MBSF MVP Design

[davidjwbbc]

Introduction

The MBSF forms part of the MBS User Services and is specified in 3GPP TS 29.580. This discussion will outline the design for an MBSF minimum viable product to fit with other components in the 5G-MAG reference tools (e.g. 5G-MAG/rt-mbs-transport-function).

What does the MBSF do?

The MBSF is responsible for accepting Distribution Session requests from the Application Provider AF/AS, validating the session and then arranging the configuration of the User Plane by talking to the MB-SMF and MBSTF. It must first create an MBS Session for each required multicast address by talking to the MB-SMF, which in turn instructs the MB-UPF to configure the User Plane for those MBS sessions. It can then create Distribution Sessions with the MBSTF which will instruct it how to perform the relevant push/pull activities and package up the received objects into a multicast stream which is tunnelled to the MB-UPF.

MVP

A full MBSF will perform the following functions and the ones that will be implemented by the MVP are marked:

Function	MVP?
Authenticate an Application Provider that is attempting to create a new MBS Session	❌
Validate that the MBS Session request will only use resources for which the Application Provider is permitted	❌
Extract the desired multicast addresses from the MBS Session request into a series of MBS Service requests to the MB-SMF and maintain a reference counted list of which MBS Services an MBS Session is using	✔️
Extract the MBS Distribution Sessions from the MBS Session request, augment with extra information from the relevant MBS Services and manage the subscriptions of the Distribution Sessions on the MBSTF(s)	✔️
Provide MBS Session update notifications	❌

An example session would look like:

sequenceDiagram
  autonumber
  box rgb(128,255,128) Application Provider
    participant AP as AF/AS
  end
  box rgb(128,255,255) 5G Core
    participant MBSF
    participant SCP
    participant MBSTF
    participant MBSMF as MB-SMF
  end

  activate AP
  activate SCP
  activate MBSF
  activate MBSMF
  activate MBSTF

  AP ->> MBSF : CreateMBSUserService
  loop For each unique multicast address in the request
    opt If there is no MBS Session for that multicast address
      MBSF ->> MBSMF : Create MBS Session and request<br/>tunnel for the multicast address
      MBSMF -->> MBSF : OK, UDP tunnel information in response
    end
  end

  loop For each MBS Distribution Session in the request
    MBSF->>SCP : On an MBSTF, create a Distribution Session<br/>request from the AP request Distribution session<br/>plus the UDP tunnel information<br/>from the MB-SMF.
    SCP->>MBSTF : Create a Distribution Session
    MBSTF -->> SCP : Response for new Distribution Session<br/>plus connection identifier header<br/>and push service URLs
    SCP -->> MBSF : Response for new Distribution Session<br/>plus connection identifier header
  end

  MBSF -->> AP : OK response for CreateMBSUserService<br/>with any push service addresses filled in

  deactivate AP
  deactivate SCP
  deactivate MBSF
  deactivate MBSMF
  deactivate MBSTF

Loading

1. The AF/AS for the Application Provider sends a request to the MBSF to create an MBS Session. The session information includes all individual MBS Distribution Sessions with the multicast address to use for each.
2. The MBSF checks its current set of established MBS Sessions to see if any of the multicast addresses, requested by the AP, have not been set up and asks the MB-SMF to create a new session for each one that needs to be created.
3. The MB-SMF (and MB-UPF) creates the new MBS sessions and passes back the UDP tunnel address (from the MB-UPF) that multicast should be sent to.
4. For each distribution session in the AP request, the MBSF creates a similar Distribution Session request with the UDP tunnelling information added (as returned from the MB-SMF in step 3), and requests that the SCP forward the request to an available MBSTF.
5. The SCP checks the recent load responses for the MBSTFs to the NRF and passes the request onto an available MBSTF.
6. The MBSTF responds to the create request. This includes any push service URLs that were requested and an NF instance identifier.
7. The SCP receives the response from the MBSTF and passes it onto the MBSF. The MBSF remembers the NF instance identifier for the MBSTF along with the distribution session information.
8. After creating all the MBS Sessions with the MB-SMF and MBS Distribution Sessions with the MBSTF(s), the MBSF responds to the AP's original request and includes any push service URLs as reported by the MBSTF(s).

By using the SCP to proxy requests to an MBSTF we can benefit from load balancing performed by the SCP. If an SCP is not present then discovery via the NRF will happen instead and the request will be made directly to the MBSTF. This is all handled by the Open5GS ogs_sbi_discover_and_send() function, and by retaining the ogs_sbi_client_t associated with the original discover and send request, further management of created sessions will communicate via the SCP with the correct MBSTF.