Application-oriented network metrology: metrics and active measurement tools

The science of communication network metrology consists of measuring the performance of networks. This article is a tutorial on application-oriented measurement tools and techniques for IP networks. First the principles of active measurements are introduced and two important active measurement initiatives are presented. Since metrology often requires precision in timing, a basic overview related to time in computers and networks will be presented, followed by a definition of principal network metrics. Metrics that will be discussed are one-way delay, delay variation, round-trip time, packet loss, packet reordering, route, and bandwidth. For each parameter, a definition is given and related measurement tools and techniques are presented. APPLICATION-ORIENTED NETWORK METROLOGY: METRICS AND ACTIVE MEASUREMENT TOOLS SECOND QUARTER 2005, VOLUME 7, NO. 2 www.comsoc. org/pubs/surveys 1 Previous works on telecommunication lines or circuit-switched networks focus on similar performance parameters (delay, jitter, etc.). However, conceptual and technical differences between these networks and packetswitched networks induce a new metrology approach. 1553-877X/05/$20.00 © 2005 IEEE IEEE Communications Surveys & Tutorials • Second Quarter 2005 3 Passive techniques are carried out by observing network traffic flows. They consist of capturing packet headers and analyzing them. The best example of a capture tool is tcpdump, which is based on the libpcap library [12]. These techniques do not add to network traffic. Passive measurement can be done on two levels: • At a microscopic level measurements are performed on each packet traveling across the measurement point. An example of collected information is packet size. • At a macroscopic level measurements are performed on flows. In this case, aggregation rules are necessary to match packets into flows. Examples of collected data are the number of flows per unit of time, flow bitrate, etc. The “Real-time Traffic Flow Measurement” (RTFM) working group of the Internet Engineering Task Force (IETF) has worked on macroscopic measurements. It has helped create a general passive measurement architecture and describe the manner in which aggregation rules are defined. Today this working group is closed. Passive measurement techniques are particularly suitable for (but not limited to) traffic engineering because they show flow dynamics and distribution. The main problem with passive measurement is the data volume. The volume of captured data can become very large on high-capacity links. Moreover, it is hard to obtain end-to-end measurements passively: the presence of traffic traveling between two measurement points cannot be ensured and matching measurements that are performed on different measurement points is difficult. Consequently, passive measurements are usually used to determine metrics characterizing one particular network element (“at-apoint” metrics). On the other hand, active measurements are performed by sending probe packets to the network. The measurement flow travels from source to destination. Upon reaching its destination, it is possible to calculate metrics by analyzing them. Active measurements can determine the end-to-end QoS experienced by a measurement flow for a particular path and then measure the QoS as it is seen by applications. Additionally, active measurements offer the flexibility to send probe packets streams with particular properties (bitrate, packet size, etc.). The main drawback of active measurements is that additional network traffic is introduced. This “intrusive” (or “invasive”) characteristic can potentially modify the properties that are trying to be measured. First, it can result in measurement errors or bias; second, it can lead to network overload. The measurement traffic thus has to be limited to avoid network disturbance and measurement errors. As previously mentioned, the major idea behind active measurements is to perform QoS monitoring and to verify SLA. However, since active measurements determine the endto-end QoS as seen by applications, this type of measurement is also useful for distributed applications (particularly when complete control of network resources is not possible). In this case, an understanding of the QoS of the network can allow applications to adapt their execution to resource fluctuations. For example, applications can adapt their throughput to the loss of the network or the size of play-out buffers to the network delay [13]. More complex application adaptation schemes have been proposed [14–19]. In this context, we will examine new developments of application-oriented metrology techniques and tools. Note that metrology for traffic engineering is beyond the scope of this article. The main objective of this article is thus to give a tutorial on application-oriented measurement tools and techniques. The first section presents active measurement generalities. It presents active measurement principles and limitations, and discusses the IETF working group dedicated to active measurement and the NIMI project. Since active measurements often need time and clock constraints to be achieved, the second section deals with timing considerations. Each of the next seven sections is devoted to the following network parameters: one-way delay, delay variation (also referred to as jitter), round-trip time, packet loss, packet reordering, route, and bandwidth measurement. Definitions, measurement techniques, and tools2 are presented for each parameter. Then we discuss the problem of “intrusiveness” that occurs with active measurements. Finally, we present our conclusions.