SIP消息路由
本文通過一個(gè)完整的SIP呼叫實(shí)例解釋SIP頭部的一些常見字段，在對(duì)這些字段的解釋的同時(shí)也闡述了SIP消息的路由過程。下圖是呼叫的消息流示意圖和所有的消息頭部(因?yàn)镾DP和消息路由無關(guān)，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?????????????????? |
???????? |------------------------------------------------->|
???????? |????????????????????????????????????????????????? |
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| F1 INVITE Alice -> atlanta.com proxy ? INVITE sip:bob@biloxi.com SIP/2.0 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds8 Max-Forwards: 70 To: Bob <sip:bob@biloxi.com> From: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sip:alice@pc33.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;branch=z9hG4bKnashds8 ?;received=192.0.2.1 To: Bob <sip:bob@biloxi.com> From: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Content-Length: 0 |
| F3 INVITE atlanta.com proxy -> biloxi.com proxy ? INVITE sip:bob@biloxi.com 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:bob@biloxi.com> From: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sip:alice@pc33.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;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:bob@biloxi.com> From: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Content-Length: 0 |
| F5 INVITE biloxi.com proxy -> Bob ? INVITE sip:bob@192.0.2.4 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:bob@biloxi.com> From: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sip:alice@pc33.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;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:bob@biloxi.com>;tag=a6c85cf From: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 Contact: <sip:bob@192.0.2.4> 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;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:bob@biloxi.com>;tag=a6c85cf From: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 Contact: <sip:bob@192.0.2.4> 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;branch=z9hG4bKnashds8 ?;received=192.0.2.1 To: Bob <sip:bob@biloxi.com>;tag=a6c85cf From: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 Contact: <sip:bob@192.0.2.4> 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;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:bob@biloxi.com>;tag=a6c85cf From: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sip:bob@192.0.2.4> 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;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:bob@biloxi.com>;tag=a6c85cf From: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sip:bob@192.0.2.4> 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;branch=z9hG4bKnashds8 ?;received=192.0.2.1 To: Bob <sip:bob@biloxi.com>;tag=a6c85cf From: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Contact: <sip:bob@192.0.2.4> Content-Type: application/sdp Content-Length: 131 |
| F12 ACK Alice -> Bob ? ACK sip:bob@192.0.2.4 SIP/2.0 Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKnashds9 Max-Forwards: 70 To: Bob <sip:bob@biloxi.com>;tag=a6c85cf From: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 ACK Content-Length: 0 |
| F13 BYE Bob -> Alice ? BYE sip:alice@pc33.atlanta.com SIP/2.0 Via: SIP/2.0/UDP 192.0.2.4;branch=z9hG4bKnashds10 Max-Forwards: 70 From: Bob <sip:bob@biloxi.com>;tag=a6c85cf To: Alice <sip:alice@atlanta.com>;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:bob@biloxi.com>;tag=a6c85cf To: Alice <sip:alice@atlanta.com>;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 231 BYE Content-Length: 0 |
1. SIP消息頭部主要字段：
·???????? Request-URI：Request消息的第一行中method后面緊跟的部分就是Request-URI(本例中的sip:bob@biloxi.com)。該值在消息經(jīng)過Proxy之后就有可能發(fā)生變化，變成消息路徑中Next Hop的URI。
·???????? To：包含的是最終用戶的Public address，消息無論被Proxy多少次該值都不會(huì)改變。當(dāng)最終用戶開始回應(yīng)第一個(gè)Response消息時(shí)(比如本例中的180 Ring)會(huì)在To中加上本地唯一的一個(gè)tag值。
·???????? From：包含Request消息發(fā)起者的URI，也不會(huì)被Proxy改變；在生成Reqeust消息時(shí)就會(huì)在后面加上一個(gè)本地唯一的tag值。
·???????? Call-Id：用來標(biāo)識(shí)一個(gè)唯一的Session，整個(gè)Session期間的所有消息的Session-Id都是相同的。
·???????? Max-Forwards：消息可被轉(zhuǎn)發(fā)的最大次數(shù)，每經(jīng)過一個(gè)Proxy，該值就會(huì)被減一。
·???????? CSeq：該屬性由一個(gè)整數(shù)和一個(gè)method名字兩部分組成；整數(shù)部分的作用是用來對(duì)同一個(gè)Session中的Request消息進(jìn)行排序的；從第一個(gè)Invite消息發(fā)出后，隨后的所有Request消息(ACK和Cancel除外)中的CSeq值都依次加一。比如Alice與Bob的通話過程中Alice想修改會(huì)話的一些屬性，于是她便發(fā)起第二個(gè)Invite消息，這個(gè)Invite消息中的CSeq就需要加一；在比如Alice給Bob發(fā)送了一個(gè)Invite消息，而Bob發(fā)回的200 OK發(fā)生延時(shí)，于是Alice又發(fā)送第二個(gè)Invite消息，此時(shí)對(duì)第一個(gè)Invite消息的200 OK到達(dá)，Alice根據(jù)200 OK中的CSeq便可以知道這是對(duì)第一個(gè)Invite消息的應(yīng)答。Request消息Cancel和Ack中的CSeq和與之對(duì)應(yīng)的Invite消息中的CSeq值是一致的。
·???????? Via：該屬性記錄了消息的路由。Request消息被生成的時(shí)候只有一個(gè)Via，那就是本地的URI，此后消息每經(jīng)過一個(gè)Proxy，Proxy都會(huì)在消息中插入一個(gè)Via記錄下自己的URI。當(dāng)最終用戶發(fā)揮Response消息時(shí)，會(huì)從Request消息中拷貝所有的Via，然后按照反序進(jìn)行路由，每經(jīng)過一個(gè)Proxy，該P(yáng)roxy就會(huì)將包含自己URI的Via刪除，這樣當(dāng)Response消息最終到達(dá)Request發(fā)起端的時(shí)候就只剩下一個(gè)Via了。
·???????? Contact：該屬性包含了用戶可以被直接找到的一個(gè)URI，Request發(fā)起方會(huì)在Request消息中加入該值，接收方會(huì)在第一個(gè)Response(該例中的180 Ring)中加入該值。利用這個(gè)URI，此后再發(fā)起Request消息時(shí)就直接發(fā)送到對(duì)方了，而無需經(jīng)過Proxy，比如本例中的F12 ACK消息，由于之前通過交換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，將ACK消息直接發(fā)送給Bob而無需經(jīng)過Proxy，注意To中仍然是原先的值。
·???????? Route and Record-Route：這兩個(gè)屬性都是有Proxy加入的；上面提到Contact屬性可以讓隨后的Reqeust消息繞過Proxy而直接發(fā)給最終用戶。有時(shí)候處于安全或者其他方面的原因，Proxy希望所有的消息都必須經(jīng)過Proxy，那么這時(shí)候Proxy就需要在經(jīng)過它的第一個(gè)Request消息中插入該屬性記錄自己的URI，這樣此后的Request消息就必須按照該屬性中指定的路徑路由。
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2. 對(duì)以上幾個(gè)頭部屬性簡(jiǎn)單概括一下：
·???????? SIP-URI是一個(gè)Hop-to-Hop的屬性，所以有可能被Proxy改變；
·???????? From和To屬性在消息的路由過程中一直保持不變；它們?cè)赗equest消息中確定，此后對(duì)端發(fā)來的所有Response消息的From和To都是原樣拷貝Request消息中的From和To；
·???????? Via是用來幫助Response消息進(jìn)行路由的，Contact是用來供隨后的Request消息進(jìn)行路由的。
參考資料：
1.? “RFC 3261”- Section 4，Section 12，Section 17，Section 24；
2.? “SIP Demystified”- Chapter 4， Chapter 5；總結(jié)
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