Aggregate Network-State Snapshot (Design Note)

Non-normative 6G research. A "network digital twin" export has no frozen Rel-20/6G Stage-3 specification; this aggregate format is project-defined. The underlying data models are TS 29.510 (NRF NFProfile) and TS 29.536 (NSACF slice quotas), which the source snapshots already follow. Research shape for education, not a conformance target. Tracking issue: #25.

What it is

A digital twin needs one coherent, read-only export of network state as its substrate. nextgcore_app::state_export (feature 6g-extensions, off by default) is an offline aggregator: it composes the state the NRF and NSACF already persist to JSON on disk into one NetworkStateSnapshot document. No live SBI plumbing, no new listening socket, no always-on task.

nrfd  --state-file nrf_state.json    ─┐  (persisted on every registry mutation)
                                      ├─► NetworkStateSnapshot::from_persisted
nsacfd --state-file nsacf_state.json ─┘      │
                                             ▼
      { captured_at_ms, nf_registrations, slice_quotas, session_summary }

Snapshot format

• captured_at_ms — capture wall-clock, ms since Unix epoch.
• nf_registrations[] — one entry per persisted NRF NFProfile: distilled nf_instance_id / nf_type / nf_status / fqdn / ipv4_addresses / service_names, plus the verbatim persisted profile document under profile (camelCase TS 29.510 keys) so nothing is lost. Profiles missing the mandatory nfInstanceId/nfType/nfStatus trio are skipped, exactly like the NRF's own lenient restore path.
• slice_quotas[] — one entry per NSACF slice: sst/sd (raw number, as persisted), max_ues/max_pdu_sessions, live current_ues / current_pdu_sessions (lengths of the persisted admission membership sets), eac_active.
• session_summary — totals across slices plus a documented note field stating the limitation: counts are NSACF per-slice admission counters; live SMF/UPF per-session enumeration is a follow-up and is not included — no session list is fabricated.

Optional/empty fields — fqdn, ipv4_addresses, service_names, and sd — are omitted from the serialized JSON entirely when absent or empty (the NSACF state doc itself persists an explicit "sd": null, but the export drops the key; deserialization accepts either form). Malformed source entries are skipped with a log::warn!, and a skipped quota row's UE/PDU counts are consequently absent from the session_summary totals.

The nf_hooks digital-twin types (NfStateSnapshot, NfStatus, NfStateDelta, SnapshotHistoryEntry) now derive serde Serialize/Deserialize, so per-NF twin snapshots can travel as JSON too.

Invocation

No NF binary changes. A cargo example (skipped entirely by default builds via required-features) does the export:

cargo run -p nextgcore-app --features 6g-extensions \
  --example export_network_state -- nrf_state.json nsacf_state.json out.json

The source files come from nextgcore-nrfd --state-file <path> (or env NEXTGCORE_NRF_STATE_FILE) and nextgcore-nsacfd --state-file <path>; both NFs persist on every mutation, so the files are current as of the last registry/admission change.

Scope limits

• Offline, read-only, point-in-time. No live cross-process SBI collection (the NWDAF nrf_collector is the seed for later live work).
• No SMF/UPF session registry exists to read — hence the honest session_summary.note instead of invented per-session data.
• Snapshot ordering follows the source docs; NRF profile order, NSACF quota-row order, and NSACF membership-set order are all nondeterministic (HashMap/HashSet iteration).

Verification

cargo build -p nextgcore-app                                  # default unchanged
cargo test  -p nextgcore-app --features 6g-extensions          # incl. state_export
cargo build -p nextgcore-app --features 6g-extensions --example export_network_state