diff --git a/src/sock.c b/src/sock.c
index 43f5f17..f8ba5be 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -66,7 +66,21 @@ static inline void *tx_slot(patty_ax25_sock *sock, size_t seq) {
     return (uint8_t *)sock->tx_slots + (i * tx_slot_size(sock));
 }
 
+static inline void tx_slot_save_len(patty_ax25_sock *sock,
+                                    size_t seq,
+                                    size_t len) {
+    size_t *size = (size_t *)tx_slot(sock, seq);
+
+    *size = len;
+}
+
+static inline void *tx_slot_buf(patty_ax25_sock *sock, size_t seq) {
+    return (uint8_t *)tx_slot(sock, seq) + sizeof(size_t);
+}
+
 static int init_bufs(patty_ax25_sock *sock) {
+    size_t i;
+
     if ((sock->tx_buf = realloc(sock->tx_buf, tx_bufsz(sock))) == NULL) {
         goto error_realloc_tx_buf;
     }
@@ -79,6 +93,10 @@ static int init_bufs(patty_ax25_sock *sock) {
         goto error_realloc_tx_slots;
     }
 
+    for (i=0; i<sock->n_window_tx; i++) {
+        tx_slot_save_len(sock, i, 0);
+    }
+
     return 0;
 
 error_realloc_tx_slots:
@@ -380,7 +398,7 @@ ssize_t patty_ax25_sock_send(patty_ax25_sock *sock,
     ssize_t encoded;
 
     uint8_t *buf = PATTY_AX25_FRAME_CONTROL_I(control)?
-        tx_slot(sock, sock->seq_send): sock->tx_buf;
+        tx_slot_buf(sock, sock->seq_send): sock->tx_buf;
 
     if (sock->iface == NULL) {
         errno = ENETDOWN;
@@ -418,6 +436,8 @@ ssize_t patty_ax25_sock_send(patty_ax25_sock *sock,
         memcpy(buf + offset, info, infolen);
 
         offset += infolen;
+
+        tx_slot_save_len(sock, sock->seq_send, offset);
     }
 
     return patty_ax25_if_send(sock->iface, buf, offset);
@@ -436,7 +456,7 @@ ssize_t patty_ax25_sock_resend(patty_ax25_sock *sock,
 
     size_t len = *((size_t *)slot);
 
-    return patty_ax25_if_send(sock->iface, buf, len);
+    return len > 0? patty_ax25_if_send(sock->iface, buf, len): 0;
 }
 
 static uint16_t control_i(patty_ax25_sock *sock, int flag) {