WebRTC SDP

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

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)

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