R. Lo Cigno

Measuring IP and TCP behavior on edge nodes with Tstat

Field measurements have always been the starting point for network design and planning; however, their statistical analysis beyond simple traffic volume estimation is not so common. In this paper we present and discuss Tstat, a tool for the collection and statistical analysis of TCP/IP traffic, which, in addition to recognized performance figures, infers TCP connection status from traces. Besides briefly discussing its rationale and use, we present part of the performance figures that can be obtained, and we highlight the insight that such figures can give on TCP/IP protocols and the Internet, thereby supporting the usefulness of a widespread use of Tstat or similar tools. Analyzing Internet traffic is difficult because a large amount of performance figures can be devised in TCP/IP networks, but also because many performance figures can be derived only if both directions of bidirectional traffic are jointly considered. Tstat automatically correlates incoming and outgoing packets. Sophisticated statistics, obtained through data correlation between incoming and outgoing traffic, give reliable estimates of the network performance also from the user perspective. Tstat computes over 80 different performance statistics at both the IP and TCP layers, allowing a good insight in the network performance. To support the latter statement, we discuss several of these figures computed on traffic measurements performed for a time period equivalent to more than three months spread during the years 2000-2003 on the access link of Politecnico di Torino.

Robust rate control for integrated services packet networks

Research on congestion-control algorithms has traditionally focused more on performance than on robustness of the closed-loop system to changes in network conditions. As the performance of the control loop is strictly connected with the quality of service, these systems are natural candidates to be approached by the optimal control theory. Unfortunately, this approach may fail in the presence of transmission delay variations, which are unavoidable in telecommunication systems.In this paper, we first show the fragility of optimal controllers and demonstrate their instability when the control delay is not known exactly. Then we propose a robust control algorithm based on a classical proportional integral derivative scheme which does not suffer from this fragility phenomenon. Its stability versus the control delay variations, as well as versus sources that transmit less than their computed share, is studied with Nyquist analysis. The control algorithm is implemented within a simulator in the framework of the asynchronous transfer mode (ATM) ABR transfer capability. The final part of the paper shows some selected results assessing the performance of the control algorithm in a realistic network environment. ABR was chosen as an example, but the control studied here can be applied in any data network to obtain a robust and reliable congestion-control scheme.

Markov models of internet traffic and a new hierarchical MMPP model

The first part of this paper gives a short tutorial survey of Internet traffic modeling, focusing on recent advances in Markov models showing pseudo-LRD (Long Range Dependence) characteristics that match those measured on the Internet. The interest in Markov models of Internet traffic, in spite of the impossibility to achieve true LRD or Self-Similarity, lies in the possibility of exploiting powerful analytical techniques to predict the network performance, which is the ultimate goal when adopting models to either study existing networks or design new ones. Then, the paper describes a new MMPP (Markov Modulated Poisson Process) traffic model that accurately approximates the LRD characteristics of Internet traffic traces over the relevant time scales. The heart of the model is based on the notion of sessions and flows, trying to mimic the real hierarchical generation of packets in the Internet. The proposed model is simple and intuitive: its parameters have a physical meaning, and the model can be tuned with only a few input parameters. Results prove that the queuing behavior of the traffic generated by the MMPP model is coherent with the one produced by real traces collected at our institution edge router under several different traffic loads. Due to its characteristics, the proposed MMPP traffic model can be used as a simple and manageable tool for IP network dimensioning, design and planning: the paper provides examples of its application in both simulative and theoretical analysis.

Efficient estimation of call blocking probabilities in cellular mobile telephony networks with customer retrials

A novel approximate technique is proposed for the estimation of call blocking probabilities in cellular mobile telephony networks where call blocking triggers customer retrials. The approximate analysis technique is based on Markovian models with state spaces whose cardinalities are proportional to the maximum number of calls that can be simultaneously in progress within cells. The accuracy of the approximate technique is assessed by comparison against results of detailed simulation experiments, results of a previously proposed Markovian analysis approach, and upper and lower bounds to the call blocking probability. Numerical results show that the proposed approximate technique is very accurate, in spite of the remarkably small state spaces of the Markovian models.

Local and global handovers for mobility management in wireless ATM networks

This article deals with the problem of virtual circuit (VC) management in wireless ATM (W-ATM) networks with mobile user terminals. In W-ATM networks, a VC terminating at a mobile user may require dynamic reestablishment during the short time span necessary for terminal handover due to its movement from one (macro)cell to another. The VC reestablishment procedure has to ensure in-sequence and loss-free delivery of the ATM cells containing user data. After a classification of the solutions proposed so far in the literature, a novel technique for the dynamic reestablishment of VCs in W-ATM networks is described, and its performance is evaluated through simulation. The proposed technique allows for a progressive upgrade of the fixed part of the ATM network and for the incremental introduction of user terminal mobility.

A detailed and accurate closed queueing network model of many interacting TCP flows

This paper presents a new analytical model for the estimation of the performance of TCP connections. The model is based on the description of the behavior of TCP-Tahoe in terms of a closed queueing network, whose solution can be obtained with very low cost, even when the number of TCP connections that interact over the underlying IP network is huge. The protocol model can be very accurate, deriving directly from the finite state machine description of the protocol. The assessment of the accuracy of the analytical model is based on comparisons against detailed simulation experiments developed with the ns-2 package. Numerical results indicate that the proposed closed queueing network model provides extremely accurate performance estimates, not only for average values, but even for distributions, in the case of the classical single-bottleneck configuration, as well as in more complex networking setups.

Measuring IP and TCP behavior on edge nodes

Tstat is a new tool for the collection and statistical analysis of TCP/IP traffic, able to infer TCP connection status from traces. Discussing its use, we present some of the performance figures that can be obtained and the insight that such figures can give on TCP/IP protocols and the Internet. While standard performance measure, such as flow dimensions, traffic distribution, etc., remain at the base of traffic evaluation, more sophisticated indices, like the out-of-order probability and gap dimension in TCP connections, obtained through data correlation between the incoming and outgoing traffic, give reliable estimates of the network performance also from the user perspective. Several of these indices are discussed on traffic measures performed for more than 2 months on the access link of our institution.

A new class of QoS routing strategies based on network graph reduction

This paper discusses a new approach to QoS routing, introducing the notion of algorithm resilience (i.e., its capability to adapt to network and load modifications) as the performance index of the algorithm itself, for a given network topology, load and traffic pattern. The new approach can be summarized as network graph reduction, i.e., a modification of the graph describing the network before the routing path is computed, in order to exclude from the path selection over-congested portions of the network. This solution leads to a class of two-step routing algorithms, where both steps are simple, hence allowing efficient implementation. Simulation experiments, run on randomly-generated topologies and traffic patterns, show that these routing algorithms outperform both the standard minimum hop algorithm and those QoS-based algorithms based on the same metrics but not using the notion of network graph reduction.

Graph Based Analysis of Mesh Overlay Streaming Systems

This paper studies fundamental properties of stream-based content distribution services. We assume the presence of an overlay network (such as those built by P2P systems) with limited degree of connectivity, and we develop a mathematical model that captures the essential features of overlay-based streaming protocols and systems. The methodology is based on stochastic graph theory, and models the streaming system as a stochastic process, whose characteristics are related to the streaming protocol. The model captures the elementary properties of the streaming system such as the number of active connections, the different play-out delay of nodes, and the probability of not receiving the stream due to node failures/misbehavior. Besides the static properties, the model is able to capture the transient behavior of the distribution graphs, i.e., the evolution of the structure over time, for instance in the initial phase of the distribution process. Contributions of this paper include a detailed definition of the methodology, its comparison with other analytical approaches and with simulative results, and a discussion of the additional insights enabled by this methodology. Results show that mesh based architectures are able to provide bounds on the receiving delay and maintain rate fluctuations due to system dynamics very low. Additionally, given the tight relationship between the stochastic process and the properties of the distribution protocol, this methodology gives basic guidelines for the design of such protocols and systems.

ATM simulation with CLASS

Abstract The paper describes an efficient, versatile and extensible software tool for the analysis of the quality of connectionless services in ATM networks. The tool is named CLASS for ConnectionLess ATM Services Simulator. CLASS is a time-driven, slotted, synchronous simulator, entirely written in standard C language. CLASS allows the performance analysis of ATM networks adopting the viewpoint of both the end-user, and the network manager; the measured performance parameters include the cell and message loss probabilities and the cell and message delay jitters. The investigation of the impact of shaping and policing techniques, and of the use of connectionless servers on the network performance is also supported. With CLASS, the network synthetic workload can be modeled choosing from a variety of traffic generators ranging from simple Poisson traffic sources to sources modelling the traffic produced when higher level protocols, like TCP, access the ATM services.