分析代碼流程
1.發送註冊請求的流程
2.接收到響應的處理流程
3.通過sip註冊和響應的流程圖，可以簡單畫出，發送和接收sip消息時，
各模塊的調用順序。這也驗證了pjsip module framework的概念。
4.SIP 消息的例子
Register：
REGISTER sip:192.168.1.180:5069 SIP/2.0
Via: SIP/2.0/UDP 192.168.1.39:5100;rport;branch=z9hG4bK3164
From: <sip:[email protected]:5069>;tag=6020
To: <sip:[email protected]:5069>
Call-ID: 14124
CSeq: 1 REGISTER
Contact: <sip:[email protected]:5100;line=be69937d61bb4b2>
Max-Forwards: 70
User-Agent: Linphone/3.6.0 (eXosip2/3.6.0)
Expires: 3600
Content-Length: 0UnRegister：
REGISTER sip:192.168.1.180:5069 SIP/2.0
Via: SIP/2.0/UDP 192.168.1.39:5100;rport;branch=z9hG4bK10883
From: <sip:[email protected]:5069>;tag=6020
To: <sip:[email protected]:5069>
Call-ID: 14124
CSeq: 2 REGISTER
Contact: <sip:[email protected]:5100;line=be69937d61bb4b2>
Max-Forwards: 70
User-Agent: Linphone/3.6.0 (eXosip2/3.6.0)
Expires: 0
Content-Length: 0