When sender sends first GTP-C message to destination, destination port is 2123. but the source port allocated locally at sender end. Messages over GTP are categorized as following:
1. Initiating message.
2. Triggered message.
3. Trigger reply message.
First message sends by sender known as initial message. And there is always a response message of initial message(triggered message). when both the protocols (GTPv2 and (GTP'v2 or GTP prime v2)) are running on same node then implementation make sure that both the protocols shall use different source ports for initiating messages, So when IP layer receives a trigger message(response message), it could forward reply message to right protocol entity based on the chosen source port.
Hope I have answered to your query properly.
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