Conference and workshop papers

Tariffs, Mechanisms and Internet Equilibria

Costas Courcoubetis and Antonis Dimakis

Eindhoven, The Netherlands

23 - 25. November 2009

3rd Network Control and Optimisation conference, NET-COOP 2009

We analyze the interplay between the demand for downloads, choice of congestion control mechanism, and tariff structure, when users have preferences over average download delay and they are charged according to the number of ECN marked packets they receive. Our model involves a timescale separation approach, where in the fast timescales active flows compete for instantaneous bandwidth share by optimally tuning congestion control parameters in a non-cooperative fashion. This is modeled by choosing utility functions within the network utility maximization framework laid down by Kelly [1]. On a slower timescale, users selfishly change their demand for downloads based on the average experienced download delay and charges incurred. We study the equilibrium of this loop of interactions from the point of view of social welfare.
For homogenous users we find that optimal equilibria are induced when they choose among linear utility functions, while this is not the case for logarithmic, i.e., proportionally fair congestion controllers. We next consider two types of users, web-browsing and bittorrent, where the latter is much less sensitive to download delay than the former. If bittorrent users choose among proportionally fair utility functions the charge induced by ECN marked packets does not provide the correct signal for service differentiation and the resulting equilibrium is suboptimal. On the other hand, if flows are charged for the volume of bits they transfer as well, then social welfare maximum is attained for the profit maximizing price per bit. This reveals a new role for bit volume pricing: to provide bittorrent users the correct incentives for choosing congestion controllers that effectively give priority to web-browsing users.