diff --git a/include/patty/ax25/sock.h b/include/patty/ax25/sock.h
index 6d1306f..13351c0 100644
--- a/include/patty/ax25/sock.h
+++ b/include/patty/ax25/sock.h
@@ -200,6 +200,8 @@ void patty_ax25_sock_vs_incr(patty_ax25_sock *sock);
 
 void patty_ax25_sock_vr_incr(patty_ax25_sock *sock);
 
+int patty_ax25_sock_flow_left(patty_ax25_sock *sock);
+
 int patty_ax25_sock_flow_full(patty_ax25_sock *sock);
 
 /*
diff --git a/src/sock.c b/src/sock.c
index 1f94bb1..6661ebf 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -535,6 +535,14 @@ void *patty_ax25_sock_assembler_read(patty_ax25_sock *sock,
     return buf + 1;
 }
 
+int patty_ax25_sock_flow_left(patty_ax25_sock *sock) {
+    if (sock->n_window_tx == 1) {
+        return 1;
+    }
+
+    return sock->vs - tx_seq(sock, sock->va + sock->n_window_tx);
+}
+
 int patty_ax25_sock_flow_full(patty_ax25_sock *sock) {
     /*
      * 6.4.1 Sending I Frames
@@ -644,11 +652,7 @@ error_invalid_type:
 }
 
 static inline uint16_t control_i(patty_ax25_sock *sock, int ns) {
-    int flag = 0;
-
-    if (sock->vs == tx_seq(sock, sock->va + sock->n_window_tx - 1)) {
-        flag = 1;
-    }
+    int flag = patty_ax25_sock_flow_left(sock) == 1? 1: 0;
 
     switch (sock->mode) {
         case PATTY_AX25_SOCK_SABM: