TEAR: TCP emulation at receivers – flow control for multimedia streaming

Congestion and flow control is an integral part of any Internet data transport protocol. It is widely accepted that the congestion avoidance mechanisms of TCP have been one of the key contributors to the success of the Internet. However, TCP is ill-suited to real-time multimedia streaming applications. Its bursty transmission, and abrupt and frequent wide rate fluctuations cause high delay jitters and sudden quality degradation of multimedia applications. For asymmetric networks such as wireless networks, cable modems, ADSL, and satellite networks, transmitting feedback for (almost) every packet received as it is done in TCP causes congestion in the reverse path. In this environment, TCP may severely underutilize the forward path throughput. Use of multicast further complicates the problem; TCP-like frequent feedback from each receiver to the sender in a large scale multicast session cause well-known scalability limitations (e.g. acknowledgment implosion). We have developed a new flow control approach for multimedia streaming, called TCP emulation at receivers (TEAR). TEAR shifts most of flow control mechanisms to receivers. In TEAR, a receiver does not send to the sender the congestion signals detected in its forward path but rather processes them immediately to determine its own appropriate receiving rate. TEAR can determine this rate using congestion signals observed at the receiver. These signals are used to emulate the TCP sender’s flow control functions at receivers.The emulation allows receivers to estimate a TCP-friendly rate for the congestion conditions observed in their forward paths. TEAR also allows receivers to adjust their receiving rates to a TCP-friendly rate without actually modulating the rates to probe for spare bandwidth, or to react to packet losses directly. Thus, the perceived rate fluctuations at the application are much more smooth than in TCP. A unicast version of TEAR is implemented. This report describes the implementation of TEAR, examine the performance of this TEAR implementation from the NS simulation and Internet experiments, and compare it with that of other TCP-friendly flow control techniques. Our preliminary tests indicate that TEAR shows superior fairness to TCP with significantly lower rate fluctuations than TCP. TEAR’s sensitivity to feedback interval is very low, so that even under high feedback latency, TEAR flows exhibit acceptable performance in terms of fairness, TCP-friendliness, and rate fluctuations. Finally, I will discuss the future extension of TEAR for multicast environments. This work is supported in part by NSF CAREER ANI-9875651.