Davide Cherubini - PhD Thesis - UniCA Eprints

2.3 Failure analysissimultaneously. In the following of this dissertation these different kinds of LSPswill be distinguished as “Implicit LSP” and “Explicit LSP”, respectively.The LSP can be a best-effort connection, in which case Label DistributionProtocol (LDP) or the earlier Tag Distribution Protocol (TDP) may be used.Alternatively, an LSP may request that bandwidth be reserved for its exclusiveuse.Once allocated, MPLS guarantees that the bandwidth is available for the entirepath. If the bandwidth is not available, then the connection request is refused.The LSP reserves bandwidth using either Resource Reservation Protocolwith Traffic Engineering extensions (RVSP-TE) or Constraint-based RoutingLDP (CR-LDP).To enable network availability, MPLS provides mechanisms to quickly findan alternate path if the primary path is no longer available (typically due to anode or link failure). This Fast Re-Route (FRR) capability is critical for allowingservice providers to offer high availability, high revenue SLAs.An MPLS router can manage multiple paths to each destination. This techniqueis known as “liberal label retention” and, requiring more memory and processorcycles, it is most appropriate for edge routers where the unique importanttask is storing labels for connections which originate/terminate on that router.Alternatively, the router may store only one path to each destination. This techniqueis called “conservative label retention” and, in the event of a failure, itrequires that the signalling protocol determines a new optimal path. This processcan take several seconds, since it may be necessary for the underlying IProuting protocols (typically iBGP, OSPF or IS-IS) to re-converge. To eliminatethis delay, it is possible to pre-define alternative IP paths through the network.2.3 Failure analysisThe backbone networks are usually well-engineered and properly provisioned,leading to very low packet losses and negligible queuing delays. This robust networkdesign is one of the reasons why the occurrence and impact of failures inthese networks have received little attention.11