The effect of initial slow-start threshold size in DCCP over large delay link networks

As an alternative to UDP, DCCP is gaining popularity as a new transport protocol for sending streaming multimedia contents in the Internet nowadays. DCCP is a new unreliable transport protocol which has built-in congestion control unlike UDP. With this feature, DCCP ensures that there is no bandwidth monopoly by certain transport protocol like UDP did for years. UDP has been proven that can eat up all the available bandwidth in the Internet while competing with other transport protocols such as TCP in carrying streaming audio and multimedia traffic in many situations. In this paper, we show that initial slow-start threshold (ssthresh) size has a significant effect to the performance of DCCP carrying VoIP traffic over large delay link. Slow-start threshold is used by congestion control implemented in DCCP´s TCP-like congestion control (CCID2) where all TCP congestion control implementations are required to support it. From our experiments, we found out that too small initial slow-start threshold value for large delay link makes the traffic throughput sent using DCCP requires longer time to become stable. The selection of suitable value of initial slow-start threshold is then vital to the stability of the network carrying streaming audio data over DCCP.