of the upper layer becomes the payload of the packet of the lower layer. This enveloping process [22] creates a packet structure where the headers relative to dif- ferent layers follow one after the other starting with the lower layer header, as shown in Figure 3.21 [21]. In Figure 3.20, it is also shown that, from an equipment point of view, the end termina- tions of an Internet trail are hosts (i.e., computers with the suitable software to implement Internet connectivity) while transit machines are called routers. Routers implement only the first two layers of the Internet model, since their role is to store arriving packets at the input, calculate the output direction for each packet on the ground of routing tables and send each packet to the correct output. Networks Fundamentals and Present Architectures Network physical link Router Router Layered model of the link Application Application Peer-to-peer Transport Transport Internet Internet Internet Internet Physical layer FIGURE 3.20 Network and layered model of a trail through an IP-based network. Application Transport header IP data Internet header Frame Frame Frame data FIGURE 3.21 header footer Progressive IP suite data encapsulation. The high-level block scheme of a router is represented in Figure 3.22. When a datagram arrives at the router, the link header and tail are seprated and used to verify the correct packet transmission, while the IP layer packet is stored in an input queue. After storing, the IP address is read and used to access the routing tables to determine the exit direction of the packet [23,24]. The output of the routing table is the output port destination. In the absence of packet conflicts, the router configures the switch fabric in order to send the packet to the correct output queue and transfer the packet there. In the output queue, the link envelope is recre- ated and the packet is launched toward the next router (forwarding process). In the presence of conflicts on an output port, a router uses a conflict solution criterion to dec