Abstract:
One embodiment of the present invention provides a system that provides a guaranteed level of availability in a communication network. During operation, the system receives a connection request that includes a specified reliability. Next, the system determines multiple candidate routes through a network to satisfy the connection request, wherein each of these candidate routes passes through a set of links that form a path through the network. The system then examines the candidate routes to identify a route that meets the specified reliability. The system only accepts the connection request if a route that meets the specified reliability is found.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to communication networks. More specifically, the present invention relates to a method and an apparatus for providing a guaranteed level of availability in a communication network. 
     2. Related Art 
     Network communication technologies provide for a wide range of interconnections and bandwidths. For example, wavelength-division multiplexing (WDM) technology enables a single optical fiber to support over one hundred wavelength channels, each of which can operate at a bandwidth of several gigabits per second (Gbps). Providing this amount of capacity through a single optical fiber has significant advantages, such as greatly reduced cost. However, a failure in such an optical network, e.g., a fiber cut or fiber conduit cut, can lead to the loss of a huge amount of data (several terabits per second (Tbps) to several petabits per second (Pbps)), and can result in penalties under a service level agreement. 
     When a connection through the WDM network is requested, a service provider and a client typically enter into a service level agreement (SLA), which provides a specified service level to the client, and provides penalties on the provider if the service level is not met. The service provider receives compensation based, inter alia, upon the service level requested and the level of penalties specified in the SLA. Upon acceptance of the SLA by both provider and client, the circuit is provisioned. 
     In the traditional connection-provisioning scheme, a shortest path (based on the link cost) between the source node and the destination node is chosen to route a given connection request. Depending on different traffic-engineering considerations, different cost functions can be applied to network links, such as a constant  1  for each link (to minimize hop distance), the length of the link (to minimize delay), the fraction of the available capacity on the link (to balance network load), etc. However, this traditional connection-provisioning scheme is unaware of any connection-availability requirements. Consequently, the route computed by a shortest-path algorithm may not satisfy the SLA, because the route may not be reliable enough. 
       FIG. 1  illustrates a shortest path through a WDM network, which does not meet the availability requirement of an SLA. A connection request is made from node  0  to node  5  and this request specifies an availability requirement of 95%. In  FIG. 1 , the integers next to the links represent the link costs. In this example, the route ( 0 ,  3 ,  4 ,  5 ), route  1 , is indicated by the dashed lines and is the least-cost path (cost=3). 
     Unfortunately, this least-cost path does not consider connection availability requirements. Referring to  FIG. 1 , the percentages next to the links represent the estimated availability of each link. Note that each of the links has an availability of 99%, except for the link between nodes  4  and  5 , which has an availability of 90%. This low availability can be the result of a number of factors, such as old fibers, fibers routed through areas of heavy construction, etc. The estimated availability of route  1  is the product of the availabilities of the links that the connection traverses. For route  1 , the availability is 88.21%, which is less than the required availability in the SLA. 
     Hence, what is needed is a method and an apparatus for providing a guaranteed level of availability in a communication network. 
     SUMMARY 
     One embodiment of the present invention provides a system that provides a guaranteed level of availability in a communication network. During operation, the system receives a connection request that includes a specified reliability. Next, the system determines multiple candidate routes through a network to satisfy the connection request, wherein each of these candidate routes passes through a set of links that form a path through the network. The system then examines the candidate routes to identify a route that meets the specified reliability. The system only accepts the connection request if a route that meets the specified reliability is found. 
     In a variation of this embodiment, the system examines the candidate routes to identify an adequate route with a minimum reliability that exceeds the specified reliability. 
     In a further variation, the system examines the candidate routes to identify a most-reliable route. 
     In a further variation, identifying the most-reliable route involves first obtaining a reliability value for each link in the candidate routes. These reliability values are derived from statistical data for the communication network. The system then computes a cost for each link by computing the negative logarithm of the reliability value for each link. The system uses these costs in a shortest-path computation to determine a least-cost route for the connection request. This least-cost route is the most reliable route. 
     In a further variation, the system uses multiple independent routes to meet the specified reliability. 
     In a further variation, the system uses multiple links between specified nodes on the route to meet the specified reliability. 
     In a further variation, the system uses multiple sub-paths between a source and a destination on the route to meet the specified reliability. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates a shortest path through a WDM network, which does not meet the availability requirement of an SLA. 
         FIG. 2  illustrates a highest availability path through a WDM network in accordance with an embodiment of the present invention. 
         FIG. 3  illustrates availability values for links of a WDM network in accordance with an embodiment of the present invention. 
         FIG. 4  illustrates multiple independent paths through a WDM network in accordance with an embodiment of the present invention. 
         FIG. 5  illustrates multiple links between nodes of a WDM network in accordance with an embodiment of the present invention. 
         FIG. 6  illustrates multiple sub-paths through a WDM network in accordance with an embodiment of the present invention. 
         FIG. 7  presents a flowchart illustrating processing of a connection request in accordance with an embodiment of the present invention. 
         FIG. 8  presents a flowchart illustrating the process of computing the route with the highest availability through a WDM network in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     The data structures and code described in this detailed description are typically stored on a computer readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. This includes, but is not limited to, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs) and DVDs (digital versatile discs or digital video discs), and computer instruction signals embodied in a transmission medium (with or without a carrier wave upon which the signals are modulated). For example, the transmission medium may include a communication network, such as the Internet. 
     Highest Availability Path 
       FIG. 2  illustrates a highest availability path through a WDM network in accordance with an embodiment of the present invention. The network illustrated in  FIG. 2  includes the same path availabilities as  FIG. 1 . The route through nodes  0 ,  1 ,  2 ,  5 , route  2 , is the path with the highest availability and is indicated by the dashed lines. The estimated availability of this route is the product of the availabilities of the individual links. In this case the estimated availability of this route is 97.03%, which is greater than the availability of 95% specified in the SLA. Route  2 , therefore, provides an acceptable path through the network, which meets the SLA guarantee. 
     Availability Values 
       FIG. 3  illustrates availability values for links of a WDM network in accordance with an embodiment of the present invention. The network in  FIG. 3  is identical to the networks of  FIGS. 1 and 2 . The availability values, however, have been converted into the negative logarithm of the availability percentages. This allows the use of a shortest path computation, such as Dijkstra&#39;s technique, to find the highest availability route between the source node and the destination node. Dijkstra&#39;s technique is well known in the art and will not be described herein. Alternatively, the path lengths can be calculated for each possible path between the source node and the destination node, and then ordered from shortest to longest. The system can then choose a path through the network that just meets the availability requirement. Doing this may save the path with the highest availability for a connection with more stringent availability requirements, or for a connection that provides higher penalties for not meeting the specified availability. 
     Multiple Independent Paths 
       FIG. 4  illustrates multiple independent paths through a WDM network in accordance with an embodiment of the present invention. Path  402  is the primary path between a source node and a destination node in the WDM network. The system may not be able to meet reliability requirements in the SLA using only path  402  because links in the best path have an estimated availability below what is required. In the example illustrated in  FIG. 4 , the system can select additional paths, such as paths  404  and  406 , to supply the required availability. 
     Multiple Links 
       FIG. 5  illustrates multiple links between nodes of a WDM network in accordance with an embodiment of the present invention. Path  502  is the primary path between a source node and a destination node in the WDM network. Path  502  includes link  504  between nodes  1  and  2 , which may be a low reliability link, causing path  502  to have insufficient reliability to meet the availability requirement of the SLA. Link  506  can be added between nodes  1  and  2  to provide the availability stated in the SLA. Note that more than one additional link can be provided between nodes  1  and  2 . 
     Multiple Sub-paths 
       FIG. 6  illustrates multiple sub-paths through a WDM network in accordance with an embodiment of the present invention. Path  602  is the primary path between a source node and a destination node in the WDM network. Path  604  provides an alternate path between the source node and node  2 . Path  606  provides an alternate path between node  1  and the destination node. Providing these secondary paths can provide sufficient availability between the source node and the destination node under most conditions encountered on the WDM network. Note that this technique of providing a secondary path between alternating nodes can be extended for longer paths. 
     Processing a Connection Request 
       FIG. 7  presents a flowchart illustrating processing of a connection request in accordance with an embodiment of the present invention. The system starts when a connection request is received that includes a specified availability in the SLA (step  702 ). Next, the system determines possible candidate routes through the WDM network (step  704 ). The system then attempts to identify a route that meets the specified availability (step  706 ). 
     If a route exists that meets the specified availability (step  708 ), the system accepts the connection request and provisions the route (step  710 ). If no satisfactory route exists at step  708 , the system denies the connection request (step  712 ). 
     Computing the Route with the Highest Availability 
       FIG. 8  presents a flowchart illustrating the process of computing the route with the highest availability through a WDM network in accordance with an embodiment of the present invention. The system starts when a connection request is received that includes a specified availability (step  802 ). Next, the system modifies the link cost on each network path to be link cost-LOG (availability) (step  804 ). The system then computes the least-cost route using the modified link cost in a shortest-path technique such as Dijkstra&#39;s technique (step  806 ). Note that steps  804  and  806  can be pre-computed prior to receiving the connection request. 
     Next, the system determines if the least-cost route satisfies the specified availability (step  808 ). Note that in an alternate embodiment, the system chooses a route that meets the specified availability but is not the least-cost route. If the least-cost route satisfies the specified availability, the system accepts the connection request (step  810 ). Otherwise, the system denies the connection request (step  812 ). 
     The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.