DDCMP

The Digital Data Communications Message Protocol (DDCMP), Version 4.0, dated March 1, 1978, is a data link control procedure developed by Digital Equipment Corporation. It is designed to ensure a reliable and error-free communication path between devices by providing data integrity and correct message sequencing over physical data links.

Key aspects and features of DDCMP include:

• Error Control: It uses 16-bit Cyclic Redundancy Checks (CRC-16) for error detection and error correction via retransmission.
• Message Sequencing: Supports pipelining, allowing up to 255 outstanding messages to be in transit.
• Versatility: DDCMP operates independently of the underlying channel's bit width (serial or parallel) and transmission characteristics (synchronous or asynchronous). It supports common operation over full-duplex, half-duplex, point-to-point, and multipoint channels.
• Data Transparency: Achieved by using a length field, allowing any bit sequence within the data field.
• Link Management: Includes a positive startup procedure to synchronize both ends of a link and a maintenance mode for diagnostic testing and system bootstrapping functions.
• Functional Organization: It is divided into three core components:
  • Framing: Responsible for locating the beginning and end of messages, encompassing bit, byte, and message synchronization.
  • Link Management: Controls transmission and reception on channels where multiple transmitters/receivers might be present (e.g., half-duplex, multipoint), using a "SELECT" flag to manage transmission ownership.
  • Message Exchange: Implements a positive acknowledgment retransmission protocol to ensure sequential, error-free data transfer, utilizing message numbering, acknowledgments (ACKs), negative acknowledgments (NAKs), and reply timers.

DDCMP serves as the Physical Link Control Layer within DECnet's Digital Network Architecture (DNA) but is designed to be used independently in various distributed computer network applications. This document primarily functions as a specification for implementers, offering detailed information on the protocol's functions, characteristics, capabilities, and operation.