diff --git a/src/server.c b/src/server.c
index 977766c..9e2d18d 100644
--- a/src/server.c
+++ b/src/server.c
@@ -1701,6 +1701,11 @@ static int handle_sock(uint32_t key,
         goto error_sock_write;
     }
 
+    /*
+     * AX.25 v2.2, Section 6.4.1 "Sending I Frames"
+     */
+    patty_timer_start(&sock->timer_t1, &server->timer, sock->n_ack);
+
 done:
     return 0;