36 research outputs found
Controlo de congestionamento em redes sem fios
Doutoramento em Engenharia ElectrotécnicaCongestion control in wireless networks is an important and open issue.
Previous research has proven the poor performance of the Transport
Control Protocol (TCP) in such networks. The factors that contribute
to the poor performance of TCP in wireless environments concern its
unsuitability to identify/detect and react properly to network events,
its TCP window based
ow control algorithm that is not suitable for
the wireless channel, and the congestion collapse due to mobility. New
rate based mechanisms have been proposed to mitigate TCP performance
in wired and wireless networks. However, these mechanisms
also present poor performance, as they lack of suitable bandwidth estimation
techniques for multi-hop wireless networks.
It is thus important to improve congestion control performance in wireless
networks, incorporating components that are suitable for wireless
environments. A congestion control scheme which provides an e -
cient and fair sharing of the underlying network capacity and available
bandwidth among multiple competing applications is crucial to the definition
of new e cient and fair congestion control schemes on wireless
multi-hop networks.
The Thesis is divided in three parts. First, we present a performance
evaluation study of several congestion control protocols against TCP,
in wireless mesh and ad-hoc networks. The obtained results show that
rate based congestion control protocols need an eficient and accurate
underlying available bandwidth estimation technique. The second part
of the Thesis presents a new link capacity and available bandwidth estimation
mechanism denoted as rt-Winf (real time wireless inference).
The estimation is performed in real-time and without the need to intrusively
inject packets in the network. Simulation results show that
rt-Winf obtains the available bandwidth and capacity estimation with
accuracy and without introducing overhead trafic in the network.
The third part of the Thesis proposes the development of new congestion
control mechanisms to address the congestion control problems
of wireless networks. These congestion control mechanisms use cross
layer information, obtained by rt-Winf, to accurately and eficiently estimate
the available bandwidth and the path capacity over a wireless
network path. Evaluation of these new proposed mechanisms, through
ns-2 simulations, shows that the cooperation between rt-Winf and the
congestion control algorithms is able to significantly increase congestion
control eficiency and network performance.O controlo de congestionamento continua a ser extremamente importante
quando se investiga o desempenho das redes sem fios. Trabalhos
anteriores mostram o mau desempenho do Transport Control Proto-
col (TCP) em redes sem fios. Os fatores que contribuem para um
pior desempenho do TCP nesse tipo de redes s~ao: a sua falta de capacidade
para identificar/detetar e reagir adequadamente a eventos da
rede; a utilização de um algoritmo de controlo de
uxo que não é adequado
para o canal sem fios; e o colapso de congestionamento devido
á mobilidade. Para colmatar este problemas foram propostos novos
mecanismos de controlo de congestionamento baseados na taxa de
transmissão. No entanto, estes mecanismos também apresentam um
pior desempenho em redes sem fios, já que não utilizam mecanismos
adequados para a avaliação da largura de banda disponível. Assim, é
importante para melhorar o desempenho do controlo de congestionamento
em redes sem fios, incluir componentes que são adequados para
esse tipo de ambientes. Um esquema de controlo de congestionamento
que permita uma partilha eficiente e justa da capacidade da rede e da
largura de banda disponível entre múltiplas aplicações concorrentes é
crucial para a definição de novos, eficientes e justos mecanismos de
controlo congestionamento para as redes sem fios.
A Tese está dividida em três partes. Primeiro, apresentamos um estudo
sobre a avaliação de desempenho de vários protocolos de controlo de
congestionamento relativamente ao TCP, em redes sem fios em malha
e ad-hoc. Os resultados obtidos mostram que os protocolos baseados
na taxa de transmissão precisam de uma técnica de avaliação da largura
de banda disponível que seja eficiente e precisa . A segunda parte da
Tese apresenta um novo mecanismo de avaliação da capacidade da
ligação e da largura de banda disponível, designada por rt-Winf (real
time wireless inference). A avaliação é realizada em tempo real e sem
a necessidade de inserir tráfego na rede. Os resultados obtidos através
de simulação e emulação mostram que o rt-Winf obtém com precisão
a largura de banda disponível e a capacidade da ligação sem sobrecarregar
a rede. A terceira parte da Tese propõe novos mecanismos de
controlo de congestionamento em redes sem fios. Estes mecanismos
de controlo de congestionamento apresentam um conjunto de caracter
ísticas novas para melhorar o seu desempenho, de entre as quais
se destaca a utilização da informação de largura de banda disponível
obtida pelo rt-Winf. Os resultados da avaliação destes mecanismos,
utilizando o simulador ns-2, permitem concluir que a cooperação entre
o rt-Winf e os algoritmos de controlo de congestionamento aumenta
significativamente o desempenho da rede
ショウヨウリョウ バッファ ニヨル ヒカリ パケット スイッチ ネットワーク ノ セッケイ ニカンスル ケンキュウ
This paper was published in Journal of Optical Networking and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://dx.doi.org/10.1364/JON.6.001116. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law
Interaction between MPI and TCP in grids.
As MPI applications are more and more resource consuming, they need to be executed on grids. The communications on the WAN interconnecting clusters mostly use TCP which suffers from WAN features: high latency, sharing between users, bandwidth smaller than the aggregate bandwidth of the nodes. In this report, we first study the interaction between MPI and TCP on grids. We show why the nature of MPI traffic raises problems while using TCP on WAN links. TCP's loss detection and congestion control mechanism can both slow down the application. Then, we propose MPI5000, a transparent applicative layer between MPI and TCP, using proxies to improve the execution of MPI applications on a grid. Proxies aim at splitting TCP connections in order to detect losses faster and avoid to return in a slowstart phase after an idle time. Finally, we test our layer on Grid'5000, the French research grid, using MPICH2. The results on the NPB (NAS Parallel Benchmarks) validate our architecture that reduces the number of idle timeout and the number of long-distance retransmissions for certain benchmarks, namely BT, SP and LU benchmarks. Using MPI5000, these applications can decrease their execution time by 35%, 28%, and, 15% respectively
Re-feedback: freedom with accountability for causing congestion in a connectionless internetwork
This dissertation concerns adding resource accountability to a simplex internetwork such as the Internet,
with only necessary but sufficient constraint on freedom. That is, both freedom for applications to evolve
new innovative behaviours while still responding responsibly to congestion; and freedom for network
providers to structure their pricing in any way, including flat pricing.
The big idea on which the research is built is a novel feedback arrangement termed ‘re-feedback’.
A general form is defined, as well as a specific proposal (re-ECN) to alter the Internet protocol so that
self-contained datagrams carry a metric of expected downstream congestion.
Congestion is chosen because of its central economic role as the marginal cost of network usage.
The aim is to ensure Internet resource allocation can be controlled either by local policies or by market
selection (or indeed local lack of any control).
The current Internet architecture is designed to only reveal path congestion to end-points, not networks.
The collective actions of self-interested consumers and providers should drive Internet resource
allocations towards maximisation of total social welfare. But without visibility of a cost-metric, network
operators are violating the architecture to improve their customer’s experience. The resulting fight
against the architecture is destroying the Internet’s simplicity and ability to evolve.
Although accountability with freedom is the goal, the focus is the congestion metric, and whether
an incentive system is possible that assures its integrity as it is passed between parties around the system,
despite proposed attacks motivated by self-interest and malice.
This dissertation defines the protocol and canonical examples of accountability mechanisms. Designs
are all derived from carefully motivated principles. The resulting system is evaluated by analysis
and simulation against the constraints and principles originally set. The mechanisms are proven to be
agnostic to specific transport behaviours, but they could not be made flow-ID-oblivious
Stable and scalable congestion control for high-speed heterogeneous networks
For any congestion control mechanisms, the most fundamental design objectives
are stability and scalability. However, achieving both properties are very challenging
in such a heterogeneous environment as the Internet. From the end-users' perspective,
heterogeneity is due to the fact that different flows have different routing paths and
therefore different communication delays, which can significantly affect stability of the
entire system. In this work, we successfully address this problem by first proving a
sufficient and necessary condition for a system to be stable under arbitrary delay. Utilizing this result, we design a series of practical congestion control protocols (MKC
and JetMax) that achieve stability regardless of delay as well as many additional
appealing properties. From the routers' perspective, the system is heterogeneous because the incoming traffic is a mixture of short- and long-lived, TCP and non-TCP
flows. This imposes a severe challenge on traditional buffer sizing mechanisms, which
are derived using the simplistic model of a single or multiple synchronized long-lived
TCP flows. To overcome this problem, we take a control-theoretic approach and
design a new intelligent buffer sizing scheme called Adaptive Buffer Sizing (ABS),
which based on the current incoming traffic, dynamically sets the optimal buffer size
under the target performance constraints. Our extensive simulation results demonstrate that ABS exhibits quick responses to changes of traffic load, scalability to a
large number of incoming flows, and robustness to generic Internet traffic
Improved algorithms for TCP congestion control
Reliable and efficient data transfer on the Internet is an important issue. Since late
70’s the protocol responsible for that has been the de facto standard TCP, which
has proven to be successful through out the years, its self-managed congestion
control algorithms have retained the stability of the Internet for decades. However,
the variety of existing new technologies such as high-speed networks (e.g. fibre
optics) with high-speed long-delay set-up (e.g. cross-Atlantic links) and wireless
technologies have posed lots of challenges to TCP congestion control algorithms.
The congestion control research community proposed solutions to most of these
challenges. This dissertation adds to the existing work by: firstly tackling the highspeed
long-delay problem of TCP, we propose enhancements to one of the existing
TCP variants (part of Linux kernel stack). We then propose our own variant:
TCP-Gentle. Secondly, tackling the challenge of differentiating the wireless loss
from congestive loss in a passive way and we propose a novel loss differentiation
algorithm which quantifies the noise in packet inter arrival times and use this
information together with the span (ratio of maximum to minimum packet inter
arrival times) to adapt the multiplicative decrease factor according to a predefined
logical formula. Finally, extending the well-known drift model of TCP to account
for wireless loss and some hypothetical cases (e.g. variable multiplicative decrease),
we have undertaken stability analysis for the new version of the model
Reducing Internet Latency : A Survey of Techniques and their Merit
Bob Briscoe, Anna Brunstrom, Andreas Petlund, David Hayes, David Ros, Ing-Jyh Tsang, Stein Gjessing, Gorry Fairhurst, Carsten Griwodz, Michael WelzlPeer reviewedPreprin
Traffic Re-engineering: Extending Resource Pooling Through the Application of Re-feedback
Parallelism pervades the Internet, yet efficiently pooling this increasing path diversity has remained elusive. With no holistic solution for resource pooling, each layer of the Internet architecture attempts to balance traffic according to its own needs, potentially at the expense of others. From the edges, traffic is implicitly pooled over multiple paths by retrieving content from different sources. Within the network, traffic is explicitly balanced across multiple links through the use of traffic engineering. This work explores how the current architecture can be realigned to facilitate resource pooling at both network and transport layers, where tension between stakeholders is strongest. The central theme of this thesis is that traffic engineering can be performed more efficiently, flexibly and robustly through the use of re-feedback. A cross-layer architecture is proposed for sharing the responsibility for resource pooling across both hosts and network. Building on this framework, two novel forms of traffic management are evaluated. Efficient pooling of traffic across paths is achieved through the development of an in-network congestion balancer, which can function in the absence of multipath transport. Network and transport mechanisms are then designed and implemented to facilitate path fail-over, greatly improving resilience without requiring receiver side cooperation. These contributions are framed by a longitudinal measurement study which provides evidence for many of the design choices taken. A methodology for scalably recovering flow metrics from passive traces is developed which in turn is systematically applied to over five years of interdomain traffic data. The resulting findings challenge traditional assumptions on the preponderance of congestion control on resource sharing, with over half of all traffic being constrained by limits other than network capacity. All of the above represent concerted attempts to rethink and reassert traffic engineering in an Internet where competing solutions for resource pooling proliferate. By delegating responsibilities currently overloading the routing architecture towards hosts and re-engineering traffic management around the core strengths of the network, the proposed architectural changes allow the tussle surrounding resource pooling to be drawn out without compromising the scalability and evolvability of the Internet