This Recommendation defines a protocol operating through the hierarchy of a multipoint communication domain. It specifies the format of protocol messages and procedures governing their exchange over a set of transport connections. The purpose of the protocol is to implement the Multipoint Communication Service defined by Recommendation T.122. Annex A defines a protocol allowing the use of multicast network services in a T.120 conference. It does this by performing data routing as part of T.125. Once it assumes responsibility for routing, it will use multicast whenever possible. This Annex handles sub-optimal networks by testing multicast pathways before depending on them. In the event that multicast does not work at some point within the conference, unicast is used in its place. This Annex is independent of the networks on which it is used. All issues that are specific to a given network are handled by the protocol stacks that lie beneath this protocol. The following changes have been incorporated into this version of the Recommendation: - the MAP protocol has been incorporated as Annex A; - MCS Protocol version 3 is specified; - Extended-Parameter MCSPDUs have been added; - Capabilities-Notification MCSPDUs have been added; - fields were added to the Send-Data and Uniform-Send-Data MCSPDUs to support changes to Recommendation T.122; - minor changes have been made to the document format.