DECnet Digital Network Architecture Phase IV Routing Layer Functional Description

This document, titled "DECnet Digital Network Architecture Phase IV: Routing Layer Functional Specification" (Order No. AA-X435A-TK, December 1983, Version 2.0.0), comprehensively details the structure, functions, interfaces, protocols, and algorithms for implementing the Routing Layer within DECnet communications networks.

Core Purpose: The Routing Layer is responsible for controlling the routing of messages (packets) between nodes in a DECnet network.

Key Architectural Concepts:

• Hierarchical Routing: Phase IV introduces hierarchical routing to support large networks, partitioning them into "areas." "Level 1 routing" manages traffic within an area, while "Level 2 routing" handles inter-area communication. Node addresses reflect this hierarchy (top 6 bits for area, bottom 10 for node ID).
• Node Types: The specification defines various node types: Phase IV endnodes (do not route), Level 1 routers (route within their area and to Level 2 routers for foreign areas), Level 2 routers (route between areas), and maintains compatibility with older Phase III routers and endnodes (though with limitations regarding Ethernet support and hierarchical addressing).
• Sublayers: The Routing Layer comprises a Control Sublayer (handles packet transmission, routing decisions, congestion control, and packet lifetime control) and an Initialization Sublayer (manages data link characteristics and node discovery).

Key Functions:

• Routing Process: Selects the best paths for packets, forwards them to their destinations, and can find alternate routes if a path fails. It maintains a Routing Data Base (connectivity, costs, minimum hops) and a Forwarding Data Base (reachability, output adjacencies).
• Congestion Control: Manages buffers at packet-switching nodes, limits packets on transmit queues, and regulates the ratio of packets received from the End Communications Layer (ECL) versus those routed through the node. It includes mechanisms like a Square Root Limiter, Originating Packet Limiter, Flusher, and Packet Size Checker.
• Packet Lifetime Control: Prevents packets from looping indefinitely by tracking their visits to nodes and discarding them if a visit limit is exceeded (Loop Detector). It also includes Node Listener (detects adjacent node activity) and Node Talker (sends Hello messages).
• Initialization: Handles the start-up procedures between adjacent nodes, exchanging control messages to verify node identities and capabilities.

Interfaces: The document specifies interfaces with:

• Network Management Layer: For configuration (setting parameters like buffer sizes, max hops, timers), monitoring (reading counters and events), and controlling routing operations.
• Data Link Layer: For transmitting and receiving data, controlling circuits (initialize, stop), and reporting errors or status changes.
• End Communications Layer: For sending data packets to and receiving them from the Routing Layer.

Design Goals & Exclusions: The Routing Layer is designed for deliverability, adaptability (to topological changes, not traffic), promptness, efficiency, robustness, stability, operator control, simplicity, and maintainability. However, it explicitly states it does not perform traffic adaptation, distinguish service classes, support source-destination routing, protect against gross operator failures, or guarantee delivery of all packets.

Message Formats: Detailed specifications are provided for various packet and control message formats, including short and long data packet headers, Initialization Messages, Verification Messages, Hello and Test Messages, and Level 1/Level 2 Routing Messages.

Appendices offer further details on addressing, network topologies, non-routing node operations, compatibility with Phase III, event counters for diagnostics, buffer management algorithms, and a comprehensive revision history outlining changes from previous DECnet versions.