Implement TS 38.473 transparency for CellGroupConfig forwarding and handle re-establishment on a different DU
This MR implements the following fixes that are relevant in RRC
procedures (e.g. re-establishment, reconfiguration):
1. Transparent CellGroupConfig forwarding
This MR implements TS 38.473 transparency requirements by ensuring that
the CellGroupConfig IE is transparently forwarded from the gNB-DU to the
UE without any re-encoding at the gNB-CU. The CU stores and forwards the
encoded bytes transparently, while the DU is responsible for all
encoding and modifications (e.g., reestablishRLC flags). Motivation
Motivation: The 3GPP TS 38.473 specification mandates that the
CellGroupConfig IE shall be transparently signaled to the UE.
Previously, the CU was re-encoding CellGroupConfig depending on the
scenario, instead of retrieving it from the DU, violating the
transparency requirement. CU may decode for internal use but must not
re-encode for forwarding. Solution
Solution: Core Principle: The CU stores the encoded CellGroupConfig
bytes received from the DU and forwards them directly to the UE without
any re-encoding. The DU handles all encoding and modifications.
Key Changes:
1. Transparent Forwarding in RRC Reconfiguration
- Modified build_RRCReconfiguration_IEs() to use pre-encoded bytes
directly
- Changed parameter from cell_group_config to cgc (encoded byte
array with buffer and size)
2. Encoded Bytes Storage
- Added byte_array_t mcg field to RRC UE structure to store encoded
CellGroupConfig
- Added store_cgc() helper function to save encoded cell group in
the RRC UE Context at gNB (from F1AP messages)
3. DU Encoding Responsibilities
- Add gNB-DU Configuration Query IE support to trigger cgc encoding
to request CellGroupConfig from DU
- DU encodes CellGroupConfig with proper reestablishRLC flags
during re-establishment
- DU removes SRB1 from rlc-BearerToAddModList during
re-establishment (SRB1 re-establishment is done before
RRCReconfiguration)
4. New functions
- Add rrc_detect_reestablishment() function to detect
re-establishment during UE Context Modification
- Extracted rrc_gNB_first_reconfiguration_after_reestablishment()
function to handle first reconfiguration after re-establishment
- Added dump_cgc() and dump_mcg() utility functions for debugging
purposes
5. Re-establishment Flow
- rrc_gNB_process_RRCReestablishmentComplete() requests
CellGroupConfig from DU via gNB_DU_Configuration_Query in UE
Context Modification Request
- DU detects re-establishment and add reestablishRLC IEs
- CU stores encoded CGC from the UE Context Modification Response
and forwards it transparently in subsequent RRC Reconfiguration
Testing
Manual Testing:
- RRC re-establishment procedure completes successfully
- RRC Reconfiguration after re-establishment includes all DU-provided information
Issues fixed
Closes issue #837
2. Trigger UE Context Setup fallback upon reestablishment on DU
different from the original one
As per 38.401 §8.7: if the UE accesses a DU different from the original
one during re-establishment, the CU must trigger UE Context Setup toward
the new DU. This refers to steps 9-10 and steps 9’-10’. Either one or
the other can be executed during re-establishment.
e.g. if this happens during handover, it shall trigger a UE Context
Setup fallback on the CU.
This MR implements the required behavior by:
- Adding a new helper function rrc_handle_f1_ho_reestab_on_target() to
handle F1 handover reestablishment on target DU. The function triggers
UE Context Setup procedure when the UE reestablishes on the target DU
during an ongoing F1 handover.
- Calling this function when detecting that the UE is reestablishing on
the target DU during handover, i.e.: (1) from
handle_rrcReestablishmentComplete(), as per 38.401 §8.7, NOTE on steps
9-10: CU gets UL RRC MESSAGE TRANSFER from a DU different than the
original (2) from rrc_CU_process_ue_modification_required, as per
38.401 §8.7, NOTE on steps 9'-10': CU gets a UE CONTEXT MODIFICATION
REQUIRED from a DU different from the original one.
Also:
- add helper rrc_fill_f1_ue_context_setup and adopt it in handover
- move DRB list filler fill_drb_to_be_setup into rrc_gNB.c and rename it
rrc_fill_f1_drb_to_setup for consistency
See also TS 38.401 Figure 8.7-1 "RRC connection reestablishment
procedure" for reference
3. DU: Handle missing old UE context in DL RRC Message Transfer
During F1 handover reestablishment, the CU may forward a DL RRC Message
Transfer to the target DU with old_gNB_DU_ue_id included, even though
the target DU does not have the old UE context yet.
The code previously used AssertFatal when the old UE context was not
found, causing a crash: "CU claims we should know UE %04x, but we don't"
According to TS 38.473, the gNB-DU shall release the old gNB-DU UE F1AP ID only:
> if the gNB-DU identifies the UE-associated logical F1-connection If
> there is no matching connection, there is nothing to release.
Fix:
- Check if old UE context exists before accessing it
- If old UE does not exist, log error, clean up any stale F1 UE data,
and return early (do not deliver RRC message since reestablishment
can't be completed)
The goal is to address a recurrent issue in the F1 pipeline, see
jenkins-oai.eurecom.fr/job/RAN-SA-Handover-CN5G/514:
Assertion (oldUE) failed!
In dl_rrc_message_transfer() /oai-ran/openair2/LAYER2/NR_MAC_gNB/mac_rrc_dl_handler.c:956
CU claims we should know UE 35ce, but we don't
This field will be adopted to trigger cgc encoding at DU,
according to the specs (see subsequent commit for details).
Changes:
- Add gNB_DU_Configuration_Query field to f1ap_ue_context_mod_req_t
- Implement encoding/decoding
- Add to cp, eq, and free functions for the new field
- Update tests to include the new field
Reference: 3GPP 38.473 v16.21.0
Mandatory fields:
Transaction ID (supported)
LMF Measurement ID (supported)
RAN Measurement ID (supported)
Add F1AP positioning measurement failure indication message enc/dec + test
Reference: 3GPP 38.473 v16.21.0
Mandatory fields:
Transaction ID (supported)
LMF Measurement ID (supported)
RAN Measurement ID (supported)
Cause (supported)
Reference: 3GPP 38.473 v16.21.0
Mandatory fields:
Transaction ID (supported)
LMF Measurement ID (supported)
RAN Measurement ID (supported)
Positioning Measurement Result List (range: 1)
> Positioning Measurement Result List Item (range: 1..64)
>> Positioning Measurement Result (supported)
>> TRP ID (supported)
Reference: 3GPP 38.473 v16.21.0
Mandatory fields:
Transaction ID (supported)
LMF Measurement ID (supported)
RAN Measurement ID (supported)
Cause (supported)
Optional fields:
Criticality Diagnostics (not supported)
Reference: 3GPP 38.473 v16.21.0
Mandatory fields:
Transcation ID (supported)
LMF Measurement ID (supported)
RAN Measurement ID (supported)
Positioning Measurement Result List (range: 0..1)
> Positioning Measurement Result List Item (range: 1..64)
>> Positioning Measurement Result (supported)
>> TRP ID (supported)
Optional fields:
Criticality Diagnostics (not supported)
Reference: 3GPP 38.473 v16.21.0
Mandatory fields:
Transcation ID (supported)
TRP Information List (range : 1)
> TRP Information Item (range : 1..65535)
>> TRP Information (supported)
Further details of TRP Information:
TRP ID
TRP Information Type Response List (range : 1)
> TRP Information Type Response Item (range : 1..64)
>> CHOICE TRP Information Type Response Item
>>> NR PCI (madatory : supported)
>>> NG-RAN CGI (madatory : supported)
>>> NR ARFCN (madatory : supported)
>>> PRS Configuration (madatory : not supported)
>>> SSB Information (madatory : not supported)
>>> SFN Initialisation Time (madatory : not supported)
>>> Spatial Direction Information (madatory : not supported)
>>> Geographical Coordinates (madatory : supported)
For now, we only use Geographical Coordinates.
Optional fields:
Criticality Diagnostics (not supported)
Reference: 3GPP 38.473 v16.21.0
Mandatory fields:
Transcation ID (supported)
TRP list (range : 0..1)
> TRP list Item (range : 1..65535)
>> TRP ID (supported)
TRP Information Type List (range : 1)
> TRP Information Type Item (range : 1..64)
>> TRP Information Type Item (supported)
While implementing NRPPA-related F1AP messages, we remarked that
extension field "UL-AoA-ExtIEs" needs to be OPTIONAL (e.g., in NRPPA it
is optional in the OAI-compiled version), but it was not marked as such.
This later version 16.21.0 fixes this.
Similarly to the commit "Use F1 UE context setup/modif Request in
stack", we do both UE context setup/modification response at once
because of some functions that are shared by the MAC message handler.
Use the new, tested F1 UE context setup request and modification
requests in the stack.
We have to do both at once, because there are some functions that are
used by handlers for both messages, notably
- handle_ue_context_drbs_setup()
- handle_ue_context_srbs_setup()
(+ and some functions called by those two). For instance, the first uses
the (new/old) type f1ap_drb_to_be_setup_t/f1ap_drb_to_setup_t which is
shared. The alternative would be to duplicate the functionality
temporarily, and I want to avoid unnecessary code churn.
Reintroduce separate message types to simplify the encoding/decoding,
including shorter names. The old types will be removed once everything
has been migrated to the new encoder/decoder, which should ensure that
everything is properly migrated to the unit-tested encoder/decoders.
Also, for each type of F1 message (F1 UE context setup request,
response, modification request, ...) introduce a separate type to avoid
to mistakenly use wrong fields, which can lead to bugs. In this regard,
the DL PDCP SN bit size in the DRB-ToBeSetup list of the UE context
setup request is optional, as it will also be used in the UE context
modification request DRB SetupMod list (which avoids quite some code
duplication). The encoder checks that this IE is present, though.
F1AP_LIB_FIND_IE() is used to look up elements in a list, but was
hitherto hardcoded to use "protocolIEs" as an intermediate struct. A
follow-up commit will use this macro where we don't have protocolIEs,
but "iE_Extensions". Hence, modify to make it work for that case (and
others) as well.
Introduce generic _F1_CHECK_EXP(), which is like the macros for checking
values (_F1_EQ_CHECK_INT() etc). Reuse the same for introducing
_F1_EQ_CHECK_OPTIONAL_IE that will be used to write the check for
optional fields more concisely.
CP_OPT_BYTE_ARRAY() FREE_OPT_BYTE_ARRAY() are macros to wrap handling of
optional byte arrays.
_F1_MALLOC() simplifies allocations and is similar to asn1cCallocOne(),
but provided to avoid confusion (since more than ASN.1 structures will
use it).
All will be used in the following commits.
Signal gNB-DU config update CellStatus from DU to CU
Add the cell status to gNB-DU configuration update message, including
test. Use the gNB-DU configuration update to signal the cell status in
the stack.
This IE was not included in a previous refactoring round, but is at
least necessary to be decoded when interoperating with the OSC O-DU.
This is preparatory work to allow NSA-like modes to be used over F1 in
the future. Notably, it fills the F1 message for UE context setup with
GTP TEID, and fills additional information to "make the F1 encoder
happy" (fill in QFI etc). Also, provide some F1 fixes to make this work.
Remove stale UE connections, if relevant.
Issue 947 and compilation improvements
This MR started with a small commit (03c7f151) to close#947. Little did
I know that I would be taken in a long journey through OAI compilation
hell.
Robert Schmidt