Introduction to Virtual Private Networks

1.5 VPN Solutions

In building a house, simply having the right materials does not mean that the house is built well. Similarly merely having the relevant technological building blocks is not enough to construct a virtual private network. A correct solution requires the right materials, and they must be put together in the right way.

Determining which kind of functions to provide at the tunnel endpoints, and how to implement these functions, is central to creating a VPN solution. When the VPN functions are implemented and integrated into an Internet device (e.g., a gateway), that device becomes a VPN device.

A VPN solution consists of multiple, appropriately configured VPN devices that are placed in the appropriate locations within the network. As with any network, after all the VPN devices are installed and configured, the network should be continually monitored and managed.

These aspects of an overall VPN solution are the topics of the chapters in Part III of this book.

VPN Gateways

The most common VPN device is the VPN gateway, which acts as the gatekeeper for network traffic to and from protected resources. Tunnels are established from the VPN gateway to other appropriate VPN devices serving as tunnel endpoints.

A VPN gateway is usually located at the corporate network perimeter, and it acts on behalf of the protected network resources within the corporate intranet to negotiate and render security services. The gateway assembles the tunneling, authentication, access control, and data security functions into a single device. The details of how these functions are integrated within a VPN gateway are specific to a vendor's implementation. Sometimes these functions can be integrated into existing router or firewall products. Sometimes a VPN gateway can be a stand-alone device that performs pure VPN functions, without firewall or dynamic routing exchange capabilities.

In general, a VPN gateway has two or more network interfaces (see Figure 1-4).

Figure 1-4 VPN gateway interfaces.

The data traffic coming from the public interfaces is termed inbound traffic. Because the inbound traffic is from the unsecure network, it is thoroughly examined according to the security policies. Usually, only the traffic from the established secure tunnels should be processed by the VPN gateway. If no secure tunnel is found, the traffic should be dropped immediately. Depending on the implementation, an alert or alarm can be generated to notify the network management station.

At least one exception exists. The gateway must process traffic whose purpose is to negotiate and establish these tunnels according to the negotiation protocols. Other exceptions may apply, as in responding to certain diagnostic requests. In general, however, the fewer exceptions there are, the more secure a gateway can be.

The data traffic coming from the private interfaces and exiting to the public interface is termed outbound traffic. Because the outbound traffic is from the private network, it is deemed secure a priori, even though many network attacks are generated within a corporate network. The outbound traffic is examined according to a set of policies on the gateway. If secure tunneling is required for the traffic, the VPN gateway first determines whether such a tunnel is already in place. If it is not, the gateway attempts to establish a new tunnel with the intended device—either another VPN gateway or simply some other device with secure tunneling capabilities. After the tunnel is established, the traffic is processed according to the tunnel rules and is sent into the tunnel. The traffic from the private interface can also be dropped if a policy cannot be found. Depending on how quickly a secure tunnel can be established, the VPN gateway may buffer the outbound packet before the secure tunnel is in place.

A VPN gateway implements some or all of the VPN technologies mentioned in the preceding section. Which functions are selected and how they are implemented is largely the choice of the device implementor. Chapter 9 is devoted to the details of issues surrounding the design, implementation, and evaluation of VPN gateways.

VPN Clients

The VPN client is software used for remote VPN access for a single computer or user. Unlike the VPN gateway—which is a specialized device and can protect multiple network resources at the same time—the VPN client software is usually installed on an individual computer and serves that computer only.

Generally, VPN client software creates a secure path from the client computer to a designated VPN gateway. The secure tunnel enables the client computer to obtain IP connectivity to access the network resources protected by that particular VPN gateway.

VPN client software also must implement the same functions as VPN gateways—tunneling, authentication, access control, and data security—although these implementations may be simpler or have fewer options. For example, the VPN software usually implements only one of the tunneling protocols. Because the remote computer does not act on behalf of any other users or resources, the access control can also be less complex.

Unlike the VPN gateway, in which all of the gateway's hardware and software is geared toward the VPN functionality, VPN client software is usually just an application running on a general-purpose operating system on the remote computer. Consequently, the client software should carefully consider its interactions with the operating system.

Today, most remote access is achieved through telephone dial-up. As broadband access (e.g., cable modems and digital subscriber lines, or DSL) deployment becomes more common, dedicated high-speed remote access will be popular. In many dial-up cases, the modem speed is relatively slow, so the VPN client software must perform IP data compression before encryption to increase the bandwidth performance.

One of the important concerns regarding VPN client software is the simplicity of installation and operation. Because client software is expected to be deployed widely on end users' machines, it must be easily installed and easily operated by regular computer users who may not know much about the operating system, software compatibility, remote access, or VPNs. A VPN gateway, on the other hand, is usually deployed on the company's corporate network site and is managed by information technology professionals.

Authentication on the client software can take several different approaches. The VPN software may have its own authentication mechanism, or it may inherit the authentication scheme from the operating system. It is often desirable to have a single sign-on for all the services on the desktop. Either digital certificates or some type of shared secret authentication scheme can be used.

In some cases, VPN client software can be designed to allow only specific application access to a server—for example, secure shell applications or a program working within a browser session. These types of clients are sometimes referred to as thin VPN clients.

Chapter 10 discusses the many details regarding the implementation and deployment of VPN client software.

VPN Network and Service Management

All networks require management to remain in good working condition; occasionally, the network topology and configuration must be changed according to business and application requirements. This management is especially important in the VPN scenario. Owing to the dynamic nature of VPNs, the networks require continuous network and service management.

We specifically use the term service management because, in many cases, a VPN is a service offered from telecommunication carriers or ISPs to their customers. The customers of the VPN service do not directly manage the VPN but rather view it as just another network service.

It is important for the service provider to guarantee a certain level of service quality according to its SLA. SLAs provide a factor that differentiates service providers. In the VPN space, SLAs are usually related to infrastructure availability and performance metrics. For example, in dedicated site-to-site intranet VPN, a certain level of availability (e.g., 99.9%) can be applied to the VPN gateways, and a certain amount of average latency between the VPN gateways can also be provided. In a remote access VPN, the modem availability and connection speed are the subjects of performance monitoring and guarantee.

Because a VPN is a secure network service, it is paramount for the network to be managed in a secure fashion. A network cannot be secure if its configuration can be altered without security checks and strong authentication mechanisms.

Traditionally, network management relies on the management information base (MIB) and the Simple Network Management Protocol (SNMP). Standard MIBs are geared toward device-specific information management. A VPN, on the other hand, by its nature, relates to more than one device. Therefore, new MIBs need to be developed (some are already under development) to address the VPN management problem.

A good VPN service management solution should take into consideration (but not be limited to) all the aspects we have mentioned. Chapter 11 addresses the many issues regarding VPN network and service management.

VPN Directions

VPN technology is still emerging, and, as with any new technology, it will experience continued developments. The performance of VPN devices will certainly improve dramatically with advances in component technology. Innovative ways to design and implement VPN functions will also be invented. Although it is impossible to predict the future, we can certainly make some observations on business and technology trends.

One such trend is the integration of VPN and firewall functions into a single device. In this way, you can manage security in a unified way rather than having separate policies and interfaces for the two devices. Additionally, incorporating routing and quality of service features into the VPN device will make it even more versatile.

Another important trend is the move toward adding intelligence to the network. In the telephone network, intelligence resides mostly within the switched telephone network; the telephone itself is a simple device. Currently in the Internet, the connected devices are computers that have substantial processing power. The routers and switches merely forward the packets without knowing what is inside them or how to process them accordingly. Having intelligence in the network enables service providers to offer value-added services to customers. We anticipate that VPNs will be among the first services to be supported by these more intelligent networks.

These and other trends in the VPN arena are discussed in Chapter 12.