This document gives information on an RTCP Packet Type and RTCP XR Block Type including associated SDP parameters for inter-destination media synchronization (IDMS). The RTCP XR Block Type, registered with IANA based on an ETSI specification, is used to collect media play- out information from participants in a group playing-out (watching, listening, etc.) a specific RTP media stream. The RTCP packet type specified by this document is used to distribute a summary of the collected information so that the participants can synchronize play- out. Typical applications for IDMS are social TV, shared service control (i.e. applications where two or more geographically separated users are watching a media stream together), distance learning, network quiz shows, multi-playing online games, etc.