建立及分析SIP-bases VoIP 穿透NAT機制

Abstract

由於NAT的發展，使用一台NAT來轉換IP位址，使NAT內部的主機可以跟NAT外部主機相互通訊，因此，解決了IPv4 IP不足的問題，但卻也帶來了另外的難題。如本研究所要介紹的VoIP穿透NAT問題。因為NAT只處理網路層及傳輸層的資訊，當屬於應用層的SIP資訊，在通過NAT時，NAT並不會修改SIP訊息(封包中應用層部分，包括SIP標頭和SIP本體)中所帶的IP位址與通訊埠，這些SIP訊息中的資訊在經過NAT之後將會發生錯誤。因此，本研究將使用三種透通技術---Session Controller、STUN以及VPN，解決VoIP在NAT環境裡所遇到的問題，並分析三種解決方案的特性(是否支援SIP UA、是否需要NAT支援、是否需要額外伺服器)、佈署的便利性(設定難易度、通訊延遲、DHCP的影響)和限制(是否可穿越對稱式NAT、是否可穿越多層NAT、是否需要額外公眾IP、SIP UA 是否需要支援對稱式RTP)。本研究部署SIP、NAT、STUN、Session Controller和VPN伺服器，構成VoIP平台，經過封包的分析，來發現VoIP實際上的問題，並且比較三種透通技術的優缺點，以期能提供企業部署VoIP平台時的參考。

Due to the development of NAT, using NAT to translate IP address can let the hosts inside NAT connect to the outside. Therefore, it solves the problem of insufficient IP, but it also brings other difficulties. For example, “the problem of VoIP traversal over NAT” that is introduced in the present research is one of them. NAT only handles the information on network and transport layer; thus when SIP information, part of the application layer, passes through NAT, NAT does not modify SIP messages (which include IP address, port in header field and body on the application layer). These SIP messages may cause errors after passing through NAT. Therefore, this research implement three kinds of transparent technologies-- Session ControllerdSTUN and VPN, to solve the problems in the NAT environment. Besides the characteristics (Whether they support SIP UA, need NAT support or need extra server), the availability of disposition (Configure difficulty, communication delay, and the influence of DHCP) and limitation (Passes through symmetric NAT and multi-storey NAT, and if SIP UA needs to support symmetric RTP) of these three schemes are also analyzed. This study disposes SIP, NAT, STUN, Session Controller and VPN servers to construct VoIP platform, and after analyzing packages, the actual problems of VoIP are discovered. Finally, this paper compares the advantages and defects of the three kinds of transparent technologies to provide enterprises with the references for disposing VoIP platform.