Virtual Private Networks (VPNs)

A virtual private network (VPN) is a network that uses primarily public telecommunication infrastructure, such as the Internet, to provide remote offices or traveling users access to a central organisational network.

VPNs typically require remote users of the network to be authenticated, and often secure data with encryption technologies to prevent disclosure of private information to unauthorized parties.
VPNs may serve any network functionality that is found on any network, such as sharing of data and access to network resources, printers, databases, websites, etc. A VPN user typically experiences the central network in a manner that is identical to being connected directly to the central network.

VPN technology via the public Internet has replaced the need to requisition and maintain expensive dedicated leased-line telecommunication circuits once typical in wide-area network installations.



Security

vpnSecure VPNs use cryptographic tunnelling protocols to provide confidentiality by blocking intercepts and packet sniffing, allowing sender authentication to block identity spoofing, and provide message integrity by preventing message alteration. Secure VPN protocols include the following: IPsec (Internet Protocol Security) was developed by the Internet Engineering Task Force (IETF), and was initially developed for IPv6, which requires it. This standards-based security protocol is also widely used with IPv4.

Layer 2 tunnelling Protocol frequently runs over IPsec. Its design meet the most security goals-authentication, integrity, and confidentiality. IPsec functions by summarizing an IP packet in conjunction with a surrounding packet, and encrypting the outcome. Transport Layer Security (SSL/TLS) can tunnel an entire network’s traffic, as it does in the OpenVPN 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 Network Address Translation and firewall rules. Datagram Transport Layer Security (DTLS), is used in Cisco’s next-generation VPN product, Cisco AnyConnect VPN, to solve the issues SSL/TLS has with tunnelling over UDP.

Microsoft Point-to-Point Encryption (MPPE) works with their Point-to-Point tunnelling Protocol and in several compatible implementations on other platforms. Microsoft introduced Secure Socket tunnelling Protocol (SSTP) in Windows Server 2008 and Windows Vista Service Pack 1. SSTP tunnels Point-to-Point Protocol (PPP) or Layer 2 tunnelling Protocol traffic through an SSL 3.0 channel. MPVPN (Multi Path Virtual Private Network). Ragula Systems Development Company owns the registered trademark “MPVPN”. Secure Shell (SSH) VPN — OpenSSH offers VPN tunnelling to secure remote connections to a network or inter-network links. This should not be confused with port forwarding. OpenSSH server provides a limited number of concurrent tunnels and the VPN feature itself does not support personal authentication.



Authentication

Tunnel endpoints must authenticate before secure VPN tunnels can be established.
User-created remote access VPNs may use passwords, biometrics, two-factor authentication or other cryptographic methods.
Network-to-network tunnels often use passwords or digital certificates, as they permanently store the key to allow the tunnel to establish automatically and without intervention from the user.


Routing

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

PPVPN Building blocks
Depending on whether the PPVPN (Provider Provisioned VPN) runs in layer 2 or layer 3, the building blocks described below may be L2 only, L3 only, or combine them both. Multiprotocol Label Switching (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.

Customer Edge device (CE)
A device at the customer premises, that 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, that 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, 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 service provider does not offer a full routed or bridged network, but provides components to 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[clarification needed] 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. 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 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, 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.



Plaintext Tunnels

Some virtual networks may not use encryption to protect the data contents. While VPNs often provide security, an unencrypted overlay network does not neatly fit within the secure or trusted categorization. For example a tunnel set up between two hosts that used Generic Routing Encapsulation (GRE) would in fact be a virtual private network, but neither secure nor trusted.
Besides the GRE example above, native plaintext tunnelling protocols include Layer 2 tunnelling Protocol (L2TP) when it is set up without IPsec and Point-to-Point tunnelling Protocol (PPTP) or Microsoft Point-to-Point Encryption (MPPE).



Trusted Delivery Networks

Trusted VPNs do not use cryptographic tunnelling, and instead rely on the security of a single provider’s network to protect the traffic.
Multi-Protocol Label Switching (MPLS) is often used to overlay VPNs, often with quality-of-service control over a trusted delivery network.
Layer 2 tunnelling Protocol (L2TP) 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) (obsolete as of 2009) and Microsoft’s Point-to-Point tunnelling Protocol (PPTP).
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.



Mobile Virtual Private Networks

Mobile VPNs are used in a setting where 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. 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, while they travel between different subnets of a mobile network. They are also used in field service management and by healthcare organizations, among other industries.

Increasingly, mobile VPNs are being adopted by mobile professionals and white-collar workers who need reliable connections. They are used for roaming 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, or fail, or even cause the computing device itself to crash.

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.
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.



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