The LDP Message Format

158 LABEL DISTRIBUTION PROTOCOLS assume that the path between the ingress LSR A and the egress LSR G, calculated using OSPF, passes through E and F. Using CR-LDP we can set up a CR-LSP that satisfies a QoS criterion, such as minimize the end-to-end delay. For instance, if LSRs B, C, and D are not heavily utilized, routing the CR-LSP through these LSRs will reduce the end-to-end delay, even though the number of hops will be higher than the E-to-F path. The following are some of the features of CR-LDP: • CR-LDP is based on LDP, and runs on top of TCP for reliability. • The CR-LDP state-machine does not require periodic refreshment. • CR-LDP permits strict and loose explicit routes. This allows the ingress LSR some degree of imperfect knowledge about the network topology see Section 6.2.3. The source LSR might also request route pinning, which fixes the path through a loosely defined route so that it does not change when a better next hop becomes available. • CR-LDP permits path preemption by assigning setupholding priorities to CR-LSPs. If a route for a high-priority CR-LSP cannot be found, then existing lower-priority CR-LSPs can be rerouted to permit the higher-priority CR-LSP to be established. • The network operator can classify network resources in various ways. CR-LDP per- mits the indication of the resource classes that can be used when a CR-LSP is being established. • As in the case of ATM, CR-LDP allows the specification of traffic parameters on a CR-LSP and how these parameters should be policed. CR-LDP depends on the following minimal LDP functionality: • Basic andor extended discovery mechanism • Label request message for downstream on demand with ordered control • Label mapping message for downstream on demand with ordered control • Notification messages • Label withdraw and release messages • Loop detection for loosely routed segments

7.2.1 CR-LSP Setup Procedure

A CR-LSP is set up using downstream on demand allocation with ordered control. Recall that in the downstream on demand allocation scheme, each LSR binds an incoming label to a FEC and creates an appropriate entry in its LFIB. However, it does not advertise its label mapping to its neighbors as in the unsolicited downstream allocation scheme. Instead, an upstream LSR obtains the label mapping by issuing a request. In the ordered control scheme, the allocation of labels proceeds backwards from the egress LSR towards the ingress LSR. Specifically, an LSR only binds a label to a FEC if it is the egress LSR for that FEC, or if it has already received a label binding for that FEC from its next hop LSR. An example of how a CR-LSP is set up is shown in Figure 7.10. Let us assume that LSR A has been requested to establish a CR-LSP to LSR E. A request to set up a CR- LSP to LSR E might originate from a management system or an application. LSR A calculates the explicit route using information provided by the management system, or the application, or from a routing table, and creates the label request message. The explicit