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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 10 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 10 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.
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