Local Area Transport (LAT) Specification

This document details the Local Area Transport (LAT) communication architecture for Ethernet local area networks, designed primarily for efficient data exchange between terminal servers and host operating systems. Its purpose is to enable interoperable implementations by prioritizing simplicity and leveraging LAN characteristics.

The original LAT architecture (version 5.0) features asymmetric master/slave connections, where terminal servers (masters) initiate connections to hosts (slaves). It supports multiplexing multiple user sessions over a single virtual circuit and relies on hosts advertising their services via multicast messages.

LAT 5.1, a significant update, introduces symmetry, allowing hosts (slaves) to initiate connections to servers (masters) for application terminals. Key enhancements include:

• Bidirectional Connection Initiation: Both masters and slaves can initiate connections.
• Service Advertising: Servers can now also advertise their offered services.
• Queued Access: Support for queued connection requests to services, improving resource management.
• Port Names: Introduction of named ports for direct user connections.
• Access Control: Clarification of Group Codes through Access Control Lists (ACLs) and Identifier Lists (IDLs).
• Compatibility: Mechanisms to ensure compatibility between LAT 5.0 and LAT 5.1 implementations.

The architecture is layered, comprising a Slot Layer responsible for user session establishment, data transfer, and multiplexing, and a Virtual Circuit Layer that establishes and maintains reliable message exchange while translating node names to Ethernet addresses. Communication is predominantly timer-based, optimizing bandwidth usage on the shared Ethernet medium. The document provides detailed specifications for naming conventions, connection solicitation processes, message formats (Start, Run, Stop messages, and various slot types), and the algorithms governing virtual circuit and session management. It operates under the assumption of a local, high-bandwidth Ethernet environment with minimal data loss or corruption.