TekSIP is a SIP Registrar and SIP Proxy for Windows with support for UDP, TCP, TLS, and WebSocket transports. TekSIP complies with RFC 3261, RFC 3263, RFC 3311, RFC 3581 and RFC 3891. It also supports NAT traversal and ENUM. If installed behind UPnP supported Internet gateway device (ADSL router e.g.), TekSIP automatically detects if it is behind a new NAT gateway and the external IP address.