摘 要
IP电话从20世纪90年代出现至今，经过近十年的发展，其可靠性和语音质量得到了极大的改善，技术日渐成熟，已经发展成为可提供语音、图像、数据以及传真等多种服务的平台，我们称这种新兴的技术为VoIP（Voice over IP）技术。VoIP以其低廉的价格和高度的灵活性受到世人广泛的欢迎和关注，VoIP是新一代网络时代发展起来的最具代表性和发展前途的应用技术之一。尽管如此，VoIP在发展的过程中也不可避免地碰到一些问题，“VoIP穿透NAT/FW的问题”就是其中的一个。
VoIP发展的关键技术之一是信令技术，信令技术用于控制电话呼叫的建立、管理和拆除以及会议参与者之间通信能力的交互，保证电话呼叫的顺利实现和通话的话音质量。H.323和SIP是目前最为广泛应用的两种VoIP信令协议，它们能够提供的信令控制功能基本相同，但是二者在设计风格、信息格式、控制模式等方面都存在着巨大的差异。H.323提出较早，技术比较成熟，现在市面上有很多H.323产品，目前它占据着VoIP控制信令技术的主流；SIP的提出相对较晚，但它以其协议结构简单灵活、分布式的控制模式以及与Internet紧密结合等优点而更加符合VoIP和未来网络发展的趋势，因此日益受到重视。
本文的主要工作是围绕着SIP应用中如何解决“VoIP穿透NAT/FW的问题”而展开的。文章首先对SIP协议进行了较为详细的介绍，然后在此基础上提出了“SIP应用层网关”的方案，该方案试图在SIP信令消息经过NAT/FW时，在应用层上对其进行修改，并通过控制NAT/FW打开必要的资源来实现VoIP应用的穿越。本文对这一方案在理论和实现两个方面进行详细的介绍。

【关键词】SIP，VoIP，H.323，SIP应用层网关
Abstract
Present in 1990s’, after nearly one decade’s development, IP telephony reaches its maturation, with its reliability and voice quality tremendously improved. Now it trends to a technical platform supporting many services such as voice, video, data and fax etc, and we call this new technology VoIP(Voice over IP). With its cheap price and obvious facility, VoIP gets great attention and popularity, and becomes one of the most representative and prospective technologies of the new network era. Despite
One of the key technologies VoIP depends on is signaling. Signaling is needed in VoIP to control the procedures of session establishment, release, management and capacity exhange between session participants. Signaling also provides assurance for call procedures along with voice quality. H.323 and SIP are the two presently most widely used signaling protocols in VoIP. These two provide similar signaling functions but quite different in design style, message format and control pattern etc. H.323 was released more early, and is maturer, also there are many H.323 applications on the market, so now H.323 occupies the mainstream of VoIP signaling. Despite of its later issuance, SIP presents the trend of VoIP and future networks, with its excellence in brief structure, distributed control pattern and close connection with Internet. SIP deserves the attention. 大量论文----德维在线 www.devay.net


[keywords] SIP，VoIP，H.323，SIP Application Level Gateway