One of the tasks of network management is to dimension the capacity of access and backbone links. Rules of thumb can be used, but they lack rigor and precision, as they fail to reliably predict whether the quality, as agreed on in the service level agreement, is actually provided. To make better predictions, a more sophisticated mathematical setup is needed. The major contribution of this article is that it presents such a setup; in this a pivotal role is played by a simple, yet versatile, formula that gives the minimum amount of capacity needed as a function of the average traffic rate, traffic variance (to be thought of as a measure of "burstiness"), as well as the required performance level. In order to apply the dimensioning formula, accurate estimates of the average traffic rate and traffic variance are needed. As opposed to the average rate, the traffic variance is rather hard to estimate; this is because measurements on small timescales are needed. We present an easily implementable remedy for this problem, in which the traffic variance is inferred from occupancy statistics of the buffer within the switch or router. To validate the resulting dimensioning procedure, we collected hundreds of traces at multiple (representative) locations, estimated for each of the traces the average traffic rate and (using the approach described above) traffic variance, and inserted these in the dimensioning formula. It turns out that the capacity estimate obtained by the procedure, is usually just a few percent off from the (empirically determined) minimally required value.
Abstract-This paper compares the performance of Web services based network monitoring to traditional, SNMP based, monitoring. The study focuses on the ifTable, and investigates performance as function of the number of retrieved objects. The following aspects are examined: bandwidth usage, CPU time, memory consumption and round trip delay. For our study several prototypes of Web services based agents were implemented; these prototypes can retrieve single ifTable elements, ifTable rows, ifTable columns or the entire ifTable. This paper presents a generic formula to calculate SNMP's bandwidth requirements; the bandwidth consumption of our prototypes was compared to that formula. The CPU time, memory consumption and round trip delay of our prototypes was compared to Net-SNMP, as well as several other SNMP agents. Our measurements show that SNMP is more efficient in cases where only a single object is retrieved; for larger number of objects Web services may be more efficient. Our study also shows that, if performance is the issue, the choice between BER (SNMP) or XML (Web services) encoding is generally not the determining factor; other choices can have stronger impact on performance.
C e n t r u m v o o r W i s k u n d e e n I n f o r m a t i c a PNA Probability, Networks and AlgorithmsProbability, Networks and Algorithms
Abstract-The Simple Network Management Protocol (SNMP)is widely deployed to monitor, control, and configure network elements. Even though the SNMP technology is well documented and understood, it remains relatively unclear how SNMP is used in practice and what the typical SNMP usage patterns are. This paper discusses how to perform large-scale SNMP traffic measurements in order to develop a better understanding of how SNMP is used in production networks. The tools described in this paper have been applied to networks ranging from large national research networks to relatively small faculty networks. The goal of the research is to provide feedback to SNMP protocol developers within the IETF, researchers working within the context of the IRTF-NMRG, as well as other researchers interested in network management in general. We believe that the results are also valuable for operators and vendors who want to optimize their management interactions or understand the traffic generated by their management software.
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