HTTP Status Codes - 1xx

1xx - Informational
This group of status codes indicates a provisional response, consisting only of the Status Line and optional headers, and is terminated by an empty line. There are no required headers for this class of status code. Since HTTP/1.0 did not define any 1xx status codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client except under experimental conditions.

100 - Continue
The client should continue with its request by sending the remainder of the 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.

101 - Switching Protocols
The client request has asked the server to switch protocols and the server is acknowledging that it will do so and comply with the client's request.

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