Instant (multimedia) messaging is a key feature for RCS as it should replace SMS service. There are more types of messages which can be sent through the IMS network.
When we will take a look on the SIP indication of capabilities by Accept header there are quite a few of supported formats:
For common Instant Messages we typically use Common Profile for Instant Messaging (CPIM). This format allows compatibility among various messaging technologies. Notifications are then sent as in Instant Message Disposition Notification (IMDN) format.
Messages can be exchanged over SIP or MSRP protocol. Sometimes also http/ws is being used for transfer of rich content.
Update: For the information about RCS Universal profile read the post GSMA Advanced Messaging – RCS Universal Profile.
RCS 5.2 defines two basic types of messaging. They are the Session Mode Messaging which is based on SIP INVITE and MSRP session and the Pager Mode Messaging which is based on SIP Message. The RCS refers to OMA CPM which is considered to be an evolution of the SMS/MMS messaging services.
Note that SIP MESSAGE can be used also for other than messaging purposes in IMS network (e. g. during eSRVCC registration)
Session Mode Messaging
Session mode does use SIP INVITE and SDP protocol to establish the session. Within the session MSRP protocol is used. This allows to split ones conversation into dialogs and use “..is typing” notifications. On the other hand more resources is needed.