An MBS User Service Description is described by a set of metadata documents that are delivered as described in clause 4.3.2 of TS 26.502. The data model defined in this clause subdivides the parameters defined in TS 26.502 and groups them into a set of metadata documents. The metadata consists of:

• A User Service Descriptions document (see clause 5.2.2) describing a set of one or more MBS User Services, and containing:
  • One of more User Service Description objects (see clause 5.2.3), each describing an MBS User Service Session that is associated with:
    • One or more Distribution Session Description objects (see clause 5.2.4), each of which references a Session Description document RFC 8866 (see clause 5.2.5) that may be packaged with the User Service Descriptions document for delivery to the MBS Client in the same User Service Bundle (see clause 5.3.4), and each of which may optionally reference an Object Repair Parameters object (see clause 5.2.7) describing the object repair parameters for the MBS Distribution Session in question.
      • Zero or more alternative Application Service Description objects (see clause 5.2.5), each of which references an Application Service Entry Point document (see clause 5.2.6) that may be packaged with the User Service Description document for delivery to the MBS Client in the same User Service Bundle (see clause 5.3.4). Additional resources referenced by the Application Service Entry Point document may also be packaged into the User Service Bundle.
    • Zero or more Service Schedule Description objects (see clause 5.2.7) advertising the delivery schedule for the MBS User Service Session.

Figure 5.1-1 illustrates the relationships between these metadata entities using UML for a User Service Descriptions document.

A User Service Description document shall contain one or more User Service Description objects, each of which describes a single MBS User Service Session. Each User Service Description object shall include at least one Distribution Service Description object describing the set of MBS Distribution Sessions currently associated with the MBS User Service Session.

• The Distribution Session Description object shall refer to one Session Description document.
• Each Distribution Session Description object may include an Object Repair Parameters object.
• Each Distribution Session Description object may include zero or more alternative Application Service Description object, optionally referencing an Application Service Entry Point document (e.g. a DASH MPD, HLS Master Playlist or HTML document) which describes the root of the Application Service associated with this MBS Distribution Session. When multiple Application Service Entry Point documents are referenced by the same Distribution Session Description (not permitted in this release), an MBS Client shall select only one on the basis of a distinct MIME content type indicated in the Application Service Description object.

Each User Service Description object may include Service Schedule Description objects. If included, the UE can expect to receive MBS User Service data during the time periods described in the Service Schedule Description object.

The following description in this clause presumes a JSON encoding of the information comprising the MBS User Service Announcement as specified in clause 5.1A. The data types in Table 5.2.1-1 from other 3GPP specifications are reused in the remainder of the present document.

The data types in Table 5.2.1-2 are defined in the present document.

The MBS User Service Descriptions document conveys one or more User Service Description objects. Table 5.2.2-1 provides the detailed semantics for the UserServiceDescriptions data type that conveys these objects.

The UserServiceDescription data type carries information about a single MBS User Service corresponding to an MBS User Service Announcement as defined in clause 4.5.7 of TS 26.502. Table 5.2.3-1 provides the detailed semantics for this data type.

The DistributionSessionDescription data type describes one MBS Distribution Session associated with an MBS User Service and carries the MBS Distribution Session Parameters as defined in clause 4.5.8 of TS 26.502. Table 5.2.4-1 provides the detailed semantics of this data type.

If the applicationServiceDescriptions array is present:

1. The distributionMethod property of the DistributionSessionDescription shall be set to OBJECT.
2. All members of the applicationServiceDescription array shall reference an Application Service Entry Point document (see clause 5.2.6A) that describes an MBS Distribution Session using the Object Distribution Method as specified in clause 6.
3. The sessionDescriptionLocator property shall point to a Session Description document (see clause 5.2.5) describing an MBS Distribution Session according to clause 6 delivering objects that are directly or indirectly referenced by the Application Service Entry Point document.

Each MBS Distribution Session shall be described by a Session Description document that shall be formatted according to RFC 8866. The Session Description document may be packaged in the same MBS User Service Bundle (see clause 5.3.1A) as the MBS User Service Descriptions document.

• The session description for the MBS Object Distribution Method is specified in clause 6.2.2
• The session description for the MBS Packet Distribution Method is specified in clause 7.2.3.

Table 5.2.6-1 provides the detailed semantics for the ApplicationServiceDescription data type.

The Application Service Entry Point document shall follow a format consistent with the value of contentType indicated as specified in clause 5.2.6. In the case of 3GP-DASH formatted content as defined in TS 26.247, the Application Service Entry Point document may be a unified media manifest which describes Representations available for both MBS reception and unicast retrieval, and this shall be used by MBS Clients compliant with the present document. In practical deployments, different subsets of the Representations described by the unified manifest document may be specified for:

• Availability via MBS delivery only,
• Availability via both unicast and MBS delivery,
• Availability via unicast only, and the Representation is redundant in MBS area coverage, i.e. the usage of these resources does not provide an improved user experience. As an example, this may be a lower bitrate Representation of a media component for which a higher bitrate is available over MBS distribution, and
• Availability always via unicast, and the Representation is supplementary in MBS area coverage, i.e. even in MBS area coverage these resources provide an improved user experience. As an example, this may be a secondary language that is only accessible over unicast.

All resources that are directly or indirectly referenced in the Application Service Entry Point document of this metadata unit that are expected to be retrieved by HTTP GET shall be delivered by at least one of the MBS Distribution Sessions associated with the MBS User Service Description.

A Service Schedule Description object describes the distribution schedule of a single instance of the MBS User Service and the availability of content via unicast delivery in terms of:

• Start/stop time point; or
• Start time point, time duration and periodicity.

The MBS Client can expect to receive MBS data during the described time period(s). The Schedule Description object may be delivered to the MBS Client prior to the MBS Distribution Session as part of the User Service Description document (see clause 5.2.2) and may be updated subsequently along with that document. The Service Schedule Description object with the highest version number shall take priority, such that schedule parameters received prior to the MBS Distribution Session are regarded as "initial defaults", and schedule parameters received during the MBS Distribution Session overwrite the earlier received schedule parameters. The MBS Distribution Session shall be available to the MBS Client during the time interval(s) announced by the session schedule. The MBS Client shall not activate reception of that MBS Distribution Session outside this time window. Schedule information received in the Schedule Description object shall take precedence over timing information that may have been received in the Session Description object (t and/or r lines in the SDP). Table 5.2.7-1 provides the detailed semantics for the ServiceScheduleDescription data type which describes a single scheduled instance of the MBS User Service Session.

Object Repair Parameters configure the Object Repair as defined in clause 6.2.4. Table 5.2.8-1 provides the detailed semantics for the ObjectRepairParameters data type.

The AvailabilityInformation data type provides additional information pertaining to the availability of the MBS Distribution Session within the 5G Network:

• The serviceArea property declares the one or more service areas in which the MBS Session corresponding to this MBS Distribution Session is currently available.
• In the case of a broadcast MBS Session corresponding to this MBS Distribution Session:
  • The mbsFSAId property identifies a preconfigured area within which, and in proximity to, the cell(s) announce the MBS Frequency Selection Area (FSA) ID and its associated frequency.
  • The nrRedCapUEInfo property indicates which classes of UE the MBS Distribution Session is suitable for consumption by.
• In the case of a broadcast MBS Session corresponding to this MBS Distribution Session, the nrParameters property indicates the one or more radio frequencies in the NG-RAN downlink on which the MBS Session is transmitted in the service area(s) identified by the serviceArea property.

Table 5.2.9-1 provides the detailed semantics for the AvailabilityInformation data type.

Table 5.2.9-2 provides the detailed semantics for the NrParameterSet data type.

In order to support the transport security for MBS, the DistributionSessionDescription object may contain an object of type SecurityDescription indicating the User Plane security parameters for the MBS Distribution Session as defined in Table 4.5.8-1 of TS 26.502. The User Plane security mechanisms between the MBS client and the MBSSF are defined in clause W.4.1.3 of TS 33.501 and clause 6 of TS 33.246.

This clause defines the delivery of User Service Descriptions, i.e. the delivery of a User Service Descriptions document, as well as additional resources (documents and possibly ancillary media objects) that are necessary to provide all necessary parameters required by an MBS Client to initiate one or more MBS User Services. The collection of all necessary resources associated with a User Service Descriptions document is referred to as a User Service Descriptions Bundle and is specified in clause 5.3.1A. A User Service Descriptions Bundle may be delivered to the MBS Client via an MBS Distribution Session at reference point MBS-4-MC (see clause 5.3.2) and/or via a regular unicast PDU Session at reference point MBS-5 (see clause 5.3.3) and/or via application-private means at reference point MBS-8.

The User Service Descriptions Bundle includes a single MBS User Service Descriptions document as well as all other associated resources that are directly referenced by the MBS User Service Description(s) included in that document or indirectly referenced by other resources in the same User Service Bundle. Each associated resource included in the User Service Bundle shall have a URL and shall have a MIME content type. The set of resources comprising a User Service Bundle shall be aggregated into a single multipart entity, referred to as a User Service Descriptions Bundle Entity. Each such resource shall be encapsulated as a distinct body part of this multipart entity. The headers and body of this entity may be conveyed by different application protocols, for example as the headers and body of an HTTP message or as a multicast transmission object. Individual body parts or the entire entity body may be compressed in transit (e.g. using GZip [41]) if the application protocol supports this. A User Service Descriptions Bundle Entity (hereafter "entity") shall conform to the following requirements:

• The Content-Type header of the entity shall be multipart/related as specified in RFC 2387.
• The root body part of the entity shall be a User Service Descriptions document as defined in clause 5.2.2.
  • The type parameter of this root body part shall be present as specified in RFC 2387 and shall be set to the MIME content type of the User Service Descriptions document specified in clause E.2.
  • The User Service Descriptions document shall be the first body part of the multipart/related entity, and hence the start parameter specified in RFC 2387 should be absent. If present, the start parameter shall be set to the content-id of the MBS User Service Description body part.
• Every other body part in the entity shall have a URI indicated in its Content-Location header per RFC 2557 so that it may be cross-referenced by other resources in the same entity. The value of this header may be an absolute URI, or it may be expressed as a path that can be resolved relative to the base URI of the enclosing body part using the rules specified in Section 5 of RFC 2557.
• All dependent resources of the User Service Descriptions described in the User Service Descriptions document shall be included as body parts in the entity. The User Service Descriptions document shall not use external references to any resource that it directly references.

In this case, one or more User Service Bundle Entities are delivered repeatedly by the MBSTF to the MBSTF Client via a suitable MBS Distribution Session at reference point MBS-4-MC using the Object Distribution Method, as defined in clause 4.2.4 of TS 26.502. This is the realisation of the MBS User Service Announcement Channel. The operating mode of this MBS Distribution Session shall be set to OBJECT_CAROUSEL and relies on an object manifest to characterize the repetition and the update pattern of the User Service Bundle Entities in the carousel. The object manifest is specified in clause 6.1.2. The objects described by this manifest shall be User Service Bundle Entities formatted as specified in clause 5.3.1A. As defined in clause 5.4 of TS 26.502, the MBSTF Client announces the arrival of an MBS User Service Description to the MBSF Client using a suitable notification at internal reference point MBS-7', and the MBSF Client may subsequently retrieve the MBS User Service Description from the MBSTF Client via internal reference point MBS-6'. It may do so using procedures equivalent to those specified for unicast retrieval at reference point MBS-5 (see clause 5.3.3). In this case, the MBSTF Client acts as a proxy for the MBS AF.

In this case, a result set of one or more User Service Descriptions Bundle Entities is retrieved by the MBSF Client from the MBS AF at reference point MBS-5 via a regular unicast PDU Session. The API at this reference point is specified in clause 9.2 and the format of the response is specified in clause 9.2.3. The OpenAPI [14] specification of the User Service Description retrieval API can be found in clause A.2.