QoS Online Routing and MPLS Multilevel Protection : A Survey

A survey of MPLS protection methods and their utilization in combination with online routing methods is presented in this article. Usually, fault management methods pre-establish backup paths to recover traffic after a failure. In addition, MPLS allows the creation of different backup types, and hence MPLS is a suitable method to support traffic-engineered networks. In this article, an introduction of several label switch path backup types and their pros and cons are pointed out. The creation of an LSP involves a routing phase, which should include QoS aspects. In a similar way, to achieve a reliable network the LSP backups must also be routed by a QoS routing method. When LSP creation requests arrive one by one (a dynamic network scenario), online routing methods are applied. The relationship between MPLS fault management and QoS online routing methods is unavoidable, in particular during the creation of LSP backups. Both aspects are discussed in this article. Several ideas on how these actual technologies could be applied together are presented and compared. QoS Online Routing and MPLS Multilevel Protection: A Survey Authorized licensed use limited to: UNIVERSITAT DE GIRONA. Downloaded on April 23,2010 at 12:18:33 UTC from IEEE Xplore. Restrictions apply. IEEE Communications Magazine • October 2003 127 activated. This section is merely an introduction to different types of LSP backups and their notification methods. Global Repair Model — In this model, an ingress node is responsible for resolving the restoration as the fault indication signal (FIS) arrives. This method needs an alternate disjoint backup path for each active path (working path). Global protection is always activated at the ingress node, irrespective of where the failure occurs along the working path. This means that failure information has to be propagated all the way back to the source node before a protection switch is activated. If no reverse LSP is created, the fault indication can only be activated as a result of failure of a path continuity test. Figure 1 shows a simple network formed by six label switch routers (LSRs) where a working path (WP = 1-3-5-6,1 solid line) and a global recovery path (GRP = 1-2-4-6, dashed line) are pre-established. In normal operation, traffic from ingress router LSR1 to egress router LSR6 is carried through the LSP working path. When a link fault is detected (e.g., between LSR3 and LSR5), traffic is switched to the LSP recovery path. The ingress node (LSR1) must be a protection switch label (PSL) to be able to switch the traffic between the working path and the recovery path. A PSL is the transmitter for both the working path traffic and its corresponding backup path traffic. It is the origin of the backup, but does not necessarily have to be an ingress node (see local repair below). A path merge LSR (PML) receives both working path traffic and its corresponding backup path traffic, and merges their traffic into a single outgoing path. As is the PSL, the PML is the destination of the recovery path, but may or may not be the destination of the working path [5]. This method has the advantage of setting up only one backup path per working path. On the other hand, this method has high cost (in terms of recovery time), especially if a path continuity test is used as the fault indication method. LSP Segment Restoration (Local Repair) — The aim of local repair is to protect a part of the working path against a link or node failure. In the local repair method, the restoration procedure simply starts from the point of failure. The protection is activated by an LSR with a PSL function along the path to a PML LSR. Figure 2 illustrates this case. As in the global model, a working path (WP = 1-3-5-6, solid line) and local recovery path (LRP = 3-4-5) are now preestablished. When a link failure occurs (3-5), LSR3, which is a PSL node, switches traffic from broken segment (3-5) to the recovery path. At the end of the RP in the PML node (LSR5), traffic is merged to the WP. Therefore, traffic is forwarded through path (1-3-4-5-6), which is larger than the recovery path in the global model. However, in normal utilization the allocated resources for the recovery path are less (i.e., RP 3-4-5 for local repair vs. RP 1-2-4-6 for global repair). This method presents the drawback of configuration of multiple backup segments (wherever protection is required), and the a priori reservation of resources leads to inefficient utilization of resources.. On the other hand, local repair offers transparency to the ingress node and faster restoration time than global mechanisms. Reverse Backup —This method can reverse traffic at the point of failure of the protected LSP back to the source switch of the protected path (ingress node) via a reverse backup LSP. As soon as a failure along the protected path is detected, the LSR at the head of the failed link reroutes incoming traffic by redirecting this traffic into the alternative LSP and traversing the path in the opposite direction to the ingress node of the LSP. Figure 3 shows an example of reverse backup utilization. LSP working and recovery paths are established as in the global model; in addition, there is a reverse recovery path ( RRP = 3-1) that reaches the ingress node. When a link failure is detected in link (3-5), the traffic is switched back to LSR1 (ingress node) through the reverse backup LSP, and then carried through the LSP recovery path as in the global model. This method is suitable in network scenarios Figure 1. The global repair model, backup LSP utilization. LSR2