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Update NAS documentation with information about NAS 5GS and OAI implementation status

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Guido Casati
2025-05-20 15:16:25 +02:00
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**5GS NAS Overview and OAI Implementation status**
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This document provides an overview of the 5G System Non-Access Stratum (5GS NAS) protocol as specified in 3GPP TS 24.501. It highlights key message types involved in Mobility and Session Management and explains how these are implemented within the OAI software stack. The document outlines the structure of the OAI codebase by detailing current support for encoding, decoding, and unit testing of specific NAS messages, and explains how the UE handles USIM simulation and message generation.
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# About 5GS NAS
The 5G System Non-Access Stratum (5GS NAS) is defined in 3GPP TS 24.501. It operates within the control plane, facilitating communication between the User Equipment (UE) and the Access and Mobility Management Function (AMF) over the N1 interface.
This tuto for 5G gNB NAS design
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Key 5GS NAS messages:
# source files
* Mobility Management (5GMM): Handles UE registration, deregistration, authentication and tracking area updates. 5GMM also manages transitions between idle and connected states.
* Session Management (5GSM): Establishes, modifies, and releases PDU sessions. Coordinates with the Session Management Function (SMF) to manage user-plane resources.
openair2/RRC/NR/nr_ngap_gNB.c: skeleton for interface with NGAP
openair3/NAS/COMMON/milenage.h: a simple milenage implementation, depend only on crypto library
openair3/NAS/COMMON/NR_NAS_defs.h: messages defined for NAS implemented in C: C struct, C enums and automatic conversion to labels for debug messages
openair3/NAS/COMMON/nr_common.c: 5G NAS common code between gNB and UE
openair3/UICC/usim_interface.c: UICC simulation for USIM messages
# OAI Implementation Status
openair3/NAS/NR_UE/ue_process_nas.c: NAS code for UE
The following tables lists implemented NAS messages and whether there is an encoder or decoder function, and if a corresponding unit test exists.
openair3/NAS/gNB/network_process_nas.c: NAS code for gNB running without 5GC
| Type | Message | Encoding | Decoding | Unit test |
|-------|-------------------------------------------|----------|----------|------------|
| 5GMM | Service Request | yes | yes | yes |
| 5GMM | Service Accept | yes | yes | yes |
| 5GMM | Service Reject | yes | yes | yes |
| 5GMM | Authentication Response | yes | no | no |
| 5GMM | Identity Response | yes | no | no |
| 5GMM | Security Mode Complete | yes | no | no |
| 5GMM | Uplink NAS Transport | yes | no | no |
| 5GMM | Registration Request | yes | yes | no |
| 5GMM | Registration Accept | yes | yes | yes |
| 5GMM | Registration Complete | yes | yes | no |
| 5GMM | Deregistration Request (UE originating) | yes | no | no |
| 5GSM | PDU Session Establishment Request | yes | no | no |
| 5GSM | PDU Session Establishment Accept | no | yes | no |
openair3/TEST/test5Gnas.c: unitary testing of NAS messages: encodes+decode a standard network entry, mixing code from UE and 5GC (or gNB in noCore)
## Code Structure
[openair3/NAS/NR_UE/nr_nas_msg.c](../openair3/NAS/NR_UE/nr_nas_msg.c):
* NAS procedures and message handlers/callbacks
* Integration with RRC (ITTI) and SDAP
* Invokes enc/dec libraries
* Handles 5GMM state and mode
[openair3/NAS/NR_UE/5GS/fgs_nas_lib.c](../openair3/NAS/NR_UE/5GS/fgs_nas_lib.c):
[openair3/NAS/NR_UE/5GS/NR_NAS_defs.h](../openair3/NAS/NR_UE/5GS/NR_NAS_defs.h):
* encoding and decoding functions for 5G NAS message headers and payloads
* relies on 5GMM/5GSM messages libs for payload encoding
[openair3/NAS/NR_UE/5GS/fgs_nas_utils.h](../openair3/NAS/NR_UE/5GS/fgs_nas_utils.h):
* NAS helpers, macros
[openair3/NAS/NR_UE/5GS/5GMM](../openair3/NAS/NR_UE/5GS/5GMM):
* encoding/decoding functions and definitions for 5GMM NAS messages payloads
[openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_lib.c](../openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_lib.c):
[openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_lib.h](../openair3/NAS/NR_UE/5GS/5GMM/MSG/fgmm_lib.h):
* encoding/decoding functions and definitions for common 5GMM IEs
[openair3/NAS/NR_UE/5GS/5GSM](../openair3/NAS/NR_UE/5GS/5GSM):
* encoding and decoding functions for 5GSM NAS messages payloads
# USIM simulation
A new USIM simulation, that parameters are in regular OAI config files
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The NAS implementation uses this possibility.
# UE side
UEprocessNAS() is the entry for all messages.
It decodes the message type and call the specific function for each messgae
## Identityrequest + IdentityResponse
When the UE receives the request, it stores the 5GC request parameters in the UE context ( UE->uicc pointer)
It calls "scheduleNAS", that would trigger a answer (seeing general archtecture, it will probly be a itti message to future RRC or NGAP thread).
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When the gNB completes RRC attach, it sends a first message to NGAP thread.
Normal processing sends NGAP initial UE message, in noCore mode, it should call: identityRequest() that encode the NAS identity request inside the gNB.
The gNB NGAP thread then should call the piece of code that forward the message to the UE as when it receives a NAS message from 5GC.
## All NAS coming from UE
When NGAP thread receives a NAS message from lower layers, it encapsulates it to forward it to the 5GC. In "noCore" mode it calls processNAS().
This function stores in the gNB the NAS data, then it should encode the next message (identity, authentication, security mode) or do nothing (registration complete).
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**Note**
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