The request requires user authentication. 
The response MUST include a WWW-Authenticate header field containing a challenge applicable to the requested resource.
The client MAY repeat the request with a suitable Authorization header field.
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 attempted authentication at least once,
then the user SHOULD be presented the entity that was given in the response,
since that entity might include relevant diagnostic information.