10.5.4 503 Service Unavailable

The server  is currently unable to handle the request due to a temporary
overloading or maintenance of the server. The implication is that this
is a temporary condition which will be alleviated after some delay. If
known, the length of the delay MAY be indicated in a Retry-After
header. If no Retry-After is given, the client SHOULD handle the
response as it would for a 500 response.

Note: The existence of the 503 status code  does not imply that a
server must use it when becoming overloaded. Some servers may
wish
to simply refuse the connection.