SDP attributes

wrtc_kit adds different attributes to the SDP sent to other client when connecting a call. These attributes send various information to the receiving client:

By adding attribute a=rtcgate to SDP the sending client informs receiver it is using wrtc_kit and understands the RTCgate protocol.

By adding attribute a=gid:groupID to SDP the sending client can tell receiver a group call's group ID.

By adding attribute a=icerestart to SDP the sending client informs receiver this SDP is sent due to ICE failure and restart.

Server may add a=rtcg_media_direct_proposed to SDP to request clients send media directly to each other. wrtc_kit may add a=rtcg_media_direct_accepted:ID to it's answer SDP to indicate it has accepted the proposal.

Server may add a=rtcg_media_direct_sdp:ID to SDP to make wrtc_kit client auto-answer call if provided ID matches previous media direct accept ID. This mechanism is used for pre-answer call scenarios.

By adding attribute a=cpbMCVD to SDP the sending client can tell what features it do or do not support:

a=cpbMCVD

• a= : standard SDP notation for extended attributes
• cpb : capabilities attribute
• M or 1 : multiple simultaneous calls supported (M) or single call support only (1)
• C or c : chat messages supported (C) or not (c )
• V or v : video calls supported (V) or not (v)
• D or d : data channel supported (D) or not (d)

If SDP contains no a=cpb attribute a=cpb1cvd (no support of any features) is assumed.