ke IP hop-by-hop forwarding), or may have an endpoint at a network egress node, enabling switching via all intermediary nodes. Two LSRs that use LDP to exchange label mapping information are known as LDP peers and they have an LDP session between them. Since in a single session each peer is able to learn about the others label mapping established by the LSR with which it is connected in the section framework, the protocol is called bidirectional. There are four sorts of LDP messages: 1. Discovery messages 2. Notification messages 3. Session messages 4. Advertisement messages Discovery messages announce and maintain the presence of a router in a network. Routers indicate their presence in a network by sending the hello messages periodically. The hello messages are transmitted as a UDP packet to the LDP port at the group multicast address for all routers on the subnet. Notification messages provide advisory information and signal error information. LDP sends notification messages to report errors and other events of interest. Session messages establish, maintain, and terminate sessions between LDP peers. When a router establishes a session with another router learned through the hello message, it uses the LDP initialization procedure over TCP transport. When the initialization proce- dure completes successfully, the two routers are LDP peers, and can exchange advertise- ment messages. Networks Fundamentals and Present Architectures Advertisement messages create, change, and delete label mappings. Requesting a label or advertising a label mapping to a peer is a decision made by the local router. This is the local mechanism creating and deleting tunnels. When a label request arrives at an LSR via an LDP packet, the router accesses routing tables, individuates the output port and the associated output label, and add the new tun- nel to the tunnels list. The tunnel is stored memorizing the association of an input label (tagged to the port from which the label request was received) to the output label tagged to the corr