3.1.5 Processing Events and Sequencing Rules

Except as noted in this section, the message processing and sequencing rules are unchanged from PPTP as specified in [RFC2637]. As a first step in establishing a PPTP call session, a control connection MUST be established between the PAC/PNS, which is a TCP session to port 1723 as specified in [RFC2637] section 1.4. Over this TCP connection, the control channel MUST first be established followed by the call session through the exchange of control messages. The ordering and retransmission of control messages is provided by the underlying TCP protocol [RFC793].

PAC and PNS MAY validate GRE packets received so that  the source IP address is the same as the peer's IP address used in PPTP control channel establishment and for when packets do not validate and are then subsequently dropped.<6>