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.
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
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
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.
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
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
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
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-
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
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
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
Loss of visibility to search engines. Search engines may not be able to index the
text content of the application.
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 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
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
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
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
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
The first major client side language and technology available with the ability to
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.
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
communications, whiteboarding, outbound webcams, opacity compositing, socket
support, all of these are implemented in different ways in different browsers, so all-
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.
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
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 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 has initiated a Web Application Formats Working Group whose mission
includes the development of such standards .
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.
IAs could use XForms to enhance their functionality.
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
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
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.
 Client-side functionalities and development tools for IA needed
range of OS and webserver functionalities.
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
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. 
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
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
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
Fedora Directory Server
Sun Directory Services
COMPARISON WITH RELATIONAL DATABASES
There are a number of things that distinguish a traditional directory service from a
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
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
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
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
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, 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
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 .
OpenDS: A next generation and open source directory service, backed by Sun
Microsystems and hosted at .
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
184.108.40.206). 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 220.127.116.11) 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
Due to its long history of security issues, several alternative
nameserver/resolver programs have been written and distributed in
HOW THE DNS WORKS IN THEORY
Domain names, arranged in a tree, cut into zones, each served by a
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
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
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
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
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
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
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.
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
Note that the term "propagation", although very widely used, does
not describe the effects of caching well. Specifically, it implies that
 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
 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
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
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.
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
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
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
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.
RFC 882 Concepts and Facilities (Deprecated by RFC 1034)
RFC 883 Domain Names: Implementation specification (Deprecated by RFC
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
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
RFC 3403 Dynamic Delegation Discovery System (DDDS) (NAPTR records)
RFC 3696 Application Techniques for Checking and Transformation of
RFC 4398 Storing Certificates in the Domain Name System
RFC 4408 Sender Policy Framework (SPF) (SPF records)
 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
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
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 18.104.22.168, but a PTR
record maps 22.214.171.124.in-addr.arpa to its canonical name,
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
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
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
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.
 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
 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)
 Billing contact
The party whom a NIC invoices.
 Name servers
Namely the authoritative name servers that host the domain name zone of
a domain name.
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.
 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 (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
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.  It allowed multiple users to log into the IBM 7094  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  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 . The ARPANET
significantly increased the popularity of e-mail, and it became the killer app of the
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"
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 firstname.lastname@example.org. An Internet e-mail address is a string of the
form email@example.com, 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
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
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
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 ) 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
Cc: carbon copy
Received: Tracking information generated by mail servers that have previously handled a
Content-Type: Information about how the message has to be displayed, usually a MIME
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
.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
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
PRIVACY PROBLEMS REGARDING E-MAIL
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.
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.
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.
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
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).
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
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
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.
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
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
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:
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.
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:
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., ). 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
FTP OVER SSH
FTP over SSH refers to the practice of tunneling a normal FTP session over an
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
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
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."
Clinton: "Please prepare to have your assistant call my secretary at +1 402 555
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
"Please prepare to have your assistant send to my fax machine +1 402
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."
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
Server: 220 Hello, this is the Acme Mail
Client: USER MB1234
Server: 331 Password required to access user
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
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
Server: 226 Listing completed. That succeeded, so the data is
now sent over the established
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
Server: 226 Transfer completed. That succeeded, so the data is
now sent over the established
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.
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.
230 Logged in.
ftp> cd Bills
250 "/home/MB1234/Bills" is new working directory.
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)
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
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
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
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
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."
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:
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.
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
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!).
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.
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
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
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. "—
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
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
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
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
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
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
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.
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 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
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
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
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
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.
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
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
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.
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.
HTTP defines eight methods (sometimes referred to as "verbs") indicating the
desired action to be performed on the identified resource.
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.
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.
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.
Uploads a representation of the specified resource.
Deletes the specified resource.
Echoes back the received request, so that a client can see what intermediate
servers are adding or changing in the request.
Returns the HTTP methods that the server supports. This can be used to check the
functionality of a web server.
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.
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.
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.
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 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.
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
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-
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).
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:
GET /encrypted-area HTTP/1.1
HTTP/1.1 426 Upgrade Required
Upgrade: TLS/1.0, HTTP/1.1
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
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.
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
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
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
Content-Type: text/html; charset=UTF-8
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.
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.
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:
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.
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.
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.
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
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,
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.
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
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
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
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.
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.
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'
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
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.
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
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
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.
In March 1982, 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.
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
DNS, TFTP, TLS/SSL, FTP, HTTP, IMAP, IRC, NNTP, POP3,
SIP, SMTP, SNMP, SSH, TELNET, ECHO, BitTorrent, RTP,
PNRP, rlogin, ENRP, …
Routing protocols like BGP, which for a variety of reasons run
over TCP, may also be considered part of the application or
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..................................
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 (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
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
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
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
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
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
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.
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
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
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
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.
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.
PuTTY is a free SSH, TELNET, rlogin, and raw TCP client for Windows, Linux,
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
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.
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.
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,
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
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.