WebRTC SDP

Abstract

WebRTC SDP

Authors

Walter Fan

Status

WIP as draft

Category

LearningNote

Updated

2024-08-21

overview

An SDP session description includes the following: * Session name and purpose * Time(s) the session is active * The media comprising the session o Information needed to receive those media (addresses, ports, formats, etc.)

An Offer/Answer Model with the SDP

  • 每当需要 Offer/Answer 交换时，发起方都会通过调用 createOffer() API 创建 Offer。

  • 然后，应用程序使用该 Offer 通过 setLocalDescription() API来设置其本地配置。offer 最终通过其首选的信令机制（例如WebSockets）发送到远程端；收到该 offer 后，远程方将使用 setRemoteDescription()API进行安置。

  • 为了完成 Offer/Answer 交换，远程方使用 createAnswer() API生成适当的 answer，使用 setLocalDescription()`API应用该`answer，并将 answer 通过信令通道发送回发起方。

启动器获得该`answer`后，它将使用 setRemoteDescription() API进行安装，并且初始设置已完成。可以重复此过程以进行其他 Offer/Answer 交换。

参见 RFC3264

RTP Profile for Audio and Video Conferences with Minimal Control

参见 RFC3551

SDP for ICE and DTLS

关于 DTLS extension to establish keys for SRTP, 请参见 RFC5764

SDP extension

  • DTLS fingerprint for SRTP (a=fingerprint)

  • RTP/RTCP Multiplexing (a=rtcp-mux)

  • RTCP Feedback support (a=rtcp-fb)

  • Host and server-reflexive candidate lines (a=candidate)

  • SRTP Setup framework parameters (a=setup)

  • RTCP attribute (a=rtcp)

  • RTP header extension indicating audio-levels from client to the mixer(a=extmap:1 * * urn:ietf:params:rtp-hdrext:ssrc-audio-level)

SDP 的两个 Plan

Plan B 和 Unified Plan

enum SdpSemantics {
"plan-b",
"unified-plan"
};


partial dictionary RTCConfiguration {
    SdpSemantics sdpSemantics;
}

Negotiating Media Multiplexing Using SDP

该扩展可以与SDP提供/应答机制一起使用，以协商使用单个传输（5元组）来发送和接收由多个SDP媒体描述所描述的媒体 5-tuple：client ip, client port, server ip, server port, protocol Allow an SDP ‘group’ attribute to contain an identification-tag that identifies a “m=” section with the port set to zero. It is to use a single transport(BUNDLE transport) for multiple media streams, defines a new SDP Grouping Framework [RFC5888] extension called ‘BUNDLE’.

请参见 RFC8843

SDP Grouping Framework

The Session Description Protocol (SDP) Grouping Framework

  • Use of “group” and “mid”

  • Flow Identification (FID)

    • For each participant, the session is defined by a particular pair of destination transport addresses

    • But a single media instance maybe sent using more than one RTP session

请参见 RFC5888

Source-Specific Media Attributes in SDP

Defines a mechanism to describe RTP sources, identified by their synchronization source (SSRC) identifier, in SDP, to associate attributes with these sources, and to express relationships among individual sources.

a=ssrc:<ssrc-id> <attribute>
a=ssrc:<ssrc-id> <attribute>:<value>
a=ssrc-group:<semantics> <ssrc-id> ...

请参见 RFC5576

SDP examples

请参见 https://tools.ietf.org/html/draft-nandakumar-rtcweb-sdp-08

参见资料