This class of status code indicates a provisional response, consisting only of the Status-Line and optional headers and 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.
A client MUST be prepared to accept one or more 1xx status responses prior to a regular response, even if the client does not expect a 100 (Continue) status message. Unexpected 1xx status responses MAY be ignored by a user agent.
Proxies MUST forward 1xx responses, unless the connection between the proxy and its client has been closed, or unless the proxy itself has requested the generation of the 1xx response. (For example, if a proxy adds an "Expect: 100-continue" field when it forwards a request, then it does not need to forward the corresponding 100 (Continue) response(s).)