Suppose you want to connect to your workplace network from home. Your workplace, however, has a security policy that does not allow “outside” IP addresses to access essential internal resources. How do you proceed, without leasing a dedicated telecommunications line to your workplace?
A virtual private network, or VPN, provides a solution; it supports creation of virtual links that join far-flung nodes via the Internet. Your home computer creates an ordinary Internet connection (TCP or UDP) to a workplace VPN server (IP-layer packet encapsulation can also be used, and avoids the timeout problems sometimes created by sending TCP packets within another TCP stream; see 7.13 Mobile IP). Each end of the connection is typically associated with a software-created virtual network interface; each of the two virtual interfaces is assigned an IP address. (Virtual interfaces are not essential; VPNs created with IPsec, 22.11 IPsec, generally omit them.) When a packet is to be sent along the virtual link, it is actually encapsulated and sent along the original Internet connection to the VPN server, wending its way through the commodity Internet; this process is called tunneling. To all intents and purposes, the virtual link behaves like any other physical link.
Tunneled packets are often encrypted as well as encapsulated, though that is a separate issue. One relatively easy-to-implement example of a tunneling mechanism is to treat a TCP home-workplace connection as a serial line and send packets over it back-to-back, using PPP with HDLC; see 220.127.116.11 HDLC and RFC 1661 (though this can lead to the above-mentioned TCP-in-TCP timeout problems).
At the workplace side, the virtual network interface in the VPN server is attached to a router or switch; at the home user’s end, the virtual network interface can now be assigned an internal workplace IP address. The home computer is now, for all intents and purposes, part of the internal workplace network.
In the diagram below, the user’s regular Internet connection is via hardware interface
eth0. A connection is established to Site A’s VPN server; a virtual interface
tun0 is created on the user’s machine which appears to be a direct link to the VPN server. The
tun0 interface is assigned a Site-A IP address. Packets sent via the
tun0 interface in fact travel over the original connection via
eth0 and the Internet.
After the VPN is set up, the home host’s
tun0 interface appears to be locally connected to Site A, and thus the home host is allowed to connect to the private area within Site A. The home host’s forwarding table will be configured so that traffic to Site A’s private addresses is routed via interface
VPNs are also commonly used to connect entire remote offices to headquarters. In this case the remote-office end of the tunnel will be at that office’s local router, and the tunnel will carry traffic for all the workstations in the remote office.
Other applications of VPNs include trying to appear geographically to be at another location, and bypassing firewall rules blocking specific TCP or UDP ports.
To improve security, it is common for the residential (or remote-office) end of the VPN connection to use the VPN connection as the default route for all traffic except that needed to maintain the VPN itself. This may require a so-called host-specific forwarding-table entry at the residential end to allow the packets that carry the VPN tunnel traffic to be routed correctly via
eth0. This routing strategy means that potential intruders cannot access the residential host – and thus the workplace internal network – through the original residential Internet access. A consequence is that if the home worker downloads a large file from a non-workplace site, it will travel first to the workplace, then back out to the Internet via the VPN connection, and finally arrive at the home.
To improve congestion response, IP packets are sometimes marked by routers that are experiencing congestion; see 14.8.3 Explicit Congestion Notification (ECN). If such marking is done to the outer, encapsulating, packet, and the marks are not transferred at the remote endpoint of the VPN to the inner, encapsulated, packet, then the marks are lost. Congestion response may suffer. RFC 6040 spells out a proper re-marking strategy in general; RFC 7296 defines re-marking for IPsec (22.11 IPsec). Older VPN protocols, however, may not support congestion re-marking.