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多媒体应用
在互联网上传输语音和视频需要一个控制平面——这是电话网一直拥有而数据网长期缺乏的。会话控制协议负责定位被叫方当前的设备、协商双方都能使用的编解码器和端口，并建立和拆除呼叫；会话描述协议以标准格式传递媒体参数，而会话发起协议则负责会合。用户由类似电子邮件的 URI 寻址，代理像 DNS 路由查找一样把呼叫邀请转发给被叫方，注册器则跟踪每个用户当前应答的位置。这种将会话控制与媒体传输分离的架构，使得前面课程中的实时流能够沿着网络提供的任意路径传输。
1. A Control Plane for Conversations
A Control Plane for Conversations
2. Invitations by URI
Invitations by URI
3. Announcing Versus Inviting
Announcing Versus Inviting
4. Control and Media, Divorced
Control and Media, Divorced
5. Two problems, one call
What two problems must an Internet phone call solve?
6. What SDP conveys
What does the session description protocol carry?
7. The person-shaped address
Why is user location the hard part of session initiation?
8. The proxy's job
What does a SIP proxy do with an INVITE?
9. The registrar
How does a callee's proxy know where the callee currently answers?
10. The divorce
Why does separating session control from media transport matter architecturally?
11. The description format
Which protocol conveys a session's media types, codecs, addresses, and ports in a standard format?
12. The invitation
Which SIP message type initiates a call toward the callee?
13. The whereabouts keeper
Which SIP entity tracks each user's current device address?
14. The one-call-many-rings trick
What does a proxy do when it sends one incoming call invitation to several registered devices at once?