The sequence of nodes and links a packet traveled is called a path. In general, a packet in a packet-switching network doesn't follow a predefined path to travel. However, for some purpuse, a network administrator may designate a path for a packet to travel. This path desigantion may be either static or dynamic. The process of finding a path for a packet is called routing. In some operational networks, network administrators may set up the routing tables in all routers according to the results of routing. Under this mechanism, packets travel acoording to the static paths designed by the routing module and are embedded in routers. Packets may also travel according to the paths that are computed by the routing module dynamically and are carried in the packets. It is much easier to asuure QoS if packets travel along designated paths.