Hi Andreas,
On Tue, Nov 05, 2013 at 07:55:29AM +0100, Andreas Eversberg wrote:
there is one issue about failed downlink TBF: the PCU does not do any retry. when the assignment is sent to the phone and it fails, there is no retry according to the specs. i assume that this is a task for the SGSN. osmo-sgsn/openggsn does not resend any LLC frame in case of LLC-DISCARDED message, if i look at the source code of libosmogb. (LLC-DISCARDED is forwarded at bssgp_rx_llc_disc() of gprs_bssgp.c, but i don't see where it is handled.)
what actually happens is that TCP layer will do the resend, but this is a problem, especially at the border of coverage where many downlink assignments get lost.
I'm not sure if I'm mixing up things, but we have three potential protocol layers that can take of reliable delivery:
1) the RLC/MAC layer between PCU and MS 2) the LLC protocol 3) the TCP layer inside user IP