Table of Contents OS/2
TCP/IP Tutorial and Technical Overview
A description of a data structure that is independent of machine-oriented
structures and encodings.
ACSE: Association Control Service Element
The method used in OSI for establishing a call between two applications.
Checks the identities and contexts of the application entities, and could apply
an authentication security check.
A gateway that is treated like a network interface, in that it is expected
to exchange routing information, and if it does not do so for a period of time,
the route associated with the gateway will be deleted.
A bit mask used to select bits from an Internet address for subnet
addressing. The mask is 32 bits long and selects the network portion of the
Internet address and one or more bits of the local portion. Sometimes called
A means for mapping Network Layer addresses onto media-specific address.
Administration Management Domain. An X.400 Message Handling System public
service carrier. Examples: MCImail and ATTmail in the U.S., British Telecom
Gold400mail in the U.K. The ADMDs in all countries worldwide together provide
the X.400 backbone. See PRMD.
In the client-server model, the part of the system that performs
information preparation and exchange on behalf of a client or server
application. See NMS, DUA, MTA.
American National Standards Institute. The U.S. standardization body. ANSI
is a member of the International Organization for Standardization (ISO).
Application Program Interface. A set of calling conventions defining how a
service is invoked through a software package.
The top-most layer in the OSI Reference Model providing such communication
services as electronic mail and file transfer.
One of the first Internet navigators, used to search for files at FTP
Address Resolution Protocol. The Internet protocol used to dynamically map
Internet addresses to physical (hardware) addresses on local area networks.
Limited to networks that support hardware broadcast.
Advanced Research Projects Agency. Now called DARPA, the U.S. government
agency that funded the ARPANET.
A packet switched network developed in the early 1970s. The "grandfather"
of today's Internet. ARPANET was decommissioned in June 1990.
Abstract Syntax Notation One. The OSI language for describing abstract
syntax. See BER.
The form of information items provided by the X.500 Directory Service. The
directory information base consists of entries, each containing one or more
attributes. Each attribute consists of a type identifier together with one or
more values. Each directory Read operation can retrieve some or all attributes
from a designated entry.
Internet (TCP/IP) terminology for a collection of gateways (routers) that
fall under one administrative entity and cooperate using a common Interior
Gateway Protocol (IGP). See subnetwork.
A process that does not require operator intervention that can be run by
the computer while the workstation is used to do another work.
A mode of program execution in which the shell does not wait for program
completion before prompting the user for another command.
The primary connectivity mechanism of a hierarchical distributed system.
All systems which have connectivity to an intermediate system on the backbone
are assured of connectivity to each other. This does not prevent systems from
setting up private arrangements with each other to bypass the backbone for
reasons of cost, performance, or security.
Characteristic of any network technology that uses a single carrier
frequency and requires all stations attached to the network to participate in
every transmission. See broadband.
Basic Encoding Rules. Standard rules for encoding data units described in
ASN.1. Sometimes incorrectly lumped under the term ASN.1, which properly
refers only to the abstract syntax description language, not the encoding
Border Gateway Protocol. A connection-oriented routing protocol (using
TCP) that was developed based upon experience using EGP. See EGP.
A format for storage or transmission of binary data in which the most
significant bit (or byte) comes first. The reverse convention is called
Because It's Time NETwork. An academic computer network based originally
on IBM mainframe systems interconnected via leased 9600 bps lines. BITNET was
eventually merged with CSNET, The Computer+Science Network (another academic
computer network) to form CREN: The Corporation for Research and Educational
Networking. See CSNET.
A device that connects two or more physical networks and forwards packets
between them. Bridges can usually be made to filter packets, that is, to
forward only certain traffic. Related devices are: repeaters which simply
forward electrical signals from one cable to another, and full-fledged routers
which make routing decisions based on several criteria.
A functional unit that connects two local area networks (LANs) that use the
same logical link control (LLC) procedure but may use different medium access
control (MAC) procedures.
Characteristic of any network that multiplexes multiple, independent
network carriers onto a single cable. This is usually done using frequency
division multiplexing. Broadband technology allows several networks to coexist
on one single cable; traffic from one network does not interfere with traffic
from another since the "conversations" happen on different frequencies in the
"ether," rather like the commercial radio system.
A packet delivery system where a copy of a given packet is given to all
hosts attached to the network. Example: Ethernet.
Berkeley Software Distribution. Term used when describing different
versions of the Berkeley UNIX software, as in "4.3BSD UNIX."
A network in which hosts are connected to networks with varying
characteristics, and the networks are interconnected by gateways (routers).
The Internet is an example of a catenet. See IONL.
Commitment, Concurrency, and Recovery. An OSI application service element
used to create atomic operations across distributed systems. Used primarily to
implement two-phase commit for transactions and nonstop operations.
a child is a process spawned by a parent process that shares resources of
A common way to describe network services and the model user processes
(programs) of those services. Examples include the name-server/name-resolver
paradigm of the DNS and file- server/file-client relationships such as NFS and
Connectionless Network Protocol. The OSI protocol for providing the OSI
Connectionless Network Service (datagram service). CLNP is the OSI equivalent
to Internet IP, and is sometimes called ISO IP.
Connectionless Transport Protocol. Provides for end-to-end Transport data
addressing (via Transport selector) and error control (via checksum), but
cannot guarantee delivery or provide flow control. The OSI equivalent of UDP.
Common Management Information Protocol. The OSI network management
CMIP Over TCP. An effort to use the OSI network management protocol to
manage TCP/IP networks.
The model of interconnection in which communication takes place without
first establishing a connection. Sometimes (imprecisely) called datagram.
Examples: LANs, Internet IP and OSI CLNP, UDP, ordinary postcards.
The model of interconnection in which communication proceeds through three
well-defined phases: connection establishment, data transfer, connection
release. Examples: X.25, Internet TCP and OSI TP4, ordinary telephone calls.
Historically, one of a set of gateways (routers) operated by the Internet
Network Operations Center at BBN. The core gateway system forms a central part
of Internet routing in that all groups must advertise paths to their networks
from a core gateway, using the Exterior Gateway Protocol (EGP). See EGP,
Corporation for Open Systems. A vendor and user group for conformance
testing, certification, and promotion of OSI products.
Cooperation for Open Systems Interconnection Networking in Europe. A
program sponsored by the European Commission, aimed at using OSI to tie
together European research networks.
See BITNET and CSNET.
Carrier Sense Multiple Access with Collision Detection. The access method
used by local area networking technologies such as Ethernet.
Computer+Science Network. A large computer network, mostly in the U.S. but
with international connections. CSNET sites include universities, research
labs, and some commercial companies. Now merged with BITNET to form CREN. See
Defense Advanced Research Projects Agency. The U.S. government agency that
funded the ARPANET.
data link layer
The OSI layer that is responsible for data transfer across a single
physical connection, or series of bridged connections, between two network
Defense Communications Agency. The government agency responsible for the
Defense Data Network (DDN).
Distributed Computing Environment. An architecture of standard programming
interfaces, conventions, and server functionalities (for example naming,
distributed file system, remote procedure call) for distributing applications
transparently across networks of heterogeneous computers. Promoted and
controlled by the Open Software Foundation (OSF), a consortium led by HP, DEC,
and IBM. See ONC.
Defense Data Network. Comprises the MILNET and several other DoD networks.
Digital Equipment Corporation's proprietary network architecture.
Domain Name System. The distributed name/address mechanism used in the
In the Internet, a part of a naming hierarchy. Syntactically, an Internet
domain name consists of a sequence of names (labels) separated by periods
(dots), for example, "tundra.mpk.ca.us." In OSI, "domain" is generally used as
an administrative partition of a complex distributed system, as in MHS Private
Management Domain (PRMD), and Directory Management Domain (DMD).
dotted decimal notation
The syntactic representation for a 32-bit integer that consists of four
8-bit numbers written in base 10 with periods (dots) separating them. Used to
represent IP addresses in the Internet as in: 188.8.131.52.
Directory System Agent. The software that provides the X.500 Directory
Service for a portion of the directory information base. Generally, each DSA is
responsible for the directory information for a single organization or
Directory User Agent. The software that accesses the X.500 Directory
Service on behalf of the directory user. The directory user may be a person or
another software element.
European Academic Research Network. A network using BITNET technology
connecting universities and research labs in Europe.
Exterior Gateway Protocol. A reachability routing protocol used by
gateways in a two-level internet. EGP is used in the Internet core system.
See core gateway.
The technique used by layered protocols in which a layer adds header
information to the protocol data unit (PDU) from the layer above. As an
example, in Internet terminology, a packet would contain a header from the
physical layer, followed by a header from the network layer (IP), followed by a
header from the transport layer (TCP), followed by the application protocol
An OSI system which contains application processes capable of communicating
through all seven layers of OSI protocols. Equivalent to Internet host.
OSI terminology for a layer protocol machine. An entity within a layer
performs the functions of the layer within a single computer system, accessing
the layer entity below and providing services to the layer entity above at
local service access points.
End system to Intermediate system protocol. The OSI protocol by which end
systems announce themselves to intermediate systems.
A 10 Mbps baseband local area network using CSMA/CD (Carrier Sense Multiple
Acess with Collision Detection). The network allows multiple stations to access
the medium at will without prior coordination; avoid contention by using
carrier sense and deference; and resolve contention by using collision
detection and retransmission.
European UNIX Network.
European UNIX Users Group.
A character other than a 7-bit ASCII character. An extended character can
be a 1-byte code point with the 8th bit set (ordinal 128-255) or a 2-byte code
point (ordinal 256 and greater).
European Workshop for Open Systems. The OSI Implementors Workshop for
Europe. See OIW.
Federation of American Research Networks.
Fiber Distributed Data Interface. An emerging high-speed networking
standard. The underlying medium is fiber optics, and the topology is a
dual-attached, counter-rotating token-ring. FDDI networks can often be spotted
by the orange fiber "cable."
The process in which an IP datagram is broken into smaller pieces to fit
the requirements of a given physical network. The reverse process is termed
reassembly. See MTU.
Federal Networking Council. The group of representatives from those federal
agencies involved in the development and use of federal networking, especially
those networks using TCP/IP, and the connected Internet. The FNC coordinates
research and engineering. Current members include representatives from DOD,
DOE, DARPA, NSF, NASA and HHS.
Federal Research Internet Coordinating Committee. Now replaced by the FNC.
File Transfer, Access, and Management. The OSI remote file service and
File Transfer Protocol. The Internet protocol (and program) used to
transfer files between hosts. See FTAM.
The original Internet term for what is now called router or more precisely,
IP router. In modern usage, the terms "gateway" and "application gateway"
refer to systems which do translation from some native format to another.
Examples include X.400 to/from RFC 822 electronic mail gateways. See router.
Internet navigation tool that provides menu access to information. Many
organizations use this tool instead of an anonymous FTP site. See Veronica.
Government OSI Profile. A U.S. Government procurement specification for
Internet Activities Board. The technical body that oversees the
development of the Internet suite of protocols (commonly referred to as
"TCP/IP"). It has two task forces (the IRTF and the IETF) each charged with
investigating a particular area.
Internet Assigned Numbers Authority. The technical body inside the IAB that
manages the Internet protocol standards. It coordinates the assignment of
values to the parameters of protocols.
Internet Control Message Protocol. The protocol used to handle errors and
control messages at the IP layer. ICMP is actually part of the IP protocol.
Internet Engineering Steering Group. The executive committee of the IETF.
Internet Engineering Task Force. One of the task forces of the IAB. The
IETF is responsible for solving short-term engineering needs of the Internet.
It has over 40 Working Groups.
Interior Gateway Protocol. The protocol used to exchange routing
information between collaborating routers in the Internet. RIP and OSPF are
examples of IGPs.
Internet Gateway Routing Protocol. A proprietary IGP used by Cisco
An OSI system which is not an end system, but which serves instead to relay
communications between end systems. See repeater, bridge, and router.
A collection of networks interconnected by a set of routers which allow
them to function as a single, large virtual network.
(note the capital "I") The largest internet in the world consisting of
large national backbone nets (such as MILNET, NSFNET, and CREN) and a myriad of
regional and local campus networks all over the world. The Internet uses the
Internet protocol suite. To be on the Internet you must have IP connectivity,
i.e., be able to TELNET to--or ping--other systems. Networks with only E-mail
connectivity are not actually classified as being on the Internet.
A 32-bit address assigned to hosts using TCP/IP. See dotted decimal
Internal Organization of the Network Layer. The OSI standard for the
detailed architecture of the network layer. Basically, it partitions the
network layer into subnetworks interconnected by convergence protocols
(equivalent to internetworking protocols), creating what Internet calls a
catenet or internet.
Internet Protocol. The network layer protocol for the Internet protocol
The fundamental unit of information passed across the Internet. Contains
source and destination addresses along with data and a number of fields which
define such things as the length of the datagram, the header checksum, and
flags to say whether the datagram can be (or has been) fragmented.
Internet Research Task Force. One of the task forces of the IAB. The
group responsible for research and development of the Internet protocol suite.
Integrated Services Digital Network. An emerging technology which is
beginning to be offered by the telephone carriers of the world. ISDN combines
voice and digital network services in a single medium making it possible to
offer customers digital data services as well as voice connections through a
single "wire." The standards that define ISDN are specified by CCITT.
Intermediate system to Intermediate system protocol. The OSI protocol by
which intermediate systems exchange routing information.
International Organization for Standardization. Best known for the
seven-layer OSI Reference Model. See OSI.
ISO Development Environment. A popular implementation of the upper layers
of OSI. Pronounced eye-so-dee-eee.
Joint Academic Network. A university network in the U.K.
Japan UNIX Network.
A popular implementation of TCP/IP and associated protocols for amateur
packet radio systems.
A popular file transfer and terminal emulation program.
A format for storage or transmission of binary data in which the least
significant byte (bit) comes first. See big-endian.
Part of an electronic mail delivery system which allows a message to be
delivered to a list of addressees. Mail exploders are used to implement
mailing lists. Users send messages to a single address (for example
email@example.com) and the mail exploder takes care of delivery to the
individual mailboxes in the list.
A machine that connects two or more electronic mail systems (especially
dissimilar mail systems on two different networks) and transfers messages
between them. Sometimes the mapping and translation can be quite complex, and
generally it requires a store-and-forward scheme whereby the message is
received from one system completely before it is transmitted to the next system
after suitable translations.
Humorous term applied to packets that turn up unexpectedly on the wrong
network because of bogus routing entries. Also used as a name for a packet
which has an altogether bogus (non-registered or ill-formed) Internet address.
Message Handling System. The system of message user agents, message
transfer agents, message stores, and access units which together provide OSI
electronic mail. MHS is specified in the CCITT X.400 Series of
Management Information Base. A collection of objects that can be accessed
via a network management protocol. See SMI.
Military Network. Originally part of the ARPANET, MILNET was partitioned
in 1984 to make it possible for military installations to have reliable network
service, while the ARPANET continued to be used for research. See DDN.
Multipurpose Internet Mail Extensions. A mail protocol that provides
support for multimedia (graphics, audio, video) as well as basic compatibility
with SMTP. MIME is described in RFCs 1521 and 1522. See SMTP.
Message Transfer Agent. An OSI application process used to store and
forward messages in the X.400 Message Handling System. Equivalent to Internet
Maximum Transmission Unit. The largest possible unit of data that can be
sent on a given physical medium. Example: the MTU of Ethernet is 1500 bytes.
A special form of broadcast where copies of the packet are delivered to
only a subset of all possible destinations. See broadcast.
A computer connected to more than one physical data link. The data links
may or may not be attached to the same network.
The process of mapping a name into the corresponding address. See DNS.
Network Basic Input Output System. The standard interface to networks on
IBM PC and compatible systems.
See Internet address or OSI Network Address.
The OSI layer that is responsible for routing, switching, and subnetwork
access across the entire OSI environment.
Network File System. A distributed file system developed by SUN
Microsystems which allows a set of computers to cooperatively access each
other's files in a transparent manner.
Network Information Center. Originally there was only one, located at SRI
International which had the task of serving the ARPANET (and later DDN)
community. Today, there are many NICs, operated by local, regional, and
national networks all over the world. Such centers provide user assistance,
document service, training, and much more.
National Institute of Standards and Technology. (Formerly NBS.) See OIW.
Network Management Station. The system responsible for managing a (portion
of a) network. The NMS talks to network management agents, which reside in the
managed nodes, via a network management protocol. See agent.
Network Operations Center. Any center given the task of managing the
operational aspects of a production network. These tasks include monitoring
and control, trouble-shooting, user assistance, and so on.
Network Service Access Point. The point at which the OSI Network Service
is made available to a transport entity. The NSAPs are identified by OSI
National Science Foundation. Sponsors of the NSFNET (National Science
Foundation Network). A collection of local, regional, and mid-level networks
in the U.S. tied together by a high-speed backbone. NSFNET provides scientists
access to a number of supercomputers across the country.
Workshop for Implementors of OSI. Frequently called NIST OIW or the NIST
Workshop, this is the North American regional forum at which OSI implementation
agreements are decided. It is equivalent to EWOS in Europe and AOW in the
Open Network Computing. A distributed applications architecture promoted
and controlled by a consortium led by Sun Microsystems.
Open Systems Interconnection. An international standardization program to
facilitate communications among computers from different manufacturers. See
OSI Network Address
The address, consisting of up to 20 octets, used to locate an OSI Transport
entity. The address is formatted into an Initial Domain Part which is
standardized for each of several addressing domains, and a Domain Specific Part
which is the responsibility of the addressing authority for that domain.
OSI Presentation Address
The address used to locate an OSI Application entity. It consists of an
OSI Network Address and up to three selectors, one each for use by the
Transport, Session, and Presentation entities.
Open Shortest Path First. A "Proposed Standard" IGP for the Internet. See
Protocol Control Information. The protocol information added by an OSI
entity to the service data unit passed down from the layer above, all together
forming a Protocol Data Unit (PDU).
Protocol Data Unit. This is OSI terminology for "packet." A PDU is a data
object exchanged by protocol machines (entities) within a given layer. PDUs
consist of both Protocol Control Information (PCI) and user data.
The OSI layer that provides the means to activate and use physical
connections for bit transmission. In plain terms, the physical layer provides
the procedures for transferring a single bit across a physical media.
Any means in the physical world for transferring signals between OSI
systems. Considered to be outside the OSI Model, and therefore sometimes
referred to as "Layer 0." The physical connector to the media can be
considered as defining the bottom interface of the physical layer, i.e., the
bottom of the OSI Reference Model.
Packet internet groper. A program used to test reachability of
destinations by sending them an ICMP echo request and waiting for a reply. The
term is used as a verb: "Ping host X to see if it is up!"
The abstraction used by Internet transport protocols to distinguish among
multiple simultaneous connections to a single destination host. See selector.
Promoting Conference for OSI. The OSI "800-pound gorilla" in Japan.
Consists of executives from the six major Japanese computer manufacturers and
Nippon Telephone and Telegraph. They set policies and commit resources to
Point-to-Point Protocol. The successor to SLIP, PPP provides
router-to-router and host-to-network connections over both synchronous and
asynchronous circuits. See SLIP.
See OSI Presentation Address.
The OSI layer that determines how Application information is represented
(i.e., encoded) while in transit between two end systems.
Private Management Domain. An X.400 Message Handling System private
organization mail system. Example: NASAmail. See ADMD.
A formal description of messages to be exchanged and rules to be followed
for two or more systems to exchange information.
The mechanism whereby one system "fronts for" another system in responding
to protocol requests. Proxy systems are used in network management to avoid
having to implement full protocol stacks in simple devices, such as modems.
The technique in which one machine, usually a router, answers ARP requests
intended for another machine. By "faking" its identity, the router accepts
responsibility for routing packets to the "real" destination. Proxy ARP allows
a site to use a single IP address with two physical networks. Subnetting would
normally be a better solution.
Packet-Switched Node. The modern term used for nodes in the ARPANET and
MILNET. These used to be called IMPs (Interface Message Processors). PSNs are
currently implemented with BBN C30 or C300 minicomputers.
Reseaux Associes pour la Recherche Europeenne. European association of
Reverse Address Resolution Protocol. The Internet protocol a diskless host
uses to find its Internet address at startup. RARP maps a physical (hardware)
address to an Internet address. See ARP.
A device which propagates electrical signals from one cable to another
without making routing decisions or providing packet filtering. In OSI
terminology, a repeater is a physical layer intermediate system. See bridge
Request For Comments. The document series, begun in 1969, which describes
the Internet suite of protocols and related experiments. Not all (in fact very
few) RFCs describe Internet standards, but all Internet standards are written
up as RFCs.
Remote File System. A distributed file system, similar to NFS, developed
by AT&T and distributed with their UNIX System V operating system. See
Routing Information Protocol. An Interior Gateway Protocol (IGP) supplied
with Berkeley UNIX.
Reseaux IP Europeens. European continental TCP/IP network operated by
EUnet. See EUnet.
A service offered by Berkeley UNIX which allows users of one machine to log
into other UNIX systems (for which they are authorized) and interact as if
their terminals were connected directly. Similar to TELNET.
Remote Operations Service Element. A lightweight RPC protocol, used in OSI
Message Handling, Directory, and Network Management application protocols.
A system responsible for making decisions about which of several paths
network (or Internet) traffic will follow. To do this it uses a routing
protocol to gain information about the network, and algorithms to choose the
best route based on several criteria known as "routing metrics." In OSI
terminology, a router is a network layer intermediate system. See gateway,
bridge and repeater.
Remote Procedure Call. An easy and popular paradigm for implementing the
client/server model of distributed computing. A request is sent to a remote
system to execute a designated procedure, using arguments supplied, and the
result returned to the caller. There are many variations and subtleties,
resulting in a variety of different RPC protocols.
Reliable Transfer Service Element. A lightweight OSI application service
used above X.25 networks to handshake application PDUs across the Session
Service and TP0. Not needed with TP4, and not recommended for use in the U.S.
except when talking to X.400 ADMDs.
Service Access Point. The point at which the services of an OSI layer are
made available to the next higher layer. The SAP is named according to the
layer providing the services: for example, transport services are provided at a
Transport SAP (TSAP) at the top of the transport layer.
The identifier used by an OSI entity to distinguish among multiple SAPs at
which it provides services to the layer above. See port.
The OSI layer that provides means for dialogue control between end systems.
Simple Gateway Management Protocol. The predecessor to SNMP. See SNMP.
Serial Line IP. An Internet protocol used to run IP over serial lines such
as telephone circuits or RS-232 cables interconnecting two systems. SLIP is
now being replaced by PPP. See PPP.
Switched Multimegabit Data Service. An emerging high-speed networking
technology to be offered by the telephone companies in the U.S.
Structure of Management Information. The rules used to define the objects
that can be accessed via a network management protocol. See MIB.
Simple Mail Transfer Protocol. The Internet electronic mail protocol.
Defined in RFC 821, with associated message format descriptions in RFC 822.
Systems Network Architecture. IBM's proprietary network architecture.
Simple Network Management Protocol. The network management protocol of
choice for TCP/IP-based internets.
Standards Promotion and Application Group. A group of European OSI
manufacturers which chooses option subsets and publishes these in a "Guide to
the Use of Standards" (GUS).
Structured Query Language. The international standard language for
defining and accessing relational databases.
See address mask.
A collection of OSI end systems and intermediate systems under the control
of a single administrative domain and utilizing a single network access
protocol. Examples: private X.25 networks, collection of bridged LANs.
Transmission Control Protocol. The major transport protocol in the
Internet suite of protocols providing reliable, connection-oriented,
full-duplex streams. Uses IP for delivery. See TP4.
The virtual terminal protocol in the Internet suite of protocols. Allows
users of one host to log into a remote host and interact as normal terminal
users of that host.
The process whereby two protocol entities synchronize during connection
OSI Transport Protocol Class 0 (Simple Class). This is the simplest OSI
Transport Protocol, useful only on top of an X.25 network (or other network
that does not lose or damage data).
OSI Transport Protocol Class 4 (Error Detection and Recovery Class). This
is the most powerful OSI Transport protocol, useful on top of any type of
network. TP4 is the OSI equivalent to TCP.
Transmitter-receiver. The physical device that connects a host interface
to a local area network, such as Ethernet. Ethernet transceivers contain
electronics that apply signals to the cable and sense collisions.
The OSI layer that is responsible for reliable end-to-end data transfer
between end systems.
User Agent. An 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.
User Datagram Protocol. A transport protocol in the Internet suite of
protocols. UDP, like TCP, uses IP for delivery; however, unlike TCP, UDP
provides for exchange of datagrams without acknowledgements or guaranteed
delivery. See CLTP.
UNIX to UNIX Copy Program. A protocol used for communication between
consenting UNIX systems.
Search tool for the gopher environment. See gopher.
World Wide Web. A global hypertext system that supports multimedia
communications on the Internet.
eXternal Data Representation. A standard for machine-independent data
structures developed by SUN Microsystems. Similar to ASN.1.
A group of computer manufacturers that promotes the development of portable
applications based on UNIX. They publish a document called the X/Open
The CCITT documents that describe data communication network standards.
Well-known ones include: X.25 Packet Switching standard, X.400 Message Handling
System, and X.500 Directory Services.
The X Window System
A popular window system developed by MIT and implemented on a number of
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