在Linux下減少因拓撲不匹配造成的額外交遞延遲

Abstract

在IEEE 802.11無線區域網路(Wireless Local Area Network)的環境下，已有許多學者專家研究如何減少因交遞造成的中斷，其中大多數皆專注於提出減少交遞的方法，並在較單純的環境下進行測試。然而也有一些研究注意到在交遞時可能會遇到網路層和連結層的拓撲不匹配的情形，而這些研究同時也展示出拓撲不匹配會造成額外長的交遞延遲。但前述研究均是在視窗系統下進行實驗，尚缺乏對Linux環境下的研究，因此本論文選定一Linux的環境，嘗試進行實驗分析，找出遭遇拓撲不匹配時Linux面臨的問題，並提出改善方法及實作之。 在分析的過程中發現到，Linux下的網路管理軟體Network Manager，其管理無線網路介面卡的狀態流程並未考慮有拓撲不匹配的情形，因此遇到如Intra-ESS Inter-Subnet此種交遞情形時，會持續的使用錯誤的網路層設定，造成使用者無法繼續使用網路服務，並且因其不會定期偵測網路狀態，便無法自行發現此異常狀態加以改正，造成此無法網路服務的時間可能長達數小時之久。而當遇到Inter-ESS Intra-Subnet此種交遞情形時，則會進行不必要的網路層交遞，造成額外的交遞延遲。 本論文針對前述狀況各別提出了建議的解決方案，使得Intra-ESS Inter-Subnet的情形可以在很短的時間內被Network Manager察覺並進一步取得新的網路層設定，使用者便可繼續正常使用無線網路服務。而Inter-ESS Intra-Subnet的解決方案可避免不必要的網路層交遞，我們推斷可以因此減少數秒的延遲。最後並以實驗證明對Intra-ESS Iner-Subnet的解決方案的可行性。

There have been many studies about reducing the delay brought by the handover in the environment of IEEE 802.11, and most of them focus on proposing new handover scheme to reduce the delay using a simple network architecture. However, some studies reveal that it is possible that the network layer topology and the link layer topology do not match. Furthermore, these studies show that this mismatching leads to longer handover delays. Nevertheless, these studies are all based on the Microsoft Windows system, with any study on Linux still yet to come. This thesis, therefore, studies a specific Linux environment by analyzing the system behavior and proposing possible solutions to the problem with which the Linux system may encounter during topology-mismatched handovers. We find out that Network Manager, one network managing software on Linux, does not consider the topology-mismatched cases at all. As a result, if the handover is Intra-ESS and Inter-Subnet, the software itself would continue using the wrong network layer settings, making the user unable to use the wireless network service. In addition, the software does not periodically check the network status, causing it to be unable to detect this misbehavior, which would result in a period of service break may last several hours. Another case is the Inter-ESS and Intra-Subnet, in which network manager will always do the unnecessary network layer handover, or DHCP process, producing an unnecessarily longer delay. This thesis provides a solution to each of the mentioned problems. In the case of Intra-ESS Inter-Subnet, after applying the solution, Network Manager is able to detect the change of network layer topology within a short period and then retrieve new network layer settings in order to bring network service back to the user. The solution to Inter-ESS Intra-Subnet cases can avoid the delay of unnecessary network layer handover, which could last several seconds. Finally, we will be using experiments to prove the viability of our solution to the Intra-ESS and Inter-Subnet cases.