Inside Macintosh: Networking With Open Transport /

Glossary

abortive disconnect  A type of disconnection that breaks a connection without the knowledge of the remote peer. An abortive disconnect can result in loss of data. See also orderly disconnect.

absolute requirement   A type of option that a protocol implementation can neither ignore nor negotiate to a partly successful value.

active peer  An endpoint provider that initiates connection requests. The use of an active peer is typical of a client-server environment in which an endpoint, the active peer, attempts to establish a connection with a passive peer, such as a file server, that listens for connection requests. See also passive peer.

address type  Anattribute that identifies the type of address format used for an Open Transport endpoint.

ADSP   AppleTalk Data Stream Protocol.

AEP Echoer  A DDP client process that implements the AppleTalk Echo Protocol (AEP).

at-least-once transaction  A type of transaction that ensures that an ATP responder receives every request directed to it at least once. These transactions are also referred to as ALO transactions. See also exactly-once transaction.

AppleTalk Echo Protocol (AEP)  An AppleTalk protocol that is a client of DDP. This protocol can measure the performance of an AppleTalk network and test for the presence of a given node.

AppleTalk internet  A number of interconnected AppleTalk networks. An AppleTalk internet can include a mix of LocalTalk, TokenTalk, EtherTalk, and FDDITalk networks, or it can consist of multiple networks of a single type, such as several LocalTalk networks. An AppleTalk internet can include both nonextended and extended networks. See also internet. Compare with Worldwide Internet.

AppleTalk Session Protocol (ASP)  A connection-oriented transaction-based AppleTalk protocol that sets up and maintains sessions between workstations and servers.

AppleTalk service provider  An Open Transport provider that gives applications access to information and services that are specific to the AppleTalk protocol stack. Applications use an AppleTalk service provider to obtain zone names and to get information about the current AppleTalk environment for a given machine.

AppleTalk Data Stream Protocol (ADSP)  A connection-oriented transactionless AppleTalk protocol that supports sessions over which applications can exchange full-duplex streams of data. In addition to ensuring reliable delivery of data, ADSP provides a peer-to-peer connection. ADSP also provides an application with a means of sending expedited attention messages.

AppleTalk Secure Data Stream Protocol (ASDSP)  An extension of ADSP to provide authentication and encryption.

AppleTalk Transaction Protocol (ATP)  A connectionless transaction-based AppleTalk protocol that allows two endpoints to execute request-and-response transactions. Either ATP endpoint can request another ATP endpoint to perform an action; the other ATP endpoint then carries out the action and transmits a response reporting the outcome.

AppleTalk Transition Queue (ATQ)  In classic AppleTalk, the AppleTalk Transition Queue (ATQ) informs applications each time certain network-related events occur, such as opening or closing an AppleTalk driver. Any applications that rely on the ATQ events, other than the miscellaneous events supported by Open Transport, must use AppleTalk backward compatibility to handle them in the classic AppleTalk manner. See also miscellaneous event.

application layer   The highest layer of the OSI model. This layer allows for the development of application software. Software written at this layer benefits from the services of all the underlying layers.

ASDSP   AppleTalk Secure Data Stream Protocol.

association-related options  Options that are tied to a particular connection, transaction, or data transmission; some of the information they contain is destined for the remote client. Compare with non-association-related options.

asymmetrical connection   A networking connection in which both ends do not have equal control over the communication. A transaction-based connection is an asymmetrical connection. Compare with symmetrical connection.

asynchronous communication  A way of coordinating serial data transfers that is the prevailing standard in the personal computer industry. This method requires each peer to agree on a clock rate before communicating.

asynchronous event   An event used to notify your application that something requires immediate attention. For example, expedited data has arrived or a disconnection request is pending. See also provider event, notifier function, completion event.

asynchronous mode   A mode of operation in which provider functions return as soon as they are queued for execution. When the function actually finishes executing, the provider issues a completion event. Compare with synchronous mode.

ATP   AppleTalk Transaction Protocol.

baud rate   The rate, in samples per second, at which a serial receiver samples a line.

best-effort delivery   A message-delivery paradigm in which the networking protocol attempts to deliver any packets that meet certain requirements, such as containing a valid destination address, but the protocol does not inform the sender when it is unable to deliver the data, nor does it attempt to recover from error conditions and data loss. Compare with reliable delivery.

binding   The process of associating an endpoint with a logical address before the endpoint can be used to transfer data. Depending on the protocol you use, you can specify this address as a symbolic name or as a network address. Address binding rules and address formats also vary with the protocol you use.

bit time   The periodic interval at which a serial receiver samples a linel.

blocking  A mode of operation in which a provider must wait for some action to complete before continuing operation when sending or receiving data. If a provider is nonblocking, any function that might have to wait, returns immediately with an error result. See also asynchronous.

blocking status     A provider's state that determines whether it is blocking. See also blocking, nonblocking.

break signal  A special signal that falls outside the character frame. The break signal occurs when the line is switched from the mark state to a space and held there for longer than a character frame.

bridge   A device that connects networking cables without examining the addresses of messages or making decisions as to the best route for a message to take. Compare with router, gateway.

canonical name  A fully qualified domain name that is not an alias.

character frame  The unit of serial communication transmission. Character frames of 7 or 8 data bits are commonly used for transmitting ASCII characters.

child port  An attribute of a port that identifies which of multiple available ports a pseudodevice uses as its transmission hardware. A port may have more than one child port, all of which can be active simultaneously.

classic AppleTalk  The Mac OS implementation of AppleTalk available before Open Transport.

Clear To Send (CTS) signal  A signal that indicates that the modem or printer is ready to send data.

client   A protocol that uses the services of an underlying protocol. For example, ADSP is a client of DDP. Also used to refer to application level code, which is a client of the network provider.

combined DDP-NBP address format  An AppleTalk address format that combines an endpoint's physical address and its NBP name. See also DDP address format, NBP address format.

completion event  A provider event used to notify your application that an asynchronous function has completed execution. See also provider event, notifier function, asynchronous.

connection   An association between two endpoints that permits the establishment and maintenance of an exclusive dialogue between the endpoints.

connectionless protocol   A networking protocol in which a node that wants to communicate with another simply sends a message without first establishing that the receiving node is prepared to receive it. Each message sent must include addressing information so that it can be delivered to its destination. Compare with connection-oriented protocol.

connection-oriented protocol   A networking protocol in which two nodes on the network that want to communicate first establish a connection. Once a connection is established, the communicating applications or processes on the nodes at either end can send and receive data without having to add addresses to the messages or repeat the handshake process. Compare with connectionless protocol. See also connection, handshake, session.

datagram    A small unit of data that includes a header portion that holds the destination address (and may contain other information, such as a checksum value), and a data portion that holds the message text. Same as packet.

Datagram Delivery Protocol (DDP)   A connectionless transactionless AppleTalk protocol that transfers data between sockets as discrete datagrams, each carrying its destination socket address. DDP provides best-effort delivery of data.

data-link layer   The layer of the OSI model that, together with the physical layer, provides for connectivity. The data-link layer contains the software that communicates directly with the physical network devices and provides for switching between physical devices.

DDP   Datagram Delivery Protocol.

DDP address format   An AppleTalk address format that indicates the physical address of an endpoint. See also combined DDP-NBP address.

DDP type   The type of protocol for DDP packets. This is used by DDP endpoints to filter incoming and outgoing data.

Data Terminal Ready (DTR) signal   A signal indicates that the computer is ready to communicate. Deasserting this signal causes the sender to suspend transmission.

default port   The port that Open Transport uses when a specific port is not indicated. The LocalTalk default port is specified in the AppleTalk control panel.

domain   A collection of hosts on a TCP/IP internet. Domains are hierarchically arranged and each can be identified by its domain name or its IP address.

domain name   A character-string name that can be used to identify a TCP/IP domain. See also fully qualified domain name.

domain name resolver   A process running on a TCP/IP network that translates between the character-string names used by people to identify nodes on the internet and the 32-bit internet addresses used by the network itself.

dynamically assigned socket   An AppleTalk socket arbitrarily assigned by DDP if you do not specify a socket number when binding an endpoint. Compare statically assigned socket.

echo request packet   A packet sent by the AEP Echoer to the target node.

echo reply packet   The packet sent in response to an echo request packet sent by the AEP Echoer.

echoer socket   The statically assigned DDP socket (socket number 4) that AEP uses to listen for echo packets.

endpoint   The communications path between your application and an endpoint provider. An endpoint consists of a set of data structures that are maintained by Open Transport and that specify the components of the endpoint provider, the provider's state, and the provider's mode of operation.

endpoint function   An Open Transport function that you can use only with endpoints. Endpoint functions create and bind endpoints, obtain information about endpoints, establish and break down connections, and transfer data. The behavior of an endpoint function is determined by the endpoint's mode of operation.

endpoint provider   An Open Transport provider that sends and receives information over a data link. See also endpoint, mapper provider, service provider.

endpoint reference   A number that Open Transport returns to you when you open an endpoint. This number identifies the instance of the endpoint provider that you have created.

endpoint state     An endpoint attribute that governs which endpoint functions you can call for the endpoint. For example, a connectionless endpoint can only transfer data while it is in the T_IDLE state; a connection-oriented endpoint can only transfer data while it is in the T_DATAXFER state.

ETSDU   See expedited transport service data unit.

event   See provider event.

exactly-once transaction   An ATP transaction that ensures that the responder receives a specific request only once. These are also referred to as XO transactions.

expedited   transport service data unit (ETSDU)   A unit of expedited data that you can use to deliver urgent data. An ETSDU is the largest piece of expedited data that an endpoint can transfer with boundaries and content preserved. Different types of endpoints permit different size ETSDUs. See also transport service data unit (TSDU).

extended network   An AppleTalk network that has a range of network numbers assigned to it and that supports multiple zones. Each node on the network has a unique network number-node ID combination to identify it.

full duplex   A networking connection in which both ends can transmit and receive data simultaneously. Compare with half duplex.

fully qualified domain name A domain name that corresponds to an internet address. The name is not abbreviated, for example www.apple.com as opposed to www.

gateway   A device that connects networking cables and that converts addresses and protocols to connect dissimilar networks. Compare with bridge, router.

general provider function     A function that you can use to manipulate any type of provider. For example, you can call the OTCloseProvider function to close any type of provider. See also provider function.

half duplex   A networking connection in which the two ends have to take turns transmitting and receiving. Compare with full duplex.

handshake   A connection-establishment process involving the exchange of predetermined signals between nodes in which each end identifies itself to the other. Also, serial handshaking is used for flow control. See also connection-oriented protocol, session.

header   The portion of a datagram that holds the destination address and may contain other information, such as a checksum value.

host   A node on a TCP/IP internet. A host that is addressable by other hosts has a host address and one or more domain names.

interface function   A function that Open Transport passes through to the underlying software modules that implement networking protocols. See also utility function.

internet   A set of networks connected by routers or gateways.

Internet     See Worldwide Internet.

internet address   A 32-bit number that uniquely identifies a host on a TCP/IP network. An internet address is commonly expressed in dotted-decimal notation (for example, "12.13.14.15"). Also called IP address.

Internet Protocol (IP)   The basic datagram-delivery protocol in the TCP/IP protocol family.

IP   Internet Protocol.

IP address   An internet address.

mail exchange   Any TCP/IP host that can accept mail for another host or for a domain. A mail exchange can be a mail server, a gateway, or just a host configured to accept and pass on mail.

mail preference value   A number used by a mail application to determine to which mail exchange to deliver a message when there is more than one that can accept mail for a particular domain. The mailer sends the mail to the mail exchange with the lowest preference value first and tries the others in turn until the mail is delivered or until the mailer deems the mail undeliverable.

mapper  The communications path between your application and a mapper provider. A mapper consists of a set of data structures, maintained by Open Transport, that specify the components of the mapper provider, the provider's state, and the provider's mode of operation.

mapper provider  An Open Transport provider that relates network addresses to network node names and can be used to register and remove node names for networks that support this ability. See also endpoint provider, mapper, service provider.

mapper reference  A number that Open Transport returns to you when you open a mapper. This number identifies the instance of the mapper provider that you have created.

mark state  An idle state in which a serial communications line.

miscellaneous event  A network-related event that may affect the operation of an Open Transport provider. In particular, these apply to AppleTalk endpoints and can include such events as opening or closing an AppleTalk driver. See also AppleTalk Transition Queue (ATQ).

mode of operation  A provider's attribute that determines whether provider functions execute synchronously or asynchronously, whether functions can wait to send or receive data, and whether data is copied.

module name   A port structure field that gives the name of the actual STREAMS module that implements the driver for a given port. Open Transport uses this name internally.

multihoming   The situation in which a single host or node is connected to two or more networks or network interface cards (NICs) at the same time.

multinode   A node ID that an application can acquire that is in addition to the standard node ID that is assigned when the node joins an AppleTalk network.

multinode address format   An AppleTalk address format that indicates the physical address of a multinode endpoint.

multinode architecture   An AppleTalk feature that allows an application to acquire node IDs that are additional to the standard node ID that is assigned to the system when the node joins an AppleTalk network.

multinode ID   An AppleTalk node ID that allows the computer running your application to appear as multiple nodes on the network even though it is only one physical entity. Each acquired multinode is in addition to the standard node ID already assigned to the computer when it joined the network as a node. The prime example of a multinode application is Apple Remote Access (ARA).

multiport identifier   A port reference parameter that distinguishes between multiple ports when a single slot supports more than one port. Typically, the hardware device in a multiport slot is either a plug-in multifunction card with multiple ports on it.

Name-Binding Protocol (NBP)   An AppleTalk protocol that maintains a mapping of logical names (like those in the Chooser) to physical socket addresses in such a way that if the node ID changes, you can continue to reliably identify services.

name registry  A register of hardware and software configuration information for Power Macintosh computers.

NBP   Name-Binding Protocol.

name   That part of an NBP name that typically identifies the user of the system or, in the case of a server, the system itself. See also type   and zone.

NBP address format   An AppleTalk address format that indicates the endpoint's NBP name.

NBP entity structure   A structure that Open Transport provides for convenient manipulation of NBP names. The NBP entity structure itself does not contain escape characters, but the function OTSetAddressFromNBPEntity inserts a backslash (\) in front of any backslash, colon (:), or at sign (@) they find in an NBP name so that mapper functions can use a correctly formatted NBP name.

NBP mapper provider   An Open Transport mapper provider that is configured as an NBP mapper.

NBP name   An endpoint's logical name, sometimes called its entity name, used in the NBP address format. The NBP name consists of three fields: name, type, and zone. See also name, type, zone.

network   A system of computers and other devices (such as printers and modems) that are connected in such a way that they can exchange data.

network layer   The layer of the OSI model immediately above the data-link layer. The network layer specifies the network routing of data packets between nodes and the communications between networks, which is referred to as internetworking.

node   An addressable physical device connected to a network. See also node ID.

node ID  An 8-bit number that identifies a node on an AppleTalk network.

noise  Environmental perturbations that can affect an electrical line. Noise can cause errors in transmission by altering voltage levels so that a bit is reversed, shortened, or lengthened.

non-association-related options     Options that are negotiated solely between the client and its endpoint provider. Such options contain no information for the remote client. Compare with association-related options.

nonblocking  A mode of operation in which a provider will wait when sending or receiving data. If a provider is nonblocking, any provider function used to send or receive data returns with an error result if it cannot complete the operation immediately. Compare with blocking.

nonextended network   An AppleTalk network that has one network number assigned to it and that supports only one zone. On nonextended networks, all nodes share the same network number and zone name, and each node has a unique node ID. Compare with extended network.

notifier function     A callback function that handles Open Transport provider events. See also provider event.

option     A value you can set for an endpoint that is of interest to a specific protocol. For example, an option might enable or disable checksums or specify the priority of a datagram. The available options and their significance are defined by each implementation of each protocol. Every option has a default value.

option negotiation     The process of trying to replace one or more default option values with other values. A negotiation might involve a client and its endpoint provider, or both a local and remote client and their endpoint providers. A successful negotiation results in obtaining exactly the option values requested, a partly successful negotiation results in getting different values for the options requested, and a failed negotiation results in not being able to change existing values at all. See also absolute requirement.

orderly disconnect   Breaking a connection with the knowledge and cooperation of the remote peer. This method of disconnection prevents loss of data. Orderly disconnects can be either remote (over-the-wire) disconnects or local disconnects. See also abortive disconnect.

OSI model   A standard reference model for network architectures. The OSI (Open Standards Interconnection) model describes a seven-layer structure for networking protocols. See also application layer, presentation layer, session layer, transport layer, network layer, datalink layer, physical layer.

packet    A small unit of data that includes a header portion that holds the destination address (and may contain other information, such as a checksum value), and a data portion that holds the message text. Same as datagram.

PAP   Printer Access Protocol.

passive peer   An endpoint provider that listens for incoming connection requests. The use of a passive peer is typical of a server environment in which a server, such as a file server, uses an endpoint to listen for connection requests from multiple remote endpoints. Endpoints throughout the network can contact the server's passive endpoint with connection requests. See also active peer.

peer-to-peer connection   See symmetrical connection.

physical layer   The layer of the OSI model that provides for physical connectivity. The communication between networked systems can be via a physical cable made of wire or optical fiber, or it can be via infrared or microwave transmission. In addition to these, the hardware can include a network interface controller (NIC), if one is used.

port   A logical entity that combines a hardware device and the software driver that acts as an interface to it. Ethernet, serial devices, and LocalTalk ports are examples of ports commonly used in Open Transport. See also child port, default port, multiport identifier, pseudodevice.

port alias   A port structure flag that identifies a default port. For example, for LocalTalk, the port alias is a port name of "ltlk." Because it has the same STREAMS module name as the default LocalTalk port, if you use the port alias in the configuration string, Open Transport can locate the default port even in those cases where a computer doesn't use the standard default of "ltlkB." See also default port.

port name   A unique name that designates the port. It is typically an abbreviation of the port's device type plus a suffix, usually numeric, for example, "enet0," "enet1," and "enet2." For historic reasons, LocalTalk and serial ports use an alphabetic suffix instead.

port reference   A 32-bit value that uniquely describes a port: its device and bus type, its physical slot number, and, where applicable, its multiport identifier.

port registry   An Open Transport registry of available ports.

port structure   A structure that contains information about a port on a system. A port structure contains each port's port reference, several sets of information flags, its port name, its STREAMS module name, and the slot ID (for ports on a PCI bus).

presentation layer   The layer of the OSI model immediately below the application layer. Protocols in this layer assume that an end-to-end path or connection already exists across the network between the two communicating parties. Protocols in this layer are concerned with the representation of data values for transfer, or the transfer syntax.

Printer Access Protocol (PAP)   An asymmetrical connection-oriented transactionless AppleTalk protocol that enables communication between client and server endpoints, allowing multiple connections at both ends. In particular, PAP is used for direct printing to AppleTalk printers.

privileged options   Options whose values can be changed only by privileged clients, although it is sometimes possible for nonprivileged clients to read the value of a privileged option. The Mac OS does not distinguish between priviledged and nonpriviledged clients; so this distinction is irrelevant to Open Transport.

protocol option   See option.

protocol stack   A set of protocols related in a hierarchical fashion, where the higher-level protocols are clients of the lower-level protocols.

provider  A layered set of STREAMS modules and drivers that provides a service to clients of Open Transport. See also endpoint provider, mapper provider, service provider.

provider event  An event Open Transport uses to notify your application that something has occurred that demands immediate attention or that an asynchronous function has completed execution. See also asynchronous event and completion event.

provider function  A function that you can call to manipulate a specific type of provider. For example, you call the OTOpenEndpoint function to open an endpoint provider. See also general provider function.

provider reference  A value that is returned to you when you open a provider and that you must pass back when you call a provider function. The data type of the provider reference depends on the type of the provider.

pseudodevice  A special type of port that is a driver that doesn't interface to a hardware device; instead, it interfaces to other device drivers. A pseudodevice uses a special device type, designated with the constant kOTPseudoDevice, and each must have a unique port reference. See also child port.

RawIP  An application interface to the IP protocol.

read-only options  Options whose values you can read but not change.

receive queue  An ADSP receiving buffer used to store incoming data until the local endpoint provider acknowledges reading it.

reliable delivery  A message-delivery paradigm in which the networking protocol includes error checking and recovery from error or loss of data. Compare with best-effort delivery.

requester  An endpoint that as part of a transaction sends a request for a service. The responder endpoint reads the request, performs the service, and sends a reply. When the requester receives the reply, the transaction is complete.

responder  An endpoint that as part of a transaction reads a requester endpoint's request, performs the service, and sends a reply.

router  A device that connects networking cables and that contains addressing and routing information that lets it determine from a packet's address the most efficient route for the packet. A packet can be passed from router to router several times before being delivered to its destination. Compare with bridge, gateway.

send-acknowledgment   status  A provider's attribute that determines whether endpoint providers that send data make an internal copy of the data before sending it and whether they notify your application when they have sent the data.

send queue  An ADSP buffer used to store outgoing data until the remote endpoint provider acknowledges receiving it.

service provider  An Open Transport provider that handles features unique to a specific protocol stack. For example, to get information about AppleTalk zones, you must open an AppleTalk service provider. See also endpoint provider, mapper provider.

session   A logical (as opposed to physical) connection between two entities on a network or internet. A session must be set up at the beginning, maintained by the periodic exchange of information, and broken down at the end. See also connection-oriented protocol.

session layer   The layer of the OSI model that serves as an interface to the transport layer, which is below it. The session layer allows for establishing a session, which is the process of setting up a connection over which a dialog between two applications or processes can occur. Some of the functions that the session layer provides for are flow control, establishment of synchronization points for checks and recovery during file transfer, full-duplex and half-duplex dialogs between processes, and aborts and restarts.

socket   A piece of software that serves as an addressable entity on a node. Endpoints exchange data with each other across an AppleTalk internet through sockets.

socket number  An 8-bit number that identifies an AppleTalk socket. Each endpoint on an AppleTalk network is associated with a unique 8-bit socket number.

space  The state into which a serial line is placed to signal data. Compare with mark.

start bit  A signal that delineates a serial line's change from the mark state to a space. The start bit triggers the synchronization necessary for asynchronous communication.

state dependence   A condition of a networking protocol or connection in which the response to a request is dependent on a previous request. For example, before a workstation application connected to a file server can read a file, it must have first issued a request to open the file.

statically assigned socket  An AppleTalk socket that is permanently reserved for a designated protocol or process. For example, socket 4 is always reserved as the echo socket, used for echoing packets across a network. Compare dynamically assigned socket.

stop bit  A signal that delineates the end of the character frame and places the serial line back into a mark state for a minimum specified time interval. This interval has one of several possible values: 1, 1.5, and 2 stop bits.

STREAMS module   A module that conforms to the STREAMS architecture. This architecture is a UNIX® standard in which protocols and other service providers are implemented as software modules that communicate between each other using messages. Open Transport software modules are implemented as STREAMS modules.

subnet   A portion of a network, which is in turn a portion of an internet.

subnet mask   A number that can be used to determine what portion of an IP address is dedicated to the host identifier and what portion identifies the subnet.

symmetrical connection   A networking connection in which both ends have equal control over the communication. Both ends can send and receive data and initiate or terminate the session. Compare with asymmetrical connection.

synchronous mode  A mode of operation in which provider functions do not return until they have finished executing. See also asynchronous mode.

TCP  Transmission Control Protocol.

TCP/IP protocol family  A set of networking protocols in wide use throughout the world for government and business applications. The TCP/IP protocol family includes TCP, UDP, and IP, among other protocols.

TCP/IP service provider  An Open Transport provider that provides an interface to the TCP/IP Domain Name Resolver (DNR).

tr  ansaction  A process during which one endpoint, the requester, sends a request for a service. The remote endpoint, called the responder, reads the request, performs the service, and sends a reply. When the requester receives the reply, the transaction is complete.

transaction-based protocol  A networking protocol that divides data transmission into transactions. Compare with transactionless protocol.

transaction ID  A number that uniquely identifies a transaction.

transactionless protocol   A networking protocol that defines how the data is to be organized and delivered from one node to another but does notdivide the data up into transactions. Compare with transaction-based protocol.

Transmission Control Protocol (TCP)  A connection-oriented data stream protocol that provides highly reliable data delivery; part of the TCP/IP protocol family.

transport independence  The independence of networking APIs from the underlying networking or transport technology.

transport layer  The layer of the OSI model that isolates some of the physical and functional aspects of a network from the upper three layers. It provides for end-to-end accountability, ensuring that all packets of data sent across the network are received and in the correct order. This process involves providing a means of identifying packet loss and supplying a retransmission mechanism. The transport layer may also provide connection and session management services.

transport service data unit (TSDU)  A unit of data that allows an endpoint to separate a data stream into discrete logical units when sending and receiving data across a connection. A TSDU is the largest piece of data that an endpoint can transfer with boundaries and content preserved. Different types of endpoints and different endpoint implementations support different size TSDUs. See also expedited transport service data unit (ETSDU).

type  That part of an NBP name that identifies the type of service that the entity provides, for example, "Mailbox" for an electronic mailbox on a server. See also name and zone.

UDP  User Datagram Protocol.

User Datagram Protocol (UDP)  A connectionless datagram protocol that provides port demultiplexing and data checksums.; part of the TCP/IP protocol family.

utility function   A function that is implemented by Open Transport itself. See also interface function.

ZIP   Zone Information Protocol.

zone  A logical grouping of nodes in an AppleTalk network or internet or that part of an NBP name that identifies the zone within the network to which the node belongs. See also name and type.

Zone Information Protocol (ZIP)  An AppleTalk protocol that maps network numbers to zone names for all networks belonging to an AppleTalk internet.