1xx - Informational
The 100 status is an interim response that is used to inform the client that the initial part of the request has been received. The client should continue by sending the remainder of the request. The server must send a final response after the request has been completed. If there is a break in communication or a further status (either succdess or fail) is not received the client should retry the request. This status can be used as a confirmation that the server is available and ready for a full request for a resource.
101 (Switching Protocols)
The server will switch protocols, for example from HTTP/1.0 to HTTP/1.1. The request will then be dealt with and a further response sent.