The Local and Global Effects of Traffic Shaping

The Internet is witnessing explosive growth in traffic due to bulk content transfers, such as multimedia and software downloads, and online sharing and backups of personal, commercial, and scientific data. Wide-area network bandwidth is expensive and this cost is forcing many ISPs to deploy middle boxes to contain bulk traffic. As a result, many Internet bottlenecks today are economic rather than physical. That is, for many links interconnecting distinct organizations, there is often plenty of available physical capacity. However, the cost of actually transmitting across these link is based on peak levels of utilization, for instance as measured by the 95% link utilization. Thus, there are incentives to perform traffic shaping across these links to limit peak levels of utilization. In light of these trends, this paper makes the following contributions. We show that appropriate inter-ISP traffic shaping mechanisms can dramatically reduce peak levels of utilization with no impact on interactive applications and only minimal degradation of bulk data transfers. This suggests that in the future, it is in the self-interest of every ISP to perform such traffic shaping at the edges of its network. Unfortunately, we show that the local incentives to perform traffic shaping will result in dramatically negative global slowdown of bulk transfers, with the degradation growing as a function of the distance between the source and destination. Essentially, geographic time zone differences and the associated offsets in the local times of peak utilization mean that bulk transfers traveling sufficiently far will always be throttled by some ISP between the source and destination. Our findings suggest that once traffic shaping becomes predominant, alternative data transfer mechanisms will be needed to efficiently deliver bulk data across the Internet.