2.2.11.2 HTTP Header Fields Introduced by the BITS Upload Protocol
BITS-Packet-Type (section 2.2.1.2): The value MUST be ACK for this message.
BITS-Session-Id (section 2.2.1.2).
BITS-Error (section 2.2.1.2): This field MUST be present if the server encountered an error processing the request and MUST NOT be present otherwise.
BITS-Error-Context (section 2.2.1.2): This field MUST be present if the server encountered an error processing the request and MUST NOT be present otherwise.