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  The User Plane Function (UPF) is one of the most important network functions (NFs) in the 5G core network. It is the second network functional unit that the Radio Network (RAN) interacts with during PDU flows in 5G (NR). As a key element in the evolution of Control Plane and User Plane Separation (CUPS), the UPF is responsible for inspecting, routing, and forwarding packets within QoS flows in subscription policies. It uses the SMF to send SDF templates via the N4 interface to enforce uplink (UL) and downlink (DL) traffic rules. When the corresponding service ends, the UPF allocates or terminates QoS flows in the PDU session.   I. User Plane Establishment When initially accessing the 5G system, the terminal (UE) needs to establish a user plane channel with the data center according to the control plane guidance for service data transmission. During this process:   When the terminal (UE) wants to access the 5G network, it first undergoes a registration process. After completing all control plane procedures, the SMF processes all session-related information during the user plane establishment phase. The AMF requests the downlink DL TEID (Terminal Equipment Identifier) ​​of all PDU sessions passed to the SMF. The SMF then selects the best UPF for the UE within the specified range and sends a session establishment request containing all parameters for the default PDU session establishment. Afterwards, a session default QoS flow (non-GBR) is created to exchange with the data network (DN) for traffic. The service traffic includes a longer route for calculating latency and maintaining traffic. Figure 1. 5G Terminal User Plane Establishment Process (Messages) [5] New UE establishment request, requires creating session context [1] Set UPF address [5] [10] Request to create session with UPF [3] Session context response [4] [5] Get default session update [3] Default QoS, AMBR [3] Add default downlink and uplink PDR rules for IMSI II. First Uplink/Downlink Data Transmission When actual data transmission (i.e., uplink or downlink data) occurs, the AMF sends another SM context request to the SMF, in which:   The SMF sends a session modification request containing information related to the requested session type. The UPF establishes a PDU session within the rules and regulations according to user requirements. The UPF then adds QoS flow mapping, sets the TEID, inserts various rules (such as PDR, FAR, URR, etc.), and some session-related policies to the PDU session. It also bills each packet exchange and adds a unique session ID to distinguish it from other PDU sessions. The UPF also adds an IMSI number to identify the UE to which the current session belongs. The session context is prepared by the UPF and sent to the AMF via the SMF, which then forwards it to the gNB. It contains information such as the UPF's local TEID, QoS context, and session release message. Figure 2.5G Terminal User Plane First Data Transmission Flow (Message) [2] QoS Policy Management (Policy Type) [2] Dynamic Rule Setting [2] Static and Dynamic Rule Update [3] Mapping FDR, PDR, QDR, BAR, URR [3] Attaching Rules to Session [3] Creating a New TEID and Inserting it into the PDR [2] Setting the TEID to be Passed to UPF [2] QoS/Bearer Management [5] Creating a Session Request [9] Updating and Creating a Session [6] Handling Rule Scheduling [7] Receiving Charging Authorization [2] Initializing Charging Credits [2] Obtaining All Active Policies [10] Setting Up UPF Session [4] Reading, Creating, Updating, and Searching for Sessions [8] Reading and Writing Sessions, and Serializing and Deserializing All Session Vectors [5] Inactive State When PDU Session Moves to Idle State [6] Handling Session Update Response [5] Process setup messages from AMF (initial request or existing PDU session) [3] Update state change notifications sent to AMF [3] Prepare responses (session context) to send to AMF for forwarding to gNB [3] Send the UPF local TEID to AMF for use by gNB [3] Send the appropriate QoS context to AMF [5] Obtain the PDU session ID from the RAT context [5] Request AMF to send a message to release the session

  Similar to previous generations of mobile communication, the services supported by the terminal (UE) are stored in the core network. The UE can only be executed by the radio network after completing authentication and encryption actions upon power-on. In 5G (NR) systems supporting NSSF (Network Slice Selection Function), after "RRC connection establishment, UE context, UE ID allocation, and security authentication," the terminal (UE) will obtain specific subscription data based on the activation status and perform user plane settings. The specific process is as follows:   I. Subscription Data Acquisition: The AMF searches for the NSSF (Network Slice Selection Function) through the N22 interface to select the best available network slice for the user's requested service. Then, it searches the UDM to retrieve all subscription data related to AM (Access Management), SM (Session Management), and UE (Terminal). The AMF connects to the UDM via the N10 interface to obtain subscription data. The process (message) is as follows: [21] Fill in the slice information in the PDU session establishment accept message [8] Obtain the AMF context based on the UE identifier [8] Obtain the SMF context from the mapping [20] Set the SMF context in the AMF context [8] The AMF creates a new UE context   ---The AMF configures the PCF (Policy Control Function) to retrieve the AM policy through the N15 interface accessible to the UE, and the SMF allocates services accordingly.   ---The AMF has collected all UE contexts, and now it creates another identifier for the UE, the AMF UE NGAP ID, to add it to the network.   II. User Plane Setup The AMF selects the SMF (which performs all session management operations in the 4G system MME (as well as SGW-C and PGW-C)) to manage all session management operations itself. Message exchange between the AMF and SMF is conducted through the N11 interface. The SMF then finds the best UPF (User Plane Function) for the UE and creates a session during the UL and DL data streams. Interaction between SMF and UPF is performed via PFCP (Packet Forwarding Control Protocol) on the N4 interface; the specific process (message) is as follows:   [3] Check the session ID of the existing PDU session [3] Send a PDU session establishment acceptance message to the UE and gNB [3] Send a PDU session resource establishment request message to the gNB [4] Process the PDU session resource establishment response [4] Process the PDU session resource release response [20] AMF processes PDU session establishment rejection [20] Send a PDU session rejection message to the UE [3] Set the session AMBR [20] Update the IP address information in the SMF context and send a downlink transmission message with a 5GMM reason to the gNB [3] [5] Retrieve the user QoS profile and UPF GTP TEID IP address from the SMF context [1] Send an activation PDU session context request message [5] Add a security header to the AMF PDU session transmission request [3] [6] Generate a new AMF NGAP UE ID [8] Notify NGAP of the new AMF NGAP ID

  Since 4G (LTE), mobile communications have implemented encryption and integrity protection during terminal (UE) access to ensure personal privacy and security during communication. The specific processes for these, along with service resources and data transmission, in the 5G (NR) system are as follows:   I. AS Security and RRC Reconfiguration: First, the AMF sends a UE Initial Context Establishment Request and Registration Acceptance Message to the gNB to update the UE context existing in the gNB. The gNB then performs the RRC reconfiguration and SMC procedures so that the UE can access the encrypted channel using derived keys (e.g., k-gNB, k-RRC, k-UP-int).   [17] AMF sends SAP [1] Update the GUTI assigned to AMF SAP [9] Process AMF AS SAP connection establishment request [9] [16] Process AMF AS SAP connection establishment rejection [9] Process AMF AS SAP connection establishment confirmation [18] Notify AMF AS SAP that it needs to send a security mode command message to the UE [9] Process AMF AS SAP security request primitive [17] Set security request when data is transmitted to the lower layer [1] Notify AS SAP that registration is rejected [10] Obtain a new security context from the upper layer [23] Encrypt/decrypt/decode Layer 3 NAS message [8] Register UE context [1] Execute registration signaling process [1] Process registration completion message [1] AMF sends registration acceptance message   II. Uplink (downlink) data transmission When the user plane is set to uplink or downlink purpose, the PDU session update message is transmitted from AMF to SMF. The specific process is as follows;   [3] Transfer gNB IP and TEID are stored in the corresponding SMF context [3] Session creation response message received from SMF [3] Prepare and send gN establishment response message to SMF via gRPC [9] QoS flow establishment list [20] Function to check if the maximum number of PDU sessions has been reached

In the 5G (NR) protocol stack, RRC (Radio Resource Control) is Layer 3, specifically responsible for the control and management of radio resource connections between the UE (UE) and gNB (gNB), including: establishing and managing connections, broadcasting system information, and processing mobility radio bearer configuration. 5G terminal RRC connections have three states: RRC_IDLE, RRC_CONNECTED, and RRC_INACTIVE; "RRC_INACTIVE" was introduced to improve battery efficiency and speed up reconnection.   I. RRC Connection Establishment Process: As shown in Figure (1), after power-on, the terminal (UE) initiates the establishment of an RRC connection with the gNB; subsequently, the gNB sends an initial NAS message to the AMF via the N2 interface, containing the RAN UE NGAP ID, UE context registration request, location information, 5G S-TMSI, and the reason for RRC establishment. Figure 1. RRC establishment process of 5G terminal (UE)   II. Initial NAS message + UE context reacquisition These parameters are the identity provided for the terminal (UE) to help the AMF obtain the UE context from the old serving AMF or by re-executing the entire process (only when the serving AMF cannot find traces of the old AMF); the whole process is completed through the N14 interface, and the specific process (message) is as follows: Figure 2. Initial NAS message and UE context of 5G terminal (UE)   [8] Release the previous registration request context [3] gNB sends the initial NAS message through the new RRC connection [23] Decode the security-protected NAS message [3][9] Process the NGAP initial UE NAS message [4] Process the initial UE message from NGAP [9] Mobility management message [16] Store the registration type in the parameters [1] Create the registration request process [9] Encode the initial NAS information message [7] Process the NAS encoded message and send it to the NGAP task [23] Decode the plain text NAS message [8] Check if there are old parameters (e.g. UE context (GUTI, IMSI, gNB ID, etc.) [3] Update AMF UE context with new gNB UE NGAP ID. Assuming the new AMF cannot find any old AMF traces in the network, it will be unable to close the NR call process. At this time, the AMF will begin identity, authentication, and security procedures for the UE in order to add a more explicit identity to the UE.

  The Access and Mobility Management Function (AMF) is a Control Plane (CU) unit in the 5G core network (CN). In a wireless network, a gNodeB must connect to the AMF before it can access 5G services. The AMF is also the only Network Functional Unit (NF) (excluding interactions with the User Plane Function (UPF) during PDU session establishment) that allows the gNodeB to communicate with the 5G core network.   I. Extended MME AMF: The AMF in 5G performs most of the functions of the MME (Mobility Management Entity) in 4G. The establishment of the terminal (UE) PDU session is performed by the Session Management Function (SMF) unit, while authentication and security-related functions are performed by the Authentication Server Function (AUSF) in 5G; thus achieving the separation of the control plane and user plane in the 5G architecture. II. AMF Functions: Its functions are defined in relevant 3GPP protocols as including:   1. Registration Management – ​​The AMF manages the registration and deregistration of the terminal (UE) in the 5G system; the terminal (UE) must complete the registration process to access 5G services. 2. Connection Management - Establishes and releases control plane (CP) signaling connections between the UE and AMF via the N1 interface. 3. Mobility Management - The AMF updates the UE's location in the network. This is achieved through the UE's periodic registration. 4. NGAP Signaling Flow - Includes paging procedures, NAS message transmission, PDU session management, UE context management, and other message transmissions.   III. 5G (NR) System Internal Interfaces (Functions) N1/N2: The AMF obtains all connection and session-related information from the UE through the N1 and N2 interfaces. N8: All user and specific UE policy rules, session-related subscription data, user data, and any other information (such as data exposed to third-party applications) are stored in the UDM. The AMF retrieves the UDM through the N8 interface. N11: This interface represents a trigger for adding, modifying, or deleting PDU sessions through the AMF on the user plane. N12: This interface simulates an AUSF within the 5G core network and provides services to the AMF through the AUSF-based N12 interface. 5G networks represent service-based interfaces, focusing on AUSF and AMF. N14: This reference point is located between two AMFs (Access and Mobility Management Functions). UE context is transmitted through this interface during handover and other processes. N15: The transmission and removal of access and mobility policies are performed through the N15 interface between the AMF and PCF. N17: An emulated Device Identity Register (EIR) is created within the 5G core network and provided to the AMF via an interface based on N5g-EIR services. This interface supports device identity verification services. N22: The AMF selects the best Network Function (NF) in the network using the NSSF. The NSSF provides Network Function Location Information to the AMF through the N22 interface. N26: This interface is used to transmit UE authentication and session management context when the UE handovers between 5G and 4G (EPS).

In 5G (NR), AMF units do not need to be interrupted or restarted when making configuration changes or updates; they only need to notify the relevant network units. For mobile terminals (UEs) within their coverage area, the changes will be notified via the gNB in ​​the radio network, and the AMF will determine whether the UE needs to re-register with the AMF. The update definition process is as follows:   I. Configuration Update Process: As shown in Figure (1), the AMF determines whether the UE needs to reconfigure or register with the AMF based on the changes. That is, when the AMF detects a change in the configuration previously sent to the UE, it will initiate the configuration update process. In response to the UE's confirmation request, the AMF will send configuration update completion information to the AMF.   Figure 1. AMF Configuration Update Notification Flowchart   II. AMF Configuration Update Interface (Message)   [12] Construct Downlink RAN ​​Configuration Transmission [13] Send Downlink RAN ​​Configuration Transmission [12] Construct Downlink RAN ​​Status Transmission [13] Send Downlink RAN ​​Status Transmission [12] RAN Configuration Update Failed [13] Send RAN Configuration Update Failed [12] RAN Configuration Update Confirmation [13] Send RAN Configuration Update Confirmation [7] Construct Configuration Update Command [8] Send Configuration Update Command [12] Construct Downlink UE-Associated NRPPA Transmission [13] Send Downlink UE-Associated NRPPA Transmission [12] Construct Downlink Non-UE-Associated NRPPA Transmission [13] Send Downlink Non-E-Associated NRPPA Transmission [9] Configuration Update Complete [12] Construct AMF Configuration Update [13] Send AMF Configuration Update

The AMF unit plays a crucial role in the 5G core network; it is responsible for processing NAS messages transmitted transparently through the RAN (gNB) from the terminal (UE). The registration and authentication and mobility management of the terminal (UE) during initial access are completed by the AMF independently or jointly with other relevant network elements, as follows:   I. The order of AMF interface and message usage for 5G terminal authentication is shown in Figure (1); Figure 1. Message usage order of UE authentication AMF interface in 5G.     [11] UE authentication request [11] UE response [17] NRF discovery AUSF [25] Initialize SCP NF instance [11] NAMF Nausf authentication request [11] 5gAKA [11] Av5gAka contains authentication vector 5gAKA method [11] Amf_ue->SUCI [11] 5g AKA confirmation URL [11] SEAF starts authentication process [11] SUPI and Kseaf [11] Authentication successful [11] (or) Authentication failed   II. Mobility Management 5G networks provide high-speed and reliable connectivity for mobile users and devices, including vehicles, smartphones, and IoT devices. During mobility, the AMF is responsible for the transmission and processing of terminal-related information. Its interface (protocol) is used as follows; Figure 2. Order of AMF interface messages used when the UE moves in 5G   [5] Process registration request [5] UE sends initial NAS message to AMF [5] Set 5GS registration type: KSI, TSC [5] AMF new GUTI [5] Copy stream number, NR-TAI, NR-CGI from ran_ue [5] Check TAI[5] The algorithm selected by AMF should be the same as the NAS security algorithm [5] 5GMM request accepted [5] 5GMM processes registration update [5] 5GMM processes service request [6] The initial NAS service request message should contain security header type, ngKSI, TMSI and security header type [6] 5GMM processes service update[17] NRF discovers AUSF [25] Initialize SCP NF instance [5][6] AMF NAUSF authentication response, then confirm [5] Identity response SUCI[6] 5GMM status registered [13] NGAP handles path switching request [13] NGAP handles switching request [13] NGAP handles switching notification [13] NGAP handles Ran configuration update [5][6] 5GMM handles UL NAS transmission [5] 5GMM handles deregistration request [5] Set 5GS deregistration type [5] AMF sbi release all sessions [5] Clear paging information [5] Clear SM context [5] Unassociate NG with NAS  

  The UPF (User Plane Function) is one of the most important units in 5GC. It is a key unit that the Radio Network (RAN) interacts with during PDU data transmission. The UPF is also an evolution of CUPS (Control Plane and User Plane Separation), responsible for inspecting, routing, and forwarding packets within QoS flows in subscription policies. It uses SDF templates sent by the SMF through the N4 interface to enforce uplink (UL) and downlink (DL) traffic rules. When the service ends, it will allocate or terminate the QoS flow in the PDU session; the order of use of UPF interface session update and deletion is as follows; please refer to the order of use of UPF interface (protocol) and terminal call in 5G.   I. Session Modification Terminal-specific QoS flow is allocated through the PDU session modification process; additional dedicated QoS flow supports traffic with higher QoS requirements (such as voice, video, game traffic, etc.); the application of session modification (update) in UPF is shown in Figure (1); Figure 1. UPF interface usage order of terminal session modification (update) in 5G   [6] N4 processes session modification request [6] Remove existing PDR [6] Update PDR [6] Update FAR [6] Update URR [6] Update QER [6] Update BAR [6] Set up GTP node [6] Set up N3 TEID and QFI [6] [7] PFCP sends session modification response [5] N4 constructs session modification response [5] PFCP request accepted [5] PDR buffer initialized [5] PDR has been created [6] Send buffered data packets to gnB (if necessary) II. Session deletion When the terminal service session ends, QoS flow will be allocated or terminated in the PDU session. The session deletion usage order in the UPF interface is as follows: Figure 2.5G Terminal deletion UPF related interface usage order   [6] N4 processes session deletion request [6][7] PFCP sends session deletion request [5][1] Session URR usage status full report [1] Last report timestamp [1] Time trigger [1] Quota validity period report [1] Capacity trigger [1] Capacity quota report [5][1] UPF session URR snapshot (total bytes, total data packets, including uplink and downlink) [6][1] UPF session deletion [1] UPF session URR account all deletion: validity period deletion, quota time deletion, threshold time deletion. [13]PDR all deleted [13]FAR all deleted [13]URR all deleted [14]QER all deleted [13]BAR all deleted [13]From SEID

User Plane Function (UPF) is one of the most important Network Functions (NFs) in the 5G core network. It is the second network function that the NR RAN interacts with during PDU flows. UPF is an evolution of CUPS (Control Plane Separation from User Plane), specifically responsible for inspecting, routing, and forwarding packets within QoS flows in subscription policies. It also uses SDF templates sent by the SMF through the N4 interface to enforce UL (Uplink) and DL (Downlink) traffic rules; when the corresponding service ends, it allocates or terminates QoS flows in the PDU session.   Figure 1.5G SMF and its interface (protocol)   I. UPF Interfaces and Protocols include the following: N4[5] After the user plane is established, the session management context and necessary parameters are transmitted from the single-mode fiber (SMF) to the user plane function (UPF). PFCP[7] Any communication between the SMF and UPF is managed by the packet forwarding PFCP (control protocol); it is one of the main protocols separating the user plane and the control plane. GTP[3] The GPRS tunneling protocol (GTP) is responsible for providing seamless interconnection and carrying traffic between roaming or home users and key network interfaces in 4G, NSA (5G non-standalone), SA (5G standalone), and mobile edge computing architectures. In 5G, GTP tunnels are also used for the N9 interface. II. Call Flow (Session Establishment and UPF Initialization) During PDU session establishment, the SMF connects to the UPF via PFCP (N4 interface). This PFCP session carries an SDF template containing information such as PDR, QFI, URR, and FAR. The UPF will allocate a default QoS (non-GBR) flow during the initial session establishment.   III. Terminal (UE) Call Interface Usage Sequence [6] N4 processes session establishment request [6] PFCP processes PDR creation [6] [12] Check existing PDI of PDR [6] [12] Check TEID [6] [12] Check source interface [6] [12] Check previous SDF filter ID [6] [12] Set all filter flags: BID, FL, SPI, TTC, FD [6] PFCP processes FAR creation [6] Create URR [6] Create BAR [6] Create QRR [6] Set N3 TEID and QFI [4] UPF Initialization [4] PFCP Context Initialization [1] Initialize UPF Context [1] Set User Plane Functional Characteristics: FTUP, EMPU, MNOP, VTIME, UPF Attribute Length [6] [7] Session Establishment Response [5] N4 Build Session Establishment Response [5] Node ID [5] PFCP Request Accepted [5] F-SEID [5] PDR Existence Checked [5] PFCP Build Message FTUP: The UP function supports the allocation/release of F-TEID. EMPU: The UP function supports sending end-of-file packets. MNOP: The UP function supports measuring the number of packets in the URR, which is performed via the "Measure Number of Packets in URR" flag. MNOP (Packet Count Measurement): When set to "1", it indicates that in flow-based measurements, in addition to measuring in bytes, the uplink/downlink/total number of packets transmitted is also requested. VTIME:UP functionality supports the quota validity period feature. If UP functionality supports the VTIME feature, it requests UP functionality to send a usage report after the validity period expires. After the quota validity period expires, if data packets are received on the UPF, the UPF should stop forwarding data packets or only allow forwarding of limited user plane traffic, depending on the operator's policy in the UP functionality. Abbreviations: FL: Flow Tag TTC: TOS (Traffic Category) SPI: Security Parameter Index FD: Flow Description BID: Bidirectional SDF Filter

1. In a 5G system, one function of SMF (Session Management Function) is to be responsible for the transmission of user control plane (CP) information; it works with UPF to manage the relevant context of terminal sessions; it is responsible for creating, updating and deleting sessions, and assigning IP addresses to each PDU session, providing all parameters and supporting various functions of UPF; the interface between SMF and other network elements is shown in Figure (1).   *Figure 1. Schematic diagram of SMF connection with other network elements (solid lines in the figure represent physical connections, and dashed lines represent logical connections).   II. Application protocols in SMF include: PFCP[2]: All communication between SMF and UPF is managed by PFCP (Packet Forwarding Control Protocol); it is one of the main protocols separating the user plane and the control plane. UDP[3]: User Datagram Protocol, a transport layer protocol that provides source and destination port addressing for multiplexing/demultiplexing of higher-level applications. This protocol is responsible for data transmission between gNB and UPF. SBI[4] (Service-Based Interface): This is an API-based communication method between network functions.   III. Terminal Session Call Flow During 5G terminal session establishment: First, the SMF registers with the NRF to locate other network functions. If a user wants to access 5G data services, a PDU session must be established with the network. The UE sends a PDU session establishment request to the core network (i.e., the AMF). The AMF selects the best SMF in the network to maintain its session-related information. After selecting the best SMF, it requests the SMF to create an SM context. The SMF obtains SM subscription data from the UDM and generates an M context. Then, the SMF and UPF initiate the PFCP session establishment process and set default values ​​for session-related parameters. Finally, the AMF sends session information to the gNB and UE to establish the default PDU session value.   Session establishment interface uses (sequential) message content: [22] Send NF registration [22] Retry sending NF registration [6] Set NF configuration file [22] Send NF discovery service AMF [5] Process PDU session establishment request [4] Build GSM PDU session establishment rejection [30] Send PDU session establishment rejection [28] HTTP POST SM context - Receive Create SM context [31] Process PDU session SM context creation [22] Send NF discovery UDM [27] Get SM context [10] Build/Set created data [2] Initialize SMF context [2] Get DNN information [4] Build GSM PDU session establishment acceptance [22] Send NF discovery PCF [10] PCF selection [24] Send SM policy association creation [29] SM policy in application decision [16] Create UPF list for selection [16] Sort UPF list by name [16] Select UPF and assign UE IP [15] Select UPF by DNN [16] Get UPF name by IP [16] Get UPF node ID by name [16] Get UPF node by IP [16] Get UPF ID by IP [18] Construct PFCP association establishment request [17] Process PFCP association establishment request [19] Send PFCP association establishment request [18] Construct PFCP session establishment request [19] Send PFCP session establishment request [20] Send PFCP request [18] PFCP creates PDR, FAR, QER, BAR [10] Add PDR to PFCP session [13] [16] Generate default data path [16] Generate data path [15] Add data path [15] Generate Terminal Equipment Identifier (TEID) [2] [10] Assign Local System Equipment Identifier (SEID) [10] Select session rule [15] Select UPF parameters [15] Add PDR, FDR, BAR, QER [29] Process session rule [3] Activate tunnel and PDR [3] Activate uplink/downlink tunnel [16] Select uplink path source [30] Activate UPF session [30] Establish PFCP session [18] Build PFCP session establishment response [19] Send PFCP session establishment response [20] Send PFCP response [18] Build PFCP association establishment response [19] Send PFCP association establishment response [2] Get user plane information [16] Get default user plane path through DNN and UPF [3] Get UPF ID, node IP, UL PDR, UL FAR [3] Copy the first data path node [25] Get UE PDU session information through HTTP [15] Get interface to get UPF interface information [15] Get UPF node through node ID [15] Get UPF IP, ID, PDR ID, FAR ID, BAR ID, QER ID [2] Get UE default path pool [30] Notify UE - send all data paths to UPF and send the results to UE [10] Send PDU address to NAS [12] Create UE data path node [2] Initialize SMF UE routing [7] Build PDU session resource establishment request transmission [8] Handle PDU session resource establishment failure transmission [8] Handling PDU session resource establishment response transmission