ents are met by extending LDP for support of constraint- based routed label switched paths (also sometimes called CR-GLSPs). Like any other GLSP, CR-GLSP is a path through a GMPLS network. The difference is that while other paths are setup based on information in routing tables or from a direct intervention of the management system, the CR-LDP constraint-based route is calculated at one point at the edge of network based on criteria including but not limited to explicit route constraints or QoS constraints. The GLSR uses CR-LDP to establish LSPs through a network by mapping network layer routing information directly to data-link layer switched paths. A Forwarding Equivalence Class (FEC) is associated with each GLSP created. This FEC specifies which type of infor- mation is mapped to that GLSP and can be used for assuring the needed QoS during the GLSP setup. Two GLSR which use CRT-LDP to exchange label mapping information are known as CR-LDP peers and they have a CR-LDP session between them. In a single session, each peer is able to learn about the other label mappings, that is, the protocol is bi-directional. Telecommunication Networks There are four types of CR-LDP messages: 1. Session messages 2. Discovery messages 3. Advertisement messages 4. Notification messages Session messages are used to manage the CR-LDP section through the standard section management steps: Section setup through handshaking between the GLSR at the section ends Section monitoring Section teardown through another specific handshaking Using discovery messages, the GLSRs announce their presence in the network by sending Hello messages periodically. This Hello message is transmitted as a UDP packet allocated into a GLSP that always exists when a physical capacity is deployed along a link in order to convey at least control plane messages. When a new session must be established, the Hello message is sent over TCP to have a better control of the session opening procedure. Apart from the Discovery messages all other messages are sent over TCP.