Virtual private network

A virtual private network (VPN) extends a across a public network, and enables users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network. Applications running across the VPN may therefore benefit from the functionality, security, and management of the private network.

VPNs may allow employees to securely access a corporate while located outside the office. They are used to securely connect geographically separated offices of an organization, creating one cohesive network. Individual Internet users may secure their transactions with a VPN, to circumvent and censorship, or to connect to for the purpose of protecting personal identity and location. However, some Internet sites block access to known VPN technology to prevent the circumvention of their geo-restrictions.

A VPN is created by establishing a virtual connection through the use of dedicated connections, virtual , or traffic . A VPN available from the public Internet can provide some of the benefits of a (WAN). From a user perspective, the resources available within the private network can be accessed remotely.

Traditional VPNs are characterized by a point-to-point topology, and they do not tend to support or connect , so services such as may not be fully supported or work as they would on a (LAN). Designers have developed VPN variants, such as (VPLS), and , to overcome this limitation.



Early data networks allowed VPN-style remote connectivity through or through connections utilizing and (ATM) virtual circuits, provisioned through a network owned and operated by . These networks are not considered true VPNs because they passively secure the data being transmitted by the creation of logical data streams. They have been replaced by VPNs based on IP and IP/ (MPLS) Networks, due to significant cost-reductions and increased bandwidth provided by new technologies such as Digital Subscriber Line (DSL) and fiber-optic networks.

VPNs can be either remote-access (connecting a computer to a network) or site-to-site (connecting two networks). In a corporate setting, remote-access VPNs allow employees to access their company’s from home or while travelling outside the office, and site-to-site VPNs allow employees in geographically disparate offices to share one cohesive virtual network. A VPN can also be used to interconnect two similar networks over a dissimilar middle network; for example, two networks over an network.

VPN systems may be classified by:

  • The used to the traffic
  • The tunnel’s termination point location, e.g., on the customer or network-provider edge
  • The type of topology of connections, such as site-to-site or network-to-network
  • The levels of security provided
  • The they present to the connecting network, such as Layer 2 circuits or Layer 3 network connectivity
  • The number of simultaneous connections

Security mechanisms

VPNs cannot make online connections completely anonymous, but they can usually increase privacy and security. To prevent disclosure of private information, VPNs typically allow only authenticated remote access using and techniques.

The VPN security model provides:

  • such that even if the network traffic is sniffed at the packet level (see and ), an attacker would only see
  • Sender authentication to prevent unauthorized users from accessing the VPN
  • Message to detect any instances of tampering with transmitted messages

Secure VPN protocols include the following:

  • () was initially developed by the (IETF) for , which was required in all standards-compliant implementations of before RFC 6434 made it only a recommendation. This standards-based security protocol is also widely used with and the . Its design meets most security goals: authentication, integrity, and confidentiality. IPsec uses encryption, encapsulating an IP packet inside an IPsec packet. De-encapsulation happens at the end of the tunnel, where the original IP packet is decrypted and forwarded to its intended destination.
  • () can an entire network’s traffic (as it does in the project and project) or secure an individual connection. A number of vendors provide remote-access VPN capabilities through SSL. An SSL VPN can connect from locations where IPsec runs into trouble with and firewall rules.
  • () – used in and in VPN to solve the issues has with tunneling over .
  • () works with the and in several compatible implementations on other platforms.
  • Microsoft () tunnels (PPP) or traffic through an 3.0 channel. (SSTP was introduced in and in Service Pack 1.)
  • Multi Path Virtual Private Network (MPVPN). Ragula Systems Development Company owns the registered “MPVPN”.
  • Secure Shell (SSH) VPN – offers VPN tunneling (distinct from ) to secure remote connections to a network or to inter-network links. OpenSSH server provides a limited number of concurrent tunnels. The VPN feature itself does not support personal authentication.


Tunnel endpoints must be authenticated before secure VPN tunnels can be established. User-created remote-access VPNs may use , , or other methods. Network-to-network tunnels often use passwords or . They permanently store the key to allow the tunnel to establish automatically, without intervention from the administrator.


can operate in a that would theoretically not be considered as a VPN, because a VPN by definition is expected to support arbitrary and changing sets of network nodes. But since most implementations support a software-defined tunnel interface, customer-provisioned VPNs often are simply defined tunnels running conventional routing protocols.

Provider-provisioned VPN building-blocks

Depending on whether a provider-provisioned VPN (PPVPN) operates in layer 2 or layer 3, the building blocks described below may be L2 only, L3 only, or combine them both. (MPLS) functionality blurs the L2-L3 identity.

RFC 4026 generalized the following terms to cover L2 and L3 VPNs, but they were introduced in RFC 2547. More information on the devices below can also be found in Lewis, Cisco Press.

Customer (C) devices

A device that is within a customer’s network and not directly connected to the service provider’s network. C devices are not aware of the VPN.

Customer Edge device (CE)

A device at the edge of the customer’s network which provides access to the PPVPN. Sometimes it’s just a demarcation point between provider and customer responsibility. Other providers 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 connects to customer networks through CE devices and presents the provider’s view of the customer site. PEs are aware of the VPNs that connect through them, and 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, 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 providers.

User-visible PPVPN services

OSI Layer 2 services

A Layer 2 technique that allow for the coexistence of multiple LAN broadcast domains, interconnected via trunks using the 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 , VLANs allow multiple tagged LANs to share common trunking. VLANs frequently comprise only customer-owned facilities. Whereas VPLS as described in the above section (OSI Layer 1 services) supports emulation of both point-to-point and point-to-multipoint topologies, the method discussed here extends Layer 2 technologies such as and LAN trunking to run over transports such as .

As used in this context, a is a Layer 2 PPVPN, rather than a private line, emulating the full functionality of a traditional (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 , but it can provide different L2 protocols at both ends. Typically, its interface is a WAN protocol such as or . 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.

Ethernet over IP tunneling

EtherIP (RFC 3378) is an Ethernet over IP tunneling protocol specification. EtherIP has only packet encapsulation mechanism. It has no confidentiality nor message integrity protection. EtherIP was introduced in the network stack and the server program.

IP-only LAN-like service (IPLS)

A subset of VPLS, the CE devices must have Layer 3 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. 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 (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 without awareness of VPNs.

Virtual router PPVPN

The virtual router architecture, 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.

Unencrypted tunnels

Some virtual networks use without encryption for protecting the privacy of data. While VPNs often do provide security, an unencrypted does not neatly fit within the secure or trusted categorization. For example, a tunnel set up between two hosts with (GRE) is a virtual private network, but neither secure nor trusted.

Native tunneling protocols include (L2TP) when it is set up without and (PPTP) or (MPPE).

Trusted delivery networks

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

  • (MPLS) often overlays VPNs, often with quality-of-service control over a trusted delivery network.
  • (L2TP) which is a standards-based replacement, and a compromise taking the good features from each, for two proprietary VPN protocols: Cisco’s (obsolete ) and Microsoft’s Point-to-Point Tunneling Protocol ().

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 among physically secure sites only, both trusted and secure models need an authentication mechanism for users to gain access to the VPN.

VPNs in mobile environments

are used in settings where an endpoint of the VPN is not fixed to a single , but instead roams across various networks such as data networks from cellular carriers or between multiple access points. Mobile VPNs have been widely used in , where they give law enforcement officers access to mission-critical applications, such as and criminal databases, while they travel between different subnets of a mobile network. They are also used in and by healthcare organizations, among other industries.

Increasingly, mobile VPNs are being adopted by mobile professionals who need reliable connections. Setting up VPN support on a router and establishing a VPN allows any networked device to have access to the entire network—all devices look like local devices with local addresses. Supported devices are not restricted to those capable of running a VPN client.

Many router manufacturers supply routers with built-in VPN clients. Some use open-source firmware such as , and , in order to support additional protocols such as .

Setting up VPN services on a router requires a deep knowledge of network security and careful installation. Minor misconfiguration of VPN connections can leave the network vulnerable. Performance will vary depending on the ISP.

Networking limitations

One major limitation of traditional VPNs is that they are point-to-point, and do not tend to support or connect . Therefore, communication, software, and networking, which are based on and broadcast , such as used in , may not be fully supported or work exactly as they would on a real . Variants on VPN, such as (VPLS), and , are designed to overcome this limitation.


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