RFC3611, "RTP Control Protocol Extended Reports (RTCP XR)" established
  a framework to allow new information to be conveyed in RTCP, 
  supplementing the original report blocks defined in RFC 3550, "RTP: A 
  Transport Protocol for Real-Time Applications".
  
  The XRBLOCK working group is chartered to work within this framework, 
  evaluating proposals for report block definitions containing new 
  metrics. The group will follow the guidelines established in RFC5968, 
  "Guidelines for Extending the RTP Control Protocol (RTCP)" and RTP 
  Monitoring Architecture being developed in the AVTCORE working group.
  The group will consider the guidelines for performance metric 
  development currently being discussed in the PMOL working group.