There are two main strategies that help provide secure connectivity between private networks and enabling network access for remote users.
Dial-up or leased line connections
A dial-up or leased line connection creates a physical connection to a port on a remote access server on a private network. However, using dial-up or leased lines to provide network access is expensive when compared to the cost of providing network access using a VPN connection.
VPN connections
VPN connections use either Point-to-Point Tunneling Protocol (PPTP) or Layer Two Tunneling Protocol/Internet Protocol security (L2TP/IPSec) over an intermediate network, such as the Internet. By using the Internet as a connection medium, VPN saves the cost of long-distance phone service and hardware costs associated with using dial-up or leased line connections. A VPN solution includes advanced security technologies such as data encryption, authentication, authorization, and Network Access Quarantine Control.
Note *Network Access Quarantine Control is used to delay remote access to a private network until the configuration of the remote access computer has been examined and validated.
Using VPN, administrators can connect remote or mobile workers (VPN clients) to private networks. Remote users can work as if their computers are physically connected to the network. To accomplish this, VPN clients can use a Connection Manager profile to initiate a connection to a VPN server. The VPN server can communicate with an Internet Authentication Service (IAS) server to authenticate and authorize a user session and maintain the connection until it is terminated by the VPN client or by the VPN server. All services typically available to a LAN-connected client (including file and print sharing, Web server access, and messaging) are enabled by VPN.
VPN clients can use standard tools to access resources. For example, clients can use Windows Explorer to make drive connections and to connect to printers. Connections are persistent: Users do not need to reconnect to network resources during their VPN sessions. Because drive letters and universal naming convention (UNC) names are fully supported by VPN, most commercial and custom applications work without modification.
VPN Scenarios
Virtual private networks are point-to-point connections across a private or public network such as the Internet. A VPN client uses special TCP/IP-based protocols, called tunneling protocols, to make a virtual call to a virtual port on a VPN server. In a typical VPN deployment, a client initiates a virtual point-to-point connection to a remote access server over the Internet. The remote access server answers the call, authenticates the caller, and transfers data between the VPN client and the organization’s private network.
To emulate a point-to-point link, data is encapsulated, or wrapped, with a header. The header provides routing information that enables the data to traverse the shared or public network to reach its endpoint. To emulate a private link, the data being sent is encrypted for confidentiality. Packets that are intercepted on the shared or public network are indecipherable without the encryption keys. The link in which the private data is encapsulated and encrypted is known as a VPN connection.
Remote Access VPN
Remote access VPN connections enable users working at home or on the road to access a server on a private network using the infrastructure provided by a public network, such as the Internet. From the user’s perspective, the VPN is a point-to-point connection between the computer (the VPN client) and an organization’s server. The exact infrastructure of the shared or public network is irrelevant because it appears logically as if the data is sent over a dedicated private link.
Site-to-Site VPN
Site-to-site VPN connections (also known as router-to-router VPN connections) enable organizations to have routed connections between separate offices or with other organizations over a public network while helping to maintain secure communications. A routed VPN connection across the Internet logically operates as a dedicated WAN link. When networks are connected over the Internet, as shown in the following figure, a router forwards packets to another router across a VPN connection. To the routers, the VPN connection operates as a data-link layer link.
A site-to-site VPN connection connects two portions of a private network. The VPN server provides a routed connection to the network to which the VPN server is attached. The calling router (the VPN client) authenticates itself to the answering router (the VPN server), and, for mutual authentication, the answering router authenticates itself to the calling router. In a site-to site VPN connection, the packets sent from either router across the VPN connection typically do not originate at the routers.
