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RTP Real-Time Transport Protocol

RTP Real-Time Transport Protocol . Speaker: Hsiao-Ting Wang Advisor: Quincy Wu Date: July 2 nd 2009. Outline. Introduction RTP session RTP header 表頭欄位 Conclusion. 應用層 (HTTP 、 RTP). 傳輸層 (TCP/UDP). 網路層 (IP). 連結層 實體層. Introduction. UDP 缺點 封包的先後順序問題 如何知道封包有沒有遺失 RTP

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RTP Real-Time Transport Protocol

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  1. RTPReal-Time Transport Protocol Speaker: Hsiao-Ting Wang Advisor: Quincy Wu Date: July 2nd 2009

  2. Outline • Introduction • RTP session • RTP header • 表頭欄位 • Conclusion

  3. 應用層(HTTP、RTP) 傳輸層(TCP/UDP) 網路層(IP) 連結層實體層 Introduction • UDP缺點 • 封包的先後順序問題 • 如何知道封包有沒有遺失 • RTP • RTP: A Transport Protocol for Real-Time Applications • It was first published in 1996 as RFC1889 which was made obsolete in 2003 by RFC 3550 • 想一想：RTP比較適合跑在哪個通訊協定上頭？ • TCP or UDP?

  4. Introduction (Cont.) • RTCP(RTP Control Protocol) • RTCP provides out-of-bandcontrol information for an RTP flow • RTCP作用是提供QoS (Quality of Service)相關的回饋(feedback) • RTCP gathers statistics on a media connection and information such as bytes sent, packets sent, lost packets, jitter, feedback and round trip delay

  5. RTP Session (RTP 會期) • Consists of a number of applications communicating with RTP • Identified by a network address and two ports • One port is used for media data • The other for RTCP (real time transfer control protocol) control data. • 想一想：因此，打網路電話時，會佔去幾個ports?

  6. RTP Session (Cont.) • Session participants can send, receive, or do both. • Each media type is transmitted in a separate session, enabling participants to choose which media types they want to receive. For example, a user may just want the audio portion of a streaming music video. • 想一想：那影像網路電話又會佔去幾個ports?

  7. RTP header • 表頭大小為 12 bytes. IP header UDP header RTP header RTP payload

  8. 表頭欄位 • Version (V) • 2-bit.目前的版本為2. • Padding (P) • 1-bit. Payload後面是否有Padding. • Extension (X) • 1-bit. 有設的話，就會多加下面的延伸表頭.

  9. 表頭欄位(cont.) • CSRC Count (CC) • 4-bit. 0~15 • 在表頭裡包含CSRC( Contributing source identifiers )的個數. • Marker (M) • 1-bit. 在RFC1890裡描述，在應用程式開始傳送封包時，第一個封包應設定這個位元. • Payload Type (PT) • 7-bit. Defined by RFC 1990. • Sequence Number • 16-bit. 每次送出一個封包就加1. • 初始值為亂數值. • 讓接收者用來偵測封包遺失與封包順序正確性.

  10. 表頭欄位(cont.) • Timestamp • 32-bit. Payload 裡的第一個sample產生時的時戳. • 可用來做jitter的計算. • 初使值由傳送端指定一亂數值. • Synchronization Source (SSRC) • 32-bit. The identifier is chosen randomly by the sender • Globally unique within an RTP session. • Contributing Source (CSRC) • 32-bit. 包含一個傳送端的SSRC值. • 此欄位只有在來源從混音器(mixer)來時，才會使用.

  11. Conclusion • RTP作用是傳送 multimedia data

  12. Reference • RTP, http://en.wikipedia.org/wiki/Real-time_Transport_Protocol • RTCP, http://en.wikipedia.org/wiki/Real-time_Transport_Control_Protocol

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