Goncalo Quadros

Experimental evaluation of a set of simple error detection mechanisms

Abstract A set of simple error detection mechanisms were evaluated through fault injection experiments in order to assess their error detection coverage and latency. All the mechanisms evaluated were implemented in an actual system and faults were physically injected on the system bus. The main selection criteria of the studied set of error detection mechanisms were the simplicity and low cost of its implementation. Results have shown that more than 75% of the injected faults were detected by these fairly simple error detection mechanisms.

An Approach to Support Traffic Classes in IP Networks

Per-hop behaviours capable of supporting different traffic classes are essential for the provision of quality of service (QoS) on the Internet according to the Differentiated Services model. This paper presents an approach for supporting different traffic classes in IP networks, proposing a new per-hop behaviour called D3 - Dynamic Degradation Distribution. The approach allows for the dynamic distribution of network resources among classes, based on the measured quality of service and on the sensitivity of classes to performance degradation, without implying any substantial change to current IP technologies. A router prototype developed according to the proposed approach is presented, along with the results of experimental tests that were performed. The test results demonstrate the feasibility and the effectiveness of the underlying ideas.

Quality of service routing in the differentiated services framework

In this paper we present a quality of service routing strategy for network where traffic differentiation follows the class-based paradigm, as in the Differentiated Services framework. This routing strategy is based on a metric of quality of service. This metric represents the impact that delay and losses verified at each router in the network have in application performance. Based on this metric, it is selected a path for each class according to the class sensitivity to delay and losses. The distribution of the metric is triggered by a relative criterion with two thresholds, and the values advertised are the moving average of the last values measured.

QoS metric for packet networks

Several approaches have been proposed to empower communication systems with quality of service (QoS) capabilities. In general, their main goal is to coherently support the end-to-end performance needs of applications, based on the establishment of, and agreement on, a set of concepts, policies and mechanisms. Regardless of the used approach, an important challenge associated with quality of service provision is the development of an efficient and flexible way to monitor QoS. The existence of an effective metric to quantify the QoS offered to classes or flows of data, and to assess the performance of communication systems, would facilitate the implementation of a QoS monitor. Such a QoS metric should be able to produce comparable measures, independently of the nature and scope of the objects to quantify, that is, should turn possible uniform QoS measures. Nevertheless, the main difficulty related to the development of such metric streams exactly from the disparate nature and scope of the things to measure. This paper discusses the above mentioned difficulties nd proposes a QoS metric intended to support QoS measurements on integrated QoS management system, which is fundamental to construct QoS-capable communication systems able to efficiently deal with the increasing variety of applications. This paper also present the main challenges found during the first approach to the QoS metric implementation. The intention was to test the metric basic concepts, to assess its feasibility, and to measure its associated overhead. The result of these overhead test are also presented.

A scheme for the quantification of congestion in communication services and systems

The characterization of congestion in communication systems is a pre-requisite for the definition of mechanisms that can be used in congestion control with the goal to minimize its effects on the performance of distributed applications. In order for those mechanisms to work properly, there is the need to quantify congestion of services and systems. This paper proposes a scheme that enables the quantification of the congestion of a particular communication service, the comparison of the congestion degree of two more services, and the quantification of the global system congestion, based on the definition of a set of system QoS parameters, of the their normal variation limits and of their degradation thresholds. The paper begins with the characterization of the congestion problem. Following, a congestion definition is presented and a congestion metric is proposed. The paper ends with an analysis of the quantification of congestion in communication systems when looked at as a set of several consecutive modules.

Stability and Scalability Issues in Hop-by-Hop Class-Based Routing

An intra-domain Quality of Service (QoS) routing protocol for the Differentiated Services framework is being developed at the University of Coimbra (UC-QoSR). The main contribution of this paper is the evaluation of the scalability and stability characteristics of the protocol on an experimental test-bed. The control of protocol overhead is achieved through a hybrid approach of metrics quantification and threshold based diffusion of routing messages. The mechanisms to avoid instability are: (i) a class-pinning mechanism to control instability due to frequent path shifts; (ii) the classification of routing messages in the class of highest priority to avoid the loss of accuracy of routing information. The results show that a hop-by-hop, link-state routing protocol, like Open Shortest Path First, can be extended to efficiently support class-based QoS traffic differentiation. The evaluation shows that scalability and stability under high loads and a large number of flows is achieved on the UC-QoSR strategy.

European success stories in industrial mathematics

Foreword.- Preface.- Introduction.- Health Biology.- Energy & Environment.- Finance & Modelling.- Automotive Industry & Manufacturing.- Aerospace & Electronics.- Services, Transport & Logistics.- Annex. Summary Report of the E.S.F. Forward Look.

An effective scheduler for IP routers

At the Communications and Telematics Laboratory of the University of Coimbra is being developed a router prototype with the aim to provide QoS to different traffic classes. One of the most important mechanisms of this router is the IP packet scheduler. It is well known that the common scheduling discipline of current routers (first come first serve) makes them useless when QoS is needed-a different type of scheduler must be used. Our first idea to overcome this problem was to use a simple, open, and available scheduler, easy to adapt to the system we wanted to implement. We thought of the WFQ discipline, and, as we are using a testbed of Intel machines running FreeBSD OS, we admitted that the WFQ/ALTQ implementation would be an interesting choice. Nevertheless, a broad set of tests carried out at our laboratory proved the contrary. Most important, these tests guided us to a deep knowledge of the problems, and causes, that can weaken the effectiveness of IP schedulers. Given the importance of that surplus information, we decided to implement our own scheduler. The idea was to take advantage of a pragmatic view of scheduling activities to construct a scheduler with the best possible characteristics (but also very simple) able to reach very good performance levels. This paper presents the scheduler that resulted from our attempts. The proposed scheduler was subject to a set of tests that proved its ability to effectively differentiate traffic classes. The results of these tests are also presented and analyzed.

The role of packet-dropping mechanisms in QoS differentiation

Many research teams are developing technologies to turn the Internet into a QoS-capable network, which is one of the biggest challenges that this communication system currently faces. Naturally, at the core of such a challenge are IP routers and the technology they use. It is a well known fact that the common packet scheduling discipline that is used in routers (first come first served) makes them useless when QoS is needed. Thus, a different type of packet scheduling must be used. One of the most referred solutions for QoS-capable systems is the weighted fair queuing (WFQ) discipline. For FreeBSD-based routers, the ALTQ implementation of the WFQ discipline is, of course, an eligible and natural choice. Given this, it is important to fully understand the characteristics and operational behaviour of such an implementation. This paper presents several tests that guide the reader to a detailed knowledge about the WFQ/ALTQ operation-its behaviour, weaknesses, and flaws-with the purpose of showing how relevant can the influence of the dropper mechanism be on the effectiveness of IP routers.

PROQOS--Dynamic SLA Management in DiffServ Space Links

Several base elements for the provision of quality of service guarantees have been developed in the recent past. Of these, the Differentiated Services (DiffServ) architecture stands out as the most promising. In spite of this, various issues remain, especially when multidomain DiffServ services are concerned. In this case, some forms of distributed management of Service Level Agreements that allow the specification, exchange, enforcement and monitoring of quality of service data must be in place. Although, again, some isolated solutions exist for each of these problems, considerable effort is necessary to make them work together. The project presented in this paper tried to assess the feasibility of providing differentiated quality of service in satellite IP networks, by developing a dynamic Service Level Agreement management solution for an IP over Digital Video Broadcast Satellite system. The functionality of the implemented system comprises system configuration, dynamic SLA negotiation, QoS monitoring and metering, SLA conformance checking, and QoS reporting to customers.