HP OpenVMS Systems Documentation

ISO Reference Model for Open Systems Interconnection: ISO's international standard for Open Systems Interconnection. Communications model that defines a hierarchical architecture of seven layers designed to make possible free interconnection between multivendor systems.

More briefly called OSI Reference Model and OSI Model.

ISODE: ISO Development Environment; public domain implementation of the upper layers of OSI.

kernel group attributes: Subset of file attributes and activity attributes that must always be defined. FTAM implementations must support kernel group attributes.

known subclass entity group: All of the entities within a particular subclass that have the same parent entity.

LAN: See local area network.

LAP: See Link Access Protocol.

LAPB: See Link Access Protocol Balanced.

LAPB module: Module that defines the X.25 level 2 protocol used to exchange frames between a DTE and a DCE.

layer: Independent, self-contained set of interconnected functions with its own characteristic purpose and protocols within the OSI Reference Model. Each layer performs functions, as specified by the architecture, for the layers above it.

Layered Environment Services (LES): DECnet-Plus software that provides a general-purpose environment for communications software; offers efficient scheduling, preserves the modularity of communications architectures, and permits the implementation of protocols in a high-level language. LES services also provide: support for layered protocols, memory management, timers, and protocol tracing.

LCN: See logical channel number.

leap seconds: Infrequent adjustment to coordinated universal time (UTC) to account for the irregularity of the earth's rotation.

leased line: See dedicated line.

LES: See Layered Environment Services.

level 1 router: Intermediate system that sends and receives data packets and routes them from one node to another within a single area. Routes messages for other areas to the nearest level 2 router.

level 2 router (area router): Intermediate system that sends and receives data packets and routes them from one node to another within its own area and also between areas and between networks.

LFDP: See long format data packet.

line: Distinct, physical path from system to system that provides direct communication.

line controller: Hardware device at each node that manages communications over each line; handles the Physical and Data Link layers. Use of these devices can significantly shorten the time required by CPUs for communications.

line sharing: Form of X.21 switched line sharing in which many clients have access to a line, but only one client has access to a single call. See also call sharing.

line speed: Maximum rate at which data can be transmitted reliably over a line; varies with the capability of the modem or hardware device that performs the transmitting.

link: Defined as:

• Communications path between two nodes.
• DECdns --- See soft link.

Link Access Protocol (LAP): X.25-defined procedure for link control in which the DTE/DCE interface is defined as operating in two-way simultaneous asynchronous response mode (ARM) with the DTE and DCE containing a primary and secondary function. See also Link Access Protocol Balanced.

Link Access Protocol Balanced (LAPB): X.25-defined procedure for link control in which the DTE/DCE interface is defined as operating in two-way asynchronous balanced mode (ABM). Provides for the reliable transfer of a packet from a host to an X.25 packet switch, which then forwards the packet on to its destination.

Modified form of HDLC that CCITT approved as the link level protocol for X.25 networks. See also Link Access Protocol.

link service access point (LSAP): ISO 8802-2 (LLC) protocol identifier that identifies a Network layer entity. For example, hexadecimal FE identifies the LLC service access point used by CLNP.

link state algorithm: DECnet-Plus routing algorithm; calculates routes based on the shared knowledge of all the end systems and intermediate systems within a DECnet-Plus routing domain. Level 1 routers share information about all nodes in their area; level 2 routers share information about all areas reachable in the domain. Intermediate systems flood adjacency information to the other intermediate systems in the network. Contrast with routing vector algorithm.

link state packet (LSP): Packet with DECnet-Plus routing (also called link state routing) control information. Intermediate systems exchange information about what adjacent nodes exist on each circuit and the cost associated with the circuit. This information is stored in LSPs and is sent to all other intermediate systems in the area. Each intermediate system collects the LSPs from all other intermediate systems in the area. From this information, each intermediate system constructs the least-cost paths through the network.

link state PDU (LSP): See link state packet.

LLC: See logical link control.

LLC2: X.25 module that defines the data link protocol used on LANs that conform to the OSI LLC type 2 standard; allows systems on a LAN to communicate with remote nodes over an X.25 network.

LNO: See Local Naming Option.

load: See downline loading.

load assist agent: Image that provides additional data required to perform a downline load to a node in an OpenVMS cluster.

load host: Node from which a system image is downline loaded to a target node. See also downline loading.

load tool: Utility used to create, modify, and administer the Local Naming Option of DECdns.

local application address: Address for incoming FTAM communications that consists only of a p-address and which accesses the local responder.

local area field (LOC-AREA): Two-octet field within the domain-specific part (DSP) that identifies a particular routing subdomain within the entire DECnet-Plus routing domain. Combines with the initial domain part (IDP) field and the preDSP field to form the area address.

local area network (LAN): High-speed, privately-owned data communications network, for example, Ethernet, that covers a limited geographical area, such as a group of buildings, a single building, or a section of a building. Contrast with wide area network.

local data: Any data stored locally by a system. Example: file data or the system information that maps to ISO service parameters.

local DTE: DTE at which the user is located.

local echo mode: Packet assembler/disassembler (PAD) mode during which characters typed at the terminal are echoed by the PAD.

local management access interface: Interface between the agent and the agent access module.

local name file: Text file containing a list of node names, synonyms, and network addresses. This information constitutes the local namespace for a DECdns clerk cache that is using the Local Naming Option.

Local namespace: A discrete, nondistributed namespace that stores name and address information locally in database files. The Local namespace is independent of DECdns and replaces functionality previously provided by the DECdns Local Naming Option. Depending on the number of address towers stored, the Local namespace is designed to scale up to at least 100,000 nodes.

Local Naming Option (LNO): DECdns option that provides a per-node, nondistributed, namespace for small-scale DECnet-Plus networks that optionally do not make use of DECdns servers.

local node: Node at which the user is located.

local server: DECdts server that synchronizes with its peers and provides its clock value to other servers and clerks on the same LAN.

local set: All of the DECdts servers in a particular LAN.

logging: Network management facility that collects network events at a logging sink, such as a file or console. See also event dispatcher.

logical channel: Logical link between a DTE and its DCE. The physical communications line between a DTE and DCE is divided into a set of logical channels.

logical channel number (LCN): Unique reference number that identifies a logical channel. A DTE recognizes a virtual circuit by its associated LCN.

logical connectivity: Ability of nodes to communicate.

logical link: Temporary connection between processes on source and destination nodes (or between two processes on the same node).

Logical Link Control (LLC): OSI protocol used on LANs to provide the Data Link layer service.

long format data packet (LFDP): Long format protocol header used for data packets on Ethernet subnetworks when communicating with DECnet Phase IV nodes. Contrast with short format data packet.

lookup: Process during which a DECdns clerk receives a request from a client application, sends the request to a DECdns server, and returns the information to the client.

loop node: Local node that is associated with a particular line and is treated as if it were a remote node. All traffic to the loop node is sent over the associated line; used for loopback testing.

low convergence: Setting that controls the degree to which DECdns attempts to keep all replicas of a directory consistent; indicates that DECdns does not immediately propagate an update, but waits for the next skulk to distribute all updates that occurred since the last one.

LSAP: See link service access point.

LSP: See link state packet.

mail: See Message Handling System and Session Control application database.

mail exploder: Part of an electronic mail delivery system that allows a message to be delivered to a list of addressees; used to implement mailing lists.

mail gateway: System that connects two or more electronic mail systems (especially dissimilar mail systems on two different networks), and transfers messages between them.

Maintenance Operations Module (MOM): Maintenance Operations Protocol that defines downline loading and upline dumping.

Maintenance Operations Protocol (MOP): DNA network management protocol used to perform functions such as downline loading, upline dumping, and circuit testing.

management access name: Another term for an entity name; label associated with an entity in order to identify or locate it for network management.

Management Access Protocol: Application layer protocol between the director and the entity.

management access relationship: Relationship between a parent entity and a child entity that indicates that the parent is capable of passing directives to its child entities; used to find entities for the purposes of managing them, except for the special cases of creating and recovering them. The management access relationships between entities are reflected in the management names of entities.

Management Event Notification (MEN) Protocol: DECnet-Plus network management protocol used for exchanging event information.

management hierarchy: Order of manageable DECnet-Plus entities defined strictly for the purpose of naming entities; does not imply or define how entities interact.

Management Information Control and Exchange (MICE) Protocol: DECnet-Plus network management protocol used to exchange management requests and data between nodes.

management model: See distributed system management model.

management namespace: Set of all entity names registered in the DECdns namespace.

master replica: First instance of a specific directory in the DECdns namespace. DECdns can create, update, and delete object entries and soft links in a master replica. The master replica is the only replica where DECdns can create child pointers and the only replica from which certain skulk operations involving new directories, deleted directories, and soft links can be performed.

maximum window: NSP and OSI transport entity characteristic for control of the number of data segments (PDUs) allowed to be transmitted over a particular transport connection before at least one acknowledgment must be returned from the destination system. If the number of PDUs already transmitted equals the maximum window and no corresponding acknowledgments have been received, transport stops sending PDUs over the transport connection and waits for an acknowledgment message.

maximum transmission unit (MTU): Largest possible unit of data that can be sent on a given physical medium. Example: The MTU of Ethernet is 1500 bytes. See also fragmentation.

medium convergence: Directory convergence setting for which DECdns attempts to propagate an update to all replicas. If the attempt fails, the next scheduled skulk makes the replicas consistent. Skulks occur at least once every 12 hours.

MEN Protocol: See Management Event Notification Protocol.

message: Message block, or a series of message blocks, that constitute a logical grouping of information; each is delimited by communications control characters.

message handling system (MHS): System of message user agents, message transfer agents, message stores, and access units which together provide OSI electronic mail; specified in the CCITT X.400 series of recommendations.

message transfer agent (MTA): OSI application process used to store and forward messages in the X.400 message handling system. Equivalent to Internet mail agent.

MHS: See message handling system.

MICE Protocol: See Management Information Control and Exchange Protocol.

modem (modulator/demodulator): Device that translates digital signals (electrical impulses) generated by a computer into analog signals (tones) that can be transmitted over telephone lines, and vice versa.

modem connect: Defined as:

• DNA --- Class of communication links governed by industry standards for modem connection.
• X.25 --- Module that defines the physical lines connecting a system to an X.25 network.

modulator/demodulator: See modem.

MOP: See Maintenance Operations Protocol.

MTA: See message transfer agent.

MTU: See maximum transmission unit.

multiaccess channel: Special type of broadcast circuit, for example, Ethernet, on which many transmitters contend for access.

multiarea network: See multiple area network.

multicast: Type of broadcast transmission in which a copy of a packet is delivered to a subset of all nodes in a subnetwork that are listening on the same address. See also broadcast.

multicast address: Address that designates a subset of nodes that are all listening for packets destined to this address.

multicast addressing: Addressing mode in which a data packet is targeted to a group of nodes that are of the same type, for example, all level 1 routers or all level 2 routers.

multicircuit end system: End system with two or more connections to the network, for example, two separate LANs; provides increased performance and reliability because traffic is split among the circuits, and if one circuit fails, connectivity is maintained.

Although traffic can be sent and received over any of the links, traffic is not forwarded from one link to another; does not perform the functions of a router.

multidrop: See multipoint circuit.

multihomed system: End system or intermediate system with more than one assigned address.

multiple area network: WAN divided into areas, with each area being a group of nodes. Nodes are grouped into areas for hierarchical routing purposes.

multiplexing: Using a single connection to carry several data streams and the mechanism for assigning these streams to that connection. For example, both NSP and the OSI transport service can multiplex several transport connections onto a single network connection.

multipoint circuit: Circuit that connects multiple systems; one is the control station and the others are tributaries. (DECnet-Plus does not support multipoint connections.) Also called multipoint link and multidrop. Contrast with point-to-point circuit. See also control station and polling.

multiprotocol routing: See integrated routing.

name resolution: DECdns process of mapping a name into the corresponding address.

name server: See DECdns server.

name service: Term for the software that manages the node name and addressing information for DECnet.

namespace: The set of names accessible to a name service. DECdns stores names in directory replicas in clearinghouses at each DECdns server. The Local namespace stores names in local database file.

namespace creation timestamp (NSCTS): Unique timestamp (with attribute name of DNS$NSCTS) automatically assigned to a namespace when installing and configuring DECdns on the first node in a new namespace.

namespace hierarchy: Logical hierarchy, or tree, of directories that exist beneath the root directory of a DECdns namespace.

namespace name: See namespace nickname.

namespace nickname: Name that a network manager or administrator assigns to a namespace when installing and configuring DECnet-Plus on the first node in a new namespace.

The following reserved namespace nicknames local: and domain: on a node full name indicate to DECnet-Plus that the information for the node is contained in the Local namespace (for local:) and in DNS/BIND (for domain:).

National Bureau of Standards: See NIST.

National Institute of Standards and Technology: See NIST.

navigation window: Navigation aid in the DECdns Browser.

NBS: National Bureau of Standards. See NIST.

NCL: See Network Control Language.

NCL script: File of NCL commands.

NCP: See Network Control Program.

NET: See network entity title.

network: Consists of two or more computer systems linked by communications hardware and software; an open network is a network of open systems; an open system is a computer system with communications software that implements formal, international open networking standards, for example, the OSI standards or RFC-compliant TCP/IP.

network address: Address that identifies a specific system on a network; can be an X.25, a hardware, or an NSAP address. Made up of a transport template name for the local DECnet-Plus system and a Network layer address of a target system. See also network service access point.

network addressing: See DECnet-Plus addressing.

network architecture: Specification of a network's functions and its parts, together with the ways in which the network is organized; specifies the layers of different functions in the network, ranging from data transmission at the lowest levels to user applications at the highest levels.

network connect block (NCB): Data structure with information needed to set up a transport connection, or to accept or reject a request to set up a transport connection.

network connection: Association, established by the Network layer, for the transfer of data between two or more entities in the Transport layer.

Network Control Language (NCL): Command-line interface to DECnet-Plus network management directors; used to manage DECnet-Plus nodes and their network components.

Network Control Program (NCP): Command-line interface for managing DECnet Phase IV nodes and their network components.

network delay: Time it takes to get a unit of data from the source of a transmission to the destination; usually refers to delay from the network and not by system-dependent application processing delays at source and destination nodes.

network diameter: Distance (number of hops) between the two nodes in the network with the greatest reachability distance. The reachability distance is the path with fewest number of hops between two nodes.

network entity title (NET): Address by which the Network layer is identified; identifies a particular system in a particular network. Is the same structure as an NSAP, but with a 0 selector field (SEL). Has two primary fields:

• Initial domain part (IDP), which consists of two subfields:
  • Authority and format identifier (AFI)
  • Initial domain identifier (IDI)
• Domain-specific part (DSP), which consists of four subfields:
  • preDSP
  • Local area (LOC-AREA)
  • Node ID
  • A 0 transport selector

When combined with a real transport selector, is transformed into an NSAP. See also network service access point and transport selector.

Network Information and Control Exchange (NICE) protocol: Protocol used to exchange DECnet Phase IV network management information.

Network layer: Layer 3 in the OSI Reference Model; permits communications between network entities in open systems on a subnetwork, intermediate systems, or both. Layer for all routing functions. Provides two types of services: CONS and CLNS. Also includes DECnet Phase IV support. Is functionally divided into two sublayers: the subnetwork independent sublayer and the subnetwork dependent sublayer. See also CONS and CLNS.

network layer protocol data unit (NPDU): Combination of a service data unit (SDU), which is the basic unit of user data that the Network layer receives from the Transport layer above, plus a protocol control information (PCI) header subsequently added by the Network layer.

network management: Services for managing a DECnet-Plus network, such as configuring and tuning the network software, monitoring network performance, maintaining network operation, and diagnosing and troubleshooting network problems. User-performed with NCL. See also Network Control Language.

network performance: How a network performs, as measured against the expectations or requirements of users, customers, designers, or implementors, or as claimed by sales and marketing personnel. The criteria for network performance include parameters such as throughput, response time, and resource utilization.

Network service: OSI service provided to entities in the Transport layer at the upper boundary of the Network layer, as defined in International Standard ISO 8348.

network service access point (NSAP): Global network address of a DECnet-Plus system; addressable point at which a network entity provides the network service to a network user; complete address that identifies both the particular network system and the transport module on that system that is to receive the data.

A DECnet-Plus address (NET) that contains a nonzero selector field (SEL). Has two primary fields:

• Initial domain part (IDP), which consists of two subfields:
  • Authority and format identifier (AFI)
  • Initial domain identifier (IDI)
• Domain-specific part (DSP), which consists of four subfields:
  • preDSP
  • Local area (LOC-AREA)
  • Node ID
  • Transport selector (SEL)

See also network entity title and transport selector.

network service data unit (NSDU): Block of data transferred between the Transport layer and the Network layer.

Network service primitive: Basic operation carried out by the network service at the request of a network user.

Network service provider (Network service, NS Provider): Software that provides a network service.

Network Services Protocol (NSP): DNA protocol that operates in the DNA Transport layer. DECnet Phase IV nodes use NSP. NSP and OSI transport can reside simultaneously on a DECnet-Plus node.

Network service user (network user): Software running above the Network layer on a given host and using the network service to communicate with some other software, possibly on another host.

network status notification: Notification with information about the state of both logical and physical links over which two tasks communicate. A nontransparent task can use this information to take appropriate action under conditions such as third-party disconnections and a partner's exiting before I/O completion.

network task: Nontransparent task that can process multiple inbound connection requests; that is, it has a declared network name or object number.

NICE protocol: See Network Information and Control Exchange protocol.

nickname: See namespace nickname.

NIST (National Institute of Standards and Technology): Standards organization of the United States government. (Formerly called the National Bureau of Standards (NBS).) See also OIW.

node: Defined as:

• DECnet-Plus --- See system.
• FTAM --- In the hierarchical file model, the structural element of a file with which a data unit is associated and whose sequential relationship to other nodes, if any, determines the file's structure.

node address: Required, unique numeric identification of each node in a network; provides addressing information for other nodes to access it. These addresses must reflect the logical network configuration (where nodes fit into the network), and may need to change as that configuration changes.