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.

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

0	S1-U eNodeB GTP-U interface
1	S1-U SGW GTP-U interface
2	S12 RNC GTP-U interface
3	S12 SGW GTP-U interface
4	S5/S8 SGW GTP-U interface
5	S5/S8 PGW GTP-U interface
6	S5/S8 SGW GTP-C interface
7	S5/S8 PGW GTP-C interface
8	S5/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)
9	S5/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)
10	S11 MME GTP-C interface
11	S11/S4 SGW GTP-C interface
12	S10 MME GTP-C interface
13	S3 MME GTP-C interface
14	S3 SGSN GTP-C interface
15	S4 SGSN GTP-U interface
16	S4 SGW GTP-U interface
17	S4 SGSN GTP-C interface
18	S16 SGSN GTP-C interface
19	eNodeB GTP-U interface for DL data forwarding
20	eNodeB GTP-U interface for UL data forwarding
21	RNC GTP-U interface for data forwarding
22	SGSN GTP-U interface for data forwarding
23	SGW GTP-U interface for DL data forwarding
24	Sm MBMS GW GTP-C interface
25	Sn MBMS GW GTP-C interface
26	Sm MME GTP-C interface
27	Sn SGSN GTP-C interface
28	SGW GTP-U interface for UL data forwarding
29	Sn SGSN GTP-U interface
30	S2b ePDG GTP-C interface
31	S2b-U ePDG GTP-U interface
32	S2b PGW GTP-C interface
33	S2b-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 1	Bit 2	Bit 3-6
IPv4 Address Present	IPv4 Address Present	Interface Type
1	1	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