The Virtual Private Network - VPN - has attracted the attention of many organizations looking to both expand their networking capabilities and reduce their costs.
The VPN can be found in workplaces and homes, where they allow employees to safely log into company networks. Telecommuters and those who travel often find a VPN a more convenient way to stay connected to the corporate intranet. No matter your current involvement with VPNs, this is a good technology to know something about. This VPN tutorial involves many interesting aspects of network protocol design, Internet security, network service outsourcing, and technology standards.

What Exactly Is A VPN?

A VPN supplies network connectivity over a possibly long physical distance. In this respect, a VPN is a form of Wide Area Network (WAN). The key feature of a VPN, however, is its ability to use public networks like the Internet rather than rely on private leased lines. VPN technologies implement restricted-access networks that utilize the same cabling and routers as a public network, and they do so without sacrificing features or basic security.
A VPN supports at least three different modes of use:

• Remote access client connections
• LAN-to-LAN internetworking
• Controlled access within an intranet

VPN Pros and Cons

Like many commercialized network technologies, a significant amount of sales and marketing hype surrounds VPN. In reality, VPNs provide just a few specific potential advantages over more traditional forms of wide-area networking. These advantages can be significant, but they do not come for free.

The potential problems with the VPN outnumber the advantages and are generally more difficult to understand. The disadvantages do not necessarily outweigh the advantages, however. From security and performance concerns, to coping with a wide range of sometimes incompatible vendor products, the decision of whether or not to use a VPN cannot be made without significant planning and preparation.

Technology Behind VPNs

Several network protocols have become popular as a result of VPN developments:

• PPTP
• L2TP
• IPsec
• SOCKS

These protocols emphasize authentication and encryption in VPNs. Authentication allows VPN clients and servers to correctly establish the identity of people on the network. Encryption allows potentially sensitive data to be hidden from the general public. Many vendors have developed VPN hardware and/or software products. Unfortunately, immature VPN standards mean that some of these products remain incompatible with each other.
 

A VPN supplies network connectivity over a possibly long physical distance. In this respect, a VPN is a form of Wide Area Network (WAN). VPNs enable file sharing, video conferencing and similar network services. Virtual private networks generally don't provide any new functionality that isn't already offered through alternative mechanisms, but a VPN implements those services more efficiently / cheaply in most cases.

A key feature of a VPN is its ability to work over both private networks as well as public networks like the Internet. Using a method called tunneling, a VPN use the same hardware infrastructure as existing Internet or intranet links. VPN technologies includes various security mechanisms to protect the virtual, private connections.

Specifically, a VPN supports at least three different modes of use:

• Internet remote access client connections
• LAN-to-LAN internetworking
• Controlled access within an intranet

Internet VPNs for Remote Access

In recent years, many organizations have increased the mobility of their workers by allowing more employees to telecommute. Employees also continue to travel and face a growing need to stay connected to their company networks. A VPN can be set up to support remote, protected access to the corporate home offices over the Internet. An Internet VPN solution uses a client/server design works as follows:

1. A remote host (client) wanting to log into the company network first connects to any public Internet Service Provider (ISP).

2. Next, the host initiates a VPN connection to the company VPN server. This connection is made via a VPN client installed on the remote host.

3. Once the connection has been established, the remote client can communicate with the internal company systems over the Internet just as if it were a local host.

Before VPNs, remote workers accessed company networks over private leased lines or through dialup remote access servers. While VPN clients and servers careful require installation of hardware and software, an Internet VPN is a superior solution in many situations.

VPNs for Internetworking

Besides using virtual private networks for remote access, a VPN can also bridge two networks together. In this mode of operation, an entire remote network (rather than just a single remote client) can join to a different company network to form an extended intranet. This solution uses a VPN server to VPN server connection.

Intranet / Local Network VPNs

Internal networks may also utilize VPN technology to implement controlled access to individual subnets within a private network. In this mode of operation, VPN clients connect to a VPN server that acts as the network gateway. This type of VPN use does not involve an Internet Service Provider (ISP) or public network cabling. However, it allows the security benefits of VPN to be deployed inside an organization. This approach has become especially popular as a way for businesses to protect their WiFi local networks.

VPN Network Scalability

The cost to an organization of building a dedicated private network may be reasonable at first but increases exponentially as the organization grows. A company with two branch offices, for example, can deploy just one dedicated line to connect the two locations, but 4 branch offices require 6 lines to directly connect them to each other, 6 branch offices need 15 lines, and so on. Internet based VPNs avoid this scalability problem by simply tapping into the the public lines and network capability readily available. Particularly for remote and international locations, an Internet VPN offers superior reach and quality of service.

Using a VPN

To use a VPN, each client must possess the appropriate networking software or hardware support on their local network and computers. When set up properly, VPN solutions are easy to use and sometimes can be made to work automatically as part of network sign on. VPN technology also works well with WiFi local area networking. Some organizations use VPNs to secure wireless connections to their local access points when working inside the office. These solutions provide strong protection without affecting performance excessively.

Limitations of a VPN

Despite their popularity, VPNs are not perfect and limitations exist as is true for any technology. Organizations should consider issues like the below when deploying and using virtual private networks in their operations:

1. VPNs require detailed understanding of network security issues and careful installation / configuration to ensure sufficient protection on a public network like the Internet. 

2. The reliability and performance of an Internet-based VPN is not under an organization's direct control. Instead, the solution relies on an ISP and their quality of service.

3. Historically, VPN products and solutions from different vendors have not always been compatible due to issues with VPN technology standards. Attempting to mix and match equipment may cause technical problems, and using equipment from one provider may not give as great a cost savings.

Virtual private network technology is based on the idea of tunneling. VPN tunneling involves establishing and maintaining a logical network connection (that may contain intermediate hops). On this connection, packets constructed in a specific VPN protocol format are encapsulated within some other base or carrier protocol, then transmitted between VPN client and server, and finally de-encapsulated on the receiving side.

For Internet-based VPNs, packets in one of several VPN protocols are encapsulated within Internet Protocol (IP) packets. VPN protocols also support authentication and encryption to keep the tunnels secure.

Types of VPN Tunneling

VPN supports two types of tunneling - voluntary and compulsory. Both types of tunneling are commonly used.
In voluntary tunneling, the VPN client manages connection setup. The client first makes a connection to the carrier network provider (an ISP in the case of Internet VPNs). Then, the VPN client application creates the tunnel to a VPN server over this live connection.

In compulsory tunneling, the carrier network provider manages VPN connection setup. When the client first makes an ordinary connection to the carrier, the carrier in turn immediately brokers a VPN connection between that client and a VPN server. From the client point of view, VPN connections are set up in just one step compared to the two-step procedure required for voluntary tunnels.

Compulsory VPN tunneling authenticates clients and associates them with specific VPN servers using logic built into the broker device. This network device is sometimes called the VPN Front End Processor (FEP), Network Access Server (NAS) or Point of Presence Server (POS). Compulsory tunneling hides the details of VPN server connectivity from the VPN clients and effectively transfers management control over the tunnels from clients to the ISP. In return, service providers must take on the additional burden of installing and maintaining FEP devices.

VPN Tunneling Protocols

Several computer network protocols have been implemented specifically for use with VPN tunnels. The three most popular VPN tunneling protocols listed below continue to compete with each other for acceptance in the industry. These protocols are generally incompatible with each other.
Point-to-Point Tunneling Protocol (PPTP)
Several corporations worked together to create the PPTP specification. People generally associate PPTP with Microsoft because nearly all flavors of Windows include built-in client support for this protocol. The initial releases of PPTP for Windows by Microsoft contained security features that some experts claimed were too weak for serious use. Microsoft continues to improve its PPTP support, though.
 Layer Two Tunneling Protocol (L2TP)
The original competitor to PPTP for VPN tunneling was L2F, a protocol implemented primarily in Cisco products. In an attempt to improve on L2F, the best features of it and PPTP were combined to create a new standard called L2TP. Like PPTP, L2TP exists at the data link layer (Layer Two) in the OSI model -- thus the origin of its name.

Internet Protocol Security (IPsec)
IPsec is actually a collection of multiple related protocols. It can be used as a complete VPN protocol solution or simply as the encryption scheme within L2TP or PPTP. IPsec exists at the network layer (Layer Three) of the OSI model.

PPTP - Point-to-Point Tunneling Protocol - extends the Point to Point Protocol (PPP) standard for traditional dial-up networking. PPTP is best suited for the remote access applications of VPNs, but it also supports LAN internetworking. PPTP operates at Layer 2 of the OSI model.

1. The PPTP client connects to their ISP using PPP dial-up networking (traditional modem or ISDN).

2. Via the broker device (described earlier), PPTP creates a TCP control connection between the VPN client and VPN server to establish a tunnel. PPTP uses TCP port 1723 for these connections.

PPTP also supports VPN connectivity via a LAN. ISP connections are not required in this case, so tunnels can be created directly as in Step 2 above.

• control messages for managing and eventually tearing down the VPN connection. Control messages pass directly between VPN client and server.
• data packets that pass through the tunnel, to or from the VPN client  

 

PPTP Security

PPTP supports authentication, encryption, and packet filtering. PPTP authentication uses PPP-based protocols like EAP, CHAP, and PAP. PPTP supports packet filtering on VPN servers. Intermediate routers and other firewalls can also be configured to selectively filter PPTP traffic. 

PPTP and PPP

• authenticating users and maintaining the remote dial-up connection
• encapsulating and encrypting IP, IPX, or NetBEUI packets

PPTP Pros and Cons

Source:compnetworking.about.com