PPTP used to be the main OSI "L6"/"VPN" protocol (before PPPoE) for ISPs using DSL and ISDN over copper wire to the home via NetBEUI, Novell IPX, as well as IP network for use with Microsoft Windows platform.

PPTP was Never formalized by an IETF RFC. A Microsoft engineer told me legal was defensively shying away from proprietary disclosures then.

Probably was the first to network reverse engineer PPTP for an ISP modem product, notably the harder but broken PPTP-MPPE, as well as PPTP-PAP MS-CHAPv1 but Cisco purchased their way into PPTP suite from MSFT ahead of me. Did code RE of Shiva PAP too, thought it was a bad design back then and disuade my product team from Shiva PAP.

Disclaimer: worked on entire network protocol suite (along with 7 other hardy network guys) toward Efficient Network ADSL modem for most Bell Core companies, including Lucent, Ascend, Southwestern Bell, Qwest. Bought out by Siemens.