A New Forwarding Policy for Load Balancing in Communication Networks

Transcription

1 A New Forwarding Policy for Load Balancing in Communication Networks Martin Heusse Yvon Kermarrec ENST de Bretagne BP 83, 985 Brest Cedex, France Abstract We present in this paper a distributed path selection procedure which objective is to balance the load in the network. It is used here in a connection oriented network and is based on selecting, at each hop along a partially computed path, any one of the available routes to the destination. As far as we know, our approach is the first distributed multi-path routing algorithm guaranteed to be loop-free after convergence, while at the same time basing its route computation on the actual load on the links of the network, as opposed to using some administrative, static, metrics. KEYWORDS: distributed routing, load balancing, network engineering. 1. Introduction In the context of the ever increasing traffic load observed in the communication networks, load balancing is a much desired feature of routing policies. Load balancing consists in distributing the load as much as possible on the network in order to delay the occurrence of congestion. On the contrary, while usual shortest path routing aims at limiting network resources consumption per flow, it does not naturally lead to load balancing. Besides, the new routing (mainly) connection-oriented frameworks such as MPLS (Multi Protocol Label Switching) or ATM are particularly tolerant to such propositions. So we propose here a new routing approach that achieves load balancing, while improving the connection establishment procedure. This routing approach is fully distributed and is a straightforward add-on to the usual routing protocols deployed on the IP or ATM networks. It is the application, in a different context, of previous investigations concerning routing and it relies on the use of a composite metric that takes into account both the path load and its length. This combination allows the routing algorithm to take into account the network load and to balance it fairly, while at the same time insuring that the connections follow valid (loop-free) paths. This paper is organized into 4 sections: section refines the context of this study and reviews current approaches. Section 3 presents the proposed algorithm while section 4 details its benefits. Section 5 concludes the paper and highlights the potentials of our approach. Approaches to the load balancing problem: basic schemes.1. A formulation of the load balancing problem Routing in a communication networks can be represented as a minimal cost multi-flow problem that consists in admitting as many flows as possible on a network while, at the same time, minimizing the global resource usage on the network. In other words, it consists in minimizing the expression: where is the number of flows on the network, is the set of uni-directional links with associated costs, is the contribution of the flow to the traffic on, and, an arbitrary cost used to take into account the fact that the traffic could not be accepted on the network. This formulation was proposed by Gondran and Minoux [3], and they showed that it is a linear programming problem. Unfortunately, in practice, the resolution of this problem is of limited use mainly for two reasons: The matrix of traffic demands cannot be determined a priori on the communication networks, and this lack forbids obtaining a useful formulation of the problem in order to optimally route the traffic. Slight shifts in traffic demands or network resources can cause significant changes to the computed routing, (1)

2 which is undesired from the network administration point of view. So it appears that proper routing of the data traffic on the networks requires approximation (or heuristic) techniques in order to produce satisfying load handling. Among these techniques, the Widest-Shortest and Shortest-Widest routing schemes have been proposed by Qingming and Steenkiste [9], and they consist in selecting, either from among the shortest paths the one with the most available bandwidth, or among the paths with the most available bandwidths, the shortest one. These two approaches are related to the minimum cost multi-flow problem in the sense that they intend to use as efficiently as possible the network resource by balancing the load on the network, while at the same time putting the emphasis on the shortest paths in order to preserve the network resource, as it is expected that this leads to accepting more traffic on the network overall. present article follows a similar approach, by basing the selection of a route on its length and its load as described in section The IP networks context The first scheme proposed for load balancing in IP networks, namely ECMP (Equal Cost MultiPath), can be found in the OSPF proposition by Moy [7], and the problem encountered is relevant of the peculiar nature of IP networks. It states that if multiple equal cost routes exist between the current router and the destination, they can all be used. Unfortunately, using (randomly) any one of multiple routes interferes with the TCP protocol and it is necessary to deal with this problem through the implantation of various mechanisms. 1 The solution, also used by Villamizar [1] with OMP (Optimized MultiPath), relies on the preservation of a stable route for each TCP flow on the network. It bases the routing decision, taken at each router, on a key which is computed as the image by a hash function calculated from the source and destination addresses and TCP ports of the packets. ECMP and OMP were both proposed to relax shortest path routing in IP networks, which identifies only one single path between any pair of nodes, no matter what the load on this path is. It is well known that this practice greatly increases the impact of congestions and does not achieve load balancing. As ECMP only proposes the usage of alternate routes with exact equal costs, it is mainly aimed at coping with parallel links in the network. It provides an even split of the traffic between the two links in this case. OMP is a relaxation of the alternate route election criterion where not only the equal cost routes are considered, but also routes 1 as described in the current IETF drafts related to ECMP for which the next router is closer than the current one from the destination. This relaxed criterion insures that the traffic follows loop-free paths since the routers that it traverses are strictly closer and closer to the destination. The traffic splitting policy is also different in OMP, as it depends on the number of distinct paths available between the source and destination nodes and is subsequently refined from measurements taken of the traffic on the links. Parts of the network can be initially (ie. before optimization) overloaded like the link D-E in figure 1. This figure also shows that the traffic split by OMP leads to a dramatic increase in the resource consumption by flows crossing the network. Here /3 of the traffic take a route twice as long as the shortest one, resulting in a global network resource consumption almost twice as large as the original one. The proposition presented in the following section aims at splitting the traffic only on the basis of the load on the available paths while still insuring that the proposed paths be loop-free..3. Hop by Hop routing of connection The classical method for avoiding the problems due to the interference between the transport and network layers raised in section., is to apply load balancing directly to transport layer connections, so that the traffic sent on them gets as much as possible a stable treatment over time. The MPLS (Multi-Protocol Label Switching) framework brings connection capabilities to IP and thus is the main tool foreseen to achieve traffic engineering (see []) and to make load balancing possible. As stated in section.1, the aim of the work presented herein is to balance the load on the network according to the actual load on the links. In this dynamic context, and occording to [10], a hop by hop (as opposed to source routing) connection establishment mechanism was chosen, since it seems more suitable to respond to changes in the network state (even changes that happen during the connection establishment process can be taken into account). 3. A new proposition 3.1. Choice of a metric For the specific problem of routing elastic flows (i.e. that can tune their rate depending on the network state, like TCP does), various policies for load balancing have been proposed. These studies are relevant in two respects : They show that for a peculiar type of traffic (elastic flows, i.e. TCP-like connections, as opposed to streams), load balancing is desirable, and they propose policies in order to enhance the network usage experience.

3 0.5 C 0.33 C 0.5 A B 0.5 D 3 6 E G 0.66 A B 0.33 D E G 0.5 F 0.33 F a) Load balancing by ECMP b) a priori load balancing by OMP Figure 1. Load split for the ECMP and OMP routing policies These policies can be used as a basis for the routing of any kind of traffic, as there is no general load balancing scheme available, and they pursue the two aims of the load balancing on the network, namely resource preservation and load repartition. The proposition in [8] for the routing of elastic flows consists in the usage of a composite path metric such that for comparable available bandwidths on two paths, the shortest one is prefered, and for two paths of roughly the same length, the widest one is chosen. The routing algorithm is in charge of computing these path metrics, and so it is permanently able to pick up the path with the greatest, to send the traffic on it. At this point, it appears that such an algorithm is not well suited for the the task of the adaptive routing of traffic in communication networks, as this load balancing technique consists in selecting, at a given time a single shortest path. It is much more scalable (and safer) to split the load beforehand on the different available routes, sending more traffic on the paths with a greater. This remark founds our approach, that will be referred to as weighted fair routing in the next sections, as it forwards randomly the load on the various available paths, with probabilities proportional to the corresponding metrics. 3.. Choice of a routing protocol 3..1 link states protocols The computation of these path metrics requires a distributed routing algorithm which can maintain and distribute link load informations to all the nodes in the network. A link state algorithm (similar to the ones used by the IS-IS, OSPF or PNNI protocols) is particularly well suited for this task as it maintains on each node a routing database that reflects the topology of the network and that can also hold estimates of the load on each link in the network. The computation and storage of these estimates is not straightforward in some networks, and it will not be described it in this paper (the interested reader will find details in [1][4] or [1]). From this routing database, each router can compute the path metrics for the best path(s) starting from each of the different neighbors of. This task complexity is in order of (as shown in [11]), where is the number of nodes in the network. 3.. Agent based routing protocols Another way of building the sets of estimates of associated to the paths emanating from a node toward each destination node is the use of agent based routing protocols that were designed for this task. A detailed description of such an algorithm can be found in [5] and [6]. 4. Proposal 4.1. Weighted fair routing An equivalent for the metric proposed in [8] is to consider as the composite path metric of a path the value () for a path of length and bandwidth at the bottleneck. 3

4 "! " For convenience, use of an estimate for along a path is, the minimum of along the links of the path, where is the available bandwidth on the link and its distance from along. differs from in the sense that it neglects the resource consumption of a flow beyond the bottleneck, given that it exponentially diminishes with the distance to the bottleneck: that is to say that it is negligible on this part of the network. These estimates give a lower bound of the bandwidth available from to the destination through each of its neighbors and it is used to fairly split the traffic between these routes. Thus, the probability of having a connection on its way to choosing the next router is of the form: 4.. Connection establishment attempts follow loopfree paths The main concern for this kind of hop by hop weighted fair routing approach is the risk of contruction of loops while routing a connection. It is mandatory to reduce this risk in order to alleviate the management load of the router from unnecessary cranbacks, which can be avoided by a proper routing. It is show hereafter that even if hop by hop multi-path routing is used, cranbacks can be very scarce with the proposed approach. It is assumed that the routers never try to forward a connection request towards the router from which it just arrived. This assumption is realistic in the sense that the routers need to keep track of such information in all of the network frameworks. With this assumption, a routing loop appears when a connection attempt is sent toward as node that it has already visited, but not the last one. It is shown below that this phenomenon is rather unlikely and can be totally avoided at low cost. The minimum configurations for a routing loop appear in figure.a and.b, when node sends back (directly or not) a connection aimed at to node. Supposing that a route (through ) is used only if the associated is such that: (3) (4) that is, if the probability associated to this path is not negligible. So in the case.a, the router sends the connection attempt towards! because #! " (5) When the connection attempt reaches, it does not send the connection toward if " $ % (6) The condition expressed by equation 6 is insured by the condition of 5 for: '& The same result can be derived from the case.b. So it is possible to avoid routing loops by considering only a subset of the next routers depending on the associated to them. A few comments can be made at this point: This policy forbids the use for load balancing a route more than one router longer than the best one if they share the same bottleneck. This is consistent with the policy used in the telephone network where only this kind of route is considered for load balancing (though in this case the network is a full mesh). With this weighted fair routing approach, the load balancing is even on the routes with comparable load and length, and it is strongly focused on resource preservation on the network, which causes the upper part (routers B,C,D,E) of the network of figure 1 to be neglected by the traffic between A and G, unless the direct path though F becomes congested, with a load approximately seven times higher on the link F-G than on E-G Implementation and stability issues This approach can be implemented at low cost as an addon to any link state algorithm, which can be OSPF for IP or PNNI for ATM. Great care has to be taken, though, regarding the stability of the algorithm, and routing updates have to remain scarce even if this is an adaptive routing algorithm. The mechanisms for the dampening of the routing updates are well-known, they mostly rely on the use of a hysteresis function to filter the link metric changes before they are taken into account by the routing algorithm. The necessity of such mechanisms was stressed in the PNNI case [1]. As suggested in section 3.., the use of agent based routing is an alternative, and would provide the same functionality with fewer stability issues Experimental results The simulator used to test this routing policy is a standard continuous time discrete event simulator developped on purpose for the design of routing algorithms. It implements a simple connection establishement mechanism and routing algorithms. 4

5 & & & & 1 1 b 1 b 1 3 d 3 d b b a) b) Figure. A routing loop occurs when a connection is routed back to a node that has already been visited. " and " are the available bandwidths on the links and. The simulation results shown in this section were obtained with a network topology similar to the one shown on figure 1. The simulation aims at illustrating two points: The low probability of occurrence of a loop on the process of establishing a connection. The traffic split in regard to the available bandwidth on the links. The considered network was allocated the same bandwidth on all the links of the network, arbitrarily set to 60 connections, except for the link whose bandwidth changes at! from 60 to 0. An average number of were fed to the network in toward, each one lasting 1. The plots shown in figure 3 show that before the bandwidth drop on link, no load is sent to the alternate route. After the bandwidth drop, the load is fairly split on the two routes in order to unload the shortest route. As the mechanism described in section 4. was used, no loop at all were detected in the process of routing the connection through the network. The routing algorithm used for this simulation was agent-based, which explains the gradual shift of load after the bandwidth drop. 5. Conclusion and perspectives This paper proposes a complete routing approach in order to achieve load balancing in the communication networks. It relies on the commonly used link-state routing algorithms but should lead to a noteworthy improvement of the network resources usage, compared to the one experienced with the previous propositions. The approach makes use of the load balancing scheme proposed for the elastic flows while at the same time providing the first routing mechanism for its application. The loop-freedom of the connection establishment attempts is proved, even though a fully distributed routing algorithm without source routing was considered. Finally, these two points were illustrated by way of simulation. References [1] ATM Forum. PNNI. [] D. Awduche, J. Malcolm, J. Agogbua, M. O Dell, and J. Mc- Manus. Requirements for traffic engineering over MPLS. IETF RFC 70. IETF, Network working group, September [3] M. Gondran and M. Minoux. Graphes et algorithmes. Eyrolles, [4] R. A. Guérin, A. Orda, and D. Williams. QoS routing mechanisms and OSPF extensions. In GLOBECOM 97, volume 3, pages IEEE, november [5] M. Heusse, S. Guerin, D. Snyers, and P. Kuntz. Adaptive agent-driven routing and load balancing in communication networks. Advances in Complex Systems, 1:34 57, [6] M. Heusse and Y. Kermarrec. Adaptive routing and load balancing of ephemeral connections. In IEEE ECUMN 000, pages , Colmar, France, september 000. [7] J. Moy. OSPF version. IETF RFC 38, april [8] S. Oueslati-Boulahia and E. Oubagha. An approach to routing elastic flows. In ITC16, Edimbourg, [9] M. Qingming and P. Steenkiste. Routing traffic with qualityof-service guarantees in integrated services networks. In Workshop on Network and Operating Systems Support for Digital Audio and Video, July [10] A. Shaikh, J. Rexford, and K. Shin. Evaluating the overheads of source-directed quality-of-service routing. In Proc. 5

6 Connections 1 10 link E - G Connections 1 10 link F - G Epoch (a) Epoch (b) Figure 3. Load on the links E-G and F-G with weighted fair routing International conference on network protocols. IEEE, october [11] D. Topkis. A k shortest path algorithm for adaptive routing in communication networks. IEEE transactions on communications, 36(7), july [1] C. Villamizar. OSPF-OMP. IETF draft, work in progress. 6