一个基于SIP的Scenario处理流程

今天在sipforum的邮件列表里看到了这样一个问题：
 A calls B
 B Rings and Times out
 Application server sends INVITE for C
 Parallelly Application server has to play announcement for A that call
 is being Forwarded.
 Once C answers A and C should get connected .
 

下面是我的回答：

A                   AS                  B                   C                    MS
|   INVITE          |                   |                   |                    |
|------------------>|   INVITE          |                   |                    |
|   100             |------------------>|                   |                    |
|<------------------|                   |                   |                    |
|                   |   100             |                   |                    |
|                   |<------------------|                   |                    |
|                   |   180             |                   |                    |
|   180             |<------------------|                   |                    |
|<------------------|                   |                   |                    |
|                   |   408             |                   |                    |
|                   |<------------------|                   |                    |
|                   |   ACK             |                   |                    |
|                   |------------------>|                   |                    |
|                   |                   |                   |                    |
|                   |   INV             |                   |                    |
|                   |----------------------------------------------------------->|
|                   |                   |                   |                    |
|                   |   100             |                   |                    |
|                   |<-----------------------------------------------------------|
|                   |   200             |                   |                    |
|                   |<-----------------------------------------------------------|
|                   |   ACK             |                   |                    |
|                   |----------------------------------------------------------->|
|   200             |                   |                   |                    |
|<------------------|                   |                   |                    |
|   ACK             |                   |                   |                    |
|------------------>|                   |                   |                    |
|                   |   INVITE          |                   |                    |
|                   |-------------------------------------->|                    |
|                   |   100             |                   |                    |
|                   |<--------------------------------------|                    |
|                   |   200             |                   |                    |
|                   |<--------------------------------------|                    |
|                   |   ACK             |                   |                    |
|                   |-------------------------------------->|                    |
|   re-INVITE       |                   |                   |                    |
|<------------------|                   |                   |                    |
|   100             |                   |                   |                    |
|------------------>|                   |                   |                    |
|   200             |                   |                   |                    |
|------------------>|                   |                   |                    |
|   ACK             |                   |                   |                    |
|<------------------|                   |                   |                    |
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|                   |            Session Media...           |                    |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|                   |                   |                   |                    |