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From Wikipedia, the free encyclopedia

A virtual private network (VPN) is a computer network that is layered on top of an underlying computer network. The private nature of a VPN means that the data travelling over the VPN is not generally visible to, or is encapsulated from, the underlying network traffic. This is done with strong encryption, as VPN's are commonly deployed to be high-security "network tunnels". Similarly, the traffic within the VPN appears to the underlying network as just another traffic stream to be passed. If you can envision secured "pipe" within the wire that is your connection, you would be well on your way to picturing a VPN deployment, if perhaps oversimplified.

In more technical terms, the link layer protocols of the virtual network are said to be tunneled through the underlying transport network.

The term VPN can be used to describe many different network configurations and protocols. As such, it can become complex when trying to generalise about the characteristics of a VPN. Some of the more common uses of VPNs are described below, along with more detail about the various classification schemes and VPN models.


VPN classifications

VPN technologies are not easily compared, due to the myriad of protocols, terminologies and marketing influences that have defined them. For example, VPN technologies can differ:

  • In the protocols they use to tunnel the traffic over the underlying network;
  • By the location of tunnel termination, such as the customer edge or network provider edge;
  • Whether they offer site-to-site or remote access connectivity;
  • In the levels of security provided;
  • By the OSI layer which they present to the connecting network, such as Layer 2 circuits or Layer 3 network connectivity.

Some classification schemes are discussed in the following sections.


Secure VPN vs Trusted VPN

The industry group 'Virtual Private Networking Consortium' have defined two types of VPN classifications, Secure VPNs and Trusted VPNs[1]. The consortium includes members such as Cisco, D-Link, Juniper and many others[2].

Secure VPNs explicitly provide mechanisms for authentication of the tunnel endpoints during tunnel setup, and encryption of the traffic in transit. Often secure VPNs are used to protect traffic when using the Internet as the underlying backbone, but equally they may be used in any environment when the security level of the underlying network differs from the traffic within the VPN.

Secure VPNs may be implemented by organizations wishing to provide remote access facilities to their employees or by organizations wishing to connect multiple networks together securely using the Internet to carry the traffic. A common use for secure VPNs is in remote access scenarios, where VPN client software on an end user system is used to connect to a remote office network securely. Secure VPN protocols include IPSec, L2TP (with IPsec for traffic encryption), SSL/TLS VPN (with SSL/TLS) or PPTP (with MPPE).

Trusted VPNs are commonly created by carriers and large organizations and are used for traffic segmentation on large core networks. They often provide quality of service guarantees and other carrier-grade features. Trusted VPNs may be implemented by network carriers wishing to multiplex multiple customer connections transparently over an existing core network or by large organizations wishing to segregate traffic flows from each other in the network. Trusted VPN protocols include MPLS, ATM or Frame Relay.

Trusted VPNs differ from secure VPNs in that they do not provide security features such as data confidentiality through encryption. Secure VPNs however do not offer the level of control of the data flows that a trusted VPN can provide such as bandwidth guarantees or routing.

From a customer perspective, a trusted VPN may act as a logical wire connecting two networks. The underlying carrier network is not visible to the customer, nor is the customer aware of the presence of other customers traversing the same backbone. Interference between customers, or interference with the backbone itself, is not possible from within a trusted VPN.

Some Internet service providers offer managed VPN service for business customers who want the security and convenience of a VPN but prefer not to undertake administering a VPN server themselves. Managed secure VPNs are again a hybrid of the two major VPN models, and are a contracted security solution that can reach into hosts. In addition to providing remote workers with secure access to their employer's internal network, other security and management services are sometimes included as part of the package. Examples include keeping anti-virus and anti-spyware programs updated on each connecting computer or ensuring particular software patches are installed before connection is permitted.

Categorization by user administrative relationships

The Internet Engineering Task Force (IETF) has categorized a variety of VPNs, some of which, such as Virtual LANs (VLAN) are the standardization responsibility of other organizations, such as the Institute of Electrical and Electronics Engineers (IEEE) Project 802, Workgroup 802.1 (architecture). Originally, wide area network (WAN) links from a telecommunications service provider interconnected network nodes within a single enterprise. With the advent of LANs, enterprises could interconnect their nodes with links that they owned. While the original WANs used dedicated lines and layer 2 multiplexed services such as Frame Relay, IP-based layer 3 networks, such as the ARPANET, Internet, military IP networks (NIPRNet, SIPRNet, JWICS, etc.), became common interconnection media. VPNs began to be defined over IP networks.[3] The military networks may themselves be implemented as VPNs on common transmission equipment, but with separate encryption and perhaps routers.

It became useful first to distinguish among different kinds of IP VPN based on the administrative relationships (rather than the technology) interconnecting the nodes. Once the relationships were defined, different technologies could be used, depending on requirements such as security and quality of service.

When an enterprise interconnects a set of nodes, all under its administrative control, through a LAN, that is termed an intranet.[4] When the interconnected nodes are under multiple administrative authorities but are hidden from the public Internet, the resulting set of nodes is called an extranet. A user organization can manage both intranets and extranets itself, or negotiate a service as a contracted (and usually customized) offering from an IP service provider. In the latter case, the user organization contracts for layer 3 services – much as it may contract for layer 1 services such as dedicated lines, or multiplexed layer 2 services such as frame relay.

IETF documents distinguish between provider-provisioned and customer-provisioned VPNs.[5] Just as an interconnected and set of providers can supply conventional WAN services, so a single service provider can supply provider-provisioned VPNs (PPVPNs), presenting a common point-of-contact to the user organization.

Internet Protocol tunnels

Some customer-managed virtual networks may not use encryption to protect the data contents. These types of overlay networks do not neatly fit within the secure or trusted categorization. An example of such an overlay network could be a GRE tunnel, set up between two hosts. This tunneling would still be a form of virtual private network yet is neither a secure nor a trusted VPN.

Examples of native plaintext tunneling protocols include GRE, L2TP (without IPsec) and PPTP (without MPPE).

Security mechanisms

Secure VPNs use cryptographic tunneling protocols to provide the intended confidentiality (blocking intercept and thus packet sniffing), sender authentication (blocking identity spoofing), and message integrity (blocking message alteration) to achieve privacy.

Secure VPN protocols include the following:

  • IPsec (Internet Protocol Security) - A standards-based security protocol developed originally for IPv6, where support is mandatory, but also widely used with IPv4. For VPNs L2TP is commonly used over IPsec.
  • Transport Layer Security (SSL/TLS) is used either for tunneling an entire network's traffic (SSL/TLS VPN), as in the OpenVPN project, or for securing individual connection. SSL has been the foundation by a number of vendors to provide remote access VPN capabilities. A practical advantage of an SSL VPN is that it can be accessed from locations that restrict external access to SSL-based e-commerce websites without IPsec implementations. SSL-based VPNs may be vulnerable to denial-of-service attacks mounted against their TCP connections because latter are inherently unauthenticated.
  • Datagram Transport Layer Security (DTLS), used by Cisco for a next generation VPN product called Cisco AnyConnect VPN. DTLS solves the issues found when tunneling TCP over TCP as is the case with SSL/TLS
  • Microsoft Point-to-Point Encryption (MPPE) by Microsoft is used with their PPTP. Several compatible implementations on other platforms also exist.
  • Secure Socket Tunneling Protocol (SSTP) by Microsoft introduced in Windows Server 2008 and Windows Vista Service Pack 1. SSTP tunnels PPP or L2TP traffic through an SSL 3.0 channel.
  • MPVPN (Multi Path Virtual Private Network). Ragula Systems Development Company owns the registered trademark "MPVPN".[6]
  • SSH VPN -- OpenSSH offers VPN tunneling to secure remote connections to a network (or inter-network links). This feature (option -w) should not be confused with port forwarding (option -L/-R/-D). OpenSSH server provides limited number of concurrent tunnels and the VPN feature itself does not support personal authentication.[7][8][9]


Tunnel endpoints are required to authenticate themselves before secure VPN tunnels can be established. End user created tunnels, such as remote access VPNs may use passwords, biometrics, two-factor authentication or other cryptographic methods. For network-to-network tunnels, passwords or digital certificates are often used, as the key must be permanently stored and not require manual intervention for the tunnel to be established automatically.


Tunneling protocols can be used in a point-to-point topology that would generally not be considered a VPN, because a VPN is expected to support arbitrary and changing sets of network nodes. Since most router implementations support software-defined tunnel interface, customer-provisioned VPNs often comprise simply a set of tunnels over which conventional routing protocols run. PPVPNs, however, need to support the coexistence of multiple VPNs, hidden from one another, but operated by the same service provider.

Building blocks

Depending on whether the PPVPN runs in layer 2 or layer 3, the building blocks described below may be L2 only, L3 only, or combinations of the two. Multiprotocol Label Switching (MPLS) functionality blurs the L2-L3 identity.

While RFC 4026 generalized these terms to cover L2 and L3 VPNs, they were introduced in RFC 2547.[10]

Customer edge device. (CE)

In general, a CE is a device, physically at the customer premises, that provides access to the PPVPN service. Some implementations treat it purely as a demarcation point between provider and customer responsibility, while others allow customers to configure it.

Provider edge device (PE)

A PE is a device or set of devices, at the edge of the provider network, which provides the provider's view of the customer site. PEs are aware of the VPNs that connect through them, and which maintain VPN state.

Provider device (P)

A P device operates inside the provider's core network, and does not directly interface to any customer endpoint. It might, for example, provide routing for many provider-operated tunnels that belong to different customers' PPVPNs. While the P device is a key part of implementing PPVPNs, it is not itself VPN-aware and does not maintain VPN state. Its principal role is allowing the service provider to scale its PPVPN offerings, as, for example, by acting as an aggregation point for multiple PEs. P-to-P connections, in such a role, often are high-capacity optical links between major locations of provider.

User-visible PPVPN services

This section deals with the types of VPN considered in the IETF; some historical names were replaced by these terms.

OSI Layer 1 services

Virtual private wire and private line services (VPWS and VPLS)

In both of these services, the provider does not offer a full routed or bridged network, but components from which the customer can build customer-administered networks. VPWS are point-to-point while VPLS can be point-to-multipoint. They can be Layer 1 emulated circuits with no data link structure.

The customer determines the overall customer VPN service, which also can involve routing, bridging, or host network elements.

An unfortunate acronym confusion can occur between Virtual Private Line Service and Virtual Private LAN Service; the context should make it clear whether "VPLS" means the layer 1 virtual private line or the layer 2 virtual private LAN.

OSI Layer 2 services

Virtual LAN

A Layer 2 technique that allows for the coexistence of multiple LAN broadcast domains, interconnected via trunks using the IEEE 802.1Q trunking protocol. Other trunking protocols have been used but have become obsolete, including Inter-Switch Link (ISL), IEEE 802.10 (originally a security protocol but a subset was introduced for trunking), and ATM LAN Emulation (LANE).

Virtual private LAN service (VPLS)

Developed by IEEE, VLANs allow multiple tagged LANs to share common trunking. VLANs frequently comprise only customer-owned facilities. The former is a layer 1 technology that supports emulation of both point-to-point and point-to-multipoint topologies. The method discussed here extends Layer 2 technologies such as 802.1d and 802.1q LAN trunking to run over transports such as Metro Ethernet.

As used in this context, a VPLS is a Layer 2 PPVPN, rather than a private line, emulating the full functionality of a traditional local area network (LAN). From a user standpoint, a VPLS makes it possible to interconnect several LAN segments over a packet-switched, or optical, provider core; a core transparent to the user, making the remote LAN segments behave as one single LAN.

In a VPLS, the provider network emulates a learning bridge, which optionally may include VLAN service.

Pseudo wire (PW)

PW is similar to VPWS, but it can provide different L2 protocols at both ends. Typically, its interface is a WAN protocol such as Asynchronous Transfer Mode or Frame Relay. In contrast, when aiming to provide the appearance of a LAN contiguous between two or more locations, the Virtual Private LAN service or IPLS would be appropriate.

IP-only LAN-like service (IPLS)

A subset of VPLS, the CE devices must have L3 capabilities; the IPLS presents packets rather than frames. It may support IPv4 or IPv6.

OSI Layer 3 PPVPN architectures

This section discusses the main architectures for PPVPNs, one where the PE disambiguates duplicate addresses in a single routing instance, and the other, virtual router, in which the PE contains a virtual router instance per VPN. The former approach, and its variants, have gained the most attention.

One of the challenges of PPVPNs involves different customers using the same address space, especially the IPv4 private address space[11]. The provider must be able to disambiguate overlapping addresses in the multiple customers' PPVPNs.


In the method defined by RFC 2547, BGP extensions advertise routes in the IPv4 VPN address family, which are of the form of 12-byte strings, beginning with an 8-byte Route Distinguisher (RD) and ending with a 4-byte IPv4 address. RDs disambiguate otherwise duplicate addresses in the same PE.

PEs understand the topology of each VPN, which are interconnected with MPLS tunnels, either directly or via P routers. In MPLS terminology, the P routers are Label Switch Routers without awareness of VPNs.

Virtual router PPVPN

The Virtual Router architecture,[12][13] as opposed to BGP/MPLS techniques, requires no modification to existing routing protocols such as BGP. By the provisioning of logically independent routing domains, the customer operating a VPN is completely responsible for the address space. In the various MPLS tunnels, the different PPVPNs are disambiguated by their label, but do not need routing distinguishers.

Virtual router architectures do not need to disambiguate addresses, because rather than a PE router having awareness of all the PPVPNs, the PE contains multiple virtual router instances, which belong to one and only one VPN.

Trusted delivery networks

Trusted VPNs do not use cryptographic tunneling, and instead rely on the security of a single provider's network to protect the traffic.

From the security standpoint, VPNs either trust the underlying delivery network, or must enforce security with mechanisms in the VPN itself. Unless the trusted delivery network runs only among physically secure sites, both trusted and secure models need an authentication mechanism for users to gain access to the VPN.

VPNs in mobile environments

Mobile VPNs handle the special circumstances when an endpoint of the VPN is not fixed to a single IP address, but instead roams across various networks such as data networks from cellular carriers or between multiple Wi-Fi access points.[17] Mobile VPNs have been widely used in public safety, where they give law enforcement officers access to mission-critical applications, such as computer-assisted dispatch and criminal databases, as they travel between different subnets of a mobile network.[18] They are also used in field service management and by healthcare organizations,[19] among other industries.

Increasingly, mobile VPNs are being adopted by mobile professionals and white-collar workers who need reliable connections.[19] They allow users to roam seamlessly across networks and in and out of wireless-coverage areas without losing application sessions or dropping the secure VPN session. A conventional VPN cannot survive such events because the network tunnel is disrupted, causing applications to disconnect, time out[17], or fail, or even cause the computing device itself to crash.[19]

Instead of logically tying the endpoint of the network tunnel to the physical IP address, each tunnel is bound to a permanently associated IP address at the device. The mobile VPN software handles the necessary network authentication and maintains the network sessions in a manner transparent to the application and the user.[17] The Host Identity Protocol (HIP), under study by the Internet Engineering Task Force, is designed to support mobility of hosts by separating the role of IP addresses for host identification from their locator functionality in an IP network. With HIP a mobile host maintains its logical connections established via the host identity identifier while associating with different IP addresses when roaming between access networks.

See also


  1. ^ [ VPN Technologies: Definitions and Requirements, VPNC Consortium, July 2008
  2. ^ VPNC Member List
  3. ^ IP Based Virtual Private Networks, RFC 2764, B. Gleeson et al.,February2000
  4. ^ Generic Requirements for Provider Provisioned Virtual Private Networks (PPVPN), RFC3809, A. Nagarajan,June 2004
  5. ^ Provider Provisioned Virtual Private Network (VPN) Terminology, RFC4026, L. Andersson and T. Madsen,March 2005
  6. ^ Trademark Applications and Registrations Retrieval (TARR)
  7. ^ OpenBSD ssh manual page, VPN section
  8. ^ Unix Toolbox section on SSH VPN
  9. ^ Ubuntu SSH VPN how-to
  10. ^ E. Rosen & Y. Rekhter (March 1999). "RFC 2547 BGP/MPLS VPNs". Internet Engineering Task Forc (IETF). 
  11. ^ Address Allocation for Private Internets, RFC 1918, Y. Rekhter et al.,February 1996
  12. ^ RFC 2917, A Core MPLS IP VPN Architecture
  13. ^ RFC 2918, K. Muthukrishnan & A. Malis (September 2000)
  14. ^ Layer Two Tunneling Protocol "L2TP", RFC 2661, W. Townsley et al.,August 1999
  15. ^ IP Based Virtual Private Networks, RFC 2341, A. Valencia et al., May 1998
  16. ^ Point-to-Point Tunneling Protocol (PPTP), RFC 2637, K. Hamzeh et al.,July 1999
  17. ^ a b c Phifer, Lisa. "Mobile VPN: Closing the Gap",, July 16, 2006.
  18. ^ Willett, Andy. "Solving the Computing Challenges of Mobile Officers",, May, 2006.
  19. ^ a b c Cheng, Roger. "Lost Connections", The Wall Street Journal, December 11, 2007.


Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary




  1. Virtual Private Network



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