§ 01
Definition
Microsoft added SSTP to Windows Server 2008 and Windows Vista SP1. It was designed so remote workers could reach a company network from hotels and offices that block other VPN ports but allow normal web traffic.
§ 02
How it works
The client opens a TLS session to the server on TCP port 443, then sends PPP frames through it. Because it looks like HTTPS at first glance, simple port-based firewalls let it through. Running everything over TCP means it can slow down on lossy networks, the classic 'TCP over TCP' problem.
§ 03
Why it matters
SSTP is still built into Windows, so it is useful when you cannot install a VPN app. It is closed-source and mainly Windows-focused, so most consumer VPNs now prefer WireGuard, OpenVPN or IKEv2.