SIP 注册过程

SIP协议包含两种类型的消息，一种是请求行用于发出邀请，而另一种则是状态行，用于标明当前通信的状态。请求行和状态行军包含三部分，其中每一部分以空格隔开，不论是请求行还是状态行均以CRLF换行符结束。在请求行中，三部分内容分别为Method 请求URI和SIP版本号。如下例所示：

Request-Line: REGISTER sip:192.169.1.150 SIP/2.0

在这里方法是REGISTER,也就是注册。空格之后的sip:192.169.1.150则是请求到达的URI目的地址，SIP/2.0则是SIP的版本号。

Status-Line: SIP/2.0 407 Proxy Authentication Required

状态行的例子如上所示，第一个部分和请求行的解释一样被作为SIP的版本号，第二部分则是一个状态码，类似于浏览器访问出现的404；最后一个部分则是对第二部分状态码进行一个简单的解释。

在上面两种行消息之后是消息头部，消息头部的组成也是以文本字符进行编码的。同样的，消息头部的每一个字段都是以CRLF换行作为结尾符。而每一个字段的格式如下：

Via: SIP/2.0/UDP 192.169.1.147:39814;rport;branch=z9hG4bK80051

上面是定义SIP消息头部字段格式的一种示例，冒号前面的是字段的名称，这里表示消息传递出去经过的路径；而第二部分则是传递的使用到的协议——基于UDP的SIP（SIP也可以基于TCP进行实现），第三部分则是起始地址的IP地址和端口号。第四部分的rport告诉服务器，哪里来的消息从哪里发送回去——IP和端口号都不会改变。第五部分则是一个标志ID，标志ID的前七个字母类似于一个魔数用于区别其他的协议。
下面看一个完整的SIP用户和用户代理之间的鉴权认证过程。

Session Initiation Protocol (REGISTER)

    Request-Line: REGISTER sip:192.169.1.150 SIP/2.0

        Method: REGISTER

        Request-URI: sip:192.169.1.150

        [Resent Packet: False]

    Message Header

        Via: SIP/2.0/UDP 192.169.1.147:39814;rport;branch=z9hG4bK80051

        Max-Forwards: 70

        To: <sip:102@192.169.1.150>

            SIP to address: sip:102@192.169.1.150

        From: <sip:102@192.169.1.150>;tag=z9hG4bK17086172

            SIP from address: sip:102@192.169.1.150

            SIP from tag: z9hG4bK17086172

        Call-ID: 281574552997@192.169.1.147

        CSeq: 1 REGISTER

            Sequence Number: 1

            Method: REGISTER

        Contact: <sip:102@192.169.1.147:39814;transport=udp>

            Contact URI: sip:102@192.169.1.147:39814;transport=udp

        Expires: 3600

        User-Agent: Sipdroid/3.6 beta

        Content-Length: 0

Session Initiation Protocol (407)

    Status-Line: SIP/2.0 407 Proxy Authentication Required

        Status-Code: 407

        [Resent Packet: False]

        [Request Frame: 1322]

        [Response Time (ms): 0]

    Message Header

        Via: SIP/2.0/UDP 192.169.1.147:39814;branch=z9hG4bK80051;received=192.169.1.147;rport=39814

        From: <sip:102@192.169.1.150>;tag=z9hG4bK17086172

            SIP from address: sip:102@192.169.1.150

            SIP from tag: z9hG4bK17086172

        To: <sip:102@192.169.1.150>;tag=13d0075e

            SIP to address: sip:102@192.169.1.150

            SIP to tag: 13d0075e

        CSeq: 1 REGISTER

            Sequence Number: 1

            Method: REGISTER

        Call-ID: 281574552997@192.169.1.147

        Allow: ACK, BYE, CANCEL, INFO, INVITE, MESSAGE, NOTIFY, OPTIONS, REFER, SUBSCRIBE

        User-Agent: miniSIPServer V16.0.7 (100 clients) build 20150324

        Proxy-Authenticate: Digest realm="myvoipapp.com",algorithm=MD5,nonce="328E3DAF06BC5374001D626160CB1C21",stale=FALSE

            Authentication Scheme: Digest

            Realm: "myvoipapp.com"

            Algorithm: MD5

            Nonce Value: "328E3DAF06BC5374001D626160CB1C21"

            Stale Flag: FALSE

        Content-Length: 0

Session Initiation Protocol (REGISTER)

    Request-Line: REGISTER sip:192.169.1.150 SIP/2.0

        Method: REGISTER

        Request-URI: sip:192.169.1.150

        [Resent Packet: False]

    Message Header

        Via: SIP/2.0/UDP 192.169.1.147:39814;rport;branch=z9hG4bK99109

        Max-Forwards: 70

        To: <sip:102@192.169.1.150>

            SIP to address: sip:102@192.169.1.150

        From: <sip:102@192.169.1.150>;tag=z9hG4bK17086172

            SIP from address: sip:102@192.169.1.150

            SIP from tag: z9hG4bK17086172

        Call-ID: 281574552997@192.169.1.147

        CSeq: 2 REGISTER

            Sequence Number: 2

            Method: REGISTER

        Contact: <sip:102@192.169.1.147:39814;transport=udp>

            Contact URI: sip:102@192.169.1.147:39814;transport=udp

        Expires: 3600

        User-Agent: Sipdroid/3.6 beta

        Proxy-Authorization: Digest username="102", realm="myvoipapp.com", nonce="328E3DAF06BC5374001D626160CB1C21", uri="sip:192.169.1.150", algorithm=MD5, response="bdf9504e7f2e0d6c685e57f9eef22f43"

            Authentication Scheme: Digest

            Username: "102"

            Realm: "myvoipapp.com"

            Nonce Value: "328E3DAF06BC5374001D626160CB1C21"

            Authentication URI: "sip:192.169.1.150"

            Algorithm: MD5

            Digest Authentication Response: "bdf9504e7f2e0d6c685e57f9eef22f43"

        Content-Length: 0

Session Initiation Protocol (200)

    Status-Line: SIP/2.0 200 OK

        Status-Code: 200

        [Resent Packet: False]

        [Request Frame: 1324]

        [Response Time (ms): 0]

    Message Header

        Via: SIP/2.0/UDP 192.169.1.147:39814;branch=z9hG4bK99109;received=192.169.1.147;rport=39814

        From: <sip:102@192.169.1.150>;tag=z9hG4bK17086172

            SIP from address: sip:102@192.169.1.150

            SIP from tag: z9hG4bK17086172

        To: <sip:102@192.169.1.150>;tag=55d70430

            SIP to address: sip:102@192.169.1.150

            SIP to tag: 55d70430

        CSeq: 2 REGISTER

            Sequence Number: 2

            Method: REGISTER

        Call-ID: 281574552997@192.169.1.147

        Allow: ACK, BYE, CANCEL, INFO, INVITE, MESSAGE, NOTIFY, OPTIONS, REFER, SUBSCRIBE

        User-Agent: miniSIPServer V16.0.7 (100 clients) build 20150324

        Contact: <sip:102@192.169.1.147:39814;transport=udp>

            Contact URI: sip:102@192.169.1.147:39814;transport=udp

        Expires: 180

        Content-Length: 0