DECnet Digital Network Architecture (Phase IV) Token Ring Data Link and Node Product Functional Specification

This document, published by Digital Equipment Corporation in July 1992, specifies the functional requirements for implementing DECnet Digital Network Architecture (Phase IV) products on IEEE 802.5 / ISO 8802-5 Token Ring Local Area Networks (LANs).

Its primary goal is to enable DECnet operation on Token Ring with minimal architectural changes to existing Ethernet-based DECnet products. To achieve this, the specification details the use of "Ethernet emulation" at the data link layer, which involves translating Ethernet frames to the 802.2 format and mapping universal multicast addresses to Token Ring's specific Functional Addresses due to hardware limitations. It mandates support for Source Routing for packets within extended Token Ring LANs, aiming for transparency to higher-level Ethernet-emulating applications.

A significant new feature, "Phase IV-Prime," is introduced, allowing DECnet Phase IV to operate using arbitrary MAC Individual Station Addresses, which improves coexistence with other network protocols. However, this also implies that routers, especially "bilingual" ones, are necessary for interoperability between Phase IV-Prime Token Ring nodes and Ethernet LANs, as direct bridging is not supported.

The document also addresses the "bit order problem," a key interoperability challenge arising from different MAC address bit representations between Token Ring and Ethernet, recommending that device drivers handle this conversion transparently.

Overall, the specification provides the technical framework for DECnet Phase IV products to function effectively and interoperate within Token Ring environments, particularly in mixed LAN setups.