Experimentally derived interactions between TCP traffic and service quality over DOCSIS cable links

We study the effect of upstream (US) and downstream (DS) rate caps and DOCSIS media access on the end-to-end performance of broadband IP services that share a typical home-access 'cable Internet' link. Our experimental study measures the performance of actual DOCSIS equipment in a typical network configuration. We observe that modestly long lived data transfers in the DS direction can create substantial latency spikes (over 100 ms) in the shared DOCSIS segment, even when the DS rate cap is one or two megabits per second. Such spikes can have a big impact on delay-sensitive applications, such as voice over IP (VoIP), online games and interactive streaming video that may be sharing the DOCSIS link. We also experimentally characterize the impact of US and DS bandwidth asymmetry, MTU sizes, and TCP window sizes in achieving maximum performance over DOCSIS links.