DECnet is a 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 transforming DEC into a networking
powerhouse in the 1980s. Initially built with three layers, it later
(1982) evolved into a seven-layer OSI-compliant networking protocol.
DECnet was built right into the DEC flagship operating system VMS
since its inception. Later Digital ported it to Ultrix, as well as
Apple Macintosh and
IBM PC running variants of
DOS and Microsoft
Windows under the name DEC Pathworks, allowing these systems to
DECnet networks of
VAX machines as terminal nodes. An
open-source version was developed for Linux.
DECnet code in the
Linux kernel was marked as orphaned on February 18, 2010.
4 External links
4.1 Inter-campus DECnet
DECnet refers to a specific set of hardware and software networking
products which implement the DIGITAL Network Architecture (DNA). The
DIGITAL Network Architecture has a set of documents which define the
network architecture in general, state the specifications for each
layer of the architecture, and describe 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 two PDP-11s running the RSX-11
operating system only, with communication over point-to-point (DDCMP)
links between nodes.
Phase II (1975) Support for networks of up to 32 nodes with
multiple, different implementations which could inter-operate with
each other. Implementations expanded to include RSTS,
TOPS-20 with communications between processors still limited to
point-to-point links only. Introduction of downline loading (MOP), and
file transfer using File Access Listener (FAL), remote file access
using Data Access Protocol (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, record access, a network management architecture, and
gateways to other types of networks including IBM’s SNA and CCITT
DECnet Phase IV protocol suite
DAP: Data Access Protocol
CTERM: Command Terminal
NICE: Network Information (and) Control Exchange
MOP: Maintenance Operation Protocol
SCP: Session Control Protocol
NSP: Network Service Protocol
DECnet Routing Protocol
DDCMP: Digital Data Communications Message Protocol
Ethernet, Token ring, HDLC, FDDI, ...
Ethernet, Token ring, FDDI, ...
Phase IV and Phase IV+ (1982). Phase IV was released initially to
RSX-11 and VMS systems, later TOPS-20, TOPS-10, ULTRIX, VAXELN, and
RSTS/E gained support. Support for networks of up to 64,449 nodes (63
areas of 1023 nodes), datalink capabilities expanded beyond
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
Telnet does in the
TCP/IP protocol stack. Digital also released a
product called the PATHWORKS client, and more commonly known as the
PATHWORKS 32 client, that implemented much of
DECnet Phase IV for DOS,
and 16 and 32 bit
Windows platforms (all the way through to
Windows Server 2003).
Phase IV implemented an 8 layer architecture similar to the OSI (7
layer) model especially at the lower levels. Since the OSI standards
were not yet fully developed at the time, many of the Phase IV
protocols remained proprietary.
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 ARP-less LAN operation because the LAN address
could be deduced from the
DECnet address. This precluded connecting
two NICs from the same
DECnet node onto the same LAN segment, however.
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.
DECnet stacks are found on Linux,
SunOS and other platforms, and Cisco and other network vendors offer
products that can cooperate with and operate within
DECnet Phase IV specifications are available.
At the same time that
DECnet Phase IV was released, the company also
released a proprietary protocol called LAT for serial terminal access
via Terminal servers. LAT shared the OSI physical and datalink layers
DECnet and LAT terminal servers used MOP for the server image
download and related bootstrap processing.
Enhancements made to
DECnet Phase IV eventually became known as DECnet
Phase IV+, although systems running this protocol remained completely
DECnet Phase IV systems.
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 an
Open Systems Interconnection
Open Systems Interconnection (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
TCP/IP were added via the IETF RFC 1006 (OSI over IP) and RFC
1859 (NSP over IP) standards (see diagram).
It was later renamed DECnet/OSI to emphasize its OSI
interconnectibility, and subsequently DECnet-Plus as
DECnet protocols were designed entirely by Digital Equipment
DECnet Phase II (and later) were open standards
with published specifications, and several implementations were
developed outside DEC, including ones for
FreeBSD and Linux.
Linux Kernel Changelog, 2010-02-18. Christine Caulfield, Orphan
DECnet "Archived copy". Archived from the original on 2011-08-11.
^ "Digital Equipment Corporation, 1957 to the present", 1978, page 53
Carl Malamud, Analyzing DECnet/OSI Phase V. Van Hostrand Reinhold,
1991. ISBN 0-442-00375-7.
James Martin, Joe Leben,
DECnet Phase V: An OSI Implementation.
Digital Press, 1992. ISBN 1-55580-769-0.
DECnet-Plus manuals for
OpenVMS are available at
DECnet Phase IV
OpenVMS manuals for
DECnet Phase IV; these Phase IV
manuals are archived on
OpenVMS Freeware V5.0 distribution, at
http://www.hp.com/go/openvms/freeware and other sites.
DECnet Phase IV architecture manuals (including DDCMP, MOP, NICE, NSP,
DAP, CTERM, routing); at http://h71000.www7.hp.com/wizard/decnet/ (the
originals are mirrored at
DECnet for Linux).
Cisco documentation of DECNet
CCNET (Computer Center Network, 1980s; more info here)