為SIP應用程式解決穿越網路位址轉換機的問題

Abstract

網路位址轉換機(Network Address Translator)普遍地用來解決IPv4位址不夠用的瓶頸；但它卻會阻礙某些應用程式或是多媒體的傳輸，這叫做「穿越網路位址轉換機的問題」。許多人研究出解決這個問題的方法，尤其是針對對稱式的網路位址轉換機，因為它是當中最難克服的一種。互相建立連線機制(Interactive Connectivity Establishment)提供一個不錯的方法可以解決所有種類的穿越問題。但我們發現到他的作法缺乏效率。 於本論文中，我們提出一種想法，就是在註冊的過程中完成大部份穿越網路位址轉換機的前置工作，來減少建立通話時檢驗路由位址可否連線的負擔，比互相建立連線機制花費更少的工作去達成相同的功能。我們也主張這個架構應該以一個類似點對點(Peer-to-Peer)的模型為基礎藉此消去對轉送伺服器的需求，以及降低對註冊伺服器的負擔，諸如此類優點。「穿洞」在我們的架構上扮演一個很重要的角色，它的意思是讓網路位址轉換機內部的主機送封包給外部的主機藉此在網路位址轉換機上製造出相對應的接口。我們也認為應該在註冊的過程中打出要給訊號端用的洞；但是在建立通話之前再打出多媒體資料要用的洞。這樣可以減去和網路位址轉換機上的接口保持聯繫、延續它的有效時間的負擔。

Network Address Translator (NAT), which is used to extend the IPv4 address space, may break Internet connections and media transmission. This is refered to as “NAT traversal problem”. Many researches have investigated this problem, especially for the symmetric NAT, which is the most difficult to traverse. Interactive Connectivity Establishment (ICE) provides a pretty good choice to solve the traversal problem, but there are some redundant steps that can be simplified for VoIP communications. In the thesis, we present a solution that accomplishes most tasks to traverse NAT during VoIP user registration, and reduces the cost in verifying allocated addresses. Our method efficiently solves each type of NAT traversal problem with less overhead than the ICE. Our NAT traversal solution is also based on a P2P-like model to eliminate the need of dedicated relay servers and to reduce the load of the SIP registrar. Hole punching, which means that an internal host sends packets to an external host to create a mapping port on the NAT, plays a significant role in our solution; a SIP UA punches a hole on the NAT for exchanging signaling during registration, and punches another for media transmission just before establishing a call. Thus our method reduces the overhead of keep-alive messages.