摘要： SIP是IETF提出的IP电话信令协议，应用于视频电话、即时通信等领域，而网络中大量存在的NAT成为SIP应用推广的最大障碍，因此SIP消息如何有效穿透NAT成为该领域的热点问题之一。文章分析了现有各类SIP穿透NAT的方案，发现其在网络拓扑和应用可扩展性上存在局限。提出了一种新的SIP透明穿透NAT方案，保持了SIP协议在应用方面的可扩展性和拓扑的灵活性，在NAT设备中也只需维护少量的地址映射信息，并对该方案的透明性予以实验验证。

关键词: SIP协议, SIP网络地址翻译, SIP应用层网关, IXP2400

Abstract: Session initiation protocol(SIP), which is delivered by IEFT for IP telephone signaling, is greatly used in video-phone, instant- messaging, and etc. recently. However what makes SIP difficult is the widely using of NAT devices in network, so how to traverse NAT efficiently becomes the SIP researching hotspot. All the existed approaches are reviewed for SIP NAT traversing, and found out their limitation in topology and flexibility. Basing on understanding on SIP, a new SIP NAT traverse approach is developed, which keeps the flexibility of SIP without assuming any topology and maintaining too much addresses mapping information. In the end, the transparency of the approach through a full test is demonstrated.

Key words: Session initiation protocol(SIP), SIP NAT, SIP ALG, IXP2400