The Delay-Friendliness of TCP

TCP has been traditionally considered unfriendly for real-time applications. Nonetheless, popular applications such as Skype use TCP due to the deployment of NATs and firewalls that prevent UDP traffic. Motivated by this observation we study the delay performance of TCP for real-time media flows. We develop an analytical performance model for the delay of TCP. We use extensive experiments to validate the model and to evaluate the impact of various TCP mechanisms on its delay performance. Based on our results, we derive the working region for VoIP and live video streaming applications and provide guidelines for delay-friendly TCP settings. Our research indicates that simple application-level schemes, such as packet splitting and parallel connections, can reduce the delay of real-time TCP flows by as much as 30\% and 90\%, respectively