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1
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The Source NG-RAN (S-RAN) sends the Handover Required message to the Source AMF (S-AMF). This message includes the following:
The Source to Target transparent container includes NG-RAN information for use in Target RAN (T-RAN), and is transparent to
5GC. It also contains the corresponding User Plane Security Enforcement information, QoS flows/DRBs information subject to
data forwarding.
If direct data forwarding is available, the SM N2 info includes Direct Forwarding Path Availability.
Direct Forwarding Path Availability indicates whether direct forwarding is available from the S-RAN to the T-RAN. This indication
from S-RAN is based on the presence of IP connectivity and security association between the S-RAN and the T-RAN.
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2
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When the S-AMF cannot serve the UE anymore, the S-AMF selects the T-AMF as described in clause 6.3.5 on "AMF Selection Function"
in TS 23.501.
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3
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The S-AMF initiates Handover resource allocation procedure by invoking the Namf_Communication_CreateUEContext service operation
towards the T-AMF.
The Namf_Communication_CreateUEContext Request includes the following:
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N2 Information
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UE context information
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SUPI
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Service area restriction
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Allowed NSSAI for each Access Type if available
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Tracing Requirements
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The list of PDU Session IDs along with the corresponding SMF information and the corresponding S-NSSAI(s), PCF ID(s), and
DNN
When the S-AMF can still serve the UE, this step and step 12 are not needed.
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4
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For each PDU session indicated by S-RAN, the AMF invokes the Nsmf_PDUSession_UpdateSMContext Request to the associated SMF.
However, if the S-NSSAI associated with PDU session is not available in the T-AMF, the T-AMF does not invoke Nsmf_PDUSession_UpdateSMContext
for this PDU session.
If the T-AMF detects that the UE moves into a restricted area based on Service area restrictions, the T‑AMF notifies that
the UE is only reachable for regulatory prioritized services to each NF consumer which has subscribed for UE reachability
event.
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5
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Based on the Target ID, the SMF checks the acceptance of N2 handover for the indicated PDU session. The SMF also checks the
UPF Selection Criteria. If the UE has moved out of the service area of the UPF connecting to NG-RAN, the SMF selects a new
intermediate UPF.
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6a
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If the SMF selects a new UPF to act as intermediate UPF for the PDU session, and the different CN Tunnel Info need to be used,
the SMF sends N4 Session Modification Request message to UPF (PDU Session Anchor (PSA)). If the SMF allocates the CN Tunnel
Info, it provides the CN Tunnel Info on N9, and the UPF (PSA) associates CN Tunnel Info with UL Packet detection rules.
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6b
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The UPF (PSA) sends an N4 Session Establishment Response message to the SMF. If the UPF (PSA) allocates CN Tunnel Info (on
N9) of UPF (PSA), it provides CN Tunnel Info (on N9) to the SMF. The UPF (PSA) associates the CN Tunnel Info (on N9) with
UL Packet detection rules provided by the SMF.
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6c
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If the SMF selects a new intermediate UPF (T-UPF) and if the T-UPF allocates the CN Tunnel Info, the SMF sends an N4 Session
Establishment Request message to the T-UPF. This request enables the Packet detection, enforcement, and reporting rules to
be installed on the T-UPF. The T-UPF receives the CN Tunnel Info (on N9) of UPF (PSA) for this PDU session, which is used
to set up N9 tunnel.
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6d
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The T-UPF sends an N4 Session Establishment Response message to the SMF with DL CN Tunnel Info and UL CN Tunnel Info (that
is, N3 tunnel info). The SMF starts a timer to release the resource of S-UPF, which is to be used in step 13a of the Execution
Phase.
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7
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If N2 handover for the PDU session is accepted, the SMF includes the N2 SM Information in the Nsmf_PDUSession_UpdateSMContext
response. The N2 SM Information contains the N3 UP address and the UL CN Tunnel ID of the UPF and the QoS parameters indicating
that the N2 SM Information is for the Target NG-RAN.
If the N2 SM information received at step 4 does not include the Direct Forwarding Path Availability and the SMF knows that
there is no indirect data forwarding connectivity between source and target, the N2 SM Information includes a Data forwarding
not possible indication.
If the N2 handover for the PDU session is not accepted as described in step 5, the SMF does not include the N2 SM Information
to avoid establishment of radio resources at the target NG-RAN. The SMF provides a reason for non-acceptance. If the SMF receives
notification from T-AMF that UE is only reachable for regulatory prioritized service, the SMF deactivates the PDU session.
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8
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The AMF supervises the Nsmf_PDUSession_UpdateSMContext Response messages from the involved SMFs. At the expiry of maximum
wait time or when all Nsmf_PDUSession_UpdateSMContext Response messages are received, the AMF continues with the N2 Handover
procedure (Handover Request message in step 9).
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9
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If the subscription information includes Tracing Requirements, the target AMF provides the target RAN with Tracing Requirements
in the Handover Request.
The Handover request includes Source to Target transparent container, N2 MM Information, N2 SM Information list, and Tracing
Requirements.
The T-AMF determines T-RAN based on Target ID. T-AMF allocates a 5G-GUTI valid for the UE in the AMF and target TAI.
N2 MM Information includes, for example, security information and Mobility Restriction List if available in the T-AMF. N2
SM Information list includes N2 SM Information for the T-RAN in the Nsmf_PDUSession_UpdateSMContext Response messages received
within allowed max delay supervised by the T-AMF in step 8.
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10
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The T-RAN sends Handover Request Acknowledge to the T-AMF. The Acknowledge message includes Target to Source transparent container,
List of PDU Sessions to Hand-over with N2 SM information, List of PDU Sessions that failed to be established with the failure
cause given in the N2 SM information element.
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11a
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The AMF sends Nsmf_PDUSession_UpdateSMContext Request (PDU Session ID, N2 SM response) to the SMF.
For each N2 SM response received from the T-RAN, the AMF sends the N2 SM response to the SMF indicated by the respective PDU
Session ID.
If no new T-UPF is selected, the SMF stores the N3 tunnel info of T-RAN from the N2 SM response if N2 handover is accepted
by T-RAN.
The SMF/UPF allocates the N3 UP address and Tunnel IDs for indirect data forwarding corresponding to the data forwarding tunnel
endpoints established by T-RAN.
If a PDU session is indicated as a rejected PDU session by the Target NG-RAN, the SMF triggers the release of this PDU session.
In all other cases of PDU Session rejection, the SMF decides whether to release the PDU session or to deactivate the UP connection
of this PDU session.
If some of the QoS Flows of a PDU Session are not accepted by the Target NG-RAN, the SMF initiates the PDU Session Modification
procedure to remove the non-accepted QoS Flows from the PDU Session(s) after the handover is completed.
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11b
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The SMF sends N4 Session Modification Request to the T-UPF. This request includes T-RAN SM N3 forwarding Information list,
and indication to allocate DL forwarding tunnel(s) for indirect forwarding.
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11c
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The T-UPF allocates Tunnel Info and returns an N4 Session Modification Response message to the SMF. The T-UPF SM N3 forwarding
info list includes T-UPF N3 address, T-UPF N3 Tunnel identifiers for forwarding data.
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11d
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The SMF sends N4 Session Modification Request to the S-UPF. This request includes T-RAN SM N3 forwarding Information list
or T-UPF SM N3 forwarding Information list, and an indication to allocate DL forwarding tunnel(s) for indirect forwarding.
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11e
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The S-UPF allocates Tunnel Info and returns an N4 Session establishment Response message to the SMF.
The S-UPF SM N3 forwarding Information list includes S-UPF N3 address and S-UPF N3 Tunnel identifiers for DL data forwarding.
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11f
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The SMF sends an Nsmf_PDUSession_UpdateSMContext Response message per PDU session to the T-AMF.
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12
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The AMF supervises the Nsmf_PDUSession_UpdateSMContext Response message from the involved SMFs. At the expiry of maximum wait
time or when all Nsmf_PDUSession_UpdateSMContext Response messages are received, the T-AMF sends the Namf_Communication_CreateUEContext
Response to the S-AMF.
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