417 Expectation Failed
A server emits
417 Expecation Failed when it encountered a
header that it didn’t understand or doesn’t support.
A client can use the
Expect header to tell the server that it requires a
certain behavior from a server.
The only expectation that (as far as I know) is standardized, is
This looks like this:
POST /foo/bar HTTP/1.1 Content-Type: application/gzip Content-Length: 12345678765 Expect: 100-continue ...
This request intends to upload a large file. The client tells the server that
it expects the server to first respond with a
100 Continue response.
If a server supports this, it will first return this
100 Continue response,
which tells a client that the request was understood, supported and probably
acceptable and it can continue uploading.
If the server did not support this feature, it must response with
HTTP/1.1 417 Expectation Failed Content-Type: text/plain We don't support 100-continue
If a server sees a different type of expectation that’s a new future standard,
or some custom extension it should always return