Defines an interface to verify whether an incoming HTTP request meets the target server's expectations.

    Expect       =  "Expect" ":" 1#expectation

    expectation  =  "100-continue" | expectation-extension
    expectation-extension =  token [ "=" ( token | quoted-string )
                             *expect-params ]
    expect-params =  ";" token [ "=" ( token | quoted-string ) ]

The Expect request-header field is used to indicate that particular server behaviors are required by the client.

A server that does not understand or is unable to comply with any of the expectation values in the Expect field of a request MUST respond with appropriate error status. The server MUST respond with a 417 (Expectation Failed) status if any of the expectations cannot be met or, if there are other problems with the request, some other 4xx status.

[Android Documentation]