NAT Inbound 連線, Web快取, 與行動環境中的Web連線切換

Abstract

在本論文中，我們提出了可動態設定的NAT (DCNAT)、混合式的Web快取管理演算法、Web瀏覽連線換手機制、以及利用網路拓樸與移動方向來進行鍊接層快速換手的機制，以便利用這些機制來支援不間斷的Web存取，與推送即時資訊至私有網路內的行動主機。 DCNAT是一個基於連接埠的NAT機制，它可以讓網際網路上的主機主動建立連線與推送即時資訊至NAT網路內的主機。DCNAT包含了一個Binding Entry Request 程序，藉由此程序，網際網路上的主機在連線至一私有網路內的主機之前，可以要求DCNAT路由器來動態地建立關於此一私有網路主機的NAT binding entry。再者，動態建立binding entry的特色使得DCNAT能夠有彈性地支援私有網路主機動態開放的連接埠與網路服務之存取。此外，網際網路上的內容提供者也能夠藉由DCNAT，主動地推送即時資訊給散佈於各個私有NAT網路的訂閱者。 為改善Web快取的效能，我們提出了混合式的Web快取管理演算法。此一混合式Web快取管理演算法將快取的儲存空間分成熱區與冷區共兩個區域，熱區採用簡單的LRU演算法，冷區採用較複雜的GreedyDual系列的演算法。這樣的混合式Web快取管理演算法，結合了SLRU、LRU、GreedyDual系列的優點，卻只須要在快取儲存空間的一部份(冷區)應用較複雜的GreedyDual系列演算法，便可以藉由冷區較周詳的管理演算法來提升快取的整體效能。因而混合式Web快取管理演算法可以達到略低於GDSP演算法的高文件命中率(DHR)與略低於LRU演算法的高位元組命中率(BHR)，但卻只需要遠低於GreedyDual系列演算法的維護代價。 為了讓使用者在更換設備後，可以繼續Web瀏覽連線，我們提出一個Web瀏覽連線換手系統。此一Web瀏覽連線換手系統可以在同質或異質的使用者設備之間切換無狀態或有狀態的瀏覽連線，讓使用者在更換設備後可以進行不中斷且無接縫的Web存取。為了能夠在使用者設備之間追蹤與交換各式各樣的連線相關資訊，我們採用的是代理伺服器為基礎的方法，再加上非必須的使用者端的輔助機制。首先，使用者利用其設備向一個稱之為UAP的代理伺服器註冊其瀏覽連線；隨後UAP便可以追蹤此瀏覽連線並記錄必要的連線資訊，以供換手使用。此外，使用者端的輔助機制可以幫助UAP來換手更詳盡的連線資訊。相較於以使用者端為基礎的方法，我們的設計有許多優點，例如：使用者設備需要較少的修改、實用性、與錯誤相容。我們已實作了一UAP主機，並在個人電腦(PC)與個人數位助理(PDA)實作了使用者端的程式。實作的結果能夠成功地在個人電腦與個人數位助理換手一有狀態的線上購物瀏覽連線。 為加速在802.11無線基地臺之間進行鍊接層換手的過程，我們利用無線基地臺拓樸、行動主機的移動方向、與行動主機的位置來計算換手的候選無線基地臺，並減少候選無線基地臺的數目。此方法另有許多優點：第一，在換手的過程中，行動主機不需要執行耗時的無線基地臺搜尋程序，便能夠直接與無線基地臺取得關聯(association)，從而進一步加快換手的速度；第二，此一計算換手的候選無線基地臺的機制可以與其它現存的快速換手機制(例如：預先驗證與預先金鑰分配)互相配合。 這四個機制一起作用，為行動環境與NAT環境的使用者提供了方便的網際網路存取。

In this thesis, we propose a Dynamically Configurable NAT (DCNAT), a hybrid web cache replacement algorithm, a web session handoff mechanism, and a Topology- and-Direction-aware link layer fast handoff scheme to support continuous web access and real-time information pushing for mobile hosts that may visit private networks. With DCNAT, a host situates in Internet can spontaneously establish inbound session and push instant information to hosts beneath NAT networks. DCNAT is a port-based NAT scheme that adopts a Binding Entry Request procedure for an Internet host to request a DCNAT router to dynamically create binding entries, for address:port translation, associated with a private host behind the DCNAT router just before the Internet host originates a communication session to the private host. Furthermore, the dynamic creation of NAT binding entries makes DCNAT very flexible in supporting inbound accesses to the ports/services opened dynamically by the private nodes behind an NAT router. Besides, with DCNAT, a content service provider can push information contents spontaneously to subscribers that are widely spreading and beneath different private networks with NAT. In order to improve web cache performance, we propose a hybrid web cache replacement algorithm that divides a cache space into two zones, a hot zone and a cold zone, and adopt a simple LRU replacement algorithm for the hot zone and a complex GD-Family replacement algorithm for the cold zone. This hybrid replacement algorithm combines the advantage of SLRU, LRU, and GD-Family algorithms but applies a complex GD-Family replacement algorithm only to a portion of the cache where strict object ordering is much significant to the overall cache performance. Therefore it can achieve a high DHR just slightly lower than that of GDSP and a high BHR just slightly lower than that of LRU while incurs much less maintenance cost, comparing with GD-Family algorithms. To resume a web browsing session after a user changes the device that he uses to browse Internet, we propose a web session handoff system that can hand over not only stateless but also stateful sessions between homogenous or heterogeneous user devices to enable uninterrupted and seamless web accesses. The proposed system adopts a proxy-based approach and an optional client-assisted scheme to track and hand over a variety of session information from one device to another. In the proposed system, a user device needs to register a session with a User Agent Proxy (UAP) and the UAP then tracks the registered session and records the information necessary to the web session handoffs. In addition to session information tracked by a UAP, the UAP can hand over more comprehensive session information by using the client-assisted scheme. Compared with client-based approaches, our design has several advantages, such as less modification to user devices, practicability, and fault tolerance. We have implemented a UAP on a PC and client programs for both PC and PDA. The implementation can successfully hand over between PC and PDA a stateful session for online shopping applications. To speed up the process of handoff between 802.11 APs, we propose a Topology-and-Direction-aware fast handoff scheme that resolves a reduced set of candidate APs according to AP topology, MN’s its moving trend and/or MN’s position. The proposed approach further benefits handoff by allowing 802.11 station reassociates directly with an AP without performing AP discovery during handoff. Besides, the topology-and-movement-aware candidate-AP generation scheme can work in conjunction with the existing fast-handoff schemes that leverage pre-authentication, proactive key distribution, or context transfer. These four approaches together provide convenient Internet access in mobile environments and NAT environments.