3.3.5.3.2 Notification Response
The back-end client MUST validate the following aspects of a received message before determining the message type:
The HTTP version MUST be 1.1.
The back-end client MUST verify that the response message satisfies the requirements in section 2.2.13. If the requirements are not satisfied, the back-end client SHOULD<27> send a 411 HTTP status code with BITS-Error 0x80070057 to the server, which will be sent to the client.
More details about message processing can be found in sections 3.3.5.2.4 and 3.3.5.2.5.