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85-514: The purpose of KMS was to let many users collaborate in creating and sharing information within large, shared hypertext, and from the very beginning, the system was designed as a true multi-user system. As a spatial hypermedia system, KMS was intended to represent all forms of explicit 'knowledge artifacts' such as presentations, documents, databases, and software programs, as well as common forms of electronic communication ( electronic mail , community bulletin boards , blogs ). The central element in

170-737: A protocol stack , often constructed per the OSI model, communications functions are divided up into protocol layers, where each layer leverages the services of the layer below it until the lowest layer controls the hardware that sends information across the media. The use of protocol layering is ubiquitous across the field of computer networking. An important example of a protocol stack is HTTP (the World Wide Web protocol) running over TCP over IP (the Internet protocols) over IEEE 802.11 (the Wi-Fi protocol). This stack

255-644: A consequence open mail relays have become rare, and many MTAs do not accept messages from open mail relays. The basic Internet message format used for email is defined by RFC   5322 , with encoding of non-ASCII data and multimedia content attachments defined in RFC 2045 through RFC 2049, collectively called Multipurpose Internet Mail Extensions or MIME . The extensions in International email apply only to email. RFC 5322 replaced RFC 2822 in 2008. Earlier, in 2001, RFC 2822 had in turn replaced RFC 822, which had been

340-555: A current Internet connection. The Post Office Protocol 3 (POP3) is a mail access protocol used by a client application to read messages from the mail server. Received messages are often deleted from the server . POP supports simple download-and-delete requirements for access to remote mailboxes (termed maildrop in the POP RFC's). POP3 allows downloading messages on a local computer and reading them even when offline. The Internet Message Access Protocol (IMAP) provides features to manage

425-499: A diverse set of networking capabilities. The protocols have a flat addressing scheme. They operate mostly at layers 1 and 2 of the OSI model. For example, MAC bridging ( IEEE 802.1D ) deals with the routing of Ethernet packets using a Spanning Tree Protocol . IEEE 802.1Q describes VLANs , and IEEE 802.1X defines a port-based network access control protocol, which forms the basis for the authentication mechanisms used in VLANs (but it

510-456: A document, search, run programs from a tree of frames starting at any frame. In KMS, links are one way and are embedded in frames. They may go from any text item, point, graphical entity, or image in the source frame to any destination frame. In addition to links, frame items can also have actions, allowing the use to activate programs that extend the intrinsic functionality of the system. A major distinction between KMS and current web practices

595-413: A few characters outside that range and base64 for arbitrary binary data. The 8BITMIME and BINARY extensions were introduced to allow transmission of mail without the need for these encodings, but many mail transport agents may not support them. In some countries, e-mail software violates RFC   5322 by sending raw non-ASCII text and several encoding schemes co-exist; as a result, by default,

680-551: A file by email. Where larger files need to be shared, various file hosting services are available and commonly used. Computer network A computer network is a set of computers sharing resources located on or provided by network nodes . Computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are made up of telecommunication network technologies based on physically wired, optical , and wireless radio-frequency methods that may be arranged in

765-437: A large corporate environment, with a proprietary protocol specific to Novell Groupwise , Lotus Notes or Microsoft Exchange Servers . Programs used by users for retrieving, reading, and managing email are called mail user agents (MUAs). When opening an email, it is marked as "read", which typically visibly distinguishes it from "unread" messages on clients' user interfaces. Email clients may allow hiding read emails from

850-440: A large, congested network into an aggregation of smaller, more efficient networks. A router is an internetworking device that forwards packets between networks by processing the addressing or routing information included in the packet. The routing information is often processed in conjunction with the routing table . A router uses its routing table to determine where to forward packets and does not require broadcasting packets which

935-410: A mail store by programs called mail delivery agents (MDAs, also sometimes called local delivery agents, LDAs). Accepting a message obliges an MTA to deliver it, and when a message cannot be delivered, that MTA must send a bounce message back to the sender, indicating the problem. Users can retrieve their messages from servers using standard protocols such as POP or IMAP , or, as is more likely in

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1020-402: A mailbox from multiple devices. Small portable devices like smartphones are increasingly used to check email while traveling and to make brief replies, larger devices with better keyboard access being used to reply at greater length. IMAP shows the headers of messages, the sender and the subject and the device needs to request to download specific messages. Usually, the mail is left in folders in

1105-440: A multi-port bridge. Switches normally have numerous ports, facilitating a star topology for devices, and for cascading additional switches. Bridges and switches operate at the data link layer (layer 2) of the OSI model and bridge traffic between two or more network segments to form a single local network. Both are devices that forward frames of data between ports based on the destination MAC address in each frame. They learn

1190-422: A notable implementation by MIT 's CTSS project in 1965. Most developers of early mainframes and minicomputers developed similar, but generally incompatible, mail applications. In 1971 the first ARPANET network mail was sent, introducing the now-familiar address syntax with the ' @ ' symbol designating the user's system address. Over a series of RFCs , conventions were refined for sending mail messages over

1275-453: A repeater hub assists with collision detection and fault isolation for the network. Hubs and repeaters in LANs have been largely obsoleted by modern network switches. Network bridges and network switches are distinct from a hub in that they only forward frames to the ports involved in the communication whereas a hub forwards to all ports. Bridges only have two ports but a switch can be thought of as

1360-418: A single failure can cause the network to fail entirely. In general, the more interconnections there are, the more robust the network is; but the more expensive it is to install. Therefore, most network diagrams are arranged by their network topology which is the map of logical interconnections of network hosts. Common topologies are: The physical layout of the nodes in a network may not necessarily reflect

1445-412: A single piece of electronic mail is called a message . The conventions for fields within emails—the "To", "From", "CC", "BCC" etc.—began with RFC-680 in 1975. An Internet email consists of an envelope and content ; the content consists of a header and a body . Computer-based messaging between users of the same system became possible after the advent of time-sharing in the early 1960s, with

1530-693: A smartphone ranges and differs dramatically across different countries. For example, in comparison to 75% of those consumers in the US who used it, only 17% in India did. As of 2010 , the number of Americans visiting email web sites had fallen 6 percent after peaking in November 2009. For persons 12 to 17, the number was down 18 percent. Young people preferred instant messaging , texting and social media . Technology writer Matt Richtel said in The New York Times that email

1615-403: A standard voice telephone line. Modems are still commonly used for telephone lines, using a digital subscriber line technology and cable television systems using DOCSIS technology. A firewall is a network device or software for controlling network security and access rules. Firewalls are inserted in connections between secure internal networks and potentially insecure external networks such as

1700-877: A transmission medium. Power line communication uses a building's power cabling to transmit data. The following classes of wired technologies are used in computer networking. Network connections can be established wirelessly using radio or other electromagnetic means of communication. The last two cases have a large round-trip delay time , which gives slow two-way communication but does not prevent sending large amounts of information (they can have high throughput). Apart from any physical transmission media, networks are built from additional basic system building blocks, such as network interface controllers , repeaters , hubs , bridges , switches , routers , modems, and firewalls . Any particular piece of equipment will frequently contain multiple building blocks and so may perform multiple functions. A network interface controller (NIC)

1785-406: A variety of network topologies . The nodes of a computer network can include personal computers , servers , networking hardware , or other specialized or general-purpose hosts . They are identified by network addresses and may have hostnames . Hostnames serve as memorable labels for the nodes and are rarely changed after initial assignment. Network addresses serve for locating and identifying

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1870-495: A variety of different sources, primarily to support circuit-switched digital telephony . However, due to its protocol neutrality and transport-oriented features, SONET/SDH also was the obvious choice for transporting Asynchronous Transfer Mode (ATM) frames. Asynchronous Transfer Mode (ATM) is a switching technique for telecommunication networks. It uses asynchronous time-division multiplexing and encodes data into small, fixed-sized cells . This differs from other protocols such as

1955-652: A virtual system of links that run on top of the Internet . Overlay networks have been used since the early days of networking, back when computers were connected via telephone lines using modems, even before data networks were developed. The most striking example of an overlay network is the Internet itself. The Internet itself was initially built as an overlay on the telephone network . Even today, each Internet node can communicate with virtually any other through an underlying mesh of sub-networks of wildly different topologies and technologies. Address resolution and routing are

2040-519: Is computer hardware that connects the computer to the network media and has the ability to process low-level network information. For example, the NIC may have a connector for plugging in a cable, or an aerial for wireless transmission and reception, and the associated circuitry. In Ethernet networks, each NIC has a unique Media Access Control (MAC) address —usually stored in the controller's permanent memory. To avoid address conflicts between network devices,

2125-530: Is a method of transmitting and receiving messages using electronic devices. It was conceived in the late–20th century as the digital version of, or counterpart to, mail (hence e- + mail ). Email is a ubiquitous and very widely used communication medium; in current use, an email address is often treated as a basic and necessary part of many processes in business, commerce, government, education, entertainment, and other spheres of daily life in most countries. Email operates across computer networks , primarily

2210-422: Is a virtual network that is built on top of another network. Nodes in the overlay network are connected by virtual or logical links. Each link corresponds to a path, perhaps through many physical links, in the underlying network. The topology of the overlay network may (and often does) differ from that of the underlying one. For example, many peer-to-peer networks are overlay networks. They are organized as nodes of

2295-553: Is also found in WLANs ) – it is what the home user sees when the user has to enter a "wireless access key". Ethernet is a family of technologies used in wired LANs. It is described by a set of standards together called IEEE 802.3 published by the Institute of Electrical and Electronics Engineers. Wireless LAN based on the IEEE 802.11 standards, also widely known as WLAN or WiFi, is probably

2380-588: Is also part of the header, as defined below. SMTP defines the trace information of a message saved in the header using the following two fields: Other fields added on top of the header by the receiving server may be called trace fields . Internet email was designed for 7-bit ASCII. Most email software is 8-bit clean , but must assume it will communicate with 7-bit servers and mail readers. The MIME standard introduced character set specifiers and two content transfer encodings to enable transmission of non-ASCII data: quoted printable for mostly 7-bit content with

2465-478: Is an electronic device that receives a network signal , cleans it of unnecessary noise and regenerates it. The signal is retransmitted at a higher power level, or to the other side of obstruction so that the signal can cover longer distances without degradation. In most twisted-pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. With fiber optics, repeaters can be tens or even hundreds of kilometers apart. Repeaters work on

2550-412: Is inefficient for very big networks. Modems (modulator-demodulator) are used to connect network nodes via wire not originally designed for digital network traffic, or for wireless. To do this one or more carrier signals are modulated by the digital signal to produce an analog signal that can be tailored to give the required properties for transmission. Early modems modulated audio signals sent over

2635-459: Is no technical restriction on the size or number of attachments. However, in practice, email clients, servers , and Internet service providers implement various limitations on the size of files, or complete email – typically to 25MB or less. Furthermore, due to technical reasons, attachment sizes as seen by these transport systems can differ from what the user sees, which can be confusing to senders when trying to assess whether they can safely send

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2720-404: Is not sending packets, the link can be filled with packets from other users, and so the cost can be shared, with relatively little interference, provided the link is not overused. Often the route a packet needs to take through a network is not immediately available. In that case, the packet is queued and waits until a link is free. The physical link technologies of packet networks typically limit

2805-494: Is often successfully used to send special sales offerings and new product information. Depending on the recipient's culture, email sent without permission—such as an "opt-in"—is likely to be viewed as unwelcome " email spam ". Many users access their personal emails from friends and family members using a personal computer in their house or apartment. Email has become used on smartphones and on all types of computers. Mobile "apps" for email increase accessibility to

2890-419: Is supplied separately to the transport protocol, SMTP , which may be extracted from the header content. The "To:" field is similar to the addressing at the top of a conventional letter delivered according to the address on the outer envelope. In the same way, the "From:" field may not be the sender. Some mail servers apply email authentication systems to messages relayed. Data pertaining to the server's activity

2975-539: Is the distinction between regular items and "Annotation Items" (text items prefaced by the "@" characters). Annotation items have the connotation of being peripheral or meta-level, relative to the rest of the contents, making it easy (and apparent) as to what was regular content versus just notes to oneself or comments by others (including by programs). Annotation items with links thus have the connotation of being arbitrary cross-references (a la "See Also...") and thus are not seen by users, and more especially agents, as part of

3060-459: Is used between the wireless router and the home user's personal computer when the user is surfing the web. There are many communication protocols, a few of which are described below. The Internet protocol suite , also called TCP/IP, is the foundation of all modern networking. It offers connection-less and connection-oriented services over an inherently unreliable network traversed by datagram transmission using Internet protocol (IP). At its core,

3145-476: The File Transfer Protocol . Proprietary electronic mail systems soon began to emerge. IBM , CompuServe and Xerox used in-house mail systems in the 1970s; CompuServe sold a commercial intraoffice mail product in 1978 to IBM and to Xerox from 1981. DEC's ALL-IN-1 and Hewlett-Packard's HPMAIL (later HP DeskManager) were released in 1982; development work on the former began in the late 1970s and

3230-457: The Institute of Electrical and Electronics Engineers (IEEE) maintains and administers MAC address uniqueness. The size of an Ethernet MAC address is six octets . The three most significant octets are reserved to identify NIC manufacturers. These manufacturers, using only their assigned prefixes, uniquely assign the three least-significant octets of every Ethernet interface they produce. A repeater

3315-444: The Internet , and also local area networks . Today's email systems are based on a store-and-forward model. Email servers accept, forward, deliver, and store messages. Neither the users nor their computers are required to be online simultaneously; they need to connect, typically to a mail server or a webmail interface to send or receive messages or download it. Originally a text-only ASCII communications medium, Internet email

3400-470: The World Wide Web , digital video and audio , shared use of application and storage servers , printers and fax machines , and use of email and instant messaging applications. Computer networking may be considered a branch of computer science , computer engineering , and telecommunications , since it relies on the theoretical and practical application of the related disciplines. Computer networking

3485-440: The Internet protocol suite or Ethernet that use variable-sized packets or frames . ATM has similarities with both circuit and packet switched networking. This makes it a good choice for a network that must handle both traditional high-throughput data traffic, and real-time, low-latency content such as voice and video. ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before

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3570-574: The Internet. Firewalls are typically configured to reject access requests from unrecognized sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase in cyber attacks . A communication protocol is a set of rules for exchanging information over a network. Communication protocols have various characteristics. They may be connection-oriented or connectionless , they may use circuit mode or packet switching, and they may use hierarchical addressing or flat addressing. In

3655-536: The KMS data model is that of screen-sized pages (called "frames") interconnected by links . The user had the option (at any time) of switching between a view of a single frame (good for large, landscape-oriented diagrams) or two side-by-side half-screen views (suitable for two portrait-sized pages). Frames are always fixed-size, meaning scrolling is not needed. The frame model is spatial rather than character based, so that text, graphics and images may always be placed anywhere in

3740-504: The URL in the To: field. Many clients also support query string parameters for the other email fields, such as its subject line or carbon copy recipients. Many email providers have a web-based email client. This allows users to log into the email account by using any compatible web browser to send and receive their email. Mail is typically not downloaded to the web client, so it cannot be read without

3825-432: The ability to be used for more frequent communication between users and allowed them to check their email and write messages throughout the day. As of 2011 , there were approximately 1.4 billion email users worldwide and 50 billion non-spam emails that were sent daily. Individuals often check emails on smartphones for both personal and work-related messages. It was found that US adults check their email more than they browse

3910-417: The ability to include in-line links and images, set apart previous messages in block quotes , wrap naturally on any display, use emphasis such as underlines and italics , and change font styles. Disadvantages include the increased size of the email, privacy concerns about web bugs , abuse of HTML email as a vector for phishing attacks and the spread of malicious software . Some e-mail clients interpret

3995-627: The actual data exchange begins. ATM still plays a role in the last mile , which is the connection between an Internet service provider and the home user. There are a number of different digital cellular standards, including: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), cdmaOne , CDMA2000 , Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN). Routing

4080-468: The association of physical ports to MAC addresses by examining the source addresses of received frames and only forward the frame when necessary. If an unknown destination MAC is targeted, the device broadcasts the request to all ports except the source, and discovers the location from the reply. Bridges and switches divide the network's collision domain but maintain a single broadcast domain. Network segmentation through bridging and switching helps break down

4165-447: The attachments. Others separate attachments from messages and save them in a specific directory. The URI scheme , as registered with the IANA, defines the mailto: scheme for SMTP email addresses. Though its use is not strictly defined, URLs of this form are intended to be used to open the new message window of the user's mail client when the URL is activated, with the address as defined by

4250-466: The body as HTML even in the absence of a Content-Type: html header field; this may cause various problems. Some web-based mailing lists recommend all posts be made in plain text, with 72 or 80 characters per line for all the above reasons, and because they have a significant number of readers using text-based email clients such as Mutt . Various informal conventions evolved for marking up plain text in email and usenet posts, which later led to

4335-441: The developed world, and it is one of the key parts of an 'e-revolution' in workplace communication (with the other key plank being widespread adoption of highspeed Internet ). A sponsored 2010 study on workplace communication found 83% of U.S. knowledge workers felt email was critical to their success and productivity at work. It has some key benefits to business and other organizations, including: Email marketing via " opt-in "

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4420-560: The development of formal languages like setext (c. 1992) and many others , the most popular of them being markdown . Some Microsoft email clients may allow rich formatting using their proprietary Rich Text Format (RTF), but this should be avoided unless the recipient is guaranteed to have a compatible email client. Messages are exchanged between hosts using the Simple Mail Transfer Protocol with software programs called mail transfer agents (MTAs); and delivered to

4505-421: The file system. Some clients save individual messages as separate files, while others use various database formats, often proprietary, for collective storage. A historical standard of storage is the mbox format. The specific format used is often indicated by special filename extensions : Some applications (like Apple Mail ) leave attachments encoded in messages for searching while also saving separate copies of

4590-399: The final restrictions on carrying commercial traffic over the Internet ended in 1995, a combination of factors made the current Internet suite of SMTP, POP3 and IMAP email protocols the standard (see Protocol Wars ). The following is a typical sequence of events that takes place when sender Alice transmits a message using a mail user agent (MUA) addressed to the email address of

4675-450: The following fields: RFC 3864 describes registration procedures for message header fields at the IANA ; it provides for permanent and provisional field names, including also fields defined for MIME, netnews, and HTTP, and referencing relevant RFCs. Common header fields for email include: The To: field may be unrelated to the addresses to which the message is delivered. The delivery list

4760-506: The frame, even overlapping one another. Another way to say this is that empty space in the frame actually denotes space, not (as in many text editors) just the absence of content. Frames being fixed in size scrolling as a form of interaction is eliminated (as the designers felt that scrolling is suboptimal) opting instead for larger aggregates such as documents and programs to be structured as hierarchies (or more generally, lattices) of hypermedia nodes. This flexibility makes it possible to create

4845-438: The hierarchical structure of the hypertext. Another major distinction between KMS and current web practices is the elimination of a separate editor mode. Navigation and editing functionality is always directly available and users can edit any frame for which they have permission. Authors can even protect frames from inadvertent editing even by themselves. Older versions of frames are saved in 'comet-like' linked list so one can see

4930-529: The history (and time spent) for any individual frame (and more generally, those in any tree). Finally, KMS contained a script programming language (akin to JavaScript) which enabled developers and users to extend the system beyond its current functionality. In keeping with the KMS philosophy of 'Everything a frame' (e.g., cursors, fillpatterns, etc., are represented as frames) so programs are also represented as hierarchies of frames; KMS dynamically read and interpreted only those program frames needed at runtime. KMS

5015-509: The inbox so the user can focus on the unread. Mail can be stored on the client , on the server side, or in both places. Standard formats for mailboxes include Maildir and mbox . Several prominent email clients use their own proprietary format and require conversion software to transfer email between them. Server-side storage is often in a proprietary format but since access is through a standard protocol such as IMAP, moving email from one server to another can be done with any MUA supporting

5100-571: The latter became the world's largest selling email system. The Simple Mail Transfer Protocol (SMTP) was implemented on the ARPANET in 1983. LAN email systems emerged in the mid-1980s. For a time in the late 1980s and early 1990s, it seemed likely that either a proprietary commercial system or the X.400 email system, part of the Government Open Systems Interconnection Profile (GOSIP), would predominate. However, once

5185-655: The limit is 998 characters. Header fields defined by RFC 5322 contain only US-ASCII characters; for encoding characters in other sets, a syntax specified in RFC 2047 may be used. In some examples, the IETF EAI working group defines some standards track extensions, replacing previous experimental extensions so UTF-8 encoded Unicode characters may be used within the header. In particular, this allows email addresses to use non-ASCII characters. Such addresses are supported by Google and Microsoft products, and promoted by some government agents. The message header must include at least

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5270-665: The literature as the physical medium ) used to link devices to form a computer network include electrical cable , optical fiber , and free space. In the OSI model , the software to handle the media is defined at layers 1 and 2 — the physical layer and the data link layer. A widely adopted family that uses copper and fiber media in local area network (LAN) technology are collectively known as Ethernet. The media and protocol standards that enable communication between networked devices over Ethernet are defined by IEEE 802.3 . Wireless LAN standards use radio waves , others use infrared signals as

5355-515: The mail server. Messaging Application Programming Interface (MAPI) is used by Microsoft Outlook to communicate to Microsoft Exchange Server —and to a range of other email server products such as Axigen Mail Server , Kerio Connect , Scalix , Zimbra , HP OpenMail , IBM Lotus Notes , Zarafa , and Bynari where vendors have added MAPI support to allow their products to be accessed directly via Outlook. Email has been widely accepted by businesses, governments and non-governmental organizations in

5440-665: The means that allow mapping of a fully connected IP overlay network to its underlying network. Another example of an overlay network is a distributed hash table , which maps keys to nodes in the network. In this case, the underlying network is an IP network, and the overlay network is a table (actually a map ) indexed by keys. Overlay networks have also been proposed as a way to improve Internet routing, such as through quality of service guarantees achieve higher-quality streaming media . Previous proposals such as IntServ , DiffServ , and IP multicast have not seen wide acceptance largely because they require modification of all routers in

5525-402: The medium for users who are out of their homes. While in the earliest years of email, users could only access email on desktop computers, in the 2010s, it is possible for users to check their email when they are away from home, whether they are across town or across the world. Alerts can also be sent to the smartphone or other devices to notify them immediately of new messages. This has given email

5610-510: The message in a non-Latin alphabet language appears in non-readable form (the only exception is a coincidence if the sender and receiver use the same encoding scheme). Therefore, for international character sets , Unicode is growing in popularity. Most modern graphic email clients allow the use of either plain text or HTML for the message body at the option of the user. HTML email messages often include an automatic-generated plain text copy for compatibility. Advantages of HTML include

5695-426: The message, as unstructured text, sometimes containing a signature block at the end. The header is separated from the body by a blank line. RFC 5322 specifies the syntax of the email header. Each email message has a header (the "header section" of the message, according to the specification), comprising a number of fields ("header fields"). Each field has a name ("field name" or "header field name"), followed by

5780-415: The most well-known member of the IEEE 802 protocol family for home users today. IEEE 802.11 shares many properties with wired Ethernet. Synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers. They were originally designed to transport circuit mode communications from

5865-412: The network needs to deliver the user data, for example, source and destination network addresses , error detection codes, and sequencing information. Typically, control information is found in packet headers and trailers , with payload data in between. With packets, the bandwidth of the transmission medium can be better shared among users than if the network were circuit switched . When one user

5950-420: The network topology. As an example, with FDDI , the network topology is a ring, but the physical topology is often a star, because all neighboring connections can be routed via a central physical location. Physical layout is not completely irrelevant, however, as common ducting and equipment locations can represent single points of failure due to issues like fires, power failures and flooding. An overlay network

6035-729: The network. On the other hand, an overlay network can be incrementally deployed on end-hosts running the overlay protocol software, without cooperation from Internet service providers . The overlay network has no control over how packets are routed in the underlying network between two overlay nodes, but it can control, for example, the sequence of overlay nodes that a message traverses before it reaches its destination . For example, Akamai Technologies manages an overlay network that provides reliable, efficient content delivery (a kind of multicast ). Academic research includes end system multicast, resilient routing and quality of service studies, among others. The transmission media (often referred to in

6120-486: The nodes by communication protocols such as the Internet Protocol . Computer networks may be classified by many criteria, including the transmission medium used to carry signals, bandwidth , communications protocols to organize network traffic , the network size, the topology, traffic control mechanisms, and organizational intent. Computer networks support many applications and services , such as access to

6205-515: The physical layer of the OSI model but still require a small amount of time to regenerate the signal. This can cause a propagation delay that affects network performance and may affect proper function. As a result, many network architectures limit the number of repeaters used in a network, e.g., the Ethernet 5-4-3 rule . An Ethernet repeater with multiple ports is known as an Ethernet hub . In addition to reconditioning and distributing network signals,

6290-456: The protocol suite defines the addressing, identification, and routing specifications for Internet Protocol Version 4 (IPv4) and for IPv6 , the next generation of the protocol with a much enlarged addressing capability. The Internet protocol suite is the defining set of protocols for the Internet. IEEE 802 is a family of IEEE standards dealing with local area networks and metropolitan area networks. The complete IEEE 802 protocol suite provides

6375-450: The protocol. Many current email users do not run MTA, MDA or MUA programs themselves, but use a web-based email platform, such as Gmail or Yahoo! Mail , that performs the same tasks. Such webmail interfaces allow users to access their mail with any standard web browser , from any computer, rather than relying on a local email client. Upon reception of email messages, email client applications save messages in operating system files in

6460-454: The recipient. In addition to this example, alternatives and complications exist in the email system: Many MTAs used to accept messages for any recipient on the Internet and do their best to deliver them. Such MTAs are called open mail relays . This was very important in the early days of the Internet when network connections were unreliable. However, this mechanism proved to be exploitable by originators of unsolicited bulk email and as

6545-705: The separator character ":", and a value ("field body" or "header field body"). Each field name begins in the first character of a new line in the header section, and begins with a non- whitespace printable character . It ends with the separator character ":". The separator is followed by the field value (the "field body"). The value can continue onto subsequent lines if those lines have space or tab as their first character. Field names and, without SMTPUTF8 , field bodies are restricted to 7-bit ASCII characters. Some non-ASCII values may be represented using MIME encoded words . Email header fields can be multi-line, with each line recommended to be no more than 78 characters, although

6630-515: The sharing of files and information, giving authorized users access to data stored on other computers. Distributed computing leverages resources from multiple computers across a network to perform tasks collaboratively. Most modern computer networks use protocols based on packet-mode transmission. A network packet is a formatted unit of data carried by a packet-switched network . Packets consist of two types of data: control information and user data (payload). The control information provides data

6715-493: The size of packets to a certain maximum transmission unit (MTU). A longer message may be fragmented before it is transferred and once the packets arrive, they are reassembled to construct the original message. The physical or geographic locations of network nodes and links generally have relatively little effect on a network, but the topology of interconnections of a network can significantly affect its throughput and reliability. With many technologies, such as bus or star networks,

6800-506: The standard for Internet email for decades. Published in 1982, RFC 822 was based on the earlier RFC 733 for the ARPANET. Internet email messages consist of two sections, "header" and "body". These are known as "content". The header is structured into fields such as From, To, CC, Subject, Date, and other information about the email. In the process of transporting email messages between systems, SMTP communicates delivery parameters and information using message header fields. The body contains

6885-434: The web or check their Facebook accounts, making email the most popular activity for users to do on their smartphones. 78% of the respondents in the study revealed that they check their email on their phone. It was also found that 30% of consumers use only their smartphone to check their email, and 91% were likely to check their email at least once per day on their smartphone. However, the percentage of consumers using email on

6970-428: Was extended by MIME to carry text in expanded character sets and multimedia content such as images. International email , with internationalized email addresses using UTF-8 , is standardized but not widely adopted. The term electronic mail has been in use with its modern meaning since 1975, and variations of the shorter E-mail have been in use since 1979: The service is often simply referred to as mail , and

7055-462: Was influenced by a wide array of technological developments and historical milestones. Computer networks enhance how users communicate with each other by using various electronic methods like email, instant messaging, online chat, voice and video calls, and video conferencing. Networks also enable the sharing of computing resources. For example, a user can print a document on a shared printer or use shared storage devices. Additionally, networks allow for

7140-551: Was like the VCR , vinyl records and film cameras —no longer cool and something older people do. A 2015 survey of Android users showed that persons 13 to 24 used messaging apps 3.5 times as much as those over 45, and were far less likely to use email. Email messages may have one or more attachments, which are additional files that are appended to the email. Typical attachments include Microsoft Word documents, PDF documents, and scanned images of paper documents. In principle, there

7225-628: Was originally written in Pascal and C , roughly 300,000 lines of code in size. A more modern, Java -based follow-on to KMS (called "Expeditee") is being developed at the Computer Science Department of the University of Waikato in New Zealand, by Rob Akscyn, one of the original developers of KMS. Electronic mail Email (short for electronic mail ; alternatively spelled e-mail )

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