SIP消息路由

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     .

               .                                       .

       Alice's  . . . . . . . . . . . . . . . . . . . .  Bob's

      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:[email protected] SIP/2.0 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8 Max-Forwards: 70 To: Bob <sip:[email protected]> From: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sip:[email protected]> 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;branch=z9hG4bKnashds8  ;received=192.0.2.1 To: Bob <sip:[email protected]> From: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Content-Length: 0

F3 INVITE atlanta.com proxy -> biloxi.com proxy   INVITE sip:[email protected] SIP/2.0 Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8  ;received=192.0.2.1 Max-Forwards: 69 To: Bob <sip:[email protected]> From: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sip:[email protected]> 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;branch=z9hG4bK77ef4c2312983.1  ;received=192.0.2.2 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8  ;received=192.0.2.1 To: Bob <sip:[email protected]> From: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Content-Length: 0

F5 INVITE biloxi.com proxy -> Bob   INVITE sip:[email protected] SIP/2.0 Via: SIP/2.0/UDP server10.biloxi.com;branch=z9hG4bK4b43c2ff8.1 Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1  ;received=192.0.2.2 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8  ;received=192.0.2.1 Max-Forwards: 68 To: Bob <sip:[email protected]> From: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sip:[email protected]> 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;branch=z9hG4bK4b43c2ff8.1  ;received=192.0.2.3 Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1  ;received=192.0.2.2 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8  ;received=192.0.2.1 To: Bob <sip:[email protected]>;tag=a6c85cf From: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 Contact: <sip:[email protected]> 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;branch=z9hG4bK77ef4c2312983.1  ;received=192.0.2.2 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8  ;received=192.0.2.1 To: Bob <sip:[email protected]>;tag=a6c85cf From: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 Contact: <sip:[email protected]> 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;branch=z9hG4bKnashds8  ;received=192.0.2.1 To: Bob <sip:[email protected]>;tag=a6c85cf From: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 Contact: <sip:[email protected]> 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;branch=z9hG4bK4b43c2ff8.1  ;received=192.0.2.3 Via: SIP/2.0/UDP bigbox3.site3.atlanta.com;branch=z9hG4bK77ef4c2312983.1  ;received=192.0.2.2 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8  ;received=192.0.2.1 To: Bob <sip:[email protected]>;tag=a6c85cf From: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sip:[email protected]> 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;branch=z9hG4bK77ef4c2312983.1  ;received=192.0.2.2 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8  ;received=192.0.2.1 To: Bob <sip:[email protected]>;tag=a6c85cf From: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sip:[email protected]> 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;branch=z9hG4bKnashds8  ;received=192.0.2.1 To: Bob <sip:[email protected]>;tag=a6c85cf From: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sip:[email protected]> Content-Type: application/sdp Content-Length: 131

F12 ACK Alice -> Bob   ACK sip:[email protected] SIP/2.0 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds9 Max-Forwards: 70 To: Bob <sip:[email protected]>;tag=a6c85cf From: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 ACK Content-Length: 0

F13 BYE Bob -> Alice   BYE sip:[email protected] SIP/2.0 Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKnashds10 Max-Forwards: 70 From: Bob <sip:[email protected]>;tag=a6c85cf To: Alice <sip:[email protected]>;tag=1928301774 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;branch=z9hG4bKnashds10 From: Bob <sip:[email protected]>;tag=a6c85cf To: Alice <sip:[email protected]>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 231 BYE Content-Length: 0

1. SIP消息头部主要字段：

·         Request-URI：Request消息的第一行中method后面紧跟的部分就是Request-URI(本例中的sip:[email protected])。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Proxy，Proxy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URI，Request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 4，Section 12，Section 17，Section 24；

2.  “SIP Demystified”- Chapter 4， Chapter 5；