diff --git a/kernel/mars_client.c b/kernel/mars_client.c
index 12ccbeb9..c59afacb 100644
--- a/kernel/mars_client.c
+++ b/kernel/mars_client.c
@@ -566,6 +566,8 @@ int receiver_thread(void *data)
 		}
 
 		switch (cmd.cmd_code & CMD_FLAG_MASK) {
+		case CMD_NOP:
+			break;
 		case CMD_NOTIFY:
 			mars_trigger();
 			break;
diff --git a/kernel/mars_net.c b/kernel/mars_net.c
index 66a6a561..da180926 100644
--- a/kernel/mars_net.c
+++ b/kernel/mars_net.c
@@ -1181,6 +1181,7 @@ EXPORT_SYMBOL_GPL(_mars_recv_struct);
 
 const struct meta mars_cmd_meta[] = {
 	META_INI_SUB(cmd_stamp, struct mars_cmd, mars_lamport_time_meta),
+	META_INI(cmd_proto, struct mars_cmd, FIELD_INT),
 	META_INI(cmd_code, struct mars_cmd, FIELD_INT),
 	META_INI(cmd_int1, struct mars_cmd, FIELD_INT),
 	META_INI(cmd_str1, struct mars_cmd, FIELD_STRING),
@@ -1191,6 +1192,7 @@ int mars_send_cmd(struct mars_socket *msock, struct mars_cmd *cmd, bool cork)
 {
 	int status;
 
+	cmd->cmd_proto = MARS_PROTO_LEVEL;
 	status = desc_send_struct(msock, cmd, mars_cmd_meta, cork);
 	return status;
 }
@@ -1200,6 +1202,8 @@ int _mars_recv_cmd(struct mars_socket *msock, struct mars_cmd *cmd, int line)
 	int status;
 
 	status = desc_recv_struct(msock, cmd, mars_cmd_meta, line);
+	if (status >= 0)
+		msock->s_remote_proto_level = cmd->cmd_proto;
 	return status;
 }
 
diff --git a/kernel/mars_net.h b/kernel/mars_net.h
index 23bec4a9..994e7d1c 100644
--- a/kernel/mars_net.h
+++ b/kernel/mars_net.h
@@ -33,6 +33,8 @@
 extern int mars_net_default_port;
 extern bool mars_net_is_alive;
 
+#define MARS_PROTO_LEVEL 1
+
 #define MAX_FIELD_LEN   32
 #define MAX_DESC_CACHE  16
 
@@ -63,6 +65,7 @@ struct mars_socket {
 	struct socket *s_socket;
 	void *s_buffer;
 	atomic_t s_count;
+	int s_remote_proto_level; /* corresponds to MARS_PROTO_LEVEL of remote site */
 	int s_pos;
 	int s_debug_nr;
 	int s_send_abort;
@@ -110,6 +113,7 @@ enum {
 
 struct mars_cmd {
 	struct lamport_time cmd_stamp; // for automatic lamport clock
+	int cmd_proto;
 	int cmd_code;
 	int cmd_int1;
 	//int cmd_int2;