摘要： 一、概述 sendmail是最重要的邮件传输代理程序。理解电子邮件的工作模式是非常重要的。一般情况下，我们把电子邮件程序分解成用户代理，传输代理和投递代理。 用户代理用来接受用户的指令，将用户的信件传送至信件传输代理，如：outlook express、foxmail等。而投递代理则从信件传输代理取得信件传送至最终用户的邮箱，如：procmail。 当用户试图发送一封电子邮件的时候，他......
摘要：这是一篇从noiile那里搬来的文章～
sip从私网到公网会遇到什么样的问题呢？
包的地址转换。
sip消息里面的sip地址转换。
sip消息里面的sdp中的rtp地址转换。
网络现存结构复杂，sip服务提供商并不一定是network提供商，很难要求客户只能使用某种方式的nat&firewall。如何找出一种可以满足各种网络的sip应用解决方案呢？
nat和fi......
例子：H.323到SIP的呼叫
in this example, shown in figure , the initial message exchange is between the calling h.323 terminal and the h.323 gatekeeper. the gatekeeper resolves the h.323 alias into an address served by the h.323/sip gateway. the acf response indicates that gatekeeper-routed signaling is required, so the q.931 and h.245 tcp connections are opened to the gatekeeper, which opens tcp connections to the gateway. the calling h.323 terminal sends a q.931 setup message to the gatekeeper, which proxies it to the h.323/sip gateway. the gateway then looks up the h.323 alias and resolves it to the sip uri of the called party. it constructs an invite from the setup message and forwards it to a sip proxy, which forwards it to the called party. note that because the setup message does not contain any media information, the invite does not contain any media information either. the called party sends a 180 ringing then a 200 ok to indicate that the call has been answered. the media information present in the sdp message body is stored by the gateway, which sends alerting and connect messages. 【推荐阅读:
防盗链的方法】h.323 to sip call
in this example, a h.323 terminal calls a sip-enabled pc through a h.323/sip gateway. the gateway does signaling translation between the protocols but allows the two end points to exchange media packets directly with each other. the full details of sip/h.323 interworking are being developed in the sip working group [4]....
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