Computer Networks · free preview
Real-Time Transport
Interactive voice and video want timeliness more than reliability, since a retransmitted syllable arrives after its moment has passed, so the real-time transport protocol rides on datagrams and gives each packet a timestamp and a sequence number, receivers smooth the network's jitter in a playback buffer whose depth trades delay against starvation, talk spurts let the playback point adapt, a companion control protocol on the adjacent port reports reception quality so senders can react to loss as congestion evidence, and the whole design follows the principle of application-level framing, that the application knows best how to packetize its own frames and conceal its own losses.
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?
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