diff --git a/src/sock.c b/src/sock.c
index 820d27b..1f94bb1 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -643,7 +643,13 @@ error_invalid_type:
     return -1;
 }
 
-static inline uint16_t control_i(patty_ax25_sock *sock, int ns, int flag) {
+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;
+    }
+
     switch (sock->mode) {
         case PATTY_AX25_SOCK_SABM:
             return ((sock->vr & 0x07) << 5)
@@ -752,7 +758,7 @@ ssize_t patty_ax25_sock_resend(patty_ax25_sock *sock, int seq) {
 
     return slot->len > 0? frame_send(sock,
                                      PATTY_AX25_FRAME_COMMAND,
-                                     control_i(sock, seq, 0),
+                                     control_i(sock, seq),
                                      sock->proto,
                                      slot + 1,
                                      slot->len): 0;
@@ -980,7 +986,7 @@ static ssize_t write_segmented(patty_ax25_sock *sock,
         size_t o = 0;
 
         uint16_t control = sock->type == PATTY_AX25_SOCK_STREAM?
-            control_i(sock, sock->vs, 0):
+            control_i(sock, sock->vs):
             control_ui(0);
 
         if (first) {
@@ -1042,7 +1048,7 @@ ssize_t patty_ax25_sock_write(patty_ax25_sock *sock,
 
             if (frame_send(sock,
                            PATTY_AX25_FRAME_COMMAND,
-                           control_i(sock, sock->vs, 0),
+                           control_i(sock, sock->vs),
                            sock->proto,
                            buf,
                            len) < 0) {