Internally, Opal transports IM as frames of T.140 text data encapsulated as per RFC 4103
Over SIP, Opal supports 4 messaging types:
- RFC 3428 page mode using MESSAGE requests outside a call
- RFC 3428 session mode using MESSAGE requests inside a call
- RFC 4975 MSRP (partially supported) signalled using "message" SIP type with transport "tcp/msrp"
- Page mode using MESSAGE requested, but signalled using "message" SDP type with transport "sip"