換手時造成的 IP 封包遺失與 TCP 傳送失序的問題,一直都是降低換手效能 的主要因素,而部份使用通道技術的換手機制,大都減少了 IP 封包遺失的問題,
但卻沒有考慮 TCP 傳送失序的問題。
在本篇論文中我們利用 Freeze-TCP 的技術來暫停換手時的 TCP 封包傳送,
無論使用 MIPv6、HMIPv6(Micro/Macro)、FMIPv6(Predictive/Reactive)或者 MMS 換手機制,都能減少換手時產生的 IP 封包遺失與 TCP 傳送失序的問題,避免兩 者的無效 TCP 封包傳送,節省無謂的頻寬浪費,並減少行動節點與網路裝置無 謂的負載,也更能提早回復正確的 TCP 傳送順序。
而 Freeze-TCP 的機制更完全不用更改 TCP 接收端,不用額外佈署網路裝 置,也不需要更改既有的換手機制,也沒有複雜的運作程序,只需在行動節點上 的 TCP 傳送端做些小更改,適時發送 TCP 標準中的零接收視窗大小廣播來凍結 (FreezeOn)對方的傳送,與換手快結束前適時的發送一個包含原來接收視窗大小 值的 TCP 告知(FreezeOff),即可回復原來的 TCP 傳送順序,而不用等到行動節 點完成換手後,等到收到對方第一個 TCP 封包才發現 TCP 失序。
結合 Freeze-TCP 的換手能以極低的成本換得更多益處,且與現有的換手機 制共同運作不互相干擾,相容於既有的行動網路架構下,能幫助行動節點更快、
更平順的完成換手程序。
近年來一種新的傳輸層通訊協定 DCCP[23]被提出,希望能在網路頻寬足夠 且穩定時,能夠追求可靠性的傳輸,而在網路頻寬較差的環境下,犧牲可靠性進 而追求即實性的傳輸,依網路的擁塞情況來決定傳送形式,本篇論文是以結合 Freeze-TCP 來增進 TCP 連線的傳送效益,未來我們可能研究如何在 DCCP 上,
實現結合如同 Freeze 的換手方式,使追求可靠性與即時性之間求得平衡的 DCCP,能在換手時維持更好的傳送效益。
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