Electronic mail is a method of exchanging messages between people using electronic devices. Invented by Ray Tomlinson, email first entered limited use in the 1960s and by the mid-1970s had taken the form now recognized as email. Email operates across computer networks, which today is the Internet; some early email systems required the author and the recipient to both be online at the same time, in common with instant messaging. Today's email systems are based on a store-and-forward model. Email servers accept, forward and store messages. Neither the users nor their computers are required to be online simultaneously. An ASCII text-only communications medium, Internet email was extended by Multipurpose Internet Mail Extensions to carry text in other character sets and multimedia content attachments. International email, with internationalized email addresses using UTF-8, has been standardized, but as of 2017 it has not been adopted; the history of modern Internet email services reaches back to the early ARPANET, with standards for encoding email messages published as early as 1973.
An email message sent in the early 1970s looks similar to a basic email sent today. Email had an important role in creating the Internet, the conversion from ARPANET to the Internet in the early 1980s produced the core of the current services; the term electronic mail was used generically for any electronic document transmission. For example, several writers in the early 1970s used the term to describe fax document transmission; as a result, it is difficult to find the first citation for the use of the term with the more specific meaning it has today. Electronic mail has been most called email or e-mail since around 1993, but variations of the spelling have been used: email is the most common form used online, is required by IETF Requests for Comments and working groups and by style guides; this spelling appears in most dictionaries. E-mail is the format that sometimes appears in edited, published American English and British English writing as reflected in the Corpus of Contemporary American English data, but is falling out of favor in some style guides.
Mail was the form used in the original protocol standard, RFC 524. The service is referred to as mail, a single piece of electronic mail is called a message. EMail is a traditional form, used in RFCs for the "Author's Address" and is expressly required "for historical reasons". E-mail is sometimes used, capitalizing the initial E as in similar abbreviations like E-piano, E-guitar, A-bomb, H-bomb. An Internet e-mail consists of an content. Computer-based mail and messaging became possible with the advent of time-sharing computers in the early 1960s, informal methods of using shared files to pass messages were soon expanded into the first mail systems. Most developers of early mainframes and minicomputers developed similar, but incompatible, mail applications. Over time, a complex web of gateways and routing systems linked many of them. Many US universities were part of the ARPANET, which aimed at software portability between its systems; that portability helped make the Simple Mail Transfer Protocol influential.
For a time in the late 1980s and early 1990s, it seemed that either a proprietary commercial system or the X.400 email system, part of the Government Open Systems Interconnection Profile, would predominate. However, once 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; the diagram to the right shows a typical sequence of events that takes place when sender Alice transmits a message using a mail user agent addressed to the email address of the recipient. The MUA formats the message in email format and uses the submission protocol, a profile of the Simple Mail Transfer Protocol, to send the message content to the local mail submission agent, in this case smtp.a.org. The MSA determines the destination address provided in the SMTP protocol, in this case firstname.lastname@example.org, a qualified domain address. The part before the @ sign is the local part of the address the username of the recipient, the part after the @ sign is a domain name.
The MSA resolves a domain name to determine the qualified domain name of the mail server in the Domain Name System. The DNS server for the domain b.org responds with any MX records listing the mail exchange servers for that domain, in this case mx.b.org, a message transfer agent server run by the recipient's ISP. smtp.a.org sends the message to mx.b.org using SMTP. This server may need to forward the message to other MTAs before the message reaches the final message delivery agent; the MDA delivers it to the mailbox of user bob. Bob's MUA picks up the message using either the Post Office Protocol or the Internet Message Access Protocol. In addition to this example and complications exist in the email system: Alice or Bob may use a client connected to a corporate email system, such as IBM Lotus Notes or Microsoft Exchange; these systems have their own internal email format and their clients communicate with the email server using a vendor-specific, proprietary protocol. The server sends or receives email via the Internet through the product's Internet mail gateway which does any necessary reformatt
Spamming is the use of messaging systems to send an unsolicited message advertising, as well as sending messages on the same site. While the most recognized form of spam is email spam, the term is applied to similar abuses in other media: instant messaging spam, Usenet newsgroup spam, Web search engine spam, spam in blogs, wiki spam, online classified ads spam, mobile phone messaging spam, Internet forum spam, junk fax transmissions, social spam, spam mobile apps, television advertising and file sharing spam, it is named after Spam, a luncheon meat, by way of a Monty Python sketch about a restaurant that has Spam in every dish and where patrons annoyingly chant "Spam!" over and over again. Spamming remains economically viable because advertisers have no operating costs beyond the management of their mailing lists, infrastructures, IP ranges, domain names, it is difficult to hold senders accountable for their mass mailings; the costs, such as lost productivity and fraud, are borne by the public and by Internet service providers, which have been forced to add extra capacity to cope with the volume.
Spamming has been the subject of legislation in many jurisdictions. A person who creates spam is called a spammer; the term spam is derived from the 1970 Spam sketch of the BBC television comedy series Monty Python's Flying Circus. The sketch, set in a cafe, has a waitress reading out a menu where every item includes Spam canned luncheon meat; as the waitress recites the Spam-filled menu, a chorus of Viking patrons drowns out all conversations with a song, repeating "Spam, Spam, Spam… Spammity Spam! Wonderful Spam!". The excessive amount of Spam mentioned, references the preponderance of it and other imported canned meat products in the United Kingdom after World War II, as the country struggled to rebuild its agricultural base. In the 1980s the term was adopted to describe certain abusive users who frequented BBSs and MUDs, who would repeat "Spam" a huge number of times to scroll other users' text off the screen. In early chat rooms services like PeopleLink and the early days of Online America, they flooded the screen with quotes from the Monty Python Spam sketch.
This was used as a tactic by insiders of a group that wanted to drive newcomers out of the room so the usual conversation could continue. It was used to prevent members of rival groups from chatting—for instance, Star Wars fans invaded Star Trek chat rooms, filling the space with blocks of text until the Star Trek fans left, it came to be used on Usenet to mean excessive multiple posting—the repeated posting of the same message. The unwanted message would appear in many, if not all newsgroups, just as Spam appeared in all the menu items in the Monty Python sketch; the first usage of this sense was by Joel Furr This use had become established—to spam Usenet was flooding newsgroups with junk messages. The word was attributed to the flood of "Make Money Fast" messages that clogged many newsgroups during the 1990s. In 1998, the New Oxford Dictionary of English, which had only defined "spam" in relation to the trademarked food product, added a second definition to its entry for "spam": "Irrelevant or inappropriate messages sent on the Internet to a large number of newsgroups or users."
There was an effort to differentiate between types of newsgroup spam. Messages that were crossposted to too many newsgroups at once – as opposed to those that were posted too – were called velveeta, but this term didn't persist. In the late 19th Century Western Union allowed telegraphic messages on its network to be sent to multiple destinations; the first recorded instance of a mass unsolicited commercial telegram is from May 1864, when some British politicians received an unsolicited telegram advertising a dentist. The earliest documented spam was a message advertising the availability of a new model of Digital Equipment Corporation computers sent by Gary Thuerk to 393 recipients on ARPANET in 1978. Rather than send a separate message to each person, the standard practice at the time, he had an assistant, Carl Gartley, write a single mass email. Reaction from the net community was fiercely negative. Spamming had been practiced as a prank by participants in multi-user dungeon games, to fill their rivals' accounts with unwanted electronic junk.
The first major commercial spam incident started on March 5, 1994, when a husband and wife team of lawyers, Laurence Canter and Martha Siegel, began using bulk Usenet posting to advertise immigration law services. The incident was termed the "Green Card spam", after the subject line of the postings. Defiant in the face of widespread condemnation, the attorneys claimed their detractors were hypocrites or "zealouts", claimed they had a free speech right to send unwanted commercial messages, labeled their opponents "anti-commerce radicals"; the couple wrote a controversial book entitled How to Make a Fortune on the Information Superhighway. An early example of nonprofit fundraising bulk posting via Usenet occurred in 1994 on behalf of CitiHope, an NGO attempting to raise funds to rescue children at risk during the Bosnian War. However, as it was a violation of their terms of service, the ISP Panix deleted all of the bulk posts from Usenet, only missing three copies. Within a few years, the focus of spamming moved chiefly to email.
By 1999, Khan C. Smith, a well known hacker at the time, had begun to commercialize the bulk email industry and rallied thousands into the business by building more friendly bulk email software and providing internet access illegally hacked from major ISPs suc
John R. Levine
John R. Levine is an Internet author and consultant specializing in email infrastructure, spam filtering, software patents, he chaired the Anti-Spam Research Group of the Internet Research Task Force, is president of CAUCE, is a member of the ICANN Stability and Security Advisory Committee, runs Taughannock Networks. He has co-authored many books, including The Internet For Dummies, UNIX For Dummies, Fighting Spam for Dummies, flex & bison, he was the mayor of the village of Trumansburg, New York, United States from March 2004 until March 2007. Levine graduated from Yale University in 1975 and earned his Ph. D. in Computer Science from Yale in 1984 with a thesis about the design and implementation of small databases. His roommate at Yale was economist Paul Krugman. Levine was a co-founder and board member of Segue Software and Senior Programmer at Javelin Software, he was a member of the R. E. S. I. S. T. O. R. S. One of the first computer clubs in the United States. Levine is the only moderator of the comp.compilers usenet group for 32 years.
Home page Weblog Biography at O'Reilly media R. E. S. I. S. T. O. R. S
A domain name is a label that identifies a network domain: a distinct group of computers under a central administration or authority. Within the Internet, domain names are formed by the procedures of the Domain Name System. Any name registered in the DNS is a domain name. Domain names are used in various networking contexts and for application-specific naming and addressing purposes. In general, a domain name represents an Internet Protocol resource, such as a personal computer used to access the Internet, a server computer hosting a web site, or the web site itself or any other service communicated via the Internet. In 2017, 330.6 million domain names had been registered. Domain names are organized in subordinate levels of the DNS root domain, nameless; the first-level set of domain names are the top-level domains, including the generic top-level domains, such as the prominent domains com, net and org, the country code top-level domains. Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names that are open for reservation by end-users who wish to connect local area networks to the Internet, create other publicly accessible Internet resources or run web sites.
The registration of these domain names is administered by domain name registrars who sell their services to the public. A qualified domain name is a domain name, specified with all labels in the hierarchy of the DNS, having no parts omitted. Labels in the Domain Name System are case-insensitive, may therefore be written in any desired capitalization method, but most domain names are written in lowercase in technical contexts. Domain names serve to identify Internet resources, such as computers and services, with a text-based label, easier to memorize than the numerical addresses used in the Internet protocols. A domain name may represent entire collections of individual instances. Individual Internet host computers use domain names as host identifiers called host names; the term host name is used for the leaf labels in the domain name system without further subordinate domain name space. Host names appear as a component in Uniform Resource Locators for Internet resources such as web sites. Domain names are used as simple identification labels to indicate ownership or control of a resource.
Such examples are the realm identifiers used in the Session Initiation Protocol, the Domain Keys used to verify DNS domains in e-mail systems, in many other Uniform Resource Identifiers. An important function of domain names is to provide recognizable and memorizable names to numerically addressed Internet resources; this abstraction allows any resource to be moved to a different physical location in the address topology of the network, globally or locally in an intranet. Such a move requires changing the IP address of a resource and the corresponding translation of this IP address to and from its domain name. Domain names are used to establish a unique identity. Organizations can choose a domain name that corresponds to their name, helping Internet users to reach them easily. A generic domain is a name that defines a general category, rather than a specific or personal instance, for example, the name of an industry, rather than a company name; some examples of generic names are books.com, music.com, travel.info.
Companies have created brands based on generic names, such generic domain names may be valuable Domain names are simply referred to as domains and domain name registrants are referred to as domain owners, although domain name registration with a registrar does not confer any legal ownership of the domain name, only an exclusive right of use for a particular duration of time. The use of domain names in commerce may subject them to trademark law; the practice of using a simple memorable abstraction of a host's numerical address on a computer network dates back to the ARPANET era, before the advent of today's commercial Internet. In the early network, each computer on the network retrieved the hosts file from a computer at SRI, which mapped computer host names to numerical addresses; the rapid growth of the network made it impossible to maintain a centrally organized hostname registry and in 1983 the Domain Name System was introduced on the ARPANET and published by the Internet Engineering Task Force as RFC 882 and RFC 883.
Today, the Internet Corporation for Assigned Names and Numbers manages the top-level development and architecture of the Internet domain name space. It authorizes domain name registrars, through which domain names may be reassigned; the domain name space consists of a tree of domain names. Each node in the tree holds information associated with the domain name; the tree sub-divides into zones beginning at the DNS root zone. A domain name consists of one or more parts, technically called labels, that are conventionally concatenated, delimited by dots, such as example.com. The right-most label conveys the top-level domain; the hierarchy of domains descends from the right to the left label in the name. For example: the label example specifies a node example.com as a subdomain of the com domain, www is a label to create www.example.com, a subdomain of example.com. Each label may contain from 1 to 63 octets; the empty label is reserved for the root node and when qualified is expressed as the empty label terminated by a dot.
The full domain name may not e
High-availability clusters are groups of computers that support server applications that can be reliably utilized with a minimum amount of down-time. They operate by using high availability software to harness redundant computers in groups or clusters that provide continued service when system components fail. Without clustering, if a server running a particular application crashes, the application will be unavailable until the crashed server is fixed. HA clustering remedies this situation by detecting hardware/software faults, restarting the application on another system without requiring administrative intervention, a process known as failover; as part of this process, clustering software may configure the node before starting the application on it. For example, appropriate file systems may need to be imported and mounted, network hardware may have to be configured, some supporting applications may need to be running as well. HA clusters are used for critical databases, file sharing on a network, business applications, customer services such as electronic commerce websites.
HA cluster implementations attempt to build redundancy into a cluster to eliminate single points of failure, including multiple network connections and data storage, redundantly connected via storage area networks. HA clusters use a heartbeat private network connection, used to monitor the health and status of each node in the cluster. One subtle but serious condition all clustering software must be able to handle is split-brain, which occurs when all of the private links go down but the cluster nodes are still running. If that happens, each node in the cluster may mistakenly decide that every other node has gone down and attempt to start services that other nodes are still running. Having duplicate instances of services may cause data corruption on the shared storage. HA clusters also use quorum witness storage to avoid this scenario. A witness device cannot be shared between two halves of a split cluster, so in the event that all cluster members cannot communicate with each other, if a member cannot access the witness, it cannot become active.
Not every application can run in a high-availability cluster environment, the necessary design decisions need to be made early in the software design phase. In order to run in a high-availability cluster environment, an application must satisfy at least the following technical requirements, the last two of which are critical to its reliable function in a cluster, are the most difficult to satisfy fully: There must be a easy way to start, force-stop, check the status of the application. In practical terms, this means the application must have a command line interface or scripts to control the application, including support for multiple instances of the application; the application must be able to use shared storage. Most the application must store as much of its state on non-volatile shared storage as possible. Important is the ability to restart on another node at the last state before failure using the saved state from the shared storage; the application must not corrupt data if it restarts from the saved state.
A number of these constraints can be minimized through the use of virtual server environments, wherein the hypervisor itself is cluster-aware and provides seamless migration of virtual machines between physical hosts -- see Microsoft Server 2012 and 2016 Failover Clusters. A key difference between this approach and running cluster-aware applications is that the latter can deal with server application crashes and support live "rolling" software upgrades while maintaining client access to the service, by having one instance provide service while another is being upgraded or repaired; this requires the cluster instances to communicate, flush caches and coordinate file access during hand-off. The most common size for an HA cluster is a two-node cluster, since, the minimum required to provide redundancy, but many clusters consist of many more, sometimes dozens of nodes; the attached diagram is a good overview of a classic HA cluster, with the caveat that it does not make any mention of quorum/witness functionality.
Such configurations can sometimes be categorized into one of the following models: Active/active — Traffic intended for the failed node is either passed onto an existing node or load balanced across the remaining nodes. This is only possible when the nodes use a homogeneous software configuration. Active/passive — Provides a redundant instance of each node, only brought online when its associated primary node fails; this configuration requires the most extra hardware. N+1 — Provides a single extra node, brought online to take over the role of the node that has failed. In the case of heterogeneous software configuration on each primary node, the extra node must be universally capable of assuming any of the roles of the primary nodes it is responsible for; this refers to clusters that have multiple services running simultaneously. N+M — In cases where a single cluster is managing many services, having only one dedicated failover node might not offer sufficient redundancy. In such cases, more than one standby servers are available.
The number of standby servers is a tradeoff between reliability requirements. N-to-1 — Allows the failover standby node to become the active one temporarily, until the original node can be restored or brought back online, at which point the services or instances must be failed-back to it