A simple stability condition for RED using TCP mean-field modeling

Congestion on the Internet is an old problem but still a subject of intensive research. The TCP protocol with its AIMD (Additive Increase and Multiplicative Decrease) behavior hides very challenging problems; one of them is to understand the interaction between a large number of users with delayed feedback. This article will focus on two modeling issues of TCP which appeared to be important to tackle concrete scenarios when implementing the model proposed in [Baccelli McDonald Reynier 02] firstly the modeling of the maximum TCP window size: this maximum can be reached quickly in many practical cases; secondly the delay structure: the usual Little-like formula behaves really poorly when queuing delays are variable, and may change dramatically the evolution of the predicted queue size, which makes it useless to study drop-tail or RED (Random Early Detection) mechanisms. Within proposed TCP modeling improvements, we are enabled to look at a concrete example where RED should be used in FIFO routers instead of letting the default drop-tail happen. We study mathematically fixed points of the window size distribution and local stability of RED. An interesting case is when RED operates at the limit when the congestion starts, it avoids unwanted loss of bandwidth and delay variations

File Sharing in P2P : Missing Block Paradigmand upload strategies

Unfinished download is a frequent issue in most peer-to-peer download architectures. For some networks, it seems to be an inherent problem, however even «smart» networks can encounter the whole file but one block downloaded case, which we call the missing block paradigm. We propose a simple and versatile model of file-sharing that applies to all block-oriented file-sharing protocols used in softwares such as MlDonkey or BitTorrent. Simulations using this model show that missing block can occur even with a popular file, and lead to some theoretical explanation. These fundamental results offer a new understanding of downloading issue in file-sharing networks, and show new strategies existing protocols could use

Stratification in P2P Networks - Application to BitTorrent

We introduce a model for decentralized networks with collaborating peers. The model is based on the stable matching theory which is applied to systems with a global ranking utility function. We consider the dynamics of peers searching for efficient collaborators and we prove that a unique stable solution exists. We prove that the system converges towards the stable solution and analyze its speed of convergence. We also study the stratification properties of the model, both when all collaborations are possible and for random possible collaborations. We present the corresponding fluid limit on the choice of collaborators in the random case. As a practical example, we study the BitTorrent Tit-for-Tat policy. For this system, our model provides an interesting insight on peer download rates and a possible way to optimize peer strategy

Enhanced Power Sources Compatible With Extended Thermal Environment

Today the race to lower the cost to orbit is stronger than ever, with the arrival of new competitors like Space-X. A way to lighten European next generation launchers (Ariane 6) is to use novel primary and secondary batteries for powering avionics, pyro systems, guidance and control electronics, as well as communication systems. Such application requires unrestricted operations at low temperature (−20°C) and safety behaviour at high temperature (70°C), in addition to a low self-discharge rate because most of the life time of the power source goes by under storage prior to launch. Several battery technologies could be used for this application. While the major challenge for Lithium primary cells is the power capability, regarding secondary Li-ion batteries it may be self-discharge. Both technologies will need a good low temperature power rate capability. CEA developed 18650 Li-ion cells and AA Li-CFx cells in order to tackle those particular challenges

Mean Field Convergence of a model of multiple TCP connections through a buffer implementing RED

RED (Random Early Detection) have been suggested when multiple TCP sessions are multiplexed through a bottleneck buffer. The idea is to detect congestion before the buffer overflows by dropping or marking packets with a probability that increases with the queue length. The objectives are an equitable distribution of packet loss, reduced synchronization together with reduced packet loss, delay, and delay variation. Baccelli, McDonald and Reynier have proposed a model for multiple TCP connections in the congestion avoidance regime multiplexed through a bottleneck buffer implementing RED. The window sizes of each TCP session evolve like independent dynamical systems coupled by the queue length at the buffer. The key idea in is to consider the histogram of window sizes as a random measure coupled with the queue. Here we prove the conjecture made in that as the number of connections tends to infinity this system converges to a deterministic mean-field limit comprising the window size density coupled with a deterministic queue

A Mean-Field Model for Multiple TCP Connections through a Buffer Implementing RED

Active queue management schemes like RED (Random Early Detection) have been suggested when multiple TCP sessions are multiplexed through a bottleneck buffer. The idea is to detect congestion before the buffer overflows and packets are lost. When the queue length reaches a certain threshold RED schemes drop/mark incoming packets with a probability that increases as the queue size increases. The objectives are an equitable distribution of packet loss, reduced delay and delay variation and improved network utilization. Here we model multiple connections maintained in the congestion avoidance regime by the RED mechanism. The window sizes of each TCP session evolve like independent dynamical systems coupled by the queue length at the buffer. We introduce a mean-field approximation to one such RED system as the number of flows tends to infinity. The deterministic limiting system is described by a transport equation. The numerical solution of the limiting system is found to provide a good description of the evolution of the distribution of the window sizes, the average queue size, the average loss rate per connection and the total throughput. TCP with RED or tail-drop may exhibit limit cycles and this causes unnecessary packet delay variation and variable loss rates. The root cause of these limit cycles is the hysteresis due to the round trip time delay in reacting to a packet loss

Acyclic Preference Systems in P2P Networks

In this work we study preference systems natural for the Peer-to-Peer paradigm. Most of them fall in three categories: global, symmetric and complementary. All these systems share an acyclicity property. As a consequence, they admit a stable (or Pareto efficient) configuration, where no participant can collaborate with better partners than their current ones. We analyze the representation of the such preference systems and show that any acyclic system can be represented with a symmetric mark matrix. This gives a method to merge acyclic preference systems and retain the acyclicity. We also consider such properties of the corresponding collaboration graph, as clustering coefficient and diameter. In particular, studying the example of preferences based on real latency measurements, we observe that its stable configuration is a small-world graph

Experiences pour évaluer le recouvrement calcul/communication des collectoves non-bloquates de MP

We present experimental results for evaluating non-blocking MPI collectives. We compute several metrics to asses the efficiency of the overlap for different MPI library, with different configurations and for different hardware.Nous présentons les résultats d’expériences pour l’évaluation des collectives non bloquantes dans MPI. Nous calculons différentes métriques pour évaluer l’efficacité du recouvrement pour différentes bibliothèques MPI avec différentes configurations et pour différents types de matériels

A locus-specific database for mutations in GDAP1 allows analysis of genotype-phenotype correlations in Charcot-Marie-Tooth diseases type 4A and 2K

<p>Abstract</p> <p>Background</p> <p>The ganglioside-induced differentiation-associated protein 1 gene (<it>GDAP1</it>), which is involved in the Charcot-Marie-Tooth disease (CMT), the most commonly inherited peripheral neuropathy, encodes a protein anchored to the mitochondrial outer membrane. The phenotypic presentations of patients carrying <it>GDAP1 </it>mutations are heterogeneous, making it difficult to determine genotype-phenotype correlations, since the majority of the mutations have been found in only a few unrelated patients. Locus-specific databases (LSDB) established in the framework of the Human Variome Project provide powerful tools for the investigation of such rare diseases.</p> <p>Methods and Results</p> <p>We report the development of a publicly accessible LSDB for the <it>GDAP1 </it>gene. The <it>GDAP1</it> LSDB has adopted the Leiden Open-source Variation Database (LOVD) software platform. This database, which now contains 57 unique variants reported in 179 cases of CMT, offers a detailed description of the molecular, clinical and electrophysiological data of the patients. The usefulness of the <it>GDAP1 </it>database is illustrated by the finding that <it>GDAP1 </it>mutations lead to primary axonal damage in CMT, with secondary demyelination in the more severe cases of the disease.</p> <p>Conclusion</p> <p>Findings of this nature should lead to a better understanding of the pathophysiology of CMT. Finally, the <it>GDAP1 </it>LSDB, which is part of the mitodyn.org portal of databases of genes incriminated in disorders involving mitochondrial dynamics and bioenergetics, should yield new insights into mitochondrial diseases.</p

One core dedicated to MPI nonblocking communication progression? A model to assess whether it is worth it

International audienceOverlapping communications with computation is an efficient way to amortize the cost of communications of an HPC application. To do so, it is possible to utilize MPI nonblocking primitives so that communications run in background alongside computation. However, these mechanisms rely on communications actually making progress in the background, which may not be true for all MPI libraries. Some MPI libraries leverage a core dedicated to communications to ensure communication progression. However, taking a core away from the application for such purpose may have a negative impact on the overall execution time. It may be difficult to know when such dedicated core is actually helpful. In this paper, we propose a model for the performance of applications using MPI nonblocking primitives running on top of an MPI library with a dedicated core for communications. This model is used to understand the compromise between computation slowdown due to the communication core not being available for computation, and the communication speed-up thanks to the dedicated core; evaluate whether nonblocking communication is actually obtaining the expected performance in the context of the given application; predict the performance of a given application if ran with a dedicated core. We describe the performance model and evaluate it on different applications. We compare the predictions of the model with actual executions