E1AP procedures documentation

- new folder dedicated to E1AP docs
- new file to document E1AP procedures

doc/E1AP/E1-design.md

@@ -75,11 +75,11 @@ sequenceDiagram
   Note over u: E1AP_CUUP_task (SCTP Handler)
   Note over u: ASN1 decoder
 ```
-
+More details about the E1AP procedures in OAI are available in this document: [E1 Procedures](./e1ap_procedures.md).
 
 # 2. Running the E1 Split
 
-The setup is assuming that all modules are running on the same machine. The user can refer to the [F1 design document](./F1-design.md) for local deployment of the DU.
+The setup is assuming that all modules are running on the same machine. The user can refer to the [F1 design document](./../F1-design.md) for local deployment of the DU.
 
 ## 2.1 Configuration File
 

doc/E1AP/e1ap_procedures.md

@@ -0,0 +1,74 @@
+<table style="border-collapse: collapse; border: none;">
+  <tr style="border-collapse: collapse; border: none;">
+    <td style="border-collapse: collapse; border: none;">
+      <a href="http://www.openairinterface.org/">
+         <img src="./images/oai_final_logo.png" alt="" border=3 height=50 width=150>
+         </img>
+      </a>
+    </td>
+    <td style="border-collapse: collapse; border: none; vertical-align: center;">
+      <b><font size = "5">E1AP Procedures</font></b>
+    </td>
+  </tr>
+</table>
+
+[[_TOC_]]
+
+# Introduction
+The E1 interface is between the gNB-CU-CP (Central Unit - Control Plane) and gNB-CU-UP (Central Unit - User Plane) nodes. This interface is governed by the E1 Application Protocol (E1AP) outlined in the 3GPP release 16 specifications, specifically in the documents:
+* 3GPP TS 38.463 - E1 Application Protocol (E1AP)
+* 3GPP TS 38.460 - E1 general aspects and principles
+* 3GPP TS 38.461 - E1 interface: layer 1 
+* 3GPP TS 38.462 - E1 interface: signaling transport
+
+The E1AP protocol consists of the following sets of functions:
+* E1 Interface Management functions
+* E1 Bearer Context Management functions
+* TEID allocation function
+
+## E1 Bearer Context Management function
+This function handles the establishment, modification, and release of E1 bearer contexts.
+* E1 Bearer Context Establishment: initiation of E1 bearer context is by gNB-CU-CP and acceptance or rejection is determined by gNB-CU-UP based on admission control criteria (e.g., resource availability).
+* E1 Bearer Context Modification: can be initiated by either gNB-CU-CP or gNB-CU-UP, with the receiving node having the authority to accept or reject the modification.
+* Release of Bearer Context: can be triggered either directly by gNB-CU-CP or following a request from gNB-CU-UP.
+* QoS-Flow to DRB Mapping Configuration: responsible for setting up and modifying the QoS-flow to DRB mapping configuration. gNB-CU-CP decides the flow-to-DRB mapping, generates SDAP and PDCP configurations, and provides them to gNB-CU-UP.
+
+# OAI implementation
+
+For the E1AP design in OAI, please refer to the [E1 Design](./E1-design.md) document.
+
+## E1 re-establishment
+
+The purpose of this procedure is to follow-up the re-establishment of RRC connection over the E1 interface. For all activated DRBs a Bearer Context Modification from CU-CP to CU-UP is necessary, according to clause 9.2.2.4 of 3GPP TS 38.463. If any modification to the bearer context is required, the CU-CP informs the CU-UP with the relevant IEs (e.g. in case of PDCP re-establishment, PDCP Configuration IE clause 9.3.1.38), Current implementation in OAI:
+
+```mermaid
+sequenceDiagram
+  participant e as UE
+  participant d as DU
+  participant c as CUCP
+  participant u as CUUP
+
+  Note over e,c: RRCReestablishment Procedure
+  e->>d: RRCReestablishmentRequest
+  Note over d: initial_ul_rrc_message_transfer_f1ap
+  d->>+c: Initial UL RRC Message Transfer (CCCH, new C-RNTI)
+  Note over c: rrc_gNB_process_initial_ul_rrc_message
+  Note over c: rrc_handle_RRCReestablishmentRequests
+  c-->>+d: DL RRC Message Transfer
+  note right of d: fallback to RRC establishment
+  d-->>-e: RRCSetup
+  e-->>d: RRCSetupComplete
+  Note over c: rrc_gNB_generate_RRCReestablishment
+  Note over c: cuup_notify_reestablishment
+  Note over c: e1apCUCP_send_BEARER_CONTEXT_MODIFICATION_REQUEST
+  c->>u: BEARER CONTEXT MODIFICATION REQUEST
+  Note over u: e1apCUUP_handle_BEARER_CONTEXT_MODIFICATION_REQUEST
+  Note over u: e1_bearer_context_modif
+  Note over u: nr_pdcp_reestablishment
+  c->>-d: DL RRC Message Transfer (old gNB DU F1AP UE ID)
+  d->>e: RRCReestablishment
+  Note over d: Fetch UE Context with old gNB DU F1AP UE ID and update C-RNTI
+  e->>d: RRCReestablishmentComplete
+  u->>c: BEARER CONTEXT MODIFICATION RESPONSE
+  Note over c: e1apCUCP_handle_BEARER_CONTEXT_MODIFICATION_RESPONSE
+```

doc/README.md

@@ -57,7 +57,7 @@ Legacy unmaintained files:
 # Designs
 
 - General software architecture notes: [SW_archi.md](./SW_archi.md)
-- [Information on E1](./E1-design.md)
+- [Information on E1](./E1AP/E1-design.md)
 - [Information on F1](./F1-design.md)
 - [Information on how NR nFAPI works](./NR_NFAPI_archi.md)
 - [Flow graph of the L1 in gNB](SW-archi-graph.md)