| 摘要: | 在高科技的社會中，隨著網路的興起、智慧型行動裝置的成熟及頻寬不斷的提升，讓現代的生活越來越便利。但因為人們過度集中於都市或移動速度過快原因，造成了頻寬的爭奪及網路換手的問題，如何解決目前正都在被廣泛的議論當中。
Stream Control Transmission Protocol(SCTP)，在2000年由IETF所提出，RFC 4960詳細地定義了SCTP。SCTP是屬於OSI網路七層架構中的傳輸層，SCTP結合TCP及UDP的優點，是一種點對點的傳輸，兼具可靠性服務的通訊協定。SCTP最為顯著特色的為多重串流(Multi-stream)與多重位址(Multi-homing)的特性。
其中，多重位址能同時擁有多組IP，並建立多條異質或同質的網路連線，也因為這特性，SCTP是適合來應用在軟式換手(soft-handover)的流程中。在原協定中所訂定的壅塞控制機制中，在換手時，新路徑的壅塞視窗(Congestion Window, CWND)因為沒有資料在傳輸，以致於CWND都處於接近初始值的數值。因此，在換手後會因為CWND的急遽下降，造成傳輸速率也瞬間急遽下降，使在換手後無法保持原有的傳輸效率。
本論文提出Fast Handover Recover - SCTP(FHR-SCTP)新的機制，基於SCTP Efficient Flow Control During Handover (SCTP-EFC)做更進階的改善。在換手後，新路徑除了繼承舊路徑的CWND外，並將其設定以慢啟動(Slow-Start)的方式來快速地到達新路徑CWND的峰值。除了保持原有的傳輸效率外，使新路徑能立即的擁有最大的傳輸量。若換手至較差通道品質的網路環境時，可能因為過大的CWND造成傳輸量過大，而導致封包掉落，因此若在換手後偵測到封包遺失時，將過大的CWND扣除遺失封包的大小，使新路徑能以可接受的最大傳輸量繼續傳輸。
本論文的實驗中，在換手後CWND遞增至最高值所花時間的部分，與SCTP及SCTP-EFC相比，提升了0%~42.5%；當FHR-SCTP換手機制在換手後CWND到達峰值時，比SCTP及SCTP-EFC多傳送0%~41.72%的資料量，由此來驗證，FHR-SCTP有較好的換手效率。
In the high-tech society, the technique of network and the mobile devices have become mature nowadays. All user usually surf the internet on the mobile device with mobile network, but all the base station have limit service range. Therefore, handover occurs all the time in our life. How to surf the internet with high efficiency during handover is a big issue.
SCTP was accepted by IETF in 2000, and SCTP included in the standard of RFC4960.SCTP, TCP and UDP are a transport protocol in OSI network structure. SCTP has advantages of TCP and UDP. The outstanding feature of SCTP is Multi-stream and Multi-homing.
Multi-homing that the devices could contain two or more networking interface is suit to implement soft-handover. However, there are still some negative effects by congestion control mechanism of SCTP during handover, the transmission rate will greatly decline during handover.
This paper presents a Fast Handover Recover - SCTP (FHR-SCTP) mechanism. It improves the mechanism that is presented by the paper of SCTP Efficient Flow Control During Handover (SCTP-EFC), and it can fast recover the CWND after handover. Besides inheriting the CWND of the old primary path to the new primary path, it sets the ssthresh of the new primary path to initial value after handover. It make the CWND of the new primary path increase by slow-start phase and quickly own maximum transmission traffic. Last, if FHR-SCTP handover to poor quality path, maybe the CWND is too large for the new primary path and makes the new primary path cannot afford the traffic volume. So, we store the TSN (Transmission sequence number) to calculate the dropped packet size before handover. If it occurs packet losing at first time after handover, we should decrease the CWND by the dropped packet size.
The result shows that FHR-SCTP is more efficient than SCTP and SCTP-EFC .FHR-SCTP improves the time of reaching the max CWND about 0.83% to 42.5% with other mechanisms after handover, and when the CWND of FHR-SCTP reaches the max CWND after handover, FHR-SCTP transmits more than 0%~41.72% packet size with SCTP and SCTP-EFC. |
