近年來隨著網路的擴張，許多的應用被移植到網際網路上，語音傳輸就是其中之一，Voice over IP以其低成本逐漸的取代傳統的電路交換(Circuit Switch)語音通話，VoIP技術將語音資料分裝成封包型態，根據網路協定來傳送這些封包，它不需要專屬路徑，在資源利用方面也更有彈性。
目前有許多VoIP軟體，但是幾乎所有的VoIP軟體都需要事先的下載及安裝，所以我們實作一個Web-based VoIP系統，使用者不管是在家中或是在外使用公用電腦，只需要進入我們的入口網站，我們系統的用戶端程式就會藉由Java Applet的方式，主動下載並在使用者端執行，離開時，只需要關掉瀏覽器即可，程式或程式資訊將不會存留在電腦中。
我們希望使用者能夠隨時隨地透過網頁介面，執行我們的程式，所以我們必須考慮所有的網路環境，而NAT的穿越就是我們必須要克服的一大困難，包括了控制訊息(SIP)、即時訊息與語音資料封包等都可能因為NAT而被阻擋，所以我們提出了一個完整的NAT穿透規劃。
我們的系統提供了一個即時訊息、語音和通話轉接的通訊平台，使用者不需要事先安裝軟體或是擔心NAT造成的問題，只需要利用一台支援JRE的電腦，即可使用我們的系統與其他人通訊。

With the development and improvement of network bandwidth, Voice over Internet Protocol (VoIP) technology begins to bloom. VoIP gradually replaces voice over telecom networks because of its low cost. For instance, VoIP technology divides voice data into different packets and transmits over Internet Protocol, so that it does not require setting up a particular path as the telecom networks does.
Although there is much VoIP software available in the market, most of them need pre-installation before using. Our web-based VoIP technology allows users to take advantage of the service with either home computers or public computers. Users only need to log on to our website, then our system will provide users the client program via Java Applet with automatic installation. Users only need to close the browser to terminate the service; the program data will not be left on the computer.
In order to provide users to use our service in anytime and anywhere, we need to recognize Network Address Translation (NAT) as one of our biggest barriers. Including Session Initiation Protocol (SIP), instant message and voice data packet could be blocked by NAT. So, we propose a complete solution of NAT traversal.
Our system, SIP Communicator, provides a communication platform to users by supporting instant message, VoIP and session transferring. Users can log on to our system via any computers that are JRE supported to communicate with other clients.

1 緒論
1.1 研究動機
1.2 研究目標
2 相關技術研究
2.1 Session Initiation Protocol (SIP)
2.1.1 SIP簡介
2.1.2 SIP組成元件
2.1.3 SIP Methods
2.1.4 SIP 運作模式
2.2 Session Description Protocol (SDP)
2.3 Network Address Translation (NAT)
2.3.1 Static NAT and Dynamic NAT
2.3.2 Network address port translation (NAPT)
2.3.2.1 Cone NAT
2.3.2.2 Symmetric NAT
2.4 SIP NAT Traversal
2.4.1 Simple Traversal of UDP through NATs (STUN)
2.4.2 Traversal Using Relay NAT (TURN)
2.5 SIP Session Mobility
3 系統元件介紹
3.1 SIP Server and Client Applet Web Server
3.2 Traversal Using Relay NAT (TURN) Server
3.3 Online Node Management Server
3.4 SIP-based Communicator
3.4.1 Application Implementation
3.4.2 NAT traversal module
3.4.3 Session Transferring
4 系統實作
4.1 系統實作簡介
4.2 伺服器實作
4.3 SIP-based Communicator實作
4.3.1 Application Implementation
4.3.2 NAT Traversal Module
4.3.2.1 NAT Discovery
4.3.2.2 NAT Traversal of SIP Message
4.3.2.3 NAT Traversal of Media Data
4.3.3 Session Transferring
5 結論與未來工作
6 參考文獻

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[2] J. Rosenberg, H. Schulzrinne, G. Camarillo “SIP: Session Initiation Protocol”, Request for Comments: 3261, Internet Engineering Task Force, June 2002.
[3] J. Rosenberg, H. Schulzrinne “An Offer/Answer Model with the Session Description Protocol (SDP)”, Request for Comments: 3264, Internet Engineering Task Force, June 2002.
[4] P. Srisuresh, K. Egevang, “Traditional IP Network Address Translator (Traditional NAT)”, Request for Comments: 3022, Internet Engineering Task Force, January 2001.
[5] J. Rosenberg, J. Weinberger, C. Huitema, R. Mahy “STUN - Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs)”, Request for Comments: 3489, Internet Engineering Task Force, March 2003.
[6] J. Rosenberg, "Traversal Using Relay NAT (TURN)", draft-rosenberg-midcom-turn-08, Internet-Draft, Internet Engineering Task Force, September 2005.
[7] H. Schulzrinne, E. Wedlund, “Application-Layer Mobility Using SIP”, Mobile Computing and Communications Review, Vol3, pp. 47-57, July 2000.
[8] R. Shacham, H. Schulzrinne, S. Thakolsri, W. Kellerer, “Session Initiation Protocol (SIP) Session Mobility”, Internet-Draft, Internet Engineering Task Force, February 2006
[9] R. Sparks, “The Session Initiation Protocol (SIP) Refer Method”, Request for Comments: 3515, Internet Engineering Task Force, April 2003.
[10] SIP Express Router. http://www.iptel.org/ser/
[11] JAIN SIP. https://jain-sip.dev.java.net/
[12] JAIN SIP Applet phone. https://jain-sip-applet-phone.dev.java.net/