diff --git a/tcp/segment.ml b/tcp/segment.ml
index 448d550f9..851d06667 100644
--- a/tcp/segment.ml
+++ b/tcp/segment.ml
@@ -96,11 +96,6 @@ module Rx(Time:V1_LWT.TIME) = struct
       try (S.max_elt q).syn
       with Not_found -> false *)
 
-  (* Determine the transmit window, from the last segment *)
-  let window q =
-    try (S.max_elt q).window
-    with Not_found -> 0
-
   let is_empty q = S.is_empty q.segs
 
   (* Given an input segment, the window information, and a receive
@@ -138,22 +133,18 @@ module Rx(Time:V1_LWT.TIME) = struct
       q.segs <- waiting;
       (* If the segment has an ACK, tell the transmit side *)
       let tx_ack =
-        if seg.ack then begin
+        if seg.ack && (Sequence.geq seg.ack_number (Window.ack_seq q.wnd)) then begin
           StateTick.tick q.state (State.Recv_ack seg.ack_number);
-          let win = window ready in
           let data_in_flight = Window.tx_inflight q.wnd in
-          let seq_has_changed = (Window.ack_seq q.wnd) <> seg.ack_number in
-          let win_has_changed = (Window.ack_win q.wnd) <> win in
-          if ((data_in_flight && (Window.ack_serviced q.wnd || not seq_has_changed)) ||
+          let ack_has_advanced = (Window.ack_seq q.wnd) <> seg.ack_number in
+          let win_has_changed = (Window.ack_win q.wnd) <> seg.window in
+          if ((data_in_flight && (Window.ack_serviced q.wnd || not ack_has_advanced)) ||
               (not data_in_flight && win_has_changed)) then begin
             Window.set_ack_serviced q.wnd false;
-            Window.set_ack_seq q.wnd seg.ack_number;
-            Window.set_ack_win q.wnd win;
-            Lwt_mvar.put q.tx_ack (seg.ack_number, win)
+            Window.set_ack_seq_win q.wnd seg.ack_number seg.window;
+            Lwt_mvar.put q.tx_ack ((Window.ack_seq q.wnd), (Window.ack_win q.wnd))
           end else begin
-             if (Sequence.gt seg.ack_number (Window.ack_seq q.wnd)) then
-               Window.set_ack_seq q.wnd seg.ack_number;
-             Window.set_ack_win q.wnd win;
+            Window.set_ack_seq_win q.wnd seg.ack_number seg.window;
             return_unit
           end
         end else return_unit in
diff --git a/tcp/window.ml b/tcp/window.ml
index 9ff05b282..a55e69fcb 100644
--- a/tcp/window.ml
+++ b/tcp/window.ml
@@ -134,10 +134,10 @@ let ack_seq t = t.ack_seq
 let ack_win t = t.ack_win
 
 let set_ack_serviced t v = t.ack_serviced <- v
-let set_ack_seq t s =
+let set_ack_seq_win t s w =
   MProf.Counter.increase count_ackd_segs (Sequence.(sub s t.ack_seq |> to_int));
-  t.ack_seq <- s
-let set_ack_win t w = t.ack_win <- w
+  t.ack_seq <- s;
+  t.ack_win <- w
 
 (* TODO: scale the window down so we can advertise it correctly with
    window scaling on the wire *)
diff --git a/tcp/window.mli b/tcp/window.mli
index 47f9405d4..8eed32430 100644
--- a/tcp/window.mli
+++ b/tcp/window.mli
@@ -44,8 +44,7 @@ val ack_seq : t -> Sequence.t
 val ack_win : t -> int
 
 val set_ack_serviced : t -> bool -> unit
-val set_ack_seq : t -> Sequence.t -> unit
-val set_ack_win : t -> int -> unit
+val set_ack_seq_win : t -> Sequence.t -> int -> unit
 
 (* rx_wnd: number of bytes we are willing to accept *)
 val rx_wnd : t -> int32