Re: [PATCH 1/1] gtp: support SGSN-side tunnels

Jonas Bonn
2017-02-06T13:33:12Z

Hi Pablo,

On 02/06/2017 12:08 PM, Pablo Neira Ayuso wrote:
> Hi Jonas,
>
> On Fri, Feb 03, 2017 at 10:12:31AM +0100, Jonas Bonn wrote:
>> The GTP-tunnel driver is explicitly GGSN-side as it searches for PDP
>> contexts based on the incoming packets _destination_ address. If we
>> want to write an SGSN, then we want to be idenityfing PDP contexts
>> based on _source_ address.
>>
>> This patch adds a "flags" argument at GTP-link creation time to specify
>> whether we are on the GGSN or SGSN side of the tunnel; this flag is then
>> used to determine which part of the IP packet to use in determining
>> the PDP context.
> So far the implementation that I saw in osmocom relies on userspace code
> to tunnel data from ME to the SSGN/SGW running on the base station.
>
> The data we get from GGSN -> SGSN needs to be places into a SN-PDU (via
> SNDCP) when sending it to the BTS, right? So I wonder how this can be
> useful given that we would need to see real IP packets coming to the
> SSGN that we tunnel into GTP.

Fair enough. The use-case I am looking at involves PGW load-testing
where the simulated load is generated locally on the SGSN so it _is_
seeing IP packets and the SNDCP is left out altogether. Perhaps this is
too pathological to warrant messing with the upstream driver... I don't
know: the symmetry does not cost much even if it's of limited use.

Couldn't the SNDCP theoretically be a separate node and push IP packets
to the SGSN, thus making this useful? Perhaps it's a stretch...

/Jonas