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Today, we will talk about GTP tunnels used in

the LTE network (LTE GTP Tunnel I)

September 26, 2013 | By Chris (tech@netmanias.com) | Netmanias Tech-Blog | www.netmanias.com

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 to eNB

A UE sends an IP packet with its destination IP address set to e.g. 74.125.71.104 (IP address of

www.google.com) to an eNB through a radio link. The original packet sent by the UE will look something like

this:

2. eNB to S-GW

Upon 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 to PGW

The 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 to PDN (www.google.com)

IP Header: SIP=UE, DIP=74.125.71.104 IP Payload

The IP packet sent by the UE

Outer IP Header: SIP=eNB, DIP=S-GW

GTP Header: TEID=XUDP HeaderOuter IP Header: SIP=eNB, DIP=S-GW IP Header: SIP=UE, DIP=74.125.71.104 IP Payload

The IP packet sent by the UEThe header added by the eNB (GTP tunnel header)

GTP Header: TEID=YUDP HeaderOuter IP Header: SIP=S-GW, DIP=P-GW IP Header: SIP=UE, DIP=74.125.71.104 IP Payload

The IP packet sent by the UEThe header added by the S-GW (GTP tunnel header)

Netmanias Tech-Blog: Today, we will talk about GTP tunnels used in the LTE network (LTE GTP Tunnel I)

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Then, 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

IP Header: SIP=UE, DIP=74.125.71.104 IP Payload

The IP packet sent by the UE

Outer IP Header: SIP=eNB, DIP=S-GW

IP packet

GTP-U tunnel

eNB S-GW P-GW

S1 GTP tunnel S5 GTP tunnel

Dst IP: InternetSrc IP : UE

Dst IP: S-GWSrc IP : eNBTEID: UL S1-TEID

Dst IP: InternetSrc IP : UE

Dst IP: P-GWSrc IP : S-GWTEID: UL S5-TEID

Dst IP: InternetSrc IP : UE

Dst IP: InternetSrc IP : UE

IP packet

GTP-U tunnel

eNB S-GW P-GW

S1 GTP tunnel S5 GTP tunnel

Dst IP: UESrc IP : Internet

Dst IP: eNBSrc IP : S-GWTEID: DL S1-TEID

Dst IP: UESrc IP : Internet

Dst IP: S-GWSrc IP : P-GWTEID: DL S5-TEID

Dst IP: UESrc IP : Internet

Dst IP: UESrc IP : Internet

(a) from the UE to the Internet

(b) from the Internet to the UE

1 2 3 4

4 3 2 1

UE

UE

Netmanias Tech-Blog: Today, we will talk about GTP tunnels used in the LTE network (LTE GTP Tunnel I)

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and one 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.

About NMC Consulting Group (www.netmanias.com)

NMC Consulting Group is an advanced and professional network consulting company, specializing in IP network areas (e.g., FTTH, Metro Ethernet and IP/MPLS), service areas (e.g., IPTV, IMS and CDN), and wireless network areas (e.g., Mobile WiMAX, LTE and Wi-Fi) since 2002. Copyright © 2002-2013 NMC Consulting Group. All rights reserved.

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