這次測試的過程中，出現困擾最多的問題是網關在呼叫過程中，沒有收到Alerting事件，導緻狀态機的紊亂。實際上，SIP協定已經定義了臨時響應的可靠性。 在SIP标準中規定，規定了在呼叫建立過程中對臨時消息可靠傳輸的定義，在其他過程中，可以借鑒這種機制，但是不是作為标準制定。如果呼叫建立需要支援臨時消息的可靠性，需要加上SIP頭——supported或者require，該頭部任選參數為100rel。require表明需要對端支援，supported表明本次呼叫不要求可靠傳輸，但本端支援，由UAS決定。如果對斷不支援，需要回420消息，并且在頭部中加上Unsupported字段，并且該字段的參數為100rel。 舉例說明，如果Invite消息中包含require或者supported字段，并且為100rel，則對端在100 Trying後，發送後續101-199事件需要設定指數類型的定時器。此時鐘和invite應答的重傳時鐘應該互相獨立。 當收到PRACK後，需要對進行比對，如果比對的話，重傳時鐘就可以中止了。所謂比對就是指PRACK和臨時響應處于同一個會話，且RAck中的方法，CSeq-num,response-num在此一對消息中都是吻合的。所謂吻合，咋一看不容易了解，通過以下的例子說明。
C->S: INVITE sip:[email protected] SIP/2.0 Via: SIP/2.0/UDP saturn.bell-tel.com Supported: 100rel From: sip:[email protected] To: sip:[email protected] Call-ID: [email protected] CSeq: 1 INVITE Subject: Come here Watson S->C: SIP/2.0 100 Trying Via: SIP/2.0/UDP saturn.bell-tel.com From: sip:[email protected] To: sip:[email protected] Call-ID: [email protected] CSeq: 1 INVITE S->C: SIP/2.0 183 Proceeding
Require: 100rel
Via: SIP/2.0/UDP saturn.bell-tel.com
RSeq: 776655
From: sip:[email protected]
To: sip:[email protected];tag=11
Call-ID: [email protected]
CSeq: 1 INVITE
Content-Type: application/sdp
v=0
s=Let's talk
b=CT:128
c=IN IP4 north.east.isi.edu
m=audio 3456 RTP/AVP 5 0 7
m=video 2232 RTP/AVP 31
C->S: PRACK sip:[email protected] SIP/2.0 RAck: 776655 1 INVITE Via: SIP/2.0/UDP saturn.bell-tel.com From: sip:[email protected] To: sip:[email protected];tag=11 Call-ID: [email protected] CSeq: 2 PRACK Content-Type: application/sdp v=0 s=Let's talk b=CT:128 c=IN IP4 machine.bell-tel.com m=audio 3456 RTP/AVP 5 0 7 m=video 2232 RTP/AVP 31 S->C: SIP/2.0 200 OK Via: SIP/2.0/UDP saturn.bell-tel.com From: sip:[email protected] To: sip:[email protected];tag=11 Call-ID: [email protected] CSeq: 2 PRACK S->C: SIP/2.0 182 Two in the Queue Via: SIP/2.0/UDP saturn.bell-tel.com Require: 100rel RSeq: 776656 From: sip:[email protected] To: sip:[email protected];tag=11 Call-ID: [email protected] CSeq: 1 INVITE S->C: SIP/2.0 182 One in the Queue Via: SIP/2.0/UDP saturn.bell-tel.com Require: 100rel RSeq: 776657 From: sip:[email protected] To: sip:[email protected];tag=11 Call-ID: [email protected] CSeq: 1 INVITE C->S: PRACK sip:[email protected] SIP/2.0 RAck: 776656 1 INVITE Via: SIP/2.0/UDP saturn.bell-tel.com From: sip:[email protected] To: sip:[email protected];tag=11 Call-ID: [email protected]saturn.bell-tel.com CSeq: 3 PRACK C->S: PRACK sip:[email protected] SIP/2.0 RAck: 776657 1 INVITE Via: SIP/2.0/UDP saturn.bell-tel.com From: sip:[email protected] To: sip:[email protected];tag=11 Call-ID: [email protected] CSeq: 4 PRACK S->C: SIP/2.0 200 OK Via: SIP/2.0/UDP saturn.bell-tel.com From: sip:[email protected] To: sip:[email protected];tag=11 Call-ID: [email protected] CSeq: 3 PRACK S->C: SIP/2.0 200 OK Via: SIP/2.0/UDP saturn.bell-tel.com From: sip:[email protected] To: sip:[email protected];tag=11 Call-ID: [email protected] CSeq: 4 PRACK S->C: SIP/2.0 200 OK Via: SIP/2.0/UDP saturn.bell-tel.com From: sip:[email protected] To: sip:[email protected];tag=11 Call-ID: [email protected] CSeq: 1 INVITE Content-Type: application/sdp v=0 s=Let's talk b=CT:128 c=IN IP4 north.east.isi.edu m=audio 3456 RTP/AVP 5 0 7 m=video 2232 RTP/AVP 31 C->S: ACK sip:[email protected] SIP/2.0 Via: SIP/2.0/UDP saturn.bell-tel.com From: sip:[email protected] To: sip:[email protected];tag=11 Call-ID: [email protected] CSeq: 1 INVITE 以上的例子摘自draft-ietf-sip-100rel-02.txt，從這個例子中可以看到PRACK發送的原理和參數要求。PRACK首先是作為一個請求，并以200 OK作為響應。PRACK發送的所針對的是一個臨時響應。 以183為例，參數為 Require: 100rel RSeq: 776655 那麼對應的PRACK的參數為
RAck: 776655 1 INVITE
CSeq: 2 PRACK 對應的200 OK的參數是 CSeq: 2 PRACK 如果收到的PRACK得不到任何的比對，需要發送481作為響應。如果得到正确的響應，應該從響應清單中删除該響應。
![Xcode中建立Target的步驟[圖]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)