18.104.22.168 Retrying a Request with the 449 Status Code
If the server does not support retrying a request with the 449 status code, it MUST NOT send the 449 Retry With status code in an HTTP response message and SHOULD treat the Ms-Echo-Reply header as an unrecognized entity header and ignore it, as specified in [RFC2616] section 7.1.
A server implementing the 449 Reply With status code should behave as an HTTP server as specified in [RFC2616]. If the server needs the client to send more information before responding to the client's request for a specific resource, the server MAY return a 449 Retry With status code.
The 449 response MUST conform to the specification of an HTTP response message, as specified in [RFC2616] section 6.
When sending a 449 status code, the server MUST include the Ms-Echo-Request header.
If the server receives a request from the client that contains an Ms-Echo-Reply header, it MUST NOT reply to such a request with a 449 Reply With status.