通信工程文献翻译——本文章摘自RFC3261文档,示例一个完整的sip协议的通话过程。

简单的示例介绍了SIP的基本实现。本节是通过自然的而非正则的示例来介绍的。

第一个例子说明了SIP的基本功能：定位一个断点，发出通讯请求，通过协商会话参数建立会话，拆卸刚才建立的会话。

图一表示一个典型的Alice和Bob两个用户间的SIP消息交易交换例子.（每一个消息采用字母”F”和一个用来指向正文的一个数字做标记）在这个例子里，Alice在她的PC上使用一个SIP的应用程序（比如说一个软的电话），呼叫Bob在Internet上的一个SIP电话。这个例子也掩饰了两个SIP代理之间，怎样为Alice和Bob建立会话连接。整个协商过程在图1中展示。

Alice 通过Bob的SIP标志“呼叫” Bob，这个SIP标志是统一分配的资源（Uniform Resource Identifier URI）称作SIP URI。它很像一个email地址，典型的SIP URI包括一个用户名和一个主机名。在这个范例中，SIP URI是sip:bob@biloxi.com,biloxi.com是Bob的SIP服务提供商。Alice有一个SIP URI: sip:alice@atlanta.com。 Alice可以输入Bob的URI，也可以直接在地址本的一个超级链接上点击一下Bob的URI。SIP也提供保密URI，称作SIPS URI。例如：sips: bob@biloxi.com。一个基于SIPS URI的通话保证这个通话是安全的，并且对呼叫者和被叫的所有的SIP消息是加密传输的（叫做TLS）。在TLS中，请求是通过加密方式传输给被叫方，但是这个加密机制是基于被叫方宿主服务器的实现的。

SIP是基于一个类似HTTP协议的请求应答的通讯模式。每一个通讯都包含对某个功能的请求，并且起码需要一个应答。在这个应答中，Alice的软电话发送一个含有Bbo的SIP URI抵制的INVITE通讯请求。INVITE是一个SIP请求的例子，表示请求方（Alice）希望服务......

This section introduces the basic operations of SIP using simple examples. 

The first example shows the basic functions of SIP: location of an end point, signal of a desire to communicate, negotiation of session parameters to establish the session, and teardown of the session once established.

Figure 1 shows a typical example of a SIP message exchange between two users, Alice and Bob.  (Each message is labeled with the letter "F" and a number for reference by the text.)  In this example, Alice uses a SIP application on her PC (referred to as a softphone) to call

Bob on his SIP phone over the Internet.  Also shown are two SIP proxy servers that act on behalf of Alice and Bob to facilitate the session establishment.  This typical arrangement is often referred to as the "SIP trapezoid" as shown by the geometric shape of the dotted lines in Figure 1.

Alice "calls" Bob using his SIP identity, a type of Uniform Resource Identifier (URI) called a SIP URI. SIP URIs are defined in Section 19.1.It has a similar form to an email address, typically containing a username and a host name.  In this case, it is sip:bob@biloxi.com, where biloxi.com is the domain of Bob's SIP service provider.  Alice has a SIP URI of sip:alice@atlanta.com. Alice might have typed in Bob's URI or perhaps clicked on a hyperlink or an entry in an address book.  SIP also provides a secure URI, called a SIPS URI.  An example would be sips:bob@biloxi.com.  A call made to a SIPS URI guarantees that secure, encrypted transport (namely TLS) is used to carry all SIP messages from the caller to the domain of the callee.  From there, the request is sent securely to the callee, but with security ......