A virtual private network extends a private network across a public network, such as the
Internet. It enables a computer to send and receive data across shared or public networks
as if it is directly connected to the private network, while benefiting from the functionality,
security and management policies of the private network. A VPN is created by establishing
a virtual point-to-point connection through the use of dedicated connections, virtual
tunneling protocols, or traffic encryptions. A virtual private network connection across
the Internet is similar to a wide area network link between sites. From a user perspective,
the extended network resources are accessed in the same way as resources available within
the private network. VPNs allow employees to securely access their
company's intranet while traveling outside the office. Similarly, VPNs securely connect
geographically separated offices of an organization, creating one cohesive network. VPN technology
is also used by Internet users to connect to proxy servers for the purpose of protecting
personal identity and location.
Type Early data networks allowed VPN-style remote
connectivity through dial-up modems or through leased line connections utilizing Frame Relay
and Asynchronous Transfer Mode virtual circuits, provisioned through a network 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. They
have been replaced by VPNs based on IP and IP/Multiprotocol Label Switching Networks,
due to significant cost-reductions and increased bandwidth provided by new technologies such
as Digital Subscriber Line and fiber-optic networks.
VPNs can be either remote-access or site-to-site. In a corporate setting, remote-access VPNs
allow employees to access their company's intranet from home or while traveling 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 IPv6 networks over an IPv4 network.
VPN systems may be classified by: the protocols used to tunnel the traffic.
the tunnel's termination point location, e.g., on the customer edge or network-provider edge.
whether they offer site-to-site or remote-access connectivity.
the levels of security provided. the OSI layer they present to the connecting
network, such as Layer 2 circuits or Layer 3 network connectivity.
Security mechanisms To prevent disclosure of private information,
VPNs typically allow only authenticated remote access and make use of encryption techniques.
VPNs provide security by the use of tunneling protocols and through security procedures
such as encryption. The VPN security model provides:
confidentiality such that even if the network traffic is sniffed at the packet level, an
attacker would only see encrypted data sender authentication to prevent unauthorized
users from accessing the VPN. message integrity to detect any instances
of tampering with transmitted messages. Secure VPN protocols include the following:
Internet Protocol Security as initially developed by the Internet Engineering Task Force for
IPv6, which was required in all standards-compliant implementations of IPv6 before RFC 6434 made
it only a recommendation. This standards-based security protocol is also widely used with
IPv4 and the Layer 2 Tunneling Protocol. 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.
Transport Layer Security can tunnel an entire network's traffic 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 - used in
Cisco AnyConnect VPN and in OpenConnect VPN to solve the issues SSL/TLS has with tunneling
over UDP. Microsoft Point-to-Point Encryption works
with the Point-to-Point Tunneling Protocol and in several compatible implementations
on other platforms. Microsoft Secure Socket Tunneling Protocol
tunnels Point-to-Point Protocol or Layer 2 Tunneling Protocol traffic through an SSL
3.0 channel. Multi Path Virtual Private Network. Ragula
Systems Development Company owns the registered trademark "MPVPN".
Secure Shell VPN - OpenSSH offers VPN tunneling 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.
Authentication Tunnel endpoints must be authenticated 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. They permanently store the key to allow the tunnel
to establish automatically, without intervention from the user.
Routing 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 Depending on whether a provider-provisioned
VPN operates in layer 2 or layer 3, the building blocks described below may be L2 only, L3
only, or combine them both. Multiprotocol label switching 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 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 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 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
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 service This section deals with the types of VPN considered
in the IETF. 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, IEEE 802.10, and ATM LAN Emulation. Virtual private LAN service
Developed by IEEE, VLANs allow multiple tagged LANs to share common trunking. VLANs frequently
comprise only customer-owned facilities. Whereas VPLS as described in the above section 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. 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 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.
Ethernet over IP tunneling EtherIP 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 FreeBSD network stack and the SoftEther
VPN server program. IP-only LAN-like service
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
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.
Unencrypted tunnels
Some virtual networks may not use encryption to protect the privacy of data. 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 would in fact be a virtual private network, but neither
secure nor trusted. Native plaintext tunneling protocols include
Layer 2 Tunneling Protocol when it is set up without IPsec and Point-to-Point Tunneling
Protocol or Microsoft Point-to-Point Encryption. Trusted delivery networks
Trusted VPNs do not use cryptographic tunneling, and instead rely on the security of a single
provider's network to protect the traffic. Multi-Protocol Label Switching often overlays
VPNs, often with quality-of-service control over a trusted delivery network.
Layer 2 Tunneling Protocol 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 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
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 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, 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.
See also Anonymizer
Opportunistic encryption Split tunneling
Mediated VPN VPNBook
nVpn.NET OpenVPN
BartVPN UT-VPN
Tinc DMVPN
Virtual Private LAN Service over MPLS Ethernet Virtual Private LAN defined by MEF
MPLS SoftEther VPN, another open-source VPN program
which supports SSL-VPN, IPsec, L2TP, OpenVPN, EtherIP and SSTP protocols listed in the Security
mechanisms section. References
Further reading Kelly, Sean. "Necessity is the mother of VPN
invention". Communication News: 26–28. ISSN 0010-3632. Archived from the original on 2001-12-17.
"VPN Buyers Guide". Communication News: 34–38. August 2001. ISSN 0010-3632.
External links