diff --git a/src/core/xfer.c b/src/core/xfer.c
index 222af52a..a755d438 100644
--- a/src/core/xfer.c
+++ b/src/core/xfer.c
@@ -104,6 +104,33 @@ size_t xfer_window ( struct interface *intf ) {
 	return len;
 }
 
+/**
+ * Report change of flow control window
+ *
+ * @v intf		Data transfer interface
+ *
+ * Note that this method is used to indicate only unsolicited changes
+ * in the flow control window.  In particular, this method must not be
+ * called as part of the response to xfer_deliver(), since that could
+ * easily lead to an infinite loop.  Callers of xfer_deliver() should
+ * assume that the flow control window will have changed without
+ * generating an xfer_window_changed() message.
+ */
+void xfer_window_changed ( struct interface *intf ) {
+	struct interface *dest;
+	xfer_window_changed_TYPE ( void * ) *op =
+		intf_get_dest_op ( intf, xfer_window_changed, &dest );
+	void *object = intf_object ( dest );
+
+	if ( op ) {
+		op ( object );
+	} else {
+		/* Default is to do nothing */
+	}
+
+	intf_put ( dest );
+}
+
 /**
  * Allocate I/O buffer
  *
diff --git a/src/include/ipxe/xfer.h b/src/include/ipxe/xfer.h
index 2aa35f8d..0c030327 100644
--- a/src/include/ipxe/xfer.h
+++ b/src/include/ipxe/xfer.h
@@ -57,6 +57,10 @@ extern size_t xfer_window ( struct interface *intf );
 #define xfer_window_TYPE( object_type ) \
 	typeof ( size_t ( object_type ) )
 
+extern void xfer_window_changed ( struct interface *intf );
+#define xfer_window_changed_TYPE( object_type ) \
+	typeof ( void ( object_type ) )
+
 extern struct io_buffer * xfer_alloc_iob ( struct interface *intf,
 					   size_t len );
 #define xfer_alloc_iob_TYPE( object_type ) \