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RRC: Allow re-establishment on different DU when physCellId differs

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When a UE re-establishes on a different DU, the physCellId in the
re-establishment request (from the old cell) will differ from the
new DU's physCellId. The previous check incorrectly treated this as
a "too fast movement" scenario and triggered fallback to RRC setup.

This fix distinguishes between two scenarios:
1. Different DU scenario: physCellId differs because UE is on a
   different DU - this is legitimate and should be allowed. UE
   Context Setup will be triggered at RRCReestablishmentComplete.
2. Same DU but different physCellId: "too fast movement" scenario
   - fallback to RRC setup as before.

The fix checks if assoc_id != ue_data.du_assoc_id to detect a
different DU scenario and allows re-establishment in that case.

This addresses the issue where re-establishment on a different DU
was incorrectly rejected, preventing proper UE Context Setup per
TS 38.401 Section 8.7.
This commit is contained in:
Guido Casati
2026-01-29 10:17:42 +01:00
parent 290306d1f8
commit 825cbcdb47
1 changed files with 19 additions and 11 deletions
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openair2/RRC/NR/rrc_gNB.c
+19 -11
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@@ -1558,17 +1558,25 @@ static void rrc_handle_RRCReestablishmentRequest(gNB_RRC_INST *rrc,
DevAssert(success);
nr_rrc_finalize_ho(UE);
} else if (physCellId != cell_info->nr_pci) {
/* UE was moving from previous cell so quickly that RRCReestablishment for previous cell was received in this cell */
LOG_I(NR_RRC,
"RRC Reestablishment Request from different physCellId (%ld) than current physCellId (%d), fallback to RRC setup\n",
physCellId,
cell_info->nr_pci);
ngap_cause = NGAP_CAUSE_RADIO_NETWORK_RELEASE_DUE_TO_NGRAN_GENERATED_REASON;
/* 38.401 8.7: "If the UE accessed from a gNB-DU other than the original
* one, the gNB-CU should trigger the UE Context Setup procedure". Let's
* assume that the old DU will trigger a release request, also freeing the
* ongoing context at the CU. Hence, create new below */
goto fallback_rrc_setup;
/* Check if this is a different DU scenario or "too fast movement" scenario */
if (assoc_id != ue_data.du_assoc_id) {
/* Different DU scenario - physCellId differs because UE is re-establishing on a different DU.
* Re-establishment is allowed, UE Context Setup will be triggered at RRCReestablishmentComplete. */
LOG_I(NR_RRC,
"UE %d: Re-establishment on different DU (physCellId %ld from old cell != %d from new DU)\n",
UE->rrc_ue_id,
physCellId,
cell_info->nr_pci);
} else {
/* Same DU but different physCellId - "too fast movement" scenario:
* UE was moving from previous cell so quickly that RRCReestablishment for previous cell was received in this cell */
LOG_I(NR_RRC,
"RRC Reestablishment Request from different physCellId (%ld) than current physCellId (%d), fallback to RRC setup\n",
physCellId,
cell_info->nr_pci);
ngap_cause = NGAP_CAUSE_RADIO_NETWORK_RELEASE_DUE_TO_NGRAN_GENERATED_REASON;
goto fallback_rrc_setup;
}
}
/* TODO: start timer in ITTI and drop UE if it does not come back */