The role of packet-dropping mechanisms in QoS differentiation

Many research teams are developing technologies to turn the Internet into a QoS-capable network, which is one of the biggest challenges that this communication system currently faces. Naturally, at the core of such a challenge are IP routers and the technology they use. It is a well known fact that the common packet scheduling discipline that is used in routers (first come first served) makes them useless when QoS is needed. Thus, a different type of packet scheduling must be used. One of the most referred solutions for QoS-capable systems is the weighted fair queuing (WFQ) discipline. For FreeBSD-based routers, the ALTQ implementation of the WFQ discipline is, of course, an eligible and natural choice. Given this, it is important to fully understand the characteristics and operational behaviour of such an implementation. This paper presents several tests that guide the reader to a detailed knowledge about the WFQ/ALTQ operation-its behaviour, weaknesses, and flaws-with the purpose of showing how relevant can the influence of the dropper mechanism be on the effectiveness of IP routers.