Why MME to MME interface in LTE is based on GTP-C why not it is based on Diameter?

Question

An UE is registered in roaming in LTE network in combined registration (in both MME and VLR).
When the UE returns to its home network in 3G network, does the home network generate both MAP-CANCEL (to the previous VLR) and CLR (to the previous MME)?
Is it implementation dependent in the Home network? (HLR & HSS)
If only MAP-CANCEL is generated, when the MME will remove the IMSI records from its internal database?

Answer 1

Wherever AAA (Authentication, Authorization and Accounting) required Diameter protocol is used. MME to MME interface is mainly used for handover signalling. Handover signalling does not come under the AAA category that's reason there is no use of having Diameter based interface.