Bandwidth Guarantees for Resilient Ethernet Networks through RSTP ...

Bandwidth Guarantees for Resilient Ethernet Networks through RSTP Port Cost Optimization Andr´as Kern, Istv´an Moldov´an and Tibor Cinkler Department of Telecommunications and Media Informatics Budapest University of Technology and Economics Magyar tud´osok krt. 2, Budapest, Hungary H-1117 email: {kern, moldovan, cinkler}@tmit.bme.hu

Abstract— Ethernet protocol is the most wide-spread protocol in the LAN environment. It is cost effective, simple, and provides high speeds, exactly what is needed in the provider network. However, deployment in the provider network imposes carriergrade requirements. Standardization bodies realized this, and they are extending its capabilities: QoS support and traffic management by 802.1Q VLANs, traffic engineering by 802.1s MSTP, OAM by 802.1ag CFM. However, with carrier-grade extensions Ethernet started to lose an important property: simplicity. Often the topology of the Ethernet aggregation is simple, tree-like, so complex protocols like MSTP are not required even when resilience is supported by adding several links. In most cases RSTP can provide the necessary restoration and loop protection capability. However, the default settings of the RSTP protocol may not provide optimal network utilization, and it is difficult to predict its behavior after a failure. In this paper we present a tool for RSTP optimization, which engineers the network for best utilization while also ensures that optimal paths will be selected after a link failure. The result of the optimization is a port cost set for all bridge interfaces. The optimization is performed offline using an ILP formulation. We show that compared to the default settings higher throughput can be achieved, while the required bandwidth is guaranteed in case of any link failure.

I. I NTRODUCTION In the next generation networks there is a clear convergence to a multi-service network offering more than best effort internet access, but also voice and video services over the same infrastructure. These new services require QoS guarantees and high availability, besides the increased bandwidth. For instance, Voice over IP (VoIP) service expects very low delay and jitter, while the video streaming services are sensitive to packet loss too. At the same time, customers have very high expectations on these services: they want to receive the same quality and availability as provided by the traditional operators over their dedicated infrastructure. Therefore, restoration times in order of seconds are anticipated, demanding resilient networks. In the Metropolitan area Ethernet drives the convergence in triple play networks. Ethernet provides very high speeds with copper or low-cost optical interfaces and its management is simple. Also, Ethernet is the predominant interface on the endpoints that need to be connected. Most of the Ethernet equipment supports the spanning tree protocols those are responsible for path selection and restoration while maintaining a loop-free logical topology. The Rapid Spanning Tree

protocol (RSTP) [1] provides low restoration times (ranging from milliseconds to several second depending on topology), while it remains simple from the management point of view. Traffic separation and QoS are provided by the Virtual LANs (VLANs) defined by IEEE802.1Q [2]. The standard adds a VLAN tag to Ethernet frames that allows 4096 VLANs and 3 priority bits that makes possible traffic class differentiation. Resilience in Metro Ethernet network can be realized by Link Aggregation (IEEE 802.3ad [3]), or by deploying dualhoming and rings topologies. Link Aggregation bundles links together provide increased bandwidth during normal operation and it protects links in the network using different physical paths for links to ensure protection. In the same time, it still requires the use of a spanning tree protocol in the network making the Link Aggregation being an auxiliary solution for protection only. On the other hand, RSTP supports restoration by automatically reconstructing the forwarding topology after a failure and it can work over arbitrary topology. However, RSTP with the default cost settings (which is based on the link speeds [2]) may not achieve good network utilization, moreover in case of failure it is difficult to predict the throughput on the restored topology. In this paper we present an optimization tool, which not only optimizes the forwarding topology, but also optimizes the tree for each link failure. This way bandwidth guarantees are given even in case of a link failure. II. T ECHNICAL BACKGROUND The simple frame forwarding scheme of Ethernet works well only with trees, but Ethernet should work over arbitrary topology in order to realize resilience and Traffic Engineering (TE). The Spanning-Tree Protocol (STP), which is proposed in initial version of IEEE 802.1D [4], is developed to address this problem. The STP is responsible for building a loop-free logical forwarding topology over the physical one providing connectivity among all nodes. The links that are not part of this tree are blocked. In case of a failure, the blocked links are activated providing a self-healing restoration mechanism. All information propagated between the switches is embedded in Bridge Protocol Data Units (BPDUs). These packets are exchanged only between adjacent bridges, and protocol events

(e.g., port state changes) are invoked by timers, thus rebuilding the topology takes considerable time. This timer based operation, which is an STP property, results in reconfiguration times up to 50 seconds and, thus, affects network performance. To achieve faster convergence Rapid Spanning Tree protocol (RSTP) [1] was introduced. It extends STP by using an active bridge-to-bridge handshaking mechanism to repair the connectivity in case of failures instead of a timer specified by the root bridge as STP. After a port receives the agreement in reply of a proposal sent earlier, it immediately changes to forwarding state. This handshake propagates quickly toward the links of the network and restores connectivity. The protocol also introduces new BPDU formats, fasten BPDUs “hello” generation (2s by default) by each bridge, and rapid transition of a port into forwarding state without reliance on any timer configuration. Besides, the RSTP keeps most of the STP parameters and can interoperate with legacy STP bridges. In RSTP (as in STP as well) the tree is constructed as follows. First, a root bridge is elected: the bridge with the smallest bridge ID will be the root. The bridge ID is composed of the MAC address of the bridge and a bridge priority that can be modified by the operator. Then every other switch selects the “closest” port to the root, which has the smallest distance to/from the root, as root port. When a bridge has two paths of equal distance to the root, the port with lower priority ID, and in case of equality of these IDs the path offered by the bridge with lower ID will be selected. Finally all bridges select their designated ports: these ports provide the fastest accesses to different segments of the access network, The other ports, which are neither root nor designated ones, will be blocked ports to avoid loops. The distance is modeled using an additive metric: a positive integer (cost) is assigned to each port. The IEEE 802.1D [4] proposed a port cost setting scheme that is based on the outgoing link speed; lower speed ports get higher, while higher speed ports get lower costs, resulting that higher speed links will be selected more frequently than the links having lower speeds. The proposed default values can be found in Table 13-3 in the standard [2]. Since both, the STP and RSTP reduce the network topology to a sole spanning tree, they will forward all packets along this tree. The utilization of redundant links is inhibited resulting in suboptimal path selection and deteriorated TE capabilities. The Multiple Spanning Tree Protocol (MSTP) [5] solves this issue by deploying more, distinct tree instances (Multiple Spanning Tree Instances: MSTIs) in the network. Each MSTI runs one (rapid) spanning tree protocol instance. VLANs are used for traffic separation, and they are also used to select the forwarding tree instance. MSTP introduces a further improvement: it divides the network into regions interconnected through a central tree instance. These regions may be used to reduce the spanning tree domain. Dividing network into some smaller MST regions will limit the impact of the failure to the MST region where it occurs. The MSTP provides a maximum of 64 spanning tree in-

stances in a region. These MSTIs may have different roots and may use different connections in the network. The desired trees can be shaped by setting appropriately the bridge and port priorities and port costs for the different MST instances. Although MSTP supports advanced TE and resilience, the drawbacks of the protocol are clear: it has increased complexity and each MST instance must have an own link cost and priority settings. The management involves a huge configuration work, which places a burden on network operators. Nevertheless, the benefits of using MSTP are clear: on the one hand, optimal resource utilization can be achieved covering alternate paths with multiple trees. On the other hand protection switching can be deployed achieving 50ms restoration times. The IEEE also recognized the shortcomings of the spanning tree protocols and it is currently working on a solution called Shortest Path Bridging [6]. The idea behind is to deploy an own shortest path tree for each bridge, where the paths between the root and other nodes are the shortest ones. Bridges use their own trees to send traffic, thus, they use the shortest path. However, the solution requires that path between two nodes is symmetrical; otherwise the bridge learning mechanism will not work. This requirement yields to modification in the spanning tree protocol. III. O PTIONS ON TE

AND

R ESILIENCE

The spanning tree protocols discussed in the previous section provide only a framework, and there is a need for an Ethernet based transport solution to deliver guaranteed services with measurable performance while still exhibiting Ethernet economics. In this section we discuss the most widespread alternatives of providing carrier grade services. A. “Plug and pray”: using the default configuration The Spanning Tree Protocols have been designed for robustness and ease of use: just plug the devices together and it should work. Using the default port cost set suggested by RSTP standard, the resulting forwarding tree will take into account link speeds. Usually the system administrator makes a simple setting to place the root of the tree by setting a low bridge priority value. However these default settings may not give surely the best forwarding topology for a traffic to be transmitted: “better organizing” the tree the traffic can be more evenly distributed realizing higher throughput. Furthermore, the restoration is performed based on the given cost set. Thus, although the restored topology is deterministic, only the connectivity is taken into account, the traffic is not. So, bottlenecks may be formed easily after restoration that deteriorates the throughput. This also implies that there is no information about the network resources after restoration. B. Layer 2 architectures To achieve optimal network utilization and predictable restoration, complex architectures have emerged. Most of them are based on the MSTP protocol and they extend the Ethernet

network with new functionalities, however, these extensions are still based on the Layer 2 standard based solutions. All these concepts define a logical overlay topology formed by set of channel over the Ethernet. These channels are either pipes or trees and they are usually identified as Virtual LANs (VLANs). 1) Viking: The first discussed framework based on MSTP is the Viking [7] that uses multiple spanning trees to select two different switching paths between end nodes. During normal operation the first path is used. In case of a failure affecting the working path, Viking switches to the backup path changing to the other spanning tree instance by changing the VLAN tag, thus changing the MST tree instance. They also provide a heuristic method to set up MST instances in order to provide load balancing and protection switching. 2) MSTP-OPT: The MSTP-OPT [8] also uses multiple trees provided by MSTP and implements an Integer Linear Program (ILP) formulation for both, the assignment of VLANs to MSTIs and forming the trees for these MSTIs. This approach takes not only the traffic demands into account, but it supports QoS and resilience requirements as well, by protection switching. The heuristic method proposed in [9] extends the scalability with a heuristic method that provides near optimal results. 3) Distributed Resilient Architecture for Ethernet: Farkas et. al [10] propose to span more static trees in the network in advance and to each tree an own VLAN ID is assigned. Since the trees are identified with VLANs, the protection can be realized by simply changing the VLAN ID of the frames. They have also proposed a failure detection method providing lower than 100 milliseconds restoration times. C. Substituting RSTP Several vendors and the IEEE proposed alternative protocols to RSTP primarily for better protection/resilience support. The Link Aggregation proposed in IEEE 802.3ad [3] enables bundling several links having the same endpoints into a single virtual one. By laying these links different physical direction, the link aggregation can be used for protection against link failures: if one of these parallel links fails, the virtual link still provides connectivity but with less bandwidth. This way, Link Aggregation provides local link protection, however, it does not handle with the demand of loop-free forwarding topology. Therefore, it is rather an auxiliary solution. The Ethernet Automatic Protection Switching (EAPS) [11] technology increases the availability of Ethernet rings. An Ethernet ring built using EAPS can have availability comparable to that provided by SDH/SONET rings, at a lower cost and with fewer constraints (e.g., on ring size). It does not limit the number of nodes in the ring, and the convergence time is independent of the number of nodes in the ring; however, this technology works well only in ring topologies. Several other enhancements emerged to reduce the restoration time of RSTP in rings: Rapid Ring Protection Protocol (Huawei) and Riverstone Networks has implemented the Rapid Ring Spanning Tree (RRST) that leverages MSTP and RSTP

to make rapid changes in port state for Ethernet switches configured in strict ring topologies. IEEE is defining another standard, the 802.17 Resilient Packet Ring (RPR)[12] for enhancing resilience of Ethernet networks. Just as its name implies, RPR is designed for ring topologies carrying packets, but with the same resilience attributes as of a typical SDH/SONET ring. This technology is new, and it is not yet widely deployed. Scaled for larger deployments, the industry expectation is that while RPR will become one of the most widely used IP networking tools, it is unlikely to be economically viable for all network sizes and application types. There are several initiations to replace the control plane of the Ethernet bridges by layer 3 mechanisms. The TRILL workgroup within IETF aims to design a solution for shortest-path frame routing in multi-hop IEEE 802.1-compliant Ethernet networks with arbitrary topologies, using an existing link-state routing protocol technology. Other works, like GELS, consider extending the Generalized Multi-Protocol Label Switching (GMPLS) signaling and routing to Layer 2 technologies. IV. O PTIMIZED RSTP CONFIGURATION Comparing the three approaches clearly shows that “carriergrade” Ethernet can be realized but at the cost of increased complexity resulted by the auxiliary protocols and network elements. Carriers often consider the 50ms recovery time of SDH/SONET as a crucial requirement. This requirement has been justified in core networks, however in access and aggregation networks 99.999% availability (often called five-nines or 5x9) is targeted by the operator. Five-nines availability means around 5 minutes of unplanned downtime per year, that can be achieved without 50ms protection switching. At the same time, the common Ethernet switches have a Mean Time Between Failure (MTBF) around 6 years, thus even in a larger topology 5x9 can be achieved by only counting on the restoration capability of RSTP. Assuming a restoration time of 2 seconds, 150 failures per a year is still within the allowed limit. Note that, the most services do not require even the fivenines availability, thus in access networks even the RSTP is able to satisfy the targeted requirements. Therefore, we consider a RSTP based architecture, where we rely on the RSTP restoration capability. Since the typical metro topologies are sparse and traffic is aggregated to one or more Edge Nodes, the capacity of the unused redundant links is usually kept for backup purposes. In such situations load balancing or TE may not be possible. Therefore, assuming one AN the sole tree of RSTP is supposed to support our goals those are as follows: • Optimize network utilization, while • Bandwidth guarantees are given in case of link failures. Since the port cost values influence the shape of the tree and, therefore, the available throughput, to reach our goals the optimization of these costs are required. For this purpose we have formulated the problem as an Integer Linear Program.

A. Problem Formulation

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B. Applicability Before detailing the proposed ILP model, we have to consider what requirements have to be fulfilled to obtain a valid and unambiguous solution. A solution is regarded as unambiguous if a common cost set exists for all failure scenarios (failure-independent cost set). Obviously, the topology must be 2-link-connected, in other words there is no such a link whose removal disconnects the topology to two or more disjoint parts. Then, let us assume an arbitrary port cost set. For this set a tree will be formed. If any link fails, a new tree will be spanned in the network using the remaining links, since the topology is 2-link-connected. However, if capacity constraints are also considered over a 2-link-connected topology we can easily construct an example that cannot be solved using one common port cost set (or even using a single tree). Figure 1 show a sample topology, where a GbE core is deployed with 100 Mbps aggregation links. Over this topology two flows are routed to node 6 from nodes 1 and 2, and each has a size of 60 Mbps. During normal operation the GbE link is able to serve both flows. If the GbE link fails between nodes 3 and 6, a new tree should be formed. Because of the applied capacity constraints

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Let us assume that the physical topology is modeled as a directed graph G(V, E, C), where V is the set of vertices where the Ethernet switches are placed. Let E be the set of directed graph edges and each edges is defined with its endpoints: ij ∈ E, where i, j ∈ V . The physical links are described with two opposite directed graph edges and each graph edge models the outgoing port of source switch. Last, but not least, a capacity function C : E → R+ is also given. Since different trees will be formed in the case of the failure of different links and/or nodes, we consider the problem as a failure-dependent protection. Therefore, we define different failure cases. By considering the protection against a single link failure one scenario is given for the failure of each physical link. Besides, a further scenario is given, describing the normal operation (i.e., no network elements are failed). Thus let F be the set of these scenarios and Ef ⊆ E be the set of links affected by failure scenario f ∈ F . This step does not limit the generality of the model: the failure of a switch can be described as the failure of all links attached to the switch. Note that, this model provides guarantees only for the considered failure situations, the shape of the tree in other cases is not controlled therefore, the tree to be formed might violate the bandwidth requirements. In Metro Aggregation Network the traffic is assumed to flow between the Access and the Edge Nodes, while the other nodes only forward it. Besides, because of administration and billing purposes direct communication between the ANs is usually prohibited. Therefore, the traffic demands are directed from the Access Nodes toward the Edge Nodes. Without loss of generality, we assume symmetrical traffic flows. So, let D = {d = (sd , td , bd )|sd ∈ AN, td ∈ EN, bd ∈ R+ } be the set of traffic demands.

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(a) Tree spanned at normal oper- (b) A failure is occurred between ation node 3 and 6 fails Fig. 1. Simple topology and traffic case, where protection cannot be realized using RSTP with common port cost.

the two flows together cannot be routed either of the alternative paths. Thus, one of the flows should be routed through node 4 while the other must use node 5. To avoid loops, selecting both alternate paths is not allowed by RSTP, thus the problem has no solution. Thus we can guarantee only 50 Mbps flows since the capacity of the alternate path is 100 Mbps. Then, we can define not only a sole tree, but an unambiguous cost set. This example also shows that the requirement of a single port cost set for all failure scenarios introduces further throughput constraint. C. ILP model of RSTP optimization The aim of the optimization is to define a suitable port cost set ensuring that the required bandwidth can be guaranteed in any failure case. The input parameters are the network topology given as graph G, the set of traffic demands (D) and the defined failure scenarios (F and sets of Ef for each scenario f ∈ F ). The result consists of the port cost set, and a set of trees, where one tree is spanned for each failure case. They are defined by variables used in the ILP model: we

f yij

is the weight assigned to graph edge e ∈ E. Since two antiparallel edges span a physical link a graph edge can be matched to a outgoing Ethernet port. Thus, these weights also define the port costs, and is a binary variable and it is set to 1.0 if and only if the tree instance uses edge ij ∈ E in failure scenario f ∈ F , otherwise it is set to 0.0.

f This variable also defines the port roles. If yij is set to 1.0, the role of the denoted outgoing port of node i will be root port, otherwise the port role can be either designated or blocked. The selection between the two roles, however, is simple. If the role of the peer port is root (y variable of the antiparallel pair of the considered edge is 1.0) the considered one will be designated. To complete the formulation two further parameters are introduced:

xfe

describes the amount of traffic flowing on edge e ∈ E in failure scenario f ∈ F (0 ≥ xfe ), and

wif

is the “distance” of node i ∈ V from the root node in failure scenario f ∈ F .

1) Objective: The objective of the ILP depends on the aim of optimization. In most cases the operator estimates the amount of traffic to be flowing and a traffic matrix is given as we assumed. Then, the operator wants to minimize the resource reservation: 

min 

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xfij  .

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The constraints are as follows: 2) Tree Rules: This group of rules describes the constraints on forming the tree. One separate tree is constructed for each failure scenario. To each switch (node) a parameter is assigned (wif ) that describes the distance to the root along the tree to be defined. If node i is the root, then the distance to the root will be 0.0 (2): wrf = 0.0

where r is the root, r ∈ N, ∀f ∈ F.

(2)

For all the other node the following consideration can be made. Let edge ij be an outgoing edge from node i to node j. If this edge is in the tree, the distance of node i will equal to the sum of the distance of node j and the port cost of edge ij. Otherwise, wif will be surely larger than wjf + wij . The two rules are formulated in (3) and (4): wif wif

f ≥ wjf + wij − D · (1 − yij )

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+ wij − ǫ · (1 −

f yij )

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∀ij ∈ Ni→ \ Ef , ∀i ∈ N, ∀f ∈ F. where D is practically infinite (e.g., 109 ) and ǫ is a small positive number (e.g., 0.005). f If edge ij is not a tree edge (yij = 0), the (3) will be trivial, since the right side will be surely negative. At the same time, (4) will satisfy the second rule. On the other hand, if the edge ij is a tree edge, the two inequalities enforce the left side to equal to the right one. These two rules, however, do not guarantee that a valid tree will be spanned. A further equation is needed that ensures that exactly one outgoing edge will be selected. In other words, one root port is defined. Obviously no root port is defined for the root node: X yef = 1.0 ∀n ∈ N \ EN, ∀f ∈ F (5) → ∀e∈Nn

A further rule is defined in order to prevent that both endpoints of a link are set to root port. To avoid this situation we must guarantee that at most one the two ys are set to 1.0: f yij

+

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(6)

3) Describing Failure Cases: The next equation formulates the different failure cases those are described as a set of failure graph edges (Ef ). Obviously, the tree instance cannot use

f these failed edges, therefore, the corresponding yij variables are surely 0.0. Thus, the sum of these variables are also 0.0: X f yij = 0.0 ∀ij ∈ Ef , ∀f ∈ F. (7) ∀ij∈Ef

4) Capacity allocation: While the previous equations define how the trees are spanned in different failure scenarios, these rules are to handle the network capacities and allocated bandwidths. Obviously, the reserved bandwidth on an unused link will be 0.0, and it must be smaller than a link capacity on a tree link (8): f xfij + xfji ≤ yij · C(ij)

∀ij ∈ E, ∀f ∈ F.

(8)

In each failure scenario, the traffic is modeled as a flow that originates at the ANs and terminates at the EN. Although this flow is directed, symmetric traffic is assumed. That is why we considered x variables in both direction (8). Due to the flow model a flow conservation rule is formulated saying that sum of the outgoing and terminating traffic equals to the incoming and entering traffic: X

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xfij +

X

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=

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xfki +

X

bd (9)

∀d:sd =i

∀i ∈ N, ∀f ∈ F. Note that, without these last two rules (8,9) the ILP formulation fully operate, but it does not take the network capacities into account. V. C ASE S TUDIES The aim of these case studies is to enlighten the main advantages of optimizing the port costs: the increased Traffic Engineering and resilience capabilities. For this purpose we have calculated the maximal throughput of the considered methods for both, protected and unprotected cases. This maximal throughput is determined by scaling the traffic: first, for each simulation case a basic traffic matrix is given in advance that expresses the space distribution of the traffic among the ANs. Then, these matrices are scaled up using a parameter called Global Traffic Level (GT L). Increasing the value of GT L the whole network load is scaled up, while the traffic ratios are kept. This scaling process is performed 12 times for each test cases using 12 randomly generated independent basic traffic matrices. Here, we assume homogenous traffic, i.e., roughly the same amount of traffic is generated for each AN. The results to be presented are average of these 12 simulations. A. Considered Topologies The evaluation is performed over typical metropolitan topologies, those are primarily used to aggregate the traffic of hundreds or thousands of users and to transmit it toward the providers or the core networks. They have usually sparse topologies: trees in the “aggregation parts”, interconnected rings or mesh in the “core part”.

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(a) Symmetric Ring Topology (SRT ) Fig. 2.

(b) Ring-Mesh Topology (RM T )

(c) Dual Homing Topology (DHT )

The assumed topologies. The speeds of all links are assumed to be 1 Gbps (GbE links).

Since over tree topologies neither TE nor resilience can be realized we focus on the core part (see Figure 2). Three topology cases are defined. The first one, the Symmetric Ring Topology (SRT ) (see Fig. 2(a)) strictly follows the previous concept and it is formed by four rings: A central and three aggregation ones and all links are GbE. The second case considers a typical metro topology that combines rings so this topology is rather a mesh (Ring-Mesh Topology (Fig. 2(b))). The third topology considers a dual homing topology (Fig. 2(c)), where each node in the lower layer is connected to two nodes in the upper layer. In all cases GbE links are used. B. Reference Methods We have discussed the main concepts of how TE and resilience can be provided in Metro Ethernet networks in Section III. Methods based on using alternative protocols are not considered here because of the assumed RSTP based architecture. The following two approaches are considered as reference methods. The first reference method is the RSTP protocol, when it uses a port cost set defined by the standard [2]. This method is referred to as RSTP with standard cost set (RST PST D ). The second reference method represents an optimized protection switching mechanism discussed in Section III-B. This protection switching method defines 1:1 dedicated protection by defining two edge disjoint trees. We have selected M ST P − OP T since this is an ILP based algorithm that provides global optimal solution. Although more trees could be used by MSTP-OPT, only 2 trees have been used in the test cases for fair comparison. VI. N UMERICAL R ESULTS A. Achieved throughput The maximal achievable throughput of the proposed port cost optimization method is compared to the two considered references over all three topology cases. Here, we do not preserve capacity for backup and we focus only on maximizing throughput. As it can be seen on Figure 3, optimizing the port costs results in increased throughput compared to the standard port cost case. The gain, however, varies between 5% up to

433% in the different topology cases. In other words, using standard based cost good forwarding topology can be created in some cases. However, in other cases, where more alternate paths with the same lengths exist, only one of these paths will be used, since the selection among these path is performed only based bridge ID of the next-hop bridge and the traffic will be concentrated to the bridge having the lowest ID. Comparing our proposal to the MSTP-OPT we can see that if MSTP is optimized with one tree the same results are obtained that is obvious since one forwarding topology is deployed in both cases. However, installing a further tree results in 1–10% higher throughput compared to RST POP T depending on the topology. With MSTP-OPT higher gain can be realized in denser topologies where more alternate paths exist. B. Effects of inhomogeneous traffic A drawback of RSTP when the standard port costs are used, is the “topology-driven” nature: neither the amount nor the distribution of the expected traffic are considered at all. Up to now the considered traffic was rather homogeneous, i.e., the difference of the largest and smallest traffic demands was 10% of parameter GT L. Since the demand sizes are obtained using uniform distribution with GT L as a mean value, a range of the possible demand sizes can be defined: bd ∈ (GT L − p · GT L, GT L + p · GT L), where p is referred to as relative range. In this paragraph, we focus on the RSTP based configuration of less homogeneous traffic. The traffic inhomogeneity is described with the help of parameter p. Therefore, we evaluated the achievable throughput over the Ring Mesh Topology considering different relative range sizes: 10%, 25%, 50% and 100% of GT L, respectively. For each cases 12 independent problem instances were generated, as well. Considering inhomogeneous traffic matrices an increased variance of the achieved throughput is expected: the larger traffic flows want to use the bottleneck links, the lower GT L values can be reached. The cost optimization can adjust the forwarding topology to distribute the traffic as even as possible that may decrease the variance of the results. Figure 4 shows calculated throughputs applying both, standard based and optimized port cost sets. MSTP-OPT provides

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Guaranteed throughput At normal operation at one link failure min avg max min avg max Topology #1: Symmetric Ring Topology RST PST D 2189 2224 2258 – RST POP T 2659 2802 2909 1233 1271 1352 M ST POP T 2921 2953 2981 1492 1495 1498 Topology #2: Ring-Mesh Topology RST PST D 1710 1733 1754 – RST POP T 1745 1762 1789 994 997 1000 M ST POP T 1747 1762 1789 994 997 1000 Topology #3: Dual Homing Topology RST PST D 1309 1333 1348 – RST POP T 3253 3478 3900 1825 1966 1999 M ST POP T 3821 3878 3979 1972 1992 1999

Fig. 4. Achieved throughput of RSTP using the standard based and optimized port costs depending on the distribution of the traffic.

the same throughputs as the optimized RSTP, that is why this reference method is not shown. The range of the achieved throughputs are depicted. For better presentation, the average values are also drawn and they are interconnected with a curve. As you can see, the difference between the two methods increases as the variance of the traffic becomes larger. Using standard cost sets a continuously decreasing throughput is shown. Comparing the variances of the results a further observation can be made: using the standard port cost set the variance steadily increases as it was expected. Optimizing cost sets decreases the variance compared to the standard cost case. The results support our expectance: the RST POP T is able to adapt the inhomogeneous traffic conditions. C. Resilience guarantees Besides the increased throughput, the behavior of the RSTP in the case of failures is also crucial. Our requirement, the at least double-connected topology ensures only the existence of the alternate paths that can be used for recovery purposes. By optimizing the port costs we can also influence the selection of these paths. The proposed ILP formulation considers various failure scenarios, thus, the provided solution gives bandwidth guarantees in the expected failure scenarios. Here, we considered the failure of at most one link. The throughput realized

is summarized in Table I. Considering protection results in roughly 50% drop of achieved throughput. This amount decreased throughput is a well-known property of the 1:1 dedicated path protection scheme. The M ST POP T reference method realized this protection, and the two methods produce about the same throughput level supporting our previous observation. D. How the trees are spanned? The previous results presented that by influencing the shape of the trees through optimizing the port costs throughput advance can be realized compared to the standard based port cost. Now, we have selected a particular solution to present how our method optimizes and gives bandwidth guarantees. In the selected example the standard based and the optimized RSTP instances span the same trees at normal operation (Figure 5(a)). Then, the same throughput can be achieved, while all links fully operate. If a link fails (e.g., between nodes 2 and 6) the restored tree instances will be different. Using the standard port cost set (Fig. 5(b)) two new links are unblocked, which detour the traffic of ANs 11 and 13 and concentrate the traffic of five ANs to a single link making it a bottleneck. Although the path through AN 10 has the same distance to root as the selected path, but its bridge ID (10) is larger than ID of the selected switch (3).

(a) Working tree (the same tree is spanned)

(b) Restored tree using standard port cost set

(c) Restored tree using optimization

Fig. 5. The trees spanned using both, the standard based and optimized cost sets at normal operation and at the failure of the link between nodes 2 and 6 in Ring-Mesh Topology.

By optimizing the port cost the traffic of ANs 11 and 13 is routed through the AN 10, thus, the traffic is more evenly distributed in the network and the bottleneck is avoided. VII. C ONCLUSION The growing extensions to make Ethernet ”carrier grade” also introduce complex architectures and increased management tasks. They provide means for traffic engineering and enhance restoration by protection switching. However in simple aggregation networks it may not be possible to exploit their advantages or simply they are not needed. On the other hand, RSTP has acceptable restoration capability which may be enough for most services, while it is standard and simple. Although it provides basic loop protection and restoration, with default settings it does not guarantee that the forwarding topology is optimal and there are no bandwidth guarantees after a failure. This paper was devoted to propose an RSTP port cost optimization method that optimizes the network utilization, while also gives a lower bound on bandwidth that is guaranteed even in case of a link failure. We also introduce “traffic-aware” configuration by considering the offered traffic and resilience requirements - without deploying complex architectures. The performance of the method was verified on several test cases. Results show that over the considered topologies, the achievable throughput can be increased compared to the standard cost based RSTP. The further significant advantage of the proposed optimization method is that not only optimizes the network throughput but also can optimize the restored topology, giving bandwidth guarantees even after a link failure. Finally we must emphasize that the optimization is applied to the basic RSTP protocol keeping the Ethernet aggregation simple. R EFERENCES [1] “Media Access Control (MAC) Bridges: Rapid Reconfiguration of Spanning Tree,” IEEE 802.1w, IEEE, 2001, incorporated into 802.1D2004. [2] “Local and Metropolitan Area Networks Virtual Bridged Local Area Networks,” IEEE 802.1Q Revision, IEEE, December 2005.

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