Modeling the interdependency of low-priority congestion control and active queue management

Recently, a negative interplay has been shown to arise when scheduling/AQM techniques and low-priority congestion control protocols are used together: namely, AQM resets the relative level of priority among congestion control protocols. This work explores this issue by (i) studying a fluid model that describes system dynamics of heterogeneous congestion control protocols competing on a bottleneck link governed by AQM and (ii) proposing a system level solution able to reinstate priorities among protocols.Comment: 9 page

Spin-phonon scattering-induced low thermal conductivity in a van der Waals layered ferromagnet Cr2_2Si2_2Te6_6

Layered van der Waals (vdW) magnets are prominent playgrounds for developing magnetoelectric, magneto-optic and spintronic devices. In spintronics, particularly in spincaloritronic applications, low thermal conductivity ($\kappa$) is highly desired. Here, by combining thermal transport measurements with density functional theory calculations, we demonstrate low $\kappa$ down to 1 W m$^{-1}$ K$^{-1}$ in a typical vdW ferromagnet Cr$_2$Si$_2$Te$_6$. In the paramagnetic state, development of magnetic fluctuations way above $T_\mathrm{c}=$ 33 K strongly reduces $\kappa$ via spin-phonon scattering, leading to low $\kappa \sim$ 1 W m$^{-1}$ K$^{-1}$ over a wide temperature range, in comparable to that of amorphous silica. In the magnetically ordered state, emergence of resonant magnon-phonon scattering limits $\kappa$ below $\sim$ 2 W m$^{-1}$ K$^{-1}$, which would be three times larger if magnetic scatterings were absent. Application of magnetic fields strongly suppresses the spin-phonon scattering, giving rise to large enhancements of $\kappa$. Our calculations well capture these complex behaviours of $\kappa$ by taking the temperature- and magnetic-field-dependent spin-phonon scattering into account. Realization of low $\kappa$ which is easily tunable by magnetic fields in Cr$_2$Si$_2$Te$_6$, may further promote spincaloritronic applications of vdW magnets. Our theoretical approach may also provide a generic understanding of spin-phonon scattering, which appears to play important roles in various systems.Comment: 14 pages, 6 figures, accepted for publication in Advanced Functional Material

The quest for low-latency at both network edges : design, analysis, simulation and experiments

Au cours de ces dernières années, les services Internet croissent considérablement ce qui crée beaucoup de nouveaux défis dans des scénarios variés. La performance globale du service dépend à son tour de la performance des multiples segments de réseau. Nous étudions deux défis représentatifs de conception dans différents segments : les deux les plus importants se trouvent sur les bords opposés la connectivité de bout en bout des chemins d’Internet, notamment, le réseau d’accès pour l’ utilisateur et le réseau de centre de données du fournisseur de services.In the recent years, the innovation of new services over Internet is considerably growing at a fast speed, which brings forward lots of new challenges under varied scenarios. The overall service performance depends in turn on the performance of multiple network segments. We investigated two representative design challenges in different segments : the two most important sit at the opposite edges of the end-to-end Internet path, namely, the end-user access network vs. the service provider data center network

La quête de latence faible sur les deux bords du réseau : conception, analyse, simulation et expériences

In the recent years, the innovation of new services over Internet is considerably growing at a fast speed, which brings forward lots of new challenges under varied scenarios. The overall service performance depends in turn on the performance of multiple network segments. We investigated two representative design challenges in different segments : the two most important sit at the opposite edges of the end-to-end Internet path, namely, the end-user access network vs. the service provider data center network.Au cours de ces dernières années, les services Internet croissent considérablement ce qui crée beaucoup de nouveaux défis dans des scénarios variés. La performance globale du service dépend à son tour de la performance des multiples segments de réseau. Nous étudions deux défis représentatifs de conception dans différents segments : les deux les plus importants se trouvent sur les bords opposés la connectivité de bout en bout des chemins d’Internet, notamment, le réseau d’accès pour l’ utilisateur et le réseau de centre de données du fournisseur de services

A control theoretic analysis of low-priority congestion controlreprioritization under AQM

Recently, a negative interplay has been shown to arise when scheduling/Active Queue Management (AQM) techniques and low-priority congestion control protocols are used together; namely, AQM resets the relative level of priority among congestion control protocols. This work explores this issue by carrying out a control-theoretic analysis of the dynamical system to prove some fundamental properties that fully characterize the reprioritization phenomenon. In particular, (i) we provide the closed-form solution of the equilibrium in the open loop (i.e., fixing a target loss probability p); (ii) we provide a stability analysis and a characterization of the reprioritization phenomenon when closing the loop with AQM (i.e., that dynamically adjusts the system loss probability). Our results are important as the characterization of the reprioritization phenomenon is not only quantitatively accurate for the specific protocols and AQM considered but also qualitatively accurate for a broader range of congestion control protocol and AQM combinations. Finally, while we find a sufficient condition to avoid the reprioritization phenomenon, we also show, at the same time, such conditions to be likely impractical: Therefore, we propose a simple and practical system-level solution that is able to reinstate priorities among protocols

Fighting the bufferbloat: on the coexistence of AQM and low priority congestion control (extended version)

International audienceNowadays, due to excessive queuing, delays on the Internet can grow longer than the round trip time between the Moon and the Earth -- for which the ``bufferbloat'' term was recently coined. Some point to active queue management (AQM) as the solution. Others propose end-to-end low-priority congestion control techniques (LPCC). Under both approaches, promising advances have been made in recent times: notable examples are CoDel for AQM, and LEDBAT for LPCC.In this paper, we warn of a potentially fateful interaction when AQM and LPCC techniques are combined: namely, AQM resets the relative level of priority between best-effort and low-priority congestion control protocols. We validate the generality of our findings by an extended set of experiments with packet-level ns2 simulation, considering 5 AQM techniques and 3 LPCC protocols, and carry on a thorough sensitivity analysis varying several parameters of the networking scenario. We complete the simulation via an experimental campaign conducted on both controlled testbeds and on the Internet, confirming the reprioritization issue to hold in the real world at least under all combination of AQM policies and LPCC protocols available in the Linux kernel. To promote cross-comparison, we make our scripts and dataset available to the research community.</p

Fighting the bufferbloat: on the coexistence of AQM and low priority congestion control

Nowadays, due to excessive queuing, delays on the Internet can grow longer than several round trips between the Moon and the Earth, for which the &quot;bufferbloat&quot; term was recently coined. Some point to active queue management (AQM) as the solution. Others propose end-to-end low-priority congestion control techniques (LPCC). Under both approaches, promising advances have been made in recent times: notable examples are CoDel for AQM, and LEDBAT for LPCC. In this paper, we warn of a potentially fateful interaction when AQM and LPCC techniques are combined: namely (i) AQM resets the relative level of priority between best effort and low-priority congestion control protocols; (ii) while reprioritization generally equalizes the priority of LPCC and TCP, we also find that some AQM settings may actually lead best effort TCP to starvation. By an extended set of experiments conducted on both controlled testbeds and on the Internet, we show the problem to hold in the real world for all tested combination of AQM policies and LPCC protocols. To further validate the generality of our findings, we complement our experiments with packet-level simulation, to cover cases of other popular AQM and LPCC that are not available in the Linux kernel. To promote cross-comparison, we make our scripts and dataset available to the research community

Interaction or Interference: can AQM and Low Priority Congestion Control Successfully Collaborate

Heterogeneity in the Internet ecosystem sometimes turns interaction into interference. Over the years, active queue management (AQM) and end-to-end low-priority congestion control (LPCC) have been proposed as alternative solutions to counter the persistently full buffer problem -- that recently became popular under the &quot;bufferbloat&quot; term.In this work, we point out the existence of a negative interplay among AQM and LPCC techniques. Intuitively, as AQM is designed to penalize the most aggressive flows it mainly hit best effort TCP: it follows that LPCC is not able to maintain its low priority, thus becoming as aggressive as TCP. By an extended set of simulation on various AQM policies and LPCC protocols, including the very recent CoDel AQM and LEDBAT LPCC proposals, we point out that this interference is quite universal and deserves further attention

A Control-Theoretic Analysis of Low-Priority Congestion Control Reprioritization under AQM

Recently, a negative interplay has been shown to arise when scheduling/Active Queue Management (AQM) techniques and low-priority congestion control protocols are used together; namely, AQM resets the relative level of priority among congestion control protocols. This work explores this issue by carrying out a control-theoretic analysis of the dynamical system to prove some fundamental properties that fully characterize the reprioritization phenomenon. In particular, (i) we provide the closed-form solution of the equilibrium in the open loop (i.e., fixing a target loss probability p); (ii) we provide a stability analysis and a characterization of the reprioritization phenomenon when closing the loop with AQM (i.e., that dynamically adjusts the system loss probability). Our results are important as the characterization of the reprioritization phenomenon is not only quantitatively accurate for the specific protocols and AQM considered but also qualitatively accurate for a broader range of congestion control protocol and AQM combinations. Finally, while we find a sufficient condition to avoid the reprioritization phenomenon, we also show, at the same time, such conditions to be likely impractical: Therefore, we propose a simple and practical system-level solution that is able to reinstate priorities among protocols