A world-wide communication network is composed of many independently operated individual networks. Traditionally, most countries have only one network operated by governments. After deregulation acts in the last decade, many countries have granted more licences to commercial networks. These networks interconnect together through interconnection links to form a world-wide network allowing signals be delivered to most locations in the world. Similarly, we assume a world-wide AlI-IP network will be deployed in the same way. For simplicity, we make following assumptions in network architecture.
An independent newtork consists of a Core network and some Stub networks. A core network covers the entire area serviced by the netowrk. A stub network, also known as Access Network, covers a small area such as a city. It can also be a wireless network such as Wi-Fi WLAN or a 2G Radio Network. In general, two terminal devices connected to the same stub network may or may not be able to communicate to each other without passing througth the core network. However, for simplicity, we assume an IP packet will travel from the near-end stub network to the core network, and to the far-end stub network. The near-end stub network is referred to as Ingress Network and the far-end is referred to as Egress Network. The core networks of all operators interconnected together through interconnection links to form the so called Backbone Network. Sending a packet to a destination that is in a different network will have to travel from the Ingress network, to several core networks (backbone), and finally to the Egress network. The edge routers of a core network, called Border Gateway (BG), perform gateway functions between stub network and the core network. For QoS purpose, an edge router may also perfrom admission control if it serves as an Ingress for a incoming traffic. On the other hand, the gateway connecting a stub network to a core network is called an Access Gateway (AG), The gateways between different core netowrks are called Inter-Domain Gateway.