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L'algorithme du seau percé (leaky bucket en anglais) permet de contrôler le nombre de paquets passant à chaque seconde par un nœud d'un réseau informatique. Il est souvent confondu à tort avec le seau à jetons. Utilisation L'algorithme du seau percé permet de contrôler le nombre de paquets par seconde passant par un nœud sur un réseau. Il est utilisé en particulier pour fluidifier des échanges irréguliers (shaping) ou pour limiter un débit (policing), mais n'est pas limité à ces seules applications. Fonctionnement Une analogie simple permet de comprendre l'algorithme :

Soit un seau percé en son fond : si le seau n'est pas vide alors le contenu s'y écoule avec un débit constant.
La taille du seau représente la quantité d'informations qui peut y être stockée, mesurée en nombre de paquets.
Lorsqu'un paquet arrive, s'il reste suffisamment d'espace dans le seau, il y est placé. Sinon, le seau déborde (paquet en excès).

Les paquets en excès sont en général je

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Stability and bounds in aggregate scheduling networks

Gianluca Rizzo

We study networks of FIFO nodes, where flows are constrained by arrival curves. A crucial question with these networks is: Can we derive a bound to the maximum delay that a packet can experience when traversing the network, and to the maximum queue size at each node? For a generic FIFO network these are still open issues: Some examples show that, contrary to common sense, no matter how low the maximum node utilization is in the network, it is possible to build an example of an unstable FIFO network. The importance of this issue lies in the necessity of hard bounds on packet delay and queue size, in order to enable QoS guarantees in these networks. For this reason we choose to tackle this problem through a deterministic approach, based on worst-case behavior. Our first result is the determination of a general method to derive sufficient conditions for the stability of a network: We show how, with a proper choice of the observed variables in the network and with the use of network calculus results, it is possible to derive the expression of an operator whose properties are associated to the stability of the network. Exploiting this method on a simple example, we first derive a generalization of the RIN result to heterogeneous settings and to leaky bucket constrained flows. Through some realistic examples, we show how this method allows networks to achieve a level of utilization which is more than three times larger than the best existing result. By applying the general method to three different variable classes, we derive some new sufficient conditions for stability, that perform largely better than all the main existing results, and we show how they can all be derived from the new sufficient conditions. Finally, we present a new formula for the computation of end-to-end delay bounds in a network of GR nodes.

EPFL2008

Chargement

Generalization of the RIN Result to Heterogeneous Networks of Aggregate Schedulers and Leaky Bucket Constrained Flows

Jean-Yves Le Boudec, Gianluca Rizzo

We consider networks of FIFO aggregate schedulers. Quite surprisingly, the natural condition (node utilization inferior to one) in general is not sufficient in these networks to ensure stability (boundedness of delay and backlog at each node). Deriving good sufficient conditions for stability and delay bounds for these networks is of fundamental importance if we want to offer quality of service guarantees in such networks as DiffServ networks, high speed switches and network-on-chips. The main existing sufficient conditions for stability in these networks are the “DiffServ bound” [1] and the Route Interference Number (RIN) result [2]. We use an algebraic approach. First, we develop a model of the network as a dynamical system, and we show how the problem can be reduced to properties of the state transition function. Second, we obtain new sufficient conditions for stability valid without any of the restrictions of the “RIN result”. We show that in practical cases, when flows are leaky bucket constrained, the new sufficient conditions perform better than existing results. We also prove that the “RIN result” can be derived as a special case from our approach. We finally derive an expression for a bound to delay at all nodes.

2007

Chargement

Generalization of the RIN Result to Heterogeneous Networks of Aggregate Schedulers and Leaky Bucket Constrained Flows

Jean-Yves Le Boudec, Gianluca Rizzo

We consider networks of FIFO aggregate schedulers. Quite surprisingly, the natural condition (node utilization inferior to one) in general is not sufficient in these networks to ensure stability (boundedness of delay and backlog at each node). Deriving good sufficient conditions for stability and delay bounds for these networks is of fundamental importance if we want to offer quality of service guarantees in such networks as DiffServ networks, high speed switches and network-on-chips. The main existing sufficient conditions for stability in these networks are the "DiffServ bound" [1] and the Route Interference Number (RIN) result [2]. We use an algebraic approach. First, we develop a model of the network as a dynamical system, and we show how the problem can be reduced to properties of the state transition function. Second, we obtain new sufficient conditions for stability valid without any of the restrictions of the "RIN result". We show that in practical cases, when flows are leaky bucket constrained, the new sufficient conditions perform better than existing results. We also prove that the "RIN result" can be derived as a special case from our approach. We finally derive an expression for a bound to delay at all nodes.

2007

Chargement

Seau à jetons

Le seau à jetons (token bucket) est un algorithme permettant de contrôler le débit passant par un nœud d'un réseau informatique. Il est souvent confondu à tort avec le seau percé (leaky bucket).

Mode de transfert asynchrone

Le mode de transfert asynchrone (en anglais Asynchronous Transfer Mode ou ATM) est un protocole de la couche « liaison de donnée» au sens du « modèle OSI » à commutation de cellules, qui a pour obje

Network scheduler

A network scheduler, also called packet scheduler, queueing discipline (qdisc) or queueing algorithm, is an arbiter on a node in a packet switching communication network. It manages the sequence of