Retrying a Request with the 449 Status Code

If the client does not support retrying a request with the 449 status code, the client SHOULD treat the 449 Retry With status code as an unrecognized status code, as specified in [RFC2616] section 6.1.1, and SHOULD treat the Ms-Echo-Request header as an unrecognized entity header and ignore it, as specified in [RFC2616] section 7.1.

When a client that supports the 449 status code extension receives a 449 status code from the server, it MUST process the entity body of the response as normal but SHOULD NOT display its contents to the user.

After the entity body has been processed, the client MUST repeat the request that generated the 449 status code to the server and MUST include the Ms-Echo-Reply header with the new request.