100 Continue

The 100 Continue status code indicates that the initial part of a
request has been received and has not yet been rejected by the
server. The server intends to send a final response after the
request has been fully received and acted upon.

When the request contains an Expect header field that includes a
100-continue expectation, the 100 response indicates that the server
wishes to receive the request payload body, as described in
Section 5.1.1. The client ought to continue sending the request and discard the 100 response.

If the request did not contain an Expect header field containing the
100-continue expectation, the client can simply discard this interim
response.