Editing DECnet (section)

==Evolution==
DECnet refers to a specific set of hardware and software networking products which implement the '''DIGITAL Network Architecture''' (DNA).  This architecture is set out in a number of documents which define the network architecture in general, state the specifications for each layer of the architecture, and describe the [[protocol stack|protocols]] which operate within each layer.  Although [[packet sniffer|network protocol analyzer tools]] tend to categorize all protocols from DIGITAL as "DECnet", strictly speaking, non-routed DIGITAL protocols such as [[Local Area Transport|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:

===1970–1980===
'''Phase I''' (1974)
Support limited to two [[PDP-11]]s running the [[RSX-11]] operating system, or a small number of [[PDP-8]]s running the [[RTS-8]] operating system, with communication over point-to-point ([[DDCMP]]) links between nodes.

'''Phase II''' (1975)<ref>{{cite web |author=Digital Equipment Corporation |date=1978 |url=http://archive.computerhistory.org/resources/text/dec/dec.digital_1957_to_the_present_(1978).1957-1978.102630349.pdf |title=Nineteen Fifty-Seven to the Present |page=53 |publisher=[[Computer History Museum]] |access-date=June 26, 2018 |archive-url=https://web.archive.org/web/20131221052641/http://archive.computerhistory.org/resources/text/dec/dec.digital_1957_to_the_present_(1978).1957-1978.102630349.pdf |archive-date=December 21, 2013 |url-status=dead }}</ref>
Support for networks of up to 32 nodes with multiple, different implementations which could inter-operate with each other. Implementations expanded to include [[RSTS/E|RSTS]], [[TOPS-10]], [[TOPS-20]] and [[VAX/VMS]]<ref>{{cite web |url=https://archive.org/download/bitsavers_decspd25.0_392688/25.03.01_7809_DECnet-VAX_V1.pdf |title=Software Product Description: DECnet-VAX, Version 1 |date=September 1978 |publisher=DEC |access-date=2023-05-23}}</ref> 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).<ref>{{cite web
   |url=http://www.bitsavers.org/pdf/dec/decnet/AA-AV51A-TK_intrPhIII_May82.pdf
   |title=Introduction to DECnet (Phase III)
   |date=May 22, 1982}}</ref> Support for networks of up to 255 nodes with 8-bit addresses, 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 [[ITU-T|CCITT]] Recommendation [[X.25]].

<div style="background:#f9f9f9;float:right;margin:15px;margin-top:0px;padding:0.2em;border: 1px solid #aaa;font-size:88%">
{| border="0" cellspacing="2" cellpadding="2"
|+ '''DECnet Phase IV protocol suite'''
|----- align="left"
! Application
|  DAP: Data Access Protocol<BR>
CTERM: Command Terminal
|----- align="left"
! Network Management
| NICE: Network Information (and) Control Exchange<br>
MOP: [[Maintenance Operations Protocol]]
|----- align="left"
! Session
| SCP: Session Control Protocol
|----- align="left"
! Transport
| NSP: Network Service Protocol
|----- align="left"
! Network
| DRP: DECnet Routing Protocol
|----- align="left"
! Data link
| DDCMP: Digital Data Communications Message Protocol<br>[[Ethernet]], [[Token Ring]], [[HDLC]], [[Fiber distributed data interface|FDDI]], ...
|----- align="left"
! Physical
| [[Ethernet]], [[Token Ring]], [[Fiber distributed data interface|FDDI]], ...
|}
</div>

===1981–1986===
'''Phase IV'''<ref>{{cite web
   |url=https://www.cisco.com/c/en/us/td/docs/ios/decnet/configuration/guide/2_xe/dec_2_xe_book/overview_decnet.pdf
   |title=Overview of DECnet}}</ref><ref>{{cite web
       |url=http://linux-decnet.sourceforge.net/docs/doc_index.html |title=The DECnet Phase IV Specifications}}</ref> and '''Phase IV+''' (1982).
Phase IV was released initially to [[RSX-11]] and [[OpenVMS|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) with 16-bit addresses, 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. 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 Microsoft Windows platforms (all the way through to Windows Server 2003).

Phase IV implemented an 8 layer architecture similar to the [[OSI model|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.

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 ARP-less LAN operation because the LAN address could be deduced from the DECnet address.  This precluded connecting two [[Network interface controller|NICs]] from the same DECnet node onto the same LAN segment, however.

The initial implementations released were for VAX/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 networks.  Full DECnet Phase IV specifications are available.

At the same time that DECnet Phase IV was released, the company also released a proprietary protocol called [[Local Area Transport|LAT]] for serial terminal access via [[Terminal server]]s. LAT shared the OSI physical and datalink layers with 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 interoperable with DECnet Phase IV systems.

===1987 & beyond===
'''Phase V'''<ref>{{cite book  |isbn=978-0-1320-2987-2
    |title=DECnet Phase V: An OSI Implementation Network
    |author1=James Martin |author1-link=James Martin (author)  |author2=Joel Leben|year=1992
 |publisher=Digital Press
 }}</ref> 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]] (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 [[Internet Engineering Task Force|IETF]] [[Request for Comments|RFC]] 1006 (OSI over IP) and RFC 1859 (NSP over IP) standards ''(see diagram)''.

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