Secure Socket Tunneling Protocol - Definition from the Hotspot Shield Glossary
SSTP is a protocol used in VPN tunneling on Microsoft Windows operating systems. Secure Socket Tunneling primarily uses SSL 3.0 (Secure Socket Layer) to send data across a VPN network - The data sent will use PPTP or L2TP protocols.
As SSL is used for transmitting encrypted data, SSTP can be used to pass through proxy servers and firewalls. During the session, clients and servers using SSTP must be authenticated during the SSL phase of transmission. Generally SSTP is used for remote access i.e. an employee accessing a network remotely from a different office or home and has no support for network to network tunnels. Performance wise, the quality of SSTP will degrade if the bandwidth is limited, and any lack of bandwidth can lead to sessions expiring.