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Doc: update RRC re-establishment documentation

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This commit is contained in:
Guido Casati
2026-01-29 10:17:42 +01:00
parent d401cf0d31
commit 3c35bad530
1 changed files with 45 additions and 10 deletions
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doc/RRC/rrc-dev.md
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@@ -175,6 +175,12 @@ with the reestablishment procedure; if any check fails, the CU-CP tries to
release the old UE at the AMF, and continues with the "normal" connection setup
(registration/service request) described above.
The re-establishment flow implements transparent forwarding of CellGroupConfig
per TS 38.473: the CU requests CellGroupConfig from the DU via UE Context
Modification Request with `gNB_DU_Configuration_Query=true`, and the DU responds
with an encoded CellGroupConfig that the CU forwards to the UE without
decode/re-encode cycles.
```mermaid
sequenceDiagram
participant ue as UE
@@ -185,24 +191,45 @@ sequenceDiagram
ue->>du: Msg1/Preamble
du->>ue: Msg2/RAR
ue->>du: Msg3 w/ RRC Reestabishment Request (old C-RNTI/PCI)
ue->>du: Msg3 w/ RRC Reestablishment Request (old C-RNTI/PCI)
Note over du: DU creates UE context for new RNTI
du->>cucp: F1AP Initial UL RRC Msg Tr (RRC Reestab Req, in Msg3)
Note over cucp: rrc_handle_RRCReestablishmentRequest()
alt UE known and at the original DU
Note over cucp: rrc_gNB_generate_RRCReestablishment()<br />SRB1 PDCP reestablishment
cucp->>du: F1AP DL RRC Msg Tr (old gNB-DU-UE ID)
Note over du: Reuse configuration of and delete old UE<br />(in dl_rrc_message_transfer())
alt UE known
opt PCI mismatch AND Association ID mismatch
Note over cucp: Detect different DU (assoc_id != du_assoc_id)<br/>Update du_assoc_id, set f1_ue_context_active=false
end
Note over cucp: rrc_gNB_generate_RRCReestablishment()<br/>SRB1 PDCP reestablishment
cucp->>du: F1AP DL RRC Msg Tr (gNB-DU-UE ID)
Note over du: dl_rrc_message_transfer()
alt Old UE found
Note over du: Reuse configuration of and delete old UE
else Old UE not found
Note over du: do nothing
end
Note over du: Set reestablishRLC=true
Note over du: - Store CellGroup in reconfigCellGroup<br/>- Remove spCellConfig from CellGroup
du->>ue: RRC Reestablishment
ue->>du: RRC Reestablishment Complete
du->>cucp: F1AP UL RRC Msg Tr (RRC Reestab Complete)
Note over cucp: handle_rrcReestablishmentComplete()
Note over cucp: cuup_notify_reestablishment()
cucp->>cuup: E1AP Bearer Context Modification Req
cucp->>ue: F1AP DL RRC Msg Transfer (RRC Reconfiguration)
cuup->>cucp: E1AP Bearer Context Modification Resp
Note over cucp: rrc_gNB_process_RRCReestablishmentComplete()<br/>Re-establish SRB2 PDCP
Note over cucp: cuup_notify_reestablishment()<br/>Notify CU-UP for DRB re-establishment
cucp->>cuup: E1AP Bearer Context Modification Req
cuup->>cucp: E1AP Bearer Context Modification Resp (asynchronous)
alt on the original DU
Note over cucp: Send UE Context Modification Request<br/>with gNB_DU_Configuration_Query=true
cucp->>du: F1AP UE Context Modification Req
du->>cucp: F1AP UE Context Modification Resp<br/>(CellGroupConfig with spCellConfig and reestablishRLC flags)
else on a different DU
Note over cucp: Detect different DU (!f1_ue_context_active)<br/>Trigger UE Context Setup on new DU
cucp->>du: F1AP UE Context Setup Request<br/>
du->>cucp: F1AP UE Context Setup Response<br/>(CellGroupConfig with spCellConfig and reestablishRLC flags)
end
Note over cucp: rrc_CU_process_ue_context_setup/modification_response()<br/>Detect re-establishment via rrc_detect_reestablishment()<br/>rrc_gNB_generate_dedicatedRRCReconfiguration()<br/>(with is_reestablishment=true)
cucp->>ue: F1AP DL RRC Msg Transfer (RRC Reconfiguration<br/>with transparent CellGroupConfig)
ue->>cucp: F1AP UL RRC Msg Transfer (RRC Reconfiguration Complete)
Note over cucp: handle_rrcReestablishmentComplete()
Note over cucp: handle_rrcReconfigurationComplete()
else Fallback*
cucp->>amf: NGAP UE Context Release Request
Note over cucp: rrc_gNB_generate_RRCSetup()
@@ -210,6 +237,14 @@ sequenceDiagram
end
```
Note on re-establishment on different DU: The "UE known and on a different DU"
case is shown in the sequence diagram above. When a UE re-establishes on a
different DU than the original one (detected by comparing `du_assoc_id` at
RRCReestablishmentRequest), the CU updates `du_assoc_id` and sets
`f1_ue_context_active=false`. At RRCReestablishmentComplete, if
`f1_ue_context_active` is false, the CU triggers UE Context Setup on the new
DU per TS 38.401 §8.7.
### Inter-DU Handover (F1)
The basic handover (HO) structure is as follows. In order to support various