Existing browsers retain authentication information until the tab or browser is closed or the user clears the history. HTTP does not provide a method for a server to direct clients to discard these cached credentials. This means that there is no effective way for a server to "log out" the user without closing the browser. This is a significant defect that requires browser manufacturers to support a 'logout' user interface element (mentioned in RFC 1945, but not implemented by most browsers) or API available to JavaScript, further extensions to HTTP, or use of existing alternative techniques such as retrieving the page over SSL/TLS with an unguessable string in the URL.