126.96.36.199.1 Header Fields
The size field indicates the total message size, including the payload.
The _msgid field is a way to indicate which action to take on the target object. The server is expected to take a particular action based on the msgid and the _idObjectSubject.
The _idObjectSubject field refers to an object that was previously created by sending a Broker_CreateObject message. The server is expected to keep references to objects until it receives a message to destroy the object.
The common header fields are specified in section 2.2.4.