Dynamic – dynamic

With LANCOM Dynamic VPN, VPN tunnels can also be set up between two gateways that both only have dynamic IP addresses. Let's modify the previous example so that in this case Gateway 1 also has a dynamic IP address. Once again, Computer A would like to connect to Computer B:

1. Gateway 1 connects to its ISP and is assigned a public, dynamic IP address.
2. It then calls Gateway 2 via ISDN to send this dynamic address. Three procedures are used to send the address:
   • As information in the LLC element of the D-channel. In the D-channel protocol of Euro-ISDN (DSS-1), the so-called LLC (Lower Layer Compatibility) element can be used to send additional information to the remote station. This transfer takes place before the B-channel connection is established. Once the address has been sent successfully, the remote station rejects the call. Charges are thus not incurred for a B-channel connection. The IP address is sent nevertheless for free in this case.
     Note: The LLC element is generally available as a standard feature in Euro-ISDN that does not require registration or activation. It may be disabled by telephone companies or individual exchanges, however. The LLC element is not available in 1TR6, the German national ISDN. The procedure described above thus will not work with 1TR6.
   • As a subaddress via the D-channel. If it is not possible to send the address via the LLC element, Gateway 1 will attempt to send the address as a so-called subaddress. Like the LLC element, the subaddress is an information element of the D-channel protocol that permits short items of information to be sent free of charge. In this case, the telephone company must enable the 'subaddressing' feature first; this is generally subject to a charge. As with the LLC element, the call is rejected by the remote station once the IP address has been transferred successfully. The connection thus remains free of charge.
   • Via the B-channel. If both attempts to send the IP address via the D-channel fail, then a conventional connection via the B-channel must be established to send the IP address. The connection is dropped immediately after the IP address has been sent. This connection is subject to the usual charges.
3. Gateway 2 connects to the ISP and receives a dynamic IP address.
4. Gateway 2 authenticates itself at Gateway 1. The static IP address of gateway 1 is known, of course.
5. Gateway 1 now knows the address of Gateway 2 and sets up the VPN tunnel to Gateway 2.