FAR Apply Action "FSSM" in PFCP Session Establishment for MBS

[jordijoangimenez]

Related 3GPP specs:

• 3GPP TS 29.244 - Release 17.9.0 (PFCP procedures and messages)
• 3GPP TS 23.247 - Release 17.7.0 (MBS procedures)

MBS Session Start for Broadcast specific case:

• 3GPP TS 23.247 ch. 7.1.1.2 - MBS Session Creation without PCC
• 3GPP TS 23.247 ch. 7.3.1 - MBS Session Start for Broadcast
• 3GPP TS 29.244 ch. 5.34.2.2 - Instructing the MB-UPF to forward MBS data using multicast and/or unicast transport

In the middle of the MBS Session Start for Broadcast, the PFCP Session Establishment over N4mb starts:

1. MB-SMF sends PFCP Session Establishment request to the MB-UPF
2. MB-UPF responds PFCP Session Establishment response to the MB-SMF

There are no more PFCP messages exchanged (no PFCP Session Modification, because no unicast transport being used in N3mb, step 8. and 12. 3GPP TS 23.247 ch. 7.3.1 - MBS Session Start for Broadcast ).

If the MB-SMF sends the PFCP Session Establishment request with MBSN4mbReq-Flags containing the flag PLLSSM (Provide Low Layer SSM), asking the MB-UPF to allocate the LL-SSM and C-TEID.

Question: Should the Apply Action be set to "FSSM" or "DROP"? multicast transport is being used in N3mb, that implies FSSM but no LL-SSM and no C-TEID is allocated for the moment, so sending MBS Multicast Parameters IE would be impossible. Also, with no PFCP Session Modification, the FAR Apply Action could not be set to "FSSM" after being set to "DROP".

Question: Should MBS Multicast Parameters IE be included by the MB-SMF in the PFCP Session Establishment request? Because there is no LL-SSM and no C-TEID allocated for now.

Possible solution:
Probably MBS Multicast Parameters should be included with Outer Header Creation with the flag ssm_c_teid but not including any IP address nor C-TEID. Then the IP Address and C-TEID are selected by the MB-UPF and added to the FAR and sent to the MB-SMF in the MBS Session N4mb Information IE, then added to the FAR in the MB-SMF.

Quote from 3GPP TS 29.244 ch. 5.34.2.2 - Instructing the MB-UPF to forward MBS data using multicast and/or unicast transport:

... including the following information in the PFCP Session Establishment Request message:
...
a Create FAR IE to provision a FAR (associated with the PDR including the above PDI or Traffic EndPoint ID) with the Apply Action set to "FSSM" with an MBS Multicast Parameters IE, when multicast transport is used over N3mb or N19mb, to forward the packets to the low layer SSM address when it is allocated; otherwise, the apply action shall be set to "DROP".