A.4 Background Download

26.5123GPP5G Media Streaming (5GMS)ProtocolsRelease 17TS

A.4.1 General

In the case of Background Download, the asset is acquired in the background, prior to viewing. Many application services offer the capability of acquiring a VoD item for later consumption. The 5GMS-Aware Application triggers the Media Session Handler to acquire the item, providing a background download network policy id.

NOTE: Here, the DASH Player is handling the acquisition, since the DASH Player contains the MPD processing and the DASH Access engine parts. Other realizations would use a separate background download agent, which is not even try to decode and render the video.

Figure A.4.1‑1 below illustrates the representation marking for background download. The MPD may be annotated with Service Descriptions clearly identifying representations intended for download. Here, Title1 should be downloaded in Full HD quality and all other titles in regular HD quality.

Figure A.4.1‑1: Background Download Representations

A.4.2 Procedure

The procedure for activating a Background Download dynamic policy is illustrated in figure A.3.2‑1 below.

Figure A.3.2‑1: Procedure for activating Background Download dynamic policy

Steps:

1. The 5GMS Application Provider interacts with the 5GMS AF to set up one or more Policy Templates (M1). Each Policy Template is identified by a Policy Template identifier and contains information about how to activate the according policy within the 5G System (e.g. N5 URLs and parameters).

2. The 5GMS Application Provider also interacts with its DASH content generation function (e.g. an MPD provider) to annotate the DASH MPD with Service Descriptions, e.g. to identify, which representation is intended for background download.

3. The 5GMS-Aware Application is configured via M8 with information about the available content catalogue (e.g. resolving MPD URLs), the available subscription identifiers (e.g. the user has a 4K subscription or the user has an SD subscription), device type identifiers.

The 5GMSd-Aware Application is configured via M8 about the available background download policy. This includes the Network Policy Id which hints a background download policy.

4. When a user selects an item via the User Interface (U1) for Background Download the 5GMS-Aware Application translates the input to the needed 5GMSd API calls.

5. The 5GMS-Aware Application provides input (via C1) on the selected presentation entry (i.e. MPD URL) and also on the Network Policy Identifier (indicating a background download policy, i.e. make the Media Session Handler request a bearer suitable for Background Download).

NOTE: C1 is an abstract interface and indicates that the 5GMS-Aware Application may either first use M6 or M7 for the interactions with the 5GMS Client.

6. The Media Session Handler uses the Network Policy Identifier to find the procedure and the parameters to activate the Dynamic Policy Instance (here a Background Download policy). The Media Session Handler has received one or more Policy Descriptions together with matching Service Access Information (via M5_1). The Media Session Handler uses the Network Policy Identifier as a key to find the correct Policy Description. The Media Session Handler should activate a Dynamic Policy in the 5GMS AF, providing Service Data Flow Template information of the media flows (audio, video, etc). The Media Session Handler can also receive information on a bit rate policing (enforcementMethod and/or enforcementBitrate), e.g. that the bit rate is actively limited.

7. The Media Session Handler activates the Dynamic Policy instance on M5, providing the Policy Template identifier and additional parameters. Upon positive response, the Media Session handler notifies the DASH Player to start the Background Download. The notification contains a Service Descriptor Filters, which is used by the DASH Player to filter policy-compliant Service Descriptions from the MPD. The Media Session Handler may receive the Service Descriptor Filters with the response or may look up the Service Descriptor Filter values by a response value (e.g. derived from a maximum bit rate indication).

The Media Session Handler may need to update the Dynamic Policy instance, depending on the selected traffic detection method. For example, when the Media Session Handler uses 5-Tuples, it needs to update the Dynamic Policy instance with every newly opened and every closed TCP connection.

8. The DASH Player fetches the MPD of the selected content.

9. The DASH Access Engine / Selection Logic (see ISO 23009-1 [32] figure K.1) selects only adaptation sets and representations according to the filter (i.e. suitable for Background Download). Here, the DASH Player fetches the MPD after the notification from the Media Session Handler.

A.4.3 Example parameters

Table A.4.3‑1: M5_1 Parameters for Policy Descriptions (used by the Media Session Handler)

Parameter	Type	Purpose	Example Values
Policy Description	Object
Network Policy Id	String	Identifies the Policy Description.	"Background Download".
Service Access Information URL	URL	References the associated Service Access Information.

Table A.4.3‑2: M5_1 Parameters for Service Access Information

Parameter	Type
Service Access Information	Object
Policy Template Id	String	Identifies the Policy Template.	"backgrounddata".
5GMS AF URL	URL	Used to invoke the 5GMS AF.
sdfMethods	[String]	Indication, which Service Data Flow Description methods are recommended to use by the media session handler.	"5-Tuple", "domainName", "TOS=xx", etc.
Mandatory M5 Request information	List	Desired bit rate, to be provided by the network for the application.	Policy Template Id, Average Bit rate, Service Data Flow Template.
M5 Response information	List	Information to the Media Session Handler on the response parameters.

Annex B (informative):
Content Hosting Configuration examples