--- includes/bootstrap.inc.old	2009-07-16 12:45:25.446318497 +0600
+++ includes/bootstrap.inc	2009-07-16 12:47:46.022302889 +0600
@@ -627,9 +627,14 @@
  * @see page_set_cache()
  */
 function drupal_page_header() {
+  $last_modified = gmdate('D, d M Y H:i:s') .' GMT';
+  # Add '_nocache' string to input of md5 function to differ logged in user and anonymous
+  $etag = '"'.md5($last_modified.'_nocache').'"';
+
   header("Expires: Sun, 19 Nov 1978 05:00:00 GMT");
-  header("Last-Modified: ". gmdate("D, d M Y H:i:s") ." GMT");
-  header("Cache-Control: store, no-cache, must-revalidate");
+  header("Last-Modified: " . $last_modified);
+  header("Etag: $etag");
+  header("Cache-Control: store, no-cache, must-revalidate, private");
   header("Cache-Control: post-check=0, pre-check=0", FALSE);
 }