There is a PPTP control connection between the IP address of the PPTP client using a dynamically allocated TCP port and the IP address of the PPTP server using the reserved TCP port 1723. The PPTP control connection carries the PPTP call control and management messages that are used to maintain the PPTP tunnel. This includes the transmission of periodic PPTP Echo-Request and PPTP Echo-Reply messages to detect a connectivity failure between the PPTP client and PPTP server. PPTP control connection packets consist of an IP header, a TCP header, a PPTP control message, and a data-link trailer and header as shown in the following figure:
Avast SecureLine and Avira Phantom VPN are run by antivirus companies as complements to their primary businesses. These services are also limited to Windows, Mac, iOS and Android and don't work without client software. But they offer few features, have a couple of dozen servers at most and don't let you pay anonymously. However, the companies are known quantities, and the services are handy for occasional travelers.