Secure Socket Tunneling Protocol (SSTP) is a form of virtual private network (VPN) tunnel that provides a mechanism to transport PPP traffic through an SSL/TLS channel. SSL/TLS provides transport-level security with key negotiation, encryption and traffic integrity checking. The use of SSL/TLS over TCP port 443 allows SSTP to pass through virtually all firewalls and proxy servers except for authenticated web proxies.
SSTP servers must be authenticated during the SSL/TLS phase. SSTP clients can optionally be authenticated during the SSL/TLS phase and must be authenticated in the PPP phase. The use of PPP allows support for common authentication methods, such as EAP-TLS and MS-CHAP.
SoftEther VPN Server, a cross-platform open-source VPN server, also supports SSTP as one of its multi-protocol capability.
Similar functionality can be obtained by using open-source solutions like OpenVPN, however on Windows a third-party client software must be installed due to the lack of native built-in VPN client.
For Windows, SSTP is available on Windows Vista SP1 and later in SEIL since its firmware version 3.50. It is fully integrated with the RRAS architecture in these operating systems, allowing its use with Winlogon or smart-card authentication, remote-access policies and the Windows VPN client. The protocol is also used by Windows Azure for Point-to-Site Virtual Network.
SSTP was intended only for remote client access, it generally does not support site-to-site VPN tunnels.
SSTP suffers from the same performance limitations as any other IP-over-TCP tunnel. In general, performance will be acceptable only as long as there is sufficient excess bandwidth on the un-tunneled network link to guarantee that the tunneled TCP timers do not expire. If this becomes untrue, performance falls off dramatically. This is known as the "TCP meltdown problem".
SSTP supports user authentication only; it does not support device authentication or computer authentication.
The following header structure is common to all types of SSTP packets:
|Bit offset||Bits 0-7||8-14||15||16-31|
The data field of the SSTP header contains an SSTP control message only when the header's Control bit C is set.
|Bit offset||Bits 0-15||16-31|
|0||Message type||Attributes count|