HP OpenVMS Systems Documentation

control station: "Master node" at the end of a multipoint circuit. The control station controls the tributaries for that circuit; responsible for data link control.

controller: See line controller.

convergence: Degree to which DECdns attempts to keep all replicas of a directory consistent. See also high convergence and low convergence.

Cooperation for Open Systems Interconnection Networking in Europe (COSINE): Program sponsored by the European Commission, aimed at using OSI to tie together European research networks.

Coordinated Universal Time (UTC): International time standard maintained by the International Time Bureau.

Corporation for Open Systems (COS): Corporation founded by U.S. computer vendors to provide a central clearinghouse for cross testing, conformance testing, certification, and promotion of their OSI products.

cost: Integer value assigned to a circuit between two adjacent nodes. The routing decision algorithm uses this value and selects paths with the lowest cost. Nodes on either end of a circuit can assign different costs to the same circuit.

cost path: Cost of each path between a source node and a destination node; sum of the costs assigned to the circuits that compose the path. DECnet-Plus routing forwards packets on the lowest-cost path even if that one does not have the fewest hops.

counters: Performance and error statistics kept for an entity by network management, such as lines and nodes.

courier: Local DECdts server that requests a time value from a randomly selected global server each time it synchronizes.

creation timestamp (CTS): Attribute (named DNS$CTS) of all clearinghouses, directories, soft links, child pointers, and object entries that contains a unique value reflecting the date and time that the name was created; actually consists of two parts: a time portion and a portion with the system identifier of the node on which the name was created. This guarantees uniqueness among timestamps generated on different nodes.

credit window: See maximum window.

CSMA (Carrier Sense, Multiple Access): Medium-access control technique for multiple-access transmission media. A station that wants to transmit, first senses the medium and then transmits only if it is idle.

CSMA-CD (Carrier Sense, Multiple Access with Collision Detect Protocol): Data link protocol used by Ethernet and ISO 8802-3 LANs; allows multiple stations to access the broadcast channel at will, avoids contention by means of carrier sense and deference, and resolves contention by means of collision detection and retransmission. Each station awaits an idle channel before transmitting and can detect overlapping transmissions by other stations.

CTF: See Common Trace Facility.

DA circuit: See dynamically assigned circuit.

DAP: See Data Access Protocol.

DAP-FTAM Gateway: DECnet-Plus for OpenVMS software that allows a DECnet-Plus for OpenVMS node to participate in an OSI network in a simple way; functions as a server that receives DAP messages through DECnet and uses that information to establish and maintain a connection with a remote FTAM system. The DECnet-Plus for OpenVMS user issues DECnet commands to perform remote file operations. See also FTAM-FTP Gateway.

Data Access Protocol (DAP): Protocol for DECnet file transfer.

data chain buffer (DCB): Descriptor that describes some or all of a protocol data unit (PDU). DCBs can be chained together to describe the entire PDU.

data circuit-terminating equipment (DCE): Network switching equipment that provides the functions needed to establish, maintain, and terminate a connection and handle the signal conversion and coding between the DTEs that use either a physical circuit or a virtual circuit. The network switching exchange to which DTEs are connected. See also data terminal equipment.

data link: Logical connection between two systems on the same circuit on which data integrity is maintained.

Data Link layer: Layer 2 in the OSI Reference Model; specifies the technique for moving data along network links between defined points on the network and for detecting and correcting errors in the Physical layer.

data link mapping (DLM): See dynamically assigned circuit.

Data Link Protocol: OSI rules and procedures that allow the data link service to be provided.

data link protocol data unit (DPDU): Packet from a data link protocol.

data network identification code (DNIC): In X.25, usually the first four digits of the DTE address that identifies the country, and distinguishes the PSDN from other PSDNs within the same country. Some networks require a leading zero to denote that a DNIC is being used.

data octet: Another term for 8 bits.

data overrun: Data blocks received that arrived too quickly to be processed by the receiver and were, therefore, lost.

data packet: See packet.

data segment: For most PSDNs, 64 bytes.

data terminal equipment (DTE): User's equipment (computer or terminal) connected to a modem (DCE) on a packet switching network; both sends and receives data. See also data circuit-terminating equipment.

data transfer facility: Facility for exchanging normal data over an OSI transport connection. The transfer occurs as a sequence of TPDUs from a sending TSAP to a receiving TSAP.

data-transfer regime: Regime that controls bulk data transfer; moves file data between FTAM peer entities.

datagram: Defined as:

• X.25 --- Self-contained packet, independent of other packets, that does not require acknowledgment, and that carries information sufficient for routing from the originating DTE without relying on earlier exchanges between the DTEs and the network.
• OSI --- Unit of data sent over the network that is handled independently of all other units of data. The unit of data (NSDU in ISO terminology) is passed between the Network layer and the Data Link layer. When a route header is added, the datagram becomes a packet (network PDU in ISO terminology). Datagrams may fail to be delivered with no notice sent to the sender.

DCE: See data circuit-terminating equipment.

DDCMP: Byte-oriented, Data Link layer, DNA protocol implemented in DECnet software; designed to provide an error-free communications path between adjacent systems. Operates over serial lines, delimits frames by a special character, and includes checksums at the link level.

DEC WANrouter system: DECnet-Plus intermediate system. Can run either the DECnet-Plus routing algorithm (link state) or the DECnet Phase IV routing algorithm (routing vector). Most DEC WANrouter products are multiprotocol routers. See also DECnet-Plus level 1 router and DECnet-Plus level 2 router.

DECdns (DIGITAL Distributed Name Service): DECnet-Plus software that provides network applications, especially DECnet-Plus, with a means of assigning unique names to network resources so a network application can find resources within the network.

DECdns clerk: Interface between client applications and DECdns servers; DECnet-Plus system running DECdns clerk software.

DECdns client: Application that interacts with a DECdns server through the DECdns clerk interface.

DECdns control program (DNSCP): Command interface that DECdns managers use to control DECdns servers and clerks, and to manage the namespace and its contents.

DECdns entity: DECdns server, clearinghouse, or clerk software on a system.

DECdns namespace: See namespace.

DECdns server: Node running DECdns server software; handles name lookup requests and maintains the contents of the clearinghouse at its node.

DECdns-defined attribute: Standard attribute that DECdns associates with names in the namespace.

DECdts (DIGITAL Distributed Time Service): DECnet-Plus software that synchronizes the clocks in networked systems.

DECdts clerk: DECdts process that synchronizes the clock for its client system by requesting time values from servers, computing a new time from the values, and supplying the computed time to client applications, such as the operating system; DECnet-Plus system running DECdts clerk software.

DECdts entity: DECdts server or clerk software on a system.

DECdts server: Node running DECdts server software; DECdts entity that synchronizes with its peers and provides its clock value to clerks and their client applications.

decision: Routing process that determines the path, or route, along which a data packet travels to reach its destination; forwards packets on the lowest-cost path even if that one does not have the fewest hops. The path that the data takes through the network is transparent to users.

DECnet: Family of DIGITAL proprietary hardware and software communications products that implement the DIGITAL Network Architecture (DNA). These products allow all DIGITAL systems to communicate with each other. See also DECnet-Plus and DECnet Phase IV.

DECnet-Plus for DIGITAL UNIX: DECnet software implementation for DIGITAL UNIX systems.

DECnet Phase IV: Family of DIGITAL proprietary hardware and software communications products that implement the DIGITAL Network Architecture (DNA) Phase IV.

DECnet Phase IV area: Area in which all level 1 routers are running the DECnet Phase IV routing vector protocol.

DECnet Phase IV end node: End node that receives and transmits data packets but does not route packets through; can receive and transmit only Phase IV-format-packets and can communicate with DECnet-Plus nodes only if their addresses are compatible with Phase IV.

DECnet Phase IV level 1 router: DECnet Phase IV router that receives and transmits data packets, forwards packets to other nodes within its same DECnet area, and forwards packets to the nearest level 2 router for out-of-area destinations. Can receive and transmit only Phase IV format packets and can communicate with DECnet-Plus nodes only if their addresses are compatible with Phase IV. Runs the DECnet Phase IV routing vector algorithm.

DECnet Phase IV level 2 router: DECnet Phase IV router that receives and transmits data packets, forwards packets to other nodes within its same DECnet area, and forwards packets for out-of-area destinations through other level 2 routers to the destination area. Can receive and transmit only Phase IV-format-packets and can communicate with DECnet-Plus nodes only if their addresses are compatible with Phase IV. Runs the DECnet Phase IV routing vector algorithm.

DECnet-Plus: Family of DIGITAL hardware and software products that implements the DIGITAL Network Architecture (DNA) Phase V, which integrates OSI and DNA protocols; compliant with OSI and compatible with DECnet Phase IV and TCP/IP.

DECnet-Plus addressing: DECnet-Plus addressing scheme that complies with the ISO 8348 Addendum 2 addressing standard; uses the concepts of global network addressing, addressing authorities, and addressing domains. ISO scheme designed to provide unique network addresses throughout the world.

DECnet-Plus area: Area in which all level 1 routers are running the DECnet-Plus link state routing protocol.

DECnet-Plus end system: DECnet-Plus system that receives and transmits data packets but cannot route packets through; can receive and transmit both Phase IV format and DECnet-Plus data packets; communicates with Phase IV nodes using Network Services Protocol (NSP) and with DECnet-Plus systems using either NSP or the OSI Transport service.

DECnet-Plus for OpenVMS: DECnet-Plus software implementation for OpenVMS systems.

DECnet-Plus level 1 router: DECnet-Plus multiprotocol router (intermediate system) that receives and transmits data packets, forwards packets to other nodes within its same area, and forwards packets to the nearest level 2 router for out-of-area destinations; can receive and transmit both Phase IV format and DECnet-Plus format data packets.

Can route OSI data and Internet Protocol (IP) packets over the same lines, conforming to the Integrated IS-IS Protocol specified in RFC 1195, "Use of OSI IS-IS for Routing in TCP/IP and Dual Environments."

When configured for multiprotocol support, can perform as an IP router in an IP network; transmits IP packets directly over the data link and does not encapsulate these packets within DECnet or OSI packets.

DECnet-Plus level 2 router: DECnet-Plus multiprotocol router (intermediate system) that receives and transmits data packets, forwards packets to other nodes within its same area, and forwards packets for out-of-area destinations through other level 2 routers to the destination area; can receive and transmit both Phase IV format and DECnet-Plus format data packets.

Can route OSI data and IP packets over the same lines, conforming to the Integrated IS-IS Protocol specified in RFC 1195, "Use of OSI IS-IS for Routing in TCP/IP and Dual Environments."

When configured for multiprotocol support, can perform as an IP router in an IP network; transmits IP packets directly over the data link and does not encapsulate these packets within DECnet or OSI packets.

decoding: Process by which the transfer syntax representation of a data value is transformed into the local representation of that value.

DED: See dynamically established data link.

dedicated line: Line for the exclusive use of a leasing customer without interchange switching arrangements. Same as leased line and private line.

delete access: Access right that grants users the ability to remove DECdns data from the namespace.

designated router: Router that solves the negative effects of excess routing information on a DECnet-Plus LAN configured with two or more routers. Creates a link state packet (LSP) that describes the entire LAN to the rest of the network and selects the router that will represent the pseudonode. (On an Ethernet or ISO 8802-3 LAN, the LAN broadcast link is considered to be a pseudonode with links to each attached system. Attached systems report their link only to this pseudonode. The designated router, on behalf of the pseudonode, constructs an LSP that lists the links to all the systems attached to the LAN at zero cost.)

destination address: Address that identifies a target system; can be an X.25, a physical, or an NSAP address.

destination service access point (DSAP): One-byte field in a logical link control (LLC) frame on a LAN that identifies the link service access point of the receiving Data Link layer client protocol.

DEUNA: Network adapter for connecting UNIBUS-based hosts to a CSMA-CD local area network.

device driver: Software associated with each physical device in the system that serves as the interface between the operating system and the device controller.

device layer: Collection of director device plug-ins that handle the details of the I/O devices that interact with the manager.

device object: Data structure in the conceptual communications area that models characteristics of real devices and helps to map display objects.

dialogue: Sequence of message exchanges between open systems that represents a single association and the set of underlying connections.

dial-up line: Telephone communications circuit established by a switched circuit connection.

DIGITAL Distributed Name Service (DECdns): See DECdns.

DIGITAL Distributed Time Service (DECdts): See DECdts.

DIGITAL Network Architecture (DNA): Set of protocols governing the format, control, and sequencing of message exchange for DECnet and DECnet-Plus implementations; defines rules for all the layers of data exchange, from the lowest level of the physical medium of transmission to the top, user interface level; also defines standard network management and network generation procedures.

DNA Phase IV comprises DIGITAL proprietary networking protocols. DNA Phase V integrates DIGITAL proprietary networking protocols and OSI open standards and protocols. See also architecture.

directive: Management operation initiated by the director (the requester, which can be a human user) and sent to the entity (the responder) that performs the function.

director: Software management system that interacts with a user, initiates management operations on behalf of the user, coordinates management activities with entities, and provides high-level management applications.

directory: Logical unit for storing object entries under one name (the directory name) in a DECdns namespace; can also contain soft links and child pointers. Each physical instance of a directory is called a replica.

disconnect abort: Method by which nontransparent tasks can de-access a logical link by means of a disconnect abort operation without deassigning the channel. This form of disconnection indicates to the receiver that not all messages sent have necessarily been received.

display object: Data structure in the conceptual communications area that models a terminal's display and keyboard operations.

distance vector routing: See routing vector algorithm.

distributed management: Form of network management in which network managers and directors are dispersed across many systems.

distributed processing: Technology that enables the distribution throughout the network of computing power and storage facilities to user work areas, such as offices, laboratories, or desks on factory floors.

distributed system: Collection of computer systems, tied together by communications networks for the purpose of sharing resources; end users do not need to be aware of the physical location of the shared resources.

distributed system management model: Framework within which DECnet-Plus network management is designed.

DLM: See data link mapping.

dlogin: Feature of both Berkeley UNIX (called rlogin) and DECnet-Plus for DIGITAL UNIX software that allows users of one system to log in to other systems. (Similar to Telnet.) Comparable to DECnet-Plus for OpenVMS software's set host and to the DECnet-Plus Virtual Terminal (VT) software.

DNA: See DIGITAL Network Architecture.

DNIC: See data network identification code.

DPDU: See data link protocol data unit.

docket: Information associated with an OpenVMS file service regime that must be preserved to provide FTAM error recovery.

document type: As defined by the FTAM protocol, a set of related statements about a file. Some of these statements relate to the intended use of the file (for example, by specifying a constraint set and a file model); the others describe its structure, scope, abstract syntax, and transfer syntax.

domain: See routing domain.

domain-specific part (DSP): Part of a network service access point (NSAP) address defined by the organization that implements the networking software.

downline loading: Transferring a copy of a system image from a load host node to a target node. Some systems, such as DEC WANrouter systems and DECserver terminal servers, automatically request a downline load of their image upon startup and reboot.

drift: Change in a clock's error rate over a specified time period.

DSP: See domain-specific part.

DTE: See data terminal equipment.

DTE address: Unique address given to a DTE.

DTE Class entity: Entity that defines a named class of DTEs; used to determine which DTE or X.25 Connector node to use when making incoming and outgoing calls.

DTSS: See DECdts and DTSS module.

DTSS module: Network management module for synchronizing and managing the system clocks in a distributed DECnet-Plus network; contains all DECdts management functions.

dynamic assignment (DA): Use of a dynamically established data link by the Network layer in such a way that a connection is made only when data is required to be transferred, and the subnetwork address to which the connection is made is determined by the destination NSAP address.

dynamic connection management (DCM): Use of a dynamically established data link by the Network layer in such a way that a connection is made only when data is required to be transferred.

dynamically assigned (DA) circuit: Routing circuit that DECnet-Plus uses to create a path between DECnet-Plus and X.25 implementations on the same node; the way through which a DECnet-Plus node running X.25 software communicates through a PSDN to another DECnet-Plus node.

dynamically established data link (DED): Connection in which routing dynamically determines the address of the destination based on receive traffic. Examples: X.25, X.21.

encapsulation: Technique used by some layered protocols in which a layer adds header information to the protocol data unit (PDU) from the layer above. As an example, DIGITAL's Internet Portal product encapsulates IP datagrams to produce DECnet packets that can be transmitted by DIGITAL routers across a DECnet WAN.

encoding: Defined as:

• Process by which a sender applies the rules of a transfer syntax to transform local data into data that another FTAM system can receive and decode.
• Process by which the local representation of a data value is transformed into the transfer syntax representation of that value.
• Actual representation of a data value in a specific transfer syntax (equivalent to "encoded form").

encoding rules: Rules that guide the encoding of local data and decoding of received data.

end node: See end system.

end system: Nonrouting system; can receive data units addressed to it and send data units to other systems on the same subnetwork but cannot relay, route, or forward data packets to other systems.

End System to Intermediate System (ES-IS) Protocol: OSI protocol implemented at the Network layer of an end system (ES) that enables communication between end systems on different subnetworks. Packets are routed to an intermediate system (IS) on the same subnetwork as the source end system, and from there are routed to the destination end system, possibly via one or more subnetworks. Also called a routing exchange protocol.

Enterprise Management Architecture (EMA): DIGITAL model for the way in which components in a distributed system are managed.

entity: Defined as:

• DECnet-Plus--- Active element in a layer. Entities in a layer communicate with other entities in the same layer, but in different systems.
• Network management --- Individual, manageable piece of a network; has attributes that describe it, a name that identifies it, and an interface that supports management operations.
• FTAM --- Instance of one or more protocol machines of a given layer operating within a process to support a specific dialogue.

entity class: Collection of entities that share the same properties and that have the same parent entity; each member of the class has a unique identifier within the class. Entity classes have class names.

entity group: Architecturally defined collection of entities. The entities in the group must have a common top entity and must all be of the same class.

entity hierarchy: Logical hierarchical tree structures of manageable entities in which child entities are below their parent entities. Children can be accessed only through their parents' agent.

entity identifier: See attribute group.

entity instance block (EIB): Logical representation of an instance of an entity to network management software; contains all values for attributes of that instance.

entity management agent access (EMAA): Internal utility of DECnet-Plus network management.

entity name: Label associated with some entities used to identify or locate them for management purposes.

entity type: Subgrouping of an entity that determines its relationship to other DECdts components: clerk or server.

epoch: Timestamp that identifies directory replicas as being part of the same set; used by DECdns when it skulks a directory, finding all replicas of the directory that are in the same epoch, and makes their contents consistent.

epoch number: Identifier that a DECdts server appends to the time values it sends to other servers. DECdts servers use time values only from other servers with whom they share epoch numbers.

equal cost path splitting: Routing process in which packets are forwarded over multiple equal cost paths to a destination node.

error: Difference between a system's clock value and the computed time.

error recovery: Procedures for recovering from the effects of detected OSI transport errors. Protocols may provide specific facilities for doing this.

error tolerance: Amount of system clock error to which the DECdts entity responds by abruptly setting the system clock to the computed time, rather than gradually adjusting the clock.

ES-IS: See end system to intermediate system protocol.

Ethernet: CSMA-CD LAN, similar to the one defined by ISO 8802-3, originally defined jointly by DIGITAL, Intel, and Xerox. Composed of passive coaxial cable or shielded twisted-pair cable with interconnections containing all active components so that no switching logic or central computer is needed to establish or control communications. A best-effort delivery system. Supports the Ethernet protocol and the IEEE 802.2 and 802.3 protocols.

Ethernet protocol: Data Link layer protocol used by Ethernet LANs.

event: Occurrence of either a normal or abnormal condition that an entity detects; network- or system-specific occurrence that can be logged. Many events are informational, for example, the arrival of a PDU. Other events report potential or current problems.