Merge remote-tracking branch 'venkatareddy453/ngap-pathswitch-and-ack-for-xnho' into integration_2026_w26

NGAP Procedures for Xn Handover: Path Switch Request Path Switch Request Acknowledge (#194)

This PR adds the following NGAP Path Switch procedures to support Xn
handover in accordance with 3GPP TS 38.413v16.2.0.

NGAP Path Switch Request: add encode and ITTI support

- Add a case in TASK_NGAP to support ITTI message from RRC to NGAP
- Add Path Switch Request message type definitions
- Implement encoder for Path Switch Request (3GPP TS 38.413v16.2.0
  §9.2.3.8)
  * RAN UE NGAP ID (M)
  * Source AMF UE NGAP ID (M)
  * User Location Information (M)
  * UE Security Capabilities (M)
  * PDU Session Resource to be Switched in Downlink List (M)
- Add required ASN.1 header includes for Path Switch Request IEs

NGAP Path Switch Request Acknowledge: add decode and handler

- Add handler at NGAP for Path Switch Request Acknowledge from AMF
- Add Path Switch Request Acknowledge message type definitions
- Implement decoder for Path Switch Request Acknowledge (3GPP TS
  38.413v16.2.0 §9.2.3.9)
  * AMF UE NGAP ID (M)
  * RAN UE NGAP ID (M)
  * Security Context (M)
  * PDU Session Resource Switched List (M)
  * Allowed NSSAI (M)
- Add required ASN.1 header includes for Path Switch Request Acknowledge
  IEs

Notes:

- Added a case NGAP_PATH_SWITCH_REQ in task_ngap to support ITTI message
  from RRC, and this will be allocated at
- RRC when the target gNB receives the RRC complete message from the UE
  during Xn handover.
- OAI CN5G does not support any of the NGAP path switch procedures
  (request, ack, and failure) as of today to test Xn Handover.
- We tested the procedures in an E2E setup at IISc with two gNBs in
  E1+F1+7.2x split containing the Xn handover code and connected to the
  same 5G core (used Open5GS & Aether-OnRamp separately), and also did
  not observe any Path Switch Failure.
- The implementation of Path Switch Failure at OAI RAN will be done
  along with the implementation of Path Switch procedures in OAI CN5G
  once the XNAP integration in OAI RAN is complete.

Acknowledgement:

> This work has been partially carried out as part of Xn Handover
development at the Indian Institute of Science (IISc), Bengaluru.

Reviewed-by: Robert Schmidt <robert.schmidt@openairinterface.org>
Reviewed-By: Guido Casati <guido.casati@openairinterface.org>

common/platform_constants.h

@@ -45,6 +45,8 @@
 
 #define NR_MAX_NB_PDU_SESSIONS (256)
 
+#define NR_MAX_NB_ALLOWED_SNSSAI (8) /* Maximum number of allowed S-NSSAI in TS 38.413 */
+
 #define MAX_DRBS_PER_UE         (32)  /* Maximum number of Data Radio Bearers per UE
                                        * defined for NGAP in TS 38.413 - maxnoofDRBs */
 #define MAX_PDUS_PER_UE         (8)   /* Maximum number of PDU Sessions per UE */