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README.md

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.
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