Proposal: Add SMF NAS supervision timers (T3591/T3592) and pending session states for reliability & NTN support

[donivtech]

Hi maintainers,

While testing SD-Core SMF for long-RTT scenarios (e.g., 5G NTN), we’ve identified that SMF currently does not implement NAS supervision timers such as T3591 (Session Modification) and T3592 (Session Release). Because of this, the SMF cannot retransmit NAS commands or detect NAS delivery failures, which can leave session procedures stuck.

To support NTN and to improve reliability in terrestrial deployments I have the following proposal:

• new SMF session states (e.g., ModificationPending, ReleasePending)
• handling of NAS PDU Session Modification/Release and associated Command/Complete
• supervision timers (T3591/T3592)
• retransmission logic

Before preparing a design or PR, I’d like to check in with maintainers to confirm:

• Whether this direction fits the roadmap
• Whether the preferred workflow is a single consolidated design issue or incremental PRs
• Whether adding new SMF states is acceptable in principle

Happy to provide a detailed design / state machine diagram after initial feedback. I am new to contributing to SD-Core SMF, so I'm happy to follow any preferred guidelines for proposing and structuring changes.
Thanks!

[gab-arrobo]

Hi @thakurajayL, any thoughts about this?