DECnet is a
group of data-communications products, including a protocol suite,
developed and supported by Digital Equipment Corporation. The first
version of DECnet,
released in 1975, allowed two directly attached PDP-11 minicomputers to
communicate. In more recent years, Digital has included support for
nonproprietary protocols, but DECnet remains the most important of
Digital's network product offerings. This chapter provides a summary of
the DECnet protocol suite, Digital's networking architectures, and the
overall operation of DECnet traffic management.
Figure 28-1 illustrates a DECnet
internetwork with routers interconnecting two LANs that contain
workstations and VAXs.
Several versions of DECnet have been
released. The first allowed two directly attached minicomputers to
communicate.
Subsequent releases expanded the DECnet
functionality by adding support for additional proprietary and standard
protocols, while remaining compatible with the immediately preceding
release. This means the protocols are backward-compatible. Currently,
two versions of DECnet are in wide use: DECnet Phase IV and DECnet/OSI.
Figure 28-1: In a DECnet-based
internetwork routers interconnect workstations and VAXs.
DECnet Phase
IV is the most widely implemented version of DECnet. However, DECnet/OSI
is the most recent release. DECnet Phase IV is based on the Phase
IV Digital Network Architecture (DNA), and it supports proprietary
Digital protocols and other proprietary and standard protocols. DECnet
Phase IV is backward-compatible with DECnet Phase III, the version that
preceded it.
DECnet/OSI (also called DECnet
Phase V) is backward-compatible with DECnet Phase IV and is the most
recent version of DECnet. This version is based on the DECnet/OSI DNA.
DECnet/OSI supports a subset of the OSI protocols, multiple proprietary
DECnet protocols, and other proprietary and standard protocols.
DECnet Phase
IV Digital Network Architecture (DNA)
The Digital Network Architecture (DNA)
is a comprehensive layered network architecture that supports a
large set of proprietary and standard protocols. The Phase IV DNA is
similar to the architecture outlined by the OSI reference model. As with
the OSI reference model, the Phase IV DNA utilizes a layered approach,
whereby specific layer functions provide services to protocol layers
above it and depend on protocol layers below it. Unlike the OSI model,
however, the Phase IV DNA is composed of eight layers. Figure 28-2
illustrates how the eight layers of the Phase
IV DNA relate to the OSI reference model.
Figure 28-2: Phase
IV consists of eight layers that map to the OSI layers.
The following section details the
functionality and role of each of these layers and identifies the
similarities between the Phase IV DNA architecture and the OSI reference
model.
The DECnet Phase IV DNA defines
an eight-layer model, as illustrated in Figure 28-2. The user
layer represents the user-network interface,
supporting user services and programs with a communicating component.
The user layer corresponds roughly to the OSI application layer. The network
management layer represents the user interface
to network-management information. This layer interacts with all the
lower layers of the DNA and corresponds roughly with the OSI application
layer. The network application layer
provides various network applications, such as remote file access and
virtual terminal access. This layer corresponds roughly to the OSI
presentation and application layers. The session
control layer manages logical link connections
between end nodes and corresponds roughly to the OSI session layer. The end
communications layer handles flow control,
segmentation, and reassembly functions and corresponds roughly to the
OSI transport layer. The routing layer
performs routing and other functions, and corresponds roughly to the OSI
network layer. The data link layer manages physical network
channels and corresponds to the OSI data link layer. The physical
layer manages hardware interfaces and determines
the electrical and mechanical functions of the physical media, and this
layer corresponds to the OSI
physical layer.
DECnet addresses are not
associated with the physical networks to which the nodes are connected.
Instead, DECnet locates hosts using area/node address pairs. An area's
value ranges from 1 to 63, inclusive. Likewise, a node
address can be between 1 and 1,023, inclusive. Therefore, each area can
have 1,023 nodes, and approximately 65,000 nodes can be addressed in a
DECnet network. Areas can span many routers, and a single cable can
support many areas. Therefore, if a node has several network interfaces,
it uses the same area/node address for each interface. Figure 28-3
illustrates a sample DECnet network with several addressable entities.
Figure 28-3: DECnet locates
hosts using area/node address pairs.
DECnet hosts do not use
manufacturer-assigned Media Access Control (MAC)--layer
addresses. Instead, network-level addresses are embedded in the
MAC-layer address according to an algorithm that multiplies the area
number by 1,024 and adds the node number to the product. The resulting
16-bit decimal address is converted to a hexadecimal number and appended
to the address AA00.0400 in byte-swapped order, with the
least-significant byte first. For example, DECnet address 12.75 becomes
12363 (base 10), which equals 304B (base 16). After this byte-swapped
address is appended to the standard DECnet MAC address prefix, the address
is AA00.0400.4B30.
DECnet/OSI
Digital Network Architecture (DNA)
The DECnet/OSI (DECnet Phase V) DNA is
very similar to the architecture outlined by the OSI
reference model. DECnet Phase V utilizes a layered approach that
achieves a high degree of flexibility in terms of support for
upper-layer protocol suites. As the following section discusses, DECnet
OSI actually allows for the support of multiple protocol suites.
The DECnet/OSI DNA defines a layered
model that implements three protocol suites: OSI, DECnet, and
Transmission Control Protocol/Internet Protocol (TCP/IP). The OSI
implementation of DECnet/OSI conforms to the seven-layer OSI reference
model and supports many of the standard OSI protocols. The Digital
implementation of DECnet/OSI provides backward compatibility with DECnet
Phase IV and supports multiple proprietary Digital protocols. The TCP/IP
implementation of DECnet/OSI supports the lower-layer TCP/IP protocols
and enables the transmission of DECnet traffic over TCP transport
protocols. Figure 28-4 illustrates the three
DECnet/OSI implementations:
Figure 28-4: The
OSI, DECnet, and TCP are all supported by DECnet/OSI DNA.
DECnet Phase IV and DECnet/OSI support a
variety of media-access implementations at the physical and data link
layers. This has contributed to the relatively wide acceptance of DECnet
in the computer networking industry. As explained in the following
sections, both DECnet Phase IV and Phase V can support many of the
common physical and data-link technologies in use today.
At the physical layer, DECnet Phase IV
and DECnet/OSI support most of the popular physical implementations,
including Ethernet/IEEE 802.3, Token Ring/IEEE 802.5, and Fiber
Distributed Data Interface (FDDI). In addition, DECnet/OSI supports
Frame Relay and X.21bis.
At the data link layer, DECnet Phase IV
and DECnet/OSI support IEEE 802.2 Logical Link Control (LLC),
Link-Access Procedure, Balanced (LAPB), Frame Relay, and High-Level Data
Link Control (HDLC). Both DECnet Phase IV and DECnet/OSI also support
the proprietary Digital data-link protocol, Digital Data Communications
Message Protocol (DDCMP), which provides point-to-point and multipoint
connections; full-duplex or half-duplex communication over synchronous
and asynchronous channels; and error correction, sequencing, and
management.
DECnet routing occurs at
the routing layer of the DNA in DECnet Phase IV and at the network layer
of the OSI model in DECnet/OSI. The routing implementations in both
DECnet Phase IV and DECnet/OSI, however, are similar.
DECnet Phase IV routing is implemented by
the DECnet Routing Protocol (DRP), which is a relatively simple and
efficient protocol whose primary function is to provide optimal path
determination through a DECnet Phase IV network. Figure
28-5 provides a sample DECnet network to illustrate how the routing
function is performed in a DECnet Phase IV network.
Figure 28-5: The DRP
determines the optimal route through a DECnet Phase IV network.
DECnet routing decisions
are based on cost, an arbitrary measure assigned by network
administrators to be used in comparing various paths through an
internetwork environment. Costs typically are based on hop count or
media bandwidth, among other measures. The lower the cost, the better
the path. When network faults occur, the DRP uses cost values to
recalculate the best paths to each destination.
DECnet/OSI routing is implemented by the
standard OSI routing protocols (ISO 8473, ISO 9542, and ISO 10589) and
by DRP. Detailed information on the OSI routing protocols can be found
in "Open
System Interconnection (OSI) Protocols."
DECnet Phase IV supports a single
transport protocol at the DNA end-communications
layer, the Network-Services Protocol (NSP).
The Network-Services
Protocol (NSP)
is a proprietary, connection-oriented, end-communications protocol
developed by Digital that is responsible for creating and terminating
connections between nodes, performing message fragmentation and
reassembly, and managing error control.
NSP also manages two types of flow
control: a simple start/stop mechanism in which the receiver tells the
sender when to terminate and resume data transmission, and a more
complex scheme in which the receiver tells the sender how many messages
it can accept.
DECnet/OSI supports NSP, three standard
OSI transport protocols, and the Transmission Control Protocol (TCP).
DECnet/OSI supports Transport Protocol
classes (TP) 0, TP2, and TP4. TP0 is the simplest OSI
connection-oriented transport protocol. Of the classic transport-layer
functions, it performs only segmentation and reassembly. This means that
TP0 will note the smallest maximum-size protocol data unit (PDU)
supported by the underlying subnetworks, and will break the transport
packet into smaller pieces that are not too big for network
transmission. TP2 can multiplex and demultiplex data streams over a
single virtual circuit. This capability makes TP2 particularly useful
over public data networks (PDNs), where each virtual circuit incurs a
separate charge. As with TP0 and TP1, TP2 also segments and reassembles
PDUs, while TP3 combines the features of TP1 and TP2. TP4, the most
popular OSI transport protocol, is similar to the Internet protocol
suite's TCP and, in fact, was based on that model. In addition to TP3's
features, TP4 provides reliable transport service and assumes a network
in which problems are not detected.
Request For Comments (RFC) 1006 and RFC
1006 Extensions define an implementation of OSI transport-layer
protocols atop the TCP. RFC 1006 defines the implementation of OSI
Transport Protocol class 0 (TP0) on top of TCP. RFC 1006 Extensions
define the
implementation of Transport Protocol class 2 (TP2) on top of TCP.
The DECnet Phase IV DNA specifies four
upper layers to provide user interaction services, network-management
capabilities, file transfer, and session management. Specifically, these
are referred to as the user layer, network-management layer,
network-application layer, and session-control layer. The upper layers
of the DECnet Phase IV architecture are discussed in more detail in the
following sections.
The DNA user
layer supports user services and programs that interact with user
applications. The end user interacts directly with these applications,
and the applications use the services and programs provided by the user
layer.
The network-management
protocol widely used in DECnet networks is the proprietary Digital
Network Information and Control Exchange (NICE) protocol. NICE is a
command-response protocol. Commands, which request an action, are issued
to a managed node or process; responses, in the form of actions, are
returned by those nodes or processes. NICE performs a variety of network
management-related functions and can be used to transfer an operating
system from a local system into a remote system, as well as enable an
unattended remote system to dump its memory to the local system.
Protocols using NICE can examine or change certain characteristics of
the network. NICE supports an event logger that automatically tracks
important network events, such as an adjacency change or a circuit-state
change. NICE supports functions that accommodate hardware and
node-to-node loop tests.
Certain network management functions can
use the Maintenance
Operations Protocol (MOP), a collection of functions that can
operate without the presence of the DNA layers between the
network-management and data link layers. This allows access to nodes
that exist in a state where only data link layer services are available
or operational.
Data-Access Protocol
(DAP), a proprietary Digital protocol, is used by DECnet Phase IV at the
network-application layer. DAP supports remote file access and remote
file transfer, services that are used by network-management layer and
user-layer applications. Other proprietary Digital protocols operating
at the network-application
layer include MAIL, which allows the exchange of mail messages, and
CTERM, which allows remote interactive terminal access.
The Session-Control
Protocol (SCP) is the DECnet Phase IV session-control
layer protocol that performs a number of functions. In particular, SCP
requests a logical link from an end device, receives logical-link
requests from end devices, accepts or rejects logical-link requests,
translates names to addresses, and terminates logical links.
The DECnet/OSI DNA is based on the OSI
reference model. DECnet/OSI supports two protocol suites at each of the
upper layers: the OSI protocols and the DECnet Phase IV protocols (for
backward compatibility). DECnet/OSI supports functionality in the
application, presentation and session layers.
DECnet/OSI implements the standard OSI
application-layer implementations, as well as standard application-layer
processes such as Common Management-Information Protocol (CMIP)
and File Transfer, Access, and Management (FTAM), among others.
DECnet/OSI also supports all the protocols implemented by DECnet Phase
IV at the user and network-management layers of the DNA, such as the Network
Information and Control Exchange (NICE) protocol.
The OSI application layer includes actual
applications, as well as application
service elements (ASEs). ASEs allows easy communication from
applications to lower layers. The three most important ASEs are
Association Control Service Element (ACSE), Remote Operations Service
Element (ROSE), and Reliable Transfer Service Element (RTSE). ACSE
associates application names with one another in preparation for
application-to-application communications. ROSE implements a generic
request-reply mechanism that permits remote operations in a manner
similar to that of remote procedure calls (RPCs). RTSE aids reliable
delivery by making session-layer constructs easy to use.
DECnet/OSI implements all the standard
OSI presentation-layer implementations. DECnet/OSI also supports all the
protocols implemented by DECnet Phase IV at the network-application
layer of the DNA. The most important of these is the Data-Access
Protocol (DAP).
The OSI presentation layer typically is
just a pass-through protocol for information from adjacent layers.
Although many people believe that Abstract
Syntax Notation 1 (ASN.1) is OSI's presentation-layer protocol, ASN.1 is
used for expressing data formats in a machine-independent format. This
allows communication between applications on diverse computer systems (ESs)
in a manner transparent to the applications.
DECnet/OSI implements all the standard
OSI session-layer implementations. DECnet/OSI also supports all the
protocols implemented by DECnet Phase IV at the session-control layer of
the DNA. The primary session-control layer protocol is the Session-Control
Protocol (SCP). The OSI session-layer protocol turns the data streams
provided by the lower four layers into sessions by implementing various
control mechanisms. These mechanisms include accounting, conversation
control, and session-parameter negotiation. Session-conversation control
is implemented by use of a token, the possession of which provides the
right to communicate. The token can be requested, and ESs can be granted
priorities that provide for unequal token use.
Figure 28-6 illustrates the complete
DECnet Phase IV and DECnet/OSI protocol suites, including the
implementation of DECnet/OSI over TCP.
Figure 28-6: DECnet Phase IV
and DECnet/OSI support the same data link and
physical layer specifications.
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