在移動式IP（Mobile IP）的網路架構下，移動節點（Mobile Node）不論移動到哪個網路只需要動態更換IP住址即可維持通訊連線，解決了傳統的行動設備在移動到不同網路之後就必須更換IP住址才可以繼續使用網際網路資源的不便。但是移動節點在換手（Handoff）時仍然會產生封包遺失（Packet Loss）、重傳（Retransmission）及傳輸效能（Throughput）下降的問題。
本論文在移動節點上偵測無線網路上的訊號雜訊比（Signal-to-Noise Ratio）來啟動Home Agent中的封包緩衝機制，以避免移動節點在Handoff時產生封包遺失還要重傳的情形，當移動節點在Handoff完成時，藉由傳送緩衝中的第一個封包給移動節點，使得移動節點回送ACK封包，避免緩衝區中重複封包（Duplicate Packets）的傳送。並且透過路徑選擇機制選擇路徑延遲較低的路徑傳送封包，增加傳輸效能。
為了驗證我們所提出的封包緩衝機制，我們在Linux平台上實作出這些機制，並且透過實驗、量測以及結果分析證明我們的機制確實可以改善移動節點在Handoff時造成的封包重傳問題與傳輸效能下降的問題。

Today’s mobile IP supports host mobility by dynamic changing IP addresses while the mobile host roaming in the Internet. However, There still exists performance problems during handoffs, such as packet loss, throughput degradation, and so on.
In this Thesis, we propose a mechanism to reduce packet loss during handoff. The packet buffering mechanism at a home agent is initiated by mobile hosts when the signal-to-noise ratio of the wireless link falls below some predefined threshold. Once the handoff has completed, the home agent immediately delivers the first packet in the buffer to the mobile host. The home agent then clears the buffered packets already received by the mobile host through the returned ACK such that no further duplicate packets are sent out. In addition, we propose a route-selection policy to reduce end-to-end transmission delay by sending out probe packets along the paths.
For the purpose of demonstration, we implement the mechanism on Linux platform. Through the measurements from the experiment, we have shown that the proposed mechanism can improve the throughput and solve the packet retransmission problem while handoffs.

第一章 導論………………………………………………………………………1
1.1 研究動機……………………………………………………………………1
1.2 研究方向與實作……………………………………………………………1
1.3 章節介紹……………………………………………………………………3

第二章 無線網路…………………………………………………………………4
2.1 Mobile IP（移動式IP）………………………………………………………4
2.2 Mobile IP網路架構…………………………………………………………5
2.2.1 Mobile 簡介…………………………………………………………5
2.2.2 Mobile IP基本運作方式………………………………………………7
2.2.3 Mobile IP 的handoff問題……………………………………………10
2.3 Mobile IP的相關研究……………………………………………………… 12
2.3.1 Hierarchical mobility management…………………………………… 13
2.3.2 Agent buffering………………………………………………………14
2.3.3 Mobile IP的架構比較………………………………………………17

第三章 封包緩衝及快速重傳機制…………………………………………20
3.1封包緩衝的時機……………………………………………………………20
3.2封包緩衝機制………………………………………………………………22
3.3快速重傳機制………………………………………………………………26
3.4 路徑選擇機制…………………………………………………………… 29
3.5在Linux平台上的實作……………………………………………………33
3.5.1偵測訊號雜訊比……………………………………………………33
3.5.2 Mobile IP隧道的建立與運作………………………………………34
3.5.3實作封包緩衝機制及快速重傳機制………………………………34

第四章 效能量測與結果分析…………………………………………………38
4.1實驗的網路拓樸……………………………………………………………38
4.2實驗數據與分析討論………………………………………………………39
4.2.1實驗的參數與環境設定……………………………………………39
4.2.2 TCP 的Packet Loss…………………………………………………40
4.2.3 TCP 的Throughput…………………………………………………41
4.2.4 UDP 連線的量測與分析……………………………………………44
4.3 路徑選擇機制對效能的增進………………………………………45
4.3.1 路徑選擇機制的實驗網路拓樸……………………………………45

第五章 結論與未來工作方向…………………………………………………48
5.1 結論………………………………………………………………………48
5.2未來工作方向………………………………………………………………49

參考文獻……………………………………………………………………………50
索引…………………………………………………………………………………53

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