The LDP PDU Format

THE CONSTRAINED-BASED ROUTING LABEL DISTRIBUTION PROTOCOL 157 Label abort, label withdraw, and label release messages An LSR A can send a label abort message to an LDP peer LSR B to abort an outstanding label request message. This might happen, for instance, if LSR A’s next hop for the FEC has changed from LSR B to a different LSR. An LSR A uses a label withdraw message to signal to an LDP peer LSR B that it cannot continue using a specific FEC-label mapping that LSR A had previously advertised. An LSR A sends a label release message to an LDP peer LSR B to signal to LSR B that LSR A no longer needs a specific FEC-label mapping that was previously requested of andor advertised by the peer.

7.2 THE CONSTRAINED-BASED ROUTING LABEL DISTRIBUTION

PROTOCOL CR-LDP CR-LDP is a label distribution protocol based on LDP. As described above, LDP can be used to set up an LSP associated with a particular FEC. CR-LDP is used to set up a unidirectional point-to-point explicitly routed LSP, referred to as the constrained-based routed label switched path CR-LSP. An LSP is set up as a result of the routing information in an IP network using the shortest path algorithm. A CR-LSP is calculated at the source LSR based on criteria not limited to routing information, such as explicit routing and QoS-based routing. The route then signaled to the other nodes along the path which obey the source’s routing instructions. This routing technique, referred to as source routing, is also used in ATM. A CR-LSP in MPLS is analogous to a connection in ATM, only it is unidirectional. The ATM signaling procedures will automatically set up a bidirectional connection between two ATM hosts, where each direction of the connection can be associated with different traffic and QoS parameters. A bidirectional CR-LSP between LSRs 1 and 2 can only be created by setting up one CR-LSP from LSR 1 to LSR 2 and a separate one from LSR 2 to LSR 1. As in the case of an LSP, a CR-LSP has an ingress and an egress LSR. CR-LSPs can be used in a variety of ways. For instance, they can be used in an IP network to do load balancing. That is, the traffic among its links can be evenly distributed by forcing some of the traffic over CR-LSPs, which pass through lesser-utilized links. CR-LSPs can also be used to create tunnels in MPLS, and introduce routes based on a QoS criterion, such as minimization of the total end-to-end delay, and maximization of throughput. For example, let us consider the MPLS network in Figure 7.9, and let us B A E C F D G Figure 7.9 An example of a CR-LSP. 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