根據(jù) RFC 3261規(guī)定IMS/SIP中支持的消息主要分為請求與應(yīng)答兩大類；它們描述了終端(UA)通信的會話過程；
1，請求消息（ Request Message）
請求類消息是由終端（UA）向代理單元（CSCF)，或通過代理單元向通信對方發(fā)起的請求（應(yīng)答或反饋）消息；其主要有以下內(nèi)容.
Request Message
消息內(nèi)容
Description
REGISTER（注冊）
終端在SIP服務(wù)器上注冊申請
A Client use this message to register an address with a SIP server
INVITE（邀請）
終端（UA）對另一個終端（UA）的會話邀請
A User or Service use this message to let another user/service participate in a session. The body of this message would include a description of the session to which the callee is being invited.
ACK（應(yīng)答）
終端（UA）對邀請的應(yīng)答
This is used only for INVITE indicating that the client has received a final response to an INVITE request
CANCEL（撤消）
對某一邀請的撤消
This is used to cancel a pending request
BYE（再見）
終端結(jié)束會話
A User Agent Client use this message to terminate the call
OPTIONS（查詢）
服務(wù)器能力查詢
This is used to query a server about its capabilities
INFO（通知）
會話進(jìn)行提醒信息
This is used for mid session signaling
除了基本請求消息外，還有一些擴(kuò)展請求消息；它們是對基本請求的更新或擴(kuò)展，其具體如下：
請求內(nèi)容
MESSAGE
信息（如短信）
Usually used to send Standalone Message (like SMS). RFC 3428
SUBSCRIBE
評閱（異步事件或內(nèi)容）
used to request asynchronous notification of an event or set of events at a later time. RFC 6665
NOTIFY
通知（早期訂閱消息反饋）
used to notify a SIP node that an event that has been requested by an earlier SUBSCRIBE method has occurred. RFC 6665
UPDATE
更新（會話進(jìn)程內(nèi)容）
used for a client to update parameters of a session (such as the set of media streams and their codecs). RFC 3311
PRACK
應(yīng)答（前期請求）
plays the same role as ACK, but this is used for provisional responses. RFC 3262
PUBLISH
（信息）發(fā)布
used for publication of presence information. RFC 3093
2.應(yīng)答消息
會話建立過程除終端（UA）、代理服務(wù)器外還涉及到其他網(wǎng)元； 應(yīng)答消息是代理服務(wù)器或終端對請求消息的回答。
Code
分類
Category
1xx
繼續(xù)
Provisional
The request has been received and processing is continuing請求已接收，處理中
2xx
成功
Success
An ACK, to indicate that the action was successfully received, understood, and accepted.應(yīng)答，進(jìn)行成功
3xx
重定向
Redirection
Further action is required to process this request需要進(jìn)一步處理請求
4xx
代理錯誤
Client Error
The request contains bad syntax and cannot be fulfilled at this server請求語句存在錯誤，服務(wù)器不能解析
5xx
服務(wù)器故障
Server Error
The server failed to fulfill an apparently valid request服務(wù)器不能接受請求
6xx
網(wǎng)絡(luò)故障
Global Failure
The request cannot be fulfilled at any server請求不能完成
原文來自： RFC 3261等
整理：kangguoying20201101
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