Bob Melander

A new end-to-end probing and analysis method for estimating bandwidth bottlenecks

We present a network friendly bandwidth measurement method, TOPP, that is based on active probing and includes analysis by segmented regression. This method can estimate two complementing available bandwidth metrics in addition to the link bandwidth of the congested link. Contrary to traditional packet pair estimates of the bottleneck link bandwidth, our estimate is not limited by the rate at which we can inject probe packets into the network. We also show that our method is able to detect bottlenecks that are invisible to methods such as the C-probe. Further more, we describe scenarios where our analysis method is able to calculate bandwidth estimates for several congested hops based on a single end-to-end probe session.

Real-Time Measurement of End-to-End Available Bandwidth using Kalman Filtering

This paper presents a new method, BART (bandwidth available in real-time), for estimating the end-to-end available bandwidth over a network path. It estimates bandwidth quasi-continuously, in real-time. The method has also been implemented as a tool. It relies on self-induced congestion, and repeatedly samples the available bandwidth of the network path with sequences of probe packet pairs, sent at randomized rates. BART requires little computation in each iteration, is lightweight with respect to memory requirements, and adds only a small amount of probe traffic. The BART method uses Kalman filtering, which enables real-time estimation (a.k.a. tracking). It maintains a current estimate, which is incrementally improved with each new measurement of the inter-packet time separations in a sequence of probe packet pairs. The measurement model has a strong non-linearity, and would not at first sight be considered suitable for Kalman filtering, but we show how this non-linearity can be handled. BART may be tuned according to the specific needs of the measurement application, such as agility vs. stability of the estimate. We have tested an implementation of BART in a physical test network with carefully controlled cross traffic, with good accuracy and agreement. Test measurements have also been performed over the Internet. We compare the performance of BART with that of pathChirp, a state-of-the-art tool for measuring end-to-end available bandwidth in real-time

Bandwidth Measurement in Wireless Networks

For active, probing-based bandwidth measurements performed on top of the unifying IP layer, it may seem reasonable to expect the measurement problem in wireless networks, such as ad-hoc networks, to be no different than the one in wired networks. However, in networks with 802.11 wireless links we show that this is not the case.

First-come-first-served packet dispersion and implications for TCP

We study the packet dispersion phenomenon that a traffic flow experiences when it passes through a router. We show that when there are competing flows and the router schedules packets first-come-first-served (FCFS), the dispersion is not described well by the bottleneck spacing effect. We therefore introduce the term FCFS-spacing effect. We also show that for a router implementing weighted fair queuing, dispersion is due either to the bottleneck spacing effect or the FCFS spacing effect. The results from real measurements and simulations corroborate each other. Finally, we discuss the implications of FCFS packet dispersion on TCPs self-clocking mechanism.

An Analysis of Active End-to-end Bandwidth Measurements in Wireless Networks

For active, probing-based bandwidth measurements performed on top of the unifying IP layer, it may seem reasonable to expect the measurement problem in wireless networks to be no different than the one in wired networks. However, in networks with 802.11 wireless bottleneck links we show that this is not the case. The results from the experiments presented in this paper show that the measured available bandwidth is dependent on the probe packet size (contrary to what is observed in wired networks). Another equally important finding is that the measured link capacity, using the well known TOPP model, is dependent on the probe packet size and on the cross-traffic intensity. The underlying reasons for the observed differences are analyzed by incorporating the characteristics of 802.11 wireless networks into the TOPP model. The extended model is applicable to other end-to-end bandwidth measurement methods as well, such as BART, Pathload and PTR.

Impact of the Ethernet Capture Effect on Bandwidth Measurements

In this paper we present impacts of the Ethernet capture effect on bandwidth measurements. We show that the unfairness caused by the Ethernet capture effect can have a severe impact on the ability to estimate available bandwidth. We define two metrics, surplus bandwidth and fair share bandwidth, that are protocol and application independent available bandwidth metrics. We propose a measurement method, the TOPP method, that measures link load with low risk of triggering an Ethernet capture effect. From TOPP measurements, we can estimate available bandwidth.