The Internet is made of a combination of physical networks connected together by internetworking devices such as routers and gateways. Referring to FIG. 1, let the Internet 1 comprise routers R1, R2, R3 and R4 connected by transmission lines L1, L2, L3, L4 and L5. Let individual computers (hosts) D1 and D2 be connected to the Internet 1 through routers R1 and R4 respectively. Similarly, let a local area network (LAN) 2 comprise hosts D3 to D6 connected to a LAN switch S, which in turn is connected to the Internet through router R2.
In a typical scenario, a host generates a message (e.g. a request or response) for transmission on the Internet 1. The message is processed by a transport layer, which adds a header and passes the resulting packet to a network layer. The network layer adds its own header (including the logical addresses of the source and destination computers) to form a network layer packet (e.g. an IP packet). The packet is then sent to a data link layer, which adds its own header (comprising the physical addresses of the source and destination hosts) and checksum and passes the resulting frame to a physical layer at which point the host (e.g. D1) transmits the frame to the nearest router (e.g. R1).
On receipt of a frame, a router (e.g. R1) strips the header and trailer from the frame and passes the packet located in the frame's payload to routing software. The routing software uses the packet header to choose an output line from the current router to forward the packet to the nearest router (e.g. R2, R3). This process is continued so that the packet is progressively moved through the Internet to its desired destination.
A virtual private network (VPN) is a private network that makes use of a public telecommunication infrastructure (e.g. the Internet), to connect sites or users together. There are two main classes of VPNs in use today, namely:
(a) a Layer 2 Ethernet VPN using the 802.1Q trunking protocol; and
(b) a Layer 2 VPN using the Multi-Protocol Labelling Standard (MPLS)/Virtual Private Line Services (VPLS) standard.
A layer 2 (L2) Ethernet VPN is used for connecting hosts that are geographically located close together (e.g. hosts in a same building). These VPNs are operated under the Ethernet protocol (with the 802.IQ trunking protocol). Thus, these VPNs are subject to the transmission distance restrictions of the Ethernet protocol. For example, a layer 2 Ethernet VPN could not be used to connect a host located in Glasgow, to a host located in Paris.
In contrast, wide area networks (WANs) permit the connection of hosts located for apart. For example, a layer 2 VPN employing the VPLS protocol encapsulates Ethernet frames in VPLS frames and transmits the resulting VPLS frames on a WAN.
Nonetheless, there remains a complete separation between the two different types of networks and the nature of the communications on each. For simplicity and clarity, an L2 Ethernet VPN will be known henceforth as a short range VPN. Similarly, an L2 VPN employing the VPLS protocol will be known as a long range VPN.