ide the order in which the packets have to be transmitted, criterion that can be as simple as a first in first out (FIFO) or as complex as a complete QoS algorithm. Telecommunication Networks Routing IP protocol tables control FIGURE 3.22 Router functional block scheme. When a packet is forwarded toward a router that has no place free in the queue on the designated input port, the packet is lost. It will be the host via a higher layer protocol (e.g., the TCP) that will report the lack of a packet and to ask the other communication end the packet retransmission. From this discussion, it results that routing through an IP network is performed via a distributed algorithm running on all the network routers; no router has a general control role and in order to reconstruct packet route there is the need to monitor contemporarily all the routers of its trail [24]. Summarizing, the four levels of the Internet model can be described as follows: Application layer: This is the layer allowing different applications to communicate for exchanging users' data or control signals; in this layer, the fact that the con- nected applications are in the same host or not is irrelevant. The communication partners are often called peers. This is where the "higher level" protocols such as Send Mail Transfer Protocol (SMTP), File Transfer Protocol (FTP), Secure Shell (SSH), Hypertext Transfer Protocol (HTTP), etc. operate. Transport layer: The transport layer manages the end-to-end communication between two network hosts, either directly on a local network or on a geographi- cal network where the hosts are connected via a set of routers. The transport layer provides a uniform networking interface that hides the actual topology of the underlying network connections. This is where flow-control, error-correction, and connection protocols, such as TCP, are located. With reference to equivalent OSI layers, transport layer protocols are considered layer 4 protocols. Internet layer: The Internet layer has the task of exchanging data packets acros