LTE GTP Tunnel I Today, we will talk about GTP tunnels used in the LTE network.

As seen in Figure (a) below, IP packets sent by an LTE device (UE) are delivered from an eNB to a P-GW through GTP tunnels. What it means is that "all IP packets that a UE sends are always delivered through an eNB to a P-GW regardless of their specified destination IP addresses (i.e., even though their destination IP addresses are different)".

Let’s find out more now.

1. UE -> eNBA UE sends an IP packet with its destination IP address set to e.g. 74.125.71.104 (IP address ofwww.google.com) to an eNB through a radio link. The original packet sent by the UE will look something like this:

 

2. eNB -> S-GWUpon receiving the IP packet from the UE, the eNB adds a GTP tunnel header, consisting of three individual headers – a GTP header, UDP header, and IP header for GTP tunneling - in front of the IP packet. Then, the IP packet (sent by the eNB to an S-GW) will be as follows:

 So, if only an IP routing network exists between the eNB and the S-GW, the routing network performs routing based on the destination IP address of the packet (i.e. the IP address of the S-GW, the destination IP address shown in the outer IP header), and then delivers the IP packet to the S-GW accordingly. 

3. S-GW -> PGWThe S-GW, upon receiving the IP packet from the eNB, modifies its GTP header and IP header

(outer IP header) as follow: 

4. P-GW -> www.google.comThen, the packet is delivered to the P-GW accordingly. The P-GW then removes all three headers (Outer IP header/UDP header/GTP header) from the packet and delivers the original packet sent by the UE to the Internet. 

 As you may notice, an explanation of a TEID (tunnel endpoint ID) included in the GTP header

has not given here. Let’s say there are 100 UEs that are connected to a S-GW and P-GW.

Since one GTP tunnel is generated per UE (more than one can be practically generated,

though), 100 GTP tunnels will be are generated. Now, the LTE network has to be able to

distinguish which GTP tunnel belongs to which UE. For this purpose, a TEID is assigned to

each UE. So, for example, the TEID is marked as TEID = UL S1-TEID (ex. 0x12345678) for

the link from the eNB to the S-GW and as TEID=UL S5-TEID (ex. 0xabcdef12) for

the link from the S-GW to the P-GW in the Figure below.

Now that TEIDs specific to UEs are used, the LTE network can distinguish its

subscribers (UEs) from one another by checking their TEIDs instead of IP addresses P-GWs

check both TEIDs and IP addresses of UEs, eNBs and S-GWs check TEIDs only).

Another thing about the TEIDs is that they are unidirectional. That is, they can only serve for

one direction, either uplink or downlink. So, for the traffic from the Internet to the UE in the

Figure (b) below, a new TEID is assigned and used for the links from the P-GW to the S-GW

and from the S-GW to the eNB.

 

In the Figure, the routes from the eNB to the S-GW, and from the S-GW to the P-GW were

named as “S1 GTP tunnel” and “S5 GTP tunnel, respectively. That was just because the

interface between an eNB and an S-GW andone between S-GW and P-GW were named as

“S1” and “S5” respectively in the LTE network reference model in 3GPP

specification. Practically there is no difference in the format of the packets used in two GTP

tunnels. Also, a letter “U” at the end of “GTP-U” stands for “user plane” and was added to

indicate that it is user data that travels through the tunnel. Likewise, a letter “C” was added

at the end of “GTP-C” to indicate that control (signaling) packets, not user (UE) data, are

delivered through the tunnel.