diff --git a/common.h b/common.h
index d9c6f58..5c7eb17 100644
--- a/common.h
+++ b/common.h
@@ -3,15 +3,20 @@
 
 #include <stdint.h>
 
-#define DATA_LEN 512
-#define TYPE_DATA 1
-#define TYPE_ACK  2
+#define TYPE_DATA 0
+#define TYPE_ACK  1
 
-typedef struct {
-    uint8_t type;      // DATA or ACK
-    uint8_t seq;       // 序号 0 / 1
-    uint16_t length;   // 数据长度
-    char data[DATA_LEN];
-} packet_t;
+#define PORT 9000
+#define BUF_SIZE 1024
+#define TIMEOUT_SEC 2
+
+/* 应用层协议数据包 */
+#pragma pack(push, 1)
+struct packet {
+    uint8_t seq;          // 停等协议序号：0 / 1
+    uint8_t type;         // 0=DATA, 1=ACK
+    char payload[BUF_SIZE - 2];
+};
+#pragma pack(pop)
 
 #endif