From Academic Kids

DECnet is a proprietary suite of network protocols created by Digital Equipment Corporation, originally released in 1975 in order to connect two PDP-11 minicomputers. It evolved into one of the first peer-to-peer network architectures, thus making DEC into a networking powerhouse in the 1980s.

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.

Digital ported it to its own Ultrix variant of UNIX, as well as Apple Macintosh computers and PCs running both DOS and Windows under the name DEC Pathworks, transforming these sytems into DECnet end-nodes on a network of VAXes. More recently, an open-source version has been developed for the Linux OS: see Linux-DECnet on Sourceforge (

Brief Overview of the Evolution of DECnet

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. Although network protocol analyzer tools tend to categorize all protocols from DIGITAL as "DECnet", strictly speaking, non-routed DIGITAL protocols such as LAT, SCS, AMDS, LAST/LAD are not DECnet protocols 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.

DECnet Phase IV protocol suite
Application FAL: File Access Listener
NML: Network Management Listener
Presentation DAP: Data Access Protocol
CTERM: Command Terminal
Session SCP: Session Control Protocol
Transport NSP: Network Service Protocol
Network DRP: DECnet Routing Protocol
Data link DDCMP: Digital Data Communications Message Protocol
MOP: Maintenance Operation Protocol
Ethernet, Token ring, HDLC, FDDI, ...
Physical Ethernet, Token ring, FDDI, ...

Phase IV and Phase IV+ (1982). Support for networks of up to 64,449 nodes (63 areas of 1023 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). CTERM allowed a user on one computer to log into another computer remotely, performing the same function that Telnet does in the TCP/IP protocol stack.

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 Ethernet implementation was unusual in that the software changed the physical address of the Ethernet interface on the network to AA-00-04-00-xx-yy where xx-yy reflected the DECnet network address of the host. This allowed router-less LAN operation because the LAN address could be deduced from the DECnet address.

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.

At the same time that DECnet Phase IV was released, the company also released a proprietory protocol called LAT for terminal access via Terminal servers.

Enhancements made to DECnet Phase IV eventually became known as DECnet Phase IV+, although systems running this protocol remained completely interoperable with DECnet Phase IV systems.

Missing image
alt:Comparison of the DECnet Phase IV and the OSI Reference Models

Phase V and Phase V+ (1987). Support for very large (architecturally unlimited) networks, a new network management model, local or distributed name service, improved performance over Phase IV. Move from a proprietary network to a Open Systems Interconnect (OSI) by integration of ISO standards to provide multi-vendor connectivity and compatibility with DNA Phase IV, the last two features resulted in a hybrid network architecture (DNA and OSI) with separate “towers” sharing an integrated transport layer. Transparent transport level links to TCP/IP were added via the IETF RFC 1006 (OSI over IP) and RFC 1859 (NSP over IP) standards (see diagram below).

It was later renamed DECnet/OSI to emphasise its OSI interconnectibility, and subsequently DECnet-Plus as TCP/IP protocols were incorporated.

alt:The DECnet Phase V+ Reference Model


  • Carl Malamud, Analyzing DECnet/OSI Phase V. Van Hostrand Reinhold, 1991. ISBN 0442003757.

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