vms-c23: DECnet Phase IV NSP logical-link connection service (rung-2)

[baron-3dl]

What

Engine rung 2 of the DECnet lane (epic vms-30e): the NSP logical-link connection service on top of the rung-2 NSP codec (vms-6986), wired into the rung-1 routing engine (vms-449d) so DECNETD.EXE can OPEN a logical link over an adjacency it already forms.

The FSM — what a logical link can now do

src/vmsdecnet/nsp/dnet_link.{c,h} — a pure, clock-injected NSP connection state machine, same discipline as the adjacency SM (no socket / thread / wall-clock / sleep):

CLOSED --connect()--> CI_SENT --rx CC--> RUN --disconnect()--> DI_SENT
   ^                     |                |                        |
   | rx CI               |                | rx DI (-> send DC)     | rx DC
   +-- CR_RCVD --accept()/CC--> RUN     CLOSED <------------------+

• Establish — Connect Initiate / Connect Confirm exchange, logical-link addresses learned both ways.
• Run — a data segment carried with its explicit acknowledgement (QUAL bit) and simple in-order sequencing; piggyback ack supported.
• Tear down — Disconnect Initiate / Disconnect Confirm handshake, clean close both ends.
• CI retransmit + give-up — an unanswered CI retransmits on the injected clock and, after the budget, abandons the link as UNREACHABLE (never a fabricated connect). Both values are oracle-informed: ~5.5 s interval (register §4.6 specimens ci: wire QEMU kernel-executive job into CI (vms-e4d) #3→dcl: SHOW DEVICE row byte-exact + create_dir leak fix (vms-b9f round 5) #4 are 5.504 s apart) and 8 retransmits (§4.6 records VAX1 giving up after 8).

Wiring + proof (open / data / ack / close)

• dnet_nsp: added Disconnect Confirm (DC, MSGFLG 0x48), classified distinct from DI. The oracle Connect-Initiate round-trip is unchanged and still byte-identical.
• dnet_engine: a minimal Phase IV long-data-packet routing header carrier (build/parse, grounded on §4.6 specimen ci: wire QEMU kernel-executive job into CI (vms-e4d) #3's header bytes) + one embedded link + link_open / accept / send / close / rx wrappers.
• DECNETD.EXE --nsp-selftest (no CAP_NET_RAW): two engines OPEN a link, move a data segment + ack, and DISCONNECT as full on-wire data frames over a real socketpair(2) — the exact build_data_frame -> wire -> link_rx path a live datalink uses.

Proven two ways (tests/vmsdecnet/test_dnet_link.c): two dnet_link FSMs drive the full choreography deterministically, and two engines exchange a logical link end-to-end over a socketpair with the payload round-tripping byte-identical.

Tests

6/6 vmsdecnet + 26/26 vmsscs ctests green, -Werror clean. Both DECNETD.EXE --self-test and --nsp-selftest PASS.

Clean-room (Rule 8) — honest scope

Only the Connect Initiate bytes are oracle-verified (the codec, §4.6). The CC / data / ack / DI / DC choreography is SPEC-DERIVED from the public DNA Phase IV NSP spec and proven only by the two-endpoint round-trip — the §4.6 capture never completed a handshake, so no completed-link bytes exist to ground it and none are fabricated. DC and the long-data header (built without the specimen's optional 0x81 padding field) are labelled spec-derived / OVMX choices. Register §6.0 records rung 2.

Scope honesty — deferred children of vms-30e (filed, not dropped)

• SET HOST / CTERM — vms-4d2
• file transfer / FAL·DAP (COPY NODE::) — vms-8c2
• live-VAX oracle bracket — vms-aac0

Different lane from CHF/libvms; no /dev/vms ioctl / kif / scsd / ovmx_init surface touched — DECnet engine is userspace over scs_datalink.

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