====== offer ======
**C → S → C {type:'offer', sdp:...}**

offer – SDP offer starting a call.

Client wanting to start a call sends an SDP offer to server, server forwards it to requested client. That client should send an [[ixnx:wrtc:rtcgate:answer|answer]] message to accept the call.

arguments:
  * sdp - [[https://en.wikipedia.org/wiki/Session_Description_Protocol|SDP]] containing streaming media communications parameters
  * from - address of caller
  * to - address to call

optional argument:
  * from_dn - display name of caller

optional argument for group calls:
  * gid - group call ID

optional arguments when transferring a call:
  * transferred_by - address of client transferring call
  * replacing - address of client old call was to