SIP Intro --- REFER method

REFER method

講到這個method，就開始有點複雜了!

因為要有以前講過的觀念，然後加上REFER與Subscrib的概念。整個例子的流程會很大、Msg的量也會增加很多! 但是，又因為有了之前的基礎，因此可以省略很多個Msg的說明，所以我就可以把整個REFER這個例子的流程簡化了很多!

首先，REFER是用在CALL Transfer上面。以下就來說明一個例子吧，假設有A,B,C三個人，A打給B，然後HOLD B，再打給C，然後把B轉接給C。最後B與C結束通訊。

A INVITE B
A HOLD B
A INVITE C
A HOLD C
A REFER B TO C
B INVITE C
C BYE A
A BYE B
...
C BYE B (or B BYE C)

而CALL Transfer又不只一種形式，其中有兩種較為常見，一種是Attended transfer，另一種為Blind transfer。前者為轉接者(發REFER的UA)，在接通兩方電話後，再做Transfer，上面的例子就是Attended transfer。而後者為先接通一方電話，再發送REFER要求對方去CALL第三方。

在此先不探討CALL Transfer的方式，我將focus在REFER這個method的使用，以及其相關概念。

下面的CALL flow是一個在Dialog外的REFER，因此其To不會有tag。而在Dialog外或內也何差異?主要差異在於在Dialog內的REFER不會有Fork的產生，但在Dialog外的REFER則允許有Fork的產生。Fork有分支的意思，在此先簡單瞭解一下，當一個Event package允許使用Fork時，就會產生數個SUBSCRIBE requests，因而就能產生multiple subscriptions，就如同分支一樣，所以才稱之為Fork。(SUBSCRIBE概念會在另一篇說明)

先來看這個session外REFER的例子。再看後面的例子的時候，可以注意一下幾個Header fields的值~

Call-ID:
CSeq:
Event:
Subscription-State:

以下是在A與B建立好通訊之後的情況

Agent A             Agent B
|                        |
|       F1 REFER         |
|----------------------->|
|    F2 202 Accepted     |
|<-----------------------|
|       F3 NOTIFY        |
|<-----------------------|
|       F4 200 OK        |
|----------------------->|
|                        |
|                        |
|                        |------->
|                        | (whatever)
|                        |<------
|                        |
|      F5 NOTIFY         |
|<-----------------------|
|      F6 200 OK         |
|----------------------->|
|                        |
|                        |

Message One (F1)
REFER sip:b@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP agenta.atlanta.example.com;branch=z9hG4bK2293940223
To:
From: ;tag=193402342
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 93809823 REFER
Max-Forwards: 70
Refer-To: (whatever URI)
Contact: sip:a@atlanta.example.com
Content-Length: 0

Message Two (F2)
SIP/2.0 202 Accepted
Via: SIP/2.0/UDP agenta.atlanta.example.com;branch=z9hG4bK2293940223
To: ;tag=4992881234
From: ;tag=193402342
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 93809823 REFER
Contact: sip:b@atlanta.example.com
Content-Length: 0

Message Three (F3)
NOTIFY sip:a@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK9922ef992-25
To: ;tag=193402342
From: ;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993402 NOTIFY
Max-Forwards: 70
Event: refer
Subscription-State: active;expires=(depends on Refer-To URI)
Contact: sip:b@atlanta.example.com
Content-Type: message/sipfrag;version=2.0
Content-Length: 20
SIP/2.0 100 Trying

Message Four (F4)
SIP/2.0 200 OK
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK9922ef992-25
To: ;tag=193402342
From: ;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993402 NOTIFY
Contact: sip:a@atlanta.example.com
Content-Length: 0

Message Five (F5)
NOTIFY sip:a@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK9323394234
To: ;tag=193402342
From: ;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993403 NOTIFY
Max-Forwards: 70
Event: refer
Subscription-State: terminated;reason=noresource
Contact: sip:b@atlanta.example.com
Content-Type: message/sipfrag;version=2.0
Content-Length: 16
SIP/2.0 200 OK

Message Six (F6)
SIP/2.0 200 OK
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK9323394234
To: ;tag=193402342
From: ;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993403 NOTIFY
Contact: sip:a@atlanta.example.com
Content-Length: 0


以下為接續上面的例子，情況為在A在現有的Dialog裡，發送第二次REFER。(第一次REFER是在Dialog外)

Agent A               Agent B
|                        |
|       F7 REFER         |
|----------------------->|
|    F8 202 Accepted     |
|<-----------------------|
|       F9 NOTIFY        |
|<-----------------------|
|      F10 200 OK        |
|----------------------->|
|                        |------->
|                        | (something different)
|                        |<------
|                        |
|       F11 NOTIFY       |
|<-----------------------|
|       F12 200 OK       |
|----------------------->|
|                        |
|                        |

Message Seven (F7)
REFER sip:b@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP agenta.atlanta.example.com;branch=z9hG4bK9390399231
To: ;tag=4992881234
From: ;tag=193402342
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 93809824 REFER
Max-Forwards: 70
Refer-To: (some different URI)
Contact: sip:a@atlanta.example.com
Content-Length: 0

Message Eight (F8)
SIP/2.0 202 Accepted
Via: SIP/2.0/UDP agenta.atlanta.example.com;branch=z9hG4bK9390399231
To: ;tag=4992881234
From: ;tag=193402342
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 93809824 REFER
Contact: sip:b@atlanta.example.com
Content-Length: 0

Message Nine (F9)
NOTIFY sip:a@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK9320394238995
To: ;tag=193402342
From: ;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993404 NOTIFY
Max-Forwards: 70
Event: refer;id=93809824
Subscription-State: active;expires=(depends on Refer-To URI)
Contact: sip:b@atlanta.example.com
Content-Type: message/sipfrag;version=2.0
Content-Length: 20
SIP/2.0 100 Trying

Message Ten (F10)
SIP/2.0 200 OK
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK9320394238995
To: ;tag=193402342
From: ;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993404 NOTIFY
Contact: sip:a@atlanta.example.com
Content-Length: 0

Message Eleven (F11)
NOTIFY sip:a@atlanta.example.com SIP/2.0
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK2994a93eb-fe
To: ;tag=193402342
From: ;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993405 NOTIFY
Max-Forwards: 70
Event: refer;id=93809824
Subscription-State: terminated;reason=noresource
Contact: sip:b@atlanta.example.com
Content-Type: message/sipfrag;version=2.0
Content-Length: 16
SIP/2.0 200 OK

Message Twelve (F12)
SIP/2.0 200 OK
Via: SIP/2.0/UDP agentb.atlanta.example.com;branch=z9hG4bK2994a93eb-fe
To: ;tag=193402342
From: ;tag=4992881234
Call-ID: 898234234@agenta.atlanta.example.com
CSeq: 1993405 NOTIFY
Contact: sip:a@atlanta.example.com
Content-Length: 0