Dean H. Lorenz

QoS routing in networks with uncertain parameters

We consider the problem of routing connections with quality of service (QoS) requirements across networks when the information available for making routing decisions is inaccurate. Such uncertainty about the actual state of a network component arises naturally in a number of different environments. The goal of the route selection process is then to identify a path that is most likely to satisfy the QoS requirements. For end-to-end delay guarantees, this problem is intractable. However, we show that by decomposing the end-to-end constraint into local delay constraints, efficient and tractable solutions can be established. Moreover, we argue that such decomposition better reflects the interoperability between the routing and reservation phases. We first consider the simpler problem of decomposing the end-to-end constraint into local constraints for a given path. We show that, for general distributions, this problem is also intractable. Nonetheless, by defining a certain class of probability distributions, which includes typical distributions, and restricting ourselves to that class, we are able to establish efficient and exact solutions. We then consider the general problem of combined path optimization and delay decomposition and present efficient solutions. Our findings are applicable also to a broader problem of finding a path that meets QoS requirements at minimal cost, where the cost of each link is some general increasing function of the QoS requirements from the link.

A simple efficient approximation scheme for the restricted shortest path problem

In this short paper we give a very simple fully polynomial approximation scheme for the restricted shortest path problem. The complexity of this @e-approximation scheme is O(|E|n(loglogn+1/@e)), which improves Hassins original result (Math. Oper. Res. 17 (1) (1992) 36) by a factor of n. Furthermore, this complexity bound is valid for any graph, regardless of the cost values. This generalizes Hassins results which apply only to acyclic graphs. Our algorithm is based on Hassins original result with two improvements. First we modify Hassins result and achieve time complexity of O(|E|n(loglog(UB/LB)+1/@e)), where UB and LB are upper and lower bounds for the problem. This modified version can be applied to general graphs with any cost values. Then we combine it with our second contribution, which shows how to find an upper and a lower bound such that UB/LB=

Optimal partition of QoS requirements on unicast paths and multicast trees

We investigate the problem of optimal resource allocation for end-to-end QoS requirements on unicast paths and multicast trees. Specifically, we consider a framework in which resource allocation is based on local QoS requirements at each network link, and associated with each link is a cost function that increases with the severity of the QoS requirement. Accordingly, the problem that we address is how to partition an end-to-end QoS requirement into local requirements, such that the overall cost is minimized. We establish efficient (polynomial) solutions for both unicast and multicast connections. These results provide the required foundations for the corresponding QoS routing schemes, which identify either paths or trees that lead to minimal overall cost. In addition, we show that our framework provides better tools for coping with other fundamental multicast problems, such as dynamic tree maintenance.

Guaranteeing High Availability Goals for Virtual Machine Placement

The placement of virtual machines (VMs) on a cluster of hosts under multiple constraints, including administrative (security, regulations) resource-oriented (capacity, energy), and QoS-oriented (performance) is a highly complex task. We define a new high-availability property for a VM, when a VM is marked as k-resilient, as long as there are up to k host failures, it should be guaranteed that it can be relocated to a non-failed host without relocating other VMs. Together with Hardware Predictive Failure Analysis and live migration, which enable VMs to be evacuated from a host before it fails, this property allows the continuous running of VMs on the cluster despite host failures. The complexity of the constraints associated with k-resiliency, which are naturally expressed by Second Order logic statements, prevented their integration into the placement computation until now. We present a novel algorithm which enables this integration by transforming the k-resiliency constraints to rules consumable by a generic Constraint Programming engine, prove that it guarantees the required resiliency and describe the implementation. We provide some preliminary results and compare our high availability support with naive solutions.

Efficient QoS partition and routing of unicast and multicast

In this paper, we study problems related to supporting unicast and multicast connections with quality of service (QoS) requirements. We investigate the problem of optimal routing and resource allocation in the context of performance dependent costs. In this context, each network element can offer several QoS guarantees, each associated with a different cost. This is a natural extension to the commonly used bi-criteria model, where each link is associated with a single delay and a single cost. This framework is simple yet strong enough to model many practical interesting networking problems. An important problems in this framework is finding a good path for a connection that minimizes the cost while retaining the end-to-end delay requirement. Once such a path (or a tree, in the multicast case) is found, one needs to partition the end-to-end QoS requirements among the links of the path (tree). We consider the case of general integer cost functions (where delays and cost are integers). As the related problem is NP complete, we concentrate on finding efficient epsiv-approximation solutions. We improve on recent previous results by Erguumln Lorenz and Orda, and Raz and Shavitt, both in terms of generality as well as in terms of complexity of the solution. In particular, we present novel approximation techniques that yield the best known complexity for the unicast QoS routing problem, and the first approximation algorithm for the QoS partition problem on trees, both for the centralized and distributed cases

Virtual Appliance Content Distribution for a Global Infrastructure Cloud Service

Cloud Computing in general and Virtualized Infrastructure Provisioning in particular, are significant trends with the potential to increase agility and lower costs of IT. An emerging cloud service is a virtual server shop, that allows cloud customers to order virtual appliances to be delivered virtually on the cloud. Like physical shops, customers want to customize the ordered products, e.g., have them pre-installed with their desired applications and pre-configured. Global cloud providers need to create customized virtual-server disk images and deliver them on time to meet the customer reservations and service level. This framework creates a new flavor of content distribution over the web, where large virtual server images need to be delivered to the target compute farms (either on the global cloud or on customer private clouds). In order to reduce provisioning time and meet reservation deadlines, one approach is to stage images on storage near the customer. This introduces an optimization problem of finding an optimal staging schedule, according to network bandwidth, pending reservations schedule, and customer value. This problem has some similarities to cache pre-filling and production-line scheduling. It combines scheduling, bandwidth considerations, and storage capacity constraints. In this paper we study the fundamental properties of this approach and formalize several flavors of the related optimization problem. We prove useful properties of the problem and then use those properties to provide exact efficient algorithms to solve it. We also derive efficient approximate solutions with proven error bounds.

IP mobility to support live migration of virtual machines across subnets

User-transparent live migration is one of the most interesting features of Virtual Machine (VM) environments. Current live-migration technologies require that the VM retains its IP network address; therefore, are typically restricted to movement within an IP subnet. The growing number of portable computing devices has led to the development of IP mobility solutions that enable uninterrupted network connectivity while moving between different IP subnets. In this paper we study the application of current network mobility approaches to VM cross-subnet live-migration. We show that although the core problems are similar, there are significant differences between these domains, in terms of both assumptions and requirements. We present a specific solution for live migration of a VM across IP subnets, and introduce a new framework for synchronizing migration and network configuration, which allows better optimization for different scenarios of live migration.

A Methodology for Processor Implementation Verification

We address the problem of verification of implementations of complex processors using architectural level automatic test program generators. A number of automatic test program generators exist, and are widely used for verification of the compliance of complex processors with their architectures. We define a four stage verification process: (1) describing the processor implementation control as a Finite State Machine (2) deriving transition coverage on the FSM using methods from formal verification (3) translation of the covering tours to constraints on test programs (4) generation of test programs for each set of constraints. This process combines a high quality and well defined theoretical method along with tools used in industrial practice. There are a number of advantages of our Method: (a) The last three stages are automated (b) Implementing the FSM model involves relatively little expert designers time (c) The method is feasible for modem superscalar processors and was studied on an enhanced PowerPC processor. We describe a formal framework for the new process, identify the obstacles that are encountered in the modeling phase, and show how to overcome them.

Optimal partition of QoS requirements for many-to-many connections

The problems related to supporting multicast connections with quality of service (QoS) requirements are studied. We investigate the problem of optimal resource allocation in the context of performance dependent costs. In this context each network element can offer several QoS guarantees, each associated with a different cost. This is a natural extension to the commonly used bi-criteria model, where each link is associated with a single delay and a single cost. This framework is simple yet strong enough to model many practical interesting networking problems. The fundamental multicast resource allocation problem under this framework is how to optimally allocate QoS requirements on the links of the multicast tree. One needs to partition the end-to-end QoS requirement along the various paths in a tree. The goal is to satisfy the end-to-end QoS requirement with minimum cost. Previous studies under this framework considered single-source multicast connections, where the end-to-end QoS requirement is specified from the source to all other multicast group members. In this paper we extend these results to the more general, and considerably harder case of multicast sessions, where the end-to-end requirement hold for every path between any two multicast group members. Our aim is to provide rigorous solutions, with proven performance guarantees, by way of algorithmic analysis. The problem under investigation is NP hard for general cost functions, thus we first present a pseudopolynomial exact solution. From this solution we derive two efficient /spl epsi/-approximate solutions. One achieves optimal cost, but may violate the end-to-end delay requirement by a factor of (1 + /spl epsi/), and the other strictly obeys the bounds and achieves a cost within a factor of (1+/spl epsi/) of the optimum. Furthermore, we present improved results for discrete cost functions, and give a simple linear-time exact polynomial solution for a specific, and practically interesting, family of convex cost functions.

DYNAMO - DirectorY, Net Archiver and MOver

The Grid communities efforts on managing and transporting data have focused on very large data sets consisting of very large elements. We are interested in leveraging the benefits of solutions such GridFTP, in particular with respect to parallel data transfer and restartability (as well as security, third party control, etc.), for moving large data sets consisting of very large numbers of small objects, e.g., moving a file system subtree. In addition, we require a solution that 1) imposes constant memory overhead on the client and server systems, 2) is independent of the actual transfer mechanism used so we can easily take advantage of advances in technologies for transferring large files, 3) works well even for very large collections of very small files and 4) is a complete solution, i.e., reproduces the directory tree at the server. In this paper, we present DYNAMO which is our tool built on GridFTP for transferring directory subtrees. In addition to describing the architecture and implementation of DYNAMO, we present performance results on a range of networks showing that we have met our goals.