diff --git a/include/patty/ax25/sock.h b/include/patty/ax25/sock.h
index 90d0ea6..94d49a4 100644
--- a/include/patty/ax25/sock.h
+++ b/include/patty/ax25/sock.h
@@ -58,16 +58,16 @@ typedef struct _patty_ax25_sock {
     unsigned int hops;
 } patty_ax25_sock;
 
-#define PATTY_AX25_SOCK_CONTROL_SABM(sock, flag) \
-    (((sock->seq_recv & 0x07) << 7) | (flag << 4) | (sock->seq_send & 0x07))
+#define PATTY_AX25_SOCK_CONTROL_SABM(sock, pf) \
+    (((sock->seq_recv & 0x07) << 7) | (pf << 4) | (sock->seq_send & 0x07))
 
-#define PATTY_AX25_SOCK_CONTROL_SABME(sock, flag) \
-    (((sock->seq_recv & 0x7f) << 15) | (flag << 7) | (sock->seq_send & 0x7f))
+#define PATTY_AX25_SOCK_CONTROL_SABME(sock, pf) \
+    (((sock->seq_recv & 0x7f) << 15) | (pf << 7) | (sock->seq_send & 0x7f))
 
-#define PATTY_AX25_SOCK_CONTROL(sock, flag) \
+#define PATTY_AX25_SOCK_CONTROL(sock, pf) \
     (sock->mode == PATTY_AX25_SOCK_SABME? \
-        PATTY_AX25_SOCK_CONTROL_SABME(sock, flag): \
-        PATTY_AX25_SOCK_CONTROL_SABM(sock, flag))
+        PATTY_AX25_SOCK_CONTROL_SABME(sock, pf): \
+        PATTY_AX25_SOCK_CONTROL_SABM(sock, pf))
 
 patty_ax25_sock *patty_ax25_sock_new(enum patty_ax25_proto proto,
                                      enum patty_ax25_sock_type type);
@@ -81,6 +81,10 @@ char *patty_ax25_sock_pty(patty_ax25_sock *sock);
 int patty_ax25_sock_bind_if(patty_ax25_sock *sock,
                             patty_ax25_if *iface);
 
+void patty_ax25_sock_seq_send_incr(patty_ax25_sock *sock);
+
+void patty_ax25_sock_seq_recv_incr(patty_ax25_sock *sock);
+
 ssize_t patty_ax25_sock_send(patty_ax25_sock *sock,
                              uint16_t control,
                              void *info,
diff --git a/src/server.c b/src/server.c
index 7552647..f7017fc 100644
--- a/src/server.c
+++ b/src/server.c
@@ -1280,7 +1280,7 @@ static int handle_info(patty_ax25_server *server,
      * TODO: Validate RX and TX sequence numbers
      */
     if (sock->seq_recv == PATTY_AX25_CONTROL_SEQ_SEND(frame->control)) {
-        sock->seq_recv++;
+        patty_ax25_sock_seq_recv_incr(sock);
     } else {
         return reply_rej(iface, frame, PATTY_AX25_FRAME_FINAL);
     }
diff --git a/src/sock.c b/src/sock.c
index 844b987..b34654a 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -138,6 +138,22 @@ error_if_promisc_add:
     return -1;
 }
 
+void patty_ax25_sock_seq_send_incr(patty_ax25_sock *sock) {
+    if (sock->mode == PATTY_AX25_SOCK_SABME) {
+        sock->seq_send = (sock->seq_send + 1) & 0x07;
+    } else {
+        sock->seq_send = (sock->seq_send + 1) & 0x7f;
+    }
+}
+
+void patty_ax25_sock_seq_recv_incr(patty_ax25_sock *sock) {
+    if (sock->mode == PATTY_AX25_SOCK_SABME) {
+        sock->seq_recv = (sock->seq_recv + 1) & 0x07;
+    } else {
+        sock->seq_recv = (sock->seq_recv + 1) & 0x7f;
+    }
+}
+
 static size_t copy_addr_to_tx_buf(patty_ax25_sock *sock) {
     void *buf = sock->iface->tx_buf;
 
@@ -247,24 +263,13 @@ ssize_t patty_ax25_sock_write(patty_ax25_sock *sock,
 
     if (sock->type == PATTY_AX25_SOCK_RAW) {
         return patty_ax25_if_send(sock->iface, buf, len);
+    } else if (sock->mode == PATTY_AX25_SOCK_DM) {
+        errno = EBADF;
+
+        goto error_invalid_mode;
     }
 
-    switch (sock->mode) {
-        case PATTY_AX25_SOCK_DM:
-            errno = EBADF;
-
-            goto error_invalid_mode;
-
-        case PATTY_AX25_SOCK_SABM:
-            control = PATTY_AX25_SOCK_CONTROL_SABM(sock, 1);
-
-            break;
-
-        case PATTY_AX25_SOCK_SABME:
-            control = PATTY_AX25_SOCK_CONTROL_SABME(sock, 1);
-
-            break;
-    }
+    control = PATTY_AX25_SOCK_CONTROL(sock, 0);
 
     if (patty_ax25_sock_send(sock, control, buf, len) < 0) {
         goto error_send;