Dropping LLC frames due the TBF destruction

Harald Welte laforge at gnumonks.org
Tue Nov 5 16:41:58 UTC 2013

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

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