What is the interface system between GSTN and the states/CBEC?

In GST regime, while taxpayer facing core services
of applying for registration, uploading of invoices, filing of
return, making tax payments shall be hosted by GST System,
all the statutory functions (such as approval of registration,
assessment of return, conducting investigation and audit
etc.) shall be conducted by the tax authorities of States and
Central governments.
Thus, the frontend shall be provided by GSTN and the
backend modules shall be developed by states and Central
Government themselves. However 24 states (termed as Model
2 states) have asked GSTN to develop their backend modules
also. The CBEC and rest of the states (Model 1) have decided
to develop and host the back-end modules themselves.