Abstract:
A method for optimization of traffic distribution in a communication network with multipath routing is provided. Distribution weightings are provided for a node of the communication network which has several alternatives or downward links for the routing to a target. The distribution weightings are modified relative to each other as a measure of the traffic loading on the downward links in order to reduce distributed traffic on highly loaded links and to increase distributed traffic on the less loaded links. An even traffic distribution on the communication network is thus achieved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2005/050087, filed Jan. 11, 2005 and claims the benefit thereof. The International Application claims the benefits of German application No. 102004003548.2 DE filed Jan. 23, 2004, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates to a method for improving traffic distribution in a communication network with multipath routing, said communication network being made up of nodes and links. The subject matter of the invention is of relevance to the field of network technologies, in particular the field of internet technology and switching technology. 
       BACKGROUND OF INVENTION 
       [0003]    So-called multipath routing plays an increasingly important role in packet-based networks, such as the IP (Internet Protocol) network, in particular. Multipath routing means that traffic to a destination is distributed over a number of routes or paths and forwarded to the destination thus. Multipath routing has the advantage that it is less susceptible to interference and frequently allows better traffic distribution. 
         [0004]    Easily the most widely used method for multipath routing in packet-based networks at present is the ECMP (Equal Cost Multi Path) method, based on the OSPF (Single Shortest Path Routing) protocol. In the context of this method a number of paths that are equivalent in the sense of a metric are defined to a destination and the traffic at a node is distributed equally to the output links leading to the destination. 
       SUMMARY OF INVENTION 
       [0005]    An object of the invention is to specify a method for optimizing traffic distribution in communication networks with multipath routing. 
         [0006]    This object is achieved by the independent claims. 
         [0007]    The invention is based on the idea of introducing distribution weightings for the distribution of traffic to a number of paths to a destination and adjusting these distribution weightings to achieve optimized traffic distribution. A distribution weighting is thereby a measure of the relative traffic load transported via a link, to which the distribution weighting is assigned. A communication network with multipath routing is thereby assumed, which is made up of nodes and links. Multipath routing thereby means that a node of the communication network has a number of outgoing links, which represent different possibilities for routing to a fixed destination. A destination is for example defined by an address or a set of addresses, with, in the case of a set of addresses, routing within the communication network being identical for said addresses. A destination can for example be defined by an edge node or edge router, to which all traffic or all data packets with specific addresses are routed. The communication network can in principle be a fixed network or a mobile network. 
         [0008]    According to the invention the distribution weightings for distribution of the traffic to the links that can be used for routing to the destination are adjusted according to the load or availability of the individual links. The load or availability is described by a parameter and, depending on the value said parameter has for a link, the distribution weighting of said link is increased or reduced in relation to the other distribution weightings. This parameter can for example be the absolute traffic load, the relative traffic load, which is also related to the link bandwidth, any traffic-dependent costs incurred with link usage, link availability, the transit time of traffic on the respective link or the load capacity of the end nodes of the respective link. 
         [0009]    The distribution weightings are adjusted such that distribution weightings of links with a higher parameter value are reduced in relation to the distribution weightings of the other links. If the parameter is defined for example by the traffic load on the respective link, the distribution weighting of a link that is more heavily loaded compared with the other links is reduced, i.e. less traffic is distributed to this link. This results in a redistribution of traffic from loaded links to less loaded links. The mean parameter value can be used as the reference point for the adjustment or modification of the distribution weightings. Depending on whether the parameter for a link has a positive or negative difference in respect of the mean value, the associated distribution weighting can be reduced or increased. This increase or reduction of distribution weightings can be carried out in proportion to the gap between the parameter for the respective link and the mean value. 
         [0010]    According to a development the distribution weightings are adjusted iteratively, with the distribution weightings being adjusted in each step. This iterative procedure can take place as follows:
       The distribution weightings are initialized with start values   A fixed number of iterations is carried out   The distribution weightings resulting after the number of iterations are used for routing to the destination in the communication network       
 
         [0014]    It can be expedient to use an attenuation variable that is a function of the number of the iteration when modifying the distribution weightings in the iterative method, resulting in a reduction in the modification of distribution weightings that increases with the number of iterations. This attenuation variable prevents situations such as the oscillation of a distribution weighting between two values. 
         [0015]    In one development of this iterative method the load on subsequent nodes is taken into account by the redistributed traffic. If during the first iteration the parameter is defined by the absolute traffic load or the relative traffic load related to the bandwidth, this can be achieved by modifying the value of the parameter for the next iteration after each iteration. The value of the parameter is then modified such that the impact of the redistribution of the traffic to subsequent nodes or links is taken into account. This modification can for example be achieved by adding a value to the parameter, which is defined by the traffic transported via the link in question to the destination, multiplied by a factor. This measure means that the traffic already transported via the respective link to the destination is taken into account. It counteracts an excessive increase in this element. If the level of all the traffic routed via a link is relatively low for example but the traffic routed to the destination makes up a large part of this, because a variable is added in proportion to the traffic routed via this link to the destination, the parameter modification means that the parameters for this value converge more quickly towards the mean value and less traffic is therefore redistributed to this link (the mean value must then be recalculated after every parameter modification). The fact that less traffic is redistributed to this link is expedient in respect of nodes or links after said link, the overall traffic load of which is not necessarily as low as that of the link in question. 
         [0016]    The method can be implemented for all nodes of the communication network, at which traffic distribution is carried out, such that traffic distribution is improved in the communication network as a whole. It is also expedient to implement the method not only for the routes to a destination but for all the different destinations within the network for routing. “Different destinations within the network” means that these destinations do not necessarily correspond precisely to the destination information used for routing the traffic. For example there are very many addresses on the internet, of which a number result in a routing within the communication network that is identical, i.e. has the same input and output nodes, in a communication network that is a sub-network of the internet. Routing for this number of addresses is expediently interpreted as a single destination in the context of the method. 
         [0017]    If the parameter is a measure of traffic loading, then the corresponding traffic loading should be known at the start of the method. The traffic volume within the network can for example be measured or calculated using the so-called traffic matrix, which shows how much traffic is to be carried between a source node and a destination node. The traffic volume within the network and therefore the traffic loading on the link can be redetermined in different phases during the method and used for further implementation of the method.
       In the case of the iterative procedure, the traffic volume can be redetermined after each iteration to modify the distribution weightings.   The traffic volume can be redetermined after determining the link costs for a node, before determining the link costs for the next node correspondingly.   The traffic volume can be redetermined, once the claimed adjustment of the links costs has been completed for all routes to a destination.   It is expedient to redetermine the traffic volume and to calculate the final traffic distribution in the network after completing the method and determining all link costs.       
 
         [0022]    The points at which and whether the traffic distribution should be recalculated during the method and used for the method are a function of the communication network, the topology of the communication network and the available computing power. The method can be implemented as software on routers, for example internet routers, which support Equal Cost Multi Path (ECMP). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0023]    The invention is described in more detail below in the context of an exemplary embodiment with reference to a FIGURE. The sole FIGURE illustrates an exemplary embodiment of an IP network having a plurality of nodes and links in accordance to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0024]    An IP network and ECMP multipath routing are assumed for the exemplary embodiment. At the start the ECMP protocol or OSPF protocol is used to calculate least-cost paths for routing within the network based on a metric. As with the ECMP method, for nodes that have two or more least-cost paths for routing that are equivalent in the sense of the metric, all or at least some of these least-cost paths are used for routing. With a number of alternative least-cost paths it is possible to limit the number of paths used, to ensure more regular conditions within the network. After calculating the paths, distribution weightings can be introduced and assigned initial values. The initial distribution weightings are set such that there is equal distribution to all possible paths. Expediently in the context of the method the distribution weightings are standardized to 1, such that the initial values for the distribution weightings at a node that has n path alternatives for a destination are equal to 1/n. 
         [0025]    In the context of the exemplary embodiment three loops are passed through. The outermost loop passes through all possible destinations for routing within the network. The second loop, which is a function of the destination, passes through all the nodes that are involved in routing to the respective destination. The third loop corresponds to an iterative modification of the distribution weightings for a specific node and a specific destination. The number of these iterations is for example 10 to 100. The traffic volume on the individual links within the network is used as the input for these iterations. This can be calculated by way of an example or by means of the traffic matrix based on known volumes of traffic going in and out at the network boundaries. The iterative adjustment of the distribution weightings is shown in more detail in the FIGURE. The FIGURE shows a node J and links, on which traffic to other nodes K 1 , K 2  and K 3  can be distributed to a specific destination. The distribution is effected according to the distribution weightings (W(J,K 1 ,D) . . . W(J,K 3 ,D). These distribution weightings are also a function of the respective destination D (outermost loop). These distribution weightings are adjusted as a function of the overall traffic transported via the respective link. This traffic is referred to as TRAF(K 1 ) . . . TRAF(K 3 ) (not shown in the FIGURE). The mean value of the traffic transported via the links to the nodes K 1  to K 3  is referred to as TRAF_AV. The new distribution weightings for K ∈ {K 1 , K 2 , K 3 } are then calculated as follows for each iteration: 
         [0000]        W ( J,K,D ) NEW   =W ( J,K,D ) OLD −(TRAF( K )−TRAF —   AV )/TRAF —   AV ×DELTA 
         [0000]    DELTA is thereby an expediently selected adjustment variable or attenuation variable, which is equal to 1: n_IT, where n_IT is equal to the number of the iteration. DELTA has the effect that modification of the distribution weightings is attenuated for the higher iterations, thereby preventing oscillations. With the above formula, the index K passes through the values K 1  to K 3 , i.e. the distribution weightings for the links leading away from the node J to the destination are adjusted. If a value of W(J,K,D) NEW &lt;0 results during the iteration, W(J,K,D)=0 is set. If W(J,K,D) NEW &gt;1 results, W(J,K,D)=1. W(J,K,D) are then standardized such that their sum is 1. The above formula produces a traffic redistribution between the links to the nodes K 1  to K 3 , which relieves the load on links with a high traffic volume and increases the load on links with a low traffic volume. Different link bandwidths can also be taken into account in the context of the exemplary embodiment. The relative traffic load on the links, in other words the traffic valve related to the link bandwidth, is then used instead of the absolute traffic. This makes it possible to take into account different link bandwidths in a simple manner. In the above formula the relative values TRAF(K)/B(K) related to the bandwidth B(K) are then used instead of TRAF(K) and TRAF_AF results as the sum over these relative values. 
         [0026]    According to a development the loading on subsequent nodes can also be taken into account as follows. To this end new values for TRAF(K) are calculated for every iteration, in that 
         [0000]      TRAF( K ) NEW =TRAF( K ) OLD +ALPHA× T ( K ),  K ∈ {K 1,  K 2,  K 3} 
         [0000]    is set. Alpha is thereby a factor between 0.5 and 2 and T(K) is the traffic of the node K already present to the destination. The values TRAF(K) new  are then used instead of the old values for the next iteration. The mean of the values TRAF(K) new  must then be similarly calculated for the next iteration. This development allows the loading of the subsequent nodes K 1  to K 3  to be taken into account by the redistribution, to prevent the local optimum of traffic distribution in the node J putting pressure on one of the nodes K 1  to K 3  due to the traffic redistribution. In other words the traffic of the individual nodes in the direction of the destination is taken into account. The modification in the context of this development means that nodes with a low overall traffic load are not loaded with too much new traffic in the direction of the destination, which must then be further distributed by the subsequent nodes. This modification also counteracts traffic to a destination being concentrated on one link that has a lower, optionally a significantly lower, level of traffic load than the other links.