Actions
Bug #6124
closedFeature #5502: MS-side LLC implementation
LLC Dummy frames received from osmo-pcu dropped due to invalid FCS
Start date:
07/31/2023
Due date:
% Done:
100%
Resolution:
Spec Reference:
Description
I usually get this warning when receiving LLC dummy fames:
20230731162447732 DLLC INFO llc_prim.c:199 Rx from lower layers: GRR-UNITDATA.indication
20230731162447732 DLLC DEBUG llc_grr.c:97 Rx GRR-UNITDATA.indication: SAPI=3 (SNDCP3), UI func=UI C/R=1 PM=1 E=0 IP=0 N(U)=0 FCS=2b2b2b
20230731162447732 DLLC NOTICE llc.c:847 LLE(ca7cb1c3/db2e1be6,SNDCP3){ASSIGNED_ADM} Dropping frame with invalid FCS 0x2b2b2b vs exp 0x561485: 43 c0 01 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b 2b
That LLC-PDU matches the llc dummy frame:
static const uint8_t llc_dummy_command[] = {
0x43, 0xc0, 0x01, 0x2b, 0x2b, 0x2b
};
Need to find out whether it's a bug in osmo-pcu or in libosmo-gprs at the MS side.
Updated by pespin 9 months ago
Relevant information can be found in 3GPP TS 44.064 6.4.2.2 "Unconfirmed Information (UI) Dummy command"
According to Figure 11a the FCS is always encoded as all-0x2b, so osmo-pcu is correct there.
Also this NOTE:
NOTE 2: The format specified for the UI Dummy command ensures that a receiving LLC entity will always discard it, since the FCS field check always fails (no matter what the length of the UI Dummy command is).
Updated by pespin 9 months ago
- Status changed from New to Feedback
- % Done changed from 0 to 90
Fixed by:
https://gerrit.osmocom.org/c/libosmo-gprs/+/34025 llc: Silently dicard UI Dummy commands
This ticket can be closed when the patch is merged.
Actions
