QoS routing granularity in MPLS networks

Abstract

This study investigates how constraint-based routing decision granularity significantly affects the scalability and blocking performance of QoS routing in an MPLS network. Coarse granularity, such as per-destination, has lower storage and computational overheads but is only suitable for best effort traffic. On the other hand, fine granularity, such as per-flow, provides lower blocking probability for bandwidth requests, but requires a huge number of states and high computational cost. To achieve cost-effective scalability, this study proposes using hybrid granularity schemes. The overflowed cache of the per-pair/flow scheme adds a per-pair cache and a per-flow cache as the routing cache, and performs well in blocking probability. The per-pair/class scheme groups the flows into several paths using routing marks, thus allowing packets to be label-forwarded with a bounded cache. INTRODUCTIONThe Internet provides users diverse and essential quality of service (QoS), particularly given the increasing demand for a wide spectrum of network services. Many services, previously only provided by traditional circuit-switched networks, can now be provided on the Internet. These services, depending on their inherent characteristics, require certain degrees of QoS guarantees. Many technologies are therefore being developed to enhance the QoS capability of IP networks. Among these technologies, differentiated services (DiffServ) [1][2][3] and multiprotocol label switching (MPLS) [4][5][6] are paving the way for tomorrow's QoS services portfolio.DiffServ is based on a simple model where traffic entering a network is classified, policed, and possibly conditioned at the edges of the network, and assigned to different behavior aggregates. Each behavior aggregate is identified by a single DS codepoint (DSCP). At the core of the network, packets are fast forwarded according to the per-hop behavior (PHB) associated with the DSCP. By assigning traffic of different classes to different DSCPs, the DiffServ network provides different forwarding treatments and thus different levels of QoS.MPLS integrates the label swapping forwarding paradigm with network layer routing. First, an explicit path, called a label switched path (LSP), is determined, and established using a signaling protocol. A label in the packet header, rather than the IP destination address, is then used for making forwarding decisions in the network. Routers that support MPLS are called label switched routers (LSRs). The labels can be assigned to represent routes of various granularities, ranging from as coarse as the destination network down to the level of each single flow. Moreover, numerous traffic engineering functions have been effectively achieved by MPLS. When MPLS is combined with DiffServ and constraint-based routing, they become powerful and complementary abstractions for QoS provisioning in IP backbone networks.Constraint-based routing is used to compute routes that are subject to multiple constraints, namely explicit route and QoS constraints. Explicit ...