搭建个人网站的步骤,微信网站游戏,wordpress教育培训,云南网站优化公司本文通过一个完整的SIP呼叫实例解释SIP头部的一些常见字段#xff0c;在对这些字段的解释的同时也阐述了SIP消息的路由过程。下图是呼叫的消息流示意图和所有的消息头部(因为SDP和消息路由无关#xff0c;故在此省略)#xff1a; atlanta.com . . . biloxi.com . prox…本文通过一个完整的SIP呼叫实例解释SIP头部的一些常见字段在对这些字段的解释的同时也阐述了SIP消息的路由过程。下图是呼叫的消息流示意图和所有的消息头部(因为SDP和消息路由无关故在此省略) atlanta.com . . . biloxi.com . proxy proxy . . . Alices . . . . . . . . . . . . . . . . . . . . Bobs softphone SIP Phone | | | | | INVITE F1 | | | |---------------| INVITE F2 | | | 100 Trying F3 |---------------| INVITE F4 | |---------------| 100 Trying F5 |---------------| | |-------------- | 180 Ringing F6 | | | 180 Ringing F7 |---------------| | 180 Ringing F8 |---------------| 200 OK F9 | |---------------| 200 OK F10 |---------------| | 200 OK F11 |---------------| | |---------------| | | | ACK F12 | |-------------------------------------------------| | Media Session | || | BYE F13 | |-------------------------------------------------| | 200 OK F14 | |-------------------------------------------------| | | F1 INVITE Alice - atlanta.com proxy INVITE sip:bobbiloxi.com SIP/2.0 Via: SIP/2.0/UDP pc33.atlanta.com;branchz9hG4bKnashds8 Max-Forwards: 70 To: Bob sip:bobbiloxi.com From: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: sip:alicepc33.atlanta.com Content-Type: application/sdp Content-Length: 142 F2 100 Trying atlanta.com proxy - Alice SIP/2.0 100 Trying Via: SIP/2.0/UDP pc33.atlanta.com;branchz9hG4bKnashds8 ;received192.0.2.1 To: Bob sip:bobbiloxi.com From: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Content-Length: 0 F3 INVITE atlanta.com proxy - biloxi.com proxy INVITE sip:bobbiloxi.com SIP/2.0 Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branchz9hG4bK77ef4c2312983.1 Via: SIP/2.0/UDP pc33.atlanta.com;branchz9hG4bKnashds8 ;received192.0.2.1 Max-Forwards: 69 To: Bob sip:bobbiloxi.com From: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: sip:alicepc33.atlanta.com Content-Type: application/sdp Content-Length: 142 F4 100 Trying biloxi.com proxy - atlanta.com proxy SIP/2.0 100 Trying Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branchz9hG4bK77ef4c2312983.1 ;received192.0.2.2 Via: SIP/2.0/UDP pc33.atlanta.com;branchz9hG4bKnashds8 ;received192.0.2.1 To: Bob sip:bobbiloxi.com From: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Content-Length: 0 F5 INVITE biloxi.com proxy - Bob INVITE sip:bob192.0.2.4 SIP/2.0 Via: SIP/2.0/UDP server10.biloxi.com;branchz9hG4bK4b43c2ff8.1 Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branchz9hG4bK77ef4c2312983.1 ;received192.0.2.2 Via: SIP/2.0/UDP pc33.atlanta.com;branchz9hG4bKnashds8 ;received192.0.2.1 Max-Forwards: 68 To: Bob sip:bobbiloxi.com From: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: sip:alicepc33.atlanta.com Content-Type: application/sdp Content-Length: 142 F6 180 Ringing Bob - biloxi.com proxy SIP/2.0 180 Ringing Via: SIP/2.0/UDP server10.biloxi.com;branchz9hG4bK4b43c2ff8.1 ;received192.0.2.3 Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branchz9hG4bK77ef4c2312983.1 ;received192.0.2.2 Via: SIP/2.0/UDP pc33.atlanta.com;branchz9hG4bKnashds8 ;received192.0.2.1 To: Bob sip:bobbiloxi.com;taga6c85cf From: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 Contact: sip:bob192.0.2.4 CSeq: 314159 INVITE Content-Length: 0 F7 180 Ringing biloxi.com proxy - atlanta.com proxy SIP/2.0 180 Ringing Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branchz9hG4bK77ef4c2312983.1 ;received192.0.2.2 Via: SIP/2.0/UDP pc33.atlanta.com;branchz9hG4bKnashds8 ;received192.0.2.1 To: Bob sip:bobbiloxi.com;taga6c85cf From: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 Contact: sip:bob192.0.2.4 CSeq: 314159 INVITE Content-Length: 0 F8 180 Ringing atlanta.com proxy - Alice SIP/2.0 180 Ringing Via: SIP/2.0/UDP pc33.atlanta.com;branchz9hG4bKnashds8 ;received192.0.2.1 To: Bob sip:bobbiloxi.com;taga6c85cf From: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 Contact: sip:bob192.0.2.4 CSeq: 314159 INVITE Content-Length: 0 F9 200 OK Bob - biloxi.com proxy SIP/2.0 200 OK Via: SIP/2.0/UDP server10.biloxi.com;branchz9hG4bK4b43c2ff8.1 ;received192.0.2.3 Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branchz9hG4bK77ef4c2312983.1 ;received192.0.2.2 Via: SIP/2.0/UDP pc33.atlanta.com;branchz9hG4bKnashds8 ;received192.0.2.1 To: Bob sip:bobbiloxi.com;taga6c85cf From: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: sip:bob192.0.2.4 Content-Type: application/sdp Content-Length: 131 F10 200 OK biloxi.com proxy - atlanta.com proxy SIP/2.0 200 OK Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branchz9hG4bK77ef4c2312983.1 ;received192.0.2.2 Via: SIP/2.0/UDP pc33.atlanta.com;branchz9hG4bKnashds8 ;received192.0.2.1 To: Bob sip:bobbiloxi.com;taga6c85cf From: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: sip:bob192.0.2.4 Content-Type: application/sdp Content-Length: 131 F11 200 OK atlanta.com proxy - Alice SIP/2.0 200 OK Via: SIP/2.0/UDP pc33.atlanta.com;branchz9hG4bKnashds8 ;received192.0.2.1 To: Bob sip:bobbiloxi.com;taga6c85cf From: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: sip:bob192.0.2.4 Content-Type: application/sdp Content-Length: 131 F12 ACK Alice - Bob ACK sip:bob192.0.2.4 SIP/2.0 Via: SIP/2.0/UDP pc33.atlanta.com;branchz9hG4bKnashds9 Max-Forwards: 70 To: Bob sip:bobbiloxi.com;taga6c85cf From: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 ACK Content-Length: 0 F13 BYE Bob - Alice BYE sip:alicepc33.atlanta.com SIP/2.0 Via: SIP/2.0/UDP 192.0.2.4;branchz9hG4bKnashds10 Max-Forwards: 70 From: Bob sip:bobbiloxi.com;taga6c85cf To: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 CSeq: 231 BYE Content-Length: 0 F14 200 OK Alice - Bob SIP/2.0 200 OK Via: SIP/2.0/UDP 192.0.2.4;branchz9hG4bKnashds10 From: Bob sip:bobbiloxi.com;taga6c85cf To: Alice sip:aliceatlanta.com;tag1928301774 Call-ID: a84b4c76e66710 CSeq: 231 BYE Content-Length: 0
1. SIP消息头部主要字段
· Request-URIRequest消息的第一行中method后面紧跟的部分就是Request-URI(本例中的sip:bobbiloxi.com)。该值在消息经过Proxy之后就有可能发生变化变成消息路径中Next Hop的URI。
· To包含的是最终用户的Public address消息无论被Proxy多少次该值都不会改变。当最终用户开始回应第一个Response消息时(比如本例中的180 Ring)会在To中加上本地唯一的一个tag值。
· From包含Request消息发起者的URI也不会被Proxy改变在生成Reqeust消息时就会在后面加上一个本地唯一的tag值。
· Call-Id用来标识一个唯一的Session整个Session期间的所有消息的Session-Id都是相同的。
· Max-Forwards消息可被转发的最大次数每经过一个Proxy该值就会被减一。
· CSeq该属性由一个整数和一个method名字两部分组成整数部分的作用是用来对同一个Session中的Request消息进行排序的从第一个Invite消息发出后随后的所有Request消息(ACK和Cancel除外)中的CSeq值都依次加一。比如Alice与Bob的通话过程中Alice想修改会话的一些属性于是她便发起第二个Invite消息这个Invite消息中的CSeq就需要加一在比如Alice给Bob发送了一个Invite消息而Bob发回的200 OK发生延时于是Alice又发送第二个Invite消息此时对第一个Invite消息的200 OK到达Alice根据200 OK中的CSeq便可以知道这是对第一个Invite消息的应答。Request消息Cancel和Ack中的CSeq和与之对应的Invite消息中的CSeq值是一致的。
· Via该属性记录了消息的路由。Request消息被生成的时候只有一个Via那就是本地的URI此后消息每经过一个ProxyProxy都会在消息中插入一个Via记录下自己的URI。当最终用户发挥Response消息时会从Request消息中拷贝所有的Via然后按照反序进行路由每经过一个Proxy该Proxy就会将包含自己URI的Via删除这样当Response消息最终到达Request发起端的时候就只剩下一个Via了。
· Contact该属性包含了用户可以被直接找到的一个URIRequest发起方会在Request消息中加入该值接收方会在第一个Response(该例中的180 Ring)中加入该值。利用这个URI此后再发起Request消息时就直接发送到对方了而无需经过Proxy比如本例中的F12 ACK消息由于之前通过交换Contact值Alice知道如何能直接找到Bob于是在第一行的SIP-URI中填上Bob所提供的Contact值将ACK消息直接发送给Bob而无需经过Proxy注意To中仍然是原先的值。
· Route and Record-Route这两个属性都是有Proxy加入的上面提到Contact属性可以让随后的Reqeust消息绕过Proxy而直接发给最终用户。有时候处于安全或者其他方面的原因Proxy希望所有的消息都必须经过Proxy那么这时候Proxy就需要在经过它的第一个Request消息中插入该属性记录自己的URI这样此后的Request消息就必须按照该属性中指定的路径路由。 2. 对以上几个头部属性简单概括一下
· SIP-URI是一个Hop-to-Hop的属性所以有可能被Proxy改变
· From和To属性在消息的路由过程中一直保持不变它们在Request消息中确定此后对端发来的所有Response消息的From和To都是原样拷贝Request消息中的From和To
· Via是用来帮助Response消息进行路由的Contact是用来供随后的Request消息进行路由的。
参考资料
1. “RFC 3261”- Section 4Section 12Section 17Section 24 2. “SIP Demystified”- Chapter 4 Chapter 5
