diff --git a/src/core/or/relay.c b/src/core/or/relay.c
index 132da21466..0eb2ba3f67 100644
--- a/src/core/or/relay.c
+++ b/src/core/or/relay.c
@@ -1548,8 +1548,11 @@ connection_edge_process_relay_cell(cell_t *cell, circuit_t *circ,
       return connection_exit_begin_conn(cell, circ);
     case RELAY_COMMAND_DATA:
       ++stats_n_data_cells_received;
-      if (( layer_hint && --layer_hint->deliver_window < 0) ||
-          (!layer_hint && --circ->deliver_window < 0)) {
+
+      /* Update our circuit-level deliver window that we received a DATA cell.
+       * If the deliver window goes below 0, we end the connection due to a
+       * protocol failure. */
+      if (sendme_circuit_data_received(circ, layer_hint) < 0) {
         log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
                "(relay data) circ deliver_window below 0. Killing.");
         if (conn) {
@@ -1560,9 +1563,8 @@ connection_edge_process_relay_cell(cell_t *cell, circuit_t *circ,
         }
         return -END_CIRC_REASON_TORPROTOCOL;
       }
-      log_debug(domain,"circ deliver_window now %d.", layer_hint ?
-                layer_hint->deliver_window : circ->deliver_window);
 
+      /* Consider sending a circuit-level SENDME cell. */
       sendme_circuit_consider_sending(circ, layer_hint);
 
       if (rh.stream_id == 0) {
@@ -1586,7 +1588,11 @@ connection_edge_process_relay_cell(cell_t *cell, circuit_t *circ,
         return 0;
       }
 
-      if (--conn->deliver_window < 0) { /* is it below 0 after decrement? */
+      /* Update our stream-level deliver window that we just received a DATA
+       * cell. Going below 0 means we have a protocol level error so the
+       * circuit is closed. */
+
+      if (sendme_stream_data_received(conn) < 0) {
         log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
                "(relay data) conn deliver_window below 0. Killing.");
         return -END_CIRC_REASON_TORPROTOCOL;
diff --git a/src/core/or/sendme.c b/src/core/or/sendme.c
index df9fc57bd6..6d9f6f7d05 100644
--- a/src/core/or/sendme.c
+++ b/src/core/or/sendme.c
@@ -180,3 +180,36 @@ sendme_process_stream_level(edge_connection_t *conn, circuit_t *circ,
             conn->package_window);
   return 0;
 }
+
+/* Called when a relay DATA cell is received on the given circuit. If
+ * layer_hint is NULL, this means we are the Exit end point else we are the
+ * Client. Update the deliver window and return its new value. */
+int
+sendme_circuit_data_received(circuit_t *circ, crypt_path_t *layer_hint)
+{
+  int deliver_window, domain;
+
+  if (CIRCUIT_IS_ORIGIN(circ)) {
+    tor_assert(layer_hint);
+    --layer_hint->deliver_window;
+    deliver_window = layer_hint->deliver_window;
+    domain = LD_APP;
+  } else {
+    tor_assert(!layer_hint);
+    --circ->deliver_window;
+    deliver_window = circ->deliver_window;
+    domain = LD_EXIT;
+  }
+
+  log_debug(domain, "Circuit deliver_window now %d.", deliver_window);
+  return deliver_window;
+}
+
+/* Called when a relay DATA cell is received for the given edge connection
+ * conn. Update the deliver window and return its new value. */
+int
+sendme_stream_data_received(edge_connection_t *conn)
+{
+  tor_assert(conn);
+  return --conn->deliver_window;
+}
diff --git a/src/core/or/sendme.h b/src/core/or/sendme.h
index 97748cf65e..6313e91216 100644
--- a/src/core/or/sendme.h
+++ b/src/core/or/sendme.h
@@ -13,13 +13,19 @@
 #include "core/or/crypt_path_st.h"
 #include "core/or/circuit_st.h"
 
+/* Sending SENDME cell. */
 void sendme_connection_edge_consider_sending(edge_connection_t *edge_conn);
 void sendme_circuit_consider_sending(circuit_t *circ,
                                      crypt_path_t *layer_hint);
 
+/* Processing SENDME cell. */
 int sendme_process_circuit_level(crypt_path_t *layer_hint,
                                  circuit_t *circ, uint16_t cell_body_len);
 int sendme_process_stream_level(edge_connection_t *conn, circuit_t *circ,
                                 uint16_t cell_body_len);
 
+/* Update deliver window functions. */
+int sendme_stream_data_received(edge_connection_t *conn);
+int sendme_circuit_data_received(circuit_t *circ, crypt_path_t *layer_hint);
+
 #endif /* !defined(TOR_SENDME_H) */