- Yes he is right, as per OSI layers, GTP is above L4 (UDP). So it is application layer L7 protocol. However, when one says GTP is at L7, we are only considering two nodes (MME and S-GW) of EPC. (SGSN and GGSN of 3G core network.)
- Just expand the horizon and consider the whole network.
- It is the GTP protocol, who helped for proper routing of user data packets.
- For UL data, GTP routes user data packets to appropriate S-GW by establishing tunnel with it. (For UL data it route user data packets from SGSN to GGSN and from RNC to SGSN. )
- For DL data, GTP route user data packets to appropriate MME . (For DL data it route user data packets from GGSN to SGSN and from SGSN to RNC. )
- For mobility case, “Modify Session Request” changes the route to new MME and new eNB. (For Mobility case, “Update PDP context request” changes the route to new SGSN. )
- Yet the exit gateway P-GW is the same, the common anchor point. (Yet the exit gateway GGSN is the same, the common anchor point. )
- So thus, GTP helps to route the user data, to appropriate node, and it also change the route for mobility case.
- Routing is done by network layer So GTP is L3 within PLMN.
- Again, if you expand the horizon further, end to end from mobile till content server in Internet cloud, then The whole wireless network (GPRS, UMTS or LTE) is just L2. It just carries the user IP packet towards ISP (Internet cloud).
Now let me recap, this L2, means the wireless network has its own stack (OSI layer), where GTP is used to route, within different nodes of core network or EPC. So GTP is L3 within L2
Again this L3 GTP is between 2 nodes. It has its own stack (OSI layer). GTP over UDP over IP over L2 over L1. So when we consider only these 2 nodes, within core networks, then yes, GTP is L7 application layer protocol.
So one line summary of long answer "Just perform: Zoom in and zoom out" :)