Cybersecurity is increasingly formalized as a board-level fiduciary responsibility, with regulatory frameworks such as NIS2 imposing explicit accountability on directors. Yet empirical evidence of how boards operationalize this responsibility within safety-critical environments remains limited. This study presents a mixed-methods case study of the Norwegian specialist healthcare sector, triangulating survey responses from 103 board members with a retrospective analysis of 594 board protocols comprising 6441 agenda items across 33 health trusts. The findings reveal a divergence between perceived and formally documented cybersecurity governance activity. While 92.2% of directors reported that cybersecurity strategy was at least somewhat aligned with organizational objectives, formal board records were dominated by informational agenda items (77.4%) rather than decision items (22.6%), with limited documented evidence of recurring evaluative deliberation. Qualitative analysis further indicates that more active board engagement was episodically triggered by audits, threat assessments, or tactical risk decisions. We conceptualize this divergence as a Strategic Illusion , referring to a condition in which perceived strategic integration exceeds documented governance intervention. Using the Five Lines of Accountability framework, the study interprets these findings as consistent with patterns of “ostensible adoption”, where governance structures formally reflect accountability expectations while substantive evaluative engagement remains unevenly institutionalized. The study contributes mechanism-based insights into how boards navigate the tension between perceived strategic involvement and auditable governance practice. It also proposes a four-pillar framework for strengthening substantive cybersecurity governance. Overall, the analysis suggests that regulatory visibility alone may be insufficient to produce substantive cybersecurity oversight without recurring evaluative engagement and independent inquiry-based monitoring.

Interconnection networks are key components in high-performance computing (HPC) systems, their performance having a strong influence on the overall system one. However, at high load, congestion and its negative effects (e.g., Head-of-line blocking) threaten the performance of the network, and so the one of the entire system. Congestion control (CC) is crucial to ensure an efficient utilization of the interconnection network during congestion situations. As one major trend is to reduce the effective wiring in interconnection networks to reduce cost and power consumption, the network will operate very close to its capacity. Thus, congestion control becomes essential. Existing CC techniques can be divided into two general approaches. One is to throttle traffic injection at the sources that contribute to congestion, and the other is to isolate the congested traffic in specially designated resources. However, both approaches have different, but non-overlapping weaknesses: injection throttling techniques have a slow reaction against congestion, while isolating traffic in special resources may lead the system to run out of those resources. In this paper we propose EcoCC, a new Efficient and Cost-Effective CC technique, that combines injection throttling and congested-flow isolation to minimize their respective drawbacks and maximize overall system performance. This new strategy is suitable for current commercial switch architectures, where it could be implemented without requiring significant complexity. Experimental results, using simulations under synthetic and real trace-based traffic patterns, show that this technique improves by up to 55 percent over some of the most successful congestion control techniques.

A prime objective of fault tolerant routing methods is the availability of multiple routing options at each hop. The methods that are currently implemented in IP networks, such as Equal-Cost Multi-Path (ECMP) and Loop Free Alternates (LFA), share the following four properties: First, they work with a hop-by-hop forwarding strategy optimized for the fault free case. Second, they do not require information associated with network faults included in the packet header. Third, they do not form forwarding loops, even under multiple failures in the network. Finally, they are compatible with standard link state routing protocols. However, ECMP and LFA give very poor fault coverage; in most cases fewer than 50% of primary next-hops are protected when using typical link weight settings. This paper presents a new routing method that combines the concept of permutation routing with interface-specific forwarding. Our method results in a routing strategy that strictly adheres to the four stated design properties. Through experiments we show that we protect more than 97% of the primary next-hops for all tested ISP networks.

Toleration of faults in the interconnection networks is of vital importance in to days huge computer installations. Still, the existing solutions are short of being satisfactory. They require that the system defaults into a routing algorithm that is inferior to the original, either in terms of performance, or in terms of the need for virtual channels, or both. Furthermore, since support for dynamic reconfiguration is not supported in current hardware, existing methods require the system to be halted while reconfiguration takes place in order to avoid deadlocks. In this paper we present a method that efficiently generates a new routing function in the presence of faults. The new routing function only reroutes the traffic that is affected by the fault, so that the performance of the original routing function is preserved to the extent possible. No specific functionality in the switches is required, we only require exactly the same number of virtual channels in the presence of faults as the original routing algorithm did. Finally, the new routing function is compatible with the old one, so that deadlock free dynamic transition between the old and the new routing function is immediately available. This means that our solution can easily be implemented on current InfiniBand platforms, e.g. through the OFED software stack. We demonstrate that the method is workable for meshes, tori and fat-trees, and that it is able to guarantee one-fault tolerance for all of these topologies.

Existing methods for routing of interconnection networks are optimized for a small group of well defined topologies. Currently, system owners and system integrators are looking for ways to connect two or more existing topologies into one, and thereby create a unified system out of several subsystems. Such integration of multiple networks is supported through e.g. The subnet routing functionality i InfiniBand. In this paper we show that the routing problem of such integration is unsolved, because the integration of two deadlock-free subsystems may lead to routing deadlock. Thereafter we present a theory and a methodology for combining multiple networks in a deadlock-free manner. Through experiments we show that the methodology is able to connect different classes of pre-defined topologies with a definable number of interconnecting links. We demonstrate that we are able to maintain the original routing strategy in the sub-networks, and still get close to full utilization of the bridging links. Finally we show that the methodology is computationally feasible.

In a loss less interconnection network, network congestion needs to be detected and resolved to ensure high performance and good utilization of network resources at high network load. If no countermeasure is taken, congestion at a node in the network will stimulate the growth of a congestion tree that not only affects contributors to congestion, but also other traffic flows in the network. Left untouched, the congestion tree will block traffic flows, lead to underutilization of network resources and result in a severe drop in network performance. The InfiniBand standard specifies a congestion control (CC) mechanism to detect and resolve congestion before a congestion tree is able to grow and, by that, hamper the network performance. The InfiniBand CC mechanism includes a rich set of parameters that can be tuned in order to achieve effective CC. Even though it has been shown that the CC mechanism, properly tuned, is able to improve both throughput and fairness in an interconnection network, it has been questioned whether the mechanism is fast enough to keep up with dynamic network traffic, and if a given set of parameter values for a topology is robust when it comes to different traffic patterns, or if the parameters need to be tuned depending on the applications in use. In this paper we address both these questions. Using the three-stage fat-tree topology from the Sun Data center InfiniBand Switch 648 as a basis, and a simulator tuned against CC capable InfiniBand hardware, we conduct a systematic study of the efficiency of the InfiniBand CC mechanism as the network traffic becomes increasingly more dynamic. Our studies show that the InfiniBand CC, even when using a single set of parameter values, performs very well as the traffic patterns becomes increasingly more dynamic, outperforming a network without CC in all cases. Our results show throughput increases varying from a few percent, to a seventeen-fold increase.

Most standard cluster interconnect technologies are flexible with respect to network topology. This has spawned a substantial amount of research on topology-agnostic routing algorithms, which make no assumption about the network structure, thus providing the flexibility needed to route on irregular networks. Actually, such an irregularity should be often interpreted as minor modifications of some regular interconnection pattern, such as those induced by faults. In fact, topology-agnostic routing algorithms are also becoming increasingly useful for networks on chip (NoCs), where faults may make the preferred 2D mesh topology irregular. Existing topology-agnostic routing algorithms were developed for varying purposes, giving them different and not always comparable properties. Details are scattered among many papers, each with distinct conditions, making comparison difficult. This paper presents a comprehensive overview of the known topology-agnostic routing algorithms. We classify these algorithms by their most important properties, and evaluate them consistently. This provides significant insight into the algorithms and their appropriateness for different on- and off-chip environments.

In loss less interconnection networks such as InfiniBand, congestion control (CC) can be an effective mechanism to achieve high performance and good utilization of network resources. The InfiniBand standard describes CC functionality for detecting and resolving congestion, but the design decisions on how to implement this functionallity is left to the hardware designer. One must be cautious when making these design decisions not to introduce fairness problems, as our study shows. In this paper we study the relationship between congestion control, switch arbitration, and fairness. Specifically, we look at fairness among different traffic flows arriving at a hot spot switch on different input ports, as CC is turned on. In addition we study the fairness among traffic flows at a switch where some flows are exclusive users of their input ports while other flows are sharing an input port (the parking lot problem). Our results show that the implementation of congestion control in a switch is vulnerable to unfairness if care is not taken. In detail, we found that a threshold hysteresis of more than one MTU is needed to resolve arbitration unfairness. Furthermore, to fully solve the parking lot problem, proper configuration of the CC parameters are required.

Existing congestion control mechanisms in interconnects can be divided into two general approaches. One is to throttle traffic injection at the sources that contribute to congestion, and the other is to isolate the congested traffic in specially designated resources. These two approaches have different, but non-overlapping weaknesses. In this paper we present in detail a method that combines injection throttling and congested-flow isolation. Through simulation studies we first demonstrate the respective flaws of the injection throttling and of flow isolation. Thereafter we show that our combined method extracts the best of both approaches in the sense that it gives fast reaction to congestion, it is scalable and it has good fairness properties with respect to the congested flows.

It is a well known fact that multiple virtual lanes can improve performance in interconnection networks, but this knowledge has had little impact on real clusters. Currently, a large number of clusters using InfiniBand is based on fat-tree topologies that can be routed deadlock-free using only one virtual lane. Consequently, all the remaining virtual lanes are left unused. In this paper we suggest an enhancement to the fat-tree algorithm that utilizes virtual lanes to improve performance when hot-spots are present. Even though the bisection bandwidth in a fat-tree is constant, hot-spots are still possible and they will degrade performance for flows not contributing to them due to head-of-line blocking. Such a situation may be alleviated through adaptive routing or congestion control, however, these methods are not yet readily available in InfiniBand technology. To remedy this problem, we have implemented an enhanced fat-tree algorithm in OpenSM that distributes traffic across all available virtual lanes without any configuration needed. We evaluated the performance of the algorithm on a small cluster and did a large-scale evaluation through simulations. In a congested environment, results show that we are able to achieve throughput increases up to 38% on a small cluster and from 221% to 757% depending on the hot-spot scenario for a 648-port simulated cluster.

Fat trees are a very common communication architecture in current large-scale parallel computers. The probability of failure in these systems increases with the number of components. We present a routing method for deterministically and adaptively routed fat trees, applicable to both distributed and source routing, that is able to handle several concurrent faults and that transparently returns to the original routing strategy once the faulty components have recovered. The method is local and dynamic, completely masking the fault from the rest of the system. It only requires a small extra functionality in the switches to handle rerouting packets around a fault. The method guarantees connectedness and deadlock and livelock freedom for up to k -1 benign simultaneous switch and/or link faults where k is half the number of ports in the switches. Our simulation experiments show a graceful degradation of performance as more faults occur. Furthermore, we demonstrate that for most fault combinations, our method will even be able to handle significantly more faults beyond the k -1 limit with high probability.

Rerouting around faulty components and migration of jobs both require reconfiguration of data structures in the Queue Pairs residing in the hosts on an InfiniBand cluster. In this paper we report an implementation of dynamic reconfiguration of such host side data-structures. Our implementation preserves the Queue Pairs, and lets the application run without being interrupted. With this implementation, we demonstrate a complete solution to fault tolerance in an InfiniBand network, where dynamic network reconfiguration to a topology-agnostic routing function is used to avoid malfunctioning components. This solution is in principle able to let applications run uninterruptedly on the cluster, as long as the topology is physically connected. Through measurements on our test-cluster we show that the increased cost of our method in setup latency is negligible, and that there is only a minor reduction in throughput during reconfiguration.

In lossless interconnection networks congestion control (CC) can be an effective mechanism to achieve high performance and good utilization of network resources. Without CC, congestion in one node may grow into a congestion tree that can degrade the performance severely. This degradation can affect not only contributors to the congestion, but also throttles innocent traffic flows in the network. The InfiniBand standard describes CC functionality for detecting and resolving congestion. The InfiniBand CC concept is rich in the way that it specifies a set of parameters that can be tuned in order to achieve effective CC. There is, however, limited experience with the InfiniBand CC mechanism. To the best of our knowledge, only a few simulation studies exist. Recently, InfiniBand CC has been implemented in hardware, and in this paper we present the first experiences with such equipment. We show that the implemented InfiniBand CC mechanism effectively resolves congestion and improves fairness by solving the parking lot problem, if the CC parameters are appropriately set. By conducting extensive testing on a selection of the CC parameters, we have explored the parameter space and found a subset of parameter values that leads to efficient CC for our test scenarios. Furthermore, we show that the InfiniBand CC increases the performance of the well known HPC Challenge benchmark in a congested network.

Computer architectures for high performance computing have traditionally been based on an assumption of one parallel application running alone on one machine. The current trend is, however, that huge computer installations offer compute power to a set of users or customers, each demanding only a subset of the available compute resources. This places new requirements on the architecture, in that it must support dynamic partitioning of the resources into several virtual servers as demand changes. We introduce a novel framework which supports flexible formation of such virtual servers while preventing interference between the communication of different virtual servers. This paper investigates the impacts of a shared interconnection network on applications running on virtual compute servers. We show that the interconnect performance supplied to each job is highly unpredictable, and that a job can experience a performance degradation of 97% when its traffic interferes with the traffic of concurrent jobs. With a minor reduction in the utilization of each processing node, this can be considerably improved through a combination of routing-containment in the interconnection network and a carefully designed resource allocation strategy.

Overlapping Reconfiguration is currently the most efficient method to reconfigure an interconnection network, but is only valid for systems that apply distributed routing. This paper proposes a solution which enables utilization of Overlapping Reconfiguration in a source routed environment. We demonstrate how a synchronized injection of tokens has a significant impact on the performance of the method. Furthermore, we propose and evaluate an optimization of the original algorithm that reduces (and in some cases even eliminates) performance issues caused by the token forwarding regime, such as increased latency and decreased throughput.

Multi-topology routing is an increasingly popular IP network management concept that allows transport of different traffic types over disjoint network paths. The concept is of particular interest for implementation of IP fast reroute (IP FRR). The authors have previously proposed an IP FRR scheme based on multi-topology routing called Multiple Routing Configurations (MRC). MRC supports guaranteed, instantaneous recovery from any single link or node failure in biconnected networks as well as from many combined failures, provided sufficient bandwidth on the surviving links. Furthermore, in MRC different failures result in routing over different network topologies, which gives a good control of the traffic distribution in the networks after a failure

As the end of Moores-law is on the horizon, power becomes a limiting factor to continuous increases in performance gains for single-core processors. Processor engineers have shifted to the multicore paradigm and many-core processors are a reality. Within the context of these multi-core chips, three key metrics point themselves out as being of major importance, performance, fault-tolerance (including yield), and power consumption. A solution that optimizes all three of these metrics is challenging. As the number of cores increases the importance of the interconnection network-on-chip (NoC) grows as well, and chip designers should aim to optimize these three key metrics in the NoC context as well. In this paper we identify and discuss the main properties that a NoC must exhibit in order to enable such optimizations. In particular, we propose the use of virtualization techniques at the NoC level. AS a major finding, we identify the implementation of routing algorithms to become a key design parameter in order to achieve an effective virtualization of the chip should also supporting broadcast within the virtualized context. The intention behind this paper is for it to serve as a position paper on the topic of virtualization for NoC and the challenges that should be met at the routing layer in order to maximize performance, fault-tolerance and power consumption in multicore chips.

Fault tolerance has become an important part of current supercomputers. Local dynamic fault tolerance is the most expedient way of tolerating faults by preconfiguring the network with multiple paths from every node/switch to every destination. In this paper we present a local shortest path dynamic fault-tolerance mechanism inspired by a solution developed for the Internet, that can be applied to any shortest path routing algorithm such as dimension ordered routing, fat tree routing, layered shortest path, etc., and provide a solution for achieving deadlock freedom in the presence of faults. Simulation results show that 1) for fat trees this yields the to this day highest throughput and lowest requirements on virtual layers for dynamic one-fault tolerance, 2) we require in general few layers to achieve deadlock freedom, and 3) for irregular topologies it gives at most a 10 times performance increase compared to FRoots.

In this paper we present a method for dynamic fault tolerant routing for SCI networks implemented on Dolphin Interconnect Solutions hardware. By dynamic fault tolerance, we mean that the interconnection network reroutes affected packets around a fault, while the rest of the network is fully functional. To the best of our knowledge this is the first reported case of dynamic fault tolerant routing available on commercial off the shelf interconnection network technology without duplicating hardware resources. The development is focused around a 2-D torus topology, and is compatible with the existing hardware, and software stack. We look into the existing mechanisms for routing in SCI. We describe how to make the nodes that detect the faulty component do routing decisions, and what changes are needed in the existing routing to enable support for local rerouting. The new routing algorithm is tested on clusters with real hardware. Our tests show that distributed databases like MySQL can run uninterruptedly while the network reacts to faults. The solution is now part of Dolphin Interconnect Solutions SCI driver, and hardware development to further decrease the reaction time is underway.

Component failures and planned component replacements cause changes in the topology and routing paths supplied by the interconnection network of a parallel processor system over time. Such changes may require the network to be reconfigured such that the existing routing function is replaced by one that enables packets to reach their intended destinations amid the changes. Efficient reconfiguration methods are desired which allow the network to function uninterruptedly over the course of the reconfiguration process while remaining free from deadlocking behavior. In this paper, we propose, evaluate, and prove the deadlock freedom of a new network reconfiguration protocol that overlaps various phases of "static" reconfiguration processes traditionally used in commercial and research systems to provide performance efficiency on par with that of recently proposed "dynamic" reconfiguration processes but without their complexity. Simulation results show that the proposed Overlapping Static Reconfiguration protocol can reduce reconfiguration time by up to 50 percent, reduce packet latency by several orders of magnitude, reduce packet dropping by an order of magnitude, and provide unhalted packet injection as compared to traditional static reconfiguration while allowing network throughput similar to dynamic reconfiguration.

Recovery of traffic in connectionless pure IP networks has traditionally been handled by a full re-convergence of the network state. This process operates in a time scale that is not compatible with new real time and highly dependable services. Recently, schemes for fast local and proactive recovery in connectionless IP networks have been proposed. All these schemes are designed to guarantee recovery of the failure of one component. As IP protocols are used to carry more highly dependable services and new wireless infrastructures are approaching, guaranteed failure coverage of more than one failure becomes necessary. In this paper we present and evaluate a scheme that guarantees to handle any two concurrent failures in a network. We are not aware of any other schemes that addresses such guarantees. We evaluate and compare it with other known recovery schemes, and we show how it gives substantially better recovery success rates than the schemes designed for one fault tolerance, also for more than two failures.

The interconnection network plays a key role in the overall performance achieved by high performance computing systems, also contributing an increasing fraction of its cost and power consumption. Current trends in interconnection network technology suggest that high-radix switches will be preferred as networks will become smaller (in terms of switch count) with the associated savings in packet latency, cost, and power consumption. Unfortunately, current switch architectures have scalability problems that prevent them from being effective when implemented with a high number of ports. In this paper, an efficient and cost-effective architecture for high-radix switches is proposed. The architecture, referred to as Partitioned Crossbar Input Queued (PCIQ), relies on three key components: a partitioned crossbar organization that allows the use of simple arbiters and crossbars, a packet-based arbiter, and a mechanism to eliminate the switch-level HOL blocking. Under uniform traffic, maximum switch efficiency is achieved. Furthermore, switch-level HOL blocking is completely eliminated under hot-spot traffic, again delivering maximum throughput. Additionally, PCIQ inherently implements an efficient congestion management technique that eliminates all the network-wide HOL blocking. On the contrary, the previously proposed architectures either show poor performance or they require significantly higher costs than PCIQ (in both components and complexity).

Fault tolerance is critical for efficient utilisation of large computer systems. Dynamic fault tolerance allows the network to remain available through the occurance of faults as opposed to static fault tolerance which requires the network to be halted to reconfigure it. Although dynamic fault tolerance may lead to less efficient solutions than static fault tolerance, it allows for a much higher availability of the system. In this paper we devise a dynamic fault tolerant adaptive routing algorithm for the fat tree, a much used interconnect topology, which relies on misrouting around link faults. We show that we are guaranteed to tolerate any combination of less than (num_switch_ports)/2 link faults without the need for additional network resources for deadlock freedom. There is also a high probability of tolerating an even larger number of link faults. Simulation results show that network performance degrades very little when faults are dynamically tolerated

Abstract — As the Internet takes an increasingly central role in our communications infrastructure, the slow convergence of routing protocols after a network failure becomes a growing problem. To assure fast recovery from link and node failures in IP networks, we present a new recovery scheme called Multiple Routing Configurations (MRC). MRC is based on keeping additional routing information in the routers, and allows packet forwarding to continue on an alternative output link immediately after the detection of a failure. Our proposed scheme guarantees recovery in all single failure scenarios, using a single mechanism to handle both link and node failures, and without knowing the root cause of the failure. MRC is strictly connectionless, and assumes only destination based hop-by-hop forwarding. It can be implemented with only minor changes to existing solutions. In this paper we present MRC, and analyze its performance with respect to scalability, backup path lengths, and load distribution after a failure. I.

An increasing amount of interconnect technologies rely on source routing to forward packets through the network. It is therefore important to develop methods for fault tolerance that are well suited for source routed networks. Dynamic fault tolerance allows the network to remain available through the occurrence of faults, as opposed to static fault tolerance which requires the network to be halted to reconfigure it. Source routing readily supports the source node choosing a different path when a fault occurs, but using this approach, packets already in the network will be lost. Local dynamic fault tolerance, where the packet is routed around the fault locally, would prevent much of the traffic being lost during failures, but this is cumbersome to achieve in source routed networks since packets encountering a fault will need to follow a path different from that encoded in the packet header. In this paper we present a mechanism to achieve local dynamic fault tolerance in source routed fat trees, a topology that has widespread use in supercomputer systems, and compare it with endpoint dynamic fault tolerance. We also show that by combining the two approaches we achieve performance superior to any of the two individually

Massively parallel computing systems are being built with thousands of nodes. The interconnection network plays a key role for the performance of such systems. However, the high number of components significantly increases the probability of failure. Additionally, failures in the interconnection network may isolate a large fraction of the machine. It is therefore critical to provide an efficient fault-tolerant mechanism to keep the system running, even in the presence of faults. This paper presents a new fault-tolerant routing methodology that does not degrade performance in the absence of faults and tolerates a reasonably large number of faults without disabling any healthy node. In order to avoid faults, for some source-destination pairs, packets are first sent to an intermediate node and then from this node to the destination node. Fully adaptive routing is used along both subpaths. The methodology assumes a static fault model and the use of a checkpoint/restart mechanism. However, there are scenarios where the faults cannot be avoided solely by using an intermediate node. Thus, we also provide some extensions to the methodology. Specifically, we propose disabling adaptive routing and/or using misrouting on a per-packet basis. We also propose the use of more than one intermediate node for some paths. The proposed fault-tolerant routing methodology is extensively evaluated in terms of fault tolerance, complexity, and performance.

Freedom from deadlock is a key issue in cut-through, wormhole, and store and forward networks, and such freedom is usually obtained through careful design of the routing algorithm. Most existing deadlock-free routing methods for irregular topologies do, however, impose severe limitations on the available routing paths. We present a method called layered routing, which gives rise to a series of routing algorithms, some of which perform considerably better than previous ones. Our method groups virtual channels into network layers and to each layer it assigns a limited set of source/destination address pairs. This separation of traffic yields a significant increase in routing efficiency. We show how the method can be used to improve the performance of irregular networks, both through load balancing and by guaranteeing shortest-path routing. The method is simple to implement, and its application does not require any features in the switches other than the existence of a modest number of virtual channels. The performance of the approach is evaluated through extensive experiments within three classes of technologies. These experiments reveal a need for virtual channels as well as an improvement in throughput for each technology class.

Existing solutions for fault-tolerant routing in interconnection networks either work for only one given regular topology, or require slow and costly network reconfigurations that do not allow full and continuous network access. In this paper, we present FRroots, a routing method for fault tolerance in topology-flexible network technologies. Our method is based on redundant paths, and can handle single dynamic faults without sending control messages other than those that are needed to inform the source nodes of the failing component. Used in a modus with local rerouting, the source nodes need not be informed and no control messages are necessary for the network to stay connected despite of a single fault. In fault-free networks under nonuniform traffic our routing method performs comparable to, or even better than, topology specific routing algorithms in regular networks like meshes and tori. FRoots does not require any other features in the switches or end nodes than a flexible routing table, and a modest number of virtual channels. For that reason, it can be directly applied to several present day technologies like InfiniBand and advanced switching

Most existing methods for network recovery are often complex and seldom used by network administrators. In this paper we present a novel approach for global and local recovery named resilient routing layers (RRL). The method is supported by algorithms, but also simple enough for a network administrator to implement by hand for reasonably sized networks. The idea in our approach is that for each node in the network there is a topology subset called a "safe layer", which can handle any traffic affected by a fault in the node itself, or any of its links. We demonstrate that our approach performs well compared to other comparable methods in a wide range of different network topologies. Particularly, we demonstrate RRLs performance for what are assumed to be the weakest parameters for our method, i.e., backup-path lengths and state information overhead. We discuss implementation issues of RRL, and demonstrate its applicability to MPLS networks.

We present a novel scheme for network recovery, named resilient routing layers (RRL). Our proposed scheme is based on calculating fully connected topology subsets, termed layers, which are used to forward traffic in case of a network failure. For the purpose of this work, the layers are created to protect against link failures only. RRL keeps pre-calculated backup routing information in the network stations. This allows local response to network failures, which gives recovery in the order of milliseconds. The main strengths of our approach are its flexibility, as it is independent of the network technology used, and its simplicity, as it offers the network operator a simple and coherent view of the resources available after a link failure. We also show that our scheme scales well for networks of several hundred nodes.

This paper develops theoretical support useful for determining deadlock properties of dynamic network reconfiguration techniques and also serves as a basis for the development of design methodologies useful for deriving deadlock-free reconfiguration techniques. It is applicable to interconnection networks typically used in multiprocessor servers, network-based computing clusters, and distributed storage systems, and also has potential application to system-on-chip networks. This theory builds on basic principles established by previous theories while pioneering new concepts fundamental to the case of dynamic network reconfiguration.

For pt.I see ibid., vol.16, no.5, p.412-427 (2005). Dynamic network reconfiguration is defined as the process of changing from one routing function to another while the network remains up and running. The main challenge is in avoiding deadlock anomalies while keeping restrictions on packet injection and forwarding minimal. Current approaches either require virtual channels in the network or they work only for a limited set of routing algorithms and/or fault patterns. In this paper, we present a methodology for devising deadlock free and dynamic transitions between old and new routing functions that is consistent with newly proposed theory [J. Duato et al., (2005)]. The methodology is independent of topology, can be applied to any deadlock-free routing function, and puts no restrictions on the routing function changes that can be supported. Furthermore, it does not require any virtual channels to guarantee deadlock freedom. This research is motivated by current trends toward using increasingly larger Internet and transaction processing servers based on clusters of PCs that have very high availability and dependability requirements, as well as other local, system, and storage area network-based computing systems.

Current massively parallel computing systems are being built with thousands of nodes, which significantly affect the probability of failure. M. E. Gomex proposed a methodology to design fault-tolerant routing algorithms for direct interconnection networks. The methodology uses a simple mechanism: for some source-destination pairs, packets are first forwarded to an intermediate node, and later, from this node to the destination node. Minimal adaptive routing is used along both subpaths. For those cases where the methodology cannot find a suitable intermediate node, it combines the use of intermediate nodes with two additional mechanisms: disabling adaptive routing and using misrouting on a per-packet basis. While the combination of these three mechanisms tolerates a large number of faults, each one requires adding some hardware support in the network and also introduces some overhead. In this paper, we perform an in-depth detailed analysis of the impact of these mechanisms on network behaviour. We analyze the impact of the three mechanisms separately and combined. The ultimate goal of this paper is to obtain a suitable combination of mechanisms that is able to meet the trade-off between fault-tolerance degree, routing complexity, and performance.

Summary form only given. Dynamic faults in irregular networks are difficult to handle when discarding uncorrupted packets is undesirable, deadlocks must be avoided, and in-order delivery of packets must be maintained. Current reconfiguration approaches involve all or many nodes to handle a fault. We introduce the LORE approach, a dynamic local reconfiguration which allows most of the network to remain oblivious of a one-fault. Only a small subnetwork, consisting of fault adjacent nodes and some nodes and links to connect these, is affected. LORE can be adapted to several topologies and several routing algorithms without introducing deadlocks, and can maintain in-order delivery in deterministic routing.

Dynamic network reconfiguration is defined as the change from one routing function to another while the network is up and running. The main challenge is avoidance of deadlocks, while keeping restrictions on packet injection and forwarding minimal. Current approaches either require virtual channels in the network, or they work only for a limited set of routing algorithms. We present a methodology for devising deadlock free and dynamic transitions between an old and a new routing function. The methodology is independent of topology and puts no restrictions on either routing function. Furthermore, it does not require any virtual channels to guarantee deadlock freedom. This research is motivated by the current trend toward using increasingly larger Internet servers based on clusters of PCs and the very high availability requirements of those as well as other local, system, and storage area network-based systems

Real-time content-based access to live video data requires content analysis applications that are able to process the video data at least as fast as the video data is made available to the application and with an acceptable error rate. Statements as this express quality of service (QoS) requirements to the application. In order to provide some level of control of the QoS provided, the video content analysis application must be scalable and resource aware so that requirements of timeliness and accuracy can be met by allocating additional processing resources.In this paper we present a general architecture of video content analysis applications including a model for specifying requirements of timeliness and accuracy. The salient features of the architecture include its combination of probabilistic knowledge-based media content analysis with QoS and distributed resource management to handle QoS requirements, and its independent scalability at multiple logical levels of distribution. We also present experimental results with an algorithm for QoS-aware selection of configurations of feature extractor and classification algorithms that can be used to balance requirements of timeliness and accuracy against available processing resources. Experiments with an implementation of a real-time motion vector based object-tracking application, demonstrate the scalability of the architecture.

Abstract. One way to reason about parallel processes is to assume that the execution of each process is subdivided into ‘small enough’ steps, and that these are executed in an interleaved fashion, thus obtaining a sequential program. The steps should be so small that for any parallel execution there will, in a suitable sense, exist a corresponding interleaved execution ending in the same state. The usual way to ensure this is to require that each step should contain at most one global access. However, if the global entities are communication channels, then larger steps may in some cases be allowed, and this may make reasoning about the programs easier. This paper explores these cases, and discusses consequences or verification and deadlock avoidance.

Exploitation of the wiring flexibility in Networks of Workstations demands configuration methods that can handle dynamic changes in irregular topologies. During reconfiguration of a network based on virtual cut-through or wormhole switching, however deadlocks in the transition phase between the old and the new routing function must be avoided. The avoidance of such deadlocks will in general make the performance of the network suffer during reconfiguration. Keeping reconfiguration time as short as possible, and leaving as much as possible of the network untouched is therefore of importance. We propose a method for dynamic reconfiguration of networks using up*/down* routing that aims at reducing the consequences of reconfiguration. This is done by identifying a restricted parr of the network, the skyline, as the only part where a full reconfiguration is necessary. This means that most of the network does not need to take part in the reconfiguration at all (other than adding entries for new nodes, and removing entries for removed nodes). Experiments show that for the most frequent configuration changes the skyline will be empty in 85-95% of the cases, leaving the whole of the network operational through the entire reconfiguration. For the most dramatic changes in topology-the addition of a link connecting two previously disjoint networks-an average of 90% of the links can start using the new routing function immediately for some topologies. Our approach is in principle orthogonal to other approaches, thus existing methods for dynamic reconfiguration can be applied in the reconfiguration of the skyline.

We consider the use of wormhole routed networks as internal interconnects of switches. The interconnection of two wormhole networks that are free from deadlocks does not necessarily produce a deadlock free network. This is problematic when wormhole networks are used as switch-internal fabrics, because it severely restricts the possibilities for coupling the switches together. This paper presents a theory that can be effectively used to control this phenomenon caused by aggregated dependencies. We show that all aggregated dependencies can be removed from wormhole networks by careful inclusion of a limited number of virtual channels.

Abstract The background for this paper is twofold: One is the definition of a caching protocol for shared memory parallel computers called SCI, and the other is the usage of rewriting techniques in program verification. The paper concentrates on a linked list system, which is a central aspect of the caching protocol. We first describe an informal proof of this system, including a rather large invariant. Thereafter we show how the list system and the invariant can both be described in the formalism of rewriting logic, and we use this to carry through a significant part of the verification mechanically, using the OBJ3 interpreter.