Retransmission Mechanisms of DCCP to Enhance QoS of Multimedia Applications

• Main Authors: Ching-neng Lai, 賴敬能
• Other Authors: none
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/88429052181060717164

博士 === 國立臺灣科技大學 === 資訊管理系 === 97 === Datagram congestion control protocol (DCCP), possessing congestion control and unreliable transmission, specially suits real-time multimedia applications. Nevertheless, losses of key packets will cause a substantial decline on quality of services (QoS) in some applications. This dissertation proposes a DCCP partial reliability extension (PR-DCCP) that can retransmit lost packets as needed. Since DCCP uses an incremental sequence number, the retransmitted packets cannot utilize their original sequence number. To solve this problem, PR-DCCP adopts sequence number compensation, which appends an offset to the retransmitted packet; thus the receiver can use the sequence number of this retransmitted packet and the attached offset so as to re-obtain the original sequence number. The simulation uses two performance metrics: decodable frame ratio (DFR) representing QoS, and useless data received ratio (UDRR) representing the bandwidth waste. These are used to evaluate different transport protocols, namely, PR-DCCP, DCCP, SCTP, TCP, and UDP. Simulation results show that PR-DCCP has the better DFR and UDRR than other transport protocols in almost all cases. For various movies, a DFR of PR-DCCP is 1.2–12.4% higher than that of DCCP; while UDRR is lower by 73.2–85.1%. Furthermore, two reliability policies to determine which packets require reliability are investigated. Finally, the comparisons between PR-DCCP and PR-SCTP are examined. Partially reliable transport protocol is able to improve the amount of the valid data received by the receiver through retransmitting key packets. Nevertheless, since most multimedia applications are time-sensitive, it has significant impact on the QoS of multimedia applications that the sender determines which packets need to be retransmitted and when these packets are retransmitted. Therefore, this dissertation further proposes Adaptive Retransmission Method (ARM). This method allows the sender to retransmit packets without referring the information in the application layer. It is according to the packet fetching rate and the Round-Trip Time (RTT) to obtain a threshold of sequence number. The sender uses this threshold to determine which packets can arrive in time and only retransmits these packets. From simulation results, in comparison with the other retransmission methods, which the lifetime (LT) of packets is set as a fixed 0.2s (LT=0.2s) or a fixed 2s (LT=2s), PR-DCCP adopting ARM can achieve the best QoS under the environments with the low bandwidth or the high propagation delay. When the environment with the low bandwidth, the DFRs of LT=0.2s and LT=2s are improved by 17.4% and 25.6%, while UDRRs are reduced by 10.2% and 25.5%, respectively. Under the condition with the long propagation delay, the DFRs of LT=0.2s and LT=2s are improved by 11.3% and 8.6%, while UDRRs are reduced by 12.3% and 7.8%, respectively.