Content for TS 23.548 Word version: 19.0.0

1… 4… 4.3 5… 6… 6.2.2.3… 6.2.3… 6.2.3.2.3… 6.2.3.2.5… 6.2.3.3… 6.3… 6.4… 6.5… 6.6 6.7… 6.7.2.4… 6.7.2.6… 6.7.3… 6.8… 7… A B… F…

6.2.2.3 EAS Re-discovery Procedure at Edge Relocation 6.2.2.4 Procedure for EAS Discovery with Dynamic PSA Distribution
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6.2.2.3 EAS Re-discovery Procedure at Edge Relocation p. 15

In order to change the PDU Session Anchor serving a PDU Session of SSC mode 2/3 for a UE, SMF triggers session continuity, service continuity and UP path management procedures as indicated in clause 4.3.5.1, 4.3.5.2 and 4.3.5.3 of TS 23.502. During these procedures, for SSC mode 2/3, it is recommended that the UE applies the following behaviour:

The UE DNS cache should be bound to the IP connection. When the UE detects the PDU Session release or new IP prefix is allocated within the PDU Session, the UE removes the old DNS cache related to old/removed IP address/prefix, for example, the old Edge Application Server address information.

With this behaviour, when the establishment of a new PDU Session triggers EAS rediscovery for an FQDN, the UE can reselect a new EAS for that FQDN.

For SSC mode 2, the procedure in clause 4.3.5.1 of TS 23.502 applies with following differences:

In step 3, when the new PDU Session has been established, UE can reselect a new EAS for the FQDN with an EAS Discovery procedure if the recommended UE behaviour has been followed.

For SSC mode 3 with multiple PDU Sessions, the procedure in clause 4.3.5.2 of TS 23.502 applies with following difference:

In step 5, the UE can reselect a new EAS for the FQDN with an EAS Discovery procedure if the recommended UE behaviour has been followed.

For SSC mode 3 with IPv6 Multi-homed PDU Session that all new traffic going via new IPv6 prefix, the procedure in clause 4.3.5.3 of TS 23.502 applies with following difference:

After steps 10-11 where SMF notifies the UE of the availability of the new IP prefix, the UE starts using it for all new traffic, including DNS Queries. The UE can reselect a new EAS for the FQDN with an EAS Discovery procedure if the recommended UE behaviour has been followed.

Then UE can reselect a new EAS for the FQDN with an EAS discovery procedure as defined in clause 6.2.2.2.

The SMF may also trigger EAS rediscovery as defined in clause 6.2.3.3 when new connection to EAS needs to be established in case the UE indicate support for this. This trigger may also be used by the SMF based on the AF trigged EAS relocation as described in clause 6.3.7.

6.2.2.4 Procedure for EAS Discovery with Dynamic PSA Distribution p. 16

5GC supports an EAS discovery procedure that allows that at PDU Session Establishment the SMF selects a Central PSA, regardless if a Local PSA is available to the SMF and then, it allows to dynamically re-anchor the PDU Session and transition to a Distributed Anchor Point connectivity model when needed. This is applicable to PDU Sessions of both SSC mode 2 and SSC mode 3.

This procedure relies on EASDF capability to influence the DNS Query of a FQDN so that the EAS Discovery considers a candidate UE topological location of a PSA further out in the network than current PSA. The PDU Session re-anchoring to the edge is performed as part of the EAS Discovery procedure.

This procedure requires that the DNS settings provided to the UE for the PDU Session are respected.

Reproduction of 3GPP TS 23.548, Fig. 6.2.2.4-1: Application server discovery with Dynamic PSA distribution using EASDF

Figure 6.2.2.4-1: Application server discovery with Dynamic PSA distribution using EASDF

The EAS Discovery procedure with Dynamic PSA distribution for both SSC mode 2 and SSC mode 3 PDU Sessions using EASDF is described in Figure 6.2.2.4-1.

The procedure is as follows:

Step 1.

PDU Session Establishment, allocation of an EASDF and sending rules to the EASDF. Steps 1-6 in the procedure in Figure 6.2.3.2.2-1 for EAS Discovery Procedure with EASDF for Session Breakout connectivity model are applied. If Dynamic PSA distribution applies to the PDU Session based on SMF local configuration, the SMF may have selected a Central PSA at PDU Session Establishment, regardless of whether a Local PSA is available.

Step 2.

The UE sends a DNS Query message for an FQDN to the EASDF via Central PSA. Steps 7-12 in the procedure in Figure 6.2.3.2.2-1 for EAS Discovery Procedure with EASDF for Session breakout Connectivity are applied. That is, the EASDF checks the DNS Query against the DNS Handling Rules in the DNS Context and reports to SMF and/or forwards to DNS for resolution as instructed by these rules. For resolution, it applies Option A or option B in the procedure in Figure 6.2.3.2.2-1 or sends the DNS Query to a pre-configured DNS server/resolver if none of them applies.

When the DNS Response is received, EASDF checks it against the DNS context matching conditions for reporting. If applicable, it reports to SMF the selected EAS and handles the DNS Response as instructed by SMF DNS handling rules: when there is a change of PDU Session Anchor per SSC mode 2 or 3, the SMF indicates to EASDF to discard the DNS Response.

When no DNS Response is sent to the UE, the UE is expected to restart the DNS Query over the new PDU Session).

For further details see clause 6.2.3.2.2.

SMF determines that the central UPF (PSA) needs to be changed to an Edge UPF (L-PSA) and it triggers one of the procedures to change the PSA of the PDU Session to a distributed anchor. Which procedure is triggered depends on the SSC mode of the PDU Session and also on SMF configuration:

Change of SSC mode 2 PDU Session Anchor with different PDU Sessions as in clause 4.3.5.1 of TS 23.502. The procedure applies with the following differences:
In step 2, the DNS context for the session is removed from EASDF as part of the PDU Session Release procedure (in step 12 of the PDU Session release procedure in clause 4.3.4.2 of TS 23.502).
In step 3, SMF selects and provisions the DNS settings for the new PDU Session as required by the procedure for EAS Discovery on Distributed anchor as described in clause 6.2.2.2.
Change of SSC mode 3 PDU Session Anchor with multiple PDU Sessions as in clause 4.3.5.2 of TS 23.502. The procedure applies with the following differences:
In step 4 in clause 4.3.5.2 of TS 23.502, SMF selects and provisions the DNS settings for the new PDU Session as required by the procedure for EAS Discovery on Distributed anchor as described in clause 6.2.2. Step 3 in clause 6.2.2.4 could happen any time after this step.
In step 6 in clause 4.3.5.2 of TS 23.502, the old DNS context for the old session and old UE IP address/prefix of UE are removed from EASDF as part of the PDU Session Release procedure (in step 12 of the PDU Session Release procedure in clause 4.3.4.2 of TS 23.502).
Change of SSC mode 3 PDU Session Anchor with IPv6 Multi-homed PDU Session as in clause 4.3.5.3 of TS 23.502. The procedure applies with the following differences:
In steps 10-11 in clause 4.3.5.3 of TS 23.502, SMF also manages the EASDF context and provides new DNS settings to the UE if needed:
- If EASDF is not going to be used, SMF sends the UE the new DNS settings in a PDU Session Modification Command and removes the EASDF context.
- If EASDF is going to be used, SMF may update existing EASDF context or it may remove it and create a new one, for example, to select a new EASDF. If a new EASDF is selected, SMF sends the UE the new DNS settings in a PDU Session Modification Command and may also send them in Router Advertisement.
  After steps 10-11 in clause 4.3.5.3 of TS 23.502, UE starts using IP@2 for all new traffic, including DNS messages, and SMF can already perform from step 3 in Figure 6.2.2.4-1.
The PDU Session establishment in this step includes the actions described above in step 1 in Figure 6.2.2.4-1 if DNS Queries should be able to trigger re-anchoring of the session to a more distributed PSA.

NOTE 1:
When new DNS settings do not involve EASDF, new DNS Query will not trigger re-anchoring of the PDU Session to a L-PSA deployed even further out in the network.

To remove the Session context in EASDF, SMF invokes Neasdf_DNSContext_Delete Request/Response.

NOTE 2:
Dynamic re-anchoring to an edge PSA implies that the UE IP address is changed from a UE IP address corresponding to the old (central) PSA to a UE IP address corresponding to the new (edge) PSA for all applications on the PDU Session.

NOTE 3:
Further re-anchoring (to a central UPF) can be triggered if activity is monitored e.g. if EC application traffic ceases. In that case, EASDF is provided again in the DNS settings for the PDU Session. New EAS Discovery will go to EASDF and be handled as described in step 1.

Step 3.

A new discovery procedure is triggered for the application over the new PSA: the UE resends a DNS Query targeting the application. (Re)discovery follows the EAS (re)Discovery procedure for distributed anchor connectivity model as in clauses 6.2.2.2 and 6.2.2.3.

Step 4.

Application traffic starts via the PDU Session Edge PSA to the EAS selected in step 3.