The DataSource schema must be given a total value in order to page correctly. However, DataSource expects this value to come from a response body field. In some cases, we must work with systems that return the total value as a custom field in the response header. We need a way to read custom response header values. Yes, the DataSource transport uses jQuery.ajax, so we can implement a custom complete function to read the response headers from the jqXHR. However, complete fires AFTER schema.total is set so it does no good.