DEC STD 121

This document, DEC Standard 121 (Version 4.0, dated March 1, 1978), defines the Digital Data Communications Message Protocol (DDCMP).

Key aspects of DDCMP:

• Purpose: It is a data link procedure designed to create a reliable, error-free, and sequentially ordered data communication path between connected devices over various data links.
• Core Functions: It ensures the correct sequencing and integrity of data transmitted.

• Features:

  • Error Handling: Employs 16-bit Cyclic Redundancy Check (CRC-16) for error detection and relies on retransmission for error correction.
  • Message Sequencing: Supports message pipelining, allowing up to 255 outstanding (unacknowledged) messages to be in transit simultaneously.
  • Versatility: Operates independently of channel bit width (serial or parallel) and transmission characteristics (synchronous or asynchronous). It functions across full-duplex, half-duplex, point-to-point, and multipoint communication channels.
  • Efficiency: Designed for simplicity and efficiency with a limited number of message formats, and ensures data transparency using a length field.
  • Operational Modes: Includes a positive startup procedure to synchronize the link and a maintenance mode for diagnostic testing and bootstrapping.

• Context: DDCMP was developed by Digital Equipment Corporation (DEC) starting in 1973 to address inefficiencies in older character-oriented protocols (like Bisync) and to operate effectively with existing DEC communication interfaces. It was developed concurrently with other link-level protocols like IBM's SDLC and ANSI's ADCCP.

• Scope: DDCMP operates at the physical link level. Higher-level protocols (e.g., DECnet's Network Services Protocol) are required for functions like routing, flow control, and device control. It is not intended to instruct users unfamiliar with basic data communications principles, nor is it optimal for scenarios with extremely high/low error rates or very short messages in some multipoint configurations.
• Audience: Primarily intended to assist individuals implementing DDCMP, but also provides general information for others needing to understand the protocol.