16th October 2004 - Kieran Mansley - kjm25@cam.ac.uk

- Add code to tcp_recved() to send an ACK (window update)
immediately, even if one is already pending, if the rcv_wnd is above a
threshold (currently TCP_WND/2)
 - This avoids waiting for a timer to expire to send a delayed ACK in
order to open the window if the stack is only receiving data.

src/core/tcp.c

@@ -445,6 +445,16 @@ tcp_recved(struct tcp_pcb *pcb, u16_t len)
      */
     tcp_ack(pcb);
   } 
+  else if (pcb->flags & TF_ACK_DELAY && pcb->rcv_wnd >= TCP_WND/2) {
+    /* If we can send a window update such that there is a full
+     * segment available in the window, do so now.  This is sort of
+     * nagle-like in its goals, and tries to hit a compromise between
+     * sending acks each time the window is updated, and only sending
+     * window updates when a timer expires.  The "threshold" used
+     * above (currently TCP_WND/2) can be tuned to be more or less
+     * aggressive  */
+    tcp_ack_now(pcb);
+  }
 
   LWIP_DEBUGF(TCP_DEBUG, ("tcp_recved: recveived %u bytes, wnd %u (%u).\n",
          len, pcb->rcv_wnd, TCP_WND - pcb->rcv_wnd));