Standalone messaging is based on the OMA CPM Pager Mode and Large Message Mode mechanisms as described in OMA CPM 
When the size of a message does not exceed 1300 bytes, it can be sent as a pager mode standalone message carried directly within the body of a SIP MESSAGE. It may be sent to one or many destinations. Using the SIP MESSAGE method, the message body is CPIM-formatted message as specified in RFC 3862
]. The SIP/IP Core provides the routing between the RCS Server and RCS Clients without establishing a SIP session.
The origination of the pager mode standalone message may request to receive a disposition status notification when the message is delivered and/or displayed to the destination of the message. If sent, these messages will follow the reverse path indicated in the message they relate to. These messages may be used for 1-to-1 or 1-to-many pager mode messaging and the notification details are carried within the body of a SIP MESSAGE.
If a standalone message is larger than 1300 bytes, large message mode is used. In large message mode, the contents of the message are not inserted into the SIP message but carried using MSRP as defined in RFC 4975
and RFC 6714
. A SIP session is established between the interested parties (sender and all receivers) with MSRP as the media Stream. The CPIM-formatted messages are then transmitted using MSRP data chunks.
Large Message Mode SIP sessions should not be confused with a chat session as no chat session is established. The SIP session is only used to transmit exactly one large message after which the SIP session is torn down. If delivery and/or display notifications need to be returned by the recipient(s), these notifications are CPIM-formatted messages carried within MSRP data chunks inside the SIP session or within SIP MESSAGEs delivered outside the SIP session as pager mode messages.
The Figure 7.13.2-1
provides the architecture for LI for pager mode standalone messaging.
In a normal deployment, the P-CSCF or S-CSCF perform the function of the RCS Server in Figure 7.13.2-1