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97-479: The X Window System ( X11 , or simply X ; stylized 𝕏 ) is a windowing system for bitmap displays, common on Unix-like operating systems. X originated as part of Project Athena at Massachusetts Institute of Technology (MIT) in 1984. The X protocol has been at version 11 (hence "X11") since September 1987. The X.Org Foundation leads the X project, with the current reference implementation, X.Org Server , available as free and open-source software under

194-402: A tunneling arrangement to accommodate the connection of dissimilar networks. For example, IP may be tunneled across an Asynchronous Transfer Mode (ATM) network. Protocol layering forms the basis of protocol design. It allows the decomposition of single, complex protocols into simpler, cooperating protocols. The protocol layers each solve a distinct class of communication problems. Together,

291-522: A window decoration is drawn around each window. The programming of both the window decoration and of available widgets inside of the window, which are graphical elements for direct user interaction, such as sliders, buttons, etc., is eased and simplified through the use of widget toolkits . The main component of any windowing system is usually called the display server , although alternative denominations such as window server or compositor are also in use. Any application that runs and presents its GUI in

388-411: A client–server model: an X server communicates with various client programs. The server accepts requests for graphical output (windows) and sends back user input (from keyboard, mouse, or touchscreen). The server may function as: This client–server terminology – the user's terminal being the server and the applications being the clients – often confuses new X users, because

485-669: A coarse hierarchy of functional layers defined in the Internet Protocol Suite . The first two cooperating protocols, the Transmission Control Protocol (TCP) and the Internet Protocol (IP) resulted from the decomposition of the original Transmission Control Program, a monolithic communication protocol, into this layered communication suite. The OSI model was developed internationally based on experience with networks that predated

582-427: A combination of both. Communicating systems use well-defined formats for exchanging various messages. Each message has an exact meaning intended to elicit a response from a range of possible responses predetermined for that particular situation. The specified behavior is typically independent of how it is to be implemented . Communication protocols have to be agreed upon by the parties involved. To reach an agreement,

679-599: A computer environment (such as ease of mechanical parsing and improved bandwidth utilization ). Network applications have various methods of encapsulating data. One method very common with Internet protocols is a text oriented representation that transmits requests and responses as lines of ASCII text, terminated by a newline character (and usually a carriage return character). Examples of protocols that use plain, human-readable text for its commands are FTP ( File Transfer Protocol ), SMTP ( Simple Mail Transfer Protocol ), early versions of HTTP ( Hypertext Transfer Protocol ), and

776-685: A connection to the user's local X server, providing display and input to the user. Alternatively, the local machine may run a small program that connects to the remote machine and starts the client application. Practical examples of remote clients include: X primarily defines protocol and graphics primitives – it deliberately contains no specification for application user-interface design, such as button, menu, or window title-bar styles. Instead, application software – such as window managers, GUI widget toolkits and desktop environments, or application-specific graphical user interfaces – define and provide such details. As

873-508: A desktop environment, which, aside from the window manager, includes various applications using a consistent user interface. Popular desktop environments include GNOME , KDE Plasma and Xfce . The UNIX 98 standard environment is the Common Desktop Environment (CDE). The freedesktop.org initiative addresses interoperability between desktops and the components needed for a competitive X desktop. The X.Org implementation

970-493: A different computer to still be fully accelerated on the X server's display. For example, in classic OpenGL (before version 3.0), display lists containing large numbers of objects could be constructed and stored entirely in the X server by a remote X client program, and each then rendered by sending a single glCallList(which) across the network. X provides no native support for audio; several projects exist to fill this niche, some also providing transparent network support. X uses

1067-400: A display server provides the services of a display and input devices. One example of a display server is the X.Org Server , which runs on top of the kernel (usually a Unix -like kernel, such as Linux or BSD ). It receives user input data (e.g. from evdev on Linux) and passes it to one of its clients. The display server also receives data from its clients; it processes the data, it does

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1164-741: A functional form of the "network transparency" feature of X, via network transmissibility of graphical services, include: Several bitmap display systems preceded X. From Xerox came the Alto (1973) and the Star (1981). From Apollo Computer came Display Manager (1981). From Apple came the Lisa (1983) and the Macintosh (1984). The Unix world had the Andrew Project (1982) and Rob Pike 's Blit terminal (1982). Carnegie Mellon University produced

1261-456: A machine rather than a human being. Binary protocols have the advantage of terseness, which translates into speed of transmission and interpretation. Binary have been used in the normative documents describing modern standards like EbXML , HTTP/2 , HTTP/3 and EDOC . An interface in UML may also be considered a binary protocol. Getting the data across a network is only part of the problem for

1358-473: A native windowing system hosts X in addition, the X system can either use its own normal desktop in a separate host window or it can run rootless , meaning the X desktop is hidden and the host windowing environment manages the geometry and appearance of the hosted X windows within the host screen. An X terminal is a thin client that only runs an X server. This architecture became popular for building inexpensive terminal parks for many users to simultaneously use

1455-612: A network protocol supporting terminal and graphics windows, the server maintaining display lists. The email in which X was introduced to the Project Athena community at MIT in June 1984 The original idea of X emerged at MIT in 1984 as a collaboration between Jim Gettys (of Project Athena ) and Bob Scheifler (of the MIT Laboratory for Computer Science ). Scheifler needed a usable display environment for debugging

1552-457: A networking protocol, the protocol software modules are interfaced with a framework implemented on the machine's operating system. This framework implements the networking functionality of the operating system. When protocol algorithms are expressed in a portable programming language the protocol software may be made operating system independent. The best-known frameworks are the TCP/IP model and

1649-417: A packet-switched network, rather than this being a service of the network itself. His team was the first to tackle the highly complex problem of providing user applications with a reliable virtual circuit service while using a best-effort service , an early contribution to what will be the Transmission Control Protocol (TCP). Bob Metcalfe and others at Xerox PARC outlined the idea of Ethernet and

1746-419: A port of X to 386-compatible PCs and, by the end of the 1990s, had become the greatest source of technical innovation in X and the de facto standard of X development. Since 2004, however, the X.Org Server, a fork of XFree86, has become predominant. While it is common to associate X with Unix, X servers also exist natively within other graphical environments. VMS Software Inc.'s OpenVMS operating system includes

1843-439: A protocol may be developed into a technical standard . A programming language describes the same for computations, so there is a close analogy between protocols and programming languages: protocols are to communication what programming languages are to computations . An alternate formulation states that protocols are to communication what algorithms are to computation . Multiple protocols often describe different aspects of

1940-730: A protocol that could both run local applications and call on remote resources. In mid-1983 an initial port of W to Unix ran at one-fifth of its speed under V; in May 1984, Scheifler replaced the synchronous protocol of W with an asynchronous protocol and the display lists with immediate mode graphics to make X version 1. X became the first windowing system environment to offer true hardware independence and vendor independence. Scheifler, Gettys and Ron Newman set to work and X progressed rapidly. They released Version 6 in January 1985. DEC, then preparing to release its first Ultrix workstation, judged X

2037-554: A protocol. The data received has to be evaluated in the context of the progress of the conversation, so a protocol must include rules describing the context. These kinds of rules are said to express the syntax of the communication. Other rules determine whether the data is meaningful for the context in which the exchange takes place. These kinds of rules are said to express the semantics of the communication. Messages are sent and received on communicating systems to establish communication. Protocols should therefore specify rules governing

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2134-565: A reference model for communication standards led to the OSI model , published in 1984. For a period in the late 1980s and early 1990s, engineers, organizations and nations became polarized over the issue of which standard , the OSI model or the Internet protocol suite, would result in the best and most robust computer networks. The information exchanged between devices through a network or other media

2231-477: A remote database being the resource for a local app, the user's graphic display and input devices become resources made available by the local X server to both local and remotely hosted X client programs who need to share the user's graphics and input devices to communicate with the user. X's network protocol is based on X command primitives. This approach allows both 2D and (through extensions like GLX) 3D operations by an X client application which might be running on

2328-544: A remote-access application called Alto Terminal, that displayed overlapping windows on the Xerox Alto, and made remote hosts (typically DEC VAX systems running Unix) responsible for handling window-exposure events and refreshing window contents as necessary. X derives its name as a successor to a pre-1983 window system called W (the letter preceding X in the English alphabet ). W ran under the V operating system . W used

2425-567: A result, there is no typical X interface and several different desktop environments have become popular among users. A window manager controls the placement and appearance of application windows. This may result in desktop interfaces reminiscent of those of Microsoft Windows or of the Apple Macintosh (examples include GNOME 2, KDE Plasma, Xfce) or have radically different controls (such as a tiling window manager, like wmii or Ratpoison ). Some interfaces such as Sugar or ChromeOS eschew

2522-478: A set of cooperating processes that manipulate shared data to communicate with each other. This communication is governed by well-understood protocols, which can be embedded in the process code itself. In contrast, because there is no shared memory , communicating systems have to communicate with each other using a shared transmission medium . Transmission is not necessarily reliable, and individual systems may use different hardware or operating systems. To implement

2619-673: A single communication. A group of protocols designed to work together is known as a protocol suite; when implemented in software they are a protocol stack . Internet communication protocols are published by the Internet Engineering Task Force (IETF). The IEEE (Institute of Electrical and Electronics Engineers) handles wired and wireless networking and the International Organization for Standardization (ISO) handles other types. The ITU-T handles telecommunications protocols and formats for

2716-456: A specification for client interoperability, has a reputation for being difficult to implement correctly. Further standards efforts such as Motif and CDE did not alleviate problems. This has frustrated users and programmers. Graphics programmers now generally address consistency of application look and feel and communication by coding to a specific desktop environment or to a specific widget toolkit, which also avoids having to deal directly with

2813-456: A standardization process. Such protocols are referred to as de facto standards . De facto standards are common in emerging markets, niche markets, or markets that are monopolized (or oligopolized ). They can hold a market in a very negative grip, especially when used to scare away competition. From a historical perspective, standardization should be seen as a measure to counteract the ill-effects of de facto standards. Positive exceptions exist;

2910-430: A transfer mechanism of a protocol is comparable to a central processing unit (CPU). The framework introduces rules that allow the programmer to design cooperating protocols independently of one another. In modern protocol design, protocols are layered to form a protocol stack. Layering is a design principle that divides the protocol design task into smaller steps, each of which accomplishes a specific part, interacting with

3007-473: A version of X with Common Desktop Environment (CDE), known as DECwindows, as its standard desktop environment. Apple originally ported X to macOS in the form of X11.app, but that has been deprecated in favor of the XQuartz implementation. Third-party servers under Apple's older operating systems in the 1990s, System 7, and Mac OS 8 and 9, included Apple's MacX and White Pine Software's eXodus. Microsoft Windows

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3104-492: A window, is a client of the display server. The display server and its clients communicate with each other over an application programming interface (API) or a communications protocol , which is usually called display server protocol, the display server being the mediator between the clients and the user. It receives all the input from the kernel , that the kernel receives from all attached input devices , such as keyboard , pointing devices , or touchscreen and transmits it to

3201-487: Is a display server, but in its current implementation it relies on a second program, the compositing window manager , to do the compositing. Examples are Mutter or KWin . Notable examples of display servers implementing the X11 display server protocol are X.Org Server , XFree86 , XQuartz and Cygwin/X , while client libraries implementing the X11 display server protocol are Xlib and XCB . Display servers that implement

3298-399: Is a key component in any graphical user interface , specifically the windowing system. The server/client relationship of a standalone display server is somewhat counterintuitive in that a "server" is usually thought of as a large, remote machine, whereas a standalone "display server" is a small local system, with most clients being executed on a larger central machine. The explanation is that

3395-432: Is device-specific and usually done by the display hardware OEM. For Apple's macOS family of operating systems, Quartz Compositor fulfils the tasks of a display server and of a window manager in the windowing system. For Microsoft Windows , from Windows Vista onward, Desktop Window Manager enables the use of hardware acceleration to render the graphical user interface. It was originally created to enable portions of

3492-436: Is generally a rectangular area of the screen. From a programmer 's point of view, a windowing system implements graphical primitives. For example: rendering fonts or drawing a line on the screen. It provides an abstraction of the graphics hardware for use by higher-level elements of the graphical interface such as a window manager. A display server protocol can be network capable or even network transparent , facilitating

3589-406: Is generally not possible. However, approaches like Virtual Network Computing (VNC), NX and Xpra allow a virtual session to be reached from different X servers (in a manner similar to GNU Screen in relation to terminals), and other applications and toolkits provide related facilities. Workarounds like x11vnc ( VNC :0 viewers ), Xpra's shadow mode and NX's nxagent shadow mode also exist to make

3686-453: Is governed by rules and conventions that can be set out in communication protocol specifications. The nature of communication, the actual data exchanged and any state -dependent behaviors, is defined by these specifications. In digital computing systems, the rules can be expressed by algorithms and data structures . Protocols are to communication what algorithms or programming languages are to computations. Operating systems usually contain

3783-474: Is no accessibility standard or accessibility guidelines for X11. Within the X11 standards process there is no working group on accessibility; however, accessibility needs are being addressed by software projects to provide these features on top of X. The Orca project adds accessibility support to the X Window System, including implementing an API ( AT-SPI ). This is coupled with GNOME's ATK to allow for accessibility features to be implemented in X programs using

3880-492: Is not shipped with support for X, but many third-party implementations exist, as free and open source software such as Cygwin/X , and proprietary products such as Exceed, MKS X/Server, Reflection X, X-Win32 and Xming . There are also Java implementations of X servers. WeirdX runs on any platform supporting Swing 1.1, and will run as an applet within most browsers. The Android X Server is an open source Java implementation that runs on Android devices. When an operating system with

3977-449: Is referred to as communicating sequential processes (CSP). Concurrency can also be modeled using finite state machines , such as Mealy and Moore machines . Mealy and Moore machines are in use as design tools in digital electronics systems encountered in the form of hardware used in telecommunication or electronic devices in general. The literature presents numerous analogies between computer communication and programming. In analogy,

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4074-478: Is rendered to a "surface"; "surfaces" are produced by applications and placed into a queue that is managed by SurfaceFlinger. Yet another Android-specific solution is "Gralloc". Gralloc handles device memory i.e. it does allocation, arbitration, it handles synchronization via Android/Linux fence file descriptors. Gralloc competes with other solutions like e.g. Mesa's Generic Buffer Management (GBM) or Nvidia's EGLStreams. The Gralloc hardware abstraction layer (HAL)

4171-419: Is that they are not capable of any input or output other than the keyboard, mouse, and display. All relevant data is assumed to exist solely on the remote server, and the X terminal user has no methods available to save or load data from a local peripheral device. Dedicated (hardware) X terminals have fallen out of use; a PC or modern thin client with an X server typically provides the same functionality at

4268-414: Is the canonical implementation of X. Owing to liberal licensing, a number of variations, both free and open source and proprietary, have appeared. Commercial Unix vendors have tended to take the reference implementation and adapt it for their hardware, usually customizing it and adding proprietary extensions. Until 2004, XFree86 provided the most common X variant on free Unix-like systems. XFree86 started as

4365-408: Is the synchronization of software for receiving and transmitting messages of communication in proper sequencing. Concurrent programming has traditionally been a topic in operating systems theory texts. Formal verification seems indispensable because concurrent programs are notorious for the hidden and sophisticated bugs they contain. A mathematical approach to the study of concurrency and communication

4462-462: Is used to allocate the buffers that underlie "surfaces". For compositing in Android, Surfaces are sent to SurfaceFlinger, which uses OpenGL ES to do the compositing. Hardware Composer HAL (HWC) was introduced in Android 3.0 and has evolved steadily over the years. Its primary purpose is to determine the most efficient way to composite buffers with the available hardware. As a HAL, its implementation

4559-684: The MIT License and similar permissive licenses. X is an architecture-independent system for remote graphical user interfaces and input device capabilities. Each person using a networked terminal has the ability to interact with the display with any type of user input device. In its standard distribution it is a complete, albeit simple, display and interface solution which delivers a standard toolkit and protocol stack for building graphical user interfaces on most Unix-like operating systems and OpenVMS , and has been ported to many other contemporary general purpose operating systems . X provides

4656-776: The National Physical Laboratory in the United Kingdom, it was written by Roger Scantlebury and Keith Bartlett for the NPL network . On the ARPANET , the starting point for host-to-host communication in 1969 was the 1822 protocol , written by Bob Kahn , which defined the transmission of messages to an IMP. The Network Control Program (NCP) for the ARPANET, developed by Steve Crocker and other graduate students including Jon Postel and Vint Cerf ,

4753-423: The OSI model . At the time the Internet was developed, abstraction layering had proven to be a successful design approach for both compiler and operating system design and, given the similarities between programming languages and communication protocols, the originally monolithic networking programs were decomposed into cooperating protocols. This gave rise to the concept of layered protocols which nowadays forms

4850-638: The PARC Universal Packet (PUP) for internetworking. Research in the early 1970s by Bob Kahn and Vint Cerf led to the formulation of the Transmission Control Program (TCP). Its RFC   675 specification was written by Cerf with Yogen Dalal and Carl Sunshine in December 1974, still a monolithic design at this time. The International Network Working Group agreed on a connectionless datagram standard which

4947-583: The classic Mac OS (version 9 and earlier), and Palm OS , contain a windowing system which is integrated with the OS. Communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any variation of a physical quantity . The protocol defines the rules, syntax , semantics , and synchronization of communication and possible error recovery methods . Protocols may be implemented by hardware , software , or

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5044-547: The finger protocol . Text-based protocols are typically optimized for human parsing and interpretation and are therefore suitable whenever human inspection of protocol contents is required, such as during debugging and during early protocol development design phases. A binary protocol utilizes all values of a byte , as opposed to a text-based protocol which only uses values corresponding to human-readable characters in ASCII encoding. Binary protocols are intended to be read by

5141-590: The public switched telephone network (PSTN). As the PSTN and Internet converge , the standards are also being driven towards convergence. The first use of the term protocol in a modern data-commutation context occurs in April 1967 in a memorandum entitled A Protocol for Use in the NPL Data Communications Network. Under the direction of Donald Davies , who pioneered packet switching at

5238-573: The Argus system. Project Athena (a joint project between DEC , MIT and IBM to provide easy access to computing resources for all students) needed a platform-independent graphics system to link together its heterogeneous multiple-vendor systems; the window system then under development in Carnegie Mellon University 's Andrew Project did not make licenses available, and no alternatives existed. The project solved this by creating

5335-547: The GNOME/GTK APIs. KDE provides a different set of accessibility software, including a text-to-speech converter and a screen magnifier. The other major desktops (LXDE, Xfce and Enlightenment) attempt to be compatible with ATK. An X client cannot generally be detached from one server and reattached to another unless its code specifically provides for it ( Emacs is one of the few common programs with this ability). As such, moving an entire session from one X server to another

5432-702: The ICCCM. X also lacks native support for user-defined stored procedures on the X server, in the manner of NeWS  – there is no Turing-complete scripting facility. Various desktop environments may thus offer their own (usually mutually incompatible) facilities. Systems built upon X may have accessibility issues that make utilization of a computer difficult for disabled users, including right click , double click , middle click , mouse-over , and focus stealing . Some X11 clients deal with accessibility issues better than others, so persons with accessibility problems are not locked out of using X11. However, there

5529-405: The Internet by tunneling the connection over an encrypted network session. An X client itself may emulate an X server by providing display services to other clients. This is known as "X nesting". Open-source clients such as Xnest and Xephyr support such X nesting. To run an X client application on a remote machine, the user may do the following: The remote X client application will then make

5626-560: The Wayland display server protocol are called Wayland compositors . Like any display server, a Wayland compositor is responsible for handling input and output for its clients and, in contrast to X11, the compositing as well. Examples are Weston , Mutter , KWin or Enlightenment . Wayland compositors communicate with Wayland clients over the Wayland display server protocol . This protocol defines that clients can directly write data into

5723-492: The X server is usually running on the computer in front of a human user, while the X client applications run anywhere on the network and communicate with the user's computer to request the rendering of graphics content and receive events from input devices including keyboards and mice. The fact that the term "server" is applied to the software in front of the user is often surprising to users accustomed to their programs being clients to services on remote computers. Here, rather than

5820-546: The X11 protocol. It was developed by Canonical and was intended to be the display server of choice for Ubuntu . As of 2017, it has been replaced with the Wayland display server for desktop editions of Ubuntu. There are implementations of the Mir display server, the libmir-server and the libmir-client libraries available under the GPLv3 . Google developed a display server called SurfaceFlinger for Android : Everything in Android

5917-456: The approval or support of a standards organization , which initiates the standardization process. The members of the standards organization agree to adhere to the work result on a voluntary basis. Often the members are in control of large market shares relevant to the protocol and in many cases, standards are enforced by law or the government because they are thought to serve an important public interest, so getting approval can be very important for

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6014-453: The bandwidth of a 100 Mbit/s network for a single client. In contrast, modern versions of X generally have extensions such as Mesa allowing local display of a local program's graphics to be optimized to bypass the network model and directly control the video card, for use of full-screen video, rendered 3D applications, and other such applications. X's design requires the clients and server to operate separately, and device independence and

6111-494: The basic framework , or primitives, for building such GUI environments: drawing and moving windows on the display and interacting with a mouse, keyboard or touchscreen. X does not mandate the user interface ; individual client programs handle this. Programs may use X's graphical abilities with no user interface. As such, the visual styling of X-based environments varies greatly; different programs may present radically different interfaces. Unlike most earlier display protocols, X

6208-448: The basis of protocol design. Systems typically do not use a single protocol to handle a transmission. Instead they use a set of cooperating protocols, sometimes called a protocol suite . Some of the best-known protocol suites are TCP/IP , IPX/SPX , X.25 , AX.25 and AppleTalk . The protocols can be arranged based on functionality in groups, for instance, there is a group of transport protocols . The functionalities are mapped onto

6305-472: The compositing and on Linux it passes the data to one of three kernel components – DRM , gem or KMS driver . The component writes the data into the framebuffer and content of the framebuffer is transmitted to the connected screen and displayed. X relies on GLX . One of the implementations of display server concept is X Window System , in particular its actually used version – X.Org Server and Xlib and XCB client libraries. The X.Org Server

6402-442: The content being carried: text-based and binary. A text-based protocol or plain text protocol represents its content in human-readable format , often in plain text encoded in a machine-readable encoding such as ASCII or UTF-8 , or in structured text-based formats such as Intel hex format , XML or JSON . The immediate human readability stands in contrast to native binary protocols which have inherent benefits for use in

6499-461: The correct client. The display server is also responsible for the output of the clients to the computer monitor . The output of sound is usually not managed by the display server, but the sound volume is usually handled through GUI applets and it is the display server who decides which applications are on top. A windowing system enables the computer user to work with several programs at the same time. Each program presents its GUI in its own window, which

6596-426: The current X-server screen available. This ability allows the user interface (mouse, keyboard, monitor) of a running application to be switched from one location to another without stopping and restarting the application. Network traffic between an X server and remote X clients is not encrypted by default. An attacker with a packet sniffer can intercept it, making it possible to view anything displayed to or sent from

6693-454: The desktop metaphor altogether, simplifying their interfaces for specialized applications. Window managers range in sophistication and complexity from the bare-bones ( e.g. , twm, the basic window manager supplied with X, or evilwm, an extremely light window manager) to the more comprehensive desktop environments such as Enlightenment and even to application-specific window managers for vertical markets such as point-of-sale. Many users use X with

6790-673: The field of computer networking, it has been historically criticized by many researchers as abstracting the protocol stack in this way may cause a higher layer to duplicate the functionality of a lower layer, a prime example being error recovery on both a per-link basis and an end-to-end basis. Commonly recurring problems in the design and implementation of communication protocols can be addressed by software design patterns . Popular formal methods of describing communication syntax are Abstract Syntax Notation One (an ISO standard) and augmented Backus–Naur form (an IETF standard). Finite-state machine models are used to formally describe

6887-466: The framebuffer using the EGL rendering API . The display server still gets to decide which window is on top and thus visible to the user and also still is responsible for passing data regarding to input devices from evdev to its clients. Wayland is used to a certain degree in some Linux desktop distributions, such as Fedora . It is also well suited for mobile computing and has been adopted, for example, by

6984-426: The horizontal message flows (and protocols) are between systems. The message flows are governed by rules, and data formats specified by protocols. The blue lines mark the boundaries of the (horizontal) protocol layers. The software supporting protocols has a layered organization and its relationship with protocol layering is shown in figure 5. To send a message on system A, the top-layer software module interacts with

7081-432: The implementation of thin clients . A display server or window server is a program whose primary task is to coordinate the input and output of its clients to and from the rest of the operating system, the hardware, and each other. The display server communicates with its clients over the display server protocol, a communications protocol , which can be network-transparent or simply network-capable. The display server

7178-643: The internet as a reference model for general communication with much stricter rules of protocol interaction and rigorous layering. Typically, application software is built upon a robust data transport layer. Underlying this transport layer is a datagram delivery and routing mechanism that is typically connectionless in the Internet. Packet relaying across networks happens over another layer that involves only network link technologies, which are often specific to certain physical layer technologies, such as Ethernet . Layering provides opportunities to exchange technologies when needed, for example, protocols are often stacked in

7275-476: The layers make up a layering scheme or model. Computations deal with algorithms and data; Communication involves protocols and messages; So the analog of a data flow diagram is some kind of message flow diagram. To visualize protocol layering and protocol suites, a diagram of the message flows in and between two systems, A and B, is shown in figure 3. The systems, A and B, both make use of the same protocol suite. The vertical flows (and protocols) are in-system and

7372-427: The layers, each layer solving a distinct class of problems relating to, for instance: application-, transport-, internet- and network interface-functions. To transmit a message, a protocol has to be selected from each layer. The selection of the next protocol is accomplished by extending the message with a protocol selector for each layer. There are two types of communication protocols, based on their representation of

7469-402: The module directly below it and hands over the message to be encapsulated. The lower module fills in the header data in accordance with the protocol it implements and interacts with the bottom module which sends the message over the communications channel to the bottom module of system B. On the receiving system B the reverse happens, so ultimately the message gets delivered in its original form to

7566-442: The new "Windows Aero" user experience, which allowed for effects such as transparency, 3D window switching and more. It is also included with Windows Server 2008, but requires the "Desktop Experience" feature and compatible graphics drivers to be installed. From Windows 8 onwards DWM can't be disabled and is software rendered if no suitable graphics card is installed. Some systems such as Microsoft Windows ( XP , 9x and earlier),

7663-419: The only windowing system likely to become available in time. DEC engineers ported X6 to DEC's QVSS display on MicroVAX . Windowing system Each currently running application is assigned a usually resizable and usually rectangular surface of the display to present its GUI to the user; these windows may overlap each other, as opposed to a tiling interface where they are not allowed to overlap. Usually

7760-470: The other parts of the protocol only in a small number of well-defined ways. Layering allows the parts of a protocol to be designed and tested without a combinatorial explosion of cases, keeping each design relatively simple. The communication protocols in use on the Internet are designed to function in diverse and complex settings. Internet protocols are designed for simplicity and modularity and fit into

7857-457: The possible interactions of the protocol. and communicating finite-state machines For communication to occur, protocols have to be selected. The rules can be expressed by algorithms and data structures. Hardware and operating system independence is enhanced by expressing the algorithms in a portable programming language. Source independence of the specification provides wider interoperability. Protocol standards are commonly created by obtaining

7954-401: The protocol, creating incompatible versions on their networks. In some cases, this was deliberately done to discourage users from using equipment from other manufacturers. There are more than 50 variants of the original bi-sync protocol. One can assume, that a standard would have prevented at least some of this from happening. In some cases, protocols gain market dominance without going through

8051-539: The protocol. The need for protocol standards can be shown by looking at what happened to the Binary Synchronous Communications (BSC) protocol invented by IBM . BSC is an early link-level protocol used to connect two separate nodes. It was originally not intended to be used in a multinode network, but doing so revealed several deficiencies of the protocol. In the absence of standardization, manufacturers and organizations felt free to enhance

8148-749: The same host. Additionally shared memory (via the MIT-SHM extension) can be employed for faster client–server communication. However, the programmer must still explicitly activate and use the shared memory extension. It is also necessary to provide fallback paths in order to stay compatible with older implementations, and in order to communicate with non-local X servers. Some people have attempted writing alternatives to and replacements for X. Historical alternatives include Sun 's NeWS and NeXT 's Display PostScript , both PostScript -based systems supporting user-definable display-side procedures, which X lacked. Current alternatives include: Additional ways to achieve

8245-517: The same large computer server to execute application programs as clients of each user's X terminal. This use is very much aligned with the original intention of the MIT project. X terminals explore the network (the local broadcast domain ) using the X Display Manager Control Protocol to generate a list of available hosts that are allowed as clients. One of the client hosts should run an X display manager . A limitation of X terminals and most thin clients

8342-475: The same, or lower, cost. The Unix-Haters Handbook (1994) devoted a full chapter to the problems of X. Why X Is Not Our Ideal Window System (1990) by Gajewska, Manasse and McCormack detailed problems in the protocol with recommendations for improvement. The lack of design guidelines in X has resulted in several vastly different interfaces, and in applications that have not always worked well together. The Inter-Client Communication Conventions Manual (ICCCM),

8439-486: The separation of client and server incur overhead. Most of the overhead comes from network round-trip delay time between client and server ( latency ) rather than from the protocol itself: the best solutions to performance issues depend on efficient application design. A common criticism of X is that its network features result in excessive complexity and decreased performance if only used locally. Modern X implementations use Unix domain sockets for efficient connections on

8536-433: The smartphone- and tablet-focused projects Tizen , Sailfish OS and AsteroidOS . An implementation of Wayland is available under the MIT License , the libwayland-client and libwayland-server libraries. There is an ongoing effort to add Wayland support to ChromeOS . The Mir display server comes with its own Mir display server protocol which is different from those used by X11 and Wayland. Mir additionally supports

8633-513: The terms appear reversed. But X takes the perspective of the application, rather than that of the end-user: X provides display and I/O services to applications, so it is a server; applications use these services, thus they are clients. The communication protocol between server and client operates network-transparently: the client and server may run on the same machine or on different ones, possibly with different architectures and operating systems. A client and server can even communicate securely over

8730-514: The top module of system B. Program translation is divided into subproblems. As a result, the translation software is layered as well, allowing the software layers to be designed independently. The same approach can be seen in the TCP/IP layering. The modules below the application layer are generally considered part of the operating system. Passing data between these modules is much less expensive than passing data between an application program and

8827-506: The transmission. In general, much of the following should be addressed: Systems engineering principles have been applied to create a set of common network protocol design principles. The design of complex protocols often involves decomposition into simpler, cooperating protocols. Such a set of cooperating protocols is sometimes called a protocol family or a protocol suite, within a conceptual framework. Communicating systems operate concurrently. An important aspect of concurrent programming

8924-406: The transport layer. The boundary between the application layer and the transport layer is called the operating system boundary. Strictly adhering to a layered model, a practice known as strict layering, is not always the best approach to networking. Strict layering can have a negative impact on the performance of an implementation. Although the use of protocol layering is today ubiquitous across

9021-499: The user's screen. The most common way to encrypt X traffic is to establish a Secure Shell (SSH) tunnel for communication. Like all thin clients , when using X across a network, bandwidth limitations can impede the use of bitmap -intensive applications that require rapidly updating large portions of the screen with low latency, such as 3D animation or photo editing. Even a relatively small uncompressed 640×480×24 bit 30 fps video stream (~211 Mbit/s) can easily outstrip

9118-415: Was first implemented in 1970. The NCP interface allowed application software to connect across the ARPANET by implementing higher-level communication protocols, an early example of the protocol layering concept. The CYCLADES network, designed by Louis Pouzin in the early 1970s was the first to implement the end-to-end principle , and make the hosts responsible for the reliable delivery of data on

9215-588: Was presented to the CCITT in 1975 but was not adopted by the CCITT nor by the ARPANET. Separate international research, particularly the work of Rémi Després , contributed to the development of the X.25 standard, based on virtual circuits , which was adopted by the CCITT in 1976. Computer manufacturers developed proprietary protocols such as IBM's Systems Network Architecture (SNA), Digital Equipment Corporation's DECnet and Xerox Network Systems . TCP software

9312-469: Was redesigned as a modular protocol stack, referred to as TCP/IP. This was installed on SATNET in 1982 and on the ARPANET in January 1983. The development of a complete Internet protocol suite by 1989, as outlined in RFC   1122 and RFC   1123 , laid the foundation for the growth of TCP/IP as a comprehensive protocol suite as the core component of the emerging Internet . International work on

9409-509: Was specifically designed to be used over network connections rather than on an integral or attached display device. X features network transparency , which means an X program running on a computer somewhere on a network (such as the Internet) can display its user interface on an X server running on some other computer on the network. The X server is typically the provider of graphics resources and keyboard/mouse events to X clients , meaning that

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