What is VPN?

Virtual private network
From Wikipedia,

A virtual private network (VPN) is a communications network tunneled through another network, and dedicated for a specific network. One common application is secure communications through the public Internet, but a VPN need not have explicit security features, such as authentication or content encryption. VPNs, for example, can be used to separate the traffic of different user communities over an underlying network with strong security features.


A VPN may have best-effort performance, or may have a defined Service Level Agreement (SLA) between the VPN customer and the VPN service provider. Generally, a VPN has a topology more complex than point-to-point. The distinguishing characteristic of VPNs are not security or performance, but that they overlay other network(s) to provide a certain functionality that is meaningful to a user community.


Business Case for Using VPN
Attractions of VPNs to enterprises include:

Shared facilities may be cheaper-- especially in capital expenditure (CAPEX)-- than traditional routed networks over dedicated facilities.
Can rapidly link enterprise offices, as well as small-and-home-office and mobile workers.
Allow customization of security and quality of service as needed for specific applications.
Can scale to meet sudden demands, especially when provider-provisioned on shared infrastructure.
Can reduce operational expenditure (OPEX) by outsourcing support and facilities.
Distributing VPNs to homes, telecommuters, and small offices may put access to sensitive information in facilities not as well protected as more traditional facilities. VPNs need to be designed and operated under well-thought-out security policies. Organizations using them must have clear security rules supported by top management. When access goes beyond traditional office facilities, where there may be no professional administrators, security must be maintained as transparently as possible to end users.

Some organizations with especially sensitive data, such as health care companies, even arrange for an employee's home to have two separate WAN connections: one for working on that employer's sensitive data and one for all other uses.[citation needed] More common is that bringing up the secure VPN cuts off Internet connectivity for any use except secure communications into the enterprise; Internet access is still possible but will go through enterprise access rather than that of the local user.

In situations in which a company or individual has legal obligations to keep information confidential, there may be legal problems, even criminal ones, as a result. Two examples are the HIPAA regulations in the U.S. with regard to health data, and the more general European Union data privacy regulations which apply to even marketing and billing information and extend to those who share that data elsewhere.


Categorizing VPNs by User Administrative Relationships
The Internet Engineering Task Force (IETF) 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, network nodes within a single enterprise were interconnected with Wide Area Network (WAN) links from a telecommunications service provider. 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 [1]. 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, not 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 interconnected a set of nodes, all under its administrative control, through an IP network, that was termed an Intranet [2]. When the interconnected nodes were under multiple administrative authorities, but were hidden from the public Internet, the resulting set of nodes was called an extranet. Both intranets and extranets could be managed by a user organization, or the service could be obtained as a contracted offering, usually customized, from an IP service provider. In the latter case, the user organization contracted for layer 3 services much as it had contracted for layer 1 services such as dedicated lines, or multiplexed layer 2 services such as frame relay.

The IETF distinguishes between provider-provisioned and customer-provisioned VPNs [3]. Much as conventional WAN services can be provided by an interconnected set of providers, provider-provisioned VPNs (PPVPNs) can be provided by a single service provider that presents a common point of contact to the user organization.


VPNs and Routing
Tunneling protocols can be used in a point-to-point topology that would generally not be considered a VPN, because a VPN is accepted to support arbitrary and changing sets of network nodes. Since most router implementations support software-defined tunnel interface, customer-provisioned VPNs are often 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 is layer 2 or layer 3, the building blocks described below may be L2 only, L3 only, or combinations of the two. MPLS functionality blurs the L2-L3 identity.

While these terms were generalized to cover L2 and L3 VPNs in RFC 4026, they were introduced in [4].


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 it to be a customer-configurable device.


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 do maintain VPN state.


Provider Device (P)
A P device is inside the provider's core network, and does not directly interface to any customer endpoint. It might, for example, be used to 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 currently considered active in the IETF; some historical names were replaced by these terms.


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.

It is the customer that determines the overall customer VPN service, which can involve routing, bridging, or host network elements.

There is an unfortunate acronym collision between Virtual Private Line Service and Virtual Private LAN Service; the context should make it clear whether the layer 1 virtual private line or the layer 2 virtual private LAN is meant.


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 are 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 are composed only of 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 is an extension of Layer 2 technologies such as 802.1d and 802.1q LAN trunking, extended to run over transports such as Metro Ethernet.

As used in this context rather than private line, a VPLS is a Layer 2 PPVPN that emulates the full functionality of a traditional Local Area Network (LAN). From the user standpoint, VPLS makes it possible to interconnect several LAN segments over a packet-switched or optical provider core, a core transparent to the customer, and makes 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 ATM or Frame Relay. In contrast, when the goal is to provide the appearance of a LAN contiguous between two or more location, 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.


L3 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 is that different customers may use the same address space, especially the IPv4 private address space[5]. The provider must be able to disambiguate overlapping addresses in the multiple customers' PPVPNs.


BGP/MPLS PPVPN
In the method defined by RFC 2547, BGP extensions are used to 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 [6], 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.


Categorizing VPN Security Models
From the security standpoint, either the underlying delivery network is trusted, or the VPN 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.

Some ISPs now 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 VPNs go beyond PPVPN scope, 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 client's computer.


Authentication before VPN Connection
A known trusted user, sometimes only when using trusted devices, can be provided with appropriate security privileges to access resources not available to general users. Servers may also need to authenticate themselves to join the VPN.

There are a wide variety of authentication mechanisms, which may be implemented in devices including firewalls, access gateways, and other devices. They may use passwords, biometrics, or cryptographic methods. Strong authentication involves using at least two authentication mechanisms. The authentication mechanism may require explicit user action, or may be embedded in the VPN client or the workstation.


Trusted Delivery Networks
Trusted VPNs (sometimes referred to APNs - Actual Private Networks)[citation needed] do not use cryptographic tunneling, and instead rely on the security of a single provider's network to protect the traffic. In a sense, these are an elaboration of traditional network and system administration work.

Multi-Protocol Label Switching (MPLS) is often used to overlay VPNs, often with quality of service control over a trusted delivery network.
Layer 2 Tunneling Protocol (L2TP)[7] which is a standards-based replacement, and a compromise taking the good features from each, for two proprietary VPN protocols: Cisco's Layer 2 Forwarding (L2F) [8] (now obsolete) and Microsoft's Point-to-Point Tunneling Protocol (PPTP) [9].

Security mechanisms in the VPN
Secure VPNs use cryptographic tunneling protocols to provide the intended confidentiality (blocking snooping and thus Packet sniffing), sender authentication (blocking identity spoofing), and message integrity (blocking message alteration) to achieve privacy. When properly chosen, implemented, and used, such techniques can provide secure communications over unsecured networks.

Secure VPN protocols include the following:

IPsec (IP security) - commonly used over IPv4, and an obligatory part of IPv6.
SSL/TLS used either for tunneling the entire network stack, as in the OpenVPN project, or for securing what is, essentially, a web proxy. SSL is a framework more often associated with e-commerce, but it has been built-upon by a number of vendors to provide remote access VPN capabilities. A major practical advantage of an SSL-based VPN is that it can be accessed from the locations that restrict external access to SSL-based e-commerce websites only, thereby preventing VPN connectivity using IPsec protocols. SSL-based VPNs are vulnerable to trivial Denial of Service attacks mounted against their TCP connections because latter are inherently unauthenticated.
OpenVPN, an open standard VPN. It is a variation of SSL-based VPN that is capable of running over UDP. Clients and servers are available for all major operating systems.
L2TPv3 (Layer 2 Tunneling Protocol version 3), a new release.
VPN Quarantine The client machine at the end of a VPN could be a threat and a source of attack; this has no connection with VPN design and is usually left to system administration efforts. There are solutions that provide VPN Quarantine services which run end point checks on the remote client while the client is kept in a quarantine zone until healthy. Microsoft ISA Server 2004/2006 together with VPN-Q 2006 from Winfrasoft or an application called QSS (Quarantine Security Suite) provide this functionality.
MPVPN (Multi Path Virtual Private Network). MPVPN is a registered trademark owned by Ragula Systems Development Company. See Trademark Applications and Registrations Retrieval (TARR)

Security and Mobility
Mobile VPNs are VPNs designed for mobile and wireless users. They integrate standards-based authentication and encryption technologies to secure data transmissions to and from devices and to protect networks from unauthorized users. Designed for wireless environments, Mobile VPNs are designed as an access solution for users that are on the move and require secure access to information and applications over a variety of wired and wireless networks. Mobile VPNs allow users to roam seamlessly across IP-based networks and in and out of wireless coverage areas without losing application sessions or dropping the secure VPN session. For instance, highway patrol officers require access to mission-critical applications in order to perform their jobs as they travel across different subnets of a mobile network, much as a cellular radio has to hand off its link to repeaters at different cell towers.