Content for  TS 23.548  Word version:  18.4.0

For PDU Session with Session Breakout connectivity model, based on UE subscription (e.g. DNN) and/or the operator's configuration, the DNS Query sent by UE may be handled by an EASDF (see clause 6.2.3.2.2), or by a local or central DNS resolver/server (see clause 6.2.3.2.3).

For the case that the UE DNS Query is to be handled by EASDF, the following applies.

• The AF may provide EAS Deployment Information to NEF which may store it in UDR, as defined in clause 6.2.3.4. SMF may retrieve EAS Deployment Information from NEF as described in clause 6.2.3.4 or has locally preconfigured information. EAS Deployment Information is used for creating DNS message handling rule on EASDF and it is not dedicated to specific UE session(s).
  EAS Deployment Information may apply to all PDU Sessions with a certain DNN, S-NSSAI and/or specific Internal Group Identifier(s).
• The SMF may provide BaselineDNSPattern to EASDF, the BaselineDNSPattern are derived from EAS Deployment Information provided by AF and are not dedicated to specific PDU Session; SMF configures EASDF with BaselineDNSPattern according to the procedures defined in clause 6.2.3.4.
  The Baseline DNS message detection template ID may be used by the EASDF to refer to Baseline DNS message detection template, and derive array of FQDN ranges and/or array of EAS IP address ranges. The Baseline DNS handling actions ID may be used by the EASDF to refer to Baseline DNS handling actions information, and derive actions related parameters.
  The Baseline DNS message detection template ID and the Baseline DNS handling actions ID are unique per SMF set when a SMF set controls an EASDF and shall be unique per SMF otherwise, within an EASDF Baseline
  BaselineDNSPattern may contain one or several items, where each item is either a Baseline DNS message detection template or a Baseline DNS handling actions information. Each BaselineDNSPattern item may be updated or deleted using Baseline DNS message detection template ID or Baseline DNS handling actions ID to identify the updated or deleted item
  • Baseline DNS message detection template
    • Baseline DNS message detection template ID
    • DNS message type = DNS Query or DNS Response:
      • If DNS message type = DNS Query:
        • Array of (FQDN ranges).
      • If DNS message type = DNS Response:
        • Array of FQDN ranges and/or array of EAS IP address ranges.
  • Baseline DNS handling actions information:
    • Baseline DNS handling actions ID:
      • ECS option.
      • Local DNS server IP address.
• During the PDU Session establishment procedure, the SMF may obtain the EAS Deployment Information from the NEF if not already retrieved (by subscription of such information to the NEF as described in clause 6.2.3.4.3) or the SMF is preconfigure with the EAS Deployment Information and the SMF selects an EASDF and provides its address to the UE as the DNS Server to be used for the PDU Session.
  The SMF configures the EASDF with DNS message handling rules to handle DNS messages related to the UE(s). The DNS message handling rule has a unique identifier and includes information used for DNS message detection and associated action(s). The DNS handling rules is defined as following:
  • Precedence of the DNS message handling rule;
  • DNS Handling Rule Identity;
  • A Baseline DNS message detection template ID and/or a DNS message detection template (optional and includes at least one of the following, if existing):
    • DNS message type = DNS Query or DNS Response:
      • If DNS message type = DNS Query:
        • Source IP address (i.e. UE IP address).
        • Array of (FQDN ranges) (optional).
      • If DNS message type = DNS Response:
        • Array of FQDN ranges and/or array of EAS IP address ranges (optional).
  • DNS message Identifier (if received from EASDF);
• Action(s) (includes at least one action); the possible actions include:
  • Reporting Action: Report DNS message content to SMF (i.e. target FQDN and if available: IP address information provided back by the DNS server). This reporting action may include reporting-once indication. If this indication is included, the EASDF reports the DNS message content to the SMF once if the DNS message detection template matches the first incoming DNS Query or DNS Response message.
  • Forwarding Action: Send the DNS message(s) to a DNS server/resolver(s) as follows:
    1. Including the information to build optional EDNS Client Subnet option to be included in the DNS message, or to be used for replacing the EDNS Client Subnet option received in the DNS Query message from the UE. (The information for the EASDF to build the EDNS Client Subnet option is either included in the DNS handling rule, or Baseline DNS handling actions ID acts as a reference to the Baseline DNS handling actions Information. This corresponds to the option A defined below.
    2. the information for the DNS message target address is either included as DNS Server Address indicated in the DNS handling rule, or the Baseline DNS handling actions ID included in the DNS handling rules refers to DNS message target address information; if no DNS Server Address is provided by the SMF in the rule, then the EASDF is to forward the DNS message to a locally preconfigured default DNS server/resolver. This corresponds to the option B defined below.
    3. Respond directly to the DNS request. In this case the EASDF is configured by the SMF not to forward the DNS Query to the DNS server, instead it creates a response based on EAS IP address provided by the SMF.
  • Control Action: Performs at least one of control actions on the DNS message(s) as follows:
    • Buffer the DNS message(s).
    • Build DNS response from DNS query with indicated IP address (e. g. common EAS). The EASDF is expected to handle the response it has built the same way as a response it has received from a remote DNS server.
    • Send the buffered DNS Response(s) message to UE.
    • Discard cached DNS Response message(s).

When the EASDF forwards a DNS message (to the UE or towards a DNS server over N6), it uses its own address as the source address of the DNS message. When the EASDF forwards the DNS message to the UE the EASDF based on configuration either replace the received EDNS Client Subnet option with the one provided by the UE (i.e. if provided by the UE) or remove any received EDNS Client Subnet. The SMF may use following information to create DNS message handling rules associated with a PDU Session:

• Local configuration associated with the (DNN, S-NSSAI, Internal Group Identifier) of the PDU Session; and/or
• EAS Deployment Information provided by the AF or preconfigured in the SMF; and/or
• Information derived from the UE location such as candidate L-PSA(s); and/or
• PDU Session information, like PDU Session L-PSA(s) and ULCL/BP; and/or
• Internal Group Identifier received in the Session Management Subscription data from the UDM; and/or
• IP address or DNAI (e.g. common EAS, common DNAI) cached locally or retrieve from UDR via PCF.
• If the FQDN in a DNS Query matches the FQDN(s) provided by the SMF in a DNS message detection template, based on instructions by SMF, one of the following options is executed by the EASDF based on a corresponding DNS message handling rule:
  • Option A: The EASDF includes or replaces an EDNS Client Subnet (ECS) option into the DNS Query message as defined in RFC 7871 and sends the DNS Query message to the DNS server for resolving the FQDN. The DNS server may resolve the EAS IP address considering the EDNS Client Subnet option and sends the DNS Response to the EASDF;
  • Option B: The EASDF sends the DNS Query message to a Local DNS server which is responsible for resolving the FQDN within the corresponding L-DN. The EASDF receives the DNS Response message from the Local DNS server.
  The SMF instructions for a matching FQDN may as well indicate EASDF to contact SMF. SMF then provides the EASDF with a DNS message handling rule;
• If the DNS Query from the UE does not match a DNS message handling rules set by the SMF, then the EASDF may simply forward the DNS Query towards a preconfigured DNS server/resolver for DNS resolution;
• When the EASDF receives a DNS Response message, the EASDF notifies the EAS information (i.e. EAS IP address(es), the EAS FQDN and if available the corresponding IP address within the ECS DNS option) to the SMF if the DNS message reporting condition provided by the SMF is met (i.e. the EAS IP address or FQDN is within the IP/FQDN range). The SMF may then select the target DNAI based on the EAS information and trigger UL CL/BP and L-PSA insertion as specified in clause 6.3.3 in TS 23.501 based on the Notification.
  The information to build the EDNS Client Subnet option or the Local DNS server address provided by the SMF to the EASDF are part of the DNS message handling rules to handle DNS Queries from the UE. This information is related to DNAI(s) for that FQDN(s) for the UE location, or in the case a common DNAI is used for the set of UEs, the information is determined based on the common DNAI of the set of UEs. The SMF may provide DNS message handling rules to handle DNS Queries from the UE to the EASDF when the SMF establishes the association with the EASDF for the UE and may update the rules at any time when the association exists. For the selection of the candidate DNAI for a FQDN for the UE, the SMF may consider the UE location, network topology, EAS Deployment Information and related policy information for the PDU Session provided as defined in clause 6.4 of TS 23.503 or be preconfigured into the SMF. After the UE mobility, if the provided Information for EDNS Client Subnet option or the Local DNS server address needs to be updated, the SMF may send an update of DNS message handling rules to the EASDF.

Once the UL CL/BP and L-PSA have been inserted, the SMF may decide that the DNS messages for the FQDN are to be handled by Local DNS resolver/server from now on. This option is further described in clause 6.2.3.2.3. To avoid EASDF sending redundant DNS message reports triggering UL CL/BP insertion corresponding to the same DNAI, the SMF may send reporting-once control information (i.e. DNS message handling rule with DNS message detection template containing EAS IP address ranges with reporting-once indication set) to EASDF to instruct the EASDF to report only once for the DNS messages matching with the DNS message detection template of the reporting-once control information for the DNS message detection template. In addition, the SMF may instruct the EASDF not to report DNS Responses to SMF corresponding to some FQDN ranges and/or EAS IP address ranges e.g. once the UL CL/BP and L-PSA have been inserted for the corresponding EAS IP address ranges for Pre-established session breakout while there is configuration for the related EASDF reporting DNS Responses. After the removal or change of the L-PSA, the SMF may instruct the EASDF to restart the reports of the DNS messages. If the SMF, based on local configuration, decides that the interaction between EASDF and DNS Server in the DN shall go via an UPF, the SMF sends corresponding N4 rules to this UPF to instruct this UPF to forward DNS message between EASDF and the external DNS server. In this case, DNS messages between EASDF and DNS Server described in this clause are transferred via this UPF transparently.

During PDU Session Release procedure, the SMF removes the DNS context by invoking Neasdf_DNSContext_Delete service.

For the case that the DNS message is to be handled by Local DNS resolver/server, the DNS Query is routed to the Local DNS resolver/server corresponding to the DNAI where the L-PSA connects. The SMF selects the Local DNS server address based on the DNAI corresponding to the inserted local PSA, local configuration and based on EAS Deployment Information in AF request as specified in clause 6.2.3.4.2. Based on the operator's configuration, one of the following options may apply when UL CL/BP and Local PSA have been inserted (during or after PDU Session Establishment):

• Option C: The SMF configures the local DNS server to the UE as new DNS server. The SMF may indicate to the UE either that for the PDU Session the use of the EDC functionality is allowed or that for the PDU Session the use of the EDC functionality is required. In addition, the SMF also configures traffic routing rule on the UL CL (including e.g. Local DNS server address) or the BP (e.g. the new IP prefix @ Local PSA) to route traffic destined to the L-DN including the DNS Query messages to the L-PSA. The L-DNS server resolves the DNS Query either locally or recursively by communicating with other DNS servers.
• Option D: If the SMF has been configured that DNS Queries for an FQDN (range) query can be locally routed on the UL CL, then the subsequent DNS Queries for the FQDN (range) will be locally routed to a Local DNS server.

Steps 2 and 3 are performed for option C: If SMF decides to remove the UL CL/BP and Local PSA as defined in clause 4.3.5.5 of TS 23.502, e.g. due to UE mobility, the SMF sends a PDU Session Modification Command to configure the new address of the DNS server on UE (e.g. to set it to the address of EASDF).

The following procedure is for selecting common DNAI with Local DNS Server/Resolver for set of UEs.

The following is the procedure for common EAS discovery for a set of UEs accessing the same application. Different UEs can be served by different SMFs. The common EAS IP address for the set of UEs may be provided by AF or determined by 5GC. AF may provide the common EAS IP via AF Traffic influence procedure as defined in clause 4.3.6.2 of TS 23.502, for this purpose AF may determine the common EAS IP address based on candidate DNAI(s) reported by SMF as described in clause 4.3.6.3 of TS 23.502. Alternatively, the common EAS IP address may be determined by NEF as defined in clause 6.2.3.2.7 and is stored in UDR as part of AF traffic influence request information.

The common DNAI for the set of UEs can be provided either by AF or determined by 5GC. When the AF determines the common DNAI, the AF provides the common DNAI for the set of UEs via AF Traffic influence procedure as defined in clause 4.3.6.2 of TS 23.502 The AF may determine the common DNAI based on candidate DNAI(s) reported by SMF as described in clause 4.3.6.3 of TS 23.502. When the 5GC determines the common DNAI, the common DNAI is determined by NEF and the NEF stores the common DNAI in UDR as part of AF traffic influence request information, as described in clause 6.2.3.2.7. The following is the procedure for discovery EAS corresponding to a Common DNAI for set of UEs accessing the same application.