HP OpenVMS Systems Documentation

Transport layer: Layer 4 in the OSI Reference Model; provides reliable, transparent transfer of data between end systems, with error recovery and flow control. Important protocol classes supported by this layer are: Class 0 (TP0), Class 2 (TP2), and Class 4 (TP4). See also TP0, TP2, and TP4.

Transport Protocol (TP): OSI protocol that provides reliable end-to-end communication between programs running on different systems. See also TP0, TP2, and TP4.

transport protocol data unit (TPDU): Unit of data, processed by a transport protocol, with transport protocol control information and, possibly, also user data.

transport selector: Field within the domain specific port (DSP) part of an NSAP that indicates to the Routing layer which transport is to receive the data. The selector value consists of two hexadecimal digits (from 0 to F). The DECnet-Plus values are 20 (for NSP transport) and 21 (for OSI transport).

Also called the NSAP selector.

Transport service: OSI service that provides a universal communications interface that is independent of the low-level communications medium and protocols; service to high-level protocols at the upper boundary of the Transport layer, as defined in International Standard ISO 8072.

transport service access point (TSAP): Unique reference that identifies a transport user. When making a transport connection, the transport user provides its TSAP. Messages sent over a transport connection include the TSAP of the transport user to which they are addressed. The transport service uses this TSAP information to route messages to their proper destination. A transport user must be associated with a TSAP in order to use the transport service.

transport service provider (transport service, TS Provider): Software that provides a transport service. Example: OSI transport.

transport service user (transport user, TS user): Software running above the Transport layer and using the transport service to communicate with other software running above the Transport layer, possibly on another host.

transport subaddress: Subaddress that identifies the transport service provider of an X.25 network; forms the last two digits of a target system's X.25 destination address.

transport template: Entity that identifies the network service to be used by an OSI transport connection, and the transport characteristics that apply to that connection.

traversal sequence: Set of rules, defined by the FTAM standard, for moving between nodes within a file, beginning with its root node.

traversing: FTAM process of moving to the next node in a sequence of nodes and, optionally, accessing information associated with that node.

tributary: One of multiple points on a multipoint line. See also polling.

tributary address: Address that a control station uses to poll a tributary. See also polling.

tributary station: Station on a multipoint line that is not a control station.

true name: Name of an entity that can be directly mapped in the namespace (without resorting to backtranslation or soft links) to an agent address.

TSAP: See transport service access point.

TT Device: Device that processes system services relating to user data, which are issued by the X.29 program, and passes them to the NV device.

UA (User Agent): OSI application process that represents a human user or organization in the X.400 message handling system. Creates, submits, and takes delivery of messages on the user's behalf.

UDP (User Datagram Protocol): TCP/IP transport protocol; a connectionless transport protocol required by RIP and SNMP. TCP/IP equivalent of OSI CLTP.

UFS: See ULTRIX File System.

UI frame: Unnumbered, HDLC, information frame, used to carry data not subject to flow control or error recovery.

ULTRIX File System (UFS): Set of UFS file-management procedures called by programs to process files.

unconfirmed event report: Event report for which the event sink does not return a response to the request, and the agent might discard the event after transmitting it.

unconfirmed service: Service with nonnegotiable parameters whose functioning is fully controlled by the requesting entity.

unique identifier (UID): Attribute of all DECdns clearinghouses, directories, soft links, child pointers, and object entries that contains a unique value reflecting the date and time that an entity and a named entity with counters was created. The timestamp 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.

universal time coordination: See Coordinated Universal Time.

unreachable node: Node to which the Network layer has determined that the path exceeds the maximum hops of the network, or the intermediate system either cannot recognize the destination address or access the destination node by a usable path.

unstructured file view: Access context for a file that recognizes only the file-contents data units in a file-access data unit (FADU).

update: Routing process that periodically updates routing information to reflect topology changes, such as circuits going on or off, and changes that network managers make to routing attributes. Level 1 routers send information about adjacent nodes to all other routers in an area in level 1 LSPs, and level 2 routers send information about adjacent areas to all other level 2 routers in level 2 LSPs.

update propagation: Immediate attempt by DECdns software to apply a change to all replicas of the directory in which the change was just made. An update propagation delivers changes in a more efficient and timely way than a skulk, which is the periodic distribution of a whole collection of changes.

update timestamp (UTS): Attribute (named DNS$UTS) that identifies the time when the most recent change was made to any attribute of a particular name in the namespace; for directories, reflects changes made only to attributes that apply to the directory as a whole (not to one of its replicas).

upline dumping: Transferring a copy of a memory image from a target node to a load host node. Some systems, such as DEC WANrouter systems and DECserver terminal servers, automatically upline dump their image upon failure.

user: Defined as:

• Service user. An OSI entity that lies directly on top of another OSI entity and that directly uses its services.
• Person who uses a command or programming interface to access services provided by an entity of the Application layer.
• DECdns --- Application or person.

user element: Part of an application entity that invokes the service primitives provided by one or more of the protocols in the Application layer and, optionally, some provided by the Presentation layer.

user entity: Part of an application process that communicates with the OSI protocols.

user interface layers: Device and style layers of the director.

UTC: See Coordinated Universal Time.

UTS: See update timestamp.

virtual circuit: Independent logical path between two entities for the purpose of exchanging data. Association between two DTEs connected to a PSDN that makes it seem as if a specific circuit were dedicated to them throughout the transmission. In reality, only a logical connection is established: actual, physical circuits are allocated according to criteria such as route availability and network traffic.

virtual file: Unambiguously named collection of structural information. The information in a virtual file includes values for the file attributes that describe a file's properties (such as its file name, size, and file-access structure) and descriptions of a file's file-access data units (FADUs). Used by FTAM software.

virtual filestore: Idealized filestore defined by the FTAM virtual filestore model to serve as an intermediary between real filestores.

virtual filestore model: Common model that describes files (alone or in organized collections) and the possible actions that FTAM software can perform on those files.

virtual terminal: Abstract representation of a real terminal, complete with terminal operations such as reading text from the keyboard, writing text to the screen, and moving the cursor. Using this model, any terminal connected to a system can access any other system regardless of the types of machines in use. See also Virtual Terminal software.

virtual terminal association: Communication established between two virtual terminal systems, one initiator and one responder. The association allows the two systems to exchange request for information and their corresponding responses. Performed by DECnet-Plus Virtual Terminal software. See also Virtual Terminal software.

Virtual Terminal software: DECnet-Plus software that allows users and applications of one host to log in to a remote host and then interact as normal terminal users of that host.

• DECnet-Plus Virtual Terminal (VT) software --- Supports the ISO Virtual Terminal Protocol for remote logins to other OSI-virtual-terminal-compliant systems, including multivendor systems. Comparable to TCP/IP's Telnet.
• DECnet-Plus for OpenVMS set host feature --- Supports remote logins to any other DECnet or DECnet-Plus system.
• DECnet-Plus for DIGITAL UNIX dlogin feature --- Supports remote logins to any other DECnet or DECnet-Plus system. Comparable to Berkeley UNIX rlogin.

VisibleString: FTAM string composed of a definable character set that includes alphanumeric characters, punctuation, and graphics.

VMScluster alias: See cluster alias.

VT: See Virtual Terminal software.

WAN: See wide area network.

wide area network (WAN): Data communications network that covers a wide geographical area, such as a country. Contrast with local area network.

wide area network device driver (WANDD) software: DECnet-Plus software that links a hardware device and layered software.

wildcard name: DECdns and network management: Name that allows the looking up of groups of entries that have common name characteristics.

window: Ordered set of consecutive data packets authorized to cross the DTE/DCE interface of the logical channel used for a switched virtual circuit (SVC) or permanent virtual circuit (PVC) and for each direction of transmission.

window size: Maximum number of packets that are allowed to be transmitted without an acknowledgment in a PSDN.

write access: Access right that grants users the ability to change DECdns data.

X Recommendations: CCITT documents that describe data communication network standards, for example, the X.25 packet switching standard, X.400 message handling system, and X.500 directory services.

X.3: CCITT recommendation that describes the operation of a packet assembler/disassembler (PAD) and PAD parameters.

X.21: CCITT recommendation that defines the physical and electrical interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) of a PSDN; includes OSI layers 1--3 functions.

X.25: CCITT recommendation that defines the interface between the data terminal equipment (DTE) and the data circuit-terminating equipment (DCE) of a PSDN; provides an OSI Network service for PSDNs. See also X.25 Protocol.

X25 Access module: Defined as:

• X.25 module that defines the interface between the X.25 protocol and applications.
• Network management module that manages the entities that constitute the X25 Access module; interfaces with the X25 Protocol, X25 Client, and X25 Server modules to provide X.25 services and functions as described in the DNA X.25 Access architecture; resides in the DNA Application layer.

X.25 address: Address of a specific DTE that allows network access on X.25 networks.

X25 Client module: Defined as:

• X.25 module that defines how an Access system operates.
• Network management module that manages the X25 Client entity; interfaces with the X25 Access module to establish communications with its X.25 Server system over a DNA Session Control connection using the X.25 GAP protocol; resides in the DNA Application layer.

X.25 gateway access: Facility that allows a user of a DECnet-Plus system not directly connected to an X.25 network to access facilities of that network through an intermediary gateway node.

X.25 line: Line supplied by a PSDN; also called DTE.

X.25 network: PSDN that can be accessed by using an interface that complies with the X.25 recommendation.

X.25 Protocol: Connection-oriented network interface protocol; negotiates services for a PSDN.

X25 Protocol module: Defined as:

• X.25 module that defines the X.25 level 3 protocol that is used to exchange packets between a DTE and a DCE; defines the DTEs, PVCs, and closed user groups recognized by an X.25 system.
• Network management module that manages the entities that constitute the X25 Protocol module; provides the X.25 packet level interface into a PSDN; resides in the DNA Network layer.

X25 Relay module: Network management module that manages the entities that constitute the X25 Relay module; interfaces with the X25 Access module to receive an incoming switched virtual call and then make an outgoing call back out through the X25 Access module; resides in the DNA Application layer.

X.25 security: X.25 utility that allows control of use of X.25 systems and protects X.25 systems from unauthorized use.

X25 Server module: Defined as:

• X.25 module that defines how a gateway system communicates with an Access system.
• Network management module that manages the entities that constitute the X25 Server module; interfaces with the X25 Access module to listen for incoming calls for its X.25 client systems, and to make outgoing calls on behalf of its X.25 clients; resides in the DNA Application layer.

X.28: CCITT recommendation that specifies how to control the packet assembler/disassembler (PAD) facilities that a user can select from a terminal; defines procedures for setting up and clearing a link between the PAD and the terminal and for setting up and clearing virtual circuits.

X.29: CCITT recommendation that specifies how to control the PAD facilities that the user can select from the remote packet-mode DTE; defines the procedures for exchanging PAD control information and PAD messages.

X.29 terminal: Remote virtual terminal.

X.75: CCITT recommendation that specifies procedures for communicating between PSDNs.

X.121: CCITT recommendation that defines the format of a DTE address.

X.400: Series of OSI standards, defined by CCITT and approved by it, that specify how networks exchange electronic mail messages.

XID frame: Frame in HDLC used to exchange operational parameters between participating stations.

X/Open Transport Interface (XTI): Transport service interface, which is independent of the transport provider, that is between the transport user (a networking application or Session layer protocol) and the transport provider; IEEE 1003.8 project.

XTI: See X/Open Transport Interface.

G.2 Acronyms