHub(conn *net.UDPConn, opts ...UDPOption) (*UDPHub, error) {
 		conn,
 		nil,
 		transport.WithUDPLogger(hub.logger, latencylog.NodeRoleServer, "hub"),
+		transport.WithUDPTXTimestampDebugLogger(hub.txTimestampDebugLogger),
 	)
 	if err != nil {
 		return nil, fmt.Errorf("server: create udp transport conn: %w", err)
 	}
 
 	hub.conn = udpConn
-
 	return hub, nil
 }
 
 // Serve 启动 UDP 接收主循环，持续读取消息并处理注册/转发。
-// 此方法会阻塞，直到底层连接关闭或发生不可恢复的错误。
 func (h *UDPHub) Serve() error {
 	return h.conn.ReceiveLoop(func(msg protocol.Message, addr *net.UDPAddr) error {
 		if err := h.handleMessage(msg, addr); err != nil {
 			log.Printf("udp hub: handle message from %s: %v", addr, err)
 		}
-		return nil // 不因为单条消息处理失败而退出主循环
+		return nil
 	})
 }
 
-// HasPeer 返回给定 ID 是否已注册到 hub。
+// HasPeer 返回给定 ID 是否已经注册到 hub。
 func (h *UDPHub) HasPeer(peerID string) bool {
 	h.mu.RLock()
 	defer h.mu.RUnlock()
@@ -83,7 +88,6 @@ func (h *UDPHub) HasPeer(peerID string) bool {
 	return ok
 }
 
-// handleMessage 处理从指定地址收到的消息。
 func (h *UDPHub) handleMessage(msg protocol.Message, addr *net.UDPAddr) error {
 	switch msg.Type {
 	case protocol.MessageTypeRegister:
@@ -101,7 +105,6 @@ func (h *UDPHub) handleMessage(msg protocol.Message, addr *net.UDPAddr) error {
 	}
 }
 
-// registerPeer 处理 peer 的注册请求。
 func (h *UDPHub) registerPeer(msg protocol.Message, addr *net.UDPAddr) error {
 	peerID := msg.From
 	if peerID == "" {
@@ -111,9 +114,7 @@ func (h *UDPHub) registerPeer(msg protocol.Message, addr *net.UDPAddr) error {
 	h.mu.Lock()
 	defer h.mu.Unlock()
 
-	// 如果同一个 peerID 从新地址注册，更新地址映射（支持 peer 重启换端口）。
 	if existingAddr, exists := h.peers[peerID]; exists {
-		// 清理旧地址的反查映射
 		delete(h.addrs, existingAddr.String())
 	}
 
@@ -123,26 +124,20 @@ func (h *UDPHub) registerPeer(msg protocol.Message, addr *net.UDPAddr) error {
 	return nil
 }
 
-// forwardMessage 转发业务消息到目标 peer。
 func (h *UDPHub) forwardMessage(msg protocol.Message, senderAddr *net.UDPAddr) error {
-	// 通过来源地址反查发送者 peerID
 	senderID := h.lookupPeerID(senderAddr)
 	if senderID == "" {
 		return h.sendErrorTo(senderAddr, msg.From, "not registered; send register first")
 	}
 
-	// server 覆盖 From，不信任客户端自报身份
 	msg.From = senderID
 
-	// 查找目标 peer 地址
 	targetAddr := h.lookupAddr(msg.To)
 	if targetAddr == nil {
 		return h.sendErrorTo(senderAddr, senderID, fmt.Sprintf("unknown target: %s", msg.To))
 	}
 
-	// 转发消息
 	if err := h.conn.SendTo(msg, targetAddr); err != nil {
-		// 转发失败，通知发送方
 		_ = h.sendErrorTo(senderAddr, senderID, fmt.Sprintf("failed to forward to %s", msg.To))
 		return fmt.Errorf("forward to %s at %s: %w", msg.To, targetAddr, err)
 	}
@@ -166,7 +161,6 @@ func (h *UDPHub) lookupAddr(peerID string) *net.UDPAddr {
 	return h.peers[peerID]
 }
 
-// sendErrorTo 向指定地址发送错误消息。
 func (h *UDPHub) sendErrorTo(addr *net.UDPAddr, to, message string) error {
 	if to == "" {
 		to = "unknown"
diff --git a/cmd/internal/transport/udp.go b/cmd/internal/transport/udp.go
index 07bba36..566d1ed 100644
--- a/cmd/internal/transport/udp.go
+++ b/cmd/internal/transport/udp.go
@@ -11,22 +11,32 @@ import (
 )
 
 // UDPConn 是对 UDP 连接的轻量封装。
-// server 侧：共享同一个 net.UDPConn，Send 时通过 peerAddr 指定对端地址。
-// peer 侧：独立的 net.UDPConn，已通过 Dial 连接到 server，Send 直接写即可。
+// server 侧共享一个 net.UDPConn，通过 SendTo 指定目标地址；
+// peer 侧使用已连接的 net.UDPConn，直接 Send 即可。
 type UDPConn struct {
 	conn     *net.UDPConn
-	peerAddr *net.UDPAddr    // server 侧为对端地址；peer 侧为 nil（连接模式下直接 Write）
-	raw      syscall.RawConn // 底层 syscall 句柄，用于 Linux socket timestamping
+	peerAddr *net.UDPAddr
+	raw      syscall.RawConn
 
 	logger                 latencylog.Logger
 	txTimestampDebugLogger TXTimestampDebugLogger
-	nodeRole               string     // 日志中记录的节点角色，例如 "server" 或 "peer"
-	nodeID                 string     // 日志中记录的节点 ID
-	writeMu                sync.Mutex // 保护 Send 的互斥锁
+	txPacketSeq            uint32
+	pendingTX              map[uint32]udpTXPendingRecord
+	nodeRole               string
+	nodeID                 string
+	writeMu                sync.Mutex
 	closeOnce              sync.Once
 	closeErr               error
 }
 
+type udpTXPendingRecord struct {
+	msg                protocol.Message
+	sendCallIndex      int
+	bytesWritten       int
+	expectedTXID       uint32
+	observedTimestamps map[string]int64
+}
+
 // UDPOption 用于为 UDPConn 注入可选行为。
 type UDPOption func(*UDPConn)
 
@@ -39,7 +49,7 @@ func WithUDPLogger(logger latencylog.Logger, nodeRole, nodeID string) UDPOption
 	}
 }
 
-// WithUDPTXTimestampDebugLogger 为 UDP 连接注入可选的 TX errqueue 调试日志器。
+// WithUDPTXTimestampDebugLogger 为 UDP 连接注入 TX errqueue 调试日志器。
 func WithUDPTXTimestampDebugLogger(logger TXTimestampDebugLogger) UDPOption {
 	return func(conn *UDPConn) {
 		conn.txTimestampDebugLogger = logger
@@ -47,13 +57,12 @@ func WithUDPTXTimestampDebugLogger(logger TXTimestampDebugLogger) UDPOption {
 }
 
 // NewUDPConn 创建 UDP transport 连接封装。
-// peerAddr 为 nil 时表示 peer 侧已连接模式（conn 已 Dial 到 server）。
-// peerAddr 非 nil 时表示 server 侧，Send 时需要指定目标地址。
 func NewUDPConn(conn *net.UDPConn, peerAddr *net.UDPAddr, opts ...UDPOption) (*UDPConn, error) {
 	udpConn := &UDPConn{
-		conn:     conn,
-		peerAddr: peerAddr,
-		logger:   latencylog.NoopLogger{},
+		conn:      conn,
+		peerAddr:  peerAddr,
+		logger:    latencylog.NoopLogger{},
+		pendingTX: make(map[uint32]udpTXPendingRecord),
 	}
 
 	for _, opt := range opts {
@@ -72,39 +81,34 @@ func NewUDPConn(conn *net.UDPConn, peerAddr *net.UDPAddr, opts ...UDPOption) (*U
 }
 
 // Send 将一条协议消息编码为 UDP 数据报并发送。
-// 多个 goroutine 可以并发调用，内部会串行化写入。
 func (c *UDPConn) Send(msg protocol.Message) error {
 	c.writeMu.Lock()
 	defer c.writeMu.Unlock()
-	latencylog.LogMessageEvent(c.logger, c.nodeRole, c.nodeID, latencylog.EventSendHandoffBegin, msg)
 
+	latencylog.LogMessageEvent(c.logger, c.nodeRole, c.nodeID, latencylog.EventSendHandoffBegin, msg)
 	if err := c.sendMessageLinux(msg); err != nil {
 		return fmt.Errorf("transport: udp send message: %w", err)
 	}
-
 	latencylog.LogMessageEvent(c.logger, c.nodeRole, c.nodeID, latencylog.EventSendHandoffEnd, msg)
 
 	return nil
 }
 
 // SendTo 将一条协议消息编码为 UDP 数据报并发送到指定地址。
-// 主要用于 server 侧向特定 peer 发送消息。
 func (c *UDPConn) SendTo(msg protocol.Message, addr *net.UDPAddr) error {
 	c.writeMu.Lock()
 	defer c.writeMu.Unlock()
-	latencylog.LogMessageEvent(c.logger, c.nodeRole, c.nodeID, latencylog.EventSendHandoffBegin, msg)
 
+	latencylog.LogMessageEvent(c.logger, c.nodeRole, c.nodeID, latencylog.EventSendHandoffBegin, msg)
 	if err := c.sendMessageToLinux(msg, addr); err != nil {
 		return fmt.Errorf("transport: udp send message to %s: %w", addr, err)
 	}
-
 	latencylog.LogMessageEvent(c.logger, c.nodeRole, c.nodeID, latencylog.EventSendHandoffEnd, msg)
 
 	return nil
 }
 
 // Receive 从 UDP 连接读取一条完整协议消息。
-// 返回解码后的消息和来源地址（peer 侧来源地址始终为 server 地址）。
 func (c *UDPConn) Receive() (protocol.Message, *net.UDPAddr, error) {
 	msg, addr, err := c.receiveMessageLinux()
 	if err != nil {
@@ -115,7 +119,6 @@ func (c *UDPConn) Receive() (protocol.Message, *net.UDPAddr, error) {
 }
 
 // ReceiveLoop 持续从 UDP 连接读取消息并交给 handler 处理。
-// handler 的第二个参数是消息来源地址。
 func (c *UDPConn) ReceiveLoop(handler func(protocol.Message, *net.UDPAddr) error) error {
 	for {
 		msg, addr, err := c.Receive()
@@ -130,8 +133,6 @@ func (c *UDPConn) ReceiveLoop(handler func(protocol.Message, *net.UDPAddr) error
 }
 
 // Close 关闭底层 UDP 连接，保证重复调用安全。
-// 注意：server 侧多个 UDPConn 共享同一个 net.UDPConn 时，
-// 只应由 UDPHub 负责关闭底层连接，不应通过此方法关闭。
 func (c *UDPConn) Close() error {
 	c.closeOnce.Do(func() {
 		c.closeErr = c.conn.Close()
diff --git a/cmd/internal/transport/udp_linux.go b/cmd/internal/transport/udp_linux.go
index 7271594..0222a61 100644
--- a/cmd/internal/transport/udp_linux.go
+++ b/cmd/internal/transport/udp_linux.go
@@ -13,7 +13,7 @@ import (
 	"omnisocketgo/cmd/internal/protocol"
 )
 
-// UDP 接收缓冲区大小，足以容纳 MaxFrameSize 加上协议头。
+// UDP 接收缓冲区需要容纳完整 datagram 和协议头。
 const udpReceiveBufferSize = protocol.MaxFrameSize + 1024
 
 // initUDPLinuxTimestamping 拿到底层 fd，并打开 Linux timestamping。
@@ -26,7 +26,6 @@ func (c *UDPConn) initUDPLinuxTimestamping() error {
 		return fmt.Errorf("transport: udp missing syscall conn")
 	}
 
-	// UDP 不需要 OPT_ID_TCP，使用标准的 OPT_ID 即可。
 	flagCandidates := []int{
 		linuxSOFTimestampingTXSched |
 			linuxSOFTimestampingTXSoftware |
@@ -68,21 +67,7 @@ func (c *UDPConn) sendMessageLinux(msg protocol.Message) error {
 		return fmt.Errorf("protocol: encode message: %w", err)
 	}
 
-	readIndex := 0
-	c.drainPendingUDPTXTimestampEvents(msg, linuxTXTimestampPhasePreSendDrain, &readIndex)
-
-	if c.peerAddr != nil {
-		if err := c.udpSendTo(payload, c.peerAddr); err != nil {
-			return err
-		}
-	} else {
-		if err := c.udpSend(payload); err != nil {
-			return err
-		}
-	}
-
-	c.collectAndLogUDPTXTimestampEvents(msg, &readIndex)
-	return nil
+	return c.sendUDPPayloadLinux(msg, payload, c.peerAddr)
 }
 
 // sendMessageToLinux 编码消息并通过 UDP 发送到指定地址，采集 TX 时间戳。
@@ -92,22 +77,65 @@ func (c *UDPConn) sendMessageToLinux(msg protocol.Message, addr *net.UDPAddr) er
 		return fmt.Errorf("protocol: encode message: %w", err)
 	}
 
-	readIndex := 0
-	c.drainPendingUDPTXTimestampEvents(msg, linuxTXTimestampPhasePreSendDrain, &readIndex)
+	return c.sendUDPPayloadLinux(msg, payload, addr)
+}
 
-	if err := c.udpSendTo(payload, addr); err != nil {
+func (c *UDPConn) sendUDPPayloadLinux(msg protocol.Message, payload []byte, addr *net.UDPAddr) error {
+	readIndex := 0
+
+	// pre-send drain 可能读到上一条消息晚到的 errqueue 事件，
+	// 这里必须先清掉，并且按 ee_data 归还给原消息，不能污染当前消息。
+	c.drainPendingUDPTXTimestampEvents(linuxTXTimestampPhasePreSendDrain, &readIndex)
+
+	chunk := c.newUDPSendChunk(len(payload))
+	var err error
+	if addr != nil {
+		err = c.udpSendTo(payload, addr)
+	} else {
+		err = c.udpSend(payload)
+	}
+	if err != nil {
 		return err
 	}
 
-	c.collectAndLogUDPTXTimestampEvents(msg, &readIndex)
+	c.commitUDPSend(msg, chunk)
+	c.collectAndLogUDPTXTimestampEvents(msg, chunk, &readIndex)
 	return nil
 }
 
+func (c *UDPConn) newUDPSendChunk(payloadLen int) txSendChunk {
+	return txSendChunk{
+		SendCallIndex:    0,
+		FrameOffsetStart: 0,
+		FrameOffsetEnd:   payloadLen - 1,
+		BytesWritten:     payloadLen,
+		ExpectedTXID:     c.txPacketSeq,
+	}
+}
+
+func (c *UDPConn) commitUDPSend(msg protocol.Message, chunk txSendChunk) {
+	// Linux 对 UDP datagram 的 ee_data 是按包递增的 ID。
+	// 这里把 ID 和原始消息元数据绑定起来，后续 drain 到的晚到事件才能记回原消息。
+	if c.txTimestampDebugLogger != nil {
+		c.pendingTX[chunk.ExpectedTXID] = udpTXPendingRecord{
+			msg:                msg,
+			sendCallIndex:      chunk.SendCallIndex,
+			bytesWritten:       chunk.BytesWritten,
+			expectedTXID:       chunk.ExpectedTXID,
+			observedTimestamps: make(map[string]int64, 2),
+		}
+		c.logUDPTXSendChunkDebugRecord(msg, chunk)
+	}
+
+	c.txPacketSeq++
+}
+
 // udpSend 通过已连接的 UDP socket 发送数据。
 func (c *UDPConn) udpSend(payload []byte) error {
 	if c.raw != nil {
 		return c.udpSendmsgRaw(payload, nil)
 	}
+
 	_, err := c.conn.Write(payload)
 	return err
 }
@@ -120,6 +148,7 @@ func (c *UDPConn) udpSendTo(payload []byte, addr *net.UDPAddr) error {
 			return c.udpSendmsgRaw(payload, sa)
 		}
 	}
+
 	_, err := c.conn.WriteToUDP(payload, addr)
 	return err
 }
@@ -213,14 +242,13 @@ func (c *UDPConn) udpRecvmsgRaw() ([]byte, *net.UDPAddr, int64, error) {
 			return nil, nil, 0, opErr
 		}
 
-		addr := sockaddrToUDPAddr(from)
-		return buf[:n], addr, rxTimeNS, nil
+		return buf[:n], sockaddrToUDPAddr(from), rxTimeNS, nil
 	}
 }
 
 // collectAndLogUDPTXTimestampEvents 采集并记录 UDP 发送的 TX 时间戳事件。
-func (c *UDPConn) collectAndLogUDPTXTimestampEvents(msg protocol.Message, readIndex *int) {
-	timestamps := c.collectUDPTXTimestampEvents(msg, readIndex)
+func (c *UDPConn) collectAndLogUDPTXTimestampEvents(msg protocol.Message, chunk txSendChunk, readIndex *int) {
+	timestamps := c.collectUDPTXTimestampEvents(msg, chunk, readIndex)
 
 	if ts, ok := timestamps[latencylog.EventATXSched]; ok {
 		latencylog.LogMessageEventAt(c.logger, c.nodeRole, c.nodeID, latencylog.EventATXSched, ts, msg)
@@ -231,13 +259,13 @@ func (c *UDPConn) collectAndLogUDPTXTimestampEvents(msg protocol.Message, readIn
 }
 
 // collectUDPTXTimestampEvents 在 errqueue 中等待 TX 时间戳。
-func (c *UDPConn) collectUDPTXTimestampEvents(msg protocol.Message, readIndex *int) map[string]int64 {
+func (c *UDPConn) collectUDPTXTimestampEvents(msg protocol.Message, chunk txSendChunk, readIndex *int) map[string]int64 {
 	if c.raw == nil {
 		return nil
 	}
 
 	deadline := time.Now().Add(linuxTXTimestampWaitTimeout)
-	timestamps := make(map[string]int64, 2)
+	observed := make([]observedTXTimestampEvent, 0, 4)
 
 	for time.Now().Before(deadline) {
 		event, err := c.recvUDPTXTimestampOnce()
@@ -251,25 +279,31 @@ func (c *UDPConn) collectUDPTXTimestampEvents(msg protocol.Message, readIndex *i
 		if event.EventName == "" || event.TSUnixNano <= 0 {
 			continue
 		}
-		*readIndex++
 
-		if isBusinessTXTimestampEventName(event.EventName) {
-			if _, exists := timestamps[event.EventName]; !exists {
-				timestamps[event.EventName] = event.TSUnixNano
-			}
-		}
+		observed = append(observed, observedTXTimestampEvent{
+			Phase:     linuxTXTimestampPhasePostSendCollect,
+			ReadIndex: *readIndex,
+			Event:     event,
+		})
+		c.recordUDPPendingEvent(event)
+		*readIndex = *readIndex + 1
 
-		if hasCompleteTXTimestampPair(timestamps) {
+		selection := selectTXTimestampEvents(observed, chunk.ExpectedTXID, true)
+		if selection.HasEvent && selection.SelectedID == chunk.ExpectedTXID && hasCompleteTXTimestampPair(selection.Timestamps) {
 			break
 		}
 	}
 
-	c.drainPendingUDPTXTimestampEvents(msg, linuxTXTimestampPhasePostSelectDrain, readIndex)
-	return timestamps
+	selection := selectTXTimestampEvents(observed, chunk.ExpectedTXID, true)
+	c.logObservedUDPTXTimestampEvents(msg, chunk, observed, selection)
+	c.releaseCompletedUDPPendingFromObserved(observed)
+	c.drainPendingUDPTXTimestampEvents(linuxTXTimestampPhasePostSelectDrain, readIndex)
+	c.releaseCompletedUDPPending(chunk.ExpectedTXID)
+	return selection.Timestamps
 }
 
 // drainPendingUDPTXTimestampEvents 清空 errqueue 中残留的时间戳事件。
-func (c *UDPConn) drainPendingUDPTXTimestampEvents(msg protocol.Message, phase string, readIndex *int) {
+func (c *UDPConn) drainPendingUDPTXTimestampEvents(phase string, readIndex *int) {
 	if c.raw == nil {
 		return
 	}
@@ -277,12 +311,27 @@ func (c *UDPConn) drainPendingUDPTXTimestampEvents(msg protocol.Message, phase s
 	for {
 		event, err := c.recvUDPTXTimestampOnce()
 		if err != nil {
+			if isWouldBlock(err) {
+				return
+			}
 			return
 		}
 		if event.EventName == "" || event.TSUnixNano <= 0 {
 			continue
 		}
-		*readIndex++
+
+		complete := c.recordUDPPendingEvent(event)
+		if msg, chunks, ok := c.lookupUDPPendingDebugContext(event.EEData); ok {
+			c.logUDPTXErrqueueDebugRecord(msg, chunks, observedTXTimestampEvent{
+				Phase:     phase,
+				ReadIndex: *readIndex,
+				Event:     event,
+			}, false)
+			if complete {
+				delete(c.pendingTX, event.EEData)
+			}
+		}
+		*readIndex = *readIndex + 1
 	}
 }
 
@@ -312,6 +361,155 @@ func (c *UDPConn) recvUDPTXTimestampOnce() (txTimestampEvent, error) {
 	return event, nil
 }
 
+func (c *UDPConn) recordUDPPendingEvent(event txTimestampEvent) bool {
+	if !isBusinessTXTimestampEventName(event.EventName) {
+		return false
+	}
+
+	record, ok := c.pendingTX[event.EEData]
+	if !ok {
+		return false
+	}
+	if record.observedTimestamps == nil {
+		record.observedTimestamps = make(map[string]int64, 2)
+	}
+	if existing, exists := record.observedTimestamps[event.EventName]; !exists || event.TSUnixNano < existing {
+		record.observedTimestamps[event.EventName] = event.TSUnixNano
+	}
+	c.pendingTX[event.EEData] = record
+
+	return hasCompleteTXTimestampPair(record.observedTimestamps)
+}
+
+func (c *UDPConn) releaseCompletedUDPPending(txID uint32) {
+	record, ok := c.pendingTX[txID]
+	if !ok {
+		return
+	}
+	if hasCompleteTXTimestampPair(record.observedTimestamps) {
+		delete(c.pendingTX, txID)
+	}
+}
+
+func (c *UDPConn) releaseCompletedUDPPendingFromObserved(observed []observedTXTimestampEvent) {
+	seen := make(map[uint32]struct{}, len(observed))
+	for _, entry := range observed {
+		if _, ok := seen[entry.Event.EEData]; ok {
+			continue
+		}
+		c.releaseCompletedUDPPending(entry.Event.EEData)
+		seen[entry.Event.EEData] = struct{}{}
+	}
+}
+
+func (c *UDPConn) lookupUDPPendingDebugContext(txID uint32) (protocol.Message, []txSendChunk, bool) {
+	record, ok := c.pendingTX[txID]
+	if !ok {
+		return protocol.Message{}, nil, false
+	}
+
+	chunk := txSendChunk{
+		SendCallIndex:    record.sendCallIndex,
+		FrameOffsetStart: 0,
+		FrameOffsetEnd:   record.bytesWritten - 1,
+		BytesWritten:     record.bytesWritten,
+		ExpectedTXID:     record.expectedTXID,
+	}
+	return record.msg, []txSendChunk{chunk}, true
+}
+
+func (c *UDPConn) logObservedUDPTXTimestampEvents(msg protocol.Message, chunk txSendChunk, observed []observedTXTimestampEvent, selection txTimestampSelection) {
+	if len(observed) == 0 {
+		return
+	}
+
+	currentChunks := []txSendChunk{chunk}
+	for _, entry := range observed {
+		recordMsg := msg
+		recordChunks := currentChunks
+		if pendingMsg, pendingChunks, ok := c.lookupUDPPendingDebugContext(entry.Event.EEData); ok {
+			recordMsg = pendingMsg
+			recordChunks = pendingChunks
+		}
+
+		// 理想情况应命中本次 expectedTXID；如果等待窗口里只看到了更高的 ee_data，
+		// 就退回到本轮实际观察到的最新事件，至少保留调试和定位线索。
+		selected := selection.HasEvent &&
+			entry.Event.EEData == selection.SelectedID &&
+			isBusinessTXTimestampEventName(entry.Event.EventName) &&
+			selection.Timestamps[entry.Event.EventName] == entry.Event.TSUnixNano
+		c.logUDPTXErrqueueDebugRecord(recordMsg, recordChunks, entry, selected)
+	}
+}
+
+func (c *UDPConn) logUDPTXSendChunkDebugRecord(msg protocol.Message, chunk txSendChunk) {
+	if c.txTimestampDebugLogger == nil {
+		return
+	}
+
+	sendCallIndex := chunk.SendCallIndex
+	frameOffsetStart := chunk.FrameOffsetStart
+	frameOffsetEnd := chunk.FrameOffsetEnd
+	bytesWritten := chunk.BytesWritten
+	expectedTXID := chunk.ExpectedTXID
+
+	record := c.newUDPTXTimestampDebugRecord(msg)
+	record.RecordType = txTimestampDebugRecordTypeSendChunk
+	record.SendCallIndex = &sendCallIndex
+	record.FrameOffsetStart = &frameOffsetStart
+	record.FrameOffsetEnd = &frameOffsetEnd
+	record.BytesWritten = &bytesWritten
+	record.ExpectedTXID = &expectedTXID
+	c.logUDPTXTimestampDebugRecord(record)
+}
+
+func (c *UDPConn) logUDPTXErrqueueDebugRecord(msg protocol.Message, chunks []txSendChunk, observed observedTXTimestampEvent, selected bool) {
+	if c.txTimestampDebugLogger == nil {
+		return
+	}
+
+	readIndex := observed.ReadIndex
+	tsUnixNano := observed.Event.TSUnixNano
+	eeInfo := observed.Event.EEInfo
+	eeData := observed.Event.EEData
+	selectedForLatency := selected
+
+	record := c.newUDPTXTimestampDebugRecord(msg)
+	record.RecordType = txTimestampDebugRecordTypeErrqueueEvent
+	record.Phase = observed.Phase
+	record.ReadIndex = &readIndex
+	record.EventName = observed.Event.EventName
+	record.TSUnixNano = &tsUnixNano
+	record.EEInfo = &eeInfo
+	record.EEData = &eeData
+	record.SelectedForLatency = &selectedForLatency
+	if matchedSendCallIndex, ok := matchTXTimestampEventToSendChunk(observed.Event.EEData, chunks); ok {
+		record.MatchedSendCallIndex = &matchedSendCallIndex
+	}
+	c.logUDPTXTimestampDebugRecord(record)
+}
+
+func (c *UDPConn) newUDPTXTimestampDebugRecord(msg protocol.Message) TXTimestampDebugRecord {
+	return TXTimestampDebugRecord{
+		NodeRole:    c.nodeRole,
+		NodeID:      c.nodeID,
+		MessageType: msg.Type,
+		MessageID:   msg.ID,
+		From:        msg.From,
+		To:          msg.To,
+		FileName:    msg.FileName,
+		BodySize:    len(msg.Body),
+	}
+}
+
+func (c *UDPConn) logUDPTXTimestampDebugRecord(record TXTimestampDebugRecord) {
+	if c.txTimestampDebugLogger == nil {
+		return
+	}
+
+	_ = c.txTimestampDebugLogger.LogTXTimestampDebugRecord(record)
+}
+
 // udpAddrToSockaddr 将 net.UDPAddr 转换为 syscall.Sockaddr。
 func udpAddrToSockaddr(addr *net.UDPAddr) syscall.Sockaddr {
 	if ip4 := addr.IP.To4(); ip4 != nil {
diff --git a/cmd/internal/transport/udp_linux_test.go b/cmd/internal/transport/udp_linux_test.go
index 71c3496..1b1d33e 100644
--- a/cmd/internal/transport/udp_linux_test.go
+++ b/cmd/internal/transport/udp_linux_test.go
@@ -11,7 +11,6 @@ import (
 	"omnisocketgo/cmd/internal/protocol"
 )
 
-// TestUDPLinuxTimestampingRecordsKernelEvents 验证 UDP 在 Linux 上能正确采集内核时间戳。
 func TestUDPLinuxTimestampingRecordsKernelEvents(t *testing.T) {
 	tests := []struct {
 		name string
@@ -45,7 +44,6 @@ func TestUDPLinuxTimestampingRecordsKernelEvents(t *testing.T) {
 			senderLogger := &recordingLogger{}
 			receiverLogger := &recordingLogger{}
 
-			// 创建 server 侧监听
 			serverAddr, err := net.ResolveUDPAddr("udp", "127.0.0.1:0")
 			if err != nil {
 				t.Fatalf("ResolveUDPAddr() error = %v", err)
@@ -65,7 +63,6 @@ func TestUDPLinuxTimestampingRecordsKernelEvents(t *testing.T) {
 			}
 			t.Cleanup(func() { _ = receiver.Close() })
 
-			// 创建 peer 侧连接
 			peerRaw, err := net.DialUDP("udp", nil, serverRaw.LocalAddr().(*net.UDPAddr))
 			if err != nil {
 				t.Fatalf("DialUDP() error = %v", err)
@@ -103,3 +100,149 @@ func TestUDPLinuxTimestampingRecordsKernelEvents(t *testing.T) {
 		})
 	}
 }
+
+func TestUDPLinuxTimestampingDebugLoggerCapturesDatagramAndErrqueueEvents(t *testing.T) {
+	debugLogger := &recordingTXTimestampDebugLogger{}
+	senderLogger := &recordingLogger{}
+	receiverLogger := &recordingLogger{}
+
+	serverAddr, err := net.ResolveUDPAddr("udp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("ResolveUDPAddr() error = %v", err)
+	}
+	serverRaw, err := net.ListenUDP("udp", serverAddr)
+	if err != nil {
+		t.Fatalf("ListenUDP() error = %v", err)
+	}
+	receiver, err := NewUDPConn(
+		serverRaw,
+		nil,
+		WithUDPLogger(receiverLogger, latencylog.NodeRolePeer, "peer-b"),
+	)
+	if err != nil {
+		_ = serverRaw.Close()
+		t.Fatalf("NewUDPConn(receiver) error = %v", err)
+	}
+	t.Cleanup(func() { _ = receiver.Close() })
+
+	peerRaw, err := net.DialUDP("udp", nil, serverRaw.LocalAddr().(*net.UDPAddr))
+	if err != nil {
+		t.Fatalf("DialUDP() error = %v", err)
+	}
+	sender, err := NewUDPConn(
+		peerRaw,
+		nil,
+		WithUDPLogger(senderLogger, latencylog.NodeRolePeer, "peer-a"),
+		WithUDPTXTimestampDebugLogger(debugLogger),
+	)
+	if err != nil {
+		_ = peerRaw.Close()
+		t.Fatalf("NewUDPConn(sender) error = %v", err)
+	}
+	t.Cleanup(func() { _ = sender.Close() })
+
+	msg := protocol.Message{
+		Type: protocol.MessageTypeText,
+		ID:   99,
+		From: "peer-a",
+		To:   "peer-b",
+		Body: []byte("hello udp debug"),
+	}
+
+	sendErr := make(chan error, 1)
+	go func() {
+		sendErr <- sender.Send(msg)
+	}()
+
+	got, _, err := receiver.Receive()
+	if err != nil {
+		t.Fatalf("Receive() error = %v", err)
+	}
+	if err := <-sendErr; err != nil {
+		t.Fatalf("Send() error = %v", err)
+	}
+	if !reflect.DeepEqual(got, msg) {
+		t.Fatalf("message mismatch: got %+v want %+v", got, msg)
+	}
+
+	assertHasEvent(t, senderLogger.Events(), latencylog.EventATXSched, msg.ID)
+	assertHasEvent(t, senderLogger.Events(), latencylog.EventATXSoftware, msg.ID)
+	assertHasEvent(t, receiverLogger.Events(), latencylog.EventBRXSoftware, msg.ID)
+
+	sendChunkRecords := debugRecordsByType(debugLogger.Records(), txTimestampDebugRecordTypeSendChunk)
+	errqueueRecords := debugRecordsByType(debugLogger.Records(), txTimestampDebugRecordTypeErrqueueEvent)
+	if len(sendChunkRecords) == 0 {
+		t.Fatal("send_chunk debug records = 0, want at least 1")
+	}
+	if len(errqueueRecords) == 0 {
+		t.Fatal("errqueue_event debug records = 0, want at least 1")
+	}
+
+	finalChunkRecord := sendChunkRecords[len(sendChunkRecords)-1]
+	if finalChunkRecord.ExpectedTXID == nil {
+		t.Fatal("final send_chunk expected_tx_id = nil, want non-nil")
+	}
+	finalExpectedTXID := *finalChunkRecord.ExpectedTXID
+
+	selectedRecords := selectedErrqueueRecords(errqueueRecords)
+	if len(selectedRecords) == 0 {
+		t.Fatal("selected errqueue debug records = 0, want at least 1")
+	}
+
+	highestObservedID := uint32(0)
+	haveHighestObservedID := false
+	haveExactFinalID := false
+	for _, record := range errqueueRecords {
+		if record.EEData == nil {
+			continue
+		}
+		if !haveHighestObservedID || *record.EEData > highestObservedID {
+			highestObservedID = *record.EEData
+			haveHighestObservedID = true
+		}
+		if *record.EEData == finalExpectedTXID && isBusinessTXTimestampRecord(record) {
+			haveExactFinalID = true
+		}
+	}
+	if !haveHighestObservedID {
+		t.Fatal("highestObservedID missing, want at least one ee_data")
+	}
+
+	wantSelectedID := highestObservedID
+	if haveExactFinalID {
+		wantSelectedID = finalExpectedTXID
+	}
+	for _, record := range selectedRecords {
+		if record.EEData == nil {
+			t.Fatalf("selected record missing ee_data: %+v", record)
+		}
+		if *record.EEData != wantSelectedID {
+			t.Fatalf("selected ee_data = %d, want %d", *record.EEData, wantSelectedID)
+		}
+	}
+
+	selectedByEventName := make(map[string]int64, len(selectedRecords))
+	for _, record := range selectedRecords {
+		if record.TSUnixNano == nil {
+			t.Fatalf("selected record missing timestamp: %+v", record)
+		}
+		selectedByEventName[record.EventName] = *record.TSUnixNano
+	}
+
+	senderEventsByName := make(map[string]int64)
+	for _, event := range senderLogger.Events() {
+		if event.MessageID != msg.ID {
+			continue
+		}
+		if !isBusinessTXTimestampEventName(event.Event) {
+			continue
+		}
+		senderEventsByName[event.Event] = event.TsUnixNano
+	}
+
+	for eventName, selectedTS := range selectedByEventName {
+		if senderEventsByName[eventName] != selectedTS {
+			t.Fatalf("sender latency event %s = %d, want %d from selected debug record", eventName, senderEventsByName[eventName], selectedTS)
+		}
+	}
+}
diff --git a/cmd/udpserver/main.go b/cmd/udpserver/main.go
index 5a938e0..01bb347 100644
--- a/cmd/udpserver/main.go
+++ b/cmd/udpserver/main.go
@@ -7,14 +7,16 @@ import (
 
 	"omnisocketgo/cmd/internal/latencylog"
 	"omnisocketgo/cmd/internal/server"
+	"omnisocketgo/cmd/internal/transport"
 )
 
 func main() {
 	listenAddr := flag.String("listen", ":9001", "UDP server listen address")
 	logPath := flag.String("latency-log", "", "optional JSONL file path for latency timestamp logs")
+	txTimestampDebugLogPath := flag.String("tx-ts-debug-log", "", "optional JSONL file path for TX errqueue debug records")
 	flag.Parse()
 
-	hubOptions := make([]server.UDPOption, 0, 1)
+	hubOptions := make([]server.UDPOption, 0, 2)
 	if *logPath != "" {
 		logger, err := latencylog.NewJSONLLogger(*logPath)
 		if err != nil {
@@ -23,6 +25,14 @@ func main() {
 		defer logger.Close()
 		hubOptions = append(hubOptions, server.WithUDPLogger(logger))
 	}
+	if *txTimestampDebugLogPath != "" {
+		logger, err := transport.NewJSONLTXTimestampDebugLogger(*txTimestampDebugLogPath)
+		if err != nil {
+			log.Fatalf("create tx timestamp debug logger %s: %v", *txTimestampDebugLogPath, err)
+		}
+		defer logger.Close()
+		hubOptions = append(hubOptions, server.WithUDPTXTimestampDebugLogger(logger))
+	}
 
 	udpAddr, err := net.ResolveUDPAddr("udp", *listenAddr)
 	if err != nil {