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EPC LTE Python RFCs & Standards

GTPv2 – F-TIED Interface Types

A quick look at the Interface Types in the F-TIED field in GTPv2

I’ve been working on a ePDG for VoWiFi access to my IMS core.

This has led to a bit of a deep dive into GTP (easy enough) and GTPv2 (Bit harder).

The Fully Qualified Tunnel Endpoint Identifier includes an information element for the Interface Type, identified by a two digit number.

Here we see S2b is 32

In the end I found the answer in 3GPP TS 29.274, but thought I’d share it here.

0S1-U eNodeB GTP-U interface
1S1-U SGW GTP-U interface
2S12 RNC GTP-U interface
3S12 SGW GTP-U interface
4S5/S8 SGW GTP-U interface
5S5/S8 PGW GTP-U interface
6S5/S8 SGW GTP-C interface
7S5/S8 PGW GTP-C interface
8S5/S8 SGW PMIPv6 interface (the 32 bit GRE key is encoded in 32 bit TEID field and since alternate CoA is
not used the control plane and user plane addresses are the same for PMIPv6)
9S5/S8 PGW PMIPv6 interface (the 32 bit GRE key is encoded in 32 bit TEID field and the control plane and
user plane addresses are the same for PMIPv6)
10S11 MME GTP-C interface
11S11/S4 SGW GTP-C interface
12S10 MME GTP-C interface
13S3 MME GTP-C interface
14S3 SGSN GTP-C interface
15S4 SGSN GTP-U interface
16S4 SGW GTP-U interface
17S4 SGSN GTP-C interface
18S16 SGSN GTP-C interface
19eNodeB GTP-U interface for DL data forwarding
20eNodeB GTP-U interface for UL data forwarding
21RNC GTP-U interface for data forwarding
22SGSN GTP-U interface for data forwarding
23SGW GTP-U interface for DL data forwarding
24Sm MBMS GW GTP-C interface
25Sn MBMS GW GTP-C interface
26Sm MME GTP-C interface
27Sn SGSN GTP-C interface
28SGW GTP-U interface for UL data forwarding
29Sn SGSN GTP-U interface
30S2b ePDG GTP-C interface
31S2b-U ePDG GTP-U interface
32S2b PGW GTP-C interface
33S2b-U PGW GTP-U interface

I also found how this data is encoded on the wire is a bit strange,

In the example above the Interface Type is 7,

This is encoded in binary which give us 111.

This is then padded to 6 bits to give us 000111.

This is prefixed by two additional bits the first denotes if IPv4 address is present, the second bit is for if IPv6 address is present.

Bit 1Bit 2Bit 3-6
IPv4 Address Present IPv4 Address PresentInterface Type
11 000111

This is then encoded to hex to give us 87

Here’s my Python example;

interface_type = int(7)
interface_type = "{0:b}".format(interface_type).zfill(6)   #Produce binary bits
ipv4ipv6 = "10" #IPv4 only
interface_type = ipv4ipv6 + interface_type #concatenate the two
interface_type  = format(int(str(interface_type), 2),"x") #convert to hex

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