GTP (GPRS Tunneling Protocol)
When a data is transferred from Radio Stack(eNB) to Core Network it goes through various Interfaces as illustrated in Figure 1 of Network Architecture and Interface page. When the data go through these interface, they get encapsulated by various tunnel protocol as in Figure 2 of Network Architecture and Interface page. GTP is a specific type of tunneling protocol by which U-plane data (user data) go through as illustrated below. In 3GPP, GTP is specified in 29.281.
Following is the GTP Header format. User Data (usually IP data) is encapsulated by a GTP packet following this header as shown in example section.
< 3GPP 29.281 - Figure 5.1-1: Outline of the GTP-U Header >
Example 1 : GTP - IP - ICMP (Echo Request)
Example 2 : GTP - IP - ICMP (Echo Reply)