From a technical standpoint DECnet was built right into the DEC flagship operating system (VAX/VMS), being the foundation for the first cluster architecture ever designed, the VAXcluster.
Initially built with four layers it later (1992) evolved into a seven layer OSI compliant networking protocol, around the time when open systems (POSIX compliant, i.e. UNIX-like) were grabbing marketshare from the proprietary OSes like VAX/VMS and AlphaVMS.
It has been ported to UNIX (Ultrix), PCs and Macs (DEC Pathworks) transforming these into DECnet end-nodes on a network of VAXes.
DECnet refers to a specific set of hardware and software networking products which implement the DIGITAL Network Architecture (DNA). The DIGITAL Network Architecture is essentially a set of documents which define the network architecture in general, states the specifications for each layer of the architecture and describes the protocols which operate within each layer. Strictly speaking, non-routed DIGITAL protocols such as LAT, SCS, AMDS, LAST/LAD are not DECnet and are not part of the DIGITAL Network
Architecture.
To trace the evolution of DECnet is to trace the development of DNA. The beginnings of DNA were in the early 1970s. DIGITAL published its first DNA specification at about the same time that IBM announced its Systems Network Architecture (SNA). Since that time, development of DNA has evolved through the following phases:
Phase I (1974)
Support limited to 2 PDP-11s running the RSX-11 operating system only, with communication over point-to-point (DDCMP) links between nodes.
Phase II (1976)
Support for networks of up to 32 nodes with multiple, different implementations which could interoperate with each other. Implementations expanded to included RSTS, TOPS-10 and TOPS-20 with communications between processors still limited to point-to-point links only. Introduction of file transfer (FAL), remote file access (DAP), task-to-task programming interfaces and network management features.
Phase III (1980).
Support for networks of up to 255 nodes over point-to point and multi-drop links. Introduction of adaptive routing capability, downline loading (MOP), record access, a network management architecture, and gateways to other types of networks including IBM’s SNA and CCITT Recommendation X.25.
Phase IV and Phase IV+ (1982).
Support for networks of up to 64,449 nodes, datalink capabilities expanded beyond DDCMP to include Ethernet local area network support as the datalink of choice, expanded adaptive routing capability to include hierarchical routing (areas, level 1 and level 2 routers), VMScluster support (cluster alias) and host services (CTERM). Phase IV implemented an 8 layer architecture similar to the OSI (7 layer) model especially at the lower levels (see diagram below). Since the OSI standards were not yet fully developed at the time, many of the Phase IV protocols remained proprietary. The initial implementations released were for VMS and RSX-11,
later this expanded to virtually every operating system DIGITAL ever shipped with the notable exception of RT-11.
Brief Overview of the Evolution of DECnet