Virtual private network

VPN classification tree based on the topology first, then on the technology used

VPN connectivity overview, showing intranet site-to-site and remote-work configurations used together

Virtual private networks may be classified into several categories:

Remote access

A host-to-network configuration is analogous to connecting a computer to a local area network. This type provides access to an enterprise network, such as an intranet. This may be employed for remote workers, or to enable a mobile worker to access necessary tools without exposing them to the public Internet.

Site-to-site

A site-to-site configuration connects two networks. This configuration expands a network across geographically disparate offices or connects a group of offices to a data center installation. The interconnecting link may run over a dissimilar intermediate network, such as two IPv6 networks connected over an IPv4 network.[5]

Extranet-based site-to-site

In the context of site-to-site configurations, the terms intranet and extranet are used to describe two different use cases.[6] An intranet site-to-site VPN describes a configuration where the sites connected by the VPN belong to the same organization, whereas an extranet site-to-site VPN joins sites belonging to multiple organizations.

Typically, individuals interact with remote access VPNs, whereas businesses tend to make use of site-to-site connections for business-to-business, cloud computing, and branch office scenarios. However, these technologies are not mutually exclusive and, in a significantly complex business network, may be combined to enable remote access to resources located at any given site, such as an ordering system that resides in a data center.

VPN systems also may be classified by:

• the tunneling protocol used to tunnel the traffic
• the tunnel's termination point location, e.g., on the customer edge 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 OSI layer they present to the connecting network, such as Layer 2 circuits or Layer 3 network connectivity
• the number of simultaneous connections

VPNs cannot make online connections completely anonymous, but they can increase privacy and security. To prevent disclosure of private information or data sniffing, VPNs typically allow only authenticated remote access using tunneling protocols and secure encryption techniques.

The VPN security model provides:

• confidentiality such that even if the network traffic is sniffed at the packet level (see network sniffer or deep packet inspection), an attacker would see only encrypted data, not the raw data
• sender authentication to prevent unauthorized users from accessing the VPN
• message integrity to detect and reject any instances of tampering with transmitted messages

The life cycle phases of an IPSec tunnel in a virtual private network

Secure VPN protocols include the following:

• Internet Protocol Security (IPsec) was initially developed by the Internet Engineering Task Force (IETF) for IPv6, and was required in all standards-compliant implementations of IPv6 before RFC 6434 made it only a recommendation.[7] This standards-based security protocol is also widely used with IPv4 and the Layer 2 Tunneling Protocol. Its design meets most security goals: availability, 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.
• Transport Layer Security (SSL/TLS) can tunnel an entire network's traffic (as it does in the OpenVPN project and SoftEther VPN project[8]) 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) – used in Cisco AnyConnect VPN and in OpenConnect VPN[9] to solve the issues TLS has with tunneling over TCP (SSL/TLS are TCP-based, and tunneling TCP over TCP can lead to big delays and connection aborts[10]).
• Microsoft Point-to-Point Encryption (MPPE) works with the Point-to-Point Tunneling Protocol and in several compatible implementations on other platforms.
• Microsoft Secure Socket Tunneling Protocol (SSTP) tunnels Point-to-Point Protocol (PPP) or Layer 2 Tunneling Protocol traffic through an SSL/TLS channel (SSTP was introduced in Windows Server 2008 and in Windows Vista Service Pack 1).
• Multi Path Virtual Private Network (MPVPN). Ragula Systems Development Company owns the registered trademark "MPVPN".[11]
• Secure Shell (SSH) VPN – OpenSSH offers VPN tunneling (distinct from port forwarding) to secure remote connections to a network, inter-network links, and remote systems. OpenSSH server provides a limited number of concurrent tunnels. The VPN feature itself does not support personal authentication.[12] SSH is more often used to remotely connect to machines or networks instead of a site to site VPN connection.
• WireGuard is a protocol. In 2020, WireGuard support was added to both the Linux[13] and Android[14] kernels, opening it up to adoption by VPN providers. By default, WireGuard utilizes the Curve25519 protocol for key exchange and ChaCha20-Poly1305 for encryption and message authentication, but also includes the ability to pre-share a symmetric key between the client and server.[15]
• Internet Key Exchange version 2 was created by Microsoft and Cisco and is used in conjunction with IPSec for encryption and authentication. Its primary use is in mobile devices, whether on 3G or 4G LTE networks, since it automatically reconnects when a connection is lost.
• OpenVPN is a free and open-source VPN protocol based on the TLS protocol. It supports perfect forward-secrecy, and most modern secure cipher suites, like AES, Serpent, TwoFish, etc. It is currently being developed and updated by OpenVPN Inc., a non-profit providing secure VPN technologies.
• Crypto IP Encapsulation (CIPE) is a free and open-source VPN implementation for tunneling IPv4 packets over UDP via encapsulation.[16] CIPE was developed for Linux operating systems by Olaf Titz, with a Windows port implemented by Damion K. Wilson.[17] Development for CIPE ended in 2002.[18]

Tunneling protocols can operate in a point-to-point network topology however, this 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 virtual, software-defined tunnel interface, customer-provisioned VPNs often are simply defined tunnels running conventional routing protocols.

Provider-provisioned VPN building blocks

Site-to-site VPN terminology

Depending on whether a provider-provisioned VPN (PPVPN) operates in Layer 2 (L2) or Layer 3 (L3), the building blocks described below may be L2 only, L3 only, or a combination of both. Multiprotocol Label Switching (MPLS) functionality blurs the L2–L3 identity.[19]

RFC 4026 generalized the following terms to cover L2 MPLS VPNs and L3 (BGP) VPNs, but they were introduced in RFC 2547.[20][21]

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 is just a demarcation point between provider and customer responsibility. Other providers allow customers to configure it.

Provider edge device (PE)

A device, or set of devices, at the edge of the provider network that 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 device that 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.

Trusted VPNs do not use cryptographic tunneling; instead, they rely on the security of a single provider's network to protect the traffic.[33]

• Multiprotocol Label Switching (MPLS) often overlays VPNs, often with quality-of-service control over a trusted delivery network.
• L2TP[34] 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)[35] (obsolete as of 2009) and Microsoft's Point-to-Point Tunneling Protocol (PPTP).[36]

From a security standpoint, a VPN must either trust the underlying delivery network or enforce security with a mechanism 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 are used in settings 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 without dropping the secure VPN session or losing application sessions.[37] Mobile VPNs are widely used in public safety where they give law-enforcement officers access to applications such as computer-assisted dispatch and criminal databases,[38] and in other organizations with similar requirements such as field service management and healthcare.[39]

A limitation of traditional VPNs is that they are point-to-point connections and do not tend to support broadcast domains; therefore, communication, software, and networking, which are based on layer 2 and broadcast packets, such as NetBIOS used in Windows networking, may not be fully supported as on a local area network. Variants on VPN such as Virtual Private LAN Service (VPLS) and layer 2 tunneling protocols are designed to overcome this limitation.[40]

• Kelly, Sean (August 2001). "Necessity is the mother of VPN invention". Communication News: 26–28. ISSN 0010-3632. Archived from the original on 17 December 2001.