Virtual private network

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

A typical site-to-site VPN.

Early data networks allowed VPN-style connections to remote sites through dial-up modem or through leased line connections utilizing X.25, Frame Relay and Asynchronous Transfer Mode (ATM) virtual circuits provided through networks owned and operated by telecommunication carriers. These networks are not considered true VPNs because they passively secure the data being transmitted by the creation of logical data streams.[3] They have been replaced by VPNs based on IP and IP/Multi-protocol Label Switching (MPLS) Networks, due to significant cost-reductions and increased bandwidth[4] provided by new technologies such as digital subscriber line (DSL)[5] and fiber-optic networks.

VPNs can be characterized as host-to-network or remote access by connecting a single computer to a network or as site-to-site for connecting two networks. In a corporate setting, remote-access VPNs allow employees to access the company's intranet from outside the office. Site-to-site VPNs allow collaborators in geographically disparate offices to share the same virtual network. A VPN can also be used to interconnect two similar networks over a dissimilar intermediate network, such as two IPv6 networks connected over an IPv4 network.[6]

VPN systems 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 usually increase privacy and security. To prevent disclosure of private information, VPNs typically allow only authenticated remote access using tunneling protocols and encryption techniques.

The VPN security model provides:

• confidentiality such that even if the network traffic is sniffed at the packet level (see network sniffer and deep packet inspection), an attacker would see only encrypted data
• sender authentication to prevent unauthorized users from accessing the VPN
• message integrity to detect 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, which 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 SSL/TLS has with tunneling over TCP (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 or to inter-network links. OpenSSH server provides a limited number of concurrent tunnels. The VPN feature itself does not support personal authentication.[12][13][14]
• WireGuard

Tunneling protocols can operate in a point-to-point network 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.

Provider-provisioned VPN building-blocks

Site-to-Site VPN terminology.

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 a combination of both. Multi-protocol label switching (MPLS) functionality blurs the L2-L3 identity.

RFC 4026 generalized the following terms to cover L2 MPLS VPNs and L3 (BGP) VPNs, but they were introduced in RFC 2547.[15][16]

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 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 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.[28]

• Multi-Protocol Label Switching (MPLS) often overlays VPNs, often with quality-of-service control over a trusted delivery network.
• L2TP[29] 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)[30] (obsolete as of 2009) and Microsoft's Point-to-Point Tunneling Protocol (PPTP).[31]

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.

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.[38]

A wide variety of (typically commercial) entities provide "VPNs" for all kinds of purposes, but depending on the provider and the application, they often do not create a true "private network" with anything meaningful on the local network. Nonetheless the term is increasingly prevalent. The general public has come to mainly use the term VPN service or just VPN specifically for a commercially marketed product or service that uses a VPN protocol to tunnel the user's internet traffic so an IP address of the service provider's server appears to the public to be the IP address of the user. Depending on the features properly implemented, the user's traffic, location and/or real IP may be hidden from the public, thereby providing the desired internet access features offered, such as Internet censorship circumvention, anonymization, and geo-unblocking. They tunnel the user's internet traffic securely only between the public internet and the user's device and there is typically no way for a user's devices connected to the same "VPN" to see each other. These VPNs can be based on typical VPN protocols or more camouflaged VPN implementations like SoftEther VPN, but proxy protocols like Shadowsocks are used as well.[39] These VPNs are usually marketed as privacy protection services.

On the client side, a common VPN setup is by design not a conventional VPN, but does typically use the operating system's VPN interfaces to capture a user's data to send through. This include virtual network adapters on computer OSes and specialized "VPN" interfaces on mobile operating systems. A less common alternative is to provide a SOCKS proxy interface.

Users must consider that when the transmitted content is not encrypted before entering a VPN, that data is visible at the receiving endpoint (usually the public VPN provider's site) regardless of whether the VPN tunnel wrapper itself is encrypted for the inter-node transport. The only secure VPN is where the participants have oversight at both ends of the entire data path, or the content is encrypted before it enters the tunnel provider.

Unapproved VPNs are reportedly illegal in China, as they can be used to circumvent the Great Firewall. (VPN refers to any protocol that tunnels traffic out elsewhere, as above.)[40]

• Anonymizer
• Dynamic Multipoint Virtual Private Network
• Ethernet VPN
• Internet privacy
• Mediated VPN
• Opportunistic encryption
• Split tunneling
• Virtual private server

• 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.