diff --git a/include/patty/ax25/sock.h b/include/patty/ax25/sock.h
index a2c17ba..0caf556 100644
--- a/include/patty/ax25/sock.h
+++ b/include/patty/ax25/sock.h
@@ -232,8 +232,9 @@ ssize_t patty_ax25_sock_recv(patty_ax25_sock *sock,
 
 ssize_t patty_ax25_sock_recv_pending(patty_ax25_sock *sock);
 
-ssize_t patty_ax25_sock_resend(patty_ax25_sock *sock,
-                               unsigned int seq);
+ssize_t patty_ax25_sock_resend(patty_ax25_sock *sock, int seq);
+
+void patty_ax25_sock_ack(patty_ax25_sock *sock, int seq);
 
 ssize_t patty_ax25_sock_send_rr(patty_ax25_sock *sock,
                                 enum patty_ax25_frame_cr cr,
diff --git a/src/server.c b/src/server.c
index c4076e9..398fd62 100644
--- a/src/server.c
+++ b/src/server.c
@@ -1484,7 +1484,7 @@ static int handle_segment(patty_ax25_server *server,
     return 0;
 
 reply_rej:
-    return patty_ax25_sock_send_rej(sock, PATTY_AX25_FRAME_RESPONSE, 1);
+    return patty_ax25_sock_send_rej(sock, PATTY_AX25_FRAME_RESPONSE, 0);
 
 error_write:
 error_sock_assembler_read:
@@ -1501,10 +1501,12 @@ static int handle_i(patty_ax25_server *server,
         return frame->pf? reply_dm(iface, frame, PATTY_AX25_FRAME_FINAL): 0;
     }
 
+    patty_ax25_sock_ack(sock, frame->nr - 1);
+
     if (sock->vr == frame->ns) {
         patty_ax25_sock_vr_incr(sock);
     } else {
-        return patty_ax25_sock_send_rej(sock, PATTY_AX25_FRAME_RESPONSE, 1);
+        return patty_ax25_sock_send_rej(sock, PATTY_AX25_FRAME_RESPONSE, 0);
     }
 
     if (frame->proto == PATTY_AX25_PROTO_FRAGMENT) {
@@ -1591,6 +1593,8 @@ static int handle_rr(patty_ax25_server *server,
         return 0;
     }
 
+    patty_ax25_sock_ack(sock, frame->nr - 1);
+
     patty_timer_stop(&sock->timer_t1);
 
     switch (frame->cr) {
@@ -1604,7 +1608,6 @@ static int handle_rr(patty_ax25_server *server,
                 patty_ax25_sock_send_rnr(sock, PATTY_AX25_FRAME_RESPONSE, 1);
 
         case PATTY_AX25_FRAME_RESPONSE:
-            sock->va      = frame->nr;
             sock->retries = sock->n_retry;
 
             fd_watch(server, sock->fd);
@@ -1627,6 +1630,8 @@ static int handle_rnr(patty_ax25_server *server,
 
     patty_timer_stop(&sock->timer_t1);
 
+    patty_ax25_sock_ack(sock, frame->nr - 1);
+
     switch (frame->cr) {
         case PATTY_AX25_FRAME_COMMAND:
             if (frame->pf == 0) {
@@ -1638,7 +1643,6 @@ static int handle_rnr(patty_ax25_server *server,
                 patty_ax25_sock_send_rnr(sock, PATTY_AX25_FRAME_RESPONSE, 1);
 
         case PATTY_AX25_FRAME_RESPONSE:
-            sock->va      = frame->nr;
             sock->flow    = PATTY_AX25_SOCK_WAIT;
             sock->retries = sock->n_retry;
 
@@ -1663,7 +1667,7 @@ static int handle_rej(patty_ax25_server *server,
         return 0;
     }
 
-    sock->va = frame->nr;
+    patty_ax25_sock_ack(sock, frame->nr - 1);
 
     end = sock->vs;
 
@@ -1690,7 +1694,7 @@ static int handle_srej(patty_ax25_server *server,
         return 0;
     }
 
-    sock->va = frame->nr;
+    patty_ax25_sock_ack(sock, frame->nr - 1);
 
     /*
      * TODO: Read the fine print of section 4.3.2.4
@@ -2124,14 +2128,18 @@ static int handle_sock(uint32_t key,
     if (sock->flow == PATTY_AX25_SOCK_WAIT) {
         sock->flow = PATTY_AX25_SOCK_READY;
     } else if (patty_ax25_sock_flow_full(sock)) {
+        int ret;
+
         sock->flow = PATTY_AX25_SOCK_WAIT;
 
         fd_clear(server, sock->fd);
 
+        ret = patty_ax25_sock_send_rr(sock, PATTY_AX25_FRAME_COMMAND, 1);
+
         patty_timer_stop(&sock->timer_t3);
         patty_timer_start(&sock->timer_t1, sock->n_ack);
 
-        return 0;
+        return ret;
     }
 
     if ((len = read(sock->fd, sock->rx_buf, sock->n_maxlen_rx)) < 0) {
diff --git a/src/sock.c b/src/sock.c
index 9d6ef11..e8a8579 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -11,6 +11,11 @@
 
 #include <patty/ax25.h>
 
+struct slot {
+    size_t len;
+    int ack;
+};
+
 static int bind_pty(patty_ax25_sock *sock) {
     char *pty;
 
@@ -52,30 +57,29 @@ static inline size_t rx_bufsz(patty_ax25_sock *sock) {
 }
 
 static inline size_t tx_slot_size(patty_ax25_sock *sock) {
-    return sizeof(size_t) + PATTY_AX25_FRAME_OVERHEAD + sock->n_maxlen_tx;
+    return sizeof(struct slot) + PATTY_AX25_FRAME_OVERHEAD + sock->n_maxlen_tx;
 }
 
 static inline size_t tx_slots_size(patty_ax25_sock *sock) {
     return sock->n_window_tx * tx_slot_size(sock);
 }
 
-static inline void *tx_slot(patty_ax25_sock *sock, size_t seq) {
+static inline int tx_seq(patty_ax25_sock *sock, int seq) {
+    return sock->mode == PATTY_AX25_SOCK_SABME?
+        (seq + 128) % 128:
+        (seq +   8) %   8;
+}
+
+static inline struct slot *tx_slot(patty_ax25_sock *sock, int seq) {
     size_t win = sock->n_window_tx,
-             i = seq % win;
+             i = tx_seq(sock, seq) % win;
 
-    return (uint8_t *)sock->tx_slots + (i * tx_slot_size(sock));
+    return (struct slot *)
+        ((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 inline void *tx_slot_buf(patty_ax25_sock *sock, int seq) {
+    return (uint8_t *)tx_slot(sock, seq) + sizeof(struct slot);
 }
 
 static int init_bufs(patty_ax25_sock *sock) {
@@ -94,7 +98,9 @@ static int init_bufs(patty_ax25_sock *sock) {
     }
 
     for (i=0; i<sock->n_window_tx; i++) {
-        tx_slot_save_len(sock, i, 0);
+        struct slot *slot = tx_slot(sock, i);
+
+        slot->len = 0;
     }
 
     return 0;
@@ -506,11 +512,7 @@ void *patty_ax25_sock_assembler_read(patty_ax25_sock *sock,
 }
 
 int patty_ax25_sock_flow_full(patty_ax25_sock *sock) {
-    if (sock->mode == PATTY_AX25_SOCK_SABME) {
-        return sock->vs == (sock->va + sock->n_window_tx) % 128? 1: 0;
-    }
-
-    return sock->vs == (sock->va + sock->n_window_tx) % 8? 1: 0;
+    return sock->vs == tx_seq(sock, sock->va + sock->n_window_tx)? 1: 0;
 }
 
 static inline int toobig(patty_ax25_sock *sock,
@@ -651,14 +653,17 @@ static ssize_t frame_send(patty_ax25_sock *sock,
 
     if (info && infolen) {
         if (PATTY_AX25_FRAME_CONTROL_I(control)) {
+            struct slot *slot = tx_slot(sock, sock->vs);
+
             buf[offset++] = proto;
+
+            slot->len = offset;
+            slot->ack = 0;
         }
 
         memcpy(buf + offset, info, infolen);
 
         offset += infolen;
-
-        tx_slot_save_len(sock, sock->vs, offset);
     }
 
     return patty_ax25_if_send(sock->iface, buf, offset);
@@ -670,14 +675,25 @@ error_noif:
     return -1;
 }
 
-ssize_t patty_ax25_sock_resend(patty_ax25_sock *sock,
-                               unsigned int seq) {
-    void *slot = tx_slot(sock, seq);
-    void *buf  = (uint8_t *)slot + sizeof(size_t);
+ssize_t patty_ax25_sock_resend(patty_ax25_sock *sock, int seq) {
+    struct slot *slot = tx_slot(sock, seq);
+    void *buf         = tx_slot_buf(sock, seq);
 
-    size_t len = *((size_t *)slot);
+    return slot->len > 0? patty_ax25_if_send(sock->iface, buf, slot->len): 0;
+}
 
-    return len > 0? patty_ax25_if_send(sock->iface, buf, len): 0;
+void patty_ax25_sock_ack(patty_ax25_sock *sock, int seq) {
+    int i;
+
+    for (i=0; i<sock->n_window_tx; i++) {
+        struct slot *slot = tx_slot(sock, seq - i);
+
+        if (slot->len > 0) {
+            slot->ack = 1;
+        }
+    }
+
+    sock->va = seq;
 }
 
 static uint16_t control_i(patty_ax25_sock *sock, int flag) {