Cellular Network Infrastructure » Core Network (CN) » OsmoSGSN

[unfortuantely this was in the sysmocom internal issue tracker, where it really didn't belong].

[16:43:57] <pespin> laforge: I really don't see how the GTP-U thing can ever work properly without the SGSN proxing the RNC<->GGSN conn
[16:44:06] <pespin> but reworking all that is probably a lot of work
[16:45:02] <pespin> there's no real way of having MT traffic working properly without proxying
[16:46:26] <pespin> at least when the mmctx MM_State_Iu goes into Idle state (RNC conn closed)
[16:48:15] <pespin> because then the only possibility is to send GTP-U data GGSN->SGSN (SGSN did PDpCtxUpdate beforehand to update the GSN addr to point to itself). And then either page and drop that packet and connect directly to the GGSN (so really shity link is established where 1st packet is always dropped), or keep proxying everything through the SGSN.
[16:48:49] <pespin> so in the end, for easiness and to avoid packet drops, it makes sense to ALWAYS proxy GTP-U traffic through SGSN
[16:49:40] <pespin> at least if one wants to support MT traffic after the RNC closed the conn
[16:50:20] <pespin> which is fine for usual cases where MS is submitting MO traffic usually, but fails for the specific case of receiving downlink data after a long idle period

So in order to be able to start paging the UE and create a new RAB, the SGSN will need to be aware of the first downlink GTP-U frame. And the only way to do that is to somehow insert itself [or a dedicated GTP-U proxy] into the user plane.