LTE MBMS: What is the use of C-TEID ?

Question

How MCE behaves differently when it receives "Time of MBMS Data Transfer" (optional IE) along with "Minimum Time to MBMS Data Transfer" which is mandatory ? What signifies their presence in the MBMS SESSION START REQUEST message ?

Answer 1

In my understanding, it is used when falling back to unicast bearer between MBMS GW and E-UTRAN, the packets will be transported through GTP tunnel

Answer 2

If I look into protocol stack of M1 interface, my interpretation is C-TEID is nothing but a common tunnel ID which is used to map data to corresponding TMGI service at eNodeB.
If eNodeB can listen for multicast IP address then it will use multicast IP address allocated by MBMS-GW and if it can't then It sends F-TIED (TEID + IP address) to MBMS-GW. After receiving F-TEID for an eNodeB, MBMS-GW uses point to point service. There is no need to allocate any user plance F-TEID at MBMS-GW, since MBMS is downlink only service.