英语翻译SIP messages are encoded using HTTP/1.1 message syntax [6],and the content of each message follows sessiondescription protocol (SDP) [7],heavily used in the context of MBONE for distributing information about MBONEsessions.In addition to

英语翻译SIP messages are encoded using HTTP/1.1 message syntax [6],and the content of each message follows sessiondescription protocol (SDP) [7],heavily used in the context of MBONE for distributing information about MBONEsessions.In addition to
英语翻译
SIP messages are encoded using HTTP/1.1 message syntax [6],and the content of each message follows session
description protocol (SDP) [7],heavily used in the context of MBONE for distributing information about MBONE
sessions.
In addition to SIP terminals which represent an IP telephones or Gateways,SIP architecture is based on four different
servers entities:
· Proxy server,which forwards requests to its final destination.Like the Gatekeeper in H.323 model,it
receives requests and sends them to the appropriate destination.In this case the Via field in the
request/response message indicates the intermediate proxies which participate in the request process.This
avoids routing loops as well as permits to force the responses to follow the same route back.Proxy servers
do not need to relay the media except in case of transcoding operations (they only need to relay control
information).
· Redirect Server,which,on the contrary to proxies,does not forward INVITE requests but responds to
clients with a redirection message that informs about the next hop.
· Registrar server,that keeps track of the current location of a user by accepting registration request
messages.Registrar server are discovered by using well-known multicast addresses or preconfigured unicast
addresses.
· Call Agent,which is a service that handles incoming and outgoing calls on behalf of a user.It is a mixture of
a proxy,a registrar and a redirect server.A call agent can perform tasks like:
o Try to find a user by redirecting the call setup message to the proper location or locations.
o Implement call redirection rules such as call forwarding on busy,call forward on no answer,etc.
o Implement call filtering with origin/time-dependant rules.
o Record unsuccessful call attempts for future reference.
o Perform any other call management task on behalf of the user.

英语翻译SIP messages are encoded using HTTP/1.1 message syntax [6],and the content of each message follows sessiondescription protocol (SDP) [7],heavily used in the context of MBONE for distributing information about MBONEsessions.In addition to
SIP的信息编码采用http/1.1讯息语法[6],而每一阶段的内容如下讯息的会话描述协议(SDP)[7] 大量使用在对mbone散发资料mbone会议.除了SIP的终端是一个IP电话或网关 SIP体系结构是基于四个不同的服务器实体:·代理服务器,其中前锋要求其最终目的地.像守门人在H.323的模式,收到请求发送到适当的目的地.在这种情况下途经现场的请求/响应消息显示,中间代理人参加这一要求的过程.这样可以避免路由环路以及许可证武力反应遵循同一路线回来.代理服务器不需要中继媒体的情况除外transcoding业务(他们只需要接力 控制信息).•重定向服务器,其中,对违反委托书,不邀请提出要求,但对客户提供redirection讯息,告知有关未来合.•注册服务器,追查到目前位置,用户在接受注册请求消息.注册服务器,发现用众所周知的组播地址或preconfigured播地址.·call剂,是一种服务,handlesincomingandoutgoingcalls代表一个用户.它是一种混合的附庸,书记官长和重定向服务器.通话剂可以执行的任务,例如:o找一个用户,重新安装电话讯息的适当位置或地点.澳实施callredirection规则,如电话转接就忙,呼叫就没有答案,等等o贯彻来电过滤与原产地/时间依赖性规则.o失败记录占线,供日后参考.澳从事任何其他通话管理任务,代表了用户的负担