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Internet applications unit1 Internet applications unit1 Document Transcript

  • INTERNET APPLICATIONS Internet applications (IA) are web applications that have the features and functionality of traditional desktop applications. IAs typically transfer the processing necessary for the user interface to the web client but keep the bulk of the data (i.e maintaining the state of the program, the data etc) back on the application server. IAS TYPICALLY: run in a web browser, or do not require software installation run locally in a secure environment called a sandbox can be "occasionally connected" wandering in and out of hot-spots or from office to office. HISTORY OF IAS The term " Internet application" was introduced in a Macromedia whitepaper in March 2002, though the concept had been around for a number of years before that under different names such as: Remote Scripting, by Microsoft, circa 1998 X Internet, by Forrester Research in October 2000 (Web) clients web application Comparison to standard web applications Traditional web applications centered all activity around a client-server architecture with a thin client. Under this system all processing is done on the server, and the client is only used to display static (in this case HTML) content. The biggest drawback with this system is that all interaction with the application must pass through the server, which requires data to be sent to the server, the server to respond, and the page to be reloaded on the client with the response. By using a client side technology which can execute instructions on the client's computer, IAs can circumvent this slow and synchronous loop for many user interactions. This difference is somewhat analogous to the difference between "terminal and mainframe" and Client-server/Fat client approaches. Internet standards have evolved slowly and continually over time to accommodate these techniques, so it is hard to draw a strict line between what constitutes an IA and what
  • does not. But all IAs share one characteristic: they introduce an intermediate layer of code, often called a client engine, between the user and the server. This client engine is usually downloaded at the beginning of the application, and may be supplemented by further code downloads as the application progresses. The client engine acts as an extension of the browser, and usually takes over responsibility for rendering the application's user interface and for server communication. What can be done in an IA may be limited by the capabilities of the system used on the client. But in general, the client engine is programmed to perform application functions that its designer believes will enhance some aspect of the user interface, or improve its responsiveness when handling certain user interactions, compared to a standard Web browser implementation. Also, while simply adding a client engine does not force an application to depart from the normal synchronous pattern of interactions between browser and server, in most IAs the client engine performs additional asynchronous communications with servers. BENEFITS Because IAs employ a client engine to interact with the user, they are: er. They can offer user-interface behaviors not obtainable using only the HTML widgets available to standard browser-based Web applications. This er functionality may include anything that can be implemented in the technology being used on the client side, including drag and drop, using a slider to change data, calculations performed only by the client and which do not need to be sent back to the server (e.g. an insurance rate calculator), etc. More responsive. The interface behaviors are typically much more responsive than those of a standard Web browser that must always interact with the server. The most sophisticated examples of IAs exhibit a look and feel approaching that of a desktop environment. Using a client engine can also produce other performance benefits: Client/Server balance. The demand for client and server computing resources is better balanced, so that the Web server need not be the workhorse that it is with a traditional Web application. This frees server resources, allowing the same server hardware to handle more client sessions concurrently.
  • Asynchronous communication. The client engine can interact with the server asynchronously -- that is, without waiting for the user to perform an interface action like clicking on a button or link. This option allows IA designers to move data between the client and the server without making the user wait. Perhaps the most common application of this is prefetching, in which an application anticipates a future need for certain data, and downloads it to the client before the user requests it, thereby speeding up a subsequent response. Google Maps uses this technique to move adjacent map segments to the client before the user scrolls their view. Network efficiency. The network traffic may also be significantly reduced because an application-specific client engine can be more intelligent than a standard Web browser when deciding what data needs to be exchanged with servers. This can speed up individual requests or responses because less data is being transferred for each interaction, and overall network load is reduced. However, use of asynchronous prefetching techniques can neutralize or even reverse this potential benefit. Because the code cannot anticipate exactly what every user will do next, it is common for such techniques to download extra data, not all of which is actually needed, to many or all clients.
  • SHORTCOMINGS AND RESTRICTIONS Shortcomings and restrictions associated with IAs are: Sandbox. Because IAs run within a sandbox, they have restricted access to system resources. If assumptions about access to resources are incorrect, IAs may fail to operate correctly. Disabled scripting. JavaScript or another scripting language is often required. If the user has disabled active scripting in their browser, the IA may not function properly, if at all. Client processing speed. To achieve platform independence, some IAs use client- side scripts written in interpreted languages such as JavaScript, with a consequential loss of performance. This is not an issue with compiled client languages such as Java, where performance is comparable to that of traditional compiled languages, or with Flash movies, in which the bulk of the operations are performed by the native code of the Flash player. Script download time. Although it does not have to be installed, the additional client-side intelligence (or client engine) of IA applications needs to be delivered by the server to the client. While much of this is usually automatically cached it needs to be transferred at least once. Depending on the size and type of delivery, script download time may be unpleasantly long. IA developers can lessen the impact of this delay by compressing the scripts, and by staging their delivery over multiple pages of an application. Loss of integrity. If the application-base is X/HTML, conflicts arise between the goal of an application (which naturally wants to be in control of its presentation and behaviour) and the goals of X/HTML (which naturally wants to give away control). The DOM interface for X/HTML makes it possible to create IAs, but by doing so makes it impossible to guarantee correct function. Because an IA client can modify the IA's basic structure and override presentation and behaviour, it can cause an irrecoverable client failure or crash. Eventually, this problem could be solved by new client-side mechanisms that granted an IA client more limited permission to modify only those resources within the scope of its application. (Standard software running natively does not have this
  • problem because by definition a program automatically possesses all rights to all its allocated resources). Loss of visibility to search engines. Search engines may not be able to index the text content of the application. MANAGEMENT COMPLICATIONS The advent of IA technologies has introduced considerable additional complexity into Web applications. Traditional Web applications built using only standard HTML, having a relatively simple software architecture and being constructed using a limited set of development options, are relatively easy to design and manage. For the person or organization using IA technology to deliver a Web application, their additional complexity makes them harder to design, test, measure, and support. Use of IA technology poses several new Service Level Management ("SLM") challenges, not all of which are completely solved today. SLM concerns are not always the focus of application developers, and are rarely if ever perceived by application users, but they are vital to the successful delivery of an online application. Aspects of the IA architecture that complicate management processes[1] are: Greater complexity makes development harder. The ability to move code to the client gives application designers and developers far more creative freedom. But this in turn makes development harder, increases the likelihood of defects (bugs) being introduced, and complicates software testing activities. These complications lengthen the software development process, regardless of the particular methodology or process being employed. Some of these issues may be mitigated through the use of a Web application framework to standardize aspects of IA design and development. However, increasing complexity in a software solution can complicate and lengthen the testing process, if it increases the number of use cases to be tested. Incomplete testing lowers the application's quality and its reliability during use. IA architecture breaks the Web page paradigm. Traditional Web applications can be viewed as a series of Web pages, each of which requires a distinct download, initiated by an HTTP GET request. This model has been characterized as the Web page paradigm. IAs invalidate this model, introducing additional asynchronous server communications to
  • support a more responsive user interface. In IAs, the time to complete a page download may no longer correspond to something a user perceives as important, because (for example) the client engine may be prefetching some of the downloaded content for future use. New measurement techniques must be devised for IAs, to permit reporting of response time quantities that reflect the user's experience. In the absence of standard tools that do this, IA developers must instrument their application code to produce the measurement data needed for SLM. Asynchronous communication makes it harder to isolate performance problems. Paradoxically, actions taken to enhance application responsiveness also make it harder to measure, understand, report on, and manage responsiveness. Some IAs do not issue any further HTTP GET requests from the browser after their first page, using asynchronous requests from the client engine to initiate all subsequent downloads. The IA client engine may be programmed to continually download new content and refresh the display, or (in applications using the Comet approach) a server-side engine can keep pushing new content to the browser over a connection that never closes. In these cases, the concept of a "page download" is no longer applicable. These complications make it harder to measure and subdivide application response times, a fundamental requirement for problem isolation and service level management. Tools designed to measure traditional Web applications may -- depending on the details of the application and the tool -- report such applications either as a single Web page per HTTP request, or as an unrelated collection of server activities. Neither conclusion reflects what is really happening at the application level. The client engine makes it harder to measure response time. For traditional Web applications, measurement software can reside either on the client machine or on a machine that is close to the server, provided that it can observe the flow of network traffic at the TCP and HTTP levels. Because these protocols are synchronous and predictable, a packet sniffer can read and interpret packet-level data, and infer the user’s experience of response time by tracking HTTP messages and the times of underlying TCP packets and acknowledgments. But the IA architecture reduces the power of the packet sniffing approach, because the client engine breaks the communication between user and server into two separate cycles operating asynchronously -- a foreground (user-
  • to-engine) cycle, and a background (engine-to-server) cycle. Both cycles are important, because neither stands alone; it is their relationship that defines application behavior. But that relationship depends only on the application design, which cannot (in general) be inferred by a measurement tool, especially one that can observe only one of the two cycles. Therefore the most complete IA measurements can only be obtained using tools that reside on the client and observe both cycles. THE CURRENT STATUS OF IA DEVELOPMENT AND ADOPTION IAs are still in the early stages of development and user adoption. There are a number of restrictions and requirements that remain: Browser adoption: Many IAs require modern web browsers in order to run. Advanced JavaScript engines must be present in the browser as IAs use techniques such as XMLHTTPRequest for client-server communication, and DOM Scripting and advanced CSS techniques to enable the user interface. Web standards: Differences between web browsers can make it difficult to write an IA that will run across all major browsers. The consistency of the Java platform, particularly after Java 1.1, makes this task much simpler for IAs written as Java applets. Development tools: Some Ajax Frameworks and products like Adobe Flex provide an integrated environment in which to build IA and B2B web applications. Accessibility concerns: Additional interactivity may require technical approaches that limit applications' accessibility. User adoption: Users expecting standard web applications may find that some accepted browser functionality (such as the "Back" button) may have somewhat different or even undesired behaviour.
  • JUSTIFICATIONS Although developing applications to run in a web browser is a much more limiting, difficult, and intricate a process than developing a regular desktop application, the efforts are often justified because: installation is not required -- updating and distributing the application is an instant, automatically handled process users can use the application from any computer with an internet connection, and usually regardless of what operating system that computer is running web-based applications are generally less prone to viral infection than running an actual executable as web usage increases, computer users are becoming less willing to go to the trouble of installing new software if a browser-based alternative is available This last point is often true even if this alternative is slower or not as feature-. A good example of this phenomenon is webmail. METHODS AND TECHNIQUES JAVASCRIPT The first major client side language and technology available with the ability to run code and installed on a majority of web clients was JavaScript. Although its uses were relatively limited at first, combined with layers and other developments in DHTML it has become possible to piece together an IA system without the use of a unified client- side solution. Ajax is a new term coined to refer to this combination of techniques and has recently been used most prominently by Google for projects such as Gmail and Google Maps. However, creating a large application in this framework is very difficult, as many different technologies must interact to make it work, and browser compatibility requires a lot of effort. In order to make the process easier, several AJAX Frameworks have been developed. The "" in " Internet applications" may also suffer from an all-JavaScript approach, because you are still bound by the media types predictably supported by the world's various deployed browsers -- video will display in different ways in different browsers
  • with an all-JavaScript approach, audio support will be unpredictable, realtime communications, whiteboarding, outbound webcams, opacity compositing, socket support, all of these are implemented in different ways in different browsers, so all- JavaScript approaches tend to cluster their "ness" around text refreshes and image refreshes. ADOBE FLASH AND APOLLO Adobe Flash is another way to build Internet Application. This technology is cross-platform and quite powerful to create application UI. Adobe Flex provides the possibility to create Flash user interface by compiling MXML, a XML based interface description language. Adobe is currently working on providing a more powerful IA platform with the product Adobe Apollo, a technology combining Flash and PDF. WINDOWS PRESENTATION FOUNDATION With the .NET 3.0 Framework, Microsoft introduced Windows Presentation Foundation (WPF) which provides a way to build single-platform applications with some similarities to IAs using XAML and languages like C# and Visual Basic. In addition, Microsoft has announced Windows Presentation Foundation/Everywhere which may eventually provide a subset of WPF functionality on devices and other platforms. ACTIVEX CONTROLS Embedding ActiveX controls into HTML is a very powerful way to develop Internet applications, however they are only guaranteed to run properly in Internet Explorer. Furthermore, since they can break the sandbox model, they are potential targets for computer viruses and malware making them high security risks. At the time of this writing, the Adobe Flash Player for Internet Explorer is implemented as an ActiveX control for Microsoft environments, as well as in multi-platform Netscape Plugin wrappers for the wider world. Only if corporations have standardized on using Internet
  • Explorer as the primary web browser, is ActiveX per se a good choice for building corporate applications. JAVA APPLETS Java applets run in standard HTML pages and generally start automatically when their web page is opened with a modern web browser. Java applets have access to the screen (inside an area designated in its page's HTML), speakers, keyboard and mouse of any computer their web page is opened on, as well as access to the Internet, and provide a sophisticated environment capable of real time applications. JAVA APPLICATIONS Java based IAs can be launched from within the browser or as free standing applications via Java Web Start. Java IAs can take advantage of the full power of the Java platform to deliver functionality, 2D & 3D graphics, and off-line capabilities, but at the cost of delayed startup. Numerous frameworks for Java IAs exist, including XUL-like XML-based frameworks such as XUI and Swixml. USER INTERFACE LANGUAGES As an alternative to HTML/XHTML new user interface markup languages can be used in IAs. For instance, the Mozilla Foundation's XML-based user interface markup language XUL - this could be used in IAs though it would be restricted to Mozilla-based browsers, since it is not a de facto or de jure standard. The W3Cs Web Clients Activity[2] has initiated a Web Application Formats Working Group whose mission includes the development of such standards [3]. IA's user interfaces can also become er through the use of scriptable SVG (though not all browsers support native SVG rendering yet) as well SMIL.
  • OTHER TECHNIQUES IAs could use XForms to enhance their functionality. Using XML and XSLT along with some XHTML, CSS and JavaScript can also be used to generate er client side UI components like data tables that can be resorted locally on the client without going back to the server. Mozilla and Internet Explorer browsers both support this. The Omnis Web Client is an ActiveX control or Netscape plug-in which can be embedded into an HTML page providing a application interface in the end-user's web browser. IA WITH REAL-TIME PUSH Traditionally, web pages have been delivered to the client only when the client requested for it. For every client request, the browser initiates an HTTP connection to the web server, which then returns the data and the connection is closed. The drawback of this approach was that the page displayed was updated only when the user explicitly refreshes the page or moves to a new page. Since transferring entire pages can take a long time, refreshing pages can introduce a long latency. DEMAND FOR LOCALISED USAGE OF IA With the increasing adoption and improvement in broadband technologies, fewer users experience poor peformance caused by remote latency. Furthermore one of the critical reasons for using an IA is that many developers are looking for a language to serve up desktop applications that is not only desktop OS neutral but also installation and system issue free. IA running in the ubiquitous web browser is a potential candidate even when used standalone or over a LAN, with the required webserver functionalities hosted locally. [edit] Client-side functionalities and development tools for IA needed With client-side functionalities like Javascript and DHTML, IA can operate on top of a range of OS and webserver functionalities.
  • DIRECTORY SERVICE A directory service is a software application — or a set of applications — that stores and organizes information about a computer network's users and network resources, and that allows network administrators to manage users' access to the resources. Additionally, directory services act as an abstraction layer between users and shared resources. A directory service should not be confused with the directory repository itself; which is the database that holds information about named objects that are managed in the directory service. In the case of the X.500 distributed directory services model, one or more namespaces (trees of objects) are used to form the directory service. The directory service provides the access interface to the data that is contained in one or more directory namespaces. The directory service interface acts as a central/common authority that can securely authenticate the system resources that manage the directory data. Like a database, a directory service is highly optimized for reads and provides advanced search on the many different attributes that can be associated with objects in a directory. The data that is stored in the directory is defined by an extendible and modifiable schema. Directory services use a distributed model for storing their information and that information is usually replicated between directory servers. [1] INTRODUCTION A simple directory service called a naming service maps the names of network resources to their respective network addresses. With the name service type of directory, a user doesn't have to remember the physical address of a network resource; providing a name will locate the resource. Each resource on the network is considered an object on the directory server. Information about a particular resource is stored as attributes of that object. Information within objects can be made secure so that only users with the available permissions are able to access it. More sophisticated directories are designed
  • with namespaces as Subscribers, Services, Devices, Entitlements, Preferences, Content and so on. This design process is highly related to Identity management. A directory service defines the namespace for the network. A namespace in this context is the term that is used to hold one or more objects as named entries. The directory design process normally has a set of rules that determine how network resources are named and identified. The rules specify that the names be unique and unambiguous. In X.500 (the directory service standards) and LDAP the name is called the distinguished name (DN) and is used to refer to a collection of attributes (relative distinguished names) which make up the name of a directory entry. A directory service is a shared information infrastructure for locating, managing, administrating, and organizing common items and network resources, which can include volumes, folders, files, printers, users, groups, devices, telephone numbers and other objects. A directory service is an important component of a NOS (Network Operating System). In the more complex cases a directory service is the central information repository for a Service Delivery Platform. For example, looking up "computers" using a directory service might yield a list of available computers and information for accessing them. Replication and Distribution have very distinct meanings in the design and management of a directory service. The term replication is used to indicate that the same directory namespace (the same objects) are copied to another directory server for redundancy and throughput reasons. The replicated namespace is governed by the same authority. The term distribution is used to indicate that multiple directory servers, that hold different namespaces, are interconnected to form a distributed directory service. Each distinct namespace can be governed by different authorities. DIRECTORY SERVICES SOFTWARE Directory services produced by different vendors and standards bodies include the following offerings: Windows NT Directory Services (NTDS) for Windows NT Active Directory for Windows 2000, Server 2003
  • Apple Open Directory in Mac OS X Server Novell eDirectory - formerly called Novell Directory Services (NDS) for Novell NetWare version 4.x-5.x OpenLDAP Fedora Directory Server Sun Directory Services COMPARISON WITH RELATIONAL DATABASES There are a number of things that distinguish a traditional directory service from a relational database. Depending on the directory application, the information is generally read more often than it is written. Hence the usual database features of transactions and rollback are not implemented in some directory systems. Data may be made redundant, but the objective is to get a faster response time during searches. Data can be organized in a strictly hierarchical manner which is sometimes seen to be problematic. To overcome the issues of deep namespaces, some directories dismantle the object namespace hierarchy in their storage mechanisms in order to optimize navigation. That is, these directories find the item based on their data attributes and then determine their namespace values as this is faster than navigating large namespaces to find the item. In terms of cardinality, traditional directories do not have many-to-many relations. Instead, such relations must be maintained explicitly using lists of distinguished names or other identifiers (similar to the cross table identifiers used in relational databases). Originally X.500 type directory information hierarchies were considered problematic against relational data designs. Today Java based object-oriented databases are being developed and XML document forms have adopted an hierarchical object model - indicating an evolution from traditional relational data engineering. A schema is defined as object classes, attributes, name bindings and knowledge (namespaces). An objectClass has:
  • Must-attributes that each of its instances must have May-attributes that can be defined for an instance, but could also be omitted when the object is created. The lack of a certain attribute is somewhat like a NULL in relational databases Attributes are sometimes multi-valued in directories allowing multiple naming attributes at one level such as machine type and serial number concatenated or multiple phone numbers for "work phone". Attributes and objectClasses are standardized throughout the industry and formally registered with the IANA for their object ID. Therefore directory applications seek to reuse much of the standard classes and attributes to maximize the benefit of existing directory server software. Object instances are slotted into namespaces. That is, each objectClass inherits from its parent objectClass (and ultimately from the root of the hierarchy) adding attributes to the must/may list. Directory services are often a central component in the security design of an IT system and have a correspondingly fine granularity regarding access control: who may operate in which manner on what information. Also see: ACLs Directory design is quite different from relational database design. With databases one tends to design a data model for the business issues and process requirements, sometimes with the online customer, service, user management, presence and system scale issues omitted. With directories however, if one is placing information into a common repository for many applications and users, then its information (and identity) design and schema must be developed around what the objects are representing in real life. In most cases, these objects represent users, address books, rosters, preferences, entitlements, products and services, devices, profiles, policies, telephone numbers, routing information, etc. In addition one must also consider the operational aspects of design in regard to performance and scale. A quick check on the operational design is to take eg. 1 million users, 50 objects each with users or applications accessing these objects up to 5000 times a second, minute, or hour (to authorize and update their service environments), and check if the server and network machinery considered can support this.
  • The major difference with databases and directories is at the system level where a database is used to automate a process with a dedicated (relational) data model, but a directory is used to hold "identified" objects that can be used by many applications in random ways. A Directory service is applied where "multi governance" (many applications and users) are, for integrity and efficiency reasons, using the same information. This approach to system design gives greater scale and flexibility so that the larger scale functions such as Service Delivery Platforms can be specified correctly. SDPs now need to support 100s of millions of objects (HSS/HLR, address books, user entitlements, VOIP telephone numbers, user and device information, etc) in real time, random ways and be managed from BSS/OSS/CRM type systems as well as the customer self care applications. See Service Delivery Platform Symptomatic of database designs is that the larger companies have hundreds (if not thousands) of them for different processes and are now trying to converge their user and service identity information and their online goods and services management, and deliver these in real time, cost effectively. So a large scale directory service should be in their solution architecture. IMPLEMENTATIONS OF DIRECTORY SERVICES Directory services were part of an Open Systems Interconnection (OSI) initiative to get everyone in the industry to agree to common network standards to provide multi- vendor interoperability. In the 1980s the ITU and ISO came up with a set of standards - X.500, for directory services, initially to support the requirements of inter-carrier electronic messaging and network name lookup. The Lightweight Directory Access Protocol, LDAP, is based on the directory information services of X.500, but uses the TCP/IP stack and a string encoding scheme of the X.509 protocol DAP, giving it more relevance on the Internet. There have been numerous forms of directory service implementations from different vendors. Among them are: NIS: The Network Information Service (NIS) protocol, originally named Yellow Pages (YP), was Sun Microsystems' implementation of a directory service for Unix
  • network environments. (Sun has, in the early 2000s, merged its iPlanet alliance Netscape and developed its LDAP-based directory service to become part of Sun ONE, now called Sun Java Enterprise.) eDirectory: This is Novell's implementation of directory services. It supports multiple architectures including Windows, NetWare, Linux and several flavours of Unix and has long been used for user administration, configuration management, and software management. eDirectory has evolved into a central component in a broader range of Identity management products. It was previously known as Novell Directory Services. Red Hat Directory Server: Red Hat released a directory service, that it acquired from AOL's Netscape Security Solutions unit[1], as a commercial product running on top of Red Hat Enterprise Linux called Red Hat Directory Server and as part of Fedora Core called Fedora Directory Server. Active Directory: Microsoft's directory service is the Active Directory which is included in the Windows 2000 and Windows Server 2003 operating system versions. Open Directory: Apple's Mac OS X Server offers a directory service called Open Directory which integrates with many open standard protocols such as LDAP and Kerberos as well as proprietary directory solutions like Active Directory and eDirectory. Apache Directory Server: Apache Software Foundation offers a directory service called ApacheDS. Oracle Internet Directory: (OID) is Oracle Corporation's directory service, which is compatible with LDAP version 3. Computer Associates Etrust Directory: Sun Java System Directory Server: Sun Microsystems' current directory service offering, found at [2]. OpenDS: A next generation and open source directory service, backed by Sun Microsystems and hosted at [3]. There are also plenty of open-source tools to create directory services, including OpenLDAP and the Kerberos (protocol), and Samba software which can act as a Domain Controller with Kerberos and LDAP backends.
  • NEXT GENERATION DIRECTORY SYSTEMS Databases have been with the IT industry since the dawn of the computer age and traditional directories for the last 20-30 years, and they will be with us in the future. However, with the larger scale, converged services and event driven (presence) systems now being developed world wide (e.g. 3G-IMS), information, identity and presence services engineering and the technologies that support it will require some evolution. This could take the form of CADS (Composite Adaptive Directory Services) and CADS supported Service Delivery Platforms, see www.wwite.com and Service Delivery Platform. CADS is an advanced directory service and contains functions for managing identity, presence, content and adaption algorithms for self-tuning and with its unique functions, greatly simplifies and enhances the design of converged services SDPs. See Service Delivery Platform
  • DOMAIN NAME SYSTEM On the Internet, the domain name system (DNS) stores and associates many types of information with domain names; most importantly, it translates domain names (computer hostnames) to IP addresses. It also lists mail exchange servers accepting e-mail for each domain. In providing a worldwide keyword-based redirection service, DNS is an essential component of contemporary Internet use. Useful for several reasons, the DNS pre-eminently makes it possible to attach easy-to-remember domain names (such as "wikipedia.org") to hard-to-remember IP addresses (such as 66.230.200.100). People take advantage of this when they recite URLs and e-mail addresses. In a subsidiary function, the domain name system makes it possible for people to assign authoritative names without needing to communicate with a central registrar each time. HISTORY OF THE DNS The practice of using a name as a more human-legible abstraction of a machine's numerical address on the network predates even TCP/IP, and goes all the way back to the ARPAnet era. Originally, each computer on the network retrieved a file called HOSTS.TXT from SRI (now SRI International) which mapped an address (such as 192.0.34.166) to a name (such as www.example.net.) The Hosts file still exists on most modern operating systems, either by default or through configuration, and allows users to specify an IP address to use for a hostname without checking the DNS. This file now serves primarily for troubleshooting DNS errors or for mapping local addresses to more organic names. Systems based on a HOSTS.TXT file have inherent limitations, because of the obvious requirement that every time a given computer's address changed, every computer that seeks to communicate with it would need an update to its Hosts file. The growth of networking called for a more scalable system: one that recorded a change in a host's address in one place only. Other hosts would learn about the change dynamically through a notification system, thus completing a globally accessible network of all hosts' names and their associated IP Addresses. Paul Mockapetris invented the DNS in 1983 and wrote the first implementation. The original specifications appear in RFC 882 and 883. In 1987, the publication of RFC 1034 and RFC 1035 updated the DNS
  • specification and made RFC 882 and RFC 883 obsolete. Several more- recent RFCs have proposed various extensions to the core DNS protocols. In 1984, four Berkeley students — Douglas Terry, Mark Painter, David Riggle and Songnian Zhou — wrote the first UNIX implementation, which was maintained by Ralph Campbell thereafter. In 1985, Kevin Dunlap of DEC significantly re-wrote the DNS implementation and renamed it BIND. Mike Karels, Phil Almquist and Paul Vixie have maintained BIND since then. BIND was ported to the Windows NT platform in the early 1990s. Due to its long history of security issues, several alternative nameserver/resolver programs have been written and distributed in recent years. HOW THE DNS WORKS IN THEORY Domain names, arranged in a tree, cut into zones, each served by a nameserver. The domain name space consists of a tree of domain names. Each node or leaf in the tree has one or more resource records, which hold information associated with the domain name. The tree sub-divides into zones. A zone consists of a collection of connected nodes authoritatively served by an authoritative DNS nameserver. (Note that a single nameserver can host several zones.)
  • When a system administrator wants to let another administrator control a part of the domain name space within his or her zone of authority, he or she can delegate control to the other administrator. This splits a part of the old zone off into a new zone, which comes under the authority of the second administrator's nameservers. The old zone becomes no longer authoritative for what comes under the authority of the new zone. A resolver looks up the information associated with nodes. A resolver knows how to communicate with name servers by sending DNS requests, and heeding DNS responses. Resolving usually entails recursing through several name servers to find the needed information. Some resolvers function simplistically and can only communicate with a single name server. These simple resolvers rely on a recursing name server to perform the work of finding information for them. UNDERSTANDING THE PARTS OF A DOMAIN NAME A domain name usually consists of two or more parts (technically labels), separated by dots. For example wikipedia.org. The rightmost label conveys the top-level domain (for example, the address en.wikipedia.org has the top-level domain org). Each label to the left specifies a subdivision or subdomain of the domain above it. Note that "subdomain" expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org comprises a subdomain of the domain wikipedia.org. In theory, this subdivision can go down to 127 levels deep, and each label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 255 characters. But in practice some domain registries have shorter limits than that. A hostname refers to a domain name that has one or more associated IP addresses. For example, the en.wikipedia.org and wikipedia.org domains are both hostnames, but the org domain is not. The DNS consists of a hierarchical set of DNS servers. Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains "beneath" it. The hierarchy of authoritative DNS servers matches the hierarchy of domains. At the top of the hierarchy stand the
  • root servers: the servers to query when looking up (resolving) a top- level domain name (TLD). THE ADDRESS RESOLUTION MECHANISM In theory a full host name may have several name segments, (e.g ahost.ofasubnet.ofabiggernet.inadomain.example). In practice, in the experience of the majority of public users of Internet services, full host names will frequently consist of just three segments (ahost.inadomain.example, and most often www.inadomain.example). For querying purposes, software interprets the name segment by segment, from right to left, using an iterative search procedure. At each step along the way, the program queries a corresponding DNS server to provide a pointer to the next server which it should consult. A DNS recurser consults three nameservers to resolve the address www.wikipedia.org. As originally envisaged, the process was as simple as: the local system is pre-configured with the known addresses of the root servers in a file of root hints, which need to be updated periodically by the local administrator from a reliable source to be kept up to date with the changes which occur over time. Query one of the root servers to find the server authoritative for the next level down (so in the case of our simple hostname, a root server would be asked for the address of a server with detailed knowledge of the example top level domain). Querying this second server for the address of a DNS server with detailed knowledge of the second-level domain (inadomain.example in our example) repeating the previous step to progress down the name, until the final step which would, rather than generating the address of the next DNS server, return the final address sought. The diagram illustrates this process for the real host www.wikipedia.org.
  • The mechanism in this simple form has a difficulty: it places a huge operating burden on the collective of root servers, with each and every search for an address starting by querying one of them. Being as critical as they are to the overall function of the system such heavy use would create an insurmountable bottleneck for trillions of queries placed every day. In practice there are two key additions to the mechanism. Firstly, the DNS resolution process allows for local recording and subsequent consultation of the results of a query (or caching) for a period of time after a successful answer (the server providing the answer initially dictates the period of validity, which may vary from just seconds to days or even weeks). In our illustration, having found a list of addresses of servers capable of answering queries about the .example domain, the local resolver will not need to make the query again until the validity of the currently known list expires, and so on for all subsequent steps. Hence having successfully resolved the address of ahost.inadomain.example it is not necessary to repeat the process for some time since the address already reached will be deemed reliable for a defined period, and resolution of anotherhost.anotherdomain.example can commence with already knowing which servers can answer queries for the .example domain. Caching significantly reduces the rate at which the most critical name servers have to respond to queries, adding the extra benefit that subsequent resolutions are not delayed by network transit times for the queries and responses. Secondly, most domestic and small-business clients "hand off" address resolution to their ISP's DNS servers to perform the look-up process, thus allowing for the greatest benefit from those same ISPs having busy local caches serving a wide variety of queries and a large number of users. CIRCULAR DEPENDENCIES AND GLUE RECORDS Name servers in delegations appear listed by name, rather than by IP address. This means that a resolving name server must issue another
  • DNS request to find out the IP address of the server to which it has been referred. Since this can introduce a circular dependency if the nameserver referred to is under the domain that it is authoritative of, it is occasionally necessary for the nameserver providing the delegation to also provide the IP address of the next nameserver. This record is called a glue record. For example, assume that the sub-domain en.wikipedia.org contains further sub-domains (such as something.en.wikipedia.org) and that the authoritative nameserver for these lives at ns1.en.wikipedia.org. A computer trying to resolve something.en.wikipedia.org will thus first have to resolve ns1.en.wikipedia.org. Since ns1 is also under the en.wikipedia.org subdomain, resolving something.en.wikipedia.org requires resolving ns1.en.wikipedia.org which is exactly the circular dependency mentioned above. The dependency is broken by the glue record in the nameserver of wikipedia.org that provides the IP address of ns1.en.wikipedia.org directly to the requestor, enabling it to bootstrap the process by figuring out where ns1.en.wikipedia.org is located. DNS IN PRACTICE When an application (such as a web browser) tries to find the IP address of a domain name, it doesn't necessarily follow all of the steps outlined in the Theory section above. We will first look at the concept of caching, and then outline the operation of DNS in "the real world." CACHING AND TIME TO LIVE Because of the huge volume of requests generated by a system like the DNS, the designers wished to provide a mechanism to reduce the load on individual DNS servers. The mechanism devised provided that when a DNS resolver (i.e. client) received a DNS response, it would cache that response for a given period of time. A value (set by the administrator of the DNS server handing out the response) called the time to live (TTL), defines that period of time. Once a response goes into cache, the resolver will consult its cached (stored) answer; only when the TTL expires (or when an administrator manually flushes the response from
  • the resolver's memory) will the resolver contact the DNS server for the same information. Generally, the Start of Authority (SOA) record specifies the time to live. The SOA record has the parameters: Serial — the zone serial number, incremented when the zone file is modified, so the slave and secondary name servers know when the zone has been changed and should be reloaded. Refresh — the number of seconds between update requests from secondary and slave name servers. Retry — the number of seconds the secondary or slave will wait before retrying when the last attempt has failed. Expire — the number of seconds a master or slave will wait before considering the data stale if it cannot reach the primary name server. Minimum — previously used to determine the minimum TTL, this offers negative caching. CACHING TIME As a noteworthy consequence of this distributed and caching architecture, changes to the DNS do not always take effect immediately and globally. This is best explained with an example: If an administrator has set a TTL of 6 hours for the host www.wikipedia.org, and then changes the IP address to which www.wikipedia.org resolves at 12:01pm, the administrator must consider that a person who cached a response with the old IP address at 12:00pm will not consult the DNS server again until 6:00pm. The period between 12:01pm and 6:00pm in this example is called caching time, which is best defined as a period of time that begins when you make a change to a DNS record and ends after the maximum amount of time specified by the TTL expires. This essentially leads to an important logistical consideration when making changes to the DNS: not everyone is necessarily seeing the same thing you're seeing. RFC 1537 helps to convey basic rules for how to set the TTL. Note that the term "propagation", although very widely used, does not describe the effects of caching well. Specifically, it implies that [1] when you make a DNS change, it somehow spreads to all other DNS servers (instead, other DNS servers check in with yours as needed), and [2] that you do not have control over the amount of time the record is cached (you control the TTL values for all DNS records in your domain,
  • except your NS records and any authoritative DNS servers that use your domain name). Some resolvers may override TTL values, as the protocol supports caching for up to 68 years or no caching at all. Negative caching (the non-existence of records) is determined by name servers authoritative for a zone which MUST include the SOA record when reporting no data of the requested type exists. The MINIMUM field of the SOA record and the TTL of the SOA itself is used to establish the TTL for the negative answer. RFC 2308 Many people incorrectly refer to a mysterious 48 hour or 72 hour propagation time when you make a DNS change. When one changes the NS records for one's domain or the IP addresses for hostnames of authoritative DNS servers using one's domain (if any), there can be a lengthy period of time before all DNS servers use the new information. This is because those records are handled by the zone parent DNS servers (for example, the .com DNS servers if your domain is example.com), which typically cache those records for 48 hours. However, those DNS changes will be immediately available for any DNS servers that do not have them cached. And, any DNS changes on your domain other than the NS records and authoritative DNS server names can be nearly instantaneous, if you choose for them to be (by lowering the TTL once or twice ahead of time, and waiting until the old TTL expires before making the change). DNS IN THE REAL WORLD
  • DNS resolving from program to OS-resolver to ISP-resolver to greater system. Users generally do not communicate directly with a DNS resolver. Instead DNS resolution takes place transparently in client applications such as web browsers (like Internet Explorer, Opera, Mozilla Firefox, Safari, Netscape Navigator, etc), mail clients (Outlook Express, Mozilla Thunderbird, etc), and other Internet applications. When a request is made which necessitates a DNS lookup, such programs send a resolution request to the local DNS resolver in the operating system which in turn handles the communications required. The DNS resolver will almost invariably have a cache (see above) containing recent lookups. If the cache can provide the answer to the request, the resolver will return the value in the cache to the program that made the request. If the cache does not contain the answer, the resolver will send the request to a designated DNS server or servers. In the case of most home users, the Internet service provider to which the machine connects will usually supply this DNS server: such a user will either configure that server's address manually or allow DHCP to set it; however, where systems administrators have configured systems to use their own DNS servers, their DNS resolvers will generally point to their own nameservers. This name server will then follow the process outlined above in DNS in theory, until it either successfully finds a result, or does not. It then returns its results to the DNS resolver; assuming it has found a result, the resolver duly caches that result for future use, and hands the result back to the software which initiated the request. BROKEN RESOLVERS
  • An additional level of complexity emerges when resolvers violate the rules of the DNS protocol. Some people have suggested that a number of large ISPs have configured their DNS servers to violate rules (presumably to allow them to run on less-expensive hardware than a fully compliant resolver), such as by disobeying TTLs, or by indicating that a domain name does not exist just because one of its name servers does not respond. As a final level of complexity, some applications such as Web browsers also have their own DNS cache, in order to reduce the use of the DNS resolver library itself. This practice can add extra difficulty to DNS debugging, as it obscures which data is fresh, or lies in which cache. These caches typically have very short caching times of the order of one minute. A notable exception is Internet Explorer; recent versions cache DNS records for half an hour. OTHER DNS APPLICATIONS The system outlined above provides a somewhat simplified scenario. The DNS includes several other functions: Hostnames and IP addresses do not necessarily match on a one-to- one basis. Many hostnames may correspond to a single IP address: combined with virtual hosting, this allows a single machine to serve many web sites. Alternatively a single hostname may correspond to many IP addresses: this can facilitate fault tolerance and load distribution, and also allows a site to move physical location seamlessly. There are many uses of DNS besides translating names to IP addresses. For instance, Mail transfer agents use DNS to find out where to deliver e-mail for a particular address. The domain to mail exchanger mapping provided by MX records accommodates another layer of fault tolerance and load distribution on top of the name to IP address mapping. Sender Policy Framework and DomainKeys instead of creating own record types were designed to take advantage of another DNS record type, the TXT record. To provide resilience in the event of computer failure, multiple DNS servers provide coverage of each domain. In particular, thirteen root servers exist worldwide. DNS programs or operating systems have the IP addresses of these servers built in. At least nominally, the USA hosts
  • all but three of the root servers. However, because many root servers actually implement anycast, where many different computers can share the same IP address to deliver a single service over a large geographic region, most of the physical (rather than nominal) root servers now operate outside the USA. The DNS uses TCP and UDP on port 53 to serve requests. Almost all DNS queries consist of a single UDP request from the client followed by a single UDP reply from the server. TCP typically comes into play only when the response data size exceeds 512 bytes, or for such tasks as zone transfer. Some operating systems such as HP-UX are known to have resolver implementations that use TCP for all queries, even when UDP would suffice. EXTENSIONS TO DNS EDNS is an extension of the DNS protocol which enhances the transport of DNS data in UDP packages, and adds support for expanding the space of request and response codes. It is described in RFC 2671. IMPLEMENTATIONS OF DNS For a commented list of DNS server-side implementations, see Comparison of DNS server software. STANDARDS RFC 882 Concepts and Facilities (Deprecated by RFC 1034) RFC 883 Domain Names: Implementation specification (Deprecated by RFC 1035) RFC 1032 Domain administrators guide RFC 1033 Domain administrators operations guide RFC 1034 Domain Names - Concepts and Facilities. RFC 1035 Domain Names - Implementation and Specification RFC 1101 DNS Encodings of Network Names and Other Types RFC 1123 Requirements for Internet Hosts -- Application and Support RFC 1183 New DNS RR Definitions RFC 1706 DNS NSAP Resource Records RFC 1876 Location Information in the DNS (LOC) RFC 1886 DNS Extensions to support IP version 6
  • RFC 1912 Common DNS Operational and Configuration Errors RFC 1995 Incremental Zone Transfer in DNS RFC 1996 A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY) RFC 2136 Dynamic Updates in the domain name system (DNS UPDATE) RFC 2181 Clarifications to the DNS Specification RFC 2182 Selection and Operation of Secondary DNS Servers RFC 2308 Negative Caching of DNS Queries (DNS NCACHE) RFC 2317 Classless IN-ADDR.ARPA delegation RFC 2671 Extension Mechanisms for DNS (EDNS0) RFC 2672 Non-Terminal DNS Name Redirection (DNAME record) RFC 2782 A DNS RR for specifying the location of services (DNS SRV) RFC 2845 Secret Key Transaction Authentication for DNS (TSIG) RFC 2874 DNS Extensions to Support IPv6 Address Aggregation and Renumbering RFC 3403 Dynamic Delegation Discovery System (DDDS) (NAPTR records) RFC 3696 Application Techniques for Checking and Transformation of Names RFC 4398 Storing Certificates in the Domain Name System RFC 4408 Sender Policy Framework (SPF) (SPF records) [edit] Types of DNS records Important categories of data stored in the DNS include the following: An A record or address record maps a hostname to a 32-bit IPv4 address. An AAAA record or IPv6 address record maps a hostname to a 128-bit IPv6 address. A CNAME record or canonical name record is an alias of one name to another. The A record that the alias is pointing to can be either local or remote - on a foreign name server. Useful when running multiple services from a single IP address, where each service has its own entry in DNS. An MX record or mail exchange record maps a domain name to a list of mail exchange servers for that domain. A PTR record or pointer record maps an IPv4 address to the canonical name for that host. Setting up a PTR record for a hostname in
  • the in-addr.arpa domain that corresponds to an IP address implements reverse DNS lookup for that address. For example (at the time of writing), www.icann.net has the IP address 192.0.34.164, but a PTR record maps 164.34.0.192.in-addr.arpa to its canonical name, referrals.icann.org. An NS record or name server record maps a domain name to a list of DNS servers authoritative for that domain. Delegations depend on NS records. An SOA record or start of authority record specifies the DNS server providing authoritative information about an Internet domain, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone. An SRV record is a generalized service location record. A TXT record allows an administrator to insert arbitrary text into a DNS record. For example, this record is used to implement the Sender Policy Framework and DomainKeys specifications. NAPTR records ("Naming Authority Pointer") are a newer type of DNS record that support regular expression based rewriting. Other types of records simply provide information (for example, a LOC record gives the physical location of a host), or experimental data (for example, a WKS record gives a list of servers offering some well known service such as HTTP or POP3 for a domain). INTERNATIONALISED DOMAIN NAMES INTERNATIONALIZED DOMAIN NAME While domain names in the DNS have no restrictions on the characters they use and can include non-ASCII characters, the same is not true for host names. Host names are the names most people see and use for things like e-mail and web browsing. Host names are restricted to a small subset of the ASCII character set that includes the Roman alphabet in upper and lower case, the digits 0 through 9, the dot, and the hyphen. (See RFC 3696 section 2 for details.) This prevented the representation of names and words of many languages natively. ICANN has approved the Punycode-based IDNA system, which maps Unicode strings into the valid DNS character set, as a workaround to this issue. Some registries have adopted IDNA.
  • SECURITY ISSUES IN DNS DNS was not originally designed with security in mind, and thus has a number of security issues. DNS responses are traditionally not cryptographically signed, leading to many attack possibilities; DNSSEC modifies DNS to add support for cryptographically signed responses. There are various extensions to support securing zone transfer information as well. Some domain names can spoof other, similar-looking domain names. For example, "paypal.com" and "paypa1.com" are different names, yet users may be unable to tell the difference. This problem is much more serious in systems that support internationalized domain names, since many characters that are different (from the point of view of ISO 10646) appear identical on typical computer screens. LEGAL USERS OF DOMAINS REGISTRANT No one in the world really "owns" a domain name except the Network Information Centre (NIC), or domain name registry.[citation needed] Most of the NICs in the world receive an annual fee from a legal user in order for the legal user to utilize the domain name (i.e. a sort of a leasing agreement exists, subject to the registry's terms and conditions). Depending on the various naming convention of the registries, legal users become commonly known as "registrants" or as "domain holders". ICANN holds a complete list of domain registries in the world. One can find the legal user of a domain name by looking in the WHOIS database held by most domain registries. For most of the more than 240 country code top-level domains (ccTLDs), the domain registries hold the authoritative WHOIS (Registrant, name servers, expiry dates etc). For instance, DENIC, Germany NIC holds the authoritative WHOIS to a .DE domain name. However, some domain registries, such as for .COM, .ORG, .INFO, etc., use a registry-registrar model. There are hundreds of Domain Name Registrars that actually perform the domain name registration with the end-user (see lists at ICANN or VeriSign). By using this method of distribution, the registry only has to manage the relationship with the registrar, and the registrar maintains the relationship with the end-
  • users, or 'registrants'. For .COM, .NET domain names, the domain registries, VeriSign holds a basic WHOIS (registrar and name servers etc). One can find the detailed WHOIS (Registrant, name servers, expiry dates etc) at the registrars. Since about 2001, most gTLD registries (.ORG, .BIZ, .INFO) have adopted a so-called "thick" registry approach, i.e. keeping the authoritative WHOIS with the various registries instead of the registrars. [edit] Administrative contact A registrant usually designates an administrative contact to manage the domain name. In practice, the administrative contact usually has the most immediate power over a domain. Management functions delegated to the administrative contacts may include (for example): the obligation to conform to the requirements of the domain registry in order to retain the right to use a domain name authorization to update the physical address, e-mail address and telephone number etc in WHOIS [edit] Technical contact A technical contact manages the name servers of a domain name. The many functions of a technical contact include: making sure the configurations of the domain name conforms to the requirements of the domain registry updating the domain zone providing the 24x7 functionality of the name servers (that leads to the accessibility of the domain name) [edit] Billing contact The party whom a NIC invoices. [edit] Name servers Namely the authoritative name servers that host the domain name zone of a domain name. [edit] Politics Many investigators have voiced criticism of the methods currently used to control ownership of domains. Critics commonly claim abuse by monopolies or near-monopolies, such as VeriSign, Inc. Particularly noteworthy was the VeriSign Site Finder system which redirected all unregistered .com and .net domains to a VeriSign webpage. Despite widespread criticism, VeriSign only reluctantly removed it after ICANN threatened to revoke its contract to administer the root name servers.
  • There is also significant disquiet regarding United States political influence over the Internet Corporation for Assigned Names and Numbers (ICANN). This was a significant issue in the attempt to create a .xxx Top-level domain and sparked greater interest in Alternative DNS roots that would be beyond the control of any single country. [edit] Truth in Domain Names Act In the United States, the "Truth in Domain Names Act", in combination with the PROTECT Act, forbids the use of a misleading domain name with the intention of attracting people into viewing a visual depiction of sexually explicit conduct on the Internet. ELECTRONIC MAIL Electronic mail (abbreviated "e-mail" or, often, "email") is a store and forward method of composing, sending, storing, and receiving messages over electronic communication systems. The term "e-mail" (as a noun or verb) applies both to the Internet e-mail system based on the Simple Mail Transfer Protocol (SMTP) and to intranet systems allowing users within one organization to e-mail each other. Often these workgroup collaboration organizations may use the Internet protocols for internal e-mail service. ORIGINS OF E-MAIL E-mail predates the Internet; existing e-mail systems were a crucial tool in creating the Internet. MIT first demonstrated the Compatible Time-Sharing System (CTSS) in 1961. [1] It allowed multiple users to log into the IBM 7094 [2] from remote dial-up terminals, and to store files online on disk. This new ability encouraged users to share information in new ways. E-mail started in 1965 as a way for multiple users of a time-sharing mainframe computer to communicate. Although the exact history is murky, among the first systems to have such a facility were SDC's Q32 and MIT's CTSS.
  • E-mail was quickly extended to become network e-mail, allowing users to pass messages between different computers. The messages could be transferred between users on different computers by 1966, but it is possible the SAGE system had something similar some time before. The ARPANET computer network made a large contribution to the evolution of e-mail. There is one report [1] which indicates experimental inter-system e-mail transfers on it shortly after its creation, in 1969. Ray Tomlinson initiated the use of the @ sign to separate the names of the user and their machine in 1971 [2]. The ARPANET significantly increased the popularity of e-mail, and it became the killer app of the ARPANET. MODERN INTERNET E-MAIL How Internet e-mail works The diagram above shows a typical sequence of events that takes place when Alice composes a message using her mail user agent (MUA). She types in, or selects
  • from an address book, the e-mail address of her correspondent. She hits the "send" button. Her MUA formats the message in Internet e-mail format and uses the Simple Mail Transfer Protocol (SMTP) to send the message to the local mail transfer agent (MTA), in this case smtp.a.org, run by Alice's Internet Service Provider (ISP). The MTA looks at the destination address provided in the SMTP protocol (not from the message header), in this case bob@b.org. An Internet e-mail address is a string of the form localpart@domain.example, which is known as a Fully Qualified Domain Address (FQDA). The part before the @ sign is the local part of the address, often the username of the recipient, and the part after the @ sign is a domain name. The MTA looks up this domain name in the Domain Name System to find the mail exchange servers accepting messages for that domain. The DNS server for the b.org domain, ns.b.org, responds with an MX record listing the mail exchange servers for that domain, in this case mx.b.org, a server run by Bob's ISP. smtp.a.org sends the message to mx.b.org using SMTP, which delivers it to the mailbox of the user bob. Bob presses the "get mail" button in his MUA, which picks up the message using the Post Office Protocol (POP3). This sequence of events applies to the majority of e-mail users. However, there are many alternative possibilities and complications to the e-mail system: Alice or Bob may use a client connected to a corporate e-mail system, such as IBM's Lotus Notes or Microsoft's Exchange. These systems often have their own internal e-mail format and their clients typically communicate with the e-mail server using a vendor-specific, proprietary protocol. The server sends or receives e-mail via the Internet through the product's Internet mail gateway which also does any necessary reformatting. If Alice and Bob work for the same company, the entire transaction may happen completely within a single corporate e-mail system. Alice may not have a MUA on her computer but instead may connect to a webmail service. Alice's computer may run its own MTA, so avoiding the transfer at step 1.
  • Bob may pick up his e-mail in many ways, for example using the Internet Message Access Protocol, by logging into mx.b.org and reading it directly, or by using a webmail service. Domains usually have several mail exchange servers so that they can continue to accept mail when the main mail exchange server is not available. Emails are not secure if email encryption is not used correctly. It used to be the case that many MTAs would accept messages for any recipient on the Internet and do their best to deliver them. Such MTAs are called open mail relays. This was important in the early days of the Internet when network connections were unreliable. If an MTA couldn't reach the destination, it could at least deliver it to a relay that was closer to the destination. The relay would have a better chance of delivering the message at a later time. However, this mechanism proved to be exploitable by people sending unsolicited bulk e-mail and as a consequence very few modern MTAs are open mail relays, and many MTAs will not accept messages from open mail relays because such messages are very likely to be spam. INTERNET E-MAIL FORMAT The format of Internet e-mail messages is defined in RFC 2822 and a series of RFCs, RFC 2045 through RFC 2049, collectively called Multipurpose Internet Mail Extensions (MIME). Although as of July 13, 2005 (see [3]) RFC 2822 is technically a proposed IETF standard and the MIME RFCs are draft IETF standards, these documents are the de facto standards for the format of Internet e-mail. Prior to the introduction of RFC 2822 in 2001 the format described by RFC 822 was the de facto standard for Internet e-mail for nearly two decades; it is still the official IETF standard. The IETF reserved the numbers 2821 and 2822 for the updated versions of RFC 821 (SMTP) and RFC 822, honoring the extreme importance of these two RFCs. RFC 822 was published in 1982 and based on the earlier RFC 733. Internet e-mail messages consist of two major sections:
  • Header - Structured into fields such as summary, sender, receiver, and other information about the e-mail Body - The message itself as unstructured text; sometimes containing a signature block at the end The header is separated from the body by a blank line. INTERNET E-MAIL HEADER The message header consists of fields, usually including at least the following: From: The e-mail address, and optionally name, of the sender of the message To: The e-mail address[es], and optionally name[s], of the receiver[s] of the message Subject: A brief summary of the contents of the message Date: The local time and date when the message was originally sent Each header field has a name and a value. RFC 2822 specifies the precise syntax. Informally, the field name starts in the first character of a line, followed by a ":", followed by the value which is continued on non-null subsequent lines that have a space or tab as their first character. Field names and values are restricted to 7-bit ASCII characters. Non-ASCII values may be represented using MIME encoded words. Note that the "To" field in the header is not necessarily related to the addresses to which the message is delivered. The actual delivery list is supplied in the SMTP protocol, not extracted from the header content. The "To" field is similar to the greeting at the top of a conventional letter which is delivered according to the address on the outer envelope. Also note that the "From" field does not have to be the real sender of the e-mail message. It is very easy to fake the "From" field and let a message seem to be from any mail address. It is possible to digitally sign e-mail, which is much harder to fake. Some Internet service providers do not relay e-mail claiming to come from a domain not hosted by them, but very few (if any) check to make sure that the person or even e-mail address named in the "From" field is the one associated with the connection. Some internet service providers apply e-mail authentication systems to e-mail being sent through their MTA to allow other MTAs to detect forged spam that might apparently appear to be from them.
  • Cc: carbon copy Received: Tracking information generated by mail servers that have previously handled a message Content-Type: Information about how the message has to be displayed, usually a MIME type Many e-mail clients present "Bcc" (Blind carbon copy, recipients not visible in the "To" field) as a header field. Since the entire header is visible to all recipients, "Bcc" is not included in the message header. Addresses added as "Bcc" are only added to the SMTP delivery list, and do not get included in the message data. IANA maintains a list of standard header fields. E-MAIL CONTENT ENCODING E-mail was originally designed for 7-bit ASCII. Much e-mail software is 8-bit clean but must assume it will be communicating with 7-bit servers and mail readers. The MIME standard introduced charset specifiers and two content transfer encodings to encode 8 bit data for transmission: quoted printable for mostly 7 bit content with a few characters outside that range and base64 for arbitrary binary data. The 8BITMIME extension was introduced to allow transmission of mail without the need for these encodings but many mail transport agents still don't support it fully. For international character sets, Unicode is growing in popularity. SAVED MESSAGE FILENAME EXTENSION Most, but not all, e-mail clients save individual messages as separate files, or allow users to do so. Different applications save e-mail files with different filename extensions. .eml - This is the default e-mail extension for Mozilla Thunderbird and is used by Microsoft Outlook Express.
  • .emlx - Used by Apple Mail. .msg - Used by Microsoft Office Outlook. MESSAGES AND MAILBOXES Messages are exchanged between hosts using the Simple Mail Transfer Protocol with software like Sendmail. Users can download their messages from servers with standard protocols such as the POP or IMAP protocols, or, as is more likely in a large corporate environment, with a proprietary protocol specific to Lotus Notes or Microsoft Exchange Servers. Mail can be stored either on the client, on the server side, or in both places. Standard formats for mailboxes include Maildir and mbox. Several prominent e-mail clients use their own proprietary format and require conversion software to transfer e- mail between them. When a message cannot be delivered, the recipient MTA must send a bounce message back to the sender, indicating the problem. SPAMMING AND E-MAIL WORMS The usefulness of e-mail is being threatened by three phenomena: spamming, phishing and e-mail worms. Spamming is unsolicited commercial e-mail. Because of the very low cost of sending e-mail, spammers can send hundreds of millions of e-mail messages each day over an inexpensive Internet connection. Hundreds of active spammers sending this volume of mail results in information overload for many computer users who receive tens or even hundreds of junk messages each day. E-mail worms use e-mail as a way of replicating themselves into vulnerable computers. Although the first e-mail worm affected UNIX computers, the problem is most common today on the more popular Microsoft Windows operating system. The combination of spam and worm programs results in users receiving a constant drizzle of junk e-mail, which reduces the usefulness of e-mail as a practical tool.
  • A number of anti-spam techniques mitigate the impact of spam. In the United States, U.S. Congress has also passed a law, the Can Spam Act of 2003, attempting to regulate such e-mail. Australia also has very strict spam laws restricting the sending of spam from an Australian ISP (http://www.aph.gov.au/library/pubs/bd/2003-04/04bd045.pdf), but its impact has been minimal since most spam comes from regimes that seem reluctant to regulate the sending of spam. PRIVACY PROBLEMS REGARDING E-MAIL E-MAIL PRIVACY E-mail privacy, without some security precautions, can be compromised because e-mail messages are generally not encrypted; E-mail messages have to go through intermediate computers before reaching their destination, meaning it is relatively easy for others to intercept and read messages; Many Internet Service Providers (ISP) store copies of your e-mail messages on their mail servers before they are delivered. The backups of these can remain up to several months on their server, even if you delete them in your mailbox; The Received: headers and other information in the email can often identify the sender, preventing anonymous communication. There are cryptography applications that can serve as a remedy to one or more of the above. For example, Virtual Private Networks or the Tor anonymity network can be used to encrypt traffic from the user machine to a safer network while GPG, PGP or S/MIME can be used for end-to-end message encryption, and SMTP STARTTLS or SMTP over Transport Layer Security/Secure Sockets Layer can be used to encrypt communications for a single mail hop between the SMTP client and the SMTP server. Another risk is that e-mail passwords might be intercepted during sign-in. One may use encrypted authentication schemes such as SASL to help prevent this.
  • FINGER In computer networking, the Name/Finger protocol and the Finger user information protocol are simple network protocols for the exchange of human-oriented status and user information. Name/Finger protocol The Name/Finger protocol is based on Request for comments document 742 (December 1977) as an interface to the name and finger programs that provide status reports on a particular computer system or a particular person at network sites. The finger program was written in 1971 by Les Earnest who created the program to solve the need of users who wanted information on other users of the network. Information on who is logged-in was useful to check the availability of a person to meet. Prior to the finger program, the only way to get this information was with a who program that showed IDs and terminal line numbers for logged-in users, and people used to run their fingers down the who list. Earnest named his program after this concept. Finger user information protocol The Finger user information protocol is based on RFC 1288 (The Finger User Information Protocol, December 1991). Typically the server side of the protocol is implemented by a program fingerd (for finger daemon), while the client side is implemented by the name and finger programs which are supposed to return a friendly, human-oriented status report on either the system at the moment or a particular person in depth. There is no required format, and the protocol consists mostly of specifying a single command line. It is most often implemented on Unix or Unix-like systems. The program would supply information such as whether a user is currently logged-on, e-mail address, full name etc. As well as standard user information, finger displays the contents of the .project and .plan files in the user's home directory. Often this file (maintained by the user) contains either useful information about the user's current activities, or alternatively all manner of humor.
  • SECURITY CONCERNS Supplying such detailed information as e-mail addresses and full names was considered acceptable and convenient in the early days of the Internet, but later was considered questionable for privacy and security reasons. Finger information has been frequently used by crackers as a way to initiate a social engineering attack on a company's computer security system. By using a finger client to get a list of a company's employee names, email addresses, phone numbers, and so on, a cracker can telephone or email someone at a company requesting information while posing as another employee. The finger daemon has also had several exploitable security holes which crackers have used to break into systems. The Morris worm exploited an overflow vulnerability in fingerd (among others) to spread. For these reasons, while finger was widely used during the early days of Internet, by the 1990s the vast majority of sites on the internet no longer offered the service. Notable exceptions include John Carmack and Justin Frankel, who until recently still updated their status information occasionally. In late 2005, John Carmack switched to using a blog, instead of his old .plan site. FILE TRANSFER PROTOCOL (FTP) FTP or File Transfer Protocol is used to connect two computers over the Internet so that the user of one computer can transfer files and perform file commands on the other computer. Specifically, FTP is a commonly used protocol for exchanging files over any network that supports the TCP/IP protocol (such as the Internet or an intranet). There are two computers involved in an FTP transfer: a server and a client. The FTP server, running FTP server software, listens on the network for connection requests from other computers. The client computer, running FTP client software, initiates a connection to the
  • server. Once connected, the client can do a number of file manipulation operations such as uploading files to the server, download files from the server, rename or delete files on the server and so on. Any software company or individual programmer is able to create FTP server or client software because the protocol is an open standard. Virtually every computer platform supports the FTP protocol. This allows any computer connected to a TCP/IP based network to manipulate files on another computer on that network regardless of which operating systems are involved (if the computers permit FTP access). OVERVIEW FTP runs exclusively over TCP. FTP servers by default listen on port 21 for incoming connections from FTP clients. A connection to this port from the FTP Client forms the control stream on which commands are passed to the FTP server from the FTP client and on occasion from the FTP server to the FTP client. For the actual file transfer to take place, a different connection is required which is called the data stream. Depending on the transfer mode, the process of setting up the data stream is different. In active mode, the FTP client opens a random port (> 1023), sends the FTP server the random port number on which it is listening over the control stream and waits for a connection from the FTP server. When the FTP server initiates the data connection to the FTP client it binds the source port to port 20 on the FTP server. In passive mode, the FTP Server opens a random port (> 1023), sends the FTP client the port on which it is listening over the control stream and waits for a connection from the FTP client. In this case the FTP client binds the source port of the connection to a random port greater than 1023. While data is being transferred via the data stream, the control stream sits idle. This can cause problems with large data transfers through firewalls which time out sessions after lengthy periods of idleness. While the file may well be successfully transferred, the control session can be disconnected by the firewall, causing an error to be generated. When FTP is used in a UNIX environment, there is an often-ignored but valuable command, "reget" (meaning "get again") that will cause an interrupted "get" command to be continued, hopefully to completion, after a communications interruption. The principle
  • is obvious—the receiving station has a record of what it got, so it can spool through the file at the sending station and re-start at the right place for a seamless splice. The converse would be "reput" but is not available. Again, the principle is obvious: The sending station does not know how much of the file was actually received, so it would not know where to start. The objectives of FTP, as outlined by its RFC, are: To promote sharing of files (computer programs and/or data). To encourage indirect or implicit use of remote computers. To shield a user from variations in file storage systems among different hosts. To transfer data reliably, and efficiently. CRITICISMS OF FTP Passwords and file contents are sent in clear text, which can be intercepted by eavesdroppers. There are protocol enhancements that circumvent this. Multiple TCP/IP connections are used, one for the control connection, and one for each download, upload, or directory listing. Firewall software needs additional logic to account for these connections. It is hard to filter active mode FTP traffic on the client side by using a firewall, since the client must open an arbitrary port in order to receive the connection. This problem is largely resolved by using passive mode FTP. It is possible to abuse the protocol's built-in proxy features to tell a server to send data to an arbitrary port of a third computer; see FXP. FTP is a high latency protocol due to the number of commands needed to initiate a transfer. No integrity check on the receiver side. If transfer is interrupted the receiver has no way to know if the received file is complete or not. It is necessary to manage this externally for example with MD5 sums or cyclic redundancy checking. No error detection. FTP relies on the underlying TCP layer for error control, which uses a weak checksum by modern standards. No date/timestamp attribute transfer. Uploaded files are given a new current timestamp, unlike other file transfer protocols such as SFTP, which allow attributes to be
  • included. There is no way in the standard FTP protocol to set the time-last-modified (or time-created) datestamp that most modern filesystems preserve. There is a draft of a proposed extension that adds new commands for this, but as of yet, most of the popular FTP servers do not support it. SECURITY PROBLEMS The original FTP specification is an inherently insecure method of transferring files because there is no method specified for transferring data in an encrypted fashion. This means that under most network configurations, user names, passwords, FTP commands and transferred files can be "sniffed" or viewed by anyone on the same network using a packet sniffer. This is a problem common to many Internet protocol specifications written prior to the creation of SSL such as HTTP, SMTP and Telnet. The common solution to this problem is to use either SFTP (SSH File Transfer Protocol), or FTPS (FTP over SSL), which adds SSL or TLS encryption to FTP as specified in RFC 4217. FTP RETURN CODES FTP server return codes indicate their status by the digits within them. A brief explanation of various digits' meanings are given below: 1yz: Positive Preliminary reply. The action requested is being initiated but there will be another reply before it begins. 2yz: Positive Completion reply. The action requested has been completed. The client may now issue a new command. 3yz: Positive Intermediate reply. The command was successful, but a further command is required before the server can act upon the request. 4yz: Transient Negative Completion reply. The command was not successful, but the client is free to try the command again as the failure is only temporary. 5yz: Permanent Negative Completion reply. The command was not successful and the client should not attempt to repeat it again. x0z: The failure was due to a syntax error. x1z: This response is a reply to a request for information. x2z: This response is a reply relating to connection information.
  • x3z: This response is a reply relating to accounting and authorization. x4z: Unspecified as yet x5z: These responses indicate the status of the Server file system vis-a-vis the requested transfer or other file system action ANONYMOUS FTP Many sites that run FTP servers enable so-called "anonymous ftp". Under this arrangement, users do not need an account on the server. The user name for anonymous access is typically 'anonymous' or 'ftp'. This account does not need a password. Although users are commonly asked to send their email addresses as their passwords for authentication, usually there is trivial or no verification, depending on the FTP server and its configuration. Internet Gopher has been suggested as an alternative to anonymous FTP, as well as Trivial File Transfer Protocol. While transferring data over the network, several data representations can be used. The two most common transfer modes are: ASCII mode Binary mode The two types differ in the way they send the data. When a file is sent using an ASCII-type transfer, the individual letters, numbers, and characters are sent using their ASCII character codes. The receiving machine saves these in a text file in the appropriate format (for example, a Unix machine saves it in a Unix format, a Macintosh saves it in a Mac format). Hence if an ASCII transfer is used it can be assumed plain text is sent, which is stored by the receiving computer in its own format. Translating between text formats entails substituting the end of line and end of file characters used on the source platform with those on the destination platform, e.g. a Windows machine receiving a file from a Unix machine will replace the line feeds with carriage return-line feed pairs. ASCII transfer is also marginally faster, as the highest-order bit is dropped from each byte in the file.[1] Sending a file in binary mode is different. The sending machine sends each file bit for bit and as such the recipient stores the bitstream as it receives it. Any form of data that is not plain text will be corrupted if this mode is not used.
  • By default, most FTP clients use ASCII mode. Some clients try to determine the required transfer-mode by inspecting the file's name or contents. The FTP specifications also list the following transfer modes: EBCDIC mode Local mode In practice, these additional transfer modes are rarely used. They are however still used by some legacy mainframe systems. FTP AND WEB BROWSERS Most recent web browsers and file managers can connect to FTP servers, although they may lack the support for protocol extensions such as FTPS. This allows manipulation of remote files over FTP through an interface similar to that used for local files. This is done via an FTP URL, which takes the form ftp(s)://<ftpserveraddress> (e.g., [2]). A password can optionally be given in the URL, e.g.: ftp(s)://<login>:<password>@<ftpserveraddress>:<port>. Most web-browsers require the use of passive mode FTP, which not all FTP servers are capable of handling. Some browsers allow only the downloading of files, but offer no way to upload files to the server. FTP OVER SSH FTP over SSH refers to the practice of tunneling a normal FTP session over an SSH connection. Because FTP uses multiple TCP connections (unusual for a TCP/IP protocol that is still in use), it is particularly difficult to tunnel over SSH. With many SSH clients, attempting to set up a tunnel for the control channel (the initial client-to-server connection on port 21) will only protect that channel; when data is transferred, the FTP software at either end will set up new TCP connections (data channels) which will bypass the SSH connection, and thus have no confidentiality, integrity protection, etc. If the FTP client is configured to use passive mode and to connect to a SOCKS server interface that many SSH clients can present for tunnelling, it is possible to run all the FTP channels over the SSH connection.
  • Otherwise, it is necessary for the SSH client software to have specific knowledge of the FTP protocol, and monitor and rewrite FTP control channel messages and autonomously open new forwardings for FTP data channels. Version 3 of SSH Communications Security's software suite, and the GPL licensed FONC are two software packages that support this mode. FTP over SSH is sometimes referred to as secure FTP; this should not be confused with other methods of securing FTP, such as with SSL/TLS (FTPS). Other methods of transferring files using SSH that are not related to FTP include SFTP and SCP; in each of these, the entire conversation (credentials and data) is always protected by the SSH protocol. AN OVERVIEW OF THE FILE TRANSFER PROTOCOL The File Transfer Protocol (FTP) was one of the first efforts to create a standard means of exchanging files over a TCP/IP network, so the FTP has been around since the 1970's. The FTP was designed with as much flexibility as possible, so it could be used over networks other than TCP/IP, as well as being engineered to have the capability with exchanging files with a broad variety of machines. The base specification is RFC 959 and is dated October 1985. There are some additional RFCs relating to FTP, but it should be noted that even as of this writing (December 2001) that most of the new additions are not in widespread use. The purpose of this document is to provide general information about how the protocol works without getting into too many technical details. RFC 959 should be consulted for details on the protocol. Control Connection -- the conversation channel The protocol can be thought of as interactive, because clients and servers actually have a conversation where they authenticate themselves and negotiate file transfers. In addition, the protocol specifies that the client and server do not exchange data on the conversation channel. Instead, clients and servers negotiate how to send data files on separate connections, with one connection for each data transfer. Note that a directory listing is considered a file transfer.
  • To illustrate, we'll just present (an admittedly contrived) example of how the FTP would work between human beings rather than computer systems. For our example, we'll assume we have a client, Carl Clinton, who wishes to transfer files from Acme Mail Service that manages his post office box. Below is a transcript of a phone call between Carl Clinton and Acme Mail Service. Clinton: (Dials the phone number for the mail service) Service: "Hello, this is the Acme Mail Service. How may I help you today?" Clinton: "Hello, this is Carl Clinton. I would like to access mailbox number MB1234." Service: "OK, Mr. Clinton, I need to verify that you may access mailbox MB1234. What is your password?" Clinton: "My password is QXJ4Z2AF." Service: "Thank you Mr. Clinton, you may proceed." Clinton: "For now, I'm only interested in looking at the bills and invoices, so look at the folder marked "bills" in my mailbox." Service: "OK." Clinton: "Please prepare to have your assistant call my secretary at +1 402 555 1234." Service: "OK." Clinton: "Now call my secretary and tell him the names of all the items in the bills folder of my mailbox. Tell me when you have finished." Server: "My assistant is calling your secretary now." Server: "My assistant has sent the names of the items." Clinton: (Receives the list from his secretary and notices a bill from Yoyodyne Systems.) "Please prepare to have your assistant send to my fax machine +1 402 555 7777." Service: "OK." Clinton: "Now fax a copy of the bill from Yoyodyne Systems." Server: "My assistant is calling your fax machine now." Server: "My assistant has finished faxing the item." Clinton: "Thank you, that is all. Good bye." Server: "Goodbye." Now let's look at how this same conversation would appear between computer systems communicating with the FTP protocol over a TCP/IP connection. Client: Connects to the FTP service at
  • port 21 on the IP address 172.16.62.36. Server: 220 Hello, this is the Acme Mail Service. Client: USER MB1234 Server: 331 Password required to access user account MB1234. Client: PASS QXJ4Z2AF Note that this password is not encrypted. The FTP is susceptible to eavesdropping! Server: 230 Logged in. Client: CWD Bills Change directory to "Bills." Server: 250 "/home/MB1234/Bills" is new working directory. Client: PORT 192,168,1,2,7,138 The client wants the server to send to port number 1930 on IP address 192.168.1.2. In this case, 192.168.1.2 is the IP address of the client machine. Server: 200 PORT command successful. Client: LIST Send the list of files in "Bills." Server: 150 Opening ASCII mode data The server now connects out connection for /bin/ls. from its port 20 on 172.16.62.36 to port 1930 on 192.168.1.2. Server: 226 Listing completed. That succeeded, so the data is now sent over the established data connection. Client: PORT 192,168,1,2,7,139 The client wants the server to send to port number 1931 on the client machine. Server: 200 PORT command successful. Client: RETR Yoyodyne.TXT Download "Yoyodyne.TXT." Server: 150 Opening ASCII mode data The server now connects out connection for Yoyodyne.TXT. from its port 20 on
  • 172.16.62.36 to port 1931 on 192.168.1.2. Server: 226 Transfer completed. That succeeded, so the data is now sent over the established data connection. Client: QUIT Server: 221 Goodbye. When using FTP, users use FTP client programs rather than directly communicating with the FTP server. Here's our same example using the stock "ftp" program which is usually installed as /usr/bin/ftp on UNIX systems (and FTP.EXE on Windows). The items the user types are in bold. ksh$ /usr/bin/ftp ftp> open ftp.acmemail.example.com Connected to ftp.acmemail.example.com (172.16.62.36). 220 Hello, this is the Acme Mail Service. Name (ftp.acmemail.example.com:root): MB1234 331 Password required to access user account MB1234. Password: QXJ4Z2AF 230 Logged in. ftp> cd Bills 250 "/home/MB1234/Bills" is new working directory. ftp> ls 200 PORT command successful. 150 Opening ASCII mode data connection for /bin/ls. -rw-r--r-- 1 ftpuser ftpusers 14886 Dec 3 15:22 Acmemail.TXT -rw-r--r-- 1 ftpuser ftpusers 317000 Dec 4 17:40 Yoyodyne.TXT 226 Listing completed. ftp> get Yoyodyne.TXT local: Yoyodyne.TXT remote: Yoyodyne.TXT 200 PORT command successful. 150 Opening ASCII mode data connection for Yoyodyne.TXT.
  • 226 Transfer completed. 317000 bytes received in 0.0262 secs (1.2e+04 Kbytes/sec) ftp> quit 221 Goodbye. As you can see, FTP is designed to allow users to browse the filesystem much like you would with a regular UNIX login shell or MS-DOS command prompt. This differs from other protocols that are transactional (i.e. HTTP), where a connection is established, clients issue a single message to a server that replies with a single reply, and the connection is closed. On the other hand, client programs can be constructed to simulate a transactional environment if they know in advance what they need to do. In effect, FTP is a stateful sequence of one or more transactions. Command primitives, result codes and textual responses The client is always responsible for initiating requests. These requests are issued with FTP command primitives, which are typically 3 or 4 characters each. For example, the command primitive to change the working directory is CWD. The server replies are specially formatted to contain a 3-digit result code first, followed by a space character, followed by descriptive text (there is also a format for multi-line responses). The protocol specifies that clients must only rely upon the numeric result code, since the descriptive text is allowed to vary (with a few exceptions). In practice, the result text is often helpful for debugging, but is generally no longer useful for end users. AUTHENTICATION Although it is not required by protocol, in effect clients must always login to the FTP server with a username and password before the server will allow the client to access the service. There is also a de facto standard for guest access, where "anonymous" (or "ftp") are used as the username and an e-mail address is customarily used as the password in a way for a polite netizen to let the server administrator know who is using the guest login. Because users do not want to divulge their e-mail addresses to protect against unsolicited bulk e-mail, this has subsequently evolved to the point where the password is just some arbitrary text.
  • TYPES OF DATA CONNECTIONS The protocol has built-in support for different types of data transfers. The two mandated types are ASCII for text (specified by the client sending "TYPE A" to the server), and "image" for binary data (specified by "TYPE I"). ASCII transfers are useful when the server machine and client machine have different standards for text. For example, MS-DOS and Microsoft Windows use a carriage return and linefeed sequence to denote an end-of-line, but UNIX systems use just a linefeed. When ASCII transfers are specified, this enables a client to always be able to translate the data into its own native text format. Binary transfers can be used for any type of raw data that requires no translation. Client programs should use binary transfers unless they know that the file in question is text. The protocol does not have any advanced support for character sets for pathnames nor file contents. There is no way to specify UNICODE, for example. For ASCII, it is 7- bit ASCII only. Unfortunately, the burden of deciding what transfer type to use is left to the client, unlike HTTP, which can inform the client what type of data is being sent. Clients often simply choose to transfer everything in binary, and perform any necessary translation after the file is downloaded. Additionally, binary transfers are inherently more efficient to send over the network since the client and server do not need to perform on-the-fly translation of the data. It should be noted that ASCII transfers are mandated by the protocol as the default transfer type unless the client requests otherwise! The PORT and PASV conundrum -- Active and Passive data connections Although it was purposely designed into the protocol as a feature, FTP's use of separate data connections cause numerous problems for things like firewalls, routers, proxies which want to restrict or delegate TCP connections, as well as things like IP stacks which want to do dynamic stateful inspection of TCP connections. The protocol does not mandate a particular port number or a direction that a data connection uses. For example, the easy way out would have been for the protocol's
  • designers to mandate that all data connections must originate from the client machine and terminate at port 20 on the server machine. Instead, for maximum flexibility, the protocol allows the client to choose one of two methods. The first method, which we'll call "Active", is where the client requests that the server originate a data connection and terminate at an IP address and port number of the client's choosing. The important thing to note here is that the server connects out to the client. Client: "Please connect to me at port 1931 on IP address 192.168.1.2, then send the data." Server: "OK" Or, the client can request that the server to assign an IP address and port number on the server side and have the client originate a connection to the server address. We call this method "Passive" and note that the client connects out to the server. Client: "Please tell me where I can get the data." Server: "Connect to me at port 4023 on 172.16.62.36." The active method uses the FTP command primitive PORT, so the first example using the actual FTP protocol would resemble this: Client: PORT 192,168,1,2,7,139 Server: 200 PORT command successful. The passive method uses the FTP command primitive PASV, so the second example using the actual FTP protocol would resemble this: Client: PASV Server: Entering Passive Mode (172,16,62,36,133,111) It should be noted that FTP servers are required to implement PORT, but are not required to implement PASV. The default has traditionally been PORT for this reason, but in practice it is now preferred to use PASV whenever possible because firewalls may be present on the client side which often cause problems. Partial data connections -- resuming downloads The protocol provides a means to only transfer a portion of a file, by having a client specify a starting offset into the file (using the REST primitive, i.e. "restart point"). If an FTP session fails while a data transfer is in progress and has to be reestablished, a client can request that the server restart the transfer at the offset the client specifies. Note that not all FTP servers support this feature.
  • DIRECTORY LISTINGS The base standard of the FTP protocol provides two types of listings, a simple name list (NLST) and a human-readable extended listing (LIST). The name list consists of lines of text, where each line contains exactly one file name and nothing else. The extended listing is not intended to be machine-readable and the protocol does not mandate any particular format. The de facto standard is for it to be in UNIX "/bin/ls -l" format, but although most servers try to emulate that format even on non-UNIX FTP servers, it is still common for servers to provide their own proprietary format. The important thing to note here is that this listing can contain any type of data and cannot be relied upon. Additionally, even those that appear in "/bin/ls -l" format cannot be relied upon for the validity of the fields. For example the date and time could be in local time or GMT. Newer FTP server implementations support a machine-readable listing primitive (MLSD) which is suitable for client programs to get reliable metadata information about files, but this feature is still relatively rare. That leaves the simple name list as the only reliable way to get filenames, but it doesn't tell a client program anything else (such as if the item is a file or a directory!). FUNCTIONAL CONCERNS Despite a rich feature set, there are some glaring omissions. For example, the base specification doesn't even provide for clients to query a file's size or modification date. However, most FTP servers in use now support a de facto extension to the specification which provides the SIZE and MDTM primitives, and even newer servers support the extremely useful MLSD and MSLT primitives which can provide a wealth of information in a standardized format. There is also no 100% accurate way for a client to determine if a particular pathname refers to a file or directory, unless MLSD or MLST is available. Since the protocol also does not provide a way to transfer an entire directory of items at once, the consequence is that there is no 100% accurate way to download an entire directory tree. The end result is that FTP is not particularly suited to "mirroring" files and directories, although FTP client programs use heuristics to make calculated guesses when possible.
  • Despite the guesswork that clients can use for determining metadata for files to download, there's little they can do for files that they upload. There is no standard way to preserve an uploaded file's modification time. FTP is platform agnostic, so there aren't standard ways to preserve platform-specific metadata such as UNIX permissions and user IDs or Mac OS file type and creator codes. Separate connections for data transfers are also a mixed blessing. For high performance it would be best to use a single connection and perform multiple data transfers before closing it. Even better would be for a method to use a single connection for both the control connection conversation and data transfers. Since each data connection uses an ephemeral (random) port number, it is possible to "run out" of connections. For details on this phenomenon, a separate article is available. SECURITY CONCERNS It is important to note that the base specification, as implemented by the vast majority of the world's FTP servers, does not have any special handling for encrypted communication of any kind. When clients login to FTP servers, they are sending clear text usernames and passwords! This means that anyone with a packet sniffer between the client and server could surreptitiously steal passwords. Besides passwords, potential attackers could not only monitor the entire conversation on the FTP control connection, they could also monitor the contents of the data transfers themselves. There have been proposals to make the FTP protocol more secure, but these proposals have not seen widespread adoption. Therefore, unless the IP protocol layer itself is secure (for example, encrypted using IPsec), FTP should not be used if sensitive login information is to be exchanged over an insecure network, or if the files containing sensitive material are being transferred over an insecure network. HISTORY OF INTERNET The History of the Internet dates back to the early development of communication networks. The idea of a computer network intended to allow general communication
  • among users of various computers has developed through a large number of stages. The melting pot of developments brought together the network of networks that we know as the Internet. This included both technological developments and the merging together of existing network infrastructure and telecommunication systems. The infrastructure of the Internet spread across the globe to create the world wide network of computers we know today. It spread throughout the Western countries before entering the developing countries, thus creating both unprecedented worldwide access to information and communications and a digital divide in access to this new infrastructure. The Internet went on to fundamentally alter the world economy, including the economic implications of the dot-com bubble. History of computing Hardware before 1960 Hardware 1960s to present Hardware in Soviet Bloc countries Operating systems Software engineering Programming languages Graphical user interface In the fifties and early sixties, prior to the widespread inter-networking that led to the Internet, most communication networks were limited by their nature to only allow communications between the stations on the network. Some networks had gateways or bridges between them, but these bridges were often limited or built specifically for a single use. One prevalent computer networking method was based on the central mainframe method, simply allowing its terminals to be connected via long leased lines. This method was used in the 1950s by Project RAND to support researchers such as Herbert Simon, in Pittsburgh, Pennsylvania, when collaborating across the continent with researchers in Santa Monica, California, on automated theorem proving and artificial intelligence. THREE TERMINALS AND AN ARPA
  • A fundamental pioneer in the call for a global network, J.C.R. Licklider, articulated the idea in his January 1960 paper, Man-Computer Symbiosis. "a network of such [computers], connected to one another by wide-band communication lines" which provided "the functions of present-day libraries together with anticipated advances in information storage and retrieval and [other] symbiotic functions. "— J.C.R. Licklider In October 1962, Licklider was appointed head of the United States Department of Defense's DARPA information processing office, and formed an informal group within DARPA to further computer research. As part of the information processing office's role, three network terminals had been installed: one for System Development Corporation in Santa Monica, one for Project Genie at the University of California, Berkeley and one for the Multics project SHOPPING at the Massachusetts Institute of Technology (MIT). Licklider's need for inter-networking would be made evident by the problems this caused. "For each of these three terminals, I had three different sets of user commands. So if I was talking online with someone at S.D.C. and I wanted to talk to someone I knew at Berkeley or M.I.T. about this, I had to get up from the S.D.C. terminal, go over and log into the other terminal and get in touch with them. I said, oh, my goodness gracious me, it's obvious what to do (But I don't want to do it): If you have these three terminals, there ought to be one terminal that goes anywhere you want to go where you have interactive computing. That idea is the arpanet." -Robert W. Taylor, co-writer with Licklider of "The Computer as a Communications Device", in an interview with the New York Times SWITCHED PACKETS At the tip of the inter-networking problem lay the issue of connecting separate physical networks to form one logical network. During the 1960s, Donald Davies (NPL), Paul Baran (RAND Corporation), and Leonard Kleinrock (MIT) developed and implemented packet switching. The notion that the Internet was developed to survive a
  • nuclear attack has its roots in the early theories developed by RAND. Baran's research had approached packet switching from studies of decentralisation to avoid combat damage compromising the entire network Networks that led to the Internet Office at ARPA, Robert Taylor intended to realize Licklider's ideas of an interconnected networking system. Bringing in Larry Roberts from MIT, he initiated a project to build such a network. The first ARPANET link was established between the University of California, Los Angeles and the Stanford Research Institute on 21 November 1969. By 5 December 1969, a 4-node network was connected by adding the University of Utah and the University of California, Santa Barbara. Building on ideas developed in alohanet, the ARPANET started in 1972 and was growing rapidly by 1981. The number of hosts had grown to 213, with a new host being added approximately every twenty days. ARPANET became the technical core of what would become the Internet, and a primary tool in developing the technologies used. ARPANET development was centered around the Request for Comments (RFC) process, still used today for proposing and distributing Internet Protocols and Systems. RFC 1, entitled "Host Software", was written by Steve Crocker from the University of California, Los Angeles, and published on April 7, 1969. These early years were documented in the 1972 film Computer Networks: The HERALDS OF RESOURCE SHARING. International collaborations on ARPANET were sparse. For various political reasons, European developers were concerned with developing the X.25 networks. Notable exceptions were the Norwegian Seismic Array (NORSAR) in 1972, followed in 1973 by Sweden with satellite links to the Tanum Earth Station and University College London. X.25 AND PUBLIC ACCESS Following on from DARPA's research, packet switching network standards were developed by the International Telecommunication Union (ITU) in the form of X.25 and
  • related standards. In 1974, X.25 formed the basis for the sercnet network between British academic and research sites, which later became JANET. The initial ITU Standard on X.25 was approved in March 1976. This standard was based on the concept of virtual circuits. The British Post Office, Western Union International and Tymnet collaborated to create the first international packet switched network, referred to as the International Packet Switched Service (IPSS), in 1978. This network grew from Europe and the US to cover Canada, Hong Kong and Australia by 1981. By the 1990s it provided a worldwide networking infrastructure. Unlike arpanet, X.25 was also commonly available for business use. X.25 would be used for the first dial-in public access networks, such as Compuserve and Tymnet. In 1979, compuserve became the first service to offer electronic mail capabilities and technical support to personal computer users. The company broke new ground again in 1980 as the first to offer real-time chat with its CB Simulator. There were also the America Online (AOL) and Prodigy dial in networks and many bulletin board system (BBS) networks such as The WELL and fidonet. Fidonet in particular was popular amongst hobbyist computer users, many of them hackers and amateur radio operators. UUCP In 1979, two students at Duke University, Tom Truscott and Jim Ellis, came up with the idea of using simple Bourne shell scripts to transfer news and messages on a serial line with nearby University of North Carolina at Chapel Hill. Following public release of the software, the mesh of UUCP hosts forwarding on the Usenet news rapidly expanded. Uucpnet, as it would later be named, also created gateways and links between fidonet and dial-up BBS hosts. UUCP networks spread quickly due to the lower costs involved, and ability to use existing leased lines, X.25 links or even ARPANET connections. By 1983 the number of UUCP hosts had grown to 550, nearly doubling to 940 in 1984. MERGING THE NETWORKS AND CREATING THE INTERNET With so many different network methods, something needed to unify them. Robert E. Kahn of DARPA and ARPANET recruited Vint Cerf of Stanford University to
  • work with him on the problem. By 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, Gerard lelann and Louis Pouzin (designer of the CYCLADES network) with important work 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. DARPA agreed to fund development of prototype software, and after several years of work, the first somewhat crude demonstration of a gateway between the Packet Radio network in the SF Bay area and the ARPANET was conducted. By November 1977 a three network demonstration was conducted including the ARPANET, the Packet Radio Network and the Atlantic Packet Satellite network—all sponsored by DARPA. Stemming from the first specifications of TCP in 1974, TCP/IP emerged in mid-late 1978 in nearly final form. By 1981, the associated standards were published as rfcs 791, 792 and 793 and adopted for use. DARPA sponsored or encouraged the development of TCP/IP implementations for many operating systems and then scheduled a migration of all hosts on all of its packet networks to TCP/IP. On 1 January 1983, TCP/IP protocols became the only approved protocol on the ARPANET, replacing the earlier NCP protocol. ARPANET TO NSFNET After the ARPANET had been up and running for several years, ARPA looked for another agency to hand off the network to; ARPA's primary business was funding cutting-edge research and development, not running a communications utility. Eventually, in July 1975, the network had been turned over to the Defense Communications Agency, also part of the Department of Defense. In 1983, the U.S. military portion of the ARPANET was broken off as a separate network, the MILNET. The networks based around the ARPANET were government funded and therefore restricted to noncommercial uses such as research; unrelated commercial use was strictly forbidden. This initially restricted connections to military sites and universities. During the 1980s, the connections expanded to more educational
  • institutions, and even to a growing number of companies such as Digital Equipment Corporation and Hewlett-Packard, which were participating in research projects or providing services to those who were. Another branch of the U.S. government, the National Science Foundation (NSF), became heavily involved in internet research and started development of a successor to ARPANET. In 1984 this resulted CSNET, the first Wide Area Network designed specifically to use TCP/IP. CSNET connected with ARPANET using TCP/IP, and ran TCP/IP over X.25, but it also supported departments without sophisticated network connections, using automated dial-up mail exchange. This grew into the nsfnet backbone, established in 1986, and intended to connect and provide access to a number of supercomputing centers established by the NSF. THE TRANSITION TOWARD AN INTERNET The term "Internet" was adopted in the first RFC published on the TCP protocol Internet Transmission Control Protocol). It was around the time when ARPANET was interlinked with nsfnet, that the term Internet came into more general use, with "an internet" meaning any network using TCP/IP. "The Internet" came to mean a global and large network using TCP/IP, which at the time meant nsfnet and ARPANET. Previously "internet" and "internetwork" had been used interchangeably, and "internet protocol" had been used to refer to other networking systems such as Xerox Network Services. As interest in wide spread networking grew and new applications for it arrived, the Internet's technologies spread throughout the rest of the world. TCP/IP's network- agnostic approach meant that it was easy to use any existing network infrastructure, such as the IPSS X.25 network, to carry Internet traffic. In 1984, University College London replaced its transatlantic satellite links with TCP/IP over IPSS. Many sites unable to link directly to the Internet started to create simple gateways to allow transfer of e-mail, at that time the most important application. Sites which only had intermittent connections used UUCP or fidonet and relied on the gateways between these networks and the Internet. Some gateway services went beyond simple e-mail peering, such as allowing access to FTP sites via UUCP or e-mail.
  • TCP/IP BECOMES WORLDWIDE The first arpanet connection outside the US was established to NORSAR in Norway in 1973, just ahead of the connection to Great Britain. These links were all converted to TCP/IP in 1982, at the same time as the rest of the Arpanet. CERN, the European internet, the link to the Pacific and beyond. Between 1984 and 1988 CERN began installation and operation of TCP/IP to interconnect its major internal computer systems, workstations, PC's and an accelerator control system. CERN continued to operate a limited self-developed system CERNET internally and several incompatible (typically proprietary) network protocols externally. There was considerable resistance in Europe towards more widespread use of TCP/IP and the CERN TCP/IP intranets remained isolated from the rest of the Internet until 1989. In 1988 Daniel Karrenberg, from CWI in Amsterdam, visited Ben Segal, CERN's TCP/IP Coordinator, looking for advice about the transition of the European side of the UUCP Usenet network (much of which ran over X.25 links) over to TCP/IP. In 1987, Ben Segal had met with Len Bosack from the then still small company Cisco about purchasing some TCP/IP routers for CERN, and was able to give Karrenberg advice and forward him on to Cisco for the appropriate hardware. This expanded the European portion of the Internet across the existing UUCP networks, and in 1989 CERN opened its first external TCP/IP connections. This coincided with the creation of Réseaux IP Européens (RIPE), initially a group of IP network administrators who met regularly to carry out co-ordination work together. Later, in 1992, RIPE was formally registered as a cooperative in Amsterdam. At the same time as the rise of internetworking in Europe, adhoc networking to ARPA and in-between Australian universities formed, based on various technologies such as X.25 and uucpnet. These were limited in their connection to the global networks, due to the cost of making individual international UUCP dial-up or X.25 connections. In 1989, Australian universities joined the push towards using IP protocols to unify their networking infrastructures. Aarnet was formed in 1989 by the Australian Vice- Chancellors' Committee and provided a dedicated IP based network for Australia.
  • The Internet began to penetrate Asia in the late 1980s. Japan, which had built the UUCP- based network JUNET in 1984, connected to nsfnet in 1989. It hosted the annual meeting of the Internet Society, INET'92, in Kobe. Singapore developed TECHNET in 1990, and Thailand gained a global Internet connection between Chulalongkorn University and UUNET in 1992. A DIGITAL DIVIDE While developed countries with technological infrastructures were joining the Internet, developing countries began to experience a digital divide separating them from the Internet. At the beginning of the 1990s, African countries relied upon X.25 IPSS and 2400 baud modem UUCP links for international and internetwork computer communications. In 1996 a USAID funded project, the Leland initative, started work on developing full Internet connectivity for the continent. Guinea, Mozambique, Madagascar and Rwanda gained satellite earth stations in 1997, followed by Côte d'Ivoire and Benin in 1998. In 1991, the People's Republic of China saw its first TCP/IP college network, Tsinghua University's TUNET. The PRC went on to make its first global Internet connection in 1994, between the Beijing Electro-Spectrometer Collaboration and Stanford University's Linear Accelerator Center. However, China went on to implement its own digital divide by implementing a country-wide content filter. OPENING THE NETWORK TO COMMERCE The interest in commercial use of the Internet became a hotly debated topic. Although commercial use was forbidden, the exact definition of commercial use could be unclear and subjective. Uucpnet and the X.25 IPSS had no such restrictions, which would eventually see the official barring of uucpnet use of ARPANET and nsfnet connections. Some UUCP links still remained connecting to these networks however, as administrators cast a blind eye to their operation.
  • During the late 1980s, the first Internet service provider (ISP) companies were formed. Companies like psinet, UUNET, Netcom, and Portal Software were formed to provide service to the regional research networks and provide alternate network access, UUCP-based email and Usenet News to the public. The first dial-up ISP, world.std.com, opened in 1989. This caused controversy amongst university users, who were outraged at the idea of noneducational use of their networks. Eventually, it was the commercial Internet service providers who brought prices low enough that junior colleges and other schools could afford to participate in the new arenas of education and research. By 1990, ARPANET had been overtaken and replaced by newer networking technologies and the project came to a close. In 1994, the nsfnet, now renamed ANSNET (Advanced Networks and Services) and allowing non-profit corporations access, lost its standing as the backbone of the Internet. Both government institutions and competing commercial providers created their own backbones and interconnections. Regional network access points (naps) became the primary interconnections between the many networks and the final commercial restrictions ended. THE IETF AND A STANDARD FOR STANDARDS The Internet has developed a significant subculture dedicated to the idea that the Internet is not owned or controlled by any one person, company, group, or organization. Nevertheless, some standardization and control is necessary for the system to function. The liberal Request for Comments (RFC) publication procedure engendered confusion about the Internet standardization process, and led to more formalization of official accepted standards. The IETF started in January of 1986 as a quarterly meeting of U.S. government funded researchers. Representatives from non-government vendors were invited starting with the fourth IETF meeting in October of that year. Acceptance of an RFC by the RFC Editor for publication does not automatically make the RFC into a standard. It may be recognized as such by the IETF only after experimentation, use, and acceptance have proved it to be worthy of that designation. Official standards are numbered with a prefix "STD" and a number, similar to the RFC
  • naming style. However, even after becoming a standard, most are still commonly referred to by their RFC number. In 1992, the Internet Society, a professional membership society, was formed and the IETF was transferred to operation under it as an independent international standards body. NIC, INTERNIC, IANA AND ICANN The first central authority to coordinate the operation of the network was the Network Information Centre (NIC) at Stanford Research Institute (SRI) in Menlo Park, California. In 1972, management of these issues was given to the newly created Internet Assigned Numbers Authority (IANA). In addition to his role as the RFC Editor, Jon Postel worked as the manager of IANA until his death in 1998. As the early ARPANET grew, hosts were referred to by names, and a HOSTS.TXT file would be distributed from SRI International to each host on the network. As the network grew, this became cumbersome. A technical solution came in the form of the Domain Name System, created by Paul Mockapetris. The Defense Data Network—Network Information Center (DDN-NIC) at SRI handled all registration services, including the top-level domains (tlds) of .mil, .gov, .edu, .org, .net, .com and .us, root nameserver administration and Internet number assignments under a United States Department of Defense contract. In 1991, the Defense Information Systems Agency (DISA) awarded the administration and maintenance of DDN-NIC (managed by SRI up until this point) to Government Systems, Inc., who subcontracted it to the small private- sector Network Solutions, Inc. Since at this point in history most of the growth on the Internet was coming from non-military sources, it was decided that the Department of Defense would no longer fund registration services outside of the .mil TLD. In 1993 the U.S. National Science Foundation, after a competitive bidding process in 1992, created the internic to manage the allocations of addresses and management of the address databases, and awarded the contract to three organizations. Registration Services would be provided by Network Solutions; Directory and Database Services would be provided by AT&T; and Information Services would be provided by General Atomics.
  • In 1998 both IANA and internic were reorganized under the control of ICANN, a California non-profit corporation contracted by the US Department of Commerce to manage a number of Internet-related tasks. The role of operating the DNS system was privatized and opened up to competition, while the central management of name allocations would be awarded on a contract tender basis. USE AND CULTURE Email and Usenet—The growth of the text forum E-mail is often called the killer application of the Internet. However, it actually predates the Internet and was a crucial tool in creating it. E-mail started in 1965 as a way for multiple users of a time-sharing mainframe computer to communicate. Although the history is unclear, among the first systems to have such a facility were SDC's Q32 and MIT's CTSS. The ARPANET computer network made a large contribution to the evolution of e-mail. There is one report indicating experimental inter-system e-mail transfers on it shortly after ARPANET's creation. In 1971 Ray Tomlinson created what was to become the standard Internet e-mail address format, using the @ sign to separate user names from host names. A number of protocols were developed to deliver e-mail among groups of time- sharing computers over alternative transmission systems, such as UUCP and IBM's VNET e-mail system. E-mail could be passed this way between a number of networks, including ARPANET, BITNET and nsfnet, as well as to hosts connected directly to other sites via UUCP. In addition, UUCP allowed the publication of text files that could be read by many others. The News software developed by Steve Daniel and Tom Truscott in 1979 was used to distribute news and bulletin board-like messages. This quickly grew into discussion groups, known as newsgroups, on a wide range of topics. On ARPANET and nsfnet similar discussion groups would form via mailing lists, discussing both technical issues and more culturally focused topics (such as science fiction, discussed on the sflovers mailing list).
  • A WORLD LIBRARY—FROM GOPHER TO THE WWW As the Internet grew through the 1980s and early 1990s, many people realized the increasing need to be able to find and organize files and information. Projects such as Gopher, WAIS, and the FTP Archive list attempted to create ways to organize distributed data. Unfortunately, these projects fell short in being able to accommodate all the existing data types and in being able to grow without bottlenecks. One of the most promising user interface paradigms during this period was hypertext. The technology had been inspired by Vannevar Bush's "memex" and developed through Ted Nelson's research on Project Xanadu and Douglas Engelbart's research on NLS. Many small self-contained hypertext systems had been created before, such as Apple Computer's hypercard. In 1991, Tim Berners-Lee was the first to develop a network-based implementation of the hypertext concept. This was after Berners-Lee had repeatedly proposed his idea to the hypertext and Internet communities at various conferences to no avail—no one would implement it for him. Working at CERN, Berners-Lee wanted a way to share information about their research. By releasing his implementation to public use, he ensured the technology would become widespread. Subsequently, Gopher became the first commonly-used hypertext interface to the Internet. While Gopher menu items were examples of hypertext, they were not commonly perceived in that way. One early popular web browser, modeled after hypercard, was violawww. Scholars generally agree, however, that the turning point for the World Wide Web began with the introduction of the Mosaic (web browser)] in 1993, a graphical browser developed by a team at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign (NCSA-UIUC), led by Marc Andreessen. Funding for Mosaic came from the High-Performance Computing and Communications Initiative, a funding program initiated by then-Senator Al Gore's High Performance Computing and Communication Act of 1991 also known as the Gore Bill. Indeed, Mosaic's graphical interface soon became more popular than Gopher, which at the time was primarily text-based, and the WWW became the preferred interface for accessing the Internet.
  • Mosaic was eventually superseded in 1994 by Andreessen's Netscape Navigator, which replaced Mosaic as the world's most popular browser. Competition from Internet Explorer and a variety of other browsers has almost completely displaced it. Another important event held on January 11, 1994, was the The Superhighway Summit at UCLA's Royce Hall. This was the "first public conference bringing together all of the major industry, government and academic leaders in the field [and] also began the national dialogue about the Information Superhighway and its implications." Finding what you need—The search engine Even before the World Wide Web, there were search engines that attempted to organize the Internet. The first of these was the Archie search engine from mcgill University in 1990, followed in 1991 by WAIS and Gopher. All three of those systems predated the invention of the World Wide Web but all continued to index the Web and the rest of the Internet for several years after the Web appeared. There are still Gopher servers as of 2006, although there are a great many more web servers. As the Web grew, search engines and Web directories were created to track pages on the Web and allow people to find things. The first full-text Web search engine was webcrawler in 1994. Before webcrawler, only Web page titles were searched. Another early search engine, Lycos, was created in 1993 as a university project, and was the first to achieve commercial success. During the late 1990s, both Web directories and Web search engines were popular—Yahoo! (founded 1995) and Altavista (founded 1995) were the respective industry leaders. By August 2001, the directory model had begun to give way to search engines, tracking the rise of Google (founded 1998), which had developed new approaches to relevancy ranking. Directory features, while still commonly available, became after- thoughts to search engines. Database size, which had been a significant marketing feature through the early 2000s, was similarly displaced by emphasis on relevancy ranking, the methods by which search engines attempt to sort the best results first. Relevancy ranking first became a major issue circa 1996, when it became apparent that it was impractical to review full lists of results. Consequently, algorithms for relevancy ranking have continuously improved. Google's pagerank method for ordering the results has received the most press,
  • but all major search engines continually refine their ranking methodologies with a view toward improving the ordering of results. As of 2006, search engine rankings are more important than ever, so much so that an industry has developed ("search engine optimizers", or "SEO") to help web-developers improve their search ranking, and an entire body of case law has developed around matters that affect search engine rankings, such as use of trademarks in metatags. The sale of search rankings by some search engines has also created controversy among librarians and consumer advocates. THE DOT-COM BUBBLE The suddenly low price of reaching millions worldwide, and the possibility of selling to or hearing from those people at the same moment when they were reached, promised to overturn established business dogma in advertising, mail-order sales, customer relationship management, and many more areas. The web was a new killer app —it could bring together unrelated buyers and sellers in seamless and low-cost ways. Visionaries around the world developed new business models, and ran to their nearest venture capitalist. Of course a proportion of the new entrepreneurs were truly talented at business administration, sales, and growth; but the majority were just people with ideas, and didn't manage the capital influx prudently. Additionally, many dot-com business plans were predicated on the assumption that by using the Internet, they would bypass the distribution channels of existing businesses and therefore not have to compete with them; when the established businesses with strong existing brands developed their own Internet presence, these hopes were shattered, and the newcomers were left attempting to break into markets dominated by larger, more established businesses. Many did not have the ability to do so. The dot-com bubble burst on March 10, 2000, when the technology heavy NASDAQ Composite index peaked at 5048.62 (intra-day peak 5132.52), more than double its value just a year before. By 2001, the bubble's deflation was running full speed. A majority of the dot-coms had ceased trading, after having burnt through their venture capital, often without ever making a gross profit.
  • RECENT TRENDS The World Wide Web has led to a widespread culture of individual self publishing and co-operative publishing. The moment to moment accounts of blogs, photo publishing Flickr and the information store of Wikipedia are a result of the open ease of creating a public website. In addition, the communication capabilities of the internet are being realised with VOIP telephone services such as Skype, Vonage, or viatalk. Increasingly complex on-demand content provision have led to the delivery of all forms of media, including those that had been found in the traditional media forms of newspapers, radio, television and movies, via the Internet. The Internet's peer-to-peer structure has also influenced social and economic theory, most notably with the rise of file sharing. HYPER TEXT TRANSFER PROTOCOL HTTP is a method used to transfer or convey information on the World Wide Web. Its original purpose was to provide a way to publish and retrieve HTML pages. Development of HTTP was coordinated by the World Wide Web Consortium and the Internet Engineering Task Force, culminating in the publication of a series of RFCs, most notably RFC 2616 (1999), which defines HTTP/1.1, the version of HTTP in common use today. HTTP is a request/response protocol between clients and servers. The originating client, such as a web browser, spider, or other end-user tool, is referred to as the user agent. The destination server, which stores or creates resources such as HTML files and images, is called the origin server. In between the user agent and origin server may be several intermediaries, such as proxies, gateways, and tunnels. An HTTP client initiates a request by establishing a Transmission Control Protocol (TCP) connection to a particular port on a remote host (port 80 by default; see
  • List of TCP and UDP port numbers). An HTTP server listening on that port waits for the client to send a request message. Upon receiving the request, the server sends back a status line, such as "HTTP/1.1 200 OK", and a message of its own, the body of which is perhaps the requested file, an error message, or some other information. REQUEST MESSAGE The request message consists of the following: Request line, such as GET /images/logo.gif HTTP/1.1, which requests the file logo.gif from the /images directory Headers, such as Accept-Language: en An empty line An optional message body The request line and headers must all end with CRLF (i.e. a carriage return followed by a line feed). The empty line must consist of only CRLF and no other whitespace. In the HTTP/1.1 protocol, all headers except Host are optional. REQUEST METHODS HTTP defines eight methods (sometimes referred to as "verbs") indicating the desired action to be performed on the identified resource. HEAD Asks for the response identical to the one that would correspond to a GET request, but without the response body. This is useful for retrieving meta-information written in response headers, without having to transport the entire content. GET Requests a representation of the specified resource. By far the most common method used on the Web today. Should not be used for operations that cause side-effects (using it for actions in web applications is a common mis-use). See 'safe methods' below. POST
  • Submits data to be processed (e.g. from an HTML form) to the identified resource. The data is included in the body of the request. This may result in the creation of a new resource or the updates of existing resources or both. PUT Uploads a representation of the specified resource. DELETE Deletes the specified resource. TRACE Echoes back the received request, so that a client can see what intermediate servers are adding or changing in the request. OPTIONS Returns the HTTP methods that the server supports. This can be used to check the functionality of a web server. CONNECT For use with a proxy that can change to being an SSL tunnel. HTTP servers are supposed to implement at least the GET and HEAD methods and, whenever possible, also the OPTIONS method. SAFE METHODS Some methods (e.g. HEAD or GET) are defined as safe, which means they are intended only for information retrieval and should not change the state of the server (in other words, they should not have side effects). Unsafe methods (such as POST, PUT and DELETE) should be displayed to the user in a special way, typically as buttons rather
  • than links, thus making the user aware of possible obligations (such as a button that causes a financial transaction). Despite the required safety of GET requests, in practice they can cause changes on the server. For example, an HTML page may use a simple hyperlink to initiate deletion of a domain database record, thus causing a change of the server's state as a side- effect of a GET request. This is discouraged, because it can cause problems for Web caching, search engines and other automated agents, who can make unintended changes on the server. Another case is that a GET request may cause the server to create a cache space. This is perfectly fine because the effect is not visible to the client and the client is not responsible for the effect. OBLIGATED METHODS Methods (eg. POST, PUT, or DELETE) that are intended to cause "real-world" effects are defined as Obligated because the client that initiates the request is responsible for such effects. IDEMPOTENT METHODS Methods GET, HEAD, PUT and DELETE are defined to be idempotent, meaning that multiple identical requests should have the same effect as a single request. Methods OPTIONS and TRACE, being safe, are inherently idempotent. HTTP VERSIONS HTTP has evolved into multiple, mostly backwards-compatible protocol versions. RFC 2145 describes the use of HTTP version numbers. Basically, the client tells in the beginning of the request the version it uses, and the server uses the same or earlier version in the response.
  • Only supports one command, GET — which does not specify the HTTP version. Does not support headers. Since this version does not support POST, the client can't pass much information to the server. HTTP/1.0 (May 1996) This is the first protocol revision to specify its version in communications and is still in wide use, especially by proxy servers. HTTP/1.1 (June 1999) Current version; persistent connections enabled by default and works well with proxies. Also supports request pipelining, allowing multiple requests to be sent at the same time, allowing the server to prepare for the workload and potentially transfer the requested resources more quickly to the client. HTTP/1.2 The initial 1995 working drafts of PEP — an Extension Mechanism for HTTP prepared by W3C and submitted to IETF were aiming to become a distinguishing feature of HTTP/1.2. In later PEP working drafts however the reference to HTTP/1.2 was removed. PEP later became subsumed by the experimental RFC 2774 — HTTP Extension Framework. STATUS CODES In HTTP/1.0 and since, the first line of the HTTP response is called the status line and includes a numeric status code (such as "404") and a textual reason phrase (such as "Not Found"). The way the user agent handles the response primarily depends on the code and secondarily on the response headers. Custom status codes can be used since, if the user agent encounters a code it does not recognize, it can use the first digit of the code to determine the general class of the response. Also, the standard reason phrases are only recommendations and can be replaced with "local equivalents" at the web developer's discretion. If the status code indicated a
  • problem, the user agent might display the reason phrase to the user to provide further information about the nature of the problem. The standard also allows the user agent to attempt to interpret the reason phrase, though this might be unwise since the standard explicitly specifies that status codes are machine-readable and reason phrases are human- readable. PERSISTENT CONNECTIONS In HTTP/0.9 and 1.0, the connection is closed after a single request/response pair. In HTTP/1.1 a keep-alive-mechanism was introduced, where a connection could be reused for more than one request. Such persistent connections improve lag perceptably, because the client does not need to re-negotiate the TCP connection after the first request has been sent. Version 1.1 of the protocol also introduced chunked encoding to allow content on persistent connections to be streamed, rather than buffered, and HTTP pipelining, which allows clients to send some types of requests before the previous response has been received, further reducing lag. HTTP SESSION STATE HTTP can occasionally pose problems for Web developers (Web Applications), because HTTP is stateless. The advantage of a stateless protocol is that hosts don't need to retain information about users between requests, but this forces the use of alternative methods for maintaining users' state, for example, when a host would like to customize content for a user who has visited before. The common method for solving this problem involves the use of sending and requesting cookies. Other methods include server side sessions, hidden variables (when current page is a form), and URL encoded parameters (such as index.php?userid=3). SECURE HTTP
  • There are currently two methods of establishing a secure HTTP connection: the https URI scheme and the HTTP 1.1 Upgrade header. The https URI scheme has been deprecated by RFC 2817, which introduced the Upgrade header; however, as browser support for the Upgrade header is nearly non-existent, the https URI scheme is still the dominant method of establishing a secure HTTP connection. HTTP 1.1 UPGRADE HEADER HTTP 1.1 introduced support for the Upgrade header. In the exchange, the client begins by making a clear-text request, which is later upgraded to TLS. Either the client or the server may request (or demand) that the connection be upgraded. The most common usage is a clear-text request by the client followed by a server demand to upgrade the connection, which looks like this: Client: GET /encrypted-area HTTP/1.1 Host: www.example.com Server: HTTP/1.1 426 Upgrade Required Upgrade: TLS/1.0, HTTP/1.1 Connection: Upgrade The server returns a 426 status-code because 400 level codes indicate a client failure (see List of HTTP status codes), which correctly alerts legacy clients that the failure was client-related. The benefits of using this method for establishing a secure connection are: that it removes messy and problematic redirection and URL rewriting on the server side, and it reduces user confusion by providing a single way to access a particular resource. SAMPLE
  • Below is a sample conversation between an HTTP client and an HTTP server running on www.example.com, port 80. Client request (followed by a blank line, so that request ends with a double newline, each in the form of a carriage return followed by a line feed): GET /index.html HTTP/1.1 Host: www.example.com The "Host" header distinguishes between various DNS names sharing a single IP address, allowing name-based virtual hosting. While optional in HTTP/1.0, it is mandatory in HTTP/1.1. Server response (followed by a blank line and text of the requested page): HTTP/1.1 200 OK Date: Mon, 23 May 2005 22:38:34 GMT Server: Apache/1.3.27 (Unix) (Red-Hat/Linux) Last-Modified: Wed, 08 Jan 2003 23:11:55 GMT Etag: "3f80f-1b6-3e1cb03b" Accept-Ranges: bytes Content-Length: 438 Connection: close Content-Type: text/html; charset=UTF-8 INTERNET ADDRESSING An IP address (Internet Protocol address) is a unique address that certain electronic devices use in order to identify and communicate with each other on a computer network utilizing the Internet Protocol standard (IP)—in simpler terms, a computer address. Any participating network device—including routers, computers,
  • time-servers, printers, Internet fax machines, and some telephones—can have their own unique address. Also, many people can find personal information through IP addresses. An IP address can also be thought of as the equivalent of a street address or a phone number (compare: VoIP (voice over (the) internet protocol)) for a computer or other network device on the Internet. Just as each street address and phone number uniquely identifies a building or telephone, an IP address can uniquely identify a specific computer or other network device on a network. An IP address can appear to be shared by multiple client devices either because they are part of a shared hosting web server environment or because a proxy server (e.g., an ISP or anonymizer service) acts as an intermediary agent on behalf of its customers, in which case the real originating IP addresses might be hidden from the server receiving a request. The analogy to telephone systems would be the use of predial numbers (proxy) and extensions (shared). IP addresses are managed and created by the Internet Assigned Numbers Authority. IANA generally allocates super-blocks to Regional Internet Registries, who in turn allocate smaller blocks to Internet service providers and enterprises. IP VERSIONS The Internet Protocol has two primary versions in use. Each version has its own definition of an IP address. Because of its prevalence, "IP address" typically refers to those defined by IPv4. IP VERSION 4 IPv4 uses 32-bit (4 byte) addresses, which limits the address space to 4,294,967,296 (232) possible unique addresses. However, many are reserved for special purposes, such as private networks (~18 million addresses) or multicast addresses (~1 million addresses). This reduces the number of addresses that can be allocated as public Internet addresses, and as the number of addresses available is consumed, an IPv4 address shortage appears to be inevitable in the long run. This limitation has helped
  • stimulate the push towards IPv6, which is currently in the early stages of deployment and is currently the only contender to replace IPv4. Example: 127.0.0.1
  • IP VERSION 5 What would be considered IPv5 existed only as an experimental non-IP real time streaming protocol called ST2, described in RFC 1819. In keeping with standard UNIX release conventions, all odd-numbered versions are considered experimental, and this version was never intended to be implemented, thus not abandoned. RSVP has replaced it to some degree. IP VERSION 6 In IPv6, the new (but not yet widely deployed) standard protocol for the Internet, addresses are 128 bits wide, which, even with a generous assignment of netblocks, will more than suffice for the foreseeable future. In theory, there would be exactly 2128, or about 3.403 × 1038 unique host interface addresses. The exact number is: 340,282,366,920,938,463,463,374,607,431,768,211,456 This large address space will be sparsely populated, which makes it possible to again encode more routing information into the addresses themselves. This enormous magnitude of available IPs will be sufficiently large for the indefinite future, even though mobile phones, cars and all types of personal devices are coming to rely on the Internet for everyday purposes. Example: 2001:0db8:85a3:08d3:1319:8a2e:0370:7334 IP VERSION 6 PRIVATE ADDRESSES Just as there are addresses for private, or internal networks in IPv4 (one example being the 192.168.0.1 - 192.168.0.254 range), there are blocks of addresses set aside in IPv6 for private addresses. Addresses starting with FE80: are called link-local addresses and are routable only on your local link area. Which means if you have several hosts connected to each other through a hub or switch then they would talk to each with their link-local IPv6 address. There was going to be an address range used for "private" addressing but that has changed and IPv6 won't include private addressing anymore. The prefix that was used for that is the FEC0: range. There are still some of these addresses in use now but no new addresses in this range will be used. These are called site-local
  • addresses and are routable within a particular site just like IPv4 private addresses. Neither of these address ranges are routable over the internet. With IPV6, virtually every device in the world can have an IP address: cars, refrigerators, lawnmowers and so on. If one's refrigerator stopped working, for example, a repair specialist could identify the problem without ever visiting in person. It might even be possible to make repairs from abroad, depending on the severity of the problem. NEWSGROUP A newsgroup is a repository usually within the Usenet system, for messages posted from many users at different locations. The term is somewhat confusing, because it is usually a discussion group. Newsgroups are technically distinct from, but functionally similar to, discussion forums on the World Wide Web. Newsreader software is used to read newsgroups. Hierarchies Newsgroups are often arranged into hierarchies, theoretically making it simpler to find related groups. The term top-level hierarchy refers to the hierarchy defined by the prefix prior to the first dot. The most commonly known hierarchies are the usenet hierarchies. So for instance newsgroup rec.arts.sf.starwars.games would be in the rec.* top-level usenet hierarchy, where the asterisk (*) is defined as a wildcard character. There were seven original major hierarchies of usenet newsgroups, known as the "Big 7": comp.* — Discussion of computer-related topics news.* — Discussion of Usenet itself sci.* — Discussion of scientific subjects rec.* — Discussion of recreational activities (e.g. games and hobbies) soc.* — Socialising and discussion of social issues. talk.* — Discussion of contentious issues such as religion and politics. misc.* — Miscellaneous discussion—anything which doesn't fit in the other hierarchies.
  • These were all created in the Great Renaming of 1986–1987, prior to which all of these newsgroups were in the net.* hierarchy. At that time there was a great controversy over what newsgroups should be allowed. Among those that the usenet cabal (who effectively ran the Big 7 at the time) did not allow were those concerning recipes, drugs, and sex. This resulted in the creation of an alt.* (short for "alternative") usenet hierarchy where these groups would be allowed. Over time the laxness of rules on newsgroup creation in alt.* compared to the Big 7 meant that many new topics could, given time, gain enough popularity to get a Big 7 newsgroup. This resulted in a rapid growth of alt.* which continues to this day. Due to the anarchistic nature with which the groups sprung up, some jokingly referred to ALT standing for "Anarchists, Lunatics and Terrorists". In 1995, humanities.* was created for the discussion of the humanities (e.g. literature, philosophy), and the Big 7 became the Big 8.
  • The alt.* hierarchy has discussion of all kinds of topics, and many hierarchies for discussion specific to a particular geographical area or in a language other than English. Before a new Big 8 newsgroup can be created, an RFD (Request For Discussion) must be posted into the newsgroup news.announce.newgroups, which is then discussed in news.groups.proposals. Once the proposal has been formalized with a name, description, charter, the Big-8 Management Board will vote on whether to create the group. If the proposal is approved by the Big-8 Management Board, the group is created. Groups are removed in a similar manner. Creating a new group in the alt.* hierarchy is not subject to the same rules; anybody can create a newsgroup, and anybody can remove them, but most news administrators will ignore these requests unless a local user requests the group by name. FURTHER HIERARCHIES There are a number of newsgroup hierarchies outside of the Big 8 (& ALT), that can be found at many news servers. These include non-English language groups, groups managed by companies or organizations about their products, geographic/local hierarchies, and even non-internet network boards routed into NNTP. Examples include (alphabetic): ba.* — Discussion in the San Francisco Bay Area ca.* — Discussion in California can.* — Canadian news groups cn.* — Chinese news groups de.* — Discussions in German england.* — Discussions (mostly) local to England, see also uk.* fidonet.* — Discussions routed from FidoNet fr.* — Discussions in French fj.* — "From Japan," discussions in Japanese gnu.* — Discussions about GNU software hawaii.* — Discussions (mostly) local to Hawaii harvard.* — Discussions (mostly) local to Harvard hp.* — Hewlett-Packard internal news groups microsoft.* — Discussions about Microsoft products
  • tw.* — Taiwan news groups uk.* — Discussions on matters in the UK Additionally, there is the free.* hierarchy, which can be considered "more alt than alt.*". There are many local sub-hierarchies within this hierarchy, usually for specific countries or cultures (such as free.it.* for Italy). TYPES OF NEWSGROUPS Typically, a newsgroup is focused on a particular topic such as "pigeon hunting". Some newsgroups allow the posting of messages on a wide variety of themes, regarding anything a member chooses to discuss as on-topic, while others keep more strictly to their particular subject, frowning on off-topic postings. The news admin (the administrator of a news server) decides how long articles are kept before being expired (deleted from the server). Usually they will be kept for one or two weeks, but some admins keep articles in local or technical newsgroups around longer than articles in other newsgroups. Newsgroups generally come in either of two types, binary or text. There is no technical difference between the two, but the naming differentiation allows users and servers with limited facilities the ability to minimize network bandwidth usage. Generally, Usenet conventions and rules are enacted with the primary intention of minimizing the overall amount of network traffic and resource usage. Newsgroups are much like the public message boards on old bulletin board systems. For those readers not familiar with this concept, envision an electronic version of the corkboard in the entrance of your local grocery store. Newsgroups frequently become cliquish and are subject to sporadic flame wars and trolling, but they can also be a valuable source of information, support and friendship, bringing people who are interested in specific subjects together from around the world. There are currently well over 100,000 Usenet newsgroups, but only 20,000 or so of those are active. Newsgroups vary in popularity, with some newsgroups only getting a few posts a month while others get several hundred (and in a few cases several thousand) messages a day. Weblogs have replaced some of the uses of newsgroups (especially because, for a while, they were less prone to spamming).
  • A website called DejaNews began archiving Usenet in the 1990s. DejaNews also provided a searchable web interface. Google bought the archive from them and made efforts to buy other Usenet archives to attempt to create a complete archive of Usenet newsgroups and postings from its early beginnings. Like DejaNews, Google has a web search interface to the archive, but Google also allows newsgroup posting. Non-Usenet newsgroups are possible and do occur, as private individuals or organizations set up their own nntp servers. Examples include the newsgroups Microsoft run to allow peer-to-peer support of their products and those at news://news.grc.com. HOW NEWSGROUPS WORK Newsgroup servers are hosted by various organizations and institutions. Most Internet Service Providers host their own News Server, or rent access to one, for their subscribers. There are also a number of companies who sell access to premium news servers. Every host of a news server maintains agreements with other news servers to regularly synchronize. In this way news servers form a network. When a user posts to one news server, the message is stored locally. That server then shares the message with the servers that are connected to it if both carry the newsgroup, and from those servers to servers that they are connected to, and so on. For newsgroups that are not widely carried, sometimes a carrier group is used as a crosspost to aid distribution. This is typically only useful for groups that have been removed or newer alt.* groups. Crossposts between hierarchies, outside of the big eight and alt, are prone to failure.
  • BINARY NEWSGROUPS While Newsgroups were not created with the intention of distributing binary files, they have proven to be quite effective for this. Due to the way they work, a file uploaded once will be spread and can then be downloaded by an unlimited number of users. More useful is the fact that every user is drawing on the bandwidth of their own news server. This means that unlike P2P technology, the user's download speed is under their own control, as opposed to under the willingness of other people to share files. In fact this is another benefit of Newsgroups: it is usually not expected that users share. If every user makes uploads then the servers would be flooded; thus it is acceptable and often encouraged for users to just leech. There were originally a number of obstacles to the transmission of binary files over Usenet. Firstly, Usenet was designed with the transmission of text in mind. Due to this, for a long period of time, it was impossible to send binary data as it was. So, a workaround, Uuencode (and later on Base64 and yEnc), was developed which mapped the binary data from the files to be transmitted (e.g. sound or video files) to text characters which would survive transmission over Usenet. At the receiver's end, the data needed to be decoded by the user's news client. Additionally, there was a limit on the size of individual posts such that large files could not be sent as single posts. To get around this, Newsreaders were developed which were able to split long files into several posts. Intelligent newsreaders at the other end could then automatically group such split files into single files, allowing the user to easily retrieve the file. These advances have meant that Usenet is used to send and receive many Gigabytes of files per day. There are two main issues that pose problems for transmitting binary files over Newsgroups. The first is completion rates and the other is Retention Rates. The business of premium News Servers is generated primarily on their ability to offer superior Completion and Retention Rates, as well as their ability to offer very fast connections to users. Completion rates are significant when users wish to download large files that are split into pieces; if any one piece is missing, it is impossible to successfully download and reassemble the desired file. To work around this, a redundancy scheme known as PAR is commonly used.
  • A number of websites exist for the purpose of keeping an index of the files posted to binary Newsgroups. MODERATED NEWSGROUPS A moderated newsgroup has one or more individuals who must approve articles before they are posted at large. A separate address is used for the submission of posts and the moderators then propagate posts which are approved for the readership. The first moderated newsgroups appeared in 1984 under mod.* according to RFC 2235, "Hobbes' Internet Timeline" TRANSMISSION CONTROL PROTOCOL The Internet protocol suite is the set of communications protocols that implements the protocol stack on which the Internet and many commercial networks run. It is part of the TCP/IP protocol suite, which is named after two of the most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were also the first two networking protocols defined. A review of TCP/IP is given under that heading. Note that todays TCP/IP networking represents a synthesis of two developments that began in the 1970's, namely LAN's (Local Area Networks) and the Internet, that revolutionalised computing. The Internet protocol suite — like many protocol suites — can be viewed as a set of layers. Each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the upper layer protocols based on using services from some lower layers. Upper layers are logically closer to the user and deal with more abstract data, relying on lower layer protocols to translate data into forms that can eventually be physically transmitted. The original TCP/IP reference model consisted of four layers, but has evolved into a five-layer model. The OSI model describes a fixed, seven-layer stack for networking protocols. Comparisons between the OSI model and TCP/IP can give further insight into the significance of the components of the IP suite. The OSI model with its increased numbers of layers provides for more flexibility. Both the OSI and the TCP/IP models are
  • 'standards' and application developers will often implement solutions without strict adherence to proposed 'division' of labour within the standard whilst providing for functionality within the application suite. This separation of 'practice' from theory often leads to confusion. HISTORY The Internet protocol suite came from work done by DARPA in the early 1970s. After building the pioneering ARPANET, DARPA started work on a number of other data transmission technologies. In 1972, Robert E. Kahn was hired at the DARPA Information Processing Technology Office, where he worked on both satellite packet networks and ground-based radio packet networks, and recognized the value of being able to communicate across them. In the spring of 1973, Vinton Cerf, the developer of the existing ARPANET Network Control Program (NCP) protocol, joined Kahn to work on open-architecture interconnection models with the goal of designing the next protocol for the ARPANET. By the summer of 1973, Kahn and Cerf 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", and it has in fact been implemented using homing pigeons.) 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 then contracted with BBN Technologies, Stanford University, and the University College London to develop operational versions of the protocol on different hardware platforms. Four versions were developed: TCP v1, TCP v2, a split into TCP v3 and IP v3 in the spring of 1978, and then stability with TCP/IP v4 — the standard protocol still in use on the Internet today. In 1975, a two-network TCP/IP communications test was performed between Stanford and University College London (UCL). In November, 1977, a three-network TCP/IP test was conducted between the U.S., UK, and Norway. Between 1978 and 1983, several other TCP/IP prototypes were developed at multiple research centres. A full switchover to TCP/IP on the ARPANET took place January 1, 1983.[1] In March 1982,[2] the US Department of Defense made TCP/IP the standard for all military computer networking. In 1985, the Internet Architecture Board held a three day workshop on TCP/IP for the computer industry, attended by 250 vendor representatives, helping popularize the protocol and leading to its increasing commercial use. On November 9, 2005 Kahn and Cerf were presented with the Presidential Medal of Freedom for their contribution to American culture.[3] Layers in the Internet protocol suite stack
  • IP suite stack showing the physical network connection of two hosts via two routers and the corresponding layers used at each hop Sample encapsulation of data within a UDP datagram within an IP packet The IP suite uses encapsulation to provide abstraction of protocols and services. Generally a protocol at a higher level uses a protocol at a lower level to help accomplish its aims. The Internet protocol stack can be roughly fitted to the four layers of the original TCP/IP model:
  • DNS, TFTP, TLS/SSL, FTP, HTTP, IMAP, IRC, NNTP, POP3, SIP, SMTP, SNMP, SSH, TELNET, ECHO, BitTorrent, RTP, PNRP, rlogin, ENRP, … 4. Application Routing protocols like BGP, which for a variety of reasons run over TCP, may also be considered part of the application or network layer. 3. Transport TCP, UDP, DCCP, SCTP, IL, RUDP, … Routing protocols like OSPF, which run over IP, are also to be considered part of the network layer, as they provide path selection. ICMP and IGMP run over IP are considered part of the 2. Internet network layer, as they provide control information. IP (IPv4, IPv6) ARP and RARP operate underneath IP but above the link layer so they belong somewhere in between. Ethernet, Wi-Fi, token ring, PPP, SLIP, FDDI, ATM, Frame 1. Network access Relay, SMDS, … .. In many modern textbooks, this model has evolved into the seven layer OSI model, where the Network access layer is split into a Data link layer on top of a Physical layer, and the Internet layer is called Network layer.................................. IMPLEMENTATIONS Today, most commercial operating systems include and install the TCP/IP stack by default. For most users, there is no need to look for implementations. TCP/IP is included in all commercial Unix systems, Mac OS X, and all free-software Unix-like systems such as Linux distributions and BSD systems, as well as Microsoft Windows. Unique implementations include Lightweight TCP/IP, an open source stack designed for embedded systems and KA9Q NOS, a stack and associated protocols for amateur packet radio systems and personal computers connected via serial lines. TELNET
  • TELNET (TELetype NETwork) is a network protocol used on the Internet or local area network (LAN) connections. It was developed in 1969 and standardized as IETF STD 8, one of the first Internet standards. It has limitations that are considered to be security risks. The term telnet also refers to software which implements the client part of the protocol. TELNET clients have been available on most Unix systems for many years and are available for virtually all platforms. Most network equipment and OSs with a TCP/IP stack support some kind of TELNET service server for their remote configuration (including ones based on Windows NT). However with recent advancements SSH has become more dominant in remote access for Unix-based machines. "To telnet" is also used as a verb meaning to establish or use a TELNET or other TCP connection, as in, "To change your password, telnet to the server and run the passwd command". Most often, a user will be telneting to a unix-like server system or a simple network device such as a switch. For example, a user might "telnet in from home to check his mail at school". In doing so, he would be using a telnet client to connect from his computer to one of his servers. Once the connection is established, he would then log in with his account information and execute operating system commands remotely on that computer, such as ls or cd. On many systems, the client may also be used to make interactive raw-TCP sessions. PROTOCOL DETAILS TELNET is a client-server protocol, based on a reliable connection-oriented transport. Typically this is TCP port 23, although TELNET predates TCP/IP and was originally run on NCP. The protocol has many extensions, some of which have been adopted as Internet Standards. IETF standards STD 27 through STD 32 define various extensions, most of which are extremely common. Other extensions are on the IETF standards track as PROPOSED STANDARDS.
  • SECURITY When TELNET was initially developed in 1969, most users of networked computers were in the computer departments of academic institutions, or at large private and government research facilities. In this environment, security was not nearly as much of a concern as it became after the bandwidth explosion of the 1990s. The rise in the number of people with access to the Internet, and by extension, the number of people attempting to crack into other people's servers made encrypted alternatives much more necessary. Experts in computer security, such as SANS Institute, and the members of the comp.os.linux.security newsgroup recommend that the use of TELNET for remote logins should be discontinued under all normal circumstances, for the following reasons: TELNET, by default, does not encrypt any data sent over the connection (including passwords), and so it is trivial to eavesdrop on the communications and use the password later for malicious purposes; anybody who has access to a router, switch, or gateway located on the network between the two hosts where TELNET is being used can intercept the packets passing by and easily obtain login and password information (and whatever else is typed) with any of several common utilities like tcpdump and Wireshark. Most implementations of TELNET lack an authentication scheme that makes it possible to ensure that communication is carried out between the two desired hosts, and not intercepted in the middle. Commonly used TELNET daemons have several vulnerabilities discovered over the years. These security-related shortcomings have seen the usage of the TELNET protocol drop rapidly, especially on the public Internet, in favor of a more secure and functional protocol called SSH, first released in 1995. SSH provides all functionality of telnet, with the addition of strong encryption to prevent sensitive data such as passwords from being intercepted, and public key authentication, to ensure that the remote computer is actually who it claims to be. As has happened with other early Internet protocols, extensions to the TELNET protocol provide TLS security and SASL authentication that address the above issues.
  • However, most TELNET implementations do not support these extensions; and there has been relatively little interest in implementing these as SSH is adequate for most purposes. The main advantage of TLS-TELNET would be the ability to use certificate-authority signed server certificates to authenticate a server host to a client that does not yet have the server key stored. In SSH, there is a weakness in that the user must trust the first session to a host when it has not yet acquired the server key. CURRENT STATUS As of the mid-2000s, while the TELNET protocol itself has been mostly superseded, TELNET clients are still used, usually when diagnosing problems, to manually "talk" to other services without specialized client software. For example, it is sometimes used in debugging network services such as an SMTP or HTTP server, by serving as a simple way to send commands to the server and examine the responses. On UNIX, though, other software such as nc (netcat) or socat are finding greater favor with some system administrators for testing purposes, as they can be called with arguments to not send any terminal control handshaking data. TELNET is still very popular in enterprise networks to access host applications, i.e. on
  • IBM MAINFRAMES. TELNET is also heavily used for MUD games played over the Internet, as well as talkers, MUSH es, MUCKs and MOOes. By using image-to-ASCII algorithms, it can also be used for primitive "video" streaming. Recently, ASCII-WM offered live broadcasts of the 2006 World Cup. TELNET can also be used as a rudimentary IRC client if one knows the protocol well enough. TELNET CLIENTS WINDOWS AbsoluteTelnet is a client for all versions of Windows, and includes telnet, SSH1, and SSH2. Hyperterminal Private Edition is another Windows telnet client, free for personal use. TeraTerm is a free telnet/SSH client for Windows that offers more features than the built-in telnet as well as offering a free SSH plug-in. Windows comes with a built in telnet client, accessible from the command prompt. Note that in Windows Vista, as of RC1, the telnet client is not installed by default, and needs to be installed as an optional Windows component. MACINTOSH Tn3270 is a free TELNET client for Macintosh designed to work with IBM mainframe systems that use the TN3270 protocol. Terminal is a TELNET capable command line interface application that comes as part of all versions of Macintosh OS X. dataComet is a full-featured Telnet & SSH application for the Macintosh. MULTIPLATFORM PuTTY is a free SSH, TELNET, rlogin, and raw TCP client for Windows, Linux, and Unix.
  • mTelnet is a free full-screen TELNET client for Windows & OS/2. Easy to use client with Zmodem download capability. Twisted Conch includes a telnet client/server implementation. IVT is a free multisession TELNET client for Windows & DOS. Also supports SSH and Kerberos (not free). Includes useful features like auto-login and scripting. UNIX TO UNIX COPY UUCP stands for Unix to Unix CoPy. The term generally refers to a suite of computer programs and protocols allowing remote execution of commands and transfer of files, email and netnews between computers. Specifically, uucp is one of the programs in the suite; it provides a user interface for requesting file copy operations. The UUCP suite also includes uux (user interface for remote command execution), uucico (communication program), uustat (reports statistics on recent activity), uuxqt (execute commands sent from remote machines), and uuname (reports the uucp name of the local system). Although UUCP was originally developed on and is most closely associated with Unix, UUCP implementations exist for several other operating systems, including Microsoft's MS-DOS, Digital's VAX/VMS, and Mac OS. TECHNOLOGY UUCP can use several different types of physical connections and link-layer protocols, but was most commonly used over dial-up connections. Before the widespread availability of Internet connectivity, computers were only connected by smaller private networks within a company or organization. They were also often equipped with modems so they could be used remotely from character-mode terminals via dial-up lines. UUCP uses the computers' modems to dial out to other computers, establishing temporary, point-to-point links between them. Each system in a UUCP network has a list of neighbor systems, with phone numbers, login names and passwords, etc. When work (file transfer or command execution requests) is queued for a neighbor system, the uucico program typically calls that system to process the work. The uucico program can also poll its
  • neighbors periodically to check for work queued on their side; this permits neighbors without dial-out capability to participate. Today, UUCP is rarely used over dial-up links, but is occasionally used over TCP/IP. One example of the current use of UUCP is in the retail industry by Epicor|CRS Retail Solutions for transferring batch files between corporate and store systems via TCP and dial-up on SCO Unix, Red Hat Linux, and Microsoft Windows (with Cygwin). The number of systems involved, as of early 2006, ran between 1500 and 2000 sites across 60 enterprises. UUCP's longevity can be attributed to its low/zero cost, extensive logging, native failover to dialup, and persistent queue management. However, this technology is anticipated to be retired in favor of Windows-only alternatives. HISTORY UUCP was originally written at AT&T Bell Laboratories, by Mike Lesk, and early versions of UUCP are sometimes referred to as System V UUCP. The original UUCP was rewritten by AT&T researchers Peter Honeyman, David A. Nowitz, and Brian E. Redman and the rewrite is referred to as HDB or HoneyDanBer uucp which was later enhanced, bug fixed, and repackaged as BNU UUCP ("Basic Network Utilities"). All of these versions had security holes which allowed some of the original internet worms to remotely execute unexpected shell commands, which inspired Ian Lance Taylor to write a new version from scratch. Taylor UUCP was released under the GNU General Public License and became the most stable and bug free version. Taylor uucp also incorporates features of all previous versions of uucp, allowing it to communicate with any other version with the greatest level of compatibility and even use similar config file formats from other versions. One surviving feature of uucp is the chat file format, largely inherited by the expect software package. UUCP FOR MAIL ROUTING The uucp and uuxqt capabilities could be used to send e-mail between machines, with suitable mail user interface and delivery agent programs. A simple uucp mail address was formed from the adjacent machine name, an exclamation mark or bang, followed by the user name on the adjacent machine. For example, the address barbox! user would refer to user user on adjacent machine barbox.
  • Mail could furthermore be routed through the network, traversing any number of intermediate nodes before arriving at its destination. Initially, this had to be done by specifying the complete path, with a list of intermediate host names separated by bangs. For example, if machine barbox is not connected to the local machine, but it is known that barbox is connected to machine foovax which does communicate with the local machine, the appropriate address to send mail to would be foovax!barbox!user. User barbox!user might publish their UUCP email address in a form such as …! bigsite!foovax!barbox!user. This directs people to route their mail to machine bigsite (presumably a well-known and well-connected machine accessible to everybody) and from there through the machine foovax to the account of user user on barbox. Many users would suggest multiple routes from various large well-known sites, providing even better and perhaps faster connection service from the mail sender. Bang paths of eight to ten machines (or hops) were not uncommon in 1981, and late-night dial-up UUCP links would cause week-long transmission times. Bang paths were often selected by both transmission time and reliability, as messages would often get lost. Some hosts went so far as to try to "rewrite" the path, sending mail via "faster" routes — this practice tended to be frowned upon. The "pseudo-domain" ending .uucp was sometimes used to designate a hostname as being reachable by UUCP networking, although this was never formally in the Internet root as a top-level domain. UUCPNET AND MAPPING UUCPNET was the name for the totality of the network of computers connected through UUCP. This network was very informal, maintained in a spirit of mutual cooperation between systems owned by thousands of private companies, universities, and so on. Often, particularly in the private sector, UUCP links were established without official approval from the companies' upper management. The UUCP network was constantly changing as new systems and dial-up links were added, others were removed, etc. The UUCP Mapping Project was a volunteer, largely successful effort to build a map of the connections between machines that were open mail relays and establish a managed namespace. Each system administrator would submit, by e-mail, a list of the
  • systems to which theirs would connect, along with a ranking for each such connection. These submitted map entries were processed by an automatic program that combined them into a single set of files describing all connections in the network. These files were then published monthly in a newsgroup dedicated to this purpose. The UUCP map files could then be used by software such as "pathalias" to compute the best route path from one machine to another for mail, and to supply this route automatically. The UUCP maps also listed contact information for the sites, and so gave sites seeking to join UUCPNET an easy way to find prospective neighbors. CONNECTIONS WITH THE INTERNET Many uucp hosts, particularly those at universities, were also connected to the Internet in its early years, and e-mail gateways between Internet SMTP-based mail and UCP mail were developed. A user at a system with UUCP connections could thereby exchange mail with Internet users, and the Internet links could be used to bypass large portions of the slow UUCP network. A "UUCP zone" was defined within the Internet domain namespace to facilitate these interfaces. With this infrastructure in place, UUCP's strength was that it permitted a site to gain Internet e-mail connectivity with only a dial-up modem link to another, cooperating computer. This was at a time when true Internet access required a leased data line providing a connection to an Internet Point of Presence, both of which were expensive and difficult to arrange. By contrast, a link to the UUCP network could usually be established with a few phone calls to the administrators of prospective neighbor systems. Neighbor systems were often close enough to avoid all but the most basic charges for telephone calls. DECLINE UUCP usage began to die out with the rise of ISPs offering inexpensive SLIP and PPP services. The UUCP Mapping Project was formally shut down in late 2000. Usenet traffic was originally transmitted using the UUCP network, and bang paths are still in use within Usenet message format Path header lines. They now have only an informational purpose, and are not used for routing, although they can be used to ensure that loops do not occur. In general, this form of e-mail address has now been superseded by the SMTP "@ notation", even by sites still using uucp.
  • Currently UUCP is used mainly over high cost links (e.g., marine satellite links). UUCP over TCP/IP (preferably encrypted, such as via the SSH protocol) can be used when a computer doesn't have any fixed IP addresses but is still willing to run a standard mail transfer agent (MTA) like Sendmail or Postfix.