Olav Lysne

Fast IP Network Recovery Using Multiple Routing Configurations

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 guarantee s 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.

Multiple routing configurations for fast IP network recovery

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). 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. 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. 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. We also show how an estimate of the traffic demands in the network can be used to improve the distribution of the recovered traffic, and thus reduce the chances of congestion when MRC is used.

Layered routing in irregular networks

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.

A routing methodology for achieving fault tolerance in direct networks

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.

A Survey and Evaluation of Topology-Agnostic Deterministic Routing Algorithms

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.

Layered shortest path (LASH) routing in irregular system area networks

In recent years we have seen a growing interest in irregular network topologies for cluster interconnects. One problem related to such topologies is that the combination of shortest path and deadlock free routing is difficult. As a result of this the existing solutions for routing in irregular networks either guarantee shortest paths relative to some constraint (like up*/down*), or have to resort to deadlock recovery through non-minimal escape channels. In this paper we propose a method that guarantees shortest path routing and in-order delivery, and that uses virtual channels for deadlock avoidance. We present a theoretical upper bound on the number of virtual channels needed, and through extensive empirical testing we demonstrate that the actual number of virtual channels is very low even for large networks.

Fast dynamic reconfiguration in irregular networks

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.

An Efficient Fault-Tolerant Routing Methodology for Meshes and Tori

In this paper we present a methodology to design fault-tolerant routing algorithms for regular direct interconnection networks. It supports fully adaptive routing, does not degrade performance in the absence of faults, and supports a reasonably large number of faults without significantly degrading performance. The methodology is mainly based on the selection of an intermediate node (if needed) for each source-destination pair. Packets are adaptively routed to the intermediate node and, at this node, without being ejected, they are adaptively forwarded to their destinations. In order to allow deadlock-free minimal adaptive routing, the methodology requires only one additional virtual channel (for a total of three), even for tori. Evaluation results for a 4 x 4 x 4 torus network show that the methodology is 5-fault tolerant. Indeed, for up to 14 link failures, the percentage of fault combinations supported is higher than 99.96%. Additionally, network throughput degrades by less than 10% when injecting three random link faults without disabling any node. In contrast, a mechanism similar to the one proposed in the BlueGene/L, that disables some network planes, would strongly degrade network throughput by 79%.

A theory for deadlock-free dynamic network reconfiguration. Part I

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.

Fast recovery from link failures using resilient routing layers

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.