Guy Leduc

A RED discard strategy for ATM networks and its performance evaluation with TCP/IP traffic

In ATM UBR networks supporting TCP traffic, optimal efficiency can only be envisaged if switches adopt a discard mechanism that operates at the packet level rather than the cell level. In this paper, we define a variant of the RED discard strategy suitable for ATM switches. An interesting feature of this ATM-RED is that it has a similar per-VC implementation complexity as the Early Packet Discard (EPD) algorithm. To study the efficiency of the ATM-RED discard strategy, we compare its performance with plain the UBR, EPD and Fair Buffer Acceptance (FBA) discard strategies by means of simulation with TCP/IP traffic. We give comparative results with respect to different performance criteria such as goodput and fairness in various environments, such as end-to-end ATM networks and IP-based networks with an ATM backbone, in both single-bottlenecked and GFC topologies.

An introduction to ET-LOTOS for the description of time-sensitive systems

Abstract Enhanced Timed-LOTOS, called ET-LOTOS, is an extension of LOTOS allowing the modelling of time-sensitive system (i.e. systems whose behaviour is influenced by the passing of time). It is the basis of the timed extension of LOTOS currently developed by ISO (1995). The purpose of this paper is to present ET-LOTOS in a tutorial style and show its applicability. The detailed study of the formal semantics is addressed in another paper. A collection of small, but realistic, examples illustrates a wide variety of time-sensitive protocol mechanisms. These examples are used to introduce and justify the extensions of our language. Finally, the basics of the formal semantics are given and a comparison is made with other timed formalisms.

Network distance prediction based on decentralized matrix factorization

Network Coordinate Systems (NCS) are promising techniques to predict unknown network distances from a limited number of measurements. Most NCS algorithms are based on metric space embedding and suffer from the inability to represent distance asymmetries and Triangle Inequality Violations (TIVs). To overcome these drawbacks, we formulate the problem of network distance prediction as guessing the missing elements of a distance matrix and solve it by matrix factorization. A distinct feature of our approach, called Decentralized Matrix Factorization (DMF), is that it is fully decentralized. The factorization of the incomplete distance matrix is collaboratively and iteratively done at all nodes with each node retrieving only a small number of distance measurements. There are no special nodes such as landmarks nor a central node where the distance measurements are collected and stored. We compare DMF with two popular NCS algorithms: Vivaldi and IDES. The former is based on metric space embedding, while the latter is also based on matrix factorization but uses landmarks. Experimental results show that DMF achieves competitive accuracy with the double advantage of having no landmarks and of being able to represent distance asymmetries and TIVs.

A framework based on implementation relations for implementing LOTOS specifications

A framework is developed for studying the implementation process, as a stepwise process in which an abstract specification is successively transformed to reach a final compilable specification adapted to the computer environment. In this context, an implementation relation is referred to as the relation which should link any “valid” implementation to its abstract formal specification. In other words, the implementation relation is intended to express formally the notion of validity. Our framework allows the exact characterization of the transformations which may take place at each step for a given implementation relation. This framework is essential for dealing with non-transitive implementation relations. In the second part of the paper, these results are exemplified in LOTOS on some existing relations, and an apparent paradox is presented. Some new results about these relations are also derived.

DMFSGD: a decentralized matrix factorization algorithm for network distance prediction

The knowledge of end-to-end network distances is essential to many Internet applications. As active probing of all pairwise distances is infeasible in large-scale networks, a natural idea is to measure a few pairs and to predict the other ones without actually measuring them. This paper formulates the prediction problem as matrix completion where the unknown entries in a pairwise distance matrix constructed from a network are to be predicted. By assuming that the distance matrix has low-rank characteristics, the problem is solvable by low-rank approximation based on matrix factorization. The new formulation circumvents the well-known drawbacks of existing approaches based on Euclidean embedding. A new algorithm, so-called Decentralized Matrix Factorization by Stochastic Gradient Descent (DMFSGD), is proposed. By letting network nodes exchange messages with each other, the algorithm is fully decentralized and only requires each node to collect and to process local measurements, with neither explicit matrix constructions nor special nodes such as landmarks and central servers. In addition, we compared comprehensively matrix factorization and Euclidean embedding to demonstrate the suitability of the former on network distance prediction. We further studied the incorporation of a robust loss function and of nonnegativity constraints. Extensive experiments on various publicly available datasets of network delays show not only the scalability and the accuracy of our approach, but also its usability in real Internet applications.

An open source traffic engineering toolbox

We present the TOTEM open source Traffic Engineering (TE) toolbox and a set of TE methods that we have designed and/or integrated. These methods cover intra-domain and inter-domain TE, IP-based and MPLS-based TE. They are suitable for network optimisation, better routing of traffic for providing QoS, load balancing, protection and restoration in case of failure, etc. The toolbox is designed to be deployed as an on-line tool in an operational network, or used off-line as an optimisation tool or as a traffic engineering simulator.

Combined intra- and inter-domain traffic engineering using hot-potato aware link weights optimization

A well-known approach to intradomain traffic engineering consists in finding the set of link weights that minimizes a network-wide objective function for a given intradomain traffic matrix. This approach is inadequate because it ignores a potential impact on interdomain routing due to hot-potato routing policies. This may result in changes in the intradomain traffic matrix that have not been anticipated by the link weights optimizer, possibly leading to degraded network performance. We propose a BGP-aware link weights optimization method that takes these hot-potato effects into account. This method uses the interdomain traffic matrix and other available BGP data, to extend the intradomain topology with external virtual nodes and links, on which all the well-tuned heuristics of a classical link weights optimizer can be applied. Our method can also optimize the traffic on the interdomain peering links.

A Formal Definition of Time in LOTOS

Abstract. Enhanced Timed-LOTOS, denoted ET-LOTOS, is an extension of LOTOS that allows the modelling of real-time behaviours. It covers all the aspects of full LOTOS, including data types, it supports both a dense and a discrete time domain and can manipulate time values as any other data values. A tutorial on ET-LOTOS, showing many application examples, has already been published [LéL97]. The present paper adds to it by providing an in-depth presentation of its theoretical aspects. The complete semantics is given and explained, and its properties are studied. In particular, we prove that the semantics is consistent and that strong bisimulation is a congruence. This requires to deal carefully with the presence of negative premises in the operational semantics, which are necessary to express urgency. ET-LOTOS is also shown to be a conservative extension of LOTOS for guarded processes, and is the basis of the timed extension of LOTOS currently developed by ISO [ISO98]. To our knowledge, this is the first in-depth study of a language that combines data types and real-time behaviours.

A machine learning approach to improve congestion control over wireless computer networks

In this paper, we present the application of machine learning techniques to the improvement of the congestion control of TCP in wired/wireless networks. TCP is sub-optimal in hybrid wired/wireless networks because it reacts in the same way to losses due to congestion and losses due to link errors. We thus propose to use machine learning techniques to build automatically a loss classifier from a database obtained by simulations of random network topologies. Several machine learning algorithms are compared for this task and the best method for this application turns out to be decision tree boosting. It outperforms ad hoc classifiers proposed in the networking literature.

How well do traffic engineering objective functions meet TE requirements

We compare and evaluate how well-known and novel network-wide objective functions for Traffic Engineering (TE) algorithms fulfil TE requirements. To compare the objective functions we model the TE problem as a linear program and solve it to optimality, thus finding for each objective function the best possible target of any heuristic TE algorithm. We show that all the objective functions are not equivalent and some are far better than others. Considering the preferences a network operator may have, we show which objective functions are adequate or not.