Dropping LLC frames due the TBF destruction

Holger Hans Peter Freyther hfreyther at sysmocom.de
Tue Nov 5 08:01:27 UTC 2013

On Tue, Nov 05, 2013 at 07:55:29AM +0100, Andreas Eversberg wrote:

dear andreas,

as usual there is more than one dimension to this problem.

1.) Dropping LLC data with no indication.
When trying to understand a problem (e.g. Samsung3 traffic stalls) one
needs to analyze what is going wrong. The "contract" for the SGSN/PCU
is that a LLC frame will be either forwarded to the MS or a discarded
message will be sent (frames, octets, etc..).

BUT with the current code this contract is broken. One simply can not
know if the LLC frame has actually reached the phone... 

2.) Retry or not to retry.

> 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.)

Well, retry or not to retry is an architectural decision. The question
is what to do with the frames we are discarding?

- Holger Freyther <hfreyther at sysmocom.de>       http://www.sysmocom.de/
* sysmocom - systems for mobile communications GmbH
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