La virtualisation des fonctions réseau constitue une révolution pour les opérateurs de télécommun... more La virtualisation des fonctions réseau constitue une révolution pour les opérateurs de télécommunications. Après avoir exposé le contexte de ce paradigme, nous allons en décrire les éléments majeurs et le fonctionnement. L’identification des enjeux en termes de fiabilité et de disponibilité sera ensuite abordée, suivie de la présentation des solutions. L’application de ces solutions sera vue à travers la preuve de concept constituée par la voix sur le protocole Internet à travers la technologie IMS. Les travaux à venir, ainsi que les tâches à traiter avant une mise en exploitation réelle, seront enfin évoqués dans la conclusion.Network functions virtualisation appears to be a revolution for telecommunications operators. After exposing the context leading to this new paradigm, we will describe its major elements and overall functioning. The identification of new challenges in terms of reliability and availability will then be treated, followed by a presentation of the mechanims to so...
ICC 2021 - IEEE International Conference on Communications
To handle the ever growing demand of resource intensive experiments distributed, network emulatio... more To handle the ever growing demand of resource intensive experiments distributed, network emulation tools such as Mininet and Maxinet have been proposed. They automatically allocate experimental resources. In this work, we show that resources are poorly allocated, leading to resource overloading and hence to dubious experimental results. This is why we propose and implement a new placement module for distributed emulation. Our algorithms take into account both link and node resources and minimize the number of physical hosts needed to carry out the emulation. Through extensive numerical evaluations, simulations, and actual experiments, we show that our placement methods outperform existing ones and allowing to re-establish trust in experimental results.
Le Centre pour la Communication Scientifique Directe - HAL - Université de Nantes, Feb 5, 2021
With the increased complexity of today's networks, emulation has become an essential tool to test... more With the increased complexity of today's networks, emulation has become an essential tool to test and validate a new proposed networking solution. As these solutions also become more and more complex with the introduction of softwarization, network function virtualization, and artificial intelligence, there is a need of scalable tools to carry out resource intensive emulations. To this end, distributed emulation has been proposed. However, distributing a network emulation over a physical platform requires to choose carefully how the experiment is run over the equipment at disposal. In this work, we evaluate the placement algorithms which were proposed for, and implemented in, existing distributed emulation tools. We show that they may lead to bad placements in which several hardware resources such as link bandwidth, CPU, and memory are overloaded. Through extensive experiments, we exhibit the impact of such placements on important network metrics such as real network bandwidth usage and emulation execution time, and show that they may lead to unreliable results and to a waste of platform resources. To deal with this issue, we propose and implement a new placement module for distributed emulation. Our algorithms take into account both link and node resources and minimize the number of physical hosts needed to carry out the emulation. Through extensive numerical evaluations, simulations, and experiments, we show that our placement methods outperform existing ones leading to reliable experiments using a minimum number of resources.
2019 IEEE 8th International Conference on Cloud Networking (CloudNet)
With the emergence of Network Function Virtualization (NFV) and Software Defined Networking (SDN)... more With the emergence of Network Function Virtualization (NFV) and Software Defined Networking (SDN) efficient network algorithms considered too hard to be put in practice in the past now have a second chance to be considered again. In this context, we rethink the network dimensioning problem with protection against Shared Risk Link Group (SLRG) failures. In this paper, we consider a path-based protection scheme with a global rerouting strategy, in which, for each failure situation, there may be a new routing of all the demands. Our optimization task is to minimize the needed amount of bandwidth. After discussing the hardness of the problem, we develop a scalable mathematical model that we handle using the Column Generation technique. Through extensive simulations on real-world IP network topologies and on random generated instances, we show the effectiveness of our method. Finally, our implementation in OpenDaylight demonstrates the feasibility of the approach and its evaluation with Mininet shows that technical implementation choices may have a dramatic impact on the time needed to reestablish the flows after a failure takes place.
2021 IEEE 7th International Conference on Network Softwarization (NetSoft)
Many companies and organizations are moving their applications from on-premises data centers to t... more Many companies and organizations are moving their applications from on-premises data centers to the cloud. The cloud infrastructures can potentially provide an infinite amount of computation (e.g., Elastic Compute) and storage (e.g., Simple Service Storage). In addition, all cloud providers propose different offers: IaaS, PaaS, and SaaS. This demo focuses on the IaaS services, presenting a simple tool to measure the network delay in a virtual infrastructure built entirely in the cloud. These measurements are useful for organizations that are moving current applications to, or creating new applications in, the cloud, but have requirements on the maximum, or average, network delay that these applications can tolerate. We present CloudTrace, a simple CLI tool that creates regional and multiregional experiments to measure delay, using Amazon AWS.
2021 IEEE Global Communications Conference (GLOBECOM), 2021
Network Function Virtualization (NFV) enables the virtualization of core-business network functio... more Network Function Virtualization (NFV) enables the virtualization of core-business network functions on top of a NFV infrastructure. NFV has gained an increasing attention in the telecommunication field these last few years. Virtual network functions (VNFs) can be represented by a set of virtual network function components (VNFCs). These VNFCs are typically designed with a redundancy scheme and need to be deployed against failures of, e.g., compute servers. However, such deployment must respect a particular resiliency mechanism for protection purposes. Therefore, choosing an efficient mapping of VNFCs to the compute servers is a challenging problem in the optimization of the software-defined, virtualization-based next generation of networks. In this paper, we model the problem of reliable VNFCs placement under anti-affinity constraints using several optimization techniques. A novel approach based on an extension of bin packing is proposed. We perform a comprehensive evaluation in terms of performance under realworld ISP networks along with synthetic traces. We show that our methods can calculate rapidly efficient solutions for large instances.
Networks have become complex systems that combine various concepts, techniques, and technologies.... more Networks have become complex systems that combine various concepts, techniques, and technologies. As a consequence, modelling or simulating them now is extremely complicated and researchers massively resort to prototyping techniques. Mininet is the most popular tool when it comes to evaluate SDN propositions. Mininet allows to emulate SDN networks on a single computer but shows its limitations with resource intensive experiments as the emulating host may become overloaded. To tackle this issue, we propose Distrinet , a distributed implementation of Mininet over multiple hosts, based on LXD/LXC, Ansible, and VXLAN tunnels. Distrinet uses the same API than Mininet, meaning that it is compatible with Mininet programs. It is generic and can deploy experiments on Linux clusters (e.g., Grid'5000), as well as on the Amazon EC2 cloud platform.
are stored in PDF, with the report number as filename. Alternatively, reports are available by po... more are stored in PDF, with the report number as filename. Alternatively, reports are available by post from the above address. ReSIST: Resilience for Survivability in IST
With the ever growing complexity of networks, researchers have to rely on test-beds to be able to... more With the ever growing complexity of networks, researchers have to rely on test-beds to be able to fully assess the quality of their propositions. In the meanwhile, Mininet offers a simple yet powerful API, the goldilocks of network emulators. We advocate that the Mininet API is the right level of abstraction for network experiments. Unfortunately it is designed to be run on a single machine. To address this issue we developed a distributed version of Mininet-Distrinet-that can be used to perform network experiments in any Linux-based testbeds, either public or private. To properly use testbed resources and avoid over-commitment that would lead to inaccurate results, Distrinet uses optimization techniques that determine how to orchestrate the experiments within the testbed. Its programmatic approach, its ability to work on various testbeds, and its optimal management of resources make Distrinet a key element to reproducible research.
2019 IEEE 8th International Conference on Cloud Networking (CloudNet), 2019
Networks have become complex systems that combine various concepts, techniques, and technologies.... more Networks have become complex systems that combine various concepts, techniques, and technologies. As a consequence, modelling or simulating them is now extremely complicated and researchers massively resort to prototyping techniques. Among other tools, Mininet is the most popular when it comes to evaluate SDN propositions. It allows to emulate SDN networks on a single computer. However, under certain circumstances experiments (e.g., resource intensive ones) may overload the host running Mininet. To tackle this issue, we propose Distrinet, a way to distribute Mininet over multiple hosts. Distrinet uses the same API than Mininet, meaning that it is compatible with Mininet programs. Distrinet is generic and can deploy experiments in Linux clusters or in the Amazon EC2 cloud. Thanks to optimization techniques, Distrinet minimizes the number of hosts required to perform an experiment given the capabilities of the hosting infrastructure, meaning that the experiment is run in a single host (as Mininet) if possible. Otherwise, it is automatically deployed on a platform using a minimum amount of resources in a Linux cluster or with a minimum cost in Amazon EC2.
This Deliverable presents the MSc Curriculum in Resilient Computing suggested by ReSIST. It inclu... more This Deliverable presents the MSc Curriculum in Resilient Computing suggested by ReSIST. It includes the description of the syllabi for all the courses in the two semesters of the first year, those for the common courses in semester 3 in the second year together with an exemplification of possible application tracks with the related courses. This MSc curriculum has been updated and completed taking advantage of a large open discussion inside and outside ReSIST. This MSc Curriculum is on-line on the official ReSIST web site, where all ...
Congrès Lambda Mu 19 de Maîtrise des Risques et Sûreté de Fonctionnement, 2015
Les arbres d'attaque sont de puissants outils utilisés pour l'analyse de la sécurité. Nous présen... more Les arbres d'attaque sont de puissants outils utilisés pour l'analyse de la sécurité. Nous présentons dans cette communication une extension du modèle d'arbre d'attaque dans le but de concevoir des stratégies de résilience de coût minimal. Notre modèle est appelé CCT (« Challenge Countermeasure Tree »). Les menaces traitées incluent à la fois les défaillances accidentelles et les attaques intentionnelles. CCT intègre des contre-mesures dans les noeuds de l'arbre d'attaque et ce, à tous les niveaux. Trois phases sont concernées par ces contre-mesures: (i) la prévention afin de réduire la probabilité d'occurrence d'une menace; (ii) la mitigation pour réduire son impact; (iii) la récupération pour retourner à un fonctionnement normal (optimal). Grâce à cette distinction, CCT constitue une bonne méthode de prise de décision qui aide à la sélection de contre-mesures efficaces et à coût minimal. Nous illustrons les caractéristiques de CCT à l'aide d'une étude de cas (présentée lors de la communication orale).
Networks became so complex and technical that it is now hard if not impossible to model or simula... more Networks became so complex and technical that it is now hard if not impossible to model or simulate them. Consequently, more and more researchers rely on prototypes emulated in controlled environments and Mininet is by far the most popular tool. Mininet implements a simple, yet powerful API to define and run network experiments on a single machine. In most cases, running experiments on one machine is adequate but for resource intensive applications one machine may not be sufficient. For that reason we propose Distrinet, a way to distribute Mininet over multiple hosts. Distrinet uses the same API than Mininet, granting full compatibility with Mininet programs. Distrinet is generic and can optimally deploy experiments in Linux clusters or in public clouds and automatically minimizes the resource consumed in the experimental infrastructure.
Congrès Lambda Mu 20 de Maîtrise des Risques et de Sûreté de Fonctionnement, Dec 1, 2016
La virtualisation des fonctions réseau constitue, aux yeux d'un grand nombre d'observateurs, une ... more La virtualisation des fonctions réseau constitue, aux yeux d'un grand nombre d'observateurs, une révolution pour les opérateurs de télécommunications. Après avoir exposé le contexte de ce nouveau paradigme, nous allons décrire les éléments majeurs et le fonctionnement global de cette nouvelle architecture. Identifier les nouveaux enjeux en termes de fiabilité et de disponibilité sera ensuite l'objet de cette communication, suivi de la présentation des solutions pour traiter ces enjeux. L'application de ces solutions sera partiellement abordée à travers la preuve de concept constituée par la voix sur le protocole Internet (VoIP, Voice over Internet Protocol) à travers la technologie IMS (IP Multimedia Subsystem). Les travaux prévus pour la suite de la preuve de concept, ainsi que les tâches qui seront à traiter avant une mise en exploitation réelle, seront enfin évoqués dans la conclusion.
2008 IEEE International Symposium on Consumer Electronics
A home gateway (HG) acts as border router by selecting the correct route to different appliances ... more A home gateway (HG) acts as border router by selecting the correct route to different appliances using devices address and supervising resource assignment between the local and access networks. From the quality of service standpoint, minimizing the number of failures for this equipment is a way to gain customer's confidence. To reach this goal, when a service terminates abnormally, the overall failure cause and impact need to be analyzed and quantified. This paper describes a fault hierarchy modeling for HGs: starting from test and field failure data, we analyzed the different failures gravity, their origin's correlation and their impact on the overall service architectural elements. The hierarchy interpreting the transition from faults to failures, a designer, or tester, can thus predict the consequences of a particular fault from this model. Our study yielded some test sets that help to trace potential failures, and to enhance the design specifications. * We comply with the fault-tolerance community who defines a fault as the source of a failure; the latest event, in turn, becomes a fault if its propagation induces another failure.
La virtualisation des fonctions réseau constitue une révolution pour les opérateurs de télécommun... more La virtualisation des fonctions réseau constitue une révolution pour les opérateurs de télécommunications. Après avoir exposé le contexte de ce paradigme, nous allons en décrire les éléments majeurs et le fonctionnement. L’identification des enjeux en termes de fiabilité et de disponibilité sera ensuite abordée, suivie de la présentation des solutions. L’application de ces solutions sera vue à travers la preuve de concept constituée par la voix sur le protocole Internet à travers la technologie IMS. Les travaux à venir, ainsi que les tâches à traiter avant une mise en exploitation réelle, seront enfin évoqués dans la conclusion.Network functions virtualisation appears to be a revolution for telecommunications operators. After exposing the context leading to this new paradigm, we will describe its major elements and overall functioning. The identification of new challenges in terms of reliability and availability will then be treated, followed by a presentation of the mechanims to so...
ICC 2021 - IEEE International Conference on Communications
To handle the ever growing demand of resource intensive experiments distributed, network emulatio... more To handle the ever growing demand of resource intensive experiments distributed, network emulation tools such as Mininet and Maxinet have been proposed. They automatically allocate experimental resources. In this work, we show that resources are poorly allocated, leading to resource overloading and hence to dubious experimental results. This is why we propose and implement a new placement module for distributed emulation. Our algorithms take into account both link and node resources and minimize the number of physical hosts needed to carry out the emulation. Through extensive numerical evaluations, simulations, and actual experiments, we show that our placement methods outperform existing ones and allowing to re-establish trust in experimental results.
Le Centre pour la Communication Scientifique Directe - HAL - Université de Nantes, Feb 5, 2021
With the increased complexity of today's networks, emulation has become an essential tool to test... more With the increased complexity of today's networks, emulation has become an essential tool to test and validate a new proposed networking solution. As these solutions also become more and more complex with the introduction of softwarization, network function virtualization, and artificial intelligence, there is a need of scalable tools to carry out resource intensive emulations. To this end, distributed emulation has been proposed. However, distributing a network emulation over a physical platform requires to choose carefully how the experiment is run over the equipment at disposal. In this work, we evaluate the placement algorithms which were proposed for, and implemented in, existing distributed emulation tools. We show that they may lead to bad placements in which several hardware resources such as link bandwidth, CPU, and memory are overloaded. Through extensive experiments, we exhibit the impact of such placements on important network metrics such as real network bandwidth usage and emulation execution time, and show that they may lead to unreliable results and to a waste of platform resources. To deal with this issue, we propose and implement a new placement module for distributed emulation. Our algorithms take into account both link and node resources and minimize the number of physical hosts needed to carry out the emulation. Through extensive numerical evaluations, simulations, and experiments, we show that our placement methods outperform existing ones leading to reliable experiments using a minimum number of resources.
2019 IEEE 8th International Conference on Cloud Networking (CloudNet)
With the emergence of Network Function Virtualization (NFV) and Software Defined Networking (SDN)... more With the emergence of Network Function Virtualization (NFV) and Software Defined Networking (SDN) efficient network algorithms considered too hard to be put in practice in the past now have a second chance to be considered again. In this context, we rethink the network dimensioning problem with protection against Shared Risk Link Group (SLRG) failures. In this paper, we consider a path-based protection scheme with a global rerouting strategy, in which, for each failure situation, there may be a new routing of all the demands. Our optimization task is to minimize the needed amount of bandwidth. After discussing the hardness of the problem, we develop a scalable mathematical model that we handle using the Column Generation technique. Through extensive simulations on real-world IP network topologies and on random generated instances, we show the effectiveness of our method. Finally, our implementation in OpenDaylight demonstrates the feasibility of the approach and its evaluation with Mininet shows that technical implementation choices may have a dramatic impact on the time needed to reestablish the flows after a failure takes place.
2021 IEEE 7th International Conference on Network Softwarization (NetSoft)
Many companies and organizations are moving their applications from on-premises data centers to t... more Many companies and organizations are moving their applications from on-premises data centers to the cloud. The cloud infrastructures can potentially provide an infinite amount of computation (e.g., Elastic Compute) and storage (e.g., Simple Service Storage). In addition, all cloud providers propose different offers: IaaS, PaaS, and SaaS. This demo focuses on the IaaS services, presenting a simple tool to measure the network delay in a virtual infrastructure built entirely in the cloud. These measurements are useful for organizations that are moving current applications to, or creating new applications in, the cloud, but have requirements on the maximum, or average, network delay that these applications can tolerate. We present CloudTrace, a simple CLI tool that creates regional and multiregional experiments to measure delay, using Amazon AWS.
2021 IEEE Global Communications Conference (GLOBECOM), 2021
Network Function Virtualization (NFV) enables the virtualization of core-business network functio... more Network Function Virtualization (NFV) enables the virtualization of core-business network functions on top of a NFV infrastructure. NFV has gained an increasing attention in the telecommunication field these last few years. Virtual network functions (VNFs) can be represented by a set of virtual network function components (VNFCs). These VNFCs are typically designed with a redundancy scheme and need to be deployed against failures of, e.g., compute servers. However, such deployment must respect a particular resiliency mechanism for protection purposes. Therefore, choosing an efficient mapping of VNFCs to the compute servers is a challenging problem in the optimization of the software-defined, virtualization-based next generation of networks. In this paper, we model the problem of reliable VNFCs placement under anti-affinity constraints using several optimization techniques. A novel approach based on an extension of bin packing is proposed. We perform a comprehensive evaluation in terms of performance under realworld ISP networks along with synthetic traces. We show that our methods can calculate rapidly efficient solutions for large instances.
Networks have become complex systems that combine various concepts, techniques, and technologies.... more Networks have become complex systems that combine various concepts, techniques, and technologies. As a consequence, modelling or simulating them now is extremely complicated and researchers massively resort to prototyping techniques. Mininet is the most popular tool when it comes to evaluate SDN propositions. Mininet allows to emulate SDN networks on a single computer but shows its limitations with resource intensive experiments as the emulating host may become overloaded. To tackle this issue, we propose Distrinet , a distributed implementation of Mininet over multiple hosts, based on LXD/LXC, Ansible, and VXLAN tunnels. Distrinet uses the same API than Mininet, meaning that it is compatible with Mininet programs. It is generic and can deploy experiments on Linux clusters (e.g., Grid'5000), as well as on the Amazon EC2 cloud platform.
are stored in PDF, with the report number as filename. Alternatively, reports are available by po... more are stored in PDF, with the report number as filename. Alternatively, reports are available by post from the above address. ReSIST: Resilience for Survivability in IST
With the ever growing complexity of networks, researchers have to rely on test-beds to be able to... more With the ever growing complexity of networks, researchers have to rely on test-beds to be able to fully assess the quality of their propositions. In the meanwhile, Mininet offers a simple yet powerful API, the goldilocks of network emulators. We advocate that the Mininet API is the right level of abstraction for network experiments. Unfortunately it is designed to be run on a single machine. To address this issue we developed a distributed version of Mininet-Distrinet-that can be used to perform network experiments in any Linux-based testbeds, either public or private. To properly use testbed resources and avoid over-commitment that would lead to inaccurate results, Distrinet uses optimization techniques that determine how to orchestrate the experiments within the testbed. Its programmatic approach, its ability to work on various testbeds, and its optimal management of resources make Distrinet a key element to reproducible research.
2019 IEEE 8th International Conference on Cloud Networking (CloudNet), 2019
Networks have become complex systems that combine various concepts, techniques, and technologies.... more Networks have become complex systems that combine various concepts, techniques, and technologies. As a consequence, modelling or simulating them is now extremely complicated and researchers massively resort to prototyping techniques. Among other tools, Mininet is the most popular when it comes to evaluate SDN propositions. It allows to emulate SDN networks on a single computer. However, under certain circumstances experiments (e.g., resource intensive ones) may overload the host running Mininet. To tackle this issue, we propose Distrinet, a way to distribute Mininet over multiple hosts. Distrinet uses the same API than Mininet, meaning that it is compatible with Mininet programs. Distrinet is generic and can deploy experiments in Linux clusters or in the Amazon EC2 cloud. Thanks to optimization techniques, Distrinet minimizes the number of hosts required to perform an experiment given the capabilities of the hosting infrastructure, meaning that the experiment is run in a single host (as Mininet) if possible. Otherwise, it is automatically deployed on a platform using a minimum amount of resources in a Linux cluster or with a minimum cost in Amazon EC2.
This Deliverable presents the MSc Curriculum in Resilient Computing suggested by ReSIST. It inclu... more This Deliverable presents the MSc Curriculum in Resilient Computing suggested by ReSIST. It includes the description of the syllabi for all the courses in the two semesters of the first year, those for the common courses in semester 3 in the second year together with an exemplification of possible application tracks with the related courses. This MSc curriculum has been updated and completed taking advantage of a large open discussion inside and outside ReSIST. This MSc Curriculum is on-line on the official ReSIST web site, where all ...
Congrès Lambda Mu 19 de Maîtrise des Risques et Sûreté de Fonctionnement, 2015
Les arbres d'attaque sont de puissants outils utilisés pour l'analyse de la sécurité. Nous présen... more Les arbres d'attaque sont de puissants outils utilisés pour l'analyse de la sécurité. Nous présentons dans cette communication une extension du modèle d'arbre d'attaque dans le but de concevoir des stratégies de résilience de coût minimal. Notre modèle est appelé CCT (« Challenge Countermeasure Tree »). Les menaces traitées incluent à la fois les défaillances accidentelles et les attaques intentionnelles. CCT intègre des contre-mesures dans les noeuds de l'arbre d'attaque et ce, à tous les niveaux. Trois phases sont concernées par ces contre-mesures: (i) la prévention afin de réduire la probabilité d'occurrence d'une menace; (ii) la mitigation pour réduire son impact; (iii) la récupération pour retourner à un fonctionnement normal (optimal). Grâce à cette distinction, CCT constitue une bonne méthode de prise de décision qui aide à la sélection de contre-mesures efficaces et à coût minimal. Nous illustrons les caractéristiques de CCT à l'aide d'une étude de cas (présentée lors de la communication orale).
Networks became so complex and technical that it is now hard if not impossible to model or simula... more Networks became so complex and technical that it is now hard if not impossible to model or simulate them. Consequently, more and more researchers rely on prototypes emulated in controlled environments and Mininet is by far the most popular tool. Mininet implements a simple, yet powerful API to define and run network experiments on a single machine. In most cases, running experiments on one machine is adequate but for resource intensive applications one machine may not be sufficient. For that reason we propose Distrinet, a way to distribute Mininet over multiple hosts. Distrinet uses the same API than Mininet, granting full compatibility with Mininet programs. Distrinet is generic and can optimally deploy experiments in Linux clusters or in public clouds and automatically minimizes the resource consumed in the experimental infrastructure.
Congrès Lambda Mu 20 de Maîtrise des Risques et de Sûreté de Fonctionnement, Dec 1, 2016
La virtualisation des fonctions réseau constitue, aux yeux d'un grand nombre d'observateurs, une ... more La virtualisation des fonctions réseau constitue, aux yeux d'un grand nombre d'observateurs, une révolution pour les opérateurs de télécommunications. Après avoir exposé le contexte de ce nouveau paradigme, nous allons décrire les éléments majeurs et le fonctionnement global de cette nouvelle architecture. Identifier les nouveaux enjeux en termes de fiabilité et de disponibilité sera ensuite l'objet de cette communication, suivi de la présentation des solutions pour traiter ces enjeux. L'application de ces solutions sera partiellement abordée à travers la preuve de concept constituée par la voix sur le protocole Internet (VoIP, Voice over Internet Protocol) à travers la technologie IMS (IP Multimedia Subsystem). Les travaux prévus pour la suite de la preuve de concept, ainsi que les tâches qui seront à traiter avant une mise en exploitation réelle, seront enfin évoqués dans la conclusion.
2008 IEEE International Symposium on Consumer Electronics
A home gateway (HG) acts as border router by selecting the correct route to different appliances ... more A home gateway (HG) acts as border router by selecting the correct route to different appliances using devices address and supervising resource assignment between the local and access networks. From the quality of service standpoint, minimizing the number of failures for this equipment is a way to gain customer's confidence. To reach this goal, when a service terminates abnormally, the overall failure cause and impact need to be analyzed and quantified. This paper describes a fault hierarchy modeling for HGs: starting from test and field failure data, we analyzed the different failures gravity, their origin's correlation and their impact on the overall service architectural elements. The hierarchy interpreting the transition from faults to failures, a designer, or tester, can thus predict the consequences of a particular fault from this model. Our study yielded some test sets that help to trace potential failures, and to enhance the design specifications. * We comply with the fault-tolerance community who defines a fault as the source of a failure; the latest event, in turn, becomes a fault if its propagation induces another failure.
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