这次测试的过程中，出现困扰最多的问题是网关在呼叫过程中，没有收到Alerting事件，导致状态机的紊乱。实际上，SIP协议已经定义了临时响应的可靠性。 在SIP标准中规定，规定了在呼叫建立过程中对临时消息可靠传输的定义，在其他过程中，可以借鉴这种机制，但是不是作为标准制定。如果呼叫建立需要支持临时消息的可靠性，需要加上SIP头——supported或者require，该头部任选参数为100rel。require表明需要对端支持，supported表明本次呼叫不要求可靠传输，但本端支持，由UAS决定。如果对断不支持，需要回420消息，并且在头部中加上Unsupported字段，并且该字段的参数为100rel。 举例说明，如果Invite消息中包含require或者supported字段，并且为100rel，则对端在100 Trying后，发送后续101-199事件需要设定指数类型的定时器。此时钟和invite应答的重传时钟应该相互独立。 当收到PRACK后，需要对进行匹配，如果匹配的话，重传时钟就可以中止了。所谓匹配就是指PRACK和临时响应处于同一个会话，且RAck中的方法，CSeq-num,response-num在此一对消息中都是吻合的。所谓吻合，咋一看不容易理解，通过以下的例子说明。
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，从这个例子中可以看到PRACK发送的原理和参数要求。PRACK首先是作为一个请求，并以200 OK作为响应。PRACK发送的所针对的是一个临时响应。 以183为例，参数为 Require: 100rel RSeq: 776655 那么对应的PRACK的参数为
RAck: 776655 1 INVITE
CSeq: 2 PRACK 对应的200 OK的参数是 CSeq: 2 PRACK 如果收到的PRACK得不到任何的匹配，需要发送481作为响应。如果得到正确的响应，应该从响应列表中删除该响应。
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