GTP-C message description in LTE enviornment.

When you come across GTP-C message then you are surely curious about the GTP-C message handling in LTE network.
GTP-C message flows between different interface to setup the signalling connection. The interfaces are S11, S5, S8 and S4.

There are two type of GTP-C messages are there.
1. Path management messages
2. Tunnel management messages.

1. Path Management Messages
ECHO request/response will be used to ping destination GTP end point as well handle the restart procedure of a GTP endpoint.
Please refer ECHO message handling in GTP-C

2. Tunnel Management Messages
Different GTP-C messages are used to manage a Tunnel. The GTP-C messages are Create Session Request/Response, Create Bearer Request/Response, Modify Bearer Request/Response, Update Bearer Request/Response, Delete Session Request/Response, Delete Bearer Request/Response etc.

Let's see the following diagram where GTP-C message is used for signalling setup.
This figure is for attaching a UE in the LTE network

Here i will explain more on the GTP-C message flow in LTE Network.
a. Create Session Request/Response
This message is used to do signalling setup between MME and SGW or SGW and PGW.

To Create a tunnel between MME and SGW, MME sends Create Session Request towards SGW with GTP-C F-TEID, Bearer Context, MME FQ-CSID and some more Informational element.

The initial Create Session request from MME will have TEID value as 0 in the GTP-C header as MME doesn't know about the SGW F-TEID.
But further Create Session Request from MME will have TEID in GTP-C header as MME knows the TEID of S-GW.

The different Informational Elements of Create Session request are:
IMSI, MSISDN, User Location Information (ULI), Serving Network, RAT Type, Indication Flags, MME/SGW F-TEID, PGW F-TEID, PDN Type, PAA, Bearer Context to be created, Bearer Context to be removed, Recovery, MME-FQ-CSID, SGW-FQ-CSID, Charging Characteristics, Private Extension.
IMSI: The 15 digit number of UE.
MSISDN: The mobile no of UE.
MME/SGW F-TEID: This IE is MME S11 GTP-C(Control plane) F-TEID if MME sends Create Session Request towards S-GW and SGW S5 GTP-C(Control plane) F-TEID if SGW sends Create Session Request towards P-GW.
Bearer Context To be Created: This informational elements is for creating a default bearer through Create Session Request. It carry some sub Informational element. The Informational elements are:

• EBI: Default bearer ID
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