Add PDU Session Release documentation

doc/E1AP/e1ap_procedures.md

@@ -72,3 +72,36 @@ sequenceDiagram
   u->>c: BEARER CONTEXT MODIFICATION RESPONSE
   Note over c: e1apCUCP_handle_BEARER_CONTEXT_MODIFICATION_RESPONSE
 ```
+
+## PDU Session Release
+
+```mermaid
+sequenceDiagram
+    participant AMF
+    participant CUCP
+    participant CUUP
+    participant DU
+    participant UE
+
+    AMF->>CUCP: NG PDU SESSION RESOURCE RELEASE COMMAND
+    Note over CUCP: ngap_gNB_handle_pdusession_release_command
+    CUCP->>CUCP: rrc_gNB_process_NGAP_PDUSESSION_RELEASE_COMMAND
+    Note over CUCP: set status PDU_SESSION_STATUS_TORELEASE
+    CUCP->>CUUP: E1 Bearer Context Modification Request
+    Note over CUUP: release_gtpu_tunnel (GTP tunnel, PDCP, SDAP)
+    CUUP->>CUCP: E1 Bearer Context Modification Response
+    Note over CUCP: rrc_gNB_send_f1_drb_release_request
+    CUCP->>DU: F1 UE Context Modification Request
+    Note over DU: handle_ue_context_drbs_release (release in MAC/RLC)
+    DU->>CUCP: F1 UE Context Modification Response
+    Note over CUCP: rrc_CU_process_ue_context_modification_response
+    Note over CUCP: replace existing CellGroupConfig
+    Note over CUCP: rrc_gNB_generate_dedicatedRRCReconfiguration
+    CUCP->>UE: RRCReconfiguration (DRB release list, NAS PDU)
+    UE->>CUCP: RRCReconfigurationComplete
+    Note over CUCP: handle_rrcReconfigurationComplete
+    Note over CUCP: rrc_gNB_send_NGAP_PDUSESSION_RELEASE_RESPONSE
+    CUCP->>AMF: NG PDU SESSION RESOURCE RELEASE RESPONSE
+    Note over CUCP: rm_drbs_by_pdusession (from stored RRC list)
+    Note over CUCP: rm_pduSession (from stored RRC list)
+```