3.3.5.1.15 Tunnel-Type
A RADIUS client MUST set the Tunnel-Type RADIUS attribute as specified in [RFC2868], with one additional enhancement: A VPN server that supports the Secure Socket Tunneling Protocol (SSTP) SHOULD use the vendor-specific value 0x00013701 for the RADIUS Tunnel-Type attribute in Access-Request messages that are sent to a RADIUS server, which indicates that the tunnels configured by it are as described in [MS-SSTP].<15>
For more details about this vendor-specific value for the Tunnel-Type RADIUS attribute, see section 2.2.2.1.