In essence a Virtual Private Network (VPN) consists in using a normally public network (IP) but at such a level of abstraction the said network is only used as a transport mechanism between two ends over which a private network has been constructed.  The inherent fact of using a public network requires security, up to the point where both concepts, VPN and security, become one, independently of whether the core of the network providing connectivity is public or private.  For this purpose, the connections between the points are often called “tunnels” thus concealing the private information being transported over the public network.

Security being a key element in these communications, it’s essential to know the different VPN techniques, understanding which is the best to apply to each scenario.  The problem arises when handling the amount of acronyms associated to the VPN, which on occasions seem as cryptic as the protocol itself it’s referring to (GRE, L2TP, IPSec, DMVPN, GDOI, SSLVPN, WebVPN, etc.).

VPN at the network layer

This consists of the implantation of the VPN at the OSI layer level 3; this means that the devices and the applications at either end of the “tunnel” can see each other (at level 3) in the same way as they can through a direct connection; consequently this type is VPN is transparent to any protocol and application:

  • GRE (Generic Router Encapsulation) and L2TP (Layer 2 Tunneling Protocol) are simple protocols that permit you to construct VPNs at the network layer, despite being both proven compatible and standardized protocols, they do not have a sufficient security level developed for them and consequently are not widely used except for an auxiliary protocol for VPN interconnection.
  • IPSec (Internet Protocol Security) is an alternative standardized, fully compatible protocol for VPNs over which a suitable security level has been developed, being the standard de facto as a security protocol and VPN and widely used in the routers at the exterior of the network
  • DMVPN (Dynamic Multipoint Virtual Private Networks) is a VPN architect based on the simultaneous use of GRE and IPSec. GRE is used for connectivity and IPSec for security.  The advantage of using GRE for connectivity is that it allows you to send routing information so the tunnel ends in the private networks talk to each other thus reducing the effort of configuration to construct VPNs for a high number of points.  DMVPN is based on a central point (Hub) that all the remote points (Spokes) connect to, and that distribute the routing information between them.  DMVPN is based on the RFC 2332 NBMA Next Hop Resolution Protocol (NHRP).
  • GETVPN (Group Encrypted Transport Virtual Private Networks) or GDOI (Group Domain Of Interpretation) is an additional mechanism to IPSec that simplifies key management.  Also based on RFC and interoperable, this is based on a central server that generates and sends keys to all the points.  GETVPN does not construct “tunnels” consequently this only operates if the host addressing is public.

Branch office routers establish VPNs at the network layer which are transparent to the local network devices although it’s also possible to find implementations (typically IPSec and L2TP) in hosts, either as part of the operating system (Windows, Linux, Android, IOS), or as additional network services developed for third parties.

VPN at the application layer

This consists of establishing the VPN without the intermediate routers or the host network stack intervening.  The base is SSL (Socket Security Layer) protocol from an HTTP session.  In the most basic version (clientless), the SSL server maintains a safe session with the HTTP browser through the public network and presents resources from the internal network (applications, file servers, etc.), in a web format to the client; this is known as HTTP Reverse Proxy.

A more advanced version (Full Network Access) downloads in the client applet creating virtual interfaces to intercept private traffic to exchange between the client and the private network, thus efficiently achieving the establishment of a VPN between the host and the private network connected to the server.

SSL is an original Netscape implementation; version 3.0 has been standardized by the IETF, now know as TLS (Transport Layer Security) 1.0, however the standardized part consists of the security protocol but not the reverse HTTP Proxy features nor the applets, which are proprietary.

Which VPN is the best?

This depends on one’s needs.  VPNs at the application layer are the solution to individually interconnect a device from any point on the public network outside the branches; for example mobile users that connect their portable devices from Internet (PCs, tablets, telephones, etc.), or even public PCs.  The VPN servers at the application layer are usually found at head offices and not in the access routers.

To establish VPNs between different company branches, the solution consists of VPNs at the network layer from the access routers in each branch office, IPSec being the standard de facto.

Teldat is world class leader in VPN technology interoperability for networks (L2TP, GRE, DMVPN, GETVPN), with ample references from large corporations and clients .We offer you all the advice you need about VPN networks. Because the best way to earn your trust, is to offer you the best service.

About the author

Marcel Gil
SD-WAN Business Line Manager

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