Computer Networks · 免费预览
实时传输
交互式语音和视频更重视及时性而非可靠性，因为重传的音节到达时其播放时机已过；因此实时传输协议运行在数据报之上，并为每个分组附加时间戳和序号。接收方在播放缓冲区中平滑网络抖动，缓冲区深度在延迟与饥饿之间权衡；语音突发使播放点得以自适应；相邻端口上的伴随控制协议报告接收质量，使发送方能将丢包视为拥塞证据而作出反应。整个设计遵循应用层成帧原则：应用最清楚如何对自身帧进行分组、如何掩盖自身丢包。
1. Timeliness over Reliability
Timeliness over Reliability
2. What the Header Carries
What the Header Carries
3. The Companion Protocol
The Companion Protocol
4. Application-Level Framing
Application-Level Framing
5. Late is worse than lost
Why does an interactive voice application prefer a lost packet over a retransmitted one?
6. The playback buffer
Packets sent every 20 milliseconds arrive with irregular spacing. How does the receiver produce smooth audio?
7. The two header fields
Which two RTP header fields let a receiver schedule playout and detect loss?
8. The control channel
What does RTCP, riding the odd port beside its data stream, actually carry?
9. Talk spurts
Why do the natural pauses in speech matter to a voice receiver's buffer management?
10. Application-level framing
What does the principle of application-level framing claim?
11. The time that matters
RTP's timestamp is expressed in units of what clock, rather than wall-clock time?
12. The port pairing
When RTP runs over UDP, the data stream uses an even-numbered port and the control protocol uses which port?
13. The smoothing device
What buffer does a real-time receiver use to convert jittery arrivals into even playback?
14. The concealing trick
When a voice packet is lost, what does the receiver do instead of retransmitting, filling the gap with silence or interpolation so playback continues?