# LAT — Local Area Transport

Reference material for the **LAT** protocol (DEC's *Local Area Transport*), the
Ethernet protocol the VT1200 firmware implements for terminal / X-session
connections to VMS and ULTRIX hosts.

See the firmware evidence in `../README.md` (`LAT: could not allocate XCB …`,
`VTELAT_COMM`, `latmain`, the `LAT_*` error table).  This folder collects the
official protocol documentation plus open-source implementations, so the
VT1200's LAT subsystem can be read against a normative spec and against working
code instead of reverse-engineered from scratch.

## Contents

```
lat/
├── manuals/     # DEC protocol documentation (PDF, scanned)
└── code/        # open-source LAT implementations
```

## Manuals (`manuals/`)

Source: bitsavers — <http://bitsavers.computerhistory.org/pdf/dec/ethernet/lat/>

| File | Document | Part no. | Date | Pages |
|------|----------|----------|------|-------|
| `LAT_Protocol_Dec83.pdf` | *Local Area Transport Architecture* (Bruce Eric Mann) — early internal architecture draft; the origin of the term "LAT" | — | Dec 1982 (rev. 1983) | 118 |
| `AA-DJ18B-TK_LAT_Architecture_Network_Managers_Guide_Jul85.pdf` | *Local Area Transport (LAT) Architecture — Network Manager's Guide* — LAT 5.0 era; configuring/managing/troubleshooting LAT, counters, performance | `AA-DJ18B-TK` | Jul 1985 | 70 |
| `AA-NL26A-TE_LAT_Specification_Jun89.pdf` | *Local Area Transport (LAT) Specification* — **the normative spec** (LAT VS.1 / "LAT 5.1"): names & translation, slot + virtual-circuit layers, connection solicitation, service classes, message formats | `AA-NL26A-TE` | Jun 1989 | 198 |

Checksums:

```
3a4b97b2d7158a1919d2d5eb425959cfa31dc03a0a2c3f62756ec9254ee5201c  AA-DJ18B-TK_LAT_Architecture_Network_Managers_Guide_Jul85.pdf
7d77520f56863b7dda92c785facf8ef9d8bd5a638406eaa1157a6ca2c055dc83  AA-NL26A-TE_LAT_Specification_Jun89.pdf
4ed56a5c5f49ce4326a677dbb77a7a990090762836e11ef6f2279596f5d67909  LAT_Protocol_Dec83.pdf
```

The June 1989 specification is the readable scan; the other two are earlier and
OCR quality is poor (image-only titles), but their figures/tables are intact.

## Code (`code/`)

### `code/latd/` — Local Area Transport Daemon

- **What:** userspace LAT implementation for Linux/BSD (login services,
  Reverse-LAT `/dev/lat/*`, `latcp`, `llogin`, `moprc`, group codes, ratings).
  Also the closest working reference for the *port/terminal side* of LAT.
- **Upstream:** <https://github.com/cguldenschuh/latd> (originally the
  Linux DECnet project, <https://sourceforge.net/projects/linux-decnet/files/latd/>).
- **Version:** 1.36 · commit `a5dd59e7c13ccd2ec6a031db53e439fc64db2c6b` (2026-09-21)
  · local `.git` retained.
- **License:** GPL-2.0 (`COPYING`).
- **Note:** this git history was created from the Debian source package
  ("Creating 1.35 git repo; from debian source"), so it holds 5 commits, not the
  full upstream history.
- **Provenance:** clean-room reverse engineering — per its own `README`:
  *"This code is a clean-room reverse-engineering effort. I have no access to
  HP technical documentation or source code for LAT and nor do I want any."*
  So it is an independent implementation to test the spec against, not derived
  from DEC source.

Key files for protocol work: `lat.h`, `lat_messages.cc` / `lat_messages.h`
(message encode/decode), `circuit.cc` (virtual circuits), `session.cc`,
`clientsession.cc` / `serversession.cc` / `reversesession.cc` /
`queuedsession.cc` (session types), `interfaces*.cc` (AF_PACKET / BPF framing),
`server.cc`, `services.cc`, `utils.cc`.

### `code/wireshark-lat/` — Wireshark LAT dissector

- **File:** `packet-lat.c` — the production packet decoder for LAT (Ethertype
  `0x6004`).
- **Upstream:** <https://gitlab.com/wireshark/wireshark> →
  `epan/dissectors/packet-lat.c` (branch `master`).
- **License:** GPL-2.0-or-later.
- **Why useful:** the dissector's own header says its field layout is taken from
  the `AA-NL26A-TE` spec in `manuals/` — a ready-made, field-by-field decoding of
  the wire format for validating captured VT1200 traffic.

## Licensing

The DEC manuals are DEC-proprietary (the 1989 spec is stamped *DIGITAL
Proprietary and Confidential*) and are kept here for local research only; do not
redistribute them.  The code folders keep their upstream licenses
(`latd` GPL-2.0, Wireshark `packet-lat.c` GPL-2.0-or-later).

## Relevance to the VT1200

- **Wire format:** `AA-NL26A-TE_LAT_Specification_Jun89.pdf` is normative.
- **Field decoding / pcap inspection:** `code/wireshark-lat/packet-lat.c`.
- **Interoperable behavior / state machines:** `code/latd/`.
- **Remaining VT1200-specific work (not covered by the above):** how the
  firmware wires LAT into the X server (`LAT X Session Service Select`), the VTE
  window service (`LAT Terminal Window Service Select`), name-server and group
  handling, and its circuit-timer policy.