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Content for  TS 23.548  Word version:  19.3.0

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6.10 Support of the Local Offloading Management  6.10.1 General  6.10.2 Procedure  6.10.2.1 General  6.10.2.2 I-SMF insertion during the PDU Session establishment  6.10.2.3 EAS Discovery Procedure with EASDF for Local Offloading Management  6.10.2.4 UE mobility for PDU session supporting Local Offloading Management  6.10.3 Policy provisioning for Local Offloading Management  6.10.3.1 General  6.10.3.2 Local offloading management policy handling 
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6.10  Support of the Local Offloading Management |R19|  p. 76

6.10.1  General  p. 76

In edge computing deployment, to reduce the impact on a centrally deployed SMF on management of the edge computing related information, 5GS may support the Local Offloading Management.
In Distributed Anchor Point and Multiple PDU Sessions connectivity models (see clause 4.3), the existing means such as URSP and SSC mode are used to support Local Offloading Management.
In Session Breakout connectivity model, the Local Offloading Management only applies to the non-roaming PDU session. For Session Breakout connectivity model, if the AMF determines that the UE is within the Local Offloading Management service area and Local Offloading Management is allowed in the SMF selection subscription data, AMF selects/re-selects an I-SMF to support Local Offloading Management. This also includes the case when the UE is within the SMF service area. For the Local Offloading Management, the I-SMF is in charge of edge computing handling and supports following functionalities:
  • EASDF configuration and DNS message handling, as defined in clauses 6.2.1 and 6.2.3, instead of SMF.
  • EASDF discovery and selection is performed by the I-SMF instead of SMF as described in clause 6.3.23 of TS 23.501.
  • Management and retrieval of EDI from NEF as described in clause 6.2.3.4.
  • Receiving of the Local Offloading Information from PCF via SMF, which indicates IP range(s) and/or FQDN(s) allowed to be routed to the local part of DN.
When the I-SMF is inserted for the Local Offloading Management, i.e. the inserted I-SMF providing Local Offloading allowed indication to SMF, the SMF does not discover, select and configure EASDF, as well as DNS message handling for the locally offloaded traffic towards the local part of DN.
The UE is unaware of whether the Local Offloading Management is applied or not for a PDU Session.
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6.10.2  Procedure  p. 76

6.10.2.1  General  p. 76

This clause describes the procedures related to Local Offloading Management.

6.10.2.2  I-SMF insertion during the PDU Session establishment  p. 77

Reproduction of 3GPP TS 23.548, Fig. 6.10.2.2-1: Procedure for PDU Session establishment supporting Local Offloading Management
Figure 6.10.2.2-1: Procedure for PDU Session establishment supporting Local Offloading Management
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Step 1.
During the Registration procedure, the AMF receives the Local Offloading Management allowed indication per DNN/S-NSSAI as part of SMF selection subscription data from the UDM in the step 14b of the procedure in the clause 4.2.2.2.2 of TS 23.502.
If Local Offloading Management is allowed for the requested DNN/S-NSSAI in the step 1 of clause 4.3.2.2.1 of TS 23.502, the AMF may discover SMF(s) that support Local Offloading Management. The AMF may use the current location of the UE as query parameter in the discovery. For the discovered SMF(s), the AMF executes as follows:
  • If the UE is not within the local offloading management service area of any of the candidate SMF(s), then an I-SMF for local offloading management is not needed and step 2a is executed.
  • If the UE is within the local offloading management service area of at least one of the candidate SMF(s), then AMF selects an I-SMF for local offloading management supporting the Local Offloading Management service area and step 2b is executed.
The AMF may also have the Local Offloading Management service area configured.
Step 2a.
Same as the step 1-6 defined in clause 6.2.3.2.2, the SMF selects an EASDF and instructs the EASDF to create DNS context for PDU session and steps 3 and 4 are skipped.
Step 2b.
The AMF selects an I-SMF that serves the UE location and local offloading management service area and supports Local Offloading Management. The PDU Session Establishment procedure as described in clause 6.7.2.2 is used with the following differences:
  • This procedure applies for non-roaming PDU session and all NFs are located in HPLMN.
  • The V-SMF and V-UPF are replaced by I-SMF and I-UPF, respectively.
  • The H-SMF and H-UPF are replaced by SMF and UPF(PSA), respectively.
  • The AMF sends a Local Offloading Management allowed indication to the I-SMF in the step 3a of clause 4.3.2.2.2 of TS 23.502 with V-SMF replaced by I-SMF.
If the I-SMF receives the Local Offloading Management allowed indication from AMF, the I-SMF:
  • Selects UL CL/BP UPF and L-PSA UPF based on UE location information.
  • Selects an EASDF.
  • Obtains the EASDF IP address based on local configuration, or invoke Neasdf_DNSContext_Create Request including the DNN, S-NSSAI and the UE IP address set to unspecified address or to a mapped address as specified in clause 7.1.2.2 to obtain the EASDF IP address.
  • May obtain EASDF DNS security information based on local configuration or via interaction with EASDF.
  • Sends Nsmf_PDUSession_Create Request (Local Offloading Management allowed indication, Offload Identifier(s) (if available, e.g. by configuration), IP address and (if exists) DNS security information of EASDF and I-SMF supported DNAI(s)). See step 6 of the procedure in Figure 4.3.2.2.2-1 of TS 23.502 with V-SMF, H-SMF replaced by I-SMF, SMF respectively.
When the SMF receives the Local Offloading Management allowed indication and allows I-SMF to perform Local Offloading Management based on SM subscription data, the SMF:
  • Skips the EASDF Discovery and Selection procedure and DNS message handling.
  • Provides Local Offloading Management allowed indicator to PCF during SM Policy Association Establishment procedure, whereby PCF provides the Local Offloading Management Policy, (i.e. FQDN/IP range(s) allowed to be routed to the local part of DN), or Offload Identifier, to SMF, as described in clause 6.4 of TS 23.503. Each Local Offloading Management Policy may also be provided with an Offload Identifier which is assigned by PCF.
  • Sends the received Local Offloading Management Policy and/or Offload Identifier to the I-SMF. SMF may send the Offload Identifier(s) alone or together with the Local Offloading Management Policy:
    • If the given I-SMF has already received the Local Offloading Management Policy corresponding to certain Offload Identifier(s), this could be indicated to the SMF in any subsequent request to another PDU Session supporting local offloading from the same I-SMF, and the SMF will in this case send only the Offload Identifier(s) as a response;
    • If the Local Offloading Management Policy for a given Offload Identifier is changed, the PCF invokes SM Policy Association Modification procedure to update Local Offloading Management Policy to SMF, together with associated Offload Identifier. For each I-SMF using the Offload Identifier, the SMF chooses one existing PDU Session applied local offloading using the Offload Identifier to update Local Offloading Management Policy and corresponding Offload Identifier to the I-SMF via PDU Session Modification procedure as described in clause 4.3.3.3 of TS 23.502;
    • During PDU Session Release procedure as described in clause 4.3.4.3 of TS 23.502, if the PDU Session is the last local offloading PDU Session using a given Offload Identifier on the I-SMF, the I-SMF may remove the Offload Identifier and corresponding Local Offloading Management Policy unless the Offload Identifier is configured at the I-SMF.
  • provides the DNS server address to be used for DNS requests related with traffic not subject to Local Offloading Management.
  • provides the PSA UPF address information (e.g. PSA UPF IP address on N6) to be used by EASDF selected by I-SMF to build EDNS Client Subnet option for target FQDN(s) of the DNS queries not subject to Local Offloading Management.
  • Sends IP address and optionally DNS security information of the EASDF received from the I-SMF to the UE via PCO.
  • Sends Local Offloading Management Allowed indication to the I-SMF.
Step 3.
The I-SMF configures the EASDF with the DNS message handling rules using the Local Offloading Control Information received from SMF.
If the I-SMF obtains the EASDF IP address in step 2, the I-SMF invokes Neasdf_DNSContext_Create Request to configure the EASDF.
The I-SMF configures the UL CL UPF to forward IP packets as per the Local Offloading Management Policy or Offload Identifier (which should include forwarding of IP packets to the EASDF) to the local PSA UPF.
Step 4.
EAS Discovery procedure with EASDF is performed as described in clause 6.10.2.3.
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6.10.2.3  EAS Discovery Procedure with EASDF for Local Offloading Management  p. 79

The EAS Discovery Procedure with EASDF for Local Offloading Management follows the same procedure as defined in clause 6.7.2.3, with the following differences:
  • General:
    • VPLMN is replaced by HPLMN;
    • V-EASDF is replaced by EASDF;
    • H-UPF is replaced by UPF;
    • H-SMF is replaced by SMF;
    • V-SMF is replaced by I-SMF;
    • V-UPF is replaced by I-UPF;
    • VPLMN Specific Offloading Information is replaced by Local Offloading Management Information.
  • Step 1:
    • Clause 6.7.2.2 is replaced by clause 6.10.2.2;
    • For option a), the I-SMF configures EASDF with the DNS server (provided by SMF) in the central part of the DN for FQDNs that are unauthorized (i.e. the target FQDN of the DNS query from UE is not part of FQDN contained in the Local Offloading Management information) for Local Offloading Management in the local part of the DN and EASDF may provide an EDNS Client options related to the PSA UPF in messages sent to this DNS server.
  • Step 3:
    • NOTE 2 and NOTE 3 are not applicable.
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6.10.2.4  UE mobility for PDU session supporting Local Offloading Management  p. 79

For a PDU session supporting Local Offloading Management, during mobility events, e.g. Hand-Over or AMF change, the mobility handling is executed same as the respective mobility handling defined in clause 4.23 of TS 23.502 and clause 6.7.2 with the following difference:
  • The retrieval of the Local Offloading Management Allowed indication either as part of UE context where source AMF provides the target AMF with whether Local Offloading Management is allowed for each PDU session of the UE or the SMF selection subscription data from UDM, the new AMF determines the new I-SMF as follows:
    • If the service area of SMF includes the new UE location and the UE location is out of local offloading management service area, the AMF removes the old I-SMF (if available).
    • If the service area of SMF does not include the new UE location and the UE location is out of local offloading management service area, the AMF follows the procedure defined in clause 4.23 of TS 23.502 without supporting Local Offloading Management with the following differences:
      • If the old I-SMF cannot serve the new UE location, the AMF performs I-SMF change procedure described in Figure 4.23.7.3.2-1 of TS 23.502. During the handover execution phase per clause 4.23.7.3.3 of TS 23.502, the new I-SMF invokes Nsmf_PDUSession_Update Request to the SMF. Then, the SMF sends PCO to the UE including central DNS server address or EASDF IP address selected by SMF.
      • If the old I-SMF can serve the new UE location, the AMF may keep the old I-SMF and send Nsmf_PDUSession_UpdateSMContext Request without local offloading management allowed indication. In this case, the old I-SMF deletes the DNS Context from the old EASDF and sends Nsmf_PDUSession_Update Request without local offloading management allowed indication to the SMF so that the SMF sends PCO to the UE including central DNS server address or EASDF IP address selected by SMF.
    • If the service area of SMF either includes or does not include the new UE location and the UE location is within local offloading management service area, the AMF changes the existing I-SMF with the new I-SMF that serves the local offloading management service area unless the existing I-SMF can serve the new UE location and local offloading management service area. If there is an old I-SMF which cannot serve the new UE location, the AMF also removes the old I-SMF. When the new I-SMF interact with SMF, if local offloading management allowed indication is received from AMF, the new I-SMF exchanges with the SMF the information as defined in clause 6.10.2.2.
    • To support the local offloading management, the selected I-SMF needs to support Local Offloading Management, serves the new UE location and local offloading management service area; and support the DNN and S-NSSAI associated with the PDU session.
  • For I-SMF insertion/change/removal/reuse, the procedure described for intra (V-)PLMN HO in clauses 6.7.2.6, 6.7.2.7, 6.7.2.8, 6.7.2.9 and 6.7.2.10 is performed with the following differences:
    • removing V-PLMN, H-PLMN, and steps defined for serving PLMN change case;
    • replacing:
      • V-SMF with I-SMF;
      • H-SMF with SMF;
      • V-EASDF with EASDF;
      • V-UPF with I-UPF;
      • HR-SBO with Local Offloading Management (including allowed indication, policy and offload identifiers);
    • enabling the anchor SMF to send PCO to the UE; and
    • not using PLMN ID in Neasdf_DNSContext_Create Request from I-SMF to retrieve EASDF IP address.
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6.10.3  Policy provisioning for Local Offloading Management  p. 80

6.10.3.1  General  p. 80

For Local Offloading Management, traffic routing related policy (e.g. Local Offloading Management Policy) need be provided to the I-SMF. AF provides traffic influence request as defined in clause 4.3.6 of TS 23.502, and procedure as defined in clause 4.23.6 of TS 23.502 is performed to provision the related information to I-SMF.
Based on the received traffic routing related policy information and EAS Deployment information, the I-SMF may (re)discover and (re)select ULCL and local PSA UPF for routing the traffic locally.
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6.10.3.2  Local offloading management policy handling  p. 81

The Local Offloading Management Policy may be dynamically generated by PCF and retrieved by SMF for Local Offloading Management as described in clause 6.10.2. The Local offloading Management Policy may be provided with a Offload Identifier. The Offload Identifier is assigned by PCF.
The SMF (including SMF which may act as I-SMF for Local Offloading Management) for Local Offloading Management may also be preconfigured with Local Offloading Management Policy and tagged with related Offload Identifier(s). In this case, the PCF only provides the Offload Identifier to the SMF to indicate the Local Offloading Management Policy applied for the PDU session.
The Offload Identifier is unique within the PLMN.
The detailed information of Local Offloading Management Policy is described in clause 6.4 of TS 23.503.The SMF sends the Local Offloading Management Information to the I-SMF.
The Local Offloading Management Information includes at least one of the following:
  • Offload Identifier;
  • IP range(s);
  • FQDN(s).
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