封包擁塞控制協定(Datagram Congestion Control Protocol, DCCP)具備擁塞控制機制及不可靠傳輸的特性，非常適合具及時性的多媒體應用。然而對某些應用而言，某些關鍵性封包的遺失恐會造成服務品質的大幅降低，因此若是能將遺失且具關鍵性的封包在其有效的時間內加以重傳，如此服務品質必能顯著提升。本論文在DCCP上提出由傳送端決定封包是否要重傳的partially reliable DCCP (PR-DCCP)，由於無可靠機制的DCCP所使用的序號會持繼增加，即使重傳時也無法使用原始的序號，因此PR-DCCP使用封包序號補償來解決此問題，其概念為重傳之封包會附加一個差距值，如此receiver可利用序號及差距值來得到原始需重傳封包的序號。模擬結果顯示PR-DCCP在Decodable Frame Ratio(DFR) 與Useless Data Received Ratio(UDRR)上都優於其他傳輸層協定DCCP、SCTP、TCP、UDP。針對各種不同的影片，PR-DCCP的DFR可比DCCP增加1.9%~14.8%，而UDRR則可降低約86.8%~90.5%。

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 paper 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.

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