The Probe Gap Model can Underestimate the Available Bandwidth of Multihop Paths

The Probe Gap Model (PGM) was proposed as a lightweight and fast available bandwidth estimation method. Measure-ment tools such as Delphi and Spruce are based on PGM. Compared to estimation methods that require multiple iter-ations with different probing rates, PGM uses a single prob-ing rate and it infers the available bandwidth from a direct relation between the input and output rates of measurement packet pairs. An important assumption behind the PGM model is that the measured path has a single bottleneck link that determines the available bandwidth of the end-to-end path. In this letter, we show that, even though PGM is accurate in the case of a single queue, it cannot estimate the available bandwidth of multi-hop paths, even if there is a single bottleneck in the path. Whether PGM is accurate or not depends on the routing of cross traffic relative to the measurement traffic. PGM is accurate when the cross traffic follows the same path with the measurement traffic. In the general case, however, PGM can significantly underestimate the available bandwidth of an end-to-end path