670 research outputs found
Modelling TCP congestion control dynamics in drop-tail environments
In this paper we study communication networks that employ drop-tail queueing and additive-increase multiplicative-decrease (AIMD)
congestion control algorithms. We show that the theory of non-negative matrices may be employed to model such networks and to derive basic
theorems concerning their behaviour
Asymptotic Approximations for TCP Compound
In this paper, we derive an approximation for throughput of TCP Compound
connections under random losses. Throughput expressions for TCP Compound under
a deterministic loss model exist in the literature. These are obtained assuming
the window sizes are continuous, i.e., a fluid behaviour is assumed. We
validate this model theoretically. We show that under the deterministic loss
model, the TCP window evolution for TCP Compound is periodic and is independent
of the initial window size. We then consider the case when packets are lost
randomly and independently of each other. We discuss Markov chain models to
analyze performance of TCP in this scenario. We use insights from the
deterministic loss model to get an appropriate scaling for the window size
process and show that these scaled processes, indexed by p, the packet error
rate, converge to a limit Markov chain process as p goes to 0. We show the
existence and uniqueness of the stationary distribution for this limit process.
Using the stationary distribution for the limit process, we obtain
approximations for throughput, under random losses, for TCP Compound when
packet error rates are small. We compare our results with ns2 simulations which
show a good match.Comment: Longer version for NCC 201
Trading link utilization for queueing delays: an adaptive approach
Understanding the relationship between queueing delays and link utilization for general traffic conditions is an important open
problem in networking research. Difficulties in understanding this relationship stem from the fact that it depends on the complex
nature of arriving traffic and the problems associated with modelling such traffic. Existing AQM schemes achieve a "low delay"
and "high utilization" by responding early to congestion without considering the exact relationship between delay and utilization.
However, in the context of exploiting the delay/utilization tradeoff, the optimal choice of a queueing scheme's control parameter
depends on the cost associated with the relative importance of queueing delay and utilization. The optimal choice of control
parameter is the one that maximizes a benefit that can be defined as the difference between utilization and cost associated with
queuing delay. We present two practical algorithms, Optimal Drop-Tail (ODT) and Optimal BLUE (OB), that are designed with
a common performance goal: namely, maximizing this benefit. Their novelty lies in fact that they maximize the benefit in an
online manner, without requiring knowledge of the traffic conditions, specific delay-utilization models, nor do they require complex
parameter estimation. Packet level ns2 simulations are given to demonstrate the efficacy of the proposed algorithms and the
framework in which they are designed
Router-based algorithms for improving internet quality of service.
We begin this thesis by generalizing some results related to a recently proposed positive system model of TCP congestion control algorithms. Then, motivated by a mean ¯eld analysis of the positive system model, a novel, stateless, queue management scheme is designed: Multi-Level Comparisons with index l (MLC(l)). In the limit, MLC(l) enforces max-min fairness in a network of TCP flows.
We go further, showing that counting past drops at a congested link provides su±cient information to enforce max-min fairness among long-lived flows and to reduce the flow completion times of short-lived flows. Analytical models are presented, and the accuracy of predictions are validated by packet level ns2 simulations.
We then move our attention to e±cient measurement and monitoring techniques. A small active counter architecture is presented that addresses the problem of accurate approximation of statistics counter values at very-high speeds that can be both updated and estimated on a per-packet basis. These algorithms are necessary in the design of router-based flow control algorithms since on-chip
Static RAM (SRAM) currently is a scarce resource, and being economical with its usage is an important task. A highly scalable method for heavy-hitter identifcation that uses our small active counters architecture is developed based on heuristic argument. Its performance is compared to several state-of-the-art algorithms and shown to out-perform them.
In the last part of the thesis we discuss the delay-utilization tradeoff in the congested Internet links.
While several groups of authors have recently analyzed this tradeoff, the lack of realistic assumption in their models and the extreme complexity in estimation of model parameters, reduces their applicability at real Internet links. We propose an adaptive scheme that regulates the available queue space to keep utilization at desired, high, level. As a consequence, in large-number-of-users regimes, sacrifcing 1-2% of bandwidth can result in queueing delays that are an order of magnitude smaller than in the standard BDP-bu®ering case. We go further and introduce an optimization framework for describing the problem of interest and propose an online algorithm for solving it
Router-based algorithms for improving internet quality of service.
We begin this thesis by generalizing some results related to a recently proposed positive system model of TCP congestion control algorithms. Then, motivated by a mean ¯eld analysis of the positive system model, a novel, stateless, queue management scheme is designed: Multi-Level Comparisons with index l (MLC(l)). In the limit, MLC(l) enforces max-min fairness in a network of TCP flows.
We go further, showing that counting past drops at a congested link provides su±cient information to enforce max-min fairness among long-lived flows and to reduce the flow completion times of short-lived flows. Analytical models are presented, and the accuracy of predictions are validated by packet level ns2 simulations.
We then move our attention to e±cient measurement and monitoring techniques. A small active counter architecture is presented that addresses the problem of accurate approximation of statistics counter values at very-high speeds that can be both updated and estimated on a per-packet basis. These algorithms are necessary in the design of router-based flow control algorithms since on-chip
Static RAM (SRAM) currently is a scarce resource, and being economical with its usage is an important task. A highly scalable method for heavy-hitter identifcation that uses our small active counters architecture is developed based on heuristic argument. Its performance is compared to several state-of-the-art algorithms and shown to out-perform them.
In the last part of the thesis we discuss the delay-utilization tradeoff in the congested Internet links.
While several groups of authors have recently analyzed this tradeoff, the lack of realistic assumption in their models and the extreme complexity in estimation of model parameters, reduces their applicability at real Internet links. We propose an adaptive scheme that regulates the available queue space to keep utilization at desired, high, level. As a consequence, in large-number-of-users regimes, sacrifcing 1-2% of bandwidth can result in queueing delays that are an order of magnitude smaller than in the standard BDP-bu®ering case. We go further and introduce an optimization framework for describing the problem of interest and propose an online algorithm for solving it
Partitioning and Invariance of AIMD Dynamics in Synchronised Networks.
In this paper we present new results on the dynamics of networks of AIMD flows. The results reveal an invariance and partitioning property that indicates potential for the design of soundly-based adaptive AIMD strategies
Mobile Networking
We point out the different performance problems that need to be addressed when considering mobility in IP networks. We also define the reference architecture and present a framework to classify the different solutions for mobility management in IP networks. The performance of the major candidate micro-mobility solutions is evaluated for both real-time (UDP) and data (TCP) traffic through simulation and by means of an analytical model. Using these models we compare the performance of different mobility management schemes for different data and real-time services and the network resources that are needed for it. We point out the problems of TCP in wireless environments and review some proposed enhancements to TCP that aim at improving TCP performance. We make a detailed study of how some of micro-mobility protocols namely Cellular IP, Hawaii and Hierarchical Mobile IP affect the behavior of TCP and their interaction with the MAC layer. We investigate the impact of handoffs on TCP by means of simulation traces that show the evolution of segments and acknowledgments during handoffs.Publicad
New RED-type TCP-AQM algorithms based on beta distribution drop functions
In recent years, Active Queue Management (AQM) mechanisms to improve the
performance of TCP/IP networks have acquired a relevant role. In this paper we
present a simple and robust RED-type algorithm together with a couple of
dynamical variants with the ability to adapt to the specific characteristics of
different network environments, as well as to the user needs. We first present
a basic version called Beta RED (BetaRED), where the user is free to adjust the
parameters according to the network conditions. The aim is to make the
parameter setting easy and intuitive so that a good performance is obtained
over a wide range of parameters. Secondly, BetaRED is used as a framework to
design two dynamic algorithms, which we will call Adaptive Beta RED (ABetaRED)
and Dynamic Beta RED (DBetaRED). In those new algorithms certain parameters are
dynamically adjusted so that the queue length remains stable around a
predetermined reference value and according to changing network traffic
conditions. Finally, we present a battery of simulations using the Network
Simulator 3 (ns-3) software with a two-fold objective: to guide the user on how
to adjust the parameters of the BetaRED mechanism, and to show a performance
comparison of ABetaRED and DBetaRED with other representative algorithms that
pursue a similar objective
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