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Packet-switched networks are built on mesh topologies in which multiple paths to a destination exist. The links in the mesh are point-to-point links joined by routers. A path to a destination may go through any number of routers, and the path may change at any time due to traffic problems or failed links. In this environment, there are two possible packet-routing methods:

• Hop-by-hop, destination-based routing    This scheme is like getting directions along the way. A packet has a destination address. Each router looks at the address and makes a routing decision about how to forward the packet. Thus, decisions are made on a hop-by-hop basis in the network until the packet reaches its destination.


• Explicit routing    This scheme relies on a network made of switch routers or ATM switches. A predefined path is specified in advance for a packet. This is a virtual circuit in the ATM world. Since the path is predefined, the packet is switched at each node, thus eliminating the need to make routing decisions at every node along the path. Explicit routing is useful for traffic engineering, QoS (Quality of Service), and the prevention of routing loops. It requires path setup in advance, something that can be done in IP networks with MPLS (Multiprotocol Label Switching). Source routing is a form of explicit routing in which end systems discover a path through the network in advance of sending packets. Constraint-based routing is a related technique that builds paths based on various constraints programmed into the network, such as bandwidth requirements for specific types of traffic.

Setting up paths implies that the network will keep state information, something that was frowned on in the past due to its potential to reduce performance. But with the need for QoS and bandwidth management rising, explicit routing is now important and can be done much more efficiently due to higher-performance devices.