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 Q0S 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 steams exactly from the disparate nature and scope ofthe things to measure (typically, throughput, transit delay or loss). This paper discusses the above mentioned difficulties and proposes a Q0S metric intended to support Q0S measurements on packet switching networks. In our opinion, the proposed metric will facilitate the development of an effective and truly integrated Q0S management system, which is fundamental to construct QoS-capable communication systems able to efficiently deal with the increasing variety of applications. This paper also presents the main challenges found during the first approach to the Q0S metric implementation. The intention was to test the metric basic concepts, to assess its feasibility, and to measure its associated overhead. The results of these overhead tests are also presented.

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This paper presents a study carried out on a WeightedFair Queuing implementation for Unix routers -the WFQ implementation of the ALTQ project, It shows the WFQ/A LTQ weaknesses and explains why we cannot expect an interesting behavior from a system using such a scheduler. The conclusions here presented are supported by a set of tests using UDP traffic only. With a tool developed in our laboratory, we were able to show that changing the classes' weights does not necessarily result on a different Quality of Service for each of the existing classes. To achieve this differentiation, the lengths of the queues which serve the scheduler (one for each class) must be increased beyond reasonable values. We found that the low-level dynamics of FreeBSD systems practically turtis WFQ schedulers useless. The same is applicable to any other work-conserving discipline. Thus, an important conclusion of this paper, is that one must design very carefully the platforms that support work conserving disciplines in order to expect adequute behaviors from those systems, in terms of QoS provision.

The cliaracterizatioii of congestion iti coiiiiiiicnicatioii systems is a pre-requisite for the dejiiitioii ofniecliniiisiiis that can be used in congestion control, with the gonl to minimize its eflects on the perforiiia17ce of clisiribi.ited applications. In order for those ~~recha~iisns to work properly, there is the need to paiitiSy coiigestioii of services and systems. This paper proposes a schetiie that eiinbles the quantijication of the cotigestioii of n particular communicatioii service, the comparisoii of the coiigestioii degree of two or more services, and the guaiitijcatioii of the global system congestioii, based on the dejiiition of a set of system QoS parameters, of their 1ior11ia1 variatioii limits and of their degivdation tliresliolcls. The pnpelbegins with the characterization of the coiigestioii problem. Following, a coiigestioii de$iiitioii is presented and a congestion metric is proposed The paper eiids with an analysis of the quaiitif cation of congestion in communication systems when looked at as a set of s e w i d consecutive modules. Causes and effects of congestionThe congestion phenomenon is associated with all communication systems of variable geometry, that is communication systems where there is a dynamic number of communication players (acting as information sources and/or destinations), wlicre M i c characteristics can change, or where the available coiiiniuiiication resources are not constant. This is the case of the majoriLy af modern communication systems, for wluch congestion control has become a major issue due to the need to support a variety of applications with and without stringent communication requirements, over a variety dF transmission technologies.From a global communication system perspective, and as a first step to the problem characterization, it can be said that congestion occurs whenever the total amount d traffic that enters a communication system in a fixed time interval is greater than the communication system outgoing flow capacity in the direction of baflic destinations, in the same time interval. Congestion affects the quality of communicating applications -leading, in extreme cases, to their termination -and results in resource wasting, as communication resources must deal with the original MIC overload as well as with the ttaffic that results from information retransmissions generated by losses due to congestion.Under working conditions, one can identify three distinct operating zones for a communication system: the Iiiieal-zoiie, the congestion zone and the collapse zone .In the linear zone the throughput increases linearly with the system load, and the transit delay remains practically constant. In this zone, the communication system is being used well below its maximum capacity and, thus, it can rapidly respond to the load variation xyithout sigilificant impact on the transit delay. From the applications point of view, communication systems should be engineered and tuned to work in this zone. Ncvertlieless, costhenefit considerations prolubit t h q lead...