====== 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:

===== wrtc_kit client =====
By adding attribute **a=rtcgate** to SDP the sending client informs receiver it is using wrtc_kit and understands the [[ixnx:wrtc:rtcgate:start|RTCgate]] protocol.

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

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

===== Media direct =====
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.

===== Capabilities =====
By adding attribute **a=cpb//MCVD//** 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 : **c**a**p**a**b**ilities 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.