Re: [v6ops] GTP questions

[Alexandre Petrescu <[e-mail]>]

Hi Mikael,

Sorry for late reply.

Le 29/09/2017 à 10:44, Mikael Abrahamsson a écrit :
> On Fri, 29 Sep 2017, Alexandre Petrescu wrote:
> 
>> I would like to ask whether by 3GPP specs the GTP packets can 
>> optionally be transported in IPv6 messages?
>> 
>> 3GPP spec "GTP" Rel 15 of September 2017 says this:
>>> UDP/IP is the only path protocol defined to transfer GTP
>>> messages in the version 1 of GTP. A User Datagram Protocol (UDP)
>>> compliant with IETF RFC 768 [13] shall be used.
>> 
>> In practice, a packet capture on PGW shows an IPv6 DHCPv6-PD 
>> Solicit message, preceded by a GTP-U Header, which is itself 
>> preceded by a "GTPU Rx PDU" which is an IPv4 UDP packet.
>> 
>> The UDPv4 port number that transports GTP packets is 2152,
>> reserved at IANA and at that 3GPP spec.
> 
> It's an implementation detail whether this is carried over IPv4 or 
> IPv6. UDP can be carried by both. If you read 29.060 it talks about 
> GTP over both IPv4 and IPv6:
> 
> "If an IPv4/IPv6 capable SGSN received IPv4 GGSN addresses from the 
> old SGSN, it shall include IPv4 addresses in the fields SGSN Address 
> for Control Plane and SGSN Address for User Traffic and IPv6 
> addresses in the fields Alternative SGSN Address for Control Plane 
> and Alternative SGSN Address for User Traffic. Otherwise, an 
> IPv4/IPv6 capable SGSN shall use only SGSN IPv6 addresses if it has 
> GGSN IPv6 addresses available. If the GGSN supports IPv6 below GTP, 
> it shall store and use the IPv6 SGSN addresses for communication
> with the SGSN and ignore the IPv4 SGSN addresses. If the GGSN
> supports only IPv4 below GTP, it shall store and use the IPv4  SGSN
> addresses for communication with the  SGSN and ignore the IPv6 SGSN
> addresses. When active contexts are being redistributed due to load
> sharing, G-PDUs that are in transit across the Gn-interface are in
> an undetermined state and may be lost."
> 
> "below GTP" seems to indicate what protocol GTP is run over.

YEs, I can agree to read it that way, but it can be a little bit of a
stretch.  GTP Rel15 Sept. 2017 clearly says only RFC768.  If it wanted 
to mean GTP/UDP/IPv6 it could have cited e.g. RFC6936.  Hence the doubt.

But yes, I agree with you that in largest part UDP works over IPv6 as 
over IPv4.

> There is a lot more text in TS 29.060 regarding this, but interested 
> parties can read it and form their own opinion.

I agree.

> To me it's clear that 3GPP has done the work to try to achieve so you
> can standards-based build a network with no IPv4 addresses used for
> infrastructure. If this works in real life in shipping products,
> that's a whole other question.

I agree that real life in shipping products is a whole other question.

I had some discussion with some people, and here are some of my 
deductions.  If I am wrong, I carry the responsibility.

Operator1 in hexagon country: the IPv6 is carried in GTP/UDP/IPv4
Operator2 in country voted out of EU: the IPv6 is carred in GTP/UDP/IPv4.

Among other cellular operators that offer IPv6 to smartphones, I wonder 
about the following:

Is T-Mobile USA carrying the smartphone's IPv6 inside GTP/UDP/IPv4?  Or 
inside GTP/UDP/IPv6?

Is Reliance JIO in India carrying the smartphone's IPv6 inside 
GTP/UDP/IPv4?  Or inside GTP/UDP/IPv6?

My gut feeling is that all do GTP/UDP/IPv4.

My intention with this discussion is twofold: find an operator that does 
GTP/UDP/IPv6 and second to deduce something for the IPv6-only 
terminology draft.

Alex