The request requires user authentication. The response MUST include a WWW-Authenticate header field ([RFC 2068], section 14.47) containing a challenge applicable to the requested resource. The client MAY repeat the request with a suitable Authorization header field ([RFC 2068], section 14.8). If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has already, at least once, attempted authentication, then the user SHOULD be presented the entity that was given in the response, since that entity might include relevant diagnostic information. HTTP access authentication is explained in "HTTP Authentication: Basic and Digest Access Authentication" .