24.3413GPPRelease 16Stage 3Support of SMS over IP networksTS
SMS over IP functionality provides the UE with the capability of sending traditional short messages over IMS network. The architecture for SMS is specified in 3GPP TS 23.040  and for SMS over IP functionality in 3GPP TS 23.204 .
4.2 SMS over IP
In order to guarantee SMS interoperability the SM-over-IP sender, the SM-over-IP receiver and the IP-SM-GW shall support encapsulation of RPDUs defined in 3GPP TS 24.011 , subclause 7.3. The SM-over-IP sender, the SM-over-IP receiver and the IP-SM-GW shall use the MIME type "application/vnd.3gpp.sms" for this purpose.
4.3 Application utilisation of SMS over IP
SMS over generic IP access can be used to support all applications and services that use SMS.
4.4 SMS over IP and subscription without MSISDN
When SMS over IP is to be delivered to a subscriber where the subscription profile does not contain an MSISDN, then there is a need for the IP-SM-GW to become aware of the IMSI during IMS registration for later correlation of short messages. In order to do so the IP-SM-GW derives the IMSI from the private user identity of the user that the UE includes in the REGISTER request at registration or if the private user identity is not available, derives the IMSI from the public user identity.
3GPP TS 23.003  defines the relationship between the private user identity and the IMSI and the relationship between the public user identity and the IMSI.