Section: 4.2.1.1 Initial Messages
Content: The UDP Destination Port number for GTPv2 Initial messages shall be 2123. It is the registered port number for GTP-C. The UDP Source Port for a GTPv2 Initial message is a locally allocated port number at the sending GTP entity. If GTPv2 and GTP' v2 modules are using the same IP address for sending messages, the implementation shall ensure that while some source port number is used by GTPv2 messages, the same source port number shall not be used by GTP' v2 messages. Otherwise, the IP interface may have difficulty to delivering a response message to the right protocol entity.