100 Continue

   The client SHOULD continue with its request. This interim response is
   used to inform the client that the initial part of the request has
   been received and has not yet been rejected by the server. The client
   SHOULD continue by sending the remainder of the request or, if the
   request has already been completed, ignore this response. The server
   MUST send a final response after the request has been completed. See
   section 8.2.3 for detailed discussion of the use and handling of this
   status code.