 operations, the label processing should be much sim- pler than the IP address effective processing and this is not always the situation. The presence of fixed tunnels conveying datagrams belonging to the same communica- tion is instead precious to perform network monitoring and QoS management. As a matter of fact, the transit of datagrams along a tunnel can be easily monitored pro- ducing statistics of errors in transmission, passing through time, loss of packets due to the transmission toward a full queue, and SO on. Telecommunication Networks Moreover, a service related tag can be applied to each MPLS label that can be used to run priority algorithms in the input queues of the routers and SO manage real-time services. In its nature MPLS would be a layer 2 technology, since it is located in between the Internet layer and the pure physical layer. As we have seen, in practice it also performs operations that would be proper of the Internet layer (i.e., layer 3). This is due to the fact that Internet layer protocols lack the needed characteristics. For this reason, often the MPLS is informally called a layer 2.5 technology. As in the case of all connection oriented protocols, the MPLS manages tunnels through three phases: 1. Tunnel setup 2. Tunnel management 3. Tunnel tier down Once a tunnel is created, the MPLS header is put in front of the Internet layer datagram; it is composed of 32 bits: 20 bits are reserved for the label, 3 bits code the priority, 1 bit indi- cates if there is a label stack, and finally 8 bits contain the time to live. All the operations of setup, management, and tear down of a tunnel, besides other func- tionalities, are performed via the Label Distribution Protocol (LDP). The LSR uses this protocol to establish tunnels through the network; this is done by mapping the network layer routing information that determines the end-to-end connec- tion request directly to data-link layer switched paths, implemented via MPLS labels. These LSPs may have an endpoint at a directly attached neighbor (li