libosmocore/lapd: Fixed handling of sequence errors at lapd_core.c

If a sequence error is received, the N(R) variable must still be used to
acknowledge previously transmitted frames.

If there are two subsequent sequence errors received, ignore it. (Ignore
every second subsequent error.) This happens if our reply with the REJ is
too slow, so the remote gets a T200 timeout and sends another frame with
a sequence error. Test showed that replying with two subsequent REJ
messages could the remote L2 process to abort. Replying too slow shouldn't
happen, but may happen over serial link between BB and LAPD.

Written-by: Andreas.Eversberg <jolly@eversberg.eu>
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
1 file changed