US 20070195701 A1 Abstract The invention is related to a method for resilient multi-paths connections between edge devices of a communication network. First are determined connection-specific traffic distribution functions for the multi-paths depending on a plausible failure pattern of active and inactive paths of the multi-path of this connection. Further is selected the traffic distribution function for a multi-path depending on the current failure pattern of active an inactive paths of the multi-path of this connection and is distributed the traffic of the connection onto the path of the corresponding multi-path pursuant to the selected traffic distribution function. An essential advantage of the invention is a significant decrease of network capacity for resilient multi-path connections. Another advantage of the invention is the short reaction time in case of an outage of one or more path of the multi-path connections.
Claims(18) 1-16. (canceled) 17. A method for providing a resilient multi-path connection between edge devices of a communication network, comprising:
calculating connection-specific traffic distribution functions of the multi-path connection based upon path failure patterns of the multi-path connection; selecting a traffic distribution function of the multi-path connection based upon the calculated connection-specific traffic distribution functions and based upon current path failure pattern of the multi-path connection; and distributing traffic into corresponding paths of the multi-path connection based upon the selected traffic distribution function. 18. The method as claimed in 19. The method as claimed in 20. The method as claimed in 21. The method as claimed in 22. The method as claimed in 23. The method as claimed in 24. The method as claimed in 25. The method as claimed in 26. The method as claimed in 27. The method as claimed in f _{g}(s)=Φp ^{0};_{g} ,s) . . . Φ(p _{g} ^{kg−1 } ,s)^{τ} withΦ(p,s) is a function indicating whether a partial (single) path p is active or inactive if a failure scenario s (set of failed network elements) occurs, and F _{g }is the failure pattern comprising a set of active and inactive partial path p_{g} ^{i }within the multi-path P_{g }for a connection g between two edge devices kg is the maximum number of partial paths p _{g} ^{i }within the multi-path P_{g}, i.e., 0<=i<kg. 28. The method as claimed in 29. The method as claimed in S is a set of all failure scenarios,
Gs is a set of all active aggregate connections in case of failure scenario s,
L
_{g}(f) is a load distribution function based upon a failure pattern f, P
_{g} ^{τ}lg(f)*c(g):^{τ} transposes a proceeding vector, this computes a vector with a fraction of a traffic rate c(g) caused by the multi-path P_{g }and a rate c(g) of a aggregate connection g, b is a vector with a capacity of links which are larger than corresponding traffic rates caused by any failure scenario and
30. The method as claimed in S is a set of all failure scenarios,
Gs is a set of all active aggregate connections in case of failure scenario s,
L
_{g}(f) is a load distribution function based upon a failure pattern f, p
_{g} ^{τ}lg(f)*C(g):^{τ} transposes a proceeding vector, this computes a vector with a fraction of a traffic rate c(g) caused by the multi-path P_{g }and a rate c(g) of a aggregate connection g, b is a vector with a capacity of links which are larger than corresponding traffic rates caused by any failure scenario and
31. An edge device for providing a resilient multi-path connection between edge devices of a communication network, comprising:
a calculator that calculates connection-specific traffic distribution functions of the multi-path connection based upon path failure patterns of the multi-path connection; a selector that selects a traffic distribution function of the multi-path connection based upon the calculated connection-specific traffic distribution functions and based upon current path failure pattern of the multi-path connection; and a distributor that distributes traffic into corresponding paths of the multi-path connection based upon the selected traffic distribution function. 32. The edge device as claimed in 33. A computer program product for providing a resilient multi-path connection between edge devices of a communication network, comprising:
a program subroutine for calculating connection-specific traffic distribution functions of the multi-path connection based upon path failure patterns of the multi-path connection; Description This application is the US National Stage of International Application No. PCT/EP2004/052034, filed Sep. 03, 2004 and claims the benefit thereof. The International Application claims the benefits of European Patent application No. 03020840.9 filed Sep. 13, 2003 and European Patent application No. 03014809.2 filed Sep. 03, 2003, all of the applications are incorporated by reference herein in their entirety. The inventive method is related to multi-path connections between edge devices in a communication network. The method is preferred provided for MPLS (Multi-Protocol Label Switching) communication networks. Carrier grade networks are expected to provide a high degree of availability although of in the networks elements can fail. This challenge arises, e.g., for virtual private networks or in the terrestrial radio access network of the universal mobile telecommunication system (UMTS). In contrast Internet Protocol (IP) technology enables a global interconnection of computer controlled devices; e.g.; hosts, servers and terminals, on a unreliable best effort basis. In time division networks, e.g., telephone networks, the reliability is ensured by hardware redundancy of the different network elements. Packet-oriented networks, e.g. the IP network, are protected against link failure by backup links or by ring shaped networks on the physical layer, e.g. SDH rings. However, these methods require a backup capacity which comprises at least the capacity of the failed network elements. In packet-switched networks, a high degree of reliability can be achieved by traffic deviation over alternative paths in case of local outages. The backup capacity may be shared among different traffic aggregates in different failure scenarios. Therefore, the required backup capacity can be reduced without compromising the failure resilience of the network. For the routing of the packets in packet switched networks there are two different routing methods. The routes in a destination address based routing used in the internet protocol (IP) are usually set up by the routing protocol like the open shortest path first protocol and the destination address transmitted in protocol header. Load balancing over multiple paths is possible if several routes to the same destination reveal the same costs. The traffic in connection-oriented routing used in MPLS networks is forwarded along virtual connections whereby each virtual connection is assigned a label. The virtual connections including the label are established or set up before the forwarding of the packets and the routes can be chosen arbitrarily, e.g. using an explicit route object in MPLS. In packet-switched networks like IP and MPLS networks, traffic is deviated over alternative paths in case of a local outage. There are basically two options for resilient mechanisms. With local path restoration in case of MPLS networks or with local rerouting in case of IP technology, a deviation path or route is only activated if a failure of the local network element is indicated. I.e., in IP networks a link failure is detected and indicated by missing hello messages of the open shortest path first protocol (OSPF). Backup capacity can be shared because no resources are bound to any aggregate before the failure is indicated. However, the reaction time of restoration mechanism especially for networks with high transmission rates is to long. To avoid long reaction time with path protection, the primary path failure is anticipated, i.e., a back up path is set up before a failure is indicated, whereby the traffic is transmitted simultaneously over the primary and the backup path. In this case a backup capacity sharing is not possible. A new approach for routing in packet-switched networks is multi-path routing. In the source network element, the traffic is distributed over the several paths wherein the paths are established connection-oriented. The distributed traffic is transmitted parallel over the paths and reassembled at the destination network element. If an outage of one or more network elements is indicated, i.e., if an inactive condition of the local network elements is indicated, the traffic is deflected to the remaining active paths. In the discussed networks, the backup capacity for resilience is not used effectively. Therefore, it is an object of the invention to amend the effectiveness of the resources in the well-known resilient networks especially in connection-oriented networks wherein the resilience of the networks should be maintained. According to the invention, a novel method for multi-path connections is provided for resilience of paths in communication networks. The method includes a calculation of connection-specific distribution functions for the multi-paths depending on path failure patterns of active an inactive paths of the multi-path of a specific connection. Furthermore, the selecting of the traffic distribution function is provided depending on the current path failure pattern and the distribution of the traffic pursuant to the selected traffic distribution function. An essential advantage of the invention is a significant decrease of network capacity for resilient multi-path connections. Another advantage of the invention is the short reaction time in case of an outage of one or more path of the multi-path connections. Preferable, the calculation of the connection-specific distribution functions for the multi-paths further depends on the topology of the communication network, the routes of the paths of the multi-path through the communication network, and both the expected traffic between the edge devices and the available link capacities. Additional, parameters may be used for this calculation to take constraints for different technologies, networks, or characteristics of determined routes into account. Pursuant further aspects of the invention, the paths of the multi-paths are logically or physically or link or node disjointed. Of particular importance are physically disjointed paths in multi-path connections because in case of a local outage of network elements the number of affected path is minimized. However, the computation of physically disjoint paths in multi-path connections depends on the topology of the network and can not always be achieved. Therefore, not all the paths of a mulit-paths may be disjoint. The sole FIGURE shows a Multi-Protocol Label Switching communication network connection with edge devices. The inventive method is related to multi-path connections between edge devices in a communication network. The method is preferred provided for MPLS (Multi-Protocol Label Switching) communication networks. MPLS equipped with edge routers ER—see FIGURE. The edge routers ER are respectively connected to terminals T. In FIGURE, between two edge routers ER a multi-path connection mc which consists of three paths p For this embodiment is assumed that multi-path connections mc are established between the depicted edge routers ER are established multi-path connections with a different number of paths p. The number of paths and the sequence of transit router TR and links L depend on the topology of the MPLS network and the expected traffic transmitted over the paths p of the network. According to the invention a calculation of connection-specific distribution functions for the multi-paths depending on path failure pattern of this multi-path connection mc is provided. In the following, the computation of the traffic distribution function is described: Basic Notation: Let X be a set of elements, then X The scalar multiplication c·v and the transpose operator Links and Nodes: The Network N=(V,E) consists of n=|V| nodes and m=|E| unidirectional links that are represented a unit vectors v The links are directed and the operators α(e The indience matrix Aε−1,{0,1} Traffic, Matrix, Paths, and Flows: The Matrix: The aggregate of all flows from an ingress router vi to an egress router v Path: A path p While cycles containing only inner nodes can be easily removed, cycles containing the start and the end node of a path are more problematic. Therefore, it is formulated a condition preventing this case. The expression v Flows: The mere path of an aggregate gεG is p Protected scenarios: A protected failure scenario is given by a vector of failed nodes s Traffic reduction: During normal operation without any failure, all aggregates gεG are inactive. If routers fail, some may disappear. There are to consider several options. No Traffic Reduction: It is assumed that failed routers lose only their transport capability for transit flows but still able to generate traffic. Therefore G Source Traffic Reduction: An aggregate flow is removed from the traffic matrix if the source node v Full traffic reduction: In contrast to above it is assumed that the traffic with a failed destination is stalled. An aggregate flow is removed from the traffic matrix if a node fails which either the source or the destination of a flow, hence
Failure indication function: The failure indication function Φ(p,s) indicates whether a path p is affected by a failure scenario s. Path p is affected by a link failure scenario s Protection alternatives: A path restoration scheme introduces a backup path q Objective function and capacity constrains: It is described the capacity of all links by a vector of edges bε(R No bandwidth reuse: In optical networks, physical resources like fibers, wavelengths, or time slots are bound to connections. If a network element fails, there might not be enough time to free the resources of a redirected connection. This is respected by the following capacity constraints:
Optimal solution summary: The free variables to be set by the optimization are
Both the primary path p Unfortunately, the path protection constraints (equation (1.11)) and the equation (1.12) are quadratic with respect to the free variables. Therefore, this description can not be solved by LP (linear program) solvers. In addition, the failure indication function Φ (p,s) cannot be transformed into a linear mapping. Thus, there is no efficient algorithm to compute the desired structures p Heuristic for path calculation: Due to the computational problems and due the difficulty of controlling the structure of multi-paths first should be calculated a suitable path layout and then should be derivate a suitable traffic distribution function. Then is calculated a link and a node disjoint multi-path structure by using an algorithm to compute the k disjoint shortest paths (kDSP). Another heuristic tries to place a primary path in preferred way for PP methods. If a primary path is given, the kDSP algorithm may be used for the computation of a link and a node disjoint multi-path for backup purposes. Another option is the computation of an optimal path layout together with traffic distribution function. This method yields a general multi-path and is, therefore, not suitable in practice. The k disjoint shortest path algorithm: Both PP method and the inventive approach require disjoint multi-paths for their path layout. A very simple solution to get disjoint paths is taking the shortest p which can be found by Dijkstra's algorithm, removing its interior node te(p) and links tr(p) from the network running Dijkstra's algorithm again. However, this procedure does not always find k disjoint paths in the network although the might be topology feasible. In contrast to online solutions, the k disjoint shortest path (kDSP) offline algorithm finds always up to k disjoint shortest path in the network if they exist. These paths may be taken as the equal paths of an safe protecting mult-path. If they are taken for layout of path protection mechanism, the shortest one of them should become the primary path and the other paths constitute the muliti-path for backup purposes. Primary path computation: minimum traffic routing: With path protection the primary path plays a distinguished role. If a network element carries a large amount of traffic and fails, this traffic has to be redistributed and requires a lot of backup capacity near the outage location. Therefore, a path layout is constructed that entails a minimum traffic load on each network element. Minimum traffic constraints: The overall traffic on all links is giver by the auxiliary vector a The value f(g) may set to 1 of only the number of aggregates is to be minimized or it may be set to c(g) if their rate should be taken into account. In the embodiment is used f(g)=c(g). Objective Function: Both the maximum traffic per network element and the overall capacity (1 The constants M Path constraints: Like above, the flow conservation rule (equation (1.5)) and the exclusion of start and end nodes from cycles (equation(1.6)) have to be respected. For a single-path solution p Therefore, p Backup path computation with kDSP: A set of disjoint single-path is required to build a backup path for a given primary path p Computation of an optimum backup path: If a primary path is given, the optimum backup path together with the corresponding traffic distribution function can be obtained by a slight modification of the LP formulation. As p For the computation of disjoint multi-paths the kDSP algorithm is used which is simple and efficient to compute. However, it does not take general into account which is a different and a hard problem. Basically, the kDSP heuristic can be substituted by any other routing scheme yielding disjoint multi-path. Computation of the traffic distribution function: If the path layout for a safe protection multi-path or a path protection mechanism is given, a suitable traffic distribution function is required. First are presented some basics for failure-dependent traffic distribution and then are derived three different traffic distribution mechanism for safe protection multi-path. Finally, an adaptation to path protection is presented. Basics for failure-dependent traffic distribution: A self protecting multi-path consists kg link an (not necessarily) node disjoint paths (expect for source and destination) p Path failure pattern f The path failure pattern is defined fg(s)ε{0,1} Traffic distribution function: For all aggregates gεG, a traffic distribution function l Furthermore, failed paths must not be used, i.e.
Finally, the vector indicating the transported traffic aggregate g over all links is calculated by P Equal traffic distribution: The traffic may be distributed equally over all working paths, i.e.
Reciprocal traffic distribution: The traffic distribution factors may be indirectly proportional to the length of the partial path (1 Optimized traffic distribution: Traffic distribution is optimal if the required capacity b to protect all aggregates gεG in all protected failure scenarios sεS is minimal. The free variables are
The objective function is given by equation (1.13). The traffic distribution function constraints in equation (1.21) and (1.22) must be respected by all l Bandwidth constraints with capacity reuse: The capacity must be large enough to accommodate the traffic in all protected failure scenarios sεS:
Bandwidth constraints without capacity reuse: Releasing capacity unnecessarily leads to waste of bandwidth if it cannot be reused by other connections. Therefore, traffic distribution factor l are removed due to router failure. Thus, the bandwidth constraints are
Note that the term f Adaptation of path protection: The adaptation of the above explained traffic distribution scheme to path protection mechanism is simple. The primary path pg is denotes together with its disjoint backup single-paths as safe protection multi-path P The approaches above for finding the structure—in our case linear programs—have to be taken also only as a favoured realization. It might be reasonable to use other or additional constraints that reflect additional networking side conditions. If the network becomes large, the solution approach with linear programs can fail due to the computational expenses. Then, faster heuristics (e.g. genetic algorithms or simulated annealing) must be applied to find the layout of the routing structures as well as a suitable load balancing to achieve suboptimal results. Apart from that there is a related problem that can be solved by the proposed forwarding paradigms. In the description above we took for the sake of simplicity a network topology, configured the structures, and dimensioned the link sizes of the network such that no quality of service (QoS) degradation is observed if traffic is rerouted in a failure case. The structures—primary path together with backup multi-path or SPM—can also be applied if the network topology is given together with the link capacities. Then, different algorithms approaches have to be taken to configure the forwarding structures and to maximize the amount of traffic that can be transported with resilience guarantees in the network. Above, the traffic amount is given and the required capacity is minimized while here link sizes are fixed and the amount of traffic is maximized. These algorithms and approximative heuristics are necessary to make best use of the network capacity in combination with our forwarding structures. Referenced by
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