2.2 Message Syntax
PPTP uses the message format specified in [RFC2637]. The PPTP control message formats are specified in [RFC2637] section 2. The GRE message format that is used to carry PPP frames is specified in [RFC2637] section 4.
All optional fields and values described in this document are sent in network-byte order unless otherwise indicated. All reserved bits MUST be set to 0 when sending messages and MUST be ignored for incoming messages.