A connection is established with the very first packet sent. Note that there can be arbitrarily many packets sent from the transmitter before requesting status, so datagrams have no upper limit. Since timers are used, this is all that is needed for a transaction service, but you can add status requests in there to make the connection shutdown happen faster.
Connections are, of course, just long lived packet exchanges.
XTP is at revision 4.0 with an addendum that strengthens some of the Multicast concepts. XTP was always a public domain protocol. PEI eventually folded for a variety of reasons, but most of the original Techical Advisory Board members continued to work on the protocol. There is a lack of public domain implementations.