The ACK-clocking mechanism of TCP flow control has a major impact on dynamic properties of Internet congestion control, for example, stability. This talk presents a new fluid model of the interaction between flows' congestion windows and the size of queues in routers. The proposed model is generic and independent of the actual source window control algorithms. Basic properties of the model and possible applications will be discussed. In particular it will be demonstrated that the model captures packet level phenomena such as traffic burstiness and that the ACK-clocking mechanism actually may be unstable for certain configurations. It will be shown that the proposed model encapsulates previous models as different orders of approximation, each valid for different time scales and configurations. Finally, the conclusions about the potential sensitivity of the stability of a window based protocol to the choice of model will be highlighted.
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