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Internet Protocol Suite

Internet protocol suite

The Internet protocol suite is the set of communications protocols that implement the protocol stack on which the Internet runs. It is sometimes called the TCP/IP protocol suite, after the two most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were also the first two defined. The Internet protocol suite can be described by analogy with the OSI model, which describes the layers of a protocol stack, not all of which correspond well with internet practice. In a protocol stack, each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the higher layers. Higher layers are logically closer to the user and deal with more abstract data, relying on lower layers to translate data into forms that can eventually be physically manipulated. The Internet model was produced as the solution to a practical engineering problem. The OSI model, on the other hand, was a more theoretical approach. Therefore, some consider the OSI model as easier to understand, and the TCP/IP model as the one that fits with actual use. Some consider it helpful to have an understanding of the OSI model before learning TCP/IP, as the same principles apply, but are easier to understand in the OSI model.

Layers in the TCP/IP stack

There is some discussion about how to map the TCP/IP model onto the OSI model. Since the TCP/IP and OSI protocol suites do not match precisely, there is no one correct answer. In addition, the OSI model is not really rich enough at the lower layers to capture the true layering; there needs to be an extra layer (the Internetworking layer) between the Transport and Network layers. Protocols specific to a particular network type, but which are run on top of the basic hardware framing, ought to be at the Network layer. Examples of such protocols are ARP and the Spanning Tree Protocol (used to keep redundant bridges idle until they are needed). However, they are local protocols and operate beneath the internetwork functionality. Admittedly, placing both groups (not to mention protocols which are logically part of the internetwork layer, but run on top of the internetwork protocol, such as ICMP) all at the same layer can be confusing, but the OSI model is not complex enough to do a better job. The following diagram attempts to show where various TCP/IP and other protocols would reside in the original OSI model: Commonly, the top three layers of the OSI model (Application, Presentation and Session) are considered as a single Application Layer in the TCP/IP suite. Because the TCP/IP suite has a comparatively lightweight session layer, consisting of opening and closing connections under TCP and RTP and providing different port numbers for different applications under TCP and UDP, these functions may be augmented by individual applications. Similarly, IP is designed around the idea of treating the network below it as a black box so it can be considered as a single layer for the purposes of discussing TCP/IP.

The link layer

The Link layer is not really part of the Internet protocol suite, but is the method used to pass packets from the Internet layer of one device to the Internet layer of another. This process can be controlled both in the software device driver for the network card, as well as on firmware or specialist chipsets. These will perform data link functions such as adding a packet header to prepare it for transmission, then actually transmit the frame over a physical medium. On the other end, the link layer will receive data frames, strip off the packet headers, and hand the received packets to the Internet layer. However, the link layer is not always so simple. It may also be a Virtual private network (VPN) or tunnel, where packets from the Internet layer, instead of being sent over a physical interface, are sent using a tunneling protocol and another (or the same) protocol suite. The VPN or tunnel is usually established ahead of time, and has special characteristics that direct transmission out a physical interface does not (for example, it may encrypt the data going over it). This recursive use of the protocol suite can be confusing since the link "layer" is now an entire network. But it is an elegant method for implementing often complex functions. (though care is needed to prevent a packet that is wrapped and sent through a tunnel being repeatedly re-wrapped and sent down the tunnel again).

The Internetwork layer

As originally defined, the Network layer solves the problem of getting packets across a single network. Examples of such protocols are X.25, and the ARPANET's Host/IMP Protocol. With the advent of the concept of internetworking, additional functionality was added to this layer, namely getting data from the source network to the destination network. This generally involves routing the packet across a network of networks, known as an internet. In the internet protocol suite, IP performs the basic task of getting packets of data from source to destination. IP can carry data for a number of different higher level protocols; these protocols are each identified by a unique IP Protocol Number. ICMP and IGMP are protocols 1 and 2, respectively. Some of the protocols carried by IP, such as ICMP (used to transmit diagnostic information about IP transmission) and IGMP (used to manage multicast data) are layered on top of IP but perform internetwork layer functions, illustrating an incompatibility between the internet and OSI models. All routing protocols, such as BGP, OSPF, and RIP are also really part of the internetwork layer, although they might seem to belong higher in the stack.

The transport layer

The protocols at the Transport layer can solve problems like reliability ("did the data reach the destination?") and ensure that data arrives in the correct order. In the TCP/IP protocol suite, transport protocols also determine which application any given data is intended for. The dynamic routing protocols which technically fit at this layer in the TCP/IP Protocol Suite (since they run over IP) are generally considered to be part of the Network layer; an example is OSPF (IP protocol number 89). TCP (IP protocol number 6) is a "reliable", connection-oriented, transport mechanism providing a reliable byte stream, which makes sure data arrives complete, undamaged, and in order. TCP tries to continuously measure how loaded the network is and throttles its sending rate in order to avoid overloading the network. Furthermore, TCP will attempt to deliver all data correctly in the specified sequence. These are its main differences from UDP, and can become disadvantageous in real-time streaming or routing applications with high internetwork layer loss rates. The newer SCTP is also a "reliable", connection-oriented, transport mechanism. It is record rather than byte oriented, and provides multiple sub-streams multiplexed over a single connection. It also provides multi-homing support, in which a connection end can be represented by multiple IP addresses (representing multiple physical interfaces), such that if one fails the connection is not interrupted. It was developed initially for telephony applications (to transport SS7 over IP), but can also be used for other applications. UDP (IP protocol number 17) is a connectionless datagram protocol. It is a "best effort" or "unreliable" protocol - not because it is particularly unreliable, but because it does not verify that packets have reached their destination, and gives no guarantee that they will arrive in order. If an Application requires these characteristics, it must provide them itself, or use TCP. UDP is typically used for applications such as streaming media (audio and video, etc) where on-time arrival is more important than reliability, or for simple query/response applications like DNS lookups, where the overhead of setting up a reliable connection is disproportionately large. DCCP is currently under development by IETF. It provides TCP's flow control semantics, while keeping UDP's datagram service model visible to the user. Both TCP and UDP are used to carry a number of higher-level applications. The applications at any given network address are distinguished by their TCP or UDP port number. By convention certain well known ports are associated with specific applications. RTP is a datagram protocol that is designed for real-time data such as streaming audio and video. RTP is a session layer that uses the UDP packet format as a basis yet is said to sit within the transport layer of the Internet protocol stack.

The application layer

The Application layer is the layer that most common network-aware programs use in order to communicate across a network with other programs. Processes that occur in this layer are application specific; data is passed from the network-aware program, in the format used internally by this application, and is encoded into a standard protocol. Some specific programs are considered to run in this layer. They provide services that directly support user applications. These programs and their corresponding protocols include HTTP (The World Wide Web), FTP (File transport), SMTP (Email), SSH (Secure remote login), DNS (Name <-> IP Address lookups) and many others. Once the data from an application has been encoded into a standard application layer protocol it will be passed down to the next layer of the IP stack. At the Transport Layer, applications will most commonly make use of TCP or UDP, and server applications are often associated with a well-known port number. Ports for server applications are officially allocated by the Internet Assigned Numbers Authority (IANA) but developers of new protocols today often choose the port numbers themselves. As it is rare to have more than a few server applications on the same system, problems with port conflicts are rare. Application software also generally allows users to specify arbitrary port numbers as runtime parameters. Client applications connecting out generally use a random port number assigned by the operating system. Applications that listen on a port and then send that port to another copy of the application via a server to set up a peer-peer link (e.g. dcc file transfers on IRC). may also use a random port but the applications usually allow specification of a specific port range to allow the ports to be mapped inwards through a router that implements network address translation.

Development

The Internet protocol suite came from work done by DARPA in the early 1970s. After doing the pioneering ARPANET, DARPA started work on a number of other data transmission technologies, including packet radio, and satellite links. Wanting to be able to communicate across them, Robert E. Kahn of DARPA recruited Vint Cerf of Stanford University to work with him on the problems of connecting multiple networks, using different access protocols. By the summer of 1973, they had soon worked out a fundamental reformulation, where the differences between network protocols were hidden by using a common internetwork protocol, and instead of the network being responsible for reliability, as in the ARPANET, the hosts became responsible. (Cerf credits Hubert Zimmerman and Louis Pouzin (designer of the CYCLADES network) with important influences on this design.) With the role of the network reduced to the bare minimum, it became possible to join almost any networks together, no matter what their characteristics were, thereby solving Kahn's initial problem. (One popular saying has it that TCP/IP, the eventual product of Cerf and Kahn's work, will run over "two tin cans and a string".) A computer called a gateway (later changed to router to avoid confusion with other types of gateway) is provided with an interface to each network, and forwards packets back and forth between them. The idea was worked out in more detailed form by Cerf's networking research group at Stanford in the 1973–74 period. (The early networking work at Xerox PARC, which produced the PARC Universal Packet protocol suite, much of which was contemporaneous, was also a significant technical influence; people moved between the two.) DARPA agreed to fund development of prototype software, and after several years of work, the first somewhat crude demonstration of what had by then become TCP/IP occurred in July 1977. On 9 November 2005 Kahn and Cerf were presented with the Presidential Medal of Freedom for their contribution to American culture. [http://news.bbc.co.uk/1/hi/technology/4415326.stm]

Implementation

- KA9Q PPJ

External links

- [http://www.showip.org/ Show your IP address]
- [http://www.itprc.com/tcpipfaq/ TCP/IP FAQ]
- [http://www.columbia.edu/~rh120/other/tcpdigest_paper.txt A Study of the ARPANET TCP/IP Digest]
- [http://www.eventhelix.com/RealtimeMantra/Networking/ TCP/IP Sequence Diagrams]
- [http://cng.ateneo.edu/cng/wyu/classes/cs197/ Ateneo Network Research Group] TCP/IP research at the Ateneo de Manila University Internet protocol suite ja:TCP/IP

Communications protocol

Hello! This article concerns communication between pairs of electronic devices. For the specific topic of computing protocols, see Protocol (computing). For protocols on two-way voice communications, see voice procedure. For the original and other meanings of the word protocol, see the disambiguation page for protocol. In the field of telecommunications, a communications protocol is the set of standard rules for data representation, signalling, authentication, and error detection required to send information over a communications channel. An example of a simple communications protocol adapted to voice communication is the case of a radio dispatcher talking to mobile stations. The communication protocols for digital computer network communication have many features intended to ensure reliable interchange of data over an imperfect communication channel.

Network protocol design principles

Systems engineering principles have been applied to create a set of common network protocol design principles. These principles include effectiveness, reliability, and resiliency.

Effectiveness

For a protocol to be effective it needs to be specified in such a way, that engineers, designers, and in some cases software developers can implement and/or use it. In human-machine systems, its design needs to facilitate routine usage by humans. Protocol layering accomplishes these objectives by dividing the protocol design into a number of smaller parts, each of which performs closely related sub-tasks, and interacts with other layers of the protocol only in a small number of well-defined ways. Protocol layering allows the parts of a protocol to be designed and tested without a combinatorial explosion of cases, keeping each design relatively simple. The implementation of a sub-task on one layer can make assumptions about the behavior and services offered by the layers beneath it. Thus, layering enables a "mix-and-match" of protocols that permit familiar protocols to be adapted to unusual circumstances. In the realm of computing, an email protocol like Simple Mail Transfer Protocol (SMTP) can be adapted to send messages to a system that follows SMTP.It can be an Aircraft having a SMTP Server to receive SMTP protocol based messages. An SMTP Client can build SMTP messages. and an SMTP Server handles the connection and interacts with client as defined through the SMTP protocol specification. RFC2821 specifies the protocol details. This paragraph informally provides some examples of layers, some required functionalities, and some protocols that implement them, all from the realm of computing protocols. A layer in charge of presentation might describe how to encode text (ie: ASCII or Unicode). A protocol like SMTP, may (among other things) describe how to inquire about electronic mail messages. A sub-task of error detection and either message correction or retransmission may be performed by the Transmission control protocol (TCP), while the related sub-task of addressing implemented by the Internet Protocol(IP) will pair up these two protocols giving rise to the familiar acronym TCP/IP. These examples may assume some point-to-point connectivity offered by point-to-point protocol (PPP) implemented in the lower-level Data link layer. At the lowest level is the sub-task involving the electrical encoding/decoding of bits into/from voltages performed in the Physical layer. These examples motivate the need to specify some software architecture or reference model that systematically places each subtask into its proper context. The reference model usually used for protocol layering is the OSI seven layer model, which can be applied to any protocol, not just the OSI protocols initially sanctioned by the International Organization for Standardization (ISO). In particular, the Internet Protocol can be analysed using the OSI model.

Reliability

Assuring reliability of data transmission involves error detection and correction, or some means of requesting retransmission. It is a truism that communication media are always faulty. The conventional measure of quality is the number of failed bits per bits transmitted. This has the wonderful feature of being a dimensionless figure of merit that can be compared across any speed or type of communication media. In telephony, links with bit error rates of 10^-4 or more are regarded as faulty (they interfere with telephone conversations), while links with a BER of 10^-5 or more should be dealt with by routine maintenance (they can be heard). Communication systems correct errors by selectively resending bad parts of a message. For example, in TCP (the internet's Transmission Control Protocol), messages are divided into packets, each of which has a checksum. When a checksum is bad, the packet is discarded. When a packet is lost, the receiver acknowledges all of the packets up to, but not including the failed packet. Eventually, the sender sees that too much time has elapsed without an acknowledgement, so it resends all of the packets that have not been acknowledged. At the same time, the sender backs off its rate of sending, in case the packet loss was caused by saturation of the path between sender and receiver. (Note: this is an over-simplification: see TCP and congestion collapse for more detail) In general, the performance of TCP is severely degraded in conditions of high packet loss (more than 0.1%), due to the need to resend packets repeatedly. For this reason, TCP/IP connections are typically either run on highly reliable fiber networks, or over a lower-level protocol with added error-detection and correction features (such as modem links with ARQ). These connections typically have uncorrected bit error rates of 10^-9 to 10^-12, ensuring high TCP/IP performance.

Resiliency

Resiliency addresses a form of network failure known as topological failure in which a communications link is cut, or degrades below usable quality. Most modern communication protocols periodically send messages to test a link. In phones, a framing bit is sent every 24 bits on T1 lines. In phone systems, when "sync is lost", fail-safe mechanisms reroute the signals around the failing equipment. In packet switched networks, the equivalent functions are performed using router update messages to detect loss of connectivity.

Standards organisations

Most recent protocols are assigned by the IETF for Internet communications, and the IEEE, or the ISO organizations for other types. The ITU-T handles telecommunications protocols and formats for the public switched telephone network (PSTN). The ITU-R handles protocols and formats for radio communications. As the PSTN. radio systems, and Internet converge, the different sets of standards are also being driven towards technological convergence.

Protocol families

A number of major protocol stacks or families exist, including the following: Proprietary standards:
- AppleTalk
- DECnet
- IPX/SPX
- SMB
- Systems Network Architecture (SNA)
- Distributed Systems Architecture (DSA) Open standards:
- Open Systems Interconnect (OSI)
- Internet protocol suite

References

- Radia Perlman: Interconnections: Bridges, Routers, Switches, and Internetworking Protocols. 2nd Edition. Addison-Wesley 1999, ISBN 0201634481. In particular Ch. 18 on "network design folklore".
- Gerard J. Holzmann: Design and Validation of Computer Protocols. Prentice Hall, 1991, ISBN 0-13-539925-4. Also available online at http://spinroot.com/spin/Doc/Book91.html

External links

- [http://www.javvin.com/protocolsuite.html Javvin's Protocol Dictionary]
- ja:通信プロトコル th:เกณฑ์วิธีการสื่อสาร

Internet

:For the more general networking concept, see internetworking. The Internet, or simply the Net, is the worldwide system of interconnected computer networks which makes information stored on it accessible. This information is transmitted by packet switching using a standardized Internet Protocol (IP) and many other protocols. It is made up of thousands of smaller commercial, academic, domestic and government networks. It carries various information and services, such as electronic mail, online chat, and the interlinked web pages and other documents of the World Wide Web.

Creation of the Internet

During the 1950s, several communications researchers realized that there was a need to allow general communication between users of various computers and communications networks. This led to research into decentralized networks, queuing theory, and packet switching. The subsequent creation of ARPANET in the United States in turn catalyzed a wave of technical developments that made it the basis for the development of the Internet. Contrary to popular myth, the DoD did not create the ARPANET so that they could communicate to the US Government after a nuclear war. The first TCP/IP wide area network was operational in 1984 when the United States' National Science Foundation (NSF) constructed a university network backbone that would later become the NSFNet. It was then followed by the opening of the network to commercial interests in 1995. Important separate networks that offered gateways into, then later merged into the Internet include Usenet, Bitnet and the various commercial and educational X.25 networks such as Compuserve and JANET. The ability of TCP/IP to work over these pre-existing communication networks allowed for a great ease of growth. Use of Internet as a phrase to describe a single global TCP/IP network originated around this time. The collective network gained a public face in the 1990s. In August 1991 CERN in Switzerland publicized the new World Wide Web project, two years after Tim Berners-Lee had begun creating HTML, HTTP and the first few web pages at CERN in Switzerland. In 1993 the Mosaic web browser version 1.0 was released, and by late 1994 there was growing public interest in the previously academic/technical Internet. By 1996 the word "Internet" was common public currency, but it referred almost entirely to the World Wide Web. Meanwhile, over the course of the decade, the Internet successfully accommodated the majority of previously existing public computer networks (although some networks such as FidoNet have remained separate). This growth is often attributed to the lack of central administration, which allows organic growth of the network, as well as the non-proprietary open nature of the Internet protocols, which encourages vendor interoperability and prevents any one company from exerting too much control over the network.

Today's Internet

FidoNets, FTP client, and Telnet client]] Apart from the complex physical connections that make up its infrastructure, the Internet is held together by bi- or multi-lateral commercial contracts (for example peering agreements) and by technical specifications or protocols that describe how to exchange data over the network. Indeed, the Internet is essentially defined by its interconnections and routing policies. In an often-cited, if perhaps gratuitously mathematical definition, Seth Breidbart once described the Internet as "the largest equivalence class in the reflexive, transitive, symmetric closure of the relationship 'can be reached by an IP packet from'". Unlike older communications systems, the Internet protocol suite was deliberately designed to be independent of the underlying physical medium. Any communications network, wired or wireless, that can carry two-way digital data can carry Internet traffic. Thus, Internet packets flow through wired networks like copper wire, coaxial cable, and fiber optic; and through wireless networks like Wi-Fi. Together, all these networks, sharing the same high-level protocols, form the Internet. The Internet protocols originate from discussions within the Internet Engineering Task Force (IETF) and its working groups, which are open to public participation and review. These committees produce documents that are known as Request for Comments documents (RFCs). Some RFCs are raised to the status of Internet Standard by the Internet Architecture Board (IAB). Some of the most used protocols in the Internet protocol suite are IP, TCP, UDP, DNS, PPP, SLIP, ICMP, POP3, IMAP, SMTP, HTTP, HTTPS, SSH, Telnet, FTP, LDAP, SSL, and TLS. Some of the popular services on the Internet that make use of these protocols are e-mail, Usenet newsgroups, file sharing, Instant Messenger, the World Wide Web, Gopher, session access, WAIS, finger, IRC, MUDs, and MUSHs. Of these, e-mail and the World Wide Web are clearly the most used, and many other services are built upon them, such as mailing lists and blogs. The Internet makes it possible to provide real-time services such as Internet radio and webcasts that can be accessed from anywhere in the world. Some other popular services of the Internet were not created this way, but were originally based on proprietary systems. These include IRC, ICQ, AIM, and Gnutella. There have been many analyses of the Internet and its structure. For example, it has been determined that the Internet IP routing structure and hypertext links of the World Wide Web are examples of scale-free networks. Similar to how the commercial Internet providers connect via Internet exchange points, research networks tend to interconnect into large subnetworks such as:
- GEANT
- Internet2
- GLORIAD These in turn are built around relatively smaller networks. See also the list of academic computer network organizations In network schematic diagrams, the Internet is often represented by a cloud symbol, into and out of which network communications can pass.

Internet culture

The Internet is also having a profound impact on work, leisure, knowledge and worldviews. worldviews]]

ICANN

The Internet Corporation for Assigned Names and Numbers (ICANN) is the authority that coordinates the assignment of unique identifiers on the Internet, including domain names, Internet protocol addresses, and protocol port and parameter numbers. A globally unified namespace (i.e., a system of names in which there is one and only one holder of each name) is essential for the Internet to function. ICANN is headquartered in Marina del Rey, California, but is overseen by an international board of directors drawn from across the Internet technical, business, academic, and non-commercial communities. The US government continues to have a privileged role in approving changes to the root zone file that lies at the heart of the domain name system. Because the Internet is a distributed network comprising many voluntarily interconnected networks, the Internet, as such, has no governing body. ICANN's role in coordinating the assignment of unique identifiers distinguishes it as perhaps the only central coordinating body on the global Internet, but the scope of its authority extends only to the Internet's systems of domain names, Internet protocol addresses, and protocol port and parameter numbers.

The World Wide Web

Through keyword-driven Internet research using search engines like Google, millions worldwide have easy, instant access to a vast and diverse amount of online information. Compared to encyclopedias and traditional libraries, the World Wide Web has enabled a sudden and extreme decentralization of information and data. Some companies and individuals have adopted the use of 'weblogs' or blogs, which are largely used as easily-updatable online diaries. Some commercial organizations encourage staff to fill them with advice on their areas of specialization in the hope that visitors will be impressed by the expert knowledge and free information, and be attracted to the corporation as a result. One example of this practice is Microsoft, via whose product developers publish their personal blogs in order to pique the public's interest in their work. For more information on the distinction between the World Wide Web and the Internet itself — as in everyday use the two are sometimes confused — see Dark internet where this is discussed in more detail.

Remote access

The Internet allows computer users to connect to other computers and information stores easily, wherever they may be across the world. They may do this with or without the use of security, authentication and encryption technologies, depending on the requirements. This is encouraging new ways of working from home, collaboration and information sharing in many industries. An accountant sitting at home can audit the books of a company based in another country, on a server situated in a third country that is remotely maintained by IT specialists in a fourth. These accounts could have been created by home-working book-keepers, in other remote locations, based on information e-mailed to them from offices all over the world. Some of these things were possible before the widespread use of the Internet, but the cost of private, leased lines would have made many of them infeasible in practice. An office worker away from his or her desk, perhaps the other side of the world on a business trip or a holiday, can open a remote desktop session into his or her normal office PC using a secure Virtual Private Network (VPN) connection via the Internet. This gives him or her complete access to all their normal files and data, including e-mail and other applications, while they are away.

Collaboration

This low-cost and nearly instantaneous sharing of ideas, knowledge and skills has revolutionized some, and given rise to whole new, areas of human activity. One example of this is the collaborative development and distribution of Free/Libre/Open-Source Software (FLOSS) such as Linux, Mozilla and OpenOffice.org. See Collaborative software.

File-sharing

A computer file can be e-mailed to customers, colleagues and friends as an attachment. It can be uploaded to a website or FTP server for easy download by others. It can be put into a "shared location" or onto a file server for instant use by colleagues. The load of bulk downloads to many users can be eased by the use of "mirror" servers or peer-to-peer networking. In any of these cases, access to the file may be controlled by user authentication; the transit of the file over the Internet may be obscured by encryption and money may change hands before or after access to the file is given. The price can be paid by the remote charging of funds from, for example a credit card whose details are also passed - hopefully fully encrypted - across the Internet. The origin and authenticity of the file received may be checked by digital signatures or by MD5 message digests. These simple features of the Internet, over a world-wide basis, are changing the basis for the production, sale and distribution of many types of product, wherever they can be reduced to a computer file for transmission. This includes all manner of office documents, publications, software products, music, photography, video, animations, graphics and the other arts. This in turn is causing seismic shifts in each of the existing industry associations, such as the RIAA and MPAA, that previously controlled the production and distribution of these products.

Streaming media and VoIP

Many existing radio and television broadcasters have provided Internet 'feeds' of their live audio and video streams (for example, the BBC). They have been joined by a range of pure Internet 'broadcasters' who never had on-air licences. This means that an Internet-connected device, such as a computer or something more specific, can be used to access on-line media in much the same way as was previously possible only with a TV or radio receiver. The range of material is much wider, from pornography to highly specialised technical web-casts. The simplest equipment can allow anybody, with little censorship or licencing control, to broadcast on a worldwide basis. Time-shift viewing or listening is not a problem as the BBC have shown with their Preview, Classic Clips and Listen Again features. Web-cams can be seen as an even lower-budget extension of this phenomenon. In this case the picture may update only slowly - perhaps once every few seconds or slower, but Internet users can watch animals around an African waterhole, ships in the Panama Canal or the traffic at a local roundabout live and in real time. Video chat rooms, video conferencing, and remote controllable webcams have become popular. Some people install webcams in their bedrooms that can be accessed by other voyeurs, often with two-way sound. VoIP stands for Voice over IP, where IP refers to the Internet Protocol that underlies all Internet communication. This phenomenon began as an optional two-way voice extension to some of the Instant Messaging systems that took off around the turn of the millennium. In recent years many people and organizations have made VoIP systems as easy to use and as convenient as a normal telephone. The benefit is that, as the actual voice traffic is carried by the Internet, VoIP is free or costs much less than an actual telephone call, especially over long distances and especially for those with always-on ADSL or DSL Internet connections anyway. The disadvantages are that it is still difficult to initiate a call with someone, unless they also have a VoIP phone or are at their computer and that there are still several competing standards that are mitigating against universal acceptance. In all of these cases, existing large organisations, that have grown accustomed to regular incomes for their services, are finding increased competition in their service areas, coming directly from the Internet. While newcomers strive to make these inroads, the traditional industries are having to adapt, adopt, complain or suffer. Meanwhile the consumer in each case most probably benefits from the increased range of services and possible price reductions. Some worry about censorship and control while others see a continuing globalisation of culture and norms.

Language

Main article: English on the Internet The most prevalent language for communication on the Internet is English. This may be due to the Internet's origins or to the growing role of English as an international language. It may also be related to the poor capability of early computers to handle characters other than those in the basic Latin alphabet (see Unicode). After English (32 % of web visitors) the most-requested languages on the world wide web are Chinese 13 %, Japanese 8 %, Spanish 6 %, German 6 % and French 4 %. (From [http://www.internetworldstats.com/stats7.htm Internet World Stats]) By continent, 33 % of the world's Internet users are based in Asia, 29 % in Europe and 23 % in North America.[http://www.internetworldstats.com/stats.htm] The Internet's technologies have developed enough in recent years that good facilities are available for development and communication in most widely used languages. However, some glitches such as mojibake still remain.

Cultural awareness

From a cultural awareness perspective, the Internet has been both an advantage and a liability. For people who are interested in other cultures it provides a significant amount of information and an interactivity that would be unavailable otherwise. However, for people who are not interested in other cultures there is some evidence indicating that the Internet enables them to avoid contact to a greater degree than ever before.

Censorship

Some countries, such as Iran and the People's Republic of China, restrict what people in their countries can see on the Internet, especially unwanted political and religious content. In the Western world, it is Germany that has the highest rate of censorship. Internet Service Providers are required by law to block some sites that contain child pornography or Nazi or Islamist propaganda. Censorship is sometimes done through government sponsored censoring filters, or by means of law or culture, making the propagation of targeted materials extremely hard. At the moment most Internet content is available regardless of where one is in the world, so long as one has the means of connecting to it.

Internet access

Germany Common methods of home access include dial-up, landline broadband (over coaxial cable, fiber optic or copper wires), Wi-Fi, satellite and cell phones. Public places to use the Internet include libraries and Internet cafes, where computers with Internet connections are available. There are also Internet access points in many public places like airport halls, in some cases just for brief use while standing. Various terms are used, such as "public Internet kiosk", "public access terminal", and "Web payphone". Many hotels now also have public terminals, though these are usually fee based. Wi-Fi provides wireless access to computer networks, and therefore can do so to the Internet itself. Hotspots providing such access include Wi-Fi-cafes, where a would-be user needs to bring their own wireless-enabled devices such as a laptop or PDA. These services may be free to all, free to customers only, or fee-based. A hotspot need not be limited to a confined location. The whole campus or park, or even the entire city can be enabled. Grassroots efforts have led to wireless community networks. Apart from Wi-Fi, there have been experiments with proprietary mobile wireless networks like Ricochet, various high-speed data services over cellular or mobile phone networks, and fixed wireless services. These services have not enjoyed widespread success due to their high cost of deployment, which is passed on to users in high usage fees. New wireless technologies such as WiMAX have the potential to alleviate these concerns and enable simple and cost effective deployment of metropolitan area networks covering large, urban areas. There is a growing trend towards wireless mesh networks, which offer a decentralized and redundant infrastructure and are often considered the future of the Internet. Broadband access over power lines was approved in 2004 in the United States in the face of stiff resistance from the amateur radio community. The problem with modulating a carrier signal onto power lines is that an above-ground power line can act as a giant antenna and jam long-distance radio frequencies used by amateurs, seafarers and others. Countries where Internet access is available to a majority of the population include Germany, India, China, Chile, Iceland, Finland, Sweden, Greece, Italy, Australia, Denmark, the United States, Canada, the United Kingdom, The Netherlands, Japan, Singapore, Taiwan, Thailand, South Korea and Norway. The use of the Internet around the world has been growing rapidly over the last decade, although the growth rate seems to have slowed somewhat after 2000. The phase of rapid growth is ending in industrialized countries, as usage becomes ubiquitous there, but the spread continues in Africa, Latin America, the Caribbean and the Middle East. However, there are still problems for many. ADSL and other broadband access are rare or nonexistent in most developing countries. Even in developed countries, high prices, mediocre performance and access restrictions often limit its uptake. Within individual countries, wide differences may exist between larger cities (often having multiple providers of broadband access) and some rural areas, where no broadband access may be available at all. The expansion of the availability of Internet access is a way to bridge the so-called digital divide.

Capitalization conventions

In formal usage, Internet is traditionally written with a capital first letter. The Internet Society, the Internet Engineering Task Force, the Internet Corporation for Assigned Names and Numbers, the World Wide Web Consortium, and several other Internet-related organizations all use this convention in their publications. In English grammar, proper nouns are capitalized. Most newspapers, newswires, periodicals, and technical journals also capitalize the term. Examples include the New York Times, the Associated Press, Time, The Times of India, Hindustan Times and Communications of the ACM. In other cases, the first letter is often written small (internet), and many people are not aware of any convention of using a capital letter. Some argue that internet is the correct form. Since 2000, a significant number of publications have switched to using internet. Among them are The Economist, the Financial Times, the London Times, and the Sydney Morning Herald. As of 2005, most publications using internet appear to be located outside of North America although one American news source, Wired News, has adopted the lowercase spelling.

Leisure

The Internet has been a major source of leisure since before the World Wide Web, with entertaining social experiments such as MOOs being conducted on university servers, and humor-related USENET groups receiving much of the main traffic. Today, many Internet forums have sections devoted to neta; short cartoons in the form of Flash movies are also popular. The pornography and gambling industries have both taken full advantage of the World Wide Web, and often provide a significant source of advertising revenue for other Web sites. Although many governments have attempted to put restrictions on both industries' use of the Internet, this has generally failed to stop their widespread popularity. One main area of leisure on the Internet is multiplayer gaming. This form of leisure creates communities, bringing people of all ages and origins to enjoy the fast-paced world of multiplayer games. These range from MMORPG to first-person shooters, from role-playing games to online gambling. This has revolutionized the way many people interact and spend their free time on the Internet. Online gaming began with services such as GameSpy and MPlayer, which players of games would typically subscribe to. Non-subscribers were limited to certain types of gameplay or certain games. With the release of Diablo by Blizzard Entertainment, gamers were treated to a built in online game service that was free of charge. With Blizzard's next game, StarCraft, the gaming world saw an explosion in the numbers of players using the Internet to play multi-player games. StarCraft may have been the first non-MMO game in which most players utilized the online gameplay as opposed to the single-player gameplay. Online gaming has progressed so much in the last 10 years that gamers earn a living from being a professional at the subject by winning tournaments and prizes as well as signing sponsor deals. Because there is a large support for certain online games, a new community has been born for people modding games, where users edit games to add a whole new element to it. This is how games such as Counter-Strike were born from the Half-Life Gaming Engine. Cyberslacking has become a serious drain on corporate resources; the average UK employee spends 57 minutes a day surfing, according to a study by Peninsula Business Services[http://news.scotsman.com/topics.cfm?tid=914&id=1001802003].

A complex system

Many computer scientists see the Internet as a "prime example of a large-scale, highly engineered, yet highly complex system" (Willinger, et al). The Internet is extremely heterogeneous. (For instance, data transfer rates and physical characteristics of connections vary widely.) The Internet exhibits "emergent phenomena" that depend on its large-scale organization. For example, data transfer rates exhibit temporal self-similarity.

Marketing

The Internet has also become a big market, and the biggest companies today have grown by taking advantage of the efficient low-cost advertising and commerce through the Internet. It is the fastest way to spread information to a vast community of people all at once. The Internet has revolutionized shopping –– a person can order a CD online and receive it in the mail within a couple of days, or download it directly in some cases.

Criticism

Many hyperlinks are outdated as time takes its toll on the existence of URL weblinks. These weblinks are often times defunct and are retained as hyperlinks for extended timeframes as a result of laziness or being busy enough to be sidetracked away from updating webpages. This is a common hoax for people who are fans in the field of what those links provide them with/to.

External links

General

- [http://www.channel101.com/ Internet TV Stations]
- [http://www.isoc.org/ The Internet Society (ISOC)]
- [http://www.techterms.org/internet.php Internet Dictionary] - Definitions of Internet-related terms
- [http://www.experienced-people.co.uk/1099-webmaster-glossary/ The Alternate Internet Glossary] (Humor)
- A [http://www.illusivecreations.com Calgary Web Design] company that has put together over 300 articles about the internet and web development. You can view them by going [http://www.illusivecreations.com/articles/ here].
- [http://www.clickz.com/stats/sectors/geographics/article.php/5911_151151 Internet access stats]
- [http://www.sharpened.net/glossary/ Glossary of Computer and Internet Terms]
- [http://scoreboard.keynote.com/scoreboard/Main.aspx?Login=Y&Username=public&Password=public Internet Health Report] from Keynote
- [http://www.internetworldstats.com/stats.htm Internet World Stats]

Articles

- [http://www.iht.com/articles/2005/09/29/business/net.php "EU and U.S. clash over control of the Net" - International Herald Tribune article by Tom Wright]
- [http://www.wired.com/wired/archive/13.08/intro.html "10 Years that changed the world" - WiReD looks back at the evolution of the Internet over last 10 years]
- [http://www.fourmilab.ch/documents/digital-imprimatur/ John Walker: The Digital Imprimatur]
- [http://www.addressingtheworld.info addressingtheworld.info] - website accompanying a book (ISBN 0742528103) on the history of DNS
- [http://computer.howstuffworks.com/internet-infrastructure.htm How Stuff Works explanation of the Infrastructure of the Internet]
- [http://www.searchandgo.com/articles/internet/net-explained-1.php Internet Explained] Seven part article explaining the origins to the present and a future look at the Internet.
- [http://www.wired.com/news/culture/0,1284,64596,00.html?tw=wn_tophead_7 "It's Just the 'internet' Now" - Wired.com article by Tony Long]

History

- [http://www.isoc.org/internet/history/brief.shtml The Internet Society History Page]
- [http://www.internetvalley.com/archives/mirrors/cerf-how-inet.txt How the Internet Came to Be]
- [http://www.zakon.org/robert/internet/timeline/ Hobbes' Internet Timeline v7.0]
- [http://www.ciolek.com/PAPERS/e-scholarship2000.html Futures and Non-futures for Scholarly Internet. ]
- [http://www.lk.cs.ucla.edu/internet_history.html History of the Internet links]
- [http://www.ietf.org/rfc/rfc801.txt RFC 801, planning the TCP/IP switchover]
- [http://www.archive.org/ Internet Archive] - A searchable database of old cached versions of websites dating back to 1996
- A list of lectures, some of which relate to the Internet, from the Massachusetts Institute of Technology is available [http://ocw.mit.edu/OcwWeb/Comparative-Media-Studies/CMS-930Media--Education--and-the-MarketplaceFall2001/VideoLectures/index.htm here]. Of particular interest is lecture #3 The Next Big Thing: Video Internet which is delivered in Real Player format. The lecture gives a brief history of networking; discusses convergence between the internet/telephone/television networks; the expansion of broadband access; makes predictions about the future of delivery of video over the internet.

References

- Walter Willinger, Ramesh Govindan, Sugih Jamin, Vern Paxson, and Scott Shenker. (2002). Scaling phenomena in the Internet. In Proceedings of the National Academy of Sciences, 99, suppl. 1, 2573 – 2580. Category:Communication Category:Digital media Category:Internet Category:Digital Revolution Category:Technology Category:Computer networks Category:Networks ko:인터넷 ms:Internet ja:インターネット simple:Internet th:อินเทอร์เน็ต fiu-vro:Internet

Internet protocol

Internet protocol may refer to:
- The Internet Protocol, a data-oriented protocol used for communicating data across a packet-switched internetwork.
- The Internet protocol suite, a set of communications protocols that implement the protocol stack on which the Internet runs.

Analogy

Analogy is either the cognitive process of transferring information from a particular subject (the analogue or source) to another particular subject (the target), or a linguistic expression corresponding to such a process. In a narrower sense, analogy is an inference or an argument from a particular to another particular, as opposed to deduction, induction and abduction, where at least one of the premises or the conclusion is general. The word analogy can also refer to the relation between the source and the target themselves, which is often, though not necessarily, a similarity, as in the biological notion of analogy. Analogy plays a significant role in problem solving, decision making, perception, memory, creativity, emotion, explanation and communication. It lies behind basic tasks such as the identification of places, objects and people, e.g. in face perception and facial recognition systems. It has been argued that analogy is "the core of cognition" (see Hofstadter in Gentner et. al. 2001). Specifically analogical language comprises exemplification, comparisons, metaphors, similes, allegories, and parables, but not metonymy. Phrases like and so on, and the like, as if and the very word like also rely on an analogical understanding by the receiver of a message including them. Analogy is important not only in ordinary language and common sense, where proverbs and idioms give many examples of its application, but also in science, philosophy and the humanities. The concepts of similarity, resemblance, homology, comparison, association, correspondence and isomorphism are closely related to analogy. In cognitive linguistics, the notion of conceptual metaphor may be equivalent to that of analogy. Analogy has been studied and discussed since classical antiquity by philosophers, scientists and lawyers. The last few decades have shown a renewed interest in analogy, most notable in cognitive science.

Models and theories of analogy

Identity of relation

In ancient Greek the word αναλογια (analogia) originally meant proportionality, in the mathematical sense, and it was indeed sometimes translated to Latin as proportio. From there analogy was understood as identity of relation between any two ordered pairs, whether of mathematical nature or not. Kant's Critique of Judgment held to this notion. Kant argued that there can be exactly the same relation between two completely different objects. The same notion of analogy was used in the US-based SAT tests, that included "analogy questions" in the form "A is to B as C is to what?" For example, "Hand is to palm as foot is to ____?" These questions were usually given in the Aristotelian format: :HAND : PALM : : FOOT : ____ It is worth noting that while most competent English speakers will immediately give the right answer to the analogy question (sole), it is quite more difficult to identify and describe the exact relation that holds both between hand and palm, and between foot and sole. This relation is not apparent in some lexical definitions of palm and sole, where the former is defined as the inner surface of the hand, and the latter as the underside of the foot. Analogy and abstraction are different cognitive processes, and analogy is often an easier one.

Shared abstraction

Greek philosophers as Plato and Aristotle actually used a wider notion of analogy. They saw analogy as a shared abstraction (Shelley 2003). Analogous objects shared an idea, a pattern, a regularity, an attribute, an effect or a function. They also accepted that comparisons, metaphors and "images" (allegories) could be used as valid arguments, and sometimes they called them analogies. Analogies should also make those abstractions easier to understand and give confidence to the ones using them. The Middle Ages saw an increased use and theorization of analogy. Roman lawyers had already used analogical reasoning and the Greek word analogia. Mediaeval lawyers distinguished analogia legis and analogia iuris (see below). In theology, analogical arguments were accepted in order to explain the attributes of God. Aquinas made a distinction between equivocal, univocal and analogical terms, the latter being those like healthy that have different but related meanings. Not only a person can be "healthy", but also the food that is good for health (see the contemporary distinction between polysemy and homonymy). Thomas Cajetan wrote an influent treatise on analogy. In all of these cases, the wide Platonic and Aristotelian notion of analogy was preserved.

Special case of induction

On the contrary, Bacon and later Mill argued that analogy be simply a special case of induction (see Shelley 2003). In their view analogy is an inductive inference from common known attributes to another probable common attribute, which is known only about the source of the analogy, in the following form: :Premise: a is C, D, E, F and G. :Premise: b is C, D, E and F. :Conclusion: b is probably G. :Alternative conclusion: every C, D, E and F is probably G. This view does not accept analogy as an autonomous mode of thought or inference, reducing it to induction. However, autonomous analogical arguments are still useful in science, philosophy and the humanities (see below), which makes this reduction philosophically uninteresting. Moreover, induction tries to achieve general conclusions, while analogy looks for particular ones.

Hidden deduction

The opposite move could also be tried, reducing analogy to deduction. It is argued that every analogical argument is partially superfluous and can be rendered as a deduction stating as a premise a (previously hidden) universal proposition which applied both to the source and the target. In this view, instead of an argument with the form: :Premise: a is analogous to b. :Premise: b is F. :Conclusion: a is plausibly F. We should have: :Hidden universal premise: all Gs are Fs. :Hidden singular premise: a is G. :Conclusion: a is F. This would mean that premises referring the source and the analogical relation are themselves superfluous. However, it is not always possible to find a true universal premise to replace the analogical premises (see Juthe 2005). And analogy is not only an argument, but also a distinct cognitive process.

Shared structure

Contemporary cognitive scientists use a wide notion of analogy, extensionally close to that of Plato and Aristotle, but framed by the structure mapping theory (See Gentner et. al. 2001). The same idea of mapping between source and target is used by conceptual metaphor theorists. Structure mapping theory concerns both psychology and computer science. According to this view, analogy depends on the mapping or alignment of the elements of source and target. The mapping takes place not only between objects, but also between relations of objects and between relations of relations. The whole mapping yields the assignment of a predicate or a relation to the target. Structure mapping theory has been applied and has found enough confirmation in psychology. It has had reasonable success in computer science and artificial intelligence (see below). Some studies extended the approach to specific subjects, such as metaphor and similarity (see Gentner et. al. 2001 and Gentner's publication page). Holyoak and Thagard (1997) developed their multiconstraint theory within structure mapping theory. They defend that the "coherence" of an analogy depends on structural consistency, semantic similarity and purpose. Structural consistency is maximal when the analogy is an isomorphism, although lower levels are admitted. Similarity demands that the mapping connects similar elements and relations of source and target, at any level of abstraction. It is maximal when there are identical relations and when connected elements have many identical attributes. An analogy achieves its purpose as much as it is fit for the problem at hand. The multiconstraint theory faces some difficulties when there are multiple sources, but these can be overcome (Shelley 2003). A problem for the multiconstraint theory arises from its concept of similarity, which, in this respect, is not obviously different from analogy itself. Computer applications demand that there are some identical attributes or relations at some level of abstraction. Human analogy does not, or at least not apparently.

High-level perception

Douglas Hofstadter and his team (see Chalmers et. al 1991) challenged the shared structure theory and mostly its applications in computer science. They argue that there is no line between perception, including high-level perception, and analogical thought. In fact, analogy occurs not only after, but also before and at the same time as high-level perception. In high-level perception, humans make representations selecting relevant information from low-level stimuli. Perception is necessary for analogy, but analogy is also necessary for high-level perception. Chalmers et. al. conclude that analogy is high-level perception. Forbus et. al. (1998) claim that this is only a metaphor. It has been argued (Morrison and Dietrich 1995) that Hofstadter's and Gentner's groups do not defend opposite views, but are instead dealing with different aspects of analogy.

Applications and types of analogy

Linguistics

- In linguistics, an analogy can be a spoken or textual comparison between two words (or sets of words) to highlight some form of semantic similarity between them. Linguistic analogies can be used to strengthen political and philosophical arguments, even when the semantic similarity is weak or non-existent (if crafted carefully for the audience).
- An analogy can also be the linguistic process that reduces word forms perceived as irregular by remaking them in the shape of more common forms that are governed by rules. For example, the English verb help once had the preterite holp and the past participle holpen. These obsolete forms have been discarded and replaced by helped by the power of analogy. However, irregular forms can sometimes be created by analogy; one example is the American English past tense form of "dive": "dove", formed on analogy with words such as drive-drove.
- Neologisms can be formed by analogy with existing words. A common example is software, formed by analogy with hardware. Another example is the humorous term underwhelm, formed by analogy with overwhelm.

Mathematics

Some types of analogies can have a precise mathematical formulation through the concept of isomorphism.

Artificial intelligence

See case-based reasoning

Anatomy

:See also: Analogy (biology) In anatomy, two anatomical structures are considered to be analogous when they serve similar functions but are not evolutionarily related, such as the legs of vertebrates and the legs of insects. Analogous structures are the result of convergent evolution and should be contrasted with homologous structures.

Law

In law, analogy is used to resolve issues on which there is no previous authority. A distinction has to be made between analogous reasoning from written law and analogy to precedent case law.

Analogies from codes and statutes

In civil law systems, where the preeminent source of law are legal codes and statutes, a lacuna (a gap) arises when a specific issue is not explicitly dealt with in written law. Judges will try to identify a provision whose purpose applies to the case at hand. That process can reach a high degree of sophistication, as judges sometimes not only look at specific provision to fill lacunae (gaps), but at several provisions (from which an underlying purpose can be inferred) or at general principles of the law to identify the legislator's value judgement from which the analogy is drawn. Besides the not very frequent filling of lacunae, analogy is very commonly used between different provisions in order to achieve substantial coherence. Analogy from previous judicial decisions is also common, although these decisions are not binding authorities.

Analogies from precedent case law

By contrast, in common law systems, where precedent cases are the primary source of law, analogies to codes and statutes are rare (since those are not seen as a coherent system, but as incursions into the common law). Analogies are thus usually drawn from precedent cases: The judge finds that the facts of another case are similar to the one at hand to an extent that the analogous application of the rule established in the previous case is justified.

Engineering

Often a physical prototype is built to model and represent some other physical object. The prototype (the "analog") is designed such that, while it is much easier and/or less expensive for someone to experiment on it. The behavior of the analog tells us something about the behavior of "the real thing". For example, wind tunnels are used to test scale models of wings and aircraft, which act as an analog to full-size wings and aircraft. For example, the MONIAC Computer (an analog computer) used the flow of water in its pipes as an analog to the flow of money in an economy.

External links and references

- [http://etext.lib.virginia.edu/cgi-local/DHI/dhi.cgi?id=dv1-09 Dictionary of the History of Ideas:] Analogy in Early Greek Thought.
- [http://etext.lib.virginia.edu/cgi-local/DHI/dhi.cgi?id=dv1-10 Dictionary of the History of Ideas:] Analogy in Patristic and Medieval Thought.
- [http://plato.stanford.edu/entries/analogy-medieval/ Stanford Encyclopedia of Philosophy:] Medieval Theories of Analogy.
- [http://www.psych.northwestern.edu/psych/people/faculty/gentner/allpubs.htm Dedre Gentner's publications page], most of them on analogy and available for download.
- Chalmers, D.J. et. al. (1991). Chalmers, D.J., French, R.M., Hofstadter, D., [http://consc.net/papers/highlevel.pdf High-Level Perception, Representation, and Analogy].
- Forbus, Kenneth et. al. (1998). [http://www.psych.northwestern.edu/psych/people/faculty/gentner/newpdfpapers/ForbusGentner98.pdf Analogy just looks like high-level perception].
- Gentner, Dedre et. al. (2001). The Analogical Mind: Perspectives from Cognitive Science. Cambridge, Massachusetts, The MIT Press.
- Itkonen, Esa (2005). Analogy as Structure and Process. Amsterdam/Philadelphia: John Benjamin's Publishing Company.
- Juthe, André (2005). [http://www.cs.hut.fi/Opinnot/T-93.850/2005/Papers/juthe2005-analogy.pdf "Argument by Analogy"], in Argumentation (2005) 19: 1–27.
- Holyoak and Thagard (1997). [http://cogsci.uwaterloo.ca/Articles/Pages/Analog.Mind.html The Analogical Mind].
- Holyoak, K.J. et. al. (1996). Mental Leaps: Analogy in Creative Thought. Cambridge, Massachusetts, The MIT Press.
- Morrison, C., and Dietrich, E. (1995). [http://eksl.cs.umass.edu/~clayton/publications/CogSci95/SM-v-HLP.pdf Structure-Mapping vs. High-level Perception].
- Shelley, Cameron (2003). Multiple analogies in Science and Philosophy. Amsterdam/Philadelphia: John Benjamin's Publishing Company. Category:Philosophical arguments Category:Cognitive science Category:Computer science Category:Semantics ja:アナロジー

Protocol stack

A protocol stack is a particular software implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols and the stack is the software implementation of them. Individual protocols within a suite are often designed with a single purpose in mind. This modularisation makes design and evaluation easier. Because each protocol module usually communicates with two others, they are commonly imagined as layers in a stack of protocols. The lowest protocol always deals with "low-level", physical interaction of the hardware. Every higher layer adds more features. User applications habitually deal only with the topmost layers. See also OSI model. In practical implementation, protocol stacks are often divided into three major sections for media, transport, and applications. A particular operating system or platform will often have two well-defined software interfaces, one between the media and transport layers, and one between the transport layers and applications. The media-to-transport interface defines how transport protocol software makes use of particular media and hardware types ("card drivers"). For example, this interface level would define how TCP/IP transport software would talk to Ethernet hardware. Examples of these interfaces include ODI and NDIS in the Microsoft Windows and DOS world. The application-to-transport interface defines how application programs make use of the transport layers. For example, this interface level would define how a web browser program would talk to TCP/IP transport software. Examples of these interfaces include Berkeley sockets and System V streams in the Unix world, and Winsock in the Microsoft world. General protocol suite description: T ~ ~ ~ T [A] [B]_____[C] Imagine three computers A, B, and C. A and B both have radio equipment, and can communicate via the airwaves using a suitable network protocol like IEEE 802.11. B and C are connected via a cable, exchanging data over that - again with the help of a protocol (for example Ethernet). However, neither of these two protocols will be able to transport information from A to C, because these computers are conceptually on different networks. One therefore needs an inter-network protocol to "connect" them. One could combine our two protocols to form a powerful third mastering both cable and wireless transmission, but we would need a different super-protocol for each possible combination of protocols. It is easier to leave the base protocols alone, and design a protocol that can work on top of any of them (the Internet Protocol is an example). This will make two stacks of two protocols each. The inter-network protocol will communicate with each of the base protocol in their simpler language. The base protocols will not talk directly to each other. A request on computer A to send a chunk of data to C is taken by the upper protocol, which (through whatever means) knows that C is reachable through B. It therefore instructs the wireless protocol to transmit the data packet to B. On this computer, the lower layer handlers will pass the packet up to the inter-network protocol, which, on recognizing that B is not the final destination, will again invoke lower-level functions. This time, the cable protocol is used to send the data to C. There the received packet is again passed to the upper protocol, which (with C being the destination) will pass it on. Often an even higher-level protocol will sit on top, and incur further processing. A commonly used protocol stack looks like this: +- - - - - -+ | HTTP | +- - - - - -+ | TCP | +- - - - - -+ | IP | +- - - - - -+ | Ethernet | +- - - - - -+ See also: Network protocol design principles Category:Computing

Spanning Tree Protocol

The spanning tree network protocol provides a loop free topology for any bridged LAN. The Spanning Tree Protocol, which is also referred to as STP, is defined in the IEEE Standard 802.1D.

History

Spanning tree is based on an algorithm invented by Radia Perlman. She also summarized the algorithm in the form of a poem, titled "Algorhyme": :I think that I shall never see :A graph more lovely than a tree. :A tree whose crucial property :Is loop-free connectivity. :A tree which must be sure to span. :So packets can reach every LAN. :First the Root must be selected :By ID it is elected. :Least cost paths from Root are traced :In the tree these paths are placed. :A mesh is made by folks like me :Then bridges find a spanning tree.

Protocol operation

Its structure corresponds to that of the spanning tree in graph theory. Networks must have only one path to any destination active at any one point in time to avoid the same frame arriving at the destination multiple times, causing dysfunction. The minimum spanning tree algorithm ensures that if multiple paths exist to the same destination then all but one will be blocked. Redundant networks can be built in this way, however the latency of spanning tree decisions must be taken into account as by default the maximum time for recalculations of the minimum spanning tree is 30 seconds. During this time no network traffic will be able to pass through ports involved in the recalculations. A root bridge is automatically selected by the algorithm as the root of the minimum spanning tree (The switches in a network will carry out elections to decide which one is the root bridge. The switch with the lowest priority wins the election and becomes the root bridge. In the event that all switches have the same priority, then the bridge ID will then be compared and the one again with the lowest wins the elections. In the event that another switch is added to the network where a root bridge is already elected then no elections are held even if the switch has the lowest priority). As with most networking protocols this is highly configurable and therefore any networking device could be forced into this role. Ports have five states:
- Listening
- Learning
- Forwarding
- Blocking
- Disabled Bridge protocol data units (BPDUs) are the frames which carry the spanning tree protocol information.

Evolutions and extensions

Per-VLAN Spanning Tree (PVST)

In Ethernet switched environments where multiple Virtual LANs exist, spanning tree can be deployed per Virtual LAN. Cisco's name for this is per VLAN spanning tree (PVST and PVST+ which is the default protocol used by Cisco switches).

Rapid Spanning Tree Protocol (RSTP)

In 1998, the IEEE introduced an evolution of the Spanning Tree Protocol: Rapid Spanning Tree Protocol (RSTP) or 802.1w. In the 2004 edition of 802.1D, STP is superseded by the RSTP.

Multiple Spanning Tree Protocol (MSTP)

The 2003 revision of the standard also rolled in the Multiple Spanning Tree Protocol (MSTP) originally defined in IEEE 802.1s and later merged into IEEE 802.1Q-2003

External links

- [http://research.sun.com/people/mybio.php?uid=28941 Radia Perlman at Sun Labs]
- [http://standards.ieee.org/getieee802/download/802.1D-2004.pdf ANSI/IEEE 802.1D-2004 standard]
- [http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm Cisco's version of 'Understanding STP']
- RFCs
- RFC 2674-1999, proposed standard, Definitions of Managed Objects for Bridges with Traffic Classes, Multicast Filtering and Virtual LAN Extensions
- RFC 1525-1993, - SBRIDGEMIB, proposed standard, Definitions of Managed Objects for Source Routing Bridges
- RFC 1493-1993 - BRIDGEMIB, draft standard, Definitions of Managed Objects for Bridges Category:Network protocols Category:Computer networks Category:Mnemonics Category:American poems

Network bridge

A network bridge connects multiple network segments (network domains) along the data link layer. It may be a physical device, such as a network switch, or it may be a virtual device using bridging. Traffic from one network is forwarded through it to another network. No routing is involved whatsoever. The bridge simply does what its name entails, by connecting two sides from adjacent networks. An example would be an office LAN using a bridge to connect down to an ISP office. A description of the Network Bridge in Windows XP is given [http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/hnw_understanding_bridge.mspx here]. A USB-to-USB network bridge is a cable that connects two computers to transfer files (without a network switch). A repeater is a similar abstract device that connects network segments at the physical layer. An Ethernet hub is a type of repeater.

Software

Computer software (or simply software) is that part of a computer system that consists of symbolicly encoded information as opposed to the physical computer equipment (hardware) which is used to store and process this information. The term is roughly synonymous with computer program but is more generic in scope. The term "software" was first used in this sense by John W. Tukey in 1957. In computer science and software engineering, computer software is all information processed by computer systems, programs and data. The concept of software was first proposed by Alan Turing in an essay.

Relationship to hardware

Computer software is so called in contrast to computer hardware, which is the physical substrate which is required to store and execute (or run) the software. In computers, software is loaded in RAM and executed on the central processing unit. At the lowest level, software consists of a machine language specific to an individual processor. Machine language consists of groups of binary values signifying processor instructions and data. Software is generally written in high-level languages that are easy and efficient for humans to use. High-level languages are compiled into machine language.

Relationship to data

Software has historically been considered an intermediary between electronic hardware and data, which the hardware processes according to instructions defined by the software. As computational science becomes increasingly complex, the distinction between software and data becomes less precise. Data has generally been considered as either the output or input of software. However, data is not the only possible output or input. For example, configuration information can also be considered input, although not necessarily considered data. The output of a particular piece of software may be the input for another piece of software. Therefore, software may be considered an interface between hardware, data, or software.

System, application and programming software

Practical computer systems divide software into three major classes: system software, application software and programming software, although the distinction is somewhat arbitrary, and often blurred. :System software helps run the computer hardware and computer system. It includes operating systems, device drivers, diagnostic tools, servers, windowing systems, utilities and more. :Application software allows a user to accomplish one or more specific tasks. Typical applications include office suites, business software, educational software, databases and computer games. Most application software has a graphical user interface (GUI). :Programming software usually provides some useful tools to help programmer to write computer programs and software using different programming language in a more convenient way. The tools include text editor, compiler, interpreter, linker, debugger, and so on. Integrated development environment (IDE) merges those tools in a software bundle, and programmer may not need to type a lot of commands for compiling, interpreter, debugging, tracing, and etc., because IDE mostly has a GUI.

Software program and library

Software program is usually the directly executable part of a software. Software libraries are software components that are used by stand-alone programs, but which cannot be executed on their own.

Users see three layers of software

Software libraries Users often see things differently than programmers. People who use modern general purpose computers (as opposed to embedded systems) usually see three layers of software performing a variety of tasks: platform, application, and user software. ; Platform software : Platform includes the basic input-output system (often described as firmware rather than software), device drivers, an operating system, and typically a graphical user interface which, in total, allow a user to interact with the computer and its peripherals (associated equipment). Platform software often comes bundled with the computer, and users may not realize that it exists or that they have a choice to use different platform software. ; Application software : Application software or Applications are what most people think of when they think of software. Typical examples include office suites and video games. Application software is often purchased separately from computer hardware. Sometimes applications are bundled with the computer, but that does not change the fact that they run as independent applications. Applications are almost always independent programs from the operating system, though they are often tailored for specific platforms. Most users think of compilers, databases, and other "system software" as applications. ; User-written software : User software tailors systems to meet the users specific needs. User software include spreadsheet templates, word processor macros, scientific simulations, graphics and animation scripts. Even email filters are a kind of user software. Users create this software themselves and often overlook how important it is. See also: Software architecture.

Software creation

Look back to Computer software

Software operation

Computer software has to be "loaded" into the computer's storage (also known as memory and RAM). Once the software is loaded, the computer is able to operate the software. Computers operate by executing the computer program. This involves passing instructions from the application software, through the system software, to the hardware which ultimately receives the instruction as machine code. Each instruction causes the computer to carry out an operation -- moving data, carrying out a computation, or altering the flow of instructions. Kinds of software by operation: computer program as executable, source code or script, configuration.

Software quality and reliability

Software reliability considers the errors, faults, and failures related to the creation and operation of software. See Software quality,