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hack: ignore empty Cell Group Config returned by DU

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Interoperability testing with srsRAN DU shows than when receiving
RRCReconfigurationCompleteIndicator in a UEContextModificationRequest
the DU replies in the UEContextModificationResponse with an empty
CellGroupConfig.

This CellGroupConfig is encoded as 2 zero bytes. Let's ignore this
message when we receive it, don't forward it to the UE in a
RRC Reconfiguration, as it should per standard (38.473 8.3.4.2):

    If the CellGroupConfig IE is included in the DU to CU RRC Information
    IE contained in the UE CONTEXT MODIFICATION RESPONSE message, the
    gNB-CU shall perform RRC Reconfiguration as described in TS 38.331 [8].
    The CellGroupConfig IE shall transparently be signaled to the UE as
    specified in TS 38.331 [8].

To be removed/reworked in the future if the interoperability
situation improves.
This commit is contained in:
Cedric Roux
2025-08-21 09:30:41 +02:00
committed by Robert Schmidt
parent 807b38ddae
commit 653ad775ea
1 changed files with 12 additions and 6 deletions
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openair2/RRC/NR/rrc_gNB.c
+12 -6
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@@ -2305,13 +2305,19 @@ static void rrc_CU_process_ue_context_modification_response(MessageDef *msg_p, i
uper_decode_complete(NULL, &asn_DEF_NR_CellGroupConfig, (void **)&cellGroupConfig, (uint8_t *)cgc->buf, cgc->len);
AssertFatal(dec_rval.code == RC_OK && dec_rval.consumed > 0, "Cell group config decode error\n");
if (UE->masterCellGroup) {
ASN_STRUCT_FREE(asn_DEF_NR_CellGroupConfig, UE->masterCellGroup);
LOG_I(RRC, "UE %04x replacing existing CellGroupConfig with new one received from DU\n", UE->rnti);
/* hack for interoperability with srsRAN: ignore cell group config if empty */
/* cell group config is empty if buffer length is 2 and content is all 0 */
bool cgc_is_empty = cgc->len == 2 && cgc->buf[0] == 0 && cgc->buf[1] == 0;
if (!cgc_is_empty) {
if (UE->masterCellGroup) {
ASN_STRUCT_FREE(asn_DEF_NR_CellGroupConfig, UE->masterCellGroup);
LOG_I(RRC, "UE %04x replacing existing CellGroupConfig with new one received from DU\n", UE->rnti);
}
UE->masterCellGroup = cellGroupConfig;
rrc_gNB_generate_dedicatedRRCReconfiguration(rrc, UE);
} else {
LOG_W(NR_RRC, "hack: UE %d: ignore empty CellGroupConfig in UEContextModificationResponse\n", UE->rrc_ue_id);
}
UE->masterCellGroup = cellGroupConfig;
rrc_gNB_generate_dedicatedRRCReconfiguration(rrc, UE);
}
// Reconfiguration should have been sent to the UE, so it will attempt the